Water is life. We all know from childhood that our body consists almost entirely of water. We drink a lot of water to stay healthy and always try to drink only clean, safe water. But why then is the water deeply purified? harmful to the body? What is demineralized water and why is it needed?
Deep purified water
Demineralized or deionized water is deeply purified water, in which the salt content is reduced. What distinguishes it from distilled water is that it contains non-electrolytes.
Today, there are many ways to obtain deionized water. Different needs require water of more or less deep purification, so different methods are used for different purposes.
Evaporation
The essence of the method is that contaminated water is evaporated. Wherein impurities remain, and pure water condenses. This method is very energy intensive, but it also allows you to remove non-electrolytic impurities.
Electrolysis
A method for purifying water under the influence of an electric field. The field acts on free ions dissolved in water and attracts them, and the water becomes cleaner.
Reverse osmosis
The cleaning principle is that water under high pressure is passed through semipermeable membrane, the smallest pores of which allow water molecules to pass through, but retain impurities. This method, in combination with the others, allows you to obtain double-distilled water, which is considered the purest to date.
Areas of use
Any water contains mineral salts, we even often buy special mineral water with a high content of certain salts.
But we also know that hard water or water with a high content of potassium and calcium salts is unsuitable for household needs. When washing, it forms a sediment, which damages washing machines, and appears on the kettle in the form of scale.
But if for everyday life we only need to slightly reduce the salt content, then for the pharmacological and food industries. Such water is needed in petrochemical plants and metal processing plants.
Another group using demineralized water is motorists. They add highly purified water to the antifreeze. Coolant contains water, but it can evaporate when the weather changes. This water is also necessary for the operation of the glass washer.
Only desalted water can be a dielectric, since salt ions in solution are capable of conducting electricity. This opens up another field of use: for research purposes. Demineralized water has found its application in the field of energy.
Recently, deionized water has become more popular than distilled water. Distillation devices wear out faster due to the presence of salts in the liquid, while demineralization is less expensive.
Harm from consuming desalinated water
If demineralized water is useful for appliances and machines, the effect on humans is not so clear. Deeply purified water can flush salts out of the body, and sometimes this is necessary. For example, it has been proven positive influence moderate consumption of demineralized water with:
- detection of deposits in the liver;
- kidney dysfunction;
- diabetes;
- allergies;
- intoxication and poisoning.
In addition to harmful impurities, water also contains useful ones, but deeply purified water is devoid of any impurities, as doctors often say: this "dead" water.
Some impurities are necessary for the body to function properly, but deionized water does not contain these impurities and does not support reactions. In addition, such water is tasteless, it is absolutely fresh and does not eliminate the feeling of thirst.
Regular consumption of highly purified water for food can lead to destruction of the mucous membrane of the gastrointestinal tract. Experiments on rats show this.
The detrimental effect on the process of mineral metabolism when drinking desalted water has been clearly proven. This water washes away minerals from biological fluids. What affects hormonal levels and red blood cell production. At the same time, the release of water from the body increases.
With frequent consumption of weak mineral water, the concentration of calcium and magnesium in the body decreases. Calcium is a building block of many bones and tissues of the body, and magnesium is necessary for more than three hundred biological processes.
It has also been proven that with regular consumption of demineralized water, the intake of toxic metals. “Dead” water has weak protective properties.
Natural water always contains various impurities, the nature and concentration of which determines its suitability for certain purposes.
Drinking water supplied by centralized domestic drinking water supply systems and water pipelines, according to GOST 2874-73, can have a total hardness of up to 10.0 mg-eq/l, and a dry residue of up to 1500 mg/l.
Naturally, such water is unsuitable for preparing titrated solutions, for performing various studies in an aqueous environment, for many preparative works involving the use of aqueous solutions, for rinsing laboratory glassware after washing, etc.
Distilled water
The method of demineralization of water by distillation (distillation) is based on the difference in the vapor pressure of water and salts dissolved in it. At not very high temperatures, it can be assumed that the salts are practically non-volatile and demineralized water can be obtained by evaporation of water and subsequent condensation of its vapor. This condensate is commonly called distilled water.
Water purified by distillation in distillation apparatuses is used in chemical laboratories in quantities greater than other substances.
According to GOST 6709-72, distilled water is a transparent, colorless, odorless liquid with pH = 5.44-6.6 and a solids content of no more than 5 mg/l.
According to the State Pharmacopoeia, the dry residue in distilled water should not exceed 1.0 mg/l, and pH = 5.0 4-6.8. In general, the requirements for the purity of distilled water according to the State Pharmacopoeia are higher than according to GOST 6709-72. Thus, the pharmacopoeia allows the content of dissolved ammonia to be no more than 0.00002%, GOST no more than 0.00005%.
Distilled water should not contain reducing substances (organic substances and inorganic reducing agents).
The clearest indicator of water purity is its electrical conductivity. According to literature data, the specific electrical conductivity of ideally pure water at 18°C is 4.4*10 V minus 10 S*m-1,
If the need for distilled water is small, water distillation can be carried out at atmospheric pressure in conventional glass installations.
Once distilled water is usually contaminated with CO2, NH3 and organic matter. If water with very low conductivity is required, the CO2 must be completely removed. To do this, a strong stream of air purified from CO2 is passed through water at 80-90 °C for 20-30 hours and then the water is distilled at a very slow air flow.
For this purpose, it is recommended to use compressed air from a cylinder or suck it in from the outside, since it is very contaminated in a chemical laboratory. Before adding air to the water, it is first passed through a washing bottle with conc. H2SO4, then through two wash bottles with conc. KOH and, finally, through a bottle of distilled water. In this case, the use of long rubber tubes should be avoided.
Most of the CO2 and organic matter can be removed by adding about 3 g of NaOH and 0.5 g of KMnO4 to 1 liter of distilled water and discarding some of the condensate at the beginning of the distillation. The bottom residue should be at least 10-15% of the load. If the condensate is subjected to secondary distillation with the addition of 3 g of KHSO4, 5 ml of 20% H3PO4 and 0.1-0.2 g of KMnO4 per liter, this ensures complete removal of NH3 and organic contaminants.
Long-term storage of distilled water in glass containers always leads to its contamination with glass leaching products. Therefore, distilled water cannot be stored for a long time.
Metal distillers
Electrically heated distillers. In Fig. 59 shows the D-4 distiller (model 737). Capacity 4 ±0.3 l/h, power consumption 3.6 kW, cooling water consumption up to 160 l/h. The weight of the device without water is 13.5 kg.
In the evaporation chamber 1, the water is heated by electric heaters 3 to a boil. The resulting steam through pipe 5 enters the condensation chamber 7, built into chamber 6, through which tap water continuously flows. The distillate flows out of condenser 8 through nipple 13.
At the beginning of operation, tap water continuously flowing through nipple 12 fills the water chamber 6 and through the drain tube 9 through the equalizer 11 fills the evaporation chamber to the set level.
In the future, as it boils away, the water will only partially enter the evaporation chamber; the main part, passing through the condenser, more precisely through its water chamber 6, will be drained through the drain tube into the equalizer and then through nipple 10 into the sewer. The hot water that flows out can be used for household needs.
The device is equipped with a level sensor 4, which protects electric heaters from burning out if the water level drops below the permissible level.
Excess steam from the evaporation chamber exits through a tube mounted in the wall of the condenser.
The device is installed on a flat horizontal surface and, using a grounding bolt 14, is connected to a common grounding circuit, to which an electrical panel is also connected.
When starting up the device for the first time, you can use distilled water for its intended purpose only after 48 hours of operation of the device.
Periodically, it is necessary to mechanically descale the electric heaters and the level sensor float.
The D-25 distiller (model 784) is designed similarly, with a capacity of 25 ±1.5 l/h and a power consumption of 18 kW.
This device has nine electric heaters - three groups of three heaters. For normal and long-term operation of the device, it is enough for six heaters to be turned on simultaneously. But this requires periodic, depending on the hardness of the feed water, mechanical descaling of the tube through which the water enters the evaporation chamber.
When initially starting up the D-25 distiller, it is recommended to use distilled water for its intended purpose after 8-10 hours of operation of the device.
Of significant interest is the apparatus for producing pyrogen-free water for injection A-10 (Fig. 60). Productivity 10 ±0.5 l/h, power consumption 7.8 kW, cooling water consumption 100-180 l/h.
In this apparatus, reagents are supplied to the evaporation chamber along with the distilled water to soften it (potassium alum Al2(SO4)3-K2SO4-24H2O) and to remove NH3 and organic contaminants (KMnO4 and Na2HPO4).
The alum solution is poured into one glass vessel of the dosing device, and the KMnO4 and Na2HPO4 solutions into another - at the rate of 0.228 g of alum, 0.152 g of KMnO4, 0.228 g of Na2HPO4 per 1 liter of pyrogen-free water.
During the initial start-up or when starting up the device after long-term preservation, the resulting pyrogen-free water can be used for laboratory needs only after 48 hours of operation of the device.
Before operating metal distillers with electric heating, you should check that all wires are connected correctly and that they are grounded. It is strictly forbidden to connect these devices to the electrical network without grounding them. In case of any malfunction, the distillers must be disconnected from the network.
The quality of distilled water depends to a certain extent on the duration of operation of the device. So, when using old distillers, the water may contain chloride ions.
The receivers must be made of neutral glass and, in order to avoid the ingress of CO2, connected to the atmosphere through calcium chloride tubes filled with soda lime granules (a mixture of NaOH and Ca(OH)2).
Fire distiller. The DT-10 distiller with a built-in firebox is designed for operation in conditions where there is no running water or electricity and allows you to obtain up to 10 liters of distilled water in 1 hour. It is a cylindrical structure made of stainless steel with a height of about 1200 mm, mounted on a base 670 mm long and 540 mm wide.
The distiller consists of a built-in firebox with combustion fittings, a 7.5-liter evaporation chamber, a 50-liter cooling chamber and a 40-liter distilled water collector.
Water is poured into the evaporation and cooling chambers manually. As water is consumed in the evaporation chamber, it is automatically replenished from the cooling chamber.
Obtaining bidistillate
Once distilled water in metal distillers always contains small amounts of foreign substances. For particularly precise work, they use re-distilled water - bidistillate. The industry mass-produces water double-distillation devices BD-2 and BD-4 with a capacity of 1.5-2.0 and 4-5 l/h, respectively.
Primary distillation occurs in the first section of the apparatus (Fig. 61). KMnO4 is added to the resulting distillate to destroy organic impurities and it is transferred to a second flask, where secondary distillation occurs, and the bidistillate is collected in a receiving flask. Heating is carried out using electric heaters; Glass water refrigerators are cooled with tap water. All glass parts are made from Pyrex glass.
Determination of quality indicators of distilled water
Determination of pH. This test is carried out by the potentiometric method with a glass electrode or, in the absence of a pH meter, by the colorimetric method.
Using a rack for colorimetry (a rack for test tubes equipped with a screen), place in four numbered identical test tubes with a diameter of about 20 mm and a capacity of 25-30 ml, clean, dry, made of colorless glass: 10 ml of test water each are placed in test tubes No. 1 and 2 , in test tube No. 3 - 10 ml of a buffer mixture corresponding to pH = 5.4, and in test tube No. 4 - 10 ml of a buffer mixture corresponding to pH = 6.6. Then 0.1 ml of a 0.04% aqueous alcohol solution of methyl red is added to test tubes No. 1 and 3 and mixed. Add 0.1 ml of a 0.04% aqueous alcohol solution of bromothymol blue to test tubes No. 2 and 4 and mix. Water is considered to comply with the standard if the contents of test tube No. 1 are not redder than the contents of test tube No. 3 (pH = 5.4), and the contents of test tube No. 2 are not bluer than the contents of test tube No. 4 (pH = 6.6).
Determination of dry residue. In a pre-calcined and weighed platinum cup, 500 ml of the test water is evaporated to dryness in a water bath. Water is added to the cup in portions as it evaporates, and the cup is protected from contamination with a safety cap. Then the cup with the dry residue is kept for 1 hour in a drying oven at 105-110 °C, cooled in a desiccator and weighed on an analytical balance.
Water is considered to comply with GOST 6709-72 if the mass of the dry residue is no more than 2.5 mg.
Determination of ammonia and ammonium salts content. 10 ml of the test water is poured into one test tube with a ground glass stopper with a capacity of about 25 ml, and 10 ml of a standard solution prepared as follows: 200 ml of distilled water is placed in a 250-300 ml conical flask, 3 ml of a 10% solution is added NaOH and boil for 30 minutes, after which the solution is cooled. Add 0.5 ml of a solution containing 0.0005 mg NH4+ to the test tube with the standard solution. Then 1 ml of ammonia reagent (see Appendix 2) is simultaneously added to both test tubes and mixed. Water is considered to comply with the standard if the color of the contents of the test tube observed after 10 minutes is no more intense than the color of the standard solution. Color comparison is made along the axis of the tubes on a white background.
Test for reducing substances. Bring 100 ml of test water to a boil, add 1 ml of 0.01 N. KMnO4 solution and 2 ml of diluted (1:5) H2SO4 and boil for 10 minutes. The pink color of the test water should be preserved.
Demineralization of fresh water by ion exchange method
During the deionization of water, the processes of H+ cationization and OH- anionization are sequentially carried out, i.e., the replacement of cations contained in water with H+ ions and anions with OH- ions. By interacting with each other, H+ and OH- ions form the H2O molecule.
The deionization method produces water with a lower salt content than conventional distillation, but does not remove non-electrolytes (organic contaminants).
The choice between distillation and deionization depends on the hardness of the source water and the costs associated with its purification. Unlike water distillation, during deionization, energy consumption is proportional to the salt content in the water being purified. Therefore, at a high concentration of salts in the source water, it is advisable to first use the distillation method, and then carry out additional purification by deionization.
Ion exchangers are solid, practically insoluble in water and organic solvents, substances of mineral or organic origin, natural and synthetic. For the purposes of water demineralization, synthetic polymer ion exchangers are of practical importance - ion exchange resins, characterized by high absorption capacity, mechanical strength and chemical resistance.
Demineralization of water can be carried out by sequentially passing tap water through a column of cation exchange resin in the H+ form, then through a column of anion exchange resin in the OH- form. The filtrate from the cation exchanger contains acids corresponding to the salts in the source water. The completeness of removal of these acids by anion exchangers depends on their basicity. Strongly basic anion exchangers remove all acids almost completely; weakly basic anion exchangers do not remove such weak acids as carbonic, silicon and boric.
If these acidic groups are acceptable in demineralized water or their salts are absent in the source water, then it is better to use weakly basic anion exchangers, since their subsequent regeneration is easier and cheaper than the regeneration of strongly basic anion exchangers.
For demineralization of water in laboratory conditions, cation exchangers of the brands KU-1, KU-2, KU-2-8chS and anion exchangers of the brands EDE-10P, AN-1, etc. are often used. Ion exchangers supplied in dry form are crushed and grains of size 0. 2-0.4 mm using a set of sieves. They are then washed with distilled water by decantation until the washing waters become completely clear. After this, the ion exchangers are transferred to glass columns of various designs.
In Fig. 62 shows a small-sized column for water demineralization. Glass beads are placed at the bottom of the column and glass wool is placed on top of them. To prevent air bubbles from getting between the ion exchanger grains, the column is filled with a mixture of ion exchanger and water. Water is released as it accumulates, but not below the level of the ion exchanger. The ion exchangers are covered with a layer of glass wool and beads on top and left under a layer of water for 12-24 hours. After draining the water from the cation exchanger, the column is filled with 2 N. HCl solution, leave for 12-24 hours, drain the HCl and wash the cation exchanger with distilled water until the methyl orange reaction is neutral. The cation exchanger, converted to the H+ form, is stored under a layer of water. Similarly, the anion exchanger is transferred to the OH form, keeping it in the column after swelling in 1 N. NaOH solution. The anion exchanger is washed with distilled water until the phenolphthalein reaction is neutral.
Demineralization of relatively large volumes of water with separate use of ion exchange filters can be carried out in a larger installation. The material for two columns with a height of 700 and a diameter of 50 mm can be glass, quartz, or transparent plastic. 550 g of the prepared ion exchanger are placed in the columns: in one - the cation exchanger in the H+ form, in the other - the anion exchanger - in the OH- form. Tap water enters the column with a cation exchange resin at a rate of 400-450 ml/min, and then passes through the column with an anion exchange resin.
Since ion exchangers are gradually saturated, it is necessary to monitor the operation of the installation. In the first portions of the filtrate passed through the cation exchanger, the acidity is determined by titration with an alkali against phenolphthalein. After about 100 liters of water have been passed through the installation, or it has operated continuously for 3.5 hours, you should take a water sample again from the cation exchange column and determine the acidity of the filtrate. If a sharp decrease in acidity is observed, the flow of water should be stopped and the ion exchangers should be regenerated.
The cation exchanger is poured from the column into a large jar with a 5% HCl solution and left overnight. Then the acid is drained, the cation exchanger is transferred to a Buchner funnel and washed with distilled water until the reaction for the Cl- ion with AgNO3 is negative. The washed cation resin is reintroduced into the column.
The anion resin is regenerated with a 5% NaOH solution, washed with water until the phenolphthalein reaction is negative, and then the column is refilled with it.
Currently, water demineralization is mostly carried out using the mixed layer method. The source water is passed through a mixture of a cation exchanger in the H+ form and a strongly or weakly basic anion exchanger in the OH- form. This method ensures the production of water of a high degree of purity, but the subsequent regeneration of ion exchangers requires a lot of labor.
To deionize water using mixed ion exchanger filters, a mixture of KU-2-8chS cation exchanger and EDE-10P anion exchanger in a volume ratio of 1.25:1 is loaded into a column with a diameter of 50 mm and a height of 600-700 mm. Plexiglas is preferred as the material for the column, and polyethylene for the supply and waste tubes.
One kilogram of ion exchanger mixture can purify up to 1000 liters of once distilled water.
Regeneration of spent mixed ion exchangers is carried out separately. The mixture of ion exchangers from the column is transferred to a Buchner funnel and sucked off until an air-dry mass is obtained. Then the ion exchangers are placed in a separating funnel of such capacity that the ion exchanger mixture occupies 1/4 of its volume. After this, add up to 3/4 volume of a 30% NaOH solution to the funnel and mix vigorously. In this case, the mixture of ion exchangers, due to their different densities (cation exchanger 1.1, anion exchanger 1.4), is divided into layers. After this, the cation exchanger and anion exchanger are washed with water and regenerated as indicated above.
In laboratories where the need for deeply demineralized water exceeds 500-600 l/day, the commercially available device Ts 1913 can be used. The estimated capacity is 200 l/h. The throughput capacity of the deionizer during the inter-regeneration period is 4000 liters. The weight of the set is 275 kg.
The demineralizer is equipped with a system for automatically shutting off the supply of tap water when its electrical resistance drops below the permissible value and float valves that allow you to automatically remove air from the columns. Regeneration of ion exchange resins is carried out by treating them directly in columns with a solution of NaOH or HCl.
To obtain clean demineralized water, so-called ion exchange filters are used (Fig. 16). Their action is based on the ability of certain substances to selectively bind cations or anions of salts. Tap water is first passed through a cation resin, which binds only cations. The result is acidic water. This water is then passed through an anion exchanger, which binds only anions. Water passed through both ion exchangers is called demineralized(i.e. does not contain mineral salts).
Figure 15. Flask for storing distilled water with protection against carbon absorption.
The quality of demineralized water is not inferior to distilled water and often corresponds to bidistillate
Ion exchangers gradually become saturated and stop working, but they are easy to regenerate, after which they can be used again. In practice, regeneration can be carried out many times and a large amount of water can be purified with the same ion exchanger. Ion exchange units are widely used not only for water purification and demineralization in industry, but also in analytical laboratories instead of devices for water distillation.
Rice. 16. Laboratory installation for producing demineralized water.
Rice. 17. Diagram of a laboratory installation for producing demineralized water: 1 - plug; 2 - glass wool; 3 - cation exchanger; 4 - three-way edge; 5 - plug; 6-anion exchanger; 7 - drain pipe.
To obtain demineralized water, you can install an installation that will produce 20-25 l/h of water. The installation (Fig. 17) consists of two tubes (columns) 70 cm high and about 5 cm in diameter. Columns can be glass, quartz, or even better, transparent plastic, such as plexiglass. 550 g of ion exchange resins are placed in the columns: a cation exchange resin (in the H+ form) is placed in one, and an anion exchange resin (in the OrT form) is placed in the other. The test tube/column with cation exchanger 3 has an outlet tube, which is connected to a water tap with a rubber tube.
The water passed through the cation exchanger is sent to the second column with an anion exchanger. The flow rate of water through both columns should be no more than 450 cm3/min. In the first portions of water passed through the cation exchanger, it is necessary to establish the acidity. A water sample is taken through a three-way valve 4 connecting the columns. Preliminary determination of water acidity is necessary for subsequent quality control of demineralized water.
Since ion exchangers are gradually saturated, it is necessary to monitor the operation of the installation. After about 100 liters of water have been passed through it or it has been running continuously for 3.5 hours, a sample of the water that has passed through the cation exchanger column should be taken. Then 25 cm3 of this water is titrated with 0.1 N. NaOH solution in methyl orange. If the acidity of the water has sharply decreased compared to the result of the first test, the flow of water should be stopped and the ion exchangers should be regenerated. To re-reinvent the cation exchanger, pour it out of the column into a large jar, fill it with a 5% HCl solution and leave it dissolved overnight. After this, the acid is compared and the cation exchanger is washed with distilled or demineralized water until the test for Cl-ions in the washing waters becomes negative. The test is done as follows: place 2-3 drops of washing water on a watch glass and add a drop of 0.01 N to it. AgN03 solution. In a negative reaction, no turbidity is formed.
The washed cation resin is reintroduced into the column. Anion resin for regeneration is poured into a large jar, filled with 2% (0.5 N) NaOH solution and left overnight. The alkali is then drained, and the anion exchanger is thoroughly washed with distilled or demineralized water until the wash water reacts neutrally when tested with phenolphthalein. . " "
It is useful to have two such installations in the laboratory: one is in operation, and the other is a backup. While one installation is being regenerated, another is in operation.
Of the ion exchange resins * produced in the USSR, ion exchangers of the KU-2, SBS, SBSR, MSF or SDV-3 brands can be used as cation exchangers.
To obtain especially pure water, the quality of which is superior to bidistillate, it is recommended to use ion exchangers KU-2 and EDE-10P**. First, ion exchangers with a grain size of about 0.5 mm are converted, respectively, into the H- and OH-forms by treating KU-2 with a 1% solution of hydrochloric acid, and EDE-10P with a 3% solution of sodium hydroxide, and the sweat is washed well. Then they are mixed in a volumetric ratio of KU-2: EDE-10P = 1.25: 1 and the mixture is placed in a plexiglass column with a diameter of about 50 mm and a height of 60-70 cm.
The bottom and top plug of the column should also be made of plexiglass, the water supply and waste tubes should be made of polyethylene or aluminum.
To obtain especially pure water, ordinary distilled water is used, which is passed through a column with a mixture of ion exchangers. One kilogram of such a mixture can purify up to 1000 liters of distilled water. Purified water should have a resistivity of 1.5-2.4*10 -7 1/(ohm*cm). This mixture of ion exchangers is not recommended for the demineralization of tap water, since the ion exchangers quickly become saturated. When the resistivity of purified water begins to decrease, water purification is stopped and the ion exchangers are regenerated. To do this, the ion exchanger mixture is poured from the column onto a sheet of filter paper, leveled, covered with another sheet of the same paper and left to dry. Or the ion exchangers from the column are poured into a porcelain Buchner funnel and sucked off until an air-dry mass is obtained.
The air-dry mass is placed in a separating funnel of an appropriate container so that the mixture of ion exchangers occupies about "D. After this, a 3% NaOH solution is added to the separating funnel, filling the funnel to approximately 3D, and quickly stirred. In this case, the ion exchangers are instantly separated. The bottom layer containing the KU-2 cation exchanger is lowered through the separating funnel tap into a vessel with water and washed repeatedly using decantation until a sample of the wash water gives a neutral reaction when adding 1-2 drops of phenolphthalein.
The top layer containing the EDE-10P anion exchanger is poured through the neck of the separating funnel into a vessel with water. Ion exchangers are regenerated as described above, each ion exchanger separately, and after that they are again used for water purification.
Demineralized water is a liquid that contains almost all types of salts. Most often it is used to ensure the efficient and normal operation of various installations and systems.
Any water, regardless of the origin of its source - surface or underground - contains mineral impurities.
Some technological processes that are used in various types of production require demineralized water.
What is it and what does it represent? It is obtained as a result of demineralization, the essence of which is the removal of magnesium and calcium salts.
Demineralized water has recently become increasingly used instead of distilled water. This is explained by the fact that electric distillers are subject to frequent breakdowns. A large amount of salts in the initial liquid leads to the formation of scale on the walls of the evaporator, which significantly worsens the quality of the water.
To desalt water, different devices are used. The essence of their work is to neutralize salts that pass through ion exchange resins. The main part of any device of this type are columns, inside of which there are anion exchangers and cation exchangers.
The activity of the latter depends on the presence of sulfonic or carboxyl groups, which can exchange H+ ions for alkaline earth and alkali metal ions. As for anion exchangers, they receive anions in exchange for hydroxyl groups OH. The design of the units has special tanks for acid and alkaline solutions, as well as distilled water.
Types of demineralization
The result of using hard water is very often scale, which can be found on the surface of heating elements, and limescale in places of direct contact. This leads to the fact that plumbing wears out very quickly, and pipes, water heaters and their parts quickly become unusable. The following methods can be used to desalinize water:
- Evaporation of water, after which the steam is concentrated. This technology is considered quite energy-intensive. In addition, scale formation occurs during operation of the evaporator.
- The essence of electrodialysis is the ability to move ions in water under the influence of voltage created by an electric field. In this case, anions or cations pass through ion-selective membranes. In addition, in the space that these membranes limit, the concentration of salts decreases.
- For professional purification, the use of reverse osmosis is considered preferable. Previously, seawater was desalinated using this method. In combination with ion exchange and filtration, this method greatly increases the possibilities of water purification. The essence of the process is that, using a thin-film semi-permeable membrane with tiny pores, the dimensions of which are almost the same as those of a water molecule, liquid, as well as carbon dioxide and hydrogen, leak inside under pressure. In this case, impurities that remain on the membrane enter the drainage.
Scope of use of demineralized water
Today, deeply desalted water has found its wide application. Very often it began to be used in heat and electricity. Also, fully demineralized water is constantly used by metalworking enterprises.
Many industrial oil and gas associations carry out their activities exclusively using water, which is previously subjected to desalination. Deep water purification from salts is also carried out for medical purposes, in the pharmaceutical and food industries: it is used to produce various medicines, injection water, soft drinks and many high-quality food products.
Water hardness and demineralization: Video
Recently, attention has been paid to using demineralized water instead of purified water. This is due to the fact that distillers, especially electric ones, often fail. Salts contained in the source water form scale on the evaporator glasses, which worsens distillation conditions and reduces water quality.
Various installations are used for desalting (demineralization) of water. The principle of their operation is based on the fact that water is freed from salts when passing it through ion exchange resins - network polymers with a gel or microporous structure, covalently bonded to ionogenic groups. The dissociation of these groups in water produces an ion pair:
An ion fixed on a polymer carrier;
Mobile – a counterion that is exchanged for ions of the same charge.
The main part of installations for water demineralization are columns filled with cation exchangers and anion exchangers.
The activity of cation exchangers is determined by the presence of a carboxyl or sulfonic group, which has the ability to exchange hydrogen ions for ions of alkali and alkaline earth metals.
Anion exchangers are network polymers capable of exchanging their hydroxyl groups for anions.
The installations also have containers for solutions of acid, alkali and distilled water, necessary for the regeneration of resins. Regeneration of cation exchangers is carried out with hydrochloric or sulfuric acid. Anion exchangers are restored with an alkali solution (2-5%).
Typically, an ion exchange installation contains 3-5 cation and anion columns. Continuity of operation is ensured by the fact that one part of the columns is in operation, the other is in regeneration.
Tap water passes through ion exchange columns, then is fed to a filter that retains particles from the destruction of ion exchange resins.
To prevent microbial contamination, the resulting water is heated to 80-90 0 C.
It is advisable to use a demineralizer in interhospital, large hospital and other pharmacies to supply demineralized water to distillers and washing rooms for washing dishes.
The productivity of the demineralizer is 200 l/hour.
8. Reverse osmosis
Reverse osmosis (hyperfiltration) is a method of separating solutions; it consists in the fact that a solution under a pressure of 3-8 MPa is supplied to a semi-permeable membrane that allows the solvent to pass through and retains, in whole or in part, molecules or ions of the dissolved substance.
This method was first proposed in 1953 by C.E. Reid for water desalination.
The driving force P of reverse osmosis is the pressure difference: osmotic pressure of the solution ( P ) and the pressure of the saline solution above the membrane (P).
P=P- P
Forward osmosis is the one-way spontaneous transfer of a solvent through a semi-permeable membrane (septum) in order to equalize the concentration of substances on both sides.
Reverse osmosis is the filtering of aqueous systems (water) from a solution through semi-permeable membranes in order to separate dissolved salts, molecules of organic substances with sizes larger than water molecules, as well as suspended impurities and colloidal particles.
Reverse osmosis plants are economical to operate and highly productive. They reliably purify water from di-, tri-, tetravalent inorganic substances, organic substances, colloids, and partly from pyrogens. The downside is that membranes are quite expensive.
The quality of water obtained by the method of ion exchange and reverse osmosis is controlled by the value of electrical conductivity.