The presence of deep copper deposits associated with primary and secondary sulfides, which have historically been processed via pyrometallurgy, bring with them some associated problems such as the production of sulfur dioxide (SO2), arsenic release and of course the high energy demand inherent to this operation. In addition, the depletion of leachable resources and the potential disuse of hydrometallurgical plants have favored the development and implementation of alternatives to conventional sulfide processing, in order to take advantage of the scalability of operations via hydrometallurgy and the feasibility of modular expansions.
Among the alternatives tested on a large scale, chloride media emerge as a migration opportunity for low metamorphism sulfide operations. In general, the addition of salt in the agglomeration stage enables the improvement of the physicochemical properties of the glomer and the bed, which ensures better recoveries of cupric ion.
The chemical reactions produced in the agglomerator drum by adding sodium chloride (NaCl) in solid form and concentrated sulfuric acid (H2SO4) are highly exothermic, i.e., they release a large amount of energy into the system, which helps accelerate the formation of sulfate bridges between the particles while generating acid gases, mainly hydrochloric acid (HCl). Although the production of gaseous HCl is beneficial to the process because it promotes the oxidation of the species, it should be noted that this gas is highly toxic and corrosive. Due to the above, it is necessary to implement a gas collection and washing system.
The representative equation for the generation of gaseous HCl is as follows:
NaCl(s) + H2SO4(l) 🡪 Na2SO4(s) + 2 HCl(g)
These gases generated in the agglomeration process must be captured by means of extraction hoods, which are located in the discharge sector of the agglomerating drum. Once captured, they must be transported through ducts by means of extractor fans to a scrubber-type scrubbing system.
The HCl and H2SO4 gases are fed to the backwash equipment in countercurrent with a recirculating solution, which generates a zone of high turbulence that favors and accelerates mass transfer from the gas to the liquid. This highly efficient procedure reduces the concentration of these compounds to values that comply with current regulations on general environmental bases and avoids the problems associated with downstream corrosion.
The acidified recirculating solution must be neutralized, for which NaOH is added as an additive to the recirculating flow, from which chloride salts are again generated in solution. To reduce the concentration of these dissolved salts and maintain a balance of reagents in the system, a purge is carried out and returned to the agglomerator drum feed, for which a process water replenishment in the scrubber must be considered.
Relevant aspects to consider are the risks of burns and irritation to the skin and respiratory tract due to the effect of acid vapors, so current regulations must be strictly complied with. On the other hand, the change of materiality in the infrastructure, piping system and equipment is essential to cushion the corrosive effect of the gases generated.
