Decarbonization

This article describes the de / decarbonization in water treatment. For the process of the same name in climate protection, see decarbonization .

As decarbonization (also: decarbonization , it is meant Entcarbonatisierung ) A method of water treatment referred to in which for reducing the water hardness , the carbonate (temporary hardness), but not the permanent hardness is reduced the water or eliminated.

Water with too high a carbonate hardness is poorly suited for use as cooling and process water without prior treatment. Heating the water leads to the precipitation of calcium carbonate ( scale ), which leads to a wide range of damage in pipe systems and on the surfaces of heat exchangers. Drinking water with a carbonate hardness well above 5-8 ° dH is disadvantageous for many practical uses.

Procedure

Various methods have been developed to partially or largely reduce the carbonate hardness of water. With some procedures only the carbonate hardness can be removed and with others also the total hardness . The main processes for decarbonization are explained below.

Decarbonization by adding acids

By adding hydrochloric or sulfuric acid , the carbonate hardness is converted into permanent hardness according to the following reaction equation:

${\displaystyle \mathrm {Ca(HCO_{3})_{2}+2HCl\longrightarrow CaCl_{2}+2CO_{2}+2H_{2}O} }$

The calcium component of the carbonate hardness, i.e. the dissolved calcium hydrogen carbonate and hydrochloric acid react to dissolved calcium chloride and dissolved carbon dioxide , the so-called free carbonic acid .

The advantage of this process is the low expenditure on equipment. Due to the free carbon dioxide that is formed, these waters are corrosive to many materials. Before it can be used as service water, the aggressive free carbon dioxide must therefore be removed as far as possible. This is done, for example, by feeding the make-up water into the cooling circuit upstream of the cooling tower.

The disadvantage of this process is that the total salt content remains practically unchanged, since the carbonate hardness is converted into non-carbonate hardness. When using sulfuric acid, the formation of the relatively poorly soluble calcium sulfate dihydrate ( gypsum ) must be taken into account, which therefore limits the use of this acid.

Lime decarbonization

In lime decarbonization, milk of lime (usually a suspension of calcium hydroxide (hydrated lime) in water) is added to the water and calcium carbonate is precipitated according to the following equation:

${\displaystyle \mathrm {Ca(HCO_{3})_{2}+Ca(OH)_{2}\longrightarrow 2CaCO_{3}\downarrow +2H_{2}O} }$

Calcium part of the carbonate hardness and calcium hydroxide react to form calcium carbonate and water.

A distinction is made between two different processes, namely long-term decarbonisation and rapid decarbonisation .

1 = container, 2 = pure water collector, 3 = raw water, 4 = pure water, 5 = sludge scraper, 6 = raw water injector, 7 = sludge discharge, 8 = milk of lime, 9 = flocculant, 10 = water level 11 = drive with motor M, 12 = sludge collector

The simplified process sketch shows a reactor for long-term decarbonization . A reactor with a water injector is shown of the various common designs. In this type, raw water is mixed with the chemicals for precipitation and with the carbonate sludge that has already formed using an injector . Instead of an injector for the mixture, other designs use agitators for this purpose. Other accessories, such as a scraper bridge, to which the internal fittings are attached with the sludge scraper, are not shown in the sketch.

In all long-term reactors , the calcium carbonate formed is precipitated as sludge. This takes place in pressureless containers which, with larger throughput quantities, consist mainly of concrete . The reactors are septic tanks / clarifiers or flocker called. Furthermore, company names of the system manufacturer , such as pulsator , are used, which refer to special properties or designs.

In addition to the milk of lime, a flocculant, for example iron (III) chloride , and a flocculant is often added. This increases and improves the clarification, i.e. the settling behavior of the undissolved substances. In the case of polluted surface water, in addition to the calcium carbonate formed, the solids and colloidal impurities of the raw water are also separated out in the sludge.

When decarbonising lime, it should be noted that predominantly only calcium carbonate is excreted. Magnesium ions are not precipitated because of the significantly greater solubility of the magnesium carbonate. With a higher addition of milk of lime, sparingly soluble magnesium hydroxide could be formed. However, this hydroxide is very fine flaky and also tends to flocculate with a delay. Magnesium is therefore not precipitated at normal temperatures.

The minimum achievable content of residual carbonate hardness after decarbonisation is <2 ° dH. However, this low residual value can only be achieved with raw water without a significant proportion of magnesium carbonate hardness. The precipitation of calcium carbonate reduces the salt content and thus partially demineralizes the water .

With long-term decarbonisation after the reactor, solids contents of 3–6 mg / l can be achieved. In the case of drinking water and further water treatment in particular, subsequent filtration takes place after decarbonisation .

With rapid decarbonisation , milk of lime is also added to the raw water. In addition, however, fine sand is periodically added as seed crystals . The calcium carbonate crystallizes on the surface of the sand and forms solid crystallization-active carbonate spheres, also called hard grains . The carbonate spheres form a floating bed in the reactor, which flows through from bottom to top . After reaching a diameter of about 10-15 mm, the balls are periodically discharged hydraulically. Below is a sketch for a conical high-speed reactor with the procedural internals and the necessary pipe connections:

1/2 = raw and pure water, 3 = raw water distribution, 4 = container, 5 = milk of lime, 6 = flocculant, 7 = fine sand suspension, 8.1 = floating bed sample, 8.2 = floating bed sample, 9 = hard grain extraction, 10 = pure water extraction funnel, 11 = Ventilation, 12 = floating bed

The decarbonization is carried out in about 8-20 m high, conical or cylindrical pressure vessels. Advantages of this method compared to the long-term method are the significantly smaller dimensions with the same system performance and the implementation under pressure. The formation of a solid waste product instead of bulky sludge with the same residual carbonate content is also an advantage. However, only raw water that is relatively low in solids can be treated. Raw water with higher contents of dissolved iron and manganese, like higher solids contents, hinder the accumulation of calcium carbonate on the surface of the carbonate spheres.

The solids content behind the reactor is higher than with long-term decarbonisation. Subsequent filtration is therefore almost always necessary before use. With this filtering, there is a further measurable reduction in the carbonate hardness due to post-reactions.

Other precipitation processes

In addition to the current ones above, there are other older processes that are hardly or only rarely used due to new developments and for economic reasons. These procedures include:

Hot desilification

During hot decilification, which normally takes place at around 65 ° C, both the carbonate hardness and the content of silicic acids in the water are reduced. The precipitation is carried out with a suspension of slaked and calcined magnohydrate . The equation for this reaction is:

${\displaystyle \mathrm {Ca(OH)_{2}\cdot MgO+Ca(HCO_{3})_{2}+SiO_{2}\longrightarrow } }$ ${\displaystyle \mathrm {\ 2\ CaCO_{3}\downarrow +MgSiO_{3}\downarrow +2\ H_{2}O} }$

Magnohydrate reacts with carbonate hardness + silica to form undissolved calcium carbonate + magnesium silicate + water

After the heated raw water has been thoroughly mixed with the magnohydrate suspension and recirculated sludge, precipitation takes place in cylindrical reactors. After the reactor, filtering is carried out, which is carried out over a silica-free mass, such as, for example, unfired dolomite. The residual contents for the carbonate hardness and the silica are values ​​of ‹2.0 ° dH and‹ 1 mg / l SiO2 reached.

Since calcium carbonate and magnesium silicate precipitate, the water is also partially desalinated during hot desilification .

Lime-soda process

One of the oldest processes for the extensive removal of carbonate and non-carbonate hardness is the soda-lime process . Further information on this under water softening .

Lye precipitation

When using sodium hydroxide solution ( caustic soda ) instead of milk of lime, comparable precipitations are possible. Both long-term and rapid decarbonization can be carried out. But since lime is cheaper than caustic soda, the economic advantages outweigh the advantages of using lime. Another disadvantage is the partial salination instead of the pure partial desalination , since part of the CO _{2 is converted} from the carbonate hardness into dissolved sodium carbonate.

Hydrogen decarbonization

Also called H-decarbonization (H of Hydrogenium / Hydrogen). This process uses cation exchange resins to reduce carbonate hardness. The alkaline earth ions of the hydrogen carbonates (= carbonate hardness) are exchanged by the acid groups of the ion exchange resins for the release of hydrogen according to the following reaction formula:

${\displaystyle \mathrm {2(CH_{2}-CH)_{x}-COOH+Ca(HCO_{3})_{2}\longrightarrow } }$ ${\displaystyle \mathrm {\ ((CH_{2}-CH)_{x}-COO)_{2}Ca+2CO_{2}+2H_{2}O} }$

The carboxylic acid group of the weakly acidic acrylate resin reacts with the calcium content of the carbonate hardness to form the calcium salt of the resin, carbon dioxide and water

The process of this decarbonization is comparable to the acid metering . The decarbonised water contains the stoichiometric content of free aggressive carbonic acid. However, since no dissolved alkaline earth compound is formed, no salination takes place. According to the reduction in carbonate hardness, the water is partially desalinated . In contrast to lime decarbonization, this process also removes any magnesium content of the carbonate hardness without any problems.

Technical applications

The processes listed are used both for the salination and partial desalination of raw water before use, and as a preliminary stage for further desalination. In particular, lime decarbonization, since hydrated lime is relatively inexpensive, is often used as a preliminary stage in larger desalination plants for economic reasons. In addition, excessive salt build-up in the wastewater is avoided, since solid calcium carbonate is separated off.

The additional water for larger service and cooling water systems in industry and large power plants is - if necessary - predominantly calcium carbonated.

Up to the development of strongly basic anion exchangers, hot decilification was the most important industrial process with which the silica content in water could be reduced. This was particularly necessary for condensation power plants. Higher silica contents in the boiler water cause silicification of the turbine blades due to the steam volatility of SiO ₂ at higher pressures. With the introduction of the strongly basic anion exchanger in the 1950s, however, this process lost much of its importance. It is now only used in special cases.

An H-decarbonization process (Carix process) was also developed and put into practice, in which the ion exchange resins are regenerated with carbonic acid (introduction of CO ₂ ).

Drinking water is also partially decarbonised centrally in large-scale plants. This is useful for waters of hardness range IV - (GH ›3.8mmol / l). Membrane desalination systems such as reverse osmosis (RO) and nanofiltration (NF) are also used for this purpose. In addition to the hardness components calcium and magnesium, the neutral salts chloride, nitrate and sulfate as well as humic substances and organic micropollutants can also be removed with this technology.

Individual evidence

1. ↑ R. Kretzer and K. Schluck; Experience in the combination of process steps for the treatment of cooling tower make-up water ; In: VGB Kraftwerkstechnik, 57, 1977, Jan. Issue 1, pp. 35-36.
2. ↑ Heinz Bedford; Central drinking water softening by means of rapid decarbonization with lime; in: cbr Fachtechnik Wasseraufbereitung , 45 years, 8/94, p. 31.
3. ↑ K. Spindler, H. Blöchl, A. Bursik; Flocculation and decarbonisation of water in a high-performance clarifier ; in: VGB Kraftwerkstechnik , June 1985, issue 6, p. 604.
4. ↑ P.Pracht; Water; in: Babcock manual ; Edition 1962, p. 59.
5. ↑ P.Pracht; Water; in: Babcock manual ; Edition 1962, p. 61
6. ↑ P. Splendor; Water; in: Babcock manual ; Edition 1962, p. 62.
7. ↑ Hagen, Klaus; Hoell, Wolfgang; in: The Carix process - a technology that has been tried and tested for many years , 2009, special issue GWF Wasser Abwasser, pp. W44 to W48, gwf special 1 2009 .

literature

• Babcock Handbook of Water , chap. Softening by the precipitation process
• VGB 1985 issue no.6, from page 600
• gwf-Wasser.Aabwasser, 1989 No. 11, from page 569
• bbr, 1994 issue No. 8, from page 28