Grindability Indices

There is no attrition test method that is accepted as being superior to others. However, there are two highly standardized and widely used test 

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Each Hardgrove machine is calibrated with standard reference samples of coal of grindability indices 40, 60, 80 and 110 which are obtainable from the US Bureau of Mines in Pittsburgh or other sources. The index is determined from the amount of fines formed if a standard sample weight (50 g) is subjected to standard grinding conditions. The harder is the coal, the lower is the index number, with 100 corresponding to an original standard soft coal. 

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. 

State County Bed Mine Hardgrove Grindability Index... 

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

FIGURE 8.3 Variation of Hardgrove grindability index with volatile matter. (From Berkowitz, 1979, p. 91.)... 

ASTM D-5003. Standard Test Method for the Hardgrove Grindability Index (HGI) of Petroleum Coke. 

Grindability index number that indicates the ease of pulverizing a coal in comparison to a reference coal. This index is helpful in estimating mill capacity. The two most common methods for determining this index are the Hardgrove grindability method and ball mill grindability method. Coals with a low index are more difficult to pulverize. 

Hardgrove grindability index weight percent of coal retained on a No. 200 sieve after treatment as specified in ASTM D-409. 

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

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. 

British Standard BS 1016 Part 20 1981—Determination of Hard-grove grindability index of hard coal. 

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