Study On Component Activity And Phase Equilibrium Of Hydrometallurgical Systems

There are a huge number of phase equilibrium problems in hydrometallurgical processes of heavy metals (HM), such as Mn, Co, Ni, Cu, Zn. Among them, the most urgent is the crystallization of calcium sulfate and magnesium sulfate, which harms production process and decrease production qualities by scaling on the wall of reactors or pipelines or crystallizing along with main crystal products as impurity. Reducing the harmless of this problem has become a world-wide difficult task. To develop a new approach to avoid the harm of calcium sulfate and magnesium sulfate, profound understanding of the complete phase diagram of hydrometallurgical systems MSO4+NSO4+H2SO4+H2O (M=Mn, Co, Ni, Cu, Zn; N=Ca, Mg) from298.15to373.15K is necessary. On account of the complexity of these phase diagrams, it is necessary to using the thermodynamic model to obtain them. Before doing this work, we have to obtain the thermodynamic model parameters by accurate water activity data of binary subsystems MSO4+H2O (M=Mn, Co, Ni, Cu, Zn) from298.15to373.15K. Up to now, a large number of measurements of water activities have been reported at298.15K for these binary subsystems. However, equivalent data at higher temperatures are rather sparse for all of them.Likewise, a method for economically separating magnesium and lithium in the salt brine with a high Mg:Li ratio has become a world-wide difficult task. Among the separation methods proposed is the method "Precipitating Mg2+by NH3(g)". However, engineers reported that several types of unexpected salts were crystallized on the walls of reactors or pipelines and harm the precipitation process of Mg(OH)2. To gain a profound understanding of the crystallization phenomena, information on the solubility phase diagram of the quaternary system MgCl2+LiCl+NH4Cl+H2O is required. To date, the phase diagram of this quaternary system has never been reported.In order to solve the above-mentioned problems, the main research contents and results are as follows:1. In this paper, an isopiestic apparatus which can determine the water activity of aqueous solution at298.15to423.15K has been established. The apparatus contains four main parts:isopiestic container, temperature control, rocking mechanism and evacuated device. After accurate testing, the sealing performance of isopiestic container and evacuated device are very well; the temperature control can reach a precision of±0.01K; while the evacuated device can decrease the pressure of the isopiestic container to10Pa.2. To verify the reliability of our established apparatus and the experimental procedures, the water activity of the reference solutions CaCl2+H2O and H2SO4+H2O at298.15and323.15K from low to high concentration were determined. At the same time, the water activity of the reference solutions CaCl2+H2O and LiCl+H2O at373.15K from low to high concentration were also determined. The results between the reference solutions are agree with each other very well. Furthermore, the water activity of the target systems MSO4+H2O (M=Mn, Co, Ni, Cu, Zn) at298.15K were determined at one high and one low concentration and compared with reliable literature data. The results obtained by our method agree very well with the literature values. In conclusion, the apparatus established by us can successfully determine the water activity of aqueous solution from298.15to373.15K.3. The water activities of the binary heavy metal sulfate aqueous systems MSO4+H2O (M=Mn, Co, Ni, Cu, Zn) were measured at323.15and373.15K using our isopiestic apparatus. The reproducibility of all the results was better than0.2%in mass fraction. The water activities for system MnSO4+H2O were obviously higher than the other four systems MSO4+H2O (M=Co, Ni, Cu, Zn).4. A Pitzer thermodynamic model was chose to describe and predict the thermodynamic properties of these binary systems MSO4+H2O (M=Mn, Co, Ni, Cu, Zn) and related ternary and quaternary systems. The binary Pitzer model parameters at373.15K were obtained by fitting the water activity data of binary systems MSO4+H2O (M=Mn, Co, Ni, Cu, Zn) at373.15K determined in this paper. Combining the accurate literature parameters at298.15K of these systems, the relation between Pitzer model parameters and temperature were obtained. By using this relation, the water activity of these systems at323.15K were predicted and compared with our experimental results, which show high agreement between them. This indicates the Pitzer model parameter relation with temperature obtained in this work is reliable.5. The methods for precise determination ions Mg2+and NH4+in the quaternary system MgCl2+LiCl+NH4C1+H2O were studied. By using the improved methods, the accurate content of Mg2+and NH4+were elaborately determined and compared with the standard concentration of the solution, which show the relative errors can be controlled within0.3%and0.2%respectively.6. Solubility isotherms as well as the corresponding solid phases of the quaternary system MgCl2+LiCl+NH4C1+H2O at298.15K have been elaborately measured by both isothermal equilibrium method and evaporation method. Four crystallization fields including two double salts (LiCl·MgCl2·7H2O(s) and NH4Cl-MgCl2·6H2O(s)), one hydrate salt (MgCl2·6H2O(s)) and one solid-solution (LiCl·H2O+NH4C1)(SS) were detected in this system. Within these fields, the formation areas of the (LiCl·H2O+NH4C1)(SS) and NH4Cl·MgCl2·6H2O(s) phases are the largest. The good agreement results between two experimental methods indicate the experimental data obtained in this paper are reliable.7. A Pitzer-Simoson-Clegg (PSC) thermodynamic model was used to simulate and predict the thermodynamic properties of this quaternary system and its sub-systems. The binary model parameters were obtained by simulating the reliable experimental solubility and water activity of the binary systems MgCl2+H2O, LiCl+H2O and NH4C1+H2O. The ternary parameters were fitted against the experimental solubility of the ternary systems MgCl2+LiCl+H2O, MgCl2+NH4C1+H2O and LiCl+NH4C1+H2O. The water activities of the ternary systems MgCl2+LiCl+H2O and LiCl+NH4C1+H2O, as well as the solubility of the quaternary system MgCl2+LiCl+NH4C1+H2O were predicted by the PSC model, the results of which were compared with available literature data and the experimental results in this work. The excellent agreement between the predicted and experimental results indicates that the solubility results obtained in this work are reliable. 8. Based on the model calculations, application examples were given to understand the crystallization phenomena in the Mg-removal process from the brine containing MgCl2and LiCl by NH3gas. Moreover, a series of equal-scale lines of water activity of the titled quaternary system has also been calculated. The calculated results help to elucidate the crystallization route in isothermally evaporating the brine containing LiCl, NH4Cl and MgCl2.