Volume 2 - Extractive Metallurgy of Lead and Zinc, …library.aimehq.org/library/books/Volume 2 -...

SECTION VII

UNIQUE PROCESSES FOR LEAD AND ZINC

Chapter 32

WAELZ TREATING OF COMPLEX ZINC-LEAD ORES,

KILN PRODUCTS LIMITED, BERG AUKAS, SOUTH WEST AFRICA.

Harry E. Cross Consu l t ing M e t a l l u r g i s t , Gold F i e l d s of South Af r i ca Limited,

Johannesburg, South Af r i ca .

and

Frank 0. Read Deputy Consul t ing M e t a l l u r g i s t , Gold F i e l d s of South A f r i c a Limited,

Johannesburg, South Af r i ca .

Abs t rac t

K i l n Products commissioned a Waelz k i l n 4 metres i n d iameter by 75 metres long, i n South West Af r i ca i n March, 1969, t o recover z i n c i n a form s u i t a b l e f o r the product ion of e l e c t r o l y t i c z inc . The raw m a t e r i a l c o n s i s t s of a mixture of z inc s i l i c a t e concen t ra tes and z inc r i c h t a i l i n g s purchased from an ad jacen t z inc mine - t h e mixture con- t a i n i n g 2% t o 25% zinc . The chemical composit ion of the ore i s t a b u l a t e d and the i n i t i a l t e s t r e s u l t s a r e descr ibed. Due t o the comparatively low temperature a t which the o re s t a r t s so f t en ing the k i l n has c e r t a i n unusual f e a t u r e s t o enab le the temperature t o be c o n t r o l l e d w i t h i n the necessary l i m i t s . A d e t a i l e d d e s c r i p t i o n of t h e k i l n , t h e s t a r t ing-up problems and p r e s e n t opera t ing cond i t ions and r e s u l t s a r e presented.

I n t r o d u c t i o n

The South West A f r i c a Company Limited have ope ra t ed a mine a t Berg Aukas, approximate ly 2 1 k i l o m e t r e s no r th -eas t of G r o o t f o n t e i n i n South West A f r i c a s i n c e 1959. Lead-zinc vanadate c o n c e n t r a t e s and l e a d and z i n c su lph ide and z i n c s i l i c a t e c o n c e n t r a t e s a r e produced by f r o t h f l o t a t i o n .

The z i n c s i l i c a t e ( w i l l e m i t e ) c o n c e n t r a t e produced is of h i g h grade bu t t h e percentage recovery i s low a s a n a p p r e c i a b l e amount of t h e z i n c s i l i c a t e i s i n t h e form of a f i n e s l ime not amenable t o f r o t h f l o t a t i o n .

An a s s o c i a t e d company, K i l n P roduc t s Limi ted , have now commissioned a Waelz k i l n on t h e same p r o p e r t y , t o r ecove r the z i n c from both t h e z i n c s i l i c a t e c o n c e n t r a t e and a n accumulated dump of z i n c bea r ing t a i l i n g s . The r e s u l t a n t z i n c oxide fume is t r a n s p o r t e d some 2 800 k i l o m e t r e s by road and r a i l , t o a n o t h e r a s s o c i a t e d company, Zinc Cor- p o r a t i o n Limi ted , s i t u a t e d n e a r Johannesburg, a s a source m a t e r i a l f o r t h e product ion of e l e c t r o l y t i c z inc .

D e s c r i p t i o n of o r e mined a t Berg Aukas

Zinc occur s i n t h e o r e mainly a s t h e m i n e r a l w i l l e m i t e 2Zn0.Si02. Minor q u a n t i t i e s a r e p r e s e n t i n t h e form of s p h a l e r i t e and d e s c l o i z i t e .

The gangue m a t e r i a l i s predominant ly dolomite .

Treatment of Ore

Approximately 13 000 t o n s of o r e a r e d e l i v e r e d t o t h e r e d u c t i o n p l a n t monthly. Approximately 1 000 t o n s of b a r r e n waste a r e d i sca rd - ed by manual s o r t i n g and t h e remaining 12 000 t o n s a r e crushed and m i l l e d i n b a l l m i l l s t o a g r i n d of approximate ly 7% minus 74 microns. A f t e r t h i c k e n i n g , a l e a d su lph ide c o n c e n t r a t e i s produced by f l o t a t i o n w i t h x a n t h a t e and A e r o f l o a t 25 , t h e z i n c b e i n g depressed by t h e addi - t i o n of z i n c s u l p h a t e and calcium cyanide . The z i n c i s then a c t i v a t e d by t h e a d d i t i o n of copper s u l p h a t e t o produce a z i n c su lph ide concen- t r a t e . The t a i l i n g s from t h i s f l o t a t i o n a r e cond i t ioned w i t h l ime and ca lc ium cyanide and a g a i n s u b j e c t e d t o f l o t a t i o n w i t h a mixture of amyl and e t h y l x a n t h a t e s and Aero f loa t 25, t o produce a l e a d vana- d a t e concen t r a t e .

The r e s u l t a n t t a i l i n g s , amounting t o some 10 000 t o n s p e r month a r e des l imed i n a 522 mm cyclone i n c l o s e d c i r c u i t w i t h a 13 m d ia- me te r t h i ckene r . The cyclone overflow product d i s c h a r g e s i n t o the th i ckene r . The f i n e s l ime overf lowing t h e t h i c k e n e r amounts t o about 2 500 t o n s p e r month c o n t a i n i n g about 18% z i n c and about 474 l ead . T h i s i s impounded i n a temporary s l i m e s dam f o r l a t e r t r ea tmen t i n

920 EXTRACTIVE METALLURGY OF LEAD AND ZINC

the Waelz k i l n .

The thickener underflow jo ins the feed t o the cyclone. The cyclone underflow i s subjected t o 2 s t a g e s of f l o t a t i o n t o produce a lead carbonate ( c e r u s s i t e ) concentra te and a z inc s i l i c a t e concentra te amounting t o about 3 000 tons per month.

For c e r u s s i t e f l o t a t i o n , sodium hydro-sulphide, i s used i n conjunc- t i o n with amyl xan tha te , Aeromine 3037 and T.E.B. f r o t h e r while wi l l emi te i s f l o a t e d with f u r t h e r a d d i t i o n s of the same reagents bu t excluding t h e xanthate .

The f l o t a t i o n t a i l i n g s containing about 6% zinc a r e discarded. The c e r u s s i t e concentra te i s smal l i n q u a n t i t y and is mixed wi th the lead sulphide concentra te .

A s z inc s i l i c a t e concentra tes a r e not popular wi th smel ters and f e t c h a much lower p r i c e per u n i t of contained z inc than do z inc sulphide concen t ra tes , i t was decided t o i n v e s t i g a t e the f e a s i b i l i t y of t r e a t i n g these concen t ra tes along with an e x i s t i n g t a i l i n g s dam i n a Waelz Ki ln t o produce a zinc oxide fume s u i t a b l e a s a feedstock f o r an e l e c t r o l y t i c z inc p lan t .

The Waelz Process

The Waelz process has i t s o r i g i n i n a process patented i n 1910 by Edward Dedolph of B r i t i s h Columbia. However, i t was not u n t i l 1923 t h a t Krupp Grusonwerk i n c o l l a b o r a t i o n with Meta l lgese l l schaf t A . G . s t a r t e d developing the process t o the s t age where i t could be app l ied on a commercial s c a l e f o r t h e f i r s t time a t Luenen i n Upper S i l e s i a i n 1925. I n 1957, according t o M . ~ o ~ a c z l , 28 Waelz Ki lns were opera t ing i n Poland t r e a t i n g over 3 000 tons of ore per day.

The process which has been w e l l descr ibed i n t h e l i t e r a t u r e 2, 3, 4 , 5 , 6 , 7 c o n s i s t s i n o u t l i n e of t r e a t i n g z inc bear ing m a t e r i a l s wi th coke o r a n t h r a c i t e i n a hor izon ta l r o t a r y k i l n a t temperatures ranging from 1 0 0 0 ° ~ t o 1500°C. The z inc i s reduced, v o l a t i l i z e d , oxidised t o zinc oxide and f i n a l l y separated from t h e exhaust gases by means of bag f i l t e r s o r e l e c t r o s t a t i c p r e c i p i t a t i o n .

I t i s commonly accepted t h a t the fol lowing r e a c t i o n s occur:-

ZnO + C --D C O + Zn (vapour) Zn (vapour) + C O + O2 4 ZnO + C02 C + o2 - C02 ZnS + CaO + C - Zn (vapour) + CaS + C O ZnS + FeO + C - Zn (Tapour) + FeS + C O 2 ( ~ n , Fe, M ~ ) O + Si02 - 2 ( ~ n , Fe, M ~ ) O . S ~ O 2 ( ~ n , Fe , M ~ ) o . s ~ o ~ + Fe Zn (vapour) + 2 ? F e , M n ) o . ~ i o ~ ZnO + A1203 - Zn0.A1203

9 . Zn0.A1203 + Fe + Zn (vapour) + Fe0.A120 l o . 3 ZnO.Fe2O + 3 C + 3 Zn0 + 2 Fe304 + 3 CO 11. ZnS + Fe?2 C U ) -+ Zn (vapour) + FeS ( C U ~ S ) 12. 2 ZnO.SiO2 + 2 C -+ 2 Zn (vapour) + SiO2 + 2 C O 13. ZnO + C O Zn (vapour) + C02 14. 2 ZnO.Si02 + 2 C O 2 Zn (vapour) + Si02 + 2 C02 15. C + C02 4 2 C O

F ig . 1

6 It i s obvious,however,from the work of Bodenstein , t h a t no chemical reac t ion takes place d i r e c t l y between s o l i d s a s shown i n equat ions 1, 3 or 4. Simultaneous reac t ions a c t u a l l y occur i n the gaseous phase a s shown i n equat ions 13 and 15 and by analogy 14 and 15. The progress of these p a r t i c u l a r reac t ions was c r i t i c a l l y examined by Truesdale and waring8 t o determine which reac t ion was r a t e con t ro l l ing . They concluded t h a t i n the temperature range 9000C t o 12000C reac t ion 13 proceeded r a t h e r more rap id ly than reac t ion 15 , while reac t ion 14, the reduc- t i o n of wil lemite by carbon monoxide, was by f a r the slowest. This i s i l l u s t r a t e d i n Fig.1 below.

P i l o t P lan t Waelz Ki ln T e s t s

A f t e r prel iminary l abora to ry s c a l e t e s t s , Messrs. F r ied Krupp Industr iebau were commissioned t o conduct a p i l o t p lan t s c a l e in- v e s t i g a t i o n i n t h e i r t e s t k i l n a t Rheinhausen near Duisburg i n West Germany. For t h i s purpose approximately 70 tons of c u r r e n t s l imes t a i l i n g s , 40 tons of m a t e r i a l reclaimed from the high grade dump, 2 tons of z inc s i l i c a t e concentra te and 30 tons of washed a n t h r a c i t e duff from the Vryheid a r e a i n Na ta l , were shipped t o t h e i r p i l o t p l a n t .

Chemical analyses of the z inc s i l i c a t e concen t ra te , reclaimed slime from a high grade dump, and c u r r e n t s l imes t a i l i n g s used f o r the t e s t a r e shown i n Table 1 below:-

Table 1. Chemical Analyses of Zinc Bearing Feed Mate r ia l s (%)

WAELZ TREATING OF COMPLEX ZINC-LEAD ORES 923

An a n a l y s i s of t h e a n t h r a c i t e duff used i s shown i n Table 11 below:-

Table 11. Analysis of Anthrac i t e Duff

Krupp's t e s t k i l n was 1.2 metres i n diameter by 14 metres long wi th a n e f f e c t i v e volume of 8.2 cubic metres. The k i l n was equipped wi th 6 thermocouples f i t t e d through t h e s h e l l and 2 s h e l l f a n s wi th s u i t - a b l e duct ing t o permit a d d i t i o n a l a i r t o be in t roduced a t any of 5 d i f f e r e n t po in t s . The i n c l i n a t i o n used was 2% and the speed of ro ta - t i o n could be va r ied from 0.5 t o 1 rev/min. The charge t o t h e k i l n c o n s i s t i n g of t h e z inc bear ing o r e , a n t h r a c i t e and recycled z inc oxide ( ' p r e o x i d e ' ) was homogeneously mixed i n batches i n a concrete mixer and the mois ture content ad jus ted t o about 1% before being f e d t o the k i l n .

Fixed Carbon ($) Vole t iles Ash Total Sulphur

Screen analysis + 10 r + 5 r - 1 o m m + 3 m - 5mm + l m - 3am - 1 1 m

Calorif ic value cal/g Ash analvsis ($1 Si02 As203 + Ti02 CaO uO Fe2O-j Ash softening temperaturn Ash melting point Ash flow point

A

The exhaust gases passed through s e t t l i n g chambers t o permit e n t r a i n - ed gangue m a t e r i a l t o s e t t l e wi th some of t h e z inc and l ead oxide i n t h e form of ' p r e o x i d e ' , which was recycled. The oxide fume was c o l l e c t e d i n a bag f i l t e r .

86 .0 5.6 8.4

1.17

0.0% 5.7

23.7 50.0 20.6

7.4

40.3 31.6 3.5 2.7

16.7

1110"~ 1250'~ 1360°c

Tes t work was conducted over a per iod of 4 weeks dur ing which time over 60 tons of z inc bear ing m a t e r i a l averaging 21.7% zinc and 3.8% l ead were t r e a t e d i n t h e k i l n . The res idue discharged averaged 2.03% z inc and 0.01% lead . The Waelz oxide produced averaged 63.3% z inc and lo.% l e a d , wi th a magnesium con ten t ranging from 0.4% t o 1.%. A t y p i c a l a n a l y s i s i s shown i n Table 111

Table 111. Tvp ica l a n a l y s i s Waelz Oxide produced i n Krupps p i l o t p l a n t

A n t h r a c i t e duff was used a s the r educ tan t a t t he r a t e of approximate ly 3% of t h e weight of the new z i n c bea r ing f eed . The follow- i n g conclus ions were reached:-

( a ) t h e burden s t a r t e d s o f t e n i n g a t about 1 1 0 0 ~ ~

( b ) t h u s t o avoid excess ive format ion of a c c r e t i o n s t h e o p e r a t i n g temperature should no t exceed l l O O ° C

( c ) i t was p o s s i b l e t o v o l a t i l i z e a t l e a s t 90% of t h e z i n c p resen t wi thout exceeding 1 1 0 0 ~ ~

( d ) as a source of a d d i t i o n a l h e a t , gas would be p r e f e r a b l e t o e i t h e r o i l o r powdered c o a l , due t o t h e s o f t e r flame ob ta inab le .

( e ) a l though a c c r e t i o n s were t o be expec ted , i t should be p o s s i b l e t o t r e a t t he raw m a t e r i a l t e s t e d by t h e Waelz P rocess , provided o p e r a t i n g c o n d i t i o n s could be s u f f i c i e n t l y c a r e f u l l y c o n t r o l l e d .

A s t h e Waelz oxide produced i n t h e t e s t k i l n had a bulk d e n s i t y of only 0.83 g/cm3 i t was obvious t h a t bu lk t r a n s p o r t by normal road and ra i lway v e h i c l e s would p resen t cons ide rab le problems. It was t h e r e f o r e decided t o p e l l e t i z e the oxide us ing a d i s c p e l l e t i z e r and adding a smal l q u a n t i t y of water only. The bulk d e n s i t y was thereby inc reased t o about 1.7 g/cm3.

A bulk sample of t h i s p e l l e t i z e d m a t e r i a l was r e tu rned t o South A f r i c a f o r t e s t i n g purposes i n a p i l o t e l e c t r o l y t i c z i n c p l a n t . Tes t work showed t h a t a l though t h e magnesium and f l u o r i n e might prove troublesome, good z i n c r e c o v e r i e s could be expected and no insuperab le d i f f i c u l t i e s were l i k e l y t o be encountered.

A s a r e s u l t of t h e above, t h e d e c i s i o n was taken t o i n s t a l l a k i l n a t Berg Aukas, w i t h a nominal c a p a c i t y of 10 000 t o n s p e r month of new z i n c bea r ing f e e d .

K i l n P roduc t s - D e s c r i p t i o n of Waele P l a n t

Messrs. Lurgi (s.A. ) ( p r o p r i e t a r y ) Limited were commissioned t o d e s i g n , supply and e r e c t a k i l n capable of t r e a t i n g 120 000 m e t r i c t o n s p e r annum of new z i n c b e a r i n g f e e d c o n s i s t i n g of 20 000 t o n s of z i n c s i l i c a t e and 100 000 t o n s of t a i l i n g s of a n a l y s i s s i m i l a r t o those shown i n Table 1.

Location

The p l a n t i s s i t u a t e d a d j a c e n t t o t h e Berg Aukas mine a t a n a l t i - t ude of 1 450 m above sea l e v e l . Maximum ambient temperature i s 40°C and t h e average annual r a i n f a l l i s 625 mm e s s e n t i a l l y w i t h i n t h e 3 months of summer. The n e a r e s t r a i l h e a d i s 2 1 k i lomet res d i s t a n t .

A f low p l a n of the p l a n t i n s t a l l e d i s shown i n Fig . 2.

K i l n

The k i l n i t s e l f i s 4 me t res i n d i amete r by 75 met res long. The s h e l l i s f a b r i c a t e d of welded mi ld s t e e l p l a t e , q u a l i t y ~ ~ ~ . 1 5 0 1 / 1 5 1 Grade 2BA, 29 mm t h i c k r e i n f o r c e d t o 60 mm f o r t h e f o u r running t y r e s and t h e d r i v i n g g e a r s e c t i o n s .

The running t y r e s , 5 160 mm i n d i amete r and 770 mm wide a r e of c a s t s t e e l , ~ ~ ~ . 5 9 2 / 1 9 5 0 Grade B q u a l i t y , welded t o the r e i n f o r c e d s e c t i o n s of t h e s h e l l . The e i g h t suppor t ing r o l l e r s , 1 300 mm i n d iamete r a r e a l s o made of c a s t s t e e l and run on r o l l e r b e a r i n g s which a r e mounted on c o n c r e t e p e d e s t a l s approximate ly 4 me t res above ground l eve 1.

The k i l n i s mounted a t a n i n c l i n a t i o n of 2% t o t h e h o r i z o n t a l . The f i r s t , t h i r d and f o u r t h s e t s of suppor t ing r o l l e r s numbered from t h e f e e d end a r e a d j u s t e d such t h a t t h e suppor t ing s u r f a c e s of each p a i r a r e p a r a l l e l t o each o t h e r but a r e i n c l i n e d t o t h e f a c e of t h e r i d i n g r i n g by a n amount of about 0.15 mm. T h i s has t h e e f f e c t of caus ing t h e k i l n t o move up the s l o p e towards t h e f eed end. A p a i r of h o r i z o n t a l r o l l e r s r o t a t i n g about a v e r t i c a l a x i s a r e r i g i d l y mounted a t No.2 r i d i n g r i n g , one on each s i d e of i t , t o r e s t r i c t t h e maximum h o r i z o n t a l movement of t h e k i l n t o about 50 mm. When No.2 r i d i n g r i n g reaches t h e t o p p o s i t i o n , o i l i s a p p l i e d t o numbers 1, 3 and 4 r i d i n g r i n g s , which causes t h e k i l n t o s l i p back t o t h e lower p o s i t i o n . By wiping o f f t h e e x c e s s l u b r i c a n t , t he k i l n begins t o c l imb a g a i n towards t h e upper p o s i t i o n . Th i s complete c y c l e i s per- formed once d a i l y .

COKE STOCKPILE COKE AND

OAS PLANT

2- PLLLETIZINO

WAE LZ OXIDE PELLETS

A N D STACK Fig. 2

The k i l n i n l e t s e c t i o n i s tapered over the f i r s t 1.1 metres from a n opening of 1.9 metres. Th i s tapered por t ion was o r i g i n a l l y l i n e d wi th s p e c i a l l y shaped tapered Lus i t e f i r e b r i c k s , but due t o severe s p a l l i n g , these were l a t e r replaced by c a s t a b l e r e f r a c t o r y approximately 150 mm i n th ickness which has s ince given good se rv ice .

The feed t o t h e k i l n i s int roduced by g r a v i t y v i a a n inc l ined s t e e l tube about 5 metres long and 500 rnm i n diameter wi th a wa l l th ickness of 25 mm. Sea l ing a i r i s blown down the feed tube t o prevent the escape of hot k i l n g a s e s , by means of a 4 kW f a n capable o f d e l i v e r i n g 2 000 I4m3/h a t a p ressure of 200 mm wg. The a c t u a l amount used i s cor i t rol led by manual adjustment of a c a l i b r a t e d b u t t e r f l y valve.

S e a l i n g of the r o t a r y k i l n i n l e t end i s achieved by means of 2 s l i p r i n g s , of c a s t i r o n and c a s t s t e e l r e s p e c t i v e l y , s l i d i n g a g a i n s t each o the r . One r i n g i s a t t ached e x t e r n a l l y t o the per iphery of t h e k i l n and the o t h e r , which i s s t a t i o n a r y , c o n s i s t s of separa te seg- ments held up a g a i n s t the r o t a t i n g r i n g by means of 16 pneumatic p i s t o n s .

The k i l n o u t l e t s e c t i o n i s double walled and provided with a l a b y r i n t h type s e a l cooled by means of a 10 kW fan . The s h e l l d iameter i s reduced by 610 mm i n two s t e p s over t h e l a s t 4 metres and a s the b r ick- l in ing th ickness i s maintained a t 229 mm throughout, the i n t e r n a l diameter of the discharge end of the k i l n i s reduced t o 2.87 metres.

E igh t a i r f a n s were o r i g i n a l l y i n s t a l l e d on the s h e l l of the k i l n a t 19 , 25, 33, 37, 42, 46, 50 and 56 metres r e s p e c t i v e l y from the feed end, f o r blowing a d d i t i o n a l a i r i n t o the k i l n v i a tubes of s p e c i a l heat r e s i s t a n t s t e e l con ta in ing 25% chromium and 12% n icke l .

The tubes a r e approximately 400 mm i n diameter wi th a wa l l th ickness of 20 mm. The discharge nozzle i s 200 mm i n diamter and designed and posi t ioned so a s t o ensure t h a t the a i r i s d i r e c t e d horizon- t a l l y a t t h e cen t re of the k i l n , counter-current t o t h e main gas f low, i . e . towards the k i l n discharge end. S i g h t g l a s s e s were subsequently f i t t e d on the bends on the a i r d u c t s between the f a n s and the tube t o permit v i s u a l examination of the tube dur ing operat ion. Each f a n has a r a t e d c a p a c i t y of 2 000 ~my/h a t a pressure of 200 watergauge, and i s d r iven by a 4 kW motor, power being suppl ied by means of copper s l i p r i n g s a t t ached t o the k i l n . The a c t u a l amount of a i r blown i n a t each po in t can be c o n t r o l l e d by manual adjustment of a c a l i b r a t e d b u t t e r f l y valve.

The k i l n was o r i g i n a l l y provided wi th 6 platinum-rhodium thermocouples f i t t e d tnyough the s h e l l and protruding a maximum of 50 mm i n s i d e t h e b r ick- l in ing . These were connected t o s l i p r i n g s near the feed end of the k i l n from where impulses were t r ansmi t t ed t o a 12 po in t temperature recorder i n the c o n t r o l room. The t ransmiss ion

t i m e s and t h e p o s i t i o n s of t h e thermocouples were s o a r r anged t h a t each thermocouple produced 2 r e c o r d i n g s p e r r e v o l u t i o n - once when t h e thermocouple was embedded i n t h e burden a t about a 7.0 o ' c l o c k p o s i t i o n and t h e o t h e r when i n t h e d i a m e t r i c a l l y oppos i t e p o s i t i o n . By t h i s means i t had been hoped t o o b t a i n bo th a burden tempera ture and a g a s tempera ture . However, i t was soon obvious t h a t t h e two t empera tu res recorded were p r a c t i c a l l y i d e n t i c a l a s t h e r e was i n s u f f i - c i e n t t ime between r ead ings f o r t h e thermocouple t o r e a c h equ i l ib r ium. I t was decided l a t e r t o i n t e r s p e r s e 5 a d d i t i o n a l thermocouples and a t t h e same t ime a l t e r t he des ign s o t h a t t h e thermocouple could be a l lowed t o p ro t rude up t o 360 mm w i t h i n t h e k i l n , i f d e s i r e d . Thermo- couples a r e t h e r e f o r e s i t e d a t t h e f o l l o w i n g p o s i t i o n s from t h e f e e d end: -

8, 20, 29, 34 , 40 , 44, 47 , 50, 53, 59 and 67 me t re s . The thermocouple a t 8 m s t i l l recorded burden and g a s t empera tu res - a l l t h e r e s t recorded burden t empera tu res only .

The k i l n o u t l e t head is of welded s t e e l c o n s t r u c t i o n l i n e d w i t h h e a t r e s i s t a n t r e f r a c t o r i e s and is mounted on a n unde rca r r i age w i t h c a s t o r s t o enab le i t t o be moved back ou t of t h e way when necessary . A l a r g e hinged door p rov ides ready a c c e s s t o t h e k i l n . A s t h e k i l n i s ope ra t ed w i t h t h i s door c l o s e d , two 100 mm d i ame te r s i g h t g l a s s e s a r e provided f o r viewing t h e c o n t e n t s of t h e k i l n d u r i n g ope ra t ion .

K i l n Dr ive

The k i l n i s d r i v e n by a c a s t s t e e l ~ ~ ~ . 5 9 2 / 1 9 5 0 Grade C g i r t h g e a r 400 mm wide, made i n two s e c t i o n s and a t t a c h e d t o t h e s h e l l by 12 t a n g e n t i a l s p r i n g p l a t e s . Two synchronized v a r i a b l e speed d.c. motors each 70 kW, w i t h t h y r i s t o r s and d iodes f o r v o l t a g e r e g u l a t i o n , o p e r a t e s e p a r a t e r e d u c t i o n gearboxes which enab le t h e k i l n speed of r o t a t i o n t o be v a r i e d over t h e range 0.25 t o 1.0 r e v o l u t i o n s p e r minute. Two a d d i t i o n a l emergency 22 kW moto r s , powered by a Deutz d i e s e l ope ra t ed 80 kVA synchronous g e n e r a t o r , o p e r a t i n g through addi - t i o n a l gearboxes a r e capab le of r o t a t i n g t h e k i l n a t 0.0625 revolu- t i o n s p e r minute. S u i t a b l e i n s t r u m e n t a t i o n e n s u r e s t h a t t h i s u n i t comes i n t o o p e r a t i o n a u t o m a t i c a l l y i n c a s e of a power f a i l u r e , t o keep t h e k i l n r o t a t i n g and t o keep c e r t a i n o t h e r e s s e n t i a l s e r v i c e s i n ope ra t ion .

Residue Disposal

The r e s i d u e d i scha rged from t h e k i l n d rops down a t r a n s v e r s e i n c l i n e d chu te l i n e d w i t h f i r e b r i c k , i n t o a quench pond from where i t i s raked up a n i n c l i n e d s t e e l covered s lope by means of a manually c o n t r o l l e d motor ized s c r a p e r winch, and d ischarged v i a a 50 mm g r i z z l e y onto a n i n c l i n e d rubber conveyor b e l t , 610 mm i n width . The s c r a p e r bucket h a s a vcd-ume of 0.7 m 3 and t h e t o t a l l i f t i s about

9 metres. The winch i s powered by a 35 kW motor.

A s o r i g i n a l l y designed, t h e discharge chute was open t o the atmosphere , which i n e f f e c t l e f t t h e discharge end of the k i l n a l s o com- p l e t e l y unsealed and permit ted an uncont ro l lab le amount of a i r t o be drawn i n t o the k i l n . Th is was even tua l ly remedied by s e a l i n g the d i s - charge chute by means of hinged h o r i z o n t a l l y overlapping s t e e l s l a t s each about 500 mrn wide, continued down t o below the water-level i n the quench pond.

An e leva ted s t e e l walkway runs the e n t i r e length of the k i l n on one s i d e , t o provide access t o the s h e l l f ans and the s h e l l thermocouples , e t c .

Gas Producer

The producer gas p l a n t , opera t ing on bituminous c o a l n u t s , nomin- a l l y 25 mrn t o 40 mm i n s i z e i s capable of d e l i v e r i n g 2 000 ~m?/h of gas a t 300°C t o 350°C wi th c a l o r i f i c value of approximately 1 800 kca l /~m3, i . e . a maximum of 3.6 x lo6 k c a l per hour, t o a burner c e n t r a l l y posi t ioned a t t h e discharge end. F lex ib le connections per- m i t t he angle of the burner t o be a l t e r e d r e l a t i v e t o the a x i s of the k i l n . Primary combustion a i r is de l ivered t o the burner by a separa te f a n wi th a maximum capac i ty of 3 500 &3/h a t 600 mm water gauge, dr iven by a 10 kW motor. The a c t u a l volume can be var ied by manual adjustment of a c a l i b r a t e d b u t t e r f l y valve, and is recorded i n t h e c o n t r o l room. The c o a l is screened immediately p r i o r t o use through a 19 mm screen t o remove any f i n e s which a r e used a s a reductant and mixed with the coke and a n t h r a c i t e . Maximum c o a l consumption is about 20 tons p e r day. A t y p i c a l a n a l y s i s of the c o a l used i s given i n Table 1 V below:-

Table 1 V . Typical Analysis of Bituminous Coal ( ~ i r Dry l3asisl

Calorific valw Ash Volatile material Flxed Carbon Total Sulphur Aah m i o n twpemtura Noiatum

6 950 calk 10.1 $ 25.9 $ 61.2 $ 0.6

lye0 C 2.8 $


The producer gas has a t y p i c a l a n a l y s i s a s shown i n Table V below:-

Table V. Typical Analysis of Producer Gas

Fig . 3

A genera l view of t h e k i l n showing t h e d r i v i n g g e a r , the a i r i n l e t d u c t s and the g a s producer p l a n t i n t h e back ground is shown i n Fig .3 .

Br ick Lining

The b r i c k s used f o r l i n i n g the k i l n measure 229 mm by 152 mm by 76 mm t aper ing down t o about 69 mm and a r e l a i d end on t o give a l i n i n g 229 mm th ick . Two q u a l i t i e s of b r i c k s were used o r i g i n a l l y ; ' S u p e r l i n f f o r the f i r s t 30 m from the feed end and ' L u s i t e ' f o r the remaining 45 m . Subsequently 'Jumbof b r i c k s were a l s o used as replacements. Typical ana lyses a r e shown i n Table V 1

Table V 1 . Typical Analyses of Super l in , Lus i t e and Jumbo Bricks

siqk

~ 1 2 0 9 ~ 0 ~ 0 ~ s Tio2k c a d n d Alkalis Porosity $ Bulk density g / a 3

Permeability ( c.g.s. units) Cold crushing strength kg/cm2

Luaite - -

44 - 46 51 - 53 1.2 - 1.7 0.5 - 0.7 0.3 - 0.5 0.3 - 0.5 Trace

I 14 - 18 2.40 1 0.10

350 - 850

Jumbo

S t e e l r i n g s made of 25 mm f l a t mild s t e e l 100 mm deep were welded i n t o the s h e l l a t about 4 t o 5 metres i n t e r v a l s t o prevent movement of the b r i c k s r e l a t i v e t o the s h e l l . A cardboard expansion j o i n t 3 mm t h i c k was i n s e r t e d a t approximately 1 .5 metre i n t e r v a l s . Relin- i n g i s now done by t h e p lan t opera to r s a t the r a t e of 2 metres pe r 2 b r i c k l a y e r s per 8 hour s h i f t , provided they a r e not kept wa i t ing f o r m a t e r i a l s . The lower h a l f of the k i l n i s l i n e d f i r s t . These b r i c k s a r e then kept i n p o s i t i o n by means of mechanical jacks while the k i l n i s r o t a t e d s u f f i c i e n t l y t o a l low the r e s t of the b r i c k s t o be l a i d .

Recovery of Oxide from Exhaust Gases

The gases emerging from t h e feed end of the k i l n pass through s e t t l i n g chambers t o remove en t ra ined s o l i d s , a coo l ing tower t o reduce the temperature and e l e c t r o s t a t i c p r e c i p i t a t o r s where t h e oxide is p r e c i p i t a t e d , before f i n a l l y being discharged t o atmosphere v i a a s t ack .

S e t t l i n g Chambers: The t h r e e f l u e g a s s e t t l i n g chambers a r e each approximately 5 m

long by 7 m wide by 6 m h igh with hopper type bottoms. They a r e con- s t r u c t e d of s t e e l p l a t e and the w a l l s a r e l i n e d w i t h 27 cm of normal duty f i r e b r i c k con ta in ing about 3% A1203. The roof i s f l a t and i s i n s u l a t e d by about 23 cm of heat r e s i s t a n t mor ta r , he ld i n place by means of s t a i n l e s s s t e e l r e i n f o r c i n g mesh. Three l a r g e doors a r e f i t t e d f o r providing access f o r i n s p e c t i o n and t h e r e a r e e i g h t ha tches each about 0.5 m2 i n s i z e which a r e designed t o blow off i n case of a n explosion i n t h e chamber. Preoxide was discharged

932 EXTRACTIVE METALLURGY OF LEAD AND Z I N C

o r i g i n a l l y from each compartment v i a a s t a r f e e d e r i n t o a pneu- m a t i c t r a n s f e r system d e l i v e r i n g i n t o a s t o r a g e s i l o which i n t u r n d i scha rged v i a s t a r f e e d e r on t o t h e main f e e d b e l t immediately ahead of t h e pugmil l .

T h i s system which had been des igned t o o p e r a t e under vacuum gave s o much t r o u b l e due t o b lockages i n t h e d e l i v e r y l i n e t h a t i t was l a t e r a l t e r e d t o o p e r a t e a s a p r e s s u r e system. T h i s was s t i l l u n s a t i s - f a c t o r y and a s r e s u l t of c o n t i n u a l b lockages and l a r g e d u s t l o s s e s , wan abandoned u n t i l i t was e v e n t u a l l y modif ied t o f u n c t i o n a s a p r e s s u r e system embodying t h e v a r i o u s r e f inemen t s d e s c r i b e d l a t e r i n t he paragraph d e a l i n g w i t h pneumatic t r a n s f e r .

A i r i s provided by a Rootes t ype blower w i t h a d isp lacement of 1500 m3/h a t a p r e s s u r e d i f f e r e n c e of 0 .41 kg/cm2. The d e l i v e r y d u c t i s 125 mm i n d i ame te r w i t h r e i n f o r c e d bends. Approximately 95% of t h e d u s t i s c o l l e c t e d i n a 17O cyc lone , approximate ly 550 mm i n d i a - m e t e r , t h e remaining 5% f i n a l l y 'being removed i n a bag f i l t e r , a l l t h i s d u s t b e i n g d i scha rged i n t o t h e s t o r a g e s i l o ahead of t h e pugmil l . The bag f i l t e r c o n t a i n s 2 8 socks w i t h a t o t a l f i l t e r i n g a r e a of app rox ima te ly 40 m2. The maximum o p e r a t i n g t empera tu re f o r t h e t ype of bags used i s approximate ly 150°C.

Coo l ing Tower: A f t e r l e a v i n g t h e s e t t l i n g chambers t h e f l u e g a s e s a r e cooled i n a n e v a p o r a t i v e c o o l e r by t h e c o n t r o l l e d i n j e c t i o n of sp ray wa te r . The c o o l i n g tower i s 6 .5 m i n d i ame te r and approximate ly 24 m i n h e i g h t , c o n s t r u c t e d of 8 mm mi ld s t e e l p l a t e .

The c o o l i n g tower was des igned t o c o o l a maximum of 65 000 Nm3/h of g a s from a n i n l e t tempera ture of about 5 5 0 ' ~ t o a n o u t l e t tempera- t u r e of about 2 0 0 ' ~ - i . e . a t empera tu re g r a d i e n t of about 350°C. Seven s e t s of s p r a y s each c o n t a i n i n g 6 n o z z l e s a r e f i t t e d a t t h e top . The nozz l e d e s i g n i s such t h a t e x c e s s wa te r which i s no t sprayed i n t o the tower i s bypassed back from t h e nozz1.e i n t o a r e t u r n wa te r mani- f o l d . The amount of wa te r sprayed i n t o t h e tower i s c o n t r o l l e d by a d j u s t i n g t h e opening of a v a l v e on t h e r e t u r n man i fo ld , t he reby in - c r e a s i n g o r d e c r e a s i n g t h e p r e s s u r e on the nozz l e s . T h i s c o n t r o l i s e f f e c t e d a u t o m a t i c a l l y from t h e tempera ture of t h e g a s l e a v i n g t h e c o o l i n g tower. Two o t h e r au toma t i c c o n t r o l s a r e provided t o o p e r a t e i n a n emergency, v i z : -

0 ( a ) i f t h e t empera tu re of t h e exhaus t g a s exceeds 300 C a n emer-

gency f l a p a t t h e t o p of t h e tower ope ra t ed by means of a motor ized winch opens t o a tmosphere , t o avoid damage t o t h e e l e c t r o s t a t i c p r e c i p i t a t o r s .

( b ) i f t h e volume of g a s p a s s i n g th rough t h e exhaus t s t a c k d rops below 30 000 Nm3/h o r i f t h e t empera tu re of t h e g a s a t t h e i n l e t t o t h e c o o l i n g tower drops below 220°C, t h e wa te r supp ly t o t h e n o z z l e s i s s h u t o f f comple te ly t o p reven t t h e accumula- t i o n of unevaporated wa te r i n t h e tower , and a s i g n a l l i g h t

f l a s h e s i n the con t ro l room. The amount of water consumed here i s normally about 110 l i t r e s per minute - maximum output of the pump is 250 l i t r e s per minute a t a maximum operat ing pressure of 40 kg/cm2. S e t t l e d dust drops i n t o a screw conveyor, the trough of which has a hinged f l a p a t the base, t o f a c i l i t a t e cleaning. By changing the d i r ec t i on of r o t a t i o n of the screw, oxide which s e t t l e s i n the tower can be d iver ted e i t h e r t o the preoxide pneumatic t r a n s f e r system o r t o the Waelz oxide system, depending on i ts qua l i ty . Provis ion i s a l s o made f o r discharging the dust on t o the f l o o r i n case of emergency.

E l e c t r o s t a t i c P r ec ip i t a t o r s : Two hor izonta l e l e c t r o s t a t i c precipi- t a t o r s designed and suppl ied by Lurgi a r e i n s t a l l e d f o r normal opera- t i o n i n p a r a l l e l , f o r a maximum gas flow of 85 000 ~m3/h. I n emergency e i t h e r p r ec ip i t a t o r can be i so l a t ed and the f u l l gas stream routed through the other . Col lec t ion e f f i c i ency of 99.5% i s guaran- teed f o r an ;inlet gas dust content of above 50 g/~m3. A t a lower in- l e t gas dus t con ten t , the cleaned gas dust content i s not t o exceed 300 mg per ~ m 3 . With only one p r e c i p i t a t o r on l i n e the co l l e c t i on e f f i c i ency drops t o about 95%.

Each p r ec ip i t a t o r i s 17.5 metres long and 5 metres wide and i s con- s t r uc t ed of 5 mm s t e e l p l a t e with s u i t a b l e s t i f f e n e r s t o withstand a negative pressure of 120 mm wg. The pos i t i ve ly charged grounded c o l l e c t i n g e lec t rodes a r e of the p l a t e type about 5 metres i n height and spaced about 250 mm apa r t with t he negat ively charged discharge e l ec t rodes suspended c e n t r a l l y between them a t about 160 mm i n t e rva l s . Mechanical rapping devices a r e provided f o r both pos i t ive and nega- t i v e electrodes. Each p r ec ip i t a t o r con ta ins th ree e l e c t r o s t a t i c f i e l d s connected i n sequence. Each power pack i s ra ted a t 95 kVA and has a cabinet type switchboard with t h y r i s t o r control and automatic power cont ro l .

The p r ec ip i t a t ed dust c o l l e c t s i n the hopper type bottoms from where i t i s removed continuously by means of Redler conveyors. A s o r i g i n a l l y suppl ied, these Redlers each discharged v i a individual s t a r feeders i n t o a pneumatic t r an s f e r system operat ing under vacuum. This arrangement gave endless t rouble due t o blockages and was l a t e r modified a s described i n the paragraph deal ing with the pneumatic t r a n s f e r system.

A i r f o r t he pneumatic t r a n s f e r system is provided by a 1 500 m3/h Rootes blower powered by a 30 kW motor. The dust de l ivery column i s 125 ma i n diameter and i s constructed of mild s t e e l with su i t ab ly re- inforced bends. Approximately 95% of the dust i s co l l e c t ed i n a cyclone and the remaining 5% i n a bag f i l t e r with a t o t a l f i l t e r i n g a rea of 40 m2. Both products g r a v i t a t e i n t o e i t h e r of two conica l bottom storage s i l o s , each with a nominal capaci ty of 100 tons.

Exhaust Fan: An a x i a l flow exhaust f a n with vane cont ro l and servo motor i s i n s t a l l e d between the p r e c i p i t a t o r s and the f i n a l

s t a c k , which i s 2.2 metres i n diameter and 26 metres i n he igh t .

The f a n w a s suppl ied by Kiihnle, Kopp and Kausch of Frankenthal- P f a l z and has a r a t e d capac i ty of 175 000 m3/h a t 2 0 0 ' ~ and 131.5 mm wg - i t i s dr iven by a 100 kW motor. I n l e t vane c o n t r o l opera tes i n conjunct ion w i t h the automatic draught c o n t r o l l e r .

Fig . 4 below shows the coo l ing tower i n the c e n t r e wi th the e l e c t r o - s t a t i c p r e c i p i t a t o r s and s e t t l i n g chambers on e i t h e r s i d e .

Fig. 4

P e l l e t i z i n g P lan t : Waelz oxide i s discharged from each s i l o v i a a s t a r f eeder and screw conveyor t o two p e l l e t i z i n g d i s c s 4 metres i n diameter. By the a d d i t i o n of about 1% of water , a s a f i n e spray, the powder i s turned i n t o p e l l e t s ranging from about 6 mrn t o 20 mm i n diameter - no o t h e r a d d i t i v e s a r e found t o be necessary. The "green" p e l l e t s , a s produced, wi thout any d ry ing , have a crushing s t r e n g t h of about 15 t o 20 kg per p e l l e t . Each d i s c i s f i t t e d wi th a 30 kW motor. The ang le of i n c l i n a t i o n can be a l t e r e d i n the range 300 t o 60° by opera t ing a handwheel, and the angle normally used i s 45O t o the hor izon ta l . The d i s c s a r e operated a t a speed of 6 revo lu t ions per minute and t h i s can be v a r i e d only by changing pu l l eys . The r a t e of output i s ' approx imate ly 8 tons per hour per p e l l e t i z i n g d i s c . The p e l l e t s a r e discharged on t o a conveyor b e l t f o r d e l i v e r y t o a s t o r a g e shed and a r e subsequently handled by means of f r o n t end loaders .

C o n t r o l Room

An a i r - cond i t ioned c o n t r o l room i s provided wi th a mimic flow d ia - gram of the p l a n t , and the necessa ry i n d i c a t i n g l i g h t s , r e c o r d e r s , swi t ches , a l a rms , e t c . t o enab le the p l a n t t o be remotely c o n t r o l l e d from t h i s p o s i t i o n . I n p a r t i c u l a r the fo l lowing ins t rumen t s a r e provided: -

( a ) A Schenk b e l t weighing machine f o r new z inc b e a r i n g f e e d , which r e c o r d s i n t h e c o n t r o l room, and i n t e g r a t e s t h e tonnage.

( b ) 2 Schenk reduc tan t p ropor t ion ing weigh f e e d e r s and record ing and i n t e g r a t i n g equipment.

( c ) K i l n draught c o n t r o l l e r and recorde r .

( d ) 12 po in t k i l n temperature r e c o r d e r .

( e ) K i l n r a t e of r o t a t i o n i n d i c a t o r and recorde r .

( f ) K i l n main d r i v e k i l o w a t t r eco rde r .

( g ) Air f low r a t e s , p r e s s u r e and temperature r e c o r d e r s , e t c . f o r pro- d u c e r g a s p l a n t .

( h ) Recorder of oxygen i n k i l n exhaust gas .

( i ) Recorder f o r g a s temperatures a t i n l e t and o u t l e t of c o o l i n g tower w i t h c o n t r o l equipment.

( j ) I n d i c a t i n g ins t rumen t s f o r m i l l i amps and v o l t a g e s i n e l e c t r o - s t a t i c p r e c i p i t a t o r s .

( k ) Exhaust g a s temperature i n d i c a t o r s .

( 1 ) High l e v e l i n d i c a t o r s f o r Waelz oxide b ins .

Operat i o n

Reductants

Unwashed a n t h r a c i t e du f f from t h e Vryheid a r e a i n Nata l and coke f i n e s from ISCOR (IRON & STEEL CORPORATION) a t Van d e r B i j l Park a r e used a s r e d u c t a n t s . Current a.nalyses a r e shown i n Table V 1 1

Table V 1 1 . Typical Analyses Unwashed A n t h r a c i t e Duff and Coke F i n e s

These products a r e r a i l e d i n bulk approximately 2 800 k i l m e t r e s t o Groo t fon te in , the n e a r e s t r a i l h e a d t o t h e k i l n , and then t r a n s - f e r r e d t o road t r u c k s by means of C a t e r p i l l a r 622B f r o n t end l o a d e r s , t o t r a v e r s e the f i n a l 2 1 k i lomet res t o the k i l n . Both products a r e s t o c k p i l e d i n bulk i n the open, the he igh t of t h e s t o c k p i l e be ing about 3 metres . A s r e q u i r e d , t h e r e d u c t a n t is removed by means of f r o n t end l o a d e r s and dropped onto a v a r i a b l e speed p l a t e f e e d e r w i t h a n e f f e c t i v e width of 1 met re , which d i scha rges onto a n i n c l i n e d conveyor b e l t 610 mm wide and d e l i v e r s onto a n e l e v a t e d r e v e r s i b l e conveyor a t a maximum r a t e of 20 tons p e r hour , which drops the product i n t o e i t h e r of 2 s t o r a g e s i l o s , each wi th a capac i ty of 150 tons . Each s t o r a g e s i l o i s equipped wi th a Schenk weigh f e e d e r which can be s e t t o a u t o m a t i c a l l y d e l i v e r a p r e s e t percentage of the weight of the new z i n c bea r ing f eed m a t e r i a l p a s s i n g over the b e l t weightometer on i t s way i n t o the k i l n . A l t e r n a t i v e l y each weigh f e e d e r can be a d j u s t e d from the c o n t r o l room t o d e l i v e r r educ tan t a t a s t eady pre-se lec ted r a t e . Each weigh f e e d e r i n d i c a t e s , i n t h e c o n t r o l room the tons p e r hour being f e d , and i n t e g r a t e s t h e tonnage. The reduc- t a n t d rops on t h e z i n c b e a r i n g f eed m a t e r i a l on t h e main f e e d b e l t which d i scha rges i n t o a pugmill t o provide homogeneous mixing of the r educ tan t wi th t h e o re .

Sice Analyais

+ 5mm - 5 + 3 m - 3 + 2 mm - 2 + 1 m - 1 + 0.5 mm - 0.5 mm

Ash Volet i les Fixed Carbon Ash fusion temp. Calorific value ca l Bulk density

Zinc b e a r i n g feed

Zinc s i l i c a t e concen t ra t e is d e l i v e r e d d a i l y t o a n open s tock-

Unwashed Anthracite

duff

1 s 22 18 2 1 11 15

9 . f i 10.5 78.2 1 4 0 0 ~ ~

7 500 0.88

Coke Fines

lllr 11 11 2 4 18 2 5

16.65 1 .1

81.6 1400°c

7 000 0.69

p i l e , u s i n g a Shawnee-Poole t r a c t o r - t r a i l e r u n i t w i t h a c a r r y i n g c a p a c i t y of 10 tons . The product a s d e l i v e r e d c o n t a i n s about 12% mois ture . It i s removed from t h e s t o c k p i l e by means of a f r o n t end l o a d e r a s r equ i red and dropped on t h e same p l a t e f e e d e r a s used i n t e r - m i t t e n t l y f o r r e d u c t a n t , f o r d e l i v e r y i n t o the z i n c s i l i c a t e s t o r a g e s i l o w i t h a c a p a c i t y of 450 tons . The s i l o i s equipped w i t h a 2 me t re d iameter t a b l e f e e d e r , r o t a t i n g a t 0 . 9 r e v o l u t i o n s p e r minute , ' capab le of d e l i v e r i n g up t o 10 t o n s p e r hour on t o t h e main f eed b e l t t o t h e pugmill .

Slime i s rec la imed from t h e s l imes dam some 500 me t res from t h e k i l n by means of f r o n t end l o a d e r s d igg ing i n from t h e s i d e . The dam i s about 6 me t res i n h e i g h t and p e r i o d i c a l l y i t h a s been considered prudent t o c o l l a p s e t h e f a c e when i t becomes too n e a r l y v e r t i c a l , by b l a s t i n g . The f r o n t end l o a d e r s d e l i v e r i n t o Shawnee-Poole t r a c t o r - t r a i l e r u n i t s a s used f o r t h e z i n c s i l i c a t e c o n c e n t r a t e , and d e l i v e r t o a d a i l y s t o c k p i l e under cover , a longs ide t h e z i n c s i l i c a t e p l a t e f eede r . The b i n above t h e f e e d e r h a s a c a p a c i t y of 90 t o n s and i s f i l l e d p e r i o d i c a l l y d u r i n g t h e course of each s h i f t from t h e s tock- p i l e a l o n g s i d e , u s i n g a f r o n t end loade r . The p l a t e f e e d e r can be a d j u s t e d t o d e l i v e r a t a s t eady r a t e ranging from about 5 t o about 25 t o n s p e r hour , on t o t h e main f e e d b e l t .

The main f e e d b e l t i s equipped w i t h a weightometer which r e c o r d s t h e r a t e of f e e d i n t o n s p e r hour and i n t e g r a t e s t h e t o t a l tonnage.

The mix tu re of z i n c s i l i c a t e , rec la imed s l i m e , r educ tan t and preoxide i s d e l i v e r e d on t o a g r i z z l e y which pe rmi t s t h e f i n e s t o d rop i n t o a pugmill wh i l e t h e p l u s 50 mm lumps, s t o n e s and f o r e i g n bod ies a r e bypassed d i r e c t l y on t o f eed b e l t r e c e i v i n g t h e mixed product from t h e pugmill . The pugmill was s u p p l i e d by Kar l Handle and Sons and i s about 2 .5 m l o n g by 0 .7 m wide. It i s equipped w i t h two p a r a l l e l s h a f t s c a r r y i n g b l a d e s which r o t a t e i n oppos i t e d i r e c t i o n s t o produce a mixing and p r o p e l l i n g motion. It i s d r i v e n by a 30 kW motor.

S t a r t up procedure

Drying ou t of t h e brickwork was commenced on 1 9 t h February , 1969. The fo l lowing procedure was adopted:-

( a ) The k i l n was r o t a t e d o n e q u a r t e r t u r n every twenty minutes , u s i n g t h e a u x i l i a r y d r i v e on ly , i . e . r a t e of r o t a t i o n : - 0.0625 rev/min.

( b ) Producer gas was burn t a t t h e d i scha rge end u s i n g t h e s t andard burne r , a t a r a t e s u f f i c i e n t t o r a i s e t h e temperature of t h e brickwork a t Numbers 1 and 3 thermocouples 67 and 59 m r e spec t - i v e l y from t h e f e e d end t o 1 5 0 ' ~ i n 24 hours , and the tempera- t u r e was then h e l d s t eady a t 1500C f o r 5 days.

( c ) Temperature was then increased a t the r a t e of 50°C per hour t o 200°C and held the re f o r one day.

( d ) S h e l l f ans were s t a r t e d up and s u f f i c i e n t a i r admitted t o main- t a i n the a i r tube temperatures a t 200°C.

( e ) When the temperature i n the s e t t l i n g chambers reached 2000C, i t was maintained the re f o r 4 days.

( f ) Cooling tower f l a p was opened and the main f a n s t a r t e d and ad- jus ted t o i t s minimum de l ive ry r a t e .

( g ) K i l n temperature was s t e a d i l y increased a t the r a t e of 50% per hour u n t i l the brickwork a t Nos. 1 and 3 thermocouples reached 7OO0C.

( h ) Ki ln was then r o t a t e d continuously on a u x i l i a r y d r ive .

( i ) Temperature was r a i s e d t o 9 0 0 ~ ~ a t the r a t e of 5 0 ' ~ per hour.

( j ) Anthrac i t e was then fed i n t o the k i l n followed by z inc bear ing feed when the temperature had reached 1 0 0 0 ~ ~ .

The above programme took 13 days t o complete. Once the brickwork had been thoroughly d r i e d out the subsequent s t a r t i n g up procedure was s t reamlined t o enable z inc bear ing feed t o be in t roduced approximately 18 hours a f t e r s t a r t i n g preheat ing.

During the f i r s t yea r of operat ion i t was necessary t o coo l the k i l n down and cease opera t ions on 13 occasions. Current s t a r t -up procedure, i f the re i s no brickwork t h a t r equ i res slow drying ou t , i s a s follows:-

( a ) The cool ing tower f l a p i s l e f t open. The ducts t o the p r e c i p i t a t o r s a r e c losed. The s h e l l f ans a r e operated with b u t t e r f l y va lves s u f f i c i e n t l y open t o allow only a t r i c k l e of a i r t o pass. The main f a n i s s t a t i o n a r y . The k i l n i s r o t a t e d 4 t u r n every 20 minutes on a u x i l i a r y d r i v e . The gas burner i s s t a r t e d up a t about ha l f maximum capaci ty . By g radua l ly inc reas ing the gas burning r a t e and c l o s i n g the coo l ing tower f l a p , the temperature of the gas a t the i n l e t t o the cool ing tower i s r a i s e d t o 200°C over a per iod of about 6 hours.

( b ) The k i l n i s then r o t a t e d cont inuously on main d r i v e a t minimum r a t e of 0.25 rev/min. The f l a p on the cool ing tower i s c losed completely, t h e duc t s t o t h e p r e c i p i t a t o r s a r e opened, the s h e l l f ans a r e opened one o r two notches and t h e main f a n i s s t a r t e d up wi th i n l e t vanes opened l e s s than 1% i n i t i a l l y .

Anthrac i t e duff i s f e d t o t h e k i l n a t the r a t e of 2 tons per hour.

( c ) About 5 hours l a t e r t h e a n t h r a c i t e reaches t h e d i scharge end of t h e k i l n where i t i s i g n i t e d by t h e gas burner. The temperature then r i s e s r a p i d l y and when t h e gas temperatures a t t h e i n l e t t o t h e coo l ing tower reaches 650°C, z inc bear ing feed is introduced, a t a n i n i t i a l r a t e of 5 tons pe r hour , wi th 50% by weight of a n t h r a c i t e and coke. Th i s i s u s u a l l y about 18 hours a f t e r t h e i n i t i a l l i g h t i n g up.

( d ) The feed r a t e i s g r a d u a l l y inc reased over t h e fol lowing 12 hours t o maximum r a t e .

R e s u l t s of Operation

Operating r e s u l t s obta ined t o d a t e a r e summarized i n Table V 1 1 1 .

Zinc bear ing feed con ta in ing about 14% z inc was in t roduced i n t o the k i l n f o r the f i r s t time on 10 th March, 1969. D i f f i c u l t y was experienced i n mainta ining t h e temperature , which even tua l ly a f t e r 7 days dropped so low t h a t i t was decided t o coo l down and inspec t t h e k i l n . Except f o r some s p a l l i n g of b r i c k s i n the feed cone and some phys ica l damage t o some of t h e thermocouples, t h e k i l n was i n good condi t ion. During t h i s campaign t h e feed t o the k i l n cons i s t ed of 8 tons pe r hour of reclaimed s l ime and 15 t o 20% of washed a n t h r a c i t e .

During t h e next 3 months t h e r e were 5 f u r t h e r campaigns ranging from 4 days t o 27 days i n length. I n each case t h e k i l n was even- t u a l l y c losed down due t o excess ive bui ldup of a c c r e t i o n s . On var ious occasions shock t reatment was t r i e d whereby t h e k i l n was r a p i d l y cooled f o r a couple of hours and then heated up aga in a s r a p i d l y a s poss ib le i n a n a t tempt t o cause t h e a c c r e t i o n s t o c rack and drop o f f . Some success was achieved wi th comparatively shor t r i n g s , but where t h e a c c r e t i o n s extended over a few metres o r more, the t reatment was q u i t e i n e f f e c t i v e . I n these cases t h e k i l n had t o be cooled down completely t o enable t h e a c c r e t i o n s t o be removed manually. The worst a c c r e t i o n s occurred dur ing t h e f o u r t h campaign i n May, 1969, when one p a r t i c u l a r a c c r e t i o n was found t o be over 8 metres long ranging i n th ickness from 1 t o 1& metres. The t o t a l weight of ac- c r e t i o n s removed from t h e k i l n on t h i s occasion exceeded 450 tons. A t y p i c a l view of a c c r e t i o n s p r ized o f f , a longside a n i n t e r n a l a i r tube , i s shown i n Fig . 5

rg

Table V 1 1 1 . Summary of operating r e s u l t s %

WAELZ TREATING OF COMPLEX ZINC-LEAD ORES

Fig . 5

The t h i c k e r the a c c r e t i o n s the longer the k i l n t akes t o cool off s u f f i c i e n t l y t o s t a r t work i n s i d e . This period ranges from about 76 hours with a comparatively c lean k i l n t o 3 days o r more. Even a f t e r 3 o r 4 days of cool ing, the t h i c k e r a c c r e t i o n s a r e s t i l l red hot ins ide . ' Jack hammers' and 'tommy b a r s ' have t o be used f o r p r i z i n g of f the a c c r e t i o n s which a r e then t rundled along t o the k i l n discharge end i n a wheel-barrow, f o r disposal .

During t h i s per iod, charac te r ized by t h e formation of a c c r e t i o n s , t h e amount of reductant used ranged from about 15% t o about 25% by weight of the z inc bear ing feed. For most of the time the discharge chute was open t o atmosphere which rendered i t p r a c t i c a l l y impossible t o prevent an excessive volume of a i r from being drawn i n t o the k i l n and cause the formation of a c c r e t i o n s near the discharge end. Con- s t a n t t rouble was a l s o experienced wi th ma1 operat ion of the cool ing tower, r e s u l t i n g i n a f lood of water pouring i n t o the cool ing tower oxide, tu rn ing i t i n t o a pas te which could not be handled by the screw conveyor o r the s t a r feeder .

Both pneumatic t r a n s f e r systems a l s o con t inua l ly blocked and rendered s teady operat ion impossible. Dust and p r e c i p i t a t o r oxide were con t inua l ly being e i t h e r dumped on the ground o r blown i n t o the atmosphere making i t impossible t o ob ta in any semblance of a metal lur- g i c a l balance.

A t t h i s s t age discharge chute was then e f f e c t i v e l y sea led , t h e percentage of reductant added was increased and the temperature of opera t ion was d e l i b e r a t e l y kept low i n a n endeavour t o prevent the formation of accre t ions . Th is r e s u l t e d i n higher z inc va lues i n t h e res idue but did prolong t h e length of t h e campaigns. The cool ing

tower and bo th pneumatic t r a n s f e r systems cont inued t o g i v e t roub le .

During campaign No.8 which s t a r t e d d u r i n g J u l y , 1969, t he q u a n t i t y of r e d u c t a n t added a s a percentage of t h e new z i n c b e a r i n g f eed was i n c r e a s e d s u b s t a n t i a l l y t o ove r 5%. T h i s enabled h i g h e r o p e r a t i n g t empera tu res t o be used wi thou t a r a p i d bu i ldup of l a r g e a c c r e t i o n s . From t h i s pe r iod onwards k i l n s toppages were brought about a lmost e n t i r e l y by mechanical breakdowns of one type o r a n o t h e r , and not p r i m a r i l y by t h e n e c e s s i t y t o remove a c c r e t i o n s . It was found t h a t a l t h o u g h a c c r e t i o n s s t i l l tended t o form, they were v e r y f r i a b l e and b r i t t l e and g e n e r a l l y f e l l down of t h e i r own accord when they grew t o o b i g because t h e burden con ta ined s o much unburnt coke. T h i s r a i s e d a n o t h e r problem, however, which p e r i o d i c a l l y caused a n i n c r e a s e i n the z i n c con ten t of t h e r e s i d u e and a l s o reduced the percentage z i n c i n t h e Waelz oxide produced. I f a r i n g formed anywhere, t h e burden dammed up behind i t u n t i l t h e r i n g c o l l a p s e d when t h e accumu- l a t i o n rushed through t h e k i l n and was d i scha rged be fo re g e t t i n g ho t enough f o r t h e z i n c t o be v o l a t i l i z e d . Fur thermore , a s a r e s u l t of t h e a d d i t i o n a l a g i t a t i o n , a g r e a t d u s t c loud was formed, much of which was swept out w i t h t h e exhaust g a s e s and p o l l u t e d t h e Waelz oxide.

A t t h i s s t a g e p e l l e t i z i n g of Waelz oxide was t e m p o r a r i l y suspended a t t h e r e q u e s t of Zinc Corpora t ion , who were having cons ide rab le d i f - f i c u l t y i n hand l ing and d i s s o l v i n g t h e p e l l e t s supp l i ed . A t l e a s t 5% of t h e p e l l e t s had broken down t o powder by t h e t ime t h e profluct had been off - loaded a t t h e i r s i l o s . Fur thermore , t h e r e were a ce r - t a i n number of o v e r s i z e p e l l e t s up t o 50 mm i n d i ame te r and ha rd lumps of agglomerated m a t e r i a l . The moi s tu re c o n t e n t was about 7%. A t K i l n P roduc t s , a s a temporary procedure , t h e Waelz oxide from t h e s t o r a g e s i l o s was d i scha rged onto t h e f i n a l p e l l e t conveyor b e l t by- p a s s i n g t h e p e l l e t i z i n g d i s c s , dropped on to t h e ground and loaded i n t o a 24 ton t i p p i n g t r u c k by means of a f r o n t end l o a d e r , f o r t r a n s - p o r t t o t h e r a i l w a y s i d i n g a t Groo t fon te in . Here t h e product was t i pped onto t h e l o a d i n g p l a t fo rm and loaded i n t o open r a i l w a y t r u c k s , l i n e d w i t h 0.15 mm polythene s h e e t i n g , w i t h t h e a i d of a f r o n t end l o a d e r . The t r u c k s were covered w i t h waterproofed t a r p a u l i n s p r i o r t o despa tch . The powder w a s found t o have a b u l k d e n s i t y of about 1.25 g/cm3, which was cons ide rab ly h i g h e r t h a n had been a n t i c i p a t e d from t h e o r i g i n a l p i l o t p l a n t work, when t h e bu lk d e n s i t y was on ly 0.83 g/cm3. Despi te t h e p r i m i t i v e hand l ing f a c i l i t i e s f o r t h i s pro- d u c t , d u s t i n g and d u s t l o s s e s were no t n e a r l y a s s e r i o u s a s had been expected .

Due t o c o n t i n u a l b lockages i n bo th t h e pneumatic t r a n s f e r systems and the r e s u l t a n t i n t e r f e r e n c e w i t h p roduc t ion , Lurgi under took t o r edes ign both systems. A s t h i s could no t be implemented f o r s e v e r a l months, v a r i o u s temporary ar rangements had t o be made, i n o r d e r t o keep producing. A l l p r eox ide , c o o l i n g tower oxide and p r e c i p i t a t o r oxide were d ischarged onto the ground and handled by means of f r o n t end l o a d e r s , t h e f i r s t two p roduc t s be ing r e t u r n e d t o t h e k i l n f e e d w i t h t h e rec la imed s l ime whi le t h e p r e c i p i t a t o r oxide was despatched

without being p e l l e t i z e d . The preoxide from t h e s e t t l i n g chambers was discharged a t a temperature i n excess of 500°C and presented the biggest problem. It was found necessary t o deposi t t h i s product on a n intermediate s tockp i le t o a l low i t t o c o o l , and then spray water onto i t t o wet i t before r e t u r n i n g i t t o the feed b e l t . Otherwise when the hot dus t was dropped onto the reclaimed slime con ta in ing about 1% of mois ture s o much steam was generated t h a t a l a r g e dust cloud r e s u l t e d .

A t t he beginning of November the pneumatic t r a n s f e r system f o r the p r e c i p i t a t o r oxide was overhauled and recommissioned temporar i ly , and the product was aga in p e l l e t i z e d . Two important a l t e r a t i o n s were made t o the pneumatic system which e l imina ted most of the e a r l i e r t r o u b l e , viz:-

1. Oxide was d e l i v e r e d i n t o t h e system a t one po in t only , i n the f i r s t ins tance , by opera t ing only one p r e c i p i t a t o r a t a time and l a t e r when opera t ing both p r e c i p i t a t o r s i n p a r a l l e l , by f i t t i n g a time switch which allowed oxide from only one p r e c i p i t a t o r t o be discharged i n t o t h e system a t a t ime, by s topping a l t e r n a t e Redler conveyors a t the bottom of the p r e c i p i t a t o r s f o r 15 min- u t e s p e l l s .

2. A pressure switch was i n s t a l l e d i n the a i r duct ad jacen t t o the blower which t r i p p e d out the Redlers whenever the pressure rose too high, t o a l low time f o r t h e system t o c l e a r i t s e l f . A s soon a s t h i s happened the pressure dropped and the Redler automatic- a l l y s t a r t e d up aga in .

With a b i t more experience and perseverance i t was found poss ib le t o produce p e l l e t s of r e l a t i v e l y uniform s i z e and by c a r e f u l supervi- s i o n of t h e handl ing, t o prevent excess ive subsequent breakage of the p e l l e t s .

A t about t h e same time temporary arrangements were made t o spray s u f f i c i e n t water onto the dust emerging from each of the 3 s e t t l i n g chambers and from the coo l ing tower, t o t u r n the product i n t o a watery s l u r r y which was allowed t o g r a v i t a t e i n t o a submerged sump and pumped i n t o s e t t l i n g paddocks nearby. A f t e r s e t t l i n g and drying f o r a few days, t h e preoxide was reclaimed by means of f r o n t end l o a d e r s and re turned t o t h e c i r c u i t wi th t h e reclaimed slime. This system, a l though i t n e c e s s i t a t e d double handl ing and rendered i t more d i f f i c u l t t o c a l c u l a t e the percentage recovery, worked reason- a b l y s a t i s f a c t o r i l y u n t i l February when i t s t a r t e d r a i n i n g heavi ly . The whole se t t l ement a r e a became a bog and i t was impossible t o use t h e f r o n t end loader . It then became necessary t o r e v e r t t o t h e e a r l i e r system of dropping the hot dus t onto the f l o o r . For a few days, however, a temporary pneumatic t r a n s f e r system was introduced t o recyc le t h e coo l ing tower oxide on ly , by discharging t h e dust p l u s c a r r i e r a i r d i r e c t l y down t h e feed chute i n t o the k i l n w i t h the new feed. Mechanically t h i s worked s a t i s f a c t o r i l y but i t was noted

d u r i n g t h i s time t h a t the z inc con ten t of t h e Waelz oxide produced was c o n s i s t e n t l y l e s s than 6 @ , p o s s i b l y due t o excess ive r e c i r c u l a - t i o n of d u s t .

During campaign No.13 i t became p r o g r e s s i v e l y more d i f f i c u l t t o ma in ta in a nega t ive p res su re a t the d ischarge end of t h e k i l n , u n t i l e v e n t u a l l y i t was necessary t o s t o p product ion and c l o s e down. Typi- c a l p r e s s u r e s a t v a r i o u s p o i n t s i n the c i r c u i t measured by means of water manometers immediately p r i o r t o t h e end of campaign No.13 a r e shown i n Table lX, wi th t y p i c a l p r e s s u r e s subsequent ly obta ined dur ing campaign No. 14.

Table 1X. Water Gauge P r e s s u r e s - mm w g

It was found t h a t t h e duct d e l i v e r i n g t h e g a s from t h e s e t t l i n g chambers t o t h e coo l ing tower was complete ly blocked w i t h d u s t which had s e t t l e d on t h e b a f f l e s w i t h i n t h e uppermost bend.

Towards ecd Camp i g n No. 13

Campaign No. 14

A few days a f t e r t h e s t a r t of campaign No.14 i t became necessa ry t o s t o p t h e f eed and a l low the k i l n t o c o o l down s u f f i c i e n t l y f o r a few days t o enable a l a r g e accumulation of oxide t o be removed from the coo l ing tower. Th i s had depos i t ed i n the tower presumably over a long pe r iod of t ime , i n t h e form of a s t a l a g m i t e , t h e base of which blanked o f f more than h a l f t h e base of t h e c o o l i n g tower. The d e p o s i t was about 10 me t res i n he igh t and had conso l ida ted t o such a n e x t e n t t h a t i t took 4 days t o remove, us ing shove l s and pickaxes. The t o t a l weight removed was n e a r l y 200 tons .

Much of t h e t r o u b l e exper ienced i n c o n t r o l l i n g the temperature i n the c o o l i n g tower i s caused by t h e hardness of t h e water used. A t y p i c a l water a n a l y s i s i s shown i n Table X .

Wain fan , s u c t i o n

- 110

- 35

I n l e t t o P r e c i p i t a -

t o r s

- 90

- 20

Cool ing Tower

- 90

- 15

S e t t l i n g

Chambers

- 10

- 1 0

K i l n Discharge

- 4

- 4

WAELZ TREATING OF COMPLEX ZINC-LEAD ORES

Table X. Analysis of Process Water a t Ki ln Products

Sca le accumulates i n t h e e n t i r e system which i n t h e f i r s t ins tance reduces t h e volume of water a v a i l a b l e a t t h e nozzles. Correct func- t i o n i n g of the nozzles i s hampered not only by t h e bui ldup of s c a l e i n t h e nozzles themselves, but a l s o by loose b i t s of s c a l e lodging i n t h e i n l e t por t s . When t h i s happens,large drops of water a r e d i s - charged which do not vaporize f a s t enough t o prevent t h e accumula- t i o n of water a t the base of the tower. Th i s wets t h e oxide and t u r n s i t i n t o a pas te which cannot be t r anspor ted by t h e equipment provided. It then becomes necessary t o open t h e f l a p s below the screw conveyor and drop t h e product onto the f l o o r . While opera t ing wi th t h e f l a p s open much of t h e coo l ing tower oxide i s a s p i r a t e d i n t o the p r e c i p i t a t o r s thus lowering t h e z inc content of the Waelz oxide.

T o t a l d i s so lved s o l i d s T o t a l a l k a l i n i t y , a s CaC03 T o t a l hardness , a s CaCO3 Calcium, a s Ca Magnesium, a s Mg Fluor ine , a s F Chlor ide , a s C 1 Su lpha te , a s SO4 PH

S t e p s a r e now being taken t o s o f t e n t h e water by the c o n t r o l l e d a d d i t i o n of l ime, followed by f i l t r a t i o n .

459 mil l igrams p e r litre 4 39 406 7.8

92.7 0.3 0.1 0.1 7.1

K i l n Gun

A s i t i s common p r a c t i c e i n the cement i n d u s t r y t o use k i l n guns f i r i n g l ead s l u g s t o break down rings, a WesternRingblaster k i l n gun was purchased. Th i s gun has a bore of approximately 23 mm and f i r e s 85 gram s o l i d l ead s l u g s a t a muzzle v e l o c i t y of 500 metres/second. It i s mounted wi th r e c o i l s p r i n g s on a s u b s t a n t i a l g u n support . It h a s been found t h a t comparatively narrow rings up t o about l m i n width can be e f f e c t i v e l y co l l apsed by c u t t i n g a l o n g i t u d i n a l s l o t r i g h t through t h e r i n g . Th i s u s u a l l y r e q u i r e s a t l e a s t 500 rounds of ammunition. The gun i s q u i t e i n e f f e c t i v e wi th rings which a r e e i t h e r much wider than t h i s o r more than about 20 metres from the discharge end of the k i l n .

Comments on D i f f i c u l t i e s Encountered

Accret ions

Samples of a c c r e t i o n s removed were submitted t o t h e l abora to ry , wi th samples of t h e wi l l emi te concentra te and reclaimed slime being charged t o t h e k i l n , f o r determinat ion of the temperature a t which these products s t a r t e d melting. The s o f t e n i n g p o i n t s a r e shown i n Table X 1 below:-

Table X 1 . Sof ten ing temperature of va r ious products

Por t ion of one t y p i c a l a c c r e t i o n approximately 14 cm t h i c k was examined mineralogical ly . It was found t o have a layered s t r u c t u r e marked by t e x t u r e and a l s o compositional d i f fe rences . The s p e c i f i c g r a v i t y ranged from 3.26 f o r t h e l a y e r ad jacen t t o t h e k i l n l i n i n g ,

Product

Vi l lea i te concentrate Reclaimed slime Accretion eru~ple A Accretion sample B Accretion sample C Kiln residue

t o 7.68 f o r t h e su r face l a y e r . The corresponding z inc con ten t s were 8.25% and 28.@ respec t ive ly . The ch ie f minerals i d e n t i f i e d were d iops ide , m e l i l i t e , p e r i c l a s e and calcium oxide, wi th unconverted wi l l emi te and g lobu les of m e t a l l i c z inc i n the l a y e r s nea re r t h e su r face . P e r i c l a s e and calcium oxide a r e obviously t h e formation products of dolomite a t e l eva ted temperatures. Diopside and m e l i l i t e a r e complex s i l i c a t e s of calcium, magnesium and aluminium. Diopside probably begins t o form a t a temperature between 6500C and 800°C and i s s t a b l e up t o about 1 3 5 0 ~ ~ . M e l i l i t e i s a high temperature mineral and begins t o form a t around 800°C t o 9 0 0 ~ ~ and i s s t a b l e up t o a temperature of about 1 4 0 0 ~ ~ .

SofteningoT~peratul*t C

1150 1150 1120 1200 1300 1150

Pneumatic t r a n s f e r systems

The pneumatic t r a n s f e r systems were designed t o work under suc- t i o n a t a maximum vacuum of about 0.5 atmospheres. I n each case a Rootes blower evacuated t h e warm a i r from a bag f i l t e r and discharged t o atmosphere. A i r was allowed t o be drawn i n t o the conveying p i p e l i n e immediately behind t h e fur thermost dust f eed ing po in t s . I n the case of the preoxide, dus t was f e d v i a s t a r f eeders

i n t o the s e t t l i n g chambers. The dus t c o l l e c t e d i n the cyclone was f e d i n t o a screw conveyor and moistened wi th water before a l s o being dropped down the feed chute. Two such systems were i n s t a l l e d , one a s a standby. The preoxide bag f i l t e r which t h u s became redundant was re- i n s t a l l e d a s a standby u n i t i n t h e Waele oxide c i r c u i t .

Thus modified, the preoxide pneumatic t r a n s f e r system has worked sploothly. However, the screw conveyor under the coo l ing tower proved t o be under c a p a c i t y and t r i p p e d out s e v e r a l t imes on overload before the s h a f t even tua l ly broke, a f t e r l e s s than two weeks i n operat ion.

The modified p r e c i p i t a t o r oxide system was s t i l l not capable of opera t ing a t the requ i red capac i ty of 8 t o n s pe r hour wi thout f r e - quent choking. Consequently t h e fol lowing f u r t h e r modi f i ca t ions were c a r r i e d out dur ing A p r i l , 1970:-

( a ) The c r o s s Redler , newly i n s t a l l e d between t h e two p r e c i p i t a t o r s , was removed and i n s t a l l e d below t h e coo l ing tower i n place of t h e screw conveyor.

( b ) Each e l e c t r o s t a t i c p r e c i p i t a t o r was provided wi th a completely separa te pneumatic t r a n s f e r system.

A i r i n l e t tubes

The o r i g i n a l a i r i n l e t tubes were cons t ruc ted of a 25% chromium, 1% n i c k e l a l l o y i n 3 p a r t s , v ie : -

( a ) the end p o r t i o n approximately 60 cm i n l eng th wi th the discharge nozzle

( b ) t h e tubu la r middle s e c t i o n approximately 130 cm long, which was spun c a s t and

(-c) t h e f l anged end s e c t i o n approximately 40 cm long.

These t h r e e p o r t i o n s were welded toge ther .

The a i r i n l e t tubes s t a r t e d f a i l i n g from about August, 1969, when No.1 a i r tube was found t o have a c i rcumferen t i a l c rack j u s t over h a l f a metre from t h e end p a r a l l e l t o t h e o r i g i n a l weld. a s can be seen i n Fig. 6

F i g . 6

T h i s was welded i n p o s i t i o n , bu t by t h e end of campaign No.8 about a month l a t e r , t h e c r a c k had opened up a g a i n and t h e m e t a l was s o s e v e r e l y c r y s t a l l i z e d t h a t i t became necessa ry t o remove t h e tube , a f t e r 146 days i n a c t u a l o p e r a t i o n . T h i s tube was not r ep laced a t t h i s s t a g e .

No.? a i r tube a c t u a l l y broke o f f somewhere nea r t h e weld a t t h e base du r ing No.9 campaign and was a l s o not r ep laced - i t had been i n o p e r a t i o n f o r 161 days. By t h i s s t a g e bo th N o s . 2 and 4 a i r t u b e s had developed c i r c u m f e r e n t i a l c r a c k s s i m i l a r t o No.1 and had t o be welded. The next s toppage w a s brought about by t h e breakage of No.4 a i r t ube . It and No.2 and 5 a i r t ubes were removed a f t e r 196 days i n a c t u a l ope ra t ion . No.2 had cracked c i r c u m f e r e n t i a l l y bu t No.5 had a c t u a l l y abraded away t o such a n e x t e n t t h a t a h o l e had appeared n e a r t h e e x t r e m i t y .

Two new t u b e s of somewhat d i f f e r e n t des ign , c a s t i n one p i ece by Scaw Meta ls from a n a l l o y c o n t a i n i n g 25% chromium and 20% n i c k e l were f i t t e d i n t h e Nos. 2 and 5 p o s i t i o n s wh i l e p o s i t i o n s Nos. 1 , 3 and 4 were blanked o f f . No.2 Scaw tube had t o be removed a f t e r on ly 25 days of o p e r a t i o n due t o s e v e r e c r a c k i n g bo th long i tud in - a l l y and c i r c u m f e r e n t i a l l y . The No.2 p o s i t i o n was then a l s o blanked o f f .

A f t e r a f u r t h e r 27 days of o p e r a t i o n i t became necessa ry t o s t o p t h e k i l n due t o b r i c k damage a t t h e f e e d end and a l s o a s eve re blockage i n t h e duc t t o t h e c o o l i n g tower. No.5 Scaw tube appeared t o be i n good c o n d i t i o n but No.6 o r i g i n a l tube a f t e r 248 days i n use had developed a sma l l c i r c u m f e r e n t i a l c r a c k and a bulge . I t was, however, dec ided t o o p e r a t e one more campaign b e f o r e r e p l a c i n g i t .

Nos. 7 and 8 tubes were s t i l l apparen t ly i n good condi t ion. It was decided t o put back one tube neare r t h e discharge end t o f a c i l i t a t e b e t t e r d i s t r i b u t i o n of a i r . A s a n experiment a new Lurgi tube was i n s t a l l e d i n the No.1 p o s i t i o n with the a i r i n l e t por t r o t a t e d through 180° so t h a t the discharge was d i r e c t e d towards the feed end. A s no r i n g s developed anywhere near No.1 f a n , i t was decided a t the end of campaign No.14 t o opera te wi th Nos. 1, 2 and 4 f a n s discharging towards t h e k i l n feed end and Nos. 7 and 8 f a n s d i s - charging i n the opposi te d i r e c t i o n . By t h i s s t age No.6 a i r tube had f a i l e d completely, a f t e r 280 days i n operat ion. Scaw tube No.5 a f t e r 84 days i n use a l s o had t o be removed due t o metal f a i l u r e .

I t i s of i n t e r e s t t o note t h a t Komley and ~ a r e e v ' l recommend, a s r e s u l t of an i n v e s t i g a t i o n a t t h e Chelyabinsk Waelz Ki ln , 40 m i n l e n g t h t r e a t i n g l each p lan t r e s i d u e s , t h a t i t i s h igh ly d e s i r a b l e t h a t a i r should be in t roduced i n t o the k i l n stagewise i n proport ion t o the oxygen demand i n t h e p a r t i c u l a r zone. I n t h e case of the above k i l n , they cons ide r t h a t 20% t o 25% of the t o t a l a i r should be suppl ied a t a po in t 21 metres from the feed end and t h a t addi- t i o n a l a i r i s 3 l s o requ i red a t 27 t o 30 metres from the feed end. Kubyshev e t a 1 r e p o r t t h a t i n the treatment of oxidized z i n c o r e s i n a k i l n a t Achisaisk , improved r e s u l t s were obtained by introduc- i n g 1 800 t o 2 500 m3/h of a i r a t a pressure of 7 t o 8 atmospheres, a t the discharge end of the k i l n , the high pressure being required t o ensure t h a t the a i r t r a v e l l e d we l l down the k i l n before the oxygen could be depleted.

Brick Lining

Right from the s t a r t t roub le was experienced w i t h the br ick- l i n i n g i n t h e feed cone t o the k i l n . I n i t i a l l y these b r i c k s s p a l l e d r a t h e r severe ly and then s t a r t e d dropping out . Patching was q u i t e i n e f f e c t i v e and e v e n t u a l l y a t the end of November, 1969, the c o n i c a l por t ion was c a s t i n p o s i t i o n using r e f r a c t o r y m a t e r i a l REFCAST S .P . , wi th t y p i c a l p r o p e r t i e s a s shown i n Table X 1 1 . The f i r s t 3 rows of b r i c k s adjacent t o the cone were a l s o replaced using Super l in b r icks . A f t e r 52 days the c a s t m a t e r i a l s t i l l appeared t o be i n good condi t ion b u t the b r i c k s had a l l f a l l e n out aga in and had t o be replaced.

T a b l e X 1 1 . R e f c a s t S.P. - T y p i c a l P r o p e r t i e s

The f i r s t s i g n s o f b r i c k f a i l u r e i n t h e main body of t h e k i l n were n o t i c e d a f t e r 196 d a y s o f o p e r a t i o n . The zone e x t e n d i n g from 42 .5 m e t r e s t o 49 .6 m e t r e s from t h e f e e d end was t h e n r e p l a c e d u s i n g L u s i t e b r i c k s as b e f o r e b u t i n c l u d i n g 2 rows of S u p e r l i n e s a t e s t . The L u s i t e b r i c k s t h a t had t o be r e p l a c e d had a l l f a i l e d i n t h e m i d d l e . The t o p 5 o f 1 0 crns o f e a c h b r i c k were s t i l l e x t r e m e l y h a r d , b u t had a c q u i r e d 3 p u r p l i s h c o l o u r and t h e zone 5 t o 1 0 crns d e e p , i n c o n t a c t w i t h t h e s t e e l s h e l l had t u r n e d n g r e y i s h - b l a c k c o l o u r b u t was a l s o s t i l l h a r d . The m i d d l e p o r t i o n was c o m p l e t e l y f r i a b l e and impregna ted w i t h s l a g , g l o j u l e s o f m e t a l end c a r b o n . T h i s i s i l l u s t r a t e d i n F i g . 7 below - t h e s u r f a c e o f t h e b r i c k s i n c o n t a c t w i t h t h e burden c a n be s e e n i n t h e t o p l e f t hand c o r n e r and t h e s t e e l s h e l l o f t h e k i l n i n t h e bot tom r i g h t hand c o r n e r .

i

S i 0 2 33.8 CaO 14.9

A1203 42.4 ago 0.1 Fe 0

2 3 6.9 Loss on ignition 0.5 T i 0 2 1.5

Brick density

A i r d r i e d 2.24 g/cm 3

F i r e d to 1250'~ 2.08 g/cm3 - A i r dr ied 425 k&lcm2 F i r e d to 1 2 5 0 ~ ~ 140 d c m 2

Linear c h a w aft= - 0.5 to 1%

.

Fig. 7

A f t e r a f u r t h e r 25 days a n o t h e r 1 .5 m e t r e s were renewed from 49.6 me t re s t o 51 .1 me t re s from t h e f e e d end. 27 days l a t e r 1.6 m e t r e s were renewed from 42.5 me t re s t o 40.9 met res from t h e f e e d end.

A t t h e end of campaign No.14, 1 .4 me t re s of t h e l i n i n g , v i z . from 42.5 t o 43.9 me t re s from f e e d end , which had p r e v i o u s l y been renewed a f t e r 196 days i n o p e r a t i o n , had t o be r ep laced f o r t h e second t ime a f t e r 84 days i n o p e r a t i o n . T h i s t ime S u p e r l i n b r i c k s were used.

The two t e s t rows of S u p e r l i n b r i c k s p r e v i o u s l y f i t t e d i n t h i s same zone s u p e r f i c i a l l y s t i l l appeared t o be i n good c o n d i t i o n . T h i s w a s confirmed by test d r i l l i n g , a s shown i n Table X .

Using a dianond d r i l l , approximate ly 70 mm i n d i ame te r , 6 t e s t h o l e s were d r i l l e d i n b r i c k s a t s e l e c t e d p o i n t s i n t h e k i l n and t h e b o r e c o r e s examined. The r e s u l t s a r e t a b u l a t e d i n Table X l l l


Tab le X 1 1 1 . Bore Cores from Br i ck L i n i n g

Prom a c r i t i c a l examinat ion of t h e type of b r i c k f a i l u r e , i t

Conditions of cony

i n colour tetndiry t~rrrds black - suspect oarbon i.pir(oation.

core broken, but no sign of carbon i.pm&ntation.

a p p e a r s t h a t t h e damage h a s been caused by t h e p e n e t r a t i o n of g a s e s fo l lowed by some chemical and o r p h y s i c a l changes w i t h i n t h e body of t,he b r i c k . Normally t h e s u r f a c e of t h e b r i c k i n c o n t a c t w i t h t h e burden w i l l have a tempera ture i n t h e r e g i o n of l000oC t o 1 2 0 0 ' ~ wh i l e t h e end i n c o n t a c t w i t h t h e s h e l l i s a t a tempera ture of 200 t o 3 0 0 ' ~ .

5

6

The carbon t i epos i t i on r e a c t i o n i s w e l l known and may occu r i n t h e tempera ture range 4 0 0 ' ~ t o 7 0 0 ' ~ a s fo l lows: -

T h i s r e a c t i o n a p p e a r s t o be c a t a l y s e d by t h e presence of i r o n o x i d e s and could r e s u l t i n t h e d e p o s i t i o n of carbon a l o n g t h e j o i n t s and i n t h e po re s of t h e b r i c k s , s e t t i n g up s t r e s s e s which cause crack- i n g of t h e l i n i n g .

32.5

22

Zinc vapour could a l s o p e n e t r a t e t he b r i c k s t o a p o i n t where t h e tempera ture i s s u f f i c i e n t l y low t o cause condensa t ion of t h e vapour. The presence of g l o b u l e s of m e t a l l i c z i n c i n t h e c e n t r e of t h e b r i c k s i s c l e a r proof t h a t t h i s has occu r red .

A f u r t h e r r e a c t i o n i n v o l v i n g z i n c vapour i s p o s s i b l e . The C02 produced by the carbon d e p o s i t i o n r e a c t i o n , whi le d i f f u s i n g back i n t o t,he k i l n could r e a c t w i t h t h e z i n c vapour p e n e t r a t i n g t h e b r i c k s and. cause re-oxidr:ti.on by the r e a c t i o n .

Lusite

Superlin

280

280

Dark grey i n colour, but no sign of carbon impmgantion.

Light grey with mauve ti- - no sign of carbon i.pmgmtion

Zn (vapour ) + C02 --+ ZnO + C O

The z i n c oxide produced by t h i s r e a c t i o n would a l s o d e p o s i t w i t h i n t h e b r i c k and behave i n t h e same f a s h i o n a s t h e carbon produced by t h e carbon d e p o s i t i o n r e a c t i o n .

F u r t h e r l a b o r a t o r y work i s a t p r e s e n t b e i n g conducted t o confirm t h e above assumpt ions . However, i t i s s i g n i f i c a n t t o n o t e t h a t t h e S u p e r l i n b r i c k s i n t h e 42 m (from f e e d end) zone have s tood up much b e t t e r t h a n t h e a d j a c e n t L u s i t e b r i c k s which had t o be r ep l aced a f t e r 84 days . I n f u t u r e Jumbo b r i c k s w i l l be used a s t h e s e have s t i l l lower p o r o s i t y and p e r m e a b i l i t y c h a r a c t e r i s t i c s and a lower i r o n con- t e n t , a s can be s e e n from Table V 1 .

Thermocouples i n K i ln

The on ly thermocouple i n t he k i l n t h a t h a s con t inued t o provide r e l i a b l e r e a d i n g s i s No.11 which i s app rox ima te ly 8 m e t r e s from t h e f e e d end , where a tempera ture of 400°c t o 500°C i s r eco rded . I t i s suspec t ed t h a t t h i s r e a d i n g i s p robab ly h i g h e r t han t h e t r u e tempera- t u r e of t h e burden and lower t h a n t h e g a s t empera tu re s a t t h i s p o i n t . I t d o e s , however, p rov ide a u s e f u l gu ide f o r o p e r a t i o n a l purposes .

The o t h e r thermocouples v e r y r a r e l y r e c o r d a r e l i a b l e tempera ture f o r more t h a n a few days ; t h e r e a f t e r t h e y e i t h e r become embedded i n a n a c c r e t i o n and t h e n r eco rd a n e r r o n e o u s l y low tempera ture o r t h e y become p h y s i c a l l y damaged by l a r g e lumps of f a l l e n a c c r e t i o n r o l l i n g around. A p o r t a b l e thermocouple i s used f o r de t e rmin ing t h e tempera ture o f t he r e s i d u e d i s c h a r g e from t h e k i l n . At h o u r l y i n t e r v a l s i t i s i n s e r t e d , f o r a few m i n u t e s , through a gu ide tube s u i t a b l y p o s i t i o n e d t o a l l ow t h e r e s i d u e t o cascade o v e r t h e end of t h e thermocouple. T h i s h a s been found t o be more r e l i a b l e t han u s i n g a n o p t i c a l pyrometer , due t o the i n t e r f e r e n c e o f t e n caused by exces- s i v e l y d u s t y c o n d i t i o n s w i t h i n t h e k i l n .

The s h e a t h s of t h e thermocouple a r e f a b r i c a t e d from a 26% C r , 5% N i a l l o y and t h e r e h a s been no ev idence of t he s h e a t h s e i t h e r b e i n g ab raded o r e roded away no r of any a c t u a l m e t a l f a i l u r e .

Suppor t ing r o l l e r s

I n mid J a n u a r y , 1970, one of t h e r o l l e r s s u p p o r t i n g t h e t h i r d r i d i n g r i n g s topped r o t a t i n g w i t h t h e k i l n . I t was found t h a t t h e s h a f t had shea red . The k i l n was o p e r a t e d wi thou t t h i s r o l l e r f o r a week, w i thou t any a p p a r e n t d e l e t e r i o u s e f f e c t s , wh i l e a new s h a f t was b e i n g f i t t e d . Dur ing A p r i l , abou t 3 months l a t e r , t h e same r o l l e r a g a i n s topped r o t a t i n g . It was found t h a t t h e r o l l e r had moved on t h e s h a f t , s o t h e whole assembly was renewed. I n v e s t i g a - t i o n s a r e p roceed ing t o e s t a b l i s h t h e cause of t he f a i l u r e .

a t 4 d i f f e r en t po in ts , i n to one common conveying pipel ine which discharged in to a cyclone, the overflow from the cyclone being f i l t e r e d i n a bag house. In the case of the Waelz oxide, oxide was delivered in to the conveying pipel ine a t 2 d i f f e r en t points v i a s t a r feeders , and was co l lec ted i n e i t h e r of two dust cyclones each s i t ua t ed imme- d i a t e ly above a storage s i l o . The cyclone exhaust gas was f i l t e r e d i n a bag f i l t e r . The systems suffered from the following defects:-

( a ) the conveying p ipe l ines choked frequently

(b ) thebends and the cyclone were very rapidly abraded away par t i - cu l a r ly i n the case of the preoxide

( c ) the cyclone dust ou t l e t choked frequently

( d ) the d u s t ou t l e t from the f i l t e r choked frequently

( e ) the f i l t e r bags themselves charred brown and appeared t o shrink and pu l l off the f i t t i n g s .

( f ) the f i l t e r bags ac tua l ly bu r s t , o f ten a t the seams

( g ) a s no s t a r feeders were f i t t e d between the cyclones and the respective storage s i l o s , these s i l o s became pa r t of the vacuum system and caused a grea t deal of i n e r t i a i n the c i r c u i t . Hence i f any inc ip ien t blockage occurred a t the feed point i n to the system, there was a considerable time l ag before the vacuum i n the system increased su f f i c i en t ly t o c l e a r the blockage. Before t h i s happened, the c i r c u i t had invariably blocked up com- p le te ly . This was pa r t i cu l a r ly bad i n the case of the Waelz oxide c i r c u i t due t o having the two 100 ton s i l o s a s par t of the vacuum system.

A s a f i r s t modification the suppliers of the equipment decided t o a l t e r both systems t o work under pressure instead of under vacuum, a s t h i s would overcome the time l ag problem. However, a s most of the s t a r feeders had not been designed t o work under pressur5, this scheme did not work e i t h e r , a s su f f i c i en t a i r leaked back through the s t a r feeders t o i n t e r f e r e with the discharge of the dust i n t o the conveying system. Dust a l s o escaped through the various s ea l s and discharged copiously i n to the atmosphere.

During March, 1970, a cross Redler was i n s t a l l e d t o pick up the oxide from the two Redlers under the e l e c t r o s t a t i c p rec ip i t a to r s and de l iver i t v i a a K i l l e r pump i n t o the pneumatic system operating under pressure. A t the same time one Redler was i n s t a l l ed t o co l l ec t the preoxide from the 3 s e t t l i n g chambers and a l so the cooling tower oxide and de l iver the combined load i n t o the pneumatic system a t one point. This a l so worked under pressure and discharged i n t o a dust co l lec t ing cyclone positioned immediately above the feed end of the k i ln . The gas leaving the top of the cyclone was discharged back $,.

Recovery of coke from t h e res idue

I n o rde r t o mainta in a g r a n u l a r , f r e e f lowing discharge from the k i l n i t has been found necessary t o ma in ta in a f r e e carbon content of about 35% i n the r e s idue . Even i f a c c r e t i o n s do form a t i s o l a t e d

' s p o t s due t o l o c a l i z e d overhea t ing , a c c r e t i o n s wi th t h i s amount of s u r p l u s coke, a r e u s u a l l y s u f f i c i e n t l y f r i a b l e t o break away of t h e i r own accord from time t o time. Under normal cond i t ions over 9% of the res idue w i l l pass through a 6 mm sc reen . A s can be seen from Table V111, the amount of r educ tan t a c t u a l l y consumed i n the k i l n i s of the o rde r of 15 t o 20% of the weight of new z inc bea r ing feed; whereas, the a c t u a l a d d i t i o n is i n t h e range of 50 t o 6%. P i l o t p l a n t s c a l e j igg ing t e s t s conducted on the re s idue showed t h a t i f t h e r e s idue were reduced t o a l l minus 6 mm i n s i z e i t should be p o s s i b l e t o recover 75% of the carbon i n the r e s i d u e i n t o a product con ta in ing 70 t o 75% carbon, which could be re-used i n t h e k i l n i f t h i s could be a t t a i n e d on a p l a n t s c a l e t h e l o s s i n the r e s idue would be equ iva len t t o about 10% of t h e weight of t h e new feed which would reduce the o v e r a l l consumptions of new coke o r a n t h r a c i t e t o about 25 t o 30%.

A s a temporary expedient a second-hand 1 metre square twin hutch Yuba j i g was i n s t a l l e d i n February, 1970, t o recover a s much coke a s p o s s i b l e . Due t o inadequate sc reen ing f a c i l i t i e s , i n s u f f i c i e n t hutch water and va r ious o t h e r obvious drawbacks, t h i s p l a n t has operated ve ry i n e f f i c i e n t l y but has never the less managed t o recover up t o 50% of the carbon i n t h e r e s idue i n t o a product conta in- i n g about 70% carbon. T h i s has been fed back i n t o the k i l n without any d e l e t e r i o u s r e s u l t s . I n the l i g h t cf the exper ience g a i n e d , a f u l l s c a l e p l a n t i s now being designed.

S t a t i s t i c a l Data

Volume of g a s discharged

Under normal opera t ing c o n d i t i o n s , i t i s es t ima ted t h a t the d i s - -

t r i b u t i o n and source of gases a r e approximately a s fo l lows i n Table X1V:-

Table X 1 V . Source of Gases Exhausted up t h e S t a c k - Typica l Cond i t ions

S e a l i n g a i r f a n d e l i v e r i n g down feed chute 2 000 k 3 / h Producer gas consumed 1 330 Water vapour ex feed 1 620 C02 from decomposition of carbonates i n feed 1 280 Water vapour ex sprays i n coo l ing tower 6 360 A i r introduced v i a s h e l l f a n s 17 170 A i r introduced a s combustion a i r N i l A i r drawn i n through end of k i l n a s r e s u l t

o f draught 16 610 Hiscel laneous a i r l e a k s n m i n n l l y es t imated 2 000 - T o t a l g a s exhausted up s t a c k 48 370 -

The a c t u a l vclme of g a s e s pazs ing through t h e k i l n , from the above f i g u r e s would be 38 000 Nm~/h. Assurnirg a n average gas tem- p e r a t u r e o f s ag 1000oC, t h e volume of g a s pas s ing through the k i l n would be a p p r o x i n a t e l y 176 000 rnJ/h, e q u i v a l e n t t o a l i n e a r v e l o c i t y o!' approxi.mn:ely 5 . 3 m p e r second.

Oxygen con ten t of k i l n gases

No f a c i l i t i e s ? r e a v s i l a b l e f o r measuring t h e oxygen c o r t e n t of the gas wi th i r , t h e k i l r i i t s e l f . The r ' igures shown i n Table XV below r e f e r t o the gas i n t h e s e t t l i n g chamber and the g a s d ischarged u ~ the s t a c k r e s p e c t i v e l y . As can be seen from Table X 1 V t h e g a s i n the s e t t l i n g chamber i n c l u d e s about 2 000 N I D S / ~ of a i r blown down t h e f eed c h u t e , which d i l u t e s 38 000 Nrn3/h of g a s e s emerging from the k i l n rind would account f o r approximate ly 1% of oxygen i n the mix tu re .

T:ible XV. ';.ypical Ana lys i s of Exhaust Gases

Stnck

7.0 15.0 Trace

-

% O2 % co2 $ co

S e t t l i n g Chamber

5.3 17.1 Trace

Labour requi rements

The p l a n t o p e r a t e s w i t h t h e f o l l o w i n g l abour complement:-

Manager P l a n t foreman ~ e c r e t a r y / s t o rekeepe r Par t - t ime ~ ~ ~ i s t / c l e r k S h i f t Opera to r s ( 2 / s h i f t ) Daysh i f t Opera to r s R e l i e f Opera tor Foreman Mechanic Other Mechanics Manual Labourers

Water, e l e c t r i c power and producer g a s consumption

When o p e r a t i n g a t t he r a t e of 10 000 t o n s of new z i n c b e a r i n g f e e d p e r month t h e consumption of w a t e r , e l e c t r i c power and producer g a s a r e a s shown i n Table X V 1 below:-

Tab le XV1. Water, e l e c t r i c power and p roduce r g a s consumption

P reox ide and c o o l i n g tower oxide and Waelz oxide

There i s no ready means of de t e rmin ing the tonnage of t he f i r s t two p roduc t s , bu t s p o t checks from time t o time have i n d i c a t e d t h a t t h e preoxide (from the s e t t l i n g chambers) amounts t o about 4 t o n s p e r hour and t h e c o o l i n g tower oxide about 2 t o n s p e r hour. Typi- c a l me ta l c o n t e n t s a r e shown i n Table X V l l below:-

por t o n new feed

1 2 2 7 180

Water ( t o n s ) E l e c t r i c power (klfh) Producer gas Xu3

Tab le X V 1 1 . T y p i c a l a n a l y s e s of preoxide and c o o l i n g oxide (%)

Der month

12 000 270 000

1 800 000

Preoxide

Cool ing tower oxide

Zinc

32.8

44.0

Lead

6.9

8 . 3

KR

3.2

2 .7

Ca

5 .3

4.6

A t y p i c a l a n a l y s i s of Waelz oxide c u r r e n t l y produced i s shown i n Table X V l l l below:-

Table X V 1 1 1 . T y p i c a l A n a l y s i s of Waelz Oxide ($1

Conc lus ions

One yea r of o p e r a t i n g expe r i ence h a s shown t h a t a s a t i s f a c t o r y e l i m i n a t i o n of z i n c can be ach ieved wi thout e x c e s s i v e t r o u b l e due t o t h e fo rma t ion of a c c r e t i o n s , t h a t a s a t i s f a c t o r y g rade of Waelz oxide can be produced, and t h a t t h e l i f e of t h e r e f r a c t o r y b r i c k l i n i n g w i l l be w i t h i n t h e expected l i m i t s .

The f a c t t h a t n e i t h e r t h e des igned r a t e of p roduc t ion nor t h e a n t i c i p a t e d pe rcen tage r ecove ry have been ma in t a ined is due l a r g e l y t o t h e inadequacy of t h e pneumatic t r a n s f e r systems. Vigorous s t e p s a r e now b e i n g t a k e n t o remedy t h i s p o s i t i o n .

Acknowledgment

Thanks a r e due t o M r . R .R.M. Cousens, Techn ica l D i r e c t o r of Gold F i e l d s of South A f r i c a Limi ted , f o r permiss ion t o p u b l i s h t h i s paper .

Re fe rences

1. Bogacz M . , "F i r e - r e f i n i n g of Zinc-Lead o r e s i n Poland," f rom M a t e r i a l s of t h e 'Techn ica l Conference i n Warsaw on Ques t ions of P roduc t ion Techniques f o r Z i n c , l e ad and Assoc i a t ed M e t a l s , ' Moscow 1957, pp. 129 - 149.

2 . Johannsen F. , "The Advance i n t h e development of t h e Waelz P roces s , " Meta l1 und E r z , 1927, 24 ( 1 7 ) 425.

3. Hoffman R . , "The Waelz P roces s , ' ' T r a n s a c t i o n s of t h e American I n s t i t u t e of Mining and M e t a l l u r g i c a l E n g i n e e r s , Vol. 7 6 , 1928.

4 . H a r r i s W .E. , "The Waelz Process , ' l American I n s t i t u t e of Mining and M e t a l l u r g i c a l Eng inee r s T r a n s a c t i o n s , Vo1.121, 1936, Rocky Mountain F i n a l Volume on Metal lurgy of l e a d and z i n c .

5. Jensen C .W. , "The Waelz Process , I1 Mining Magazine, Vol. 92 , 1955, PP. 73 - 79.

6 . Bodenste in M . , "The Mechanism of the M e t a l l u r g i c a l p roduc t ion of Zinc," T r a n s a c t i o n s of t h e American E lec t rochemica l S o c i e t y (1927) , 51, 449.

7. Komlev G . A . and Gareev V.N., "Chemism of t h e Waelz method of p rocess ing Zinc Cakes," Tsvetnge met all^ The Sov ie t Journa l of Non-ferrous Meta l s , March 1964, pp. 22 - 29.

8. Truesda le E.C. and Waring R . K . , "Re la t ive Ra tes of Reac t ions involved i n Reduction of Zinc Ores ," AIME T r a n s a c t i o n s , I n s t i t u t e of Meta ls D i v i s i o n 152, 1942, pp. 303 - ,315.

9 . Kubyshev e t a l . , "Waelz t r ea tmen t of Achisacsk oxidized z i n c o r e s i n h i g h c a p a c i t y furnaces ,"Tszvetnge Meta l lx , Vol. 9 , September 1968, pp. 36 - 40.

Volume 2 - Extractive Metallurgy of Lead and Zinc, …library.aimehq.org/library/books/Volume 2 -...

Documents

Transcript of Volume 2 - Extractive Metallurgy of Lead and Zinc, …library.aimehq.org/library/books/Volume 2 -...