Separation of cobalt and nickel by ozone oxidation

Hydrometal/urgy, 30 ( 1992) 483-497

483

Elsevier Science Publishers B. Y., Amsterdam

Separation of cobalt and nickel by ozone oxidation T. Nishimura and Y. Umetsu Research Institute ofMineral Dressing and Metallurgy (SENKEN). Tohoku University. Aoba-ku. Sendai. Japan (Revised version accepted January 2, 1992)

ABSTRACT Nishimura, T. and Umetsu, Y., 1992. Separation of cobalt and nickel by ozone oxidation. In: W.e. Cooper and D.B. Dreisinger (Editors), Hydrometallurgy, Theory and Practice. Proceedings ofthe Ernest Peters International Symposium. Hydrometallurgy, 30: 483-497. The utilization of ozone for the separation of cobalt from nickel sulfate solutions was investigated by determining the oxidation rate for Co(lI) and Ni(II) ions under various ozonation conditions at 60· e. The oxidation reaction was observed to follow a first order rate with respect to the ozone partial pressure of the 0 3-0 2 mixture gas and to be promoted considerably by vigorous agitation. The oxidation rates were virtually constant down to a fairly low concentration of the oxidizable ions. Nickel ion was found to be oxidized more easily at lower pH in the mixed sulfate solutions than in solutions of a single sulfate. At pH 2.5-5.0, ozone oxidation seems to be effective to separate cobalt ions selectively from nickel sulfate solutions, due to the extremely slow oxidation of the nickel ion in comparison with cobalt.

INTRODUCTION

Ozone is well known to be a powerful oxidant in both the gaseous and aqueous phases. Recently, ozone has been used in various fields, such as disinfection of drinking-water supplies, decomposition of organic contaminants in the treatment of industrial waste effiuents, cleaning of the sewer pipe system of large buildings, and as a deodorizer for refrigerators. In hydrometallurgical processes, however, the application of ozone has been attempted only to a limited extent [1-3]. In our previous works [4-9], ozone oxidation was investigated for the removal or separation of manganous ion as manganese dioxide in weakly and highly acidic sulfate solutions. Ozone was found to be capable of producing manganese dioxide precipitate even at a sulfuric acid concentration as high Correspondence to: T. Nishimura, Tohoku University, Research Institute of Mineral Dressing and Metallurgy, Aoba-ku, Sendai, Japan.

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

484

T. NISHIMURA AND Y. UMETSU

as 5.0 AI. The oxidation rate [4,5] and characteristics of manganese dioxide produced by ozonation (morphology of the particle [6], chemical and physical properties [7,8], and discharge behavior as battery-active material [9] were affected by reaction temperature and acidity of the solution. An appropriate set of ozonation conditions is expected to enable us not only to eliminate manganous ion from acidic solution but also to produce a fine powder of manganese dioxide with interesting characteristics. Separation of cobalt from nickel has been one of the important themes in hydro metallurgy to produce nickel of high purity. Currently, oxidative separation techniques are operating on an industrial scale, as well as solvent extraction methods. Direct oxidation of cobaltous ion with chlorine and precipitation of cobaltic hydroxide [10] and electrolytic oxidation [10] are typical examples of industrial practice. In addition, strong oxidants such as persulfate [11], Caro's acid [12] and ozone [13] have been tested to remove cobalt from solutions of nickel sulfate. For better understanding and development of a cobalt-nickel separation method, detailed data on the oxidation rates for the target ions are required. In this work, ozone oxidation of cobalt and nickel ions has been determined under various conditions in solutions of a single sulfate and mixed solutions, of cobalt and nickel sulfates. The effects of agitation, ozone partial pressure of the feed gas and pH on the oxidative precipitation are presented. EXPERIMENTAL

The stock and test solutions were prepared from analytical reagent grade sulfates, CoS04 06H 20 and NiS0 4 07H 20. The experimental apparatus is shown in Fig. 1. Ozone was generated from dry oxygen by corona discharge in a four-electrode laboratory ozone generator. The oxygen gas was fed into the discharge chambers at a flow rate of 500 ml/min. In order to keep the ozone partial pressure of the ozone-oxygen mixture gas, the discharge current was kept constant at a predetermined value during ozonation operation. The ozone content of the mixture gas stream was determined by potassium iodide scrubbing, followed by sodium thiosulfate titration. In each run, the sulfate solution was charged in a cylindrical, round-bottom glass reactor of 500 ml capacity placed in a thermostatically controlled water bath. After the operating temperature (60°C in the present work) was reached, the solution pH was adjusted to the desired value with sulfuric acid and potassium hydroxide solutions. Then the ozone-oxygen mixture gas was sparged, at time zero, into the solution through a disperser, a glass ball filter, positioned below the glass impeller. During the oxidation run, the pH was kept constant by adding potassium hydroxide solution (0.50 M) using an auto-burette. The accumulated volume

SEPARATION OF COBALT AND NICKEL BY OZONE OXIDATION

485

dried oxygen

- = = : ; b 1Ii====i1

pH electrode

Pt electrode

Fig. 1. Schematic diagram of apparatus with ozonizer.

of the potassium hydroxide solution was used to monitor the oxidation reaction. The samples for analysis were withdrawn at selected time intervals. The precipitate was filtered off and the filtrate was subjected to analysis for cobalt or nickel, after reduction to the divalent state, by EDTA titration and atomic absorption spectrometry, depending on the concentration levels. For the solutions containing both cobalt and nickel, inductively coupled plasma (ICP) spectrometry was employed to determine their concentrations. The oxidation-reduction potential (ORP) was measured using a platinum electrode. The observed ORP values are presented in the figures as E(V) (Ag/AgCl). RESULTS AND DISCUSSION

Ozonation in solutions ofa single sulfate During the ozone oxidation of cobalt and nickel ions, hydrogen ions are generated. To sustain the oxidation reaction at constant pH, the hydrogen ions liberated must be consumed by the addition of a neutralizer, a 0.5 AI KOH solution. Figure 2 shows the decrease in the concentration of cobalt and nickel in the solution with ozonation time. Here, the circles indicate the concentration determined by chemical analysis and the solid line demonstrates the value based on the consumption ofKOH needed to keep the pH constant. It can be clearly seen that both values are in excellent agreement. This indicates that the oxidation reaction can be observed by continuous monitoring of the volume of 0.5 AI KOH solution added to maintain the pH constant at a set value.

486

T. NISHIMURA AND Y. UMETSU

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Fig. 3. Variation in ORP and cobalt concentration with time at different stirring speeds; pH 5.0; feed gas Po)=8.9x 10- 3 atm; gas flow rate =500 ml/min. (a) Oxidation-reduction potential. (b) Cobalt concentration.

SEPARATION OF COBALT AND NICKEL BY OZONE OXIDATION

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Fig. 4. Variation in ORP and nickel concentration with time at various stirring speeds; pH 6.5; feed gas Po,=8.9X 10- 3 atm; gas flow rate =500 ml/min. (a) Oxidation-reduction potential. (b) Nickel concentration.

As reported in our previous publications, the agitation was found to be one of the important factors affecting the ozone oxidation of manganous ion [4,5] and arsenious acid ion [14] in aqueous solutions. Figure 3 shows the variation of the cobalt concentration with time under various agitation conditions, expressed in terms of rotation velocity of the glass impeller, at pH 5.0 and an ozone partial pressure of8.9x 10- 3 atm in the feed gas. The oxidation-reduction potential (ORP) for each run is also presented. An increase in the stirring speed of the impeller up to 2000 rpm gave rise to a remarkable enhancement of the oxidation reaction. A further increase in the oxidation rate was not observed at stirring speeds above 2500 rpm. Under any agitation condition considered here, the reaction followed a zero order dependency with respect to the cobalt ion concentration of the solution, the rates being virtually constant down to a fairly low level of concentration. As shown in Fig. 4, a similar effect of agitation on the oxidation was determined for the solutions

488

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Fig. 5. Effect of stirring speed on precipitation rate; feed gas POJ=8.9X 10- 3 atm, gas flow rate =500 ml/min. 1 2 _._- -

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Fig, 6. Variation in ORP and cobalt concentration with time at various partial pressures of ozone; pH 5.0. (a) Oxidation-reduction potential. (b) Cobalt concentration.

SEPARATION OF COBALT AND NICKEL BY OZONE OXIDATION

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Fig. 7. Variation in ORP and nickel concentration with time at various partial pressures of ozone; pH 6.5. (a) Oxidation-reduction potential. (b) Nickel concentration.

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490

T. NISHIMURA AND Y. UMETSU

of nickel sulfate. It was noticed that the oxidation of nickel ion was not completed to so Iowa concentration level as in the case of the cobalt ion, even at a very high stirring speed. The oxidation rate, -d[M]/dt, (where [M] = concentration of cobalt or nickel ion remaining in the solution), was calculated from the slope of the linear portion of the individual curves showing a fall in the cobalt or nickel ion concentration with reaction time. In Fig. 5, the effect of stirring speed on the oxidation rate of cobalt ion at pH 5.0 and nickel ion at pH 6.5 is summarized. The significant dependence shown, of the the oxidation rate on stirring speed up to 2500 rpm suggests that mass transfer plays an important role in the ozone-oxidation reaction. In the subsequent experiments, a stirring speed of2500 rpm was employed. The oxidation of cobalt ion at different ozone partial pressures is presented in Fig. 6, where the partial pressure of the feed gas was varied from 5.8 Xl 0- 3 to 22.7x 10- 3 atm. A rise in the ozone pressure of the feed gas considerably enhanced the oxidation reaction. At each ozone partial pressure, it was observed that the reaction proceeded almost linearly with time down to very low cobalt concentrations. The oxidation of nickel ion, as shown in Fig. 7, took place in a similar manner to cobalt ion down to a low concentration, whereupon the reaction rate was very much reduced. At a low ozone pressures of around 5.8 X 10- 3 atm, the oxidation started after a short time. The effect of ozone partial pressure on the oxidation rate is illustrated in Fig. 8. The plot of the log of the oxidation rate versus the log of ozone partial pressure for both metal ions has a slope of approximately 1. This indicates that the ozone oxidation of cobalt and nickel is first order with respect to the ozone partial pressure of the feed gas. The acidity or acid concentration of the solution is generally considered to be a factor affecting the rate of the oxidation reaction, particularly when some solid oxide phase is involved. Figure 9 shows the oxidation of cobalt at different solution pH's. At pH 6.0 and 5.0, the cobalt conc:entration decreased in a linear manner with oxidation time. The most interesting observation is the appearance of a very long induction period prior to the start of the oxidative precipitation of cobalt ions at pH 3.0, followed by a rapid fall in concentration. A line, indicated as "seeded", presents the oxidation where the precipitate, prepared at pH 3.0 in a separate operation, was added as seed material. In this run, the oxidation reaction was initiated after a very short induction period and then proceeded at an almost constant reaction rate. Thus, the seeding seems to be helpful to start the reaction at low pH values, but the presence of the solid reaction product results in no detectable acceleration of the subsequent oxidation. Also, it is noticeable that a change in pH leads to significantly different ORP's during the oxidation. Figure 10 shows that the oxidation of nickel was affected in a much

491

SEPARATION OF COBALT AND NICKEL BY OZONE OXIDATION

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more complicated manner than the oxidation of cobalt. From this figure, it is obvious that the oxidation of nickel takes place at higher pH values than with the cobalt ion and that the reaction rate is affected by pH in the range 5.57.0. As the pH was raised, the oxidation reaction was enhanced drastically and proceeded to lower nickel ion concentrations. The oxidation rates for cobalt and nickel ions are presented against solution pH in Fig. 11. The oxidation of cobalt was observed to occur even at a pH of around 2.0 and then to increase gradually as the pH was raised. On the other hand, the oxidation of nickel ions could be detected above pH 5.5 and the reaction rate increased rapidly with rising pH, above 5.8 up to 6.8. As described above, cobalt and nickel ions are oxidized to be precipitated by ozone at quite different reaction rates in spite of a similarity in their thermodynamic properties. In order to facilitate a better understanding of the overall reactions of ozone oxidation, the precipitates from the solutions of a

492

T. NISHIMURA AND Y. UMETSU

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Time (min) Fig. 10. Variation in ORP and nickel concentration with time at various pH values; feed gas Po, =8.9 X 10- 3 atm. (a) Oxidation-reduction potential. (b) Hickel concentration. co

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493

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28 (Cu-Ka) Fig. 12. Typical X-ray diffraction patterns of (a) CoOOH and (b) NiOOH produced by ozonation. 1.2 ..---.----,r---..,----r---.--""T(a):J

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0.0 L- - - - J- -~400---(6)-0---&'80--1~OO:---~12~O~ 0 20 Time (min) Fig. 13. Variation in ORP and metal concentration in solution of cobalt and/or nickel sulfate with time; pH 5.0; feed gas Po,=8.9x 10- 3 atm. (a) Oxidation-reduction potential. (b) Concentration of individual metal.

494

T. NISHIMURA AND Y. UMETSU

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::;: 0.4

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20

40

60

80

100

Fig. 14. Variation in ORP and metal concentration in solution of cobalt and/or nickel sulfate with time; pH 6.5; feed gas Po,=8.9X 10- 3 atm. (a) Oxidation-reduction potential. (b) Concentration of individual metal.

single sulfate were submitted to X-ray diffraction. Typical X-ray diffraction patterns of the precipitates are shown in Fig. 12. Though the peaks are broad, the X-ray diffraction patterns identify the reaction product from nickel sulfate solution to be the Ptype of NiOOH. X-ray diffraction patterns for the reaction product for cobalt shows that CoOOH is the most probable species in the cobalt-bearing solid. Summarizing the results described above, the overall reactions for the ozone oxidation ofCo2+ and Nj2+ can be expressed as:

and:

SEPARATION OF COBALT AND NICKEL BY OZONE OXIDATION

t ,,

10 _

solution of Co or Ni sulfate

I

solution of Co and Ni sulfates ~

495

:,

}

...~ 6

Co__ ' ,

I

b .,...

°1~~~2--~3--~4~~~5-a~--~7· pH Fig. 15. Effect of pH on precipitation rate in solution of cobalt and/or nickel sulfate.

Ozonation in solutions ofa suI/ate mixture Figure 13 presents the variation of the cobalt and nickel ion concentrations in the mixed sulfate solution with time, at pH 5.0, together with those in their individual single sulfate solutions. The solid lines and the circles show for the oxidation in the solutions of a single sulfate: cobalt sulfate, and nickel sulfate, respectively. The broken line and the dots show the oxidation in solutions containing both metal ions. In the mixed sulfate solution, the oxidation of cobalt ion took place in a manner similar to that in single sulfate solution. While nickel ion in its single sulfate solution was not oxidized to a detectable extent in single sulfate solution at pH 5.0, the nickel ion concentration decreased gradually in the mixed solution. As presented in Fig. 14, at pH 6.5 both ions were oxidized and precipitated from their single sulfate solution at a considerable rate. At this pH, the decrease in both the cobalt and nickel concentrations is faster than in the solution of the single sulfate. It is interesting to note that the oxidation of nickel ion proceeded more favorably in the mixed solution. The oxidation rates for cobalt and nickel ions are plotted against pH in Fig. 15. The oxidation of nickel ion is promoted markedly in the mixed solution at pH values higher than 5.5 The Ni-Co molar ratio of the precipitate is another important factor in considering the separation of these ions. Figure 16 shows the molar ratio of the precipitate after a 2 h ozonation of the solution with an initial concentration of 1.0 gil of each metal ion. At lower pH values cobalt ions were preferentially oxidized and the molar ratio stayed fairly low. As the pH was raised the oxidation rate of nickel also increased and a significant amount of basic nickel

496

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T. NISHIMURA AND Y. UMETSU

0.8

'u G>

Q. .E

0.6

0

~ :;; 0.4 '0 E

0

g 0.2 :z

0

1

2

3

4

5

6

7

pH

Fig. 16. Ni/Co molar ratio in the precipitates from solution containing cobalt of 1.0 gil and nickel of 1.0 gil at various pH values; feed gaspo,=8.9x 10- 3 atm, reaction time=2 h.

oxide was precipitated when the oxidative precipitation of cobalt was completed. CONCLUSIONS

The oxidative separation of cobalt and nickel in their sulfate solutions with ozone was investigated by determining the oxidation rates for the individual ions at 60°C. The ozonation-precipitation reaction follows zero-order kinetics, with respect to the concentration of the oxidizable metal ions in solution, and a firstorder rate, with respect to the ozone partial pressure of the O 2-0 3 feed gas. A long induction period, followed by a rapid decrease in the cobalt concentration was detected in the oxidative precipitation of cobalt at low pH values. This induction period was shortened significantly or eliminated by seeding. In single sulfate solutions, cobalt ion is oxidized afld precipitated more easily at lower pH value than nickel ions. However, oxidative precipitation of nickel increases at lower pH values in the mixed sulfate solution compared to the single sulfate solution. The separation of cobalt and nickel by ozone oxidation is completed favorably at pH values between 2.5 and 4.0. The X-ray diffraction of the reaction products showed the peaks as coming from CoOOH and NiOOH. REFERENCES I Nesmeyanova, G.M. and Vikulov, A.I., J. Appl. Chem. USSR, 398 (1965): 24-28. 2 Mouret, P., Parley, G. and Pottier, P., German Pat. I 096887 (1961). 3 Subramanian, K.N. and Samson, P., In: Physical Chemistry of Process Metallurgy: The Richardson Conference. Inst. Min. Metall., London (1974), pp. 49-54.

SEPARATION OF COBALT AND NICKEL BY OZONE OX IDAnON

497

4 Nishimura, T. and Umetsu, Y., J. Min. Mater. Process. Inst. Japan, 107 (1991 ): 556-561. 5 Nishimura, T. and Umetsu, Y., In: Z. Kozuka, T. Ori, K. Morinaga (Editors), Rare Metals '90. Min. Mater. Process. Inst. Japan (1990), pp. 117-120. 6 Nishimura, T. and Umetsu, Y., J. Min. Mater. Process. Inst. Japan, 107 (1991): 805-810. 7 Nishimura, T. and Umetsu, Y., J. Min. Mater. Process. Inst. Japan (1992) (in press). 8 Nishimura, T. and Umetsu, Y., J. Min. Mater. Process. Inst. Japan (1992) (in press). 9 Nishimura, T. and Umetsu, Y., J. Min. Mater. Process. Inst. Japan (1992) (submitted). 10 Coussement, M., De Schepper, A. and Standaert, R., In: K. Osseo-Asare and J.D. Miller (Editors), Hydrometallurgy, Research, Development and Plant Practice. Metall. Soc. AIME, Warrendale, Pa. ( 1982), pp. 569-585. II Cmojevich, R., Wilkinson, D.H. and Blanco, J.L., In: K. Osseo-Asare and J.D. Miller (Editors), Hydrometallurgy, Research, Development and Plant Practice. Metall. Soc. AIME, Warrendale, Pa. (1982), pp. 941-953. 12 Mobbs, D.B. and Mounsey, D.M., Trans. Inst. Min. Metall. Sect. C, 90 (1981): 103-110. 13 Nikolic, c., Queneau, P.B., Sherwood, W.G., Barlow, C.B. and Simons, C.S., CIM Bull., 71 (1978) 121-127. 14 Nishimura, T. and Tozawa, K., J. Min. Met. Inst. Jpn. 104 (1988): 549-553.