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The hydrated titanium dioxide produced by the hydrolysis of titanium liquid is a relatively viscous suspension. It precipitates slowly precipitates white hydrated titanium dioxide. It is composed of 1000 hydrated titanium dioxide with a particle size of 0.6~1.2μm. The gel formed has about 20 crystallites (crystal seeds) of 2 nm in size on each colloidal structure. The chemical composition is generally: TiO 2 170~200g/L, H 2 SO 4 300~400g. /L, FeSO 4 ·7H 2 O200~250g/L, the above composition varies slightly with the process. Although the suspension can be separated by conventional solid-liquid separation, the surface of the hydrated titanium dioxide adsorbs a large amount of free acid (mother liquor), and the adsorbed free acid cannot be removed by a general solid-liquid separation operation. separating them clean and free acid which contains a large amount of ferrous sulfate and chromium sulfate, vanadium, manganese, copper, lead and other metals, the presence of these impurities will seriously affect the optical properties and pigment properties of the product. The purpose of water washing is to use hydrated titanium dioxide particles insoluble in water, and some low-valent metal impurities are soluble in water. The free acid, unhydrolyzed titanium liquid and iron salt are adsorbed on the surface of hydrated titanium dioxide by water washing. The main soluble impurity ions are washed away with water to achieve the purpose of purifying hydrated titanium dioxide.
Industrial water washing operations usually use centrifuges, vacuum leaf filters and vacuum drum filters. Among them, the filtration and washing operations for hydrated titanium dioxide are mainly vacuum filter machines at home and abroad, because in this production process, washing is more important than filtration. Therefore, it is necessary to select a liquid-solid separation filtration and washing device that has a sufficient washing function.
Vacuum leaf filter, also known as Moor filter, can be divided into two types: pressure and vacuum. The titanium dioxide factory mainly uses vacuum leaf filter. The blade of the machine is rectangular, each piece has a filtering area of ​​2~5m 2 (two sides), and has a wall groove and a hole on the top, the groove ratio is generally not less than 50%, and 2~4 vacuum tubes are connected inside each blade, 10 ~30 leaf filters are grouped together, and the vacuum tube in each blade is connected with the vacuum manifold of the upper part of the group. Each blade surface is covered (or pasted, embedded) with acid-resistant industrial filter cloth, and the material of the blade is mostly hard polymer. Made of vinyl chloride (PVC), polypropylene, ABS, hard rubber, wood and other materials with good acid resistance.
When performing the water washing operation of metatitanic acid (hydrated titanium dioxide), the blade is first hoisted into the titanic acid slurry tank (sucking tank or upper tank), and the metatitanic acid slurry should be cooled to below 60 ° C in advance. In order to avoid excessive temperature damage to the blade and the corrosion-resistant material of the lining in the slurry tank. After the vacuum is turned on, under the action of the negative pressure, the filtrate in the slurry passes through the filter cloth through the collecting tube (vacuum tube) in the blade, and is pumped to the automatic pouring tank, while the metatitanic acid particles are trapped on the filter cloth to form a filter. cake. When adding the slurry, keep adding the slurry continuously, keep the blade always immersed in the slurry. When the filter cake reaches a certain thickness, while holding the vacuum, lift the whole set of blades, and then drain it into a clear water pool. Continuous washing operation is carried out while continuously adding fresh water and keeping the blades completely immersed in water. After washing for a period of time, collect a small amount of filter droplets and add 1% potassium ferricyanide solution. If it is blue, it needs to continue to be washed. If it is blue, it will be washed. If it is yellow, it will reach the standard of washing before bleaching (Fe 2 O 3 0.01%) ).
Although the vacuum leaf filter is an old-fashioned gap filtration operation device, it has a simple structure, low cost, large production capacity, no transmission parts, convenient maintenance, and has the functions of filtration and washing, especially The washing of the filter cake is sufficient, and the whole blade is immersed in water during the washing operation, which prevents the filter cake (meta-titanate) from contacting the air and avoids the low-cost iron (ferrous sulfate) plasma in the filter cake in the air. It is oxidized to a high price and cannot be washed off. Therefore, it is widely used in the titanium dioxide industry at home and abroad. The disadvantage is that it consumes more water than a vacuum drum filter.
When selecting and manufacturing the blades of the vacuum leaf filter, the material of the filter should be considered to be corrosion-resistant and temperature-resistant, so as not to cause deformation under long-term operation under vacuum, and secondly, increase the opening ratio of each leaf filter. In the past, the outer sheet punched on a hard PVC board was not used because most of the factory was low in opening rate and easily deformed. The vacuum collecting tube (vacuum tube) should be placed at the bottom of the blade to ensure that the filtrate and water inside the blade are cleaned after the filtration and washing operations are completed.
Although the structure of the vacuum leaf filter is simple, its liquid discharge system is relatively complicated, and the performance of the liquid discharge system directly affects the smooth operation of the water washing process. There are two kinds of drainage equipment used in China's current titanium dioxide factory. [next]
(1) Automatic liquid pouring tank
The automatic liquid pouring tank in Fig. 1 is also called vacuum automatic liquid pouring tank, automatic pouring water tank or vacuum liquid suction tank, etc. In the past, it was a liquid discharging equipment commonly used in various titanium dioxide factories in China.
The tank is generally vertical, and the inside is divided into upper and lower tanks. The intermediate partition plate has a lower material port and an upper floating valve, and the lower floating valve communicates with the upper tank body through the communication pipe. In the vacuum state, the bottom liquid discharge and the gas port are sucked by the rubber plate. Under the action of the vacuum force, the filtrate from the leaf filter machine continuously enters the upper tank and flows through the lower feed port on the intermediate partition plate to the lower tank. As the liquid level of the lower tank rises continuously, the lower float valve floats to seal the bottom of the connecting pipe. When the liquid level of the lower tank continues to rise to the position of the floating valve, the floating valve also blocks the cutting port, and the upper and lower tanks are completely isolated. The upper tank continues to maintain a negative pressure state, the filtrate drawn in can only remain in the upper tank, and the lower tank is sealed off from the vacuum due to the sealing of the connecting tube, and the rubber sheet on the exhaust port automatically falls off under atmospheric pressure, and the lower tank is often In the state of pressure, the filtrate is pushed by gravity to push the rubber plate on the bottom drain port, and the filtrate is automatically discharged (some factories install a ejector on the floating valve. When the float valve rises to a certain position, the ejector pin puts the vent on the vent The rubber sheet is topped to make the lower tank form a normal pressure drain). When the filtrate of the lower tank is discharged to the vicinity of the lower float valve, the lower float valve is dropped by the gravity, and the connecting tube introduces the upper vacuum into the lower tank, and the lower tank re-absorbs the rubber plate on the liquid discharge port and the air outlet under the action of the vacuum force. Sustained, the upper floating valve is dropped by the gravity of the upper tank level and the negative pressure of the lower tank, and the liquid of the upper tank re-flows into the lower tank to start the liquid discharging operation of the next cycle.
The material of the automatic pouring tank is generally selected from hard PVC board or steel lined rubber, steel lined glass steel and the like. The internal operation and drainage of the equipment are gaps, but the process of water filtration and filtrate collection is continuous. There is no need to manually switch valves, but each vacuum filter must be equipped with an automatic liquid pouring tank.
Because the old automatic liquid draining tank needs to be equipped with an automatic liquid pouring tank, the washing section of a medium-sized titanium dioxide factory is sometimes equipped with more than 10 sets, and the floating valve, the lower floating valve, the exhaust port, etc. The moving parts often fail to repair, and the vacuum is usually destroyed once in a short time during each draining process. In 1985, Nanjing Grease Chemical Plant developed an automatic horizontal continuous drainage system suitable for the process layout of most titanium dioxide factories in China, which has been applied in many titanium dioxide factories in China. [next]
(2) Horizontal automatic continuous drainage system
The horizontal automatic continuous drainage system has the following advantages compared with the automatic liquid pouring tank.
a. 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;
b. Continuously draining the liquid with a centrifugal pump, without damaging the vacuum when draining, can save vacuum energy consumption, and the technical key is to solve the normal operation of the centrifugal pump under vacuum;
c. Horizontal arrangement, the inlet of the drainer can be lower than or equal to the height of the filtrate of the leaf filter, reducing the reactive power loss and saving the vacuum energy consumption, because the vacuum loss increases by about 10% for each lm increase;
d. It can be operated automatically and continuously. The amount of maintenance is very small. Since there is no need to configure one automatic liquid pouring tank for each vacuum filter, the number of equipment can be greatly reduced, the floor space is small, and the economic benefits are obvious. It is suitable for the process arrangement of low-level washing of domestic titanium dioxide factory.
See Figure 2 for the operation of the system.
The filtrate drawn from the vacuum leaf filter enters the separation tank, and a main vacuum tube at the upper part of the separation tank is connected with a vacuum system (vacuum pump), the auxiliary vacuum tube is connected as a balance tube and a splitter, and the filtrate is sent from the outlet of the bottom of the tank to the centrifugal pump first. The shunt above 10m is then discharged from the overflow trough below the atmospheric leg, so that the inlet and outlet of the centrifugal pump are under vacuum, and the absolute pressure on both sides is equal, which solves the problem of the centrifugal pump operating under vacuum. 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.