Flotation deinking with magnesium oxide as the alkali source

Concerns over the environmental problems of highly saline effluent and high sodium adsorption ratios from recycled fibre (RCF) plants and paper mills has meant that paper mills need to find alternative strategies to reduce the problem. One solution is to reduce the amount of sodium hydroxide used or replace it with an alternative alkali such as magnesium oxide (MgO). This paper describes the results of laboratory work and mill scale trials undertaken to assess the viability of MgO as an alternative alkali source in flotation deinking. Laboratory experiments, using a Lamort deinking unit, showed that the deinking efficiency and final pulp brightness of the MgO system were not as sensitive to changes in chemical levels as the NaOH system. Increasing the soap addition in the pulper did appear to improve the brightness and ink removal efficiency of the process. We also investigated a dual alkali system of MgO and NaOH with promising results in the laboratory. A number of mill trials in which MgO partially replaced NaOH showed potential for reducing the salinity and high SAR from the mill. The RCF pulp brightness was slightly lower compared to normal operation with NaOH as the alkali source, but this did not appear to significantly affect the final paper brightness.