International Journal of Mineral Processing, 13 (1984) 105--115 105 Elsevier Science Publishers B,V., Amsterdam -- Printed in The Netherlands

THE F L O T A T I O N OF C H R Y S O C O L L A BY M E R C A P T A N

F.F. APLAN 1 and D.W. FUERSTENAU ~

1 Mineral Processing Section, The Pennsylvania State University, University Park, PA 16802 (U.S.A.) Department of Materials Science and Mineral Engineering, University of California,

Berkeley, CA 94720 (U.S.A.)

(Received January 5, 1983; revised and accepted October 10, 1983)

ABSTRACT

Aplan, F.F. and Fuerstenau, D.W., 1984. The flotation of chrysocolla by mercaptan. Int. J. Miner. Process., 13: 105--115.

Laboratory experiments have demonstrated that chrysocolla and malachite can be floated with a mercaptan as collector. In contrast, even when used in large quantities, the higher xanthate homologs (hexyl, dodecyl) will float malachite but not chrysocolla. The flotation of chrysocolla with mercaptan is readily accomplished in a pristine system, but in the presence of finely ground gangue particles, additions of the mercaptan to the grinding mill gave superior recoveries to those achieved when the mercaptan is added to the flotation cell. A model for the attachment of the mercaptan to the chrysocolla surface is proposed which involves the reaction of molecular mercaptan with the copper sites. This results in the formation of copper mercaptide at the surface and the splitting off of a molecule of water. By extension, this model should describe the reaction of mercaptan with any base metal oxide or sulfide mineral where the metal mercaptide is relatively insoluble.

INTRODUCTION

Six general m e thods have been repor ted for the f lo ta t ion of oxidized minerals o f the base metals:

(1) Sulf idizat ion fo l lowed by f lo ta t ion with a su lphydry l col lector such as xan tha te (Gaudin, 1932, 1957). Mercap tobenzo th iazo le is par t icular ly eff icacious here (Jaekel, 1959).

(2) Soap f lo ta t ion (Gaudin, 1932, 1957). (3) Leach-prec ip i ta t ion-f lo ta t ion (LPF) (Milliken and Goodwin , 1943) . (4) Act ivat ion with a heavy-metal ion fol lowed by f lo ta t ion with an anionic

col lec tor such as su lphydryl , soap, or sul fonate (Gaudin and Anderson , 1930; Fuers tenau and Palmer, 1976).

(5) Chelating agents and dyes (Gutzei t , 1946; L u d t and DeWitt, 1949; Fuers tenau and Palmer, 1976).

(6) Selected su lphydry l collectors, especially mercap tobenzo th i azo l e and

106

the mercaptans (Gaudin, 1932, 1957; Gaudin and Anderson, 1930; Gaudin and Martin, 1930; Gaudin, 1938). The first three methods have long been used commercially, especially for ores rather rich in base metal oxides. Procedure 4 has been studied exten- sively while Method 5 remains a laboratory curiosity. The direct use of su lphyd~ l collectors (Method 6) has met with indifferent success at best. This paper will demonstrate that the use of mercaptans can be applied specifically to the flotation of the copper silicate chrysocolla which appears to be generally less floatable than the copper carbonates.

The need to develop reagents to float, especially small quantities of oxidized base metal minerals, is great. White and Rule (1971) sampled the tailings sites of twelve western U.S. copper concentrators and found them to contain an average of 0.068% oxide copper (range 0.011--0.239). Thus, these tailings contain, on average, about 0.7 kg of copper per tonne existing in oxide forms. Recovery of only a port ion of this copper, which has already borne the expensive mining and grinding costs, could have a major impact on the economic viability of these properties.

The recently revived interest in the use of mercaptans as sulfide collectors (Anonymous, 1976; Wiechers, 1978; Shaw, 1981) has prompted a re-examina- tion of experiments performed by the authors some 25 years ago in Professor Gaudin's laboratory at MIT. These data, plus an evaluation of the older litera- ture, indicate that a major advantage of the mercaptan may lie not so much in its use as a sulfide mineral collector but as a collector for oxidized base-metal minerals.

The use of mercaptan as collector for sulfide minerals has long been known, and both Gaudin (1932) and Wark (1938) in the first editions of their flotation texts have discussed their use as collectors. A commercial collector, Barnac, reported to be a mercaptan of relatively high chain length, was used in the 1930's (Cornell, 1937). Gaudin and his associates (1928, 1930, 1932, 1938) were perhaps the first to make an extensive study of mercaptan as a collector for such minerals as chalcocite, azurite, malachite, and smithsonite. They preferred a collector containing approximately six carbon atoms (benzyl, amyl, heptyl). Gaty et al. (1946) showed that mercaptan could be used as a collector for calamine, and Gaudin and Harris (1954) demonstrated by adsorption experiments that it could be an effective collector for sphalerite, willemite and zincite.

For more than a decade, one of the authors (Aplan, 1983) has used Tergitol 12-M-10 (an ethylene oxide adduct of "dodey l" or, more properly, t r ibutyl mercaptan) as a collector for malachite in a laboratory flotation experiment. Podobnik and Harris (1978) proposed the use of a polyalkene oxide adduct of a mercaptan as a froth enhancer and observed somewhat higher copper recoveries through its use. Quite likely, this material performs some collection fucntion as well.

Aplan and de Bruyn (1963) demonstrated that hexyl mercaptan is a powerful collector for gold with nearly complete recovery at ~ 10 -s mol 1-1

107

and significant recovery in solutions containing ~ 10 -6 mol 1-'. It was found to be strongly adsorbed onto gold from either aqueous solution or from the gas phase. Mercaptans containing roughly six carbon atoms combine the advantage of a relatively long hydrocarbon chain with reasonable solu- bility in water [hexyl mercaptan solubility in water is 3.2 × 10 -4 mol 1-1 (Yarboff, 1940)]. The use of a longer hydrocarbon chain should result in a stronger collector based on the principles previously elucidated by the authors (Aplan and Fuerstenau, 1962), but there is undoubtedly some trade-off between water solubility and collector strength as the hydrocarbon chain is increased. Various devices can be used to mitigate the insolubility problem of the higher homologs, such as introducing the collector into the grinding circuit, incorporating it within a carrier liquid, and adding it as a dispersion or emulsion. Use of these techniques has long been established practice for the slightly soluble collector, thiocarbanilid (Taggart, 1945) and, more recently, they have apparently been used to introduce the sparingly soluble dodecyl mercaptan into the flotation circuit (Anonymous, 1976; Wiechers, 1978; Shaw, 1981).

Eyring and Wadsworth (1956) showed conclusively that during mercaptan adsorption onto the zinc minerals sphalerite and willemite, the S-H bond in the mercaptan is destroyed upon adsorption and a zinc mercaptan salt is formed at the mineral surface. These experiments therefore indicate that the mercaptan has a specific affinity for the metal ion in the mineral. Since mercaptan is known to form very insoluble mercaptides with heavy metal ions in solution, it seems reasonable that it should serve as a collector for mineral containing heavy metal cations, even the recalcitrant silicates. If this hypothesis is correct, the hydrated copper silicate, chrysocolla, should respond to flotation using a mercaptan as collector. The objective of the present investigation is to evaluate the floatability of chrysocolla with hexyl mercaptan as the collector.

MATERIALS AND METHODS

The chrysocolla used in these experiments was from Miami, Arizona, and was obtained from Ward's Natural Science Establishment, Rochester, New York. The material was hand-picked, crushed in a mortar and pestle, and the 48 × 65-mesh fraction selected for experimentation. The mineral was cleaned by rinsing repeatedly in distilled water and was found to be non-floatable in the absence of collector.

Berry and Mason (1959) suggested that chrysocolla contains theoretically 45.2% CuO, 34.3% SiO2 and 20.5% H20. Chrysocolla is known to be highly variable in composition, and the present sample is no exception. Analysis of the chrysocolla used in the present experiments indicated that it contained 27.4% CuO (21.9% Cu), 59.3%, Insol, and 3.3% CO2. Assuming that all of the CO2 oc- curs in malachite, CuCO3 • Cu(OH)2, the mineral sample would then contain

17% malachite. Macro- and microscopic observation of the chrysocolla pieces

108

showed that the color varied from fairly dark blue-green to a very pale bluish green (almost colorless), indicating a wide variation in copper content from piece to piece. For example, the float and non-float fractions from several of the experiments assayed about 25 and 7% copper, respectively, although chrysocolla was supposedly the only mineral present.

The hexyl mercaptan collector, Eastman-grade n-hexanethiol, was further purified by double distillation and stored under nitrogen to insure that no impurities resulting from its oxidation would be formed. Its refractive index agreed well with that reported in the literature (Yarboff, 1940). The hexyl and dodecyl xanthates were prepared by the method of Foster (1928).

The flotation cell and technique used in this work were essentially those described by Fuerstenau et al. (1957). The Hallimond tube was slightly modified for the present investigation by the insertion of a 1-cm glass frit for bubble introduction in place of a single capillary. Sized chrysocolla was conditioned at the desired mercaptan concentration by agitating 5 g of the mineral for 15 min in 100 ml of solution containing the mercaptan or xanthate collector in distilled water. The contents of the flask were then transferred to the Hallimond tube and floated for 40 s at a flow rate of 35 cm 3 of nitrogen per minute.

A few additional experiments were carried out in a 500-g Fagergren flotation laboratory machine in order to delineate the flotation characteristics on a somewhat larger scale.

EXPERIMENTAL RESULTS AND DISCUSSION

Until the late sixties, chrysocolla was considered to be a mixture of a crystalline copper silicate phase dispersed in an amorphous silica hydrogel, with a composit ion that can be expressed in terms of CuO, SiO2 and H20. Recent articles by van Oosterwyck-Gastuche (1970) and van Oosterwyck- Gastuche and Gregoire (1970) reveal that chrysocolla is a definite mineral having an or thorhombic unit cell, and a characteristic fibrous structure. Van Oosterwyck-Gastuche has proposed a combination sheet and chain silicate structure of nominal composit ion Cu4 (Si40,0) (OH)6 • n H20 where n = 0, 2 or 4. This structure is shown in Fig. 1. A problem in chrysocolla flotation is the leaching of surface copper from the mineral before the flotation collector can be adsorbed.

To study the effect of hexyl mercaptan concentration on the flotation of chrysocolla, the essentially pure mineral was first floated in the modified HaUimond tube. In Fig. 2, the weight recovery of mineral is presented as a function of the initial concentration of hexyl mercaptan in solution. With the more concentrated solutions, the bulk of the chrysocolla floated within 5 s although 40 s were used as the standard flotation time. Increasing the flotation time beyond 40 s would have increased recovery only slightly. Note from Fig. 2 that chrysocoUa floats easily at hexyl mercaptan concen- trations above about 10 -4 mol 1-1. Since color variations indicated that the

109

copper content of the chrysocolla varied, each of the products was analyzed for copper and the metallurgical recovery of copper calculated. These data are plotted in Fig. 3 where it may be seen that copper recoveries exceeding 92% were obtained.

In the two best tests, approximately 79% of the weight was recovered at 25% Cu, 53% Insol, and 4.1% CO2, giving a copper recovery of 92.5%. The tailings assayed about 7.6% Cu and 83% Insol. The presence of 4.1% CO2 in the concentrate, equivalent to all of the CO2 in the head sample, indicates that the concentrate contains ~ 21% malachite, and this accounts for essentially all of the malachite calculated to be present in the ore. Thus, it appears that the mercaptan is an excellent collector for malachite as well as for chrysocolla.

To compare the flotation of chrysocolla with mercaptan and xanthate as collectors, experiments were carried out using the potassium salts of

© 0 E

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(o)

OCTAHEORAL FIBER

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Fig. 1. Structure of chrysocolla after Van Oosterwyck-Oastuehe and Gregoire (1970).

110

tO0 I J t ' I i

CHRYSOCOLLA

I 0 RsSM

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hexyl and dodecyl xanthate and procedures similar to those with mercaptan. However, when dodecyl xanthate was employed as collector, a 60-s flotation time was used instead of 40 s used for the hexyl compounds. As indicated by Fig. 2, even at very high collector concentrations of hexyl xanthate (up to 4.6 × 10 -~ mol 1-1), only slightly over 20% of the total weight of the sample floated, which approximates the amount of malachite in the head sample. Examination of the floated material, under conditions where maxi- mum recovery with xanthate was achieved (above ~ 10 -4 mol 1-1), showed the product to be green colored, whereas the tailings exhibited predomi- nantly the bluish-green shades typical of chrysocolla. The green color was then observed to develop on some of the particles during conditioning, typical of the xanthate-malachite reaction product. These green particles effervesced vigorously in acid, demonstrating that they were indeed mala- chite. Use of dodecyl xanthate to float the material gave a maximum weight recovery of 36% at a collector concentration of 10 -3 mol 1-1. Apparently this very high dosage of the higher xanthate either floats a small amount of the better quality chrysocolla and/or some locked malachite-chrysocolla particles. At a collector concentration of 10:4 mol 1-1 of dodecyl xanthate, where a 22% weight recovery was achieved, most of the material floated in the first 30 s, substantially slower than that achieved with hexyl mercaptan at a similar concentration level. As with mercaptan, the incipient concen- tration for flotation of the floatable material occurs at ~ 10 -4 mol 1-1 with

111

either of the two xanthates. Use of the xanthate was seen to turn the solu- tions yellow during the conditioning step -- faintly yellow in 10 -4 hexyl or 10 -~ mol 1-1 dodecyl xanthate solutions but strongly yellow, almost a precipitate, in the 10 -a tool 1 -~ dodecyl xanthate solution. The ionic xanthate in reacting with solubilized copper in solution thus reacts differently than does the mercaptan.

As with the material floated with hexyl mercaptan, assays were made on the products of the hexyl xanthate flotation and the recoveries calculated. These results are presented graphically in Fig. 3. At concentrations above 10 -4 mol 1-1 slightly more than 20% of the weight was floated, giving copper recoveries of about 33%. Analysis of the concentrates showed them to contain about 35% Cu, 39.0% Insol, and 11.6% CO2, whereas the tails from the latter tests averaged about 18% Cu, 65.9% Insol, and 1.3% CO2. The 11.6% CO2 is equivalent to ~ 58% malachite and would represent most of the copper recovery and much of the malachite present in the head sample. Thus, hexyl xanthate floats mostly malachite and but little chrysocolla.

IOO ! I I r ! i

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80 / I 0 RsSH / I I Z~ R s X (b

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>- 60 / & R6SH --~

c~ L.d > © U 4C Ld

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Fig. 3. Copper recovery from the flotation of ehrysoeolla with hexyl mereaptan and potassium hexyl xanthate.

To check the flotation of chrysocolla at a larger scale, a few tests were carried out with a Fagergren laboratory flotation machine having a cell volume of 2.2 1. A 5-min flotation time was used in each step. The results of these tests are summarized in Table I. In Test A, chrysocolla was floated in the absence of gangue. No flotation occurred with the addit ion!of 7 mg 1-1 (6 × 10 -s mol 1-1 ) of mercaptan but after a total of 25 mg1-1 (2 .1×

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10 -4 mol 1 -~) had been added, the chrysocolla floated well. These results agree well with those of Fig. 2. Tests B and C included the complication of the finely ground particles produced during grinding being present, namely flotation was carried out without prior desliming. The use of a total of 36 mg 1-' (3 X 10 -4 mol 1-1) of mercaptan, added stagewise, resulted in the flotation of about 75% of the chrysocolla, much of it after the first addition of the mercaptan. In test C, high solids conditioning in the pebble mill was used, and the flotation results improved markedly. Over 90% of the chrysocolla was recovered with an addition of 18 mg 1 -~ (1.5 X 10 -4 mol 1 -~) of mercaptan, and the use of an additional amount of mercaptan gave essentially 100% chrysocolla flotation.

These results indicate that the flotation of chrysocolla must result from the chemical reaction of the mercaptan at the surface to form a stable complex with copper. By analogy with the results of Eyring and Wadsworth (1956) for the interaction of mercaptan with zinc minerals, molecular mercaptan (HSR) probably reacts with hydroxylated surface copper ions as follows:

-Si-O-Cu-OH(surf) + HSR(aq) -* -Si-O-Cu-SR(surf) + H20

The mechanism for the adsorption of mercaptan onto malachite could be similar to that of chrysocolla and the process could be extended to the at tachment of mercaptan to any base-metal oxide or sulfide mineral. Collec- tion apparently does not result from ionic adsorption as would be expected with the xanthate. The formation of a well-anchored, stable surface com- pound occurs with mercaptan but not with xanthate; even when the long- chained dodecyl xanthate is used.

CONCLUSIONS

The effective flotation of oxidized base-metal minerals, especially the silicates, has long eluded flotation practitioners. The presence of these minerals in an orebody, most particularly when present in only small quanti- ties, has traditionally been ignored or they are recovered by tedious, compli- cated, rather ineffective and/or relatively expensive procedures. The experi- ments reported here indicate that chrysocolla and malachite can be floated by use of mercaptan. By inference from these experiments and from the literature, mercaptans should be effective collectors for the flotation of most base metal oxide minerals.

The results indicate that both chrysocolla and malachite can be floated with mercaptan whereas even very large quantities of the higher (hexyl, dodecyl) xanthate homologs will not float chrysocolla. The xanthates do, however, float malachite at rather high concentrations. In the presence of finely ground gangue particles, the flotation of chrysocolla with mercaptan is not as effective as in a pristine system. The use of high solids conditioning (that is, addition of the collector to the grinding mill) obviates this problem.

114

The effectiveness of this procedure may be due to concentration effects or to improved particle-collector contact resulting from better dispersion of the collector or the lesser dilution of the pulp.

A model for the action of the mercaptan in chrysocolla flotation is proposed wherein molecular mercaptan reacts with the hydroxylated copper in the chrysocolla lattice forming a copper mercaptide to perform the collecting function, splitting off a molecule of water. The authors believe that this model can be extended to describing the at tachment of mercaptan to any base metal oxide or sulfide mineral where the metal mercaptide is relatively insoluble.

ACKNOWLEDGEMENTS

The authors wish to thank Mr. G.D. Seele for assistance with some of the Hallimond tube experiments and Mr. H.J. Modi for running the experiments using dodecyl xanthate. Both were graduate assistants in Mineral Engineering, MIT at the time of these experiments.

REFERENCES

Anonymous, 1976. Chem. Eng., 83(13): 68--69. Aplan, F.F. , 1983. Laboratory experiment in Mn Pr 502, "F lo ta t ion and Agglomeration".

The Pennsylvania State University (unpubl.). Aplan, F.F. and de Bruyn, P.L., 1963. Adsorption of hexyl mercaptan on gold. Trans.

AIME, 229: 235--242. Aplan, F.F. and Fuerstenau, D.W., 1962. Principles of non-metallic mineral flotation.

In: D.W. Fuerstenau (Editor), Froth Flota t ion -- 50th Anniversary Volume, Chapter 7. AIME, New York, N.Y., pp. 170--213.

Berry, L.G. and Mason, B., 1959. Mineralogy, W.H. Freeman Co., San Francisco, Calif., 564 pp.

Cornell, R.L., 1937. Characteristics of flotation reagents. Min. J. Ariz., 21(13): 7, 41--42. Eyring, E.M. and Wadsworth, M.E., 1956. Differential infrared spectra of adsorbed

monolayers n-hexanethiol on Zn minerals. Trans. AIME, 205: 531--535. Foster, L.S., 1928. Preparation of xanthates and other organic thiocarbonates. TP 2,

Engineering Experiment Station, University of Utah. Fuerstenau, D.W., Metzger, P.H. and Seele, G.D., 1957. How to use this modified Hallimond

tube for better flotation testing. Eng. Min. J., 158(3): 93--95. Fuerstenau, M.C. and Palmer, B.R., 1976. Anionic flotation of oxides and silicates.

In: M.C. Fuerstenau (Editor), Flotat ion -- A.M. Gaudin Memorial Volume. AIME, New York, N.Y., pp. 148--196.

Gaty, F., de Rycker, H. and Thyssen, H., 1946. Revue Universelle des Mines de la Metal- lurgie, Belgium, 89: 153--166.

Gaudin, A.M., 1932 (1st Ed.); 1957 (2rid Ed.). Flotat ion. McGraw-Hill, New York. Gaudin, A.M., 1938. U.S. Patent #2,125,337. Gaudin, A.M. and Anderson, A.E., 1930. Flotat ion Fundamentals, Part 5, TP No. 9.

Engineering Experiment Station, University of Utah. Gaudin, A.M. and Harris, D.L., 1954. Adsorption of a mercaptan on zinc minerals.

Trans. AIME, 199: 925--930. Gaudin, A.M. and Martin, J.S., 1928. Flotat ion Fundamentals, Part 3, TP No. 5. Engi-

neering Experiment Station, University of Utah.

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Gutzeit, G., 1946. Chelate-forming organic compounds as flotation reagents. Trans. AIME, 169: 272--286.

Jaekel, J.A., 1959. New guides to chrysocolla flotation. Min. World, 21(8): 44--46. Ludt, R.W. and de Witt, C.C., 1949. The flotation of copper silicate from silica. Trans.

AIME. 184: 49--51 ,330. Milliken, F. and Goodwin, R., 1943. Ohio Copper Company trailings re-treatment plant.

Trans. AIME, 153: 609--618. Podobnik, D.M. and Harris, G.H., 1978. U.S. Patent #4,130,477. Shaw, D.R., 1981. Dodecyl mercaptan: a superior collector for sulfide ores. Trans. AIME,

270: 686--693. Taggart, A.F., 1945, Handbook of Mineral Dressing. John Wiley, New York, N.Y., Sec-

tion 12, p. 10. Van Oosterwyck-Gastuche, M.C., 1970. La structure de la chrysocolle., C.R. Acad. Sci.

Paris, S~r, D, 271 : 1837--1840. Van Oosterwyck-Gastuche, M.C. and Gregoire, C., 1970. Electron microscopy and

diffraction identification of some copper silicates. Inst. Mineral Assoc., Proc. Gen. Meet., 7th, pp. 196--205.

Wark, I.W., 1938. Principles of Flotat ion ( l s t Edition). Australasia Institute of Mining and Metallurgy. Melbourne.

White, J.C. and Rule, A.R., 1971. Distribution of sulfide and oxide copper in copper mill tailings. USBM RI 7498.

Wiechers, A., South African Patents 74-02,871, 1974; 75-07,090, 1977; 76-07,089, 1978.

Yarboff, D.L., 1940. Extraction of Mercaptans with Alkaline Solutions. IEC 32: 257.

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