A PROCESS AND SYSTEM TO RECOVER CLEAN COAL FROM HIGH ASH COAL FINES USING GRAVITY CONCENTRATION

FIELD OF INVENTION:

[001] The present invention relates to beneficiation process of coal fines. More specifically, the present invention relates to beneficiation of coal fines of size in the range of 0.25 mm to 2 mm for raw coal fines and 0.15 mm to 1mm for washery tailings using classification, gravity separation process followed by dewatering to produce less ash containing clean coal fines, which are used in coke making process before charging it to blast furnace.
BACKGROUND AND PRIOR ART AND PROBLEM IN PRIOR ART:
[002] Typical coal washeries are designed with two popular circuits, one is dense media cyclone (DMC) to treat coarse Run of Mine (ROM) coal in the size range of 0.5 mm to 13mm and second circuit is froth flotation (FF) to treat fine (ROM) coal in the size range of 0 to 0.5 mm. However, the ROM coal quality deteriorates with increase in the mine depth, as with increase in mine depth the coal contains high ash which leads to poor washability characteristics. Hence, in conventional circuits the organic efficiency of the fines in the size range of 0.25 mm – 2 mm is very low (~80%). It results in high losses of clean coal in the coal rejects (in DMC circuit) & tailings (in FF circuit). Since, this fraction is neither large enough for DMC circuit nor small enough for FF circuit, beneficiation of this size coal fraction in existing circuits with high yield at target ash content is not efficient. To address this problem, lot of conventional methods such as spiral, small diameter dense media cyclones and air tables were explored. However, it is observed that either of these existing technologies is not able to produce clean coal with less ash content and high yield effectively.
[003] Moreover, as the depth of mine increases the content of non-reactive maceral groups such as inertinite and mineral matter which are not suitable for steel industry for coke making process, also increases in the ROM coal. The content of non-reactive maceral group also got high in coal tailing. Due to lack of proper beneficiation process, these coal tailings are sent to power plants for power generation or brick kilns. Inertinite rich coal fines have high specific gravity due to which these fines are not able to beneficiate in the Froth Flotation circuit, due to poor hydrophobic characteristics. Whereas, due to its fines nature, Inertinite rich coal fines cannot be treated in the large diameter dense media cyclones, due to high Ecart probable error (Ep) which results into low organic efficiency of coal preparation plant. In the other scenario, these tailings are grounded, and re-flotation was carried out, however with increase in fines content the selectivity decreases. Also, the process is high energy intensive process.
[004] IN200802538I1 describes the beneficiation process for the high ash coking coal fines by the use of spiral concentrator for the size fraction of 0.1 mm to 0.5 mm and froth flotation process for the size fraction of below 0.1 mm. IN201403485I1 describes the dry beneficiation of non-coking coal fines in the size range of 1mm to 0.1 mm using air table. US6889842B2 describes the dry beneficiation of coal fines using multiple number of air table for different size ranges of top size of 3.8 cm. US6126705A describes the process of recovery of clean coal particles from tailings trough heat treatment for the size fraction of 2 mm to 0.075 mm to recover semi coke or coke and oil agglomeration process for the below 0.075 mm ultra-fines. WO2008069849A1 describes the method for beneficiation of ultra-fine coal using spiral concentration and water only cyclone. US5794791A describes the fine coal cleaning process using specially designed dense media cyclone. US5819945A describes a method for separation of fine coal particles (0 to 0.6 mm) in a bimodal (two component) dense medium in dense media cyclone. US4282088A describes the beneficiation process, first the coal was de-slimed up to 0.1 mm and for the fine coal in the size range of 10 mm to 0.1 mm is treated in the autogenously dense media fluidized bed separator using water as media.
[005] The present invention provides a beneficiation process with the objective to produce less ash containing clean coal around 8-15% for raw coal fines and 15-20% for washery tailings of size in the range of 0.25 mm to 2 mm and 0.15 mm to 1 mm respectively.
OBJECTS OF THE INVENTION:
[006] It is therefore the object of the invention to overcome the aforementioned and other drawbacks in prior method/product/apparatus.
[007] The principal objective of the present invention is to develop a process to beneficiate high ash ROM coal fines of size in the range of 0.25 mm to 2mm.
[008] Another object of the present invention is to develop a process to beneficiate coal tailings (flotation rejects) in the size range of 0.15 mm to 1mm.
[009] Another object of the present invention is to propose a beneficiation method to produce the clean coal with high reactive macerals (i.e. vitrinite group maceral) from less vitrinite maceral content.
[0010] Yet another object of the present invention is to develop a beneficiation process of high ash coal fines via classification, gravity concentration followed by dewatering.
[0011] Yet another object of the present invention is to develop a beneficiation process to improve organic efficiency of coal washery by beneficiating through coal fines in the dedicated circuit.
[0012] Yet another object of the present invention is to utilize the coal washery rejects/tailings in the coke making of metallurgical (steel/iron) industry though beneficiation.
[0013] These and other objects and advantages of the present subject matter will be apparent to a person skilled in the art after consideration of the following detailed description taken into consideration with accompanying drawings in which preferred embodiments of the present subject matter are illustrated.
SUMMARY OF THE INVENTION:
[0014] One or more drawbacks of conventional coal beneficiation process, and additional advantages are provided through the process as claimed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be a part of the claimed disclosure.
[0015] According to the process of current invention, high ash ROM coal in the size range of 0 to 13 mm is collected from coal washery. This material is screened using 2 mm screen to separate coarser fraction i.e. 2 mm to 13 mm. Hydro cyclone is used to de-slime the ROM coal and to generate suitable feed in the size range of 0.25 mm to 2mm for beneficiation process.
[0016] Fine ROM coal (0.25 mm - 2mm) generated after de-sliming is mixed with water using a mechanical stirrer and further mixing of coal particles and water through pump and sump combination can be used to ensure desired percentage of coal particles in the slurry. Peristatic pump is used to pump the feed slurry from mixing tank to the advance fluidized bed separator i.e. Reflux Classifier (RC). In this experimentation RC 100 is employed for gravity concentration of coal fines. Reflux Classifier is a combination of fluidization section, autogenous bed (no foreign material such as magnetite used to generate the desired bed density setpoint) and inclined channels. The RC unit has better flexibility to operate at desired setpoint to obtain target ash with maximum yield. The fluidization water enters from the bottom of the device and rises against the falling/incoming coal particles. Initially, the underflow valve of the Reflux Classifier is closed to obtain the desired bed density setpoint and the underflow valve will open for continuous discharge of under-flow after obtaining desired bed density setpoint. The coal particles with higher settling velocities than rising fluidization water settles in the bed and the coal particles with lighter or equivalent settling velocities will report to the over-flow as a concentrate. In this operation, the concentrate contains less ash hence it termed as clean coal and under-flow contains high ash, so it is termed as rejects. The fluidization water rate can be adjusted using needle valve according to feed coal washability characteristics. After, attaining the steady state (i.e. no significant difference between the setpoint and present value) the timely samples from product steams is collected separately and same were decanted, dried and sent for chemical analysis. In the experimentation, the gravity concentration of Reflux Classifier, depends on three important parameters such as feed rate, bed setpoint and fluidization water rate. Hence these three parameters were adjusted/manipulate to obtain maximum yield with targeted ash content of concentrate. The concentrate can be used as raw material for coke making before charging it to blast furnace.
[0017] In an embodiment, the bed density setpoint is maintained in the range of 1100-1450 kg/m3.
[0018] In an embodiment, the ROM coal fines are processed using Reflux Classifier is 0.4 mX0.4 m in cross section with a height of about 1.5 m. The length of the inclined channels is 1 m with 6 mm spacing between the inclined plates.
[0019] In an embodiment, the slurry fed in to the Reflux Classifier has a solid concentration of 15 to 30 Wt.%.
[0020] In an embodiment, the coal slurry is de-slimmed using a hydro cyclone has a solid concentration in the range of 5-15 Wt%.
[0021] In an embodiment, the ash content in the concentrate is in the range of 8 -16%.
[0022] In an embodiment, the concentrate is recovered with concentrate yield in range of 30 to 80 wt% from ROM coal fines.
[0023] In an embodiment, the fluidization water rate of Reflux Classifier is maintained in the range of 1.7 to 4.9 LPM.
[0024] In an embodiment, the Reflux Classifier comprises of 0.4X0.4 m cross section and aspect ratio of inclined plates (height to vertical space ratio) of greater than 150.
[0025] In an embodiment, the slurry fed in to the Reflux Classifier has a solid concentration of 15 to 25 Wt.%.
[0026] It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.
[0027] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0029] Fig. 1 illustrates size analysis of the coal.
[0030] Fig. 2 illustrates block diagram of process of the current invention.
[0031] Fig. 3 illustrates schematic diagram of Reflux Classifier.
[0032] Fig. 4 illustrates effect of feed rate on Reflux Classifier Performance
[0033] Fig. 5 illustrates effect of bed setpoint on RC Performance.
[0034] Fig. 6 illustrates effect of fluidization water rate on Reflux Classifier Performance.
[0035] Fig. 7 illustrates size analysis of the coal tailings.
[0036] Fig. 8 illustrates a block diagram of process of the current invention for coal tailings.
[0037] Fig. 9 illustrates a block diagram of system of current invention.
[0038] Fig. 10 illustrates a block diagram of steps used in the process disclosed in present invention
[0039] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0040] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.
[0041] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such system, or assembly, or device. In other words, one or more elements in a system or device proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or device.
[0042] The present subject matter relates to a process to recover clean coal with less ash content from high ash coal fines using gravity concentration.
[0043] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.
[0044] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[0045] This invention is towards developing a method to recover carbon values from high ash ROM coal fines of size in the size range of 0.25 mm to 2mm. The block diagram of process flow of different steps involved is shown in Fig.2.
[0046] According to the process of the present invention, at step 101 high ash Run of Mine (ROM) coal having its size in the range of 0 to 13 mm is collected from the coal washery. This coal from the washery is subjected to wet screening using 2 mm screen to separate coarser fraction having size in the range of 2 mm to 13 mm (herein termed as over-flow 1 (O/F1)) from 0 to 2 mm finer fraction (herein in termed as under-flow 1 (U/F1)). In wet screening water is added to screen to increase its capacity and improve its sizing efficiency.
[0047] At step 102a, the over-flow 1 (O/F1) is sent to existing Dense Media Cyclone Circuit (DMC) (902) and at step 102b, the under-flow 1 (U/F1) is sent to Hydro Cyclone (903) for de-slimming purpose.
[0048] The solid concentration of Under-flow (U/F1) is 5-15 Wt. %. The aim of hydro cyclone (903) is size classification to generate de-slimmed feed. If under-flow 1 (U/F1) is not subjected to de-slimming and are directly feed to Reflux Classifier (906) the particles of coal which are very fine in nature were carry over by the raising fluidization water and reports to overflow i.e. concentrate. This result in high ash content in the concentrate. Due to the fine nature of these particles they experience zero mass and do not participate in gravity concentration. Hence for effective gravity concentration of the Reflux Classifier (906), the feed must be de-slimmed to obtain concentrate with less ash content. In present process these fine particles are removed prior to gravity concentration. Process condition and geometry of the hydro cyclone (903) are selected to keep the above requirement in mind.
[0049] According to one embodiment of the invention hydro cyclone (903) is operated with a target cut size close to 0.25 mm. de-slimming process reduces the further load and improves the overall efficiency of the process.
[0050] At step 102b, the hydro cyclone (903) during de-slimming further separates the finer fraction having size in the range of 0 to 0.25 mm (herein termed as over-flow 2 (O/F2)) from coarser fraction having size in the range of 0.25 mm to 2 mm (herein termed as under-flow 2 (U/F2)).
[0051] At step 103a, the over-flow 2 (O/F2) fraction is sent to existing froth flotation circuit (904) and at step 103b, the under-flow 2 (U/F2) is used to make coal slurry which is fed to the Reflux Classifier (906) at step 104. The slurry of coal is prepared by mixing under-flow 2 (U/F2) fraction of coal with water using a mechanical stirrer in a mixing tank (905). To obtain desired particles of coal in slurry, coal particles and water is mixed through pump and sump combination. Ideally the coal slurry fed in the Reflux Classifier has solid concentration in range of 15-30%. The prepared slurry is pumped to Reflux Classifier (906) via peristatic pump (914).
[0052] The reflux classifier (906) unit is a combination of fluidization section, autogenous bed (no foreign material such as magnetite used to generate the desired bed density setpoint) and inclined channels. The reflux classifier (906) has better flexibility to operate at desired setpoint to obtain target ash with maximum yield. The fluidization water enters from the bottom of the device and rises against the falling/incoming coal particles. Initially, the underflow valve of the reflux classifier (906) is closed to obtain the desired bed density setpoint and it will open for continuous discharge of under-flow after obtaining the desired bed density setpoint. The coal particles with higher settling velocities than rising fluidization water settles in the bed and the coal particles with lighter or equivalent settling velocities will report to the over-flow as a concentrate. In this operation, the concentrate contains less ash hence it termed as clean coal and under-flow contains high ash, so it is termed as rejects. The fluidization water rate can be adjusted using needle valve according to feed coal washability characteristics.
[0053] At step 104, the reflux classifier further separate the coal slurry in concentrate fraction (O/F3) having less ash content and reject fraction (U/F3) having high ash content. At step 105, the over-flow 3 (O/F3) fraction is sent to thickener (912) and dewatering screen (907) to remove excess water. After removing excess water the clean coal (910) (herein referred as overflow 4 (O/F4)) fraction is recovered and the ash containing coal (herein referred as under-flow 4 (U/F4)) fraction is sent to clarified tank (909) At step 106, the reject (herein referred as under-flow 3 (U/F3)) fraction of the reflux classifier (906) is send to thickener (908) and dewatering screen (913) for removing excess water. After removing excess water the reject of U/F3 of step 104, (herein referred as under-flow 5 (U/F 5)) fraction is removed from the process and the ash containing coal (herein referred as over-flow 5 (O/F5)) fraction is sent to the clarified tank (909).
[0054] . At step 107, the stored under-flow 4 (U/F4) and over-flow 5 (O/F5) is sent to reflux classifier (906) for recirculation.
[0055] One embodiment of the present invention is shown in figure 8. Reference may be made to figure 8 where current process is adapted for coal tailing. Herein coal tailing of size in the range of 0.15 mm to 1 mm are sent to hydro cyclone (903) for de-slimming. The ash containing fraction (herein referred to as over-flow (O/F6)) is discarded and the coal and ash containing fraction herein referred to as under-flow 6 (U/F6)) is used to prepare coal slurry for Reflux Classifier (906). Coal slurry is prepared in mixing tank where water from water storage is mixed with under-flow 6 (U/F6) to prepare slurry. This slurry is fed to reflux classifier (906). The concentrate fraction (herein referred to as over-flow 7 (O/F7)) from reflux classifier (906) is the fraction having less ash which is sent to thickner (912) and dewatering unit (907) to recover clean coal (910). The reject fraction containing more ash (herein referred to as under-flow 7 (U/F7)) is sent to thickner (908) and dewatering unit (913) for recovery of rejects.
[0056] Reference may be made to figure: 9, coal having size in the range of 0 to 13 mm is collected from coal washery and subjected to wet screening in a wet screening unit (901). Wet screening units are designed to efficiently screen out fines and classify oversize particles. Wet screening unit (901) use 2 mm screen to separate coarser fraction over-flow 1 (O/F1)) from finer fraction under-flow 1 (U/F1)). The O/F1 fraction is send to dense media cyclone circuit (902), whereas the U/F1 fraction is send to Hydro cyclone (903). Hydro cyclone (903) is used to de-slime the U/F1 fraction into finer fraction over-flow 2 (O/F2)) from coarser fraction under-flow 2 (U/F2)). The over-flow 2 (O/F2) fraction is then sent to existing froth flotation circuit (904) and the under-flow 2 (U/F2) is used to make coal slurry. Coal slurry is prepared in the mixing tank (905) and transferred to reflux classifier (906) via peristatic pump (914). Reflux Classifier (906) is a combination of lamella settler, autogenous dense medium separator and fluidized bed separator. The fluidized bed separator acts as the density-based separator whereas the Lamella section acts the classification zone in the Reflux Classifier (906). Lamella section contains multiple number of parallel inclined plates. Fluidization section provides uniform flow of particles to each inclined plate. In these inclined plates, particles of finer sizes or of lesser density are carried through the channels by the fluidization liquid to the concentrate. The coarser size particles or higher density particles, having higher settling velocity, settle short distances within the channels, form sediment layers and rapidly slide down to the previous section, i.e. the fluidization zone. This phenomenon develops a reflux action because of fluidized particles segregating onto the inclined plates and returning to the fluidized zone below. This self-recycling effect eliminates the misplacement of materials, thus enhancing the product quality. The Boycott phenomenon in the lamella of inclined channels makes the reflux classifier high throughput equipment. As the channel is inclined, the effective area available for settling is increased from the cross-sectional area of the channel to the projected area of the channel height. Since each lamella consists of multiple inclined channels the effective area available for segregation or the classification capacity of the lamella is proportionally increased. The over-flow 3 (O/F3) fraction is sent to thickner (912) and dewatering screen (907) to remove excess water. The under-flow 3 (U/F3) fraction of the reflux classifier is send to thickener (908) and dewatering screen (913) for removing excess water. The clean coal (910) (herein referred as overflow 4 (O/F4)) fraction is recovered and the ash containing coal (herein referred as under-flow 4 (U/F4)) fraction is sent to clarified tank (909). Also, the reject (911) (herein referred as under-flow 5 (U/F 5)) fraction is removed from the process and the ash containing coal (herein referred as over-flow 5 (O/F5) fraction is sent to the clarified tank (909). The stored under-flow 4 (U/F4) and over-flow 5 (O/F5) is sent to reflux classifier (906) for recirculation.
[0057] Experiments were done on the process disclosed for recovery of clean coal with less ash content form high ash coal fines. They are discussed below:
Case Study: 1 Beneficiation of ROM Coal fines using Reflux Classifier
[0058] Representative high ash ROM coal sample of size fraction 0 to 13 mm was collected from the raw coal stock pile at West Bokaro coal washery of TATA Steel. This sample was screened using 2 mm size screen to remove oversize fraction of 2 mm to 13 mm, which was treated in exiting dense media cyclone (DMC). The under-flow of the screen was mixed with water in an agitator with mechanical stirrer. The coal slurry was pumped to hydro cyclone, for de-sliming. The over-flow fraction 0 to 0.25 mm was sent for the treatment of froth flotation circuit and the under-flow of hydro cyclone is used as feed material in this invention. The size analysis of de-slimmed coal sample is shown in Fig.1. From the size analysis result, it is evident that the ash distributed uniformly across all sizes. The ash content of the air-dried feed was 28% and the average particle size (D50) was greater than 700µm.

Fig 1 Size Analysis of the Coal
[0059] The proximate analysis of the coal was carried out by following ASTM standard procedures using a Thermogravimetric analyzer (TGA 701) supplied by Leco. The ultimate analysis was carried out by following American Society for Testing Materials (ASTM) standard procedures using a Leco 628 CHN/S elemental analyzer. The chemical analysis (proximate and ultimate) results are shown in Table.1. Proximate analysis shows that, the sample was found to contain 1.4% moisture, 21% volatile matter and 50% fixed carbon.
Table 1: Chemical Analysis of the Coal
Proximate Analysis Ultimate Analysis
Parameter Weight, % Element Weight, %
Fixed Carbon 49.8 C 61.7
Volatile Matter 20.8 H 3.6
Ash 28 N 1.1
Moisture 1.4 S 1.00
O 32.6

[0060] Feed slurry of 15 wt% solids concentration was prepared in a 400-litre feed preparation tank and a peristatic pump was used to feed the slurry to the RC. Prior to experimentation, the pump was calibrated with coal slurry and a calibration curve was obtained for feed flow rate vs rotational speed (rpm). The fluidization water was supplied at the base of the device and the flow rate was monitored using a flow meter. The laboratory unit has the flexibility to adjust the fluidization water velocity with a valve. The process flow sheet of gravity concentration of coal fines using Reflux Classifier is shown in Fig.3.
[0061] In the experimentation, the gravity concentration of RC, depends on three important parameters such as feed rate, bed setpoint and fluidization water rate. Hence these three parameters were adjusted/manipulate to obtain maximum yield with targeted ash content of concentrate. The concentrate can be used as raw material for coke making before charging it to blast furnace.
[0062] The separation efficiency of Reflux Classifier was investigated by varying the feed flow rate from 5 to 8.4 lpm with an increment of 1.7 lpm. During the experimentation, the other two variables, set point density (bed density) and fluidization water velocity were kept constant at 1450 kg/m3 and 3.3 lpm respectively. The results are shown in Fig 4 and it is evident that the concentrate ash content was 18.8, 18.8 and 20% for feed rates of 5, 6.7 and 8.4 lpm respectively. The corresponding rejects ash content was 63.8, 62 and 58% respectively. The experimental measured yields were 79.7, 78.7 and 78.9% for the feed rates of 5, 6.7 and 8.4 lpm. The corresponding combustible material recoveries were 53.5, 52.8 and 56.4%.
[0063] Another set of experiments were carried out and the RC was operated at 1100, 1200 and 1300 kg/m3 setpoints by keeping other two variables (feed rate and fluidization water) constant at 6.7 lpm and 1.7 lpm respectively. The experimental results are as shown in Fig 5, and it is evident that, the respective concentrate ash was 7.9, 11.2 and 12.4%. The corresponding rejects ash was 37, 42 and 45%. The concentrate and rejects ash content were found to increase with increasing the bed setpoint. The experimental measured yields were 31, 45.3 and 52.1% for the bed setpoint of 1100, 1200 and 1300 kg/m3 respectively.
[0064] The final set of experiments were carried out with different fluidization water flowrates from 1.7 to 4.9 lpm. The other two variables, feed rate and bed setpoint was kept constant at 6.7 lpm and 1200 kg/m3 respectively. The experimental results are shown in Fig 6. It is evident that the concentrate ash content varied from 11.2 12.4 and 16% for the fluidization water velocities of 1.7, 3.3 and 4.9 lpm respectively. The corresponding rejects ash content were 42, 44.5 and 51 %. The experimental measured yields were 45.4, 51.4 and 65.6% for the water fluidization velocities of 1.7, 3.3 and 4.9 lpm and the corresponding combustible material recoveries were 18.1, 22.8 and 37.5%. Maximum clean coal achieved at the following condition. The results of Reflux Classifier tests are presented in Table 2.
[0065] Table 2. Optimum condition of operating parameters of Reflux Classifier for coal tailings sample
Operating parameters Value Range
Feed Rate (LPM) 6.7 5-8.4
Bed Setpoint (Kg/m3) 1200 1100-1450
Fluidization water Rate (LPM) 4.9 1.7-4.9

Petrography Characterization
[0066] The organic micro constituents of coal are termed as macerals and based on their reflectance level these can be categorized by petrographic analysis into three groups: vitrinite, liptinite and inertinite. During the carbonization stage in coke making process, only vitrinite and liptinite are reactive to become plastic, while inertinite remains inactive. Hence first two macerals are termed as reactive and later is called non-reactive macerals. In the current research work petrography analysis was carried out for one set of reflux classifier products (clean coal and rejects) and the corresponding feed samples. Macerals were identified, using Leica DM6000 M fully automated microscopy system, as per the International Committee for Coal and Organic Petrology procedure.
[0067] Petrography analysis of the product and feed samples is shown in Table 3. It is observed that, the coal rank (Ro) was 1.1, and V-distribution varies in the range of V9 to V12. The vitrinite enriched from 47.4 to 63.2% in the concentrate and the mineral matter content decreased from 20% to 4.5% simultaneously. The mineral matter content enriched from 20% to 67.7% in the rejects stream. It can be concluded that, reactive macerals were enriched through the gravity separation of coal fines.
[0068] Table 3: Petrography Analysis of Reflux Classifier Products
S. No Maceral Feed Concentrate Rejects
1 Ro 1.1
2 Vitrinite 47.4 63.2 14.3
3 Inertinite 31.8 32.3 18
4 Liptinite 0.7 0 0
5 Mineral Matter 20.1 4.5 67.7

Case Study: 2
Recovery carbon values from coal washery tailings using Reflux Classifier

[0069] The coal washery tailings sample was collected from Washery#2, West Bokaro. The coal tailings were de-slimmed i.e. removed -150 µm coal particles using hydro cyclone. The de-slimmed coal size distribution is shown in Fig. 8.
It is evident that the ash distributed uniformly across all sizes and increases as size decreases. The ash content of the air-dried feed was 37.5% and the average particle size (D50) was greater than 500µm.

[0070] During the operation, Reflux Classifier is operated with the targeted concentrate ash of less than 18% from the feed ash of 37.5% of de-slimmed coal tailings. De-sliming process using hydro cyclone reduces the load for further process and improves the efficiency of the process. The optimum conditions of operating parameters used to beneficiate the coal tailings in Reflux Classifier operation is shown in Table 4. After optimizing the Reflux Classifier parameters, tests on Reflux Classifier are conducted to produce concentrate with less ash content. The results of Reflux Classifier tests are presented in Table 5.
[0071] Table 4. Optimum condition of operating parameters of Reflux Classifier for coal tailings sample
Operating parameters Value Range
Feed Rate (LPM) 6.7 5-8.4
Bed Setpoint (Kg/m3) 1350 1100-1400
Fluidization water Rate(LPM) 3.5 1.7-4.9

Table 5. Optimum test result of Reflux Classifier for coal tailings
Description wt% Ash%
Concentrate 40 17.25
Rejects 60 51.01
Total 100 37.5
[0072] Petrography analysis of the product and feed samples of is shown in Table 6. It is observed that, the coal rank (Ro) is 0.98. The vitrinite enriched from 44.6 to 62.7% in the concentrate and the mineral matter content decreased from 8.4% to 4.2% simultaneously. The mineral matter content enriched from 8.4% to 56% in the rejects stream. It can be concluded that, reactive macerals were enriched through the gravity separation of coal tailings, which can be utilized in coke making before fed to blast furnace.
[0073] Table 6: Petrography Analysis of coal tailings in Reflux Classifier Products
S. No Maceral Feed Concentrate Rejects
1 Ro 0.98
2 Vitrinite 44.6 62.7 14.6
3 Inertinite 46.4 28.6 25.5
4 Liptinite 0.6 4.5 3.9
5 Mineral Matter 8.4 4.2 56

[0074] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
[0075] It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that “consist essentially of” or “consist of” the recited feature.
[0076] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

We claim:

1. A process to recover clean coal from high ash coal fines using gravity concentration, the process comprises:
separating coal from mine into coarser fraction of coal (O/F1) and finer fraction of coal (U/F1) by wet screening;
sending the coarser fraction (O/F1) to dense media cyclone circuit (902);
sending the finer fraction (U/F1) to hydro cyclone (903) for de-sliming wherein the hydro cyclone (903) separate the finer fraction (U/F1) into finer fraction (O/F2) and coarser fraction (U/F2);
sending the finer fraction (O/F2) to froth flotation circuit (904);
mixing the coarse fraction (U/F2) with water to form coal slurry in a mixing tank (905);
feeding the coal slurry to reflux classifier (906) for gravity concentration, wherein the reflux classifier (906) further separates the coal slurry in concentrate fraction (O/F3) having less ash content and reject fraction (U/F3) having high ash content;
sending concentrate fraction (O/F3) of the reflux classifier (906) to thickener (912) and dewatering screen (907) to remove excess water wherein after removing excess water clean coal fraction (O/F4) is recovered and ash containing coal fraction (U/F4) is sent to clarified tank (909);
sending reject fraction (U/F3) of the reflux classifier (906) to thickener (908) and dewatering screen (913) to remove excess water wherein after removing excess water reject fraction (U/F5) is removed from the process and ash containing coal fraction (O/F5) is sent to clarified tank (909);
sending ash containing coal fraction (U/F4 and O/F5) stored in clarified tank (909) to reflux classifier (906) for recirculation.
2. The process as claimed in claim 1, wherein 2 mm opening screen is used in wet screening to separate coarser fraction of coal (O/F1) and finer fraction of coal (U/F1).
3. The process as claimed in claim 1, wherein mechanical stirrer and pump and sump combination is used to ensure 15 to 30 Wt. percentage of coal particles in slurry.
4. The process as claimed in claim 1, wherein concentration of coarser fraction of coal for de-sliming in hydro cyclone (903) is 5-15 Wt.%
5. The process as claimed in claim 1, wherein peristatic pump (914) is used to pump the coal slurry from mixing tank to the reflux classifier.
6. The process as claimed in claim 1, wherein operating parameter of Reflux Classifier are feed rate, bed density setpoint and fluidization water rate.
7. The process as claimed in claim 1, wherein bed density setpoint is maintained in the range of 1100-1450 Kg/m3.
8. The process as claimed in claim 1, wherein fluidization water rate of reflux classifier is maintained in the range of 1.7 to 4.9 LPM.
9. The process as claimed in claim 1, wherein feed rate in reflux classifier is maintained in the range of 5 to 9 LPM.
10. The process as claimed in claim 1, wherein the ash content in concentration (O/F3) is in the range of 8-16%.
11. The process as claimed in claim 1, wherein concentrate is recovered with concentrate yield in range of 30 to 80% from ROM coal fines.
12. A process to recover clean coal from washery tailings using gravity concentration, the process comprises:
Sending coal tailing of the size 0.15 mm to 1 mm to hydro cyclone (903) for de-sliming, wherein after de-sliming hydro cyclone (903) produce high ash containing fraction (O/F6) and ash containing coal fraction (U/F6);
Discarding high ash containing fraction (O/F6);
Preparing coal slurry by mixing water and ash containing coal (U/F6) in a mixing tank;
Feeding the coal slurry in reflux classifier (906), wherein the reflux classifier (906) produce concentrate fraction (O/F7) having less ash and reject fraction (U/F7) having high ash content;
Sending concentrate fraction (O/F7) to thickener (912) and dewatering unit (907) to recover clean coal;
Sending reject fraction (U/F7) to thickener (908) and dewatering unit (913) to recover reject.
13. A system to recover clean coal from high ash coal fines using gravity concentration, the system comprises:
a wet screening unit (901) to screen coarser fraction (O/F1) from finer fraction (U/F1) of coal;
a dense media cyclone circuit (902) for processing coarser fraction (O/F1) of coal;
a hydro cyclone (903) for de-sliming finer fraction (U/F1) into coarser fraction (U/F2) and finer fraction (O/F2);
a froth flotation circuit (904) for processing finer fraction (O/F2);
a mixing tank (905) for preparation of coal slurry from coarser fraction (U/F2);
a reflux classifier (906) for processing of coal slurry and producing concentrate fraction (O/F3) and reject fraction (U/F3);
a plurality of thickeners (912, 908) and dewatering screens (907, 913) for removal of excess water from concentrate fraction (O/F3) and reject fraction (U/F3); and
a clarified tank (909) for storing ash containing coal fraction (U/F4, O/F5) for recirculating in reflux classifier (906).
14. The system as claimed in claim 13, wherein reflux classifier comprises:
a lamella settler having multiple number of parallel inclined plate;
a fluidized bed separator for density separation, wherein the fluidized bed provides uniform flow of particles to each inclined plate of the lamella settler; and
an autogenous dense medium separator.