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The Underfiring and Overfiring Phenomenon of Lime

Lime production is a complex thermal decomposition process in which the calcium carbonate turns into calcium oxide. Because of the different kiln shape, equipment, quality and composition of raw materials, many kinds of problems may happen during the process, in which the most common seen is underfiring. We explore its reason mainly from aspects like raw materials, operation and temperature control.

1.Too big granularity: the calcination speed of limestone is determined by its granularity and the temperature of contact surface. Under the certain temperature, the speed is controlled by its granularity. The bigger the materials are, the slower they are calcinated. This is because the heat conductivity coefficient of lime is smaller than the calcinations speed. Lime layer increases gradually in thickness makes it more difficult for hot air to enter the rocks inside and slow down the calcination process. So many big lime is not burned fully. The granularity for common shaft kiln should be controlled within 40-80mm, and can be broadened within 50-150mm for kiln with big capacity.

2. Too small fuel ratio or low calorific value of fuel: the fuel ratio used in mixed kiln is related with its technical parameter. The heat value is normally above 5500 and the granularity should be controlled in a certain degree.

3. Unreasonable air supply: the calcination of every fuel is indispensable from three factors: fuel, oxide and fire. The air supply is in direct relationship with fire: more air supply, bigger fire there is. Underfiring phenomenon will appear where air supply is not enough.

4. Overfiring: Quick lime quality depends on two aspects: first, the amount of calcium oxide and magnesium oxide, the second is the underfiring and overfiring rate of quick lime. Underfiring means the lime is not calcinated fully, while the overfiring means that too much calcination makes it have a large density. The overfired lime has low activity, which makes it difficult to differentiate later.

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