The system for the HT process is shown in Fig. 1. Before the HT process, the fly ash was pre-treated by water washing or diluted nitric acid washing step. In the water washing step, the raw fly ash was mixed with distilled water according to the ratio of S = 2:1 and then stirred for 0.5 h. In the acid washing step, raw ash and distilled water were mixed according to the ratio of 2 L/kg and
14.4 M nitric acid was pipeted into the mixture to decrease the pH value of the mixture to 6.20 via agitation for several minutes. In total, 0.192 mol/kg of H+ was consumed. To prevent the excessive acid addition, the amount of nitric acid should be less than acid- neutralization-capacity (ANC) of raw fly ash when pH was 7.0. ANC was measured according to characterization of waste – Leaching
behaviour tests – Influence of pH on leaching with initial acid addi- tion(PrEN14429). After pre-treatment, the mixture was filtrated to separate solid phase from liquid phase and the solid phase would be used in the hydrothermal process.