WO2004046424A2

WO2004046424A2 - Metal amalgam decomposer for mercury cathode electrolytic cells

Info

Publication number: WO2004046424A2
Application number: PCT/EP2003/012745
Authority: WO
Inventors: Dario Oldani; Salvatore Peragine
Original assignee: De Nora Elettrodi S.P.A.
Priority date: 2002-11-15
Filing date: 2003-11-14
Publication date: 2004-06-03
Also published as: WO2004046424A3; AU2003292022A1; AU2003292022A8; ITMI20022420A1

Abstract

It is described an amalgam decomposer for mercury cathode electrolytic cells, provided with a liquid distributor (8) consisting of an arrangement of conduits comprising an external annulus (15), one or more internal annuli (16) and a central distributing tube (17) suitable for being crossed in sequence by a water flow. The distributor of the invention is suited to the upward feed of a graphite pack (7) crossed by the amalgam under gravity comprising at least one gas diffuser of anyone of the preceding claims. Arrangements of graphite packs disposed as a bundle of tubes, each fed by a distributor of the invention are also provided.

Description

METAL AMALGAM DECOMPOSER FOR MERCURY CATHODE ELECTROLYTIC CELLS

DESCRIPTION OF THE INVENTION

The invention relates to the field of electrolytic plants with mercury cathode, in particular to electrolysis plants for the production of alkali obtained by discharge of a metal ion on a mercury cathode with formation of metal amalgam, and subsequent water phase decomposition of the amalgam to obtain an alkali solution with simultaneous evolution of hydrogen. The most common industrial case to which reference will be made for the sake of simplicity is that of mercury chlor- alkali plants, which in the most common case carry out the electrolysis of sodium or potassium chloride with formation of the relevant amalgam, and thus of the relevant hydroxide upon decomposition.

In particular, the invention relates to a novel metal amalgam decomposition unit design.

In the above indicated processes of electrolysis with mercury cathode, the metal ions contained in the electrolysed solution (for instance Na+ ions from sodium brine) are discharged on a negatively charged mercury bed flowing on the cell bottom; the product metal is bonded to the mercury forming an amalgam (in the mentioned case, a sodium amalgam) which is extracted from the cell outlet end-box and fed to the decomposer. As it is known to the experts of the field, the amalgam decomposer is a fixed bed reactor, with a filler usually made of graphite, in which the amalgam flows under gravity to the bottom; a liquid flow, generally pure water, is fed countercurrent from the bottom (hence in the upward direction) causing the amalgam splitting and the production of hydroxide with hydrogen evolution, according to the reaction:

2 Na(Hg)_x + 2 H₂0 → H₂ +2 NaOH + x Hg The critical aspect of this reaction is in the homogeneous compenetration of the two countercurrent flows of amalgam and water; in particular, water easily tends to form preferential paths within the graphite pack and the interaction with the metal amalgam is liable to be non optimal. A first consequence lies in the fact that, to obtain a complete decomposition of the amalgam, quite tall decomposing units are needed, which are not always compatible with the encumbrance requirements of the plants to which they are destined. A second, even more serious consequence, is in the fact that the inhomogeneous water feeding tends to create high local concentrations of alkali, prevalently along the decomposer walls; this gives often rise to corrosion problems of the steel mantle delimiting the decomposer itself.

It is an object of the present invention to provide a metal amalgam decomposer overcoming the limitations of the prior art, in particular allowing a uniform flow of water countercurrent to the metal amalgam and an efficient decomposition of the latter also with graphite packs of limited height.

Under one aspect, the invention consists of a distributing element for a metal amalgam decomposer comprising an arrangement of conduits formed by a multiplicity of distributing annuli with calibrated holes, suitable for being crossed by the liquid from the outermost to the innermost, and by a central dead-end distributing tube, also provided with calibrated holes.

Under a second aspect, the invention consists of a metal amalgam decomposer formed by a lower mercury outlet section wherein the distributor of the invention is positioned, a central section with a graphite pack fed from the top with the metal amalgam to be decomposed, and an upper section containing the hydrogen stripping and alkaline hydroxide outlet ducts.

Under another aspect, the invention consists of a metal amalgam decomposer comprising a multiplicity of graphite packs disposed in a tube bundle, for instance according to a radial arrangement, each bundle containing a distributor of the invention in the lower mercury outlet section.

The invention will be described making use of the annexed figures, having a merely exemplifying scope not limiting the extent of the invention. - Figure 1 schematically represents the metal amalgam decomposer of the invention coupled to a mercury cathode electrolytic cell.

- Figure 2 represents the liquid distributing element of the invention.

- Figure 3 represents a radial arrangement of three liquid distributing elements of the invention.

- Figure 4 represents an exploded view of a decomposer comprising three radially arranged graphite packs according to the invention.

In figure 1 the connection of a metal amalgam decomposer (1) with a mercury cathode electrolytic cell (2), for instance a chlor-alkali cell is shown. From the outlet end-box (3) of the mercury cell (2) the cathodic product to be decomposed, consisting of the alkali metal (for example sodium) in amalgam phase, is discharged through the duct (4), along the direction indicated by the arrow (5); it is then fed to the decomposer (1) in correspondence of a perforated distributing plate (6), wherefrom it spreads under gravity through a graphite pack (7), crossed countercurrent by a flow of liquid, generally consisting of pure water, fed by means of the distributing element of the invention (8) along the direction indicated by the arrow (9). The mercury recovered from, the deamalgamation reaction is collected at the bottom, along the direction of the arrow (10). The arrow (11) indicates the outlet of the hydrogen produced by the deamalgamation, discharged through the nozzle (12), while the arrow (13) indicates the outlet of the alkali hydroxide solution (for instance caustic soda) through the nozzle (14). In the decomposers of the prior art, the water is fed from the bottom without any particular precaution, and requires a certain vertical development of the graphite pack (7) wherein the reaction takes place to achieve the deamalgamation. In the decomposer of the invention, the water is fed in a much more homogeneous fashion because of the distributing element (8), shown in detail in figure 2. It consists of an arrangement of conduits comprising at least two concentric distributing annuli, namely one outer distributing annulus (15) and at least one inner distributing annulus (16), connected by a central dead-end distributing tube (17); both the annuli and the central distributing tube are provided with calibrated holes (18) feeding the graphite pack (7) in a controlled fashion. It is an essential feature of the invention that the distributing annuli be crossed by the liquid in sequence starting from the external one (15) and proceeding inwards. In a preferred embodiment, each annulus is divided in two generally symmetrical sectors, for example delimited by the central distributing tube (17). According to a particularly preferred embodiment, the distributing element is configured so that it is crossed by the liquid, proceeding inwards, according to a flow pattern splitting at the inlet of the first annulus between the two sectors into which the latter is subdivided, rejoining in the central tube, splitting again at the inlet of the subsequent annulus between the two relevant sectors and so on. For instance, in the case of the distributing element of figure 2, comprising two annuli, the flow of liquid is first fed to the outer distributing annulus (15), symmetrically splitting between the two semicircular sectors composing the same, rejoining again in the central tube (17), splitting again in half between the two semicircular sectors in the inner distributing annulus (16), finally rejoining in the dead-end central tube (17). Along this path, the liquid is progressively fed in a controlled fashion through the calibrated holes (18) that are present along the whole path. In this way, the fed liquid is uniformly distributed on each annulus, but with decreasing flow-rate going from the outside to the inside of the decomposer; such aimed distribution, directed to provide more water to the zones of larger development, serves to ensure an alkali concentration uniformity even in the peripheral zones of the graphite pack.

In some cases, the dimensional constraints, especially in terms of decomposer height, make the device of the invention not yet sufficient to guarantee the required decomposition efficiency. This case applies especially when pre-existing plants are retrofitted, for instance to improve their performances. The distributing element of the invention allows operating with elements of even lower height, and wider base surface, when coupled with other equivalent ones, for example according to a radial arrangement. In that case, it is possible to operate with a multiplicity of graphite packs (7) disposed according to a tube bundle provided with upper and lower tube plates, each pack (7) being fed from the bottom with a distributing element (8) of the invention. In figure 3 a coplanar radial arrangement of three distributing elements (8) of the invention is shown. The configuration of a decomposer adopting a design of this type is shown in the exploded view of figure 4: the various elements of the decomposer (1) shown therein are identified by the same reference numerals employed in the previous figures. It is hence indicated as (4) the feeding of metal amalgam coming from the mercury cathode cell, crossing the graphite packs (7) through the relevant perforated distributing plates (6), while (8) indicates the distributing elements fed with water or other decomposing liquid. The deamalgamated mercury is discharged at (10), while (12) is the withdrawal nozzle of the hydrogen and (14) that of the alkali solution.

The above description is not to be understood as limiting the invention, which may be practiced according to different embodiments without departing from the scopes thereof, and whose extent is univocally defined by the appended claims.

In the description and the claims of the present application, the term "comprise" and variations thereof such as "comprising" and "comprises" are not intended to exclude the presence of other elements or additional components.

Claims

1. A distributing element for a metal amalgam decomposer comprising an arrangement of conduits formed by outer distributing annulus, at least one inner distributing annulus and a central dead-end distributing tube, said distributing annuli and said distributing tube provided with calibrated holes, wherein said arrangement of conduits is suitable to be crossed by a flow of liquid which crosses sequentially said outer distributing annulus and said at least one inner distributing annulus, feeding the decomposer with a decreasing flow-rate from the outside to the inside.

2. The distributing element of claim 1 wherein said outer distributing annulus and said at least one inner distributing annulus are subdivided in at least two distinct sectors.

3. The element of claim 2, wherein said arrangement of conduits is suitable for being crossed by said flow of liquid through the subsequent steps of: splitting of said flow between said at least two distinct sectors of said outer distributing annulus rejoining of said flow splitting of said flow between said at least two distinct sectors of said inner distributing annulus rejoining of said flow in said central distributing tube.

4. A metal amalgam decomposer comprising at least one distributing element of the previous claims.

5. The decomposer of claim 4 comprising a radial arrangement of said distributing elements.

6. The decomposer of claim 4 or 5 comprising a lower mercury outlet section, said lower section containing the at least one distributing element fed with water, a central section containing at least one graphite pack fed from the top with the metal amalgam, an upper section containing the hydrogen stripping and the metal hydroxide outlet ducts.

7. The decomposer of claim 6 wherein the metal amalgam is fed through a perforated distributing plate positioned in the upper part of said central section.

8. The decomposer of claim 6 or 7 comprising three of said distributing elements radially disposed in correspondence of three of said graphite packs housed in a tube bundle provided with upper and lower tube plates.

9. A process of decomposition of metal amalgam coming from a mercury-type electrolyser, comprising feeding said metal amalgam to a decomposer of claims 4 to 8 and the production of metal hydroxide and hydrogen.

10. The process of claim 9 wherein said metal amalgam crosses said decomposer from top to bottom, countercurrent to a flow of water fed through said at least one distributing element..

11. The process of claim 10 wherein said metal amalgam is a sodium or potassium amalgam and said metal hydroxide is respectively caustic soda or potash.

12. A metal amalgam decomposer containing the distinctive elements of the description and the figures.