1. Decomposition methods of tungsten concentrate: pyrometallurgical method and wet method.
(1) sodium carbonate sintering method is often used for pyrolysis. The method is that wolframite concentrate and sodium carbonate are put into a rotary kiln and sintered at 800-900℃. It is also necessary to treat scheelite concentrate with quartz sand to obtain calcium orthosilicate with low solubility, and the sintering temperature is about 1000℃. After sintering for about two hours, the decomposition rate of concentrate can reach 98 ~ 99.5%. The sintered material is leached with water at 80 ~ 90℃ and filtered to obtain sodium tungstate solution and insoluble residue.
(2) Wet method is divided into alkali decomposition method and acid decomposition method. When decomposing wolframite concentrate, sodium hydroxide solution is used for leaching at1/kloc-0 ~130℃ or higher. Scheelite concentrate is leached with sodium carbonate solution in autoclave at 200 ~ 230℃, or decomposed with hydrochloric acid at 90℃ to obtain solid crude tungstic acid. The decomposition rate of tungsten concentrate by wet treatment can reach 98 ~ 99%.
Purification of tungsten compounds
Impurities such as silicon, phosphorus and arsenic contained in sodium tungstate solution exist in the form of sodium silicate, sodium hydrogen phosphate and sodium hydrogen arsenate respectively. Boil the solution and neutralize it with dilute hydrochloric acid. When the pH value of the solution is 8 ~ 9, sodium silicate is hydrolyzed into silicic acid coagulation and precipitation, and magnesium chloride and ammonium chloride solution are added to make phosphorus and arsenic generate magnesium ammonium phosphate and magnesium ammonium arsenate with little solubility and precipitate and remove them.
When sodium sulfide is added to the sodium tungstate solution, molybdenum forms sodium thiomolybdate before tungsten and is neutralized with hydrochloric acid, so that when the pH of the solution is 2.5 ~ 3.0, molybdenum precipitates out and is removed as insoluble molybdenum trisulfide. Calcium chloride solution was added to the purified sodium tungstate solution to obtain calcium tungstate precipitate, which was decomposed by hydrochloric acid to obtain industrial tungstic acid, and calcined at 700 ~ 800℃ to obtain industrial pure tungsten trioxide.
If chemically pure tungsten trioxide is prepared, industrial tungstic acid can be dissolved in ammonia water to obtain ammonium tungstate solution, and impurities such as silicon are left in the slag. The flaky ammonium paratungstate [5(NH) O 12WO 5H O] crystal was obtained after evaporation and crystallization of the solution. Because the solubility of ammonium paratungstate is greater than that of ammonium paratungstate, the molybdenum content of ammonium paratungstate crystal decreases after crystallization. Ammonium paratungstate is dried and calcined at 500 ~ 800℃ to obtain chemically pure tungsten trioxide. In 1970s, sodium tungstate solution was transformed into ammonium tungstate solution by tertiary amine method, which simplified the process flow and improved the recovery rate of tungsten.
Second, the preparation of tungsten powder
Tungsten powder is molded, sintered and melted to obtain dense tungsten. Powder metallurgy is used in industry, that is, tungsten powder or tungsten powder with additives is molded-pre-sintered-vertical melting sintering and isostatic pressing molding-and indirectly sintered into strips (doped strips and alloy strips). The former process is often used for batch production of small strips, while high-purity tungsten products are generally melted by electron beam.
3. The tungsten rod is made of tungsten powder, mainly through two processes: forming and sintering:
① molding. There are two methods: molding (mechanical forming) and isostatic pressing. The former is generally only used to press blanks with smaller size and unit weight, which is simple to operate and can accurately control the size and appearance quality of blanks, but the density of pressed blanks is small and uneven. The isostatic pressing method can press single and large-sized blanks (round, rectangular and rectangular in shape), tube blanks and complex parts, and the pressed blanks have high and uniform density. However, the size and appearance quality of the compact are worse than that of the molding method, so it is generally necessary to carry out mechanical processing and molding before sintering.
② Sintering. There are vertical melting sintering (direct sintering) and indirect sintering. Before sintering, the compact should be pre-sintered to remove the forming agent. Vertical melting sintering is only used for small blank sintering. Indirect sintering method can be divided into hydrogen protection sintering and vacuum sintering, which can sinter large-size tungsten blanks or tungsten parts to be processed.