Production of Titanium Dioxide by Sulfuric Acid Method--Calcination of Metatitanic Acid (I)

A, partial titanate main chemical components
According to the multiple chemical analysis of metatitanic acid produced in a certain mine, the composition is shown in Table 1.

Second, the purpose of the titanic acid satin burning and satin burning process
When titanium is precipitated from titanium liquid in the form of white metatitanic acid, it contains not only a large amount of water but also a certain amount of sulfuric acid. The purpose of calcination is to dehydrate and desulfurize metatitanic acid at high temperature and form iron white powder with a certain crystal form and a certain quality index.
The salt-treated metatitanic acid slurry is sent to a storage tank of a calcining process, and then extruded into an internal combustion rotary kiln by a squeeze pump for calcination. If production welding, metallurgy, ceramics, other capacitor grade titanium dioxide, mostly by pressure hydrolysis method, the metatitanic acid obtained coarse particles, as reasons of fuel economy can be used after drying centrifuge, solid delivery device It is sent directly to the combustion process, and then the barium titanate block is added to the kiln for calcination by a bucket elevator. The rotary kiln is driven by a variable speed motor, which can be adjusted according to the requirements of process conditions such as slurry and temperature. In the kiln, countercurrent heating is used, and the metatitanic acid added from the tail end of the kiln is brought up to a certain height with the rotation of the rotary kiln, and then continuously rises and falls due to its own rotation. Since the kiln body has a slope of 2%-4%, the material will fall once every liter and then advance to a certain distance in the direction of the kiln head. The metatitanic acid is moved to the kiln by gravity. The fuel and combustion air are compared. The low kiln head end enters the kiln, and the high temperature gas generated by the combustion flows from the kiln head to the kiln tail, and forms a countercurrent operation with the metatitanic acid slurry. This is how the metatitanic acid is sent from the kiln tail to the kiln head, and a series of physical and chemical changes such as dehydration, desulfurization, crystal transformation and particle growth are completed in the process of gradually increasing temperature, and a certain crystalline titanium dioxide product is formed. The titanium dioxide powder is cooled by the cooling drum of the kiln head, and then sent to the powder storage bucket through the bucket elevator for preparation for crushing.
The metatitanic acid that enters the rotary kiln is not suctioned by a leaf filter, but is pumped with a drum filter and pumped by a screw. [next]
Third, the area divided by the satin burning and the temperature range of each area
The calcined rotary kiln can be divided into three areas: a drying zone, a crystal transformation zone and a particle growth zone. The temperature in the kiln and the size of the kiln, the distance between the material from the center of the heat source and the position of the temperature point of the thermocouple are related to the distance from the heat source point. The temperature varies from the different plants to the temperature range of each area of ​​the calcined rotary kiln. .

4. Changes in metatitanic acid in the drying zone of rotary kiln
In the dry zone, metatitanic acid undergoes changes in dehydration and desulfurization. This change can be expressed as:
Ti0 ₂ ·xH ₂ O·yS0 ₃ ==Ti0 ₂ +xH ₂ O↑+yS0 ₃ ↑
1 Dehydration a. Dehumidification Water Wet water is the water attached to the surface of the meta-titanic acid particles and entrained in the interstices. The combination of this part of water and Ti0 ₂ is not strong and evaporates between 100-200 ° C;
b. Dehydration combined water is the water bound to the inside of the metatitanic acid molecule. The combination of water and Ti0 ₂ is relatively strong, and it can be removed between 200-300 °C.
2 In the metatitanic acid slurry produced by the desulfurization hydrolysis, most of the sulfuric acid contained is a free acid, which can be removed by washing with water. However, sulfuric acid, which accounts for 7%-8% of the total amount of metatitanic acid, is strongly bonded to metatitanic acid in the form of S0 ₃ . Because of the conditions of metatitanic acid formation and the entrained impurities, it contains 500 sulfuric acid. Between -800 ° C, it can be decomposed into S0 ₃ and SO ₂ gas and removed.
Due to the large water content of metatitanic acid, the heat required to evaporate water is more than 5 times higher than that of S0 ₃ . Because of the high water content, the smoke from the chimney of the calcining kiln is mainly water vapor, and the water vapor meets the atmosphere and condenses into fog. As a result, S0 ₃ gas and water vapor also become acid mist when it is cold. Titanium dioxide in the flue gas is relatively small.
V. Changes in metamorphic acid in the crystal conversion zone of rotary kiln
Generally, the metatitanic acid obtained by the sulfuric acid method is an anatase crystal. After calcination at a lower temperature, all of the obtained anatase titanium dioxide was obtained. This anatase type is generally stable below 900 °C. When the temperature exceeds 950 ° C, the conversion to the rutile crystal form begins. The pure anatase crystal form must be at a high temperature above 1200 °C to fully convert to a rutile crystal form. When calcined at such a high temperature, Ti0 _{2 is} easily sintered. For this reason, various rutile type conversion promoters must be added to lower the temperature of the crystal form conversion to between 800 and 860 ° C to grow anatase crystals. Smoothly and completely convert to rutile crystals.
6. Changes of metatitanic acid in the growth zone of rotary kiln particles
The aggregation of fine crystals into pigment particles requires a certain amount of energy. The higher the calcination temperature, the faster the particles will grow. At 600 ° C or lower, the particle growth rate is very slow. When the temperature exceeds 600 ° C, the particle growth rate starts to increase. When the temperature reaches 900 ° C, the rate of particle growth is greatly increased. If the calcination temperature is raised to 1000 ° C, the aggregated particles will have a diameter of 1 μm. The most suitable particle size as the pigment titanium dioxide is 0.2 - 0.35 μm, that is, the particle diameter should be half the wavelength of the visible light. If the particle diameter is smaller than the half wavelength of the visible light wave, the pigment particles will become transparent; if the particle diameter is larger than the half wavelength of the visible light wave, the white pigment will exhibit a red phase. To this end, the temperature of this zone should be controlled between 860-950 °C according to different conditions, so that the grown crystals coalesce into pigment particles.
7. The adverse effects of the reaction of TiO ₂ and potassium sulfate at high temperatures on the finished product and how to avoid this effect
In the production of pigment titanium dioxide, potassium carbonate is often added for salt treatment. Potassium carbonate reacts with sulfuric acid in ferric acid to form potassium sulfate. During calcination, potassium sulfate and titanium dioxide can react at high temperature to form potassium metatrate. This potassium metatrate will hydrolyze in the presence of water to form potassium hydroxide. It is precisely because of the appearance of alkaline substances such as potassium hydroxide that the pH value of the aqueous solution of titanium dioxide is weakly acidic or neutral or weakly alkaline, and the end point of calcination is controlled by this change. Its series of reactions are as follows: [next]

This phenomenon occurs in general fuel combustion, and the powder at the end of the kiln is gray. Sometimes, due to poor ventilation in the kiln, the calcined exhaust gas cannot be discharged in time, and the incomplete combustion of the fuel may also cause CO to be generated. Further, in the case of metatitanic acid, if a large amount of reducing trivalent titanium is contained, sufficient oxygen oxidation is not obtained in the kiln, and Ti ₂ 0 ₃ is easily formed to cause a gray phase of the titanium white powder.
In order to avoid the adverse effects of reducing substances such as CO and trivalent titanium, it is necessary to ensure that the fuel is completely burned in the kiln and has a certain oxidizing atmosphere, which requires accurate control of the ratio of fuel to combustion air. Usually the proportion of fuel (coal gas) and combustion air in 1: (3.2-3.3), when using diesel oil or heavy oil as fuel, the excess air coefficient is controlled to 1.2, i.e. 20% excess air. Increase the amount of air in the kiln head, strengthen the ventilation in the kiln, maintain sufficient oxidizing atmosphere in the kiln, prevent the generation of reducing substances, or oxidize the original reducing substances, so that titanium exists in the highest valence state, such titanium dioxide It is white. However, increasing the amount of air intake will reduce the kiln temperature, which in turn will increase the amount of oil, resulting in increased fuel consumption. Increasing the amount of air without increasing the amount of oil will definitely lower the temperature of the kiln head. Although this can increase the whiteness of the product, the achromatic power will decrease and the oil absorption will increase. This kind of contradiction requires the factories to weigh the pros and cons according to their respective production conditions and quality requirements, and take corresponding measures.
9. Method for improving the capacity of rotary kiln calcined products and reducing heat consumption
Under the premise of ensuring the quality of the material, the more heat is obtained per unit time, the faster the heating of the material and the higher the production capacity of the kiln. At the same time, because the heat obtained by the material is large, the lost heat is relatively reduced, so that the utilization rate of the heat energy is high and the heat consumption is small. Increasing production capacity and reducing heat consumption is mainly to control the appropriate air excess coefficient. The air excess coefficient is too small, and the fuel combustion is incomplete, which is not conducive to maintaining the oxidizing atmosphere in the kiln; the excess air coefficient is too large, so that the kiln's production capacity is reduced, and the exhaust gas is taken away. The heat is relatively increased. Therefore, under the premise of complete combustion of the fuel and maintaining the oxidizing atmosphere in the kiln, the excess coefficient of the air can be minimized; the temperature of the human kiln of the fuel and the combustion air can be increased as much as possible; the heating area of ​​the material and the surface area of ​​the kiln should be enlarged; Device; uses a fuel with a high calorific value.
X. The adverse effects of impurities contained in metatitanic acid during calcination
The addition of a positive catalyst to metatitanic acid increases the conversion rate and correspondingly lowers the conversion temperature. The inhibitory effect of the anion contained in metatitanic acid on the conversion is related to the volume of the anion. The order of inhibition is: Cl ^- <SO ₄ ^2- <PO ₄ ^3- (ie, the conversion temperature of the crystal form containing chloride is lower) containing high sulfate containing higher phosphate salt). For some colored metal oxides such as copper oxide, cobalt oxide, nickel oxide, chromium oxide, manganese oxide, etc. can promote the conversion, especially the role of the maximum copper oxide, tungsten oxide, sodium oxide impede. [next]
When the amount of additive (in terms of oxide) is about 1 mol, the material is calcined at 900 ° C for 3 h, and tungsten oxide and sodium oxide have no effect on particle growth; chromium oxide has little effect on particle growth; nickel oxide, oxidized diamond , manganese oxide particles have to promote moderate growth; copper oxide, iron oxide on the grain growth promoting effect was remarkable; molybdenum oxide has a particularly significant role in promoting grain growth.
XI. Influence of temperature of rotary kiln head and kiln tail on product quality
The kiln head temperature is usually called the high temperature zone temperature, which determines the conversion of the Ti0 ₂ crystal form and the growth of the pigment particles, and is an important factor affecting the performance of the titanium dioxide pigment. In general, the higher the temperature of the kiln head, the faster and more complete the crystal transformation of Ti0 ₂ and the growth of the particles. However, if the temperature of the kiln head is too high, the material is easily sintered, the calcined product particles are hardened, and the color becomes yellowish and gray. At about 810 °C, the achromatic power increases with the increase of temperature, but after rising to a certain temperature, the achromatic power will drop sharply, generally between 850-950 °C.
The kiln tail temperature directly affects various changes in the drying zone, and is also a sign of good ventilation in the kiln. The kiln tail has a high temperature and good ventilation, and the exhaust gas after calcination is easy to discharge. However, the ventilation is too large, and the heat taken away by the exhaust gas and the white dust of the titanium dioxide increase, causing a certain loss. At the same time, the moisture is removed prematurely, and it is easy to sinter in the high temperature zone. The temperature at the kiln tail is low, not only the ventilation in the kiln is not good, but also the dehydration of the metatitanic acid in the drying zone and the incomplete desulfurization, so that the phenomenon of entrainment of raw materials in the calcined product occurs. Generally controlled between 150-300 °C.
12. Effect of length of material staying in rotary kiln on product quality
Since the Ti0 ₂ pigment particles are formed in the later stage of calcination, the residence time of the materials in the kiln has a decisive influence on the crystal transformation, the size and shape of the particles. In the industry, the particles formed by calcination are required to be round and regular, so that the formation of the crystal form and the growth of the crystal grains cannot be too fast, and it is necessary to have sufficient time for the crystal lattice to be arranged neatly and gradually grown. At the same time, the calcined products are mostly aggregates, and it is necessary to have sufficient calcination time. The temperature is correspondingly low and the calcination time is appropriately extended, and the sintering and coalescence phenomenon can be correspondingly reduced. However, too long residence time of the material in the kiln will affect the output. If the residence time of the material in the kiln is too long and the calcination temperature is high, the material will be sintered, the calcined product particles will be hardened, the color will turn yellow and gray, and the whiteness will decrease; when the material stays in the kiln for too short, it will burn. Impervious, the calcined product appears pinch phenomenon, the crystal transformation is incomplete, the achromatic power decreases, the oil absorption increases, and the pH value decreases. 5-9之间之间。 The kiln speed is controlled between 3. 5-9r / min. That is, the material stays in the kiln for about 10 hours.
Thirteen, in the rotary kiln calcination should try to avoid particle sintering phenomenon
If the calcination time of the material in the kiln is too long and the temperature is high, the size of the sintering growth of the pellet becomes infinite, generally 1.0-10 μm. The sinter has a large space volume or forms horny particles. In addition to hardening the granules, the color becomes yellowish and gray, the suction amount is also increased, the decoloring power and the hiding power are lowered, and the optical properties and the gloss of the paint film are deteriorated. Therefore, it is necessary to avoid sintering during the calcination.
14. Influence of thickness of material layer in calcining kiln on product quality
Too much feed into the calcining kiln, the material layer is too thick, the various changes of the material in the kiln are not complete, and the forged products appear to be sandwiched, which is not conducive to desulfurization and crystal transformation, and the decolorizing power is reduced. Increase, the pH value decreases; if the amount of feed is too small, the material layer in the kiln is thin, it is easy to cause the material to be sintered, the calcined product particles become hard, the color becomes yellow and gray, the whiteness is reduced, and the output is reduced, waste The heat increases energy consumption, increases costs and reduces efficiency. The theory and the amount of feed are often expressed in terms of the filling factor N of the kiln:

Where N is the fill factor;
S ₁ — cross-sectional area of ​​material accumulation in the kiln, m ² ;
S ₂ - effective cross-sectional area of ​​the kiln, m ² .
5 吨。 Preferably, the kiln of the kiln is about 5. 5-8. 5 tons is suitable. Of course, the amount of feed is also different due to the different types of titanium dioxide. The amount of non-pigmented titanium dioxide can be more than doubled compared with the amount of titanium dioxide produced.

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