What is generally defined the "lime production process", actually takes care of burning and sub-products processing obtained from different types of minerals available in nature.
|Hydraulic Limestone||CaCO3 MgCO3SiO2 Fe2O3 Al2O3|
The above mentioned chemical schema, for clarity of explanation, in nature doesn’t find a such clear division in the minerals that are used industrially, which are a more or less complex mixture of the types mentioned.
This mix of possible chemical composition of the starting mineral, gives a bigger complexity of the “real” material behaviour both during its work and in terms of chemical and physical characteristics of the finished products of the productive process.
CHEMICAL BASE OF THE LIME PRODUCTION PROCESS
The lime and its derivates production process can be summarised principally by the following three chemical reactions groups:
|The chemical reaction of the limestone and dolomia decarbonation|
|CaCO3||+||Kcal||↔||CaO||+||CO2||(+ 760 kcal/kg)|
|MgCO3||+||Kcal||↔||MgO||+||CO2 ↑||(+ 723 kcal/kg)|
|CaCO3 MgCO3||+||Kcal||↔||CaO MgO||+||2CO2 ↑||(+ 723 kcal/kg)|
|The hydration reaction of calcium and magnesium oxides|
|CaO||+||H2O||↔||Ca(OH)2||+||Kcal||(- 273 kcal/kg)|
|CaO MgO||+||H2O||↔||Ca(OH)2 MgO||+||Kcal|
|CaO MgO||+||2H2O||↔||Ca(OH)2 Mg(OH)2||+||Kcal||(- 211 kcal/kg)|
|The recarbonation reaction of the calcium and magnesium oxides|
The first group of reported reaction summarises the decarbonation process of the three main typology of carbonate existing in nature.
They represent the reaction that, industrially, arrives in the limestone burning kiln when starting from the mineral coming from the quarry and supplying a prefixed heat quantity, it's obtained the carbonate dissociation (CaCO3) in an oxide (CaO) and in carbon dioxide (CO2).
The reaction is endothermic and to develop it needs a caloric contribution obtained by the use of a fuel.
They are necessary 760 kcal/kg to decarbonate the CaCO3 and 723 kcal/kg for the MgCO3.
(Note: here following, except particular note, we will refer to the case of calcium carbonate and/or to the lime oxide for simplicity of explanation, but in the case of the other mentioned components, the mechanisms by which the reaction happens, are quite similar).
The limestone dissociation happens in five following steps:
- In the kiln pre-heating zone, the heat generated by the fuel is transferred by the combustion gases to the mineral to decarbonate which goes from room temperature till to 800° C.
- At the temperature of 800°C, the carbon dioxide pressure produced by the dissociation is equal to the partial pressure of the CO2 in the gases that cross the kiln. In the moment in which the temperature begins to rise up over the decarbonation value, the superficial coating of the limestone starts its decarbonation. When the superficial temperature has reached about 900°C a coating of 0,5 mm has decarbonated (5% of total mass).
- At the temperature of 900°C, the partial pressure of CO2 is bigger of 1 atmosphere and the process of decarbonation can proceed from the surface to the limestone piece center.
- For all the decarbonation process the mineral temperature remains almost constant and if for any reasons tied to the industrial process the temperature increases over the decarbonation point or the limestone chemical composition used reduces the decarbonation temperature, the crystalline structure of the produced lime begins to collapse(sintering).
- Finished the decarbonation the lime is cooled with the air in entry in the kiln that rising up the shaft pre-heated, it will become the combustion air.
- Limestone dimension and form (minimum dimension).
This factor engraves on the migration time of the CO2 from the center of the mass in decarbonation to the surface.
- Size distribution of the used limestone.
A different size distribution of the limestone gives a different surface of exchange between limestone and warm gases and so a different heat transfer speed.
- Limestone chemical composition.
The impurities or magnesium carbonate presence give a variation in the theoretic temperature at which the decarbonation starts.
- Gases speed in the kiln inside and their temperatures.
The speed of the heating fluid and the temperature gradient are some factors that engrave on the transfer speed of the energy from the gases to the mineral.
- Gases pressure in the kiln inside.
Higher pressures ask bigger pressure from the part of the CO2 to can migrate in the limestone inside.
With the word hydration or lime slaking it's intended the process in which the lime oxide (CaO) transforms itself in lime hydroxide (Ca(OH)2).
From the chemical point of view it’s one only typology of reaction but from the industrial point of view it’s possible to have two different process typologies..
One is the hydration process itself, where the oxide is reacted with the water stoichiometric quantity (32% H2O) obtaining an hydroxide under powder form containing max 1,5% of free water.
The other is the slaking process where the lime oxide is made to react with a water quantity much higher than the stoichiometric content obtaining a lime hydroxide in suspension with concentrations values much variable in function of the required use.
The hydration chemical reaction is very elementary but the kinetics of the reaction, linked to the crystallization and the agglomeration are much more complex and depend not only by physical and chemical characteristics of the oxide to slake but also by the modalities by which this simple reaction is carried out and therefore they have been developed two different methods of slacking indicated.
At temperatures inferior to 350°C the oxide reacts completely with the water giving origin to an exothermic reaction with the develop of kcal/kg. CaO, at higher temperature the reaction arrives in opposite way generating the reaction water separation.
The magnesium oxide is few reactive with the water and in normal conditions only the 25% of it reacts.
To obtain the MgO complete hydration it’s necessary that the reaction arrives at a temperature over the 100°C and using equipments in pressure.
The main factors that influence on the hydration are:
- The reactivity of the used oxide (T60)
- The apparent density of the lime oxide
- The size distribution and the oxide top cut
- The carbonate percentage (L.O.I.)
- The sulphur percentage
- The magnesium oxide percentage
- The temperature at which happens the hydration process
- The mixing system efficacy between oxide and water
Both systems by the schematic point of view are mixers in which thanks to the mechanic agitation, it’s obtained an intimate contact between the lime oxide and the reaction water.
The recarbonation reaction represents the lime life cycle last reaction, it controls the process that transforms the lime in carbonate, re-absorbing the CO2 from the ambient and conferring to the lime, the chemical and physical characteristics that had the starting limestone.
This is the fundamental reaction that allows the lime, once applied on the wall, to set hardening itself.
The man has learned to control such reaction at his use for the PCC production (Calcium Carbonate Precipitated), process in which checking the parameters that influence the recarbonation, it’s possible to modify the size and the produced calcium carbonate morphology.
At the ambient conditions the lime recarbonation is very modest, but starting from 290°C and till 600°C the recarbonation speed increases quickly, increasing in an exponential way the CaO with the CO2 affinity.
This reaction speed is furthermore increased in a significant way both by the specific surface of the oxide and by the rapidity by which the oxide is mixed with the carbon dioxide.
We want to note how the magnesium oxide has recarbonation times significantly longer than the calcium oxide.
The water presence both like humidity and steam, allows to activate a quick recarbonation also at ambient temperature and pressure because the water behaves in the reaction like the reaction inhibitor.