Soap retention on pulp fibers during recycling : a mechanistic study using a packed bed reactor

A hypothetical mechanism for the loss of papermaking and printing potential of recycled paper is the adsorption and entrapment of molecules of low surface energy such as oil and soap, on the wood fibers during recycling.A sensitive technique to quantify the retention of contaminants on the pulp is to follow the dynamics of retention of a concentration pulse generated at the entrance (breakthrough curve, BTC) of a packed bed of fibers. Using this technique, we showed that sodium salts of fatty acids undergo ion-exchange with the calcium present in the pulp. Soap prepared by mixing equimolar amounts of sodium oleate and calcium chloride (1 Meq) do not adsorb on pulp fibers. However, retention of the soap particles increases as the Calcium chloride concentration is increased to 0.164 and 1.48 mmol/L. This is due to a charge screening mechanism expected in the presence of an electrolyte.In the presence of a trivalent cation, such as Al$ sp{+3}$, the mechanism becomes more complicated as Al$ sp{+3}$ ions undergo adsorption or ion-exchange with the Ca$ sp{+2}$; this renders the soap particles positive. (Abstract shortened by UMI.