Recovering Metals From Scrap Must for Sustainable Development
India has modest reserves of iron ore, bauxite and coal, but very little deposits of other non-ferrous metals. Considering the increasing need in the country for iron, aluminium and other metals during future years. It is apprehended that the mineral resources of the country may be exhausted soon. In view of this, it is essential to conserve these as much as possible and supplement the requirements of various metals and alloys by recovering the metal values from secondary sources.
Metal scrap is of two types – new and old. Discarded metal in different forms, generated within an industrial setting either in the production or fabrication stage, is now called new scrap. The metal scrap obtained after use in different sectors of human activities and rejected in the form of discarded infrastructure, automobiles, industrial equipment, consumer goods etc. is called old scrap.
New scrap, which is produced in industries, is generally least contaminated with other elements and can be easily purified and reused. But old scrap is heterogeneous in nature and contains a large amount of different types of alloying elements, plastics, glass, wood etc. Therefore, old scrap has to be processed, first by different physio-chemical routes, to remove unwanted non-metallic material and then by chemical processes to separate and recover the principal metal as well as the alloying metals.
Some of the advantages in processing scrap and rejects and bringing back to the main stream are summarised below:
- By utilising scrap and rejected metallic material, it is possible to conserve their respective mineral resources in the mines for meeting future demand.
- Instead of reprocessing, if the metal scrap and rejects are dumped in the earth crust, particularly electronic and nuclear waste, a lot of environmental problems arise.
- Processing metallic waste to produce reusable metals and alloys, involves least expenditure compared to extracting metals from the minerals through mining the minerals, beneficiating the low-grade ones, extracting the metals and finally producing the required alloys.
- It is also attractive to extract metal from scrap from the energy point of view. For example, steel produced from iron ore consumes nearly 3½ times more energy than steel from scrap; similarly, producing copper from its ore requires 5 to 7 times more energy than that by recovering the metal from its scrap and aluminium, obtained from bauxite requires nearly 20 times more energy than that from recycling old scrap.
- In addition to this, elaborate infrastructural facilities, extra water, energy and space etc. required for extracting metal from mineral is much more compared to producing it from metallic waste. In view of this, all over the world, efforts are being made to recover metal values from scrap and rejects generated in different sectors of human activities, particularly the municipal solid waste stream and industrial waste.
The major sources of old scrap and waste is municipal solid waste. Rejected steel from old buildings, bridges, rail roads, steel containers, packaging material, rejected aluminium structures, cans, old automobiles, lead acid batteries, old electrical and electronic equipment etc. are some of the important ones. It is reported that during 1995, in the United States of America, about 31% of durable steel goods of 2.4 million metric ton (MMT), 54% of steel containers and packaging material (1.4 MMT) was recovered. Similarly, 0.9 MMT of aluminium cans, 0.825 MMT led acid batteries, 6.6 MMT of discarded containers and 2.2 MMT of packaging material was also recovered. At present, with further increase in industrialization in USA and other parts of the world, huge quantities of old scrap, electrical and electronic rejects etc., are being generated. Their metal values should be recovered and reused in an environment- friendly manner.
Similarly, in industries, a large amount of secondary metal scrap, dross and dust is generated. This should be recovered. Some of the important ones are nickel, chromium, cobalt, copper, vanadium, molybdenum, copper and many other metals from flu dust, filter cakes, mill scales, grindings, nickel cadmium batteries and used catalysts. For example, ash from coal combustion contains considerable amount of oxide of silicon, iron, aluminium and other metallic oxides. Iron oxide can be separated magnetically, the ash can also be melted to produce ferro silicon and aluminium rich slag. The secondary sources containing metals also include slags, dross and flu dust. For example, Electric Arc Furnace (EAF) dust contains considerable amount of metal oxide like zinc, lead and cadmium; the copper converter slag contains appreciate amounts of nickel and cobalt along with copper; the tin extraction slag contains a large amount of refractory metals like niobium, tantalum, zirconium, titanium etc., along with some tin.
The largest single source of scrap metal comes from discarded automobiles. This scrap contains a lot of iron and steel as well as some non-ferrous metals, mainly aluminium. In 1993, iron and steel comprised 90% of the weight of the old scrap recycled in USA, while it represented only 42% of the metal value. In value terms, aluminum follows with 26%, copper at 15%, and gold with 10% of old scrap value.
Like stainless steel, non-ferrous metals and alloys are also used considerably in various sectors. For example, about 55% each of magnesium and zinc are used to produce aluminium alloys for various purposes, similarly, 13% of the total tin is used as a coating material for corrosion resistance, and along with copper, 20% of zinc and 10% of tin is also used for producing brass and bronze alloys respectively. As a result, in due course, such non-ferrous metals and alloys are generated as scrap. Therefore, it is essential to recover metals for reusing from this scrap.
In India, with a growing demand for various types of metals and alloys, sincere efforts should be made to systematically collect scrap and other metals containing waste from the industrial sector as well as from municipal waste for recovery of all metal values. As the deposits of the minerals, particularly of non-ferrous metals, are very scanty in our country, it would be highly essential for our sustainable development to recover non-ferrous metals both from domestic as well as imported scrap and metallic waste by using the best available technology.
(The writer is former Director General, Council of Scientific & Industrial Research, India, former, Planning Board Member, Government of Odisha, founder Chairman, Institute of Advance Technology & Environmental Studies (IATES) and founder president, Natural Resources Development Foundation)