African Mining May 2020 | Page 43

OPERATIONS • Barnabas Mboweni In the confines of the man-made underground caves and mining pits, water is continually intruding through fissures, seepage, rain, and even accidental at times from pipe burst and leaks from service lines. inspection of the cave will rely on the timely completion of the mine dewatering activities. Critical considerations As a starting point, various considerations must be undertaken, most critical of these will be: 1. The vertical distance between the highest point discharge line on surface and the lowest point to be pumped underground. 2. The rate at which the mine is to be de-flooded (currently expected to be around 1 000 cubic meters per hour; enough to fill an Olympic-size swimming pool in 2.5 hours). 3. Available power at the site. These factors will determine the method to be used, discharge pipe sizes, which at a depth of 1 024m, would require a pipe capable of handling over 100 Bar (10MPa) without taking into account any friction head losses through the line. This is close to 100 times the normal atmospheric pressure at sea level (101.325 KPa). Fortunately, the mine (Prieska) is equipped with both a vertical shaft and a series of declines running all the way to the bottom level of the mine. This allows for a few viable options that can be considered for the dewatering of the mine. The 400m landing stations along the decline also provide the perfect opportunity for intermediate pumping stations to form a cascading system. This can reduce the pumping heads to about 40 Bar per station instead of over 100 Bar from the bottom straight to surface. First option would be to go through the vertical shaft. This would entail the use of submersible turbine multistage pumps www. africanmining.co.za African Mining Publication African Mining African Mining • May 2020 • 41