coal beneficiation - an overview | sciencedirect topics
Coal preparation, or beneficiation, is a series of operations that remove mineral matter (i.e., ash) from coal. Preparation relies on different mechanical operations (not discussed in detail here) to perform the separation, such as size reduction, size classification, cleaning, dewatering and drying, waste disposal, and pollution control. Coal preparation processes, which are physical processes, are designed mainly to provide ash removal, energy enhancement, and product standardization . Sulfur reduction is achieved because the ash material removed contains pyritic sulfur. Coal cleaning is used for moderate sulfur dioxide emissions control, as physical coal cleaning is not effective in removing organically bound sulfur. Chemical coal cleaning processes are being developed to remove the organic sulfur; however, these are not used on a commercial scale. An added benefit of coal cleaning is that several trace elements, including antimony, arsenic, cobalt, mercury, and selenium, are generally associated with pyritic sulfur in raw coal and they, too, are reduced through the cleaning process. As the inert material is removed, the volatile matter content, fixed carbon content, and heating value increase, thereby producing a higher quality coal. The moisture content, a result of residual water from the cleaning process, can also increase, which lowers the heating value, but this reduction is usually minimal and has little impact on coal quality. Coal cleaning does add additional cost to the coal price; however, among the several benefits of reducing the ash content are lower sulfur content; less ash to be disposed of; lower transportation costs, as more carbon and less ash is transported (coal cleaning is usually done at the mine and not the power plant); and increases in power plant peaking capacity, rated capacity, and availability . Developing circumstances are making coal cleaning more economical and a potential sulfur control technology and include :
Coal preparation, or beneficiation, is a series of operations that remove mineral matter (i.e., ash) from coal. Preparation relies on different mechanical operations, which will not be discussed in detail, to perform the separation, such as size reduction, size classification, cleaning, dewatering and drying, waste disposal, and pollution control. Coal preparation processes, which are physical processes, are designed mainly to provide ash removal, energy enhancement, and product standardization (Elliot, 1989). Sulfur reduction is achieved because the ash material removed contains pyritic sulfur. Coal cleaning is used for moderate sulfur dioxide emissions control as physical coal cleaning is not effective in removing organically-bound sulfur. Chemical coal cleaning processes are being developed to remove the organic sulfur, but these are not used on a commercial scale. An added benefit of coal cleaning is that several trace elements, including antimony, arsenic, cobalt, mercury, and selenium, are generally associated with pyritic sulfur in raw coal, and they too are reduced through the cleaning process. As the inert material is removed, the volatile matter content, fixed carbon content, and heating value increase, thereby producing a higher-quality coal. The moisture content, from residual water from the cleaning process, can also increase; this lowers the heating value, but it is usually minimal so as to have little impact on coal quality. Coal cleaning does add additional cost to the coal price; however, there are several benefits to reducing the ash content which includes lower sulfur content, less ash to be disposed, lower transportation costs since more carbon and less ash is transported (since coal cleaning is usually done at the mine and not the power plant), and increases in power plant peaking capacity, rated capacity, and availability (Harrison, 2003). Developing circumstances are making coal cleaning more economical and a potential sulfur control technology, and they include the following (Elliot, 1989):
Coal beneficiation, or coal preparation as it is also termed, refers to the processes through which inorganic impurities are separated from raw mined coal, thereby providing improved combustion characteristics to the fuel produced. The separation processes used are primarily based on exploiting the physical differences between the organic (i.e., coal) and inorganic (i.e., ash) components. Given the low unit value of coal, it is imperative for these separation processes to be both efficient and cost effective. The most commonly used processes are jig washing, density separation, sizing, and froth flotation. Typical configurations divide the run of mine coal into size fractions and utilize different separation processes for each size fraction (Luttrell, Barbee, & Stanley, 2003).
Density separation exploits the differences in density between the organic and inorganic components found in mined coal. As previously described, coal typically is comprised of an assemblage of macerals and inorganic material. Macerals containing primarily organic matter generally have a density of <1.4g/cm3, and as the amount of ash associated with the macerals increases, the density of the particles also increases, because the primary composition of ash associated with coal is essentially the weathered products of quartz (density 2.65g/cm3). Thus particles in the density range of 1.61.8g/cm3 have a higher ash content. Pyrite (FeS2), another commonly associated mineral, has a much higher density of 5.0g/cm3. Given the difference in density between the desired material (coal) and undesired material (ash and pyrite), density separation can be an efficient approach for producing low-ash coal, provided the high-ash content particles are liberated from the low-ash particles.
Density separation processes employed in coal preparation are typically performed in a medium suspension of fine ground (45m) magnetite (Fe3O4) dispersed in water. Magnetite is added to the suspension to maintain the desired medium density. For example, if the medium density is maintained at a density of 1.45g/cm3, all particles with lower density will float to the top of a separation vessel while the higher density particles sink. The float- and- sink products are separately removed and washed on an appropriately sized screen. Magnetite particles are recovered from washwater with magnetic separators and recycled back into the process. Dense medium separation of coarse particles (>50mm) is typically accomplished in vessels, while intermediate-size particles (501mm) are treated in cyclones. The operating principles of dense medium cyclones are essentially the same as those of conventional cyclone sizing processes; however, with dense medium cyclones, the fluid density can be increased to the desired separation density by the addition of magnetite. Jig washing employs similar separation principles, but rather than adjusting the medium density, particles are separated in a water medium that is pulsated pneumatically or hydraulically. The pulsation of the jigging motion stratifies particles based on density. Lighter particles migrate to the top of the particle bed, and denser particles migrate to the bottom, thus producing a separation based on particle density. The choice between using jigging or dense medium separation is generally made depending on the amount of near-gravity material, or the amount of material within 0.1 specific gravity units of the desired separation specific gravity. With 07% of the feed near gravity, almost any separation process will work effectively, though jigs are commonly employed under these conditions. With 7%10% near-gravity material, jigs operate with decreased efficiency, and so dense medium separation processes are appropriate. With >10% near gravity material, dense medium separation processes have application, but the process needs to be more closely controlled. With >25% near-gravity material, dense medium separation is very difficult, but can still find application in limited situations (Wills, 2006).
Size separation processes are the simplest to implement. These processes exploit distinct difference in sizes between coal and ash particles. If, for example, the coal to be processed is coarse while the ash is fine, then efficient separation can be achieved by a simple screening at the appropriate size. The same is true for the converse (i.e., coarse ash and fine coal). As this approach is so simple, it is used wherever possible; however, it is dependent upon the size distribution of the coal and ash particles. When particles are too small to screen efficiently, the size difference between coal and ash particles is exploited using classifying cyclones.
For fine particles (<150m), dense medium separation and sizing do not produce efficient separations. These particles are separated by flotation, which exploits differences in particle hydrophobicity. Most bituminous and higher-rank coals have some natural hydrophobic properties, while ash particles are hydrophilic. Coal hydrophobicity can be increased by selective adsorption of small quantities (100200g/tonne) of nonpolar collectors, such as diesel or fuel oil. The coal/ash suspension (1015% solids w/w) is agitated in a tank or cell, and air bubbles are introduced at the bottom of the cell. Surface-active agents, such as short-chain alcohols, are typically added to increase bubble surface area by reducing surface tension at the air/liquid interface, thus producing copious amounts of small air bubbles. Hydrophobic coal particles adsorb onto the rising air bubble and are transported to the top of the cell, where they coalesce and form a stable froth layer. The froth layer overflows the cell or is removed by mechanical scrapers while ash particles remain in suspension and are withdrawn from the cell. Flotation cells used in coal preparation are either mechanically agitated or column flotation cells with no agitator.
Economics of coal beneficiation using oil agglomeration approach is very sensitive to the quality and price of oil used. As it was shown in (23), the cost of oil comprised about 31 percent of the total product cost of the beneficiation plant or $14 per ton of coal processed. Total process capital and fixed costs comprised about 8 percent and cost of electricity for coal grinding about 2 percent of the total product cost. (No. 2 fuel oil at a rate of 10 percent on the dry-ash-free-coal weight and oil price of $200/t were considered. Energy consumption for coal grinding of 30 kWhr/t of feed at c2.75/kWh and coal cost of $26/t were assumed).
It is obvious from above that at the oil and coal prices presented, the oil agglomeration approach considered for coal beneficiation is uneconomical, unless oil consumption is drastically reduced, for example, by economic recovery of oil from beneficiated coal, or oil cost may be mostly written off as in the case of coal beneficiation integrated with a process where oil would be utilized together with the coal. Also, reduction of coal cost would substantially improve the economics.
On the other hand, as it was discussed earlier, direct liquefaction (hydrogenation) of a coal with reduced ash content may substantially increase liquid product yields. Preliminary calculations have shown, that for Canadian conditions, the expected overall liquid product cost of an integrated direct coal liquefaction plant producing 25,000 bbl/day of syncrude would be in the $4050/bbl range for lignite at a cost of $10-15/t having ash content of about 8-10 percent on dry coal weight.
The hard coal beneficiation process in mechanical preparation plants generates coarse, small or fines rejects and coal tailings slurries. The tailings are the finest grain size, with the majority below ~0.25mm, whereby material sized below 0.035mm makes up to 60% share in the slurry composition. Depending on the quality parameters (ash and sulphur content, calorific value, etc.), such slurries can be transferred as an ingredient to energy mixtures, or are dumped in earth settlers of individual mines. Most slurries to date have been collected in settlers, as there were no customers interested in buying them at the time they were produced. Dumped slurries were therefore treated as waste from coal preparation processes. Most of this waste is actually a potentially viable energy source. For this reason, in recent years, the interest in combustion options has increased as other fossil energy sources have increased in delivered cost. There is also interest in using coal tailings in construction products and engineering projects.
Some coal tailings are transferred to preparation plants for recovery of coal contained in the waste. Currently about 9% of generated waste is utilized in this way. The residue after the recovery of coal is re-dumped or used, for example in hydraulic backfilling or the building construction materials industry. Energy generation from coal tailings is covered in more detail in the sub-section below.
Coal tailings are quite commonly used in the manufacture of construction products for the building industry as an essential raw material for obtaining slate aggregate, i.e., a lightweight building construction aggregate used in the manufacture of lightweight concrete, as well as an essential raw material or component for the production of various building construction elements, such as bricks or roofing tiles. Currently, only about 0.5% of generated waste is utilized in this way. The waste is also added to the charge in the production of cement, in order to adjust the main module of cement clinkers. Coal tailings may also be useful for the production of refractory materials, but only if they have a high content of Al2O3.
Attempts have been made to recover metal concentrates from coal tailings, including aluminium, iron, titanium, germanium and gallium. Fine coal waste can also, after mixing with a compound fertilizer and peat, be used for biological reclamation and restoration of the fertility of devastated land, or reclamation of soil.
Flotation tailings wastes, a specific type of tailings, have not yet found an industrial application due to a number of factors including significant thixotropy, high humidity and difficulties in transport. However, such wastes can be used as a material for filling abandoned workings in mines or to seal the surface stockpiles. Post-flotation wastes from beneficiation of coking coals with calorific value more than 5 000kJ/kg can be used as fuel for the production of building construction ceramics, and after further beneficiation as an additive to energy fuel.
As no commercial coal beneficiation is perfectly efficient, some indices are required to measure the efficiency of the process. The best way of indicating the efficiency of a density separation device is the distribution of the partition curve (Fig. 7.4), which was first proposed by Tromp (1937). This curve depends on the equipment used, the relative density or cut-off point and the size range of the feed coal. Various simpler measures of efficiency have been defined but none are as accurate in predicting the performance of a density separation device as the Tromp curve. It denotes the probability of a particle reporting with the floats to its specific gravity. The distribution numbers are marked on the vertical axis against the various specific gravity fractions shown on the horizontal axis. Thus, if the vertical axis has value x, then the corresponding value of the horizontal axis is denoted as dx. The partition density is denoted by d50 the distribution number is 50, in this condition the particle will have an equal chance of floating or sinking. The Tromp curve is nothing but an error curve, the steeper the curve, the most efficient is the separation. To measure the inclination of the curve, Terra introduced Ecart Probable (Ep) which is defined as,
When, Ep = 0, the curve becomes a straight vertical line at the specific gravity of separation the efficiency of separation will be 100%. The Ep value does not consider the tails of the Tromp curve that are above the distribution number 75 and below the distribution number 25. The larger tails in the Tromp curve result in lower yield at the desired ash.
The efficiency at any relative density is defined as (Sarkar and Das, 1978) the recovery % of clean coal (ash % of raw coal ash % of clean coal) divided by the recovery % of float coal (ash % of raw coal ash % of float coal).
It may be noted that the most widely accepted measure of the efficiency with which a cleaning device separates coal from impurities is referred to as probable error (Ep) and Ep/dp which is sometimes called the generalised probable error (Gottfried and Jacobsen, 1977).
Currently, the wet coal beneficiation process is the predominant method for coal upgrading. The wet beneficiation processes include heavy media separation, cyclone (water only), froth flotation, and spiral separation [23,24]. The use of these technologies depends on the particle size of the feed and the quality of the product required. The quality of product and the recovery from the wet method is generally better than those obtained from the dry beneficiation method . Slimes and acidic water generated from the wet process require tailings ponds. Dewatering of the washed coal may cause leaching out of pollutants, which in turn can cause ground water pollution if not managed properly. Wet cleaning is mostly used for metallurgical coals, whereas there is a general trend to use dry beneficiation for thermal coals.
As noted, one of the fundamental reasons for coal beneficiation is the reduction of ash yield and deleterious minerals and elements with an inorganic affinity. The partitioning of major, minor, and trace elements depends on the degree of liberation of the minerals, their inorganic versus organic association, and the specific gravity of the separation. Mineral matter occurring as discrete bands and lenses within the coal can often be removed easily, but that disseminated within the coal matrix or within the organic compounds of the macerals will be more difficult to remove by simple density separation and may require extensive (and expensive) grinding to beneficiate. In low rank coals, dissolved salts or inorganic elements incorporated within the organic compounds of the macerals are common.
An overview of analytical methods used to determine inorganics in coal is given by Huggins (2002), and mineral matter in coal is presented in Chapter 2 of this book. A common method for determining whether a mineral or element will partition during beneficiation is through analysis of the float/sink fractions for different size fractions (Querol et al., 2001). As stated in Huggins (2002), the higher the organic affinity, the more the element reports to light-specific gravity fractions, and hence, the more it is associated with the organic fraction of the coal. One would assume that these lighter fractions would be dominated by vitrain, but that is not always the case. Various studies (Zubovic, 1966; Gluskoter et al., 1977; Cavallaro et al., 1978; Fiene et al., 1978; and Kuhn et al., 1980) suggest that the organic affinity of many elements varies significantly from coal to coal. More direct methods of analyzing maceral separates or scanning electron microscopy will assist in characterizing this variability for specific macerals and minerals.
Mitchell and McCabe (1937), Helfinstine et al. (1971, 1974), Cavallaro et al. (1976), and, more recently, Mastalerz and Padgett (1999) studied the ash and sulfur partitioning of (generally) high-S Pennsylvanian Illinois Basin coals. Because of the fine nature of much of the pyrite and an organic association of about half of the total S, the S in the clean product was generally above 2%.
Finkelman (1994b) discussed the associations of the hazardous trace elements. His work was based both on his own research (Finkelman, 1981) and on comprehensive works by others (e.g., Gluskoter et al., 1977; Raask, 1985b; Eskenazy, 1989; and Swaine, 1990). Akers and Dospoy (1994) demonstrated the magnitude of element reduction through a number of coal beneficiation schemes and DeVito et al. (1994) examined the trends in a large collection of coal company data from the Illinois Basin and the Northern Appalachians. Summaries of the associations of elements and the estimated ease of removal by conventional coal cleaning are shown in Table 3.1. As shown in the table, because of the varying modes of occurrence and the fine mineral associations, removal of trace elements by coal beneficiation can be quite inefficient. Further studies of the association of trace elements in coals have been conducted by Senior et al. (2000a) and Palmer et al. (2004) and their partitioning by size and gravity separation by W. Wang et al. (2006). Specific studies (and reviews of other studies) have been conducted for As (Kolker et al., 2000b; Yudovich and Ketris, 2005a), Hg (Yudovich and Ketris, 2005b,c; Brownfield et al., 2005; Wang M. et al., 2006), and Se (Yudovich and Ketris, 2006a). It should also be noted that trace elements are not uniformly distributed in minerals, such as As in pyrite (Ruppert et al., 2005) and Hg in pyrite (Hower and Robertson, 2003, citing unpublished work from 2000 by same authors).
Source: Fuel Processing Technology 39, M. S. DeVito, L. W. Rosendale, and V. B. Conrad, Comparison of trace element contents of raw and clean commercial coals, 87106, copyright 1994, with permission from Elsevier.
Not all element concentrations will be reduced by beneficiation. Organic sulfur is an obvious example of an element that will not easily be eliminated in beneficiation. Hower et al. (1998) noted an increase in total S from run-of-mine to clean Eastern Kentucky coals, since organic sulfur will go with the product rather than high density reject coal. Similarly, chlorine (associations reviewed by Spears, 2005; Yudovich and Ketris, 2006b) is generally associated with the organic fraction; therefore, removal of the diluent mineral matter increases relative Cl concentration.
Coal oxidation has an important influence on some relevant properties in relation with the coal beneficiation and utilization. For example its influence on plastic properties, which can be completely destroyed by the effect of air oxidation is well known . The reactivity of chars and cokes produced can be substantially modified by preoxidation of coal , and is strongly influenced by the conditions and the extent of oxidation and by coal rank. In this work a study was made in order to contribute to a better understanding of the effect of coal preoxidation on the reactivity of the chars produced. Textural properties of the chars and the gasified materials were determined an a study of their relation with kinetic parameters was carried out.
mining and beneficiation | ownerteamconsultation
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Koos has over 38 years experience in multidisciplinary mining, mineral beneficiation and the cement industry in various executive, projects, operational, engineering, business development and consulting roles. He is a registered professional engineer with a degree in Mechanical Engineering. More...
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This is the second of a two-part series of articles on the hydraulic fracturing of rock, also known as fracking. This is a technology that everyone has an opinion on, but few take the trouble to understand what its all about. The two parts are as follows: Part 1:...
This is the first of a two-part series of articles on the hydraulic fracturing of rock, also known as fracking. This is a technology that everyone has an opinion on, but few take the trouble to understand what its all about. The two parts are as follows: Part 1:...
what is beneficiation?
In the mineral industry, beneficiation is a process which is designed to improve the yield from a deposit of ore. This increases the potential profits available from the ore, and allows a company to increase the overall profitability of a mine and its business in a particular area. A number of processes are used to accomplish beneficiation objectives, and several companies which make mining equipment have lines of products which are designed to help companies get more out of their ore.
The goal of beneficiation is to eliminate inefficiency and waste by ensuring that as much recoverable material as possible is extracted from ore. A number of techniques can be used for this, often starting with grinding the ore into particles. Once ground, the particles can be sifted and sorted to extract usable material and set waste aside. For example, the particles may be suspended in water to allow various components to separate out, making it easy to access usable ore.
For rare resources, beneficiation is critical, because it takes advantage of every scrap of material available. This practice can also make a marginal mining facility more practical than it might otherwise be, and may in fact be used to extract ore from a facility previously believed to be exhausted. The potential for beneficiation is also considered when evaluating sites of prospective mines, to determine whether or not the expense of mine operation will be outweighed by the products of the mine.
People concerned with sustainable development and ethical business practices also use the term beneficiation, but in a slightly different way. Rather than meaning that the maximum potential of a resource has been exploited, beneficiation refers to business practices which benefit the communities where products are mined, harvested, and otherwise taken. Historically, major companies have tended to enter small communities, take resources, and then leave, with no benefit to the populace.
This practice of exploiting a community and then leaving has become frowned upon as a form of exploitation of people and national governments, making beneficiation increasingly popular. With beneficiation, a company does things like moving some of its operations to the country where a product is harvested or mined, giving back to the community, and doing more work to keep some of the profits and benefits in country. For example, if a company is mining opals, it might open a facility for cutting and polishing opals near the mine, rather than shipping them overseas for processing, to create more job opportunities for the local community. Likewise, a company taking timber might operate a mill near the forest rather than shipping raw timber overseas.
Ever since she began contributing to the site several years ago, Mary has embraced the
exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and
spends her free time reading, cooking, and exploring the great outdoors.
Ever since she began contributing to the site several years ago, Mary has embraced the
exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and
spends her free time reading, cooking, and exploring the great outdoors.
benefits of beneficiation - st equipment & technology (stet)
In the mining industry, the standard definition of beneficiation is any process that improves (benefits) the economic value of mined ore by removing valueless materials, resulting in a higher grade of product.
At ST Equipment & Technology, our definition differs slightly in that we not only improve the value of the product coming out of our proprietary triboelectric belt separator process, we can also collect all of the useful materials separated out, to provide you with a variety of grades of product.
So, the bottom line is your bottom line. You get more from your current operations, through a process that generates additional potential income via a very fast, economic and ecofriendly process. Even better, this process can be done on-site, so you dont have to spend a fortune trucking materials to a separation facility.
With ST Equipment & Technologys proprietary triboelectrostatic belt separator process we reclaim more from what you are currently processing. This method provides you with myriad levels of your final product,