Effect of Free Silica

Effect of Free silica/Quartz silica in Kiln feed 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. 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 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.