Potential controlled flotation and depression of copper sulphides and oxides using hydrosulphide in non-xanthate systems

203

Potential controlled flotation and depression of copper sulphides and oxides using hydrosulphide in non-xanthate systems D.R. Nagaraj and A. Gorken American Cyanamid Company, P.O. Box 60, Stamford, Connecticut 06904, U.S.A.

ABSTRACT The complete behavior of flotation and depression of copper sulfldes and oxides as a function of potential modified by sodium hydrosulfide in certain non-xanthate collector systems was studied using a Sulfide Ion Electrode (SIE). Detailed flotation tests on three diverse copper ores, using different types of collectors, and two Cu-Mo concentrates have revealed that the optimum potentials for copper flotation are in the range of -400 to -600 mV (vs. Ag/AgCl reference). Sulfide flotation was found to be insensitive to sulfldlzatlon up to approximately -600 mV, whereas flotation of sulfidized oxides increased gradually and reached an optimum in the range of -600 to -650 mV. The onset of copper depression for both ores and Cu-Mo concentrates occurred at potentials more negative than -600mV - the actual value being dictated largely by ore mineralogy - and was complete above -700 mV. These cut-off potentials are in agreement with those reported in the literature for pure copper sulfldes and for Cu-Mo systems. Thus the potentials used for Cu-Mo separations are merely part of the same curve of flotation vs. potential that can be generated for the copper ores. The results obtained in this study for different collector systems are generally consistent with those reported in the literature for xanthate system, and support the view that operating a circuit on the basis of SIE potential should eliminate the uncertainty of dosing NaHS, should ensure consistent metallurgy, and perhaps minimize the necessity of making changes in collector dosage, pH etc. in response to changes in feed composition or circuit metallurgy. KEYWORDS Flotation; Depression; Minerals Processing; Copper Sulfides; Copper Oxides; Sulfldlzatlon; Sulfide Ion Electrode; Potential control; Redox Potential; Thionocarbamate; Dithiophosphate; Dithlophosphinate INTRODUCTION The role of electrochemical processes and the associated redox potentials in sulfide flotation or depression is well documented In the literature (Woods and Richardson, 1986 and references therein). It is also recognized that for a given pH, each mineral can be characterized by a potential range in which optimum flotation occurs. Such potential ranges have been established for many sulfide minerals in xanthate collector system (Woods and Richardson, 1986). There Is not much literature available for other collector systems. The potentials in question can

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be modified either electrochemically or chemically. The latter is more practical, and sulfide ions (either as sodium sulfide or hydrosulfide), which are potential determining for sulfide minerals, are most commonly used for this purpose; for example, in the important processes of sulfidization-flotation of oxides and Cu-Mo separations. Careful monitoring and control of sulfide ion addition are required for the success of these processes in plant operations in terms of both metallurgical performance and economics. Jones and Woodcock (1978a, 1978b, 1986) used a sulfide ion electrode successfully to arrive at the optimum potential values for sulfidization-flotation of mixed sulfide-oxide copper ores using sodium sulfide and a xanthate collector. Detailed studies by these investigators have revealed the many advantages of the technique they called controlled potential sulfidization (CPS). Faster flotation of copper minerals and improved concentrate grades were obtained in roughers and cleaners. A potential of -500 mV (with respect to SCE) was found to be optimum for sulfidization and was the best balance between copper recovery, concentrate grade, and sodium sulfide consumption. Significantly better overall copper recoveries were obtained with the use of CPS than with conventional sulfidization techniques. There are numerous sulfide flotation plants that use collectors other than xanthate. Many of these plants treat mixed sulfide-oxide feeds and invariably the oxide recovery is rather poor. Potential controlled sulfidization would be an attractive solution in such plants to significantly improve copper recoveries. There is currently no detailed data available suggesting optimum conditions for sulfidization in non-xanthate systems. A similar situation exists for the Cu-Mo systems; if conditions for controlled depression are established, significant savings in depressant dosages may be possible. The major objective of the present study was to investigate the complete behavior of copper flotation and depression as a function of hydrosulfide modified potentials using a sulfide ion electrode (SIE) for different ores and collector types. The aim was not only to establish optimum conditions for copper flotation from ores and depression in Cu-Mo systems, but also to identify problems, if any, that may arise from collectors or in the use of SIE. The feasibility of using nitrogen as a flotation gas instead of air In the entire potential range was also determined. The use of nitrogen should minimize undesirable oxidation of sodium hydrosulfide by air (therefore reducing consumption) and deleterious effects, if any, on flotation arising from the oxidation products. EXPERIMENTAL Three copper ores from different parts of the world - Canada, Southeast Asia and India - were used in this study. Their characteristics are given in Table 1. Two Cu-Mo concentrates from South America were also included (Table 1). The general experimental details were similar to those outlined by Jones and Woodcock (1978b, 1986). Potentials (vs. Ag/AgCl reference) were measured using an Orion Ag„S electrode (Model 94-16) and controlled by the addition of sodium hydrosulfide solutions of concentrations 5, 10 or 35%, as required, using a syringe pump. Sodium hydrosulfide is sometimes preferred In the industry over sodium sulfide because of Its ease of handling, often lower cost and smaller effect on pH. The potential of the pulp was continuously adjusted to within ± 10 mV of the target value. The SIE was checked periodically for its performance by immersing in a fresh 10% sodium sulfide solution prepared by using reagent grade Na„S.9H90; the potential registered was approximately -875 mV. The electrode was also gently polished occasionally using a fine abrasive paper. Platinum electrode potentials were also measured using an Orion combination electrode for comparison

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TABLE 1. Details of the Ores And Concentrates Tested SAMPLE

MINERALOGY

.

GRIND

TOTAL Cu

OXIDE Cu

CANADA

0.550

0.190

Bo,Cp,Dg,Cc Cv Mai,Nat.Cu Cup,Azur (Cp,Bo & Py commonly rimmed by Dg & Cc); also has clays

40% -200 Mesh

INDIA

3.100

0.320

Cp,Dg,Cc,Cv Bo Mai,Cup Nat.Cu,Azur (Dg,Cc & Cv rimming commonly on Cp & Py; Cup capping on & commonly locked with sulfides)

45% -200 Mesh

SOUTH EAST ASIA

0.660

Cp; massive sulfide; mostly Po Cp extensively locked with Po

50% -200 Mesh

Concentrate A SOUTH AMERICA Concentrate B SOUTH AMERICA

Λ

MAJOR

MINOR

Cu 22.6

Mo 0.51

Cp

Dg,Cc,Cv

25.0

0.45

Cc,Dg,Cv,Bo

Cp

* In approximate order of decreasing abundance; + contained significant amounts of pyrite; // Fe 30%; S 18%; Abbreviations: Azur - azurite; Bo - bornite; Cc chalcocite; Cp - chalcopyrite; Cup - cuprite; Cv - covellite; Dg - digenite; Mai - malachite; Nat.Cu - native copper; Po - pyrrhotite; Py - pyrite. with the SIE values. The platinum electrode was calibrated against the standard ferrous/ferric couple. Ore flotation tests were conducted In a Denver D12 cell with 500 g. charges. Three stages of flotation were conducted at each potential value. In each stage, the pulp was sulfidized for 5 min., conditioned for 2 min. with a standard amount of collector and frother, and floated for 3 min. with nitrogen gas. The pulp pH was initially adjusted to 8.5 in all of the tests by using lime, but no subsequent adjustment of pH was made. pH values were In the range of 9.8-10.6, for example, corresponding to the potential range of -400 to -600 mV. The collectors used were selected on the basis of our previous experience and the mineralogy of the.ore in question (Nagaraj and co-workers, 1986a, 1989; Rickleton, 1981). Thus AERO 4037 promoter, a dithiophosphate and thlonocarbamate formulation, was used for the Canadian ore (25 g/t in each stage was found to be the suitable dosage). A 50:50 blend of AERO 3418A promoter (a dlthiophosphinate) and AERO 3894 promoter (a thlonocarbamate) was used for the Indian copper ore (two dosage levels - 5g/t and 25g/t In each stage). Ethoxycarbonyl alkyl thlourea was used (lOg/t in each stage) for the Southeast Asian ore. Flotation tests In the Cu-Mo systems were conducted using 250 or 500 g. concentrates. Tests were carried out on both fresh (tests on-slte) and aged samples (In the laboratory). The procedure for the latter was similar to that used previously (Nagaraj co-workers, 1986b) except that three stages of 3 min. conditioning with Trademark of American Cyanamid Company.

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NaHS at a constant SIE potential and 2 min. flotation in each stage were used. Only the results for the aged samples are presented in this paper. Thionocarbamate was the major collector that had been used to produce these two concentrates in the plant, though a small amount of sodium isopropyl xanthate had also been used for concentrate B. Slime coating on the SIE was a problem with the Canadian ore, which necessitated frequent cleaning of the electrode surface during tests. Fine molybdenite coating was also a problem, though much less serious than clays, in the Cu-Mo tests. RESULTS Copper Ore Flotation Mixed sulfide-oxide ores. Copper recoveries (total of three stages) for sulfide, oxide and total Cu as a function of SIE potential for the Canadian ore are shown in Fig. 1. Total copper recovery was found to be optimum in the potential range of -400 to -600 mV; it decreased sharply at potentials above (i.e. more negative than) -600 mV, and slightly at potentials below (i.e. more positive than) -400 mV. Sulfide copper recovery was unaffected in the potential range of -200 to -600 mV, and it decreased sharply above -600 mV · Oxide copper recovery, on the other hand, was optimum in the potential range of -400 to -650 mV. The results used for Fig. 1 have been replotted in Fig. 2, 3 and 4 to reflect recoveries of sulfide, oxide and total copper, respectively, obtained at the end of each stage of flotation. Sulfide flotation was very rapid and about 90% occurred in the first stage (Fig. 2). The recoveries in each stage were almost constant in the potential range of -200 to -600 mV and decreased sharply above -600 mV. The stagewise oxide recovery (Fig. 3) showed considerable dependence on potential. The first stage recoveries gave a clear indication of the rate of flotation of sulfidized oxides as a function of SIE potential; the maximum rate was observed at potentials close to -600 mV. The recoveries at other potentials increased in subsequent stages and the cumulative recoveries at the end of the third stage were almost constant in the range of -400 to -650 mV. There was some indication that oxide recovery may have reached a plateau in the range of -400 to -500 mV. The stagewise, total copper recoveries (Fig. 4) reflected essentially the contributions from the stagewise oxide copper recoveries. Concentrate copper grades obtained at the end of each stage are plotted in Fig. 5 as a function of SIE potential for this Canadian ore. Although there were significant differences between grades of the first stage concentrates (best grades, 27-33 % Cu, obtained at -400 and -500 mV), there were essentially no differences in cumulative grades between concentrates obtained at the end of second and third stage as a function of potential. This indicates that sulfidization had virtually no effect on flotation of pyrite which would have been the main diluent in the concentrate. The copper grade decreased sharply above -600 mV as a result of depression of copper minerals. The sulfide and oxide recoveries obtained as a function of SIE potential at two dosage levels of collector for the Indian ore are presented in Fig. 6. The sulfide recoveries, as in the case of the Canadian ore, were essentially constant as a function of potential, but extended up to -650 mV, decreased sharply above -650 mV, and were not affected by the increased collector dosage. The oxide recovery, however, increased gradually in this potential range, reached optimum values in the range of -500 to -650 mV, followed by a sharp decrease above -650 mV, and was higher at the higher collector dosage. The oxide recovery appeared to reach a plateau in the potential range of -300 to -400 at both collector dosages.

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Overall concentrate grades (results not shown) were The stagewlse oxide recoveries (shown in Fig. 7 for indicated that the rate of flotation increased with recovery was more pronounced in the early stages of

almost constant up to -650 mV. the higher collector dosage) potential. The plateau in flotation.

Massive sulfide ore. The effect of SIE potential on the stagewlse grade vs. recovery behavior for the Southeast Asian massive sulfide ore Is shown in Fig. 8. The natural SIE potential for this ore was -250 mV. Sulfidization to a potential of -500 mV had almost no effect on the grade-recovery behavior, but further additions of NaHS to potentials more negative than -600 mV decreased copper

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flotation sharply. Although pyrrhotite was the major component of this massive sulflde ore, its flotation was not significantly affected by sulfIdlzatlon since the concentrate grades remained almost constant (w> 8% Cu) up to -600 mV. SIE vs. Pt Electrode Potentials The Pt electrode potentials measured during sulfIdlzatlon are plotted against the simultaneously measured SIE potentials in Fig. 9 (values are plotted for Indian

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Fig. 12. Copper recovery as a function of SIE potential for two SOUTH AMERICAN Cu-Mo concentrates (A & B ) .

ore only; similar values were observed with other ores). There was considerable scatter in the Pt electrode potentials at any given SIE potential and this scatter increased as the SIE potential increased; maximum scatter being observed in the range of -500 to -650 mV (SIE). This observation differs from that of Jones and Woodcock (1978b,1986) who stated that the Pt electrode would be less sensitive at low sulfide concentrations. Even when stable readings were observed with the SIE, the Pt potential would fluctuate at high SIE, but frequently would gradually decay or drift to more positive values.

I

PROCESSING OF COMPLEX ORES

210 SIE v s . pH

The unadjusted pH values measured during sulfidization are plotted as a function of SIE potential for the Indian ore in Fig. 10. Also plotted in Fig. 10 are the SIE potentials measured in the pulp when pH was continuously adjusted using lime in the absence of NaHS. In the optimum flotation region for copper (approximately -400 to -600 mV) the pH values were in the range of 9.8 to 10.6. There was also considerable scatter in the pH values at any given SIE value. In the absence of NaHS, the SIE behaved similarly to any 'inert1 electrode and gave a straight line dependence on pH (Natarajan and Iwasaki, 1972). SIE Potential vs. NaHS Consumption in kg/t The total NaHS consumption in kg/t is plotted as a function of SIE potentials for the Canadian and Indian ores in Fig. 11. The total NaHS consumption (for three stages of flotation) up to the onset of depression was about 0.85 kg/t for the Canadian ore and about 3 kg/t for the Indian ore, and 11.3 and 18.6 kg/t, respectively, to reach a potential of -750 mV. At all SIE potentials, the Indian ore consumed more NaHS because it had higher amounts of malachite, cuprite and tarnished sulfides. Collectorless Flotation Copper recoveries obtained at -500 mV with sulfidization in the absence of any collector for all the three ores are compared in Table 2 with those obtained with collector. Collectorless flotation was observed for all the three ores, but the total copper recoveries were well below those obtained with collector. Even with the Southeast Asian ore for which collectorless flotation was maximum (71%), 30 g/t of the strong collector was required to obtain additional 13 units of copper. TABLE 2. Flotation With and Without Collector at -500 mV ORE

COPPER RECOVERY, % WITHOUT COLLECTOR SULFIDE OXIDE TOTAL

CANADIAN INDIAN S.E.ASIAN

72.3 49.9 71.2

46.0 42.7

63.2 49.2 71.2

SULFIDE 99.0 98.3 84.1

WITH COLLECTOR OXIDE TOTAL DOSAGE,g/t 67.2 85.8

88.1 96.9 84.1

75 75 15

Cu-Mo Separations Copper flotation as a function of SIE potential for the two South American Cu-Mo concentrates is presented in Fig. 12. The onset of copper depression occurred at about -600 mV for Concentrate A which had predominantly chalcopyrite, and at -650 mV for Concentrate B which had predominantly chalcocite. These values for the onset of depression are in excellent agreement with those observed for the copper ore flotation (both Canadian and Indian; see depression regime in Fig. 1 & 6). Complete depression of copper minerals occurred at -700 mV for both concentrates. This depression, however, was short-lived and could not be sustained for, say, more than 4 minutes. At -725 mV, depression could be sustained for up to 10 minutes, and longer at -750 mV, suggesting that at these potentials, copper depression should continue well into the Cu-Mo cleaners. This potential vs. copper depression behavior has been confirmed for other Cu-Mo systems from different parts of the world and on fresh Cu-Mo concentrates (results not shown here).

PROCESSING OF COMPLEX ORES

211

DISCUSSION The general behavior observed in this study for copper flotation vs. SIE potential modified by NaHS is approximately the same for all of the three ores despite the different mineralogies involved and the different families of collectors used. This behavior is also consistent with that reported in the literature for the flotation of copper sulfides using xanthate (Woods and Richardson, 1986 and references therein), though slight differences exist as might be expected, between collectors, in the actual magnitude of the potential ranges for optimum flotation. Such a similarity in behavior between xanthates and the other collectors suggests that there is a common factor in terms of adsorption mechanism, though the strength of adsorption may be different. The results obtained in this study, while consistent with those of Jones and Woodcock (1978b, 1986) for the xanthate system, also demonstrate the validity and usefulness of controlled sulfidization in the non-xanthate systems studied. The very rapid and almost constant flotation of sulfides up to -600 mV with sulfldization is noteworthy. This has two implications: first, that sulfidization can be practiced safely in a wide potential range and second, that a "sulfide prefloat" is not necessary during sulfidization flotation of mixed sulfide-oxide ores (which was also pointed out by Jones and Woodcock, 1986). One might suspect collectorless flotation, especially after sulfidization (Yoon, 1981; Shannon and Trahar, 1986), as the major reason for this sulfide flotation behavior In two different collector systems. The results in Table 2 indicate, however, that collectorless flotation (which Includes effects of sulfidization, frother and particle size) could account for only 50-80% of sulfide copper (same for total) floated for the three ores. The role of the collector is, therefore, still very significant. The SIE potential for the onset of sulfide copper depression in the Canadian ore, which had predominantly bornite, chalcopyrite, digenite and chalcoclte (In that order) Is consistent with the SIE potential observed for the Cu-Mo concentrate A (Fig. 12) which had predominantly chalcopyrite. Kritskii and Mashevskii (1970) used -650 to -660 mV for chalcopyrite depression. The more negative potential for the onset of depression observed for the Indian ore which had abundant chalcopyrite may be attributed to extensive rimming of chalcopyrite by the other copper minerals whose depression would occur at more negative potentials, such as In the case of Concentrate B which had chalcoclte as the predominant mineral. Surface composition (not that of bulk) is the one that is relevant as far as flotation Is concerned. The difference in cut-off potentials between Canadian and Indian ores could also be the result of using different collectors since it Is known that some collectors, such as dlthlophosphinates, adsorb more strongly than others. The contribution from collector, however, does not appear to be very significant in view of a similar difference in cut-off potentials between Concentrates A and B which were produced using the same collector, assuming that a small amount of xanthate used for B would not shift the potential to more negative values in view of lower cut-off potentials observed by Jones and Woodcock (1978b) for xanthates. Richardson and Walker (1985) reported the following order for onset of depression in xanthate system: chalcoclte (-300 mV vs. Pt and SCE), bornite (-175 mV), chalcopyrite (-20 mV), and pyrite (+100 mV). It is interesting that the order for onset of depression observed in this study is consistent with the above, though the actual potential values (after converting from Pt to SIE based on Fig. 9) are quite different, which should be expected between different collectors. The onset of depression of sulfldized oxide copper minerals should be closer to that of chalcoclte (or digenite) than that of chalcopyrite (that is, shifted to more negative potentials) since after sulfIdization the oxides should present surfaces similar to chalcoclte or digenite. This is Indeed the case (compare data for Canadian ore, Indian ore and Cu-Mo concentrates).

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PROCESSING OF COMPLEX ORES

The presence of a plateau in the oxide recovery vs. SIE potential for the Indian ore (and possibly for the Canadian ore) may be attributed to contributions from tarnished sulfides, locked sulfide-oxide particles and/or from differences in sulfidization behavior of the different oxide minerals present in the ore. A similar plateau appears to exist even in the results of Jones and Woodcock (Fig. 7 in 1978b) though this was not mentioned. It is not known how much significance can be attributed to these plateau. Unlike the Canadian ore which contained predominantly malachite and native copper (in that order), the Indian ore contained cuprite as a major mineral along with malachite (Table 1). In view of the fact that cuprite is a heavier (S.Gr. 6) and richer (88.8% Cu) mineral than malachite (S.Gr. 4 and 57.4% Cu), it can be speculated that oxide recovery at lower potentials (-300 and -400 mV) may correspond to malachite flotation and at higher potentials it may correspond to increasing contribution from cuprite flotation. The best overall oxide copper recovery from the Canadian ore 0*70%) was significantly less than that from the Indian ore 0*86%), although mineral liberation was similar for both ores. Contribution to this lower recovery from Canadian ore may come from poorer flotation of native copper or the presence of clays. Mineralogical analysis of the tails should confirm this. Limitations of the SIE. In order to test the scope of SIE, its response to three representative non-hydrosulfide depressants, viz. thioglycolate, raercaptoethanol and a proprietary product (Nagaraj, and co-workers, 1986b, 1986c) was compared with that for NaHS in Cu-Mo pulps. The results indicated that the SIE responded to only sodium hydrosulfide, and the change in SIE potential even with high dosages of the non-hydrosulfide depressants (which gave good copper depression) was not large enough (often merely 50 mV) to warrant its use over a Pt electrode. This may be viewed as a shortcoming of the SIE in systems that do not contain sulfide ions. SIE was also found to be insensitive to aeration of pulps when there was no sulfide ions present, and to the addltLon of other depressants (or modifiers) such as sodium metabisulfite or SO«. Choice of Collector type. Although the potential ranges for optimum flotation were approximately the same for different collectors used, the choice of a single collector or a collector blend will still depend on the mineralogy of each orebody in question and on on other factors that are ore and plant specific. The results may be qualitatively similar with different classes of thiol collectors, but the actual magnitude of the recoveries, dosages used, potential ranges etc. may still depend on the collector(s) used; for example, of the many collectors tested, Ser and co-workers (1970) found that AERO 3302 promoter was one of the better collectors for sulfidized oxides. Role of pH. The role of pH in arriving at the optimum potential ranges could not be studied since the pH value of the pulp was mostly dictated by the NaHS dosage and the amount of oxide minerals that would consume NaHS, and to some extent the amount of gangue minerals that would consume OH ions. It is conceivable that pH, as an independent variable, may influence the actual magnitude of copper recoveries and, therefore, affect the width of the optimum potential range. If, however, an optimum dosage of collector is used, such an influence may be minimal. Indeed Ser et al. (1970) found no major influence of pH during conventional sulfidization in the narrow range of Interest here. PRACTICAL ASPECTS AND CONCLUDING REMARKS Flotation and depression of copper sulfides and oxides from mixed sulfide-oxide ores, massive sulfide ore and from Cu-Mo concentrates in certain non-xanthate systems were studied as a function of potential modified by NaHS using a sulfide ion electrode (SIE). The SIE potential values for optimum copper flotation were

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In the range of -400 to -600 mV vs. Ag/AgCl reference. This potential range is not very different from that reported in the literature for a xanthate system, which suggests that the factors that determine the choice of a collector should not be influenced by the proposal to incorporate controlled sulfidlzatlon in a circuit. The onset of depression of sulfldes and sulfldized oxides occurred in the range of -600 to -650 mV (for both ores and Cu-Mo concentrates) and the actual value was largely dependent on the copper mineralogy. Copper depression was complete at -700 mV, but its sustenance increased as the potential increased to -750 mV. The results obtained in this study suggest that operating a circuit in the optimum potential range should ensure consistent metallurgy irrespective of the nature of the oxide and sulfide minerals, should eliminate the uncertainty in dosing NaHS, and perhaps minimize the necessity of making changes In collector dosage, pH etc. solely on the basis of on-stream analyzers. The results have also established that nitrogen could be used as the flotation gas (it is already used in many Cu-Mo plants). This should result in reduced NaHS consumption. The SIE was found to be a convenient tool even in the non-xanthate systems studied, as no serious ill-effects attributable to the collectors were observed. ACKNOWLEDGMENTS The authors wish to acknowledge the help provided by their colleagues in the Research and Technical Service Groups; special thanks to Ms.J. Coe, Mr. B. Dennis and Mr. P. Riccio. The support of the Management of American Cyanamid Company and the permission to publish this work are also acknowledged. REFERENCES Jones, M. H., and, J.T. Woodcock (1978a). Trans. Instn. Min. Metall. (Sect. C ) , 7£, C99-C106. Jones, M.H., and J.T. Woodcock (1978b). Proc. Australas. Inst. Min. Metall., No. 266, 11-19. Jones, M.H., K.Y. Wong, and J.T. Woodcock (1986). In L.E. Fielding and A.R. Gordon (Ed.), Proc. 13th CMMI Congress, Metallurgy, CMMI and Australas. Inst. Min. Met., Australia, pp. 33-40. Kritskii, E.L., and G.N. Mashevskii (1970). Obogashch. Rud., j_5, No.1-2, 287-292. Nagaraj, D.R., and co-workers (1986a). In L.E. Fielding and A.R. Gordon (Ed.), Proc. 13th CMMI Congress, Metallurgy, CMMI and Australas. Inst. Min. Met., Australia, pp. 49-57. Nagaraj, D.R., and co-workers (1986b). Trans. Instn. Min. Met., (Sect. C ) , 95, C17-C26. Nagaraj, D.R., and co-workers (1986c). Discussion. Trans. Instn. Mln. Met., (Sect. £ h 95^, C176-C177. Nagaraj, D.R., C. Basilio, and R.H. Yoon (1989), Soc. Min. Engrs. Annual Meeting, Las Vegas, Nevada, Preprint No. 89-306. Natarajan, K.A., and I. Iwasaki (1972). Trans. Soc. Min. Engrs., 252, 317-324. Richardson, P.E., and G.W. Walker (1985). Proc. XV Int. Mineral Process. Congr., Cannes, France, vol. II, pp. 198-210. Rickleton, W. A. (1981). Canadian Patent 1,105,156. Ser, F., and co-workers (1970). Rudy, 18, No. 5, 167-174. Shannon, L.K., and W.J. Trahar (1986). In P. Somasundaran (Ed.), Advances in Mineral Processing, Society of Mining Engineers, Colorado, Ch. 24, pp. 408-425. Woods, R, and P.E. Richardson (1986). In P. Somasundaran (Ed.), Advances in Mineral Processing, Society of Mining Engineers, Colorado, Ch. 9, pp. 154-170. Yoon, R.H. (1981). Int. J. Miner. Process., £, 31-48.