Zinc smelting is the process of converting zinc concentrates (ores that contain zinc) into pure zinc.
The most common zinc concentrate processed is zinc sulfide, which is obtained by concentrating sphalerite using the froth flotation method. Secondary (recycled) zinc material, such as zinc oxide, is also processed with the zinc sulfide. Approximately 30% of all zinc produced is from recycled sources.
There are two methods of smelting zinc: the pyrometallurgical process and the electrolysis process. Both methods are still used. Both of these processes share the same first step: roasting.
Roasting is a process of oxidizing zinc sulfide concentrates at high temperatures into an impure zinc oxide, called "Zinc Calcine". The chemical reactions taking place during the process are:
Approximately 90% of zinc in concentrates are oxidized to zinc oxide, but at the roasting temperatures around 10% of the zinc reacts with the iron impurities of the zinc sulfide concentrates to form zinc ferrite. A byproduct of roasting is sulfur dioxide, which is further processed into sulfuric acid, a commodity.
The process of roasting varies based on the type of roaster used. There are three types of roasters: multiple-hearth, suspension, and fluidized-bed.
The electrolysis process consists of 4 steps: leaching, purification, electrolysis, and melting and casting.
The basic leaching chemical formula that drives this process is:
This is achieved in practice though a process called double leaching. The calcine is first leached in a neutral or slightly acidic solution (of sulfuric acid) in order to leach the zinc out of the zinc oxide. The remaining calcine is then leached in strong sulfuric acid to leach the rest of the zinc out of the zinc oxide and zinc ferrite. The result of this process is a solid and a liquid; the liquid contains the zinc and is often called leach product; the solid is called leach residue and contains precious metals (usually lead and silver) which are sold as a by-product. There is also iron in the leach product from the strong acid leach, which is removed in an intermediate step, in the form of goethite, jarosite, and haematite. There is still cadmium, copper, arsenic, antimony, cobalt, germanium, nickel, and thallium in the leach product. Therefore it needs to be purified.
The purification process utilizes the cementation process to further purify the zinc. It uses zinc dust and steam to remove copper, cadmium, cobalt, and nickel, which would interfere with the electrolysis process. After purification, concentrations of these impurities are limited to less than 0.05 milligram per liter (4x10-7 pounds per gallon). Purification is usually conducted in large agitated tanks. The process takes place at temperatures ranging from 40 to 85 °C (104 to 185 °F), and pressures ranging from atmospheric to . The by-products are sold for further refining.
The zinc sulfate solution must be very pure for electrowinning to be at all efficient. Impurities can change the decomposition voltage enough to where the electrolysis cell produces largely hydrogen gas rather than zinc metal.
Zinc is extracted from the purified zinc sulfate solution by electrowinning, which is a specialized form of electrolysis. The process works by passing an electric current through the solution in a series of cells. This causes the zinc to deposits on the cathodes (aluminium sheets) and oxygen to form at the anodes. Sulfuric acid is also formed in the process and reused in the leaching process. Every 24 to 48 hours, each cell is shut down, the zinc-coated cathodes are removed and rinsed, and the zinc is mechanically stripped from the aluminum plates.
Electrolytic zinc smelters contain as many as several hundred cells. A portion of the electrical energy is converted into heat, which increases the temperature of the electrolyte. Electrolytic cells operate at temperature ranges from 30 to 35°C (86 to 95°F) and at atmospheric pressure. A portion of the electrolyte is continuously circulated through the cooling towers both to cool and concentrate the electrolyte through evaporation of water. The cooled and concentrated electrolyte is then recycled to the cells. This process accounts for approximately 1/3 of all the energy usage when smelting zinc.
There are two common processes for electrowinning the metal: the low current density process, and the Tainton high current density process. The former uses a 10% sulfuric acid solution as the electolyte, with current density of 270–325 amperes per square meter. The latter uses 22–28% sulfuric acid solution as the electrolyte with a current density of about 1,000 amperes per square meter. The latter gives better purity and has higher production capacity per volume of electrolyte, but has the disadvantage of running hotter and being more corrosive to the vessel in which it is done. In either of the electrolytic processes, each metric ton of zinc production expends about of electric power.
The four types of commercial pyrometallurgical processes are the Belgian-type horizontal retort process, the New Jersey Zinc continuous vertical-retort process, the blast furnace process and the St. Joseph Minerals Corporation's process.
The process begins with a downdraft sintering operation. The sinter, which is a mixture of roaster calcine and EAF calcine, is loaded onto a gate type conveyor and then combustions gasses are pumped through the sinter. The carbon in the combustion gases react with some the impurities, such as lead, cadmium, and halides. These impurities are driven off into filtration bags. The sinter after this process, called product sinter, usually has a composition of 48% zinc, 8% iron, 5% aluminium, 4% silicon, 2.5% calcium, and smaller quantities of magnesium, lead, and other metals. The sinter product is then charged with coke into an electric retort furnace. A pair of graphite electrodes from the top and bottom furnace produce current flow through the mixture. The coke provides electrical resistance to the mixture in order to heat the mixture to and produce carbon monoxide. These conditions allow for the following chemical reaction to occur:
The zinc vapor and carbon dioxide pass to a vacuum condenser, where zinc is recovered by bubbling through a molten zinc bath. Over 95% of the zinc vapor leaving the retort is condensed to liquid zinc. The carbon dioxide is regenerated with carbon, and the carbon monoxide is recycled back to the retort furnace.
This process was developed by the Imperial Smelting Corporation at Avonmouth, England, in order to increase production, increase efficiency, and decrease labor and maintenance costs. L. J. Derham proposed using a spray of molten lead droplets to rapidly chill and absorb the zinc vapor, despite the high concentration of carbon dioxide. The mixture is then cooled, where the zinc separates from the lead. The first plant using this design opened up in 1950. One of the advantages of this process is that it can co-produce lead bullion and copper dross. In 1990, it accounted for 12% of the world's zinc production.
The process starts by charging solid sinter and heated coke into the top of the blast furnace. Preheated air at 190-1050 °C is blown into the bottom of the of the furnace. Zinc vapor and sulfides leave through the top and enter the condenser. Slag and lead collect at the bottom of the furnace and are tapped off regularly. The zinc is scrubbed from the vapor in the condenser via liquid lead. The liquid zinc is separated from the lead in the cooling circuit. Approximately of lead are required each year for this process, however this process recovers 25% more lead from the starting ores than other processes.
This process begins by roasting concentrates that are mixed with coal and briquetted in two stages. The briquettes are then heated in a autogenous coker at and then charged into the retort. There are three reasons to briquette the calcine: to ensure free downward movement of the charge; to permit heat transfer across a practical size cross-section; to allow adequate porosity for the passage of reduced zinc vapor to the top of the retort. The reduced zinc vapor that is collected at the top of the retort is then condensed to a liquid.
Overpelt improved upon this design by using only one large condensation chamber, instead of many small ones, as it was originally designed. This allowed for the carbon monoxide to be recirculated into the furnaces for heating the retorts.
This process was licensed to the Imperial Smelting Company (ISC), in Avonmouth, England, which had a large vertical retort (VR) plant in production for many years. It was used until the mid 1970's when it was superseded by the company's Imperial Smelting Furnace (ISF) plant. The VR plant was demolished in 1975.