The ion exchange method is a classic technique for preparing ammonium metatungstate (AMT), widely used in industrial production. The core of this method is to replace sodium or ammonium ions in the tungstate solution with hydrogen ions using cation exchange resins, ultimately producing AMT. The specific steps are as follows:
- Raw Material Dissolution: Dissolve ammonium paratungstate (APT) or sodium tungstate in water to form a tungstate solution. If using APT, since its solubility is low, a small amount of ammonia water (NH₃·H₂O) is usually added to assist in dissolution.
- Ion Exchange: Pass the solution through a column packed with strong acid cation exchange resins (e.g., H⁺ resins). The resin adsorbs NH₄⁺ or Na⁺ from the solution, releasing H⁺, which transforms the tungstate ions (WO₄²⁻) into poly-tungstate forms.
- Concentration and Crystallization: Adjust the pH of the solution (typically controlled between 2-4), then heat to concentrate. After cooling, AMT crystals are precipitated.
- Separation and Drying: Separate the crystals by filtration or centrifugation, then dry to obtain the final product.
The advantage of this method is that it yields high purity with low impurity content (such as Na, Mo), making it suitable for producing high-quality AMT. The downside is that resin regeneration requires acid washing, increasing the cost of waste liquid treatment. In industry, the ion exchange method is often used for the production of catalyst-grade AMT due to its strict purity requirements.
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