It is known to recover nickel and/or copper from sulfidic ores by comminuting the ore to a finely divided state, subjecting the comminuted ore to froth flotation to upgrade the metal content thereof, and roasting the concentrate in an oxidizing atmosphere to remove the sulfur therefrom as SO.sub.2, followed by the reduction of the oxidized concentrate at an elevated temperature with a carbonaceous material to form molten nickel which is cast to provide pig nickel for use in the manufacture of stainless steel.
The foregoing method has certain disadvantages particularly with respect to the formation of SO.sub.2 which is undesirable. Unless the SO.sub.2 is converted to sulfuric acid on site, the SO.sub.2 effluent released into the atmosphere presents environmental problems.
Nickel-containing sulfidic minerals and laterites are the two major raw material sources for nickel. The laterites are abundantly distributed throughout the world.
However, laterites unlike nickel sulfide ores, cannot be concentrated by froth flotation or magnetically. Compared to other ores, the nickel content is low, for example, may range from 0.5 to 1.5% nickel by weight, with the exception of deposits in New Caledonia and Indonesia in which the amount of nickel is of the order of up to about 3% by weight which is quite high.
It has been predicted that by the end of this century, laterites will become the major source for the production of nickel.
The conventional process for recovering nickel from lateritic ores is somewhat energy intensive in that the nickel is extracted from the ore by high pressure leaching at elevated temperature in an autoclave.
For example, one process for recovering nickel and cobalt from lateritic ores is the well known Moa Bay process involving acid leaching at elevated temperatures and pressures at which iron oxide and aluminum oxysulfate are substantially insoluble.
In the Moa Bay process, lateritic ore at minus 20 mesh (95% passing 325 mesh U.S. Standard) is pulped to approximately 45% solids and the nickel and cobalt selectively leached with sufficient sulfuric acid at elevated temperature and pressure (e.g. 230.degree. C. to 250.degree. C. and 405 to 580 psia) to solubilize about 95% each of nickel and cobalt in about 60 to 90 minutes. After pressure let down, the leached pulp is washed by countercurrent decantation with the washed pulp going to tailings. The leach solution pH, which is quite low (e.g., between 0 and 0.5), is then neutralized with coral mud to a pH of about 2.4 in a series of four tanks at a total retention time of about 20 minutes and the thus-treated product liquor (containing about 5.65 gpl Ni, 0.8 gpl Fe and 2.3 gpl Al), after solid-liquid separation, is then subjected to sulfide precipitation. The leach liquor is preheated and the sulfide precipitation carried out using H.sub.2 S as the precipitating reagent in an autoclave at about 120.degree. C. (250.degree. F.) and a pressure of about 150 psig.
In the original scheme for treating the mixed sulfides, the sulfide precipitate was washed and thickened to a solids content of 65%. It was then oxidized in an autoclave at about 177.degree. C. (350.degree. F.) and a pressure of about 700 psig.
The solution containing nickel and cobalt was then neutralized with ammonia to a pH (5.35) sufficient to precipitate any residual iron, aluminum, and chromium present using air as an oxidizing agent.
The precipitate was thereafter separated from the solution and the nickel and cobalt solution then adjusted to a pH of about 1.5. H.sub.2 S was added to precipitate selectively any copper, lead and zinc present. The precipitate was separated from the solution by filtration and the nickel recovered by various methods. One method comprised treating the nickel-containing solution with hydrogen at elevated temperature and pressure to produce nickel powder.
The aforementioned method, as stated hereinbefore, had certain economic disadvantages. The conversion of mixed nickel-cobalt sulfide into salable separate nickel and cobalt products was very expensive and there was no market for mixed sulfide precipitates.
It is known to subject gold-bearing sulfide ore to oxidative bioleaching. Such methods are disclosed in U.S. Pat. No. 4,729,788, No. 5,127,942 and No. 5,244,493. The sulfidic material is ground, placed in heaps or piles or pulped or slurried and bioleached to oxidize the sulfide mineral using bacteria at temperatures of about 15.degree. C. to about 40.degree. C. The sulfide particle containing gold occluded within it is biooxidized to physically free up the gold for removal by cyanide leaching or other types of leaching.
Attempts to use bioleaching to recover base metals, such as nickel, have not been attractive enough to warrant the building of a commercial plant, particularly since technology was not in place economically at the time for recovering nickel from the solution which were quite dilute at best, except for the use of solvent extraction by means of which more concentrated solutions could be produced for the subsequent recovery of nickel. The recovery of nickel from low grade bioleach solutions by solvent extraction has its problems in that (i) there are organic solvents that preferentially extract nickel from mixed ferric iron-nickel containing solutions such as shown hereinafter in Example 2 and (ii) the micro-organisms present in the bioleach solutions tend to adversely affect the separation of the organic phase from the aqueous phase.
The problem with ferric iron is that either the ferric ion will preferentially load on organic solvents, such as DEPHA(di-2-ethyl hexyl phosphoric acid) or it will oxidize the active ingredient in organic solvents such as Cyanex 272; 301 and 302. These reagents are sold by the American Cyanamid Company with the following active ingredients: phosphoric, phosphonic and phosphinic acids.
"Third phase" formation during solvent extractions sometimes limits the application of solvent extraction in leaching operations, particularly in a bioleaching circuit because bacteria and organic solvents are not compatible. For example, Thiobacillus ferroxidans is in essence a sulfur-loving bacteria which presents problems in solvent extraction, particularly when the organic solvent contains sulfur, such as in di-nonyl-naphthyl sulfonic acid. Since many nickel sulfide ore bodies have a metal cut-off grade of around 0.2% to 0.5% Ni, it at once becomes apparent that a method is needed to enable the production of nickel solutions of sufficiently high concentration from which the nickel can be recovered economically. Thus, low grade nickel ores, in essence, could then be treated the same as a high grade ore with the same economical advantages.
Recent work conducted in the bioleaching of ores has indicated that low grade ores can be economically leached using bacteria as a means for effecting the dissolution of metal, e.g., nickel and/or cobalt, into an aqueous acid solution.
An advantage of bioleaching, while time dependent, is the fact that it is not energy and cost intensive. The pregnant solution obtained, however, is quite dilute.
One bioleaching method proposed for the recovery of nickel from sulfide ores is disclosed in Canadian Patent No. 2,065,491 which issued on Oct. 9, 1992.
According to the Canadian patent, a method disclosed comprises crushing the sulfide ore which is thereafter formed into a heap and the ore heap percolated with an iron sulfate solution which, optionally, carries bacteria, such as Thiobacillus ferroxidans, Thiobacillus thiooxidans or Leptospirillum ferroxidans. By virtue of the oxidation of the sulfide ore, the generation of sulfuric acid occurs, thus forming a sulfate solution.
Sulfuric acid or an alkali, such as lime, is added to the solution, if necessary, to control the pH to a range of about 1.2 to 3, preferably from 2.3 to 2.5.
An anaerobic bacterium is added to the sulfate solution to cause the precipitation of the dissolved metal as an insoluble sulfide, thus upgrading the metal into a highly concentrated form which then must be treated to recover the metal, e.g., nickel.
To bring about sulfide generation of the dissolved metal (e.g., nickel), a bacterium, referred to as Desulforvivrio Desulfuricans, may be added to the solution. After the nickel sulfide precipitates, it is separated from the solution to provide a concentrate high in nickel which must be further treated, such as by high pressure leaching at an elevated temperature in the presence of sulfuric acid to produce a nickel sulfate solution from which the nickel is extracted by known conventional methods.
In a paper entitled "The Solubilization of Nickel, Cobalt and Iron From Laterites by Means of Organic Chelating agents" (Denis I. McKenzie et al, International Journal of Mineral Processing' 21 (1987) P.275-292), a group of carboxylic acids were mentioned as chelating agents, including Oxalic Acid, Citric Acid, Tartaric Acid, among others. The efficacy of the organic acids at natural pH were compared to H.sub.2 SO.sub.4 (15 mM final concentration). Over a 456 hour period, using 15 mM concentrations of acids with 2 grams of ore (West Australian ore) in 150 ml of H.sub.2 O, Oxalic, Citric and Tartaric acids compared somewhat favorably with H.sub.2 SO.sub.4. Amount of nickel dissolved in ppm was 30.3 for H.sub.2 SO.sub.4, 18.5 for Oxalic acid, 20.2 for Citric acid and 16.3 for Tartaric.
The same acids employed on Indonesian ore showed that the nickel dissolved amounted to the following: Citric Acid 863 ppm, Tartaric Acid--708 ppm, Oxalic Acid--318 ppm, etc.
In a paper entitled "Microbial Leaching of Nickel from Low Grade Greek Laterites," Mineral Bioprocessing, TMS, 1991 page 191-205, the authors indicate a variety of heterotrophic micro-organisms that can produce such organic acids. They include: asperigillus and penicillia concentrations of around 40 grams of organic acid which were readily produced by these microorganisms.
Close to 70% of the Ni and less than 5% of the Fe were solubilized after a 51-day leaching period from a laterite ore containing about 1% Ni and 30% Fe. This work also showed an improved extraction when the organisms plus the culture medium were mixed with the laterite ore. An explanation was given: "Once the organisms attach themselves to the surface of the mineral grains, a high metal concentration gradient is experienced which could be toxic to the organisms spurring them to produce more citric acid (possibly as a defense response) which subsequently leaches out more ions from the mineral grains." If the toxic metal were to be removed from the solution, as proposed in accordance with the present invention, either during the leaching process or interrupted by it, the leaching kinetics will be enhanced significantly so long as a low level of the toxic metal is maintained.
One method of heap leaching with nutrient solutions containing at least one micro-organisms include those selected from the group consisting of the fungi Aspergillus Niger, Penicillium Sp., Aspergillus Sp., Penicillium Simplicissimus and the bacteria Enterobacter Spp., Bacillus Spp., and Achromobacter Spp.
It would be desirable to provide a process for bioleaching relatively low grade as well as relative high grade nickel-containing lateritic ores and sulfidic ores or concentrates thereof in combination with a novel method for concentrating the nickel ions in solution from which nickel is economically recovered.