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 beneficiation (CB) is an effective pre-combustion PM emission control method.Density based CB can effectively reduce ash and trace elements (TEs) in raw coalRemoval efficiency of TEs in the raw coal varies depending on coal samples.By-products of CB (middling and slime) should not be used as an energy sourceFuture research directions of CB as pre-combustion PM control method is discussed.
Fly ash emitted from the coal-fired power plant is the major contributor of the outdoor airborne particulate matters (PMs). Coal beneficiation, an industrial process to improve the quality of raw coal by removing ash-bearing components, can be a cost-effective sustainable and clean technology to reduce the emission of hazardous trace metals. As the removal efficiency of mineral matters and heavy metals within the coal is depend highly on the raw coal and the employed beneficiation process, a wide range of case studies at laboratory- and industrial-scale, published in the last 20 years, are reviewed in this study. Our review indicates that the coal beneficiation processes can effectively reduce content of heavy metals of fly ash, encouraging the use of clean coal to reduce pollutants emission.
Dense-medium cyclones have been used for beneficiation of fine particles of coal. In this study, the usability of cyclones in the beneficiation of tailings of a coal preparation plant was investigated. For this purpose, separation tests were conducted using spiral concentrator and heavy medium cyclones with the specific weight of medium 1.31.8 (g/cm3) on different grading fractions of tailing in an industrial scale (the weight of tail sample was five tons). Spiral concentrator was utilized to beneficiate particles smaller than 1mm. In order to evaluate the efficiency of cyclones, sink and float experiments using a specific weight of 1.3, 1.5, 1.7 and 1.9g/cm3, were conducted on a pilot scale. Based on the obtained results, the recovery of floated materials in cyclones with the specific weight of 1.40, 1.47 and 1.55g/cm3 are 17.75%, 33.80%, and 50%, respectively. Also, the cut point (50), which is the relative density at which particles report equally to the both products are 1.40, 1.67 and 1.86g/cm3. The probable errors of separation for defined specific weights for cyclones are 0.080, 0.085 and 0.030, respectively. Also, the coefficients of variation was calculated to be 0.20, 0.12 and 0.03. Finally, it could be said that the performance of a cyclone with a heavy medium of 1.40g/cm3 specific weight is desirable compared with other specific weights.
Coal is specified as one of the most important energy resources in the world. Approximately 28% of the energy of the world is provided by coal (BP Statistical Review of World Energy; 2017). Considerable amounts of coal particles are accumulated in the tailing dams of washing plants which can lead to serious environmental problems. Recovery of these particles from tailings has several economic and environmental advantages. Maintaining natural resources and reducing the amount of material discharged to the dams are the most important ones (Ashghari et al. 2018). Various investigations have been explored environmental impacts of coal tailing piles on air, soil and groundwater (Meck et al. 2006; Battioui 2013; Kotsiopoulos and Harrison 2017). It is reported in some cases acid mine drainage (AMD) of coal tailing dams contained an amount of sulfates, nitrates, chlorides and heavy metals higher than the average value defined by the World Organization of Health (WHO) (Battioui 2013). These AMDs can cause harmful effects on groundwater quality, river flows and ecology their deposits proximity (Sengupta 1993; Simate and Ndlovu 2014; Kefeni et al. 2017; Peiravi et al. 2017). Moreover, cone shaped damps of coal tailing can potentially be a source of self-ignition and possible explosion (Siboni et al. 2004; Adiansyah et al. 2017). Offering a solution to recover them, can reduce the volume of tailing and increase the number of productions and efficiency. In fact, producing coal from tailing is cost-effective, economical and environment friendly. Reprocessing tailings of coal preparation plants is a new approach to coal washing industry.
Gravity separation and flotation are the most common techniques in coal processing and recovery of coal from tailings in large scale (Wills 2011; John et al. 2002). Heavy media separation method is one of the gravity separation methods, which was patented in 1858 by Henry Bessemer (Napier et al. 2006). This method is so advantageous due to the high capacity and efficiency of separation. One of heavy media separators is hydrocyclone which was developed in the 1950s and in the chemical industries due to the simple design, suitable performance and high capacity (Delgadillo and Rajamani 2005). The heavy media cyclone is used extensively in coal processing and in the primary treatment of metal ores such as Pb and Zn. The modern cyclone for coal preparation is the most effective option for size fraction of 0.550mm (Chu et al. 2012). In this separator, the centrifugal force causes the heavy particles such as dust or ashes to move to the wall of the cyclone where the particles get down because of the axial velocity and discharge through the underflow of the cyclone (Chu et al. 2009). The heavy media cyclones are installed inclined or upside down (Rayner and Napier-Munn 2002).
In order to determine the specific weight of the heavy media liquid of the cyclones, the results of heavy liquid tests are used in different specific weights. Also, evaluation of the separation method or the operation of a gravity separator is usually based on the decomposition of the sinkfloat, and the washing ability curves(Gupta and Yan 2006). In an ideal separation process, all the particles with a specific weight less than separation density are recovered to concentrate or the light product (coal), and all the particles with a specific weight more than separation density are introduced to tailings or the heavy product(Majumder and Barnwall 2011). It can be stated that there is no ideal separation in any of the separators, and some of the materials are mistakenly divided.
Parameters that influence the mistake of particle splitting are the geometry of separators, machine mechanisms and settings, the composition and feed rate, and product crop, media rheological properties, and relative separation density. In addition, the time required to separate a particle and the settling rate of the particles are effective in the recovery. The performance of a gravity separator in coal treating is commonly determined by plotting a Tromp (distribution) curve which is basically a plot of partition coefficients in term of average specific gravity (Mohanta and Mishra 2009). The separation efficiency can be obtained from the slope of the distribution curve (or the curvilinear curvature). This curve depends on the size of the particles and the type of separator and it is also independent of the operation of sinking and floating (Burt 1984). In Fig.1, the distribution curve is shown in two ideal and realistic modes. According to the shape, when the curve slope is increasing in the 50% distribution coefficient, the curve is changed from the realistic mode to the ideal one (vertical slope). This shows that increase in the efficiency of a separator. The greater the slope of the distribution curve is plotted, the better performance of the device is observed(Farzanegan et al. 2013).
In this study, reprocessing of tailings from the Anjir Tange processing plant using a heavy media cyclone was investigated. Gravity separation tests by the heavy media cyclone and sinkfloat were performed in both industrial and laboratory scale for different size fractions. Finally, the efficiency of heavy media cyclone devices has been evaluated using by Tromp curves. The Tromp curve is an indicative of actual performance of the separation unit since it is independent of feed quality. In case of coal washing, the degree of misplacement is directly depended to the amount particles with a specific weight close to gravity of separation (near-gravity material). However, the coal containing high near-gravity material can be effectively processed by choosing the right process and correct operating parameters.
Coal tailings samples were collected from the coal preparation plant of Anjir Tange which actively produces washed coke in central AlborzIran. Three methods of hand sorting, gravity, and flotation are applied in thia plant to supply coke. The tailings aforementioned processes are accumulated in two depots of flotation tailings and the other processing methods. The mass of the piled tailings is 2 million tons with the average ashes between 40% and 45%. According to studies, 70% of tailings are produced from jig process with the 44% valuable coal and 56% ashes.
In order to investigate the possibility of reprocessing of tailings from Anjir Tange coal preparation plant by using heavy media cyclones, approximately five tons of tailings sample from tailing dumps were tested with heavy liquids. The spiral tests were also performed in an industrial scale. After the classification of the sample in various grain fractions, heavy media tests were conducted in specific weights of 1.4, 1.5, 1.6, 1.7 and 1.8g/cm3.
Based on the results of these experiments, the specific weight of the media were selected, and the heavy medium cyclone tests were carried out on an industrial scale with specific weights of media 1.40, 1.47 and 1.55g/cm3. Then, to investigate the efficiency of heavy media cyclones, on both products of tailings, sinkfloat experiments were carried out in specific weights of 1.3, 1.5, 1.7, 1.9g/cm3. Finally, the efficiency of cyclones has been investigated by plotting the corresponding distribution curves.
The particle size distribution (PSD) and the amount of ash in each size fractions of the sample are presented in the Figs.2 and 3. Giving the results of the sieve analysis and the amount of the ash of the coarse-grained tailing, about 50% of the total samples were in the range of 112mm. The maximum amount of ash is belongs to the fraction of 1225mm with the ash content of 69.70%, which is 21.90% of the total weight of sample. In Fig.3b, the results of the sieve analysis and ash content of the flotation tailings are shown. According to this analysis, about 80% of particles are larger than 40 micrometers with a cumulative ash value of 23.80%.
In order to investigate the reprocessing capability of the tailings by gravity separation method, heavy liquid tests were carried out in the specific gravity of 1.4, 1.5, 1.6, 1.7 and 1.8g/cm3. The results of the heavy media tests in each size fraction are presented and discussed in the following bullet points:
Particle size greater than 25mm Fig.4a shows the heavy media test result for this size fraction. Based on the results, using a solution with a specific weight of 1.5g/cm3, a concentrate with 10% recovery and 16.40% ash content and a final tailing with 70.20% recovery and 73% ash content was obtained.
Size fractions of 1225mm according to the results of the analysis, the ash content of the feed is 69.70% and 21.90%. Based on the results of sink and float experiments (Fig.4b), in a solution with a specific weight of 1.5g/cm3, a concentrate with 16% ash and 8.80% recovery was achieved. In specific weight of 1.6g/cm3, a product with 14.40% weight percentage of feed and 21.40% ash with 75.60% recovery to the tail and 60.60% ash was obtained.
Particle size fraction of 112mm This fraction makes up 48.90% of the feed weight with 60.60% ash. Based on Fig.4c, in the specific weight of media 1.6g/cm3, the weight percent of the concentrate is 24.30% and the ash content is 15.10%. In a solution with a specific weight of 1.7g/cm3, a product with 28.50% recovery, and 19.20% ash was obtained; in this case, the final tailings was 69.90% of the feed weight with 78.20% ash.
Particle size greater than 1mm the particles which are larger than 1mm contain 87% of the total feed weight. The results of the heavy liquid test on this size fraction are shown in Fig.4d. According to the figure, in a specific weight of 1.5g/cm3, the weight percent of the concentrate is 15.20% with 12.20% ash. Concentrates with 21.40% and 29.50% recovery and 18.50% and 22.60% ash content, respectively were achieved in special weights of 1.6 and 1.7g/cm3. The final tail in the specific weight of 1.7g/cm3 is 71.70% recovery to the tail and 78% ash.
The fraction with the particle size less than 1mm This fraction consists 12.80% of the feed weight with 54.80% ash. 12.40% of this fraction is between 1.00 and 0.15mm, and 0.40% is less than 150m. In this part, the primary operation was successful and the particles with a size less than 150m were removed by using the industrial spiral. The results of this experiment are presented in Fig.5. According to the results, a product with 32% recovery and 15.30% ash was obtained, which is considered as a desirable result.
After preparing thin blade from sample size fraction larger than 12mm, microscopic studies was conducted and degrees of liberation was obtained. It was observed that coal has a good purity in this fraction, but on the other hand, it has a high degree of contamination with the tail. Despite the unusually crumbling of the coal, to increase the recovery and reduce the amount of ash, this section was dimensioned from the grinding sample, and then a heavy liquid test was performed on fraction of particles which is larger than 1mm. The results of crushing and heavy liquid tests are presented in Figs.6 and 7. According to the results, grinding has not had a desirable effect on reducing the amount of ash in this fraction.
According to the results, by carrying out heavy media test on the tailings of Anjir Tange coal preparation plant, the products of different qualities are produced. The particles which are in the size fraction larger than 1mm generated three products with 7% ash and 19% recovery, 12% ash and 30% recovery, and 45% ash with the recovery of 22.50%. In the case of combining the spiral product and the fraction larger than 1mm, concentrate with 18% ash and 48.50% recovery is obtained which constitutes about 22.50% of the feed weight.
To conduct a large-scale heavy media test on an industrial scale, the feedstock was analyzed by using a 20mm sieve mesh size. Based on the results, 50% of the feed was smaller than 20mm and about 20% of it was removed from the circuit during the irrigation stage. Eventually, about 18 tons of feed was introduced into the heavy duty cyclone by the DMS (Dense Medium Separation) process (GSZs feed method). Based on the results (Table1), after the separation of materials in the cyclone, a concentrate with a weight of approximately 6 tons was obtained which consists 10% of the total feed and 20% of the feed to the cyclone.
The results of sink and float experiments are used to select the specific weight of fluid in the hydro-cyclone apparatus. In this research, based on the results of heavy media tests performed at industrial scale, the specific weight of the hydro-cyclone medium was selected and the gravity separation tests of coal tailings has been done in specific weights of 1.40, 1.47 and 1.55g/cm3 at industrial scale.
At the entrance of the heavy media cyclone devices, there is a sieve that assumes particles that are smaller than the crater as tailings. Based on the analysis of the gradient carried out on the concentrate and tailings of each of the heavy media cyclones (Fig.8), it was observed that more than 70% of the particles are larger than 2mm, so it can be said that the size of the span which is embedded at the entrance of the heavy media cyclones is 2mm. Therefore, heavy media cyclone experiments were performed on particles 215mm in size. The results of these experiments are given in Table2 for specific weights of 1.40, 1.47 and 1.55g/cm3.
As noted above, the evaluation of the separation method or the performance of a gravity separation device is usually based on the decomposition of the sinking and floating and the washing ability curves. For this purpose, for assessing the efficiency of heavy media cyclones in different specific weights, sink and float experiments were carried out on concentrates and tailings of hydro-cyclone on a laboratory scale. Heavy liquid solutions required are obtained from dissolving zinc chloride in water. Based on the similar experience obtained from the processing factories, Eps probability error rate for this plant was considered 0.05 and the weight of especially heavy liquids were calculated using Eq.(1):
Due to limitations in the preparation of heavy media solutions as well as their cost, the value of n was considered to be 2 and liquids with specific weight of 1.3, 1.5, 1.7, 1.9g/cm3. The results of sink and float experiments for the feed of heavy media cyclones in two particle fractions larger than 2mm and 12mm are shown in Fig.9. Also in Fig.10, the results of analyzing the amount of ash in the feed with the size greater than 1mm at each specific gravity are presented.
In Fig.11, the results of the heavy liquid tests are shown for concentrate and tailing of heavy media cyclone devices with the size of larger than 2mm. The amount of floated concentrate of the cyclone with a specific weight of 1.40g/cm3 in a solution of 1.4g/cm3 is about 80% of the feed weight. The proportion of floated coal in the cyclone with the specific weight of 1.47g/cm3 in a solution of 1.5g/cm3 is 68%, this amount for a cyclone with a specific weight of 1.55g/cm3 is 50%.
In order to carry out more precise studies, sink and floating experiments were also performed on size fraction of 12mm concentrates and tailings of cyclones. Figures12 and 13 show the results of these experiments. The heavy liquid test does not have a good result for particles of less than 2mm in size. Therefore, the experiment was not conducted on this fraction.
According to the results of the sink and floating experiments and analyzing the amount of ash, the amount of floated concentrate of the cyclone with a specific weight of 1.40g/cm3 in a solution of 1.4g/cm3 is over 95% of the feed weight and the ash contents less than 12%. The specific gravity of the cyclone floated coal is 1.47g/cm3 in a solution of 1.5g/cm3 is about 78% with 15%, and this value for the cyclone with a specific weight of 1.55g/cm3 is 68% with 20% of ash.
The performance of gravity separation equipments is evaluated by distribution curves. Determining the distribution curves for controlling gravity separation processes is important and can provide a proper correction for controlling a given process, and it is possible to simulate and predict the results (Ferrara and Bevilacqua 1995). The distribution curve is obtained by calculating the distribution coefficient by the average density in each fraction. In order to plot the distribution curve, weighting the concentration of concentrate and sinkfloat tests for concentrate and tailings should be performed. The weight recovery of the concentrate can be obtained through direct weighting or using mass balance equations. With regard to weight recovery, as well as sink and floating data, the feed can be restored. The data from sink and floating experiments for cyclones with specific weights of media 1.40, 1.47 and 1.55g/cm3 are presented in Tables3, 4 and 5 for plotting the distribution curve.
According to the data obtained from the sink and floating experiments on the concentrate and tailings of the heavy medium cyclone of Anjir Tange and Eqs.(2) and (3), at a specific weight of 1.40g/cm3, the efficiency of floated materials is 17.75% and efficiency of submerged material is 82.25%. Also, using the specific weight of 1.47g/cm3, these values are 33.80% and 66.20%, respectively. The efficiency of floated materials in cyclones with a specific weight of medium 1.55g/cm3 was also 50 percent, and the efficiency of submerged materials was also calculated to be 50 percent. Then, the separation curves or Tromp curves were plotted with a distribution coefficient relative to the mean density range for the three specified weights (Fig.14).
From the Tromp curve many common performance parameters can be estimated. These parameters include (1) cut point (50), which is the relative density at which particles report equally to both products; (2) probable error (Eps), which is half of the specific gravity interval between 25% and 75% partition values; and (3) imperfection, which relates to the shape of the partition curve (Mohanta and Mishra 2009).
According to the obtained Tromp curves (Fig.14), d50 at the density of 1.40g/cm3 is equal to 1.40g/cm3, and at 1.47 and 1.55g/cm3, this value is 1.67 and 1.86g/cm3, respectively. The actual separation curve shows that the efficiency of the particles with their density at the separation density is the highest, and the efficiency is reduced for particles whose density is close to the separation density (the separation limit). Therefore, given the equalization of the separation density (d50) at a specific weight of 1.40g/cm3, it can be claimed that the performance of the machine was more appropriate at this particular specific weight.
The probability error is the characteristic of a process and separators with less Ep are evaluated as effective separators. The variation of Eps dispersion criterion to centralized variation is also a violation factor. The violation coefficient, independent of the separation density, is used as an auxiliary method for comparing separation processes and is defined as the Eq.(5):
As noted above, the slope of the curve is a parameter to measure the degree of separation and indicate the separation accuracy. The slope of the distribution curve of Fig.14 is approximately linear in the distance between the distribution coefficients of 25% and 75%, so this slope can be used to represent the efficiency. Also, the slope in the density of 1.40g/cm3 relative to other densities indicates a better performance of the device in this particular weight. The linear distribution of the distribution curve (the distance between the distribution coefficients of 25% and 75%) is closer to the straight line, the Ep value is smaller and the separation efficiency is greater. In an ideal separation, this line is straight and the Ep value is zero. In this study, the calculated value of Ep for a heavy media cyclone with a density of 1.40g/cm3 equals to 0.080 and at the specific weight of 1.47, this value is 0.085. Also, this value is 0.030 for a specific weight of 1.55g/cm3. According to the Eq.(5), the coefficient of variations are also calculated for the Tromp curve at the specific weights of 1.40, 1.47 and 1.55g/cm3as 0.20, 0.12 and 0.03, respectively.
Based on the results of industrial scale experiments, by performing heavy medium tests, products with various qualities are obtained. In the size fraction larger than 1mm, the recovery will be 19% for the product with 7% ash, the product with 12% of the feed weight will have 30% recovery, and the recovery of the product with 22.50% ash is 45%. In the case of combining products with the size of larger than 1mm and spiral products (particles smaller than 1mm), concentrate with 18% ash and 48.50% recovery is produced, which is 22.50% of the feed weight. According to these experiments, specific weights of media 1.40, 1.47 and 1.55g/cm3 was considered appropriate for cyclones. The performance of hydro cyclones was investigated by conducting sink and floating experiments. According to the obtained data, in specific weights of media 1.40, 1.47 and 1.55g/cm3, the efficiency of floated materials were 17.75, 33.80 and 50 percent, and the values of sunk materials were above 82.25, 66.20 and 50%. According to the curvatures of the tromp d50, the separation curve in the density of 1.40g/cm3 is 1.00g/cm3 and in the density of 1.47 and 1.55g/cm3, this value is 1.67 and 1.86g/cm3. According to the equalization of the separation density (d50) at a specific weight of 1.40g/cm3, it can be claimed that the function of the machine is suitable for this specific weight. The calculated probability error value for a heavy medium cyclone with a density of 1.40g/cm3 is 0.080 and at the specific weight of 1.47, this error is 0.085. Also, this value is 0.030 for a specific weight of 1.55g/cm3. The value of the imperfection coefficient for the tromp curve of Anjir Tange coal preparation plant at specific weights of 1.40, 1.47 and 1.55g/cm3 are 0.20, 0.12 and 0.03, respectively.
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