Calcium carbonate

Three types of calcium carbonate-containing rock are excavated

and used by industry. They are limestone, chalk and dolomite. Limestone and

chalk are both forms of calcium carbonate and dolomite is a mixture of calcium

and magnesium carbonates. All have impurities such as clay but some rocks

are over 97% pure. Limestone and other products derived from it are used

extensively in the construction industry and to neutralize acidic compounds in a

variety of contexts.

In the chemical industry, large quantities of limestone are heated to ca 1500 K

to form calcium oxide, known as quicklime:

Water can be added to lime to form calcium hydroxide. The process is known

as 'slaking'. Solid calcium hydroxide is known as slaked lime or hydrated lime,

and solutions and suspensions in water as milk of lime.

The term lime is often used to cover quicklime, slaked lime (hydrated lime) and

milk of lime.

For a particular use, an appropriate choice is made from the four: limestone,

quicklime, slaked lime or milk of lime. In many uses, lime reacts more quickly

than limestone but is more expensive, because a high temperature is required

to produce it from limestone.

Uses of limestone and lime

Figure 1 Principal uses of limestone and lime.

The principal uses, by far, of limestone and lime are in the construction

industry and cement making. They are also used in the chemical and

metallurgical industries and in agriculture.

On a worldwide basis, the proportions of lime used in different industries are:

60% metallurgy (mainly steel manufacture, slag formation and its use in the blast

furnace)

25% construction (for example, it is used with asphalt in road paving, to

stabilize soils and in making mortar and plaster)

15% for chemical and industrial uses (for example to make bleaches used in

the manufacture of paper, to make precipitated calcium carbonate, a fine powder

used in coatings for paper and paints, and in refining sugar to remove colloidal

impurities) and for environmental uses (for example, with soda ash by both

municipal authorities and industry, to soften water (remove calcium and

magnesium ions), in the treatment of sewage to remove colloidal particles and in

flue gas desulfurization)

However, these proportions vary widely from country to country. For example,

in the US, the proportions are:

38% metallurgy (mainly steel manufacture)

31% environmental uses (for example, with soda ash, by both municiipal

authorities and industry to soften water (remove calcium and magnesium ions), in

the treatment of sewage, to remove colloidal particles and in flue gas

desulfurization

22% chemical and industrial uses (for example to make bleaches used in the

manufacture of paper, to make precipitated calcium carbonate, a fine powder

used in coatings for paper and paints and in refining sugar to remove colloidal

impurities)

8% construction uses (for example, it is used with asphalt in road paving, to

stabilize soils and in making mortar and plaster)

1% others

Data from the US Geological Survey, 2012

The uses are described below, in more detail, in terms of different industries.

In the construction industry

Limestone has been used as a building material since the Stone Age. Indeed,

the largest use of limestone and the various forms of lime is still in the

construction industry, particularly in road building and building projects, from

vast in size, bridges and skyscrapers, to houses. Large lumps of calcium

carbonate are often used where sizeable quantities of aggregate are needed,

for example for the foundations of roads.

Lime is often used to make soil firmer. It reacts with clay minerals in the soil to

form cement-like compounds (for example calcium silicate and calcium

aluminate (calcium aluminosilicate)), Figure 2.

Figure 2(a) Clay particles are surrounded by

water, allowing them to be aligned and able to

slide easily. This results in a clay soil with a

low strength.

Figure 32b) When lime is added, the amount of

water around the clay particles is reduced. The

clay particles are no longer able to slide easily

and the soil is strengthened.

The strengthening of soil enables the construction of buildings by giving a

more stable foundation. Lime is also used on building sites to allow large

vehicles to move more easily (Figure 3).

Figure 3 The wet soil has been made

harder by the addition of lime. This

earth-moving equipment is able to

move around easily.

By kind permission of Singleton

Birch.

Limestone is also the main constituent of cement and concrete.

In cement making

Cement is made by heating a mixture of limestone and substances such as

clays (which contain silica, alumina and iron(III) oxide) in kilns at high

temperatures until it almost fuses. The mixture is then cooled and ground to a

fine powder and mixed with calcium sulfate (gypsum). This is cement. It is

essentially a mixture of calcium aluminosilicate and calcium sulfate. When it is

mixed with water, chemical reactions occur to form a hard solid, impervious to

water.

About 3.6 billion tonnes of cement is produced annually, of which China

accounts for 2.1 billion tonnes. Cement powder is usually mixed with sand and

aggregate (gravel, granite) and when needed, they are mixed with water to

form concrete.

Figures 4 and 5 Builders and masons, prior to the use of modern cement, used a mixture of lime

(calcium hydroxide), sand and water, known as lime mortar or simply mortar. It has been used for over

6000 years, since the buildings of Ancient Greece and Rome. When these buildings are renovated, lime

mortar is used rather than cement. In these photographs of York Minster, a relatively modern building

dating back only 900 years, lime mortar has always been used in renovating the stonework that has to

be replaced because of weathering over the centuries.

By kind permission of Jessica Waddington.

In industry and the environment

Many lakes have become too acidic because of aerial pollution (acid rain), for

example in the US, Scandinavia and Scotland. The lakes are sprayed with very

finely powdered calcium carbonate (Figure 6). Another effective way to treat

this problem is to apply the powdered limestone to unplanted areas near to the

sources of the streams leading to the lakes.

Figure 6 Powdered limestone is being sprayed over a lake near Hemsjö, in Southern Sweden.

By kind permission of Rickard Gillberg.

Limestone and the various forms of lime are used in large quantities to clean

up the environment, by neutralising acids. For example, limestone and

quicklime are used to remove sulfur dioxide produced in the burning of coal in

power stations. Even 'clean' coal can contain about 1% sulfur.

The gaseous effluents, flue gases, from the burning of the coal are passed

through a spray of very finely ground limestone or quicklime suspended in

water. The sulfur dioxide, being an acid, reacts with them, for example:

The resulting calcium sulfite collects at the base of the absorber and

compressed air is blown into this residue. The calcium sulfite reacts with the air

to form calcium sulfate (gypsum), used to make, for example, plaster board

and cement.

Very fine and pure calcium carbonate is used as filler in plastics and paper. A

filler is a substance that gives bulk but does not alter the properties of the

substance to which it is added and is also inert. Calcium carbonate when very

finely crushed (less than 2 microns) is used in paints to give a 'matt' finish.

Figure 7 Uses of limestone.

Calcium carbonate is also used:

to make sodium carbonate by the Solvay process

in the blast furnace to make iron

in the manufacture of glass

The uses are further summarized in Figure 7.

In agriculture

Crushed limestone and lime in all its forms are used to neutralize acids in the

soil and so create the optimum soil conditions for crop growth. They also help

to break down clays as described above, improving the soil structure, thus

improving drainage and reducing soil erosion. Further, they provide a source of

calcium ions that are an important plant nutrient.

Annual production of limestone

Data for the annual production of calcium carbonate are not readily available.

Approximately 1 billion tonnes of its two principal ores, limestone and dolomite,

are mined annually in the US. Given the relative amounts of lime that is used in

different countries, an estimate of worldwide mining of calcium carbonate is 15

billion tonnes a year.

Annual production of lime (calcium oxide and

calcium hydroxide)

World 348 million tonnes

United Sates 19 million tonnes

Europe 27 million tonnes

China 220 million tonnes

Rest of Asia 32 million tonnes

Data from the US Geological Survey, 2012

Manufacture of calcium oxide (quicklime)

Calcium carbonate (limestone) is heated to form calcium oxide (quicklime) and

carbon dioxide:

It is an endothermic reaction and the equilibrium lies far to the left at low

temperatures. Only at about 1200 K does the partial pressure of carbon

dioxide exceed atmospheric pressure and the decomposition proceeds to

completion.

Quicklime is produced in refractory-lined kilns. Many designs are used, but the

most common are based on the Vertical Shaft Kiln (Figure 8).

The kiln is made of steel, lined with refractory bricks. The limestone is fed in

from the top and air is either sucked by fans or pushed by roots type blowers,

through the kiln from the bottom (counter flow). The fuel is fed through the

sides of the kiln, using about 8-10 lances inserted around the kiln.

The lime kiln consists of three principle zones:

in the preheating zone, the heat in the combustion products, including carbon

dioxide from the dissociation of limestone, is used to preheat the limestone to

1200 K

in the burning zone the limestone is decomposed to quicklime at gas

temperatures of 1500 K, a process known as calcining

in the cooling zone, the heat in the quicklime leaving the burning zone is used to

preheat the air required for combustion to 600-750 K.

The fuels used vary, depending on what is available. Many kilns use natural

gas but in others oil is pumped in. In others, solids, such as finely powdered

coal is pressurized and pumped in so that it acts as a fluid.

If there is not enough air to complete combustion of the fuel, more is fed in

directly to the burning zone.

Figure 8 Manufacture of lime: The vertical shaft kiln.

Many recently introduced kilns have two parallel units (Figure 9). They are

known as two-shaft kilns. In the simple single kiln, described above, the air is

pumped up the kiln as the limestone flows down. This is known as a counter

flow system. In the two-shaft kiln, known as the Parallel Flow Regenerative

(PFR) lime kiln, the air and combustion gases travel parallel to the limestone.

The fuel is injected just above the burning zone and the limestone absorbs

most of the heat released by the fuel and so the temperature of the burning

zone can be reduced to 1400 K, the temperature of the decomposition of

limestone.

Figure 9 Manufacture of lime: The Parallel

Flow Regenerative (PFR) lime kiln.

By kind permission of Maerz Ofenbau AG.

Figure 10 A Parallel Flow Regenerative (PFR)

lime kiln in Mexico.

By kind permission of Maerz Ofenbau AG.

In the diagram, the left-hand shaft is described as in burning mode. The

exhaust gases cross over into the right-hand shaft, which is said to be in non-

burning mode.

These hot gases travel up in counterflow to the stone, heating it up ready for

the right-hand shaft to become the burning shaft and the left-hand shaft

becomes non-burning.

Each shaft cycles through the burning and non-burning mode about every 10

minutes.

There is a large reduction in the fuel used by the two-shaft kiln compared to

the single shaft kiln.

Shaft kilns have capacities for producing up to 800 tonnes of lime per day.

Quicklime is generally sold either as granules or as a finely ground powder.

Manufacture of calcium hydroxide (hydrated lime)

The reaction of quicklime with water is exothermic.

Powdered calcium hydroxide is produced by hydrating quicklime with a

controlled excess of water to make a dry product. Milk of lime is prepared by

slaking quicklime with an excess of water.

Date last amended: 28th November 2013