Electrolysis of hydrochloric acid

The electrolysis cell used is similar in construction to the membrane cell, with PVC-cloth acting as the diaphragm. The bipolar electrodes are graphite. Small quantities of platinum group compounds may be added to the cathode to 

23% hydrochloric acid is depleted on graphite electrodes forming Ho + Cl to ca. 20%. Anode and cathode chambers are separated with a PVC-cloth diaphragm 

In the dual interests of waste disposal and manufacturing economics, this process is currently used to manufacture ca. 350 10 t/a chlorine. 

Principles. Hydrochloric acid (22wt% HCl) is fed into the cells in two separate circuits, a catholyte circuit and an anolyte circuit. During electrolysis the concentration is reduced to ca. 17 %, and the temperature increases from 65 to 80 °C. A part of the depleted acid is separated from the catholyte stream, concentrated in the absorption plant to ca. 30 %, and fed back into the main stream. The electrolyzer is bipolar, with pairs of electrodes arranged like the leaves of a filter press. A diaphragm or membrane (e.g., Nafion 430) separates the anode compartment from the cathode compartment to prevent mixing of the gaseous products. 

The reversible standard decomposition potential of hydrochloric acid is 1.358 V, made up of the anode potential, the discharge of chloride ions with formation of chlorine, and the cathode potential, the discharge of hydroxonium (HsO ) ions with formation of hydrogen. In practice ( 15 % HCl, 70 C), the decomposition potential is 1.16 V. 

The conductivity of hydrochloric acid is maximized at a concentration of 18.5 wt%. High temperatures improve the conductivity, but to avoid increased vapor pressure of HCl and material problems, the temperature is kept below 85 C. Modem cells have a voltage of ca. 1.90 V at 4.8 kA/m, corresponding to an energy consumption of 1400-1500 kW h per tonne of chlorine. 

DeNora and General Electric are developing an electrolyzer with a solid polymer electrolyte (SPE) based on Nafion [196]. In addition to a voltage savings of 20%, it is hoped that completely chloride-ffee hydrogen gas can be produced. 

In the absorption column, the hydrogen chloride gas is absorbed adiabatically by depleted hydrochloric acid from the catholyte. In the upper section of the column, an absorber removes the remaining hydrogen chloride and the water vapor by absorption 

---

Electrolysis of hydrochloric acid yields hydrogen at the cathode and oxygen at the anode from the dilute acid, but chlorine at the anode (of carbon) from the concentrated acid. Electrolysis of the concentrated acid is used on the large scale to recover chlorine. 

The electrolysis of hydrochloric acid.—When cone, hydrochloric acid is electrolyzed, a... 

Electrolytic processes for the perchlorates.—F. von Stadion found that if an aq. soln. of chlorine dioxide be included in Volta s circuit, at first very little gas is developed, but after some hours, oxygen and chlorine appear at the anode, and hydrogen at the cathode. The volume of hydrogen so obtained is nearly twice that of the oxygen. After some time the soln. is decolorized, and transformed into perchloric acid. In 1857, A. Riche 18 prepared perchloric acid by the electrolysis of hydrochloric acid, or of an aq. soln. of chlorine and ten years earlier, H. Kolbe prepared potassium perchlorate by the electrolysis of an aq. soln. of potassium chloride—acidified with sulphuric acid—and of potassium trichloro-methyl-sulphonate. H. Kolbe (1846), a pioneer in the electrolytic preparation of compounds, specially noted that the formation of perchloric acid is always preceded by that of chloric acid, and stated ... 

Perchloric Acid. Several techniques have been employed in the manufacture of perchloric acid, including thermal decomposition of chloric acid, anodic oxidation of chloric add. irradiation of chlorine dioxide solutions, electrolysis of hydrochloric acid, oxidation of hypochlorites by ozone, ion exchange, and electiodialysis of perchlorate salts. 

Summary Chlorine gas is readily prepared by the electrolysis of hydrochloric acid. The reaction generates heat, so the flask or container should be submerged in a cold-water bath. 

Starting with a mixture of hydrogen and chlorine—-prepared by the electrolysis of hydrochloric acid in darkness—no movement was observed for 600.seconds after the commencement of exposure, and after that the time occupied by the liquid in moving over the 1st, 2nd, 3rd, 4th, and 5th divisions of the scale was respectively 480,165, 130, 95, and 93 seconds, and thereafter it moved regularly at the same rate. These results are graphed in Fig. 6, as average velocities per second. Starting from the moment the mixture is illumi-... 

Derivation (1) Electrolysis of sodium chloride brine in either diaphragm or mercury-cathode cells chlorine is released at the anode. (2) Fused-salt electrolysis of sodium or magnesium chloride. (3) Electrolysis of hydrochloric acid. (4) Oxidation of hydrogen chloride with nitrogen oxide as catalyst and absorption of steam with sulfuric acid ( KeloChlor process). No by-product caustic is produced. 

Figure 4.3.2 Balance sheet for electrolysis of hydrochloric acid solution. (a) Cell schematic, (b) Various contributions to the current when lOe are passed in the external circuit per unit time.

There are no any data in the literature on using of other material, besides graphite, for industry electrolysis of hydrochloric acid. 

The electrochemical preparation of an HgPbClg solution is based upon the electrolysis of hydrochloric acid with a Pb cathode and two anodes. One anode is made of lead and dissolves, yielding Pb + ions. The other anode, which is not attacked, is made of carbon. Further oxidation of the ions to Pb + takes place at the carbon electrode. 

F.M. Berkey, Electrolysis of Hydrochloric Acid Solutions. In J.S. Sconce (ed.). Chlorine Its Manufacture, Properties and Uses, ACS Monograph 154, Robert E. Krieger Publishing Co., Huntington, NY... 

Alternatively, the electrolysis of hydrochloric acid solutions is also used to produce chlorine. Some historical details mentioned in this chapter are taken from a detailed review of the chlor-alkali industry. ... 

As discussed in the Introduction section, three main types of electrolytic cells have been used for the large scale production of chlorine and caustic soda mercury, diaphragm and membrane cells. The main difference in these technologies lies in the manner by which the chlorine gas and the sodium hydroxide are prevented from mixing with each other to ensure generation of pure product. Alternatively, the electrolysis of hydrochloric acid solutions is also used to produce chlorine. Individual electrolysis cells can be electrically wired in parallel (monopolar electrolysers) or in series (bipolar electrolysers). 

Worldwide, 94% of the chlorine is made by the electrolysis of brine. Sodium hydroxide and hydrogen are prodnced as by-products. A few plants produce potassium hydroxide as well as, or instead of, sodinm hydroxide by the electrolysis of potassium chloride solutions. Abont 3% of the chlorine is made by other processes. These include electrolysis of hydrochloric acid electrolysis of molten sodinm or magnesium chloride, which, respectively, produce sodium or magnesium metal as well as chlorine " nitric acid oxidation of potassinm chloride to make nitrosyl chloride, which is further oxidized by oxygen to make potassium nitrate, and the oxidation of hydrochloric acid directly with oxygen or air using a catalyst, or indirectly throngh the formation and snbsequent oxidation of metal chlorides. ... 

Gallone, P. Messner, G. (1965) Direct electrolysis of hydrochloric acid. Electrochem. Technol., 3(11-12), 321-326. 

Electrolysis of Hydrochloric Acid. Development of this process was conducted initially at Bitterfield in the I.G. Farben Industrie Plant (26). The German patent application covering this innovation was dated October 15, 1942. It took 14 years before this work became known and taken up by De Nora in Milan. The original HCl electrolysis cell constructed on this concept had vertical bipolar electrodes. An extensive study made of possible diaphragm materials had resulted in a choice that gave satisfaction since the last electrolyser was operated by I.G. Farben continuously for fifteen months, but the details of manufacture of the diaphragm were never published. 

Other electrochemical processes in which chlorine is produced include the electrolysis of hydrochloric acid and the electrolysis of molten alkali metal and alkaline earth metal chlorides, in which the chlorine is a byproduct. Purely chemical methods of chlorine production are currently insignificant. 

The installation at Corpus Christi operated at 1.4 MPa and 120 -180 C. On account of the aggressive nature of the chemicals, expensive materials, such as tantalum-plated equipment and pipes, must be used. For outputs of 250 - 3001 of chlorine per day, this process can be more economical than the electrolysis of hydrochloric acid, depending on local conditions. 

---