Solution vs conventional

The Saskatchewan prairie has a reputation as a largely featureless flatland, but potash mines must use different extraction methods depending on where they are located. On the plains, it is the conditions below the ground that determine what technique will be most appropriate to reach the potash deposits and bring them to the surface.

Conventional mining

“There are two ways that potash is mined in Saskatchewan,” said Jason Berenyi, assistant chief geologist in charge of Minerals and Northern Geology at the Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy. “Most of our mines are underground at a depth of about one kilometre. These are referred to as conventional mines, where the potash is extracted by mining machines which cut long, large, uniform tunnels through the potash-bearing units. The small pieces of potash are then transported on a system of underground conveyer belts to mine shafts where the ore is hoisted to the surface and then processed in a plant.”

Solution mining

The other potash extraction method is solution mining. “Solution mining doesn’t have any underground component — no personnel or equipment go underground,” Berenyi said. “It involves drilling down into the target formation that contains the potash units, and then following a two stage process to extract the potash.” In the first stage of mining, engineers create a cavern by pumping down water to dissolve out the salt and the potash, which are both water-soluble.

Once the cavern is complete, engineers move to the second stage of mining, in which a sodium-saturated brine is injected from the surface into the cavern. “Basically, they’re trying absorb the potassium without bringing up any more salt,” said Berenyi. The brine pumped down is potassium-deficient but sodium-enriched and designed to absorb as much of the potassium from the walls of the caverns as possible. The efficiency of potassium extraction depends on the amount of time that the brine is in the ground, the temperature of the brine and, of course, the amount of potassium accessible in the rock.

When the potash has been absorbed into the brine, it is pumped to the surface and processed. Said Berenyi, “They bring the impregnated liquid up and precipitate out the potash either in a cooling pond or through mechanical processes like evaporation circuits.”

Conventional mining is, by a wide margin, the more common method in this province. “In Saskatchewan, we have 10 potash mines currently,” said Berenyi. “Eight of those are conventional, underground mines with shafts.”

Different appearance, same potash

Potash extracted in Saskatchewan tends to have a different colour depending on the method used to mine it. “The potash that’s recovered from solution mining tends to not be pink,” Berenyi said. “You know how people associate potash with a pink colour? That pink colour actually has nothing to do with the sylvite or the carnallite [the two most common potash minerals in the province] — the pink is an iron impurity in the mineral. That’s not to say that the pink potash isn’t good or pure, it’s just that the ore contains some iron, so it has that distinctive colour.”

That cosmetic distinction aside, the unprocessed potash brought to the surface by the two methods is of differing composition. Berenyi said that solution mining typically produces potash of a higher initial purity, though both types of mines further process the potash onsite, and the product leaves the mine in a more concentrated form than when it first reached the surface.

Water danger

Each method, moreover, has its own demands and strengths that make it more or less appropriate to a particular location. In Saskatchewan, moisture has long been a confounding agent in operating potash mines. “Flooding is always the biggest concern of an underground potash miner,” said Berenyi. “It’s safe to say that water is one of their biggest threats.”

“It’s a problem with other types of mines too, but it’s more of a concern with potash, or any salt mine really, because those minerals are soluble compounds,” Berenyi said. “So, if you start getting water ingress, it can quickly get out of control because you’ll dissolve the walls of your drifts and you start getting the water migrating to all sorts of areas.”

For example, in the late 1980s, unexpected flooding forced one Saskatchewan mine to change its mining method entirely. “The Patience Lake mine near Saskatoon used to be a conventional, underground mine until it flooded and, after repeated attempts to try and save it, the previous operators gave up and just let it flood,” said Berenyi. “Since then, it has been operating as a solution mine. Because the caverns already existed, they were able to take advantage of the fluid sitting in the caverns to absorb the potassium.”

Deposit depth

An even more significant factor in determining the appropriate potash extraction technique is the depth of the deposit. “I know that, when a lot of people picture Saskatchewan and they picture the potash, they think it’s a uniform layer across the whole province, but that’s not actually the case,” Berenyi said. “The potash is hosted in three layers, or geologic members, in a thick salt unit throughout the southern part of the Saskatchewan and, to a lesser extent, in a fourth lens in the far southeast corner of the province that’s not actually exploited.

“The three potash-bearing geologic members are the Patience Lake member, the Belle Plaine member and the Esterhazy member. They’re actually stacked on top of each other like pancakes. The Patience Lake member is the one closest to the surface, then the Belle Plaine is below that and the Esterhazy is below that one. They all have different geographic distributions throughout the province so, for instance, the Patience Lake is more widely distributed on the west side of the province and tapers out towards the east side, whereas the Belle Plaine is a little bit further out east than that and the Esterhazy is concentrated on the east side of the province.”

“As a result of that, you can get to potash in the Saskatoon area at about 1,000 m below the surface but, as you travel southwest, it gets deeper and deeper,” said Berenyi. “So, for instance, the potash around Regina would be about 1.5 km deep, and then, by the time you’re down into the US border area, you’re looking at closer to a depth of 3 km to reach the potash. While it’s relatively flat-lying on a local scale — miners always say that the deposits are flat and they just follow them along — on a regional scale, it’s actually dipping quite a bit to the southwest.”

Pressure and temperature

Conventional mining is most effective in shallower deposits. “Conventional underground potash mining is typically only done at depths of no more than 1,200 metres — any deeper than that and the temperature and pressure underground are such that the walls of the drifts start to pinch shut because potash and salt start to become ductile,” Berenyi said. However, these same conditions become increasingly more favourable for solution mining the deeper the formations get.

As previously explained, solution mining relies on the ability of the water injected into the formation to act as a solvent, an ability that increases with heat. “You have to equate it to something like a glass of water: if you have a cold glass of water, you might be able to stir in a teaspoon of salt but, if you warm that up, you might be able to absorb two teaspoons of salt,” Berenyi said. “It’s a similar situation with the recovery of the potassium.”

It is the specific details of this process that remain a mystery to outside observers. “Unfortunately, the exact amount of time, and the exact temperature and the exact sodium-potassium balance are all closely-guarded secrets by the industry,” said Berenyi. “They’ve been doing it for years, so they’ve refined it quite well.”

Costs

Another mystery is the relative capital construction cost of one mining method versus the other. Saskatchewan’s current potash mines were all initiated decades ago, in a different financial climate and with different technology. “All of these new companies that are trying to do this now are hiring consultants to come in and look at their projects and produce feasibility studies, and the feasibility studies are what tells them what their construction and operating costs should be,” Berenyi said. “Only one company, K + S Potash Canada, has officially started construction a new mine and they’ve revised their cost estimates upwards since they started building.”

That said, cost cannot be the only consideration in determining which method is used, as the land places demands on the effort that must be addressed regardless of the expense. “The depth is the main thing, and then your geothermal gradient/formation temperature and things like that are going to influence how efficient you’re going to be at extracting the potassium,” said Berenyi. “You can’t just put in a potash mine anywhere — it’s always going to be dictated by the geology.”