Lightweight aggregate from flyash and sewage sludge

Abstract

A method is shown for producing a lightweight aggregate by treating flyash and sewage sludge. The flyash and sewage sludge are mixed together and then agglomerated into pellets, with or without the use of a binder. The pellets may be coated and then are dried. The dried pellets are introduced into a rotary kiln in a direction that is co-current with the flow of fuel and air through the kiln. The pellets in the kiln will be indurated and will experience complete calcination as well as varying degrees of pyrolizing and sintering. The product of the kiln is a nodular material having a low density but with a hard and porous structure. The product of the kiln is feed to a cooler. The flyash sewage sludge mixture has a significant fuel value that is usable in the kiln. Furthermore, the fuel valve available in the kiln off-gases may be used for drying the materials.

We claim: 
     
       1. A method of producing a lighweight aggregate, comprising the steps of: mixing flyash and sewage sludge;   agglomerating the resulting mixture;   drying the agglomerate; .[.and.].   .Iadd.creating a flow of hot gases along the length of a rotary kiln;   passing the dried pellets through the rotary kiln co-current with the flow of hot gases; and .Iaddend.   indurating the agglomerate .Iadd.while .Iaddend.in .[.a.]. .Iadd.the .Iaddend.rotary kiln to a temperature below the melting point of the agglomerate.   
     
     
       2. A method in accordance with claim 1 wherein the agglomerating is accomplished by pelletizing the mixture. 
     
     
       3. A method in accordance with claim 2 together with the step of coating the pellets prior to heating of the pellets to produce an outer shell on the pellets to prevent sticking of the pellets as they are heated in the kiln. 
     
     
       4. A method in accordance with claim 1 wherein the flyash and sludge are mixed with a binder. 
     
     
       5. A method in accordance with claim 1 wherein the agglomerated mixture is heated up to about 800°-1200° C. 
     
     
       6. A method of treating flyash and sewage sludge for subsequent use, comprising the steps of: mixing flyash and sewage sludge in the proportions of about .[.35-99%.]. .Iadd.35-80% .Iaddend.dry weight flyash with about .[.1-65% .]. .Iadd.20-65% .Iaddend.dry weight sludge;   agglomerating the mixture;   drying the agglomerated mixture to a moisture content of less than about 5% by weight;   heating the dried agglomerated mixture in a rotary kiln to form a nodular product; and   cooling the product of the kiln.   
     
     
       7. A method in accordance with claim 6 wherein the flyash and sludge are mixed with a binder in the proportion of 0-20% by dry weight. 
     
     
       8. A method in accordance with claim 6 wherein .Iadd.a flow of hot gases is created along the length of the rotary kiln and .Iaddend.the dried agglomerated mixture is passed through the kiln co-current with the flow of hot gases in the kiln. 
     
     
       9. A method of treating flyash and sewage sludge, comprising: mixing flyash with sewage sludge having a high fuel value;   agglomerating the mixture;   drying the agglomerated mixture;   heating the agglomerated mixture to a temperature less than the melting point of the mixture in a rotary kiln using the agglomerated mixture as the principal source of fuel in the kiln to form a porous nodular product; and   recovering the nodular product from the kiln.   
     
     
       10. A method in accordance with claim 9 wherein additional fuel is added to the kiln in an amount that is a small fraction of the total fuel required. 
     
     
       11. A method in accordance with claim 9 together with the step of recovering the heat content of the off-gases of the kiln. 
     
     
       12. A method in accordance with claim 9 together with the steps of recovering the off-gases from the kiln and using the heat value of the off-gases to dry the agglomerated mixture before it is introduced into the rotary kiln. 
     
     
       13. A method of producing a lightweight aggregate, comprising the steps of: mixing flyash and sewage sludge in the proportions of about 35-99% dry weight flyash with about 1-65% dry weight sludge;   pelletizing the resulting mixture;   drying the pellets to a moisture content of less .[.then.]. .Iadd.than .Iaddend.about 5% by weight;   .Iadd.creating a flow of hot gases along the length of a rotary kiln; .Iaddend.   passing the dried pellets through .[.a.]. .Iadd.the .Iaddend.rotary kin co-current with the flow of hot gases to form a nodular product; and   cooling the product of the kiln.   
     
     
       14. A method in accordance with claim 13 wherein the flyash and sludge are mixed with a binder in a proportion of up to about 20% by dry weight. 
     
     
       15. A method in accordance with claim 13 wherein the pellets are coated prior to being dried. 
     
     
       16. A method in accordance with claim 13 wherein the kiln is heated up to about 800°-1200° C. 
     
     
       17. A method in accordance with claim 13 wherein the heat content of the kiln off-gases is recovered. 
     
     
       18. A method in accordance with claim 17 wherein the recovered heat content is used to supply heat value for drying the sludge prior to mixing and for drying the pellets. 
     
     
       19. A method in accordance with claim 13 wherein the heat removed from the product during cooling is used to dry the agglomerated mixture. .Iadd. 
     
     
       20.  A method of treating flyash and sewage sludge, comprising the steps of: mixing flyash and sewage sludge;   pelletizing the resulting mixture;   creating a flow of hot gases along the length of a rotary kiln;   passing the pellets through the rotary kiln co-current with the flow of hot gases to form a nodular product; and   adding air to the kiln sufficient to completely combust the pellets within the kiln while heating the pellets to a temperature that is below the melting point of the pellets. .Iaddend. .Iadd.   
     
     
       21.  A method in accordance with claim 20 wherein the flyash and sludge are mixed in the proportions of about 35-80% dry weight flyash with about 20-65% dry weight sludge. .Iaddend. .Iadd. 
     
     
       22.  A method of treating flyash and sewage sludge, comprising the steps of: mixing flyash and sewage sludge;   pelletizing the resulting mixture;   introducing fuel through a burner into one end of a rotary kiln with the result that hot gases will flow within the kiln from the one end toward the other end of the kiln;   introducing air separate from air introduced with the fuel into the one end of the kiln sufficient to completely combust the pellets in the kiln; and   introducing the pellets into the one end of the kiln and passing the pellets through the kiln co-current with the flow of hot gases to form a nodular product while heating the pellets to a temperature below the melting point of the pellets. .Iaddend. .Iadd.23. A method of treating flyash and sewage sludge, comprising the steps of:   mixing flyash and sewage sludge;   pelletizing the resulting mixture;   introducing fuel and air into a rotary kiln to create a flow of hot gases through the kiln and to maintain a substantially flat temperature profile along the length of the kiln and wherein the temperature is below the melting point of the pellets; and   passing the pellets through the rotary kiln co-current with the flow of hot gases. .Iaddend.