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Prevention of Acid Rock Drainage (ARD) formation from fine coal and tailings fractions by sulphide removal: The role of bioflotation reagents
Expanded Title:Coal mining produces sulphide-containing ultrafine waste tailings which pose an Acid Rock Drainage (ARD) generation risk. A two-stage froth flotation process developed at UCT aims to further process such wastes: the first stage produces a desulphurised, saleable coal concentrate and the second stage produces a low volume, high-sulphide concentrate and a high volume benign tailings which is safe for disposal without containment. Hampering the implementation of this treatment technique are: (1) the high cost of the oleic acid collector required in the first flotation stage and (2) the toxicity of the xanthate collector used in the second stage which presents issues with the final product stream handling and use. In this study, microorganisms and bioproducts were investigated as xanthate replacements. Potential microorganisms and bioproducts were identified, based on evidence that they are attracted to (attach/modify) surfaces either coal or pyrite, the main sulphide mineral present in the coal wastes. All bioflotation options were required to be non-pathogenic and non-toxic. Mycobacterium phlei, raw algal lipids (RAL) and derived fatty acid methyl esters (FAME) were selected as potential coal bioflotation reagents. Eight micro-organisms were identified as potential pyrite bioflotation reagents. The performance of these bioflotation reagent options was tested using attachment experiments (for the microorganisms) and by monitoring surface charge changes, followed by batch flotation tests. Both algal bioproducts (RAL and FAME) and M. phlei demonstrated excellent technical potential as selective coal (first-stage) bioflotation reagents. In the best case for M. phlei, the same overall flotation yield (37 – 40%) was achieved (with or without frother) as the chemical reagent equivalent for coal ultrafine waste tailings and a concentrate sulphur recovery of 20 – 26%, which is comparable to the chemical system. However, the microbial system was only able to upgrade the coal feed from 51% to 57% combustibles, compared to 71% for the chemical system. The RAL and FAME potential was tested using an ultrafine coal waste tailings feed. The overall yield was 28 ± 1% for the RAL and 30 ± 2% for the FAME, only marginally below the 35 ± 3% achieved by the chemical system. The FAME system had the best combustible recovery, increasing the combustible content from 52% to 79%, compared to 73% for the RAL system and 77 ± 2% for the chemical system. The FAME system was also best at desulphurising the coal, with only 20% of the sulphur recovered to the concentrate, compared to 28% for the RAL system and 25% for the chemical system. Though the FAME bioflotation reagent performed best, one advantage of the RAL bioproduct was that it created a stable froth in the absence of MIBC, thereby potentially allowing for replacement of both the chemical collector and frother. For the second stage flotation, only P. polymyxa yielded competitive pyrite flotation results, with all other options achieving overall yields of <1% compared to 95% for the chemical system, despite good initial attachment results. In the most successful bioflotation with P. polymyxa, 81% pyrite recovery was achieved within 2 minutes. The pyrite recovery was lower at only 7%. The economics of producing a bioflotation and implementing reagent were examined by modifying the chemical system process flowsheet and financial analysis for the M. phlei example. M. phlei bioflotation (using current growth data and assuming on site production) was found to be more expensive than the chemical process. Thus, it is recommended optimisation of M. phlei production is required before bioflotation implementation can be considered. The RAL or FAME options also require further investigation of large-scale production facilities before this option can be costed. In summary, technically competitive and non-toxic bioflotation options were successfully identified and demonstrated for both stages of the two stage froth flotation process for the desulphurisation of ultrafine coal waste tailings, but a preliminary financial analysis showed that their production and usage results in a financially less attractive operation.
Date Published:25/10/2018
Document Type:Research Report
Document Subjects:Mine water - Mine water treatment
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:2389/1/18
ISBN No:978-0-6392-0000-2
Authors:Fagan-Endres MA; Msipa W; Chiodza K; Harrison TL
Project No:K5/2389
Document Size:2 023 KB
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