IS METAL RECYCLING SUSTAINABLE? By Steven Wright1*, Sharif Jahanshahi1, Frank Jorgensen1 and David Brennan2 1
CSIRO Minerals, Box 312, Clayton South, Vic. 3169 Australia
2
Department of Chemical Engineering, Monash University, Clayton, Vic. 3800 Australia
ABSTRACT The recycling of metals can be very efficient with careful selection of the material being recycled. The volume and value of recycled ferrous scrap has lead to the development of steel mini-mills which in turn has caused major changes to the steel industry. Although much smaller in volume, the recycling of lead is very significant, with battery recycling accounting for 60% of annual world production.
This high
recycle rate can be attributed to the relatively uniform feed-stock from lead-acid batteries. The recycling rates of other metals are considerably lower due to longer use and non-uniform product usage that results in less efficient segregation and processing. It is clear that recycling of metals has advantages but its sustainability is uncertain. What are the key benefits of metal recycling? Metal recycling has technical, economic and social impacts. It is relatively easy to assess recycling on a technical basis, but the social impacts are harder to evaluate. This paper examines some of the issues in evaluating the sustainability of metal recycling.
INTRODUCTION It is arguable that for as long as human society has been able to smelt metals from their ores and produce useful tools, metal remelting or recycling has occurred. Recycling is seen as one of the ways for the minerals sector to be sustainable, but is metal recycling per se sustainable? What are the criteria to judge the sustainability of recycling? There is a consensus that sustainability or sustainable development is “development that meets the needs of the present without compromising the ability of *
Corresponding author, Tel: 61-3-9545 8665, Fax: 61-3- 9562 8919, E-mail:
[email protected]
future generations to meet their own needs." (Brundtland Commission, 1987; UN Commission on Environment and Development, 1992). Sustainability is also linked to the concept of the triple bottom line where Economics, Environment and Society are all of equal importance to the health and wealth of the world’s people. Tools such as Life Cycle Assessment or Analysis (LCA) are being developed to assess some of the technical and environmental penalties of manufacturing/production processes (Burgess and Brennan, 2001). As yet, the tools or methods for evaluating the social impacts of processes are in their infancy, but progress is being made in this area (Tateda, Ike and Fujita, 1997; Azapagic and Perdan, 2000). Irrespective of the discipline of the assessors, bringing together the economic, technical and social assessments will be very difficult. The aim of this paper is to set the scene and look at some of the sustainability issues with metal recycling, especially compared to primary metal production. We hope to show that the metals recycling sector is moving towards sustainability, but there are issues to be dealt with before the sector can be classed as truly sustainable. Generally, the commonly used metals are recycled either at primary smelters or at typically
smaller
operations
(secondary
smelters)
using
well-established
technologies. In short, the metal recycling operations in the smelter are either: •
Remelt/refining/casting processes; or
•
Reduction and refining processes.
Iron is recycled in the basic oxygen steel converter (BOS) or electric arc furnace (EAF) from scrap steel/iron.
Scrap steel is also the primary source for recycled
manganese, nickel and chromium and turned back into stainless steels. Recycled aluminium is remelted in rotary or reverberatory furnaces primarily from scrap and used beverage containers. Copper is recycled from a range of scrap including wires, pipes, brasses and printed circuit boards in a range of furnaces including the PierceSmith converter. The recycling of zinc and lead is different to the above metals as they are recycled primarily as oxides, zinc as oxide fume produced from the remelting of brasses, or from recycling galvanised scrap. The zinc oxide fumes are usually recycled at a primary lead/zinc smelter.
Metallic zinc diecast alloys are
recycled with copper alloys to make copper base alloys. Recycled lead is produced
from scrap car batteries where the lead oxide pastes and the metal grids are remelted and smelted back to metal.
Precious metals are either recycled at
specialised operations or at primary lead/zinc or copper smelters where the gold, silver and platinum can be extracted as part of the existing process circuit. The recycling rates of the commonly used metals vary significantly as shown in Figure 1 and Table 1. Iron is recycled in the greatest quantity and as a percentage of US production, typical of most western countries production, the recycling rate is around 55%, (US Geological Survey Minerals Yearbook, 1999). The average lifetime of steel goods before it becomes post consumer scrap is estimated to be 14 years (Michaelis and Jackson, 2000). The average lifetime of a car is 7 years and the lead acid battery life is estimated to be 4.5 years (Kircher, 1989).
Lead, although
produced in a lot smaller quantities than steel, has a greater recycling rate, with 62% of lead production coming from secondary lead. This is due to lead mainly being used in a single commodity, the lead acid battery (Ahmed, 1996). Aluminium if used in beverage cans has a very short lifetime, but if used in other commodities has a similar lifetime to steel before becoming post consumer scrap (Reuter, 1998). A common feature in the recycling of these metals is that the energy required to collect and prepare the metals for smelting is significantly less than that required to mine, crush and upgrade the ore for primary production (Kellogg, 1977; Reuter, 1998; Szekely, 1996). Mining and mineral processing steps can produce significant quantities of solid wastes and emissions to air and water, the extent of which will depend upon the ore quality and mining method. Open pit mining requires removal of overburden, while underground mining requires removal of country rock for shafts and drives. Mining can have problems due to acid mine drainage. Table 2 shows the amount of ore required to produce one ton of metal based on typical concentrate and ore analysis (Szekely, 1996).
Recycling avoids further generation of large
volumes of mining wastes and the scrap has a much higher metal content than most primary smelter feeds. In order to evaluate the sustainability of metal recycling, it is firstly worth discussing the processing methods, the energy and environmental penalties for the commonly recycled metals, lead, aluminium, copper and steel by way of example.
CASE STUDIES Recycling of Lead Acid Batteries As mentioned above, the trend for lead use is now tied to the demands for the lead acid battery. The car battery is the major use for lead, and the demand for lead matches the world demand for automobiles. The potential environmental and health dangers of the lead acid battery have encouraged development of procedures for battery recycling (Hagen, 1999; Quirijen, 1999; Wilson, 1993) and recycling rates of batteries in most countries is between 95 and 97% (Ainley, 1995). The lead acid battery is made up of a number of materials (lead, plastic, acid,) and some of the problems that arise when separating and recycling these composite materials have been discussed in the literature (Ahmed, 1996; Bied-Charreton, 1993; Dahodwalla and Herat, 2000; Frost, 1999; Jolly and Rhin, 1994). Modern battery recyclers produce as by-products polypropylene chips, wallboard grade gypsum and a low leaching discard slag (Phillips and Lim, 1998; Worcester and Sankovitch, 1997; Ahmed, 1996). In primary metal production, lead concentrates are oxidised to form a sinter or a slag depending upon the process. To avoid sulphur release to the atmosphere, the SO2 produced has to be captured as acid, and either used in other processes or converted into gypsum for disposal. The capture of SO2 to make acid is not total, and some SO2 is still released from fugitive emissions. Toxic metals such as Hg, As, and Cd also have to be stripped from the acid and disposed of.
The sulphide
oxidation step is avoided with secondary lead production. The energy required in the smelting step to produce metal from battery paste is similar to that required for primary or virgin metal production, but overall it is only 36% of that required to produce virgin metal (Kassem, 1992). In primary production, the impurity elements need to be removed in a series of complex refining steps, whereas the purity of lead and lead alloys produced from recycled batteries meets the requirements of battery manufacturers for modern batteries. (Prengaman, 1993; Ramus and Hawkins, 1993; Frost, 1999). The lead fumes that are produced during smelting are recycled back to the furnace.
One of the issues that need to be addressed is the environmental stability of the final or discard slag produced. The volume of slag produced from recycling batteries is small and traditionally the industry has used a low melting point Na2O-FeOx slag and also produced a small quantity of Na2S-FeS matte (Queneau, Cregar, Mickey, 1990). The discard slags are dark in colour due to entrapped matte and can be ecotoxic (Coya, Marañón, Sastre, 2000). Entrained metal prills in the slag can also be easily leached, and so separation of metal and slag is extremely important in producing an environmentally stable discard slag. In the late 80’s to 90’s, smelters installing new technologies shifted to slags based on a CaO-FeO-SiO2 slag chemistry which pass the requirements of the Toxicity Characteristic Leaching Procedure (TCLP) for disposal in landfill (Ramus and Hawkins, 1993). These slags also have a capacity for toxic elements (Jahanshahi et al, 1994) and assist the refining during secondary metal production. Recycling of Aluminium Compared with virgin aluminium metal production, the volumes of waste streams from secondary production are much smaller.
Comparison by way of Life Cycle
Assessment (LCA) (Reuter, 1998) for aluminium, shows that secondary aluminium production requires around 5-10% of the energy required for producing primary aluminium. 20 tonnes of useable secondary aluminium can be produced with the same amount of energy required to produce 1 tonne of primary aluminium (Pownall, 1988). Recycling has a significant reduction of most upstream impacts for aluminium as the Bayer and Hall-Herault processes are avoided. Less than 100 kg of solid waste per tonne of aluminium is produced from recycled metal compared with just over three tonne of solid waste, mainly red mud from primary production. Another big advantage of recycling over primary production relates to avoiding the production of spent pot lining (SPL) which is a waste product from aluminium electrolysis. The costs of stabilising red mud and treating SPL are significant. The main solid waste from secondary aluminium production is a NaCl-KCl salt slag (Utigard, Roy and Friesen, 2001; Netchaef, 1999) which can be recycled to recover the salts, leaving an insoluble aluminium oxide-hydroxide product. This is disposed in landfill but has potential to be used in refractories (Pereira, Couto, and Labrincha, 2000). There is also a small quantity of flue dust generated which requires disposal, usually in secure
landfill.
Another byproduct is hydrocarbons from oil contaminated machine shop
scrap which is centrifuged to drop the oil content to 5%. The recovered oil is then sent to a waste oil processor for further treatment. Recycle of Copper and Brass As with aluminium, the volume of solid wastes produced from recycling copper metal is very much smaller than from primary production (Reuter, 1998). A small volume of borate slag is produced during melting of the scrap along with a fume containing zinc, lead and tin.
Volumes of slag and fume produced in primary production are
considerably greater, but the greatest contribution is from the mine and concentrator tailings. Secondary copper produced from melting of scrap copper and fire refining generally contains too many impurities to be used as an electrical conductor without purification or electro-refining (Biswas & Davenport, 1978). Fire refined secondary copper usually contains far less impurities than blister copper, typically between 100 and 800 ppm of impurities (Esparducer et al, 1999). These impurities (As, Sb, Bi, etc) have disposal costs and recycling reduces the volume of impurities and these disposal costs. Recycled copper if refined to cathode quality copper, requires almost the same amount of energy for the refining process as blister copper (Reuter, 1998), but compared with virgin material, there is a large energy saving from avoiding the mining step. However if scrap copper is used to make brass and gunmetal alloys, the energy used compared with primary copper production is very much reduced. Borate slags containing Cu, Pb, Sn and Zn produced in secondary copper production are used in cement production. In order to produce the alloys to specification, the molten alloy is fire refined with oxygen and a small quantity of Zn/Pb fume is generated. This fume is recycled by sending it to a lead/zinc smelter for further processing. Steel Recycling Steel scrap is recycled in the steel converter or the electric arc furnace (EAF). In the latter, electrical energy is required to melt the scrap, while in the converter, the charge to the furnace is a mixture of hot metal and scrap and the heat is supplied from oxidation of carbon and silicon in the hot metal. It is calculated that the energy requirements to produce 1 tonne of finished steel is 11.6 GJ t-1 for the EAF route, and 19.4 GJ t-1 for the blast-furnace and BOS route (Michaelis & Jackson, 2000). It is
also estimated that in recycling steel, 0.1 to 0.2 tonne of solid waste products are produced in the EAF compared with 0.4 to-0.6 tonne from primary production via the BF & BOS route (Szekely, 1996) per tonne of steel. The volume of solid waste in producing steel is very much smaller than most of the other major metals, as steel is produced mainly from high grade iron ores containing very little gangue material. The main pollution problems for steel making via the BF and BOS route are the release of particulate, NOx and SOx emissions from the sinter plant and volatile organic compounds from the coke ovens (Szekely, 1996). As the EAF does not need coke and does not use sinter feed, the volume of potential pollutants produced directly from recycling steel is less than the conventional process. There are three solid waste product streams from the EAF and the associated rolling mills which are also common to the BF/BOS route for steel production. These are slags, fume or dust and mill scales. The granulated slag is used in cements or as road aggregate (Prosser, 1994; Szekely, 1996) and should be considered a byproduct. Mill scales and internally generated scrap are recycled back to the furnace. EAF dusts are produced at a rate of between 10 and 20 kg per tonne of steel and typically contain 13-30% ZnO, 0.15-6% PbO, and 28-38% metallic Fe and 4-15% CaO (Hagni, Hagni and Demars, 1991). Dusts from specialty steel recyclers may also contain 2-12% Cr and 1-5%Ni (Szekely, 1995). Recycling of the dust has been extensively studied and many process routes have been proposed (Beckovich, 1996). The predominant use of zinc is in galvanising with 48% of annual global production in 1998 used to protect steel, but only 6% of Zn from secondary sources originate from steelmaking dusts and, it appears that most steelmaking dusts are sent to landfill (Viklund-White, 2000). A Life Cycle Assessment of recycling galvanised steel via several processes found that recycling zinc did not necessarily decrease the potential impact on global warming or acidification, the magnitude of which was very dependent on the process route. A major portion of the energy required was for the reduction of zinc oxide to metal, whether it is by carbothermic reduction or electrolysis. However compared with primary production, where ore is mined and concentrated, and the concentrate calcined, less solid waste is produced, less
energy is required and the acidification potential is decreased (Viklund-White, 2000). In Australia, the EAF dusts from steelmaking are sent to lead-zinc smelters for further treatment.
SUSTAINABILITY ISSUES FACING METALS RECYCLERS The recycling sector is very segmented, with secondary smelters targeting easy to process scrap to maintain profit margins, ie copper and brass, used aluminium beverage cans, stainless steel scrap, etc. Recycling metals does not imply that product quality must suffer. Most steel produced currently is either low carbon or low alloy material and has very good recyclability in the EAF (Szekely, 1996). Aluminium cans are designed to be recyclable. Recycled lead meets the purity requirements of battery manufacturers. Scrap which is difficult to process and requires several processing steps, still ends up at primary smelters. Hence the future of recycling is also linked with primary metal production. Although furnace fumes and dusts are collected at secondary smelters, the operations are typically too small to justify the capital costs to treat the fumes and they are also sent to primary smelters capable of handling a wide range of feed materials. Primary smelters are well equipped to recycle complex metallic composite materials such as electronic scrap in sequential unit operations and the refining facilities enable the extraction of all the values including the precious metals (Lehner and Vikdahl, 1998, Lehner and Vikdahl, 2000). Looking at zero emissions and wastes as one of the criteria for assessing sustainability, the volume of solid wastes produced per tonne of recycled metal compares very favourably with that for new or virgin metal production, whether it be at a secondary or primary smelter. This is because the greatest source of solid waste from virgin metal production is from the mines. About 60% of total energy used in the production of most metals are consumed when crushing and grinding the ores. Thus recycling has very favourable energy consumption per tonne of metal produced and lower greenhouse gas production than primary metal production. As recycled scrap has hardly any gangue contaminants, recycling to a primary smelter does not mean there is an associated increase in slag volumes. In the
secondary smelter, slag chemistries have the potential to be adjusted to improve environmental stability, and for Al, Cu, Fe and Pb, the by-product slags can either be used in cements, road aggregates or disposed of as non-hazardous landfill. Spent refractories are another solid waste produced during metal production and common to both primary and secondary metal production. However as the slag volumes are smaller in secondary production and the operating temperatures can be lower, the wear rate of the refractories can be significantly slower for secondary production. The productivity of the furnace is greater due to less down time and the tonnes of waste refractory generated per tonne of metal processed is lower. Due to a region of slag/metal infiltration in the refractory it will not be possible to achieve very high recycling rates of refractories, unless there is an effective separation step which can discriminate between contaminated and uncontaminated refractories. In a study of spent refractory wastes in Missouri, it was found that 7500 t of spent refractories from 150 metal related companies was disposed of annually with 99% disposed to landfill (Smith, Fang and Peaslee, 1999). The largest refractory waste streams were aluminosilicate bricks from the carbon anode baking furnace for aluminium production, and spent MgO-C and dolomite, both steelmaking ladle refractories. The aluminosilicate bricks can be recycled into Portland cements, and the dolomitic bricks have potential to be used as a soil conditioner. It appears it may be possible to recover and recycle portions of spent high alumina refractories used in stainless steel production, but more work is required in this area. There has been significant effort into researching recycling steel-making and copper-making refractories (both magnesia-chrome) and producing brick with similar performance to virgin bricks (Noga, 1994; Maginnis and Bennett, 1995). It is estimated that 10-20% of used steelmaking refractories are being recycled in Japan and Germany (Bennet, Kwong and Sikich, 1995, Nakamura et al, 1999).
Clearly there is need for
improvement. The last major waste from secondary smelters is CO2 emissions.
As described
earlier, recycling operations are either a melting step or a combined melting and reduction step.
Compared with new or virgin metal production, there are
considerable energy savings. Table 3 compares the energy required to produce 1 tonne of metal from primary and secondary production. For steel there is nearly a
six-fold decrease in CO2 emissions when scrap is melted in the EAF compared with the BF/BOS route. Aluminium and copper are currently melted in both reverberatory and rotary furnaces, both of which can be oil or natural gas fired.
Unless
recuperators are fitted to the reverberatory furnace, the efficiency is only 8-10% (Pownall, 1988) and the rotary furnaces are much more efficient, both in melting and refining.
Lead battery recyclers are also replacing older technologies with more
efficient furnaces based on rotary or Isasmelt technologies (Ramus & Hawkins, 1993; Jolly and Rhin, 1994) where natural gas, diesel or oil fuels can be used. Innovations such as the transportation of hot molten alloy from the smelter to the foundry will improve the thermal efficiency and reduce greenhouse gases. As discussed in the introduction, the environment, economics and society are all of equal importance in sustainable development. On the basis of this triple bottom line model of sustainability, metal recycling has a much lower environmental impact than primary metal production. The authors are beginning to investigate some of the economic and social aspects of metals recycling, and raise the following points as a basis for future investigation and research. Scrap Availability and Quality Recycling of scrap now involves very large tonnages, and a lot of local scrap is processed overseas where labour costs are cheaper. This has meant that there can be shortages in the supply of local scrap. In Australia, the total volume of scrap recycled is a small fraction of the global volume recycled (1999 Minerals yearbook). Scrap arrives at the smelter already roughly sorted to type, eg, Cu, brass, Al, steel, depending upon the metal being recycled. The preparation of the scrap for addition to the furnaces is usually kept to a minimum, typically baling and weighing into storage bins (Pownall, 1988). In the case of aluminium, cans are crushed into small bales. In Australia and other western countries, occupational heath and safety standards have decreased the permissible levels of repetitious manual handling tasks. It is therefore not practical to disassemble complex machinery using manual labour into component alloy types. Contamination of the scrap with debris material or other metals which escaped the initial sorting is also an issue for recyclers. There is a
double financial penalty from debris. The recycler has not only paid for the debris; the cost of removal and disposal of debris can be significant. In Germany, the cost of disposing of the residual shredder waste from scrapping cars exceeds the value of the scrap recovered. As a consequence cars are not shredded in the Ruhr region, but shredded in other countries where disposal costs are significantly cheaper (Anon, 1995); the recovered shredded scrap is then sent back to Germany to be smelted. Global Competition The decline in local manufacturing has also meant a declining demand from traditional markets for alloys. Local metal production industry also has to compete with imported alloys or components with cheaper production costs, or compete with other producers in export markets. In common with primary metals producers, this has tended to decrease profit margins. Technology Metal recycling operations are typically a lot smaller in scale than primary smelter operations. Refining steps are kept to a minimum and the operations rely on costeffective technology to achieve productivity and product quality.
Efficiency
improvements could be realised throughout the operations, from improving feed quality by better separation of metals, reducing manual handling, rapid and cheap on-line analysis, through to improved safety in transport and handling of molten products. Social Currently there is broad public support for all types of recycling, whether it is paper, plastics, glass or metals.
Collection of both industrial metallic scrap and post
consumer scrap from households is well supported with collection rates increasing. In the United States, 90% of automobiles are recycled and the recycling rate of whitegoods increased from 4% in 1985 to 62% in 1993 (Szekely, 1996). The world requires metals, which have to be supplied from either primary or secondary production. In most of the world apart from Europe, mines are not close to major urban centres, and usually have associated satellite towns, which supply the infrastructure for the mine, and the services required for the mine-workers and their
families. In Australia, mining communities have made significant contributions to social policy development (Drew, 1993). Mining companies are continuing to be at the forefront of social policy development through negotiations with indigenous communities since the Mabo and Wik decisions (Mercer, 1997). Due to the remote locations of mining communities, they are noted for their self reliance and community spirit, but mining towns often have a relatively large population of transitory male workers, who work at the mine for a couple of years and then move on. The social pressures on such communities have been well recognised. Although it is hard to gauge, metal recycling must have had an impact on the number of mines, miners and mining towns, and possibly how mining communities are managed. The desire to keep skilled workers may have encouraged fly in/out type of mining operations, where workers are flown from larger towns or cities to the mines to work. In terms of judging the social impacts of recycling, the relative workplace safety of mining compared with scrap collection and sorting may be a factor to consider. Workplace injuries have huge social and economic impacts, not only on the injured and their families but also on other workers. Any workplace can be hazardous, but mines are potentially very hazardous workplaces. It could be argued that scrap collection and sorting is potentially a lot safer on the basis of not needing explosives alone, but as most scrap is generated in urban centres, even the proximity of major heath services and expertise may reduce the magnitude of workplace injuries when they occur. An analysis of accident types and duration would help in assessing the social impacts. Compared with primary smelters, secondary smelters do not require large sites, but rather just enough for scrap storage, the furnaces and gas cleaning, and product metal. They are generally located close to the centres where scrap is generated. The secondary smelters get scrap direct from manufacturers or from scrap metal merchants, who do the primary sorting. As metallic scrap is generally not dusty, secondary smelters can operate in the urban environment, and close to manufacturing. This is leading to developments such as transporting molten alloys to foundries for casting into product. The Basel convention on the trans-boundary movement of hazardous wastes across national borders and between countries is also influencing recycling, particularly lead.
The used lead acid battery is classified as a hazardous waste under annex VIII of the convention, with shipment prevented from annex VII countries to non-annex VII countries (Stone, 2000).
The convention places emphasis on local recycling at
possibly a greater cost, with transport of used batteries between countries restricted to some degree (Stone, 2000; Elmer, 1996). By way of example, Australia can no longer export battery scrap to non-OECD countries. In countries with a strong and growing demand for batteries, which have banned the import of battery scrap, there have been undesirable socio-economic and environmental effects, including an expansion of backyard smelters and possible illegal shipments of scrap.
CONCLUDING REMARKS Local recyclers are under economic pressure like most of the metals sector, and there may be rationalisation with the number of producers declining. Globally, the metals recycling sector is strong, but the economics relies on cheap scrap collection or government regulation to expedite scrap collection. Many of the procedures for collecting and processing scrap are in place. One of the key aims for sustainability would be to further improve scrap collection and sorting; this could involve regulation or use of economic instruments by governments. Metal recycling is a very important component of world metal production with very large tonnages produced annually from recycled metal. Secondary metal can be of similar (or better) quality to virgin metal. There are significant energy and waste savings from avoiding mining, crushing and grinding and flotation alone. The metal content of scrap is also higher than in the feed to primary smelters. Toxic impurity elements are not a large problem, and the major contaminants are tramp elements arising from scrap pieces that are difficult to separate. This difficulty would diminish through better product design. The energy requirements for melting/smelting secondary scrap and refining are generally less than primary metal, depending upon the desired product quality. The volume of carbon dioxide emitted during scrap recycling is also less. Furthermore, for aluminium and copper and lead, the melting furnaces operate at a lower temperature than primary production, use natural gas as a fuel, and enjoy longer refractory life. For metals, which are smelted from sulphide ores, recycling eliminates
the potential problems with sulphur dioxide emission or capture, as well as related emissions or purification problems derived from heavy metal impurities. Metal recycling, in reducing the demand for primary metal, slows down the depletion of resources. Lengthening the life of resources is a key element to the concept of sustainability. Metal recycling meets a lot of the other requirements of sustainability. Compared with primary production, recycling is very environmentally favourable, as it reduces upstream loads associated with primary metal production. Environmental regulations are directly and indirectly impacting upon recycling. Reducing access to landfill for hazardous materials is encouraging recycling and also discouraging primary production where separated impurity elements have to be locked away in environmentally stable materials. Metal recycling also fits well into the urban environment, where scrap generation and collection, recycling and remanufacture operations are situated in close proximity. This paper has focussed on the recycling of major metals. Recycling of metals that normally occur in ore bodies at low concentrations is even more favourable due to energy and environmental savings, especially if the element has a high value such as silver, indium, platinum or gold.
Regulation is encouraging the recycle of toxic
elements such as cadmium. This paper has shown that metal recycling meets many of the criteria for sustainability, but there are still weaknesses. For every metal there are still wastes or potential issues to be solved. Disposal or recycling of refractories is one such issue. Cleaner production and the use of materials and manufacturing techniques to make recycling easier are other issues. So is the removal of tramp elements. Ensuring metal products can be easily recycled should be an objective for all product developers and designers. Further improving recycling rates is a very worthwhile goal as the earth has a fixed supply of metals and limitations in its ability to withstand impacts from mining and primary processing.
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Table 1.
1999 production (kt) of selected metals in the US from primary and
recycled sources .
Aluminium
Annual Production (kt) 9940
Primary sourced metal (kt) 6190
Secondary sourced metal (kt) 3750
Chromium
558
440
118
Copper
4080
2750
1330
130000
59000
71000
Lead
1790
700
1090
Magnesium
232
144.7
87.3
Nickel
211
140
71
Tin
57.2
40.9
16.3
Titanium
39.8
17.9
21.9
Zinc
1610
1204
406
Iron & Steel
Table 2.
Estimated tonnes of ore to produce 1 tonne of refined metal
Metal
ore (t)
Al
6.5
Au
115000
Cu
140
Fe
1.6
Ti
4.5
Zn
7.2
Table 3.
Comparison of the energy required (GJ) for 1 tonne of metal from
primary and secondary sources. Metal
Al
Cu
Fe
Pb
Zn
Primary
250*
68.0§
19.4†
39‡
16.4#
Secondary
15
18.0§§
11.6††
12‡‡
18.0##
* § §§ † †† ‡ ‡‡ #
##
Based on production from Bauxite Based on cathode quality copper. Concentrate derived from open pit mining, shipped to a smelter using a flash smelting technology. Based on cathode quality copper. Blast furnace and basic oxygen steelmaking route Electric arc furnace Based on the mining of a 2% Pb ore, flotation, sintering and blast furnace route Secondary lead produced using battery breaking technology and a rotary furnace Based on an average production of 85% from electrolytic zinc, 15% from the Imperial Smelting Furnace Based on treating a 35% Zn electric arc furnace dust in a Waelz process
Figure 1: 1999 production of selected metals in the US from primary and recycled sources .
