Treatment of galvanic sludge in an arc thermal plasma reactor

Abstract

The performance of arc thermal plasma in treating galvanic sludge was investigated. The inductively coupled plasma optical emission spectrometry analysis conducted to characterise the raw galvanic sludge reveals the presence of chromium (Cr), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), lead (Pb) and zinc (Zn). Product gas was measured at varying applied current of 170 A to 190 A and residence time of 2 to 5 minutes. The fourier transform infrared spectrometry analysis of the gaseous product identified the presence of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethene (C2H4), ethyne (C2H2) and water (H2O) in different concentrations. In general, the trend in CO and CO2 concentrations initially shows an increase as the applied current was increased and thereafter decreases. Low concentrations of hydrocarbon were persistent, which is due to the low amount of hydrocarbon in the galvanic sludge. From the analysis the concentration of CH4, C2H4 and C2H2 revealed were 46 ppm, 39 ppm and 73 ppm respectively. A quadratic model for CO concentration in the gaseous product was developed. The developed model using the response surface methodology showed a good fit with correlation coefficients of 99.63%. The optimum operating conditions for the treatment were 170.4 A, 20.13 L/min and 3.12 minutes for applied current, plasma gas flow rate and residence time respectively. The optimum response of CO obtained from the optimization of the reaction system was 1622 ppm with desirability of 1. Treatment at applied current of 170 A revealed metals were either within the toxicity leaching characteristic limit or not detected. The concentration of the metals found in the treated sludge were Cr (0.410 mg/l), Cu (0.350 mg/l), Fe (4.0 mg/l), Zn (0.8 mg/l) while magnesium, nickel and lead were not detected. The applied current of 170 A represents the minimum applied current sufficient for immobilisation of metals contained in the galvanic sludge. The economic analysis of arc thermal plasma for treatment of galvanic sludge reveals a profit margin of RM 5.89 per kg of sludge. The cost of arc thermal plasma treatment is RM 6.49 which is far cheaper than the cost of treatment (RM 27.811) using incineration. Furthermore, a positive return on investment of 5.81% would be realized in the fifth year of operation. Thermal plasma treatment technique is, therefore, a viable alternative method for treating galvanic sludge.