Potash Stories from 3000 Feet Down – Part 1

Between 1985 and 1989 I worked at the Mosaic K1 potash mine in Saskatchewan, Canada.  I had roles that included mine engineer, chief mine engineer and production foreman.   This was relatively early in my work career so it represented an important learning period of time for me.

Each of these roles gave me a different perspective about a mining operation.   In this two part blog, I’ll share some stories relating to the uniqueness of potash mining.

The K1 mine is 15 kilometres outside the town of Esterhazy; about 2.5 hours east of Regina.  The mine is located at a depth of around 3,000 feet below surface, hence the title of this blog.

I will be referring to imperial units in this text because when the mine started production in the early 1960’s the maps and units were imperial and continued on through the 80’s.   At my previous job with Syncrude we used metric units, so it took some time to mentally recalculate back.

Geologically, the province wide Esterhazy Member was being mined, named after the town.  The layer was about 8 ft high.  In other regions of Saskatchewan they are mining a different potash layer (the Patience Lake Member) which is located a few feet higher up. These mines have slightly different mining conditions than we experienced in Esterhazy.  An example of their conditions is shown in the photo, where a parting layer can result in a roof collapse.

The Mosaic K1 and K2 mine shafts are interconnected at depth 6 miles.  The total expanse of the K1 and K2 underground workings probably extends laterally over 18 miles.  Back in the 80’s they said there were over 1,000 miles of tunnel underground and probably a lot more now since the mines are still operating.  There is now a new K3 shaft which allows access to more distant potash reserves.

Potash, also termed sylvanite, mainly consists of intermixed halite (rock salt) and sylvite (potash) crystals. There are also minor amounts of insolubles (clays) and sometimes carnallite (a magnesium salt).  Sylvite (KCl) is the pay dirt, salt is the gangue mineral.

The production rooms being mined were about 75 ft. wide and 5,000 ft. long.  That is quite a large span, not likely possible in many other rock types other than salt rock.  Overall, the mine would extract about 40-45% of the potash, leaving behind the rest in the form of large pillars to support the overlying rock mass.

The mines use continuous miners or borers. In the 80’s they were Marietta miners and now they use borers from Prairie Machine (read more here).   When I was there they were starting to transition from two-rotor to four rotor machines.   The small machines would carve out ~350 tph, and the large ones could do over ~700 tph.

All borers are directly connected to the massive underground conveyor network that moves the ore from the cutter head to the shaft in a continuous process.

Mining excavates an 8 ft. high room consisting of three layers of interest.  A 4.3 ft. layer of high grade ore, a 1.7 ft. layer of low grade ore, and a 2.5 ft. layer of moderate grade potash.  This adds up to 8.5 ft., so the miners try to take the best 8 feet.  The best head grade would be delivered if the borer cut exactly to the top of the high grade 4.3 ft. seam. One didn’t want to overcut into overlying salt or undercut and leave high grade behind in the roof.  I’ll have more on how grade control was done in Part 2.

The air temperature underground is about 25C, so it’s quite a pleasant short sleeve climate.  Around the cutting face, the electric motors, and cutter head, temps can reach into the high 30 degree range. Hot but better than freezing. The mine was dry with very low humidity.   Hence metal didn’t rust.  However, bring the metal to surface and it would rust in no time.

Potash is termed a plastic rock, in that it will deform slowly under stress.  Hard rock will build up stresses and erupt violently in a “rock burst”.  Potash will go with the flow and deform.  Pillars will compress vertically and expand horizontally.  Room heights can decrease over time as much as 6 inches over several weeks in higher stress areas.

Interestingly when the borer shut down and all was quiet, one could walk to the freshly cut face.   You could hear the potash walls gently crackling like rice krispies, as the potash expanded into the opening.  After all, a few moments earlier the potash was being laterally confined under a vertical stress of 3,000 psi. That confinement is suddenly gone.

Roof closure was always an issue.  In panels that were almost mined out, all the ground stress was being carried by the unmined ground.  When advancing into this ground, roof closure could be so rapid that once a borer finished a 4 to 6 week room, the miner operator would need to cut a few inches off  the roof to get back to the front of the room.  It took a couple of days to get the borer out, rather than during a regular shift.

In very high stress areas you could see the floor heaving up as pillars were compressing laterally. That’s when you really knew that mining panel was done and operations should relocate to a new area.

I recall inspecting some of the very old workings from 20 years past.  The rooms looked like new except the roof had compressed down to 5 ft. in height.   We’d have to drive leaning sideways out of the jeep because the steering wheel was literally scraping the roof.

The biggest fear in potash mining is water inflow and mine flooding. There can be a water bearing layer about 50-70 feet above the mining level.  A layer of salt rock separates the two.

If you have groundwater flowing through a dissolvable material, it’s no surprise that the pathway will keep getting larger and larger. That’s why any sign of water seepage is taken seriously and causes the engineering team to leap into action (more on that in Part 2).

At least one potash mine in Saskatchewan has flooded.  A couple of others have come close but have managed to deal with the water inflows.

Barren areas where the sylvite has been leached away and replaced with pure halite are called salt horses.   I never knew why and still don’t.

Exploration core holes could be a mile apart so salt horses would be encountered unexpectedly.  They could be 20 or 2,000 feet in size; you never knew which.   They were a nuisance and an economic penalty.  If you tried to tunnel through them, you would send a lot of uneconomic salt up the shaft to the mill (which they did not like).

Conversely, pulling out and abandoning a room was a painful decision because it took that borer off line for a couple of days to relocate. It’s an unplanned outage, which is not good if another borer happens to be moving at the same time.

On occasion, a borer would abandon the room and move to the next room 50 feet away and could pass by the same area without seeing the salt horse.  Upon reflection, perhaps we should have persevered in the previous room and gone another 20 ft. and we might have been through it.  Often the mill manager hollering about the mill getting too much salt was a deciding factor in whether to abandon a room or keep cutting.

For a year or so,  I also worked as a production foreman.  It was an interesting role. How does a young mining engineer four years out of school tell  guys working underground for 20 years what they need to do?

This also leads to the time when I wrecked a Toyota Land Cruiser underground.   I’ll leave that account to Part 2.  I’ve rambled long enough for today.  You can read Part 2 at this link “Potash Stories from 3000 Feet Down – Part 2″.

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