Anthracite - Properties and Applications
What is anthracite?
Anthracite is a form of coal that is densely packed and has a high carbon content, relatively few impurities, and the second highest calorific content of any type of coal with only graphite beating it. Anthracite is not described as a bituminous coal, therefore it does not contain elevated levels of volatile organics or oily substances. The principle uses of anthracite in the industrial sector include power production as well as the fields of metallurgy and water purification, in addition to a burgeoning presence in the refractory materials space.
Representing only one percent of all of the coal in the world, anthracite is mined at scale in only a select number of countries such as China (the world’s leading producer), Russia, Vietnam, Canada, South Africa and the United States. Anthracite can often be distinguished from other types of coal due to its semi metallic appearance which reflects light in a manner similar to the mineraloid (and fellow ignite) jet, a substance used in jewellery products.
The high carbon content of anthracite makes it particularly suitable for filtration.
Physical Properties of Anthracite
Colour Black
Dry Bulk Density 800 kg m-3
Specific Gravity 1.6 ± 0.05 g cm-3
Effective Size 1.2 - 2.4 mm
Uniformity Coefficient 1.5 - 1.7
Bond Work Index 20 - 22 kWh T-1 (BBH)
Anthracite As A Water Filtration Mechanism
How does a media filter such as anthracite work?
Anthracite coal filter media and others work through a combination of chemical composition as well as using the laws of gravity and physics to purify contaminated water.
A filter media is any kind of substance - such as crushed gravel, sand, anthracite, or even glass - which acts as a sieve from which solids are removed. The finer the grain of your filter media, the more granular it can get in removing these impurities.Whilst a small particle size of anthracite filter will result in better filtration of smaller contaminants, such small sizes can easily be compacted to a more solid bed, which can lead to overpressure situations. This can be avoided by the use of the correctly sized anthracite particles, a suitable operation flow rate/pressure and a well provisioned backwashing programme to release contaminants from the filter bed.
Because substances can readily dissolve in water but do not readily dissolve entirely, substances that are not completely dissolved in water can be “caught” in the filter media with only the water passing through at the end of the filter.
It is not hard to imagine that anthracite filters have myriad uses in the modern world from managing runoff from a storm to keeping water clean and drinkable. It is this last concern that has anthracite water treatments gaining traction in many arid parts of the world where access to fresh water is limited and the resource itself is scarce. In fact, anthracite filter media suppliers around the world are pushing it as a solution to many impending water crises in different locations across the globe.
The only question is: Are anthracite water treatment filter media effective at removing the impurities found in seawater which are myriad and microscopic?
Performance, Operating Characteristics And Backwashing
Compared to sand, anthracite is more tolerant of higher operating flow rates and backwashing rates. These are important characteristics when large volumes of liquid are to be purified. Across grain sizes of 1.4 to 2.5 mm, backwashing rates of 55 to 60 m3 h-1 can be achieved at 20 °C. Such high backwashing rates are testament to ground anthracite’s ability to be a reliable performer in filtration, with only minimal downtime. Furthermore, the superior hardness of anthracite means that physical abrasion is minimised and thus filter lifetime is further extended.
Desalination
Is Using Anthracite for Water Filtration Effective at Removing Impurities in Salt Water?
While there is little question about anthracite’s effectiveness as a filter, it is not the only option when it comes to desalination. Many industrial water purification systems, be those for desalination or otherwise, rely on anthracite as a filtration medium. Not only is it an effective medium for desalination but is performatively on par with processed expanded clay (another popular material in the desalination space).
Research along the coast of northern Greece has shown that in a direct comparison of filtration using anthracite and using process expanded clay, in every case scenario, the two media performed at nearly identical levels, with optimal filtration performance achieved in the summer months because of what was attributed to higher aggregations of material caused by hotter temperatures.
The only differences in performance arose from the coagulant material used in each scenario.
How Anthracite Water Treatment Methods Work with Salt Water
Because the impurities in salt water are quite tiny indeed, filter media such as anthracite often work in tandem with a coagulant substance for optimal performance and filtration of water.
Coagulants are chemicals that aid in the creation of what are called aggregates or clumps of impurities that are larger in sum than they are in their parts. When a coagulant helps create larger aggregates, this makes the impurities easier to remove for filter media like sand and anthracite.
To look at it a different way: using coagulants with anthracite makes anthracite more effective at removing the impurities in salt water by increasing their weight and surface area. Research findings show that not only was anthracite water treatment effective at desalination across a wide range of performance environments (temperature, etc.) but also that it was comparable to the clay material in building up water resistance or what is called hydrostatic head.
The process begins with a pretreatment process which is where untreated or “raw” seawater is drawn and algae and organic materials are removed from the water. This is accomplished by pumping the raw seawater into multimedia filter tanks consisting of a layer of anthracite, sand, and then gravel at the bottom. Water moves from the top of the tank out through the bottom, passing through the anthracite, sand and gravel layers as it does so. The resulting liquid is largely free of contaminants, including all at the macro scale, at this point.
The treated salt water that results from this process then goes through another treatment which is often called microfiltration. What microfiltration refers to is the removal of small matter in the treated seawater. At this point, the water is ready for a process referred to as reverse osmosis, as detailed below.
It should be noted that coagulants are not used before every example of reverse osmosis (RO). Anthracite filters, in more depth below, with sand are part of pre-RO treatment, without which the RO process would be inefficient.
Proposed as a solution to the problem that 4 billion people - or nearly half of the world’s population - will be short of potable water by 2030, anthracite filter media suppliers will be working in concert with others such as sand and gravel presents one among many possible solutions to this problem. Given its demonstrated efficacy in this area, the use of anthracite water treatment methods is expected to grow in the coming decades.
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