Sulfur Rejection Research Articles

Separation of pyritic sulfur from ultrafine coal by pyrolysis & wet high magnetic separation

ABSTRACT Coal plays a major role in the economic development of many countries, especially in metallurgical industries and conventional power generation plants. The removal of sulfur from coal has recently become even more critically important. Existence of sulfur compounds in coal limits its industrial applications due to environmental as well as technical problems. Thus, high sulfur coal is usually upgraded by desulfurization through physical, chemical, and biotechnological processes. In this study, lowering of pyritic sulfur took place after thermal treatment at 500–700°C (pyrolysis) using a wet high-intensity magnetic separation process. The samples were subjected to size reduction to −25 µm and characterized with X-ray diffraction (XRD), proximate and ultimate analysis, and differential thermal analysis (DTA). Then, the experiments were performed based on central composite rotatable design (CCRD) and response surface methodology (RSM) after thermal treatment of coal containing 3.30% total sulfur. Statistical analysis was performed to examine the significance of the effect of different operating parameters. A maximum sulfur rejection of 90.10% with the lowest sulfur content of 0.59 was obtained at 8.38% solid concentration, 4.59 g/min feed rate, and 7810 gauss magnetic field intensity. The confirmation tests at the optimum conditions produced 0.54–0.56% total sulfur with 90.25–92.33% sulfur rejection.

Particles motion simulation and application exploration of three-stage cone water-only cyclone

ABSTRACT In this study, the particle motion simulation and application exploration of three-stage cone water-only cyclone (TWOC) were performed. The discrete phase model was applied to simulate the particles motion during the separation process of TWOC. The particle distribution curve of light products of TWOC obtained by the simulation is well consistent with that of the validation experiment, demonstrating the credibility of the particles motion simulation. The separation characteristic of this cyclone was clarified. The cone part is the main separation region, and the axial separation position of particles is deepened with the growth of the particle density. The “T-F” flow, consisting of the TWOC separation and flotation, was designed to separate the coal-slime efficiently. The separation experiments indicate that the pyrite sulfur rejection rate and the ash rejection rate of the NO.1 experiment are all greater than 79.00%. The “C-T-F” flow, consisting of the crushing process and the “T-F” flow, was designed to improve the desulfuration performance of the high-sulfur lump-coal. The crushing size was determined as 3 mm, and the pyrite sulfur rejection rate of the “C-T-F” flow is greater than 90.00%. The superiorities of the two separation flows promote the practical application of TWOC.

Designing coal preparation circuit for cleaning high ash and high sulfur Punjab (Pakistan) coal to meet end-user requirements

ABSTRACT Modern coal preparation plants are designed to achieve maximum overall clean coal yield with an acceptable amount of impurities to meet end-user requirements. LIMN is commonly used to design and simulate coal processing circuits based on size and density distribution data of coal. In this study, particle distribution data for size, density, and floatability were established for Punjab coal after crushing it to 38 mm. Punjab coal contains high amounts of ash (34.70%) and sulfur (8.34%). Float-sink tests conducted on several size fractions disclosed that 16% target ash could be achieved at an overall yield of 63% with ash and sulfur rejection values in the range of 72.63–94.18% and 73.01–88.45%, respectively. However, the sulfur limit of 1.0% would be difficult to meet. Dell release analysis performed on – 0.25 mm fraction revealed poor flotation response of Punjab coal. Several process flowsheets were designed and simulated using LIMN to generate data for yield, ash and sulfur. End-User Beneficiation Number (EBN) was introduced and utilized for determining the best processing option that can provide maximum yield while meeting the quality requirements of the end-user. Economic analysis involving CAPEX, OPEX, and payback period validated the merit of EBN.

Cleaning of high pyritic sulfur fine coal via flotation

Large quantities of fine coal have been produced during mining and processing activities of coals. Use of these fine coals after cleaning is important in terms of recovering of economically valuable energy source and prevention of environmental pollution. In the present study, amenability of a high ash-sulfur coal sample to flotation process was investigated. Coal particle size and collector dosage were selected as variables. Increasing coal particle size affected sulfur and ash rejections adversely. On the other hand, more combustible matter was recovered at coarser particle sizes. Maximum combustible recovery, ash rejection, pyritic sulfur rejection and sulfate sulfur rejection were achieved to be 82.56%, 82.84%, 90.04% and 81.45%, respectively. Polished section examinations and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis confirmed the rejection of significant level of ash and sulfur from the coal.

Purifying hazardous elements in Yiluo fine coal by dry high-gradient magnetic separation

ABSTRACTDry magnetic separation of different sizes of Yiluo fine coal was carried out to purify deleterious elements. It was found that dry magnetic separation can effectively reduce a variety of harmful trace elements in raw coal. Regardless of coal particle size, the rejection efficiency of Cd, Hg, As, and Mn is 39.07%–57.16%, 33.89%–49.94%, 30.49%–47.52%, and 27.10%–52.28%, respectively. Relatively high removal efficiency of Pb, Cu, and Se is obtained, which is 26.23%–35.32%, 27.25%–39.41%, and 25.05%–32.84%, respectively. Moreover, the results show that sulfur rejection of 33.87% and reductions in ash of 41.19% are obtained with particle size of 80–120 mesh, while the combustible recovery reaches 83.68%.

Role of recovery sieve size in upgrading of fine coal via oil agglomeration technique

Although recovery sieve size has vital importance for the success of the oil agglomeration technique for upgrading of fine coals, its effect on the process has been generally omitted in previous studies. Therefore, the present study was undertaken to fill the gap in this area and draw attention to the importance of recovery sieve size. An oxidized fine coal sample was subjected to oil agglomeration process and effect of separation sieve size on the success of the process with regard to combustible recovery and ash–sulphur rejection was investigated. 72.90% of ash, 90.90% of pyritic sulphur, and 87.75% of sulphate sulphur were removed from the coal with combustible recovery of 76.53%. Increase in recovery sieve size had positive effect on removals of ash and sulphur. Recovery sieve size was found to be extremely important for the performance of the process in that combustible recovery reduced sharply with increasing sieve sizes. It was concluded that recovery sieve size should be the same size with the top feed size.

Effect of particle size on cleaning of high-sulphur fine coal by oil agglomeration

An oxidized fine coal sample with high sulphur content was subjected to oil agglomeration process in order to determine the effect of coal particle size (−0.5, −0.25, −0.125mm) on the performance of the process at different oil dosages (5, 10, 15, 20%). Maximum combustible recovery, ash rejection, pyritic sulphur rejection and sulphate sulphur rejection were achieved to be 75.23%, 97.56%, 99.23% and 98.15%, respectively. Increase in coal particle size affected the ash and pyritic sulphur rejections adversely. However, higher combustible recovery was obtained at coarser coal particle sizes. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis and polished section examination of feed coal, clean coal and tailings support the success in rejection of considerable amount of ash and sulphur sources from the coal. Microscopic examination also showed an increase in agglomerate size with increases in coal particle size and oil dosage.

Increasing coal quality by oil agglomeration after ultrasonic treatment

In the present study, oxidized and high sulphur fine coal was subjected to oil agglomeration process after ultrasonic treatment. Power and time of ultrasonic treatment were selected as variable. Combustible recovery, ash rejection, pyritic sulphur rejection, ash separation efficiency and pyritic sulphur separation efficiency of the agglomeration process with and without ultrasonic treatment were determined. In addition, calorific value of the clean coal produced by agglomeration process was measured. In the agglomeration process without ultrasonic treatment, calorific value, combustible recovery, ash rejection and pyritic sulphur rejection were obtained to be 6518kcal/kg, 63.78%, 75.19% and 92.64%, respectively. Ultrasonic treatment enhanced the performance of oil agglomeration process. By the application of ultrasonic treatment before agglomeration process, calorific value, combustible recovery, ash rejection and pyritic sulphur rejection were increased to maximally 6939kcal/kg, 66.13%, 87.24% and 97.44%, respectively. In the study, changes on the surface structure of the coal after ultrasonic treatment were also examined. Particle breakage, formation of cracks and cavities, and altering of surfaces into fresh-clean surfaces were observed. Increase in time and power of ultrasonic treatment had slight positive effect on ash and pyritic sulphur rejections.

Use of ultrasonic emulsification in oil agglomeration for coal cleaning

Emulsification is one of the most important stages in coal cleaning by using oil agglomeration. Reduction in size of oil droplets and increase in total surface area provided by emulsification result in coverage of entire coal surface with oil, thereby reducing the oil dosage and affecting rapid agglomeration. Ultrasonic emulsification which has been used recently in many sectors such as pharmaceutical, cosmetic, food, chemical industries may be a beneficial and promising operation in oil agglomeration of coal. In the present study, an oxidized fine coal with high ash and pyritic sulfur content was cleaned by oil agglomeration process after ultrasonic emulsification and the effects of power level and time of ultrasonic treatment on the performance of the agglomeration process were determined. Ash and pyritic sulfur rejections that was found to be 50.38% and 85.28%, respectively, at agglomeration test without ultrasonic emulsification was increased to maximally 56.89% and 88.69% by using ultrasonic emulsification before agglomeration. While increase in power of ultrasonic treatment did not affect ash and pyritic sulfur removals considerably, ultrasonic treatment time had positive affect at higher power levels. Contrary to expectations, ultrasonic emulsification affected the combustible recovery and separation efficiencies adversely.

Desulphurization and deashing of oxidized fine coal by Knelson concentrator

In order to efficiently clean fine coals, enhanced gravity separators, such as Knelson concentrator, have been developed. Although the Knelson concentrator has not been applied yet in industrial scale for fine coal cleaning, different investigations have been conducted in recent years. In this study, an oxidized coal with high sulphur was subjected to cleaning process by Knelson concentrator after being classified into the size fractions of −106μm, −300+106μm and −500+300μm. Experimental variables were coal particle size, bowl speed and fluidizing water pressure. Maximum combustible matter recovery, pyritic sulphur rejection, and ash rejection were achieved to be 99.13%, 91.60%, and 60.94%, respectively. The highest separation efficiency for the pyritic sulphur and ash was determined to be 67.91% and 39.53%, respectively. These corresponded to the removal of 70.45% of pyritic sulphur with combustible matter recovery of 97.46% and removal of 45.96% of the ash with combustible matter recovery of 93.57%. Coal particle size, bowl speed and fluidizing water pressure were determined to affect the separation performance with a close interaction between them.

Effect of pH, air velocity and frother concentration on combustible recovery, ash and sulphur rejection using column flotation

This study shows the effect of pH, air velocity (Jg) and frother concentration (FC) on combustible recovery, ash and sulphur rejection for four Colombian bituminous coals: Cerrejón (Guajira), Nechí (Antioquia), Jagua (Cesar) and Guachinte (Valle del Cauca) using column flotation. For the experimental development a 23 factorial central composite experimental design was used. The coal samples were processed in a pH range of 4 to 10, Jg in the range of 0.7 to 2.1cm/s and FC in the range 2.1 to 6.3kg/t. Results showed that Cerrejón and Jagua coal presented combustible recovery higher than 50%, (57.6% and 54.7% respectively), at acidic pH (pH 5), with Jg of 1cm/s and 3.2 to 4.2kg/t of FC, respectively, while Guachinte coal presented a maximum combustible recovery of 56.4% at alkali pH (pH 9), Jg of 1cm/s and 3.2kg/t of FC and Nechí coal obtained the highest combustible recovery (46.8%) at pH 7 with 2.1kg/t of FC and Jg of 1.4cm/s. For Guachinte and Nechí coal ash and sulphur rejection were high (90.3% and 88.5%) and (71.3% and 73.8%) respectively at acidic conditions (pH 4 and 5) with Jg in the range of 1.4 to 1.8cm/s, whereas Cerrejón and Jagua coals exhibited high values of ash (85.4% and 94.7%) and sulphur rejection (81.1% and 85.1%) at alkali conditions (pH 9 and 10) with Jg in the range of 1.0 to 1.4cm/s. In general, it was observed that Cerrejón and Jagua coals presented higher values of combustible recovery under acidic conditions than Nechí coal and Guachinte coal.

Beneficiation of Fine Coal Using the Air Table

The increased mechanization in the underground coal-mining industry has increased the volume of fine size coal and waste (refuse) in the mined coal. Processing of run-of-mine (ROM) coal is generally done using water away from mine and in some cases the coal has to be transported a long distance to the preparation plant. Dry processing of coal can be economical as it will not utilize water and no dewatering or drying of the product will be required. The goal of this study was to develop a dry separation process for processing of coal finer than 6.3 mm . The coal sample from a mine located in Western Kentucky was used for the study. Statistical design experiments were conducted to assess the effects of operating parameters of the dry separator on product yield for a given ash content. Tests conducted with 6.3 × 3.35 mm (and 3.35 × 1.4 mm size fractions showed that the air table was able to reduce the ash from 27% to 10%–12% ash with a clean coal yield of 75%–80%. The ash rejection was about 77%–80% with a combustible recovery of around 95% indicating excellent separation efficiency. The pyritic sulfur rejection was 43%. The heat content of the 6.3 × 1.4 mm (1/4 inch × 14 mesh) coal fraction increased from 23997 kJ/kg to 29595 kJ/kg. The pyritic sulfur was reduced by about 33% with a product yield above 80%.

Performance Optimization of the FGX Dry Separator for Cleaning High-Sulfur Coal

The main goal of the present study was not only to deshale (remove pure rock) raw coal extracted from Illinois mines but also to assess the maximum ash separation efficiency and sulfur rejection achievable using the FGX Dry Separator for cleaning raw coals of varying cleaning characteristics. A Model FGX-1 Dry Separator with feed throughput capacity of 10 tph was extensively tested at the Illinois Coal Development Park using multiple coal samples having distinctly different cleaning characteristics. Statistically designed experimental programs were conducted to indentify critical process variables and to optimize FGX Dry Separator performance by systematic adjustments of critical process variable parameters. The coal-cleaning performance of the FGX Dry Separator was evaluated for the particle size range of 4.75–63.5 mm in most cases, although FGX Dry Separator feed consisted of nominal 7minus;63.5 mm run-of-mine coals. Deck vibration frequency, longitudinal deck angle, feeder frequency, and baffle plate height were identified as critical process variables for the FGX Dry Separator. The best cleaning performance obtained from the FGX Dry Separator was described by specific gravity of separation (SG50) and probable error (Ep) values of 1.98 and 0.17, respectively. For a relatively easy-to-clean coal (having a Cleaning Index of 0.72), only about 0.42% of the clean coal (i.e., 1.6 float fraction) present in the feed was lost to the tailings stream. For a relatively difficult-to-clean coal (having a Cleaning Index of 0.53), about 0.98% of the clean coal present in the feed was lost to the tailings stream. The positive impact of having fine materials in the FGX feed stream was also noted in this study. A modified log-logistic partition model was developed using experimental data reported in literature and validated using new experimental data generated in this study. The results showed that this model could be effectively used to predict the FGX Dry Separator coal-cleaning performance.

Experimental studies on hydrophobic flocculation of coal fines in aqueous solutions and flotation of flocculated coal

Hydrophobic flocculation of fine bituminous coal particles in aqueous solutions under mechanical conditioning and without any surfactants has been experimentally studied through the measurements of aggregative efficiency, zeta potential and contact angle in this work. The results have shown that the hydrophobic coal fines strongly aggregated in a wide pH range (3.5?9.5), even though the particle surfaces were highly charged. This hydrophobic flocculation closely correlated with particle hydrophobicity, having a stronger aggregation for more hydrophobic coal fines. It reaches the maximum degree only if a sufficient kinetic energy is applied to the coal slurry. Also, it has been found that the hydrophobic flocculation could be greatly enhanced by adding a little amount of nonpolar oil. In addition, the flotation of flocculated coal fines was tested on the fine Prince coal for the deep elimination of ash and pyrite from coals in this work. An ultraclean coal with 1.3% ash remaining was produced with 87% combustible recovery. The ash rejection and pyritic sulfur rejection were about 93% and 66%, respectively.

Cyclo-microbubble Column Flotation of Fine Coal

Cyclo-microbubble flotation column (CMFC) is an advanced column flotation technology for fine coal cleaning developed by China University of Mining and Technology (CUMT). It combines cyclone separation with column flotation to enhance pyritic sulfur rejection and separation efficiency. A specially designed external bubble generator is employed to efficiently precipitate fine bubbles on particle surface. A set of screen plates inside the column produces nearly plug-flow condition, which is preferred for flotation process. The CMFC technology has been successfully employed to recover fine coal from discarded waste ponds and replace conventional mechanical cells. Typical commercial testing results are described and analyzed. The column is very effective in cleaning particles down to 45 μm. Laboratory- and pilot-scale testing of CMFC has also demonstrated that the CMFC process can produce a superclean coal product of 1.5∼1.6% ash content from a 9.8% ash feed.

Altair jig: an in-plant evaluation for fine coal cleaning

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.

An experimental investigation on column flotation circuit configuration

The integration of flotation columns into existing flotation circuits of conventional mechanical cells is becoming increasingly common in industry. Nevertheless, the study of column circuit configuration is scarce and problems relating to the selection of the appropriate circuit have not been addressed. Various column circuits for flotation of coal fines have been experimentally examined and theoretically analyzed in the present study to reveal the effect of circuit configuration on the overall circuit performance evaluated using separation efficiency and separation curves. The flotation circuits examined in this study included single–stage (SS), rougher–scavenger (RS), rougher–cleaner (RC), rougher–scavenger–scavenger cleaner (RSSC), rougher–scavenger–cleaner (RSC), and rougher–cleaner–recleaner (RCRC). The flotation tests were carried out with the Pittsburgh No. 8, Illinois No. 6 and Upper Freeport coal samples that were precleaned using conventional flotation. It has been demonstrated that the overall separation efficiency of the column flotation system was rather insensitive to circuit arrangements due to the unique characteristics of the unit flotation column distinguished from the conventional flotation cell. The flotation separation efficiency changed with ash or sulfur rejection and the Pittsburgh No. 8 and Illinois No. 6 coal samples showed a stronger dependence of separation efficiency on ash rejection than the Upper Freeport coal sample. The experimental results were shown to be in good agreement with theoretical circuit analysis conclusions.

Desulfurization of coal by microbial flotation in a semicontinuous system

A microbiological flotation system that facilitates the removal of pyritic sulfur from coal is reported. The chemoautotrophic bacterium Thiobacillus ferrooxidans is utilized as a pyrite-selective biological surfactant that augments the flotation-mediated rejection of pyritic sulfur. The bacteria were produced by continuous cultivation in 50-L tanks, where it was possible to produce 3.8 × 1012 cells/day when the dilution rate was optimal and the pH was 1.6, a condition that prevented ferric precipitation. Prior to flotation, the bacteria were mixed with coal, which allowed cells to selectively adhere to the pyrite. The adherence of bacteria made the pyrite surfaces more hydrophilic, which facilitated separation by increasing the tendency of pyrite to sink, while the hydrophobic coal floated. When a synthetic coal/pyrite mixture (10.1% sulfur) was subjected to microbial flotation, the level of sulfur rejection was stable during the entire period of operation (i.e., one hour). In the absence of bacteria, flotation reduced the sulfur content of a coal/pyrite mixture to within the range of 6.7% to 7.4%, with the combustible recoveries ranging from 93.9% to 99.0%. In the presence of bacteria, the sulfur content of cleaned coal was in the range of 1.3% to 1.4%, with the combustible recoveries ranging from 80.8% to 86.3%. Bacterial activity accounted for 57% of the total sulfur rejected through flotation. The desulfurization capacity of the system was further confirmed by testing it with run-of-mine coal. The pyritic sulfur content was reduced from 2.9% in the feed coal to around 1.2% in the froth product, while 80% of the combustible recovery was retained. Microbial flotation removed 60% of the pyritic sulfur from the feed. These findings demonstrated that microbial flotation could be utilized for the removal of pyritic sulfur from coal in a semicontinuous system.

Oil Agglomeration for Recovery of Coal Fines: Effect of Vegetable Oil and Bacterial Pretreatment

In the present study, the technique of oil agglomeration has been explored with the aim of employing the process for recovery of coal fines from washery/coal industry effluents as well as deashing of coal. Both edible and non-edible vegetable oils have been tested for agglomeration of five different types of Indian coals widely used in power and metallurgical sectors. The yield of agglomerates ranged from 40.0 to 87.5% and ash rejections from 13.5 to 62.0% using different coal-oil combinations. Bacterial pretreatment of coal with mixed culture prior to oil agglomeration improved the selectivity of vegetable oils resulting in higher ash rejections (59.0−76.0%). Also, pretreatment of high sulphur coal with Thiobacillus ferrooxidans culture resulted in significant enhancement in pyritic sulphur rejection from 69.0 to 98.5%. Various process parameters have been optimized for vegetable oil agglomeration. Higher rates of biodegradation observed with vegetable oils as compared to that with petroleum fractions further supported their use as environmentally safe agglomerants.

High capacity fine coal cleaning using an enhanced gravity concentrator

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.