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Effect of crystallisation on the reaction kinetics of nickel reduction by hydrogen

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Abstract

The reaction kinetics of precipitation from aqueous solution is not only a function of the concentration of reagents but also depends on the properties of the solid particles formed in the process. These property changes arise from the increasing influence of surface properties in comparison to volumetric bulk-properties as the particle size decreases. The ratio α of the active surface area to the actual surface area of the particles in the system is used in this work to evaluate the reaction activity of the particles. The investigation of the reaction kinetics of nickel reduction without sampling during the process of the reaction was successfully carried out in the experiment. The overall kinetics of nickel reduction have been suggested, where the constants relate to the main processes of nickel precipitation from the solution. The significant agglomeration reduces the deactivation of the nickel particles in the precipitation process, while breakage and crystal growth decrease the activation. The activations of dry and wet seeds are insignificantly different.

Introduction

Precipitation is an extensively industrialised process. It is also called reactive crystallisation, typically with high supersaturations which very often induce nucleation and agglomeration besides crystal growth, breakage and attrition (Bratland et al., 1997). This results in difficulty in precisely controlling the process in order to obtain the required crystal morphology and size distribution of the final product. The main factors to be controlled in precipitation process are the supersaturation of the solute in solution (Manth et al., 1996) and the depletion of the supersaturation in a batch/semi-batch process. The reaction kinetics of the precipitation process directly influence the depletion of the supersaturation.

Studies of the kinetics of precipitation processes usually focus on the particle shape and size distribution. In addition, the population balance equation is frequently used (McCoy, 2002) to trace the evolution of the particle size distribution (PSD). In contrast, although some works have investigated the reaction kinetics of a precipitation process (Skoufadis et al., 2003, Saarinen et al., 1998, Yang et al., 1990), a few have focussed on the reaction kinetics of the precipitation from melt solution (Robson, 2004), the publications are related to the effect of crystallisation process on the reaction kinetics of batch precipitation from aqueous solution. For example, the influence of the characteristics of nickel particles on the reaction kinetics of nickel reduction from aqueous solution by hydrogen has not been investigated and is not reflected in the kinetic equations used (Saarinen et al., 1998).

The industrialised precipitation process of production of nickel metal powder from aqueous solution by pressure reduction with hydrogen is operated in an agitated autoclave in semi-batch mode, in which the presence of nickel particles is necessary for the reaction to proceed (Saarinen et al., 1998, Yang et al., 1990), in order to catalyse the reaction and/or provide the surface for the deposition of the product in solution. The effect of the crystallisation in the nickel reduction process on the reaction kinetics was investigated in this work.

Section snippets

Overall kinetics of nickel reduction

The overall reaction of nickel reduction is as follows (Saarinen et al., 1998):Ni(NH3)nSO4+H2Ni0+(NH4)2SO4+(n-2)NH3.In the reaction, a nickel solid is necessary on which nickel atoms can deposit. The rate of reduction is affected by temperature, hydrogen pressure, seed addition, agitation speed, the amount of alkali used, the morphology and the concentration of nickel particles and pH of the solution (Saarinen et al., 1998).

As a heterogeneous reaction process, the suggested mechanism for the

Materials and methods

The experimental process followed in this work was similar to that carried out industrially. Metallic nickel was produced by the reduction of ammoniacal nickel sulphate solution by hydrogen under high pressure in an agitated laboratory autoclave. The round-bottomed autoclave was constructed from 316 stainless steel with maximum pressure and temperature ratings of 15 Mpa and 350C, respectively. Internal features were a removable draft tube, a removable set of four baffles (8 mm width and 90

Results

Two experimental repetitions of the dropping time of hydrogen pressure from 28 bars to 25 bars versus reaction time are shown in Fig. 1. It can be seen that the reduction rate in densification D2 was the fastest, followed by D1 and D3. In D4, the reaction was the slowest, and took four times longer than in D2 to achieve the final conversion.

Fig. 2 shows the size distributions of the original seed and final nickel particles of the densifications D1, D2 and D3. According to Figs. 1 and 2, the

Conclusion

The precipitation process in nickel reduction critically influences the kinetics of the reaction, and thus results in the variation of the overall reaction kinetics. The ratio α of the active surface area to the actual surface area of the particles in system is used in this work to evaluate the reaction activation of the particles. The overall kinetics of nickel reduction has been suggested, with constants relating to the main processes of nickel precipitation.

The investigation of the reaction

Acknowledgments

The authors would like to acknowledge the financial support of the University Research Commission (URC) at the University of Cape Town (UCT) and of the National Research Foundation (NRF).

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