US3785949A - Electrolysis cell with liquid electrode - Google Patents

Abstract

AN ELECTRLYSIS CELL OF THE TYPE EMPOLYING A LIQUID ELECTRODE, SUCH AS MERCURY CATHODE IS USED FOR THE ALKALINE CHLORIDE ELECTRLYSIS PROCESS. THE BOTTOM OF THE CELL IS MADE UP OF AN INECPENSIVE MATERIAL SUCH AS ALUMINUM OR ALUMINUM ALLOY TO WHICH IS BONDED A THIN LAYER HAVING SURFACE PROPERTIES SUCH AS WETTABILITY, CORROSION RESISTANCE, AND LITTLE TENDENCY TO FOULING. THE THIN LAYER MAY BE A NOBLE METAL, A CORROSION-RESISTANT METAL, SUCH AS NICKEL, A NON-METALLIC CONDUCTOR OR A SEMI-CONDUCTOR, OR AN ELECTRICALLY CONDUCTING CERAMIC MATERIAL.

Description

" United States Patent 3,785,949 ELECTROLYSIS CELL WITH LIQUID ELECTRODE Hermann Matthey, Herdekce-Kirchende, Stefan Payer,

Holzen-Hochsten, and Eberhard Zirngiebl, Cologne- Stammheim, Germany, assignors to Friedrich Uhde GmbH, Dortmund, Germany No Drawing. Filed Mar. 8, 1972, Ser. No. 232,917 Int. Cl. C22d 1/04; B01k 3/04 US. Cl. 204-219 3 Claims ABSTRACT OF THE DISCLOSURE An electrolysis cell of the type employing a liquid electrode, such as mercury cathode is used for the alkaline chloride electrolysis process. The bottom of the cell is made up of an inexpensive material such as aluminum or aluminum alloy to which is bonded a thin layer having surface properties such as wettability, corrosion resistance, and little tendency to fouling. The thin layer may be a noble metal, a corrosion-resistant metal, such as nickel, a non-metallic conductor or a semi-conductor, or an electrically conducting ceramic material.

BACKGROUND OF THE INVENTION The present invention relates to an electrolysis cell using a liquid electrode, preferably a mercury cathode, said cell being used, for example, for the alkaline chloride electrolysis process. An electrolysis cell of this type is shown in US. Pat. 3,445,373, issued May 20, 1969, and entitled Mercury Cathode Electrolysis Cell, and also in the patent application of Luciano Mose, Ser. No. 824,655, filed May 14, 1969, now U.S. Pat. No. 3,692,658, and entitled Current Supply for Electrolysis Cells. Cells of this type incorporate an area, which receives the stream of the liquid electrode or which serves for moving across the electrolyte a film of electrode liquid, part of which adheres to and part of which leaves said area. A typical example of the latter type is the Honsberg cell which, however, is no longer used in modern practice; consequently, considerations are restricted to those cells in which the liquid cathode passes along on a plate that constitutes at the same time the bottom of the cell trough. Normally, the electric current is discharged from the liquid cathode through this bottom. The bottom is required to have the following surface characteristics:

(1) Good wettability by the electrode liquid: During operation, the bottom within the cell must remain fully covered by electrode liquid to avoid undesirable reactions with the bottom material. This condition shall be safeguarded even with a thin film of the-generally expensive--electrode liquid. If, for example, the steel bottom of a mercury cell is not covered with mercury at any point during operation, the reaction of the electrolyte with the steel produces hydrogen that might cause explosions in the cell or in the piping system if it is present in certain definite quantities. I

(2) Adequate resistance against attack by the electrode liquid: Referring to the mercury process, for example, the bottom cannot be fabricated from any metals or alloys that have a strong tendency to amalgam formation, such as aluminum or brass, and would, therefore, be subject to destruction.

(3) Little tendency to fouling, i.e. adhesion of viscous mixtures of electrode liquid and substances of the electrolyte or of the equipment (for example, what is called amalgam butter).

(4) Little tendency of the material of construction of the bottom to forming such mixtures with the electrode liquid.

(5) Good general corrosion resistance to acids, caustic solutions, and brines: Cell bottoms whose surface conwhich means loss of production.

stitutes the carrier surface for the electrode liquid are required to possess further properties which, as opposed to the properties listed above, do not depend on the quality of the surface but on the bottom material itself. The following of these properties may be listed by way of example:

('6) Good electric conductivity: The electric current passes from the liquid cathode through the carrier surface into the bottom and further laterally through the outgoing busbar to the adjacent cell, i.e. the full current must pass across this bottom. For economic considerations, the various electrolysis cells-up to or more-are connected in series. The electric current passes from the incoming conductors through the anodes across the electrolyte to the liquid cathode and further through the bottom to the outgoing conductors which, in turn, are the feeders of the adjacent cell. In order to reduce the length of busbars and obtain short current paths, the cells are arranged for close spacings with the longitudinal axes of the cells running parallely to one another. With this arrangement, the electric current passes from the cathode liquid almost vertically into the bottom and further in a horizontal direction to the outgoing conductors. Current intensity in the bottom is subject to practically linear increase across the width of the bottom until it reaches its peak value at the bottom end. A high electric resistance of the bottom is bound to cause energy losses and heat formation.

(7) Sufficient mechanical strength: Deformations caused by mechanical or thermal influence might disturb the cathode flux.

(8) Sufiicient capability of heat dissipation: This is necessary to avoid hot spots on the cell bottom in the event of non-uniform load or of short circuits.

Referring to alkaline chloride electrolysis cells for the mercury process it is common practice today to use steel bottoms for carrying the mercury film. Steel bottoms combine the advantages of relatively good wettability and high mechanical strength and are comparatively inexpensive. Their disadvantage is, however, that their general corrosion resistance and electric conductivity are low. Fouling of cell bottoms is bound to affect the economy of the cells and tends to cause short circuits. The latter are praticularly undesirable for cells equipped with titanium anodes because, for economic considerations, the anodes must be operated with the smallest possible clearance to the cathode while being more sensitive yet to short circuits than are graphite anodes. Cleaning the cell bottom can be performed only when the cell is shut down,

The electric conductivity of the steel at operating temperature of the cell is so poor that thick-walled cell bottoms only can ensure economical operation in View of the high cell loads, current densities, and cell sizes commonly employed in modern process facilities. For a cell width of 2.5 m. and a current density of 15 ka./m. and on the basis of usual steel and electricity prices, the calculated optimum bottom thickness, referred to lowest operating costs, is 100 to mm. as compared to the thickness of 45 mm. commonly applied today. In calculating these figures, consideration has been given to the operating costs of the bottom that can be influenced by the bottom wall-thickness, i.e. the sum of capital expenses for the bottom material and the cost of power for discharging the electric current across the bottom. Referring to a cell with a cathode surface of 30 m. and to an economical bottom wall-thickness the weight of the bottom would be 20 to 35 tons. This figure itself reflects the problems of transportation and field assembly. Moreover, it is obvious that operating costs for cell bottoms of solid steel are markedly higher than for those of a more suitable material with better conductivity because in the majority of cases steel is not the most economical material for electric conductors.

Another disadvantage of commonly used steel bottoms is found at the time of shutting down individual cells. For a shutdown, the cell bottoms are normally shortcircuited, i.e. the total flow of electric current is carried from one neighboring cell directly to one side of the bottom and further across the total width of the bottom and across discharge conductors to the other neighboring cell. This procedure is bound to cause energy losses in the bottom with consequent heating of the bottom. Heat formation per unit area is proportional to the square of cell width and specific current density. Considering the increasing cell dimensions and current densities the equilibrium temperatures referring to electric energy input and thermal energy dissipation by convection and radiation are above the limits imposed by material resistance, rising mercury vaporization and health hazards. Under extreme conditions, calculated equilibrium temperatures exceed 200 C.

As far as electric conductivity is concerned, aluminum would be a good material for cell bottoms; however, its surface does not have the necessary properties for resistance to amalgam, which would destroy it immediately. Attempts have already been made to solve the corrosion and wetting problems of cell bottoms by using bottoms of rubberlined steel with inserted mushroom-shaped steel feeders whose surface was slightly lower than the rest of the rubberlined cell bottom surface to form circular recesses in the cell bottom. The current transfer surface of the steel was, therefore, permanently covered with mercury held up in the recess so that no hydrogen could be evolved during operation on unwetted areas and no corrosion was encountered during periods of shutdown. Deposits and fouling were practically eliminated.

Disadvantages of this method were found in the disturbance of the mercury flow through the recesses and the more elevated electric resistance in discharging the electric current across the relatively thin mercury layer of poor conductivity to the steel inserts. The unevenness of the mercury surface required a greater clearance between anode and cathode, and the two disadvantages combined to cause higher energy costs which makes this design of cell bottoms uneconomical particularly because of the high current densities commonly applied in modern facilities.

SUMMARY OF THE INVENTION The object of the invention is to provide, for a liquid electrode, a carrier, which offers good wettability, corrosion resistance, little tendency to fouling, good electric and thermal conductivity, and reduced weight.

PREFERRED EMBODIMENTS OF THE INVENTION The cell bottom according to this invention consists of two different materials of construction, either material providing optimum properties for specific requirements. Requirements relating to mechanical strength as well as electric and thermal conductivity, all of which depend on the cross-sectional area, are satisfied by a suitable inexpensive material, such as aluminum or aluminum alloy, while surface properties, such as wettability, corrosion resistance and little tendency to fouling are safeguarded by a relatively thin layer of a more suitable material such as noble metal, a corrosion-resistant metal, such as nickle, a nonmetallic conductor or a semi-conductor, or an electrically conducting ceramic material, which, because of the reduced thickness, may be allowed to be several times more expensive than steel and need not absolutely possess excellent conductivity and high mechanical strength. The two layers must be joined by a well-conducting bond. This can be achieved, for example, by explosion cladding, cladding by rolling, soldering, bonding by means of electrically conducting glues, electroplating or coating with ceramics, or by any combination of these procedures.

Example 1 For a cell of 30 m. cathode area operated at a current density of 15 ka./m. use is made of a combination-type bottom consisting of an aluminum plate of 30 mm. thickness and a steel plate of 10 mm. thickness. The two plates are bonded together by means of a well-conducting glue (max. specific resistance 0.03 ohm cm.). As compared to a steel bottom of 40 mm. thickness, this bottom features the following advantages:

Percent Weight 50 Electrical conductivity 560 Operating costs at current prices 30 At the same time, the rigidity or the apparent modulus of elasticity of the bonded assembly is still about 52.5% of the modulus of elasticity of pure steel.

Example 2 The cell bottom is a 39.5 mm. thick aluminum plate to which a 0.5 mm. thick sheet of a high-percentage nickel alloy or of pure nickel is bonded with electrically conducting glue. As compared to a steel bottom of the same thickness, the weight is 30%, the electrical conductivity 700%, the operating costs about 35%, and the apparent modulus of elasticity 33 /3 The bottom is corrosion-resistant and has little tendency to fouling because its surface remains absolutely even. Temperature differences arising in the bottom as a consequence of short circuits or non-uniform load are rapidly balanced.

What we claim is:

1. In a mercury cathode electrolytic cell, as used for the alkaline chloride electrolysis process, having a bottom through which electric current is discharged from the mercury cathode to the anodes of the laterally adjoining cell, the improvement comprising a horizontally disposed cell bottom,

said bottom being of two layers, and one layer being a carrier material of aluminum or aluminum alloy, and

the other layer of said bottom being thin and of electrical contact material having a thickness of less than 1 mm. to be wetted by the mercury cathode and possessing adequate resistance to attack thereby, said thin layer being of good general corrosion resistance and having little tendency to fouling.

2. In an electrolysis cell according to claim 1, in which said thin layer comprises a noble metal.

3. In an electrolysi cell according to claim 1, in which a said thin layer comprises a nickel or nickel base alloy.

References Cited UNITED STATES PATENTS 3,499,829 3/1970 Messner et a1 204-250 X 3,679,570 7/1972 King et al. 204250 X 3,689,397 9/1972 Norton 204250 3,679,570 7/1972 King et al. 204-250 X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. 01. X.R.