GB2086941A - Recovery of materials from low concentrations - Google Patents

Abstract

A process for separating recoverable metals from a material in which the recoverable metals are in a low concentration includes repeated contacting of a molten slag containing the recoverable metal with molten collector metals. The slag is maintained in the molten state between the repeated contactings so as to avoid the necessity for re- melting the slag. The process may also include separating the slag from the enriched collector metal at sites adjacent to or at the interface between the molten slag and collector metal, to promote intimate contact between the slag and collector metal at the removal sites. Apparatus for carrying out the process preferably includes a pair of electric furnaces 10 and 12 between which molten slag 22, 22' and collector metals 20, 20' are transferred by slag conduit 32 and transfer means 24, the furnaces preferably being plasma gun furnaces. <IMAGE>

Description

SPECIFICATION Multiple pass smelting process The present invention relates to a process for recovering recoverable metals from a material, which may be an ore or a recycled material, in which the recoverable metal is dispersed, usually in very low concentration. The process may be generally described as a smelting operation in which the material containing the recoverable metal is melted and recoverable metal is dissolved into a molten collector metal. The recoverable metal is thereafter won from the collector metal by conventional metal refining techniques.

In addition to ores, an increasingly significant source of valuable recoverable metals resides in materials such as refractory oxides containing platinum group metals, for example, spent catalyst, electronic scrap, catalyst manufacturing scrap and rejects and, in general, any materials in which metals, particularly precious metals such as platinum group metals, gold and silver, are dispersed in a base or carrier material in highly dilute form. A large and growing scrap material of this type is spent auto exhaust catalyst and spent chemical and petroleum processing catalyst, all of which essentially comprises a refractory oxide carrier such as alumina, cordierite, mullite or the like on which one or more precious metals are dispersed.Typically, these catalysts contain one or more platinum group metals dispersed in highly dilute concentration relative to the mass of the supporting carrier. The carrier may be a monolithic honeycomb-type body, typically made of cordierite (an alumina-silica-magnesia material) or may comprise individual beads typically made of alumina. The cordierite monolith usually has a surface coating predominantly of gamma alumina to provide a high surface area film on which the catalytic material, usually platinum plus one or more other platinum group metals, is dispersed.

The catalyst may contain other components such as a base metal catalytic material, e.g., nickel or nickel oxide, and rare earth oxides to help stabilize the high surface area alumina. For example, see U.S. Patents 3,565,830 and 4,157,316, for catalyst compositions typical of the type being described. The total content of precious metal, in this case platinum group metals, present in some compositions is very small, often being on the order of 0.02 to 1% by weight of the total weight of the catalyst. Similarly low concentrations of precious metals are found in ores from which the metal is to be won.

In recovering metals such as platinum group metals from a solid material containing the metal in dilute form, it is conventional practice to melt the solid material with a flux component and contact the molten slag containing the recoverable metal with a collector metal into which the recoverable metal (e.g., one or more platinum group metals, gold, silver, etc.) dissolves or otherwise migrates. For example, when a molten slag containing cordierite and/or alumina and platinum group metals, such as is obtained from a catalyst, and a suitable flux material is contacted with molten iron or copper as the collector metal, the platinum group metals migrate, whether by dissolving or some other mechanism, from the slag, into the collector metal. For convenience, the term "dissolve" will be used to describe this phenomenon.

The recoverable metal is separated from the collector metal in a separate, conventional process. In order to enhance recovery, it is often necessary to run the treated slag a second time to recover residual recoverable metals not recovered in the first pass. In conventional practice, this is done by cooling and pulverizing the slag discharged from the process, separating any metal prills from it, if desired, and charging the crushed slag to a furnace for a second independent melting and smelting operation. (Prills are small occlusions of collector metal trapped in the slag.) In addition to the steps of cooling and pulverizing the solidified slag, separating prills, and handling the material, the conventional practice requires energy input to re-melt the slag.

Generally, the multiple pass concept, i.e., one or more repeated smeltings of the material discharged from the primary smelting, is applicable to any furnace melting or smelting operation, regardless of the heat source, but has been found to be particularly applicable to a process for secondary recovery of platinum group metals dispersed on a support member in dilute form, i.e., in small quantities relative to the mass of the carrier. As used herein, "platinum group metals" means platinum, rhodium, ruthenium, palladium, osmium and iridium.For example, it is particularly applicable to, but not limited to, recovery of platinum group metals from spent catalyst having a platinum group metal dispersed on monolithic or bead supports, such as auto exhaust gas catalysts, platinum on alumina petroleum reforming catalysts, rhodium on alumina and ruthenium on alumina catalysts used for chemical processing, intermediate refined platinum group metals, catalysts, etc. The invention may be accomplished by the utilization of furnaces heated by plasma guns or other suitable heat source, as described below. U.S.

Patents 3,394,242, 3,783,167, 4,037,043 and 4,006,284, for example, show typical plasma gun furnaces.

The present invention provides an improvement in a process for separating a recoverable metal from a starting material in which the recoverable metal is dispersed. The process comprises melting the starting material, melting a collector metal, contacting the starting material and collector metal in the molten state to enrich the collector metal with recoverable metal, and separating the resulting residual material and resulting enriched collector metal from each other.The improvement comprises maintaining the residual material in the molten state after the separation of the residual material and enriched collector metal and at least until the following contacting step which comprises contacting the residual material in the molten state with a molten fresh collector metal to enrich the fresh collector metal with recoverable metal, and separating from each other the resulting residual material and resulting enriched fresh collector metal obtained from the second contacting step.

The process of the invention may also include maintaining the residual material resulting from the fresh collector metal contacting step in the molten state after separation of it from the enriched fresh collector metal, and repeating on it one or more times the above-described contacting and separating operations with additional fresh collector metal. As used herein, "fresh" collector metal means collector metal whose concentrations of recoverable metal is lower than that of enriched collector metal. Preferably, fresh collector metal is substantially free of recoverable metal prior to being enriched by contact with the molten material.

The molten starting material and molten collector metal are substantially immiscible in the molten state whereby an interface is formed between them and, in one apsect, the residual material is withdrawn in the molten state at a plurality of sites disposed at or adjacent the interface whereby the molten residual material being separated from the molten collector metal is constrained to flow in intimate contact with the collector metal at the plurality of sites.

The process of the invention may further include combining the enriched collector metal with the enriched fresh collector metal to obtain a product collector metal, and the starting material may comprise a molten slag of selected viscosity obtained by combining the starting material with a flux material in proportions selected to impart the selected viscosity to the slag. The material may prefereably comprise a catalyst, such as a spent catalyst, comprising a carrier material having one or more catalytic metals dispersed thereon, the catalytic metals including at least one platinum group metal. The carrier material may be a refractory metal oxide carrier material such as alumina, cordierite, and mixtures thereof, the flux material may preferably be lime and the collector metal preferably iron.

In one preferred aspect, the invention provides a process for separating a recoverable metal from a starting material in which the recoverable metal is dispersed, the process comprising the following steps. Melting the starting material and a collector metal in a first furnace and contacting therein the resulting molten starting material and collector metal whereby to enrich the collector metal with the recoverable metal. Transferring molten residual material resulting from the abovedescribed contacting step from the first furnace to a second furnace and introducing fresh collector metal into the second furnace, melting and contacting it herein with the molten residual material whereby to enrich the fresh collector metal with recoverable metal, and withdrawing enriched collector metal from the first furnace and withdrawing enriched fresh collector metal from the second furnace.

A suitable apparatus for carrying out the process of the invention is described and claimed in our co-pending British Patent Appiication No.

8132818, and comprises the following.

A first furnace has a heat source and a refractory crucible, means for admitting a feed and a collector metal therein, and means for withdrawing molten collector metal therefrom. A second furnace having a heat source, a refractory crucible, means for introducing a fresh collector metal therein and a spent slag discharge means for withdrawing molten slag from the second furnace. A slag transfer means is provided for transferring molten slag from the first furnace to the second furnace, the slag transfer means including means to maintain the slag in the molten state during such transfer. The means to maintain the slag in the molten state may comprise thermal insulation means to help maintain it at an elevated temperature during transfer and/or heating means to heat the slag during transfer.The slag transfer means may be a conduit or a tip ladle or the like may be utilized to transfer the slag. A collector metal transfer means for transferring collector metal from the second furnace to the first furnace is also provided. The collector metal may be transferred back to the first furnace in the molten or cooied, solid state. If molten, means similar to those employed to transfer molten slag may be employed.

In a preferred embodiment, the apparatus may include slag transfer means including a plurality of conduit entry ends disposed at or adjacent to the interface between molten slag and molten collector metal contained within the furnace, whereby molten slag entering the entry ends is constrained to flow in intimate contact with molten collector metal at each of the plurality of entry ends.

In another preferred embodiment, the apparatus comprises a furnace having a heat source, a refractory crucible having a floor portion and an upper portion, means for introducing the feed and the collector metal into the crucible, means for withdrawing molten collector metal from the floor portion of said crucible to exteriorly of the furnace, and means for withdrawing molten slag from the crucible to exteriorly of the furnace.

The improvement comprises that the means for withdrawing molten slag comprises a plurality of slag conduits each having an entry end disposed at or adjacent the juncture of the upper and floor portions of the crucible, the juncture being defined by the interface between the charge of molten slag and the charge of molten collector metal, whereby molten slag flowing into the entry ends of the slag conduits is constrained to flow in intimate contact with the molten collector metal at the plurality of sites.

The collector metal may partially enter the entry ends and/or be disturbed in the vicinity of the entry ends, so that reference to the slag flowing in intimate contact with the molten metal includes flowing of the slag around, through and/or over disturbed or displaced portions of the molten collector metal. The heat source is preferably a plasma gun.

In accordance with another aspect of the invention, there is provided a process for separating a recoverable metal from a feed in which the recoverable metal is dispersed, the process comprising melting the feed in a furnace to form a charge of molten slag, melting a collector metal in the furnace to form a charge of molten collector metal of greater density than and immiscible with the molten slag, contacting the molten slag with the molten collector metal in a crucible of the furnace, the crucible having a floor portion within which the charge of molten collector metal accumulates and an upper portion within which the charge of molten slag accumulates, the charge of molten slag being in contact with and supported by the charge of molten collector metal whereby the collector metal is enriched by migration of recoverable metal from the slag to the collector metal, the improvement comprising withdrawing molten slag from a plurality of sites located at or adjacent to the interface between the molten slag and said collector metal whereby molten slag being removed from said furnace is constrained to flow in intimate contact with the molten collector metal at the plurality of sites.

The invention in general may be said to provide, in one aspect, for maintaining the treated slag from a primary furnace in the molten state and contacting it while still molten with a fresh charge of collector metal. The treated slag is accordingly treated second time, by contacting it with a fresh collector metal which, being lower in concentration of recoverable metal than is the primary collector metal, is better able to extract the relativeiy low residual values of recoverable metal in the once-treated slag.The twice treated slag may be treated one or more additional times in a similar manner The multiple contacting of the molten slag may be carried out in a single furnace, as by withdrawing enriched collector metal from the furnace and admitting a fresh charge of collector metal in the furnace while the molten slag is maintained within the furnace.

Alternatively, the molten slag may be withdrawn and held in the molten state in a separate holding vessel or compartment while the fresh collector metal charge is introduced. Alternatively, two furnaces (or dual crucibles or compartments within a single furnace enclosure) may be employed generally in the manner as described above.

The process may be operated substantially continuously or in a batch-wise manner and preferably product collector metal is withdrawn in a batch-wise manner.

The starting material may be a spent catalyst or production scrap from catalyst manufacture or virgin ore or ore concentrates and the like. That is, the process is not limited to secondary recovery from scrap or spent materials.

In general, another aspect of the invention, which could also be utilized in a single or otherwise conventional furnace operation, is the provision as described above of multiple slag withdrawal conduits having their entry ends at or adjacent the slag-collector metal interface. For the withdrawal of a given amount of slag, the use of a plurality of withdrawal sites at or adjacent the interface will increase the quantity of molten collector metal which is subject to intimate, flowing contact with the molten slag. This results in enhanced dissolving or migration of recoverable metal from the slag into the collector metal, particularly since the surface of the collector metal may be somewhat displaced or disturbed in the vicinity of the flowing exiting slag at the withdrawal sites, further enhancing the contact.

The invention is further described with reference to the accompanying drawings, in which: Fig. 1 is a schematic view in elevation of a preferred embodiment of an apparatus for carrying out the invention; Fig. 2 is a schematic partial view in elevation of a portion of another preferred embodiment of the apparatus for carrying out the invention.

Referring now to Fig. 1 , there is shown a pair of refractory furnaces comprising a primary furnace 10 and a secondary furnace 12. In the illustrated specific embodiment, both furnaces are of generally similar construction although obviously, they need not necessarily be. Each furnace has a heat source, comprising a plasma gun 14 mounted in the top cover 1 6 of primary furnace 10 and a plasma gun 14' mounted in the top cover 16' of secondary furnace 12. Any suitable heat source, such as an electric arc, could be employed. As is known in the art, plasma guns operate by passing an inert gas such as argon through an electric arc to form a high temperature plasma, and usually employ an anode spaced from the plasma gun. In furnaces 10 and 12, the anodes would normally be located in the bottom of the furnace enciosure.The anode and other associated equipment for the plasma gun, being well known in the art, are not illustrated or further described. The body of primary furnace 10 comprises a furnace enclosure 18 (18' in the case of secondary furnace 12) which is shown schematically in the drawings. As is known in the art, such furnace enclosures normally comprise an outer shell, usually made of a high mechanical strength, highly heat resistant material, which outer shell is lined with refractory material to define in effect a crucible to receive the molten material. Such details of construction, being well known in the art, are not shown in or further described with respect to Fig. 1.As illustrated, the interior of furnace enclosure 1 8 in effect describes a crucible having a floor portion 21 A in which a molten collector metal 20 accumulates, and an upper portion 21 in which a molten slag material 22 accumulates. Secondary furnace 1 2 similarly has a crucible defined by the interior of its enclosure 18', the crucible having an upper portion 21' in which its molten slag material 22' accumulates and a floor portion 21 A' in which its molten collector metal 20' accumulates. Molten collector metal 20' may be referred to as secondary collector metal.

A collector metal transfer means 24 is shown schematically and serves to transport secondary collector metal 20' into the pool of collector metal accumulated in floor portion 21 A of furnace 10.

Secondary collector metal may have been cooled and transported into furnace 10 in solid form for re-melting therein. It may be introduced with the feed or included in the collector metal charge upon start-up of furnace 10. Alternatively, it may be transported while still in the molten state by any suitable tip-ladle or conduit means. Primary furnace 10 has means for withdrawing enriched molten collector metal therefrom, the means comprising product collector metal discharge 30.

The withdrawal of product collector metal from discharge 30 (and from corresponding discharge 30' of secondary furnace 12) may be accomplished by any suitable means. Thus, the molten metal may be discharged into a tip ladle or other means to transport it for pouring into a mold or other chilling device, or for recycling it to primary furnace 10. Alternatively, a conduit may be employed for such withdrawal or recycling. If it is desired to recycle collector metal in molten form, the tip-ladle, conduit or other means will have suitable insulating and/or heating means to avoid solidification of the molten metal.

Secondary furnace 12 also has a collector metal inlet 26' through which fresh collector metal is introduced as indicated by the arrow 28'.

Enriched collector metal discharge 30' of secondary furnace 12 provides the connection to secondary collector metal conduit 24.

A slag conduit 32 connects primary furnace 10 in liquid flow communication with secondary furnace 12 and has an entry end 32A disposed within the first or primary furnace 10 and a discharge end 32B disposed within the second or secondary furnace 12. It will be noted that entry end 32A of slag conduit 32 is disposed at or adjacent the interface between molten slag material 22 and molten collector metal 20. Entry end 32 is preferably disposed slightly below the interface so that there is a limited entry of molten slag into conduit 32. Slag conduit 32 is heated by any suitable means to maintain a temperature therein which is sufficiently high to prevent solidification of slag therein.

A second slag conduit 33 is also provided, and for clarity of illustration only a portion of it adjacent its entry end (un-numbered) is shown in Fig. 1. Like slag conduit 32, slag conduit 33 has its entry end disposed at or immediately adjacent the interface between molten slag 22 and molten collector metal 20. Slag conduit 33 may be a separate conduit leading to secondary furnace 12, or it may merge through suitable header means or otherwise, with slag conduit 32. Additional slag conduit means to provide a plurality of withdrawal sites of molten slag 22 from primary furnace 10 may be provided, as desired, with each preferably having its respective entry end at or adjacent the interface between molten slag 22 and collector metal 20.Means other than the illustrated conduit, such as a tip-ladle loaded from the outlet of a truncated version of conduit 32 may be used to transport molten slag to secondary furnace 12.

For example, conduit 32 may be ended at flange connection 28 and plugged or fitted with means suitable to open it as desired to discharge molten slag into a suitable receptacle.

Secondary furnace 12 has a spent slag discharge means comprising a spent slag discharge conduit 34 having an entry end 34A and a discharge end 34B. Preferably, entry end 34A of spent slag discharge conduit 34 is disposed at or adjacent the interface between molten slag 22' and molten fresh collector metal 20'. A plurality of spent slag discharges may be provided.

It will be noted that generally, entry end 32A of slag conduit 32 (and the entry end of slag conduit 33) is disposed at an elevation above that of enriched collector metal discharge 30. Similarly, the entry end 34A of spent slag discharge conduit 34 is disposed at an elevation above that of enriched collector metal discharge 30'. Primary or first furnace 10 has means for introducing a feed therein, said means comprising a feed entry 36 through which a feed material is introduced as indicated by arrow 38. A second feed entry 37 may also be provided in primary furnace 10 for adding additional flux or other material.

In operation, a feed material is introduced into primary furnace 10 via feed entry 36 and may comprise a mixture of a material containing a recoverable metal dispersed thereon and a suitable flux material. The collector metal is usually introduced into the furnace first and melted therein before the introduction of the feed.

Make-up collector metal may be introduced through an opening such as second feed inlet 37 if it is desired to expose the collector metal more directiy to the heat generated by the plasma arc emanating from plasma gun 14. It will be appreciated by those skilled in the art that plasma gun 14 will provide an intense high temperature arc emanating from its tip disposed within furnace enclosure 1 8.

Due to the intense heat generated within primary furnace 10, the feed material melts to provide and/or maintain molten slag 22, and the collector metal melts to provide and/cr maintain molten collector metal 20. The size of the respective charges are controlled so that the interface between molten slag 22 and molten collector metal 20 is disposed at or immediately adjacent entry ends 32A of slag conduit 32 and the other slag conduit 33. Recoverable metal contained in the molten slag dissolves or otherwise migrates into molten collector metal 20 to enrich the same with the recoverable metal.

Residual material comprising molten slag from which at least a portion of the recoverable metal has been separated by migration into the molten collector metal is transported in molten form via slag conduit 32 to secondary furnace 12. Fresh collector metal will usually have been introduced into secondary furnace 12 prior to commencement of the operation and melted therein. Alternatively, or in addition, fresh collector metal or make-up fresh collector metal may be introduced via second feed 37' of secondary furnace 12.

Molten slag 22', which having already surrendered a portion of its total content of recoverable metal in primary furnace 10 may be considered to be a residual material, is now contacted with fresh collector metal 20' and additional recoverable metal migrates from molten slag 22' into fresh collector metal 20'. Additional flux material may be introduced in furnace 12 to control viscosity of the slag, if desired. Spent slag is removed via spent slag discharge conduit 34.

Fresh collector metal, enriched by recoverable metal dissolved or otherwise migrated into it from molten slag 22', is passed via secondary collector metal transfer means 24 into floor portion 21 A of primary furnace 10. When the content of recoverable metal in the pool of enriched collector metal accumulating in floor portion 21 A of primary furnace 10 is at an appropriately high level, product collector metal is withdrawn via enriched collector metal discharge conduit 30 as indicated by arrow 31.

Withdrawal of product collector metal may be on an intermittent batch basis or it may be continuous.

By providing the entry ends of slag conduit 32, 33 at or immediately adjacent, preferably, somewhat below, the interface between the molten slag and molten collector metal, as indicated by the un-numbered curved arrows within furnace enclosure 18, molten slag entering slag conduits 32, 33 is constrained to flow over and around in intimate contact with the surface of molten collector metal 20 at a plurality of sites.

The same technique is preferably employed with spent slag discharge conduit 34. By withdrawing slag at or immediately adjacent the molten metal interface, good flow contact between the exiting slag and the molten metal is enhanced, and this is believed to increase the migration of recoverable metal from the slag into the molten collector metal by enhancing the intimacy and area of contact therebetween. The provision of a plurality of such withdrawal sites increases the total areas in which such intimate contact is promoted and this may be better appreciated with respect to the discussion of Fig. 2.

Fig. 2 is a partial schematic view in which the crucible portion 38 of a furnace which may be of a type similar to that schematically illustrated in Fig. 1, is seen to have a floor portion 39 and an upper portion 40. The juncture between floor portion 39 and upper portion 40 is defined by the interface between molten collector metal 42 and molten slag material 44. Slag material 44 is less dense than and immiscible with molten collector metal 42 so that it floats upon it. This is generally true for the molten slag and collector metals employed.A pair of slag conduits 46, 46' are disposed within crucible portion 38 and, in this embodiment, are seen to have entry ends 46a, 46a' of enlarged diameter relative to the balance of conduits 46, 46'. Entry ends 46a and 46a' are seen to be disposed at or adjacent the juncture or interface between molten collector metal 42 and molten slag 44 and in fact, as is preferable, slightly below the interface. As the pressure head of molten slag 44 increases, molten slag 44 is forced up through slag conduits 46, 46' and outwardly therefrom as indicated by the arrows showing direction of slag flow.It will be noted that the surface of molten collector metal 42 is disturbed in the immediate vicinity of entry ends 46a and 46a' and this is due to the collector metal being forced a short distance upwardly into or towards the path of flow through slag conduits 46, 46'.

Collector metal 42, being of much greater density than molten slag 44, is not forced upwardly and outwardly through slag conduits 46, 46', but is only partly or initially forced therein and falls back under the effect of gravity. This disturbance, however, is useful in insuring and enhancing intimate flow contact between the exiting molten slag 44 and collector metal 42 in the vicinity of entry ends 46a and 46a'. Small globules of molten collector metal may, in fact, be entrained by the exiting slag and this may result in the prills of collector metal often found in the solidified slag.

As indicated by the curved flow direction arrows within crucible portion 38, the exiting slag is constrained to flow around and through the disturbed surface portion of molten collector metal 42 and the provision of a plurality of withdrawal sites increases the amount of such intimate flow contact between the molten slag and collector metal. Reference in the claims to flow slag being in intimate contact with the collector metal includes slag flow through and around portions of the collector metal disturbed in the vicinity of the entry ends of the slag conduits as illustrated in Figure 2.

With respect to use of a process of the type illustrated in Fig. 1 for recovery of platinum group metals from a feed comprising spent auto exhaust catalyst, it is contemplated that the ultimate eventual concentration of platinum group metals in the primary collector metal may be as high as 10% or more, e.g., 15-20% and the eventual concentration of platinum group metals in the secondary collector metal may be less than 10% e.g., 5-10%. Batch-wise withdrawal of product collector material may be called for, in order to withdraw during appropriate intervals when the amount of platinum group metal in the collector metal pool at the bottom of the primary furnace is sufficiently high. On the other hand, the entire process may be operated in a continuous manner, or continuously with batch-wise removal of product collector metal.

Claims (14)

1. A process for separating a recoverable metal from a starting material containing dispersed recoverable metal, by contacting molten starting material and molten collector metal to enrich said collector metal with recoverable metal, and separating resulting residual material and resulting enriched collector metal from each other, which comprises: (a) maintaining the residual material in the molten state after said separation of the residual material and enriched collector metal, (b) contacting the molten residual material from step (a) with molten fresh collector metal, to enrich the fresh collector metal with recoverable metal, and (c) separating from each other resulting residual material and resulting enriched fresh collector metal obtained from step (b).

2. A process as claimed in claim 1 which comprises maintaining the residual material resulting from step (b) in the molten state after separation thereof from the enriched fresh collector metal, and repeating thereon one or more times the contacting and separating operations of steps (b) and (c).

3. A process as claimed in claim 1 or 2 wherein the starting material comprises a catalyst comprising a carrier material having one or more catalytic metals dispersed thereon, said catalytic metals including at least one platinum group metal.

4. A process as claimed in claim 3 wherein the carrier material comprises a refractory metal oxide.

5. A process as claimed in claim 4 wherein the carrier material is alumina, cordierite, or a mixture thereof

6. A process as claimed in any preceding claim wherein the collector metal is iron.

7. A process as claimed in any preceding claim wherein the starting material forms part of a molten slag of selected viscosity obtained by combining the starting material with a flux material in proportions selected to impart said selected viscosity to said slag.

8. A process as claimed in claim 7 wherein the flux material is lime.

9. A process as claimed in any preceding claim which comprises combining at least a portion of the enriched collector metal with at least a portion of the enriched fresh collector metal to obtain a product collector metal.

10. A process as claimed in any preceding claim which comprises: (a) melting the starting material and collector metal in a first furnace and contacting therein the resulting molten starting material and collector metal whereby to enrich the collector metal with recoverable metal; (b) transferring molten residual material resulting from step (a) from the first furnace to a second furnace; (c) introducing fresh collector metal into the second furnace, melting and contacting it therein with molten residual material, whereby to enrich the fresh collector metal with recoverable metal; and, (d) withdrawing enriched collector metal from the first furnace and withdrawing enriched fresh collector metal from the second furnace.

11. A process as claimed in claim 10 wherein the enriched fresh collector metal withdrawn from the second furnace is transferred to the first furnace and therein combined with the enriched collector metal to form a product collector metal, which is withdrawn from the first furnace.

12. A process as claimed in any preceding claim wherein the starting material and collector metal are immiscible in the molten state whereby an interface is formed between them, and the molten residual material is withdrawn at a plurality of sites disposed at or adjacent to the interface, whereby residual material being separated from the molten collector metal is constrained to flow in intimate contact with the collector metal at the plurality of sites.

13. A process as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

14. Enriched collector metal obtained by a process as claimed in any of the preceding claims.