Published in -Research Needs in Mineral Processing,'Somasundaran, P., and Fuerstenau, D. W., eds., NSFWorkshop Report, 1976.

FINE PARTICLES TREATMENT

P. Somasundaran

Mineral products which ~njoy an ever increasing demandsuffer'from the dilemma of both a decrease in the grade ofeconomically accessible ores and also an increase in the de-qree of fineness of value minerals in the ore. Hence. ifth~increasing minerals d~and is to be met. technology torecover the value minerals from finely dispersed or~s andwastes is urgently needed. Such technology should alsonecessarily be within the energy. environmental and econom-ical constraints.

Mineral values lost as slime from the world's metallicores are actuaily of immense proportion in several cases. Forexample, roughly one-third of the phosphate mined in Florida,one-half of the tin mined in Bolivia, and one-fifth of the~rld tungsten are lost as fines. Copper, uranium, fluorspar,bar,ite, zinc and iron come as close seconds in the same cate-gory. As long as new or modified technology is not developed,since more and more finely dispersed ore bodies will have tobe mined and processed in the future, these staggering wasteproportions and concanitant environmental problems will onlycontinue to increase. Indeed, fines must ideally be producedonly for the purpose of liberation of values fromwaste&.During the mining and milling of ores such as phosphate rock,how~ver, excessive fines are produced than what' is requiredfor liberation. A reevaluation of mining, transportat~on, andmilling techniques is warranted to determine whether the prob-:-lem of fines could be at least partially resolved by reducingits production itself.

In addition to the problem of unnecessary genesis offines duting mining and milling, there are three major prob-lem areas in fine particle processing:

a) Fine Grinding for liberation of values and accompany-ing energy consumption. ~

b) Recovery of mineral valQes from the fines.c) Processing of fine wastes for disposal in an environ-

mentally acceptable form.

Technology for processing fine particles for recoveryof mineral values or for disposal is largely undeveloped. Con-ventional mineral processing techniques fail in the sub-sieverange. Thus flotation process is not used for beneficiatingminus 150 mesh phosphate rock fraction that contains almost

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one-third of the valuable phospha~e. Similar problems exist,among others, with cassiterite, taconite, and pentlandite oresDevelopment of new technology demands an understanding of theproperties of particles that predominate in the fine si~erange and the mechanisms of the dependence of various mineralprocessing operations on particle size. Equally important,specificity of some of the current fine separation techniquesneed to be understood lest their utility would remain limitedto a system or two for which they have been successful.

FIVE PROBLEMATIC CHARACTERISTICS OF FINES

As particles become smaller, changes in five propertiesbegin to influence their processing in the manner lis~ed be-lCM.

The three major problem areas in fine particle treatmentare examined below in the light of the above changes andtypical research needs are indicated.

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PREPARATION AND CHARACTERIZATION OF FINESProblem Area 1:- .The extent of fine qrindinq and the enerqy required to

liberate mineral values from ores will be less if fracturedurinq comminution is more of interqranular type than of theusual indiscri.inate type. To devise processes that willyield interqranular fracture, research is needed to elucid.te~ch~~s-9-! fr~e on pply~ry~alline and polymineral~imens. -

A major problem in the preparation of fines by grindingis the existence of a practical grind limit, which is oftenthe result of a physical equilibrium created between theprocesses of size reduction and reaggregation of fines. Ex-cept for some work in West Germany on the tensile p.ropertiesof a9gregat$s of ground minerals products, there has beenalmost no work to understand the mechanisms of the processesdetermining the grind 1imit.

Chemical additives have been tested in the past tofacilitate fine grinding. For example, significant in-crease in mill capacity has been reported in the presenceof chemical additives for the industrial scale grinding ofcement clinkers in West Germany and Yugoslavia. The mechanismof the role of these additives is not, however, established.ainly because the available data is of limited use as a re-sul t of lack of monitoring and control of important systemvariables in the experimentation involved therein. Evidently,there is need for some careful research on this im rtant as-oect 0 chem~cal ~t~ves on CO8m nut on processes.

After the particles are prepared, their proper character-ization in terms of surface chemical composition, structure,heterogeneity, etc. r is often needed. However, at presentno mineral processing laboratory at least in the academicworld in the U.S., is fully equipped with all the necessary.modern facilities for scanning electron microscopy, ESCA,SrMS, Raman, and other useful spectroscopic techniques. Itmust be pointed out that equally important here is the deter-mination of which technique to use and which not to use. Useof Auger, for example, can reportedly yield tOtally misleadingresults when used for studying non-conducting minerals.

Problem Area 2: BENEFICIATION OF _FINE MINERALS

Techniques that will be discussed for possible use inthis area are shown below:

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Conventional Flotation (Mpdified)

The conventional froth flotation can be extended to lowersize ranges possibly by three means; 1) use of flotation agentswhich adsorb specifically, e.g.~ use of oxines in the flotationof oxidized lead and zinc ores; 2) use of variables such as tempera-ture as a process parameter, e.q.,.hot flotation of finely groundiron ore by Cleveland Cliff ];ron Company; 3) use of techniqueswhich permit refluxinq, e.g., column flotation.

Attempts for developing flotation agents that adsorb speci-fically miqht involve synthesis of reagents that chemisorb morestrongly and selectively. Reagents that adsorb by hydrogen bond-ing or even by electrostatic bonding can probably be ruled outfor use as collectors in systems containing fines.

A parameter, the potential of which is largely unrecognizedis temperature. Furthecmore, mechanisms involved in determiningthe effects of temperature during conditioning are yet to beascertained. For example, it was recently found at the HenryKr~ School of Mines that hot-conditioning ,of hematite witholeate was .beneficial only below an ionic strength of 2xIO-3N:above this, temperature had a detrimental effect. The reasonsare largely unknown.

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In column flotation, the countercurrent flow of bubblesand mineral slurry ensures excellent particle-bubble contact,the resultant mineralized bubble being continously cleaned bythe countercurrent flow of water. It has been reported togive results that are superior to conventional flotation forundeslimed uranium ore, fine graphite containing clay, and-100 mesh copper ore. It gave poorer results for the copperore in the upper size range. Also, use of columns that aretaller than 28 feet in height has been found to be ineffective.

Flocc:-Flotation

In the case of flocc-flotation, interactions of floccu-lants with other dissolved species in the system such as celciumare not in general understood. Usoni et al., for example,found the flotation of sphalerite and smithsonite to be en-hanced by the addition of Separan and certain Jierofloc andNalco flocculating agents up to certain concentrations andto be depressed above them. The reasons for this are highlyspeculative. Major problems in selectively flocculating min-erals are further described below.

Selective Fl~_lation

A technology, with enormous potential, is selective floc-culation, accompanied by flotation as described above or pos-sibly elutriation, filtration, etc. Advantages and disad-vantages of various combinations are not, however, known.Examples of noteworthy work in the past include separationsof iron ore and heavy metal ores from silica and clay usingstarch. Usoni et al. during their study on the selective pro-perties of anionic, cationic and nonionic polymers as floccu-lants for several minerals individually and then in combinationwith each other found prediction of selective flocculation onthe basis of the ~esults from single mineral tests to agreewith the results obtained for certain mixtures such as pyrite-quartzusing a nonionic polymer but to fail with those obtained forcertain other mixtures Such as smithsonite-quartz. The reason forthis discrepancy is not established. This problem (analogicalto that in the flotation area) needs careful and systematic:study in the area of wet selective separation. In addition,developments, again along the same line as for flotation, ofnew specific reagents with cOmplexing or chelating ligandsshould pr'ove fruitful.

~_l~raflotation (Carrier Flotation)

This technique has been used by Engelhard Minerals andChemicals for more than ten years on a conmlercial scale toremove anatase from kaolin. While the anatase does not floatby itself it is cofloated when an auxiliary material, calcite,

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is added. To date this technique has not been commerciallyused for beneficiating any other system. It was in facttried for phosphate slimes by the same inventors withoutsuccess. Basic research on mechanisms of ultraflotation,which appears to have a lot of potential, is strongly suq-gested. -

Spherical Aqglaneration

In this process, the fines are tumbled in an aqueous so-lution containing an immiscible liquid which forms capillarybridges between reagentized particles and causes their aggre-gation. Since Stock's original observation in 1952 of thisphenomenon with barium sulfate precicipates in benzene con-taining a small amount of water, it has been examined mainlyby Puddington et al. -for agglaneration of graphite, chalk, zincsulfide, coal, iron ore and tin ore suspensions in aqueous so-lutions. Farnand et al. have claimed good separation of ea~hcanponent fran a mixture of zinc sulfide, calcium carbonate- andgraphite in water with nitrobenzene as binding liquid by step-~'ise selective agglomeration. Even though the role of a numberof process variables have been studied, many questions of theeffects and interactions of such variables on Grade and recover,a lomerate stren th, ros~t and the o~ recover, rema~n un-answere.--

Flotation Processes Usinq Oil.

The flotation processes using oil/water interface for col-lection of particles are emulsion flotation and liquid-liquidextraction. In the former the reagentized particles are col-lected by oil/water emulsion droplets and by aeration of thesystem, whereas in the latter removal of the particles collectedat the interface is achieved mostly by phase-separation. Theonly current commercial use of emulsion flotation, to the author'sknowledge, is that of the separation of apatitefram iron ore atLKAB, Malmberget, Sweden. Basic work is needed here on molecularand ionic interactions at various oil-solid interfaces since thesuccess of these techniques for fines is dependent essentiallyon such interactions. Also, understand in of mechanisms thatdetermine size distribution of ro ets or var ous m neral col-'~~c~~;~~~u~1.on/o1.~_sy~~ems w1.l,! D~_ne!prul s nce a control 01.t appears to prov1.de the opportun1.ty to optimize recovery andgrade (the former "being higher with smaller bubbles and viceversa)..Fine Bubble Flotations (Vacuum, Pressure release, Electro-flotation, Foam separ!tlons) .

Inability to control the bubble size during the conven-tional flotation operation is the basic handicap of it in the

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fine region. Methods tha~ can generate fine bubbles in thecell include vacuum flotation, pressure release flotation,electroflotation and various foam separations. '

In the first two processes the generation of ~ubble canbe expected to occur preferentially on hydrophobic sites onthe minera~s and possibly enhance the selectivity. The air-pockets in crevices and pores could also act as nucleationsite for the bubbles: this of course can be detrimental. Theelectroflotation. reportedly used in Russia in various in-dustries. is particularly attractive since it can generatebubbles that are extremely fine and homogeneous in size and-since they fortunately resist coalescence possibly due tosimilar charges on the bubbles. It . rationin combination with the conventiona externala a 1. allse r1. .

Techniques Using Miscellaneous Field Forces

Use of differences in density. magnetic. electrostaticand electrophoretic properties of fine particles have beenattempted by various workers for beneficiation purposes.

It is doubtful whether the gravity techniques would findany major application in this area, even though Bartles-Mozleytables with 40 fiber-glass decks per frame have been reportedlyused for recovering cassiterite particles down to 8 microns.Both lower-intensity and high-intensity wet magnetic separatorshave also been reportedly used for separation in the sub-sievesize range. the former for treating 91\ minus 325 mesh magnetictaconites and the ~atter mainly for purification of kaolin(71\ minus 2 microns). High-gradient magnetic separation hasbeen recently successful in the beneficiation of semitaconites,oxidized taconites, coal and kaolin, with commercial applica-tion reported for treating the last. Research needs exist hereon the use of extraneous reagents to treat the ore or to qener-ate areas of higher magnetic susceptibility and on the applica-tion of superconducting magnets even though problems would existhere due to the washing cycles.

Advantages of reported application of the electrostaticseparators in the subsieve range with the help of pulsed coronadischarge. high rotor speeds and external electric field andof the fluidized bed electrostatic devices result from the factthat these are dry processes avoiding the introduction of oftenundesired water into the system.

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Electrophoresis and electropermeation (electrodialysisusing charged mesh in place of membranes) suffers from thedifficulty in the scale-up of these processes for any siqni-ficant capacity maintaining the required quiescent hydrody-namic conditions.

Chemical Processes-

Below a size range, hydrometallurgical processes alone orin combination with the processes discussed above might be thebest choice for treatment of fines.. 'l1le size cut will varyfrom ore to ore and will have to be estimated by taking intoconsideration, in addition to recovery, also the energy, en-vironmental and economical factors. Research needs in hydro-metallurgy have been discussed elsewhere in this program andwill not be dealt with here further.

Problem Area 3: DEWATERING AND CLARIFICATION

Treatment of slimes and sludges produced during processingoperations for disposal remains as a serious problem in severalareas. An example of this is the problem caused by the phos-phatic slimes. A solution to this problem needs to meet tworequirements: quick return of clarified water for recirculationand discharge of slime in a tractable and environmentally accept-able form. There is a need here to determine the causes for thepertinacious behavior of the slime. The electrostatic repulsionbetween charged clay particles was usually thought to be the'reason, but recent results at the Henry Krumb School of Minesindicate the problem to be more complex.. During the effortsto improve the dewatering process,- it was also found that theaddition of coarse particles enhance the dewatering rate by asmuch as 30 to 50 times but only if the particles are at leastpartially hydrophilic.*. Continued work in this alone to under-stand the reasons involved miaht orovide answers to the auestionon the nast av~or 0 8 ~mes an ~n ~cat~ons as to w at couone t, . - at al anvt ~na.

*S6masundaran, P., Smith, Jr., E.L., and Harris, C.C.. .MechaniSmof Water Separation from Phosphatic Clay Suspensions.,49th NationalColloid Symposium; Potsdam, New York, Abstract, pp. 33-34, June,1975.

**Somasundaran, P., Smith, Jr., E.L., and Harris, C.C.." .Dewater-ing of Phosphate Slimes Using Coarse Additives., KIth InternationalMineral Processing Congress, Cagliari, Paper 49, April 1975.

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Even in the area of clarification of aqueous colloidalsuspensions using the familiar filtration technique, therole of interfacial charge of the filter medium on filtra-tion efficiency falls into the category of "unresearchedtopics." Filtration of clay suspensions by sand bed hasbeen found to become more efficient on treating the bedwith catonic amine and to become less efficient on treatingit with ferrocyanides. Research need exists here to deter-mine the role of process variables, type of surfactants andpolymers, etc., on the deposition of colloids and the natureof their distribution in the channels. Clarification of non-aqueous media is also an extremely difficult problem aggra-vated by lack of basic research work needed for proper hand-ling of non-aqueous suspensions. The removal of particulatesfrom liquified coal is a vital example that indicates themagnitude of this problem.'

SU~y

The above discussion clearly shows that there is a dearthof technology for beneficiating fines; application of the fewknown processes are limited and their mechanisms are yet to bedetermined. Research needs in this underdeveloPed area areenormous. A few selected needs are:

1. Study of mechanisms of fracture in polycrystallineand polymineral systems 1 also of the effect ofchemical additives in fine grinding.

2. Proper characterization of surfaces of fines pro-duced--facilities and technique adaptations areneeded for this.

Development of specifically adsorbinq reaqents.'Study of the effect of variables such as temperature

Elucidation of mechanisms of selective flocculation,flocc-flotation, ultraflotation, etc.

5 Determination of effects and interactions in electro~flotation, spherical agglomeration, liquid-liquidextraction, foam separation, etc., and mechanismsinvolved.' .

6. Investiqations on the mechanisms of dewaterinq ofslimes.

7 St~dy of surface effects in filtration and clarifi-cation.

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"FINE PARTICLES TREATMENT--DISCUSSION

R. D. Macdonald

Professor Somasundaran has made an excellent analysi~of the fine particle size problem, pointing out correctlythat very large quantities of valuable minerals are lost,probably forever, because of man's current inability to con-centrate fine slime-sized minerals.

One aspect of this problem is its very high cost. Byfar the largest cost of recovering any valuable mineral isthe cost of mining it. Costs of concentration, smelting,refining, and purification are relatively minor compared tothe cost of mining, and as the grade of ore goes down, Sothat you have to mine two tons of rock to get the amount ofvalue you used to get from one ton of rock the economiccrunch .starts to hurt. If you now must reject a quarter,or a third or more of what you mine, because you don't knowhow to handle it, the costs per unit of value actually re-covered rise to cover the cost of mining the minerals lostas fines.

From an economic point of view one might argue that thebest way to make low-grade minerals available to man wouldbe to work on decreasing the cost of mining, even if we mustin ignorance continue t9 waste much of the fine sized mineralsFortunately for us that is not the subject of this symposium.If you want a really tough job just try reducing the cost ofmining.

Professor Somasundaran has furthermore presented a mostcomplete discussion of the factors which affect the concen-tration of fine particles including the mechanism of grinding,froth flotation and its vari~tions, gravity separation, mag-netic and electrostatic separation, and other methods ofminor importance. In his discussion he suggests literallydozens of areas of study, most of which could conceivably re-sult in improvements in concentration methods, particularlythe synthesis of new and more effective flotation reagents,study of the effect of hot flotation, ultraflotation, the pro-duction of ~xtremely fine bubbles, and selective flocculation.

Two aspects which possibly did not get enough emphasisare the use of increased temperature and the use of ion-ex-change materials and activated charcoal. During the past year,at a mill in northern Canada the recovery of liberated sphal-erite has been improved from a deplorable 25 percent to a re-latively ~espectible 65 percent largely by increasing the tem-perature of the zinc cleaners to about 85~. The subject couldstand a great deal more study at this operation alone.

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At two mills in Africa activated charcoal is addeddirectly to flotation pulps, in one case to improve a lead~zinc separation, in the other to control a wild froth. Theseare empirical uses not well understood, but worthy of con-tinued development.

One of the new methods of considerable practical po-tential is the high gradient magnetic separator which oper-ates with much higher magnetic fields than were availablepreviously. It is no longer a laboratorycuriosity.andappears to be nearly ready for larger scale industrial use,where it is economic to do so. Its development has beenhampered by the high cost of both laboratory and industrialequipment.

I should like to call your attention to an old approach,namely heavy-liquid separation. It was my fortune in about1958 to work on the cormnercial deve1opment of this processwith the late Professor A. M. Gaudin, teacher and friend ofmany of us in this room.

Professor Gaudin proposed that inasmuch as acetylenetetrabromide of Sp.Gr. 2.96 is now available from Israel intonnage quantities at a reasonable cost currently in the'range of SO - 60 cents a pound, it is now possible to re-consider gravity concentration by heavy liquids. The keyto his proposal was the observation that residual heavyliquid can be nearly completely washed fran both float andsink by multiple repulping'and washing with simple shortchain alcohols. He envisioned a standard countercurrentdecantation system for washing. Laboratory studies withmanganese ores and with iron ores indicated that the pro-ducts could be cleaned of acetylene tetrabromide down toan acceptable economic limit, and economic calculationsshowed that the entire operation could be profitable, in-.cluding distillation towers for recovery. of alcohol, andoperation of the entire system in equipment with a con-trolled atmosphere to avoid loss of volatile solvent.

How, you may ask, does this affect fine particle se-paration? Actually, I don't know, but let me remind youthat heavy fine particles, down to 'bug dust size wi~t settlethrough heavy liquids under centrifugal force, and fine par-ticles of specific gravity lesa than the liquid will rise. tothe top, even in a centrifuge. Can this approach be made towo~k with commercial centrifuges on tonnage quantities? AgainI don't know, but I think so. Unfortunately it will CO$t alot of money to find out because pilot scale trials are re-quired to demonstrate the method.

I intend to work on this approach myself in the future,and I hope SOme of you will too.

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In conclusion, let me suggest that perhaps we have cometo the place where it is no longer effective for individualgraduate students or even individual industrial companies towork on problems of the magnitude we have been discussing.Perhaps it is time for cooperative research funded jointlyby gove~nment and industry, a proposition not without itsproblems, both legal and corporate. Establishment of suchan effort is not an easy thing to do, but it could be mostworth-while. Starting now is not too soon.

FLOOR DISCUSSION

The fine particles probl~s in mineral processing areof a very serious nature and call for immediate research workboth at fundamental and applied levels. There is a large num-ber of basioc areas that need study here. Identification of thelower size limits for various techniques and of the basic rea-sons for the existence of such limits is an example. Thus itis important to establish whether in the case of flotation,there is a lower size limit below which, for reasons that arenot known yet, the process fails. If there is such a limit,it would be of fundamental importance to establish whether itis set by reduced particle-bubble attachment or reduced levi-tation of particle-bubble assembly or some other fa~tor. Onefactor that is responsible for poor efficiency of separationof fines by flotation 'appears to be the poor reagent selecti-vity in that size range. If that is indeed a major reason, itis important to find out how the efficiency can be increasedby changes in chemical or physical environments in flotationsystems.

For the sedimentation and dewatering of fine suspensionsthe role of forces other than gravity deserves increased atten-tion since gravity does become ineffective in several of thesecases. It is also important to recognize that the problem offine particle sedimentation is a consequence, in addition tothat of its colloidal size, of its peculiar mineralogicalcharacteristics. Understanding of the role of these character-istics on such properties as settling can be helpful for sol-ving, particularly, problems of dewatering of slimes and sludges.An effective way to solve such dewatering problems ,is, indeed,to develop dry proc~sses. A careful examination of the possi-bility of using dry mineral beneficiation methods possibly incombination with pyro and electrometallurgical techniques iswarranted. A systematic attempt to understand the mechanismsof generation of fines during milling can possibly reduce theproduct'ion of fines. It will be helpful to note here that thisis often controlled by the mechanism of size reduction. Thuswhile single particle crushing can yield Gaudin-Schuhman

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distribution modulus of 1, ball milling yields a product withmodulus of, say, 0.7 and a classifier-grinding circuit a pro-duct with that of about 0.5. This essentially means that theamount of fines or slimes produced in the last two cases ismore than that in single-particle crushing. The problem thenis to determine mechanisms by which results obtained duringsingle particle crushing tests can be translated to multipleparticle milling systems and thereby produce less fines. Basicresearch on this aspect is important both from the point ofview of alleviating the fine particles proble~ as well asachieving reduced energy consumption during milling.

RESEARCH NEEDS

1. Elucidation of mechanisms of potential processes,i.e. selective flocculation, carrier flotation, oilflotation, etc.

2. State of ag9regation of wet and dry particle systems.

Energetics and mechanism of adherence of ultrafinesolids to solid surfaces in gas/solid systems (dustcoating) 1 energetic requirements for dry desliming1modification of host solid properties by the presenceof adherent finer particles.

3.

4. Study of fundamental reasons behind lower size limitsof applicability of separation and sizing devices; de-termination of whether these reasons are intrinsic andinviolable. .

5. Determination of effects and interactions in fine-bubbleflotation (electro flotation, liquid extraction, foamseparation, spherical agglomeration, etc.), and mechanismsinvolved.

6. Use of high gradient magnetic separation and heavy liquidseparation in centrifugal field.

7. Study of mechanisms of fracture in polycrystalline andpolymineral systems; also the effect of chemical addi-tives in fine grinding.

8. Development and adaptation of techniques for character-ization of fine surfaces.

9. Investigations on the mechanisms of slime dewatering.

Study of surface effects in filtration and clarification.

Study of dry mineral concentration techniques...

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