Manufacturing Process of Lime: How to Manufacture Fat Lime?

Following three distinct operations are involved in the manufacture process of of fat lime: 1. Collection of Lime Stones 2. Calcination of Lime Stones 3. Slaking of Burnt Lime.

1. Collection of Lime Stones:

The lime stones of required quality are collected at site of work. For fat lime, the percentage of impurities in lime stones should not exceed 5 per cent. It is desirable to use comparatively pure carbonate of lime in the manufacturing process of fat lime.

2. Calcination of Lime Stones:

The calcination or burning of lime stones to bright red heat is the next important operation. The fuel required for calcination of lime stones may consist of charcoal, coal, firewood or coal ashes. The initial firing is achieved with the help of few chips of dry wood or cow-dung cakes. As in case of bricks, the burning of lime stones can be achieved either in clamps or kilns. The clamps are temporary structures.

The kilns are permanent structures and they may be of intermittent type or continuous type.

The burning of lime stones is thus carried out in one of the following:

(i) Clamps

(ii) Intermittent kilns

(iii) Continuous kilns.

(i) Clamps:

The ground is levelled and cleaned. The lime stones and fuel are placed in alternate layers, if fuel is wood. But if fuel is of coal or charcoal, the lime stones and fuel are mixed together and placed in a heap form.

Fig. 5-1 shows a typical clamp with dimensions as 6 m X 3.60 m x 3.60 m. The sloping sides are covered with mud plaster and attempt is thus made to preserve as much heat as possible. It is then fired from bottom. A small opening is provided at top for draught. When the blue flame at top disappears, it indicates the completion of the burning of lime. The clamp is then allowed to cool down and the pieces of quick lime are handpicked subsequently.

The clamp burning is adopted to manufacture lime on a small scale because of its following disadvantages:

(a) The clamp burning proves to be uneconomical to manufacture lime on a large scale.

(b) The loss of heat is considerable. The mud plaster cracks by the heat from inside and allows heat to escape.

(c) The quality of lime produced by the clamp burning is not good.

(d) The quantity of fuel required is more and hence it is practised where lime stones and fuel are abundant.

(ii) Intermittent Kilns:

These are of various patterns and their sizes and shapes vary as per prevailing practice in the locality. The two important types of intermittent kilns is discussed here.

Fig. 5-2 shows the section of intermittent kiln in which alternate layers of limestone and fuel are arranged. Such a kiln is known as the intermittent flame kiln. The horizontal and vertical flues are suitably formed and top of kiln is covered with un-burnt material. The kiln is ignited from bottom and lime stones are allowed to burn for about 3 days or so. The kiln is then cooled and unloaded. The process is then repeated.

Fig. 5-3 shows the section of an intermittent kiln in which fuel is not allowed to come into contact with lime stones. A rough arch of selected big pieces of lime stones is formed and smaller pieces of lime stones are packed over this arch. The fuel is placed below the arch and when it is ignited, only flame comes into contact with lime stones.

Such a kiln is known as the intermittent flare kiln. When lime stones are sufficiently burnt, the kiln is cooled and unloaded. The process is then repeated. This type of kiln is easy to manage. The flare kiln produces lime of better quality because the lime stones are not in contact with the fuel and the finished product is not mixed with ashes.

There is considerable wastage of time in intermittent kilns as every operation includes loading, burning, cooling and unloading. The supply of lime is also not continuously guaranteed. Hence such kilns are used to manufacture lime on moderate scale.

(iii) Continuous Kilns:

These are also of various patterns and their sizes and shapes vary as per prevailing practice in the locality. The two important types of continuous kilns is discussed here.

Fig. 5-4 shows the section of continuous kiln in which mixture of lime stones and fuel is fed from the top. Such a kiln is also known as the continuous flame kiln.

Fig. 5-4 shows the continuous kiln of Roof capacity about 4 m³ per day. It is in the form of a cylinder with diameters at top, middle and bottom as about 1.80 m, 2.30 m and 1.40 m respectively. The widening of middle portion is done to accommodate hot gases of combustion. The bottom is covered by grating with holes.

After burning, the lime is collected at the bottom and it is removed through access shaft. The kiln is partly under the ground and partly above the ground. A loading platform is provided at the top.

The inside surface of kiln is covered with fire-brick lining. To facilitate the fall of calcined particles, the grating may be raked or cleaned through the rake hole. As the level of material in kiln falls, the required quantity of mixture of lime stones and fuel is fed from the top. A roof may be provided at the top to protect the kiln.

Fig. 5-5 shows the section of continuous kiln in which fuel is not allowed to come into contact with lime stones. It is also known as the continuous flare kiln.

This kiln consists of two sections – upper and lower. The upper section serves as storage of lime stones. The lower portion is provided with fire-brick lining. While starting the kiln, a small quantity of fuel is mixed with limestone and ignited. The fuel is then fed through shafts around the upper and lower sections of kiln.

The feeding of lime stones is done from opening at top. The removal of calcined material is done through a grating placed at the bottom of kiln. A roof is provided at the top to protect the kiln.

There is considerable saving of time and fuel in case of continuous kilns, but the initial cost is high. Hence these kilns are adopted to manufacture lime on a large scale.

Following facts should be remembered in the process of burning of lime stones:

(a) The bright red colour of stone indicates that the burning is complete. The burnt lime stones should be withdrawn from the kiln as soon as CO₂ is driven off. The dark red colour of stones indicates the presence of CO₂ in the stones. But when the CO₂ passes off completely, the colour changes to brilliant white even without flame or blaze.

(b) The burning should be such that it does not result into over-burning or under-burning.

(c) The heating should be gradual. The sudden heating results in the blowing of stones to pieces due to quick release of moisture and carbon dioxide.

(d) The imperfectly calcined lime does not slake with water and is referred to as the dead-burnt lime.

(e) The lime stones should be broken into suitable sizes before they are burnt. For proper burning, the fat lime stones are to be broken into lumps of size 200 mm to 250 mm and the hydraulic lime stones into lumps of size 75 mm to 100 mm.

(f) The quantity of fuel required in each case should be carefully decided. It is found that about 60 N of fuel is sufficient for burning 1 kN of CaCO₃.

(g) The burning should be kept uniform for several hours. For successful burning, the temperature should be maintained at 800°C with a rapid draught of inert gases through the hot lime stones.

3. Slaking of Burnt Lime:

The quick lime which is obtained by burning of lime stones slakes when exposed to the atmosphere. This is known as the natural slaking or air slaking and it is a very slow process. Hence the slaking is achieved by adding water to quick lime.

Following are the two methods of slaking:

(i) Slaking to paste

(ii) Slaking to powder.

(i) Slaking to Paste:

In this method, the quick lime is spread in a layer of 150 mm depth in a wooden or masonry basin. The water in sufficient quantity is then added so as to submerge quick lime. It is found that the quantity of water required is about 2½ to 3 times the volume of quick lime. The excess water retards slaking and little water results in unsatisfactory slaking.

The water should be added at a time and it should not be added after the temperature has risen. The basin is covered with wooden planks to preserve heat and to ensure proper slaking of the entire mass of quick lime. The stirring is not necessary and slaking is completed in about ten minutes or so.

(ii) Slaking to Powder:

In this method, the quick lime is slaked to powder form.

This may be achieved in one of the following two ways:

(a) The quick lime is broken into pieces of size not more than 50 mm. It is then carried in a basket and the basket is immersed in water for few seconds. It is then taken out and thrown on a wooden or masonry platform in a heap form. The quick lime crumbles and falls as powder form. The period for which basket is to be immersed in water is to be determined from experience.

(b) In this arrangement, the quick lime is spread in layer of 150 mm depth on a wooden or masonry platform. The water is then sprinkled over this layer from a water-can or vessel fitted with a rose or perforated nozzle. The quick lime swells, crumbles and falls as powder form. This method is generally used to slake quick lime obtained from the shells.

It is to be noted that over-burnt or under-burnt lime stones do not slake easily. Hence such undesirable pieces should be removed before slaking. It is also necessary to convert all lumps into powder or pulp form. It is observed that one part of fat quick lime is converted into about 114 parts in paste form and about two parts in powder form.