An Improved Method of X-Ray Fluorescence Analysis of Chrome Containing Refractories

Subrata Banerjee, Ben G. Olsen and Melvin K. Hess

General Refractories Co., Research Center, Baltimore, Maryland 21203, USA

An improved x-ray fluorescence method has been developed for the analysis of chrome containing refractory materials (raw materials, in process and 6nal products). It involves the fusion of the powdered samples with a mixture of sodium hexametaphosphate and lithium metaborate in the presence of lanthanum oxide into a glass disc followed by polishing of the disc before exposure to the x-rays. Three British standards, two South A€rican Standards and one NBS Standard were used for standardizing the method. SiO,, Fez03, AlzO,, CaO MgO and Cr203 were used in the standardization because these compounds constitute the majority of the materials under analysis. Linear regression analysis of the data showed that all the lines were linear, the coefficient of determination being equal or dose to 1.00 with a mean error of less than *t3.5%, excepting CaO which had a mean error of +8.6% because of its low concentration and the larger variance in its standard value.

INTRODUCTION

It has been suggested in previous papers'.' that x-ray fluorescence (XRF) is the most important tool in the analysis of refractory oxide materials. The composition of different types of refractories varies so much from one extreme to the other, that no single method is applicable to all the materials. A judicial combination of different methods has enabled us to analyze all types of refractories-from silica and alumina-silicate to high alumina, from magnesia and magnesia-chrome to high chrome-magnesia.

Chromite ore by itself is one of the most difficult materials to analyze-some specific chromite ores pose problems at every step of wet analysis (ASTM C-572). Magnesite chrome refractories form different types of phases with SiOz and CaO depending on the ratio of CaO/SiO,, temperature and atmosphere of firing. X- ray analysis is very sensitive to these mineralogical differences particularly with regard to CaO and SiOz analysis which, incidentally, are the most important constituents in controlling refractory properties. Furthermore, the raw materials used have a significant effect on the analysis although the ultimate composi- tion may be the same. For these reasons chrome containing refractory materials could not be analyzed by direct measurement of briquettes made from fine ground samples.

Attempts have been made to use theoretical methods of calculating-coefficients from different in- terelemental effects were not successful, primarily be- cause of the serious effect of small mineralogical changes as well as different raw materials, although the ultimate chemistry may be the same. Hence the most practical and realistic approach is the fusion of the material into a glass disc to eliminate the mineralogical effects.

Chromite ores have been known to dissolve in phosphoric acid, so work has been carried out on the

fusion of chrome containing materials in phosphate g l a ~ s . ~ , ~ The use of plain phosphate created problems of occasional cracking of the disc. The present method involves the use of a mixture of phosphate and borate which was very successful. A dilution factor of 15 and the use of lanthamum oxide as a heavy absorber alleviated almost all the problems of mineralogical, interelemental and absorption/enhancement factors.

EXPERIMENTAL ~~~ ~~

All standard samples (-100 or -200 mesh) were taken out of the bottles directly and mixed with sodium hexametaphosphate, lithium metaborate and lanthanum oxide (dried at 400 "C for 1 h) in the ratio of 0.5 : 6 : 1 : 0.25, respectively, in a mixerlmill. To facilitate non-wettability three drops of HBr was added to each sample, which was placed in a Pt/5% Au crucible and heated slowly on burners until the mater- ial was fused. The crucibles with the molten glass were then transferred to a furnace, preheated at 1125 "C, and heated for 40 min with occasional swirling every 10-15 min to ensure complete homogenization. The molten glass was then poured into preheated disc molds (to fit directly in the spectrometer) 30 mm (1- ;sin) diameter, made of Pt/5% Au placed on wire stands (about 1-2 in high) to facilitate uniform air cooling of the glass disc. The discs along with the small beads remaining in the crucible were reweighed to make corrections for any loss occurring during the fusion. This factor was then introduced in the final percentage weight calculation. The cooled discs were then ground and polished (70~l.m diamond power) and run in the spectrometer. All the samples were run duplicate.

Analyses were carried out in a Siemens x-ray spec- trometer SRS-1 with a Cr target at 35 kv and 55 mA. A Ti filter was used for Cr determination, and the flow

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X-RAY SPECTROMETRY, VOL. 11, NO. 1, 1982 25

S. BANERJEE, B. G. OLSEN AND M. K. HESS

Table 1. Regression equations for the analyzed oxides

Percentage Oxide range

Si02 0.6- 5.0 Fe203 7.0-28.0

10.0-30.0 CaO 0.3- 2.0 MgO 10.0-62.0 Cr203 13.0-49.0

Equation (R = ratio of x-ray

intensities of samplelreference

3.70R- 2.33 31.97R- 3.21 16.59R- 1.36

1.30R- 0.52 24.86R- 14.45 49.91R- 3.27

Percent Standard relative deviation deviation

0.11 3.4 0.47 2.9 0.25 1.5 0.06 8.6 0.59 2.0 0.46 1.4

Coefficient of

determination

0.996 0.996 1 .ooo 0.989 1 .ooo 1 .ooo

Table 2. Accuracy

Oxide 1

SiO, 1.05 Fez03 27.45

15.02 CaO 0.18 MgO 1 1.47 Cr203 46.31

2 3 4

1.04 1.10 1.19 27.14 27.32 27.13 15.02 15.21 15.18 0.17 0.18 0.17

11.75 11.41 11.74 46.39 46.11 46.39

proportional counter was stabilized by a gas density compensator. Three analyzing crystals were used: KAP for Mg, PET for A1 and Si and LiF 200 for Ca, Fe and Cr. All measurments were made on a ratio basis with reference to one 'reference' sample with similar composition. Duplicate samples were run for each standard.

~~ ~~

RESULTS AND DISCUSSION

The method has been standardized by analyzing the following available standard reference samples: British Standards BCS-308-chromite ore, BCS-369-burned magnesia-chrome refractory, BCS-370-burned magnesia-chrome refractory; South African Standards SARM-8-chromite ore, SARM-9-chromite ore; NBS Standard 103a-chromite ore. These provided a

Standard deviation 5 SD %RSD

1.07 0.06 5.50 27.26 0.1 1 0.40 15.11 0.09 0.58 0.18 0.01 3.41

11.84 0.20 1.72 46.30 0.12 0.26

good range of those analyte concentrations that are generally handled in the refractory industry. Linear regression equations were derived for the calibration lines of the individual oxides, along with their error estimates and coefficients of determination which are given in Table 1. It is apparent from this table that the error estimates or standard deviations are very low, except for CaO which has the lowest concentration. Actually the certified analyzed values for CaO have a much larger variation compared to other component oxides.

The accuracy of the method, given in Table 2, was determined by running a chromite ore from five fused discs. The standard deviation of each component was small.

The certified values and the values calculated from the present method along with their percent relative deviation are given in Table 3. Calibration lines for SiO,, Fe203, A1,03, CaO, MgO and Cr203 are shown

Table 3. Analysis of Chrome-containing refractory standards

BCS-369 BCS-270 BCS-308 Relative Relative Relative

Certified X-ray deviation Certified X-ray deviation Certified X-ray deviation Oxide analysis analysis f%) analysis analysis (%) analysis analysis f%)

Fe203 10.30 10.56 1.4 7.23 7.05 2.5 17.00 17.07 0.4

CaO 1.17 1.14 2.6 1.54 1.51 1.9 0.34 0.35 2.9 MgO 53.50 53.04 0.8 62.80 62.10 1.1 16.40 16.14 1.6 cr203 17.15 17.09 0.3 13.41 13.49 0.6 41.50 41.26 0.6 Total 99.42 99.09 100.27 99.20 98.89 98.76

SiO, 2.59 2.52 2.7 3.01 2.95 2.0 4.25 4.30 1.2

A1203 14.71 14.74 0.2 12.28 12.10 1.5 19.40 19.64 1.2

NBS-lOa SARMB SARM-9

SiO, 4.63 4.56 1.5 4.30 4.31 0.3 0.62 0.66 6.4 Fe203 13.81 13.77 0.3 20.17 20.20 0.0 27.75 27.63 0.4 A1203 29.96 29.87 0.3 1.56 10.62 0.6 15.17 15.17 0.0 CaO 0.69 0.76 10.1 0.27 0.25 7.4 0.16 0.15 6.2 MgO 18.54 18.46 0.4 14.73 14.92 1.3 10.82 11.03 1.9 Cr203 32.06 32.14 0.2 48.97 49.02 0.1 46.45 46.57 0.3 Total 99.69 99.56 99.0 99.32 100.97 101.21

26 X-RAY SPECTROMETRY, VOL. 11, NO. 1, 1982 @ Heyden & Son Ltd, 1982

XRF ANALYSIS OF CHROME CONTAINING REFRACTORIES

I I I 1 I 2 3 4 5

% SIOZ

Figure 1. X-ray gross intensity ratio of sample to reference versus percentage of SiO, in sample.

10 20 30 0

% Fe,03

Figure 2. X-ray gross intensity ratio of sample to reference versus percentage of Fe,O, in sample.

2 .o

Figure 3. X-ray gross intensity ratio of sample to reference versus percentage of AI,O, in sample.

in Figs 1-6, respectively. The horizontal lines within the circles indicate the variance in the certified values, and vertical lines indicate the variance in the observed intensity between two individual runs.

Consistency of the starting material has been found to be very important in this method. The present work was carried out with 'Baker' grade sodium hexameta- phosphate, lithium metaborate of 99% purity and

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 70 coo

Figure 4. X-ray gross intensity ratio of sample to reference versus percentage of CaO in sample.

I I I I I I I 10 20 30 40 50 60 70

%MgO

Figure 5. X-ray gross intensity ratio of sample to reference versus percentage of MgO in sample.

% Cr203

Figure 6. X-ray gross intensity ratio of sample to reference versus percentage of Cr,O, in sample.

lathanum oxide, 99.9% pure. Drying of lanthanum oxide prior to use at 400 "C for 1 h is essential.

The introduction of lithium metaborate aided fusion and helped prevent breakage of the glass discs. After casting in the molds, the discs had to be polished because the surface of the molds were pitted and became buckled after long usage. If new molds were available with clean, smooth faces, the polishing step

@ Heyden & Son Ltd, 1982 X-RAY SPECTROMETRY, VOL. 11, NO. 1, 1982 27

S. BANERJEE, B. G. OLSEN AND M. K. HESS

could be omitted and the disc could be run directly after casting.

Homogenization was of importance in the fusion process and intermittent swirling of the molten glass way essential. Both SiO, and CaO, which are minor ingredients and are very important for the refractory properties, can produce wide variance due to a lack of homogeneity invalidating the analysis. Furthermore, the occurrence of partial devitrification can be pre- vented by proper homogenization.

In addition, care must be taken to prevent contami- nation during the polishing process.

CONCLUSION Chrome containing refractory materials have been analyzed successfully with a new method by XRF involving fusion of the sample into a glass disc with sodium hexametaphosphate and lithium metaborate. Calibration lines were established from six standard reference materials. The most important factor in the process was the homogenization of the glass during fusion and the prevention of contamination during polishing, which would otherwise have led to inconsis- tencies in the values of minor components.

REFERENCES

1. S. Banerjee and B. G. Olsen, Adv. X-Ray Anal. 18,317 (1974). 2. S. Banerjee and B. G. Olsen, Appl. Spectrosc. 32 (6). 576

3. R. P. Lucas and J. R. Ryan, paper presented at the American

Received 10 February 1981; accepted 24 July 1981

@ Heyden & Son Ltd, 1982 (1978).

Ceramic Society Meeting, Washington DC (April 1975).

28 X-RAY SPECTROMETRY, VOL. 11, NO. 1, 1982 @Heyden & Son Ltd, 1982