Burning in a Kiln

Burning in a kiln – formation of cement clinker 

 Because the raw ingredients are not completely melted, the mix must be agitated to ensure that the clinker forms with a uniform composition. This is accomplished by using a long cylindrical cylindrica l kiln that slopes downward and rotates slowly

 A variety of fuels can be used, used, including pulverized coal or coke, coke, natural natural gas, lignite, lignite, and fuel oil. These fuels create varying types and amounts of of ash, which tend to have compositions similar to some of the aluminosilicate ingredients ingredient s in the raw mix. Since the ash combines with the raw mix inside the kiln, this must be taken into account in order to correctly predict the cement compassion. There is also an increasing trend to use waste products as part of the fuel, for example old tires. n the best!case scenario, this saves money on fuel, reduces "# $ emissions, and provides a safe method of disposal.  a kiln%without a preheater& is typically about '() m long and * m in diameter, has a downward slope of +!-, and rotates at '!$ revolutions revolutions per minute. !!!!!!!!!!!!! orient kiln ( m long / m diameter speed 0 +.* to / 123 max

The raw mix enters at the upper end of the kiln and slowly works its way downward to the minutes, undergoing several different hottest area at the bottom over a period of 60-90 minutes, reactions as the temperature increases. t is important that the mix move slowly enough to allow each reaction to be completed at the appropriate temperature. Because the initial reactions are endothermic (energy absorbing), it is difficult to heat the mix up to a higher temperature until a gien reaction is complete!

Dehydration Dehydration zone (up to ~ 450˚C): This is simply the eaporation and remoal of the free "ater . 4ven in the 5dry process6 there is some adsorbed moisture in the raw mix. Although the energy. n temperatures re7uired to do this are not high, this re#uires significant time and energy. the wet process, the dehydration zone would re7uire up to half the length of the kiln, while the dry process re7uires a somewhat shorter distance. Calcination zone (450˚C – 900˚C): At about 600$% the bound "ater is drien out of the clays, and by 900$% the calcium carbonate is decomposed, releasing carbon dioxide. dioxide. By

the end of the calcination zone, the mix consists of oxides of the four main elements which are ready to undergo further reaction into cement minerals. Because calcination does not involve melting, the mix is still a free-flo"ing po"der at this point!

Solid-state reaction zone (900˚ - !00˚C) & solid!state reactions begin to occur. %a' and reactie silica combine to form small crystals of % (dicalcium silicate), one of the four main cement minerals. n addition, intermediate calcium aluminates and calcium ferrite compounds form. These play an important role in the clinkering process as fluxing agents, in that they melt at a relatively low temperature of 8 '+))9", allowing a significant increase in the rate of reaction. :ithout these fluxing agents, the formation of the calcium silicate cement minerals would be slow and difficult. n fact, the formation of fluxing agents is the primary reason that portland %calcium silicate& cements contain aluminum and iron at all. The final aluminum! and iron!containing cement minerals %" + A and " A;& in a portland cement contribute little to the final properties. As the mix passes through solid!state reaction zone it becomes 5sticky6 due to the tendency for ad