US3755109A - Electrolysis of alkali metal chlorides - Google Patents

Abstract

Disturbances occurring in chloride-caustic cells using the amalgam method are caused by felty coatings which form on the bottom of the cells. These disturbances can be avoided by treating the mercury (after it has left the decomposer and before or after it has entered the electrolytic cell) with an aqueous solution which contains chlorine and/or ferric chloride.

Description

United States Patent n91 Opp et al.

[ ELECTROLYSIS OF ALKALI METAL CHLORIDES [75] lnventors: Karl Opp, Heidelberg; Ewald Wygasch, Ludwigshafen; Gotthard Csizi, Bad Duerkheim, all of Germany [73] Assignee: Badische Anilln & Soda-Fabrik Aktiengesellschaft, Ludwigshafen/Rhein, Germany [22] Filed: Apr. 21, 1971 [21] Appl. No.: 136,213

[30] Foreign Application Priority Data Apr. 27, 1970 Germany P 20 20 480.2

[52] U.S. Cl 204/99, 204/140, [51] Int. Cl ..C0ld 1/08 C23b/-l/00 [58] Field of Search 204/99, 128, 140

[4 1' Aug.2 8, 1973 [56] References Cited UNITED STATES PATENTS 3,639,118 2/1972 O'Grady 204/140 X 3,364,128 l/l968 Williston et a1 204/140 Primary Examinr.lohn H. Mack Assistant Examiner-D. R. Valentine Attorney-Johnston, Root, O'Keeffe, Keil, Thompson & Shurtleff 57 ABSTRACT Disturbances occurring in chloride-caustic cells using .the amalgam method are caused by felty coatings 4 Claims, No Drawings ELECTROLYSIS F ALKALI METAL CHLORIDES In the electrolytic production of'chlorine and analkali metal hydroxide by the mercury method, cells are used whose bottom consists of iron and whichamustbe covered completely with mercury. In order to=achieve this, the bottom is amalgamated prior tosetting'the cell in operation so that it is made wettable by mercury; and complete coveringof the bottom'is achievedeven'wlien the layer of mercury isnot very thick. The covering of mercury is essential during operation of the cell? because of the risk of therisk of the formation: of hydrogen.

Onthe other hand; for reasons of keeping the comsumptionof energy at. a minimum; the cell potential should be at an optimumelow, he. the distance between* the anodes and the'mercury film shouldbe'assmall and' above all as uniform as possible. Itis-unfavor-able if; owing to local variations in the thlCkHBSSlOf the mercury layer, the mean distance betweenanode and cathode hasto bemarkedly increased. such' an uneven distribution of mercury may result in the mercury film being split apartand thenhydrogen will: be formedas mentioned above. Inchlorine-caustic cells'as designed according to the present state oftheartzthe two disturbances outlined above are caused by thedeposition of traces of iron inthe mercury. Iron'is oficourse'insoluble in mercury but forms asuspensiom offine" crystals of iron in the mercury. These particle'areretained in' the cell by magnetic forcessothatafterafew weeks? operation a felty coatingrforms on=the-bottom ofthecell. Short-circuits occur owing to the localretentionof' mercury thuscaused and'it'is'necessary to'raise the anodes. This increase in the meandistance between-the electrodes causes an-increase inthe cell-potential. The latest remedy for this is-to open the. cells atregularintervals-ofi to 8 weeksand toremove the. iron amalgam mechanically fromthe bottom. This-operation notonly causes considerable assembly costs: but alsoconsiderable losses in output.

In order to obviate this difficulty, attempts have been made to incline. the cells more steeply in the hope that the mercury would flow sufficientlyrapidly to carry the iron with it. This hope has only been partly realized because the thin layer of mercury cannot be accelerated beyond a certain extent even at a very great inclination. Moreover the magnetic forces also become greater as the current densities are increased. The said .efi'ect can be delayed by the said measures but cannot be prevented.

The present invention has as an objectthe prevention of the felty coating on the bottom of the cells in'the electrolysis of alkali metal chloridesaccording to the amalgam process.

We have found that the said object is achieved in the electrolysis of alkali metal chlorides by treating the mercury (after the alkali metal amalgam has been destroyed) with an aqueous solutioncontaining chlorine and/or ferric chloride.

Treatment of the mercury according to this invention is carried out after it has left the decomposer and before it enters the electrolytic cell. or inside the latter. It may be carried out for example in packed towers into which the two liquids (mercury and the solution containingchlorine or ferric chloride) ar introduced. The treatment may also be carried out inside the electrolytic cell itself, for example by appropriate subdivision of the same.

Since intense mixing of the two liquids is" not neces sary for the removal of iron from the mercury, how ever, it is possible according to a preferred erri'bodi'- ment of the invention to'carryout thetreati'ne'rit" of thie mercury by providing a" covering of the aboveme'ri"-' aqueous solution is-fedi For a given amount of me'rcury" being circulated ,.the amount of aqueous'soluti onfed in is such' thatby its'content'of active chlorineandlor'fe'r ricchloride, the iron' cbntent'of the'mercur-yfo'ragiv'en residence time does not fall below OiQ-l percent weight. If thetreatment is carriedfurtl'ien-there-is' a risk that mercury may be dissolved in-appreciable-quanfi ties. Provided thatthe' iron content-inthe recyclecl' mer cury is notallowed'to rise'above aboutOlO I to 0:02pm centby weight, thefelt'formationreferredto above-can'- be'reliably avoided; Thedurationof the" treatment ac-- cording to the'inventionwill" naturally depend on: the amountof'mercury tobet'reated and thusdepends oh' thesize of the electrolytic-cell; lt'amountsas-a rirle'to l to 1 '0 percent'of the residence time of the-mercury'in the electrolytic cell;

Chlorine containin'g brine havingthe highest possib'le content of activechlorineisadvantageously-used as-tliechlorinewontaining liquidi lron chlbride is also used in theform ofa-solution of-the highest possib le" concentra' tion'to' which" it'is advantageous to add from ol5 tb 15% byweight'of-HCI. v 1 The following Example illustrates the "invention? EXAMPLE a. 4'cubic meters per'hourof-m'ercur'y (5'4141 tons) is circulated througha" i kiloampere mercu y; cell. Mercury leaving thedecompjoser'hasan' iron content of'0.0 l 6% by weight. Prior'to reentryinto'the el cittrolytic cell, the mercury is treated with" l6=cubic me ters per hour-of a chlorine-containing sodium chloridebrine having a content'of active chlorine 'of340rri'gll covering it therewith in a channel having a IengthjOf S" meters (residence time 22 seconds).

At the end of the channel the mercury-is drawn with an iron content of 0.01 2% by weight. The chlorirre' content of the brine has been decreased to 50 mg of active chlorine per liter.

The cell can be operated for months at a time without} the occurrence of the disturbances described bate.

mercuryare achieved under otherwise the same condi ric chloride solution containing hydro-chloric acid'gplil 2) and having acontent of 40 g of Feper litet;

The aqueous solution contains about l8 g of Fe f per.

liter and 32.88'g of Fe+++ per We claim: I v l. A process for the electrolysisof an alkali metal chloride by the amalgam method-in a cell having a'rnercury cathode, decomposition of the alkali metal alfhal liter after the treatment.

gam thus formed in a decomposer cell vinto mercury and caustic alkali solution and return of the mercurytb" theelectrolytic cell wherein after the-decomposition off the alkali metal amalgam the resultant mercurytreated with'an aqueous solution whichcontains' chlo rine, and/or ferric chloride, said treatment'bei'n'gidonf trolled so that the iron content of the mercury "is not de' creased below 0.01% by weight. i

b. Thesame results as regards the iron content of tl'i'e 2. A process as claimed in claim 1 wherein the treatsolution is supplied so that it covers the mercury. ment is carried out by covering the mercury with a 4. A process as set forth in vclaim 1 wherein said aque layer of the aqueous solution.

3. A process as claimed in claim 1 wherein recycled mercury flows through a channel to which the aqueous 5 ous solution containsferric chloride.

t a t e

Claims (3)

1. 2. A process as claImed in claim 1 wherein the treatment is carried out by covering the mercury with a layer of the aqueous solution.
2. 3. A process as claimed in claim 1 wherein recycled mercury flows through a channel to which the aqueous solution is supplied so that it covers the mercury.
3. 4. A process as set forth in claim 1 wherein said aqueous solution contains ferric chloride.