ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 4, с. 542-547
^ СПЕКТРОСКОПИЯ ^^^^^^^^^^^^
АТОМОВ И МОЛЕКУЛ
УДК 546.9
A RAPID AND PRACTICAL STRATEGY FOR THE DETERMINATION
OF PLATINUM, PALLADIUM, RUTHENIUM, RHODIUM, IRIDIUM AND GOLD IN LARGE AMOUNTS OF ULTRABASIC ROCK BY INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION SPECTROMETRY COMBINED WITH ULTRASOUND EXTRACTION
© 2015 г. Gai Zhang and Min Tian
School of Materials and Chemical Engineering, Xi'an Technological University, 710032 Xi'an, China
E-mail: gaizhang@aliyun.com Received June 14, 2014
This proposed method regulated the determination of platinum, palladium, ruthenium, rhodium, iridium and gold in platinum-group ores by nickel sulfide fire assay-inductively coupled plasma optical emission spectrometry (ICP-OES) combined with ultrasound extraction for the first time. The quantitative limits were 0.013-0.023 ^g/g. The samples were fused to separate the platinum-group elements from matrix. The nickel sulfide button was then dissolved with hydrochloric acid and the insoluble platinum-group sulfide residue was dissolved with aqua regia by ultrasound bath and finally determined by ICP-OES. The proposed method has been applied into the determination of platinum-group element and gold in large amounts of ultrabasic rocks from the Great Dyke of Zimbabwe.
DOI: 10.7868/S0030403415040042
INTRODUCTION
The Great Dyke of Zimbabwe is alayered mafic intrusion of igneous, metal-bearing rock that has been dated to approximately 2.5 billion years old. This geological feature extends more than 550 kilometers (342 miles) northeast to southwest across the center of Zimbabwe, varying from 3 to 12 kilometers (2—8 miles) in width.
The Great Dyke is a strategic economic resource with significant quantities of chrome and platinum. Chromite occurs to the base of the Ultramafic Sequence and is mined throughout the dyke [1]. Below the Ultramafic-Mafic sequences' contact, and in the uppermost pyroxenite (bronzitite and websterite) units are economic concentrations of nickel, copper, cobalt, gold, and platinum group elements (PGE). The base metals occur as disseminated inter-cumulus Fe—Ni—Cu sulfides within an interval referred to as the base metal subzone, below which is a sublayer enriched in platinum group metals called the PGE subzone. The base metal and PGE subzones together make up the Main Sulphide Zone (MSZ) [2].
Platinum group metals are currently mined at Ngezi Mine south of Selous by Zimplats of the Impala Platinum Group, at Unki Mine near Shurugwi by Anglo American and at Mimosa Mine near Zvishavane by Zimasco for Impala Platinum. The Hartley Platinum Mine near Makwiro is not currently operating [3, 4]. Oxidized, near-surface ores are a promising future
source for platinum-group metals because of their easy accessibility. They constitute a resource of ca. 160— 400 Mt, but mining has so far been hampered due to insufficient recovery rates [5, 6].
It is well know, Platinum group metals are usually found in copper/nickel ultrabasic rocks and are also produced commercially as a by-product of nickel/copper ore processing. South Africa and Zimbabwe, with vast platinum ore deposits in the Merensky Reef of the Bushveld complex, is the world's largest producer of platinum [7, 8]. The huge quantities of nickel/copper ore processed makes up for the fact that platinum makes up only two parts per million of the ore. Because of the low concentration level and the in-homogeneous distribution of these elements in rocks, a large sample size is commonly taken for preconcen-tration prior to instrumental determination.
Different procedures and methods for the enrichment and determination of PGEs and Au in geological samples have been proposed. The methods are mainly carried out by nickel sulfide fire assay combined with laser ablation-inductively coupled plasma-mass spectrometry [9], lead fire assay combined with inductively coupled plasma-optical emission spectrometry [10, 11], high-pressure acid digestion combined with inductively coupled plasma-mass spectrometry (ICP-MS) [12], lead fire assay combined with laser ablation-ICPMS [13], Fe-Ni Sulfide Fire Assay combined with ICP-MS and Negative Thermal Ionization-
Mass Spectrometry [14], liquid extraction combined with ICP—OES [15], solvent extraction combined with ICP—MS [16] gamma-ray spectrometry [17], lead fire assay combined with glow discharge-MS and spark-OES [18], nickel sulfide fire assay combined with neutron activation analysis [19], sodium peroxide fusion combined with ICP-MS [20].
In these methods, the main enrichment step include nickel sulfide fire assay and lead fire assay. Lead fire assay was widely used as an effective enrichment way of gold, palladium and platinum. However, the enrichment ability for ruthenium, rhodium and iridium were not good [21]. Nickel sulfide fire assay (NiS-FA) method coupled with tellurium coprecipi-tation has been widely applied for determining gold and PGEs in geological samples. NiS-FA gained popularity as a preconcentration method for the determination of gold and PGE. NiS-FA allows for the use of a large sample size. Sample sizes, used by different laboratories, range from 10 to 50 g.
The main ways of detection are combined with ICP—OES and ICP-MS. Basically, the sample introduction system and plasma of ICP—OES and ICP-MS look similar. In ICP—OES, the optical spectrum with a typical range of 165—800 nm is viewed and measured, either sequentially or simultaneously. The simultaneous ICP—OES can be faster for a large numbers of elements, but more expensive, than sequential ICP—OES.
For routine analyses, ICP—OES has matured in automation to the point where relatively unskilled personnel can use methods created by the ICP—OES specialist, similar in ease of use as flame atomic absorption spectrometry (AAS). Until recently, ICP—MS was still the domain of the specialist chemist making fine adjustments before performing routine analysis. Recent ICP—OES spectrometers have been able to analyze routinely up to 10% total dissolved solids (TDS) and even up to 30% for simple salt solutions. Although the analysis of 0.5% TDS for ICP—MS may be possible for a limited time-scale, most chemists are happier with 0.2% maximum TDS. The short-term (in-run) precision of ICP-MS is generally 1—3%. This is improved routinely by use of multiple internal standards. The longer term precision (over a period of hours) is still <5% relative standard difference (RSD). The use of isotope dilution can give results of very high precision and accuracy, although the cost can be prohibitive for routine analysis, due to the cost of the standards. ICP—OES has generally in-run precision of 0.3—2% RSD and again less than 3% RSD over several hours (for some spectrometers <1% for 4 hours). According to the above, ICP—OES was adopted to the route analysis for the large amount of rocks.
In this work, a rapid and practical strategy for the determination of platinum, palladium, ruthenium, rhodium, iridium and gold by ICP—OES combined with nickel sulfide fire assay and ultrasound-assisted
extraction was reported firstly. Gold and PGE were enriched into a NiS button by nickel sulphide fire assay. The button was dissolved by HCl and then Te was used to coprecipitate the dissolved gold and PGE. Finally, the separated gold and PGE by filtration were extracted with aqua regia by ultrasound wave and determined by ICP—OES. Comparing the conventional heating extraction, ultrasound wave extraction not only could effectively escape the loss of the solution volume in the extraction process, but also could reduce the time of extraction. Moreover, the method by ICP— OES has wider linear range, better precision, better toleration of (TDS) and lower cost compared to ICP-MS. The proposed method has been applied into the determination of platinum-group element and gold in large amounts of ultrabasic rocks from the Great Dyke of Zimbabwe.
EXPERIMENTAL Apparatus
A simultaneous inductively coupled plasma optical emission spectrometer (iCAP MFC 6300 Radial, Thermo Fisher Scientific) was used in the study employing a high-resolution Echelle optical system and charge injection device (CID) array detector, with an extended spectral range of 166—847 nm and axial plasma view. Appropriate solutions were introduced into the ICP plasma using an ordinary sample introduction system including a quartz nebulizer, a cyclonic spray chamber and a quartz torch. The ICP-OES operating conditions were listed in Table 1.
Reagents
Analytical-grade anhydrous sodium tetraborate, anhydrous sodium carbonate and silica, laboratory grade sublimed sulfur, guaranteed grade HCl and HNO3 and deionized water were used. The nickel powder was self-purified. The solution of 0.5 mg/mL in 3 mol/L HCl was prepared from analytical reagent grade Na2TeO4 • 2H2O.
SnCl2 solution of 1 mol/L in 6 mol/L HCl was prepared from analytical-grade SnCl2 • 2H2O. All solutions were prepared in pure water (distilled and passed through a Milli-Q water purification system from Mil-lipore, Milford, MA, USA).
Standard Solution
The preparation of single element stock standard solution: (1) platinum and palladium standard solution: 0.1000 g spectrograde platinum, palladium metal was accurately weighed into a 100 mL beaker. 20 mL aqua regia was added to the beaker and heated on a hotplate to dissolve. 0.2 g NaCl was added into the solution that was then evaporated to dry in boiling-water bath. The HNO3 was removed by adding (2 + 1)
Table 1. The operating parameters for Thermo 6300R ICP-OES
Operating parameter Parameter value
Excitation frequency, MHz 27.12
Radio frequency forward power, W 1150
Plasma view Axial
Detector CID-86 chip
Spectral range, nm 166-847
Spectral resolution 7 pm at 200 nm
Torch and Center tube Quartz
Spray chamber Cyclonic spray chamber
Nebulize Quartz concentric nebulizer
Plasma gas flow rate, L/min 12.00
Auxiliary gas flow rate, L/min 1.00
Carrier gas flow rate, L/min 0.4
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