The Science of Recycling

Background

The reclamation of common metals in the UK has long been established. Very little is recovered from domestic waste. Most cost-effective, however, is the recovery of tin and steel scrap from food cans. Recycling is increasing as local authorities and the public become aware of its advantages.

Recycling of steel has to be carried out at the integrated steel works of the BSC. This limits the amount of scrap recycled. Aluminium

Although not dating as far back as ferrous metal recycling, the recovery of aluminium is also a well-established industry which is on the increase.

Technically the smelters can recycle a wide variety of scrap, but in practice the amounts and types processed are limited by a combination of factors. One of the most important is the presence of contaminants. These may be present as loose materials, bonded or physically attached to the aluminium, or as alloying elements.

Some are removed during the high-temperature processes, but they can cause problems. Some plastics and organics, when combusted, give off highly toxic vapours. Their removal is expensive.

Recovery is made cost-effective by the relatively low temperature required (660 degrees C, the melting point of aluminium), compared with that of the electrolytic extraction from bauxite, which approaches 1000 degrees C. 150,000 KWh of electricity are needed to produce just 1 tonne of primary aluminium. The extraction of aluminium from recycled sources uses only about 5% of the energy which would be required to extract the metal from its ore. Iron

Scrap steel is added to the steel converters before cast iron from the blast furnaces is poured in. This is to break up the flow of the iron, which could fracture the converter if it hit the bottom directly. It also serves to control the temperature during the process.

The programme illustrates the process of the recycling and recasting of cast iron to meet customers' requirements. An electric induction furnace with a capacity of approximately 1 tonne, operating marginally above the melting point of iron, is charged with scrap iron. When this has melted, a charge of magnesium alloy is added. This alters the structure of the carbon present from flat plates to spherical drops. These make the iron much more flexible and improve its casting qualities. Since the charge is almost pure iron (about 97%), the process is both efficient and economic compared with the blast furnace, which uses ores containing about 65% iron. Plastics

In 1995, plastic waste in the UK was estimated to be approximately 2.4 million tonnes annually. In that year 112,500 tonnes of plastics were recycled: this is only about 5% of the amount produced. The UK was a net importer of nearly 2 million tonnes per year of commodity plastics.