Grindability

The grindability of coal (i.e., the ease with which coal may be ground fine enough for use as pulverized fuel) is a composite physical property embracing other specific properties such as hardness. 

Strength, tenacity, and fracture. Several methods of estimating relative grindability utilize a porcelain jar mill in which each coal may be ground for, say, 400 revolutions and the amount of new surface is estimated from screen analyses of the feed and of the ground product. Coals are then rated in grindability by comparing the amount of new surface found in the test with that obtained for a standard coal. 

Pennsylvania Cambria Lower Kittanning (bituminous) Bird No. 2 109 

Source Baughman, G.L., Synthetic Fuels Data Handbook, 2nd edn., Cameron Enginea s, Inc., Denva-, CO, 1978, p. 169. 

FIGURE 9.8 Variation of the hardgrove grindability index with volatile matter yield. (From Berkowitz, N., 

State County Bed Mine Hardgrove Grindability Index 

FIGURE 8.2 Calibration chart for the Hardgrove grindability index. (From ASTM, 2004.) 

Variations in the data that fall outside the experimental allowable limits may be due to factors that originate during sample preparation and handling, such as (1) the sample moisture may not have been in equilibrium with the laboratory atmosphere (2) the sample may have been overdried or under-air-dried (3) excessive dust loss may have occurred during screening due to a loose-fitting 

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R. M. Hardgrove, The delation Tetween Tu/verising Capacity, Power and Grindability, ASME, Chicago, lU., 1993, pp. 6—27. 

Pliability. (Also see Grindability, Sec. 8.) This is the tendency of the material to break into smaller sizes in the course of handhng. There are quantitative tests specially devised for certain materials such as coal which can be used to estimate this property. Abrasiveness of one ingredient upon another should also be considered. 

Grindability is a measure of the rate of grinding of material in a particular mill (discussed later). 

Chandler [Bull. Br Coal Util. Re.s. A.s.soc., 29(10), 333 (11), 371 (1965)] finds no good correlation of grindabihty measured on 11 coals with roh crushing and attrition, and so these methods should be used with caution. The Bond grindability method is described in the subsection Capacity and Power Consumption. ... 

Manufacturers of various types of mills maintain laboratories in which grindability tests are made to determine the suitability of their machines. When grindability comparisons are made on sm equipment of the manufacturers own class, there is a basis for scale-up to commercial equipment. This is better than relying on a grindabihty index obtained in a ball mih to estimate the size and capacity of different types such as hammer or jet mihs. 

A guide to the. selection of equipment may be based on ieed.size and hardness (see subsec tion Grindability ) as shown in Table 20-7. It should be emphasized that Table 20-7 is merely a guide and that exceptions can be found in practice. 

The net production in wet grinding in the Bond grindability test varies from 145 to 200 percent of that in diy grinding depending on mesh [Maxson, Cadena, and Bond, Trans. Am. Int. Min. Metall. Pet. Eng., 112, 130-145, 161 (1934)]. Ball mills have a large field of apph-cation for wet grinding in closed circuit with size classifiers, which also perform advantageously wet. 

TABLE 20-31 Ball-Mill Grindability of Various Phosphate Rocks... 

Pulverization occurs by impact, attrition, or crushing. The capacity of a pulverizer depends on the grindability of the coal and the fineness desired, as shown by Fig. 27-19. Capacity can also be seriously reduced by excessive moisture in the coal, but it can be restored by increasing the temperature of the primary air. Figure 27-20 indicates the temperatures needed. For PC boilers, the coal size usually is 65 to... 

FIG. 27-19 Variation of pulverizer capacity with the grindability of the coal and the fineness to which the coal is ground. (Bahcock [Pg.2385]

Fluid coking has several characteristics that make it undesirable for most petroleum coke markets. These characteristics are high sulfur content, low volatility, poor crystalline structure, and low grindability index. 

Other Tests. The so-called tumbler tests are usually used for testing material like coke, coal, iron ore pellets or tablets. They can be divided into drum tests and ball mill type tests. The latter type is used to derive both the Hardgrove Index and the Bond s Work Index, which are often used to classify the material friability as described in Sec. 3. They are generally more suited to coarse material. The Hardgrove Grindability test requires an initial size range form 595 to 1190 microns. 

Austin and Klimpel 1 11 have reviewed these three laws and their applicability, and Cutting(12) has described laboratory work to assess grindability using rod mill tests. 

Cutting, G. W. The Chemical Engineer No. 325 (October 1977) 702-704. Grindability assessments using laboratory rod mill tests. 

Philip L. Walker Hardgrove grindability data for many Pennsylvania anthracites have been determined and reported on in the Proceedings of the 1956 Anthracite Conference held at Pennsylvania State University. 

Other important chemical and physical tests performed to characterize coal include (I) Heating value (Btu content) (2) sulfur forms (31 ash fusibility temperatures (4) ash analysis (5) trace elements (6) free swelling index and (7) hardgrove grindability. 

Grindability Dustiness index Relative amount of work needed to pulverize coal Amount of dust produced when coal is handled... 

Considerable information is available on the properties of coal that relate to its hardness, such as friability and grindability, but little is known of the hardness of coal as an intrinsic property. However, hardness can also be equated... 

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