Preparation of a Sulfate-Based Complex Coagulant from Boiler Slag and its Application in Domestic Sewage Treatment

This paper presents a new way of recycling aluminum and iron in boiler slag derived from coal combustion plants for the production of a sulfate-based complex coagulant containing ferric sulfate and aluminum sulfate. The boiler slag sample was determined for more complete characterization by means of scanning electron microscopy(SEM), X-ray diffraction(XRD), X-ray fluorescence(XRF) and other techniques. An analysis for the boiler slag sample collected from Baotou Steel Plant located in Inner Mongolia, PR China showed that the quantity of iron and aluminum oxides, in general, accounted for about 35% of the boiler slag. XRD analysis indicates that predominate minerals such as kaolinite, quartz, calcium silicide, hematate and metakoalin exist in this boiler slag. This boiler salg was evaluated to determine the efficiency of converting the iron and aluminum components of the material into a sulfate-based complex coagulant when heated with sulfuric acid at different temperatures and reaction times. The maximum concentrations of Fe3+ and Al3+ in the complex coagulant prepared from the boiler slag were obtained at 130 and after 3 h of reaction time. These concentrations were 0.04 M Fe3+ and 0.46 M Al3+, respectively. The corresponding conversion efficiencies of aluminum and iron were 66.2 and 95.0% in the boiler slag, respectively. Finally, the prepared sulfate-based complex coagulant proved to be an effective agent for reducing the turbidity and chemical oxygen demand(COD) of a typical domestic sewage sample.