Although heap leaching is by now well established in the mining industry, the process remains limited by low recoveries with different rate-limiting factors that are not clearly understood. In this study, three large particle size classes (+191-25, +9.51-16, +4.75/-5 mm) were prepared from a sphalerite ore by two different methods of comminution (HPGR and cone crusher). The particles were then packed into leach reactors that were operated continuously for 11 months with well-mixed internal circulation of the leach solution. Characterization of the residue of the leach reactors indicated that there are areas within the ore particles where although sphalerite grains are accessible to the solution, they remain unreacted. X-ray tomography and QEMSCAN(R) analysis of the selected samples before, during and after leaching, showed increased leaching of sphalerite grains associated with pyrite due to galvanic interactions. Mineral chemistry (Fe, Mn content of sphalerite) and jarosite precipitation were also investigated as factors influencing sphalerite leaching. (C) 2013 Elsevier Ltd. All rights reserved.