Particles grindability

Instruments have been described that employ (3-ray attenuation [10-15]. The aecuracy of these devices is limited by their sensitivity to changes in feed density [16]. In order to calibrate it is necessary to re-circulate slurry samples in a closed loop at a number of dilutions for each slurry system, sieve analyses being carried on representative sub-samples. The signals from a scintillation counter can be used to control mill feed rate in order to compensate for changes in feed ore, grindability and feed particle size. Accuracies of 2% to 3% have been reported on Cornish granite, nepheline systems and copper and iron pulps in the size range 20 to 105 pm [17,18]. 

Crindability Methods Laboratory experiments on single particles have been used to correlate grindability. In the past it has usually... 

Unlike fracturing, this approach should cause only moderate disruption of the coal matrix and might not lead to complete comminution, but this method could be envisioned as a prelude to coal grinding. Removal of material from structural boundaries would yield a more friable, "soft" coal structure. The leaching of fine mineral particles from the coal matrix results in a more porous structure and should also improve the coal s grindability. The prior removal of some mineral matter would also reduce the coal s abrasiveness. 

In the test method (ASTM D-5003), the coke sample is crushed to produce a high yield of particles passing a No. 16 sieve and retained on a No. 30 sieve. These particles are reduced in the Hardgrove grindabiUty machine according to the test method for coal (ASTM D-409). The quantity of particles retained on a No. 200 sieve is used to calculate the Hardgrove grindability index of the sample. Both this test method and the test method for coal (ASTM D-409) produce the same results on petroleum coke samples. 

Grindability (in its many different definitions) is a composite material property, and depends on many primary material properties (e.g. particle hardness, bulk and shear moduli of elasticity) as well as its flow properties and other conditions like moisture content, humidity of the atmosphere or material composition (rank or ash content of coal, for example). It also depends on the type of mill used for its evaluation. There have been some attempts made by several authors to find correlations relating different measures of grindability the reader is referred to the literature for details of these54. 

Applicability to the manufacturing process means that the drying agent must he commercially available at a reasonable price, grindable to a fine particle size for dispersion In the resin, dispersible In commercial compounding equipment, and sufficiently thermally stable for can manufacture. 

In addition to its chanical properties, the efficient use of a coal also requires a knowledge of its physical properties, such as its density (which is dependent on a combination of rank and mineral matter content), hardness, and grindability (both related to coal composition and rank). Other properties include its abrasion index (derived mainly from coarse-grained quartz) and the particle size distribution. Float-sink testing may also be included with the analysis process. This involves separating the (crushed) coal into different density fractions as a basis for assessing its response to coal preparation processes. Float-sink techniques may also be used to provide a coal sample that represents the expected end product of a preparation plant, in order to assess the quality of the coal that will actually be sold or used rather than the in situ or ROM material represented by an untreated (raw) coal sample. 

The various constituents of Portland clinker differ in their grindability. In general, the grindability declines in the order calcimn sulfate>alite> almninate>ferrite>belite. As a consequence, different cement particle size fractions may vary in their phase composition. 

The Hardgrove Grindability Index was originally developed by Babcock and Wilcox to measure the relative ease of pulverizing coal. This ASTM D409-71 method has since been employed for characterizing other particles as well. The method processes 50 g of air-dried particles screened to a size of 16 x 30 mesh (1180 X 600 pm) in a small ball-and-race mill for 60 revolutions. The amount of material (1 200) passing the 200 mesh (75 pm) screen is then measured. The result is then compared with a standard index to arrive at the HGI or the HGI is calculated from the equation... 

The values of the HGI usually range from 15 to 140. The higher the HGI, the higher is the grindability of the material. The HGI has been found to correlate with the attrition characteristics of the particles in fluidized beds and in pneumatic transport lines (Davuluri and Knowlton, 1998). The HGI does not directly relate to hardness. For example, some materials such as plastics are difficult to grind. 

In clinker grinding, the gypsum, being more readily grindable, tends to be concentrated in the finer particle size fractions of the product. So does any fly-ash that may be added, whereas blastfurnace slag becomes concentrated in the coarser fractions. 

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