A Greener Method to Extract APT

Ammonium paratungstate (APT) is an important intermediate in tungsten metallurgy and tungsten chemicals. The common material to prepare APT is a crude sodium tungstate solution obtained by treating a tungsten raw material (including a mineral raw material and a secondary raw material). The crude solution mainly contains sodium tungstate Na₂WO₄, and impurities such as NaOH, P, As, Si, Sn and so on.

In order to exact the APT from it, two goals should be completed: the first one is purification or impurity removal; the other is transformation, by transforming sodium salt into ammonium salt. The processes to reach the goals are by strong base anion exchange resin or Ammonium magnesium salt method are adopted to remove impurities. In which the strong base anion exchange process is more widely used.

The advantages of the traditional ion exchange process are numerous, but the shortcomings are well known. The water consumption is large, and the ion exchange wastewater cannot be recovered, resulting in a large amount of waste water, and it is also difficult to treat harmful impurities such as P, As, Si, and Sn in the solution. Therefore, different researchers have tried to improve the ion exchange method.

In recent years, Chinese researdchers have made great achievements in this field. It is reported that one research team in China has improved and developed a greener way to extract APT through ion exchange that greatly reduce the amount of wastewater discharged.

The major process are as follows:

1. Concentration and crystallization of crude Na₂WO₄ solution

The sodium tungstate crystal is supersaturated by Na₂WO₄, and impurities such as P, As, Si, and Sn are retained in the alkali solution, and the alkali decomposition process of the mineral raw material is returned. During the alkaline decomposition process, these impurities such as P, As, Si, and Sn will react with scheelite (or Ca(OH)₂) to form insoluble calcium salts and solidify into alkali decomposition slag.

2. Dissolution and acidification of sodium tungstate crystals

The sodium tungstate crystals were dissolved to obtain a solution containing WO3 30-500 g/L while acidifying to pH 2.0-6.0. At this time, WO4 2- in the solution is polymerized into a tungstate ion H2W12O40 6-. The acid used for acidifying the Na2WO4 solution is an inorganic acid such as sulfuric acid, hydrochloric acid or nitric acid.

3. Anion exchange resin adsorption

The adsorption is carried out by using a macroporous weakly base anion exchange resin, and H2W12O40 6- is exchanged with SO4 2- of the resin phase to enter the resin phase.

5. Evaporation crystallization

The extracted (NH4)2WO4 solution was subjected to evaporation of crystals by removing molybdenum to obtain APT. Evaporation crystallization is carried out under normal pressure or under a negative pressure of 50 kPa, and the resin is converted with an acid and returned to adsorption.

Due to the use of macroporous weak base anion exchange resin, the affinity with tungsten anion is strong, and the allowable pre-crossing liquid concentration is 3-25 times that of the strong base anion exchange process under alkaline conditions. Therefore, the volume of the discharged liquid after discharge is only 1/3-1/25. At the same time, it basically contains no harmful impurities, so it is easy to be treated. At the same time, the high concentration of Na2WO4 solution also leads to the reduction of the volume of the equipment workshop, and the valuable substance NaOH can be recycled and turned into waste.

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