Recovery of Scandium, Nickel and Cobalt from Hydrometallurgical Waste of Laterite

Recovery of new-energy critical metals including scandium, nickel and cobalt as well as copper and zinc from a neutralization residue produced in laterite hydrometallurgical process has been studied. Effect of leaching parameters such as acid consumption, solution pH and temperature has been investigated. It was found that scandium, nickel, cobalt and copper could be recovered at high efficiencies from the residues by selective leaching using sulphuric acid solutions under ambient conditions, while the co-leaching of impurities including iron, aluminium and silicon was low under the optimal conditions. The nickel, cobalt, copper and zinc in the leaching solution could be further concentrated into mixed sulphides and separated from impurities by sulphide precipitation.


Introduction
Laterite has become the main resource of nickel recovery in the world.About 0.12-3.0%Ni with 50-85% of Fe2O3, 4.0-18% of Al2O3, 0.2-5.0% of MgO, 0.6-1.0% of CaO, and 0.3-2.5% of MnO are contained in limonitic laterite produced at different sites [1].The Ramu NiCo Project in PNG (Papua New Guinea) contains a total identified resource of 143.2 Mt of nickeliferous laterite [2].It was designed to produce a mixed hydroxide product containing 31,150 tonnes of nickel and 3,300 tonnes of cobalt annually.The laterite-contained metals including scandium, nickel and cobalt are called as new-energy critical metals.Nickel and zinc were added in the draft 2021 U.S. critical minerals list, besides scandium and cobalt had been included on the 2018 list.In 2021, the average annual price of nickel was increased by 30% comparing with that in 2020, which was probably attributed to the increased application of nickel in electric vehicle batteries and continued strong demand for stainless steel.The price of scandium oxide was US$2.2 per gram, and the principal uses for scandium were in aluminum-scandium alloys and solid oxide fuel cells (SOFCs) [3].The laterite ore used in the Ramu project in PNG is a laterite with about 1% of nickel and about 0.1% of cobalt.The impurity contents of the laterite are high, such as about 40% of iron, about 3% of silicon and about 2% of aluminium.Most of the iron and aluminium impurities have been rejected in the HPAL (high pressure acid leaching) hydrometallurgical process [4,5].However, the concentrations of iron and aluminium in the pregnant leaching liquor are still too high to precipitate nickel and cobalt into a mixed-hydroxide product.Currently, the impurities are removed from the solution system by neutralisation and precipitation with limestone and calcium hydroxide, resulting in the formation of large amounts of neutralisation residues.About 80,000 tons of neutralisation residues were produced as hydrometallurgical wastes in RAMU project annually.The residues contain high contents of impurities including iron, aluminium and silicon as well as calcium.Although, in order to reduce the loss of nickel and cobalt by precipitation, the precipitation pH value had been strictly controlled and was below their precipitation pH value of about 7.0.It was found that some valuable metals including nickel, cobalt and copper were co-precipitated with the impurities in some degree.Similarly, it was reported that nickel and cobalt were present in the precipitate from synthetic laterite leach solutions even at a pH value as low as 4.0 [6][7][8].The loss of the valuable metals is significant, considering the large amount of such neutralisation residue in the world.However, it is difficult to recover the values from the laterite neutralisation residues, considering the value contents are much lower than the main impurity elements, such as iron.In most of the industrial plants, the laterite neutralisation residues were wastes which are disposed in hazardous landfill sites after stabilization treatments.In order to recover the valuable metals in an economical way, it is necessary to keep the material and energy consumptions in low levels, and it is desirable to have a high recovery of the value metals while a low recovery of the impurities by controlling their dissolutions.Thus, currently the metal values are not recovered in most of the laterite processing plants including the Ramu hydrometallurgical plant.In this article, the recovery of the valuable metals from the neutralisation residue has been studied.The effect of leaching parameters such as acid consumption and pH has been investigated.The scandium, nickel, cobalt and copper can be recovered at high efficiencies from the residues by selective leaching using sulphuric acid solutions under ambient conditions, and the co-leaching of iron was low under optimal conditions.A hydrometallurgical method for the recovery of scandium, nickel, cobalt and copper from the laterite-originated waste by selective leaching with sulphuric acid and subsequent sulphide precipitation was proposed.

Experimental 2.1. Materials
The laterite neutralisation residue used in this work is from the pilot production of the RAMU plant in Papua New Guinea.The chemical composition of the neutralisation residue sample used for this work is listed in Table 1. ) were mixed with 210 mL of de-ionised water in a 500 mL beaker covered with plastic film.The mixture was stirred mechanically at 300 rpm and heated on an electrical hotplate with the temperature controller set at 90℃ for 1 hour, and then a certain amount of concentrated sulphuric acid was added.The reaction mixture was heated and stirred continuously for 1 h.The leachate was filtrated using vacuum filtration with a paper filter, and the filtrate liquor was polish-filtered using a 0.45 μm PVDF membrane filter for sample analysis.The filter residue was re-mixed with 80 mL of de-ionised water, heated at 90℃, stirred for 10 minutes and filtered.The pH values of aqueous phases were determined with a ROSS Sure-flow electrode (model 8172BN) and a Hanna portable pH meter (HI 9025).The element concentrations were determined using inductively coupled plasma atomic emission spectroscopy (Optima 5300DV, Perkin Elmer).The electron microscope (EM) pictures were determined with a Mitutoyo microscope (Hyper MF-U).

Residue Characterization
The calcium content of the residue was about 17.92% (Table 1), indicating that the residue mainly contains calcium compounds.It is consisted with the fact that the residue was formed as a product of neutralisation process using limestone and calcium hydroxide, with formation of calcium sulphates.The other main constituents are iron (5.87%), aluminium (4.87%), and silicon (1.527%).The content of nickel, cobalt and copper was about 1.14%, 0.042% and 0.196%, respectively.The residue also contained low amounts of magnesium and manganese.
The electron microscope (SEM) pictures of the residue show the rod-like crystals and particulate matter.It is assumed that the crystals and particulates are composed with calcium sulphate hydrates (gypsum) and compounds of the other main elements, including iron, aluminium and silicon, respectively, since the former is readily to form by crystallisation, while the latter form amorphous aggregates of hydroxides.Some of the crystals are transparent and others are translucent or opaque, indicating the presence of impurities.The size of the crystals generally has dozens of micrometres in width and hundreds of micrometers in length.The residues generated through precipitation within a pH range of about 4.0-6.0 in the RAMU pilot production, so as to precipitate the dissolved nickel, cobalt and copper sulphates as mixed hydroxides, associated with iron and aluminium [9].Probably, a small amount of the mixed hydroxides could be co-precipitated with calcium sulphates.At the same time, a fraction of the nickel, cobalt and copper sulphates could be co-precipitated with the mixed hydroxides and calcium sulphates by the entrainment and adsorption.Hence, it is assumed that nickel, cobalt and copper are present, as both hydroxides and sulphates in the precipitate, together with iron and aluminium hydroxides and calcium sulphates.The value metals including nickel, cobalt and copper are generally contained in the amorphous aggregates of hydroxides with iron and aluminium, while a certain fraction of them is associated with the gypsum crystals.

Effect of Sulphuric Acid/Ore Ratio on pH Values of Leachates
A number of acid/ore ratios (mass/mass) were tested at different pH values of leachates for metal leaching.As shown in Figure 2, the pH value of both the first leachates and the second leachates decreased with the increase of the sulphuric acid/ore ratio.The pH values were in the range of about 2-4 when the sulphuric acid/ore ratio was in a range from 0.25: 1 to 0.07: 1 (Figure 2).It was found that the pH values of the second leachates using water for leaching were slightly higher than the corresponding pH values of the first leachates using sulphuric acid solutions, indicating that the residual acid from the first leaching was fractionally used for the second 1-1 W: 0.082 mm L: 0.246 mm 1-2 W: 0.053 mm L: 0.072 mm leaching, and most of the residual acid was transferred to the new leachate solution.The second leaching can be considered as a washing procedure and no more acid was necessary to be added.

Figure 2.
Effect of sulphuric acid/ore ratio (mass/mass) on pH values of leachates.

Effect of Leach pH on the Metal Concentration in the Solutions
As shown in Table 1, the contents of nickel, cobalt and copper in the neutralisation residue were relatively low comparing to calcium, iron, aluminium and silicon.Thus, calcium, iron, aluminium and silicon can be predominantly interferential elements and are the main impurities for the recovery of the valuable metals including nickel, cobalt and copper using the low-sulphuric acid method in this study.Although the content of calcium was about 18%, the concentrations of calcium in the solutions were relatively low and in the range from 0.0005g/L to 0.0037g/L, which is similar to the data reported in literature that the low saturated solubility of calcium suphate in solutions with low concentration of sulphuric acid [10].Hence, the separation of nickel, cobalt and copper over calcium has been achieved by the low-sulphuric acid method.It was found that after a single contact of the first leaching the concentrations of metal ions including nickel, cobalt, copper, iron, aluminium and silicon in the pregnant leaching solutions decreased with the increasing pH of the solutions (Figure 3), which is in agreement with the previous findings that the concentrations of metals including nickel, cobalt, copper, iron and aluminium decreased with the neutralization pH for precipitation from laterite leach solutions [6].As one of the main elements for recovery, nickel has the highest content in the residue and the highest concentration in the pregnant leaching solutions over those of cobalt and copper.The concentrations of nickel ions were significantly higher than most of the impurity metals including iron and silicon except aluminium in the pH range studied (2.0-3.7)(Figure 3).For example, at pH about 3.22, the nickel concentration was about 2.38g/L, while the concentrations of iron and silicon ions were only about 0.33g/L and 0.025g/L, respectively.In the pH range from about 2.8 to about 3.7, the concentrations of metal ions in the pregnant leaching solutions were in the following order: aluminium(III)> nickel(II)> magnesium(II)> copper(II)~ silicon(IV)~ manganese(II) > iron(III)> cobalt(II)> calcium(II).The molar concentration ratios of nickel(II) ions over other metal ions in the pregnant leaching solutions with the first leaching were showed in Table 2.It was found that the concentration ratios of nickel ions over iron, aluminium and silicon at lower pH, were significantly lower than those with higher pH.For example, the Ni/Fe, Ni/Al, Ni/Si ratios were 1.61, 0.15 and 1.21 at pH about 2.06, which were improved to be 6.83, 0.29 and 3.04, respectively, at pH around 3.36.Hence, the optimal selective extractions and separation of nickel over iron, aluminium and silicon can be achieved in a relatively high pH range, such as the pH from 2.82 to 3.68.However, the pH should be controlled in a relatively low pH level in order to achieve high concentration of nickel in the leaching solution.The concentrations of aluminium in the pregnant leaching solutions linearly depends on the pH from pH about 2.06 to about 3.68, indicating the aluminium hydroxides complexes in the neutralisation residue were gradually dissolved by sulphuric acid.It showed that the concentration of the iron ions was in small amount at low acidity of H2SO4 with pH above 3.0, but became significantly higher after the pH was below about 2.8 (Figure 3).For example, the concentrations of iron ions were about 0.33g/L and 2.22g/L at pH about 3.36 and 2.06, respectively.This behavior is consistent with the enhanced hydrolysis of iron(III) ions at the higher pH values and comparing well with the pH of 2.90 at 25℃ for iron(III) hydrolysis given by Baes and Mesmer [11].Thus, in order to avoid the significant dissolution of the impurities, such as ferric and aluminium hydroxides, the leachate pH should be controlled in a comparatively high level, and the reaction conditions benefit for the hydrolysis of metals ions, which has versa effect on dissolution.
Thereafter, a pH in a range of 2.8-3.0 is desirable considering the above data with relatively low dissolution of impurities, such as ferric and aluminium hydroxides, while with relatively high dissolution of nickel, cobalt and copper complexes.Furthermore, a relatively high temperature, such as 90℃ used in this study, is beneficial to improve the metal hydrolysis, considering the initial hydrolysis pH of metals ions, including iron(III) and Al(III) ions, can shift to lower pH with increasing temperature [12,13].

Effect of acid/ore ratio on recovery efficiency of leaching
The total recovery percentages of metals with two contacts of leaching are in the following order: Mn>Sc~ Co> Cu >Ni >Al~ Si> Fe> Ca (Figure 4).The complete dissolution of manganese was readily achieved with low to high amounts of sulphuric acid.The leaching percentage values of calcium were very low (all lower than 0.4%) in all the acid/ore ratio range studied, most possibly owing to the low dissolution of gypsum under our test conditions.It was found that the iron dissolution was low and the leaching percentages slightly increased from 1.54% to 3.61% in the acid/ore ratio range from 0.07:1 to 0.19:1 (Figure 4).The leaching percentages of scandium, nickel, copper and cobalt all significantly increased with the increase of acid/ore ratio.After contacting with acid leaching and water leaching, the complete dissolutions of scandium, nickel, copper and cobalt were all achieved when the acid/ore ratio was close to 0.19: 1.However, the dissolutions of impurities including aluminium and silicon reached to a relatively high level at the same time.Actually, an acid/ore ratio around 0.16: 1 was sufficient for leaching out about 94.79% of nickel, about 96.46% of copper and about 100% of cobalt from the residue while the leaching percentage of the impurities were much lower.Thus, the optimised sulphuric acid/ore ratio was selected to be about 0.16: 1, considering the relatively high recovery of the valuable metals including scandium, nickel, copper and cobalt and the relatively low leaching of impurities elements including iron, aluminium and silicon.

Recovery of the Valuable Metals from Solution by Sulphide Precipitation
The method with mixed sulphide precipitations was used to recover the valuable metals from the laterite PLS solution in this study, due to the strong combination of divalent transition metals including nickel, copper and cobalt and the rejection of aluminium, magnesium and calcium.The PLS solutions from the above leaching tests were blended and about 1.0 liter of the blended solution was used for a preliminary precipitation test with 0.61 mol/L of potassium sulphide solution.About 1.4 liter of a barren solution was obtained with an end pH point of about 4.10 after precipitation.Based on the metal concentrations in the original blended solution and barren solution, the precipitation percentages of the metals including nickel, copper and cobalt were calculated.It was showed that 99.6% of nickel, 99.2% of cobalt and 99.9% of copper can be preferentially precipitated as concentrate solids while only about 9.2% of aluminium, 5.2% of calcium, 11.4% of manganese and 10.9% of magnesium were co-precipitated.Although the iron precipitation percentage was about 47.8%, the iron amount in the concentrate solid can be low considering the iron concentration in the PLS solution was only about 0.22g/L (Table 3).The scandium ions with low concentrations in the barren solution could be recovered by solvent extraction with various organic extractants [14,15].

Conceptual Flow-sheet for Recovery of from Neutralisation Residue
Based on the above results, a conceptual flow-sheet for recovery of scandium, nickel, cobalt, copper and zinc from neutralisation residues of laterite hydrometallurgy can be proposed as: first, selective leaching of scandium, nickel, cobalt and copper from neutralisation residues into PLS with low concentration of sulphuric acid solutions, and separating from the main impurities including iron, aluminium and silicon; second, selectively precipitating of nickel, cobalt, copper and zinc from the leachates into concentrates with sulphides such as sodium sulphide and hydrogen sulphide, to make efficient separations from the impurities including scandium, manganese, magnesium and aluminium (Figure 5).

Conclusions
The recovery of scandium, nickel, cobalt, copper and zinc from a neutralisation residue of laterite hydrometallurgy of RAMU project has been studied.The results show that scandium, nickel, cobalt and copper can be recovered at high efficiencies from the residues by selective leaching using sulphuric acid solutions under ambient conditions, and the co-leaching of iron is low under the optimal conditions.In the pH range from about 2.8 to about 3.7, the concentrations of metal ions in the pregnant leaching solutions are in the following order: nickel (II)> copper (II)> iron (III)> cobalt (II)> calcium (II).The total recovery percentages of metals with two contacts of leaching are in the following order: Mn>Sc~ Co> Cu >Ni >Al~ Si> Fe> Ca.The mixed leaching solution is further purified with sulphide precipitation processes using sulphide salt.A conceptual flowsheet to recover scandium, nickel, cobalt, copper and zinc from laterite neutralisation residues has been proposed.These findings make practicable larger-scale evaluation of valuable metals recovery from nickel laterite neutralisation residue, reveal the fundamental behaviour of the impurity rejection process, and potentially provide a process enabling guide further industrial development of laterite-processing production with high quality.

Figure 1 .
Figure 1.EM picture of the residue.

Figure 3 .
Figure 3.Effect of leaching pH on the metal concentration in the solutions.

Figure 4 .
Figure 4. Effect of sulphuric acid/ore ratio on total recovery percentages.

Figure 5 .
Figure 5.A conceptual flow sheet for metal recovery from neutralisation residue

Table 1 .
Composition of the neutralisation residue.

Table 2 .
Effect of pH on molar concentration ratio of metal ions in the PLS

Table 3 .
Recovery of the valuable metals from solution by sulphide precipitation.