Nickel Smelting and Refining
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Multilateral Investment Guarantee Agency Environmental Guidelines for Nickel Smelting and Refining Industry Description and Practices flash smelting. Flash smelting is the most common process but electric smelting is used Primary nickel is produced from two very for more complex raw materials when increased different ores—lateritic and sulfidic. Lateritic flexibility is needed. Both processes use dried ores are normally found in tropical climates concentrates. Electric smelting requires a where weathering will, with time, extract and roasting step ahead of the smelter to reduce deposit it in layers at varying depths below the sulfur content and volatiles. Older nickel surface. Lateritic ores are excavated using large smelting processes, such as blast or earth moving equipment and are screened to reverberatory furnaces, are no longer acceptable remove boulders. Sulfidic ores, often found in because of low energy efficiencies and because conjunction with copper bearing ores, are of environmental concerns. mined from underground. Following is a In flash smelting, dry sulfide ore containing description of the processing steps used for the less than 1% moisture, is fed to the furnace two types of ores. along with pre-heated air, oxygen-enriched air (30-40% oxygen), or pure oxygen. Iron and Lateritic ore processing sulfur are oxidized, and the heat that results from exothermic reactions is adequate to smelt Lateritic ores have a high percentage of free and concentrate to produce a liquid matte (up to combined moisture which must be removed. 45% nickel) and a fluid slag. Furnace matte still Drying removes fee moisture and chemically contains iron and sulfur that are oxidized in the bound water is removed by a reduction furnace converting step to sulfur dioxide and iron oxide which also reduces the nickel oxide. Lateritic by injecting air or oxygen into the molten bath. ores have no significant fuel value and the Oxides form a slag which is skimmed off. Slags electric furnace is required to obtain the high are processed in an electric furnace prior to temperatures that are required to accommodate discard to recover nickel. Process gases are the high magnesia content of the ore. Some cooled followed by removal of particulates by laterite smelters add sulfur to the furnace to gas cleaning devices. produce a matte for processing. Other furnaces are operated to reduce iron content sufficiently Nickel refining to produce ferronickel products (the route adopted by most laterite nickel operators). Various processes are used to refine nickel Hydrometallurgical processes based on matte. Fluid bed roasting and a chlorine- ammonia or sulfuric acid leach are also used. hydrogen reduction will produce high grade Ammonia leach is usually applied to the ore nickel oxides (95%+ nickel). Vapor processes, after the reduction roast step. such as the carbonyl process, can be used to produce high purity nickel pellets. In this Sulfidic ore processing process, copper and precious metals remain as a pyrophoric residue requiring separate Modern technology uses electric smelting or treatment. Electrical cells equipped with inert 435 436 Nickel Smelting and Refining cathodes is the most common technology used process used for nickel. Chloride electrolytes are for nickel refining. Electrowinning, where nickel used although sulfuric acid solutions are used is removed form solution in cells equipped with more often. inert anodes is the more common refining slag granulation, but the majority should be Waste Characteristics recycled. Air emissions Solid wastes and sludges Sulfur dioxide (SO2) is a major air pollutant The smelter contributes a slag that is a dense emitted in the roasting, smelting and converting silicate. Sludges that require disposal will result of sulfide ores (nickel sulfide concentrates where neutralized process effluents produce a contain 6-20% nickel and up to 30% sulfur). precipitate. Sulfur dioxide releases can be as high as 4 metric tons of sulfur dioxide per metric ton of Pollution Prevention and Control nickel produced, before controls. Reverberatory furnaces and electric furnaces produce sulfur Pollution prevention is always preferred to the dioxide concentrations of 0.5-2.0% while flash use of end-of-pipe pollution control facilities. furnaces produce sulfur dioxide concentrations Therefore every attempt should be made to of over 10%—a distinct advantage for the incorporate cleaner production processes and conversion of the sulfur dioxide to sulfuric acid. facilities to limit, at source, the quantity of Particulate emission loads for various process pollutants generated. steps include: 2.0-5.0 kilograms per metric ton The choice of flash smelting over older (kg/t) for the multiple hearth roaster; 0.5-2.0 technologies is the most significant means to kg/t for the fluid bed roaster; 0.2-1.0 kg/t for reduce pollution at source. the electric furnace; 1.0-2.0 kg/t for the Pierce- Sulfur dioxide emissions can be controlled by: Smith converter; and 0.4 kg/t for the dryer • recovery as sulfuric acid; upstream of the flash furnace. Ammonia and • recovery as liquid SO2 (absorption of clean hydrogen sulfide are pollutants associated with dry off gas in water or chemically absorbed by the ammonia leach process; hydrogen sulfide ammonium bisulfite or dimethyl aniline); emissions are associated with acid leaching • recovery as elemental sulfur (using processes. Highly toxic nickel carbonyl is a reductants such as hydrocarbons, carbon or contaminant of concern in the carbonyl refining hydrogen sulfide). process. Various process off gases contain fine dust particles and volatilized impurities. Toxic nickel carbonyl gas is normally not Fugitive emissions will occur at furnace emitted from the refining process because it is openings, launders, casting molds and ladles decomposed in decomposer towers. However that are carrying molten product. The transport very strict precautions throughout the refining and handling of ores and concentrates produce process are required to prevent the escape of the wind borne dust. nickel carbonyl into the workplace. Continuous monitoring for the gas with automatic isolation Liquid effluents of any area of the plant where the gas is detected is required. Worker protection against Pyrometallurgical processes for processing liquid nickel carbonyl contact with skin is sulfidic ores are generally dry and effluents are afforded by impervious clothing. of minor importance although wet electrostatic Prevention measures for reducing emissions precipitators (ESPs) are often used for gas of particulate matter include encapsulation of treatment and a resulting wastewater could furnaces and conveyors to avoid fugitive have high metal concentrations. Process bleed emissions. Covered storage of raw materials streams may contain antimony, arsenic or should be considered. mercury. Large quantities of water are used for 437 Nickel Smelting and Refining Wet scrubbimg should be avoided and must be justified in the EA and acceptable to cooling waters should be recirculated. Storm MIGA. waters should be collected and used in the The following guidelines present emission process. Process water used to transport levels normally acceptable to the World Bank granulated slag should be recycled. To the Group in making decisions regarding provision extent possible, all process effluents should be of World Bank Group assistance, including returned to the process. MIGA guarantees; any deviations from these levels must be described in the project Treatment Technologies documentation. The guidelines are expressed as The discharge of particulate matter emitted concentrations to facilitate monitoring. Dilution during drying, screening, roasting, smelting and of air emissions or effluents to achieve these converting is controlled by using cyclones guidelines is unacceptable. followed by wet scrubbers, ESPs, or bag filters. All of the maximum levels should be Fabric filters may require reduction of gas achieved for at least 95% of the time that the temperatures by, for example, or dilution with plant or unit is operating, to be calculated as a low temperature gases from hoods used for proportion of annual operating hours. fugitive dust control. Preference should be given to the use of fabric filters over wet Air Emissions scrubbers. Liquid effluents are used to slurry tailings to For nickel smelters the following air emissions the tailings ponds which act as a reservoir for levels should be achieved: the storage and recycle of plant process water. However, there may be a need to treat bleed Air Emissions from Nickel Smelting streams of some process effluents to prevent a Parameter Maximum value build-up of various impurities. Solid wastes milligrams per normal from nickel sulfide ores often contain other cubic meter (mg/Nm3) metals such as copper, and precious metals, and Particulates 20 consideration should be given to further processing for their recovery. Slag can be used Nickel 1 as construction material after nickel recovery as Sulfur dioxide 2 kg/t sulfuric acid appropriate (e.g. return of converter slag to the furnace). Sanitary sewage effluents require Liquid Effluents treatment in a separate facility or discharged to a municipal sewer. For nickel smelters the following effluent Modern plants using good industrial emissions levels should be achieved: practices are able to achieve the following in terms of pollutant loads: The double contact Effluents from Nickel Smelting double absorption plant should emit no more Parameter Maximum value than