The main components of acid mine water are free acid, sulphate, and Fe2+. Limestone is the most cost-effective alkali that can be used for neutralization. The purpose of this investigation was to identify conditions where Fe2+ is removed with limestone and simultaneously oxidized with oxygen to Fe3+, in a polyvinyl chloride pipe under pressure. Gypsum scaling is prevented by passing rubber balls through the pipe of the so-called Oxygen-Pipe-Neutralization (OPeN) process pilot plant. Two synthetic waters were treated: (A) acid mine water containing 123 mg L−1 Fe2+ representing gold mine water, and (B) acid mine water containing 6,032 mg L−1 Fe2+ representing coal mine water. Batch studies were carried out in a pipe reactor and showed that the rate of Fe2+ oxidation depended on the Fe2+concentration, oxygen pressure, amount of recycled sludge, limestone dosage and the mixing rate. Continuous studies in an OPeN process pilot plant resulted in 100% removal of total acidity from synthetic coal mine water and a 98% removal from synthetic gold mine water. Fe2+ was removed completely as precipitated Fe(OH)3 from both synthetic coal and gold mine water at around pH 7 at 200 and 100 kPa oxygen pressure, respectively.