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A Changing Environment: Reflections on 50 Years of Hydrometallurgy
by
Emeritus Professor John Monhemius
Royal School of Mines
Imperial College London
ABSTRACT
Looking back over the past half century, it can be seen that the growth in the
importance of hydrometallurgy for the production of non-ferrous and
precious metals almost exactly parallels the rise of the environmental
movement and its principal NGOs, such as Greenpeace and Friends of the
Earth. The paper will review important landmarks in the development of the
science and technology of hydrometallurgy and show how many of these
have been influenced by pressures on the industry brought about by
environmentalists. Successful innovations as well as commercial failures
will be considered and lessons drawn to guide future developments.
________________
2014 is a landmark year for me, because it is the 50 th anniversary of the start
of my career in hydrometallurgy, which began in September 1964 when I
arrived at the University of British Columbia to start an MSc degree in
Metallurgical Engineering. Following a six day crossing of the Atlantic by
ship from Southampton to Montreal, where I was astounded to learn I was
less than half way to Vancouver, and then an eight hour flight over endless
Canadian forests, I finally arrived at UBC’s beautiful campus overlooking
the Strait of Georgia. I had chosen to study at UBC because at that time it
was the leading university in North America and probably the world for
teaching and research in hydrometallurgy. The main attraction was
Professor Frank Forward, who was a world expert in pressure
hydrometallurgy and who had been one of the main driving forces behind
the development of Sherritt Gordon’s revolutionary pressure leaching and
hydrogen reduction process for nickel sulphide ores. Forward was ably
assisted at UBC by two lieuetenants, Ernie Peters and Ian Warren, who both
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went on to make their own successful careers in academic hydrometallurgy,
although Ian Warren’s was cut short by his untimely early death. The
current heir to this great tradition is of course our conference chairman,
David Dreisinger, who is very successfully continuing to ensure that UBC
remains to this day a world-class centre for teaching and research in
hydrometallurgy.
One of the major driving forces for the changes in technology that have
taken place in the mineral industry over the past half century has been “the
environment”. The period in question, i.e. the last 5 decades, coincides
almost exactly with the birth and growth of today’s worldwide
environmental movement, and the NGOs with which we are all now very
familiar – names such as Greenpeace, FoE, Sierra Club, etc.
As a matter of fact, while I was a student at UBC in the mid-60s, there was a
fierce local controversy about a proposed copper mine on Vancouver Island.
The copper ore body was situated on the edge of a deep fjiord at the north
end of the island and the mining company proposed to dispose of the tailings
in the bottom of the fjiord. There was much local opposition to this
proposal, as people were very concerned about pollution and its effects on
the Pacific salmon, which spawn in the fjiord. The opposition was strong
enough to persuade the Provincial government to set up the first ever public
enquiry into a mining project held in British Columbia, at which all the
issues, particularly the environmental issues, were thoroughly examined.
One of the main opponents of this project was a PhD student at UBC, named
Patrick Moore (no relation to the late TV astronomer), who argued the
scientific case against the Island project at the public enquiry. Following
this experience, Patrick Moore went on to be a founder member of
Greenpeace and to become its first executive director. Thus it appears that
the mineral industry was inadvertently responsible for the birth of the
environmental protest movement. Another first for the industry!
In spite of the opposition, the Island Copper mine was eventually built and it
operated successfully for 25 years, under a strict environmental monitoring
regime, which showed that there was no significant detrimental impact on
the ecology of the fjiord. The mine eventually ran out of ore and it was
closed down about 16 years ago in a very innovative and unique manner,
which involved blasting a channel through to the fiord and filling the mine
with sea water.
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Figure 1
The Island Copper Mine, Vancouver Island, BC.
Thus the mineral industry has been under the scrutiny of the environmental
activists for at least 50 years and the pressures brought to bear by these wellorganized groups has had profound effects on the development of
technology in the industry. Back in the 1960s, in the early days of the
environmental movement, nobody was worried about CO2 and global
warming. Instead it was SO2 and acid rain that was making the headlines in
those days. Dying forests and acidified lakes in countries like Canada and
Scandinavia were blamed on the large quantities of SO2 that were pouring
into the atmosphere from the industries of the USA and Western Europe.
One of the most easily identified sources of SO2 pollution were the
metallurgical plants where sulphide minerals were smelted to produce metals
such as copper, lead and zinc. In these processes, the sulphur was oxidized
to form SO2 gas, which in many smelters in those days was disposed of
directly into the atmosphere through tall chimnies.
The responses of the smelting companies to the political and regulatory
pressures brought upon them varied around the world. Some bit the bullet
and made the large capital investments necessary to install acid plants to
convert the SO2 to sulphuric acid. The fortunate smelters, who were sited
close to industrial areas, were able to sell their sulphuric acid to the chemical
industry and thus to make some sort of return on their investments. More
remotely sited smelters, however, had essentially to give away their acid, or
even pay to have it transported away from their sites.
Other companies had more half-hearted responses and exhibited a type of
corporate denial. One of the most notorious of these was INCO in Canada.
INCO’s nickel smelting activities in the Sudbury district in Ontario since the
early years of the 20th century had been largely responsible for the
devastation of the ecology over a radius of many miles around the town of
Sudbury. The company’s grudging response to pressures brought upon it to
improve its environmental performance was to build one of the world’s
highest smoke stacks, the INCO Super Stack, which rose some 400 metres
above the town. This was an extreme example of the “dilute and disperse”
solution to environmental pollution. In other words, the philosophy was to
“spread it thinly over a large area in the hope that nobody notices”.
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Figure 2.
The INCO Super Stack, Sudbury, Ontario.
Other responses by the smelting industry to the Clean Air Acts introduced in
various countries ranged from the extremes of shutting down old-fashioned
smelters, to completely rebuilding them using modern, clean, smelting
technologies. Both of these responses occurred in the United States. Many
of the old US copper smelters are now consigned to history, but on the other
hand, one of the most modern copper smelter in the world was built 1995 by
Rio Tinto on the shores of the Great Salt Lake in Utah on the site of the old
Garfield smelter, which smelted ore from the near-by, massive Bingham
Canyon mine. When it opened, the new smelter was claimed to be the
cleanest in the world, capturing at least 99.9% of the sulphur that enters in
the feed. Such environmental performance, however, does not come cheaply
– the smelter cost Rio Tinto close to one billion dollars nearly 20 years ago.
Figure 3.
Rio Tinto’s Utah Smelter
Another response of the copper industry to the environmental pressures
brought upon its smelters was to consider radically different technologies for
the production of its copper. The Holy Grail was a technology that did not
involve the production of SO2 gas as a co-product with the copper. This
objective can be achieved in two ways:- (i) use copper oxide minerals
instead of copper sulphide minerals, or (ii) turn the sulphur into water
soluble sulphate instead of gaseous SO2. Both of these routes involve the
use of hydrometallurgy instead of the traditional pyrometallurgical smelting
process and I want briefly to look at each of these options in turn.
Prior to the 1960s, copper oxide minerals were spurned as ore minerals
basically because they are not suitable for smelting, as they lack the sulphur
content that acts as the fuel to generate the high temperatures needed for the
smelting process. However, it is precisely the lack of sulphur that makes
these minerals attractive from an environmental point of view. If there is no
sulphur in the mineral, there is no sulphur disposal problem when they are
processed. What was missing at that time was an enabling technology,
which would allow these minerals to be economically processed for their
copper content.
The key to this conundrum was solvent extraction, a process that had been
born in the midst of the second world war, in the race to produce nuclear
grade uranium for the atomic bomb project. For twenty years after the war,
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solvent extraction remained exclusively in the hands of the nuclear industry,
where it was used in the production of a range of exotic metals that were
needed for the construction of nuclear reactors.
When I was an
undergraduate, the conventional wisdom was that solvent extraction was a
process that could only be contemplated for high value metals and it was not
economically possible to use it for common base metals like copper.
Happily, not everybody believed this conventional wisdom and there were
certain individuals who were prepared to “think out of the box”, to use a
modern cliché.
One of these individuals was a young and wealthy American called Maxie
Anderson. Maxie was an entrepreneur out of the Richard Branson mold, a
risk-taker, whose passion was ballooning. In fact, Maxie is best known for
being the first person to cross the Atlantic Ocean in a balloon. He undertook
this epic journey, together with two other American companions, in 1978
and later he and his team were awarded a Congressional Medal of Honour
by the US Congress to mark their achievement. Tragically only a few years
later, Maxie was killed in a ballooning accident in the Alps in 1983, when he
was aged only 49.
Figure 4.
Maxie Anderson’s Memorial Plaque
Before these ballooning adventures, Maxie Anderson had already played a
key role in a development that has transformed the copper industry over the
last half century. In the 1960s, Maxie owned a small copper mine in
Arizona, called the Bluebird Mine. This became the site of the first
application of solvent extraction outside the nuclear industry. For this to
happen, however, required other individuals to also be working against
conventional wisdom. One of these individuals important to this story was
another American from the deep South, a Texan called Joe House. I never
had the opportunity to meet Maxie Anderson, but I met Joe House on many
occasions – he was a delightful character, full of American charm and with a
wry sense of humour.
Joe worked for General Mills, the American food giant, in a division of the
company that supplied solvent extraction reagents to the nuclear industry.
(The reason for this apparently strange combination was that the organic
chemicals from which the solvent extraction reagents were made were
themselves by-products from the processing of certain foodstuffs). Joe
realized, ahead of anybody else, that there was a good market opportunity in
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the copper industry for a solvent extraction reagent that was selective for
copper, and so he set the research chemists at General Mills the task of
designing a reagent which could extract copper from a solution while
leaving behind any other metals dissolved in that solution.
After a number of false starts, the General Mills chemists came up with a
reagent that would do exactly what Joe wanted and it was given the code
name LIX 64N. L/I/X stood for Liquid/Ion /Exchange and 64 for the year in
which it was invented – 1964. The reason for the “N” is not so obvious –
perhaps it was the Nth compound that they had tried!
Joe House’s big problem was now to get his new invention tested. The
metallurgical industry is notorious for its desire to be second to install new
processes. This is where Maxie Anderson came into the story: he owned a
copper mine, he was young and wealthy, and he enjoyed a challenge. Not
having shareholders and a board of directors to worry about, Maxie was
willing to put up his own money to fund the very risky step of introducing a
radically different technology into an industry where the basic methods of
making copper had not changed significantly since the Bronze Age. This
technology, which is now known the world over as SX-EW (Solvent
Extraction-Electrowinning) was installed in Maxie’s Bluebird Mine in
Miami, Arizona and the new plant began operations in 1968, in the heady
days of flower power, student protests and the anti-Vietnam war movement.
This plant was quite small, not much more than a large pilot plant, producing
about 6000T of copper per year, less than a month’s production from a
conventional, medium-sized copper smelter. However its influence on the
copper industry has been profound. From those small beginnings has grown
a technology that now accounts for about one fifth of primary copper
production world-wide and this proportion continues to increase steadily.
Figure 5.
Bluebird Mine, Miami, Arizona.
Only six years after the start-up of the Bluebird plant, two very large SXEW copper plants were opened, one in Zambia on the Copper Belt and the
other in Arizona, not very far from the Bluebird mine. The Zambian plant at
Nchanga was the biggest in the world when it began operations in 1974 and
it remains the biggest, even today, with a rated capacity of 120,000 T/a of
copper metal, although it is many years since it produced at that capacity.
Both these large plants were designed in the UK by Davy-Power Gas in
Stockton-on-Tees. The Anamax plant was half the capacity of the Zambian
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plant, although the equipment, particularly the mixer-settlers, was the same
size in both plants. The difference was that the Zambian plant had twice as
many mixer-settlers as the American plant.
A couple of other landmark plants in the SX-EW story were built in Chile,
the biggest producer of copper in the world. The Pudahuel plant near
Santiago, which was opened in 1980, was the first SX-EW built in Chile.
This was a privately owned enterprise and it was not until some six years
later that CODELCO, the nationalized corporation that ran most of the major
copper mines in Chile at that time, finally took the plunge and opened its
first SX-EW plant. In 1986, almost two decades after the Bluebird mine had
blazed the trail, the SX-EW plant at El Teniente, CODELCO’s giant
underground copper mine, south of Santiago, began production.
Figure 6
Stripping mixer-settlers at El Teniente SX-EW plant, Chile
The resounding success of the SX-EW process in the copper industry is due
to a number of factors, but one of the most important of these is the excellent
environmental performance of the process.
The process is almost
completely closed cycle and it theoretically requires only copper oxide ore
and electric power to produce ultra pure copper at the mine site. Virtually
all these plants use heap leaching to dissolve the copper from the ore with
sulphuric acid. Thus the only wastes from the process are the heaps of
barren leached rock that are left once the copper has been extracted.
Although not terribly sightly, these heaps are physically stable and
chemically inert and so they can be safely abandoned without posing any
lasting threat to the environment.
Figure 7
Closed-cycle flowsheet of Copper SX-EW process
Let us move on now from a success story to a less successful tale, again
featuring the copper industry. I said earlier that the response of the copper
industry to the Clean Air Acts that prevented it from pouring SO 2 into the
atmosphere was two-pronged. The first was to turn to copper oxide
minerals, which contain no sulphur, and that route led to the SX-EW
process. The second prong was to look for other processes for treating
conventional copper sulphide minerals, but which did not produce SO 2 gas.
Thus began the search for the “hydrometallurgical copper smelter”. From
the mid 60s, for about twenty years, a great deal of research and
development was carried out in universities, research institutes and
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companies in pursuit of a process that could compete head-on with the
smelters for the production of copper from copper sulphide minerals, in
particular, chalcopyrite.
Many different process routes were investigated. Nearly all of them were
hydrometallurgical, involving some form of leaching to attack and dissolve
the copper from the sulphide minerals. During dissolution, sulphur was
oxidized, either to sulphate, which being soluble had to be removed from the
leach liquor at some point in the process, usually by precipitation as calcium
sulphate, or a better option was to oxidize sulphur only as far as the
elemental stage, which being a solid, could be recovered as such. Many
different leaching options were investigated, some of them in considerable
detail. Sulphate, chloride, and nitrate systems were used. Virtually every
conceivable oxidizing agent was tried, with ferric iron being the most
popular.
Pressure oxidation was also tried and the first serious
investigations of bio-leaching using oxidizing bacteria were directed towards
dissolving copper sulphide minerals. Although not commercially successful
in those days, the knowledge gained in these studies of bacterial leaching
and pressure leaching was put to good use a decade or so later in commercial
processes for the treatment of sulphidic gold ores, which we shall return to
shortly.
Although it resulted in hundreds of scientific papers and many scientific
conferences around the world, much of this huge body of research work on
copper sulphide leaching did not get beyond the laboratory bench. A few of
the better-developed processes got as far as the pilot plant stage, but only
two made it to full-scale production plants. These were the Arbiter process
and the CLEAR process, which started up at industrial scale in 1974 and
1978, respectively. The Arbiter process, named after Nathaniel Arbiter, the
technical director of the Anaconda company that developed the process, was
based on ammonia chemistry. It involved leaching of chalcopyrite with an
ammoniacal solution with pure oxygen as oxidant. Copper was recovered
from the leach liquor by solvent extraction and electrowinning. The CLEAR
process involved the leaching of chalcopyrite concentrates in ferric chloride
solutions and electrowinning copper as a powder in a one electron process
from cuprous chloride solutions.
Although commercial plants were built for both these processes, neither of
them stood the test of time and both plants were shut down after only a few
years of operation. Neither was able to compete economically against the
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copper smelters, which themselves had undergone modernizations to
improve their environmental performance. So in spite of all this research
and development work, pyrometallurgical processing of copper sulphide
concentrates, using modern flash smelting technology, still reigns supreme
in today’s copper industry. However, “what goes around, comes around,”
and we find that now there is renewed interest in bioleaching and pressure
leaching of copper sulphide concentrates. We shall return to this subject a
little later.
The world-wide economic slump of the early 1980s, brought about by the
OPEC-led oil price rises in the 1970s, resulted in an era of low base metal
prices. This swiftly brought an end to any further technology developments
in the copper industry as it battened down to try to ride out the economic
storms. As there was no money to be made in base metals, much of the
mining industry turned its attention to gold, to try to maintain its cash flows
and keep its shareholders happy.
Now, when I was an undergraduate student in the 60s, gold metallurgy was
incredibly boring. There was the cyanide process (i.e. cyanide leaching with
Merrill-Crowe zinc cementation for gold recovery) and that was it. South
Africa was the dominant gold producer in the west and South African gold
extraction technology had not changed much for half a century since the first
world war. However, things began to change in the gold industry in the mid
1970s. The pace of technological change was slow at first, but it began to
quicken as more and more of the industry turned to gold mining as a source
of economic salvation in the face of low base metal prices. The American
gold industry led the way in technological innovations. Heap leaching,
which had proved to be so successful for copper ores, was first used for gold
ore leaching in Nevada in the early 1970s. At about the same time, the
world’s first carbon-in-pulp plant was started up in 1974 at the Homestake
gold mine in South Dakota. Carbon-in-pulp, which was the first major
change to the original cyanide process since it was invented in 1888, had
been developed in the 1950s by Zadra and co-workers in the USBM
laboratories, but it took 20 years before it finally came into industrial use.
Surprisingly, given the conservative nature of the South African gold
industry, it was only four years later in 1978 that the first CIP plant was
installed in South Africa at the Western Areas gold mine.
Figure 8.
Western Areas CIP plant, Johannesburg, South Africa.
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The pace of change continued to increase through the early 1980s as more
and more new gold mines were opened up in countries like Australia, Brazil
and the USA. Within about ten years, the CIP process had displaced the
traditional zinc cementation process as the industry standard method of gold
extraction and also gold heap leaching became equally well accepted
throughout the mining world.
The boom in gold exploration in the 1980’s turned up many refractory gold
ores, which would not respond to standard cyanidation, usually because the
gold was physically locked inside sulphide minerals. In the past, these types
of gold ore had tended to be overlooked or ignored, because the only process
available to treat them was roasting, which eliminated the sulphur and
arsenic, and left the gold in a cyanide soluble form. However, arsenic and
sulphur in the roaster exhaust gases made this process generally
unacceptable in the environmentally conscious 1980s and so other methods
of treating refractory gold ores had to be developed. Here as has already
been mentioned, technology originally developed for the copper industry,
but never used there, was adopted by the gold industry for treatment of these
difficult refractory ores.
The successful technologies were bioleaching and pressure oxidation. Both
of these techniques lead to the oxidation of sulphur and arsenic in the ore to
form water soluble sulphate and arsenate, respectively, thus eliminating the
air pollution caused by roasting. In bioleaching, oxidation is catalysed by
bacteria. This is a relatively slow process, taking several days, but gold
recoveries from the treated ores are generally good. This process was
pioneered in South Africa in the mid 80s and since then has been installed in
at least 10 gold plants around the world.
Pressure oxidation achieves the same result as bioleaching, i.e. conversion of
sulphur and arsenic to sulphate and arsenate, but it does it very much more
quickly by using high temperatures, around 200°C. This reduces reaction
times to about 1 hour, but this is achieved only at the cost of heavy capital
investment in pressure autoclaves, which can cope with the high water
vapour pressures at this temperature. The first commercial gold pressure
oxidation plant was started up in 1985 at the McLaughlin mine on the
California/Nevada border. Again, once the risk had been taken and the
process had been shown to work on a large-scale, others soon followed. One
of the largest gold pressure oxidation plants ever built was at the Goldstrike
mine near Elko in Nevada. This plant was opened in 1993.
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Figure 9.
Pressure oxidation plant at Goldstrike Mine, Nevada.
The success of pressure oxidation for the treatment of gold ores then
encouraged other parts of the industry to reexamine autoclave technology.
In the case of nickel, pressure acid leaching of nickel laterite ores is already
at commercial scale at several plants in Australia and elsewhere. Even the
copper industry, apparently undaunted by the vast amounts of time and
money spent in the 1970s in unsuccessful attemps to develop
hydrometallurgical alternatives to the copper smelter, has turned with
renewed interest to the pressure leaching of copper sulphide concentrates.
This work has not yet reached full industrial fruition, but two or three semicommercial plants are currently in operation.
Autoclaves are no longer considered to be exotic beasts. They are proving
themselves to be rugged workhorses, which can operate in long campaigns,
comparable with smelting furnaces. The day when the “hydrometallurgical
smelter” becomes a reality is coming ever closer. It will be very interesting
to see whether the second wave of attempts to displace the dominance of the
copper smelters by hydrometallurgical processes will be any more successful
than was the first wave.
Looking back over the past half century of progress in extraction metallurgy,
it is clear that much has changed, but even more has stayed the same. This
is perhaps not surprising in a technology whose history stretches back
thousands of years, to the start of the Copper Age.
Certainly,
hydrometallurgy has become of age and it now sits as an equal partner with
pyrometallurgy and electrometallurgy in the toolkit of today’s extraction
metallurgists.
In this business, there are always fresh challenges. Each new ore body has
its unique features, which often require new technical solutions. Even if the
technology is available, market demands are always changing. Metal prices
are generally in a long-term downward trend, in spite of short-term
fluctuations, whereas specifications in terms of purity and performance seem
always to be increasing. Couple this with ever-tightening environmental
constraints and it is clear that there is much to do in the future. No new
mineral project is ever quite the same as a previous one. For people like
myself, who chose to become minerals engineers, the profession has
provided us with fascinating careers, always challenging, never dull. We
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have each been privileged to play a small part in the on-going development
of the mineral industry, which always has been, and always will be, the
bedrock of the world’s economy.
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