Abstract
SYNOPSIS The neutralization of acid mine drainage (AMD) with coal discards in percolating columns was investigated as a potential precursor to lime neutralization. The neutralizing capacity of three coal samples, A (70% ash), B (25.3% ash), and C (28.9% ash, estimated), sourced from three South African coal mines, was determined at different crush sizes (-40 mm, -12.5 mm and -6.3 mm). AMD solution obtained from another local coal mine was percolated over the coal samples packed in 1 m and 6 m columns, until the pH of the accumulated drainage solution measured approximately pH 7. Samples B and C, with an alkalinity content equivalent to 2.3% CaCO3, achieved neutralizing capacities of 2112 L AMD and 929 L AMD per ton coal respectively, at a -6.3 mm crush size. Sample A, with an alkalinity content equivalent to 0.48% CaCO3, neutralized only 282 L AMD per ton coal at the same crush size. An economic analysis was performed to compare neutralization with waste coal against lime neutralization in tanks. The analysis was based on a rate of AMD generation of 1750 m3/d, a neutralizing capacity of 1.4 m3 AMD per ton coal, with capital costs estimated at R18 million for lime neutralization and R27.6 million for coal neutralization. Operating costs were estimated at R24 million for lime neutralization and R9 million for coal neutralization. AMD neutralization with suitable waste coal may therefore be less expensive than neutralization with lime. Keywords: acid mine drainage, neutralization, waste coal.
Highlights
Approximately 6.9 billion tons of coal is extracted annually and used for electricity generation, steel manufacture, cement manufacture, and conversion to liquid fuel (World Coal Association, 2017)
Amortised capital (10%, 10 years) neutralization in sample B were more liberated, and more exposed to the percolating solution
➤ Neutralizing capacities increased with decreasing crush size, with sample B achieving capacities of between 804 L acid mine drainage (AMD) per ton coal and 2112 L AMD per ton coal at –40 mm and –6.3 mm, respectively
Summary
Approximately 6.9 billion tons of coal is extracted annually and used for electricity generation, steel manufacture, cement manufacture, and conversion to liquid fuel (World Coal Association, 2017). The coal samples were loaded into 1 m and 6 m columns, and irrigated with AMD solution (pH 2.5–2.8). Representative sub-samples were split out in 25 kg charges for chemical analysis and column neutralization test work. The irrigated solution drained down through the packed coal bed by gravity (with lateral flow by conduction), and was collected at the column base through an enclosed perforated plate, with a drainage hole (fitted with a ‘pigtailed’ tube), for daily collection of drained solution. The daily drainage solutions were collected from the base of the columns (Figure 1a) and the following parameters were measured: mass, volume, SG, redox potential, pH, and temperature. The final accumulated drainage solutions were analysed by ICP-OES and AAS
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