RETENTION OF FATTY-ACID SOAPS DURING RECYCLING .1. A STUDY USING PACKED-BEDS OF PULP FIBERS

Paper made of recycled fibres often has different properties than pape r made entirely of virgin pulp. A hypothetical mechanism for the loss of papermaking and printing potential of recycled paper is the adsorpt ion and entrapment of molecules of low surface energy (such as oil and soap) on the wood fibres during recycling. This hypothesis was tested with a packed bed of wood fibres by following the dynamics of retenti on of a concentration pulse generated at the entrance (breakthrough cu rve, ETC). This study investigated the retention of model deinking che micals in packed beds. The variables studied include the type of fibre (kraft, TMP), the type of adsorbate (calcium and sodium oleate), and the ionic strength. The objectives of this study were threefold: to de velop a sensitive technique to quantify contaminant retention during r ecycling; to elucidate the mechanisms of soap retention by the fibres; and to assess the effects of soap retention on product quality A sens itive technique to quantify the retention of contaminants on the pulp is to measure the area under a ETC of a packed bed of fibres. It was s hown that sodium salts of fatty acids underwent ion exchange with the calcium present in the pulp. The availability of calcium and other ion s within the pulp for ion exchange is diffusion controlled. When tap w ater was used during washing stages, calcium ions from the tap water w ere retained by the pulp, replacing some of the sodium ions. Soaps pre pared by mixing equimolar amounts of sodium oleate and calcium chlorid e (1 meq) did not adsorb on pulp fibres. This behaviour goes according to theory as the negatively charged soap particles are repelled from the negatively charged fibres. However; complete retention of the part icles occurred at high excess calcium concentration (10 meq). Dynamic contact angle and surface tension experiments confirm the important ro le of soap retention in sizing and carry-over phenomena.