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Determination of platinum in ores by a combined fire-assay and flameless atomic-absorption method
Tdm~ra,Vol. 24, pp. 421-424. Pergamon Press, 1977. Printedin Great Britain.
DETERMINATION OF PLATINUM IN ORES BY A COMBINED FIRE-ASSAY AND FLAMELESS ATOMIC-ABSORPTION METHOD R. J. COOMBES and A.
CHOW
Chemistry Department, University of Manitoba, Winnipeg, Manitoba, Canada and R. WAGEMAN Freshwater Institute, Winnipeg, Manitoba, Canada (Received
27 September
1976. Accepted
30 December 1976)
Summary-A combined fire-assay and 5ameless atomic-absorption procedure for the determination of platinum in ores is described. Silver beads obtained by cupellation are dissolved in aqua regia and made up to standard volume with 6M hydrochloric acid, then 50-p] aliquots are injected into a carbon tube. From 0.5 to 5ppm platinum can be determined with a precision of approximately 5%. The procedure tolerates other platinum metals and gold in the amounts present in the ores analysed.
Much work has been done on the determination of the noble metals by flameless atomic absorption.r4 Most of this work however, has been concerned with studies of optimum conditions and interferences from various metals and inorganic acids and very few practical applications have been reported. Janouskova et aL5 determined platinum in reforming catalysts and good agreement with conventional flame atomicabsorption analysis was reported. Bratzel et aL6 determined gold and silver in geological and metallurgical samples in the n&/g range with a carbon-rod atomizer. The results showed good agreement with certified values obtained by a combined fire-assay and flame atomic-absorption procedure. As part of a comparative study of methods for the determination of platinum in ores conducted in this laboratory, a combined fire-assay and flameless atomic-absorption method was developed and results were compared with previous analyses obtained from independent sources. EXPERIMENTAC Appurutus
Lindberg Hevi-duty 20-kW electric furnace. Magnesia cupels produced by Leonard Light Industries (Benoni, South Africa). Crucibles produced by A. P. Green Refractories (Ontario, Canada), Perkin-Elmer 403 Atomic Absorption Spectrophotometer fitted with the HGA-70 flameless atomization device and a deuterium backgroundcorrector. Intensitron platinum hollow-cathode lamp. An Eppendorf 50-g pipette with disposable tips was used for all injections. Reagents
solution was filtered into a I-litre flask and made up to the mark with O.lM hydrochloric acid. The solution was standardized with thiophenol,7 and a second solution was standardized sp&trophotometrically8 against the first, calibrated glassware being used. P~l~~i~ and g&d so~~rions (1 ~/mI). Prepared by dissolving the pure metals in aqua regia, evaporating to dryness several times with concentrated hydrochloric acid and making up to volume in 0.1 M hydrochloric acid. Rhodium solution (1 mg/ml). Prepared by dissolving 0.5838 g of sodium hexachlororhodate (Alfa Inorganics) in 100 ml of O.lM hydrochloric acid. iridium solution (1 mg/mQ Prepared by dissolving 0.7292g of sodium chloroiridate (Johnson, Mattbey) in 100 ml of O.lM hydrochloric acid. Silwr nitrate solutions (2 mgjml and 20 mg/mI). Prepared by dissolving 0.3148 and 3.148 g of silver nitrate (Mallinckrodt, analytical reagent) in 100 ml of doubly distilled, doubly demineralized water. Lead foil, 0.004~in. thick.
Wavelength Slit setting Lamp current Drying time Charring time Atomization time Drying temperature Charring temperature Atomization temperature Water flow Argon flow Chart speed
265.9 nm
4 28 mA 40 set 35 set 15sec 100 ‘1100” 2600” (1Ov) 3 l./min 3-5 arbitrary setting 51 mm/min
Several graphite tubes were used in this work. They had all been used to some extent before the platinum injections. Before each analysis the tube was fired to m~imum temperature for 2-3 sec. Platinum solutions were then injected. In all cases consistent results were obtained immediately.
PZ&num solution (1 mg/ml). Prepared by dissolving 1 g Flameless atomic-absorption analysis of platinum standards of pure platinum wire (Johnson, Matthey and Mallory) Aqueous solutions of platinum were “fired” to determine in aqua regia. Nitrous oxides were removed by repeated evaporations with concentrated hydrochloric acid. The the optimum concentration range. This was found to be 421
422
R. J. COOMBES, A. CHOW and R.
0.5-5 ppm for 50-~1 injections. A slight curvature of the calibration was obtained at higher concentrations. Platinum standards were prepared by adding pl quantities of stock solutions of silver and platinum to a volumetric flask. A measured volume of nitric acid was added and the solution made up to the mark with hydrochloric acid (1 + 1). Final concentrations of silver and nitric acid were 2 mg and 0.5 ml respectively per 10 ml of solution. Standards were prepared in this manner as it was expected that platinum would be extracted from ores by the classical fire-assay process and concentrated in a 2-mg silver bead. Nitric acid is required to dissolve the silver bead and the hydrochloric acid concentration must be high (>6M) to keep silver in solution. These standards were fired and the calibration curve was found to be linear over the same range as for aqueous solutions. However, the peak-heights had been depressed by approximately 50%. The calibration curves are shown in Fig. 1. The lines for the 6M acid solutions were drawn by the least-squares method. At least 4 injections were made at each concentration. Peak-heights were found to vary considerably from tube to tube, thus it was necessary to draw calibratidn curves for each analysis. The results for the 6M acid solutions were compared with those for the same solutions containing 2 mg of silver per 10 ml of solution. Silver depresses the platinum absorbance by 3%. Increasing the silver concentration to 4 mg/lO ml gave no further depression due to silver. Blanks containing only silver in 6M hydrochloric acid were analysed and no signal was obtained at the platinum wavelength. Acid effects on platinum absorbance
The effects of the addition of sulphuric and phosphoric acids on the determination of platinum in hydrochloric acid were studied. It has been shown in a previous study’ that the addition of these acids to nitric acid and hydrochloric acid solutions of silver greatly improves the precision for synthetic samples. The addition of nitric acid to a silver bead containing platinum results in the dissolution of all the silver and some of the platinum. This has been confirmed during this investigation on the fire-assay for platinum and nitric acid alone could not be used as a medium for platinum under these circumstances. Sulphuric and phosphoric acids (lO%v/v) were added to a 4-ppm platinum solution containing silver, nitric acid and hydrochloric acid in the concentrations stated above. These solutions were analysed and the results are shown in Table 1. Each acid reduces both peak-height and precision and this is especially marked in the case of sulphuric
WAGEMAN
acid. Peak-heights were initially consistent at 56mm, but then became alternately short (- 51 mm) and then long (- 68 mm), giving the large coefficient of variation. Both acids emitted dense fumes during volatilization and charring time with phosphoric acid present had to be increased to allow argon gas to clear the tube of these fumes. As a consequence of these results neither sulljhuric nor phosphoric acids was added to platinum solutions. Interelement
effects
Mutual interferences between noble metals in flameless atomic absorption have been well documented.3*4 All the interferences listed have been observed when the interfering element is in a large excess, except for one example, that of ruthenium and platinum (1: 1 ratio). However, in the platinum ores analysed, ruthenium is either absent or occurs at a much lower level than does platinum. Also there is strong evidence to show that losses of ruthenium are high during the cupellation process” and thus the effect of this metal on platinum absorbance was not studied. For similar reasons osmium was also not studied. Solutions were prepared in a similar manner to that described above, but containing 50 pg of platinum, 25 pg of palladium, 5 pg of gold, 5 pg of rhodium and 2 pg of iridium. These represent the maximum amounts of each metal in the ores to be analysed. The solutions were analysed and the results compared with those for a standard 50-pg platinum solution. The peak-heights were identical, indicating that there is no interference at these levels. Blanks containing the same concentrations of the noble metals with the exception of platinum were also analysed, but no signal was observed at the platinum wavelength. Base-metal interference was not tested. The only base metal likely to be present in microgram quantities in a silver bead from cupellation is lead. Lead is more volatile than silver and so it is expected that all the lead would be volatilized with silver during the charring cycle. Flameless
atomic-absorption
analysis of synthetic samples
The method was tested by synthetic samples prepared by salting lead boats and cupelling to produce a silver bead. Aliquots of standard platinum solutions were added to a boat-shaped container made from a 12.5 x 12.5cm square of lead foil of O.OlOcm thickness. One ml of 2mgJml silver solution was added and the boats were placed in a drying oven for several hr. The boats were then folded into the form of a cube (- 25 g), and quickly added to magnesia cupels placed in a furnace at 960”. These cupels had been heated for at least 10min before use. The furnace door was kent closed for 5 min to allow the lead to melt and then dpened by placing a 0.6-cm
200 z
t
Platinum concentration,
ppm
Fig. 1. Flameless atomic-absorption calibration curve for aqueous and hydrochloric acid solutions containing 0.5-5 ppm platinum. A, H,O: 5-mV scale. B, 50% HCl + 5% HN03 + 10 mg Ag/50 ml: 2-mV scale. C, 50% HCl + 5% HN03 + 10 mg Ag/50 ml: 5-mV scale. Bars represent standard deviations at the 95% confidence level.
Determination
of platinum in ores
423
Table 1. Acid effects on platinum absorbance
Acid medium 50% HCl + 5% HNO, 50% HCl + 5% HNO, + 10% HsPO, 50% HCI + 5% HNOs + 10% H2S04
Peak-height,* mm
Coefficient of variation, %
86.4 81.6 59.3
3.5 3.6 12.5
* Average of at least 11 measurements. thick steel plate under it. Under these conditions the driving of lead occurred at the rate of approximately 1 g/mm. When driving was complete the furnace door was closed and the cupellation continued for a further 5 min. Cupels were then slowly withdrawn from the furnace and allowed to cool. When cool the silver beads were transferred to a lo-ml volumetric flask, 0.5 ml of concentrated nitric acid was added and the flask heated by immersion in a beaker of hot water until dissolution of the bead was complete. Then 5ml of concentrated hydrochloric acid were added and the flask was heated once more to dissolve the silver. The solution was left to cool overnight, and made up to the mark with distilled water. Samples and standards were then analysed, 2 or 3 samples being fired between each standard. A calibration curve was drawn (by the leastsquares method) and used to calculate the recoveries of platinum (Table 2). Blanks were run each time but no platinum was detected in them. Combinedjre-assay for ores
and jlameless atomic-absorption method
Ores for analysis were designated by name or number and originated from the Merensky Reef, South Africa. These ores, which had been crushed fine enough to pass through sieves of 100 and 200 mesh, had previously been analysed for platinum metals and go1d.“~‘2 The entire ore sample was placed on a large cellophane sheet and rolled or tumbled by lifting alternate corners of the sheet. After 25 min of mixing, the ore was spread out evenly on the cellophane and marked off into 2.5~cm squares. The sample for assay was then obtained by taking small portions from each of the squares. The weights of the ores taken were l-2 g of float concentrate and 3-7 g for ores S4 and USBM 31. Float concentrate and USBM 31 samples were roasted at 700” for 1 hr before fire-assaying. Each sample was added to a small-size polyethylene “sandwich” bag containing 85 g of PbO, 21.1 g of Na,COs, 4.5 g of CaO and 2 g of flour. To prevent the flux from becoming too basic, approximately log of silica were added to each ore. Silica (15 g) was used as a blank ore. Silver (2 mg, added as silver nitrate solution) was added to bags containing ores S4 and USBM 31, and 20mg of silver to the float concentrate samples. Each bag was placed in a crucible pot and the pot in a drying oven at 70” for several hr. After drying, lumps were broken up and the mixture thoroughly shaken for 4-5 min. The crucible pots were placed in the furnace at 950” and heated to 1200” at the maximum rate of the furnace. This norTable 2. Flameless atomic-absorption Platinum added, ,Q 10.0 20.1 40.2 50.2
of synthetic samples
Platinum recovered, w* 10.1 * 20.5 f 37.9 + 46.6 f
0.5t 1.35 1.4t 2.02t
* f values quoted at the 95% confidence level. t Average of at least 3 injections for each of 5 samples. 5 Average of at least 3 injections for each of 4 samples.
Table 3. Analysis of ores by combined fire-assay and flameless atomic absorption s4”
Averages Previous analysis
Concentration of platinum, ppmjj Float concentrate” USBM311’
5.43 4.61* 5.61 5.61 5.50 5.58 5.46 4.92 5.44 f 0.22t 5.233
77.0 143.5* 103.7* 76.2 68.2 71.9 78.4 73.2 74.2 f 4.0t 82.61
4.56 4.25 5.04 4.25 4.88 4.69 4.28 4.63 4.57 f 0.28t 4.9
* Values. rejected at the 95% confidence level. t + values quoted at the 95% confidence level. $ Analysed by combined fire-assay and flame atomic absorption. NIM Johannesburg, S.A. 5 Average of at least 3 injections. 11 Average result from 8 laboratories taking part in a “round-robin” analysis included fire-assay/flame atomic absorption, fire-assay/emission spectrography and fireassay/spectrophotometry. United States Bureau of Mines, Reno, Nevada. mally required 75-90 min. After being held at this temperature for 15 min the crucibles were removed from the furnace and the molten contents poured into conical iron moulds. When cool, slags could be separated from the lead buttons by gentle tapping with a small hammer. Lead buttons were then cupelled to a silver bead as described above. Silver beads from ores S4 and USBM 31 were placed in lo-ml volumetric flasks and the beads from the float concentrate in lOO-ml volumetric flasks. The beads were dissolved and the solutions made up to contain 5% nitric acid and 50% hydrochloric acid (v/v). As with synthetic samples, ore samples and standards were analysed so that 2 or 3 samples were fired between each standard. The ore samples were chosen at random for each firing. A calibration curve was again drawn by the least-squares method and the results of these analyses are shown in Table 3.
CONCLUSION The flameless atomic-absorption method has several advantages. Volatile interferences can be removed by selective volatilization. Palladium, gold, rhodium and iridium (in the amounts found in the ores analysed) do not affect the platinum signal and hence neither prior separation nor the addition of releasing agents is necessary. The method has higher sensitivity and is as fast or faster than most other methods of analysis for platinum in ores. Thus the
424
R. J. COOMBES,A. CHOW and R. WAGEMAN
flameless atomic-absorption technique offers a feasible alternative to spectrophotometric determinations when highest accuracy and precision are not required or when a very low metal concentration is to be determined. The combined fire-assay and Aameless atomicabsorption method has been shown to be quantitative for platinum in ores and from 0.5-5 ppm of platinum can be determined with a precision of approximately 5%. Results for ores S4 and USBM 31 showed good agreement with independent analyses, while the result for the float concentrate indicated a low, but acceptable, recovery; possibly some platinum was volatilized with silver during the charring stage or was not completely atomized at 2600”. This procedure provides both a concentration and an isolation step (fireassay), and is combined with a highly selective and sensitive method of analysis. Thus, this method should be applicable to many samples having a variety of matrices and platinum concentrations, but will find particular use for very dilute samples.
REFERENCES
1. E. Adriaenssens and P. Knoop, Anal. Chim. Acta, 1973, 6l?, 37. 2. I. Rubeska and J. Stupar, At. A&sorption newsletter, 1966, 5, 69. 3. D. C. G. Pearton and R. C. Mallet, Nat. Inst. Metalho-gy Report No. 1598, March 1974. 4. B. D. Guerin, J. S. African Chem. Inst., 1972, XXV. 5. J. Janouskova, M. Nehasilova and V. Sychra, At. Absorption Newsletter, 1973, 12, 161. 6. M. P. Bratzel, C. L. Chakrabarti, R. E. Sturg~~ M. W. McIntyre and H. Agemain, Anal. Chem., 1972, 44, 372. 7. J. E. Currah, W. A. McBryde, A. J. Cruikshank and F. E. Beamish, fnd. Eng. Chem., Anal. Ed., 1946, IS,
120. 8. G. H. Ayres and A. S. Meyer Jr., Anal. Chem., 1951, 23, 299. 9. R. C. Mallet and S. Kellerman, Nat. Inst. metallurgy Rept. No. 1669, Oct. 1974. 10. F. E. Beamish, The A~iyt~a~ Chemistry of the Noble ~etu~s, Pergamon, New York, 1966. 11. National Institute of Metallurgy, Johannesburg, S. Africa. 12. United States Bureau of Mines, Reno, Nevada.
DETERMINATION OF PLATINUM IN ORES BY A COMBINED FIRE-ASSAY AND FLAMELESS ATOMIC-ABSORPTION METHOD R. J. COOMBES and A.
CHOW
Chemistry Department, University of Manitoba, Winnipeg, Manitoba, Canada and R. WAGEMAN Freshwater Institute, Winnipeg, Manitoba, Canada (Received
27 September
1976. Accepted
30 December 1976)
Summary-A combined fire-assay and 5ameless atomic-absorption procedure for the determination of platinum in ores is described. Silver beads obtained by cupellation are dissolved in aqua regia and made up to standard volume with 6M hydrochloric acid, then 50-p] aliquots are injected into a carbon tube. From 0.5 to 5ppm platinum can be determined with a precision of approximately 5%. The procedure tolerates other platinum metals and gold in the amounts present in the ores analysed.
Much work has been done on the determination of the noble metals by flameless atomic absorption.r4 Most of this work however, has been concerned with studies of optimum conditions and interferences from various metals and inorganic acids and very few practical applications have been reported. Janouskova et aL5 determined platinum in reforming catalysts and good agreement with conventional flame atomicabsorption analysis was reported. Bratzel et aL6 determined gold and silver in geological and metallurgical samples in the n&/g range with a carbon-rod atomizer. The results showed good agreement with certified values obtained by a combined fire-assay and flame atomic-absorption procedure. As part of a comparative study of methods for the determination of platinum in ores conducted in this laboratory, a combined fire-assay and flameless atomic-absorption method was developed and results were compared with previous analyses obtained from independent sources. EXPERIMENTAC Appurutus
Lindberg Hevi-duty 20-kW electric furnace. Magnesia cupels produced by Leonard Light Industries (Benoni, South Africa). Crucibles produced by A. P. Green Refractories (Ontario, Canada), Perkin-Elmer 403 Atomic Absorption Spectrophotometer fitted with the HGA-70 flameless atomization device and a deuterium backgroundcorrector. Intensitron platinum hollow-cathode lamp. An Eppendorf 50-g pipette with disposable tips was used for all injections. Reagents
solution was filtered into a I-litre flask and made up to the mark with O.lM hydrochloric acid. The solution was standardized with thiophenol,7 and a second solution was standardized sp&trophotometrically8 against the first, calibrated glassware being used. P~l~~i~ and g&d so~~rions (1 ~/mI). Prepared by dissolving the pure metals in aqua regia, evaporating to dryness several times with concentrated hydrochloric acid and making up to volume in 0.1 M hydrochloric acid. Rhodium solution (1 mg/ml). Prepared by dissolving 0.5838 g of sodium hexachlororhodate (Alfa Inorganics) in 100 ml of O.lM hydrochloric acid. iridium solution (1 mg/mQ Prepared by dissolving 0.7292g of sodium chloroiridate (Johnson, Mattbey) in 100 ml of O.lM hydrochloric acid. Silwr nitrate solutions (2 mgjml and 20 mg/mI). Prepared by dissolving 0.3148 and 3.148 g of silver nitrate (Mallinckrodt, analytical reagent) in 100 ml of doubly distilled, doubly demineralized water. Lead foil, 0.004~in. thick.
Wavelength Slit setting Lamp current Drying time Charring time Atomization time Drying temperature Charring temperature Atomization temperature Water flow Argon flow Chart speed
265.9 nm
4 28 mA 40 set 35 set 15sec 100 ‘1100” 2600” (1Ov) 3 l./min 3-5 arbitrary setting 51 mm/min
Several graphite tubes were used in this work. They had all been used to some extent before the platinum injections. Before each analysis the tube was fired to m~imum temperature for 2-3 sec. Platinum solutions were then injected. In all cases consistent results were obtained immediately.
PZ&num solution (1 mg/ml). Prepared by dissolving 1 g Flameless atomic-absorption analysis of platinum standards of pure platinum wire (Johnson, Matthey and Mallory) Aqueous solutions of platinum were “fired” to determine in aqua regia. Nitrous oxides were removed by repeated evaporations with concentrated hydrochloric acid. The the optimum concentration range. This was found to be 421
422
R. J. COOMBES, A. CHOW and R.
0.5-5 ppm for 50-~1 injections. A slight curvature of the calibration was obtained at higher concentrations. Platinum standards were prepared by adding pl quantities of stock solutions of silver and platinum to a volumetric flask. A measured volume of nitric acid was added and the solution made up to the mark with hydrochloric acid (1 + 1). Final concentrations of silver and nitric acid were 2 mg and 0.5 ml respectively per 10 ml of solution. Standards were prepared in this manner as it was expected that platinum would be extracted from ores by the classical fire-assay process and concentrated in a 2-mg silver bead. Nitric acid is required to dissolve the silver bead and the hydrochloric acid concentration must be high (>6M) to keep silver in solution. These standards were fired and the calibration curve was found to be linear over the same range as for aqueous solutions. However, the peak-heights had been depressed by approximately 50%. The calibration curves are shown in Fig. 1. The lines for the 6M acid solutions were drawn by the least-squares method. At least 4 injections were made at each concentration. Peak-heights were found to vary considerably from tube to tube, thus it was necessary to draw calibratidn curves for each analysis. The results for the 6M acid solutions were compared with those for the same solutions containing 2 mg of silver per 10 ml of solution. Silver depresses the platinum absorbance by 3%. Increasing the silver concentration to 4 mg/lO ml gave no further depression due to silver. Blanks containing only silver in 6M hydrochloric acid were analysed and no signal was obtained at the platinum wavelength. Acid effects on platinum absorbance
The effects of the addition of sulphuric and phosphoric acids on the determination of platinum in hydrochloric acid were studied. It has been shown in a previous study’ that the addition of these acids to nitric acid and hydrochloric acid solutions of silver greatly improves the precision for synthetic samples. The addition of nitric acid to a silver bead containing platinum results in the dissolution of all the silver and some of the platinum. This has been confirmed during this investigation on the fire-assay for platinum and nitric acid alone could not be used as a medium for platinum under these circumstances. Sulphuric and phosphoric acids (lO%v/v) were added to a 4-ppm platinum solution containing silver, nitric acid and hydrochloric acid in the concentrations stated above. These solutions were analysed and the results are shown in Table 1. Each acid reduces both peak-height and precision and this is especially marked in the case of sulphuric
WAGEMAN
acid. Peak-heights were initially consistent at 56mm, but then became alternately short (- 51 mm) and then long (- 68 mm), giving the large coefficient of variation. Both acids emitted dense fumes during volatilization and charring time with phosphoric acid present had to be increased to allow argon gas to clear the tube of these fumes. As a consequence of these results neither sulljhuric nor phosphoric acids was added to platinum solutions. Interelement
effects
Mutual interferences between noble metals in flameless atomic absorption have been well documented.3*4 All the interferences listed have been observed when the interfering element is in a large excess, except for one example, that of ruthenium and platinum (1: 1 ratio). However, in the platinum ores analysed, ruthenium is either absent or occurs at a much lower level than does platinum. Also there is strong evidence to show that losses of ruthenium are high during the cupellation process” and thus the effect of this metal on platinum absorbance was not studied. For similar reasons osmium was also not studied. Solutions were prepared in a similar manner to that described above, but containing 50 pg of platinum, 25 pg of palladium, 5 pg of gold, 5 pg of rhodium and 2 pg of iridium. These represent the maximum amounts of each metal in the ores to be analysed. The solutions were analysed and the results compared with those for a standard 50-pg platinum solution. The peak-heights were identical, indicating that there is no interference at these levels. Blanks containing the same concentrations of the noble metals with the exception of platinum were also analysed, but no signal was observed at the platinum wavelength. Base-metal interference was not tested. The only base metal likely to be present in microgram quantities in a silver bead from cupellation is lead. Lead is more volatile than silver and so it is expected that all the lead would be volatilized with silver during the charring cycle. Flameless
atomic-absorption
analysis of synthetic samples
The method was tested by synthetic samples prepared by salting lead boats and cupelling to produce a silver bead. Aliquots of standard platinum solutions were added to a boat-shaped container made from a 12.5 x 12.5cm square of lead foil of O.OlOcm thickness. One ml of 2mgJml silver solution was added and the boats were placed in a drying oven for several hr. The boats were then folded into the form of a cube (- 25 g), and quickly added to magnesia cupels placed in a furnace at 960”. These cupels had been heated for at least 10min before use. The furnace door was kent closed for 5 min to allow the lead to melt and then dpened by placing a 0.6-cm
200 z
t
Platinum concentration,
ppm
Fig. 1. Flameless atomic-absorption calibration curve for aqueous and hydrochloric acid solutions containing 0.5-5 ppm platinum. A, H,O: 5-mV scale. B, 50% HCl + 5% HN03 + 10 mg Ag/50 ml: 2-mV scale. C, 50% HCl + 5% HN03 + 10 mg Ag/50 ml: 5-mV scale. Bars represent standard deviations at the 95% confidence level.
Determination
of platinum in ores
423
Table 1. Acid effects on platinum absorbance
Acid medium 50% HCl + 5% HNO, 50% HCl + 5% HNO, + 10% HsPO, 50% HCI + 5% HNOs + 10% H2S04
Peak-height,* mm
Coefficient of variation, %
86.4 81.6 59.3
3.5 3.6 12.5
* Average of at least 11 measurements. thick steel plate under it. Under these conditions the driving of lead occurred at the rate of approximately 1 g/mm. When driving was complete the furnace door was closed and the cupellation continued for a further 5 min. Cupels were then slowly withdrawn from the furnace and allowed to cool. When cool the silver beads were transferred to a lo-ml volumetric flask, 0.5 ml of concentrated nitric acid was added and the flask heated by immersion in a beaker of hot water until dissolution of the bead was complete. Then 5ml of concentrated hydrochloric acid were added and the flask was heated once more to dissolve the silver. The solution was left to cool overnight, and made up to the mark with distilled water. Samples and standards were then analysed, 2 or 3 samples being fired between each standard. A calibration curve was drawn (by the leastsquares method) and used to calculate the recoveries of platinum (Table 2). Blanks were run each time but no platinum was detected in them. Combinedjre-assay for ores
and jlameless atomic-absorption method
Ores for analysis were designated by name or number and originated from the Merensky Reef, South Africa. These ores, which had been crushed fine enough to pass through sieves of 100 and 200 mesh, had previously been analysed for platinum metals and go1d.“~‘2 The entire ore sample was placed on a large cellophane sheet and rolled or tumbled by lifting alternate corners of the sheet. After 25 min of mixing, the ore was spread out evenly on the cellophane and marked off into 2.5~cm squares. The sample for assay was then obtained by taking small portions from each of the squares. The weights of the ores taken were l-2 g of float concentrate and 3-7 g for ores S4 and USBM 31. Float concentrate and USBM 31 samples were roasted at 700” for 1 hr before fire-assaying. Each sample was added to a small-size polyethylene “sandwich” bag containing 85 g of PbO, 21.1 g of Na,COs, 4.5 g of CaO and 2 g of flour. To prevent the flux from becoming too basic, approximately log of silica were added to each ore. Silica (15 g) was used as a blank ore. Silver (2 mg, added as silver nitrate solution) was added to bags containing ores S4 and USBM 31, and 20mg of silver to the float concentrate samples. Each bag was placed in a crucible pot and the pot in a drying oven at 70” for several hr. After drying, lumps were broken up and the mixture thoroughly shaken for 4-5 min. The crucible pots were placed in the furnace at 950” and heated to 1200” at the maximum rate of the furnace. This norTable 2. Flameless atomic-absorption Platinum added, ,Q 10.0 20.1 40.2 50.2
of synthetic samples
Platinum recovered, w* 10.1 * 20.5 f 37.9 + 46.6 f
0.5t 1.35 1.4t 2.02t
* f values quoted at the 95% confidence level. t Average of at least 3 injections for each of 5 samples. 5 Average of at least 3 injections for each of 4 samples.
Table 3. Analysis of ores by combined fire-assay and flameless atomic absorption s4”
Averages Previous analysis
Concentration of platinum, ppmjj Float concentrate” USBM311’
5.43 4.61* 5.61 5.61 5.50 5.58 5.46 4.92 5.44 f 0.22t 5.233
77.0 143.5* 103.7* 76.2 68.2 71.9 78.4 73.2 74.2 f 4.0t 82.61
4.56 4.25 5.04 4.25 4.88 4.69 4.28 4.63 4.57 f 0.28t 4.9
* Values. rejected at the 95% confidence level. t + values quoted at the 95% confidence level. $ Analysed by combined fire-assay and flame atomic absorption. NIM Johannesburg, S.A. 5 Average of at least 3 injections. 11 Average result from 8 laboratories taking part in a “round-robin” analysis included fire-assay/flame atomic absorption, fire-assay/emission spectrography and fireassay/spectrophotometry. United States Bureau of Mines, Reno, Nevada. mally required 75-90 min. After being held at this temperature for 15 min the crucibles were removed from the furnace and the molten contents poured into conical iron moulds. When cool, slags could be separated from the lead buttons by gentle tapping with a small hammer. Lead buttons were then cupelled to a silver bead as described above. Silver beads from ores S4 and USBM 31 were placed in lo-ml volumetric flasks and the beads from the float concentrate in lOO-ml volumetric flasks. The beads were dissolved and the solutions made up to contain 5% nitric acid and 50% hydrochloric acid (v/v). As with synthetic samples, ore samples and standards were analysed so that 2 or 3 samples were fired between each standard. The ore samples were chosen at random for each firing. A calibration curve was again drawn by the least-squares method and the results of these analyses are shown in Table 3.
CONCLUSION The flameless atomic-absorption method has several advantages. Volatile interferences can be removed by selective volatilization. Palladium, gold, rhodium and iridium (in the amounts found in the ores analysed) do not affect the platinum signal and hence neither prior separation nor the addition of releasing agents is necessary. The method has higher sensitivity and is as fast or faster than most other methods of analysis for platinum in ores. Thus the
424
R. J. COOMBES,A. CHOW and R. WAGEMAN
flameless atomic-absorption technique offers a feasible alternative to spectrophotometric determinations when highest accuracy and precision are not required or when a very low metal concentration is to be determined. The combined fire-assay and Aameless atomicabsorption method has been shown to be quantitative for platinum in ores and from 0.5-5 ppm of platinum can be determined with a precision of approximately 5%. Results for ores S4 and USBM 31 showed good agreement with independent analyses, while the result for the float concentrate indicated a low, but acceptable, recovery; possibly some platinum was volatilized with silver during the charring stage or was not completely atomized at 2600”. This procedure provides both a concentration and an isolation step (fireassay), and is combined with a highly selective and sensitive method of analysis. Thus, this method should be applicable to many samples having a variety of matrices and platinum concentrations, but will find particular use for very dilute samples.
REFERENCES
1. E. Adriaenssens and P. Knoop, Anal. Chim. Acta, 1973, 6l?, 37. 2. I. Rubeska and J. Stupar, At. A&sorption newsletter, 1966, 5, 69. 3. D. C. G. Pearton and R. C. Mallet, Nat. Inst. Metalho-gy Report No. 1598, March 1974. 4. B. D. Guerin, J. S. African Chem. Inst., 1972, XXV. 5. J. Janouskova, M. Nehasilova and V. Sychra, At. Absorption Newsletter, 1973, 12, 161. 6. M. P. Bratzel, C. L. Chakrabarti, R. E. Sturg~~ M. W. McIntyre and H. Agemain, Anal. Chem., 1972, 44, 372. 7. J. E. Currah, W. A. McBryde, A. J. Cruikshank and F. E. Beamish, fnd. Eng. Chem., Anal. Ed., 1946, IS,
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