Don’t slag off slag

Graham Ellicott pleads for a fairer deal for the man made mineral with an unfortunate name.

Where there’s muck there’s brass, goes the old phrase from Yorkshire in the north of England: there’s money to be made from waste products by those prepared to deal with them. The concept has turned out to be especially true applied to slag, which in the past was viewed as a waste material. The term relates back to the tapping of waste from a metallurgical process.

Today, although “slag” may be vituperative in the vernacular, this versatile recycled material turns up in all sorts of valuable applications. The first slags date from around 3500BC from the production of bronze. Since then, slags have been produced from a wide variety of other metal manufacturing processes, and have been increasingly used in many different products and markets. As such they often compete with naturally occurring industrial minerals: the use of slag must rank as one of the earliest examples of the recycling of waste.

Blast furnace slag is perhaps the best known of all of the slags. In the furnace, iron ore, coke and limestone are mixed together and large volumes of hot air are injected into this mixture. The hot air and coke react together to form carbon monoxide and carbon dioxide that in turn chemically react with the iron oxides to reduce the latter material to molten iron. During this process the heat melts the limestone and reacts with impurities such as sulphur to form molten slag.

The molten products are tapped separately and the slag is either allowed to air cool to form crystalline blast furnace slag, or it is passed through a trough of high pressure, high volume water sprays, where the heat energy contained in the molten slag causes it to explode and instantly form slag which is in granules, typically 2-3mm in size.