Production of Titanium Dioxide by Sulfuric Acid Method--Sintering of Titanium Dioxide

First, the optical properties of the pigment titanium white powder mainly depend on the particle characteristics and surface properties
The main optical property of titanium dioxide for pigments is the opacity when it is dispersed in a medium and applied to a surface (as a coating). Important optical properties that are interrelated to opacity to varying degrees are brightness, whiteness, hue, achromatic and opaque forces. The opacity of titanium dioxide is highly dependent on its refractive index and particle properties for light. Particle characteristics include particle size, particle size distribution, and particle shape. In addition, the ability of titanium dioxide to disperse in various media is also an important factor. Because in most cases, the optical properties of the pigment can be fully manifested, it depends on the dispersibility of the titanium dioxide, and the dispersibility is often closely related to the particle characteristics and surface properties.
Second, the effect of particle characteristics of titanium dioxide on pigment properties
The effect of particle characteristics on pigment properties has the following three aspects.
1. Effect of particle size on pigment properties For a certain wavelength of human light, when the pigment particle size is half wavelength, the scattering rate of light is the highest, that is, the opacity is the highest. The wavelength of visible light is 0.4-0.7 μm, so theoretically the optimum particle size of the pigment particles is 0.2-0.35 μm. If the particle size is too small, diffraction of light occurs, resulting in a decrease in opacity. And the pigment properties have a great relationship with the scattering coefficient. When the scattering coefficient is the highest at a particle diameter of 0.2 μm, the color reduction and hiding power of the pigment are optimal, and the whiteness and gloss are the best. However, the performance of various pigments sometimes contradicts each other. For example, when the particle size becomes small, the weather resistance decreases accordingly. Therefore, for some pigments with high weather resistance, the particle size should be appropriately larger.
2. Effect of particle size distribution on pigment properties Titanium dioxide with excellent pigment properties should have a particle size distribution width as narrow as possible, that is, the particle size should be between 0.2-0.3 μm, and below 0.1 μm. And 0.37 μm or more is almost non-existent.
3. Effect of particle shape on pigment properties The particle shape of pigment also has an effect on light scattering. Generally, when the particles are circular, the scattering rate is the highest, and the particles with smoothness and smoothness have the best pigment performance. It is necessary to avoid horn-like particles as much as possible because they are not only poor in optical properties, but also have a large volume of space when stacked together, and the corresponding oil absorption is high.
Third, the reason why titanium dioxide needs to be crushed and the method of crushing
The calcined titanium dioxide is mostly agglomerates of particles, which need to be pulverized in order to achieve the particle size requirements of the pigment standard, thereby obtaining the highest possible opacity and other pigment properties.
Pigment titanium dioxide has strict requirements on particle size and particle size distribution, so it is necessary to choose suitable crushing equipment and process flow. The method of pulverizing titanium dioxide can be classified into wet pulverization and dry pulverization. Wet pulverization such as wet ball milling and sanding are carried out in medium water; dry pulverization includes Raymond mill, hammer mill, centrifugal mill (universal mill), fluid energy mill (air jet mill) and the like. The pulverization may be carried out by a single grinding device, or may be used in combination of two or more kinds of grinding devices. For example, the wrought material may be firstly pulverized by Raymond grinding, then pulverized by airflow, or may be pulverized by the same equipment. , such as secondary airflow crushing. The choice of the comminution process depends primarily on the needs of the titanium dioxide variety. Production of non-pigmented products, such as welding, metallurgy, enamel, the capacitor grade titanium dioxide, does not emphasize the individual particle size, only requires 320 mesh sieve residue can not exceed a certain range, only once after calcination was dry The pulverization, such as centrifugal grinding or Raymond grinding, can meet the requirements, and some anatase pigment titanium dioxide is also subjected to dry pulverization only once.
Fourth, the principle of centrifugal mill grinding, the factors affecting the grinding effect and the method to obtain the ideal fineness
The grinding of the centrifugal pulverizer is an action of centrifugal force by means of the material rotating at a high speed in the pulverizer, and is an operation of overcoming and splitting the cohesive force inside the solid material. The operation of splitting a large piece of material into small pieces of material is called crushing, and the operation of turning a small piece of material into a fine powder is called pulverization.
In the process of pulverizing the calcined material by the centrifugal pulverizer, there are three kinds of impact forms of the material in the machine: one is the impact of the calcined product particles and the casing; the other is the impact of the calcined product particles and the cutter; the third is the calcination The impact between the particles, the former two types of impact. The fineness of the milling depends on the extent of these impacts. The more chances of impact, the finer the fineness of titanium dioxide, and the amount of material impact in the centrifugal mill is determined by the time the material stays in the machine and the centrifugal force obtained by the material. The distance between the cutter and the casing is small, the air intake is small, the material stays for a long time, the power of the main motor of the crusher is large, and the centrifugal force obtained by the material is large. In the production, as long as the control of the smaller spacing, the appropriate amount of air intake and the amount of feed, the desired fineness can be obtained. [next]
Five, Raymond mill crushing process
Raymond mills are also known as ring roll mills, and the titanium dioxide industry uses roller balance ring rolls. Raymond mill can be used to pulverize non-pigmented titanium dioxide products, and can also be used for one-time pulverization and primary comminution of pigment titanium dioxide. The process flow of Raymond mill crushing titanium dioxide is shown in Figure 1. The calcined material is connected to the body of the Raymond mill from the storage hopper. The machine has a vertical axis. There are 2-6 pendulums hanging freely on the cross beam intersecting the top of the shaft and with hanging rolls. In addition, it rotates around the vertical axis with the pendulum. When the vertical axis is rotated, the centrifugal force causes the suspension roller to abut against the stationary annular gasket. The material passes between the suspension rolls and the liner. Large and unpulverized material falls on the bottom of the machine and is thrown back by the wheel on the surface of the lining ring in front of the fast-moving suspension roll. The pulverized material is carried by the air stream in the upper separator. The coarse particles are separated and returned to the grinding zone for re-grinding. The fine titanium dioxide is introduced into the cyclone with the airflow, and the separated titanium dioxide is discharged from the lower star feeder, and the fineness of the finished product can be adjusted by the amount of blowing air. To control, the air is returned to the bottom of the Raymond machine through the blower to form a closed loop. If the primary cyclone separator does not meet the requirements, a two-stage cyclone separator can be installed or connected in series with the bag filter.

Sixth, Raymond mill has advantages compared with centrifugal mill
The Raymond mill has the following five advantages over the centrifugal mill.
① Raymond mill can grind only titanium dioxide, titanium iron ore can be ground.
2Because the separator is attached inside the Raymond mill, the fineness can be adjusted, so the fineness of the mill is good, the product size is uniform, and the coarse particles are few.
3Because the Raymond machine achieves the purpose of crushing by pressing and grinding the material by the centrifugal force of the balance wheel, the shape of the crushed titanium dioxide particles is relatively smooth, the particles are firmer, and the oil absorption of the finished titanium dioxide powder is significantly reduced, whiteness and The color reduction power has also improved. However, if the titanium dioxide before pulverization is too soft, the pulverized titanium white powder is highly viscous, and it is easy to block the pipe, and it is easy to cause agglomeration. The solution is to increase the wind pressure.
4 large output, electricity saving. A five-roll Raymond mill produces about 1 ton per hour, while a Φ600mm centrifugal mill pulverizes pigmented titanium dioxide with an output of only 160-300 kg per hour. 5至吨。 The pulverized non-pigmented titanium dioxide production can reach 1. 5-2.0 tons.
5Because the body of the centrifugal pulverizer is mostly made of cast iron, when the TiO2 powder is pulverized, the body wear is serious. The iron pollution sometimes increases the iron content in the titanium dioxide by about 0.02%, which seriously affects the color and luster of the titanium dioxide. Some units use the method of depositing tungsten carbide electrodes on the inner wall to reduce wear and achieve certain effects, but the pollution is still very large, and the material wear of Raymond mill is much less.
7. Working principle of airflow pulverizer, pulverizing medium and flat airflow pulverizer commonly used in titanium dioxide industry
The airflow pulverizer, also known as fluid energy mill, is an ultrafine pulverizing equipment with superior performance. The materials in the jet mill are crushed by violent collision and grinding with each other under the action of high-speed airflow (equal to or exceeding the speed of sound). The jet mill commonly used in the titanium dioxide industry is available in both flat and ring forms, and the pulverizing medium (gas) used is superheated steam or high pressure air or inert gas. [next]
The flat airflow pulverizer is composed of an upper and lower cover, a feeding pipe, a nozzle ring and a nozzle, a discharge pipe and an exhaust pipe. The working principle is as follows: when the jet mill is working, the high-pressure working fluid is usually high-pressure steam, which enters the crushing chamber through the nozzle to form a supersonic jet stream of up to several hundred meters per second or even kilometers per second, and the material passes through the Wen. The pipe sucks the crushing chamber, and the jet stream carries the material particles to rotate at a very high speed in the crushing chamber. In such a powerful cyclone flow, the hydrodynamic characteristics are different everywhere, that is, there is a gradient in the radial direction. In addition to the strong airflow, there are many small vortex flows near the circumference of the pulverizing chamber, so that the materials are highly turbulent, where the materials collide with each other with huge momentum (accounting for about 80% of the pulverization). It is also rubbed against the circumferential wall (about 20%) to complete the pulverization process. This zone is called the comminution zone (see Figure 2).

Between the comminution zone and the central collection zone, the working fluid forms a main cyclone flow at a relatively high flow rate. The pulverized material moves here in a laminar flow pattern with the main cyclone flow, so that under the action of the centripetal force, a classification action occurs, and this zone is called a classification zone. The boundary between the comminution zone and the grading zone is called a grading circle. The axis of the nozzle, which is the axis of the jet stream, is tangent to this circle. The qualified particles that have been separated are collected in the orbit of the Archimedes spiral, overcoming the centrifugal force and running the collection area.
Eight, the process of airflow crushing
Figure 3 is a flow chart of jet milling titanium dioxide. The high-pressure steam generated from the boiler room (generally 0.637-1.176MPa gauge pressure) is heated to a certain temperature (250-350 °C) through the superheater, and enters the crushing chamber of the jet mill through the flowmeter. A small amount of steam leads the Venturi Riga. The material is used to suck the material in the hopper into the crushing chamber. The pulverized product is partially collected from the lower barrel of the main body and discharged through the star feeder, and partially enters the cyclone and the bag filter with the airflow, and the collected titanium dioxide is discharged from the discharge drum and the bag. Out. If the fine titanium dioxide remains in the airflow passing through the bag, the water spray tower can be re-installed and the exhaust gas is vented.

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Nine, the choice of airflow crushing process conditions
1. Feeding amount The amount of the jet mill is closely related to the working fluid, the pulverized material, and the required fineness. Different sizes of jet mills have an optimal feed amount for a certain material. At this time, the impact velocity of the material particles can be maximized, and the concentration of the materials in the working medium can be moderate, thereby making the crushing effect. In the best condition. Φ280mm flat airflow pulverizer, the working medium is superheated steam with a pressure of 0.784MPa or more, and the amount of pulverized anatase titanium dioxide is about 65-80kg per hour. As the diameter of the jet mill increases, the yield increases rapidly. A 40% increase in diameter and a 200% increase in production. In general, reducing the amount of feed causes the pulverization process of the particles to be greatly accelerated, the residence time in the pulverization chamber is lengthened, and the collision opportunity is increased, thereby increasing the fineness of the particles. In order to make the quality of the pulverized product stable, it is especially necessary to pay attention to the uniformity of the feeding. It is generally considered that the variation of the feeding amount should be no more than ± 2%.
2. Feed particle size feed particles should not be too large, for hard materials, the maximum feed size is 4-14 mesh, the minimum is 20-100 mesh; soft and brittle materials, the maximum is 2-5 mesh, the minimum is 10 -100 mesh.
3. Working fluid, pressure and temperature It can be used as the working fluid of the jet mill with superheated steam, compressed air and inert gas. In general, when crushing ore-type materials, superheated steam is mostly used; when pulverizing pharmaceutical materials, compressed air is mostly used; inert gas is generally used only in special occasions where materials must be protected from oxidation.
Superheated steam is generally preferred over compressed air because it is readily available and inexpensive, and costs about 1/2 of that of compressed air. The steam pressure can be very high, and the kinetic energy provided is much higher than that of air, and there is no pollution problem of the air compressor lubricating oil. When the titanium dioxide is pulverized by the superheated steam, the sticking phenomenon is not easy to occur, and the cohesion phenomenon of the titanium white powder is small and long. No agglomeration occurs during transportation and storage. Therefore, any material that can be pulverized with steam generally does not use compressed air. In the titanium dioxide industry, the jet mill of Φ200mm or more uses superheated steam as the working fluid, and the jet mill of Φ200mm or less uses high-pressure air because it is difficult to solve the steam condensation problem.
The pressure of the working fluid is the main parameter for generating the velocity of the jet stream, and is one of the most important parameters affecting the crushing effect. For the shrinking nozzle, after the pressure is increased to the critical value, the airflow speed is no longer increased, but the airflow density can be increased, so the kinetic energy of the airflow is increased, thereby strengthening the pulverization process. Generally, the higher the pressure of the working fluid, the better, but The industry is limited by equipment, and most of them use superheated steam with a pressure of 0.98 MPa.
4. The steam-solid ratio is the important parameter affecting the pulverization effect. Due to the different types, specifications, working conditions and materials of the jet mill, the steam-solid ratio varies greatly. The steam-solid ratio reported in the literature can be 0.5-1.0): Change between 1. This has a great relationship with the particle size of the original material. For example, when the coarse crushed by the Raymond mill and the air powder after the coating, the steam-solid ratio is generally (1.2-2.5):1. The best steam-solid ratio of pulverized untreated anatase titanium dioxide is (0.8-1.2):1. In general, the use of high steam-solid ratio can make the pigment performance of titanium dioxide fully exerted, which is beneficial to improve the achromatic power and reduce the oil absorption, but it will impair the dispersibility of the pigment; if the steam-solid ratio is too low, the dispersion performance is good. The performance of the pigment is declining. Whether it is particle size distribution or pigment performance, the flow pulverization → surface coating treatment → air flow pulverization process is the best, and the first pulverization is more desirable with high steam-solid ratio.
5. Separation and capture of the material after pulverization Most of the primary dust collection of the pulverized material is a cyclone separator. There are two kinds of dust collection methods: wet method and dry method. The wet method can recover the entrained product by steam condensation, or use another sprinkler. Although the effect of wet dust collection is good, the collected slurry must be dried again. Therefore, the process is long, the area is large, and the equipment is large, which is currently less used. Dry dust collection generally uses two levels of cyclone to collect dust and then use a pulse backflush bag to collect dust. Dry dust collection must be such that the temperature of the dust collection system is 14-22 ° C above the steam dew point to avoid sticking and clogging.
6. Chemical reaction in the jet mill Many materials are coated with a material at the same time as the airflow is pulverized. For example, the US cyanide company has proposed adding a small amount of aluminum isopropoxide to titanium dioxide. The aluminum isopropoxide is first dissolved in carbon tetrachloride, and then mixed with titanium dioxide and then introduced into a jet mill. At the high temperature of the superheated steam, aluminum isopropoxide is hydrolyzed to precipitate aluminum hydroxide coated on the surface of the titanium dioxide.
Another treatment carried out by the jet mill is the addition of a grinding aid. Since the material in the jet mill is highly turbulent, even if a small amount of additives are added, it can be highly uniformly mixed in the finished product. In order to improve the dispersibility of titanium dioxide, it is commonly used in the jet mill to adsorb certain materials. The organic compound method is carried out by organic coating. [next]
Ten, the advantages of airflow crushing compared with mechanical crushing
Airflow pulverization has the following six advantages over mechanical pulverization.
1 The particle size and particle size distribution of the material after jet milling is much better than mechanical crushing. When it is necessary to pulverize the material to submicron fineness and narrower particle size distribution, jet pulverization is one of the most important and effective means. The ideal particle size of the pigment titanium dioxide is about 0.2 μm, and the width distribution width requirement is very narrow. Generally, mechanical pulverization cannot achieve this, and this can only be achieved by airflow pulverization. Figure 4 shows the particle size distribution curve of the material after airflow pulverization and mechanical pulverization. The average particle diameters of the two curves are equal, but the difference in particle size distribution is very different. The airflow pulverization particle size distribution width is very narrow, that is, the particle size is uniform and regular after pulverization. Its pigment properties are excellent, which is difficult to achieve with general mechanical pulverization.

2 Generally, the mechanical pulverization cycle is long and the efficiency is low. Sometimes, although the pulverization is repeated for many times, the required fineness cannot be achieved.
3 Mechanical pulverization generates a lot of heat during the long-term grinding process, which is not suitable for heat sensitive materials.
4 The particle size distribution of the product of air jet pulverization rarely changes with time, and the quality of the product is relatively stable. In general, mechanical pulverization not only has a wide particle size distribution, but also changes every day, causing product quality fluctuations.
5 The airflow pulverizing machine has little wear and is not easy to cause pollution to the pulverized material.
6 Airflow pulverization In the material smashing fashion, some simple chemical reactions can be carried out, which is extremely valuable for the titanium dioxide industry. This property has been used to perform surface treatment while pulverizing.
XI, the choice of titanium dioxide crushing process
Reasonable selection of the crushing process is a very complicated problem, and it must be considered comprehensively based on product variety, quality requirements and technical and economic indicators. Although airflow pulverization is the most effective pulverization, it has high operating cost and consumes more energy, and the operating cost of mechanical pulverization is lower. Therefore, mechanical pulverization such as centrifugal pulverization and Raymond pulverization is still commonly used in primary pulverization. When pulverizing anatase type titanium dioxide, the process of Raymond mill-water selection~encapsulation-drying-Raymond mill (or airflow pulverization) is used, the quality of the product is still good; when rutile TiO2 is crushed, due to The hardness is relatively high, and the process of using Raymond mill-water to select a coating-drying-airflow crushing is relatively high. Of course, some manufacturers also directly burn the calcined material.
12. Requirements for fineness of various titanium dioxide varieties
Due to the different uses of various titanium dioxides, their fineness requirements for milling are also different (see table).

13. Moisture absorption of anatase pigment titanium dioxide and measures to prevent moisture absorption
The anatase pigment titanium dioxide has a large hygroscopicity. It can be seen from Fig. 5 that the exposure to air is 2%, that is, water absorption is 0.5%, and the titanium dioxide has a water content requirement of only 0.5%, so that the cooled titanium dioxide is pulverized. If it is packaged for more than 2 hours, it is likely to cause excessive moisture absorption. To this end, the crusher must be quickly crushed and quickly packaged. After packaging, it is still necessary to prevent moisture and pressure.

If the non-pigment grade titanium dioxide is produced, it will be the finished product after pulverization; if it is produced without the film of BA-0101 anatase pigment titanium dioxide or the production of non-coated rutile pigment titanium dioxide (such as Liuzhou factory), it will be crushed. After that, it will become a finished product; if it is a BA-0102 anatase pigment titanium dioxide (such as Shanghai Coking Plant) or a coated BA-0103 rutile pigment titanium dioxide, it needs to be surface treated after pulverization.