Stepwise Treatment Of Waste Acidderived From Titanium Dioxide Production And Comprehensive Utilization Of The Products

In the process of producing titanium dioxide by sulfuric acid technology,a large amount of waste acid is generated,which is high in acidity and contains a large number of iron ions and other metal ions.Therefore,it is difficult to be recovered and reused.Effective treatment of this waste acid is of great significance to the sustainable development of titanium dioxide industry.For harmless treatment and resource utilization of the waste acid,this paper proposed a stepwise processing strategy to treat it into common wastewater,and sulfate and iron ions were recycled to produce white gypsum,schwertmannite,and biochar supported zero-valent iron magnetic composite（Fe⁰/Fe₃O₄@BC）.Moreover,schwertmannite and Fe⁰/Fe₃O₄@BC composite were applied to treat other wastewater,achieve the purpose of treating waste with waste.The main contents and results of this study are as follows:（1）After three treating steps of initial neutralization with lime,chemical mineralization,and complete neutralization with sodium hydroxide,94.6%of SO₄^2?,100%of Fe,and most of other metals in the waste acid were recycled.The treated water sample was neutral with low content of metal ions,which could be discharged as common wastewater to general sewage treatment plant for further treatment.The white gypsum produced in the initial neutralization with lime treating step contained less impurity and exhibited high purity and complete crystal,which was consistent with natural gypsum.The schwertmannite produced in the chemical mineralization treating step showed ball-with-whiskers microstructure,and its structural characteristics were consistent with those formed in nature.It had a large specific surface area（130.4m²/g）.The molar ratio of Fe/S was 4.58,and the chemical formula could be determined as Fe₈O₈（OH）_4.5（SO₄）_1.75.Fe⁰/Fe₃O₄@BC composite was produced from high-temperature reductive calcination of hydroxides@lignocellulose complex formed during complete neutralization treating step.The analysis results showed that the particles such as Fe⁰and metal oxides were supported on the network structure of porous biochar.The composite had large specific surface area（335.3 m²/g）,high porosity,and superparamagnetic properties.（2）The catalytic degradation of organic dye methyl orange（MO）with schwertmannite as heterogeneous Fenton-like catalyst was studied.The results showed that schwertmannite could catalyze H₂O₂to produce active free radical·OH in a wide pH range,which effectively oxidized and degraded MO into H₂O and CO₂.The schwertmannite exhibited good reusability,and the degradation efficiency of MO was still above 95%after 5 cycles.（3）The removal effect of Cr⁶⁺with Fe⁰/Fe₃O₄@BC composite as adsorbent was studied.The results showed that under the conditions of pH=3,NB initial concentration of 100 mg/L,and adsorbent dosage of 2 g/L,the equilibrium adsorption capacity of Cr⁶⁺onto Fe⁰/Fe₃O₄@BC composite was 67.74 mg/g.The adsorption process conformed to the pseudo-second order kinetic model,indicating that the removal of Cr⁶⁺by Fe⁰/Fe₃O₄@BC composite was mainly via chemical adsorption.XPS analysis of the composite before and after adsorption showed that Cr⁶⁺was reduced to Cr³⁺by Fe⁰,and most Cr³⁺and Fe³⁺formed Cr³⁺/Fe³⁺hydroxides precipitation on the surface of the composite.（4）The removal effect of nitrobenzene（NB）with Fe⁰/Fe₃O₄@BC composite was studied.The results showed that removal efficiency of NB could reach100%after 60 min under the conditions of pH=3,composite dosage of 2 g/L,temperature of 30℃,and rotating speed of 170 r/min.The results of molality concentration change analysis showed that the reduction rate of NB to aniline（AN）was about 65%,indicating that porous biochar in Fe⁰/Fe₃O₄@BC composite physically adsorbed part of NB or reduced it to other intermediates.In conclusion,the stepwise treatment strategy is a technology with wide application prospects for the treatment of highly acidic industrial wastewater and comprehensive utilization of related products.