Ammonium Paratungstate Preparing - Tube Type Multi Hearth Furnace

Thermal degradation is a mainstream technology which prepares ammonium paratungstate (APT) by using ammonium metatungstate (AMT) as raw material.

Currently, the pyrolysis equipment industry in China is mostly rotary calciner which has simple structure and is suitable for mass production. However, it has also some disadvantages which mainly reflected in the low accuracy of control to temperature and the pyrolysis conversion rate is generally around 80%. There are many patents in the United States that present significant improvements of the pyrolysis equipment. The most well-known equipment is Tube type multi hearth furnace.

Similar to metallurgical multi hearth roaster, the tube type multiple hearth furnace contains only two moving parts: tray and fans. The tray, fixed to the inside support of the machine, is shaped like a vertical tube and can rotate slowly. Fed into the dryer through the feed chute, wet materials move layer by layer from top to bottom, forming a thin layer on each layer. When the tray rotates, the material is driven by a blade fixed inside the machine, and it is paved along the same height. After the tray rotates a circle, a fixed blade on the second floor paves the material. If the tray continues to rotate, the material thickness remains the same, but it will fall into the secondary tray along the radial slit, thus a new cycle begins. Heat flow, such as hot air is fed into the machine through by a long catheter secured to the edge. Controlling the size of the air flow valve is achieved by controlling valve adjustment. Driven by vertical centrifugal fan in the tower, the flow of air is finally discharged through the tower top.

Pyrolytic parameter such as temperature, residence time, bed thickness, pyrolysis number determines the quality of AMT. Usually, the pyrolysis temperature is controlled at 200一400℃, and 260一370℃ are the best. After pyrolysis, the APT decomposition is carried out in the non ionic water pulping with pre heating to 60℃-80℃, and the weight ratio of solid to liquid is 20:80 to 40:60.

APT decomposition should be evaporated and crystallized again. In general, the hydration temperature is 90—100℃ and the hydrated acidity should be controlled at pH=3 - 4.2. After four hours, the decomposition substance is evaporated in the atmospheric evaporator. And then after the swallow, the concentrated liquid is cooled, crystallized and filtered to remove the insoluble matter. Finally, the AMT crystal is obtained. According to the calculation of WO3 content, the direct yield of AMT is more than 95%, which is better than that of rotary calciner.

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