Volume 3 Number 1 Dennis R. Dinger 1 November 2004
An Update
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The topic in this issue was originally an answer to a specific question by a subscriber. It was then suggested that I share this with everyone.
Grinding Gages
Introduction
Grinding gages provide a simple way to learn the fineness of a particular grind in a mill or the fineness of any particulate suspension. It is a quick and dirty method to determine the size of the largest particles in the suspension sample. For example, after a suspension or body has been milled in a ball mill for several hours, this type of gage can provide the mill operators with information about the coarse end of the suspension being milled. It does not (it cannot) provide information about the whole distribution, but it can show how far the feed particles have been reduced in size.
For example, when we were studying coal/water slurries, we knew the coal slurries were within proper specifications when 98.5% of all particles in the slurries were finer than 300 micrometers. We used a 300 micrometer sieve and a technician with a calibrated eyeball. He could put some slurry on the sieve, wash it through with water, and look at the oversize remaining to tell whether the ball milling operation had reached completion. We could also have developed a procedure using one of these grinding gages which would also have provided similar information. Both types of tests (calibrated eyeball with sieves, or use of a grinding gage) are operator sensitive. Both types of tests provide slightly different information, but they both characterize the coarse end of distributions. So we could have used either technique -- as we wished.
The Device
The grinding gage, sometimes known as a Hegman Gage, is also known as a grind gage, a fineness-of-grind gage, a chocolate gage, etc. (see T.C. Patton, Paint Flow & Pigment Dispersion, 2nd ed., John Wiley & Sons, New York, 1979, pp. 501-507.) It is a relatively simple device consisting of a heavy block of hardened steel with a sloping channel (from deep at one end to zero depth at the other) milled into the top surface. Also required is a flat scraper blade to pull the suspension sample across the face of the gage. According to Patton, the coatings industries follow ASTM test method D1210-64 when using this type of gage. Figure 1 shows a sketch of a grinding gage.
Figure 1. A Gringing Gage
The different types of gages vary by the lengths and depths of the sloped channel milled into the face of the gage. The chocolate gage, for example, covers the depths and sizes of importance to the proper milling of cocoa beans.
How is it used?
A suspension sample is placed on the face of the gage at the deep end of the channel. The suspension is then dragged (smeared) up the face of the gage toward the shallow end of the channel. The theory behind its use goes as follows: when the gage size is properly chosen, the coarsest particles in the suspension will sit in the channel and as the scraper is dragged across the top of the gage, it will not contact large particles sitting down in the channel until the diameters of the largest particles and the depth of the channel are similar. As the scraper blade is pulled towards the shallow end of the channel, the tops of the coarse particles will be closer and closer to the blade. Eventually as the channel becomes shallower, coarse particles sitting on the bottom of the channel will protrude above the surface of the gage where they will be contacted by the scraper blade. At that point, the coarse particles will be dragged along towards the shallow end of the gage. When this happens, the suspension will no longer fill the channel, but the suspension filling the channel will be disrupted (streaky) and eventually, the channel will be scraped clean (and empty) as the particles collide and as remaining suspension is pulled along by the scraper blade.
The depths of the channel are marked on the surface of the gage. The depths at which the particles begin to interact with the scraper blade give indication of the sizes of the largest particles remaining in the suspension. The depths, which correspond to particle sizes, can be read from the gage scales.
As mentioned above, this process is operator sensitive. But with practice, it can be performed reproducibly by different mill operators, technicians, and/or engineers.
What does it tell about the suspension?
This technique is sensitive to the size of the coarsest particles in suspension. In an ideal world, a 100 micrometer diameter particle can sit in a 100 micrometer deep channel without being contacted by a scraper blade pulled across the surface of the gage. It is likely, however, that 100 micrometer diameter (and slightly smaller particles) will be contacted by the scraper blade at the 100 micrometer channel depth because (1) the particles are not sitting perfectly on the bottom of the channel, (2) they are not exactly spherical, and/or (3) even because the suspension is slightly dilatant. There are several reasons why a scraper blade will interact with coarse particles and drag the channel clean -- but this is where standardized test procedures apply. Test procedures must be standardized so results are comparable. The velocity at which the blade is pulled across the gage must be comparable from test to test.
When solids contents are high and particles in suspension are crowded, the speed at which the scraper blade is pulled across the surface of the gage is critical! The blade cannot be pulled across the surface at a high velocity or particles will become mechanically linked and even fine particles can link together and act as if they are coarse. This will occur when the suspension is at least slightly dilatant and when dilatancy is present, blade speed must be slooooowwwwww! This is where operator variations can come into the picture. And when this occurs, a fast swipe of the scraper blade across the surface of the gage will indicate a coarser particles in suspension than are actually present.
This test is also affected by the state of flocculation/deflocculation of the suspension being tested. Deflocculated suspensions which are somewhat (or fully) dilatant will typically give coarser readings than flocculated and partially flocculated suspensions which are shear thinning.
If the test is done properly, this test will show the size of the coarsest particles in a distribution. If the whole distribution is finer than 100 micrometers, but it contains one remaining particle of the 1/4" feed material, the 1/4" particle will be dragged down the channel by the scraper blade. Unless the channel is greater than 1/4" deep (which is not usually the case because channel depths are more typically in the 100s of micrometer range down to 0 micrometer depths) the test will not indicate the size of the coarse particle. When the coarsest particles are larger than the maximum depth of the gage, the gage won't give any indication of coarsest particle diameter other than to show that it is coarser than the maximum size the gage can measure. In this example, one would only learn that some particles are coarser than the deepest depth of the channel which was 100 micrometers.
On the other hand, if all particles are finer than 100 micrometers (with no really coarse feed particles remaining), a gage which goes from 100 micrometers down to zero should indicate the size of the largest particles in suspension. It won't tell how many are present and it will typically indicate sizes that are slightly larger than the sizes of the largest particles that are actually present. The magnitude of the error in this measurement can be determined using a few tests by another particle size measurement method.
Grinding gages will give an indication of the coarsest particles remaining in the distribution -- that is, they will give indications of the DL values of the distributions.
Summary
Grinding gages provide the means to perform simple, quick tests which indicate the size of the coarsest particles in a distribution. These gages are frequently used in the paint and pigment industries and in the chocolate industry, but they are certainly also applicable to the ceramics industries and any other industries that mill, produce, and/or utilize particulate suspensions.
Miscellany
Suggested topics .... Please continue to send your ideas or questions for future topics. Thanks. Until next time ...
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