Potash at the Milestone Project area occurs conformably within Middle Devonian-age sedimentary rocks, and is found in total thicknesses ranging from approximately 30 to 40 m at a depth of approximately 1,350 to 1,450 m. Evaporites are generally formed by seawater flowing into landlocked basins, followed by the evaporation of the seawater and precipitation of the dissolved salts. Progressive solar distillation of these salt-rich brines results in sequentially precipitated beds of limestone (CaCO3), dolomite (CaCO3·MgCO3), anhydrite (CaSO4), halite (NaCl), carnallite (KCl·MgCl2·6H2O), sylvite (KCl), kieserite (MgSO4.H2O), and other calcium and magnesium salts.

In the KLSA 008 Lease area, the Esterhazy, Belle Plaine, and Patience Lake Members are present. Also present is the “White Bear Marker Beds,” which is a distinctive unit of thin interbedded clay, halite, and sylvinite beds between the Belle Plaine and Esterhazy Members but is of insufficient thickness and grade to be attractive for mining.

The typical sylvinite interval within the Prairie Evaporite Formation consists of a mass of interlocked sylvite crystals that range from pink to translucent, and which may be rimmed by greenish-grey clay or bright-red iron insolubles, with minor halite randomly disseminated throughout the interval. Local large (greater than 2.0–2.5 centimeters [cm]) cubic translucent to cloudy halite crystals may be present within the sylvite groundmass, and overall, the sylvinite ranges from a dusky brownish-red color (lower grade, 23%–27% K2O grade with an increase in the amount of insolubles) to a bright, almost translucent pinkish-orange color (high grade, 30%+ K2O grade). The intervening barren beds typically consist of brownish-red, vitreous to translucent halite with minor sylvite and increased insolubles content.

Following are the thickness and weighted average potash, carnallite (as carnallite mineral [KCl.MgCl2.6H2O]), and insolubles grades for the individual potash members based upon averages of the values for 10 wells:

* Patience Lake Member: 10.55 m averaging 17.97% K2O, 0.64% carnallite, and 10.92% insolubles.

* Belle Plaine Member: 4.40 m averaging 18.53% K2O, 0.56% carnallite, and 4.32% insolubles.

* Esterhazy Member: 5.68 m averaging 20.96% K2O, 4.00% carnallite, and 3.96% insolubles.

* Patience Lake and Belle Plaine Members including waste interbeds: 17.89 m averaging 14.42% K2O, 0.58% carnallite, and 8.46% insolubles .

* Patience Lake and Belle Plaine Members excluding interbeds: 14.98 m averaging 18.15% K2O, 0.61% carnallite, and 8.83% insolubles.

Carnallite is present to a limited degree in all potash members of the Prairie Evaporite Formation; no significant concentrations of carnallite have been detected in the Patience Lake or Belle Plaine Members in KLSA 008. However, drilling by WPX has confirmed that portions of the Esterhazy Member on the KLSA 008 Lease area contain excess carnallite, as shown on Figure 7. Note that the minimum carnallite grades are located in the Pilot Study area west of well M 008. This differs from the expectations of Holter (1969) and Fuzesy (1982), who have shown elevated carnallite concentrations in the Esterhazy Member within the entire KLSA 008 Lease area. Carnallite is also present as a separate unit 6 to 10 m below the Esterhazy Member in wells M 004 and M 005.

The Milestone Project will extract potash from the Prairie Evaporite Formation by means of solution mining. Selection of site-specific cavern dimensions is based on depth, in-situ temperature, and rock mechanics considerations.

The mining method contemplated in the Pilot Study uses secondary solution mining consisting of selective KCl solution mining from horizontally drilled wells utilizing a NaCl-saturated injection brine at a temperature of 100°C, so that solvent temperature at the caverns is higher than the in-situ formation temperature. The injection brine will preferentially dissolve KCl into solution leaving the NaCl and insolubles in place. KCl crystal production is then achieved through cooling crystallization processes.

Secondary solution mining employs injection and production wells that are directionally drilled to be tangential to the bottom of the potash seam. The target geological horizon for the Pilot Study is the Esterhazy Member with a thickness of 5.3 m and a grade of 39.5 KCl at a depth of approximately 1,770 m below the surface. The production well is designed to intersect with the injection well at the bottom of the potash seam with both wells cased to the intersection point. The injection well is extended horizontally, a distance of 800 m from the junction point. An injection tubular is inserted into the injection well and carried to near the outward end of the injection well.

With the tubing in place, brine at 100°C and saturated with respect to NaCl (but not KCl) is injected through the injection tubular. The temperature was chosen to maximize the KCl concentration in the production brine. During solution mining, solvent is injected through the injection pipe to reach the far end of the horizontal well, and flows back toward the point of intersection of the injection and recovery wells through the annulus between the pipe and horizontal hole. The hole will be gradually enlarged into a horizontal cavern as KCl in the potash bed is dissolved and transported to surface through the recovery well. Vertical growth of the cavern is limited by the salt interbed immediately above the Esterhazy Member. When the roof span at the injection point has reached 100 m, the injection tubular is retracted some distance and dissolution is restarted. This process is repeated until the entire 800 m length has cross sectional dimensions of 100-m wide by 5.3-m wide, marking cavern completion. This development of the cavern with time is shown schematically in Figures 14a through 14c.

The production brine returns to surface through a production well and is transported through a 2-km pipeline to the processing plant where the brine is cooled to 12ºC in a single train of barometric cooling crystallizers. The crystallizers are cooled via a recirculating cooling loop using mechanical refrigeration and by a recycle of the crystallization circuit mother liquor.

KCl (MOP) is precipitated on cooling in the crystallizers because KCl solubility decreases with falling temperature whereas that of NaCl remains almost constant. The 12ºC crystallizer mother liquor, which has been depleted of KCl, is re-heated to 100ºC and pumped back to the solution mining caverns as solvent in a closed loop that supplies the caverns through a central brine/solvent distribution system.

A purge stream is required in the crystallization circuit to bleed off MgCl2 in the brine due to carnallite in the mining horizon. The purge stream is directed to a purge pond, from whence the contents are pumped into a disposal well.

The crystallizer circuit underflow is pumped to a single centrifuge. The centrifuge dewaters the slurry to approximately 96 wt% solids. The centrate (decanted mother liquor) is recycled back to the crystallizers in a heat recovery circuit and combined with the mining solvent.

The centrifuge cake is fed by gravity to the inlet of a natural-gas-fired fluid-bed dryer for drying of the KCl product before it is finally fed to a loadout bin to load trucks for transportation to market.