Introduction

Blast Furnace Slag (BFS), Steel Furnace Slag (SFS), Electric Arc Furnace Slag (EAFS), Melter Slag and KOBM are all manufacturing co-products of iron and steel production processes.

They [slag] develop in the molten state during the manufacture process and as such their quality and consistency at formation reflect directly the quality of the iron or steel that is formed at the same time. Being lighter than their metal co-products, they are separated in the molten form by gravity. Analogous to removing the cream from the top of milk given slag lower specific gravity.

Molten BFS is formed in a blast furnace along with molten iron from iron ore in the reducing presence of heated air, coke and limestone. The resulting molten slag and iron, once tapped (removed) from the furnace are subject to further processing.

For molten BFS, the initial processing options are to allow it solidify slowly by air cooling to form rock slag.  Air cooled BFS can be crushed like normal quarry products for use as aggregates in concrete and pavement manufacture.

Molten BFS alternative initial processing options are to rapidly quench it to form Granulated Blast Furnace Slag or GBFS which looks similar to coarse river sand.  

This solidified slag product can be further processed or ground to form Ground Granulated Blast Furnace Slag or GGBFS used as a cement replacement for a range of end user products in the construction industry.

SFS is the co-product of steel formed in the Basic Oxygen System.  The slag is tapped (removed) from the vessel after the exothermic refinement of molten iron and recycled steel in the presence of fluxes and oxygen.

EAFS is the co-product of steel formed in an Electric Arc Furnace.  In the process, steel scrap and fluxes are added to a refractory lined cup-shaped vessel. This vessel has a lid through which carbon electrodes are passed. An arc is induced between the scrap and electrodes and the resultant heat generated melts scrap and fluxes. Steel and slag are also separated similarly to the SFS process.

Iron is mined using conventional earthmoving equipment before being separated magnetically, by creating a slurry and running it over magnetic drums. This is followed by gravimetric separation through a series of cones and spiral separators, where the heavier iron-bearing materials gravitate towards the centre, while residual clays and silts gravitate outwards. The slurry is then pumped 18 kilometres to the steel mill through an underground pipeline, where it is finally dewatered and stockpiled.

To convert the iron, a direct reduction process is used, adding coal and limestone to the irons before pre-heating them in four multi-hearth furnaces. This drives off the volatile constituents of the coal.  The material then enters one of four rotary kilns where the direct reduction takes place over a period of eight hours.

The directly reduced product is then melted in one of two large electric melters. It is from this stage of the process that SteelServ Ltd obtains about 250,000 tonnes per annum of "melter" slag. The chemistry of New Zealand’s melter slag differs from other variants of slag. This product consists of a high percentage of titanium and quantities of magnesium oxide and alumina typically higher than the industry norm. In contrast, the material has a characteristically low amount of silica, calcium oxide and sulphur.

New Zealand Steel uses a KOBM Oxygen Steel Converter vessel.  The vessel is charged with the molten iron from the melters and a small proportion of scrap before refining begins using a top lance and bottom blown tuyeres to produce. The remaining steel making operation follows conventional practices, apart from the chemistry of the slag, which again differs from international equivalents due to irons and source.

KOBM slag is high in fines and cannot be used as an aggregate for road making or surfacings. KOBM can be used, however, as a lime substitute in stabilising clay sub-bases, as an additive to cement manufacture and as a soil conditioner for horticultural farming.