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Direct synthesis of nickel powders from NiO slurry by hydrothermal hydrogen reduction process
Hydrometallurgy 102 (2010) 101–104
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Direct synthesis of nickel powders from NiO slurry by hydrothermal hydrogen reduction process Jong-Gwan Ahn a, Hoang Tri Hai b,⁎, Dong-Jin Kim a, Je-Shin Park a, Sang-Bae Kim a a b
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea Department of Electronic Engineering, Tohoku University, Sendai 980-8576, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2009 Received in revised form 8 February 2010 Accepted 8 February 2010 Available online 12 February 2010 Keywords: Nickel powder Hydrogen reduction Nickel oxide slurry Acidic solution Nickel sulfate
a b s t r a c t High purity nickel powder (99.5% Ni) was prepared by direct hydrogen reduction of commercial NiO powder in an acidic slurry in an autoclave with a yield of approximately 100% at 250 °C, initial pH 5.4 and 300 psi partial hydrogen pressure. It was found that the initial pH of the solution and the addition of NiSO4 play critical roles in the NiO/Ni powder conversion process. They provide an adequate acidic media and a source counter ions for dissolving NiO into Ni2+ ions, while the subsequent reduction of Ni2+ ions to of SO2− 4 metallic nickel proceeds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nickel powders have attracted a great deal of attention over past decades due to their specific properties such as magnetism, thermal resistance and chemical activity; and have a wide range of applications including batteries, hard alloys, catalysts etc. (Kisher et al., 1995; Shukla et al., 2001; Bricknell, 1986; Borowiecki et al., 1997). The preparation of nickel powders from slurry compounds such as NiO, Ni(OH)2, NiCO3, NiC2O4, etc. are important because these compounds exist in industry as middle products, used catalysts and pure minerals obtained from ores. The conversion of these compounds into metallic nickel powders can be performed by conventional hydrometallurgical processes, often from ammoniacal media (Burkin, 1987; Yamasaki and Liang, 1993; Kutty et al., 1982; Saarinen et al., 1996a; Saarinen et al., 1996b; Agrawal et al., 2006; Wang et al., 1998); but little has been found so far for processing NiO slurry. This may be due to the inert and refractory properties of NiO compounds. It has been well known that a polyol process has successfully prepared metal powders from the corresponding metal oxides or hydroxides directly (Sinha and Sharma, 2002; Fievet et al., 1989a; Fievet et al., 1989b). However, for inert compound such as NiO, long induction times of several days at high temperature are generally required, resulting in high energy consumption. Thus there is a need to develop a simpler and more efficient hydrometallurgical process for NiO slurry.
In this work, a preliminary evaluation of the feasibility of preparing nickel powder by direct hydrogen reduction of analytical grade NiO slurry has been made and the influence of some key parameters on the conversion has been investigated. 2. Theoretical background The reaction of metal oxide slurries to metal powders proceeds via a dissolution-crystallization process rather than a solid phase transformation (Sinha and Sharma, 2002; Fievet et al., 1989a; Fievet et al., 1989b). Fig. 1 shows the potential-pH diagram for Ni–H2O system at 25 °C (Pourbaix, 1974) which indicates that Ni3O4 (implied for Ni2O3. NiO) can be readily dissolved in acidic solution at pH b6, while the Ni2+ ions can be reduced to Ni0 by H2 at pH N4. Theoretically, this suggests that the NiO slurry–Ni powder conversion can be carried out by hydrogen reduction in acidic solution in the pH range of 4–6. Therefore, nickel powders should be synthesized from NiO by the following scheme: (i) Dissolution of NiO into Ni2+ ions; þ
2þ
NiO þ 2H →Ni
þ H2 O
ð1Þ
(ii) Reduction of Ni2+ ions into metallic Nio species; 2þ
Ni
o
þ
þ H2 →Ni þ 2H
ð2Þ
(iii) Nucleation and growth; ⁎ Corresponding author. Tel.: + 81 22 795 7134; fax: + 81 22 263 9402. E-mail address: [email protected] (H.T. Hai). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.02.005
0
0
nNi ↔ðNi Þnucl:
ð3Þ
102
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
water and dried at 40 °C in nitrogen gas. Because of the inert properties of NiO, the yield of the conversion was simply calculated through determining the weight of the residual NiO after dissolving the product in HCl solution. In most cases, the fraction reduced was inferred from the change of hydrogen pressure and this was consistent with that determined through the weight of the residual NiO. The products were characterized by X-ray diffraction (XRD, X-Ray Diffractometer MAX 2500 h, Rigaku), and scanning electron microscopy (SEM; JSM-6380LA, JEOL). The purity of the as-prepared nickel powders was determined by an atomic adsorption spectroscopy (AAs, AAnalyst400, PerkinElmer) and an inductive coupled plasma mass spectrometer (ICP-MS, ELAN 5000, PerkinElmer SCIEX). The pH of the solution was measured by pH meter (EC-PH510). 4. Results and discussion 4.1. Effect of initial pH The effect of the initial pH on the conversion was investigated at three different values of pH = 3.2, 5.4 and 9.2. Fig. 2 shows (a) the
Fig. 1. Potential-pH diagram for Ni–H2O system at 25 °C and 1 atm.
0
0
0
0
0
ðNi Þnucl: þ mNi →ðNi Þparticl:
ð4Þ
0
ðNi Þparticl: þ ðNi Þparticl: →ðNi Þbigger particl:
ð5Þ
Additionally, by looking in detail at the proposed conversion scheme, the dissolution reaction (1) can occur in acidic medium if the system is provided a source of anions to neutralize the Ni2+ ions. Accordingly, the effects of pH and concentration of counter-anions were considered and systematically investigated. 3. Experimental The hydrogen reduction experiments were carried out in an autoclave equipped with a 350 mL glass lined reactor (5 cm× 18 cm H) and an agitator having a 4.5 cm diameter 4-blade marine type impeller rotated at 650 rpm. The reactor was charged with 250 mL H2O, NiO powder and several additive agents, such as NiSO4, K2HPO4 and NaH2PO4, together with a constant amount of polyvinyl-pyrrolidone (PVP) about 4 g (or 16 g/L) to protect against agglomeration. Herein, NiSO4 acts as a source of SO2− 4 anions, whilst K2HPO4 and NaH2PO4 are buffer agents to adjust pH of the system and PVP is used to prevent the aggregation of the produced nickel particles. Table 1 summarizes the hydrothermal reduction conditions performed in the present work. All chemicals used were supplied by Junsei Chemical Co., Ltd, Japan. Once the reaction temperature reached 250 °C, hydrogen gas with partial pressure of 300 psi was introduced into the reactor after flushing a few times with N2 gas. The decrease in pressure and its lower steady value indicates the start and end of the reaction. The change of hydrogen pressure with reaction time was recorded periodically to calculate the reduction fraction. After completion of the reaction, the product was filtered, washed three times by distilled Table 1 Experimental conditions for hydrogen reduction of NiO. Factors
Conditions
Temperature PH2 Initial pH NiO concentration NiSO4 concentration PVP concentration K2HPO4 + NaH2PO4 (pH controllers)
250 °C 150–300 psi 3–10 0.32–0.64 mol/L 0–0.016 mol/L 16 g/L Changeable
Fig. 2. (a) Fraction reduced as a function of time at different initial pH. (b) XRD patterns of the products obtained at different initial pH.
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
103
fraction reduced as a function of reaction time; and (b) XRD patterns of the corresponding products. The fraction reduced gradually increased and reached a steady state of 10% and 62% at initial pH 9.2 and 3.2, respectively. The low value can be explained by the low solubility of NiO in basic solution, while partial reduction in acidic media is due to the low reduction potential of hydrogen as inferred from Fig. 1. The final pH values measured for both cases were pH 7.4 and 0.7, respectively. However, at an initial pH of 5.4, the fraction reduced rapidly increased and the conversion is complete after 9 min. This is reasonable because at this pH value, NiO is reasonably well dissolved to give Ni2+ ions and the subsequent reduction of Ni2+ to Nio can proceed. The final pH measured under this condition is pH 3.1 which is lower than pH 4 as inferred in Fig. 1, but the 100% yield of this conversion can be understood because the reduction potential of hydrogen is promoted by not only pH, but also the temperature. The XRD patterns of the final products shown in Fig. 2b which strongly support the reactions performed at different initial pH (Fig. 2a). Indeed, only well-defined Ni0 crystalline peaks are observed in the XRD pattern of the product obtained at initial pH 5.4, whereas at initial pH 3.2 and 9.2, both Ni0 and NiO crystalline peaks exist in the diffraction patterns. At initial pH 9.2, the product is dominated by the NiO phase, while the portion of NiO is similar to that of Ni0 in the product obtained at initial pH 3.2 as judged from their diffraction intensities. According, the optimum value of initial pH lies within the range of pH 4–6 for complete conversion. 4.2. Effect of NiSO4 concentration The effect of NiSO4 on the conversion is understood due to its impact on the dissolution of NiO under hydrothermal conditions and can be plausibly explained by the following scheme:(iv) Dissociation of NiSO4 and subsequent reduction of Ni2+ ions in aqueous solution: NiSO4 ↔Ni Ni
2þ
2þ
2−
þ SO4 0
ð6Þ
þ
þ H2 →Ni þ 2H
ð7Þ
(v) Dissolution of NiO and subsequent reduction of the corresponding Ni2+ ions: 2−
NiO þ SO4 Ni
2þ
þ
2þ
þ 2H →Ni 0
þ
þ H2 →Ni þ 2H
2−
þ SO4
þ H2 O
ð8Þ
Fig. 3. (a) Fraction reduced as a function of time at different NiSO4 concentrations. (b) XRD patterns of the products obtained at different concentrations of NiSO4.
ð9Þ
The SO2− 4 anions dissociated from Eq. (6) act as counter-anions to neutralize the Ni2+ ions dissolved from the solid NiO in Eq. (8); thus allowing the dissolution of NiO. In order to verify this mechanism, the hydrothermal reactions were performed at different concentrations of NiSO4 in range of 0–0.016 mol/L, whilst the initial pH of 5.4 and other parameters were fixed. Fig. 3 shows (a) the fraction reduced and reaction time dependence, and (b) XRD patterns of the corresponding products. Without NiSO4, the fraction reduced is quite negligible (approximately 8% even after 30 min), whereas it rapidly increases with an increase in NiSO4 concentration. For instance, the fraction reduced reaches 45% with 0.008 mol/L NiSO4 addition and 100% after just 12 min, with 0.016 mol/L NiSO4 added. The low conversion yields at low NiSO4 concentrations can be attributed to the low solubility of NiO rather than a low reduction potential of hydrogen because the final pH values were 5.1 and 3.8, respectively. The 100% yield achieved at 0.016 mol/L NiSO4 is clearly indicated by only Nio crystalline peaks observed in the XRD pattern (Fig. 3b), while the incomplete conversions at 0 and 0.008 mol/L NiSO4 are well demonstrated by the presences of both Nio and NiO diffraction peaks.
Accordingly, addition of NiSO4 as a source of SO−2 counter ions is 4 essential for the conversion of NiO to Ni powder and the above mechanism of NiSO4 on the conversion is plausible. The impact of SO2− anions on this conversion was further proven 4 by using H2SO4 instead of NiSO4 wherein the initial pH was maintained at 5.4 by addition of K2HPO4 buffer agent. The same effect as of NiSO4 was observed, but the phosphorus impurity was significant in the metal powder product. 4.3. Characteristics of metallic nickel product Fig. 4 shows the SEM images of (a) raw NiO and (b) Ni powders prepared from 0.32 mol/L NiO powder produced under standard conditions (0.016 mol/L NiSO4, 250 °C, pH 5.4 and 300 psi partial hydrogen pressure). The morphologies of two powders are clearly different. NiO powder consists of primary sharply-edged shaped submicron size particles in a highly aggregated form. After hydrogen reduction, the original NiO particles converted into spherical-grain metallic Ni particles with a high degree of aggregation. The chemical analysis revealed that in most cases, the purity of the nickel powder was very high (about 99.5%).
104
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
of preparing nickel powder by direct hydrogen reduction of NiO slurry. Therefore, further studies with various additives are needed to control the size and dispersion of nickel particles suitable for specific applications. 5. Conclusions High purity nickel powder (99.5% Ni) was successfully prepared by direct hydrogen reduction of pure commercial NiO powder in acidic slurry in autoclave with a yield of ∼ 100% at 250 °C. The initial pH of solution and the addition of NiSO4 play critical roles in the conversion process. They provide an adequate acidic media and a source of SO2− 4 counter ions for dissolving NiO into Ni2+ ions, while the subsequent reduction of Ni2+ ions to metallic nickel can proceed. For converting 0.32 mol/L NiO into metallic nickel, the optimum range of initial pH, as theoretically and experimentally determined, was pH 4–6, with the addition of 0.016 mol/L NiSO4 at 250 °C and 300 psi partial hydrogen pressure. Acknowledgments This research was supported by the General Research Project of the Korea Institute of Geosciences and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea (MKE). References
Fig. 4. Surface morphologies of (a) NiO raw powder; (b) and (c) nickel powders produced at NiO concentration of 0.32 mol/L and 0.64 mol/L, respectively.
In order to preliminarily evaluate the scale-up ability of this process, a double amount (0.64 mol/L) of NiO powder was added to the system for the conversion experiment. The change in hydrogen pressures inferred that the conversion was completed after about 20 min. However, as shown in SEM image (Fig. 4c), even stronger aggregation occurred. This can be understood due to high probability of particle collision at high temperature, pressure, and particle density. This study is just a preliminary evaluation of the feasibility
Agrawal, A., Kumar, V., Pandey, B.D., Sahu, K.K., 2006. A comprehensive review on the hydrometallurgical processes for the production of nickel and copper powders by hydrogen reduction. Materials Research Bulletin 41, 879–892. Bricknell, R.H., 1986. The structure and properties of a nickel-based super-alloy produced by osprey atomization deposition. Metallurgical and Materials Transaction A 17, 583–591. Borowiecki, T., Ebiowski, A.G., Ska, B.S., 1997. Effect of small MoO3 addition on the properties of nickel catalysts for the steam reforming of hydrocarbons. Applied Catalysis A: General 153, 141–156. Burkin, A.R., 1987. Extractive Metallurgy of Nickel. John Wiley and Sons, New York. Fievet, F., Lagier, J.P., Figlarz, M., 1989a. Preparing monodisperse metal powders in micrometer and sub-micrometer size by the polyol process. MRS Bulletin 14, 29–34. Fievet, F., Lagier, J.P., Blin, B., Beaudoin, B., Figlarz, M., 1989b. Homogeneous and heterogeneous nucleations in the polyol process for the preparation of micron and submicron size metal particles. Solid State Ionics 32–33, 198–205. Kisher, H., Gessmann, T., Wurschum, R., Kronmuller, H., Schaefer, H.E., 1995. Magnetic properties of high purity nano-crystalline nickel. Nanostructured Materials 6, 925–928. Kutty, T.R.N., Tareen, J.A.K., Basavalingu, B., Uttaraju, B., 1982. Low-temperature hydrothermal reduction of metal hydroxides to metal powders. Materials Letters 1, 67–70. Pourbaix, M., 1974. Atlas of Electrochemical Equilibria in Aqueous Solutions2nd English edition. National Association of Corrosion Engineers, Houston, Texas. Saarinen, T., Fugleberg, S., Lindfors, L., 1996a. Pressure reduction of nickel by hydrogen from hydroxide slurries. Hydrometallurgy 43, 117–127. Saarinen, T., Lindfors, L., Fugleberg, S., 1996b. Study of a nickel hydroxide sulphate precipitate obtained during hydrogen reduction of nickel hydroxide slurries. Hydrometallurgy 43, 129–142. Shukla, A.K., Venugopahan, S., Hariprakash, B., 2001. Nickel-based rechargeable batteries. Journal of Power Sources 100, 125–148. Sinha, A., Sharma, B.P., 2002. Preparation of copper powder by glycerol process. Materials Research Bulletin 37, 407–416. Wang, C., Zhang, X.M., Qian, X.F., Xie, Y., Wang, W.Z., Qian, Y.T., 1998. Preparation of nanocrystalline nickel powders through hydrothermal-reduction method. Materials Research Bulletin 33, 1747–1751. Yamasaki, N., Liang, H., 1993. Reduction kinetics of Ni(OH)2 to nickel powders and preparation under hydrothermal conditions. Metallurgical and Materials Transactions B B24, 557–561.
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Direct synthesis of nickel powders from NiO slurry by hydrothermal hydrogen reduction process Jong-Gwan Ahn a, Hoang Tri Hai b,⁎, Dong-Jin Kim a, Je-Shin Park a, Sang-Bae Kim a a b
Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea Department of Electronic Engineering, Tohoku University, Sendai 980-8576, Japan
a r t i c l e
i n f o
Article history: Received 22 October 2009 Received in revised form 8 February 2010 Accepted 8 February 2010 Available online 12 February 2010 Keywords: Nickel powder Hydrogen reduction Nickel oxide slurry Acidic solution Nickel sulfate
a b s t r a c t High purity nickel powder (99.5% Ni) was prepared by direct hydrogen reduction of commercial NiO powder in an acidic slurry in an autoclave with a yield of approximately 100% at 250 °C, initial pH 5.4 and 300 psi partial hydrogen pressure. It was found that the initial pH of the solution and the addition of NiSO4 play critical roles in the NiO/Ni powder conversion process. They provide an adequate acidic media and a source counter ions for dissolving NiO into Ni2+ ions, while the subsequent reduction of Ni2+ ions to of SO2− 4 metallic nickel proceeds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The nickel powders have attracted a great deal of attention over past decades due to their specific properties such as magnetism, thermal resistance and chemical activity; and have a wide range of applications including batteries, hard alloys, catalysts etc. (Kisher et al., 1995; Shukla et al., 2001; Bricknell, 1986; Borowiecki et al., 1997). The preparation of nickel powders from slurry compounds such as NiO, Ni(OH)2, NiCO3, NiC2O4, etc. are important because these compounds exist in industry as middle products, used catalysts and pure minerals obtained from ores. The conversion of these compounds into metallic nickel powders can be performed by conventional hydrometallurgical processes, often from ammoniacal media (Burkin, 1987; Yamasaki and Liang, 1993; Kutty et al., 1982; Saarinen et al., 1996a; Saarinen et al., 1996b; Agrawal et al., 2006; Wang et al., 1998); but little has been found so far for processing NiO slurry. This may be due to the inert and refractory properties of NiO compounds. It has been well known that a polyol process has successfully prepared metal powders from the corresponding metal oxides or hydroxides directly (Sinha and Sharma, 2002; Fievet et al., 1989a; Fievet et al., 1989b). However, for inert compound such as NiO, long induction times of several days at high temperature are generally required, resulting in high energy consumption. Thus there is a need to develop a simpler and more efficient hydrometallurgical process for NiO slurry.
In this work, a preliminary evaluation of the feasibility of preparing nickel powder by direct hydrogen reduction of analytical grade NiO slurry has been made and the influence of some key parameters on the conversion has been investigated. 2. Theoretical background The reaction of metal oxide slurries to metal powders proceeds via a dissolution-crystallization process rather than a solid phase transformation (Sinha and Sharma, 2002; Fievet et al., 1989a; Fievet et al., 1989b). Fig. 1 shows the potential-pH diagram for Ni–H2O system at 25 °C (Pourbaix, 1974) which indicates that Ni3O4 (implied for Ni2O3. NiO) can be readily dissolved in acidic solution at pH b6, while the Ni2+ ions can be reduced to Ni0 by H2 at pH N4. Theoretically, this suggests that the NiO slurry–Ni powder conversion can be carried out by hydrogen reduction in acidic solution in the pH range of 4–6. Therefore, nickel powders should be synthesized from NiO by the following scheme: (i) Dissolution of NiO into Ni2+ ions; þ
2þ
NiO þ 2H →Ni
þ H2 O
ð1Þ
(ii) Reduction of Ni2+ ions into metallic Nio species; 2þ
Ni
o
þ
þ H2 →Ni þ 2H
ð2Þ
(iii) Nucleation and growth; ⁎ Corresponding author. Tel.: + 81 22 795 7134; fax: + 81 22 263 9402. E-mail address: [email protected] (H.T. Hai). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.02.005
0
0
nNi ↔ðNi Þnucl:
ð3Þ
102
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
water and dried at 40 °C in nitrogen gas. Because of the inert properties of NiO, the yield of the conversion was simply calculated through determining the weight of the residual NiO after dissolving the product in HCl solution. In most cases, the fraction reduced was inferred from the change of hydrogen pressure and this was consistent with that determined through the weight of the residual NiO. The products were characterized by X-ray diffraction (XRD, X-Ray Diffractometer MAX 2500 h, Rigaku), and scanning electron microscopy (SEM; JSM-6380LA, JEOL). The purity of the as-prepared nickel powders was determined by an atomic adsorption spectroscopy (AAs, AAnalyst400, PerkinElmer) and an inductive coupled plasma mass spectrometer (ICP-MS, ELAN 5000, PerkinElmer SCIEX). The pH of the solution was measured by pH meter (EC-PH510). 4. Results and discussion 4.1. Effect of initial pH The effect of the initial pH on the conversion was investigated at three different values of pH = 3.2, 5.4 and 9.2. Fig. 2 shows (a) the
Fig. 1. Potential-pH diagram for Ni–H2O system at 25 °C and 1 atm.
0
0
0
0
0
ðNi Þnucl: þ mNi →ðNi Þparticl:
ð4Þ
0
ðNi Þparticl: þ ðNi Þparticl: →ðNi Þbigger particl:
ð5Þ
Additionally, by looking in detail at the proposed conversion scheme, the dissolution reaction (1) can occur in acidic medium if the system is provided a source of anions to neutralize the Ni2+ ions. Accordingly, the effects of pH and concentration of counter-anions were considered and systematically investigated. 3. Experimental The hydrogen reduction experiments were carried out in an autoclave equipped with a 350 mL glass lined reactor (5 cm× 18 cm H) and an agitator having a 4.5 cm diameter 4-blade marine type impeller rotated at 650 rpm. The reactor was charged with 250 mL H2O, NiO powder and several additive agents, such as NiSO4, K2HPO4 and NaH2PO4, together with a constant amount of polyvinyl-pyrrolidone (PVP) about 4 g (or 16 g/L) to protect against agglomeration. Herein, NiSO4 acts as a source of SO2− 4 anions, whilst K2HPO4 and NaH2PO4 are buffer agents to adjust pH of the system and PVP is used to prevent the aggregation of the produced nickel particles. Table 1 summarizes the hydrothermal reduction conditions performed in the present work. All chemicals used were supplied by Junsei Chemical Co., Ltd, Japan. Once the reaction temperature reached 250 °C, hydrogen gas with partial pressure of 300 psi was introduced into the reactor after flushing a few times with N2 gas. The decrease in pressure and its lower steady value indicates the start and end of the reaction. The change of hydrogen pressure with reaction time was recorded periodically to calculate the reduction fraction. After completion of the reaction, the product was filtered, washed three times by distilled Table 1 Experimental conditions for hydrogen reduction of NiO. Factors
Conditions
Temperature PH2 Initial pH NiO concentration NiSO4 concentration PVP concentration K2HPO4 + NaH2PO4 (pH controllers)
250 °C 150–300 psi 3–10 0.32–0.64 mol/L 0–0.016 mol/L 16 g/L Changeable
Fig. 2. (a) Fraction reduced as a function of time at different initial pH. (b) XRD patterns of the products obtained at different initial pH.
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
103
fraction reduced as a function of reaction time; and (b) XRD patterns of the corresponding products. The fraction reduced gradually increased and reached a steady state of 10% and 62% at initial pH 9.2 and 3.2, respectively. The low value can be explained by the low solubility of NiO in basic solution, while partial reduction in acidic media is due to the low reduction potential of hydrogen as inferred from Fig. 1. The final pH values measured for both cases were pH 7.4 and 0.7, respectively. However, at an initial pH of 5.4, the fraction reduced rapidly increased and the conversion is complete after 9 min. This is reasonable because at this pH value, NiO is reasonably well dissolved to give Ni2+ ions and the subsequent reduction of Ni2+ to Nio can proceed. The final pH measured under this condition is pH 3.1 which is lower than pH 4 as inferred in Fig. 1, but the 100% yield of this conversion can be understood because the reduction potential of hydrogen is promoted by not only pH, but also the temperature. The XRD patterns of the final products shown in Fig. 2b which strongly support the reactions performed at different initial pH (Fig. 2a). Indeed, only well-defined Ni0 crystalline peaks are observed in the XRD pattern of the product obtained at initial pH 5.4, whereas at initial pH 3.2 and 9.2, both Ni0 and NiO crystalline peaks exist in the diffraction patterns. At initial pH 9.2, the product is dominated by the NiO phase, while the portion of NiO is similar to that of Ni0 in the product obtained at initial pH 3.2 as judged from their diffraction intensities. According, the optimum value of initial pH lies within the range of pH 4–6 for complete conversion. 4.2. Effect of NiSO4 concentration The effect of NiSO4 on the conversion is understood due to its impact on the dissolution of NiO under hydrothermal conditions and can be plausibly explained by the following scheme:(iv) Dissociation of NiSO4 and subsequent reduction of Ni2+ ions in aqueous solution: NiSO4 ↔Ni Ni
2þ
2þ
2−
þ SO4 0
ð6Þ
þ
þ H2 →Ni þ 2H
ð7Þ
(v) Dissolution of NiO and subsequent reduction of the corresponding Ni2+ ions: 2−
NiO þ SO4 Ni
2þ
þ
2þ
þ 2H →Ni 0
þ
þ H2 →Ni þ 2H
2−
þ SO4
þ H2 O
ð8Þ
Fig. 3. (a) Fraction reduced as a function of time at different NiSO4 concentrations. (b) XRD patterns of the products obtained at different concentrations of NiSO4.
ð9Þ
The SO2− 4 anions dissociated from Eq. (6) act as counter-anions to neutralize the Ni2+ ions dissolved from the solid NiO in Eq. (8); thus allowing the dissolution of NiO. In order to verify this mechanism, the hydrothermal reactions were performed at different concentrations of NiSO4 in range of 0–0.016 mol/L, whilst the initial pH of 5.4 and other parameters were fixed. Fig. 3 shows (a) the fraction reduced and reaction time dependence, and (b) XRD patterns of the corresponding products. Without NiSO4, the fraction reduced is quite negligible (approximately 8% even after 30 min), whereas it rapidly increases with an increase in NiSO4 concentration. For instance, the fraction reduced reaches 45% with 0.008 mol/L NiSO4 addition and 100% after just 12 min, with 0.016 mol/L NiSO4 added. The low conversion yields at low NiSO4 concentrations can be attributed to the low solubility of NiO rather than a low reduction potential of hydrogen because the final pH values were 5.1 and 3.8, respectively. The 100% yield achieved at 0.016 mol/L NiSO4 is clearly indicated by only Nio crystalline peaks observed in the XRD pattern (Fig. 3b), while the incomplete conversions at 0 and 0.008 mol/L NiSO4 are well demonstrated by the presences of both Nio and NiO diffraction peaks.
Accordingly, addition of NiSO4 as a source of SO−2 counter ions is 4 essential for the conversion of NiO to Ni powder and the above mechanism of NiSO4 on the conversion is plausible. The impact of SO2− anions on this conversion was further proven 4 by using H2SO4 instead of NiSO4 wherein the initial pH was maintained at 5.4 by addition of K2HPO4 buffer agent. The same effect as of NiSO4 was observed, but the phosphorus impurity was significant in the metal powder product. 4.3. Characteristics of metallic nickel product Fig. 4 shows the SEM images of (a) raw NiO and (b) Ni powders prepared from 0.32 mol/L NiO powder produced under standard conditions (0.016 mol/L NiSO4, 250 °C, pH 5.4 and 300 psi partial hydrogen pressure). The morphologies of two powders are clearly different. NiO powder consists of primary sharply-edged shaped submicron size particles in a highly aggregated form. After hydrogen reduction, the original NiO particles converted into spherical-grain metallic Ni particles with a high degree of aggregation. The chemical analysis revealed that in most cases, the purity of the nickel powder was very high (about 99.5%).
104
J.-G. Ahn et al. / Hydrometallurgy 102 (2010) 101–104
of preparing nickel powder by direct hydrogen reduction of NiO slurry. Therefore, further studies with various additives are needed to control the size and dispersion of nickel particles suitable for specific applications. 5. Conclusions High purity nickel powder (99.5% Ni) was successfully prepared by direct hydrogen reduction of pure commercial NiO powder in acidic slurry in autoclave with a yield of ∼ 100% at 250 °C. The initial pH of solution and the addition of NiSO4 play critical roles in the conversion process. They provide an adequate acidic media and a source of SO2− 4 counter ions for dissolving NiO into Ni2+ ions, while the subsequent reduction of Ni2+ ions to metallic nickel can proceed. For converting 0.32 mol/L NiO into metallic nickel, the optimum range of initial pH, as theoretically and experimentally determined, was pH 4–6, with the addition of 0.016 mol/L NiSO4 at 250 °C and 300 psi partial hydrogen pressure. Acknowledgments This research was supported by the General Research Project of the Korea Institute of Geosciences and Mineral Resources (KIGAM) funded by the Ministry of Knowledge Economy of Korea (MKE). References
Fig. 4. Surface morphologies of (a) NiO raw powder; (b) and (c) nickel powders produced at NiO concentration of 0.32 mol/L and 0.64 mol/L, respectively.
In order to preliminarily evaluate the scale-up ability of this process, a double amount (0.64 mol/L) of NiO powder was added to the system for the conversion experiment. The change in hydrogen pressures inferred that the conversion was completed after about 20 min. However, as shown in SEM image (Fig. 4c), even stronger aggregation occurred. This can be understood due to high probability of particle collision at high temperature, pressure, and particle density. This study is just a preliminary evaluation of the feasibility
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