High pressure acid leaching of refractory lateritic ores comprising nickel, cobalt and scandium and recovery of scandium from pregnant leach solution and purification precipitates

FIELD OF THE INVENTION The present invention relates to the use thereof in processing refractory lateritic nickel ores by means of hydrometallurgical methods.

In particular, the present invention relates to high pressure acid leaching of refractory lateritic ores comprising nickel, cobalt and scandium and recovery of scandium from pregnant leach solution and purification precipitates.

PRIOR ART

Scandium transition metal is typically obtained as a by-product during the production of uranium, tungsten and some rare earth elements. However, studies carried out in recent years show that some lateritic nickel and cobalt ores comprising a considerable amount of scandium would be new promising scandium sources in the near future. Leaching process basically is one of the hydrometallurgical enrichment methods. http://tr.wikipedia.org/wiki/Hidrometalurji In leaching process, valuable metals contained in the material are transferred to the solution phase by means of some chemicals such as acids and bases. http://tr.wikipedia.0rg/wiki/Asithttp://tr.wikipedia.org/wik i/Ba2

Lateritic ores can be classified in two types as ordinary laterites and refractory laterites which are difficult to leach. One method of producing scandium, nickel and cobalt from lateritic ores is leaching the ores and recovering the desired target metals from the pregnant leach solution by means of hydrometallurgical techniques. Refractory ores are poorly leachable ores where removing the target metals therefrom is difficult. In the high pressure acid leaching process (HPAL), extraction rates of refractory ores are significantly lower as compared to normal lateritic ores and this leads to low leaching efficiencies. The reason for low leaching efficiencies is due to more complex mineralogical structures of refractory lateritic nickel ores.

In the conventional high pressure acid leaching process with process steps as shown in Figure 3, an ordinary lateritic ore is leached at a temperature of about 250-255°C for a period of 60-90 minutes with a sulphuric acid/ore mass ratio of 0.2-0.4. Under these process conditions, 95-98% of nickel and cobalt and 90-95% of scandium pass into the leach solution. However, with the same process parameters, only 80-85% of nickel and cobalt and 75-85% of scandium can be transferred from refractory lateritic ores into the leach solution.

In the literature, one of the patents on the subject matter is the patent application CN 2011/10028466. This invention relates to provision of recovering scandium from the red slurry residue resulting from the Bayer process using the mixture of sulfonated kerosene, P507 and tributylphosphate as extraction medium.

Another application on the subject matter is the patent application EP96308320. This invention relates to recovering scandium from an ordinary laterite oxide ore containing a large amount of iron and nickel and a small amount of scandium by means of recovery and purification processes of entire nickel and scandium content via selective leaching.

The aforementioned inventions cannot provide solutions to the existing problems and cannot provide recovery of scandium from refractory lateritic ores which are difficult to leach as compared to the ordinary lateritic ores.

Consequently, recovery of scandium from refractory lateritic ores is improved, thus, new structures, which would eliminate the aforementioned drawbacks and offer solutions to the existing systems are needed. BRIEF DESCRIPTION OF THE INVENTION

The present invention, in order to eliminate the aforementioned disadvantages and provide new advantages to the respective technical field, relates to positively affecting the leaching behavior of refractory lateritic ores in terms of nickel, cobalt and scandium recovery as well as relates to recovery of scandium from the pregnant leach solution by means of using various organic reagents or by means of solvent extraction subsequent to releaching of purification precipitates.

The main object of the present invention is to increase extraction rates of nickel, cobalt and scandium by means of applying new methods during leaching of refractory lateritic nickel ores comprising nickel, cobalt and scandium metals using the HPAL method. Thus, nickel, cobalt and scandium leaching efficiency of refractory lateritic ores becomes closer to the leaching efficiency of ordinary lateritic ores.

Another object of the present invention is, after the pregnant leach solution (PLS) comprising nickel, cobalt and scandium is obtained, to separate and recover these metals from the pregnant leach solution or purification precipitates of this solution or by means of solvent extraction subsequent to releaching process, applying novel methods.

To achieve the aforementioned objects, refractory lateritic nickel ores are subjected to the following process steps: preparing a leach solution for high pressure acid leaching process, subjecting it to high pressure acid leaching process with leach solution, recovering scandium from the pregnant leach solution resulting from said leaching process by means of solvent extraction method.

To achieve the objects of the present invention, in the leach solution preparation step, at least one of the following or mixtures thereof is used: sulphuric acid and HCI, FeS0 ₄ , Cu ₂ 0 or S.

To achieve the objects of the present invention, in the ore preparation and slurrying process, it is subjected to beneficiation process and particle size thereof is reduced. In said beneficiation process following are performed: coarse ore particles having a low nickel, cobalt and scandium content and comprising mainly quartz are removed from the system by means of surface washing and sieving; they are ground so as to provide solid-liquid contact of undersize particles; and water amount in the thickener is adjusted for obtaining the desired solid-liquid ratio.

To achieve the objects of the present invention, prepared slurry is passed through the preheaters, slurry and leach solution are fed to the autoclave, nickel, cobalt and scandium comprised in the slurry is transferred to the leach solution and said pregnant leach solution is flashed in the flash tanks. Steam generated in the flash tanks is recycled to the preheaters.

To achieve the objects of the present invention, said pregnant leach solution is subjected to a recycle leaching process by means of releaching and mixing nickel, cobalt and scandium metals comprised in the precipitates generated after iron precipitation II and MHP (II). Following this process, in the continuous countercurrent decantation thickener system I, precipitate-solution separation is carried out and leaching residues are removed from the system. To achieve the objects of the present invention, said scandium is separated from the pregnant leach solution remaining after the removal of leaching residues from the system by means of direct solvent extraction process. In said solvent extraction process, organic reagents, modifiers and diluents are used for separating scandium from the other elements and extracting it into the organic phase. Scandium and derivatives thereof are obtained by means of acid stripping and precipitation of separated scandium.

To achieve the objects of the present invention, after feeding the pregnant leach solution, from which scandium and impurities are separated, back to the system, nickel and cobalt are obtained by means of precipitation processes. Said precipitation processes comprise the following process steps:

iron (Fe) precipitation I and neutralization of said solution with CaC0 ₃ , precipitate- solution separation in the thickener system II, iron (Fe) precipitation II with CaC0 ₃ , precipitate-solution separation in the thickener, MHP(I) precipitation with MgO, washing and filtration, MHP(II) precipitation with Ca(OH) ₂ , precipitate-solution separation in the thickener, Mn precipitation with Ca(OH) ₂ , precipitate-solution separation in the thickener, Mg precipitation with Ca(OH) ₂ , precipitate-solution separation in the thickener. Water from the last thickener is fed back to the thickener system I and II.

To achieve the objects of the present invention, in the step of obtaining scandium from the pregnant leach solution resulting from said leaching process by means of solvent extraction method, scandium is obtained from the purification precipitates resulting from Fe precipitation process of the pregnant leach solution remaining after the removal of leaching residues from the system by means of subjecting the new leach solution obtained by releaching the precipitates to solvent extraction process.

To achieve the objects of the present invention, recycle leaching process is applied to said pregnant leach solution by means of releaching and mixing the target metals comprised in the precipitate resulting from MHP(II), precipitate-solution separation is carried out in the continuous countercurrent decantation thickener system I and leaching residues are removed from the system. To achieve the objects of the present invention, scandium, along with iron and the other impurities, is precipitated using CaC0 ₃ and water mixture from the pregnant leach solution remaining after the removal of leaching residues from the system. Pregnant leach solution subjected to purification processes is added to the thickener system II overflow solution.

To achieve the objects of the present invention, the precipitate comprising scandium is taken from the thickener system II underflow and releached with a mineral acid. After the releaching process, precipitate-solution separation is carried out. Following precipitate-solution separation, organic reagents, modifiers and diluents are used for separating scandium from the other elements and extracting it into the organic phase. Scandium and derivatives thereof are obtained by means of acid stripping and precipitation of separated scandium. To achieve the objects of the present invention, thickener II overflow is subjected to the following process steps and nickel and cobalt are obtained: MHP(I) precipitation with MgO, washing and filtration, MHP(II) precipitation with Ca(OH) ₂ , precipitate- solution separation in the thickener, Mn precipitation with Ca(OH) ₂ , precipitate- solution separation in the thickener, Mg precipitation with Ca(OH) ₂ and precipitate- solution separation in the thickener. Water from said last thickener is fed back to the thickener system I and II.

The structural and the characteristic features and all advantages of the present invention will be understood more clearly with the following figures and the detailed description written by referring to said figures and therefore, the evaluation needs to be done by taking said figures and the detailed description into consideration.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a flow diagram of the process according to the present invention and process steps carried out for scandium recovery

Figure 2 is an alternative flow diagram of the process according to the present invention and process steps carried out for scandium recovery

Figure 3 is a flow diagram of the prior art

ABBREVIATIONS HPAL: High Pressure Acid Leaching

PLS: Pregnant Leach Solution

ORP: Oxidation-Reduction Potential

CCD: Continuous Countercurrent Decantation Thickener System

MHP: Mixed Hydroxide Precipitate

DETAILED DESCRIPTION OF THE INVENTION In this detailed description, novelty according to the present invention is described with examples only for a better understanding of the subject without constituting any restrictive effect. High pressure acid leaching process of the refractory lateritic nickel ores starts with ore preparation step. In this step, coarse ore particles typically having a low nickel, cobalt and scandium content and comprising mainly quartz are removed from the system by means of surface washing and sieving. Then, undersize particles are ground so as to provide the optimal solid-liquid contact thereof and water amount in the thickener (systems performing solid-liquid separation) is adjusted for obtaining the desired solid-liquid ratio. Generated slurry is fed to the autoclave after the preheating step.

Simultaneously, a leach solution is prepared in order to increase the recovery efficiency of the target metals from the slurry. Conventionally, refractory lateritic nickel ores comprising nickel, cobalt and scandium are leached with sufficient amount of sulphuric acid for sufficient duration and at sufficient temperatures in order to optimally transfer said metals into the pregnant leach solution (PLS). According to the results of leaching tests, transfer of these metals into the leach solution gives better results at higher leaching temperatures, for longer leaching durations and smaller particle sizes. After the ore preparation, slurry is fed to the autoclave together with sufficient amount of sulphuric acid and additives such as HCI, FeS0 ₄ , CU2O or S in order to increase the efficiency of the leaching process, wherein temperature of the autoclave is further increased by steam injection. The autoclave pressure is about 5 MPa (megapascals), the autoclave temperature value is about 255°C and the pH value is smaller than 1.

To recover the target metals in higher amounts by means of this process, basically, two alternative original methods are integrated to the conventional pressure acid leaching procedure. In the first one, in order to decompose difficult to leach refractory minerals comprised in the lateritic ore, a hydrochloric and sulphuric acid mixture is prepared by some amount of HCI (5%-10% by mass) to the initial leach solution. Extractions of metals into the leach solution are increased by addition of some amount of HCI into the leach solution. The second method is based on changing the oxidation-reduction potential (ORP) of the leach solution in order to extract higher amounts of metal into the leach solution. Tests performed have demonstrated that leach solutions with lower oxidation-reduction potentials are more effective in obtaining pregnant leach solutions with higher concentrations. In order to obtain a PLS with a lower oxidation-reduction potential, various additives such as FeSO ₄ , Cu ₂ O or S can be used. Actual acid pressure leaching tests demonstrate that addition of each of these additives into the leach solution is effective in transfer of the metal cations into the pregnant leach solution at higher rates. As a result, in order to increase transfer amounts of said target metals into the leach solution, additives such as HCI, FeSO ₄ , Cu ₂ O or S should be added to the sulphuric acid.

After the ore with said additives is subjected to high pressure acid leaching process in the autoclave, it is passed through a number of tanks referred to as flash tanks in order to gradually lower the temperature and pressure of the pregnant leach solution. Steam generated during the flashing process is recycled to the preheaters for reuse. Pregnant leach solution comprising significant amounts of free acid, in order to neutralize this free acid as well as in order to provide recovery of the metals by releaching the precipitates generated in the subsequent steps of the process referred to as second iron precipitation and MHP(II) and comprising target metals Ni, Co, Sc, is mixed with some of the precipitates resulting from these steps. After the recycle leaching process, precipitate-solution separation is carried out in the continuous countercurrent decantation thickeners (CCD 1), washed leaching residues are sent to the residue storage area after neutralization and removed from the system.

In a preferred embodiment of the present invention, as shown in Figure 1 , after the precipitate-solution separation of the pregnant leach solution constituting the CCD 1 overflow is carried out, it is subjected to direct solvent extraction process for recovering the scandium from the solution. At this stage, phosphoric, phosphonic, phosphinic acid or amine-based organic solvents such as commercial D2EHPA, Cyanex 272, Cyanex 923, Ion 801 , Ion 290, trioctylamine, tripropylamine, trioctylmethylammonium chloride, tridodecylamine, tributyl phosphate (TBP) and PC- 88-A are used. Said organic solvents are used together with suitable modifiers such as TFE (tetrafluoroethylene), ethanol, methanol, acetonitrile, octanol and decanol and diluents such as kerosene, Shellsol 140, Shellsol AB, Shellsol 2046 and Solvesso 150. Use of diluents provides some advantages. Optimum temperatures at which organic reagents are used for solvent extraction are generally higher than the ignition temperature of these organic reagents. However, organic diluents being used enable this ignition temperature to increase by forming a mixture with the organic reagent, thus, enable solvent extraction to be carried out at a temperature higher than the ignition temperature of the organic reagent. Another purpose of using an organic diluent is to facilitate pumping through the pipes by decreasing the viscosity of the organic reagents having a high viscosity and to reduce operational costs in the industrial applications. After the solvent extraction process, scandium concentrated inside the organic reagent is selectively stripped again with a suitable acid from this organic reagent. Scandium comprised in said stripping solution is obtained by means of precipitation thereof in the form of scandium oxalate/carbonate/oxide/hydroxide. Stripped organic substance is recycled for reuse in the extraction process. In another preferred embodiment of the present invention, as shown in Figure 2, precipitate - solution separation of the precipitate leaving the MHP-II step is carried out in the CCD 1 after releaching thereof. Leaching residues are collected from the underflow of the countercurrent decantation thickeners, neutralized and removed from the system. Slurry comprising the mixture of CaCO ₃ and water is added to the overhead flow solution of the CCD 1 in order to precipitate a large portion of iron and scandium and to neutralize the solution. During this step, precipitating highest possible amount of scandium and thereby concentrating scandium by 5-6 times more as compared to the system inlet concentration is intended. This precipitate comprising scandium is separated in the CCD 2 (thickener II). The precipitate to be removed under normal conditions from the system as residue is utilized for recovery of scandium. The precipitate obtained from the underflow of the CCD 2 is releached with a mineral acid such as sulphuric acid, hydrochloric acid or hydrofluoric acid. Then, separation and recovery of scandium from the new leach solution with said organic reagents is provided by means of solvent extraction process following the precipitate-solution separation. Scandium concentrated inside the organic reagent is selectively stripped from the organic phase with an acid solution, then, precipitated from the stripping solution and obtained in the form of scandium oxalate/carbonate/oxide/hydroxide. Organic reagent provided with stripped scandium therein is recycled to be reused in the solvent extraction process.

According to the first alternative (Figure 1), after obtaining the scandium, solution is neutralized by addition of slurry comprising the mixture of CaCO ₃ and water, with pH increase, iron is precipitated during neutralization and removed from the solution by means of precipitate - solution separation. The iron precipitation process applied at this stage is referred to as the first iron precipitation step. Precipitation process is carried out at a pH value of about 3 and at a temperature value of about 90°C. After the neutralization and first iron precipitation process, pH value of the solution is further increased again by the addition of CaCO ₃ slurry. Depending on the increase of pH value of the solution, iron still being present in the solution is precipitated together with some amount of aluminum, chromium, nickel and cobalt and solution is further purified in this step referred to as the second iron precipitation. The second iron precipitation process is carried out at a pH value of about 4-4.5 and at a temperature value of 60°C. Since the precipitate generated from the second iron precipitation comprises a significant amount of nickel and cobalt, this solid precipitate is recycled to the previously mentioned releaching process step. Solution purified in the first and second iron precipitation steps, being ready for nickel and cobalt recovery is first mixed with the fresh slurry comprising the mixture of MgO and water, thus, the product comprising high amounts of nickel and cobalt, having a commercial value and referred to as MHP-I is obtained. The liquid solution remaining after MHP-I precipitation is mixed with the slurry comprising the mixture of Ca(OH) ₂ and water and precipitation of the nickel and cobalt remaining in this solution as a product referred to as MHP-II is provided. This product having lower values than MHP-I in terms of nickel and cobalt concentration can either be sold or can be recycled to the recycle leaching step in order to recover the nickel and cobalt comprised therein. Solution remaining after obtaining the scandium, nickel and cobalt and comprising substantially manganese and magnesium is mixed with the Ca(OH) ₂ slurry, precipitation of said elements as Mn(OH) ₂ , MnO ₂ and Mg(OH) ₂ is provided, thus, obtaining washing water free of impurities to be used in the countercurrent decantation thickener is intended.

If the other alternative (Figure 2) is followed, in order to recover the scandium comprised in the precipitate generated after the iron precipitation process carried out at a pH value of 4-4.5 and a temperature value of 60°, the solution exiting through the overflow of the second series of thickeners after being separated from the solution in the second series of thickeners is subjected to MHP-I precipitation process. Then, similar purification and precipitation steps are applied as in the case of the other aforementioned alternative.