Post crushing of titanium dioxide

Titanium dioxide pigment was again dried agglomerate together into coarse particles, the agglomerated coarse particles and flocculation Although it is easy pulverization, but only to such a re-crushed to coarse particles after wet milling the pigment precursor in substantially Size, and as far as possible, the particle size distribution is narrow, so as to fully exert the optical properties and pigment properties of the pigment. Generally, the mechanical pulverizer can be finely powdered, but it is difficult to meet the requirements in terms of particle size distribution, particle shape, product purity, etc. Currently, the best method is to use a steam (gas) flow pulverizer to complete the steam flow. The pulverizer can pulverize the solid material to a sub-micron level, and the particle size distribution is narrow, the pollution is small, no heat is generated during the pulverization process, and a simple chemical reaction can be performed in the air pulverization surface. One of the important advances in the titanium dioxide industry in the 20th century was the use of jet mills as a means of final comminution of titanium dioxide. Since this is the last comminution in production, the pulverized material is not an aggregate after calcination, so this pulverization process is called post-grinding or pulverization, de-coagulation, dispersion, and the like.

As the name suggests, airflow crushing is performed by the energy of the airflow. It is also called Fluid Energy mill or Jet mill. The airflow can use compressed air, inert gas, superheated steam, etc. The crushing of titanium dioxide usually uses overheating. Steam is therefore often written as a steam mill. The airflow pulverizer has many structural forms, such as flat type, circulating tube type, counter-type type, single-jet type, etc., and the pulverization of titanium dioxide is flat. Our flat jet mill can produce, the key is to choose the lining material, general wear-resistant hard alloy or corundum, flat jet mill in Sri Lanka only US foreign Tiffany special company (Sturtevant mill Go.) Of (Micronizer) is used in the titanium dioxide industry. The company's 1.05m (42-inch) flat jet mill can pulverize 3t/h titanium dioxide. The average particle size is 0.25μm, which consumes 288°C. , 1.05MPa superheated steam 3.63t / h.

The structure of the flat jet mill is shown in the above figure. Its production capacity depends on the diameter of the crushing chamber. The diameter of the industrial flat jet mill is ф280~1000mm. There are 12~24 nozzles inside, and the angle of the nozzle is off. a is generally 32 ° ~ 40 °, it consists of upper and lower cover, feeding tube, intake pipe, grinding ring, nozzle, air outlet pipe and discharge pipe. The crushing chamber is large and the nozzles are sometimes different. Layer layout. Its working principle is to make the high-pressure superheated steam enter the crushing chamber from the nozzle in a tangential direction at a speed of 200~500m/s, forming a high-speed rotating tangential circular airflow in the crushing chamber, and between the tangent circle and the crushing outdoor circle. There are many small swirls, which make the fluid highly turbulent, and the material is strongly pulverized in this high-speed airflow. The particles are pulverized in a few milliseconds, and 80% of the collisions are comminuted with each other. The friction pulverization accounts for 20%. Because it is mainly collided with each other, its particle shape is smoother, the particle size distribution is narrower, and the degree of contamination of the product is also small.

The superheated steam is selected as the working medium gas source, mainly because the steam is easy to obtain and cheap, and the pressure of the steaming working medium is much higher than that of the compressed air and is also easy to be improved, so that its critical speed is high, the kinetic energy is large, and the superheated steam is simultaneously The viscosity is lower than air, and it does not carry static electricity, which can reduce the cohesion of materials. Compared with compressed air, the gas-solid ratio required for superheated steaming is much smaller, and the compressed air also needs to solve the oil and condensate. The problem is that titanium dioxide factories at home and abroad use superheated steam to pulverize titanium dioxide. The main factors affecting the effect of steam pulverization are as follows.

a . Gas-solid ratio, gas-solid ratio during pulverization is not only an important technical parameter, but also an important economic indicator. The gas-solid ratio is too small, because the lack of kinetic energy of the gas flow will affect the fineness of the product; the gas-solid ratio is too high, which not only wastes energy, but also deteriorates the dispersion properties of certain pigments. When the superheated steam is used as the working medium, the hard material after calcination is pulverized, and the gas-solid ratio is generally controlled at 2 to 4:1; the material after the pulverized surface treatment is generally controlled at 1 to 2:1.

b . Feed particle size, feed particle size is not as important as gas-solid ratio, but there are strict requirements on the feed particle size when crushing hard materials. For titanium white powder, it is better to control 100~200 mesh when crushing calcined materials. For better; the material after smashing the surface treatment is generally 40~70 mesh, and the maximum is no more than 2~5 mesh.

c . The temperature of the working fluid, because the temperature is increased, the flow rate of the gas can be increased. Taking air as an example: the critical speed at room temperature is 320 m/s. When the temperature is raised to 480 ° C, the critical speed can be increased to 500 m/s. The kinetic energy is increased by 150%, so increasing the temperature of the working fluid is advantageous for the pulverization effect. The temperature of superheated steam when pulverizing titanium dioxide is generally about 300~400 °C, and the temperature is usually high when pulverizing and calcining, and the material after pulverizing the surface treatment is lower because some surface treatment agents, especially organic surface treatment agents, are not resistant to high temperature. Sometimes, it only needs to overheat 100 °C based on the original steam temperature.

d . The pressure of the working fluid, the pressure of the working fluid is the main parameter for generating the velocity of the jet stream, and is also the main parameter affecting the fineness of the crushing. The kinetic energy of the working fluid jet is proportional to the square of its mass and proportional to the square of its velocity. Therefore, the higher the pressure, the faster the velocity, the greater the kinetic energy. When the steam pressure increases to a certain value, it passes through the nozzle. Although the flow velocity of the airflow is no longer increased, the pressure is increased, and as the density of the airflow increases, the same kinetic energy is correspondingly increased. As for the high pressure selected during pulverization, depending on the pulverizability and fineness requirements of the material, when the titanium dioxide is pulverized by superheated steam, the vapor pressure is generally 0.8 to 1.7 MPa, and the general pulverized calcined material is higher, after the pulverized surface treatment. The material can be lower.

e . Crushing auxiliaries, selecting the right pulverizing agent, not only can improve the pulverization efficiency, but also improve the dispersion performance of the product in the medium. Most of the organic surfactants added during the surface treatment of titanium dioxide have pulverization aids. function, an inorganic grinding aid is generally used hexametaphosphate, sodium phosphate and sodium pyrophosphate (potassium) and the like.