35. The influence of calcination factors on product quality
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The water content of metatitanic acid determines the completeness of dehydration and desulfurization of the material in the drying zone. The water content is too high. Because the dehydration and desulfurization are incomplete, the material enters the high temperature zone in the form of agglomerates or large particles, and the residence time of the high temperature zone is very short. Therefore, the burning of the products is not transparent and is prone to pinching. The product's color reduction power decreases, the oil absorption increases, and the pH value decreases. If the water content is too low, the material is dehydrated in the drying zone, and the desulfurization is sufficient. The material enters the high temperature zone in the form of powder, which is easy to be sintered and hardened, and the color becomes yellow and gray. The whiteness is lowered. Generally, the pigment titanium dioxide production control water content is about 65%. The non-pigmented titanium dioxide production control water content is about 50%.
16. Effect of particle size on product quality during calcination
The titanate particle size is an important factor affecting desulfurization when producing titanium dioxide. The particle size is small, the desulfurization is easy and complete, the material does not flow to the kiln head, and the obtained calcined product has a white color; the granularity is large, and the desulfurization is difficult, because the sulfur-containing titanium dioxide has strong fluidity at high temperature. The material is easy to flow to the kiln head. The size of the titanic acid particle size is mainly formed from the hydrolysis process of the titanium liquid. Therefore, the hydrolysis process conditions must be strictly controlled according to the different requirements of the variety to obtain the titanic acid with suitable particle size.
17. Normal state of movement of materials in the kiln during calcination
The state of motion of the material in the kiln can be measured by the cumulative height H of the material in motion. The larger the H value is, the smaller the material is, the smaller the H value is, and the material is rolled into the kiln head, indicating that the kiln temperature is too low, and the raw material will be mixed into the calcined product; Calcination is normal when the H value is moderate and the material is uniformly granulated into the kiln head. In the production of non-pigmented titanium dioxide, the state of motion of the material in the kiln can clearly reflect the level of sulfur in the calcined product. When the material in the kiln is “liquid†flowing toward the kiln head, the calcined product contains extremely high sulfur. When the H value is large and the material is pushed slowly to the kiln head, the sulfur content of the calcined product is extremely low. The material in the kiln has a "boiling" shape before entering the high temperature zone. When the material enters the high temperature zone, the material does not flow to the kiln head. The sulfur content of the satin product can meet the requirements of the production process.
18. Determination of the pH value of the calcined product and the reasons for the red, yellow, yellow-green and blue colors of the falling products in the porcelain plate using the universal pH test.
The method for determining the pH value of the calcined product is as follows: a small amount of the falling product is taken out from the kiln head feeding port by using the sampled ladle, placed in a porcelain crucible, and placed on a colorimetric porcelain plate, and a few drops of the universal pH test are dropped. The liquid, and then compared with the standard sample, the pH value of the falling product can be measured.
The universal pH test droplets appear red on the falling material of the porcelain plate, indicating that the pH value is below 6, indicating that the material is acidic, hard to burn, and high in sulfur. Cause may be: lack of salt-treated calcium carbonate was added ①When adding phosphoric acid or excessive; ② calcination temperature is too low; ③ material residence time in the kiln is too short; ④ excessive feeding, the material layer is too thick; partial ⑤ The water content of the titanic acid is too large; 6 the metatitanic acid particles are large. The sample appeared yellow, indicating near neutral; yellow-green, indicating that the pH value is between 6.5 and 7.5, indicating that the sulfur content is acceptable, the calcination is suitable; the blue color indicates alkaline, which has reached the requirement or slightly exceeded the requirement. If the sample appears red, yellowish green, and blue at the same time, it indicates that the calcination is uneven, and there are both burnt and burnt, and burnt. Since the finished product of titanium dioxide requires a pH of 6.5-8, it is satisfactory to burn to the yellow with green.
X. Pre-kiln analysis of non-pigment grade titanium dioxide
In the production of electrode grade and enamel grade titanium dioxide, the pre-kiln analysis mainly relies on controlling the sulfur content. Most of the sulfur was measured using a calcination Method iodine. The sulfur content of the electrode-grade titanium dioxide is ≤0. 05%, the pre-kiln control should be below 0.04%; the enamel-grade titanium dioxide containing S0 3 is ≤0.15%, and the kiln should be controlled below 0.12%. To ensure the quality of the finished product, but the sulfur content should not be too low, when the product is too low, the color of the product is poor, and it will consume more fuel. As to the metallurgical grade titanium dioxide production due to the calcination at a high temperature of temperature control is 1250 deg.] C, low sulfur products falling kiln, sulfur moot measured before kiln. [next]
Twenty, judge the satin burning situation from the appearance and color of the falling kiln products
The normal color of the falling kiln is white and soft, and the pigment-grade titanium dioxide falling kiln is granular; the non-pigment-grade titanium white powder is mostly hydrolyzed by atmospheric pressure, and the falling kiln is bonded into small particles, and the color of the falling kiln is white. The sulfur content is also low. If the powdered material falls, most of the metatitanic acid particles are too thick, and the sulfur content is too high, so that the kiln temperature has to be increased accordingly, and the product tends to be gray. If the falling kiln product is yellow, it is mainly caused by the impurity iron in the meta-titanic acid not washed in the water washing process.
Twenty-one, judge the calcination from the observation of the situation in the kiln
When the calcination in the rotary kiln is normal, the material layer is stable and the space above is relatively clear. If there is a lot of smoke in the upper space, pay attention to the passage in the kiln and check if the kiln flue is blocked. In the production of non-pigment grade titanium dioxide, if the layer is found to slide forward, or even slip to the lower opening, commonly known as running powder or rushing kiln head. This indicates that the metatitanic acid particles are too coarse and have a high sulfur content. Although they have reached the firing zone, they still maintain a free-flowing powder state, and the powder that is washed out is often fine. Under normal circumstances, there is no large piece of material visible from the kiln head. If large materials are found, indicating that the material dewatering zone is prolonged, the amount of feed should be appropriately reduced, the speed of the kiln should be slowed down, the kiln temperature should be increased or the ventilation should be enhanced, or used in conjunction with each other.
Twenty-two, the work of opening the kiln and ignition
Before the kiln is opened, the kiln body and its ancillary systems should be inspected comprehensively, and the empty car test should be carried out to eliminate hidden dangers. In order to avoid impurities contaminating titanium dioxide products, new kiln, kiln overhaul, replacement of lining refractory bricks or exchange of varieties, the kiln must be cleaned. After changing the bricks, it must pass through a dry day and night to ignite the kiln.
Before ignition, first open some gray box doors, then turn the kiln body, start the blower, input air into the kiln, and discharge the combustible gas that may remain in the kiln. The igniter is introduced from the kiln head observation port and aligned with the nozzle to open the fuel gas cock. At this time, the operator must face the kiln head to avoid foreign matter or dust rushing out of the kiln, or due to improper gas ratio explosion. Hit. If you can't stop at a time, you should cut off the flammable gas immediately, but do not stop the air transportation. You must wait for the remaining flammable gas in the kiln to exhaust, and then ignite, otherwise it will cause an explosion accident. If liquid fuel is used, the oil pump can be started to drive the circulating oil, the ignition igniter is placed in front of the nozzle, the oil is slowly ventilated, and the oil and gas ratio is appropriately adjusted.
Two + three, rotary kiln should be baked before feeding
The oven is designed to remove moisture from the liner and raise the temperature of the liner evenly to the operating temperature. When drying the kiln, it is necessary to prevent the temperature from rising too fast, so as to avoid cracking and damage of the lining. The length of the drying kiln should be determined according to how many bricks are changed. The new kiln or all of the brick kiln is replaced, and the kiln is relatively large. The kiln time is about 2-3 days and nights, and it gradually heats up to about 350 °C; The drying time is about 10-16h; the kiln without changing bricks has a drying time of about 6-8h. After the kiln, the kiln temperature gradually rises below the normal working temperature (100-200 °C) and can be put into production.
Twenty-four, matters needing attention in stopping the kiln
The steel casing of the calcined rotary kiln is easily deformed when heated at a high temperature. Whether it is the planned shutdown kiln or unplanned equipment or power supply, gas supply system failure suddenly stop the kiln, after the flameout must continue to rotate the kiln body and blast until cooling. Otherwise, the heated kiln body will undergo bending deformation under the huge self-gravity of the kiln, resulting in a major equipment accident.
In the planned stop kiln, the material in the kiln can be maintained until all the calcination is completed. Before stopping the kiln, the material should be gradually reduced, and the materials should be discharged as much as possible before stopping the kiln. As the material in the kiln is gradually reduced, the amount of fuel and air should be reduced accordingly. When the material is basically cleared, the material is extinguished, and the kiln body is idling and cooled. Stop the kiln. In case of sudden stop of the kiln outside the plan, the material in the kiln has not been burnt through. In order not to affect the equipment maintenance, it must be completely discharged, and then burned back later.
25. Construction and operation procedures of a reverberatory furnace for calcining metatitanic acid
The use of reverberatory furnace for calcination of metatitanic acid, although its labor intensity is large, but because of its low investment, small footprint, easy to operate, many small iron powder factories, especially the production of low-grade enamel, metallurgy, welding grade titanium dioxide It is still used in factories with low technical requirements. The structure is made of refractory bricks, which are divided into upper, middle and lower layers, indirectly heated, and the hot gas heats the refractory plate, and then transfers the heat to the material layer through the refractory plate. Materials and flue gas countercurrent operation, lower heat to burn coal, produced by the flue, and finally discharged from the chimney to the middle, the upper run. The metatitanic acid feed is placed in the upper layer for drying, and then dehydrated, then the intermediate layer is preheated, and then the lower layer is calcined, and the desulfurization, crystal transformation and particle growth are completed in the middle layer and the lower layer. After the lower layer sampling inspection is passed, the lower layer can be discharged from the lower layer. After the discharging, the middle layer material is raked to the lower layer, and then the upper layer material is raked to the middle layer, and finally the metatitanic acid is transported to the upper layer by a belt conveyor. [next]
Twenty-six, the composition of the rotary kiln and its role
The rotary kiln is mainly composed of a cylinder, a supporting device (including a rolling ring, a supporting roller and a retaining wheel), a transmission device, a sealing device and other ancillary equipment (feeding and burning equipment, etc.).
The cylinder is made of 18-28mm thick high-quality steel plate and is lined with refractory bricks to prevent the cylinder from being subjected to high temperature and reduce heat loss. During the production, the material rotates to a certain height with the cylinder wall inside the cylinder, and it is moved forward due to the action of gravity, and the temperature of the material is gradually increased by the high temperature airflow in the cylinder body, and finally the temperature of the satin burning required by the process is reached.
The rolling ring of the supporting device supports the entire rotating cylinder so that it can rotate on the supporting wheel, so it must have sufficient rigidity and durability, and can be made of cast steel or forged steel; the supporting wheel supports the rotary kiln; The function is to prevent the upper and lower axial movement of the cylinder, generally on both sides of the rolling ring.
The function of the transmission device is to drive the cylinder to rotate at the rotation speed required by the process. Because the rotary kiln rotates at a slower speed, a reducer is required. For the occasions where the speed needs to be adjusted frequently, a continuously variable transmission can be used, and the rotation speed of the cylinder is slow, so the large gear mounted on the cylinder is subjected to a large circumferential force and can be made of cast iron.
The sealing device is for preventing the harmful gas such as S0 3 and the Ti 2 2 dust emitted during the calcination of the metatitanic acid from escaping, and the inside of the drum is operated under a negative pressure, so there must be a sealing device between the drum and the chambers at both ends thereof. Its structural requirements are that it does not leak or affect the normal operation of the drum.
Other ancillary equipment include charging devices and combustion devices. For the metatitanic acid slurry with higher water content, a squeeze pump is generally used, and the metatitanic acid is introduced into the kiln through the extrusion pump and the feed tube. For the dry material dewatered by the centrifuge, equipment such as hoist and rotary feeder can be used, and even manual unloading and feeding are used. As the fuel for the rotary kiln, there are gas, diesel and heavy oil, and the corresponding gas burner and heavy oil burner. In order to prevent contamination of titanium dioxide, a combustion chamber can be provided when diesel and heavy oil are used as fuel.
Twenty-seven, the role of the combustion chamber in the rotary kiln head
Whether the rotary kiln has a separate combustion chamber has a great impact on product quality. Since the flame temperature can be as high as about 1300 ° C, such as the flame and the material contact, the temperature difference between the combustion gas and the material is very large, and the temperature of the fired material itself is high, and sintering is inevitably generated. It is generally believed that the temperature difference between the maximum temperature of the combustion gas and the material should not exceed 200 ° C. Therefore, a combustion chamber is installed to fully burn the fuel and then diluted with air to a certain temperature before being contacted with the material of the kiln to improve the output and quality. benefit. A 48m long rotary kiln, the combustion chamber is about 6m, the resulting combustion is like the size of the rice grain, the color is white and soft.
Twenty-eight, the effect of the size and length of the rotary kiln on combustion
The rotary kiln for calcining metatitanic acid should be short and thick, the inner diameter is from 1.0-4.2m, the ratio of length to diameter is generally (12-20):1, the ratio of small diameter is larger, and the proportion of larger diameter is smaller. . The kiln with a diameter of 2.4-4.2 m has a length of about 48 m; when the diameter is less than 2.4 m, the length is also shorter. In general, production is proportional to diameter. When the diameter is large, the material filling factor is correspondingly lower, which is good for improving product quality. Since the dehydration of metatitanic acid consumes more than half of the heat, the fine and long kiln must have sufficient heat for dehydration, and the temperature of the kiln must be increased accordingly, or the amount of air added. The increase in temperature tends to cause over-burning, and the addition of atmospheric amount causes the linear velocity of the material to increase, resulting in an increase in resistance and an increase in dust loss, which is disadvantageous for calcination.
Twenty-nine, fuel combustion produces heat transfer
In the rotary kiln, there is a complex heat exchange process between the high temperature gas stream, the kiln wall and the material. The high-temperature airflow generated by the combustion of the fuel radiates heat to the material and the kiln wall; the heated kiln wall radiates heat to the material and the gas; the high-temperature gas transfers heat to the surface of the material and the kiln wall when flowing in the kiln; and the kiln in contact with the material The wall transfers heat to the material by conduction. Generally speaking, in the dehydration zone with lower temperature, the heat transfer is mainly convection and conduction. With the increase of temperature, the heat transfer mode is mainly based on radiation.
30. Requirements for burning liquid fuels and devices used therefor
Titanium dioxide has high requirements on whiteness and must be avoided due to incomplete combustion of liquid fuel. In order to complete the combustion of the fuel, it is desirable to make the fuel droplets heat up quickly and in close contact with the oxygen in order to complete the decomposition process of the fuel at a lower temperature and completely burn the decomposed hydrocarbons. To do this, the liquid fuel needs to be atomized into very fine oil droplets. The finer the fuel is atomized, the larger the surface area in contact with the oxygen in the air, and the faster and more complete the fuel burns. The atomization process of the fuel is done in the nozzle, which disperses the fuel into fine droplets on one side and evenly mixes the droplets with the air. The nozzle has a mechanical nozzle, a high pressure steam-air nozzle and a low pressure air nozzle. In the production of titanium dioxide, more manufacturers use C-shaped low-pressure nozzles, and there is a flared tube outside the nozzle to improve the atomization, and air is used as an atomizing agent. [next]
31. Combustion device for combustion of gaseous fuel
In the combustion of rotary kiln gas fuel, a mixed nozzle is often used. In order to improve the combustion intensity, a blower is used to force the air to be sent to a nozzle to mix with the gas for combustion. The burning speed and the degree of combustion of the gas combustion depend on the mixing process of the gas and the air. The better the mixing, the more complete the combustion and the shorter the flame length. The advantages of the hybrid nozzle are high productivity, compact structure, low pressure gas, high thermal index (less than 2% chemical incomplete combustion); the disadvantage is that the blower is required to supply air, the flame is longer, and the combustion chamber is also Larger.
Thirty-two, rotary kiln calcined with metatitanic acid is not an ideal calcining equipment
At present, calcining equipment uses rotary kiln, but the rotary kiln for calcining metatitanic acid is not an ideal and perfect equipment because it can not create good heat transfer conditions for the heated powdery material. The heat transfer of the rotary kiln basically relies on the hot air flow in the kiln to heat the exposed material surface and to heat the material close to the kiln wall by the hot kiln wall. However, a large amount of material between the two materials cannot directly contact the hot gas stream or the kiln wall, and even in the same section, the temperature is not uniform. The filling kiln has a filling factor of only 10%-20%, so space utilization and thermal efficiency are not high. In addition, when the titanic acid is calcined, it gradually pulverizes into a powdery material as the temperature rises, the thickness of the material is very uneven, and the material layer is relatively thick. To make the material burn through, the temperature of the hot gas flow needs to be higher. The calcination temperature is high, and as a result, when the large particles are burned through, the small particles have already burned over the head, so the product tends to be a mixture of materials that are burnt and overcooked. Sintering of some materials is also difficult to avoid.
Foreign studies on titanium dioxide boiling calcination have been carried out since the 1960s, and have long been used in production practice. Boiling roasting has been widely used in metallurgical and chemical fields. With boiling calcination, the material is highly turbulent in the bubbling bed, and the gas flow can be almost in contact with the surface of each particle, so mass transfer and heat transfer efficiency are greatly improved.
Thirty-three, the cause of color change of calcined titanium dioxide and its method of preventing color change
When producing anatase type titanium dioxide, it often encounters calcined falling kiln products or after being pulverized into titanium white powder, the surface layer is exposed to sunlight, and the color change phenomenon occurs, and the original white color changes into light gray or light yellow. There are two reasons for this: 1 Some of the anatase crystals are converted into rutile crystals; 2 impurities such as iron, chromium , copper , and nickel enter the rutile crystal lattice, causing lattice deformation. There are several ways to prevent: 1 use a fixed point source to control the content of impurities in the titanium ore; 2 bleach the metatitanic acid to reduce the content of iron impurities; 3 increase the amount of phosphate in the salt treatment agent to mask the iron Impurities; 4 adjust the atmosphere in the calcining kiln to avoid the production of rutile crystals; 5 semi-finished products from the rotary kiln exist in the form of strong aggregated large crystals, which must be slowly cooled to relax the crystal lattice of Ti02. When the color change phenomenon occurs, the treatment methods are as follows: 1 lowering the calcination temperature, because the transformation of the anatase crystal form into the rutile crystal form must be carried out at a higher temperature, and it is difficult to form a rutile crystal when the kiln temperature is lowered; However, the kiln temperature can not be lowered too low, the kiln temperature is too low, the dehydration is incomplete, the pH value is too low, which will cause the product's color reduction power to decrease; 2 in the salt treatment, more potassium carbonate is added to lower the desulfurization temperature and accelerate the sharpness. The growth rate of the titanium type microcrystals attenuates the color change phenomenon which occurs due to sintering caused by high temperature.
34. Reasons for abnormal phenomena in calcination and methods of treatment
The abnormal phenomena, causes and treatment methods of calcination are shown in Table 3. [next] 
The effect of calcination factors on product quality is shown in Table 4. [next]
Production of Titanium Dioxide by Sulfuric Acid Method--Calcination of Metatitanic Acid (II)
Fifth, the partial titanate water content affect the level of product quality