Development of a combined waterjet plasma scrubber for tetrafluoromethane and by-product removal



Perfluorocompounds (PFCs) have been extensively used for plasma etching and chemical vapor deposition (CVD) gases for the semiconductor manufacturing processes. PFCs have significant effects on global warming, and have very long atmospheric lifetimes. Moreover, the wastewater, including fluorine, would be caused by groundwater pollution. The long-term consumption of water containing excessive fluoride can lead to fluorosis of the teeth and bones. In this study, a waterjet gliding arc plasma system in which plasma is combined with waterjet was developed, and the optimum operating conditions for efficient CF4 destruction were investigated by enlarging the discharge region and producing a large amount of OH radicals. The CF4 decomposed by the waterjet plasma produces hydrogen fluoride (HF) and is dissolved in the waterjet. The present system in the work utilized electrocoagulation and calcium hydroxide, which is very effective for getting rid of HF from wastewater. For the experimental parameters, the waterjet flow rate and input power were used for the waterjet plasma scrubber, the initial pH and current density for the electrocoagulation process, and the reaction pH and Ca(OH)2 injection volume for the calcium hydroxide process. It was found in the parametric experiment that the total CF4 decomposition efficiency was 99.2%; the removal efficiencies of CO2 and HF, which are the by-products in the gas, were 69.4 and 78%, respectively; and the removal efficiency of HF, which is the by-product in the liquid, was 99.3%. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 229–237, 2014