Kinetics of dissolution of synthetic heazlewoodite (Ni3S2) in nitric acid solutions

Hydrometallurgy, 14 (1985) 67--81

67

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

KINETICS OF DISSOLUTION OF SYNTHETIC (Ni3S2) I N N I T R I C A C I D S O L U T I O N S

HEAZLEWOODITE

W~ADYSLAWA MULAK

Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroc].aw, Wybrze~e Wyspia~iskiego 27, 50-370 WrocJaw (Poland) (Received March 19, 1984;accepted in revised form November 24, 1984)

ABSTRACT Mulak, W., 1985. Kinetics of dissolution of synthetic heazlewoodite (Ni3S2) in nitric acid solutions. Hydrometallurgy, 14: 67--81. The dissolution rate of heazlewoodite in nitric acid solution has been determined. The effects of nitric acid concentration, temperature, particle size, stirring intensity and addition of Cu ~ ions have been investigated. Solid residues after leaching were examined by SEM, X-ray diffraction and chemical analysis. In the solutions containing less than 2.0 M HNO~, dissolution was observed to be completely inhibited after 30 rain leaching time, and the rate of hydrogen sulphide production was faster than its oxidation to S O and HSO~. In 3 M HNO 3, an abrupt increase in dissolution rate of Ni3S 2 was found. Two different regions of the dissolution of heazlewoodite were observed below and above 50°C. At temperatures below 50°C, the dissolution rate was very slow, even in 3.0 M HNO~ solution, and H2S gas was evolved. Above 50°C, the dissolution rate rapidly increased. Over the temperature interval 60--90°C in 3.0 M HNO~ dissolution followed a linear rate law, and the activation energy was found to be 42.1 kJ tool -~. Most of the oxidized sulphide ion was found in the solution as sulphate. The leaching rate was independent of stirring speed. The rate-controlling step of the Ni~S~ dissolution is the oxidation of hydrogen sulphide to elemental sulphur or sulphate ions on the Ni3S 2 surface. Addition of small amounts of Cu 2÷ ions to the nitric acid acted as catalyst for the dissolution of Ni~S~. Bubbling air through the leach suspension increased the dissolution rate of Ni~S~ in solutions containing less than 2.0 M HNO~.

INTRODUCTION H e a z l e w o o d i t e (Ni3S2) is t h e m a i n n i c k e l s u l p h i d e o f n i c k e l - - c o p p e r m a t t e s . P r e v i o u s s t u d i e s o n a c i d d i s s o l u t i o n o f Ni3S2 h a v e b e e n c o n d u c t e d b o t h in n o n - o x i d a t i v e a n d o x i d a t i v e systems. A g o o d e x a m p l e o f n o n - o x i d a t i v e acid l e a c h i n g is t h e F a l c o n b r i d g e p r o cess, i n w h i c h n i c k e l is d i s s o l v e d s e l e c t i v e l y f r o m a n i c k e l - - c o p p e r s u l p h i d e c o n v e r t e r m a t t e u s i n g s t r o n g h y d r o c h l o r i c a c i d ( T h o r n h i l l e t al., 1 9 7 1 ) . S i n e v e t al. ( 1 9 7 5 ) i n v e s t i g a t e d t h e m e c h a n i s m o f d i s s o l u t i o n o f Ni3S2 i n h y d r o chloric a n d s u l p h u r i c acids u n d e r a t m o s p h e r i c pressure at 90°C. T h e y f o u n d

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68 that millerite (/~-NiS) was formed as an intermediate phase in b o t h leaching solutions. Hydrochloric acid leaching was more rapid than t h a t in sulphuric acid. A fundamental investigation of the non-oxidative dissolution in aqueous acid solutions of nickel sulphides of different stoichiometric compositions, in which a rotating ring
69 some experiments was absorbed in lead acetate and sulphur contents were determined. Leach residues were examined by optical microscopy, X-ray diffraction, scanning electron microscopy and chemical analysis. RESULTS AND DISCUSSION

Effect o f nitric acid concentration To investigate the effect of nitric acid concentration on the rate of heazlewoodite dissolution, experiments were carried out with varying initial concentrations of HNO3 (from 0.5 to 3.0 M) at a constant temperature of 80°C, and particle size < 6 3 tzm. The results are shown in Fig. 1. In the dissolution of Ni3S2 using solutions containing less than 2.0 M of HNO3, retardation of the dissolution rate was observed after 30 min of leaching time. During the whole leaching process H~S gas was evolved. At a concentration of 3.0 M, an abrupt increase in the dissolution rate was observed. The reason for the increase in the dissolution rate at higher HNO3 concentration is the considerable oxidation of sulphide to sulphate ions. The influence of nitric acid concentration on the proportions of various 1.0

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70

forms of sulphidic ions (H2S, SO~- and S 2-) in the residue is shown in Table 1. At concentrations below 2.0 M HNOz, small amounts of sulphide ion are oxidized to sulphate (ca. 7%), whereas in 3.0 M HNO3 the amount is ca. 80%. TABLE 1

Effect o f initial HNO 3 c o n c e n t r a t i o n on the fraction of sulphide ions evolved as H2S , oxidized to SO~- and in solid residue after 2 h dissolution of Ni3S 2 (fraction < 6 3 pro) at 80°C) Initial [ HNOz ] (tool 1-1)

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Fig. 2. E f f e c t o f t e m p e r a t u r e on fraction of nickel e x t r a c t e d f r o m Ni3S ~ in 3.0 M HNO3.

71

Effect of temperature T h e leaching was p e r f o r m e d w i t h i n t h e t e m p e r a t u r e range 30--90°C with an initial nitric acid c o n c e n t r a t i o n o f 3.0 M, at c o n s t a n t stirring speed o f 500 min -1, and particle size < 6 3 p m . T h e dissolution curves are s h o w n in Fig. 2. It has b e e n s h o w n t h a t u n d e r c o n d i t i o n s w h e r e t h e p r o d u c t s o f t h e r e a c t i o n are soluble or, in general, leave t h e surface o f reacting particles, the r e a c t i o n o f particulates can be f o r m a l i s e d in t e r m s o f t h e f r a c t i o n r e a c t e d , ~ (Wadsw o r t h , 1979). This relation assumes t h a t t h e rate o f t h e process is c o n t r o l l e d b y t h e area o f t h e interface, w h i c h goes o n decreasing w i t h t h e progress in dissolution o f a particle. With this a s s u m p t i o n t h e f u n c t i o n 1 - (1 - ~)1/3 s h o u l d be linear w i t h time, t: ~)l/a = kt

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w h e r e k (min -1) is a c o n s t a n t . T h e variation o f 1 - (1 - a) 1/8 with t i m e is s h o w n in Fig. 3 for d i f f e r e n t t e m p e r a t u r e s in t h e range 60--90°C. T h e rate c o n s t a n t s were calculated f r o m t h e slopes o f each straight line. -

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A p l o t o f log h against 1/T is s h o w n in Fig. 4, f r o m w h i c h t h e activation e n e r g y (EA) was f o u n d t o be 42.1 + 0.8 k J mo1-1. The value o f t h e a c t i v a t i o n e n e r g y seems t o indicate t h a t t h e chemical r e a c t i o n o n the surface is the slowest (and h e n c e t h e r a t e , d e t e r m i n i n g ) stage o f dissolution o f Ni~S2 (Peters, 1973). T h e dissolution rate w i t h i n t h e t e m p e r a t u r e range 30--50°C is strikingly low. T h e reason f o r this seems t o be t h e covering o f t h e sulphide surface and t h e s a t u r a t i o n o f t h e leaching s o l u t i o n with H2S gas. A t t e m p e r a t u r e s lower

72 than 50°C, the rate o f hydr ogen sulphide p r o d u c t i o n is faster t han its oxidation rate to elemental sulphur or sulphate ions. Table 2 shows the effect of t e m p e r a t u r e on the fraction o f sulphide ion oxidized to sulphate after 2 h dissolution in 3.0 M HNO3. At the leaching t e m p e r a t u r e o f 50°C, only 1.6% o f sulphide is oxidized to sulphate, whereas at 90°C this fraction is 91%. An abrupt increase in oxi da t i on of sulphide ions to sulphate at temperatures above 50°C was also f o u n d in the dissolution o f NiS in acid dichromate solution {Mulak, 1983). 2.1

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Effect of particle size Th e effect of particle size on dissolution rate was studied by using two size fractions in 3.0 M HNO3 ( < 6 3 ; 63--71 t~m), and f o u r size fractions in 2.0 M HNO3 (< 63; 5 6 - - 6 3 ; 6 3 - - 7 1 ; 7 1 - - 1 0 0 p m ) at 80°C. Figure 5 shows the results o f the study. During the leaching in 2.0 M HNOz, the influence of

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particle size was noticed only for ca. 30 min; after that time the retardation of dissolution by H2S gas evolution became dominant. Influence of particle size (and consequently of the surface area of particles) on the rate of nickel extraction has been observed more distinctly during leaching in 3.0 M HNO3 at 80°C, because H2S which covers the sulphide surface is oxidized to elemental sulphur and sulphate. After 2 h leaching of the fraction 63--71/~m, 54% of Ni was extracted, whereas leaching of a smaller fraction size (<65 pm) over the same time interval showed nickel extraction to be 86%.

Effect of stirring rate The effect of stirring speed on nickel extraction from heazlewoodite was studied in 3.0 M HNO3 at 80°C and at revolution rates from 250 to 1000 min -1. The dissolution rate was observed to be independent of stirring speed• This indicates that the rate is chemically controlled, which is also in agreem e n t with the calculated activation energy.

74

Effect of the addition of copper ions In the presence of small quantities of Cu 2÷ ions (1 mmol/1) no hydrogen sulphide is evolved. Figure 6 (curves 2 and 3) shows the effect of addition of Cu 2÷ ions at the beginning of leaching and after one hour of leaching in 2.0 M HNO3 at 80°C. The addition of Cu 2÷ ions to 2.0 M HNO3 accelerated the dissolution. The dissolution rate of Ni3S: in 2.0 M HNO3 at 80°C was also greatly enhanced by the bubbling of air through the suspension during the leaching {Fig. 6, curve 4). The air flow (0.2 1 min -1) was passed through a gas train to give adequate humidity before the dissolution experiment, so that no significant volume change in the aqueous solution during the dissolution experiment was anticipated. Increase of the dissolution rate of Ni3S2 by the passing o f air causes a lowering of concentration of H2S in the solution and a corresponding shift in the equilibrium between H2S dissolved and absorbed on the surface o f the sulphide. The very distinct differences in the quantity and morphology of the sul1.0 I- - - - l

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Time, rain Fig. 6. Effect of copper ion addition and bubbling air through the suspension on nickel extraction from Ni~S 2 in 2 . 0 M HNO~ at 80°C: 1 - - no addition; 2 - - w i t h 1 m M C u ~÷ addition after 1 h o f leaching; 3 - - w i t h 1 m M C u :÷ addition at the beginning o f leaching; 4 - - with bubbling o f air.

75

Fig. 7. SEM p h o t o g r a p h s o f Ni~S~ leach residue ( f r a c t i o n 6 3 - - 7 1 ~ m ) a f t e r 2 h leaching in 2.0 M H N O 3 at 80°C, ~ -- o . 2 0 ; (a) Ni3S ~ grain, × 8 0 0 ; (b) f r a g m e n t o f t h e surface o f this grain, × 3 2 0 0 .

76

Fig. 8. SEM p h o t o g r a p h s o f Ni3S: leach residue ( f r a c t i o n 6 3 - - 7 1 ~ m ) a f t e r 2 h leaching in 2.0 M H N O 3 at 80°C w i t h Cu 2÷ a d d i t i o n , a = 0 . 5 6 ; ( a ) Ni3S2 grain, × 8 0 0 ; ( b ) f r a g m e n t o f t h e surface o f this grain, × 3 2 0 0 .

77

Fig. 9. SEM p h o t o g r a p h s o f Ni3S ~ leach residue ( f r a c t i o n < 6 3 u m ) after 2 h leaching in 2.0 M HNO 3 at 80°C w i t h b u b b l i n g o f air, a = 0.62; (a) Ni3S 2 grain, × 1 2 0 0 ; ( b ) f r a g m e n t o f t h e surface o f this grain, × 5000.

78

Fig. 10. SEM p h o t o g r a p h s o f Ni3S 2 leach residue ( f r a c t i o n 6 3 - - 7 1 urn) a f t e r 2 h leaching in 3.0 M H N O 3 a t 80°C, a = 0 . 5 4 ; (a) Ni3S 2 grain, × 8 0 0 ; (b) f r a g m e n t o f t h e surface o f this grain, x 3 2 0 0 .

79 phur which is formed on the surface of Ni3S~ during leaching under varying conditions are shown b y the scanning electron microscope photographs in Figs. 7--10. Figures 7 (a) and (b) show SEM photographs of Ni3S2 (fraction 63--71 um) solid residue after 2 h leaching in 2.0 M HNO3 at 80°C (20% of Ni was leached). The sulphur surface created in this case is a weakly developed b u t rather porous layer. Under the same leaching conditions 56% Ni was extracted in a Cu2+-catalysed reaction, and the sulphur which was formed on the surface of Ni3S2 is very porous with easily recognizable, separate sulphur crystallites (Figs. 8 (a) and (b)). The SEM photographs of the surface of the grain of Ni3S2 residue (fraction < 6 3 t~m) after 2 h leaching in 2.0 M HNO3 at 80°C with air bubbling through the suspension are shown in Figs. 9 (a) and (b) (62% Ni leached). Sulphur which covers the surface of Ni3S2 is formed as inflated (probably with H2S) bubbles and their residues (collapsed outer sulphur envelopes), densely distributed on the Ni3S~ surface. The surface of sulphur which forms on some areas of the sulphide surface during the leaching of Ni~S2 (fraction 63--71 um) in 3.0 M HNO3 at 80°C, is similar to that after leaching with air bubbling through the suspension. Other areas of the surface of Ni3S~ are covered with very porous sulphur deposits (Figs. 10 (a) and (b)). The fraction of Ni extracted in this experiment was the same as that in the leaching reaction catalysed with Cu 2+ ions in 2.0 M HNO3 solution.

Mechanism of chemical leaching Examination of the leach residues by optical microscopy, X-ray diffraction, scanning electron microscopy and chemical analysis showed that in the leaching of Ni3S2 two intermediate phases occur: millerite and elemental sulphur. The unleached Ni3S2 was covered with NiS and S O in selected areas. These areas have not yet been identified but it seems likely that they are grain boundaries and crystal imperfections. During the leaching of Ni3S2, the sulphide is evolved as hydrogen sulphide gas and converted to elemental sulphur and sulphate ion. The amounts of these formed depends on temperature of leaching and nitric acid concentration. The dissolution of Ni~S2 proceeds by the following steps: I II III IV

3Ni3S2+2NO;+14H +=6Ni 2++3NiS+3H2S+4H20+2NO 3NiS+2NO;+8H ÷=3Ni 2÷+2NO+4H20+3S O 3H2S+2NO;+2H ÷=3S O+4H20+2NO SO + 3 N O ; + 2 H + = H S O ~ + 3 N O + H 2 0

The rate-determining step during dissolution appears to be the oxidation of H2S occurring on the heazlewoodite surface (step III). Addition of copper ions removes hydrogen sulphide from the Ni3S2 surface, and the copper sulphide formed as an intermediate product is subsequently oxidized to elemental sulphur and sulphates.

80 T h e d e t a i l e d m e c h a n i s m o f t h e d i s s o l u t i o n o f Ni3S2 in t h e p r e s e n c e o f Ag ÷, Cu 2÷ a n d Fe 3÷ ions will b e discussed in a f u t u r e p u b l i c a t i o n . It was f o u n d b y e l e c t r o n m i c r o s c o p y t h a t t h e s u l p h u r d e p o s i t f o r m e d in t h e c o p p e r c a t a l y s e d r e a c t i o n is v e r y p o r o u s (Fig. 8). CONCLUSIONS F r o m t h e a b o v e e x p e r i m e n t a l d a t a o n t h e nitric acid d i s s o l u t i o n o~ heazlew o o d i t e (Ni3S2), t h e f o l l o w i n g c o n c l u s i o n s can be d r a w n : (1) D u r i n g t h e leaching o f Ni3S2 at a c o n c e n t r a t i o n o f HNO3 l o w e r t h a n 2.0 M, t h e d i s s o l u t i o n was o b s e r v e d to be c o m p l e t e l y i n h i b i t e d a f t e r 30 m i n leaching. T h e rate o f H2S gas p r o d u c t i o n was f a s t e r t h a n its o x i d a t i o n to S O a n d HSO~. In 3.0 M H N O 3 an a b r u p t increase in the d i s s o l u t i o n rate o f Ni3S2 was o b s e r v e d . (2) T w o d i s t i n c t regions o f d i s s o l u t i o n o f h e a z l e w o o d i t e w e r e e s t a b l i s h e d b e l o w a n d a b o v e 50°C. B e l o w 50°C, t h e d i s s o l u t i o n rate is v e r y slow even in 3.0 M HNO3, a n d h y d r o g e n sulphide is evolved. T h e leaching at t e m p e r a t u r e s b e t w e e n 60 a n d 90°C follows a linear law; t h e a c t i v a t i o n e n e r g y is f o u n d to be 42.1 k J m o l -~. Most o f t h e o x i d i z e d sulphide ion is f o u n d in t h e s o l u t i o n as s u l p h a t e . (3) T h e leaching rate is i n d e p e n d e n t o f stirring speed. T h e r a t e - c o n t r o l l i n g step o f t h e d i s s o l u t i o n o f Ni3S: is s u p p o s e d t o be t h e o x i d a t i o n o f h y d r o g e n sulphide t o e l e m e n t a l s u l p h u r or s u l p h a t e ions o n t h e Ni3S2 surface. (4) A d d i t i o n o f small a m o u n t s o f Cu 2÷ ions t o t h e nitric acid acts as a c a t a l y s t f o r t h e d i s s o l u t i o n o f Ni3S2. (5) A d e c r e a s e in H2S c o n c e n t r a t i o n in s o l u t i o n a c h i e v e d b y b u b b l i n g air t h r o u g h t h e leach s u s p e n s i o n increases the d i s s o l u t i o n r a t e o f Ni3S2 in 2.0 M HNO3 s o l u t i o n .

REFERENCES Filmer, A.O. and Nicol, M.J., 1980. The non-oxidative dissolution of nickel sulphides in aqueous acidic solutions. J.S. Afr. Inst. Min. Metall., 80: 415--424. Gerlach, J., Pawlek, F. and Rictesel, H., 1970. Pressure leaching of nickel sulphides. Erzmetall, 23: 486--492. Ghali, F.L. and Girard, B., 1978. Ferric chloride leaching of nickel sulphide concentrates. Hydrometallurgy, 3: 355--371. Kryukova, V.N. and Tseft, A.L., 1964. Complex hydrometallurgical processing of nickel matte. Tr. Inst. Metall. Obogashch., Alma Ata, 11 : 3--9. Kullerud, G. and Yung, R.A., 1962. The Ni--S system and related minerals. J. Petrol., 3: 126--175. Mulak, W., 1983. Kinetics of dissolution of synthetic millerite (~-NiS) in acidic potassium dichromate solutions. Hydrometallurgy, 11 : 79--89. Peters, E., 1973. The physical chemistry of hydrometallurgy. Symposium on Hydrometallurgy, A.I.M.E., New York, pp. 205--228. Sinev, L.A., Soboleva, T.R. and Sharmo, E.A., 1975. Mechanism of the acid dissolution of heazlewoodite (in Russian). Izv. Vyssh. Ucheb. Zaved., Tsvetn. Metall., 4: 35--39.

81 Thornhill, P.G., Wigstol, E. and Weert, G. van, 1971. The Falconbridge matte leach process. J. Metals, 23: 13--18. Wadsworth, M.E., 1979. Hydrometallurgical processes, in: Sohn, H.Y. and Wadsworth, M.E. (Eds.), Rate Processes of Extractive Metallurgy. Plenum Press, New York, 1979, pp. 133--186.