Raw-mix powders in cement plants are supposed to be homogeneous in every sense. But, even perfect blending can not achieve homogeneity in the chemical composition in different particle size intervals. Because a mixture of raw materials is ground to produce raw-mix, coarser size ranges may be richer in lime with finer size ranges being richer in silica if a siliceous raw material (like shale) softer than the run-of-mine limestone is used as one of the raw materials.
FRACO is a software to analyse such heterogeneity in otherwise 'homogeneous' raw-mixes and its implication on clinker quality, liquid formation in kiln and burning itself. It is a powerful tool, extremely useful as a companion software to RAWMEAL. It is not at all sufficient to have a 'good' raw-mix design arrived at using a software like RAWMEAL alone. The raw-mix so designed should also be good from the chemical homogeneity angle, which can be predicted only by FRACO.
In one case, it was found that the raw-mix C3S (not clinker) of fines below 53 microns was only between 16 to 45 whereas in coarser fractions, it was above 80. The LSF of raw-mix varied from 100 to 160 for fines to coarse sizes. The minimum burning temperatures (MBT) required were higher for coarser fractions by as much as 250 deg.C over that for finer fractions. In such cases, it may be better to use low-grade limestone from the same mine than using outside shale, although the same may be harder than shale. Similarly, while the use of an additive such as laterite may be predicted to be better from raw-mix design angle by RAWMEAL, it may be shown to be undesirable from the chemical homogeneity angle by FRACO. Therefore, RAWMEAL and FRACO should be used together in any optimisation exercise.
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