Shorter CommunicationEffect of crystallisation on the reaction kinetics of nickel reduction by hydrogen
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):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 , respectively. Internal features were a removable draft tube, a removable set of four baffles (8 mm width and
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 was the fastest, followed by and . In , the reaction was the slowest, and took four times longer than in to achieve the final conversion.
Fig. 2 shows the size distributions of the original seed and final nickel particles of the densifications , and . 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 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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