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

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

Sample	Supplier
Certified Reference Material - FeCr (SRM 64c)	NIST
Certified Reference Material - Chromite ore (SARM 8)	Mintek
Certified Reference Material - FeCr slag (SARM 77)	Mintek
FeCr sample	Mining industry
Chromite ore sample	Mining industry
FeCr slag sample	Stainless 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. 

Figure 2. PerkinElmer® Optima™ 7300 DV ICP-OES

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

Nebulizer	Gem Tip Cross flow
Spray chamber	Scott
Injector	Alumina
RF	1500 W
Argon Flow rate	Plasma: 16.0 L/min  Nebulizer: 0.8 L/min  Auxiliary: 0.4L/min
Shear gas	100 psi
Sample flow rate	1.0 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

Element	Wavelength	Viewing mode	MDL (mg/Kg)
Al	394,401	Axial	63
Ca	315,887	Axial	1000
Co	228,616	Axial	38
Cr	283,563	Radial	250
Cu	224,700	Axial	25
Fe	238,204	Radial	375
Mg	279,077	Radial	63
Mn	257,610	Radial	25
Ni	231,604	Axial	125
P	178,221	Axial	125
S	180,669	Axial	625
Si	212,412	Radial	63
Ti	334,940	Axial	63
V	290,880	Axial	5

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

Element	Wavelength	Average Experimental values (%) n=10	Certified values (%)	Accuracy (%)	Precision (%)
Al	394,401	BDL	-	-	-
Ca	315,887	BDL	-	-	-
Co	228,616	0,05	0,05	107	2
Cr	283,563	69,03	68,0	102	23
Cu	224,700	BQL	0,005	(60)	(15)
Fe	238,204	25,87	24,98	104	3
Mg	279,077	BDL	-	-	-
Mn	257,610	0,16	0,16	101	2
Ni	231,604	0,43	0,43	100	2
P	178,221	BQL	0,02	(86)	(7)
S	180,669	BDL	0,07	-	-
Si	212,412	1,28	1,22	105	3
Ti	334,940	BQL	0,02	(63)	(5)
V	290,880	0,15	0,15	102	2

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

Element	Wavelength	Average Experimental values (%) n=10	Certified values (%)	Accuracy (%)	Precision (%)
Al	394,401	5,81	5,56	105	2
Ca	315,887	BQL	0,19	(90)	(18)
Co	228,616	0,03	-	-	4
Cr	283,563	35,50	33,5	106	2
Cu	224,700	BDL	-	-	-
Fe	238,204	15,20	14,13	108	2
Mg	279,077	8,98	8,86	101	2
Mn	257,610	0,21	-	-	2
Ni	231,604	0,15	-	-	3
P	178,221	BDL	0,004	-	-
S	180,669	BDL	0,03	-	-
Si	212,412	2,12	2,01	106	2
Ti	334,940	0,14	0,14	95	2
V	290,880	0,07	0,08	90	2

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

Element	Wavelength	Average Experimental values (%) n=1	Certified values (%)	Accuracy (%)	Precision (%)
Al	394,401	15,34	14,55	105	5
Ca	315,887	2,48	2,60	95	2
Co	228,616	BQL	-	-	(8)
Cr	283,563	9,39	8,55	110	3
Cu	224,700	BDL	-	-	-
Fe	238,204	5,98	5,31	113	4
Mg	279,077	13,94	13,86	101	2
Mn	257,610	0,16	-	-	2
Ni	231,604	BQL	-	-	(7)
P	178,221	BDL	-	-	-
S	180,669	0,17	(0.32)	(54)	5
Si	212,412	12,81	12,5	102	2
Ti	334,940	0,36	-	-	1
V	290,880	0,06	-	-	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 )

Element	Wavelength	#771 (%)	#775 (%)	#784 (%)	SRM 64c (%)	SARM 8 (%)	SARM 77 (%)
Al	394,401	101	96	109	101	99	92
Ca	315,887	96	109	99	114	97	112
Co	228,616	103	101	99	98	102	100
Cr	283,563	107	105	107	107	108	103
Cu	224,700	99	89	94	99	101	98
Fe	238,204	110	107	108	107	103	101
Mg	279,077	102	97	94	91	94	93
Mn	257,610	105	104	108	105	107	110
Ni	231,604	109	112	110	96	102	100
P	178,221	114	57	113	96	84	106
S	180,669	97	95	107	110	66	86
Si	212,412	102	53	106	91	105	91
Ti	334,940	107	105	106	105	103	102
V	290,880	105	105	105	99	103	97

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.