DEFINITION

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

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

In water, sodium hydroxide dissolves with the release of a large amount of heat due to the formation of hydrates.

Sodium hydroxide should be stored in well-sealed vessels, as it readily absorbs carbon dioxide from the air, gradually converting to sodium carbonate.

Rice. 1. Sodium hydroxide. Appearance.

Obtaining 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 ions 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 for producing sodium hydroxide, sometimes an older method is still 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.

The sodium hydroxide solution changes the color of the indicators, so, for example, when litmus, phenolphthalein or methyl orange is added to the solution of this alkali, their color will turn blue, raspberry 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.

Application of sodium hydroxide

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

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

· Precautions when handling sodium hydroxide · Literature & middot

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

Chemical methods for obtaining sodium hydroxide

The chemical methods for obtaining sodium hydroxide include lime and ferritic.

Chemical methods for producing sodium hydroxide have significant drawbacks: many energy carriers are consumed, the resulting caustic soda is heavily contaminated with impurities.

Today, these methods are almost completely superseded by electrochemical production methods.

Lime method

The lime method of obtaining sodium hydroxide consists in the interaction of a solution of soda with slaked lime at a temperature of about 80 ° C. This process is called causticization; it follows the reaction:

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

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

Ferritic method

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

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

Reaction 1 is the process of sintering soda ash with iron oxide at a temperature of 1100-1200 ° C. In addition, speck is formed - sodium ferrite 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 Obtaining Sodium Hydroxide

Electrochemically sodium hydroxide is obtained electrolysis of halite solutions(a mineral consisting mainly of sodium chloride) 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 industrial practice, all three methods of obtaining chlorine and caustic are used with a clear tendency to an increase in the proportion of membrane electrolysis.

In Russia, approximately 35% of all produced caustic soda 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 the production of chlorine and alkali: BUT- anode, IN- insulators, WITH- cathode, D- space filled with gases (above the anode - chlorine, above the cathode - hydrogen), M- diaphragm

The simplest of the electrochemical methods, in terms of organizing the process 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 applied to the steel cathode mesh, into which, in some cases, a small amount of polymer fibers is added.

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

Counterflow is a very important feature of the diaphragm electrolyser design. It is thanks to the countercurrent flow directed from the anode space to the cathode space through the porous diaphragm that it becomes possible to separately obtain lye and chlorine. The countercurrent flow is designed to counteract the diffusion and migration of OH - ions into the anode space. If the amount of counterflow is insufficient, then hypochlorite ion (ClO -) begins to form in large quantities in the anode space, which, afterwards, can be oxidized at the anode to chlorate ion ClO 3 -. The formation of chlorate ion seriously reduces the chlorine flux efficiency and is a major by-product in this sodium hydroxide method. 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 ingress of phosgene impurities into the chlorine.

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

As an anode in diaphragm electrolyzers, graphite or carbon electrodes can be used. To date, they have been mainly replaced by titanium anodes with ruthenium-titanium oxide coating (ORTA anodes) or other low-consumption ones.

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

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, flaking or granulation.

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

The loss of anolyte is replenished by adding fresh brine, obtained by underground leaching of salt layers, mineral brines such as bischofite, previously purified from impurities or by dissolving halite. Fresh brine, before mixing it with reverse 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 fed either to the production of chlorine-containing products or to liquefaction.

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

Membrane method

The membrane method for the production of sodium hydroxide is the most energy efficient, at the same time, it is 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 more pure liquors than in the case of the diaphragm method. Therefore, in a membrane electrolyzer, unlike a diaphragm one, there is not one stream, but two.

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

The alkali obtained by membrane electrolysis is practically not inferior in quality to that obtained by the method using the mercury cathode and slowly replaces the alkali obtained by the mercury method.

At the same time, the feeding salt solution (both fresh and circulating) and water are preliminarily purified as much as possible from any impurities. This thorough cleaning is determined by the high cost of polymeric 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 plants. For the same reason, membrane plants require the most sophisticated automatic monitoring and control systems.

Mercury liquid cathode method

Among the electrochemical methods for producing lye, the most effective method is electrolysis with a mercury cathode. Alkalis 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 in comparison with the membrane method, the organization of the process for obtaining alkali by 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 flow of mercury pumped by a pump. Anodes - graphite, carbon or low-wear (ORTA, TDMA or others). Together with mercury, a stream of sodium chloride feed 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 saturated with fresh halite, the impurities introduced with it are removed from it, and in addition washed out from the anodes and structural materials, and returned to the electrolysis. Before additional saturation, chlorine dissolved in it is extracted from the anolyte.

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

The amalgam flows continuously from the electrolyser to the amalgam decomposer. The decomposer is also continuously supplied with highly purified water. 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, practically does not contain impurities. Mercury is almost completely freed from sodium and returned to the electrolyzer. Hydrogen is removed for purification.

However, complete cleaning of the alkali solution from mercury residues is practically impossible, therefore this method is associated with leakage of metallic mercury and its vapors.

The growing requirements for the environmental safety of production and the high cost of metallic mercury lead to the gradual replacement of the mercury method by methods of obtaining alkali with a solid cathode, especially the membrane method.

Laboratory methods of obtaining

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

The chemical methods for obtaining sodium hydroxide include lime and ferritic.

Chemical methods for producing sodium hydroxide have significant drawbacks: many energy carriers are consumed, the resulting caustic soda is heavily contaminated with impurities.

Today, these methods are almost completely superseded by electrochemical production methods.

Lime method

The lime method of obtaining sodium hydroxide consists in the interaction of a solution of soda with slaked lime at a temperature of about 80 ° C. This process is called causticization; it follows the reaction:

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

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

Ferritic method

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

Na 2 CO 3 + Fe 2 O 3 = 2NaFeО 2 + CO 2

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

Reaction 1 is the process of sintering soda ash with iron oxide at a temperature of 1100-1200 ° C. In addition, a speck is formed - sodium ferrite 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 Obtaining Sodium Hydroxide

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

2NaCl + 2H 2 O ± 2e - → 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 industrial practice, all three methods of obtaining chlorine and caustic are used with a clear tendency to an increase in the proportion 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 the 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 factories in our country annually emit tens of millions of cubic meters of nitrogen oxides into the atmosphere, which are a strong and dangerous poison. 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. Up to 40 million tons of sulfuric acid can be produced from this amount of sulfur.

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

Flue gases from factory chimneys and power plants annually emit several billion cubic meters of carbon dioxide into the atmosphere. This gas can be used to produce efficient carbon-containing fertilizers.

The given examples show what huge material values ​​are thrown into the atmosphere with gaseous emissions.

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

Although in recent years domestic industrial enterprises have not been operating at full capacity, the problem of combating harmful emissions is very acute. And given the general ecological situation on the planet, it is necessary to take the most urgent and most radical measures to clean the exhaust 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 their catalytic action. Poisoning of heterogeneous catalysts occurs as a result of the adsorption of the poison or the product of its chemical transformation on the catalyst surface. Poisoning can 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 fails to restore the activity of the catalyst. To prevent poisoning, the reaction mixture supplied to the catalyst is thoroughly cleaned. Among the most common To. I. for metal catalysts are 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+). Acidic catalysts are usually poisoned with base impurities, and basic ones - with acid impurities.

8. Obtaining nitrous gases.

Nitrogen oxides released after bleaching are condensed in water and brine condensers and used to prepare a crude 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 (the saturated vapor pressure of N 2 O 4 at 21.5 ° C over 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 a reduced temperature. 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 (vapor pressure) at the condensation temperature (–10 ° C) is 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 a nitrous gas at a temperature of –10 ° C begins at a pressure of 0.327 MPa. The degree of condensation increases sharply with an increase in pressure to 1.96 MPa; with a further increase in pressure, the degree of condensation changes insignificantly.

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

Under the 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 lowers the crystallization temperature. So the mixture, which has 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. Obtaining 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.). A 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 PO 4 ) 2 + 2CaSO 4 .

Simple superphosphate- gray powder, almost non-caking, medium dispersible; in fertilizer 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 of the preparation of simple superphosphate is shown in Fig. The main processes take place in the first three stages: mixing of raw materials, formation and solidification of superphosphate pulp, ripening of superphosphate in the warehouse.

Rice. Functional diagram of the production of simple superphosphate

To obtain a marketable product of a higher quality, superphosphate, after ripening, is subjected to neutralization 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. 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 a small amount of ballast, that is, it reduces transport costs, storage costs, packaging

Double superphosphate is produced by the action of sulfuric acid H 2 SO 4 on natural phosphates. In Russia, the flow method is mainly used: 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 by the chamber method. The 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 crystallization water, after cooling it becomes porous and brittle. At 280-320 ° C, orthophosphates transform into meta-, pyro- and polyphosphates, which are in assimilable and partially water-soluble forms. It melts at 980 ° C, transforming after cooling into a glassy product, in which 60-70% of metaphosphates are citrate-soluble. Double Superphosphate contains 43-49% of assimilable phosphoric anhydride (phosphorus pentoxide) Р 2 О 5 (37-43% water-soluble), 3.5-6.5% free phosphoric acid Н 3 РО 4 (2.5-4.6% Р 2 О 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

Block 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 - receiving flue gases (in the furnace); 8 - screening of dry product; 9 - grinding of a coarse fraction; 10 - separation of small and medium (marketable) fractions on the second screen; 11 - mixing of crushed coarse fraction and fine; 12 - ammonization (neutralization) of residual phosphoric acid; 13 - purification of gases containing ammonia and dust; 14 - cooling of the neutralized commercial fraction of double superphosphate;

10.Production of extraction orthophosphoric acid

Obtaining extraction phosphoric acid

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

Fig 1. Scheme of phosphorus production: 1 - bunkers of raw materials; 2 - mixer; 3 - ring feeder; 4 - charge hopper; 5 - electric furnace; 6 - slag ladle; 7 - ladle for ferrophosphorus; 8 - electrostatic precipitator; 5 - capacitor; 10 - collection of liquid phosphorus; 11 - sump

The extraction method (allows 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 capturing fog 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 a film. The oxidation state of P under industrial conditions is determined by the temperature in the oxidation zone, diffusion of components, and other factors. The second stage of obtaining thermal phosphoric acid - hydration of P4O10 - is carried out by absorption with acid (water) or by interaction of P4O10 vapors with water vapor. Hydration (P4O10 + 6H2O4H3PO4) proceeds through the stages of formation of polyphosphoric acids. The composition and concentration of the resulting products depend on the temperature and partial pressure of water vapor.

All stages of the process are combined in one apparatus, except for the collection of fog, 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 ways of producing thermal phosphoric acid: evaporative, circulation-evaporative, heat-exchange-evaporative.

Evaporation systems based on the removal of heat by 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 allow combining the stages of combustion of P, cooling of the gas phase with circulating acid and hydration of P4O10 in one apparatus. The disadvantage of this 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 the combustion and cooling towers, as well as through the evaporation of water from the gas phase; a significant advantage of the system is the absence of acid circulation loops with pumping and refrigeration equipment.

Domestic enterprises operate technological schemes with a circulation-evaporative cooling method (two-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, which ensures uniform, finely dispersed atomization of a jet of liquid P and its complete combustion without the formation of lower oxides.

The technological scheme of the installation with a capacity of 60 thousand tons per year of 100% H3PO4 is shown in Fig. 2. Molten yellow phosphorus is sprayed with heated air at a pressure of up to 700 kPa through a nozzle in the combustion tower, sprayed with circulating acid. The acid heated in the tower is cooled by circulating water in plate heat exchangers. The production acid containing 73-75% H3PO4 is removed from the circulation loop to the warehouse. Additionally, the cooling of gases from the combustion tower and the absorption of acid are carried out in the cooling (hydration) tower, which reduces the afterbirth, the temperature load on the electrostatic precipitator and promotes effective gas cleaning. Heat removal in the hydration tower is carried out by circulating 50% H3PO4 cooled in plate heat exchangers. The gases from the hydration tower after cleaning from the H3PO4 fog in the plate electrostatic precipitator are emitted into the atmosphere. For 1 ton of 100% H3PO4 320 kg of P. is consumed.

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

11. Catalysts for the oxidation of sulfur dioxide to 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 noticed that iron oxide, vanadium pentoxide and especially finely crushed platinum accelerate the oxidation reaction of sulfur dioxide to sulfuric anhydride. These substances are catalysts for the oxidation of sulfur dioxide. So, for example, at 400 ° C in the presence of platinized asbestos (that is, asbestos, on the surface of which finely crushed platinum is applied), almost 100% of sulfur dioxide is oxidized by atmospheric oxygen to sulfuric anhydride. At a higher temperature, the yield of sulfuric anhydride decreases, since the reverse reaction is accelerated - the reaction of decomposition of sulfuric anhydride into sulfur dioxide and oxygen. At 1000 ° C, sulfuric anhydride decomposes almost completely into the starting materials. Thus, the main conditions for the implementation of 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 cleaned of impurities that inhibit the action of catalysts; Sulfur dioxide and air must be dried as moisture reduces the yield of sulfuric anhydride.

Introduction .

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

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

When working with chlorine, hydrogen chloride, hydrochloric acid and caustic soda, it is necessary to strictly observe safety rules: inhalation of chlorine causes a sharp cough and suffocation, inflammation of the mucous membranes of the respiratory tract, pulmonary edema, and later the formation of inflammatory foci in the lungs.

Even if it is negligible in the air, hydrogen chloride causes irritation in the nose and throat, tingling in the chest, hoarseness and choking. In chronic poisoning with small concentrations of it, teeth are especially affected, the enamel of which is rapidly destroyed.

Hydrochloric acid poisoning is very similar with chlorine poisoning.

Chemical methods for producing sodium hydroxide.

The chemical methods for producing sodium hydroxide include lime and ferritic.

The lime-based method for producing sodium hydroxide consists in the interaction of a solution of soda with lime milk 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 sediment

According to reaction (1), a solution of sodium hydroxide and a precipitate of calcium carbonate are obtained. The calcium carbonate is separated from the solution, which is evaporated to a molten product containing about 92% NaOH. The molten NaOH is poured into iron drums where it solidifies.

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 sediment

reaction (2) shows the process of sintering soda ash with iron oxide at a temperature of 1100-1200 ° C. In this case, a sinter - 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 drawbacks: a large amount of fuel is consumed, the resulting caustic soda is contaminated with impurities, the maintenance of apparatus is laborious, etc. At present, these methods are almost completely replaced by the electrochemical method of production.

The concept of electrolysis and electrochemical processes.

Electrochemical processes are chemical processes that occur in aqueous solutions or melts under the influence of a constant electric current.

Solutions and molten salts, solutions of acids and alkalis, called electrolytes, refer to conductors of the second kind, 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 an electric current passes through an electrolyte, a discharge of ions occurs on 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 obtain a number of chemical products - chlorine, hydrogen, oxygen, alkalis, etc. It should be noted that by electrolysis, chemical products of high purity are obtained, in some cases unattainable by chemical methods of their production.

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

Raw materials for sodium hydroxide production.

For the production of sodium hydroxide, chlorine, hydrogen, a solution of table salt is used, which is subjected to electrolysis. Table salt occurs 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 regions. Some lakes in our country are also rich in salt.

In the summer, water evaporates from the surface of the lakes, and table salt falls out in the form of crystals. This salt is called self-settling. Seawater contains up to 35 g / l of sodium chloride. In places with a hot climate, where intensive evaporation of water occurs, concentrated solutions of sodium chloride are formed, from which it crystallizes. In the bowels of the earth, in layers of salt, underground waters flow, which dissolve NaCl and form underground brines that go out through boreholes to the surface.

Salt solutions, regardless of the way they are obtained, contain impurities of calcium and magnesium salts and, before they are transferred to the electrolysis department, are purified from these salts. Cleaning is necessary because poorly soluble calcium and magnesium hydroxides can form during the electrolysis process, which disrupt the normal course of electrolysis.

The brines are cleaned with a solution of soda and lime milk. In addition to chemical cleaning, solutions are freed from mechanical impurities by settling and filtration.

The electrolysis of sodium chloride solutions 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 electrolysers used for the equipment of modern large chlorine shops must have high productivity, simple design, be compact, operate reliably and stably.

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

It makes it possible to obtain sodium hydroxide, chlorine and hydrogen in one apparatus (electrolyzer). When a direct electric current passes through an aqueous solution of sodium chloride, chlorine evolution can be expected:

2CI - - 2eÞ C1 2 (a)

as well as oxygen:

20H - - 2eÞ 1 / 2О 2 + Н 2 О (b)

H 2 0-2eÞ1 / 2О 2 + 2H +

The normal electrode potential of the OH - ions discharge is + 0.41 in, and the normal electrode potential of the discharge of chlorine ions is + 1.36 in. 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 potential of the discharge of hydroxyl ions will be

Equilibrium potential of the discharge, chlorine ions at a NaCl concentration in the solution of 4.6 g-eq / l is equal to

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

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

The release of chlorine is facilitated with an increase in the concentration of NaCl in the solution due to a decrease in the value of the equilibrium potential. This is one of the reasons for the use in electrolysis of concentrated sodium chloride solutions containing 310-315 g / l.

At the cathode in an alkaline solution, a discharge of water molecules occurs according to the equation

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

Hydrogen atoms after recombination are released in the form of molecular hydrogen

2H Þ H 2 (g)

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

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

that is, chlorine is formed at the anode, and hydrogen and sodium hydroxide at the cathode.

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

In the case of diffusion of alkali (OH - ions) to the anode or displacement of cathodic and anode products, hypochlorous and hydrochloric acids are neutralized with 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 to ClO 3 -. Consequently, due to side processes during electrolysis, hypochlorite, sodium chloride and 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 deteriorate the electrolysis performance.

To reduce the occurrence of side reactions, it is necessary to create conditions that prevent mixing of the cathode and anode products. These include the separation of the cathode and anode spaces by a diaphragm and filtration of the electrolyte through the diaphragm in the direction opposite to the movement of OH - ions towards the anode. Such diaphragms are called filter diaphragms and are made of asbestos.

Introduction

You came to a store looking to buy unscented soap. Naturally, in order to understand which products from this assortment have a smell and which do not, you take each bottle of soap in your hands and read its composition and properties. Finally, we chose the right one, but while looking at various soap formulations, we noticed a strange tendency - almost all the bottles read: "The structure of the soap contains sodium hydroxide." This is the standard story of most people getting to know sodium hydroxide. Some half of the people will "spit and forget", and some will want to know more about him. For them today I will tell you what kind of substance it is.

Definition

Sodium hydroxide (formula NaOH) is the most abundant 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 they were given separate names. There is also an "inverted" name for the substance being described now - sodium hydroxide, which is usually used in Ukrainian translations.

Properties

As I 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 will “rush” and “jump” on its surface with a hiss (photo). And this will continue until he completely dissolves in her. If, after the completion of the reaction, you lower your hand into the resulting solution, then it will be soapy to the touch. To find out how strong an alkali is, indicators are dipped into it - phenolphthalein or methyl orange. Phenolphthalein in it becomes crimson, and methyl orange - yellow. In sodium hydroxide, as in all alkalis, hydroxide ions are present. The more of them in the solution, the brighter the color of the indicators and the stronger the alkali.

Receiving

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

Application

The delignification of cellulose, the production of cardboard, paper, fibreboard and man-made fibers is not complete without sodium hydroxide. And when it reacts with fats, soaps, 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 branches of its application.

Conclusion

Now you know everything about sodium hydroxide. As you can see, it brings a great benefit to a person - both in industry and in everyday life.