Production of Titanium Dioxide by Sulfuric Acid Method--Purification and Concentration of Titanium Solution

    First, the purpose of titanium liquid purification
After settling to remove most of the residue was freeze-crystalline ferrous sulfate crystals are removed, but still contains some liquid titanium incomplete settling over trace with the solid phase, which solid fine impurity particles, the ferrous sulfate When the crystal is coarsely filtered, the filter cloth can be left in the titanium liquid, and the surface may have a certain electric charge, so that it remains in the titanium liquid, which may cause the following two aspects of damage.
1. Affect the particle structure of the hydrolyzate, causing difficulty in washing the metatitanic acid and poor pigment properties of the product.
The process of hydrolysis of titanium liquid is to first form a batch of crystal centers, which gradually grow and eventually become solid and precipitate. The crystallization center is a colloidal substance that has a certain influence on the particle structure of the hydrolyzate. In order to obtain a desired hydrolyzate, the crystallization center must be prepared as required. The colloidal solid phase impurities in the titanium solution act as a crystallization center when the titanium solution is hydrolyzed, but this crystallization center is not satisfactory and is a poor or irregular crystallization center, which affects hydrolysis. The granular structure of the product makes the shape irregular, the surface of the particles is not smooth, and the edges are angular, which causes poor washing effect of the metatitanic acid and also affects the pigment properties of the product.
2. Adsorption of metal ions, the finished product contamination, reduce the purity, the influence whiteness.
Since these solid phase impurities are charged colloidal particles, they adsorb certain metal ions. When they become the center of crystallization during the hydrolysis process, these adsorbed metal ions enter the finished product of the titanic acid in the hydrolyzed product, and finally are mixed into the finished product, thereby reducing the purity of the finished product. The incorporation of certain colored harmful metal ions such as vanadium , chromium , manganese , lead , diamonds, etc., also affects the whiteness of the product.
For this reason, in the production of pigment titanium dioxide, it is necessary to perform a plate and frame press filter to remove these solid phase impurities.
Second, the filter aid and its role in purification filtration
An auxiliary powdery substance capable of changing the particle size distribution of the filter material and its filtration characteristics to improve the filtration effect is called a filter aid. The filter aid is a kind of loose, porous and adsorbable. It has small particles, narrow particle size distribution, harder particles, dispersibility, suspension, chemical stability and porosity of the filter cake. The price is low. A source of abundant material. It has the function of filtering out fine particles, preventing the filter medium from clogging, prolonging the operation time of the filtration equipment , and obtaining a completely clarified filtrate.
After the sedimentation and slag removal and the titanium liquid filtered by ferrous sulfate, there is still a very fine colloidal solid phase impurity. If the plate frame pressure filtration is carried out by the general method, the filtration is easy to pass through during the initial filtration, and the obtained filtrate is still not clear, and as the filtration proceeds, these colloidal solid phase impurities will gradually accumulate in the pores of the filter layer, resulting in filtration. The layer is clogged, making it difficult for the titanium liquid to continue to pass, and the filtration speed is greatly reduced. In order to overcome the above difficulties, the filter aid can be formulated into a suspension and a plate frame press before the plate frame is filtered, so that the filter cloth is first adhered with a filter aid, and then the titanium solution is added to the plate frame. Pressure filtration, so that the solid phase impurities of the colloid can be prevented from clogging the filter layer, so that the pressure filtration process can proceed smoothly.
Third, the characteristics, quality requirements and dosage of various common filter aids
Commonly used filter aids are charcoal powder, diatomaceous earth , chaff ash and pulp.
Charcoal powder is a black powder obtained by grinding wood charcoal and then pulverizing it. The charcoal powder is loose and porous, and the particles are approximately spherical, which is favorable for filtration and has a certain specific surface area and can adsorb colloidal impurities. The quality requirement of charcoal is that the ignition loss should be >90%, without Fe 2 O 3 , MnO 2 , etc., so as not to reduce the trivalent titanium content of the titanium liquid, and the fineness is required to be 150 mesh sieve residue of 2% per square The metering area of ​​the filter is about 0. 5kg, and the filter filter layer is 1-2mm thick.
Diatomaceous earth is a residue deposit of ancient single-celled diatom microbes. It is used as a filter aid for natural or processed products. Its main components are Si0 2 , A1 2 O 3 , Fe 2 O 3 , CaO, MgO. Etc., wherein the content of Si0 2 is about 80%-85%, the acid resistance is good, the particle size is 20-150μm, the relative density is 2.1-2.5, the color is various, the white is the purest, and the dark earth-like opaque luster is required. The fineness is 150 mesh, 0.5 kg per square meter of filter area, and the thickness of the filter layer is 1 mm.
The rice hull ash contains more than 90% of Si0 2 and has active adsorption, and is composed of porous silicic acid. The usage amount is 1-2kg per square meter of filter area. [next]
Fourth, the structure and working principle of the plate and frame filter press The plate and frame filter press is composed of 10-30 pairs of filter plates and filter frames alternately arranged. A filter cloth is placed between each pair of filter plates and the filter frame, and is placed on a support frame, and is provided with a pressing device and a collecting tray. The pressing device can be operated manually or electrically, so that the filter plate and the filter frame are in close contact without leaking. The collecting tray is used to collect the titanium liquid dripping between the plates.
The filter plate and the filter frame have a hole at each of the upper corner and the lower corner of the one side. After the plate frame is arranged, the holes are connected to each other to form two holes. The upper hole is a flow passage for the titanium liquid to be filtered, and communicates with the inside of the filter frame. The middle of the box is empty. The lower channel is the channel through which the filtered titanium liquid flows out, communicates with the liquid collecting port of the filter plate, the filter plate is smooth on four sides, the middle portion has a straight groove for the filtrate to flow, the groove and the lower channel communicate, and the filtrate is collected. Discharged in the drain channel.
When the plate and frame filter press is working, the filter frame is located between the two filter plates to form a filter chamber. After the slurry titanium solution flows into the filter chamber, the solid particles are trapped on the filter cloth, and the clear liquid passes through the filter cloth. The groove on the filter plate flows down, flows to the passage port, and finally collects in the drain pipe to be discharged.
Fifth, the process operation process of plate and frame pressure filtration
The operation of the plate and frame filter press is to first adjust the filter aid to a suspension with light waste acid or water, stir it evenly through compressed air, and then use a pump to hit the plate filter press of the filter cloth for circulation filtration. Under the action of pressure, the filter aid forms a uniform filter layer on the filter cloth until the circulating liquid is clarified. Then, the titanium liquid to be filtered is pumped and filtered, until the filtrate reaches the requirement, the circulation is stopped immediately, and the normal continuous filtration operation is performed. Obtain a qualified liquid titanium liquid, and send it to a qualified titanium liquid storage tank for concentration. The solid content of the titanium liquid should be <0.06g/L.
For the continuous filtration and the filtration pressure is getting larger and larger, the flow rate of the filtrate is getting less and less, indicating that the pores of the filter cloth have been blocked. At this time, the filtration should be stopped. After disassembling, wash the filter cloth with a small amount of washing powder, and then reinstall it. Re-filter according to the above steps.
Cloth filter media used in the acid-long fiber polyester cloth # 240 as well. After the polyester filter cloth is used, it can be regenerated by soaking in a dilute alkali solution.
6. Removal of dissolved impurities
According to the behavior of dissolved impurities in the hydrolysis of titanium liquid, it can be divided into two categories: one type of impurities is in the hydrolysis process of titanium liquid, because the acidity of titanium liquid is not hydrolyzed, no precipitation occurs, for example, ferrous sulfate is still It exists in the hydrolyzed mother liquor to remain in a dissolved state. Such impurities do not need to be purified and removed in the titanium solution in advance, and can be removed by washing with metatitanic acid; another type of impurities may be hydrolyzed together with titanium to form a precipitate during the hydrolysis reaction of the titanium liquid, or they may be precipitated, or they may be hydrolyzed with titanium. It will be co-precipitated by adsorption of metatitanic acid. If such impurities are harmless to the quality of the final titanium dioxide, then it is not necessary to purify and remove. For example, ruthenium is harmless to pigmented titanium dioxide because barium sulphate is hydrolyzed to hydrolyze to form metasilicate and precipitate with metatitanic acid. When satin is burned, the bismuth acid is also decomposed to form a white antimony trioxide (also known as ruthenium white) mixed in the titanium dioxide, which has no effect on the properties of the finished product, and thus may not be removed. If it can affect the quality of titanium dioxide, it must be removed beforehand. Metal ions such as lead, diamond, vanadium, chromium, and manganese must be removed beforehand. The removal method can be subjected to targeted purification treatment according to its reaction characteristics in a sulfuric acid solution, for example, some heavy metals such as bismuth, lead, diamond, etc., and their sulfides are generally insoluble in an acidic medium. In the method of purifying and removing impurities, a small amount of sodium sulfide may be added before the titanium solution obtained by acid desorption to discharge, and the heavy metal ions are converted into insoluble sulfides. Its reaction formula is as follows:

The resulting heavy metal sulfide precipitate is removed together with the residue upon settling. Part of sodium sulfide case of acid generated unreacted hydrogen sulfide gas from the chimney vent drain pan acid hydrolysis. [next]
7. Concentration of titanium liquid and reasons for concentration of titanium coating
The water in the titanium solution is a solvent and is volatile. Titanium oxysulfate, titanium sulphate and ferrous sulphate in the titanium solution are solute and are non-volatile. By means of the heating action, the solvent (water) in the titanium liquid is gradually vaporized and volatilized to be removed, and the solute concentration is gradually increased. This process is called concentration.
Based on the titanium liquid having a low concentration, the particles of the metatitanic acid are coarser, and the pigment of the obtained titanium white powder is inferior. In order to make the titanic acid particles obtained by hydrolysis of the titanium liquid fine and uniform, and to obtain titanium white powder with superior pigment properties, it is necessary to concentrate the titanium liquid having a lower concentration by crystal filtration and plate and frame filtration until concentration. It is until Ti0 2 (200±5) g/L (for pressurized hydrolysis) or 215-230 g/L (for normal pressure hydrolysis). When a non-pigment titanium white powder is produced, the titanium liquid can be directly subjected to atmospheric pressure hydrolysis after removing the ferrous sulfate crystal without concentration.
8. Reasons for the vacuum concentration of titanium liquid and its requirements
Since the solution is at the temperature above the boiling point, its vaporization volatilization rate is the fastest. The boiling point of the titanium liquid is between 104 and 114 °C. If heated at a high temperature for a long time, the stability of the titanium liquid will drop rapidly. Since the critical temperature for hydrolysis of titanium liquid is only 80 ° C, hydrolysis will occur above 80 ° C, which will lead to early hydrolysis of titanium liquid, which is unsuitable for the manufacture of high quality pigment titanium dioxide. Therefore, the boiling point of the titanium liquid must be lowered to below 80 °C. To achieve this, vacuum concentration must be used because the boiling point of the liquid is proportional to the pressure, and the lower the pressure, the lower the boiling point. In order to make the pressure low, it is necessary to carry out concentration under a reduced pressure of vacuuming, and the higher the degree of vacuum, the lower the boiling point. To control the concentration temperature not to exceed 75 ° C, the vacuum must be above 8 * 10 4 Pa.
9. Construction and concentration process of single-effect vacuum concentration tank
Concentration in vacuo is mono-concentrated and continuously concentrated. The structure of the single-effect concentrating tank is generally a steel enamel reaction pot or a steel tank lined with an acid-resistant ceramic plate, lead bismuth, etc., which is heated by a serpentine tube or a jacket, and has a vapor-liquid separation device and a feed valve at the top of the tank. There is a discharge valve at the bottom, and the tank is also equipped with a level gauge, a thermometer and a vacuum gauge. The process of concentration is as follows: the titanium liquid is heated in the infusion tank by the indirect steam of the serpentine tube, reaches the boiling point under vacuum, the water in the titanium liquid begins to vaporize, is discharged from the top of the concentration tank, and enters the vapor-liquid separator. The steam that has been initially separated enters the condenser from the bottom and flows upwards. The cooling water is sprayed from the top and is in full contact with the steam. After condensation, it is discharged from the pneumatic pipe to the outside of the water-sealing system. The uncondensed gas , discharged from the top of the condenser, through the vapor-liquid separation device to the vacuum buffer tank, and then exhausted to the atmosphere through the vacuum pump.
X. Construction of continuous vacuum concentrator and its concentration process
A continuous film vacuum concentration is used for the film concentrator. The structure is a one-way rise-up type consisting of a feed, an evaporation chamber and a separator. The vertical steel pipe of the outer casing is an evaporation chamber, and the pipe has a row of thin titanium tubes, the lower part of which is a feeding chamber for the titanium liquid, and the feeding chamber has a device for distributing the liquid, so that the titanium liquid is uniformly introduced into each other. Inside the tube, there is steam on the outside, heat exchange occurs in the walls of each evaporation column, and the top is a separator. The separator has a liquid trapping device to separate the vapor and liquid. In order to prevent temperature stress between the tube and the tube, there is a thermal compensation device on the housing. There are also a sight glass, a vacuum gauge, a thermometer, and a flow meter on the evaporator (see figure). [next]

The process of concentrating the titanium liquid in the thin film evaporator is as follows: the titanium liquid is fed into the titanium tube evaporator from the feeding chamber at the bottom of the film concentrator via a flow meter, where the steam outside the tube is heated, and the evaporated water vapor is at a high speed. Ascending in the tube, the titanium liquid is pulled into a film by the high-speed gas stream, rises to the top of the evaporator in a very short time, and is vapor-liquid separated in the separator, and the concentrated titanium liquid enters the titanium liquid water-sealing tank through the overflow pipe. The steam enters the mixing condenser, mixes and condenses with the cooling water, and is discharged to the outside of the system through the air pressure pipe. The non-condensable gas is discharged from the top of the condenser and enters the vacuum through the vapor-liquid separation device. The buffer tank is finally discharged to the atmosphere by a vacuum pump. [next]
XI, the advantages of continuous concentrator
Compared to single-effect vacuum concentrating tanks, continuous vacuum concentrating membrane concentrators have the following six advantages.
1 operation is continuous, simpler than intermittent operation, and easy to automate.
2 High concentration efficiency, a Φ360mm film concentrator, its production capacity is equivalent to 4 single-effect concentrating tanks with a volume of 3000L.
3 Because the titanium liquid has a large heating area in the column tube, the heat is evenly distributed, so that the energy consumption is small.
4 equipment footprint is small.
5 Titanium liquid has a short heating time. The rate at which the titanium liquid rises in the tube can generally reach 20-25 m/s, and the actual heating time is only 0.08 to 0.20 s, which is very beneficial to maintain the stability of the titanium liquid. When the single-effect concentrating tank is concentrated, the heating time of the titanium liquid is often as long as several hours, which causes the stability of the titanium liquid to decrease.
6 The concentrated titanium liquid obtained has a high concentration. When concentrating with a single-effect concentrating tank, as the water evaporates, the liquid level in the tank decreases continuously, the heat exchange area becomes smaller and smaller, and the evaporation becomes slower and slower. After a certain liquid level, the heating is not worth the loss. The concentration of titanium solution is low, and the concentration of concentrated titanium solution is also low, which may not meet the requirements of the index. The membrane concentrator is concentrated, and only the feed amount and the heating vapor pressure are adjusted, and even if the concentration of the dilute titanium liquid is low, it can be concentrated into a concentrated titanium liquid having a higher concentration.
12. Operation of vacuum concentration of membrane concentrator
1 Inject the qualified titanium liquid to be concentrated into the titanium liquid high-position tank, and inject a certain amount of titanium liquid into the concentrated titanium liquid water-sealing tank to keep the liquid seal.
2 Turn on the vacuum pump, turn on the cooling water, and adjust the flow rate of the cooling water and adjust the vacuum to 8*10 4 Pa or more.
③ meter below the valve open, so the liquid titanium into titanium tube evaporator, 6m 2 concentrator feed quantity control 1-1. 5m 3 / h. 5-3米3 /h。 The 14m 2 concentrator is controlled at 1. 5-3m 3 /h. The imported titanium liquid evaporator has a heat exchange area of ​​260 m 2 , which can meet the production requirements of 15,000 tons/year titanium dioxide.
4 Turn on the steam valve and keep it under vacuum at a vapor pressure of 4.9*10 4 -7.8*10 4 Pa to control the temperature of the titanium solution between 70-75 °C.
5 Determine the concentration of titanium solution on time so that the total titanium content meets the index requirements.
6 When the concentration stops, first close the steam valve, let the titanium liquid continue to feed for 3-5min, close the feed valve immediately after the temperature drops slightly, and then turn off the cooling water and vacuum pump.
7 Turn on the discharge valve under the concentrator, put the titanium liquid in the concentrator, and then add the unconcentrated light titanium liquid to the concentrated concentrated titanium solution to make the titanium liquid index hydrolyzed. Process requirements.
Thirteen, vacuum concentration should pay attention to matters
1 Operate in strict accordance with the operating procedures.
2 Strictly control the steam pressure, vacuum, feed amount, feed concentration and concentration temperature to ensure that the concentrated titanium liquid meets the index requirements.
3 If the amount of cooling water in the concentration process is too large, the cooling water is not completely discharged from the air pressure pipe, so that the cooling water level in the condenser is higher than the inlet pipe of the condensed steam, and enters the top of the concentrator from the overflow pipe. The flow of people into the concentrated titanium liquid water-sealed pool causes the concentration of the titanium liquid to drop sharply, failing to achieve the purpose of concentration. To this end, it is necessary to prevent the cooling water from flowing into the titanium liquid water sealing tank.
4 If the system leaks more seriously or the pumping capacity is insufficient or the steam is too large when heated or the cooling water is not enough, the vacuum will be reduced. The vacuum should be kept above 8*10 4 Pa to prevent the above phenomenon.
5 If the flow rate of the titanium liquid is too large, the concentrator overflow pipe is too late to be discharged or the concentrator overflow pipe is blocked or the overflow pipe is leaking, etc., the titanium liquid is introduced into the condensation system, and the above situation must be prevented.
6 If the vacuum is reduced or the temperature is too high or the cooling water is reversed during concentration, the stability of the titanium liquid will decrease and the early hydrolysis will occur, causing irreparable damage. It is necessary to prevent such accidents.

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