The properties and uses of metallic chromium
Metal chromium is mainly used in the production of nickel based and cobalt based high-temperature alloys, aluminum alloys, titanium alloys, resistance alloys, corrosion-resistant alloys, iron-based heat-resistant alloys, and stainless steel. There are two types of metal chromium produced in industrial production. One is aluminum thermite chromium, which is block shaped, silver bright, metallic luster, and contains Cr>98%. Impurities are required according to different uses; Another type is electrolytic chromium, which is sheet-like and has a dark brown surface. After hydrogen refining, the surface is bright and contains Cr>99%.
purpose
Chromium is a brittle metal that cannot be used as a metal material alone, but when combined with iron, nickel, cobalt, titanium, aluminum, copper, and other alloys, it becomes an engineering material with heat resistance, thermal strength, wear resistance, and special properties. Chromium metal is used as an alloying agent in the production of various nickel or cobalt based high-temperature alloys, titanium alloys, aluminum based alloys, resistance alloys, and copper alloys. Part of it is used to produce stainless steel and heat-resistant steel. These materials are widely used in industries such as aviation, aerospace, nuclear reactors, automobiles, shipbuilding, chemical, and military.
Production process
The main chromium resource in nature is chromite ore. The production of metallic chromium uses chromium oxide as the raw material, so the entire process is divided into two steps. First, use chromite ore as raw material to produce chromium oxide. Then, aluminum is used to reduce chromium oxide (i.e. aluminothermic method) to smelt metallic chromium. The first step in producing chromium oxide from chromite ore is alkaline roasting, which converts insoluble Cr3+chromium salts into water-soluble Cr6+chromium salts, namely Na2CrO4. After water leaching, sodium chromate (Na2CrO4) aqueous solution is obtained. There are three industrial methods for preparing chromium oxide from sodium chromate aqueous solution:
(1) Ammonium chloride reduction method. Add sulfuric acid to transform Na2CrO4 into a solution of sodium dichromate (Na2Cr2O7). After concentration, Na2SO4 crystals precipitate. Cool the mother liquor to 25 ℃ to obtain sodium dichromate crystals. Mix sodium dichromate crystals with ammonium chloride and reduce at 700-800 ℃ to obtain Cr2O3 and NaCl; After washing with water to remove NaCl, chromium oxide is obtained, which is then oxidized and calcined for desulfurization at about 1200 ℃ to obtain chromium oxide.
(2) Chromic anhydride thermal decomposition method. React sodium dichromate crystals with concentrated sulfuric acid to form chromic anhydride (CrO3), which is then thermally decomposed into chromium oxide in a high-temperature calcination furnace.
(3) Chromium oxide thermal decomposition method. Add sulfur powder or sodium sulfide solution to sodium chromate solution to reduce Cr6+to Cr3+, which is the precipitation of Cr (0H) 3. Then it is calcined at high temperature to form chromium oxide. The production of chromium oxide using chromium hydroxide method has a short process flow; High chromium recovery rate; Not producing corrosive gases, which is beneficial for the maintenance of factories and equipment; Save a large amount of sulfuric acid and chemical raw materials; The by-product Haibo (Na2S2O3) can be recycled and reused; Advantages such as cost reduction. The following diagram is a process flowchart for smelting metallic chromium.