Page 14 - Energize December 2022

INDUSTRY NEWS

Study underway into novel ‘green’ zinc ore

refining process

concept study into the “What we proposed was to go back to the drawing board and rethink the entire
development of a mini-scale, process in terms of its basic chemistry,” explains Professor Petersen. While using an
A simple, green and energy- ammonia medium for base metal processing has been well established, it has yet to be
efficient process to recover pure zinc applied to zinc refining. “The particularly interesting thing about ammonia is that it does
from polymetallic South African ore not dissolve the iron, which simply remains behind.”
concentrates is forging ahead at the Thus, the focus of the concept study, backed by the International Zinc
University of Cape Town’s (UCT’s) Association (IZA) Africa, is direct alkaline ammonia processing of the ore
Department of Chemical “This is an concentrate to dissolve zinc and sulphur in a benign form and leave the
Engineering under the exciting project that iron in the residue. Novel solvent extraction then produces a very
leadership of Professor takes a whole new look at pure zinc stream that can be recovered through electrowinning.
Jochen Petersen. zinc refining in the light of the UCT’s Department of Chemical Engineering has already proven
Processing zinc the effectiveness of this process on the copper mineral
into a pure form need to use low power processes chalcopyrite.
is complicated by and work clean and green.” “It works beautifully in that it can extract the copper almost
the fact that the ore – Simon Norton, Executive selectively, which of course is of great interest in the context of
invariably contains Director, International Zinc zinc refining,” adds Professor Petersen. “The zinc still needs to be
other chemical elements Association, Africa recovered, but that is relatively straightforward. It would need to be
such as iron and sulphur. transferred back into an acid -based system, achievable through solvent
Traditionally, the zinc ore extraction or Ion exchange. We are still exploring these different separation
concentrate is roasted in hot air to techniques,” reveals Professor Petersen. “The important thing to understand is that
remove the sulphur as sulphur dioxide the ammonia itself is not consumed and simply acts as a complexing agent. Once the zinc
(SO 2), which is then captured. However, is recovered, the ammonia is liberated and can then be recycled.”
the elevated temperatures required for The electrowinning required here operates at low voltage and direct current,
roasting fuses some of the zinc and iron which presents the exciting possibility of linking the process directly to using electricity
into an insoluble chemical compound generated by solar power. What is more, the production of green hydrogen by electrolysis
(zinc ferrite). Hot sulphuric acid leaching can also be used to produce the oxygen required in the leach process.
then follows the roasting step to convert “The idea of twinning the metallurgical production with the concept of solar
the zinc into a water-based solution from
which it is then recovered as pure zinc
metal by a process called electrowinning.
“In this process there is always a
significant loss of zinc while a lot of
the iron is dissolved at the same time,”
notes Professor Petersen. Separating
the iron has always proven problematic,
especially as it cannot be used in any
kind of iron smelting process and
invariably ends up in waste dumps. As
some of the zinc is captured in the iron
precipitation process, disposal of the
residue poses both an environmental
hazard and a loss of valuable metal.
In addition, producing high-grade
zinc through electrowinning is energy
intensive.

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