Preparation Of High Purity Ferrous Sulfate From Waste Sulfuric Acid By Eutectic Freezing Crystallization

Ferrous sulfate is an important chemical raw material.The crude ferrous sulfate containing numerous impurities is often treated as industrial waste.At the same time,as the raw material for industrial production of ferrous sulfate,nearly 40%of waste sulfuric acid has adopted neutralization treatment means.In order to resource utilization of industrial waste sulfuric acid,adding iron material and cooling crystallization to prepare ferrous sulfate is a more common resource utilization process,but the disadvantage of this process is that the recovery rate of ferrous sulfate is low and a large amount of residual mother liquor will be produced,which still needs to be post-treated.Eutectic freezing crystallization is a new water treatment technology,which realizes the simultaneous precipitation of ice crystals and solid salts and the concentration of pollutants through the principle of solid-liquid phase equilibrium.However,eutectic freezing crystallization method has not been applied to the preparation of ferrous sulfate from waste sulfuric acid.This thesis aims to resourcefully utilize waste sulfuric acid by preparing crude ferrous sulfate by eutectic freezing crystallization technology,which is purified to prepare feed additive grade high-purity ferrous sulfate(GB34465-2017)for resourceful recycling.In this paper,the effects of various process parameters on the crystalline quality and recovery of ferrous sulfate during eutectic freezing crystallization were investigated,and a pilot plant for the preparation and purification of ferrous sulfate by eutectic freezing crystallization was designed on this basis.Firstly,the study was conducted to investigate the preparation of ferrous sulfate from waste sulfuric acid by eutectic freezing crystallization method for resourceful utilization of waste sulfuric acid.In this chapter,the impurity content and regulation techniques in ferrous sulfate crystals were investigated in combination with Visual MINTEQ software simulations and experimental studies.The Visual MINTEQ software results showed that when the pH was controlled at 6.3-6.5,the chromium element was mainly removed as Cr(OH)3 was removed,and the removal rate of chromium and ferrous sulfate loss reached 70%and 10%,respectively.The solubility and super-soubility of ferrous sulfate in this system were measured by the equilibrium method and FBRM technique,respectively,and the dissolution curves of ferrous sulfate in waste sulfuric acid were plotted by equation fitting to determine its mesostasis zone.The recoveries were in the range of 38.5%-43.2%by cooling crystallization(25℃-50℃)and increased to more than 60%by freezing co ncentration co-crystallization(-8℃-25℃),and the elemental chromium content was reduced from 67 mg·kg-1 to 19 mg·kg-1,which reached the standard of water treatment agent ferrous sulfate class Ⅱ.Secondly,crude ferrous sulfate was investigated to purify crude ferrous sulfate by eutectic freezing crystallization method.Visual MINTEQ calculations showed that when pH was controlled at 4,titanium element was removed mainly as TiO2-2H2O with 99.9%removal rate.The addition of 0.01 mol of sodium sulfide to 500 mL of mother liquor was able to remove more than 90%of zinc and nickel ions,and 60%of cobalt ions.In this section,the effects of supersaturation,freezing temperature,stirring rate,and addition of crystalline species on the crystallization yield and quality during the eutectic freezing crystallization process were investigated.The results of response surface analysis were obtained that the supersaturation degree of ferrous sulfate had the most significant effect on its crystallization purity,followed by freezing temperature,stirring rate and crystal seed addition,respectively,and the best experimental conditions were obtained,and the purity reached more than 99.7%in accordance with the standard of feed additive ferrous sulfate(GB 34465-2017).Finally,a pilot test for the purification of ferrous sulfate by eutectic freezing crystallization method was carried out.Firstly,the design and calculation of the reactor were carried out,and a three-phase separator for separating ferrous sulfate,ice crystals and concentrate mixture was successfully built.The pilot crystallization reactor was designed with a capacity of 0.5 t/d.The effect of stirring rate on the ice crystal effluent was investigated,and the results showed that the ice crystal effluent had the highest quality and the conductivity removal rate was over 90%when the stirring rate of the three-phase separator was 120 r/min.The effect of maturation time on the crystallization of ferrous sulfate was investigated,and the results showed that the purity of ferrous sulfate increased from 98%to 99.5%as the maturation time increased from 3 h to 6 h.Through energy consumption analysis,it was calculated that the electricity consumption per ton of wastewater was 26.88 kWh,which was 40.3%saving compared with the evaporation crystallization electricity consumption.It provides a reference for promoting the industrial application of eutectic freezing crystallization method for resourceful use of waste sulfuric acid to prepare high purity ferrous sulfate,which has some practical significance.