Introduction

You came to the store, trying to buy unscented soap. Naturally, in order to understand which products from this range have a smell and which do not, you pick up each bottle of soap and read its composition and properties. Finally, we chose the right one, but while looking at various soap compositions, we noticed a strange trend - on almost all the bottles it was written: “The structure of the soap contains sodium hydroxide.” This is the standard story of most people's introduction to sodium hydroxide. Some half of the people will “spit and forget,” and some will want to know more about him. It is for them that today I will tell you what this substance is.

Definition

Sodium hydroxide (formula NaOH) is the most common alkali in the world. For reference: alkali is a base that is highly soluble in water.

Name

In different sources it can be called sodium hydroxide, caustic soda, caustic soda, caustic soda or caustic alkali. Although the name "caustic alkali" can be applied to all substances in this group. Only in the 18th century were they given separate names. There is also an “inverted” name for the substance now described - sodium hydroxide, usually used in Ukrainian translations.

Properties

As I already said, sodium hydroxide is highly soluble in water. If you put even a small piece of it in a glass of water, after a few seconds it will ignite and hissingly “scurry” and “jump” across its surface (photo). And this will continue until he completely dissolves in it. If, after the reaction is complete, you dip your hand into the resulting solution, it will be soapy to the touch. To find out how strong the alkali is, indicators - phenolphthalein or methyl orange - are dipped into it. Phenolphthalein in it becomes crimson in color, and methyl orange turns yellow. Sodium hydroxide, like all alkalis, contains hydroxide ions. The more of them in the solution, the brighter the color of the indicators and the stronger the alkali.

Receipt

There are two ways to obtain sodium hydroxide: chemical and electrochemical. Let's take a closer look at each of them.

Application

Delignification of cellulose, production of cardboard, paper, fiberboard and man-made fibers cannot be done without sodium hydroxide. And when it reacts with fats, soap, shampoos and other detergents are obtained. In chemistry, it is used as a reagent or catalyst in many reactions. Sodium hydroxide is also known as food additive E524. And this is not all the areas of its application.

Conclusion

Now you know everything about sodium hydroxide. As you can see, it brings very great benefits to people - both in industry and in everyday life.

Chemical methods for producing sodium hydroxide include lime and ferrite.

Chemical methods for producing sodium hydroxide have significant disadvantages: a lot of energy carriers are consumed, and the resulting caustic soda is heavily contaminated with impurities.

Today, these methods have been almost completely replaced by electrochemical production methods.

Lime method

The lime method for producing sodium hydroxide involves reacting a solution of soda with slaked lime at a temperature of about 80 °C. This process is called causticization; it goes through the reaction:

Na 2 CO 3 + Ca(OH) 2 = 2NaOH + CaCO 3

The reaction results in a solution of sodium hydroxide and a precipitate of calcium carbonate. Calcium carbonate is separated from the solution, which is evaporated to obtain a molten product containing about 92% wt. NaOH. NaOH is then melted and poured into iron drums, where it solidifies.

Ferrite method

The ferrite method for producing sodium hydroxide consists of two stages:

Na 2 CO 3 +Fe 2 ABOUT 3 = 2NaFeO 2 + CO 2

2NaFeО 2 + xH 2 O = 2NaOH + Fe 2 O 3 *xH 2 ABOUT

Reaction 1 is a process of sintering soda ash with iron oxide at a temperature of 1100-1200 °C. In addition, sintered sodium ferrite is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction 2; a solution of sodium hydroxide and a precipitate of Fe 2 O 3 *xH 2 O are obtained, which, after separating it from the solution, is returned to the process. The resulting alkali solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% of the mass. NaOH, and then a solid product is obtained in the form of granules or flakes.

Electrochemical methods for producing sodium hydroxide

Electrochemically sodium hydroxide is obtained electrolysis of halite solutions(a mineral consisting mainly of sodium chloride NaCl) with the simultaneous production of hydrogen chloride. This process can be represented by the summary formula:

2NaCl + 2H 2 About ±2е - → H 2 +Cl 2 + 2NaOH

Caustic alkali and chlorine are produced by three electrochemical methods. Two of them are electrolysis with a solid cathode (diaphragm and membrane methods), the third is electrolysis with a liquid mercury cathode (mercury method).

In world production practice, all three methods for producing chlorine and caustic soda are used, with a clear tendency to increase the share of membrane electrolysis.

7. Purification of sulfur dioxide from catalytic poisons.

Gaseous emissions have a very adverse effect on the environmental situation in the locations of these industrial enterprises, and also worsen sanitary and hygienic working conditions. Aggressive mass emissions include nitrogen oxides, hydrogen sulfide, sulfur dioxide, carbon dioxide and many other gases.

For example, nitric acid, sulfuric acid and other plants in our country annually emit tens of millions of cubic meters of nitrogen oxides, which are a strong and dangerous poison, into the atmosphere. Thousands of tons of nitric acid could be produced from these nitrogen oxides.

An equally important task is the purification of gases from sulfur dioxide. The total amount of sulfur that is emitted into the atmosphere in our country only in the form of sulfur dioxide is about 16 million tons . in year. From this amount of sulfur, up to 40 million tons of sulfuric acid can be produced.

A significant amount of sulfur, mainly in the form of hydrogen sulfide, is contained in coke oven gas.

Several billion cubic meters of carbon dioxide are released into the atmosphere every year with flue gases from factory chimneys and power plants. This gas can be used to produce effective carbon-based fertilizers.

The above examples show what enormous material values ​​are released into the atmosphere through gaseous emissions.

But these emissions cause more serious damage in that they poison the air in cities and enterprises: toxic gases destroy vegetation, have an extremely harmful effect on the health of people and animals, destroy metal structures and corrode equipment.

Although in last years Domestic industrial enterprises are not operating at full capacity, but the problem of combating harmful emissions is very acute. And given the general environmental situation on the planet, it is necessary to take the most urgent and most radical measures to clean up emission gases from harmful impurities.

Catalytic poisons

contact poisons, substances that cause “poisoning” of catalysts (See. Catalysts) (usually heterogeneous), i.e., reducing their catalytic activity or completely stopping the catalytic effect. Poisoning of heterogeneous catalysts occurs as a result of adsorption of the poison or the product of its chemical transformation on the surface of the catalyst. Poisoning may be reversible or irreversible. Thus, in the reaction of ammonia synthesis on an iron catalyst, oxygen and its compounds poison Fe reversibly; in this case, when exposed to a pure mixture of N 2 + H 2, the surface of the catalyst is freed from oxygen and poisoning is reduced. Sulfur compounds poison Fe irreversibly; the action of a pure mixture cannot restore the activity of the catalyst. To prevent poisoning, the reacting mixture entering the catalyst is thoroughly cleaned. Among the most common K. i. for metal catalysts include substances containing oxygen (H 2 O, CO, CO 2), sulfur (H 2 S, CS 2, C 2 H 2 SH, etc.), Se, Te, N, P, As, Sb, as well as unsaturated hydrocarbons (C 2 H 4, C 2 H 2) and metal ions (Cu 2+, Sn 2+, Hg 2+, Fe 2+, Co 2+, Ni 2+). Acid catalysts are usually poisoned by base impurities, and basic catalysts by acid impurities.

8. Obtaining nitrous gases.

The nitrogen oxides released after bleaching are condensed in water and brine condensers and used to prepare the raw mixture. Since the boiling point of N 2 O 4 is 20.6 ° C at a pressure of 0.1 MPa, under these conditions, gaseous NO 2 can be completely condensed (saturated vapor pressure of N 2 O 4 at 21.5 ° C above liquid N 2 O 4 equal to 0.098 MPa, i.e. less than atmospheric). Another way to obtain liquid nitrogen oxides is to condense them under pressure and at low temperatures. If we recall that during contact oxidation of NH 3 at atmospheric pressure, the concentration of nitrogen oxides is no more than 11% by volume, their partial pressure corresponds to 83.5 mm Hg. The pressure of nitrogen oxides above the liquid (vapour elasticity) at the condensation temperature (–10 °C) is equal to 152 mm Hg. This means that without increasing the condensation pressure, liquid nitrogen oxides cannot be obtained from these gases; therefore, the condensation of nitrogen oxides from such nitrous gas at a temperature of –10 ° C begins at a pressure of 0.327 MPa. The degree of condensation increases sharply with increasing pressure up to 1.96 MPa; with a further increase in pressure, the degree of condensation changes slightly.

Processing of nitrous gas (i.e. after conversion of NH 3) into liquid nitrogen oxides is ineffective, because even at P = 2.94 MPa, the degree of condensation is 68.3%.

In conditions of condensation of pure N 2 O 4, cooling should not be carried out below a temperature of –10 ° C, because at –10.8 °C N 2 O 4 crystallizes. The presence of impurities NO, NO 2, H 2 O reduces the crystallization temperature. Thus, a mixture with the composition N 2 O 4 +5% N 2 O 3 crystallizes at –15.8 °C.

The resulting liquid nitrogen oxides are stored in steel tanks.

9. Preparation of simple and double superphosphate

"Superphosphate" is a mixture of Ca(H 2 PO 4) 2 *H 2 O and CaSO 4. The most common simple mineral phosphorus fertilizer. Phosphorus in superphosphate is present mainly in the form of monocalcium phosphate and free phosphoric acid. The fertilizer contains gypsum and other impurities (iron and aluminum phosphates, silica, fluorine compounds, etc.). Simple superphosphate is obtained from phosphorites by treating them with sulfuric acid according to the reaction:

Ca 3 (RO 4 ) 2 + 2H 2 SO 4 = Ca(H 2 P.O. 4 ) 2 + 2CaSO 4 .

Simple superphosphate- gray powder, almost non-caking, moderately dispersed; in the fertilizer there is 14-19.5% P 2 O 5 assimilated by plants. The essence of the production of simple superphosphate is the transformation of natural fluorapatite, insoluble in water and soil solutions, into soluble compounds, mainly monocalcium phosphate Ca(H 2 PO 4) 2. The decomposition process can be represented by the following summary equation:

2Ca 5 F(PO 4) 3 +7H 2 SO 4 +3H 2 O=3Ca(H 2 PO 4) 2 *H 2 O]+7+2HF; (1) ΔН= - 227.4 kJ.

In practice, during the production of simple superphosphate, decomposition occurs in two stages. In the first stage, about 70% of apatite reacts with sulfuric acid. This produces phosphoric acid and calcium sulfate hemihydrate:

Ca 5 F(PO 4) 3 +5H 2 SO 4 +2.5H 2 O = 5(CaSO 4 *0.5H 2 O) +3H3PO 4 +HF (2)

The functional diagram for the production of simple superphosphate is shown in Fig. The main processes take place in the first three stages: mixing of raw materials, formation and hardening of superphosphate pulp, ripening of superphosphate in the warehouse.

Rice. Functional diagram of the production of simple superphosphate

To obtain a commercial product of higher quality, superphosphate, after ripening, is neutralized with solid additives (limestone, phosphate rock, etc.) and granulated.

Double superphosphate- concentrated phosphorus fertilizer. The main phosphorus-containing component is calcium dihydrogen orthophosphate monohydrate Ca(H 2 PO 4) 2 H 2 O. It usually also contains other calcium and magnesium phosphates. Compared to simple phosphate, it does not contain ballast - CaSO 4. The main advantage of double superphosphate is the small amount of ballast, that is, it reduces transportation costs, storage costs, and packaging

Double superphosphate is produced by the action of sulfuric acid H 2 SO 4 on natural phosphates. In Russia, they mainly use the in-line method: decomposition of raw materials, followed by granulation and drying of the resulting pulp in a drum granulator-dryer. Commercial double superphosphate from the surface is neutralized with chalk or NH 3 to obtain a standard product. A certain amount of double superphosphate is produced in a chamber method. Phosphorus-containing components are basically the same as in simple superphosphate, but in larger quantities, and the CaSO 4 content is 3-5%. When heated above 135-140 °C, double superphosphate begins to decompose and melt in the water of crystallization, and after cooling it becomes porous and brittle. At 280-320 °C, orthophosphates transform into meta-, pyro- and polyphosphates, which are in digestible and partially water-soluble forms. It melts at 980 °C, turning after cooling into a glassy product in which 60-70% of metaphosphates are citrate-soluble. Double Superphosphate contains 43-49% digestible phosphorus anhydride (phosphorus pentoxide) P 2 O 5 (37-43% water-soluble), 3.5-6.5% free phosphoric acid H 3 PO 4 (2.5-4.6% R 2 O 5):

Ca 3 (PO 4) 2 + 2H 2 SO 4 = Ca(H 2 PO 4) 2 + 2CaSO 4

There is also a method for decomposing phosphorus-containing raw materials with phosphoric acid:

Ca 5 (PO 4) 3 F + 7H 3 PO 4 = 5Ca(H 2 PO 4) 2 + HF

Flow diagram of the technological process for the production of double superphosphate: 1 - mixing of crushed phosphorite and phosphoric acid; 2 - decomposition of stage I phosphorite; 3 - decomposition of stage II phosphorite; 4 - pulp granulation; 5 - purification of phosphorus-containing gases from dust; 6 - drying of pulp granules; 7 - production of flue gases (in the furnace); 8 - dry product screening; 9 - grinding of large fractions; 10 - separation of fine and medium (commodity) fractions on the second screen; 11 - mixing of crushed coarse and fine fractions; 12 - ammoniation (neutralization) of residual phosphoric acid; 13 - purification of gases containing ammonia and dust; 14 - cooling of the neutralized commercial fraction of double superphosphate;

10.Preparation of extraction orthophosphoric acid

Preparation of extraction phosphoric acid

Immediately before obtaining EPA, phosphorus is obtained using a special technology

Figure 1. Phosphorus production diagram: 1 - raw material bunkers; 2 - mixer; 3 - ring feeder; 4 - charge hopper; 5 - electric furnace; 6 - ladle for slag; 7 - ladle for ferrophosphorus; 8 - electric precipitator; 5 - capacitor; 10 - collection of liquid phosphorus; 11 - settling tank

The extraction method (allows for the production of the purest phosphoric acid) includes the main stages: combustion (oxidation) of elemental phosphorus in excess air, hydration and absorption of the resulting P4O10, condensation of phosphoric acid and capture of mist from the gas phase. There are two ways to obtain P4O10: oxidation of P vapor (rarely used in industry) and oxidation of liquid P in the form of drops or films. The degree of oxidation of P under industrial conditions is determined by the temperature in the oxidation zone, diffusion of components and other factors. The second stage of producing thermal phosphoric acid - hydration of P4O10 - is carried out by absorption with acid (water) or by interaction of P4O10 vapor with water vapor. Hydration (P4O10 + 6H2O4H3PO4) proceeds through the stages of formation of polyphosphoric acids. The composition and concentration of the products formed depend on the temperature and partial pressure of water vapor.

All stages of the process are combined in one apparatus, except for fog collection, which is always carried out in a separate apparatus. In industry, circuits of two or three main devices are usually used. Depending on the principle of gas cooling, there are three methods for producing thermal phosphoric acid: evaporation, circulation-evaporation, heat exchange-evaporation.

Evaporative systems based on heat removal during the evaporation of water or dilute phosphoric acid are the simplest in hardware design. However, due to the relatively large volume of waste gases, the use of such systems is advisable only in installations of small unit capacity.

Circulation-evaporation systems make it possible to combine in one apparatus the stages of combustion of P, cooling of the gas phase with circulating acid and hydration of P4O10. The disadvantage of the scheme is the need to cool large volumes of acid. Heat exchange and evaporation systems combine two methods of heat removal: through the wall of combustion and cooling towers, as well as by evaporating water from the gas phase; A significant advantage of the system is the absence of acid circulation circuits with pumping and refrigeration equipment.

Domestic enterprises use technological schemes with a circulation-evaporative cooling method (double-tower system). Distinctive features of the scheme: the presence of an additional tower for gas cooling, the use of efficient plate heat exchangers in the circulation circuits; the use of a high-performance nozzle for combustion of P, providing uniform fine atomization of a jet of liquid P and its complete combustion without the formation of lower oxides.

The technological diagram of a plant with a capacity of 60 thousand tons per year of 100% H3PO4 is shown in Fig. 2. Molten yellow phosphorus is atomized by heated air under pressure up to 700 kPa through a nozzle in a combustion tower irrigated with circulating acid. The acid heated in the tower is cooled by circulating water in plate heat exchangers. Product acid containing 73-75% H3PO4 is removed from the circulation loop to the warehouse. In addition, cooling of gases from the combustion tower and absorption of acid is carried out in the cooling (hydration) tower, which reduces the afterbirth, the temperature load on the electrostatic precipitator and promotes effective gas purification. Heat removal in the hydration tower is carried out by circulating 50% H3PO4, cooled in plate heat exchangers. Gases from the hydration tower, after being purified from H3PO4 mist in a plate electrostatic precipitator, are released into the atmosphere. 1 ton of 100% H3PO4 consumes 320 kg P.

Rice. 2. Double-tower circulation scheme for the production of extraction H3PO4: 1 - acidic water collector; 2 - phosphorus storage; 3.9 - circulation collectors; 4.10 - submersible pumps; 5.11 - plate heat exchangers; 6 - combustion tower; 7 - phosphorus nozzle; 8 - hydration tower; 12 - electric precipitator; 13 - fan.

11. Catalysts for the oxidation of sulfur dioxide into sulfuric anhydride. Contacting

Sulfuric anhydride is produced by the oxidation of sulfur dioxide with atmospheric oxygen:

2SO2 + O2 ↔ 2SO3,

This is a reversible reaction.

It has long been noted that iron oxide, vanadium pentoxide and especially finely crushed platinum accelerate the oxidation of sulfur dioxide into sulfuric anhydride. These substances are catalysts for the oxidation reaction of sulfur dioxide. For example, at 400° C in the presence of platinized asbestos (i.e., asbestos on the surface of which finely crushed platinum is applied), almost 100% of sulfur dioxide is oxidized by atmospheric oxygen into sulfuric anhydride. With more high temperature the yield of sulfuric anhydride decreases, as the reverse reaction accelerates - the decomposition of sulfuric anhydride into sulfur dioxide and oxygen. At 1000° C, sulfuric anhydride almost completely decomposes into its original substances. Thus, the main conditions for the synthesis of sulfuric anhydride are the use of catalysts and heating to a certain, not too high temperature.

The synthesis of sulfuric anhydride also requires compliance with two more conditions: sulfur dioxide must be purified from impurities that inhibit the action of catalysts; sulfur dioxide and air must be dried, since moisture reduces the yield of sulfuric anhydride.

Introduction .

Sodium hydroxide or caustic soda (NaOH), chlorine, hydrochloric acid HC1 and hydrogen are currently produced industrially by the electrolysis of sodium chloride solution.

Caustic soda or sodium hydroxide - a strong alkali, commonly called caustic soda, is used in soap making, in the production of alumina - an intermediate product for the production of aluminum metal, in the paint and varnish industry, oil refining industry, in the production of rayon, in the organic synthesis industry and other sectors of the national economy.

When working with chlorine, hydrogen chloride, hydrochloric acid and caustic soda, you must strictly follow safety rules: inhalation of chlorine causes a sharp cough and suffocation, inflammation of the mucous membranes respiratory tract, pulmonary edema, and subsequently the formation of inflammatory foci in the lungs.

Hydrogen chloride, even at low levels in the air, causes irritation in the nose and larynx, tingling in the chest, hoarseness and suffocation. In case of chronic poisoning with low concentrations, teeth are especially affected, the enamel of which is quickly destroyed.

Hydrochloric acid poisoning is very similar With chlorine poisoning.

Chemical methods for producing sodium hydroxide.

Chemical methods for producing sodium hydroxide include lime and ferrite.

The lime method for producing sodium hydroxide involves reacting a solution of soda with milk of lime at a temperature of about 80°C. This process is called causticization; it is described by the reaction

Na 2 C0 3 + Ca (OH) 2 = 2NaOH + CaC0 3 (1)

solution precipitate

Reaction (1) produces a solution of sodium hydroxide and a precipitate of calcium carbonate. The calcium carbonate is separated from the solution, which is evaporated to produce a molten product containing about 92% NaOH. Molten NaOH is poured into iron drums where it hardens.

The ferritic method is described by two reactions:

Na 2 C0 3 + Fe 2 0 3 = Na 2 0 Fe 2 0 3 + C0 2 (2)

sodium ferrite

Na 2 0 Fe 2 0 3 -f H 2 0 = 2 NaOH + Fe 2 O 3 (3)

solution precipitate

reaction (2) shows the process of sintering soda ash with iron oxide at a temperature of 1100-1200°C. In this case, sintered sodium ferrite is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction (3); a solution of sodium hydroxide and a precipitate of Fe 2 O 3 are obtained, which, after separating it from the solution, is returned to the process. The solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% NaOH.

Chemical methods for producing sodium hydroxide have significant disadvantages: a large amount of fuel is consumed, the resulting caustic soda is contaminated with impurities, maintenance of the devices is labor-intensive, etc. Currently, these methods are almost completely replaced by the electrochemical production method.

The concept of electrolysis and electrochemical processes.

Electrochemical processes are chemical processes occurring in aqueous solutions or melts under the influence of direct electric current.

Solutions and molten salts, solutions of acids and alkalis, called electrolytes, belong to the second type of conductors in which the transfer of electric current is carried out by ions. (In conductors of the first kind, for example metals, current is carried by electrons.) When electric current passes through an electrolyte, ions are discharged at the electrodes and the corresponding substances are released. This process is called electrolysis. The apparatus in which electrolysis is carried out is called an electrolyzer or electrolytic bath.

Electrolysis is used to produce a number of chemical products - chlorine, hydrogen, oxygen, alkalis, etc. It should be noted that electrolysis produces chemical products of a high degree of purity, which in some cases is unattainable using chemical methods of their production.

The disadvantages of electrochemical processes include the high energy consumption during electrolysis, which increases the cost of the resulting products. In this regard, it is advisable to carry out electrochemical processes only on the basis of cheap electrical energy.

Raw materials for the production of sodium hydroxide.

To produce sodium hydroxide, chlorine, and hydrogen, a solution of table salt is used, which is subjected to electrolysis. Table salt is found in nature in the form of underground deposits of rock salt, in the waters of lakes and seas, and in the form of natural brines or solutions. Rock salt deposits are located in the Donbass, the Urals, Siberia, Transcaucasia and other areas. Some lakes in our country are also rich in salt.

In the summer, water evaporates from the surface of the lakes, and table salt precipitates in the form of crystals. This type of salt is called self-settling salt. IN sea ​​water contains up to 35 g/l sodium chloride. In places with a hot climate, where intense evaporation of water occurs, concentrated solutions of sodium chloride are formed, from which it crystallizes. In the depths of the earth, in salt layers, underground water flows, which dissolves NaCl and forms underground brines that emerge through boreholes to the surface.

Solutions of table salt, regardless of the route of their preparation, contain impurities of calcium and magnesium salts and, before they are transferred to the electrolysis workshop, are purified from these salts. Cleaning is necessary because poorly soluble calcium and magnesium hydroxides can be formed during the electrolysis process, which disrupt the normal course of electrolysis.

Cleaning brines is done with a solution of soda and lime milk. In addition to chemical purification, solutions are freed from mechanical impurities by settling and filtration.

Electrolysis of solutions of table salt is carried out in baths with a solid iron (steel) cathode and with diaphragms and in baths with a liquid mercury cathode. In any case, industrial electrolyzers used to equip modern large chlorine shops must have high performance, a simple design, be compact, operate reliably and steadily.

Electrolysis of sodium chloride solutions in baths with a steel cathode and graphite anode .

Makes it possible to produce sodium hydroxide, chlorine and hydrogen in one apparatus (electrolyzer). When a direct electric current passes through an aqueous solution of sodium chloride, one can expect the release of chlorine:

2CI - - 2eÞ C1 2 (a)

as well as oxygen:

20N - - 2eÞ 1/2O 2 + H 2 O(b)

H 2 0-2eÞ1/2О 2 + 2H +

The normal electrode potential for the discharge of OH - ions is + 0.41 V, and the normal electrode potential for the discharge of chlorine ions is + 1.36 V. In a neutral saturated solution of sodium chloride, the concentration of hydroxyl ions is about 1 10 - 7 g-eq/l. At 25° C, the equilibrium discharge potential of hydroxyl ions will be

Equilibrium discharge potential, chlorine ions at a NaCl concentration in solution of 4.6 g-eq/l equals

Consequently, oxygen should be discharged first at the anode with low overvoltage.

However, on graphite anodes the oxygen overvoltage is much higher than the chlorine overpotential and therefore mainly the discharge of C1 ions will occur on them - with the release of chlorine gas according to reaction (a).

The release of chlorine is facilitated by increasing the concentration of NaCI in the solution due to a decrease in the value of the equilibrium potential. This is one of the reasons for using concentrated solutions of sodium chloride containing 310-315 g/l.

At the cathode in an alkaline solution, water molecules are discharged according to the equation

H 2 0 + e = H + OH - (c)

Hydrogen atoms, after recombination, are released as molecular hydrogen

2Н Þ Н 2 (g)

The discharge of sodium ions from aqueous solutions on a solid cathode is impossible due to their higher discharge potential compared to hydrogen. Therefore, the hydroxide ions remaining in the solution form an alkali solution with sodium ions.

The decomposition process of NaCl can be expressed in this way by the following reactions:

i.e., chlorine is formed at the anode, and hydrogen and sodium hydroxide are formed at the cathode.

During electrolysis, along with the main processes described, side processes can also occur, one of which is described by equation (b). In addition, chlorine released at the anode is partially dissolved in the electrolyte and hydrolyzed according to the reaction

In the case of diffusion of alkali (OH - ions) to the anode or displacement of cathodic and anodic products, hypochlorous and hydrochloric acids are neutralized by alkali to form hypochlorite and sodium chloride:

HOC1 + NaOH = NaOCl + H 2 0

HC1 + NaOH = NaCl + H 2 0

ClO - ions at the anode are easily oxidized into ClO 3 -. Consequently, due to side processes during electrolysis, sodium hypochlorite, chloride and sodium chlorate will be formed, which will lead to a decrease in current efficiency and energy efficiency. In an alkaline environment, the release of oxygen at the anode is facilitated, which will also worsen electrolysis performance.

To reduce leakage adverse reactions, conditions should be created to prevent the mixing of cathode and anodic products. These include the separation of the cathode and anode spaces by a diaphragm and the filtration of the electrolyte through the diaphragm in the direction opposite to the movement of OH - ions to the anode. Such diaphragms are called filter diaphragms and are made of asbestos.

· Precautions when handling sodium hydroxide · Literature ·

Sodium hydroxide can be produced industrially by chemical and electrochemical methods.

Chemical methods for producing sodium hydroxide

Chemical methods for producing sodium hydroxide include lime and ferrite.

Chemical methods for producing sodium hydroxide have significant disadvantages: a lot of energy carriers are consumed, and the resulting caustic soda is heavily contaminated with impurities.

Today, these methods have been almost completely replaced by electrochemical production methods.

Lime method

The lime method for producing sodium hydroxide involves reacting a solution of soda with slaked lime at a temperature of about 80 °C. This process is called causticization; it goes through the reaction:

Na 2 CO 3 + Ca (OH) 2 = 2NaOH + CaCO 3

The reaction results in a solution of sodium hydroxide and a precipitate of calcium carbonate. Calcium carbonate is separated from the solution, which is evaporated to obtain a molten product containing about 92% wt. NaOH. NaOH is then melted and poured into iron drums, where it solidifies.

Ferrite method

The ferrite method for producing sodium hydroxide consists of two stages:

1. Na 2 CO 3 + Fe 2 O 3 = 2NaFeO 2 + CO 2
2. 2NaFeO 2 + xH 2 O = 2NaOH + Fe 2 O 3 * xH 2 O

Reaction 1 is a process of sintering soda ash with iron oxide at a temperature of 1100-1200 °C. In addition, sintered sodium ferrite is formed and carbon dioxide is released. Next, the cake is treated (leached) with water according to reaction 2; a solution of sodium hydroxide and a precipitate of Fe 2 O 3 *xH 2 O are obtained, which, after separating it from the solution, is returned to the process. The resulting alkali solution contains about 400 g/l NaOH. It is evaporated to obtain a product containing about 92% of the mass. NaOH, and then a solid product is obtained in the form of granules or flakes.

Electrochemical methods for producing sodium hydroxide

Electrochemically sodium hydroxide is obtained electrolysis of halite solutions(a mineral consisting mainly of sodium chloride NaCl) with the simultaneous production of hydrogen and chlorine. This process can be represented by the summary formula:

Caustic alkali and chlorine are produced by three electrochemical methods. Two of them are electrolysis with a solid cathode (diaphragm and membrane methods), the third is electrolysis with a liquid mercury cathode (mercury method).

In world production practice, all three methods for producing chlorine and caustic soda are used, with a clear tendency to increase the share of membrane electrolysis.

In Russia, approximately 35% of all caustic soda produced is produced by electrolysis with a mercury cathode and 65% by electrolysis with a solid cathode.

Diaphragm method

Diagram of an old diaphragm electrolyzer for producing chlorine and alkalis: A- anode, IN- insulators, WITH- cathode, D- space filled with gases (above the anode - chlorine, above the cathode - hydrogen), M- aperture

The simplest of the electrochemical methods, in terms of process organization and construction materials for the electrolyzer, is the diaphragm method for producing sodium hydroxide.

The salt solution in the diaphragm electrolyzer is continuously fed into the anode space and flows through, usually an asbestos diaphragm coated on a steel cathode mesh, to which, in some cases, a small amount of polymer fibers is added.

In many electrolyzer designs, the cathode is completely immersed under a layer of anolyte (electrolyte from the anode space), and the hydrogen released on the cathode grid is removed from under the cathode using gas outlet pipes, without penetrating through the diaphragm into the anode space due to countercurrent.

Counterflow is a very important feature of the diaphragm electrolyzer design. It is thanks to the countercurrent flow directed from the anode space to the cathode space through a porous diaphragm that it becomes possible to separately produce alkalis and chlorine. The countercurrent flow is designed to counteract the diffusion and migration of OH - ions into the anode space. If the countercurrent is insufficient, then hypochlorite ion (ClO -) begins to form in the anode space in large quantities, which can then be oxidized at the anode to the chlorate ion ClO 3 -. The formation of chlorate ion seriously reduces the chlorine current yield and is a major by-product in this method of producing sodium hydroxide. The release of oxygen is also harmful, which in addition leads to the destruction of the anodes and, if they are made of carbon materials, the release of phosgene impurities into the chlorine.

Anode: 2Cl - 2e → Cl 2 - main process 2H 2 O - 2e - → O 2 +4H + Cathode: 2H 2 O + 2e → H 2 + 2OH - main process ClO - + H 2 O + 2e - → Cl - + 2OH - СlО 3 - + 3Н 2 O + 6е - → Сl - + 6ОН -

Graphite or carbon electrodes can be used as an anode in diaphragm electrolyzers. Today, they have mainly been replaced by titanium anodes with ruthenium-titanium oxide coating (ORTA anodes) or other low-consumable ones.

At the next stage, the electrolytic lye is evaporated and the NaOH content in it is adjusted to a commercial concentration of 42-50% by weight. in accordance with the standard.

Table salt, sodium sulfate and other impurities, when their concentration in the solution increases above their solubility limit, precipitate. The caustic alkali solution is decanted from the sediment and transferred as a finished product to a warehouse or the evaporation stage is continued to obtain a solid product, followed by melting, flakiness or granulation.

Reverse salt, that is, table salt that has crystallized into sediment, is returned back to the process, preparing the so-called reverse brine from it. In order to avoid the accumulation of impurities in solutions, impurities are separated from it before preparing the reverse brine.

The loss of anolyte is compensated by the addition of fresh brine obtained by underground leaching of salt layers, mineral brines such as bischofite, previously cleared of impurities, or by dissolving halite. Before mixing it with return brine, fresh brine is cleaned of mechanical suspensions and a significant part of calcium and magnesium ions.

The resulting chlorine is separated from water vapor, compressed and supplied either for the production of chlorine-containing products or for liquefaction.

Due to its relative simplicity and low cost, the diaphragm method for producing sodium hydroxide is currently widely used in industry.

Membrane method

The membrane method for producing sodium hydroxide is the most energy efficient, but at the same time difficult to organize and operate.

From the point of view of electrochemical processes, the membrane method is similar to the diaphragm method, but the anode and cathode spaces are completely separated by a cation exchange membrane impermeable to anions. Thanks to this property, it becomes possible to obtain cleaner liquors than in the case of the diaphragm method. Therefore, in a membrane electrolyzer, unlike a diaphragm electrolyzer, there is not one flow, but two.

As in the diaphragm method, a flow of salt solution enters the anode space. And in the cathode - deionized water. From the cathode space flows a stream of depleted anolyte, which also contains impurities of hypochlorite and chlorate ions and chlorine, and from the anodic space flows alkali and hydrogen, practically free of impurities and close to commercial concentration, which reduces energy costs for their evaporation and purification.

The alkali produced by membrane electrolysis is almost as good in quality as that produced by the mercury cathode method and is slowly replacing the alkali produced by the mercury method.

At the same time, the feeding salt solution (both fresh and recycled) and water are preliminarily purified as much as possible from any impurities. Such thorough cleaning is determined by the high cost of polymer cation exchange membranes and their vulnerability to impurities in the feed solution.

In addition, the limited geometric shape and, in addition, the low mechanical strength and thermal stability of ion exchange membranes, for the most part, determine the relatively complex designs of membrane electrolysis installations. For the same reason, membrane installations require the most sophisticated automatic monitoring and control systems.

Mercury method with liquid cathode

Among the electrochemical methods for producing alkalis, the most effective way is electrolysis with a mercury cathode. Liquors obtained by electrolysis with a liquid mercury cathode are much cleaner than those obtained by the diaphragm method (for some industries this is critical). For example, in the production of artificial fibers, only high-purity caustic can be used), and compared to the membrane method, the organization of the process for producing alkali using the mercury method is much simpler.

The installation for mercury electrolysis consists of an electrolyzer, an amalgam decomposer and a mercury pump, interconnected by mercury-conducting communications.

The cathode of the electrolyzer is a stream of mercury pumped by a pump. Anodes - graphite, carbon or low-wear (ORTA, TDMA or others). Together with the mercury, a feed stream of table salt continuously flows through the electrolyzer.

At the anode, chlorine ions from the electrolyte are oxidized, and chlorine is released:

Chlorine and anolyte are removed from the electrolyzer. The anolyte leaving the electrolyzer is additionally saturated with fresh halite, the impurities introduced with it, and also washed out from the anodes and structural materials, are removed from it, and returned for electrolysis. Before saturation, the chlorine dissolved in it is removed from the anolyte.

At the cathode, sodium ions are reduced, which form a weak solution of sodium in mercury (sodium amalgam):

The amalgam continuously flows from the electrolyser to the amalgam decomposer. Highly purified water is also continuously supplied to the decomposer. In it, sodium amalgam, as a result of a spontaneous chemical process, is almost completely decomposed by water with the formation of mercury, caustic solution and hydrogen:

The caustic solution obtained in this way, which is a commercial product, contains practically no impurities. The mercury is almost completely freed from sodium and returned to the electrolyzer. Hydrogen is removed for purification.

However, complete purification of an alkali solution from mercury residues is practically impossible, therefore this method is associated with leaks of metallic mercury and its vapors.

Increasing requirements for environmental safety of production and the high cost of metallic mercury are leading to the gradual displacement of the mercury method by methods of producing alkali with a solid cathode, especially the membrane method.

Laboratory methods of obtaining

In the laboratory, sodium hydroxide is sometimes obtained by chemical methods, but more often a small diaphragm or membrane type electrolyzer is used.

DEFINITION

Sodium hydroxide forms hard white, very hygroscopic crystals, melting at 322 o C.

Due to its strong corrosive effect on fabrics, skin, paper and other organic substances, it is called caustic soda. In engineering, sodium hydroxide is often called caustic soda.

Sodium hydroxide dissolves in water, releasing a large amount of heat due to the formation of hydrates.

Sodium hydroxide should be stored in well-sealed containers, as it easily absorbs carbon dioxide from the air, gradually turning into sodium carbonate.

Rice. 1. Sodium hydroxide. Appearance.

Preparation of sodium hydroxide

The main method for producing sodium hydroxide is electrolysis of an aqueous solution of sodium chloride. During electrolysis, hydrogen ions are discharged at the cathode and at the same time sodium and hydroxide ions accumulate near the cathode, i.e. sodium hydroxide is obtained; Chlorine is released at the anode.

2NaCl + 2H 2 O = H 2 + Cl 2 + 2NaOH.

In addition to the electrolytic method of producing sodium hydroxide, sometimes an older method is also used - boiling a solution of soda with slaked lime:

Chemical properties of sodium hydroxide

Sodium hydroxide reacts with acids to form salts and water (neutralization reaction):

NaOH + HCl = NaCl + H 2 O;

2NaOH + H 2 SO 4 = Na 2 SO 4 + H 2 O.

A solution of sodium hydroxide changes the color of indicators, for example, when litmus, phenolphthalein or methyl orange are added to a solution of this alkali, their color will become blue, crimson and yellow, respectively.

Sodium hydroxide reacts with salt solutions (if they contain a metal capable of forming an insoluble base) and acidic oxides:

Fe 2 (SO 4) 3 + 6NaOH = 2Fe(OH) 3 ↓ + 3Na 2 SO 4;

2NaOH + CO 2 = Na 2 CO 3 + H 2 O.

Applications of sodium hydroxide

Sodium hydroxide is one of the most important products of the basic chemical industry. It is consumed in large quantities to purify petroleum products; Sodium hydroxide is widely used in soap, paper, textile and other industries, as well as in the production of artificial fiber.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2