Estimating the longevity of limestone drains in treating acid mine drainage containing high concentrations of iron

Abstract

Limestone drains are often implemented in the treatment of acid mine drainage (AMD), but when the AMD contains high levels of dissolved Fe their lifetime is dependent on the rate of precipitation of Fe hydroxide on the limestone surface. This study used a small-scale laboratory experiment to define the longevity of a limestone drain by determining the thickness of the Fe coating encapsulating the limestone particles when the system lost its maximum neutralising potential. Synthetic AMD (100 mg/L Fe, pH 4–4.8) was pumped through a column containing limestone particles for 1110 h, when the effluent pH had dropped from a maximum of 6.45–4.9. The decline in neutralisation during the experiment was due to the formation of Fe hydroxide coatings on the limestone grains. These coatings are composed of lepidocrocite/goethite in three distinct layers: an initial thick porous orange layer, overlain by a dense dark brown crust, succeeded by a layer of loosely-bound, porous orange globules. After 744 h, a marked increase in the rate of pH decline occurred, and the system was regarded as having effectively failed. At this time the Fe hydroxide crust effectively encapsulated the limestone grains, forming a diffusion barrier that slowed down limestone dissolution. Between the coating and the limestone substrate was a 60 μm wide void, so that agitation of the limestone sample would readily remove the coating from the limestone surface. In the experimental system, the time for sufficient Fe hydroxide to precipitate on the limestone to cause a distinct decline in the rate of neutralisation was defined by: t ( years ) = surface area ( m 2 ) × 4.4 fraction Fe retained in system × [ Fe 2 + ] ( mg / L ) × flowrate ( L / h ) . At this time the limestone in the drain would need to be replaced or mechanically agitated to remove the Fe hydroxide coating, to allow the system to continue functioning at its maximum potential. Application of this formula to the field situation shows that even when the influent Fe concentrations are moderately high, limestone drains will continue to function well for one to several years. Thus, passive limestone systems can be used in AMD treatment when the influent Fe concentration is considerably greater than 1 mg/L, the currently recommended limit, particularly given that the Fe precipitates armouring the limestone grains may be loosely bound and relatively easily dislodged. Therefore, limestone drains are more widely applicable than presently realised.