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CHAPTER 2

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CHAPTER -2

MAGNESITE - AN OVERVIEW2.1. NATURE OF MAGNESITE

Metals occur i n nature, some times free ( i . e i nuncombined state), but mostly in the combined state,( i . e as compounds with other elements). When a me t a l i sfound f r ee , it is s a id to occur a s ' n a t i v e f . Metals f ound i nn a t i v e s t a t e a r e copper, silver, gold , mercury and platinum.other s a re found in the combined state1. Magnesite is foundin a combined state.

4

Th e Ch em i c a l name o f m a g n e s i t e is m a g n e s i u m~ a r b o n a t e (M gC 0 ~ ) .i . e ) Mag n e s i t e i s a c a rb o n a t e o fmaganesium. When pure, it contains 52.4 % carbon d i - ox i de( CO z ) , and 47.6% magnesium oxide (MgO) But pure magnesiteis rarely found in nature. It is usually associated withsmall percentage of calcium, silicon, iron and aluminium,totals going u p t o 10%. These elements are ca l led impuritiesZ@

Magesite mineral can be compared with paddy.

Paddy Raw magnesiteRice .LwCIw-.I Processed magnesite or calcinedmagnesiteFood ---I-I- Refractory bricks/ramming mass/ f e ttling mass/ gunning mix , etc.,

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Paddy has limited use, It can be used as a seed only.Similarly, the uses of raw rnagnesite are very muchlimited. From paddy, we get rice. Rice cannot be used assuch. It is used to produce various food items, Similarlywhen raw magnesite is processed, it gives calcined magnesite.The calcined magnesite, as such, finds no much use. It isused to produce other usable products.

The rice is converted into food products which can beconsumed as such. Similarly, the,clacined magnesite is usedto produce refractory bricks, gunning mass, ramming mass,fettling mass and many other products which find many utses.Thus, there is a one bo one relationship between paddy andmagnesite.

For a clear understanding of the subject ofmagnesite, a few terms (used in this report) are explainedbelow.

MINERALSA mineral is a material dugout from earth a s such. It

is a compound (i.e) i t + i s combination of more than one

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A mineral must have the followingcharacteristics. The material must be inorganic, not formedform any process of animal or plant life, f a l l withinspecific l i m i t s of chemical composition and have a regular

4internal arrangement of atoms. . .Magnesite is a mineral asit satisfies the above characteristics.

2 . 2 . 2 : OREOre is an aggregation of the minerals and gangue

5from which one or more metals may be extracted at a profit .Magnesite is an ore as rnagesium metal can be profitablyextracted from it.

2.2,3: GANGUEGangue minerals are the associated non-metallic

materials of deposit. They may,..be ntroduced mine ra l sor the enclosing rock and are usually discarded in thetreatment of the ore. The gangue in customary usageincludes only non-metallic minerals such as p y r i t e , whichi s u s u a l l y discarded a s worthless. certain ganguematerials, however, may at times, be collected as by-products and utilised. For example rock gangue may beutilised for road metal, quartz f o r abrasive etc., I n thecase of rnagnesite deposit, silicate, calcium, etc., are t h egangue materials.

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2 . 2 . 4 : MINING

T h e process of taking out the ore from the e a r t his called mining. The magnesite ore is taken out f r o mmothe r earth by mining. The first step in mining is tolocate o r find the minerals. This is called prospecting orexploting. Next the best way of mining is p l a nn ed and used.Finally, the safety and h e a l t h of the mi n e r s must be cared

7for -.

2.2.5: METALLURGYThe process crf extracting the metals from t h e i r o r e s

and refining t h e m is c a l l e d metallurgy .8

Note: 1. The process used for the metallurgy of ametal depends on the n a t u r e of t h e ore from which themetal is being extracted and also on the properties of themetal. Thus it i s n o t p o s s i b l e t o chalk out a universalmethod f o r extraction of all the metals.

Note: Consideration t h e extractionmagnesium metal from magnesite is beyond the scope of thisstudy.

2.2.6: REFRACTORIES

These are substances which c an w i t h s t a n d hightemperature in the furnace without melting and does

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no t react with t h e material poured in it. $Alumina ( A 1 2 0 3 ) ,Chromite ( F e O Cr203) Magnesite (Mg COj) , T i t a n i a ( T iOZ ) ,Carborundum (Si C ) , Zirconia (Zr02) a r e t h e importantrefractory materials. 9

The refractory industry consumes 90-95% ofmagnesite production in India ( ~ n d i a n ineral year books,Nagpur) .

2.2.7: CALCINATION

Calcination is t h e p r o c e s s o f heating the orelightly. It does the following things.

i) It removes the volatile impurities like c a r b o ndi-oxide (C02), matter, moisture etc., from the ore.

ii) It removes water from the hydrated oxide ore.iii) It removes C 0 2 from a carbonate ore.10

Magnesite is heated lightly to g e t calcinedmagnesite,

Mg C o g Calcination MgO + C 0 2 .---------->Sintering refers t o h e a t i n g of the o r e at a veryhigh temperature range of 1600 - 2 0 0 0 ~ . Magnesite issintered at a temperature range of 1600 - 1800 C to getdead burned magnesite. (Sintering i s also called calciningin usage)

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INDUSTRIAL MINERAL

It is a term of t h e trade only. A .mineral to bean industrial mineral must satisfy the following: (i) thenature of t h e substance m u s t be inorganic, (ii) it must beavailable in a combined state, (iii) one or more metalsmust be extracted from it, profitably, and (iv) someother products must also be produced from it. As magnesitesqtisfies a11 these conditions, it is a n industxialmineral. O t h e r industrial minerals a r e alurnina(Al2O3.2H20)carnallite ( K C 1 - M g C12.6H20), magnetite ( l ? e 3 0 4 ) , hematite(Fe203) siderite(Fe GO3) etc.

2.3: MAGNESITE FAMILY

Though t h e raw magnesite has no much use as such. Thevarious products produced from magnesite have various uses.Hence it is essential to know t h e various products producedfrom magnesite. These products may collectively be called"rnagnesite family". Chart 2.1 given on next Rage will give aclear picture ab o u t the magnesite f a m i ly .

2.3.1: SE PARAT ION O F MAGNESITE FROM SOIL

Magnesite b e a r i n g earth is first loosened bydrilling and blasting. The run-of-mines is separated intoraw magnesite (also called crude magnesite) and spoils by

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the method of hand picking. Spoils are nothing but the soiland rocks which are obtained along with the magnesite.Spoils are w a s t e and hence have no use.

2 . 3 . 2 : GRADING O F MAGNESITEThe magnesite ore is chipped and dressed by hand to

get rid of the surface impurities associated with lt. Theraw magnesite is then graded into refractory and non-refractory grades by visual appearance and they are stackedseparately.

2 . 3 . 3 : MAGNESIUM METAL

M a g n e s i u m c a n b e o b t a i n e d from the calcinedmagnesite either by electrolysis or by thermal reductionme t h o d s . ''

2.3.4: REFRACTORY GRADE OF MAGNESITE

The refractory grade magnesite is ased to produceDBM and CCM. Magnesite is calcined at a sufficiently hightemperature ( 1600 - 1800 O C) to drive of the C02 and tofrom a dense, hard crystalline variety of magnesia,c a l l e d periclase, which reacts neither with C02 nor wa t e rat ordinary temperatures. This MgO is called 'Dead-burned magnesitef (DBM) 12 The raw magnesite fired at700 O to 1000 OC produces a more porous and reactive

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material known as Caustic Calcined Magnesite (CCM) l3 Therefractory grade magnesite is used only for these purposes.

The DBM is used to produce ramming mass, gunningmass, ettling mass and refectory bricks. Differentchemicals in different proportions are added to DBM toproduce these products. These products are calledmonolithics. They are produced depending upon thechemicals added to DBM by simple production process ofcrushing.

Refractory bricks are manufactured by mixingdifferent chemicals and additives to DBM and passing themixture through different stages of production process.Caustic calcined magnesite, also called lightly calcinedmagnesite (LCM) and simply called calcined magnesite isused as such in many industries for various purposes.

2.3.5: NON-REFRACTORY GRADE OF MAGNESITEThe non-refractory grade magnesite is pulvarised

and used by some industries as such. When the non-refractory grade magnesite is calcined at a temperature r angeof 700' - ~ O O O O C , it also gets converted into causticcalcined magnesite. But this product will be of lower qualitythan the one obtained from calcining refractory grademagnesite; hence the use of this calcined magnesite will alsodiffer.

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2 . 3 . 6 : IMPROVING THE QUALITY OF LOW-GRADE MAGNESITE

The quality o f low-grade magnesite can be improvedby physical or chemical beneficiation methods. Themagnesite thus converted into high quality, is c a l l e dHigh Q u a l i t y Sinter Magnesia (HQSM). This HQSM can be usedto manufacture high quality refractory bricks.

-a

Thus t h e magnesite family consists of rawmagnesite, magnesium metal, calcined magnesite, DBM,ramming mass, gunning mass, fettling mass, refractorybricks, HQSM, etc.,

Note: (i) The use of the various magnesite productsare explained latter in this c h a p t e r .

(ii) The process of separation of magnesiteinto refractory and non-refractory grade and the productionprocess of the various products of the magnesite familyhave been explained in detail in the c h a p t e r on 'Mining andProduction'.

2 . 4 : PROPERTIES OF MAGNESITE

The raw magnesite has b o t h physical andchemical properties. An account of these properties are givenbelow: -

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2.4.1: PHYSICAL PROPERTIES

The following a r e t h e physical properties ofmagnesite.

(i) COLOUR: The colour ranges from chalk whiteto grayish white and pinkish white and sometimes takesgreenish or brown colour.

(ii) HARDNESS: Varies from 3.5 to 5 in the Moh'ss c a l e . 14

(iii) SPECIFIC GRAVITY: T h e s p e c i f i c g r a v i t yranges from 3.0 to 3.12 15. It is comparatively a lightmineral. 16

(iv) 'LUSTRE: The lustre of magnesite ranges fromearthly dull in the case of amorphous variety to vitreous incrystalline variety. l7

(v ) STREAK : White. l8(vi) CLEAVAGE : Perfect.l9(vii) FRACTURE: ~o nc ho id al(viii) STATE : Solid state(ix) CRYSTAL SYSTEM AND HABITS: Trigonal,

rhombohedra; massive, fibrous, granular, compact. 21( x )QUALITY : I t i s he t e r og e n e ou s in quality.22

2.4.2: CHEMICAL PROPERTIES

The chemical properties of raw magnesite are verymuch limited but t h e calcined magnesite has many chemical

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properties. Hence here we give the important chemicalp r o p e r t i e s of raw magnesite and t h a t of t h e calcinedmagnesite in t h e a p p r o p r i a t e p l ace .

( i) CHEMICAL COMPOSITION: The chemicalcomposition of magnesite varies from place to place andhence it is not possible to give a g e n e r a l chemicalcomposition of magnesite. However, the usual grade ofrnagnesite treated for calcination will contain thefollowing composition, 2 3

A1203CaO

(ii) MELGING POINT: Around 2800' C. 24(iii) HEAT CONDUCTIVITY: Very low.

2 . 5 : FORMATION, OCCURRENCE AND STRUCTURE

Magnesite is formed in different ways. It occursin different varieties and t h e s t r u c t u r e of magnesitealso varies from variety to variety.

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2.5.1: FORMATION OF MAGNESITE

Magnesite is found in nature u s u a l l y as secondarydeposit formed due t o i) alteration of ultramafic rocks(mostly serpenti ne) as is found in Chalk hills, Salem, andDodkanya in Karnataka; ii) replacement deposit in carbonaterock as found in Almora(U.P) in dolomite rock; iii) beddeddeposits as found in Manchuria and South Nevada; and i v ) asa v e i n Thus t h e r e are four varieties ofmagnesite based on the process of formation. commercialdeposits are found mainly in (i) to ( i i i ) types ofdeposits. In India, bedded or vein filling deposits havenot been located. 2 6

2.5.2: OCCURRENCE OF MAGNESITEMagnesite has three modes of occurrences as d e s c r i b e d

below: dolomite l i m e s t o n e(Ex.Washington, Austria, M a n c h u r i a , Czechoslovakia, andQuebec); (ii) veins (Ex. alifornia, Greece, India, Russia,Yugoslavia and Nevada) ; (iii) sedimentary beds (Ex. evada) . 7

The' replacement deposits y i e l d the c r y s t a l l i n evariety and have resulted from progressive r e p l a c e m e n t(rarely complete) of lime stone or dolomite by Mg C03through hydrothermal solutions, This forms bedded deposits,l e n s l i k e or irregular in shape and of l a r g e size. Theygenerally contain some ferrous iron. 28

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The veins contain t h e hard amorphous varietyand occupy fractures of crushed zones in serpentine orultrabasic rocks. They result from the breakdown ofserpentine by hydrothermal solutions, accompanied by therelease of silica, which forms opal or chalcedony. 29

The sedimentary 'depositsare probably formedby evaporation but they are so interbedded with dolomiteand other rocks to make their extracting unprofitable. 30

2 . 5 . 3 : STRUCTURE OF MAGNESITE

There are three kinds of magnesite, namely,(i) cryptocrystalline (ii) crystalline, ' and (iii)amorphous.31. This classification is on the basis of t h estructure af magnesite.

Magnesite is formed by the alteration ofdolomitic limestone by magnesium-bearing solution associatedwith the intrusive rocks. The crystalline varieties ofmagnesite are the products of such alterations. Thecrypto crystalline varieties are the products of alterationof serpentine and of similar magnesium bearing rocks.Magnesite of this type is found extensively in an areawhere there are serpentine rocks and it occurs in fissuresor in the sheer-zones thoroughly mixed'up with serpentine,along with some opal and chalced ony.32 Magnesite resultingf r o m the alteration of serpentinous rock is amorphous innature. 3 3

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2 . 6 : EFFECTS OF IMPURITIES

The magnesite is associated with various' objectionableconstituents, called impurities, like silica in the form oftalc, lime in the form of calcite, lime-stone dolomite andiron as ~ederite.~* he effects of impurities are givenbelow.

2.6.1 SILICA AND LIME

Silica and lime form fluid silicate with relatively lowmelting point and greatly reduce the strength of magnesitebricks in which they are present. When much silica ispresent, dicalcium silicate (2Ca0 S i O Z ) forms and as thisdisintegrates on cooling, it tends to break down the wholemass. Magnesium calcium silicate containing 20% of magnesium,30% of lime and 50% of silica has a melting point of 1 3 5 0 ~ ~only. 5 High silica in magnesite causes ring or wdllformation inside the kiln (rotary) resulting in chocking.Keeping silica under control is the major problem for themagnesite industry. 3 6 At present, the silica in magnesiteis controlled by selective mining, efficient manualdressing, crushing and screening.

The silica present usually forms a much lessrefractory product with any lime or iron oxide present.However, a small proportion of lime is considered favorable

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as without it, magnesium ferrite is not readily formed.When dicalcium ferrite is formed. It produces an entecticswith magnesia which melts at 1 3 8 0 ~ ~o that th e whole masscan sinter at or slightly below 1 4 0 0 ~ ~ .hen a mixture ofmagnesia and iron oxide is heated to 1 5 0 0 ~ ~ - 1 5 5 0 ~ ~ ,nentectic melting at that temperature is formed. his andthe silicates which are molten at this temperature farm aviscous fluid which instead of assisting, hinders theformation of crystals of periclase. 7

2 . 6 . 2 : I R O N OXIDE

The presence of iron oxide is not regardedcommercially as an impurity because rnagnesite which is almostfree from iron oxide is very difficult t o sinter. If this ispresent in the range of 2 t o 4% , it a c t s * a s a convenientf l u x and can t h u s produce a w e l l sintered product. Ifpresent in large percentage, it is undesirable because itreduces t h e refractoriness of the product. 38

2 . 7 : SPECIAL FEATURES OF MAGNESITE

In this section,. some of the special features ofmagnesite are given.

2.7.1: PURITYTh e purity of magnesite is decided on the basis of

its silica content. If the silica content is more, the

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magnesite is not pure. On the otherhand, if it is less, thenthe meterial is said to be pure (ie, of high grade) . TheChalk Hills rnagnesite (Salem dt., TN) is said to be of highgrade compared to Alrnora (U. ) magnesite. 3 9

2.7.2: PURIFICATION

When the raw magnesite is heated, the carbondioxidein it goes out; but the silica does not decompose. Silica canbe reduced only by physical or chemical benefication of theraw magnesite. When silica cantent is reduced, the purity ofthe magnesite is improved/increased.-2 . 7 . 3 : VARYING TEMPERATURES

The temperature range to produce CCM and DBMvaries according to the quality of t h e input r a wmagnesite. The temperature r e q u i r e d is determined by testingthe raw magnesite in the chemical l a b o r a t o r y to give theo u t p u t a t a desired level of purity.2.7.4: EFFECTS OF H E A T I N G ON CHEMICAL PROPERTIES OFMAGNESITE

When raw magnesite is heated around 8 0 0 ~ ~tbecomes f r e e from carbondioxide. When the magnesite is

trf u r t h e r h e a t e d to 1 6 0 0 ~ ~ ~h e m e t e r i a l becomes morestronger. In calcination, t h e r e is a slight possibility ofthe presence of some chemical properties whereas insintering t h e meterial (DBM) becomes i n e r t .

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2 - 7 . 5 : NO EFFECT ON FURTHER HEATINGOnce the magnesite is heated and cooled down, i t

becomes inactive and hence it will not get any effect onfurther heating. So calcined magnesite, obtained at atemperature range of 700 - 1 0 0 0 ~ ~a n n o t b e u sed a s t h ei n p u t in t h e manufacture of DBM, which is obtained at atemperature range 1400 - 1 6 0 0 ~ ~ .2 . . 7 .7 : THE PROPERTY OF D O U B L I N G OF S I L I C A ( S i 0 2 ) INCALCINATION

Magnesite is a compound o f M g , CaO, S i O z , F e O ,A1203, C02 e t c . Of these elements, C02 is a g a s s i o u s elementwhereas the others are solids. When magnesite i s h e a t e d , thesilica content (the impurity which determines the qualityof the magnesite) gets doubled in proportion to MgO . Thiscan be understood from the following:

L e t t h e Si02 c o n t e n t i n the r a w magnesite be 6%( i . e ) 100 grams of magnesite contains 6 grams of S i O Z . L e tl o s s on i g n i t i o n ( L O 1 ) be 5 0 % ~(usually it is 49-52%). When

grams raw magnesite hea t e d , grams andother impure gases w i l l escape due t o lass on i g n i t i o n andh en ce after heating the weight of the c a l c i n e d magnesite

8will be around 50 grams o n l y . As silica i s a solid, it willnot decompose upon h e a t i n g . H e n c e the same 6 grams of silicawill r emain with t h e calcined magnesite, even after heating( i , e ) i n the quantum of 50 grams of calcined magnesite, 6grams of silica w i l l b e p r e s e n t . I n otherwords, t h e silica

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content in the calcined magnesite expressed as a pe r c e n t a g ewill be 12. This means t h a t t h e portion of silica in calcinedmagnesite has doubled compared to that of r a w r n a g n e s i t e .

In DBM also, the presence of silica shall be thesame (doubled) as i n the case of calcined magvesite, because,when h e a t i n g i s increased t o 1600c, n o t h i n g goes out exceptthat the meterial becomes s t r o n g e r .

T h i s i s further conf i rmed from the following: l1Whilemanufacturing the high g r a d e basic r e f r e c t o r i e s , S i O Zc o n t e n t should be resonably low in DBM. T h e IS1 has-prescribed that the S i O , c o n t en t i n DBM and m a g n e s i t e bricksshould not exceed 5.5%. In order t o achieve this requirement,the raw magneslte should not contain more than 2.5% S i 0 , l 1 4 0

2 . 8 : USES OF MAGNESITE PRODUCTS

The uses of various r n a g n e s i t e products are g i v e nbelow:

2.8.1: R A W MAGNESITE

T h e r a w rnagnes i t e -has h e following applications.(i) On addition of sulphuric acid, it forms magnesium

s u l p h a t e , commonly known as epsum saltf Epsom salt i sused i n t h e manufacture of artificial silk, tanning and inpharmaceuticals. Each kilogram of rayon produced consumesabout 5 kgs of epsom salt. 2

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(ii) Pure magnesite free from iron is used inthe glass i n d u s t r y to control the viscosity and improve theworkability of t h e glass.43

(iii) Raw magnesite is used in the electrodeindustry t o p r o t e c t t h e molten metal from aerial exidationby way of forming slag and layer of C02. 4 4

(iv) Raw magnesite is used as a constituent ofv i t r e o u s enamal f r i t and ceramic glaze frit in ceramicindustry. 5

(v) The paper and pulp industry uses magnesiteto produce magnesium bi-sulphate which acts as cookingliquor. 46 (vi) High grade raw magnesite is used in thepharmaceutical industry.

(vii) Raw m a g n e s i t e is used a s a very goodfiller agent in many p r o d u c t s .

(viii) Small chips of low grade magnesite areused in the manufacture of mosaic tiles to giveattraction t o the floor. 7 ,

2.8.3 : CAUSTIC CALCINED MAGNESITE (CCM)CCM is useful for the following purposes:(i ) MANUFACTURE OF SORE CEMENT : Sorel cement,

discovered by Sorel in 1867, requires magnesia as a rawmaterial. 49 W h e n pulverised CCM is treated with a strongsolution of magnesium chloride, sore1 cement, calledmagnesium oxychloride, is obtained. This cement expands

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very slightly after setting. It appears to be warm in touchin cold weather; it is also noiseless and therefore usedas flooring material in hospitals, hotels, ships anddocks, It is flexible, durable, non-shrinking and dustless. 0This cement is used as wood preserver, particularly forrailway sleepers. It is also used in the manufacture ofgrinding stone and abrasive wheels, Emery powder is mixedcarefully with sore1 cement for manufacturing durableabrasives.

(ii) FERTILIZER : CCM with 90% MgO is used infertilizer. It increases crop production. Usually magnesiaalone is not added to the soil, but it is introduced in thesoil along with superphosphate, as a chemically balancedconstituent of complex fertilizer 51

(iii) PAPER : CCM is very useful in manufacturingpaper pulp from wood and bamboo. It is capable o f p r o d u c i n ghigh quality p a p e r ,

For the manufacture of cigarette papers,magnesia is considered as an excellent ingradient forharmless smoking.

(iv) GLASS : By virtue of its chemical purity,it is suitable for the manufacture of colourless andscientific glassware. It improves the chemical resistanceof the finished product. It is also used as opacifier.

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( v ) RUBBER : It is used extensively as apowerful accelerator in the manufacture of rubber. Itincreases resilence and tensil strength. It acts as acuring agent in the process of rubber manufacture. It isalso used for the manufacture of synthetic rubber. It is verymuch perferred for hard rubber moulding.

(vi) CHEMICALS : It is used in the manufacture ofmagnesium based chemicals.

(vii) AEROPLANE : Because of lightness andstrength of the magnesium alloy, it is being increasinglyused in the manufacture of aeroplane parts. 5 2

(viii) R I C E M I L LS : Magnesia cement is used inrice mills in conjunction with emery powder for hullingand polishing the rice. 5 3

(ix) PHARMACEUTICALS : Magnesia is . used in anumber of medical preparations such as magnesia pondezosa(heavy) magnesia levis(1ight) and milk of magnesia. 54

( x ) COSMETICS : It is alss used in t h epreparation of cosmetics.-

(xi) STEAM PLANTS : Lightly calcined magnesia isincorporated with asbestos in boiler lagging compositions,producing a permanent and efficient covering which greatlyimproves the thermal efficiency of any steam plant to whichit is applied.55

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(xii) PLASTICS: High purity (99% MgO) magnesiaproduced by a chemical d i r e c t acidification process isused in elastomer plastics and thermo plastics to impartflame retardent properties. 56

(xiii) ANIMAL FEED: Caustic calcined magnesia isused in the preparation of animal feed.

(xiv) AS A POLISHING MATERIAL : Magnesia powderis used as a polishing material in preparing metallurigicaland ore-mineral test pieces examined under the relfectingmicroscope.

2.8.4. DEAD BURNED*MAGNES I T E (DBM)The principal use of rnagnesite is in the manufacture

of refractory material which is used for furnace lining.When DBM is mixed w i t h c h r om i t e i n different proportions,chrome-mag bricks and mag-chrome bricks (depending upon theratio of magnesite and chrome) are manufactured.

DBM is desired chiefly as a basic refractory forbasic slags in metallurgical furnaces, for kiln and for usewith corrosive materials. It i s most suited for use inindustrial processes, where a n acidic r e a c t i o n isundesirable. 57 It is an ideal mineral for manufacture ofsuper duty basic refractory bricks.

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Magnesia bricks are used for bo t t oms and sides offurnaces employed in iron and steel industries; theconstruction of bottom and side walls of electric furnaces;the construction of refining furnaces for gold, silver, andplatinum; the construction of melting furnaes for lead,antimony and copper; all other construction furnaces wherehigh temperatures and high resistance to metalic ox ides andslags are required; and covering fire clay bricks.58

2.8.5: FUSED MAGNESIA

Fused magnesia is manufactured by fusing purecalcined magnesia in a high temperature electric furnacewithin a temperature range of 2550 - 3 0 0 0 ~ ~ .he fusedproduct looks like the normal mineral periclase, which hasa fusion point of 2 8 0 0 ~ ~ . The product is h i g h l yresistant to chemical changes which take place in ametallurgical furnace at high temperatures. It is used int h e shape of m o u l d e d vessels a n d also as compressedmaterial during the melting operation of non-ferrous metalsand alloys s u c h as lead, copper, zinc and tin. Fusedmagnesia bricks can withstand very high temperature andhence used extensively in alloy steel manufacture and non-ferrous metal refining f u r n a c e s . It is commonly used forimmersion heaters, resistance elements, melting pots,annealing chamber and laboratory-wave. 9

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Due to t h e non-availability of h i g h temperatureelectric furnaces in India, fused magnesia is not producedand hence imported for our metallurgical industries.60

2.8.6: RFiMMING MASSAs already stated, ramming mass i s produced from DBM

by mixing some chemicals and then crushing the mixture. Itwill be of coa r s e variety after crushing. It i s used a s theintermittant binder between two bricks in the construction ofthe k i l n w i t h the magnesite bricks. After t h e construction ofthe kiln, a thin coating is given on the inside surface ofthe walls of the kilns with ramming mass t o make thesurface uniform.

FETTLING MASS

Fettling mass is finer in variety than rammingmass.When the furnace is put into operation, due to wear andtear, patches may occur on the inner surface of the w a l l s . Toset-right these patches, the kiln o p e r a t i o n c a n n o t bestopped b e c a u s e , it will result in loss of production andhigh cost in f u e l consumption. Fettling mass is made intopaste form by adding water and then it is thrown on thepatches through the outlet of the kiln with shavels, Thechemicals mixed with the DBM to produce fettling mass havethe sticking property and hence when t h rown on t h e patches,it immediately sticks on the walls and makes the surfacealmost uniform.

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2 . 8 . 8 : GUNNING MASS

W h e n t h e patches on t h e i - n s i d e walls of t h e k i l nc o u l d n o t be set r i g h t with f e t t l i n g mass, g u n n i n g mass isu s e d ,

Gunning mass is finer i n variety. It c a n b e m i x e d w i t hw a t e r easily. A special equipment called air-gun is used t ospray t h i s mixture on t h e patches. The e q u i p m e n t is m a d e tohung f r o m t h e t o p o f t h e kiln an d t h e m i x e r is allowed tof l o w through t h e equ i pment when operated. T h e position oft h e e qu i pm e n t m a y be adjusted with t h e help o f a crane.P ow e rf u l b i n a c u l a r i s - u s e d t o l o c a t e t h e patches and thenthe equipment is p l a c e d in t h e p r o p e r position so that whensprayed, t h e m i x t u r e w i l l s t i c k on the p a t c h and make itright. Thus gunning mix is used to s e t right all patchesthat occur on t h e surface of t h e i n n e r s i d e of the kiln.

Note: (i) Ramming mass, fettling mass and gunningm a s s a r e g e n e r a l l y called 'monolithics'.

( i i ) They m a y a l s o b e called ramming mix,fettling mix and gunning mix, respectively.

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REFERENCESMadan R.D., Modern Inorganic Chemistry, S.Chand andCompany., (P) Ltd., New Delhi; 1987, P. 53.Sinha R.K, A Treatise on Industrial Minerals of I n d i a ,~ l l i e d ublishers ( P ) Ltd, Bombay, 1969, P.383.Venkatesh.V., Status note on the Magnesite deposits andoccurrences of Tamil Nadu, Handbook on Magnesite, TANMAG,Salem, 1985, P.1.Britanica ~ unior Encyclopedia 10/M, P.206.Venkatesh, V. Op. Cit, P. 1.Madan R.D. Op. Cit, P.753.Britanica Junior Encyclopedia, Op, Cit, P.210.Madan R.D. Op. Cit, P.756.ibid, P.977.i b i d , P.758.ibid, P.918.Kuriacose, J.C., and Rajaram J, Chemistry in Engneeringand Technology, Volume 2, T a t a - M c G r o w Hill PublishingCompany Ltd., New Delhi, 1984, P.609.Gopal,V., Magnesite Industry in Tamil Nadu, Handbook onMagnesite, Tanmag, Salem 1985. P. 90.Dep. S. Industrial Minerals and Rocks of India (Ahandbook of economic geology). Allied Publishers (P)Ltd.,New Delhi, 1980, P.340.Monograph on Magnesite, Indian Bureau of Mines, Nagpur,August 1990, P.34.Krishnasamy S, Indian Mineral Resources, Oxford and IBHPublishing Company, New Delhi, 1972, P.258,Monograph on Magnesite, Op. C i t , P.34.Whitten DGA, and Brooks JRV, A Pengion Dictionary ofGeology, Penguin Books, Great Britain, 1974, Appendix.ibid, Appendix

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20. ibid, Appendix21. ibid, Appendix22. Hariharan,M., Some observations of a field geologist.Handbook on Magnesite, Tanmag, Salem, 1985, P.54.23. Sinha R - K . , Op. Cit, P , 386.24. Hariharan, M. O p . it, P . 53.25. Sinha R.K., Op. Cit, P. 3 8 3 .26. ibid, P . 383.2 7 . Bateman, Alan M., ~conomic M i n e r a l D e p o s i t s , AsiaPublishing House, Bombay, 1962, P. 724.2 8 . ibid, P. 724.29. ibid, P. 724.30. Jenson M . L * , and Bateman A.M., Economic ~ineralDeposits, John Wiley and sons, New Yark, 3rd Edition,P.516.31. D e p , S., Op. Cit, P. 340.32. ibid, P.344.33. Sinha R.K., O p . Cit, P.353.34. Monograph on Magnesite, Op. Cit, P.6

ibid, P . 6 .Tayb, M.A.K., Handbook on Magnesite, Tanmag, Salem,1985, Preface.Monograph an ~agnesite, p. Cit, P.7.ibid, P.7,An Account of Salem Magnesite Deposits, Burn StandardCompany Ltd., Salem 1981, P. 3.

A11 Accoun t af Salem magnesite, O p . Cit, P. 2 .Sinha R.K., Op. Cit, P , 385.

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Monograph on Magnesite, Op . Cit, P . 5 .i b i d , P.4,i b i d , P.4.i b i d , P.4.sinha R . K . , Op. C i t , P. 387.D e p , S . , Op. Cit, P. 344-345.i b i d , P.342.J e n s o n M.L., an d Bateman A .M . , O p . C i t , P . 5 1 6 .D e p , S . , Op. Cit, P. 341.An Account of Salem Magnesite, O p . C i t , P . 4 .i b i d , P . 4 .i b i d , P . 4 .ibid, P.3.Gerry C l a r k e , ' Magnesia a round the w o r l d - ( H a r d t i m e si n s o f t markets - again ) ',Industrial M i n e r a l s , M o n th ly ,London, A p r i l 1 9 9 2 , P.53.A n Accoun t of Salem ~agnesi te, p . Cit, P . 5 .Uppal M.M., A Text book of Engneering C h e m i s t r y , Khannapublishers, New D e L h i , 1979, P . 352,D e p , S., Op. Cit, P. 342.sinha R.K., Op. Cit, P. 3 8 6 .An Accoun t Of Salem magnesite, Op. Cit, P.5.