Acid biodesulphurisation of coal discards using a heap leaching approach

Tambwe, Olivier

Acid biodesulphurisation of coal discards using a heap leaching approach

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Chemical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

The extraction and processing of coal generate substantial amount of wastes, which include waste rock, coal discards, and coal fines, posing significant environmental risks. The presence of sulphidic minerals, particularly pyrite, in these wastes can lead to the formation of acidic runoffs, known as acid rock drainage (ARD), when exposed to moisture and oxygen. While physical separation can effectively remove sulphides from fine coal waste, it is more challenging for coal discards and waste rock due to their low liberation. This necessitates either costly physical liberation processes followed by separation or removal through chemical reactions. The management of ARD pollution involves both prevention and treatment strategies. ARD prevention focuses on minimizing the interaction between oxidants and sulphide-bearing minerals to reduce the formation and release of acidic compounds. Whilst ARD treatment aims at neutralizing the acidic effluent produced and precipitating dissolved metals. Treatment methods are divided into active, and passive approaches based on available resources and pollution severity. Active treatment requires continuous use of alkaline chemicals, incurring significant long-term expenses. Passive treatment systems utilize natural chemical and biological processes to mitigate acidity and precipitate metal pollutants. Heap bioleaching, a technology commonly used in recovering base metals from low-grade metal sulphide ores, offers a sustainable approach to accelerate the reaction of pyritic fractions within coal discards. This method enables the removal of sulphides in solution under controlled conditions, thereby mitigating ARD over the mine's lifespan. Heap bioleaching is cost effective and readily implementable and so holds promise as an economical alternative to traditional ARD treatment methods. Moreover, the accelerated removal of sulphides through bioleaching could yield environmentally benign coal discards with potential value. The desulphurization process in heap bioleaching is sustained by microorganisms capable of oxidizing iron and sulphur. Iron oxidizers facilitate the regeneration of the leaching agent Fe3+ from Fe2+, while sulphur-oxidizing microorganisms produce sulphuric acid from elemental sulphur and sulphides. These reactions maintain highly acidic conditions, promoting the continuous oxidation and leaching of pyrite. This thesis presents a proof-of-concept experiment aiming to test the feasibility and efficacy of accelerated desulphurisation as an ARD prevention method. The study was conducted in laboratory-scale columns simulating waste heaps over a period of 380 days, using coal discards from the Emalahleni region of Mpumalanga, South Africa. The results demonstrated successful sulphur removal with 50% desulphurisation achieved within 250 days, with a terminal duration of approximately 600 days. Post-leaching characterization of the discards showed a significant reduction in the acid generating potential. With static tests displaying a gradual decline in the acid producing potential over time and categorizing the leached discard as non-acid forming after 380 days of irrigation. Biokinetic tests confirmed the significant depletion of the pyritic content in the coal discards further supporting the discards reduced risk of ARD generation. These findings underscored the potential of heap bioleaching as a viable strategy for desulphurising of high sulphur-containing coal discards during the mine's operational lifespan, potentially enabling their repurposing as saleable coal or safer disposal methods.