Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998
Industry Description and Practices flexibility is needed. Both processes use dried concentrates. Electric smelting requires a roast- Primary nickel is produced from two very dif- ing step before smelting to reduce sulfur content ferent ores, lateritic and sulfidic. Lateritic ores are and volatiles. Older nickel-smelting processes, normally found in tropical climates where weath- such as blast or reverberatory furnaces, are no ering, with time, extracts and deposits the ore in longer acceptable because of low energy efficien- layers at varying depths below the surface. Lat- cies and environmental concerns. eritic ores are excavated using large earth-mov- In flash smelting, dry sulfide ore containing ing equipment and are screened to remove less than 1% moisture is fed to the furnace along boulders. Sulfidic ores, often found in conjunc- with preheated air, oxygen-enriched air (30–40% tion with copper-bearing ores, are mined from oxygen), or pure oxygen. Iron and sulfur are oxi- underground. Following is a description of the dized. The heat that results from exothermic re- processing steps used for the two types of ores. actions is adequate to smelt concentrate, producing a liquid matte (up to 45% nickel) and Lateritic Ore Processing a fluid slag. Furnace matte still contains iron and sulfur, and these are oxidized in the converting Lateritic ores have a high percentage of free and step to sulfur dioxide and iron oxide by injecting combined moisture, which must be removed. air or oxygen into the molten bath. Oxides form Drying removes free moisture; chemically bound a slag, which is skimmed off. Slags are processed water is removed by a reduction furnace, which in an electric furnace prior to discard to recover also reduces the nickel oxide. Lateritic ores have nickel. Process gases are cooled, and particulates no significant fuel value, and an electric furnace are then removed by gas-cleaning devices. is needed to obtain the high temperatures re- quired to accommodate the high magnesia con- Nickel Refining tent of the ore. Some laterite smelters add sulfur to the furnace to produce a matte for processing. Various processes are used to refine nickel matte. Most laterite nickel processers run the furnaces Fluid bed roasting and chlorine-hydrogen reduc- so as to reduce the iron content sufficiently to tion produce high-grade nickel oxides (more than produce ferronickel products. Hydrometallurgi- 95% nickel). Vapor processes such as the carbo- cal processes based on ammonia or sulfuric acid nyl process can be used to produce high-purity leach are also used. Ammonia leach is usually nickel pellets. In this process, copper and precious applied to the ore after the reduction roast step. metals remain as a pyrophoric residue that re- quires separate treatment. Use of electrical cells Sulfidic Ore Processing equipped with inert cathodes is the most com- mon technology for nickel refining. Electro- Flash smelting is the most common process in winning, in which nickel is removed from modern technology, but electric smelting is used solution in cells equipped with inert anodes, is for more complex raw materials when increased the more common refining process. Sulfuric acid
349 350 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES solutions or, less commonly, chloride electrolytes Solid Wastes and Sludges are used. The smelter contributes a slag that is a dense sili- Waste Characteristics cate. Sludges that require disposal will result when neutralized process effluents produce a Air Emissions precipitate.
Sulfur dioxide (SO2)is a major air pollutant emit- Pollution Prevention and Control ted in the roasting, smelting, and converting of sulfide ores. (Nickel sulfide concentrates contain Pollution prevention is always preferred to the
6–20% nickel and up to 30% sulfur.) SO2 releases use of end-of-pipe pollution control facilities. can be as high as 4 metric tons (t) of sulfur diox- Therefore, every attempt should be made to ide per metric ton of nickel produced, before con- incorporate cleaner production processes and fa- trols. Reverberatory furnaces and electric cilities to limit, at source, the quantity of pollut- furnaces produce SO2 concentrations of 0.5–2.0%, ants generated. while flash furnaces produce SO2 concentrations The choice of flash smelting over older tech- of over 10%—a distinct advantage for the con- nologies is the most significant means of reduc- version of the sulfur dioxide to sulfuric acid. Par- ing pollution at source. ticulate emission loads for various process steps Sulfur dioxide emissions can be controlled by: include 2.0–5.0 kilograms per metric ton (kg/t) • Recovery as sulfuric acid for the multiple hearth roaster; 0.5–2.0 kg/t for • Recovery as liquid sulfur dioxide (absorption the fluid bed roaster; 0.2–1.0 kg/t for the electric of clean, dry off-gas in water or chemical furnace; 1.0–2.0 kg/t for the Pierce-Smith con- absorbtion by ammonium bisulfite or dimethyl verter; and 0.4 kg/t for the dryer upstream of the aniline) flash furnace. Ammonia and hydrogen sulfide are • Recovery as elemental sulfur, using reductants pollutants associated with the ammonia leach such as hydrocarbons, carbon, or hydrogen process; hydrogen sulfide emissions are associ- sulfide ated with acid leaching processes. Highly toxic nickel carbonyl is a contaminant of concern in Toxic nickel carbonyl gas is normally not emit- the carbonyl refining process. Various process off- ted from the refining process because it is broken gases contain fine dust particles and volatilized down in decomposer towers. However, very strict impurities. Fugitive emissions occur at furnace precautions throughout the refining process are openings, launders, casting molds, and ladles required to prevent the escape of the nickel car- that carry molten product. The transport and bonyl into the workplace. Continuous monitor- handling of ores and concentrates produce wind- ing for the gas, with automatic isolation of any borne dust. area of the plant where the gas is detected, is re- quired. Impervious clothing is used to protect Liquid Effluents workers against contact of liquid nickel carbonyl with skin. Pyrometallurgical processes for processing Preventive measures for reducing emissions sulfidic ores are generally dry, and effluents are of particulate matter include encapsulation of of minor importance, although wet electrostatic furnaces and conveyors to avoid fugitive emis- precipitators (ESPs) are often used for gas treat- sions. Covered storage of raw materials should ment, and the resulting wastewater could have be considered. high metal concentrations. Process bleed streams Wet scrubbing should be avoided, and cool- may contain antimony, arsenic, or mercury. Large ing waters should be recirculated. Stormwaters quantities of water are used for slag granulation, should be collected and used in the process. Pro- but most of this water should be recycled. cess water used to transport granulated slag Nickel Smelting and Refining 351 should be recycled. To the extent possible, all pro- Group in making decisions regarding provision cess effluents should be returned to the process. of World Bank Group assistance. Any deviations from these levels must be described in the World Treatment Technologies Bank Group project documentation. The emis- sions levels given here can be consistently achieved The discharge of particulate matter emitted dur- by well-designed, well-operated, and well-main- ing drying, screening, roasting, smelting, and tained pollution control systems. converting is controlled by using cyclones fol- The guidelines are expressed as concentrations lowed by wet scrubbers, ESPs, or bag filters. Fab- to facilitate monitoring. Dilution of air emissions ric filters may require reduction of gas or effluents to achieve these guidelines is un- temperatures by, for example, dilution with low- acceptable. temperature gases from hoods used for fugitive All of the maximum levels should be achieved dust control. Preference should be given to the for at least 95% of the time that the plant or unit use of fabric filters over wet scrubbers. is operating, to be calculated as a proportion of Liquid effluents are used to slurry tailings to annual operating hours. the tailings ponds, which act as a reservoir for the storage and recycle of plant process water. Air Emissions However, there may be a need to treat bleed streams of some process effluents to prevent a The air emissions levels presented in Table 1 buildup of various impurities. Solid wastes from should be achieved. nickel sulfide ores often contain other metals such as copper and precious metals, and con- Liquid Effluents sideration should be given to further process- ing for their recovery. Slag can be used as The effluent emissions levels presented in Table construction material after nickel recovery, as 2 should be achieved. appropriate (e.g., return of converter slag to the furnace). Sanitary sewage effluents require treat- ment in a separate facility or discharge to a mu- Table 1. Air Emissions from Nickel Smelting nicipal sewer. (milligrams per normal cubic meter, unless otherwise specified) Modern plants using good industrial practices are able to achieve the pollutant loads described Parameter Maximum value below: The double-contact, double-absorption PM 20 plant should emit no more than 0.2 kg of sulfur Nickel 1 dioxide per metric ton of sulfuric acid produced Sulfur dioxide 2 kg/t sulfuric acid (based on a conversion efficiency of 99.7%).
Emission Guidelines Table 2. Effluents from Nickel Smelting Emissions levels for the design and operation of (milligrams per liter, except for pH) each project must be established through the en- Parameter Maximum value vironmental assessment (EA) process on the ba- sis of country legislation and the Pollution Prevention pH 6–9 and Abatement Handbook, as applied to local con- TSS 50 ditions. The emissions levels selected must be Nickel 0.5 justified in the EA and acceptable to the World Iron 3.5 Total metals 10 Bank Group. The following guidelines present emissions Note: Effluent requirements are for direct discharge to surface levels normally acceptable to the World Bank waters. 352 PROJECT GUIDELINES: INDUSTRY SECTOR GUIDELINES
Ambient Noise furnaces should only be used where regenera- tive energy is available. Noise abatement measures should achieve either • Choose oxygen enrichment processes that al- the levels given below or a maximum increase in low higher SO2 concentrations in smelter gases background levels of 3 decibels (measured on the to assist in sulfur recovery. A scale) [dB(A)]. Measurements are to be taken • Recover as much sulfur dioxide as possible by at noise receptors located outside the project producing sulfuric acid, liquid sulfur dioxide, property boundary. or other sulfur products. • Reuse process waters, recirculate cooling wa- Maximum allowable log ters, and use stormwater for the process. equivalent (hourly • Enclose processes and conveyors to mini- measurements), in dB(A) mize fugitive emissions; cover raw material Day Night storage. Receptor (07:00–22:00) (22:00–07:00)
Residential, Sources institutional, educational 55 45 Bounicore, Anthony J., and Wayne T. Davis, eds. 1992. Industrial, Air Pollution Engineering Manual. New York: Van commercial 70 70 Nostrand Reinhold. UNEP (United Nations Environment Programme). Key Issues 1993. Environmental Management of Nickel Production. A Technical Guide. Technical Report 15. Paris. The key production and control practices that will World Bank. 1995. “Industrial Pollution Prevention and lead to compliance with emission requirements Abatement: Nickel Smelting and Refining.” Draft can be summarized as follows: Technical Background Document. Environment • Use flash smelting for sulfidic ores; electric Department, Washington, D.C.