Surface chemicals concepts of flotation de-inking

Abstract

This review outlines the important parameters, which influence the flotation de-inking and discusses the surface chemical aspects of the process. Although, it has been established increasing temperature and pH facilitate the release of ink particles from the fibre during pulping (prior to flotation), it has not yet been completely established to what extent these parameters increase or decrease the efficiency of the primary flotation step. In fact, increasing temperature appears to decrease the flotation rate and also an increase in pH can retard the flotation due to a reduction in capture efficiency between the air bubbles and the ink particles decreasing the flotation in the cell. In addition, the size, shape and roughness of the ink particles influence this bubble/particle capture mechanism. Bubble frequency and bubble size is influenced by surface tension (type and concentration of frother) but X-ray studies also indicate that the fibre consistency can influence the bubble shape and flow patterns causing channelling and re-circulation of bubble flow in the cell. Tests with different gases (oxygen and nitrogen instead of air) show no significant gains in optical and mechanical properties of the fibre. Fatty acids with higher chain length and lower degree of saturation are less soluble and ensure lower carry-over but less foaming and also less fibre recovery. The primary mechanisms of fatty acid flotation involves precipitation of calcium soap, followed by micro-encapsulation of ink through a hetero-coagulation mechanism, followed by the bubble/ink particle capture step. In the use of nonionic surfactant, cloud point and HLB are important parameters, which influence brightness, washing and flotation efficiency. Optimum flotation occurs with slight excess of fatty acid to reduce surface tension and optimum calcium levels ensuring that most of the calcium is removed in the process. A lower calcium level gives lower stock loss but a high calcium has a detrimental effect, causing scaling and deposition. A critical nonionic/fatty acid balance is needed to minimize stock loss during the flotation. New chemicals need to be developed to increase selectivity, reduce entrainment and increase the process efficiency. Also, the underlying knowledge linking structural/function relationship for de-inking chemicals in relationship to the heterogenity at the pulps needs to be established. Further progress could help in the treatment of higher fibre content pulps, reducing water and chemical consumption and reducing redeposition of the ink on the fibre.