There is much value in a large range of solid and gaseous waste materials generated in the process of making steel. Their scientific recycling offers multiple benefits to steelmakers, as it helps keep the environment clean. Commercial gains from waste recycling and the privilege of wearing a ‘friends of environment’ badge are reasons for progressive steelmakers like SAIL and Tata Steel to pursue zero waste target. In an era of high cost of raw materials and energy and the government ever raising the bar on clean environment norms, the steel industry remains under pressure to invest in technologies for reduction in waste generation and optimum recycling of metallics derived from slag and sludge.
Unarguably the most environment friendly group in the industry, Tata Steel has been able to achieve an 85 per cent rate of recycling of solid waste generated from the blast furnace (BF) areas, where hot metal is made, to rolling mills wherefrom finished steel products come, either in its own plant or by way of selling as input materials to other industries. Pressure is building on steelmakers to send minimum waste to landfills because of growing space constraint and environment hazards. Simila-rly, as all industries will have to make do with lesser water, steelmakers, big users of water, will only at their own peril not commit themselves to total recirculation, except for what is lost to evaporation.
“We need a holistic appr-oach to waste management and recycling of anything of value extractable from waste materials. The exercise, ideally, should start before iron ore and coke are fed into BF. Beneficiated iron ore will have much reduced gangue content and washed coal will give coke with low ash. Feeding of such clean materials in BFs will offer twin benefits. As BF energy efficiency and productivity will improve, it will be possible to bring down slag generation from a high of 400 kg a tonne of hot metal to 250 kg, which is the case with many steel plants abroad,” says SAIL chairman Chandra Sekhar Verma.
In fact, in the industry’s scheme of things, beneficiation should get high priority, as that would permit use of low grade iron ore and fines. Seeing all these as waste materials will be considered a sacrilege in many parts of the world, including China, where ore with 25 per cent iron and less is beneficiated.
In fact, it is time our steelmakers with captive iron ore mines and merchant miners had made it a practice to recover the mineral from slime in tailings dams, an environment hazard. SAIL wants a third party to recover and then beneficiated iron ore from slime and run a one-million-tonne (mt) pellet plant downstream on a conversion cost basis. The steel lobby had the government’s ear when export duty on lump ore and fines was raised to 30 per cent and basic customs levy on ore beneficiation and pellet plants reduced to 2.5 per cent from 7.5 per cent. All this was done to conserve a natural resource for local use. The ball is now in the industry’s court to rapidly step up pelletisation capacity from the present 18 mt. Recycling in the steel industry has got to start from this point.
Let’s have a quick run through all the areas in a steel plant where waste is generated but offering scope for recovery of metallics, gases and heat for recycling. Slag that comes as BF residue blends well in a granulated form with clinker and gypsum for production of cement. This way around 90 per cent of slag gets recycled earning revenue for the steelmaker. Slag, which comes out of steel melting shop (SMS)-basic oxygen furnace (BOF), is of much higher value, for it contains 10 per cent steel. So at SMS-BOF, generation of 150 kg slag for one tonne of steel will allow extraction of 15 kg of steel scrap through magnetic separation. Recycling here happens by way of melting scrap in furnaces. Post extraction of steel, SMS-BOF slag finds use in road construction and making of railway ballast. Going a step forward, some mills in China and South Korea are granulating this slag for making tiles and cement and for use in mine stowing.
In economy of water use and its recycling, Tata Steel, SAIL and some others are not far behind best global practices. The ones in the forefront of conservation are doing with about 3.75 cubic metres of water per tonne of crude steel. At the same time, there are many over-indulgent steelmakers, particularly in the induction furnace sector, which use six cubic metres of water and more per tonne of steel. Gases available in BF and SMS-BOF as by-products are recycled in the plant as fuel. Dusts separated from such gases have in them iron and they go into the making of briquettes fed into a BF. “Responsible steelmakers remain vigorous in their pursuit of waste recycling. To the extent there is recycling of metallics and coal, the mining intensity will be less giving a boost to resource conservation. Our target is to ensure no valuable materials find their way to landfills,” says Verma.
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