B. K. Parekh

Enhanced ultrafine coal dewatering using flocculation filtration processes

Abstract Ultrafine coal (−150 μm) can be effectively cleaned using advanced separation techniques such as column flotation, however, dewatering it to below 20 percent moisture level using the conventional dewatering techniques is difficult. A comparative flocculation filtration study was performed for enhancing dewatering of ultrafine coal using vacuum, hyperbaric, and centrifugal filters. The cationic and anionic flocculants were added into the slurry individually or in combinations. Vacuum filtration results showed that use of flocculants increased filtration rate by several times and/or substantially reduced cake moisture. Combined use of anionic and cationic flocculants showed further improvement. Addition of flocculants significantly increased filtration rate of hyperbaric filtration and reduced cake moisture in centrifugal filtration. Anionic flocculant was more effective in enhancing fine coal dewatering than cationic flocculant in vacuum filtration while cationic flocculant was more effective in high shear centrifugal filtration. A new approach on using flocculants in vacuum filtration is proposed for enhanced fine coal dewatering.

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%.

Apex water injection for improved hydrocyclone classification efficiency

Abstract 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.

Briquetting of Coal Fines and Sawdust Part I: Binder and Briquetting-Parameters Evaluations

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 conditions exerted the greatest influence on briquette quality.

Picobubble Column Flotation of Fine Coal

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 coal by up to 40%, depending on the feed rate, collector dosage, and other flotation conditions.

Estimation of surface free energy of pyrites by contact angle measurements

The surface properties of coal-pyrite play a major role in determining its separation from coal in processes such as flotation. The solution pH is an important parameter in determining the surface properties of both coal and coal-pyrite such as surface free energy and zeta-potential. In the present investigation, the effect of pH on the surface free energy of pyrites from different sources was studied. The surface free energy of solids is made up of two components, i.e. the dispersive surface free energy and the acid-base interaction energy. Various methods have been used by previous researchers to evaluate these two components for different solids. In the present study, a new approach was developed and used to study the surface free energy of pyrite surfaces. Results indicate that the dispersion surface free energy of various pyrites is independent of pH while the acid-base interaction energy is strongly dependent on the pH. The acid-base interaction energy is different for each pyrite sample and also the change with pH varies with the type of pyrite. Coal-pyrite was found to be more hydrophobic than ore-pyrite which may be attributed to the presence of carbon in coal-pyrites. The acid-base interaction energy varied little with pH for coal pyrites than ore-pyrite. Comparison of acid-base interaction energy with zeta-potential measurements shows a good correlation between the minimum in acid-base interaction energy and the pHpzc.

Upgrading Low-Rank Coal Using a Dry, Density-Based Separator Technology

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 densities between 1.58 and 1.88 gm/cm3, respectively. Effective separations were achieved using air table separators for particle sizes larger than 1 mm.

Paste Thickening of Fine Coal Refuse

The coal industry is being subjected to increasing public scrutiny with regards to its effect on the environment and impact on public health and safety. Recently, disposal and storage of fine coal waste slurry has drawn considerable public attention. This article discusses the emerging paste thickening technology as a possible solution to the fine coal waste slurry disposal problem. Paste-thickening studies were conducted on thickener underflow slurry from a central Appalachia preparation plant. Initial experiments were conducted with a laboratory scale T-Floc apparatus to optimize flocculant dosages to obtain maximum settling flux and underflow solids concentration. Results showed that the addition of anionic flocculant (400 g/t) to the slurry followed by cationic flocculant (100 g/t) provided the highest settling flux (3.85 tonnes/hr/m2) and solids concentration (35% by weight). Pilot-scale paste thickening tests were conducted using a Dorr-Oliver Eimco Deepcone™ thickener. The thickener concentrated the solids from 10% to 50% by weight using about 450 g/t of anionic and 150 g/t of cationic flocculants. The thickened paste had a yield stress of about 165 Pa that is sufficiently low to allow transport using a conventional positive displacement pump to a disposal area. The clarity of the overflow stream was similar to that currently obtained with a conventional thickener.

A parametric study of column flotation for fine coal cleaning

Recovery of fine coal is becoming an important and integral part ofcoal cleaning plants. The froth flotation technique is widely used for recovery of fine coals; however, the technique becomes inefficient when excessive amounts of ultra-fine clay material is present. The counter-current column addresses this problem by providing a stream of wash water to remove fine clays from the froth. The experimental results reported herein examine the effect of column operational parameters (feed rate, frother concentration, airflow, column height, pulp density and wash water) on cleaning of fine coal. The study was conducted using the Plackctt-Burman experimental design followed by a 23 factorial design. The study identifies frother concentration, column height and airflow as the most significant parameters and enabled characterization of the magnitude and interaction of the effects. In general, for the same amount of frother concentration column flotation provided a lower ash clean coal product compared to the conv...

Pilot-Scale Evaluation of Hyperbaric Filtration of Ultra Fine Clean Coal

A pilot scale evaluation of continuous pressure filtration was conducted on minus 200 mesh clean coal recovered by column flotation using an Andritz-Ruthner trailer mounted hyperbaric filter. Results showed that increasing vessel pressure for a given cake formation angle (CFA), increased cake thickness and throughput, while decreasing cake moisture. Baseline testing concluded that filter operating parameters-of 3 bar pressure, 1.5 rpm filter speed and 165∗ CFA produced a 15.7 mm thick cake with a cake moisture of 23.7 percent. The air consumption to achieve these results was 242 NmJ/l (452 cfm/t) and the resulting throughput was 793 kg/mVhr (163 Ib/ftVhr). Using these operating parameters, either nonionic or cationic surfactants effectively reduced cake moisture to below 20 percent at dosages as low as 0.25 kg/t. The use of flocculants increased throughput but also increased cake moisture.