Avaliação do processo de eletrocoagulação aplicado a efluentes cianídricos da indústria galvânica

Abstract

The growing need for preservation and/or maintenance of water available for human consumption generates an endless search for alternative and efficient methods for the treatment of industrial effluents. The aim of the present work was to evaluate the process of electrocoagulation (EC) for the treatment of hydrocyanic galvanic effluent. The following process variables were evaluated, using statistical analysis: electrolysis time, amount of supporting electrolyte (NaCl), applied current density, distance between electrodes and electrode surface area. It was used a synthetic solution simulating the hydrocyanic galvanic effluent, an reactor built with transparent acrylic and aluminum electrodes connected to a continuous current source for generation of coagulating agent, coupled with an polarity inverter. The mass loss of the electrodes was determined by gravimetric method and the amount of aluminum remaining in the effluent by atomic absorption. The electrochemical behavior of aluminum electrodes in the hydrocyanic galvanic effluent was evaluated by potentiostatic polarization curves. The morphology and chemical elements present in the aluminum electrodes after treatment were evaluated by analysis of scanning electron microscopy and energy dispersive spectroscopy. The galvanic sludge generated in the process was characterized by the techniques of X-ray diffraction and infrared spectroscopy. The following optimal operating conditions were obtained in experiment named ECO: electrolysis time of 30 minutes, addition of 5.0 g/L NaCl, current density of 8 mA/cm², 1 cm between electrodes and surface area of 104 cm²/L and the removal results of 99.55% total cyanide, 22.49% nickel ions, 52.66% copper ions and 100,00% zinc ions. The results of turbidity, TSS, COD, pH and removal of zinc parameters, meet the amounts subject to disposal of treated effluent, not being obtained for total cyanide, copper and nickel ions parameters. It was obtained 8.50 mg/L of aluminum in the ECO experiment, amenable to disposal and increased by 20,00% the efficiency removal of nickel and copper ions and total cyanide using the polarity inverter. The efficiency of EC and alkaline chlorination removal were similar for total cyanide and zinc ions, obtaining removal greater than 70,00% and 40,00% for nickel and copper ions by the alkaline chlorination. SEM analysis showed pitting corrosion in the external faces of the aluminum electrodes and general corrosion in internal faces. EDS analysis indicated the presence of aluminum oxide and copper on the surface of the electrodes. The electrochemical results showed that the higher the amount of NaCl added to the galvanic effluent, the higher the anodic dissolution of aluminum electrodes, the higher the current density in the circuit and lower the necessary potential to be applied. Regarding the galvanic sludge, in the analyzes of XRD and FT-IR were identified coagulating agents in the form of hydroxides of aluminum, and the FT-IR analyzes proved the presence of the metals copper, nickel and zinc, cyanide and cyanate. The obtained removal results are attributed to the synergy of EC process and the electrolytic oxidation of the metal cyanide complexes as function of applied potential difference. The EC process was efficient for removing contaminants from hydrocyanic galvanic effluent and resulted in final parameters amenable of disposal according to current regulations.