Obviously this is a massive fuck up. It will be interesting to read the technical reports on why it happened, assuming they are made public. I believe that the damage can be fixed, especially if the rock is a benign (non-acid generating, low in heavy metals) as first reports are indicating. It will be a massive excavation project, but hopefully the mine is able / allowed to re-open and ongoing production could fund the clean up.
quote:Originally posted by Marc
I'm sure Farmer will be on soon to explain how this created new wealth.
In all seriousness since Farmer works in that industry, I would be very interested on his take on this.
I will respond to you Marc. Steve on the other hand can go fuck himself.
There are two sides to this. Geotechnical issues and environmental issues.
On the geotechnical side, I expect that it will all come down to water management. I have read that their water discharge permit was delayed. A certain amount of water naturally percolates through an impoundment (provided that it is not lined), and the remainder of the water balance must be made up via discharge. Those dikes are meant to have tailings stacked up against them, not standing water. From what I can see in the pictures, there was a fair bit of standing water.
Environmentally, Prother is correct. Sediment is the real issue. Whatever water washed out is downstream and diluted. The sediment on the other hand will have to be remediated. I would expect a massive excavation project, at least within the lakes and creek beds. It sounds like a lot ended up well outside the existing creek bed, and depending on the chemistry of it the best option may be to simply re-contour those areas and re-vegetate.
I pulled up pictures of the failure during our weekly managers meeting this afternoon. I think it served as a good example of why we need to maintain our focus on all aspects of the operation, not just production.
There seems to be a lot of bad information out there on what exactly a tailings impoundment is, what it does, and what is in it. To help understand a tailings impoundment, which is the final stage in the process, I'll do my best to explain the process for a traditional flotation mill, which is what Mt Polley is as far as I am able to gather. This is the most common type of processing for sulphide orebodies. Run-of-mine rock is crushed to the size of gravel (typically in 2+ stages) and then put into mills, which are large rotating steel drums. Water is added to help the material flow. Depending on the rock characteristics, they will use steel rods, balls, or the rock itself to grind the rock to a find powder, similar to flour in size. Mt Polley has a P80 of 50 microns, meaning 80% of the tailings would pass through a 50 micron sieve. That's pretty damn fine. We grind to somewhere around 125 microns.
Once ground up, two types of compounds are added to the slurry. Frothers reduce the surface tension of the water, making air bubble in the water smaller (therefor more surface area per unit volume of air), and collectors attach to the surface of sulphide minerals, make them hydrophobic, which allows them to float to the surface. The slurry is pumped into large tanks, or flotation cells, which have large mixing blades to agitate the mixture, and air lines to add bubbles. The sulphide minerals (which contain the metals) float to the top and are skinned off. There are typically several different steps to separate as much of the metals from the rest of the rock. The sulphides are then dried, and shipped to a smelter as what is called a concentrate. This concentrate may make up less than 1% of the total ore mined in a large, low grade open pit (let alone any waste which isn't sent to the crusher, which was somewhere around 2-3X more material than the ore at Mt Polley). What remains after the sulphides are removed is what is known as tailings.
Tailings are often pumped to a traditional impoundment in a slurry at somewhere >50% solids. That is still a lot of water to deal with. Other disposal methods include backfill systems (more common in underground operations or operations with old pits that can be filled), paste tailings / dry stack, which eliminates much of the water but can create issues with dust, and subaqueous storage, which is actually the most environmentally friendly solution for potentially acid generating tailings, as if you put the tailings deep enough underwater in a low oxygen environment, it prevents the formation of acidophiles, bacteria which act as a catalyst in the formation of acid and the dissolution of metals.
The idea behind a tailings impoundment is that you build a starter dike, typically along the edge of a hill so that you don't need to build it around all 4 sides, and fill the area with tailings. You then continually add "lifts", where the next dike is built either on top of existing tailings or the existing dike, depending on geotechnical considerations. Water percolates though the impoundment and is either collected and treated or allowed to drain naturally if it meets water quality standards.
So all tailings is, is a very fine powder of ground up rock that was already in the area. Most of the compounds added during flotation are removed with the concentrate, and what remains often breaks down in sunlight at the impoundment. These are not "catch all dumps" for any waste products on site, as I have heard people claim. They are not garbage dumps. They are not used to dispose of toxic chemicals.
I have been asked why Quesnel Lake had such a vibrant green / blue color when the tailings / water flowed into it. That is due to the same refraction of light by fine rock powder that makes glacial lakes look so green.