The alkaline digestion of reject fibre in deinking sludge as a precursor to filler recovery by wet air oxidation

Waste deinking sludge produced in the recycling of paper comprises similar quantities of reject fibre and minerals (filler and coating) with very much smaller quantities of ink particles and organic polymeric material (stickies). The sludge is currently disposed of by land spreading, or by incineration and land filling the resulting ash. Both are environmentally undesirable and the valuable mineral components are lost. Hydrothermal oxidation at elevated temperature and pressure, using hydrogen peroxide, offers an attractive approach in removing the waste fibre, ink colorants and stickies. This yields the minerals as a free flowing white powder in an essentially unaltered form, which can potentially be re-used as a filler material. However, as the majority of the oxidant is consumed in oxidising the waste fibre an alternative two-stage process has been developed. In the first stage the fibre and stickies are removed by alkaline hydrothermal hydrolysis using 1 molar NaOH at 250°C for 2 hours, which yields an amber coloured solution and a grey solid. Chromatographic separation and analysis has shown that the solution contains mainly low molecular mass polar carboxylic acids, notably lactic and formic acid, as the most abundant followed by glycolic, 3,4-dideoxypentonic and 2-hydroxybutanoic acid, formed largely from the hydrolysis of the cellulose. There is also a non-polar component formed from the hydrolysis of lignin and stickies. The grey solid comprises mainly calcium carbonate and kaolinite coated with ink colorants. In the second stage this grey powder is subjected to hydrothermal oxidation also at 250°C for up to 2 hours using a very much reduced quantity of hydrogen peroxide. The ink colorants are removed to yield a free flowing white powder, which is a mix of the original filler and coating minerals. The physical and optical properties of this powder suggest it can be re-used as filler in newsprint and similar paper grades.