MINISTRY OF EDUCATION AND SCIENCE OF RUSSIA Federal state budgetary educational
institution "Southwestern State University"
Department of General and Inorganic Chemistry
I APPROVED First Vice-Rector – Vice-Rector for Academic Affairs
E.A. Kudryashov “___”____________2012
WATER HARDNESS AND METHODS OF SOFTENING IT
Guidelines for independent work in the discipline "Chemistry" for students of non-chemical specialties
UDC 546 Compiled by: I. V. Savenkova, F.F. Niyazi
Reviewer: Candidate of Chemical Sciences, Associate Professor V. S. Maltseva
Water hardness and methods of softening it: Guidelines for independent work in the discipline "Chemistry" for students of non-chemical specialties / South-West. state University; Compiled by: I.V. Savenkova, F.F. Niyazi Kursk, 2012. 18 p.
Methodological materials on assessing water hardness and methods for softening it are presented, laboratory work on this topic and individual assignments for students are presented.
Intended for students of non-chemical specialties.
QUESTIONS FOR INDEPENDENT PREPARATION
1. Water hardness and the reasons for its formation. Units of measurement of hardness.
2. Types of hardness: temporary, permanent, total, carbonate and non-carbonate. What ions are they caused by?
3. The effect of hardness on water pH.
4. Negative consequences of using hard water in industry.
5. Basic methods of softening industrial waters. What guides their selection?
6. Thermal method of water softening. Its advantages and disadvantages.
7. Reagent methods used to soften water. What chemical processes occur when water is softened by the method: a) liming; b) phosphating; c) soda; d) adding sodium hydroxide?
8. Water softening using the ion exchange method.
9. Ion exchange capacity of cation exchanger and anion exchanger. In what units is it expressed? What factors does it depend on?
10. Why is it washed with sodium chloride solution and then with water to regenerate the cation exchanger? Is it possible to regenerate a cation exchanger by washing it with a solution of magnesium chloride?
Bibliography
1. Korovin N.V. General chemistry. M.: Higher. school, 2007
2. Problems and exercises in general chemistry / Ed. N.V. Korovina. M.: Higher. school, 2004
3. Glinka N.L. Problems and exercises in general chemistry. M.: Integral-press, 2002
4. Akhmetov N.S. General and inorganic chemistry. M.: Higher. school,
Natural water is a complex multicomponent system that contains various organic and inorganic compounds in dissolved form.
1) Major ions.
Cations: Na+, Ca2+, Mg2+, K+ (less often Fe2+, Fe3+, Mn2+); Anions: HCO3 -, SO4 2-, Cl-, CO3 2- (less commonly HSiO3 -, SO3 2-, S2 O3 2-).
2) Dissolved gases.
Most often dissolved in water are: carbon dioxide, oxygen, nitrogen, hydrogen sulfide, methane, etc.
3) Biogenic substances.
TO Nutrients include those compounds that arise in connection with the vital activity of organisms. They contain various forms of nitrogen (ammonia, nitrite, nitrate), phosphorus, silicon, and iron.
4) Microelements.
TO These include elements that are contained in water in quantities smaller 10-3 %.
5) Organic substances.
These can be various kinds of plant and animal organisms, microorganisms and products of their interaction with the environment.
Natural waters vary greatly in the total content of dissolved salts and in the relative abundance of various ions. This difference can significantly affect the properties of water and,
hence, its application in various fields. Ca2+ and Mg2+ ions give specific properties to water,
the presence of which determines water hardness.
Water hardness is one of the technological indicators adopted to characterize the composition and quality of natural waters,
which is characterized by the content of the number of millimole equivalents of Ca2+ and Mg2+ ions in 1 liter of water. One milliequivalent of hardness corresponds to the content of 20.04 mg/l Ca2+ or 12.16 mg/l Mg2+ in water, which corresponds to the equivalent mass of these ions.
These ions appear in natural waters as a result
interaction with limestones or as a result of gypsum dissolution. CaCO3 + H2 O + CO2 = Ca2+ + 2HCO3 -
The hardness of natural waters varies widely. Water whose hardness is less than 4 meq/l of Ca2+ and Mg2+ ions is characterized as soft, from 4 to 8 – moderately hard, from 8 to 12
– hard and more than 12 meq/l – very hard.
For example, precipitation water is the softest (0.07-0.1 meq/l), and the hardness of ocean water is 130 meq/l.
There are several types of hardness: general, temporary, permanent, carbonate and non-carbonate.
General hardness is the total concentration of Ca2+, Mg2+ ions in water, expressed in meq/l.
Constant hardness - part of the total hardness remaining after boiling water at atmospheric pressure for a certain time.
Temporary hardness – part of the total hardness that is removed by boiling water at atmospheric pressure for a certain time. It is equal to the difference between the total and constant stiffness.
Carbonate hardness– part of the total stiffness,
equivalent concentration of calcium and magnesium bicarbonates. Non-carbonate hardness- part of the total stiffness equal to
difference between total and carbonate hardness.
Example 1. 5 m 3 of water contains 250 g of calcium ions and 135 g of magnesium ions. Determine the total hardness of water.
Solution . Let's find the content of calcium and magnesium ions (in mg/l) in
250 1000 / 5 1000 = 50 (mg/l) Ca2+ ions
And 135 1000 / 5 1000 = 27 (mg/l) Mg ions 2+ .
1 meq of hardness corresponds to the content of 20.04 mg/l of ions. Ca2+ or 12.16 mg/l Mg2+ ions; hence,
F = 50/20.04 + 27/12.16 = 4.715 (meq/l).
Answer: The water is moderately hard.
Example 2. Calculate the carbonate hardness of water, knowing that for titration of 100 ml of this water containing calcium bicarbonate,
it took 6.25 ml, 0.08 N HC1 solution. Give the equation for the corresponding reaction.
Solution: We solve the problem using the law of equivalents for solutions.
Let's calculate the normality of the calcium bicarbonate solution: N1 = 6.25 0.08 ⁄ 100 = 0.005 n
Therefore, 1 liter of water contains 0.005 1000 = 5 meq of calcium bicarbonate.
Answer: F=5mEq/l
Ca2+ and Mg2+ ions are not dangerous, but their significant content in water leads to excessive soap consumption, deterioration of the taste of products, etc. When heated and, especially when water evaporates, the salts of these metals form a layer of scale, which reduces the heat transfer coefficients in cooling and heating systems, which is extremely undesirable.
The use of natural water in technology requires its preliminary purification. The process leading to a decrease in water hardness is called water softening.
Water softening methods can be divided into three main groups:
1) thermal softening of water; 2) reagent softening methods; 3) water softening using the ion exchange method.
1. Thermal method of water softening
Temporary or carbonate hardness, is eliminated by heating water to 70-80°C and subsequent filtration. When heated, the following reactions occur:
Ca(HCO3)2 = CaCO3 + CO2 + H2O
Mg(HCO3)2 = MgCO3 + CO2 + H2O
However, it is impossible to completely eliminate carbonate hardness using the thermal method, since CaCO3, although slightly, is soluble in water. The solubility of MgCO3 is quite high, so magnesium bicarbonate immediately reacts with water, i.e.
a hydrolysis process is observed and instead of MgCO3, it precipitates
Mg(OH)2:
MgC03 + H2 O = Mg(OH)2 + CO2
Thermal softening of water is associated with significant costs, so it is used only when the water must be subjected to appropriate heating.
2. Reagent water softening.
Reagent water softening consists in the fact that when introduced into
water of special reagents, calcium and magnesium cations dissolved in it are transformed into practically insoluble compounds, which precipitate. Depending on the reagents used, water softening methods are classified into lime, lime-soda, alkaline, phosphate and barium.
2.1. Lime method.
This method is used to partially remove carbonate hardness from water.
When slaked lime is added to water in the form of lime milk, calcium bicarbonate salts are precipitated in the form of carbonates:
Ca(HCO3)2 + Ca(OH)2 = 2CaCO3 + 2H2 O, Further introduction of lime into water leads to hydrolysis
magnesium salts and the formation of poorly soluble magnesium hydroxide, which precipitates at pH≥ 10.2...10.3:
Mg(HCO3)2 + Ca(OH)2 = MgCO3 + CaCO3 + CO2 + 2H2 O MgCO3 + Ca(OH)2 = Mg(OH)2 + CaCO3,
Non-carbonate magnesium hardness is also removed from water by liming, provided that the water pH is not lower than 10.2 (at other water pH values, magnesium hydroxide does not precipitate):
MgSO4 + Ca(OH)2 = Mg(OH)2 + CaSO4
MgCl2 + Ca(OH)2 = Mg(OH)2 + CaCl2
The above equations show that magnesium hardness is eliminated, but the value of the total hardness remains unchanged, since magnesium hardness is replaced by calcium, non-carbonate hardness. Therefore, this method can only be used to soften water with a high value. carbonate hardness.
Elimination of temporary hardness by neutralizing bicarbonates with slaked lime is used extremely rarely, because a) fine sediments are difficult to settle, and particle enlargement is required; b) a large amount of finely dispersed organic substances prevents the formation of sediment.
2.2.Lime-soda
This method is used to simultaneously reduce carbonate and non-carbonate hardness when deep softening of water is not required.
The chemistry of the process is described by the reactions: MgS04 + Na2 CO3 = MgCO3↓ + Na2 SO4 CaCl2 + Na2 CO3 = CaCO3 + 2NaCl
(For reaction equations for eliminating carbonate hardness using lime, see above in paragraph 2.1.).
After adding reagents to water, the instantaneous formation of colloidal compounds CaCO3 and Mg(OH)2 occurs, but their transition from a colloidal state to a coarsely dispersed one, i.e. It takes a long time to reach the state in which they precipitate. Therefore, the lime-soda method is often combined with the thermal method. For example, this combination is used to soften water, which is used to power low-pressure boilers, to feed heating networks, etc.
The depth of water softening with the lime-soda method is correspondingly equal: without heating the water, the hardness is reduced to
1…2mEq/l;
when water is heated to 80...90o C, the hardness decreases to
0.2…0.4 meq/l.
2.3. Alkaline method.
This method of water softening is described by the following chemical reaction equations:
Ca(HCO3 )2 + 2NaOH = CaCO3 ↓ + Na2 CO3 + H2 O
Mg(HCO3 )2 + 2NaOH = Mg(OH)2 ↓ + Na2 CO3 + H2 O + CO2
CaSO4 + Na2 CO3 = CaCO3 ↓ + Na2 SO4
CaCl2 + Na2 CO3 = CaCO3 ↓ + 2NaCl
CO2 +NaOH = Na2CO3 + H2O
MgSO4 + 2NaOH = Mg(OH)2 ↓ + Na2 SO4
MgCl2 + 2NaOH = Mg(OH)2 ↓ + 2NaCl
From the given reaction equations it follows:
1) Sodium hydroxide (NaOH) in the process of water softening is used to eliminate carbonate hardness and neutralize carbon dioxide dissolved in water.
2) soda (Na 2 CO3), formed during the decomposition of bicarbonates and neutralization of carbon dioxide, is used to remove non-carbonate hardness.
The depth of water softening with the alkaline method is the same as with the lime-soda method, i.e. the value of residual hardness is almost 1 meq/l, and when heating softened water -
0.2…0.4 meq/l.
2.4. Phosphate method.
This method of water softening is the most effective reagent method. The chemistry of the process of water softening with sodium phosphate is described by the following reaction equations:
3CaS04 + 2Na3 P04 = Caz (PO4 )2 ↓ + Na2 SO4 3MgCl2 + 2Na3 PO4 = Mg3 (PO4 )2 ↓ + 6NaCl 3Ca(HCO3 )2 + 2Na3 PO4 = Ca3 (PO4 )2 ↓ + 6NaHCO3 3Mg(HCO3 )2 + 2Na3 PO4 = Mg3 (PO4 )2 ↓+ 6NaHCO3
As can be seen from the above reaction equations, the essence of the method is the formation of calcium and magnesium salts of phosphoric acid, which have low solubility in water and therefore precipitate quite completely.
Phosphate softening is usually carried out by heating water to 105...1500 C, achieving a reduction in hardness to 0.02...0.03 meq/l. Due to the high cost of sodium phosphate, the phosphate method is usually used to soften water previously softened with lime and soda. This method is used, for example, to prepare feed water for medium and high pressure boilers (588...980 MPa).
2.5.Barium method.
Water softening is based on the introduction of barium hydroxide or barium aluminate into it and the formation of practically insoluble compounds of calcium and magnesium, as well as barium sulfate. The chemistry of the process is described by the following reaction equations:
CaSO4 + Ba(OH)2 = Ca(OH)2 ↓ + BaSO4 ↓
CaCl2 + BaAl2 O4 = BaCl2 + CaAl2 O4 ↓
Ca(HCO3 )2 + BaAl2 O4 = CaAl2 O4 ↓ + BaCO3 ↓ + H2 O + CO2
(Similar reaction equations can be written for magnesium salts).
The barium method of water softening is very expensive, and barium salts are poisonous, so it is advisable to use it for partial desalination of water due to the extraction of sulfates.
Example 3. Water hardness is 5.4 meq of calcium ions in 1 liter of water. What amount of sodium phosphate Na3 P04 must be taken to reduce the hardness of 1 ton of water to almost zero.
Solution: We solve the problem using the formula
F = m / E V, (1)
where m is the mass of a substance that causes water hardness or is used to eliminate water hardness, g;
E is the equivalent mass of this substance; g/mol; V – volume of water, l.
E (Na3 PO4 ) = M (Na3 PO4 ) / n V, |
where n is the number of metal ions; B is the valence of the metal.
E(Na3PO4) = 164 / 3 =54.7 (g/mol)
From equation (1) we express the mass
m = F E V = 5.4 54.7 1000 = 295.38 (g) Answer: m = 295.38 g.
3. Ion exchange methods
The cation exchange method of water softening is based on the ability of some practically insoluble substances in water, called cation exchangers, to exchange the active groups of cations contained in them (sodium, hydrogen, etc.) for calcium or magnesium cations found in water.
Currently, ion exchange resins, which are produced on the basis of synthetic polymers, are widely used. Ion exchange resins are networked, three-dimensional polymers that are insoluble in water, but have limited swelling in it and contain groups capable of exchanging ions
The water to be softened is filtered through a layer of cation resin, while calcium and magnesium cations pass from the water into the cation resin, and into the water
Many people have heard about softening hard water and are trying to order a softener for their water treatment. Is this so important and necessary?
The physiological norm of hardness is specified in SanPiN 2.1.4.1116-02 for bottled water and is from 1.5 to 3.5 mmol/l. Household appliances require even softer water to prevent scale formation.
There are two types of hardness:
Carbonate (temporary)- called because it is eliminated by boiling.
Non-carbonate (permanent)- called because when boiling, hardness is not eliminated, but when evaporated, a light white, slightly soluble precipitate such as calcium or magnesium sulfate forms in the form of scale on the walls of the vessel. Salts MgCl2, CaCl2, MgSO4 contained in water with constant hardness cause corrosion of steel structures and accelerate the wear and tear of water heating and heating equipment. When hard water is used for water heating equipment and heating equipment, scale is formed from calcium and magnesium carbonates, gypsum and other salts. The formation of scale makes it difficult to heat water and causes an increase in electricity and fuel consumption.
In hard water, meat, vegetables, and cereals do not cook well, and tea does not brew well. When washing fabrics (as when washing your hair), the insoluble compounds formed are deposited on the surface of the threads and gradually destroy the fibers.
Water softening is the process of removing hardness cations from it, i.e. calcium and magnesium.
Thermal method is based on heating water to a temperature above its boiling point, distilling it or freezing it in order to eliminate calcium carbonate and magnesium carbonate. Due to the use of this method, the residual hardness of water is no more than 0.7 mmol/l. Therefore, the thermal method is used for technical needs, in particular when using water used to feed low-pressure boilers, as well as in combination with reagent methods.
When softening water reagent methods they use reagents that, when interacting with calcium and magnesium, form poorly soluble compounds with their subsequent separation in illuminators, thin-layer sedimentation tanks and lighting filters. Lime, soda ash, sodium and barium hydroxides and other substances are used as precipitating reagents. The choice of reagents depends on the quality of the source water and the conditions of its further use. When using reagent methods, the residual hardness of water will be up to 0.7 mg/l. In accordance with the recommendations of the “Building Codes and Rules” (SN and P), reagent methods are mainly used to soften surface water, when water clarification is also required.
Water softening based on different diffusion rates of these substances through a semi-permeable membrane, separating concentrated and dilute solutions. Water softening by dialysis is carried out in membrane devices with nitro- and cellulose acetate film membranes. As a result of using this method, the residual water hardness will be up to 0.01 mg/l and below. The negative side of the dialysis method is the high cost of membrane devices.
Magnetic water treatment- Commonly used to combat scale formation. The essence of the method is that when water crosses magnetic lines of force, scale formers are released not on the heating surface, but in the mass of water. The resulting loose sediments (sludge) are removed by blowing.
Received the greatest practical application ion exchange method water softening. The essence of the ion exchange method lies in the ability of ion exchange materials (ion exchangers) to absorb positive or negative ions from water in exchange for an equivalent amount of ion exchanger ions. Depending on the composition, there are mineral and organic cation exchangers, which, in turn, are divided into substances of natural and artificial origin. In water treatment technology, organic cation exchangers of artificial origin, so-called ion exchange resins, are widely used. The quality of ion exchange resins is characterized by their physical properties, chemical and thermal resistance, working capacity, etc. In water softening installations, it uses ion exchange resins based on the use of a cation exchange resin in the Na-form and an anion exchange resin in the Cl-form, i.e. uses the sodium-chlorine ionization method. This method consists of the following stages: sodium cationization and chlorine cationization. At the sodium cationization stage, calcium and magnesium ions, which give water hardness, are replaced with sodium ions.
As a result, the treated water is softened, and calcium and magnesium form an insoluble polymer. When sodium-cationized water is passed through a chlorine-anoion, exchange reactions of anions contained in Na-cationized water for chlorine ions occur and the alkalinity of the treated water decreases. To restore the properties of the ion exchange resin (regeneration), a solution of table salt is used. Thus, deep softening of water is achieved (up to 0.03 ... 0.05 mmol/l). When using the sodium-chlorine ionization method, only one reagent is consumed - table salt, no corrosion protection of equipment, pipelines and special fittings is required, the amount of equipment is reduced, and control of the operation and operation of the water softening unit is simplified. The result is increased reliability and reduced cost of the water softener. Just drink this softened one all the time
Water softening refers to the process of removing hardness cations from it, i.e. Ca and Mg. Water softening is carried out using the following methods:
1) thermal softening, based on heating water, distilling it or freezing it;
2) reagent, in which hardness ions present in water are bound by various reagents into practically insoluble compounds;
3) ion exchange, based on filtering softened water through special materials that exchange sodium or hydrogen ions included in their composition for calcium and magnesium cations;
4) dialysis;
5) combined, representing various combinations of the listed methods.
The choice of water softening method is determined by its quality, the required depth of softening and technical and economic considerations.
Thermal method of water softening.
It is advisable to use when using carbonate waters used to feed low-pressure boilers, as well as in combination with reagent methods of water softening. It is based on a shift in carbon dioxide equilibrium when water is heated towards the formation of calcium carbonate
Ca(HCO 3) 2 → CaCO 3 ↓+CO 2 + H 2 O
The equilibrium is shifted due to a decrease in the solubility of CO 2 caused by an increase in temperature and pressure. Boiling can completely remove CO 2 and thereby significantly reduce carbonate hardness. In addition, the hardness determined by calcium sulfate is reduced. However, it is not possible to completely remove this hardness, since calcium carbonate is still soluble in water (18 mg/l). A thermal softener is used for this method. The residence time of water in it is 30-45 minutes.
Reagent softening methods.
They are based on the treatment of water with reagents that form poorly soluble compounds Mg(OH) 2, CaCO 3, Ca 3 (PO 4) 2 and others with calcium and magnesium, followed by their separation in clarifiers. Lime, soda ash, sodium and barium hydroxides and other substances are used as reagents.
Water softening by liming is used for high carbonate and low non-carbonate hardness. Lime is used as a reagent, which is introduced in the form of a suspension into preheated water. When dissolved, lime enriches water with OH - and Ca +2 ions, which leads to the binding of water-soluble CO 2 to form CO 3 -2 and the transition of HCO 3 to CO 2.
CO 2 + 2 OH - →CO 3 -2 + H 2 O; HCO3 - +OH - → CO 3 –2 + H 2 O
An increase in the concentration of CO 3 –2 in the treated water and the presence of Ca + 2 ions in it, taking into account those introduced with lime, leads to the precipitation of CaCO 3
Ca +2 + CO 3 –2 → CaCO 3 ↓.
To speed up the process, coagulation is used simultaneously with liming.
The dose of lime is determined by the formula:
D i = 28([CO 2 ] /22 +2 F k - [Ca +2 ]/20 + D k /e k + 0.5)
D k – dose of coagulant, e – equivalent mass of the active substance of the coagulant,
The expression D k / e k - is taken with the sign - if the coagulant is introduced before the lime and + if together or after.
Deeper softening of water can be achieved by heating it, adding an excess reagent - a precipitant, and creating contact between the softened water and the previously formed sediment.
Phosphating is used to soften water. Residual hardness is reduced to 0.02-0.03 mg*eq/l. Phosphating also achieves greater stability of water, reduces its corrosive effect on metal pipelines and prevents carbonate deposits on the inner surface of pipe walls. Sodium hexametaphosphate and sodium tripolyphosphate are used as a phosphating reagent. The phosphate softening method using trisodium phosphate is the most effective reagent method. The chemistry of the process is described by the equation:
3Ca(HCO 3) 2 /3 Mg(HCO 3) 2 + 2 Na 3 PO 4 = Ca 3 (PO 4) 2 / Mg 3 (PO 4) 2 +6 NaHCO 3.
Phosphate softening is carried out by heating water to 105–150 0 C. The resulting precipitates Ca 3 (PO 4) 2 and Mg 3 (PO 4) 2 well adsorb colloids and silicic acid from softened water, so this method is used to prepare feed water for boilers medium and high pressure.
Water softening by dialysis.
Dialysis is a method of separating solutes that differ significantly in molecular weight. It is based on different rates of diffusion of these substances through a semi-permeable membrane that separates concentrated and dilute solutions. Dialysis is carried out in membrane devices with nitro- and cellulose acetate membranes. The effectiveness of a semi-permeable membrane is determined by the high values of selectivity and water permeability, which it must maintain over a long period of operation.
Magnetic water treatment.
Currently, magnetic water treatment is successfully used to combat scale formation and encrustation. Its essence lies in the action of a magnetic field on ions of salts soluble in water. Under the influence of a magnetic field, polarization and deformation of ions occurs, accompanied by a decrease in their hydration, increasing the likelihood of their approach and the formation of crystallization centers. The essence of the method is that when water crosses magnetic lines of force, scale formers are released not on the heating surface, but in the mass of water. The resulting loose sediments are removed by blowing.
Water softening by cationization.
The essence of ion exchange lies in the ability of ion exchangers to absorb positive and negative ions from water in exchange for an equivalent amount of ion exchanger ions. The process of water treatment using the ion exchange method, which results in the exchange of cations, is called cationization.
Cation exchangers swell in water and increase in volume. The energy of entry of various cations into the cation exchanger according to the magnitude of their dynamic activity can be characterized by the following series:
Na< NН 4+ < К + < Мg +2 < Са +2 < Аl +3 E p = (Q* F i)/(a*h к), where Ж and – water hardness; Q – amount of softened water, m3; a – area of the cation exchange filter, m2; h k – height of the cation exchanger layer, m. The duration of the filter operation is determined by the formula: T k = E r * h k / V k * F i. where Vk is the water filtration rate. Organic cation exchangers are used in water treatment technology. They contain functional chemical active groups, H + of which can be replaced by other cations: quaternary amines NH 3 OH, sulfo groups HSO 3, carboxyl groups COOH. The HSO 3 group has strong acidic properties, and COOH has weak acidic properties. Depending on the content of functional groups, cation exchangers are divided into weakly acidic and strongly acidic. Strong acids exchange cations in alkaline, neutral and acidic environments, weak acids exchange cations only in alkaline environments. The quality of cation exchangers is characterized by their physical properties, chemical and thermal resistance, and working exchange capacity. The fractional composition characterizes the operational properties of the cation exchanger. The working exchange capacity depends on the type of cations being extracted, the ratio of salts in softened water, pH, the height of the cation exchanger layer, filter volume, operating mode, and specific consumption of the regenerating reagent. Sodium cationization. This method is used to soften water with a suspended solids content n/b 8 mg/l and color n/b 30 0. Water hardness is reduced with single-stage cationization to 0.05–0.1, with two-stage cationization – to 0.01 mg*eq/l. The sodium cationization process is described by the following equations: 2 Na[K] + Ca(HCO 3) 2 / Mg(HCO 3) 2 ↔Ca[K] 2 / Mg[K] 2 +2 NaHCO 3 2 Na[K] + CaCl 2 / Mg Cl 2 ↔Ca[K] 2 / Mg[K] 2 + 2 NaCl, where [K] is the insoluble polymer matrix. After the working exchange capacity of the cation exchanger is depleted, it loses its ability to soften water and must be regenerated. The process of water softening using cation exchange filters consists of the following operations: Filtering water through a layer of cation exchange resin until the maximum permissible hardness in the filtrate is reached; Loosening the cation exchanger layer with an ascending flow of water; Draining the water cushion to avoid dilution of the regeneration solution; Regeneration of cation resin by filtering the appropriate solution; Washing the cation exchanger. The choice of method is dictated by the requirements for softened water, the properties of the source water and technical and economic considerations. Regeneration is carried out with a 5% sodium chloride solution in the amount of 1.2 m 3 solution per 1 m 3 of resin, then the residual amount in the form of an 8% solution. The regeneration process is described by the following reaction: Ca[K] 2 / Mg[K] 2 + 2 NaCl↔2 Na[K] + CaCl 2 / Mg Cl 2 Sodium chloride is used because of its availability, low cost, and also because it produces highly soluble salts CaCl 2 and MgCl 2, which are easily removed with the regeneration solution and water. Hydrogen-sodium cationite water softening. Treatment of water by H-cationization is based on filtering it through a layer of cation exchanger containing hydrogen as exchange ions. 2 H[K] + Ca(HCO 3) 2 / Mg(HCO 3) 2 ↔Ca[K] 2 / Mg[K] 2 +2H 2 O +CO 2 2 H[K] + NaCl↔2 Na[K] + HCl; 2 Н[К] + Na 2 SO 4 ↔2 Na[К] + Н 2 SO 4 During H-cationization of water, its pH significantly decreases due to acids formed in the filtrate. The CO2 released during H-cationization can be removed by degassing and mineral acids will remain in the solution in quantities equivalent to the content of SO 4 -2 and Cl - in the source water. From the above reactions it is clear that the alkalinity of water does not change during ion exchange. Therefore, by proportionally mixing the acidic filtrate after H-cation exchange filters with the alkaline filtrate after Na-cation exchange filters, you can obtain softened water with different alkalinity. This is the essence and advantages of H-Na – cationization. Parallel, sequential and mixed Н-Nа – cationization are used. In parallel, 1 part of the water goes through the Na-cation exchange filter, the other through the H-cation exchange filter. The resulting waters are mixed in such proportions that the alkalinity does not exceed 0.4 mg*eq/l. With sequential filtering, part of the water is passed through the N-cation exchange filter, then mixed with the rest of the water and fed to the Na-cation exchange filter. This makes it possible to more fully utilize the exchange capacity of the H-cation exchanger and reduce acid consumption for regeneration. Mixed cationization is carried out in one filter, loaded at the top with H-cation exchanger, and at the bottom with Na-cation exchanger. A high level of hardness provokes the formation of scale and impairs the effectiveness of detergents. In such unfavorable conditions, the risk of damage to the functional components of heating equipment and other equipment increases. Operating costs and the costs of complying with sanitary and hygienic rules are increasing. Modern manufacturers offer different water softening methods and corresponding equipment sets. Choosing the best option will not be difficult after reading this publication. There is useful information here that will help you implement the project inexpensively and quickly. The overall level of rigidity is determined as the sum of the permanent and temporary components. As a rule, the first part has little practical significance, so it can be excluded from the review. The second is determined by the concentration of magnesium and calcium cations. When heated, these chemicals are converted into an insoluble sediment called scale. They are the ones who clog technical ducts, which is accompanied by a deterioration in boiler performance. Such formations are characterized by porosity and low thermal conductivity. When accumulated on the surface of the heating element, this layer blocks normal heat removal. If you do not use an effective method for softening hard water, your washing machine or other equipment with a heating element will be damaged due to scale. In practice, they solve the problems of reducing the level of rigidity, or completely eliminating harmful phenomena. The second option is better! It involves reliable protection of expensive products, effective prevention and prevention of emergency situations. The principle of operation of these methods of water softening is clear from the general definition. Every person knows that when boiling (heating) a layer of scale actively forms on the walls of the kettle. Once the procedure is completed, the stiffness will be reduced. The theoretical simplicity of the method is the only advantage. A detailed study of the issue reveals the following shortcomings: It should be remembered that at the finishing stage it is necessary to remove stubborn scale. These are labor-intensive work operations that can ruin the working container. From the above descriptions we can draw an intermediate conclusion. Removing harmful compounds using chemicals, ion exchange, boiling and membrane filtration requires complex engineering challenges. This will be written below. Costs increase accordingly. Polyphosphate compounds are more effective. They are inexpensive, but reliably block the negative process. The method can be considered ideal if it were not for contamination of the liquid. Electromagnetic processing technology does not have these disadvantages. Exposure to a strong field changes the shape of scale particles. The created needle-like protrusions do not allow them to combine into large fractions. This blocks the process of scale formation. To obtain a field of optimal power and configuration, a high-frequency generator of electromagnetic oscillations is used. It works according to a special algorithm that does not cause an “addictive” effect. A decrease in the positive effect is observed when working with permanent magnets. When studying current market offers, you should pay attention to modern high-quality models of electromagnetic water treatment devices: In any case, you need to choose a manufacturer who has solid experience in the relevant field of activity! A technique well known to specialized specialists is adding slaked lime to the solution. Chemical reactions bind calcium and magnesium molecules with the subsequent formation of an insoluble precipitate. As it accumulates at the bottom of the working tank, it is removed. Small suspended particles are retained through the phosphate method. A similar technology is used to reduce the non-carbonate component using soda. The main disadvantage of this and other methods in this category is the contamination of the liquid with chemicals. In order for such processing to be safe, optimal dosages must be strictly observed and all important stages must be carefully monitored. High-quality reproduction of the technology at home is not possible without excessive difficulties and costs. It is used at municipal and collective water treatment stations of the professional category. However, one “chemical” technique has become popular in everyday life. Researchers have discovered that polyphosphate compounds form shells around tiny insoluble fractions. They prevent them from combining into large particles and attaching to pipe walls and external surfaces of heating devices. Manufacturers of phosphate washing powders take advantage of this useful property. Specialized flow-through containers are also used in which polyphosphate salts are placed. The devices are mounted on the inlet pipe in front of boilers and washing machines. The method is not suitable for preparing drinking water. Is the problem solved? One should not make hasty conclusions. The filtration technique is really good, but only for processing 180-220 liters/day. This is the performance of the series at a reasonable cost. This amount is not enough for a single shower or to satisfy other household needs. To increase productivity, several membranes are installed in parallel. To operate the kit, you have to increase the pressure with a special pumping station. Such water filtration equipment is expensive and takes up a lot of space. The benefits are given in the following list: It is necessary to emphasize the availability of the regeneration mixture. This is an inexpensive solution of ordinary table salt (good purity). As before, here are the nuances that deserve mention for a full analysis of water softening using the ion exchange method: Treatment with vibrations of the corresponding frequency range is used to reduce the level of rigidity. At the same time, the layer of old scale is destroyed, which is useful for cleaning pipes without aggressive chemical compounds. Ultrasound is used with professional precautions to clean and protect industrial equipment. Large elements of these structures and threaded connections have better resistance to strong vibration effects. The optimal method is selected taking into account the actual conditions of future operation. Experienced specialists advise creating a general project with mechanical and other filters to accurately coordinate all functional components. In a city apartment you can count on maintaining acceptable quality of hard water. The corresponding obligations are specified in the contract with the supplying organization. However, at home, accidents on highways and pressure surges cannot be ruled out. To protect against these negative influences, a phosphate or mechanical filter with a pressure regulator and control pressure gauges is installed at the inlet. It is necessary to emphasize the advantages of the electromagnetic converter, taking into account the features of objects in this category: For autonomous suburban water supply, prudent owners prefer to use an artesian well. This source provides a high degree of purification through natural filtration. But at great depths, the concentration of impurities washed out of rocks increases. Among them are salt compounds in fairly high concentrations. In a private home it is easier to find free space for technological equipment. Here you can install kits for water softening using the ion exchange method. The necessary engineering networks are installed in the premises. We must not forget about good insulation. It is necessary to maintain the temperature regime set by the manufacturer. Chlorine and other chemical compounds that could damage the existing backfill should be removed. Softening water means removing calcium and magnesium from it. The total hardness of water supplied by water pipes for household and drinking needs should not exceed 7 mEq/dm3, and in special cases, in agreement with the sanitary and epidemiological service authorities, no more than 10 mEq/dm3. The hardness level of steam generator feed water can reach 0.05 mEq/dm3. Depending on the quality of the source water and the desired effect of reducing hardness, reagent, thermochemical, ion exchange softening methods or various combinations of them are used. Reagent softening. Reagent methods are based on the ability of Ca2+ and Mg2+ cations to form insoluble and slightly soluble compounds when treating water with reagents. The most commonly used reagents are lime and soda. Decarbonization of water only by liming is used in cases where a simultaneous reduction in water hardness and alkalinity is required. Lime, together with soda, is used to soften water, which contains calcium and magnesium in combination with anions of strong acids. The theoretical limit of water softening is determined by the solubility of calcium carbonate and magnesium hydroxide. The solubility of calcium carbonate in a monosolution at a temperature of 0°C is 0.15 mEq/dm3, and at a temperature of 80°C - 0.03 mEq/dm3; for magnesium hydroxide - 0.4 and 0.2 mEq/dm3, respectively. Both CaCO3 and Mg(OH)2 have the ability to form supersaturated solutions, which only very slowly approach an equilibrium state even when in contact with the solid phase of the resulting precipitate. In practice, it is not advisable to keep water in water softeners for a long time until an equilibrium state occurs. Therefore, water softened by liming (if the hardness is all carbonate) or the lime-soda method usually has a residual hardness of at least 0.5-1 mEq/dm3. The depth of softening depends on the presence in the treated water of an excess of precipitated ions and precipitating reagents. So, at 40°C, the salt content of water is up to 800 mg/dm3, the presence of Ca2+ ions in it in an amount of 0.7-1.0; 1-3 and > 3 mEq/dm3, residual carbonate hardness in the absence of crystallization retarders usually does not exceed 0.5-0.8; 0.6-0.7 and 0.5-0.6 mg-eq/dm3, respectively, and< 1,2; Щгидр < 0,4 и Жо6щ < 1,0 мг-экв/дм3. При солесодержании 800-2000 мг/дм3 Щ0бЩ = 2,0-2,2 мг-экв/дм3, Щгидр < 0,5-0,8 мг-экв/дм3 и Жобщ < 2,0 мг-экв/дм3. Здесь в подстрочнике «общ» и «гидр» обозначают соответственно «общая» и «гидратная». It should be noted that water softened by liming or the lime-soda method is usually supersaturated with calcium carbonate and has a very high pH. Therefore, to increase the accuracy of reagent dosing, it is necessary, in addition to automatic control in proportion to the flow rate of the treated water, to adjust the dose also according to pH. It is also possible to adjust the dose depending on the electrical conductivity of the treated water, if the content of SO^, SG and NO3 is stable and low. With small fluctuations in the dosage of lime, Mg2+ plays a buffering role: with an increase in the dosage of lime, the amount of Mg2+ transferred to the sediment increases (thereby worsening its properties), while maintaining the alkalinity of the softened water at an approximately constant level. The softening process is controlled by the pH value, which should be > 10 due to the need to remove Mg2+ from water, or, less accurately, by the value of hydrate alkalinity, calculated based on titration of water samples with acid in the presence of phenolphthalein and methyl orange indicators. It should be noted that the process of reagent water softening can be monitored by its electrical conductivity. When lime is added to water and bicarbonates transform into carbonates that precipitate, the electrical conductivity of the treated water changes. In accordance with the conductometric titration curve, at the moment of complete neutralization of carbonate hardness salts, the electrical conductivity reaches a minimum value. With a further increase in reagent additions, the electrical conductivity increases due to the excess of the reagent. Thus, the optimal dose of lime milk introduced into softened water is characterized by the minimum value of the water’s electrical conductivity. With increasing water temperature, chemical reactions and crystallization of CaCO3 and Mg(OH)2 sediments accelerate. Temperature fluctuations worsen deposition conditions. Coagulation improves the precipitation of CaCO3 + Mg(OH)2. Due to the high pH of the softener, only iron-based coagulants and sodium aluminate are used. For 1 mole of FeS04, 4 mg of 02 is required in water. Air entering the clarifier leads to sedimentation and removal of sediment with the softened water. The supersaturation of water with air can be determined by determining the oxygen content in the water after the air separator using an iodometric method and comparing the results obtained with the tabulated ones for given temperatures. Thermochemical softening consists of heating water above 100°C and using lime and soda, less often sodium hydroxide and soda. As a result of thermochemical softening, calcium hardness can be reduced to 0.2 mEq/dm3, and magnesium hardness to 0.1 mEq/dm3. The thermochemical method is often combined with phosphate softening of water. Di- or trisodium phosphate is used as phosphate reagents. As a result of phosphate softening, it is possible to obtain water with a residual hardness of 0.04-0.05 mEq/dm3. Sulfate hardness is removed with barium carbonate, hydroxide or barium aluminate. Appropriate analytical controls are required to ensure that the water softening processes described above are carried out correctly. Recommended tests and frequency of their performance are given in table. 1.7. The following rules can serve as a useful guide to ensure a good softening effect: 1) hydrate alkalinity should exceed magnesian hardness by approximately 0.4 mEq/dm3 in an unheated process and by 0.2 mEq/dm3 in a heated process; 2) carbonate alkalinity should exceed calcium hardness by approximately 1.2 mEq/dm3 in an unheated process and by approximately 0.8 mEq/dm3 in a heated process. Since some poorly soluble salts may precipitate during long-term storage, and NaOH turns into Na2C03, you should not use data from average samples of softened water. Also, due to the presence of leaks of the CaCO3 and Mg(OH)2 suspension into softened water, it must be additionally filtered through crushed anthracite. In this case, quartz sand is an undesirable material due to the fact that it can enrich the water with silicic acid compounds. Ionite softening. It is carried out mainly using Na+-, H+- and NHj-forms. In the process of water softening by Na-cation, the calcium and magnesium content in water can be reduced to very small values. The total alkalinity will not change, the dry residue increases slightly as a result of replacing one calcium ion in water, having a molecular weight of 40.08, with two sodium ions (mass 2 x 22.99 = 45.98). Water Water quality indicators Frequency of analyzes Mandatory Additional Original Free carbon dioxide, total hardness, calcium, magnesium, total alkalinity Sulfates, solids, pH, silicon, chlorides At least once a week, and hardness and alkalinity - daily Softened Lime-soda softening Total hardness, pH, total and phenolphthalein alkalinity, suspended solids Sulfates, dry residue, calcium, magnesium, silicon. aluminum, chlorides For periodic devices - with each new dose of reagents; for continuous devices - daily, although more frequent analysis may be required if the quality of the source water changes significantly Heated phosphate softening Total hardness, phenolphthalein alkalinity, excess phosphates When filtering through a cation exchange resin in the H-form, all cations of dissolved salts (including cations of hardness salts) will be sorbed on its grains; an equivalent amount of H+ ions will pass into the water; salts dissolved in water will be converted into the corresponding acids. The acidity of water passing through an H-cation exchanger filter, which is loaded with a strong basic cation exchanger, will be equal to the sum of the concentrations of strong acid salts in the source water. Regeneration of H-cation exchanger filters with acid in an amount insufficient to completely displace hardness cations from the cation exchanger (“hungry” regeneration), allows in the operating cycle to reduce the alkalinity of water to 0.4-0.5 mEq/dm3, without reducing its non-carbonate hardness . If the presence of sodium and potassium carbonates is not allowed in softened water, but the presence of ammonium ions is allowed, then instead of H-Na cationization, NH4-Na-Ka ionization can be used. Water softened by cationization turns out to be more corrosive than the original one, due to the complete absence of calcium bicarbonate in it, which, under certain conditions, can form a protective layer of calcium carbonate on the surface of the metal in contact with water. When monitoring the quality of filtrate from cation exchange plants, special attention is paid to determining indicators that are in one way or another related to the concept of water hardness and alkalinity: total and carbonate hardness, carbonate and hydrate alkalinity, the content of calcium and magnesium salts, total salt content, pH value, anion content. During the operation of cation exchangers, it is additionally necessary to periodically check the absorption or removal of organic substances from them by the filtrate. Water desalination refers to the process of reducing the salts dissolved in it to the required value. A distinction is made between partial and complete desalting. A special case of water desalination is desalination, as a result of which the salt content in purified water does not exceed 1000 mg/dm3 - the maximum permissible concentration of all salts in drinking water. The most common methods of water desalination include ion exchange, electrodialysis, reverse osmosis and distillation. Desalting allows you to almost completely remove from water substances that can completely or partially dissociate (for example, salts and silicic acid); Non-electrolytes may remain in the water. Sometimes there is also a slight decrease in color associated with the absorption of acidic organic substances by ion exchangers and membranes. Since desalting removes those substances that conduct electrical substances, an indicator of the quality of treated water is usually its electrical conductivity, expressed in µS/cm. The calculated value of this parameter at 18°C in “ultrapure” water is 0.037 µS/cm. However, under production conditions it is still possible to obtain “ultrapure” water with a specific electrical conductivity of 0.1 - 1.0 µS/cm. The main criterion for assessing the quality of water treatment and the ion-exchange capacity of filters is often taken to be the electrical conductivity of water, the threshold value of which is established based on experimental data. For example, the electrical conductivity of water after a cation exchanger should be less than 240, after a weakly basic anion exchanger - 50-220 and after a strongly basic anion exchanger< 20 мкСм/см. Превышение этих значений указывает на истощение ионообменных смол до контрольного уровня и на необходимость их регенерации. Since existing drinking water quality standards mostly regulate the maximum permissible concentrations of macro- and microcomponents of its composition, desalinated water generally meets current regulatory requirements. However, in connection with the ever-expanding involvement of desalinated waters in centralized drinking water supply systems, there is a need for additional standardization of the minimum required concentrations of the most important hygienic quality indicators: calcium content, bicarbonates, total salt content, sodium, potassium, etc. As modern medical and physiological studies show research, insufficient content of hardness salts in desalinated water (less than 1.5 mEq/dm3) can lead to metabolic disorders and cardiovascular diseases in the body of people who drink such soft water for a long time.Basic definitions
Method 1: Heat
Method 2: Electromagnetic field treatment
Chemical methods of water softening
Filtration
The desired effect can be obtained by reducing the size of the cells to the size of molecules. Such microscopic ducts are created in reverse osmosis membranes. They are capable of passing only clean water. The contaminated liquid accumulates in front of the barrier and is removed into the drain.Water softening using ion exchange method
Reduce primary and operating costs with the help of equipment in this category. A special backfill is used that retains calcium and magnesium ions. At the same time, the liquid is filled with harmless sodium compounds.Ultrasonic exposure
Which water softening methods are suitable for different properties?
