Hydrated titanium dioxide calcination

Hydrous titanium oxide is calcined into the process of titanium dioxide, the process requires this step is: (a) desulfurizing the material by dehydration became neutral; (b) is preferably obtained so that the desired crystal form conversion of 100%; (c The particle growth size is uniform and uniform, and the pigment grade titanium dioxide is required to be between 0.2 and 0.3 μm; (d) the shape of the particle is preferably approximately spherical; (e) the titanium dioxide formed after calcination is required to have no lattice defects, physical and chemical properties. stable.

Calcination of hydrated titanium dioxide is a strong endothermic reaction. It is generally carried out in a rotary kiln in the industry. It uses direct internal heating and its chemical reaction is as follows:

However, the calcination of hydrated titanium dioxide is not the process of heating dehydration and desulfurization in the above reaction, it also involves the growth, aggregation and crystal transformation of TiO ₂ particles, so the various physical properties of titanium dioxide with the increase of calcination temperature It also changes.

Hydrous titania is generally between 150 ~ 300 ℃ process is off free water and water of crystallization, is about 650 ℃ desulfurization process begins anatase to rutile conversion during 700 ~ 950 ℃, an alkali metal catalyst (salt In the presence of a treating agent, the conversion temperature can be lowered and the conversion rate can be increased.

The relative density of titanium dioxide during calcination varies with the crystal structure, from 3.92 (anatase) at 600 °C to 4.25 rutile at 1000-1200 °C. The conversion temperature of rutile can be reduced after the addition of accelerator. Up to 850~900 °C.

The refractive index also changes with the change of the calcination temperature, and the refractive index of the amorphous hydrated titanium oxide 1.8 can be converted into 2.55 in the anatase form and 2.71 in the rutile type by calcination.

The particle size of titanium dioxide is also constantly changing during the calcination process. The hydrated titanium dioxide is usually an aggregate of microcrystalline colloids of 0.6-0.7 μm. They are composed of crystallites of 3-10 mμm and increase continuously during calcination to 750 °C. When these microcrystals generally grow to 0.2~0.4μm, the surface area of ​​the particles is reduced to 1/10~1/20, and the size of these pigment particles does not change much after being converted into a certain crystal form. However, the calcination continues for a long period of time, and the particles will further aggregate together to become large particles.

As a result of the calcination, the titanium dioxide obtains the necessary pigment properties (achromatic power, hiding power, etc.), while the photochemical activity of titanium dioxide is weakened, the solubility in the acid is lowered, and the chemical property tends to be stable. The above figure is the differential thermal analysis of hydrated titanium dioxide and quartz . It can be seen from the figure that the endothermic process due to dehydration occurs at 150 ° C, and the endothermic process of desulfurization occurs at 650 ° C. The exothermic process below 900 ° C is Due to the reduction in the surface area of ​​the particles, the turning point at 900 ° C is an anatase type to a rutile type. It is also seen from the figure that the total heat required for water evaporation is much greater than the heat required for desulfurization. Therefore, reducing the moisture content in the material before calcination is one of the important measures for energy saving in the calcination operation. In the figure, 1 has formed a complete curve beyond the frame; 2 is TiO ₂ ; 3 is quartz.

Calcination is the most demanding place in the production of titanium dioxide after hydrolysis. Although it does not have much chemical reaction mechanism (some simple chemical reactions during salt treatment), most of the calcination process is physical changes such as relative density and refraction. Rate, hiding power, achromatic power, oil absorption, crystal form, particle size, etc. These physical changes directly affect the optical properties and pigment properties of the finished product. It is related to calcination temperature, calcination time, calcination strength, calcination atmosphere, and directly It is affected by the variety and dosage of the salt treatment agent. [next]

1. Calcination process of hydrated titanium dioxide

A rotary kiln calcination is generally high quality aluminum refractory lined steel, is generally not used silica brick, the silicon content in silica brick makes the product increased, the aspect ratio of the rotary kiln is generally 12 to 20: 1, such as domestic Commonly used Ñ„2400 × 38000mm, Ñ„2800 × 50000mm and so on. Heating within the heating reverse, more coal and fuel gas, natural gas. Liquefied gas, diesel oil, heavy oil, low-carbon hydrocarbons (C ₉ or C ₁₀ ), etc., the kiln head is the discharge and heating part, the kiln tail is the feeding part and the exhaust gas discharge outlet and is equipped with a baffle or shrinkage section to prevent the material from flowing backwards. . The kiln body is mostly straight, and the thin and long kiln structure can have enough heat and time to dehydrate and desulfurize, and ensure the time of particle growth and crystal transformation. There are also shaped kiln such as: retaining ring in different parts of the kiln, narrowing section at the kiln tail, large section of kiln head, etc. The rotary kiln for pigment grade titanium dioxide is generally equipped with a combustion chamber to avoid incomplete combustion. Fuel pollution products. The arrangement of the rotary kiln is generally high in tail height and low in head. The slope is usually 2%~5%. The rotation speed is 3~7min per revolution. The filling factor of the material is 10%~20%, and the residence time of the material is generally 8~16h. The feed of hydrated titanium dioxide often uses reciprocating squeeze pump, hose pump, screw pump, screw propeller, etc., and the material slowly moves forward under the action of rotary stirring and gravity. The first half of the kiln head, kiln tail and kiln There are several temperature measuring points in the department, and some also have sampling ports to keep track of the calcination of materials in the kiln.

After the material enters the rotary kiln, it is the dehydration process first. Theoretically, the free water can evaporate after exceeding 100 °C, but the hydrated titanium dioxide also contains a large amount of chemically bound water, so the dehydration process is generally in the range of 100 to 300 °C. According to the reason, the desulfurization process should be discharged after the water is removed. In fact, due to the combination of chemical bonds, a mixture of some acid and various sulfur oxides is always discharged together with the water vapor during dehydration.

A large amount of sulfate is adsorbed in hydrated titanium dioxide, which needs to be removed by calcination. Generally, the desulfurization temperature is 500-800 ° C (usually around 650 ° C), and the potassium salt desulfurization temperature can be as low as 480 ° C. The addition of aluminum salt can prolong the desulfurization time. The temperature required for desulfurization is also high, and as the sulfur is depleted, the titanium dioxide changes from acidic to neutral. Since a large amount of H ₂ O, SO ₃ , and CO ₂ (products upon decomposition of the potassium salt) are released during calcination, the agglomerated material becomes loose and dispersed in a state of dispersed particles. The delay in desulfurization time or the deflation of sulfur will affect the growth of titanium dioxide particles and the transformation of crystal form.

After dehydration and desulfurization, the hydrated titanium dioxide is gradually moved to the high temperature region of particle growth and crystal transformation with the rotation in the rotary kiln. In this range, the first amorphous crystalline hydrated titanium dioxide is converted into anatase titanium dioxide ( The hydrated titanium dioxide formed by hydrolysis of titanium tetrachloride is directly converted into rutile type), and the particles begin to grow. When the temperature reaches 600 ° C, the particles begin to increase significantly until the pigment particles of about 0.2 to 0.4 μm are formed, to about 950 ° C. The anatase type begins to be converted into a rutile type. If a rutile-type accelerator (ZnO, TiO ₂ sol, etc.) is added, the conversion temperature can be lowered to about 850 ° C, but the calcination is carried out at a high temperature for a long time, and these 0.2 to 0.4 μm The basic pigment particles will further increase. When the temperature reaches 1000 °C, the particles can grow up to 1 μm. Sometimes in the high temperature region, the pigment particles will not be grown and will be sintered together to form coarse particles, which may be some low melting salts. After melting, the titanium dioxide particles are sintered together. Finally, the material falls into the cooling kiln (cylinder) and is sent to the pulverizing process by air cooling or water cooling.

After calcination, the material is an aggregate of titanium dioxide pigment particles with high temperature, which needs to be slowly cooled to relax the crystal, which can reduce the lattice defects, otherwise the titanium dioxide pigment may be discolored or even undergo color-to-color interconversion. Generally, it can be cooled to 40 ° C, and the temperature is too low to easily absorb moisture in the air.

2. Calcination temperature and calcination strength

The calcination temperature and calcination intensity affect the most important factors of calcined product quality. Calcination strength includes calcination temperature and calcination time as a function of calcination temperature and calcination time. Grasping the calcination temperature and calcination time not only has good whiteness, high color-removing power, high hiding power, low oil absorption and good weather resistance, and the product particles are moderately soft and hard. Otherwise, it is difficult to balance all the above pigment properties, so it is calcined. When operating, not only must the material reach a certain temperature, but also ensure that the material stays in this area for a long time, not too long or too short.

Generally, the hiding power, the achromatic power and the weather resistance increase as the calcination temperature increases, and the oil absorption decreases as the calcination temperature increases. However, if the temperature is too high or the residence time is too long in the high temperature range, the particles become hard and the whiteness decreases yellowish and gray; the oil absorption increases due to the large voids in the coarse particles sintered together; the achromatic force and the hiding power may be due to The further growth of the elementary particles leads to a decrease in the ability to reflect and scatter light, and too high a temperature sometimes causes the titanium dioxide lattice to deoxidize with a gray phase. On the contrary, if the calcination temperature is low, the surface may have some advantages for the whiteness of the dry powder, and the particles are also soft and pulverized, but it is not good for improving the achromatic power and the hiding power.

The growth and crystal form conversion of titanium dioxide pigment particles occur in the late stage of calcination after dehydration and desulfurization (high temperature zone near the kiln head), and the growth of particles and the lattice rearrangement during crystal transformation require a certain time. And temperature, that is, a certain calcination strength is required. If the calcination temperature meets the requirements, or the residence time is too short, the material will not burn through. The pH test solution can clearly see that the unfired sandwich raw material is mixed therein. The product can be found to have low crystal transformation rate and oil absorption. High volume, pH, achromatic power and hiding power are low, and the pigment performance is not good; if a relatively low temperature is adopted, the method of prolonging the residence time can alleviate the above disadvantages, but the yield is too low, and the kiln head is close to the flame. Where the calcination time is too long at high temperature, material overheating may occur, and the combustion chamber may be placed in front of the kiln head to avoid direct contact of the high temperature flame with the material, which greatly contributes to the quality of the product, so the calcination temperature and calcination are correctly grasped. Time is the basic guarantee for improving product quality. [next]

3. Calcination atmosphere and temperature gradient

The calcination of titanium dioxide needs to be carried out in an oxidizing atmosphere. Since titanium dioxide is an n-type semiconductor, the smaller the partial pressure of oxygen during calcination, the more likely it is to cause lattice defects, and the faster the gold-red petrochemical, which causes the rutile type to be mixed into the anatase product. And reduce the whiteness of the product.

The calcination atmosphere is mainly determined by the ratio of combustion air during combustion, such as the ventilation in the kiln and the wind extraction condition of the chimney. The atmosphere in the kiln is good. The whiteness and luster of the burned product are good. The condition of ventilation can be expressed by the pressure difference in the kiln. The calcination atmosphere in the large kiln is good, so that the H ₂ O, SO ₃ and the discharge are exhausted during calcination, but the pressure difference is too large. The heat loss is large, and the dust carried away by the exhaust gas is also more. The proportion of excess air is generally ≥20%. When the gas is used as fuel, the excess ratio is even higher. Due to the air mixing mode and mixing ratio of many burners and combustion nozzles, the manufacturer has been set at the factory and should not be adjusted casually. They are all regulated by the method of fan supply. The combustion air used in general burners and nozzles is called primary air. It is directly related to whether the fuel is completely burned. The size of the baffle opening of the kiln head observation hole, the furnace door and the discharge opening can be supplemented by the natural wind, also called the third wind. It is helpful to reduce the over-burning phenomenon of the kiln head. Therefore, correctly grasping and adjusting the ratio of these three kinds of air volume is an important means to optimize the calcination operation.

The temperature gradient refers to the temperature gradient between the rotary kiln head and the kiln tail. The kiln head temperature directly determines the calcination strength of the product, which has a great influence on the pH, whiteness, achromatic power, hiding power, oil absorption, crystal form conversion, particle size and aggregation degree of the product, but the temperature of the kiln tail cannot be ignored. It affects the time of dehydration and desulfurization, as well as the residence time and calcination strength of the material. The temperature at the kiln tail is generally required to be between 250 and 400 ° C. The temperature of the kiln head is different depending on the location of the temperature measurement point and whether there is a combustion chamber. The temperature range indicated by each factory is slightly different.

For pigment-grade titanium dioxide, the most important is the temperature gradient from the kiln head discharge site (such as the kiln length of 38m, ie 5~6m from the kiln head) and the temperature control of 50 °C before the expected crystal transformation temperature. The residence time is very important, because this segment is the region where the titanium dioxide particles grow and crystal form, which has a very important influence on the size, shape and pigment properties of the final titanium dioxide particles. Generally, the heating rate of this section should not exceed 100 °C. For example, when calcining anatase titanium dioxide, the temperature of the kiln head is 920 °C, then the temperature at 5 m from one point should not exceed 820 °C, so some factories are in this section. There are 4~5 temperature measuring points on the kiln. It is not important whether there is temperature measurement point in the middle and back of the kiln. The key is that the temperature at 5~6m before the kiln head, kiln tail and kiln head must be strict. grasp. The adjustment of the temperature gradient is mainly solved by controlling the temperature of the combustion chamber and the kiln head and controlling the flow of the kiln air.

4. Effect of salt treatment agent and other impurities on calcination

Since the titanium dioxide pigment is very sensitive to the calcination temperature and calcination time during calcination, it is possible to adjust the variety and amount of the salt treatment agent while strictly controlling the calcination temperature and the calcination time, and not only can mutually compensate, but also obtain excellent and complete pigment properties. salt treatment agent particularly the role of potassium carbonate, phosphorous acid, zinc oxide, titania sol most obvious.

Increasing the amount of potassium carbonate can significantly reduce the negative effect of excessive calcination strength, making the particles soft and white, but not good for oil absorption; phosphoric acid has the same effect, but the effect is not as obvious as potassium carbonate; zinc oxide and titanium dioxide sol are very good Rutile type conversion accelerator, which can significantly reduce the temperature required for rutile conversion, especially the titanium dioxide sol can make the titanium dioxide particles grow relatively regular and become rounded particles, but excessive addition of zinc oxide will make the coating yellow Thickening, this can make up for the shortcomings of zinc oxide with aluminum salt, but the temperature at the time of calcination is higher than that of zinc oxide.

Some impurity ions in metatitanic acid have a certain influence on the growth and crystal form conversion of titanium dioxide particles. Some negative ions (anions) hinder the growth of particles and the transformation of crystal forms, and their effects are related to the volume of anions. The order of hindrance is Cl ^- <SO ₄ ^2- <PO ₄ ^3- . Some metal oxides not only affect the whiteness of the product, but also promote the growth of the particles and the transformation of the crystal form. Their copper oxide and cobalt oxide , promoted nickel oxide, manganese oxide, chromium oxide, copper oxide, wherein the maximum.

5. Control and analysis of product quality during calcination

If the calcined product has good color and low achromatic power, this indicates that the temperature gradient is large and the heating rate is fast, and the temperature gradient should be tried to be gentle;

If the calcined product has poor color, low achromatic power and hard particles, this is a signal of excessive calcination. The calcination temperature should be lowered to lower the temperature at the first point. After the color is improved, the temperature gradient is adjusted to gradually increase the decolorizing power. ;

If the calcined product is poor in color but has good achromatic power, this indicates that the temperature at the first point is high, and the temperature at the first point is gradually lowered while maintaining the color reduction force, and then the temperature gradient is adjusted.

A heavy principle of the calcination operation is “stable operation”. When there is a problem, do not rush to move it. It is not possible to adjust the temperature, feed volume and air volume frequently. The temperature is adjusted at least 1 hour for each adjustment, and the first adjustment is not obvious. Before the effect, the second adjustment cannot be made immediately, and the amplitude of each adjustment cannot exceed ±5 °C.

If a sudden drop in mass occurs during the calcination operation, it is likely to be a problem with the calcination operation itself or a change in the salt treatment formulation (or uneven salt treatment). If the product quality is slowly decreasing, the problem is generally in the case of metatitanic acid. Its own quality problems, including the effects of previous hydrolysis, seed crystals, and the quality of titanium fluid.

The rotation speed of the rotary kiln, the filling amount of the material, and the moisture content of the material also have an influence on the quality of the calcined product. The adjustment of these parameters cannot be too frequent or too large. For example, the adjustment of the feed amount should not exceed 2 times a day. Do not >> 5%.

Check the quality of the calcined product during normal operation, mainly to observe the appearance of the product, the softness or looseness ratio of the particles, the pH value, the crystal form conversion rate of the achromatic force, etc., and then adjust the temperature, temperature gradient and advance according to the above results. The amount of the material, etc., wherein the crystal form conversion rate is determined by a special X-ray diffractometer. Recently, the British Titanium Dioxide Group, introduced in European Patent Application Publication No. 0776222A ₂ , installed a Raman spectrometer in the kiln head of the rotary kiln, and observed the crystal transformation process according to different lines of anatase and rutile types, so that it can be monitored at any time. The crystal form conversion of the material inside.

6. Precautions for opening and closing of rotary kiln

The rotary kiln for calcination is the most expensive equipment for the production of titanium dioxide in the production of titanium dioxide, and should be carefully operated and carefully maintained. The selection of new kiln refractory bricks and the quality of the furnace are very important. The operation of the kiln before driving is also very important. The refractory bricks used for the furnace should be properly kept, not damp, nor damaged. It is best to dry naturally for 2 weeks. Masonry, so as to remove the moisture inside the refractory bricks as much as possible, so that the temperature of the refractory bricks rises evenly, avoiding the evaporation of water too quickly when the temperature rises, causing the refractory bricks to crack. When igniting the kiln for the first time, the temperature in the kiln should not exceed 100 °C, then increase to 400 °C at a rate of 10 °C per shift (8h), and then it can be heated at a rate of 50 °C per shift. When repairing and replacing some refractory bricks at ordinary times The rate of temperature rise can be relatively fast.

When the kiln is ignited, the kiln body should be rotated first, and the fan should be blown into the kiln to inject fresh air to avoid the explosion of the flammable gas in the kiln. If the ignition fails, the fuel valve should be closed immediately and the kiln should be continuously sucked into the kiln. Air, after the flammable gas is discharged, can be ignited for the second time.

When planning to stop the kiln, slowly reduce the amount of feed. When the material is finished, slowly reduce the amount of fuel and air. After all the materials are finished, slowly reduce the temperature. After the flame is turned off, the kiln still needs to keep rotating until it is close. After normal temperature, the rotation can be stopped. Even if it is not used for a long time, the kiln body should be rotated regularly to prevent deformation due to heavy effects.

When an emergency shutdown is required due to an accident (such as power outage, gas outage, or fuel system failure), the fuel valve and the feed pump should be immediately shut down to maintain ventilation. Try to use the backup power supply or mechanical transmission to keep the kiln body rotating to prevent high temperature. The long-term static stationary kiln body is deformed. At this time, the unfired materials should be stored separately and should not be mixed into normal products.