Silica removal in industrial effluents with high silica content and low hardness.
High silica content of de-inked paper mill effluents is limiting their regeneration and reuse after membrane treatments such as reverse osmosis (RO). Silica removal during softening processes is a common treatment; however, the effluent from the paper mill studied has a low hardness content, which makes the addition of magnesium compounds necessary to increase silica removal. Two soluble magnesium compounds (MgCl₂∙6H₂O and MgSO₄∙7H₂O) were tested at five dosages (250-1,500 mg/L) and different initial pH values. High removal rates (80-90%) were obtained with both products at the highest pH tested (11.5). With these removal efficiencies, it is possible to work at high RO recoveries (75-85%) without silica scaling. Although pH regulation significantly increased the conductivity of the waters (at pH 11.5 from 2.1 to 3.7-4.0 mS/cm), this could be partially solved by using Ca(OH)₂ instead of NaOH as pH regulator (final conductivity around 3.0 mS/cm). Maximum chemical oxygen demand (COD) removal obtained with caustic soda was lower than with lime (15 vs. 30%). Additionally, the combined use of a polyaluminum coagulant during the softening process was studied; the coagulant, however, did not significantly improve silica removal, obtaining a maximum increase of only 10%.
Department of Chemical Engineering, Faculty of Chemistry, Complutense University of Madrid, Avda. Complutense s/n, 28040, Madrid, Spain.
Isabel
I
Latour
Ruben
R
Miranda
Angeles
A
Blanco
D006863:Hydrogen-Ion Concentration; D007220:Industrial Waste / Q000032:analysis*; D008274:Magnesium / Q000737:chemistry; D009995:Osmosis; D010209:Paper; D012822:Silicon Dioxide / Q000737:chemistry / Q000302:isolation & purification*; D014865:Waste Disposal, Fluid / Q000379:methods*
D007220:Industrial Waste; D012822:Silicon Dioxide; D008274:Magnesium