Physico-Chemical Aspects of ... Use of Additives

8/12/2019 Physico-Chemical Aspects of ... Use of Additives

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Powder TechnololY, 38 (1984) 275 - 293 275

Physico-Chemical Aspects of Grinding: a Review of Use of AdditivesH. EL-SHALLMontana College of Mineral Science and Technology, Butte, MT 59701 (U.S.A.)and P. SOMASUNDARANHenry Krumb School of Mines, Columbia University, New York, NY 10027 (U.s.A.)(Received August 20, 1982;in revised form August 1,1983)

SUMMARYThe efficiency of energy utilization intumbling mills is discussed n terms of com-ponent processes aking place inside the mill.The past reports on the effect of physico-

chemical parameters of the environment onmechanical properties and grindability ofmaterialsare reviewed.Reported mechanismsexplaining such effects are analyzed andpossible ways to improve the grinding effi-ciency, through the use of chemical additives,are also discussed.EFFICIENCY AND ENERGY CONSUMPTION INORE GRINDING

Grinding is an important industrial opera-tion that is used for the size reduction ofmaterials, production of large surface areaand/or liberation of valuable minerals from.their matrices. In addition to mineral proces-sing, t is widely used n the manufacture o:fcement, pigments and paints, ceramics,pharmaceuticals,and cereals. However, theefficiency of this operation is very low [1].In mineral beneficiation, grinding is alsothe most energy~onsuming process. Energyconsumed n ore grinding in various mills ascompiled by Hartley et al. [2] is shown inTable 1. It is to be noted that the energyconsumed n grinding alone representsup to70% of the energy or the whole beneficiationprocess seeTable 2 reproduced from a 1934article [3]). Corresponding figures for thepresent operations can be expected to bemuch higher. The decrease n ore grade andconcomitant increase n the degreeof finenessof values n the ore increasealso the need for

r

fine and ultrafine grinding. Consequently, heenergy consumption is now higher, and thegrinding cost represents a relatively higher..portion of the total beneficiation cost. There-lore, it becomesan important task to improvethe energy utilization inside the grinding mill.A scheme or the flow of energy and itSutilization in tumbling mills is shown in Fig. 1[ 4 - 6]. It can be seen hat grinding energy sexpended for 1) elastic deformation, 2)plastic deformation, 3) lattice rearrangementsand mechano-chemical eactions, and 4) newsurface energy. External to the particles,energy can be expended for 1) frictionbetween the particles and the grinding mediaas well as between particles and particles, 2)~ound energy, 3) kinetic energy of theproducts, and 4) deformation and wear of thegrinding media. The schemeshown in Fig. 1,

and the data given n Tables 3 and 4 suggestthat the actual energy needed for fracture(i.e. to produce new surface area) is only asmall fraction (less than 1%) of the totalenergy input to the grinding mill. A greatproportion of the energy input (more than75%) is lost as he~t, probably due to friction,non-productive collisions, elastic and plasticdeformation, etc. However, as considered byPiret [8], the energy expended n elastic andplastic deformations might be necessaryactivation energy for subsequent fractureprocess. If this deformation energy is in-. cluded, grinding efficiency can be consideredto be 20 - 50% [6].The above discussion suggests hat energyutilization can be improved by using suitablemeans or minimizing the crushing media andthe interparticle and pulp losses.Evidently, aclear understanding of the grinding com-ponent processes nd their dependenceon the

0032-5910/84/$3.00 ()Elsevier Sequoia/Printed in The Netherlands


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TABLElEnergyconsumedn grindingat variousmills [2JConcentrate produced Plant En(k'- 8.7.

78.911.7.10.10.1311107176815209574677714471079419

CuCuCuCuCuCuCuCu-FeCu-FeCu-FeCu-FeCu-NiCu-NiCu-NiPb-Zn, Cu-Zn, ZnPb-Zn. Cu-Zn, ZnPb-Zn, Cu-Zn, ZnPb-Zn,Cu-Zn,ZnPb-Zn, Cu-Zn, ZnPb-Zn, Cu-Zn, ZnPb,ZnPb,ZnPb,ZnPb,ZnPb,ZnPb,ZnPb,ZnMoAuAuAuAuAuAu

Calumet & Hecla, MichiganAnaconda Co., MontanaP. D. Lavendar Pit, ArizonaMulfulira, N. RhodesiaSilver BellGalena (Idaho)Copper CitiesCyprus Mines, CyprusKeretti, FinlandBoliden, SwedenSan Manuel, ArizonaKotalahti. FinlandInco-Copper Cliff, OntarioInco-Creighton. OntarioTennesseeCopperSt. Joseph Indian CreekA.S. & R BuchansA.S. & R PageIdaradoGilmanSt. Joseph Lead. MontanaBroken Hill. AustraliaNew Broken Hill, AustraliaNorth Broken Hill, AustraliaBroken Hill South. AustraliaSoc. Algerienne, MoroccoBunker Hill, IdahoClimax ColoradoCarlin Gold Mining CompanyBenguet ConsolidatedCortez Gold MinesHomestake Mining CompanyRosacio Dominicana, Dominican Rep.Hogon~Suyoc Mines, Incorporated

,environment in the mill is needed n order toidentify possibly ways for improving thegrinding efficiency.

GRINDING COMPONENT ROCESSES

of the products to the dischargeend of themill. Any parameter affecting any or allof these processeswill affect the grindingefficiency.Breakageof a particle can be achieved fthe particle is captured in the grinding zoneand subjected to a fruitful breaking action.Probability of breakage.P (overall breakage)., is thus the product of probabilities or theabove two basic processes 2]:

P (overall breakage)=P (capture) X P (breakageupon capture)The probability of capture. P (capture). isthe probability that a particle will be capturedin a grinding zone and is expected to depend.

Two major simultaneous component pro-cesses, amely pulp flow and stress applica-tion, are involved in grinding operations.Specifically, theseprocessesnclude transportof material to the grinding zone, and subject-ing the material to the grinding actions,leading possibly to propagation and/orinitiation of cracks, ollowed by the transport

ergyNh/t).5.5.0.2.1.2.0.9.7.3.2.5.0.2.2.8.8.8.8.9.5.1.3.0.3.9.7.9.11

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277TABLE 2Energy consumed in grinding in different plantscompared with the total energy needed for the wholebeneficiation [3]

TABLEExperimentalmeasurementsf crushingand surfaceenergies 6 ]Material Crushingenergy(erg/cm2)

Surfaceenergy(erg/cm2)Power consumedin grinding('Yoplant total)

Plant\. -

Quartz (8iOvGlass (8iOVCalcite (CaCOJ)Halite (NaCl)Barite (8a804)

11700082000324002610071600

920121011002761250305060355645575959

TABLE 4Distribution of energy in a ball mill (7 J. Reproducedby permission from G. C. Lowrison, Crushing andGrinding, Butterworths, London, 1979.

38653646

Energy distribution Energyconsumpti,(%)

A Copper oresHaydenHarmonyUnited VerdeCopper QueenOld DominionArther and MagnaEngeloWalkerBritanniaB Lead and zinc oresChief consolidatedTyboHughesville

MorningC Gold oresSpring HillPorcupineKirkland Lake564470

Bolt frictionGear lossesHeat losses rom drumHeat absorbed by air circulationTheoretical energy for size reduction

4.38.06.431.00.697.6

among other things, on the fluidity and theparticle transport in the mill and the floccula.tiODof particles. The probability of breakage,provided that capture has occurred, P (break.age upon capture), is related to the particlestrength. Additives can affect grinding rates

fR>ss ~Y JII'Ur- -t ICTICI~(,E.~rRICTICI 1K.E.AlIIF-.t.. ,tlASTlC III> PlASTIC:I~F~TICIl~

T-ISSI(lllJ)SSES -I8Ttt:r EtERGY INPUT ro KILL .It,

.t

losSf;s IN CR\$1ING IEDIANI> OF CRUSHI/; SIF~S IQr&a;y INPUr TO TIE OMGE

~~ -i ICTIOOHrEII9MTIClE NIl P\U~S K.E. AICIP.E.: ~I R~TI(JI - __~TEI SINTESIAT 100 ; IbT~F~ EI6IGVCllUATI\t: B8GY IIAIr 10 TIEINOIVlnk PARTIa.ESOF TIE OMGE

fRulTlSS Ir-=TS NC> STA~SifRlCTlm t-.E. Il P.E.

ElASTIC NIl PLASTIC~TlmfNew;y AESI1.TIIG IN fRACTIR.

(Nt) ~~TE DISINTESlATlOO)

ii,t- FRK.TIIIE tRICTIOI I.t,* NG> .E,-J tLAsrlC Nt> PLASTICTI~TIOI :l~ICAL flfK:rlOlS :~ J-~ =t0l J ma

~S~ACE ~YFig. 1. Energy utilization in model tumbling mill: gross energy >N > C> H> F [4J. (-Residual kinetic energyafter possible secondary fracture has occurred.\


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279

9800

\ 9400":"sa59000ron~i 8600E0u ..

~ pHFig. 2. Effect of organic and inorganic additives onthe compressive strength of sandstone (ref. 15). "10-4 N dodecylammonium chloride; 8, 10-4 N AlCl);6, 10-4 N Na1COJ.itself was perfomled. It should be noted thatthere was also no control or monitoring ofcritical parameters such as pH and ionicstrength in this work.In a similar work, Wang [15] examinedthe effect of organic (dodecylammoniumchloride) and inorganic (AlCI) and Na2CO)electrolytes on the compressivestrength ofsandstone. Their results showed that theeffect of the additives was pH dependent, asshown in Fig. 2. The investigatorsattributedthis to reduction in surface energy upon the

adsorption of such additives on the solidsurface.Other reports [16] have shown a decreasein the compressive trength of some rocks dueto the presenceof water (seeTable 5). How-ever, no explanation was offered for theseobservations.Vutukuri [17] reported the tensile strengthof quartzite immersed n 0.06% aluminumchloride solution to be 11.1% ower than thatin water. However, the data (given n Table 6)have a statistical variance of 30%, whichmakes the conclusion questionable. In an-other investigation, Tweeton and co-workers[18] were, however, unable to find any effectof aluminum chloride when used as anadditive at a concentration of 0.05 ppm onthe tensile strength of fused-quartz hread.There have been many other investigationsof the influence of environment on mechani-cal properties of materials [19 - 29]. In all ofthese investigations, the decrease n thefracture strength of glass-like materialsdue to different environments has beenfound to range from a few per cent to 300%.Hammond and Ravitz [29], for example, intheir study of the effect of saturated vaporsof water, n-propyl alcohol, acetone andbenzene, on the brittle fracture of silica,observed water vapor to cause a 40 - 50%decrease n the fracture strength from itsvalue in vacuum. In general, their data indi-cated the environments with polar moleculesto causemaximum effects.These authors attributed the observedeffects to stresscorrosion cracking [30, 31].This mechanism s related to that of fractureof metals when simultaneously stressedandexposed to certain chemical environments.

\TABLE5Effect of water on the compressi ve strength of rocks [16)

Compressivestrength (lbf/in2)ype of rock Saturated/dry(%)Dry Saturated244389386199941661422862225501235437204

12241251'13715425369298

504412583671125680

BasaltBasalt with sandstoneDiabaseDolomiteGraniteGniessLimestoneQuartzite

9718067478335820

.3

.0

.6

.0

.7

.3

.3

.2


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280TABLEEffect of AlCl] solutionson the tensilestrengthof quartzite [17]Concentration('Yo) Number ofexperiments Mean tensile strength(lbf/in2) Standard deviation('Yo) Percentage changecompared with water0 (i.e. water)0.020.040.060.10

2030303030

20062008202717831788

3230373034

-G. 1-1.111.110.9

TABLE7Specificenvironments nown to cause tress-corrosionracking 31Metal Aqueous environment CommentsCarbon steelsMild steel NO - OH-3 . Also (1) anhydrous liquid NHJ. (2)SbCIJ. HCl. AlClJ in hydrocarbonMedium strength

Ultra-high strengthStainlesssteels:AusteniticFerritic

HCNH,%O,H'%a-, Br-, OH-

Martensitic

a-brass Cupric ammonium complex.NH3 or amine atmospherein presence of H2O and O2

Immune above 50% Ni.Susceptible to hydrogen cracking orblistering. Immune to CI-, OH-,N03-.Susceptible to hydrogen cracking.Also, when heat treated to highhardness, o H2O and variousaqueous solutions.Nitrates may reduce to NH4+ causingscc. S02 is reported to causescc,but probably causes ntergranularcorrosion instead.H2O3./3 + 'Y brassAluminum alloys:

4 - 0% Zn-AlCommercial. containing Cu or Mg H2ONaCl solutions. variousaqueous solutionsH2Oagnesium alloysTitanium alloys:8% AI, 1% Mo, 1% V, Balance Ti Cl-, Br-, 1- Also H2O, CH3OH, CCl4.methylenechloride, trichlorethylene at roomtemperature; NaCl at >250 c.N2O..% AI, 4% V, Balance TiGold alloys:Cu-AuAg-Au Immune above 40% Au.eCI).Aqua Regia, N~OH. HNO)the surrounding environment [13, 32 - 381.In this regard, Rehbinder [13] classifiedhardness educers nto two groups. The firstgroup contained inorganic salts such as NaCI,NaOH, Na2CO3,MgCl2, CaCl2and AICIJ, andthe second group contained organic chemicalswith polar molecules. The effect of theadditives in the first group has been ound to

Uhlig [31] observedcracking of this kind tobe present only when the metal was undertensile stress n a damaging environment.Table 7 lists specific environments observedby Uhlig to causestress-corrosion racking ofmetals.In addition to strength properties, materialhardnesshas been known to be affected by


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281a sharp decrease n the surface energy of thesolid in contact with the environment. Besidesinteracting with the crack walls, the environ-ment atoms create a force compatible withthe interatomic bond strength, which pro-motes crack propagation.In addition to mechanical properties,environment has been found to affect sizereduction of materials. The following para-graphs summarize the published reports, inthis regard.

,

increase apidly with the level of addition,reachinga maximum at a given evel for eachreagent. For example, concentrations of0.01% to 0.05% AICl3 were reported byRehbinder [13] to produce maximum effecton quartzite rocks. The influence of organicchemicals are, on the other hand, found todepend mainly on the molecular weight. Forexample, in a homologous seriesof organicacids, hardness eduction has been found toincreasewith increase n molecular weight.Subsequentwork by Montrove [34] on theeffect of xylene and benzeneon the hardnessof muscovite showed his mineral to be harderin such environmen~ than in air.Interestingly, even water and moist air havebeen reported by Westbrook and Jorgensen[35, 36] to decreasehardnessof differentmaterials such as oxides, silicates, sulfides,fluorides, carbides,and carbonates.

Westwood and co-workers have done acomprehensivestudy on the effect of dif-ferent environmen~ on the hardnessof MgO[37] and CaF2 38]. They found the hardnessof these wo minerals to increaseor decrease,depending on the surrounding environment.The authors ascribed he observedeffec~ toadsorption-induced changes in the flowproperties of the near-surface region ascontrolled by the movement of surfacedislocations.In other investigations 39,40], the sameresearcherscorrelated the observed effec~with the surface charge properties of thematerials in different environmen~. Theyfound MgO and quartz to be harder (i.e. lessdislocation mobility) at their correspondingisoelectric poin~. In contrast, Engelmannet al. [41] used the oscillating pendulumtechnique to measure he hardnessof granitesample n AlCl3 and oleylammonium acetatesolutions, and fo~nd that granite was easilypenetrated at the isoelectric point.In a recent investigation, Shchukin andYushchenko [42] studied environment-sensitivemechanicalbehavior of materials onan atomic scale using a molecular dynamic(MD) simulation of strain and failure of acrystal. They have undertaken a series ofexperimen~ to reveal the environment-induced qualitative changes n the atomicpicture of strain and failure. The resul~indicated that adsorption-active atoms oftencausedbrittle cracking. This was attributed to

Effect of the environment on grindingSize reduction of solids in grinding pro-cessess achieved by subjecting particles todifferent stresses,eading ultimately to thefracture of particles.Fracture may proceed within the particleitself (intragranular fracture) or along thegrain boundaries (intergranular fracture).While intragranular fracture is sufficient forsize reduction processes,t is intergranularfracture that is required for liberation. Gener-ally, fracture process nvolves rupturing ofchemical bonds to create new surfaces.Anyphenomenon hat can help in breakingof thechemical bonds and retard rejoining of theruptured surfacescan be expected o help thegrinding process 1].Grinding has, in the past, usually beenconsidered as a physical processcontrolledonly by the mechanical conditions of thegrinding system. Researchersn the field didnot pay as much attention to the effects ofthe physico-chemical parameters of thegrinding environment on the grinding effi-ciency. Only after Rehbinder [13] hadreported his findings on the effect of chemi-cals in enhancing he mechanical failure ofmaterials, did researchersbegan to studycomminution processes n the presence ofchemical additives.Lowrison [7] has compiled a list of addi-tives used in grinding investigations togetherwith the changes laimed (seeTable 8). Thedata suggest hat as much as a 20-fold increasein grinding rate can be obtained by usingchemical additives. It is the purpose of thefollowing discussion to review reportedeffects of environment on grinding processesand mechanisms hat are responsible or theseeffects.


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282_TABLE 8Additives used in comminution (7). Reproduced by permission from G. C. Lowrison, Crushing and Grinding,Butterworths, London, 1979.

Material ground Wetor dryAdditive % added Grindingratefactor*DDD

Water 0.060.060.04 1.~1.3Ceramic enamelsMarbleCement clinker

Alcohols and phenolsMethanolIsopentanol-1.20.1.20.o.

:~.-0.010.25

D

QuartzQuartzIron powderQuartzSoda lime glassIron powderCementsec-OctanolSeries normal alcoholsGlycero.lPhenols and polyphenois

GypsumCement clinker D.2-0.020.02

KetonesAcetoneAminesTriethanolamineFIotigan (C12-C14amine ex coconut oil)

-QuartziteLimestone 2...2t.?

SuIphonic acidsArylalkyl suIphonic acid (RDA) 0.06 GraphiteCement 1.3Fatty acidsOleic acid 1.1.003 LimestoneZinc bIen de

QuartzPumiceLimestoneQuartzLimestoneLimestone

1.27utyric acidStearic acid 100000005

2.02.0

Sodium oleateVinsol resin (calcium stearate)Sodium stearate 1.2Aluminium stearateCaprilic acidMarine oilBeef tallowWool grease

DolomiteCement clinkerCementChrome ore-magnesiteChrome ore-magnesite DD 1.21.1LimestoneGypsum

0.11.01.0Cement clinkerQuartziteQuartzite

DWW1.331.401.80

Doda lime glass1.23Quartz1.3

Other carboxylic acidsNapthenic acidSodium napthenateSodium sulphonapthenateHydrocarbonsn-alkanes, variousEstersAmylacetateOthersCarbon black 0.080.32 CementCement

(continued)

290405

.0

.1

.1.05

.10

.15

.5

.5

.0


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283TABLE 8 (continued)Additive % added Material ground Wetor dry Grindingratefactor.

Sodium silicateSodium hydroxide 1.16\ --1S1- 1--

-o.o.o.o.

Sodium carbonateSodium chlorideCarbon dioxidewAluminum chloride -

1.3Ammonium carbonate

Clay slipMagnesiteLimestoneLimestoneQuartziteMagnesiteDolomiteQuartziteCarbon blackGraphiteTalcMicaVermicullitePumiceLead zinc ore

-0.01-0.0410.00.02Hardwood pitchSodium polymetaphosphate (Calgon)

SulphurQuartzite

DW-

1.65KaolinThalium chloride--Grinding rate factor =new surface produced with additive/new surface produced without additive.

Grinding in waterWet grinding is generally more efficientthan dry grinding r 43 -46J.This has beenattributed mainly to chemical reactionsbetween broken surface bonds and watermolecules 46J. On the basis of this consi-deration, water vapor should also be expectedto cause similar effects since it can easilypenetrate to the crack tip. Ziegler r47J hasfound that water vapor can increase theefficiency of cement dry grinding in a smallvibratory mill, but he also observedorganicvapor to produce such effects. Additionalevidencehas been provided by Locher et at.r48 J, who have reported that the grindingrate of soda ime glasswas higher in humid airthan in vacuum. In contrast to the chemicaleffects discussed bove, nvestigatorshave alsoconsidered physical factors such as reducedcushioning n water of coarseparticles and thegrinding media by the fines as well as effects ofviscosity and specific gravity [1, 12,49 - 76].

\

Grinding in organic liquidsThere have been several eports indicatingthe grinding in organic liquids to be moreefficient than in water [46, 51 - 57]. Forexample, Kiesskalt [51] has obtained a12-fold increase n surface area or grindingin organic liquids, such as isoamyl alcohol,

from that in water. In a similar investigation,Engelhardt [53, 54] studied the effect ofalcohol on quartz grinding. His results in-dicated that in the caseof alcohol, lessenergywas needed o produce the samesurface areathan that required in water.In 1956, Ziegler [47] reported a 40%increase n the capacity of cement grindingmills by the use of organic liquids such asethylene glycol, propylene glycol, butyleneglycol and triethanol amine. Since then,organic iquids have been reportedly used ngrinding of cement commercially [52]. It wassuggested 52, 55] that the vapors of theorganic iquids reduced the forces of adhesionbetween he particles and that led to reducedagglomeration and enhanced material flowand, in turn, to improved grinding.In another investigation, Lin and Metzmager[46] ground quartz in different environments.Their data showed as much as a 25% ncreasein grinding rate in carbon tetrachloride andmethyl cyclohexane than in nitrogen. Grind-ing in water was more efficient than in anyof the other environments tested. Inter-estingly, when water was added to the organicliquids, the efficiency of grinding was restoredto its value in water alone.Other organic liquids have been used bySavage t al. [57] to grind silicon carbide in a

.5

.0

.2

.55

008020803


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284TABLE9Effect of surfaceactiveagents n grindingof quartzite and imestone 59 ]Feed Additive Additive concentration

(%)Percentage increasein new surface

Flotigam Puartzite 00.0050.010.030.050.11.000.0080.00170.0030.0060.061.33

+1+1+1+++'

+- i"-',-,'~'C;

~~c~++'

Limestone Oleic acid

Limestone Sodium oleate 0o.o.O.1.0O.O.O.O.

Limestone Flotigam P

laboratory vibratory mill. They obtained a100% increase in the grinding rate in benzenein comparison with that in water alone, whilethe effect of ethanol was considerably less.Effect of surfactant addition to grindingenvironmentsThere have been several reports in thepast that grinding efficiency can be influencedby the addition of surface active agents ntothe grinding mill. For example, Flotigam PC12-C14) mine from coconut oil) was oundby von Szantho [58] to produce as much as a100% increase n the surface area of quartzand 75% ncrease n the surface of limestonewhen the reagentconcentrations were in therange of 0.005 to 0.02% (seeTable 9). How-ever, above he concentration of 0.02% of theadditive, the effect was noticed to be reduced.Similar results were obtained with an oleicacid/limestone system except that in thiscasedetrimental effects were observedabovea concentration of 0.003% oleic acid. On theother hand, sodium oleate decreased hegrinding rate of limestone at all levels of

addition (Table 9). It should be mentionedhere that the method used for measuring hesurface area of the ground product was notdescribed n the paper. In addition, it appearsthat there was no proper control of importantvariablessuch as pH and ionic strength in thiswork. The observed decrease n surface areaabove a certain concentration of the additivescould be due to experimental artifacts intro-duced by possible aggregationof the groundproducts. There is no discussion n the paperon the state of dispersion of the groundproducts or on any microscopic examinationof the products.In a similar investigation, Gilbert andHughes 59] studied the effect of additions ofArmac T (a mixture of saturated and un-saturated CI6 and CI8 amine acetate) on theball milling of quartz. In this case, he addi-tive was ound to produce, at a concentrationof 0.01%, a 4 to 12% decreasen the amountof minus 200 mesh produced in the pH rangeof 2 to 5. An increase n the surfactantconcentration from 0.01% to 0.1% decreasedthe grinding efficiency further at all pH

003142371655800+710176199035

-184920295674+9

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.7

.4

.4

.8

.3

.3

.6

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.9

.7

.8

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.2

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040850005010205


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285

\

(a)

surfactant. The importance of proper disper-sion of the ground products has been verifiedrecently by EI-Shall et al. [60]. Using micro-scopic examination of selectedsize fractions,the authors have found that sodium silicatewas not a sufficiently effective dispersant orthe product ground in 10-1 mol/I aminesolution (see Fig. 3(a. The grinding resultsin this case suggested nitially the use ofamine to be detrimental (see Fig. 4). How-ever, after proper dispersion of the groundproducts using alcohol/acetone washes seeFig. 3(b, amine was observed o have actedas a grinding aid.Other reports of the effect of surfactantaddition include that of Kukolev et al. [61,62], who obtained improved grinding effi-ciency of alumina on using organo-siliconeata concentration of 0.005%. Malati and hisco-workers [63] and EI-Shall et al. [64]have independently produced finer productsby adding oleic acid to the wet milling ofhematite.The influence of dodecylammonium chlo-ride on grinding of quartz in a porcelain ball

(b)Fig. 3. Photomicrographs of (-14 +20 mesh) sizefraction of quartz ground in 10-1 molll amine in astainless steel mill [60]; (a), dispersed using 8 g/lsodium silicate solution; (b), dispersed using 8 g/lsodium silicate solution and then washed with alcoholand acetone.

,values.For example, he level of addition of0.1% at pH 5.0 produced as much as a 36%decreasen the amount of 200 mesh producedas compared with that produced in wateralone. The authors have not offered anyexplanation for the observedeffects. Soma-sundaran and Lin [1] have examined thesedata and suggestedhat physical adsorption ofArmac T (cationic surfactant) on the quartzsurface (negative above pH 2.0) could havedecreased he charge on the particles andthereby causedan increase n their floccula-tion. According to them, it is not clearwhether the reported low grinding efficiencywas due to any experimental artifact intro-duced by such aggregation of the groundproducts, or the direct result of a change ninterfacial properties brought about by the


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286ore grinding, due to the possibility of thepresenceof extraneous onic species.Recently, EI-Shall et al. [64] have ob-tained finer products in the neutral andalkaline pH range, on adding dodecylam-monium chloride during quartz grinding in astainlesssteel ball mill. Detrimental effectswere obtained in the acidic solutions, asshown in Fig. 5. The authors correlated theirresults with the effect of amine on interfacialproperties of quartz (zeta potential andflocculation characteristics) under simulatedchemical conditions. Ferric salts were addedin this case o simulate the iron released romthe balls and the mill during grinding. This isvery important since ferric ions can evenreverse he charge on quartz particles andconsequently affect the flocculation proper-ties, as shown in Figs. 6 and 7.

mill has been investigated by Ryncarz andLaskowski [65]. The effect of the additivewas found, in this case, o depend on itsconcentration and the pH of the solution. Theauthors correlated their grinding results withthe zeta potential of quartz in amine solu-tions. They concluded that grindability wasminimum under conditions of zero zetapotential. It should be noted that the investi-gators have not taken into account thepossibility of surface contamination byaluminum species rom the porcelain mill andconsequent effects on the interfacial proper-ties of the ground material. Aluminum speciesat concentrations as low as 10-5 molll havebeen found to reverse he zeta potential ofquartz fines [66]. Adsorption of cationiccollectors (e.g. amine) can indeed be pre-vented under such conditions. In this regard,Ryncarz and Laskowski [65] have pointedout that grinding was extremely sensitive tothe concentration of iron speciespresent intheir grinding system. Although the authorshave concluded that wet grinding of a givenmineral could be improved by the properchoice of additives, they have also warnedthat it would appear o be almost impossibleto predict any effects accurately in the caseof..

+30+25+20

1E~'0'CI.5fu~

~ ~I10

Effect of addition of inorganic electrolytes togrinding environmentsThe effect of inorganic electrolytes ongrinding of minerals has been nvestigated byseveralworkers [58, 77, 78]. Although resultsreported in the literature are often contra.dictory to each other, grinding has n generalbeen found to be more efficient in the pres-ence of inorganic electrolytes [19].In 1948, Kukolev and Melnishenko [79]studied the effect of caustic soda and soap onthe ball milling of magnesite MgCO3)anddolomite (MgCO3.CaCO3). hey found causticsoda to be beneficial to the grinding ofmagnesite,with no effect on the grinding ofdolomite. Soap, on the other hand, was foundto help the grinding of dolomite, with noeffect on magnesite.The authors attributedthe observed effects to the difference inadsorption of sodium hydroxide on the twominerals. Somasundaranand Lin [1] havenoted that there is no reason for sodiumhydroxide not to have the same adsorptioncapacity on both minerals. They supportedtheir argument with the help of the reportedbeneficial effects of the above eagentson thegrinding of limestone, which is chemicallysimilar to dolomite [80,81].In another work, Frangiskos and Smith[80] investigated the influence of sodiumhydroxide and sodium carbonate on thegrinding of limestone using a laboratory drop-weight mill. Theseworkers obtained improvedgrinding efficiency in the presence of the

I ' lonIc~3x1O-'M(KNOJ) i-1&1 . . . . . . I0 2 4 . 8 10 12 14pH

Fig. 5. Effect of amine on the amount of -200 meahproduced by wet ball milling of quartz [64]. DDACIconcentration: 6,10-5 mol/l; 0,10-3 mol/I; 'Y, 10-1mol/I. Ionic strength: 3 X 10-2 mol/I (KNO3)'

+15

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28780

+40

\~

+20

'>oS: 0cui...N -20

-40

-60 2 . . 10pH

Fig. 6. Effect of 10-4 molll FeClJ on zeta potential of quartz as a function of pH [67].", water; e, FeClJ. Ionicstrength: 3 X 10-1. molll (NaCl.).above additives. Ghosh and co-workers [81]have subsequently repeated some of Fran-giskos and Smith's experiments. Their dataalso ndicated increasedbeneficial effects butwith a maximum at an additive concentrationof 0.02%. Increaseof the concentration to0.04% produced less beneficial effects. Theauthors failed to give any explanation for theobserved maximum. The presence of suchmaximum has also been reported by otherworkers [58], who obtained poor grindingon increasing additive concentration abovecertain levels. Additional evidence s providedby Halasyamaniand co-workers [82, 83] intheir investigation of the influence of pH onthe grinding rate of quartz and calcite in asteel ball mill using HCI and NaCH. Maximumgrinding rate of quartz was obtained aroundpH 7.0. In this case, he authors explained theobserved esults as due to increased loccula-tion of quartz in the neutral pH rangecom-pared with the acidic or the alkaline pH range.They claimed that the flocculated material inthe mill was in a better position to receiveimpacts, leading to improved grinding. Itshould be noted that these investigators didnot verify whether the quartz was flocculated.

,

Actually, it is not clear why quartz shouldflocculate at any pH other than close to itspoint of zero charge, .e. pH -2.0 [84]. Inanother work, Mallikarjunan et at. [83]reported an increase n surface area of calcitewith decreasingpH in the pH range of 8 to 4,whereas he surface areaof quartz was oundto decreasewith pH in the same pH range.This observation could, however, have beendue to better dispersion of the particles sincethe direction of pH change for surface areaincreasewas away from the point of zerocharge for both quartz and calcite (2.0 and10.5, respectively) [84,85].Savage t at. [57] studied the effect ofsodium chloride and various salts of multi-valent ions on the grinding of silicon carbide,using water or ethanol as he grinding fluid.They classified the sal~ into flocculants(MgCI2, CaCI2,and FeCl2, and FeCI3) anddispersants NaCI and N~207)' The authorsconcluded that in both water and ethanol,dispersantssignificantly improved the grind-ing rate.It has been reported by various workersthat multivalent ions do affect the grindingefficiency of minerals. For example, Fran-


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288In addition to the works discussedabove,higher efficiency of grinding has been re-ported in mineral processingplants using seawater to make up the pulp instead of theordinary tap water [86 - 88].

~E~~....

2 4 8 8 10 12pHFig, 7, Effect of 10-4 mol{l FeCI) on the turbidity ofquartz slimes as a function of pH [67). e, 10-4 mol IIFeCI) blank test; " water; " FeCI), Ionic strength3 X 10-2 mol{l (NaCl).

giskos and Smith [80] have reported anincrease of as much as 50% and 15% insurface area of ground quartz due to additionof 2.0 mol/ AlCl] and CUS04. respectively.Furthermore. they found iron-stained quartzcrystals to grind faster than the clear ones. Onthis basis, hey have suggestedhat ferric ionsmight have affected the fracture of quartzduring grinding. It should be noted that therewas no indication as o whether these nvesti-gators have controlled the pH or the ionicstrength of the grinding solution. Thus, noconclusion can actually be drawn about themechanisms nvolved in such systems.Recently, Hartley et al. [2] have studiedthe effect of sodium hydroxide on wet ballmilling of taconite ore. They obtained anincrease n surface areawhen the feed was ofnarrow size range;however, when they used afeed of complete size distribution. no effectwas obtained. They assumed he ineffec-tivenessof the additives n the latter case obe the result of changes n viscosity due to thepresenceof fines in the feed.

Effect of the physical properties of thegrinding environmentAs mentioned earlier, transport of thematerial through the grinding mill is one ofthe important component processes n thegrinding operation. The material transport inthe mill dependsprimarily on physical proper-ties of the pulp, such as pulp fluidity, state ofdispersion or flocculation of the pulp, densityof the solids and density of the medium. Inother words, theseproperties determine howwell particles are transported to the grindingzone itself. In addition, such properties can beexpected to have an effect on the hydrody-namic behavior of particles as well as on thegrinding media such as balls or rods, and con-sequently, on the grinding performance [1].It has been reported [57, 89 - 92] that pulpviscosity can influence the grinding of ores.Schweyer [89] found grinding in pebble millsto be dependent on the viscosity of themedium up to a certain grinding time, andthen to be independent of it. In a similarwork, Hockings et al. [90] studied the effectof pulp viscosity (using com syrup to controlthe viscosity) on the rate of production ofquartzite fines in a rod mill and found theincrease n the pulp viscosity from 1 to1000 cP to causeas much as a 50% decreasein the grinding rate when the grinding wasconducted for 1 min. The observedeffect wasof a smaller magnitude for 2, 3, and 4 mingrinding (see Fig. 8). Additional results in thisarea have been reported by Savage t al. [57],who ground silicon carbide in liquids ofdifferent viscositiesand the results indicatedfaster grinding rates in liquids of lowerviscosities.Fluidity modifiers and dispersants havebeen reported to produce both an increaseand a decrease n grinding efficiency [2,67,68 - 75,92]. For example Hartley, et al. [2]have obtained a marked decrease n grindingefficiency of molybdenum ore when theyadded sodium tripolyphosphate to theirgrinding circuit. On the other hand, Hannaand Gamal [92] found that sodium silicateenhancedgrinding of limestone.


15/19

2891.00: ~=:~ ~ ~8 0.301.20~ 0.80GO~ ~~f 0.602'~I -~ i~ ~ 040.-;tE ~~-

u 0.20

~.5E.,;&j

o~ 0.100.07'0S o.~

~

~ 0.2 mgf,-/ XFS-4272

No~-

0.01~ 50 200 500 1000 2000Size I_IFig. 9. Back-calculated S curves for taconite oreslurries ground in 8 in batch ball mill showing effectof grinding additives [72 J. All slurry densities at84 wt.%. +, experimental (one-size fraction method);-, computer back-calculation.

0.6 mg/g XFS-4272

~ 0.03I'" 0.02 . .. ... ., 5 10 50 100 SOO 000 5000VilCooity u, CP.Fig. 8. Cumulative percentage finer than the startingsize va; viscosity for rod mill grinding of 10 X 14 meshquartzite in corn syrup-water mixtures for variousgrinding times; after Hockings et al. [90 J." 2405;0,180 s;6, 1205;0,60 s.

~Ji~ i':~~,,9 t"v"'" ,

,

C;",c:i;,~,~ '1'7Vt~.~_. ~ ~ +4 & I +2 - ---

~~J -i' Steady state (no aidl'2' Polymer added, Poly , shut off~ Polymer addedt FI8d ,... increase~ +105 +

&i-5

~. ...~~.~ 1~-OJ

~I.10" . . , . . , , . , , ,7 8 9 10 11N- 1 2 3 4 5 6l' '2' l' -a"tTimeFig. 10. Typical response curve for industrial-scalemill {72].

..

Unfortunately, most of the past work isnot of much practical use due to lack ofproper control of relevant variables such assolution, pH, ionic strength, etc., and possiblyowing to experimental artifacts that can ariseout of agglomeration of products that werenot properly dispersedbefore size analysisafter contacting with surfactants.An additional experimental problem can bethe frothing of the pulp in the presenceofsurfactants during grinding and the conse-quent change n the hydrodynamic character-istics of the grinding environment, as well aspossible entrapment of the mineral particlesin the froth (see Fig. I ) [60]. The entrappedparticles can remain levitated and escape hegrinding actions, with resultant reduction ingrinding efficiency. This possibility has notbeen taken into consideration by the pastworkers.

In 1970, Klimpel and co-workers [68 - 75]have initiated a researchprogram aiming atidentifying possible grinding aid chemicals.They have conducted several aboratory andindustrial scale ests using fluidity modifiers.Examples of the results obtained in thesetests are shown in Figs. 9 and 10. The datagiven in Fig. 9 suggestan increase n thespecific rate of breakagedue to the grindingaid addition. Similarly, the industrial datashown in Fig. 10 indicate that a finer productcould be obtained with approximately con-stant feed rate whenever the aid is added.Also, a constant grinding size s achievedwithhigher feed rates, due to addition of thegrinding aid.

The behavior of these reagentshas beencharacterized by measuring their effectson the rheology (viscosity) of the groundproducts. Correlation of rheological data andlaboratory grinding results [74,75] has madeit possible or the investigators to identifyslurry conditions when chemical additiveswould increase rates of breakage. In thisregard, they have concluded that chemicalsthat can work as grinding aids should main-tain pseudoplastic behavior in the slurrywithout yield stress,or reduce the yield stressin a densepseudoplasticslurry.On the basisof the above discussions,onecan conclude that comminution processescan, in general, be enhanced by the use ofinorganic and organic additives under selectedconditions. Indeed, n order to fully developand utilize such effects, it is necessary oestablish the mechanisms nvolved in sucheffects.


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290

Fig. 11. Photograph showing frothing during millingin 10-1 molll amine at 78% critical speed, elapsedtime oCgrinding = 58 [60).Unlessprecautions are taken to avoid or toaccount for such experimental artifacts andproblems, observations,even hough real, canbe misleading. In addition, without a full

understanding of all the interfacial propertiesof the systems under study, comminutionresults are likely to be attributed directly tothe effect of additives on fracture.The above review of the past results clearlyshows hat most of the past works have nottaken into account the above-mentionedproblems, and therefore no definite mecha-nisms could be derived on the basisof suchresults.Reported mechanismsMost of the past results of the effect ofchemical additives on comminution of~aterials have been interpreted in terms oftwo major mechanisms.The first is called'Rehbinder's effect' [93J. It is basedon thereduction of surface free energy of solidsinduced by the adsorption of surface activeagents. Since fracture of materials in com-minution processesnvolves creation of newsurfaces, the amount of energy requiredshould be proportional to the surface freeenergy of the created surfaces. f the surfacefree energy could somehow be reduced, thenthe energy consumption in creating the samesurface areacould be expected to be less.Onthis basis,adsorption of surface active agentsduring grinding should be expected to im-prove the grinding efficiency. However, asdiscussedearlier, the actual amount of energyneeded o create new surface representsonlyabout 1% of the energy nput to the commi-nution system. In addition, the effect of

chemical additives on other important param-eters and properties of the system such asplastic deformation, crack initiation and/orpropagation, flocculation and dispersion, etc.,are neglected n this mechanism.Primarily, presenceor initiation of cracks sthe prerequisite factor for fracture to occur.Additionally, parameterssuch as crack length,crack tip radius, plastic flow at the crack tip,propagation of the cracks, etc., will determinethe strength of materials. The effect ofchemical additives on such parameters, inaddition to their effect on the surface freeenergy, should therefore be noted.The second mechanism, based on adsorp-tion-induced mobility of near surface disloca-tions, was proposed by Westwood et al.[37 - 40], who studied the effect of variousadditives on the hardnessof different crystal-line and non-crystalline materials. Theycorrelated their hardnessmeasurementsanddrilling rates with near-surfacedislocationmobility and interpreted 'Rehbinder's effect'as a result of changes n the electronic statesnear the surface, and point and line defectscausedby the adsorption of the additive onthe surface of the solid. Such changescaninfluence specific interactions between dis-locations and point defects which control thedislocation mobility and hence he hardness.However, this mechanism cannot be usedexclusively to explain the effect of theenvironment on grinding, because mportantproperties such as pulp fluidity, flocculation,and dispersion, etc., are not considered.Chemisorption or formation of activatedcomplexes between the additive moleculesand the crack surface has been considered byseveralworkers, including Somasundaran ndLin [1], to be important to the fractureprocess.Other investigators [31] have consideredenhancedcorrosion due to stressapplication(stress-corrosioncracking) as a mechanismresponsible especially for the fracture ofmetals. The mechanism responsible forstress-corrosion cracking is not, however,clear at present, particularly becauseof theinconsistencies n reported results.Another mechanism considered by someinvestigators 44,57,67,68 - 75, 92] is basedmainly on the role of reagents n dispersionand, hence, he flow of particles in the millduring comminution processes.


17/19

291REFERENCES

.

As discussedearlier, grinding is an integralprocess composed of several si~ultaneoussub-processes, nd chemical additives canaffect these sub-processes ue to one or allof the previously discussed mechanisms.Identification of the role of each mechanismis possible only if the relevant properties ofthe solid and of the solution in contact withthe solid during its fracture are simulta-neously determined. The experimentallyaccessibleproperties, in this regard, includezeta potential, surface tension, pH, ionicstrength, temperature, chemical compositionof the solution, and frothing and flocculationcharacteristics of the system. Also, it isnecessary o control and monitor pretreat-ment of the material to be ground and itsphysical as well as structural and surfaceproperties. Finally, the machine character-istics such as speed,size, and mechanisms fstress application and other related param-eters ike loading, material filling, etc., mustbe taken into account.It is clear from the above review thatchemical additives can influence grinding fora number of reasons:a) modification of the flow of pulp in thegrinding mill;b) influence on reagglomeration of thefreshly produced fines;c) modification of frothing characteristics ofthe pulp during grinding;d) influence of interactions between mineralparticles and balls and mill wall and amongparticles themselves (due to changes nfrictional characteristics);e) influence on the strength of the materialdue to: 1) effect on crack initiation andcrack extension energy, and 2) retardation ofrejoining or sealing of the freshly createdcracks.Elucidation of the mechanismsgoverningthe effect of environment on grinding de-pends essentially on identification of theeffects on all the above factors and inter-actions between them.

ACKNOWLEDGEMENTThe support of the American Iron andSteel Institute is gratefully acknowledged.

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29259 L. A. Gilbert and T. H. Hughes, SymposiumZerkleinern I, verlag Chern. Duseldorf, Germany,1962, pp. 170 - 193.60 H. El-Shall, S. Vidarage and P. Somasundaran,Int. J. Mini. Proc., 6 (1979) 105.61 G. V. Kukolev, I. Ya. Piven, R. G. Zaika, D. V.Minkov, L. V. Vinogradova and T. S. Makarova,Refractories (USSR), 33 (1968) 465.62 G. V. Kukolev, I. Ya. Piven, V. M. Martynova,

D. B. Minkov, A. S. Yudina, T. V. Malikova andB. V. Parkhaev, Refractories (USSR), 33 (1968)741.63 M. A. Malati, M. K. Orphy and A. A. Yousef,Powder Technol., 2 (1968) 21.64 H. El-Shall, A. Gorken and P. Somasundaran,Effect of Chemical Additives on Wet Grinding ofIron Ore Minerals, paper presented at the XIIIInt. MinI. Proc. Cong., Warsaw (June 1979).65 A. Ryncarz and J. Laskowiski, Powder Technol.,18 (1977) 179.66 M. C. Fuerstenau, D. A. Elgillani and J. D. Miller,Tran,. AIME, 247 (March 1970) 11.67 H. El-Shall, D.E.Sc. Thesis, Columbia University,New York (1980).68 R. Klimpel and W. Manfroy, Development ofChemical Grinding Aids and their Effect onSelection-for-Breakage and Breakage DistributionParameters in the Wet-Grinding of Ores, paperpresented at XU Int. Min. Proc. Cong., Sao Paulo,Brazil (1977).69 R. R. Klimpel and W. Manfroy, in S. Ramani(Ed.), 14th Int. Sym. on Application of Com-puters in the Mineral Industry, AIME, New York,1977, pp. 197 - 206.70 R. R. Klimpel and W. Manfroy, Ind. Eng. Chem.Process Des. Dev., 17 (1978) 518.71 R. R. Klimpel and R. Samuels, Proc. AnnualMeeting of Canadian Mineral Processors,Ottawa,1979.72 R. R. Klimpel, in P. Somasundaran (Ed.), Proc.

Symp. on Fine Particles Processing, Las Vegas,1979, AIME, New York, 1980, p. 1129.73 M. Katzer, R. Klimpel and J. Sewel, MiningEngineering, 33 (1981) 1471.74 R. R. Klimpel and L. G. Austin, Powder Technol.,31 (1982) 239.75 R. R. Klimpel,Powder Technol., 32 (1982) 267.76 U.S. Patent 4,126, 276;4, 126, 277;4, 126,278;9,136,830;4,162,059;4,162,045.77 J. H. Brown, M.I.T. Progress Report, N.Y.O.,7172 (1955).78 M. H. Stanzyk and I. L. Feld, U.S. Bureau ofMines Report 7168 (1968).79 G. V. Kukolevand L. G. Melnishenko, FireproofMater. (Moscow), 13 (1948) 447.80 A. Z. Frangiskos and H. G. Smith, Trans. Miner.Dressing Congr., Stockholm, Sweden (1957),pp. 67 .84.81 S. K. Ghosh, C. C. Harris and A. Jowett, Nature,188 (1960) 1182.82 R. Mallikarjunan, K. M. Pai and P. Halasyamani,Trans. Indian In,t. Metal, 18 (1965) 79.83 P. Halasyamani, S. Venkatachalam and R. Malli-karjunan, Ind. Eng. Chem. ProcessDes. Develop.,7 (1968) 79.

30 W. A. Weyl, J. Am. Gram. Soc., V.. 32 (1949) 367.31 H. H. Uhlig, in H. Liebowitz (Ed.), Fracture: AnAdvanced Treatise, Academic Press, New York,1971, pp. 646 - 675.32 V. I. Lichtman, P. A. Rehbinder and G. V.Karpenko, Effects of a Surface Active Medium onthe Deformation of Metals, H.M. StationaryOffice, London, England, 1958.33 V. I. Lichtman, E. D. Shchuken and P. A. Reh-binder, Physico-Chemical Mechanics of Metals,Academy of Sciences,Moscow, USSR, 1962.34 M. I. Montrov, Zh. Eksp. Tear. Fiz., 4 (1934) .1957.35 J. H. Westbrook and P. J. Jorgensen, Trans.AIME, 233 (1965)425.36 J. H. Westbrook and P. J. Jorgensen, Am. Miner-alogist, 53 (1968) 1899-37 A. R. C. Westwood, D. L. Goldheim and R. G.Lye, Phil. Mag., 16 (1967) 505.38 A. R. C. Westwood and D. L. Goldheim, J. Appl.Phys., 39 (1968) 3401.39 N. H. MacMillan, R. D. Huntington and A. R. C.Westwood, 7th Rept. to Office of Naval Research,Contract No. 0014-70-G-0300 (July 1973) 15.

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53 W. V. Engelhardt, Nachr. Akad. Wiss.Gottingen,Math. Phys. KI., No.2 (1942).54 W. V. Engelhardt, Naturwissenschaften, 33(1946)195.55 W. Fischer, Zem. Kalk Gops, 20 (1967) 138.56 H. Schneider, Zem. Kalk Gops, 22 (1969) 193.57 K. I. Savage,L. G. Austin and S. C. Sun, Trans.AIME, 225 (March 1974) 89.58 E. von Szantho, Z. fur Erzberglass und Metall-huttenwesen, 2 (1949) 353.


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29390 W. A. Hockings, M. E. Volin and A. L. Mular,Trans. AIME. 232 (1965) 59.91 B. Clarke and J. A. Kitchner, British Chern. Eng.,13 (July 1968) 991.92 K. M. Hanna and A. E. Gamal, Powder Technol..17 (1977) 19.93 P. A. Rehbinder, L. A. Schreiner and K. F.Zhiyach, Hardness Reducers in Drilling - A

Physico-Chemical Method of Facilitating theMechanical Destruction of Rocks During Drilling.Akad Nauk, Moscow, USSR (1974), Trans. byCouncil for Scientific and Industrial Research(CSIR), Melbourne. Australia. 1948, p. 163.

~

84 P. Somasundaran, n G. Goldfinger (Ed.), CleanSurfaces: Their Preparation and Characterizationfor Interfacial Studies, Marcell Dekker, NewYork, 1970, p. 285.85 P. Somasundaran and G. E. Agar, J. ColloidInterface Sci., 24 (1967) 433.86 M. Rey and P. RaCfinot, Mining World, 19 (1966)18.87 M. K. Orphy, A. A. YouseC and H. S. Hanna,Min. Mineral Eng., 4 (October 1968) 48.88 I. J. Lin, J. Au. Eng. Arch. (Israel), 27 (1969) 27.89 H. E. Schweyer, Ind. Eng. Chem., 34 (1942)1060.

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Physico-Chemical Aspects of ... Use of Additives