Points far above the Sherwood line indicate underexploited metallic waste streams. Aluminum, although not shown in this Sherwood diagram, has been very heavily recycled in recent years. In the case of aluminum, the energy cost of electrolytic extraction from the ore is much higher than the energy cost of recycling used soda cans.
Why is there so much waste, especially of iron, steel, and precious metals, in the metal industry, which has such a long tradition of recycling? The barriers to industrial recycling of metals can be classified into six interrelated areas: technical hurdles, economic barriers, information barriers, organizational obstacles, regulatory issues, and legal concerns. When recycling is technically feasible, it may be economically unsound. When it is technically and economically satisfactory, a lack of information may block its adoption. Even when the requisite information is at hand, organizational problems can still stymie implementation. Finally, when all else is satisfactory, a recycling scheme can founder on the rocks of regulatory or other legal barriers.
The suitability of the material for an intended reuse is a key technical concern. Metals, metal compounds, and organic materials make up a large fraction of industrial products. The metals are relatively easy to reprocess and reuse. In many cases, however, organic materials are best thought of as energy stored in chemical bonds (largely carbon bonds) rather than as reusable materials; they are generally exothermically oxidizable. The choice between recycling the material and burning it as fuel or otherwise extracting its chemical energy might be made on the basis of comparative market values.
Waste and product materials sometimes contain unwanted ''tramp" elements. These contaminants can ruin the reuse potential of the materials or make handling difficult or dangerous; purification is often problematic. As products are redesigned for newer more cyclical material use, some of the material problems may be eliminated through smarter design. However, it will not always be possible to "design out" problematic materials. For example, zinc is often used to coat steel to prevent corrosion. It can interfere with the desirable properties of new steel forged from melted recycled scrap steel. Steel mills therefore limit the permissible content of zinc in the scrap they buy or they pay less for scrap with more than a threshold concentration of zinc. Such matters are generally handled by scrap dealers, who blend zinc-free and zinc-coated scrap in the mix they sell to the mills. In the long run, this practice may lead to a zinc contamination problem in the steel-scrap recycling business, but there are no good, inexpensive substitutes for zinc as an anticorrosion coating. The blending process is sometimes regarded as a case of sham recycling, because the zinc is not being recycled. One could say that it is being disposed of in the steel. Such sham recycling, however, may be preferable to uncontrolled releases of zinc to the environment.