Activated Carbon Properties and ASTM
1. Adsorption Properties
The effectiveness of activated carbon usually specified by the amount of a certain test chemicals it can adsorb per weight of activated carbon used. For activated carbon used for filtering air and gases, the test chemical used is usually Carbon Tetrachloride, commonly designated as CTC. For activated carbon used in filtering water and liquids, the chemical used is usually Iodine mixed with water.
1a. Iodine Number
This number is tested using ASTM4607. A known quantity of activated carbon is ground up into powder and mixed into a standard solution of Iodine in water. After mixing, a certain amount of the Iodine is adsorbed. By finding out how much Iodine is left the solution, we can determine how much Iodine has been adsorbed by the amount of activated carbon used. This number is usually expressed as milligrams of Iodine adsorbed per gram of activated carbon used. An Iodine number of 900 for an activated carbon is a good grade. A more expensive grade of activated carbon has an Iodine Number of greater than 1000.
1b. Surface Area
When looked under the microscope, a granule of activated carbon is full of tiny holes. It is these holes which give the activated carbon the large surface area. The larger the area, the more it can adsorb. Typically a teaspoonful of activated carbon has enough surface area to cover a football field about 1000m2/gram.To measure the surface area of all these tiny holes, a test method, known as the BET method is used. The BET measures the area by determining the amount of liquid nitrogen needed to evenly cover all the surface in these holes. The BET method is used in activated carbon research and is not a practical method to be used a quality control tool.The Iodine number and the CTC number, both of which measure the adsorption capacity, are ordinarily used to monitor quality during production.
2. Physical Properties
2a. Bulk Density
This is a property of activated carbon that is required by the engineers to find out how many kgs of activated carbon must be used to fill up a certain volume of tank or cartridge. It is known as the Apparent Density. A typical Bulk Density number to use for activated carbon is 450- 500 grams/liter (28-30 lbs/cu.ft).Bulk density is affected by the raw material used and the degree of activation. The density does not effect the effectiveness of the activated carbon measured in adsorption per unit weight, but will have an effect on adsorption per unit volume.The standard test method for bulk or apparent density of activated carbon is ASTM D2854.
2b. Hardness Number
The harder the activated carbon, the less it will crumble into fine particles during handling and use. To define this property, a certain amount of activated carbon is put into a pan, together with some steel balls, and shaken for a defined period of time. That is why this property is often known as Ball-Pan Hardness. Weighing the carbon before and after the shaking determines the amount of loss.The percentage of the original carbon that is left after the shaking is the Hardness Number. 95% and above is a good Hardness number, and coconut shell activated carbon is the hardest. The standard test method for Ball-Pan Hardness of activated carbon is ASTM D3802.
Activated carbon is made by passing hot air and steam over coal in a kiln. Sometimes the resulting activated carbon is washed in a bath of acid, rinsed and then dried again. There is bound to be some amount of moisture left in the activated carbon upon final drying. A practical limit for the level of moisture present in the activated carbon, when packed at the factory is 3%. The standard test method of Moisture in activated carbon in ASTM D2867.
3. Total Ash Content
Activated carbon is made from coal, which is 85-95% carbon, the reminder is made up of various minerals. These minerals remains as ash when the activated carbon is incinerated. To test for the Ash Content, a known quantity of dry activated carbon is ground up into powder and heated in a hot kiln until all the carbon has been burnt off. The weight of the remaining ash, expressed as a percentage of the original weight of activated carbon, is the Ash Content of the activated carbon. Coconut shell activated carbon has usually lower ash content. The standard test method for the Ash Content is ASTM D2866.
4. pH Value
The pH value of activated carbon is a measure of whether it is acidic or basic. The pH of a neutral substance, such as pure water has a value of 7. A pH of less than 7 means the substance is acidic and a pH of greater than 7 means it is basic.
Activated carbon, when coming out of the kiln tends to be a bit basic, with pH of around 8. Too high a pH indicates too much contaminants. Too low a pH, especially for acid washed activated carbon, means that the acid has not been properly rinsed away. Most of the activated carbon are specified for a pH of 6-8. The standard test method of pH of activated carbon is ASTM D3838.
5. Particle Size Distribution
Different particle sizes of activated carbon offer different flow resistance to air or water it is filtering. Typically, for filtering air or gases, the larger granule sizes are used, and for filtering water or fluids, the smaller granules sizes are used.
Granules sizes are rated by the size of the sieve used to separate the particles. A #4 sieve has holes size of 1/4 inch and therefore, accounting for the dimension of the wires which make-up the sieve, each hole is then slightly smaller than 1/4 of a inch. Similarly a #8 sieve will have holes size slightly less then 1/8 of an inch. An activated carbon sized 4 X 8 contains granules that can pass through a #4 sieve but not through a #8 sieve and will have granules ranging from slightly under 1/4 to slightly under 1/8 inch.
The proportion of the larger granules vs the smaller granules can be measured by using intermediate sieve sizes. The proportions of these intermediate sizes can be used to calculate the Uniformity Coefficient, which is a measure of how the fluids would flow around the activated carbon due the distribution of the various granule sizes.Common sizes for water the particles sizes are:
8 X 20, 12 X 40, 14 X 40 and 20 X 50 etc.
The standard test method for particle size distribution is ASTM D2862.