RU2366762C1 - Method of sodium hydroxide preparation - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention refers to the production of inorganic substances and can be used at preparation of sodium hydroxide. The method of sodium hydroxide preparation includes usage of reagent water solutions and solid inert electrodes separated with diaphragm. Anolyte is sulphide water solution with concentration 180-200 g/l, katholyte is water solution of NaOH with concentration 100-120 g/l barbotated with technical oxygen with rate not less than 0.2 l/min/l of katholyte.

EFFECT: power consumption decrease, excluding of gaseous chlorine formation.

1 tbl, 9 ex

Description

The invention relates to the production of inorganic substances and can be used to obtain caustic soda.

A known method for the electrolytic production of caustic soda in the cathode (mercury melt) - electrolyte (aqueous sodium chloride) - anode (insoluble solid electrode) system. At the same time, sodium reduction reactions proceed at the cathode:

and the formation of intermetallic soluble in free mercury. In turn, the oxidation of the chlorine ion with the formation of gaseous chlorine proceeds at the anode:

The electrochemical process is characterized by a current load on the bath 170-190 kA, voltage 4.4-4.6 V, current density 5.58-6.3 kA / m ² , current output 96% and is carried out at a temperature of 50-80 ° С .

As sodium accumulates in the mercury amalgam (0.25% by weight), it is removed from the electrolysis process and supplied to the anodic oxidation of sodium under the conditions of operation of the short-circuited element with the immersion of the electrode (cathode) in the mercury phase. The electrolyte is an aqueous solution of NaOH. Anodic oxidation is completed by deep extraction of sodium in an aqueous solution with the accumulation of NaOH in the latter. The specified process is accompanied by the decomposition of water in the cathode space:

Accumulating hydrogen ions are reduced on a solid (graphite) cathode with hydrogen evolution:

The result is an aqueous solution of NaOH with a concentration of the latter 620-700 g / DM ³ supplied to evaporation. In general, according to the technology, the energy consumption per ton of NaOH is 3100-3200 kWh [N.P. Fedotiev. Applied electrochemistry. / N.P. Fedotiev, A.F. Alabyshev et al. L .: Goskhimizdat. - 1962. - 640 p.].

The disadvantages of the method include multi-operation, high energy consumption per ton of dry alkali, the need for utilization of chlorine and hydrogen.

A known method of electrolytic production of NaOH using aqueous solutions of NaCl (260-310 g / DM ³ ) and solid electrodes separated by a diaphragm made of sheet asbestos. Chemical and electrochemical reactions in the cathode space are associated with the decomposition of water (equation 3) and the reduction of hydrogen (equation 4). At the anode, the oxidation of the chlorine ion (equation 2) proceeds with the formation of gaseous chlorine. Sodium ions accumulating in the anode space diffuse through the pores of the diaphragm into the catholyte with the formation of alkali:

The electrochemical process is characterized by a current load on the electrolyzer of 5-30 kA, a bath voltage of 3.3-3.9 V, a cathode current density of 0.42-1 kA / m ² . The final concentration of NaOH in the electrolyte is 120-140 g / dm ³ . The process is carried out at a temperature of 50-80 ° C. The catholyte solution is evaporated to obtain dry NaOH. The energy consumption per ton of NaOH is 2600-2800 kWh. In the production of one ton of alkali, 0.865 tons of gaseous chlorine and 245-250 m ^{3 of} hydrogen are simultaneously obtained. These products are characterized by environmental hazards. A mixture containing 4% (wt.) Hydrogen in chlorine is explosive [N.P. Fedotiev. Applied electrochemistry. / N.P. Fedotiev, A.F. Alabyshev et al. L .: Goskhimizdat. - 1962. - 640 p.].

The disadvantages of the method include:

- the accumulation of gaseous chlorine and hydrogen, which must be disposed of, and the complexity of their separation;

- high energy consumption due to the increased voltage on the bath, due primarily to the high depolarization potentials of the cathode and anode;

The aim of the invention is to reduce energy consumption for the implementation of electrolysis, as well as the elimination of the formation of gaseous products - chlorine and hydrogen.

This goal is achieved when implementing the method of electrolytic production of NaOH using aqueous solutions of reagents and solid inert electrodes separated by a diaphragm, characterized in that an anolyte is used an aqueous solution of sodium sulfide with a concentration of 180-200 g / dm ³ , and as a catholyte - an aqueous solution NaOH concentration of 100-120 g / DM ³ through which technical oxygen is bubbled, the flow rate of which is not less than 0.2 DM ³ / min per 1 DM ^{3 of} catholyte.

The process is carried out in an electrochemical system anode (graphite) - an aqueous solution of Na ₂ S - a diaphragm - an aqueous solution of NaOH - gaseous oxygen - a cathode (graphite). The oxidation reaction of sulfide sulfur and the formation of water-soluble sodium polysulfide proceeds at the anode:

In turn, in catholyte, oxygen dissolves, oxygen is reduced on the electrode to form an oxygen anion:

which in turn is bound by water:

Sodium ions released during the oxidation of sulfide sulfur are transported to the cathode space through the diaphragm and the catholyte is enriched in NaOH.

The course of oxygen reduction (8) in an alkaline medium together with the formation of a hydroxyl ion (9) is characterized by an equilibrium potential of +0.1 V. Given the overvoltage, this cathode depolarization potential will be -0.4 V. In turn, taking into account the equilibrium potential and overvoltage the anodic oxidation of sulfide sulfur (depolarization of the anode) will be +0.3 V. Hence, the voltage of depolarization of the electrodes will be approximately 0.6-0.7 V, and the voltage on the bath is 1.7-1.8 V.

The electrolysis in this system is characterized by a current efficiency of NaOH at the level of 97-98% and energy consumption for the production of a ton of alkali in a solution of 1300-1500 kW · h.

An essential feature of the proposed method is the use of a set of reagents responsible for the depolarization of the electrodes. In the case of the anode, this is sodium sulfide; in the case of the cathode, oxygen dissolved in catholyte. The actual concentration of sodium sulfide in the anolyte should be 180-200 g / dm ³ . The actual concentration of NaOH in catholyte is 100-120 g / dm ³ when sparging through the last technical oxygen, the flow rate of which (in the cathode space) is at least 0.2 dm ³ / min per 1 dm ^{3 of} catholyte. These signs allow you to attribute the content of the proposed method to the criterion of "novelty."

The method is described in the examples.

The electrolytic cell is divided by a diaphragm made of sheet asbestos with a thickness of 4 mm, has in the bottom of the anode and cathode chambers pipes for draining electrolytes, which are pumped into pressure tanks located above the cell. The electrodes are made of graphite. The value of their wetted surface is 240 cm ² . The electrolyte circulation rate of 0.5 DM ³ / min. The temperature of the electrolytes is 60 ± 0.5 ° C. The working current density in the prototype and the claimed method was changed from 350 to 500 A / m ² . The duration of electrolysis is 3 hours. In the experiments according to the claimed method, technical oxygen was bubbled through the cathode space at a rate of 0.05-0.25 dm ³ / min, which at a given circulation speed is 0.1-0.5 dm ³ / min per 1 dm ^{3 of} electrolyte.

Anolyte:

- an aqueous solution of NaCl 300 g / DM ³ (prototype);

- an aqueous solution of Na ₂ S from 160 to 200 g / DM ³ (the inventive method).

Catholyte:

- an aqueous solution of NaOH - 100 g / DM ³ (according to the prototype and the claimed method).

Volumes of electrolytes 5 dm ³ each.

Experience 1 (prototype)

The NaCl content in the anolyte is 300 g / dm ³ . The catholyte is represented by an aqueous solution of NaOH (100 g / dm ³ ). During the electrolysis, a constant concentration of NaCl in the solution is maintained. Current density 420 A / m ² , bath voltage 3.7 V.

In the electrolysis process, the duration of which is 3 hours, a catholyte with a NaOH content of 108.6 g / dm ^{3 was} obtained, the NaOH increment was 42.9 g. The current yield of NaOH was 96%. The amount of chlorine ion consumed in the anode process was 39.7 g.

Experience 2 (by the present method)

As the anolyte used an aqueous solution of sodium sulfide with a concentration of 200 g / DM ³ . Catholyte - an aqueous solution of alkali (NaOH) - 100 g / DM ³ . After three hours of electrolysis at a current density of 420 A / m ² (bath voltage 1.8 V) and sparging through the cathode space of technical oxygen at a flow rate of 0.15 dm ³ / min (0.3 dm ³ / min per 1 dm ³ solution) a catholyte with a NaOH content of 108.74 g / dm ^{3 was} obtained, which corresponds to an alkali increment of 43.7 g. Based on electrochemical calculations, it was determined that the cathode current output was 97.8%.

Under electrolysis conditions, anolyte with a Na ₂ S content of 196 g / dm ^{3 was} obtained. Taking into account the electrochemical equivalent of sulfur (0.597 g / Ah), it was determined that the theoretical oxidation of sulfide sulfur to elemental should be 17.2 g. The practical decrease in sulfide sulfur was 17.7 g, which corresponds to a current efficiency of 96%.

Experience 3 (by the present method)

Other things being equal (experiment 2), the current density was maintained equal to 630 A / m ² .

Experience 4 (by the present method)

Other things being equal (experiment 2), the current density was maintained equal to 840 A / m ² .

Experience 5 (by the present method)

Other things being equal (experiment 2), the initial concentration of Na ₂ S is 180 g / dm ³ .

Experience 6 (by the present method)

Other things being equal (experiment 2), the initial concentration of Na ₂ S is 160 g / dm ³ .

Experience 7 (by the present method).

Other things being equal (experiment 2), the initial concentration of Na ₂ S is 180 g / dm ³ . The oxygen supply rate is 0.1 dm ³ / min (0.2 dm ³ / min per 1 dm ^{3 of} electrolyte).

Experience 8 (by the present method)

Other things being equal (experiment 2), the initial concentration of Na ₂ S is 180 g / dm ³ . The oxygen feed rate is 0.05 dm ³ / min (0.1 dm ³ / min per 1 dm ^{3 of} electrolyte).

Experience 9 (by the present method)

Other things being equal (experiment 2), the initial concentration of Na ₂ S is 180 g / dm ³ . The oxygen supply rate is 0.25 dm ³ / min (0.5 dm ³ / min per 1 dm ^{3 of} electrolyte).

The results of experiments 2-9 No. Raft current
A / m ² Voltage on the bath, V Oxygen consumption, dm ³ / min per 1 dm ³ electrolyte The initial concentration of Na ₂ S in the anolyte, g / DM ³ The final concentration of NaOH in catholyte, g / DM ³ The increment of the mass of NaOH, g Current efficiency NaOH,% 2 420 1.8 0.3 200 108.6 43.1 96.2 3 630 2.1 0.3 200 112.8 64.3 96.9 four 840 2.2 0.3 200 117.2 86.0 95.7 5 420 1.8 0.3 180 108.7 43,4 93.7 6 420 1.8 0.3 160 108,0 40.1 89.5 7 420 1.8 0.2 180 108.7 43.6 97.3 8 420 1.8 0.1 180 107.9 39.38 87.9 9 420 1.8 0.5 180 108,5 43,4 96.9

From the table it follows that the optimal conditions for the implementation of the method of producing caustic soda are the content of Na ₂ S in the anolyte 180-200 g / dm ³ , the amount of oxygen supplied is not less than 0.2 dm ³ / min per 1 dm ^{3 of} electrolyte with a NaOH content of 100 catholyte -120 g / dm ³ . Operating current density 420-800 A / m ² . In this case, the energy consumption for obtaining 1 ton of NaOH (in solution) will be 1300-1500 kW · h.

The implementation of the method completely eliminates the formation of gaseous products - chlorine and hydrogen, and thereby simplifies the design of the cell.

Claims (1)

A method of producing caustic soda using aqueous solutions of reagents and solid inert electrodes separated by a diaphragm, characterized in that an anolyte is an aqueous solution of sodium sulfide with a concentration of 180-200 g / dm ³ , and as a catholyte is an aqueous solution of NaOH with a concentration of 100-120 g / dm ³ through which technical oxygen is bubbled, the flow rate of which is at least 0.2 dm ³ / min per 1 dm ^{3 of} catholyte.