Production of Titanium Dioxide by Sulfuric Acid Method--Separation and Washing of Metatitanic Acid (1)

First, the partial separation of the mother liquor titanate
The hydrolyzate is cooled in a cooling tank by coil water to about 40 ° C, that is, it is sent to the upper tank, the leaf filter is turned on to the vacuum line, and the entire set of blades is immersed in the metatitanic acid slurry. The vacuum leaf filter consists of 10-15 blades each arranged in parallel. Each blade is covered with a filter cloth. Before the top sheet is opened, the bottom mixer or compressed air is started to stir the slurry evenly. Due to the pressure difference of the vacuum, the filtrate gradually permeates the filter cloth, and the metatitanic acid is deposited on the surface of the blade. As the slurry continues to be applied, the liquid level in the tank decreases accordingly, and the titanic acid slurry needs to be continuously added to maintain the liquid level. height. When the titanate filter cake is deposited to a thickness of 25 to 35 mm, the entire set of blades is lifted by an electric crane, and after being slightly drained, the water is washed in a water washing tank. At this time, more than half of the mother liquor in the metatitanic acid has been separated. The mother liquor is pumped into the aspiration tank and automatically discharged. The mother liquor in the previous period is called concentrated acid, and is recycled to the acid hydrolysis process; the mother liquor washed once to the settling tank to recover a small amount of suspended metatitanic acid in the diluted acid.
Second, the purpose and operation of partial titanic acid washing
The purpose of the water washing is to perform liquid-solid separation using water insolubility of metatitanic acid and water solubility of impurity ions, thereby removing a large amount of non-titanium soluble impurities and free sulfuric acid in the mother liquid other than metatitanic acid, thereby obtaining relatively pure metatitanic acid.
Since the impurity content directly affects the hue of the titanium dioxide product, for example, the finished product contains 0.009% Fe ₂ O ₃ , which affects the whiteness of the anatase pigment titanium dioxide, and 0.003% of Fe ₂ 0 ₃ will make gold. The red stone pigment titanium dioxide product exhibits a yellow phase. The reason is that iron is converted to reddish brown Fe ₂ 0 ₃ during calcination. It can be seen that the quality of the water wash has a great impact on the quality of the product.
The water washing operation is to lift the blade after one water washing (1-2h) and put it into the secondary washing tank for water washing. Estimate the approximate time of washing according to different varieties. The pressurized hydrolyzed pigment titanium dioxide is washed for about 8 hours, and the atmospheric pressure hydrolyzed pigment titanium dioxide is washed for about 4 hours. That is, using a partial vacuum removal method from the pumping main pipe, take a small amount of water washing liquid, and use 0.5% red blood salt (potassium ferricyanide) test solution for qualitative test. If it is blue, it means that there are still many ferrous ions. Continue to wash; if it is green, it means it will be washed; if it is yellow, it means it has been washed. Can also be tested with ammonium thiocyanate test solution, first wash the water with hydrogen peroxide to oxidize the ferrous ions into high-iron ions, and then add the ammonium thiocyanate test solution, such as red or brown, indicating that it needs to continue washing; It is colorless, indicating that it has been washed. After washing the metatitanic acid, the whole set of blades can be lifted and placed in the upper part of the beating tank to prepare the scraping material. At this time, the filter cake contains a lot of water, and the vacuum can be maintained until a large number of cracks appear on the surface, so that the water content is further reduced. Then, the vacuum is cut off, and the metatitanic acid block material falls into the beating tank. Place some water in the pulping tank in advance and start the horizontal mixer to stir. The metatitanic acid remaining on the filter cloth can be manually scraped off with a plastic shovel or washed with a high pressure water gun. After the material is evenly stirred, it is pumped to the bleaching process to further remove iron. Some manufacturers do not have a bleaching process and send them directly for salt treatment.
Third, the conditions and precautions for the separation of metatitanic acid and water washing control
The separation and washing of metatitanic acid requires control of the following eight conditions:
1 slurry temperature is maintained at 25-40 ° C;
2 The degree of vacuum is maintained at ≥5.3*10 ⁴ Pa;
3 filter cake thickness is maintained at 25-35mm;
4 filming time is about 1.5h;
5 filter cake water content ≤ 65%;
6 metatitanic acid iron content ≤ 0.01%;
7 washing time, pressurized hydrolysis slurry about 8 ~ 12h, atmospheric pressure hydrolysis slurry about 3 ~ 6h;
8 water washing process recovery rate ≥ 94%;
Need to pay attention to the following items:
1 When adjusting the top plate, pay attention to adjust the slurry feeding amount. Always keep the liquid level just above the upper end of the filter. It is necessary to prevent the slurry from being too small to make the suction sheet thin and thick, and to prevent overfilling and overflowing the outside of the tank;
2 The washing tank should regularly clean the sediment deposited on the bottom of the tank;
3 frequently clean and regularly change the filter cloth;
4 to prevent water and water shortage, keep the water clean and not turbid. [next]
Fourth, method for separating and washing water of titanic acid and common equipment thereof
There are two methods for separating and washing water with metatitanic acid.
1. Straight-air filtration washing method This method uses the pressure difference caused by vacuuming to suck the filtrate through the filter medium and adsorb the solid on the surface of the filter medium. The water is continuously removed by passing the dissolved impurity ions through the filter layer while washing.
Commonly used equipment is a leaf filter, a vacuum rotary filter, and the like. It is widely used as a leaf filter. The leaf filter has a small volume, a large filtration area, and the washing time can be arbitrarily controlled. It can be applied to sediments of various particle sizes.
2. Centrifugal Filtration Washing Method This method uses the centrifugal force generated by high-speed rotation to cause a pressure difference on both sides of the filter medium, and the filtrate is taken out of the filter layer to achieve the purpose of solid-liquid separation and solid water washing. This method can often be used to separate and wash relatively coarse titanic acid obtained by hydrolysis of non-pigment-grade atmospheric pressure.
Commonly used equipment is a centrifuge, which is fast and easy to filter and wash with a centrifuge, but is labor intensive.
Fifth, the structure of the leaf filter and its working principle
The leaf filter consists of a blade and a suction tank.
Each set of blades of the leaf filter generally consists of 10-30 blades, each of which has a filtration area of ​​2-5 m ² on both sides, arranged in parallel, and connected to the vacuum line. The blade is made of a PVC or polypropylene hard plastic plate with a thickness of about 5 mm and a certain thick plastic tube. The plastic plate is covered with a small hole with a diameter of about 5 mm, and the outer bread is filtered with a polyester cloth bag. The working principle is that a material having a plurality of capillary pores is used as a medium, and under the action of a vacuum, the solution is passed through the small hole, and the solid is intercepted, thereby achieving the purpose of solid-liquid separation and solid water washing. The introduction molar filter (also known as leaf filter) monolithic filtration area of 6m ^2, the area of each filter to reach 180m ^2.
Sixth, the structure, working principle and advantages and disadvantages of the vacuum aspirating tank
The vacuum aspirating tank is matched with the leaf filter. It is made of polyvinyl chloride hard plastic or steel plate lining rubber. It is divided into upper and lower tanks. It consists of a connecting pipe, an upper floating valve and a lower floating valve.
The working principle of the vacuum aspirating tank is as follows: When vacuuming, the inlet port and the discharge port are blocked by the rubber sheet. The filtrate drawn from the blade continuously flows into the upper tank, and flows through the lower liquid port to the lower tank. As the water level in the lower tank continuously rises, the lower floating valve gradually floats, and the lower end of the connecting pipe is blocked, at this time, The next two tanks are completely isolated, the upper tank continues to maintain a vacuum, the drawn filtrate can only stay in the upper tank, and the lower tank has been separated from the vacuum system, and a small amount of air is constantly flowing from the inlet edge to make the vacuum in the lower tank. Gradually, after a certain degree, the rubber plate of the air inlet and the discharge port will automatically open due to the pressure of the filtrate, so that the pressure of the lower tank rapidly rises to normal pressure, and the filtrate in the tank is discharged by gravity. When the liquid level drops below the lower float valve, the lower float valve falls because the suction force is less than its self-gravity, and the lower tank is reconnected to the vacuum system by the communication tube. When the lower tank has a certain vacuum, the floating valve is also due to self-gravity. And automatically fall, the next cycle operation (see Figure 1). [next]

The internal operation and drainage of the vacuum aspirator are intermittent, but the water filtration and the collection and filtration of the filtrate are continuous, and there is no need to manually switch the valve. The disadvantage is that each vacuum leaf filter must be equipped with a vacuum automatic liquid pouring tank, and several moving parts such as the floating valve, the lower floating valve and the exhaust port often fail to be repaired. Every time the draining process, the vacuum is destroyed once in a short time.
Seven, horizontal automatic continuous drainage system operation and its advantages
The operation of the horizontal automatic continuous drainage system is as follows: the filtrate drawn from the vacuum leaf filter enters the separation tank, a main vacuum tube at the upper part of the separation tank is connected with a vacuum system (vacuum pump), and the secondary vacuum tube is connected as a balance tube to the flow divider, and the filtrate The outlet of the bottom of the tank is sent to the splitter above lom by the centrifugal pump, and then discharged from the overflow tank, so that the inlet and outlet of the centrifugal pump are under vacuum, the absolute pressure on both sides is equal, and the centrifugal pump is solved in vacuum. Operational problems in the state. The separation tank is provided with a liquid level controller and a centrifugal pump interlocking to prevent the liquid level from being too high to pump the filtrate into the vacuum system, or the liquid level is too low to cause the centrifugal pump to run empty. The horizontal washing automatic continuous drainage system is shown in Figure 2. [next]

This horizontal automatic continuous liquid discharge system has the following advantages as compared with the above vacuum liquid suction tank.
1. Multiple vacuum leaf filters can share a horizontal automatic continuous drainer. If the classification of washed waste acid is not considered, in principle, a horizontal automatic continuous drainer can meet the production needs.
2. The centrifugal pump is used for continuous external discharge, which does not destroy the vacuum when draining, which can save vacuum energy consumption. The key technology is to solve the normal operation of the centrifugal pump under vacuum.
3. Horizontal arrangement, the inlet of the drain can be lower than or equal to the outlet height of the filtrate of the leaf filter, reduce the reactive power loss, and save the vacuum energy consumption, because the vacuum loss increases by about 10% for each lm increase.
4. It can be operated automatically and continuously, and the amount of maintenance is very small. Since it is not necessary to configure an automatic liquid pouring tank for each vacuum filter. Therefore, the number of equipment can be greatly reduced, the floor space is small, and the economic benefits are obvious. [next]
8. Construction and operation procedures of SS type three-legged upper discharge centrifuge
This centrifuge is still used in small non-pigment titanium dioxide plants. The structure is mainly composed of a drum, a main shaft, a chassis, a casing, a cover, a brake, a base, a centrifugal clutch and an electric motor. The drum is supported on the chassis by a spindle, and the spindle is vertically mounted in the bearing housing. The chassis is suspended by three booms on the column foot, and the column foot is connected to the frame by bolts. The centrifuge drives the power through a centrifugal clutch and a V-belt. The metatitanic acid slurry is added to the drum at full speed to ensure uniform fabric. The feed is stopped as soon as the feed rate reaches the loading limit. Under the action of centrifugal force, the liquid phase waste sulfuric acid in the suspended ferric acid slurry is discharged through the filter cloth to form a filtrate, and the solid phase metatitanic acid is collected in the drum to form a filter cake, and then added to the upper mouth. The tap water is washed to the qualified one, that is, the machine is manually discharged from the upper part of the drum. Since the entire process is in an acidic environment, all parts in contact with the material are made of stainless steel.
The use of such a centrifuge for the filtration and washing of the metatitanic acid slurry, although having a certain labor intensity, but the filtration and washing speed is fast, the metatitanic acid particles obtained by atmospheric pressure hydrolysis of the enamel and the electrode non-pigment titanium dioxide In larger cases, it is still applicable. The vacuum equipment that uses the centrifuge without negative pressure has less investment, easy to use, quick effect, convenient use, small footprint, less water content of the filter cake, and faster forging of the metatitanic acid. One such centrifuge can produce more than 100 tons of titanium dioxide per year. The operating procedure is as follows.
1 According to the size of the drum, make the filter bag with acid-resistant polyester cloth.
2 Start the stirrer that stores the high-position tank of the metatitanic acid slurry, and stir the slurry evenly.
3 Start the centrifuge, extend the rubber tube at the outlet of the high level tank into the centrifuge, and open the discharge valve to allow the slurry to flow into the centrifuge.
4 The slurry is centrifuged to remove the mother liquor (waste acid solution) through the filter cloth, and the solid metatitanic acid is blocked by the filter cloth in the filter bag on the inner wall of the drum to form a filter cake. The waste acid extracted from the waste is connected to the waste acid pool. After clarification, the upper waste acid is sent to the acid hydrolysis section for use, and the metatitanic acid filtered at the bottom of the waste acid tank is sent back to the titanic acid slurry tank.
5 When the filter cake reaches a certain thickness, stop adding the titanic acid slurry and change the tap water for washing. The tap water is subjected to centrifugal force, and the soluble impurities are taken away through the filter cake. When the water is washed to the washing water, the red blood salt test liquid is yellow.
6 Stop adding water and continue to dry. When the water outlet is seen to flow out, stop the filter cake from the upper mouth for use in the next process.

After use,stainless steel mesh element can be cleaned, please use a professional cleaning device. And note that the cleaning effect which is the left service life can not be calculated accurately