Reduction of binary sulfate mixtures containing CESO

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The reduction of a binary sulfate mixture cannot be predicted from the behavior of the individual .... the tests. Aluminum sulfate was chosen as a representative.

Reduction of binary sulfate mixtures containing CESO, by Hz FATHIHABASHI A N D SIIAHEER A. MIKI-IAIL Department of Mining atld Metallurgy, Lacal Unicersity, Quebec, P.Q., Canada G I K 7P4

Received March 12, I976 FATHIHABASHI and SHAHEER A. MIKHAIL. Can. 3. Chem. 54, 3651 (1976). The reduction of a binary sulfate mixture cannot be predicted from the behavior of the individual components. Thus, while CuS04 is reduced to Cu at 400 ' C and NiS04 is reduced to Ni3S2, the sulfate mixture yields Cu, Ni3S2, and Cu2S. Also while FeSO, is completely stable in H Zat 400 'C, (Cu,Fe) S o 4yields Cu and Cu5FeS4.The formation of Cu2S in the first case and CuSFeS4in the second was unexpected. On the other hand, (Cu,Mn)S04 is stable in H 2 up to 550 "C although pure CuS04 is contpletely reduced at 400 ' C . CoS04 also interferes when reduced in presence of CuS04, while Na2S04, MgS04, A12(S04)3,ZnS04, CdS04 do not interfere within limited temperature range. Of these only Na2S04 forms a complex sulfate with CuS04. No Cu2S04 was identified when CuS04 was reduced in presence of other sulfates although it is a n intermediate product during the reduction of pure CuS04. FATHIHABASHI et SHAHLER A. MIKHAIL. Can. J. Chem. 54, 3651 (1976). Le cours de la riduction d'un melange binaire de sulfates ne peut pas Ctre predit partir du comportement des composCs individuels. Par exemple, alors que le CuS04 est rCduit en Cu a 400 "C et le NiS04 est rCduit 8 Ni&, le melange de sulfates conduit 8 du Cu, du Ni3S2 et du Cu2S. Aussi alors que le FeS04 est complktement stable vis-8-vis du H2 a 400 "C, (Cu,Fe)S04 conduit a du Cu et du Cu5FeS4. La formation du Cu2S dans le premier cas et du CusFeS4 dans le second Ctait inattendue. Par ailleurs (Cu,Mn)S04 est stable vis-a.vis du H z jusqu'a 550 'C m&mesi le CuS04 pur est complkternent rCduit B 400 "C. Le CoS04 interfere aussi lorsqu'il est rCduit en presence de CuS04, alors que Na2S04, MgS04, f%12(S04)3,ZnS04, CdS04 n'interfkrent pas 8 I'intCrieur des limites de tempirature. Parmi ces composCs, il n'y a que le Na2S04 qui forme un sulfate complexe avec le CuS04. On n'a pas pu identifier de Cu2S04 quand le CuS04 a CtC rtduit en presence d'autres sulfates quoiqu'il soit un produit intermediaire lors de la reduction du CuS04 pur. [Traduit par le journal]

Introduction Studies on the reduction of CuS04 were undertaken in connection with a proposed method for copper recovery which would compete with other methods, e.g., electrolysis or precipitation by HZ under pressure (l,2). In this process, CuS04.nH20 is crystallized from solution, dehydrated, then reduced by H2 at 350-380 "C or by CO at about 500 "C to yield metallic copper according to the overall equations:

Solutions containing copper may be obtained by leaching low-grade oxide ore, sulfide concentrates, or scrap metal. It is evident that the purity of the copper produced will depend on the behavior of impurities in solution during crystallization and reduction. Sulfates that are insoiuble in CuS04 solution such as CaS04, BaS04, PbS04, and Ag2S04, or those that undergo

hydrolysis, e.g., Ti(S04I2 and SnS04, will not interfere during the crystallization process and therefore can be eliminated. It is also expected that the sulfates that accompany CuS04, but are reduced at a much higher temperature such as MgS04 and Na2SO4 (starting reduction temperature 670 and 700 "C respectively (3)) would not cause contamination problems. However, it is not certain whether the formation of solid solutions or complex sulfates will influence the reduction process. Also, it is not known what would be the product of reduction of CuSO4 containing impurities such as NiS04, CdS04, CoSO4,ZnS04, Al2(S04I3,FeS0'{, and MnS04 whose starting temperature of reduction is in the range 340-540°C (3); can tfrc products of reduction be predicted from the behavior of the individual sulfate? Solubility data of binary sulfates containing Gus04 as one component are available in literature (4) and from these data it can be concluded that during crystallization the following cases


CAN. J. CHEM. VOL. 54, 1976

will arise for the hydrated crystals: (I) Sulfates that form complex salts with CuS04: Na2S04, K2S04,Wb2S04, (NH4)2S04, and T12S04. (2) Sulfates that form solid solutions with CuS04: MgS04, ZnS04, MnSO4, NiS04, CaS04, and FeS04. (3) Sulfates that do not form complex salts or solid solutions with CuS04: Li2S04, BeS04, A12(S04)3, and CdS04. On dehydration. it is not certain that the above classification hill still be valid since the con~plexsulfates may decompose, and the solid solutions may interact or breakdown due to a difference in structure in the anhydrous form. The present work was therefore undertaken to study the role of metal sulfates during the crystallization of an aqueous solution of CuS04 and their effect on its dehydration and reduction.

Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were also used. The TGA instrument was a Fisher 10-560-100 assembly with a Cahn R G electrobalance. The heating system consisted of a vertical tube furnace connected to the model 360 linear temperature programmer which provides a linear heating rate. A heating rate of 10 "C,'min was used. An X-Y recorder Honeywell Electronik 19 was connected to the system. The sample was about 20mg. The DTA instrument was a Stanton model 6-25 with a non-magnetic stainless steel sample holder, and platinum-rhodium crucibles with control base recess. Calcined A1203 was the reference material.

Results and Discussion Cr~~stalliza fiorz and Dehj.dration of Binar): S~llfates When the binary sulfates were crystallized from solutioll. hydrated crystals were obtained, which in most cases were homogeneous on microscopic examination but sometimes had Experimental colors different from the mechanical mixture of Finely ground reagent grade sulfates were used in all the individual hydrated components. For exthe tests. Aluminum sulfate was chosen as a representative ample crystals obtained from the CuS04-FeS04of the group of sulfates that do not form complex salts or H 2 0 system were very pale blue although solid solution, Na2S04as a representative of the group of sulfates that form complex salts, and finally all sulfates CuS04.5H20 is dark blue and FeS04.7H20 is pale green. Further, in some cases when the that form solid solutions with CuSO, were studied. The general procedure for preparing a sample was as hydrated crystals were heated in inert atmosfollows. A solution nearly saturated with respect to phere at 300 OC to remove the water, the color CuS04 was prepared, then crystals of the other sulfate of the anhydrous mixture was unrelated to the added to it portionwise with continuous stirring until thc color of the individual anhydrous salts. Thus solution became saturated with respect to the added anhydrous C L I S Oand ~ FeS04 are hhite, but the sulfate. Dissolution was usually conducted while heating, and the crystals were obtained on cooling to room anhydrous mixture obtained by dehydrating the temperature. In the case of CuS04-FeSQ4 special pre- mixture obtained from solution was vi01et.~This cautions had to be taken to prevent the reaction: intense coloration can be an indication of a partial valence change during heating, probably The sulfate mixture was prepared at room temperature' to Cu+ and Fe3+. No attempt at present is made and was deaerated and kept under inert atmosphere to to identify these species; this will be the subject prevent oxidation of Fe2+ ion; also few drops of H2S04 of a future study. More data on the color of the were added to prevent its hydrolysis. Crystallization took piace by cooling the solution in an ice bath. The crystals sulfate systems studied is given in Table 1. were collected, dried between tissue paper, and kept in When the anhydrous sulfate mixture was desiccator. They were later dehydrated by heating in inert examined by X-ray diffraction method, there atmosphere then examined by X-ray diKraction method. was a difficulty in identifying mixed crystals Reduction by Hz was performed at different temperatures because most of the anhydrous sulfates studied and the products were examined by X-ray diffraction. A Worelco X-ray diffractometer with CuK, radiation showed nearly similar X-ray diffraction patterns. was used. It consisted of: Diffraction Unit type 12045- The following statement. however, can be made: 12046, Wide Range Goniometer type 42201-42202, ( I ) An anhydrous complex sulfate was confirmed Electronic Circuit Panel type 12038-12049, and AMR Focusing Monochromator Model 3-202 with a graphite in the case of CuSO4-Na2S04. (2) The presence of anhydrous solid solution was positive in the crystal. The diffraction patterns of reaction products obtained were compared with those in the ASTM card index and their contents were classified qualitatively as major, intermediate, and minor depending on the height of the major peaks. 'Oxidation of Fez- by Cu'+ takes place to a minor extent on boiling.

'The same violet color was obtained when chalcopyrite, CuFeSz. was oxidi~eclby concentrated H2S04 at 200 "C to form anhydrous sulfates, a process once seriously considered by the Anaconda Company in Tucson, Arizona and Treadwell Corporation for recovering copper from concentrates.




TABLE 1. Color of the individual sulfates and their binary mixtures with CuS04 Color of the Color of the Color of the Color of the binary hydrated binary anhydrous hydrated anhydrous mixture with CuSQ4 sulfate mixture withCuS04 sulfate White White

(5H20) dark blue (2H20) pale blue (7H20) pale green

Pink Dark blue Yellow White White White White White

(HzO) pink (7H20) red (6H20) green (7H20) white (7H20) white (10H20) white (7H20) white (19Hz0) white

(rzH20) pale blue (2HzO) beige Very pale blue Purple Verj pale green Very pale blue Very pale blue Turquoise White blue* Pale blue


violet Canary yellow Purp!e White White White White White White

*Heterogeneous mixture as re~ealedb j microscopic examination.

case of CuS04-FeS04 and CuS04-MnS04.3 (3) The presence of anhydrous solid sollition was doubtful in case of CuS04-CoS04, CuS04NiS04, CuS04-ZnS04, and CuS04-MgS04. (4) No anhydrous solid solutions were present in the systems CuS04-A12(S04)3 and CuS04-CdS04.

with CuS04 were positively identified. These were CuS04-FeS04 and CuS04-MnS04.

CuS04-FeS04 Crystals obtained from a solution saturated with CuS04 and FeS04 gave a distinctive X-ray diffraction pattern. This material is identified here Reduction of Bina~ySu~fcifes as (Cu,Fe)S04.nH20. When heated in argon, The simplest systems in which no complex another crystalline form was obtained at 200 "C: sulfates or solid solutions are formed were having the composition (Cu,Fe)S04.nzFH20.The studied first; these are CuS04-A12(S04)3 and amount of water of crystallization was deterCuS04-CdS04. It was found that A12(S04)3 had mined experimentally and m was found to be no effect on the reduction of CuS04 up to equal to 2. The latter material was beige in color 400 "C, and CdS04 had no efTect up to 320 "C with a slight orange tint. Since its X-ray diffracbut traces of CdS were formed at 400 "C. In both tion pattern is not indexed by the ASTM systerr,, cases metallic Cu was obtained together with the it is given here in Table 2. When heated further unreacted sulfate which could be removed by to 300 "C, it lost its water of crystallization to leaching with water. ibrm (Cu,Fe)S04 which has a vio!et color. The effect of formation of complex sulfates was The results of the thermal decomposition of studied next; this is the case of CuS04.Na2S04. (Cu,Fe)S04.nH20 are summarized in Table 3. It. It was found that CuS04 was reduced selectively can be seen that the anhydrous material is stable at 300-400 "C, i.e., Na2S04 had no effect on the in inert atmosphere up to 500 "C.Above this reduction and the complex sulfate behaved as if temperature, there is a gradual decomposition to it were a mechanical mixture of CuS04 and give CuS04 and Fe203. In the region 600 to Na2S04. This is contrary to expectation since the 700°C, CuS04 gradually decomposes to CuO formation of a con~plexsulfate was thought to while Fe203 is partially converted to Fe304. No influence the reduction because a new crystal DTA peaks were observed in the temperature structure was formed. The salt CuS04.Ma2S04is range 500 to 700 "C but rather a gradual endostable in inert atmosphere up to 470 "C. thermic effect (Fig. 1). Also it is remarkable that The effect of formation of solid solutions was no oxysulfate, CuO.CuS04, is formed as is the studied in two systems in which solid solutions case when pure CuS04 is decomposed. TGA curve for heating the crystals in argon is shown 3Although CuSQ4 and FeS04 are not isostructural and in Fig. 2. are therefore not expected to form a solid solution over When heated in H2 atmosphere, (@rr,Fe)the whole range, yet, apparently there is a limited soluS04.2H20was stable up to 2.10 "C (Table 4 and bility of one in the other. + '


CAN. J. CHEM. VOL. 54. 1976

TABLE 2. >(-Ray diffraction pattern of (Cu,Fe)S04.2H20 (Cu K, radiation)

Fig. 1). Above this temperature, however, the fo!lowing observations were made: ( I ) At 210 "C the crystals lost their water of hydration to form (Cu,Fe)S04 as indicated by the X-ray pattern. No Cu2SOl was identified at 230 "C as in the case whe~lpure CuSO4 was reduced. (2) At 300 "C the individual diffraction patterns of CuS04 and FeS04 appeared as hell as that of Cu. Under the

microscope Cu appeared as whiskers. The material was soluble in water leaving Cu metal. (3) Bornite, CusFeS4, appeared at 350 "C among the reduction products. (4) The sulfates disappeared completely at 425 "C and two new phases appeared, namely, Fe304and CuFeSz in addition to bornite and Cu although the temperature at which pure FeS04 starts to be influenced by H2 is 440 "C and the products of reduction are Fe304 and FeS. ( 5 ) At 500 "C. CuFeSz decomposed and the reduction products were: Fe, CusFeS4, Cu, and FeS. For comparative purposes, three sets of experiments were conducted on a mechanical mixture of CuS04 and FeS04. The first set consisted of the reduction of different CuS04:FeSO4 ratios by H 2 at 500 "C. Different samples were prepared with molar ratios ranging between pure CuS04 and pure FeS04. It was found that in addition to Cu and Fe, bornite formation was evident in all experiments. Equimolar mixture of CuS04 and FeS04 was chosen to carry out the

ARGON 705OC e x o t h e r m i c



1. 360°C






FIG. I. Differential thermal anaiysis curves of (Cu,Fe)S04.nH20 crystals in argon and in Hz.


TABLE 3. Thermal decomposition of (Cu,Fe)SO,.rrH~Oin argon* -Reaction products Temperature (=C) Major Intermediate Minor Color (Cu.Fe)SQ4.2H20 (Cu,Fe)S04 (Cu,Fe)SO, CuS04 Fe304.CuO

200 300 500 600 700

Fe203 FeZ03

Beige Violet Violet Red-brown Red-black

*Heating in a boat for 1 11. Products identified b) X-1.3). diffracrioii analisis

TABLE 4. R e d ~ ~ c t i oofn (Cu,Fe)SO4.nH20 by Hz in a boat for I hq

Reduction products



Major (Cu,Fe)S04.2H20 (Cu,Fe)SQ4 (Cu,Fe)S04 Cu Cu Cu Fe304,CuFeS2 ~ e ~ 0 , Fe. CujFeS4


,Minor CuS04 Cu Cu

FeS04, CuS04 FeS04, CuSO, FeS04 CujFeS4 CuFeSz, CujFeS4 Cu, FeS

Cu5FeS4 CuS04, CujFeS, Cu Cu

Color Beige Beige Beige Black Biack Black Black Black Black

"Products identified ti) X-ra) diffrdction nnal>sis.

second set of experiments to sttidy the kinetics of the reduction at 50Q°C. It was found that bornite formation was evident in case of the mechanical mixture as well. The third set of experiments was carried out to study the effect of reduction temperature. The results showed that u p to 400 "C the mixture behaved in an additive way; thus Cu2S/34 was identified when reduction took place at 280°C, the product u a s white in color and showed X-ray diffraction patterns due t o CuS04 and FeSQ4. At 500"C, however, bornite formation Lvas evident. It seems that it is not a question of presence of solid soiution because a i-i~echanica!mixture of CuS04 and FeS04 behaves similarly to the solid

FIG. 2. Therniogravimetric analysis curve for the thermal decomposition of (Cu,Fe)S04.nHz0 in argon.

solution with the exception that a slightly higher temperature is needed for the bornite phase to appear. This shows that in this case the solid solutions are iess stable than the individual sulfates. Probably the proposed reaction in the solid state between Cu2+ and Fez+ lending t o the partial formation of Cu- and Fe3+ is responsible for the early appearance of metallic Cu during heating in H2, because of the ease of reduction of Cu- to Cu. Since not much information is availabie on the crystals (Cu,Fe)S04.ilH20, DTA curves are presented in Fig. 3 for their decomposition in air. It can be seen that decomposition starts a t 595 OC, but there are two sma!l changes in the heating curve at 222 and 520 "C which were reproducible and could not be explained, as well as a small endatherm starting a t 290°C, which is apparently due to the loss of the last molecule of H20. In this respect the behavior of this material is different from the component sulfates. Thus, the exothermic peak that starts a t 475 "C due to the formation of ferric oxysulfate, h;e20(S0& (Fig. 36, is absent, and decomposition goes directly to F e 2 0 3 a t about 400 "C while CuS04 remains unreacted.


CAN. J. CHEM. VOL. 54, 1976

400°C, the binary mixture CuS04-CoS04 is already reduced at 380°C, i.e., the reduction temperature of CoS04 is lowered by at least 20 "C in the presence of CuS04. Similar observation was found for the reduction of the binary mixt~lreCuS04-NiS04 with the difference that beside the expected products Cu and Ni3S2, some Cu2S was also formed. On the other hand, when the binary mixtures CuS04-ZnSOj and CuS04-MgS04 were reacted with H2 a t 380 OC, only CuSO4 was reduced, i.e., ZnSO4 and MgS04 do not interfere. Since the systems CuSO4-NiSO4 and CuS04FeSOj proved to be the most complex with reduction products and the reduction temperatures different from those of the individual sulfates, it was decided to search for another system that might lead to similar reduction behavior. The system FeS04-NiSO4 was chosen since it is known that hydrated solid solutions are formed in this system. It was found that there was a similarity in the X-ray diffraction patterns between the crystals obtained by crystallizing then dehydrating a FeSO4-NiS04 mixture and the individual anhydrous sulfates and it could not be ascertained whether or not anhydrous solid solutions existed. The results of reducing these crystals are shown in Table 5 , from which it can be seen that the reduction temperature of FeS04 is decreased by at least 40 "C in presence of NiS04 and that the complex sulfide pentlandite, (Fe,Ni)9S8, was formed and no FeS could be detected although the latter is a major reduction product of FeS04.


FIG. 3. DiFferential thermal analysis curve for heating (Cu,Fe)S04.nH20 and FeS04 in air.

CuSO4-MnS04 The reduction of pure MnS04 is noticeable at 540°C. When the solid solution was heated in HZ at 400 OC, excess CuS04 in the mixture was reduced to Cu while excess MnS04 remained unchanged together with the solid solution. At 500°C, the reduction products were the same except traces of Cul 9gS appeared. At 550 "C the solid solution was still present. It is evident, therefore, that CuS04 bound with MnS04 in the solid solution is resistant to reduction. Contrary to the previous case, this example shows that solid solutions can be more stable than at least one of the individual sulfates. Other Syster?~s It was not possible to ascertain on basis of X-ray diffraction whether or not solid solutions existed in the following systems: CuS04-C'oS04, CuS04-NiS04, CuS04-ZnSO4, and CuSO4-MgSO4 because of the similarity of the patterns of the individual sulfates. For this reason, they are presented together. While pure CoS04 is reduced to CogSg at

Conclusions 41) The X-ray diffraction patterns of anhydrous Fel', Co, Ni, Mn, Zn, and Cd sulfates resemble to a great extent that of CuSO4 and in most cases it was not possible to decide whether

TABLE 5. Reduction of FeS04-NiS04(mixed) crystals in H2* Reduction products


("c) 300 400 500



FeSB,, NiSQ4 Ni& Ni3S2 (F~,NI)~SS

'Products identified by X-ray diffraction analysis.

FeS04 -.




Green Black Brass

(Fe,Ni)gS8 Fe



anhydrous solid solutions were formed or not CuS04 and Na2S04 form a complex salt but the when CuS04 and another sulfate were crystal- presence of one sulfate does not influence the lized from solution, then dehydrated. Anhydrous other during reduction. On the other hand, when solid solutions were only positively identified in solid solutions are formed the reduction products the systems CuS04-FeS04 and CuS04-MnS04 may be different from that of the individual while an anhydrous complex sulfate was identi- components as indicated earlier. fied in the case of CuS04-Na2S04. (6) There are indications that the anhydrous (2) The reduction of a sulfate mixture pre- solid solution (Cu1',~e")S04 may be partly pared by crystallization from aqueous solution transformed into (Cu1,Fe1")S04; this is based on then dehydrated may be different from that of the color change, X-ray diffraction, behaviour the individual anhydrous sulfates. When CuS04 during oxidation, reduction, and thermal deis reduced at 380°C in presence of another composition. sulfate it was found that the following sulfates (7) T o get pure metallic Cu by reduction of d o not influence the reduction : Na2S04, MgS04, CuS04, the impurities Fe, Co, and Ni must be A12(S04)3, ZnS04, and CdS04. On the other absent from the solution before obtaining the hand CoS04, NiS04, FeS04, MnS04 interfere in sulfate crystals. Other sulfates such as Na2S04, the following manner: C0S04 and NiS04 are M3S04, A12(S0&, ZnS04, CdS04, and MnS04 reduced a t a lower temperature than expected may be tolerated and would be removed from and in the latter case a Cu2S phase appears. metallic Cu by leaching with water, provided the FeS04 is also reduced at a lower temperature reduction temperature does not exceed 380 "C. than expected and a complex sulfide phase, CujFeS4, is formed. MnS04 forms a stable solid Acknowledgements solution with CuS04 which is reduced at a temperature higher than that expected. The financial aid of the National Research (3) The formation of a complex sulfide phase Council of Canada is greatly appreciated. A disduring the reduction of a sulfate mixture was also cussion with Professor F . Claisse was very useful. observed during the reduction of FeS04-NiS04. I n this case pentlandite (Fe,Ni)9S8 was observed. 1. F. HABASHI and R. DUGDALE. Met. Trans. 4 (9,1429 (4) N o Cu2S04 was formed as intermediate (1973). Can. J. Chem. Eng. 52, product during the reduction of CuS04 in ad- 2. K. Vo VANand F. HABASHI. 369 (1974). mixture with another sulfate although it was identified during the reduction of pure CuS04. 3. F. HABASHI,S. A. MIKHAIL,and K. Vo VAN.Can. J. Chem. This issue. ( 5 ) The formation of solid solutions or com- 4. Gmelins Handbuch der Anorganischen Chemie, plex salts might be expected t o alter conditions; Volume 60, Kupfer, Part B3, Verlag Chemie, Weinit does in some cases but not in every case. Thus, heim, 1965.

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