WO2025229293A1

WO2025229293A1 - A method of treating a platinum group metal containing liquor to remove arsenic

Info

Publication number: WO2025229293A1
Application number: PCT/GB2025/050692
Authority: WO
Inventors: Neil Davidson; Dikivitila N'DEKPLOMAN; Dipen PANDYA; Hazel Mary REID; Stephen SPELLER; Michael David TURNER JONES; Rebecca Louise WITTON
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2024-05-03
Filing date: 2025-04-01
Publication date: 2025-11-06
Anticipated expiration: 2026-11-03
Also published as: GB2640776A; GB202406249D0; GB202504862D0

Abstract

A method of treating a platinum group metal containing liquor to remove arsenic, the method comprising: adjusting an MCI concentration of the platinum group metal containing liquor such that the platinum group metal containing liquor has an MCI molarity of at least 6 M; adding a reducing agent to the platinum group metal containing liquor to reduce an oxidation- reduction-potential (ORP) value of the platinum group metal containing liquor to less than 625 mV; and after adjusting the MCI concentration and adding the reducing agent, subjecting the platinum group metal containing liquor to a distillation process to remove arsenic.

Description

A METHOD OF TREATING A PLATINUM GROUP METAL CONTAINING LIQUOR TO REMOVE ARSENIC

Field

The present specification relates to a method of treating a platinum group metal (PGM) containing liquor to remove arsenic. The methodology is applicable in platinum group metal refineries in order to prevent build-up of arsenic in a PGM refining circuit.

Background

Platinum group metal refining processes can typically involve one or more pyrometallurgical processes, including smelting, to separate platinum group metals from other materials in the feed, followed by acid dissolution of the platinum group metals and a series of hydrometallurgical processes in a refining circuit to separate and purify the individual platinum group metals (platinum, palladium, rhodium, iridium, and/or ruthenium). Such processes are known in the art.

Platinum group metal feed materials for the refining processes can vary in nature. For example, input feeds for smelting can be in the form of platinum group metal alloys such as iron, copper or nickel- based alloys comprising one or more of iron, copper and nickel and one or more platinum group metals. Alternatively, the input feed may be another type of platinum group metal material such as a spent platinum group metal catalyst material. Such materials can be input to the refining processes in powdered (particulate/granulated) form.

It has been found that certain platinum group metal alloy feed materials provided for refining of the platinum group metals contain arsenic carried from the source material with collector elements such as iron, copper or nickel. Arsenic can also be present in other types of platinum group metal feed materials such as spent catalysts.

It has been found that during hydrometallurgical processing in a PGM refining circuit, arsenic can build up in the refining circuit contaminating PGM liquors. Such a build-up of arsenic can present environmental health and safety (EHS) issues. Furthermore, such a build-up of arsenic has also been found to be detrimental to the functional performance of a PGM refining circuit. For example, arsenic build-up in a PGM refining circuit can lead to issues such as blockages and/or reduced filtration capacity. As well as causing process delays, removing the blockages exposes the operators to toxic and sensitising material which is an EHS concern. Furthermore, arsenic retention in the acid can make it difficult to re-use the acid where high purity acid is needed, such as in chromatography process steps (e.g., Pt/Pd chromatography) and other PGM separation techniques. Accordingly, there are both EHS and technical/financial concerns associated with arsenic build-up in a PGM refining circuit. As such, an efficient, cost effective, safe, and environmentally friendly method for arsenic removal from PGM liquors in a PGM refining circuit is required which does not adversely affect the performance (i.e., PGM refining capabilities) of the PGM refining circuit. Additionally, it is desirable for such a method to be capable of producing an arsenic rich solution which could be sold on for end-uses which require arsenic.

It is an aim of the present specification to address this problem.

Summary According to the present specification there is provided a method of treating a platinum group metal containing liquor to remove arsenic, the method comprising: adjusting an HCI concentration of the platinum group metal containing liquor such that the platinum group metal containing liquor has an HCI molarity of at least 6 M, 6.5 M, 7 M, 7.5 M, or 8 M (optionally no more than 10 M, 9.5 M, 9 M, or 8.5 M, e.g., between 7.5 M and 8.5 M); adding a reducing agent (e.g., copper (I) chloride or iron (II) chloride) to the platinum group metal containing liquor to reduce an oxidation-reduction-potential (ORP) value of the platinum group metal containing liquor to less than 625 mV, 600 mV, 550 mV, 500 mV, 450 mV, 400 mV, or 350 mV (optionally no less than 100 mV, 200 mV, 300 mV, or 350 mV, e.g. between 550 mV and 350 mV) and after adjusting the HCI concentration and adding the reducing agent, subjecting the platinum group metal containing liquor to a distillation process to remove arsenic (e.g., by heating the platinum group metal containing liquor to distil off the arsenic).

It has been found that if the PGM liquor has a low HCI molarity and/or a high ORP, arsenic cannot be efficiently removed from the PGM liquor via a distillation process. In contrast, the present methodology which combines: (i) an adjustment of HCI molarity to, for example, around 8 M by addition of HCI; and (ii) a reduction in the ORP of the PGM liquor (e.g., to between 550 mV and 350 mV) by addition of a reducing agent. This method has been found to provide a highly efficient process for arsenic to be selectively removed from a PGM liquor via a distillation process.

The present method can therefore be implemented in a PGM refining circuit to prevent build-up of arsenic within the circuit. As such, EHS and functional performance issues associated with such a buildup of arsenic in a PGM refining circuit are addressed. Furthermore, the method enables platinum group metal feed materials which are higher in arsenic content to be safely and reliable refined to recover the platinum group metals, thus enabling platinum group metals to be recovered safely from sources which would otherwise not be processible due to environmental, safety, and/or functional performance concerns.

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 shows a graph for Example 1 indicating how the % mass of As remaining in the bulk solution varies against the % volume of solution removed by distillation;

Figure 2 shows a graph for Example 2 indicating the concentration of As distilled from the PGM liquor with excess CuCI against total volume distilled (Cu:Pt ratio is almost 3:1; ORP was 363.2 mV);

Figure 3 shows a graph for Example 2 indicating the concentration of As distilled from 8 M PGM liquor with controlled (lower) CuCI addition against total volume distilled (ORP = 588 eV);

Figure 4 shows a graph for Example 2 indicating the concentration of As distilled from 8 M PGM liquor with 1:1 Cu:Pt ratio against total volume distilled (ORP = 401.4 mV);

Figure 5 shows a graph For Example 3 indicating the relationship between the redox potential of the PGM liquor to the percentage arsenic removal achieved; Figure 6 shows a graph for Example 3 indicating the relationship between the mass of CuCI added to the percentage arsenic removal achieved;

Figure 7 shows a graph for Example 4 illustrating the As removal during distillation - a total of 91.5 wt% As was removed from the PGM liquor with a 1:1 ratio Cu:Pt (ORP = 301.5mV);

Figure 8 shows a graph for Example 5 illustrating the As removal during distillation (ORP = 518.9 mV); and

Figure 9 shows a graph for Example 6 illustrating the As removal during distillation when using FeCL as a reductant as an alternative to CuCI.

Detailed Description

As described in the summary section, the present specification provides a method of treating a platinum group metal containing liquor to remove arsenic, the method comprising: adjusting an HCI concentration of the platinum group metal containing liquor such that the platinum group metal containing liquor has an HCI molarity of at least 6 M; adding a reducing agent to the platinum group metal containing liquor to reduce an oxidation- reduction-potential (ORP) value of the platinum group metal containing liquor to less than 625 mV; and after adjusting the HCI concentration and adding the reducing agent, subjecting the platinum group metal containing liquor to a distillation process to remove arsenic.

The step of adjusting the HCI concentration of the platinum group metal containing liquor may comprise increasing the HCI concentration, e.g., by addition of HCI. For example, the step of adjusting the HCI concentration of the platinum group metal containing liquor may comprise adjusting the molarity of HCI to: at least 6.0 M, 6.5 M, 7 M, 7.5 M, or 8 M; no more than 10 M, 9.5 M, 9 M, or 8.5 M; or within a range defined by any combination of the aforementioned lower and upper limits, e.g., between 7.5 M and 8.5 M. In this regard, it has been found that efficiency of arsenic removal via distillation of a PGM liquor can be increased by increasing the HCI molarity of the PGM liquor to within the stated values.

The step of adding the reducing agent to the platinum group metal containing liquor may comprise reducing the ORP value of the platinum group metal containing liquor to be: no more than 625 mV, 600 mV, 550 mV, 500 mV, 450 mV, 400 mV, or 350 mV; no less than 100 mV, 200 mV, 300 mV, or 350 mV; or within a range defined by any combination of the aforementioned upper and lower limits, e.g., to between 550 mV and 350 mV. In this regard, it has been found that efficiency of arsenic removal via distillation of a PGM liquor can be increased by reducing the ORP of the PGM liquor to within the stated values.

Combining suitable adjustments of both the HCI molarity and the ORP of a PGM liquor prior to distillation can provide a highly efficient process for removing arsenic from the PGM liquor.

The reducing agent can be added to the platinum group metal containing liquor before or after the HCI acid adjustment. Alternatively, the reducing agent can be added to the platinum group metal containing liquor at the same time as the HCI acid adjustment by adding an HCI solution of the reducing agent which both reduces the ORP value and adjusts the HCI molarity of the platinum group metal containing liquor. The reducing agent can be select from one or more of a metal chloride, a transition metal chloride, copper (I) chloride, and/or iron (ii) chloride. Copper (I) chloride is particularly preferred. When the reducing agent is copper (i) chloride, it can be added to the platinum group metal containing liquor to achieve a Cu:Pt ratio of: at least 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, or 1:1; no more than 5:1, 4:1, 3:1, 2:1, 1.5:1, 1.2:1, 1.1, 0.9:1, 0.8:1, 0.7:1, 0.6:1, or 0.5:1; or within a range defined by any combination of the aforementioned lower and upper limits.

In this regard, it has been found that for certain PGM refining circuits/processes unduly decreasing the ORP of the PGM liquor to very low values and/or unduly increasing the Cu:Pt ratio by adding too much reducing agent can adversely affect certain down-stream PGM refining steps. As such, in certain embodiments the ORP reduction and/or Cu:Pt increase via addition of the reducing agent is controlled to not unduly reduce the ORP and/or increase the Cu:Pt ratio beyond the point at which a sufficient arsenic removal rate is achieved by the method. Additionally, or alternatively, after removal of the arsenic via distillation, the PGM liquor can be treated to increase its ORP value and/or reduce its Cu:Pt ratio. Such a treatment to increase the ORP or reduce the Cu:Pt ratio of the platinum group metal containing liquor may involve blending the platinum group metal containing liquor with another platinum group metal containing liquor having a lower concentration of Cu and/or a higher ORP. For example, the treatment to reduce the Cu:Pt ratio may involve reducing the Cu:Pt ratio to be no more than 0.9:1, 0.8:1, 0.7:1, 0.6:1, 0.5:1, 0.4:1, or 0.3:1. In this way, any adverse effects of decreasing the ORP and/or increasing the Cu:Pt ratio can be mitigated if this is required to efficiently perform certain downstream PGM refining steps.

The distillation process itself may comprise heating the platinum group metal containing liquor to remove arsenic. For example, the platinum group metal containing liquor may be heated to a temperature of at least 90°C, 100°C, 105°C, or 110°C; no more than 150°C, 140°C, 130°C, or 120°C; or within a range defined by any combination of the aforementioned lower and upper limits. The optimal temperature is sufficient to distil off the arsenic while not being so high that significant amounts of HCI are distilled from the PGM liquor unduly lowering the HCI molarity of the PGM liquor. However, it may be noted that if the distillation is performed under modified pressure conditions, then the optimal temperature for distillation can be different.

During distillation, if the HCI molarity is significantly reduced, then HCI can be added during the distillation process to maintain the HCI molarity (within the previously defined values) during the distillation process.

After distillation to remove arsenic, and subsequent treatment to increase ORP and/or reduce Cu:Pt ratio of the PGM liquor if required for further process steps, the PGM liquor is processed to separate and recover one or more platinum group metals. These subsequent PGM refining steps can be achieved using methods known in the art.

Examples

1. Investigation into As distillation at different acidities

It is of importance to certain PGM refining flowsheets that arsenic can be dealt with as it tends to distil off in the acid during boil down and this can make it difficult to re-use the acid where high purity acid is needed (e.g., chromatography). It has been found that in certain PGM flowsheets, As is known to cause problems in both Pt/Pd chromatography and other separations, and so it would be beneficial if it could be removed before these processes as it could impact on the purities of the PGM streams. As is present as As(V) in the dissolve liquor. During solvent extraction and subsequent boil down steps, at least some of the As(V) is reduced to As(lll). AsCU is volatile and so some of it boils off during this boil down. However, the As removal is far from complete and so it has the potential to spread itself over the remainder of the flowsheet. In order to improve the efficiency of the As removal, it is important to understand the reduction kinetics of As(V) to As(lll) conversion and more particularly the conditions required to maximise As(lll) removal via distillation.

Preparation of Reagents

An initial feed was prepared comprising 250 mL of 10 gL ¹ As (III) in 6 M HCI by weighing 3.3 g of As (III) oxide into a 500 mL round bottom flask and adding 200 mL 6 M HCI. The mixture was stirred and heated to reflux for 1 hour to dissolve the As (III) oxide before cooling. The resultant solution was transferred to a 250 mL volumetric flask and made up to volume with 6 M HCI.

The above feed solution was used to make the following test solutions having different HCI molarities:

2 M HCI: 50 mL of feed solution was pipetted into a 250 mL volumetric flask along with 33.3 mL of 6 M HCI and made up to volume with deionized water.

4 M HCI: 50 mL of feed solution was pipetted into a 250 mL volumetric flask along with 116.7 mL of 6 M HCI and made up to volume with deionized water.

6 M HCI: 50 mL of feed solution was pipetted into a 250 mL volumetric flask and made up to volume with 6 M HCI.

8 M HCI: 50 mL of feed was pipetted into a 250 mL volumetric flask along with 90.1mL of concentrated HCI and made up to volume with 6 M HCI.

Distillations

235 mL of each of the aforementioned test solutions was poured into a 250 mL 3-necked round flask and heated to reflux with stirring (condenser fitted). 10 mL distillate samples were collected until 15 samples had been collected.

Acidities of the samples were measured by titrating 1 mL samples to an end point of 2 with 1 M Sodium Hydroxide. Each of the samples was also subjected to ICP-ES (inductively coupled plasma emission spectroscopy) analysis of arsenic.

In addition to the above, the 8M HCI solution was also subjected to distillation using a nitrogen sparge during heating (0.4 litres per minute) to determine if a nitrogen sparge improved the distillation process in terms of As removal.

Results & Discussion

The data was processed to calculate the mass of As remaining in solution. This was done by multiplying the concentration of As in the bulk with the volume of the bulk solution at the same point in time. The values were scaled so that they started at 100 %. Results are illustrated in Figure 1 which shows a graph indicating how the % mass of As remaining in the bulk solution varies against the % volume of solution removed by distillation.

The graph shown in Figure 1 clearly indicates that the As distillation efficiency is related to the acidity of the starting acid with the fastest removal occurring from the 8 M HCI solution. The addition of the N2 sparge does not seem to have had a large effect, and the slightly different shape in the profile may be due to the fact that the distillation was much slower with the N2 sparge, as the gas flow was effectively cooling the vessel.

The results suggest that it is more efficient to adjust the acidity of a PGM liquor to around 8 M [HCI] before conducting distillation as this will allow a more efficient As removal. Furthermore, an additional option is to continuously add concentrated HCI to the liquor as it is boiled down to keep the acidity of the bulk solution high as the distillation progresses. If acid is added to the bulk solution at the same rate as it is boiled off, then the HCI concentration can be maintained. This work suggests that it is more important to keep the acidity high, rather than avoiding letting the As concentration in the bulk drop due to dilution. Acidity measurement results show that it is possible to maintain the acidity of the bulk solution during distillation by the addition of concentrated HCI. This is predicted to be the best way to maximise the acidity in the boiling solution and to maximise the As removal rate.

2. As distillation from PGM liquors

Example 1 showed that it is possible to remove As from PGM liquors by distillation. That work was carried out using synthetic solutions and found that it is possible to remove As(lll), rather than As(V), particularly AsCU. The results of those trials found that an HCI acid concentration of around 8 M is advantageous to distil off As. These Example 2 trials aimed to remove As from real PGM liquors from a PGM refining flowsheet, including those which comprise As(V) species.

Distillation from PGM liquor 1

As mentioned above, the As species that will be removed by distillation tends to be As (III) rather than As (V). It is known that the majority of As in PGM liquor 1 is AsCI₅.

The method used for this trial was:

- Measure the initial acid concentration of the feed as received from the refinery.

- Adjust to 8 M HCI.

- Use 250 mL in a round bottomed flask with a condenser attached to collect the distillate.

- Heat to 110°C and collect 25 mL samples of distillate for ICP-OES analysis.

Results & Discussion

The distillation process reduced the amount of As in the PGM liquor by only 10.2 wt% (i.e., approximately 90 wt% of the arsenic remained in the PGM liquor after the distillation process). The reason for this low level of As removal was considered to be due to only around 10% of the As in the PGM liquor being in the form of As(lll), with the remainder being in the form of As(V). As such, it was considered that it would be advantageous to use a reductant to convert the As speciation from As (V) to As (III) prior to distillation and that this could be useful combined with also adjusting the molarity of HCI in the PGM liquor as previously described to provide an optimized process for removing As from PGM refinery liquors.

Distillation from 8 M PGM liquor using a reductant in combination with acid adjustment

The method used in this trial was similar to the method described above, but also included the use of a reductant. The method used for this trial was:

- Measure the initial acid concentration and ORP of the feed as received from the refinery.

- Adjust to 8 M HCI using a 3 M solution CuCI (reductant) dissolved in 37% hydrochloric acid. - Measure the acid concentration and ORP of the feed after adjusting.

- Heat to 110 °C and collect 25 mL samples of distillate to send for ICP-OES analysis.

In relation to the above, a number of possible reductants were considered including the use of oxalic acid, sodium formate solution, iron (II) chloride, and copper (I) chloride. It was decided that the use of copper (I) chloride would be most suitable, but there would be the option of using iron (II) chloride depending on the assay of the liquor as received.

This trial used an excess of CuCI to identify whether the reductant was powerful enough to reduce As (V) to As(lll). A 3 M CuCI solution in 37% HCI was used to increase the acidity of the feed to 8 M as suggested by results of Example 1. 3 M CuCI in 37% HCI was made up by adding 74.25 g of solid CuCI to 250 mL 37% HCI. Other work had indicated that to reduce As (V) to As (III), an ORP less than 625 mV is required in HCI matrices, so the ORP measurement in this trial was considered important. The initial ORP of the initial PGM liquor feed was 797.8 mV at 18.8°C, which is much higher than the desired value below 625 mV. Furthermore, the acidity was found to be 6.35 M, rather than 8 M. So, the feed prep was carried out by having 159 mL of feed as received and 111.4 mL of 3 M CuCI in 37% HCI. This solution was stirred for 30 minutes, where a sample was then taken to measure the acid concentration and ORP. The acidity was found to be 8.04 M and the ORP was 363.2 mV at 19.1°C (i.e., within the targeted value ranges for both HCI molarity and ORP). A sample of this liquor was taken for analysis by ICP-OES and found to contain 7600 mgL ¹ As. Heating was started and samples were taken every 25 mL. A sample of liquor was taken at the end of the trial to accurate determine the mass of arsenic after the distillation was complete.

Results & Discussion

Figure 2 shows a graph indicating the concentration of As distilled from the PGM liquor with excess CuCI against total volume distilled. The trial showed that there was a large concentration of As distilled off in the initial fraction, and there was a further removal of As throughout the distillation. The total amount of As removed by the distillation process was 71.8 wt% of the total As in solution. This indicated that the reduction of As from As (V) to As(lll) was successful. Furthermore, there was still 1 gL ¹ of As being distilled off in the final distillate sample, so the method is suitable for achieving higher levels of As removal if desired.

This level of removal was extremely high, but the amount of Cu added was a concern. There is currently a specification in a subsequent PGM refinery step for the mass ratio of Cu to Pt to be no more than 0.3:1. The adjusted liquor Cu:Pt ratio is almost 3:1, so exceeds the refinery specification for subsequent processing of the PGM liquor. Accordingly, the addition of Cu needs to be managed or the Cu needs to be removed prior to the subsequent PGM refining step which has a lower Cu:Pt specification. As such, using a lower amount of CuCI was investigated to assess whether sufficient As can be removed to prevent undue build-up in the refinery without unduly increasing Cu levels beyond specifications for subsequent PGM refining steps.

8 M PGM liquor distillation with controlled CuCI addition

The same PGM liquor that was used in the previous trial was used. The addition of CuCI as a reductant was controlled to only reduce the ORP of the PGM liquor to below 625 mV. The remainder of the method was the same as the method for the previous trial, to allow a fair comparison.

The method used for this trial was: - Measure the initial acid concentration and ORP of the feed as received from the refinery.

- Adjust to 8 M HCI using 37% HCI.

- Add 3 M CuCI dropwise until the ORP is below 625 mV.

- Measure the acid concentration of the feed after adjusting.

As the same liquor was used, the same dilution to get the feed to an 8 M acid concentration was completed, so 159 mL of feed was mixed with 111.4 mL of 37% HCI. The ORP of this adjusted feed was 799 mV. CuCI was added dropwise while constantly measuring the ORP. After 3.5 mL of 3 M CuCI was added, the ORP was at 588 mV. The acidity was measured after this adjustment and found to be 7.90 M, which was considered to be close enough to the desired concentration of 8 M to start the trial. A sample of the liquor was taken for analysis by ICP-OES which found an As concentration of 8103 mgL' ¹. Heating was started and samples were taken every 25 mL. In line with the previous trials, a liquor sample was taken after the trial was complete to identify the mass of As distilled.

Results & Discussion

Figure 3 shows a graph indicating the concentration of As distilled from 8 M PGM liquor with controlled CuCI addition against total volume distilled. This trial followed a similar trend as the previous distillation trial with 8 M liquor but with excess CuCI added. This trial, however, did not remove anywhere near as much As in total. The total amount of As removed by distillation was only 2 wt% of the original amount of As in solution. This indicated that the controlled reduction of As (V) to As (III) was incomplete.

Due to this result, further discussions were had to consider the maximum possible ratio of Cu to Pt that would be acceptable in the PGM refinery. After discussions, it was identified that it would be possible to use a 1:1 Cu to Pt ratio within the liquor to reduce As effectively and then this high Cu feed could then be bled into the next steps of the PGM refining process.

8 M PGM liquor distillation with Cu to Pt ratio of 1:1

The same PGM liquor that had been used for the previous trials was used for this experiment. The same experimental method steps were also used to allow a fair comparison between the trials. The only difference is how much CuCI is required.

From the basis of the trial with excess CuCI added, it is possible to calculate the amount of CuCI which needs to be added. 111.4 mL of 3 M CuCI in 37% HCI was added in that trial and the ratio was found to be 3:1. If a third of this volume is used (~37 mL) and is diluted with 74 mL of 37% HCI, this should give a 1 M CuCI solution which will give a 1:1 Cu to Pt ratio.

The only other change for this trial was that the feed would be made back up to volume at the end of the trial. There had been issues with solids dropping out of solution, which had made the analysis of the As concentration in the liquor after boiling down very difficult.

The method used for this trial due to this calculation was:

- Measure the initial acid concentration and ORP of the feed as received from the PGM refinery. - Adjust to 8 M HCI using 37 mL of 3 M CuCI in 37% HCI solution and 74 mL of 37% HCI.

- Measure the acid concentration of the feed after adjusting.

- Measure the ORP of the feed after adjusting.

- Top up liquor after heating using 6 M HCI to original volume.

- Submit liquor and distillate samples for analysis by ICP-OES

Once again, the same liquor was used, the same dilution to get the feed to an 8 M acid concentration was completed, so 159 mL of feed was mixed with 111.4 mL of 37% HCI was added. Following this, 37 mL of 3 M CuCI was mixed with 74 mL 37% HCI and this was added to the above feed to give the desired Cu:Pt concentration ratio of 1:1. The initial ORP of the feed was 797.1 mV and this was reduced to 401.4 mV. This value was low enough to reduce As(V) to As(lll) as in the previous trials. The acidity was then measured before starting the distillation and found to be 8.08 M, which was determined to be close enough to the desired concentration of 8 M. A sample of the liquor was taken for analysis by ICP- OES and found an As concentration of 7681 mgL ¹. Heating was started and samples were taken every 25 mL. In line with the previous trials, a liquor sample was taken after the trial was complete to identify the mass of As distilled. The liquor sample at the end was found to be close to forming solids so 80 mL 6 M HCI was added to ensure that no solids were formed. This was also noted in the previous trials.

Results & Discussion

Figure 4 shows a graph indicating the concentration of As distilled from 8 M PGM liquor with 1:1 Cu:Pt ratio against total volume distilled. A similar trend was seen in this trial as the previous ones, where there was the majority of the As distilled in the first 25 mL, but this trial saw much more As distilled. A total of 1768 mg of As was distilled in this trial, which is 81.6 wt% of the total As. This showed that a 1:1 Cu:Pt ratio was successful at removing large concentrations of As. Given that such large amounts of As are removed at a 1:1 Cu:Pt ratio, it will be possible to use a lower ratio between 1:1 and 0.3:1 while still removing significant amounts of As.

3. Arsenic distillation for PGM chemicals production plant

This example encompasses the trial work to further develop an arsenic removal distillation process and agree process set points for the scale-up into a plant batch process.

Experimental plan

The use of copper (I) chloride to reduce As(V) to As(lll) has been found to be the most effective reducing agent. Example 1 showed that for optimum arsenic removal, an initial acid concentration of 8 M HCI is required. Following this, in Example 2 the effect of Cu:Pt ratio was explored which showed 2 % arsenic removal at a Cu:Pt ratio of 0.3:1 and a 82 % arsenic removal at a Cu:Pt ratio of 1:1.

The aims of this example were as follows:

A. Repeat of Example 2 method to demonstrate reliability of the method.

B. Confirm the acceptable Cu:Pt ratio for the downstream PGM processing steps.

C. Produce data required for vessel sizing for scale-up to a plant batch process. Experimental method

PGM liquor from the refinery was obtained. The liquor was then adjusted to 8 M HCI using 37 % HCI. For the reduction, solid and liquid reductant addition was trialled. For solid addition, CuCI powder was added to a 250 mL solution to target the desired Cu:Pt ratio. For liquid addition, a solution of 25.1 g/L CuCI in 37 % HCI was added to a volume of the feed solution to make it up to 250 mL. The liquor was then heated to 110 °C and distilled until ~ 40 % of liquor was removed. 10 mL samples were taken from the feed vessel after every 20 mL of distillate collected to measure the arsenic removal. Additions of CuCI reductant were varied to investigate the effect on As removal by changing reductant levels and changing ORP.

Results & Discussion

Figure 5 shows a graph indicating the relationship between the redox potential of the PGM liquor to the percentage arsenic removal achieved. Figure 6 shows a graph indicating the relationship between the mass of CuCI added to the percentage arsenic removal achieved.

Figure 5 shows a general trend that greater arsenic removal is achieved at lower redox potentials. However, there are two anomalous results where there was no arsenic removed. The liquid stock solution used in these experiments was left to stand overnight before use. Therefore, there may have been some oxidation from Cu⁺ to Cu²⁺. CuCI is light and air sensitive. It needs to be stored under nitrogen to prevent gradual oxidation from Cu(l)CI to Cu( 11 )CI 2. The oxidation is indicated by a colour change from white to green. However, a small amount of Cu²⁺ could colour the solid powder. The Cu( I )CI powder was a light green colour indicating some premature oxidation.

Figure 6 shows a trend where the more CuCI which is added, the more As is removed. However, greater arsenic removal was achieved when for solid addition compared to liquid reductant addition. This may also be due to gradual oxidation of the CuCI which must be accounted for when scaling up the process to plant scale.

At least 35 wt% As removal is desired to achieve downstream PGM refining specifications. This laboratory work has shown that this removal can be achieved when the redox potential of the feed is reduced to < 450 mV. Copper (I) chloride was used as a reductant in this experiment. However there is an issue with the gradual oxidation of Cu(l) to Cu(ll) which makes the reductant ineffective. This is especially prevalent as a stock solution. However, when kept as a solid there is also gradual oxidation at the surface if not kept under an inert atmosphere. As such, for the process scale-up to production, it is important to maintain the reductant in a primarily Cu(l) state. Previous experiments have investigated the use of alternative reducing agents for this process. However, copper (I) chloride was the most suitable.

Furthermore, the downstream Cu:Pt specification is 0.3:1 although to achieve the required redox potential this limit is exceeded. To prevent disruption to the downstream process, the feeds which have undergone this process can be blended with other feeds to reduce the Cu/Pt ratio. It has been noticed that the downstream process can handle a Cu/Pt ratio of 0.5 without significant disruption. Therefore, selective feed blending can be planned as part of the process scale-up.

4. Arsenic distillation from PGM liquors with CuCI

35% removal of As is desirable to meeting PGM refining specifications, although higher removal is useful. Previous examples have shown that it is possible to remove As using distillation once reduced from As(V) to As(lll). Ideal conditions are indicated as a concentration of 8M acid and ORP below 625 mV, though more likely to be around a 400 mV value. Issues to consider include: CuCI oxidation to CuCL; how that affects the reduction of arsenic; whether direct solid addition provides better reduction vs freshly made solution; how much of an impact storing CuCI solution has on reduction ability; and whether it can be stored under inert atmosphere to prevent or limit oxidation.

Method plan

- Measure acid concentration and ORP of PGM feed as received.

- Adjust to ~8 M HCI using 1 M solution of CuCI in HCI in a round bottom flask with a condenser attached to collect distillate.

- Remeasure concentration and ORP and record these. Concentration should be around 8M now and ORP should be below 625mV.

- Heat to 110°C and collect 25mL samples of distillate to send for ICP-OES analysis.

- Final liquor may need topping up with 6M HCI prior to submission to analytical to ensure all possible solids are in solution.

Experimental

Acid concentration and redox potential of the initial PGM feed were measured at 4.92 M HCI and 813.9mV. 10.05 g of CuCI was combined with lOOmL of HCI to make a ~1M solution CuCI to use as a reductant and to increase the acidity to the ideal 8 M. 65mL of CuCI/HCI solution was added to 155mL of PGM feed, and then further HCI additions made to achieve a HCI molarity of 8.06M. No more acid was added at this time and the total CuCI added was 10.05g. The redox of the final solution was 301.5mV. Apparatus was set up for distillation and heating started. The internal temperature measured by the thermometer situated next to the distillation arm read in the range of 100-105°. The apparatus was left to distil, and samples were taken. 6M HCI was added to the remaining solution to the original level to prevent any solid dropout and maintain original viscosity.

Results and Discussion

Figure 7 shows a graph illustrating the As removal during distillation. A total of 91.5 wt% As was removed from the PGM liquor with a 1:1 ratio Cu:Pt.

5. Arsenic distillation from PGM liquor using CuCI as a reductant

The aim of this Example was to repeat Example 4 but with a reduced Cu:Pt ratio.

Method

150 mL of PGM liquor was measured using a measuring cylinder into a 500 mL 3 necked round bottom flask. The redox potential and the acidity measurement were taken. 3.51 g of CuCI was weighed and combined with 100 mL ~37% HCI and left to stir on a cold hotplate for approx. 10 minutes to ensure complete dissolution. The CuCI/HCI mixture was added to the PGM liquor in the round bottom flask, and all associated glassware was washed into the same vessel using 100 mL 37% HCI. The resultant liquor was left to stir for approx. 20 minutes before further acidity and redox measurements were taken. The round bottom flask was fitted with equipment for distillation, and the heating mantle set to 180°C, with a condensate temperature of 100-110°C shown on the internal thermometer. 3 x 20 mL samples of distillate were taken, followed by a 30 mL sample, and a final 20 mL sample. The PGM liquor was transferred to a measuring cylinder (240 mL) and combined with 90 mL 6.5 M HCI that had been used to rinse the vessel to ensure any solids would return to solution. A 5 mL sample was taken and filtered through a 0.45 pm SFCA syringe filter before submitting to analytical. Results and Discussion

The initial acidity and redox potential pre-CuCI addition were recorded as 5.12 M and 878.1 mV respectively, changing to 8.02 M and 518.9 mV once the 200 mL HCI and 3.51 g CuCI had been added. The redox potential is theoretically low enough to work (previous work suggested less than 625 mV will be suitable for this reaction to proceed), though previous trials have dropped the redox potential to ~400 mV and below. The earlier trial at 588 mV did not work well in respect of As removal, though the volume of CuCI added was very low in comparison.

Figure 8 shows a graph illustrating the As removal during distillation. The method yielded a total removal of 595.59 mg As. Data indicates that most of the As distils off in the first 1-2 samples worth (roughly 1 hour of distillation).

6. Arsenic distillation from PGM liquor using FeCI₂ as a reductant

The aim of this example was to demonstrate the use of FeCL as a reductant as an alternative to CuCI.

Method

500 mL PGM liquor was acid adjusted from 4.85 M to 8 M with 410 mL ~37% HCI. 150 mL of liquor was added to a 500 mL round bottom flask and the redox value was recorded as 663.3 mV. A solution of FeCL in 6 M HCI was added slowly until the redox value was reduced to 407 mV after 80 mL. The heating mantle temperature was set to 185°C, reaching an internal vapour temperature of 105°C. the distillate was collected in the following portions: 10 mL, 10 mL, 18 mL, 21.5 mL, and 22 mL. Once the distillation had been operating for ~1 hour, it was switched off and allowed to cool before filtering to remove any precipitates.

The five distillate samples, a sample of the PGM liquor pre-FeCL, a sample post-FeCL, and a sample post distillation were sent for analysis. There were no precipitates present on the filter paper.

Results and discussion

The initial acidity and redox potential were recorded to be 4.85 M and 663.3 mv respectively, changing to 8 M and 407 mv post adjustment, with 410 mL HCI and FeCL. FeCL was deemed to be an adequate reductant, removing 49.12% of the As present in solution. On comparison with CuCI, the results are not as good, but can be classed as acceptable. Additionally, it was noted that there was still a significant concentration of As in the later distillate samples, suggesting that if this experiment was conducted for a longer time period, then more As would distil off. Figure 9 shows the gradual removal of As through the distillation process.

While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of treating a platinum group metal containing liquor to remove arsenic, the method comprising: adjusting an HCI concentration of the platinum group metal containing liquor such that the platinum group metal containing liquor has an HCI molarity of at least 6 M; adding a reducing agent to the platinum group metal containing liquor to reduce an oxidation- reduction-potential (ORP) value of the platinum group metal containing liquor to less than 625 mV; and after adjusting the HCI concentration and adding the reducing agent, subjecting the platinum group metal containing liquor to a distillation process to remove arsenic.

2. A method according to claim 1, wherein the step of adjusting the HCI concentration of the platinum group metal containing liquor comprises increasing the HCI concentration.

3. A method according to claim 1 or 2, wherein the step of adjusting the HCI concentration of the platinum group metal containing liquor comprises adjusting the molarity of HCI to: at least 6.5 M, 7 M, 7.5 M, or 8 M; no more than 10 M, 9.5 M, 9 M, or 8.5 M; or within a range defined by any combination of the aforementioned lower and upper limits.

4. A method according to any preceding claim, wherein the step of adding the reducing agent to the platinum group metal containing liquor comprises reducing the ORP value of the platinum group metal containing liquor to be: no more than 600 mV, 550 mV, 500 mV, 450 mV, 400 mV, or 350 mV; no less than 100 mV, 200 mV, 300 mV, or 350 mV; or within a range defined by any combination of the aforementioned upper and lower limits.

5. A method according to any preceding claim, wherein the reducing agent is added to the platinum group metal containing liquor before or after the HCI acid adjustment.

6. A method according to any one of claims 1 to 4, wherein the reducing agent is added to the platinum group metal containing liquor at the same time as the HCI acid adjustment by adding an HCI solution of the reducing agent which both reduces the ORP value and adjusts the HCI molarity of the platinum group metal containing liquor.

7. A method according to any preceding claim, wherein the distillation process comprises heating the platinum group metal containing liquor to remove arsenic.

8. A method according to claim 7, wherein the platinum group metal containing liquor is heated to a temperature of at least 90°C, 100°C, 105°C, or 110°C; no more than 150°C, 140°C, 130°C, or 120°C; or within a range defined by any combination of the aforementioned lower and upper limits.

9. A method according to any preceding claim, wherein the reducing agent is select from one or more of a metal chloride, a transition metal chloride, copper (I) chloride, and/or iron (ii) chloride.

10. A method according to claim 9, wherein the reducing agent is copper (i) chloride which is added to the platinum group metal containing liquor to achieve a Cu:Pt ratio of: at least 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, or 1:1; no more than 5:1, 4:1, 3:1, 2:1, 1.5:1, 1.2:1, 1.1, 0.9:1, 0.8:1, 0.7:1, 0.6:1, or 0.5:1; or within a range defined by any combination of the aforementioned lower and upper limits.

11. A method according to claim 10, wherein, after the distillation process, the platinum group metal containing liquor is treated to reduce the Cu:Pt ratio.

12. A method according to claim 11, wherein the treatment to reduce the Cu:Pt ratio involves blending the platinum group metal containing liquor with another platinum group metal containing liquor having a lower concentration of Cu.

13. A method according to claim 11 or 12, wherein the treatment to reduce the Cu:Pt ratio involves reducing the Cu:Pt ratio to be no more than 0.9:1, 0.8:1, 0.7:1, 0.6:1, 0.5:1, 0.4:1, or 0.3:1.

14. A method according to any preceding claim, wherein HCI is added during the distillation process to maintain the HCI molarity during the distillation process.

15. A method according to any preceding claim, wherein after removing the arsenic from the platinum group metal containing liquor, the platinum group metal containing liquor is processed to separate and recover one or more platinum group metals.