THE FORT SASKATCHEWAN REFINERY
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.
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 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 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 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.
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