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Coal Preparation

Efficient Reagents for Disposal of Coal Tailings: Pilot Scale Studies

Coal Preparation » Dewatering

Published: May 23Project Number: C33039

Get ReportAuthor: Ngoc Nguyen and Anh Nguyen | University of Queensland

Sustainable disposal of tailings is a pressing issue in the coal industry. Conventional tailings storage facilities (TSFs) pose a significant risk of catastrophic liquefaction failure. Dry stacking via co-disposal presents a new tailings management strategy in which fine coal tailings are dewatered and disposed of through a mixture with coarse rejects. The dewatering of fine coal tailings from the under-thickener flow is implemented using mechanical dewatering devices such as belt press or centrifuge facilities. When clays are present in tailings, it is difficult to capture fine particles and achieve necessary (low) moisture for effective disposal. Moreover, the tailings properties vary greatly from one to another mine, leading to difficulty in the decision-making process for the dewatering of fine coal tailings.

In previous project C28054, the positive effects of chemicals on the handleability and disposal stability of fine and co-disposed rejects was demonstrated. This project delivers pilot-scale studies on efficient reagents for the disposal of coal tailings. In this project homebuilt approaches and devices were used to effectively test and screen the effects of 39 commercial reagents (from four vendors) on the dewatering of fine coal tailings.

Based on the screening results, the six most effective reagents were selected for pilot-scale dewatering experiments using a solid bowl centrifuge (SBC) and a Netzsch filter press as an alternative to a belt press filter. In each pilot-scale experiment, the filtrate was characterised for solid content and turbidity. The resulting cakes were characterised for rheological properties, moisture content, and slump to evaluate the suitability for the post dewatering handling, transport, and disposal of the cakes via dry stack. Fine cakes were also mixed with coarse rejects, measured the slump, and characterised for a co-disposal strategy.

The results showed that the properties of coal tailings samples vary greatly from one site to another one. This variation in tailings properties was found to cause significant impacts on the dewatering performance, the quality of filtrate, the moisture, and the rheological properties of the resulting cakes. In particular, the types and mass fraction of clays present significant negative impacts on the dewatering technology. Under similar dewatering conditions, the high content of swelling clays and finer particle sizes in tailings A led to higher moisture content (between 37 and 39 wt%), higher stickiness and lower yield stress of cakes A in comparison with cakes B (28 and 32 wt% of moisture) when these tailings were dewatered using SBC facility. Commercial flocculants show a large variation in the effect on the dewatering process as well as the rheology (stickiness and yield stress) of the resulting cakes, which in turn affects the ability to handle and dispose of the cakes via dry stack. Suitably selected reagents (e.g., the flocculant #4 in this report) could provide substantial benefits to dewatering technology. Such reagents helped to achieve a fast dewatering rate, low-moisture cakes with low stickiness and high yield stress. These cakes had solid-like behaviours which are easy to handle and transport of the cakes and suitable for the dry stack. Types of dewatering equipment also played essential roles. Good combinations of flocculant and coal tailings that worked well for SBC dewatering experiments showed no significant advantages in the dewatering using the Netzsch filter press. Generally, SBC produced more consistent and predictable dewatering performance compared to the Netzsch filter press. Investigations of co-disposal with coarse rejects based on a “bucket rheometer” showed that mixing fine cakes with coarse rejects led to remarkably improved stability of the dump and demonstrated a very feasible strategy for dry stacking.

These results provide important knowledge for the decision making process in the development of advanced tailings management via dry stack.

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