The kinetics of lime dissolution in acid mine drainage neutralization
BEng Chemical (UCT)
Acid mine drainage (AMD) contains high concentrations of dissolved heavy metals and sulphates, with pH values as low as 2.5. Such acidic wastewater streams pose a threat to plant and aquatic life. It is therefore important to treat acidic effluents before their disposal to the environment. The treatment of AMD involves neutralization using a caustic material or basic solution to adjust pH to acceptable environmental standards. Some of the commonly used bases or alkaline solutions for neutralization are sodium hydroxide (NaOH), calcium oxide (lime, quicklime or CaO), slaked lime (Ca(OH)2), limestone (CaCO3), and soda ash (Na2CO3), with lime being the most commonly used. Lime has been adopted because it is cheap, safe to handle and reacts readily with all types of acids from the strongest to weakest, whether organic or inorganic.
The lime neutralisation process involves two steps: slaking or reaction of quicklime (calcium oxide) with water to form hydrated lime, which is a solid-liquid reaction, and reaction of hydrated lime with an acidic solution to be treated. It has been established from literature that the reaction of quicklime with water is an exothermic reaction; therefore, there are many factors that are anticipated to affect this process. This includes thermodynamic factors, mass transport and phase transition. It is therefore important to establish a strong base of understanding for lime dissolution kinetics.
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