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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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.

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.

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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.

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.

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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.

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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.

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