Analysis of a Coal Preparation Plant. Part 1. Changes in Water Quality

1 Analysis of a coal preparation plant. Part 2 1. Changes in water quality, coal seam, 3 and plant performance 4 5 Ghislain Bournival 1, Masataka Yoshida 1,3, Nicholas Cox 2, Noel Lambert 2, Seher Ata 1* 6 7 1 School of Minerals and Energy Resources Engineering, University of New South Wales, Sydney, 8 NSW, 2052, Australia 9 2 Clean Process Technologies Pty Ltd, 700 Standen Drive, Lower Belford, NSW, 2335, Australia 10 3 Mitsubishi Development Pty Ltd, Level 16, 480 Queen Street, Brisbane, QLD, 4000, Australia 11 * To whom correspondence should be addressed 12 Email: [email protected] 13 14 15 1 16 Abstract 17 Coal preparation plants are under increasing pressure to reduce their consumption of fresh water 18 leading to the use of recycle water. Recycled water generally contains a large quantity of dissolved 19 inorganic electrolytes, which affect coal flotation. This paper reports the conductivity and pH 20 measurements of water in tailings stream of an industrial scale flotation cell covering a period of 21 approximately two years. The different coal seams processed at the site were also compared and the 22 influence the water quality on the overall yield of coal assessed. This study presents the large 23 variation in inorganic content in recycled water observed for a wash plant and determine its possible 24 effect on coal preparation. The maximum daily temperature was found be an important factor 25 controlling the amount of inorganic electrolytes in the water and, very significantly, the water 26 management system implemented. The overall plant performance was not significantly affected by the 27 use of water containing high concentrations of inorganic electrolytes. The effect of the quantity of 28 ions in the water on the flotation process is explored in Part 2. 29 30 Keywords: coal, coal preparation, flotation, water quality, water recycling, inorganic electrolytes 31 1. Introduction 32 Mining operations have been known to impact the quality of natural water streams [1]. The quality of 33 the water from coal mining could have an adverse effect on aquatic life, on the deterioration of 34 concrete and metal structures, and result in increased costs of water treatment. These environmental 35 effects may be problematic considering the shortage of water supply around coal mines [2]. To limit 36 the environmental impact and reduce their fresh water usage, coal preparation plants have developed 37 water usage strategies. The most common strategy is to recycle process water. However, other sources 38 of water with high salt content such as bore water and sea water may also be used [3]. Wang and Peng 39 [4] and Bournival, Muin, Lambert and Ata [5] have reported compositional analyses of water samples 40 from Australian coal preparation plants. They have found a wide range of water conductivities 41 reporting values from 0.24 to 12.86 mS/cm. The pH was mostly in the alkaline range but acidic water 42 with a pH as low as 2.6 has been noted. The main elements found in the water are presented in Table 43 1. It is surprising to find that iron is a relatively low impurity in the process water while pyritic iron is 44 a major impurity in coals. In all cases sodium was to the major cation in process water in coal 45 preparation plants. 46 Table 1. Characteristics of process water from some Australian coal preparation plants. Bournival, Muin, Lambert and Wang and Peng [4] Ata [5] Parameter Bournival, Zhang and Ata [6] * Minimum Maximum Minimum Maximum pH 7.1 9.2 2.6 8.8 Conductivity, mS/cm 0.24 12.86 3.80 10.95 Na, mg/L 385 3100 586 1212 K, mg/L 3.4 54 7.65 33.8 Ca, mg/L 6 365 4.11 470 Mg, mg/L 3 180 2.9 458 Fe, mg/L -- -- 0 30.8 Si, mg/L -- -- 2.32 49.4 Cl, mg/L 333 2360 423 1011 2 SO4, mg/L 57 4800 177 5084 47 * Both references combine a total of 7 different sites of which one site was analysed at two different times for a total of 8 48 analyses. The data in the table represent a small sample of the 125 coal mining sites in Australia listed in 2015 [7] and a 49 small sample in time. 50 The chemical composition of the water used in minerals and coal processing plants is determined by 51 the sources in which the water is derived, properties of the feed going through the plant (e.g. soluble 52 minerals like potash), processing routes applied in upgrading the ore and retention time of the 53 recycled water in the system. As a result of changes in chemical composition, mineral processing 54 operations requiring chemically demanding processes such as froth flotation and leaching may be 55 affected [8, 9]. The constituents present in the water may affect, through adverse or synergistic 56 interaction, the effectiveness of chemical reagents used for beneficiating minerals or coals e.g. [5, 10- 57 15]. 58 Another important factor concerning water quality in the flotation of coal is pH. The pH of the water 59 in a coal preparation plant may vary greatly [5, 16] and has been shown to affect the combustible 60 recovery of coal through a number of mechanisms. The pH of the pulp influences the adsorption of 61 ionic collectors [17] as well as directly altering the surface property of the coal. The wettability of 62 coal varies with surface composition [18-20], which can be controlled by changing the surface groups 63 through the pH [21]. The pH of the slurry affects the charge of the coal particles with the value of the 64 isoelectric point being dependent on the impurities in the coal [22-24]. A change in the charge of the 65 coal has been shown to modify the interaction forces between a particle and a bubble, which affects 66 flotation efficiency [25-27] and the particle-particle interaction (i.e. rheological properties) [28, 29]. It 67 has also been found that specific clay slimes may coat the coal particles and prevent their attachment 68 to air bubbles [10, 30-32]. This phenomenon is regulated by the differences in charge between the 69 coal particles and the clays, which is controlled by the pH of the pulp. As well, the quantity of ions 70 dissolved in the slurry from the coal is strongly dependent on the pH of the slurry. Thus a sudden 71 change in the pH from acidic, with a high content of dissolved metal ions, to alkaline led to the 72 precipitation of inorganic electrolytes, which decreased the combustible recovery of coal [33, 34]. 73 Therefore in assessing the quality of the water the pH ought to be included along with any measure of 74 ionic strength (e.g. concentration of salts, conductivity). 75 Most studies investigating the impact of water chemical composition on coal combustible recovery 76 used either artificially prepared solutions in a laboratory environment or water collected from a 77 preparation plant at a specific time. The chemical composition of such samples is very well 78 characterized but does not capture changes in the day to day water quality and how those changes 79 affect the operation of the wash plant. This paper presents the quality (i.e. conductivity, pH) of the 80 process water measured for the equivalent of two years over a three year period in a coal preparation 81 plant in the Hunter Valley in NSW, Australia. However, these measurements were analyzed post hoc. 82 As such they were not obtained through a rigorous experimental campaign, which normally 83 accompanies planned experiments. Despite this shortcoming, it is believed that the analysis presented 84 here is relevant due to the period of time it represents and because it is unbiased since the data was not 85 produced to fit a particular experimental protocol or aimed at getting a specific outcome. In addition, 86 it represents the largest such dataset published to the best of the authors’ knowledge. The first part of 87 this study presents a survey of a typical coal preparation plant with an emphasis on the variations in 88 water quality. The effect of water quality on flotation is presented in the second part of this study [35]. 89 In addition to developing correlations between the different measured factors the current work 90 reinforces the importance of instrumentation and controls in mineral processing plants as discussed by 3 91 Shean and Cilliers [36] as well as demonstrating the presence of strong and uncontrolled variations in 92 mineral and coal processes [37]. 93 2. Experimental Procedure 94 2.1 Coal preparation plant 95 The coal handling and preparation plant from which the data was obtained was located in NSW, 96 Australia. The plant processes mostly coal of thermal quality as well as some semi soft coking coal. 97 As such the mineralogy changes considerably for the different seams washed. A total of 5 different 98 seams were washed in the period of time considered, including several splits for each of the seams. 99 A schematic representation of the washing circuit is presented in Figure 1. The run-of-mine is crushed 100 to 50 mm. The oversize of the screened product (+ 1 mm) is washed by a series of two dense medium 101 cyclones (DMC). The cleaned coal is de-watered in a vibrating basket centrifuge. The finer fraction is 102 de-slimed using a hydrocyclone. The hydrocyclone oversize is cleaned using a spiral. The cleaned 103 product from the spiral is sized using a sieve bend (S/B) of size 150 μm.

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