Understanding the effect of carbonization process on the morphology of light calcium carbonate
In the industry, carbonization is generally used to produce light calcium carbonate. The carbonization reaction system in the carbonization process is a gas-liquid-solid three-phase system, which mainly includes the following sections:
(1) Calcination: lime and coal are mixed uniformly in a certain ratio, and then calcined by mixing shaft kiln to produce calcium oxide and carbon dioxide;
(2) Digestion: calcined lime is removed from the digester and then digested into water to form lime milk, which is refined;
(3) Carbonization: After the lime milk is refined, it is carbonized with the kiln gas at a certain temperature and concentration;
(4) The filter press is dehydrated, dried, pulverized, and packaged.
The flow chart of light calcium carbonate production and carbonization process is as follows:
Light calcium carbonate production, carbonization process flow chart â–²
In the carbonization process, the morphology and size of the light calcium carbonate particles will not only be affected by the crystal form control agent (please see the article "[Calcium carbonate industry] crystal shape control agent on the morphology of light calcium carbonate"), and It is affected by carbonization process conditions such as carbonization temperature, calcium hydroxide concentration, reaction stirring speed, and CO2 aeration.
1The effect of carbonization temperature on the morphology of calcium carbonate
From the thermodynamic point of view, as the temperature increases, the solubility of CO2 and the solubility product of Ca(OH)2 in the system will decrease, resulting in a decrease in the supersaturation of calcium carbonate in the liquid phase, which reduces the nucleation of crystals. The growth rate is not conducive to the progress of the carbonization reaction.
From a kinetic point of view, the increase in temperature accelerates the reaction of CO2 with OH- to form CO32- and increases the mass transfer coefficient of the entire diffusion step, which is advantageous for increasing the carbonization rate.
In actual production, when the temperature is high, the crystal nucleation rate is lower than the growth rate, and the crystal tends to grow too large to form a larger particle size of calcium carbonate crystal; in addition, if the temperature is too high, the growth activation energy of each crystal plane of the crystal occurs. The change causes the morphology of the crystal to change, and the particles aggregate and grow to form a secondary particle size with a larger particle size, which is not conducive to the control of the crystal morphology.
Effect of 2 Ca(OH)2 Concentration on Morphology of Calcium Carbonate
During the carbonization reaction, calcium carbonate nucleation and crystal growth are a competitive relationship, but generally the nucleation predominates in the early stage of carbonization reaction, and crystal growth dominates in the middle stage of carbonization reaction. The nucleation rate increases, and the calcium carbonate particle size becomes smaller; while the growth rate increases, the calcium carbonate crystal grain size becomes larger.
The high concentration of calcium hydroxide or Ca2+ in the system is beneficial to the improvement of the nucleation rate and the growth rate, which is beneficial to increase the rate of the entire carbonization reaction.
In actual production, when the concentration of calcium hydroxide and Ca2+ in the system is too high, the carbonization reaction rate is too high, and the reaction generates a large amount of heat, which causes the temperature of the system to rise, and it is easy to generate large-size crystals, and the quality of the calcium carbonate product is easily deteriorated; When the concentration of calcium hydroxide or Ca2+ is too low, the nucleation rate is lowered, and the energy consumption of the product is increased, which is not suitable for industrial applications.
Thus, by controlling the concentration and initial temperature of the calcium hydroxide slurry, calcium carbonates of different morphologies were prepared without the addition of a crystal form controlling agent.
3 Effect of stirring speed on the morphology of calcium carbonate
In the secondary nucleation process, a certain range of agitation speed is advantageous for forming particles of small particle size. Therefore, under certain conditions, as the stirring rate increases, the collision energy of the microcrystalline particles in the system increases, and some of the microcrystals which are not strongly adsorbed are easily scattered due to collision, so that large crystals are not formed.
At the same time, also help to strengthen the stirring distribution of carbon dioxide gas, stirring with the accelerated rate of liquid carbon dioxide bubbles in a smaller diameter, increase in carbon dioxide gas and calcium hydroxide slurry contact area, and to improve the solubility of carbon dioxide in the system The concentration of CO32-ion.
4 Effect of CO2 aeration on the morphology of calcium carbonate
The solubility of carbon dioxide in the slurry is related to its volume concentration and flow rate, which directly affects the concentration of CO32- in the system, thereby changing the chemical potential of Ca2+ or CO32- ions.
The higher the concentration of CO32-, the larger the nucleation rate of crystal, the easier to obtain crystals with smaller particle size; the lower the concentration of CO32-, the smaller the nucleation rate of the system, the crystal growth of calcium carbonate is dominant, which is beneficial to obtain larger particle size. In addition, when the volume concentration of carbon dioxide is too large, the supersaturation in the system increases, the number of locally generated nuclei is too large, and the crystal nuclei are easily collided and agglomerated, which makes the particle size of the prepared calcium carbonate. The distribution becomes wider.
When the volume concentration of carbon dioxide is constant, the gas flow rate is increased, the mass transfer rate of carbon dioxide is increased, the carbonization reaction time can be shortened, and the nucleation growth time of the calcium carbonate crystallites formed in the nucleation stage is shortened, so that the prepared calcium carbonate crystal particles are obtained. The diameter is small and the settlement volume increases.
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