Ammonium Paratungstate Preparation -- Evaporation Crystallization
Evaporation crystallization method is the most commonly used method of preparation of ammonium paratungstate, this method of technology and equipment is simple, easy to large-scale, have a certain role in the purification at the same time,it has been favored by industrial production.
When the ammonium tungstate solution heated, the ammonia concentration decreased, (NH4) 2WO4 solution concentration gradually increased, ammonium paratungstate crystallization, the reaction is:
12(NH4)2WO4==5(NH4)2O·12WO3·5H2O+14NH3↑+2H2O
In industrial production, the degree of evaporation crystallization is generally based on the amount of molybdenum allowed in the solution and the product. There are usually two ways of batch intermittent operation and continuous crystal operation.
The batch intermittent operation was carried out in a jacketed heated stirred tank, and a solution of (NH4) 2WO4 having a density of 1.16-1.28 g / cm3 was added to the reactor and stirred to boil the boiling ammonia.The reactor roof is generally installed with the vacuum system connected to the pipeline, discharged the ammonia to outside of the reactor in time to cooling and recovery. Solution pH value decreased to 7.0-7.7, separated out APT.
Field sampling measure mother liquid density is 1.06 1.02 g/cm3, namely stop heating and cooling for 0.5 hours,The material is discharged to a vacuum filter and filtered and washed. The resulting wet APT is dried at a temperature of 90 ° C to 120 ° C and then sieved to obtain an APT product.
The continuous crystallization operation is carried out in a continuous mold. Material liquid outer heater after heated to a certain temperature and then sent steam into the room, ammonia evaporation and gives some supersaturated solution in the steam, the supersaturated solution by the middle tube into the crystallization of chamber, with which contact with the crystal crystal, crystal send continuous filter to filter washing, part of the return to the original solution crystallization mother liquor mixed circulation, mixing part into the batch operations last reactor for secondary crystallization.
The discontinuous operation is simple and easy to recycle, but the distribution range of APT particle size is too large. On the contrary, the continuous crystallization method has the same composition and particle size. The process is continuous and the quality is stable, but the crystallization rate is not high, so it needs more time.
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