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Coal and coal combustion byproducts are potential candidates for alternative resources of REEs (rare earth elements). The Illinois Basin is a major coal-producing district in the USA, but little work is available on speciation of REEs in the basin’s coals. In this study, a total number of 382 samples from known locations in the Illinois Basin coal beds were acquired, analyzed for their REE contents, and investigated for any spatial trends or chemical correlations. This study includes Springfield, Herrin, and Baker coal beds—the most heavily mined coals in the basin. An average coal ash from the basin contains 320 ppm REE, with Baker being the highest (407 ppm on average). However, the distribution of REEs in the coal beds was laterally and vertically heterogeneous. A number of trends were found in the REE variations. The most important trend was the relationships of the REE concentrations and fractionations with the distance from the syndepositional sandstones. The total REE concentrations were higher in the proximity of the sandstones, likely due to the increased deposition of detrital material. This increase was the most significant in Springfield coal (90%) and least in Baker (6%). Those coal samples from the Springfield and Herrin coal beds distal from the syndepositional sandstones had a heavy to light REE ratio (H/L ratio) of 0.40 on average, whereas in proximal locations, the ratio was dropped to an average of 0.30. This trend was not significant in the Baker coal bed. In general, higher mineral matter in the coals was associated with higher REEs. In coal preparation plants, refuse rocks can have up to 360 ppm REE. However, the ashes of the cleanest coals had higher REE levels. Such ashes also have a higher ratio of heavy to light REEs. The highest REE concentrations were found in the claystones and the shales associated with the coal beds. Since the naturally occurring calcite, quartz, and pyrite samples were depleted in REEs, an association of REEs with the clay minerals can be suggested.

Recovery of rare earth elements (REE) from coal-related resources has recently received significant interest due to supply concerns and their key roles in critical industries and defense-related technologies. An integrated flowsheet consisting of sorting, crushing/grinding, physical separation, acid leaching, solvent extraction, and selective precipitation was designed to achieve continuous production of rare earth products from various coal-related resources such as coarse refuse and acid mine drainage. A pilot plant was constructed that enabled continuous testing of the circuitry on a range of different coal-based feedstocks. Despite achieving significant success in producing high-grade rare earth oxide products, low REE recovery values and high operational costs necessitated a re-evaluation of the process flowsheet. This article describes the circuitry improvements that were based on both laboratory and pilot plant test results. Roasting as a pre-leach treatment was found to significantly improve the leaching rate and overall REE recovery values. After roasting West Kentucky No. 13 and Fire Clay coal refuse materials at 600 °C, REE recovery increased by 60 and 40 absolute percentage points, respectively. Rare earth concentration in the pregnant leachate solution (PLS) was increased to around 700 ppm prior to solvent extraction using a staged precipitation method. REEs in the precipitate were re-dissolved, purified, and extracted from the PLS using solvent extraction and selective precipitation to values of around 80%. As a result of the improvements, chemical consumption in the acid leaching and solvent extraction processes was significantly reduced. A techno-economical assessment of the improved flowsheet indicated significant commercialization potential.

A field study was undertaken to experimentally measure the concentrations of rare earth elements (REEs) contained in the process streams generated by a group of 20 coal preparation plants located in the eastern USA. For each site, representative samples of clean coal product, coarse refuse, and fine refuse were collected. Each sample was then partitioned into preselected size and density classes by wet screening/sieving and float-sink testing. The resultant products were dried and subjected to laboratory analyses to determine ash contents and rare earth element concentrations including Yttrium and Scandium. A detailed analysis of the database generated by this exercise showed that coal-based products from these preparation plants contained significant quantities of rare earth elements. In particular, the coarse refuse streams currently discarded by the 20 plants examined were found to contain a sufficient tonnage of REEs to satisfy the current domestic demand for these important elements. The data also showed a strong positive correlation between ash content and REE concentration, which suggested that the primary association of REEs in eastern USA bituminous coal sources is likely within fractions containing inorganic impurities. This association was well described using a simple power equation relating ash content and REE concentration. The ratio of heavy-to-light rare earth elements was discovered to be significantly higher in organically rich fractions of clean coal, suggesting that mineral impurities intimately associated with carbonaceous matter have elevated concentrations of heavy rare earth elements. A similar trend was observed for a grouping of rare earth elements (Y, Nd, Eu, Tb, and Dy) that are likely subject to near-term supply shortages. Finally, the database showed that a linear correlation existed between La for many REEs of interest, although there were several notable exceptions for some high-value REEs (Lu, Pr, and Tb).

Selective depression of calcite in a monazite-calcite flotation system was systematically studied using micro-flotation and electrokinetic tests as well as X-ray photoelectron spectroscopy (XPS) characterization and solution chemistry calculations. Monazite was found to be depressed by both sodium silicate and sodium hexametaphosphate when calcium ions appeared in solution. It was hypothesized that co-adsorption of calcium species (CaHCO3+ and CaCO3(aq)) and Si ( OH ) 4 as well as SiO ( OH ) 3 - via hydrogen bonding formed a compact layer over the monazite surface, thereby reducing octanohydroxamic acid adsorption by steric hindrance. The use of citric acid or EDTA formed soluble chelates (e.g. CaCit− and CaEdta2−) with the calcium ions which negated the flotation suppression effect. An XPS study indicated that both EDTA and citric acid chemically adsorbed on the monazite surface, while the adsorption on the calcite surface was achieved mainly through hydrogen bonding. EDTA was more effective than citric acid due to two reasons: (i) citric acid itself presented stronger depression effects and (ii) citric acid has weaker chelating properties for calcium. Using 2.5 × 10−4 M octanohydroxamic acid, 0.05 g/L sodium silicate and 6 × 10−5 M EDTA, monazite recovery of more than 90% was achieved while recovering only 20% of the calcite.

The majority of rare earth elements (REEs) existing in the feed to coal preparation plants report to the coarse refuse streams which are transported for permanent storage in contained piles. In this study, an integrated flowsheet was developed based on laboratory test data which combines physical separation, pyrite bio-oxidization, heap leaching, selective precipitation and solvent extraction processes. The test data was obtained from (1) characterization of a number of natural leachate and solid samples collected from different preparation plants which process coals from a number of coal seams and (2) laboratory acid leaching and selective precipitation tests results. The highly-valued critical REEs (i.e., Y, Nd, Eu, Tb and Dy) were selectively leached from the refuse samples in the natural environment due to the acid generated by pyrite oxidization. The leachate samples were evaporated to remove water and obtain residual solids (i.e., dissolved solids in the leachates). The total REE content in the dissolved solids from a given leachate sample was 380 ppm, which was higher than the REE content of the coarse refuse material that generated the leachate (322 ppm). Acid leaching tests recovered as much as 80% of the total REEs from the coarse refuse samples using a 1.2 M sulfuric acid solution. Afterwards, the pH of the leachate was increased in a step-wise fashion which resulted in the production of precipitates containing 0.3–1.1% total REEs. A significant amount of contaminants, such as Fe, Al, and Ca, were eliminated in the sequential precipitation process, which allowed further upgrading using oxalic acid precipitation and/or solvent extraction. In the proposed flowsheet, the coarse refuse is arranged in heap leach pads and the acid needed for REE leaching is primarily produced from pyrite bio-oxidization, which enhances the selectivity of REE recovery and significantly reduces the cost. The successful application of the flowsheet would result in significant benefits to both the coal and rare earth industries.

Leaching recovery of rare earth elements (REEs) from two natural kaolinite samples were investigated in this study. Using standard leach conditions without pretreatment, only 10% and 56% of the total REEs were leached from the raw samples, respectively. However, pretreatment by calcination at temperatures between 500 °C and 900 °C increased the leaching recovery of total REEs to values as high as 93% for both samples. Mineralogical characterization and acid leaching results suggested that the decomposition of the kaolinite structure occurs as a function of calcination temperature. Comparisons between the REEs and the major constituent elements of kaolinite showed that their leaching characteristics contrasted. Moreover, sequential chemical extraction results suggested that the fractions of REEs occurring in an ion-adsorption form were negligible. Therefore, the REEs were primarily associated with REE-bearing minerals of which, a portion was encapsulated in kaolinite aggregates while the remaining existed as liberated particles. These association characteristics were proven through SEM-EDX analysis. Based on the mineralogy characterization, acid leaching, and sequential chemical extraction results, it was concluded that the REEs have complicated modes of occurrence and association characteristics with kaolinite. The positive impacts of calcination on the leaching recovery of REEs are likely due to two mechanisms, i.e., (1) liberation of the encapsulated REE-bearing minerals through expansion of the kaolinite aggregates, and (2) conversion of the REE-bearing minerals into more soluble forms.

The present study concentrated on a novel froth flotation process with the addition of hydrophobic magnetic particles. The effects of the concentration of hydrophobic particles, coal type, and the ...

Rare earth elements (REEs) found in coal are in the form of minerals, ion-adsorbed onto clay surfaces or inner layers, or organically bound. Rare earth minerals such as monazite exist in coal and have grain sizes smaller than 5 µm. In this study, froth flotation was successful in concentrating rare earth minerals existing in a thickener underflow material derived from Fire Clay seam coal that contained around 431 ppm of total rare earth elements (TREE) on a dry ash basis. Conditioning with fatty acid followed by processing using multiple stages of conventional flotation produced a final concentrate containing 2,300 ppm TREE. Using a laboratory flotation column to limit hydraulic entrainment, the TREE content was further enriched to around 4,700 ppm, which equated to an enrichment ratio of 10:1..

The dissolution of minerals provides elevated concentrations of alkaline earth metal ions such as calcium (Ca 2+ ) in water which may have a significant influence on flotation performance. The effects of Ca 2+ ions dissolved from calcite surfaces on the flotation performance of a monazite-calcite system have been investigated using micro-flotation tests followed by fundamental collector adsorption studies. The results indicate that specially adsorbed CaOH + via hydrogen bonding competed with hydroxamic acid for the phosphate-oxygen active sites on the monazite surface and decreased its hydrophobicity due to hydration. Electrostatically adsorbed CaOH + on monazite increased the adsorption density of hydroxamic acid through a chelating interaction. At higher dosages of calcium and hydroxamic acid, bulk and surface precipitation of calcium-hydroxamate occurs in the monazite-calcium-hydroxamic acid system.

The effect of hydrophobic and magnetic plastic particles having a contact angle of around 83° on flotation performance was evaluated using coal particles of varying degrees of floatability. The magnetic plastic material were recovered by a low intensity magnetic separator and recycled back to the flotation feed for re-use. Flotation rate tests conducted on coal using a conventional cell proved that combustible recovery and flotation rate were significantly enhanced with the addition of the plastic particles, especially for difficult-to-float coals, which was corroborated by flotation column tests. Carrying capacity and particle size-by-size flotation tests further showed that the magnetic plastic particles preferentially increased the recovery of coarse particles by as much as 35 absolute percentage points due to froth stabilization which reduced the selective detachment of coarse and/or weakly hydrophobic particles. The enhanced flotation recovery was attributed to the influence on liquid drainage rate in the froth zone, froth stability, bubble coalescence and flotation rates.

The modern air-based density separator achieves density-based separation for particle sizes greater than 6 mm. Parametric studies, conducted with a modified laboratory-scale unit, operating at an optimal feed rate of 200 kg/hr to upgrade low-rank coal, have demonstrated the applicability to clean finer size fractions up to 1 mm. However, there is a lack of fundamental understanding of the separation process that can be provided utilizing numerical modeling techniques. The separation process was numerically modeled using K-Epsilon and RSM turbulence formulations and validated using experimental dataset. The results prove that the effect of fine coal vortices forming around the riffles act as a transport mechanism for higher density particle movement across the table deck resulting in 43% displacement of the high-density particles to the product side. The velocity and vector plots show high local variances of air speeds and pressure near the feed end and an increase in feed rate results in a drop in...

Acid mine leachate (AML) may be a significant source of rare earth and other critical elements needed for the manufacturing of advanced electronics and renewable energy technologies. A systematic study was performed on a natural leachate collected from a coal preparation plant that treated bituminous coal containing elevated amounts of pyrite. The leachate contained 1.96 ppm total REEs, 2.52 ppm Zn, 2.15 ppm Ni, 1.22 ppm Cu, 0.77 ppm Co, and 25 ppm Mn. Sequential precipitation using simple pH control generated a pre-concentrate containing 0.82% total REEs, 1.08% Zn, 0.91% Ni, 0.50% Cu, 0.34% Co, 7.1% Mn. The pre-concentrate was further treated by re-dissolution using a 10 M HNO3 solution, which resulted in a solution pH of 1.5. The addition of oxalic acid to selectively precipitate the REEs followed by roasting of the precipitate generated a product containing >98% rare earth oxides. More than 95% of the Cu and Zn were recovered from the residual liquid using Na2S at pH 2 and 3, respectively. The CuS and ZnS contents in the corresponding concentrates were 60% and 58%, respectively. The majority of the Co and Ni remained in the residual liquid. Based on these findings, a process flowsheet was developed to recover the REEs and critical metals from AML.

Maceral separation is one possible technique to expand coking coal resources, either by increasing the blending ratio of non- or slightly caking coal or rationally utilizing the coking coal according to the different behaviors of macerals in coking. The success of maceral enrichment for a given coal usingfiotation is dependent on the differences in flotation kinetic rate among macerals. This paper presents theflotation rates and zeta potentials of the vitrinite-rich and inertinite- rich maceral concentrates obtained from six coal samples of different ranks — from high-volatile B bituminous coal with mean random vitrinite reflectance (Rrandom) of 0.64 percent to low-volatile bituminous coal with Rrandom of 1.73 percent — using a float-sink procedure. The flotation results show that vitrinite-rich concentrates can be produced in tap water using froth flotation only with frother for the high-volatile B and A bituminous coal samples, but there was no good performance for the high-volatile C bituminous and medium- to low-volatile bituminous coal samples. The zeta potential results show that froth flotation could probably separate the high-volatile C bituminous coal samples well by adjusting pH, but there was no good performance for the higher-rank coal samples.

Previous research indicates that coal and coal by-products are a potential source of critical elements including rare earth elements (REE) with estimated amounts in the range of 50 million metric tons. Despite the proven presence of elevated REE concentrations, commercial extraction and recovery have not been realized. This article provides a review of the abundance, mode of occurrence, and recovery methods of rare earth elements in coal and coal by-products. The feasibility of using established REE extraction and recovery technologies is discussed along with issues associated with their use with coal resources.

Getting magnetite concentrate with extremely high Fe grade using reverse flotation method has become increasingly important due to the enhanced requirement of iron making industry. The coagulation in magnetite-concentrate flotation system may impose negative effect on flotation performance. This paper combined the DLVO calculation and microscope observation to study the coagulation behavior in the system. The result shows that the coagulation between fine magnetite and coarse silica particle is very likely to occur with energy barrier 1.5 × 10 −18 J at pH 10.0, which is much smaller than 1.6 × 10 −17 J and 3.3 × 10 −17 J for fine magnetite and fine magnetite-coarse magnetite particles. The coating of fine magnetite particle on silica surface was observed under microscope. Mechanical scrubbing increases the concentrate Fe grade and recovery by 1.5% and 8.5% respectively. Magnetite concentrate of 67.9% Fe grade and 71.5% Fe recovery was finally obtained.

Low-rank coal such as the coal in the Powder River Basin (PRB) is typically direct shipped without any need for upgrading. Due to the lack of on-site processing capabilities, coal that is mixed with out-of-seam dilution during the mining process is typically left in the mine pit. In some cases, the loss could amount to 5% of the total reserve. Research conducted on laboratory and pilot-scale pneumatic air table separators indicates that sufficient upgrading can be achieved on the +1 mm fraction of the reject material to meet typical end-user specifications. Low-rank coals are especially susceptible to upgrading by density-based processes due its naturally lower density relative to higher rank coals. For example, a PRB coal containing 26% feed ash was reduced to 7% ash content with a combustible recovery of 83% on a dry basis from a coal source that was reject from the mining process. Partition curve data revealed the achievement of relatively low Ep values in the range of 0.12 to 0.22 with separation dens...

Fine coal cleaning circuits in operating preparation plants worldwide vary significantly in the technologies used for upgrading and dewatering coal as well as the overall circuit layout. The differences are often due to the cleanability characteristics of the coal and the varying market specifications. This article presents the use of plant optimization methodology based on the constant incremental quality concept toward selection of the fine coal cleaning and dewatering technologies required for maximizing plant yield. The applications of the technologies in various circuit layouts are presented with respect to targeted markets and discussed using case studies.

The modes of occurrence associated with rare earth elements (REEs) in bituminous coal sources were evaluated along with their potential recovery by calcination treatment at 600 ℃ followed by sequential extraction. The coal samples were collected from three different resources, i.e., Fire Clay, West Kentucky No. 13, and Illinois No. 6 seams. Sequential extraction tests indicated that the majority of REEs in the calcined samples occurred as metal oxides especially for the West Kentucky No. 13 calcined material (54% of total REEs), which is distinct from untreated coal and combustion byproducts from pulverized coal boilers reported in literature. In addition, heavy REEs were more likely associated with easily dissolvable forms (i.e., ion-exchangeable, carbonates, and metal oxides). The calcined samples were leached under weak acidic conditions with and without adding ammonium sulfate to recover REEs. About 13%, 24%, and 20% of the REEs were extracted from the Fire Clay, West Kentucky No. 13, and Illinois No. 6 materials, respectively, by leaching at pH 4.0. The addition of 1 M ammonium sulfate further increased the recovery values to 18%, 45%, and 32%, respectively. In addition, about 64% of HREEs were extracted from the West Kentucky No. 13 calcined material under this condition. As such, a significant portion of the REEs can be easily extracted from the samples obtained by calcining the coals at 600 ℃.

Numerous studies have been conducted describing the challenges with oxidized coal flotation. Common industrial collectors such as fuel oil are typically ineffective under natural conditions for the flotation of oxidized coal sources such as the Coalburg seam from the Central Appalachia coalfields of the U.S. Low plant flotation recovery values of less than 30% were realized in part due to the weak surface hydrophobicity and the clay coating that results from a reduced isoelectric point of the coal and low slurry pH values. Elevated pH values and the use of a model collector containing carboxylic groups improved flotation recovery values by nearly 45 absolute percentage points. Operating at pH values around 7.5 to disperse the clay slimes and provide more surface active dimer complexes produced from a carboxyl collector resulted in excellent flotation recovery values for a hard-to-float coal. Due to the frothing properties of the carboxyl collector, column flotation with froth washing was needed to provide product ash values as low as 7.5%.

Froth flotation is the most widely used method of separating fine coal, especially coal with coking properties. However, froth flotation is not efficient for treating idtrafine coal and coarse coal particles. Our previous fundamental test residts demonstrated that cavitation-generated nanobubbles around 700 tun in diameter can significantly improve coal and phosphate flotation performance over a wide particle size range. In this study, the effect of nanobubbles on the flotation performance of different particle size fractions of coal was investigated using a bank of 10-L flotation cells, a specially designed 50-nun inside diameter column and a 152-nun inside diameter column. It was found that nanobubbles that were either directly formed on or subsequently attached to the surface of coal particles during the froth flotation process improved flotation efficiency. The use of nanobubbles in a bank of mechanical cells flotation and column flotation significantly increased the flotation recovery at a given product grade. Nanobubbles increased the flotation rate constants of different coal particle sizes The presence of nanobubbles in flotation slurry could extend the lower and the upper particle size limits for effective coal flotation.

Coal cleaning is often conducted using wet physical separation processes such as heavy medium vessels or spirals at coal preparation plants to remove impurities such as ash, sulfur, and mercury. However, the resultant clean coal product still contains a significant amount of impurity due to the fact that impurities are not well liberated from coal particles ranging from several millimeters to inches in size at which wet cleaning processes take place. A cleaner coal product can be obtained if a dry process is avaialble to further clean pulverized and thus better liberated fine coal at the power plant prior to its combustion. In this study, a novel rotary triboelectrostatic separator (RTS) was investigated for its application to dry cleaning of fine coal samples acquired from the power plants in the state of Illinois. The proprietary RTS is characterized by an innovative high-efficiency rotary charger, charger electrification, laminar air flow, etc. Compared to existing triboelectrostatic separators, the RT...

The CAVEX dense medium cyclone (DMC) was developed in the later part of the 1990s as a result of the expertise developed by Weir engineers in slurry pumping. The inlet area of the cyclone is designed to minimize turbulence and to reduce wear at the feed entry point, which provides more energy for particle separation at a given feed pressure. A parametric study was performed on a 150 mm diameter unit to quantify separation efficiency as a function of feed pressure, apex diameter, medium density, and cone angle. The added energy in the cyclone was confirmed by comparing the stability of the medium in the CAVEX unit with that provided by a common commercial unit having the same dimensions. A 500 mm unit was installed in parallel with an identically sized industrial unit in an operating preparation plant treating 12 × 1 mm coal. The separation efficiency values achieved by the CAVEX DMC were found to be higher than those obtained by the standard industrial unit and the amount of improvement increased with a d...

This article presents an experimental study of rotary tribo-electrostatic separation (RTS) as an alternative approach for beneficiating steam coal mainly used in South African power plants. An RTS with an octagonal charger developed at the University of Kentucky, USA has been used in beneficiating two types of South African coals containing nearly 37% and 32% ash, respectively. System parameters, such as applied charger potential, separation chamber voltage, rotation speed of the copper-plated rotor, and splitter distances were investigated for their effects on the separation performance. It was found that better separation was observed at 5000 rpm rotation speed, no applied potential to the charger, and 25 KV separation voltage. The RTS process reduced the ash content of the −177 µm coal fraction by nearly 14.9% for the Klipfontein coal and 12.2% for the Liketh Townlands coal, with corresponding combustible recovery values of 10.7% and 8.9%, respectively. Total sulphur content was also reduced from 2.1% ...

The coal industry usually discards fine-size (−150 microns) coal because of its high-moisture content and handling problems. One avenue for utilization is to either pelletize or briquette this material. However, industry has not adopted this route due in large part to significant drying and binder costs. In an effort to reduce these costs, compacting and briquetting studies were conducted to determine the effect of combining a coarse (1.18 × 0.15 mm) spiral separator product with a fine coal flotation product (−150 microns), with and without adding sawdust. Maximizing the packing density of the coal and wood waste mixture could potentially reduce the binder requirement by minimizing the void space as well as reducing shipping costs. Accordingly, work reported here focused on evaluating the impact of the particle-size distribution of different blends of fine and coarse coal, with and without sawdust and/or binder. The modified Proctor density of compacted blends along with the porosity and compressive stre...

In response to concerns regarding cost and future supply of magnetite, a study has been performed to evaluate the potential of alternative materials that can be used to generate a dense medium for coal-cleaning applications. Alternative materials included waste steel slag, fine sand, and high-density material existing in run-of-mine feed. Under certain conditions, each of the materials tested provided separation efficiencies that meet industrial standards including probable error values of around 0.03. Dense medium derived from the non-magnetite sources achieved organic efficiency values exceeding the 95% level over a medium density range from 1.3 RD to 1.6 RD with lower than normal density offsets. Lower cost, coarse magnetite provided excellent separation efficiencies when medium density values above 1.6 RD are sufficient to meet product grade requirements.

Various technical and economic aspects relating to the briquetting of fine coal with sawdust have been evaluated with the results for two segments of that study presented here: binder and briquetting-parameter evaluations. Approximately 50 potential binder formulations were subjected to a series of screening evaluations to identify three formulations that were the most cost effective for briquetting fine coal with sawdust. Two of the binders, guar gum and wheat starch, were selected as most suitable for the pulverized coal market while the third formulation, lignosulfonate/lime, was targeted for the stoker market. Following binder selection, a number of briquetting parameters including binder and sawdust concentration, sawdust type, briquetting pressure and dwell time, coal and sawdust particle size, clay content, moisture content, and cure temperature and cure time were evaluated. Briquetting pressure and dwell time have the least impact while binder and sawdust concentrations, sawdust type, and curing c...

Froth flotation is widely used in the coal industry to clean −28 mesh (0.6 mm) or −100 mesh (0.15 mm) fine coal. A successful recovery of particles by flotation depends on efficient particle-bubble collision and attachment with minimal subsequent particle detachment from bubble. Flotation is effective in a narrow size range, nominally 10–100 µm, beyond which the flotation efficiency drops sharply. A fundamental analysis has shown that use of picobubbles can significantly improve the flotation recovery of particles by increasing the probability of collision and attachment and reducing the probability of detachment. A specially designed column with a picobubble generator has been developed for enhanced recovery of fine coal particles. Picobubbles were produced based on the hydrodynamic cavitation principle. Experimental results have shown that the use of picobubbles in a 5-cm diameter column flotation increased the combustible recovery of a highly floatable coal by up to 10% and that of a poorly floatable c...

The potential of dry cleaning coal of varying ranks using a pneumatic table concentrator has been evaluated as part of an ongoing investigation. The evaluation has been performed at several sites throughout the United States where coal is extracted from surface open cast, highwall and underground operations as well as from coarse reject. The treated coals varied in feed ash content (i.e., 7–70%). Regardless of the mineral matter type, pure rock removal into the reject stream was achieved in all applications with little coal loss. Field data obtained when treating 50 × 6 mm run-of-mine bituminous coal indicate that 70–90% of the >2.0 Relative Density (RD) rock can be rejected. As a result, a clean product having acceptable market quality was generated from several coal sources including lignite, sub-bituminous, and bituminous coals. This article provides an overview of data from recent field testing of the dry air table technology and discusses the potential implementation strategy for the various sites ev...

A two-stage classification circuit using 150 mm diameter gMax cyclones was installed and evaluated in a coal preparation plant in an effort to achieve a clean coal product without the use of froth flotation. Particle size separations of around 37 μm were achieved while limiting ultrafine bypass to less than 10% in the circuit underflow stream. As a result, approximately 81% of the ash-bearing material in the circuit feed was rejected to the circuit overflow stream. The feed ash content was reduced from around 50% to values in the range of 22–30% in the circuit underflow stream with a mass recovery of about 30%. Further reductions in the coarse product ash content were limited due to the particle density effect and the remaining presence of a significant quantity of high-ash slime material in the coarse product. The typical D 50 for the coal particles was 40 μm while the estimated value for mineral matter was 17 μm. Based on the findings of the study, the use of classification to recover a low-ash, coarse fraction in the feed of a fine coal circuit is limited by the density effect regardless of the ability to eliminate ultrafine bypass.

The conventional circuit fine coal circuit typically incorporates spiral concentrators to treat nominal 1 × 0.15 mm material. Pilot and in-plant studies have been performed to determine the operating parameter values needed to achieve optimum separation performances when extending the lower limit of a conventional spiral concentrator to 44 μm. Based on experimental and empirical data, a feed solids concentration of about 12% by weight is required with a feed rate of around 60 l/min per start. Under these conditions, 60% of the ash-forming minerals and 48% of the sulfur was rejected from the 210 × 44 μm size fraction of a given coal source.

Hindered-bed separators are recognized as low-cost, high-capacity devices for both classification and density separation; however, since their inception there have been few significant advances in the fundamental technology. Recently, Eriez has shown through modeling and pilot-scale testing that the innovative approach to feed presentation offered in the CrossFlow teeter-bed separator provides improved metallurgy when compared to traditional hindered-bed classifiers or single-stage coal spirals. This design feature prevents excess water from entering the separation chamber and disrupting the overall fluidization flow rate within the teeter zone. Most recently, a side-by-side industrial-scale evaluation verified that this technology improves overall efficiency and simultaneously reduces the separation cut point. With regards to coal processing, data from full-scale units indicate that the CrossFlow offers good separation efficiency, high unit capacity and metallurgical results consistent with laboratory- and pilot-scale separators.

The selectivity in flotation columns involving the separation of particles of varying degrees of floatability is based on differential flotation rates in the collection zone, reflux action between the froth and collection zones, and differential detachment rates in the froth zone. Using well-known theoretical models describing the separation process and experimental data, froth zone and overall flotation recovery values were quantified for particles in an anthracite coal that have a wide range of floatability potential. For highly floatable particles, froth recovery had a very minimal impact on overall recovery while the recovery of weakly floatable material was decreased substantially by reductions in froth recovery values. In addition, under carrying-capacity limiting conditions, selectivity was enhanced by the preferential detachment of the weakly floatable material. Based on this concept, highly floatable material was added directly into the froth zone when treating the anthracite coal. The enriched froth phase reduced the product ash content of the anthracite product by five absolute percentage points while maintaining a constant recovery value.

Coal extraction typically results in the recovery of pure rock that ranges from small to very large quantities, depending on seam thickness, existence of above-seam draw rock or in-seam partings and other characteristics. The removal of pure rock may allow for the recovery of three times the amount in middling coal particles, which has significant economic benefits. In an industrial example, a 150% increase in revenue was realized from deshaling a low-ash, run-of-mine coal and then blending it with a processed clean coal product. Deshaling is the process of removing relatively pure rock from coal, which normally involves a high-density separation in a gravity-based process. The removal of the relatively pure rock near the point of extraction has the potential to further enhance the economics of an operation due to reduced-materials handling and refuse storage costs.

Equipment wear and corrosion have significant adverse impacts on the operating cost and efficiency of mineral processing plants. In this paper, advanced surface-enhancement technologies are investigated to decrease the wear and corrosion rates of metal surfaces. Enhanced metal surfaces are achieved through surface conversion, enrichment and coating by use of an advanced high-density infrared (HDI) plasma arc lamp. Studies were done to determine the effects of process parameters on surface properties and identify optimum conditions. The micro structure of the enhanced metal surfaces was characterized by SEM, EDS and microindentation. Laboratory wear testing was carried out to investigate the wear performance, and a viable coating candidate was determined.

Equipment wear and corrosion have significant adverse impact on the operating cost and efficiency of mineral processing plants. In this study, advanced surface enhancement technologies were investigated to improve the wear resistance of metal surface of mineral processing equipment. Enhanced metal surfaces were achieved through surface conversion, enrichment, and coating by use of advanced high-density-infrared (HDI) technology. Attempts were made to determine the effects of process parameters on surface properties and corresponding optimum conditions. The microstructure of the enhanced metal surfaces was characterized by SEM, EDS, and microindentation. Laboratory dry sliding wear testing was carried out to investigate the wear performance and the viable coating candidate was determined.

There is a significant amount of energy in the waste generated by the coal and timber industries. In the coal industry there is an energy loss mainly due to the discharge of fine coal to the waste impoundment. Despite the fact that the fine coal has the greatest cleaning potential, dewatering, handling and transportation concerns prevent its use as product in many cases. Likewise, sawdust generated from wood production has energy value but is only used as industrial boiler fuel when the transportation distance is relatively short. Wood waste is often discarded into cumbersome landfills due to uneconomical transportation alternatives. Recent research found that these two waste materials can be combined to provide a high-energy fuel, containing around 30.2 MJ/kg (13,000 Btu/lb), that can be easily handled and transported to end users.

The improved selectivity between particles of varying degrees of hydrophobicity in flotation froths has been well documented in literature, especially in the deep froths utilized in flotation columns. The phenomenon is believed to be due to the selective detachment process whereby the least hydrophobic particles are released from the bubble surface upon bubble coalescence. To quantify the selective detachment process, column flotation experiments were performed under various operating conditions that provided varying amounts of reflux between the froth and collection zones. Entrainment was eliminated by the use of relatively coarse 250 × 75 micron material. The flotation column incorporated the ability to provide instantaneous stoppage of the process streams and separation between the collection and froth zones after ensuring steady-state operation of the column. The samples collected from the two zones and process streams were evaluated to quantify the flotation rate distribution of the particles comprising each sample. The flotation rate was used as an indicator of the degree of hydrophobicity and thus a relative measure of the binding force between the particle and bubble in the froth zone. The flotation rate data was used as input into well known flotation models to obtain the froth zone recovery rate and the quantity of material that refluxes between the collection and froth zones.

Coal preparation plants treat a majority of the run-of-mine coal using various gravity processes, which are known for their low cost and high process efficiency values. Due to the inefficiencies of conventional gravity-based processes in treating −1 mm coal, froth flotation processes are traditionally employed at a relatively high cost. To address cost and separation performance issues, several enhanced gravity separation technologies have been developed and evaluated which may allow effective gravity-based separation to particle sizes as small as 25 μm. The Altair centrifugal jig is an enhanced gravity technology, whose suitability for fine coal cleaning has been demonstrated through an in-plant study as reported in this publication. A relatively low specific gravity cut-point of 1.50 with a probable error value of 0.11 over a wide particle size range 1 mm ×45 μm is indicative of the excellent separation performance achievable from the Altair jig. Tests performed with and without ragging material were performed with the goal that the latter would provide enhanced throughput capacities. However, although the performance was close to the theoretical limits over the broad range of product grades generated, the no-ragging experiments resulted in a significant loss in coal recovery under the given conditions. Overall, the centrifugal jig achieved 80% ash rejection and 50% total sulfur rejection while recovering nearly 80% of the combustibles.

Classifying cyclones are a widely used device for achieving ultrafine particle size separations in industrial applications. However, inherent deficiencies include particle density effects in multi-component suspensions and ultrafine particle short-circuiting to the underflow stream due to hydraulic entrainment. A detailed in-plant test program has been conducted to evaluate the benefits of tangential water injection into the apex portion of a classification cyclone for the removal or minimization of the ultrafine by-pass. Based on models developed using the test results, ultrafine bypass can be reduced by 50% or more while maintaining the same corrected d 50 value through complex manipulation of operating and geometric parameters. In a subsequent test, the by-pass was reduced from 15% to about 7% while maintaining a d 50(c) value of 26 microns. However, the magnitude of the benefit in by-pass reduction is subject to the geometric parameters of the conventional classifying cyclone.

Because of increasing production of fine coal during mining over the past century and because of inefficient fine-coal recovery technologies, a vast reserve of high-quality coal now exists in refuse ponds. A novel fine-coal circuit, consisting of a hindered-bed classifier, an enhanced gravity concentrator and a flotation column, was evaluated for the recovery of fine coal from refuse ponds. The treatment of a pond derived from Pittsburgh No. 8 seam coal resulted in the production of a premium fuel containing less than 5% ash and a calorific value of about 30,170 kJ/kg with 60% mass yield. Results from the treatment of two refuse pond materials are presented in this paper.

Dense-medium separators have proven to be the most efficient processes for removing the undesirable gangue material from run-of-mine coal. The application of high-pressure feed injection into dense-medium cyclones to provide an elevated centrifugal force has recently been found to allow efficient separation performances for the treatment of fine coal (i.e., A test prograrn has been conducted to evaluate the potential cleaning of 1000 x 44 μm fine coal using dense-medium in an enhanced gravity separator (EGS), which mechanically generates enhanced gravity field. Test results indicate that the use of dense-medium in an EGS resulted in an 8% weight unit increase in mass yield compared to the use of water-only for the treatment of an easy-to-clean Illinois No. 6 fine coal sample. For a difficult-to-clean coal, the mass yield improvement was significantly greater at nearly 20% by weight. From the treatment offour different coal samples, organic efficiency values greater than 90% were obtained over the entire range of product quality values. These findings are reflective of the highly efficiency, low density separations provided by the dense-medium as indicated by probable error values below 0.05.

A comparative evaluation has been conducted using three leading advanced flotation technologies, which utilize different types of bubble-particle attachment environment. Based on a statistical evaluation of the test data the Packed-Column technology, which provides a near plug-flow flotation environment due to the presence of corrugated packing material in the cell, produced the best separation performance due to its ability to support an extremely deep froth zone. However, because of the absence of an air sparging system and the consequent larger bubbles, the froth carrying capacity was the mininum with the Packed-Column technology. On the other hand, the throughput capacity achieved by the Jameson Cell technology, which has a self air-inducing co-current system that provides an intimate bubble-particle attachment environment characterized by an extremely high air fraction and ultrafine bubbles, was found to be maximum. The Microcel™ technology achieved its maximum carrying capacity while providing a high energy recovery with a reasonably low reagent consumption.

Recent research has shown that enhanced gravity separators provide the opportunity to effectively clean 1 mm x 37 micron coal. A detailed experimental program was performed on a continuous Falcon Concentrator, which was found to achieve separations at mass throughput capacity values in excess of 75 tonnes/hour. The apparent particle separation density for 1 mm x 75 micron coal was about 1.60 with relatively high efficiency ( Ep = 0.12). Typical ash and sulfur rejection values of 85% and 70%, respectively, were achieved while recovering 85% of the combustible material. Metallurgical and efficiency data from the evaluation are presented in this publication.

An enhanced gravity separator (EGS) commercially known as the Falcon Concentrator has been evaluated for its ability to treat fine coal. From a comparison with other EGS technologies, the magnitude of the applied centrifugal force was found to determine the overall separation performance and throughput capacity. Using the relatively high g-forces supplied by a continuous 25-cm diameter Falcon unit, a low density cut point of 1.6 was achieved for the 210 x 37 μm size fraction of a fine coal sample which is significantly lower than that achieved by other EGS units. As a result, ash rejection values between 60% to 75% have been obtained from the treatment of several fine coal samples while recovering greater than 85% of the combustibles, which corresponds to an organic efficiency of about 90%. In addition, the sulfur rejection values achieved on various particle size fractions comprising a -⊥ mm coal sample were superior to that achieved by a spiral concentrator and a flotation column. Due to an insufficient centrifugal force, the metallurgical performances achieved on the basis of ash rejection for the −37 μm size fraction of all coal samples were insignificant. The ability to achieve efficient, low gravity cut points on fine coal and the operational simplicity of the Falcon Concentrator indicate potential for near term application in coal processing plants.

Past studies have found that the froth flotation process is efficient in recovering heavy middling particles, which negatively affects the recovery-grade curve achieved for fine coal recovery. Experimental and theoretical results indicate that the recovery of middling particles can be reduced from 25% to 10% using flotation columns in a multi-stage cleaning approach, thereby improving the overall separation performance. Tests conducted on a −48 mesh coal sample found a 7% improvement in mass yield using a rougher-cleaner column circuit while producing a flotation concentrate containing 8% ash. Similarfindings were obtained from the treatment of a −65 mesh coal sample using a different flotation column technology. To fundamentalty evaluate the use of multi-stage cleaning, a continuous column model has been developed which incorporates selective and non-selective froth drop-back, and feed component flotation kinetics. For kinetic rate limiting conditions, separation performance was found to substantially improve with the use of multi-stage cleaning, which agrees with the experimental findings. Under carrying capacity conditions, the improvement is a function of the degree of selectivity in the detachment process. Economic justification of the use of multi-stage column treatment must be based on enhancements in product grade and/or mass yield.

Froth flotation is now the most commonly used method for cleaning fine-coal fractions in coal-preparation plants. Past research and industrial practice have demonstrated that froth flotation achieves excellent separation efficiencies for fine coals having only a small fraction of middling particles. However, the efficiency of froth flotation decreases sharply as the concentration of middling particles increases. This is due to the flotation of high-ash particles that have a small amount of hydrophobic coal present at the surface. Because gravity-based processes are much more efficient for the treatment of middlings, the application of centrifugal washing for fine-coal cleaning was studied with an emphasis on the primary cleaning of fine-circuit feed. Results are shown of a parametric study that used a semibatch Falcon centrifugal concentrator. The results of the study revealed that separation performance is a function of particle size, bowl geometry, feed solids content and feed rate. 10 refs., 5 figs.