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 (Part 3 of 5)

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.

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.

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.

For part four, click here: http://bit.ly/2h1I8V3

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