Milk of lime is mixed with the raw melter liquor. Carbon dioxide (CO2) recovered from the boilers is bubbled through the mixture. The carbon dioxide reacts with the lime to form chalk which attracts the waxes, gums, resins and other impurities in the. The reaction is Ca(OH)2 + CO2 ------------------> CaCO3 + H2O The mixture (carbonated liquor) is then passes through filter presses working in parallel which the liquor will be filtered. This removes the chalk and about half of the colour, together with virtually all the fine debris and solid matter. The resulting pressed liquor is then ready for the next stage.