Calcium hydroxide is a kind of white powder solid, which is generated by the reaction of lime and water. Its saturated water solution is called the whitewash. Calcium hydroxide is a binary alkali of connectivity with alkali and has corrosion on skin and fabric. Calcium hydroxide has wide application in the industry and is common building materials. It’s also used as a fungicide and chemical raw materials.

 

Scheelite (CaWO₄) is a kind of tungsten ore. Scheelite is the main raw material of smelting tungsten. In the traditional tungsten smelting process, after purifying and removing impurity of sodium tungstate solution decomposed by sodium hydroxide, using calcium chloride as precipitant can get the artificial scheelite. While soluble calcium chloride can obtain higher  Ca²⁺ ion concentration and the rate of scheelite precipitation, NaCl precipitation mother liquor can't return tungsten decomposition process as decomposition reagent. Because Na⁺ and Cl^-chemical property is lively, it’s difficult to form insoluble compounds to realize precipitation separation. Precipitation mother liquor is unable to achieve closed-circuit circulation and it must be opened to discharge.

 

However, using calcium hydroxide as precipitation agent, using sodium tungstate calcium hydroxide causticizing-precipitating scheelite method can increase the recovery ratio of tungsten and realize the mother liquor closed cycle. Test steps are as follows:

1. Add the high tungsten concentration of sodium tungstate solution to a certain amount of pure water to dilute the settingWO₃ concentration. Measure a certain volume of sodium tungstate solution into the constant temperature magnetic stirrer or the reaction kettle and add quantitative calcium hydroxide. Start the test according to the set temperature and stirring speed. After reaching the temperature, keep time to the soaking time. 

2. Wash the artificial scheelite precipitated out with pure water until washing liquid pH is neutral. Put artificial scheelite through filtering inside the drum wind drying oven to dry, weigh and make the test analysis according to the requirements.

 

The conclusions can be drawn as follows:

1. The process of causticizing-precipitating scheelite by calcium hydroxide from sodium tungstate solution is the technical way of the realization of recycling NaOH reagent for tungsten ore decomposition, and wolframite and scheelite and wolframite mixed ore green smelting. 

2. When the calcium hydroxide dosage is 1.4 times of the theoretical amount, temperature 100℃, tungsten concentration of sodium tungstate solution is 105g/L, the holding time is 2 h, the stirring speed is 350 r/min, The precipitation rate of scheelite can reach above 96%. 

3. Comparing with traditional process calcium chloride used as a precipitating agent, calcium hydroxide precipitating scheelite requires larger times of the theoretical amount, longer reaction time and faster stirring speed.