Pressure filtration properties of sludge generated in the electrochemical treatment of mining waters

Abstract

In this study, batch electrocoagulation (EC) experiments were performed with synthetic mining water in various conditions in a laboratory-scale 1L reactor. The process was scaled up and the selected results were verified with both synthetic and real mining water in a 70 L reactor. The generated solids were characterized by XRD, SEM, and a laser diffraction particle size analyzer. After preconcentration by settling and decantation, the EC solids were separated by constant pressure filtration at 2–6 bar. In order to improve the separation, various filter aids were used in body-feed and precoat modes. The results show that the overall removal efficiency was the highest with consumable electrode pairs such as Fe/Fe, Al/Al and Fe/Al, and the highest treatment efficiency was achieved with Fe/Al electrodes where 100/100% of the nitrate and 96/87% of the sulfate were removed in small/large-scale experiments. Depending on the dissolved electrode material, different solid species were formed: crystalline primary particles with a minor degree of agglomeration were observed in Fe/Fe slurry, whereas aluminium-containing solids (Al/Al and Fe/Al) were mainly amorphous agglomerates. High values of average specific cake resistances (αav = 2·1012 - 4·1013), average porosities (>90%) and moisture contents (>68 wt%) of filter cakes were obtained for all filtered samples. The highest values of the above-mentioned cake characteristics were observed for aluminium-based solids, which might be explained by its highly amorphous structure. The application of filter aids improved the filterability of the sludges by reducing the average specific cake resistance by as much as 95–96% in the body-feed mode and by 84% in the precoat mode.