It seems that the melting point of tungsten is the highest in a single metal element. How can such a metal be refined and melted, and what is it used for?

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The tungsten smelting process includes the decomposition of concentrate, the preparation of tungsten powder and dense tungsten.
1. Tungsten concentrate decomposition method: fire method and wet method.
① Sodium carbonate sintering method is commonly used for fire decomposition. In this method, wolframite concentrate and sodium carbonate are placed in a rotary kiln and sintered at 800-900°C. When processing scheelite concentrate, it is necessary to add quartz sand, the purpose is to obtain calcium orthosilicate with low solubility, and the sintering temperature is about 1000°C. After about 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.
② Wet method is divided into alkali decomposition method and acid decomposition method. When decomposing wolframite concentrate, it is leached with sodium hydroxide solution at a temperature of 110-130 ℃ or higher. Scheelite concentrate is leached in an autoclave at 200-230 ℃ with sodium carbonate solution, or decomposed with hydrochloric acid at 90 ℃ to obtain solid crude tungstic acid. The decomposition rate of wet processing tungsten concentrate can reach 98-99%.
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 to condense and precipitate, and 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.
Adding sodium sulfide 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 (CaWO) precipitation, decompose calcium tungstate precipitation with hydrochloric acid to obtain industrial tungstic acid, which is calcined at 700-800 ℃ to obtain industrial pure three Tungsten oxide.
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 [5(NH) O 12WO 5H O] 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 ℃ to obtain chemically pure tungsten trioxide. In the 1970s, the tertiary amine (RN) method or method was used to convert sodium tungstate solution into ammonium tungstate solution, which simplified the process and improved the recovery rate of tungsten.
2. Preparation of tungsten powder
Tungsten powder undergoes forming, sintering, melting and other treatments to obtain dense tungsten. Industrially used powder metallurgy method, that is, tungsten powder or tungsten powder containing additive components is used for compression molding-pre-sintering-vertical melting sintering and isostatic pressing-indirect sintering into strips (doped strips, alloy strips) Of the two processes, the former process is often used for mass production of small strips, while high-purity tungsten products are generally smelted by electron beams.
3. Tungsten bars are made from tungsten powder, which mainly undergoes two processes: forming and sintering:
① Forming. There are two methods of pressing (mechanical forming) and isostatic pressing. The former is generally only used for compacting billets with small size and unit weight. The operation is relatively easy, and the size and appearance quality of the billets can be controlled more accurately, but the density of the compacts is small and uneven. The isostatic pressing method can press single-weight and larger billets (round, rectangular and rectangular shapes), tube blanks and complex parts, with a large and uniform compact density. However, the size and appearance quality of the compact is worse than that of the molding method, and it is generally subjected to mechanical processing and shaping before sintering.
②Sintering. There are vertical melting sintering (direct sintering method) and indirect sintering method. Before sintering, the compact must be pre-fired to remove the forming agent. Vertical melting sintering is only used for small billet sintering. The indirect sintering method is divided into hydrogen protection sintering and vacuum sintering process, which can sinter large-size tungsten blanks or tungsten parts that need to be machined.