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.
The metal scraps are two types, the new and old. The discarded metals in different forms generated within an industrial setting either in production stage or fabrication stage, are called new scraps. The metal scraps obtained after use in different sectors of human activities and rejected in the form of discarded infrastructures, automobiles, industrial equipment, consumer goods etc, are called old scraps. The new scraps which are produced in the industries, are generally least contaminated with other elements and can be easily purified and reused. On the other hand, the old scraps are heterogeneous in nature and contain a large amount of different type of alloying elements, plastics, glass, wood etc. Therefore, these old scraps have to be processed first by different physico – chemical routes to remove the unwanted non metallic materials and then by chemical processes to separate and recover the principal metal as well as the alloying metals.
Some of the advantages in processing scraps and rejects and bringing back to the main stream are summarized below:-
- By utilizing the scraps and rejected metallic materials, it is possible to conserve their respective mineral resources in the mines for meeting our future demands.
- Instead of reprocessing, if the metal scraps and rejects are dumped in the earth crust, particularly the electronic and nuclear wastes, a lot of environmental problems arise.
- Processing the metallic wastes to produce reusable metals and alloys, involves least expenditure compared to extracting the 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 the metals from scraps from energy point of view. For example, the steel produced from iron ore consumes nearly 3 ½ times more energy than steel from the scrap; similarly, producing copper from its ore requires 5 to 7 times more energy than that by recovering the metal from it’s scrap and aluminium obtained from bauxite requires nearly 20 times more energy than that from recycling the old scrap.
- In addition to these, elaborate infrastructural facilities, extra water, energy and space etc required for extracting the metal from the mineral, are much more compared to producing it from the metallic wastes. In view of these, all over the world, efforts are being made to recover the metal values from scraps and rejects generated in different sectors of human activities particularly the municipal solid waste stream and industrial wastes.
The major sources of old scraps and wastes are municipal solid wastes. Rejected steel from old buildings, bridges, rail roads, steel containers, packaging materials, rejected aluminium structures, cans, old automobiles, lead acid batteries, old electrical and electronic equipments etc., are some of the important ones. It is reported that, during 1995, in United States of America, about 31% of durable steel goods of 2.4 million metric ton (MMT), 54% of steel containers and packaging materials (1.4 MMT) were 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 materials were also recovered. At present with further increase in industrialization, in U.S. A and also in other parts of the world huge quantities of old scraps, electrical and electronic rejects etc., are being generated, and these metal values should be recovered and reused in an environment friendly manner.
Similarly, in industries, large amounts of secondary metal scraps, dross and dusts are generated and these 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 amounts of the oxides of silicon, iron, aluminium as well as other metallic oxides. The iron oxides can be separated magnetically, the ash can also be melted to produce ferro silicon and the aluminium rich slag. The secondary sources containing metals also include the slags, dross and flue dust. For example, the Electric Arc Furnace (EAF) dust contains considerable amounts of metal oxides like zinc, lead and cadmium, the copper converter slag contains appreciate amounts of nickel and cobalt along with copper, the tin extraction slag contains large amounts of refractory metals like niobium, tantalum, zirconium, titanium etc., along with some tin.
The largest single source of scrap metals come from discarded automobiles. These scraps contain 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 the U.S.A. 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, the 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 are also used for producing brass and bronze alloys respectively .As a result, in due course such non ferrous metals and alloys are generated as scraps. Therefore, from these scraps, it is essential to recover the metals for reusing.
In India, with growing demand for various types of metals and alloys, sincere efforts should be made to systematically collect the scraps and other metal containing wastes from the industrial sector as well as from municipal wastes for recovery of all the 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 the non-ferrous metals both from domestic as well as imported scraps and metallic wastes wastes, by using best available technology.