LEACHING & SX / EW
“ As chalcopyrite is a semiconductor, the crystal lattice can react with small amounts of copper. We used the very element we were leaching to act as a catalyst, and then strained the chalcopyrite crystal lattice. And in that way, we made the entire particle more vulnerable to chemical attack,” she adds.
The team discovered that small levels of doping accelerated the leach kinetics of the chalcopyrite and could significantly shorten the required residence time within the total leach circuit significantly.
“ Other technologies have relied on fine grinding to increase the surface area of the particles. While fine grinding is effective, it also requires a lot of energy. We needed to come up with a low-energy process,” she explains.
To ensure continued leaching, despite the passivation layer, the team developed a mechano-chemical leach reactor, a Stirred Media Reactor( SMRt). This is 30 times milder than a regular stirred media mill, and 100 times milder than that of regular grinding. The primary leach vessels are linked to this satellite reactor, and the leaching materials circulate between the two.
The SMRt gently scrubs the surface of the particles causing abrasion of passivating films and exposing reactive mineral surfaces. The abrasion of the particle surfaces is balanced to match the leach rate of the particles, so that the soft sulphur layer is removed with minimal energy. The reactor also harnesses free radicals. At the freshly fractured mineral’ s surface, a number of transitory, high-energy surface states are produced; including the likelihood of surface bound free radicals producing reactive oxygen species. With half-lives of seconds to minutes, the benefits to be gained from the generation of these highly-reactive surface species are lost if the processes of grinding and chemical leaching are performed separately, Rocks explains.“ People have really underestimated the reactivity of fresh mineral surfaces,” she says.
Thanks to the low grinding rates of the SMRt the team could combine mechanical and chemical energy and take advantage of the free radicals. The unstable species on the mineral surface quickly react with the ferric lixiviant and result in faster copper leach kinetics, reducing overall energy consumption.
In December 2016, the ROL process team succeeded in leaching chalcopyrite in an integrated pilot plant, combining leach and SX- EW. Roy is excited:“ We produced 99.9 % pure copper cathode on a continuous basis, with full recycle process streams. Next step is to work with customers on a large scale demonstration plant”. ROL performance summary: n Leaches chalcopyrite in less than six hours n Leaches arsenic containing minerals in 15-30 hours n Process low grade concentrates between 7 and 20 % Cu n Integrates with existing SX-EW technologies – FLSmidth can retrofit existing leach infrastructure by adding SMRt reactors n Operates at atmospheric pressures and 80-90 ° C- reduces CAPEX as no pressure vessels required n Is amenable to a feed particle size P90 of 40- 60 μm- so no up-front fine grinding required n No catalyst required- reduces OPEX and no catalyst recovery, regeneration or recycling unit operations required n Can easily be scaled from 5,000 t / y cathode up to 100,000 t / y cathode n Is autogenously heated and net consumption of acid is neutral n Recovery of other metals like zinc or lead that may be penalised by smelter can turn to revenue. Further ability to extract gold and silver n Treat arsenic bearing concentrates on site, reducing smelter penalties and avoid risk of future regulations on transport of high arsenic levels n Concentrates with high arsenic bearing minerals like enargite or arsenopyrite, have higher Cu grades, and also higher amount of gold or silver than clean concentrates. This higher value can be unlocked if the ore can be processed without smelters. This also expands‘ usable reserves’.