In a coal based thermal power plant, the initial process in the power generation is Coal Handling. So in this article i will discuss the overall processes carried out at a Coal Handling plant in a coal based thermal power generating station.
The huge amount of coal is usually supplied through railways. A railway siding line is taken into the power station and the coal is delivered in the storage yard. The coal is unloaded from the point of delivery by means of wagon tippler. It is rack and pinion type. The coal is taken from the unloading site to dead storage by belt conveyors. The belt deliver the coal to 0m level to the pent house and further moves to transfer point 8.
The transfer points are used to transfer coal to the next belt. The belt elevates the coal to breaker house. It consists of a rotary machine, which rotates the coal and separates the light dust from it through the action of gravity and transfer this dust to reject bin house through belt.
The belt further elevates the coal to the transfer point 7 and it reaches the crusher through belt. In the crusher a high-speed 3-phase induction motor is used to crush the coal to a size of 50mm so as to be suitable for milling system. Coal rises from crusher house and reaches the dead storage by passing through transfer point 8.
A series of such switches are arranged in series at a 1m distance on the side of conveyor belt. The power supply to rotor of the conveyor belt is established only if all switches in series are connected.
The transmitter consists of a high frequency oscillator, which produces a oscillations of 1500 Hz at 15V. The receiver receives this frequency signal. If there is any presence of metal in the coal. Then this frequency is disturbed and a tripping signal is send to relay to stop the conveyor belt.
After hand picking foreign material, coal is transported to the Crush house by conveyor belts where it is crushed to small pieces of about 20 mm diameter. The crushed coal is then transported to the store yard. Coal is transported to bowl mills by coal feeders.
This table is rotated with the help of a motor. There are three large steel rollers, which are spaced 120 apart. When there is no coal, these rollers do not rotate but when the coal is fed to the table it packs up between rollers and the table and this forces the rollers to rotate.
This crushed coal is taken away to the furnace through coal pipes with the help of hot and cold air mixture from P.A Fan. P.A Fan takes atmospheric air, a part of which is sent to Air pre-heaters for heating while a part goes directly to the mill for temperature control.
The super heaters are located inside the furnace and the steam is superheated (540C) and finally it goes to turbine. Flue gasses from the furnace are extracted by induced draft fan, which maintains balance draft in the furnaces with forced draft fan. These flue gasses emit their heat energy to various super heaters in the pant house and finally pass through air pre-heaters and goes to electrostatic precipitator where the ash particles are extracted. Electrostatic precipitator consists of metal plates, which are electrically charged.
Regular mechanical hammers blows cause the accumulation of ash to fall to the bottom of the precipitator where the bottom of the precipitator where they are collected in a hopper for disposal. This ash is mixed with water to form slurry and is pumped to ash pond.
What is Dome Valve ? | Principle, Features and Valves Dome Valve is Used in the Pneumatic Phase Dense Conveying Application. An isolation valve used in many applications where conventional valves like ball valve, butterfly valve and slide gate.
Anyone know what the Average cost per square foot it is to build a coal prep plant? I have an 11 story 1000 Tons per hour Plant that has been idle for a few years and needs to updated Im trying to figure out what it would cost would be to upgrade it, or tear it down and rebuild it. Would appreciate anyones input for a guess of what the cost per square foot is for building prep plants.
Sir, If there is any job for Electrical Operation or Electrical Maintenance, Can you please refer me for any job?? I need a job, I am working in a Coal handling plant in electrical isolation, please do something for me, I will be thankful to you very much.
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The gradual depletion of high-grade mineral deposits and the necessity for development of lower-grade deposits, together with the increased demand for nonmetallic minerals, has increased the importance of the ore-dressing field during recent years. There is strong need for development of new ore-concentration methods and machines to keep pace with the constant changing of marketing specifications and requirements resulting owing to the many-new uses made of mineral products by the metallurgical industry.
Many ores are not readily amenable to concentration by gravity, flotation, or other conventional methods. For example, the dry separation of minerals having a small specific gravity difference is an extremely difficult problem. Therefore, the Bureau of Mines has conducted research to develop new treatment procedures and to develop new types of ore-dressing equipment. As a part of this program, studies were made and equipment was developed to separate minerals according to physical characteristics other than specific gravity.
Each of the separators has a high-frequency vibrating deck that can beneficiate comminuted ore to make a split of the ore minerals on a basis of their shape or size rather than difference in specific gravity. In addition to a vibrating deck separator B includes several electrodes, both grounded and high-voltage, which add electrostatic forces for further mineral beneficiation. Separator C combines an electromagnetic armature with a vibrating deck in order to separate magnetic from nonmagnetic ore particles.
The machine (figs. 1 and 2) consists of a base and a vibrating deck motivated by a solenoid-type vibrator which is controlled by a variable transformer. The deck may have a flat or curved longitudinal section and can be constructed of a variety of materials; fir plywood was found to be suitable for the laboratory model. The deck and base are connected by three equal-length flat springs set in parallel planes. The base is supported on ball and socket joints, so that the separator may be adjusted to any desired angle as governed by the nature of the material being treated. The vibrator mechanism is mounted on the base between two of the springs.
The laboratory model of the separator is designed and balanced to vibrate at a natural frequency of 66 cycles (120 impulses per second), the frequency of the electromagnetic system. For its construction to operate at any other frequency it would be necessary to change the design of deck load or spring dimensions.
The forces controlling the separation of minerals on the deck are variable and are balanced by adjustment of deck slope and the amplitude and intensity of the deck vibration. Once adjusted for. a particular size and type of feed, the machine requires little attention.
A vibrating feeder is used to discharge the ore to the deck. Each mineral particle. moves across the sloping deck in the direction of the resultant of the forces acting upon it. The spring throw of the tilted deck causes a differential movement that conveys the tabular mineral grains up the deck slope and, in addition, retards the downward movement of the rounded grains. Friction between the deck and the tabular particles must be sufficient to prevent downward slipping between impulses.
The shape separator products are collected from three sides of the deck. The flake travels to the top end, the thick tabular grains to the side, and the round particles roll to the bottom or lower end.
Grading according to shape is positive. Very little sweeping of flake occurs from the downward movement of the rounded grains. Therefore, a particle repassed over the deck will return to its respective bin. In general, best results are obtained when dry, closely sieve-sized ore is fed to the separator. The ore should be crushed in such a manner as to produce a low percentage of fines and to cause as little damage as possible to the shape characteristics of the liberated grains. Products that contain locked middling particles can be reground and retreated with finer fractions of the ore.
Vermiculite ore from Montana, which contained, in order of their abundance, vermiculite, pyroxene, hornblende, nepheline, and chlorite, was treated on the shape separator. A sample of ore was crushed to minus 10-mesh and closely screen-sized. Each fraction was treated separately to make a concentrate, middling, and tailing. The middlings were dry-ground, sized, and added to the respective finer sizes prior to shape-separation, treatment. The final middling was rejected with the tailing. The minus 100-mesh material was not treated. Microscopic grain counts were used in reporting the percentage of vermiculite in each product as chemical assays were not indicative of the mineral composition of the test products. Data are shown in table 1.
A flotation concentrate, made from a Montana graphite ore, was treated on the vibrating-deck shape separator in an effort to increase the grade of the product, maintain maximum flake size, and to produce a uniform thin-flake concentrate. The graphite in the ore occurs as books of graphite flakes with siliceous gangue included between the plates. Repeated grinding and cleaning by flotation produced high-grade concentrates, but the flake size was extremely small.
A sample of rougher flotation concentrate was screen-sized, and each fraction was treated separately on the shape separator. The tailing and middling products from each fraction were dry-ground in a pebble mill to separate the flakes from the thick tabular books with a minimum of flake breakage. The reground product was sized and added to the respective finer sizes before treatment. Results are shown in table 2.
By shape tabling, the grade of the graphite concentrate was increased from 50.4 to 90.9 percent C with a recovery of 94.8 percent of the graphitic carbon in the flotation concentrate. Over 91 percent of the ash and nearly 95 percent of the silica were rejected. Over half of the recovered graphite was plus 60-mesh, and the flakes were uniformly thin.
The vibrating-deck shape separator has been successfully employed in the laboratory for numerous separations of flat particles from rounded or angular material. From sized fractions of feed, muscovite mica has been separated from feldspar and quartz; flake graphite from quartz, feldspar, and ether gangue minerals; slate from coal; molybdenite from pyrite and chalcopyrite; vermiculite from; pyroxene, hornblende, and other gangue-minerals; and wood chips, grass. roots, and other carbonaceous matter from foundry sand.
In addition, a product containing free mineral grains of uniform shape can be sized by the separator with the elimination of two difficulties of sieve-sizing blinding of sieve-openings and mesh wear. Feldspar sand products and clay samples have been sized by this method.
The electrostatic mineral shape separator is a machine on which the liberated minerals of a comminuted ore are beneficiated by the surface action of a high frequency vibrating deck and by an electrostatic field maintained at the deck edges. It is designed to segregate those mineral particles that differ from others by shape, size, and electrical conductivity. Whenever there is little shape difference between the liberated mineral particles of an ore, the deck mainly sizes the sample as it progresses toward the electrostatic field.
The separator consists of a deck, base, and vibrator system similar to those of the vibrating deck-shape separator. Three edges of the deck are covered with grounded copper sheeting. High-voltage needle-type electrodes are placed opposite and several inches from the deck edges. In the laboratory model of the separator, the needle electrodes are charged with 60- cycle, pulsating, direct current obtained from a transformer and electronic tube rectifier set; the voltage employed is usually 50,000 volts.
Ore particles, fed to the deck by a vibrating feeder, are separated by size and shape and, as they leave the deck, momentarily contact one of the grounded copper electrodes. Some of the particles are charged with polarity opposite that on the high-voltage needle electrodes and consequently are attracted across the intervening space to the needles, where they discharge on contact and fall to the concentrate bins directly below. Any particles that retain a charge like that of the needle electrodes are repelled under the deck edge to other bins. The remaining particles, which are not effected by the electrostatic field, are caught in bins directly below the deck edges. As the distribution of particles by shape or size is closely controlled by the vibrating deck, the electrostatic products are high in grade.
Figure 3 shows the construction of the deck, and figure 4 shows one sot of high-voltage needle electrodes in place. Other sots of needle electrodes may be placed opposite the top and bottom edges of the deck when they arc needed to complete an ore separation.
A minus 16-mesh tailing product from a commercial vermiculite mill was treated to recover a plus 35-mesh high-grade vermiculite concentrate. One sample was screened on 35-mesh, and the oversize was treated on the laboratory electrostatic mineral shape separator. One pass over the machine produced a finished concentrate, a middling, and a finished tailing. The middling was retreated once to give a small amount of additional concentrate and tailing and to reduce the bulk of middling. A second sample was treated in a laboratory elutriator-type air classifier. In both tests, microscopic grain counts were used to determine the percentage of vermiculite present in the various products. Data are summarized in table 3.
By treatment on the electrostatic mineral shape separator 80.5 percent of the vermiculite in the feed was recovered at 95 percent vermiculite grade. Air separation treatment of the same product recovered 67.7 percent of the vermiculite at slightly lower grade; also, more closely sized fractions of. the ore were required to give concentrates of comparable grade.
The chief advantage of the separator over conventional electrostatic separators is that it, in itself, gives a preferential sizing of material prior to and during electrostatic separation, thus precluding close screen- sizing of feed when high grade concentrates are a requisite.
This is a dry magnetic ore separator on which liberated minerals of a pulverized ore are concentrated by a high-frequency vibrating deck and by a progressing magnetic field. It is designed to separate particles according to their magnetic susceptibility, size, and shape.
The machine has two principal parts the high-frequency vibrating deck, which controls the mechanical movement of the ore, and the direct-current magnetic rotating armature that develops the progressing field for ore separations.
As shown in figure 5 (a longitudinal section of the separator), the deck is a rigid construction of a series of flat surfaces arranged one above the others, each slightly narrower than the one below. The deck is made of non- magnetic and preferably nonconducting materials, as eddy current interference is high if electrical conductors are used. Fir plywood or pressed- wood sheets were found satisfactory for construction of the laboratory model.
On the laboratory machine, the deck is mounted to a heavy wooden base by means of two parallel sheet-steel springs. The lower end of the springs are anchored solidly to a heavy block, which in turn is bolted to the base. The deck is fastened to the springs with hinges having machine-fitted joints. Deck vibration is caused by the vibrator composed of an electromagnet and plunger fastened solidly to the base and the top end of the roar spring, respectively. The vibrator operates on a half-wave rectified current at 60 cycles per second.
The armature is made of a four-pole Armco iron core, four coils, two roller bearings, two slip-ring contacts, and a pulley drive. The coils contain approximately 550 turns of No. 16-gage, single, cotton-covered, enameled magnet wire held on the core with No. 11-gage nonmagnetic chromel wire. They are connected in series and have a total of 15.8 ohms resistance on a 110-volt, direct-current circuit. The armature has alternating north and south poles, so that when it is rotated a progressing magnetic field is created.
Unsized ore is fed to the top plane of the deck with a vibrating feeder. The mineral grains are all moved by the differential movement of the deck in the direction of the spring throw. The magnetic particles are affected by the armature rotating immediately below the deck. Clockwise rotation of the armature causes counterclockwise rotation of the magnetic ore grains and moves them in the direction opposite that of the deck vibration.
Separation is controlled by the tilt of the deck, the rotation speed of the armature, and the intensity of vibration of the deck. These factors are regulated to maintain sufficient friction between ore particles and the deck to move the grains in the proper direction and to break up chaining and clamping of highly magnetic particles reducing the entrainment of gangue and forming a fluid bed of easily separated ore grains.
Figure 6 is a photograph taken during beneficiation of a finely crushed, unsized magnetite ore. The ore layer on the deck of the separator is so dispersed by the progressing magnetic field and vibrating deck that the magnetite concentrate produced entrains very little gangue.
Figure 7 shows the clumping and chaining of magnetite in a stationery magnetic field. For this photograph the current was left on in the rotor coils but was switched off all the other separator controls.
Samples of magnetite ore were ground in a ball mill to minus 100-mesh, deslimed by decantation, and dried. One sample of sand was passed once over the progressing field magnetic separator; a second sample was given like treatment on a disk- type, high -intensity, dry magnetic separator. In both tests the slime fraction was rejected. The slime contained 15.9 percent of the original weight of the ore at a grade of 21.7 percent Fe and represented a loss of 7.9 percent of the total iron. Comparative data are shown in table 4.
As is shown in table 4, the progressing field magnetic separator made a cleaner concentrate than the disk-type separator with about the same recovery. In addition, the progressing field machine treated the material at the rate of 160 pounds per hour, as compared with 36 pounds per hour for the disk- type separator.
The progressing field magnetic separator has been found successful in the laboratory for the separation of magnetic from nonmagnetic particles in unsized ore. The capacity is fairly large, end, when properly controlled, the concentrates are virtually free of entrained gangue.
Three new dry-mineral concentrating machines have been developed at the Intermountain Experiment Station and tested in the laboratory. They were found to be satisfactory for specific types of mineral separation.
The vibrating-deck mineral-shape- separator is designed to separate flat or tabular particles from rounded or angular material. Flake material such as graphite, molybdenite, and mica has been separated successfully from quartz, feldspar, or other gangue materials.
The electrostatic shape separator combines sizing, shape separation, and separation according to difference in electrical conductivity of particles. The chief advantage of this machine over conventional-type electrostatic separators is that it sizes the material prior to separation and produces high-grade concentrates without screen sizing the feed.
The progressing field magnetic separator has been used successfully to separate magnetic from nonmagnetic particles in unsized ore. It is especially effective in the range of finer sizes and the capacity is large in comparison to standard laboratory magnetic separators.
SGS uses mineral separation in situations ranging from small-scale projects to full-scale pilot plants. Our capabilities include wet and dry magnetic separation in both low and high intensity magnetic separations with field strengths up to 25,000 gauss. Our equipment base includes both bench and pilot scale equipment:
Magnetic separation is a well-established separation technique and has become increasingly popular as new equipment on the market enhances the range of separations possible. It is an attractive process choice because of low capital and operating costs and the lack of chemicals to cause environmental concerns.
Application: non-ferrous metal and black metal, such as iron ore, zircon, cassiterite(tin), ilmenite, rutile, primary limonite, beach placer, manganese ore, titanium, tantalum-niobium, monazite and primary retiles.
The ARC type Electrostatic Separator is mainly used for separating a small number of metallic conductor minerals from nonmetallic materials, especially for separating and purifying placer ores. Application of Magnetic Separator 1. Selection of heavy minerals from coastal places, such as sandy gold. 2. beneficiation of nonferrous and rare metallic minerals such as Scheelite and Cassiterite, wolframite and Scheelite, primary ilmenite and gangue. 3. The concentration of Ferrous minerals, such as ultrapure iron concentrates, chromite, manganese. 4. Separation of non-metallic minerals such as graphite, quartz, feldspar, asbestos, apatite. 5. Separation of non-ferrous and non-metallic materials such as copper, tin, aluminum and plastic from waste. Types of magnetic separators:drum magnetic separator, roll separator, tube separator, ARC type magnetic separator. We supply the magnetic separator design service, manufacturer customized magnetic separation equipment since 1985, worth your trust.
Magnetic Separator Working Principle: 1. The machine is equipped with chassis and it should be placed vertically when installed. 2. It should be grounded well. The ground line should be buried with an angle iron or water pipe into a 2-3 meters wet underground place. 3. High-voltage cables should be in the plastic tube to prevent leakage. 4. Ore storage heating wire should be 4.5 square millimeter cable. There are high and low-temperature switches in the control cabinet. Operating Method 1) check that there is no foreign matter in the electric separator first, avoid the connection of the negative electrode tube and the positive electrode plate; 2) voltage regulator KNOB must be back to zero before slowly boost voltage, boost voltage to see if the microammeter current is abnormal, such as current is too large, please check the cause of leakage; 3) when the required voltage is set, start feeding work; 4) every work hours, should open the door to clean steel pipe and steel dust, dust must use a long plastic brush, remember not to close the steel pipe, in order to avoid electric shock; 5) at the end of the work or inspection of the equipment must be Knob to zero, and then with a discharge rod steel pipe and steel plate directly connected to discharge. (discharge rod is made of Long Plastic Rod, front end ties up about 20cm bare copper wire) the discharge will see Bright High Voltage ARC; 6) when the continuous discharge occurs in the working process, the voltage should be lowered in time to eliminate the fault.
In this chapter the theories underlying magnetic, electrostatic and conductive properties of minerals and their use to separate minerals from their gangue constituents are explained on the basis of their atomic structures. The design of commercial equipments based on this concept and their operation are described for both dry and wet conditions.
Electrostatic separation is a beneficiation technique that exploits the differences in conductivity between different minerals to achieve separation (Higashiyama and Asano, 2007; Kelly and Spottiswood, 1989a,b,c).
Electrostatic separation works on the natural conductivity properties between minerals in feed. Separation is between economic ore constituents, noneconomic contaminants, and gangue. The common units are high-tension plate and screen electrostatic separator. The electrostatic plate separators work by passing a stream of particles over a charged anode. The electrostatic minerals lose electrons to the plate and are pulled away from other particles due to induced attraction to the anode. The dry stream of moving particles is preferred between 75 and 250m, with close size distribution and uniformity of shape for efficient separation. It is used for separating monazite, spinel, sillimanite, tourmaline, garnet, zircon, rutile, and ilmenite from heavy beach/stream placer sand. The electrostatic technique with local modification is extensively used in Australia, Indonesia, Malaysia, and India bordering Indian Ocean for separation of mineral sands.
There are three distinct stages in electrostatic separation processes: particle charging, separation at the grounded surface, and separation caused by the trajectory of the particles. Particle charging can occur by three possible mechanisms: contacting of dissimilar materials, ion bombardment, and induction.
Contacting, followed by separation, of dissimilar materials results in one being positively charged and the other negatively charged. Bulk movement with repeated contacts is necessary for sufficient charging.
Charging by ion bombardment occurs as the air between the particle and electrode conducts by a corona discharge. If the particle is a nonconductor, it does not lose charge while in contact with the grounded rotor and is held to the surface by its own image force Fi. This force represents the attraction between the charged nonconducting particle and the grounded surface, the latter being equivalent to a similar charge of opposite sign in a mirror image position behind the surface. The force is given by
Induction charging of a particle occurs on a grounded surface in the presence of an electric field. Both the conducting and nonconducting particles initially develop opposite charges on opposing faces, but because the conducting particle loses one of these charges to the grounded rotor, it develops an equipotential surface and experiences an electrical force Fe away from the rotor given by
The separation that occurs at a grounded surface then results from the differing forces on the particles. With high tension separators, the important forces are the image force Fi and the centrifugal force. Equating these forces yields the pinning factor
Because all electrostatic separators give incomplete separations, multiple stages are used. The probability of collection of any component at each stage tends to remain constant, so that the process can be modeled by a probability relationship such as Eq. (24). The probability p will have some relationship to the parameters described earlier.
As a first step in the refining process, water, inorganic salts, suspended solids, and water-soluble trace metal contaminants are removed by desalting using chemical or electrostatic separation. This process is usually considered a part of the crude distillation unit. The desalted crude is continuously drawn from the top of the settling tanks and sent to the crude fractionation unit. Distillation of crude oil into straight-run cuts occurs in atmospheric and vacuum towers. The main fractions obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils, and residue. The composition of the products is directly related to the characteristics of the crude processed. Desalted crude is processed in a vertical distillation column at pressures slightly above atmospheric and at temperatures ranging from 345 to 370C (heating above these temperatures may cause undesirable thermal cracking). In order to further distill the residue from atmospheric distillation at higher temperatures, reduced pressure is required to prevent thermal cracking. Vacuum distillation resembles atmospheric distillation except that larger diameter columns are used to maintain comparable vapor velocities at the reduced pressures.
Sieving is carried out in the physical recycling process to classify the different sized particles based on the various sizes of sieve apertures to the desired particulate size for separation. Sieving is not only been utilized to prepare a uniformly sized feed but also to upgrade metal contents (Kaya, 2016). The screening is essential as the particle size and shape of metals are different from that of plastics and ceramics. Rotating screen is used mainly for metal recovery in WEEE recycling process.
Based on the variation in shape, density, and electric conductivity of metallic and nonmetallic materials in WEEE electrostatic separation are considered as a promising way to recover metals from pulverized WPCBs. Recycling industry basically used shape separation by tilted plate and sieves. Copper recovery is promising by an inclined conveyor with a vibrating plate from electric cable waste, printed circuit board scrap, and waste television and personal computers in Japan (Cui and Forssberg, 2003).
Magnetic, electrostatic, and density separation are mechanical separation techniques that have been widely used in urban mining of WEEE. Low-intensity drum separators are the standard method of magnetic separation for the recovery of ferromagnetic metals from nonferrous metals and other nonmagnetic wastes (Hsu et al., 2019). Magnetic separation is in general performed first, followed by shredding or grinding to fine particle size, and after that electrostatic separation is applied. High-intensity separators are used for possible separation of copper alloys from the waste matrix (Veit et al., 2005). Through an intense magnetic field, copper alloys with relatively high mass susceptibility (Al multicompound bronze), copper alloys with medium mass susceptibility (Mn multicompound bronze, special brass) and copper alloys with low mass susceptibility and/or diamagnetic material behavior (Sn and Sn multicompound bronze, Pb and Pb multicompound bronze, and brass with low Fe content) can be separated (Cui and Forssberg, 2003).
Electrostatic separation is considered as advantageous compared to the other physical techniques as it is smooth operation, less hazardous, and requires less energy (Lu and Xu, 2016). Electrostatic separation is based upon electrical conductivity and separates the nonconductive materials from the conductive ones. Although the electrostatic separators were initially recovered nonferrous metals from automobile scrap or municipal solid waste, now widely used for WEEE utilized explicitly for the recovery of copper or aluminum from chopped electric wires and cables and recovery of copper and precious metals from printed circuit board scrap (Lu and Xu, 2016). It has been observed that the multistage process is needed to separate conductors from nonconductors (Hsu et al., 2019). Both corona discharging and eddy current-based electrostatic separation have received significant attention in the separation of ferrous and nonferrous metals and the separation of plastics from the plastic and metal mixture. Particle size has become a limiting factor, along with the sticking effect of larger particles in terms of corona separation, whereas eddy current-based electrostatic separation depends on the flow of the particles (Cui and Forssberg, 2003).
Gravity separation is considered as the best physical separation option for nonmetals from the metals by different specific gravities. Density separation is dependent on the density and the size of the components. Viscous liquids such as tetrabromoethane can serve as the separation medium where the metals can be separated from the plastics or ceramics. Conventional gravity separators that are used in E-waste recycling are water or airflow tables, dense media separation, and sifting. Density separation techniques that have extensively been used in the mineral processing industry are now applied into E-waste recycling as WEEE consists of many plastics, with a density less than 2.0g/cm3; light metal, primarily Al and glass, with a density of 2.7g/cm3; and heavy metals, predominantly Cu and ferromagnetic, with a density more than 7g/cm3 (Kaya, 2016). The enriched fractions are treated by chemical techniques: pyrometallurgical and hydrometallurgical processes after mechanical/physical treatments in order to extract precious metals.
The idea of UCC is that the ash-free coal (AFC) can be used more efficiently to provide electricity either via direct firing into the gas turbine followed by a steam cycle or via catalytic gasification followed by fuel cell. Chemical cleaning of coal to obtain more orless AFC can be broadly classified into two: physical and chemical. Fig.2.9 shows the physical and chemical coal cleaning pathways and the characteristics of the cleaned coal . In physical cleaning, there is no involvement of chemicals and thecoal structure does not change. Physical cleaning involves the removal of minerals by methods such as gravity separation, froth flotation, electrostatic separation, magnetic separation, oil agglomeration, and air dense medium fluidization . In the air dense mediumfluidized bed process, the lighter coal particles containing fewer minerals floats on the surface of the bed and the particles with the higher mineral content sink.
Even though physical coal cleaning is easy and low cost, it does have the problem that only impurities in the larger sized particles can be cleaned (e.g., gravity separation requires particle size >0.5mm). Physical cleaning cannot remove finely distributed minerals and chemically bounded species within a coal matrix.
Chemical cleaning methods are better than physical methods because chemical processes can remove finely distributed minerals and organically bounded chemical species. The chemical cleaning methods can be divided into two categories: use of chemicals that dissolve minerals, usually called as UCC, and solvents for dissolving the coal-like matter, usually called AFC. In UCC, there is no change in the coal structure of the product, whereas in AFC, the coal structure is changed. In UCC, the ash content ranges between 0.1% and 5%, whereas in AFC, ash content is less than 0.1%. The details of the production process, yield advantages, and disadvantages of these processes have been presented in our recent review . Extraction of organic matters using NMP (N-methyl-2-pyrrolidone) to produce AFC also reduces sulfur; in particular, inorganic sulfur is completely removed .
Sorting processes are most often required in order to provide type sorted fractions for recompounding processes. Dry sorting processes apply mechanical (sieves) or electrostatic forces or they sort by means of spectroscopic detection systems. Sieves or air classifiers are applied to reduce the amount of fine particles in waste plastic fractions, since the fines contain small metal particles from grinding processes and dust collected on plastic surfaces in the use or reclamation phase. These foreign materials may worsen the mechanical properties of recycled plastics. However, sieving or air classification have no impact on the FR system.
Electrostatic separators are applied to separate mixtures of two or three different polymers by means of electrostatic forces . If one of the three plastics in the input stream contains (B)FR, this process may separate FR equipped from other plastics. However, such defined input streams are most likely found in postindustrial waste and are generally not present in real postconsumer e-waste. Therefore, electrostatic separation of postconsumer plastics in e-waste is only applied to fractions obtained by preceding processes (e.g. sink and float fractions).
Manual spectroscopic detection devices have been applied in recycling operations of dismantling plants, whereas these techniques are applied to comparably large plastic parts (unground casings, etc.). NIR and midinfrared MIR can be applied to sort plastic parts according to their polymer types unless their color is too dark to produce analyzable spectra. ABS and ABS equipped with TBBPA can be differentiated by NIR, whereas ABS and ABS equipped with other BFR cannot be distinguished by this technique. Therefore, NIR is not a powerful tool to sort FR and FR-free polymers.
In contrast, bromine or phosphorous can be detected by sliding spark spectroscopy (SSS), which can be performed by low cost manual devices. Thus, the combination of SSS and NIR provides information suitable for an effective sorting of FR-free polymers. As both instrument scan plastics within seconds, their application in dismantling plants appears to be a reasonable and low cost recycling approach for light plastics.
However, from an economic point of view, automated sensor-based sorting isconsidered to be more effective than manual approaches. These techniques are applied to coarsely ground plastic waste and subject particles with 550mm to the sensor system e.g. on a band conveyer. An NIR camera or an alternative sensor system analyzes the particles coming across and controls a blowing device that may then blow a particle out of a waste stream, if the sensor results indicate thepresence (or absence) of a given polymer. These sorting techniques are rather effective; however, they work best with highly concentrated target polymers (>80%). As long as a complex mixture is to be segregated, the sorting will have to be done in several consecutive sorting steps in order to produce reasonably pure polymer fractions. This is necessary since the mechanical sorting by blowing off particles always leads to a small amount of falsely sorted particles. This is exemplified in Table 5, which provides experimental data on the automated NIR separation of ground gray monitor casings. Whereas the separation of ABS and PS has not been completely optimized in this example, PC/ABS can well be segregated from ABS and PS. However, there is a small percentage of 6% ABS and PS particles, which have been sorted falsely into the PC/ABS fraction.
An innovative approach applies laser spectroscopy to sort plastics automatically by a sensor technique that produces analyzable spectra even for black polymers . Thus, NIR and laser spectroscopy provide two powerful sorting tools with respect to polymer type. XRT is also used as detector in automated sorting systems and can differentiate polymers of different specific atomic densities. As the optical density is significantly increased by BFR, e.g. it has been shown that XRT may separate BFR-free polymers from BFR polymers. As for the manual approach, a combination of two automated sensor-based technologies appears to be capable of sorting pure and BFR-free polymer fractions from WEEE.
Electrostatic separation is a beneficiation technique that exploits the differences in conductivity between different minerals to achieve separation (Higashiyama and Asano, 2007; Kelly and Spottiswood, 1989a,b,c). Electrostatic separation techniques are typically only used when alternative processing techniques will not suffice, as the comminution steps in mineral processing flowsheets are generally wet processes and the energy requirements to drive off all moisture prior to electrostatic separation can be significant (Kelly and Spottiswood, 1989a). In the context of rare earth mineral processing, the typical use of electrostatic separation is in the separation of monazite and xenotime from gangue minerals with similar specific gravity and magnetic properties (Ferron et al., 1991; Zhang and Edwards, 2012). A specific example of this is when xenotime, which is more strongly paramagnetic than monazite, is concentrated with ilmenite after magnetic separation of heavy mineral sands (Gupta and Krishnamurthy, 1992). In this case the only means by which xenotime may be removed from the ilmenite is via electrostatic separation, as ilmenite is conductive but xenotime is not (Gupta and Krishnamurthy, 1992).
Electrostatic separation is a valuable technique for heavy mineral sand beneficiation, and the successful application of this process to separate ultrafine (<37m) coal particles may present an opportunity to treat the fines produced in many currently operating mineral processing circuits that account for significant rare earth losses (Higashiyama and Asano, 2007). Unfortunately, all electrostatic separation techniques (drum-type, belt-type, plate-type etc.) suffer from the requirement that the feed material must be completely dry (Higashiyama and Asano, 2007). Aside from the heavy mineral sand deposits, almost all other discovered rare earth deposits (aside from the ion-adsorbed clays in southern China) require some form of comminution prior to separation and these grinding operations are heavily reliant on slurried feed material. The energy costs associated with completely drying a ground ore prior to an electrostatic separation step are likely to be far too cost-prohibitive for such a process to be applied on an industrial scale.
Electrostatic separation can be applied in ilmenite upgrading because of the conductivity difference between ilmenite and its co-existing minerals such as titanaugite and zircon. High-tension (electrical) and electrostatic plate separators are commonly employed for this purpose (Australia, 2019). The successful industrial application is reported in Australia for the selective separation of ilmenite, rutile, zircon and monazite by means of electrostatic separation, together with magnetite separation in view of their different magnetic and electrical properties at various elevated temperatures (Australia, 2019; Farjana et al., 2018; Jones, 2009). As a combined processing technology, rare earth drum magnets are first used to remove ilmenite from mined heavy mineral concentrate feed, from which most of the ilmenite can be recovered. Then non-magnetic minerals are processed by electrostatic separation to obtain conductor minerals (rutile and leucoxene) and non-conductors (consisting of zircon, kyanite, quartz, monazite and staurolite). Subsequently, the non-conductors are treated by gravity separation to remove the lower specific gravity material (quartz, kyanite, garnet, and staurolite) from the higher specific gravity zircon. After gravity separation, electrostatic separation is used again to remove residual conductors from the zircon and induced roll magnets are used to separate traces of monazite and staurolite from zircon.
As far as the authors knowledge, no report on the current application of electrostatic separation in ilmenite industry can be found in China. Electrostatic separation has been used to prepare high purity ilmenite and titanaugite in labs, and also used for theoretical studies on separating ilmenite from its associated gangue minerals such as quartz and zircon. Ziemski and Holtham (2005) used electrostatic separation to separate titanium minerals (ilmenite and zircon) using a new theory of particle discharge in high tension roll (HTR) separation, and the influence of particle bed and size was considered. Triboelectric separation of ilmenite from quartz is also successful in lab experiments, provided optimum airflow rate, feed rate and voltage (Yang et al., 2018). Overall, there are some limitations impeding the popularization and industrial use of electrostatic separation in ilmenite dressing, such as high requirement for feedstock quality in terms of moisture content (completely dry) and uniform granularity, safe operation for workers (dust problem) and limited handling capacity (<5 t/h).