Environmental Code of Practice for base metals smelters and refineries: chapter 2

2: Operational Activities

This section describes the major activities involved in the operation of base metals smelters and refineries. It is not intended to be an all-inclusive list of operational activities of potential environmental significance, nor are all activities and techniques necessarily applicable to all base metals smelters and refineries. Rather, the intent is to identify the nature and scope of activities addressed in the Code with emphasis on those activities that relate to the possible adverse environmental impacts and to the mitigative measures that are discussed in Sections 3.0 and 4.0.

The main processes and common techniques involved in the extraction and refining of base metals generally proceed as shown in Figure 2.

Figure 2: Processes involved in the extraction and refining of base metals28

Figure 2: Processes involved in the extraction and refining of base metals

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The key metal recovery technologies that are used to produce refined metals are:

  1. Pyrometallurgical technologies, which separate desired metals from other less desirable or undesirable materials in the molten state at very high temperatures. These processes capitalize on the differences between constituent oxidation potential, melting point, vapour pressure, density, and/or miscibility when melted.
  2. Hydrometallurgical technologies, which differ from pyrometallurgical processes in that the desired metals are separated from undesirables using techniques that capitalize on differences between constituent solubilities and/or electrochemical properties in acid or basic solutions at temperatures generally below 300°C.
  3. Vapo-metallurgical technologies, which apply to the Inco Carbonyl Process, whereby nickel alloys are treated with carbon monoxide gas to form nickel carbonyl.

Sulphur dioxide can be captured during pyrometallurgical processing and recovered as sulphuric acid.

Primary smelting and refining processes produce metals directly from ores, while secondary smelting and refining processes produce metals from scrap and process wastes. Most primary smelters have the technical capability to supplement primary feed with recyclable materials. Several have done so. Examples of scrap feedstock include post-consumer goods, such as telephone and computer components, metal parts, bars, turnings, sheets, and wire that is off-specification or worn out. Lead has the largest and most developed recycling component, resulting primarily from the relatively short product life of lead acid batteries and the relative ease with which batteries can be segregated at source for collection and recycling.

Base metals include copper, lead, nickel and zinc. Depending upon the origin of the ore or scrap metal and its residual metals content, various metals, such as gold, silver, indium, germanium, cadmium, bismuth, and selenium, may be recovered as co-products. A general overview of the major processes currently employed by the base metals smelting sector is given in this section. Canadian site-specific flow sheets and process descriptions for existing facilities can be found in the report Review of Environmental Releases for the Base MetalsSmelting Sector, prepared for Environment Canada by Hatch Associates Ltd. and dated November 3, 2000.

2.1 Pretreatment

Pretreatment of feed materials includes drying of slurry concentrates, milling, sorting and separation of scrap material, and feed proportioning. Pretreatment is conducted to ensure that feeds are in appropriate condition and proportions for initial processing.

2.2 Roasting

Roasting is the conventional technique used in the pyrometallurgical processing of copper, nickel, and zinc sulphide concentrates. During roasting, the sulphur is removed by adding air and simultaneously heating and drying the concentrate to achieve sulphur content favourable for smelting. The sulphur is released as sulphur dioxide. Roasting to completion eliminates the sulphur and produces the metal oxide for reduction by carbon or carbon monoxide or for leaching in a sulphuric acid solution followed by electrowinning. Incomplete roasting is used to remove excess sulphur in copper and nickel sulphides in preparation for the matte smelting process. The sulphur dioxide in the process off-gas is usually recovered as sulphuric acid or, sometimes, liquefied sulphur dioxide.

2.3 Smelting

Smelting serves two functions: first, to melt the concentrates to a molten state, and second, to separate the metal of value from other less desirable metals, impurities, and gangue materials.

Concentrates are fed to the furnace along with fluxing agents, fuel (oil, natural gas), and oxygen (in the form of air, pure oxygen, or oxygen-enriched air). High temperatures from combustion and oxidation in the smelting furnace cause the feed materials to melt. Separation of the metal of value from other impurities and gangue materials occurs through fluxing, where the siliceous fluxing agent forms a silica-iron-sulphur slag. Some impurities (e.g., sulphur, some metal compounds) are also separated from the metal of value through oxidation and volatilization.

The resulting product from a smelter is a molten matte or bullion containing a high concentration of the metal of value.

Layers of matte or bullion and slag are tapped from the furnace, or, in the case of continuous processes (e.g., Mitsubishi process), the materials travel through covered gravity-flow launders to the next processing stage. Primary and secondary releases of process off-gases are captured through direct ductwork from the furnace and/or overhead canopies. The collected off-gases are treated by a gas conditioning system, which may include removal of sulphur dioxide, particulate matter, fume, etc. Smelter slags are treated or "cleaned" to recover any remaining metal of value.29

2.4 Converting

Converting is used primarily for copper and nickel matte processing and serves to remove residual sulphur and iron in the matte from the smelter. Converters also have the capability of processing high-grade scrap materials. Both continuous and batch converting processes are used in Canada. Air or oxygen-enriched air is blown through the matte, generating off-gases containing sulphur dioxide and volatile metals such as lead and zinc. Continuous converting allows for better capture of process off-gases and a consistent and/or higher concentration of sulphur dioxide, enabling capture of the sulphur dioxide through the production of sulphuric acid. Converting produces blister copper, named for the blisters of air/oxygen trapped in the molten material. Slag from the converter typically has a high copper concentration and can be returned to the smelting furnace for recovery of the copper.

2.5 Fire Refining (Anode Refining)

Prior to final casting or electrorefining, impurities must be further removed from metals. This process is sometimes used in the production of copper in Canada. Fire refining lowers the sulphur and oxygen levels in blister copper and removes the impurities as slag or volatile products. Reverberatory or rotary furnaces are used. First, air is blown through the molten mixture to oxidize the copper and volatilize the sulphur impurities, producing a small amount of slag. Sodium carbonate flux may be added to remove arsenic and antimony. Then the copper is reduced by a process known as "poling" with green wood poles or by feeding ammonia or natural gas to remove the oxygen, forming the purer copper to be cast as anodes.

2.6 Electorefining

Electrorefining produces a purified metal from a less pure metal. This process is used to refine copper, nickel, and lead in Canada. The metal to be purified is cast as an anode and placed in an electrolytic cell. A current is applied, and the metal is dissolved into an acidic aqueous electrolyte or molten salt. The pure metal is electroplated or deposited on the starter plates, which act as the cathodes. Metallic impurities either dissolve in the electrolyte or precipitate out and form a sludge. These anode slimes contain precious metals such as silver, gold, and tellurium and are recovered. The cathode deposits are washed, then cast into bars, ingots, or slabs for sale.

2.7 Carbonyl Refining

The carbonyl process is used for refining crude nickel oxide. Carbon monoxide and the crude nickel react to form nickel carbonyl at high pressure. The volatile and highly toxic nickel carbonyl is refined by separation of solid impurities. With further heating, the carbon monoxide separates, and high-purity nickel powder or pellets are formed. The solid impurities contain copper and precious metals, which are recovered. The generated carbon monoxide off-gases are recycled back to the process.

2.8 Leaching

Leaching requires the use of an acid or other solvent to dissolve the metal content of ores and concentrates before refining and electrowinning. Leaching is usually conducted using material in the form of an oxide, either an oxidic ore or an oxide produced by roasting. Sulphidic ores can also be leached, but require conditions that promote oxidation of the ore or concentrate, such as high pressure, presence of bacteria, and/or the addition of oxygen, chlorine, or ferric chloride. The pregnant leach solution is processed by solvent extraction and is purified. The purified solution is then used for electrowinning and refining of the metal.

2.9 Electrowinning

Electrowinning is used to capture metal dissolved in the pregnant solution produced during leaching (purified electrolyte). Electrowinning is used for refining zinc, copper, nickel, and cobalt in Canada. The process is conducted in tank houses, with electrolytic cells containing the purified electrolyte, inert anodes, and starter cathodes of the pure metal (for copper refining) or permanent cathodes of stainless steel or aluminum. Electric current is passed through the cell, and the dissolved metal ions (metal of value) are deposited onto the cathode. Oxygen gas, acid mist, and spent electrolyte (acid) are generated through the electrowinning process. The spent electrolyte is returned to the leaching process. After a sufficient time lapse, the cathodes are removed. Either the cathodes are sold directly or the metal is stripped from the cathode, melted, and cast.

2.10 Casting

The casting process involves melting the metal and passing it through a holding furnace and into the caster, where billets, blocks, slabs or cakes, and rods are produced. Casting can be done continuously or in batches. Stationary casting uses a casting wheel with a series of moulds, which can be on the circumference of a rotating table that passes through a series of cooling water jets. Continuous casting involves the production of a continuous bar or rod for reduction to wire. The bar or rod is cut into shape using shears or by casting in special side dam blocks spaced in defined intervals in the caster. The billets can be heated, then extruded and drawn into tubes. Slabs or cakes are preheated and rolled into sheets and strips. Ingots are produced using a fixed mould casting process.

2.11 Process Off-Gas Conditioning

Off-gases from smelting facilities typically contain sulphur dioxide, particulate matter, fume (e.g., volatile metals), and other pollutants of concern, such as carbon dioxide, nitrogen oxides, and organics. Off-gases are treated to remove sulphur dioxide, particulate matter, and/or other pollutants before being released to ambient air.

For removal of particulate matter and dust, cyclones are used to remove medium- to large-sized particles. Cyclones are not considered sufficient control devices on their own to remove particulate matter. Other control devices with greater dust removal efficiencies include electrostatic precipitators, either hot or wet, and fabric filter baghouses. Hot electrostatic precipitators can withstand higher off-gas temperatures than fabric filter baghouses. However, baghouses can achieve greater dust collection efficiencies than hot electrostatic precipitators.30 Scrubbers are also used to remove both dust and soluble or acidic gases from the off-gas stream.

Process off-gases with a minimum sulphur dioxide concentration of 5-7% can be used for the manufacture of sulphuric acid and thus can remove the sulphur dioxide from the off-gas stream. Double-contact acid plants are able to achieve a higher rate of conversion of sulphur dioxide in the process off-gas to sulphuric acid compared with single-contact acid plants.

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