Steel Without Slag?
Borate vitrification offers new possibilities
December 1995

Steel is made through the removal of non-ferrous impurities from iron. This is accomplished either in an oxygen furnace or electric arc furnace in the presence of carbon, a small proportion of which is essential to the final alloy. Traces of elements such as nickel, chromium, and molybdenum may be added to improve or adjust the steel's final properties. The original impurities, for example, silica, alumina, sulphur, phosphorus, are removed by lime-based flux agents, and form a molten slag which floats on the surface of the steel melt, until it is poured off. As it cools, the slag expands, and forms a dusty calcium silicate residue which is perceived today as a potential pollutant. Slag can be bad news for steel makers and the environment.

Slag cooling and disposal have presented particular problems. The conventional treatment is to pour molten slag into a pot, dump it in a disposal yard, and leave it to cool naturally, perhaps assisted by water spraying, after which it can be crushed by a bulldozer. This is a noisy, dusty process with some potential risk of steam explosions. While such practices may not conflict with health and safety regulations, many modern steel makers have sought to improve working practices and conditions in this area.

Since steel cannot, under current technology, be produced without slag, the re-use of slag is an attractive proposition on both economic and environmental grounds. Certain slags can indeed provide viable raw materials for cement, paving slabs, even soil conditioners. However, the reducing slags from electric arc furnaces and from secondary - ladle - refining are particularly difficult to re-use. They have a strong tendency to weather, and form powders of extremely fine, dusty consistency. This is because the calcium silicate in the slag expands so much that it destroys the crystalloid form of the other slag components.

Slag collapse and de-crystallization can be prevented by adding certain expensive, hazardous, or esoteric elements to the slag, including gold, silver, tellurium, and beryllium. Technically they must be elements whose ion radius is either greater than that of calcium or less than that of silicon. However, these slags can also be stabilized by boron, an element which is safe, relatively inexpensive, and readily available, in the form of pentahydrate borax (Neobor®) or anhydrous borax (Dehybor®). This has recently been demonstrated very successfully by Astec Irie, a company which supplies technology, know-how, and maintenance services to the steel and metal processing industries. It is located in Kyushu, Japan's southern island.

Astec Irie has developed a slag modifier, Kailex-B, based on Dehybor (45 percent) and a special silicate. This can be added to the slag pot or to the slag as it is poured off. Alternatively it is introduced to the furnace just before steel tapping, so that there is no chance of boron pick-up by the metal itself. Because it can strengthen steel, but reduces its ductility, very small amounts of boron are introduced to certain special steels; however, most steels do not require boron.

Kailex-B induces the slag to form a vitreous mass rather than a fine dust or powder. The vitrified slag - which cools into roughly egg-sized pieces - is safe to transport, powder- free, and suitable for re-cycling. As little as 0.3 percent boron in the slag will effect sufficient vitrification.

In conjunction with Nippon Steel Corporation, Astec has also improved the slag cooling and treatment process. Molten slag is rapidly cooled on a special steel pan, then transferred to a water pool for secondary cooling. This improves the working environment, increases productivity in the slag treatment area by some 75 percent, and eliminates the need for bulldozing or primary crushing.

The Astec Irie process has already been adopted outside Japan at Bohsan Works in China. In the longer term, it is now seen as the basis for treating huge amounts of furnace slag from oxygen converters around the world, as well as for modifying electric furnace slag.