Humic acid as a fouling agent in filtration

Abstract

Humic acid is a degradation product of lignin, carbohydrate and protein. It is contained in soils and therefore it is often present in surface waters in small amounts, which vary with the seasons. Besides giving the waters a yellowish to brownish color, the humic acid can also cause fouling problems in filtration of surface waters. In the present study humic acid was studied in different types of filtration processes because it seemed to be a problem at many mill sites using humic acid containing process waters. Humic acid was studied as a model substance in pressure and membrane filtration at different low concentrations. In the pressure filtration experiments it was added to a copper concentrate, containing as much iron and sulfur as copper. In membrane filtration it was filtered together with iron chloride. Also means to measure humic acid were investigated. In pressure filtration either capillary disc filters made of aluminum and silica oxides or filters made of different types of woven cloth were used. In membrane filtration the experiments were made with organic nano- and ultrafiltration membranes and with inorganic ultrafiltration membranes. The experiments were made on the laboratory scale. The humic acid could be analyzed using SEM-EDAX methods or with UV/VIS spectrophotometric methods. The studies showed that humic acid was most harmful in capillary pressure filtration. Even at small concentrations the filter was blocked and the cake did not form properly when added to the ore concentrates to be filtered. The flux decreased greatly with an addition of 10 ppm, and the filter was almost completely blocked when 100 ppm were added. In pressure filtration using cloth the harmful effect was not as remarkable. In membrane filtration humic acid did not disturb filtration as much as in pressure filtration even though it was used together with iron chloride. The effect of humic acid was a bit larger in ultrafiltration than in nanofiltration. The reason for the fouling tendency of humic acid seems to be its binding tendency to multivalent salts. The capillary filters used in pressure filtration are positively charged containing Al and Si. The humic acid is either not charged or somewhat negatively charged and probably forms chelates with the metals. It can be seen as a gel-like layer on the filter, which disturbs the formation of the cake and blocks the pores of the filter.