Although the nickel market has registered only modest growth lately due to unfavorable economic conditions, process equipment supplier Outotec noted that analysts expect an upturn in the future. Outotec, in fact, said it has constantly been developing new, sustainable, and economically viable nickel processing methods—and its nickel matte chloride leaching process is designed to leach matte in atmospheric OKTOP reactors using hydrochloric acid and oxygen, as well as ammonia for pH control. This innovative approach, according to the company, produces good overall metal recovery, enables the regeneration of expensive chemicals using cheaper chemicals and also minimizes waste and byproduct output. Particularly suited for small-capacity stand-alone refineries, Outotec claims it offers low capital expenditure and reduced operating costs.
Outotec described the matte leaching process as an extremely flexible method that can be used for treating a wide variety of raw material feeds. In addition to nickel matte, the process can easily be modified for treating different concentrates and intermediates. The atmospheric operating environment helps to minimize initial capital investment, and Outotec can supply the majority of the key equipment, testing and automation solutions, as well as provide ongoing support to ensure maximum uptime and faster return on investment.
Outotec’s history in chloride hydrometallurgy began in the 1980s when the technique was originally developed for copper recovery. It has since been extended to nickel, zinc and gold processing.
Despite the fact that nickel raw materials are increasingly coming from laterite sources, the company noted that the continuing treatment of sulfide ore in smelters means that nickel matte production will continue far into the future. Outotec has developed a calcium chloride-based processing route for nickel matte and other nickel-containing raw materials, which incorporates two-stage leaching and iron precipitation stages that make the recovery of precious metals such as gold and silver possible.
The heart of the process is the regeneration of ammonia and hydrochloric acid, which cuts operational expenses while maintaining metal recovery. As a closed process, it also minimizes waste and byproducts, and therefore has minimal environmental impact.
Because the Outotec process is based on a calcium chloride solution, it enables easy acid and base regeneration. Base metals are leached in oxidative conditions at atmospheric pressure, iron is precipitated with limestone, and base metals are purified by solvent extraction, using ammonia as the neutralization agent. Organic reagent entrainment and dissolved reagents are removed from the aqueous solution with copper, cobalt, and nickel solvent extractions, and ammonium chloride is regenerated into ammonia. The water balance is controlled by evaporation after ammonia. Calcium chloride is regenerated into hydrochloric acid and the solution is returned to leaching. In total, the leaching process takes about 10 to 15 hours to complete. The leaching residue weight is about 25% of that of the matte fed into the process.
Iron is precipitated using slaked lime or limestone at around pH 2 while oxygen is fed to the reactors. Under these conditions, iron precipitates as goethite. Controlled sulfur removal at this stage significantly decreases scaling in the downstream process. About five hours of residence time is sufficient for good removal of iron and sulfur. Both iron and sulfur can be precipitated to a very low concentration without significant loss of valuable metals.
Copper is purified from the process solution by solvent extraction, using conventional hydroxyoxime-based copper extractants, with ammonia being used to control pH in order to improve extraction. Impurities are removed from the organic phase in scrubbing stages before the copper is stripped to a copper sulfate electrolyte, while electrowinning is done by conventional means. Cobalt is also extracted by solvent extraction. Impurities are scrubbed with diluted hydrochloric acid before stripping and the cobalt is then precipitated from the rich stripping solution.
Nickel is recovered from the process solution by solvent extraction, using the reagent Versatic 10, with ammonia again being used to maintain a pH of 5 to 5.3. Unlike sulfate-based extraction processes, there is no significant gypsum precipitation since most of the sulfates have already been precipitated as part of the iron removal stage and the process solution has been diluted slightly due to the pH control. The loaded organic is scrubbed with the diluted anolyte from nickel electrowinning to reduce calcium loading and, in turn, gypsum precipitation. Nickel is recovered by electrowinning.
After nickel solvent extraction, the raffinate is a concentrated calcium chloride solution containing a significant amount of ammonium chloride. Ammonium is regenerated into ammonia by adding calcium hydroxide. Because sulfates are not present in the Outotec process, there is no significant gypsum precipitation during ammonia regeneration. The solution and steam are fed to a stripping column, where ammonia is recovered in the gas phase. The ammonia gas is condensed back to an ammonia solution, which is recycled for use as a neutralization chemical in solvent extraction. This helps to produce a more concentrated ammonia solution and enables better overall water-balance control.
The final process step is hydrochloric acid regeneration, where calcium chloride solution is reacted with sulfuric acid to produce pure gypsum precipitate. Hydrochloric acid stays in the solution and is returned to the leaching stage. The typical hydrochloric acid concentration in the solution is between 120 and 150 g/L. Acid regeneration takes place in multiple reactors, with a total residence time of four to five hours.
Outotec’s new TankCell e630 has an effective volume of 630 m3 in its standard configuration, with almost 700 m3 of volume possible in other configurations. It is, according to the company, the largest flotation cell on the market, designed to meet demand for increased capacity combined with improved production and energy efficiency.
The new cell features Outotec’s FloatForce mixing technology, claimed to enable superior pumping performance as well as better air dispersion and mixing capability, improved efficiency and longer component wear life. Compared with an equivalent plant build using the smaller Outotec TankCell e300, total capital cost can be reduced by as much as 10% to 20% when the larger cell is installed. Typical installed power for the e630 is 500 kW (600 hp) and specific power consumption is lower than that of smaller TankCells. Outotec recommends using a variable frequency drive to enable optimization of both metallurgy and power consumption in these high-capacity production units.
Its large capacity makes the Outotec TankCell e630 particularly suitable for rougher and scavenger duties in gold and base metal applications. The unit has a diameter of 11 m and a lip height of approximately 7 m.