CA2175270A1 - Anode structure for electrowinning metals by anodic processes different from oxygen evolution and for electrochemical processes with separator - Google Patents

Abstract

An anode structure for cells divided by a separator of membrane or diaphragm type, characterised by consisting of a substantially box body within which feed, discharge and hydraulic sealing means for a catholyte or anolyte solution are provided, said separator facing a perforated anode plate to which it remains adhering at every point by the effect of a hydraulic head created by the level of said solution relative to the level of the solution contained in said cell.

Description

- ` ~ 1 75270 ANODE STRUCTURE FOR ELECTROWINNING METALS BY ANODIC PROCESSES
DIFFERENT FROM OXYGEN EVOLUTION AND FOR ELECTROCHEMICAL PROCESSES
WITH SEPARATOR

Current equipment for electrowinning metals from solutions of their ions are essentially of two types:
- Undivided cells. These consist of a tank containing the electrolyte in which the electrodes, ie anodes and cathodes, are immersed without any separation element interposed between them.
-10 - Divided cells. These consist of tanks in which the electrodes, ie anodes and cathodes, are immersed with separation elements such as diaphragms and/or membranes interposed between them.
In all current electrowinning applications, the separation system is implemented using a separator supported by an electrochemically inert structure independent of the anode and cathode, within which an electrode is housed.
An example of a cell with separator is that known as the Falconbridge. In this cell, used for recovering nickel from nickel chloride solutions, the cathodes are immersed in a tank and between them there are interposed anode frames in which the anode is inserted. In practice this frame is a box closed by a porous diaphragm of controlled porosity. The anode compartment is fed by applying vacuum to the compartment. Because of the pressure difference the electrolyte passes from the cathode compartment to the anode compartment through the diaphragm. The only feed to the cell is that to the tank in which the cathodes are immersed, the electrolyte being discharged from the anode compartment, from which the chlorine produced in this compartment also leaves.
This type of cell has the drawback of not being able to be equipped either with microporous diaphragms, because material passage through these is very limited, or with membranes, through which material passage is practically zero.
Other systems exist in which the tank is divided by separators, but these are always inserted into usually plastic structures which support them. These separator support structures cause very large voltage drops.
With reference to the particular case of the use of membranes, through which only ion exchange takes place, there are other drawbacks such as difficulties in applying the membrane to the support structure, difficulties in forming the hydraulic seal between the cathode and anode compartments, and difficulties in optimizing the distance between the anode and cathode.
An object of the present invention is to eliminate the aforesaid problems of the known art.
A further object is to achieve the advantage of simplified construction and operation, and reduced energy consumption.
These objects and further advantages which will be more apparent from the description given hereinafter are attained by an anode ~ 115270 structure for cells divided by a separator of membrane or diaphragm type, characterised by consisting of a substantially box body within which feed, discharge and hydraulic sealing means for a catholyte or anolyte solution are provided, said separator facing a perforated anode plate to which it remains adhering at every point by the effect of a hydraulic head created by the level of said solution relative to the level of the solution contained in said cell.
The characteristics and advantages of the invention will be more apparent from the description of embodiments thereof given hereinafter by way of non-limiting example with reference to the figures of the accompanying drawings.
Figure 1 is a partly cut-away front elevation of an anode structure according to the invention.
Figure 2 is a section therethrough on the line II-II of Figure 1.
Figure 3 shows a modified embodiment of the invention in a view similar to Figure 1.
Figure 4 shows the modified embodiment of Figure 3 in a view similar to Figure 2.
Figure 5 is an elevation of the anode structure of Figure 1 used in an electrochemical cell, this latter being shown in cross-section.
Figure 6 is a partly sectional view of Figure 5 taken on the line VI-VI.
Figure 7 shows the modified embodiment of Fi~ure 3 in a view similar to Figure 5.
Figure 8 shows the modified embodiment of Figures 3 and 4 in a Z~1S27~

view similar to Figure 6.
As stated, Figures l, 2, 5 and 6 show a first embodiment of the invention.
Specifically, Figures l, 2, 5 and 6 show an anode structure according to the invention in the case of an extractable internal cathode inserted into an open anode compartment, particularly suitable for metal recovery in which the anodic reaction is not oxygen development but the oxidation of a solution component which can be easily reduced at the cathode or can react with the product of the cathodic reaction.
Hence, Figures 1, 2, 5 and 6 relate to that embodiment of the invention applicable to metal electrowinning processes without gas development.
With reference to these figures, an anode structure of the invention, indicated overall by 20, consists of a substantially box body 1 delimited by a frame of upperly open U-section, and provided with a series of holes along its entire perimeter.
On each side of the frame of the box body 1 there are fixed, in the stated order, a perimetral gasket 2, a diaphragm or membrane separator 3, a perforated anode plate 4 and, upperly, current-carrying bars 11.
As the structure 20 is upperly open, in this case stiffening side members 22 are also provided in correspondence with said current-carrying bars 11.
Said parts are fixed by screws 5 inserted through the holes in the frame of the box body 1, so that the assembly structured in this manner forms a hydraulically sealed container for the catholyte 217~27() solution.
Internally, the frame comprises longitudinal rails 16 for housing a cathode 12 which can be independently inserted and extracted from the open upper end of the frame.
The frame of the box body 1 also comprises external guides 7 for housing the anode structure in the tank containing the anolyte solution.
The frame of the box body 1 is also provided with a feed pipe 8 and a discharge pipe 9 for the catholyte solution.
Figures 5 and 6 show the application of several anode structures 20 of said Figures 1 and 2 within an electrochemical cell comprising a tank 14 for circulating the anolyte solution, which is fed by the pipe 17 and discharged by the pipe 18.
The pipe 18 is at a lower level than the discharge pipe 9 of the anode structure 20 inserted in the tank 14, so that a hydraulic head is created in this manner between said levels under operating conditions.
As shown in Figures 5 and 6, the cathodes 12 are inserted from above into the anode structures along the longitudinal seats 16.
In operation, the effect of the hydraulic head created by the difference in level between the catholyte solution within the anode structure 20 and the anolyte solution within the tank 14 causes the separator 3 to remain adhering to the inner face of the anode plate 4 relative to the box body 1.
Figures 3 and 4 show a modified embodiment of the invention, in the typical configuration applicable to metal electrowinning with gas development.

In the embodiment of Figures 3 and 4, the anode structure indicated overall by 21 differs from the embodiment described in relation to Figures 1 and 2 in that the top of the frame delimiting the box body 1 has no aperture neither is there any internal guide for positioning the cathode, which in this case is applied external to the box body.
Guides 23 (Figure 8) for positioning the cathodes 12 are therefore in this case positioned on the inner walls of the tank 14.
The anode structure 21 is composed, in the stated order, of an upperly closed frame for the box body 1, a first perimetral gasket layer 2, the perforated anode plate 4, a further gasket 2, then the separator, consisting of the diaphragm or membrane 3, and a fixing rim 13.
The box body frame comprises holes as usual, for enabling all the parts to be fixed together by screws 5 so that the assembly structured in this manner forms a hydraulically sealed container for the anolyte solution.
The frame of the box body 1 again comprises external guides 7 for housing the anode structure in the tank which in this case contains the catholyte solution.
The frame of the box body 1 is also provided with a feed pipe 8 and a discharge pipe 9 for the anolyte solution.
The current carrying bars 11 are again in this case at the top outside the anode structure 21.
Figures 7 and 8 show the application of an electrolytic cell consisting of a tank 14 for containing the catholyte solution in which a plurality of anode structures 21 are inserted.

, As can be seen, in this case a series of cathodes 12 are inserted outside the anode structure 21 into the cell delimited by the tank 14.
The cathodes 12 hence face the anode structure 21 at the separator 3, which in this case constitutes the most outer element of the anode structure.
As can be seen from Figures 7 and 8, the discharge 9 for the anolyte solution from the inside of the anode structure 21 is positioned at a lower level than the discharge 18 for the catholyte solution from the inside of the tank 14.
Hence a hydraulic head is again formed, but this time inverted relative to the hydraulic head formed in the structure shown in Figures 5 and 6.
The result is that again in this case the surface of the separator 3 perfectly adheres at all points by the effect of the head to the anode plate 4, which in this case faces the interior of the box body from the inside of the separator 3.
Again in this second embodiment, the effect of the described hydraulic head ensures perfect adhesion between the separator 3 and the anode plate 4 so that no separator support element is required in addition to the anode plate.
From the aforegoing and from the described embodiments, it can be noted that the basic advantages of the anode structure according to the invention are firstly a simplified construction because a support for the separator (membrane or diaphragm) is no longer required, as this function is performed directly by the perforated anode plates.

`` 2175270 Evidently a membrane can be used as the separator element between the anode and cathode which itself forms a hydraulic seal, without the need for using the classical filter press configuration.
The following should also be noted:
The extreme ease of replacement or maintenance of an individual separator (membrane or diaphragm) in that each anode structure can be easily extracted from the tank in which it is immersed without the cell in question having to be excluded from the production cycle.
Annulment of the distance between the separator (membrane or diaphragm) and anode, and hence the possibility of reducing the total distance between the electrodes with advantages in reducing ohmic drops resulting in lesser energy consumption.
Versatility of use of the anode structure for different processes in that it can be formed with internal separators (membrane or diaphragm) (internal cathode) or with external separators (membrane or diaphragm) (external cathode) as described.
Versatility of material use. The materials of the various components can be easily varied as required, to adapt to the various process requirements and in particular to be compatible with the physical and chemical characteristics of the electrolytes.
The following is a general indication of usable materials:
Frames: thermoplastic and thermosetting resins.
Electrodes: carbon materials, metal alloys and metals with or without catalytic coatings.
Gaskets: elastomers or fluoroelastomers.

Nuts and bolts: thermoplastic and thermosetting resins, metal alloys and metals.
EXAMPLE
An applicational example of the anode structure of the invention, with perforated graphite anode plates, is the recovery of Pb by treating PbS (galena) with solutions containing the couple Fe2~/Fe3~ .
This process enables Pb to be deposited in the presence of ferrous ions while simultaneously oxidizing the ferrous ions to ferric, which are then used to dissolve the starting material.
This type of electrolysis allows operation with an anodic potential less than that found with electrowinning using an insoluble anode with oxygen development. This results in a considerable energy saving per unit of metal produced.
The presence of iron in solution precludes the use of an undivided cell because of the simultaneous anodic oxidation of the ferrous ions to ferric and the cathodic reduction of the ferric ion (produced at the anode) to ferrous. These reactions prejudice the progress of the most important reaction of the process, ie the cathodic deposition of metals.
For this reason, the two compartments must be kept separated using a microporous baffle so that when the metal is deposited in the presence of the ferrous ion at the cathode, the catholyte is fed into the anode compartment for oxidizing the ferrous ion to ferric without it being able to return to the cathode compartment.
Moreover, one of the main characteristics of metal deposition is the possible formation of dendrites which, notwithstanding the use of additives to prevent its formation, tend to grow and orientate towards the anode until they succeed in damaging the separator.
For this reason the separators must be accessible so that their maintenance and/or replacement are as easy as possible and do not compromise the cell operation.
In the cell used in the aforedescribed process and containing the anode structure of the invention (with an internal steel cathode and graphite anodes), e~uipped with electrodes having an area of 0.5 m2, a solution of the following composition circulates within 10 the pack:
pb2+ 90.0 g/l Fe2+ 50.0 g/l Fe3+ 0.0 g/l Free HBF 100.0 g/l Pb is deposited on the cathode contained in the anode structure with a current density of 250 A/m2 and a Faraday efficiency exceeding 97%.
The solution leaving the pack is fed into the tank containing the anode structure where, via the outer part of the anodes defining the pack and which support the separator, Fe2+ is oxidized to Fe3+
with a Faraday efficiency of 100%. The solution circulating within this tank has the following composition:
pb2+ 90.0 g/l Fe2+ 32.0 g/l Fe3+ 18.0 g/l Free HBF 71.7 g/l Initially it was thought to use solid graphite slabs. Under these 2 1 7527~) conditions a cell voltage of 2.75 V was used. This means that Pb was produced with an energy consumption of 0.73 KWh/kg.
By perforating the graphite anodes with 20 mm diameter holes to achieve a voids percentage of 12% the cell voltage fell to 2.4 V.
This means that Pb was produced with an energy consumption of 0.64 KWh/kg.
By increasing the holes in the graphite until a voids percentage of 25% was achieved, the cell voltage fell to 2.0 V. This means that Pb was produced with an energy consumption of 0.53 KWh/kg.
This value is very close to that achieved in a cell with non-supported separators in which the voltage is about 1.95 V, but this configuration, which is very sensitive to the head difference between the electrolytes, caused two types of problem:
- If the anolyte is higher than the catholyte the separator, in approaching and adhering to the cathode, becomes trapped within the deposit and for this reason tears when the cathodes are removed to recover the deposited Pb. In this case there is no variation in the cell voltage.
- If the catholyte is higher than the anolyte the separator, in approaching and adhering to the anode, masks a part of it, so increasing the anodic current density causing considerable increase in the cell voltage, which can even exceed 3 V with increase in energy consumption and the presence of anode corrosion. This Pb production had previously been implemented in a divided cell in which the separators were supported by a plastic structure perforated with about 50% of voids which enabled the cell to produce Pb with a cell voltage of about 2.8 V. As the ~ 1 75~ 70 Faraday efficiency and the electrolyte composition were as aforestated, Pb was produced with an energy consumption of 0.747 KWh/kg.
The use of the anode structure of the invention has resulted in simplified operations and reduced energy consumption. The main characteristics of the invention applied to this process can be summarized as follows:
- The support effect provided by the perforated graphite anodes (to reduce ohmic losses and ensure contact between the anolyte and catholyte) strongly reduces ohmic drops because of the non-provision of supports, and the anode chamber, the size of which is determined by the distance of two anode structures, can be reduced to the minimum necessary.
- The diaphragm is maintained against the anodes by a hydraulic head difference which is easily controllable.
- In the case of diaphragm breakage, it is not necessary to halt the cell because only that compartment concerned with the breakage is removed, whereas the rest of the elements continue to operate normally. In this respect the operation of a cell equipped with the anode structure of the invention is totally similar to that of a classical undivided cell used in electrowinning.
The cell voltage is very similar to that obtained with the unsupported diaphragm as deposition is very little penalized by this assembly.
The anode structure of the invention can be used in any electrochemical process requiring the use of a cell with separator (membrane or diaphragm).

Claims (8)

1. An anode structure for cells divided by a separator of membrane or diaphragm type, characterised by consisting of a substantially box body within which feed, discharge and hydraulic sealing means for a catholyte or anolyte solution are provided, said separator facing a perforated anode plate to which it remains adhering at every point by the effect of a hydraulic head created by the level of said solution relative to the level of the solution contained in said cell.

2. An anode structure as claimed in claim 1, characterised in that said substantially box body is open on its upper side to allow the insertion of a cathode into its interior, said separator facing said anode plate which is in a position internal to it within the box body, so as to be faced by the opposite side to said cathode when in the operating position, said level of catholyte solution in the anode structure being higher than said level of anolyte solution contained in said cell.

3. An anode structure as claimed in claim 1, characterised in that said box body is closed on its upper side, said separator facing said anode plate which is in a position external to it within the box body, so as to be faced by the opposite side to a cathode positioned external to the box body, said level of anolyte solution in the anode structure being lower than said level of catholyte solution contained in said cell.

4. An anode structure as claimed in claim 2, characterised in that on each side of the frame of said box body there are fixed, in the stated order, a perimetral gasket, a separator of membrane or diaphragm type, a perforated anode plate and, upperly, current-carrying bars.

5. An anode structure as claimed in claim 3, characterised in that on each side of the frame of said box body there are fixed, in the stated order, a perimetral gasket, a perforated anode plate and a separator of membrane or diaphragm type, a fixing rim being positioned externally.

6. An electrochemical cell characterised by comprising an anode structure in accordance with claim 1.

7. An electrochemical cell as claimed in claim 6, characterised by comprising cathodes inserted within each of said anode structures.

8. An electrochemical cell as claimed in claim 6, characterised by comprising cathodes fixed external to each anode structure.