Rotary kiln is a major energy consumer in the production of titanium dioxide. Its energy consumption accounts for about 20% of the total energy consumption of titanium dioxide production. It is also one of the chemical operation units with low thermal energy utilization. In the 1980s, the coating research institute, the third design institute and some titanium dioxide factories of the Ministry of Chemical Industry of the Ministry of Organizations and Chemistry of the Ministry of Chemical Industry carried out on-site tests on a rotary kiln with a heat source of light diesel oil, a lining of 15 cm refractory brick, a leaf filter for dehydration of metatitanic acid and an anatase type of product in Zhenjiang for nearly a week. The energy consumption per ton of product was 1423.6. Kilo-coke (340 * 104 calories), of which the moisture evaporation of metatitanic acid accounts for 30.84% of energy consumption, the heat dissipation of kiln body accounts for 39.53%, the heat taken by flue gas accounts for 22.44% (the temperature of flue gas is about 250 C), the heat taken by finished products accounts for 6.94%, and the first three items account for 92.81% of total energy consumption. Recently, according to the test results of the above 80's, Shandong Institute re-converted the annual production of 40,000 tons of rotary kiln (using natural gas heating, diaphragm filter press dehydration, flue gas temperature about 380 C, the product is still anatase type). The results show that the energy consumption of the product is 1034.4 kilocoke per ton, the moisture evaporation of metatitanic acid accounts for 20.1%, and the kiln body is scattered. Heat accounted for 24.97%, heat from flue gas accounted for 39.58%, and heat from finished products accounted for 7.85%. The utilization rate of heat energy has been improved obviously, but the first three items still account for 84.65%. The heat of flue gas and the heat taken by the product are higher than that of the small kiln in the 1980s. This is mainly due to the high tail temperature (380-400 C) of the present operation process and the large output.
In recent years, waste heat utilization of tail gas is very common in the industry. The most widely used is that the flue gas is pre-concentrated after high temperature electrostatic precipitation, and then used for waste acid concentration. Thermal energy utilization rate is relatively good. Several other enterprises can reduce the consumption of natural gas by introducing foreign technology. After high temperature electrostatic precipitation, some of the flue gas will be returned to the kiln for secondary hot air use. The remaining flue gas will be supplied to waste acid pre-concentration. The waste heat utilization rate of the flue gas of the two methods mentioned above can be more than 50%. Others directly feed the flue gas into the waste heat boiler, generating 0.8 MPa steam for use in the system, but it is difficult to operate directly using the flue gas. It is better to use the heat transfer oil for the waste heat boiler after heat exchange, and the operation is relatively stable. The simplest and least expensive method is to use secondary water as cooling water for electric defogging and high hilly area. While the flue gas temperature is lowered, the wastewater is heated and then used for washing. Of course, the thermal efficiency of this method is also relatively low.
Measures to Reduce Heat Dissipation of Kiln Body
Ti-Cons Company of Germany has introduced that in a rotary kiln with a diameter of 3 200 *55 000, the heat loss on the surface of the rotary kiln shell can be reduced by 126 *104 kJ (30.1 *104 kcal), equivalent to 25% by lining one layer of 152 mm insulation brick and one layer of 152 mm refractory brick. At present, domestic conditional enterprises have changed the original single-layer firebrick design to double-layer brick (one-layer firebrick, one-layer insulation brick). According to the size of the kiln body, some adopt 150 mm firebrick + 150 mm insulation brick or 150 mm firebrick + 50 mm insulation brick, which has obvious effect of reducing the heat dissipation of the kiln body.
In the 1980s, titanium dioxide mills in Shanghai, Nanjing and other places used for reference the insulation measures of steel-making furnaces in iron and steel works. A layer of aluminium silicate fiber insulation felt was lined between kiln and refractory bricks. Its insulation effect was very obvious. People could hardly feel too much radiation heat under the kiln head. They could even touch the shell with their hands. Too loose and soft, the refractory bricks are displaced and discarded after long-term operation of rotary kiln. If the problem of fixing insulation felt and bricks can be solved, this method can greatly reduce the heat loss of the kiln body.
Recovery of heat from products
The temperature of calcined titanium dioxide is more than 800 degrees. The heat taken away by calcined titanium dioxide accounts for 7%~8% of the total energy consumption. In the past, most of the cooling kilns in domestic titanium dioxide enterprises used water cooling. Because of the unreasonable internal design of the cooling cylinder, the heat exchange effect is not good, and most of the factory water has not been recovered. After the introduction of Eastern European technology in the 1990s, almost all the cooling kilns were air-cooled. After the heat exchange between cold air and hot materials, the kiln could be raised to about 90 degrees Celsius and returned to the rotary kiln for primary air use. However, from the thermodynamic point of view, the recovery of waste heat is not complete. Taking natural gas as fuel, for example, only 38% of the heat energy is recovered, and using gas as fuel, only 54% of the heat energy is recovered. It is suggested that air cooling and water cooling should be carried out simultaneously in the future, and the heated cooling water can be used for washing.