Tungsten concentrates decomposition method has fire method and wet method.
Purification of tungsten compound
The silicon, phosphorus and arsenic impurities in sodium tungstate solution are presenting in sodium silicate, sodium hydrogen phosphate and sodium hydrogen arsenate state. Boiling solution and neutralized with dilute hydrochloric acid, when the solution pH is 8-9, the sodium silicate hydrolyze into silicate condensation precipitation, add magnesium chloride and ammonium chloride solution, make the phosphorus, arsenic generate a very small solubility of magnesium ammonium phosphate and arsenate ammonium magnesium precipitation removed. Add sodium sulfide to the sodium tungstate solution, molybdenum first form the thiosulfate molybdate with tungsten, neutralized with hydrochloric acid, make the solution pH from 2.5 to 3.0, molybdenum was formed of insoluble precipitates molybdenum trisulfide removed. In the purified sodium tungstate solution add calcium chloride solution to obtain calcium tungstate (CaWO) precipitation (ie synthetic scheelite), with hydrochloric acid decompose calcium tungstate precipitation gets the industrial tungsten acid, at 700 ~ 800 calcination, get industrial pure tungsten trioxide. If preparing chemically pure tungsten trioxide, can be dissolved industrial tungstic acid in aqueous ammonia, obtained ammonium tungstate solution, silicon and other impurities remain in the slag. The solution through evaporated and crystalline treatment, obtains a sheet-like ammonium paratungstate crystal. As the solubility of ammonium paramolybdate greater than ammonium paratungstate, after crystallization, ammonium paratungstate crystal was reduced molybdenum content. After the ammonium paratungstate dried, calcined at 500 ~ 800, can be obtained chemically pure tungsten trioxide. In 70s, use tertiary amine method to make sodium tungstate solution convert into ammonium tungstate solution, simplifying the process and improve the recovery of tungsten.
Preparation of tungsten powder
In industry, use the method of hydrogen reducing tungsten trioxide or ammonium paratungstate to prepare tungsten powder. Reduction process depends on the product tungsten powder particle size, particle size composition and oxygen requirements. Hydrogen reduces tungsten trioxide preparing tungsten powder is generally taken two steps: first in the 550 ~ 800 reduce tungsten trioxide into tungsten dioxide, and then at 750 ~ 900 reduce tungsten dioxide into tungsten powder. Also be the first pass into hydrogen or not to reduce paratungstate into blue tungsten oxide (blue tungsten), and then use hydrogen to reduce into tungsten powder. Tungsten powder’s particle size, particle size composition is tungsten powder’s important indicator of quality. The reduction reaction was conducted in the rotary tube furnace or electric furnace.
Dense tungsten preparation
Tungsten powder through forming, sintering, melting and other processing obtains dense tungsten. The forming is taking tungsten powder into the steel die, pressed into the billet or a compact with hydraulic pressure. Large briquettes use liquid and other static pressure method molding, can be obtained more uniform density compact. Tungsten billet sintered divided into two steps: first in the 1100 ~ 1200 temperature sintering, then the current directly through the billet for vertical melting (ie, high temperature sintering). The density of through vertical melting tungsten was not to17-19 g/cm. Small shaped, different shaped and large shaped tungsten compacts sintered usually use radiant heating or induction heating method to achieve the sintering required high temperature, at this time, do not have to completely separate operation low-temperature sintering and high-temperature sintering. While preparing large tungsten ingots usually use a vacuum or inert gas protection arc melting method and an electron beam melting method. Prepare high purity dense tungsten is usually use electron beam melting purified. Electron beam melting also can be obtained tungsten single crystals, the purity was up to 99.99% or more.
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