Crucible Test for Lime Reactivity in Slags

The rapid acceptance of basic oxygen furnace (BOF) steelmaking has caused a veritable revolution within the steel industry and also the lime industry as a major supplier of a raw material. The speed of the process requires a lime product which goes into solution rapidly to form a slag. Field observations of the BOF steelmaking process showed major differences in the rate of slag formation with various commercial lime products. Operating practices could be adjusted to take poor lime into solution but would have a detrimental effect on the furnace refractories and metallurgical processing. The slagmaking reactions are discussed.

The ASTM water reactivity test for lime would differentiate between the lime products observed to react differently in BOF slagmaking. However, arguments were made that this test did not simulate slagmaking or lime solution rate in a slag. A crucible test was devised in which minus 5 plus 6 mesh lime sample is mixed with a lime deficient slag and heated for various lengths of time. The free lime is determined in the fired samples and plotted against time. This gives an indication of solution rate. A correlation was found between this crucible test and the ASTM water reactivity test for commercial lime products of interest. There are ways to lower water reactivity without decreasing slag solution rate, notably through prereaction with other slagmaking ingredients. The crucible test has been used to evaluate these experimental products.

The water reactivity test is adapted more easily to plant quality control than is the crucible test. Since the correlation existed between the two for products of interest, the water reactivity test was selected for control of incoming lime product. Specifications were established for reactivity, loss on ignition, sulfur, and calcium oxide content. The specifications are realistic and can be met by commericial suppliers of lime.