Dissolution made easy using peroxide fusions for ICP-OES analyses for chromite ores, ferrochromes and chromium slags

Although there are more than ten known chromium minerals, only one is a source of commercial importance. This mineral is known as chromite and has the theoretical composition FeCr2O4 containing 68 % of chromic oxide (Cr2O3), in which the proportions of Mg2+, Fe2+ and Cr3+, Al3+, Fe3+ may vary considerably.

The main product generated by chromite is ferrochrome, a major player in the steel industry. In particular the stainless steel industry uses more than 90 % of the world’s chromite output. The mining and steel industries must assess the quality of the chromite ore to optimize the grade of their stainless steel production. As a result, the chemical analyses of the chromite ore as well as its final and waste products are mandatory. Metal analysis traditionally uses AA or ICP-OES to measure the metal contents in the ores and industrial products. However, the traditional dissolution method for chromite and ferrochrome is a multi-step, multi-acid digestion which requires the use of HNO3, HF and HClO4 and can take between 1 to 3 hours.

Knowing the risks associated with the use of HF and HClO4, many laboratories look for alternative methods to obtain full dissolution of their samples while optimizing their uptime and productivity. As will be demonstrated, sodium peroxide fusions are a quick, safe and efficient alternative for the dissolution of these specific samples.

Introduction

Although there are more than ten known chromium minerals, only one is a source of commercial importance. This mineral is known as chromite and has the theoretical composition FeCr2O4 containing 68 % of chromic oxide (Cr2O3), in which the proportions of Mg2+, Fe2+ and Cr3+, Al3+, Fe3+ may vary considerably[1].

The main product generated by chromite is ferrochrome, a major player in the steel industry[2]. In particular the stainless steel industry uses more than 90 % of the world’s chromite output. The mining and steel industries must assess the quality of the chromite ore to optimize the grade of their stainless steel production. As a result, the chemical analyses of the chromite ore as well as its final and waste products are mandatory. Metal analysis traditionally uses AA or ICP-OES to measure the metal contents in the ores and industrial products. However, the traditional dissolution method for chromite and ferrochrome is a multi-step, multi-acid digestion which requires the use of HNO3, HF and HClO4 and can take between 1 to 3 hours[3,4].

Knowing the risks associated with the use of HF and HClO4, many laboratories look for alternative methods to obtain full dissolution of their samples while optimizing their uptime and productivity. As will be demonstrated, sodium peroxide fusions are a quick, safe and efficient alternative for the dissolution of these specific samples.

Experimental

Sample preparation

Peroxide fusions can be performed either manually or with auto­mated systems. Although both methods are efficient, the auto­mated systems have the advantage of increasing productivity, improving safety, maintaining repeatable preparation conditions, avoiding spattering as well as cross-contamination. In this project, fusions were performed using a Claisse® Peroxide™ Fluxer (Figure 1).

The Claisse® Peroxide™ Fluxer is a 6 position gas Fluxer specifi­cally designed to do multiple and repetitive peroxide fusions with excellent repeatability and reproducibility. It’s special de­sign and finish makes it optimal to endure repetitive exposure to the aggressive environment generated by peroxide fusions. Combined with the specially designed high-stability burners, the Peroxide™ fluxer offers stable temperature allowing for controlled fusions.

Different samples and reference materials were used to vali­date the developed method (Table 1). 

Table 1. List of Reference Materials and Samples Used to Validate the Developed Method
SampleSupplier
Certified Reference Material - FeCr (SRM 64c)NIST
Certified Reference Material - Chromite ore (SARM 8)Mintek
Certified Reference Material - FeCr slag (SARM 77)Mintek
FeCr sampleMining industry
Chromite ore sampleMining industry
FeCr slag sampleStainless steel industry

In a zirconium crucible, 0,2 g of finely ground sample at less than 100 μm was mixed with 0.5 g of Sodium Carbonate (Na2CO3) and 3,0 g of Sodium peroxide (Na2O2). The crucible containing the mixture was placed on the Claisse® Peroxide™ Fluxer, fused at 560ºC for 3,5 minutes, then cooled by the fluxer fans for 4 minutes.

The cooled zirconium crucible was then placed in a funnel over a 250 mL volumetric flask. Around 10 mL of warm (70ºC) ultra-filtered deionized water is added to the crucible followed by 25 mL of HNO3. The dissolution reaction takes less than a minute after which time the crucible is tipped over and rinsed out with deionized ultra-filtered water. 25 mL of HCl is then added in the flask, then brought up to volume with diluted acid, and brought to the ICP-OES for analysis.

Instrumentation

The measurements were performed using the PerkinElmer® Optima™ 7300 DV ICP-OES instrument (Figure 2). It is equipped with the WinLab32™ for ICP Version 4,0 software. On this particular instrument, the ICP torch is mounted in a horizontal orientation in the instrument’s shielded torch box, but can be viewed either axially or radially.

A Scott Spray Chamber with a Gem Tip Cross Flow Nebulizer (Figure 3) was selected as an introduction system because of its proven reliability, robustness and capability to handle high dissolved solids. The viewing mode is user-selectable on an element-by-element basis. A shear gas flow (compressed air) eliminates the cool plasma tail and allows a direct observation of the plasma’s normal analytical zone, thus minimizing che­mical matrix effects when the axial-view mode is employed. 
Fig 2.bmp
Figure 2. PerkinElmer® Optima™ 7300 DV ICP-OES
Fig 3.bmp
Figure 3. Scott Spray Chamber with Gem Tip Cross Flow Nebulizer

By combining a SCD detector and an Echelle optical system, the Optima™ 7300 DV ICP-OES can measure all the wavelengths simultaneously. Its wavelength flexibility allows the end users to easily add new elements or wavelengths as their program changes. Another benefit of using the Optima for fusion samples includes a 40 MHz free-running solid state RF generator designed to operate between 750 to 1500 W in 1 W increments. High RF power is required to generate a robust plasma, essential for precise analysis of high matrix samples such as fusion samples[4] (refer to Table 2 for detailed Optima™ 7300 DV operating parameters).

Table 2. Optima™ 7300 DV operating parameters
NebulizerGem Tip Cross flow
Spray chamberScott
InjectorAlumina
RF1500 W

Argon Flow rate

Plasma: 16.0 L/min 
Nebulizer: 0.8 L/min 
Auxiliary: 0.4L/min

Shear gas100 psi
Sample flow rate1.0 mL/min

Results and discussion

The following criteria were taken into consideration in selec­ting the wavelength: (a) the freedom from spectral interfe­rences; (b) the different sensitivities and expected concentration in the samples. The most sensitive line was not always used in order to avoid spectral interferences and to remain in the linear range. Observed interferences were compensated for by modifying the processing parameters (e.g. adjusting the background correction points, applying multi-component spectral fittings (MSF) or inter-elemental corrections (IEC)). 

Method detection limits (MDLs) were based on ten replicate measurements of a series of low concentration or diluted sample solutions. The MDL was calculated by multiplying the standard deviation of the ten replicate measurements by 3[5]:

MDL = 3 x S10 x CDF

Where: S10 = Std Deviation of the ten replicates 

CDF = Corrected Dilution Factor 

Table 3 demonstrates analytes of interest with selected wavelengths, viewing modes and method detection limits (MDL).

Table 3. Analytes of interest with selected wavelengths, method detection limits (MDL) and viewing modes
ElementWavelengthViewing modeMDL (mg/Kg)
Al394,401Axial63
Ca315,887Axial1000
Co228,616Axial38
Cr283,563Radial250
Cu224,700Axial25
Fe238,204Radial375
Mg279,077Radial63
Mn257,610Radial25
Ni231,604Axial125
P178,221Axial125
S180,669Axial625
Si212,412Radial 
63
Ti334,940Axial63
V290,880Axial5

The accuracy and precision of the method was evalua­ted. The accuracy were determined by calculating the elemental recovery of certified reference materials (CRMs). The precision was determined by preparing and measuring 10 replicates of the various CRMs. The results for each CRM are presented in Tables 4, 5 and 6. The accuracy and precision obtained demonstrates that the developed method performed very well.

Table 4. Accuracy and precision measurements on NIST SRM 64c
ElementWavelengthAverage Experimental values (%) n=10Certified values (%)Accuracy (%)Precision (%)
Al394,401BDL---
Ca315,887BDL---
Co228,6160,050,051072
Cr283,56369,0368,010223
Cu224,700BQL0,005(60)(15)
Fe238,20425,8724,981043
Mg279,077BDL---
Mn257,6100,160,161012
Ni231,6040,430,431002
P178,221BQL0,02(86)(7)
S180,669BDL0,07--
Si212,4121,281,221053
Ti334,940BQL0,02(63)(5)
V290,8800,150,15 
1022
In parenthesis and italic = informative values          BDL = below detection limit 
Corrected dilution factor = 1250                                 BQL = below quantification limit
Table 5. Accuracy and precision measurements on Mintek SARM 8
ElementWavelengthAverage Experimental values (%) n=10Certified values (%)Accuracy (%)Precision (%)
Al394,4015,815,561052
Ca315,887BQL0,19(90)(18)
Co228,6160,03--4
Cr283,56335,5033,51062
Cu224,700BDL---
Fe238,20415,2014,131082
Mg279,0778,988,861012
Mn257,6100,21--2
Ni231,6040,15--3
P178,221BDL0,004--
S180,669BDL0,03--
Si212,4122,122,011062
Ti334,9400,140,14952
V290,8800,070,08 
902

In parenthesis and italic = informative values          BDL = below detection limit 
Corrected dilution factor = 1250                                 BQL = below quantification limit

Table 6. Accuracy and precision measurements on Mintek SARM77
ElementWavelengthAverage Experimental values (%) n=1Certified values (%)Accuracy (%)Precision (%)
Al394,40115,3414,551055
Ca315,8872,482,60952
Co228,616BQL--(8)
Cr283,5639,398,551103
Cu224,700BDL---
Fe238,2045,985,311134
Mg279,07713,9413,861012
Mn257,6100,16--2
Ni231,604BQL--(7)
P178,221BDL---
S180,6690,17(0.32)(54)5
Si212,41212,8112,51022
Ti334,9400,36--1
V290,8800,06--2

In parenthesis and italic = informative values          BDL = below detection limit 
Corrected dilution factor = 1250                                 BQL = below quantification limit 

Table 7. Recovery results on pre-fusion spikes ( n = 5 )
ElementWavelength#771 (%)#775 (%)#784 (%)SRM 64c (%)SARM 8 (%)SARM 77 (%)
Al394,401101961091019992
Ca315,887961099911497112
Co228,6161031019998102100
Cr283,563107105107107108103
Cu224,7009989949910198
Fe238,204110107108107103101
Mg279,0771029794919493
Mn257,610105104108105107110
Ni231,60410911211096102100
P178,221114571139684106
S180,66997951071106686
Si212,412102531069110591
Ti334,940107105106105103102
V290,8801051051059910397
Comments: 
• Spike concentration = 50 to 100 % more than the concentrations in the samples and CRM solutions (Corrected dilution factor: 2500). 
• If concentrations < MDL, addition of ±10 times the MDL value.

Conclusion

Peroxide fusions combined with the simultaneous ICP-OES (Optima™ 7300 DV) have the analytical capabilities to perform the analysis of chromite ore, ferrochrome and chromium slag samples with good accuracy, precision and speed of analysis. The analytical method developed was robust and fulfills the requirements normally set for the analysis of high matrix samples such as fusion samples. Metal components were measured at low and high concentrations in a variety of samples and reference materials, demonstrating good accuracy. The sodium peroxide fusion approach to dissolution of chromite ore, ferrochrome and chromium slag samples is an excellent alternative to other harsh, incomplete and time consuming acid digestions. 

References

1. http://minerals.usgs.gov/minerals/pubs/commodity/chromium/chrommcs06.pdf, Papp, John F. "Mineral Commodity Summary 2006: Chromium". United States Geological Survey. 
2. Determination of major and minor elements in ferroalloys by inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom., 1988, 3, 1101-1103 Authors: M. Vaamonde, R. M. Alonso, J. García and J. Izaga 
3. Analysis of Ferroalloys by Atomic Absorption Spectrometry, Applied Spectroscopy, Vol. 24, Issue 6, pp. 576-579 (1970) Authors: D. C. Smith, J. R. Johnson and G. C. Soth 
4. Trace Metal Characterization of Soils using the Optima 7300DV ICP-OES, PerkinElmer® Inc., Application Note, Author: Praveen Sarojam, PhD. 
5. Protocole pour la validation d'une méthode d'analyse en chimie, Centre d'Expertise en Analyse Environnementale du Québec, Ministère de l'Environnement, Document de référence DR-12-VMC, Édition courante, 21 pages.

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