Evaluation of the time-dependent neutralising behaviours of MSWI bottom ash and steel slag

A long-time batch pH titration was used to investigate the time-dependent neutralising behaviours of municipal solid waste incineration (MSWI) bottom ash and steel slag. The neutralising processes of the solid wastes were evaluated by using reaction path modelling. The experiments showed that the acid neutralising capacity increased with reaction time. Thus, a short-time pH titration should lead to a large underestimation of the neutralising capacity since relatively slow matrix dissolution reactions of the solid waste may give a considerable contribution in a long-time process. The characteristics of the two investigated wastes differed in their neutralising processes. The differences appeared in the pH-dependent behaviour as well as in the distribution of the acid neutralising capacity at different pH levels. The pH-dependent behaviour was closely related to the dissolution kinetics of the waste matrix. The key factor of the neutralising processes was the dissolution of calcium-containing minerals or materials. In addition, the secondary minerals, which may precipitate and later redissolve, were also important to the neutralising processes.