HUMIC-ACID AS A FOULING-AGENT IN FILTRATION

Humic acid is a degradation product of lignin, carbohydrate and protei n. It is contained in soils and therefore it is often present in surfa ce waters in small amounts, which vary with the seasons. Besides givin g the waters a yellowish to brownish color, the humic acid can also ca use fouling problems in filtration of surface waters; In the present s tudy 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 substanc e in pressure and membrane filtration at different low concentrations. In the pressure filtration experiments it was added to a copper conce ntrate, containing as much iron and sulfur as copper. In membrane filt ration it was filtered together with iron chloride. Also means to meas ure humic acid were investigated. In pressure filtration either capill ary 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 ma de on the laboratory scale. The humic acid could be analyzed using SEM -EDAX methods or with UV/VIS spectrophotometric methods. The studies s howed that humic acid was most harmful in capillary pressure filtratio n. Even at small concentrations the filter was blocked and the cake di d 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 fi ltration using cloth the harmful effect was not as remarkable. In memb rane filtration humic acid did not disturb filtration as much as in pr essure 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 seem s to be its binding tendency to multivalent salts. The capillary filte rs used in pressure filtration are positively charged containing Al an d Si. The humic acid is either not charged or somewhat negatively char ged and probably forms chelates with the metals. It can be seen as a g el-like layer on the filter, which disturbs the formation of the cake and blocks the pores of the filter.