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atomic-absorption method for the determination of platinum, palladium and gold in ores and concentrates: A modification of the tin-collection scheme
A RAPID FIRE-ASSAY/ATOMI~-A~SOR~ION METHOD FOR THE DETERMINATION OF PLATINUM, PALLADIUM AND GOLD IN ORES AND CONCENTRATES: A MODIFICATION OF THE TIN-COLLECTION SCHEME* F. E, MO~OU~~Y
and G. H. FAYE
Mine& Sciences Laboratories, Canada Centre for Mineral and Energy Technology, Mines and Resuur~es+ Ottawa, Canada
Department of Energy,
tin-collection scheme of fire-assaying has been simplified to permit the rapid and accurate determination of platinum, palladium and gold in ores and related materials. The presence of tellurium in the charge ensures that the precious metals remain insoluble during the parting of the tin button with hydrochloric acid. The residue is easily collected and dissolved and the resultant solution analysed for the precious metals by AAS. The accuracy of the method has been established by application to five diverse certified reference materials.
Summary-The
The tin-dlection scheme I4 forthe dete~nat~on of the precious metals in ores and concentrates has been rtsed in our iaboratories for ap~rox~ate~y 10 years. Its accuracy, reliability and versatility have been wetl proved in interlaboratory studies.5-7 When this scheme is applied to the determination of gold, silver, and the platinum-group metals. solutions derived from tin assay buttons are treated by rather tedious ion-exchange and extraction processes and the analysis is finished either spectrophotometri-
tally or by atomic-absorption spectrophotometry (AAS). However, it is often necessary to determine only platinum, palladium and possibly gold. and a simplified procedure would be desirable. The basis For such a method was recognized severaf years ago when it was established that the intermetaliic compounds formed by platinum, palladium, and gold with tin are essentially insoluble in the hydrochloric acid used to part the tin-assay button.‘-’ However, it was also observed that copper and nickel tend to increase the solubility of the precious metals during the parting operation. Tellurium has been used to precipitate platinum, palladium, rhodium and iridium by reduction’ and it is rknown to form intermetallic compounds such as PtTe,, PdTe,.’ Tellurium was therefore added to the fire-assay charge to assess it as a “carrier” or “tixing agent” for platinum, p~~adium and gold. The results showed that an easily coflected insoluble residue containing the three precious metals is formed quantitatively during the decomposition of the tinassay button in hydrochloric acid, This forms the basis of the proposed method, which is rapid, accurate, and widely applicable. * Crown Copyrights reserved
Furnaces. A lS-kW GIobar type with suitable tbermocouple and temperature controller, capable of accommodating 6 40-g assay crucibles and maintainmg their temperature at 1250”. A Jetrus “Handy-Melt” portable electric furnace. (J&us Technical Products Corp. New Hyde Park. N.Y.) or slmrlar. The Jelrus is a small vertical furnace equipped with removable graphite crucibles, used m this work for melting tin-base assay buttons before thalr granulation in water. It is recommended that after 4 6 months of relatively constant use the bottom of the crucibles be examined for small holes.
Tin (IV) oxide.B.D.R.
It has consistently 200 ngjg.
reagent-grade is preferred because given a goId btank value of 1%
~~~~~~~rnpowder. Reagent grade. Standard soturions of piuf~~~~ p~ll~~~rn arrd yoid. Pre-
pared by dxssolving accurately weighed quantities of Johnson Matthey “Specpure” sponge in aqua regia. Each solution is evaporated to dryness, then the residues are dissolved m concentrated hydrochloric acid and the solution evaporated to dryness again. this being repeated several times. Finally, the salts are dissolved in. and diluted to volume with, 1M hydrochloric acid. The gold solution is standardtzed gravimetrically by the classical fire-assay procedure with lead, and the platinum and palladium soiutions are standardized s~trophotometrical~y. Mixed ~ffdrni~o~~~ s~~h~te sofurion.” Prepared by dissolvrng 98 g ofCu%&SH& and 57 g of 3CdSZf,.8H28!? in 500 ml of i2M hvdr~hIor~~ acid and 300 ml of water. lotlowed by d&&on to I Ike with water Fiux ,far jire-assul;. SnU2 30 g R&Q3 50 gp Na,B.& 10 g flour 35 g. Te 25 mg, silica IO-20 g accordmg to the amount of silica in the sample, make enough flux for a sample up to 1 assay-ton (29.17 g) in size. Proccdurrs
Although aspects of certain of the following procedures have been described previously,’ they are repeated here for completeness and for the convenience of the reader. 377
378
P. E. MOLOUGHNEY and G. H. FAKE
Pretreatment of samples Roasting. Before the crucible fusion procedure, all samples except those of copper-nickel matte are roasted at 75NKlO” for approximately 1 hr to decompose sulphides and volatilize arsenic and antimony. The sample is placed on a shallow fire-clay dish and stirred intermittently during the roastmg process. In cases where only a few grams of material (particularly sulphides) are to be roasted, the sample IS placed on a bed of silica to prevent possible loss of the resultant calcine to the surface of the dish (the silica is included as part of that required in the flux above). Leaching of copper-nickel matte. Leaching is performed to remove the bulk of the copper and nickel from the residue of precious metals. The sample, weighing up to 2 assay-tons, is placed in a 1500-ml beaker and treated with 25 g of ammonium chloride and 10&200 ml of 12M hydrochloric acid. The sample is heated until the amount of insoluble matter appears not to exceed 2-3 g. With large samples, it may be necessary to treat the residue once or twice more with fresh acid after intervening filtrations. The combined sample solution (approximately 100 ml) 1s diluted with an equal volume of water and the solution containing most of the nickel and copper is filtered through a moderately fast paper. The solids are completely washed onto the paper with dilute (- 5”“) hydrochloric acid. The washed residue and paper are dried at - 110 for about 1 hr and then mixed with the recommended assay flux for fusion. Chromite. Chromite is not completely decomposed during the fusion process, and samples containing an appreciable proportion of it must be subjected to a pretreatment that will decompose It. This can be done by sintering with sodium peroxide. The sample is mixed with 1.5 times its weight of sodium peroxide, then placed on a 10-g bed of silica in a roasting dish and roasted at 700” for about 1 hr. The sinter cake and underlying silica are ground together in a mortar and mixed with the flux for the crucible fusion process. The weights of sodium peroxide and silica are subtracted from the weights of sodium carbonate and silica, respectively. in the flux described above. Procedure for preparation of huttons Powdered samples. For powdered samples, the standard assay practice of blending the samples with the flux on glazed paper and transferring the charge to a “40-gram” crucible is followed. Soltltions (synthetic samples for method development). When solutions are to be mixed with the flux, approximately one-third of the flux is placed m the crucible and a 30-cm square of thin, commercial wrapping-film 1s pressed into the crucible to form an envelope, and then the remainder of the flux is transferred into this envelope. With a spatula, a cavity IS formed in the bed of flux and the sample solution is transferred slowly into the depression so as to avoid wetting the film or crucible walls. The crucible is then heated m a drying oven at 110” for at least 2 hr. After drying, the material in the wrapping film is ground in a mortar, mixed well, and placed back m the film in the crucible. It is to be noted that, after drying. the salted portion of the charge is lumpy and difficult to pulverize and mix with the rest of the charge. This could lead to occasional spurious results (Table 2). Fusion. The crucible 1s placed in the assay furnace at 1250’ for about 90 mm to fuse the charge. At the completion of the fusion period, the melt should not be viscous or lumpy nor should there be extensive crust formation at the top of the melt. The melt is poured into a conical steel mould and, when it is cool, the tm button is separated from adhering slag by tapping with a small hammer. Granulation of burtons. The button is placed m the crucible of the Jelrus furnace, from which air 1s purged by nitrogen delivered through a ceramic tube placed directly
over the button. The temperature 1s increased until the button melts (6O@looo” depending upon composition). then the melt is poured into a pail of water to granulate the alloy. Any large pieces are easily reduced in Size with metal shears. Analysis of granulated tin buttons. Each sample of granulated tin alloy is treated with 150 ml of 12M hydrochloric acid in a covered 600-m] beaker and heated until the excess of tin has dissolved and vigorous evolution of bubbles from the residue has ceased. A further 15-25-m] portion of acid is added and the sample is boiled for approximately 10 min. Water is added to give a volume of approximately 400 ml and the residue 1s allowed to settle The supernatant solution is decanted through a filter pad The resldue in the beaker is washed several times. by decantatlon. with 15”~ v/v hydrochloric acid, the washings being passed through the filter pad. The residue m the beaker 1s treated with a mixture of I5 ml of 12M hydrochloric acid and 5 ml of 30”,, hydrogen peroxide, and the beaker is heated gently for a few minutes to ensure complete dissolution of the residue. The residue on the filter pad is eluted with 20 ml of a 3: 1 mixture of 8M hydrochloric acid and 30”, hydrogen peroxide. and added to the beaker. Approximately 50 mg of sodium chloride are added. and the sample solution is evaporated to dryness. When the evolution of fumes has nearly ceased. the beaker IS removed from the evaporator and the sides are washed with - 10 ml of a 7:2 mixture of hydrochloric and hydrobromic acids. The sample is again evaporated to dryness to volatilize the remaining tin. The beaker is cooled, 10-15 ml of 12M hydrochloric acid are added and, while the beaker is being swirled. 309,” hydrogen peroxide 1s cautiously added until it is evident that an excess is present. The beaker is heated for a few minutes, then, after cooling, the sides are washed with water. After filtration of the solution through a fast paper into a 400-ml beaker and washing of the paper several times with 15”,, hydrochloric acid, approximately 5 ml of aqua regia are added and the solution IS evaporated to approximately 1 ml. To the cooled sample solution, 5 ml of cadmmm-copper sulphate solution 1s added and the mixture 1s transferred to a 25-m] volumetric flask and diluted to volume with water. The platinum, palladium and gold content of the sample is then determined by AAS. Any silver, rhodium. ruthenium or iridium remaining in the solution will not interfere. NOTE. For milligram amounts of the precious metals. the solution obtained after the volatilization of tm 1s filtered into a 10@500-ml flask and diluted to volume with 15”, hydrochloric acid to prevent hydrolysis. An aliquot is taken and treated by the procedure given above. Calibration for AAS. Calibration curves for gold and palladium are linear in the ranges 0.2-3 ppm and 0.4-3 ppm respectively. However. for the determmatlon of platinum it is necessary to use the interpolative method with standard solutions Correction for gold in stunnic oxide. Because all batches of stannic oxide tested in this laboratory were found to contain gold, it is deemed necessary to carry a blank through the analytical scheme. RESULTS
AND DISCUSSION
Effect of tellurium on precious
metals during the detrr-
mination by AAS To determine by tellurium precious which
the magnitude in the
metals, the
range.
of possible
determination
synthetic
Te:precious
an appreciable
AAS
solutions metal
Each
ratio solution
interference of the
were
prepared
was
varied
was
three in over
evaporated
Platmum, palladium and gold Table 1. Effect of tellunum on AAS deter~nation Tellunum added, my
Platinum Added
jig Found
28 40 73 29 43 73 28 44 72 28 40 66
28 42 71 28 42 71 28 42 71 28 42 7t
3 6 12
loo
to approximately 1 ml and then conditioned for atomization by the procedures described above. The results, given in Table 1, show that tellurium has a moderate depressant effect on the determination of the precious metals when the Te:precious metal ratio is approximately 1OOO:l. However, in subsequent tests (Table 2) it was established that the amount of tellurium required (i.e., _ 15 mg) to act as an effective carrier during the fusion of the charge would ultimately result in a lower ratio in the final solution for atom~ation and, consequently, not interfere. The results in Table 1 also confirm that cadmium-copper sulphate buffer solution prevents interelement interference among the three precious metals,” at least in the ranges investigated. Eficiency of tellurium as carrier parting of tin button
during fusion
and
To establish the efficiency of tellurium as a “carrier” during fusion and button-parting steps, a number of fire-assay charges were salted with tellurium, platinum, palladium and gold, and 0.5 g each of copper and nickel oxides to simulate ore samples. These samples were fused to produce tin buttons which were each analysed for the precious metals according to the procedures given above. The results of these tests, given in Table 2, show that when tellurium is not present, the recovery of the precious metals from the tin button is incomplete in most cases, Because it is known that collection of the precious
379
of platinum, palladium and gold
Palladium, w Added Found 16 31 62 62 31 16 62 16 31 62 16 3t
16 31 61 62 31 17 62 16 32 57 15 29
Cold. jq Added Found 10
20 30 10 20 30 10 20 30 30 10 20
10 20 30 10 20 30 10 20 29 28 9 19
metals by tin is quantitative, the losses no doubt occurred during the parting operation. In earlier work, the hydrochloric acid-stannous chloride solution obtained on dissolution of the tin button was not boiled with additional fresh acid to reduce the amount of undissolved copper. Under those conditions, quantitative recoveries of platinum, palladium and gold were often achieved. In the present method, the parting step is protracted to ensure that there is less than approx~ately @I g of copper in the residue. This mount, when diluted to 25 ml will not interfere subsequently in the AAS determination of the precious metals. (For example, by the procedure given above, less than OQ5 g of copper remained in the residue obtained on the parting of a granulated tin button that originally contained 0.75 g of copper.) The results in Table 2 indicate that when 15-20 mg of tellurium are added to the charge, sufficient tellurium enters the button to act as a carrier for at least 7 mg of combined platinum, palladium and gold during the hydrochloric acid parting step. Of course, in practice, most samples would contain substantially less than milligram amounts of the precious metals (e.g., Table 3). In certain experiments with blank charges containing 50-200 mg of tellurium, it was found that a significant proportion of finely divided tellurium remained at the surface of the melt during fusion and subsequently stained the crucible wall at the top of the
Table 2. Effect of tellurium on recovery of precious metals from synthettc samples Tellurium added mg
6 12 18 100 18 100
Platinum, pg Added Found
71 71 1540 28 71 42 42 1540 1540
71 67 1200 28 71 48 40 1520 1550
Paltadium, w Added Found
62 62 2180 16 62 :: 2180 2180
54 56 1500 15 62 31 31 2120 2100
Gold /~g Added Found
40 40 3000 30 10 20 20 3ooo 3cKlo
29 35 2800 30 12 19 19 2800 3ooo
380
P. E. MCXOUGHNEY and G. H. FA~X Table 3. Application of proposed method to certified reference materials
Relative std. devn, 0,
Average of within lab rel. std. devn. of certif$ng Iabs., O<>
Mean, Ppt”
Std. devn., Wm
3.02, 326, 3.05, 3.05, 3.26
3.12
0.10
3.2
3-05
15.7
Pt Pd AU
2. 81, 3*16, 298, 3.26, 2.98 1238, 13.4, 12.2. 12.2, 12.8 0.65, 082, 0% 0.65, 0.58
3.05 12.7 0.65
@17 048 0.10
56 3.8 15.0
2.98 12.7 065
11.3 7.2 20.2
14.58
Pi Pd An
566, 566, 5.87. 5.87, 5.73 8.64. 8@, 858, 8.58, 8.23 1.96, 199, 1.92, 1.99, 1.96
5-76 8.47 196
0.10 0.17 0‘03
1.7 20 I.5
5.83 8.23 1-78
52 7-9 9.7
Gold ore MA-114
14.58
An
t7.5, 17.5, 16.8, 18.5, $6-8
0.70
4%
If-8
3-O
South African Oi???
14%
Pt Pd Au
3.77, 3.84, 367, 3.77, 3.74 1.58, l*SS, 1+1, 1.51, 1.58 031, @31, 0.27. 0.31, 0.27 --
0.06 0.03 0.02
1.6 1.9 6”5
3.74 1.53 0.31
Sampfe wt., .ti
Element
Results, ppm
Magnetite cone. PTA-1 5
1458
Pt
Flotation cont. PTC-I6
14.58
Cu-Ni matte PTM-I””
Sample
17.4 3.74 1.54 0.31
Certified value, PPm
5,5* 9.4% 18.0”
* Estimated from Table 15 of Ref. 7.
melt. I~I these tests the solubility of tellurium in tin may have been exceeded. In any case, using the recommended quantity of 25 mg ensures that sufficient tellurium ult~a~ly appears in the button to act as an effective carrier for the precious metals. The finely divided, amorphous, te&kan-precious metals residues from several synthetic samples were examined by microscopical methods and analysed by electron-microprobe techniques, and no discrete tellurium compounds could be identified. A substantial fraction of the silver and rhodium that may be in the tin button will dissolve during parting, even when tellurium is present. Results from previous work,“’ l2 suggest that the same would be true for iridium and ruthenium. In any case, none of these metals, in the ~n~ntrations expected in the parting residue, will interfere uftimately with the AAS determination of pfatinum, palladium or gold.3* lo
The proposed method was applied to four certified reference materials prepared by the Canadian Certified Reference Materials Project, and also to a certified ore recently prepared by the National Institute for Metallurgy of South Africa. The analyses were performed in quintuplicate by the appropriate procedures described above, and the results are given in Table 3. With the possible exception of the gold value for matte sample PTM-1, all results are in excelIent agreement with the certified or recommended values for the five reference materials. Moreover. except for
gold in MA-l, the coefficient of variation of the results by the proposd method is Iower than the average of the coefficients of variation for the laboratories which participated in the certification of the reference materials. CONCLUSION The proposed modification of the tin-collection method, in which tellurium acts to prevent dissolution of platinum, palladium and gold during parting of the button in hydrochloric acid, offers several advantages over alternative fire-assay methods. It is simple and rapid; a set of samples can easily be carried from the fusion step through to completion within a working day. It is widely applicable and uses a flus of essentiaIly &xed composition for all type of sample. 30 major separation steps are required to isolate and separate the precious metals after the fusion, e.g., scorifications, ion-exchange or solvent extraction procedures, therefore potential losses are minimized. Its accuracy has been well established by analysis of a number of diverse certified reference materials. Acknowledgement-The authors are grateful to J.H.G. Laflamme of CANMET for the electron-microprobe analyses performed in connection with the development of the method. REmERENCES 1. G. H. Faye and W. R. Inman, Ant& Ckem., 1961, 33. 278. 2. fdem, ibid., 1961, 33, 1914.
Platinum, palladium and gold 3. G. H. Faye and P. E. Moloughney,
Tulunta, 1972. 19,
269. 4. ldem, J. S. AfLican Chem. Inst., 1915 25, 166. 5. R. C. McAdam, R. Sutarno and P. E. Moloughney, Mines Branch Tech. Bull. TB 138, Department of
Energy. __ Mines and Resources, Ottawa, 1971.
6. Zdem, Mines Brunch Tech. Bull. 176, 1973. 7. T. W. Steele, J. Levin, and I. Copelowitz. S. African Natl. Inst. Metall. Rep. No. 1696, 1975. 8. A. D. Westland and F. E. Beanush, Mikrochim. Acta,
1957. 625. 9. L. J. Cabrl, Minerals SCI. and Engng., 1972, 4. No. 3.
3.
381
10. M. M. Schnepfe and F. S. Grlmaldi, Tulanta, 1969, 16, 891. 11. G. H. Faye, W. R. Imnan and P. E. Moloughney, Anal. Chem., 1964, 36, 366. 12. G H Faye. ibid.. 1965. 37, 696. 13. R. C. McAdam, R. Sutarno and P. E. Moloughney, Mines Branch Tech. Bull. 182, Department of Energy, Mines and Resources, Ottawa, 1973. 14. G. H. Faye, W. S. Bowman and R. Sutarno, Mineral Sci. Tech. Rep. MSL 75-29, Canada Centre for Mineral and Energy Technology, Ottawa, 1975.
and G. H. FAYE
Mine& Sciences Laboratories, Canada Centre for Mineral and Energy Technology, Mines and Resuur~es+ Ottawa, Canada
Department of Energy,
tin-collection scheme of fire-assaying has been simplified to permit the rapid and accurate determination of platinum, palladium and gold in ores and related materials. The presence of tellurium in the charge ensures that the precious metals remain insoluble during the parting of the tin button with hydrochloric acid. The residue is easily collected and dissolved and the resultant solution analysed for the precious metals by AAS. The accuracy of the method has been established by application to five diverse certified reference materials.
Summary-The
The tin-dlection scheme I4 forthe dete~nat~on of the precious metals in ores and concentrates has been rtsed in our iaboratories for ap~rox~ate~y 10 years. Its accuracy, reliability and versatility have been wetl proved in interlaboratory studies.5-7 When this scheme is applied to the determination of gold, silver, and the platinum-group metals. solutions derived from tin assay buttons are treated by rather tedious ion-exchange and extraction processes and the analysis is finished either spectrophotometri-
tally or by atomic-absorption spectrophotometry (AAS). However, it is often necessary to determine only platinum, palladium and possibly gold. and a simplified procedure would be desirable. The basis For such a method was recognized severaf years ago when it was established that the intermetaliic compounds formed by platinum, palladium, and gold with tin are essentially insoluble in the hydrochloric acid used to part the tin-assay button.‘-’ However, it was also observed that copper and nickel tend to increase the solubility of the precious metals during the parting operation. Tellurium has been used to precipitate platinum, palladium, rhodium and iridium by reduction’ and it is rknown to form intermetallic compounds such as PtTe,, PdTe,.’ Tellurium was therefore added to the fire-assay charge to assess it as a “carrier” or “tixing agent” for platinum, p~~adium and gold. The results showed that an easily coflected insoluble residue containing the three precious metals is formed quantitatively during the decomposition of the tinassay button in hydrochloric acid, This forms the basis of the proposed method, which is rapid, accurate, and widely applicable. * Crown Copyrights reserved
Furnaces. A lS-kW GIobar type with suitable tbermocouple and temperature controller, capable of accommodating 6 40-g assay crucibles and maintainmg their temperature at 1250”. A Jetrus “Handy-Melt” portable electric furnace. (J&us Technical Products Corp. New Hyde Park. N.Y.) or slmrlar. The Jelrus is a small vertical furnace equipped with removable graphite crucibles, used m this work for melting tin-base assay buttons before thalr granulation in water. It is recommended that after 4 6 months of relatively constant use the bottom of the crucibles be examined for small holes.
Tin (IV) oxide.B.D.R.
It has consistently 200 ngjg.
reagent-grade is preferred because given a goId btank value of 1%
~~~~~~~rnpowder. Reagent grade. Standard soturions of piuf~~~~ p~ll~~~rn arrd yoid. Pre-
pared by dxssolving accurately weighed quantities of Johnson Matthey “Specpure” sponge in aqua regia. Each solution is evaporated to dryness, then the residues are dissolved m concentrated hydrochloric acid and the solution evaporated to dryness again. this being repeated several times. Finally, the salts are dissolved in. and diluted to volume with, 1M hydrochloric acid. The gold solution is standardtzed gravimetrically by the classical fire-assay procedure with lead, and the platinum and palladium soiutions are standardized s~trophotometrical~y. Mixed ~ffdrni~o~~~ s~~h~te sofurion.” Prepared by dissolvrng 98 g ofCu%&SH& and 57 g of 3CdSZf,.8H28!? in 500 ml of i2M hvdr~hIor~~ acid and 300 ml of water. lotlowed by d&&on to I Ike with water Fiux ,far jire-assul;. SnU2 30 g R&Q3 50 gp Na,B.& 10 g flour 35 g. Te 25 mg, silica IO-20 g accordmg to the amount of silica in the sample, make enough flux for a sample up to 1 assay-ton (29.17 g) in size. Proccdurrs
Although aspects of certain of the following procedures have been described previously,’ they are repeated here for completeness and for the convenience of the reader. 377
378
P. E. MOLOUGHNEY and G. H. FAKE
Pretreatment of samples Roasting. Before the crucible fusion procedure, all samples except those of copper-nickel matte are roasted at 75NKlO” for approximately 1 hr to decompose sulphides and volatilize arsenic and antimony. The sample is placed on a shallow fire-clay dish and stirred intermittently during the roastmg process. In cases where only a few grams of material (particularly sulphides) are to be roasted, the sample IS placed on a bed of silica to prevent possible loss of the resultant calcine to the surface of the dish (the silica is included as part of that required in the flux above). Leaching of copper-nickel matte. Leaching is performed to remove the bulk of the copper and nickel from the residue of precious metals. The sample, weighing up to 2 assay-tons, is placed in a 1500-ml beaker and treated with 25 g of ammonium chloride and 10&200 ml of 12M hydrochloric acid. The sample is heated until the amount of insoluble matter appears not to exceed 2-3 g. With large samples, it may be necessary to treat the residue once or twice more with fresh acid after intervening filtrations. The combined sample solution (approximately 100 ml) 1s diluted with an equal volume of water and the solution containing most of the nickel and copper is filtered through a moderately fast paper. The solids are completely washed onto the paper with dilute (- 5”“) hydrochloric acid. The washed residue and paper are dried at - 110 for about 1 hr and then mixed with the recommended assay flux for fusion. Chromite. Chromite is not completely decomposed during the fusion process, and samples containing an appreciable proportion of it must be subjected to a pretreatment that will decompose It. This can be done by sintering with sodium peroxide. The sample is mixed with 1.5 times its weight of sodium peroxide, then placed on a 10-g bed of silica in a roasting dish and roasted at 700” for about 1 hr. The sinter cake and underlying silica are ground together in a mortar and mixed with the flux for the crucible fusion process. The weights of sodium peroxide and silica are subtracted from the weights of sodium carbonate and silica, respectively. in the flux described above. Procedure for preparation of huttons Powdered samples. For powdered samples, the standard assay practice of blending the samples with the flux on glazed paper and transferring the charge to a “40-gram” crucible is followed. Soltltions (synthetic samples for method development). When solutions are to be mixed with the flux, approximately one-third of the flux is placed m the crucible and a 30-cm square of thin, commercial wrapping-film 1s pressed into the crucible to form an envelope, and then the remainder of the flux is transferred into this envelope. With a spatula, a cavity IS formed in the bed of flux and the sample solution is transferred slowly into the depression so as to avoid wetting the film or crucible walls. The crucible is then heated m a drying oven at 110” for at least 2 hr. After drying, the material in the wrapping film is ground in a mortar, mixed well, and placed back m the film in the crucible. It is to be noted that, after drying. the salted portion of the charge is lumpy and difficult to pulverize and mix with the rest of the charge. This could lead to occasional spurious results (Table 2). Fusion. The crucible 1s placed in the assay furnace at 1250’ for about 90 mm to fuse the charge. At the completion of the fusion period, the melt should not be viscous or lumpy nor should there be extensive crust formation at the top of the melt. The melt is poured into a conical steel mould and, when it is cool, the tm button is separated from adhering slag by tapping with a small hammer. Granulation of burtons. The button is placed m the crucible of the Jelrus furnace, from which air 1s purged by nitrogen delivered through a ceramic tube placed directly
over the button. The temperature 1s increased until the button melts (6O@looo” depending upon composition). then the melt is poured into a pail of water to granulate the alloy. Any large pieces are easily reduced in Size with metal shears. Analysis of granulated tin buttons. Each sample of granulated tin alloy is treated with 150 ml of 12M hydrochloric acid in a covered 600-m] beaker and heated until the excess of tin has dissolved and vigorous evolution of bubbles from the residue has ceased. A further 15-25-m] portion of acid is added and the sample is boiled for approximately 10 min. Water is added to give a volume of approximately 400 ml and the residue 1s allowed to settle The supernatant solution is decanted through a filter pad The resldue in the beaker is washed several times. by decantatlon. with 15”~ v/v hydrochloric acid, the washings being passed through the filter pad. The residue m the beaker 1s treated with a mixture of I5 ml of 12M hydrochloric acid and 5 ml of 30”,, hydrogen peroxide, and the beaker is heated gently for a few minutes to ensure complete dissolution of the residue. The residue on the filter pad is eluted with 20 ml of a 3: 1 mixture of 8M hydrochloric acid and 30”, hydrogen peroxide. and added to the beaker. Approximately 50 mg of sodium chloride are added. and the sample solution is evaporated to dryness. When the evolution of fumes has nearly ceased. the beaker IS removed from the evaporator and the sides are washed with - 10 ml of a 7:2 mixture of hydrochloric and hydrobromic acids. The sample is again evaporated to dryness to volatilize the remaining tin. The beaker is cooled, 10-15 ml of 12M hydrochloric acid are added and, while the beaker is being swirled. 309,” hydrogen peroxide 1s cautiously added until it is evident that an excess is present. The beaker is heated for a few minutes, then, after cooling, the sides are washed with water. After filtration of the solution through a fast paper into a 400-ml beaker and washing of the paper several times with 15”,, hydrochloric acid, approximately 5 ml of aqua regia are added and the solution IS evaporated to approximately 1 ml. To the cooled sample solution, 5 ml of cadmmm-copper sulphate solution 1s added and the mixture 1s transferred to a 25-m] volumetric flask and diluted to volume with water. The platinum, palladium and gold content of the sample is then determined by AAS. Any silver, rhodium. ruthenium or iridium remaining in the solution will not interfere. NOTE. For milligram amounts of the precious metals. the solution obtained after the volatilization of tm 1s filtered into a 10@500-ml flask and diluted to volume with 15”, hydrochloric acid to prevent hydrolysis. An aliquot is taken and treated by the procedure given above. Calibration for AAS. Calibration curves for gold and palladium are linear in the ranges 0.2-3 ppm and 0.4-3 ppm respectively. However. for the determmatlon of platinum it is necessary to use the interpolative method with standard solutions Correction for gold in stunnic oxide. Because all batches of stannic oxide tested in this laboratory were found to contain gold, it is deemed necessary to carry a blank through the analytical scheme. RESULTS
AND DISCUSSION
Effect of tellurium on precious
metals during the detrr-
mination by AAS To determine by tellurium precious which
the magnitude in the
metals, the
range.
of possible
determination
synthetic
Te:precious
an appreciable
AAS
solutions metal
Each
ratio solution
interference of the
were
prepared
was
varied
was
three in over
evaporated
Platmum, palladium and gold Table 1. Effect of tellunum on AAS deter~nation Tellunum added, my
Platinum Added
jig Found
28 40 73 29 43 73 28 44 72 28 40 66
28 42 71 28 42 71 28 42 71 28 42 7t
3 6 12
loo
to approximately 1 ml and then conditioned for atomization by the procedures described above. The results, given in Table 1, show that tellurium has a moderate depressant effect on the determination of the precious metals when the Te:precious metal ratio is approximately 1OOO:l. However, in subsequent tests (Table 2) it was established that the amount of tellurium required (i.e., _ 15 mg) to act as an effective carrier during the fusion of the charge would ultimately result in a lower ratio in the final solution for atom~ation and, consequently, not interfere. The results in Table 1 also confirm that cadmium-copper sulphate buffer solution prevents interelement interference among the three precious metals,” at least in the ranges investigated. Eficiency of tellurium as carrier parting of tin button
during fusion
and
To establish the efficiency of tellurium as a “carrier” during fusion and button-parting steps, a number of fire-assay charges were salted with tellurium, platinum, palladium and gold, and 0.5 g each of copper and nickel oxides to simulate ore samples. These samples were fused to produce tin buttons which were each analysed for the precious metals according to the procedures given above. The results of these tests, given in Table 2, show that when tellurium is not present, the recovery of the precious metals from the tin button is incomplete in most cases, Because it is known that collection of the precious
379
of platinum, palladium and gold
Palladium, w Added Found 16 31 62 62 31 16 62 16 31 62 16 3t
16 31 61 62 31 17 62 16 32 57 15 29
Cold. jq Added Found 10
20 30 10 20 30 10 20 30 30 10 20
10 20 30 10 20 30 10 20 29 28 9 19
metals by tin is quantitative, the losses no doubt occurred during the parting operation. In earlier work, the hydrochloric acid-stannous chloride solution obtained on dissolution of the tin button was not boiled with additional fresh acid to reduce the amount of undissolved copper. Under those conditions, quantitative recoveries of platinum, palladium and gold were often achieved. In the present method, the parting step is protracted to ensure that there is less than approx~ately @I g of copper in the residue. This mount, when diluted to 25 ml will not interfere subsequently in the AAS determination of the precious metals. (For example, by the procedure given above, less than OQ5 g of copper remained in the residue obtained on the parting of a granulated tin button that originally contained 0.75 g of copper.) The results in Table 2 indicate that when 15-20 mg of tellurium are added to the charge, sufficient tellurium enters the button to act as a carrier for at least 7 mg of combined platinum, palladium and gold during the hydrochloric acid parting step. Of course, in practice, most samples would contain substantially less than milligram amounts of the precious metals (e.g., Table 3). In certain experiments with blank charges containing 50-200 mg of tellurium, it was found that a significant proportion of finely divided tellurium remained at the surface of the melt during fusion and subsequently stained the crucible wall at the top of the
Table 2. Effect of tellurium on recovery of precious metals from synthettc samples Tellurium added mg
6 12 18 100 18 100
Platinum, pg Added Found
71 71 1540 28 71 42 42 1540 1540
71 67 1200 28 71 48 40 1520 1550
Paltadium, w Added Found
62 62 2180 16 62 :: 2180 2180
54 56 1500 15 62 31 31 2120 2100
Gold /~g Added Found
40 40 3000 30 10 20 20 3ooo 3cKlo
29 35 2800 30 12 19 19 2800 3ooo
380
P. E. MCXOUGHNEY and G. H. FA~X Table 3. Application of proposed method to certified reference materials
Relative std. devn, 0,
Average of within lab rel. std. devn. of certif$ng Iabs., O<>
Mean, Ppt”
Std. devn., Wm
3.02, 326, 3.05, 3.05, 3.26
3.12
0.10
3.2
3-05
15.7
Pt Pd AU
2. 81, 3*16, 298, 3.26, 2.98 1238, 13.4, 12.2. 12.2, 12.8 0.65, 082, 0% 0.65, 0.58
3.05 12.7 0.65
@17 048 0.10
56 3.8 15.0
2.98 12.7 065
11.3 7.2 20.2
14.58
Pi Pd An
566, 566, 5.87. 5.87, 5.73 8.64. 8@, 858, 8.58, 8.23 1.96, 199, 1.92, 1.99, 1.96
5-76 8.47 196
0.10 0.17 0‘03
1.7 20 I.5
5.83 8.23 1-78
52 7-9 9.7
Gold ore MA-114
14.58
An
t7.5, 17.5, 16.8, 18.5, $6-8
0.70
4%
If-8
3-O
South African Oi???
14%
Pt Pd Au
3.77, 3.84, 367, 3.77, 3.74 1.58, l*SS, 1+1, 1.51, 1.58 031, @31, 0.27. 0.31, 0.27 --
0.06 0.03 0.02
1.6 1.9 6”5
3.74 1.53 0.31
Sampfe wt., .ti
Element
Results, ppm
Magnetite cone. PTA-1 5
1458
Pt
Flotation cont. PTC-I6
14.58
Cu-Ni matte PTM-I””
Sample
17.4 3.74 1.54 0.31
Certified value, PPm
5,5* 9.4% 18.0”
* Estimated from Table 15 of Ref. 7.
melt. I~I these tests the solubility of tellurium in tin may have been exceeded. In any case, using the recommended quantity of 25 mg ensures that sufficient tellurium ult~a~ly appears in the button to act as an effective carrier for the precious metals. The finely divided, amorphous, te&kan-precious metals residues from several synthetic samples were examined by microscopical methods and analysed by electron-microprobe techniques, and no discrete tellurium compounds could be identified. A substantial fraction of the silver and rhodium that may be in the tin button will dissolve during parting, even when tellurium is present. Results from previous work,“’ l2 suggest that the same would be true for iridium and ruthenium. In any case, none of these metals, in the ~n~ntrations expected in the parting residue, will interfere uftimately with the AAS determination of pfatinum, palladium or gold.3* lo
The proposed method was applied to four certified reference materials prepared by the Canadian Certified Reference Materials Project, and also to a certified ore recently prepared by the National Institute for Metallurgy of South Africa. The analyses were performed in quintuplicate by the appropriate procedures described above, and the results are given in Table 3. With the possible exception of the gold value for matte sample PTM-1, all results are in excelIent agreement with the certified or recommended values for the five reference materials. Moreover. except for
gold in MA-l, the coefficient of variation of the results by the proposd method is Iower than the average of the coefficients of variation for the laboratories which participated in the certification of the reference materials. CONCLUSION The proposed modification of the tin-collection method, in which tellurium acts to prevent dissolution of platinum, palladium and gold during parting of the button in hydrochloric acid, offers several advantages over alternative fire-assay methods. It is simple and rapid; a set of samples can easily be carried from the fusion step through to completion within a working day. It is widely applicable and uses a flus of essentiaIly &xed composition for all type of sample. 30 major separation steps are required to isolate and separate the precious metals after the fusion, e.g., scorifications, ion-exchange or solvent extraction procedures, therefore potential losses are minimized. Its accuracy has been well established by analysis of a number of diverse certified reference materials. Acknowledgement-The authors are grateful to J.H.G. Laflamme of CANMET for the electron-microprobe analyses performed in connection with the development of the method. REmERENCES 1. G. H. Faye and W. R. Inman, Ant& Ckem., 1961, 33. 278. 2. fdem, ibid., 1961, 33, 1914.
Platinum, palladium and gold 3. G. H. Faye and P. E. Moloughney,
Tulunta, 1972. 19,
269. 4. ldem, J. S. AfLican Chem. Inst., 1915 25, 166. 5. R. C. McAdam, R. Sutarno and P. E. Moloughney, Mines Branch Tech. Bull. TB 138, Department of
Energy. __ Mines and Resources, Ottawa, 1971.
6. Zdem, Mines Brunch Tech. Bull. 176, 1973. 7. T. W. Steele, J. Levin, and I. Copelowitz. S. African Natl. Inst. Metall. Rep. No. 1696, 1975. 8. A. D. Westland and F. E. Beanush, Mikrochim. Acta,
1957. 625. 9. L. J. Cabrl, Minerals SCI. and Engng., 1972, 4. No. 3.
3.
381
10. M. M. Schnepfe and F. S. Grlmaldi, Tulanta, 1969, 16, 891. 11. G. H. Faye, W. R. Imnan and P. E. Moloughney, Anal. Chem., 1964, 36, 366. 12. G H Faye. ibid.. 1965. 37, 696. 13. R. C. McAdam, R. Sutarno and P. E. Moloughney, Mines Branch Tech. Bull. 182, Department of Energy, Mines and Resources, Ottawa, 1973. 14. G. H. Faye, W. S. Bowman and R. Sutarno, Mineral Sci. Tech. Rep. MSL 75-29, Canada Centre for Mineral and Energy Technology, Ottawa, 1975.