Method of treating a platinum group metal material

Abstract

A method of removing arsenic from a platinum group metal material containing arsenic, comprising mixing the platinum group metal material with a metal carbonate; heating the mixture in the presence of

Description

Field This specification relates to methods of treating platinum group metal materials. The methodology is particularly useful for treating platinum group metal materials which are then provided as a feed into a platinum group metal refining process to recover one or more platinum group metals. 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 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. The present specification relates to a method of pretreating such platinum group metal feed materials prior to refining although it will be appreciated that the methodology could also be applied to an intermediate platinum group metal material as part of a multi-step refining process Summary 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. This is problematic because certain environmental permits limit the arsenic levels allowed on a platinum group metal refinery site. Accordingly, such platinum group metal feed materials cannot be taken onto the platinum group metal refinery site for this reason. Furthermore, the present inventors are not aware of any previously published method in the platinum group metal recycling / refining field for removal of arsenic from such platinum group metal feed materials. It has been found that some arsenic can be removed by thermal processing of platinum group metal feed materials, the arsenic and arsenic compounds being removed via sublimation. However, removal rates are low, at least for certain types of platinum group metal feed materials, and do not reduce the arsenic content below environmentally allowed limits. In light of the above, the present inventors have developed a more efficient method of removing arsenic from a platinum group metal materials containing arsenic. The method comprises: mixing the platinum group metal material with a metal carbonate to produce a mixture of the platinum group metal material and the metal carbonate; heating the mixture in the presence of oxygen to produce a metal arsenic oxide; washing the mixture with water to dissolve the metal arsenic oxide; and separating the platinum group metal material from the water containing dissolved metal arsenic oxide. This methodology has been found to provide a more efficient removal of arsenic from platinum group metal feed materials, thus lowering the arsenic concentration below environmental limits and thereby allowing such treated feed materials to be refined to recover the platinum group metals. The methodology thus enables platinum group metals to be recovered safely from sources which would otherwise not be processible due to environmental 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 arsenic removal rates from platinum group metal materials using different additives illustrating that sodium carbonate performs best in the method of removing arsenic from platinum group metal feeds; and Figure 2 illustrates the trend for increased arsenic removal rate on moving to higher sodium carbonate content. Detailed Description As indicated in the summary section, the present specification provides a method of removing arsenic from a platinum group metal material containing arsenic, the method comprising: mixing the platinum group metal material with a metal carbonate to produce a mixture of the platinum group metal material and the metal carbonate; heating the mixture in the presence of oxygen to produce a metal arsenic oxide; washing the mixture with water to dissolve the metal arsenic oxide; and separating the platinum group metal material from the water containing dissolved metal arsenic oxide. The platinum group metal material can then be provided as a feed to a platinum group metal refining process to recover one or more platinum group metals. The platinum group metal material can be an alloy, optionally an iron, copper or nickel-based alloy comprising one or more of iron, copper and nickel and one or more platinum group metals. The iron, copper and / or nickel may constitute the major component of the alloy, e.g., >50% by weight. Alternatively, the platinum group metal material can be another type of material such as a spent platinum group metal catalyst material. The platinum group metals may include one or more of platinum, palladium, rhodium, iridium and ruthenium. The platinum group metal(s) component may constitute a minor component of the platinum group metal material, e.g., at least 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, or 5% or more by weight. Typical platinum group metal collector alloys may comprise 2 to 5 wt% of platinum group metal. Spent catalyst material may contain lower quantities of platinum group metal, e.g., 0.1 to 0.2 wt%. As previously indicated, it has been found that collector elements for platinum group metals such as iron, copper and nickel can carry arsenic from the source material and the present methodology has been found to efficiently remove the arsenic from such alloys enabling them to be subjected to platinum group metal refining and recovery. The methodology can also be applied to other platinum group metal feeds such as spent catalyst which have been contaminated with arsenic. The platinum group metal material (e.g., alloy) can be in the form of a powder having a particle size of less than 0.5 mm. 0.3 mm, 0.2 mm, 0.15 mm, or 0.125 mm. It has been found that arsenic removal via sublimation is particularly problematic for small particle size platinum group metal feed powders (e.g., particles after 120 mesh processing or 0.125 mm diameter) as the feed material is more likely to sinter thus preventing sublimation of arsenic and arsenic compounds. In contrast, the present methodology has been found to efficiently remove the arsenic from such platinum group metal materials enabling them to be subjected to platinum group metal refining and recovery. The metal carbonate can be selected from one or more of NazCOa, CaCOa, MgCOa, or Cuz(OH)zCO3, preferably NazCOs. A mass ratio of metal carbonate to platinum group metal material may be at least 0.4 : 1, 0.6 : 1, 0.8 : 1, 1 : 1, or 1.2 : 1 for NazCOs or molar equivalent mass ratios for other metal carbonates where the reaction equation for NazCOs is given by: 4As + 6NazCOa + 5Oz -> 4NaaAsO4 + 6COa. Increasing the amount of metal carbonate relative to platinum group metal material has been found to increase the rate of arsenic removal from the platinum group metal material. However, in practice it is desired to not unduly add excessive quantities of metal carbonate and thus the mass ratio of metal carbonate to platinum group metal material may be no more than 3 : 1, 2 :1, or 1.5 : 1 for NazCOs or molar equivalent mass ratios for other metal carbonates. It is also envisaged that the mass ratio may lie in a range defined by any combination of the aforementioned lower and upper limits. In the heating step, the mixture of metal carbonate and platinum group metal material may be heated to a temperature of: at least 500°C, 550°C, 600°C, 700°C or 800°C; no more than 1500°C, 1300°C, 1000°C, 900°C or 800°C; or within a range defined by any combination of the aforementioned lower and upper limits. A temperature in a range 600°C to 800°C has been found to be particularly preferred. Furthermore, the mixture may be heated for a time of at least 1 hour, 2 hours, 3 hours, or 4 hours. Heating for an increased length of time can achieve higher arsenic removal but in practice it is desired to not heat for unduly excessive time periods and it has been found that the majority of arsenic can be removed by heatingfor a time period of around 4 hours. Optionally, the heating period is no longer than 10 hours, 8 hours or 6 hours or falls in a range defined by any combination of the aforementioned lower and upper limits. The heating step can be performed in a furnace selected from an arc furnace, a plasma electric furnace, an induction furnace, a cupellation furnace or a muffle furnace. A cupellation furnace has been found to be preferrable in obtaining higher arsenic removal rates. Such a furnace has better air flow than, say, a muffle furnace which has restricted air flow within the furnace chamber. It has also been found to be advantageous to ensure that the heating is performed in an oxygen rich (oxidizing) environment and that the oxygen rich environment is maintained during heating, e.g., by having a suitable air flow in the furnace chamber. That is, the mixture is heated in a furnace which maintains a sufficiently high oxygen level to maintain production of the metal arsenic oxide during heating. It has also been found that arsenic removal rates can be increased by spreading out the mixture of metal carbonate and platinum group metal material in a wider, shallower container introduced into the furnace for the heating step. That is, the mixture is heated in a container having a diameter greater than its height, thereby increasing contact area with the oxygen during heating. Finally, as previously indicated, after separation of the platinum group metal material from the water containing dissolved metal arsenic oxide, the platinum group metal material can be provided as a feed to a platinum group metal refining process to recover one or more platinum group metals. In relation to the above, we have noted two publications for removal of arsenic from other types of material: (1) "Selective removal and leaching kinetics of Arsenic from electric dust collection of silver furnace by alkaline roasting - water leaching" by WANG Xiaoyang, WANG Wenxiang, FANG Hongsheng, MAChengrong, WU Kefu, China Academic Journal Electronic Publishing House, Vol. 41 No. 3(sum 183), June 2022; and (2) "Removal of arsenic from arsenic-bearing copper ore" by LI Huimeng, SHI Lihua, China Academic Journal Electronic Publishing House, Vol. 40 No. 4(sum 178), Aug 2021. However, there is no suggestion in these documents of the problems relating to arsenic in certain platinum group metal feeds such as platinum group metal alloys based on iron, copper, or nickel and no suggestion of the presently claimed solution which enables platinum group metals to be recovered safely from sources which would otherwise not be processible due to environmental concerns. There is also no disclosure or suggestion of preferred features according to the present specification such as the use of a cupellation furnace to improve arsenic removal from platinum group metal feeds. Examples Several different platinum group metal (PGM) feed powders containing arsenic have been tested: PGM alloy batch no. Arsenic content (ppm) 1028-1 3202 ESR-23-01 771 ESR-23-02 725 ESR-23-03 710 Samples of the PGM alloy feed powders (e.g., 50 grams) were mixed with sodium carbonate (lab grade NajCOs - dehydrated) in a 3D mixer for 20 minutes to ensure thorough mixing. Different mixing (mass) ratios of sodium carbonate to PGM feed alloy have been tested including PGM feed : sodium carbonate mass ratios of 1:1.2,1: 0.6, and 1: 0.4. Samples of the PGM alloy feed powders were also mixed with CaO and dolomite rather than a carbonate. After mixing, the samples were heated in a furnace at designated temperatures and durations (e.g., 600°C or 700°C; 2 hours or 4 hours). Different furnaces were also tested having different air flows (e.g., a cupellation furnace having high air flow versus a muffle furnace having restricted air flow). After heating, and once material was cooled down, the mixtures were left in water for dissolving overnight then filtered and analysed for arsenic content. Results are provided in the table below. Exp. No. Alloy batch No. Alloy Mass(g) Additive Type Additive mass (g) Mixing ratio Heating Conditions Remaining As (ppm) Removal rate (%) 1-1 1028-1 50 CaO 5 1:0.1 Cupellation 700°C*2h 2450 23.49% 1-2 ESR-23-01 50 504 34.63% 1-3 ESR-23-02 50 481 33.66% 1-4 ESR-23-03 50 482 32.11% 2-1 1028-1 50 Dolomite 5 1:0.1 Cupellation 700°C*2h 2519 21.33% 2-2 ESR-23-01 50 603 21.79% 2-3 ESR-23-02 50 534 26.34% 2-4 ESR-23-03 50 549 22.68% 3-1 1028-1 50 Na2CO3 (dehydrated) 60 1:1.2 Cupellation 600°C*2h 769 75.98% 3-2 ESR-23-01 50 105 86.38% 3-3 ESR-23-02 50 54 92.55% 3-4 ESR-23-03 50 112 84.23% 4-1 1028-1 50 Na2CO3 (dehydrated) 20 1:0.4 Cupellation 600°C*2h 2711 15.33% 4-2 ESR-23-01 50 572 25.81% 5-1 1028-1 50 Na2CO3 (dehydrated) 30 1:0.6 Cupellation 600°C*2h 1182 63.09% 5-2 ESR-23-01 50 347 54.99% 6-1 1028-1 25 Na2CO3 (dehydrated) 15 1:0.6 Larger surface area, 600°C*2h 339 89.41% 6-2 ESR-23-01 25 86 88.85% 7-1 1028-1 25 Na2CO3 (dehydrated) +Dolomite 10+5 1:0.4 : 0.2 Larger surface area, 600°C*2h 396 87.63% 7-2 ESR-23-01 25 288 62.65% 8-1 1028-1 25 Na2CO3 (dehydrated) 30 1:1.2 Cupellation 600°C*4h 250 92.19% 8-2 ESR-23-01 25 140 81.84% 9-1 1028-1 25 Na2CO3 (dehydrated) 30 1:1.2 Muffle furnace 600°C*2h 1755 45.19% 9-2 ESR-23-01 25 404 44.28% As shown by experiments 3-1 to 5-2, for different arsenic content of feed material (ranging between 700 ppm to 3200 ppm), arsenic removal rates range between 75% - 92% for a 1:1.2 mass ratio, 55% - 63% for a 1 : 0.6 mass ratio, and 15% - 26% for a 1 : 0.4 mass ratio of PGM feed alloy to sodium carbonate content. This indicates that the arsenic removal rate increases with increasing addition of sodium carbonate. To promote a high surface area so the reaction mixture was in contact with more oxygen, a flatter container with a larger surface area was trialled in the cupellation furnace yielding an increase in arsenic removal. As shown by experiments 6-1 to 7-2, the result for a 1 : 0.6 mass ratio is approximately 89% and for a 1:0.4 mass ratio is in a range 87% - 62%. This shows that arsenic removal rates can be increased by spreading the reaction mixture over a larger area container within the furnace. A trial was also conducted in a muffle furnace which has restricted air flow in the furnace chamber with the same temperature condition and duration, for a 1:1.2 mass ratio, which showed a result for arsenic removal of around 45%. This shows that the cupellation furnace performs better than the muffle furnace. See experiments 9-1 and 9-2 versus experiments 3-1 and 3-2. To check the impact of heating duration, a trial with heating time extended to 4 hour under the same temperature in the cupellation furnace, for a mixing ratio of 1 : 1.2. The arsenic removal rate was in a range 92%-81% which is increased compared to a 2 hour heating time. See experiments 8-1 and 8-2 versus experiments 3-1 and 3-2. The CaO and dolomite additives performed much less well than the sodium carbonate additive as can be seen in the results for experiments 1-1 to 2-4. Figure 1 illustrates the results for the different additives with sodium carbonate performing much better for removal of arsenic from all of the PGM feed batches. Figure 2 illustrates the trend for increased arsenic removal rate on moving to higher sodium carbonate content for two of the PGM feed batches. 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. It should be noted that the claimed features can be combined in any way and multiple claim dependencies are envisaged where formally acceptable.

Claims

1. A method of removing arsenic from a platinum group metal material containing arsenic, the method comprising:mixing the platinum group metal material with a metal carbonate to produce a mixture of the platinum group metal material and the metal carbonate;heating the mixture in the presence of oxygen to produce a metal arsenic oxide;washing the mixture with water to dissolve the metal arsenic oxide; andseparating the platinum group metal material from the water containing dissolved metal arsenic oxide.

2. A method according to claim 1,wherein the platinum group metal material is a platinum group metal alloy.

3. A method according to claim 2,wherein the platinum group metal alloy is an iron, copper or nickel-based alloy comprising one or more of iron, copper and nickel and one or more platinum group metals.

4. A method according to claim 1,wherein the platinum group metal material is a spent catalyst material comprising one or more platinum group metals.

5. A method according to claim 1,wherein the platinum group metal material contains at least 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, or 5% or more by weight of platinum group metal.

6. A method according to claim 1,wherein the platinum group metal material is in the form of a powder having a particle size of less than 0.5 mm. 0.3 mm, 0.2 mm, 0.15 mm, or 0.125 mm.

7. A method according to claim 1,wherein the metal carbonate is selected from one or more of Na2CO3, CaCO3, MgCO3, or Cu2(OH)2CO3.

8. A method according to claim 1,wherein a mass ratio of metal carbonate to platinum group metal material is: at least 0.4 : 1, 0.6 : 1, 0.8 :1,1:1, or 1.2 : 1 for NajCOs or molar equivalent mass ratios for other metal carbonates; no more than 3 : 1, 2 : 1, or 1.5 : 1 for NazCOs or molar equivalent mass ratios for other metal carbonates; or within a range defined by any combination or these limits.

9. A method according to claim 1,wherein the mixture is heated to a temperature of: at least 500°C, 550°C, 600°C, 700°C or 800°C; no more than 1500°C, 1300°C, 1000°C, 900°C or 800°C; or within a range defined by any combination of these limits.

10. A method according to claim 1,wherein the mixture is heated for a time of at least 1 hour, 2 hours, 3 hours, or 4 hours.

11. A method according to claim 1,wherein the mixture is heated in a furnace selected from an arc furnace, a plasma electric furnace, an induction furnace, a cupellation furnace or a muffle furnace.

12. A method according to claim 11,wherein the furnace is a cupellation furnace.

13. A method according to claim 1,wherein the mixture is heated in a furnace which maintains a sufficiently high oxygen level to maintain production of the metal arsenic oxide during heating.

14. A method according to claim 1,wherein the mixture is heated in a container having a diameter greater than its height, thereby increasing contact area with the oxygen during heating.

15. A method according to claim 1,wherein after separation of the platinum group metal material from the water containing dissolved metal arsenic oxide, the platinum group metal material is provided as a feed to a platinum group metal refining process to recover one or more platinum group metals.

Images