My research work is related to the calcination of limestone. I have a complex question and would like to consult with experts in this field. Could you please advise which kiln to choose (a twin-shaft kiln or a rotary kiln) if high-quality lime CaO+MgO of at least 90% is required, with lime reactivity T60 of less than 2 minutes? The raw limestone has CaO > 53%, SiO2 < 2%, MgO < 0.8%, Fe3O4 < 0.2%, and Al2O3 < 0.9%. The compressive strength of the raw limestone is 20-60 MPa. One of my academic supervisors mentioned that, based on his experience, to achieve high-quality lime in a twin-shaft kiln, the compressive strength needs to be above 60 MPa. Therefore, it seems that the rotary kiln might be the better option. However, I would like to hear from other experts as well. Could you please share your experience on which kiln is better to choose and the role that compressive strength plays in kiln selection?
Hi Olga,
I don’t know the reason of your research, I mean why calcining only Calcium Carbonate (or marble that should be better for crystallography reasons)? Limestone is pure rock and it’s calcined usually in vertical kiln at medium temperature, usually from 850-900°C, to produce lime (but still CaO, so quick lime, very dangerous if not estinguished with water to obtain the final binder called natural hydrate lime). From your analysis the Lost of Ignition should be higher than 41-42% that is the average for every calcium carbonate source, the Magnesium mineral like dolomite or Magnesite can explain this fact (dolomite has L.o.I. of 45-47%, while Magnesite till 48-50%).
Why Using higher temperatures? and why using more expensive kilns if the result is just hydrate lime?
Other strange data you named is about the strength of the carbonatic source. Obviously there are different calcium carbonates and their compressive strength depend on their diagenesis (the way have been formed). So marble that is metamorphic rock can achieve 60-70 MPa while normal sedimentary carbonate (calcite) is the weakest and aragonite that comes from organogenesis is little better (mainly about flexural ability not the compressive).
The compressive strength of the final hydrate lime depends on the purity of the carbonate source and on the correct amount of water to estinguish the quick lime (CaO) till the production of the hydrate Ca(OH)2, that will react with the CO2 generating finally new CaCO3. The mechanical resistance class of this pure lime is 3,5 MPa after 28days. The same class if you calcine aragonite from shells like Romans did for centuries (but the performances of Roman cements depended on other parameters, not for sure the quality of the lime). Probably little better using marble but should be demonstrated by trials.
Without any pozzolanic reinforcement the mechanical strength will remain weak even changing type of kiln.
Hoping to be useful for your studies.
Best regards
Dr. Alex Reggiani
CTO GeoMITS
In shaft kilns, mainly coarse fractions of limestone are fired. In rotary kilns, both coarse and fine fractions of limestone can be fired. The quality of lime depends on the quality of the limestone, i.e. the grade of the raw material used. The extent to which CO2 gases are removed from the kiln during firing is also an influencing factor on the quality of the lime. The quality is also affected by over- or under-burning of limestone in the kilns. It is possible that if less durable limestone is used during firing in shaft kilns, the pieces will be crushed into smaller pieces. This will cause the smaller pieces to be sandwiched between the larger pieces, which will form a stronger obstacle to the passage of gases through the limestone layers. This will adversely affect the firing process and, of course, the quality of the lime.
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