The Prairie Evaporite Formation forms part of the Elk Point Basin, a sub-basin of the Williston Basin centred on the northwest corner of North Dakota. The deposits are all sedimentary with the potash minerals representing the final stages of evaporation of a shallow inland sea. The depositional model described by Garrett as sequential flow during evaporation, suggests the Saskatchewan Sub-Basin, the Central Alberta Sub-Basin, and the Northern Alberta Sub-Basin were cut off from the seas by the Presqu’lle barrier reef. Periodic ruptures or overflowing of the reef due to tectonic changes allowed sea water into the evaporating Elk Point Basin. Similar barriers at the Peace River Arch and the Meadow Lake Escarpment further restricted the amounts of water passing through into the Central Alberta and Saskatchewan Sub-Basins. By the time brines made it into Saskatchewan, most of the salt (halite) had been deposited in Alberta and the brines were highly concentrated in potash and carnallitic salts. The potash salts are confined to the Saskatchewan Sub-Basin.

The mining method employed at Agrium VPO is a room and pillar method utilizing long rooms and sequence delays to allow for stress relief.

In the mine, a fleet of 10 borer style miners are used to mechanically excavate the rock and load it directly onto a series of interconnected conveyor belts. The broken ore is then transported to #1 Shaft where it is hoisted from underground to surface at a capacity of 1800 tph and fed to the mill. The mine is accessed using a fleet of 4x4 trucks and a network of roads that stretches 11 km north, 11 km south and 14km east of #1 Shaft. The borer miners are 3.35 m high, 5.5 m wide and use two, three armed rotors to cut the rock. The miners can advance at about 30 cm (1 ft) per minute and will mine tunnels up to 2,200 m long and 10.2 m wide. These boring machines are supported by 7 drumcutter style miners and associated auxiliary equipment such as scoop trams, bolters, and cutting machines used for rehabilitation. The potash ore being mined contains about 40% potassium chloride (potash), 55% sodium chloride (common salt) and 5% insolubles.

The ore will be hoisted from underground at a nominal rate of 1,800 tph (typically operating for 16 hours per day).

The purpose of the crushing circuit is to prepare the ore for the downstream process by reducing the material size to 95% less than 9.5mm (-3/8”). This is the optimal size for the liberation of the salt and insoluble material in this ore body.

Slimes separation is required to remove the fine potash particles in the primary cyclone overflow from the unwanted insoluble material. This flow is passed over trash screens to remove larger foreign material that can impede the downstream process. The slurry that passes through the trash screens is sent to the inclined plate hydro-classifier to complete the slimes separation process. The current process uses insoluble (or slimes) flotation to achieve this step and hydro-classification has been proven throughout the potash industry to be a much more efficient and cost effective method.

Proper management of saturated process brine is an essential component for sustaining the desired recovery. This is accomplished through the efficient reuse of saturated process brine through the efficient operation of the slimes thickener and the use of tailings cyclones to recover process brine and recycle it back to the process.

Fines and Coarse Rougher Flotation are the first stages of potash recovery. The split between the two circuits occurs at the secondary deslime screens with the +20 mesh material reporting to coarse rougher flotation and the -20 mesh reporting to the fines rougher flotation.

The fines flotation concentrate (along with the coarse rougher concentrate screen undersize) then goes through cleaner flotation. The concentrate from these conventional flotation cells goes onto re-cleaner flotation.

Scavenger flotation is used to recover the fine potash particles from the hydro-classifier underflow, the cleaner flotation tails (existing plant), and the product centrifuge effluent cyclone overflow (both new and existing circuits).

The coarse rougher flotation circuit is required to maximize the production of coarse material and minimize the generation of finer material. However, in the tailings of the coarse rougher flotation circuit there is a significant amount of potash that isn’t sufficiently liberated. The regrind circuit is designed to recover this material by screening, crushing and re-floating the desirable potash from the coarse rougher tails.

The crystallization circuit is required to maximize overall recovery. This circuit converts low grade fine material to high grade product of sufficient size though the use of differential crystallization. The existing crystallization will continue to be used, taking its feed from the concentrate of the scavenger column flotation cells and the fine dust from the product loading operations.