In many places iron tailing is stored in ponds or discarded even though its iron content is good enough (>45%) to extract iron from it. Size of particles is the only consideration or iron content too taken care of while deciding tailing as waste.
Tailings are discarded based on the cost of upgrading them at the time they are mined. While the iron discarded in tailings can in principle be recovered, in practice the value of the iron concentrate made is not high enough to warrant the added expense. This may be because the iron oxide particles are too small to be easily separated from the gangue minerals, or because the iron being discarded is in a different mineral than the iron being recovered. For example, if a plant is recovering magnetite by magnetic separation, it is likely to be discarding hematite, goethite, siderite, and other iron minerals that would require a different process in order to recover.
As technology changes, it is perfectly reasonable for these tailings to become attractive as a source of iron and be re-mined. There are companies that have specialized in doing exactly this, like Magnetation LLC.
I agree with Tim's explanation. When mining companies see a commodity becoming scarce, the options such as re-mining are being examined. Many a places worldwide the companies are thinking to mine the old tailing, such as Century Zinc mine in Australia and so on. People are also trying to recover critical metals, such as REEs, PGMs and other elements of interest from tailing.
In essence, the technology, cost, commodity price and depletion concerns are going to determine whether we're likely to re-mine the tailings or not.
I agree with both explanation , in case of tailings o which I carried out experiments problem how to extract REE's or even rodium is under intensive research but till now re-mining is uneconomical, to Cu content is about 0,2% but pracies of copper are stil low, maybe in the future
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