EP2403980A1

EP2403980A1 - Method of electrowinning a metal and an electrolysis system

Info

Publication number: EP2403980A1
Application number: EP10738243A
Authority: EP
Inventors: Ville Nieminen; Henri Virtanen
Original assignee: Outotec Oyj
Current assignee: Metso Corp
Priority date: 2009-02-03
Filing date: 2010-02-02
Publication date: 2012-01-11
Anticipated expiration: 2030-02-02
Also published as: EA201190121A1; AU2010210040A1; CA2751302C; EP2403980B1; WO2010089452A1; BRPI1008826A2; FI122595B; AU2010210040B2; CN102308029A; FI20095094A0; FI20095094L; EA020315B1; CA2751302A1; CN102308029B; EP2403980A4; BRPI1008826B1

Abstract

A method and a system of electrowinning, in an electrolytic tank, a metal from an electrolyte that contains a metallic salt. In the method, a diaphragm fabric is selected as the material of the anode bag from a group of diaphragm fabrics with different permeabilities, the dependences of the permeability of the diaphragm fabric on the viscosity of the catholyte and the pressure difference used are defined. On the basis of the defined dependences, the exit velocity of the anolyte from the anode bag is adjusted by the current density that is used, the pressure difference and the viscosity of the catholyte, so that an acid content of the anolyte of at least 50 g/1 is obtained. In the system, a fabric wherein the dependences of its permeability on the viscosity of the catholyte and the pressure difference used are known is selected as the diaphragm fabric of the anode bag from a group of diaphragm fabrics with different permeabilities. The exit velocity of the anolyte is adjusted by the current density used, the pressure difference and the viscosity of the catholyte, so that the acid content of the anolyte that is removed from the anode bag is at least 50 g/1.

Description

METHOD OF ELECTROWINNING A METAL AND AN ELECTROLYSIS SYSTEM

FIELD OF THE INVENTION

The invention relates to a method defined in the preamble of Claim 1. The invention further relates to an electrolysis system defined in the preamble of Claim 11.

BACKGROUND OF THE INVENTION

In electrolysis, a metal that is dissolved in an electrolyte is electrowon. Electrowinning takes place in an electrolytic tank that contains a number of anodes and a number of cathodes that are arranged in and alternating manner. When an electric current is conducted to the system in sulphate-based electrolysis, metal is precipitated on the surface of the cathode and, when the water decomposes, acid and oxygen are formed on the anodes, according to the reaction equations (1) and (2) :

Anodic reaction: H₂O → |2H]⁺ + ¹^O₂ + 2e^~ (1) (1) Cathodic reaction: Me²⁺ + ze^~ -> Me

Me = metal, such as Ni, Co, Mn or Cu wherein z = the charge of a metal ion

A diaphragm technique is used in electrowinning metals, which in the electrochemical series are more ignoble than hydrogen. The overpotential of the reduction of these metals, which include for example nickel, cobalt and manganese, 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 ca- tholyte space to the anolyte space. Generally, the cathodes in the sulphate-based electrolysis are placed in a diaphragm space.

When using anode bags, each anode is arranged inside an anode bag that is made of the material that permeates the electrolyte in a controlled manner. The anode bag defines an anodic space in its inside and the cathodes are in the free cathodic space that surrounds the anode bags. Due to the formation of acid, the pH of the anolyte in the anode bag is lower (in the order of pH 1 or lower) than the pH of the catholyte in the ca- thodic space (in the order of pH 3-4) . The electrolyte flows continuously from the cathodic space into the anodic space inside the anode bag. The anolyte is an electrolyte that surrounds the anode and the catholyte is an electrolyte that surrounds the cathode. The electrolyte is fed into the cathodic space and removed by overflow. The anolyte is continuously removed from each anode bag. The flow of electrolyte is provided by means of a pressure difference between the anodic and cathodic spaces, including a hydrostatic pressure (caused by the difference of height between the anolyte and catholyte surfaces) , and this prevents the back diffusion of protons into the catholyte space.

Even though the anode bag technique has been used com- mercially for a short time in the Cawse Nickel Refinery in Australia, the electrowinning process of nickel used therein is no longer in operation.

The Anglo Platinum Base Metal Refinery, which is lo- cated in South Africa, has tested the anode bag technique ["Nickel Electrowinning Tankhouse Developments at Anglo Platinum Base Metal Refinery", Authors: LJ Bry- son, NJ Graham, EP Bogosi, DL Erasmus proceedings of ALTA 2008 - Nickel/Cobalt, Copper & Uranium Conference, June 16-18 2008] . According to this article, the process criteria regarding the anolyte acid concentration were the main stumbling stones in the implementation. The anode bags functioned under low pressure, which was needed to remove both aerosols and anolyte from the anode bag, resulting in pulling a low acid concentration through the anode bags, which further caused a significant addition in the amount of nickel electrolyte that was circulated. In other words, due to the low anolyte acid concentration, the anode bag technique tested in the Anglo Platinum Base Metal Re- finery was not economically attractive.

Production of nickel is more economic, if a higher acid concentration of the anolyte than before is achieved in the electrowinning process of nickel. The high acid content of the anolyte is useful in dissolution. The high anolyte acid concentration can be provided by a suitable selection of a diaphragm fabric and by controlling the process parameters, namely the viscosity of the electrolyte and the permeability of the wall of the anode bag that is formed from the diaphragm fabric that permeates the electrolyte in a controlled manner, the permeability depending on the viscosity of the electrolyte.

The problem is that it . is difficult to select the correct diaphragm fabric of the anode bag, since manufacturers offer fabrics that have different gradual permeabilities and they express the permeability of the fabric in permeability of water only (a specific vis- cosity) and at one specific pressure difference (a specific height of a water column) . When a high anolyte acid content is to be achieved, it is impossible to select the correct fabric on the basis of the above information, as it is not known, what the permeability of the fabric to the electrolyte is and how the viscosity of the electrolyte and the pressure difference on the different sides of the fabric influence the permeability.

OBJECT OF THE INVENTION

The object of the invention is to eliminate the disadvantages mentioned above.

In particular, the purpose of the invention is to dis- close a method and a system, which enable the selection of the diaphragm fabric of the anode bag and the adjustment of process parameters, which is made on the basis of the selected fabric, so that a desired degree of anolyte acid concentration is obtained in the an- odic space, which makes the electrowinning of metal a profitable business.

SUMMARY OF THE INVENTION

The method according to the invention is characterized in that, which is disclosed in Claim 1. The electrolysis system according to the invention is characterized in that, which is disclosed in Claim 11.

According to the invention, a diaphragm fabric is selected in the method as the material of the anode bag from a group of diaphragm fabrics with different permeabilities, the dependences of the permeability of the diaphragm fabric on the viscosity of the catholyte and the pressure difference used are defined, and on the basis of the defined dependences, the exit veloc- ity of the anolyte from the anode bag is adjusted by- means of the current density used, the pressure difference and the viscosity of the catholyte, so that an acid content of the anolyte of at least 50 g/1 is ob- tained.

According to the invention, a fabric wherein the dependences of its permeability on the viscosity of the catholyte and the pressure difference used are known is selected as the diaphragm fabric of the anode bag in the system from a group of diaphragm fabrics with different permeabilities. The exit velocity of the anolyte is adjusted by the current density used, the pressure difference and the viscosity of the ca- tholyte, so that the acid content of the anolyte that is removed from the anode bag is at least 50 g/1.

It is essential for the invention that the flow can primarily be defined on the basis of the density of the fabric, i.e., the permeability of the electrolyte which, in addition to the properties of the fabric, is defined by the pressure difference and the viscosity.

•The anolyte acid concentration can be controlled by using a tighter fabric for a less viscous electrolyte, which decreases the flow velocity from the cathodic space into the anodic space inside the anode bag, and vice versa, i.e., using a less tight .fabric for a more viscous electrolyte. The viscosity of the electrolyte, in turn, can be adjusted to a minor degree by the concentration of the metallic salt in the catholyte, whilst the other parameters remain constant. This adjustment does not necessarily have significance for the process. For example,- the variation of the Ni con- tent in the process can be as little as about 10 g/1. The concentration of metallic salt in the catholyte, in turn, can be adjusted by adjusting the circulation speed of the catholyte. Fig. 1 shows the block flow chart of the process.

The acid concentration thus settles on a specific level, which depends on the exit velocity of the electrolyte (which is as high as the flow of electrolyte into the anode bag) and the flow used. E.g., in a process, wherein the total flow that is fed into one electrolytic tank is 16000 A and the flow of electrolyte into the anode bags totals 0.33 rrvVh, the anolyte acid concentration is about 80 g/1.

In an application of the method, the concentration of the metallic salt in the catholyte and/or the viscosity of the catholyte are measured, and the circulation speed of the catholyte is increased, if the metallic salt concentration in the catholyte and/or the viscosity of the catholyte exceed a predefined limit value, and the circulation speed of the catholyte is reduced, if the metallic salt concentration in the catholyte and/or the viscosity of the catholyte are below the predefined limit value.

In an application of the method, the pressure difference is adjusted by changing the difference of height between the fluid levels of the anolyte and the catholyte .

In an application of the method, the anolyte is removed from each anode bag by overflow so that, to provide a hydrostatic pressure, the level of the anolyte is kept lower than the level of the catholyte. This provides a sufficiently high hydrostatic pressure, which causes a flow of electrolyte from the catholyte side into the anode bags, preventing the migration of protons from the anolyte back to the catholyte side, i.e., from the anodic space into the cathodic space.

In an application of the method, the anolyte is re- - moved from each anode bag by means of an overflow pipe, its head defining the level of the anolyte in the anode bag. The position of the overflow pipe head inside the anode bag very accurately defines the maximum level of anolyte in the anode bag. Damage of the anode bag is easy to observe, as the level of anolyte in the anode bag then rises higher than normal and the flow of anolyte to the overflow pipe increases to an exceptional degree, which is easy to observe and start the correcting measures accordingly.

In an application of the method, the anolyte is conveyed from the overflow pipes to a collector and further to a collecting tank.

In an application of the method, the anolyte is removed from each anode bag by means of a first suction pipe.

In an application of the method, oxygen and/or acid fog are sucked from the anode bag.

In an application of the method, the oxygen and/or acid fog are sucked from the anode bag through the first suction pipe, a second suction pipe and/or the overflow pipe.

In an application of the method, the metal to be elec- trowon is nickel, cobalt or manganese, and the metallic salt is the sulphate of the respective metal. In an application of the system, the system includes a means of measuring the concentration of metallic salt in the catholyte, and a means of adjusting the circulation speed of the catholyte on the basis of the measured concentration.

In an application of the system, the means of removing the anolyte include an overflow pipe for each anode bag, which overflow pipe opens in the area of the up- per part of the anode bag, defining the level of the anolyte in the anode bag, so that the level of the anolyte is lower than that of the catholyte.

In an application of the system, the system includes a collector for receiving the anolyte that is collected by the overflow pipes.

In an application of the system, the system includes a collecting tank for receiving the anolyte from the collector.

In an application of the system, the anode bag includes a first suction pipe for sucking the anolyte and, possibly, oxygen and/or acid fog from the anode bag.

In an application of the system, the anode bag includes a second suction pipe for sucking the anolyte, oxygen and/or acid fog from the anode bag.

LIST OF FIGURES

In the following, the invention is described in detail by means of application examples and with reference to the appended drawing, wherein

Fig. 1 shows the block flow chart of the process; Fig. 2 shows schematically the cross section of part of the electrolytic tank that belongs to one application of the electrolysis system according to the in- vention;

Fig. 3 shows the cross section II-II of the tank of Fig. 1;

Figs. 4-6 show the permeability values that are measured at three different heights of the fluid levels (i.e., three different pressure differences) of a first diaphragm fabric (Fabric 1) at five different viscosity values of the catholyte, and the dependence graphs of the viscosity and permeability that are defined on the basis of the measured values;

Figs. 7-9 show the permeability values that are measured at three different heights of the fluid levels (i.e., three different pressure differences) of a second diaphragm fabric (Fabric 2) at five different viscosity values of the catholyte, and the dependence graphs of the viscosity and permeability that are defined on the basis of the measured values; and

Fig. 10 shows the effect of the difference of fluid levels (the pressure difference) on the permeability of Fabric 1 and Fabric 2. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 is a schematic general view of the electrolysis system that is suitable for electrowinning a metal, such as nickel, cobalt or manganese from an electrolyte that contains salts of the said metal. The elec- trolyte is brought into the electrolytic tank from a circulation container and the catholyte is removed from the tank by overflow back to the circulation con- tainer. A rich electrolyte is added to the circulation container. Anolyte is removed from the electrolytic tank from inside the anode bags and conveyed further to dissolution. Oxygen and acid fog are removed from the anode bags to purification and the oxygen is further conducted to dissolution. In tanks, where permanent cathodes are used and on the surfaces of which a sufficient amount of metal to be electrowon has precipitated, the permanent cathodes are periodically re- moved from the electrolytic tank; metal is removed from the surface of the cathode and the permanent cathodes are returned to the tank again. In tanks, where starter sheets are used and on the surfaces of which a sufficient amount of metal to be electrowon has precipitated, the starter sheets are removed periodically from the electrolytic tank, and new starter sheets are placed in the tank.

Figs. 2 and 3 show part of an electrolytic tank 1. The tank 1 contains a number of anodes 2 and a number of cathodes 3 that are arranged alternately. The anodes 2 are lead anodes, alloys of lead, DSA anodes (Dimen- sionally Stable Anodes) or titanium anodes. The cathodes 3 are preferably either permanent cathodes, which are manufactured from acid-resistant special steel, titanium, or nickel starter sheets that are made for the purpose are used.

Referring to Fig. 3, the anodes 2 are inside anode bags 4 that permeate the electrolyte in a controlled manner. The cathodes 3 are freely inside the tank. The anode bag 4 defines an inner anodic space 5 inside of it and an outer free cathodic space 6 outside of it, where the cathodes 3 are. Metal is precipitated on the surface of the cathodes 3 and oxygen and acid are gen^¬ erated on the anodes 2. The pH of the anolyte 7 in the anode bag 3 is lower (pH < 1) than that of the catholyte 8 (pH of about 3- 4) in the cathodic space 6. The electrolyte flows con- tinuously from the cathodic space to the anodic space inside the anode bag to constitute the anolyte. The system comprises a means of feeding the catholyte into the cathodic space and removing it from the tank by overflow or suction (not shown) .

For removing the anolyte separately from each anode bag, an overflow pipe 9 is arranged for each anode bag 4, its head 10 opening in the area of the upper part of the anode bag 4. The position of the head 10 de- fines the level of the anolyte in the anode bag, so that the level of the anolyte is lower than the level of the catholyte by a distance H. The difference H of the levels of the catholyte and anolyte causes a pressure difference. The electrolyte flows through the diaphragm fabric at a flow rate, which depends on its permeation properties and which can be 1-100 l/m²/h.

Fig. 2 also shows that the anode bag 4 can include a first suction pipe 12 for removing the anolyte/oxygen/acid fog by suction. It can further include a second suction pipe 13 for removing the anolyte/oxygen/acid fog from the anode bag by suction. The bag may be sealed and impervious to gas above the fluid level .

To remove the anolyte, oxygen and acid fog from the anode bag, there can be three arrangements: 1) The anolyte is removed by overflow through the overflow pipe 9 and oxygen and acid fog are removed by the first suction pipe 12 or through the overflow pipe 9 as well; 2) The anolyte is removed by the second suction pipe 13, by which the oxygen and acid fog are also removed;

3) The anolyte is removed by the second suction pipe 13, and the first suction pipe 12 is used to remove both the anolyte and oxygen and acid fog.

For the process, the diaphragm fabric material of the anode bag is selected on the basis of an anolyte acid concentration of a desired degree, so that the permeability of the diaphragm fabric

X = f(Δp, τ)

wherein

Δp is the pressure difference

τ is the viscosity of the catholyte

Correspondingly, the acid concentration of the anolyte C = f (X, I)

wherein

I is the guantity of electric current used

For the process, a high anolyte acid concentration can be selected, and to achieve this, a suitable diaphragm fabric material can be selected. In the process, efforts are made to keep the viscosity of the catholyte the same by adjusting the circulation speed of the catholyte. A desired permeability of the diaphragm fabric is achieved by a diaphragm fabric, which is made of polymer or microfibre, and by a correct selection of filaments, weave and coating. The permeability of the electrolyte can also be optimised by calendering. By selecting a suitable fabric, the desired acid content can be achieved; the high acid content cannot be achieved by other parameters, unless the fabric is dense enough.

EXSJyIPLE 1

To prove that a high anolyte acid concentration can be achieved, two bench-scale tests were conducted for the electrowinning of nickel by using anode bag fabrics, which had different permeabilities of the electrolyte. Two diaphragm fabrics A and B were used, which had the following permeabilities of the electrolyte: 280 ml/h for A and 2070 ml/h for B per anode bag for an electrolyte, which had a NiSO^ concentration of about 70 g/1. Three anode, bags (lead anodes) and two cathodes (starter sheets) were accommodated in a cell, the volume of which was 27 litres. The anolyte was removed by overflow and the overflow of the cathode was adjusted at the differences of height H of 30 and 20 mm, caus- ing hydrostatic pressures, for the bag fabrics A and B, respectively. The catholyte was circulated and fresh electrolyte was added to the tank in an amount as large as the amount of anolyte that was removed from the anode bags. The temperature of the electro- lyte was 57 ⁰C. The current density was 200 A/m² and the cell current for A was 23 A, and for B, 22 A. The circulation speed of the catholyte was 6 1/h. For the electrolyte, the nickel concentration of which was 106 g/1, the total removal of anolyte from the cell during the test was 0.34 1/h for the bag fabric A and 3.4 1/h for the bag fabric B. The end result was that the anolyte acid concentrations were 122 g/1 for A and 28 g/1 for B at the end of the tests. The test shows that the high anolyte acid concentration can be reached by- using correct process parameters.

Figs. 4-6 show, for the two different fabrics, Fabric 1 and Fabric 2 (which is not the same fabric as in the example above) , the permeability values measured at three different heights of the fluid levels (i.e., three different pressure differences) at five different viscosity values of the catholyte, and the dependence graphs of viscosity and permeability defined on the basis of the measured values.

The graphs include the permeability of the electrolyte (litres per hour and square meter) as a function of viscosity (cP i.e., mPa/s) . The graphs are adapted for a damping exponential function, which comprises three parameters y₀, a and b:

/(X) = y₀ +ae-^h" η is the viscosity or the X-axis and X is the permeability or the Y-axis.

In adjusting the parameters, the following values were obtained for them:

Fabric 1

H (cm) 10 15 19

Yo 32.8 45.4 50.6 a 1183.7 1292.9 611.3 b 2.8 2.5 1.6

Fabric 2

H (cm) 10 15 19

Yo 7.7 13.4 14.5 a 68.2 163.0 144.8 b 1.1 1.5 1.2 Fig. 10 shows the effect of the difference H of the fluid levels (i.e., the pressure difference) of Fabric 1 and Fabric 2 on the permeability at a specific viscosity value 2.73 cP, when the Ni content of the ca- tholyte is 87 g/1.

The invention is not limited to the application examples described above only, but many modifications are possible within the inventive idea defined by the claims.

Claims

CLAIMS :

1. A method of electrowinning a metal from an electrolyte that contains a metallic salt in an electrolytic tank (1) , which comprises a number of anodes (2) and a number of cathodes (3) that are arranged alternately, each anode (2) being arranged inside an anode bag (4), the anode bag consisting of a material that permeates the electrolyte in a controlled manner and defining inside it an anodic space (5) and outside it a free cathodic space, where the cathodes (3) are located, whereby metal is precipitated on the surface of the cathode and acid and oxygen are generated on the anodes, and the hydrogen ions generated on the anode are prevented from entering through the diaphragm fabric by means of a pressure difference, so that the electrolyte flows constantly from the cathodic space into the anodic space inside the anode bag; and the ca- tholyte is fed into the cathodic space of the electro- lytic tank, removed therefrom as an overflow and re- circulated back into the cathodic space, and the anolyte is removed separately from each anode bag as an overflow or by suction, characterised in that, as the material of the anode bag, a diaphragm fabric is selected from a group of diaphragm fabrics with different permeabilities, the dependences of the permeability of the diaphragm fabric on the viscosity of the catholyte and the pressure difference used are defined and, on the basis of these defined dependences, the exit velocity of the anolyte from the anode bag is adjusted by the current density used, the pressure difference and the viscosity of the catholyte, so that an acid content of the anolyte of at least 50 g/1 is obtained.

2. A method according to Claim 1, characterised in that the concentration of metallic salt in the catholyte (8) and/or the viscosity of the catholyte are measured, and the circulation speed of the catholyte is increased, if the metallic salt concentration in the catholyte and/or the viscosity exceed a predefined limit value, and the circulation speed of the catholyte is reduced, if the metallic salt concentration in the catholyte and/or the viscosity of the catholyte are below the predefined limit value.

3. A method according to Claim 1 or 2, characterised in that the pressure difference is adjusted by changing the difference of height between the fluid levels of the anolyte and the catholyte.

4. A method according to any of Claims 1-3, characterised in that the anolyte (7) is removed from each anode bag (4) by overflow so that, to provide hydro- static pressure, the level of the anolyte is kept lower than the level of the catholyte.

5. A method according to Claim 4, characterised in that the anolyte (7) is removed from each anode bag (4) by means of an overflow pipe (9) , its head (10) defining the level of the anolyte in the anode bag.

6. A method according to any of Claims 1-5, characterised in that the anolyte (7) is conveyed from the overflow pipes (9) to a collector (11) .

7. A method according to any of Claims 1-6, characterised in that the anolyte (7) is removed from each anode bag (4) by means of a first suction pipe (12) .

8. A method according to any of Claims 1-7, characterised in that oxygen and/or acid fog are sucked from the anode bag (4) .

9. A method according to Claim 8, characterised in that the oxygen and/or acid fog are sucked from the anode bag (4) through the first suction pipe (12), a second suction pipe (13) and/or the overflow pipe (9) .

10. A method according to any of Claims 1-9, characterised in that the metal to be electrowon is nickel, cobalt or manganese, and the metallic salt is the sulphate of the respective metal.

11. An electrolysis system for electrowinning a metal ^■from an electrolyte that contains a metallic salt, comprising electrolytic tanks (1), each of which contains a number of anodes (2) and a number of cathodes (3) arranged alternatively, each anode (2) being ar- ranged inside an anode bag (4), which comprises a wall that is formed from a diaphragm fabric that permeates the electrolyte in a controlled manner, the wall limiting an anode space (5) in its inside and a cathodic space in its outside, the cathodes (3) being located in the space outside, whereby metal is precipitated on the surface of the cathode and acid and oxygen are generated on the anodes, and the hydrogen ions generated on the anode are prevented from escaping through the diaphragm fabric by means of a pressure differ- ence, so that the electrolyte flows constantly from the cathodic space into the anodic space inside the anode bag; and the catholyte is fed into the cathodic space of the electrolytic tank, removed therefrom as an overflow and recirculated back into the cathodic space, and the anolyte is removed separately from each anode bag as an overflow or by suction; the system comprising a means of feeding the catholyte into the cathodic space, removing it as an overflow or by suction and recirculating it back to the cathodic space, and a means of separately removing the anolyte from each anode bag, characterised in that a fabric wherein the dependences of its permeability on the. viscosity of the catholyte and the pressure difference used are known is selected as the diaphragm fabric of the anode bag from a number of diaphragm fabrics that have dif- ferent permeabilities; and that the exit velocity of the anolyte is adjusted by the current density used, the pressure difference and the viscosity of the catholyte, so that the acid content of the anolyte that is removed from the anode bag is at least 50 g/1.

12. A system according to Claim 11, characterised in that it includes a means of measuring the concentration of metallic salt in the catholyte, and a means of adjusting the circulation speed of the catholyte on the basis of the measured concentration.

13. A system according to Claim 11 or 12, characterised in that the means of removing the anolyte (7) includes an overflow pipe (9) for each anode bag (4), which overflow pipe opens in the area of the upper part of the anode bag, defining the level of the anolyte in the anode bag, so that the level of the anolyte is lower than that of the catholyte.

14. A system according to any of Claims 11-13, characterised in that it comprises a collector (11) for receiving the anolyte (7) that is collected by the overflow pipes (9) .

15. A system according to any of Claims 11-14, charac- terised in that the anode bag (4) includes a first suction pipe (12) for sucking the anolyte, oxygen and/or acid fog from the anode bag.

16. A system according to any of Claims 11-15, characterised in that the anode bag (4) includes a second suction pipe (13) for sucking the anolyte, oxygen and/or acid fog from the anode bag.