Coal Washing And Zinc Chloride: A Crucial Combination
Coal as mined is not a worthy material for modern combustion, owing to its large quantity of sulfur compounds amongst others. When burned, these release toxic gases. Coal washing reduces the amount of these gases released on burning - by helping to remove them from the coal. The addition of zinc chloride to a coal beneficiation process can lead to the production of activated carbon products.
An Introduction To Coal Washing
Coal washing - also known as coal benefication - is a process by which mined coal is treated with water to remove sulfur and other impurities before it is burned as a fuel. Coal washing is part of coal preparation, which includes other steps such as crushing and grading. Impurities can include soil and other rocks, in addition to the sulfur compounds. Once washed, the coal will have an overall lower ash content, meaning it is easier to transport and is a higher quality fuel(1). Ash content can be reduced from around 40 - 45% to as low as 30%. As part of the process, coal needs to be dewatered, which is commonly achieved through centrifugation, slurry screening or conventional filtration, depending on the coal type.
High sulfur content fuels are poor environmental performers - releasing oxides of sulfur into the atmosphere, which lead to air pollution and acid rain.
One concern with coal washing is the amount of water it uses. Values of circa 45 cubic metres per tonne of coal are commonplace. That is to say, it takes 45 tonnes of water to wash one tonne of coal. Coal washing refers to the entire spectrum of coal improving processes.
Washability
Washability refers to the testing of a particular substance in order to determine ideal conditions to remove impurities, which is calculated on a basis of density. Current standard methods for calculating this, in the case of coal, include dropping a sample into a liquid of known density and measuring how long it takes to sink. These are referred to as ‘float and sink’ tests. Coal has a density of around 1,300 kg m-3, whereas mineral matter (i.e. impurities present, ‘ash’) often have densities exceeding 2,000 kg m-3(2). It can therefore be inferred that a higher density coal sample will have a higher proportion of ash - because ash is more dense than coal(3). The standard for the known density liquids tend to be organic in nature - perchloroethylene, bromoform and tetrabromoethane. The disadvantage to these is that they are all toxic and they are all volatile, relative to water. Concentrated or saturated solutions of zinc chloride in water are becoming popular alternatives to organic solvents for washability testing(4). The difference in moisture contents is well known and accounted for, with surface effects such as solubility on the side of the coal are minimal(5). Zinc chloride solutions tend to be utilised for float and sink tests in the 1,200 to 1,800 kg m-3 regime(6).
Heavy Media Separation
Based on the principles established in washability testing, heavy media separation is a method used to remove certain compounds from a mixture. It relies on the specific gravity of a material being higher or lower than that of the liquid into which it has been placed. Modulation of this principle can result in a sample being added to a liquid, with high density material sinking and low density material floating(7). A simple extension of the very process used to determine washability is the process used to separate heavy media in coal washing. The major sulfur containing mineral within coal is pyrite, which in heavy media separation, sinks.
Oftentimes, a combination of saturated zinc chloride solution with another solvent is used to ensure a certain density, and therefore a specific separation behaviour. Such an example is in the separation of highly bituminous coal from a mine in Turkey, where an isopropyl alcohol, carbon tetrachloride and zinc chloride solution with a specific gravity of 1,400 kg m-3 was used(8). The coal - although of relatively poor quality - floated and was completely separated from the ash and other mineral content, which sank.
Using a range of zinc chloride solutions at densities ranging from 1,100 to 1,750 kg m-3 were used to probe the effects of the froth floatation process on washing lignite. The frothing process is a development of the conventional heavy media separation process, using kerosene to enhance the collection of the desired organic material - which floats. Removal of more than 90% of sulfur compounds from the lignite were reported, albeit with little effect arising from zinc chloride density in solution(9). It seemed not to matter greatly how much ZnCl2 was present in this study.
Crucially, both of these examples show the potential for improvement of lower quality coals like lignite. This is important as over 50% of the world’s remaining coal deposits are examples of lower quality coals(10).
Zinc Chloride And Carbon: Reactivity
Not to be outdone with properties and uses based solely on physico-chemical separation techniques, coal and zinc chloride are reactive with one another. The specific dehydrogenation of coal when exposed to zinc chloride accounts for some 12% of the hydrogen contained within the coal itself(11). This is particularly interesting as such a dehydrogenation may occur at temperatures below that at which pyrolysis would normally occur(12). It is this reactive behaviour which leads, in part, to the production of activated carbon compounds from coal. It should be noted that in the pyrolysis of coals with higher than average sulfur content, zinc sulfide can be formed, which effectively reduces desulfurization(13).
Production Of Activated Carbon Compounds
Activated carbon (also known as activated charcoal) is a form of carbon that has been processed in such a way that it has many small, low volume pores. The presence of such pores create a massive surface area - which is then available for chemical reactions or adsorption processes. It is commonly derived from charcoal, but there are methods for manufacturing it from less high quality carbon sources, such as bituminous coal, using compounds such as zinc oxide.
Zinc chloride is key to the activation of coal, where it primarily behaves as a dehydrating agent post-carbonisation (relatively low temperature heating). Pore volume evolution is increased with a greater amount of zinc chloride activation, making for a more active end product(14). The process is to mix ground or bituminous coal with a concentrated solution of zinc chloride before leaving the resulting slurry to dry at 110 °C for 14 hours - with this method being particularly liked due to providing uniform dehydration(15). The carbon produced had well developed and - crucially - uniform porosity throughout. Sulfur content was significantly reduced - with the zinc chloride solution providing a washing effect.
For activated carbons, an increase in particle size of the coal precursor leads to a reduction in the porosity of the resulting carbon material(16), even when produced under incredibly forcing temperatures. It is therefore imperative that sufficient washing occurs prior.
In the preparation of activated carbons from coal based humic acids (i.e. those from soils), it has been shown that a ratio of 2:1 of zinc chloride to humic acid is the most effective for producing activated carbons that are oxygen enriched(17), at a temperature of 500 °C, for use as electrodes. The zinc chloride treatment of the coal was found to be crucial to ensuring the porosity required. Uses of activated carbon compounds produced with zinc chloride indicate the removal of elemental mercury from flue gases, where the high surface area carbon removed 91.4% of mercury from the gas flow - with zinc chloride being responsible for ensuring effective adsorption(18).
After Coal Washing
As mentioned, coal washing uses vast quantities of water - and the process doesn’t ensure that no ‘good’ coal material isn’t leached into the wastewater. Coal washing wastewater, like many other industrial waste streams, may present an opportunity to increase overall plant efficiency through a treatment process. Most of the suspended or dissolved composition of the wastewater is fly ash. Coagulants based on fly ash and non-toxic salts of calcium(19) have been developed that are able to remove in excess of 99% of suspended solids and residual metal ions from coal washing wastewater. Recovered fly ash from coal can be used to improve anaerobic digester processes(20). Treatment of wastewater means it can be released into conventional sewers.
Summary
- Coal washing is the overall process of treating coal to remove impurities and add value
- Sulfur, often as pyrite, is the most important contaminant to remove as burning it leads to severe air pollution
- Zinc chloride solutions are used for washability testing, to determine how best to separate coal materials from each other, and are replacing toxic and/or organic solvents
- Heavy media separation is the predominant method for separating coal contents, relying on density. Zinc chloride solution is used as a medium
- Zinc chloride and carbon are reactive under certain conditions and ZnCl2 is used extensively in the production of activated carbons, even from low quality coals such as lignite
References
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