Physical cleaning

Generally, precombustion coal cleaning is achieved by the use of physical techniques, some of which have been used for more than a century. Physical cleaning methods typically separate undesirable matter from coal by relying on differences in densities or variations in surface properties. 

Physical cleaning can remove only matter that is physically distinct from the coal, such as small dirt particles, rocks, and pyrite. Physical cleaning methods cannot remove sulfur that is chemically combined with the coal (organic sulfur), nor can they remove nitrogen from the coal. Currently, physical cleaning can remove 30%-50% of the pyritic sulfur and about 60% of the ash-forming minerals in coal. 

Advanced physical cleaning techniques are expected to be more effective than older techniques (Feibus et al., 1986). And increased efficiency can be achieved by grinding the coal to a much smaller size consistency whereupon the coal will release more of the pyrite and other mineral matter. Thermal treatment can be used to reduce moisture and modify surface characteristics to prevent reabsorption. New coal-cleaning processes can remove more than 90% of the pyritic sulfur and undesirable minerals from the coal. 

Removing organic sulfur that is chemically bound to the coal is a more difficult task than removing pyritic sulfur through physical means (Wheelock, 1977). Currently, chemical and biological processes are being used to remove organic sulfur. 

Chemical or biological coal cleaning appears to be capable of removing as much as 90% of the total sulfur (pyritic and organic) in coal. Some chemical techniques also can remove 99% of the ash. 

---

Nitrogen, unlike pyritic sulfur, is mosdy chemically bound in organic molecules in the coal and therefore not removable by physical cleaning methods. The nitrogen content in most U.S. coals ranges from 0.5—2.0 wt %. 

Minor elements contribute >1 wt % to the ash trace elements contribute <0.1 wt %. The degree of de-ashing achievable by physical cleaning depends on the distribution of mineral matter in the coal. In some cases, a considerable amount of the mineral matter can be removed in other cases, especially where the mineral matter is distributed throughout the coal as microscopic particles, deashing by physical cleaning is not practical. 

Table 3. Effects of Physical Cleaning on Sulfur Reduction in Coal ...

Apart from physically cleaning tanks periodically, dispersants that promote homogeneity are employed. These often take the form of dispersant packages, which are polymer blends containing asphaltene and wax dispersants and other problem-specific additives. In addition, low-priced general-purpose dispersants, microbiocides, and antioxidants form part of the package. 

It is important to fully understand the physical cleaning cycle and the implications this has on the application of the methodologies. The same cleaning cycle is followed for both the mixing vessels and the intermediate storage vessels. 

After each growth cycle, the reactors must be opened, the wafers removed, and the lower portion of the reactor physically cleaned. The lower quartz reactor and the bottom plate (base plate) are scraped clean using a metal tool, and the particulate material (mixture of GaAs, GaAsP, arsenic oxides and phosphorus oxides) is collected in a metal container positioned below the vertical reactor. 

The most logical response, therefore, to using a makeup water source with high chlorides is to be scrupulous in maintaining clean metal surfaces in the cooling system. This can take the form of more frequent physical cleaning, the use of sidestream filters, polymer dispersants, and biodispersants. 

The disinfected water is circulated for 1 to 4 hours, then drained and the system physically cleaned. Biodispersants may be employed. 

Before reusing a mold, it should be physically cleaned to remove any grime or protective materials. Buildup on the surface should be cleaned off without damaging the surface of the mold. Location pins must be checked and repaired if needed. Any damage caused during demolding also must be repaired if in a critical area. The repairs must be such that they do not leave any marks on the surface of the part. 

Table IV lists the moisture and ash-free sulfur values for the eight coals before and after they were physically cleaned to reduce the mineral matter. The moisture and dry ash values for the cleaned coals are also listed. One can see from the results of the cleaning that the sulfur contents of the eight coals were substantially reduced.

Trace Element Removal During Physical Cleaning. Comnercial coal cleaning processes employ physical means for beneficiation and are aimed at removing ash forming minerals and sulfur, although removal of the mineral matter also results in reduced levels of some trace elements. Trace element extraction efficiencies for various physical cleaning processes have been reported. 

Physical cleaning of various coals by oil agglomeration reduced levels of As, Cr, Pb, Mn, Mo, Ni, and V by 50-80%, while levels of some other trace elements were reduced by lesser amounts (20). Oil agglomeration appeared to be more effective at removing trace elements than the wet concentrating table or float/sink density separations. This may be related to an increase in the liberation of mineral matter associated with grinding to produce the relatively fine particle sizes required in the oil agglomeration technique. 

Based on literature data, physical cleaning methods generally seem to be less effective at trace element removal then chemical methods. 

The technical feasibility of such an approach is demonstrated in Table IV, where data obtained on a sample of Illinois No. 5 seam cleaned coal from an Illinois Basin operating mine was processed by supercritical extraction. The cleaned coal had a total sulfur content of 1.5 % processing this physically cleaned coal for 1 hour at 350C in methanol with 5% K0H reduced the total sulfur concentration to 0.75 % The solid product, which retained 56 % of the original volatile matter concentration, exhibited a value of 1.1 lb SC / million BTU, thus meeting the existing new performance standard of 1.2 lb SO2/ million BTU. 

Lin, Y. A. (Ed.), Physical Cleaning of Coal—Present and Developing Methods, Marcel Dekker, New York, 1982. 

---