Secondary Sources

Natural resources are called primary sources. As the grades of ores become poorer in some countries, availability of some ores drops and finally, as the amounts of metals produced increase, recycling has become a more important source of metals. There are various general sources for secondary metal production:

• Manufacturing scrap: Rejects, shavings, turnings, metal production scrap (the latter is called “home scrap” – metal that is recycled within the producing plant). “New scrap is manufacturing scarp that is sent off-site for recycling.”
• Obsolete (or “old”) scrap: Cars, beverage cans (53.8 billion aluminum cans in the U.S. in 2002 were recycled; about 53% of the total produced), dead batteries (80% of the lead used in lead-acid batteries is recycled), photographic film, catalytic converters, old rail, computers (for precious metals), etc.
• Metallurgical wastes: Flue dusts, effluent streams, off gases, tailings etc. Treatment may be required by environmental regulations, often resulting in saleable products, but often at a loss.

Some statistics for recycling in the U.S.A. are shown in Table 7. It can be seen that this is a multi-billion dollar business; roughly $15,000,000,000 in 2002 in the U.S. alone for the metals shown. The table also indicates the growth in recycling since 1980. Some other metals that are recycled include antimony, beryllium, bismuth, cobalt, gold (4.6 tonnes in 1980), mercury, PGM, selenium, silver, tantalum, and tungsten. Recycling of complicated scarp materials (e.g. circuit boards) is an area of research and hydrometallurgical methods may have a role to play.

In many instances the energy required to recover a metal from scrap may be much less than that required for primary metal extraction from ores. This is illustrated in Table 8. One of the major costs associated with metal production is reduction from ions (formally, as in carbon reduction of iron ore, or actually as in electrolysis, e.g. of aqueous Zn+2) to the metallic state. The energy required to produce aluminum metal from Al(OH)3 is very high compared to that to produce lead from PbS. This is evident from the standard reduction potentials, Eo, which will be discussed later. Thus Eo for Al+3 + 3e- = Al is -1.68 V, whereas that for Pb+2 + 2e- = Pb is only -0.13 V. (Recall for now that Eo is related to Go. The larger a positive Go, the more energy is required to make a process proceed. Other metals may require energy-intensive production processes due to low grades. Titanium, which is not listed in the table, is the most energy intensive metal because of the low grades of ores, complex chemical processing and a high negative reduction potential (Eo for TiO2 to Ti is -1.08 V.) Recycling may also be driven by legislation, e.g. lead due to its high toxicity.