Conventional underground mining

With conventional underground mining, once the ore level is attained, extraction can encompass several standard mining methods, modified as necessary for specific subsurface situations. 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.

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Continuous miner, cutter blade assembly

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Continuous miner, feed assembly moving cut ore to conveyer

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.

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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.

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Lanigan Mine (Nutrien), Saskatchewan

Whatever the chosen underground 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 planned but the current low price for potash and a short term oversupply is slowing the development of any new mine.


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