“Most sites have a lot of Fe in their acid mine drainage.” True. But how do you treat it? I do not know. But the answer is on the web. This is what I found.

First I did the obvious and went to the InfoMine Library. There I found these papers by the correspondent, Bernard Aubé, who first alerted me to my ignorance:

Next I looked at his website. It passes my criterion for an informative site: case histories and project descriptions. Here is my edit of one that caught my attention—it epitomizes the essence of consulting engineering, and technology advance:

Blackbird Mine is a closed cobalt operation in Idaho, U.S. The water treatment plant using a High-Density Sludge (HDS) process neutralizes the mine drainage and removes Cu and Fe from solution. To address Cu discharge concentrations, we made physico-chemical measurements, identified key operational issues, and recommended improvements which when implemented brought the effluent Cu concentration down significantly. Producing a high-density sludge remains an issue. Recommendations to address this are being implemented.

Blackbird Mine Treatment Plant

To check my conclusions re treating Fe, I went to the Pennsylvania Department of Environmental Protection. I have found they consistently post high quality informative materials and have grown to trust their conclusions on most mining topics. This search also took me to a treasure trove of information: the abstracts of papers from the Mine Water Treatment Technology Conference in August 2005.

And the answer to treating Fe in acid mine drainage? Here is what Bernard Aubé writes—he is the authority I never will be:

Classic lime treatment remains the standard process for treating acid mine drainage with significant Fe concentrations. Emerging technologies to recover heavy metals such as Ni, Cu, and Zn apply to sites where the concentrations of these metals are considerably higher than that of Fe. Fe sulphides or hydroxides have little to no commercial value. Removing the Fe is therefore strictly a cost issue.

If, for example, one wants to recover Cu or Zn from a stream that contains significant Fe, one may want to precipitate the Fe first, as it theoretically will form a hydroxide at a lower pH if in ferric form. The iron is typically mostly ferrous, and would therefore require oxidation. When completed at a low pH, it either requires very long retention times, or a strong oxidant such as peroxide to convert the ferrous iron to ferric. Either option is expensive.

Then, when precipitation is completed, the other heavy metals are going to disappear from solution as well. This is due to the high adsorption capacity of ferric hydroxides. If the initial Fe concentration is higher than that of other heavy metals, expect most of the heavy metals to be contained in the iron sludge. It is possible in some cases to do an acid wash and remove most of the heavy metals without dissolving the Fe, but this adds yet another step that will increase treatment costs.

Essentially, it all comes down to costs. The primary requirement at most mine sites is the release of excess water from the site with a water quality that meets discharge guidelines. A secondary objective is to do this as economically as possible. By recovering metals, it may be possible to partially off-set process costs. But when Fe is present in significant concentrations, the process complexity required to recover other metals is such that the total costs are higher than simple lime treatment, including the off-setting revenue of heavy metal recovery.