New Green Process For Comprehensive Utilization Of Ferrous Sulfate And Relevant Fundamental Investigation

Ferrous sulfate heptahydrate FeSO₄·7H₂O is a major waste produced in titanium dioxide industry by the sulfate process.This process generates more than 3 tons of FeSO₄·7H₂O per ton TiO2 produced.In China,about 10 million tons of FeSO₄·7H₂O are generated annually from TiO2 industry.Most of them are disposed without recycling,which has resulted in serious pollution and become a bottleneck for the development of titanium pigment plants.In this work,on one side,it is proposed to improve the shape and purity of FeSO₄·7H₂O by optimizing the operation conditions;on the other side,a new process for production of H₂SO₄ and Fe₂O₃ from FeSO₄·7H₂O was provided.The H₂SO₄ solution recovered from it can take up about 50%of the total amount of H₂SO₄ consumed in the sulfate process.The main results are listed as following:（1）Anew process was provided for recovery of H₂SO₄ and Fe₂O₃ from FeSO₄·7H₂O.In the new process,HCl was added into the FeSO₄ solution and FeCl₂·4H₂O was produced because of common ion effect.Then Fe₂O₃ was obtained by calcination of FeCl₂·4H₂O at 380℃.Concentrated H₂SO₄ can be obtained by evaporation of the filtrate to reuse in the titanium dioxide production.A laboratory-scale test was carried out with about 70%recovery of ferrous source for a single cycle,indicating the feasibility of the new process.（2）Experimental apparatus were established for measuring the vapor-liquid equilibrium and solid-liquid equilibrium.The phase equilibrium of the following systems were investigated:1)the vapor-liquid equilibrium of the FeSO₄-Fe₂（SO₄）₃-H₂SO₄-H₂O system at 30、60、90 and 101.3 kPa;2)the solubility of FeSO₄-7H₂O in the Ti（SO₄）₂-H;2SO4-H₂O system at 278-298 K with Ti（SO₄）₂ and H₂SO₄ concentrations of 0.16-1.2 mol kg^-1 and 1.6-3.7 mol kg^-1 respectively;3)the solubility of FeSO₄·7H₂O in the MgSO4-H₂SO₄-H₂O system at 298-308 K with MgSO4 and H₂SO₄ concentrations of 0.2-1.3 mol kg^-1 and 2.9-3.6 mol kg^-1 respectively;4)the solubility of FeCl₂-4H₂O in the H₂SO₄-HCl-H₂O system with HC1 and H₂SO₄ concentrations of less than 11 and 4 mol·kg^-1.（3）Speciation-based chemical models of the FeSO₄-Fe₂（SO₄）₃-H₂SO₄-H₂O,FeSO₄-MgSO4-Ti（SO₄）₂-H₂SO₄-H₂O and FeCl₂-H₂SO₄-HCl—H₂O systems were established,considering the phase and the dissociation equilibrium of species.The activity of species were calculated with Bromley-Zemaitis model,the MSE model and the electrolyte NRTL model.From the calculation results with the new model,it can be found that with increasing temperature and the concentration of H₂SO₄,the number of sulphate ions complexed with ferric increases from one（FeSO₄⁺）to two（Fe（SO4）2）in H₂SO₄ solutions.And FeCl⁺ can be formed for Fe（Π）in HCl solution,which relative concentration increases with the increase of HCl concentration.（4）Mass balance of the new process has been calculated with the new model in the OLI platform.Experiments were carried out to verify the feasibility.The predicted yield of FeCl₂·4H₂O was generally in accordance with the experimental data.And H₂SO₄ and Fe₂O₃ were also obtained.Moreover,the calcination of FeCl₂·4H₂O（380℃）can save much more energy than the calcination of FeSO₄·7H₂O（650℃）.