Elementos de la autogestión

- Boynton, R.S., "Chemistry and Technology of Lime and Limestone"

John Wiley &. Sons, 1980, ISBN 0-471-02771-5.

- Oates, J.A.H., "Lime and Limestone"

Wiley-VCH, 1998, ISBN 3-527-29527-5

- European Commission, "Integrated Pollution Prevention and Control (IPPC)" - Reference Document on Best Available Techniques in the Cement and Lime Manufacturing Industries (adopted Dec 2001)

- Schiele, E., Berens L.W., "Kalk – Herstellung, Eigenschaften, Verwendung"

(in German), Stahleisen m.b.H., 1972, ISBN 3-514-00115-4 3.2 Calcination of limestone and dolomite

3.2.1 Limestone

Limestone is found widely throughout the world and is an essential raw material for many industries.

3.2.2 Formation of limestone

Limestone is one of the most widely distributed sedimentary rocks throughout the world. Commercially used limestone is mainly of organic origin. Deposits were formed by the building-up of fossiliferous marine sediments in oceans consisting of shells and skeletons of plants and animals. Some of these sediments were deposited by natural chemical reaction. Calcium bicarbonate was produced by the extremely slow

dissolution of calcium carbonate fossils through the solvent action of carbon dioxide, which was subsequently re-precipitated in carbonate form. Layer by layer of these deposits form massive beds of limestone.

3.2.3 Mineralogical composition

Limestone and dolomite can be composed of the following four minerals, characterized by the following physical data:

Chemical

Aragonite CaCO3 100.1 2.94 3.5-4.0 orthorhombic

Dolomite CaMg(CO3)2 92.2 2.84 3.4-4.0 rhombohedral

Magnesite MgCO3 84.3 3.00 5.0-4.5 rhombohedral

Dolomite and calcite play the main role as industrial minerals.

- Pure limestone (calcite and aragonite) is 100% calcium carbonate.

- Pure dolomite contains 54.3% CaCO₃ and 45.7% MgCO₃ (30.4% CaO, 21.8% MgO, and 47.8% CO ).

INFORMATION

Limestone and dolomite used for industrial purposes include:

- Pure calcite with 97–99% CaCO₃

- Pure dolomite with 40–43% MgCO₃ and 57–60% CaCO₃

Impurities in these limestone and dolomite rocks are usually between 1 and 3%.

3.2.4 Impurities

Impurities in limestone are classified as homogeneous and heterogeneous.

Silica and alumina

- Homogeneous impurities such as clay, silt, sand, and other forms of silica like quartz are well dispersed throughout the formation.

- Heterogeneous impurities, which are found, for example, as siliceous pieces or nodules of sand, chert or flint are loosely embedded in the limestone.

Iron

- The third major impurity is homogeneously distributed after the limestone has started to form iron carbonate by chemically replacing calcium with iron. This frequently occurs in oolitic limestone.

- It is heterogeneously distributed as iron sulphide or iron oxide in minerals like pyrite, limonite, and hematite.

Phosphorous and sulphur

- They usually occur only in small quantities.

Manganese, copper, titanium

- These and further impurities are virtually negligible and considered as trace elements in the pure stone.

3.2.5 Mineral structure and grain size

Limestone is crystalline. The grain size (to be distinguished from particle size)

increases with the amount of re-crystallization that has occurred during the formation of the deposit.

The crystalline structure varies greatly in density and hardness.

Micro <4 × 10^-6 m Fine 4 – 50 × 10^-6 m Medium 50 – 250 × 10^-6 m

Coarse >250 × 10^-6 m (up to about 1000 × 10^-6 m)

The particle shape depends partly on the microstructure of the grain, but also on the crushing characteristics of the crushing machine.

NOTE

Cubic or spherical shapes of limestone particles are usually preferred for lime kilns.

Avoid processing layered or flat limestone particles whenever possible.

3.2.6 Porosity and density

The porosity of limestone particles varies considerably depending on the degree of compaction and structure of the limestone. It is defined as the ratio of the void volume V_v and the total volume V_tot. The void volume V_v comprises both accessible and inaccessible pores. The figure below illustrates different kinds of pores.

Fig. 3 Different kinds of pores (typical)

Item Description 1 solid pore 2 inaccessible pore 3 accessible pore

Density is defined as the ratio of mass m and volume V of a particular particle.

The solid or specific density (D) considers the volume of the pure solid without any void volume.

The apparent density (D_s) considers the volume of the solid with the inaccessible space.

The apparent porosity (P_s) describes the accessible volume as the difference of that part of the specific density minus the apparent density with the amount of inaccessible space. D = Specific density

Some data regarding apparent porosities and apparent densities of commonly used types of limestone is provided in the table below.

Industrial limestone shows a wide range of apparent porosities (0.1 to 40%) and densities (1.50 to 2.90 g/cm³) caused by the different forming conditions and levels of re-crystallization.

Apparent porosity [%] Apparent density [g/cm³] dried at 110 °C

Dense limestone 0.1 to 3.0 up to 2.7

Marble 0.1 to 2.0 2.7 to 2.8

Chalk 15 to >40 1.5 to 2.3

3.2.7 Bulk density and particle size

Bulk density is the mass per unit volume of a solid, including the voids in a bulk sample of the material.

Bulk density depends largely on the apparent density of the limestone, its particle size distribution, and on the particle shape.

Crushed, screened limestone with a size ratio of 2:1 generally has a bulk density of 1.3 to 1.6 g / cm³.

Crushed, unscreened limestone has a bulk density of 1.6 to 1.75 g / cm³. 3.2.8 Thermal dissociation of carbonate

Thermal dissociation is the most important characteristic of limestone.

All carbonate rocks dissociate at high temperatures, forming oxides and CO₂ gas.

For example:

CaCO₃ + heat = CaO + CO₂

The dissociation temperature may be reduced by several hundred degrees due to higher amounts of impurities, such as SiO₂, Al₂O₃, and Fe₂O₃ in the limestone. The effect of SiO₂ (silica) is shown in the following figure.

Fig. 4 p,t diagram of the CaO-SiO2 system (typical)

Curve Element Temperature range

1 CaCO3 + SiO2 400–590 °C

2 CaCO3 + 2CaO·SiO2 400–750 °C

3 CaCO3 650–890 °C

3.2.9 Mechanical strength and abrasion resistance

Pore volume and pore distribution give the limestone a specific structure, which results in different apparent densities. They have a direct influence on the mechanical

properties of the limestone.

Mechanical strength and abrasion resistance of the limestone must be sufficiently high to avoid breakage. Breakage of limestone particles during handling or passage through the kiln causes the generation of fine lime and compresses the stone packing in the kiln. The gas flow and heat transfer may thus be adversely affected causing

downgrading of the quality of the quicklime (also see Influence of feed size on the retention time in this chapter).

The compressive strength varies from 10 MPa for some types of marl and chalk to 200 MPa for some types of marble.

INFORMATION

The compressive strength of limestone to be burnt in a Maerz lime kiln should generally not be lower than 30 MPa.

3.2.10 Data and properties of limestone

The following table lists some fundamental data and properties of limestone.

Properties Data

Expansion coefficient 5 x 10-6 K-1 at 20°C. Total expansion of limestone during heating up from 20 to 800°C is approx. 2-2.5%.

Thermal conductivity Limestone at 130 °C

Dolomitic limestone at 123 °C

1.6341 W / mK 1.4246 W / mK

Integrated specific heat

CaCO3

Chemical properties Limestone and dolomite are unaffected by CO2-free water. Decomposition can only occur at very high temperatures or by reaction with strong acids.