LEACHING & SX / EW accelerate the kinetics of leaching of refractory minerals( such as chalcopyrite but also of the principal contributor of heat, iron and acid namely pyrite) which would render heap bioleaching a viable alternative for copper extraction from low-grade whole chalcopyrite ore.
The authors conclude that“ at least for the case study presented here it seems that the desired preservation of heat has not occurred to the same extent as predicted. This possibility needs to be accounted for in any future designs of high-temperature heap bioleaching processes.
“ Furthermore, temperature is not the only parameter governing effective kinetics of extraction, and is not throughout the entire process the most important. In this case study the limited availability of acid has governed the rate of extraction during the earlier part of the process. Furthermore, the modelling results suggest that diffusional mass transfer limitations became the limiting factor of the process by the time that 40 to 50 % copper extraction had occurred.
“ Ore heaps exhibit limited permeability to air flow, which can be expected to be further reduced the more saturated it becomes in solution. Apart from limiting the benefit that could therefore be had from heat transfer from the solution to the gaseous phase to preserve heat within the heap, the case of column 1 illustrated that the availability of oxygen could also become rate-limiting.
“ Therefore, the design of heaps intended for high-temperature heap bioleaching need to incorporate means to ensure adequate supplies of acid and oxygen and to prevent, delay or mitigate insofar as possible diffusional mass transfer limitations, throughout the full duration of the process.
“ To date the technology has not been commercially adopted with the lack of flat ground at Darehzare rendering it expensive, and the enhancement of the resource by continued exploration having shifted the economically optimal process from heap leaching to concentrate production.
“ However interest in it remains and it is currently being considered for low-grade primary sulphide ore leaching at Sarcheshmeh.”
Chelating resins
In his ALTA presentation Selective Chelating Ion Exchange Resins in Base Metal Recovery and Refining, Stefan Neufeind( LANXESS) commented that“ the commercial introduction of chelating ion exchange( IX) resins in the middle of last century represented a groundbreaking milestone for the application of solid based adsorbers in the treatment of aqueous streams. Hitherto, the properties of‘ classical’ anion and cation exchange resins had been limited to the separation of negatively and positively charged ions or the preferred adsorption of multivalent cations / anions. Henceforth, the‘ new’ chelating resins bearing iminodiacetic acid( IDA) functional groups allowed to distinguish between two or more types of transition metals with the same ionic charge and to selectively capture only one species.”
His paper gives a comprehensive overview on the use of IDA, aminomethylphosphonic acid( AMPA), bispicolylamine( BiPA), thiourea and other chelating resins at different stages of metal winning, ie:( i) Primary extraction / separation of base metals from leachates to yield concentrates;( ii) Removal of trace impurities from concentrates prior to EW and precipitation steps;( iii) Elimination of heavy metals from mine waste waters to reduce the ecological footprint of a mine( iv) Recovery of trace amounts of metals from residual ores in order to manage the legacy of mining activities.
For example, LANXESS’ s Lewatit ® MonoPlus TP 207 is a weakly acidic, macroporous cation exchange resin with chelating iminodiaceticacid groups designed for the selective extraction of heavy metal cations from weakly acidic to weakly basic solutions. Divalent cations are removed from neutralised waters in the following order( decreasing affinity):
Copper > vanadium( VO 2 + 2
) > uranium( UO + 2 > lead > nickel > zinc > cadmium > cobalt > iron( II) > beryllium > manganese >> calcium > magnesium > strontium > barium >>> sodium.
The monodisperse, uniform sized beads of MonoPlus TP 207 are mechanically and osmotically more stable than ion exchange resin beads with heterodisperse bead size distribution. Additionally they offer superior kinetic behaviour which leads to faster uptake of cations and a better utilisation of capacity.
Among other applications Lewatit MonoPlus TP 207 is used for the removal nickel, cobalt and copper from various hydrometallurgical streams.
Lewatit ® MDS TP 220 is especially suitable for the use in the purification of cobalt electrolytes( cobalt / nickel separation), separation of nickel / copper from ferric solutions, the recovery of copper from strongly acidic solutions( pH < 2) and the adsorption of heavy metals( eg copper) from solutions containing strong chelating agents like EDTA.
The arsenic problem
An increasing proportion of the known remaining copper deposits of the world contain high levels of arsenic. Many new projects under prefeasibility study have the combination of high quality copper and high arsenic. In Chile, where more than 20 % of the global copper deposits are based, half the mines are contending with increased arsenic in the orebodies, and that proportion is projected to increase in the coming years.
Most smelters reject concentrates containing arsenic. Others impose significant treatment cost penalties for concentrates containing more than 0.1 % As. This currently impacts more than 20 % of the world’ s copper concentrate output, FLSmidth reports.“ Since 2014, the volume of copper
NOVEMBER 2017 | International Mining 39