Refractory carbon-bonded magnesia brick and process for producing it

Abstract

The disclosure relates to a process for producing a refractory, ceramically fired, carbon-bonded magnesia brick whose matrix is more than 70% by weight, in particular from 80 to 98% by weight, of MgO grains and also a carbon framework binder matrix resulting from carbonization, and pores, wherein the MgO grains are fixed by means of carbon bonding of the carbon framework and at least 30%, in particular from 50 to 100%, of the MgO grains have at least one sintering bridge resulting from the ceramic firing.

1 . A process for producing a magnesia brick, the process comprising the steps of:
 preparing a blend of the following constituents:
 from 72% to 95% by weight of granular magnesia composed of primary magnesia grains, 
 from 2% to 4% by weight of a carbon binder, in particular pitch or synthetic resin, and 
 from 0% to 25% by weight of graphite, 
   shaping of a green body from the batch produced by pressing,   heating the green body at temperatures in the range from 150° C. to 600° C. until the binder has carbonized and carbon bonding of the magnesia grains has occurred, thereby, forming a tempered blank,   ceramic firing of the tempered blank in a reducing atmosphere above 1200° C., wherein the firing conditions, such as a reducing atmosphere, the duration of firing, and the temperature control are set in such a way that, under the action of the carbon existing in the brick, vaporous magnesium is formed from magnesia in a manner distributed homogeneously in the brick as a whole as a result of redox reactions in accordance with the following reaction:
   MgO+C→Mg+CO
 
   and the vaporous magnesium is oxidized with oxygen available in the brick to form secondary magnesia and accumulates directly on existing magnesia grains and forms sintering bridges between magnesia grains until at least 30% of the existing primary magnesia grains are sintered together with at least one other primary magnesia grain in each case by way of at least one sintering bridge of secondary magnesia.   
     
     
         2 . The process of  claim 1 , wherein the step of ceramic firing is carried out until more than 50% of the secondary magnesia grains that are present have more than two secondary magnesia sintering bridges. 
     
     
         3 . The process of  claim 1 , wherein the steps of heating and ceramic firing are carried out in direct succession in the same firing apparatus. 
     
     
         4 . The process of  claim 1 , wherein the step of firing is carried out in the range from 1200° C. to 1500° C. 
     
     
         5 . The process of  claim 1 , wherein in the step of preparing a blend, the blend comprises the following:
 83%-95% by weight of fused magnesia (MgO content=97.5% by weight);   2%-14% by weight of floc graphite; and   2%-4% by weight of pitch binder.   
     
     
         6 . The process of  claim 1 , wherein the in the step of preparing a blend, the blend comprises the following:
 72%-95% by weight of fused magnesia (MgO content=97.5% by weight);   2%-25% by weight of floc graphite; and   2%-4% by weight of phenolic resin.   
     
     
         7 . The process of  claim 1 , wherein the step of heating is carried out at a heating rate in the range from 0.5° C. to 5° C./minute, up to a temperature in the range from 1200° C. to 1500° C. 
     
     
         8 . The process of  claim 1 , wherein the step of firing is carried out for 2 to 10 hours. 
     
     
         9 . The process of  claim 1 , wherein cooling is carried out at cooling rates in the range from 0.5° C. to 5° C./minute. 
     
     
         10 . The process of  claim 9 , wherein cooling is carried out at cooling rates in the range from 2 to 3° C./minute. 
     
     
         11 . The process of  claim 7 , wherein the step of heating is carried out at a heating rate of in the range of 2° C. to 3° C./minute and in the range of 1300° C. to 1400° C. 
     
     
         12 . The process of  claim 8 , wherein firing is carried out for 5 to 6 hours. 
     
     
         13 . The process of  claim 1 , wherein the MgO grains have grain sizes in the range from powder fineness up to 8 mm. 
     
     
         14 . The process of  claim 1 , wherein the MgO grains comprise the following grain size distributions: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Grain size (mm) 
                   Proportion (% by weight) 
                 
                     
                     
                 
                     
                   2-4 
                   25-40 
                 
                     
                   1-2 
                   15-30 
                 
                     
                   0.063-1    
                   10-25 
                 
                     
                   <0.063 
                    15-30.