Coal Preparation

Value-Added Products from Coal Tailings

Coal Preparation » General

Published: June 22Project Number: C29057

Get ReportAuthor: Utsab Katwal, Atousa Khazaie, Mingtiem Yong, Soheil Jahandari, Mohammad Babla, David Osborne, Maroun Rahme, Alistair Harriman, Zhonghua Chen, Zhong Tao | Western Sydney University

The quantity of coal tailings (CT) currently discharged annually in Qld and NSW is estimated to be between 14 and 16 million dry tonnes with the majority going into tailings storage facilities (TSF). Due to the continuous mining activities over the last 50 years, the existing tailings storage in Australia has reached an order of 500 million tonnes.

Currently, over 200 million tonnes of aggregates and 4.9 million tonnes of fertilisers are used in Australia per annum. As the demand for aggregates and soil conditioners is very high, a large proportion of tailings could be potentially consumed for the production of aggregates and soil conditioners to be used locally in mining areas and surrounding communities. This project aimed to develop a new tailings management solution by using tailings to produce road subbase aggregates and agricultural soil conditioners that can generate revenue from coal wastes and reduce the impact of CT on our environment. For this purpose, coal tailings from four mining sites were collected and investigated further.

To investigate the feasibility of producing road subbase aggregates from coal tailings, characterisation tests were conducted on CT samples collected from all four different mining sites. From the microstructural analysis tests, including scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis and X-ray diffraction analysis (XRD), it was found that the coal tailings were mostly composed of silica and alumina, which are essential elements for geopolymerisation process. The XRD analysis reveals that the phase compositions of the tailings were quartz, kaolinite, muscovite and other clay minerals. According to the sieve, hydrometer, and the Atterberg test results, all coal tailings are slightly/medium plastic finely grained soil. CT1 and CT3 are very similar, whereas CT2 has much more coarse-grained particles. Fine coal tailings with D 60 < 75 microns are more suitable for pelletisation. Based on the characterisation results, CT1 was finally selected for developing coal tailings-based road subbase aggregates.

To identify the optimum mix design for the production of coal tailings-based aggregates (CTBAs), various mix designs were developed, in which different amounts of binders and activators were added to coal tailings. The compressive strength results show that the combination of 3% slag and 2% sodium silicate provided the highest compressive strength of 4.2 MPa after 28 days of curing. Therefore, this mix design was selected for the production of aggregates in the following stages. Trials were further conducted to optimise the pelletising parameters, such as the tilt angle, rotation speed, duration of pelletisation, and moisture content. The optimised manufacturing parameters were selected for bulk production of aggregates in the characterisation tests. The freshly-made aggregates are strong enough to be discharged into a hopper or conveyor to be either stockpiled or trucked to the desired destination.

The wet strength tests showed that 40% of natural aggregates could be replaced by CTBAs for road subbase applications. To increase the replacement level, surface treatment was conducted on CTBAs using different waterproofing/coating materials to reduce the water absorption and improve the wet strength. The results indicated that the coated tailings-based aggregates through either cold glazing or epoxy resin could successfully pass all the requirements of Australian standards to be used in road subbase. The CTBAs can be mixed with natural aggregates using a grader or rotary mixer on site. Initial cost analysis showed that the manufactured tailings-based aggregates are very economical ($30 for uncoated aggregates and $52 for coated aggregates).

To investigate the possibility of using coal tailings as soil conditioners, elemental characterisation tests were carried out on raw coal tailing samples. The tailings from sites 1 and 2 have ~3% of macronutrients, high carbon (C), and low heavy metals and metalloids (As, Cd, Se, Cu, Zn, Pb). Heavy metal contents in CT from both sites CT1 and CT2 were also below safe levels. However, the contents of some heavy metals were relatively high in coal tailings from sites 3 and 4, which are not suitable for producing soil conditioners. Nonetheless, they may still be used for aggregate production provided that the environmental safety is proved by leaching tests. Then germination tests in growth chambers and two greenhouse trials were conducted to investigate the effect of raw coal tailings from site 2 on the growth of tomato plants (cv. Black Krim). Although some improvements in germination rate were observed but the plants were adversely affected after 3 weeks of treatment, especially for the samples, with a higher application rate (20%). The potential reason could be the high value of pH of raw coal tailings (8.0 to 8.5), which is significantly higher than the required pH for tomato plants (5.5 to 6.5). This necessitates the further amendments of raw coal tailings to be used as a soil conditioner.

Trials were performed with several chemicals to reduce the alkaline pH of coal tailings, and it was found that iron sulphate (0.8% by weight of dry raw coal tailing) was effective in lowering pH to the required range of 5.5 to 6.5. Meanwhile, small amounts of gypsum, sulphate of potash and chicken manure were added to the mix to enhance the nutrient and beneficial microbial activity of the soil conditioner. To minimise the potential health hazard resulting from dust during application in the field, and to assist in storage and transportation, it was necessary to produce soil conditioner pellets. The pellets with sizes ranging from 1.18 to 6.7 mm were considered to be suitable for soil conditioners. Accordingly, further investigations were carried out to optimise the pellet production parameters, such as the binder type, moisture content, tilt angle, and rotation speed. The pH of the final product was found to be within the target range of 5.5 to 6.5.

One greenhouse trial was then conducted to check the effect of coal tailings-based soil conditioner (CTSC) pellets on the growth and yield of Black Krim tomato plants. In the greenhouse trial, 20 tomato plants were grown with four different treatments (control, commercial soil conditioner, and CTSC with two different application rates). Greenhouse test results indicated that the plant leaves, petioles, and flower numbers were improved for the plants treated with CTSC pellets. The yield of tomato fruits, however, was limited inside the closed greenhouse, which could be attributed to the absence of insect pollinators that usually help pollination in the field. Elemental analysis was also conducted to determine heavy metal con ents in both coal tailings and leaves of tomato plants. The test results indicate that there is no contamination hazard of Cu, Zn, Cr, and Pb elements by applying CTSC to the soil.

A field trial was further conducted to check the effectiveness of coal tailings-based soil conditioners in at the experimental farm of Hawkesbury Campus, Western Sydney University. Its effectiveness was compared to that of a commercial soil conditioner (CSC), which also mainly consists of carbon. Both soil conditioners were applied at a rate of 25 tonnes ha-1 in a completely randomized block design with three blocks of CTSC treatment and three blocks of CSC treatment. Three control blocks without any soil conditioners were also included in the design for comparison.. Accordingly, there were three replicates of each group (Control, SCT treatment, and CTSC treatment) in different blocks. In each block, 41 plants were grown, resulting in a total of 369 plants in the nine blocks. The treatment of soil conditioners was applied in two stages. In the first stage, 50% of treatment was applied after transferring tomato plants in the field for 4 weeks. The remaining 50% of treatment was applied in Week 8. Plant morphology results revealed that while the plant heights were similar for all groups, the flower number and fruit numbers were higher for plants treated with CTSC pellets. For CTSC-treated plants, 11% increase in tomato fruit yield was observed compared to the control.


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