CALCIUM SOAPS IN FLOTATION DEINKING - FUNDAMENTAL-STUDIES USING SURFACE FORCE AND COAGULATION TECHNIQUES

The surface force technique was shown to give useful information conce rning the interaction of fatty acid flotation collectors and calcium a ctivator with a negatively charged mica substrate at high pH. Since th e surface of ink particles under deinking conditions are enriched main ly by negatively charged groups, these experiments enabled some detail s of the fundamental mechanisms involved in deinking flotation to be c larified. The preliminary results (carried out at relatively low calci um and fatty acid concentrations) may be summarized as follows: (a) At pH > 10, the negatively charged surface generated a long range DLVO d ouble-layer repulsion and the potential at the mica/electrolyte interf ace could be estimated. However, at short distances (15 Angstrom from mica contact), a repulsive non-DLVO hydration barrier was detected due to adsorbed Na+. (b) On addition of CaCl2, the Na+ was replaced by th e specific adsorption of the less strongly hydrated Ca(OH)(+) solution species or more strongly hydrolyzable calcium species. This resulted in the elimination of the hydration forces and mica-mica contact. On a ddition of fatty acid, no change in the force profile was detected sug gesting no calcium was removed from the surface and there was no evide nce of calcium soap formation in the surface region. This result impli es that under alkali conditions, the calcium does not induce a bridgin g mechanism in the presence of fatty acid (below the calcium soap prec ipitation level). In fact, the calcium could only operate as bridging agents if they can specifically bind to the surface, as well as to the carboxylated fatty acid. The ''calcium dehydration/destabilization me chanism'' was verified by coagulation studies with quartz suspensions. At higher fatty acid and calcium concentrations, calcium soap was pre cipitated in bulk solution. It was suggested that microencapsulation o f the ink particles with hydrophobic species occurs through heterocoag ulation with the bulk precipitated calcium soap particles. (C) 1997 El sevier Science B.V.