Nickel and cobalt occur together in many ores of both the oxide type and the sulfide type. Extraction by sulfuric acid leaching is a common practice in treating ores such as those found in Moa Bay in Cuba. Such leaching results in aqueous sulfate solutions containing the nickel and cobalt, together with other leachable constituents of the ore. It is commercially desirable that a nickel product as low as possible in cobalt and a cobalt product as low as possible in nickel be provided from such leach solutions. Of course, both the nickel product and the cobalt product should be as low as possible in other impurities which may be copresent in the sulfate solution.
The separation of nickel and cobalt from such sulfate solutions has been studied by many investigators for years. In the commercialized Outokumpu process, a portion of the Ni-Co solution is neutralized with NaOH, and nickelous hydroxide is precipitated. The precipitate is then electrolytically oxidized to nickelic hydroxide. The nickelic hydroxide is filtered, washed, and used for precipitation of cobaltic hydroxide from the main solution stream to result in a pure nickel solution. However, the cobaltic hydroxide that is formed usually has a cobalt-to-nickel ratio of about 1:1 to 5:1 and must be further processed to generate pure cobalt solution. Commercially, a Pentammine process is used for that purpose. This process consists of dissolution of Co-Ni hydroxide, ammoniation of the resulting solution, oxidation of Co(II) to Co(III), and removal of Ni(II) by Pentammine crystallization and ion exchange. The purified cobaltic solution is then reduced to the cobaltous state prior to cobalt recovery by electrowinning or hydrogen reduction. The overall process is complicated and costly. Recently, an ammonium persulfate process was developed by AMAX to replace the Outokumpu process. However, a simple process for replacing the Pentammine process is still needed.
Solvent extraction to remove cobalt from nickel in sulfate solution using esters, salts, and other organic derivatives of phosphoric acid as selective extractants is known. For improving the separation factor between cobalt and nickel, new cobalt extractants such as derivatives of phosphonic and phosphinic acids have been developed. Using these extractants, cobalt can be removed from nickel sulfate solutions readily, but purified cobalt solution cannot be generated directly from the loaded extractants. The cobalt-loaded phosphonic or phosphinic extractant usually has a cobalt-to-nickel ratio of about 5 to 50. In order to increase the ratio to 10,000, a cobalt-rich solution must be used to scrub the loaded extractant within a very limited pH range. There are several disadvantages in performing the scrubbing: (1) The scrubbed aqueous usually is recycled to the extraction stage. The amount of cobalt recycled may be as high as about 50 percent of the cobalt in the original feed solution. (2) For effective extraction of cobalt in the extraction stage, the organic-to-aqueous ratio (O/A) usually is about 0.5 to 1. At this O/A ratio, a large quantity of organic must be processed through the extraction, as well as the scrubbing stage. Thus, the capital and operating costs are high. (3) The pH control for selective extraction of cobalt in the loading stage and selective scrubbing of nickel in the scrubbing stage is very critical. Usually, such close pH control is not easy to implement in an integrated circuit.
It is to be appreciated that the leach solutions resulting from sulfuric acid leaching of oxide ores tend to be relatively dilute, e.g., such leach solutions may contain only about six grams per liter of nickel and about 0.6 grams per liter of cobalt even when leaching is conducted at elevated temperature and pressure using 98% sulfuric acid at 450.degree.-500.degree. F. Treatment of such dilute solutions by solvent extraction would not be economic because of the great volume of solution involved. Accordingly, it is desirable to concentrate the metal content of the solution beforehand. This may be done by precipitating the nickel and cobalt as sulfides under conditions which inhibit precipitation of iron as sulfide. The precipitate is separated from the spent solution and may be redissolved in sulfuric acid to yield a sulfate solution containing about 50 to 150 gpl nickel and 1 to 5.0 gpl cobalt.
Those skilled in the art know that reagents which display a selective affinity for cobalt over nickel will still coextract some nickel and possibly other metal ions such as copper or iron while extracting cobalt and that nickel-selective extractants will coextract some cobalt and possibly other metal ions. It is also known that the reagents used in solvent extraction are usually employed in solution in a water-immiscible organic solvent such as kerosene. In addition, it is known that the cobalt-selective extractants are comparatively more selective than are the nickel-selective extractants.
The invention is directed to separation of cobalt and nickel from a sulfate solution containing both ions in various proportions wherein the amount of cobalt recycle is significantly reduced as compared to solvent extraction processes wherein scrubbing of the loaded organic is employed in the conventional manner.