Ink detachment and redeposition in alkali flotation deinking systems

The effective detachment of ink from the fibre surface and the prevention of its redeposition onto the surface before its removal in the flotation cell is important in deinking old newsprint (ONP). Magnesium Oxide (MgO) has been found to be a suitable replacement for sodium hydroxide (NaOH) in flotation deinking in the laboratory but not as effective in mill trials. The reason for the poorer performance is discussed in this paper. The use of NaOH in the deinking cell was found to produce higher ISO brightness, due to its contribution to peroxide bleaching of the fibres, than when MgO is used as the alkali source. Traditional brightness measurements are thus a poor indication of the deinking efficiency. Measurements of R700 or ERIC are needed to better assess the effect of the different alkalis and other factors on deinking efficiency. MgO has been found to cause more redeposition of ink onto the fibres than when NaOH is used as the alkali. To overcome this, the addition of soap to the pulper when using MgO is important. The addition of silicate and calcium were found to have a detrimental effect and appear to contribute to ink redeposition. Increasing the surface area of the MgO was found to increase its hydration and also improved the deinking efficiency of the MgO. The addition of the MgO as a hydrated slurry was also found to increase deinking efficiency.