Steel Yourselves
February 2000

Boron tames steel slag
Steel mills have never been known for being the most attractive of places. They can be very hot, noisy, and dirty. The process dictates that this be so – making refined, tough steel from impure, weak 'pig-iron' involves really heavy machinery, rivers of molten metal, piles of ingots and all the paraphernalia of heating, cooling and moving it all around the site.

And there's slag, mountains of it. Destined maybe for road-building or cement manufacture, but at the steel mill, it's a real nuisance. Upon cooling, some slags form dust. When the weather is dry, it is blown by the wind; when it is wet, it becomes a sea of mud.

But slag is not just an unfortunate, inevitable, consequence of steel making. It is more than that – it is a necessity.

Turning 'pig iron' into steel is simple (but chemically quite complex). Get the unwanted impurities like silica, phosphorus and manganese out of raw iron, and desirable elements in – to achieve the right proportion of carbon, and for specialty steels, introduce chromium, tungsten, nickel, and others.

Floating on the molten metal in the furnace is slag, which at the start of this high-temperature chemistry is in fact a mixture of key reagents which have been put there deliberately. The interface between steel and slag is where the important chemical reactions happen.

After the process has finished, steel can be tapped off to be cast, rolled or forged. But before then, having given up its useful content to the steel, the slag also molten at this stage and containing all the impurities, has to be poured off. Now waste material, it is carted away to cool, either naturally or by being sprayed with water, and then to be stored.

A mixture mostly of silica and calcium oxide, slag's physical chemistry can be odd. Upon cooling, slags from some processes form weak rock-like material which slowly disintegrates, forming dust and mud which is difficult to deal with. The dust forms from what are called 'falling slags', because of density changes in the slag as it cools.

But add a specialized borate to the molten slag, and this all changes. In plant trials, the destructive density changes have been shown to be absent.

The slag cools in a stable way and the result is a slag off-take in the form of fist-sized rocks. The benefits are that there is hardly any dust to blow around the site and beyond, meaning a safer working environment and being a better neighbor. There is no mud. There is better handling with no water cooling – in most cases this means a reduction in costs resulting from lower energy consumption and easier disposal. And the slag, dusty no more but rock-like, is a far more useful construction material.