Minerals are highly valuable natural resource as these are the principal raw materials used for producing various types of metals like iron and steel, aluminium, copper, nickel, lead, zinc etc and their alloys and also harnessing energy in nuclear power plants by using uranium and plutonium as fuel.
These metals and their alloys are being used in increasing quantities in different sectors like transport, buildings, industrial infrastructures etc. to improve our standard of living. Therefore, more and more minerals are being extracted from increasing number of mines in different parts of India. The mineral resource being non-replenishable in nature, the apprehension is that within a few decades, most of these will be exhausted from the earth’s crust.
In India, most of the minerals — particularly those of non-ferrous metals like copper, nickel, cobalt, lead, zinc, niobium, tantalum, tungsten, molybdenum etc. — are very scanty. Thus, it is essential to conserve these minerals and supplement our requirements of various metals and alloys by recovering the metal values as much as possible from secondary sources and other wastes.
The secondary sources of the metals are of two types — new and old scraps. 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, industrial equipment including consumer goods, electrical and electronic wastes (E-wastes) etc, are called old scraps.
The major sources of old scrap 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.
Similarly, in metal industries, large amounts of metal scraps and wastes are generated, and hence these should be recovered. Some of the important ones are nickel, chromium, iron, lead, zinc and many other metals from flu dust, filter cakes, mill scales, grindings, nickel-cadmium batteries and used catalysts.
Ash from coal combustion contains considerable amounts of the oxides of silicon, iron, aluminium as well as other metallic oxides. The wastes containing metals also include the slags, dross and flue dust. For example, the Electric Arc Furnace (EAF) dust contains considerable amount of metal oxides of zinc, lead and cadmium; the copper converter slag contains appreciable amounts of nickel and cobalt along with copper; tin extraction slag contains large amounts of high-valued refractory metals like niobium, tantalum, zirconium, titanium etc. along with some tin.
The largest single source of scrap metal comes from discarded automobiles. These scraps contain a lot of iron and steel as well as some non-ferrous metals, mainly aluminium. Increasing amounts of waste electric and electronic equipments are being discarded all over the world. For example, more than 50 million tons (MT) of electric wastes (E-wastes) was generated globally in 2009. In India, 146.2 thousand tons of E-wastes was generated in 2005. These E-Wastes are very hazardous in nature but contain valuable metals like gold, silver, platinum and palladium.
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 which are used for various purposes. Similarly, 13% of the total tin is used as coating material for corrosion resistance along with copper, 20% of zinc and 10% of tin are also used for producing brass and bronze, respectively .As a result, such non-ferrous metals and alloys are generated as scraps. Therefore, from all these scraps and wastes it is essential to recover the metals.
The new scraps produced in industries are generally least contaminated with other elements and can be easily purified and reused. On the other hand, the old scraps and wastes are heterogeneous in nature and contain large amounts of different types of alloying elements, plastics, glass, wood etc. Therefore, these old scraps and wastes 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 wastes and bringing back the metals to the mainstream are as follows:
- By collecting and processing used scraps and waste metallic materials, it is possible to conserve their respective mineral resources in the mines for meeting future demands.
- Instead of reprocessing, if the metal scraps and metal containing wastes are dumped in the earth’s crust, particularly the electronic and nuclear wastes, a lot of environmental problems would arise.
- Processing the scraps to produce reusable metals and alloys involves least expenditure compared to extracting the metals from minerals through mining the minerals, beneficiating the low-grade ones, extracting the metals and finally producing required alloys.
- It is also attractive to extract the metals from scraps, from the energy point of view. For example, the steel produced from iron ore consumes nearly three-and-a-half 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 the old scrap.
- In addition to these, elaborate infrastructural facilities, extra water, energy and space etc. required for extracting the metals from minerals, are much more compared to producing it from scraps and wastes.
In view of these factors, efforts are being made all over the world to recover the metal values from scraps and wastes generated in different sectors of human activities, particularly the municipal solid waste stream and industrial wastes.
In recent years, India imports a lot of metals and alloys for various purposes. Over Rs 900 billion worth of non-ferrous metals were imported by India in fiscal year 2019. There has been a consistent increase in value of imports of non-ferrous metals since fiscal year 2011.
In view of this, consolidated efforts should be made to systematically collect all types of scraps and wastes from internal sources as well as import valuable non-ferrous metal scraps including E-wastes at a much cheaper rate and process these by using environment-friendly and best-available technology to recover the metal values, particularly non-ferrous ones. This will help considerably to conserve our limited mineral resources while meeting our requirements of metals, particularly non-ferrous ones, at a much lesser cost.