FIELD OF INVENTION

This invention relates to a plant layout and related process for extracting fine chrome from a PGM tails stream to achieve a chrome concentrate sufficient to meet international market metallurgical yield grade requirements. Currently, the PGM tails stream flows into a tails dam, and so by extracting this fine chrome an additional revenue stream could be unlocked and would also reduce the environmental liability on existing tails dams.

SUMMARY OF INVENTION

According to a first aspect of the invention, there is provided a plant layout for extracting fine chrome from a PGM tails stream, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry, the plant layout comprising: a conditioning arrangement to adjust the pH of the mineral slurry and to dose the mineral slurry with a collector reagent; a first flotation stage comprising at least one rougher chrome flotation cell for floating and extracting chrome concentrate from the conditioned mineral slurry; a scavenger spiral arrangement to receive the rougher tailings from the first flotation stage as an input and to output extracted chrome concentrate, a first middling concentrate and first tailings, with the first middling concentrate and first tailings then being discharged; a cleaner spiral arrangement to receive the extracted chrome concentrate from the first flotation stage and the scavenger spiral arrangement as an input and to output a final grade chrome concentrate for storage, a second middling concentrate and second tailings, with the second tailings then being discharged together with the first middling concentrate and first tailings from the scavenger spiral arrangement; and a second flotation stage comprising at least one (but typically a plurality of, to maximise the concentrates recovered) recleaner chrome flotation cells for floating and extracting chrome concentrate from the second middling concentrate from the cleaner spiral arrangement, with the extracted chrome concentrate then being pumped as a further input into the cleaner spiral arrangement, and with the rougher tailings from the second flotation stage then being pumped as a further input into the scavenger spiral arrangement.

The cleaner spiral arrangement is used to not only upgrade the chrome concentrate but also to control plant grade by adjusting the final product to ensure it is on grade, typically around 40% yield grade.

According to a second aspect of the invention, there is provided a method of extracting fine chrome from a PGM tails stream, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry, the method comprising: conditioning the mineral slurry by adjusting the pH of the mineral slurry (by adding sulphuric acid to the fine fraction slurry) and dosing the mineral slurry with a collector reagent; floating and extracting chrome concentrate from the conditioned mineral slurry, using a first flotation stage comprising at least one rougher chrome flotation cell; using a scavenger spiral arrangement to receive the rougher tailings from the first flotation stage as an input and to output extracted chrome concentrate, a first middling concentrate and first tailings, with the first middling concentrate and first tailings then being discharged; using a cleaner spiral arrangement to receive the extracted chrome concentrate from the first flotation stage and the scavenger spiral arrangement as an input and to output a final grade chrome concentrate for storage, a second middling concentrate and second tailings, with the second tailings then being discharged together with the first middling concentrate and first tailings from the scavenger spiral arrangement; and floating and extracting chrome concentrate from the second middling concentrate from the cleaner spiral arrangement using a second flotation stage comprising at least one recleaner chrome flotation cell; and pumping the extracted chrome concentrate to define a further input into the cleaner spiral arrangement; and pumping the rougher tailings from the second flotation stage to define a further input into the scavenger spiral arrangement.

According to a third aspect of the invention, there is provided a plant layout for extracting fine chrome from a PGM tails stream, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry, the plant layout comprising: a first spiral concentrator arrangement to receive the fine fraction mineral slurry as an input and to output an intermediate grade chrome concentrate, a first intermediate middling concentrate and first tailings, the first intermediate middling concentrate and first tailings being combined to define a flotation feed; a flotation stage comprising at least one chrome flotation cell for receiving the flotation feed and floating and extracting chrome concentrate from the flotation feed, with the tailings from the flotation stage being discharged into a tailings sump; a second spiral concentrator arrangement to receive the intermediate grade chrome concentrate from the first spiral arrangement as an input and to output a first final grade chrome concentrate for subsequent storage in a final grade concentrate sump, a second intermediate middling concentrate and second tailings, the second tailings being sent to the flotation stage; and a third spiral concentrator arrangement to receive the extracted chrome concentrate from the flotation stage as an input and to output a second final grade chrome concentrate for subsequent storage in the final grade concentrate sump (together with the first final grade chrome concentrate), a third intermediate middling concentrate and third tailings, the third tailings being sent to the flotation stage.

In an embodiment, the flotation stage comprises: a primary chrome flotation cell for receiving the flotation feed and floating and extracting chrome concentrate from the flotation feed, with the tailings from the primary chrome flotation cell being discharged into the tailings sump; and a secondary chrome flotation cell for receiving the second tailings from the second spiral concentrator arrangement and the third tailings from the third spiral concentrator arrangement, wherein the extracted chrome concentrate from both the primary and secondary flotation cells is sent to the third spiral concentrator arrangement.

In an embodiment, the second intermediate middling concentrate from the second spiral concentrator arrangement and the third intermediate middling concentrate from the third spiral concentrator arrangement are sent to a middling concentrate cleaner, with the cleaned middling concentrate then being sent to the second spiral concentrator arrangement (in addition to the intermediate grade chrome concentrate from the first spiral arrangement).

In an embodiment, the plant layout includes a conditioning arrangement, between the floatation feed of the first spiral concentrator arrangement and the primary chrome flotation cell, to adjust the pH of the mineral slurry and to dose the mineral slurry with a collector reagent.

According to a fourth aspect of the invention, there is provided a method of extracting fine chrome from a PGM tails stream, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry, the method comprising: using a first spiral concentrator arrangement to receive the fine fraction mineral slurry as an input and to output an intermediate grade chrome concentrate, a first intermediate middling concentrate and first tailings; combining the first intermediate middling concentrate and first tailings to define a flotation feed; floating and extracting chrome concentrate from the flotation feed, using a flotation stage comprising at least one chrome flotation cell; discharging the tailings from the flotation stage into a tailings sump; using a second spiral concentrator arrangement to receive the intermediate grade chrome concentrate from the first spiral arrangement as an input and to output a first final grade chrome concentrate, a second intermediate middling concentrate and second tailings; storing the first final grade chrome concentrate in a final grade concentrate sump and pumping the second tailings to the flotation stage; and using a third spiral concentrator arrangement to receive the extracted chrome concentrate from the flotation stage as an input and to output a second final grade chrome concentrate, a third intermediate middling concentrate and third tailings; storing the second final grade chrome concentrate in the final grade concentrate sump; and pumping the third tailings to the flotation stage.

In an embodiment, the flotation stage comprises: a primary chrome flotation cell for receiving the flotation feed and floating and extracting chrome concentrate from the flotation feed, with the method including discharging the tailings from the primary chrome flotation cell into the tailings sump; and a secondary chrome flotation cell for receiving the second tailings from the second spiral concentrator arrangement and the third tailings from the third spiral concentrator arrangement, with the method including pumping the extracted chrome concentrate from both the primary and secondary flotation cells to the third spiral concentrator arrangement.

In an embodiment, the method comprises: pumping the second intermediate middling concentrate from the second spiral concentrator arrangement and the third intermediate middling concentrate from the third spiral concentrator arrangement to a middling concentrate cleaner; cleaning the middling concentrate cleaner; and pumping the cleaned middling concentrate to the second spiral concentrator arrangement. BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will become apparent from the following description of two examples described with reference to the accompanying drawings, wherein:

Figure 1 shows a schematic representation of a plant layout for extracting fine chrome from a PGM tails stream, according to a first embodiment of the invention; and

Figure 2 shows a schematic representation of a plant layout for extracting fine chrome from a PGM tails stream, according to a second embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring first to Figure 1 , a plant layout 10 is shown for extracting fine chrome from a PGM tails stream, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry 12.

The plant layout 10 comprises a conditioning arrangement 14 to adjust the pH of the mineral slurry (by adding sulphuric acid to the fine fraction slurry, as indicated by arrow 16) and to dose the mineral slurry with a collector reagent, as indicated by arrow 18.

In order to get the collector reagent to adhere to the fine chrome particle in the mineral slurry, the pH of the mineral slurry needs to be reduced to 2.5. This is crucial as adhesion of the collector reagent cannot occur if this is not done. The collector reagent typically adsorbs to the surfaces of the desired minerals, making them hydrophobic, which in turn allows them to attach to air bubbles. This pH adjustment is instant and can be compared to a steel pickle and passivation process. Due to the instant nature of the process, the collector is added as soon as possible before the pH starts increasing. The result is activated chrome, ready for flotation in the next step.

The plant layout 10 comprises a first froth flotation stage 20 comprising a plurality of rougher chrome flotation cells 22 for floating and extracting chrome concentrate from the conditioned mineral slurry.

Chrome froth flotation is highly versatile for physically separating all size fraction particles based on the ability of air bubbles to selectively adhere to specific surfaces in the mineral slurry. The flotation cells 22 are essentially agitated tanks into which finely-dispersed air bubbles are introduced. The desired hydrophobic mineral will then attach to the air bubbles and float to the top of the flotation cell, where it will be skimmed off as a mineral-laden froth.

A scavenger spiral arrangement 24 is provided to receive the rougher tailings from the first flotation stage 20 as an input, as indicated by flow line 26, and to output extracted chrome concentrate 28, a first middling concentrate 30 and first tailings 32. The first middling concentrate 30 and first tailings 32 may then be discharged, as indicated by flow lines 34, 36.

The plant layout 10 further comprises a cleaner spiral arrangement 38 to receive the extracted chrome concentrate overflow from the first flotation stage, as indicated by flow lines 40, 42, and the scavenger spiral arrangement 24, as indicated by flow line 44, as an input. The cleaner spiral arrangement 38 is arranged to output a final grade chrome concentrate 46 for storage 48, a second middling concentrate 50 and second tailings 52. The second tailings 52 is then being discharged together with the first middling concentrate 34 and first tailings 32 from the scavenger spiral arrangement 24, as indicated by flow lines 54, 36, respectively.

The spiral arrangements 24, 38 are typically spiral concentrators and are well known devices for the concentration of low-grade ores and industrial minerals in slurry form. These devices 24, 38 work on a combination of solid particle density and its hydrodynamic dragging properties. The device 24, 38 comprises a single or double helical conduit or sluice wrapped around a central collection column. The device 24, 38 has a wash water channel and a series of concentrate removal ports placed at regular intervals, corresponding to the extracted chrome concentrate 28, 46, middling concentrate 30, 50 and tailings outputs 32, 52 mentioned above. In one version, separation is achieved by stratification of material caused by a combined effect of centrifugal force, differential settling, and heavy particle migration through the bed to the inner part of the conduit. Two or more spirals are constructed around one central column to increase the amount of material that can be processed by a single integrated unit.

The plant layout 10 further comprises a second froth flotation stage 56 comprising a plurality of recleaner chrome flotation cells 58 for floating and extracting chrome concentrate, as indicated by flow lines 60, from the second middling concentrate 50 from the cleaner spiral arrangement 38. The extracted chrome concentrate is then pumped (i.e. recycled) as a further input into the cleaner spiral arrangement 38, as indicated by flow line 62.

The rougher tailings from the second flotation stage 56 is then pumped as a further input into the scavenger spiral arrangement 24, as indicated by flow line 64.

The cleaner spiral arrangement 38 is used to not only upgrade the chrome concentrate but also to control plant grade by adjusting the final product to ensure it is on grade, typically around 40% yield grade.

Turning now to Figure 2, another plant layout 100 is shown for extracting fine chrome from a PGM tails stream 102, typically after the PGM tails stream has undergone a desliming process to yield a fine fraction mineral slurry. The plant layout 100 comprises a first course spiral concentrator arrangement 104 to receive the fine fraction mineral slurry as an input (corresponding to stream 1 in Figure 2 and with reference to Table 1 below), via collector 105. The first course spiral concentrator arrangement 104 outputs an intermediate grade chrome concentrate (corresponding to stream 2 in Figure 2, Table 1 ), a first intermediate middling concentrate (corresponding to stream 3 in Figure 2, Table 1 ) and first tailings (corresponding to stream 4 in Figure 2). The first intermediate middling concentrate and first tailings are combined to define a flotation feed (corresponding ultimately to stream 5 in Figure 2, Table 2 below).

Table 1 : Spiral Plant Mass Balance example

The plant layout 100 further comprises a flotation stage 108 comprising at least one chrome froth flotation cell 110, 112 for receiving the flotation feed (corresponding to stream 5 in Figure 2, Table 2). The flotation stage 108 is arranged to float and extract chrome concentrate from the flotation feed, with the tailings from the flotation stage 108 being discharged into a tailings sump 114 (corresponding to streams 7 and 14 in Figure 2, Table 2).

The plant layout 100 further comprises a second cleaner spiral concentrator arrangement 116 to receive the intermediate grade chrome concentrate from the first course spiral concentrator arrangement 104 as an input (corresponding to stream 8 in Figure 2, Table 2). The second cleaner spiral concentrator arrangement 116 outputs a first final grade chrome concentrate (corresponding to stream 9 in Figure 2, Table 2) for subsequent storage in a final grade concentrate sump 118. The second cleaner spiral concentrator arrangement 116 further outputs a second intermediate middling concentrate and second tailings. The second tailings are then sent (pumped) to the flotation stage 108 (corresponding to stream 11 in Figure 2, Table 2, which in turn defines a first component of stream 12 in Figure 2, Table 2).

The plant layout 100 further comprises a third fins spiral concentrator arrangement 120 to receive the extracted chrome concentrate from the flotation stage 108 (corresponding to streams 6 and 13 in Figure 2, Table 2, via collector 122, to define stream 15) as an input. The third fins spiral concentrator arrangement 120 is arranged to output a second final grade chrome concentrate (corresponding to stream 16 in Figure 2, Table 2) for subsequent storage in the final grade concentrate sump 118 (together with the first final grade chrome concentrate (corresponding to stream 9 in Figure 2, Table 2).

The third fins spiral concentrator arrangement 120 is arranged to also output a third intermediate middling concentrate and third tailings, the third tailings being sent (pumped) to the flotation stage 108 (corresponding to stream 18 in Figure 2, Table 2), which in turn defines a second component of stream 12 in Figure 2.

In an embodiment, the flotation stage 108 comprises a primary chrome froth flotation cell 110 and a secondary chrome froth flotation cell 112.

The primary chrome froth flotation cell 110 receives the flotation feed (corresponding to stream 5 in Figure 2, Table 2) and floats to extract chrome concentrate from the flotation feed. The tailings from the primary chrome froth flotation cell 110 are discharged into the tailings sump 114 (corresponding to stream 7 in Figure 2, Table 2), as indicated above.

The secondary chrome froth flotation cell 112 receives the second tailings from the second cleaner spiral concentrator arrangement 116 (corresponding to stream 11 in Figure 2, Table 2) and the third tailings from the third fins spiral concentrator arrangement 120 (corresponding to stream 18 in Figure 2, Table 2) to define stream 12 in Figure 2. The extracted chrome concentrate from both the primary and secondary flotation cells 110, 112 (corresponding to streams 6 and 13 in Figure 2, Table 2), is combined in collector 122 and sent (pumped) to the third fins spiral concentrator arrangement 120 (corresponding to stream 15 in Figure 2, Table 2). In an embodiment, the second intermediate middling concentrate from the second cleaner spiral concentrator arrangement 116 (corresponding to stream 10 in Figure 2, Table 2) and the third intermediate middling concentrate from the third fins spiral concentrator arrangement 120 (corresponding to stream 17 in Figure 2, Table 2) are combined and sent (pumped) to a middling concentrate cleaner 124. The cleaned middling concentrate is then sent (pumped) to the second cleaner spiral concentrator arrangement 116 (in addition to the intermediate grade chrome concentrate from the first course spiral concentrator arrangement 104, corresponding to stream 2 in Figure 2, Table 1 ) to define combined stream 8 in Figure 2, Table 2 via collector 126. Table 2: Spiral Plant Mass Balance example (contd)

The recovery calculation is set out below, which shows an overall yield of 35%:

In an embodiment, the plant layout includes a conditioning arrangement, between the floatation feed of the first (course) spiral concentrator arrangement and the primary chrome (froth) flotation cell, to adjust the pH of the mineral slurry and to dose the mineral slurry with a collector reagent.

The invention extends to a method of extracting fine chrome from a PGM tails stream, which largely mirrors the layout described above with reference to Figure 2. At a high level, the method comprises using a first spiral concentrator arrangement to receive a fine fraction mineral slurry as an input and to output an intermediate grade chrome concentrate, a first intermediate middling concentrate and first tailings; and combining the first intermediate middling concentrate and first tailings to define a flotation feed.

The method then float so as to extract chrome concentrate from the flotation feed, using a flotation stage comprising at least one chrome flotation cell, with the tailings from the flotation stage then being discharged into a tailings sump.

The method further comprises using a second spiral concentrator arrangement to receive the intermediate grade chrome concentrate from the first spiral arrangement as an input and to output a first final grade chrome concentrate, a second intermediate middling concentrate and second tailings. The first final grade chrome concentrate is then stored in a final grade concentrate sump, with the second tailings then being pumped to the flotation stage.

The method further comprises using a third spiral concentrator arrangement to receive the extracted chrome concentrate from the flotation stage as an input and to output a second final grade chrome concentrate, a third intermediate middling concentrate and third tailings. The second final grade chrome concentrate is then stored in the final grade concentrate sump, with the third tailings then being pumped to the flotation stage.

In an embodiment, the flotation stage comprises a primary chrome flotation cell and a secondary chrome flotation cell. The primary chrome flotation cell receives the flotation feed and floats and extracts chrome concentrate from the flotation feed, with the method including discharging the tailings from the primary chrome flotation cell into the tailings sump.

The secondary chrome flotation cell receives the second tailings from the second spiral concentrator arrangement and the third tailings from the third spiral concentrator arrangement, with the method including pumping the extracted chrome concentrate from both the primary and secondary flotation cells to the third spiral concentrator arrangement.

In an embodiment, the method comprises pumping the second intermediate middling concentrate from the second spiral concentrator arrangement and the third intermediate middling concentrate from the third spiral concentrator arrangement to a middling concentrate cleaner; cleaning the middling concentrate cleaner; and pumping the cleaned middling concentrate to the second spiral concentrator arrangement.

The present invention thus provides an efficient way of extracting high grade fine chrome from a PGM tails stream to achieve a chrome concentrate sufficient to meet international market metallurgical yield grade requirements.