| Titanium dioxide is an important white pigment and widely used in various industries. At present, sulfuric acid method is basically used to produce Titanium dioxide in China. In this way,8～10tons of waste acid containing20%sulfate acid and5%～8%ferrous sulfate is discharged when1ton of Titanium dioxide is produced. The recycling governance of concentrated waste acid is of great significance to enterprise development and environmental safety. In this paper, the removal of ferrous sulfate from concentrated waste acid was studied by solvent-out crystallzation in order to solve the bottleneck problem in the waste acid concentration and recycling of resources.Firstly, in order to determine the feasibility of using solventing-out crystallzation in the removal of ferrous sulfate from the waste acid, an organic solvent (marked SOA) was chosen and the solubility of ferrous sulfate in pure water, sulfuric acid solution, SOA-sulfuric acid-water mixed solution were determined through laser detection technology. The results show that the solubility of ferrous sulfate in sulfuric acid solution can be effectively reduced by solventing-out crystallization using SOA as eluent, so it is feasible to remove ferrous sulfate in the concentrated waste acid by solventing-out crystallization. Based on the solubility data, the concentration of ferrous sulfate in waste acid can decrease from260g/L to5.01g/L and the theoretical removal rate can be reach over95%when the temperature is10℃and mass ratio of SOA to water is1:1by solventing-out crystallization. Based on the solubility and supersolubility data of ferrous sulfate, operating conditions in solventing-out crystallization were optimized for the simulated waste acid. The results show that the optimum process conditions are as follows:the speed of agitator is150r/min, the adding ratio of seeding is1%, the eluent is added by continuously drip and the crystalline curing time is60min. Under these conditions, the actual removal rate of ferrous sulfate can reach91.3%by solventing-out crystallization.For the sulfuric acid solution after solventing-out crystallization, a preliminary design about the solvent recycling distillation tower was conducted and the energy consumption in the eluent recycling and the evaporation concentration of sulfuric acid were calculated with Aspen simulation software. A total energy consumption of4.27×106kJ is needed to dispose of1ton of the waste acid. An integrated governance process was designed and compared with the direct concentration technology. The results show that the solventing-out crystallization technology has great advantages both in technical performance and economic benefits. |
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