Silicon is one of the very common elements, however, it rarely appears in nature in the form of simple substance. It widely exists in rock, gravel and dust in the form of complex silicate or silica. Phosphorus distributes widely within the biosphere and lists the top ten in earth crust and belongs to the second class in seawater concentration. Arsenic is the nonmetallic element which distributes widely in nature and the twentieth element constituting the earth crust. 

 

With the development and utilization of tungsten resources, ingredients of tungsten are more and more complex. This increases the difficulty of the beneficiation and metallurgy from the processing and disposing of raw material. And with the high speed development of science and technology, the user puts forward higher requirements on the product purity. Sodium tungstate solution through pyrogenic process of tungsten concentrate and alkaline decomposition usually contains silicon, phosphorus, sulfur, arsenic, molybdenum and other impurities. Sodium tungstate solution including impurities needs to be disposed by removing impurities to ensure the quality of the products.

 

This article describes how to remove silicon, phosphorus and arsenic from sodium tungstate solution. At present, neutralizing hydrolysis method to remove silicon and magnesium ammonium salt or magnesium salt method to remove arsenic and phosphorus are often used in industry. The specific method is as follows:

1. Add 1:3 dilute hydrochloric acid to sodium tungstate solution which has been heated to boiling. Neutralize free alkali to 4 -5 g/L which is the equivalent of pH = 13-14. After adding the hydrochloric acid, let the solution boil for 30 min. Later, parts of silicic acid coagulate and precipitate out.

2. Without heating, slowly add ammonium chloride and avoid the ammonia escaping intense. Addition amount of ammonium chloride should be twice of the alkali amount in neutralization solution. After cooling, clarification, filtering, the removing silicon solution should be sent to remove arsenic phosphorus. When removing phosphorus and arsenic, MgCL₂ also have the effect of removing silicon to drop the silicon content below the quality standard.

3. Add dilute hydrochloric acid slowly to the hot alkaline sodium tungstate solution under constantly stirring to neutralize the free alkali of Na₂WO₄solution and NaOH of Na₂SiO₃hydrolysis. And pH of Na₂WO₄ solution is 11, at this time only a fraction of Na₂SiO₃ hydrolysis, the hydrolysis rate is only 50%. Because in the following removing the arsenic and phosphorus, such alkalinity condition is necessary and the rest of the silicon can be removed in the process of removing phosphorus and arsenic.

4. Under the circumstance of neutralizing to basicity 0.8 1.0 g/L, add the specific gravity of 1.16 1.18 MgCL₂solution slowly to the hot sodium tungstate solution. At this time, phosphorus, arsenic and silicon will generate precipitation respectively. After adding the MgCL₂, boil it for 30 minutes and take samples to analyze alkalinity and arsenic. After analysis to determine the qualification, then let it clarify and filter. At first, filter the supernatant, then filter the bottom of the filter pulp. Wash the residue with water 2-3 times and take out the residue to analyze. When WO3 content is less than 0.5%, the residue is qualified.