2025, the concentration of thallium in Chenzhou-Leiyang cross-city section of Leishui in Hunan Province was 0.13 micrograms/liter, which was abnormal (referring to the standard limit of 0.1 micrograms/liter for specific items of centralized drinking water surface water sources in the Environmental Quality Standard for Surface Water). The incident has aroused widespread concern from all walks of life. Under the guidance of the Emergency Response Office of the Ministry of Ecology and Environment, Hunan Province, Chenzhou and Hengyang quickly launched emergency response, upstream and downstream linkage, and actively responded. Now the pollution sources have been controlled.
It is reported that on March 17, after verification, the source of pollution was identified as a cement production enterprise located in Suxian District, Chenzhou City. The main reason is that when the enterprise dismantled the kiln of the old production line recently, the dust in the kiln was washed by rainwater and flowed into the water body of the external environment through the rainwater discharge outlet.
Data show that thallium in cement kilns mainly comes from two aspects.
1. Raw materials and fuels: Thallium mostly exists in sulfur ores of lead, zinc, iron, copper and other metals in a dispersed state. If the raw materials for cement production, such as limestone, clay and iron ore, come from thallium-containing mining areas, cement enterprises may bring in thallium by using tailings from some metal mines or metal smelting waste residues as raw materials; For example, in this incident, the cement plant is located in an important non-ferrous metal mining area, and the high content of thallium in raw materials may lead to element enrichment. In addition, fuels such as coal may also contain thallium, which enters the cement kiln system during combustion.
2. Co-disposal of wastes: If the cement kiln adopts co-disposal of hazardous wastes or other industrial wastes containing thallium, the thallium in these wastes will enter the cement kiln along with the disposal process. For example, some electronic waste and electroplating sludge may contain thallium, which will exist in the cement kiln after co-disposal.
In general, most of the thallium in the cement kiln will form internal circulation in the preheater and condense in the kiln ash to form external circulation, and generally do not enter the clinker and flue gas. However, if the management is not good, such as the recent thallium pollution incident in Leishui, Hunan Province, it is because when the enterprise dismantled the kiln of the old production line, the dust containing thallium in the kiln was washed by rainwater and then flowed into the environmental water body.
Based on the above two reasons, experts suggest that cement enterprises should focus on monitoring the content of heavy metals when dismantling the original kilns, especially those whose raw materials come from heavy metal mining areas or co-disposal of hazardous wastes.
In fact, cement kilns have high temperature (long residence time), high alkali, oxygen enrichment, multiple fluid flow fields, heat recovery, batching, solid solution.. It has become the preferred way to absorb, dispose and comprehensively utilize all kinds of industrial wastes, domestic garbage and harmful substances. Therefore, in recent years, a wave of co-disposal of solid hazardous wastes in cement kilns has been set off in China. However, in this process, cement enterprises need to pay special attention to heavy metal pollution.
The Maximum Allowable Emission Concentration of Air Pollutants from Cement Kiln for Co-processing of Solid Wastes shows that the maximum allowable emission concentration limit of thallium, cadmium, lead, arsenic and their compounds is 1mg/m ³; the maximum allowable emission concentration limit of beryllium, chromium, tin, antimony, copper, cobalt, manganese, nickel, vanadium and their compounds is 0.5mg/m ³. The reference limits of heavy metal content in
cement kiln raw materials are: arsenic 28mg/kg, lead 67mg/kg, cadmium 1mg/kg, chromium 98mg/kg, copper 65mg/kg, nickel 66mg/kg, zinc 361mg/kg and manganese 384mg/kg. For details, refer to Pollution Control Standard for Co-disposal of Solid Wastes in Cement Kilns, Technical Specification for Co-disposal of Solid Wastes in Cement Kilns, Technical Specification for Environmental Protection of Co-disposal of Solid Wastes in Cement Kilns, Identification Standard for Hazardous Wastes, Identification of Leaching Toxicity, etc. Wu Xiaomei, an associate professor at
South China University of Technology, said that the problem of heavy metals is one of the factors that must be considered in the process of co-disposal of cement kilns. Different heavy metals have different volatile characteristics at high temperatures, and the tracking and monitoring of flue gas emissions should be strengthened in the production process. The results show that the volatility of lead and cadmium in raw meal is strong in the high temperature stage of cement kiln. Most of arsenic, barium, cadmium, copper, lead and zinc in the clinker are dissolved in the silicate minerals, and the solid solubility of chromium and manganese in the silicate minerals and intermediates is equivalent.
She suggested that in order to strengthen basic research and improve various standards, safety evaluation methods and mechanisms for heavy metal dissolution in cement (mortar products) should be established, with special attention to barium and thallium . The form, type and total amount of heavy metals brought in are different, and the limit of safe solidification of cement is also different. Is there an effective way to avoid heavy metal pollution in cement kiln co-disposal technology
at this stage? Wu Xiaomei believes that,
on the one hand, it depends on basic research to determine the safety evaluation methods and requirements for the migration and dissolution characteristics of heavy metals in the process of co-disposal of different heavy metals in cement kilns and the use of cement produced by this clinker. Second, when some heavy metals cause harm to cement products or environmental safety, the standards need to be further improved. These standards include specifying the types of hazardous wastes, which can be disposed of directly in cement kilns, which can only be disposed of in small quantities, and what amounts may be unsafe. Thirdly, there is no safety standard for the dissolution of heavy metals in buildings. With the large-scale application of co-disposal technology in cement kilns in the future, the standard in this respect needs to be improved urgently.
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