Manufacturing of the Lime

The manufacturing of the lime is done by following the process of collection of the limestones, calcination of the limestone, and then slaking of the limestone.
Lime is of the three types. It is fat lime, hydraulic lime, and poor lime. If we have to prepare the fat lime then the quality of the limestone should be such that it contains about 90 to 95% of the limestone and should contain less than 5% of the impurities.

Collection of the Limestone:  

The stones of the required quality are collected at the site of the manufacturing. For fat lime, the pure form of the limestone may be used and for the hydraulic lime, the kankar (contains 30% of clay) may be used. The burning of the limestone may be done either in the clamp or it may be done in the kilns. Fuel used for the burning of limestone: wood, coal, coke, coal gas, charcoal, cinder & oil, etc.

Clamp Burning:

Clamps are temporary structures and they are used when the demand of the limestone is not much and the fuel is available in abundance.  The limestone and the fuel are stacked in alternate layers and so the stack is formed whose height is kept less than 3.6m in general. The sides are sloped for stability purposes. The clamp is covered with mud plaster to avoid the escape of the heat during the process of burning. A firing place is kept at the bottom of the clamp for firing.

Demerits/ disadvantages: 

 In this method, the fuel is wasted as the burnt product often contains a high percentage of the un-burnt and over-burnt limestone. The face near the outside side of the clamp is more liable to remain un-burnt because it is in the contact with the air and the limestone in the heart of the clamp is more liable to overburning.
Clamp burning is un-economical as a lot of heat is lost through the mud plaster which cracks by the heat from the inside and gives an inferior quality of lime(as the lime produced gets partly slacked by the entry of the moist air through cracks in mud plaster and falls into powder form and mixes with the fuel ashes which can not be separated.
It is not a continuous process of burning and can not be performed in the rainy season.

Kiln Burning: 

The kilns used for the burning of the limestone are of two types, whose selection and size may vary.

(1)Intermittent Kilns
(2) continuous Kilns

 (1) Intermittent Kilns

They are formed on the sides of the hill or on the bank side of a river. An arch over the furnace is formed with the limestone and the small pieces are then arranged on the top of the arch. Here the charge does not come in direct contact with the fuel and consequently, the out-turn is less associated with the impurities(ashes, etc.). Each charge takes 2 days for burning and 1 day for cooling before the kiln can be recharged. About 40kg of fuel is required for the burning of about 3 cubic meters of limestone.

(2) Continuous Kilns

They are used when lime is to be manufactured on large scale. The kiln is a vertical structure lined with fire clay bricks. It is usually built on a slope to facilitate the continuous removal of lime. Alternate layers of limestone and coal(sometimes wood) are charged from the top and lime is removed from an opening at the ground level. Fire is lighted in the kiln and as the calcination progresses lime is continuously withdrawn and fresh change is fed from the top. The air required for the combustion enters the same opening through which the lime is removed, thus it gets heated while the lime gets cooled.  Lime obtained from such kilns is not of high purity as it gets mixed with the ash of fuel. Its thermal efficiency is however higher than that of other furnaces.