Discovering the lost aggregate opportunity: this 12th and last segment of our 12-part series examines how new technology makes digital imaging analysis of product samples a more accurate science

Pit & Quarry, Feb, 2004 by Neil Steinorth

Measuring particle size with digital imaging is not a new concept. My first exposure to the principle and the opportunity to see an instrument in operation was during the late 80s while attending an American Institute of Mining, Metallurgical and Petroleum Engineers symposium in Duluth, Minn.

The hardware necessary for digital imaging analysis is minimal. The basic requirements are a strobe light source, a camera for high-speed video imaging and a computer running with the appropriate software program.

The validity of information derived from any sample is only as good as the sample itself. That is, the sample must be representative of the material from which it is obtained. For gradation determination by digital imaging, the volume of the sample must be adequate to include a sufficient number of particles in the various size categories to allow an accurate gradation computation relative to a specification.

Generally, the larger the sample, the more likely it is representative of the entire lot. Identification and measurement of oversize or undersize non-conforming particles is crucial even though they only represent a very small percentage of the total number of particles in a sample. Therefore, it is important to measure a representative number of particles in a sample.

One method used to accomplish the particle separation needed for individual measurement and size classification is to feed the sample from a mechanical feeder into free fall past the field of view of the video camera, at a rate (software controlled) slow enough to assure separation and definition.

Old problems

One of the limitations of the early models was the slow speed of the computers used to process the digital imaging information obtained by the camera. Only a very limited number of frames per second could be processed. Given the acceleration of the particles due to gravity and the relatively small field of view governed by optical restrictions, essentially the camera was taking a sample of the sample.

The likelihood of missing one or more non-conforming particles was great. Recent developments in high-speed processing with the new computers allows for the measurement of virtually every particle in a representative sample.

New solution

The current successful application of this methodology at the Presque Isle Quarry--Great Lakes Division, Lafarge North America--is feasible because a team of professionals with expertise in this field recognized the unique application, and were willing to learn the requirements and to develop an enhanced aggregate software application to fit industry needs.

The instrument chosen at the Presque Isle Quarry is the PartAn 1001F Maxi online version particle size analyzer manufactured by Norsk Hydro of Porsgrunn, Norway and distributed by Sci-Tec Inc., Worthington, Ohio. The PartAn processes between 15 and 50 frames per second, measuring and computing the area, perimeter and major and minor chord, that is, length and thickness of every analyzed particle.

After calibrating the optics in the instrument, there is an option in the software to correlate the product classes that will be run for gradation analysis. For example, the first step in making a correlation for an ASTM 57 would be to select the sieve sizes to match the specification for 57s.

The second step is to decide how the particles are to be classified. That is, by area diameter, perimeter diameter, length or thickness. To classify the particles, as a mechanical shaker would sort them, all our correlations are developed using the thickness option.

The software is instructed to perform a linear correlation for the designated size fractions. Then a sample of ASTM 57s, with a known percent retained in each size fraction, is run through the instrument using the linear correlation. The mechanical analysis percents retained are then entered and the program is instructed to correlate the results of the digital imaging analysis performed with the known percents retained in the different size fractions.

The finished correlation is saved for performing gradation analysis of unknown samples of ASTM 57s or other products that have the same general particle size range and size fractions.

In Michigan

Presque Isle Quarry serves diverse markets on the Great Lakes, shipping to both the metallurgical and aggregate industries. Using this methodology, correlations have been successfully established for a large number of varied limestone products.

For all washed products with more than 90 percent of the particles retained on a 3/8-in. sieve, the gradation analysis measured and computed using the PartAn has been proven to be accurate. Literally hundreds of instrument gradation results have been compared with mechanical analysis run on the same samples.

For products containing a relatively large proportion of particles smaller than 3/8-in., such as dense graded aggregates, a key variable affecting the gradation accuracy is surface moisture content of a sample. The same degree of accuracy has been proven, for products that fall into this size classification, when the sample has been dried to sufficiently reduce the surface moisture below a critical level.