| Method for nickel and cobalt recovery from laterite ores by reaction with concentrated acid and water leaching -> Monitor Keywords |
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Method for nickel and cobalt recovery from laterite ores by reaction with concentrated acid and water leachingRelated Patent Categories: Chemistry Of Inorganic Compounds, Treating Mixture To Obtain Metal Containing Compound, Iron Group Metal (fe, Co, Or Ni), Forming Insoluble Substance In LiquidMethod for nickel and cobalt recovery from laterite ores by reaction with concentrated acid and water leaching description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060002835, Method for nickel and cobalt recovery from laterite ores by reaction with concentrated acid and water leaching. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATION [0001] This application claims the benefit of U.S. provisional patent application No. 60/583243 filed Jun. 28, 2004, the disclosure of which is hereby incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates to the hydrometallurgical processing of nickeliferous laterite ore, and in particular to a method for acid leaching both the limonite fraction and the saprolite fraction of such ores in a single process. BACKGROUND OF THE INVENTION [0003] Laterite ores are formed by the in-situ weathering of nickel-bearing ultramafic rocks near or at the surface of the earth in tropical environments by the action of naturally acidic meteoric waters over geologic time. They consist of a variety of clay, oxide and silicate minerals, some enriched in nickel and/or cobalt, and this distinguishes them from the other major class of nickel ores, the sulfide ores. The latter consist typically of sulfide minerals of iron, nickel and cobalt, often with copper and minor precious metals, and are associated with mafic-ultramafic magmatic intrusions in the earth's crust. [0004] The weathering process typically creates a layered deposit, with the products of complete or most extensive weathering occurring near the surface and these grading into the products of lesser degrees of weathering as depth is increased and finally terminating in unweathered rock at some greater depth. The highly weathered layer usually contains most of its contained nickel microscopically distributed within very finely divided goethite particles. Goethite is an oxyhydroxide of ferric iron with the chemical formula FeOOH. This layer is usually given the name limonite, and typically contains a high proportion of iron. [0005] Cobalt is usually associated with the limonite layer and is usually predominantly associated with oxidized manganese minerals (Mn(III) and/or Mn(IV) containing oxides and hydroxides), often called asbolane or manganese wad. [0006] The lesser weathered layers typically contain increasing proportions of their contained nickel in various magnesium silicate minerals, such as, for example, serpentine. Serpentine is a silicate mineral of magnesium which has the chemical formula 3MgO*2SiO.sub.2*2H.sub.2O. It is believed that nickel substitutes for some of the magnesium in serpentine. Magnesium may also be substituted by other divalent metals, for example, ferrous iron (Fe.sup.2+). There may be many other silicate minerals that also host nickel in the incompletely weathered zones. The partially weathered, high-magnesium bearing zone is often given the name saprolite, or garnierite. ("Garnierite" is also used to describe a particular apple-green colored magnesium-nickel silicate mineral of variable composition.) [0007] In some deposits there is another zone typically located between the limonite and saprolite that consists predominantly of nontronite clays; these are silicates of magnesium, iron and aluminum that may also be nickeliferous. In most deposits located in the (current) tropics, the nontronite zone is largely absent. [0008] It should be noted also that none of the weathering zones are homogeneous in mineralogical or chemical composition, nor is the boundary between the zones parallel to the earth's surface. However, there is usually a fairly sharp transition from ore of high iron and relatively low magnesium contents to ore of a relatively high magnesium content and lower, although variable, iron content, which occurs over vertical distances of 1 to 3 m within a laterite deposit. [0009] For illustration purposes only, typical chemical compositions of limonite and saprolite are as follows: Limonite: 1.0-1.8% Ni, 0.05-0.3% Co, 35-50% Fe, 0.2-3.5% Mg Saprolite: 1.2-3.5% Ni, 0.02-0.07% Co, 7-20% Fe, 10-20% Mg [0010] Each zone also contains typically significant concentrations of aluminum, manganese and chromium, as well as trace concentrations of other heavy metals such as copper and zinc in a variety of other minerals. [0011] A challenging aspect of nickel recovery from laterite ores is that the nickel values typically can not be concentrated substantially by physical means, that is, so-called ore dressing techniques, prior to chemical processing to separate the metal values. This renders the processing of laterites expensive, and means to lower the costs of processing laterites have been sought for many decades. [0012] Also, because of the distinct mineralogical and chemical composition of limonite and saprolite ores, these ores usually are not amenable to processing by the same process technique. [0013] One known acid leaching process for nickel laterites is the so-called High Pressure Acid Leaching (HPAL) process (see, for example pages 437-453 in "The Winning of Nickel Its Geology, Mining and Extractive Metallurgy," by Joseph R. Boldt, Longmans Canada Ltd. 1967). This process was first employed at Moa Bay in Cuba in the late 1950s and additional plants were constructed in Western Australia in the late 1990s. [0014] The process utilizes sulfuric acid leaching at high temperature, typically 250.degree. C., and high pressure; the associated steam pressure at 250.degree. C. is approx. 570 psi. At this temperature, the nickeliferous minerals in the ore are nearly completely solubilized. The dissolved iron is rapidly precipitated as hematite (Fe.sub.2O.sub.3) at the high temperature employed because this compound is largely insoluble even in slightly acidic solutions at this temperature. The nickel remains in solution and after cooling, the leach residue containing iron is separated from the nickel-bearing solution by thickening in a series of wash thickeners, a so-called counter-current decantation (CCD) circuit. Thus, the primary objective of the leaching process, which is separation of nickel from iron, is achieved. [0015] A major disadvantage of the HPAL process is that it requires sophisticated high-temperature autoclaves and associated equipment which are expensive, both to install and to maintain. In addition, the HPAL process also consumes more sulfuric acid than is required to stoichiometrically dissolve the non-ferrous metals content of the ore because at high temperature most of the sulfate ions provided by sulfuric acid are tied up as bisulfate ions (HSO.sub.4.sup.-). In other words, sulfuric acid (H.sub.2SO.sub.4) only dissociates to release a single proton (H.sup.+) at high temperature. On cooling and neutralization of the leach liquor the bisulfate ions decompose to sulfate (SO.sub.4.sup.2.sup.-) and another proton. The latter proton (acid) is therefore not utilized fully for leaching and results in excess sulfuric acid which must be neutralized, for example with limestone. [0016] Another disadvantage of the HPAL process is that it is limited to treating largely limonite-type feeds because the presence of saprolite will cause a large, and often uneconomic, increase in sulfuric acid consumption due to the leaching of magnesium from saprolite. This is exacerbated by the bisulfate "shift" problem at high temperature, which is described above. [0017] U.S. Pat. No. 4,097,575 describes an improvement to the HPAL process which constitutes roasting saprolite ore below about 820.degree. C. in order to render the ore more reactive with sulfuric acid and then using the roaster calcine to neutralize excess acid in the discharge of an autoclave wherein pressure leaching of limonite ore occurs. Nickel contained in the saprolite ore is largely dissolved during this neutralization. The reported advantages of this process are that it better utilizes the sulfuric acid added during pressure leaching of limonite, it reduces the consumption of limestone or other costly neutralizing agents to treat the autoclave discharge liquor, and it achieves the capability of treating both the limonite and saprolite fractions of a typical nickel laterite orebody. Disadvantages of the process are that it still requires the use of expensive autoclaves for leaching limonite, and it requires a roasting process for saprolite ore, which is expensive both in capital and operating cost terms. [0018] U.S. Pat. No. 6,379,636 B2 describes a further improvement to the process described in U.S. Pat. No. 4,097,575 wherein the saprolite roasting step is eliminated and the saprolite in "raw" form is used to neutralize the excess acid in the autoclave discharge solution. In addition, more acid could be added to the discharge to increase the amount of saprolite that could be leached. However, this process still requires the use of expensive autoclaves. [0019] Several processes have also been described that utilize acid leaching at atmospheric pressure only, eliminating the disadvantages of pressure leaching described above. U.S. Pat. No. 3,793,432 describes an atmospheric leaching process for laterite ore, in which the ore is reacted with sulfuric acid at or below the boiling point and the precipitation of dissolved iron is achieved by the addition of an iron precipitating agent such as ammonium, sodium, potassium or lithium ions. Although not stated explicitly, all of the examples cited in the specification employed limonite ore samples, as evidenced by the high iron content and low magnesium content of the feed ore. While this process overcomes the disadvantages of pressure leaching, it has other disadvantages. First, the precipitation of iron would be as a jarosite compound, which is a thermodynamically unstable compound of iron that decomposes over time to release sulfuric acid, thus causing environmental problems. (Although jarosite is not stated explicitly it would be apparent to one skilled in the art that jarosite will precipitate at the conditions outlined in the examples.) Jarosite contains two moles of sulfate for every three moles of iron and thus this compound represents substantial excess consumption of sulfuric acid to provide the necessary sulfate ions. [0020] Second, the nickel extractions from the ore were apparently relatively low. While extractions were not stated explicitly, based on the nickel content of the residue and the fact that the residue weight must be more than the weight of the original ore because jarosite contains a lower percentage of iron than the original ore and virtually all of the iron remained in the residue, nickel extractions were usually in the 60-65% range. Third, there is a requirement for very long leach times, of the order of 4-5 days. Fourth, there is a need to add relatively expensive iron precipitating agents such as potassium carbonate, sodium carbonate or the like. [0021] U.S. Pat. Nos. 6,261,527 B1 and 6,680,035 B2 describe another atmospheric leaching process in which limonite ore is first "totally" leached with strong sulfuric acid, i.e. both nickel and iron are substantially dissolved from goethite, and then saprolite ore is leached in the resulting limonite leach slurry while simultaneously precipitating iron as jarosite by the addition of a jarosite precipitating agent. This process also has the disadvantages of producing jarosite, requiring separate mining and preparation of the limonite and saprolite fractions of the ore, and being limited to a narrow range of saprolite to limonite ratios. The latter disadvantage is due to the fact that the quantity of saprolite that can be leached effectively is determined by the quantity of ferric iron in the limonite leach solution. Continue reading about Method for nickel and cobalt recovery from laterite ores by reaction with concentrated acid and water leaching... 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