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The Use Of Chromium Powder In Industry

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Chromium symbol Cr, silver-white metal, belongs to group VIB in the periodic table. Chromium has an atomic number of 24 and an atomic weight of 51.9961. It is a body-centered cubic crystal. The common valences are +3, +6 and +2. The oxidation number is 10. In 1797, French chemist L.N.Vauquelin discovered a new element in Siberian red lead ore (chrome-lead ore) and reduced it with carbon the following year to obtain metallic chromium. Because chromium can generate beautiful multi-colored compounds, it is named chromium according to the Greek word chroma (color).
Basic Information
Chromium is a silver-white metal with extremely hard quality and corrosion resistance. The density is 7.20 g/cm³. The melting point is 1857±20℃, and the boiling point is 2672℃. Valence +2, +3 and +6. The ionization energy is 6.766 electron volts. Metallic chromium is generally characterized by surface passivation in acid. Once depassivated, it is easily dissolved in almost all acids. It is oxidized by water vapor at high temperature and oxidized by carbon monoxide at 1000°C. At high temperatures, chromium reacts with nitrogen and is attacked by alkali. Soluble in strong alkali solution. Chromium has high corrosion resistance, and it oxidizes slowly in the air, even in the state of red heat. not soluble in water. It can be protected by plating on metal.

Industrial use
The chromium ore used in industry is chromite, which belongs to spinel (MgO·Al2O3) and magnetite (FeO·Fe2O3), and its general chemical formula is (Fe, Mg) O·(Cr, Fe, Al2O3). As the divalent elements (Mg2+, Fe2+, Zn2+) and trivalent elements (Al3+, Fe3+, Cr3+) replace each other, various ores of different compositions can appear. In addition to the main components FeO and Cr2O3, it generally contains different components of MgO, Al2O3 and other impurities. The structure and composition of the ore have a significant impact on the use. For example, chromium spinel is more difficult to reduce than chromite (FeO·Cr2O3); chromium ore containing serpentine, if the volatile content is greater than 2%, use it to make chrome refractory bricks When heated to 1000°C, it will burst due to the release of crystal water. The United States consumed 917,000 tons of chromite in 1978, and its use is distributed as follows: 61% for metallurgy, 21% for chemical industry, and 18% for refractory materials. The price of chrome ore in the London market in 1981: Turkish ore (48%Cr2O3, Cr/Fe=3) 130-135 US dollars/ton, South Africa (Azania) chrome ore (44%Cr2O3) 60-70 US dollars/ton.
The toxicity of chromium is related to its existing valence state. Hexavalent chromium is 100 times more toxic than trivalent chromium, and is easily absorbed by the human body and accumulates in the body. Trivalent chromium and hexavalent chromium can be converted into each other. Natural water does not contain chromium; the average concentration of chromium in seawater is 0.05μg/l; it is even lower in drinking water. The pollution sources of chromium include sewage discharged from the processing of chromium-containing ore, metal surface treatment, leather tanning, and printing and dyeing.
Chromium is an essential trace element for human body. Trivalent chromium is an element beneficial to the human body, while hexavalent chromium is toxic. The human body's absorption and utilization rate of inorganic chromium is extremely low, less than 1%; the human body's utilization rate of organic chromium can reach 10-25%. The content of chromium in natural foods is relatively low and all exist in trivalent form.