+44 (0) 1269 595925
The Durability of anthracite filter media has been described by several methods each of which has its strengths and weaknesses.
Hardness - Moh Scale
Hardness on the Moh Scale compares the hardness of anthracite with other minerals. An average of many comparisons is made to obtain a valve. The higher the value the harder the anthracite. The Moh's hardness test is a very subjective test as anthracite does not have a completely uniform internal structure. The minimum value should be above 2.8.
Grindability - Hardgrove Index
The hardness of anthracite may be high on the Moh scale, but if it fractures easily it will appear soft. The Hardgrove Index of grindability goes some way to address this. Basically the anthracite is hit a set number of times by steel balls and the degrdation measured. A figure is then calculated to give the Hardgrove Index. The lower the number the less friable the anthracite. No fogure is given by any standard but a reasonable result would be between 50 and 80 Hardgrove.
Crushing Modulus
Neither hardness or grindability completely describes the resistance of anthracite not to breakdown in the filter. The crushing Modulus was developed to over come this. In this test the anthracite filter media is crushed in a test chamber by a hydraulic ram and a figure calculated from the results. A minimimum value of 105bar must be obtained. The major draw back of this method is that a different value is obtained for the same anthracite but of different hydraulic size. The smaller the hydraulic size the higher the crushing modulus.
Backwash Test
The backwash test is probably the best way to determine the durability of anthracite as it mimics the actual conditions under which the anthracite will operate. This test determines the loss of anthracite due to particle breakage over a period of 100 hours backwashing [approx 3 years of operation].It must be noted, however, that the amount of anthracit loss will diminish over time as the sharp corners and edges of the anthracite are slowly worn away. The second 100 hours will show a better result than the first. A 2% loss in anthracite over 100 hours backwashing is acceptable.