Pulverizing and Blending: Key Points About Grinding

Preface

Product Information

Technical Information

Appendix

Solid Reference Material

Pulverizing and Blending Key Points About Grinding

Contamination
Processing a sample always contaminates it. Successful analysis depends on recognizing the sources of contamination and controlling them. When the contaminants are known and can be quantified, analytical results can be refined accordingly

As the grinding container is the major source of contamination, its selection is critical. In general, one’s objective should be to minimize contamination levels while avoiding elements which will interfere with analysis. An example is the grinding of steel slags in a tungsten carbide container: tungsten carbide grinds rapidly, and the expected low-level contaminants of tungsten, carbon, and cobalt are not generally looked for in these slags. SPEX SamplePrep’s selection of grinding container materials gives you maximum flexibility in choosing the best approach for your samples and analytical ims.

Major, minor, and trace elements predictably found in SPEX SamplePrep grinding containers are listed in Chart A, and more detailed information is usually available upon request. However, strictly speaking, almost no two grinding containers will have exactly the same elemental profile. There are many different steels, carbides, and ceramics, each with specific compositions. Often the formulas are proprietary, so that type of tungsten carbide engineered to have specific properties will have a different makeup from two different manufacturers. In addition, there are inevitable variations from batch to batch of the same material, both in the exact proportions of the major elements and in trace element composition.

Because of these variations in grinding container composition, we strongly recommend determining the exact elemental profile of your individual grinding containers, preferably with your own analytical equipment and techniques. The simplest approach is to grind samples of known composition and see what is added by grinding. Lacking known samples, one may grind portions of a single sample for increasing lengths of time, and check to see which elements increase in proportion to grinding time.Once the contributed impurities and their proportions are known for a grinding container, the resulting profile can be fitted to the analytical results, regardless of the actual contamination level. (While this level is important, it clearly will vary with the composition and condition of the grinding container, the size, hardness, and toughness of the sample, and grinding time.)

Cleaning Containers
Grinding containers should be cleaned between sample runs to avoid cross-contamination, and the procedure can be as simple or as complex as your analytical objectives warrant. In some applications simple wipedown with ethanol may suffice; another practical approach is to brush out container, then briefly grind an expendable portion of the next sample and discard it.

For more thorough cleaning one may grind one or more batches of pure quartz sand, and then wash the container thoroughly. In extreme cases, such as the plating of container walls with a malleable metal, chemical cleaning or multiple grinds with quartz may be necessary

An effective single-step grinding procedure for most grinding containers is to grind pure quartz sand together with hot water and detergent, then rinse and dry the container. Drying is speeded by the use of blow-dryer or similar ppliance. A safety advantage of this cleaning method is that it controls respirable airborne dust.

A cleaning procedure is easily evaluated by grinding and analyzing a known sample, or even by checking the impurities appearing in successive batches of ground quartz sand. It should be noted that grinding containers become more difficult to clean with age because of progressive pitting and scratching of the grinding surfaces.

Hardened steel containers can rust.While iron oxide coatings can be removed by warm dilute oxalic acid solution or abrasive cleaning, we recommend that steel containers be thoroughly dried after cleaning and, if stored, kept in a plastic bag with a desiccating agent.

Respiratory Protection
The general objective of sample grinding is, of course, to convert an inhomogeneous solid to a fine, homogeneous powder. Inevitably, some of this powder is released into the environment, usually during the emptying or cleaning of the grinding container. We strongly recommend the wearing of n approved dust-mask during this procedure, and suggest the use of laboratory fume hood. Even harmless rock and cement samples could become potentially harmful to the respiratory system when finely pulverized.

Grinding Aids
When samples agglomerate or “cake” during grinding, further particle size reduction is clearly inhibited. Caking can result from moisture, heat, static charge accumulation, the fusing of particles under pressure, and other causes. Many of the techniques which make sample preparation an “art” are devoted to getting around caking, and we can only hint at the possibilities.

Slurry grinding is an obvious approach; if particles remain in suspension during grinding, they are unlikely to cake. Water, alcohol, or other liquids are added to the sample before grinding, and removed afterwards. Although slurry grinding is a reasonably reliable way of grinding a sample to micron-sized particles, it is sloppy and time-consuming, requires a leakproof grinding cont- ainer, and adds extra steps to one’s sample preparation procedure.

Dry grinding is simpler and quicker, but requires much more careful matching of the technique to the sample. If the caking is due to moisture, as in many soils and cements, the sample can be dried before grinding. Other samples can be successfully ground with a variety of additives. Dry soaps/detergents are lubricants, and some also include an abrasive; graphite is an anti-static agent as well as a lubricant; there are many proprietary grinding aids as well, which may contain an abrasive, a lubricant and a binding agent. Other grinding aids include polyvinyl alcohol, phenyl acetate and aspirin.

Howard Kanare of Construction Technology Laboratories, Inc. has publicized the use of propylene glycol (one drop for up to ten grams of sample, roughly 0.3 wt.%) for laboratory fine grinding of Portland cement and many minerals. In a swing mill such as the SPEX SamplePrep Shatterbox, oven-dried samples can be ground quickly to less than ten microns without agglomeration or sticking to the mill walls. Propylene glycol must be used safely, after consulting the material safety data sheet; it is available as aPrepAid product.

Vertrel XF™, a DuPont product sold as a cleaning fluid, is finding increased acceptance as a grinding aid. A fluorocarbon fluid, it prevents sample caking during grinding, and quickly evaporates from an open grinding container without leaving any residue. Our experience is that the “grindability” of almost any sample is enhanced by the use of Vertrel XF, contamination is lowered, and the grinding container is easier to clean. Typical XRF samples such as cement, rock, clinker, and similar material can be routinely ground below 10 microns.

The technique that follows was pioneered by John Anzelmo and colleagues at Bruker AXS and is here adapted for SPEX SamplePrep, LLC. equipment: in an 8501 Hardened Steel Grinding Container load together 10 grams of sample, 2.5 grams of 3644 Ultrabind binder, and 7 ml of Vertrel XF. Grind in an 8515 Shatterbox for 2.0 minutes, then open the grinding container in a hood until the Vertrel XF has evaporated. Prepare a 3614 40mm Pellet Die with a flared 3617 Spec-Cap, and transfer the ground sample/binder powder to the die. Press at 20 tons for 0.3 minutes in a 3630 X-Press.

Reference
Anzelmo, John; Seyfarth, Alexander; and Arias, Larry (2001). Approaching a Universial Sample Prperation Method for XRF Analysis of Powder Materials. Advanced in X-Ray Analysis, Vol. 44, JCPDS- International Centre for Diffraction Data, Newtown Square PA.