Alkali resistant ceramic media

Abstract

The present invention relates to novel ceramic media or ceramic media coatings comprising mainly magnesia, silicon dioxide, and alumina, with forsterite and spinel as the dominant crystalline phases, which show high resistance to alkali attack at high temperature. Ceramic materials having these characteristics are particularly well suited for use as heat-exchange media in regenerative thermal oxidizers (RTOs) for the wood process industry.

I claim:  
     
         1 ) An alkali-resistant material comprising: 
 20 to 80% by weight of MgO;    10 to 50% by weight of SiO 2 ;    5 to 30% by weight Al 2 O 3 ; and    1 to 10% by weight Fe 2 O 3 , CaO, and alkali oxides.    
     
     
         2 ) The alkali-resistant material of  claim 1  wherein the dominant crystalline phases are forsterite and spinel.  
     
     
         3 ) The alkali-resistant material of  claim 1  wherein the alkali oxide is Ka 2 O.  
     
     
         4 ) The alkali-resistant material of  claim 1  wherein the alkali oxide is Na 2 O.  
     
     
         5 ) The alkali-resistant material of  claim 1  wherein the MgO is derived from oxides of magnesium.  
     
     
         6 ) The alkali-resistant material of  claim 1  wherein the MgO is derived from talc.  
     
     
         7 ) The alkali-resistant material of  claim 1  wherein the SiO 2  is derived from clay.  
     
     
         8 ) The alkali-resistant material of  claim 1  wherein the Al 2 O 3  is derived from clay.  
     
     
         9 ) A process for the production of an alkali-resistant ceramic body comprising the steps of: 
 a) grinding together a mixture comprising 0% to 50% by weight of light magnesia, 0% to 85% by weight of roasted magnesia, 10% to 60% by weight of ceramic clay, 0% to 15% by weight of limestone, 0% to 3% by weight of water glass, 0% to 3% by weight of carboxymethyl cellulose, 0% to 30% by weight of talc, and 0% to 30% by weight of calcium or barium carbonate to a particle size of less than 50 microns for 80% of the particles;    b) mixing the ground mixture with water to produce a paste containing less than 30% by weight of water;    c) shaping the paste to a desired shape;    d) drying the shaped product at a temperature greater than 100 degrees centigrade to make it suitable for firing in a kiln; and    e) firing the dried shaped product in a kiln at 1,250 to 1,450 degrees centigrade.    
     
     
         10 ) A process for coating a conventional ceramic body with an alkali-resistant ceramic coating comprising the steps of: 
 i) preparing an alkali resistant ceramic coating by grinding together 75% to 85% by weight of roasted magnesia, 5% to 10% by weight of ceramic clay, 0% to 15% by weight of limestone, 0% to 2% by weight of water glass, and 0.5% to 2% by weight of carboxymethyl cellulose to a particle size of less than 40 microns for 80% of the particles;    ii) mixing the ground mixture with water to produce a paste containing less than 30% by weight of water;    iii) applying a coating of the alkali resistant material paste to the surface of the conventional ceramic body;    iv) drying the coated ceramic body at a temperature greater than 100 degrees centigrade; and    v) firing the dried coated ceramic body in a kiln at 1,200 to 1,400 degrees centigrade.    
     
     
         11 ) An alkali-resistant ceramic body comprising: 
 20 to 80% by weight of MgO;    10 to 50% by weight of SiO 2 ;    5 to 30% by weight Al 2 O 3 ; and    1 to 10% by weight Fe 2 O 3 , CaO, and alkali oxides.    
     
     
         12 ) The alkali-resistant ceramic body of  claim 11  wherein the dominant crystalline phases are forsterite and spinel.  
     
     
         13 ) The alkali-resistant ceramic body of  claim 11  wherein the alkali oxide is Ka 2 O.  
     
     
         14 ) The alkali-resistant ceramic body of  claim 11  wherein the alkali oxide is Na 2 O.  
     
     
         15 ) The alkali-resistant ceramic body of  claim 11  wherein the MgO is derived from oxides of magnesium.  
     
     
         16 ) The alkali-resistant ceramic body of  claim 11  wherein the MgO is derived from talc.  
     
     
         17 ) The alkali-resistant ceramic body of  claim 11  wherein the SiO 2  is derived from clay.  
     
     
         18 ) The alkali-resistant ceramic body of  claim 11  wherein the Al 2 O 3  is derived from clay.  
     
     
         19 ) The alkali-resistant ceramic body of  claim 11  wherein the porosity is less than 5% by volume.  
     
     
         20 ) The alkali-resistant ceramic body of  claim 11  wherein the water absorption is less than 5% by weight  
     
     
         21 ) The alkali-resistant ceramic body of  claim 11  wherein the compressive strength of the ceramic material is greater than 2×10 8  Newtons per square meter.  
     
     
         22 ) The alkali-resistant ceramic body of  claim 11  wherein the loss in weight of the alkali-resistant ceramic body is less than one percent when exposed to molten potassium carbonate.  
     
     
         23 ) The alkali-resistant ceramic body of  claim 11  wherein the gain in weight of the alkali-resistant ceramic body is less than one percent when exposed to wood ash at greater than 800 degrees centigrade.  
     
     
         24 ) A ceramic body coated with an alkali-resistant material, the alkali resistant coating comprising: 
 20 to 80% by weight of MgO;    10 to 50% by weight of SiO 2 ;    5 to 30% by weight Al 2 O 3 ;    and 1 to 10% by weight Fe 2 O 3 , CaO, and alkali oxides.    
     
     
         25 ) The ceramic body of  claim 24  wherein the dominant crystalline phases of the alkali resistant coating are forsterite and spinel.  
     
     
         26 ) The ceramic body of  claim 24  wherein the porosity of the alkali resistant coating is less than 5% by volume.  
     
     
         27 ) The ceramic body of  claim 24  wherein the water absorption of the alkali resistant coating is less than 5% by weight.  
     
     
         28 ) The ceramic body of  claim 24  wherein the alkali oxide is Ka 2 O.  
     
     
         29 ) The ceramic body of  claim 24  wherein the alkali oxide is Na 2 O.  
     
     
         30 ) The alkali-resistant material of  claim 24  wherein the MgO is derived from oxides of magnesium.  
     
     
         31 ) The alkali-resistant material of  claim 24  wherein the MgO is derived from talc.  
     
     
         32 ) The alkali-resistant material of  claim 24  wherein the SiO 2  is derived from clay.  
     
     
         33 ) The alkali-resistant material of  claim 24  wherein the Al 2 O 3  is derived from clay.