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Economic and operating conditions make it important to provide a simple, low cost, efficient method for recovering fine coal from washery waste. Not only is the water pollution problem a serious one, but refuse storage and disposal in many areas is becoming limited and more difficult. Many breakers and washeries efficiently handle the coarser sizes, but waste the coal fines. This problem is assuming major importance due to the increase in the amount of coal fines being produced by the mechanization of coal mining.
Flotation offers a very satisfactory low-cost method for recovering a fine, low ash, clean coal product at a profit. Often this fine coal, when combined with the cleaned, coarser fractions, results in an over-all superior product, low in ash and sulphur, giving maximum profit returns per unit mined.
Generally a very simple flotation flowsheet, as illustrated above, will be suitable for recovering the lowash coal present in waste from coarse recovery washeries.Assuming the fines are approximately all minus 20 mesh and in a water slurry of about 20% to 25% solids, the first step is to condition with a reagent which will promote flotation of the fine coal particles. Kerosene, fuel oil, coal tars and similar hydrocarbons will accomplish this effectively when added to thecoal slurry in a (Patented) Super Agitator and Conditioner. A frothing agent such as pine oil, alcohol frother, or cresylic acid added to the slurry as it discharges from the conditioner is also used. The separation between low ash coal and high ash refuse is efficiently accomplished in a Sub-A Flotation Machine. As the amount of clean coal floated represents a high percentage of the initial feed, provision is made to remove the cleaned coal from both sides of the cell. Fine coal is dewatered with a Disc Filter, as the Flotation Machine can usually be regulated to produce a product low in ash and with proper density for direct filtration.
It is highly desirable to extend the range of coal flotation to include the coarser Sizes. Not only will this simplify general washery practice but will result in a superior product having desirable marketing characteristics for metallurgical and steam power plant uses. It is now possible to efficiently recover coal by flotation through the entire size range beginning at about 4 mesh down to fines, minus 200 mesh.
With the flowsheet as outlined for coarse coal recovery, the feed is first deslimed for removal of high ash slimes and excess water. The hydroclassifier underflow is conditioned at 40% to 45% solids with kerosene or fuel oil and diluted with water to 20%-25% solids prior to flotation. If pyrite and coarse high ash material are present, it is often helpful to pass the conditioned pulp over a Mineral Jig for removal of a portion of these impurities. Hindered settling in the jig against a rising pulsating water column classifies out the high gravity impurities and eliminates them from the flotation circuit. Water requirements are low and feed density to flotation can easily be maintained at the proper level.
The Sub-A (Lasseter Type) Flotation Machine has proved successful for treating coarse coal with the flowsheet as indicated. A frother of the alcohol type is generally added to the flotation feed after conditioning with kerosene. Floated coal will collect in a heavy dense matte at the cell surface and as raked off, will contain up to 60% solids. Mechanical dewatering is usually not necessary. Natural drainage, dewatering on porous bottom screw conveyors, and vibrating screen dewatering are all being used successfully in coarse coal recovery circuits.
Flotation, with the Sub-A gravity flow principle, provides the ideal way to treat coal fines even as coarse as 3/16 top size. According to reports from plants operating for the production of metallurgical coke, each percent ash in the coal carries a penalty of 2$ per ton of coal. Thus there is a considerable margin for operating costs in a fine flotation cleaning method that will materially lower the ash of the cleaned coal. Further convincing evidence that ash removal from coal is of major importance is found in the weekly magazine of metal working, Steel, January 29, 1951, reporting on a modern coal preparation plant. The report states that a 1% reduction in ash content of coal means a reduction of 30 cents in cost of pig iron. One large plant reduces the ash from 7% to 3.5% by cleaning, thus cutting the cost of producing pig iron a dollar or more per ton.
A coal flotation machine must not only be able to handle a coarse as well as a fine feed, but it must also be simple to operate. Gravity circulation permits the treatment of difficult unclassified feeds.
High cost of mining makes it very important from a profit standpoint to recover all of the low ash coal, both coarse and fines. With the present trend toward mechanization, more fines are produced in mining. In many operations it is no longer economical to discard these fines to waste even though ash contiminants render the fines unmarketable without additional cleaning.
Water conservation, stream pollution and refuse storage are also factors which must be taken into consideration along with marketing requirements for the clean coal product. Flotation offers an efficient and low-cost method for recovering coal fines at a profit. In many cases floated coal fines can be blended with the coarser fractions without affecting ash, moisture or size limitations. This is being done successfully in coking coal operations. Fine coal is also being used extensively in steam plants for electric power generation.
The above flowsheets are based on existing small coal flotation plants. They illustrate clearly the simplicity and feasibility of adding Sub-A Coal Flotation as an additional process to small washing plants.
Because of its limited output, treatment must be very simple and operating costs kept to a minimum. At the washery, illustrated by flowsheet A, the entire mine output is sold for coking coal. Mining the relatively narrow seam produces a product with 15 to 20% ash, although the coal when cleaned will carry only 3 to 3% ash. This low ash coal brings a premiumprice, so it is an economic necessity to remove the impurities.
The mine run coal is crushed to a size for coking coal requirements. The entire production is treated over a coal jig which removes as waste primarily the coarse refuse. The coarse clean coal passes over the jig along with the fines and is elevated to a wedge bar stationary screen with 1 millimeter openings for dewatering. The coarse clean coal passing over the screen discharges by gravity into a storage bin. The fine coal, along with clay and its high ash fractions and water averaging 15 to 18% solids, discharges by gravity into a (Patented) Super Agitator and Conditioner. Kerosene and pine oil are added and the conditioned slurry or pulp then is introduced into the Sub-A Coal Flotation Machine.
The low ash coal product removed from the Sub-A Coal Flotation Cells contains 35 to 40% solids and is transferred to the coarse coal storage bin through a Vertical Concentrate pump. The flotation coal mixes with the coarse product which allows for adequate drainage and minimum loss of fines.
In the operation as illustrated by flowsheet B, approximately 15 tons per hour of coal flotation concentrate are produced. This installation requires more control to meet specifications and consequently a more elaborate system is necessary.
Screen undersize and water containing fines from the gravity separator are thickened in a centrifugal or cyclone separator to give the proper water-to-solids ratio for subsequent treatment. The effluent from the cyclone contains collodial slimes and high ash fines in addition to the bulk of the water from screening and gravity systems. Thickened coal fines from the cyclone pass over a Mineral Jig which removes a high ash refuse and free pyrite down as fine as 150 to 200 mesh.
The coal fines passing over the Jig are conditioned with reagents in the (Patented) Super Agitator and Conditioner and subjected to flotation treatment in a 6-cell Sub-A Coal Flotation Machine at approximately 20-25% solids. Double overflow of froth is used due to the low ratio of concentration and the high weight percentage of floatable coal recovered by flotation.
The coal flotation product at 35% solids is dewatered by a Disc Filter. Coarse coal from the gravity section and fine coal from the flotation section are blended and transferred by rail to the coke plant.
In some cases the coarse and fine coal are dewatered by Dillon Vibrating Screens. The coarser fractions of coal are first added to the screen to form a bed and flotation fines are added on top of this bed for dewatering. Where operating conditions are favorable, this system is preferred to other means of dewatering as it assures a well blended product low in moisture and uniform in ash content.
Effluent from the cyclone, high ash jig refuse and flotation tailing refuse are thickened in a Thickener to conserve and re-use water. Thickener refuse is disposed of without contaminating local streams.
Sub-A Coal Flotation with its gravity flow principle and selective action makes it possible to recover low ash coal from 1/8 down to minus 200 mesh. If an appreciable amount of recoverable coal is plus 20 mesh in size, the Sub-A Lasseter Type Coal Flotation Machine should be used. It is no longer necessary to use a complex system for fine coal recovery. Flotation will effectively handle the entire fine size range at low cost and produce a low ash marketable product.
In the washing of coal the problem exists in having to clean the fines in an economical and efficient manner without an excessively complex flowsheet. Mechanized mining creates fines not considered as problems in older methods of selective mining and underground loading. In many cases the minus 1/8 inch fines require cleaning to lower the ash content and frequently it is also necessary to reclaim all of the water for re-use in the washing system. Most plants use a closed water system to conserve water and comply with anti-stream pollution regulations.
Flotation offers a means for handling the entire size range minus 1/8 inch x 0. Efficient recovery of the fines at a low ashcontent is accomplished in a relatively simple flowsheet. Thesubstantial amount of coarser sizes in the concentrates aids in subsequent dewatering either by vacuum filters or dewatering screens.
In the flowsheet shown mine run coal after proper size reduction treatment is passed over heavy duty screening equipment to removethe minus 1/8 inch fines. Wet screening down to 10 or 12 mesh offers no particular problem. Water sprays are generally employed to thoroughly wash the fines from the coarse coal and prepare it for treatment. A surge tank or a thickener ahead of the conditioning and flotation section may be necessary to provide a uniform feed rate both as to solids content and density.
The coarse coal is washed and up graded in a conventional manner through heavy media or coal jigs to produce a clean coal and a coarse refuse. Any fines due to degradation through the coarse cleaning system is collected, partially dewatered and combined with the fines from the screening section.
Minus 1/8 inch x 0 coal fines are conditioned with the required amount of fuel oil or kerosene (approximately 1 to 3 lbs/ton) to thoroughly activate the low ash coal particles and render them floatable. Density in the conditioner should be as high as possible; however, for the open circuit system as shown it very likely will be maintained between 20 to 25% solids. A Super Agitator and Conditioner is preferred for this service since any froth accumulation on the surface is drawn down the standpipe and thoroughly dispersed throughout the pulp. This also aids in the most effective use of reagents.
The discharge from the conditioner at 20% solids is floated in a Sub-A Flotation Machine of the free flow type for handling coarse solids. Some dilution water may be necessary to maintain the feed density at 20% solids. A frother such as pine oil, cresylic acid, or one of the higher alcohols is added to the head of the flotation circuit at the rate of about 0.5-1.5 lbs/ton.
In the primary flotation section a high recovery of the coal fines minus 28 mesh is secured. In addition some of the more readily floatable coarse coal, low in ash, is also recovered. However, ability of the machine to handle all 1/8 inch feed permits recovery of coal over wide range of mesh sizes, thus improving filtering and handling characteristics. This coal, if not clean enough, is refloated in cleaner cells and middlings are recycled back to the feed. Clean coal will contain about 35% solids which is ideal for vacuum filtration. A Agitator Type Disc Filter is used as solids are effectively kept in suspension giving uniform distribution of cake for greater dewatering.
Generally the refuse from the primary flotation cells will contain a very high ash content in the -28 or-35 mesh size fraction. By screening the refuse the excess water and undersize high ash fines are eliminated while screen oversize is re-treated by flotation. This screening need not be highly efficient since only a partial sizing is satisfactory. Handling the coal in this manner reduces size degradation to a minimum.
The coarse coal from the foregoing dewatering and screening step is repulped to about 40% solids and conditioned with reagents. The conditioned pulp after dilution to 25 to 28% solids is floated in a second bank of flotation cells. The coarse coal in the absence of fines will form a dense, heavy matte at the surface of the cells. For this type of flotation, slow moving rakes are provided to remove the coal as final concentrate. This clean coal will generally contain over 50% solids, thus making it ideal for dewatering over vibrating screens or on a horizontal or top feed vacuum filter. In some plants where moisture is not too critical a screw conveyor with wedge bar bottom sections is used for the dewatering step.
The refuse from the coarse coal flotation cells may still contain some coal notresponsive to flotation recovery but low enough in ash to be saved. In such cases the refuse can be screened and the oversize fraction jigged or tabled. The tonnage at this point is usually only a very small percentage of the initial fines so the equipment requirements for this gravity section are moderate.
All refuse in the 1/8 inch x 0 coal recovery section is collected in a thickener for water reclamation. The thickened refuse or sludge underflow may be pumped to waste ponds, or if water is in short supply, filtration of this refuse may be necessary.
Coal flotation concentrates produced in this primary section are filtered direct and the filtrate is re-cycled back to the flotation cells for re-use. This filtrate is high in reagent content and is particularly useful as dilution water. Generally the density of the coal from the primary cells will contain about 35% solids and thus does not require thickening ahead of filtration.
Coal from the coarse flotation and the gravity section, if employed, can be readily dewatered over screens or horizontal or top feed filters. In some cases it may be possible to divert part or all of this coal to the filter handling the fines provided it is equipped with proper agitation equipment and a high displacement vacuum system. Some of the new synthetic filter bag fabrics such as Saran and nylon materially aid in securing high filter rates and low final moisture content.
Sub-A Coal Flotation Systems have been successful for recovery of both coarse and fine coal. It is important, however, to employ a two-stage circuit for maximum efficiency in saving the plus 28 mesh fraction which is normally the most difficult to float. The development of the free flow and Type M flotation cells offers a means for efficiently handling coarse coal in a size range heretofore reserved for other more complex systems.
Ash and sulphur content is desired to be as low as, or lower than, for regular lump coal. Generally, for anthracite, not over 13 per cent ash is desired. Bituminous coal operations usually limit ash to not more than 8 per cent in the fines.
Flotation or gravity concentration are generally applied only to washery fines that otherwise would not be saleable and which generally have to be impounded to prevent stream pollution. Because of the low price secured, the expense of treatment must be held to a minimum. Pyrite and coarser ash-forming content may need to be removed by gravity treatment.
Kerosene or fuel oil with pine oil, or alcohol, frother are the more common reagents used. Cresylic acid frother may sometimes be advantageous. Fine pyrite, if free, may be rejected with the high-ash refuse by addition of lime to the flotation feed.
Under proper conditions, coal as coarse as 10 mesh maybe effectively floated with kerosene and pine oil. For this coarse flotation it is generally necessary to classify out the high-ash 200 mesh slimes ahead of flotation.
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.