Ore extraction methods
There are three basic types of potash extraction in use today: 1) Conventional mining that, once the ore level is attained, can encompasses several standard mining methods, modified as necessary for specific subsurface situations, 2) Solution mining of ancient evaporites and 3) Solar processing of modern lake brines (Figure). Solution mining typically requires slightly lower upfront capital expenditure compared to conventional underground mining techniques, which requires significant mine development prior to production. Conversely, operating costs for underground mining techniques are generally lower, compared to solution mining and lake brine extraction processing methods, which can both be energy intensive.
For potash beds that are relatively flat and uniform in thickness, or for cutting entries to the ore zone, boring machines with two or four cutting arms have proven to be an effective and economic mining method. For moderately inclined, undulating and/ or thickening-thinning potash seams, continuous miners with cutting “drums” mounted on one or two moveable arms are the most effective (Figure). In general, two types of continuous mining machines are used: Borer miners and Drum miners. Borer miners apply uniform cutting pressure, have fixed cutting heads, and possess a higher capacity, though they only cut a fixed seam thickness and width. Drum miners have rotating cutting heads that cut sideways against the face and despite their lower capacity, are better able to adapt to changing ore thicknesses. For highly variable ore bodies, or ore with potential rock-bursts or other special needs, drilling and blasting are still required. Blasting is most effective in managing mine sites with large variances in ore thickness, and requires less initial capital and maintenance than continuous mining, though generally results in higher costs overall.
Many potash mine configurations are conventional room and pillar designs. Stress-relief mining (multiple entries with outer yield pillars) has proven highly effective in deeper room and pillar mines, where there is a need to deal with plastic flow (of the ore and the adjacent salt host) or with unstable roof (mine back) conditions. The “room and pillar” method progresses along the potash seam, while pillars and timber are usually left standing to support the potash mine roof. Worldwide, in many shallower operations, where the original pillars collapse rapidly enough for the waste heaps to support the weight of the back, miners can return to extract ore from the pillars.
Whatever the chosen production method, establishing a new conventional potash mine is associated with setup costs well in excess of a billion or more dollars (US$), so new players in the market are rare. The costs are high as the entry shaft for a new mine must be completed without water entry and is usually done via ground freezing. Once the vertical shaft is completed it is protected from water-bearing rocks by the use of tubbings, which are curved segments bolted together and sealed to form stacked rings that line the shaft. They are usually made from cast iron or steel-reinforced concrete. Such tubbed shafts generally have a diameter of 5 – 7 m, and can extend down to depths exceeding 1000 m. Because of the very high costs involved in entry construction for a new potash mine, the proved amount of ore should be sufficient for at least 20 years of production (subject to a given mill size, mill recovery rate for a given ore depth and the density and origin of salt “horses”). Kogel et al. (2006) states the initial plant or mill annual capacity should be at least 300,000 t K2O, in order to compete with a number of established plants with annual capacity in excess of 1 Mt. High lead-in costs mean no significant greenfield potash project involving a conventional mine has been completed in the past three decades, although some brownfield expansion has taken place. There are a number of new operations p[anned but the current low price for potash and a short term oversupply is slowing the development of any new mine.
Solution Mining of Potash
Solution mining entails the injection of brine solutions into underground potash-bearing salt seams. The solution dissolves soluble potash-bearing minerals from the seam and the pregnant potash-bearing solution is then recovered to the surface for processing. Solution mining techniques, focused on caverns in halite, are discussed in detail in Chapter 13, Warren (2016). Solution mining can substitute for conventional shaft mining in some potash deposits at depths of more than 1,100m, which is the current limit for conventional potash mining. It is not an all-encompassing mining alternative to be used whenever potash zones are too deep or too variable for conventional mining methods. Currently, there are six active and a number of planned solution mining operations, focused specifically on potash recovery (Warren, 2016). Thickness, mineralogy, and structure/ore-continuity, as well as other technical considerations of the mining operation, must be evaluated to determine the suitability of solution mining.
As well as purpose-designed facilities, a solution mining approach has been successfully employed in situations where a conventional underground potash mines was accidentally flooded and became unworkable. Of the three current potash solution mining plants in the North Americas, two recover brine from flooded mine, one in Canada (PCS-Patience Lake) and the other (Moab) in the United States. Water is injected as brine and circulated throughout the flooded mine workings, dissolving potash and salt from the original pillars and walls. The resulting dense brine sinks to the bottom of the water-filled mine cavity, is pumped back to the surface and into an evaporation pond or set of crystallizers. High capacity submersible pumps are used in these operations, for example in the PCS-Patience Lake solution mine each is pumping about 9,000 litres per minute.
Intrepid's Moab Potash solution operation, Utah, USA
Patience Lake, PCS solution mine, Saskatchewan, Canada
Lake brine processing and solution chemistry
Potash is extracted from modern lake brines in two forms; MOP (muriate of potash) and SOP (sulphate of potash). In both cases, due to elevated levels of Mg or SO4, sylvite is typically not the precipitated phase, making the production of sylvite from these facilities relatively expensive compared to conventional mines targeting sylvite ore from ancient bedded deposits. The largest set of lake brine potash operations is extracting sylvite from the processing of carnallitite precipitates in man-made solar ponds at the southern end of the Dead Sea. This and other lake brine operations, along with their brine chemistries, are discussed in detail on a separate page (Dead Sea).
Climate is critical for effective solar evaporation or cryogenic production from surface ponds, this is true whether ponds are fed modern lake brines or brines pumped from solution mine cavities. Brine brought to the surface at the Moab mine (Köppen climate BSk) evaporates completely in single ponds to precipitate a mixture of sylvite and halite. Because of its cool steppe arid climate, blue dyes added to pond brines increase solar energy adsorption and so increase brine temperature and rates of evaporation. The operations at Ogden and Wendover, Utah (Köppen Climate, Csa and BWh, respectively), use Great Salt Lake water (Ogden) or nearsurface brines and multiple ponds on the Bonneville saltflat (Wendover). Multiple fractionation ponds are used to establish brine densities for various K minerals at different stages. Efficient solar evaporation occurs during only 3 months of the year (June, July, and August) in the Utah region, with more than half the local evaporation occurring in July. During this 3-month period, low rainfall must combine with sufficient wind to keep the brine mixed and to enhance evaporation. In the potash-harvesting ponds at Moab and Wendover, both sylvite and halite drop precipitate out of the liquor. The harvest is then sent to a standard flotation-type beneficiation plant to separate the two salts.
In the Middle East, the two potash operations at the south end of the Dead Sea benefit from a hot arid climate that promotes evaporation throughout the year (Köppen climate, BSh) whereby carnallite and halite drop out of solution in harvesting ponds. In the nearby processing plant, MgCl2 is leached away with freshwater, leaving a mixture of sylvite and halite. On the Israeli side, sylvite is separated from halite in three different processing plants. The oldest plant uses flotation technology. A more recent capacity expansion added a process that uses dissolution-recrystallisation. The newest plant uses a proprietary cold-crystallisation process that uses a low-heat crystallizer to produce KCl and makes the extraction process more energy and cost efficient.
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