The carbon used in filters is a form of charcoal. It is made by heating wood or coal under controlled conditions (temperature and pressure) so that volatile materials are turned into gas and removed. What remains is mostly carbon (and some silicon where the raw material is wood) containing tiny pores where the material which turned into gas used to be. The charcoal has the same shape as the wood or coal it was made from – but it is much lighter due to the pores created.
The property of carbon we are interested in is its unique ability to bind all sorts of chemicals, but in particular, organic chemicals including poisonous gas and odorous compounds. Activated carbon is particularly good at this job because the tiny pores left within the structure make a very large surface area relative to the weight of the carbon.
What is activated carbon?
Activated carbon is a very porous material made from carbonaceous material, such as peat, wood, coal, coconut shells etc. The choice of the raw material and the method of activation determines the specific properties of activated carbon and, as such, the effectiveness of the activated carbon under certain conditions.
There are two methods to make carbon active:
1. Chemical activation
By adding a chemical substance (eg phosphoric acid, zinc chloride) the pores of the raw material are 'opened' and further activated at temperatures of 400-700 C. This method renders a product with predominantly large (macro) pores, mainly suitable to adsorbing large molecules, such as colour compounds from liquids.
2. Thermal activation
The raw material is heated to 900-1000 C and under addition of steam (to prevent carbonisation) a product with predominantly medium (meso) and small (micro) pores is obtained. By varying the process conditions (temperatures, steam dosing, kiln retention time, etc., the size, ratio and number of the pores can be influenced.
From these methods, a semi - product is derived which will then undergo various aftertreatments, depending on the final application. These treatments may consist of:
· Grinding to powdered carbon
· Sieving to obtain broken type carbon of different mesh sizes
· Washing with water or acid to obtain a pure product
· Mixing with inert binding material to produce extrudates (pellets with a fixed diameter)
Can Filters use a particular type of thermally activated carbon that is steamed under pressure and extruded (CKV – 4). The extrudates possess ideal qualities for volatile organic compound (odour) filtration as CKV-4 is specifically developed for this purpose.
Claims have been made that, " Extruded or machine compressed coal/carbons are also extremely hard but are used in the gold industry and have no place in air filtration whatsoever. Machine compressed carbons also tend to have very small pore size and generally only contain micropores."
This statement is scientifically incorrect.
Activated carbon is also used to absorb organic components from liquids. The gold industry is a good example where gold is dissolved (or technically "lixiviated") by cyanide (an organic chemical), then the gold/cyanide complex is absorbed onto activated carbon. When carbon is used for extraction from liquids, diffusivity of liquids makes a big difference, so the pelletised particles are less dense and pores are much bigger. Carbon pellets used in liquid extraction will still work for air extraction; they just won’t last as long because they are not as dense.
Claims are also made about the benefits of natural versus "synthetic" raw materials. This is somewhat misleading. Nature is very nice, but look at a natural forest. Every tree is a different shape, different size and different age. All natural products are variable. The main objective of using technology is to improve on nature (eg soil versus hydroponics). In this case, modern manufacturing methods not only make activated carbon uniform, efficient and predictable, but let us develop specific products for specific purposes.
Put simply, machine extruded carbon comes in different forms and the most efficient activated carbons, whether it be for chemical or odour filtration (liquid or gas) are machine extruded.
It is also worthwhile remembering we breath air (a gas), not solids or liquids. Everything we can smell is volatile, meaning it evaporates (converts to gas) and mixes with the air we breathe in. Claims have been made that "we are attempting to filter ‘organic particles’ not water, gasses or chemical compounds." This is an odd statement. Particles are solid or liquid, not gas. Organic compounds can be solid (like coal), liquid (like petrol), or gas (like methane).
Ever since chemical warfare started with mustard gas in World War I, people have used activated carbon filters to purify air of toxic volatile substances. They are still used today, for instance in masks used to protect workers spraying insecticides, and by fire fighters facing toxic fumes.
The process by which organic molecules bind to carbon does not involve chemistry – it is a physical binding caused by a force called Van der Waal’s Force. Put simply, it is the molecular structure of organic compounds which stick to carbon in the same way Velcro works – the hook bits stick to the furry bits. The carbon is the hook and the organic compounds are the furry bits.
The quality of different activated carbons used for air filtration is determined by the quality of the raw material used to make the carbon, and by the manufacturing process. The modern world requires reliable performance and high efficiency, whereas virtually all natural products like coal and wood are variable, and charcoal is very light. To improve both the performance and efficiency of carbon filters, manufacturing involves blending and grinding the charcoal down to a fine powder, then extruding the powder into pellets. These pellets have a much higher density than charcoal, thus you get much more filtration (more hooks) into a smaller space. All gas masks now use pelletised carbon because they are more efficient and offer much greater protection time.
The performance of a carbon filter is determined by the quality and amount of carbon present, by the activation process used, by the flow pattern of air through the filter, and by the moisture in the air. A given amount of carbon can only absorb a certain amount of volatile organic chemicals/compounds, so clearly the more carbon present (ie. the denser or heavier the filter) the longer it will work. Airflow through filters is fairly easy to achieve because air has very high "diffusivity" meaning there is little resistance to flow even through very tiny holes.
The holes in carbon are referred to as macropores (large), mesopores (medium) and micropores (small). Absorption mainly takes place in the smaller (micro) pores. It is here that the attraction forces (hooks) are most concentrated. The larger pores, although they have some absorption capacity, mainly act to conduct air or fluid to the micropores,
High humidity reduces air filter performance because carbon particles become coated with water, and water reduces the diffusivity into the pellets. However if performance drops off because a filter becomes wet, it is relatively simple to improve performance by driving warm, dry air through the filter to evaporate the water, dry out the pores within the carbon and re-expose the hooks.
Once a carbon filter has become completely filled with organic compounds, it is like a saturated sponge that cannot hold any more water. You can wring out a sponge and use it again. You can also clean out a carbon filter, but this requires dissolving and digesting the bound organic compounds, which usually requires hot, caustic solutions and solvents. The high cost of recycling carbon versus the cheap cost of manufacturing carbon usually means it makes more sense to simply replace filters, or replace the carbon, rather than recycle the saturated/spent product. The exception is large-scale industrial processes where handling hazardous caustics and solvents is routine.
