Pollution Analysis And Environmental Risk Assessment Of Waste By-product In The Sulphate Process Of Titanium Dioxide Production

A lot of waste by-products will be produced in the titanium dioxide production process,such as sulphate method process will produce 1300 t/a waste acid,13000 t/a acidic wastewater,and 1700 t/a titanium gypsum.Titanium gypsum,with CaSO4·2H2O as the main component,is not only massive,but contains many kinds of heavy metal impurities(e.g.,Cr,Ni,As,Pb etc.).The long time stacking of titanium gypsum except for taking up a lot of land,may pollute the environment,but until now our country hasn't establish accurate qualitative of titanium gypsum,related treatment and management standard.In this paper,we study the whole titanium dioxide production process by three methods,such as literature review,expert consultation,and on-the-spot investigation.Mentioned with production process,wastewater treatment process,sludge characteristics,and characteristics of the risk assessment of sludge lifecycle,we analysis the nodes,chemical composition,treatments of waste acid and wastewater,and the disposal condition,chemical and hazardous characteristics analysis and environmental risk assessment of titanium gypsum.In order to guide the treatment and disposal of sludge in the industry,the character of titanium gypsum and its potential risks to the environment,were clarified to provide a theoretical basis for the formulation of relevant management policies.The results show that:(1)At present,the utilization rate of waste acid in sulfuric acid titanium dioxide industry has been greatly improved,which is as high as 52.1%.The waste acid has been successfully applied to recycle in concentrated method,leach the ore,prepare the polymeric ferric sulfate(water purifying agent)and prepare the calcium superphosphate or ammonium sulfate fertilizer.While these methods still have their limitations,such as the high energy consume in the concentrated process and limited market of polymerized ferric sulfate and fertilizer etc.In addition,comparing the waste acid utilization in North China,Jiangsu and Zhejiang and Panzhihua three areas,we found that in Jiangsu and Zhejiang area of the utilization of waste acidis highest up to 61.2%,while in North China and Panzhihua area the utilization rate is low,is 25.7%and 17.8%respectively.Moreover,it is found that there are a large number of metal.salts,which may be the main factors that restrict the reuse.Therefore,the researchers should actively develop practical waste acid utilization technology.(2)Through the study of acidic waste water,it is found that the amount of harmful heavy metals is not much.At present,the wastewater treatment methods used in the treatment of sulfate method are alkaline powder neutralization process,the specific process of the enterprise is slightly different,but all can achieve better results.In addition to manganese,other elements in the external drainage can meet the"integrated wastewater discharge standard"(GB 8978-1996)regulations.Due to high manganese content in wastewater,many enterprises in drainage Mn2+ exceed the standard.(3)There are still 69.8%of the titanium gypsum slag stockpiled in the disposal field,while its comprehensive utilization method is used for cement retarder,brick or building materials made of white gypsum.As these three methods have limitations,so the comprehensive utilization of titanium gypsum is not very optimistic.In addition,difference analysis on comprehensive utilization of titanium gypsum shows that,the utilization rate of titanium gypsum is high in North Jiangsu and Zhejiang regions,while there is no titanium gypsum resource utilization in Panzhihua area.(4)The risk characteristics of titanium gypsum shows that,the heavy metal content in the leaching solution did not exceed the specified limit from "identification standard for hazardous wastes"(GB 5085.3-2007),and is lower than the maximum allowable emission concentration from "integrated wastewater discharge standard"(GB 8978-1996).Its corrosively pH value is also in the standard range(6-9),so in the preliminary judgment,titanium gypsum belongs to the class of general industrial solid waste disposal and management,which can refer to the general industrial solid waste.(5)The to The environmental risk assessment of enterprise's titanium gypsum show that only No.12 enterprise has moderate Cu pollution calculated by the geological accumulation index,6 companies have a slight pollution situation calculated by Nemerow index data,and all the enterprises are low risk by the calculation method of potential ecological risk index.Above all,the harm of titanium gypsum to the environment is not serious,but the government and industry should in time develop a unified evaluation standard of titanium gypsum,which is also helpful to the disposal and utilization of titanium gypsum.