Slag corrosion and thermal shock in the reaction zone of zinc oxide rotary kilns

Zinc oxide rotary kilns — including kilns that volatilise zinc from slags and sludges — concentrate the reaction in a high-temperature span. The brick face meets molten slag, gas and a continually tumbling burden, so thermal shock, erosion and chemical attack are superimposed on the same band. The load in the cooler span is much lighter, and the mouths at each end are closer to the thermal shock and impact that a castable is good at.

Infiltration of slag along pores and grain boundaries is the characteristic mode of damage in these kilns. Once the matrix is wetted, aggregates begin to dissolve or to come away, the hot face loosens, and spalling follows. A slag-resistant working lining therefore needs a low open porosity, and a bond that remains tough at high temperature: a brick that is too rigid tends to let a crack run through once it has started. Laying a spalling-resistant wear brick together with a more flexible corundum slag-resistant brick, and using a masonry method that spreads stress, is a way of treating slag attack and thermal shock in the same band.

The raw materials decide the slag. Iron-water slag, nickel-iron slag, lead slag and zinc leach residue differ in basicity, iron content and residual carbon; the high-temperature reaction zone sometimes takes a magnesia-alumina-chrome spinel, to resist attack by FeO, PbO and sulphur dioxide. The arrangement has to follow the raw materials; the zoning that works with one slag should not be carried across to a kiln on a different feed.