silica (quartz vs silicate minerals)
Good day experts.
Can you tell me what are the implications (if any) of using some silica in the quartz form rather than the silicate mineral form in terms of kiln operation and also in the C3S and C2S formation reactions. What is the maximum amount of quartz silica that can be used instead of silicate minerals in the kiln feed slurry? Thanks.
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Re: silica (quartz vs silicate minerals)
Although silicate minerals are generally considered easier to burn than pure quartz, the maximum amount of quartz that can be used really depends on the particle size of the quartz. All other factors being equal, significant amounts of quartz greater than 32 microns will have a negative effect on the burnability of the rawmix (higher clinker free lime, numerous large C2S clusters etc).
The degree of free lime increase directly attributed to coarse quartz is in the order of about 0.6% for every 1% of +32um quartz.
Coarse quartz can be minimized in a variety of ways;-
1. Use of raw materials and silica correctives that are naturally high in silica while low in +32um quartz. (eg diatomite, high silicate clays, calcium silicate wastes etc)
2. Grind the rawmix to a sufficient fineness so that +32um quartz content is minimised, while at the same time not excessively increasing milling costs.
3. Grind the quartz component separately to maximize its fineness. (eg separate sand grinding) Note: This has occupational health risks associated with respirable quartz.
At our plant we have either investigated or trialled all these things and found that the best solution was a combination of 1. and 2. Separate sand grinding should only be regarded as a last resort because of the high cost of a separate grinding circuit and the inherent health risks with processing and handling fine quartz.
We have also found that complicated and costly XRD mineralogical or microscopic analysis is not necessary to control the coarse quartz fraction in the raw meal. You can do this by simple chemical analysis of the +32um wet sieve fraction. The SiO2 content of this fraction will give you a good estimate of the coarse quartz content, especially if you use sand as a silica corrective.
We succeeded in reducing our quartz sand corrective usage rate by almost 50% and the coarse quartz fraction in raw meal by over 30% using these methods.
I hope this helps,