Hardness is a measure of the mechanical strength of activated carbon. The y hardness index is an important parameter when selecting activated carbon for a specific application, as it can affect the performance of the carbon, the life of the adsorption system, and the generation of fines or dust, which can cause operational problems or reduce the efficiency of the adsorption process. Hardness is usually expressed as a percentage or index, which indicates the fraction of activated carbon that remains intact after a standardised wear test.
Common tests to measure the hardness of activated carbon include the ball disc hardness test (also known as the ball abrasion test) and the ASTM D3802 hardness test.
In the ball pan hardness test, a sample of activated carbon is subjected to a specific number of rotations in a ball mill containing a certain number of steel balls. Upon completion of the test, the amount of carbon left on the specified sieve is used to calculate the hardness percentage.
In the ASTM D3802 hardness test, the activated carbon is subjected to a specified number of rotations in a rotating drum with a specified number of steel rods. At the end of the test, the carbon particles remaining on the specified sieve are weighed and the hardness index is calculated as a percentage of the initial mass remaining.
The hardness of activated carbon can be influenced by various factors such as the raw material, the activation process and the particle size. In general, activated carbon from harder materials has a higher hardness value (e.g. coconut shell). Activation processes and post-treatment steps can also affect hardness, with processes leading to more crystalline structures or impregnation with chemicals usually resulting in harder carbon.
In applications where activated carbon is subjected to mechanical stress, such as in water treatment systems with backwashing, air filters with pulsed airflow or packed beds with high flow rates, a higher hardness is desired to minimise the production of fines and to maintain the performance of the carbon over time. In gold recovery in gold mines, where coconut shell activated carbon is used, there are also high requirements for activated carbon hardness.