FLOTATION PERFORMANCE ANALYSIS ON COAL WASHERY REJECTS

Coal washery rejected coals and coal slurries have better options in froth flotation as a separation process. In this study, coal slurry received from Sudamdih coal washery, Jharkhand, India, examined for cleaning through fixed cell flotation. The Petrography study of the feed sample showed the presence of vitrinite, semi vitrinite and liptinite and inertinite as major minerals. General full factorial statistical design package (Minitab V17) was used to develop the regression models for the responses like froth height, froth density, recovery and ash content of clean coal. Results showed that experimental responses like froth height, froth density, recovery and ash content were found to be more sensitive to the frother dosage. The coefficient of correlation (R 2 ) values between the experimental and the predicted values of the flotation responses was found to be >0.98 for all the models. Further, flotation tests were conducted for varying pulp density and its effect on recovery, froth height, ash content, tests with five levels of pulp density (8, 9, 10, 11, 12 and 13%solids by weight). It found that the coal slurry sample from the study area could be cleaned with sufficient efficiency. The cleaned coal is suitable for powder coal consuming industries.


Introduction:
A large amount of coal fines has been generating during modern mechanized coal mining techniques and coal washing plants, which has serious effects not only on the environment but also on sustainable mining practices [1]. Processing of finer size coal has always been a problem and costlier than the cleaning of coarse coal [2]. The best available technique for cleaning of fine size coal with particle size <0.5mm is the froth flotation technique [3][4][5]. The coal particles subjected to treatment with suitable liquid hydrocarbon reagents to alter the hydrophobicity, and enhance recovery and /or improve selectivity [6][7], but fine particles show lower flotation rate, resulting in low flotation recovery [8]. Generally, in the coal flotation, froth can affect both the recovery and the grade, because it promotes the selective drainage of minerals back to the pulp zone [9]. Drainage in the froth zone contributes significantly to the increase of selectivity of the flotation process. Therefore, the froth should have enough stability and height, to allow the drainage of the entrained hydrophilic particles back into the pulp and simultaneously support the hydrophobic particles, inhibiting their drainage and contributing to the total selectivity of the process. In most of the Indian coal washeries, diesel oils were used as a collector with a combination of different frothers. The type and dosage of collector and frother have significant effects on coal flotation performance [10]. Recovery of cleaner coal from coal fines will be an attractive approach for improving the economic performance of coal washery. Some researchers have done considerable researchers to improve the performance of coal flotation applying advanced techniques like packed column, Jameson cell, column cell and microcell flotation [11].
Many works attempted to recover coal fines from coal washeries. However, there is a wide scope to improve the flotation performance of coal fines generated during washing. Keeping in view of the latest developments in the cleaning of coal fines, studies were initiated on the flotation of coal fines.
One of the most effective techniques to study process behaviour is the factorial design of experiments with an analysis of variance [12][13][14][15][16]. The present study aims to recover clean coal from the discarded coal slurry of coal washery located in Jharkhand, India, by using the flotation technique. The general full factorial experimental design method was used to investigate the influence of collector and frother dosages on recovery and ash content of clean coal (froth product). Consequently, the effects of collector dosage and frother dosage, on the froth height, froth density, recovery and the ash content of the clean coal are discussed using 3D surface plots.
Further, the study also attempted to understand the effect of pulp density on the froth height and froth density.
Experimental results are analyzed using response surface methodology (RSM) is a wellestablished technique, used to obtain the optimal conditions by using a regression model [17].
Works of literature indicate the use of RSM for modeling of processes such as coal processing and flocculation [18&19].

Materials:
The discarded coal slurry samples composed of coal fines (<0.5 mm) taken from a coal washery, Jharkand, India.tTe coal-slurry sample was filtered, dried and disaggregated for use in the flotation tests. Flotation experiments were carried out using commercially available diesel oil as a collector and pine oil as a frother. Figure 1 shows the size wise ash analysis of the dried coal sample. The feed sample was subjected to characterization studies such as proximate analysis and specific gravity analysis.

Flotation Experiments.
WemcoFagergrenflotatioanmachine with 3.5L of volumetric capacity as shown in figure 2was used for flotation studies. A total of 100 g of coal was mixed with 300 ml of water and conditioned with diesel oil (collector) in a flotation cell for 2 min. The pH of the slurry was adjusted to 8. Another 600-ml make-up water was added and conditioned further with a predetermined quantity of pine oil (frother). The conditioning time for all the reagents was 2 min in each stage, and then flotation was carried out by releasing the air at 1500 rpm for 5 min. froth height is measured with a scale. Froth density was measured by weighing the 500ml beaker filled with froth. From Eq(1)froth density is calculated. Where: Weight of beaker filled with froth W2 = Weight of empty beaker V= Volume of the beaker (500ml) A general full 2 factors 4 level factorial design was used to obtain the response pattern and then to establish a model. The regression model can approximate a mathematical relationship between the two independent factors as follows: Where, Y is the predicted response function (froth height, froth density, recovery and ash content of float coal).a0 is a model co-efficient. a1, a2, a3 are the coefficients.x1 and x2arecoded values of flotation test variables (collector and frother dosage). The computer program (Minitab V.17)was used for the determination of the coefficients of Eq. (2)by regression analysis of the experimental data. The actual and coded values of the variables, collector dosage (kg/t) and frother dosage (kg/t), used in this design are given inTable 3. The responses are analyzed using 3D surface plots to know the effect of collector and frother dosages.

Characterization studies
Sieve analysis carried out using 1000−75μmsieveaperture sizes. The feed particle size distribution with size-wise ash content is shown in figure 2. About 80% of the particles are found to be less than 150 μm size fractions. While about 40% of the particles found tobe coarser than 500 μmparticle sizes (Fig. 2). About12% of particles were found to be of a size less than 75μm.
The proximate and ultimate analysis of the feed coal slurry sample on a dry basis is as shown in Table 1. The average density of coal slurry was found to be 1.63g/cm 3 and a gross calorific value of 5130 kcal/kg.

Flotation studies
The shows the linear relationship with the frother dosage with the fixed collector dosage of 0.48 kg/ton.

Effect of Pulp Density on Flotation Responses.
Thicker the pulp, the smaller is the cell volume required in flotation plants. In general flotation, industries prefer to carry out the process with pulp as dense as possible for their economics with good selectivity and operating conditions. Usually, industrial flotation for minerals is carried out with pulps of densities between 20 and 40% solids by weight. However, the pulp density may be as high as 55% or more, and as low as 8% [20]. Depending upon the rank, type, ash content and size distribution of the coal feed. The best condition for coal flotation has been suggested as 10% solids by weight [21].
Subsequently, tests with six levels of pulp density (8,9,10,11,12 and 13 % solids by weight) were carried out. Figures.10-12shows the results of the effect of pulp density on the froth height, ash content and recovery of clean coal. Figure 10 shows  of pulp density later the curve tends to reach its minimum. It is believed that the reason for the decrease of recovery with high pulp density is reduced turbulence, caused by dense pulp [22].
The absence of sufficient air bubbles to float the increased number of particles could be another reason for less recovery with denser pulp. However, the maximum recovery of >90% is achieved at minimum collector and frother dosages of 0.32kg/t lead to 24.32 ash%. Hence, collector and frother dosages of 0.32kg/t, 0.48 kg/t respectively at 10% pulp density attains the optimum recovery of 74%. Figure 12 depicts the effect of pulp density on ash content (%) of clean coal. Figure 12 indicates that increasing frother concentration results in higher recoveries along with increases in the ash content value of clean. There is a clear separation of low ash products obtained from lower collector dosages i.e up to 0.32 Kg/t. The ash content of the cleaner product is increased when the collector dosage beyond the 0.32 Kg/t. However, the pulp density of the feed has not altered the ash content of cleaner coal. From the figure 12, shows the horizontal lines showing almost parallel to each test conditions with varying pulp density.

Conclusions
Coal slurry sample received from the coal washery, India with <0.5mm particle size containing ash content of 35.50% with volatile matter content of 20.80% and fixed carbon of 42.90% on dry basis analysis. Petrography study showed the presence of vitrinite, semi vitrinite and liptinite and inertinite as major minerals. General full factorial statistical design package (Minitab V17) was used to develop the regression models for the responses like froth height, froth density, recovery and ash content of clean coal. Response function predictions determined by the regression analysis were found to be in good agreement with the experimental results.
Flotation responses like froth height, froth density, recovery and ash content were found to be more sensitive to the frother dosage. The coefficient of correlation (R 2 ) values between the experimental and the predicted values of the flotation responses was found to be >0.98 for all the models.
Subsequently, flotation tests were conducted to study the effect of pulp density on the recovery, froth height, ash content, tests with five levels of pulp density (8, 9, 10, 11, 12 and 13%solids by weight) were carried out. With increasing pulp density, the recovery increased but above optimum densities, approximately 10% for recovery, they decreased as the density increased. Due to the lack of good selectivity, the ash content of the clean coal also increased with higher frother dosage. However, pulp density appears less effect on ash content. It is believed that the reason for the decrease of recovery with high pulp density is reduced turbulence, caused by dense pulp. Insufficient air bubbles to float the increased number of particles could be another reason for less recovery with denser pulp. It found that the coal slurry sample from the study area could be cleaned with sufficient efficiency. The clean coal can be utilized effectively in powder coal consuming industries.