Fresh or stockpiled
cement kiln dust is moistened with sufficient water so that the amount of total free and combined water relative to dust is about 3 parts water to 1 part dust by
mass, or less. The wet solids are treated with
carbon dioxide to convert compounds, such as
calcium hydroxide, to carbonates, such as
calcium carbonate. The degree of
carbonation is controlled so that the
solubility of
calcium becomes minimum for the dust being treated; this is also when hydroxyl and
bicarbonate ions in solution are about at their minima. As the
carbonation reactions occur, the water combined in hydroxides is released as
free water so that the mixture becomes a
slurry and the potentially soluble alkalies and
sulfate (and any chlorides present) are released to the
liquid phase. The solids are separated from the liquid, and the solids, which may be washed, provide a material suitable for return as feed to the
kiln. The liquid, which contains the dissolved alkali compounds, is recycled to reclaim additional dust or treated to recover alkali salts when the alkali salts are sufficiently concentrated.While any source of
carbon dioxide may be used, the preferred source is exit gases from the
kiln. The gases are conditioned by condensation of water and removal of
ammonium compounds, such as
sulfate and
chloride. The conditioning condensate may be treated to recover useful byproduct salts.