Washing and drying of titanium dioxide pigment

(1) water wash
After the titanium dioxide surface-treated once again become flocculated state, and which contains many water-soluble salt, are primarily generated by the surface treatment out salts (e.g. Na 2 SO 4), was added before the calcination treatment agent and a salt The dispersing agent or the like added during the grinding and dispersing, if not removed, will affect the coating application properties of the product such as storage stability, grinding dispersibility, weather resistance and the like. Since these salts are soluble in water, they can be removed by washing. Although it is ideal to wash all the ions of the salt, it is actually difficult to do.
The surface treatment of titanium dioxide washing equipment can use vacuum leaf filter and drum filter, but the large factory one film uses vacuum drum filter, this filter can be operated in a single operation, or several units can be operated in series The machine integrates filtration, washing and unloading (or even squeezing) into one, with high automation, low water consumption and good washing effect. The filtering area of ​​single machine is 5~40m 2 .
The water quality during washing is very high. The deionized water is used at home and abroad. The resistivity of the water should not be lower than 50,000 Ω·cm. The special varieties such as titanium dioxide for chemical fiber should be no less than 20×10 4 Ω·cm. The filtrate was controlled to have no sulfate (measured by a 10% BaCl 2 solution), or the resistivity of the filter cake was not less than 8000 Ω·cm.
(2) Drying
The washed titanium dioxide filter cake generally does not exceed 200 ° C during drying, and is usually controlled at 120 to 150 ° C. This maintains the structure of the hydrated oxide in the coating because it can only be removed at this temperature. The surface water cannot be removed from the combined water. The excessive drying temperature not only carbonizes and decomposes the organic coating agent, but also crystallizes the hydrated aluminum oxide, causing the titanium dioxide particles to coagulate and reduce the dispersibility.
One type of drying device is a belt dryer, and the other is a spray dryer. The belt dryer is to pass the filter cake on the stainless steel perforated track or stainless steel wire mesh belt through the extruder, and slowly pass through the dryer under the action of the transmission device, and indirectly heat the air through the air blower to force the hot air to pass through the material from the upper part through the blower. The layer is subjected to a top-down cycle to dry the material. The characteristics of the machine are large contact area between the hot air and the material, the drying time is short, the operation is stable during drying, the dust is small, the production capacity is large, and the dried material is granular, which facilitates the subsequent steam pulverization operation, and the disadvantage is equipment. It has a large area, many circulating fans and high power consumption.
Spray drying is generally carried out using a centrifugal spray dryer. The titanium dioxide slurry (50% to 55%) is centrifugally dispersed into mist particles (fine particles) in a high-speed dispersion disk of 10000 to 1500 r/min, so that it is countercurrent to the drying chamber. The hot air is contacted for drying. Since the material is dispersed very finely, the evaporation area is large, and the moisture in the material is evaporated in an instant (a few seconds). The drying time of the material in the spray dryer is extremely short, which can avoid overheating of the material. The dried material is powdery, but the moisture content in the material is stricter and the operation control is complicated.

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