I've always held the belief that mining is really a large-scale exercise in waste management (with the added benefit of gaining value from the ore being extracted!)...as natural ore deposits diminish in concentrations and we are having to dig deeper and deeper to access natural ore bodies, will we find a flipping point where recovery from man-made waste streams will become more economical than extracting from the earth?
When the US Geological Society went looking for gold and other metals in unconventional places, they found that they "hit scientific pay dirt, so to speak...greatly exceed naturally occurring soil metal concentrations," in some cases. This article by USGS speaks specifically to the concentrations of various elements found in sewage wastes and mine waste rock piles, so I thought I'd both summarize their points, and suggest a few more that should garner a bit more focus.
The USGS article noted that just over half of biosolids in the US are currently used as fertilizers, but the rest go to landfills, or are incinerated. Investigations, however, indicated the presence of gold, silver, platinum, copper, zinc and other precious and industrial metals in those biosolids. Sources of most metals are still being explored, but some metal concentrations greatly exceed natural soil concentrations, for example, "about one part per million of gold," which "would be similar to the concentrations measured in low-grade, currently sub-economic gold deposits." What efforts and costs are associated with separating these metals out before the final processing steps to prepare for fertilizer use?
Mine Waste Rock
As indicated by USGS, "waste rock could contain metals with concentrations that were too low to be economically recoverable at the time or metals that weren’t of interest then, but that now have new high-tech applications. At many old inactive mining sites, waste-rock piles and tunnels driven into the hills can be sources of mine drainage waters that may contain high levels of environmentally detrimental, but potentially useful, metals." Historic sources, typically managed as abandoned sites and liabilities (if even documented, as some are not), require funding for appropriate cleanup and closure, which typically comes from federal agencies, and indirectly from tax-payers. Perhaps many of these could in fact be their own little 'gold mines' (forgive the pun!) to help pay for their own management.
The USGS article focused on mine waste rock specifically, which could have included tailings, but lets just make it clear - past processing methods are not as efficient as those of today, and a lot was left behind, including metals that were not of interest, and in fact, this still happens today. Many studies and pilot tests have been done to show the value of re-processing old tailings, and yet it is still such an infrequent activity - perhaps another topic would be to discuss how ownership and legacy liabilities could be managed to facilitate more of this. (A direct add-value to doing this would be that those old deposits would be in a 'cleaner' state after recovery of metals, and there would be potential to geochemically stabilize the rest at the same time, essentially removing the source of those legacy liabilities!)
Yes, this is being done, but not very frequently - why not go after both the energy that can be obtained, as well as the buried metals of yesteryear (and what people still don't recycle!)...
Water Treatment Facilities
Let's face it - there are many sources of drinking water that require some cleanup before consumption, as well as sources used for industrial or agricultural purposes. This can be due to natural geologic influences on sub-surface aquifers, but can also be caused by not-so-natural influences, such as historic contaminant sources, and discharges from industry. Particular sources might be from those industrial waste waters that are being cleaned up for re-use on operating sites.
When we strip 'contaminants' out of those sources, are we checking to see how much of the resultant waste stream might be of value, if we just took that extra step to separate it from the rest? After all, it's already being handled, (and otherwise disposed).
All in all, there are many sources of waste that we, as a society, should be extracting value from - not just for that value, but also to reduce the need for waste disposal sites in general, and to reduce our draw on natural resources that are rapidly diminishing. The population continues to grow, society is demanding equality of life for all, and consumption behaviours are not going away - we will need all that we can get, including our wastes!