FROM WASTE TO RESOURCE: A CASE STUDY OF HEAVY-METAL SLUDGE BY MICROWAVE TREATMENT

This study employed the microwave technique to treat hazardous heavy-metal sludge and generate a valuable resource. A specially designed microwave oven (max 3000 W, 2.45 GHz) was utilized as the microwave radiation source. Conventional and microwave acid extraction were used to remove copper ions from heavy-metal sludge. Experimental findings demonstrate that the efficiency of microwave acid extraction was significantly better than conventional acid extraction. Increasing microwave power and reducing the solid-to-liquid ratio during microwave acid extraction increased copper extraction efficiency; however, sludge particle size did not significantly affect the copper extraction efficiency. The copper fraction in extracted sludge was predominantly inorganic matter- and residual-bounds after microwave/sulfuric-acid extraction, yielding an extracted sludge that was more stable than raw sludge. Moreover, this work utilized activated carbon (AC) and Na2HPO4 as stabilizing agents in microwave treatment to stabilize extracted sludge. Experimental results reveal that the copper stabilization ratio reached 94% and the toxic characteristic leaching procedure yielded a concentration of copper < 5 mg L -1 after 30 min of microwave (800 W)/AC/Na2HPO4 treatment at a sludge/AC/liquid ratio of 0.5/0.5/1. Additionally, copper leaching declined as the AC addition ratios increased in the microwave/AC/Na2HPO4 system. The pickled copper solution was precipitated by adding NaOH. The precipitates were treated via the microwave hydrothermal method to recover copper oxides. The purity of the CuO generated by the microwave hydrothermal method was 76-80%.

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