Method of removing fluoride from quicklime and hydrated lime

Abstract

A method is shown for producing food grade quicklime and hydrated lime. Raw limestone feed stock is calcined in a kiln to produce quicklime which is then slaked to produce hydrated lime product. The present invention involves the discovery that, by increasing the calcination temperature a selected amount, the concentration of fluoride in the quicklime can be dramatically reduced to a level below what is required for food grade quicklime and hydrated lime.

1 . A method of producing food grade quicklime and hydrated lime, the method comprising the steps of: 
 passing a raw limestone feed stock to a calcining kiln;    calcincing the raw limestone feed at a selected discharge temperature to thereby produce a quicklime product;    passing the quicklime product to a hydration stage where the quicklime is reacted with an aqueous slaking medium to produce a hydrated lime;    wherein the selected discharge temperature which is used in calcincing the raw limestone feed stock exceeds the normal discharge temperature customarily used in the industry by a selected amount, the selected amount of temperature increase being effective to reduce the concentration of fluoride in the quicklime and in the ultimate hydrated lime product to a level below what is required for food grade hydrated lime.    
   
   
       2 . The method of  claim 1 , wherein the kiln is a horizontal or vertical kiln which produces hot combustion gases during the calcining process and wherein the hot combustion gases are not passed over the quicklime to improve energy efficiency as in the case of a regenerative kiln.  
   
   
       3 . The method of  claim 1 , wherein the selected discharge calcining temperature is at least 100° C. higher than the discharge temperature customarily used in the industry.  
   
   
       4 . The method of  claim 1 , wherein the selected discharge calcining temperature is above 1100° C. but below 1300° C.  
   
   
       5 . The method of  claim 1 , wherein the selected discharge calcining temperature is approximately 1200° C.  
   
   
       6 . The method  claim 1 , wherein the selected discharge calcining temperatures is effective to produce a quicklime product having a fluoride content of 150 ppm or below.  
   
   
       7 . The method of  claim 1 , wherein the selected discharge calcining temperature is effective to produce an ultimate hydrated lime product having a fluoride content of 50 ppm or below.  
   
   
       8 . The method of  claim 7 , wherein the hydrated lime product is further processed by passing the product through a classification and grinding steps to produce a more uniform product.