Field The present invention relates to a method and apparatus for removing fluoride compounds from a gas stream, in particular from fumes emitted from rare earth metal electrolytic cells. The present invention also relates to a method and system for recycling fluoride compounds back to rare earth metal electrolytic cells. Background

Rare earth metals, in particular the lower atomic weight metals of the lanthanide series (e.g. lanthanum, cerium, neodymium and praseodymium), are commercially produced in an electrolytic cell containing a molten electrolyte comprising a mixture of alkali metal fluorides and rare earth metal fluoride.

The electrolytic cell operates at a temperature about 20-50°C above the freezing point of the electrolyte, at which high temperatures (>1000°C) the rare earth metal is produced in liquid phase. For example, in neodymium production the operating temperature of the electrolytic cell is around 1050°C, while the freezing point of both the metal and the electrolyte is around 1020°C. The operating temperature of the electrolytic cell is determined, in part, by the composition of the electrolyte.

As a consequence of the high operating temperatures and reactions occurring within the electrolyte, the molten electrolyte emits fumes containing fluoride compounds into the head space of the electrolytic cell. The fluoride emissions released from the cell include:

Particulates and vapour species which are emitted by virtue of the species' vapour pressure. The vapour pressure of the particulate vapour species is strongly dependent on the operating temperature of the electrolytic cell. The vapour species may include the alkali metal fluorides (e.g. LiF) or rare earth metal fluorides (e.g. NdF ₃ ), or more volatile ionic species formed within the electrolyte such as Li(NdF ₄ ).

Hydrogen fluoride gas (HF) formed by the reaction of fluoride species in the electrolyte or its vapours in contact with water. Contact with water may arise from the air humidity, or from moisture introduced to the electrolytic cell along with the metal oxide raw material feed.

Gaseous and particulate fluoride emissions are regulated by operating licence to levels well below the primary emission level from the process. These regulations are primarily intended to protect the environment (particularly flora) from the well known harmful effects of excessive fluoride exposure.

Air-borne fluoride emissions (fumes, vapours and dusts) are also known to cause respiratory disease in susceptible smelter employees where exposure is excessive.

Furthermore, the fluoride emissions must be compensated for by make-up additions to the electrolytic cell to maintain the electrolyte composition and volume in the electrolytic cell. This provides a strong economic incentive to capture and return the emissions to the process without generation of waste streams. For example, make-up addition of neodymium fluoride to compensate for fluoride emissions is known to represent around 10% by weight of the feed rate of neodymium oxide. This represents a substantial cost burden to the rare earth metal production process. In some rare earth metals electrolytic facilities, it is known to employ wet scrubbing whereby the gases collected from the head space of the electrolytic cells are passed through or over caustic lime solution. The fluorides are collected as insoluble calcium fluoride but these compounds are not suitable for recycling back to the electrolytic cell. Composite metal oxide adsorbents for fluoride removal from water, including drinking water and waste water, are known. For example, US Patent No. 7,786,038 describes a mixed metal oxide adsorbent comprising largely aluminium oxide and/or magnesium oxide with minor parts of transition metal and rare earth metal oxides. Said adsorbent is provided with a series of metal hydroxide and polyhydroxy metal oxide hydrates. The presence of the surface hydroxy groups as functional groups makes the adsorbent have a strong affinity for aqueous fluoride ions by means of chemical bond chemical adsorption, electrostatic adsorption and ion exchange adsorption processes. Quite different physical and chemical mechanisms are demanded of an adsorbent to remove molecular fluoride compounds and particulates from the gas phase.

Similarly, the mixed metal oxide adsorbent described in US7,786,038 would be entirely unsuitable to use as an adsorbent for fluorides which, once spent, could be recycled back to the electrolytic cell. The presence of transition metals and aluminium and/or magnesium in the spent adsorbent would contaminate the rare earth metal fluoride electrolyte in the cell. Fluoride emissions are also generated by other industrial processes and environmental concerns about venting fluoride-containing emissions to the atmosphere similarly apply.

Therefore there is a need for alternative or improved methods and systems for removing fluoride compounds from industrial emissions and for recycling the fluoride compounds back to said electrolytic cells.

Summary According to a first aspect, there is provided a method of removing fluoride compounds entrained in a gas stream, the method comprising contacting the gas stream with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the gas stream. In one embodiment, the method may further comprise the step of recovering the fluoride compounds by separating the adsorbent charged with fluoride compounds from the gas stream.

According to a second aspect, there is provided an apparatus for removing fluoride compounds entrained in a gas stream, the apparatus comprising:

- an adsorption zone configured for contacting the gas stream with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the gas stream;

- the adsorption zone having at least one inlet to receive the gas stream and the adsorbent in the adsorption zone, and an outlet for egress of the adsorbent charged with fluoride compounds and the fluoride compound-depleted gas.

In one embodiment, the apparatus may further comprise a separator for separating the adsorbent charged with fluoride compounds from the fluoride compound-depleted gas.

According to a third aspect, there is provided a method of recycling fluoride

compounds emitted from an electrolytic cell, the method comprising: - contacting the emissions with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the emissions;

- recovering the adsorbent charged with fluoride compounds by separating said charged adsorbent from the fluoride compound-depleted emissions; and,

- feeding the separated charged adsorbent to the electrolytic cell.

According to a fourth aspect, there is provided a system for recycling fluoride compounds arising from emissions of an electrolytic cell, the system comprising:

- an apparatus comprising an adsorption zone configured for contacting the emissions with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the emissions;

- the adsorption zone having at least one inlet to receive the emissions and the adsorbent in the adsorption zone, and an outlet for egress of the adsorbent charged with fluoride compounds and the fluoride compound-depleted emissions;

- a separator for separating said charged adsorbent from said depleted emissions; and,

- a delivery means to feed the separated charged adsorbent to the electrolytic cell. According to a fifth aspect, there is provided an adsorbent for fluoride compounds comprising one or more rare earth metal oxides.

According to a further aspect, there is provided use of one or more rare earth metal oxides as a fluoride adsorbent.

Brief Description of the Drawings

Notwithstanding any other forms which may fall within the scope of the system and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of an apparatus for recovering fluoride compounds entrained in a gas stream, the apparatus having been adapted for use in a system to recycle recovered fluoride compounds to an electrolytic cell for production of rare earth metals. Detailed Description

In one aspect, the present application relates to a method of removing fluoride compounds entrained in a gas stream.

Fluoride compounds

The term 'fluoride compound' is used broadly to refer to any molecular or ionic chemical compound containing fluoride (F ^" ). The fluoride compounds may be gas phase or ultra-fine particulates, for example resulting from sublimation and subsequent condensation of molten fluoride salts. Examples of 'fluoride compounds' include, but are not limited to, hydrogen fluoride (HF), metal fluoride salts including alkali metal fluorides such as LiF, KF, alkaline earth metal fluorides such as CaF ₂ , BaF ₂ , transition metal fluorides such as AIF ₃ and GaF ₃ , rare earth metal fluorides (MF ₃ ), and mixed metal fluoride species such as Li(MF ₄ ), where M is a rare earth metal. It will be appreciated that a reference to a fluoride compound may also encompass a mixed halide compound containing fluoride and one or more of another halide, such as chloride, bromide or iodide.

The fluoride compounds are generated at temperatures sufficiently high (> 1000 °C) by virtue of the gas partial pressure of (relatively) volatile fluoride compounds and/or sublimation of fluorides compounds capable of subliming at high temperatures.

The fluoride compounds may be emitted as fumes from industrial processes, including electrolytic cells charged with fluoride-containing electrolytes and welding processes where fluorides are used as electrode coatings and flux materials for welding low- and high-alloy steels, or as exhaust gases from industrial processes such as glass-making, enamel and steel production.

In particular, fluoride compounds as defined above, may be emitted by rare earth metal electrolytic cells. Rare earth metal electrolytic cells comprise an electrolytic bath containing a molten mixed alkali metal fluoride and rare earth metal fluoride electrolyte with corresponding cathodes and anodes for the purposes of producing rare earth metals. As described previously, the operating temperatures of rare earth metal electrolytic cells generate fumes containing fluoride compounds.

The reference to 'entrained in a gas stream' refers to gaseous or particulate fluoride compounds being incorporated into and carried along by the flow of a gas stream. The gas stream may comprise a flow of gas emissions from an industrial process which inherently entrains the fluoride compounds produced as part of the industrial process generating such emissions. Alternatively, the gas stream may be a positive pressure flow of gas which is applied to the fluoride compound emissions (e.g. the fumes residing in a head space above an electrolytic cell) to mix with and entrain the fluoride compounds and direct them away from the source of the emissions (e.g. the electrolytic cell). The positive pressure may be applied by techniques well known to those skilled in the art, such as with a pump. In another variation, a negative pressure may be applied to the fluoride compound emissions in a manner to generate a gas stream with entrained fluoride compounds therein and thereby draw the fluoride compounds away from the source of the emissions. The negative pressure may be applied by techniques well known to those skilled in the art, such as with a suction pump. It will be appreciated that the gas stream may additionally have entrained therein other gaseous and/or particulate non-fluoride compounds that are generated along with or separate to the fluoride compounds

Removing fluoride compounds

The method of removing fluoride compounds entrained in a gas stream comprises contacting the gas stream with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the gas stream.

Adsorbent

The adsorbent comprises one or more rare earth metal oxides. The term 'rare earth metal oxide' broadly refers to any oxide or any hydrated precursors of such oxides of a rare earth metal, including rare earth metal hydroxides. Rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium. Scandium and yttrium are considered rare earth metals since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. The lanthanides include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

The one or more rare earth metal oxides may be selected from a group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbuium oxide, dysprosium oxide, holmium oxide, erbirum oxide, thulium oxide, ytterbium oxide, and lutetium oxide. In one embodiment, the adsorbent may be scandium oxide. In another embodiment the adsorbent may be yttrium oxide. In another embodiment the adsorbent may be lanthanum oxide. In another embodiment the adsorbent may be cerium oxide. In another embodiment the adsorbent may be praseodymium oxide. In another embodiment the adsorbent may be neodymium oxide. In another embodiment the adsorbent may be promethium oxide. In another embodiment the adsorbent may be samarium oxide. In another embodiment the adsorbent may be europium oxide. In another embodiment the adsorbent may be gadolinium oxide. In another embodiment the adsorbent may be terbium oxide. In another embodiment the adsorbent may be dysprosium oxide. In another embodiment the adsorbent may be holmium oxide. In another embodiment the adsorbent may be erbium oxide. In another embodiment the adsorbent may be thulium oxide. In another embodiment the adsorbent may be ytterbium oxide. In another embodiment the adsorbent may be lutetium oxide.

In another embodiment, the adsorbent may be scandium oxide and one or more other rare earth metal oxides selected from the group comprising yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be yttrium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be lanthanum oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be cerium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be praseodymium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be neodymium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide. In another embodiment, the adsorbent may be promethium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be samarium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be europium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be gadolinium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide. In another embodiment, the adsorbent may be terbium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be dysprosium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, holmium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be holmium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, erbium oxide, thulium oxide, ytterbium oxide, or lutetium oxide. In another embodiment, the adsorbent may be erbium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, thulium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be thulium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, ytterbium oxide, or lutetium oxide.

In another embodiment, the adsorbent may be ytterbium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, or lutetium oxide. In another embodiment, the adsorbent may be lutetium oxide and one or more other rare earth metal oxides selected from the group comprising scandium oxide, yttrium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, promethium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, or ytterbium oxide.

In one embodiment, the adsorbent may comprise at least 85 mole percent of one or more rare earth metal oxides. In another embodiment, the adsorbent may comprise at least 90 mole percent of one or more rare earth metal oxides. In a further

embodiment, the adsorbent may comprise at least 95 mole percent of one or more rare earth metal oxides. In a still further embodiment, the adsorbent may consist of one or more rare earth metal oxides. In another embodiment, the adsorbent may consist essentially of one or more rare earth metal oxides. It will be appreciated that the adsorbent may have minor amounts of impurities.

In certain embodiments wherein the adsorbent is employed to remove fluoride compounds emitted by electrolytic cells used in the production of one or more rare earth metals, the adsorbent may comprise one or more rare earth metal oxides of the one or more rare earth metals produced by the electrolytic cell. In embodiments where two or more rare earth metals are produced by the electrolytic cell, it will be

appreciated that the relative mole percentages of the two or more rare earth metal oxides comprising the adsorbent may substantially correspond to the relative mole percentages of the two or more rare earth metals produced by the electrolytic cell. The adsorbent may be a particulate material. The adsorbent may be in the form of a powder. Alternatively, the adsorbent may be in the form of ultrafine particles. The adsorbent may have a d ₅₀ of about 45 micron to about 150 micron.

The adsorbent may be prepared by calcining one or more rare earth metal carbonates or phosphates. Temperatures and periods at which these materials are calcined may be selected to obtain a desired degree of porosity, surface area, adsorptive capacity, and concentration of residual bound hydroxyl groups on the available surface area of the rare earth metal oxides.

It will be appreciated that adsorption of the fluoride compounds on the adsorbent may be by physical adsorption or by chemisorption processes. The term 'adsorb' also refers to any chemical reactions between the adsorbed fluoride species and the surface of the adsorbent which may proceed subsequent to physical adsorption or chemisorption processes. For example, the fluoride compounds may react with the rare earth metal oxide or its hydrated precursors to form a hydroxy fluoride phase on the surface of the adsorbent.

Contacting the gas stream with the adsorbent

Contacting the gas stream with the adsorbent may comprise passing the gas stream through the adsorbent. The adsorbent may be suspended or fluidised in an adsorption zone through which the gas stream is caused to flow. The gas stream may flow upwardly or horizontally from one end of the adsorption zone to an opposing end thereof, depending on the spatial orientation of the adsorption zone. The adsorbent may be homogenously dispersed in the adsorption zone. The adsorbent may be prevented from moving under gravity beyond a predetermined threshold. The flow rate of the gas stream in the adsorption zone may be selected to ensure a desired degree of gas mass transfer to achieve effective gas scrubbing.

The residence time of the gas stream in the adsorption zone may be selected to ensure a desired degree of gas mass transfer to achieve effective gas scrubbing.

The concentration of the adsorbent in the adsorption zone may be selected to ensure a desired degree of gas mass transfer to achieve effective gas scrubbing.

The residence time of the adsorbent in the adsorption zone may be selected to ensure a desired degree of gas mass transfer to achieve effective gas scrubbing.

The voidage of the adsorption zone may be selected to ensure a desired degree of gas mass transfer to achieve effective gas scrubbing. Separating the adsorbent

When the fluoride compound content of the adsorbent is compatible with its adsorptive capacity, the adsorbent charged with fluoride compounds ('spent adsorbent') may be separated from the resulting fluoride-depleted gas stream in a separator. It will be appreciated that the adsorbent charged with fluoride compounds may comprise a suitable feed or be suitable for supplementing the feed of an electrolytic cell for the production of rare earth metals. This would reduce the need for make-up addition of rare earth metal fluorides into the electrolytic cell to compensate for loss of rare earth metal fluorides and fluorides in fluoride emissions, thereby reducing the substantial cost burden placed on the electrolytic production of rare earth metals currently presented by conventional systems and methods for management of fluoride emissions in electrolytic rare earth metal production facilities.

Accordingly, the method of the present invention may be adapted to recycle fluoride compounds arising from emissions of an electrolytic cell. Recycling fluoride compounds

The method of recycling fluoride compounds emitted from an electrolytic cell comprises the steps of:

contacting the emissions with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the emissions;

recovering the adsorbent charged with fluoride compounds by separating said charged adsorbent from the fluoride compound-depleted emissions; and,

feeding the separated charged adsorbent to the electrolytic cell.

The emissions of the electrolytic cell containing the fluoride compounds may be entrained in a gas stream prior to contacting with said adsorbent and separating the charged adsorbent, as has been described previously.

Feeding the separated charged adsorbent to the electrolytic cell

The separated charged adsorbent may be collected from the separator and fed to the electrolytic cell, optionally via a feed hopper, by conventional techniques as will be understood by a person skilled in the art.

The separated charged adsorbent may be fed to the electrolytic cell separately from the conventional feed material. Alternatively, the separated charged adsorbent may be blended in a desired concentration with the conventional feed material prior to feeding the electrolytic cell.

Apparatus for removing fluoride compounds

The apparatus for removing fluoride compounds entrained in a gas stream comprises: an adsorption zone configured for contacting the gas stream with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the gas stream; the adsorption zone having at least one inlet to receive the gas stream and the adsorbent in the adsorption zone, and an outlet for egress of the adsorbent charged with fluoride compounds and the fluoride compound-depleted gas. The apparatus may further comprise a separator for separating the adsorbent charged with fluoride compounds from the fluoride compound-depleted gas.

It will be appreciated that a flow path of the gas stream in which the fluoride

compounds are entrained will be configured to convey the fluoride compounds to the adsorption zone.

Adsorption zone

The term "adsorption zone" refers generally to a zone of an apparatus in which an adsorption reaction occurs. This zone may comprise a column, duct or portion thereof, a structure or a vessel to contain the adsorbent as described previously for passing the gas stream through the adsorbent.

The adsorption zone may be configured to have a volume, length and orientation to contain a predetermined volume of adsorbent. The adsorption zone may be configured to have a volume, length and orientation to provide a sufficient residence time therein for both the gas stream and the adsorbent so that the fluoride compounds may be adsorbed onto the adsorbent in the adsorption zone.

Separator

The separator may be any separator suitable for separating the adsorbent from the gas stream, as will be understood by the person skilled in the art. Examples of suitable separators include, but are not limited to, cyclones, bag filter arrangements, filter-cloth separators, gravity separators, and so forth. The system for recovering fluoride compounds may be adapted for recycling recovered fluoride compounds to the electrolytic cell from which they may have been originally generated as fumes.

System for recycling fluoride compounds

The system for recycling fluoride compounds emitted from an electrolytic cell comprises:

- an apparatus comprising an adsorption zone configured for contacting the emissions with an adsorbent comprising one or more rare earth metal oxides to adsorb and remove the fluoride compounds from the emissions;

- the adsorption zone having at least one inlet to receive the emissions and the adsorbent in the adsorption zone, and an outlet for egress of the adsorbent charged with fluoride compounds and the fluoride compound-depleted emissions;

- a separator for separating said charged adsorbent from said depleted emissions; and,

- a means to feed the separated charged adsorbent to the electrolytic cell. The system may further comprise means to convey the emissions to the adsorption zone. Said means may be in the form of a gas stream.

The gas stream may comprise a positive pressure gas flow configured in fluid communication with a head space of the electrolytic cell in a manner to mix with and carry the fluoride compounds emitted in the fumes from the electrolytic cell.

In an alternative embodiment, the gas stream may be generated by a negative pressure configured in fluid communication with and applied to the head space of the electrolytic cell to generate a flow of gas which incorporates and carries the fluoride compounds.

It will be appreciated that a flow path of the gas stream may be configured to convey the fluoride compounds from the head space of the electrolytic cell to the adsorption zone.

The gas stream is subsequently contacted with the adsorbent in the adsorption zone and separated from the adsorption zone by known separators when the adsorbent has achieved a predetermined absorptive capacity. The separated spent adsorbent is charged with adsorbed fluoride compounds and subsequently used as a feed material (or make-up feed) in the electrolytic cell.

Referring to Figure 1 , one embodiment of the apparatus 10 for removing fluoride compounds entrained in a gas stream is now described. The apparatus 10 has been adapted for recycling recovered fluoride compounds to an electrolytic cell (not shown). The apparatus 10 includes an adsorption zone 12 in the form of an adsorption column 14. The adsorption column 14 is provided with a first inlet 16 at a lower end 18 thereof to receive a gas stream with entrained fluoride compounds. The first inlet 16 is in fluid communication with a head space above an electrolytic cell (not shown) which may be as previously described. In this embodiment, the adsorption column 14 is vertically oriented and is configured for the gas stream to flow upwardly through the column 14 to an outlet 20 disposed at an opposing upper end 22 of the column 14.

The column 14 is also provided with a second inlet 24 disposed proximal to the first inlet 16 to receive an adsorbent. The adsorbent may be delivered into the column 14 as a free flowing stream of particulate material or in a fluidized stream. In the latter state, the gas stream flowing through the column 14 may suspend or fluidize the adsorbent entering the column 14. The adsorbent and the gas stream are caused to mix and contact in the column 14 for a predetermined residence time in the column 14 to ensure effective gas mass transfer, achieve a desired absorptive capacity, thereby producing a scrubbed gas (from which the fluoride compounds have been removed) and an adsorbent charged with fluoride compounds ('spent adsorbent').

The apparatus 10 further includes a separator 26, in the form of a 'baghouse' filter, in fluid communication with the outlet 22 of the adsorption column 14. The separator 26 receives a mixture of scrubbed gas and spent adsorbent and separates the scrubbed gas and the spent adsorbent. It will be appreciated by persons skilled in the art that the separator 26 may take one of several suitable forms.

The separator 26 includes an outlet 28 for scrubbed gas. The scrubbed gas may be vented to atmosphere or directed to other treatment processes for removal of further pollutants.

In this particular embodiment, the separator 26 is configured for spent adsorbent to collect under gravity in a lower portion 30 of the separator 26. An outlet 32 for the spent adsorbent is disposed in the lower portion 30 of the separator. The outlet 32 may be in fluid communication with a feed hopper (not shown) via conduit 34. The feed hopper is configured to feed spent adsorbent to the electrolytic cell. The outlet 32 may additionally, or alternatively, be in fluid communication via conduit 36 with a hopper or an adsorbent regenerator for storage and/or regeneration of the spent adsorbent.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.