DE2020480A1 - Process for the electrolysis of alkali chlorides - Google Patents

Description

Badische Anilin- & Soda-Fabrik AG ^ _n ^ n / η η

Our reference: 0.Z.26 7H Ki / AR 6700 Ludwigshafen, 23-4.1970

Process for the electrolysis of alkali chlorides

In the electrolytic production of chlorine and alkali hydroxide by the mercury process, cells are used whose The soil consists of iron and must be completely covered with mercury during the electrolysis. To achieve that, the Soil amalgamated before commissioning the cell and thus for. Made mercury wettable, since you can then also at low Layer thickness reaches a complete coverage of the soil. During operation, the mercury coverage must be due to the risk of hydrogen formation must be maintained.

For reasons of the lowest possible energy consumption on the other hand, the line voltage must be optimally low, i.e. that there must be a small and, above all, as uniform as possible distance between the anodes and the mercury film. Unfavorable it is when the mean distance between anode and due to locally occurring increased mercury layer ceilings Cathode must be greatly enlarged. By such an unequal The mercury distribution can tear the mercury film, which, as stated, forms hydrogen. at Chlor-alkali electrolysis cells that are state-of-the-art The two above-mentioned disturbances are caused by the trace-wise deposition of iron in the mercury evoked. Iron is insoluble in mercury, but forms a suspension of fine iron crystals in the Mercury, these particles are held in the cell by magnetic forces, which causes them to disappear after a few weeks of operation Form felt-like coverings on the cell floor. Due to the local back pressure of the mercury that occurs as a result short circuits that make it necessary to pull up the anodes. This increase in the mean distance between the electrodes it also causes an increase in cell voltage.

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As a remedy, according to the current state of the art, the cells are opened at regular intervals of 6 to 8 weeks and mechanically removed the iron amalgam from the soil. In addition to significant assembly costs, this operation also incurs one not inconsiderable production loss.

Solutions have therefore been sought in order to master this difficulty: For example, attempts have been made to make the cells steeper tend in the hope that the mercury will flow fast enough to carry the iron with it. This measure has however, it has only partially proven itself, since the thin mercury layer does not have a specific one even with a very great inclination * Measure can be accelerated. In addition, the increasing current densities also increase the magnetic forces. The effect described could be delayed by this measure, but not prevented.

The present invention was based on the object of the formation of the felt-like deposits during the electrolysis of alkali chlorides after the amalgam process on the bottoms of the cells impede.

It has now been found that this object is achieved in the electrolysis of alkali chlorides by the amalgam process can be that the mercury after the decomposition of the Na "amalgam with aqueous chlorine and / or iron (III) chloride containing Solutions is dealt with.

The treatment of the mercury according to the invention is after Leaving the decomposition cell and carried out in front of or in the electrolysis cell. You can for example in with SHiIlkkörper loaded towers in which the two liquids mercury and the one containing chlorine or ferric chloride Solution initiated. However, the treatment can also be carried out in the electrolysis cell, e.g. by means of appropriate Subdivide yourself.

However, there is an intensive mixing of the two liquids for a removal of the iron from the mercury not is required, you can according to a preferred embodiment

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form of the method according to the invention, the treatment of the mercury carry out so that this is covered with the above-mentioned oxidizing liquids. this happens E.g. in such a way that the mercury after the decomposer is allowed to pass a channel into which the aqueous solution is fed will. For a given amount of mercury in circulation, this becomes so much fed in aqueous solution that the iron content of the mercury due to its content of active chlorine or iron (III) chloride for a given residence time does not fall below 0.01% by weight. If the treatment is continued, there is a risk that noticeable quantities of mercury will be dissolved. If ensures that the iron content of the circulated mercury does not exceed about 0.01 to 0.02 wt. $, see above the fungus formation described above can be avoided with certainty. The duration of the treatment according to the invention is based naturally depends on the amount of mercury to be treated and thus depends on the size of the electrolysis cell. It amounts to usually 1 to 10 $ of the residence time of the mercury in the electrolytic cell.

Chlorine-containing brines with the highest possible active chlorine content are advantageously used as the chlorine-containing liquid. Even the iron chloride is used in the form of a solution which is as concentrated as possible, to which 0.5 to 5% by weight of HCl is expediently added are.

example

a) Through a 100 kA mercury electrolysis cell every hour 4 m of mercury (54.4 t) are pumped in a circuit. To Leaving the decomposer cell, the mercury has an iron content of 0.016 wt. #. It will re-enter into the electrolysis cell with 16 m per hour of a chlorine-containing sodium chloride brine with an active chlorine content of 340 mg / 1 by overlaying in a 5 m long channel (residence time 22 seconds).

At the end of the channel, the mercury with an iron content of 0.012% by weight is drawn off, while the chlorine content of the brine _{has decreased to 50 mg of active Cl 2} / l.

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The cell can be operated for months without the malfunctions described above.

b) When using 200 l / h of a hydrochloric acid ferric chloride solution (pH = 2) with a content of 40 g Fe / l, refer to the iron content of mercury under otherwise identical conditions the same results as under a) described, achieved. After the treatment, the aqueous solution contains about 18 g Fe / l and 32.88 g Fe / l.

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Claims (1)

- 5 - ': O.Z. 26th Claim Process for the electrolysis of alkali chlorides by the amalgam process in cells with mercury cathodes, decomposition of the
Sodium amalgam formed in this process in decomposition cells in mercury and sodium hydroxide solution and recycling of the mercury into the electrolytic cells, characterized in that the mercury is treated with solutions containing chlorine and / or iron (III) chloride after the decomposition of the sodium amalgam. Badiache Anilin- & Soda-Pabrik AG 109846/1538