Goode 1990 (Goldstrike autoclave plant)

8/7/2019 Goode 1990 (Goldstrike autoclave plant)

1/11

Chapter 18

THE GOLDSTRIKE AUTOCLAVE PLANTDESIGN, COMMISSIONING AND OPERATION

K.G. ThomasSenior Vlce President , American Barr ick Resources Corp., Toronto, ONH. Pi eter se

Assi st . Vlce President-Engineering, American Ba rr lc k Resources Corp., Elko, NVR. WilliamsAutoclave General Foreman, Ba rr lc k Go ldst rl ke Mines, Inc. , Elko, NVK.S. FraserSenior Me ta ll ur gi st , K ilb orn Limited , Toronto, ONJ R. GoodeManager of Metallurgy, Kl lbo rn Limited , Toronto, ON

ABSTRACT i s about 40 km (25 miles) northwest ofthe town o f Car l ln which I s situat ed onThe Goldst rike autoclave pl an t i s In te rs ta te Highway 80. The s i t e I sdesigned f o r the pressure ox ida ti on o f about 1700 m (5576 f t ) above sea lev el1363 t / d (1500 stpd) o f re fr ac to ry gold I n th e Tuscarora Mountains.ore. The p lant was commissioned i nea rl y February 1990 over a one week I n i t i a l Gold str ike Operationsperiod - an unusually short period for apl an t o f t h i s complexity. The rapid Shallow, low-grade mine ra liz at ion wasp lan t s ta rt -up i s a t t r i bu ted t o ce r tain discovered by diamond d r l l i n g I n thedetailed design and control aspects of Goldstr lke area I n the 1960's. A heapthe plant. leach operation was establishe d I n the

mid-1970's by th e previous owners.This paper discusses key features o fthe autoclave plant with emphasis on the The Post depos it was discovered I ndesign of the splash/flash system of 1982. Gold production by min ing andheat recovery and the control systems. heap leaching reached 1 t/a (40,000Commissioni ng and p lan t operations over oz/a) .the f 1 s t three months are reviewed.American Barrick purchased theproper ty I n 1987 and star te d anBACKGROUND aggressive program of deep d r i l l i n g .This led t o the discovery of the DeepLocatlon Post/Betze deposit and establi hed

suf f i c ien t reserves to j us t i fy anThe Goldstrlke property i s located I n i n i t i a l 4100 t/ d (4500 stpd) m i 11 and aEureka County i n north-cent ra l Nevada on 5 500 000 t / a ( 6 m i l l i o n tons/a) heapthe Ca rl in geologica l trend. The mine leach operation.181


2/11


3/11

GOLDSTRIKE AUTOCLAVE PLANT DESIGN 183produced the data o f Table 2 . Flotat lontests showed only 75% gold recovery t o abulk, 20% mass pull, concentrate.

I PROCESS RECOVERY -% ICyanidation 10 t o 20Chlorlnatlon 15 t o 40Flotatlon,Roast 50 t o 60with Talls LeachWhole Ore Roast 75 t o 85

' Pressure OX I ~ . 88 t o 95Ifable 2 Recovery frm Goldstrike O re -I n i t i a l Test Data

Prellmlnary economlc analyses clearlyshowed that pressure ox idat ion was thepreferred process. I n t h i s operationthe gold-bearing sulphldes are oxidizedusing oxygen gas under elevated pressureand temperature. Oxidation destroys thesulphide structure and exposes the goldthereby maklng It more amenable t ocyanldation. Detailed study o f theprocess was i l l ted.Definitive Tests and Studies

D r l l l data and orebody model l i n gestablished the typical ore compositlonsunmarlzed i n Table 3 .The metallurgical team designed ade ta iled program o f testwork. Ani n i t l a l serles of bench scale test s wasundertaken by Lakefield Research,Ontario, Canada. A program o f p i l o t

pla nt tests was then designed andseveral laboratories Invi ted to makeproposals. The work was awarded t oHazen Research, Golden, W.Principal f lndlngs of the definitivebench and p i l o t scale te sts were:

7 g/ t (0 .2 oz/st)Si lver 1 g/ t ( .02 oz/st)CopperLeadZincI ron

I Sulphur - Total 2.5 %Sulphur - Sulphlde 2 . 1 % ICarbon - Total 1.36 %Carbon - as C 4 3 .5 %Carbon - Organlc 0 .75 %I Mercury 26 PPmArsenic 0.15 %I ITable 3 Goldstrike Ore Analysisoz/st). Fine sulphides (20 micrometres)assay as high as 170 g/t (5 oz/st).Thickening. Tests and prel minarystudies showed that an autoclave feedcontaining at least 40% solids wasdesirable. This would require 50%sollds i n the feed t o the pre-heatsystem. A thickener would therefore berequired t o partially dewater thegr inding cyclone overflow. Tests showedthat a hlgh r t e type th i c ener sizedsuitable. -3 5o r 8 .8 t/d. (0.9 stpd/f t ) would beCarbonate Reduction, Tests Indicatedth at the Goldstrlke ore was preferablyoxidized i n the presence o f 10 t o 15 g/Lo f fre e sulphuric acld. Oxidation o fthe sulphides produces acid butcarbonates i n the ore consume acld. Toensure that oxidation takes place underacid conditions, excess carbonate mustbe destroyed before the ore enters theautoclave.P The batch testsMlneraloav. Pyri te and marcaslte are established that the rate o f oxldatlonthe maln sulphldes with minor quantities Increased with free acid content of theof arsenopyrite, py rrho ti te and slu rry . Tem rature variations i n thechalcopyri te. Most of the gold occurs range of 185IY (365OF) t o 225'~ ( 4 3 7 ' ~ )as 0 .5 micrometre grains i n pyr it e and had only minor impact on the overall

marcasi e. Coarse sulphides (200 rate of oxidation, other factors beingmicrometres) contain up t o 16 g / t (0 .5 constant. Design resldence times o f


4/11

184 ADVA NCES IN GOLD AN D SILVER PROCESSING

75 minutes were required for 95%sulphlde oxidation at an oxygen partialpressure of 345 kPa (5 0 psi) .Neutrallzatlon Severalautoclave plants separate the oxidizeds lur ry in t o l iq u i d and so l id f rac t ionsby thickening. Acid ic solu tion s areneutralized and precipitated solidsdiscarded. Oxidized so li ds are washedfree of solute and neutralized beforeCIL treatment. On some ores t h i streatment y elds improved gold recovery.I t i s also a means of cooling theautoclave product pr io r t o CILprocessing.

The separate treatment rou te req ui resexpensive, acid-proof thickeners and twoseparate neut ral iza tion ci rc ui ts . Testson Goldstrike ore showed that a bulkneutralization step would work well.Design requirements were established.P i l o t Plant. The basic autoclave pl antflowsheet was established from the benchtest data and preliminary economicanalyses. Major elements o f th eproposed plan t were then tes ted i n acontinuous p i o t operation.

During the p i l o t plant operation, at o t a l o f 3 t of Goldstrike ore wereprocessed through a 24 L capacityautoclave. Six t e s t campaigns werecompleted processing up t o 10 kg/hduring runs of about 60 h duration each.0 Throughout thedevelopment period various mini-studieswere conducted t o guide t he testwork andpermit selectlon of the final design.The economic re su lt s o f a typ ic al studyare presented i n Table 4 . Plantoperabi l i ty , av ai l abi l i t y , capi ta l andoperating costs featured i n theselectlon process as well as economicfactors.

A spreadsheet template was developedt o solve t he complex materia l andthermal balances around the autoclave.This proved essential t o the rapidassessment o f al te rn at iv e pla ntconfigurations.

I PLANT CONFIGURATION COST 13 l ines, 6 compartment 71horizontal , splash/f lashheat exchanger (HX)

I 3 l ines, 6 compartmenthorizontal, double pipe HX 69 I4 11nes, 5 compartment 74horizontal, splash/flash HX2 l ines, 7 ver t ica l po ts

1 per 1 ne, splash/flash HX 68 12 l ines, 6 ver t i ca l pots 69per line, splash/flash HX2 l ines, 8 ver t i ca l potsper line, double pipe HX 65 1I

Comparing Type of Vessel and HeatExchanger. 1988 data, 4500 t /d plant.Final Design Criteria

The various test programs andcomparati e studies were used t o developthe general flowsheet of Figure 2. Thefi na l process design c r i t e r i a aresumnarized i n Table 5 .

kigure 2 Ooldstri ke Autoclave Ci rc ui t I


5/11

GOLDSTRIKE AUTOCLAVE PLANT DESIGN

I UNITS DATA 1GenerslAverage Feed Rate t / d 1360stpd 1500P lan t Ava i l ab11 t y X 85ThickenerCyclone O/F S o li d s X 38Thick ene r Type Hig h RateFeed Soli s X 12U/F Solids X 50.0U n i t Ra te t /d . m1 8.8P essu re O x lda t l onReaction Temp. O C 225"F 437Oxygen Pres su re Kpa 345

PSI 50Design React. Time min 75Su lph lde O x lda t l on X 95Oxygen Consumed kg /kg S 3.1Ox y g e n U t i l i z a t i o n X 60Vessel Canp artments 5Heat Recoverv - Tmce ra tu resSystem Feed OC 32Vessel Feed OC 177Approach Temp. OC 3Peak CIL Feed O C 27Heat Recovery - SvstenogMain System Spla sh/Flas hNumber o f Stages 3F i na l Coo l ing Sh e l l and TubeI ITab le 5 S i m p l i f i e d D esign C r i t e r i a f o rG o ld s t r l k e Phase 1 Autoc lave P lan t .

PLANT DESIGNFeed Source and Preparation

Su lph ide o re i s campa igned th roughthe c rush ing p lan t and s tockp i l edsepara te ly . The G o ld s t r l k e g r in d in gp l a n t i s o pe ra te d on o x id e o re f o r a bo ut18 h/d and su lph ide ore f o r 6 h/d. Theg r i n d i n g m i11s are f lushed out betweenor e changes. Cyclone ov erf lo w i s pumpedt o t h e 30 m (100 f t ) diameter pre-ox id a t io n th icke ner when su lph ides a rebeing processed.During about 18 h every day thet h i c k e n e r u n d e rf lo w i s r e c i r c u l a t e d .When new fe ed i s en te rin g th e system,the th i ckene r unde r fl ow i s advanced t o

th e f i r s t o f two 12 m d ia . x 13 m (40'd ia . x 42 ') ac id u la t lon /surg e tanks.S u l p h u r i c a c i d i s added t o d e s tr o ys u f f i c i e n t ca rbonate t o g i ve a m inimumr e q u i r e d f r e e a c i d i t y l e v e l i n th eau toc lave d ischarge . A i r i s i n je c te di n t o t he a c i d u la t io n ta nk s t o s t r i pcarbon dio xide . The second tank als oacts as a su rge tank p rov id ing enoughfeed t o keep the au toc lave p lan to p e ra t i o n a l when th e g r i n d i n g c i r c u i t i sp rocess ing ox ide o re .Sp lash /Fl ash Heat Exchange

O re s l u r r y i s pre -h ea te d t o 1 7 7 ' ~( 3 5 0 " ~ ) n a s e r i e s o f t h r e e s pla shcondensers. The heat source I s l i v esteam released from the au toc laved is c ha rge i n f l a s h v es se ls .

A l l three condensers are 2.3 min te rn a l d iameter (1.d.) by 9.1 m s i d ew a l l he igh t (7 .6 'i .d . x 30 ' ) . Thec arb on s t e e l v e ss e ls a re l i n e d w i t h a c i db r i c k and p ro v id e d w i t h s p l as h t r a y s t omaximize contact between steam and feeds l u r r y .The b ot ta n o f each vessel ac ts as apumpbox fo r th e in te r -s tage f w d pumps.These co ns is t o f a s i n g l e opera t ing pumpbetween th e f i r s t and second sp lashtowe r and a two-stage system between th esecond and t h i r d tower. A1 1 in te r-st ag epumps are 6" x 4" Denver Orion units.The h igh p ressure ( t h i r d ) condenserpro du ct i s pumped by two Geho ZPM 600s lu r ry pumps in t o the au toc lave . Eachpump can d e l iver approx ima te l y 60% o f

t h e r e q u i r e d f e e d ra t e t o t h e a u to c la v e.When one pump i s sh ut down f o r va lv emaintenance o r oth er work, th e secondpump i s op era ted a t maximum speed.Therefore planned shutdowns do nots e r i o u s ly a f fe c t p l a n t a v ai la b 1 i t y .Autoc 1eve

The autoclave vessel i s 4.12 m 1.d.by 18.5 m tangent t o tangent (13.5 ' 1.d.x 60.9'). It has a p rop r i e ta rym em bra ne /b rick l i n i n g s u i t a b l e f o r a c i ds e rv ic e . The v es se l i s i l l u s t r a t e d I nFigure 3.


6/11

ADVA NCES IN GOLD A ND SILVER PROCESSING

I pressure from the s lu rr y. As the

SLURRY-/ LAGITATORSLURRYINLET MANHOLE OUTLETPLAN (TYP)'OR

E L E V A T I O NFigure 3 General Arrangement of Autoclave

Each o f the f i v e compartments i sprovided wl th a 93 kW (125 hp) mixermechanism. Shafts are equipped wl th adouble mechanical seal. A sophisticatedseal water cooling/fl ushing system I sprovided. Each mixer i s mounted on aflange sized t o permit removal o f t hemechanism, shaft and impeller withoutdismantling the assembly.

Oxygen i s introduced t o eachcompartment through a dip tube extendingt o beneath the impeller.Steam can be admitted through asecond plpe t o pre-heat the vessel o r

provide supplementary heat i f sul phidele ve ls are low. Cooling water can beadded through the same plpe t o l i m i t thetemperature i f necessary. Th ls may berequired I n the la st few compartments -especial ly i f sulphur levels are veryhigh.Slurry Cooling

Slurry exits the autoclave at about225' ~ (437'~). A target temperature of2 7 ' ~ 8 0'~ ) was set f o r t he CIL feed t ominimize carbon loading problems andcyanide consumption.

An i n i t i a l s lur ry cooling stageinvolves the controlled release of

pressure I s lowered, pa rt o f th e wateri n the sl urr y i s evaporated therebycooling the slurry.

Control e d pressure reduct on takesplace i n a series o f three fl as h tanks.Each i s provided wlt h a r es tr ic ti ono r i f i c e (choke) through which the s lu rr ypasses. Each choke i s sized f o r apredetermined pressure drop selected t ogive th e requi red thermal balance. Asan example, the f i r s t (high pressure)fl as h tank I s equipped wlt h a 71 mm(2.8") choke designed t o create apressure drop o f 2.2 HPa (320 ps i ) a tthe design flow rate. I n contrast thelast stage unit has a 89 mm (3.5") chokeand a pressure drop o f only 275 kPa (40psig).A l l three flash tanks are 3 m 1.d. by3.7 m tangent t o tangent (10' 1.d. x12') and have a side ou tl et f o r sl ur rydischarge. Thls ensures tha t there i s apool o f s l ur r y I n the vessel at a l ltimes t o pro tec t the br ick l i n i ng o f the

f lash tank from direct slurryimpingement. Thls feature has beenmissed from other desi gns and resu ltedi n rapid fa i lu r e o f the l i n i ng due toabrasion effects.The steam from each f l ash tank i sducted t o the corresponding splashcondenser t o preheat the feed t o th eautoclave. A t one poi nt i n the design,the team considered the use of cyclonest o prevent the carry-over o f sl ur ry

drop le ts i n the steam. However t h i sconcept was reject ed i n favour of la rgerflash tanks and steam ducts.The larger flash tanks also providegreater inter-stage surge capacity andimprove circuit control.The configuration of a flash/splashu n i t i s i l l u s t r a t e d i n Figure 4.Flash evaporation i s not eff ec ti vebelow the bo il in g po int o f water.

Therefore shell-and-tube heat exchangersare used to reduce the slurrytemperature t o th e desired CIL feedtemperature. A cooling tower i s used t odispose o f low grade heat.


7/11

GOLDSTRIKE AUTOCLAVE PLANT DESIGN 187

FROM FEED

"3SPLASHCONDEN

TO HIGH PRESSURESPLASH CONDENSERFEED PUMPS

I ~ l g u r e Arrangmmt o f ~ n t e m i a t ;Pressure Splash/Flash Cool ng/HeatingSysten

Neutrallzatlon and CILCooled slurry I s pumped t o the f l r s tof three neutrallzatlon tanks connectedI n series. The acld I n the slu rry I sneutral ized wlt h l lme slurry . It wasIntended t o pump neutral sl ur ry t o theCIL c l rc u lt when the grinding cl r cu l t I shandl lng sulphldes. As noted below,t h l s s trategy has been revised.

Reagents and Ut l l l t l e sFlocculant. A packaged flocculantsystem i s used f o r the preparat ion anddlspenslng of reagent used f o r thethickener. Deslgn capaclty I s 250 kg/d(530 lb/d).

Concentrated acid I sdisplaced from road tankers by dry a l rinto a 330 t capaclty storage tank. ItI s dispensed from a loop l l ne t o theac ldul at lon tanks. Acid consumptlonvaries wlth ore carbonate content.Design ac ld handllng capaclt y I s 39 t/ d(43 stpd).Oxvnen. Big Three Indust r ia l Gas, I nco f Houston, TX own and operate the 160

t/d (175 stpd) cryogenic plant locatedadjacent t o the autoclave plant.The oxygen plant Includes 530 t (580s t ) o f l lq ul d oxygen storage capaclty asa precautlon.agalnst oxygen productionproblems. Up t o 8 t / d (9 stpd) ofl l q u ld oxygen can be produced fo rstorage.Hlgh pressure oxygen I s del lvered t othe autoclave through a blow-out tankand other sa fety devlces t o ellmlnatethe posslbl l l ty of slurry belng blownback t o the oxygen plan t.

Clme. Lime I s stored I n a 230 t (250st ) capaclty s l l o before slaklng I n adetention type un lt . Slurry contalnlng15% so li ds I s held I n a 7.6 dla. x 7.6 m(25' x 25') storage tank and dl s tr lb ut edthrough a loop llne.Steam. Steam I s provlded from an 18 t /h(40,000 lb/h) capaclty, 4.1 MPa (600psl) bol er . The water qua ll ty a t theGoldstr lke s l te i s poor. An elaboratewater treatmant pl ant I s provldedIncluding c l a r l f ers, polymer traps,reverse osmosl s and demlneral lzers.

A blow-out tank and other safetydevlces are provlded t o prevent back-f low of s lurr y t o the boi le r .Alr. The autoclave p lant I s provldedwl th hlgh pressure Instrument and plan ta l r compressors and a low pressureprocess a1r compressor.Plant Layout

'The arrangement o f the autoclaveplant was developed by a team thatIncluded several operators wi th e a rl ie rautoclave pl an t experlence. The teamhad a clear understanding of theoperating and malntenance problemscreated by unsatisfactory layout atother autoclave plants.The team ensured tha t good craneaccess was provlded t o a l l pumps,

agl tat ors , large s lu rr y valves and otherhlgh maintenance Items.


8/11

188 ADVANCES IN GOLD AN D SILVER PROCESSINGAn important fa c to r i n the design wasthe fact that the p lant w i l l be expandedi n l a t e r phases o f t he Development Plan.It should also be noted that theGoldstr ike s i t e i s extremely restr icted .This required a compromise between goodaccess t o equipment and a compact p lan t.The re su lt in g layout i s shown i n Figures5 and 6.

SPLASHCONDENSERS

AGITATOR SPLASH/ R E M ~ V - J ONDENSE f ?r:z /AUTOCLAVET

I IFigure 6 Pla nt Section a t High PressureFlash Tanks

Mater ials o f ConstructionAcidic pressure oxidation presents

special , and po te nt ia l 1y severe,corrosion and abrasion problems.Mater ials sehcted f o r the Goldst rikepl an t are presented i n Table 6.

ITEM WETTED SURFACEThickener Painted SteelAc idu la tio n Tk. Chlorobutyl RubberAcidulation Agit. Chlorobutyl RubberSplash Tks. Acid-proof Br ic kInter-stage Pumps High C r I ronGeho - Housing CD4MCu~ e h o valves CD4MCuGeho - Diaphragm EDPMAutoclave Shel l Acid-proof Br ic kIn jec t i on Tubes Hastell oy C276Auto. Ag lt . Shaf t Ti Grade 12Auto. Ag lt . Imp. T i Grade 5Flash Tank Walls Acid-proof Br ic kFlash Valve Body T i Grade 2Flash Choke CeramicFlash-Splash Steam DuctsHigh Pressure T iMedium Pressure 20Cb-3LOW pressure 316 Stainle ss SteelCooler Feed Pumps Chlorobutyl RubberCooler Shel l Carbon SteelCooler Tubes 316 Stainless SteelNeut. Tank Ch lorobuty l RubberNeut. Tank Ag it . Chlorobu ty l Rubber

Table 6 Materials of Construction

CONTROL SYSTEMSDi st ri bu te d Control System

The autoclave pla nt i s provided wi tha Bailey Network 90 di st ri bu te d contro lsystem (DCS). The DCS i s provided w i thtwo operator stations providing 15screens of process graphics, alarms,trends and remote start/stop f unct ons.An engineering worksta tion i s providedfor system configuration.Two process co nt ro l u n i t s areprovided, one f o r t he autoc lave and onefo r t he anc il l a ry c i rcu i t s . A t o ta l o f


9/11

GOLDSTR IKE AUTOCLAVE PLANT DESIGNabout 300 d isc re te and 100 analogueloops are handled by the system.Th icken ing and Ac id i f i ca t i on

Thickener underf low pumping ra te I sgoverned by the underf low densi ty .F loccu lant i s added a t a pre-set ra te .A cid i s added t o t h e a c i d u l a t i o ntanks a t a pre-set ra te determined bythe lev e l o f carbona te i n t he feed andt h e o r e f l o w r a t e .

Splash/Flash SystemE f f e c t i v e c o n t r o l o f t h e s p l as h /f la s hsystem I s c r u c l a l t o t h e o p e r a ti on o ft he c i r c u i t .The dynamics of the system are qui tecomplex mak ing cont ro l po tent ia l lyd l f f l c u l t . Fo r example, an inc rease i nt h e f lo w o f c o l d s l u r r y t o a s pla shcondenser w i l l cause more s t e m t ocondense and reduce t h e pressure i n th econdenser. T hi s reduces th e head acrossthe condenser feed pump which causes thef l o w t o i nc re a s e even f u r t h e r .The p rimary con t ro l l e d va r ia b le i nthe sys tem I s t he f l o w ra t e between t heintermediate and high pressure splashcondenser. Flow ra te i s sensed w l t h amagnet ic f lowmeter and control led by ava ri a b le speed pump. The le ve l i n th ehig h pressure splash condenser i s usedt o co n t ro l t he speed o f t he au toc lavefeed pump.A cascade le ve l / f low system i s usedt o c o n t ro l t h e r a t e a t which s l u r r y i spumped f rom t he low pressure t o t heinte rm ed iat e pres sure condenser. Theleve l i n t he in te rmed ia te condenser i sused t o mod if y t he s e t po in t on a f l owcont ro l loop on the ln termedla tecondenser feed pump.The feed t o the low pressurecondenser I s governed by the s lu rr yl e v e l I n t h a t v es se l.The low press ure condenser I s

operated a t atmospheric pressure. Theother two splash condensers are providedw l th p ressu re co n t ro l l ed ven t va lves .

The rather complex control systemdescr ibed above worked f lawless ly . Ital lowed an ext remely rapid and t rouble-f re e c m l s s i o n i n g .Cont ro l o f the pre-heat sys tem i sconsiderably eased through carefuldes ign o f th e system geometry. Thevessels are e levated t o ensure a h ighhead ove r su ct io n o f each pump. 'Thisprov ides a measure o f pass ive f lo wcontrol and ensures that the pumpsuc t ion requ i remen ts a re sa t i s f i ed .R e s t r i c t i o n o r i f i c e s i n t he i nt er -s ta g epip e 1 ne s steepen th e system curves andfu r t h e r improve co ntr o l . The surgecapac i t y p rov ided i n t he bo ttom o f t hev es se ls a l s o improves c i r c u i t s t a b i l i t y .

AutoclaveOxygen i s admi t ted t o th e autoclavea t a pre-set , co nt ro l ed ra te . Ther eq u l r e d f e ed r a t e I s c a l c u l a t e d fro mt h e s u lp h ld e s u lp hu r l e v e l i n t h eautoclave feed assuming 60% u t i l i z a t i o nand a consumption o f 3.125 kg o f oxygenper kg o f sulphide sulphur . Anadd i t ion a l , f i xe d amount o f oxygen I s

added t o compensate f o r vent losses.As i n e r t gases, such as ni t ro ge n andcarbon diox ide, accumulate i n th eautoclave atmosphere, th e pre ssure tendst o In cre as e. A ven t va lve I s used t oremove iner ts and contro l autoc lavepressure. Attempts t o use an on -l in eoxygen analyzer have so far met wlthon ly l im i t e d success.Steam i s a dded t o t h e f i r s t andsecond autoc lave compartments t o co nt ro lth e temperature t o pre-set leve ls .Water i s au tomat i ca l l y added t o t he la s tth re e compartments t o co nt ro ltemperatures.A nuclear source and detector arrayd ete rm in e th e l e v e l i n t h e l a s tcompartment. The choke va lv e on th ef i r s t f l as h ta nk i s m odulated t o h o ldthe des i red leve l .

Sampl 1ngThe autoc lave p lan t i s prov ided w l t hsampl ing systems t o a l low proper


10/11

ADVANC ES IN GOLD AND SILVER PROCESSINGassessment o f operat ing performance.Sampling systems generally consist of aprimary t h i e f sampler followed by asecondary moving pipe type sampler.Energy dissipating tanks have beenprovided between the two samplers.

CONSTRUCTION AND COMMISSIONINGGeneral Schedule

Authorization f o r the autoclavepro jec t was given i n mid-September,1988. The order fo r th e longestdel iv ery item, the autoclave was awardedimmediately. The autoclave was receivedon s i t e i n mid-June, 1989. In st al la ti onof the l i n i n g system and aci d cur ing wascompleted a t t he end o f September 1989.

The in s ta ll a t io n o f process andservi ce p ipi ng and the inspection andcleaning o f th e oxygen systems werecompleted i n mid-January 1990.Slurry was f l r s t admi tted t o theautoclave i n l a t e January and continuousoperation achieved on February 2, 1990.Production data from that date arediscussed below.

Check-out and Comissionlng ProblemsThere were remarkably few problemsduring the check-out and comnissioningperiod. The main problems were asfol lows:

Agi tat or Seals. The double mechanicalseals supplied with the autoclaveag it at or s were of a modified design.Retaining pins were inadequate fo r th etorque and sheared. The problem waseliminated by increasing the strengthand the number o f pins . The addition ofglycol t o the closed-loop cool ing f l u i dsystem also helped by acting as alubri cant a t the seal face.

B a l l Valves. Trunnion mounted b a l lvalves are installed as block valvesaround the autoclave and t o preventback-flow from th e high-pressure splashcondenser. I n i t i a l operations withthese valves indic ated very high

operating torque requirements after afew va lve cycles. These problems wereaccomnodated during the sta rt-up. Thevalves have since been modified by theaddition of hard surfacing on thetrunnions and bea ring surfaces and th eaddi t ion of lubr icant t o the t runnion.TrWD Oversize. E ar li er operation ofthe p lan t was made di f f i c u l t by thepresence o f coarse ore pa rt ic le s.Construction debris was also a minorannoyance. Problems were el im in ate d bythe re- rou tin g o f sump li n es and th eaddition of second stage trash screenson the plant feed .Feed Di lu ti on . Feed t o the autoclavepre-heat c i r c u i t was of lower thandesired s lu rr y density dur ing the f l r s tfew days o f operation. Di lu ti on wasreduced by re-r ou ting o f sump lin es .St ra iner Blockage. Perfo rated basketstralners are provided on the suctionlines of the autoclave feed pumps.These are intended t o p rot ect thepos i t l ve displacement pumps from damageby coarse debris . During sta rtu p, t hestralners required frequent cleaningbecause of tramp oversize.

More rece ntl y the st ra in er s haveagai n requi red frequent clean1ng. Theentrapped mate rial i s scale. Thisoriginates i n the high pressure splashcondenser, becomes detached from thewalls and entrapped by the strainer.Under some c i cumstances, pa r t a11yblocked strainer baskets have resultedi n erosion per forat ion of the stra in er .OPERATING RESULTS

Plant ThroughputThe pl ant throughput has increased asre st ri ct io ns have been Id en ti f i ed andeliminated. Da il y tonnages i n excess o f1820 t / d (2000 stpd) have been comnon.Plant availabil i ty has also shown asteady improvement. Seven-day running

averages i n excess o f 96% have beenachieved. The average a v a i l a b i l i t y overthe f i r s t t hree months was 77%.


11/11

Goode 1990 (Goldstrike autoclave plant)

Documents

Transcript of Goode 1990 (Goldstrike autoclave plant)