Sunday, September 16, 2012

Kiln burning continued..

When lime of high purity is required continuous separate feed kilns are used. The kiln is similar to the kiln described above except that two or four fireplaces are provided by the side of the kiln near the bottom. In the fireplaces coal or wood are burnt and the hot gases of combustion move up the shaft of the kiln. The advantage of using wood as a fuel is that the long flames are produced and the temperature of the limestone is not raised too much. When coal is burnt same steam is introduced which maintains a low temperature and forms water gas. The water gas so produced burns and heats the kiln up to a greater height. Lime stone is supplied to the kiln from the top and the lime removed from the bottom.
The Rotary kiln used for the burning lime is similar to that used for cement manufacturing but is much smaller in size and is worked at a lower temperature. It consists of an inclined steel cylinder 1.5 to 2.5m in diameter and 15m to 45m long, lined with refractory bricks. Limestone crushed to much smaller pieces is charged at the upper end. Kiln is fired with coal, oil or producer gas and daily out turn is nearly 70 to 80 tonnes if lime. Due to better control on the operation product obtained is of superior quality. Rotary kilns are widely used in highly industrialized countries and for making lime for chemical industries.
Precautions:
1. The quantity of fuel to be used should be carefully worked out.
2. Before burning the limestone they should be broken into suitable sizes.
3. Quick heating of limestone may blow the limestone due to quick release of moisture and carbon dioxide so the burning should be gradual.
5. Over burning and under burning of the lime should be avoided as the over burnt or under burnt lime does not slake and is known as the dead lime.

Note: This content is prepared with the help of some books written on the topic, if you want the reference names please leave a comment. 

Manufacturing of the Lime

Manufacturing of the lime:
The manufacturing of the lime is done by following the process of collection of the lime stones, calcination of the limestone, and then slaking of the limestone.
Lime is of the three types. It is fat lime, hydraulic lime and the 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 the 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 the alternate layers and so the stack is formed whose height is kept less than 3.6m in general. The sides are sloped for the stability purpose. The clamp is covered with the 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 over burning.
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 the size may vary.
(1)Intermittent Kilns
(2) continuous Kilns

(1) Intermittent kilns are formed on the sides of the hill or on the bank side of a river. An arch over the furnance is formed with the limestone and the small pieces are then arranged on the top of 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 metre of the limestone.
(2) Continuous Kilns  are used when lime is to be manufactured on large scale. The kiln is vertical structure lined with fire clay bricks. It is usually built on a slope to facilitate 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. 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 ash of fuel. Its thermal efficiency is however higher than of other furnances. 

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