Cement, Energy and Environment

back to the burning zone. Inevitably, however, some condensation will happen on the sides of the kiln or preheater. If alkali and sulphate are balanced such that just alkali sulphates form as buildup the deposits that are relatively soft and can be easily removed by air cannon. As the proportion of sulphate Increases, the double calcium and potassium salt known as calcium langbelnlte will form. This Is a harder deposit and can cause significant problems. As in all situations where high sulphur levels are present in a kiln, the solution Is to maintain sufficiently high oxygen levels so that sulphur Is present as S0 4 and is not reduced to S0 3 . CaS03 present in the burning zone will volatilize at lower temperatures than CaS0 4 and will cause more severe problems with buildup. Clinker nodules and free lime Alkali sulphate which remains in the clinker as a liquid through the burning process is the very last of the main constituents to crystallize. Because the rest of the crystals are already in place, alkali sulphates are present in the pore spaces as seen in Figure 2. As a consequence of this, it is the more porous clinker nodules which tend to contain the most abundant alkali sulphates. Harder burning of clinker reduces the litre weight (bulk density) and therefore encourages volatilization of alkali sulphates through lack of internal space tor the liquid. Trials separating out fine and coarse clinker nodules have confirmed that finer nodules contain lower alkali levels than coarser nodules, due to higher surface available for alkali volatilization . Where screening for size is practical and there is an alternative outlet for the higher alkali fraction of clinker, this has proved to be a satisfactory method of reducing alkali levels in certain products from a cement factory. When attempting to reduce free lime to a target level in clinker there may be a temptation to increase the burning temperature, but increasing litre weight will considerably exacerbate any issues with regard to volatile cycles and other, more fundamental solutions to high free lime such as finer milling of raw feed or reducing the variability of raw feed chemistry are more satisfactory. Summary Most alkali in cement comes from the clinker raw materials, specifically from the clay component, therefore a key to minimizing alkali is by minimizing the clay component. This can be done by changing the chemical parameters or by finding alternative raw materials. Increasing the Limes Saturation Factor of the clinker uses a higher proportion of limestone and a lower amount of clay and therefore reduces the alkali input. Similarly, raising the silica ratio uses a higher amount of sand and less clay or shale, raising the silica ratio will reduce the alkali input. Decreasing the ratio of alumina to iron uses a higher proportion of iron oxide and lower amount of clay and therefore reduces the alkali input. Unfortunately, each of these actions makes the feed more difficult to bum and creates higher risk of build up. Some alternatives to clay are available which may decrease the alkali input to the kiln in the event of straight replacement. Fly ash will frequently have a high alkali content on chemical analysis, but it needs to be remembered that there will be no further toss from fly ash on burning, so the effect may not be as great as it may at first appear. Usually, fly ash will have a lower silica ratio that natural clays and shales, so the requirement for further sand needs to be considered. The desire to reduce alkali needs to be balanced against the costs of doing so. These may not be immediately apparent and might include the extra costs associated with harder burning of the feed. Whether the cost is acceptable will depend primarily on the particular market circumstances. Courtesy: International Cement Review, December 2009, Pp 80 - 82. 20