Re-evaluating the traditional production cycle
Materials produced from minerals are essential for society and will be essential during the transition to sustainability. What a sustainable world might be like is open to speculation, but it is hard to imagine any acceptable form in which mineral-based materials would not have an ongoing and important role. In the transition to sustainability, it will be necessary to progressively close the materials cycle in ways that reduce the demand for virgin minerals and reduce the quantity of wastes. How the primary minerals and metals industry can contribute to this is the focus of this article.
The well-known materials cycle (Figure 1) provides a useful reference point. Figure 1 focuses on the materials and products of interest; however, it fails to show the waste that is produced at every stage of the materials cycle, not just at the end-of-life stage of a product.
Wastes are conventionally discharged to the environment or stored in purpose built facilities and this approach provides the conventional definition of wastes as substances to be disposed of. However, a more useful definition is that wastes are substances produced as part of the materials cycle for which there are no present uses. This definition entertains the possibility that wastes may in future, or under different circumstances, be useful. Hence, wastes are now often referred to as co-products.
Wastes and the waste hierarchy
Our present throw-away culture was largely the result of the increasing availability of cheap energy and materials and of cheap landfill. There has been a renewed focus on minimising waste production only since the latter half of the 20th century, when the ever-increasing quantities of waste from energy and materials production and consumption began to have wide-scale environmental impacts.
The conventional waste hierarchy – reduce, reuse, recycle (the three Rs) – lists waste management strategies in decreasing order of desirability. It focuses on the reduction or minimisation of waste and was developed with manufactured products, building and construction products, and domestic waste in mind. The focus on minimising waste started shifting towards eliminating wastes at the source in the 1990s and the European Union led the way. Directive 91/156/EEC of 1991 established the hierarchy of waste prevention, recovery, safe disposal. Importantly, it addressed things and co-products that have had no previous use (such as mining and processing wastes) as well as those that have had a useful life. After several revisions, Directive 2008/98/EC established the present hierarchy:
- preparation for reuse
- other recovery (eg energy recovery)
Directive 2008/98/EC excludes wastes resulting from the prospecting, extraction, treatment and storage of mineral resources and the working of quarries, which had been included in earlier versions, since these are covered specifically by Directive 2006/21/ EC, which follows a similar hierarchy.
Mining and processing wastes are of two types: those produced directly in the production of a mineral or metal commodity (at the mine, smelter, etc), and those produced indirectly (in the production of inputs used at the mine, smelter, etc) (Table 1). The total waste associated with the production of a metal or mineral commodity is the sum of the direct and indirect wastes.
The quantities of wastes produced in the materials production stage of Figure 1 are very large (Figure 2). Solids make up the largest quantity and can range from several times the mass of valuable element for abundant elements (eg iron and aluminium ores) to many thousands, and even millions, of times for some scarce elements.
The total quantity of directly produced wastes decreases along the value chain from mining to manufacturing to recycling. The largest quantities of solid and liquid wastes are produced during mining and beneficiation, while the major quantities of gaseous wastes are produced during high-temperature chemical processing, particularly smelting of metals and cement manufacture.
Wastes from mining and beneficiation have the largest potential environmental impact on land and water, and chemical processing wastes have the largest potential impact on the atmosphere.
Reducing and eliminating wastes and their impacts
Figure 3 illustrates the historical trend in approaches to addressing the environmental impact of wastes. Company behaviour has moved in recent decades from complying with regulations to corporate social responsibility. In the next decades, it will need to move progressively to ‘closing the loop’ strategies to dramatically reduce the quantities of wastes. The drivers for change have moved from being almost exclusively profit to include regulations, stakeholders and increasingly to changing social values. In parallel, the materials cycle focus has shifted from a narrow focus on products towards including co-products. Increasingly, focus will shift to the entire materials cycle and, ultimately, to the entire economy.
Cleaner production is the continuous application of an integrated preventive environmental strategy to processes, products, and services to increase overall efficiency, and reduce risks to humans and the environment (UNEP, 2015). For extraction and processing, cleaner production involves one or a combination of the following:
- conserving raw materials, water and energy
- eliminating toxic and dangerous raw materials
- reducing the quantity and toxicity of emissions and wastes at source during the production process.
Cleaner production differs from pollution control in that pollution control is an after-the-event, react-and-treat or end-of-pipe approach while cleaner production attempts to anticipate and prevent. Cleaner production aims to minimise or avoid practices such as waste treatment (including stabilisation, encapsulation and detoxification), waste dilution to comply with regulations (eg releasing contaminated water into rivers or streams during periods of high flow, blending arsenic residues with flotation tailings), and transferring hazardous or toxic substances from one medium to another (eg wet-scrubbing gases and then disposing of the contaminants as waste water).
Wastes as co-products
Use of mining and processing wastes as raw materials for another process is not new and some wastes from mining and processing are used to produce saleable products. Sulfur dioxide in smelter gases is routinely converted to sulfuric acid. Slags from iron and steel production are used as aggregate materials, as a raw material in cement manufacturing and as direct additions to concrete. However, mining and processing wastes are a largely untapped resource and there are many other potential applications.
However, not all, or even most, mining wastes can be used productively, because the quantities of waste rock and overburden are so large that there are insufficient bulk applications, even in construction projects. Furthermore, mines are often located in remote and/or sparsely populated areas and the transportation of low-value construction products (sand, aggregate) to populated areas for infrastructure projects is uneconomic. Hence, the focus is necessarily on wastes from mines and processing operations close to populated areas and/or on higher-value products which can be transported economically over long distances.
Some examples to illustrate the possibilities include utilisation of red mud from Bayer processing of bauxite (Cooling, 2007; Jahanshahi et al, 2007), spent pot lining from aluminium smelting (Mansfield et al, 2002), fly ash from power generation, slags from smelting operations and the use of wastes in geopolymer concrete (Davidovits, 2008). There are many other examples in the literature.
Waste reduction through re-engineering aims to minimise the quantity of waste produced or to produce a by-product in a form that can be used more readily. This involves process modification or even completely redesigning the flow sheet. There are three broad approaches:
- flow sheet simplification
- use of novel equipment
- use of novel processing conditions.
While one of these approaches may predominate, a major technological development usually combines aspects of two or all three. Some examples are given in Table 2, though many more could be mentioned.
Closing the loop
Figure 4(a) shows the conventional open-loop production system we are most familiar with. A company takes in new (or raw) materials and processes them using energy, and generates products and wastes. The products and wastes are considered as externalities and their environmental impacts are borne largely by society as a whole. Some recycling of end-of-life products may take place. Figure 4(b) shows a closed-loop production system. Note, however, it is not quite closed. New materials and energy still come from outside the system and certain wastes still leave it, but products and process wastes remain within the system. Responsibility for products and for process wastes, and for the impacts of their use, is borne within the system.
The unusable wastes which leave a closed-loop system are of three main types:
- wastes generated during the extraction of new materials (eg overburden and waste rock from mining)
- wastes that escape from the recycling loop (since some loss is inherent in recovering and recycling materials)
- wastes lost through the use of products (eg by being discarded to landfill or incinerated).
Closing the loop in practice is difficult. It means recognising that the industrial system consists of much more than separate stages of extraction, processing, manufacture and disposal, and that the stages are linked across time, distance and economic sectors (Frosch, 1995). In many respects, a closed-loop system operates like a natural ecosystem.
The evolution, or planned implementation, of a set of interrelated symbiotic links among groups of firms gives rise to an industrial ecosystem. When these symbiotic links occur within a relatively defined area the links are often referred to as regional synergies and the area as an eco-industrial park. There are many eco-industrial parks and they are rapidly growing in number and complexity. Kalundborg (Denmark), Humberside (United Kingdom), Moerdijk and Rotterdam (the Netherlands) and Kwinana (Australia) are frequently cited examples (Kurip, 2007).
An alternative way of viewing the materials cycle in Figure 1 is to consider it as consisting of three interrelated components (Figure 5):
- the resources from which materials are obtained
- the materials themselves
- the goods, products and infrastructure that contain materials.
At any time there is a stock of each of these, which may increase or decrease. The sustainability challenge posed by non-renewable resources can only be addressed through an integrated strategy for managing these stocks. This is the concept of stewardship. Such an integrated strategy is summarised in Table 3.
There is already a degree of integration between the three forms of stock but greater integration, both within the minerals industry and between the minerals industry and other industry sectors, is possible. This can be achieved using technologies and business strategies that link aspects of two or more stocks in complementary ways, for example:
- Closed-loop processing. Industrial ecology principles can integrate industry sectors, commodities, products, wastes, etc and are potentially the most powerful integrating strategies.
- Recycling and reuse. Examples include the integration of primary smelters with metal recycling (particularly of electronic wastes) and the use of cement kilns in urban areas to consume secondary and waste materials that would otherwise be sent to landfill.
- Vertical integration. Vertically integrated companies have scope for managing the use of primary metals and other commodities through the materials cycle and for optimising extraction processes along the value-adding chain. Vertical integration was common among the large minerals companies until recent decades, but the advantage has been lost through the business strategy of focusing on mining as the core business. As a result, many companies are now less able to respond to changing expectations, to move into areas of technological growth, and to identify new business opportunities.
- Mine-site processing. The integration of mining, milling and smelting/leaching by optimising the combined value chain rather than optimising each step independently is easier when the three stages are performed at the mine-site. Wastes can be more easily managed and opportunities for new industries, based on extracting value from tailings, slags and other co-products, can be better realised. As with vertical integration, there has been a trend away from this in recent decades, with most large mining companies now preferring to do the minimum processing needed to produce a tradable commodity (usually a concentrate).
Zero waste – barriers and drivers
Large financial investments are needed for major changes in technologies. Established technologies have been refined over many years and operations usually give financial returns long after the capital costs have been depreciated. The introduction of new technologies introduces production risks which can, and often do, prove very costly. As a result, minerals companies are reluctant to introduce new technologies unless it can be done in an incremental way with minimum risk to production. Furthermore, the relatively low cost of disposal of mining and mineral processing wastes in most mineral resource rich countries is a disincentive to do anything other than discard them.
The technical barriers are often less important than financial and other barriers. Frequently, technical solutions are available or can be developed but are considered too costly, risky or difficult to implement. Developing the right technology at the right time and in such a way that it can be introduced with minimum risk to production has proved a challenge in the minerals industry.
There is a large degree of entwinement between minerals companies and other industry sectors such as power generation, cement production, infrastructure (roads, rail, ports) and suppliers of reagents and other consumables. Technological changes in one area have implications that flow through the entire system. The co-production of multiple products (due to the complex nature of many mineral deposits), and the need to sell these to different markets with differing and changing demand cycles, adds another layer of complexity. These constrain the changes that can be made easily, cheaply and with little risk.
Companies often perceive themselves as in the business of making a particular commodity (alumina, iron ore, steel, aluminium, gold, etc). All other materials created in making their product are seen as wastes to be disposed of as cheaply as possible. Changing the culture of a company so that it perceives the resource in its entirety as its greatest asset is a challenge which no minerals company has yet come near to tackling. Internal reward systems and the compartmentalisation of company activities discourage the interactions necessary to achieve the change.
Often regulations fail to promote closed-loop systems and may actually discourage or prevent them. Of particular concern are regulations relating to the use of wastes or co-products as substitutes for virgin materials, and the assignment of liabilities. In many countries, defining a material as a waste or secondary raw material has consequences for what uses are permitted, what administrative procedures apply to its transport, export and processing, and what costs will be incurred (Pongracz, 2002).
Historically, governments have responded to community expectations for better environmental outcomes through regulatory responses. However, in many situations this has failed or proved very expensive. Market-based instruments (MBIs) are increasingly being used for the management of natural resources and the environment. MBIs focus on achieving outcomes through the self-interest of companies and individuals. MBIs have two potential financial advantages over more traditional instruments (Whitten et al, 2004). They allow different companies to make different adjustments in response to their unique business structures and opportunities, and they provide companies with an incentive to discover cheaper ways to achieve outcomes.
It is now generally accepted that a combination of increased regulation and use of MBIs will drive stewardship initiatives in the next decades. Extended producer responsibility, integrated product policy and other product-focused policies are being used to assign responsibility to companies along the materials cycle to manage environmental impacts and optimise resource recovery and recycling. Schemes such as the Basel Convention of 1989 (www.basel.int) and the more recent REACH Directive (EC, 2007) seek to limit the impacts of toxic materials on human health and the environment. Regulatory controls on end-of-life vehicles and waste electrical and electronic equipment have been introduced in a number of countries. This pressures manufacturers and material suppliers to consider how materials selection will facilitate easier recycling.
The environmental challenges posed by non-renewable mineral resource extraction and use need to be addressed within the broader context of sustainability through an integrated strategy for managing the stocks of resources from which materials are obtained, the materials themselves, and the goods, products and infrastructure that contain materials.
Vision 2050 (WBCSD, 2010), produced by the World Business Council for Sustainable Development, is a major development in business thinking. It envisages by 2050 ‘a planet of around nine billion people, all living well – with enough food, clean water, sanitation, shelter, mobility, education and health to make for wellness – within the limits of what this small, fragile planet can supply and renew, every day’. The pathway to achieve this vision involves fundamental changes in governance structures, economic frameworks, and business and human behaviour. It involves incorporating the cost of externalities (carbon, ecosystem services, water), halving carbon emissions worldwide (based on 2005 levels), and achieving a four- to 10-fold improvement in the use of resources and materials.
The International Council on Mining and Metals has adopted some of the principles of sustainability and corporate social responsibility but the industry is yet to fully incorporate sustainability thinking within its business models at all levels. The inevitable closing of the materials cycle will create new opportunities for mining and minerals companies prepared to adopt new business models.
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