I took your 2.5 X 5 KM resource size and, with a total depth 357 M, I came up with 4.5 billion M3 of material. A single 5000 M3/hr BWE would take 100 years to excavate the entire site.
If we imagine 2 or 3 of these working a site, it is not inconceivable to see how they will be able to extract your calculated 250 million tons of KCL within 25 - 30 years.
The beauty of open pit mining is that it is basically low tech, low cost. Nothing fancy, just BFI (brute force and ignorance).
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