The biological removal of calcium SI-om a bauxite ore in the presence of the soil bacterium Paenibacillus polymyxa is demonstrated. A column bioreactor that call be either-operated in a fluidized-bed (FB) mode for coarser particles or in a total-recycle-slurry (TRS) mode for finer particles was designed fabricated and rested for this purpose. For the efficient removal of calcium from the ore to the required levels, leach solution should not be allowed to accumulate in the reactor: Therefore, the solution needs to be drained out periodically. Cascade leaching was found to be more efficient. A direct correlation between pH changes in the leach solution and calcium solubility could be made. More than 85% of the calcium could be removed through cascade leaching.