|Title of Invention||
ANODIC STRUCTURE FOR MERCURY CATHODE ELECTROLYTIC CELLS
|Abstract||The invention relates to an anode for chlorine evolution in a mercury cathode chlor-alkali electrolytic process, consisting in a current-distributing frame and a grid array of titanium or titanium alloy or other valve metal or valve metal alloy comprising a multiplicity of generally parallel blades fixed to a multiplicity of supporting elements, the blades having a thickness comprised between 0.2 and 1 millimetres and a height comprised between 8 and 20 millimetres, the distance between one blade and the next being comprised between 1.5 and 2.5 millimetres, wherein at least the main vertical surfaces of the blades are provided with an electrocatalytic coating for chlorine avolution and wherein the surface facing the mercury cathode of the blades are mechanically machined to define a plane having a tolerance not higher than 0.2 millimetres.|
DESCRIPTION OF THE INVENTION
The present invention is directed to a new type of metallic structure (hereafter
called grid array) for gas evolving electrochemical reactions, and in particular for
the anodic reaction of chlorine evolution in a mercury cathode cell for the
electrolysis of sodium chloride with production of chlorine and sodium
hydroxide. The scope of the invention is on one hand the reduction of the
energetic consumption of the electrolysis cell, and on the other hand the
reduction of the cost for restoring the electrocatalytic coating for chlorine
evolution when the latter results deactivated.
The production of chlorine and sodium hydroxide (chlor-alkali), about 45 millions
of tons of chlorine per year, is carried out in electrolytic cells of different kinds,
among which the mercury cathode electrolytic cell is of particular relevance,
accounting for a production of about 12 millions of tons of chlorine per year.
In fig. 1 a typical structure of a cell of this kind is outlined, consisting in an iron
case (1) on whose bottom (2) the mercury amalgam (3) constituting the cathode
flows. The anode is made by a multiplicity of electrodes shaped as a grid array
(4), supported by mobile frames (5), preferably controlled by microprocessors
having the purpose of regulating the interpolar gap, which may vary during the
As 12 millions of ton chlorine/year are produced in cells at the following average
Current density: 10 kA/m2
Anode/cathode voltage: 4.05 V
Faradaic yield: 96%
Energy consumption: 3185 kWh/tonCl2
this kind of technology involves a consumption of about 38 millions of MWh / year.
In consideration of the high amount of involved energy and of the continuous increase in the cost of electricity, the cell technology has been remarkably enhanced in the course of the years, with the aim of reducing the energetic consumption, representing the most relevant item in the production costs. Among the numerous technological innovations which contributed the most to decrease the energetic consumption, the replacement of the graphite consumable anodes with the metallic anodes has to be emphasised: the latter are typically made of titanium or other valve metal, coated with electrocatalytic material generally based on noble metals and/or oxides thereof. This type of anode, an example of which is given in US 3,711,385, is still commercialised under the trade-mark DSA® by De Nora Elettrodi S.p.A, Italy. It consists in a metallic structure comprising one frame and one grid array, overlapped and mutually welded or somehow fixed; the frame performs the function of mechanical support and of element of direct electric current distribution to the surface of the grid array, which is coated with an electrocatalytic film specific for the chlorine evolution reaction, and constitutes the anodic active surface.
The geometry of the grid array plays a role of great importance on the efficiency of the electrolysis process and on the energetic consumption of a cell as it influences, in a determining way, both the voltage and the faradaic yield thereof. In fact, the anode/cathode voltage of a cell, expressed as Volts, can be calculated by means of the relationship: V anode/cathode = 3.15 + Kf x J
wherein J is the current density impressed to carry out the electrolytic process, expressed as kA/m2, and the Kf term (o "Key factor") incorporates all the components of resistive origin. The most important factors of such resistive components, namely the ohmic drop within the anodic structure, the ohmic drop in the electrolyte due to the bubble effect, and the ohmic drop in the electrolyte due to the interpolar gap, all depend from the anodic geometry; it is one of the main objects of the invention, in particular, to minimise the two latter factors. The bubble effect is a measure of the increase of ohmic resistance in the electrolyte due to the gas bubbles developing on the anodic surface of the grid array and interrupting the electric continuity within the electrolyte itself. In particular, the bubble effect mainly depends on the number and size of the gas bubbles that are generated upon the anodic surface of the grid array and stagnate on the immediate vicinity thereof between the anode and the cathode; it further depends on the bubble ascending velocity, and on the descending velocity of the degassed electrolyte.
In summary, the bubble effect depends from the actual current density on the anodic surface (which determines the amount of bubbles developing per unit time), from the grid array geometry (which determines the ratio between actual working surface whereupon the gas is evolved and projected surface, as well as the gas withdrawal resistance), and from the optional added devices directed to improve the fluid dynamics. In particular, it is a first object of the present invention to provide an anodic grid array geometry producing a bubble effect minimisation.
Even in the absence of bubble effect, the ohmic drop within the electrolyte is directly proportional to the interpolar gap, so that it is extremely important to
bring the anodic surface as close as possible to the mercury cathode, adjusting the gap between anode and cathode surfaces in a progressive fashion. It is however necessary to maintain a certain margin of safety, to avoid the mercury touching in some points the anodic surface, causing hazardous short-circuiting phenomena. For this reason, it will be possible to maintain as lower the interpolar gap as better is the planarity of the anodic structure. It is a further object of the present invention to provide an anodic grid array geometry with enhanced planarity characteristics with respect to the prior art. In the most recent industrial cells, operating in ideal conditions, the Kf is normally comprised between 0.065 and 0.085 V m2/kA, depending on the cell size, the type of anode and the system of interpolar gap adjustment the cell is equipped with, whereof:
~ 0.0070 - 0.0080 V rrrVkA are attributable to the ohmic drop within the anodic structure.
- 0.0310 - 0.0410 V m2/kA are attributable to the bubble effect in
correspondence of the anodic surface.
- 0.0270 - 0.0360 V m2/kA are attributable to the ohmic drop in the electrolyte,
as a function of the interpolar gap.
In other words, about 10% of Kf is attributable to the anode structure, about
50% to the bubble effect, and the remaining 40% to the interpolar gap.
For a given cell and at given process conditions, the minimum obtainable Kf is
therefore a property of the anode, to a large extent attributable to the grid array
characteristics (in the order of about 90%), as it depends from the width of the
region affected by the bubble effect and from the planarity of the grid array
For this reason, since the introduction of the metallic anodes, the grid array has
been the object of several inventions, among which are recalled for their
- The cited metallic anode of US 3,711,385, which in the earliest industrial
embodiments comprised a grid array made of meshes, or more commonly of a
multiplicity of titanium rods of about 3 mm diameter and 4.5 mm pitch, disposed
in parallel, and supported by a current distributing frame, in its turn made of
rectangular titanium conductors. Although enjoying a remarkable success at the
time of its introduction, this configuration presented some big limitations, due
both to the bubble effect and to the shielding effect of the rods on the cathode
surface, with consequent difficulties of electrolyte circulation and of gas
withdrawal when operating at high current density and reduced interpolar gap.
The best known industrial results with this type of anode, commonly called "Rod
type anode", operating at 10 kA/m2, are the following:
Anode/ cathode voltage: 4.00 V
Kf: 0.085 V m2/kA
Faradaic yield: -96%
Energy consumption: 3146 kWh/ton Cl2
With the purpose of overcoming these drawbacks, US Patent 4,263,107
discloses hydrodynamic baffles, mounted on the upper part of the grid array,
which generate convective motions so as to reduce the bubble effect, improve
the fluid dynamics and ensure an effective renewal of the electrolyte.
The shielding effect of the rods has been subsequently reduced with the
introduction of the invention disclosed in US 4,364,811 according to which a
grid array made of a multiplicity of rectangular strips, about 1.5 mm thick, 5 mm
high and 4.0 mm spaced, defined as blades, arranged vertically respect to the
cathode, was coupled to a frame of the prior art. The best known industrial
results with this type of anode, operating at 10 kA/m2, are the following:
Anode/cathode voltage: 3.90 V
Kf: 0.075 V m2/kA
Faradaic yield: ~96%
Energy consumption: 3067 kWh/ton Cl2
Even better results were obtained by coupling the hydrodynamic means of US
4,263,107 to a grid array made of triangular strips, with their vertex facing the
mercury cathode, as disclosed in the Italian Patent n° 1.194.397. This new
configuration, in which said triangular strips have, as typical dimensions, 2.2
mm base, 3.7 mm height, rounded vertex of 0.5 mm diameter and pitch
(intended as distance between the axis of two consecutive strips) 3.5 mm, has
brought to an important reduction of the bubble effect and of the shielding effect
of the rods, and to a sensible improvement of the fluid dynamics.
The best industrial results obtained with this type of anode, still commercialised
by De Nora Elettrodi S.p.A. under the trade-mark RUNNER®, operating at 10
kA/m2, are the following:
Anode/cathode voltage: 3.80 V
Kf: 0.065 m2/kA
Faradaic yield: -96%
Energy consumption: 2988 kWh/ton CI2
An alternative solution was proposed in US Patent US 5,589,044, which
discloses a frame similar to the previous ones, coupled to a grid array made of
a multiplicity of rectangular strips and specially configured with the purpose of
increasing the actual surface in correspondence of the vertical sides, and of decreasing the bubble stagnation effect on the surface facing the cathode. Although the results obtained with this kind of grid array are better than those obtained with the grid array of US 4,364,811, they still remain inferior to those obtained with the grid array of IT 1.194.397.
The above described grid array configurations of the prior art, different in terms of hydrodynamic properties, bubble effect and shielding effect on the cathode, have however two different aspects in common:
- the overall anodic surface planarity is hampered by the fact that the
tolerances relative to the multiplicity of elements constituting the grid array
(rods, blades or strips) and to the welds needed to fix the latter to the frame
add up to the tolerances related to the frame itself. For all the grid arrays of
the prior art, the typical tolerances along the anodic surface range between
0.5 and 1 mm, although recurring to rather controlled and sophisticated (and
thus costly) machining.
- the restoration of the catalytic properties of exhausted electrodic structures
(to be repeated with cycles ranging from 2 to 5 years, depending on the
operative conditions of the plant) involves complex and very expensive
working consisting in the removal of the exhausted coating with mechanical
(sandblasting) and chemical (etching) means frequently producing
mechanical distortions; in some cases therefore, additional working is
needed for restoring the planarity of the grid array, before (or after) providing
a novel catalytic coating. The performances of a reactivated anode are
virtually never equivalent to those of an anode of new construction, both
because the planarity reinstatement is never perfect, and because the
removal of the exhausted coating cannot be sometimes completed, or because the material constituting the grid array itself undergoes morphological changes that are not totally reversible. Finally, it is mandatory to remove the whole electrocatalytic coating even if just part of it has been consumed, in order to recover the full operability of the active surface. This involves a remarkable and useless consumption of matter consisting in extremely expensive precious metals such as ruthenium, indium', platinum, and so on.
It is an object of the invention to provide a new grid array configuration which makes possible to overcome the problems of the prior art arising from the bubble effect, the fluid dynamics, the planarity of the anodic surface, the drawbacks associated to the reactivation of the exhausted elements. The invention will be described making reference to fig. 2, which represents an axonometric view of an anodic grid array.
The latter comprises a multiplicity of blades (6) of a vatve metal, for instance pure or alloyed titanium, generally parallel to each other, orthogonally fixed to a multiplicity of supporting elements, for instance rods (7), preferably made of the same valve metal as the blades (6); on the fatter an electrocatalytic coating specific for the chlorine evolution reaction is preferably applied. In a preferred embodiment, the electrocatalytic coating is applied at least on the vertical walls of said blades, or at least on a portion thereof. The electrocatalytic coating is applied only on part of the grid array surface or on the whole surface thereof as known in the art.
The grid array of the invention must be fixed on a frame either new or used, having the function of mechanical support and of current conduction/distribution
to the grid array itself. The size of the new grid array may vary according to the dimensions of the frame to which it has to be fixed and of the size of the cell in which it has to be installed. As a mere example, a type of frame according to the prior art foresees the use of grid array surfaces of about 700 mm x 800 mm. The thickness of the blades (6) is comprised between 0.2 and 1 mm, and a particularly preferred value is 0.3 - 0.5 mm. The height of the blades is comprised between 8 and 20 mm, preferably 12 mm. The gap between two adjacent blades is comprised between 1.5 and 2.5 mm, and preferably 2.0 mm. In one preferred embodiment, for a grid array with a 700 mm x 800 mm surface, the blades (6) are bonded by means of 4 titanium rods of 2 - 3 mm diameter orthogonally welded to the upper part thereof, acting as the supporting elements (7). The number, the dimensions and the nature of the supporting elements (7) may however vary depending on the grid array dimensions, the type of current-distributing frame and other considerations associated to the process parameters.
The disclosed configuration proved to be surprisingly effective in terms of bubble effect minimisation and fluid dynamics enhancement. Moreover, the particular geometry of the blades has a positive effect on the electrode planarity, simultaneously eliminating the need to proceed with costly and harmful reactivations. In fact:
- The particular type of grid array, with long and spaced apart blades catalytically activated on the vertical walls, allows the grinding of its surface after the assemblage and activation. In other words, instead of adding the tolerances of the frame, of the elements constituting the grid array and of the relevant welds to determine the total tolerance, it is now possible to
assemble the pre-activated grid array to the frame and proceed with an overall grinding (or other equivalent mechanical machining) of the surface that has to face the cathode, with overall tolerances not exceeding 0.2 mm (± 0.1 mm). This allows maintaining extremely reduced interpolar gaps without the risk of hazardous and detrimental local short-circuiting phenomena. The described operation entails the abrasion of the blade material with elimination of the catalytic coating on the surface facing the mercury cathode (corresponding to the thickness of the blades). This elimination does not constitute a problem, since the effective catalytic coating is practically just the one deposited on the vertical walls of the blades.
- The length of the blades gives rise to the fact that only part of the activated vertical surface constitutes the active working surface, and thus only part of the catalytic coating is subjected to consumption; once said part of the coating, corresponding to a few millimetres of blade, results exhausted, instead of subjecting the anode to reactivation, with all the above described associated drawbacks, it is sufficient to proceed with a new grinding that removes the exhausted portion. This procedure allows sensible savings in electrocatalytic coating and a significant reduction of manufacturing time, considering in particular that it may be repeated more than once, resulting in a consistent increase of the anode overall lifetime, in extremely reduced manufacturing costs, and in anode/cathode cell voltages almost unvaried along the whole anode lifecycle.
At a current density of 10 kA/m2, a grid array of 700 mm x 800 mm having the above disclosed preferred dimensions (blades 12 mm high, 0.3 mm thick, 2.0
mm spaced apart), coupled to the hydrodynamic means for the generation of
convective motions disclosed in US 4,263,107, gave the following results:
Anode/cathode voltage: 3.60 V
Kf: 0.045 m2/kA
Faradaic yield: ~ 96%
Energy consumption: 2832 kWh/ton CI2
with a overall energy saving of about 150 kWh/ton CI2 compared to the best performances of the RUNNER® electrode and of about 250 kWh/ton Cl2 compared to the blade anodes of the prior art (according to the disclosure of US 4,364,811). Without wishing the invention to be bound to any particular theory, the following reasons may be hypothesised for these utterly surprising performances:
- A lower blade surface faces the mercury: 0.3 mm, according to the example
of the invention, versus 1.5 and 0.5 mm respectively of the grid array of US
4,364,81 land of the RUNNER® electrode in their known industrial
embodiments; this factor allows the downright lessening of bubble
stagnation in the portion of anodic surface facing the cathode, thereby
improving the faradaic yield at the minimum interpolar gap.
- A lower ohmic drop in the electrolyte is experienced as a lower interpolar
gap may be maintained, due to the better planarity of the grid array surface,
delimited by the longitudinal parts of the blades, which faces the cathode,
having a maximum tolerance of 0.2 mm versus 0.5 - 1 mm obtainable, in the
best cases, with the grid arrays of the prior art.
- A greater actual anodic surface is provided on one hand to by the increased
number of blades for unit projected surface (2.3 mm grid pitch according to
the disclosed example, against 3.5 - 4.0 mm of the prior art), on the other hand by the decreased bubble stagnation effect, which affects this factor as well.
Improved fluid dynamics due to the height of the blades: 12 mm according to the example, versus 5.0 and 3.7 mm typical respectively of the grid arrays of US 4,364,811 and IT 1.194.397 (RUNNER®). The increase in height gives rise to an efficient "chimney effect" with quick renewal of the electrolyte upon the electrode surface, and to a further reduction of the bubble effect due to a higher bubble ascending velocity.
1. An anode for chlorine evolution in a mercury cathode chlor-alkali
electrolytic process, consisting in a current-distributing frame and a grid
array of titanium or titanium alloy or other valve metal or vaive metal alloy
comprising a multiplicity of generally parallel blades fixed to a multiplicity
of supporting elements, the blades having a thickness comprised between
0.2 and 1 millimetres and a height comprised between 8 and 20
millimetres, the distance between one blade and the next being comprised
between 1.5 and 2.5 millimetres, wherein at least the main vertical
surfaces of the blades are provided with an electrocatalytic coating for
chlorine evolution and wherein the surfaces facing the mercury cathode of
the blades are mechanically machined to define a plane having a tolerance
not higher than 0.2 millimetres.
2. The anode as claimed in claim 1, wherein the blades have a thickness
comprised between 0.3 and 0.5 millimetres.
3. The anode as claimed in one of claims 1 or 2, wherein the upper surface
of the grid array comprises hydrodynamic means for the generation of
4. The anode as claimed in one: of claims 1 to 3, wherein the supporting
elements are made by rods.
5. The anode as claimed in one of claims 1 to 3, wherein the blades have a
height of 12 millimetres, the gap between adjacent blades is 2.0
millimetres, and the supporting elements are rods of diametre comprised
between 2 and 3 millimetres orthogonally fixed to the upper surface of
6. The anode as claimed in one of claims 1 to 5, wherein the multiplicity of
blades is fixed to the multiplicity of supporting elements by welding,
7. Method for the production of the anode as claimed in one of claims 1 to 6,
comprising the activation of the blades with said electrocatalytic coating,
the assemblage of the grid array to the frame, and a final mechanical
machining of the surface of said grid array to be faced to the mercury
cathode suited to ensure the planarity of said surface.
8. The method as claimed in claim 7, wherein said mechanical machining is a
9. Method for the reactivation of the anode as claimed in one of claims 1 to
6, comprising mechanically removing a portion of said blades in
correspondence of which said electrocatalytic coating is exhausted.
10.The method as claimed in claim 9, wherein said mechanical removal is carried out by grinding.
11.A mercury cathode sodium chloride cell for the production of chlorine, comprising an anode as claimed in claims 1 to 6.
12. An anode for chlorine evolution in a mercury cathode chlor-alkali
electrolytic process as substantially described herein with reference to the accompanying figures.
The invention relates to an anode for chlorine evolution in a mercury cathode chlor-alkali electrolytic process, consisting in a current-distributing frame and a grid array of titanium or titanium alloy or other valve metal or valve metal alloy comprising a multiplicity of generally parallel blades fixed to a multiplicity of supporting elements, the blades having a thickness comprised between 0.2 and 1 millimetres and a height comprised between 8 and 20 millimetres, the distance between one blade and the next being comprised between 1.5 and 2.5 millimetres, wherein at least the main vertical surfaces of the blades are provided
with an electrocatalytic coating for chlorine avolution and wherein the surface
facing the mercury cathode of the blades are mechanically machined to define a plane having a tolerance not higher than 0.2 millimetres.
|Indian Patent Application Number||00975/KOLNP/2003|
|PG Journal Number||51/2007|
|Date of Filing||30-Jul-2003|
|Name of Patentee||DE NORA ELETTRODI S. P.A.|
|Applicant Address||VIA DEI CANZI 1, I-20134 MILAN|
|PCT International Classification Number||C 25 B 11/02|
|PCT International Application Number||PCT/EP02/03468|
|PCT International Filing date||2002-03-27|