The alkaline digestion of reject organic material in de-inking sludge as a precursor to filler recovery by hydrothermal oxidation

Abstract

With the increasing cost of virgin pulp for papermaking, the emphasis on using recovered fibre is growing worldwide. The recycling of paper leads to problems with a waste sludge from the de-inking step. Disposal costs for the sludge are increasing dramatically meaning alternative methods of treatments of the sludge are needed. The sludge contains kaolinite, calcite, and other minerals used as fillers in paper, that have potential economic values if recovered. The sludge also contains waste fibre, glues, coating polymers, and other organic compounds classed as "stickies". During this study, themal-alkali conditions were used to digest and solubilise the organic components to enable separation from the minerals. It is important for environmental reasons that the organic components of the solution are characterised. A novel separation method has been developed using a reverse-phase chromatography column, which allows separation of the organic degradation products to enable characterisation. Using GC analysis, the compounds not adsorbed on the column have been shown to be hydroxy acids derived from the degradation of cellulose. NMR analysis of the compounds that can be extracted, showed that they are mainly fatty acids, degradation products from lignin and coating polymers. XRD analysis of the minerals showed that while the calcite and other minor minerals can be recovered unreacted, the kaolinite was solubilised and re-crystallised as a zeolite, cancrinite. Further treatment of mineral with hydrothermal conditions produced a white powder of almost commercially acceptable optical properties.