METHOD AND APPARATUS FOR ELECTROCHEMICAL RECOVERY OF MERCURY FROM SOLUTIONS

Cross-Reference to Related Applications

This application claims priority from UK application number 1012711.6 filed on July 29, 2010.

Field

The present invention relates to a method and apparatus for

electrochemically removing mercury from solutions.

Description of Prior Art

In prior art systems, when an ore is roasted, the necessary processing solutions may release gases, which may then be treated to recover mercury and particulates, the resulting scrubber bleed solution may then be treated to separate the liquids from the solids, which are then taken to a leaching process. Then the liquids may be mixed with Zinc, in a mix tank, and the solids and liquids are separated and through a chemical reaction mercury is extracted as a mercurous chloride precipitate and the supernatant solution is disposed of.

PCT/GBOO/01388 to Gilroy, filed April 12, 2000, describes a cylindrical electrochemical cell comprising a cylindrical flow through cathode for the electrochemical deposition of mercury and gallium. The cathode surrounded by a cylindrical anode, the anode and cathode compartments of the cell are separated by a proton conducting membrane. US patent 5,292,412 to Pitton, Issued on March 8, 1994, describes metal alloy porous electrodes, and electrochemical cells wherein solution flow is directed across the face of the electrodes.

The abstract of SU 1668483 to Barmashenko, discloses the use of a carbon felt cathode to electrochemically precipitate mercury.

The abstract of SU1760780 to Vsesoyuznyj discloses a flow through carbon fiber cathode to deposit mercury.

Summary of Invention

In an embodiment there is disclosed a substantially flat, flow through electrode.

In an alternative embodiment the electrode may have a surface area of at least about 500m ² per 1 m ² of geometric surface.

In alternative embodiments there are disclosed a cathode according to an embodiment and a carbon felt electrode according to an embodiment.

In an alternative embodiment there is disclosed an electrochemical cell comprising an electrode according to an embodiment.

In an alternative embodiment there is disclosed a cathode according to an embodiment for electrochemically depositing from solution a metal substantially liquid at room temperature.

In an alternative embodiment the metal is mercury.

In an alternative embodiment there is disclosed an electrochemical cell comprising a cathode according to an embodiment and an anode, wherein the cathode and the anode may each comprise substantially flat geometrical surfaces and the substantially flat cathode and anode geometrical surfaces are mutually opposed and are substantially uniformly distanced.

In an alternative embodiment the distance between the electrode geometrical surfaces is less than about 2 cm and the cathode and the anode are in direct fluid contact.

In an alternative embodiment distance between the anode and cathode is less than about 1 cm.

In an alternative embodiment the electrochemical cell further comprises a solution inlet positioned to direct at least a portion of the solution to flow through the cathode.

In an alternative embodiment the electrochemical cell is configured so that substantially all of the solution flows through the cathode.

In an alternative embodiment the electrochemical cell may further comprise a collector for collecting the metal under gravity induced flow when the metal is electrochemically deposited at the cathode.

In an alternative embodiment there is disclosed an apparatus for treating a scrubber bleed solution, the apparatus comprising the electrode according to an embodiment.

In an alternative embodiment there is disclosed an apparatus for treating a scrubber bleed solution, the apparatus comprising the electrochemical cell according to an embodiment.

In an alternative embodiment there is disclosed an apparatus comprising a plurality of electrochemical cells according to embodiments.

In an alternative embodiment there is disclosed a method for recovering from solution a metal substantially liquid at room temperature, the method comprising collecting metal electrochemically deposited at a cathode according to an embodiment.

In an alternative embodiment there is disclosed a method for recovering mercury from solution, the method comprising collecting mercury electrochemically deposited at a cathode according to an embodiment.

In an alternative embodiment there is disclosed a method for recovering from a solution a metal substantially liquid at room temperature, the method comprising the step of electrolytically depositing the metal at a flow through cathode positioned in the solution, wherein the solution directly contacts both the cathode and a corresponding anode.

In an alternative embodiment the cathode and the anode each have a substantially flat geometric surface and the anode geometric surface and the cathode geometric surface are mutually opposed and substantially uniformly separated by a distance of less than about 2cm. In an alternative embodiment the cathode may have a surface area of at least about 500m ² per 1 m ² of geometric surface.

In an alternative embodiment the cathode may be a carbon fibre cathode or may be a carbon cathode or may comprise carbon, pyrolyzed parylene C (PCC), .carbon foam, carbon nanofoam, carbon coatings, carbon films, carbon pastes, carbon beads, carbon microbeads, carbon microtubes, carbon nanotubes, graphite, graphene, pyrolytic graphite, highly oriented pyrolytic graphite, randomly oriented graphite, carbon black, carbon fiber, evaporate a- C, a-C:H, pyrolyzed photoresist film, boron doped diamond, or N-doped amorphous tetrahedral carbon. In an alternative embodiment the current density between the anode and the cathode may be less than about 10V per m ² of geometrical cathode surface. In an alternative embodiment the solution may be a scrubber bleed solution.

In an alternative embodiment the metal may be mercury.

In an alternative embodiment there is disclosed a continuous flow method according to an embodiment. In an alternative embodiment there is disclosed an electrochemical cell adapted to receive an electrode according to an embodiment and comprising a solution inlet adapted to direct an electrolyte to flow through the electrode.

In an alternative embodiment there is disclosed an electrochemical cell adapted to receive the electrode according to an embodiment and comprising a solution inlet adapted to direct an electrolyte to flow through the electrode.

Brief Description of the Drawings

FIG. 1 is a perspective view of an electrode according to a first embodiment.

FIG. 2 is a schematic representation of a general process comprising the bleed and destruction systems according to an embodiment.

FIG. 3 is a schematic representation of the recovery system according to an embodiment.

FIG. 4 is a side view of the interior of an electro-chemical cell in accordance with a first embodiment.

FIG. 5 is an end view of the interior of an electrochemical cell according to FIG. 4, taken at right angles to FIG. 4.

FIG. 6 is a top plan view of the interior of the cell according to FIGS. 4 and 5.

FIGS. 7A and 7B are enlarged views of portions of FIG. 4.

FIGS. 8A and 8B are enlarged views of portions of FIG 5.

FIG. 9 is a cut away sectional view of a second embodiment FIG. 10 is a sectional view of the embodiment according to FIG. 10.

Description of Embodiments

Definitions:

In this disclosure the term "pore" or "pores" means any interstice, space, passage, channel, opening, perforation, cavity or similar structure by means of which a substance may pass through a structure.

In this disclosure the term "porosity" or "porousness" refers to the ratio or relative relationship of the number or volume of pores in a substance, structure or mass relative to the total geometrical volume or geometrical area as defined by the gross external dimensions of the substance or structure or mass of the substance or structure. Porosity is generally referred to as a ratio of the volume of the pores relative to the gross geometrical volume of the structure. In particular embodiments electrodes or substances may be porous and may be highly porous. A porous electrode, which may be highly porous and may be a cathode, may be or may be greater than, about 20%, great than about 25%, greater than about 30%, greater than about 35%, greater than about 40%, greater than about 45%, greater than about 50%, greater than about 55%, greater than about 60%, greater than about 65%, greater than about 70%, greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 95% porous or more, or may be in a range delimited by values of about 40%, about 45%, about 50%, about 55%, about 60%, about 65%, about 70%, about 75%, about 80%, about 85%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%. about 97%, about 98%, about 99% about 99% or greater porosity.

In this disclosure the term "highly porous" indicates a porosity of greater than about 50%, greater than about 55%, greater than about 60%, greater than abut 65%, greater than about 70%, greater than about 75%, greater than about 80%, greater than about 85%, greater than about 90%, greater than about 95% or more, or and may indicate a range of porosity delimited by values of about 70%, about 75%, about 80%, about 85%, about 90%, about 95%, about 95%, about 97%, about 98%, about 99%, or greater porosity.

In this disclosure the term "felt" means a structure formed from matted or compressed fibers and "carbon felt" means a felt or structure formed from matted or compressed carbon fiber. The manufacture, handling and purchase of suitably formed felts, including carbon felts, will be readily understood and achieved by those skilled in the art.

In this disclosure, electrodes may be constructed in any conventional shapes or materials all of which will be readily identified, understood and adopted by those skilled in the art. In particular embodiments electrodes may be anodes or cathodes or both, and may be constructed to present an enlarged surface area, relative to the geometrical volume or surface of the electrode material itself. By way of example but without limitation, a cathode may be porous and may be constructed to present a large geometrical surface area by shaping the cathode in the form of one or more flattened or curved plates, sheets or membranes, or as a plurality of wires, threads, fibers or tubes, or the electrode may comprise any other structure or conformation suited to present an increased surface area for contact with an electrolyte which may be a scrubber solution. In particular embodiments a cathode may comprise an electrically conductive felt or reticulated material, non-limiting examples of which include a conductive felt, mesh or net of any kind and examples include carbon felt or reticulated carbon. In embodiments the carbon felt may be formed of carbon fibres, which fibres may be formed, for example, by the carbonization and/or graphitization of synthetic polymer fibres, for example, polyacrylonitrile or ester fibres. The felt may be formed from a pad of such carbon fibres and the pad may be compressible. In embodiments, the fibres may suitably have a diameter of the order of about 6 to 8 microns, especially about 6 microns. But in alternative embodiments a range of alternative materials will be readily identified and adopted by those skilled in the art and where used fibers may have diameters of, or of greater than or less than about 0.001 , 0.005, 0.01 , 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 , 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, or more microns, micrometers, or millimeters. It will be understood that an electrode having the desired properties may be made in a form other than a felt or fibrous structure or may comprise a coated fibrous or non-fibrous structure and may comprise a solid or semi solid structure with pores provided therethrough and may comprise or may comprise an electrolytic surface that comprises one or more of carbon, pyrolyzed parylene C (PCC), .carbon foam, carbon nanofoam, carbon coatings, carbon films, carbon pastes, carbon beads, carbon microbeads, carbon microtubes, carbon nanotubes, graphite, graphene, pyrolytic graphite, highly oriented pyrolytic graphite, randomly oriented graphite, carbon black, carbon fiber, evaporate a-C, a-C:H, pyrolyzed photoresist film, boron doped diamond, N-doped amorphous tetrahedral carbon and other materials which will be readily apparent to those skilled in the art who will readily select between them and adapt their compositions and structures for particular purposes. It will be understood that electrodes may comprise or have associated therewith or may be shaped to cooperate with, suitable supporting frames, clips, mountings or other structures which may maintain the structure, integrity or position of the electrodes. Such structures and frames may include but are in no way limited to internal and external frames of metals, plastics, carbon, and any other material of sufficient strength and rigidity to maintain the desired geometrical shape of the electrode in operation. This may also comprise the provision of a net or mesh interwoven with the electrode material or covering and containing the electrode material.

In this disclosure the statement that an electrode or a geometrical surface of an electrode is flat or substantially flat or generally flat means that at least one geometrical surface of the electrode is generally planar, or has only limited curvature or divergence from a plane. It will be recognised however that, while such a surface may be generally flat, some degree of irregularity or lack of smoothness may be permissible in embodiments, and those skilled in the art will readily determine the degree of smoothness or the tolerance that is necessary or desirable for acceptable or desirable performance of electrodes, electrode pairs and electrochemical cells according to embodiments.

In this disclosure reference to a geometric surface or geometric surface area of an electrode or structure means the external surface or gross external shape of the electrode or structure and is to be distinguished from more general references to surface and surface area of an electrode, which, unless the context otherwise requires, indicate the potentially reactive surface of the electrode at which electrical contact between the electrode and an electrolyte may occur. Thus where an electrode is porous, the total surface area of the electrode includes surface area presented within the pores. By way of example and not limitation, if an electrode has the general conformation of a square plate, then the geometrical surface area of the electrode will be defined by the areas of such square faces and any plate edges. It will therefore be apparent that in the case of a porous electrode, the total surface area of the electrode will be substantially greater than the geometrical surface of the electrode. It will be further understood that reference to the geometrical volume of an electrode refers to the volume defined by the external dimensions of an electrode.

In this disclosure an indication that an electrode has a high surface area to volume ratio means that the area of electrode surface that is potentially available to electrically contact an electrolyte is high relative to the external geometrical volume of the electrode or area, as defined by the external dimensions of the electrode. It will be further understood that in an electrode with a high surface area to volume ratio, the ratio of total surface for contact with the electrolyte to geometrical surface of the electrode will be greater than 1 :1. In embodiments such a high surface area to volume ratio may be, or may be greater than, about 1 ,000:1 , 2,000:1 , 3,000:1 , 4,000:1 , 5,000:1 , 6,000:1 , 7,0001 , 8,000:1 , 9,000:1 or 10,000:1 , 15m000:1 , 20,000:1 , 25,000:1. Similarly the ratio may be between about 50:1 and about 1000:1 , about 100:1 and about 1000: 1 , about 200: 1 and about 1000: 1 , about 300: 1 and about 1000: 1 , about 400:1 and about 1000:1 , about 100:1 and about 900:1 , about 100:1 and 800:1 , about 100:1 and about 700:1 , about 100:1 and about 600:1. Likewise, in embodiments, a high surface area to volume ratio may have any value within these ranges.

In particular embodiments the geometrical shape and size of an electrode may be of any desired dimensions, but in particular embodiments the dimensions will be chosen to reduce the current density between opposed anodes and cathodes. Thus in particular embodiments the geometrical surface area of a cathode or the opposed geometrical surfaces of a cathode/anode pair may be chosen to be as large as practicable to reduce the unit current flow between electrodes over a given area. Thus in particular embodiments any one of the edges of the substantially flat surface of a cathode or anode may be greater than about 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, or more meters or greater than about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10 or more feet. In embodiments the geometrical surface area of a cathode or anode may be greater than about 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 100 or more square meters.

In particular embodiments a cathode may have about 20 m ² of geometric surface area, although greater or lesser surface areas and particular geometrical dimensions can be selected and will be readily selected from by those skilled in the art. Where a cathode has a geometrical surface of about 20 m ² then it may provide a total cathode surface of about 250,000 m ² but different ratios of total surface to geometric surface will be readily created and selected amongst by those skilled in the art. It will be appreciated that the current flow per unit area may depend primarily on the opposed geometrical surfaces of opposed or paired electrodes rather than by the entire external surfaces of the electrodes.

In this disclosure an "electrochemical cell" also referred to as a "cell" means any device designed to pass electrical current between an anode and a cathode through an electrolyte liquid. In embodiments the electrolyte may be a solution, may be a scrubber bleed solution, or may be derived from a scrubber bleed solution. In one embodiment, a typical electrochemical cell for treating a scrubber bleed solution may have dimensions of about 5 ft. x 4 ft. x 6 ft although it will be understood that any convenient dimensions may be chosen to suit operational requirements and to accommodate desired electrode dimensions, and such possible dimensions will be readily selected from by those skilled in the art to suit particular operational requirements.

In embodiments a cell may be operated with a potential difference across the cell of about 10V, and the gap between cathode and anode may be less than about 2cm, and in some cases about 1 cm or less than about 1cm. Alternative voltages will be readily selected by those skilled in the art to suit particular cell properties and dimensions, and by way of example, in alternative

embodiments a potential difference of up to or greater than about 1 V, 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V, 11V, 12V, 13V, 14V, 15V, 16V, 17V, 18V, 19V, 20V or greater may be applied to the cell.

In particular embodiments, the rate of flow of an electrolyte, which may be a scrubber bleed solution, through an electrochemical cell may be relatively low and may be less than about 50,000 cm ² /sec/m ² , and may be less than about 45,000, 40,000, 35,000, 30,000, 25,000, 20,000, 15,000, 14,000, 13,000, 12,000, 1 1 ,000, 10,000, 9,000, 8,000, 7,000, 6,000, 5,000, 4,000, 3,000, 2,000, 1 ,000 or fewer cm ² /sec/m ² . It will be understood that in alternative embodiments the flow rate of electrolyte through an electrochemical cell of an embodiment may be above or below about 1 ,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000, 10,000, 11 ,000, 12,000, 13,000, 14,000, 15,000, 16,000, 17,000, 18,000, 19,000, 20,000, 21 ,000, 22,000, 23,000, 24,000, 25,000, 26,000, 27,000, 28,000, 29,000, 30,000, 31 ,000, 32,000, 33,000, 34,000, 35,000, 36,000, 37,000, 38,000, 39,000, 40,000, 45,000, 50,000 cm ³ /sec/m ² and those skilled in the art will readily adjust the flow rate to suit particular purposes and to achieve desirable performance parameters with particular cell geometries and electrolyte compositions. In particular

embodiments the pressure difference between the inlet and outlet of the cell may be very low and may be close to zero.

In this disclosure the term "hydrogen potential" of an electrode, and particularly a cathode means the reduction potential of the electrode. In embodiments cathodes may have high hydrogen potentials and may have hydrogen potentials of up to or greater than or about 2000mV (two thousand millivolts). Those skilled in the art will understand that the hydrogen potential of an electrode may be modified by the electrolyte in which it is immersed.

In this disclosure, reference to a metal that is "liquid at room temperature" or is "substantially" or "normally" liquid at room temperature, means that the metal is or, is normally, or is in part, liquid at room temperature or temperatures approximating room temperature, or at a temperature of greater than about 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, 11°C, 12 °C, 13 °C, 14°C, 15 °C, 16 °C, 17 °C, 18 °C, 19 °C, 20 °C, or 21 °C, and less than about 95 °C, 90 °C, 85°C, 80 °C, 75 °C, 70 °C, 65 °C, 60 °C, 55 °C, 50 °C, 45 °C, 40 °C, or 45C. In particular embodiments the metal may be Mercury. It will be further understood that the stipulation that a metal is normally or substantially liquid at room temperature, does not thereby require that the operation of an

electrochemical cell according to an embodiment is carried out at a

temperature that approximates room temperature. Thus for a reaction in aqueous solution, a cell may potentially operate at any temperature between the freezing and boiling points of the aqueous solution, or of the metal substantially liquid at room temperature, subject to any operational limits that may be imposed by a user for safety reasons or for any other reasons. All such adjustments and determinations will be readily made by those skilled in the art. Similarly, in any embodiment wherein an electrochemical cell is operated using an electrolyte that is not an aqueous solution, the cell may be operated at any temperature that is consistent with the safe operation of the cell.

In this disclosure the term "substantially liquid" or "normally liquid" or "liquid" where used to describe a metal, indicates the ability of the metal to flow, and encompasses the full range of possible viscosities that may be compatible with the operation of a cell or electrode according to embodiments disclosed herein.

In this disclosure paired anodes and cathodes of embodiments may present mutually opposed geometrical surfaces that are separated by a distance. In particular embodiments the opposed anode and cathode surfaces may be separated by a distance of less than about 2.0cm, 1.9cm, 1.8cm, 1.7cm, 1.6cm, 1.5cm, 1.4cm, 1.3cm, 1.2cm, 1.1cm, 1.0cm, 0.95cm, 0.90cm, 0.85cm, 0.80cm, 0.75cm, 0.70cm, 0.65cm, 0.60cm, 0.55cm, 0.50cm. In embodiments such separation distance may be substantially constant over the opposed area of the electrode surfaces. By "substantially constant" is meant that the separation distance may vary from point to point to an extent that does not prevent the effective or desired operation of an electrochemical cell according to embodiments. It will be appreciated that if the separation of the opposed plates becomes less uniform, this may affect the performance of the cell in ways that will be readily understood and managed by those skilled in the art who will readily determine acceptable parameters for an electrode pair for particular applications and will understand when an electrode is in need of repair or replacement. In particular embodiments where an electrode has a geometrical surface of about 20 square meters, then the separation distance between opposed anode and cathode faces may be between about 0.5cm and about 2cm.

In this disclosure the statement that an electrolyte or a solution or a liquid flows or may, or may in part, flow "through" an electrode, which may be a cathode, indicates that the electrode is, or is in part, porous so that the electrolyte, solution or liquid is able to pass through such pores from one side of the electrode to another side of the electrode, without having to flow around the geometrical surface of the electrode.

In this disclosure "scrubber bleed solution" means the solution generated from processing of off-gases from a range of processes, including roasting of ores and tailings. A scrubber bleed solution may typically have an initial mercury concentration of up to 500 ppm. However, in particular embodiments it will be understood that a scrubber bleed solution may contain any concentration of Mercury or of any other liquid normally or substantially liquid at room temperature and may contain more or less than about 50ppm, 100ppm, 150ppm, 200ppm, 250ppm, 300ppm, 350ppm, 400ppm, 450ppm, 500ppm, 550ppm, 600ppm, 650ppm, 700ppm, 750ppm, 800ppm, 850ppm, 900ppm, 950ppm, lOOOppm or greater of either alone or in combination with any other metals or other chemical components. Further it will be understood that an electrolyte or electrolyte solution may contain similar or greater or lesser concentrations of Mercury.

In this disclosure reference to a "continuous flow", "recycling" or "recirculation" of electrolyte means that the electrolyte is continuously circulated through one or more electrochemical cells, and may be supplemented, replenished, added to, diluted or otherwise modified during such recirculation process. This is to be distinguished from a batch process wherein the electrochemical cells or their associated containing structures are drained, or the electrolytic process temporarily or permanently halted after the processing of each batch of electrolyte. The use of a continuous flow process may mean that the extraction of mercury or other metal substantially liquid at room temperature may be continued for extended periods, for example in embodiments the processing may be continued for 24 hours a day, 7 days a week, or for such period as may be necessary or desirable thus allowing the continuous processing of solution in response to its source. A continuous flow process may be desirable for the processing of a relatively dilute electrolyte. It will be understood that even when electrolyte is processed in a continuous flow manner, it may be necessary or desirable to shut the process down from time to time to permit maintenance and adjustments to the apparatus or process.

In this disclosure, in alternative embodiments, a low current density means a current density of less than about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 3, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, or 50V per square meter.

In this disclosure, a "collector" "collection device" or the like, unless the context requires otherwise, refers to a device or design feature for collecting deposited metal normally liquid at room temperature may comprise any suitable structure or structures, including trays, pipes, channels, grooves, valves or other devices or constructions. Those skilled in the art will readily recognise and construct a wide range of suitable collectors and adapt and implement them into embodiments of the subject matter disclosed.

A statement in this disclosure that an electrode pair are in direct electrical contact through an electrolyte, or are both in direct contact with the electrolyte, indicates that the two electrodes are not separated by a membrane or other divider, such as a proton conductive membrane and that the same electrolyte solution is in direct physical contact with both electrodes.

In this disclosure a scrubber solution may be derived from any source including but not limited to the roasting of mineral ores including tailings. In particular but non limiting embodiments the ore may be gold ore.

In this disclosure an ore or a mineral ore, refers to any ore, rock or tailings, that may be processed to extract components therefrom. Embodiments are hereafter generally described with reference to FIGS. 1 through 10.

First Embodiment

In a general form of the first embodiment, generally designated 400 and illustrated in FIG. 1 there is disclosed a substantially flat, flow through electrode assembly which may be a cathode.

FIG. 1 shows a simplified diagram of an electrode for use in the first

embodiment, which electrode may be a cathode. The electrode 400 may comprise a porous electrode body 412. The porous electrode 412 may be confined and supported by supporting grids 416 and 418. Grid 416

comprising vertical components 411 and horizontal components 423 and grid 418 comprising vertical components 415 and horizontal components 414. Metal feeder sheet 413 may be secured between porous cathode material 412 and grid 418 or may be positioned external to the grid. It will be understood that a variety of alternative arrangements of the components and a variety of alternative materials may be selected for particular requirements and will be readily understood and implemented by those skilled in the art. When mounted in a suitable electrochemical cell it will be understood that additional supporting structures, gaskets, and wiring to apply an electrical current to the electrode, may all be incorporated in the electrode. For simplicity and clarity these details are omitted from FIG. 1 but will be readily understood by those skilled in the art and such structures and the arrangement of an electrode within a cell are further explained elsewhere in this disclosure with reference to FIGS. 4 through 8.

It will be seen that the feeder sheet 13 comprises a plurality of openings 433 distributed upon the sheet to allow electrolyte to access and flow through the electrode 412. As will be seen particularly from FIG. 1 , the geometric surface of the electrode body 412 is defined by spatial dimensions X, Y, Z. This is to be distinguished from the actual total surface area of the electrode which may include pores, and notwithstanding the geometric surface of the electrode, the electrode may have a total surface area of at least about 500m ² for each 1 m ² of geometric surface and may be a cathode. The porous electrode 412 may be made of or may comprise carbon felt or may comprise other materials. In an alternative embodiment the electrode may be a cathode and may be used for depositing from solution a metal substantially liquid at room temperature, and the metal may be mercury.

A cathode 412 according to the first embodiment may be porous or otherwise constructed to present a large surface area and may be in the form of an electrically conductive felt or reticulated material, especially a carbon felt or reticulated carbon. A carbon felt or reticulated carbon may provide a high porosity particularly in excess of 90%; the high porosity provides a high cathode surface area and in embodiments this may provide an increased reaction rate and may be suitable for use with dilute solutions. Where used, carbon felt may be formed of carbon fibres, which may be formed form the carbonization and/or graphitization of synthetic polymer fibres, for example, polyacrylonitrile or ester fibres. In particular embodiments the fibres may suitably have a diameter of the order of 6 to 8 microns, especially about 6 microns. Since a pad of fibres may be mechanically fragile, it may be mechanically supported in a frame or assembly which serves to hold the felt in a planar state. The felt may be supported under compression so as to have a planarity whereby a constant gap or separating distance is maintained between the cathode and an opposed anode. The cathode should, of course, be resistant to materials and conduction which may be present in the cell where it is used, which materials and conditions may be extreme and may include exposure to the presence of chlorine and chloride ions, heat, cold, acidity, alkalinity, oxidation and reduction conditions. Carbon felt and reticulated carbon are generally suited to these requirements. In embodiments the material of the cathode may have a high hydrogen over potential which may allow mercury or other metal normally or substantially liquid at room temperature to be deposited in preference to liberation of hydrogen at the cathode 412.

FIGS 4 through 8 illustrate an electrochemical cell generally designated 100 comprising an electrode according to the general design of the first

embodiment. As will be seen from FIGS. 4 through 8, in use a cathode 12 according to embodiments may be paired with an anode 10 in a suitable electrochemical cell 100. An anode 10 is generally essentially impermeable to the electrolyte solution, which may be or may comprise scrubber bleed solution or a treated extract or derivative thereof. In particular, an anode 10 may comprise a dimensionally stable electrode and may typically have a core of titanium sheet coated with a metal oxide, for example, one or more oxides of tantalum, iridium and platinum. A range of alternative suitable materials will be readily identified and implemented by those skilled in the art. Without limitation, any forms of inert or relatively inert and conductive metal and/or metal oxide may be suitable for use as or in anodes according to

embodiments. The anode 10 suitably may have a low oxygen overpotential such that hydroxide ions are discharged liberating oxygen in preference to chloride ions releasing chlorine.

The cathode 12 and the anode 10 each may comprise substantially flat geometrical surfaces and the substantially flat cathode and anode geometrical surfaces 210, 212 are mutually opposed and are substantially uniformly distanced 214. In particular embodiments the distance between electrode geometrical surfaces 210, 212 is less than about 2 cm and the cathode 12 and the anode 10 are in direct fluid contact, and in embodiments the distance 214 is about 1cm or less than about 1cm. Electrochemical cells 100 according to embodiments may further comprise a solution inlet 102 positioned to direct at least a portion of the electrolyte solution to flow through the cathode 12. The cell, which may comprise one or more collecting trays 100, may be configured so that substantially all of the solution flows through the cathode 12. Electrochemical cells may also comprise one or more collectors 220 for collecting a metal substantially liquid at room temperature under gravity induced flow when the metal is

electrochemically deposited at the cathode 12. The inflow passage 17 may be angled or shaped so that any deposited metal is guided to collect at a collection point 222, or any trays 220. The collector structure itself may have any suitable design and may be a simple drain to allow the deposited metal to flow out of the cell 100 to be harvested in a suitable container.

A cathode and anode may form an electrolysis electrode assembly 106 in a cell 100. In particular embodiments of a cell 100 electrode assemblies 106, each assembly having a cathode 12 and an anode 10 whose opposed faces are separated by a distance 214 therebetween. A flow path for flow of solution being treated extends from an inlet 102 to an outlet 104 of the cell. The flow path provides a contact time between the flowing solution and the electrode assemblies 106 sufficient for deposition of the mercury metal at the porous cathode 12. The flow path may, in particular, comprise an inflow passage 17 and an outflow passage extending across the cell in opposed generally parallel arrangement, with the electrode assemblies 106. These may be provided or in multiples and in embodiments a plurality of electrode

assemblies 106 is arranged extending in spaced apart relationship between the inflow 102 and outflow 104 passages and generally perpendicular thereto. A plurality of discrete branch passages between the electrode assemblies may communicate with the inflow passage 17 into the plurality of branch passages and from there through an adjacent porous cathode 12 into the gap 214 between such cathode 12 and its anode 10, the gap forms a gap passage communicating with the outflow passage and the solution flows along the gap passage against the anode 10 and into the outflow passage 2 and from there exits from the cell 100.

Suitably a gas passage is maintained as small as possible, for example, 1 cm or less. In this way a plurality of discrete treatment flow paths is formed within the cell 100 thereby maximizing the electrochemically active surface area of the cell per cell volume.

In a further variant of the embodiment, there is disclosed apparatus for treating a scrubber bleed solution, the apparatus comprising the electrode according to embodiments. In embodiments the apparatus may comprise a plurality of electrochemical cells. The electrochemical cells may be connected in series or in parallel and the scrubber bleed solution may be recycled through them with periodic or ongoing addition of fresh scrubber bleed solution.

In further detail, the embodiment illustrated in FIG. 1 and FIGS. 4 through 8 and is described as follows. Electrochemical cell 100 has an inlet 102 and an outlet 104. Inlet 102 communicates with and inflow passage 17 and outlet 104 is in communication with an outflow passage 2.

A plurality of electrode assemblies 106 is housed in cell 100 including a pair of end electrode assemblies 108, 110 and a plurality of intermediate electrode assemblies 112. Each of end assemblies 108 and 110 includes a cathode assembly 114 supporting a porous cathode 12 spaced form a dimensionally stable anode 10.

Each of intermediate electrode assemblies 1 12 includes a pair of cathode assemblies 114 each supporting a porous cathode 12 spaced from a single dimensionally stable anode 10, therebetween. Flow passages 16 are defined between adjacent intermediate electrode assemblies 112 and between intermediate electrode assemblies 112 and end electrode assemblies 108 and 110, respectively. The flow passages 16 communicate with in-flow passage 17 but are closed adjacent out-flow passage 2 by caps 4. The electrode assemblies 106 are pressed together as an assembly between a pair of end supports 160 comprising inner end plates 1 , suitably of PVC, and outer end plates 18, suitably of steel. Cell 100 includes a plurality of cathode supports 19 each comprising cathode feeder 3 suitably in box section of mild steel sheet. With particular reference to FIGS. 7A and 7B which show an enlargement of details A and B of FIG. 4, each cathode assembly 114 comprises a frame 6 and a porous cathode 12 suitably a carbon fibre felt supported between a grid 116 and a grid 118. A metal feeder sheet 13, conveniently an expanded stainless steel sheet having a plurality of orifices, is supported between cathode 12 and grid 118 but alternatively the grid 116 may be positioned between the feeder 13 and electrode body 12 so long as suitable electrical contact is maintained between electrode body 12 and feeder sheet 13. Grid 116 is conveniently of PVC and comprises a plurality of spaced apart vertical members 11 and a plurality of spaced apart horizontal members 23.

Grid 118 is conveniently of mild steel and comprises a plurality of spaced apart vertical members 5 and a plurality of spaced apart horizontal members 14. The grids 116 and 118 hold the porous cathode 12 with a required degree of planarity, in spaced relationship with anode 10.

With particular reference to FIGS. 4 and 5, and FIGS. 7A and 7B outer gaskets 7 are disposed between feeder sheets 13 and frame 6 adjacent outflow line 2 and outer gaskets 15 are similarly disposed adjacent in-flow passage 17. Gaskets 8 and 9 are disposed between opposed sides of anode 0 and grids 116 in the vicinity of out-flow passage 2 and in-flow passage 17. The gaskets 7, 8, 9 and 15 are suitably of neoprene but a range of other suitable materials will be readily identified, selected from, and used by those skilled in the art. With particular reference to FIGS. 7A, 7B and 8A and 8B, the cathode supports 19 provide electrical contacts and are suitably of mild steel. A plurality of insulation and anode supports 20 suitably of PVC, house anode feeds 21, suitably of copper or other electrically conductive metal, connected to each anode 10.

With particular reference to FIG. 8, a mesh 22 suitably of polypropylene is disposed between porous cathode 12 and grid 116. With particular reference to FIG. 7, a plurality of flow paths 120 in parallel are identified by flow line arrows.

Thus in one embodiment as shown generally in FIGS. 4 and 5, the cell 100 consists of ten (10) cathodes 12 and eleven (1 ) anodes 10. The cathodes 12 and anodes 10 are pressed together between the end plates 1 and the resulting assembly is compressed between end plates 18 which thus provide the rigidity which cell 100 needs to ensure a uniform separation of each anode 10 and is associated cathode 12.

Each cathode 12 has opposed porous surfaces, the geometric integrity and planarity of which is maintained by there being contained between the three-dimensional grids 116 and 118.

The cathode assembly 114 may comprise a cathode feeder 3 suitably of box section mild steel, which acts as a frame and a means of distributing electrical current to grid 118. The expanded metal feeder sheet 13 is welded onto the vertical member 5 of grid 118. Feeder sheet 13 acts as a current distributor to the porous cathode 12 and as a physical constrain to ensure that the porous cathode 12 retains a uniform thickness. The porous cathode 12 is pressed against the feeder sheet 13 by the three-dimensional grid 116, suitably of PVC; the polypropylene mesh 22 between grid 116 and cathode 12 ensures the planarity of the surface of cathode 12 facing anode 10. Grid 116 is attached to frame 6, suitably of PVC, which is suitably bolted through feeder sheet 13 to a corresponding frame 6 of the adjacent cathode assembly 114. Gasket 7 is interposed between frame 6 and feeder sheet 13. The electrode assemblies 106 are supported by the cathode feeders 3 which act as electrical contacts and by supports 44 which also conduct current to the electrode .

The anode 10 suitably consists of a titanium sheet coated with one or more metal oxides to produce a dimensionally stable anode. In particular embodiments, suitable anode types include those produced by El-tech

Corporation of Cleveland, Ohio. The current to the anodes 10 is distributed by anode feeds 21 , suitably four copper strips riveted on each face at either side of an anode 10. The anode 10 is supported by the insulation and anode supports 20 on either side which also electrically insulate the anode 10 from the cathodic bussing.

In operation the scrubber solution after it has been passed through a mercury recovery system is now termed scrubber bleed solution. The scrubber bleed solution enters the electrochemical cell 00 through the inlet 102 and passes into the in-flow passage 17.

The solution passes through the cell 100 in parallel flow paths 120 the distribution of the solution through the cell 100 is governed by the pressure drop associated with each possible flow path 120. The only significant pressure drop in the system is that across the face of the porous cathode material of cathode 12. This ensures that each electrode receives a similar flow of solution. Any imbalance is self correcting as an increase in flow through any electrode will result in greater deposition and a consequent rise in the pressure across the electrode. From the in-flow passage 17 the solution passes up into the flow passage 16 where its exit is blocked at the top by cap 4. The solution exits through the faces of the cathode 12 through the grid 118, the expanded feeder sheet 13, the porous cathode 12, the polypropylene mesh 22 and into the grid 116.

The solution passes up the grid 116 between the anode and the cathode 12 and into the out-flow passage 2 from where it exits via outlet 104 in the endplates 1 and 18.

Second Embodiment

A second embodiment of an electrochemical cell is shown in FIGS. 9 and 10 and is generally designated 300. In the embodiment the cell 300 comprises body 301 holding a single anode 310 and a single cathode 312 separated by a distance 314. It will be seen that the interior 304 of the cell forces the electrolye which may be or derive from a scrubber bleed solution and may enter the cell through an inflow 302 leading into inflow chamber 317, to flow through the cathode 312 and around the anode 310 passing to outflow chamber 320 and then exiting the cell 300 through outflow 304. It will be appreciated that the cell 300 incorporates suitable brackets and mountings to hold the cathode 312 and anode 310 in place and comprises an electrical supply to apply a current through the electrodes and electrolyte. Mountings are generally designated 350 and 352, but for simplicity the detail of such mountings and of any power supply is omitted from FIGS. 1 , 9 and 10. A range of suitable methods and materials for the mounting and application of a potential difference to the electrodes will be readily apparent and will be readily implemented by those skilled in the art.

The bottom 306 of cathode 312 is engaged by a drainage channel or collector 340 with a collection point 342 for any deposited metal that is substantially liquid at room temperature. In operation, as electrolyte flows through the cathode 312, which may be a carbon felt cathode or of any other construction, such as the construction illustrated in FIG. 1 and described above, the metal, which may be mercury, is deposited at the cathode, and flows under gravity down through the openings in the cathode to accumulate in collection channel 340, whence it is collected for further use at the collection point at end 342 of the channel 340. The collection channel may be of any suitable size but in an embodiment may be about 1.25 inches in diameter or may be narrower or wider.

It will be appreciated that in this embodiment, it may be of particular

importance to restrict the flow rate of solution through the cell in order to prevent undue pressure on the cathode structure. While one form of the second embodiment is illustrated with reference to FIGS. 9 and 10 it will be appreciated that applied current, electrolyte flow rate, inlet and outlet positions, and other parameters may be adjusted according to required performance parameters in ways readily apparent to those skilled in the art. Examples of possible modifications are provided in the description of the first embodiment.

Third Embodiment

In an third embodiment there is disclosed a method for recovering from a solution a metal substantially liquid at room temperature, the method

comprising the step of electrolytically depositing the metal at a flow through cathode. In one embodiment of the third embodiment here is disclosed apparatus and a method for treating a scrubber bleed solution which may be generated from off gases, or from the processing of mineral ores. A general embodiment of a method for treating off gases from ore processing, is shown in FIG. 2 and FIG. 3. It will be seen that in some cases, for clarity, features shown in one of the FIGS. 2 and 3 may be omitted in the other.

With reference to FIG. 2, when an ore is roasted, the necessary processing solutions 500 may release gases, which may then be treated 510 to recover mercury and particulates, the resulting scrubber bleed solutions 660 may then be treated at 670 to separate the liquids from the solids, which are then taken to a leaching process 671. Then the liquid is mixed with necessary

conditioning reagents 545, in a mix tank 550, then the conditioned solution is passed through an electrochemical cell 680 so that mercury liquid 681 can be recovered, and outflow solution 580 may be returned to the process cycle 700 which may be operated on a continuous flow basis.

With reference to FIG. 3, which shows the general process according to FIG. 2 in the context of associated quench towers, scrubbers and the like, gas from ore roasting, 599 is introduced to a quench tower 600 along with introduced treatment solutions 602, bleed solution 605 is drawn off for processing and output 602 from the quench tower 600. The output from quench tower 600 is conveyed to a particulate scrubber 610, mixed with treatment solutions 612, and bleed liquids drawn off at 615 while output 621 from the scrubber 610 flows to the sulphur dioxide scrubber 620 to be mixed with treatment solutions 622. Bleed solution 625 is drawn off and the output 621 from the scrubber 620 flows to first mercury scrubber 630. Treatment solutions 632 is introduced and the output from the first mercury scrubber 630 flows to second mercury scrubber 640 to be mixed with treatment solution 642. The output 641 from the second mercury scrubber 640 flows to tails scrubber 650 for treatment with solutions 652, gas is vented at 651 and bleed solution 655 is removed for processing.

The bleed solution from the first and second mercury scrubbers 630, 640, is combined in a common feed 660 leading into a separation tank 670 where solids 671 are removed for processing.

The scrubber bleed solution from this separation process flows to an electrochemical cell 680, where mercury or other metals substantially liquid at room temperature are electrochemically deposited and are harvested. In the case of mercury metal this is typically in solutions the form of mercuric ions. The electrochemical cell 680 may be constructed or operated according to the first or second or other embodiments. In flowing through the porous cathode, mercuric ions are discharged electrochemically within the porous structure and the treated solution flowing from cell is thus poor in mercury metal ions. As the porous cathode becomes loaded with deposited mercury metal, the mercury will coalesce and become a free flowing liquid gathering at the base of the electrode. A suitable collector for collecting the liquid mercury is provided for at the base of the cathode or cathodes or more generally in the base of the cell 680. The scrubber bleed solution to the cell can be temporarily interrupted without terminating the operation of the process

A surge tank 690 is provided for the used electrolyte solution, make up solution or additives 691 are added as needed, and the resulting solution 700 is returned to scrubbers 600, 610, 620, 630, 640, 650 as necessary or desirable. Thus in operation the scrubbing process may be operated on an essentially or substantially continuous flow basis. In a continuous operation scrubber bleed solution may be continuously cycled from the roaster or combustion process scrubber to the electrochemical cell and treated solution recycled to the process.

In one embodiment, a typical cell 680 for treating scrubber bleed solution may have dimensions of about 5 ft. x 4 ft. x 6 ft. The cathode may have about 20 m ² geometric surface area, while the porosity of the cathode may provide a total cathode surface of about 250,000 m ² . The cell may be operated with a potential difference across the cell of about 10V, the gap between cathode and anode may be less than about 2cm, and in some cases about 1 cm or less than about 1cm. Such a cell may be operable treat a scrubber bleed solution flow of 20,000 cm ³ /sec/m ² . The scrubber bleed solution may typically have an initial mercury concentration of up to 500 ppm.

It will be appreciated that the specifics of dimensions, flow rates, surface areas, volume and other parameters of a cell can be readily modified and adapted by those skilled in the art all consistent with the embodiments disclosed herein. For example cells may be made larger, smaller, or shaped for particular applications, the numbers and arrangements of electrodes may be modified and a wide range of other adaptations will be recognised and implemented by those skilled in the art.

The rate of flow of scrubber bleed solution may be maintained low and in particular embodiments may typically be between about 5,000 cm ³ /sec/m ² to 20,000 cm ³ /sec/m ² , depending on the concentration of mercury metal. At high concentrations of mercury metal the flow rate may preferably be maintained at the at the lower end of the 5,000 cm3/sec/m2 to 20,000 cm3/sec/m2 range. In one embodiment the pressure drop between the inflow passage and the outflow passage is effectively zero, so that the pressure drop through the porous cathode governs the flow rate, this flow rate being governed by the porosity.

Alternative Embodiments

In an embodiment there is disclosed a method and apparatus for the removal of a metal which may be mercury from a solution which may be a scrubber solution, whereby the resulting solution can be recycled. In the case of a scrubber solution it may be recycled to the mill process. In some

embodiments the scrubber solutions may be created by the roasting of ore. In an alternative embodiment the mercury is electrochemically deposited and subsequently collected as metallic mercury. In a further alternative

embodiment the scrubber solution may flow through a porous cathode and across the surface of an anode of an electrochemical cell. In a further alternative embodiment embodiment a potential difference is maintained between the cathode and the anode to effect electrochemical deposition of the mercury metal in the porous cathode. In a further alternative embodiment the porous cathode may have a high electrochemically active surface area per unit volume and in embodiments this be achieved by employment of a cathode material of high porosity in excess of 90%. In embodiments the cathode can be loaded with a high level of mercury metal which is collected by allowing the mercury to exit the electrode at its base by gravity induced flow. In a further alternative embodiment the cathode materials may have a substantially evenly distributed or substantially homogenous, or substantially uniform porosity. In embodiments the porosity should permit loading of the cathode with a mercury level of more than 0.5 g cm ³ . The embodiments and examples presented herein are illustrative of the general nature of the subject matter disclosed and are not limiting. It will be understood by those skilled in the art how these embodiments can be readily modified and/or adapted for various applications and in various ways without departing from the spirit and scope of the subject matter disclosed. The subject matter hereof is to be understood to include without limitation all alternative embodiments and equivalents. Phrases, words and terms employed herein are illustrative and are not limiting. Where permissible by law, all references cited herein are incorporated by reference in their entirety. It will be appreciated that any aspects of the different embodiments disclosed herein may be combined in a range of possible alternative embodiments, and alternative combinations of features, all of which varied combinations of features are to be understood to form a part of the subject matter claimed. Particular embodiments may alternatively comprise or consist of or exclude any one or more of the elements disclosed.