The effects of thermal-alkaline pretreatment on dewatered activated sludge (DAS) solubilisation and subsequent high-solid anaerobic digestion were studied by response surface methodology (RSM) from 105 to 135 °C and between 5 and 35 mg alkaline/g total solid (TS) DAS. Soluble chemical oxygen demand (SCOD), soluble carbohydrates, and protein concentrations were significantly enhanced in thermal-alkaline pretreated DAS samples. Daily methane yield increased at the middle of digestion, and cumulative methane yield (CMY) significantly increased after thermal-alkaline pretreatment. A first-order linear model of temperature and alkaline was significant for SCOD by RSM, and the determination coefficient (R 2 ) was 94.62%. The quadratic model of temperature and alkaline was also significant for methane yield. R 2 of 99.80% confirmed that the model used in this study fit the experimental variables very well. Using the model, the optimum pretreatment condition of methane yield was obtained at 134.95 °C and 23.77 mg alkali. Therefore, RSM was an effective tool in predicting the DAS pretreatment condition for optimum methane yield.
Experiments were conducted to investigate the process of aluminum and lithium extraction from high-alumina coal fly ash (HCFA) generated from coal-fired power plants located in northern China. The presence of mullite and other aluminosilicates lead to low reactivity of coal fly ash. An activation pretreatment that destroys an inert composition of coal is necessary. The activation roasting of coal fly ash using sodium chloride and a subsequent leaching process were performed in this research. The results showed that almost no aluminum and lithium were dissolved under direct water leaching, while about 7% and 10% of those were leached into the acid solution respectively. Adding NaCl enhanced the atmospheric pressure leaching of aluminum and lithium with a leaching rate around 50%. Phase analysis and equilibrium calculations results showed that the roasting reaction between the HCFA and NaCl occurred, which led to generation of main new phase NaAlSi3O8. The pressure extraction efficiencies of aluminum and lithium were increased to about 93% and 98%, respectively. The implications of the findings provide an alternative process for recovering aluminum and lithium from readily available high-alumina coal fly ash.
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