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Precipitation of Nickel Hydroxide by Hydrolytic Stripping of Nickel Phosphinate Dissolved in Organic Solvent Yasuhiro Konishi,* Dai Satoh, and Katuya Takano Department of Chemical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
This paper describes a hydrolytic stripping, in which the nickel-loaded phosphinic acid extractant (Cyanex 272) in aliphatic hydrocarbon was directly hydrolyzed by water at elevated temperatures to strip and recover nickel as hydroxide precipitates. This recovery route to nickel hydroxide is a combined process of the stripping and precipitation stages in a conventional solvent extraction. When contacted with water at 170-220 °C and 0.8-2.5 MPa for 2 h, the nickel phosphinate solution gave precipitates of nickel hydroxide. The resulting hydroxide powders were submicron sized agglomerates of very fine crystallites, which were readily calcined to nickel oxides at 280 °C in air within 24 h. The rates of hydroxide precipitation in a batch autoclave were sensitive to processing parameters such as liquid-phase stirring speed, temperature, and organic-phase compositions. Introduction Solvent extraction has been widely used for recovering nickel from leach liquors derived from various raw materials such as lateritic nickel ores, sludge, scrap, and spent catalysts. Many studies have addressed the solvent extraction behavior of nickel with various regents such as carboxylic acids and phosphorus acids. In typical solvent extraction processes, the nickel is stripped from loaded solvent extractant using concentrated aqueous solutions of inorganic acids. The nickel stripped in the aqueous solution can be either recovered as nickel metal by electrowinning or crystallized to obtain insoluble product such as nickel sulfate. Alternatively, nickel ions in a loaded solvent extractant (tertiary monocarboxylic acid) can be directly recovered as hydroxide powders by hydrolysis with water at 200 °C. Such a process is called hydrolytic stripping, in which metal-loaded solvent extractants are hydrolyzed directly by water at 140-210 °C, to precipitate oxides or hydroxides.1-8 Doyle-Garner and Monhemius4 first demonstrated that powders of nickel hydroxide, Ni(OH)2, are directly recovered from nickel loaded carboxylate solutions when contacted with water at 200 °C for 3 h. The overall reaction for the hydrolytic stripping can be represented as
(RCOO)2Ni(org) + 2H2O(aq) ) Ni(OH)2(s) + 2RCOOH(org) (1) where RCOOH represents the free carboxylic acid. Because the carboxylic acid extractant is stable enough to withstand under the mild hydrothermal conditions and is regenerated by reaction 1, hydrolytic stripping could replace both a stripping stage and a precipitation stage, eliminating the reagents costs at each stage and simplifying the process flow sheet. However, the properties of the resulting precipitates have been poorly characterized, and little attention has been directed * To whom correspondence should be addressed. Phone: 81-722-54-9297. Fax: 81-722-54-9911. E-mail: yasuhiro@ chemeng.osakafu-u.ac.jp.
toward the influence of process conditions on the kinetics of stripping and precipitation. Moreover, no reports have appeared concerning the hydrolytic stripping for nickel-loaded organophosphorus extractant systems, although various organophosphorus extractants such as alkyl phosphoric, alkyl phosphonic, and alkyl phosphinic acids have been used extensively for the extraction of nickel.9-13 This paper describes the precipitation of nickel hydroxide (oxide precursor) by direct hydrolysis of nickelloaded phosphinic acid extractant (Cyanex 272) in an organic solvent with water at temperatures up to 220 °C. The effects of process conditions on the precipitation rate and the particle size of hydroxide powders were examined. Experimental Section Materials. The phosphinic acid extractant used in this work was commercially available Cyanex 272, bis(2,4,4-trimethylpentyl)phosphinic acid (Cytec Industries Inc., West Paterson, New Jersey). The commercial Cyanex 272 extractant contains at least 85% organophosphinic acid and is only slightly soluble in water (16 g/m3 at pH 2.6). The Cyanex 272 was diluted to desired concentration levels using commercial Shellsol D70, an aliphatic hydrocarbon diluent (Shell Chemicals Ltd., Tokyo, Japan). These organic materials were used as received, with no further purification. Nickel phosphinate solutions were prepared by solvent extraction from an aqueous nickel sulfate solution. During the extraction operation, the aqueous solution pH was adjusted from 6.5 to 7.0 by the addition of dilute sodium hydroxide solution. The nickel-loaded organic solutions were washed with distilled water to remove residual anions and then passed through glass fiber paper and phase-separating paper to remove physically entrained water. The initial concentrations of nickel in the organic solution were from 0.074 to 0.268 kmol/m3, and the initial concentrations of free phosphinic acid were from 0.19 to 1.20 kmol/m3. Apparatus and Procedure. A stainless steel autoclave lined with glass was used to perform hydrolytic
10.1021/ie010349a CCC: $22.00 © 2002 American Chemical Society Published on Web 07/16/2002
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stripping tests at elevated temperatures and pressures. The autoclave was 7-cm i.d. and 16-cm height, and a six-blade turbine stirrer of 5.0-cm diameter was placed 3 cm above the bottom of the vessel. A 100-cm3 volume of the nickel loaded organic solution was charged into the round-bottomed autoclave with an equal volume of distilled water. The stirrer was at the liquid-liquid interface. The organic and aqueous solutions were sufficiently mixed at room temperature by the stirrer, and nitrogen was continuously bubbled into the solution for 20 min. After that, the organic and aqueous phases were heated in the autoclave and maintained at a reaction temperature for 2 h. The experimental temperature was varied from 170 to 220 °C. The time required to reach the reaction temperatures of 170-220 °C was from 10 to 20 min. Because of the low volatility of the organic solutions, the total pressure in the autoclave was approximately equal to the saturated steam pressure at the operating temperature, that is, 0.8 MPa at 170 °C and 2.5 MPa at 220 °C. The starting solution was mixed at three different stirring speeds of 100, 300, or 500 rpm. A solution sample of 5 cm3 was withdrawn from the autoclave and centrifuged for analysis. The sampling was performed 4 cm above the bottom of the autoclave. The organic samples were mixed with 6 kmol/m3 hydrochloric acid solution to strip completely the nickel species in the organic phase, and the aqueous solutions were analyzed for nickel by ion chromatography (LC6A chromatograph, Shimadzu). Because the aqueous solution was completely free from nickel ions during the hydrolytic stripping process, the percentages of hydroxide precipitation were determined from the concentration of nickel in the organic solution at any time divided by the initial organic-phase nickel concentration. The resulting precipitates were filtered, washed with distilled water and acetone, and dried for 5 h at 50 °C. The precipitates were characterized by X-ray diffraction (XRD) analysis. The particle size and morphology were observed with scanning electron microscopy (SEM). Aside from hydrolytic stripping runs, experiments were carried out to check the thermal stability of the phosphinic acid extractant, Cyanex 272, at elevated temperatures and pressures. The solvent extraction behavior of nickel with the phosphinic acid reagent was investigated, before and after the extraction reagent is used for the hydrolytic stripping tests at 200 °C and 1.6 MPa for 2 h. Solvent extraction tests were done using a 0.34 kmol/m3 solution of phosphinic acid in aliphatic hydrocarbon diluent, that is, 12.5% (vol/vol) Cyanex 272 in Shellsol D70. The aqueous phase was 0.001 kmol/m3 nickel sulfate solution, and the solution pH was adjusted gradually with a dilute sodium hydroxide solution, in the presence of the organic solution. The aqueous solutions were mixed with an equal volume of the organic solution and equilibrated overnight at 25 °C, and then equilibrium pH was measured. The extent of nickel extraction was determined by stripping the organic solutions with 6 kmol/m3 hydrochloric acid solution and measured nickel concentrations by ion chromatography. Results and Discussion Identification of Resulting Precipitates. The nickel-loaded organic solutions gave precipitation when contacted with distilled water at different temperatures and pressures, 170-220 °C and 0.8-2.5 MPa. The
Figure 1. X-ray diffraction patterns of precipitates from nickel phosphinate solution with water at 200 °C and 1.6 MPa: (a) precipitate dried at 50 °C for 5 h; (b) precipitate dried at 280 °C for 24 h. Conditions: 0.074 kmol/m3 nickel phosphinate, 0.19 kmol/ m3 free phosphinic acid, and 500 rpm: (O) Ni(OH)2 (JCPDS 140117), (b) NiO (JCPDS 44-1159).
resulting precipitates were identified by XRD analysis. As shown in Figure 1a, the observed XRD patterns were consistent with standard patterns of nickel hydroxide (Ni(OH)2) (JCPDS 14-0117), and the sharpness of peaks indicated the highly crystalline nature of the hydroxide product. There were no peaks due to unidentified crystalline phases. Assuming that the nickel species in the phosphinate solution exists predominantly as Ni(HA2)2(HA)2,13 the overall reaction for the precipitation of nickel hydroxide can be represented in the simplified form as
Ni(HA2)2(HA)2(org) + 2H2O(aq) ) Ni(OH)2(s) + 3(HA)2(org) (2) where (HA)2 denotes the free phosphinic acid (Cyanex 272) dimer. Figure 2a,b shows the electron micrographs of nickel hydroxide powders precipitated directly from nickel phosphinate solution at two different liquid-phase stirring speeds. The nickel hydroxide powders precipitated at 100 rpm were micrometer agglomerates of elongated, fine crystallites (Figure 2a). On the other hand, the hydroxide powders precipitated at s stirring speed of 500 rpm were submicron sized agglomerates of tabular, fine crystallites (Figure 2b). There appears to be a decrease in particle size with increasing the liquid-phase stirring speed. It is likely that vigorous stirring of the organic and aqueous solutions leads to the rapid formation of many nuclei or break up growing particles. In addition, microscopic examination revealed that the particle size of nickel hydroxide is little affected by other operating variables such as the temperatures (170-220 °C), initial nickel phosphinate concentrations (0.0740.268 kmol/m3), and initial free phosphinic acid (0.190.52 kmol/m3). The resulting hydroxides, which were precipitated at 200 °C and 500 rpm, were air-dried in an oven at 280 °C for 24 h. Figure 1b shows that the XRD patterns of hydroxide powders treated at 280 °C were characteristic
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Figure 2. Scanning electron micrographs for (a) nickel hydroxide precipitated from organic solution of 0.074 kmol/m3 nickel phosphinate and 0.19 kmol/m3 free phosphinic acid at 200 °C, 1.6 MPa, and 100 rpm; (b) nickel hydroxide precipitated from organic solution of 0.074 kmol/m3 nickel phosphinate and 0.19 kmol/m3 free phosphinic acid at 200 °C, 1.6 MPa, and 500 rpm; (c) nickel oxide obtained by calcining for 24 h at 280 °C nickel hydroxide precipitated at 500 rpm.
Figure 3. Solvent extraction behavior of nickel with phosphinic acid extractant: (b) freshly prepared extractant, (O) completely regenerated extractant by hydrolytic stripping (once), (0) completely regenerated extractant by hydrolytic stripping (twice).
of nickel oxide (NiO) (JCPDS 44-1159). This demonstrates that the nickel oxide products can be readily derived by the calcination, heating the hydroxide precipitates, and driving off water at 280 °C for 24 h. Figure 2c shows the oxide particles formed by calcining nickel hydroxide. Comparing Figure 2, parts b and c, it is clear that the particle size and morphology were not changed appreciably by the calcination. Figure 3 shows the results of the solvent extraction tests, in which the organic solution of extractant Cyanex 272 in Shellsol D70 was used before and after the hydrolytic stripping tests. When the nickel-loading organic solution was treated at 200 °C and 1.6 MPa for 2 h, the nickel extracted in the organic solution was entirely stripped and precipitated. After the hydrolytic stripping test, the organic solution was used again in the next extraction test. The cycle of stripping-extraction was repeated twice. The observed values of percentage extraction were reproduced within (5% on repeated runs. As shown in Figure 3, the ability of the Cyanex 272 to extract nickel scarcely lowered even after the hydrothermal treatment at 200 °C and 1.6 MPa. This indicates that Cyanex 272 extractant has a suitable thermostability for the hydrolytic stripping. Rate of Hydroxide Precipitation. Figure 4 shows rate data for the precipitation of nickel hydroxide at different liquid-phase stirring speeds at 200 °C and 1.6 MPa. The precipitation rate was markedly enhanced as the stirring speed was increased from 100 to 500 rpm. At a stirring speed of 500 rpm, the nickel was completely stripped and precipitated from the organic phosphinate
Figure 4. Rate data for precipitation of nickel hydroxide at different liquid-phase stirring speeds: (b) 500 rpm; (9) 300 rpm; (2) 100 rpm. Conditions: 200 °C, 1.6 MPa, 0.074 kmol/m3 nickel phosphinate, 0.19 kmol/m3 free phosphinic acid.
solution within 120 min. The effect of liquid-phase stirring speed in an emulsion system probably reflects slower transfer of dissolved water across an aqueousorganic interface at the lower stirring speed; this is because a decrease in the liquid-phase stirring speed generally results in a lowering of both overall mass transfer coefficient and interfacial area. In addition, because a decrease in the liquid-phase stirring intensity reduces the rates of primary nucleation and secondary nucleation in crystallization operation,14 it is likely that a change in the formation rate of primary particles (nuclei) is responsible for the precipitation rate of nickel hydroxide. Figure 5 shows rate data for the precipitation of nickel hydroxide at different hydrothermal temperatures between 170 and 220 °C (0.8 and 2.5 MPa). There was a marked increase in the precipitation rate of hydroxide powders as the operating temperature was changed from 170 to 220 °C. The hydrolytic stripping process involves hydrolysis reaction of nickel complex present in the organic solution and homogeneous nucleation which gives primary crystallites, followed by secondary nucleation and growth of either the crystallites or the agglomerates. An increase in temperature generally permits an increase in the rate of hydrolysis reaction
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Figure 5. Rate data for precipitation of nickel hydroxide at different temperatures: (b) 220 °C and 2.5 MPa; (9) 200 °C and 1.6 MPa; (2) 180 °C and 1.0 MPa; ([) 170 °C and 0.8 MPa. Conditions: 0.074 kmol/m3 nickel phosphinate, 0.19 kmol/m3 free phosphinic acid, and 300 rpm.
Figure 6. Rate data for precipitation of nickel hydroxide at different organic-phase concentrations of initial free phosphinic acid: (b) 0.19 kmol/m3 free phosphinic acid; (2) 0.52 kmol/m3 free phosphinic acid; (9) 1.2 kmol/m3 free phosphinic acid. Conditions: 200 °C, 1.6 MPa, 0.074 kmol/m3 nickel phosphinate, and 500 rpm.
in the organic solution. In addition, it is accepted that the solubility of water in an organic solvent increases with an increase in temperature. Furthermore, previous work15 on crystallization from solution has indicated that the rates of primary nucleation and secondary nucleation increase with temperature. Because temperature has a significant effect on the individual stages such as hydrolysis reaction and nucleation, the overall (observed) rate of nickel precipitation appears to be determined in a complex way. Figures 6 and 7 show that the precipitation rates of nickel hydroxide powders are influenced by the initial organic-phase compositions. As shown by Figure 6, a marked decrease in the precipitation rate occurred when the initial concentration of free phosphinic acid was increased 0.19-1.2 kmol/m3. An increase in the free
Figure 7. Rate data for precipitation of nickel hydroxide at different organic-phase concentrations of initial nickel phosphinate: (b) 0.074 kmol/m3 nickel phosphinate; ([) 0.147 kmol/m3 nickel phosphinate; (2) 0.268 kmol/m3 nickel phosphinate. Conditions: 200 °C, 1.6 MPa, 0.19 kmol/m3 free phosphinic acid, and 500 rpm.
phosphinic acid concentration shifts the position of equilibrium in eq 2 to the left, which will lead to suppress the precipitation of hydroxide. Because the overall precipitation process depletes the nickel phosphinate in the organic phase and produces the free phosphinic acid, the hydrolytic stripping system appears to be approaching equilibrium as the hydroxide precipitation proceeds. Therefore, the precipitation of hydroxide was initially rapid and then gradually decreased the rate, as shown by Figures 4-6. In Figure 7, the precipitation rate decreased as the initial concentration of nickel phosphinate in the organic phase increased from 0.074 to 0.268 kmol/m3. The effect of initial nickel phosphinate concentration on the precipitation rate is a result of two competing factors. First, an increase in nickel phosphinate concentration shifts the position of equilibrium in eq 2 to the right, promoting the precipitation of hydroxide. On the other hand, at higher initial concentrations of initial nickel phosphinate, higher concentrations of free phosphinic acid are produced by the precipitation of nickel hydroxide, which will suppress the hydroxide precipitation. The two effects therefore act in opposite directions. The increased free phosphinic acid effect dominates and the precipitation rate decreases with increasing the initial nickel concentration in the organic solution. Conclusions When contacted with water at 170-220 °C and 0.82.5 MPa for 2 h, the nickel phosphinate solutions were hydrolyzed and precipitate submicron crystalline powders of nickel hydroxide. The resulting hydroxide powders were calcined to nickel oxides at 280 °C in air within 24 h. The extraction ability of the phosphinic acid scarcely lowered, even after the hydrolytic stripping test at 200 °C, indicating that the phosphinic acid extractant is stable enough to withstand the mild hydrothermal conditions. Moreover, the precipitation rates of hydroxide powders were markedly increased with increasing liquid-phase stirring speed and temperature and decreasing free phosphinic acid concentration.
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Acknowledgment The authors thank Mr. Hiroshi Doi, Japan New Metals Co., Ltd., Osaka, Japan, for his assistance in characterizing the precipitated powders. Literature Cited (1) Monhemius, A. J.; Thorsen, G. The Application of Hydrolytic Stripping to the Iron Problem in Hydrometallurgy. Proceedings of International Solvent Extraction Conference (ISEC′80), Liege, Belgium, 1980; Vol. 3, p 80. (2) Thorsen, G.; Monhemius, A. J. U.S. Patent 4,282,189, 1981. (3) Monhemius, A. J.; Teixeria, L. A. C.; Thorsen, G. The Precipitation of Hematite from Iron-Loaded Versatic Acid Solutions by Hydrolytic Stripping. In Hydrometallurgical Process Fundamentals; Bautista, R. G., Ed.; Plenum Press: New York, 1984; p 647. (4) Doyle-Garner, F. M.; Monhemius, A. J. Hydrolytic Stripping of Single and Mixed Metal-Versatic Solutions. Metall. Trans. B 1985, 16B, 671. (5) Doyle, F. M.; Ye, W. ZrO2 Powders from Zirconium(IV) carboxylates. J. Met. 1987, 39 (7), 34. (6) Doyle, F. M.; Monhemius, A. J. Kinetics and Mechanisms of Precipitation of Nickel Ferrite by Hydrolytic Stripping of Iron (III)-Nickel Carboxylate Solutions. Hydrometallurgy 1994, 35, 251. (7) Konishi, Y.; Kawamura, T.; Asai, S. Preparation and Characterization of Ultrafine Nickel Ferrite Powders by Hydrolysis of Iron(III)-Nickel Carboxylate Dissolved in Organic Solvent. Ind. Eng. Chem. Res. 1996, 35, 320.
(8) Konishi, Y.; Murai, T.; Asai, S. Preparation and Characterization of Fine Ceria Powders by Hydrolysis of Cerium (III) Carboxylate Dissolved in Organic Solvent. Ind. Eng. Chem. Res. 1997, 36, 2641. (9) Preston, J. S. Solvent Extraction of Cobalt and Nickel by Organophosphorus Acids, I. Comparison of Phosphoric, Phosphonic and Phosphinic Acid Systems. Hydrometallurgy 1982, 9, 115. (10) Rickelton, W. A.; Flett, D. S.; West, D. W. Cobalt-Nickel Separation by Solvent Extraction with Bis(2,4,4-trimethylpentyl)phosphinic Acid. Solvent Extr. Ion Exch. 1984, 2, 815. (11) Danesi, P.; Reichley-Yinger, L.; Mason, G.; Kaplan, L.; Horwitz, E. P.; Diamond, H. Selectivity-Structure Trends in the Extraction of Co(II) and Ni(II) by Dialkylphosphoric, Alkyl Alkylphosphonic and Dialkylphosphinic Acids. Solvent Extr. Ion. Exch. 1985, 3, 435. (12) Yuan, C.; Xu, Q.; Yuan, S.; Long, H.; Shen, D.; Jiang, Y.; Feng, H.; Wu, F.; Chen, W. A Quantitative Structure-Reactivity Study of Mono-Basic Organophosphorus Acids in Cobalt and Nickel Extraction. Solvent Extr. Ion Exch. 1988, 6, 393. (13) Sole, K. C.; Hiskey, J. B. Solvent Extraction Characteristics of Thiosubstituted Organophosphinic Acid Extractants. Hydrometallurgy 1992, 30, 345. (14) Garside, J. Industrial Crystallization from Solution. Chem. Eng. Sci. 1985, 40, 3. (15) Randolph, A. D.; Larson, M. A. Theory of Particulate Processes; Academic Press: San Diego, CA, 1988; Chapter 5.
Received for review April 19, 2001 Revised manuscript received April 22, 2002 Accepted April 22, 2002 IE010349A
