WO2001073165A1

WO2001073165A1 - Removal of manganese from electrolytes

Info

Publication number: WO2001073165A1
Application number: PCT/ZA2000/000093
Authority: WO
Inventors: Michael John Thom
Original assignee: Michael John Thom
Priority date: 1999-05-05
Filing date: 2000-05-05
Publication date: 2001-10-04
Also published as: AU5050100A

Abstract

The invention provides a process for the extraction of manganese ions from an electrolyte in an electrolytic cell, which includes the step of: (a) generating an oxidising agent at the surface of an anode; and (b) causing the oxidising agent to be displaced from the surface to react with the manganese ions in the electrolyte so that the manganese ions are oxidised. The process provides for the extraction of manganese from an electrolyte while zinc is removed from the electrolyte at a cathode. The invention also provides an anode and an electrolytic cell to be used in the process.

Description

REMOVAL OF MANGANESE FROM ELECTROLYTES

BACKGROUND OF THE INVENTION

This invention relates to the removal of manganese from electrolytes used in the electrowinning of base metals. In particular it applies to the sulphuric acid based electrolytes used in the electrowinning of zinc. More specifically the invention relates to the continuous removal of manganese as hydrated manganese dioxide or MnO₂.x(H₂O), where x is any value from 0,5 to 6, during the electrolysis process of extracting zinc i.e. MnO₂.H₂O.MnO₂ to MnO₂.6H₂O.

The invention is however not limited only to processes involving the electrowinning of base metals as the invention also relates to the removal of manganese from electrolytes prior to electrolysis of the base metals.

In nature zinc may occur in several forms. According to the applicant's knowledge the most important zinc bearing minerals are:

Sphalerite (ZnS), Smithsonite (ZnCO₃)_, Hydrozincite (2ZnCO₃.3Zn(OH)₂), Hemimorphite (Zn Si₂O₇(OH)₂.H₂O), Willemite (Zn₂SiO₄), Goslarite (ZnSO₄.7H₂O), Zincite (ZnO), and Franklinite ((Zn,Fe,Mn)₂O₄ or (Fe,Mn)₂O₄).

In the electrowinning of zinc the ore is usually roasted and the zinc then extracted with sulphuric acid. Oxidised ores, such as zincite, may be acid leached without prior roasting. Either way the resulting liquor is never pure and contains many metallic impurities such as iron, cadmium, cobalt, arsenic, antimony, lead, germanium, selenium, manganese and magnesium. Some of these impurities may only be present in parts per million and yet be harmful e.g. germanium. The elements more electro-negative than zinc such as magnesium do not interfere with the electrolysis in any way but they do "take up space" i.e. they consume sulphuric acid that could have extracted zinc. Mg, Fe and Mn may be present at grams per litre levels in the leach and of these Fe must be almost completely removed. Magnesium is left in the electrolyte and manganese is tolerated up to 10 grams per litre in some plants but is a nuisance at higher levels as it tends to coat onto the anode as the dioxide and manganese tends to accumulate in recycled electrolytes if not removed. A small amount of manganese in the electrolyte might be beneficial when lead or silver-lead anodes are used as it prevents the dissolution of lead and extends the life of the anodes. The ideal level of manganese in the electrolyte is in the order of 2 to 3 grams per litre.

Many zinc deposits contain manganese, much of which may report in the purified acid electrolyte. The GAMSBERG zinc ore deposit in Namaqualand, South Africa, is a prime example. The high manganese content of the sphalerite at this deposit makes the recovery of zinc by conventional hydro-metallurgical methods difficult. One approach to the manganese problem is to extract or leach zinc from the ore without leaching any of the manganese, however this appears to be a difficult task.

The invention aims to provide a relatively simple and cost effective process and apparatus for the separation of manganese from electrolytes.

SUMMARY OF THE INVENTION

The invention provides a process for the extraction of manganese ions from an electrolyte in an electrolytic cell, which includes the steps of:

(a) generating an oxidising agent at the surface of an anode; and

(b) causing the oxidising agent to be displaced from the surface to react with the manganese ions in the electrolyte so that the manganese ions are oxidised.

The oxidising agent may be contained in a space between 1 micron and 30mm from the surface of the anode.

The oxidising agent may be generated in a cation-depleted layer or zone at the surface of the anode.

The oxidising agent may be in any form and preferably is in the form of nascent oxygen (O) or any complex thereof. The oxidising agent may be caused to be displaced in any appropriate manner. In a preferred process of the invention the oxidising agent is caused to be displaced by hydraulic action of the electrolyte.

The hydraulic action of the electrolyte may be caused by any appropriate means and may for example be generated by gaseous flow in the electrolyte.

The gaseous flow in the electrolyte may be as a result of oxygen rising against gravity to a surface of the electrolyte. The gaseous flow may be as a result of the flow of any other gas such as air.

The gaseous flow may be against or adjacent the surface of the anode.

The hydraulic action may also be generated by means of a hydraulic pump.

In another form of the invention the oxidising agent is caused to be disturbed by movement of the anode. The movement of the anode may be caused in any appropriate way such as vibrating the anode.

In another form of the invention the oxidising agent is caused to be disturbed by ultrasonic sound waves.

The extraction of manganese ions from the electrolyte may be regulated by regulating the degree of disturbance of the cation depletion layer containing the oxidising agent.

The process may include the step of employing a catalyst to increase the oxidation rate of the manganese ions. Any appropriate catalyst may be used and may include any one of lead, silver, manganese or palladium or any mixture of these or any of their oxides.

The process may include the step of extracting a slurry which includes electrolyte rich in oxidised manganese, from the electrolytic cell.

The process may include the step of separating oxygen (O₂) from the slurry in a liquid/gaseous separation stage. The process may include the step of filtering the slurry to separate the oxidised manganese from the electrolyte.

The process may include the step of returning the electrolyte, with a reduced concentration of manganese, to the electrolytic cell or to a leach plant.

The process may include the step of locating a barrier between the anode and a cathode of the electrolytic cell. The barrier may be of any construction and may be conductive and electrically isolated from the anode and the cathode.

The process may include the step of extracting zinc from the electrolyte. The extraction of zinc from the electrolyte may occur simultaneously with the extraction of manganese from the electrolyte.

The invention also provides for an anode for an electrolytic cell used in any one of the abovementioned processes which includes an anode plate and a jacket which at least partially covers and is spaced from the anode plate.

The jacket may include a plurality of apertures through which an electrolyte and electric current can flow.

The jacket may include an open ended lower end section and a sealed upper section.

The jacket may include means for extracting an electrolyte from between the anode plate and the jacket. The means for extracting the electrolyte may be in any appropriate form and preferably is in the form of a pipe.

The anode plate may be from any appropriate material but is preferably made of a hardened alloy of lead containing silver.

The jacket is preferably spaced between 2mm and 30mm from a surface of the anode plate.

The invention further extends to an electrolytic cell which is used in any one of the aforementioned processes which includes an anode and a cathode located in an electrolyte and a barrier between and spaced from the anode and the cathode. The barrier may be of any construction and preferably includes a plurality of apertures through which the electrolyte can flow.

Preferably the barrier is electrically conductive and may be electrically isolated from the anode and the cathode.

The potential in the electrolyte between the cathode and the barrier may be less than the potential in the electrolyte between the barrier and the anode. Preferably the potential of the barrier is between 1 ,2 volts and 2,3 volts to minimise the formation of oxygen and the deposit of zinc at the barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which:

FIGURE 1 illustrates a block diagram of a manganese recovery process according to the invention;

FIGURE 2 is a front view of an anode as used in the process of Figure 1 ;

FIGURE 3 is a sectioned side view of the anode of Figure 2;

FIGURE 4 is an enlarged view of the bottom end of the anode of Figure 3;

FIGURE 5 is a diagrammatic sectioned side view of part of an electrolytic cell in use in the process of Figure 1 ;

FIGURE 6 is a sectioned side view of an anode according to another form of the invention;

FIGURE 7 is a sectioned side view of an electrolytic cell according to another form of the invention showing the voltage potential between the various components of the electrolytic cell;

FIGURE 8 is an anode according to yet another form of the invention; and FIGURES 9, 10 and 11 are various graphs illustrating the recovery of manganese from an electrolyte.

DESCRIPTION OF PREFERRED EMBODIMENTS

The separation of manganese from a sulphuric acid electrolyte containing zinc according to the invention is described. It is to be appreciated that the invention is not limited to sulphuric acid or zinc electrolytes.

Figure 1 shows a schematic block diagram of a process 10 for the extraction of hydrated manganese dioxide precipitate from an electrolyte 14 according to the invention.

The electrolyte 14 is a typical zinc electrolyte high in manganese and has the following general composition:

Zn 56 grams per .

Mn 16 grams per .

Mg 12 grams per .

H₂SO 150 grams per -

This electrolyte is treated by one of two methods:

(a) The manganese is oxidised by some agent, flocculated and filtered off and the purified electrolyte is sent to electrolysing tanks for further chemical extractions; or

(b) The electrolyte, high in manganese, is fed directly to an electrolysing plant and the manganese is removed while the electrolysis and recovery of zinc is taking place.

The second method takes place during the electrowinning of zinc from the electrolyte without the manganese interfering with the zinc extraction process.

The recovery of zinc by electrolysis is well known and is not further described.

According to the first mentioned method the manganese ions are oxidised in an electrolytic plant 15 and a slurry 12 is extracted from the electrolyte 14. The slurry 12 contains electrolyte which is rich in MnO₂ or MnO₂.x(H₂O) precipitate where x is any value from 0,5 to 6 and more commonly is 2 or 4. The manganese ions are oxidised by any number of oxidising agents 18 such as potassium or sodium chlorate, any of the perchlorates, sodium, potassium or ammonium persulphate, sodium periodate or potassium periodate under the right conditions.

Some of these oxidising agents require a catalyst, such as sodium persulphate which will only oxidise Mn⁺² in the presence of a trace of sliver (Ag⁺). Sodium periodate on the other hand will oxidise Mn⁺² without the use of a catalyst. In the case of both the periodate and the persulphate the Mn⁺² is oxidised to the permanganate ion MnO⁴ (i.e. Mn⁺⁷ ) which is easily reduced to MnO₂ .

In fact the permanganate ion may even be reduced to MnO₂ by the manganous ion, but this occurs slowly at room temperature.

All of the above reagents either require heat, catalysts, are relatively expensive, may be dangerous to handle or may be hazardous to store. Most of the above reagents introduce sodium or potassium ions into the electrolyte 14.

Oxidising agents that are free of cations that will reside in the electrolyte after oxidation are superior. The following are of particular interest:

Hydrogen peroxide H₂O₂

Organic peroxides various

Ozone gas O₃

Nascent oxygen O

Bromide and chlorine are powerful oxidising reagents but are not suitable as any bromides or chlorides introduced into the electrolyte would be harmful to the anodes and cathodes. The organic peroxides are expensive and hydrogen peroxide and ozone are costly to generate and may require heat to reach the required oxidation potential. According to the invention nascent oxygen O is generated which is used to oxidise Mn +2 to Mn⁺⁴.

In a first form of the invention, ozone is formed by blowing air or oxygen (O₂) 20 through an electric arc 22. The following reactions take place:

O₂ O + O;

O₂ + O ϋ O₃

The overall reaction is :- 3O₂ ^ 2O₃ but the yield of O₃ is small.

The mixture of oxygen and ozone is bubbled through the electrolyte 14 and the manganese is oxidised and the solution is stirred at the same time.

Alternatively, as is shown in Figures 2 to 5, the electrolyte 14 is hydraulically pumped past an anode 26 and a cathode 28 carrying a high current which passes between the anode 26 and cathode 28. The anode 26 and cathode 24 may be of any construction and in this example are two plates described in detail herein below.

The anode 26 and cathode 28 could also form the nozzle of a non-conductive tube which carries the electrolyte 14. Either way a reaction 30 results at the anode 26 where anions such as the divalent sulphate ion are drawn to the anode 26 and electrons are donated by these anions to the anode 26. The opposite reaction takes place at the cathode 28 where electrons are donated to cations, thus maintaining the flow of current. At the anode 26 the following reaction 30 takes place.

2(SO₄)^"2 -4e + 2H₂O = 2H₂SO₄ +O +O

O + O = O₂

The overall reaction at the anode 26 is:

2H₂0 - 4e = 4H⁺ + O₂ This reaction 30 may however proceed by several routes, for example the electrolyte 14 also contains the (HSO₄)^" anion which will donate an electron to the anode 26.

The species of interest is the nascent oxygen O which has a short half-life. It is a highly active atom which carries no charge and is therefore neither attracted to nor repelled from a surface 32 of the anode 26. The anode 26 has a thin surface boundary or cation depleted layer (CDL) 34 of electrolyte depleted of cations but anion enriched. Within this cation depleted layer 34 the atoms of nascent oxygen move around until one nascent oxygen atom collides with another to form oxygen gas, O₂. The oxygen molecules combine to form bubbles which rise to the surface of the electrolyte and escape to atmosphere.

As the nascent oxygen is formed at the surface of the anode 26 and the nascent oxygen atoms remain in the thin surface boundary they remain in a region between approximately 1 micron and 1mm from the surface of the anode.

If a nascent oxygen atom collides with a manganous ion it is immediately absorbed to form an unstable intermediate manganese complex 36 probably (MnO)⁺² or (Mn⁺⁴O^"2)⁺². Alternatively, the nascent oxygen atom collides with a hydrated manganese dioxide molecule to form an unstable complex of unknown composition but probably MnO₃.xH₂O. Whatever the composition is, the complex is a very powerful oxidising agent and readily reacts with the manganous ion, even at room temperature. This extremely reactive manganese complex 36 immediately reacts with one of the other species in the electrolyte most likely a water molecule to form a manganese dioxide precipitate. The slurry 12 extracted from the electrolyte is rich in this manganese dioxide precipitate.

The following reactions thus occur:

1. O (nascent) + Mn⁺² = (MnO)⁺²

2. (MnO)⁺² + H₂O = MnO₂ + 2H⁺

3. MnO₂ + xH₂O = MnO_{2 .} xH₂O where x is any value from 0,5 to 6 and commonly is 2 or 4. Because the nascent oxygen atom is free to move in the electrolyte it is purely a question of probability whether it reacts with another nascent oxygen or a manganous ion. The cation depleted layer 34 has a high relative concentration of nascent oxygen atoms. Because of the depletion of manganous ions in the cation depleted layer 34, the probability of the nascent oxygen atom reacting with Mn⁺² is approximately 200 times less than the probability of reacting with another nascent oxygen atom.

The cation depleted layer 34 surrounding the anode 26 is disturbed at step 38 (see Figure 1) causing the nascent oxygen in the cation depleted layer 34 to be displaced from the surface 32 of the anode 26 to facilitate the mixing of the cation depleted and nascent oxygen rich layer 34 with the manganous ion rich electrolyte 14, and so considerably increasing the probability of the nascent oxygen atoms reacting with the divalent manganese ions in the electrolyte 14. Various methods to facilitate the disturbance and displacement of the nascent oxygen through the electrolyte are described below.

Tests carried out at a current density of 600 amps per square meter and a manganese concentration of 12 grams per litre in a standard electrolyte have given results where 70% of the nascent atoms react with manganese and only 30% form oxygen. The manganese anode efficiency is therefore increased from 0,5% to 70% as a result of the disturbance of the cation-depleted zone 34 and the displacement of the nascent oxygen.

The MnO₂.x(H₂O) contained in the slurry 12 is flocculated, precipitates and is filtered off in a recovery stage 40. Oxygen which is formed escapes from the electrolyte 14 and is simultaneously trapped and recovered in the recovery stage 40.

The slurry 12 with manganese dioxide and oxygen rich electrolyte is pumped from the electrolytic cell 14 by a pump 42. The MnO₂.x(H₂O) and O₂ are separated in a liquid/gas separator 44. The recovered O₂ 46 is used in any appropriate way and is for example used in a pyro plant.

A spent electrolyte(S.E.) 48 with precipitated MnO₂.xH₂O is recovered from the liquid/gas separator 44 and is piped to a settling tank 50 and through a filter 52 where manganese dioxide 54 and electrolyte 56 in the form of ZnSO₄ and H₂SO₄ solution are separated. The electrolyte 56 is returned to the electrolytic cell 14 or preferably to a leach plant 57 for the leaching of zinc.

The manganese dioxide 54 is for example used in the pyro plant or for the oxidation of ferrous iron Fe⁺² to ferric iron Fe⁺³.

This method has been extensively tested and found to give manganese anodic efficiencies of between 20% and 70% depending on the manganese concentration in the electrolytic cell 14.

It is preferable to continuously remove the manganese from the electrolytic cell 14 while zinc 58 is simultaneously being recovered in the electrolytic cell 14.

The precipitate MnO₂.x(H₂O) has been found not to adversely affect the electrolysis process or the quality of zinc produced in an electrolytic cell.

In addition to the reactions explained above the nascent oxygen may react in many other ways but the end result is always the oxidation of Mn⁺² to MnO₂.x(H₂O). For example:

O (nascent) + H₂O = H₂O₂ (activated)

H₂O₂ ^* + Mn⁺² = MnO₂ + 2H⁺

MnO₂ + xH₂O = MnO₂.x(H₂O)

Since the above reactions take place at only 20°C the hydrogen peroxide formed must be in some activated form or the reaction is catalysed by the nascent oxygen itself. The nascent oxygen, the hydrogen peroxide formed or any activated manganese complex formed are all by-products of the main process being the electrowinning of zinc and are therefore generated at little or no cost. All reagents are generated in the electrolyte 14 as required and there is no need for the storage or addition of the reagents.

If however required, catalysts 60 could be used in the process. Silver and palladium or any of their oxides are generally good catalysts and aid in the oxidation of manganese. A silver-lead anode 26 would therefore be preferable as the oxidation process may be enhanced as the following reactions show: O (nascent) + 2Ag = Ag₂O

Ag₂0 + Mn⁺² + H₂O = 2Ag + MnO₂ + 2H⁺

Figures 2, 3 and 4 show an anode 26 from a silver-lead alloy. The anode 26 has a header-bar 62, to which is attached an anode plate 64. A non-conductive cover or jacket 66, which could be made from formica, glass, plastic, P.T.F.E. or the like, covers the anode plate 64 and is fitted close to the outer surfaces 32 of the anode plate 54. The spacing 68 between the outer surfaces 32 and the jacket 66 is between 2 and 30mm and is conveniently in the region of 15mm to 25mm.

The jacket 66 has regularly spaced holes or apertures 70 which have a diameter of about 1mm and are spaced by 3mm from each other with the holes occupying between 5% to 20% of the surface area of the jacket 66. The holes 70 allow the passage and flow of current and electrolyte through the jacket 66. The sides of the jacket are joined across the ends of the anode and the bottom end 72 of the jacket 66 is open ended. The jacket 66 is fitted with a suction pipe 74 at its sealed upper end 76.

It is to be understood that any number of pipes 74 of any appropriate size could be used through which the electrolyte 14 is drawn by means of the pump 42. The suction of the electrolyte 14 through the pipe 74 causes a hydraulic disturbance in the space 68 between the anode 26 and the jacket 66. The flow rate in this space 68 is in the order of between 1cm and 12cm per second.

Figure 5 illustrates an electrolytic cell 78 in use, with the cathode 28, electrolyte 14, anode 26 and jacket 66. Suction by means of the pump 42 and pipe 74 cause an upward flow 80 in the space 68 between the anode 26 and the jacket 66. The flow 80 is against or adjacent the surface 32 of the anode 26.

Electrolyte 14 which is rich in manganous ions passes through the holes 70 and moves into the space 68 between the jacket 66 and anode 26 together with electrolyte which is driven into the open end 74 of the anode jacket. The upward flow 80 is caused not only by the pump action but also by rising oxygen bubbles. The speed at which the bubbles rise in the space 68 through the electrolyte is proportional to the square root of the bubble diameter, for bubbles of 6mm diameter or less. Bubbles larger than about 10mm diameter are not spheroidal and so have a higher coefficient of drag and generate far more drag turbulence in the electrolyte than the smaller bubbles. These bubbles are therefore more effective in stirring and stripping the cation depleted layer from the surface 32 of the anode. The nascent oxygen atoms in the cation depleted layer 34 are displaced and mixed with the electrolyte 14 rich in manganous ions to react in the described manner.

The jacket 66 concentrates the oxidised manganese and oxygen formed. The slurry 12 pumped from the jacket 66 thus has a high concentration of oxidised manganese and oxygen. The jacket 66 further restricts the flow 80 and disturbance of the nascent oxygen to close proximity of the anode plate 64. The nascent oxygen is therefore contained in the space 68 which is between 1 micron and 30mm from the surface of the anode 26.

Current or electrons 82 flow from the cathode 28 to the anode 26. The current flow 82 is constricted by the jacket 66 and concentrated through the holes 70. This causes a slight loss of conductivity between the cathode 28 and the anode 26. The loss of conductivity is compensated for by decreasing the distance between the cathode 28 and the anode 26 to about four to six times that of the thickness of the jacket material.

Zinc 84 is deposited on the cathode 28 in the usual manner.

Figure 6 shows an alternative way for causing flow and turbulence in the space 68. Here hydraulic flow 86 is generated at an anode 26A only by oxygen bubbles that rise to the surface of the electrolyte.

The hydraulic flow 86 causes the same displacement of the cation depletion layer containing the nascent oxygen as described in relation to the anode 26. The disadvantage of this free running anode 26A is that the oxygen is released into atmosphere causing acid mist spray 88 whereas the anode 26 illustrated in Figures 2 and 3 do not produce acid spray. As stated above the oxygen generated by the anode 26 and 26A could be recovered and could, for example, be sold or piped to a pyrometallurgical section of a plant to be used in flame enrichment.

Figure 7 shows another electrolytic cell 90 where conductive plates 92 or barriers are strategically placed between an anode 26B and cathodes 28B. The conductive plates 92 are made from a silver-lead alloy or from any other suitable metal or alloy. The conductive plates 92 are isolated from both the cathode 28B and the anode 26B and have holes 70 similar to those of the jacket 66.

The potential between the cathodes 28B and an outer surfaces 94 of the conductive plates 92 is below the required voltage for the formation of nascent oxygen. The potential difference between the cathode 28B and the anode 26B (usually about 3,6 volts) is sufficient to allow the formation of nascent oxygen 100.

It is to be noted that the potential at the inner surface 98 of the plates 92 is substantially the same as at the outer surface 94 as the conductive plates 92 are thin and the material from which they are made is a better conductor of electricity than the electrolyte 14.

The cell potential 96 at various positions horizontally across the cell, as shown in the graph of Figure 7, is between 0 volts at the surface of the cathodes 28B where zinc is deposited and 1 ,9 volts at the outer surface 94 of the plates 92. As the decomposition voltage of zinc sulphate is about 2,3 volts no zinc is deposited on the conductive plates 92. The potential 96 between the inner surface 98 of the plates and the anode 26B is between the 1 ,9 volts at the inner surface 98 and approximately 3,6 volts at the anode 26B.

Flow and turbulence between the plates 92 and the anode 26B are caused by the external pump 42 and rising oxygen results in the precipitation of the manganese oxide as discussed above. Again, as with the jacket 66, the conductive plates 92 localise the nascent oxygen 100, manganese oxide and oxygen to close proximity of the anode 26B which facilitates removal of the MnO₂ rich electrolyte. Figure 8 is another example of an anode 26C. The anode 26C is similar to the anode 26A shown in Figure 6 and the differences between the anode 26C and anode 26A are discussed using similar reference numerals for similar components.

The jacket 66 on the anode 26C has vent holes 102A, 102B on either side of the anode plate 64. A top portion 104 of the jacket 66 is without holes 70. Glass wool 106 however fills the space 68 at the top portion 104. Glass or plastic balls 108 are located in the space 68 below the glass wool 106.

The balls 108 and glass wool 106 prevents a substantial amount of acid spray 88. Droplets of manganese sludge 110 form above the level of the electrolyte 14 and exit through the vent holes 102A, 102B. In this way the concentration of free manganous ions in the electrolyte 14 is reduced.

Figure 9 shows three graphs indicating the reduction of manganese content of an electrolyte as a function of time in trials conducted using the apparatus and process of the invention. Manganese was successfully reduced from 24, 12 and 8,1 grams per litre to 7, 5 and 0,5 grams per litre respectively, in less than six hours.

Figure 10 is a graph showing the number of grams of manganese per litre removed from the electrolyte measured and calculated against FARADAYS passed. The decrease in concentration is calculated according to the equation:

Mn concentration (g/.) = 24,3 [1-e - °^65837F] where F is FARADAYS and e = 2,718, the base of the system of natural logarithms. It is to be noted that this equation applies only to the graph of Figure 10.

The encircled dots are actual measurements of manganese concentration plotted against FARADAYS passed and the dotted curve shows the calculated results.

Figures 9 and 10 illustrate that rate of removal of manganese is proportional (or very nearly so) to the manganese concentration in the electrolyte.

Figure 11 shows a curve of anodic manganese efficiency expressed as a percentage, versus manganese concentration in grams per litre. It is to be understood that the cation depletion layer which contains the oxidising agent, nascent oxygen, may be caused to be disturbed by various means. For example by moving, disturbing or vibrating the anode within the electrolyte or by means of ultrasonic sound waves.

It is also to be understood that the rate of extraction of manganese ions from the electrolyte can be regulated by regulating the disturbance of the cation depleted layer containing the nascent oxygen.

It is further to be understood that oxidised manganese ions in the electrolyte may be used as a catalyst for the further oxidisation of manganese ions in the electrolyte. In this respect as is shown in Figure 1 the electrolyte 14 is seeded in step 120 by returning some of the slurry 12 to the electrolyte 14.

Claims

1. A process for the extraction of manganese ions from an electrolyte in an electrolytic cell, which includes the steps of:

(a) generating an oxidising agent at the surface of an anode; and

2. A process according to claim 1 wherein the oxidising agent is contained in a space between 1 micron and 30mm from the surface of the anode.

3. A process according to either one of claims 1 or 2 wherein the oxidising agent is generated in a cation-depleted layer at the surface of the anode.

4. A process according to any one of claims 1 , 2 or 3 wherein the oxidising agent is in the form of nascent oxygen (O) or any complex thereof.

5. A process according to any one of claims 1 to 4 wherein the oxidising agent is caused to be displaced by hydraulic action of the electrolyte.

6. A process according to claim 5 wherein the hydraulic action in the electrolyte is generated by gaseous flow in the electrolyte.

7. A process according to claim 6 wherein the gaseous flow is as a result of bubbles of oxygen or air rising to the surface of the electrolyte.

8. A process according to claim 6 or 7 wherein the gaseous flow is against or adjacent the surface of the anode.

9. A process according to any one of claims 5 to 8 wherein the hydraulic action is generated by means of a hydraulic pump.

10. A process according to any one of claims 1 to 9 wherein the oxidising agent is caused to be displaced by movement of the anode.

11. A process according to any one of claims 1 to 10 wherein the oxidising agent is caused to be displaced by ultrasonic sound waves.

12. A process according to any one of claims 1 to 11 wherein the extraction of manganese ions from the electrolyte is regulated by regulating the displacement of the oxidising agent.

13. A process according to any one of claims 1 to 12 which includes the step of employing a catalyst to increase the oxidation rate of the manganese ions.

14. A process according to claim 13 wherein the catalyst includes any one of Pb, Pd, Ag and Mn or any of their oxides.

15. A process according to any one of claims 1 to 14 which includes the step of extracting a slurry which includes electrolyte rich in oxidised manganese from the electrolytic cell.

16. A process according to claim 15 which includes the step of separating oxygen (O₂) from the electrolyte in a liquid/gaseous separation stage.

17. A process according to claim 15 or 16 which includes the step of filtering the slurry to separate the oxidised manganese from the electrolyte.

18. A process according to claim 15, 16 or 17 which includes the step of returning the electrolyte, with a reduced concentration of manganese, to the electrolytic cell or to a leach plant.

19. A process according to any one of claims 1 to 18 which includes the step of locating a barrier between the anode and a cathode of the electrolytic cell.

20. A process according to claim 19 wherein the barrier is electrically conductive and electrically isolated from the anode and the cathode.

21. A process according to any one of claims 1 to 20 which includes the step of extracting zinc from the electrolyte.

22. A process according to claim 21 wherein the extraction of zinc occurs simultaneously with the extraction of the manganese ions from the electrolyte.

23. An anode for an electrolytic cell used in a process according to any one of claims 1 to 22 which includes an anode plate and a jacket which at least partially covers and is spaced from the anode plate.

24. An anode according to claim 23 wherein the jacket includes a plurality of apertures through which an electrolyte and electric current can flow.

25. An anode according to claim 23 or 24 wherein the jacket includes an open ended lower end section and a sealed upper end section.

26. An anode according to claim 23, 24 or 25 wherein the jacket includes a means for extracting electrolyte from between the anode plate and the jacket.

27. An anode according to claim 26 wherein the electrolyte extraction means is in the form of a pipe.

28. An anode according to any one of claims 23 to 27 wherein the anode plate contains lead or silver metal.

29. An anode according to any one of claims 23 to 28 wherein the jacket is spaced between 2mm and 30mm from a surface of the anode plate.

30. An electrolytic cell used in a process according to any one of claims 1 to 22 which includes an anode and a cathode which is located in an electrolyte and a barrier between and spaced from the anode and the cathode.

31. An electrolytic cell according to claim 30 wherein the barrier includes a plurality of apertures through which the electrolyte and electric current can flow.

32. An electrolytic cell according to either one of claims 30 or 31 wherein the barrier is electrically conductive.

33. An electrolytic cell according to claim 32 wherein the barrier is electrically isolated from the anode and the cathode.

34. An electrolytic cell according to any one of claims 30 to 33 wherein the potential in the electrolyte between the cathode and the barrier is less than the potential in the electrolyte between the barrier and the anode.

35. An electrolytic cell according to claim 34 wherein the potential of the barrier is between 1 ,2 volts and 2,3 volts to minimise the formation of oxygen and the deposit of zinc at the barrier.