Agglomerate (steel industry)

This product is obtained by burning coal previously mixed with iron ore and oxides.

The primitive method, which consisted of a grindstone grate, was abandoned towards the end of the 19th century because it was too fuel-intensive.

[L 1] Shaft furnaces then replaced them, their much higher efficiency being due both to the confinement of the reaction and to counter-current operation (the solids sink and the gases rise).

Today, it plays an essential role in the blending of different ores and, above all, in the incorporation of mineral wastes of varying iron content.

[2] It's also a way of getting rid of zinc, the element that "poisons" blast furnaces, as its vaporization temperature of 907°C corresponds to that of a well-conducted roast.

[L 7][notes 2] On the other hand, agglomerate is an abrasive product that damages blast furnace vessels, especially if these are not designed for absorber,[SF 1] and is above all fragile.

Repeated handling degrades its grain size and generates fines, making it unsuitable for packaging at sites far from blast furnaces: pellets are therefore preferable.

Cold resistance, particularly to crushing, can be improved by increasing the energy input during sintering.

However, in addition to increasing the cost of agglomerate production, reducibility deteriorates when mechanical strength is sought.

It is also in this range that mechanical strength is best (and also, the slag's fusibility is the worst, which complicates its removal from the blast furnace).

Above an ib value of 2.6, the proportion of molten agglomerate increases, clogging the pores and slowing down chemical reactions between gases and oxides.

As for acid agglomerates with an ib index of less than 1, softening begins as soon as only around 15% of the ore has been reduced.

[6] The optimum basicity index is therefore determined according to the ore used, the technical characteristics of the blast furnace, the intended use of the cast iron and the desired qualities.