Pyrite oxidation under circumneutral pH conditions 1 Institute of Mineralogy and Geology, University of Miskolc, Hungary; moriczferi@gmail.com 2 Institute of Mineralogy and Geology, University of Miskolc, Hungary; askmf@uni-miskolc.hu 3 Kjeøy Research and Education Center, Norway; ifwalder@kjeoy.no PYRITE OXIDATION UNDER CIRCUMNEUTRAL pH CONDITIONS FERENC MÓRICZ 1 , FERENC MÁDAI 2 , INGAR F. WALDER 3 1. Introduction In the last two decades, the effect of Acid Rock Drainage (ARD) has become the leading environmental problem in the mining of sulphidic type ores. In sulphidic mine wastes a principal weathering process is the sulphide oxidation, resulting in low pH and mobilizing heavy metal contamination. Usually the extracted sulphidic ores consist of different metal sulphides with various oxidation rates. The most common, iron-bearing sulphides – pyrite, pyrrhotite – usually became part of the mining waste and are stored in mine waste facilities where these minerals are exposed to atmospheric water and oxygen that results in quick oxidation processes. However, if neutralizing minerals are available, they can buffer the acid production and hence reduce the heavy metal mobilization. From a mining waste management point of view, a crucial question is whether there are enough available buffering minerals that can consume acidity. Regulations require that the neutralisation potential should exceed by 3-4 times the acid producing potential to consider the waste material safe from ARD production. However, this does not mean that pyrite oxidation stops even under these conditions. In the porous waste rock material in presence of water and oxygen, pyrite grain surfaces start oxidizing, thus creating an acidic micro- environment. Calcite, as a quick neutralizing mineral, is able to maintain a pH range. The net chemical reaction does not show acid production, but sulphide oxidation with possible metal leaching takes place. Neutral rock drainage (NRD) was detected at some mining waste facilities (e.g. mines in the Kola Peninsula, Russia (high-F), the Schwaz and Brixlegg Mines, Austria (high-Sb), the Endako Mine, BC, Canada (high-Mo)), where weathering and consequent metal leaching takes place under circumneutral conditions [1]. The focus of the current work is to prove whether the ARD effects and pyrite oxidation could happen in the zone of neutral pH and to estimate the oxidation rate. Moreover, the authors would like to emphasize the importance of proper waste management and planning of mining waste facilities. This research work was completed to prove whether Equation (1) takes place, to demonstrate the presence of the oxidation process, even if the discharging pH is not acidic.     ) g ( ) aq ( ) s ( ) s ( O O H CaCO FeS 2 2 3 2 8 9 3 2 (1/a) ) g ( ) aq ( ) s ( ) s ( CO SO O H * CaSO ) OH ( Fe 2 2 4 2 4 3 3 2 3 2      (1/b)