Characteristics Of Heavy Metal Emissions And Its Release Mechanism From Non-ferrous Smelting In South-central China

The central and southern region is an important non-ferrous metal industrial base in China,but its industry spatial distribution pattern and pollution emission characteristics are still uncertain due to the large number of enterprises,complex production processes and different end treatment standards.Based on the distribution pattern of non-ferrous smelting capacity and the change characteristics of the number of enterprises,this study analyzed the spatial distribution and aggregation characteristics of non-ferrous smelting enterprises in central and southern China,estimated the spatial distribution of typical heavy metal emissions in non-ferrous smelting industry in central and southern China,and identified the hot areas of heavy metal emissions in central and southern China.On this basis,through the field investigation of a typical lead smelting enterprise,the production and discharge nodes as well as the end treatment measures of three kinds of industrial wastes in lead smelting process were clarified,the flow direction,distribution and circulation of typical heavy metal elements in each production process were analyzed by element flow analysis method,and meanwhile their impact on the environment was assessed.On the basis of identifying the main sources of heavy metal pollution,the pollution characteristics of smelting dust and related heavy metal migration and release were studied,in order to provide theoretical reference for the prevention and control of heavy metal pollution in non-ferrous metal smelting.The main results are as follows:(1)The results of the analysis of the change in the number of nonferrous metal smelting capacity and enterprises show that the central south region is important non-ferrous metal production base in China with Hunan and Henan province as an important part of the central south region.The distribution of nonferrous smelting enterprises is present dual-core,Hunan nonferrous smelting enterprise distribution is in northwest-southeast direction as a whole,with relatively weak polarization characteristics and randomness distribution.Meanwhile,the overall distribution direction of non-ferrous metal smelting enterprises in Henan province is northeast to southwest,and the polarization characteristics are obvious,and the distribution is significant in the central and western regions.The nonferrous smelting enterprises in Hunan province are concentrated in the eastern regions represented by Chenzhou and Hengyang,with potential pollution dominated by lead and arsenic;The distribution of non-ferrous smelting enterprises in Henan is mainly concentrated in the central and western regions of Luoyang,Anyang and Xuchang.The overall distribution is high in the west and low in the east,with potential pollution dominated by lead.(2)Based on the preliminary research results of distribution of nonferrous smelting enterprises and heavy metal pollution discharge in central and southern China,a typical lead smelting enterprise was selected as the further research district.Through field research,21 production and discharge nodes in the lead smelting process were identified and the output of three industrial wastes in the lead smelting process was calculated.The flow of lead and its associated metals was analyzed through the element flow.The results showed that the main outflow channels of lead elements were water quenching slag and secondary zinc oxide,which accounted for more than 90% of lead loss.Arsenic and cadmium are discharged through fuming slag,reverberator slag and anode slime,while mercury is mainly discharged through exhaust gas and dust generated in oxygen-rich bottom blowing process.Life cycle assessment results showed the environmental impacts of lead smelting include human carcinogenic toxicity,human noncarcinogenic toxicity and terrestrial ecotoxicity.Sensitivity analyses show that reducing direct waste discharges provides the greatest environmental benefit in terms of human carcinogenic and human non-carcinogenic toxicity.(3)Lead smelting dust has high heavy metal content and leaching concentration exceeds the standard,which belongs to the dangerous waste categories.The acid extractable fraction of zinc and arsenic elements in bottom-blowing furnace dust accounts for a higher percentage,and the acid extractable fraction of cadmium in reduction fuming furnace dust is the highest,which is more hazardous to the environment.Isotopic analysis shows that smelting dust contributes 97.12% to the source of lead in the soil and is a main sources of heavy metals in the soil.Quantitative mineral analysis indicates that the main mineral phases of heavy metals are transformed after the dust is deposited into the surrounding soil by atmospheric transport,and the process of mineral dissolution/precipitation transformation is accompanied by heavy metal release.Through phreeqc geochemical modelling surmise the potential mineral-controlled phases of heavy metal release under different p H conditions.The result shows that the release of heavy metals is mainly controlled by sulphate minerals when the p H is acidic,while the release of heavy metals is controlled by sulphate minerals when the p H is alkaline.alkaline values,the formation of alkali hydroxides or alkali chlorides immobilise heavy metals reducing their leaching.The release of the heavy metal arsenic is related to the adsorption of iron hydroxide,with the strongest adsorption and lowest leaching of arsenic under neutral conditions;under alkaline conditions,arsenic is mostly present as arsenite or arsenite ions and the adsorption is weakened,increasing the leaching of arsenic.There are 30 figures,13 tables and 173 references in this paper.