Hydrometallurgy, 28 (1992) 373-379 Elsevier Science Publishers B.V., Amsterdam

373

Kinetics of zinc and cobalt sulphide precipitation and its application in hydrornetallurgical

separation

P.K. Mishra and R.P. Das Regional Rcsearch Laboratory, Bhubaneswar, India

(Received April 15, 1991; revised version accepted August 19, 1991 )

ABSTRACT

Mishra, P.K. aad Das, R.P., 1992. Kinetics of zinc and cobalt sulphides and its application in hydro- metallurgical separation. Hydrometallurgy, 28: 373-379.

Zinc and cobalt sulphides are precipitated from an ammoniacal solution containing ammonium sulphate by controlled addition of sodium sulphide solution. The kiaetics of the reaction have been studied. Zinc shows first order kinetics whereas cobalt shows three kinetic regions with an induction period in the first region. This difference is exploited to obtain differential precipitation of sulphides.

INTRODUCTION

Environmental protection is a subject of interest especially to mineral in- dustries having potential water pollution problems. Hydrometallurgical pro- cesses also present potential waste treatment problems. Often waste streams contain a number of metal ions present at trace level. Several methods have been suggested for bringing about significant reduction in the quantity of metals released to the environment. Some of these processes are oxidation, boiling, steam stripping, acidification, sulphide precipitation and hydrogen reduction. In all these processes final metal recovery is highly dependent on the interaction of dissolved metal with residual solids and the hydrothermal behaviour of aqueous metal species. Among the chemical precipitation meth- ods, precipitation of metal hydroxide is most conventional, but it suffers from shortcomings, such as high solubilities, the amphoteric properties of metal hydroxides and ineffectiveness in the presence of the chelating agents that are commonly used in metal finishing operations. Sulphide precipitation of met-

Correspondence to: Dr. P.K. Mishra, Special Materials Division, Regional Research Laboratory, Bhubaneswar-751013, Orissa, India.

0304-386X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

374 P.K. MISHRA AND R.P. DAS

als is a viable alternative process because of the possible high degree of metal removal over a broad pH range. Moreover, it has several advantages over hydroxide precipitation, such as the low solubility and high stability of metal sulphides. Most of the metals can, therefore, be precipitated as sulphides even in the presence of chelating agents. In addition, metal sulphide sludges have better dewatering characteristics than hydroxides. Precipitation also has wide applications in hydrometallurgy [ 1-4 ], water treatment [ 5-9 ] and the pro- duction of sulphide films for semiconductor devices [ 10,11 ] and cathod ray tubes [ 12 ].

The solubility products of metal sulphides are given in handbooks [ 13 ]. However, the solid product produced by precipitation is often amorphous and its solubility depends on the method of precipitation, particle size and age [ 14,15 ]. A comparison of solubilities of metal hydroxides and metal sul- phides is given in the literature [ 5 ]. The important metal sulphide reactions are

H2S,HS- 1 [H + ( 1 )

M 2 + + S 2- -, MS (solid) where M 2 + is any bivalent metal ion. With sulphide precipitation a residual concentration less than 0. l mg/i can be achieved at 4 < pH < 12. Currently, two methods, which differ in the technique of delivering sulphide ions, are available [ 16 ] namely, the soluble sulpilide method and the sparingly soluble sulphide method.

In the soluble sulphide method, sulphidising agents, such as sodium sul- phide, hydrogen sulphide, calcium sulphide, ammonium sulphide, thio- acetamide [l 7] and thio-urca [l l,l 8 ], are used. An extensive literature is available for sodium sulphide as a bulk precipitant of copper and mercury from strongly acidic solution [ 2 ], but literature on the selective precipitation of these species is scarce. Some problematic combinations are cobalt-nickel; cobalt-manganese and cobalt-zinc separation from dilute solution. Ammo- nia leaching in hydrometallurgical processes is usually conducted in oxidising environments. In a cobalt-containing leach liquor the dissolved cobalt is pres- ent essentially as cobaltic-amines. The present paper studies the kinetics of zinc and cobalt sulphide precipitation from their ammoniacal solution (pres- ent as zinc (II) and cobalt (III)) containing ammonium sulphate. The Eh- pH analysis of C o - S - H 2 0 and Zn-S-H20 indicates the existence ofboth CoS and ZnS under reducing conditions in the alkaline pH range [ 19,20 ].

EXPERIMENTAL METHODS

Synthetic metal amine solutions containing ammonium sulphate were pre- pared using analytical grade zinc sulphate, cobalt sulphate, ammonia and am- monium sulphate. The cobalt-containing solution was aerated for conversion

KINETICS OF ZINC AND COBALT SULPHIDE PRECIPITATION 375

of cobaltous to cobaltic ions in the presence of activated carbon. Complete conversion of Co 2+ to Co 3+ was tested by solvent extraction [21 ]. Tests were conducted in l and 2 1 reaction vessels. Mixing was accomplished by means of a PTFE impeller. The pH was constantly monitored with a Toshniwal pH meter and recorded. A constant temperature water bath provided tempera- ture control within +_0.02°C whenever necessary. Kinetics of the reactions were studied at 35°C, pH 9.6, with metal concentrations varying from 50 to 80 mg/l and a sulphide ion concentration of 1.2 g/l. Sodium sulphide was added continuously. At a preselected time the flow was stopped and the re- action allowed to continue until further changes in concentration were small. Samples were withdrawn at regular intervals and immediately filtered through cellulose nitrate membranes with a pore size of 0.45/zm. The concentrations of zinc and cobalt in the filtrate were determined using a Perkin-Elmer Atomic Absorption Spectrophotometer.

RESULTS AND DISCUSSION

The kinetics of reactions between metals and sulphide ions in ammoniacal solution containing ammonium sulphate were studied by the controlled ad- dition of sulphide ions. After the flow of sulphide ions was stopped after var- ious lengths of time, an attempt was made to fit the data with a kinetic expres- sion-eqn. (2). A first-order relationship adequately simulated the slow approach to equilibrium in the case of zinc sulphide precipitation:

d Cm(t.)/dt=kmCm(t) (2)

where: Cm (t) = the zinc concentration in solution at time t; t = the time elapsed after the flow of sulphide ions was halted; km(=k[S 2- ] ) = a constant.

Regression analysis was applied to the data obtained. A pseudo first-order rate constant was found of 8.57 × 10- 5 s- ~. An example of a first order plot is shown in Fig. 1.

Cobalt sulphide precipitation exhibits different behaviour in three distinct kinetic regions. The first region is an induction period of a few minutes, this is followed by a rapid decline in the cobalt concentration and, finally, by a very slow approach to equilibrium. Figure 2 shows the zinc and cobalt con- centrations at various lengths of time after stopping the flow.

Thermodynamically, the separation of cobalt and zinc as sulphides is dif- ficult because of the co-existence of zinc and cobalt sulphides in the alkaline pH range under reducing conditions. The precipitation kinetics of cobalt sul- phide show an induction period of a few minutes during which ZnS is precip- itated. Based on this discovery, it is possible to separate zinc from cobalt by a two-stage precipitation. Experiments were carried out in synthetic solutions

376 P.K. MISHRA AND R.P. DAS

-0"06

- 0 "06

- 0 "10

-0"12

- 0 '14

- 0 ' 16

-0118

Q

[]

I I I I I

10 20 30

T ime (min)

Fig. I. First order plot for zinc sulphide precipitation. Flow stopped after 10 min. Time (mir l j

20 L ,.,o

~0

E u 30 C 0

gl 20

0

5O

I I I I I I I

-~0

E

,3 t - O

' e " w

, I D

0

20u

0 /. 8 12 16 20 2¢ 28

Time (ra in)

Fig. 2. Zinc concentrations in solution after the flow of sodium sulphide was stopped at 2.5, 5.0, 7.5, !0 and 20 rain and cobalt at 2.5, 5.0, 7.5, 15, 20, 40 and 60 min, respectively. @=cobalt; * = z i n c .

KINETICS OF ZINC AND COBALT SULPHIDE PRECIPITATION 377

Eh =-9"6mV ICobolt & Zinc] PH = 9"/,8 Roffinote

,,,

pptn. (1)

Sodium sulphide solution (dil.)

I

Eh='182mV l S/L sep~ I ,-Zinc Sulphide (97"99 ~) ( 2 0"/. C0S)

S~dium sulphide solution (dil.)

[ pptn.(2) [

+

r + I E h =-356mV S/L sep~ -------,-Cobolt sulphide tpure)

66g

Solution for disposol.

Fig. 3. Tentative flow sheet for zinc and cobalt separation as their sulphides at different poten- tials: Co=Zn= 50 rag/I, sodium sulphide of 1.2 g/l.

C:I LU > 0 LU n." LU ¢D <~ I - - z LU re" ILl n

~00

8O

6O

40

2O

/

// I I I I , . A i

50 100 150 200 250 300 350 400

- E h ( m V )

Fig. 4. Plot of percentage of zinc and cobalt removed versus potential (mV)' Co = Zn = 50 roB/ ! and sodium sulphide of 1.2 g/l.

378 P.K. MISHRA AND R.P. DAS

TABLE 1

X-ray diffraction data for the precipitate obtained in stage 2 at 35 °C

d (Intensity) experimental

d (Intensity) literature, CogSs

1.753(100) 1.76 (100) 1.8922 (25) 1.92 (20) 1.2968 (32) 1.29 (35) 2.O86 (55) 2.986 (37.8) !.9658 (30) 1.92 (20)

in the pH range 9.25-9.75 and the same behaviour is shown. Figure 3, shows a tentative flowsheet for a two-stage precipitation process followed by sepa- ration at controlled potential. The percentage of zinc and cobalt sulphide re- moved versus potential (mV) is shown in Fig. 4. This shows that at - 185 mV more than 90% of zinc is separated, with little loss of cobalt; while at - 3 5 6 mV, 92% of both cobalt and zinc are precipitated as sulphides. How- ever, after zinc removal in the first stage, pure cobalt sulphide is obtained in the second stage. The precipitates obtained at stage 1 and stage 2 (Fig. 3) were analysed by chemical analysis and X-ray diffraction (XRD) (Table 1 ). The chemical analysis showed the precipitates to be zinc sulphide and cobalt sulphide. XRD data for precipitate obtained in stage 2 are given in Table 1.

CONCLUSION

The precipitation of zinc sulphide from its ammoniacal solution shows first order kinetics. The rate constant is found to be 8.57 × l 0- 5 s-~. Cobalt shows different behaviour: an induction period of few minutes is followed by a rapid decline in the cobalt concentration, after which there is a gradual decrease in concentration beyond 3 h. If a two-stage precipitation method is used, 90% of the zinc can be separated from cobalt during the induction period.

REFERENCES

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KINETICS OF ZINC AND COBALT SULPHIDE PRECIPITATION 379

6 Van den Steen, A., Polloni, J.M., Kalala, B. and Shungen, T., Development of cobalt sul- phate solution purification by sulphide precipitation. In: Proc. Symp. Extractive Metal- lurgy of Nickel and Cobalt (Phoenix, Ariz., Jan. 25-28), Metallurgical Soc., Pennsylvania (1988).

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