1999-11-11_REVISION - M1977208 - Laserfiche WebLink

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Cement kiln dust (CKD) is a by-product of cement manufacturing. It is an inorganic material collected in the air pollution control devices of portland cement manufacturing plants and is the finely-divided particulate matter carried from the cement kiln by exhaust gases. The dust is composed of variable mixtures of calcined and uncalcined feed materials, fuel combustion byproducts. condensed alkali compounds, and fine cement clinker formed during the high temperature processing. Alkalis may be concentrated in the dust through volatilization in the high temperature zones in the kiln then condensed in the exhaust gases as they pass through the dust collection system. The composition of CKD varies depending on production conditions and the nature of the raw material and fuel. The actual form of the components may typically be: Calcium Carbonate (CaCO3) 10 percent Available Lime (as CaO) 30-50 percent Potash (as Na and K salts) 6-10 percent Chloride 4 percent Remembering the stages of cement clinkering in the kiln, it can be seen that CKD example above is made up of the raw materials for clinker manufacture, which are limestone (CaCO3) and calcined limestone or lime (CaO). It makes sense that the kiln exhaust would contain dust made up of pre clinker raw materials, because by the time clinker is formed the material is molten and would not produce much dust. Most cement plants recycle their kiln dust into the kiln. However, complete recycling is not possible due to a build up of undesirable elements as discussed below. CKD is not portland cement and does not exhibit the complex cementitious properties of portland cement. For most CKD, the chemical and physical behavior is dominated by the lime (CaO) component. CKD contains little calcium silicate, which is the basic ingredient of cement. Continuous recycling of CKD concentrates alkalis (potassium and sodium), chloride, and sulfate, which adversely affect the clinkering process and force the kiln operator to shunt a certain percentage of CKD. Recycled CKD also concentrates volatile metals making recycled CKD disposal potentially more likely to release metals to the environment. This memo distinguishes between what is termed "fresh" CKD and "weathered" CKD. These materials differ in their physical properties and in their response to leach testing, metals mobility, and effects on pH. These differences are discussed. Some CKD may contain lazge quantities of calcined feed materials, alkalis and sulfur compounds, or both, while others may be primarily composed of uncalcined raw feed. Many CKDs are easily compacted and reactive with small quantities of water to form a low strength cementitious mass; some may show only mild reactivity and be non-consolidating. The following discussion of "fresh" and "weathered" CKD assumes a limey CKD. "Weathered" CKD. As discussed above, most CKD is largely composed of lime. Lime is a simple cementing material produced by driving water and carbon dioxide off limestone (calcining). Its cementing properties arise from the reabsorption of the liquid and gas that has been expelled and the formation of chemical compounds similar to the original limestone raw material. Left exposed to the atmosphere, CKD will react, cure and harden. Its ultimate composition will become limestone. Thus, in that regard, weathered CKD is r:or CKD at all. It is limestone.