REMOVING IMPURITIES FROM AN ELECTROLYTE

Abstract

It is disclosed a purifier assembly and method for removing impurities from an electrolytic bath before using the same with an electrolytic cell for making a metal, such as aluminum or aluminium. The assembly comprises a purification tank, located upstream the cell, for containing the bath; and at least one row, preferably at least two rows, of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes. The rows of vertically oriented cathodes and anodes are configured in size to be inserted into the tank. The purifier assembly is configured to maintain an anode-to-cathode distance (ACD) between the cathodes and anodes. The purifier is particularly adapted for removing sulfur, phosphorus, iron, and/or gallium from cryolite for the eco-friendly production of aluminum with a cell using oxygen-evolving or inert anodes, which preferably requires a purer bath.

Description

TECHNICAL FIELD

The present patent application generally relates to the purification of a molten electrolytic bath of an electrolytic cell, the electrolytic cell being used, for instance, for the production of a metal, such as aluminum.

BACKGROUND

Aluminum metal, also called aluminium, is produced by electrolysis of alumina, also known as aluminium oxide (IUPAC), in an electrolytic bath of molten electrolyte at about 750-1000° C. contained in a number of electrolytic cells. The cells have a crucible comprising a steel shell, containing a carbonaceous cathode material, steel current conducting bars and refractory insulation materials capable of containing the electrolyte, at least one cathode and at least one anode.

The direct current that passes through the anodes, the electrolyte and cathodes causes alumina reduction reactions, and is also capable of maintaining the electrolytic bath at the target operating temperature by the Joule effect. The electrolysis cell is regularly supplied with alumina so as to compensate for consumption of alumina caused by electrolysis reactions.

In the traditional Hall-Heroult process, the anodes are made of carbon and are consumed during the electrolytic reaction. The anodes need to be replaced after 3 to 4 weeks. Consumption of the carbonaceous material releases large quantities of carbon dioxide in the atmosphere.

Aluminum producers have been searching for anodes made of non-consumable materials, called “inert anodes” or “oxygen evolving anode”, to avoid environmental problems and costs associated with manufacturing and use of anodes made of carbonaceous material. Several materials have been proposed, particularly ceramic materials (such as SnO₂ and ferrites), metallic materials and composite materials such as materials known as “cermets” containing a ceramic phase and a metallic phase, particularly nickel ferrites containing a metallic copper-based phase.

Problems encountered in the development of inert anodes for the production of aluminum by electrolysis lie not only in the choice and manufacturing of the material from which the anode is made, but also in the use of an electrolytic bath comprising a high purity electrolyte which has been purified to remove unwanted electrochemically active impurities, such as sulfur, phosphorus, iron, nickel, chromium, copper, and gallium.

Existing solutions would require an excessive amount of space to implement in a commercial bath melter.

There is thus a need for a new purifier system, particularly adapted for removing impurities from electrolytic baths or electrolytes of electrolytic cells using vertical anode and cathode arrangement, in particular with inert or oxygen-evolving anodes.

SUMMARY

The shortcomings of the prior art are generally mitigated by a new purifier assembly and method for purifying the electrolytic bath of an electrolytic cell typically used for the electrolytic production of a metal, such as aluminum.

It is first disclosed a purifier assembly for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the purifier assembly comprising: a purification tank located upstream an electrolytic cell and configured to contain at least a portion of the electrolytic bath; and at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes, wherein the at least one row of vertically oriented cathodes and anodes is configured in size to be inserted into the purification tank; and wherein the purifier assembly is configured to provide and maintain an anode-to-cathode distance (ACD) between each of said vertically oriented cathodes or anodes.

According to a preferred embodiment, the purifier assembly further comprises at least two parallel rows of alternating vertically oriented cathodes and anodes, each anode or cathode of one row being adjacent respectively to another anode or cathode of adjacent parallel rows, forming as such an array of alternating columns of vertically oriented cathodes and anodes.

According to a preferred embodiment, the purifier assembly further comprising: at least one anode connection rail configured to be operatively connected to the power supply, each of the anode connection rail being configured to support one row of vertically oriented anodes and to electrically connect the anodes one to each other in a parallel arrangement; and at least one a cathode connection rail configured to be operatively connected to the power supply, each cathode connection rail being configured to support one row of vertically oriented cathodes and to electrically connect the cathodes one to each other in a parallel arrangement. Preferably, the at least one anode and cathode connection rails are configured to be independently moved at a desired position relative to the electrolytic bath in the purification tank. More preferably, the at least one cathode connection rail is configured to entirely plunge the cathodes below a bath-vapor interface of the electrolytic bath.

According to a preferred embodiment, each of the anodes or cathodes comprises a longitudinal stem having one end connected to an anode body or a cathode plate, and an opposite end configured to be operatively connected to the anode or cathode connection rails respectively. Preferably, each of the anode bodies and cathode plates have a top extremity connected to their respective stems, the top extremities of the cathodes being located below the top extremities of the anodes when the row of electrodes is plunged into the electrolytic bath in order to have the cathode plates entirely plunged into the bath.

According to a preferred embodiment, the anode body comprises carbon or graphite, the cathode plates comprises steel or stainless steel, and the longitudinal stem comprises steel or stainless steel.

According to a preferred embodiment, each of the anodes and cathodes further comprises a protective sleeve around the longitudinal stem for protecting the longitudinal stem from corrosion. Preferably, the longitudinal sleeve comprises: a metal oxide of the metal to be produced, the metal oxide being aluminum oxide when the metal to be produced is aluminum; a semi-noble metal, such as copper; or silicon carbide (SiC).

According to a preferred embodiment, the purifier assembly further comprises a supporting structure for securing the at least one row of alternating vertically oriented cathodes and anodes at a position relative to the purification tank. Preferably, the connection rails are secured to the supporting structure for reinforcing and stabilizing the position of anodes and cathodes.

According to a preferred embodiment, the supporting structure is configured in size to close a top opening of the purification tank and seal the tank. Preferably, the supporting structure is configured to provide insulation, to be resistant to corrosion and to prevent gas from escaping the purification tank when the top opening is closed, the purification tank being then equipped with a gas outlet for safely collecting anode gas.

According to a preferred embodiment, the purification tank may belongs to a melter, the melter being used for melting a dry (solid) bath of said electrolytic bath.

According to a preferred embodiment, the at least one row of alternating vertically oriented cathodes and anodes forms a compact array with an ACD ranging from about 1 cm to about 5 cm, preferably about 2.5 cm.

According to a preferred embodiment, the power supply comprises a DC rectifier.

According to a preferred embodiment, the metal to produce by the electrolytic cell is aluminum, the electrolytic bath then comprising cryolite and additives, and the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof.

It is also disclosed a method for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the method comprising the steps of:

• • injecting at least a portion the electrolytic bath to purify into a purification tank located upstream an electrolytic cell;
  • positioning into the purification tank at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes, the at least one row of alternating vertically oriented cathodes and anodes being configured in size to be inserted into the purification tank and to provide and maintain an anode-to-cathode distance (ACD) between each of the vertically oriented cathodes or anodes; and
  • applying the electric current between the anodes and cathodes for a period of time to remove impurities from the electrolytic bath.

According to a preferred embodiment, each of the cathodes comprises a cathode plate, the cathodes being positioned in the electrolytic bath so as to be entirely submerged in the electrolytic bath.

According to a preferred embodiment, when the electrolytic bath to purify is a dry electrolytic bath, the method further comprises the step of:

• • melting the dry bath before injecting at least the portion the electrolytic bath into the purification tank; or
  • melting the dry bath directly in the purification tank.

According to a preferred embodiment, the electric current is a direct current applied using a DC rectifier.

According to a preferred embodiment, applying the electric current comprises measuring an amount of impurities present in the bath before adapting a total electric charge passing through a purifier assembly comprising said purification tank and said at least one row of alternating vertically oriented cathodes and anodes. Preferably, the total charge is about 0.1 to about 10 Ampere-hours (Ah) per kilogram of electrolytic bath to purify, more preferably about 0.3 to about 4.0 Ampere-hours (Ah) per kilogram of electrolytic bath to purify.

According to a preferred embodiment, a current density of the cathodes is about to about 0.4 A/cm², preferably about 0.1 to about 0.3 A/cm².

According to a preferred embodiment, the voltage across a purifier assembly comprising the row of alternating vertically oriented cathodes and anodes and the purification tank is between about 0.5 V and about 2.5 V.

According to a preferred embodiment, the period of time is between about 1 to about 150 hours, preferably between about 24 to about 96 hours.

According to a preferred embodiment, the method further comprises increasing current density for either increasing a specific electrical charge (Ah/kg bath), reducing the period of time of purification, or a combination thereof.

According to a preferred embodiment, the method further comprises removing the cathodes from the purification tank and removing solid impurities accumulated on the cathodes.

According to a preferred embodiment, the metal to produce by the electrolytic cell is aluminum, the bath then comprising cryolite and additives, the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof. Preferably, the additives comprises sodium fluoride, aluminum fluoride, calcium fluoride, and/or aluminum oxide.

It is also disclosed a purifier assembly for purifying an electrolytic bath located in a purification tank located upstream an electrolytic cell, the bath once purified being used in said electrolytic cell for the making of a metal. The purifier assembly comprises alternating anodes and cathodes in a compact array, the compact array being configured in size to be inserted into the purification tank; and the purifier assembly being configured to be operatively connected to a power supply located outside the purification tank for providing an electric current to the anodes and cathodes.

It is also disclosed a method for purifying an electrolytic bath of an electrolytic cell, the method comprising the steps of:

• • a) charging the electrolytic bath into a purification tank;
  • b) inserting the anodes and cathodes of the purifier assembly as disclosed herein into the tank until the anodes and cathodes are plunged, at least partially, into the electrolytic bath; and
  • c) applying a current between the anodes and cathodes for a given period of time to remove impurities from the bath.

The invention is particularly adapted for the purification of electrolytic bath of electrolytic cell using inert or oxygen-evolving anodes requiring a bath of higher purity. Purifying electrolytic bath by immersing and passing DC current through a graphite or carbon anode to a steel or metallic cathode for a period of time, has been found adequate to deposit as solid metals and compounds, various impurities, such as Fe, Ni, Cr, Cu, S and P on the cathodes, as well as liberate additional impurities, such as sulfur dioxide gas from the anode.

Other and further aspects and advantages of the present invention will be better understood upon the reading of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying drawings in which:

FIG. 1 is a schematic illustration of a purifier assembly according to a preferred embodiment;

FIG. 2 is a schematic illustration of a purifier assembly located upstream the electrolytic cell according to a preferred embodiment;

FIG. 3 is a three-dimensional illustration of an anode according to a preferred embodiment;

FIG. 4 is a three-dimensional illustration of a cathode according to a preferred embodiment;

FIG. 5 is a three-dimensional illustration of electrodes of a purifier assembly according to a preferred embodiment;

FIG. 6A is a top isometric illustration of a purifier assembly according to another preferred embodiment;

FIG. 6B is a bottom isometric illustration of the electrodes of the purifier assembly illustrated on FIG. 6A;

FIG. 6C is a top isometric illustration the electrodes and guardrails illustrated on FIGS. 6A and 6B;

FIG. 6D is a bottom isometric illustration the electrodes and guardrails illustrated on FIG. 6C;

FIG. 7A is an isometric illustration the electrodes and guardrails of a purifier assembly according to another preferred embodiment;

FIG. 7B is a front plan view of the electrodes and guardrails of the purifier assembly illustrated on FIG. 7A;

FIG. 7C is a closer view of the guardrails illustrated on FIGS. 7A and 7B;

FIG. 7D is a closer view of the electrodes 7 illustrated on FIGS. 7A and 7B;

FIG. 8 is a flowchart illustrating the method for removing impurities from an electrolyte assembly according to another preferred embodiment;

FIG. 9 is a top isometric illustration of two rows of electrodes according to another preferred embodiment; and

FIG. 10 is a cross-sectional view of a purifier assembly with the two rows of electrodes illustrated on FIG. 9 inserted into a purification tank according to another preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel purifier assembly and method will be described hereinafter. Although the invention is described in terms of specific illustrative embodiments, it is to be understood that the embodiments described herein are by way of example only and that the scope of the invention is not intended to be limited thereby.

The terminology used herein is in accordance with definitions set out below.

As used herein % or wt. % means weight % unless otherwise indicated. When used herein % refers to weight % as compared to the total weight percent of the phase or composition that is being discussed.

By “about”, it is meant that the value of distance (cm), weight % (wt. %), amperage (A), voltage (V), time (second, hour, or day), resistance, volume or temperature (° C.) can vary within a certain range depending on the margin of error of the method or device used to evaluate such distance, weight %, time, resistance, volume or temperature. A margin of error of 10% is generally accepted.

The description which follows, and the embodiments described therein are provided by way of illustration of an example of particular embodiments of principles and aspects of the present invention. These examples are provided for the purposes of explanation and not of limitation, of those principles of the invention. In the description that follows, like parts and/or steps are marked throughout the specification and the drawing with the same respective reference numerals.

As aforesaid, the invention as disclosed herein is directed to an apparatus, also named purifier assembly, for removing impurities from an electrolytic bath or electrolyte of an electrolytic cell. The electrolytic cell is preferably used for the making of metals, such as aluminum. When aluminum is the metal to produce, the electrolyte then comprises cryolite and the impurities to remove comprise sulfur, phosphorus, iron, gallium or mixture thereof

FIGS. 1 and 2 are schematic illustrations of a purifying system in accordance with the principle of the invention.

The purifier assembly 100 allows purifying or removing impurities from an electrolytic bath 102 located in a purification tank 104 located upstream an electrolytic cell 200 (FIG. 2). An inlet 101 can be provided to introduce the electrolyte in the tank 104. The bath 102 once purified is transferred to the electrolytic cell 200 for the making of a metal. The bath 102 once purified can be transferred 109 to the cell 200 by siphoning it into a crucible before pouring it into the cell. Other way for transferring the electrolyte once purified from the purifier assembly to the electrolytic cell can be considered.

As shown in FIGS. 1 and 2, the bath forms a bath-vapor interface 103. As aforesaid, when the metal to produce is aluminum, the electrolytic bath typically comprises cryolite and additives. Preferably, the additives comprises sodium fluoride, aluminum fluoride, calcium fluoride, and/or aluminum oxide.

The purifier assembly 100 may comprise one or more rows of alternating vertically oriented anodes 112 and cathodes 114, preferably forming a compact array of one row (see e.g. FIGS. 6 and 7), two rows (see e.g. FIG. 5), or more rows of alternating anodes and cathodes. The electrodes are configured in size to be inserted or plunged into a purification tank 104. The purifier assembly 100 can then be operatively connected to a power supply 120, preferably located outside the purification tank 104 and operatively connected the anodes and cathodes for providing an electric current to the anodes and cathodes.

The purifier assembly as disclosed herein comprises anodes and cathodes, preferably in a compact array. The anode's body of the anodes can comprises carbon, preferably graphite. The cathodes are typically cathode plates of steel or stainless steel. All the anodes are electrically connected to each other in a parallel arrangement. Likewise, the cathodes are preferably connected to each other in a parallel arrangement. The purifier assembly as disclosed herein is configured to be operatively connected to the power supply, preferably comprising a DC rectifier providing a direct current.

As illustrated in FIGS. 1, 5-7, each anode 112 of one row 113 may be connected to an anode connection rail or guardrails 116 whereas each cathode 114 of the same row is connected to a cathode connection rail or guardrails 118. The anode and cathode connection rail 116, 118 are operatively connected to the power supply 120, preferably a DC rectifier, for generating a direct current that will pass through the anodes 112 and cathodes 114 submerged in the purification tank 110. Preferably, the cathodes are fully submerged into the bath to avoid or limit corrosion thereof.

According to a preferred embodiment, as the one shown on FIGS. 5 and 9, the purifier assembly 100 comprises two rows 113, 115 of multiple parallel alternating anodes 112 and cathodes 114 in a compact array.

As used herein, “compact array” refers to an anode-to-cathode distance (ACD) of up to about 5 cm, preferably between 1 cm and 3 cm, and even more preferably about 2.5 cm. The anode-cathode distance is nominally about 2.5 cm, but can be varied.

According to a preferred embodiment, the anode connection rail 116 is configured to support and electrically connect the anodes 112 to each other in a parallel arrangement. The anode and cathode connection rails are preferably configured to be independently moved at a desired position relative to the electrolytic bath in the purification tank. As illustrated on FIG. 9, each of the anode bodies 121 and cathode plates 141 have a top extremity 121a, 141a connected to their respective stems 122, 142. The top extremities of the cathodes 121a is positioned below the top extremities of the anodes 141a when the row of electrodes is plunged into the electrolytic bath in order to have the cathode plates entirely plunged into the bath or below the bath-vapor interface. This advantageously protect the cathodes plates, preferably made of steel as detailed herein below, from corrosion of the vapors above the interface.

According to a preferred embodiment illustrated on FIG. 3, each anode 112 comprises an anode body 121 and a longitudinal stem 122 for electrically connecting each anode body to the anode connection rail 116. The longitudinal stem 122 may be made of any suitable conductive material, and may be threaded for securely and electrically connecting the longitudinal stem 122 to the connection rail 116 using, for example, stainless steal fasteners. The anode body 121 may comprises carbon or graphite.

As illustrated on FIGS. 6C and 7C for instance, an anode connection rail 116 may comprise a plurality of openings 117, configured to receive and maintain the stem of the anodes. Preferably, the openings 117 are located along the rail 116 about 2 cm to 10 cm from another opening so as to maintain an anode-to-cathode distance (ACD), for instance of about 1 cm to 5 cm when the anode connection rail 116 and the cathode connection rail 118 are “sandwiched” together in the purifying assembly. As illustrated on FIG. 7C, the rails can be each a one peace element with lateral extensions 116b, 118b allowing the openings 117, 119 of the rails 116, 118 to intermingle in a way that all the openings of both rails are aligned to form the electrode row 113. Another embodiment of the rails is illustrated on FIG. 6C for instance, where each rail comprises a main longitudinal element 116a with extension 116b hingely fixed thereto, the opening 117 being located at the end of the extension 116b.

The center-to-center distance between two anode openings may be twice the ACD plus the thickness of one anode body 121 and one cathode plate 141. It may be the same for the cathode rails as discussed herein after. The distance between the surfaces or faces of two anodes is typically twice the ACD plus the thickness of the cathode plate 141 located between two anode bodies 121.

The anode 112 may further comprise a sleeve 123 for encapsulating and protecting the longitudinal stem 122 from the electrolytic bath and vapors, preferably adjacent the anode plate. The sleeve is preferably cylindrical and may be removably secured onto a supporting structure, or separator plate, using fasteners or the like. The sleeve is preferably made of a metal oxide of the metal to be produced. For example, an anode for the production of aluminum will comprise a sleeve made of a metal oxide that comprises aluminum oxide, or alumina, or also silicon containing material, such as silicon carbide. Another option would be to use copper for the sleeve.

According to a preferred embodiment, the anodes are sacrificial anodes, preferably comprising carbon or graphite. The anode can be connected to the stem 122 using, for instance, a threaded connection 124 as visible on FIG. 6D for instance where one anode is transparently illustrated to show the connection in the anode body.

According to a preferred embodiment, the purifier assembly also comprises a cathode connection rail 118 configured similarly as the anode connection rail 116, for supporting and electrically connecting the cathodes to each other in a parallel arrangement along the row 113.

According to a preferred embodiment illustrated on FIG. 4, each cathode 114 preferably comprises a cathode plate 141 and a longitudinal stem 142 for electrically connecting each cathode plate to the cathode connection rail 118. The longitudinal stem 142 may be made of any suitable conductive material, preferably steel, and may be threaded for securely and electrically connecting the longitudinal stem to the connection rail using, for example, stainless steal fasteners. The cathode connection rail 118 comprises a plurality of openings 119, each about 2 cm to 10 cm from another opening so as to maintain an anode-to-cathode distance (ACD) of about 1 cm to 5 cm when the anode connection rail 116 and the cathode connection rail 118 are “sandwiched” together in the purifying assembly. As for the anode rails, the center-to-center distance between two cathodes openings of the cathode rail may be twice the ACD plus the thickness of one anode body 121 and one cathode plate 141. The distance between two cathode plate surfaces may be twice the ACD plus the thickness of one anode.

The cathode 114 may further comprise a sleeve 143 for encapsulating and protecting the longitudinal stem 142 from the electrolytic bath in a region of the longitudinal stem adjacent the cathode plate. The sleeve 143 is preferably cylindrical and may be removably secured onto a supporting structure, or separator plate, using fasteners or the like. The sleeve is preferably made of a metal oxide of the metal to be produced in downstream applications. For example, a cathode for the production of aluminium will comprise a sleeve made of metal oxide that comprises aluminum oxide, or alumina. A semi-noble metal, such as copper, and also silicon containing material, such as silicon carbide, have also been found to work well as a sleeve material.

According to a preferred embodiment, the cathode plate 141 comprises steel, preferably stainless steel. When the plate 141 and the stem 142 are made of the same metal or alloy, such as steel, the two can be connected together using continuous welding 144, as visible on FIG. 4. Other material for the cathode can used as long as the material is conductive to electricity and resistant to corrosion in contact with the electrolytic bath and gas. Non-limiting examples are steel, nickel, copper, or carbon.

According to another embodiment shown on FIG. 5, the electrodes of the purifier assembly 100 may be mounted onto a supporting structure 150 for securing the electrodes 112, 114 at a desired position relative to the purification tank 104. The supporting structure may comprises a bridge 152, such as the one illustrated on FIG. 6A, 6B or 7A, 7B, for moving and supporting the electrodes over the tank 104. The array of electrodes 112, 114 may be mounted onto the supporting structure using fasteners and the threaded longitudinal stem. The anodes and cathodes are therefore secured to both the guardrail 116, 118 and the supporting structure 150. The guardrails 116, 118 may also be secured onto the supporting structure 150 to reinforce and stabilize the position of the electrodes 112, 114.

According to another embodiment, the purifier assembly is configured for varying the anode-cathode distance (ACD). As illustrated on FIG. 6D, the supporting element 150 may comprises elongated slots 154 to allow lateral movements of the stems. The central electrode 114c, preferably a cathode, can be fixed as a reference point, whereas the other cathodes and anodes can be laterally movable to adjust the ACD.

As illustrated on FIGS. 2 and 10, the supporting structure 150 of the purifier assembly 100 may also be configured in size to close a top opening 105 of the tank 104 of the purifier 100. The supporting structure 150 may also provide insulation, be resistant to corrosion and prevent gas from escaping the tank. As illustrated on FIG. 2, the tank 104 can be equipped with a gas outlet 107 for safely collecting the anode gas.

As illustrated on FIGS. 9, the supporting structure 150 of the purifier assembly may also comprises several attachment elements 156 for attaching the supporting structure and electrodes and moving them in and out the tank 104.

A method for removing impurities from an electrolytic bath or electrolyte of an electrolytic cell is also disclosed.

Referring first to FIG. 2, a raw non-purified bath 102 of electrolyte is transferred into the purification tank 104, for instance via the inlet 101. The electrodes 112, 114 are plunged in the raw bath 102. Preferably, the cathode plates of the cathodes 114 of the purifier assembly 100 are entirely submerged in the bath 102, limiting as such the corrosion of the top end of the cathode plates that would be above the bath-vapor interface 103 during purification. The power supply 120, such as the direct current (DC) rectifier, is then activated to supply direct current to the anodes and cathodes submerged in the raw bath. The electrical current is then applied for a period of time until the impurities deposit onto the cathode surface or are released with the anode gas (e.g. sulfur dioxide, hydrogen sulfide, carbonyl sulfide, etc.). The purified bath 102 may be then transferred 109 into the electrolysis cell 200 for downstream applications, for example, aluminum production.

According to a preferred embodiment, such as the one illustrated on FIG. 8, the method 1000 for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal comprise the steps of:

• • injecting 1010 at least a portion the electrolytic bath to purify into a purification tank located upstream an electrolytic cell;
  • positioning 1020 into the purification tank at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes, the at least one row of alternating vertically oriented cathodes and anodes being configured in size to be inserted into the purification tank and to provide and maintain an anode-to-cathode distance (ACD) between each of the vertically oriented cathodes or anodes; and
  • applying the electric current 1030 between the anodes and cathodes for a period of time to remove impurities from the electrolytic bath.

According to a preferred embodiment. when the electrolytic bath to purify is a dry electrolytic bath (e.g. solid cryolite), the method 1000 may further comprise the step of:

• • melting the dry bath 1040 before injecting at least the portion the electrolytic bath into the purification tank; or
  • melting the dry bath 1050 directly in the purification tank.

According to a preferred embodiment, the method comprises, before the step of charging a dry bath, the step of mounting the purifier assembly onto a first supporting structure for securing the assembly at a desired position.

The impurities to be removed may comprise, but are not limited to, sulfur, phosphorus, iron, nickel, chromium, copper, and/or gallium. The impurities accumulated on the surfaces of the cathodes are periodically removed therefrom, for instance by scraping. The gas impurities are safely recovered from the tank via the gas outlet 107.

According to a preferred embodiment, the amount of current to be applied will depend on several parameters such as the number of electrodes of the purifier assembly, typically selected in function of the size of the purification tank and the amount of electrolyte to purify. For example, a constant direct current is applied with a total charge of preferably about 0.1 to 10, preferably from about 0.3 to 4.0, Ampere-hours per kilogram of bath to purify. Preferably, the current density is between about 0.004 to about 0.3 A/cm², preferably from 0.1 to about 0.3 A/cm², and the voltage across the purifying assembly may be between about 0.5 V and about 2.5 V.

According to a preferred embodiment, the period of time for removing the impurities may be 1 hour to about 150 hours, more preferably between about 24 to about 96 hours.

According to a preferred embodiment, applying the electric current may comprises measuring an amount of impurities present in the bath before adapting a total charge of current passing between the cathodes and anodes.

According to a preferred embodiment, the method 1000 comprises removing the cathodes after purification 1060 to clean and remove solid impurities therefrom.

According to a preferred embodiment, the method further comprises adding the purified bath to the electrolytic cell prior to start electrolysis.

By the use of multiple electrodes in one or more row, the present invention provides a method of achieving the desired amperage and electrode surface area in a much smaller cross-section. This makes it possible to install the electrodes of the purifier through an insulated lid of a commercial-sized bath melter within a reasonably-sized work port.

The compact nature of the purifier assembly makes this invention feasible for scale-up, and makes easy for the electrical connections of the electrodes and the ability to install through a work port in an insulated lid.

EXAMPLES

Example 1

The experimental purifying unit described below has produced the basic metric for the design base for the pilot unit. The guiding metric used is Ampere Hours/Kilogram Bath. The experimental unit comprised one flat, steel plate cathode and one cylindrical graphite anode. Since there were only one anode and one cathode, the cathode and anode current densities were calculated based only on the area of the side facing the opposite electrode (i.e. 50% the total submerged surface area).

TABLE 1
Experimental Purifying Unit Parameters
Total weight of	658	kg	The submerged	365	cm2
the bath			area of the cathode
	Active area is
	approx. 50% of the
Voltage	1.6	V	sub. area	182	cm2
	Minimum	Average	Maximum
Current	5	A	7 A	10	A
Days of operation	2	3	4
Total charge	0.36	0.77	1.46
(Amp · Hrs/kgBath)	Ah · kg−1	Ah · kg−1	Ah · kg−1
Current density	0.027	0.038	0.055
for the cathodes	A · cm−2	A · cm−1	A · cm−2

Example 2

Transforming the physical design of the experimental unit to a commercial design required a compact design with a smaller footprint. The new design comprises a compact plate (as opposed to round) array of electrodes, to achieve the required surface area.

TABLE 2
Commercial Unit Calculated Parameters
	Total weight of the	16 400	kg
	bath
	Total Ah	12 567	Ah
	(Amp · Hours)
	Minimum	Average	Maximum
	Required Current	131	A	174 A	262 A
	[Related to days
	and current]
	Days of operation	2	3	4
	Total charge	0.38	0.77	1.53
	(Amp · Hrs/kgBath)	Ah · kg−1	Ah · kg−1	Ah · kg−1
	Current density	0.0046	0.0062	0.0092
		A · cm−2	A · cm−2	A · cm−2

To test the commercial design, a prototype bath purifier was constructed, comprising an array of 4 anodes and 3 cathodes, each electrode being rectangular in shape 19 cm tall and 6.35 cm wide, with a 2.54 cm ACD. The anodes comprised a fine-grained graphite, 40 mm thick and the cathodes comprised carbon steel, 10 mm thick. The purifier was immersed in a molten bath containing impurities to a level of about 2.5 cm below the electrode to rod connection. The molten bath is preferably a molten salt composed of sodium fluoride, aluminum fluoride and calcium fluoride and is typically the same that is used in the electrolytic cell. A DC rectifier used to power the purifier was set for 7 A constant current and run for 72 hrs. The rectifier voltage initially started at 1.15 V and reduced to about 0.7 V after 12 hours. Bath samples were taken at 12 hour intervals and analyzed for impurities. The results are given below in Table 3 in parts per million (ppm).

TABLE 3
Concentration of impurities in the bath at 12
hour intervals for 72 hours of purification
Time	Impurities (ppm)
(hours)	P	S	Ca	Mn	Fe	Cu	Ga
0	35.0	3.44	282	2.9	99	6.2	24
12	38.7	2.54	233	3.4	154	7.8	16
24	28.2	5.07	274	4.0	89	13.9	16
36	18.4	3.86	240	4.5	94	4.9	18
48	24.3	1.58	222	4.6	75	10.7	12
60	11.5	<1	265	5.6	62	6.1	15
72	6.4	<9	220	5.5	39	7.0	14

Iron, sulfur, phosphorus and gallium were significantly reduced in the purified bath. The majority of the phosphorus removed from the unpurified bath accumulated on the cathode growth. The cathode growth refers to the deposit accumulation onto the cathode surface during operation. Fe and Ga were also found on the cathode growth. In contrast, the majority of the sulfur was released as sulfur dioxide at the anode since none of the sulfur removed from the unpurified bath was found on the cathode surface. Calcium (Ca) was not electrolyzed at the applied voltage whereas Mn and Cu were not significantly changed. Taken together, this test demonstrates the feasibility of removing multiple impurities from an unpurified bath using the purifier assembly as disclosed herein.

While illustrative and presently preferred embodiments of the invention have been described in detail hereinabove, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.

Claims

1. A purifier assembly for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the purifier assembly comprising: a purification tank located upstream an electrolytic cell and configured to contain at least a portion of the electrolytic bath; andat least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes,wherein the at least one row of vertically oriented cathodes and anodes is configured in size to be inserted into the purification tank; andwherein the purifier assembly is configured to provide and maintain an anode-to-cathode distance (ACD) between each of said vertically oriented cathodes or anodes.

2. The purifier assembly of claim 1, comprising at least two parallel rows of alternating vertically oriented cathodes and anodes, each anode or cathode of one row being adjacent respectively to another anode or cathode of adjacent parallel rows, forming as such an array of alternating columns of vertically oriented cathodes and anodes.

3. The purifier assembly of claim 1, further comprising: at least one anode connection rail configured to be operatively connected to the power supply, each of the anode connection rail being configured to support one row of vertically oriented anodes and to electrically connect the anodes one to each other in a parallel arrangement; andat least one a cathode connection rail configured to be operatively connected to the power supply, each cathode connection rail being configured to support one row of vertically oriented cathodes and to electrically connect the cathodes one to each other in a parallel arrangement.

4. The purifier assembly of claim 3, wherein the at least one anode and cathode connection rails are configured to be independently moved at a desired position relative to the electrolytic bath in the purification tank.

5. The purifier assembly of claim 4, wherein the at least one cathode connection rail is configured to entirely plunge the cathodes below a bath-vapor interface of the electrolytic bath.

6. The purifier assembly of claim 3, wherein each of the anodes or cathodes comprises a longitudinal stem having one end connected to an anode body or a cathode plate, and an opposite end configured to be operatively connected to the anode or cathode connection rails respectively.

7. The purifier assembly of claim 6, wherein each of the anode bodies and cathode plates have a top extremity connected to their respective stems, the top extremities of the cathodes being located below the top extremities of the anodes when the row of electrodes is plunged into the electrolytic bath in order to have the cathode plates entirely plunged into the bath.

8. The purifier assembly of claim 7, wherein the anode body comprises carbon or graphite, the cathode plates comprises steel or stainless steel, and the longitudinal stem comprises steel or stainless steel.

9. The purifier assembly of claim 7, wherein each of the anodes and cathodes further comprises a protective sleeve around the longitudinal stem for protecting the longitudinal stem from corrosion.

10. The purifier assembly of claim 9, wherein the longitudinal sleeve comprises: a metal oxide of the metal to be produced, the metal oxide being aluminum oxide when the metal to be produced is aluminum;a semi-noble metal, such as copper; orsilicon carbide (SiC).

11. The purifier assembly of claim 3, further comprising a supporting structure for securing the at least one row of alternating vertically oriented cathodes and anodes at a position relative to the purification tank.

12. The purifier assembly of claim 11, wherein the connection rails are secured to the supporting structure for reinforcing and stabilizing the position of anodes and cathodes.

13. The purifier assembly of claim 11 or 12, wherein the supporting structure is configured in size to close a top opening of the purification tank and seal the tank.

14. The purifier assembly of claim 13, wherein the supporting structure is configured to provide insulation, to be resistant to corrosion and to prevent gas from escaping the purification tank when the top opening is closed, the purification tank being then equipped with a gas outlet for safely collecting anode gas.

15. The purifier assembly of claim 13, wherein the purification tank belongs to a melter, the melter being used for melting a dry bath of said electrolytic bath.

16. The purifier assembly of claim 1, wherein the at least one row of alternating vertically oriented cathodes and anodes forms a compact array with an ACD ranging from about 1 cm to about 5 cm.

17. The purifier assembly of claim 16, wherein the ACD is about 2.5 cm.

18. The purifier assembly of claim 1, wherein the power supply comprises a DC rectifier.

19. The purifier assembly of claim 1, wherein the metal to produce by the electrolytic cell is aluminum, the electrolytic bath then comprising cryolite and the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof.

20. A method for removing impurities from an electrolytic bath before using the same in an electrolytic cell for the making of a metal, the method comprising the steps of: injecting at least a portion the electrolytic bath to purify into a purification tank located upstream an electrolytic cell;positioning into the purification tank at least one row of alternating vertically oriented cathodes and anodes configured to be operatively connected to a power supply for providing an electric current to the anodes and cathodes, the at least one row of alternating vertically oriented cathodes and anodes being configured in size to be inserted into the purification tank and to provide and maintain an anode-to-cathode distance (ACD) between each of the vertically oriented cathodes or anodes; andapplying the electric current between the anodes and cathodes for a period of time to remove impurities from the electrolytic bath.

21. The method of claim 20, wherein each of the cathodes comprises a cathode plate, the cathodes being positioned in the electrolytic bath so as to be entirely submerged in the electrolytic bath.

22. The method of claim 20, wherein when the electrolytic bath to purify is a dry electrolytic bath, the method further comprises the step of: melting the dry bath before injecting at least the portion the electrolytic bath into the purification tank; ormelting the dry bath directly in the purification tank.

23. The method of claim 20, wherein the electric current is a direct current applied using a DC rectifier.

24. The method of claim 20, wherein applying the electric current comprises measuring an amount of impurities present in the bath before adapting a total electric charge passing through a purifier assembly comprising said purification tank and said at least one row of alternating vertically oriented cathodes and anodes.

25. The method of claim 24, wherein the total charge is about 0.1 to about Ampere-hours (Ah) per kilogram of electrolytic bath to purify.

26. The method of claim 25, wherein the total charge is about 0.3 to about 4.0 Ampere-hours (Ah) per kilogram of electrolytic bath to purify.

27. The method of claim 20, wherein a current density of the cathodes is about 0.004 to about 0.4 A/cm2.

28. The method of claim 27, wherein the current density of the cathodes is about 0.1 to about 0.3 A/cm2.

29. The method of claim 20, wherein the voltage across a purifier assembly comprising the row of alternating vertically oriented cathodes and anodes and the purification tank is between about 0.5 V and about 2.5 V.

30. The method of claim 20, wherein the period of time is between about 1 to about 150 hours.

31. The method of claim 30, wherein the period of time is between about 24 to about 96 hours.

32. The method of claim 20, further comprising increasing current density for either increasing a specific electrical charge (Ah/kg bath), reducing the period of time of purification, or a combination thereof.

33. The method of claim 20, wherein the method further comprises removing the cathodes from the purification tank and removing solid impurities accumulated on the cathodes.

34. The method of claim 20, wherein the metal to produce by the electrolytic cell is aluminum, the bath then comprising cryolite and additives, the impurities to remove comprising sulfur, phosphorus, iron, nickel, chromium, copper, gallium or mixture thereof.

35. The method of claim 34, wherein the additives comprises sodium fluoride, aluminum fluoride, calcium fluoride, and/or aluminum oxide.