In this specification"platinum group metals"means the group of metals, also known as the"platinum metals", consisting of platinum, palladium, ruthenium, osmium, rhodium and iridium.

According to a first aspect of the invention there is provided a process for the extraction of platinum group metals (PGM), the process including the steps of: smelting a PGM concentrate to provide a melt comprising a PGM-containing matte, a spinel-rich slag and an oxide-rich slag ; and enriching the matte with regard to PGM in a conversion step by selective oxidation of undesired constituents of the matte, the process including, as intermediate steps, after the smelting step and prior to the conversion step for enriching the PGM-containing matte : separating the matte and the spinel-rich slag from the oxide-rich slag ; and separating the spinel-rich slag from the matte.

The word"spinel"in the specification is to be understood to refer to, and to include, magnetite-based spinels and other spinels such as chromite-based spinels, which are not reduced and fluxed during smelting and are dissolved in the molten matte and/or in the molten slag, forming an intermediate spinel-rich layer in the melt, between a lowermost matte layer and an uppermost oxide-rich slag layer. Such spinels are also known in the art as"mush","magnetite","bottom"or"false bottom".

The process may include, as a preliminary step, prior to the smelting step, of subjecting a particulate PGM-containing ore to flotation to produce a PGM concentrate and oxide-containing tailings which are separated from each other, the PGM concentrate being employed as a feed to the smelting step.

The ore may be of the type known as Upper Group 2 (UG2) which has a chromite matrix. Instead, the ore may be a mixture of the UG2 ore and a Merensky ore which contains chromite levels of less than 1% by mass. While the process can be used for any chromite content, it is envisaged that the process will usually be used for treatment of PGM-containing ores having a chromite content of 0.01-50% by mass, typically 30-45% by mass, e. g. 40-45% by mass, the PGM concentrate produced by such ore typically having a chromite content of 0.1-10% by mass, e. g.

2-6% by mass.

Separating the matte and the spinel-rich slag from the oxide-rich slag may comprise:

separating the oxide-rich slag in the melt from the spinel-rich slag in the melt ; and withdrawing the spinel-rich slag and the matte from the melt.

More particularly, separating the matte and the spinel-rich slag from the oxide-rich slag may comprise allowing the melt to separate into an uppermost oxide- rich slag layer, a lowermost matte layer and an intermediate spinel-rich slag layer, tapping spinel-rich slag and matte at a low level from the melt, and tapping oxide-rich slag at a high level from the melt.

As will be appreciated, during smelting of the PGM concentrate in the furnace, as a result of the molten slag's having a lower density than that of the molten matte, the furnace contents will comprise a lowermost layer of molten matte and an uppermost layer of molten slag. Thus, the molten matte is typically tapped from the furnace via a discharge opening or tap-hole provided at a low level in the furnace, into one or more ladles prior to being transferred to a converter for the enriching in the conversion step, the molten slag being tapped from the furnace via a discharge opening or tap-hole provided at a high level in the furnace, for discarding to waste or transfer to a re-treatment facility. The spinel-rich layer typically forms as an intermediate viscous layer between the molten slag layer and the molten matte layer, as indicated above.

The intermediate step of separating the spinel-rich slag from the matte may include allowing the spinel-rich slag to solidify in contact with the matte while keeping the matte molten, and drawing the molten matte from the solidified spinel-rich slag.

More particularly, the process may include arranging two or more ladles in series, such that each ladle is in fluid communication with an adjacent ladle.

The molten matte, having a higher density and a lower melting point than that of the spinel-rich slag, will collect at the bottom of each ladle, prior to being transferred to the conversion stage. Conveniently, the spinel-rich slag may be reprocessed, separately from the smelting step, to recover the PGM residue therefrom, the oxide-rich slag being discarded.

In an embodiment of the invention, the intermediate steps may include arranging two ladles adjacent and in fluid communication with each other, namely an upstream ladle having a spout, such as a so-called teapot spout, through which molten matte may be discharged, and a downstream ladle, at a lower level than the upstream ladle, to permit discharging of molten matte from the upstream ladle via the spout, to be received therein. Thus, the spinel-rich portion of the slag together with the molten matte may be tapped from the furnace, for example via a launder, into the upstream ladle, the spinel-rich portion of the slag being allowed to solidify and the molten matte being allowed to collect in the upstream ladle until the upstream ladle is full.

The intermediate steps may further include discharging the molten matte from the upstream ladle into the downstream ladle via the spout, as more molten spinel-rich slag and molten matte are tapped from the furnace into the upstream ladle, until the amount of solidified slag therein prevents further tapping from the furnace.

According to a second aspect of the invention there is provided an installation for use in the extraction PGM, the installation comprising: a smelting stage for receiving PGM concentrate and for smelting the concentrate to obtain a PGM matte, a spinel-rich slag and an oxide-rich slag ; and a conversion stage for receiving PGM matte from the smelting stage and enriching it with regard to its PGM content, the installation including a separation stage between the smelting stage and the conversion stage for separating slag from matte fed from the smelting stage to the conversion stage.

The installation may include a flotation stage for receiving a particulate PGM-containing ore, carrying out a flotation step on the PGM-containing ore to obtain a PGM concentrate, and for feeding the PGM concentrate to the smelting stage.

The smelting stage may comprise a furnace having a low level discharge opening or tap-hole for tapping molten matte and spinel-rich slag from the furnace

(which tap-hole may be specifically installed for this purpose) and a high level discharge for tapping an oxide-rich slag from the furnace.

The separation stage may comprise at least one ladle for receiving molten matte and spinel-rich slag from the furnace and for separating the spinel-rich slag from the matte by allowing the slag to solidify therein. As mentioned above, the separation stage preferably comprises a plurality of ladles arranged in series for discharging molten matte from one ladle to another along the series. Each ladle in the series may include a spout for facilitating the discharging of the molten matte from the ladle. The spout may be a so-called teapot spout.

A launder may be provided between the smelting stage and the first ladle in the series, for conveying spinel-rich slag and molten matte to the first ladle, the launder including a dam or weir for diverting part of the spinel-rich slag (fed with the molten matte from the smelting stage) away from the series of ladles. Thus, with this arrangement, spinel-rich slag may be trapped in the dam, the spinel-rich slag being removable therefrom manually.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings. In the drawings: Figure 1 shows a schematic flow diagram of part of an installation for carrying out the process for the extraction of platinum group metals (PGM) from an ore such as a chromite ore, in accordance with the invention; and

Figure 2 shows a schematic sectional side elevation of the separation stage in the installation of Figure 1.

Referring to the drawings, reference numeral 10 generally refers to an installation for carrying out the process for the extraction of platinum group metals (PGM) from an ore such as a chromite ore, containing PGM, in accordance with the invention.

The installation 10 comprises a flotation stage 12 for carrying out a flotation step on a particulate PGM-containing ore to obtain a PGM concentrate, a smelting stage 14 connected by a flow line 16 to the flotation stage 12 for receiving PGM concentrate from the flotation stage 12 and for smelting the concentrate to obtain a PGM matte and a slag. A conversion stage 18 is connected by a flow line 20 to the smelting stage 14 for receiving PGM matte from the smelting stage 14 and enriching it with regard to its PGM content.

The installation 10 also includes a separation stage 22 (see also Figure 2) between the smelting stage 14 and the conversion stage 18 for separating a spinel- rich slag from the PGM matte passing from stage 14 to stage 18. The separation stage 22 is connected to the smelting stage 14 by a launder 24 (illustrated in Figure 1 merely by a flow line) and to the conversion stage 18 by the flow line 20.

Further, the installation 10 includes. a slag-cleaning furnace 30 for cleaning converter slag from the conversion stage 18 to recover PGM residue trapped therein.

The smelting stage 14 comprises a smelting furnace 32 of a type known in the art for smelting PGM-containing ore. The furnace 32 has a low level discharge opening or tap-hole 34 and a high level discharge opening or tap-hole 36.

The conversion stage 18 comprises a reactor 40 of the type known in the art.

The separating stage 22 comprises an upstream ladle 42 and a downstream ladle 44 adjacent each other as shown in Figure 2. The downstream ladle 44 is arranged at a lower level than the upstream ladle 42 to permit discharging of molten matte from the upstream ladle 42 to the downstream ladle 44. The upstream ladle 42 has a so-called teapot spout 46 for facilitating the discharging of molten matte into the downstream ladle 44.

In use, particulate PGM containing ore such as the Upper Group 2 (UG2) ore is fed along a feed line 11 into the flotation stage 12 wherein a PGM concentrate is physically separated by flotation from oxide-containing tailings or waste rock. The oxide-containing tailings or waste rock is discarded via flow line 33 and the

PGM concentrate is dried in a drying stage (not shown) prior to being fed to the furnace 14 via the flow line 16.

In the furnace 32, the PGM concentrate is heated in conventional fashion to a temperature of at least 1150°C, typically 1400°C or higher, to melt oxide- and suphide-containing components of the PGM concentrate. An oxide-rich portion of the molten concentrate, having a lower density than the matte, separates out and forms an uppermost slag layer indicated by reference numeral 35, with the matte forming a lowermost layer, indicated in the drawing by reference numeral 37. A spinel- rich slag forms an intermediate layer between the oxide-rich layer 35 and the matte layer 37, and is indicated in the drawings by reference numeral 41. To enhance separation of the oxide-containing portion of the concentrate from the matte, fluxes of a type known in the art for this purpose, are added to the furnace 32 via flow line 31.

The oxide-rich slag 35 is tapped from the furnace 32 via the discharge opening or tap-hole 36 and discarded via flow line 38.

The spinel-rich slag layer 41 together with the matte layer 37 are tapped from the furnace 32 via the lower discharge opening or tap-hole 34 into the upstream ladle 42 via the launder 24. The launder 24 includes a dam or weir 25 (illustrated in Figure 1 by a flow line) for diverting part of the spinel-rich slag, fed from the furnace 32 with the molten matte, away from the separation stage 22.

The upstream ladle 42 is allowed to fill with matte and spinel-rich slag.

The spinel-rich slag, having a higher melting point and lower density than the matte, is allowed to solidify as a crust in the top of the upstream ladle 42 and the molten matte with a higher density and lower melting point than the slag is allowed to collect at the bottom of the ladle 42. As the spinel-rich slag and the matte are discharged from the furnace 32 into the upstream ladle 42, ladle 42 eventually fills with molten matte and solidified slag as more of the spinel-rich slag solidifies, and the molten matte then starts to discharge from the ladle 42 via the spout 46 into the downstream ladle 44.

The matte in the downstream ladle 44 is then transferred to the conversion stage 18 via the flow line 20. After the matte has been transferred to the conversion stage 18, the ladles 42 and 44 are upended, bumped on a suitable bumping block to remove any solidified spinel-rich slag therein, the solidified slag being reprocessed to recover PGM by being sent via flow line 21 to a crushing unit (not shown) for crushing the solidified slag, the crushed slag from the crushing unit being recycled to the flotation stage 12. After the removal of any solidified slag, the ladles 42 and 44 are then re-positioned in the separation stage 22.

In the conversion stage, the matte is fed to the reactor 40 operated in conventional fashion at a temperature of at least 1050 °C. Selective oxidation of iron in the matte and of sulphur associated with the iron takes place in the converter 40, enriching the matte in the converter 40 relative to matte in the furnace 32 with regard to PGM. Suitable fluxes are introduced to the converter 40 via flow line 43 to modify

the iron oxides. The enriched matte, commonly known in the art as so-called white matte is discharged from the converter 40 via flow line 48, sulphur being removed as sulphur dioxide via flow line 50, and converter slag containing entrained droplets of valuable PGM-containing matte being sent to the slag cleaning stage 30 via flow line 52.

At the slag cleaning stage 30 which comprises a furnace, the converter slag is cleaned using a suitable method known in the art for this purpose, with the valuable matte recovered being returned to the converter 40 via flow line 54, the cleaned converter slag being discarded via flow line 56. Alternatively this slag from the cleaned furnace 30 may be cooled and crushed prior to being subjected to flotation at the flotation stage 12 for recycling any residual PGM therein to the smelting furnace 32.

An advantage of the process of extracting PGM from an ore in accordance with the present invention, as described and illustrated by the drawings herein, is that it permits unrestricted exploitation of a variety of ores, because undesired contaminants that would otherwise build up in furnace 32 in the form of a spinel-rich slag, are removed from the furnace 32 together with the matte. The removal of the spinel-rich slag in accordance with the present invention allows the furnace 32 to be operated at lower temperatures, resulting in extended furnace life and reduction of furnace operating costs while increasing PGM extraction. It is a further advantage of the present invention that the risk of catastrophic failure of the furnace lining of the smelting furnace 32 is reduced.