The Manufacturing Of Zinc oxide (ZnO)
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Zinc oxide (ZnO) is manufactured by oxidizing zinc vapor in burners in which the concentration of zinc vapor and the flow of air are controlled to produce the desired particle size and shape. The hot gases and particulate oxide or fume pass through tubular coolers, and then the zinc oxide is separated in a baghouse. The purity of the zinc oxide depends upon the source of the zinc vapor.
In the indirect process, zinc metal vapor for burning is produced in several ways, one of which involves horizontal retorts. Since the entire vapor is burned in a combustion chamber, the purity of the oxide depends on that of the zinc feed. Oxide of the highest purity requires special high-grade zinc, and less-pure products are made by blending in Prime Western and even scrap zinc. In the direct process, four or more firebrick furnaces having common walls are charged in cyclic fashion. Coal that is hot from the previous charge is first spread on the grate and, after ignition, a damp, well-blended mixture of zinc ore or zinc-containing material and coal is added. The bed is maintained in a reducing condition with carbon monoxide to produce zinc and lead, if present. Metal vapors are drawn into a chamber above the furnace, where combustion air oxidizes them to pigment. The hot pigmentgas stream enters a cooling duct common to the whole block and, in this way, the product becomes a uniform blend. Traveling-grate furnaces can also be employed. In this process, anthracite briquettes are fed to a depth of about 15 cm. After ignition by the previous charge, the coal briquettes are covered by ore/coal briquettes. The latter are dried with waste heat from the furnace. Zinc vapor evolves and burns in a combustion chamber, and the spent clinker falls into containers for removal. A pigment-grade zinc oxide rotary kiln uses high temperature to produce pigment-quality zinc oxide and makes possible higher recovery than a grate furnace.
Other processes include an electrothermic process, an electric-arc vaporizer process, and the slag fuming process. Zinc oxide, as an amphoteric material, reacts with acids to form zinc salts and with strong alkali to form zincates. In the vulcanization of rubber, the chemical role of zinc oxide is complex and the free oxide is required, probably as an activator. Zinc oxide reacts with organic acids to produce zinc soaps and also reacts with carbon dioxide in moist air to form oxycarbonate. Acidic gases, e.g., hydrogen sulfide, sulfur dioxide, and chlorine, react with zinc oxide, and carbon monoxide or hydrogen reduce it to the metal. At high temperatures, zinc oxide replaces sodium oxide in silicate glasses. An important biochemical property of the oxide is its fungicidal/mildewstatic action. It is also soluble in body fluids and soils. Zinc oxide of high purity is required for pharmaceutical, photoconductive, and certain other uses, and is manufactured by the indirect process. Less-pure zinc oxide is manufactured by the direct process, by which impure zinc oxide is reduced to zinc vapor that is then burned.
In the indirect process, zinc metal vapor for burning is produced in several ways, one of which involves horizontal retorts. Since the entire vapor is burned in a combustion chamber, the purity of the oxide depends on that of the zinc feed. Oxide of the highest purity requires special high-grade zinc, and less-pure products are made by blending in Prime Western and even scrap zinc. In the direct process, four or more firebrick furnaces having common walls are charged in cyclic fashion. Coal that is hot from the previous charge is first spread on the grate and, after ignition, a damp, well-blended mixture of zinc ore or zinc-containing material and coal is added. The bed is maintained in a reducing condition with carbon monoxide to produce zinc and lead, if present. Metal vapors are drawn into a chamber above the furnace, where combustion air oxidizes them to pigment. The hot pigmentgas stream enters a cooling duct common to the whole block and, in this way, the product becomes a uniform blend. Traveling-grate furnaces can also be employed. In this process, anthracite briquettes are fed to a depth of about 15 cm. After ignition by the previous charge, the coal briquettes are covered by ore/coal briquettes. The latter are dried with waste heat from the furnace. Zinc vapor evolves and burns in a combustion chamber, and the spent clinker falls into containers for removal. A pigment-grade zinc oxide rotary kiln uses high temperature to produce pigment-quality zinc oxide and makes possible higher recovery than a grate furnace.
Other processes include an electrothermic process, an electric-arc vaporizer process, and the slag fuming process. Zinc oxide, as an amphoteric material, reacts with acids to form zinc salts and with strong alkali to form zincates. In the vulcanization of rubber, the chemical role of zinc oxide is complex and the free oxide is required, probably as an activator. Zinc oxide reacts with organic acids to produce zinc soaps and also reacts with carbon dioxide in moist air to form oxycarbonate. Acidic gases, e.g., hydrogen sulfide, sulfur dioxide, and chlorine, react with zinc oxide, and carbon monoxide or hydrogen reduce it to the metal. At high temperatures, zinc oxide replaces sodium oxide in silicate glasses. An important biochemical property of the oxide is its fungicidal/mildewstatic action. It is also soluble in body fluids and soils. Zinc oxide of high purity is required for pharmaceutical, photoconductive, and certain other uses, and is manufactured by the indirect process. Less-pure zinc oxide is manufactured by the direct process, by which impure zinc oxide is reduced to zinc vapor that is then burned.
