Pretreatment of wastewater from heavy-oil-pyrolysis-gas-making system

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.