A History of Sherritt

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A History of Sherritt – Fifty Years of Pressure

Hydrometallurgy at Fort Saskatchewan – by M. E. Chalkley,

P. Cordingley, G. Freeman, J. Budac, R. Krentz and H.

Scheie

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INTRODUCTION

The Beginning

In July 1927, Sherritt Gordon Mines Limited was incorporated, and named after Carl Sherritt and the

Gordon family. Carl Sherritt was an American citizen who worked as a teamster on the construction of

the Hudson’s Bay railroad. He later became a trapper and prospector and staked copper prospects in the

Cold Lake area of Manitoba. J. Peter Gordon was a civil engineer who also worked on the railroad

construction and later became interested in mining developments in the area.

The formation of the company was largely due to the efforts of Eldon Brown, a young mining engineer,

with the financial backing of Thayer and Halstead Lindsley and the Gordon family (1).

The Discovery of Nickel at Lynn Lake

In 1941, a Sherritt Gordon prospector named Austin McVeigh sampled an outcrop of sulphide-bearing

rock near Lynn Lake that assayed 1.5% nickel and 1.0% copper (2). It was wartime and Sherritt Gordon

could neither afford the men nor the equipment necessary to stake and drill the area. The discovery was

kept secret until after the war.

In the summer of 1945, McVeigh started staking in a six mile square area which covered all of the known

magnetic anomalies and McVeigh’s original nickel-copper find. A diamond drill was flown in but

drilling on the strongest magnetic anomalies found only magnetite. In September, the drill was moved to

test several weak magnetic anomalies close to Lynn Lake and by the end of the month, an intersection

with good ore grade had been made.

By early 1946, Sherritt Gordon had staked 344 claims in a block approximately 10 miles long and 2 to 3

miles wide, completely hemmed in by outside staking. Sherritt Gordon had secured all of the nickel

deposits.

The drilling program continued through 1946 and in January 1947, a high-grade orebody was discovered,

finally establishing the presence of sufficient ore at Lynn Lake to warrant production. By 1946, Eldon

Brown was president and managing director of Sherritt Gordon Mines Limited. At this time it was

determined that the copper mine developed from Carl Sherritt’s original claim at Sherridon, had sufficient

reserves for only four more years of production. Eldon Brown decided that the mining and milling plant

from Sherridon would be utilized to equip the new mine at Lynn Lake. The story of how not only the

mining equipment but also an entire town, including 258 buildings, was moved 265 km over winter ice

and trails (3) is a story of tremendous achievement unto itself.

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PRESSURE HYDROMETALLURGY

The Early Years

Diamond drill cores from the Lynn Lake exploration program were subjected to flotation testwork at the

mill laboratory at Sherridon. Preliminary results were favourable. Samples of the nickel concentrate

were sent to Professor Frank Forward at the Department of Mining and Metallurgy at the University of

British Columbia (UBC) for a preliminary evaluation. In the meantime, Eldon Brown approached the

major nickel producers, International Nickel and Falconbridge Nickel, with a view to reaching an

agreement on custom treatment of the nickel concentrate. Neither of these companies was prepared to

enter into a long-term commitment needed to bring the mine to production.

The only possibility to make the Lynn Lake Project viable was to establish a fully integrated operation to

produce concentrate and process it to refined nickel metal. At that time, conventional nickel technology

was to smelt concentrates and produce nickel metal by electrorefining. The relatively small ore reserves,

the remote location and the lack of local supplies of fuel and power made that option unattractive. These

circumstances, and the progress being made at UBC led to the development of pressure hydrometallurgy.

Following a meeting in early 1947 between Eldon Brown and Frank Forward, to discuss possible methods

of treating the Lynn Lake nickel concentrate, Eldon Brown stated “We can always smelt and

electrolytically refine but if there is a prospect of another method being devised, we should do everything

possible to find it.”

Sherritt Gordon provided funds for a laboratory program at UBC. Initial efforts focused on adapting the

Caron Process, used at Nicaro, Cuba, to the treatment of roasted Lynn Lake concentrate to produce nickel

oxide. Results of this work were presented at the CIMM Annual Meeting in Vancouver in April, 1948

(4). While the flowsheet was shown to be technically feasible, it was unattractive as it did not produce

finished metal and offered no financial advantage over smelting and electrolytic refining.

In the course of some of the leaching experiments, it was observed that if certain conditions of

temperature, oxygen partial pressure and ammonia concentration were maintained, the nickel, copper and

cobalt present in the concentrate, could be dissolved without prior roasting and reduction. Iron remained

as an insoluble hydrated oxide and sulphur was converted into a soluble form. It was also found that

ammonium carbonate could be replaced by ammonia and ammonium sulphate.

The next step of the investigation was to develop simple and inexpensive methods of separating and

recovering the metals and ammonium salts from the leach solution. The flowsheet contemplated at that

time involved acidification of the leach solution with sulphuric acid after boiling off the free ammonia to

yield a nickel ammonium sulphate, which could be separated by filtration. The remaining solution would

then be heated to decompose “polysulphides” and precipitate copper sulphide. Finally, ammonium

sulphate could be recovered from the barren solution by evaporation. By the end of 1948, the results of

the testwork were sufficiently promising to justify planning and building a small pilot plant.

In December 1948, a meeting was set up between Sherritt Gordon and the Chemical Construction

Corporation (Chemico), a subsidiary of American Cyanamid, who were familiar with the design and

engineering of hydrometallurgical operations, and who were also conducting research in the

hydrometallurgical field. Of particular interest to Sherritt was the fact that the Chemico engineers had

established that nickel could be deposited from ammoniacal solutions as a metal foil or plate on the

reaction vessel walls by reduction with hydrogen under pressure. It was this fortunate turn of events that

brought Sherritt Gordon and Chemico together, and the piloting that followed, that gave birth to

commercial pressure hydrometallurgy.

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Another major milestone in the development process was the hiring of Vladimir Mackiw by Sherritt

Gordon in April 1949. Dr. Mackiw played an instrumental role in the subsequent stages of process

development. The association between Sherritt and Dr. Mackiw was to last for the next 52 years, during

which time the more than 40 patents issued under his name made a significant contribution to the

development of pressure hydrometallurgy. His later roles with the Company included Director of

Research and Development, Vice President, and subsequently Executive Vice President.

The first pilot plant, with a capacity of 275 kg of nickel concentrate per day, was located at the Bureau of

Mines in Ottawa. In a reciprocal agreement, Chemico designed the pilot plant and provided some

technical assistance to the operation while Sherritt Gordon provided engineering staff to participate in

further testwork on hydrogen reduction at the Cyanamid Laboratories in Connecticut. The Ottawa plant

operated intermittently over a period of fourteen months and consisted of a co-current leach, filtration,

precipitation of nickel ammonium sulphate, and copper sulphide precipitation. Initially oxygen was used

under pressure to oxidize the concentrate, but was later replaced by air with a suitable ammonia recovery

unit to scrub the autoclave vent gas.

The major development was the discovery by Mackiw (5) that copper could be removed first from the

leach solution as copper sulphide by modifying the leach to a countercurrent system and ensuring that

trithionate and thiosulphate ions were present in adequate amounts to allow copper to be precipitated by

boiling the solution at atmospheric pressure. By this method, a solution could be produced from which

pure nickel could be recovered, thus simplifying the process by eliminating the sulphuric acid addition

and the nickel ammonium sulphate precipitation step.

By the spring of 1950, it was apparent that the process was technically and economically feasible.

However, many aspects of both chemistry and engineering required further examination before a

commercial plant could be designed with competence. The decision was made to expand the pilot plant

and a disused foundry in Ottawa was acquired and converted. A horizontal, mechanically agitated

autoclave was installed for the second stage leach, while a vertical autoclave was used in the first stage.

The second pilot plant operated on a semi-continuous basis for three months in early 1951, treating 275

kg/day of nickel concentrate. The pilot work on the hydrogen reduction of nickel was transferred to

Ottawa at this time, and high-pressure batch reduction autoclaves were included in the circuit. Following

completion of the pilot run, it was recommended that individual unit operations should be tested on a

larger scale to provide engineering data for scale up purposes.

The next technical challenge was the chemistry of the nickel reduction step. Chemico had successfully

demonstrated a procedure for depositing nickel onto fine seed particles. The challenge was to develop an

economic process for the generation of seed. The Sherritt Gordon team set out to find a method of

producing seed by direct reduction from solution. In the absence of seed, deposition of nickel occurred

only on the vessel walls. The key to success laid in finding a soluble catalyst that would initiate self-

nucleation in a nickel-containing solution. It was found that a solution of the desired composition of

nucleation could be produced economically and simply by using second-stage leach liquor to redissolve

the small amount of plating from the walls of the reduction autoclave.

The final challenge was to resolve the issue of high sulphur content in the nickel powder. The high

sulphur level was traced to the presence of residual unsaturated sulphur species, such as thiosulphate,

polythionates and sulphamate. Treatment of the reduction feed solution at elevated temperature with

oxygen resulted in the oxidation of all unsaturated sulphur species, and the hydrolysis of sulphamate to

sulphate.

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A third pilot plant, incorporating continuous countercurrent leaching, copper removal, and nickel

reduction was operated continuously for about five weeks in early 1952. The design for the refinery was

frozen in July 1952, with completion of construction scheduled for December 1953.

Towards the end of 1952, a fourth pilot plant was assembled in Ottawa. This circuit was a small-scale

replica of the commercial plant and was operated with the objective of identifying and overcoming

problems likely to be encountered during plant start up. The pilot plant operated for about five months,

processed 120 tonnes of concentrate and produced 10 tonnes of nickel metal.

The selection of Fort Saskatchewan as the plant site was influenced by several important factors,

including the availability of an abundant supply of natural gas for making ammonia, access to the North

Saskatchewan River for water supply and the Canadian National Railway for transportation, and was

within easy reach of a large centre of population in Edmonton. Construction work began in May 1952 on

a section and a quarter east of the town and took about two and a half years. The ammonia plant was

completed in April 1954, the leaching section was completed in May 1954, and the ammonium sulphate

plant in July 1954.

THE FORT SASKATCHEWAN REFINERY

Commissioning

Leaching of concentrate started on May 24, 1954. By June 19, the leach circuit was filled and by July 15

feed liquor was available for the metal recovery section. On July 21, 1954, the first nickel metal was

produced and met specifications. The plant reached 90% of design capacity by the end of 1954 and

operated at design capacity during 1955.

Ongoing Development of the Ammonia Leach Process

Through the years, as feed sources to the refinery changed and developments were made and

implemented, the configuration of the leach stages and autoclaves was altered many times. However, the

basic function and operation of the ammonia leach has remained remarkably constant. The dissolution of

metal values combined with the simultaneous oxidation of sulphur forms the basis for the chemistry of

the ammonia leach.

In the ammonia leach nickel, cobalt, copper and zinc are leached into solution. Iron, if present in reactive

form, upon dissolution is immediately hydrolysed and precipitated as hydrated iron oxide. The iron oxide

tailings are removed by thickening and filtration and discarded. Sulphur chemistry is complex, as sulphur

may exist as any of several intermediate oxidation states as well as the fully oxidized ammonium sulphate

and sulphamate.

The typical feed in the early years was a concentrate from the Sherritt Gordon mine at Lynn Lake. It

contained approximately 10% Ni, 2% Cu and 0.5% Co, the remainder being iron, sulphur and gangue.

The leach process operated initially with two countercurrent stages, which were referred to as the

adjustment and final leach. Two adjustment stage autoclaves, operated in parallel, were fed with fresh

concentrate and solution recycled from the second stage. The adjustment autoclaves were operated at

temperatures between 70 and 90ºC and pressures between 690 and 1 035 kPa. The extent of the leach

was controlled to maintain a balance between dissolution of the metals and the production of unsaturated

sulphur compounds required in the subsequent copper precipitation step. The second, or final stage,

consisted of six autoclaves operated in parallel trains each with three autoclaves. The residue was cleaned

in a wash stage in which the residue was repulped to recover as much of the entrained leach solution as

possible.

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The copper in the discharge solution from the adjustment leach was removed by first reacting the copper

with unsaturated sulphur compounds already in solution then by adding hydrogen sulphide to remove any

residual copper.

One of the first improvements to the process was the addition of elemental sulphur and sulphur dioxide,

which react to increase the level of unsaturated sulphur compounds at the copper removal step. This

removed the requirement for a separate precipitation step with hydrogen sulphide, and also gave the leach

more flexibility to maximize metals extraction, as the leach could be allowed to proceed further, knowing

that any deficiency in unsaturated sulphur compounds could be rectified by adding more sulphur and

sulphur dioxide in the copper removal step. This facilitated the subsequent installation of indirect

heating, yielding increased nickel concentration and a proportional increase in downstream production

capacity.

Thickeners and disk filters were used for solid-liquid separation. The disk filters were notoriously

difficult to operate and one of the thickeners had a structural failure in the mid 1970’s. Over time the

failed thickener along with the disk filters were replaced by lamella thickeners and conventional plate as

well as frame filter presses. These changes led ultimately to a significant change in autoclave

configuration in 1992.

When the Lynn Lake mine was shut down in 1976, Sherritt was forced to look elsewhere for their main

supply of feed. The refinery had been augmenting mine output by treating small amounts of mattes and

secondary materials since the early 1960’s. For the next fifteen years, numerous types of feed were

processed. This included everything from concentrate from the Inco mines in Thompson, Manitoba to

mattes and spent catalyst. Some of the feeds came from places as distant as Australia, South Africa and

the Philippines.

The change in feedstock yielded a windfall in leaching capacity, as they did not contain as much

leachable iron. “Sulphide oxidation” capacity previously consumed in leaching the pyrrhotite content of

Lynn Lake concentrate became available to leach additional nickel sulphide units. By the mid-1980’s

nickel refining capacity was around 20 000 tonnes per year, at which level sourcing sufficient feeds from

third parties was an increasingly difficult task.

In 1991, Sherritt started to obtain feed as a mixed sulphide from the Pedro Sotto Alba plant in Moa,

Cuba. The analysis of this feed is significantly different to the concentrate feed for which the plant was

originally designed, containing about 55% Ni, 5.5% Co, 1% Fe, 1% Zn and 0.03% Cu. The higher cobalt

and lower iron content inspired the development of a new nickel–cobalt separation technology (6), with

attendant changes in the leaching steps. Today, Moa sulphides account for 95% of the refinery feed.

Leaching, now known as the Hexammine leach, is performed using a three-stage configuration, consisting

of first, second and final stages. In this configuration, solution from the final stage is fed to the first and

second stages. Solids liquid separation is performed after each stage, and lamella underflow is fed to the

following stage. Leach residue from the final stage is filtered and washed in a Larox filter and is

impounded in the former tailings pond area as a dry cake. The solutions from the first and second stages

are combined and passed through polishing filters to provide feed for the cobalt separation circuit which

is designed to separate cobalt from the mixed nickel cobalt leach solution.

Leaching conditions are carefully controlled to maximize the formation of the cobaltic hexammine

complex. This is a crucial aspect of the pressure leach, as the efficiency of the subsequent cobalt

separation depends on the fact that cobalt exists primarily as the cobaltic hexammine complex in the leach

solution. The requirement for improved formation of cobaltic hexammine was met in part by increasing

the temperature of the leach. One of the tradeoffs was that the concentration of unsaturated sulphur

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compounds in the leach product liquor decreased and as a result more sulphur and sulphur dioxide were

required in the feed to copper boil. Higher leach temperatures also significantly increased the leach rate.

In 2000, the leach operation was modified again to increase the leach capacity and the utilization of the

reactant air. The first stage now uses five autoclaves with the first two autoclaves operating in parallel.

The second stage utilizes two autoclaves and the final stage now uses only one autoclave. The first stage

autoclaves operate at temperatures between 110 and 120ºC, with pressures between 790 and 895 kPa.

The ammonia pressure leach was originally designed with eight autoclaves to process 7 700 tonnes per

year of nickel contained in a concentrate grading 10.5% nickel plus cobalt. The same eight autoclaves

are now processing approximately 34 000 tonnes per year of nickel contained in a mixed sulphide grading

60.5% nickel plus cobalt.

Nickel Reduction

Nickel powder is produced at Sherritt by hydrogen reduction of purified aqueous ammoniacal nickel

ammonium sulphate solution, using basic procedures developed during the pilot plant operations in the

early 1950’s (7). The steady production increases have been achieved with only minor modifications to

the procedures and the installation of larger sized equipment (8).

Prior to the nickel reduction step, product liquor from the leaching process undergoes several purification

stages. Cobaltic hexammine is removed in the Cobalt Separation plant. Ammonia and copper removal is

now achieved in three stages: an air stripping column, in which a portion of the compressed air supply for

the pressure leach autoclaves is contacted counter-currently with the pre-heated solution from Cobalt

Separation, a packed distillation column with a steam heated thermosyphon reboiler, known as the Nickel

Boil, and the original copper boil distillation train, consisting of four pots and the reboiler (9). Copper

sulphide precipitates by reaction with unsaturated sulphur compounds produced primarily by the injection

of sulphur and sulphur dioxide. Sulphuric acid is added to adjust the ammonia/metal molar ratio to a

nominal 2:1 in the reboiler.

The copper free nickel diammine solution still contains small concentrations of unsaturated sulphur

compounds and ammonium sulphamate. The solution is heated to 246 degrees celcius to destroy

ammonium sulphamate by hydrolysis. Air is injected to destroy compounds such as ammonium

thiosulphate and ammonium trithionate by oxidation. These reactions take place in a column reactor with

about 20 minutes retention time. The circuit where these two reactions occur is known as Oxydrolysis.

The column reactor currently used was installed in 1985, allowing the two original oxydrolysis autoclaves

previously used for this step to be used for additional nickel reduction capacity. Operating practice is to

prepare the nickel reduction feed solution with a slight deficiency in ammonia (ammonia/metal molar

ratio of 1.95:1); this effectively stops the reduction before conditions encouraging co-precipitation of

cobalt occur.

In the nickel reduction autoclaves, pure nickel powder is reduced from purified nickel leach solution at

about 185oC with about 3 000 kPa hydrogen overpressure. After a nucleation step using a catalyst to

produce fine nickel seed powder of about 10 microns in size, batches of nickel solution, referred to as

densifications, are charged to the vessels for reduction onto the seed powder. Each densification deposits

a layer of nickel on the seed particle surfaces, and about 60 densifications are required to grow seed into

product nickel. Each densification requires about one hour to complete. Product nickel powder is

discharged, washed, dried and further processed before shipping.

While nickel reduction has been practiced commercially for over 50 years, it remains a unit operation

where operating practice is considerably lower than theoretical optimum. The actual amount of operating

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time nickel reduction autoclaves spend chemically reducing nickel from solution is only 30 to 50% (10).

The remaining operating time is consumed by “non-productive” batch operating steps: filling the

autoclave with fresh solution, settling powder from spent solution, discharging the settled solution, and

rinsing the autoclave. A viable continuous reduction process capable of producing high quality nickel

product remains a research project.

From the beginning of operations, much of the powder was compacted into briquettes weighing about 65g

each that, after sintering, analyzed less than 0.02% sulphur. By 1962 production had increased to about

14 500 tonnes, with the installation of additional reduction equipment. Today both sintered and

unsintered briquettes representing about 90% of the present production, are bagged in 2 tonne bags for

shipment to customers in Europe and Asia.

Sherritt nickel reduction technology, now licensed by Dynatec Corporation, is presently used at WMC

and Minara Resources in Australia, Impala Platinum in South Africa and OMG in Finland.

Coinage and Specialty Metal Powders

The research group, headed by Vladimir Mackiw, discovered that the Sherritt nickel hydrogen reduction

process, with the controlled addition of organic additives, could produce nickel powder with unique

physical properties.

Sherritt scientists soon produced a nickel powder that could be easily compacted, sintered and rolled into

high-density strip, leading to the construction of a rolling mill in 1961 to produce strip and blanks for

domestic and international coinage. Today all Canadians and much of the world’s population have daily

contact with Sherritt produced nickel in the form of coinage. The most famous Canadian coin developed

by Sherritt is the one-dollar coin, commonly known as the “loonie”.

In addition to the development of coinage strip, Sherritt scientists also discovered that both metallic and

non-metallic particulates could be activated and coated with a continuous layer of nickel using the Sherritt

nickel reduction process.

Every conceivable material that could survive the reduction process was coated with nickel in the

laboratory. Successful commercial production of nickel/aluminium, nickel/graphite and nickel/carbides

powders soon occurred, developing into a thriving specialty materials business producing dozens of high-

tech materials for aerospace and electronics applications. Pratt & Whitney, United Technologies, General

Electric, Rolls Royce and NASA were customers and frequent visitors to the plant site. In the laboratory,

research staff produced thousands of specially requested mono and composite powder samples, from a

few grams to several kilograms in size, for testing by commercial and university laboratories worldwide.

Corporate restructuring during the 1990’s removed the business of producing value added nickel products

from the refinery business unit. Today, products include commodity nickel powders, sintered and

unsintered briquettes only.

History and Development of Cobalt Production

As Sherritt was developing the hydrometallurgical process for refining nickel, they were also faced with

the question of how to separate cobalt from nickel, and then what to do with the cobalt. The selection of

hydrogen reduction technology to produce metallic nickel powder also provided Sherritt with a primary

nickel-cobalt separation step. As long as the ratio of nickel to cobalt is large, nickel can be selectively

reduced with hydrogen without reducing cobalt.

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The Lynn Lake concentrate, with typical ore grades of 10% nickel and 0.5% cobalt, yielded nickel

reduction feed solution with relatively low cobalt content (nickel/cobalt ratio greater than 30:1). Since the

relatively small amount of nickel and cobalt remaining in the solution after nickel reduction could be

precipitated from solution with hydrogen sulphide to yield a saleable intermediate nickel-cobalt sulphide

product, development and construction of the nickel refinery was able to proceed without a final answer

as to how to handle the cobalt.

Many alternative cobalt flowsheets were studied. The Ottawa pilot plant was closed in 1955 and some of

the pilot plant equipment was shipped to Fort Saskatchewan where it was used in the assembly of a

“commercial sized” cobalt refinery. Output of this plant, at less than 150 tonnes of cobalt per year, was

so low that it was only utilized for commercial cobalt production for part of the year, and used for pilot

scale development of other hydrometallurgical processes during the remainder of the year. Refining of

nickel-cobalt sulphides, utilizing an acid leach of the sulphides, began on June 16, 1955.

The initial cobalt refining flowsheet, known as the Preferential Nickel Reduction process, produced cobalt

metal containing 0.4% nickel. The process required close operating control and involved stopping the

reduction process before equilibrium conditions were achieved, as equilibrium would result in higher

nickel content in the cobalt powder. In addition to the cobalt metal product, the process also generated a

mixed nickel-cobalt metal product containing 80% nickel and 20% cobalt that was recycled to the nickel

refinery feed or sold as an impure product. However, Sherritt was not satisfied with this process.

By 1958, Sherritt had begun to use the Soluble Cobaltic Ammine process, which Chemico and Sherritt

had developed based on the patents of Chemico’s Schaufelberger (11). In this process, purified acid leach

solution is ammoniated and oxidized at elevated temperature and pressure. Cobalt is oxidized from the

divalent cobaltous state to cobaltic pentammine sulphate with nickel remaining in the divalent state as

nickel ammine sulphate. Subsequent acidification permitted quantitative removal of nickel (and any

remaining cobaltous) as precipitated nickel ammonium sulphate while cobaltic pentammine sulphate

remained in solution.

The filtered cobalt solution was concentrated by evaporation before a small portion of the cobaltic was

reduced back to the cobaltous state by reaction with a small addition of metallic cobalt powder and

sulphuric acid. The cobaltous crystallized out as a cobaltous ammonium sulphate salt, carrying with it the

remainder of the nickel. Cobalt to nickel ratios of 1 500:1 were achieved by this method. The nickel-free

cobaltic pentammine sulphate was converted to cobaltous ammine sulphate by reaction with pure metallic

cobalt powder before reduction with hydrogen to metallic cobalt.

Both the Preferential Nickel Reduction process and the Soluble Cobaltic Ammine process are described in

detail in the literature (12). The Soluble Cobaltic Ammine process was also used by the National Lead

Co, at their nickel, copper and cobalt refinery at Fredericktown, Missouri, USA, and was installed by

Freeport Nickel Company at Port Nickel, Louisiana, USA. This process, while somewhat complex and

recycle-intensive, served Sherritt well and was used until 1992 when it was replaced with the Cobaltic

Hexammine process.

As described earlier, when Sherritt began treating Moa sulphides, the proportion of cobalt reporting to

nickel reduction increased dramatically. Without additional change in the flowsheet, this would have

presented the unacceptable dilemma of accepting either higher cobalt content in the nickel product, or

reduced nickel reduction capacity due to the need to leave higher levels of nickel unreduced in the end

solution. As with the preferential Nickel Reduction Process, process control would be a significant

challenge.

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The development of the new cobalt separation process, located prior to nickel reduction in the flowsheet,

provided a suitable solution to this dilemma. Today, leach solution is fed to the cobalt separation plant,

where cobalt is separated and purified as crystalline cobaltic hexammine sulphate. The initial step of this

separation involves the precipitation of a cobalt-nickel hexammine salt, which is controlled by the

concentration of ammonium sulphate and ammonia in the leach solution.

The cobalt-nickel salt is then dissolved in water, where the relatively insoluble cobaltic hexammine

sulphate precipitates, while the more soluble nickel hexammine sulphate remains in solution. This

selective nickel removal from the cobalt-nickel hexammine salt results in the separation of a purified

cobalt salt. It is this selective removal of nickel and the separation of a pure crystalline cobalt salt,

repeated in two purification stages, that forms the basis for Sherritt’s ability to produce very high purity

cobalt metal (10 000:1 Co:Ni).

The nickel rich solutions are then directed to the nickel plant for further solution purification steps and

ultimately nickel powder production. The cobalt content of this stream, at a nickel/cobalt ratio of greater

than 20:1, is suitable for feed to the existing nickel reduction flowsheet without compromising nickel

product quality or production capacity.

The cobalt hexammine salt is re-crystallized to further remove any low levels of nickel contamination.

After washing and filtration, the re-crystallized cobalt salt is dissolved in an ammonium sulphate

solution.

Cobaltic present in the pure cobaltic hexammine solution is converted to a cobaltous ammine, before

precipitation as a metal powder in a reduction autoclave. Cobalt powder is produced in a reduction cycle

comprising one nucleation reduction, in which fine cobalt powder of sufficient surface area to function as

catalyst is prepared, followed by up to 60 densifications. The purpose of the densification reductions is

to deposit cobalt metal onto a seed particle until desired powder density, chemical composition and screen

fraction of the powder have been achieved. The reduction of cobalt to metal powder is very similar to

that of nickel reduction.

At the completion of a reduction cycle, the product slurry is discharged from the autoclave to a flash tank

and the cobalt powder is transferred to a vacuum pan filter. Cobalt powder is washed and cooled

thoroughly with water. Care must be taken during washing to prevent contact with air, as the powder

oxidizes rapidly. The washed powder is dried in a jacketed tumble drier under vacuum. The powder

product is canned and sold as such, or if required, is briquetted and sintered using similar techniques as

for nickel briquettes.

Pressure Hydrometallurgy at Moa

The acid pressure leach process for the treatment of low magnesium content lateritic ore has been in

operation at the Pedro Sotto Alba plant in Moa, Holguin, Cuba since 1959. The plant was originally

constructed by Chemico for Moa Bay Mining Company, a subsidiary of Freeport Sulphur, but was taken

over by the Cuban government in 1960. The plant recommenced operations in 1961, under Cuban

management.

Under Cuban management the production at Moa gradually increased and improvements were made to

the recovery of nickel and cobalt. In December 1994, Sherritt Inc. and General Nickel Co. S.A.

announced the formation of a combined enterprise that included the Moa plant, now known as Moa

Nickel S.A. The nickel and cobalt sulphides produced by Moa Nickel S.A. (13) are transported to the

nickel and cobalt refinery at Fort Saskatchewan, Alberta, Canada now known as “Corefco” (The Cobalt

Refinery Company Inc.), a second combined enterprise company, for processing to pure metal products.

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At Moa, Nickel limonite ore is processed in a high-pressure acid leach to selectively dissolve nickel and

cobalt from the ore. Concentrated sulphuric acid is the lixiviant.

The ore consists entirely of mixed oxides; sulphides, sulphates and carbonates are not found in the ore.

The ore typically contains 1.3% Ni, 0.12% Co, 0.55% Mg, 4.8% Al, 4.3% Si and 2% Cr.

Extensive laboratory investigations demonstrated that high extractions of nickel and cobalt could be

obtained with a minimum dissolution of iron and C. At°aluminum by operating the leach at temperatures

in the range 230 to 260 these temperatures, acid consumption is minimized since the majority of the

dissolved iron and aluminum hydrolyze and precipitate with the liberation of the acid initially used to

dissolve these species.

The leach circuit consists of five independent trains, each having four Pachuca type vertical reactors

connected in series. Ore thickener underflow pumps deliver slurry to a direct contact slurry preheater

where the slurry is heated by low pressure (100 kPa) steam to between 70 and 80 degrees celcius The

preheated slurry is delivered to a direct contact heater where the slurry is heated to 246 degrees

celcius with high-pressure (4 500 kPa) steam. This steam is a combination of reactor vent gas and fresh

steam supplied from the powerhouse and the acid plants. The use of direct steam for heating the feed

slurry results in a reduction in the solids content of the slurry.

Each Pachuca reactor contains a draft tube and is agitated by the injection of high-pressure steam.

Concentrated sulphuric acid is fed to the first reactor. Slurry passes through the reactors in the train by

overflow pipes.

Slurry overflows from the fourth reactor through two parallel slurry coolers C. The low-pressure (100

kPa) steam°where the temperature is reduced to 135 used in the slurry preheaters is generated in these

coolers. The slurry discharges to flash tanks and then to the counter current decantation (CCD) wash

circuit. Low-pressure steam from the flash tanks is used to preheat the solution entering the sulphide

precipitation autoclaves.

The parameter that has the biggest impact on metal extractions is the acid addition, which in turn sets the

free acid concentration in the leach discharge solution. Metal extraction increases with increasing acid

addition. Acid addition also affects the sedimentation characteristics of the leach discharge slurry, with

the sedimentation of the slurry improving at higher acid concentrations in the leach discharge solution.

Aluminum is the largest consumer of acid.

Since the plant commenced operation, metal extractions have averaged 94% and values in excess of 95%

are now regularly obtained with good control of acid addition. The metal concentrations in the leach

discharge solution are largely influenced by the solids content of the ore slurry feeding the leach reactors.

The nickel concentration is generally in the range 6.5 to 7.0 g/L and the free acid concentration is in the

range 30 to 35 g/L.

A major feature of acid leaching of limonite ores is the formation of scale in the reactor. The scale builds

up to a thickness of 75 to 100 mm over a period of three to four months. Periodically each leach train is

shut down and the scale is removed by pneumatic hammers. The majority of the brickwork in the leach

reactors is the original lining installed in 1958. Small repairs to the brickwork are carried out as required

when the leach train is shut down for scale removal.

Leach residue is separated from the metal-rich leach solution, raw liquor, in a seven-stage CCD circuit.

Raw liquor is contacted with hydrogen sulphide gas to precipitate copper as copper sulphide and reduce

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ferric iron to the ferrous state, and hexavalent chromium to the trivalent state. Limestone mud is then

added to the raw liquor to neutralize excess free acidity and raise the solution pH to 2.5.

Nickel and cobalt are selectively precipitated from the neutral solution by hydrogen sulphide gas in the

sulphide precipitation plant. Zinc also precipitates, but magnesium, manganese, iron and aluminum

remain in solution. The precipitation reaction is carried out in four, three-compartment, mechanically

agitated, horizontal autoclaves.

C with flash tank exhaust steam from°The neutral solution is preheated to 82 the leach plant, and is then

heated by 100 kPa steam, generated in the leach C and 1°C. The autoclaves operate at 121°plant coolers,

to between 116 and 121 034 kPa. A continuous vent is maintained in each autoclave to remove inerts and

control the hydrogen sulphide concentration at 60 to 70%. The vent gas is cooled, enriched with fresh

hydrogen sulphide and used in reduction. Slurry discharges to the flash tank with the vent gas from the

flash tank cooled by direct contact with water. The gas is compressed and recycled to the autoclaves with

makeup of fresh gas. The water from the vent gas cooler is discharged with the precipitation thickener

overflow solution as waste liquor.

The slurry from the flash tank, which has a solids content of about 3%, is fed to one of two parallel

thickeners. Flocculant, Magnafloc 455, is added to reduce the suspended solids content of the overflow

solution. The solids settle rapidly and an underflow solids content of up to 60% solids is obtained. This

slurry is stored in tanks before transportation to the washing and drying section at the port, where the

sulphides are bagged for shipment to the refinery in Fort Saskatchewan.

Three of the original autoclaves and linings are still in use. The autoclave linings are brick and rubber.

All autoclave internals are Hastelloy C or titanium. After about 1 200 h of operation, an autoclave is shut

down to remove scale buildup from the walls. The scale, which is chemically the same mixed sulphide, is

recovered, ground and shipped together with the precipitated mixed sulphide product.

Application of Sherritt’s Pressure Hydrometallurgical Technology to Other Metals

Much of Sherritt’s metallurgical and product technology developed over the last 50 years can be traced

back to work done during the development of the ammonia leach process. Pressure leaching of sulphide

ores and concentrates, using continuous horizontal autoclaves, provided the basis for a thriving pressure

hydrometallurgical process licensing business which offered processes for treating nickel mattes and

concentrates, zinc concentrates, and refractory gold ores and concentrates. The nickel reduction process

perfected in the Ottawa pilot plant was subsequently licensed worldwide.

During the early 1950’s, following the successful commissioning of the nickel refinery at Fort

Saskatchewan, Sherritt utilized its laboratory and pilot plant facilities in Ottawa to look for other potential

applications for pressure leaching processes in the metals industry (14). Laboratory tests were carried out

on the pressure leaching of uranium ores and on the pressure oxidation of refractory gold ores, where the

oxidative pressure treatment proved an excellent method for oxidizing pyrite and arsenopyrite to liberate

the gold for subsequent recovery.

Two additional leaching plants were built by Chemico to treat cobalt concentrates in the aftermath of the

Korean War, when the cobalt price was artificially high, but both plants became uneconomic as the price

of cobalt declined, and closed in the early 1960s. A fourth pressure leaching plant was the Port Nickel

plant, constructed by Freeport to treat the nickel-cobalt sulphide from Moa.

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In 1957, Sherritt purchased the interests of Chemico in all patents in the chemical metallurgical field in

which Sherritt was involved. These patents covered inventions, which were made or developed during the

period 1947 to 1956, leaving Sherritt as sole owner of jointly developed technologies. This included an

assignment of the royalties from Freeport for the use of these patents in its plants under construction at

Moa. An unfavourable development in 1960 was the action of the Cuban Government in taking over the

Moa nickel operation. This deprived Sherritt of a source of revenue and made the license agreement

covering the operation of this plant of doubtful value. The Port Nickel plant was shut down and

mothballed, until refurbished by Amax in 1974 as a refinery for nickel-copper matte.

During 1957, the Research and Development group demonstrated the feasibility of treating nickel matte

by the Sherritt leaching and reduction processes (15). This was very important because it widened the

potential supply of additional feed materials for the refinery at Fort Saskatchewan. By the early 1960s the

only surviving operating pressure leach plant was Sherritt’s nickel and cobalt refinery at Fort

Saskatchewan. Sherritt, under the technical leadership of Vladimir Mackiw, continued to develop the

acid pressure leaching technology with a series of pilot plant campaigns on a variety of nickel and nickel-

copper mattes.

These efforts paid off with overseas process license sales. The acid pressure leach for nickel-cobalt

sulphides was licensed to the Finnish nickel company, Outokumpu, in 1967. Two years later the

ammonia refining process for nickel concentrates was licensed to Western Mining Corporation in

Australia, and a new acid leaching process for separating platinum group metals from nickel and copper

sulphides was developed for Impala Platinum in South Africa. By 1991, the latter process has been

licensed to five major platinum producers in South Africa. In 1996, the first North American plant, based

on the flowsheet at the Western Platinum refinery was commissioned for Stillwater Mining Company in

Montana, U.S.A.

By 1960, while always giving priority to technical assistance to the refinery operation, the Research and

Development Division was broadening its activities to include treatment processes for zinc and lead. In

1961, the first major custom pilot plant operation was undertaken, for Marinduque Iron Mines Agents,

Inc., on copper-zinc concentrates from its mines in the Philippines, using the ammonia leaching process.

The operation demonstrated the production of high purity copper powder, zinc carbonate and ammonium

sulphate, but a commercial plant was never built.

Eldon Brown and Vladimir Mackiw had always believed that the eventual world supply of nickel would

come from lateritic ores because the deposits of sulphide nickel ore would eventually be depleted. It was

thus with Brown’s encouragement that Sherritt Research and Development in 1962 had started bench

work on a laterite treatment process which by 1963 was ready for piloting. A new much larger pilot plant,

or demonstration plant, was designed and built.

By 1969, Marinduque Mining and Industrial Corporation of the Philippines had acquired sufficient ore

reserves from the Philippine government to justify a plant to produce 34 000 tonnes of nickel metal per

year on Nonoc Island. Sherritt agreed to license its laterite process and know-how to Marinduque and to

supply technical personnel and advice on the management and operation of the plant. A 9 000 tonne

sample of lateritic ore was shipped to Fort Saskatchewan for the pilot plant. Bechtel Corporation was

retained by Marinduque to design the commercial plant that was built at Nonoc Island. Sherritt engineers

were hired as consultants to Bechtel on the design and construction. In 1973 and 1974, Sherritt staff

members and their families were transferred to the Philippines to participate in the final stage of

construction and plant start-up. Sherritt Gordon acquired a 10% interest in the Marinduque Mining and

Industrial Corporation.

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Subsequently, demonstration runs were made for Le Nickel, the French nickel laterite mining company in

New Caledonia, and for P.T. Pacific Nickel Indonesia on their laterite ore from Gag Island.

More recently in the 1990s, Outokumpu installed hydrogen reduction facilities for nickel production at

the Harjavalta refinery in Finland (now owned by OMG). Further, Anaconda Nickel (now Minara

Resources) in Western Australia adopted the Sherritt pressure acid leach (PAL) process as used at Moa,

as well as hydrogen reduction for nickel and cobalt recovery (16).

In the 1970s Sherritt’s process research and development resources were spread more widely to include

the treatment of both zinc and copper concentrates. The successful zinc pressure leach process took many

years of persistent and patient effort to reach commercial application. This process, in which zinc

sulphide is pressure leached in sulphuric acid solution at 150oC in an oxygen atmosphere to produce zinc

sulphate solution and elemental sulphur, is an environmentally attractive alternative to fluid bed roasting,

which produces sulphur dioxide.

The initial laboratory testwork, which identified the potential of the process, was carried out for Sherritt

by Frank Forward and Herb Veltman at U.B.C. in 1958. However the problems presented by the

behaviour of elemental sulphur, which is molten at the leaching temperature, both in the leach itself, and

during further treatment of the leach residue, were not fully resolved until the 1970s. The process was

piloted in 1976, and the first commercial plant started up at Trail, B.C. in 1981. The second commercial

plant was also in Canada, at the Kidd Creek operation at Timmins, Ontario that was commissioned in

1983. The first overseas plant was built by Ruhr Zink in Germany in 1991. Following that, a third

Canadian zinc pressure leach plant was commissioned for Hudson Bay Mining and Smelting at Flin Flon,

Manitoba in 1993, the first commercial application of the Sherritt two-stage zinc pressure leach process

(17).

A technically and economically viable hydrometallurgical process for the treatment of copper sulphides

has long been sought, primarily because of the concern for the environment with regard to sulphur

dioxide emissions. Since the late 1950s, Sherritt, as well as others, has expended considerable effort in

the field of copper hydrometallurgy and had developed two potential processes both of which were

successfully piloted at Fort Saskatchewan. One was based on ammoniacal pressure oxidation leaching,

followed by recovery of the copper from solution as refined copper powder by hydrogen reduction, and

produces ammonium sulphate as a byproduct. The other was based on sulphuric acid oxidation leaching

and produces elemental sulphur as a byproduct. Both processes, particularly the latter, are applicable to

chalcopyrite, the most abundant and one of the most refractory copper sulphides (18).

In 1971, an intensive and cooperative research program was initiated jointly by Sherritt and Cominco to

develop a versatile hydrometallurgical process for producing refined copper and recovering sulphur and

other metal concentrates as byproducts from a variety of chalcopyrite containing copper concentrates.

After laboratory development of a process that appeared to be economically competitive with smelting,

the Government of Canada and the two companies supported the construction, in 1975, and operation, in

1976, of a comprehensive pilot plant facility. The process that resulted from this work is now known as

the Sherritt-Cominco (S-C) copper process (19).

Sherritt’s interest in the pressure oxidation of pyrite containing ores and concentrates was revived in the

late 1970s, in the work carried out with Anglo American Corporation of South Africa to develop a

process for the pressure leaching of pyrite containing uranium ores. Extensive piloting was carried out

and a commercial plant was constructed, but never operated due to a depressed uranium market. Sherritt

was engaged from 1977 to 1980, by the Key Lake Mining Corporation, to assist in the development of a

process for the very high-grade uranium-nickel-arsenic ores that this company had in northern

Saskatchewan. The process that was evolved for this application relied on mild pressure leaching in

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dilute sulphuric acid in the second stage of a two-stage system. Milling of the ore commenced in October

1983 and full production was achieved, and surpassed, by May 1984 (20).

Pressure oxidation of refractory gold ores finally came into its own in the 1980s as a healthy gold price

encouraged the exploitation of refractory gold ore deposits (21). Sherritt has played a significant part in

the commercialization of pressure oxidation for refractory gold ores. Several pressure oxidation plants

have been established since 1985 in the U.S.A. and Brazil (22). Canada’s first installation, at Campbell

Red Lake in Ontario was commissioned in 1991 (23). Two plants were also commissioned in Papua New

Guinea, the Porgera operation with startup in 1991 and further expansion in 1994, and the Lihir plant in

1997.

During the last 50 years, from conceptual research at the University of British Columbia, Sherritt Gordon

Limited, the Chemical Construction Corp. and, later, many other locations, pressure hydrometallurgy has

become well established worldwide. It has provided the mining industry with very versatile tools to solve

some of its most difficult problems in metal extraction and recovery from ores, concentrates and other

intermediate products. Pressure hydrometallurgy has now become a standard unit operation throughout

the industry.

REFERENCES

1. D.G.E. Kerfoot, “Historical Metallurgy – The Development of the Sherritt Ammonia Pressure Leach

Process”, CIM Bulletin, Vol. 82, No. 926, 1989, 136-141.

2. J.R. Boldt Jr. and P. Queneau, The Winning of Nickel, Longmans Canada Ltd., Toronto, Canada, 1967,

299-314.

3. Manitoba Industry Economic Development and Mines. http://www.gov.mb.ca/itm/mrd/min-

ed/minfacts/history.pdf

4. F. A. Forward, C.S. Samis and V. Kudyk, “A Method for Adapting the Ammonia-Leaching Process to

the Recovery of Copper and Nickel from Sulphide Ore and Concentrate”, C.I.M. Trans Vol. LI, 1948,

181.

5. V. N. Mackiw, R.L. Benoit, R. J. Loree and N. Yoshida, “Simultaneous Distillation of Ammonia and

Separation of Copper from nickel-bearing solutions”, Chem. Eng. Vol. 54, 1958, 79-85.

6. D.G.E. Kerfoot, “Process for the Separation of Cobalt from Nickel”, Canadian Patent, No. 2,068,982, 3

October 2000.

7. B. Benson and N. Colvin, “Plant Practice in the Production of Nickel by Hydrogen Reduction”,

Proceedings of the AIME International Symposium on Hydrometallurgy, Dallas, Texas, February 1963,

735-752.

8. V. N. Mackiw, W.C. Lin and W. Kunda, “Reduction of Nickel by Hydrogen from Ammoniacal Nickel

Sulphate Solutions”, J. Metals Vol. 9, 1957, 786-793.

9. D.G.E. Kerfoot and P.D. Cordingley, “The Acid Pressure Leach Process for Nickel and Cobalt

Laterite. Part II: Review of Operations at Fort Saskatchewan”, Nickel Cobalt 97, Vol. 1:

Hydrometallurgy and Refining, W. C. Cooper and I. Mihaylov, Eds., CIM, Montreal, 1997, 355-370.

10. B. Willis and J. Von Essen, “Precipitation of Nickel Metal by Hydrogen reduction: A new

perspective”, ALTA2000 Nickel-Cobalt –6 Conference, Perth, Australia, May 15- 18, 2000, 1-12.

11. (a) F. A. Schaufelberger, “Separation of Nickel and Cobalt Metal from Acidic Solution”, U.S. Patent,

No. 2,694,005, 9 November 1954.

(b) F. A. Schaufelberger and P. J. McGauley, “Separation of Nickel and Cobalt Metal from Ammine

Solution”, U.S. Patent, No. 2,694,006, 9 November 1954.

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(c) F. A. Schaufelberger and A. M. Czikk, “Cobalt Pentammine Separation”, U.S. Patent, No. 2,767,054,

16 October 1956.

(d) F. A. Schaufelberger, “Cobalt Pentammine Sulfate Separation”, U.S. Patent, No. 2,767,055, 16

October 1956.

12. V.N. Mackiw and T.W. Benz, “Application of Pressure Metallurgy to the Production of Metallic

Cobalt”, Extractive Metallurgy of Copper, Nickel and Cobalt, AIME International Symposium, P.

Queneau, Ed., Interscience Publishers, New York, 1961, 503-534.

13. M.E. Chalkley and I.L. Toirac, “The Acid Pressure Leach Process for Nickel and Cobalt Laterite.

Part I: Review of Operations at Moa”, Nickel Cobalt 97,Vol. 1: Hydrometallurgy and Refining, W. C.

Cooper and I. Mihaylov, Eds., CIM, Montreal, 1997, 341-353.

14. V.N. Mackiw, R.M. Berezowsky and D.G.E. Kerfoot, “Recovery of Nonferrous Metals by Pressure

Hydrometallurgy”, 41st Canadian Chemical Engineering Conference, Vancouver, Canada, Oct 6-9, 1991.

15. M.J.H. Ruscoe, “Sherritt Research: A History of Achievement”, unpublished paper, Fort

Saskatchewan, Alberta, Canada, December 5, 1996.

16. G. Motteram, M. Ryan and R. Weizenbach, “Application of the Pressure Acid Leach Process to

Western Australian Nickel/Cobalt Laterites”, Nickel Cobalt 97,Vol. 1: Hydrometallurgy and Refining,

W. C. Cooper and I. Mihaylov, Eds., CIM, Montreal, 1997, 391-407.

17. E. Ozberk, W.A. Jankola, M. Vecchiarelli and B.D. Krysa, “Commercial operations of the Sherritt

zinc pressure leach process”, Hydrometallurgy, Vol. 39, 1995, 49-52.

18. W. Kunda and R. Hitesman, “The Reduction of Copper to Powder from its Aqueous Ammine

Ammonium Sulphate System using Hydrogen under Pressure”, Paper presented at the American Inst. Of

Chemical Engineers 64th Annual Meeting, Nov.28 – Dec. 2, 1971.

19. G.M. Swinkels and R.M.G.S. Berezowsky, “The Sherritt-Cominco Copper Process, Part 1: The

Process”, Paper presented at the 16th Conference of Metallurgists of the Metallurgical Society of CIM,

Vancouver, British Columbia, Aug. 21-25, 1977.

20. D.R. Weir and I.M.Masters, “The Key Lake Uranium Process, Part 1: Uranium Extraction”, Paper

presented at the CIM Conference of Metallurgists, Halifax, N.S., Aug. 24-28, 1980.

21. D.R. Weir and R.M.G.S. Berezowsky, “Recovery of Gold from Refractory Auriferous Iron-

Containing Sulphidic Ore”, U.S. Patent, No. 4,571,264, 18 February 1986.

22. R.M.G.S. Berezowsky and D.R. Weir, “Refractory Gold: The Role of Pressure Oxidation”, Gold

Forum on Technology and Practices – World Gold ’89, R.B. Bhappu and R.J. Hardin, Eds., Littleton,

CO: SME 1989, 295-304.

23. J. Frostiak, R. Raudsepp and R.F. Stauffer, “The Application of Pressure Oxidation at the Campbell

Red Lake Mine”, Randol Gold Forum ’90, Squaw Valley, CA, Sept. 13-15, 1990

A History of Sherritt

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Transcript of A History of Sherritt