The hardness and resistance to attrition of activated carbons is becoming more and more important. The loosely applied term of “hardness” is somewhat difficult to measure on activated carbon.
Three forces can mechanically degrade an activated carbon – impact, crushing and attrition. Of these three, the force of attrition, or abrasion, is the most common cause of degradation in actual end use. At the present time, there are two commonly used methods available to evaluate a carbon’s hardness.
The first of these is the Ball-pan Hardness Test.
A screened, weighed sample of carbon is placed in a special hardness pan with a number of stainless steel balls and subjected to combined rotating and tapping action for ½ hour. The particle size degradation is measured by determining the weight of carbon retained on a sieve (with an opening closest to one half the opening of the sieve defining the minimum nominal particle size of the original sample). The ball-pan hardness method has been used widely in the past and has a broad history in the activated carbon industry for measuring the property loosely described as “hardness”. In this context, the test is useful in establishing a measurable characteristic, conceding that it does not actually measure in-service resistance to degradation, it can be used to establish comparability of differing batches of the same material. This test actually applies all of the three forces mentioned earlier, in a variable manner determined by the size, shape and density of the particles.
The second method used is the Stirring Bar Abrasion Test.
In this procedure, a sample of carbon is placed in a cylindrical vessel where an inverted T-shaped stirrer is turning rapidly at a controlled rate. The percentage reduction in average particle size, resulting from the Tbar action, is recorded after 1 hour. This method measures attrition of the carbon, as long as the particle size is smaller than a 12 mesh. There is evidence showing that the results of this method are influenced by particle geometry.
Whichever of these tests is performed on carbon it is generally accepted that granular coconut based carbons show the least rate of physical degradation.
This is possible due to two factors.First, granular coconut carbon is produced from pieces of raw coconut shell whereas, most other carbons are produced from reconstituted powders. In consequence, carbons other than coconut based types, can only breakdown to a powder or dust. Coconut carbon essentially chips and breaks into smaller pieces and thus degradation to powder, is a relatively lengthy process. Second, as outlined earlier, the coconut carbon structure is different to other types, producing a material of relatively high density and physical strength.