7 Zinc Processing
Zinc is the fourth most widely used metal, following iron, aluminum, and copper. Zinc is mined mostly in Canada, the former USSR, Australia, Peru, Mexico and the US. In 1993, about 50% of the zinc mined came from Alaska. Tennessee, New York and Missouri are the top producers of zinc metal.
The US is the worlds largest consumer of zinc. Eighty percent is consumed in slab format, while 20% is consumed in compounds. Most zinc is used in the galvanizing steel process. Other uses include the automotive, construction, electrical and machinery industries. Zinc compounds include agricultural chemicals, paints, pharmaceuticals, and rubber.
Zinc concentration is usually done at the mine site, prior to reaching the zinc processing plant. The concentration includes crushing and flotation techniques. At the zinc processing plant, the zinc is first reduced using pyrometallurgical methods, including distillation, or hydrometallurgical methods, including electrowinning, calcination, leaching, or purification. The electrowinning process is most commonly used.
Electrowinning uses an electrolytic cell to reduce the zinc. An electric current is run from a lead-silver anode through a zinc solution. The zinc deposits on an aluminum cathode and is harvested. The zinc is then melted and cast into ingots.
Primary zinc production produces air emissions, process wastes and solid-phase wastes. Air emissions come primarily from the zinc roasting process and consist primarily of sulfur dioxide emissions. Most emissions are recovered on site in sulfuric acid production plants, where sulfuric acid is produced.
Zinc roasters also produce particulate emissions. Particulate air emissions from primary zinc production often contain cadmium, lead and other compounds, depending on inputs. The slurry formed from the emissions control equipment is K066 hazardous waste.
The electrowinning process produces waste heat. Rather than letting the hot gas escape into the environment, some is recovered and sent to cooling towers where the steam is collected for reuse.
Wastewater is produced from leaching, purification and electrowinning. The water is usually treated and discharged. Reuse opportunities may be available.
Solid wastes include acid plant slurries, sludge from electrolytic cells and copper cakes, a by-product of zinc production, from the purification cells. Much of the waste is considered RCRA hazardous waste. Anode slime from electrolytic cells consists of impurities not captured prior to the electrowinning process. The composition usually makes the slime a RCRA hazardous waste. Copper cakes are captured and sold to copper processing plants.
Secondary zinc production uses process scrap from zinc slabs, zinc oxides and zinc dust. Selective melting may also be used to capture zinc if the zinc is mixed with other non-ferrous metals with higher melting points. Zinc is also often recovered from the furnace dust of galvanized steel making plants. Using pyrometallurgical refinement techniques, the zinc can be recovered.
Once obtained, secondary zinc first undergoes a separation process. Magnetic separation, sink-float and hand sorting are usually used to remove the zinc from unwanted components.
After separation, the zinc is melted with new scrap from brass plants, rolling zinc clippings or die casting. The zinc is melted in a kettle, crucible, reverberatory furnace or electric induction furnace. Flux is used to trap impurities and produces dross that is skimmed from the surface of the molten zinc. The zinc is then either poured into molds or sent to refiners.
High quality scrap from dross, diecastings, and other zinc rich sources usually can be remelted without further refinement. The recovered metal can become galvanized brighten or alloy materials in copper, aluminum, magnesium, iron, lead, cadmium or tin production. Zinc helps to make the metals stronger.
Medium to low grade skims, oxide dusts, ash and residues containing zinc require more refinement before melting. These may undergo reduction, or distillation using pyrometallurgical processes. The reduction upgrades the zinc for further processing to reach the desired standards.
Secondary zinc processing produces air emissions and solid waste emissions. Air emissions come from sweating and melting. The emissions include particulates, zinc fumes, volatile metals, flux fumes and smoke, rubber, plastics and zinc scrap. Incomplete combustion products are also emitted, but are eliminated when passed through an after burner. Particulates are collected in emissions control equipment such as baghouses. The particulates are often refined for the metals.
In distillation and oxidation processes, zinc oxides in the form of dust are produced. The oxides are collected in baghouse emissions control systems.
Air emissions are also common from the pyrometallurgical processes. If simple remelting of the zinc is required, the emissions are not high. However, if the zinc requires reduction or other refinement, emissions are likely. The lower the quality of zinc scrap, the more air emission produced in the process. Air emissions are usually collected in ventilation systems. The emissions control dust is usually sold as fertilizer or animal feed.
Solid waste is present in the form of slag. The slag from secondary zinc production usually contains copper, aluminum, iron and lead. Slag is generated during pyrometallurgical processes and may be hazardous.
USEPA, Profile of The Nonferrous Metals Industry. EPA 310-R-95-010. This document discusses uses, processes and pollution prevention opportunities for the zinc processing industry.