A novel dry method for slag granulation could help the cement, iron and steel industries develop sustainable practices and derive value from waste.

About 300 kilograms of slag is generated for every tonne of iron produced. Slag wastes are usually air-cooled in large pits or water-granulated, and then sent to landfills or used in low-level applications such as road base materials.

Some blast furnace operations water-quench discharged slags to produce glassy granules, which can be used in cement.

"The new, dry way of treating slags offers a number of benefits over the traditional wet method," says CSIRO Minerals Project Leader Dongsheng Xie.

The process involves feeding molten slag on to a rotary disc. This disc spins at high speed, breaking the slag into small droplets and rapidly solidifying them to produce glassy granules with similar properties to those produced by wet granulation.

With these properties, the granules can be used in Portland cement, which is a key constituent of concrete - the most consumed mineral product on earth (at about 1.7 billion tonnes annually).

Producing one tonne of Portland cement consumes about 3000MJ of electrical and thermal energy and emits about 900kg of carbon dioxide, mainly due to the decomposition of limestone in the cement kiln.

Granulated slag can substitute for up to 70 per cent of the Portland cement, leading to significant energy savings and reduction in greenhouse gas emissions.

"The wet granulation method involves high capital costs," says Dr Xie. "And it doesn't contribute to sustainable practices because it consumes excessive amounts of water, doesn't recover heat, and generates environmental problems such as acid mist.

"With a lower capital cost and benefits in heat recovery and reduced air pollution, dry granulation is an attractive alternative to conventional wet granulation."

The CSIRO team has built a pilot-scale facility and tested the dry method with several slags from ferrous and non-ferrous industries.

"With a good understanding of how slag properties influence the granulation process, we can tailor-design processes for a variety of slag wastes," says Dr Xie.

"This process is likely to become part of our future work at the Centre for Sustainable Resource Processing."