Titanium dioxide used as a pigment has excellent optical properties and stable chemical properties. Titanium dioxide has very high requirements on particle size, particle size distribution, and purity. Generally, the particle size of titanium dioxide is based on the wavelength range of visible light, which is between 0.15m and 0.35m. And as a white basic pigment, it is very sensitive to the increase of impurities, especially iron impurities, and the increase in crushing is required to be less than 5 ppm. In addition, titanium dioxide is required to have good dispersibility in different coating systems. Therefore, the general mechanical crushing equipment is difficult to meet the requirements, so the final grinding (finished product grinding) of titanium dioxide is currently selected by jet mills at home and abroad.

According to the grinding requirements of titanium dioxide: narrow particle size distribution, less increase in inclusions, good dispersibility, etc., and material characteristics of titanium dioxide: high viscosity, poor fluidity, fine particle size and easy to adhere to the wall, etc. At present, domestic and foreign titanium dioxide manufacturers choose to have self-distribution The flat-type (also known as horizontal disc type) jet mill with high-level function is used as the final grinding equipment for titanium dioxide; and superheated steam is used as the grinding working medium. Because steam is easily available and cheap, the pressure of the steam working medium is much higher than that of compressed air and is also easy to increase, so the flow energy of steam is greater than that of compressed air. At the same time, the cleanliness of the superheated steam is higher than that of the compressed air, the viscosity is low, and there is no static electricity, and while grinding, it can eliminate the static electricity generated by the collision and friction of the material, and reduce the secondary cohesion of the powdered material. In addition, grinding under high temperature conditions can improve the application dispersibility of titanium dioxide and increase the fluidity of titanium dioxide. The use of superheated steam has low energy consumption, which is only 30% to 65% of compressed air. In addition, using a flat jet mill, organic additives can be added to organically modify the surface of the titanium dioxide while pulverizing, so as to increase the dispersibility of the titanium dioxide in different application systems.

With the rapid development of the titanium dioxide industry, the requirements for equipment are getting higher and higher. Under the premise of meeting the process conditions and quality requirements, the large-scale and systematization of equipment is particularly important. Airflow grinding is also continuously improved with the development of titanium dioxide. The production capacity of the gas powder machine has also increased from 1.2t/h to 1.5 t/h at the beginning to the current 2.5 t/h to 3.5 t/h. The production capacity of the gas powder system has also increased from a single line of 10,000 t/a to the current single line 2 Ten thousand t/a, the collection method has also been changed from the relatively backward wet collection to the advanced dry collection, which greatly improves the one-time yield and reduces waste. With energy saving and emission reduction, the higher the requirements for cost reduction, the more reasonable the configuration of the gas-powder system, and the full utilization of the waste heat of the exhaust gas. In the past, the gas and powder collection method was mainly wet collection, that is, the materials from the gas and powder machine first enter the cyclone for vapor-solid separation, and the separated materials are discharged by the star unloader at the bottom of the cyclone for cooling and packaging. The separated material enters the spray tower with the air flow for spray cooling and collection. The material collected by the spray tower is in the form of slurry, which must be settled, filtered and dried before returning to the gas powder machine. The one-time yield of this process is very low, up to 90%, the energy consumption is large, the exhaust heat cannot be used, and it has been basically eliminated. The current gas and powder collection method is mainly dry collection, that is, the material from the gas and powder machine first enters the high-temperature bag filter for vapor-solid separation. The current surface-coated high-temperature resistant filter material has a separation rate of more than 99.5%. The discharged materials are discharged through the star discharger at the lower part of the high-temperature bag filter for cooling and packaging. The separated high-temperature tail gas is discharged from the clean air chamber at the upper part of the high-temperature bag filter and enters the tail gas waste heat recovery device for waste heat utilization.

Factors affecting jet milling equipment

(1) Jet mill: As the most important equipment for jet milling, the quality of the air powder machine directly determines the quality of the product. The gas powder machine is required to have reasonable design, excellent production, high impact kinetic energy, good classification effect, wear resistance and high temperature resistance. Therefore, it is very important to choose a gas powder machine.

(2) Steam quality: The grinding working fluid of jet milling is superheated steam. If the steam quality does not meet the crushing requirements, it will seriously affect the quality of gas and powder. Generally, the requirements of the gas powder engine for steam are: the pressure is 1.6 to 2.0 MPa, and the temperature is between 290°C and 310°C. If the temperature and pressure are lower than the requirements, it will cause low impact kinetic energy, decreased grinding force, insufficient heat in the system, and materials easily damp, which will affect the grinding effect, block the system and make it unable to operate normally; if the temperature and pressure are too high, it will The equipment in the system causes damage.

(3) Process control: Airflow grinding requires stable and continuous operation. The fluctuation of steam and the fluctuation of feed volume should be controlled within a certain range, and must be adjusted slowly during adjustment, and it is strictly forbidden to increase or decrease. In addition, once the gas-powder system is normal, it should maintain continuous operation and avoid frequent startup and shutdown. Furthermore, the operating procedures should be strictly followed when starting and stopping.

(4) System monitoring: In order to ensure the normal operation of the system, necessary monitoring equipment must be installed in a reasonable position of the system to make timely adjustments according to changes in the situation.

 

 

Article source: China Powder Network