AU2010210040B2

AU2010210040B2 - Method of electrowinning a metal and an electrolysis system

Info

Publication number: AU2010210040B2
Application number: AU2010210040A
Authority: AU
Inventors: Ville Nieminen; Henri Virtanen
Original assignee: Metso Outotec Finland Oy
Current assignee: Metso Finland Oy
Priority date: 2009-02-03
Filing date: 2010-02-02
Publication date: 2014-04-17
Anticipated expiration: 2030-02-02
Also published as: CA2751302C; EA020315B1; FI20095094A; AU2010210040A1; CA2751302A1; EP2403980B1; CN102308029B; BRPI1008826B1; EP2403980A4; EP2403980A1; WO2010089452A1; FI20095094A0; FI122595B; EA201190121A1; BRPI1008826A2; CN102308029A

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

WO 2010/089452 PCT/F12010/050058 1 METHOD OF ELECTROWINNING A METAL AND AN ELECTROLYSIS SYSTEM FIELD OF THE INVENTION 5 The invention relates to a method defined in the pre amble of Claim 1. The invention further relates to an electrolysis system defined in the preamble of Claim 11. 10 BACKGROUND OF THE INVENTION In electrolysis, a metal that is dissolved in an elec trolyte is electrowon. Electrowinning takes place in an electrolytic tank that contains a number of anodes 15 and a number of cathodes that are arranged in and al ternating manner. When an electric current is con ducted 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 20 formed on the anodes, according to the reaction equa tions (1) and (2): Anodic reaction: H 2 0 - *H] + 02 + 2e- (1) (1) Cathodic reaction: Mez+ + z- -> Me 25 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, 30 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 35 be avoided by separating the anolyte and the catholyte from each other by a material that permeates the elec- WO 2010/089452 PCT/F12010/050058 2 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 5 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 10 defines an anodic space in its inside and the cathodes are in the free cathodic space that surrounds the an ode 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 15 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 20 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 25 (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 30 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 35 cated in South Africa, has tested the anode bag tech nique ["Nickel Electrowinning Tankhouse Developments at WO 2010/089452 PCT/F12010/050058 3 Anglo Platinum Base Metal Refinery", Authors: LJ Bry son, NJ Graham, EP Bogosi, DL Erasmus proceedings of ALTA 2008 - Nickel/Cobalt, Copper & Uranium Confer ence, June 16-18 2008]. According to this article, the 5 process criteria regarding the anolyte acid concentra tion were the main stumbling stones in the implementa tion. 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 10 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 15 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 20 high acid content of the anolyte is useful in dissolu tion. The high anolyte acid concentration can be pro vided by a suitable selection of a diaphragm fabric and by controlling the process parameters, namely the viscosity of the electrolyte and the permeability of 25 the wall of the anode bag that is formed from the dia phragm fabric that permeates the electrolyte in a con trolled manner, the permeability depending on the vis cosity of the electrolyte. 30 The problem is that it is difficult to select the cor rect diaphragm fabric of the anode bag, since manufac turers offer fabrics that have different gradual per meabilities and they express the permeability of the fabric in permeability of water only (a specific vis 35 cosity) and at one specific pressure difference (a specific height of a water column). When a high 4 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 5 how the viscosity of the electrolyte and the pressure difference on the different sides of the fabric influence the permeability. OBJECT OF THE INVENTION 10 The object of the invention is to alleviate at least one of the disadvantages mentioned above. In particular, the purpose of the invention is to 15 disclose 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 20 the anodic space, which makes the electrowinning of metal a profitable business. SUMMARY OF THE INVENTION 25 In an embodiment of the present invention, there is provided a method of electrowinning a metal from an electrolyte that contains a metallic salt in an electrolytic tank, which comprises a number of anodes 30 and a number of cathodes that are arranged alternately, each anode being arranged inside an anode bag, the anode bag consisting of a material that permeates the electrolyte in a controlled manner and defining inside the anode bag an anodic space and 35 outside the anode bag a free cathodic space, where the cathodes 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 4a 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 5 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 10 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 15 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 20 the anolyte of at least 50 g/l is obtained. In another embodiment of the present invention there is provided an electrolysis system for electrowinning a metal from an electrolyte that contains a metallic 25 salt, comprising electrolytic tanks, each of which contains a number of anodes and a number of cathodes arranged alternatively, each anode being arranged inside an anode bag, which comprises a wall that is formed from a diaphragm fabric that permeates the 30 electrolyte in a controlled manner, the wall limiting an anode space inside the wall and a cathodic space outside the wall, the cathodes being located in the space outside, whereby metal is precipitated on the surface of the cathode and acid and oxygen are 35 generated on the anodes, and the hydrogen ions generated on the anode are prevented from escaping 4b 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 catholyte is fed into the 5 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 10 catholyte into the cathodic space, removing the catholyte as an overflow or by suction and recirculating the catholyte back to the cathodic space, and a means of separately removing the anolyte from each anode bag, characterised in that a fabric 15 wherein the dependences of the permeability of the fabric 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 different permeabilities; 20 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/l. 25 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 30 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 51B24031 (GHMaters) P87611.AU WO 2010/089452 PCT/F12010/050058 5 ity of the anolyte from the anode bag is adjusted by means of the current density used, the pressure dif ference and the viscosity of the catholyte, so that an acid content of the anolyte of at least 50 g/l is ob 5 tained. According to the invention, a fabric wherein the de pendences of its permeability on the viscosity of the catholyte and the pressure difference used are known 10 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 15 tholyte, so that the acid content of the anolyte that is removed from the anode bag is at least 50 g/l. It is essential for the invention that the flow can primarily be defined on the basis of the density of 20 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 25 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, 30 in turn, can be adjusted to a minor degree by the con centration of the metallic salt in the catholyte, whilst the other parameters remain constant. This ad justment does not necessarily have significance for the process. For example, the variation of the Ni con 35 tent in the process can be as little as about 10 g/l. The concentration of metallic salt in the catholyte, WO 2010/089452 PCT/F12010/050058 6 in turn, can be adjusted by adjusting the circulation speed of the catholyte. Fig. 1 shows the block flow chart of the process. 5 The acid concentration thus settles on a specific level, which depends on the exit velocity of the elec trolyte (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 10 electrolytic tank is 16000 A and the flow of electro lyte into the anode bags totals 0.33 m 3 /h, the anolyte acid concentration is about 80 g/l. In an application of the method, the concentration of 15 the metallic salt in the catholyte and/or the viscos ity 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 viscos ity of the catholyte exceed a predefined limit value, 20 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. 25 In an application of the method, the pressure differ ence is adjusted by changing the difference of height between the fluid levels of the anolyte and the ca tholyte. 30 In an application of the method, the anolyte is re moved from each anode bag by overflow so that, to pro vide a hydrostatic pressure, the level of the anolyte is kept lower than the level of the catholyte. This provides a sufficiently high hydrostatic pressure, 35 which causes a flow of electrolyte from the catholyte side into the anode bags, preventing the migration of WO 2010/089452 PCTIF2010/050058 7 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 5- 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 maxi mum level of anolyte in the anode bag. Damage of the 10 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. 15 In an application of the method, the anolyte is con veyed from the overflow pipes to a collector and fur ther to a collecting tank. 20 In an application of the method, the anolyte is re moved from each anode bag by means of a first suction pipe. In an application of the method, oxygen and/or acid 25 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 30 overflow pipe. In an application of the method, the metal to be elec trowon is nickel, cobalt or manganese, and the metal lic salt is the sulphate of the respective metal. 35 WO 2010/089452 PCT/F12010/050058 8 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 circu lation speed of the catholyte on the basis of the 5 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 10 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 15 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 20 collector. In an application of the system, the anode bag in cludes a first suction pipe for sucking the anolyte and, possibly, oxygen and/or acid fog from the anode 25 bag. In an application of the system, the anode bag in cludes a second suction pipe for sucking the anolyte, oxygen and/or acid fog from the anode bag. 30 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 35 Fig. 1 shows the block flow chart of the process; WO 2010/089452 PCT/F12010/050058 9 Fig. 2 shows schematically the cross section of part of the electrolytic tank that belongs to one applica tion of the electrolysis system according to the in 5 vention; Fig. 3 shows the cross section II-II of the tank of Fig. 1; 10 Figs. 4-6 show the permeability values that are meas ured 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 15 graphs of the viscosity and permeability that are de fined on the basis of the measured values; Figs. 7-9 show the permeability values that are meas ured at three different heights of the fluid levels 20 (i.e., three different pressure differences) of a sec ond diaphragm fabric (Fabric 2) at five different vis cosity values of the catholyte, and the dependence graphs of the viscosity and permeability that are de fined on the basis of the measured values; and 25 Fig. 10 shows the effect of the difference of fluid levels (the pressure difference) on the permeability of Fabric 1 and Fabric 2. 30 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 electro lyte that contains salts of the said metal. The elec 35 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- WO 2010/089452 PCT/F12010/050058 10 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 5 the anode bags to purification and the oxygen is fur ther conducted to dissolution. In tanks, where perma nent cathodes are used and on the surfaces of which a sufficient amount of metal to be electrowon has pre cipitated, the permanent cathodes are periodically re 10 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 15 has precipitated, the starter sheets are removed peri odically 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 20 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 cath odes 3 are preferably either permanent cathodes, which 25 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 30 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 35 surface of the cathodes 3 and oxygen and acid are gen erated on the anodes 2.

WO 2010/089452 PCT/F12010/050058 11 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 5 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). 10 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 15 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 pres sure difference. The electrolyte flows through the 20 diaphragm fabric at a flow rate, which depends on its permeation properties and which can be 1-100 1/m 2 /h. Fig. 2 also shows that the anode bag 4 can include a first suction pipe 12 for removing the 25 anolyte/oxygen/acid fog by suction. It can further in clude 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. 30 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 35 the first suction pipe 12 or through the overflow pipe 9 as well; .

WO 2010/089452 PCT/F12010/050058 12 2) The anolyte is removed by the second suction pipe 13, by which the oxygen and acid fog are also removed; 5 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 10 anode bag is selected on the basis of an anolyte acid concentration of a desired degree, so that the perme ability of the diaphragm fabric X = f(8p, T) 15 wherein Ap is the pressure difference 20 T is the viscosity of the catholyte Correspondingly, the acid concentration of the anolyte C = f(X, I) 25 wherein I is the quantity of electric current used For the process, a high anolyte acid concentration can 30 be selected, and to achieve this, a suitable diaphragm fabric material can be selected. In the process, ef forts are made to keep the viscosity of the catholyte the same by adjusting the circulation speed of the ca tholyte. 35 WO 2010/089452 PCT/F12010/050058 13 A desired permeability of the diaphragm fabric is achieved by a diaphragm fabric, which is made of poly mer or microfibre, and by a correct selection of fila ments, weave and coating. The permeability of the 5 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. 10 EXAMPLE 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, 15 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 elec trolyte, which had a NiSO 4 concentration of about 70 20 g/l. Three anode. bags (lead anodes) and two cathodes (starter sheets) were accommodated in a cell, the vol ume 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 25 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 30 lyte was 57 0 C. The current density was 200 A/m 2 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/l, the total removal of anolyte from the cell during 35 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 WO 2010/089452 PCT/F12010/050058 14 anolyte acid concentrations were 122 g/l for A and 28 g/l 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. 5 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., 10 three different pressure differences) at five differ ent viscosity values of the catholyte, and the depend ence graphs of viscosity and permeability defined on the basis of the measured values. 15 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 yo, a and b: 20 f(X)=yO+ ae-b' q is the viscosity or the X-axis and X is the perme ability or the Y-axis. 25 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 15 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 catholyte is 87 g/l. 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. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

1. A method of electrowinning a metal from an electrolyte that contains a metallic salt in an electrolytic tank, which comprises a number of anodes and a number of cathodes that are arranged alternately, each anode being arranged inside an anode bag, the anode bag consisting of a material that permeates the electrolyte in a controlled manner and defining inside the anode bag an anodic space and outside the anode bag a free cathodic space, where the cathodes 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 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, 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/l is obtained. 17

2. A method according to Claim 1, characterised in that the concentration of 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 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 one of Claims 1-3, characterised in that the anolyte is removed from each anode bag by overflow so that, to provide hydrostatic 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 is removed from each anode bag by means of an overflow pipe, having a head defining the level of the anolyte in the anode bag.

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

7. A method according to any one of Claims 1-6, characterised in that the anolyte is removed from each anode bag by means of a first suction pipe. 18

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

9. A method according to Claim 8, characterised in that 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.

10. A method according to any one 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, each of which contains a number of anodes and a number of cathodes arranged alternatively, each anode being arranged inside an anode bag, 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 inside the wall and a cathodic space outside the wall, the cathodes 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 difference, 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 19 means of feeding the catholyte into the cathodic space, removing the catholyte as an overflow or by suction and recirculating the catholyte 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 the permeability of the fabric 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 different 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/l.

12. A system according to Claim 11, characterised in that 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.

13. A system according to Claim 11 or 12, characterised in that the means of removing the anolyte includes an overflow pipe for each anode bag, 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 one of Claims 11-13, characterised in that the system comprises a collector for receiving the anolyte that is collected by the overflow pipes.

15. A system according to any one of Claims 11-14, characterised in that the anode bag includes a first 20 suction pipe for sucking the anolyte, oxygen and/or acid fog from the anode bag.

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

17. A method of electrowinning a metal from an electrolyte that contains a metallic salt in an electrolytic tank substantially as herein described with referenced to the accompanying figures.

18. An electrolysis system for electrowinning a metal from an electrolyte that contains a metallic salt substantially as herein described with reference to the accompanying figures.