Cleaning coal chemical treatment

Some mineral products are employed essentially in the form in which they are mined, with only shaping, crushing, cleaning or other treatments that do not change their compositions. Coal, in most of its uses, is a typical example. Most minerals are processed, however, to yield usable products - metals, chemicals or other materials from which most of the items of utility are derived. As regards coal, it may be pointed out that it is often referred to as a mineral, but unlike a true mineral, it has no fixed chemical formula. 

Physical methods can effectively remove some trace constituents from coal, especially if deep cleaning methods are employed (7 ). However, such methods do not adequately remove finely disseminated minerals or organically bound elements, thereby necessitating chemical treatments for removing such components. 

Ten alternatives for correcting or preventing water pollution from acid drainage produced by piles of coal-cleaning wastes are compared. Options 1, 2 and 10 involve pretreatments of the waste before it is disposed to the pile. Options 3 to 6 refer to treatments undertaken as the pile is being formed. Options 7 to 9 refer to chemical treatment of the pile effluent. All costs are based on 1985 dollars. 

The removal of both mineral matter and sulfur species to very low values would provide premium solid fuels and possibly new chemical feedstocks. Several techniques are being explored to achieve these goals. The mineral matter in a physically cleaned coal can be further reduced by the solubilization of the aluminosilicate minerals. This can technically be accomplished with the use of alkaline and then acid treatments. A variety of studies are under way to define the conditions required for effective removal of the mineral matter and establish the amount of sulfur reduction that can be accomplished. Others involve the use of fine grinding to liberate the coal from the mineral matter. Then an agglomerant is used to separate the coal matter from the aqueous phase containing suspended mineral matter. A new approach uses microwave energy to selectively decompose the clays into species that can be solubilized and removed. Still another technique involves treatment with carbon dioxide to reduce the particle size and permit the liberation of the mineral matter. Over the next few years these will be studied further and it is hoped that coal will become available in a form with less of these interesting, but not entirely desirable mineral species. 

Indeed, since the passage of the original Clean Air Act of 1970, subsequently amended in November 1990, coal preparation efforts in the United States have emphasized development of technology for the reduction of sulfur. One approach to reducing sulfur emissions is accomplished by coal conversion, the direct thermal and/or chemical treatment of coal to produce virtually sulfur-free liquid fuels (Chapters 18 and 19) or gaseous fuels (Chapters 20 and 21) the sulfur can be recovered as a by-product of the conversion process (Chapter 23). 

Nevertheless, clean coal technologies reduce emissions of several pollutants, reduce waste, and iuCTease the amount of energy gained from each ton of coal (Nordstrand et al., 2(X)8 Ranco and Diaz, 2(X)9). They include various chemical and physical treatments applied pre- or postcombustion and may be broadly divided into processes relating either to (1) combustion efficiency or (2) poUntion control. 

Apart from the provision of various permutations of (chemical-based) boiler water programs, it is common to find water treatment companies supplying value adding chemicals and services in other boiler plant-related areas where their expertise in applied chemical technology can deliver additional economic benefit. Such areas typically include cleaning services for boiler waterside and fireside and the provision of fuel treatments and combustion additives, dust suppressants (for coal and ash handling), acids, and cleaner products. 

It is important that the tube surfaces be kept clean to avoid the initiation of corrosion. Regular waterside inspections and, if necessary, chemical cleaning of high-pressure equipment is recommended. The level of chloride that may be tolerated in such boilers during steady operation depends on the type of treatment employed. Where all-volatile alkaline treatments (AVT) are used, then the chloride levels should be lower than where nonvolatile alkalis (NVAT), such as sodium hydroxide and sodium phosphate, are used. The value may vary, depending on whether the boiler is coal-fired or oil-fired. 

In our own work, the effectiveness was examined of trace element removal by two chemical coal cleaning methods being studied for possible large-scale application. One method involved treatment with aqueous Na,C03 solutions, and the other involved treatment with molten NaOH/KOH mixtures. In each case, acid washes were used as subsequent cleaning steps to decrease the ash content, followed by water washes in the final step. Although the primary objective of these treatments was to remove sulfur and ash-forming minerals, the extraction of trace elements from these coals was examined as an additional benefit of chemical coal cleaning. 

Use Glass manufacture, chemicals, pulp and paper manufacture, sodium compounds, soaps and detergents, water treatment, aluminum production, textileprocessing, cleaning preparations, petroleum refining, sealing ponds from leakage (sodium ions bind to clay particles, which swell to seal leaks), catalyst in coal liquefaction. 

The chemical methods are not effective in removing major amounts of organic sulfur. Thus, removal of total sulfur to produce a clean-burning fuel is very difficult. It is envisioned that the optimum desulfurization of coal will include physical, chemical, and microbial treatment. The raw coal will be ground fine to liberate most of the pyritic sulfur and then processed by using the column... 

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