It was found that the process of pressure-hydrolysis failed for the treatment of a wastewater containing cyanide and nickel from heavy-oil-pyrolysis-gas-making system. The analysis of the wastewater showed the existence of Ni2+,CN,NH3, Ca2+ and HCO3. Therefore, the coordinate compounds of [Ni(CN)4]2− and [Ni(NH3)n]2+ could be expected. Considering the Ca2+ and potential CO32− transformed from HCO3, it was not surprising that white precipitate blocked the pipeline within the pressure-hydrolysis process under the condition of high pH. A pretreatment step of calcium removal was suggested. Various factors were investigated and optimal operational conditions were concluded. After calcium was removed, the treatment processes were operated successfully and satisfactory results obtained. The residual concentrations of Ni2+ and CN could be reduced to meet the discharging standards.

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