Brought to you by Stefan Sädbom from Lovisagruvan AB
The FutuRaM project (funded by Horizon Europe) is investigating several different waste streams for secondary raw materials, with a focus on critical raw materials, that may eventually be recovered and recycled. One of the largest waste streams is the one from mining, accounting for nearly 64 % of the total annual waste generated within EU.
As long as the mining operation is in production there is an infrastructure and, of course, a financial incentive to recover as many valuable materials from the mined rock as possible, and it is “relatively” easy to add low grade material into an existing enrichment plant on site to recover more valuables.
Historical Mining Waste
When it comes to extraction of valuable materials from historical mining waste, the conditions are quite different. Not only is the site abandoned by the mining company – meaning that ownership, permits and responsibilities are unknown or distributed between different parties – but the site has been more or less remediated, re-vegetated or has a new and different use.
It is very often not entirely clear who is in control of the secondary resource or how it is possible to gain control. Before the legal issues are dealt with, one must question if there is a secondary resource worth the effort to resolve the legal and permitting issues?
Some fundamental knowledge that must be gained is; what is the commodity, what is the content/grade, is it recoverable by technical processes and what kind of impact would extraction cause etc. Finally, taking all factors into account, is it worth it?
Crucial Questions to Ask
The first part, (commodity, content/grade, amount etc.) is a set of questions that require a special study, often raising similar queries that challenge resource geologists exploring and defining the grade, content and value of primary mineral resources hidden in the ground.
A fundamental difference though is that minerals in the ground are distributed along the “logics of geology”, i.e. minerals do most often occur in mineralogical/ petrological associations that are not entirely random. Picking a sample out of solid ground and examining it, it is often possible to predict – with some degree of certainty – what the adjacent sample might look like and what it may contain.
However, once the rocks have been mined, blasted or crushed, so to say handled and processed by destroying a big portion of its association with its “neighbouring piece”; each piece ends up with a – not entirely – random association with the neighbouring mining waste rock specimen as well. Long story short, pick one sample and you may not be able to guess the composition of the adjacent piece with the same level of accuracy as when you picked it directly from the bedrock.
Another factor to take into consideration is that the average of all pieces in the mining rock waste dump does not correspond to the average of pieces whilst they were still in the ground, “in situ”.
Mining has removed some or several of the minerals of interest through a series of more or less well documented technological/chemical processes. The settings for those processes have also changed over time, due to changes in labour costs, chemicals, energy or the value of the commodity extracted as a product. I.e. mining rock waste, although mined from an in theory “homogenous” orebody, will be different depending on when, how, and why different processes were used for the extraction of the valuable component. Mining from the same orebody by handpicking in the 15th century would have created different mining rock waste compared to a fully industrialised mine process in the 1920’s, or 2020’s.
Sampling strategies applied to any are of historic mining rock waste will thus need to either be so holistic that it only reports gross averages or need to be so detailed that it can separate areas of materials with different history and consequently different grade/volume/size fraction locations for different “batches” of mining rock waste. One “batch” might, from the perspective of potential recovery, be very difficult whilst the other may be relatively straight forward.
Sampling – Would You Pick the “Shiny Piece or Not”?
Imagine a mining rock waste area with a surface of 100,000 square meters, distributed as an even 10-meter-thick layer. Your eye can spot more than 40,000,000 rock specimens at just the surface level – some are shiny, others not…
Continue imagining that your mission is to determine the average grade of a valuable component that is distributed in these rocks (might be vanadium, rare earth elements, phosphorus etc.) and occur in, say, 40% of all specimens. How many (and how large of a) sample(s) would you need to collect before you have a good idea of the average grade in that waste mining rock waste? If you have an idea, consider how your sampling strategy would change if the valuable component were present as extremely valuable hazelnut sized nuggets (such as gold or diamonds (although not critical materials) or rare earth minerals…
Would you pick the “shiny piece” or not, and how many specimens, how large of a volume, would you collect to be sure you can estimate the average grade?
Sampling of mining waste rock is merely one of the many challenges we in the FutuRaM project or any entrepreneur engaging in evaluation of possible extraction of critical raw materials must face. We don’t have all the answers, but we try!