Primary Copper Smeltinghttp://www.ltlu.com/ 2009-4-26 13:27:48
[Brief] Primary Copper Smelting-gangtou copper smelting furnace Manufacturer
Primary Copper Smelting
Copper ore is produced in 13 states. In 1989, Arizona produced 60 percent of the total U. S. ore. Fourteen domestic mines accounted for more than 95 percent of the 1.45 megagrams (Mg)(1.6 millon tons) of ore produced in 1991.Copper is produced in the U. S. primarily by pyrometallurgical smelting methods. Pyrometallurgical techniques use heat to separate copper from copper sulfide ore concentrates. Process steps include mining, concentration, roasting, smelting, converting, and finally fire and electrolytic refining.
2 Process Description2-4
Mining produces ores with less than 1 percent copper. Concentration is accomplished at the mine sites by crushing, grinding, and flotation purification, resulting in ore with 15 to 35 percent copper. A continuous process called floatation, which uses water, various flotation chemicals, and compressed air, separates the ore into fractions. Depending upon the chemicals used, some minerals float to the surface and are removed in a foam of air bubbles, while others sink and are reprocessed. Pine oils, cresylic acid, and long-chain alcohols are used for the flotation of copper ores. The flotation concentrates are then dewatered by clarification and filtration, resulting in 10 to 15 percent water, 25 percent sulfur, 25 percent iron, and varying quantities of arsenic, antimony, bismuth, cadmium, lead, selenium, magnesium, aluminum, cobalt, tin, nickel, tellurium, silver, gold, and palladium.
A typical pyrometallurgical copper smelting process, as illustrated in Figure 12.3-1, includes 4 steps: roasting, smelting, concentrating, and fire refining. Ore concentration is roasted to reduce impurities, including sulfur, antimony, arsenic, and lead. The roasted product, calcine, serves as a dried and heated charge for the smelting furnace. Smelting of roasted (calcine feed) or unroasted (green feed) ore concentrate produces matte, a molten mixture of copper sulfide (Cu2S), iron sulfide (FeS), and some heavy metals. Converting the matte yields a high-grade blister copper, with 98.5 to 99.5 percent copper. Typically, blister copper is then fire-refined in an anode furnace, cast into anodes, and sent to an electrolytic refinery for further impurity elimination.
Roasting is performed in copper smelters prior to charging reverberatory furnaces. In roasting, charge material of copper concentrate mixed with a siliceous flux (often a low-grade copper ore) is heated in air to about 650°C (1200°F), eliminating 20 to 50 percent of the sulfur as sulfur dioxide (SO2). Portions of impurities such as antimony, arsenic, and lead are driven off, and some iron is converted to iron oxide. Roasters are either multiple hearth or fluidized bed; multiple hearth roasters accept moist concentrate, whereas fluidized bed roasters are fed finely ground material. Both roaster types have self-generating energy by the exothermic oxidation of hydrogen sulfide, shown in the reaction below.
H2S O2 ® SO2 H2O Thermal energy (1)
In the smelting process, either hot calcine from the roaster or raw unroasted concentrate is melted with siliceous flux in a smelting furnace to produce copper matte. The required heat comes from partial oxidation of the sulfide charge and from burning external fuel. Most of the iron and
some of the impurities in the charge oxidize with the fluxes to form a slag on top of the molten bath, which is periodically removed and discarded. Copper matte remains in the furnace until tapped. Matte ranges from 35 to 65 percent copper, with 45 percent the most common. The copper content percentage is referred to as the matte grade. The 4 smelting furnace technologies used in the U. S. are reverberatory, electric, Noranda, and flash.
The reverberatory furnace smelting operation is a continuous process, with frequent charging and periodic tapping of matte, as well as skimming slag. Heat is supplied by natural gas, with conversion to oil during gas restrictions. Furnace temperature may exceed 1500°C (2730°F), with the heat being transmitted by radiation from the burner flame, furnace walls, and roof into the charge of roasted and unroasted materials mixed with flux. Stable copper sulfide (Cu2S) and stable FeS form the matte with excess sulfur leaving as sulfur dioxide.
Electric arc furnace smelters generate heat with carbon electrodes that are lowered through the furnace roof and submerged in the slag layer of the molten bath. The feed consists of dried concentrates or calcine. The chemical and physical changes occurring in the molten bath are similar to those occurring in the molten bath of a reverberatory furnace. The matte and slag tapping practices are also similar.
The Noranda process, as originally designed, allowed the continuous production of blister copper in a single vessel by effectively combining roasting, smelting, and converting into 1 operation. Metallurgical problems, however, led to the operation of these reactors for the production of copper matte. The Noranda process uses heat generated by the exothermic oxidation of hydrogen sulfide. Additional heat is supplied by oil burners or by coal mixed with the ore concentrates. Figure 12.3-2 illustrates the Noranda process reactor.
Flash furnace smelting combines the operations of roasting and smelting to produce a highgrade copper matte from concentrates and flux. In flash smelting, dried ore concentrates and finely ground fluxes are injected together with oxygen and preheated air (or a mixture of both), into a furnace maintained at approximately 1000°C (1830°F). As with the Noranda process reactor, and in contrast to reverberatory and electric furnaces, flash furnaces use the heat generated from partial oxidation of their sulfide charge to provide much or all of the required heat.
Slag produced by flash furnace operations contains significantly higher amounts of copper than reverberatory or electric furnaces. Flash furnace slag is treated in a slag cleaning furnace with coke or iron sulfide. Because copper has a higher affinity for sulfur than oxygen, the copper in the slag (as copper oxide) is converted to copper sulfide. The copper sulfide is removed and the remaining slag is discarded.
Converting produces blister copper by eliminating the remaining iron and sulfur present in the matte. All but one U. S. smelter uses Pierce-Smith converters, which are refractory-lined cylindrical steel shells mounted on trunnions at either end, and rotated about the major axis for charging and pouring. An opening in the center of the converter functions as a mouth through which molten matte, siliceous flux, and scrap copper are charged and gaseous products are vented. Air, or oxygen-rich air, is blown through the molten matte. Iron sulfide is oxidized to form iron oxide (FeO) and SO2. Blowing and slag skimming continue until an adequate amount of relatively pure Cu2S, called "white metal", accumulates in the bottom of the converter. A final air blast (final blow) oxidizes the copper sulfide to SO2, and blister copper forms, containing 98 to 99 percent coppers. The blister copper is removed from the converter for subsequent refining. The SO2 produced throughout the operation is vented to pollution control devices.
One domestic smelter uses Hoboken converters. The Hoboken converter, unlike the Pierce- Smith converter, is fitted with an inverted u-shaped side flue at one end to siphon gases from the interior of the converter directly to an offgas collection system. The siphon results in a slight vacuum at the converter mouth.
Impurities in blister copper may include gold, silver, antimony, arsenic, bismuth, iron, lead, nickel, selenium, sulfur, tellurium, and zinc. Fire refining and electrolytic refining are used to purify blister copper even further. In fire refining, blister copper is usually mixed with flux and charged into the furnace, which is maintained at 1100°C (2010°F). Air is blown through the molten mixture to oxidize the copper and any remaining impurities. The impurities are removed as slag. The remaining copper oxide is then subjected to a reducing atmosphere to form purer copper. The fire-refined copper is then cast into anodes for even further purification by electrolytic refining.
Electrolytic refining separates copper from impurities by electrolysis in a solution containing copper sulfate (Cu2SO4) and sulfuric acid (H2SO4). The copper anode is dissolved and deposited at the cathode. As the copper anode dissolves, metallic impurities precipitate and form a sludge. Cathode copper, 99.95 to 99.96 percent pure, is then cast into bars, ingots, or slabs.
3 Emissions And Controls
Emissions from primary copper smelters are principally particulate matter and sulfur oxides (SOx). Emissions are generated from the roasters, smelting furnaces, and converters. Fugitive emissions are generated during material handling operations.
Roasters, smelting furnaces, and converters are sources of both particulate matter and SOx. Copper and iron oxides are the primary constituents of the particulate matter, but other oxides, such as arsenic, antimony, cadmium, lead, mercury, and zinc, may also be present, along with metallic sulfates and sulfuric acid mist. Fuel combustion products also contribute to the particulate emissions from multiple hearth roasters and reverberatory furnaces.
Gas effluent from roasters usually are sent to an electrostatic precipitator (ESP) or spray chamber/ESP system or are combined with smelter furnace gas effluent before particulate collection. Overall, the hot ESPs remove only 20 to 80 percent of the total particulate (condensed and vapor) present in the gas. Cold ESPs may remove more than 95 percent of the total particulate present in the gas. Particulate collection systems for smelting furnaces are similar to those for roasters. Reverberatory furnace off-gases are usually routed through waste heat boilers and low-velocity balloon flues to recover large particles and heat, then are routed through an ESP or spray chamber/ESP system.
In the standard Pierce-Smith converter, flue gases are captured during the blowing phase by the primary hood over the converter mouth. To prevent the hood from binding to the converter with splashing molten metal, a gap exists between the hood and the vessel. During charging and pouring operations, significant fugitives may be emitted when the hood is removed to allow crane access. Converter off-gases are treated in ESPs to remove particulate matter, and in sulfuric acid plants to remove SO2.
Remaining smelter operations process material containing very little sulfur, resulting in insignificant SO2 emissions. Particulate may be emitted from fire refining operations. Electrolytic refining does not produce emissions unless the associated sulfuric acid tanks are open to the atmosphere. Crushing and grinding systems used in ore, flux, and slag processing also contribute to fugitive dust problems.
Control of SO2 from smelters is commonly performed in a sulfuric acid plant. Use of a sulfuric acid plant to treat copper smelter effluent gas streams requires that particulate-free gas containing minimum SO2 concentrations, usually of at least 3 percent SO2, be maintained. Table 12.3-1 shows typical average SO2 concentrations from the various smelter units. Additional information on the operation of sulfuric acid plants is discussed in Section 8.10 of this document. Sulfuric acid plants also treat converter gas effluent. Some multiple hearth and all fluidized bed roasters use sulfuric acid plants. Reverberatory furnace effluent contains minimal SO2 and is usually released directly to the atmosphere with no SO2 reduction. Effluent from the other types of smelter furnaces contain higher concentrations of SO2 and are treated in sulfuric acid plants before being vented. Single-contact sulfuric acid plants achieve 92.5 to 98 percent conversion of plant effluent gas. Double-contact acid plants collect from 98 to more than 99 percent of the SO2, emitting about 500 parts per million (ppm) SO2. Absorption of the SO2 in dimethylaniline (DMA) solution has also been used in domestic smelters to produce liquid SO2.
Particular emissions vary depending upon configuration of the smelting equipment. Tables 12.3-2 and 12.3-3 give the emission factors for various smelter configurations, and Tables 12.3-4, 12.3-5, 12.3-6, 12.3-7, 12.3-8, and 12.3-9 give size-specific emission factors for those copper production processes where information is available.
Roasting, smelting, converting, fire refining, and slag cleaning are potential fugitive emission sources. Tables 12.3-10 and 12.3-11 present fugitive emission factors for these sources. Tables 12.3-12, 12.3-13, 12.3-14, 12.3-15, 12.3-16, and 12.3-17 present cumulative size-specific particulate emission factors for fugitive emissions from reverberatory furnace matte tapping, slag tapping, and converter slag and copper blow operations. The actual quantities of emissions from these sources depend on the type and condition of the equipment and on the smelter operating techniques.
Fugitive emissions are generated during the discharge and transfer of hot calcine from multiple hearth roasters. Fluid bed roasting is a closed loop operation, and has negligible fugitive emissions. Matte tapping and slag skimming operations are sources of fugitive emissions from smelting furnaces. Fugitive emissions can also result from charging of a smelting furnace or from leaks, depending upon the furnace type and condition.