The challenge
Water-intensive slag waste processing
Slag is a high volume by-product from the metal smelting process. Globally, hundreds of millions of tonnes of molten slags are produced each year.
Slag is commonly air-cooled in large pits and landfilled or used for road-based materials after crushing and screening. In some modern integrated steelworks, molten blast furnace slag is granulated using water jets — producing a glassy product that can be used as valuable feed in cement manufacturing by replacing ordinary Portland cement and significantly reducing the greenhouse gas emissions associated with concrete production.
However, water granulation consumes large volumes of water and may generate acid mist causing air pollution. More importantly, the wet process does not recover the large amount of high-grade heat contained in molten slag. On cooling from around 1500 °C to ambient temperature, one tonne of molten slag releases about 1.8 gigajoules (GJ) of heat. This could amount to 800 petajoules (PJ) of heat globally from blast furnace slags alone and, if recovered and utilised, could potentially reduce greenhouse gas emissions by up to 60 million tonnes each year.
Our response
Recover heat and create valuable waste products
We developed a new, integrated dry granulation and heat recovery process which promotes sustainability and full value recovery.
Molten slag is atomised under centrifugal forces exerted by a spinning disc to produce droplets which are then quenched and solidified using air to recover the heat. This method produces glassy slag for cement production and simultaneously recovers waste heat as hot air, which can be used onsite for drying, preheating or steam generation.
Compared with water granulation, dry granulation provides a much more sustainable approach, overcoming major shortcomings of the existing method through saving water, eliminating sulphur emissions and recovering high-value waste heat.
The results
A sustainable process to lower water use and increase value
The dry granulation process is destined to replace the conventional water granulation process, delivering sustainable, environmentally friendly and full value recovering process with benefits of:
- waste heat recovery
- huge savings in water use
- reduced air pollution
- lower capital costs.
By converting low value air-cooled blast furnace slag into a high value material for the cement/construction industry, this process turns a waste product into wealth.
We are partnering with Beijing MCC Equipment Research & Design Corporation (MCCE) to commercialise the dry slag granulation technology. A 20-tonne-per-hour demonstration plant was commissioned at an industrial site in China in 2019.