This thesis concerns the acidic drainage which can emanate from mineral, and mineral waste heaps; and in particular, acid drainage produced by oxidation of iron sulphides contained in rejects of a coal washery. The drainage, high in acidity, sulphate, and dissolved iron, is a serious pollution hazard and must be treated before disposal. Beneficial aspects of acid drainage include the removal of sulphur from coal prior to combustion, thus eliminating the expense of stack gas scrubbing. Metals can also be recovered from low grade ores by leaching in this manner.
The mechanisms by which the metal sulphides, such as pyrite (FeS2), contained in the waste are oxidized and the factors controlling the reaction are reviewed. Bacteria catalyze the oxidation of ferrous ion to ferric, which in turn do the actual pyrite oxidation. Oxidation takes place during a relatively dry stage, after which the oxidation products are removed by flushing when water infiltrates the heap.
The study is separated into two major sections, studies on sulphide oxidation rate, and studies on mass transport of the oxidation products from the reaction site to the heap boundaries. An experimental method is developed for prediction of the actual rate and extent of sulphide oxidation. The results of these experiments showed a constant oxidation rate over time, until pyrite availability was reduced to such an extent that the rate dropped. The oxidation rate is strongly dependent on temperature over the range that the experiments were carried out (25 to 37° C).
The hydrodynamics during flushing were studied using tracer experiments in a column packed with waste material. Dispersion due to channeling proved to be negligible in comparison to that caused by mass transfer between mobile and immobile liquid phases.
Two models are developed to describe transfer of the oxidation products from the reaction site to the mobile liquid, and hence, transported out of the heap. The first model proposed transfer of precipitated oxidation products across a stagnant film and into the mobile liquid, while the second assumes the oxidation products are dissolved in a stagnant liquid phase, and are transported through the stagnant zone to the dynamic liquid phase. The limiting cases for very high product concentration in the first model, and very high dissolved product concentration in the second model, proved to be identical and also were best in predicting the effluent concentrations from column experiments.
Examples of how the models can be used to predict the fraction of the drainage requiring treatment, or the period for which drainage must be treated, are given.
Treatment of only a fraction of the total drainage results in reduced costs in chemicals, capital equipment, and land requirements.