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Guidance Document for Management of Wastes from the Base Metals Smelting Sector

Summary

Environment Canada and Health Canada have joint responsibility for the effective management of substances declared toxic under the Canadian Environmental Protection Act 1999 (CEPA 1999). This provides for actions, including regulations, relating to the quantity or concentration of a toxic substance that may be released to the environment.

The production of base metals can result in the releases of pollutants (e.g., sulphur dioxide, heavy metals such as cadmium, lead and mercury) to the environment. Released pollutants can be controlled by air pollution control devices or waste water treatment plants. Collected dusts and sludges can have appreciable amounts of heavy metals. There are other materials (e.g., slags) which are produced as part of the smelting and refining process. Minimizing or avoiding the creation of these wastes will lead to reduced impacts to the environment and human health. Where avoidance is not possible, the proper management of these wastes is required.

The determination of what is a waste and what can be recovered will vary from facility to facility depending upon a number of technical, environmental and economic factors.

Definitions

For the purposes of this report, the following definitions apply. The various definitions are schematically shown in Figure 1.

Figure 1: Process Schematic Showing Waste Generation

Figure 1: Process Schematic Showing Waste Generation

The selection of these definitions does not over-ride any regulatory requirements by international, federal, provincial, territorial or municipal agencies. It is recognized that the classification of wastes/residues/by-products does vary between jurisdictions.

Residues are defined as "materials which are generated as a result of industrial processes which can be recycled/recovered or disposed of."

By-products are defined as residues that are in a state that can be recovered, reused or recycled in accordance with environmentally sound management practices.

"However, residues and by-products are essentially recyclable materials. This is the nomenclature that is used for the purpose of imports, exports, transits and domestic movements in Canada."1

Wastes are, therefore, defined as residues and/or by-products that are not recovered, reused or recycled and are discarded. This is consistent with the Basel Convention definition, where wastes are defined as: "wastes are substances or objects which are disposed of or are intended to be disposed of or are required to be disposed of by the provisions of the national law."2

Recovery is defined as the reclamation of valuable material and/or energy from residues. Recovery, the umbrella term, can be accomplished by recycling or re-use of the residue.

  • Recycling is defined as measures which return specific residues to a previous stage of a cyclic process. The residues may or may not undergo treatment prior to recycling.
  • Re-use is defined as the successive use of residues by subsequent users. Between subsequent users, the residues may or may not undergo treatment.

Wastes Generated

The production of non-ferrous metals from primary and secondary material results in the generation of a wide variety of wastes. They are a result of the metals separation that is necessary for the production of pure metals from complex sources. These wastes arise from the different stages of processing as well as from the off-gas and water treatment systems. The following paragraphs described the key wastes which result from the production of non-ferrous metals.

Slag is produced by the reaction of slag-forming elements (e.g. iron) in the ore with added fluxes. In the smelting process, the slag is liquid and has a different density than the melted metal and can, therefore, be tapped off separately. It is either rapidly quenched with water to form granules that can be re-used or discarded or alternatively the slag is transported in a liquid state to a cleaning operation or slag dump.

Drosses and skimmings result from the oxidation of metals at the bath surface or by reactions with fireproof material used as furnace linings.

Spent linings and refractories result when refractory material falls out of the furnace linings or when the furnace lining has to be replaced completely.

Pollution abatement systems wastes and residues include flue gas dust and sludge recovered from the air pollution control equipment as well as other solid materials like spent filter material. Sulphuric acid and liquid sulphur dioxide are also wastes/residues from pollution abatement systems.

Liquid effluent treatment wastes and residues generally result from treatment of waste water streams. Process water from processing usually requires cleaning in a wastewater treatment plant. The cleaning takes place by neutralization and precipitation of specific ions. The main wastes/residues from these effluent treatment systems are gypsum (CaSO4), and metal hydroxides and sulphides.

Hydrometallurgical wastes and residues include sludge generated in the leaching process, while purification and electrolysis processes can generate metal rich solids such as anode slime.

Other wastes and residues typically produced are oils and greases and industrial scrap such as steel and wood wastes.

Management Options

The following table details typical management options for BMS residues.

Table 1: Typical Wastes and Residue Management Options
SourceAssociated MetalsWaste/ResidueAvailable Options

Raw materials handling

 

All metalsDust, SweepingsFeed for main process
Smelting furnaceAll metalsSlagConstruction material after slag treatment if/as necessary, abrasive industry, refractory material, mine backfill, slag dump
Converting furnaceCopperSlagRecycle to smelting
Refining furnacesCopperSlagRecycle to smelting
 LeadSkimmingsRecovery of other valuable metals
 Precious metalsSkimmings and slagInternal recycle
Slag TreatmentCopper and NickelCleaned slagConstruction material, abrasive industry, refractory material, mine backfill, slag dump
Melting furnaceAll metalsSkimmingsRecycle to process after treatment
  Skimmings and salt slagMetal recovery, recovery of salt and other material
Electro-refiningCopperElectrolyte bleedRecovery of Nickel
  Anode ScrapRecycle to smelter if contaminated, or to melting furnace if clean
  Anode SlimeRecovery of precious metals
Electro-winningZinc, Nickel, Cobalt, Precious Metals

Spent electrolyte

 

Re-use in leaching process or disposal

 

LeachingZincIron wastesSafe disposal, re-use of liquor
 CopperWastesSafe disposal
 Nickel/cobaltCopper/Iron wastes/residuesRecovery, disposal
Sulphuric acid plantAll metalsCatalystRegeneration
  Acid sludgesSafe disposal
  Weak acidNeutralization, safe disposal of sludge, discharge of water
Furnace liningsAll metalsRefractoryUsed as slagging agent, disposal
PicklingCopperSpent acidRecovery
Dry abatement systemsMost-using fabric filters or ESPs

Filter dust

 

Recycle to process, recovery of other metals
Wet abatement systemsMost-using scrubbers or wet ESPs

Filter sludge

 

Return to process or recovery of other metals (mercury). Disposal
Wastewater treatment sludgeMostHydroxide or sulphide sludge

Recycle or Safe disposal

 

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Best Techniques for Minimization of Wastes and Residues Generated

Slag, Skimmings and Drosses - The amount of slag, skimmings and drosses produced is dependent on the impurities in the raw materials and typically cannot be significantly reduced. In some cases, the selection of cleaner raw materials lead to reduced generation of these residues. The limited availability and higher costs of these concentrates means this is not a global solution. It should be noted that higher grade concentrates could mean that more impurities have been removed during the milling process and that increased residues (i.e., tailings) would result at this earlier stage of the metals production.

The generation of slags, skimmings and drosses can be minimized by optimizing furnace operations.

Spent Linings and Refractories - Spent linings and refractories cannot be avoided, but can be minimized by the following measures:

  • Careful construction of the brick lining of the furnace;
  • Continuous use of furnace to minimize variations in temperature;
  • Shorten impact time of fluxing agents;
  • Avoidance of aggressive fluxing agents;
  • Careful cleaning of furnaces and crucibles; and
  • Reduction of furnace agitation.

Wastes and Residues from Pollution Abatement Systems - Volumes of flue gas produced can often be reduced by using oxygen or oxygen enriched air for combustion instead of atmospheric air. The lower off-gas flow results in a reduction of wastes and residues generated.

The quantity of spent filter bags may be decreased by using modern filter materials that are more robust.

Wastes and Residues from Effluent Treatment - The amount of wastewater that is generated by a wet scrubber can be reduced if the water is recycled back to the scrubber. A bleed stream is still required to avoid a build-up of material.

Wastes and Residues from Hydrometallurgical Processes - The amount of jarosite generated can be minimized by optimizing the flotation process at the mill to increase zinc recovery and the extent of iron rejection, thereby reducing the quantity of iron waste generated.

Wastes/residues from zinc refining can also be reduced by switching to the goethite process for zinc refining. The goethite process uses a different precipitation technique and the resulting residue from the goethite process is approximately 25 to 30% less by volume than the residue generated from the traditional jarosite process.


Recommendations

This report represents an analysis of information on the generation of wastes from the Canadian Base Metals Smelting Sector and potential techniques for the minimization and management of these wastes. The recommendations establish a general direction and are, therefore, general in nature. The recommendations need to be tailored by each facility to reflect facility-specific considerations. When applied to a specific facility, these recommendations should lead to successful minimization, recycle and re-use of residues and minimization of wastes.

Best Techniques for Recycling

Efforts should be made to recycle residues internally.

Dusts resulting from off-gas treatment systems should be recycled to smelting furnaces.

Slags should be treated to recover metals which can be recycled to the process.

Best Techniques for Re-use

Re-use options should be identified for all residues which cannot be recycled on site. Not all materials will be re-usable but consideration should be given to re-use options.

Alternative uses of residues should be explored and marketed where feasible.

As recommended in the Base Metals Smelting Sector Strategic Options Report, barriers to the re-use of residues should be reviewed and modified to increase the level of re-use of residues.

Best Techniques for Disposal

Disposal should only be considered when there are no possibilities for recycle or re-use.

Wastes should be disposed of in an environmentally-sound manner.

Wastes should be fixed, if required, to eliminate/reduce leaching potential prior to disposal. The leaching potential should be determined using standard leaching protocols as identified in the applicable regulations.

Consideration should be given to the eventual decommissioning of on-site disposal areas.


1 Environment Canada, Waste Management Branch. Communication from J. Wittwer to M.A. Crichton, February 2004.

2 Basel Convention Glossary of Terms

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