August 28, 2005

Successful Recycling 

I have already expressed my skepticism regarding the recycling of plastics, especially when this is seen as a way to Save the World. But other materials can and are recycled with much more success.

One of the best ones is steel, but this is true for almost all metals.

Most metals (excluding the precious ones) are normally found as oxides and other salts, and in order to extract the metal a reduction step is required - after having mined and processed the ore, which can require pretty big amounts of energy. Iron oxides can be reduced using carbon under the form of coal in monstruous facilities called blast furnaces. Coal is reasonably cheap, but steelmaking consumes huge amounts of coal, and oil and natural gas in side-operations. Scrap steel is already reduced, so it takes only a relatively small quantity of carbon to melt and process it in special furnaces which can use the fuel directly, or are electrically fired.

Copper is produced in a similar fashion, as are other metals. Aluminium is different: its oxides cannot be reduced with carbon, so an electrolytic process has been developed to reduce molten aluminium salts. It's neat, but it requires an awful lot of electricity. Other useful metals such as Nickel and Titanium require peculiar processes too.

It is pretty easy to separate steel from other metals using magnets, and scrap metals do not require to be cleaned before processing, because at their melting temperatures, even for low-melting metals, all impurities will burn and/or melt forming slag that floats on the molten bath. Given that the formation of some slag is already taken into account (indeed, slag is often necessary), there are no particular problems with the presence of impurities. Different metals may stratify spontaneously in the melt due to their density (aluminium tend to go in the slag) and can thus be separated.

Steel is already so cheap that (big) scrap steel traders often don't bother to deal with small quantities, but if you've got a steel bridge to knock down, or an old ship to dispose of, they'll be happy to help. When I was younger and skint, I made a little extra money by recovering copper and a bit of lead from old, scrapped electrical cables. Metals are also much denser than plastics (if you exclude unpressed aluminium cans, which traders are unwilling to accept) so transportation is less of a problem.

Glass (the common variety) is produced by melting together siliceous sand and soda (plus a small amount of other additives) at 1200 C: eventual impurities will again form slag. The situation is a bit more complicated, tho: in order to produce recycled clear glass you have to use rather pure clear glass as a raw material, and separating glass by colour is feasible but not really trivial. Mixed glass will produce a green or brown material, which has a quite vast market anyway. Recycling of waste and defective products is something that glassworks already do internally from a long time.

Also paper can be recycled with good results. Paper is made mainly from wood (nowadays, fir or poplar trees that are cultivated for that specific reason) which is first ground with water in a fine pulp; the pulp is then bleached, if required added of fillers etc and finally pressed into a thin sheet while being dried. Scrap paper is pulped and the pulp processed in the same way - it only rquires a little more bleaching. Recycled paper is usually greysh or brownish, but this is not a big problem for packaging purposes. If a better look is required, a sheet of virgin paper and one of recycled material can be pressed together. Also paper is being recycled from a long time; factories, printhouses, returned/defective magazines and books are all sources of sizable amounts of scrap paper and cardboard that can be pressed in bales, shipped to a paper mill and promptly recycled.

Update 30/08: There is another reason that makes recycling metals and glass more successful than recycling plastics. While polymers always undergo a certain degradation during recycling, metals do not - because they are elements, not molecules. Glass is made of molecules, but these are thermally stable. The crystal structure of steel will be lost with melting, but if a prticular crystal structure is required (in the case of high performance steels or aluminium alloys) it will be produced subjecting the alloys to the appropriate thermo-mechanical treatments, whatever the origin of the same alloys is. Finally, glass has no crystal structure to speak of.


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