Method of treating cement kiln dust for recovery and recycle

Abstract

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.

Dependencies are inferred from the claim text.