Development And Application Of A Partial Equilibrium Model For The Precipitation Reaction Systems

Precipitation is widely used in the industry of powder production, metal ions separation and purification. In order to optimize the operating parameters and improve the product quality, a partial equilibrium model was proposed in the thesis for describing the relationships among the reaction components in the precipitation reaction process. The new model assumes that the precipitation reactions should consist of two steps including a liquid-phase reaction step followed by a solid-formation step, in which the first step is considered fast enough to make the liquid phase reaction system in a partial equilibrium state which can be described by ionic equilibrium thermodynamics. Various precipitation systems were chosen for systematic research on the model construction, prediction verification, and practical application.For the ideal precipitation systems of M+-P-and M+-P--H+(M+metal ion, P-precipitant, H+hydrogen ion), partical equilibrium models based on their reaction mechanisms were investigated. Compared with the calculation results of the rigorous kinetic models, the prediction results of the reactant component concentrations by the partial equilibrium models were found to be affected by the reaction rate constants and yield of precipitation. The prediction was found to be more accurate with higher reaction rate constants. For both of the ideal systems, the prediction deviation from the kinetic model was found to be less than1%under the condition that the liquid-phase reactant constants are100times of the solid-formation reaction constant and the yield of precipitation is more than0.6%at the same time.To further study the prediction accuracy of the partial equilibrium models, two convenient-for-detection practical systems were chosen and experiments were performed. For the system of Co(II)-H2C2O4-K2C2O4, pH and the yield of precipitation were both introduced into the partial equilibrium model for simulation of the relationship between them. The simulation results correspond well to the experimental results. Research results also indicate that the accuracy of the partial equilibrium model is improved after the transformation into a semi-empirical equation or the introduction of Davies activity correction. For the system of Ba(Ac)2-H2C2O4-HAc, parameters including pH, Ba total concentration [Ba]1and conductivity k were all introduced into the partial equilibrium model to model the three relationships of pH-[Ba]T, K-[Ba]T, and pH-K. The simulation results are in good agreement with the experimental results.Based on the above verifications, five practical precipitation systems were adopted to investigate the application of the partial equilibrium model in powder manufacturing, separation and purification, and online process monitoring.Firstly, the monometallic precipitation system of NiCl2-H2C2O4-K2C2O4and the co-precipitation system of Ni(II)-Co(II)-C2O42--H2O were studied to investigate the practical application of partial equilibrium models in the preparation of powders. Normally stable pH, appropriate supersaturation, and high metallic recovery rate are required to obtain narrowly distributed and monoshaped powders economically. To achieve such requirements, corresponding partial equilibrium models were constructed to simulate the effects of process operation conditions such as feed solution composition, proportion, operating mode and feeding mode on process pH, supersaturation and yield of precipitation with the goal of finding proper operating conditions for producing narrowly distributed and monoshaped powders with high yield of precipitation. Under such optimized operating conditions, experiments were carried out to prepare high quality nickel oxalate powders and nickel-cobalt oxalate powders.Afterwards, the system of Ni(II)-Mn(II)-NH3-CO32--H2O was employed to investigate the practical application of partial equilibrium model in separation and purification of metal ions. To achieve effective separation of manganese by selective precipitation from the nickel manganese mixture solution in the absence of the formation and colloidal manganese hydroxide and fine-sized manganese carbonate, a low supersturation level of nickel carbonate and manganese hydroxide with a suitable supersaturation level of manganese supersaturation are commonly desirable. To satisfy such a requirement, partial equilibrium models were established and simulations were carried out to determine the optimum process conditions such as feed solution concentration, proportion, and feeding modes for separation. Under such optimized conditions, experiments were performed to achieve the effective separation of nickel and manganese with the loss percentage for nickel as low as1.0%and the yield of precipitation for manganese as high as99.7%.Lastly, the precipitation systems of Co(NO3)2-H2C2O4-K2C2O4and Ca(OH)2-CO2-H2O were adopted to investigate the practical application of partial equilibrium model in the online monitoring of the precipitation processes. It was shown that by the input of the online monitored pH data, the evolution of yield of precipitation and supersaturation for the system of Co(N03)2-H2C204-K2C204can be calculated by a partial equilibrium model. And by the input of the both online monitored pH and conductivity data, the evolution of yield of precipitation and supersaturation of the more complicated system of Ca(OH)2-CO2-H2O can also be calculated by a partial equilibrium model. It can be therefore considered that the partial equilibrium models provide possible opportunity for the application of simple and practical devices including the pH meter and (or) conductivity meter for online inspection of the main components and key process parameters in precipitation. Such applications show great theoretical and practical significance in obtaining information on evolution of the precipitation system and in optimization of the precipitation processes.