Calcium carbonate - Calcium carbonate Three types of calcium carbonate-containing rock are excavated

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  • 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

    http://www.essentialchemicalindustry.org/metals/steel.html#steel http://www.essentialchemicalindustry.org/metals/iron.html#blast_furnace http://www.essentialchemicalindustry.org/metals/iron.html#blast_furnace

  • 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.

    http://www.essentialchemicalindustry.org/chemicals/calcium-carbonate.html#flue_gas

  • 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.

    http://www.essentialchemicalindustry.org/polymers/polymers-an-overview.html#filler http://www.essentialchemicalindustry.org/chemicals/sodium-carbonate.html#sodium_carbonate http://www.essentialchemicalindustry.org/metals/iron.html#blast_furnace

  • 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