Typical kiln residence times for raw materials range from one to three hours. Figure 2 shows the kiln's temperature profile. Energy typically comes from coal, as mentioned. But it is not uncommon today to see used tires, spent solvents, blended paint wastes, used oil, petroleum residues, wood chips, agricultural wastes such as rice hulls, and many more sources of energy routinely in use. The silicon, aluminum, and iron needed as cement process ingredients do not always have to come from clay, shale, and sand found in a quarry.
Power plant fly ash and boiler ash have raw material characteristics.
Spent aluminum potliner is both an energy source and a raw material substitute candidate. A cement kiln produces no ash. The inorganic components from the fuel bond with the inorganic compounds of the clinker or kiln dust. Lead, barium, chromium, nickel, and other metals become a fixed part of the cement structure. As the fuel burns, the heat causes the molecules of fuel, air, and raw materials to break apart.