Calcium-carbonate precipitate can be produced by the reaction of an aqueous solution of sodium carbonate and calcium oxid
e. the by-product is aqueous sodium hydroxid
e. following decantation, the slurry leaving the precipitation tank is 5 wt% calcium carbonate, 0.1 wt% sodium hydroxide, and the balance water. one hundred thousand ib/h of this slurry is fed to a two-stage, continuous, countercurrent washing system to be washed with 20,000 ib/h of fresh water. the underflow from each thickener will contain 20 wt% solids. the amount of sodium hydroxide in final underflow is approximately equal to