Performance of steel slag leach beds in acid mine drainage treatment

• Performance of 12 steel slag leach beds (SLBs) was monitored quarterly for one year.
• SLBs have potential, but current SLB designs limit alkalinity loading performance.
• Plugging of SLBs by calcium carbonate precipitates negatively affects flow rates.
• SLB alkalinity production decreases with time according to a decay model.
• Design changes should allow SLBs to meet performance targets.

Steel slag leach beds are a popular choice for acid mine drainage treatment in southeastern Ohio. Large amounts of alkalinity leached from the surface of steel slag, a by-product of steel manufacturing, have been added to acid mine drainage-affected streams to neutralize pH and precipitate metals. Results from steel slag leach beds are promising, but alkalinity production has decreased significantly over time in all beds. To determine the cause of the decrease, the effluent flow and chemical characteristics of twelve steel slag leach beds were monitored over a one year period and compared to historical data. Alkalinity production fell and remained below design expectations for two main reasons. First, declining effluent flow rates led to low alkalinity loadings. Thick layers of precipitates in the effluent piping contributed to low flow rates. Precipitates also formed within the slag itself, further lowering flow rates. Recommendations to limit precipitation, improve flow and alkalinity loading rates, included using a low-alkalinity influent, and minimizing carbon dioxide contact with the alkaline effluent. Declining alkalinity loadings were also caused by an inherent limitation in the amount of readily dissolvable calcium compounds on the surface of the slag particles. Past and present data were used to model the declining alkalinity production capabilities of slag with time. All steel slag leach beds studied except one were initially capable of high alkalinity loadings, but typically lost more than 75% of peak alkalinity production within 50 empty bed volumes. Similar results were obtained in the laboratory with slag columns.