M. K.
SEN ( NCP)
Effect of Free silica/Quartz silica in Kilnfeed
Free silica/quartz should have no adverse effect below a particle size of about 32um.
During reaction in the kiln, large quartz grains react at their surface with nearby CaO
grains to form C₂S. The reaction in the absence of liquid phase occurs by solid-solid
reaction with CaO particles, so the smaller the particle sizes of both silica and
calcite in the kiln feed, the quicker the reaction.
If the quartz particle is large enough a wall of C₂S forms around the remaining
quartz, isolating it from surrounding CaO. i.e. 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
dissolved in the liquid phase to form C₃S, ie.C₂S + CaO --> 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, leaving behind a dense core of C₂S. This results in one
equivalent 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.
However all agree that coarse quartz and calcite have a negative effect on
burnability.
Also I have sent a quick and simple method for the estimation of +32um SiO2 and
+90um CaCO3
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˃˃THANKS˂˂
M. K. SEN (OFFCIER) NCP
