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Coal Preparation

Integrated Investigation of Solid Capture and Moisture Reduction of Clay Tailings

Coal Preparation » Dewatering

Published: July 20Project Number: C27065

Get ReportAuthor: Cuong Nguyen, Van Nguyen, Andrew Doi, Majid Ejtemaei and Anh Nguyen | University of Queensland

Dewatering of fine coal tailings is a big challenge to the coal industry. These dewatering and handling problems are caused by the complex surface properties and gelation due to the swelling characteristics of smectite-type clays, which can result in high yield stress, high dosages of dewatering aids, low settling rates, and poor supernatant clarity. Besides, high salinity process water can affect adversely the flocculation of the coal tailings which are flocculated using conventional polyacrylamide (PAM) flocculants.

This project conducted an integral investigation of the effect of chemical and mechanical treatments on the efficacy of dewatering fine coal tailings, focusing on solid capture and moisture reduction, by undertaking the following tasks:

  • Systematically studied mineralogy, surface chemistry, and rheological properties of mineral particles in coal tailings;
  • Scanned the effect of chemicals (salts and flocculants) on the performance of fine coal tailings dewatering;
  • Designed a two-step flocculation procedure with salt-resistant flocculants and demonstrating its superiority in dewatering fine coal tailings;
  • Validated the advancement of the optimal chemicals and our new flocculation procedure to dewatering fine coal tailings using a Centribaric centrifuge and a pilot-scale solid bowl centrifuge.

Kaolinite and montmorillonite (two main clays in coal tailings), Na+, Ca2+, and Mg2+ cations (three main cations in the process water), and Al3+ (a popular agent for water treatment) were applied for the fundamental studies. The high negative surface charge of the pure clays was considerably neutralized by the multivalent ions, Al3+ in particular. A set of conventional flocculants with different functional charge groups, molecular weights, and charge densities was employed to flocculate the pure clays and coal tailing samples. The obtained results indicated that anionic flocculants with high MW and low charge density were the most suitable chemicals for settling kaolinite while settling montmorillonite was almost independent of flocculants. The most important observation was the inhibitory effect of salts on the flocculation performance of anionic flocculants. This observation highlighted the need for applying salt-resistant flocculants for dewatering fine coal tailings. The newly-designed flocculants (mPAMs) were applied to a two-step procedure (coagulation and flocculation) and compared to the performance of the optimal anionic PAM. Interestingly, the settling rate of kaolinite suspensions treated by mPAM with low charge density (mPAM1) was less dependent on the suspension salinity. The two-step procedure significantly enhanced the settling rates of both high-salinity suspensions of pure kaolinite and pure montmorillonite in comparison with conventional PAMs. Studies on three coal tailings samples using this method by a lab-scale Centribaric showed that the optimal mPAM1 dosage for sample A and sample B was 200 g/t, while sample C would work well with 300 g/t mPAM1. Dewatering the samples by this method using a pilot-scale solid bowl centrifuge demonstrated an enhancement in the solid recovery and the moisture content reduction. Specifically, with the same dosage of aPAM, mPAM1 applied in the two-step procedure could increase the solid recovery by ~1 %, 3.7 %, and 0.3 %, and reduce the moisture content of dewatered tailings by 4.4 %, 1.8 %, and 4 % for sample A, B, and C, respectively. The collective results of this project provide a comprehensive understanding of the effect of salts and flocculants with various molecular structures on dewatering fine coal tailings. A salt-resistant flocculant and a new dewatering approach were developed and can be applied in coal preparation plants to increase the revenue and reduce the environmental impacts of the coal tailings.

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