Application of Multiple Linear Regressions and Taguchi Design Method in Clean Coal Recovery from Lignite Fine Coal Tailings: Comparison of Multi-Gravity Separator (MGS) and Falcon Concentrator PDF

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APPLICATION OF MULTIPLE LINEAR REGRESSIONS AND TAGUCHI DESIGN METHOD IN CLEAN COAL RECOVERY FROM LIGNITE FINE COAL TAILINGS: COMPARISON OF MULTI-GRAVITY SEPARATOR (MGS) AND FALCON CONCENTRATOR Muhammed Fatih Can, Eyüp Sabah Afyon Kocatepe University, Mining Engineering Department, 03200 Afyonkarahis...

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APPLICATION OF MULTIPLE LINEAR REGRESSIONS AND TAGUCHI DESIGN METHOD IN CLEAN COAL RECOVERY FROM LIGNITE FINE COAL TAILINGS: COMPARISON OF MULTI-GRAVITY SEPARATOR (MGS) AND FALCON CONCENTRATOR Muhammed Fatih Can, Eyüp Sabah

Afyon Kocatepe University, Mining Engineering Department, 03200 Afyonkarahisar, Turkey

ABSTRACT: Recovery of fine coals from coal preparation tailings and recycle of processing water are of both economic and environmental incentives, not only preserving natural resources but also reducing environmental consequences of discharging large volume of tailings. Recent developments in the use of various gravity equipments in fine-coal beneficiation have been discussed and their relative merits have been compared. In this study, the application of multiple linear regressions and Taguchi experimental design method for modeling and optimizing of some operations variables of Multi-Gravity Separator (MGS) and Falcon concentrator for lignite coal cleaning was discussed. The variables considered in this study include the pulp solid ratio, drum speed, tilt angle, shaking amplitude, wash water rate, feed rate for MGS, and the gravity force, solids rate, flow rate, water pressure for Falcon concentrator. The positive and negative effects of variables and the interaction between variables ash content and recovery of clean coal were determined. The predicted values were found to be in good agreement with experimental values (R2 values of 0.807 and 0.944 for ash content and combustible recovery of clean coal, respectively) for MGS. However, the match of predicted values with the actual data points indicates a poor fit (R2 value of 0.577 and 0.399 for ash content and combustible recovery values, respectively) of the equation for Falcon concentrator.

Keywords: Coal cleaning, multi-gravity separator (MGS), Falcon concentrator, regression analysis, Taguchi method 1. INTRODUCTION Coal is a fossil fuel which is the most abundant and geographically scattered all around the world. In world general 861 billion tones proved reserve of coal at end-2011 is determined. In other words, this means the world has coal reserves those can sustain 112 years (1). However coal has this much long consume period, apparent oil and natural gas reserves consumption period is estimated between 40 to 60 years according to current production rates (2). For this reason coal is one of the world’s most important sources of energy, fuelling almost 40% of electricity worldwide. In many countries this figure is much higher: Poland relies on coal for over 94% of its electricity; South Africa for 92%; China for 77%; and Australia for 76%. Coal has been the world’s fastest growing energy source in recent years – faster than gas, oil, nuclear, hydro and renewable (3). As the demand on clean coal increase with developing environmental standarts, an increase of modern coal preparation plant numbers is observed in latest years. Modern coal preparation plants are consisting of coarse, medium and fine coal cleaning circuits. Fine coal has size distribution below 28 meshes (0.589 mm) and designated as slime in coal cleaning circuits. Generally, this size fractioned material must be processed in all coal preparation plants. In past year’s coals of below 28 meshes (0.589 mm) size discharge was economic, but with swiftly developing technologies fine coals are also utilized.

Also today’s mining processes produces more fine materials as the mechanization applications have more frequently applied. Therefore it is a must to recycle the coal in fines. In this manner recovered fine coals are prevented to deploy in waste and avoid a series of problem in waste disposals. Today 5% flotation and 95% gravimetric processes are used to prepare the produced coal all around the world. Gravimetric processes applied for coarse coal preparation have low cost, high capacity and selectivity. Gravimetric processes are also utilized for cleaning fine coal. However, some other problems arouse within the used separators. Only with developing technologies, some advanced gravity separation equipments as hydrocylones, Multi Gravity Separator (MGS), Falcon and Knelson concentrator. Taguchi approach provides a new experimental strategy in which a modified and standardized form of design of experiment (DOE) is used. This technique helps to study effect of many factors (variables) on the desired quality characteristic most economically. By studying the effect of individual factors on the results, the best factor combination can be determined (4). Taguchi designs experiments using specially constructed tables known as “orthogonal array” (OA). The use of these tables makes the design of experiments very easy and consistent and it requires relatively lesser number of experimental trials to study the entire parameter space. As a result, time, cost, and labor saving can be

Muhammed Fatih Can, Eyüp Sabah / The Journal of ORE DRESSING ® 2012

achieved (5). Recently, several optimization approaches have been proposed for the optimization of multiple responses (6-10). Statistical regression has many applications (11-12), in many manufacturing processes the behavior of processes is usually vague and the observed data is irregular, hence the statistical regression models have an unnaturally wide possibility range (13). The present work is aimed to optimize of some operating parameters of MGS (pulp solid ratio, drum speed, tilt angle, shaking amplitude, wash water rate, feed rate) and Falcon concentrator (gravity force, solids rate, flow rate, water pressure) based on Taguchi method, which have been predicted to play a very significant role in this concept. Additionally, during optimization of these operating parameters, a comparative study of MGS and Falcon concentrator performance for maximum ash rejection and coal recovery from Tunçbilek/Kütahya coal plant tailings could be revealed with the mathematical software package (Minitab 15). 2. EXPERIMENTAL STUDIES 2.1 Materials Lignite coal tailings obtained from Tunçbilek Coal Preparation Plant of G.L.I of Turkish Coal Enterprises (Kütahya-Turkey) were used in this study. The samples were taken from slurry waste with standard of TS ISO 5667-10 (14). 2.2 Methods 2.2.1 Characterization tests A number of qualitative and quantitative analysis techniques were used to characterize the coal tailings. The chemical composition of the tailings was defined by X-ray fluorescence (XRF). The mineral composition of the tailings was determined by X-ray diffraction (XRD) method using a Rigaku-Giger Flex analyzer. The particle size distribution of the tailings was obtained using a Retsch AS200 Sieve Shaker and Fritsch-Analysette 22 Particle Size Analyzer. The specific gravity of the tailings was determined by Quantachrome Ultrapycnometer 1000. The ash and sulfur contents of the tailings were determined according to ISO 1171 and ISO 351, respectively. In addition, the calorific value was determined based on ISO 1928. Following the characterization tests, the beneficiation studies were immediately initiated before any physical and chemical decomposition of tailings.

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2.3 Test Procedure Before MGS and Falcon Concentrator experiments, classification tests were performed using a hydrocyclone. The aim of this test was to separate the clay and/or carbonate minerals from the coal. Since large amounts of fine and ultra fine particles in the tailings cause a low cut point, a small diameter hydrocyclone (44 mm) was selected for the experiments. Underflow products were concentrated using a Multi-Gravity separator (MGS) or Falcon Concentrator. For each MGS and Falcon Concentrator tests, 2000 g of the dry coal sample was used. The effects of varying operating parameters such as drum speed, tilt angle, shaking amplitude, wash water rate, pulp feed rate and pulp solid ratio was investigated for MGS. The shaking frequency of the MGS was fixed at 4.9 cps for all the experiments. To obtain required solids ratio in feed, measured quantities of solids and water were mixed in the slurry tank. The MGS variables were adjusted at necessary levels. The feed slurry was pumped into the MGS drum at the required flow rate using the peristaltic pump while the MGS was in operation. Samples from the clean coal and tailing streams were collected at steady-state condition. The samples were filtered, dried and analyzed for ash content and combustible recovery. When changing a parameter in each test, the other parameters were kept constant and the optimization results were used in the other tests. However, a laboratory-scale Falcon SB-40 concentrator was used for the tests. Once the optimum hydrocyclone conditions were identified and applied, the concentrate was further processed in the Falcon concentrator. Ash rejection was achieved by optimizing variables such as gravity force, water pressure, pulp solids ratio and feed rate. The classification and concentration test results were analyzed with Minitab 15 for MGS and Falcon Concentrator. The model included a set of equations relating the variables, ash content and combustible recovery in the clean coal. 3. RESULTS AND DISCUSSIONS 3.1 Characteristics of tailings The results of chemical analysis for the samples were presented in Table 1. As seen in Table 1, LOI of the samples are about 37% of Tunçbilek which means that the amount of organic material (coal) in the tailings was significant when compared to the amount of inorganic impurities. When Figure 1 was examined, 80% of the Tunçbilek coal tailing was...