Limescale

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Limescale build-up inside a pipe reduces both liquid flow through the pipe and thermal conduction from the liquid to the outer pipe shell. Both effects will reduce the pipe's overall thermal efficiency when used as a heat exchanger. Limescale-in-pipe.jpg
Limescale build-up inside a pipe reduces both liquid flow through the pipe and thermal conduction from the liquid to the outer pipe shell. Both effects will reduce the pipe's overall thermal efficiency when used as a heat exchanger.

Limescale is a hard, chalky deposit, consisting mainly of calcium carbonate (CaCO3). It often builds up inside kettles, boilers, and pipework, especially that for hot water. It is also often found as a similar deposit on the inner surfaces of old pipes and other surfaces where "hard water" has flowed. Limescale also forms as travertine or tufa in hard water springs.

Contents

The color varies from off-white through a range of greys and pink or reddish browns, depending on the other minerals present. Iron compounds give the reddish-browns.

In addition to being unsightly and hard to clean, limescale can seriously damage or impair the operation of various plumbing and heating components. [1] Descaling agents are commonly used to remove limescale. Prevention of fouling by scale build-up relies on the technologies of water softening or other water treatment.

This column in the Bad Munstereifel church in Germany is made from the calcium carbonate deposits that built up in the Roman Eifel Aqueduct over several centuries of use. Eifelmarmor01.jpg
This column in the Bad Münstereifel church in Germany is made from the calcium carbonate deposits that built up in the Roman Eifel Aqueduct over several centuries of use.

Chemical composition

The type found deposited on the heating elements of water heaters consists mainly of calcium carbonate (CaCO3). Hard water contains calcium (and often magnesium) bicarbonate or similar ions. Calcium, magnesium, and carbonate ions dissolve from rocks through which rainwater percolates before collection. Calcium salts, such as calcium carbonate and calcium bicarbonate (Ca(HCO3)2), are more soluble in hot water than cold water; thus, heating water does not cause calcium carbonate to precipitate per se. However, there is an equilibrium between dissolved calcium bicarbonate and dissolved calcium carbonate as represented by the chemical equation

Ca2+ + 2 HCO
3
Ca2+ + CO2−
3
+ CO2 + H2O

Note that CO2 is dissolved in the water. Carbon dioxide dissolved in water (aq) tends to equilibrate with carbon dioxide in the gaseous state (g):

CO2(aq) CO2(g)

The equilibrium of CO2 moves to the right, toward gaseous CO2, when water temperature rises or pressure falls. When water that contains dissolved calcium carbonate is warmed, CO2 leaves the water as gas, this reduces the amount involved in the reaction causing the equilibrium of bicarbonate and carbonate to rebalance to the right, increasing the concentration of dissolved carbonate. As the concentration of carbonate increases, calcium carbonate precipitates as the salt: Ca2+ + CO2−
3
→ CaCO3.

In pipes as limescale and in surface deposits of calcite as travertine or tufa the primary driver of calcite formation is the exsolution of gas. When heating hard water on the stove, these gas bubbles form on the surface of the pan prior to boiling. Gas exsolution can also occur when the confining pressure is released such as when you take the top off a beer bottle or where subsurface water is flowed into an atmospheric pressure tank.

As new cold water with dissolved calcium carbonate/bicarbonate is added and heated, the process continues: CO2 gas is again removed, carbonate concentration increases, and more calcium carbonate precipitates.

Scale is often colored because of the presence of iron-containing compounds. The three main iron compounds are wustite (FeO), hematite (Fe2O3), and magnetite (Fe3O4).

As a stone

The Roman Eifel aqueduct was completed around 80 AD and broken and largely destroyed by Germanic tribes in 260. By the Middle Ages the limestone-like limescale accretions from the inside of the aqueduct were particularly desirable as a building material, called "Eifel marble" in an area with little natural stone. In the course of operation of the aqueduct, many sections had a layer as thick as 20 centimetres (7.9 in). The material had a consistency similar to brown marble and was easily removable from the aqueduct. Upon polishing, it showed veins, and it could also be used like a stone board when cut flat. This artificial stone found use throughout the Rhineland and was very popular for columns, window frames, and even altars. Use of "Eifel marble" can be seen as far east as Paderborn and Hildesheim, where it was used in the cathedrals. Roskilde Cathedral in Denmark is the northernmost location of its use, where several gravestones are made of it. [2]

Trade to the west took it to England as a high-status export material in the 11th and 12th centuries, where it was made into columns for a number of Norman English Cathedrals. The impressive polished brown stone was known for many years as 'Onyx Marble'. Its origin and nature was a mystery to people studying the stonework at Canterbury Cathedral, until its source was identified in 2011. [3] It is used there as columns supporting the cloister roof, alternating with columns of Purbeck Marble. These large cathedral cloisters needed several hundred such columns around an open quadrangle, which must have been supplied by a well-organized extraction and transport operation. The Eifel deposits, now called Calcareous sinter or calc-sinter (since it is neither onyx nor marble), have also been identified at Rochester [4] and in the now lost Romanesque cloister at Norwich [5] as well as the Infirmary Cloisters, Chapter House windows, and Treasury doorway at Canterbury. [6]

Soap scum forms when calcium cations from hard water combine with soap, which would dissolve in soft water. This precipitates out in a thin film on the interior surfaces of baths, sinks, and drainage pipes.

See also

Related Research Articles

Carbonate Salt of carbonic acid

A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula CO2−
3
. The word carbonate may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.

Stalactite Elongated mineral formation hanging down from a cave ceiling

A stalactite (, ; from the Greek 'stalaktos' via stalassein is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble and that can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, minerals, mud, peat, pitch, sand, sinter, and amberat. A stalactite is not necessarily a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves.

Calcium carbonate Chemical compound

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite and is the main component of eggshells, gastropod shells, shellfish skeletons and pearls. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

Sodium carbonate Chemical compound

Sodium carbonate, Na2CO3·10H2O, (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions in water. Historically, it was extracted from the ashes of plants growing in sodium-rich soils. Because the ashes of these sodium-rich plants were noticeably different from ashes of wood (once used to produce potash), sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process.

Stalagmite Elongate mineral formation found on a cave floor

A stalagmite is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically composed of calcium carbonate, but may consist of lava, mud, peat, pitch, sand, sinter, and amberat.

Travertine Form of limestone deposited by mineral springs

Travertine is a form of terrestrial limestone deposited around mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, cream-colored, and even rusty varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites, stalagmites, and other speleothems. It is frequently used in Italy and elsewhere as a building material.

Hard water Water that has a high mineral content

Hard water is water that has high mineral content. Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates.

Calcium sulfate Laboratory and industrial chemical

Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.

The Solvay process or ammonia-soda process is the major industrial process for the production of sodium carbonate (soda ash, Na2CO3). The ammonia-soda process was developed into its modern form by the Belgian chemist Ernest Solvay during the 1860s. The ingredients for this are readily available and inexpensive: salt brine (from inland sources or from the sea) and limestone (from quarries). The worldwide production of soda ash in 2005 was estimated at 42 million tonnes, which is more than six kilograms (13 lb) per year for each person on Earth. Solvay-based chemical plants now produce roughly three-quarters of this supply, with the remaining being mined from natural deposits. This method superseded the Leblanc process.

Eifel Aqueduct

The Eifel Aqueduct was one of the longest aqueducts of the Roman Empire.

Flowstone Archaeological phenomenon

Flowstones are composed of sheetlike deposits of calcite or other carbonate minerals, formed where water flows down the walls or along the floors of a cave. They are typically found in "solution caves", in limestone, where they are the most common speleothem. However, they may form in any type of cave where water enters that has picked up dissolved minerals. Flowstones are formed via the degassing of vadose percolation waters.

Water softening Removing positive ions from hard water

Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extends the lifetime of plumbing by reducing or eliminating scale build-up in pipes and fittings. Water softening is usually achieved using lime softening or ion-exchange resins but is increasingly being accomplished using nanofiltration or reverse osmosis membranes.

Calcium bicarbonate, also called calcium hydrogencarbonate, has the chemical formula Ca(HCO3)2. The term does not refer to a known solid compound; it exists only in aqueous solution containing calcium (Ca2+), bicarbonate (HCO
3
), and carbonate (CO2−
3
) ions, together with dissolved carbon dioxide (CO2). The relative concentrations of these carbon-containing species depend on the pH; bicarbonate predominates within the range 6.36–10.25 in fresh water.

Carbonate hardness, is a measure of the water hardness caused by the presence of carbonate and bicarbonate anions. Carbonate hardness is usually expressed either in degrees KH (dKH), or in parts per million calcium carbonate. One dKH is equal to 17.848 mg/l (ppm) CaCO
3
, e.g. one dKH corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate(CaCO
3
) per litre of water. Both measurements are usually expressed as mg/l CaCO
3
– meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions.

Speleogenesis is the origin and development of caves, the primary process that determines essential features of the hydrogeology of karst and guides its evolution. It often deals with the development of caves through limestone, caused by the presence of water with carbon dioxide dissolved within it, producing carbonic acid which permits the dissociation of the calcium carbonate in the limestone.

Alkali soil Soil type with pH > 8.5

Alkali, or Alkaline, soils are clay soils with high pH, a poor soil structure and a low infiltration capacity. Often they have a hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their unfavorable physico-chemical properties mainly to the dominating presence of sodium carbonate, which causes the soil to swell and difficult to clarify/settle. They derive their name from the alkali metal group of elements, to which sodium belongs, and which can induce basicity. Sometimes these soils are also referred to as alkaline sodic soils.
Alkaline soils are basic, but not all basic soils are alkaline.

Descaling agent

A descaling agent or chemical descaler is a liquid chemical substance used to remove limescale from metal surfaces in contact with hot water, such as in boilers, water heaters, and kettles. Limescale is either white or brown in colour due to the presence of iron compounds. Glass surfaces may also exhibit scaling stains, as can many ceramic surfaces present in bathrooms and kitchen, and descaling agents can be used safely to remove those stains without affecting the substrate since both ceramics and glass are unreactive to most acids.

Lime softening is a type of water treatment used for water softening, which uses the addition of limewater to remove hardness by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter by flocculation.

Calcareous sinter is a freshwater calcium carbonate deposit, also known as calc-sinter. Deposits are characterised by low porosity and well-developed lamination, often forming crusts or sedimentary rock layers. Calcareous sinter should not be confused with siliceous sinter, which the term sinter more frequently refers to. It has been suggested that the term "sinter" should be restricted to siliceous spring deposits and be dropped for calcareous deposits entirely.

Oilfield scale inhibition is the process of preventing the formation of scale from blocking or hindering fluid flow through pipelines, valves, and pumps used in oil production and processing. Scale inhibitors (SIs) are a class of specialty chemicals that are used to slow or prevent scaling in water systems. Oilfield scaling is the precipitation and accumulation of insoluble crystals (salts) from a mixture of incompatible aqueous phases in oil processing systems. Scale is a common term in the oil industry used to describe solid deposits that grow over time, blocking and hindering fluid flow through pipelines, valves, pumps etc. with significant reduction in production rates and equipment damages. Scaling represents a major challenge for flow assurance in the oil and gas industry. Examples of oilfield scales are calcium carbonate (limescale), iron sulfides, barium sulfate and strontium sulfate. Scale inhibition encompasses the processes or techniques employed to treat scaling problems.

References

  1. Hermann Weingärtner, "Water" in Ullmann's Encyclopedia of Industrial Chemistry , December 2006, Wiley–VCH, Weinheim. doi : 10.1002/14356007.a28_001
  2. Tegethoff, F. Wolfgang; Rohleder, Johannes; Kroker, Evelyn. Calcium Carbonate: From the Cretaceous Period Into the 21st Century . Birkhäuser, 2001. ISBN   3-7643-6425-4
  3. C. Wilson (2015). "Canterbury Cathedral's Mystery "Marble": A Double Imposture Unmasked'". In P. Fergusson (ed.). Canterbury Cathedral Priory in the Age of Becket. New Haven and London. pp. 156–60.
  4. John McNeill (2015). "The Romanesque Cloister in England". Journal of the British Archaeological Association. 168: 34–76. doi:10.1179/0068128815Z.00000000038. S2CID   194154048.
  5. R.B. Harris (2019). "Reconstructing the romanesque cloister of Norwich cathedral". The Antiquaries Journal. Cambridge University Press. 99: 133–159. doi:10.1017/S0003581519000118. S2CID   203298501.
  6. Geoff Downer (2019). "calc-sinter or Onyx Marble". canterbury-archaeology.org.uk. Canterbury Historical & Archaeological Society (CHAS).