Screen Bowl Centrifuge Research Articles

Screen bowl centrifuge: a high-efficiency particle size separator

Over the years, screen bowl centrifuges have been widely used for dewatering fine coal in coal preparation plants in the United States and elsewhere. Its popularity is attributed to its relatively low cost, its high capacity of providing low moisture content product and its relative ease of operation and maintenance. It is generally recognized in the engineering and scientific communities that screen bowl centrifuges provide some degree of particle size separation while dewatering fine coal in a common application. However, the extent of differential partitioning of coarse and fine particles achievable by a screen bowl centrifuge has not been systematically studied in the past. The present investigation was aimed at conducting a parametric study using a statistically designed experimental program to better understand and optimize the size classification performance of a screen bowl centrifuge. A continuously operating screen bowl centrifuge having a bowl diameter of 0.5 m was used for this study at the Illinois Coal Development Park. Three key operating parameters, i.e., feed flow rate, feed solid content and pool depth, were varied to conduct a total of 17 experiments using a three-level factorial test matrix. Some of the best size separation performances achieved in this study may be described as having an imperfection value of 0.13 at an effective separation size (d 50c ) of 38 µm and an imperfection value of 0.27 at an effective separation size (d 50c ) of 2.8 µm. Due to an effective separation of ultrafine high ash materials, the ash content of the screen bowl feed was reduced from 22.3% to a minimum of 8.84% with a combustible recovery of 84.1% and an ash rejection of 71.6%. A higher combustible recovery of 92.1% was achieved at a product ash content of 12.5% with a d 50c of 2.8 µm and imperfection of 0.27.

Evaluation of an enhanced gravity-based fine-coal circuit for high-sulfur coal

One of the main objectives of this study was to evaluate a fine-coal cleaning circuit using an enhanced gravity separator specifically for a high sulfur coal application. The evaluation not only included testing of individual unit operations used for fine-coal classification, cleaning and dewatering, but also included testing of the complete circuit simultaneously. At a scale of nearly 2 t/h, two alternative circuits were evaluated to clean a minus 0.6-mm coal stream utilizing a 150-mm-diameter classifying cyclone, a linear screen having a projected surface area of 0.5 m{sup 2}, an enhanced gravity separator having a bowl diameter of 250 mm and a screen-bowl centrifuge having a bowl diameter of 500 mm. The cleaning and dewatering components of both circuits were the same; however, one circuit used a classifying cyclone whereas the other used a linear screen as the classification device. An industrial size coal spiral was used to clean the 2- x 0.6-mm coal size fraction for each circuit to estimate the performance of a complete fine-coal circuit cleaning a minus 2-mm particle size coal stream. The 'linear screen + enhanced gravity separator + screen-bowl circuit' provided superior sulfur and ash-cleaning performance to the alternative circuit that used a classifyingmore » cyclone in place of the linear screen. Based on these test data, it was estimated that the use of the recommended circuit to treat 50 t/h of minus 2-mm size coal having feed ash and sulfur contents of 33.9% and 3.28%, respectively, may produce nearly 28.3 t/h of clean coal with product ash and sulfur contents of 9.15% and 1.61 %, respectively.« less

Challenges in Dewatering of Indian Medium Coking Coals: Case Studies with Chemical Reagents

At the West Bokaro Washery of Tata Steel, India, the coarse coal is dewatered in a vibrating basket centrifuge and fine clean coal is dewatered in a screen bowl centrifuge. Due to the higher amount of fine fraction (0.038 mm) treated in the flotation circuit of the washery (about 9% of ROM), the moisture content of the clean coal fines is high. A polyacrylate surfactant reduced the fine coal moisture content by 8% and it was found to be the best among the various reagents tested in the laboratory. The chemical was found very effective for dewatering the coarse clean coal. Laboratory and plant trials showed that through treating only the coarse clean coal by the surfactant, the moisture in the composite clean coal could be reduced from 12.0% to 9.8%. The optimum dosage of the reagent was 0.24 kg/t dry coal. Laboratory-scale filtration using coagulant and surfactant increased filtration rate of centrifuge centrate. Also, the final moisture content was found to be 10% less than that obtained without addition of any reagents. For improving the clarity of thickener overflow, anionic polyacrylate based flocculant was found better than present plant flocculant based on polyamide. The optimum dosage of the recommended flocculant was 0.012–0.016 kg/t of dry fine coal.

Screen Bowl Centrifuge Dewatering Process: A Parameteric Study

Screen bowl centrifugation process is widely used for fine coal dewatering due to its relatively low cost, high capacity of providing low‐moisture content product as well as relative ease of operation and maintenance. However, screen bowl centrifuge tends to lose a significant amount of ultrafine clean coal to the main effluent and screen‐drain streams. This study aims at obtaining a better understanding of the parameter main effects and interaction effects with an ultimate goal of improving the dewatering performance of a screen bowl centrifuge. A three‐level factorial experimental design has been utilized to conduct a test program using a continuously operated screen bowl centrifuge having a bowl diameter of 0.5 meter. The results of this study indicate that moisture content of the clean coal product is affected by both feed solid content and the pool depth maintained in the centrifuge. On the other hand, clean coal recovery to the product launder was found to be a function of feed solid content and volumetric feed flow rate but independent of the pool depth. The interaction effect of feed flow rate and feed solid content also appeared to have a significant effect on the clean coal recovery.

In-situ reslurrying and dewatering in screenbowl centrifugues

An innovative patented cake washing technology has been implemented in a continuous-feed screenbowl centrifuge to effectively remove cake impurities. At high wash rates the cake is reslurried and deliquored in sequential stages in-situ in a screenbowl centrifuge to remove residual impurities left in the cake. At low wash rates the new technology increases contact between the cake and the wash liquid promoting cake washing by dissolution and diffusion. The new reslurrying technology was developed in the laboratory with a pilot screenbowl centrifuge on model glass beads and a highly porous polymer. Field tests on sodium carbonate monohydrate crystals using the pilot screenbowl centrifuge with the optimal reslurrying design demonstrated better washing performance with higher cake purity and lower wash liquid.

Centrifuge dewatering and reconstitution of fine coal: A pilot-plant application of the GranuFlow Process

A continuous pilot-scale test of the GranuFlow Process was conducted using a screen-bowl centrifuge for the dewatering and reconstitution of column-flotation concentrate at a coal preparation plant in Virginia. In this test, a slipstream of the fine clean-coal slurry from the column-flotation concentrate was treated with a bitumen emulsion before dewatering. The treated products from the screen-bowl centrifuge appeared to be dry and in afree-flowing granular form, while the untreated products were wet, sticky and difficult to handle. Test results indicated that the average moisture contents of the dewatered coal products were 35.7%, 35.5%, 32.6%, 29.9% and 26.5% (by weight) with Orimulsion additions of 0.0%, 0.7%, 3.2%, 4.8% and 6.4% (by weight), respectively. The handleability and dust reduction of the dewatered coal products were also vastly improved.

Centrifugal Dewatering and Reconstitution of Fine Coal: The GranuFlow Process

This paper describes the continuous bench-scale testing of the GranuFlow Process, developed and patented by the Pittsburgh Energy Technology Center (PETC) of the U.S. Department of Energy. Testing was conducted on both high-g force solid-bowl and screen-bowl centrifuges for dewatering and reconstitution of fine-clean-coal slurry at 300 Ib/h (136 kg/hr) in PETC7's Coal Preparation Process Research Facility. In the GranuFlow Process, fine-clean-coal slurry was treated with a bitumen emulsion before dewatering in both centrifuges. In one example, the treated products from a solid-bowl centrifuge appeared to be dry and in a free-flowing granular form, while the untreated products were wet, lumpy, sticky, and difficult to handle. Specifically, test results from the solid-bowl centrifuge indicated that the moisture content, handleability, and dust reduction of the dewatered coal product improved as the addition of emulsion increased from 2 wt% to 8 wt%. The improvement in handleability was most visible for the ...

Preliminary study of correlations between fine coal characteristics and properties and their dewatering behaviour

The dewatering performance of 25 fine coal samples taken from 21 Australian coal preparation plants has been studied. Coal equilibrium moistures, surface properties, quantity of superfine particles, mineral matter and water properties influence the behaviour of filters or screen bowl centrifuges more directly than do any other coal properties. The equilibrium moisture, ranging from 1.3 percent to 5.8 percent, has a strong influence on filter cake moisture, but a weak effect on product moisture from screen bowl centrifuges and other fine coal centrifuges. The surface properties of coal samples were analysed by gas (N 2) adsorption. Specific surface area is usually less than 2.0 m 2/g for coal samples. The total pore volume is in the range of 0.0013 to 0.0121 cclg. These surface properties appear to have a significant correlation with the centrifuge product moisture rather than filter cake moisture. The quantity of superfine material (−0.063 mm), is strongly correlated with the product moisture of screen bowl centrifuges, while the measured zeta potential is significantly correlated with the filter cake moisture. Kaolin appears to have a much stronger effect on filter cake moisture than do any other ash minerals. Water conductivity and total dissolved solids in fine coal samples varied over wide ranges: from 1050 to 9960 μS/cm and from 852 to 7380 mg/l respectively. Surprisingly, both parameters are found to have quite a strong influence on filter cake and centrifuge product moisture. The product moisture increases as the water conductivity and total dissolved solids in the water increase.

Plant Experience in Improving Coking Coal Product Moisture at Curragh

With lower moisture specifications becoming the norm on coal sales contracts, Curragh Queensland Mining Limited has found itself under increasing pressure to reduce product moisture levels out of its preparation plant. This paper reports the efforts by site staff to identify and implement methods of achieving the goal. It describes the results of surveys carried out to highlight which area would produce the greatest benefit from attention and explains why the fine coal product was chosen. Results of surveys and testwork designed to pinpoint the aspects of the operation of the screen bowl centrifuges used for dewatering this product which could be modified are shown. The implementation of these changes is discussed and the resultant moisture reduction shown.