The tungsten smelting process includes the decomposition of concentrate, the purification of tungsten compounds, the preparation of tungsten powder and dense tungsten.
Tungsten concentrate decomposition methods include fire method and wet method
① The sodium carbonate sintering method is commonly used for fire decomposition. In this method, wolframite concentrate and sodium carbonate are sintered together in a rotary kiln at 800-900°C. The main chemical reactions are:
When processing scheelite concentrate, it is necessary to add quartz sand to obtain calcium orthosilicate with low solubility. The sintering temperature is about 1000℃. The main chemical reaction is: 2CaWO4+2Na2CO3+SiO2─→2Na2WO4+2CaO+SiO2+2CO2 after approx. After two hours of sintering, the concentrate decomposition rate can reach 98-99.5 percent. The sintered material is leached with water at 80-90°C, and filtered to obtain sodium tungstate solution and insoluble residue.
1. Wet method is divided into alkali decomposition method and acid decomposition method. When decomposing wolframite concentrate, leaching with sodium hydroxide solution at 110～130℃ or higher temperature, the main chemical reaction is: (Fe,Mn)WO4+2NaOH─→Na2WO4+(Fe,Mn)(OH)2 The scheelite concentrate is leached in an autoclave with sodium carbonate solution at 200～230℃. The main chemical reaction is: CaWO4+Na2CO3─→Na2WO4+CaCO3 or decomposed with hydrochloric acid at 90℃ to obtain solid crude tungstic acid: CaWO4+ 2HCl─→H2WO4↓+CaCl2
The decomposition rate of wet processing tungsten concentrate can reach 98-99%.
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Tungsten compound purification
Impurities such as silicon, phosphorus and arsenic contained in the sodium tungstate solution are in the state of sodium silicate, sodium hydrogen phosphate and sodium hydrogen arsenate, respectively. The solution is boiled and neutralized with dilute hydrochloric acid. When the pH of the solution is 8-9, sodium silicate is hydrolyzed into silicic acid and precipitated. Magnesium chloride and ammonium chloride solutions are added to make phosphorus and arsenic produce magnesium ammonium phosphate and arsenic acid with little solubility. The magnesium ammonium precipitate is removed. When sodium sulfide is added to the sodium tungstate solution, molybdenum forms sodium thiomolybdate before tungsten, which is neutralized with hydrochloric acid so that when the pH of the solution is 2.5-3.0, molybdenum becomes insoluble molybdenum trisulfide precipitated and removed. Add calcium chloride solution to the purified sodium tungstate solution to obtain calcium tungstate (CaWO4) precipitation (ie artificial white tungsten). Use hydrochloric acid to decompose calcium tungstate precipitation to obtain industrial tungstic acid. Tungstic acid is calcined at 700～800℃ , You get 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 remain in the slag. The solution is evaporated and crystallized to obtain flake ammonium paratungstate crystals. Since the solubility of ammonium paramolybdate is greater than that of ammonium paratungstate, after crystallization, the molybdenum content of the ammonium paratungstate crystal decreases. After the ammonium paratungstate is dried, it is calcined at 500-800°C to obtain chemically pure tungsten trioxide. In the 1970s, the tertiary amine method was used to convert sodium tungstate solution into ammonium tungstate solution, which simplified the process and improved the recovery rate of tungsten.
Tungsten powder production
Industrially, tungsten powder is prepared by hydrogen reduction of tungsten trioxide or ammonium paratungstate. The reduction process depends on the requirements for the particle size, particle size composition and oxygen content of the product tungsten powder. Hydrogen reduction of tungsten trioxide to prepare tungsten powder is generally divided into two steps: first reduce tungsten trioxide to tungsten dioxide at 550-800℃, and then reduce tungsten dioxide to tungsten powder at 750-900℃. The ammonium paratungstate can also be reduced to blue tungsten oxide (blue tungsten) with hydrogen or no hydrogen, and then reduced to tungsten powder with hydrogen. The particle size and composition of tungsten powder are important quality indicators of tungsten powder. The reduction is carried out in a tubular electric furnace or a rotary electric furnace.
Preparation of dense tungsten
Tungsten powder undergoes forming, sintering, melting and other treatments to obtain dense tungsten. Forming is to put tungsten powder into a steel die and press it into a billet or block with a hydraulic press. Large-scale briquettes are formed by liquid isostatic pressing, and briquettes with a relatively uniform density can be obtained. The sintering of the tungsten billet is divided into two steps: first sintering at a low temperature of 1100 to 1200 ℃, and then the current is directly passed through the billet for vertical melting (ie, high temperature sintering). The density of the tungsten rod after sagging reaches 17-19 g/cm3. The sintering of small, special-shaped and large tungsten compacts usually uses radiant heating or induction heating to achieve the high temperature required for sintering. At this time, it is not necessary to completely separate low-temperature sintering and high-temperature sintering. When making large tungsten ingots, vacuum or inert gas-protected arc melting and electron beam melting are usually used. The production of high-purity dense tungsten is usually purified by electron beam melting or zone melting. The latter method can obtain tungsten single crystals with a purity of over 99.99%.
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