WO2013079788A1

WO2013079788A1 - Frame and electrolysis system

Info

Publication number: WO2013079788A1
Application number: PCT/FI2012/051172
Authority: WO
Inventors: Lauri Nordlund; Harri VANHATALO; Ville Nieminen; Henri K. Virtanen; Rauno Luoma; Noora KAAKKOLAMPI; Heikki AALTONEN; Jouni Unkuri
Original assignee: Outotec Oyj
Priority date: 2011-11-28
Filing date: 2012-11-28
Publication date: 2013-06-06
Also published as: ZA201404605B; BR112014012718A2; EP2785895B1; EP2785895A4; AU2012343685B2; EA026265B1; CN103958741B; CA2853076C; CA2853076A1; FI125637B; AU2012343685A1; FI20116193A; BR112014012718A8; EP2785895A1; CN103958741A; BR112014012718B1; EA201490765A1

Abstract

The invention concerns a frame (1) to which a bag (2) of diaphragm fabric used in an electrolysis process is attachable and to the inside of said bag an anode plate (3) is adaptable. The frame (1) comprises a horizontal top beam (4) having a first end (5) and a second end (6), an upper side (7) and a lower side (8), the anode plate being sealably attachable in relation to the top beam at the longitudinal vertical middle plane of the top beam. A vertical first beam (9) having a first upper end (10) is connected to the first end (5) of the top beam. A vertical second beam (11) having a second upper end (12) is connected to the second end (6) of the top beam. The top beam (4) comprises a cavity (13) which opens downwardly and ex- tends along the length of the lower side (8) of the top beam. The invention also concerns an electrolysis system for electrowinning a metal from an electrolyte that contains a metallic salt. The system comprises electrolytic cells (20), each of said cells having means for keeping the electrolyte at a predetermined level (L). A number of anode plates (3) and cathode plates (21) are immersed in the electrolyte in an alternating and consecutive manner. Each anode plate (3) is arranged inside the frame (1) onto which a bag (2) of diaphragm fabric is attached to form an anode bag assembly (22. The cavity (13)in the top beam (4)collects pure oxygen generated on the anode plate during the electrolysis process.

Description

FRAME AND ELECTROLYSIS SYSTEM FIELD OF THE INVENTION

The present invention relates to a frame to which a bag of diaphragm fabric used in an electrolysis pro^¬ cess is attachable and to the inside of said bag an anode plate is adaptable. Further, the invention re^¬ lates to an electrolysis system for electrowinning a metal from an electrolyte that contains a metallic salt, the system comprising electrolytic cells, each of said cells having means for keeping the electrolyte at a predetermined level and a number of anode plates and cathode plates immersed in the electrolyte in an alternating and consecutive manner, and wherein each anode plate is arranged inside a frame onto which a bag of diaphragm fabric is attached to form an anode bag assembly.

BACKGROUND OF THE INVENTION

Electrowinning is a process where a metal dissolved in an electrolyte is reduced on a cathode by means of electric current. Electrowinning takes place in an electrolytic cell that contains a number of anodes and a number of cathodes arranged in and alternating man- ner.

When an electric current is conducted to the electrol^¬ ysis system, metal is precipitated on the surface of the cathode and oxygen is generated on the anodes when the water decomposes, acid and oxygen are formed on the anodes, according to the reaction equations (1) and (2 ) :

Anodic reaction: H₂0 → |2Η + ½0₂ + 2e^" (1) (1)

Cathodic reaction: Me^z+ + ze^" -> Me (2) Me = metal, such as Ni, Co, Mn or Cu

wherein z = the charge of a metal ion

A diaphragm technique can be used in electrowinning metals, which in the electrochemical series are less noble than hydrogen. The overpotential of the reduction of these metals, which include nickel, cobalt and man^¬ ganese, is higher than that of hydrogen, which is why the development of hydrogen at a low pH should be avoided by separating the anolyte and the catholyte from each other by a material that permeates the elec^¬ trolyte in a controlled manner, such as a diaphragm fabric, and the electrolyte should flow from the catho^¬ lyte compartment to the anolyte compartment. Each anode is arranged inside an anode bag that is made of a dia^¬ phragm material that permeates the electrolyte in a controlled manner. The anode bag defines the anodic compartment on the inside thereof and the cathodes lie in the free cathodic compartment that surrounds the anode bags. The anolyte is an electrolyte that sur^¬ rounds the anode inside the bag and the catholyte is an electrolyte that surrounds the cathode.

The pH of the anolyte in the anode bag is lower (in the order of 1) than that of the catholyte in the ca^¬ thodic space (in the order of 3-4), because the elec^¬ trolyte continuously flows from the cathodic compart^¬ ment to the anodic compartment inside the anode bag. The electrolyte is fed into the cathodic compartment and removed as an overflow. The anolyte and oxygen is continuously removed from each anode bag. The pH of the catholyte does not decrease, because the electro^¬ lyte flows in the manner as described. This is typical for a sulphate based electrolysis, but not for a chlo- ride based electrolysis. The flow of electrolyte is provided by means of a pressure difference between the anodic and cathodic compartments, including a hydrostatic pressure (caused by the difference of height between the anolyte and catholyte surfaces) , and this prevents the back diffu^¬ sion of hydrogen ions into the catholyte compartment.

In prior art, the publication ZA 9810968 (Filtaquip (Proprietary) Limited) discloses a frame assembly. The Filtaquip frame comprises a horizontal top beam having a first end and a second end, an upper side and a low^¬ er side, the anode plate being sealably attachable in a slot of the top beam at the longitudinal vertical middle plane of the top beam, a vertical first beam having a first upper end which is connected to the first end of the top beam, and a vertical second beam having a second upper end which is connected to the second end of the top beam. A horizontal bottom beam is connected to the lower ends of the vertical beams to form a rigid rectangular frame. The bag is sealed about the top beam by a caulking cord which is tied around the bag to locate the cord in the caulking groove of the top beam. The frame is provided with a suction tube, through which oxygen and the anolyte can be sucked out of the bag by underpressure. The publi^¬ cation ZA 9810968 also mentions that the bag can be made to be non-porous above the electrolyte level to further prevent of escape of gas therefrom. Even though the Filtaquip anode bag technique has been used commercially for a short time in the Cawse Nickel Refinery in Australia, the sulphate based elec- trowinning process of nickel used therein is no longer in operation. There are significant drawbacks with the above- described prior art system.

In tests it has been proved that the above-described anode bag assembly is not gas-tight and leakage of am^¬ bient air into the bag can occur. This causes that the underpressure in the parallelly connected anode bag frame assemblies becomes uneven. Therefore, in some of the bags the acid concentration of the anolyte is low while in some of the bags the acid concentration is so high that the anodes may start to dissolve. The anolyte which is sucked from the anode bags is used in the leaching process. As the sucked anolyte has a low acid concentration due to said lack of gas-tightness of the anode bag assemblies, more rich electrolyte, which has a high acid concentration, must be produced otherwise for the needs of the leaching process. This causes additional costs. Further, due to the leakage of ambient air into the anode bags, pure oxygen, which also would be suitable to be used in the leaching process, cannot be ob^¬ tained. Oxygen would be regarded to be sufficiently "pure" for this purpose if the amount of oxygen in the recovered gas would be more than 80%.

Therefore, an object of the present invention is to alleviate the problems described above and to intro^¬ duce a frame and an electrolysis system that allow re- covery of anolyte with high acid concentration and recovery of pure oxygen which can be used in an acid- oxygen pressure leaching process. If excess oxygen is produced, it could be sold for outside users. Further, the object of the invention is to introduce a frame which is easy to assemble, repair and maintain. In the nickel electrowinning process the aim is to ob^¬ tain a high acid concentration of the anolyte. Also the aim is to suppress acid mist and nickel emissions in the tankhouse environment because these emissions are detrimental to health and the operators have to use breathing masks in the tankhouse environment. How^¬ ever, with the above described prior art frame that is not gas-tight it is impossible to reach these aims. Therefore, the object of the invention is also to in- troduce a frame and an electrolysis system that allow reaching of high acid concentration of the anolyte and low acid mist concentration and nickel emissions in the tankhouse environment. SUMMARY OF THE INVENTION

A first aspect of the present invention is a frame to which a bag of diaphragm fabric used in an electroly^¬ sis process is attachable and to the inside of said bag an anode plate is adaptable. The frame comprises a horizontal top beam having a first end and a second end, an upper side and a lower side, the anode plate being sealably attachable in relation to the top beam at the longitudinal vertical middle plane of the top beam. Further, the frame comprises a vertical first beam having a first upper end which is connected to the first end of the top beam, and a vertical second beam having a second upper end which is connected to the second end of the top beam. According to the in^¬ vention the top beam comprises a cavity which opens downwardly and extends along the length of the lower side of the top beam, so as to collect into the cavity pure oxygen generated on the anode plate during the electrolysis process. A second aspect of the present invention is an elec^¬ trolysis system for electrowinning a metal from an electrolyte that contains a metallic salt, said system comprising electrolytic cells, each of said cells hav^¬ ing means for keeping the electrolyte at a predeter^¬ mined level and a number of anode plates and cathode plates immersed in the electrolyte in an alternating and consecutive manner, and wherein each anode plate is arranged inside a frame onto which a bag of dia^¬ phragm fabric is attached to form an anode bag assem^¬ bly. The frame comprises a horizontal top beam having a first end and a second end, an upper side and a low^¬ er side, the anode plate being sealably attachable in relation to the top beam at the longitudinal vertical middle plane of the top beam, a vertical first beam having a first upper end which is connected to the first end of the top beam, and a vertical second beam having a second upper end which is connected to the second end of the top beam. According to the invention the top beam comprises a cavity which opens downwardly and extends along the length of the lower side of the top beam and below the predetermined electrolyte sur^¬ face level, so as to collect into the cavity pure oxy^¬ gen generated on the anode plate during the electroly^¬ sis process and to prevent any leakage of ambient air into the cavity.

The advantage of the invention is that the downwardly opening cavity of the top beam extending below the electrolyte level acts as an "air lock" and prevents any ambient air from contaminating the pure oxygen. The pure oxygen collected to the cavity can be sucked away. With the anode frame according to invention it is possible to obtain 99% pure oxygen. Pure oxygen can then be used in an acid-oxygen pressure leaching process of sulphide ores. Since pure oxygen is valuable, this saves a lot of costs. Also the anolyte which is sucked away from the anode bags has a high acid con^¬ centration and can be used in the leaching process. The need to produce more acid for the leaching process otherwise in an acid plant is reduced. If oxygen is produced in excess, it can be sold for outside users and additional income can be obtained. The frame ac- cording to the invention is easy to assemble, repair and maintain. In the nickel electrowinning process the frame and electrolysis system of the invention allow obtaining pure oxygen, reaching of high acid concentration of the anolyte and low acid mist concentration and nickel emissions in the tankhouse environment.

In an embodiment of the frame and the electrolysis system, the top beam comprises downwardly extending skirt flanges which are integral with the top beam, and which skirt flanges laterally limit the inner space of the cavity.

In an embodiment of the frame and the electrolysis system, the top beam comprises a pair of first skirt flanges each disposed at a lateral distance from the longitudinal vertical middle plane of the top beam.

In an embodiment of the frame and the electrolysis system, the top beam comprises a pair of second skirt flanges each disposed adjacent to the longitudinal vertical middle plane of the top beam.

In an embodiment of the frame and the electrolysis system, the skirt flanges are dimensioned to extend a distance below a predetermined electrolyte level when the frame is placed in an electrolysis cell having said predetermined electrolyte level.

In an embodiment of the frame and the electrolysis system, the second skirt flanges extend at a lower level than the first skirt flanges. This is because the anolyte level is kept lower than the catholyte level .

In an embodiment of the frame and the electrolysis system, the top beam comprises a slot located at the longitudinal vertical middle plane of the top beam, said slot being adapted to receive the anode plate through the slot. In an embodiment of the frame and the electrolysis system, the top beam comprises a sealing placed around the edges of the slot.

In an embodiment of the frame and the electrolysis system, the slot is adapted with a play to receive the anode plate detachably.

In an embodiment of the frame and the electrolysis system, the top beam is permanently fixed to the anode plate at the slot.

In an embodiment of the frame and the electrolysis system, the top beam comprises a suction pipe having a lower end which opens to the cavity at the first end of the top beam for removal of the oxygen and electro^¬ lyte from the cavity by suction.

In an embodiment of the frame and the electrolysis system, the lower end of the suction pipe is beveled.

In an embodiment of the electrolysis system, the means for keeping the electrolyte surface level at the pre^¬ determined level comprise an overflow outlet for re^¬ moval of the electrolyte from the cell by overflow.

In an embodiment of the electrolysis system, the anode bag assembly comprises: - a hanging rod,

the anode plate having the upper end of which fixed to the hanging rod,

- the top beam having the slot through which the anode plate is adapted to extend,

- the vertical first beam having the first upper end which is connected to the first end of the top beam, and a first lower end,

- the vertical second beam having a second upper end which is connected to the second end of the top beam, and a second lower end,

the bag of diaphragm fabric arranged to surround and accommodate the frame and the anode plate, the vertical first and second beams (, ) and a part of the top beam,

- a bottom beam arranged on or inside the bag to abut against the first and second lower ends of the vertical first and second beams, and

one or more of strapping bands strapped vertically around the hanging rod, the bag and the bottom beam to hold the anode assembly together.

In an embodiment of the electrolysis system, the anode bag assembly comprises one or more of spacers attached to the bottom beam for spacing the anode assembly from the adjacent cathode plates.

In an embodiment of the electrolysis system, the spac^¬ er is a U-shaped piece of electrically insulating ma- terial having a bottom part adapted to be placed against the lower side of the bottom beam, and U- branches which extend to a length above the bottom beam. It is to be understood that the aspects and embodi^¬ ments of the invention described above may be used in any combination with each other. Several of the as- pects and embodiments may be combined together to form a further embodiment of the invention. A frame or an electrolysis system which is an aspect of the inven^¬ tion may comprise at least one of the embodiments of the invention described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to pro^¬ vide a further understanding of the invention and con- stitute a part of this specification, illustrate em^¬ bodiments of the invention and together with the de^¬ scription help to explain the principles of the inven^¬ tion. In the drawings: Fig. 1 is an explosion view of an anode bag assembly including a frame according to one embodiment of the invention, an anode plate and a bag of diafragm fab^¬ ric, a bottom beam and spacers, Fig. 2 is a cross section II-II of the top beam from Fig. 1,

Fig. 3 is a cross section similar to Fig. 2 with an anode plate installed in the slot of the top beam,

Fig. 4 is an enlarged view of detail A from Fig. 1,

Fig. 5 shows schematically the cross section of the end parts of the electrolytic cell of one embodiment of the electrolysis system according to the invention;

Fig. 6 shows one embodiment of the anode bag assembly assembled from the parts shown in Fig 1 and bound to^¬ gether with strapping bands,

Fig. 7 is a cross section VII-VII from Fig. 6, and Fig. 8 shows another embodiment of the anode bag as^¬ sembly assembled from the parts shown in Fig 1 and bound together with strapping bands. DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodi^¬ ments of the present invention, examples of which are illustrated in the accompanying drawings. Figure 1 shows an exploded view of the anode bag as^¬ sembly 22 usable in an electrolytic process for elec- trowinning a metal from an electrolyte that contains a metallic salt. In particular, the anode bag assembly 22 of the described embodiment is usable in sulphate based nickel electrowinning . The anode bag assembly 22 of Fig. 1 is shown in an assembled state in Figs. 5 to 7. The anode bag assembly 22 comprises a frame 1 to which an anode plate 3 is adaptable. A bag 2 of dia^¬ phragm fabric can be attached on the frame 1. The an- ode plate 3 thus remains inside the bag 2.

The frame 1 comprises a horizontal top beam 4 having a first end 5 and a second end 6, an upper side 7 and a lower side 8. The anode plate 3 is sealably attachable in relation to the top beam 4 by insertion through a slot 16 located at the longitudinal vertical middle plane of the top beam 4. The frame 1 further comprises a vertical first beam 9 having a first upper end 10 which is connected, eg. by a bolt joint (not shown), to the first end 5 of the top beam 4. The frame 1 fur^¬ ther comprises a vertical second beam 11 having a se^¬ cond upper end 12 which is connected, eg. by a bolt joint (not shown), to the second end 6 of the top beam 4.

The top beam 4 comprises a cavity 13 which opens down^¬ wardly and extends along the length of the lower side 8 of the top beam 4. When the anode bag assembly 22 is installed in the electrolytic cell 20 as shown in Fig. 5, the cavity 13 can collect the oxygen generated on the anode plate 3 during the elecrolytic process. In use, leakage of air into the cavity 13 will be pre^¬ vented, because the top beam 4 will be partially im^¬ mersed in the electrolyte.

Reference is made to Figs. 2 and 3 showing the cross section of the top beam 4. The top beam 4 comprises downwardly extending skirt flanges 14, 15 which are integral, ie. uniform rigid material, with the top beam 4. The skirt flanges 14, 15 laterally limit the inner space of the cavity 13. The top beam 4 compris- es a pair of first skirt flanges 14 each disposed at a lateral distance from the longitudinal vertical middle plane T of the top beam 4. The top beam 4 also com^¬ prises a pair of second skirt flanges 15 each disposed adjacent to the longitudinal vertical middle plane T of the top beam. The skirt flanges 14, 15 are dimen^¬ sioned to extend a distance below a predetermined electrolyte level L when the frame 1 is placed in an electrolysis cell 20 having said predetermined elec^¬ trolyte level L (see also Fig. 5) . The second skirt flanges 15 extend at a lower level than the first skirt flanges 14. This is because the anolyte level Li inside the bag is kept a height H (see Fig. 4) which is lower than the catholyte level L in the cathodic compartment outside the bag.

Referring to Figs. 1 to 3, the top beam 4 comprises a slot 16 located at the longitudinal vertical middle plane T of the top beam. The slot 16 is adapted to re^¬ ceive the anode plate 3 through the slot with a small play to allow insertion. The top beam 4 comprises a ring-like sealing 17 placed around the edges of the slot 16. When the anode plate 3 is placed in the slot 16, a hanging rod 24 at the upper end of the anode plate will be pressed against the sealing 17 as shown in Fig. 3. The sealing 17 prevents leakage of air to the cavity 13 via the slot 16. In another embodiment (not-shown) the top beam 4 may be permanently fixed, eg. adhesively, to the anode plate at the slot 16,

With reference to Fig. 4, the top beam 4 comprises a suction pipe 18 having a lower end 19 which opens to the cavity 13 at the first end 5 of the top beam 4 for removal of the oxygen and electrolyte from the cavity 13 by suction. The lower end 19 of the suction pipe 18 is beveled at an angle which is preferably about 45° .

Fig. 5 is schematic cross-sectional illustration of the electrolytic cell 20. Rich electrolyte is fed to the cell via an inlet manifold 100 locating at the bottom of the cell 20. The level L of the electrolyte is maintained by an overflow outlet 23 for removal of the electrolyte from the cell 20 by overflow at the othe end of the cell.

A number of anode plates 3 and cathode plates 21 are immersed in the electrolyte in an alternating and con^¬ secutive manner. Each anode plate 3 is arranged inside a frame 1 onto which a bag 2 of diaphragm fabric is attached to form an anode bag assembly 22 which is shown in an exploded view in Fig. 1 and assembled to- gether by strapping bands 28 in Fig. 6.

Referring to Figs. 6 and 7 the anode bag assembly 22 comprises a hanging rod 24. The anode plate 3 has the upper end fixed to the hanging rod 24. The anode plate 3 extends through the slot of the top beam 4. The structure of the frame 1 is described above with reference to Figs. 1 to 4. The vertical first beam 9 has the first upper end 10 connected to the first end 5 of the top beam 4, and a first lower end 25. The vertical second beam 11 has a second upper end 12 which is connected to the second end 6 of the top beam 4, and a second lower end 26. The bag 2 of diaphragm fabric is arranged to surround and accommodate the frame 1 and the anode plate 3, the vertical first and second beams 9, 10 and a part of the top beam 4. A bottom beam 27 is arranged on the bag 2 to abut against the first and second lower ends 25, 26 of the vertical first and second beams 9, 11. Three strapping bands 28 are strapped vertically around the hanging rod 24, the bag 2 and the bottom beam 27 to hold the anode assembly together.

As shown in Figs. 6 and 7 the anode bag assembly 22 comprises three spacers 29 attached to the bottom beam 27 for spacing the anode assembly 22 from the adjacent cathode plates 21. The spacers 29 also act as guides for the strapping bands 28.

Fig. 7 shows that spacer 29 is a U-shaped piece of electrically insulating material having a bottom part 30 adapted to be placed against the lower side of the bottom beam 27, and U-branches 31, 32 which extend to a length above the bottom beam 27.

Fig. 8 shows another embodiment of the anode bag as^¬ sembly 22. The anode plate 3 has the upper end fixed to the hanging rod 24. The anode plate 3 extends through the slot of the top beam 4. The structure of the frame 1 is described above with reference to Figs. 1 to 4. The vertical first beam 9 has the first upper end 10 connected to the first end 5 of the top beam 4, and a first lower end 25. The vertical second beam 11 has a second upper end 12 which is connected to the second end 6 of the top beam 4, and a second lower end 26. A bottom beam 27 is arranged to abut against the first and second lower ends 25, 26 of the vertical first and second beams 9, 11. The bag 2 of diaphragm fabric is arranged to surround and accommodate the frame 1 and the anode plate 3, the vertical first and second beams 9, 10, the bottom beam 27 and a part of the top beam 4. Thus, the difference of the embodiment of Fig. 8 in relation to the embodiment of Figs. 6 and 7 is that in Fig. 8 also the bottom beam 27 is inside the bag 2. Three spacers 29 are attached to the bottom beam 27 on the outer surface of the bag 2 for spacing the anode assembly 22 from the adjacent cathode plates 21. The spacers 29 also act as guides for the strap^¬ ping bands 28. Three strapping bands 28 are strapped vertically around the hanging rod 24, the bag 2 and the bottom beam 27 to hold the anode assembly togeth^¬ er .

Example

In order to show the gas impermeability of the anode bag frame of the invention, the gas content inside the anode bag during the electrowinning operation of nickel was analyzed. At the current density of 200 Am-2 gas was suctioned from the bag and gas content ana- lyzed. CO₂ was analyzed with FTIR Gasmet 5 gas ana^¬ lyzer, O₂ with gas analyzer O₂ Vaisala OMT355 and ¾ with Siemens CALOMAT 6. The gas contents were as fol^¬ lows: 0₂ 98%, H₂ 1.3% and C0₂ 0.2%. Thus, the mathemat^¬ ically obtained content of ₂ was very low being less than 0.5%. The air content was almost negligible indi^¬ cating that the bag is practically gas impermeable above the electrolyte level. In addition, oxygen con^¬ tent was very high and it could be recycled to be used elsewhere .

While the present inventions have been described in connection with a number of exemplary embodiments, and implementations, the present inventions are not so limited, but rather cover various modifications, and equivalent arrangements, which fall within the purview of prospective claims.

Claims

1. A frame (1) to which a bag (2) of diaphragm fabric used in an electrolysis process is attachable and to the inside of said bag an anode plate (3) is adapta^¬ ble, the frame comprising

- a horizontal top beam (4) having a first end (5) and a second end (6), an upper side (7) and a lower side (8), the anode plate being sealably attach- able in relation to the top beam at the longitudinal vertical middle plane of the top beam,

- a vertical first beam (9) having a first upper end (10) which is connected to the first end

(5) of the top beam, and

- a vertical second beam (11) having a second upper end (12) which is connected to the second end

(6) of the top beam, characterised in that the top beam (4) comprises a cavity (13) which opens downward^¬ ly and extends along the length of the lower side (8) of the top beam, so as to collect into the cavity pure oxygen generated on the anode plate during the elec^¬ trolysis process.

2. The frame according to claim 1, characterised in that the top beam (4) comprises downwardly extending skirt flanges (14, 15) which are integral with the top beam, and which skirt flanges laterally limit the in^¬ ner space of the cavity (13) .

3. The frame according to claim 2, characterised in that the top beam (4) comprises a pair of first skirt flanges (14) each disposed at a lateral distance from the longitudinal vertical middle plane (T) of the top beam.

4. The frame according to claims 2 or 3, characterised in that the top beam (4) comprises a pair of second skirt flanges (15) each disposed adjacent to the lon^¬ gitudinal vertical middle plane (T) of the top beam.

5. The frame according to one or more of claims 2 to 4, characterised in that the skirt flanges (14, 15) are dimensioned to extend a distance below a predeter^¬ mined electrolyte level (L) when the frame (1) is placed in an electrolysis cell having said predeter^¬ mined electrolyte level.

6. The frame according to claim 5, characterised in that the second skirt flanges (15) extend at a lower level than the first skirt flanges (14) .

7. The frame according to one or more of claims 1 to 6, characterised in that the top beam (4) comprises a slot (16) located at the longitudinal vertical middle plane (T) of the top beam, said slot being adapted to receive the anode plate (3) through the slot.

8. The frame according to claim 7, characterised in that the top beam (4) comprises a sealing (17) placed around the edges of the slot (16) .

9. The frame according to claim 7 or 8, characterised in that the slot (16) is adapted with a play to re^¬ ceive the anode plate (3) detachably.

10. The frame according to claim 7, characterised in that the top beam (4) is permanently fixed to the an^¬ ode plate at the slot (16) .

11. The frame according to one or more of claims 1 to 10, characterised in that the top beam (4) comprises a suction pipe (18) having a lower end (19) which opens to the cavity (13) at the first end (5) of the top beam (4) for removal of the oxygen and electrolyte from the cavity by suction.

12. The frame according to claim 11, characterised in that the lower end (19) of the suction pipe (18) is beveled .

13. An electrolysis system for electrowinning a metal from an electrolyte that contains a metallic salt, said system comprising electrolytic cells (20), each of said cells having means for keeping the electrolyte at a predetermined level (L) and a number of anode plates (3) and cathode plates (21) immersed in the electrolyte in an alternating and consecutive manner, and wherein each anode plate (3) is arranged inside a frame (1) onto which a bag (2) of diaphragm fabric is attached to form an anode bag assembly (22), the frame (1) comprising

- a horizontal top beam (4) having a first end (5) and a second end (6), an upper side (7) and a lower side (8), the anode plate (3) being sealably at^¬ tachable in relation to the top beam (4) at the longitudinal vertical middle plane (T) of the top beam (4),

(5) of the top beam (4), and

(6) of the top beam (4), characterised in that the top beam (4) comprises a cavity (13) which opens downward^¬ ly and extends along the length of the lower side (8) of the top beam (4) and below the predetermined elec^¬ trolyte surface level (L) , so as to collect into the cavity (13) pure oxygen generated on the anode plate during the electrolysis process and to prevent any leakage of ambient air into the cavity.

14. The electrolysis system according to claim 13, characterised in that the top beam (4) comprises down^¬ wardly extending skirt flanges (14, 15) which are in^¬ tegral with the top beam, and which skirt flanges laterally limit the inner space of the cavity (13) .

15. The electrolysis system according to claim 14, characterised in that the top beam (4) comprises a pair of first skirt flanges (14) each disposed at a lateral distance from the longitudinal vertical middle plane (T) of the top beam.

16. The electrolysis system according to claims 14 or 15, characterised in that the top beam (4) comprises a pair of second skirt flanges (15) each disposed adja^¬ cent to the longitudinal vertical middle plane (T) of the top beam.

17. The electrolysis system according to one or more of claims 13 to 16, characterised in that the skirt flanges (14, 15) are dimensioned to extend a distance below a predetermined electrolyte level (L) when the frame (1) is placed in an electrolysis cell having said predetermined electrolyte level.

18. The electrolysis system according to claim 17, characterised in that the second skirt flanges (15) extend at a lower level than the first skirt flanges (14) .

19. The electrolysis system according to one or more of claims 13 to 18, characterised in that the top beam (4) comprises a slot (16) located at the longitudinal vertical middle plane (T) of the top beam, said slot being adapted to receive the anode plate (3) through the slot.

20. The electrolysis system according claim 19, characterised in that the top beam (4) comprises a sealing (17) placed around the edges of the slot (16) .

21. The electrolysis system according to claim 19, characterised in that the the slot (16) is adapted with a play to receive the anode plate (3) detachably.

22. The electrolysis system according to claim 19, characterised in that the top beam (4) is permanently fixed to the anode plate (3) at the slot (16) .

23. The electrolysis system according to one or more of claims 13 to 22, characterised in that the top beam (4) comprises a suction pipe (18) having a lower end

(19) which opens to the cavity (13) at the first end

(5) of the top beam (4) for removal of the oxygen and electrolyte from the cavity by suction.

24. The electrolysis system according to claim 23, characterised in that the lower end (19) of the suc^¬ tion pipe (18) is beveled.

25. The electrolysis system according to one or more of claims 13 to 24, characterised in that the means for keeping the electrolyte surface level at the pre^¬ determined level (L) comprise an overflow outlet (23) for removal of the electrolyte from the cell (20) by overflow .

26. The electrolysis system according to one or more of claims 13 to 25, characterised in that the anode bag assembly (21) comprises:

- a hanging rod (24),

- the anode plate (3) having the upper end of which fixed to the hanging rod (24), the top beam (4) having the slot (16) through which the anode plate is adapted to extend, the vertical first beam (9) having the first upper end (10) which is connected to the first end (5) of the top beam (4), and a first lower end (25) ,

- the vertical second beam (11) having a se^¬ cond upper end (12) which is connected to the second end (6) of the top beam (4), and a second lower end (26),

- the bag (2) of diaphragm fabric arranged to surround and accommodate the frame (1) and the anode plate (3), the vertical first and second beams (9, 10) and a part of the top beam (4),

- a bottom beam (27) arranged on or inside the bag (2) to abut against the first and second lower ends (25, 26) of the vertical first and second beams (9, 11), and

one or more of strapping bands (28) strapped vertically around the hanging rod (2), the bag (2) and the bottom beam (27) to hold the anode assembly together.

27. The electrolysis system according claim 26, char- acterised in that the anode bag assembly (22) compris^¬ es one or more of spacers (29) attached to the bottom beam (27) for spacing the anode assembly (22) from the adjacent cathode plates (21).

28. The electrolysis system according claim 27, characterised in that the spacer (29) is a U-shaped piece of electrically insulating material having

- a bottom part (30) adapted to be placed against the lower side of the bottom beam (27), and

- U-branches (31, 32) which extend to a length above the bottom beam (27) .