WO1993006261A1

WO1993006261A1 - Electrowinning metals from solutions

Info

Publication number: WO1993006261A1
Application number: PCT/AU1992/000506
Authority: WO
Inventors: Neal Barr
Original assignee: Spunboa Pty Ltd
Priority date: 1991-09-23
Filing date: 1992-09-23
Publication date: 1993-04-01

Abstract

A membrane electrolysis apparatus for electrowinning metals from acidic metal salt solutions having a unit cell comprising an anodic compartment with an anode located therein containing an anolyte solution, a cathodic compartment on either side of the anodic compartment, each cathodic compartment having a cathode located therein and containing an acidic metal salt solution constituting a catholyte solution, and a pair of intermediate compartments located respectively between the anodic compartment and a respective one of the cathodic compartments, the anodic compartment being separated from each of the adjacent intermediate compartments by a cation exchange membrane and each cathodic compartment being separated from the adjacent intermediate compartment by an anion exchange membrane, said unit cell including electrical circuitry adapted to maintain an electrolysis reaction so as to plate metals on each cathode and generate an acid solution in said intermediate compartments, and wherein said anode consists of two anode members each located adjacent a respective one of said cation exchange membranes, the acid solution generated in the intermediate compartment being substantially free of metal contaminants.

Description

ELECTROWINNING METALS FROM SOLUTIONS

The present invention is concerned with the treatment of acidic metal salt solutions to remove metals and, more particularly, with electrowinning metals from these solutions.

Metal objects that are to ^• be subject to further treatment such as galvanizing must have a clean surface. This can be achieved by immersion of the surface in dilute sulphuric acid (H2SO4) or hydrochloric acid (HCl) followed by washing. This process is known as "pickling" and the acid solution is called the "pickle solution" or pickle liquor.

Pickling is initially slow at the start of pickling with relatively pure acid. The rate rises rapidly however as the concentration of iron rises then falls off as the acidity is exhausted. Pickle times of 1 to 10 minutes are commonly used in hydrochloric acid. A 20% HCl solution pickles in 2 minutes and an 8% solution in 10 minutes at 34 degrees C. The pickle time is economically important as it is the rate limiting step in the galvanizing process, a typical hot dip galvanizing plant having 3 pickle baths to 1 each of rinse, preflux, zinc and chromate baths.

In the galvanizing industry the composition of waste pickle solution is typically, 70 g/1 zinc, 70 g/1 iron and 6% residual hydrochloric acid. This is the approximate equivalent of 230 g/1 or 23% hydrochloric acid. This final composition is achieved by starting with a fresh pickle solution of 21% HCl and topping up with fresh concentrated HCl solution at a later stage when the pickle liquor has lost a considerable portion of its acid.

For convenience, the present invention will be primarily described in relation to a process and apparatus for regenerating pickle solution and recovering dissolved metals from waste pickle solution, but it is to be understood that the invention is not so limited and may be applied to other systems such as mine waters and solutions generated by ore leaching and generally to any acidic metal salt solution and, in particular, to any acidic mixed metal/iron salt solution. Typically, these solutions may contain iron mixed with zinc, copper, nickel, lead, cadmium, silver or gold in the presence of chloride, sulfate, acetate or nitrate.

The proposed treatment system is one in which the metals are electrowon from the waste pickle liquor, or other acidic metal salt solutions.

Various methods for the treatment of waste pickle liquor to remove iron have been proposed in the prior art.

For example. United States patent No. 4,177,119 describes a multi-step process which involves:-

(1) converting Fe^÷ ions in the waste to Fe^+ ions;

(2) extracting the Fe^⁺ ions with an organic solvent to form an Fe CI3 - complex in the solvent and an aqueous raffinate;

(3) extracting Fe~-⁺ from the solvent; and

(4) subjecting the raffinate to diaphragm electrolysis to recover hydrochloric acid therefrom. In this process iron is won by solvent extraction. Electrowinning of metals from solution is well known and the recovery of iron from waste pickle liquor has been proposed, for example, in United Kingdom Specification No. 794,368. That specification describes an electrolytic process in which an electrolysis cell is divided into anode compartment and a cathode compartment by an anion selective membrane. The pickle liquor is introduced to the cathode compartment and a current is applied to the cell to deposit iron on the cathode. Nonetheless, unless the cell is run until the iron in the pickle solution is entirely exhausted the pickle solution is still contaminated to some degree and can only be re-used as fresh pickle liquid. It is not feasible to remove all of the iron since, as the concentration of iron in the solution decreases the voltage applied to the cell must be increased to maintain the reaction, and once iron concentration is quite low the voltage required is very high and is not economical. Furthermore, in time the anolyte solution becomes contaminated with chloride which is transported across the anion exchange membrane from the catholyte solution. This results in the production of chlorine gas at the anode which is undesirable and may also damage the anode. The system is therefore suitable only for sulfate systems.

United States Patent No. 3,072,545 describes another electrolytic process of regenerating spent pickle liquor employing an electrolytic cell divided into three compartments. The cell comprises a cathode compartment, a centre compartment and an anode compartment which is separated from the centre compartment by a fluid-permeable porous diaphragm, the centre compartment being separated from the anode compartment by a cation exchange membrane. Spent pickle liquor is introduced to the cathode compartment whereupon when the cell is operated iron plates onto the cathode. In operation, the fluid in the cathode compartment must be maintained at a greater pressure than the fluid in the centre compartment to cause some of the spent pickle liquor to flow into the centre compartment through the porous diaphragm to maintain the reaction. This is inconvenient and at least some leakage of metal ions into the centre compartment can be expected, so contamination of the regenerated acid is likely.

In United States Patent No. 4,058,441 an electrolytic cell is proposed, which cell is suitable for the regeneration of hydrochloric acid that is spent after use for pickling iron. The cell has an anodic compartment, a cathodic compartment and an intermediate compartment. The anodic compartment is separated from the intermediate compartment by a cation - selective membrane and the cathodic compartment is separated from the intermediate compartment by a an anion - selective membrane. It is apparent from the passage at column 1 lines 38 to 46 that spent pickle solution is introduced both into the cathodic compartment and into the intermediate compartment simultaneously. The resultant regenerated acid stream from the intermediate compartment is thus contaminated with iron and is only suitable for re-use as fresh pickle liquor. The method thus constitutes a method of acid replenishment only and is not envisaged as a full treatment method. The specification also provides only a general disclosure of the apparatus used but seems to envisage only a single anode and is largely silent on constructional details of the cell, for example, the means of introducing fluid to the cell and its flow through the cell.

In addition, none of the above prior art envisages electrowinning metals from an acidic mixed metal salt solution such as waste pickle liquor from the galvanizing industry. The galvanizing industry is unusual in that the pickle liquor also becomes contaminated with zinc, whereas in most industries iron is the only significant contaminant.

In particular, electrowinning from a solution containing iron, another metal such as zinc and hydrochloric acid presents a few problems. These are:

1. Efficiency and thus energy loss due to cyclic redox reactions of Fe(II)/Fe(III) .

2. The generation of chlorine

3. Mixed metal or alloy plating.

Due to 1 above, the treatment of the liquor in a simple cell is not feasible. The anodic generation of chlorine from waste liquor does not in practice happen immediately as the anodic half reaction of lowest overvoltage is the oxidation of Fe(II) to Fe(III). Even if the limiting current density for ferrous oxidation is exceeded and chlorine is produced it does not reach the atmosphere as it reacts in solution with Fe(II) to form Fe(III) and chloride. Until the level of Fe(II) in solution is low, chlorine is thus not likely to be liberated. However, in cells with low fluid flow and poor mixing the above limiting conditions are easily exceeded close to the anode and chlorine generation is evident.

Mixtures of iron and zinc generally plate out together in conventional systems since the overpotential for reduction of the two metals is similar under the conditions employed in those systems.

It is an object of the present invention to provide means for electrowinning metals from acidic metal salt solutions, particularly mixed metal salt solutions, while generating an acid solution substantially free from metallic contaminants.

Accordingly, in one broad aspect, the invention resides in a membrane electrolysis apparatus for electrowinning metals from acidic metal salt solutions having a unit cell comprising an anodic compartment with an anode located therein containing an anolyte solution, a cathodic compartment on either side of the anodic compartment each cathodic compartment having a cathode located therein and containing an acidic metal salt solution constituting a catholyte solution, and a pair of intermediate compartments located respectively between the anodic compartment and a respective one of the cathodic compartments, the anodic compartment being separated from each of the adjacent intermediate compartments by a cation exchange membrane and each cathodic compartment being separated from the adjacent intermediate compartment by an anion exchange membrane, said unit cell including electrical circuitry adapted to maintain an electrolysis reaction so as to plate metals on each cathode and generate an acid solution in said intermediate compartments, and wherein said anode consists of two anodic members each located adjacent a respective one of said cation exchange membranes, the acid solution generated in the intermediate compartment being substantially free of metallic contaminants.

In one form of the invention, the apparatus has a plurality of unit cells each as described above arranged in series, each cathodic compartment of a unit cell also forming a part of the adjacent unit cell, the apparatus therefore having a repeating sequence of a cathodic compartment, an intermediate compartment, an anodic compartment and another intermediate compartment, each cathodic compartment being in fluid communication with the adjacent cathodic compartment, a first cathodic compartment having an inlet for the introduction of catholyte solution and a second cathodic compartment having an outlet, and each intermediate compartment being in fluid communication with the adjacent intermediate compartment, a first intermediate compartment having an inlet for the introduction of acid solution and a second intermediate compartment having an outlet, whereby there is a continuous flow of catholyte solution in one direction which progressively plates metal on each cathode as said catholyte solution proceeds through the apparatus, and a continuous flow of acid solution in the other direction which progressively increases in concentration as it proceeds through the apparatus.

Alternatively, the apparatus has a plurality of unit cells each as described above, each cathodic compartment of each unit cell being in fluid communication with all other cathodic compartments, the apparatus having a repeating sequence of a cathodic compartment, an intermediate compartment, an anodic compartment and another intermediate compartment, the catholyte solution being provided as a bath which has substantially the same composition in all of the cathodic compartments at any time. A plurality of such apparatus can be arranged in hydraulic series with the catholyte solution being introduced in batches to each of the apparatus in sequence.

Apparatus in accordance with the invention may be used to treat a wide variety of acidic metal salt solutions but is particularly suitable for use to treat waste pickle solution, including pickle solution that is contaminated by zinc as well as by iron. The apparatus may nonetheless be used to remove metals from acidic metal salt solutions from other sources. Preferably, the acidic metal salt solutions include iron salts and, most preferably, salts of another metal such as zinc, copper, nickel, lead, cadmium, silver or gold in the presence of chloride, sulfate, acetate or nitrate.

The anode consists of two anodic members except where the apparatus has anodic compartments at its extremities, which is preferred, whereupon the anode in those compartments is a single anodic member.

The anode or each anodic member may be made of any suitable material. Preferably, they are made of mixed metal oxide coated titanium. This type of anode is often referred to as a dimensionally stable anode (DSA) .

The cathode can be made of any material compatible with the solution that can accept the plated metal. In a preferred embodiment, the cathode is constructed of steel, stainless steel, titanium or carbon.

In a typical use the catholyte solution is waste pickle solution containing zinc ions, iron ions, hydrogen ions and chloride or sulfate ions.

It is preferred that the anolyte solution provide good conductivity. It is further preferred that the anode is an oxygen half-cell thus the anolyte ought to be able to support the oxygen generation half reaction. A dilute aqueous solution of sulfuric acid or hydrochloric acid is most preferred.

The concentration of the anolyte solution is chosen to minimise water flux across the membrane. The concentration necessary will depend on the strength of the intermembrane electrolyte but generally about 5% w/v sulfuric acid solution is suitable.

The anolyte may also be a chloride containing solution such as dilute hydrochloric acid. In this case chlorine gas will be generated at the anode.

Preferably, the acid solution in the intermediate compartment is an aqueous acid compatible with the anions that are transported into it during operation of the electrolytic cell. Most preferably, it is hydrochloric acid so that pickle solution or a solution which on further concentration may be used as pickle solution is generated by migration of hydrogen and chloride ions into the intermediate compartment.

Since a cation exchange membrane is provided in the unit cell between the anodic compartment and each of the adjacent intermediate compartments, it is expected that hydrogen ions generated by the oxygen half-cell reaction will be conducted into the intermediate compartment while migration of anions, such as the chloride ion, into the anodic compartment will be minimised. Acid production is thus maximised.

Likewise, since an anion exchange membrane is provided in the unit cell to separate each cathodic compartment from the adjacent intermediate compartment, it is expected that chloride or sulfate ions in the catholyte solution will be conducted into the intermediate compartment while migration of hydrogens ion from the acid solution into the cathodic compartment will be minimised. Thus, a weak hydrochloric or sulfuric acid solution in the intermediate compartment is concentrated and iron and zinc are plated on the cathode during the operation of the cell. Surprisingly, it has been found that substantially all of the zinc in the catholyte solution plates out before iron starts to plate out. Hydrogen is also evolved from the cathode.

Optionally, the acid solution produced in the intermediate compartment can be further concentrated in additional process steps. The acid solution is substantially free from contamination by iron, zinc or other metals and may be used for any purpose to which acids solutions may be put including being returned to the pickle tank as fresh pickle solution. In contrast, in the prior art described above the acid solution had to be returned to the pickle tank since it has significant residual metal contamination and is not suitable for many other purposes.

In addition, the solution produced in the intermediate chamber, being essentially iron free, is suitable for secondary electrolysis to produce chlorine and hydrogen. The chlorine and hydrogen can be combined to form gaseous HCl and full strength concentrated hydrochloric acid of high purity.

In a particularly preferred embodiment of the invention each anodic compartment and each intermediate compartment has a manifold inlet and a manifold outlet, said manifold inlet comprising a plurality of nozzle-shaped inlets at spaced intervals along the bottom side of each said compartment, said manifold outlet comprising a plurality of nozzle-shaped inlets at spaced intervals along the top side of each said compartment. Optionally each intermediate compartment contains a spacer member which may be a matrix of expanded open cell foam or other porous material.

The spacer member allows the operation of the intermediate compartment at lower pressure than the adjacent compartments while maintaining the ion exchange membranes well spaced from each other and effectively parallel and planar. It also serves to induce, together with the inlet/outlet arrangement described above, plug flow in the circulated acid solution.

Advantageously, a metal complexing agent is made available in the catholyte solution to reduce the amount of chloride that is complexed by the metals. Preferably, a neutralising agent such as zinc oxide is added to increase the concentration of hydroxide ions in the catholyte solution, the hydroxide ions tending to replace chloride ions in metal complexes.

In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings in which :-

Fig 1 is a perspective view of a membrane electrolysis apparatus in accordance with the present invention;

Fig 2 is an exploded view of a composite membrane-anode structure;

Fig 3 is a section along line A-A in Fig 1;

Fig 4 is a section along line B-B in Fig 2;

Fig 5 is a schematic representation of a membrane electrolysis apparatus in accordance with the present invention which has a plurality of unit cells arranged in series; and

Fig 6 is a schematic representation of a membrane electrolysis system which has a plurality of apparatus in accordance with the present invention arranged in hydraulic series utilising catholyte recirculation. Referring now to Fig 1, it can be seen that the membrane electrolysis apparatus comprises a casing 12 which includes a plurality of composite membrane-anode structures 10 that are separately removable from the casing. A plurality of cathodes 11 are positioned within the container 12 in alternating arrangement with the composite membrane- anode structures, a cathodic compartment as described above being constituted by the area surrounding a cathode 11 between adjacent composite membrane-anode structure 10. The cathodic compartments may be isolated from one another or be in fluid communication. Cathode 11 is a solid steel plate which can be removed from the apparatus to recover metal plated thereon.

The apparatus further includes a plurality of inlets 13 at the bottom of the apparatus each in fluid communication with a cathodic compartment and a plurality of outlets 14 at the top of the apparatus each in fluid communication with a cathodic compartment. An acidic metal salt solution such as waste pickle solution is introduced to the cathodic compartment through the inlet 13 and the treated solution is conducted out of the cathodic compartment through outlet 14. As described later with reference to Figs 5 and 6, the inlets 13 and outlets 14 may be connected so that the apparatus includes a plurality of unit cells in series or to circulate catholyte solution where all of the catholyte compartments are in fluid communication.

The composite membrane-anode structure 10 is best described with reference to Fig 2. Each composite membrane- anode structure 10 includes a membrane support 15 holding an anionic membrane 16 against the intermediate compartment (which is filled with open cell expanded foam 18 and surrounded by frame 19) and a cation exchange membrane 20 constituting an intermediate compartment. Adjacent but spaced slightly from the cation exchange membrane 20 is first anodic member 21, frame 22 and second anodic member 23 which together make up a composite anode, the anodic compartment being constituted by the area between cation exchange membrane 20 and cation exchange membrane 24. Another intermediate compartment constituted by cation exchange membrane 24, frame 25, anion exchange membrane 26 and membrane support 27 completes the composite membrane anode structure 10. Gaskets or sealing components are used between each of the components.

Each of the frames 19, 25 include an outlet 28, 29 which constitutes an outlet from the intermediate compartment. Reference to Fig 1 shows that the frames 19, 25 also an include inlets 30, 31 that are similar in external appearance to the outlets 28, 29. The inlets 30, 31 and outlets 28, 29 are described later with reference to Fig 7.

The intermediate compartments also contain open cell expanded foam 18 or some other suitably rigid porous material which prevents anion exchange membrane 16 from touching cation exchange membrane 20. Fluid flow through the intermediate compartment creates a negative pressure therein and thus the membranes tend to be drawn inwardly and may otherwise touch. The open cell expanded foam 18 also serves to create a plug flow in the compartment.

Frame 22 also includes an outlet 32 constituting an outlet from the anodic compartment. A similar inlet 33 is located in the other side of the frame 22. The anodic members 21, 23 are grids of a metal oxide coated titanium.

Referring now to Fig 3, the section shows a unit cell as described above. The unit cell consists of anodic compartment 34, intermediate compartments 35, 36 and cathodic compartments 37, 38. Anodic compartment 34 contains a pair of anodic members 21, 23 which are located as close as possible to cation exchange membranes 20, 24 respectively to minimise the voltage of the unit cell.

As described earlier, the cation exchange membranes 20, 24 separate the anodic compartment 24 from the intermediate compartments 35, 36. The cation exchange membranes are permeable to cations and, as shown in Fig 3, permit the passage of hydrogen ions from the anodic compartment 34 into the intermediate compartments 35, 36. The cathodic compartments 37, 38 are separated from the intermediate compartments 35, 36 by anion exchange membranes 16, 26 which are permeable to anions and, as shown in Fig 3, permit the passage of chloride ions from the cathodic compartments 37, 38 into the intermediate compartments 34, 35. A hydrochloric acid solution is thus generated in the intermediate compartments 34, 35 when a current is applied to the apparatus and its concentration increases while the current continues to be applied. Application of the current also causes positive ions, such as Zn2+, Fe^+ and H⁺ in waste pickle solution, the catholyte solution, to migrate to the cathode. This results in zinc and iron plating on the cathodes 11 and in hydrogen evolution. Oxygen is generated at the anodic members 21, 23 as a result of the oxygen half-reaction, which also produce hydrogen ions.

The anolyte solution is dilute sulphuric acid. This ensures that the current efficiencies of oxygen production and concomitant acid generation are near 100%. Inefficiency in current utilisation arises from contamination of the anolyte solution by chloride, iron or other species that can support an anodic half reaction that competes with the preferred half reaction, which is the oxygen-generation half reaction. In practice chloride is the only measurable contaminant causing considerable effect.

Chloride contamination originates from the leakage of chloride ions across the cation exchange membranes 20, 24 from the hydrochloric acid solution in the intermediate compartment. In operation this does not lead to an efficiency fall of greater than 2%.

The cation exchange membranes 20, 24 are in this case FLEMION, NAFION or NEOSEPTA CMX. As illustrated in Fig. 3 it is intended that the cation exchange membranes 20, 24 pass only hydrogen ions into the intermediate compartments 35, 36 while resisting the passage of anions in the other direction. As the hydrogen ion has a much higher mobility than other positive ions this is the most likely outcome, although some inefficiency arises as a result of transport of anions in the opposite direction as discussed earlier.

The anion exchange membranes are available under the name NEOSEPTA AMH or SELEMION AMT. The anion exchange membranes 16, 26 allow the passage of chloride ions from the catholyte solution into the intermediate compartments 35, 36 whilst resisting hydrogen ions migrating from the intermediate comparts 35, 36 to the cathodic compartments 37, 38.

The chloride ions in the waste pickle liquor migrate across the anion exchange membranes 16, 26 into the intermediate compartments 35, 36 to regenerate fresh pickle solution. However, much of the chloride in waste pickle solution is complexed by the metal ions also present in the solution and is not available for transport across the membrane except in the form of complex metal chloride ions. It is, therefore, advantageous to increase the free chloride level in the waste pickle liquid to aid in transporting chloride across the membrane.

One possible way to achieve this is to add a second metal complexing agent that will compete successfully against chloride to complex the metal. A convenient agent is the hydroxide ion as an increase in the concentration of hydroxide ion can easily be achieved by neutralisation or dilution of the waste pickle liquid. A suitable neutralisation agent is zinc oxide. This may be in the form of ash from the zinc bath used in the galvanizing process.

Fig 4 illustrates a manifold system used for the inlet and outlet to the intermediate compartments and to the anodic compartment, with the modification in the latter case that the inlet and outlet are located on the side.

In Fig 4, inlet 31 is in fluid communication with conduit 39 which directs fluid, in this case acid solution, to a plurality of nozzle-shaped inlets 40 to the intermediate compartment. As the intermediate compartment includes a spacer member 18 which in this case is expanded foam, plug flow is induced through the compartment to the outlets 41. It is preferred that the outlets 41 be larger in size than the inlets 40 to prevent back pressure. The outlets 41 are in fluid communication with conduit 42 which leads to outlet 29. It should be noted that in the ^'anodic compartment a spacer member is preferably not used so as to induce continuously stirred or well-mixed flow. This has the advantage that the anolyte solution is well mixed so the current density in the compartment is uniform. Lack of a spacer member in the anodic compartment also facilitate the flushing of generated gas bubbles from the system.

Fig 5 shows schematically the connections made in the membrane electroysis apparatus. It can be seen that the catholytic solution is held in holding tank 43 then passes through line 44 into the first cathodic compartment [indicated thus (-)] whereupon it passes through line 45 into the next cathodic compartment, line 46 into the next and so on until it exits via line 47. When valve 50 is opened the treated catholyte enters line 48 and thence the first intermediate compartment via line 49 and is then passed through the apparatus as the acid solution in the opposite direction to the catholyte solution into holding tank 43.

The system may be run either with each individual cathodic compartment separated from the others, for example with an inflatable gasket separating the bath into individual compartments, or with all the cathodic compartments in fluid communication. In the former case the system runs as distinct cells in hydraulic series but in the latter case the system serves only to circulate and/or recirculate catholyte solution through the system to maintain even current density.

Fig 6 illustrates a situation where a plurality of the membrane electrolysis apparatus are run in hydraulic series to treat waste pickle solution.

The process stream 52 issuing from the pickle tank 51 consists of spent pickle solution, containing dissolved iron and zinc as well as unused acid, for treatment. The process stream 52 passes to a holding tank 53, the contents of which are conducted to the catholyte chamber or chambers of the membrane electrolysis apparatus 54, in which the dissolved metal is won at the cathode and a hydrochloric acid solution is generated as described above. A zinc/iron alloy which is high in zinc content is plated out. The hydrochloric acid solution is conducted from the apparatus 54 in line 60.

The zinc and acid depleted process stream 55 passes to a second holding tank 56, the contents of which are conducted to the catholyte chamber or chambers of the membrane electrolysis apparatus 57 in which the dissolved metal is won at the cathode and a further hydrochloric acid stream 59 is generated. The predominant metal won is iron.

The process stream 58, depleted in zinc, iron and acid now represents treated liquor which can be subsequently disposed of or retained for acid replenishment.

It will be clear to those practised in the art that the flow chart Fig. 6 is basic and numerous variations in both flow patterns for the process stream and electrical lay-out for combining cells and permutations and combinations of these can be suggested or presented which are acceptable. EXAMPLE 1 A cell of:

1 sheet steel cathode of working area 2 x 0.001 m^,

2 x 0.0225 ι_2 sheets of Neosepta AMH anion exchange membrane providing 2 x 0.0144 ^ working area in assembled cell,

2 x 0.0225 m^ sheets of Neosepta CMX cation exchange membrane providing 2 x 0.0144 m-- working area in assembled cell, and

2 x dimensionally stable grid anodes, each of working area 0.013 ra.2.

The parts were assembled into a cell by using frames and seals, clamped together with the components, to give;

1 central cathodic compartment equipped with top port for cathode insertion/removal and ports for liquid inlet and outlet,

2 x Intermediate compartments equipped with liquid inlet and outlet ports, and

2 x anodic compartments equipped with top ports for anode insertion.

The anode/cathode spacing was 70 mm. The cathode/anion exchange membrane spacing was 23 mm. The cation exchange membrane/anion exchange membrane spacing was 33 mm.

A reservoir was charged with a 1:1 mixture of waste pickle liquor and water, the composition of the waste pickle being;

4.1% w/v HCl as residual acid,

75.7 g/1 zinc,

93.2 g/1 iron.

The anolyte was 5% v/v sulphuric acid and the concentration of the acid solution (otherwise known as the membralyte) was maintained within the range 0.2% to 1.2% w/v during operation. Catholyte recirculation rate was 12 1/hr and membralyte flow rate was 150 1/hr divided between the two chambers.

Using a cathode current density of 600 Αm^~~- and a corresponding membrane ion current density of 416 Am^~2 the solution was recirculated through the cathode chamber of the cell to produce a solution of composition; pH = 2,

0.1 g/1 zinc,

18.7 g/1 iron.

Cell voltage ranged from 15 volts at start down to 6.5 Volts. Current densities were reduced to half the above values and the solution further treated to produce a solution of composition; pH -= 3, 26 ppm zinc, 4.5 g/1 iron.

Cell voltage ranged from 5.3 Volts at the beginning of this treatment period to 8.1 volts at conclusion.

Overall, for each litre of waste pickle liquor treated; the power consumption was 1.48 kWhrs, 196 g of HCl was produced in the membralyte stream, 61.6 g of zinc chloride was produced in the membrane stream,

40g of zinc and 84 g of iron were recovered in the form of zinc/iron alloys of varying composition. The transport of iron across the anion exchange membranes was effectively zero throughout the treatment. EXAMPLE 2

In a cell similar to that described in example 1, a waste pickle liquor of composition; 83.6 g/1 iron, 0.6% w/v HCl, 50g/l NaCl, was treated at a cathodic current density of 900 Αm^~~- , a membralyte HCl concentration range of 2.55 to 2.72%, to produce a solution of composition; 78.6 g/1 iron, 0.36% w/v HCl, 50 g/1 NaCl. Overall, iron plating current efficiency was 39%, acid destruction in catholyte current efficiency was 15% and membralyte HCl regeneration current efficiency was 53%.

Cell voltage was in the range 5.6 to 5.4. EXAMPLE 3

In a cell similar to that described in example 1, a waste pickle liquor of composition;

66.2 g/1 iron,

0.07% w/v HCl,

50 g/1 NaCl, was treated at a cathodic current density of 200 Am-2, a membralyte HCl concentration range of 0.9 to 1.88%, to produce a solution of composition:

51.1 g/1 iron,

0.02% w/v HCl,

50 g/1 NaCl.

Overall, iron plating current efficiency was 84%, acid destruction in catholyte current efficiency was 6% and membralyte HCl regeneration current efficiency was 91%.

Cell voltage was in the range 3.2 to 3.4 volts. EXAMPLE 4

15.1 g/1 iron,

0.03% w/v HCl,

50 g/1 NaCl, was treated at a cathodic current density of 200 Am^~2, a membralyte HCl concentration range of 0.5 to 0.9%, to produce a solution of composition; 0.22 g/1 iron, pH = 6.95, 50 g/1 NaCl.

Overall, the acid destruction in catholyte current efficiency was 1.5% and membralyte HCl regeneration current efficiency was 92%. Iron removed from the catholyte was in the form of metal or in the form of an iron (II) hydroxide precipitate and overall current efficiency for removal of iron from catholyte was close to 100%.

Cell voltage was in the range 3.5 to 3.7 volts. An increase in current density at this stage to 600 Am^~2 increased pH quickly and after sedimentation gave a supernatant liquor with an iron content of 62 ppm. EXAMPLE 5

In a cell similar to that described in example 1, a waste pickle liquor of composition; 52.5 g/1 zinc, 1.26 w/v HCl, 39 g/1 NaCl, was treated at a cathodic current density of 900 Am-2, a membralyte HCl concentration range of 0.7 to 1.2% to produce a solution of composition; 31.76 g/1 iron, 0.2% w/v HCl, 39 g/1 NaCl.

Overall, zinc plating current efficiency was 94.5%, acid destruction in catholyte current efficiency was 38% and membralyte HCl regeneration current efficiency was 59%. In addition the current efficiency of zinc transport into the membralyte was 11% (based on the passage of [ZnC13]^2-).

Cell voltage was in the range 5.6 to 5.4.

Whilst the above has been given by way of illustrative example of the present invention, many variations and modifications thereto will be apparent to those skilled in the art without departing from the broad ambit and scope of the invention as herein set forth.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A membrane electrolysis apparatus for electrowinning metals from acidic metal salt solutions having a unit cell comprising an anodic compartment with an anode located therein containing an anolyte solution, a cathodic compartment on either side of the anodic compartment each cathodic compartment having a cathode located therein and containing an acidic metal salt solution constituting a catholyte solution, and a pair of intermediate compartments located respectively between the anodic compartment and a respective one of the cathodic compartments, the anodic compartment being separated from each of the adjacent intermediate compartments by a cation exchange membrane and each cathodic compartment being separated from the adjacent intermediate compartment by an anion exchange membrane, said unit cell including electrical circuitry adapted to maintain an electrolysis reaction so as to plate metals on each cathode and generate an acid solution in said intermediate compartments, and wherein said anode consists of two anode members each located adjacent a respective one of said cation exchange membranes, the acid solution generated in the intermediate compartment being substantially free of metal contaminants.

2. A membrane electrolysis apparatus having a plurality of unit cells, each as defined in Claim 1 and arranged in series, each cathodic compartment of a unit cell also forming a part of the adjacent unit cell, the apparatus therefore having a repeating sequence of a cathodic compartment, an intermediate compartment, an anodic compartment and another intermediate compartment, each cathodic compartment being in fluid communication with the adjacent cathodic compartment, a first cathodic compartment having an inlet for the introduction of catholyte solution and a second cathodic compartment having an outlet, and each intermediate compartment being in fluid communication with the adjacent intermediate compartment, a first intermediate compartment having an inlet for the introduction of acid solution and a second intermediate compartment having an outlet, whereby there is a continuous flow of catholyte solution in one direction which progressively plates metal on each cathode as said catholyte solution proceeds through the apparatus, and a continuous flow of acid solution in the other direction which progressively increases in concentration as it proceeds through the apparatus.

3. A membrane electrolysis apparatus as defined in claim 2 wherein the catholyte solution contains a mixture of iron and another metal and the other metal plates predominantly in the cathodic compartments close to the inlet and the iron plates predominantly in the cathodic compartments close to the outlet, plating of the other metal being substantially complete before the iron begins to plate.

4. A membrane electrolysis apparatus as defined in claim 3 wherein the other metal is zinc.

5. A membrane electrolysis apparatus as defined in claim 4 wherein the catholyte is waste pickle solution, contaminated with zinc, having been used as a pre-treatment in a galvanizing process.

6. A membrane electrolysis apparatus as defined in any one of claims 2 to 5 wherein the treated catholyte solution from the outlet is introduced to the first intermediate compartment as the acid solution.

7. A membrane electrolysis apparatus having a plurality of unit cells, each as defined in claim 1, each cathodic compartment of each unit cell being in fluid communication with all other cathodic compartments, the apparatus having a repeating sequence of a cathodic compartment, an intermediate compartment, an anodic compartment and another intermediate compartment, the catholyte solution being provided as a bath which has substantially the same composition in all of the cathodic compartments at any time.

8. A membrane electrolysis apparatus as defined in any one of the preceeding claims wherein the acid solution is hydrochloric or sulfuric acid and is suitable, optionally with an additional concentration step, for use as fresh pickle solution.

9. A membrane electrolysis apparatus according to any one of the preceding claims wherein a metal complexing agent is made available in the catholyte solution to reduce the amount of chloride that is complexed by the metals.

10. A membrane electrolysis apparatus as defined in claim 9 wherein a neutralising agent, preferably zinc oxide, is added to increase the concentration of the hydroxide ion in the catholyte solution.

11. A membrane electrolysis apparatus according to any one of the preceeding claims wherein each intermediate compartment contains a spacer member.

12. A membrane electrolysis apparatus according to any one of the preceding claims wherein each anodic compartment and each intermediate compartment has a manifold inlet and a manifold outlet, said manifold inlet comprising a plurality of nozzle-shaped inlets at spaced intervals along the bottom side of each said compartment, said manifold outlet comprising a plurality of nozzle-shaped inlets at spaced intervals along the top side of each said compartment.

13. A membrane electrolysis system having a plurality of membrane electrolysis apparatus as defined in claim 7 arranged in hydraulic series whereby the catholyte solution is introduced in batches to each of said apparatus in sequence.

14. A membrane electrolysis system as defined in claim 13 wherein the catholyte solution contains a mixture of iron and another metal and the other metal plates out predominantly from the batches early in the sequence and the iron plates out predominately from the batches late in the sequence, plating of the other metal being substantially complete before the iron begins to plate out.

15. A membrane electrolysis system as defined in claim 14 wherein the other metal is zinc.

16. A membrane electrolysis system as defined in claim 15 wherein the catholyte is waste pickle solution, contaminated with zinc, having been used as a pre-treatment in a galvanizing process.