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Making steel is a dirty business. It is energy-, resource- and carbon-intensive. For every tonne of steel, industry estimates show, some 1.8 tonnes of carbon dioxide is spewed into the atmosphere. As a consequence, the steel industry is reckoned as among the top emitters of greenhouse gases in the world. Production of steel also involves a variety of valuable natural resources, from iron ore to manganese to hematite. In recent years, the industry has made great strides in reducing its environmental footprints by transitioning away from fossil fuels, improving process efficiency and reusing materials from waste, such as steel scrap recycling. However, achieving circularity is hampered by the large volumes of waste that the industry generates in the form of steel slag. And the problem lies in its very composition.
Production of virgin steel typically follows a two-step process—first, pig iron is extracted from iron ore in the blast furnace, and then refined in a basic oxygen furnace (BOF) or Linz-Donawitz converter to produce steel. During these steps, large volumes of molten impurities, or slag, are generated that need to be skimmed off. According to “Indian Minerals Yearbook 2022”, of the Union Ministry of Mines, 300 to 540 kg of blast furnace or iron slag is produced from a tonne of pig or crude iron, whereas 150 to 200 kg of BOF or steel slag is obtained from a tonne of liquid steel. These waste materials resemble crushed stone and sand, but differ in terms of chemical and mineral compositions, which de-termine their usability.
Blast furnace or iron slag has cementitious properties. It is therefore almost entirely utilised by cement industries as a low-cost replacement for clinker (a mix of limestone and minerals). It is also used as road metals and railway ballast and to make slag wool, an insulation material. But finding uses for steel slag has been notoriously difficult.
Sector experts say that BOF or steel slag is not ideal for construction applications because of free lime content, which reacts with moisture to form calcium hydroxide, and results in volume expansion or swelling and subsequent formation of cracks in roads and buildings. High phosphorous content and specific gravity or density also prolongs the setting time of concrete and lowers its strength. Over 15 million tonnes of steel slag is stockpiled in slag yards of industries across the country, shows an analysis of data with the Central Pollution Control Board (CPCB). If disposed to landfills, heavy metals present in the slag risk contaminating the groundwater.
To prevent environmental pollution and improve circularity, several companies in India are innovating to make steel slag usable, primarily by the ageing treatment.
One such effort is by the Bhilai Steel Plant in Chhattisgarh. The plant under the Steel Authority of India Ltd (SAIL) generates 2 million tonnes of steel slag in a year. Of this, 50-60 per cent is used within the unit in laying kachha roads. To utilise the stockpiled steel slag, the plant in 2023 initiated a pilot project to make paver blocks, by replacing stone chips with steel slag. To ensure the quality, the paver blocks use a mix of cement, sand and steel slag in the proportion of 1:2:4, and are subjected to ageing. First the demoulded blocks are kept in a shelter, away from direct sun and winds, for 24 hours and then cured with water for 14-21 days for complete moisturisation. The blocks are then dried in the natural atmosphere and sent for use. Officials at the plant claim the plant produces 1,000 blocks a day, which helps it save Rs 1.25 million in a year, and reduce the dependence on natural aggregates, such as crushed stones and sand.
The Bhilai Steel Plant is also conducting a trial to replace river sand with steel slag at runners—connected channels at a blast furnace that convey the molten metal to storage vessels. Runner sand prevents molten metal from sticking to the earth material. If successful, officials claim, steel slag will help save the plant Rs 29 million in a year and conserve 25,000 tonnes of river sand in a year.
At Tata Steel Plant in Jamshedpur, the metal elements in steel slag are recovered for selling or reuse. To use the dis-carded non-metallic components, the company has set up an open steam ageing plant with two pits of 1,300 tonnes capacity each, lined with steam pipes. Steel slag in the pits is exposed to steam for 72 hours continuously. Officials part of the project say the steam maturing quickens the process of natural ageing that may take up to three months. The ma-tured slag, sold under brand name Tata Aggreto, has less than 2 per cent expandable properties and can be used in construction. “Tata Aggreto helps conserve 16,500 million tonnes of natural aggregates for every kilometre of national highway. It also has no environmental side effects,” says an official with Tata Steel, Jamshedpur. Using steel slag, the plant has developed another product, Tata Nirman, which it claims can be used to replace sand in the making of fly ash bricks and partially replace limestone in clinker in cement.
Taking a step forward, JSW Steel Ltd at Vijaynagar has developed technologies that can use steel slag to produce aggregates suitable for construction and slag fines that can be used in cement making as well as in agricultural applications (such as neutralising acidic soil to improve nutrient availability). In 2022, JSW set up a steel slag-to-sand facility, dubbed the first in the world. D Satish Kumar, associate vice president, research and development at JSW Steel, says that apart from recycling 0.27 million tonnes of steel slag a year, the project is expected to reduce 100 tonnes of CO2-equivalent of greenhouse gas emissions a year from the use of vehicles for slag disposal within the plant; and reduce the demand for river sand by 0.2-0.3 million tonnes.
The Bureau of Indian Standards and Union Ministry of Steel need to set standards for steel slag usage.
Ensure that the plants utilise the current generation slags within one year and legacy slags by September 2027.
Steam ageing should be considered for maturing of steel slag. The government should prepare a model of supply chain to ensure that matured steel slag is used for road construction.
This article was originally published as part of the cover story The Circularity Argument, published in the February 1-15, 2025 print edition of Down To Earth. The cover story offers a snapshot of industrial initiatives to deal with 10 high-volume wastes such as steel slag, municipal solid waste, fly ash, FGD gypsum, red mud, bagasse, press mud, biomass, hazardous waste, stone slurry and waste paper.