Recovery of copper powder from copper concentrates and from solutions of copper(II) sulfates using sulfur dioxide and aqueous acetonitrile

HydrometaUurgy, 6 (1981) 239--260

239

Elsevier Scientific Publishing Company - - Printed in the Netherlands

RECOVERY OF COPPER POWDER FROM COPPER CONCENTRATES AND FROM SOLUTIONS OF COPPER(H) SULFATES USING SULFUR DIOXIDE AND AQUEOUS ACETONITRILE

A.J. PARKER and D.M. MUIR

Mineral Chemistry Research Unit, Murdoch University, Murdoch, W.A. 6150 (Australia) (Received November 7, 1979; accepted in revised form June 23, 1980)

ABSTRACT Parker, A.J. and Muir, D.M., 1981. Recovery of copper powder from copper concentrates and from solutions of copper(II) sulfates using sulfur dioxide and aqueous acetonitrile. Hydrometallurgy, 6: 239--260. Copper concentrates can be sulfation roasted and leached with water to produce impure solutions of copper(II) sulfate. Copper sulfites are precipitated from aqueous copper(II) sulfate solutions with soluble salts of sulfurous acid like (NH4)2SO 3 or Na2SO3. The water insoluble copper sulfites dissolve in acetonitrile--water (AN/H20) and reduce Cu 2+ to give acidic solutions containing up to 2.4 M Cu + as Cu2SO4. Removal of acetonitrile by steam and disproportionation, gives up to 75 g pure particulate copper per litre of such solutions. Conditions for the recovery of Chevreul's salt from solution using different sulfite bases and for its dissolution in CuSO4/AN/H20 are determined. It is shown that both temperature and the molar ratios of Chevreul's salt and CuSO4 are important in the efficiency and stoichiometry of the dissolution reaction. Methods of separating copper and nickel from solutions of copper(II) and nickel sulfates, and of recovering copper from dilute (<0.1 M) solutions of copper(II) sulfate, are suggested. A flow-sheet which combines these reactions and methods to r~cover copper from sulfation roasted chalcopyrite concentrates is proposed.

INTRODUCTION

Sulfur dioxide is generated from copper sulfides by roasting. Other products o f roasting include copper sulfate, copper oxides, sometimes copper ferrites and iron oxides [1--4]. Copper oxides can be obtained readily b y roasting chalcopyrite with lime b u t no sulfur dioxide is evolved. The calcines from roasting can be leached to give copper(II) sulfate solutions. Copper is obtained from solutions of soluble salts of copper, such as copper(II) sulfate, by electrowinning or b y cementation with iron. There are a number of objections to electrowinning as a means of copper recovery from solution. They include the substantial energy requirements, the capital and operating costs of tankhouses, the acid mist formed at anodes and the occasional p o o r quality of cathodes if solutions are impure.

0304-386X/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

240

Cementation gives very poor quality copper and it is difficult to dispose of the resulting iron(II) sulfate solutions. Supplies of iron, suitable for cementation, are not always available at economic prices. For obvious reasons, therefore, there is interest in the prospect of reducing soluble copper salts to copper with a reductant like sulfur dioxide, especially as part of a roast--leach--reduction system for copper concentrates [ 5, 6]. Attempts at direction reduction with SO2 as in eqn. (1) have given poor yields of copper and/or created problems with handling acidic solutions at high temperatures and pressures [7 ]. CuSO4 + SO2 + 2H20 ~ Cu + 2H2SO4

(1)

More sophisticated work, by Arbiter, Milligan and McClincy [7], and by Arbiter and Milligan [ 8], has used copper sulfites as intermediates for SO2 reduction of copper(II) salts. Copper sulfites can be precipitated from solutions of soluble copper(II) salts by the addition of soluble salts of sulfurous acid [5,9,10] (e.g. Na2SO3, Mg(HSO3)2, NaHSO3, (NH4)2SO3;eqns. (2) and (3)) Copper ammonium sulfites are precipitated by adding SO2 to solutions of copper(II) salts in ammonia, e.g. Cu(NH3)4SO4 [7, 8,11], or by adding (NH4)2SO3 to solutions of copper(II) sulfate such that the molar ratio of (NH4)2SO3 : Cu2+ is 2:1 (eqn. (4)). 3CuSO4 + 6NaHSO3 ~ Cu2SO3CuSO3" 2H20 + 3SO2 + 3Na2SO4 + H2SO4 (2) 3CUSO4 + 3(NH4)2SO3 -+ Cu2SO3CuSO3" 2H20 + 3(NH4)2SO4 + H2SO

(3)

2CuSO4 + 4(NH4)2SO3 ~ Cu2SO3(NH4)2SO3 + 3(NH4)2SO4 + SO2

(4)

Copper sulfites can be produced also from insoluble oxidized copper, such as CuO, Cu20, CuCO3, etc. by treating the solid slurry with SO2, preferably at above-ambient temperatures [10]. For best results in the precipitation of copper sulfites, it is necessary to maintain a pH above 2 and preferably below 4.5. Insoluble copper sulfites do not form in moderately acidic solutions (i.e. those containing substantial amounts of sulfurous acid) and co-precipitation of copper oxides or other metal sulfites and oxides occur at high pH. The potential--pH diagram for the Cu--SO2--H20 system for unit activity of dissolved sulfur dioxide and for 0.01 activity of all aqueous copper species is shown in Fig. 1. Copper sulfites and copper ammonium sulfites have been converted with varying degrees of difficulty and efficiency to copper oxides by calcining [6] and to particulate copper by acidification (eqn. (5)) [11], or by heating in an autoclave at temperatures between 100° and 150° (eqns. (6) and (7)) [7,8,11]. Cu2SO3CuSO3" 2H20 + 2H2SO4 -* Cu + 2CuSO4+ 2H2SO3 + 2H20

(5)

heat -+ 3Cu + 2H2SO4

(6)

Cu2SO3CuSO3- 2H20 Cu2SO3(NH4)2SO3

heat ~ 2Cu + (NH4)2SO4 + SO2 150 °

(7)

241 I "50

\

/'25 I'00 Cu 2 ÷

0-7"5 -

C~O t'cJ

0.500 "25"

000. -

0"25"

-0.50 -2"00

oloo

~'oo

4'oo

61oo

e'.oo ~o!oo ~2'oo 14.oo

Fig. 1. Potential--pH diagram for the Cu--SO2--H20 system. (P. Duby and S. Fountoulakis, Private communication.) Activity S02 = 1; dissolved copper species = 0.01. Some mixed copper(I) and copper(II) sulfites, e.g. Chevreul's salt, generate highly corrosive sulfuric acid at 150 ° when autoclaved (eqn. (6)). The acid problem has been avoided by Arbiter, Milligan and McClincy [7] by autoclaving copper(I) a m m o n i u m sulfites (eqn. (7)). A disadvantage of autoclaving is that m a n y easily reduced metal ions (e.g. antimony, bismuth or silver), or anions like thiosulfate or chloride m a y co-precipitate with the copper sulfite and remain with the copper product after autoclaving. Likewise, of course, impurities like calcium and lead m a y precipitate as sulfites and remain as water insoluble sulfates after autoclaving. Arbiter and MiUigan report that most of the Bi, Pb, Cd and Zn, present in an ammonia leach solution, coprecipitates with copper a m m o n i u m sulfite [8]. Between 1--10% of this zinc remains with the copper powder after autoclaving in acid solution [8]. We have proposed what appears to be a more flexible m e t h o d than autoclaving, calcining or acid treatment for obtaining particulate copper from copper sulfites [ 12,13], in that a greater opportunity exists to remove impurities from various feedstocks and to control the size and shape of the copper powder product. Copper sulfites are dissolved in acetonitrile--water containing copper(II) sulfate to produce acidic copper(I) sulfate solutions (eqns. (8) and (9)). These solutions are then thermally or electrochemically disproportionated [14] (eqn. (10)) to copper and copper(II) sulfate. The nitrile, water and copper(II) sulfate recycle to dissolve more copper sulfite after pH adjustment. Cu2SO3CuSO3- 2H20 + 3CuSO4

AN/H20 ~ 3Cu2SO4 + 2H2SO4

(8)

242 Cu2SOa'H20 + 2CUSO4

Cu2SO4

AN/H20 -+ 2Cu2SOa + H2SO4

distil AN -~ Cu + CuSO4 o r electrolyse

(9)

(I0)

If the copper sulfite is Cu2SO3(NH4)2SO3, as produced in the Arbiter process, then it is preferable to convert this to Cu~SO3 or Chevreul's salt with separation of (NH4)2SO4 before leachingwith CuSO4 in acetonitrile--water. High concentrations of ammoniumsulfate reduce the solubility of acetonitrile in water. The two processes of dissolution of copper sulfites and disproportionation of copper(I) ions offer two steps for separating and removingimpurities from solution which are not available when copper sulfites are autoclaveddirectly [7, 8]. Thus, nitrile-refining(eqns. (8--10)) of crude copper sulfites has the potential to give pure copper with good control of the powder characteristics, starting from a wide range of feed materials. This paper describes some observations and conditions required for the precipitation of Chevreul's salt and other copper sulfites from dilute and concentrated copper(II) sulfate solutions, as produced from leaching low grade copper oxide ores or sulfation roasted chalcopyrite concentrates. It also considers the separation of copper sulfites from solutions containing both copper and nickel sulfates, as produced from sulfation roasting of copper-nickel concentrates. We report the overall stoichiometry and rate of the reaction between Chevreul's salt and solutions of CuSO4 in aqueous acetonitrile to produce solutions of Cu2SO4 suitable for thermal disproportionation and recovery of copper. It is shown that impure copper sulfites are invariably precipitated from impure solutions and that, under laboratory conditions, higher purity copper powders are produced by redissolution of the crude sulfite followed by thermal disproportionation, than by direct autoclaving of the crude sulfite. Some notional overall processes based on these observations are suggested. EXPERIMENTAL Copper(II) sulfate solutions and impure solutions containing iron, magnesium and zinc were prepared from their hydrated sulfate salts (Analar). Ammonium sulfite solution (3.2 M) was prepared by passing SO2 into 50% concentrated ammonia solution until the pH was 7. It was subsequently analysed for ammonia by Kjeldahl's method and for sulfite by titration against standard KMnO4 solution. Solutions of sodium sulfite (1.82 M) and bisulfite (1.8 M) were prepared by passing SO2 into standard solutions of NaOH until the pH was 7 and 2 respectively; they were similarly analysed for sulfite ion. The solubility of Na2SO3 in water at 25 ° was found to be about 1.8 M. Chevreul's salt was precipitated from copper(II) sulfate solutions (500 ml) by slowly adding known volumes of stock solutions of (NH4)2SO3, Na2SO3 or

243

Na(HSO3)2, to well-stirred solutions of CuSO4 maintained at fixed temperatures in a water bath. It was necessary to ensure that the molar ratio of sulfite: Cu 2÷ was less than 1.5:1 to obtain Chevreul's salt, or else double salts like copper ammonium sulfites were produced. At 25 ° the initial precipitate was yellow-brown and gelatinous but this slowly crystallised into well-defined brick-red crystals of Cu2SO3CuSO3" 2H20 on standing. At temperatures of 50 ° and above, the precipitation of Chevreul's salt was complete in less than 2 minutes. Copper sulfites were analysed for copper by digesting with H2SO4/HNO3 to dryness and titrating against standard KI/Na2S203. X-ray diffraction pat~ terns of the precipitated sulfites conformed to the ASTM index for Chevreul's salt. Iron, nickel and magnesium were estimated by atomic absorption spectroscopy. The stoichiometry and kinetics of the reaction of Chevreul's salt with CuSO4 in acetonitrfle--water were determined from the analysis of total Cu, Cu ÷, 802 and H2SO4. Total copper was analysed by digestion with H2SO4/HNO3 to dryness, followed by titration with standard KI/Na2S203. The total Cu ÷ and SO2 was determined by titration with standard KMnO4. Sulfurous acid was analysed by distillation of SO2 from acidified aliquots into excess KMnO4 and back titration of the distillate with standard FeSO4. Concentrations of Cu ÷ were therefore obtained by difference and confirmed semi-quantitatively by distilling acetonitrile from acidified solutions and weighing the copper powder produced. Sulfuric acid was estimated by titrating aliquots, from which SO2 had been distilled, with standard NaOH solution to a pH of 4.5, where Cu(OH}2 begins to precipitate. Solutions of copper(I) sulfate were produced by stirring at 600 rpm a mixture of Chevreul's salt, copper(II} sulfate and acetonitrile in a sealed, water-jacketed, 1-1itre glass reaction vessel. For optimum rate of reaction, the acetonitrile was added slowly over five minutes to a stirred slurry of Chevreul's salt and an aqueous copper(II) sulfate solution at pH 2--3. All solutions containing copper(I) sulfate were kept under an atmosphere of nitrogen to prevent undue oxidation of Cu ÷ to Cu 2÷ by air. For a continuous recycling process, it is necessary to neutralise recirculating acidic copper(II) sulfate solutions prior to reaction with Chevreul's salt to prevent a build up of H2SO4. Tests confirmed that calcium carbonate was a suitable base. A hot solution of copper(II) sulfate at 70 °, containing Cu 2÷ (~60 g 1-1) and H2SO4, reacted with CaCO3 to give readily filterable CaSO4- 2H20. After washing the gypsum with three portions of hot water, the gypsum contained 1.5% Cu. The solubility of calcium in the hot copper(II) sulfate solution was 400 ppm but this dropped to 41 ppm after addition of acetonitrile (30% v/v} and Chevreul's salt. When the filtered copper(I) sulfate solution was thermally disproportionated, calcium remained in the aqueous 'bottoms' solution and the precipitated copper powder contained no detectable calcium (<3 ppm). Copper powders were precipitated by distilling solutions of copper(I) sulfate

244 (initially pH 2--3) at ~ 6 0 ° and 200 m m Hg in a hot water-jacketed reaction vessel, equipped with stirrer and condenser. Steam was injected beneath the solution through a steam lance to assist distillation of volatile acetonitrile and to make up for water carried over with the acetonitrile vapour. Solutions were slowly stirred during disproportionation to keep the copper powder discrete and friable. Polyacrylamide and other proprietary addition agents prevented agglomeration of the copper powder during disproportionation. Discrete microcrystalline copper powder was readily washed with water to remove copper(II) sulfate and other impurities. It was analysed by digestion with HNO3/H2SO4 followed by atomic absorption analysis of a 5% copper solution. Typically, powders precipitated from solutions containing 10 g 1Fe :+, Zn 2+, Ni ~+, or Mg 2+, contained < 1 0 ppm of the respective impurity metal. Copper powders were also produced by directly autoclaving solutions of copper(II) sulfate (500 ml) and 3.2 M (NH4)2SO3 to which polyacrylamide was added. The mixture was charged into a 1-1itre Autoclave Engineers Ltd. Titanium Autoclave fitted with a baffle and stirred at 1200 rpm. After 2 hours at 150 °, the autoclave was cooled and discharged to give a slurry o f copper powder in a solution of (NH4)2SO4 containing some SO2. The copper powders were analysed as above. RESULTS AND DISCUSSION

1. Precipitation of copper sulfites (a) From concentrated (0.5--1 M) copper(II) sulfate solutions With careful control of pH and of proportions of reactants, very pure Chevreul's salt can be precipitated from impure solutions of CuSO4 by addition of solutions ofjVlg(HSO3)2. 3CUSO4 + 4Mg(HSO3)2 -+ Cu2SO3CuSO3"2H20 + 4MgSO4 + 5SO2 + 2H20

(11)

The procedure has been described by Esdaile [5] and his work, together with some other known chemistry based on the solubilities of certain sulfites, bisulfites and sulfates, has been confirmed by our experiments. The m e t h o d is suited to continuous operation where the SO2 is available as a hot, dilute, contaminated, stack gas. In the work by Esdaile [ 5], magnesium bisulfite is regenerated from the magnesium sulfate produced in reaction (11) by contacting with Ca(HSO3)2. solutions, which in turn are produced by absorbing SO2 in lime or limestone slurries (eqn. (12)). MgSO4 + Ca(HSO3)2 -~ CaSO4-2H20 + Mg(HSO3)2

(12)

The simple way of precipitating copper sulfites is to add a m m o n i u m sulfite to the copper sulfate solution, or to add NH3 and then SO2 to precipitate copper sulfites [11] as in eqn. (3). This requires a source of pure SO2. Ammonia is regenerated from the separated ammonium sulfate solution by a con-

245 v e n t i o n a l t r e a t m e n t w i t h lime or, in s o m e areas, t h e a m m o n i u m sulfate m i g h t be r e c o v e r e d and sold f o r a s a t i s f a c t o r y r e t u r n . Clearly a v a r i e t y o f o p t i o n s are o p e n , b u t all involve k n o w n t e c h n o l o g y , w h i c h is developing r a p i d l y because o f t h e n e e d t o scrub SO2 f r o m stack gases, b e t h e y f r o m smelters or coal-burning utilities [ 1 7 ] . T h e c h o i c e o f soluble sulfite f o r c o p p e r sulfite p r e c i p i t a t i o n will d e p e n d o n f a c t o r s w h i c h d i f f e r greatly at d i f f e r e n t locations, e.g. selling price o f a m m o n i u m sulfate o r c o s t o f suitable base. T h e e f f e c t o f using e i t h e r bisulfite o r sulfite o r using s o d i u m or a m m o n i u m salts was theref o r e investigated. T h e results o f a d d i n g solutions o f NaHSO3 or (NH4)2SO3 t o i m p u r e solutions with p r e c i p i t a t i o n o f Chevreul's salt are s u m m a r i s e d in Fig. 2. M u c h o f t h e same principles w o u l d a p p l y t o p r e c i p i t a t i o n b y o t h e r salts o f sulfite o r bisulfite ion, e.g. Ni(HSO3)2, Mg(HSO3)2 and Na2SO3,*whose sulfates, as well as bisulfates, are soluble. I00

I

I

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I

Cu

:p,~ o* O

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•

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pptd. as sulphite

• 1I iI ,

• Zn

II / / I /

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3.0

molar ratio

- -

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4O

CNH~SO3 No HSO3 : Cu504

Fig. 2. Precipitation of Chevreul's salt and other sulfites from impure solutions of copper(II) sulfate by addition of solutions of ammonium sulfite or sodium bisulfite. Effect of moles of sulfite on recovery and purity of Chevreul's salt (Ref. [13]). Conditions: 75°C, 15 minutes. Key: ®, 0.5 M CuSO 4 containing 0.2 M Fe 2÷ precipitated lJy NaHSO3; 0 1.0 M CuSO 4 containing 0.2 M Fe 2+, 0.1 M Zn 2÷ and 0.2 M Mg2÷ precipitated by (NH4)2SOs; e, 0.5 M CuSO4 precipitated by (NH4)2SO3; Y, 0.08 M CuSO 4 precipitated by (NH4)2SO3; 9, Zn; ~Tv, Fe;~, Mg.

246 TABLE 1 S o l u b i l i t y o f s o m e metal s u l f i t e s a at 25 ° (in g m o l 1-1)

Cu~SO3CuSO3.2H20 b Cu2SO 3 - H 2 0 CaSO 3 ZnSO3 FeSO~ NiSO3 MgSO3

pH 2

pH 3

pH 4

pH 5

0.015 0.09 0.20 ~ 1.0 ----

0.01 0.03 0.04 0.10 ~1.0 ---

<0.01 <0.01 <0.01 0.04 0.20 /> 1.0 /> 1.0

---0.03 ~0.02 0.20 0.40

aRef. [19]. b A t 75 ° .

At 75°C, by adding two moles of NaHSO3 per mole of CuSO4, according to the stoichiometry of eqn. (2) only about 70% of the copper is precipitated as Chevreul's salt from a 0.5 M solution of CuSO4 containing 0.2 M ir.on(II) sulfate (Fig. 2). Only 5% of the iron is precipitated with the Chevreul's salt and the final pH is 1.5. With three moles of NaHSO3 per mole of CuSO4, the pH rises to about 3 and 92% of the copper is precipitated. However, 45% of the iron is precipitated also. The precipitation of copper, iron and zinc sulfites is pH dependent (Table 1). Thus there may be advantages in precipitating copper from a b o u t 0.5 M solutions in two stages; the first at pH 1.6 with bisulfite salts, to precipitate relatively pure Chevreul's salt containing at least 70% o f the copper, the second at pH 4 to precipitate all b u t 1--2% of the original soluble copper as crude Chevreul's salt. The overall stoichiometry for precipitation requires 8 moles of NaHSO3 per three moles of CuSO4 if all the acid is neutralised (eqn. (2)). The results of carrying o u t precipitation with ammonium sulfite, rather than NaHSO3, are summarised in Fig. 2 also. Much the same amount of Cu 2÷ is precipitated as Chevreul's salt, using half the number of moles o f (NH4)2SO3 compared with NaHSO3. This is because it is the sulfite ion rather than SO2 that is the reductant; therefore the addition of SO2 to convert sulfite to bisulfite plays no part in the reaction and is evolved from the barren solution. The pH of the final solution, and hence recovery of Chevreul's salt, is similar in both cases because the pH is determined by the H2SO4 produced (eqns. (2) and (3)). As shown in Fig. 2, the recovery of copper as sulfite is also similar from dilute or concentrated solutions. For solutions of CuSO4, between 0.08 and 1.0 M, the percentage of copper precipitated is independent of both the initial concentration of CuSO4 and of the temperature in the range of 65--90°C. When excess NaHSO3 or (NH4)2SO 3 is used, or when base is added to bring the pH to 4--5, over 95% of the copper is precipitated but, as expected from Table 1, other salts such as iron(II) and zinc sulfites, co-precipitate with Chevreul's salt. Double salts analysing as Cu2SO3-MSO3 precipitate at pH > 4 when 1.5 moles of sulfite ion per mole of CuSO4 are used.

247

(b) Precipitation of Chevreul's salt from dilute (0. 02--0.1 M) solutions of CuS04 Streams Containing <0.1 M copper as copper(II) sulfate are c o m m o n in the copper industry. The low solubility of copper sulfites at pH > 3 in water and their ready conversion to pure particulate copper by dissolution in acetonitrile--water followed by thermal disproportionation, offers an alternative to cementation, to direct electrowinning with special cathodes or to solvent extraction--electrowinning, for winning pure copper from dilute copper solutions. The results of adding soluble sulfite salt solutions to dilute copper(II) sulfate solutions at 25 ° are shown-in Fig. 3. The resulting copper sulfites conrain 49% copper, which suggests salts like Cu2SO3CuSO3" 2H20 and (CuOH)2Cu2SO3" 5CuSO3.4H20, but Chevreul's salt is the only salt readily identified by X-ray diffraction. Although 75 ° is an optimum temperature for Chevreul's salt formation, ambient temperatures are the most desirable, in terms of energy and process costs, for treating large volumes of dilute copper(II) solutions. As shown in Fig. 3, up to 95% of the copper is precipitated at 25 ° at pH 5 from 0.08 M solutions of CuSO4 when solutions of Na2SO3 or (NH4)2SO3 are added. The

% Cu pptd as Sulfite

Ideol /

(Sfoichiometric]

//~/~'~ ~8 /

0

o

o'.5

v

l!O

~

/!~

~

rNH~SO,

I

z.o

m

mole rotlo SO~3 Cu t÷ Fig. 3. Precipitation o f c o p p e r sulfites f r o m dilute CuSO4 solution at 25 ° using (NH4)2SO 3 or Na2SO 3 solution. Key: CuS04, 0.08 M at pH 4; × , using (NH4)2SO3 (3.2 M); -, using Na2SO3 (1.8 M); o , , after a d d i t i o n o f N H 3 or N a O H to adjust pH to 5; I, red crystals o f Cu2SO3CuSO 3 - 2 H 2 0 (Cu = 47--49%) precipitated using (NH4)~SO 3. Orange crystals of Cu2(OH)~Cu2SO3 • 5CuSO 3 - 4 H 2 0 (Cu ffi 46--50%; ref. [8] ) precipitated using Na2SO3; II, fawn crystals o f Cu2SO3(NH4)2SO 3 (Cu = 39%) or Cu,(NH4)3(S03) , • 1 2 H 2 0 (Cu = 29%) or Cu2SO3Na2SO 3 (Cu = 38%) are precipitated.

248

nature of the copper sulfite depends on the precise pH and the temperature, as well as the molar ratio of soluble sulfite to copper(II) sulfate. At 25°C, a gelatinous suspension is formed instantly when Na2SO3 or (NH4)2SO3 is added to dilute CuSO4; b u t after 2 hours this precipitate reverts to a crystalline, readily filterable copper sulfite (cf. eqn. (3)). The pH of the solution is usually between 3 and 3.5. As noted, it is necessary to bring the pH to about 5 by addition of excess soluble sulfite salt or other base, to achieve maximum precipitation of copper as copper sulfite from dilute solutions of CuSO4. It was found that sodium sulfite is more effective than the same molar proportion of ammonium sulfite at precipitating copper from dilute CuSO4 solutions (Fig. 3). When 1.33 moles SOl- were added per mole Cu 2*, 90% of the copper precipitated using Na2SO3, whereas 70% precipitated using (NH4)2SO3. This could be because sulfite ion has a hig_her activity, being less ion-paired with sodium ion than with ammonium ion, or because the pH of the final solution is slightly higher with sodium than with ammonium salts. In summary, a proportion of between 1.2--1.4 moles of sulfite ion per mole of CuSO4 is recommended for precipitating copper sulfite from dilute CuSO4 solutions at 25°C, with pH adjusted to a b o u t 5. Some purification is achieved by washing with sulfurous acid at pH 2--3 after the precipitate has crystallised to Chevreul's salt. This removes any co-precipitated FeSO3 or ZnSO3 which is soluble at this pH (Table 1), unlike Chevreul's salt. Unlike that at 25°C, a readily filterable crystalline Chevreul's salt is precipitated instantaneously at 75°C from dilute (0.1 M) CuSO4 solutions by 0.1 M sodium or ammonium sulfite solutions, as shown in Fig. 2. Precipitation of copper at 75°is more effective (90% precipitated at pH 3) than at 25°C, but heating dilute solutions to 75 ° may n o t always be justified because of heating costs.

(c) Separation o f copper and nickel As an example of a specific application of copper sulfite precipitation at a lower pH than most other metal sulfites, we describe a method of separating copper and nickel from a mixture of their sulfates in solution. Such mixtures are c o m m o n in the nickel processing industry, b u t it is difficult to apply conventional methods like H2S precipitation of CuS or solvent extraction of Cu 2÷ to concentrated copper solutions. We therefore use nickel bisulfite to precipitate the copper from solution. Nickel sulfite is only slightly soluble in water at pH > 4 . 5 to give a solution containing at least 0.5 M nickel (Table 1). Nickel bisulfite is formed b y mixing calcium bisulfite with nickel sulfate. Gypsum is discarded after washing with H2SO3 to recover any nickel. NiSO4 + Ca(HSO3)2

H20 -,

CaSO4" 2H20 + Ni(HSO3)2

(13)

A solution of NiSO3/Ni(HSO3)2 containing 0.9 M SO2 and 0.5 M Ni 2÷ readily

249

precipitated 80--90% of copper as Chevreurs salt from a solution containing 0.5 M NiSO4 and 0.5 M CuSO4 at 60--80 ° at pH 2.0--2.7. Details of the precipitation of copper as Chevreul's salt at 70°C from aqueous solutions containing 0.5 M CuSO4 and 0.5 M NiSO4 are given in Table 2. Either equimolar or 50% excess proportions of nickel bisulfite were added to precipitate the Chevreurs salt. With equimolar proportions, 68% of the copper was precipitated at a pH of 2.0; with a 50% excess of Ni(HSO3)2 84% of the copper was precipitated as Chevreul's salt at pH 1.5--1.9. This salt contained ~0.5% nickel. When the precipitated Chevreul's salt was removed and the pH of the solution raised by adding nickel carbonate, up to 99% of the copper was precipitated, but the second precipitate contained between 1 and 12% nickel in the pH range 2.5-3.8. Only part of this nickel could be removed by washing with sulfurous acid at pH 1.8. However, as shown in Table 2, ff precipitation of Chevreul's salt from 0.5 M CuSO4 and 0.5 M NiSO4 with equimolar Ni(HSO3)2 is carried out with pH adjustment to 2.9, the overall Chevreul's salt (i.e. combination of the three stages) contains ~ 1--3% nickel and represents a precipitation of 96--98% of the copper. This purity is quite satisfactory for subsequent conTABLE 2 Yield a n d p u r i t y o f c o p p e r sulfite p r e c i p i t a t e d f r o m N i S O 4 / C u S O , s o l u t i o n s a using s o l u t i o n s o f Ni(HSO3)2 b a t 75°C Moles SO~ per m o l e Cu 2÷

Stage

1.0

1 2 (pH adjust) 3 (pH adjust)

2.0 2.7 c 2.9 c

68 84 98

1.0

1 2 3 4 1 2 1 2

2.5 5.9 c 3.8 e 1.8 e 1.5 2.0 c 1.9 2.4 c

93 99 99 94 f 80 93 84 96

1.5 1.5

(pH adjust) (SO2 w a s h ) (SO2 w a s h ) (pH adjust) (pH adjust)

Final pH

% Cu p p t d . from solution

C h e v r e u l ' s salt % Cu

49.6 47.7 d 41 d Overall 48.0 --34 39 d -45 -45

% Ni 0.4 1.0 d 5,9 d 1.3 --11.6 6.3 d -0.2 -1.7

a Initial s o l u t i o n c o n t a i n e d 0.5 M NiSO4 a n d 0.5 M CuSO4. b T h e s o l u t i o n c o n t a i n e d 0 . 7 5 M SO~ as 0 . 3 7 5 M Ni(HSO3) 2 a t p H 3.5 cNiCO~ was a d d e d t o raise t h e pH. ' dpurity of incremented copper precipitate after removing the previous precipitate and a d j u s t i n g t h e p H t o t h e value s h o w n . e T h e p r e c i p i t a t e d c o p p e r sulfite was w a s h e d w i t h s u l f u r o u s acid t o dissolve excess NiCO s a n d l o w e r t h e p H f r o m 5.9 t o t h e value s h o w n . f R e c o v e r y o f c o p p e r varies w i t h p u l p d e n s i t y b e c a u s e s o l u b i l i t y o f c o p p e r sulfite in this s o l u t i o n is a b o u t 0 . 0 0 5 M as Cu.

250 NoOH (

>

" SOZ

] Cu 2. J _ -,

I

Ni{HS03)2 _

CHEVi~3 i SALT

NiL" I PPTN IM-SE~RA~NI

I =

I

=

~ ]CoS03ZH20 i BISULPHITE GIENERATION

I.s~" ! SEF~RAT~N

Fig. 4. Method of separating copper and nickel from solution by selective precipitation of Chevreul's salt using solutions of Ni(HSO3)2 produced via exchange with Ca(HSO3)2. version to pure copper powder via thermal disproportionation since nickel is left in solution. A single-stage precipitation with 50% excess Ni(HSO~)2 followed by pH adjustment to pH 2--2.5 gave 0.2--1.7% nickel in the Chevreul's salt with 93--96% copper recovery. The remaining copper would be recovered from solution by cementation onto iron or nickel. These reactions suggest the m e t h o d of Fig. 4 for selectively separating reasonably pure solid copper sulfites from a mixture of CuSO4 and NiSO4 in water. As discussed later in this paper, the separated copper sulfites are readily converted to copper by dissolution in acetonitrile--water, followed by thermal disproportionation. The copper-free nickel sulfate solution can be converted to nickel by conventional procedures, e.g. electrowinning, after conventional removal (e.g. cementation) of the small amount of copper (<0.03 M) which is not precipitated as sulfite.

2. Dissolution of copper sulfites in acetonitrile--water (a) Effect of added copper(II) sulfate Copper sulfites are reduced to copper(I) salts and dissolve readily in water, provided that there is at least 4 moles of acetonitrile per mole of potential copper(I) ion present. In the absence of acetonitrile, the solubility of Chevreul's salt at pH 2, is about 0.015 g mol 1-' at 25 ° (Table 1); but a 0.5 M Cu ÷ solution, as copper(I) sulfite and sulfate, (eqn. (14)) is formed in the presence of acetonitrile. 2Cu2SO3"CuSO3"2H20

AN/H20 -*

Cu2SO4 + 2CuHSO3 + Cu2SO3 + 3H20 (14)

If copper(II) sulfate is added to copper sulfites in the presence of acetonitrile, it is reduced by the sulfite and bisulfite ions to copper(1) sulfate (e~n. (15)) provided that the pH is above 1, so that much SO2 is present as SO3- or HSO;.

251 Undissociated sulfurous acid does n o t reduce CuSO4. Reaction (15) gives up to 2.4 M Cu ÷ solution as copper(I) sulfate. 5CuSO4 + 3Cu2SO3-CuSO3-2H20 -~ 7Cu~SO4 + 2H2SO4 + 2SO2

(15)

The solubility of copper(II) sulfate in water--acetonitrile mixtures decreases with increasing concentrations of acetonitrile (e.g. ~ 0 . 2 M Cu 2÷ in 30% v/v AN/H20 at 50 °) so that care must be taken n o t to precipitate copper(II) sulfate, when mixing Chevreul's salt, acetonitrile, water and copper(II) sulfate, if a fast reaction is required. Precipitated CuSO4- 5H20 redissolves slowly, so it is preferable to add CuSO4 solution in water to Chevreul's salt, then slowly add the acetonitrile, with stirring, to the slurry. This avoids precipitation o f CuSO4- 5H20 and rapidly generates Cu2SO4 solutions.

(b ) Rate of dissolution of Chevreul's salt in Cu2+/AN/H20 The rate o f reaction (15) between excess solid CuSO4" 5H20 and excess solid Chevreul's salt in 40% v/v acetonitrile--water at 25°C was followed b y measuring with time, the total a m o u n t copper entering solution. The term "excess" refers to the ability of the solution to dissolve CuSO4 and to stabilise Cu2SO4 Both depend on the proportion of acetonitrile present. Separate measurements o f solubility o f CuSO4- 5H20 and Chevreul's salt showed that, in the absence of reaction between them, one would expect 0.07 M copper as copper(II) and copper(I) in a saturated solution above a mixture o f the t w o salts at 25°C. At 25°C, reaction (15) had a half-life of 15 minutes and was 90% complete in 45 minutes. At completion, 1.5 M total soluble copper was in solution. On raising the temperature to 50°C, the concentration of copper in solution was 2 M at completion of reaction (15) and at 65°C it was 2.5 M.-Although the reaction was more than 90% complete in 30 minutes at both 25 ° and 65 °, higher temperatures drive the reaction with SO~- to the right (Fig. 5).

(c) Stoichiometry of reaction (15) and effect of acetonitrile concentration The soluble products from reaction (15), in equilibrium with the solid reactants, were determined at 50 ° and 65 °. The solution was analysed after 2 hours reaction time for sulfuric acid, sulfurous acid, copper(I) sulfate and total soluble copper. Details are shown in Fig. 6. The total copper (Cu :÷ + Cu 2*) is effectively constant at 2 M at 50 ° and 2.4 M at 65 ° in the range 20--50% v/v acetonitrile--water. However, the concentration of Cu ÷ .ncreases markedly with increasing concentration of acetonitrile and temperature, whilst the concentration of Cu 2÷ decreases. The concentration of Cu 2* is determined by the solubility of CuSO4.5H20, which decreases with increasing concentration of acetonitrile. As the acetonitrile concentration increases, there is also a corresponding increase in the concentration of sulfuric acid but little change in the concentration o f SO2. The reason for the change in stoichiometry is that the reduction o f Cu 2* to Cu ÷ (by SO2) is favoured by increasing acetonitrile and temperature (eqn. 16) [14].

252 2.5

•

65°

2.0" Total copper

M. 1.0

~

•

50 °

o --

-- C'

o

25°

0.5'

o

~

~ Time-

~

hours

Fig. 5. E f f e c t of temperature on the rate and e x t e n t of reaction between c o p p e r ( I I ) sulfate

and Chevreul's salt in 40% v/v acetonitrile. 2.s!

$ ~,--65"

I

I

2.01 ~

1.5

i

-0.4M.

v

H2SO,

1.0-

& SO2

Hcu+, 0.5-

01

0.2

2so4

,b

3b ~o

5b

v,/v C H 3 C N

Fig. 6. E f f e c t o f temperature and c o n c e n t r a t i o n o f acetonitrile o n the reaction b e t w e e n e x c e s s c o p p e r ( I I ) sulfate and e x c e s s Chevreul's salt in a q u e o u s acetonitrile. A n a l y s i s o f final s o l u t i o n after 2 hours reaction. K e y : full line at 65°; dashed line at 50 °.

253

2Cu804 + 802 + 2H20

AN/H20 -~ Cu2804 + 2H2804

(16)

Chevreul's salt reacts with this acid at pH < 2 to produce further Cu2804 and SO2, Cu2SO3CuSO3"2H20

+ 2H2SO4 -~ Cu2SO4 + CuSO4 + 2SO2 + 4H~O

(17)

but the reaction does not go to completion because SO~- buffers the solution. The combination of eqns. (16) and (17), as they reach equilibrium, is thus represented by eqn. (15). The stoichiometry of the reaction in 4 0 % v/v A N / H 2 0 at 50 ° was established by studying the effect of varying the molar proportions of copper(II) sulfate and Chevreul's salt from 6:1 through to 0:1 (Table 3). The total copper added was maintained constant (1.57 moles) as was the volume of solution. Only that proportion corresponding to the stoichiometry of the reaction completely dissolved and reacted; for all other combinations studied either excess C u S O 4 . 5 H 2 0 or Chevreul's salt was present at the completion of reaction. Thus the optimum concentration of Cu + was obtained with a molar proportion of about 5CuSO4 to 3Cu2SO3CuSO3- 2H20. It is 0f.practical value that the highest concentration of Cu ÷ which was produced by reaction of C u S O 4 . 5 H ~ O (excess) with Chevreul's salt (excess) was 2.4 M Cu + as Cu2SO4. This solution was produced in 50% v/v A N at 65 ° and also contained 0.09 M SO2 and 0.40 M H2SO4.

(d) Effect o f varying molar ratios o f Chevreul's salt and C u S 0 4 . 5 H 2 0 on reaction products With a proportion of 3:1 or more CuSO4" 5H20 to Chevreul's salt, the concentration of sulfurous acid at completion of reaction is low (~<0.05 M) and the sulfuric acid concentration is high (0.20 M), as required by eqns. (16) TABLE 3 E f f e c t o f m o l a r ratio o f c o p p e r ( I I ) sulfate a n d Chevreul's salt o n t h e s o l u t i o n c o m p o s i t i o n a f t e r r e a c t i o n in 40% A N / H 2 0 a t 50 ° for 2 h o u r s Molar r a t i o a CuSO 4 : Cu~SO3CuSO 3 • 2 H 2 0

Total copper

Cu ÷ (M)

H~SO4 (M)

0.52 1.33 1.51 1.27 0.86

0.03 0.10 0.24 0.22 0.20

SO2 (M)

(M) 0:1 b 1:1 b 3:2 3:1 c 6:1 c

0.52 1.40 1.57 1.50 1.08

0.03 0.15 0.12 0.05 <0.005

a T o t a l c o p p e r a d d e d as C u S O 4 . 5 H 2 0 a n d Cu2SO3CuSO 3 • 2H20 = 1.57 m o l e s (100 g 1-2) in all reactions. b S o m e CuSO3CuSO ~ • 2 H 2 0 d o e s n o t r e a c t or dissolve. c S o m e C u S 0 4 . 5 H 2 0 d o e s n o t r e a c t o r dissolve.

254 and (17). Thus as the proportion of Chevreul's salt to CuSO4.5H20 increases, the proportion of sulfurous acid increases and that of H2SO4 decreases because the sulfite reacts with the acid. Clearly, using a ratio of 3 moles copper sulfate per mole of Chevreul's salt is the most efficient way of generating a Cu2SO4 solution, high in Cu ÷ and low in sulfurous acid. To summarize, an important point shown in Table 3; by varying the ratio of copper sulfate to Chevreul's salt in 40% v/v acetonitrile-water, the amount of sulfurous acid produced from one litre of solution containing 1.57 moles of total copper can vary from < 0 . 0 0 5 M to 0.15 M. The amount of H2SO4 produced varies from 0.24 M to 0.07 M. In a continuous process whereby solutions are recycled, it would be necessary to remove SO2 or H2SO4. It should be noted that calcium carbonate is a base which both neutralises H2SO4 (eqn. (17)) and adjusts the pH of the solution to 3 allowing SO2 to reduce Cu 2÷ to Cu ÷ (eqn. (16)). Experiments established that calcium sulfate is readily deposited as gypsum in acetonitrile-water and that calcium does n o t contaminate the copper recovered in subsequent steps (see Experimental).

3. Recovery and purity of copper Comparison of copper produced from impure copper sulfites by autoclaving or by redissolution and thermal disproportionation The most direct m e t h o d of obtaining copper is to add ammonium sulfite (directly produced from an SO2 scrubber) to a copper(II) sulfate solution. The solution is then charged and heated in an autoclave without the need to isolate the intermediate copper sulfite. However, this approach does not allow co-precipitated impurities to be removed from the system and they may appear with the copper after autoclaving. In our experience, it is more difficult to control the size, shape and degree of agglomeration of the copper particles, which is important if high purity copper particles are to be precipitated from impure solutions. Agglomerated copper particles trap or occlude solution containing water soluble impurities and this is not able to be washed free from the copper. It is our experience that copper powders produced in a stirred autoclave are well rounded particles resulting from crystal growth and attrition, b u t microscopic examination of cross sections of these particles often show cavities which trap the solution. By contrast, copper powders produced by thermal disproportionation of Cu2SO4 solution can be more readily obtained as smaller more discrete micro-crystalline particles which can be easily washed free of impurities (Fig. 7). A comparison of the proposed method of obtaining copper powder with this direct autoclave approach was carried out using synthetic solutions of impure copper(II) sulfate containing water soluble impurities and ammonium sulfite (Table 4). In one set of experiments, the two solutions were charged into an autoclave, in such proportions as to give Cu2SO3(NH4)2SO3 as an intermediate, and were thermally decomposed directly. In the other set of

255

Fig. 7. Scanning electron micrograph of copper powder produced by thermal disproportionation of Cu2SO4 solution at pH 2.8. No additives. Magnification: × 1250.

experiments, crude Chevreurs salt was isolated from the solutions after mixing, then subsequently dissolved in CuSOJAN/H20 and thermally disproportionated as described above. One experiment used a pure solution of CuSO4/AN/I-I20 to redissolve the crude Chevreul's salt, the other used a synthetic impure solution of CuSO4/AN/H20 in order to test the purity of the copper powder subsequently precipitated by disproportionation. The results in Table 4 clearly show that copper powders produced via thermal disproportionation of impure Cu2SO4 solution are purer than powders produced via thermal decomposition of copper sulfites in impure solutions under the conditions of our experiments. The reason would appear to reside in the particle size and shape. Particles from the autoclave were well rounded with occlusions, whereas particles from distillation were~smaller, more discrete and microcrystalline. It is recognised that additives, such as polyacrylamide, chloride, ion, thiourea, etc., play an important role in determining the particle size and shape of nickel powders produced by hydrogen reduction in an autoclave [15]. However, it is not possible to extrapolate results from nickel powder production to copper powder production because the kinetics and mechanism of crystallisation differ. Whereas nickel powders usually require nucleation and seeding, copper crystals are nucleated spontaneously in solution as well as upon active copper surfaces [16]. Nevertheless, it would appear that the role of polyacrylamides in preventing agglomeration

256

TABLE 4 Recovery of co pper from impure CuSO, solution by r e d u c t i o n with (NH,),SO 3 Comparison of copper recovery and copper powder p uri t y by direct a ut oc l a vi ng of a CuSO4-ammonium sulfite m i x t u r e k at 1 5 0 ° a n d by p r e c i p i t a t i o n o f C h e v r e u l ' s salt, dissolut i o n in C u S O , / A N / H 2 0 and thermal d i s p r o p o r t i o n a t i o n , all at ~ 75 °. (a) Autoclave a Solution analysis

Cu Mg Zn Fe pH

Initialb (g l - ' )

Final e ( g l ')

32 5 5 5 3.0

0.35 4.0 4.0 4.0 1.8 d

% Cu pptd: Copper product:

Copper powder (ppm)

-5

45 30

Solution analysis Initialb (g 1-')

Final c ( g l ')

64 4.1 4.1 4.1

1.5 2.7 2.7 2.7 1.5

Well r o u n d e d small granules, size 50--150 ~m

Impure leach solution

% Cu pptd: Copper product:

solution f

Impure CuSOJAN/H~O solution g Copper powder m (ppm)

Solution analysis . . . . Final h Initialb (gJ ') ( g l ')

64 5 5 10 3.0

~- 5 <5 -~. 10

20 24 9 9 3

~90% k

18 250 370

Well rounded granules and agglomerates, average size 75--200 ~m

Solution analysis -Initialb Final c ( g l ') ( g l ') 5.2 3.6 2.6 4.0 2.8

--

97% l

99% l

(b) Via Chevreul's salt e and d i s p r o p o r t i o n a t i o n of Cu2SO4

Cu Mg Zn Fe pH

Copper pow de r (ppm)

22 i 24 9 9 2.0

Copper powder n (ppm)

7 4 <5

~90% j,!

Discrete fine grained microcrystalline, size mostly ( 53 a m

90%£ ! Microcrystalline particles, size m o s t l y < - 5 3 um

aBy ad dition of 125 ml or 250 ml 3 . 2 M (NH,)2SO3 to 500 ml of 0.5 or 1 M CuSO~ solution and heating at 150 ° for 2 hours. Mole ratio (NH4)2SO3:Cu ~÷ is 1.6:1. bContalns 100 p p m polyacrylamide. c Solution diluted with respect to initial solution by a ddi t i on of (NH,)2SO , solution, dContained 0.24 moles H:SO, buffered with (NH4),SO . eBy a d d i t i o n of 200 ml 3.2 M (NH4)2SO 3 to 600 ml 1 M CuSO4. f Prepared by dissolution of Cu2SO3CuSO:, • 2H~O in CuSO4/AN/H20 (see text). g I m p u r e so lution used to dissolve Cu2SO3CuSO 3 • 2H20, also c ont a i ne d 20% v/v AN/H20. Ref. [11]. hAfter distillation of AN and r e a d j u s t m e n t to original volume. i Some increase in Cu c o n c e n t r a t i o n due to o x i d a t i o n of Cu + during disproportionation. J For thermal d i s p r o p o r t i o n a t i o n only. Remaining copper as CuSO~ is recycled. kis Chevreul's salt. l As copper powder. m A f t e r dissolution of crude Chevreul's salt in pure CuSO4/AN/H20 and distillation. nAfter dissolution of crude Chevreul's salt in impure C u S O J A N / H 2 0 s ol ut i on (specified) and distillation.

257

and plating of the metal onto vessels is common to both copper and nickel. Certainly our experience is that less pure copper is produced by thermal disproportionation from impure solutions, if larger particles are produced in the absence of polyacrylamides. Table 4 also shows that the recovery of copper from direct autoclaving of solutions is higher than the recovery via sulfite precipitation. However, sulfite precipitation would be linked with cementation to recover virtuslly all copper from the original CuS04 solution. As described above, a less pure Chevreul’s salt is obtained as the last fraction of copper is precipitated from solution and it may be preferable to recover this remaining copper by cementation. Cement copper, like Chevreul’s salt, reacts readily with CuS04 in AN/H?0 to produce further C&SO+ Thus, in an overall process, the recovery of copper is >99% in both cases. 4. Processes The precipitation of Chevreul’s salt (eqn. (2)), its dissolution in the acetonitrile-water containing copper(I1) sulfate (eqn. (15)), and thermal disproportionation of the resulting solution of C&SO4 .by distillation of acetonitrile, has suggested a roast-leach-disproportionate (RLD) process for obtaining pure copper from chalcopyrite concentrates [ 131 which is summarized by Fig. 8. NH? VENT

1

I

~WkS03_

CNVRNCS

H20_ -

,/ ,

SO2

________

pREc%;ATlON $

SO2

&JH$lzSQ h I

SEWATI)N

Cu29J3cuSO3

I

CuFeS2 -

>

SULPHATING ROAST

/,

ACID ;- - - NEUTRALISATION

---__A

I 4

LIME

BOIL

NH3

REGENERATE

$

SEWRATITION

-

H20

jcusq : BLEED

I GYPSUM

CUSOL 1, Fe203

II

LEACH S/L

SEPARATION CUSOL SOLUTION

I

Fe203

Fig. 8. Roast-leach-disproportionate process for the recovery of copper powder from chalcopyrite via Chevreul’s salt, using a water soluble base to recover SO, (Ref. [ 131). ._ Overall reaction: SCuFeS, + (15/2)0, + 4Ca(OH), + 4H,O + 2Cu + 4CaSO,.2H,O + Fe,O,.

258

The chemistry outlined in this paper indicates that under proper conditions of pH and temperature, this same flow sheet can be applied to mixed c o p p e r nickel or impure copper sulfites or to the recovery of copper from dilute copper sulfate solutions. Details of the thermal disproportionation of Cu2SO4 in acidic acetonitrile-water solutions to give copper and CuSO4 have been given by Parker et al. [14] i Thermal disproportionation involves stripping roughly 50: 50 acetonitrile-water vapour from the solution at 80--95 °, preferably by injection o f wet steam at 100°C. After a few minutes particulate copper commences to precipitate. Precipitation and stripping can be complete in times of less than 30 minutes and the latent heat of vapourization of the acetonitrile--water distillate is less than that of an equivalent weight of water. The distillate can be fractionated to give the 80% v/v acetonitrile--water azeotrope, with return of water to the steam generator. CONCLUSIONS

The application and advantages of the RLD process compared with more conventional, processes have been previously discussed [ 13 ]. Recently, Foster et al. [18] have set o u t and described the process steps in detail a n d have estimated the capital and operating costs for a 50,000 t p y greenfield copper plant. It was concluded that the RLD process required 55% less energy and 30% less capital than modern pyrometallurgical methods. The precipitation of copper from solution as water insoluble copper sulfites offers an alternative m e t h o d to solvent extraction/electrowinning for the recovery of pure copper from impure slightly acidic solutions. It is especially suited to more concentrated solutions (t>0.1 M Cu 2÷) or solutions containing quite high concentrations of Ni 2÷, where the only suitable alternative methods of recovery are electro-~nning or H2 reduction. Copper sulfites precipitate rapidly as readily filterable crystals using a range of water soluble sulfite solutions as may be produced from an SOs scrubber, except at ambient temperatures the kinetics of crystallisation is slow. The recovery and purity o f the sulfite is det~ermined by the mole ratio of sulfite ion to copper(II) ion and the pH of the solution. Copper may be recovered from copper sulfites either by thermal decomposition in an autoclave, or by dissolution with CuSO4 in aqueous acetonitrile and thermal disproportionation of the resulting Cu2SO4 solution. Thermal disproportionation appears to offer a greater control over the purity and physical characteristics of the copper powder than autoclaving and a!lows a further stage to remove impurities. Chevreul's salt dissolves readily in copper(II) sulfate solution containing acetonitrile to give solutions suitable for thermal disproportionation b u t the exact composition of these solutions depends on the relative proportions of copper(I) sulfite, copper(II) sulfate and acetonitrile and on the temperature of the reaction. Thermal disproportionation is a low energy process for recovering copper

259 c o m p a r e d w i t h e l e c t r o w i n n i n g o r H2 r e d u c t i o n , especially w h e n w a s t e s t e a m f r o m w a s t e h e a t boilers o r p o w e r g e n e r a t i o n p l a n t s is u s e d as a h e a t s o u r c e f o r distillation. T h e overall m e t h o d o f r e c o v e r y o f c o p p e r f r o m c o p p e r ( I I ) s u l f a t e s o l u t i o n is t h e r e f o r e a p p r o p r i a t e f o r p r o c e s s e s involving s u l f a t i o n r o a s t e d c h a l c o p y r i t e o r c o p p e r / n i c k e l sulfides, w h e r e b o t h SO2 ( f o r sulfite p r e c i p i t a t i o n ) a n d w a s t e h e a t a n d s t e a m ( f o r t h e r m a l d i s p r o p o r t i o n a t i o n ) are g e n e r a t e d f r o m t h e roaster. ACKNOWLEDGEMENTS We are i n d e b t e d t o t h e West A u s t r a l i a n G o v e r n m e n t f o r financial s u p p o r t f o r this p r o j e c t a n d t o Y.C. S m a r t a n d B.W. Clare f o r t e c h n i c a l a n d a n a l y t i c a l assistance.

REFERENCES 1 Shirts, M.B., Blook, P.A. and McKinney, W.A., Double roast--leach--electrowinning process for chalcopyrite concentrates. U.S. Bureau of Mines, Report 7996, 1974. 2 Whitehead, A.B. and Uric, R.W., Fluidized bed roasting of copper concentrates. Proc. Australas. Inst. Min. Metall., 199 (1961) 51--58. 3 Esdaile, J.D., The sulphating roasting of copper sulphide concentrates. Proc. Australas. Inst. Min. Metall., 227 (1968) 39--44. 4 Thomas, R.O. and Hopkins, D.W., Control chart for sulphation roasting of sulphides. Trans. Inst. Min. Metall. Section C: Mineral Process. Ext. Metall., 83 (1973) C243-245. 5 Esdaile, J.D., Process for recovery of copper, U.S. Patent 3494764 (1970). 6 Palperi, M. and Aaltoner, O., Fluid-bed sulphatizing roasting and leaching at the Outokumpu plant. TMS--AIME meeting 1970, Preprint No. A70-55 (1970) 15. 7 Arbiter, N., Milligan, D. and McClincy, R., Metal production from copper ammine solution with sulfur dioxide. I. Chem. E. Symp. Ser., 42 (1975) 1--9. 8 Arbiter, N. and Milligan, D.A., Reduction of copper ammine solutions to metal with sulfur dioxide. In: Extractive Metallurgy of Copper (Vol. II), Yannopoulos, J.C. and Agarwal, J.C. (Eds.), Met. Soc. A.I.M.E., New York, 1976. 9 Mellor, D.P., Comprehensive treatise on inorganic and theoretical chemistry (Vol. 10), Longmans, London, 1969, pp. 273--278. 10 Hori, S. and Okabe, T., Wet process for manufacturing metallic copper, U.S. Patent 357896 (1971). 11 Chupungo, E.L., Quintos, J.A. and Godinez, W.A., Chemical recovery of metallic copper from sulfate solutions, U.S. Patent 3148051 (1964). 12 Parker, A.J. and Muir, D.M., Copper from copper concentrates via solutions of cuprous sulfate in acetonitrile--water solutions. In: Extractive Metallurgy of Copper (Vol. II), Yannopoulos, J.C. and Agarwal, J.C. (Eds.), Met. Soc. A.I.M.E., New York, 1976; 13 Muir, D.M. and Parker, A.J., Low energy processes for the production of pure copper from crude copper, copper sulphate and chalcopyrite by use of aqueous acetonitrile. Advances in Extractive Metallurgy, Jones, M.J. (Ed.), I.M.M., London, 1977, pp. 191--196. 14 Parker, A.J., Muir, D.M., Sharp, J.H., Waghorne, W.E., Alexander, R. and Giles, D.E., Compositions containing copper salts, their methods of production and the recovery and purification of copper, British patent 1381666 (1975).

260

15 Kunda, W., Evans, D.J.I. and MacKiw, V.N., Effect of addition agents on the properties of nickel powders produced by hydrogen reduction. In: Modern Developments in Powder Metallurgy (Vol. I), Fundamentals and Methods, Hauser, H.H. and Roll, K.H. (Eds.), Plenum Press, N e w York, 1966. 16 Burkin, A.R., The Chemistry of HydrometaUurgical Processes, Spon, London, 1966. 17 Yan, C.J., Evaluating environmental impacts of gas desulfurization processes. Eng. Sci. and Tech., 10 (1976) 54--57. 18 Foster, E.P., Taschler, D.R. and Lowe, D.F., Nitrile metallurgical processes for copper concentrates. J. of Metals, 31(1979)23--28. 19 Esdaile,J.D. and Walters, G.W., Studies of sulfiteprocesses for the treatment of McArthur River ore, C S I R O Australia Report R17, 1969.