Limestone calcination

The calcination of limestone is a simple chemical process:

CaCO₃ + heat = CaO + CO₂

The calcination temperature depends upon several factors. At the atmospheric pressure and with a CO2 concentration of 25%, the dissociation starts at about 810°C. Some components in fact (e.g. NaHCO3 or KHCO3) dissociate at lower temperatures (200-300°C) already in the preheating zone; some other like NaCO3 or K2CO3 dissociate at 800-900°C in the calcining zone, this forming Na2O and K2O which sublime to Na and K vapours at approx. 1200°C.
However experience and a well proven design are the secrets for a successful lime burning project as several factors influence the choice of a lime kiln technology: the available raw material quality, the fuel and the required characteristics of the finished product.

TSR kilns models:

• Cim-RT
• Cim-TD
• Cim-RD
• FinerStone models

Annular kiln Twin-C

Single shaft kiln ABC

Link to the Uses of Lime...

  Single shaft kiln ABC
 

When simplicity, lower capital investment and easy maintenance are the main issues, the ABC Kiln is the right kiln. ABC is the Cimprogetti’s vertical single shaft kiln with countercurrent flow arrangement and it is the latest step of evolution in single shaft kiln.

Daily output (tons)

from 40 to 150

Fuels

oil, natural gas & lean gases

Operating principles of
a single shaft kiln

The kiln is loaded from the top through a system of hoppers and gates, which ensure the tightness of the kiln and a uniform distribution of the material over the shaft section.
The upper part of the kiln operates as pre-heating zone and is sized to achieve the best heat exchange between flue gases and limestone, thus recovering as much heat as possible.
The middle part of the kiln is the burning zone and it is designed to ensure a uniform calcination of the material and features a group of burners installed peripherally along the kiln side (plus one central burner, column type, that is provided for best calcination of the central part of kiln feed, if required by the size of the kiln section). The lower part of the kiln is for cooling the calcined lime.
In this type of kiln the lime cooling air, which represent the major part of the combustion air, is sucked from the kiln bottom so as to be pre-heated by the contact with the calcined lime which as a result is cooled down to a lower temperature.
The lime is extracted by means of electromagnetic vibrators that discharge into a weighing hopper: in this way the uniform descent of the material is guaranteed across the whole all the section of the shaft.
During operation, the kiln is kept in negative pressure by a centrifugal fan sucking the combustion gases and the dissociated carbon dioxide.
A flue gas filtering unit is included at kiln outlet. This unit is composed of a high efficiency cyclone, installed before of the kiln main fan to retain the bigger solid particles, an air-to-air heat exchanger, a bag filter for flue gases dedusting and a tail fan. If required, on the kiln stack, a motor driven valve may be installed to allow the adjustment of the flue gas stream to any downstream plant (i.e. PCC, soda ash, etc.).
A second ventilator recycles part of the flue gases to the burners in order to protect from excess heat the refractory lining in the area where the burners themselves are installed and to improve the penetration of the flame into the limestone mass.