Technology Profile No. 2: Production of lime*

* This technology has been developed by the Central Building Research Institute (CBRI), Roorkee, India

Production of lime is a simple process in which limestone is calcined at elevated temperatures. Theoretically, 900°C is a sufficiently high temperature to carry out the process. However, in practice, it has been found necessary to maintain the temperature at a much higher level than this to complete the chemical reaction. In the absence of adequate temperature over sufficient time, the lime produced will be of inferior quality: it might be underburnt or overburnt. The success of the process, therefore, lies in maintaining proper conditions for calcination.

Kiln design

Lime kilns of various designs have been used. However, vertical-shaft types are thermally the most efficient. Consequently, their use results in savings in fuel. In India, different types of kiln have been employed through the ages, but investigations carried out at the Central Building Research Institute (CBRI) have shown that most of the traditional designs produce an inferior quality of product with a higher consumption of fuel. CBRI, in recent years, has developed lime kilns of several types, which are being offered for exploitation by the industry. The smallest kiln has about a 5 ton/day capacity below which hardly any efficiency can be expected. Figure 1 shows a lime kiln developed by CBRI.

Some salient features of the kilns

(a) The kilns are of brick or stone masonry;

(b) The kiln designs ensure smooth running and periodic withdrawal of lime;

(c) The kilns lend themselves to a fair degree of instrumentation, if required;

(d) They work on natural draft and have an arrangement for their control;

(e) They work continuously but can be adapted for day working only;

(f) They are thermally efficient, and heat losses are minimal;

(g) They produce a uniform quality lime, by avoiding overburning or underburning;

(h) Under standard working conditions, these kilns produce very little core or unburnt material;

(i) They can be operated by trained unskilled labour;

(j) Contamination of lime with fuel is minimal.

Raw material and chemical composition

The impurities in limestone are primarily SiO₂, Al₂O₃ and Fe₂O₃. They are non-volatile in nature and remain as contaminants in the lime produced. Limestone generally contains some MgCO₃ also. Calcite stone usually contains CaCO₃ exceeding 95 per cent, and dolomitic stone has an MgCO₃ content of 35-40 per cent. In the burning operation, the carbonates are converted to their corresponding oxides. Dolomitic lime is used largely in refractories where a high MgO content is essential.

The principal reactions involved in the calcination of calcitic and dolomitic limestones are:

CaCO₃ ® CaO + CO₂
(Calcite)

CaCO₃ MgCO₃ ® CaO.MgO. + 2CO₂
(Dolomite)

The average dissociation temperatures for the above two types of limestone at atmospheric pressure are 900° and 725°C respectively. Certain materials, such as sulphur dioxide, present in the stone or fuel tend to react with lime and oxygen to form CaSO₄ which is unstable at high temperatures. Al₂O₃ and SiO₂ combine with CaO and MgO to form various silicates and aluminates at very high temperatures. These compounds are water-insoluble and are undesirable, as they decrease oxide values and also coat the lime particles and so reduce its reactivity.

The reaction of quicklime is also influenced by high operating temperatures and retention times. With an increase in temperature, the reaction rate increases, and, consequently, the reaction time decreases. However, the maintenance of high temperatures beyond an optimum limit causes overburning of the lime.

Production process

Limestone is broken to a size of about 75 to 125 mm, and coal to half this size. These are mixed in mixers near the kiln. To initiate fire, a layer of firewood is first laid. Above this, some steam coal is spread. Thereafter, the kiln is filled with previously mixed coal and limestone. Generally, the coal requirement is 12-15 per cent that of limestone, but this varies, depending upon the type of limestone and the quality of coal. Fire is introduced from the bottom and rises. Charging and discharging are so adjusted that the firing zone is maintained in the middle of the kiln.

Figure 1. Lime kiln.

Figure 2. Production process of quicklime.

Scheme for the production of quicklime

(a) The manufacturing process is shown in a flow chart in figure 2.

(b) Production scale

(i) Rate of production

10 tons per day of three shifts,
3000 tons per year of 300 working days

(ii) Inputs