Potash

Potash may be mined from North Dakota in beds of sylvite (potassium chloride) or sylvinite (mixtures of potassium chloride and sodium chloride). Approximately 50 billion tons of potash occur in the Prairie Formation (Devonian) in North Dakota. These evaporites were deposited in a trough that extends from the Northwest Territories in Canada to northeastern Montana and northwestern North Dakota. Potash beds occupy an area of 11,000 square miles that extends from the Montana border to central Bottineau County and from the Canadian border to central Dunn County. This salt interval reaches its maximum thickness of over 500 feet in Burke County. The potash portion of the salt section has a gross thickness of 83 feet.

Thick salt deposits accumulated over the wide expanse of the Elk Point Basin during the Middle Devonian some 400 million years ago, when waters extended down to the northwestern portion of modern-day North Dakota. At times, when water circulation was impeded, potassium salts would precipitate out of dense brine and accumulate into distinct layers within the larger salt body. These potash-rich zones are found in the subsurface in areas of Saskatchewan, Manitoba, Montana, and North Dakota in a rock unit named the Prairie (Evaporite) Formation.

The Prairie Formation was deposited in broad horizontal layers, but due to the structural development of the Williston Basin, the Prairie Formation dips inward toward the center of the basin. As a result, the Prairie Formation is closer to the surface in Saskatchewan than it is in North Dakota. Before 2014, it was assumed that the potash layers in North Dakota were extensions of the Esterhazy, Belle Plaine, and Patience Lake Members which are mined in Saskatchewan. At the time, the potash layer referred to as the Mountrail Member in North Dakota had not been correlated across the Canadian border but was considered likely to be the stratigraphic equivalent of the Patience Lake Member (Anderson & Swinehart, 1979). Subsequently, an NDGS investigation of the Prairie Formation made these cross-border correlations and found that the Mountrail member was a distinct layer that does not extend into Saskatchewan and overlies the Patience Lake member. It was also found that the White Bear Member, present in the southeastern corner of Saskatchewan, but thinning to the status of a marker bed in the Canadian mining region (Fuzesy, 1982), is the most prominent of the potash-containing beds in North Dakota. Finally, a sixth, thin potash bed of very limited areal extent was identified stratigraphically above the other members and was named the White Lake Member.

The main reason that North Dakota's potash deposits have not been utilized is because these same beds are found at much shallower depths which enable the use of both conventional mining as well as solution mining in southern Saskatchewan. There are currently eleven potash mines in Saskatchewan. Mosiac operates a large potash plant that processes brines from a series of solution wells near Belle Plain, Saskatchewan. It is the oldest and largest potash solution mine in the world. Potash is primarily used in the manufacture of fertilizer although testing is currently being done to determine the feasibility of a potash-based water-softening salt.

A typical potash solution mine involves a process of injecting fresh water and recovering brine from a set of two vertical wells to form caverns in the salt below a potash deposit. The caverns are enlarged until open communication between the two wells exists and sumps are created below the wells where insoluble impurities can be held. Once the wells are connected, hot brine is injected into one well where it dissolves potash from the roof and walls before it is extracted through the other well. The process works its way up through the potash layer with an injected oil blanket used to prevent uncontrolled vertical migration. Back at the surface, the potash-containing brine is recrystallized in ponds by natural or mechanical cooling, with both potash and salt (primarily NaCl) being brought to the surface.