GB2067599A - Recovery of Pt group metals - Google Patents

Abstract

In a process for recovery of platinum group metals (PGMs) from refractory ceramic substrates containing an aluminium silicate and/or alumina, for example, wash- coated auto emission control catalysts and Pt reforming catalysts, a charge, in divided form, containing the substrate, one or more fluxes and a collector material is heated in a high heat intensity furnace preferably of the plasma arc type, to a temperature of at least 1420 DEG C to produce a molten metallic phase including a substantial proportion of the said metals and a molten slag phase. These are then separated and the platinum group metals subsequently extracted from the metallic phase. The preferred fluxes are selected from CaO, CaF2, BaO, Fe2O3, MgO, SiO2 and TiO2, and the preferred collector material is selected from Fe, Ni, Cu, Co, Pb, Zn and Al or mixtures thereof.

Description

SPECIFICATION A process for the recovery of platinum group metals from refractory ceramic substrates This invention relates to the recovery of platinum group metals which may be present in artefacts made from refractory materials, that is to say, to the secondary refining of such metals as opposed to primary refining -- from the ore.

The high melting point of refractory based substrates which contains platinum group metals (PGM), in particular alumina substrates, presents a severe slag problem when attempts are made using conventional pyrometallurgical processes.

Blast and reverberating furnaces normally operate at temperatures in the range 1250-1 3500C which is considerably below the melting point of alumina. At these temperatures, therefore, it is necessary to include in in the charge, slags such as Wollastonite or Olivine in order to dissolve the alumina but only 15% alumina can be dissolved without detriment to the melting point and viscosity.

Many refractory ceramic substrates, however, have an alumina content much higher than 1 5%.

Automobile emission control catalysts with aluminium-silicate (e.g. cordierite and mullite) substrates contain 35% alumina and up to 46% when wash coated. To apply conventional pyrornetaliurgical process to such catalysts therefore necessitates the use of sufficient flux to maintain an upper limit of 15% Awl203 but this is economically non-viable, not least because the loss of PGM in the increased quantities of slag is unacceptable.

What we now propose, in accordance with the present invention, is a process for the recovery of platinum group metals deposited on or contained in a refractory ceramic substrate containing an aluminium silicate and/or alumina, comprising preparing, in divided form, a charge containing the refractory substrate bearing the said metals, one or more fluxes, and a collector material or collector material precursor, for the metal or metals to be recovered, heating the charge to a temperature of at least 14200C to produce a molten metallic phase containing a substantial proportion of the said metal or metals, and a molten slag phase containing flux, ceramic residues and the remainder of the said metals, separating the two phases, and separating the platinum group metals from the metallic phase.

The flux or fluxes are preferably selected from the groups consisting of CaO, CaF2, BaO, Fe203, MgO, SiO2 and TiO2.

By heating a charge containing cordierite to at least 14200 (i.e. above the melting point of the cordierite), the amount of slag needed to dissolve the alumina can be reduced. In the case of substrates having an alumina wash coat or which consists of alumina, (e.g. a Pt reforming catalyst) at least some of the alumina, which is a precursor for aluminium-silicate, can be converted to an aluminium-silicate and so can become molten at the operating temperature, by adding to the charge, a SiO2 and/or a MgO, flux.

With an alumina substrate acceptable recovery is possible without the addition of SiO2 and/or MgO, but only by including in the charge a flux content approximately equal in weight, to the alumina content.

If the substrate contains another aluminium silicate it may be necessary to operate at a higher temperature. Examples of other aluminium silicates are mullite, sillimanite, petalite, spodumine and andalusite.

In addition, however, the use of operating temperatures higher than heretofore applied in secondary refining, enables the use of the high temperature fluxes specified whereby the alumina present, even where the substrate is wash coated, can be dissolved without excessive quantities of flux.

The melting point of aluminium-silicates such as mullite (aluminium-silicate) or cordierite (magnesium-aluminium silicate) is in the region of 1 4200C but with the addition of an alumina washcoat the melting point of the substrate is increased to about 1 6500 C. In order to produce a fluid slag with a low viscosity and hence optimise recovery, the operating temperature needs to be about 1000C above the melting point of the substrate and we, therefore, prefer operating temperatures in the range 1 500-1 7500C. Such temperatures can be achieved using high heat intensity furnaces, for example, submerged electric arc furnaces and plasma-arc furnaces, which latter furnaces are known for primary refining but not for secondary refining.

Because submerged electric arc furnaces produce undesirable agitation of the charge, plasma arc furnaces are to be preferred and we have tested a variety of different types inter alia furnaces incorporating expanded plasma systems, furnaces having a static gun, extended arc furnaces including a transferred plasma arc furnace. Only furnaces adapted for batch operation have been tried but a continuous operation furnace with provision for continuous removal of the slag and/or metallic phases could be used. The process of this invention is operable with all types of high heat intensity furnaces tested though with varying degrees of success. It is considered, however, that the differences in recovery obtained were related primarily to the charge formulation and only to a limited extent upon the type of furnace used.

Suitable gases for the plasma are argon, helium, or nitrogen and we have found that it is even possible under some circumstances to use air, which is considerably cheaper than the alternatives. With air, however, there is a tendency to oxidise iron with attendant loss of iron into the slag.

The divided charge is fed gradually into the furnace through the plasma arc and, in batch processes, it is desirable to continue the discharge of the plasma arc for a minimum holding period after passing the entire charge therethrough, the minimum holding period preferably being from 5 to 30 mins.

Automobile emission control catalysts may be in one of two basic forms namely a monolithic structure or in pelletized form. The charge is prepared by mixing in suitable proportions the catalyst material reduced to a finely divided form by crushing or otherwise or, depending upon the pellet size, in the form of pellets, with the flux or fluxes and collector material or materials. If desired a monolithic catalyst or large pellets may be reduced by (e.g.) crushing and mixed with the selected flux(es) and collector material(s), the mixture then being compacted to produce pellets. A binder material preferably in an amount of 2% by wt of the mixture, can be used to ensure adequate green strength.

The grain size of the charge is selected inter alia to ensure intimate contact between the catalyst material, the fluxes and the collector and to avoid undue losses by entrainment in the gas flow through the plasma furnace.

It is preferred to reduce the catalyst to the range minus 10-minus 200 mesh but in trials we have conducted, the best results have been obtained at minus 8 mesh (2.8 mm).

Satisfactory results have, however, been obtained using raw autocat pellets measuring 3 mm x 6 mm and pellets compacted from a finely divided mixture of the refractory substrate, flux and collector, having a diameter of 8t/ As for the collector material, this is preferably present in an amount of 210% by weight (of the refractory material) and in the preferred embodiment, iron is used either in the form of iron powder or filings or cast iron shavings. Alternatively, the iron can be produced in situ by the addition of an oxide or iron, such as Hematite and a reducing agent such as carbon, to the charge. Other collector materials may be used, e.g. copper, nickel, cobalt, lead, aluminium or mixtures thereof.

The choice of flux or fluxes, which may be present in an amount of up to 100% by weight (of the refractory material content), depends to a large extent upon the particular refractory material from which the PGM's are to be recovered. As stated above MgO and/or SiO2 are useful to convert alumina to aluminium silicate (e.g. cordierite). We have found that CaO is particularly efficient in producing good recovery and is a preferred component of the flux. Another preferred flux component is CaF2 which has a high solubility in the refractory oxide, alumina.

The operating temperature of the plasma should be kept to the minimum consistent with the production of a low viscosity slag and satisfactory recovery of platinum group metal. Both Fe203 and CaO are beneficial in lowering the melting point and viscosity of the slag.

Separation of the slag and metallic phases, and separation of the platinum group metals from the metallic phase, after cooling thereof, may be effected by any suitable method known to those skilled in the art.

EXAMPLE 1 TO 10 Preparation of Charge Automobile emission control catalysts hereinafter referred to as "autocat" consisting of Pt, Rh, and NiO deposited upon an Awl203 washcoated cordierite ceramic refractory were jaw crushed to minus 8 mesh (2.8 mm). An analysis of the "as received material and the crushed catalyst" showed very good agreement, which suggests that any fines that were lost during the crushing operation did not contain a significantly higher proportion of platinum group metals. Samples of the crushed catalyst from three of the trials were analysed, the results were as follows: % by weight Pt Rh Ni Trial 6 0.21 0.026 0.67 Trial 9 0.21 0.025 0.55 Trial 10 0.19 0.023 0.58 The relevant figures were added as powders from standard laboratory reagents, the lime was added as Ca(OH)2. The iron collector was added as iron sponge or as gray cast iron shavings with a particle size similar to the crushed catalyst. The whole charge was handmixed and fed into a hopper.

Plasma Because of the smail scale of the operation a static plasma arc furnace was used and melting was carried out either in salamander, suprex or graphite crucibles.

In the initial trial the charge from the hopper was screw fed into the crucible via three plastic tubes. The tubes were kept cool by the passage of argon. In order to maintain the plasma a flow of argon is passed through the water cooled plasma gun. With this relatively high gas velocity, it is possible that any fines in the charge might be blown through the system. In order to minimise such losses all subsequent trials used a single feed tube, thus reducing the argon throughput.

A known weight of charge was put into the hopper for each trial. A total of ten trials detailed in the following examples were carried out nine with a nominal charge of 5 kg catalyst with the last trial at the 10 kg scale. The feed rate to the plasma was 0.5 kg minus in all the trials. When all the charge had melted, a fluid melt was maintained for a minimum holding time. After the requisite holding period the power was switched off, the refractory insulating box removed and the crucible and contents withdrawn. The products were physically removed from the crucible in each trial. The slag was broken into pieces with a hammer and any visible metal prills removed. The balance of the slag was crushed and spiit riffled to give an assay sample.The brittle metallic collector button was broken and tema milled to produce a representative sample for assay. The platinum group metal recoveries were calculated on the weight and assays of the melted products.

No attempts were made to collect the fume from the trials, and the exit gases were allowed to burn and escape to the atmosphere. The exact loss due to fume was not established; however, a sample of the fume that had condensed in the exhaust tube over several runs and could not, therefore, be associated with any particular trial was analysed. The results showed that it contained 0.05% Pt and 0.008% Rh.

The results achieved in the Examples 1 to 10 are set out in Table 1. It will be seen that direct melting of the autocat at 1 700CC with 5% Fe gave poor coalescence of the collector and resulted in numerous prills in the slag. The addition of 30% MgO+SiO2 to convert the Awl203 washcoat to cordierite reduced the operating temperature to 1550PC and gave recoveries of 93.8% Pt and 98.9% Rh.

Of the fluxes used CaO gave the least amount of fume, an operating temperature similar to cordierite, and good coalescence of the collector with recoveries of 94.3% Pt and 98.5% Rh.

TABLE 1

Holding % Recovery Wt. % Flux Temp Time Visual Ex. Charge Flux Added Collector C* (Min) Examination Pt Rh Remarks 1 5 kg Autocat - 5% cast iron 1700 15 Poor separation, - - Graphite crucible crushed to minus shavings large no. prills 8 mesh (2.8 mm) 2 As Ex. 1 + MgO. 30 5% Fe powder 1540 None Numerous prills - SiO2 to convert Al2O3 washcoat to cordierite 3 As Ex. 2 30 Ditto 1540 15 Good separation, 93.8 98.9 minimal prills 4 As Ex. 2 30 2% cast iron 1550 30 Ditto 91.7 98 Effect of reducing iron shavings collector to 2% and increasing holding period 5 As Ex. 2 30 5% cast iron Variable 30 Several large prills, - - Several operational shavings poor coalescence problems 6 As Ex. 2 + CaO 30% MgOSiO2 Ditto 1470 15 Clean separtion 83.9 94.5 Poor mass balance (probably 10% CaO almost complete due to lock up/contamination coalescence in charge mechanism) 7 As Ex. 1 + Fe2O3 10 Ditto 1560 15 Fair separation, - - Large amount of fume, All + Fe powder 3.3 large amount of Fe2O3 added appears to have iron collector been reduced by Fe to FeO present 8 As Ex. 1 + CaO 10 Ditto 1550 15 Good separation a) 94.3 98.5 b) 96.1 Slag treated on low intensity mangetic separator 9 As Ex. 1 + CaF2 10 Ditto 1550 15 Good separation 91.9 98.1 Lot of fume evolved 10 As Ex. 1 + CaO 10 2% cast iron 1550 15 Good separation 86.8 96.3 Crucible leaked near end of (10 kg charge) trial resulting in some loss slag.

*Optical pyrometer.

The slag from Example 8 (autoemission control catalyst + 10% CaO flux) was ground to 80% minus 100 mesh, mixed with water to form a slurry containing 1 5% solids and passed through a low intensity (1200 gauss) wet drum magnetic separator. A magnetic concentrate totalling 2% of the input material was obtained. The assays of the products were as follows: Pt % Rh % Fe Input slag 0.01 0.0003 Magnetic concentrate 0.187 6.58 Discard slag 0.0085 - 0.37 Recalculated head 0.012 The platinum recovery from the slag after magnetic scavenging was 31%.

Changes in rhodium concentrate at these low levels was not taken into account. The overall platinum recovery from the catalyst after magnetic scavenging was increased from 94.3% to 96.19/0.

Because of the low concentration of platinum group metals in the magnetic concentrate, it would probably be returned with the feed to the plasma furnace.

EXAMPLES 11 TO 15 A summary af these examples, which were conducted using the same furnace as for Examples 1 to 10 but with a Pt/Pd containing monolith autocat, is set out in Table 2.

As before the monolith autocat was ground or crushed to minus 8 mesh but the autocat and Pt reforming catalyst pellets were mixed with the appropriate fluxes and iron collector, as received. The CaO was added as lime (Ca(OH)2) and CaF2 and MgO were commercially available powders.

The recovery of platinum in Examples 11 to 14 was very similar to platinum recovery in Examples 1 to 10. The recovery of palladium on the other hand is slightly lower than the recovery of rhodium (i.e.

96.6% as compared to 98.5%).

Recoveries of 95.3% Pt and 96.6% Pd were obtained when cordierite based autocatalyst monoliths were smelted at approximately 1 5000C in a static expanded plasma arc furnace with 10% CaO flux additions and an iron collector. Increasing the weight of iron in the charge from 5% to 10% reduced the residual PGMs in the slag from 0.013% to 0.007%. The dust collected accounted for 1.8% of the charge and represented 0.7% of the platinum and 1.5% of the palladium in the input material.

In order to achieve a comparable smelting temperature with pure alumina substrates the amount of fluxes required constitutes 50% of the charge. Although the level of PGMs in the Al2O3-CaO-MgO- CaF2 slag were of the same order (0.009%) as the monolith trials, the increase in weight of the fluxes resulted in a PGM recovery of only 60% with 4 wt% iron collector. Increasing the latter to 10% and recirculating the Fe-PGM, in order to achieve a reasonable concentration of PGMs in the bullion, should improve the recovery. The results of the initial smelting trials with platinum reforming catalyst using a CaO-MgO flux addition and 2.5% iron collector showed a recovery of 95% Pt and produced a bullion containing 16.78% Pt.

TABLE 2

Feed Holding % Recovery Flux Rate Temp Time Visual Ex. Charge (wt %) Collector kg/min C (Min) - Examination Pt Rh Rd Remarks 11 5 kg Autocat 10 5% cast iron 1 1500 15 Good coalescence, - - - Suprex crucible crushed to minus turnings separation from 8 mesh + CaO slag easy 12 As Ex. 11 10 10% Fe 1 1510 15 Ditto 95.3 96.6 Collector Fe-PGM from Ex. 11 + 5% virgin Fe.

Superstar crucible 13 4.9 kg autocatalyst 100 2.5 cast iron 1 1420 15 Large prills Salamander crucible Alumina pellets turnings trapped in slag.

(3 mm x 6 mm) + - - CaO poor coalescence Probably slag too viscous = 25% loss in wt of collector 14 2.5 kg autocatalyst 100 5% Fe 0.7 1475 15 a number of prills 60 62 Salamander crucible pellets in base of slag 1.5 kg CaO 0.5 kg MgO 0.5 kg CaF2 15 5.0 kg Pt reforming 100 2.5% cast iron 0.5 1500 15 good coalescence 95 - - Supres crucible catalyst 1.7 mm dia turnings only two large pellets prills 3.9 kg CaO 1.0 mgO EXAMPLES 16 TO 20 These Examples relate to trials conducted in an extended arc furnace supplied by University of Toronto, Canada, using Pt/Pd containing refractory substrates, the substrate in each case being ground to minus 60 mesh.

The best recoveries of 74% Pt and 70% Pd were achieved with only 2.6% Fe collector and a slag composition similar to that used in the expanded plasma arc furnace. The residual PGM content of the siag was 0.006. Full results are set out in Table 3.

EXAMPLE 21 This example relates to a trial conducted in a furnace supplied by Technology Application Services Corporation, of North Carolina, U.S.A. The trial was conducted using a pelletised charge including a crushed cordierite monolith, flux and collector. The result is set out at the foot of Table 3.

TABLE 3

Feed Holding % Recovery Flux Rate Temp Time Visual Ex. Charge (wt %) Collector kg/min C (Min) Examination Pt Rh Rd Remarks 16 1 kg Autocatalyst 10 10% iron 0.25 1600 15 good coalescence + CaO powder but a number of prills 17 500g Autocatalyst 100 10% 0.25 1600 5 good coalescence 76 53 Considerable loss of pellets fewer prills than collector and gain in 250 g CaO in Ex. 16 weight of slag 250 g SiO2 18 500 g Autocatalyst 100 10% iron 0.25 1600 5 As Ex. 17 pellets shot 300 g CaO 100 g CaF 100 g MgO 19 1.0 kg monolith 10 20% iron 0.25 1600 8 good coalescence autocatalyst shot few prills 100 g CaO 65 62.5 20 500 g Autocatalyst 100 10% iron 0.25 1600 8 53 35 pellets shot 400 g CaO 100 g MgO 21 Pellitised 10 10% Fe 1 1500 15 97 97 Charge autocat monolith 13.67 kg + CaO + 2% binder

Claims (21)

1. A process for the recovery of platinum group metals deposited on or contained in a refractory ceramic substrate containing an aluminium-silicate and/or alumina comprising preparing, in divided form, a charge containing the refractory ceramic substrate bearing the said metals, one or more fluxes, and a collector material or collector material precursor, for the metal or metals to be recovered, heating the charge to a temperature of at least 1 4200C to produce a molten metallic phase containing a substantial proportion of the said metal or metals formerly deposited on or contained in the substrate, and a molten slag phase containing flux, ceramic residues and the remainder of the said metals, separating the two phases, and separating the platinum group metals from the metallic phase.

2. A process according to claim 1, wherein the flux or fluxes are selected from the group consisting of CaO, CaF2, BaO, Fe2O3, MgO, SiO2 and TiO2.

3. A process according to claim 1 or claim 2 wherein the charge is heated to a temperature in the range 1 500-1 7500C.

4. A process according to any one of the preceding claims wherein the charge is heated in a high intensity heating furnace.

5. A process according to claim 4 wherein the said furnace is a plasma arc furnace.

6. A process according to claim 5 wherein the gas for the plasma arc furnace is selected from the group consisting of Argon, Helium, Nitrogen and Air.

7. A process according to claim 2 wherein the flux content of the charge is up to 100% by wt of the refractory ceramic substrate content of the charge.

8. A process according to claim 7 wherein the flux content of the charge is 10% by wt of the refractory ceramic substrate content of the charge.

9. A process according to any one of the preceding claims wherein the collector material content of the charge is 2 to 10% of the refractory substrate content of the charge.

1 0. A process according to any one of the preceding claims wherein the collector material is selected from the group iron, nickel, copper, cobalt, lead, zinc and aluminium, or mixtures thereof.

11. A process according to claim 10 wherein the collector material comprises iron powder or filings, iron sponge or cast iron shavings.

12. A process according to any one of claims 1 to 11 wherein the refractory ceramic substrate is reduced to finely divided form by crushing or the like.

1 3. A process according to claim 12 wherein the finely divided substrate has a mesh size in the range minus 10 to minus 200 mesh.

14. A process according to claim 1 2 wherein the substrate in finely divided form is mixed with the flux or fluxes and collector material, and the charge so produced is compacted to form charge pellets.

1 5. A process according to claim 1 4 wherein 2% by wt of a binder material is added to the finely divided charge.

1 6. A process according to any one of claims 1 to 11 wherein the substrate is in the form of autoemission control catalyst pellets.

1 7. A process according to any one of the preceding claims wherein the substrate is an alumina wash-coated aluminium silicate.

1 8. A process according to claim 1 7 wherein the flux comprises MgO and/or SiO2 whereby during heating at least some of the alumina wash-coat is converted to an aluminium silicate.

1 9. A process according to claim 17 or claim 18 wherein the aluminium silicate is cordierite.

20. A process according to claim 1 wherein the substrate consists of alumina and the flux content of the charge is approximately equal in weight to the alumina content.

21. A process according to claim 5 and which is a batch process, wherein the divided charge is fed into the furnace through the plasma arc and wherein discharge of the plasma arc is continued for a minimum holding period after passing the entire charge therethrough.