Table Of Content

P1: GRB/LSX P2: ZCK Final Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 Coal Preparation Robert A. Meyers Janusz S. Laskowski Ramtech Limited University of British Columbia Anthony D. Walters Kilborn Engineering Limited I. Coal Characteristics Related to Coal Preparation II. Breaking, Crushing, and Grinding III. Coal Screening IV. Float-and-Sink Analysis V. Dense-Medium Separation VI. Separation in a Water Medium VII. Flotation VIII. Separation Efficiency IX. Ancillary Operations X. Coal Preparation Flowsheets XI. On-Line Analysis XII. Research into New Beneficiation Processes GLOSSARY washability, by which the coal is separated into various density fractions. Dense media (or heavy liquids) Fluids used in dense- Floatability Description of the behavior of coal particles medium separation of coal and gangue particles by in froth flotation. their relative densities. The medium can be any suitable Hardgrove grindability index Measure of the ease by fluid, but in commercial coal preparation operations, it which the size of a coal can be reduced. Values decrease is usually a suspension of fine magnetite in water. with increasing resistance to grinding. Density (gravity) separation Separation methods based Metallurgical coal Coal used in the manufacture of coke on differences in density of separated minerals, such as for the steel industry. dense-medium separation and jigging. Near-density material Percentage of material in the feed Float-and-sink tests Tests carried out to determine coal within ±0.1 density range from the separation density. . 79 P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 80 Coal Preparation Organic efficiency (recovery efficiency) Measure of I. COAL CHARACTERISTICS RELATED separating efficiency, calculated as TO COAL PREPARATION actual clean coal yield Coal, an organic sedimentary rock, contains combustible × 100 theoretical clean coal yield organic matter in the form of macerals and inorganic matter mostly in the form of minerals. with both yields at the same ash content. Coal preparation upgrades raw coal by reducing its con- Probable error Ep (Ecart probable moyen) Measure tent of impurities (the inorganic matter). The most com- of separating efficiency of a separator (jig, cyclone, mon criterion of processing quality is that of ash, which is etc.). Calculated from a Tromp curve. not removed as such from coal during beneficiation pro- RRB particle size distribution Particle size distribu- cesses, but particles with a lower inorganic matter con- tion developed by Rossin and Rammler and found by tent are separated from those with a higher inorganic mat- Bennett to be useful in describing the particle size dis- ter content. The constituents of ash do not occur as such tribution of run-of-mine coal. in coal but are formed as a result of chemical changes Separation cut point δ50 Density of particles report- that take place in mineral matter during the combustion ing equally to floating and sinking fractions (heavy process. The ash is sometimes defined as all elements media overflow and underflow in the dense media in coal except carbon, hydrogen, nitrogen, oxygen, and cyclone). The δ50 is also referred to as partition sulphur. density. Coal is heterogeneous at a number of levels. At the Separation density δs Actual density of the dense simplest level it is a mixture of organic and inorganic medium. phases, but because the mineral matter of coal originated Thermal coals Coals used as a fuel, mainly for power in the inorganic constituents of the precursor plant, from generation. These are lower-rank coals (high volatile other organic materials, and from the inorganic compo- bituminous, subbituminous, and liginites). nents transported to the coal bed, its textures and libera- Tromp curve (partition curve, distribution curve) tion characteristics differ. The levels of heterogeneity can Most widely used method of determining graphi- then be set out as follows (Table I): cally the value of Ep as a measure of separation efficiency. Synonyms: partition curve, distribution 1. At the seam level, a large portion of mineral matter in curve. coal arises from the inclusion during mining of roof Washability curves Graphical presentation of the float- or floor rock. sink test results. Two types of plots are in use: Henry– 2. At the ply and lithotype level, the mineral matter may Reinhard washability curves and M-curves. occur as deposits in cracks and cleats or as veins. Washing of coal (cleaning, beneficiation) Term denot- 3. At the macerals level, the mineral matter may be ing the most important coal preparation unit opera- present in the form of very finely disseminated tion in which coal particles are separated from inor- discrete mineral matter particles. ganic gangue in the processes based on differences in 4. At the submicroscopic level, the mineral matter may density (gravity methods) or surface properties (flota- be present as strongly, chemically bonded elements. tion). Cleaning then increases the heating value of raw coal. Even in the ROM coal, a large portion of both coal and shale is already liberated to permit immediate concentration. This is so with heterogeneity level 1 and to some COAL PREPARATION is the stage in coal production— extent with level 2; at heterogeneity level 3 only crushing preceding its end use as a fuel, reductant, or conver- and very fine grinding can liberate mineral matter, while at sion plant feed—at which the run-of-mine (ROM) coal, level 4, which includes chemically bonded elements and consisting of particles, different in size and mineralogi- probably syngenetic mineral matter, separation is possible cal composition, is made into a clean, graded, and con- only by chemical methods. sistent product suitable for the market; coal prepara- Recent findings indicate that most of the mineral matter tion includes physical processes that upgrade the quality in coal down to the micron particle-size range is indeed of coal by regulating its size and reducing the content a distinct separable phase that can be liberated by fine of mineral matter (expressed as ash, sulfur, etc.). The crushing and grinding. major unit operations are screening, cleaning (wash- The terms extraneous mineral matter and inherent ing, beneficiation), crushing, and mechanical and thermal mineral matter were usually used to describe an ash- dewatering. forming material, separable and nonseparable from coal P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 Coal Preparation 81 a TABLE I Coal Inorganic Impurities Type Origin Examples Physical separation Strongly chemically From coal-forming organic Organic sulphur, nitrogen No bonded elements tissue material Adsorbed and weakly Ash-forming components in pure water, Various salts Very limited bonded groups adsorbed on the coal surface Mineral matter Minerals washed or blown into the peat Clays, quartz Partly separable by physical methods a. Epiclastic during its formation b. Syngenetic Incorporated into coal from the very Pyrite, siderite, some clay Intimately intergrown with coal macerals earliest peat-accumulation stage minerals c. Epigenetic Stage subsequent to syngenetic; migration Carbonates, pyrite, kaolinite Vein type mineralization; epigenetic of the minerals-forming solutions minerals concentrated along cleats, through coal fractures preferentially exposed during breakage; separable by physical methods a Adapted from Cook, A. C. (1981). Sep. Sci. Technol. 16(10), 1545. Physical cleaning methods can remove inorganic sul- by physical methods. Traditionally in coal preparation pro- fates (gypsum) and most of the coarse pyrite; the finely cesses, only the mineral matter at the first and, to some disseminated microcrystalline pyrite and organic sulfur extent, the second levels of heterogeneity was liberated, are usually not separable by such processes. This means the rest remained unliberated and, left with the cleaned that in the case of coal containing 70% of sulfur in pyritic coal, contributed to the inherent mineral matter. Recent form and 30% as organic sulfur, the physical cleaning can very fine grinding, which also liberates the mineral mat- reduce the sulfur content by about 50%. ter at the third level of heterogeneity, has changed the old meanings of the terms inherent and extraneous. The content of the “true” inherent part of ash-forming material II. BREAKING, CRUSHING, (i.e., the part left in coal after liberating and removing AND GRINDING the mineral matter at the first, second, and third levels of heterogeneity) is usually less than 1%. The primary objectives of crushing coal are In recent years, the emphasis in coal preparation has been placed on reducing sulfur content of coal and on re- 1. Reduction of the top size of ROM coal to make it covering the combustible material. Sulfur in coal is present suitable for the treatment process in both organic and inorganic forms. The dominant form 2. liberation of coal from middlings, and of inorganic sulfur is pyrite, but marcasite has also been 3. size reduction of clean coal to meet market reported in many coals. Pyrite occurs as discrete particles, specification. often of microscopic size. It comprises 30–70% of total sulfur in most coals. Other forms of inorganic sulfur that may be present are gypsum and iron sulfates. The sulfate Size reduction of coal plays a major role in enabling level in fresh unoxidized coals is generally less than 0.2%. ROM coal to be used to the fullest possible extent for Organic sulfur in coal is believed to be contained in power generation, production of coke, and production of groups such as synthetic fuels. ROM coal is the “as-received” coal from the mining process. Because the types of mining processes are varied, and size reduction actually begins at the face COAL COAL in the mining operation, it is quite understandable that S the characteristics of the products from these various pro- THIOPHENE cesses differ widely. The type of mining process directly affects the top size and particle size distribution of the R S R′ R SH R SS R′ mine product. During the beneficiation of coal, the problem of treating ORGANIC MERCAPTAN ORGANIC SULFIDE DISULFIDE middlings sometimes arises. This material is not of sufficient quality to be included with the high-quality clean 2T.h5e%owrg/awn.ic sulfur content in coals range from 0.5 to coal product, yet it contains potentially recoverable coal. If this material is simply recirculated through the cleaning P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 82 Coal Preparation circuit, little or no quality upgrading can be achieved. Lib- rections. Normally, one roll is fixed while the other roll is eration can be accomplished if the nominal top size of the movable against spring pressure. This permits the passage material is reduced, which permits recovery of the coal of tramp material without damage to the unit. The drive in the cleaning unit. Hammer mills, ring mills, impactors, units are normally equipped with shear pins for overload and roll crushers in open or closed circuits can be used to protection. reduce the top size of the middlings. Normally, reducing Hammer Mills. The swinging hammer mill (instead the nominal top size to the range 20–6 mm is practiced. of having teeth as on a single-roll crusher) has hammers Breaking is the term applied to size operations on large that are mounted on a rotating shaft so that they have a material (say +75 mm) and crushing to particle size re- swinging movement. duction below 75 mm; the term grinding covers the size reduction of material to below about 6 mm. However, these B. Grinding terms are loosely employed. A general term for all equipment is size reduction equipment, and because the term The most common grindability index used in conjunction comminution means size reduction, another general term with coal size reduction, the Hardgrove Grindability In- for the equipment is comminution equipment. dex, is determined by grinding 50 g of 16 × 30 mesh dried coal in a standardized ball and race mill for 60 revolutions A. Breaking and Crushing at an upper grinding ring speed of 20 rpm. Then the sample is removed and sieved at 200 mesh to determine W , the Primary breakers that treat ROM coal are usually rotary amount of material passing through the 200 mesh sieve. breakers, jaw crushers, or roll crushers; some of these are The index is calculated from the following formula: listed below. HGI = 13.6 + 6.93W. (1) Rotary Breakers (Fig. 1). The rotary breaker serves two functions—namely, reduction in top size of ROM and re- From the above formula, it can be deduced that as jection of oversize rock. It is an autogenous size-reduction the resistance of the coal to grinding increases, the HGI device in which the feed material acts as crushing media. decreases. The HGI can be used to predict the particle size Jaw Crusher. This type of primary crusher is usually distribution, that is, the Rossin–Rammler–Bennett curve used for crushing shale to reduce it to a size suitable for [see Eq. (4)]. handling. The distribution modulus (m) and the size modulus d 63.2 Roll Crusher. For a given reduction ratio, single-roll must be known to determine the size distribution of a crushers are capable of reducing ROM material to a prod- particular coal. uct with a top size in the range of 200–18 mm in a single It has been shown that for Australian coals, the distri- pass, depending upon the top size of the feed coal. Double- bution modulus can be calculated from the HGI by the roll crushers consist of two rolls that rotate in opposite di- following equation: FIGURE 1 Cutaway view of rotary breaker. P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 Coal Preparation 83 −1.54 HGI = 35.5m . (2) The value of d63.2 is a function of the degree of breakage that the coal has undergone during and after mining. Having selected d63.2 and the value of m from the HGI, one can predict the size distribution. For the use of coal at power stations and for treatment FIGURE 3 Passage of a spherical particle through a square by some of the newer beneficiation techniques (e.g., for aperature. coal/water slurries), grinding is employed to further reduce the top size and produce material with a given par- through. In the simplest case (Fig. 3), a spherical particle ticle size distribution. The principal equipment used for of diameter d will only pass through if it does not touch coal grinding is the following: the sides of the aperture. The condition for passing is that the center of the sphere falls within the inner square, side 1. air-swept ball mills, a − d. The probability P of passing is thus given by the ra- 2. roll or ball-and-race type mills, tio between the areas of the inner and outer squares, that is, 3. air-swept hammer mills, and 2 2 2 P = (a − d) /a = (1 − d/a) . (3) 4. wet overflow ball mills. The assumption that passage will be achieved only if there is no contact with the aperture sides is too re- III. COAL SCREENING strictive. They can and do collide with the screen deck while passing through. In particular, the following factors Sizing is one of the most important unit operations in coal contribute to the probability of passing: preparation and is defined as the separation of a heterogeneous mixture of particle sizes into fractions in which all 1. Percentage open area particles range between a certain maximum and minimum 2. Particle shape size. Screening operations are performed for the following 3. Angle of approach purposes: 4. Screen deck area 5. Bed motion 1. Scalping off large run-of-mine coal for initial size 6. Size distribution of feed. reduction 2. Sizing of raw coal for cleaning in different processes To select the correct screen for an application in a coal 3. Removal of magnetic (dense medium) from clean preparation plant, a detailed knowledge of the size dis- coal and refuse tribution of the feed is necessary. Size distributions are 4. Dewatering usually presented graphically, and it is useful to use a 5. Separation of product coal into commercial sizes straight-line plot, because curve fitting and subsequent interpolation and extrapolation can be carried out with The size of an irregular particle is defined as the smallest greater confidence. In addition, if a function can be found aperture through which that particle will pass. that gives an acceptable straight-line graph, the function In a practical screening operation the feed material itself or the parameter derived from it can be used to de- forms a bed on the screen deck (Fig. 2) and is subjected scribe the size distribution. This facilitates data compari- to mechanical agitation so that the particles repeatedly son, transfer, or storage, and also offers major advantages approach the deck and are given the opportunity of passing in computer modeling or control. The particle size distribution used most commonly in coal preparation is the Rossin–Rammler–Bennett, m F(d) = 100(1 − exp[(−d/d63.2) ], (4) where F(d) is the cumulative percent passing on size d, d63.2 is the size modulus (d63.2 is that aperture through which 63.2% of the sample would pass), and m is the distribution modulus (the slope of the curve on the RRB graph paper). A sample of the RRB plot is shown in Fig. 4. In Fig. 4 one can read off the slope m; the scale for FIGURE 2 Four-mesh wire screen. S · d63.2 is also provided. If d63.2 is the particle size (in P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 84 Coal Preparation FIGURE 4 Rossin–Rammler–Bennett net with additional scales. Specific surface S is in square meters per cubic decimeter, d63.2 in millimeters. millimeters) belonging to F(d) = 63.2%, then the scales introduction of very large screens (5.5 m wide, 6.4 m 2 3 furnish the specific surface S in m /dm . long). Combined with large tonnage [1000 metric The types of screens used in coal preparation plants tons per hour (tph) per screen], screens have been generally fall into the following categories: developed (banana screens) with varying slope: first ◦ ◦ ◦ section 34 , second section 24 , and third section 12 . 1. Fixed screens and grizzlies (for coarse coal). 5. Resonance screens. These have been designed to save 2. Fixed sieves (for fine coal). These are used for energy consumption. The screen deck is mounted on dewatering and/or size separation of slurried coal or flexible hanger strips and attached to a balance frame, rejects. The sieve bend is the most commonly used in which is three or four times heavier than the screen coal preparation plants for this application (Fig. 5). itself. 3. Shaking screens. These are normally operated by 6. Electromechanical screens. This type of screen camshafts with eccentric bearings. They can be operates with a high-frequency motion of very small mounted horizontally or inclined, and operate at low throw. The motion is usually caused by a moving speeds with fairly long strokes (speeds up to 300 rpm magnet which strikes a stop. with strokes of 1–3 in). 4. Vibrating screens. These are the most commonly used A. Recent Developments screens in coal preparation and are to be found in virtually all aspects of operations. A summary of their A new process for the screening of raw coal of high application is shown in Table II. A recent moisture content at fine sizes has been developed by the development in vibrating screens has been the National Coal Board. Their Rotating Probability Screen P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 Coal Preparation 85 FIGURE 5 Sieve bend. (Fig. 6) has the advantage that the effective screening aper- coal consists of some proportion of both coal and shale ture can be changed while the screen is running. The screen particles, already sufficiently liberated, together with coal “deck” is made up of small diameter stainless steel rods ra- particles with inclusions of gangue (i.e., bands of shale). diating from a central hub. The hub is rotated, and the coal Commercially cleaned coal contains only very dissemi- falls onto the rotating spokes. The undersize coal passes nated impurities and has a density ranging from 1.2 to through the spokes; the oversize coal is deflected over 1.6. Carbonaceous shale density ranges from 2.0 to 2.6, them. The speed of rotation dictates the screening aper- and pure shale, clay, and sandstone have a density of ture. The ability to screen coal with high levels of moisture about 2.6. The density of pyrite is about 5.0. The dif- has taken precedence over the accuracy of size separation. ference in density between pure coal and these impurities, A screening operation by which the proportion of under- in a liberated state, is sufficient to enable an almost com- flow product can be controlled while the machine is in plete separation to be achieved fairly easily. However, it operation represents an important advance in the prepara- has been shown that the inorganic impurity content, and tion of blended coals in treatment plants where the fines hence, the ash content, ranges from pure coal containing are not cleaned. only microscopic impurities to shale, which is entirely free from carbonaceous matter. Generally speaking, the mineral matter content of any coal particle is proportional IV. FLOAT-AND-SINK ANALYSIS to its density and inversely proportional to its calorific value. Most coal cleaning processes that are used to remove For the prediction of concentration results (design of inorganic impurities from coal are based on the gravity flowsheet) and for the control of preparation plant opera- separation of the coal from its associated gangue. ROM tions, a measure of concentrating operations is needed. P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 86 Coal Preparation TABLE II Vibrating-Screen Applications in Coal Preparation Plants Number Installation a Type of decks angle Aperture Screen deck type Accessories ◦ ◦ Run-of-mine scalper Single 17 –25 6 in. Manganese skid bars, Feed box with liners, extra high AR perforated plate side plates, drive guard with skid bars enclosures ◦ ◦ Raw-coal sizing screen Double 17 –25 1 in. AR steel perforated Dust enclosures,drive plate, polyurethane, rubber guard enclosures 5 in. Polyurethane, wire, 304 Feed box with liners 16 stainless steel profile deck, rubber Pre-wet screen Double Horizontal 1 in. Wire, polyurethane, rubber Water spray bar, side plate drip angles, drive guard 1 mm Stainless steel profile deck, enclosures, feed box liners polyurethane Dense-medium drain and Double Horizontal 1 in. Wire, polyurethane, rubber Side plate drip angles, spray bars, rinse screen (coarse coal) shower box cross flow screen 1 mm 304 stainless steel profile or sieve bend, drip lip angles, deck, polyurethane drive guard enclosures Dewatering screen Single Horizontal 1 mm 304 stainless steel profile Sieve bend or cross flow screen, (coarse coal) deck, polyurethane dam, discharge drip lip angles, drive guard enclosures Desliming screen Single Horizontal 0.5 mm 304 stainless steel profile Sieve bend or cross flow screen, deck, polyurethane spray bars, shower box, drive guard enclosures ◦ Classifying screen Single 28 100 mesh Stainless steel woven wire Three-way slurry distributor and (fine coal) sandwich screens feel system Dense-medium drain and Single Horizontal 0.5 mm 304 stainless steel profile Sieve bend or cross flow screen, rinse screen (fine coal) deck, polyurethane spray bars, shower box, drip lip angles, drive guard enclosures Dewatering screen (fine coal) Single Horizontal 0.5 mm 304 stainless steel profile Sieve bend or cross flow screen, ◦ ◦ or 27 –29 deck or woven wire, dam, drip lip angles, drive rubber, polyurethane guard enclosures a Typical application. The best known means of investigating and predicting In the example shown in Fig. 7, five heavy liquids with theoretical beneficiation results are the so-called washabil- densities from 1.3 to 1.8 are used. The weight yields of six ity curves, which represent graphically the experimental density fractions are calculated (γ1, γ2, . . . , γ6), and their separation data obtained under ideal conditions in ash contents are determined (λ1, λ2, . . . , λ6). The results so-called float–sink tests. Float-and-sink analysis is also are set out graphically in a series of curves referred to as used to determine the Tromp curve, which measures the washability curves (Henry–Reinhard washability curves practical results of a density separation. The practical re- or mean-value curve, M-curve, introduced by Mayer). sults of separation can then be compared with the ideal The construction of the primary washability curve and a measure of efficiency calculated. (Henry–Reinhard plot) is shown in Fig. 8. It is noteworthy The principle of float–sink testing procedure is as that the area below the primary curve (shaded) represents follows: A weighed amount of a given size fraction is ash in the sample. The shaded area, changed into the rect- gradually introduced into the heavy liquid of the lowest angle of the same surface area, gives the mean-ash content density. The floating fraction is separated from the fraction in the sample (α = 16.44% in our example). that sinks. The procedure is repeated successively with The shape of the primary curve reveals the proportions liquids extending over the desired range of densities. The of raw coal within various limits of ash content and so fraction that sinks in the liquid of highest density is also shows the proportions of free impurities and middlings obtained. The weight and ash contents of each density present. Because the relative ease or difficulty of cleaning fraction are determined. a raw coal to give the theoretical yield and ash content P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 Coal Preparation 87 FIGURE 6 Rotating probability screen. depends on the proportion of middlings, the shape of the L the easier the cleaning process will be. (This is further primary curve also indicates whether the coal is easy or illustrated in Fig. 10.) difficult to clean. Figure 10 shows four different cases: (a) ideal separa- The construction of the mean-value curve (M-curve), tion, (b) easy cleanability, (c) difficult cleanability, and also referred to as Mayer’s curve, is shown in Fig. 9. The (d) separation impossible. point where the curve intersects the abscissa gives the The primary washability curve for the difficult-to-clean average ash content of the raw coal (α). coal (c) exhibits only a gradual change in slope revealing The shape of the primary washability curve and the a large proportion of middlings. M-curve is an indication of the ease or difficulty of clean- It can be seen from the example that the further the ing the coal. The more the shape approximates the letter M-curve is from the line connecting the zero yield point FIGURE 7 Float–sink analysis procedure. P1: GRB/LSX P2: ZCK Final Encyclopedia of Physical Science and Technology EN003H-867 June 13, 2001 20:13 88 Coal Preparation FIGURE 8 Construction of the primary washability curve (Henry–Reinhard washability diagram). FIGURE 9 Construction of the M-value curve.