Development of Precipitation Processes for Removal of Scale Formers from Sea Water

摘要

By reaction with phosphoric acid and ammonia, it has been proven technically feasible on a continuous pilot plant basis to precipitate from sea water essentially all of the magnesium and, as required, up to 99% of the calcium present. The free ammonia released in the evaporator cannot be appreciably minimized by conventional distilling plant deaerators. Soda ash (Na2CO3) may be substituted for some or all of the ammonia to greatly minimize ammoniation of distillate and create a high brine pH. Magnesium and calcium are precipitated as magnesium ammonium phosphate and dicalcium phosphate. Assuming that all descaling costs were absorbed by the sale of the slurry, manufacturing cost of the wet cake (dry basis) is estimated at $55/ton at 590 tons per day production, concurrently with the complete descaling of 15,000,000 gallons per day of sea water. It was shown by pilot plant operation that in the presence of recycled CaCO3 seeds, addition of magnesium carbonate trihydrate (MgCO3.3H2O) followed by lime will precipitate up to 65% of calcium from raw sea water. (Author)