Method for simultaneously scrubbing cement kiln exhaust gas and producing useful by-products therefrom

Abstract

This invention relates to an apparatus and method for simultaneously scrubbing acidic exhaust gases and removing metal salts from alkaline waste dust from a cement plant or the like thereby producing useful products from what would otherwise be wasted. In essence the two wastes are combined in water and the resulting solution and precipitate separated in a tank. The solution is useful both to cool incoming waste gas and as a fertilizer, whilst the precipitate is suitable as kiln feed stock.

I claim: 
     
       1. A method of simultaneously scrubbing acidic oxides of sulfur, nitrogen and carbon from the hot exhaust stream from a cement kiln and utilizing acid derived therefrom to produce useful products from the carbonates and oxides of alkali and alkaline earth metals contained as solids in cement kiln dust comprising said carbonates and oxides and insoluble silicates, aluminates and iron compounds, which comprises (a) mixing said dust with water to form a slurry comprising undissolved solids and a solution of the soluble compounds in said dust, and   (b) passing said gas stream into said slurry whereby said acidic oxides contained therein will react with water to produce acids.   the latter will react with said carbonates and oxides of alkali and alkaline earth metals to yield a solution of alkali and alkaline earth metal salts comprising principally the cationic components, calcium, potassium, magnesium and sodium, and anionic components, carbonate, sulfate and nitrate, and a precipitate of alkali and alkaline earth metal salts with insoluble silicates, aluminates and iron compounds, and   the gas of said gas stream after passing through said slurry emerges as scrubbed exhaust.     
     
     
       2. The method of claim 1 wherein said gas stream is dehumidified by cooling prior to said passing step. 
     
     
       3. The method of claim 2 wherein said alkali salt solution is used to cool and dehumidify said gas stream. 
     
     
       4. The method of claim 1 wherein said precipitate together with insoluble silicates, aluminates and iron compounds is added to the feed stock of a cement plant to produce cement. 
     
     
       5. The method of claim 3 wherein heat from said exhaust gas stream is used to remove water from said alkali salt solution. 
     
     
       6. The method of claim 5 wherein said heat is derived in part from each of the hot exhaust gas, the latent heat of moisture in said exhaust, the hydrating reaction between dust and water and from compressing said gas prior to said passing step. 
     
     
       7. The method of claim 1 wherein the dissolved solids contained in said alkali salt solution are suitable for use as a fertilizer. 
     
     
       8. The method of claim 1 wherein said solution of alkali and alkaline earth metal salts is separated from said precipitate and insoluble silicates, aluminates and iron compounds by allowing the latter to settle therefrom. 
     
     
       9. The method of claim 1 including the step of transferring said solution of alkali and alkaline earth metal salts and said precipitate with said insolubles to a settling tank wherein said salt solution is separated from said precipitate and insoluble silicates, aluminates and iron compounds by allowing the latter to settle therefrom. 
     
     
       10. The method of claim 9 including the step of passing the separated salt solution through a heat exchanger wherein it extracts heat from said exhaust gas before the latter is passed into said slurry. 
     
     
       11. The method of claim 1 including the step of agitating said salt solution and precipitate to prevent settling of the solids during said reactions. .Iadd. 
     
     
       12.  A method of simultaneously scrubbing acidic oxides of sulfur, nitrogen and carbon from the hot exhaust stream from a cement kiln or boiler and utilizing acids derived therefrom to produce useful products from cement kiln dust containing as solids carbonates and oxides of alkali and alkaline earth metals along with insoluble silicates, aluminates and iron compounds, which comprises (a) mixing said dust with water to form a slurry comprising the undissolved components and a solution of the soluble components in said dust, and   (b) reacting said gas stream with said slurry whereby   said acidic oxides contained in said stream will react with water to produce acids,   the latter will react with said carbonates and oxides of alkali and alkaline earth metals to yield a solution of alkali and alkaline earth metal salts comprising principally the cationic components, calcium, potassium, magnesium and sodium, and anionic components, carbonate, sulfate and nitrate, and a precipitate of alkali and alkaline earth metal salts with insoluble silicates, aluminates and iron compounds,   the gas of said gas stream after contact with said slurry emerging as scrubbed exhaust. .Iadd.   
     
     
       13.  The method of claim 12 wherein said gas stream is passed through said slurry to react therewith. .Iaddend. .Iadd. 
     
     
       14.  The method of claim 13 including the step of agitating said slurry to prevent settling of the solids during said reactions. .Iaddend. .Iadd.15. The method of claim 14 wherein said slurry is agitated by stirring. .Iaddend. .Iadd.16. The method of claim 14 wherein said slurry is agitated by recirculation. .Iaddend. .Iadd.17. The method of claim 12 including the step of separating said solution of alkali and alkaline earth metal salts from said precipitate and insoluble silicates, aluminates and iron compounds. .Iaddend. .Iadd.18. The method of claim 17 wherein said separating step comprises allowing said precipitate and insoluble silicates, aluminates and iron compounds to settle out of said solution. .Iaddend. .Iadd.19. The method of claim 18 wherein said solution of alkali and alkaline earth metal salts and said precipitate with said insolubles are transferred to a settling tank wherein said salt solution is separated from said precipitate and insoluble silicates, aluminates and iron 
     
     
        compounds by allowing the latter to settle therefrom. .Iaddend. .Iadd.20. The method of claim 17 wherein said separated precipitate together with insoluble silicates, aluminates and iron compounds is added to the feed stock of a cement plant to produce cement. .Iaddend. .Iadd.21. The method of claim 12 including the step of cooling and dehumidifying said gas stream prior to passing it into contact with said slurry. .Iaddend. .Iadd.22. The method of claim 17 including the further step of using said separated solution to cool and dehumidify said gas stream prior to passing said gas stream into contact with said slurry. .Iaddend. .Iadd.23. The method of claim 22 including the step of using heat from the gaseous components of said exhaust gas stream to evaporate water from said separated solution of alkali and alkaline earth metal salts to produce a precipitate useful as fertilizer. .Iaddend. .Iadd.24. The method of claim 23 wherein said heat is derived in part from each of the hot exhaust gas, the latent heat of moisture in said exhaust, the hydration reaction between dust and water and from compressing said gas prior to said passing step. .Iaddend. .Iadd.25. The method of claim 22 wherein said separated solution and exhaust gas stream are passed through a heat exchanger wherein said solution extracts heat from said exhaust gas before the 
     
     
        latter is passed into contact with said slurry. .Iaddend. .Iadd.26.  The method of claim 23 wherein said separated solution and exhaust gas stream are passed through a heat exchanger wherein heat from the gaseous components of said stream evaporates water from said separated solution. .Iaddend.