Separating the good from the slag

By Angela Beggs

Let me just say that in my primary school days the term slag had a completely different meaning than it does here now – one more closely related to the production of saliva than that of steel.

If a classmate turned to me and declared they have a pilot plant that processes 100 kilograms of slag per minute, I’d be running for cover with my lunch box strapped to my head as protection.

Thankfully, this slag story has a far more promising and dryer ending than the last scenario could have.

In the steel industry, slag is a high volume by-product from the metal smelting process. A steelworks that produces one million tonnes of steel a year also produces around a quarter of a million tonnes of molten slag.

Slag is poured onto a spinning disk and broken up into droplets.

Slag is commonly air-cooled in large pits and then used as a material for road works, but another cooling process uses water to produce granules for cement production. This process reduces the greenhouse gas emissions involved, but it consumes large amounts of water and can cause nasty air pollution.

Dry granulation, on the other hand, saves the water that would have been lost to the atmosphere, estimated at up to 60 gigalitres (that’s 60 billion litres!) a year for the steel industry globally.

In dry granulation, molten slag is poured on to a spinning disc and broken up into droplets. When the droplets contact the air, a heat exchange takes place in which the air becomes hot and the slag cold. The process captures the heat and uses it for other purposes, like electricity production.

All of this while reducing the world steel industry’s gas emissions by around 60 million tonnes per year.

Our dry slag team is now working with leading international engineering companies to conduct industrial scale piloting that will involve processing one tonne of slag per minute.

Now, where’s my lunch box?