Limestone Calcination

Limestone is among the most extensively used and least expensive alkalising agents globally. It is frequently used in chemical operations in the form of slaked or calcium hydroxide. The heat breakdown of limestone into quick Limestone and carbon dioxide is referred to as “Limestone Calcination.” It’s often referred to as “Calcination.” Fairly basic chemical processes characterise limestone decomposition. 

When working with limestone, complexity is seen, producing a difference in crystallography and microstructure. The breakdown kinetics of granular and lumped Limestone has been quite complicated. As a result, the feasibility of developing a unified theory on Calcination is restricted. This module will help us learn about Limestone Calcination. 

What is Calcination?

Calcination is a metallurgical process in which metals are burnt at high temperatures and pressure or restricted air supply to remove volatile compounds and transform the metals into their oxides. In other words, Calcination refers to the process of reducing a deceased human corpse to a remnant that is not as significant as the deposit that results from cremation.

The exact Chemistry definition of Calcination refers to the thermal decomposition of carbonate minerals, such as calcium carbonate (this is one of the important minerals in the limestone family) that helps create a compound called calcium oxide in a kiln made up of cement.

Several systems also contain alternatives for crushing capacity before, during, and after calcination, depending on the desired products. Commercial calcination equipment includes- flashing calciners, kettle calciners, rotary calciners, fluidised calciners for beta plaster and autoclave calciners for alpha plaster. The various calcination systems have varying capacity, residence periods, and efficiency and produce diverse plaster characteristics, which govern their best application.

The Process and Chemical reaction of Limestone Calcination

When limestone is calcined to form unslaked lime, the thermal breakdown of calcium carbonate can be represented as-.

CaCO3 (100 g) + Heat ⇆  CaO (56g) + CO2 (44g)

Calcite dissociation pressure varies with temperature. At around 900°C, it achieves 1 atm (101.3 kPa). Compared to prior studies, which indicated a temperature of 898°C, a recent study has shown a value of 902.5°C.

Limestone Calcination is followed by heat dissociation. In the literature, the heat of dissociation of calcite is equivalent to 25°C and is measured to range between 695 and 834 kcal/kg of CaO. Boynton reported an average figure of 770 kcal/kg, while Schwarzkopf reported a rate of 754 kcal/kg.

Calcination is a balanced reaction. CaCO3 breaks down to Limestone if the atmospheric air pressure of CO2 is less than the partial equilibrium pressure at a particular temperature. If the air pressure of CO2 surpasses this equilibrium point, any Limestone created is converted back to carbonate.

Thus, the decomposition rate is determined by the partial pressure of CO2, the reaction temperature, and the particle size. Even in the absence of CO2, the pace of the reaction becomes extremely slow at 700 degrees Celsius and atmospheric pressure. Chemical reactivity varies amongst limestone sources, not only because of changes in the crystalline structure but also due to the composition of the impurities.

CaCO3 calcination is a highly endothermic process that requires around 755 Mcal of heat input and generates one ton of Limestone(CaO). When the temperature rises over the temperature of dissociation of carbonates in the limestone, the reaction begins. This is usually between 780 and 1340 degrees Celsius.

Once the reaction begins, the temperature must be kept above the dissociation point, and the CO2 produced must be removed. CaCO3 dissociation occurs gradually from the particle’s outer surface inside, leaving a porous layer of CaO, the desired result.

As a result, the process is dependent on a sufficient firing temperature of approximately 800 deg C to make sure breakdown and a good contact time, i.e. holding the lime/limestone for an adequately long period of time at temperatures ranging from 1,000 deg C to 1,200 deg C to regulate its reactivity.

Uses of Limestone after Calcination

Limestone is one of the most fundamental raw materials used in the steel industry, and it is utilised in both ironmaking and steelmaking processes and auxiliary activities. The vast majority of limestone is used in steel production for fluxing impurities inside the steel manufacturing furnace and several other steelmaking processes. Limestone is also used in various amounts in the sintering process to prepare iron ore, desulphurisation of pig iron, acid neutralising, and water treatment facilities.

Parameters affecting the Limestone Calcination

Limestone reactivity measures how quickly limestone reacts in the presence of water. Slaking the Limestone in water is the test procedure used to determine the reactivity of crushed lime. Limestone reactivity is affected by various factors linked to the raw material and the process. These are the parameters-

• Limestone Crystalline Structure 
• Burning Time and Temperature 
• Limestone Impurities 
• Kiln Type 
• Fuel

Conclusion

This module will explain and help a student know all about the topic of Calcination. Calcination is one of the key processes in the field of Chemistry that a student interested in chemistry must know. Calcined substances possess a lot of applications today. This module has successfully discussed Limestone Calcination, limestone calcination importance and various other topics related to Limestone Calcination.

Calcination plays an important role in Chemistry and in the industrial field of Chemistry hence a student must study this topic in detail and carefully. The above module thus covers all the concepts related to the topic of Limestone Calcination to make studying easy and less time consuming.