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Mechanism of Lime Hydration Process and the Decisive Factors

2023-12-17 11:30:47

Lime Hydration Mechanism
Quicklime mixed with water produces calcium hydroxide. The mixing reaction produces Ca2+ and OH, from which solid phase calcium hydroxide crystallizes. The lime hydration reaction mechanism is as follows:

CaO +H2O →Ca(OH)2

Ca(OH)2  Ca2+ +2OH(solid surface)

Ca2+ +2OH → Ca(OH)2 (crystal)

Hydrated Lime Plant

At the beginning of the reaction, under low agitating intensity, the entire hydration process is mainly controlled by the main liquid phase diffusion process of Ca+ or OH on the CaO solid surface. As the agitation intensity increases, the influence of the chemical reaction begins to increase, driven by both diffusion and reaction. Because as the agitation rate increases, the diffusion liquid film on the solid surface becomes thinner, speeding up the diffusion rate of the reaction product, causing the apparent hydration rate of lime to increase accordingly, and the hydration process begins to be dominated by both diffusion and reaction.

It is well known that the main factors affecting the hydration rate are agitation, quicklime particle size and hydration water temperature.

1.)The impact of agitation on the lime hydration process:
Agitation intensity is a critical value. It must not only fully mix quicklime and water to prevent lime from agglomerating, but also reduce the heat loss of the hydrators and reduce the wear of the mixing blades. Using high-performance mixing paddles can achieve higher mixing effects at low rotational speeds, while adding a wear-resistant layer to the paddles. When the stirring paddle is working, it is necessary to ensure that the solution is fully mixed and to reduce energy consumption and equipment wear.

2.) Effect of quicklime size on lime hydration process:
As the particle size of quicklime decreases, the apparent hydration rate of lime is significantly accelerated, and the total temperature rise also increases with the decrease of particle size. The reduction of quicklime particle size can increase the apparent hydration rate of lime and the apparent hydration conversion rate of lime. Because the smaller the particle size, the larger the reaction surface area of CaO, which can speed up the reaction and the diffusion of reaction products. At the same time, the reaction releases a large amount of heat, causing the temperature of the solution to rise rapidly, thereby speeding up the hydration process of lime, and the result is hydrogen oxidation. Calcium crystal nuclei form quickly and crystal nuclei grow slowly.

3.) Effect of hydration water temperature on lime hydration process:
The hydration process accelerates as the temperature of the hydration water increases, and it is best to preheat the water before hydration. When the temperature of the water is between 25C and 45C, the apparent hydration rate does not change much, which is more suitable for lime hydration; when the temperature of the water is lower than 25C, it is not conducive to the lime hydration process. The total hydration time changes at different water temperatures. The apparent lime hydration rate decreased with the increase of temperature, and the apparent hydration rate increased with the increase of hydration temperature. The mixing, diffusion and reaction of water and lime will release a large amount of heat, which will rapidly increase the temperature of the solution and promote the diffusion of the product, thus accelerating the hydration process of lime.

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