Steelmaking slag has been used in the coastal area for the purpose of the environmental improvement. Short-term alkali elution behavior induced by free CaO in the steelmaking slag was investigated by a batch and continuous vessel. In this study, long-term alkali elution experiments were done in an open channel vessel with a slag box to make clear the decreasing mechanism of the alkali elution from steelmaking slag into seawater. The experimental period was 44 d (1056 h). The pH values increased just in the beginning of the experiment, and then gradually decreased in about 100 min. They showed almost the same value as that of seawater in 1200 min. As the experimental time passed, the white deposits on the steelmaking slag layer, which were composed of Mg[OH] 2 and CaCO 3 , spread to the lower slag zone in the slag box. The white deposit zone of larger slag size was more rapidly diffused downward than that of smaller one due to larger voidage in the slag layer. While carrying out the batch test with the used slag by the long-term experiment, the alkali elution rate of slag layer on the open channel flow was 0.03-0.11 times decreased compared with the unused slag, whereas that of the lower positioned slag layer was 0.10-0.38 times decreased. From these results, it was found that the alkali elution rate was reduced by the white deposit on the steelmaking slag.

The alkali elution behavior of steelmaking slag in seawater was kinetically investigated and simulated under continuous flow in an open channel vessel with packed bed of steelmaking slag. Two types of steelmaking slags, viz. decarburization slag and dephosphorization slag, were used in this study. The alkali elution rate of decarburization slag was larger than that of dephosphorization slag due to larger free CaO content. The pH value for dephosphorization slag was almost the same as the seawater pH value in 3-4 days, whereas that for decarburization slag was stabilized in 3 days although the pH value was slightly larger than that of seawater. The capacity coefficients of alkali elution for dephosphorization and decarburization slags decreased together in an exponential manner with time. Based on a regression equation on the mass transfer capacity coefficient change with time, the alkali elution behavior was simulated and the calculated results agreed well with experimental ones. The temporal pH change was predicted by changing slag surface area and seawater flow rate as a parameter. According to the simulation results for dephosphorization slag, the seawater pH value did not reach a high level in the ocean area.