In water filtration, activated carbon blocks remove contaminants and impurities by absorbing them from the water. Due to the high surface area of activated carbon, it contains many small pores that can trap organic or volatile compounds. Carbon filters made from powdered carbon blocks are more efficient than granular activated carbon because the blocks are denser, slowing down the passage of water through them.

Methods for Manufacturing Activated Carbon Blocks

There are two main processes used for manufacturing the carbon block known as extrusion and compression molding.

Extrusion

In the extrusion process, a homogeneous mixture of carbon and polymeric binder is introduced to form a continuous porous block which is tubular-shaped. This block is then twisted into an auger type screw by melting the mixture and forcing it through a die section.  Heat is applied to the mixture in order to make the binder melt. The screw in a barrel pushes the plastic mixture out of a die to shape it. The blocks are now left to cool down and solidify. Afterward, they are cut to length to assemble into water cartridges.

Compression Molding

In this type of molding, the same mixture mentioned above is added to individual molds and sintered at a temperature below the binder’s melting point. This material is compressed under high pressure and heated until it finally hardens. By varying the carbon and binder mesh sizes, we can create different amounts of porosity in carbon blocks which is why they are rated by micron size from 0.5 microns to 20 microns. After cooling, the blocks are sometimes trimmed down before they get packaged into filter cartridges. When comparing the two processes, one should consider the pros and cons of each in terms of manufacturing costs.

Compression molding tends to be more labor-intensive and has a slower production rate than extrusion. However, it does allow for a wider choice of thermoplastic binder materials including finely powdered carbon, thereby leading to blocks that can withstand higher pressure and ensure better performance in filtering water. In addition, compression molding allows the manufacturer to be more flexible about offering blocks that have customized dimensions and characteristics. Extrusion is more suitable for restocking inventory as it lowers the cost of materials and production. Both compression molding and extrusion produces blocks with similar properties. Either way, the purpose of activated carbon blocks is to attract negatively-charged chemical contaminants.

Why RFT uses compression molding for carbon blocks:

Activated carbon source materials

Activated carbon is made from carbonaceous material such as coconut, coal  and wood. The source material used to produce activated carbon has a big impact on the quality and performance of the block. While coal and wood-based activated carbon filters are cheaper based on their quality and flow rate, coconut-based filters are the best choice for water purification. The micro-pores of coconut-based activated carbon are smaller than activated carbon made from coal or wood and therefore do a better job of trapping contaminants.

In a 2014 paper titled ‘A study of coconut shell – activated carbon for filtration and its comparison
with sand filtration’ the Basha Research Centre reported the following:

Activated carbons produced from coconut shells typically have a tighter, more microporous pore structure than their coal-based counterparts. This is due to the inherent pore structure of the raw material coconut shell as compared to raw material coals. This microporosity lends itself towards certain applications where activated carbon is used.

 

Also, coconut shell-based carbons tend to be harder,more resistant to abrasion, and lower in ash than similar grades of coal-based carbons. The major advantage with the coconut shell activated carbon is that it is an outstanding material for applications requiring taste, odor and dissolved organic chemical removal from water with suspended particle present. In addition, its balanced pore structure gives a more efficient adsorption range and it imparts a high “polish” to the filtered water.

Physical properties of activated carbon

Ash content:

In coal and wood-based activated carbon the ash content can be between 10 – 15 % while in coconut filters it will be between 3 – 5 %. As the presence of ash reduces the overall activity of activated carbon, the lower the ash content, the better the product.

Hardness and density:

Coconut-based activated carbon has higher density which provides greater volume activity and it is harder which gives it better resistance to attrition.

Longevity:

The hardness and density attributes of coconut-shell activated carbon gives it greater longevity.