Cement rotary kiln is the main equipment of cement clinker dry and wet production line. Rotary kiln is widely used in gold treatment, chemical industry, building refractories, environmental protection and other industries. A large number of research and practice show that the cement rotary kiln has high combustion temperature, long residence time in the kiln, negative pressure operation and stable working conditions. A large number of harmful substances are reduced in the kiln, and the exhaust gas discharge meets the requirements of non-environmental protection, and the waste residue is not discharged outward. At the same time, this kind of disposal process is carried out synchronously by using cement production process, and the disposal cost is low. Therefore, it is considered as a reasonable disposal method by foreign experts.
Cement rotary kiln is composed of cylinder, transmission device, supporting device of supporting wheel and baffle wheel, lining of kiln, sealing of kiln tail, kiln head and fuel device, etc.
According to the different heating methods, it can be divided into internal heating and external heating.
The material of internal heating rotary kiln is directly in contact with flame and flue gas. The atmosphere of oxidation or reduction in the furnace can be adjusted. Refractory bricks are laid on the inner wall of the cylinder. The inner lining of the head and tail cover is high-strength and wear-resistant castable. The flexible seal between the head and tail cover and the cylinder can completely prevent dust and leakage. It can adopt multi-stage support, so that the length of the cylinder can reach 60 meters, and the maximum service temperature can reach 1600 C. The output is large, and the suitable fuels are liquefied gas, natural gas, gas, fuel oil, pulverized coal, coal, etc.
The material of external heating rotary kiln does not contact with flame and flue gas directly. The heat source is between the furnace and the cylinder. The material is heated by heat transfer through the heat-resistant steel cylinder wall. The inner lining of the furnace is made of refractory fibers, which can save energy and reduce consumption. Limited by heating mode and material of the cylinder, the length of the cylinder is generally less than 15 meters, and the heating temperature is less than 1200 C. The output is small, suitable heating methods are: electric heating, gas, fuel oil, coal and so on.
1. Treatment of Hazardous Waste in Cement Rotary Kiln
According to the Law of the People’s Republic of China on the Prevention and Control of Environmental Pollution by Solid Wastes: Wastes with one or more hazardous characteristics, such as corrosiveness, toxicity, flammability, reactivity or infectivity, and wastes with hazardous characteristics, which may have harmful effects on the environment or human health, are among the hazardous wastes. For example: medical wastes, pesticide wastes, heat treatment cyanide wastes, waste emulsifiers, fuel and paint wastes, explosive wastes, and wastes containing hazardous chemical components, such as zinc-containing batteries as we often mentioned, and a large number of hazardous wastes produced in industrial production and scientific experiments. Knowing these, do you think that dangerous waste originally was around us? We are alarmed by its wide source and huge quantity. The damage of dangerous waste to human body and ecological environment has been paid attention to by all countries in the world. Some people call dangerous waste “political waste”. From this, we can see that dangerous waste has brought troubles to people’s lives. Good disposal and reuse of hazardous wastes has become the top priority in building a green, low-carbon, environmentally friendly and energy-saving life.
After a long time of continuous exploration and experiments, many methods of hazardous waste treatment have been summarized, such as burial, incineration, neutralization, extraction, magnetic separation, hydrolysis, precipitation and so on by chemical or physical methods. Among them, cement rotary kiln technology has become the most effective method for hazardous waste treatment and reuse because of its unique advantages. It has been widely used. Cement rotary kiln has high incineration temperature, stable state, more thorough waste combustion, cement rotary kiln is in a negative pressure state in operation, which avoids harmful gas spillover when dealing with hazardous waste, and because the rotary kiln has good tightness, it will not cause dust pollution and is conducive to environmental protection. Hazardous waste contains chlorine, sulphur, fluorine and other chemical components, which are completely absorbed in high temperature treatment and converted into non-toxic calcium chloride, calcium sulfate, calcium fluoride and so on. It is directly mixed in cement clinker, which has been proved that the quality of cement will not be adversely affected. When the waste is burned, it produces a certain amount of heat and saves coal-fired raw materials and production costs.
The advantages of cement rotary kiln in dealing with hazardous waste are: environmental protection, energy saving, safety, and certain economic benefits.
In addition to burning cement clinker, cement rotary kiln is also used for burning clay, limestone and slag drying. In refractory production, rotary kiln is used for burning raw materials to stabilize its size, increase its strength and reprocess. In nonferrous and ferrous metallurgy, iron, aluminium, copper, zinc, tin, nickel, tungsten and chromium are sintered and roasted in rotary kiln. For example, aluminium hydroxide is roasted to alumina in aluminium production; pellets are produced for blast furnace ironmaking with aluminium hydroxide in ironmaking; iron ore is directly reduced by SL/RN method and Krupp method abroad; tin and lead are extracted by chlorination volatilization roasting method. In the process of mineral processing, the lean iron ore is magnetized by rotary kiln, so that the original weak magnetism of the ore is changed to strong magnetism, which is beneficial to magnetic separation. In chemical industry, baking soda, phosphate fertilizer and barium sulfide are produced in rotary kiln. This method has the advantages of low energy consumption, less electricity consumption, no use of sulphuric acid and the availability of medium and low grade phosphate rock, and is quickly popularized. In addition, in environmental protection, the use of cement kiln incineration of hazardous waste, garbage, not only to reduce waste, harmless, but also to use waste as fuel, save pulverized coal, achieve waste recycling.