Canadian Mineralogist Vol. 18,pp. 365-372(1980) OXIDATIONOF SULFIDEMINERALS. V. GALENA,SPHALERITE AND CHALCOGITE H.F. STEGBR eup L.E. DESJARDINS Mineral SciencesLaboratories, Canada Centre lor Mineral and Energy Technology, Department ol Energy, Mines and Resources,Ottawa, Ontaio KIA OGI AssrRecr long-term stability of sulfide-bearing ores and concentrates.Part of this study was concerned Samples of galena, sphalerite and cbalcocite were with the nature of the products and kinetics of oxidized at 52oC and, 68Vo of relative humidity the oxidation of the commonly encountered periods to five weeks, and the prqd- in air for up sulfide minerals. The oxidation of pyrite, chal- for metal and sulfur-bearing ucts were analyzed pyrrhotite at 52oC and, 687o of. species. Galena is. oxidized to PbSOa, sphalerite copyrite and (RH) has already been in- to ZnSO. * FezO, if iron-bearing, and chalcocite relative humidity to CuO and CuS. The oxidation of galena and vestigated (Steger & Desjardins 1978). This re- sphalerite proceeds according to a linear rate potr summarizes the results of the study 9f tle law: that of chalcocite leads to the formation of a bxidation of galena, sphalerite and chalcocite coherent product layer impenetrable to Oz and HrO under the same conditions. vapor. The air oxidation of galena at relatively low without Keywords: air oxidation, oxidation products, sul- temperatures has been investigated fide minerals, galena, sphalerite, chalcocite. reaching a consensuson the nature of the oxida- tion product. Hagihara (1952), using a re- Sourvrlrnp flection electron-diffraction technique, and Kirkwood & Nutting (1965), using a trans- Nous avons 6tudi6 I'oxydation dans I'air d'6chan- mission electron-diffraction technique, found tillons de galdne, de sphal6rite et de chalcocite i this product to be PbSOo,whereas Leia et al. 52"C et 687o d'humidit6 relative, pour des p6- (1963), using reflection infrared spectroscopy' s'6talant sur semaines. On a d6termin6 riodes cinq concluded that it was PbSbOr.Using transmis- la concentration des m6taux et des complexes du spectroscopy, however, Greenler soufre dans les produits. La galbne se transforme sion infrared en PbSOa, la sphalErite en ZnSO, (et Fe2O. si (1962) identified PbSOq PbSO' and PbC0,l l'€chantillon est ferrifdre), et la chalcocite en CuO in the oxidation products of galena. All these et CuS. L'oxydation de la gallne et de la sphal6rite workers identified the oxidation product by est une r6action du premier ordre. La chalcocite en comparing its electron-diffraction pattern or s'oxydant forme une gaine coh6rente imperm6able infrared spectrum with that of PbSOa,PbSrO", d I'oxygdne et i la vapeur d'eau. etc. A recently developed method (Steger & 1977), whereby sulfate and thio- (Traduit par la R6dacuon) Desjardins sulfate in the oxidation products of sulfide min- Mots-cl6s: oxydation dans l'air, produits d'oxyda- erals are separatedchemically and their quanti' tion, sulfures, galdne, sphal6rite, chalcocite. ty is determined, gives a more direct identifi- cation. INrrooucrroN The authors could not find any reference to a study of the air oxidation at low temperature The Canadian Certified Reference Materials of sphalerite. In several references (e.9., Khris- Project (CCRMP) offers ores and consentrates toforov et aL. 1969) that pertain to the oxida- for sale to the analytical community for use tion of sphalerite ores during storageothe oxi- as certified reference materials. Since many of dation of sphalerite to ZnSOa is assumed to these materials have a high sulfide content, take place. A study to establish the nature of they are potentially'susceptible to air oxidation the oxidation at relatively low temperatures rn storage, with the accompanying risk of of such an economically important mineral as rendering the certified element-values invalid. sphalerite was long overdue. A study was therefore initiated to assess the Chalcocite, CuaS, is a secondary copper sul- fide rnineral of minor commercial importance. A study of its oxidation was undertaken to Crown Copyright Reserved compare a "metal-rich" mineral with "metal- 365 366 THE CANADIAN MINERALOGIST poor" minerals, e.9., pyrrhotite, pyrite and min- 1.92:1 is inconsistent with chalcocite, but elec- erals having approximatd equal metal and sul- tron-microprobe analysis and X-ray-diffraction fur, like chalcopyrite, galena and sphalerite. The studies confirmed that the mineral is chalcocite oxidation of chalcocite at low temperatures has Cu,S. The low Cu:S ratio is due to the bornite. not been studied. Precipitated CurS was shown to oxidize at ?AO"C to CuO, CuS and CUSO4 Oxidation ol mineral samples (Lefevre et al. 1957). The procedure of Steger & Desjardins (1978) The literature is full of references to electro- was followed. For each mineral, - 16 g were chemical, hydrometallurgical and geological in- weighed into a tared 90 x 50 cm crystallization vestigations of the oxidation, electrochemical dish, which was then placed in a controlled dissolution and oxidative dissolution of sulfide temperature-humidity chamber (Blue M. Elec- minerals. As important as these references may tric Co., Blue Island, Illinois). The conditions be for an understanding of certain phenomena, of 52 +- 1'C and 68 -+ 3% RH were moni- they are not relevant to this study, merely tored with a dry bulb - wet bulb therrnometer because a liquid aqueoru phase, whether rain, combination. The samples were removed from water or acidic leach solution, is involved. In this chamber after selected time intervals, put air oxidation, water in the vapor phase is in- in a desiccator over Drierite@ for 18-20 hours volved. The presence of a liquid phase alters to remove adsorbed water, weighed to deter- both the reaction mechanism and kinetics either mine I wt. (the change in weight due to oxi- by the dissolution of soluble reaction products dation), and then thoroughly mixed rnanually. or by allowing electrochemical oxidation mech- After the removal of subeamples of - 2 g, the anisms to occur (Bailey 1977). mineral samples were reweighed and returned to the chamber. The value of 7\ wt. is reported ExPERIMENTAL only for chalcocite. Although the values of I wt. for galena and sphalerite showed a tendency Minerals to increase with oxidation period, they were erratic, especially for the longer periods. This The 37-74 pm size fractions, prepared from is attributable. to th'e fact that the potential ac- crushed massive specimens of galena (Galena, cumulative error, due to the multifude of Kansas), sphalerite (Ottawa Co., Oklahoma) weighings, is significant with respect to the and chalcocite (Superioq Arizona), were low I wt. resulting from the small extent of cleaned according to the procedure of Steger oxidation undergone by galena and sphalerite. & Desjardins (1978). Another sample of sphale- These errors are estimated to be 0.M g/mole rite (Montauban, Qu6bec) required further and 0.02 g/mole for galena and sphalerite, treatment with a Frantz separator to lemove respectively, for the S-week oxidation period. pyrrhotite. Microscopic examination indicated The oxidation conditions of 52"C and 68/o traces of chalcocite, digenite and gangue in RH are, of course, relativd extreme compared the galena and approxirnately 2% gangue plus with ambient conditions, but are nevertheless traces of pyrite and chalcopyrite in both sam- necessary to promote oxidation to an extent ples of sphalerite. The shalcocite contained sufficient for a study lasting months rather than traces of chalcopyrite and pyrite and 2.5% yearc. bornite (mean of two determinations by quanti- parameters tative image-analysis). The elemental analysis Determination ol of oxidation and suggested empirical formulae of the min- The oxidation products were analyzed for erals are given in Table l. The Cu:S ratio of lead for galena, zinc for both sphalerites, and copper for chalcocite by selective extraction with a 15% ammonium acetate, 3% acetic acid gT'I]FIDE TABLE ]-. RESUIJTg OF CUEMICAI] ATIAI]YSIS OT I'IINERAI,I' solution (Steger 1977). All subsamples were l4lneraL E!.4etal 89 llrplrIcal analyzed for elemental sulfur by the volatiliza- tion--spectrophotometric ,method of Steger cal-ena Pb, 85.71 13.39 PbS (L976b). The entire suite of initial and oxidized sphalellte z^. 56.61 32'69 zto.g:4F"0.146s (high lron) Fe, A.26 samples of chalcocite was analyzed for sulfate Des- sphalerlte 2n,65.31 32'L6 zoo.999F and tliosulfate by the method of Steger & o.oo1s (1q lrcn) Fe, 0.09 jardins (1977), in which the sulfur-bearing Chelcoclte cq, 78.32 20.63 Cu2s anions are separated from tle sulfide minerals Cu5l'eg4 by ion exchange with sulfide ion. The sulfate is determined directly by precipitation with ex- O)CDATION OF GALENA, SPHALERITE AND CIIALC(rcITB 367 cess Ba2+. Another portion of the sulfur-anion TABIJ 2. RESULTS OF ANAIYSIS OF OXIDIZED GAI,ENA sample is oxidized and precipitated with Ba3+ 10' Tlme to give the sulfate above and the sulfate re- (hr) ,o1' s" sulting from the oxidation of non-sulfate sulfur- (0.07) (0.12) 0.37 (0.r5) bearing oxidation products, such as thiosulfate. Unoxl-d. 0.36 r.12 Because of the small extent of oxidation in 72 0.44 r.37 0.30 galena and the two sphalerites, the quantities t6B 0. s8 of sulfate and thiosulfate are approximately the 240 0.54 0.30 same as those suggested by the reproducibility 336 0.71 0.15 of this method; only the unoxidized and S-week- s04 0.92 L.A2 0.30 oxidized samples were analyzed for sulfate and 672 1.17 L-97 0.30 thiosulfate. Because no thiosulfate was found 0.1s (see below), the sulfate in the oxidation prod- 840 1.40 uct(s) in all sampleswas determined as follows. A 150-ml aliquot of the ammonium acetate - TABI.E 3. RESUI]TS OT'ANAI,YgXS OF OXIDIZED acetic acid extract, prepared to determine tle HIGH-IRON SPI$IARTTE metal in the oxidation product(s), was boiled to dryness and twice redissolved in 5 ml each Time (hr) zn(rr ) of concentrated hydrochloric and nitric acid and "o;- then boiled to dryness to remove acetate.