The neutralization of acid mine drainage (AMD) by limestone proved to be effective in various laboratory tests and real-scale applications worldwide. The present contribution describes the results of an experimental approach intended to test the limestone’s efficiency under laboratory conditions. Two open channels have been built, with the length of 1 and 2 m respectively. In the first channel (1 m long), 2.6 kg of limestone (granulometry 5-10 mm) were used at two different flow rates: 25 ml/min and 50 ml/min. For the second channel (2 m long), 5.2 kg of limestone were used at the same flow rates. In each experiment the samples were collected every hour for the determination of ions’ concentrations. The results of all the experiments have shown that in the first hour, the pH value increased from 2.3 up to 4.04-6.26. After 5 hours of contact, the pH value increased constantly between 3.04 and 5.82, in relation with the channel’s length and flow rates. In regard to the flow rate, the highest efficiency in terms of neutralization was observed for the lower flow rate (25mL / min) associated with a pH value of 6.26 for the first hour and 4.55 after 8 hours. Another important parameter associated with the experiments was the limestone grain size. The results showed that larger grain size (20-40 mm) are less effective in neutralizing acidic water compared to the case when a smaller grain size (5-10 mm) was used. The heavy metals (Cd, Fe) and sulphate concentration decreased in the first couple of hours of interaction. Following this study, it can be concluded that the contact time between water and limestone is of high importance. By adjusting the drain length and the water flow rate, there is an increase in the contact time. Efficiency of Cd and Fe retention was observed especially in the first hour of experiments. The same was also noticed for the ability of the limestone to remove SO₄^2-.