Recovery of Process Water of Effluent from Metallurgical Industry by Association of Dissolved Air Flotation and Biodemulsifier

Abstract

A method of treating a metalworking industry effluent composed by water, cutting fluid and graphite has been developed. The recovery of components of an effluent from the machining process becomes complex due to the existence of solid materials mixed to water-oil emulsions, as is the case of graphite. The graphite is distributed in the form of very small particles emulsified in water and distributed in both phases (aqueous and organic), making the treatment process very difficult. In general, treatment of an effluent of this type requires a preliminary step to breaks the emulsion so that the graphite dispersed in the liquid medium can be recovered. Thus, the treatment was performed in a laboratory-scale phase separation prototype, constructed with recirculation tanks, pumps, hydrocyclones and a continuous decanter. Initially, the effluent was acidified with sulfuric acid to break the solid-liquid emulsion. Subsequently, the solid phase was recovered with the aid of a hydrocyclone that originated a discharge by the equipment overflow of a mixture of oily wastewater and cutting fluid. Then the water-oil mixture was separated in the continuous decanter, yielding a cutting fluid stream and an aqueous phase containing residual oil contaminated with 120 ppm of oil. The aqueous phase was then subjected to a dissolved air flotation (DAF) process with a biodemulsifier as the collector in a laboratory prototype. To define appropriate conditions to obtain an aqueous phase with a level of contamination that would allow its return to the process, the conditions of application of DAF were determined with experiments outlined by a Central Composite Rotational Design (CCRD) technique. A statistical model for the DAF prototype operating conditions was obtained by response surface methodology (RSM). Concentrations of residual oil 12% below the Brazilian environmental agency regulations for disposal in water bodies, which is 20 ppm, were found. The separation efficiency reached values higher than 97%.