| Summary: | Hydrometallurgical extraction technologies provide a process route for resource recovery of
valuable metals from both primary as well as secondary resources. In this study, the possibility
of treating coal fly ash (CFA), a residue formed as a result of coal combustion in coal-fired
power plants, was investigated. Eskom CFA contains significant amounts of alumina typically,
26-31%, in two dissimilar phases, namely amorphous and crystalline mullite, which may be
processed separately. Due to its high silica content, however, CFA cannot be treated through the
Bayer process route. Therefore, a leach-sinter-leach process was formulated that employed a
two-step acid leach technique to extract alumina from CFA using sulphuric acid.
In the preliminary test work, the effect of parameters on CFA leaching characteristics was
investigated. From the experimental results, appropriate factor levels were found to be 6M acid
concentration, 6 hours leaching time, 75°C temperature and 1:4 solid to liquid ratio. Calcium
sulphate precipitate formation was found to inhibit aluminium extraction and activation energybased
kinetic results showed that aluminium extraction from CFA was a product diffusion layer
controlled mechanism.
By leaching the CFA, and using design of experiments (DOE) and response surface methodology
strategy for screening and optimization of significant factors, it was found that temperature and
leaching time significantly influence the aluminium extraction process. The theoretical optimum
conditions established from the statistically based optimization model, for a maximum
aluminium extraction of 23.9%, was found to be a temperature of 82°C and a leaching time of
10.2 hrs.
Using the optimum conditions, the first stage leaching was done, followed by sintering at
1150°C for 180 minutes to liberate the mullite phase aluminium and then second stage leaching.
An aluminium extraction of 24.8%, representing 89.3% extraction from the CFA amorphous
phase, was obtained from first stage leaching. The second stage leaching yielded an aluminium
extraction of 84.3%. A combination of the two leaching stages gave a total aluminium extraction
of 88.2%.
This work has shown that by employing a leach-sinter-leach method based on a two-step acid
leach technique, CFA can be optimally leached.
|
|---|
