Virgin stock extraction efficiency depends on availability (scarcity), economies of scale, material concentration (for example ore grade), and technological efficiency (including energy demand), among other aspects. It is increasingly apparent that for several critical virgin resources around the world that availability and material concentration are reducing. See Fig. (1) for an example of copper ore grades over time.

These availability and material concentrations are inter-coupled and impact technological efficiency and energy use. See Fig. (2) for an example of the increased energy demand for Cu production as ore grade diminishes.

Given the assumptions that 1) available virgin ore grade over time is decreasing, and in turn increasing the energy and overall cost to extract virgin materials and 2) technological development is steadily decreasing the cost for extraction of substances in secondary stocks; we have prepared Fig. (3) as an illustrative example. While in reality, the curves for extraction costs may be more or less dramatic depending on factors such as technological development (virgin and secondary). However, the cost trends are currently headed in this general direction [3R. Dobbs, J. Oppenheim, F. Thompson, M. Brinkman, and M. Zornes, "Resource revolution: Meeting the world’s energy, materials, food, and water needs", Report-McKinsey Global Institute, 2011., 9UNEP, Environmental Risks and Challenges of Anthropogenic Metals Flows and Cycles, 2013., 11B. K. Reck, and T. E. Graedel, "Challenges in metal recycling", Science, vol. 337, no. 6095, pp. 690-695, 2012.
[http://dx.doi.org/10.1126/science.1217501] ].

Fig. (3) The hypothetical time trends for cost of virgin and secondary stock extraction along with material grade.

Secondary stock extraction is not necessarily viable when the trend curves intersect in Fig. (3). The point at which secondary stock extraction is viable is illustrated further in Fig. (4) looking at market values.

Fig. (4) The value of secondary stock extraction costs (VB, VC, VD) and the value of alternative recycling/treatment options (VA) in relation to the market value of substituted primary material (Horizontal axis).

Waste materials and by-products are a resource with several alternative markets or uses. These markets may be, for example, energy recovery, use as additives (such as ashes in stabilized soil), landfill, ballast material, etc. When deciding which pathway a waste resource will follow the responsible organization may compare the various markets, their value, and the related production costs in preparing the raw stock for such a market. Other aspects such as access to markets, legal ability including permits, and company strategy may also play a role. Fig. (4) illustrates how a decision for performing secondary material extraction can be compared to alternative material markets. When the value of the virgin stock being substituted is higher, the viability of extracting secondary stock increases. The point at which extracting secondary stock is a preferential option compared to alternative material markets depends on the production (extraction) costs of the secondary stock. These costs vary depending on economies of scale, if landfilling costs are incurred in the stockpiling of secondary stock, etc. The circles in Fig. (4) mark the point at which stock extraction is a competitive alternative to alternative markets.

V_A = Market value of alternative recycling/treatment options such as road base aggregate, recycled fuel, landfilling, etc. after processing costs. These can have a positive or negative economic value.

V_B, V_C&V_D = Value of extracting secondary stock after various production costs. V_B = stock accumulation through longer term stockpiling allowing lower production cost, and no stockpiling fees (such as landfill tax). V_C = the stockpiling of material is not taxed as landfilling. V_D = the stockpiling of material incurs landfilling fees, increasing the production cost.

One can also infer from Fig. (4), that if V_A (the value of alternative market delivery) were to go down via market forces or steering mechanisms (such as taxes on waste to energy or mixing into ballasts), that secondary material extraction would become competitive at even lower market values for the extracted material. Conversely, if new technologies, market forces, or steering mechanisms were to make alternative markets more valuable (less negative or more positive) it would take even higher extracted material values to enable extraction.

It is not assumed that all secondary stocks will be suitable for viable extraction in the near future. However, some hints as to which materials may be more critical in the near future is given by the US Geological Survey and the McKinsey Global Institute [3R. Dobbs, J. Oppenheim, F. Thompson, M. Brinkman, and M. Zornes, "Resource revolution: Meeting the world’s energy, materials, food, and water needs", Report-McKinsey Global Institute, 2011.] in Table 1. This table touches on many of the market drivers mentioned in the beginning of this chapter including: market supply (left columns), availability of substitutes to the virgin material, and key industrial demand (right two columns). Elasticity of demand is in turn related to availability of substitutes and key industrial demand.

Table 1 Potential shortages of materials and the possible economic impact. US Geological Survey data adapted from [3R. Dobbs, J. Oppenheim, F. Thompson, M. Brinkman, and M. Zornes, "Resource revolution: Meeting the world’s energy, materials, food, and water needs", Report-McKinsey Global Institute, 2011.].

As can been seen in Table 1, there are several materials with a short projected number of productive virgin extraction years left that are of key importance to production. There are also materials with a longer expected virgin supply such as phosphate that have a key importance to production, high lack of substitutes, and yet currently a low recyclability. Stockpiling secondary materials that include content of such materials could be a responsible high value recycling strategy.

Typically recycling and waste management plants operate on a scale of less than 1 million ton/year. Mining and extraction of virgin materials typically operate on a several million ton per year scale. The largest Waste to Energy plant in Sweden has a capacity of 480 kton/year. Compared to virgin material, waste has a far more complex composition and hence, in general, more unit operations are needed per ton material produced; this to drives up OPEX (Operational expenditure) and CAPEX (Capital expenditure). Another important issue of scale arises when selling the materials extracted as the buyer needs guarantee enough volume of product can be delivered. Stockpiling - or the creation of manmade deposits - is a tool to achieve enough scale to enable extraction of resources [12S.J. Callan, and J.M. Thomas, "Economies of scale and scope: A cost analysis of municipal solid waste services", Land Econ., vol. 77, no. 4, pp. 548-560, 2001.
[http://dx.doi.org/10.2307/3146940] , 13X. Wang, G. Gaustad, C.W. Babbitt, and K. Richa, "Economies of scale for future lithium-ion battery recycling infrastructure", Resour. Conserv. Recycling, vol. 83, pp. 53-62, 2014.
[http://dx.doi.org/10.1016/j.resconrec.2013.11.009] ]. This is illustrated in Fig. (4), in axis V_B. As long as the supply of extracted secondary materials to market is not a large percentage of total market (and prices are relatively inflexible), diseconomies of scale should not be of concern.

Table 1
Potential shortages of materials and the possible economic impact. US Geological Survey data adapted from [3R. Dobbs, J. Oppenheim, F. Thompson, M. Brinkman, and M. Zornes, "Resource revolution: Meeting the world’s energy, materials, food, and water needs", Report-McKinsey Global Institute, 2011.].

Although society has set up a number of policies, some main barriers for increased use of recycled aggregates still remain. Taking the example of the use of secondary materials in recycled aggregates some recent barriers are:

1. Legal issues - are recycled aggregates a waste or a product? If considered a waste, generally the legal process needed (permits, applications, quality control) are to slow to cope with the pace of the construction industry.
2. Economic issues - the value of the virgin material is low and the supply of recycled aggregate is low compared to demand in each construction project. For example, when a road is built, the specific need will often be much greater than the possible local production of recycled aggregates.

Continuing with the example of use of waste materials in recycled aggregates, a substantial issue with the continued viability of such markets is raised when assessing the risk for having to remediate such material in the future. Might such markets be spreading materials into society that will have to be painstakingly sanitized in the future? When comparing old and new clean-up levels, the following general observations can be made:

1. The number of pollutants included in the lists of clean-up levels tend to increase over time
2. The clean-up level for a specific contaminant is often reduced over time

As can be seen in Table 2, the upper limit values for several substances is diminishing over time.

Table 2
The Development of general Swedish clean-up levels (not comprehensive).

In a short perspective, road base recycling may make sense to the producer of the waste since it’s probably the most economic option (when compared to high landfill taxes) and it makes some sense for society since natural resources may be saved. The levels of contaminants in the waste used for road base recycling may well be within what today is considered safe levels or that can be accounted for in a site specific scenario. However, in the long run, given PPP and that clean-up levels tend to be changed over time, it’s likely that some of the road base recycling projects will need to be remediated in the future and that the cost for such remediation will be directed towards the Polluter. Hence, if a waste is used for road base recycling today, someone (the Polluter or Society) is taking a financial risk. This risk is rarely accounted for when comparing different options of handling waste. From the analysis above one can conclude that if recycling of waste is done the wrong way it may cause a burden for future generations, hence it may not be not a sustainable solution. It can also be concluded that there is a need for better guidance as to what wastes may be recycled for what uses.