A method of refining platinum group metal

Abstract

A method of refining platinum group metal containing material in a platinum group metal refinery, the method comprising: placing the platinum group metal containing material in a furnace having a flue; thermally processing the platinum group metal containing material in the furnace; capturing flue dust passing through the flue of the furnace during thermal processing of the platinum group metal containing material, said flue dust comprising platinum group metal and base metal; treating the captured flue dust in sulphuric acid to leach the base metal into solution producing a sulphuric acid leach liquor comprising the base metal and a solid residue comprising the platinum group metal; separating the solid residue comprising the platinum group metal from the leach liquor comprising the base metal; and recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery to recover the platinum group metal.

Description

[0001] A METHOD OF REFINING PLATINUM GROUP METAL

[0002] Field

[0003] The present specification relates to a method of refining platinum group metal and particular one which improves the productivity and efficiency of refining operations over extended periods of operation.

[0004] Background

[0005] 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.

[0006] 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.

[0007] During pyrometallurgical processing, platinum group metal containing material is thermally processed, e.g., smelted, in a furnace. Dust is produced during thermal processing which passes out through the flue of the furnace. This dust comprises platinum group metal. As such it is desired to capture this dust and recycle it into the refinery to recover the platinum group metal in the dust and thus prevent losses. However, this process of recycling flue dust has been found to create issues within the refinery circuit which are addressed by the present specification.

[0008] Summary

[0009] It has been found that capturing flue dust from a platinum group metal refinery furnace and recycling the flue dust into a platinum group metal refining circuit leads to a build-up of deleterious elements in the refining circuit, which is detrimental to the productivity and efficiency of refining operations over extended periods of operation. These deleterious elements comprise base metals such as one or more of the following: As, Bi, Te, Se, Zn, Cd, Cu, Na, Ni, and Fe. As such, to solve this problem the present specification provides a method of refining platinum group metal containing material in a platinum group metal refinery, the method comprising: placing the platinum group metal containing material in a furnace having a flue; thermally processing the platinum group metal containing material in the furnace; capturing flue dust passing through the flue of the furnace during thermal processing of the platinum group metal containing material, said flue dust comprising platinum group metal and base metal; treating the captured flue dust in sulphuric acid to leach the base metal into solution producing a sulphuric acid leach liquor comprising the base metal and a solid residue comprising the platinum group metal; separating the solid residue comprising the platinum group metal from the leach liquor comprising the base metal; and recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery to recover the platinum group metal.

[0010] In relation to the above, key features to achieve this method are: (i) the realization that if platinum group metal containing flue dust is captured and recycled into a platinum group metal refinery then deleterious base metals in the flue dust can build-up over time in the refinery leading to a reduction in the productivity and efficiency of refining operations; and (ii) the finding that this problem can be address by treating the captured flue dust in sulphuric acid to leach the base metal into solution producing a sulphuric acid leach liquor comprising the base metal and a solid residue comprising the platinum group metal which can be separated and safely recycled back into the platinum group metal refinery while avoiding, or at least reducing, build-up of deleterious base metals. As such, the method is capable of improving the productivity and efficiency of refining operations over extended periods of operation while still avoiding losses of platinum group metal in flue dust from refinery furnaces. Sulphuric acid has been found to be particularly useful in this method as it can efficiently leach a wide range of elements which can be problematic if they build up in a platinum group metal refinery while retaining the platinum group metal within the solid residue such that it can be separated and safely processed within the refinery.

[0011] Brief Description of the Drawings

[0012] 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.

[0013] Figure 1 shows a flow diagram of a method of refining platinum group metal containing material in a platinum group metal refinery according to the present specification.

[0014] Figures 2a and 2b show the average composition of furnace flue dust (measured using a combination of ICP fusion and XRF analysis).

[0015] Figure 3 shows a leach profile for an 85 g / l acid leach showing deleterious element concentration on the left axis and free acidity on the right axis.

[0016] Figure 4 shows a leach profile for an 85 g / l acid leach showing copper, iron, silicon and calcium concentration on the left axis and free acidity on the right axis.

[0017] Figure 5 shows a leach profile for an 85 g / l acid leach showing aluminium, zinc, sodium and indium concentration on the left axis and free acidity on the right axis.

[0018] Figure 6 shows deleterious element solubility as a function of free acidity measured by ICP-OES spectroscopy (bismuth, tellurium, selenium and arsenic).

[0019] Figure 7 shows platinum group metal solubility as a function of free acidity at filtration measured by ICP-MS spectroscopy. Figure 8 shows concentration of elements vs. free acidity at filtration (copper, silicon, iron, calcium).

[0020] Figures 9a and 9b show analysis results for 85 g / l acid leached flue dust residue vs. initial flue dust (first columns = initial flue dust; second columns = after leaching) indicating that at this acid concentration deleterious elements such as such as arsenic, tellurium, bismuth and selenium can be reduced while increasing the relative amounts of PGM in the leached flue dust residue.

[0021] Figure 10 shows results for filtration with and without flocculant added indicating that the flocculated slurry filters at least 2 % times more quickly than when no flocculant is added, and the filtration rate remains linear over the course of the experiment in contrast to when no flocculant is added where the filtration rate decreases over time.

[0022] Figure 11 shows a graph indicating concentration (mg / l) of elements versus washing volume (ml).

[0023] Figure 12 shows the percentage of elements leached on repeated re-use of the leach liquor for leaching of flue dust.

[0024] Figure 13 shows the percentage of base metal / deleterious elements leached between 1-4 cycles when re-using the leach liquor as the leaching reagent (from ICP OES analysis of the filtrates).

[0025] Figure 14 shows the concentration of leached PGM in the filtrate after 1, 2, 3 and 4 cycles when reusing the leach liquor as the leaching reagent (from ICPMS analysis of the filtrates).

[0026] Figure 15 shows the percentage of PGM leached after 1, 2, 3 and 4 cycles when re-using the leach liquor as the leaching reagent (from ICPMS analysis of the filtrates).

[0027] Detailed Description

[0028] As described in the summary section and illustrated in Figure 1, the present specification provides a method of refining platinum group metal containing material in a platinum group metal refinery, the method comprising: placing the platinum group metal containing material in a furnace having a flue; thermally processing the platinum group metal containing material in the furnace (e.g., smelting); capturing flue dust passing through the flue of the furnace (e.g., using a filter within the flue duct) during thermal processing of the platinum group metal containing material, said flue dust comprising platinum group metal and base metal; treating the captured flue dust in sulphuric acid to leach the base metal into solution producing a sulphuric acid leach liquor comprising the base metal and a solid residue comprising the platinum group metal; separating (e.g., by filtering) the solid residue comprising the platinum group metal from the leach liquor comprising the base metal; and recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery to recover the platinum group metal.

[0029] The base metal which is leached from the flue dust may include one or more of As, Bi, Te, Se, Zn, Cd, Cu, Na, Ni, and Fe. Furthermore, one or more non-metal elements in the flue dust can also be leached from the flue dust into the sulphuric acid leach liquor, e.g., Cl. Such elements can build-up over time in the refinery if the flue dust is recycled leading to a reduction in the productivity and efficiency of refining operations. It has been found that introducing a sulphuric acid leach into the process can effectively remove these elements from the flue dust while retaining the platinum group metal within the solid residue such that it can be separated and safely processed within the refinery.

[0030] Suitable conditions for the flue dust leaching process may include one or more of the following: the flue dust is treated in the sulphuric acid at a temperature: of at least 20°C, 30°C, 40°C, or 50°C; of no more than 80°C, 70°C, 60°C, or 50°C; or within a range defined by any combination of the aforementioned lower and upper limits; the sulphuric acid is added to the flue dust to give a free acid concentration: of at least 50 gl'1, 60 gl1, 70 gl1, or 75 gl1; of no more than 120 gl1, 110 gl1, 100 gl1, or 90 gl1; or within a range defined by any combination of the aforementioned lower and upper limits; the flue dust is treated in the sulphuric acid for a time period: of at least 10 minutes, 20 minutes, 40 minutes, or 60 minutes; of no more than 5 hours, 3 hours, 2 hours, or 1 hour; or within a range defined by any combination of the aforementioned lower and upper limits; during treatment of the flue dust in the sulphuric acid, the sulphuric acid is stirred with the flue dust forming a suspension in the sulphuric acid; a flocculant (e.g., a non-ionic polyacrylamide flocculant) is added to the mixture of flue dust and sulphuric acid to aid separation (e.g., by filtering) of the solid residue comprising the platinum group metal from the leach liquor comprising the base metal (e.g., at a flocculant concentration of: at least 1, 5, 10, or 12 ppm; no more than 100, 50, 30, or 20 ppm; or within a range defined by any combination of the aforementioned lower and upper limits); after separation of the solid residue comprising the platinum group metal from the leach liquor comprising the base metal, the leach liquor is recycled and re-used one or more times for further leaching of base metal from the flue dust (e.g., the leach liquor may be re-used in the leaching process at least 1, 2, or 3 times, no more than 6, 5, or 4 time, or within a range defined by any combination of the aforementioned lower and upper limits).

[0031] In relation to the above, conditions can be selected such that deleterious elements are efficiently leached from the flue dust into sulphuric acid while the platinum group metal is retained within the solid residue such that it can be separated and safely processed within the refinery.

[0032] It has been noted that in the steel making industry, dust from electric arc furnaces used to manufacture steel has been found to contain non-ferrous metals such as zinc, lead, cadmium, and chromium and that these non-ferrous metals make the materials hazardous wastes. As such, processes have been developed to treat the steel manufacturing flue dust to generate a non-hazardous residue that can be used as landfill. These processes use a sulphuric acid leach to remove hazardous metals, particularly zinc, and thus produce a non-hazardous waste residue which can be disposed of or used in other applications such as in the cement industry. See, for example: "Electric arc furnace flue dusts: characterization and leaching with sulphuric acid", M Cruells, A Roca, C Nunez, Hydrometallurgy, Volume 31, Issue 3, November 1992, Pages 201-212; and "Hydrometallurgical Treatment of EAF Dust by Direct Sulphuric Acid Leaching at Atmospheric Pressure", V. Montenegro, S. Agatzini-Leonardou, P. Oustadakis & P. Tsakiridis, Waste and Biomass Valorization (2016) 7, 1531-1548. However, while these processes use sulphuric acid to leach flue dust in a steel making process to produce a waste residue for disposal which is non-hazardous, there is no suggestion of the problem of deleterious element build up in a platinum group metal refinery when capturing and recycling flue dust into the PGM refinery circuit or that leaching of the platinum group metal flue dust with sulphuric acid can effectively remove deleterious elements without removing platinum group metal, thereby enabling the treated solid residue to be recycled back into the PGM refinery processes to recover the platinum group metal. As such, there is no suggestion of the present method for refining platinum group metal or its advantages.

[0033] Advantageously, after leaching and separating the solid residue from the leach liquor, the solid residue can be washed to remove residual sulphuric acid prior to recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery. Water can be used for washing of the solid residue. Optionally, the water is heater to a temperature of: at least 40, 50, or 60°C; of no more than 100, 90, or 80°C; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, equipment used to separate the solid residue from the leach liquor can also be washed to remove residual sulphuric acid prior to re-use in separating a subsequent batch of solid residue from leach liquor.

[0034] Once the flue dust has been processed in the aforementioned manner, the remaining solid residue can be safely returned to a smelting process within the refinery.

[0035] In summary, the present specification provides a PGM refining method which includes leaching of deleterious elements (e.g., As, Bi, Te, Se, Zn, Cd, Cu, Cl, Na, Ni,Fe) from furnace flue dusts using sulphuric acid in a stirred reactor (e.g., at a temperature of 20 to 70°C) followed by filtration and recycling of solid residue back into the PGM refining processes. Flue dust from furnace operations containing platinum group metals (PGMs) and mixed base metals (deleterious elements) are loaded into a stirred reactor containing water. The suspension is heated to a required set point and an aliquot of sulphuric acid is added to a concentration that will achieve efficient leaching of deleterious elements and retention of PGMs within the suspended solids. After a defined leaching time the solids are separated from solution via a filtration process with recycling of the leached solid for further PGM processing and disposal of the sulphuric acid leach liquor. Sulphuric acid leaching is more effective and gives a broader range of element removal than a previous washing method using just water, while retaining PGM in the solid residue for further refining.

[0036] In an example of a method according to the present specification, flue dust is transferred into a stirred reactor containing water, the suspension is heated to 50°C, and sulphuric acid added to give a free acid concentration of 75 - 90 g / l. The suspension is leached for up to 1 hour followed by filtration to separate the solids from the sulphuric acid leach liquor. Hot water is used to wash the solids and for removal of residual acid from the filtration equipment. The damp solids are returned to smelting processes for further treatment. Removal of deleterious metals prevents build-up of these elements in the refining circuit, which are otherwise detrimental to the production and efficiency of refining operations.

[0037] Further details and examples of the present methodology are set out in the description of the development work provided below.

[0038] Flue Dust Acid Leaching - Experimental Work

[0039] Overview

[0040] Flue dust acid leaching is a hydrometallurgical process for the removal of deleterious elements, such as arsenic, tellurium, bismuth and selenium, from the platinum group metal refining circuit. Sulphuric acid leaching is a relatively low-cost process. During leaching insoluble components such as lead, silver and the platinum group metals remain within the residue whilst base metals and deleterious elements at least partially deport to the filtrate. This specification details the conditions required to ensure sufficient deleterious element removal whilst minimizing platinum group metal loss, yielding a residue which is easily filterable. The key findings are summarised below:

[0041] • Flue dust leaching was found to be highly acid dependent; a higher free acid concentration at filtration leads to improved deleterious element removal.

[0042] • This provides a paradox since the concentration of sulphuric acid also influences the solubility of Ir, Rh, and Ru. With higher acidity more PGM loss to effluent occurs.

[0043] • An initial 85 g / l free sulphuric acid concentration was found to provide good deleterious element removal, with between 30-70% of the total As, Te, Bi, Se, Cu and Zn deporting to the effluent.

[0044] • PGM losses to effluent per metric tonne of flue dust were low.

[0045] • Slow filtration was found to be caused by two different factors: (i) the precipitation of Bi and Te due to pH changes during a leach; and (ii) the partial solubilisation of silicon, which then reprecipitates as colloidal silica over time.

[0046] • The addition of NP10, a non-ionic polyacrylamide flocculant solution, at a concentration of 15 ppm prior to filtration improves the filtration rate by at least 2.5 times. Higher flocculant dosing increases the filtration rate further.

[0047] • The precipitation of silicon colloids can be avoided by filtering quickly, aided by the addition NP10 and by keeping acidity high.

[0048] • Recycling of the leach liquor can be accomplished three times at a 1:10 solid / liquid ratio.

[0049] • Therefore, four overall cycles of flue dust leaching can be carried out using the same liquor before the ionic strength becomes too high and colloidal precipitation occurs.

[0050] Objectives

[0051] Objectives of this work included the following:

[0052] • Provide conditions to maximise deleterious element removal without undue loss of PGM.

[0053] • Define the cause of the slow filtration of acid leached flue dust slurry.

[0054] • Develop a mitigation strategy.

[0055] Flue dust

[0056] Flue dust is produced when elements which form oxides or those which are present as volatile compounds, are heated in the presence of oxygen. Abatement technologies collect these fine particulate dusts which are then recycled back through the smelting circuit. This practice however leads to a loop and a build-up of unwanted elements which damage refractories, reduce capacity, contaminate melts, and increase overall smelting costs.

[0057] Figures 2a and 2b show the average composition of furnace flue dust (measured using a combination of ICP fusion and XRF analysis). In Figure 2a, base metal content is shown with several particularly deleterious elements specifically targeted by flue dust acid leaching highlighted (Te, Bi, Se, and As). PGM's and Au content is shown in Figure 2b expressed as troy ounces per metric tonne measured using ICP fusion, with osmium measured by XRF. The high value of flue dust is attributed particularly to the platinum and palladium content.

[0058] Leaching

[0059] The addition of sulphuric acid to a slurry of flue dust will selectively remove some base and amphoteric oxides. The percentage removal will be limited not only by the amount of solid present and concentration of free acid but may also be further complicated by kinetic differences and thermodynamic factors, such as the presence of insoluble metal sulphides. The presence of a large amount of free chloride in flue dust slurry (see Figure 2a) is likely to result in elements dissolving as partially chlorinated species.

[0060] Noble metals

[0061] Dissolution of gold, silver, platinum and palladium should not occur in dilute sulphuric acid unless they are very finely divided. The dissolution of ruthenium, iridium and rhodium is expected at trace levels.

[0062] Base metals and deleterious elements

[0063] Copper oxide will solubilize to blue Cu2+species. Copper metal will resist dissolution unless strongly oxidising conditions are employed.

[0064] Selenium dissolves to a colourless F SeOa complex but readily reduces back to selenium metal if oxidising conditions are not maintained.

[0065] Tellurium metal will dissolve to a colourless Te(OH)3+cation, which is in pH dependent equilibrium with white solid TeO2. This results in the precipitation of TeO2if the acidity of the leach decreases. The solubility of tellurium in this system is therefore dependent on the pH.

[0066] Bismuth oxide will solubilize to form Bi3+which readily hydrolyses to BiOH+and BiOH2+species on pH increases. Due to the presence of free chloride, the BiOH+cation reacts to form a white insoluble oxychloride, BiOCI. Bismuth solubility is therefore similar to tellurium in that the pH must be kept low to prevent the formation of precipitates. A red precipitate of bismuth (VI) oxide will form when Bi3+species are exposed to air for long periods of time.

[0067] Arsenic (III) oxide is soluble in water to around 22g / l and yields the colourless H3ASO3 complex in moderately oxidising acidic solution. Oxidation to the arsenate complex is possible by atmospheric oxygen.

[0068] Silica, SiO2, is highly insoluble. Solubility of the amorphous form is 0.16 g / l in pure water. It can only undergo attack by hydrofluoric acid or by strong base to form SiOs2-. Acidification of a solution of SiOs2-yields a hydrated crystalline amorphous solid in either gel or colloid form. The exact nature of the silica will depend on agitation, time, acid addition rate and temperature.

[0069] Experimental

[0070] Leaching

[0071] Leaching of flue dust in the laboratory was carried out at atmospheric pressure. Experimental conditions are summarised below:

[0072] • Overhead mixing at 500 RPM using a 45° angle pitched blade down-flow impeller of diameter 40 mm.

[0073] • Temperature regulation by PTFE thermocouple connected to a hotplate. • 1 litre or 500 ml baffled glass reactor. Vessel was lidded to minimize evaporative losses, but not sealed.

[0074] • Solids were delivered by a funnel port on the lid of the vessel to give a solid / liquid ratio of 1:5 or 1:10.

[0075] • Sulphuric acid; 77% wt. / wt., SG (specific gravity) 1.69, was added above surface, using Marprene™ tubing and a peristaltic pump.

[0076] • Flocculant was added through the same port.

[0077] • Slurry was pumped out of the vessel by a peristaltic pump and filtered as detailed below.

[0078] • Sampling was made either after filtration, or samples were taken using syringe filters with 0.45pm retention.

[0079] The flue dust used in all experiments was from a refinery batch and was leached as received. Elemental analysis of the flue dust is shown below. Complete analysis is from a combination of pukka assays, ICP fusion results, XRF and LECO analysis.

[0080] Settling rate experiments

[0081] Flue dust leach slurry was mixed, aliquots were then transferred to graduated measuring cylinders, a known volume of flocculant or water was then added. The cylinder was stoppered and inverted fifteen times. A stopwatch was then started, and the time taken for the slurry to settle to a volume recorded.

[0082] Filtration

[0083] The equipment used to determine the filtration rates was a Millipore™ filtration unit with a graduated receiver flask. The filter media was filter cloth. The cloth was carefully cut so that it covered the porous frit of the filtration unit. The vacuum was maintained by carefully sealing the unit using Teflon tape and clamping. A pressure reading was recorded using a digital vacuum gauge. Filtration technical data is provided below:

[0084] • Diameter = 50 or 40mm

[0085] • Area = 1963 or 1256 mm2

[0086] • Filter cloth = Polynova™ 95520K

[0087] • Pre coat (if used) = Harborlite™ 900 (lg per 250ml filtrate)

[0088] • Vacuum = 860-950 mBar less than atmospheric pressure.

[0089] The receiver flask and filter unit were weighed empty and after filtration to obtain accurate masses of the filtrates and residues.

[0090] Initial experiments

[0091] Flue dust leach slurry was generated for initial flocculation and filtration screening by a series of leaches at varying acidities. Experimental details are shown in the table below for three different experiments carried out.

[0092] The mass of the filtrate collected at the end of each experiment was then used to calculate the deportments of different elements. The residues were dried, weighed and then assayed by sodium peroxide fusion and XRF spectroscopy. This was to ensure the deportments calculated from ICP analysis were reliable.

[0093] Settling rate experiments

[0094] A variety of different flocculants at different concentrations were added to flue dust leach slurry from Experiment 1 in a series of multiple trials following the procedure detailed previously.

[0095] Filtration rate experiments

[0096] The flocculant which gave the fastest settling during the filtration trials was then used to treat a larger volume of slurry and the filtration rate was measured. The table below summarises filtration rate experiments.

[0097] Washing trials

[0098] The flue dust leach residue from Experiment 3 which had been treated with NP10 prior to filtration was then washed using hot demineralised water (60-80°C) in 100 ml portions, with filtrate collection under vacuum. After each portion of water added, the washings were collected, volume recorded, and elemental composition determined by ICP-OES spectroscopy.

[0099] Recycle experiments

[0100] Calculated acid addition - recycle trial #1

[0101] Experiments were carried out where, after leaching and filtration, the filtrate is recycled to the reactor and another batch of flue dust leached using the filtrate.

[0102] Flue dust was suspended in demineralised water at 500 RPM at 0.1 solid / liquid ratio. 77% w / w sulphuric acid was then charged to the vessel using a peristaltic pump to give an 85 g / l acid concentration. The temperature was then set to 50°C using the temperature controller. The slurry was then allowed to leach for an hour before NP100.5% solution was added. The slurry was pumped to a filter of area 12.56 cm2. The filtrate was then weighed, sampled for ICP, acidity and specific gravity (SG), reweighed and returned to the reactor. The solid residue was weighed and dried.

[0103] A calculated amount of acid (6 ml) was then added to maintain the acidity at 85g / L and the cycle continued until the 3rdcycle, whereby filtration rate slowed considerably. The experimental conditions are summarised in the table below for recycle trial #1.

[0104] Measured acid addition recycle trial #2

[0105] The same experiment was repeated using a smaller charge size, adding in extra water volume to simulate the addition of washings, adding extra acid based on the free acid measurement and adding 25% more flocculant. A summary of the conditions used for each cycle are shown in the table below for recycle trial #2.

[0106] Results

[0107] Initial experiments

[0108] Deleterious elements solubility - Leach profile

[0109] Flue dust was suspended in demineralised water prior to acidification. This was to prevent the flue dust being charged directly to an acidic solution which may increase PGM losses. This also results in the rate of acid addition controlling the rate of dissolution making the process safer.

[0110] The leach profile for an 85 g / l initial sulphuric acid leach is shown in Figure 3 indicating deleterious elements concentration on the left axis and free acidity on the right axis. The first data point at time t=0 corresponds to an aqueous slurry pre acid addition. The second data point at t=50 minutes corresponds to after acid addition. The third data point corresponds to a sample taken at temperature T = 50°C. The 4thdata point at t=134, T=50°C is a sample taken after leaching for an hour has occurred. This data point was used for deportment calculations. The samples at t=2OO, T=25°C and t=300, T=22°C are to show there is minimal precipitation on cooling and flocculation.

[0111] Figure 4 shows a leach profile for an 85 g / l acid leach showing copper, iron, silicon and calcium concentration on the left axis and free acidity on the right axis. Figure 5 shows a leach profile for an 85 g / l acid leach showing aluminium, zinc, sodium and indium concentration on the left axis and free acidity on the right axis.

[0112] The amount of acid charged does not correspond to the measured acidity, due to acidity being partially neutralized as the flue dust leaches. The deleterious elements are leached to the filtrate immediately on acidification and remain in solution for at least 4 hours, providing the acidity remains constant. The leach profile for calcium displays a kinetic factor whereby the calcium is solubilised initially in water, further dissolution occurs on acidification, but slowly re-precipitates out of solution in a linear fashion. It is also apparent that iron leaching requires time and temperature, whereas copper and silica leaching is immediate on acidification.

[0113] There appears to be little difference in the samples taken after acid addition. This confirms the assumption that leaching occurs quickly, is most probably mass transfer controlled, and temperature is irrelevant except for iron which appears to have slower kinetics. The leach was heated to 50°C to simulate conditions on the plant whereby the acid addition creates a significant exotherm due to less efficient heat transfer. The results shown here indicate that heating to 50°C and leaching for an hour provides little benefit in terms of leaching efficiency, but in order to simulate the plant, heating and leaching for an hour is necessary. There was also little change in the concentration of the elements on cooling to room temperature (points 4 and 5) and before and after flocculation (points 5 and 6). Indicating that once leached the elements stay in solution for long enough to allow for a solid liquid separation to take place.

[0114] Correlation between acidity and solubility

[0115] The solubility of elements within flue dust at different acidities was also investigated. Three leaches were carried out as detailed previously, with sampling after 1 hour at 50°C. The free acidity at the end of the leach was markedly different to the calculated initial acidity added.

[0116] Figure 6 shows deleterious element solubility as a function of free acidity measured by ICP-OES spectroscopy. Results shown in Figure 6 illustrate that bismuth and tellurium solubility is limited by free acidity more so than selenium and arsenic. Figure 7 shows platinum group metal solubility as a function of free acidity at filtration measured by ICP-MS spectroscopy. Au and Pt were not detected. With higher free acidity more PGM loss is encountered as show in Figure 7. This paradox means that flue dust leachate is advantageously filtered between 50-70 g / l free acidity to give acceptable deleterious element removal without undue PGM loss. Figure 8 shows concentration of elements vs. free acidity at filtration illustrating that copper (and zinc - not shown) leaching appears to be independent of free acidity, with removal at around 70% regardless of acid concentration. Silicon appears to be to be inversely proportional to acidity. Figure 8 shows that iron solubility was influenced by the free acidity, but this effect is not evident for copper or other base metals.

[0117] These results imply that the acidity of flue dust slurry at filtration affects the solubility of the deleterious elements more so than leach time. Ideally the acidity of the leach liquor would be high enough to solubilize large amounts of the deleterious elements, but this would lead to large ruthenium losses to the effluent. As such, a balance must be struck to leach deleterious elements sufficiently from flue dust to maintain efficient operation of the PGM refining circuit when the flue dust is recycled into the circuit without undue leaching of PGM from the flue dust.

[0118] Precious metal leaching

[0119] The leaching of Ir, Rh and Ru is unavoidable, even under low acid conditions. No trace of Pt or Au was detected in leach liquors under any conditions by ICP-MS spectroscopy. The amount of Ir, Rh, Ru and Pd leached was calculated as a percentage from the pukka assay. Results are shown in the table below. Leaching is expressed as a percentage of the total element content at different acidities with sol id : liquid (S:L) constant at 1:5, T=50°, sample time = 60 minutes after acid addition. Data is from ICP- OES / MS analysis of filtrate.

[0120] The leached residue

[0121] The solid residues from the leaches at 50, 85 and 120 g / l acid, were dried at 105°C overnight. The dry residue was crushed and sent for full scan solid analysis (ICP Fusion and XRF). The analysis of the 85 g / l acid leached residue is shown in Figure 9a and 9b vs. the initial flue dust (first columns = initial flue dust; second columns = after leaching).

[0122] As expected, elements which are not leached by sulphuric acid such as Pb, Ag, Zr and the PGM's increase in relative concentration after leaching. This upgrades values on a dry basis within the residue by around 20% for an acid leach at 85 g / l. The concentration of sulphur within the residue also increases after leaching, possibly as a result of the formation of insoluble metal sulphides / sulphates phase or entrainment within the filter cake.

[0123] Critically, it is possible to reduce the concentration of elements which are deleterious to the PGM refining processes, such as arsenic, tellurium, bismuth and selenium, without significant leaching of PGMs. While a greater reduction could be achieved using more concentrated acid, a balance is struck to reduce elements which are deleterious to the PGM refining process when the residue is recycled back into the refinery without undue loss of PGMs during the leaching process.

[0124] Comparison of leaching efficiency by ICP-OES analysis and solid assay

[0125] A comparison of the leaching efficiencies generated from data collected by ICP analysis of the filtrates vs. analysis of the dry solid leached residues was carried out and is shown in the table below. Data which was collected from solid analysis was normalised with respect to silver to account for mass changes on leaching.

[0126] Although the percentage leached figures show slight variation, the trend that increasing acidity leads to higher deleterious element leach efficiencies is still evident.

[0127] Flocculation of flue dust leach slurry

[0128] Settled volume screening

[0129] The table below shows settled solid volume after 45 minutes after different flocculant additions for 100 ml homogenous slurry aliquots from Experiment 1.

[0130] Settling rate at 20 ppm flocculant

[0131] The rate of settling was then measured. Goldcrest NP10, non-ionic polyacrylamide flocculant, produced the fastest settling solids at a 20 ppm dosing level. The table below shows a summary of settling rates for different flocculants used.

[0132] Filtration of flue dust leach slurry

[0133] Slurry generated in Experiment 2, the initial 85 g / l leach, which had a free acidity after leaching of 52g / l acid, was used to measure filtration rates. The same settling rate experiment as described previously was carried out, with effective flocculant concentration of 15ppm. The table below summarizes the settling rate from Experiment 2.

[0134] From the above data several conclusions were drawn:

[0135] • Settling of Flue dust slurry is faster when the free acidity is higher regardless as to whether or not a flocculant is added.

[0136] • NP10 flocculant improves the settling time by at least 2 % times when added at 15 ppm and by around 7 times when added at 20 ppm.

[0137] • At doses below 10 ppm NP10 has no measured effect.

[0138] Filtration rate experiments were carried out using the experimental detailed previously. No filter aid was used in this filtration trial and the cloth was replaced between each trial. The first pass through of the slurry from Experiment 2 was used to build up a cake on the cloth, the filtrate was then collected, ensuring the cake did not crack and remained moist. It was then passed back through the cake and the rate at which it collected was measured. A summary of the results of the filtration rate experiments is shown in the table below. Figure 10 shows results for filtration with and without flocculant added. Initial filtration rates were excluded as since they produced a rate which was much faster than the bulk of the filtration.

[0139] From the filtration trials it was clear that the flocculated slurry filters at least 2 % times more quickly than when no flocculant is added, and the filtration rate remains linear over the course of the experiment. When no flocculant is added the filtration rate decreases over time.

[0140] White precipitate which forms on water addition

[0141] A white precipitate forms when flue dust slurry or filtrate comes into contact with water. Such a white precipitate was formed by adding 7.5 ml of water to 2.5 ml of flue dust filtrate from Experiment 2. The slurry was then filtered using 0.45 um retention syringe filters. The difference in ICP-OES assay between diluted filtrate and undiluted filtrate was then calculated, since the dilution factor is known, the composition of the precipitate could be deduced as shown in the table below.

[0142] Previous attempts to separate the precipitate in sufficient quantity to analyse by other techniques had proved unsuccessful due to the large volume of sample required.

[0143] Washing of flue dust leach slurry

[0144] Figure 11 shows a graph indicating concentration (mg / l) of elements versus washing volume(ml). Initially low concentrations of elements found in wash liquor were because the cake had cracked and channelling occurred. Once the cake had resettled the filtration rate slowed somewhat but washing was more complete. The optimal wash volume is likely to be less than 20% of the volume of the filtrate. Since washing under vacuum and on a filter press are different, washing volumes can be optimised on plant.

[0145] Recycling leach liguor experiments - Run 1

[0146] This experiment investigated re-use of leach liquor in subsequent flue dust leaching. The experiment was sub-optimal as too little acid was added causing an increase in the amount of dissolved silica which then precipitated halfway through filtration on the 3rdcycle. All filtrations were carried out using the same apparatus which had an area of 12.56 cm2. The table below shows a summary of conditions used for the different cycles.

[0147] The percentage leached was calculated from measuring the concentration of elements within the filtrate at each stage, normalising for mass, then subtracting the mass of elements deported to the filtrate from each previous leach cycle.

[0148] Figure 12 shows the percentage of elements leached at each cycle. From Figure 12 it is apparent that the leaching of Bi and Te are highly dependent on free acidity. As the acidity dropped between cycles, a lower percentage of these elements leached. The amount of As, Se, Cu, Zn and Fe leached also appears dependent on free acidity. Ca leaching displays some complex behaviour, whereby instead of leaching, precipitation out of solution occurs on the 2ndand third cycle.

[0149] ICP analysis of the filtrate which filtered and the slurry which did not filter, confirmed the only notable differences were in calcium and silicon concentrations as shown in the table below.

[0150] The dissolved silica concentration increased halfway through filtration. Cooling between the reactor and filtration unit occurred and this may explain the drop in the calcium concentration, since calcium sulphate is only marginally soluble. The slurry from the 3rdcycle was re acidified and re treated with NP10 at 60 ppm to allow for its filtration.

[0151] Recycling leach liquor experiments - Run 2

[0152] The flue dust leach recycling experiment was carried out on a smaller scale such that smaller volumes of flue dust slurry could be filtered through the same filter more quickly. This meant that the filtrate could be returned to the reactor and pumped back to the filter so that more solids could be removed from the reactor. This is a closer approximation as to what happens on the plant whereby the filter press is set to recycle to the tank. The addition of water is to simulate the addition of washings to the filtrate and to increase the volume back to 250 ml between cycles to make the results comparable since volume remains constant. At a small scale the volume of filtrate lost to the cake is large, approximately 10%. This is because once filtration is complete the cake cracks, meaning drying by vacuum becomes less effective. In addition, only the filtrate was sampled to prevent large losses of volume due to multiple samples being taken. The table below shows a summary of conditions used during the washing cycles and filtration rates achieved.

[0153] Figure 13 shows the percentage of base metal / deleterious elements leached between 1-4 cycles from ICP OES analysis of the filtrates. The percentage of base metals leached is in line with expectations of between 30-70% leach efficiency. The 5thleach is excluded since the filtrate did not filter.

[0154] Figure 14 shows the concentration of leached PGM in the filtrate after 1, 2, 3 and 4 cycles from ICPMS analysis of the filtrates. Figure 15 shows the percentage of PGM leached after 1, 2, 3 and 4 cycles from ICPMS analysis of the filtrates The amount of precious metal leached decreases on increasing the number of cycles.

[0155] Residue results

[0156] The table below shows percentage composition calculated from filtrate ICP OES analysis and residue analysis after each cycle. Solid results are normalised on silver.

[0157] A good correlation between deportments calculated from the filtrate and solid exists across the first three cycles. The fourth cycle however does not share the same relationship - deleterious element removal is far lower than expected by analysis of the solid residue, especially Bismuth and tellurium. This could be due to sampling of the residue or analytical error. Conclusions

[0158] The following conclusions can be drawn from this work:

[0159] • Filtration of flue dust leach slurry is improved by filtering at high acidity, this is to maintain the solubility of tellurium and bismuth.

[0160] • The deportment of deleterious elements to the filtrate is increased by leaching initially at 85g / l free sulphuric acid which gave at least a 30% removal of As, Bi, Te and Se.

[0161] • PGM losses are restricted to Ru, Ir and Rh.

[0162] • The value of PGM loss per metric tonne of flue dust is limited using this approach.

[0163] • Adding NP10 flocculant improves the settling time and filtration rate of flue dust leach slurry by at least 2% times at 15 ppm flocculant and by a factor of 6 at 20 ppm.

[0164] • The solubility of silica in flue dust leach slurry is thermodynamically unstable and precipitation of silica colloids over time results. This limits the number of cycles which the liquor can be recycled (advantageously no more than three).

[0165] • 4 metric tonnes of flue dust per 10,000 litres is the maximum amount of flue dust that can be processed, without silicon colloid precipitation and subsequent filtration problems occurring.

[0166] A plant method for leaching flue dust in dilute sulphuric acid based on the conditions which gave good leach efficiency and filterability in the laboratory has been developed. The process can remove between 30-70% As, Bi, Te, Se, Zn, Cd, and Cu from flue dust by sulphuric acid leaching with minimal losses of PGMs. The method processes the flue dust to provide a dry filter press cake product for recycling to smelting.

[0167] 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

Claims1. A method of refining platinum group metal containing material in a platinum group metal refinery, the method comprising: placing the platinum group metal containing material in a furnace having a flue; thermally processing the platinum group metal containing material in the furnace; capturing flue dust passing through the flue of the furnace during thermal processing of the platinum group metal containing material, said flue dust comprising platinum group metal and base metal; treating the captured flue dust in sulphuric acid to leach the base metal into solution producing a sulphuric acid leach liquor comprising the base metal and a solid residue comprising the platinum group metal; separating the solid residue comprising the platinum group metal from the leach liquor comprising the base metal; and recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery to recover the platinum group metal.

2. A method according to claim 1, wherein the base metal which is leached from the flue dust includes one or more of As, Bi, Te, Se, Zn, Cd, Cu, Na, Ni, and Fe.

3. A method according to claim 1 or 2, wherein one or more non-metal elements in the flue dust are also leached from the flue dust into the sulphuric acid leach liquor, optionally Cl.

4. A method according to any preceding claim, wherein the thermal processing of the platinum group metal containing material in the furnace comprises smelting of the platinum group metal containing material.

5. A method according to any preceding claim, wherein the flue dust is captured by a filter within the flue duct.

6. A method according to any preceding claim,wherein the flue dust is treated in the sulphuric acid at a temperature: of at least 20°C, 30°C, 40°C, or 50°C; of no more than 80°C, 70°C, 60°C, or 50°C; or within a range defined by any combination of the aforementioned lower and upper limits.

7. A method according to any preceding claim, wherein the sulphuric acid is added to the flue dust to give a free acid concentration: of at least 50 gl1, 60 gl1, 70 gl1, or 75 gl1; of no more than 120 gl1, 110 gl1, 100 gl1, or 90 gl1; or within a range defined by any combination of the aforementioned lower and upper limits.

8. A method according to any preceding claim, wherein the flue dust is treated in the sulphuric acid for a time period: of at least 10 minutes, 20 minutes, 40 minutes, or 60 minutes; of no more than 5 hours, 3 hours, 2 hours, or 1 hour; or within a range defined by any combination of the aforementioned lower and upper limits.

9. A method according to any preceding claim, wherein, during treatment of the flue dust in the sulphuric acid, the sulphuric acid is stirred with the flue dust forming a suspension in the sulphuric acid.

10. A method according to any preceding claim, wherein a flocculant is provided in the mixture of flue dust and sulphuric acid to aid separation of the solid residue comprising the platinum group metal from the leach liquor comprising the base metal.

11. A method according to any preceding claim, wherein the step of separating the solid residue from the leach liquor comprises filtering of the solid residue from the leach liquor.

12. A method according to any preceding claim, wherein after the step of separating the solid residue from the leach liquor, the leach liquor is recycled and re-used one or more times for further leaching of base metal from flue dust.

13. A method according to any preceding claim, wherein after the step of separating the solid residue from the leach liquor, the solid residue is washed, optionally with water, to remove residual sulphuric acid prior to recycling the solid residue comprising the platinum group metal for further processing within the platinum group metal refinery.

14. A method according to any preceding claim, wherein the step of recycling the solid residue for further processing within the platinum group metal refinery to recover the platinum group metal comprises returning the solid residue to a smelting process.

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