A study of a nickel hydroxide sulphate precipitate obtained during hydrogen reduction of nickel hydroxide slurries

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hydrometallurgy ELSEVIER

Hydrometallurgy 43 (1996) 129-142

A study of a nickel hydroxide sulphate precipitate obtained during hydrogen reduction of nickel hydroxide slurries Thomas Saarinen a, Lars-Eric Lindfors a,*, Sigmund Fugleberg b a Laboratory of Industrial Chemistry, ,~bo Akademi University, Biskopsgatan 8, FIN-20500 Turku, Finland Outokumpu Research Oy, FIN-28101 Pori, Finland

Received 27 May 1995; accepted 22 October 1995

Abstract A nickel sulphate hydroxide precipitate that is insoluble in sulphuric acid medium at pH 2.0 was detected during hydrogen reduction experiments on nickel slurries. This precipitate always appeared when a slurry, obtained by neutralising nickel sulphate by addition of sodium hydroxide, was being reduced by hydrogen gas at an elevated temperature, 160°C, and a total pressure of 21 bar, the hydrogen partial pressure being 15 bar. The properties of this precipitate were studied. This secondary precipitate proved to be stable for months when suspended in sulphuric acid at pH 2.0. When a similar slurry made by adding nickel sulphate to sodium hydroxide (only the addition order of solutions changed) was reduced under the same conditions, no such 'insoluble' precipitate occurred. The stable secondary precipitate was found to disappear by further hydrogen reduction and it also dissolved at a temperature of 80°C at pH 4.

1. Introduction Metallic nickel powder can be precipitated from aqueous solutions by pressure reduction with hydrogen [2-7]. The results o f an investigation [1] of the precipitation of metallic nickel powder by hydrogen reduction of a hydroxide slurry, obtained by neutralising nickel sulphate with sodium hydroxide in the temperature region 130-160°C, revealed that the time required for complete reduction o f the hydroxide was dependent on the neutralisation procedure. It was found that the time required to reduce the same

* Corresponding author. Fax + 358 21 2654479. 0304-386X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0304-386X(95)00 103-4

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

amount of nickel from a suspension could be reduced by up to 50% in the best of cases simply by changing the order of addition of the two solutions, nickel sulphate and sodium hydroxide, during the neutralisation stage preceding the hydrogen reduction. The shortest reduction times were achieved when nickel sulphate was added to sodium hydroxide. Consequently, the addition order affects the composition of the precipitate in a decisive manner. To clarify these reduction results the two precipitates, Precipitate 1 (sodium hydroxide added to nickel sulphate) and Precipitate 2 (nickel sulphate added to sodium hydroxide), were studied. During these examinations a nickel sulphate hydroxide precipitate, which is insoluble in sulphuric acid at pH values around 2.0 at room temperature, was detected. The results of an investigation of this precipitate at a temperature of 160°C are reported in this paper.

2. Experimental 2.1. Apparatus The hydrogen reduction experiments were performed in a 600 ml titanium autoclave, Parr Series 4560 Bench Top Mini reactor. The reactor was stirred by a magnetically driven axial flow-type impeller above a straight-blade impeller. The stirrer speed was adjusted to 690 min-1 for the hydrogen reduction tests. The degree of agitation was high enough to permit hydrogen to be entrained from the gas phase into the slurry during reduction. Local concentration gradients in the suspension might occur during the neutralisation stage, where the agitation speed was 400 min -1, as this speed was considered high enough to give good mixing but, at the same time, avoided bubbles of gas being driven into the suspension. The autoclave was heated by an electrical jacket and the reactor temperature controlled by a regulator.

2.2. Procedure Technical grade nickel sulphate, NiSO4.6H20, supplied by Outokumpu Oy, was dissolved in distilled water. The pH of this nickel sulphate solution was 4.6. The sodium hydroxide solution was prepared by dissolving pellets (E. Merck) in distilled water. In all hydrogenation runs, 90% of the stoichiometric amount of sodium hydroxide required to precipitate all nickel as hydroxide was added. The experimental procedure was initiated by loading one of the two solutions into the autoclave and heating it to 60°C. After this, the other solution was pumped to the reactor for 60 min, resulting in precipitation of a nickel sulphate hydroxide complex. After neutralisation, the autoclave was heated to the reduction temperature, 160°C. When the temperature in the reactor had stabilised, after about 40 min, the autoclave was pressurised with hydrogen to give the predetermined total pressure of 21 bar. Hydrogen gas was introduced into the slurry by a tube, with its outlet near the bottom of the reactor, and a continuous flow of hydrogen through the vessel was maintained by venting a small hydrogen stream into the atmosphere during the experiment. The flow of hydrogen was kept at the same constant level in all experiments.

T. Saarinen et aL / Hydrometallurgy 43 (1996) 129-142

131

2.3. Analysis Samples of the suspension were collected at the bottom of the reactor through the hydrogen inlet tube. By using this technique there was no problem caused by remains of suspension from previous sampling in the tube, as it was purged by the continuous flow of hydrogen. Samples of the unreduced slurries from each of the series were analysed and samples of the precipitates were also taken at different stages during the hydrogen reduction. Metallic nickel was removed from the partially reduced precipitates by a magnet. It was very difficult to separate all of the nickel and thus some metallic nickel interfered with the analysis of the samples. When basic salts with different solubility occurred, the easily soluble portion dissolved in sulphuric acid solution at pH 2.0 at room temperature and the remaining portion was labelled 'insoluble' precipitate. These precipitates were investigated by chemical analysis, TGA and X-ray spectrometry. The pH values of the solutions were measured and the concentrations of nickel in ionic form were analysed by an X-Met 880 Fluorescence Analyser.

3. Results and discussion

When analysing samples from the hydrogen reduction test series, a nickel hydroxide sulphate precipitate that was insoluble at low pH values was detected. It was noticed that this precipitate was stable for months in a sulphuric acid solution at pH 2.0 and room temperature. As normal basic nickel sulphates should readily dissolve at pH below 5, the present investigation was carried out. As mentioned in the introduction, two series of hydrogen reduction tests were performed. The only parameter changed between these series was the order in which the solutions of sodium hydroxide and nickel sulphate were mixed during neutralisation prior to hydrogenation (Table 1). The insoluble precipitate discussed only appears when the suspension made by adding sodium hydroxide to nickel sulphate has been reduced for some time by hydrogen at elevated temperature. In the other series the precipitates were completely soluble within a few minutes at pH values of 3-4. The two partially reduced precipitates from Series 1 and Series 2 were titrated at a constant pH value of 2.0 at 25°C with a sulphuric acid solution with the concentration of 50 g/1. The consumption of acid is plotted on the y axis in Fig. 1, where the difference in solubility between the two precipitates can be seen. After about 300 s, the precipitate in Series 2 was completely dissolved, whereas some of the precipitate from Series 1 remained insoluble. The colour of this nickel precipitate is green-grey.

Table 1 Neutralisation procedures and solubility of precipitate obtained Neutralisation procedure

Unredueed precipitate

Partially reduced precipitate 160°C, 15 bar H 2

NaOH ~ NiSO 4 (Series 1) NiSO 4 =, NaOH (Series 2)

soluble soluble

insoluble soluble

T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

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A similar titration at 80°C and a pH value of 4.0 was also carried out. The results are shown in Fig. 2. The precipitate from Series 2 was immediately dissolved. At this temperature, the precipitate from Series 1 also dissolved completely, but at a much lower rate.

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142 3.1. N e u t r a l i s a f i o n

During neutralisation, nickel sulphate and sodium hydroxide react forming solid NiSO 4 Ni(OH) 2 complexes, according to: (x + y)NiSO 4 + 2y NaOH ~ y Ni(OH)2 *xNiSO 4 + yNazSO 4

(1)

The composition of this basic nickel sulphate complex can vary, depending on many factors, of which the most important is the pH of the solution during precipitation. The effect of the pH was investigated by adding the solutions to each other in a different order. A Pourbaix diagram of nickel in the aqueous environment (Fig. 3) shows in which form nickel is expected to be as a function of the potential, E, and the pH value. It can be seen that, if the pH is high, nickel is in the form of solid nickel hydroxide; if the pH

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

is low, the nickel is in ionic form in solution. In this diagram only Ni(OH) 2 has been considered as the solid phase, but at lower pH values basic nickel sulphates also will occur. The composition of the basic salts (i.e., the sulphate to hydroxide ratio) is dependent on the pH [9] so that it decreases with increasing pH and will be zero (which means pure nickel hydroxide) at a certain pH value. In Series 1 the initial pH of the bulk solution was 4.6 and rose by the addition of sodium hydroxide. In Series 2 the initial pH was 12.7 and decreased by the addition of the nickel sulphate. Accordingly, it is to be expected that the structure and composition of the two precipitates in these two series differ from each other: Series 2 should contain more hydroxide in the precipitate than Series 1. The composition will also depend upon the concentration of the solutions, precipitation temperature, agitation rate and phenomena of an occasional nature [8,10].

3.2. Investigations of unreduced suspensions The solubility of the unreduced nickel precipitates was investigated as follows: 50 ml of a nickel sulphate solution with a concentration of 0.68 m o l / l was mixed with 45 ml of a sodium hydroxide solution of 1.36 m o l / l at 60°C with 30 rain addition time. In Series l, the initial pH of the nickel sulphate solutions was 4.6, after the addition of nickel hydroxide the pH was 9.3. In Series 2, the nickel sulphate was added to the sodium hydroxide and the final pH was 8.75. After neutralisation the suspensions were cooled and titrated with sulphuric acid solution (100 g / l ) at pH 4.0 until the precipitates were dissolved. Both precipitates dissolved and no difference in behaviour was noticed. The amount of acid necessary for complete dissolution was about 30.8 ml in both series. Before the chemical analyses were performed, the nickel slurries were heated to 140°C, rapidly cooled, filtered and washed, in order to define the conditions prior to the reduction. Analysis of the suspensions before reduction produced the results shown in Table 2. As can be seen from the results, the only difference between the precipitates that might be of importance for the solubility is that the sulphur content in Series 1 is higher than that in Series 2. These findings are supported by TGA analyses in [1]. The data in Table 2 reveal that the x:y ratio (Eq. (1)) is 1:20 for Series l and 1:30 for Series 2, when sodium in the precipitate is assumed to be a s N a E S O 4. The XRD diagram of the unreduced, dried precipitate from Series 2 (Fig. 4a) shows peaks typical of theophrastite. The diffraction line for the precipitate from Series 1 (Fig. 4b) is similar. Only the peak at the x-value 18 is missing and has been replaced with two different peaks at x values of 17 and 21. This shows that the phase composition of the two precipitates is different. Table 2 Composition of unreduced solutions and precipitates Element

Ni Na S

Series 1

Series 2

Solution (g/l)

Precipitate (%)

Solution (g/l)

Precipitate (%)

2.14 13.9

60.8 0.2 1.8

0.43 14.8

54.4 0.3 1.2

T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142 2

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3.3. Investigations of reduced precipitates Nickel requires a solid surface onto which it can precipitate during hydrogen reduction. The solid nickel hydroxide sulphate precipitated during the neutralisation stage probably plays the role of such a 'catalyst' and provides the nuclei onto which nickel precipitates. As there was no nucleation seed added before the hydrogen reduction

136

T. Saarinen et al./ Hydrometallurgy 43 (1996) 129-142

was commenced, or during the process, the reason for the stability of the insoluble precipitate cannot be due to any addition agents. In order to investigate the precipitates, the suspensions were reduced by hydrogen, at a temperature of 160°C and a hydrogen pressure of 15 bar. The pH of the suspension was around 8 at the beginning of the reduction. The reduction was interrupted at two stages and the remaining precipitates were examined. The first sample was taken from Series 1, where sodium hydroxide was added to nickel sulphate, when the slurry had been reduced so that about one third of the nickel was in metal form. Most of the metallic nickel was removed by magnetic separation. The XRD curves in Fig. 4b and Fig. 5 show that the phase composition of the unreduced precipitate has changed drastically during the reduction. The spectra in Fig. 5 represent the precipitate that has a component that is insoluble at a pH value of 2.0. Only fractions of this precipitate dissolved with the addition of sulphuric acid (100 g / l ) at pH 2.0 and most of it remained insoluble. The exact portion of this insoluble precipitate in relation to the total amount of precipitate is hard to estimate because it is difficult to determine the quantity of reduced nickel exactly and thereby the amount of precipitate. The share of insoluble/total amount of precipitate also seemed to vary from experiment to experiment. The insoluble part of the nickel hydroxide sulphate precipitation was analysed chemically and by XRD. The XRD diagram in Fig. 6 shows that there is an apparent difference in the phase composition of this precipitate compared to the initial unleached precipitate of Sample 1 (Fig. 5) At pH 2.0 the insoluble precipitate contained 52% Ni, 0.15% Na and 5.1% S, corresponding to an x:y ratio of 1:5.7. The precipitate thus contains some water. The structure of the precipitate is hard to determine because there is nickel present in both metallic and ionic form. Sample 2 was taken from Series 1 when the slurry had been reduced so that about two thirds of the nickel was in metallic form. From the XRD curve (Fig. 7) it can be seen that there have not been any major changes in the phase composition of the

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142 i

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Fig. 6. XRD spectra of a nickel hydroxide sulphate precipitate, obtained by adding sodium hydroxide to nickel sulphate, which has been reduced to one third with hydrogen. The sample has been leached at pH 2.0. precipitate during further reduction with hydrogen. When this precipitate was mixed in sulphuric acid at a pH value of 2.0 it again remained partially undissolved. The X R D diagram in Fig. 8 shows almost the same peaks as the ones from Sample 1 in Fig. 7. One difference, however, is that the peak at the x value 40 seems to be higher in this case and the nickel metal peaks at the x values 44 and 52 are different, which is the consequence o f various success in the separation of metallic nickel. Chemical analysis of this insoluble precipitate revealed that it contained 50% Ni, 0.14% Na and 4.9% S, which gives an x:y ratio o f 1:5.7. The result is the same as for

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T. Saarinen et al./Hydrometallurgy 43 (1996) 129-142

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Fig. 8. XRD spectra of a nickel hydroxide sulphate precipitate, obtainedby adding sodium hydroxideto nickel sulphate, which has been reduced to two thirds with hydrogen. The samplewas leached at pH 2.0. Sample 1. This shows that the molar ratio between nickel and sulphur in the 'insoluble' part of the precipitate remains constant during the reduction. It can also be noticed from Samples 1 and 2 that the sulphur content in the precipitate has increased from 1.8% in the unreduced to about 5% in the insoluble part of the precipitate during the hydrogen reduction. It can be presumed that a new basic sulphate has been formed, possibly because some nickel hydroxide has been reduced to nickel, so that the relative share of the remaining sulphate has increased. The precipitate that was stable at pH 2.0 was filtered, washed and suspended in distilled water and the stability was studied at various temperatures. The pH was decreased to 1.98 by the addition of sulphuric acid (100 g / l ) and continuously measured at 25°C. The results are presented in Fig. 9, where it can be seen that the pH remains at 2.0 and that the precipitate, consequently, did not dissolve. Another portion of this precipitate suspended in water was heated to 80°C. During heating the pH value decreased from 7.82 to 6.77. At 80°C the pH was adjusted to 2.00, again by the addition of sulphuric acid (100 g/l). The pH, measured on-line, rose rapidly, indicating that the precipitate started to dissolve. In approximately 10 min all of the precipitate was dissolved and the solution became clear (Fig. 10). As the first sample did not dissolve at 25°C, it was decided to heat the slurry slowly, l ° C / m i n , and measure the pH. The result is shown in Fig. 11. At 70°C the precipitate started to dissolve and the suspension gradually became clear after extra addition of sulphuric acid. Sample 3 was taken from Series 2 (where nickel sulphate was added to sodium hydroxide) when approximately half of the nickel ions had been reduced to metallic nickel. XRD of this precipitate, shown in Fig. 12, shows that the spectra of the reduced precipitate in Series 2 is almost identical to that which was unreduced (Fig. 4a). Some theophrastite has been reduced to metallic nickel, seen at x value 45 in the XRD. Most

T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

139

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Time, min

Fig. 9. pH versus time for the suspended insoluble precipitateat 25°C. of the metallic nickel had been removed before analysis and the peak is therefore small. This partially reduced precipitate dissolved rapidly at pH values of 3 - 4 by the addition of sulphuric acid. Chemically, this precipitate consisted of 65% Ni, < 0.05% Na and 0.93% S. In this case the molar ratio of nickel to sulphur has not changed during reduction. The nickel powder produced in the two series was investigated by XRD in order to examine if there were any structural or chemical differences between the powders. The

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Time, min Fig. 10. pH versus time for the suspended insoluble precipitate at 80°C.

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

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Fig. 11. pH versus temperature for the suspended insoluble precipitate when the temperature was raised l°C/min. diffraction peaks of the powders, Fig. 13, show that the powders obtained are identical. The small difference between the peaks is due to the slightly varying amount of nickel powder in the sample cup. Additional tests and analyses of the insoluble precipitates were carried out, from which it can be concluded that the precipitate does not contain sulphide. The insoluble

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Fig. 12. XRD spectra of a nickel hydroxide sulphate precipitate obtained by adding nickel sulphate to sodium hydroxide, which has been reduced by hydrogen.

T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

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precipitate is also formed at other temperatures than 160°C, and this precipitate disappears with further hydrogen reduction.

4. Conclusions When a nickel sulphate solution is neutralised by the addition of sodium hydroxide and the slurry obtained is reduced by hydrogen at elevated temperature and pressure, a stable precipitate which has not been reported in the literature is formed. This nickel hydroxide sulphate precipitate proved to be very stable in sulphuric acid medium at pH 2.0 and 25°C. XRD investigations showed that the phase composition of the solid changed drastically through hydrogen reduction. Chemical analysis revealed that the sulphur content of the precipitate increased from around 1.5% prior to reduction to about 5% during the hydrogen reduction. The precipitate disappears by completion of the hydrogen reduction and it also dissolves when the temperature of the solution at pH 2.0 exceeds 70°C. In [ 1] it was stated that the time required to reduce a batch of nickel ions by hydrogen gas could be affected by using different neutralisation procedures. The detection of this insoluble precipitate, in the case where nickel sulphate was neutralised by the addition of sodium hydroxide, probably explains why the reduction time depends on the neutralisation order. The reduction results indicate that the reduction mechanisms are different in the two systems.

A c k n o w l e d g e m e n t s

The support of this work by Outokumpu Oy is gratefully acknowledged.

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T. Saarinen et al. / Hydrometallurgy 43 (1996) 129-142

References [1] Saarinen, T., Fugleberg, S. and Lindfors, L.-E., Pressure reduction of nickel by hydrogen from hydroxide slurries. Hydrometallurgy, 43 (1996): 117-127, this issue. [2] Mackiw, V.N., Kunda, W. and Lin, W.C. Reduction of nickel by hydrogen from ammoniacal nickel sulfate solutions. Trans. AIME, 209 (1957): 786-93. [3] Needs, C.R.S. and Burkin, A.R., Kinetics of reduction of nickel in aqueous ammoniacal ammonium sulphate solutions by hydrogen, leaching and reduction. In: Hydrometallurgy (1975), pp. 91-96. [4] Mackiw, V.N., Kunda, W. and Evans, D.J.T., Effect of addition agents on the properties of nickel powders produced by hydrogen reduction. In: Modem Developments in Powder Metallurgy (1966), pp. 15-49. [5] Courtney, W.G and Schaufelberger, F.A., Observations on the kinetics of the reduction of NiSO4 in aqueous solutions. In: Physical Chemical Process Metallurgy. Met. Soc. Conf., 8 (1961): 1277-1290. [6] Burkin, A.R., Extractive Metallurgy of Nickel. Wiley, New York (1987). [7] Yongxiang, Y., Taskinen, P. and Lilius, K., Pressure hydrogen reduction of nickel, cobalt and copper from their aqueous systems. Helsinki Univ. Technology (1990). [8] Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 9. Mack, Easton, Pa. (1952), pp. 291-299. [9] Gmelins Handbuch der Anorganische Chemie, Nickel. Teil B, Liefemng 2. Verlag Chemic GMBH, Weinheim (1966), 732 pp. [10] Weast, R.C., C.R.C Handbook of Chemistry and Physics. C.R.C. Press, Cleveland, Ohio, 58th ed. (1978).