Carbonatation
Encyclopedia
Carbonatation is a chemical reaction
in which calcium hydroxide
reacts with carbon dioxide
and forms insoluble calcium carbonate
:
The process of forming a carbonate is sometimes referred to as "carbonation", although this term usually refers to the process of dissolving carbon dioxide in water
.
Carbonatation is a slow process that occurs in concrete
where lime (calcium hydroxide) in the cement
reacts with carbon dioxide from the air and forms calcium carbonate.
The water in the pores of Portland cement concrete is normally alkaline with a pH
in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel rebar
is passivated and is protected from corrosion. According to the Pourbaix diagram
for iron, the metal is passive when the pH is above 9.5.
The carbon dioxide in the air reacts with the alkali
in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonatate the cement in the concrete from the moment the object is made. This carbonatation process will start at the surface, then slowly move deeper and deeper into the concrete. The rate of carbonatation is dependent on the relative humidity of the concrete - a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete.
Eventually this may lead to corrosion of the rebar and damage to the construction.
from sugar beet
s.
It involves the introduction of milk of lime
(calcium hydroxide
suspension
) and carbon dioxide enriched gas into the "raw juice" (the sugar rich liquid prepared from the diffusion stage of the process) to form calcium carbonate and precipitate impurities that are then removed. The whole process takes place in "carbonatation tanks" and processing time varies from 20 minutes to an hour.
Carbonatation involves the following effects:
The target is a large particle that naturally settles rapidly to leave a clear juice. The juice at the end is approximately 15 °Bx
and 90% sucrose
. The pH
of the thin juice produced is a balance between removing as much calcium
from the solution and the expected pH drop across later processing. If the juice goes acid
ic in the crystallisation
stages then sucrose rapidly breaks down to glucose
and fructose
; not only do glucose and fructose affect crystallisation but they are molassagenic
taking equivalent amounts of sucrose on to the molasses stage.
The carbon dioxide bubbled through the mixture forms calcium carbonate. The non-sugar solids are incorporated into the calcium carbonate particles and removed by natural (or assisted) sedimentation
in tanks.
There are several systems of carbonatation, named from the companies that first developed them. They differ in how the lime is introduced, the temperature and duration of each stage, and the separation of the solids from the liquid.
Both DDS and RT processes are favoured by Europe
an factories. The carbonatation system is generally matched to the diffusion scheme; juice from RT diffusers being processed by the RT carbonatation.
The clear juice from carbonatation is generally known as "thin juice". it may undergo pH adjustment with soda ash and addition of sulfur
("sulfitation") prior to the next stage which is concentration by multiple effect evaporation.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
in which calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
reacts with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and forms insoluble calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
:
- Ca(OH)2 + CO2 → CaCO3 + H2O
The process of forming a carbonate is sometimes referred to as "carbonation", although this term usually refers to the process of dissolving carbon dioxide in water
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...
.
Concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
where lime (calcium hydroxide) in the cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
reacts with carbon dioxide from the air and forms calcium carbonate.
The water in the pores of Portland cement concrete is normally alkaline with a pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
in the range of 12.5 to 13.5. This highly alkaline environment is one in which the steel rebar
Rebar
A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
is passivated and is protected from corrosion. According to the Pourbaix diagram
Pourbaix diagram
In chemistry, a Pourbaix diagram, also known as a potential/pH diagram, maps out possible stable phases of an aqueous electrochemical system. Predominant ion boundaries are represented by lines. As such a Pourbaix diagram can be read much like a standard phase diagram with a different set of axes...
for iron, the metal is passive when the pH is above 9.5.
The carbon dioxide in the air reacts with the alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonatate the cement in the concrete from the moment the object is made. This carbonatation process will start at the surface, then slowly move deeper and deeper into the concrete. The rate of carbonatation is dependent on the relative humidity of the concrete - a 50% relative humidity being optimal. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete.
Eventually this may lead to corrosion of the rebar and damage to the construction.
Sugar refining
The carbonatation process is used in the production of sugarSugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
from sugar beet
Sugar beet
Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a high concentration of sucrose. It is grown commercially for sugar production. Sugar beets and other B...
s.
It involves the introduction of milk of lime
Lime water
Limewater is the common name for saturated calcium hydroxide solution. It is sparsely soluble. Its chemical formula is Ca2. Since calcium hydroxide is only sparsely soluble, i.e. ca. 1.5 g per liter at 25 °C, there is no visible distinction to clear water. Attentive observers will notice a slightly...
(calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
suspension
Suspension (chemistry)
In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
) and carbon dioxide enriched gas into the "raw juice" (the sugar rich liquid prepared from the diffusion stage of the process) to form calcium carbonate and precipitate impurities that are then removed. The whole process takes place in "carbonatation tanks" and processing time varies from 20 minutes to an hour.
Carbonatation involves the following effects:
- The increase in alkalinityAlkalinityAlkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
coagulatesCoagulationCoagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s in the juice. - Calcium carbonate absorbs colourants
- Alkalinity destroys some monosaccharideMonosaccharideMonosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
sugars, mostly glucoseGlucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and fructoseFructoseFructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
The target is a large particle that naturally settles rapidly to leave a clear juice. The juice at the end is approximately 15 °Bx
Brix
Degrees Brix is the sugar content of an aqueous solution.One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by weight . If the solution contains dissolved solids other than pure sucrose, then the °Bx is only approximate the...
and 90% sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
. The pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the thin juice produced is a balance between removing as much calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
from the solution and the expected pH drop across later processing. If the juice goes acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ic in the crystallisation
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
stages then sucrose rapidly breaks down to glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...
; not only do glucose and fructose affect crystallisation but they are molassagenic
Molasses
Molasses is a viscous by-product of the processing of sugar cane, grapes or sugar beets into sugar. The word molasses comes from the Portuguese word melaço, which ultimately comes from mel, the Latin word for "honey". The quality of molasses depends on the maturity of the sugar cane or sugar beet,...
taking equivalent amounts of sucrose on to the molasses stage.
The carbon dioxide bubbled through the mixture forms calcium carbonate. The non-sugar solids are incorporated into the calcium carbonate particles and removed by natural (or assisted) sedimentation
Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
in tanks.
There are several systems of carbonatation, named from the companies that first developed them. They differ in how the lime is introduced, the temperature and duration of each stage, and the separation of the solids from the liquid.
- DorrJohn V. N. DorrDr John Van Nostrand Dorr was an industrial chemist active in early to middle twentieth century. He attended Rutgers University, where he was admitted to the Zeta Psi fraternity, graduating in 1896. He was awarded the Franklin Institute's John Scott Medal in 1916...
(also Dorr-Oliver) - a continuous process using two tanks with recycling ("1st carbonatation") to build up particle size for natural flocculationFlocculationFlocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
. The recycling ratio is about 7:1. The particles are separated under gravity in a thickening stage in a vessel called a clarifier. The clear juice is then gassed further in another tank ("2nd carbonatation") and filtered. The concentrated mud (underflow) from the clarifier is filtered and/or pressed to recover more liquid. The Dorr process is low in maintenance and man-power but susceptible filtration problems when frost damaged beet are processed. It is favoured in the UK and the USA. - DDSDDSD.D.S. refers to the academic degree of Doctor of Dental Surgery.DDS may refer to:-Computer and information science:*Data Design System, DDS-CAD software vendor*Data Description Specifications, AS400 programming...
(Det Danske Sukkerfabrik - "The Danish Sugarfactory") -- multistage process involving pre-liming where the pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the juice is gradually increased to start precipitation of proteins, followed by addition of further lime and gas. The particles are removed at each stage by filtration. - RT (Raffinerie Tirlemontoise -Sugar refinery of TienenSugar refinery of TienenThe Sugar refinery of Tienen , a subsidiary of Raffinerie Tirlemontoise Group , is a Belgian sugar producing company...
) - another multistage process with a pre-liming stage. Particles also removed by filtration.
Both DDS and RT processes are favoured by Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
an factories. The carbonatation system is generally matched to the diffusion scheme; juice from RT diffusers being processed by the RT carbonatation.
The clear juice from carbonatation is generally known as "thin juice". it may undergo pH adjustment with soda ash and addition of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
("sulfitation") prior to the next stage which is concentration by multiple effect evaporation.
See also
- Phosphotation — a similar process used in sugarcane processing.