Effects of recirculation cells in cement kilns
As the raw material passes through the cement production process, recirculation cells are created. Due to the heterogenous nature of the fine materials, the composition in each cell is not uniform and as such, recirculation cells can impact on the final product.
Figure 1: example of the relationship of hotmeal LSF to clinker LSF
The manufacture of Portland cement clinker is necessarily concerned with the handling of very fine grains or dusts at various stages of production. First the raw materials are milled to a condition with most grains measuring less than 45µm. In dry-process systems the dusts must be manoeuvred through a series of cyclones and ducts before entering the rotary kiln to be nodulised and fired to form ceramic balls. These are then milled again to produce cement grains of similar size to those of the kiln feed.
The firing process depends on the passing of the fine materials in one direction and the passage of hot gases in the opposite direction. During this process at each preheater stage dust particles are picked up in the gas stream and returned through the system resulting in a series of recirculation cells. Because not all of the fine materials are the same size or the same density, each time a recirculation cell is set up there is segregation of the different components of the kiln feed, meaning that in each of the cells the overall composition of the material passing through is likely to vary from that of the original feed. From the entry of the feed close to the final fan, through a number of cyclones then through the rotary kiln and possibly through a tertiary air duct, there is ample scope for dust production and therefore, blockages.
