Why are Activated Carbons Different?
As mentioned earlier, because activated carbons can be made from any carbonaceous raw material, differences will exist in the finished product as shown in Table 1. Domestically, most carbons are manufactured from coals. These include, in order of decreasing quality, metallurgical-grade bituminous coal, a lower ranked sub-bituminous coal and lignite. The base raw material and pretreatment steps prior to activation can affect many of the physical and activity characteristics of activated carbon. These different properties make some carbons better suited than others for specific applications.
Carbons made from lignite tend to have a large pore diameter (higher molasses number) that makes them better suited for the removal of large color body molecules from liquids. Bituminous coal activated carbons have a broad range of pore diameters. Since these carbons have both a fine and wide pore diameter, they are well suited for general de-chlorination and the removal of a wider variety of organic chemical contaminants from water, including the larger color bodies.
Some physical properties can be important in determining which carbon is best suited for a specific application. For instance, the abrasion resistance of activated carbons can be important if the carbon is to be used in an application where frequent back-washing will be required.
Generally, coal-based activated carbons show an increase in the abrasion number (therefore. increased abrasion resistance) when going from the softer lignite carbons to the bituminous coal carbons (Table 1).
|
|
Bituminous |
Sub-Bituminous |
Lignite |
|
Iodine Number
|
1,000 |
1,000 |
600 |
|
Molasses Number
|
235 |
230 |
300 |
|
Abrasion Number
|
80 |
75 |
60 |
|
Bulk Density as packed in column pounds / ft3
|
26 |
25 |
23 |
|
Volume Activity
|
26,000 |
25,000 |
13,800 |
|
% Ash
|
6.7 |
12.3 |
20.1 |
|
* % Phosphorus On Carbon
|
<.05% |
1-5% |
<.05% |
|
|
|
|
|
|
* Part of ash that can form a precipitate in hard water areas. |
|||
Density can also be a major consideration for specific applications. As Table 1 shows the densities of activated carbons also vary with the raw material. Fewer pounds of carbon with a low density will fit into a given container as compared to a carbon with a high density. This is significant because, while a container may require less carbon weight of a low-density carbon to make a volume fill. Its contaminant removal performance may be severely reduced as compared to a higher density carbon.
The concept of volume activity then becomes important when evaluating carbons. A simple calculation for determining the volume activity of carbons is to multiply the bulk density by the Iodine number. Thus, two containers having the same volume with carbons having the same Iodine activity (measured in milligrams Iodine per gram carbon) but different densities will have significantly different total surface areas (volume activity) available for adsorption.
Table 1 presents volume activity data for carbons made from three different raw materials. These volume activities have been calculated for a standard volume of one cubic foot. Differences in volume activity are evident when Iodine activities are the same but bulk densities are different (bituminous vs. sub-bituminous), and even more dramatic when both Iodine activities and bulk densities are different (bituminous vs. lignite).
Ash content can play an important role in home water filter applications. Activated carbons made from high ash content coals, such as sub-bituminous or lignite generally have some sort of chemical acid treatment step to reduce the inorganic constituents that cannot be activated. Although rinsed acid residuals can remain with the activated carbon after treatment. Table 1 shows the high phosphorus level on sub-bituminous-based activated carbon as a result of phosphoric acid pre-treatment. These residuals can some times be released during use and combined with metal ions in water to form magnesium or calcium precipitates (e.g., magnesium phosphate, calcium phosphate). When this happens, cloudy water can be produced.
