APPROACHING A UNIVERSAL SAMPLE PREPARATION … · Page 1 of 6 APPROACHING A UNIVERSAL SAMPLE PREPARATION METHOD FOR XRF ANALYSIS OF POWDER MATERIALS John Anzelmo, Alexander Seyfarth,

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APPROACHING A UNIVERSAL SAMPLE PREPARATION METHODFOR XRF ANALYSIS OF POWDER MATERIALS

John Anzelmo, Alexander Seyfarth, Larry AriasBruker AXS, Inc. Madison, Wisconsin

Abstract

It is well known that the preparation of metals and liquids for XRF analysis is fast, easy, andusually requires little strategy. The preparation of powder samples such as silicates, carbonates,slags, cements, ferro-alloys and other powdered materials however requires careful planning inthe way of additives, lubricants, binders, dilution, backing and holders. The ratio of binder tosample must include forethought regarding the homogeneity of the briquette, the stability of thebriquette, the cleaning and cleanliness of the grinding vessel, and the performance requirementsof the analysis. The ideal situation would be to have one set of preparation materials, with onlyone sample to additive ratio suffice for all powder preparations. Various preparation materialsand strategies are examined, and their usefulness toward achieving a universal preparationmethod considered.

Introduction

The word “approaching” was chosen very carefully when deciding on the title of this paper,because there is no universal preparation method for pressed powders. However, it is possible togreatly reduce the number of different methods in many laboratories. Many laboratories usewidely varying techniques in their selection of preparation methods, when it may have beenpossible for that laboratory to use only one method.

The use of a grinding aid or lubricant is the one material that is not made use of in manylaboratories. This material has many properties which make the consideration of otherparameters in the preparation method unnecessary. The grinding aid Vertrel XFTM (1) wasreported in a previous paper(2) to have these properties in the preparation of cement, clinker andrelated raw materials. This lubricant, which replaces previously used Freon TF, is a clearcolorless liquid, which is not retained in the pressed pellet as shown by scans for fluorine infinished sample briquettes. Vertrel XF is the trade name for 2,3 Dihydroperfluoropentane, and thematerial Safety Data Sheet(1) (MSDS) for this product should be consulted for properties andcharacteristics of this material. It is not intended for this method to be selected as a universalmethod. But, rather, it is hoped that the concepts presented here will be applied, and morethought given to reducing the number of variables considered when preparing various materials.

Discussion

There are many variables (Table 1) to consider when designing a method for preparation of amaterial for XRF analysis. Many times, the container material chosen is W/C because of theneed to eliminate iron contamination when using steel grinding media. A grinding aid acts as alubricant, and minimizes if not eliminates, the iron contamination, so that a hardened steelcontainer can be used. The binder to sample dilution is another area where multiple methods can

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This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website – www.dxcicdd.com

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be reduced. Typical binders stick to the grinding vessel and components unless the proper ratiois determined, making many trial and error experiments necessary. The use of Vertrel XF alongwith a Whatman CF11 fibrous cellulose(3) minimizes the need to vary dilution ratios. The lowash content (typically ~ 50 ppm) of this binder, used in a dilution ratio of 20% insures that themost contamination possible from the binder is 10 ppm total. No expanding of briquettes hasbeen observed when using the the formula listed in Table 2, even after years.

After grinding, the mixture exists as a slurry. A gentle draft for ~15 seconds will vaporize theliquid. The remaining powder can be brushed from the container and components very easily,almost dust free. The container and components need only a gentle wiping with clean,disposable, paper towels to remove contamination for the next grind.

Table 1. Sample Preparation Variables1. Grinder Type (manual, mixer, ring and puck disk mill)

- Particle size distribution desired- Grinding time allowed or desired

2. Container Material (Fe/Cr, W/C, ZrO, Si/N, Al2O3)- Weight, brittleness, expense

3. Grinder cleanliness and container condition (rust outside inserts)4. Workbench contamination5. Sample volume6. Container/Ring/Puck/Lid condition7. Component dimensions and pitting8. Binder type and dilution9. Grinding Aid – Lubricant10. Dust11. Sample contamination12. Ease of cleaning

Table 2. Proposed “General Method” for Sample PreparationØ Disc Mill (ring and puck type)Ø Hardened steel container and componentsØ 10.00x grams of sampleØ 2.50x grams Whatman CF11 cellulose as binderØ 7 ml Dupont Vertral XFØ 90 second grinding timeØ 15 second press at 40, 000 lbs. in 40mm die moldØ Tapered Aluminum pellet cup

The elimination of contamination for subsequent sample preparations is of paramountimportance. Typical binders leave material stuck to the components that is difficult to remove,even after thorough wiping (Figure 1). Washing with water is then necessary, causing loss oftime.In some cases, such as when using Boric acid as a binder (Fig. 2), this effect is quite noticeable.The sample preparation method shown in Table 2 makes cleaning the grinding vessel andcomponents easy, dustless, and contamination free, while making a homogenous, stable, sturdy


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pellet. After the liquid has been removed from the freshly ground slurry, most materials appearevenly and smoothly distributed over the grinding media (Fig. 3). After the material is brushedout, and the vessel and components are wiped clean with a paper towel, the vessel appearsspotless as shown in Figure 4. In many critical applications where tight precision and accuracyspecifications must be met, the smallest amount of contamination from the vessel or previoussamples will not allow the method to pass. Table 3 shows the accuracy and precisionrequirements of such a method, ASTM C-114 Qualification(2).

Table 3. ASTM C-114 Qualification Specifications

The instrumentation used to generate data for this study was the S4 Explorer (Figure 5).

Many other materials (Table 4) were evaluated for preparation using the “General Method”proposed, and all had excellent results except gypsum, which is well know for its preparationproblems unless pressed as is.

Table 4. Various materials types prepared successfully with proposed “General Method”

Titanium, Iron, Copper Ores,Corrundum, Mullite, Kyanite, Etc.CokeFeMo, FeSiKaolin and lower quality claysCement, LimestoneSaltsQuartz, SilicatesSlags

Component

Max Difference between

Duplicates on alternate

days

Max Difference of the Average

of Duplicates from SRM Certificate

SiO2 0.16 ±0.2Al2O3 0.20 ±0.2Fe2O3 0.10 ±0.10CaO 0.20 ±0.3MgO 0.16 ±0.2SO3 0.10 ±0.1

Na2O 0.03 ±0.05K2O 0.03 ±0.05TiO2 0.02 ±0.03P2O5 0.03 ±0.03ZnO 0.03 ±0.03

Mn2O3 0.03 ±0.03


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Figure 1. Components after brushing – typical binder

Figure 2. Components after brushing – boric acid binder


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Figure 3. Material after grinding with lubricant

Figure 4. Components after brushing – lubricant used with binder


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Figure 5. S4 Explorer XRF Spectrometer

Conclusion

The use of a grinding aid or lubricant, together with the proper binder and dilution, can eliminatemost of the variations in sample preparation for widely varying materials. A lubricant utilizedduring the grinding process can eliminate wear and contamination, reduce dust, contribute tostable, homogenous briquettes, and reduce operator fatigue. The authors have no affiliation withthe vendors mentioned in this article, and only mention specific names because these materialsare known to produce the desired results.

References:

1. MSDS – VERTREL XF, Dupont Chemicals, Engineering and Product Safety, P.O. Box 80709, Chestnut Run, Wilmington, DE 19880-0709, December 29, 1998

2. Anzelmo, Seyfarth, Arias, XRF Analysis of Cement – ASTM C-114 Qualification,Presented at the Denver X-ray Conference, Steamboat Springs, CO, August 4, 1999

3. Whatman CF11 fibrous, cellulose powder, Whatman International Ltd., Springfield Mill,James Whatman Way, Maidstone, Kent, ME142LE England

4. Uhlig, Mauser, Granacher, XRF Spectrometers For Multi-Element Analysis, WorldCement, June, 1999


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APPROACHING A UNIVERSAL SAMPLE PREPARATION … · Page 1 of 6 APPROACHING A UNIVERSAL SAMPLE PREPARATION METHOD FOR XRF ANALYSIS OF POWDER MATERIALS John Anzelmo, Alexander Seyfarth,