Ammonium Metatungstate Preparation -- Ion Exchange Method
Ion Exchange method is the process of preparing ammonium metatungstate. It is made from ammonium wolframate solution or ammonium paratungstate.
This method is also one of the main methods for preparing ammonium metatungstate (AMT) at present.
The main mechanism of ion exchange method is that use NH4+ in H+ solution of strong acid or weak acid type H+ cation resin to process ion exchange. Under the condition of pH 3.5, H+ enters solution , cook for 5 hours at 98 °C. Converting ammonium tungstate into stable ammonium metatungstate solution. Then after evaporation and concentration, ammonium tungstate was converted to stable ammonium metatungstate solution. Again after evaporation and concentration, AMT crystals were prepared by freeze crystallization or spray drying. The resin saturated by NH4+ is recycled after treatment with hydrochloric acid solution.
Because of the simple process and convenient control, the ion exchange method can avoid the formation of APT. It does not bring new pollutants into the process, it has gained popularity because of process continuity. The main drawback is that a small amount of colloidal tungstic acid may be produced during the ammonium removal process and plug the resin layer and resin pores. In addition, the tungsten liquid entrained by the resin must be removed.
The resin should be regenerated by acid, which will increase the operation step and bring troubles to the production. Of course, there is no denying that the production of AMT by ion exchange method has the potential for development.
Recently, some scholars have proposed a new method of making high-purity ammonium metatungstate (AMT). The method adopts cation interchange under the low pH condition. A mixture of ammonium paratungstate and water is exchanged with a cationic material to obtain a high-purity ammonium metatungstate product.
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