Hello Raza,
The 'Free silica' in question is quartz or other hard high-silica mineral. Since quartz is very hard it is much more difficult to grind than limestone and is therefore concentrated mostly in the coarse fractions of the kiln feed.
The quartz in clays is already very fine <~5um) and normally poses no problem, however the limestone component can sometimes contain thick (>1cm) seams of pure quartz which has been deposited over long periods of time in cracks in the limestone by silica-rich ground waters.
Such large quartz seams are very difficult to grind and the resultant particle size of quartz grains in the raw meal produced from such material is therefore increased significantly.
During reaction in the kiln, large quartz grains react at their surface with nearby CaO grains to form C2S. In the burning zone, the reaction occurs by diffusion of the CaO dissolved in the liquid phase. If the quartz particle is big enough a wall of C2S forms around the remaining quartz, isolating it from the liquid phase and preventing any more diffusion of CaO. ie the reaction SiO2 + 2CaO -> C2S becomes stalled at this point and, no matter how hard the clinker is burned, this free silica can never all react in time. This leaves two equivalents of unreacted CaO lime behind which increases the free lime of the clinker.
A similar situation occurs if the large quartz grains are just small enough to completely react into C2S. The next step is for this C2S to react with further CaO to form C3S. Again, if the C2S cluster resulting from a coarse quartz particle is large enough, a wall of C3S will form around the cluster preventing CaO diffusion through to the centre of the cluster. This results in three equivalents of free lime being left behind.
Essentially the coarse quartz has substantially increased the burnability of the kiln feed. This situation cannot be detected in the chemical analysis of the kiln feed which is normally performed at a cement plant. It must be determined by the analysis of critical size fractions of the kiln feed.
Depending on kiln conditions/chemical targets/raw meal particle size distribution and raw material mineralogy the point at which quartz and calcite grains are likely to become problematic will vary from plant to plant.
The actual amount of unreacted lime remaining depends on the following drivers;-
–Specific reaction area (the area of contact between the grains)
–Local oversaturation (grain size of individual minerals)
–Ambient conditions (pressure, temperature, and burning time)
–Diffusion coefficient of CaO through the liquid phase (composition of the liquid phase)
–Amount of liquid phase formed during burning
–Supply and demand of CaO
If we assume that kiln conditions such as pressure, temperature and burning time can be kept relatively constant, the rest of these drivers can be condensed into just four main parameters;-
Silica Ratio
Lime Saturation Factor
Amount of oversized quartz grains (>32um)
Amount of oversized calcite grains. (>90um)
Having measured these parameters and performed burnability tests on many different raw meals, a mathematical relationship such as the one below can be obtained by regression analysis;-
Free Lime (burnability test) = w * Quartz(>32um) + x * Calcite(>90u) + y * LSF + z * SR - c
The coefficients w,x,y,z and c will be slightly different for each plant and the critical size criteria for oversized quartz and calcite often vary from researcher to researcher. Some maintain that quartz >45um and calcite >125um are critical.
For further information, see a previous post on this topic located here:-
http://tinyurl.com/23bj5c5
Hope this helps,
Ted.
