Nickel and Cobalt Extraction from Greek Laterites Using Nitrate Solutions ^†

Abstract

The extraction of nickel and cobalt from a limonitic laterite sample, derived from a mine area in the Greek island of Euboea, was studied via an acid agitation leach process with nitric acid as the leaching agent, to determine the optimal conditions at which the highest possible extractions of nickel and cobalt were obtained in the pregnant solution. Two series of experiments were carried out. In the first series, the extractability of metals was studied by varying the leaching temperature at values of 60 °C, 80 °C and 100 °C. For the second series of tests, the metals’ extraction at different values of initial nitric acid concentrations of 1M, 2M and 4M was examined. Based on the results, the effect of temperature is characterized as particularly significant, as with its increase, the final recoveries of nickel and cobalt were particularly high. The variation in acid concentration had a significant effect but not like that of temperature. For extraction conditions of an S/L ratio of 20%, a temperature of 100 °C and a 2M HNO₃ concentration, the highest recoveries of nickel and cobalt were obtained, namely 94.4% and 83.6%, respectively. Iron in all tests did not exceed dissolutions of more than 7.2% in the pregnant solution.

1. Introduction

Nickel and cobalt are among the critical and strategic elements with a plethora of applications in modern times. Not only the shift in energy orientation but also the extinction of rich reserves have turned the research interest to hydrometallurgical routes of extracting the two metals over conventional pyrometallurgical ones. Several inorganic and organic acids have been investigated at the lab and pilot scale or applied industrially for leaching nickel and cobalt from laterites either at ambient temperature or at high pressure in autoclaves.

The use of nitric acid as a leaching agent has several advantages given that it can treat all the types of nickel lateritic ores, and in addition, it could be regenerated and recycled back to the leaching stage after treatment of the pregnant leaching liquor, as has been demonstrated by the Direct Nickel (DNi) process [1]. Apart from the DNi process, there are several other studies investigated at laboratory scale using HNO₃ as a leaching agent to extract Ni and Co from laterites, including leaching of limonitic laterites from Indonesia [2], leaching of two types of lateritic nickel ores, one limonitic and one nondronitic from Gördes and Manisha in Turkey, respectively [3], at ambient conditions, pressure leaching of saprolitic-type ore from the Yuanjiang region of China [4], limonite ore leaching from the Indonesian island of Sulawesi [5], and leaching of oxidized nickel-rich ore (1.80% w/w Ni) from Turkey [6].

In the present study, the recovery of nickel and cobalt from a laterite ore of Euboea, Greece, was investigated via a hydrometallurgical leaching process, in a sample derived from the homogenized piles of ores to be fed to the Rotary Kilns of LARCO SA (Larymna, Greece). The effect of several parameters including retention time, temperature and pulp density on nickel and cobalt extraction, as well as iron co-extraction, was investigated, aiming at identifying the leaching parameters leading to maximum nickel and cobalt extraction with the lowest iron co-extraction.

2. Materials and Methods

The laterite ore was mined at a mine on Euboea island in the area of Vrysakia. After obtaining a representative sample of the ore, the necessary crushing and grinding operations followed. In respect to the reduction in the size of coarse laterite, a laboratory jaw crusher was used, into which about 10 kg of the ore was fed. A LabTechnics LM2 Laboratory Pulverising Disc Mill was used for grinding. The obtained sample was less than 250 μm in size. Chemical analysis was performed using acid digestion followed by atomic absorption spectroscopy (AAS, PinAAcle 900T, PerkinElmer, Akron, OH, USA) and ICP-OES (Optima 7000, PerkinElmer, Akron, OH, USA) analyses. Mineralogical analysis was performed by XRD analysis (Bruker AXS, Karlsruhe, Germany, D8 Focus).

Leaching tests then followed, under atmospheric pressure using nitric acid solutions as the reagent, to investigate the extraction rates of nickel and cobalt. All the experiments were carried out in a 0.5 L capacity five-necked, angled, round, glass split reactor. Mechanical stirring was employed, providing continuous agitation throughout the tests. Heating was rendered by a heating mantle, while a Pt-100 sensor was used for the regulation of temperature. Vapor condenser and water circulation systems were also adjusted.

The effect of leaching temperature (60, 80 and 100 °C) and initial nitric acid concentration (1, 2 and 4M) was investigated. For the central conditions, values of 80 °C and 4M regarding the previous parameters were chosen, respectively. Every batch lasted a total of 300 min under continuous agitation of 500 rpm. The volume of solution (400 mL), the granulometry of the material (<250 μm) and the solid to liquid ratio (200 g/L) were kept constant. Pulp samples were obtained at specific times; they were filtered, and the obtained solutions were kept for further chemical analysis by FAAS and ICP-OES. The final solution was filtered under vacuum.

As it can be seen in

Table 1. Chemical analysis of the laterite sample.

Element	Acid Digestion/AAS (% w/w)
Ca	0.62
Fe	43.57
Mg	2.07
Ni	1.24
Co	0.06
Cr	2.06
Mn	0.16
Al	1.99
Si	10.64

, the ore appears to have a medium Ni content (1.24%) and is characterized as quite poor in Co (0.06%).

As seen in Figure 1, the main mineral phases indicated by the XRD pattern are clinochlore (Mg₅Al(AlSi₃O₁₀)(OH)₈), quartz (SiO₂), hematite (Fe₂O₃) and magnesium–chromium oxide (MgCr₂O₄).

3. Results and Discussion

3.1. Effect of Leaching Temperature

Increases in temperature seem to have a very strong effect on the extractability of Ni (Figure 2a). At a temperature of 100 °C, almost 70% of the metal has already reacted with the acid, during the first 30 min. From this moment onwards, the kinetics of extraction continue at a steady rate until 5 h. The maximum recovery is observed at 4 h, reaching 94.40%. This is the highest Ni recovery found among all extraction tests. On the contrary, at central conditions, where the temperature is equal to 80 °C, there is a lower extraction of Ni, with its final recovery after 5 h at almost 81%. The percentage values of Ni for each time sampled during extraction at 60 °C are markedly lower. The final maximum recovery achieved was equal to 50.18%.

The increase in temperature seems to have also a significant effect on the extractability of Co (Figure 2b). At 100 °C, 60% of the metal has been dissolved, during the first 30 min. Then, as with Ni, the kinetics of extraction evolve at a more uniform rate. The maximum recovery is achieved in 4 h with a percentage that reaches 83.57%. This value is the highest for Co compared to all other experimental tests that took place. At a temperature of 60 °C, the maximum cobalt extraction is less than half that of 100 °C (about 40.25%). Extraction at 80 °C gave maximum recovery for Co (67.64%) in the 5 h reaction time.

Fe also exhibits increased solubilization as it is extracted at a higher temperature. The maximum recovery values for 60, 80 and 100 °C were 3.57%, 5.51% and 7.18%, respectively.

3.2. Effect of Initial HNO₃ Concentration

Increased initial acid concentration affects the extractability of Ni. As can be seen in Figure 3a, after 2 h, the curves of 2M and 1M have an ascending trend, while that of 4M reaches a plateau. At 4M nitric acid concentration, a higher percentage of dissolution of the metal occurs compared to the other two acidity values, reaching extraction rates of 83.34% in 3 h. The extraction at a HNO₃ concentration of 2M progressively increases with time, giving a final nickel recovery rate of 80.79% in 5 h, close to that obtained using the 4M HNO₃ solution. For an acid concentration of 1M, there is a noticeably lower extractability than the other values, with a final recovery of 52.72%.

Co seems to be extracted with similar kinetics to Ni (Figure 3b). Specifically, for a concentration of 4M at 2 h, its dissolution rate reaches 72.5%. From this point on, the extractability progresses at a fairly slow pace, thus giving the maximum recovery of about 77%, at 3 h. Both extraction with an acid concentration of 2M and that with 1M seem to yield the same Co recovery until the first 1 h. From this time onwards, at central conditions, extractability is observed with greater acceleration compared to 1M, resulting in the final recovery of Co estimated at 67.64% compared to 46.31% at low concentration.

4. Conclusions

The development of hydrometallurgical processes for the treatment of low-grade ores for the recovery of various metals has turned the scientific community to the study of nickel and cobalt ore extraction by various methods. In this study, leaching experiments under atmospheric pressure of a nickel laterite from Vrysakia, Euboea, with a solution of nitric acid (HNO₃) was studied. The ore contained 1.24% Ni, 0.06% Co and 43.57% Fe. Nickel and cobalt occurred mainly inside the complex of phyllosilicate minerals (Mg₅Al(AlSi₃O₁₀)(OH)₈), whereas iron was detected in the form of hematite (Fe₂O₃).

At a leaching temperature of 100 °C, a pulp density of 20%, an initial HNO₃ concentration of 2M, a stirring speed of 500 rpm, an extraction time of 5 h and an ore grain size of <250 μm, maximum recoveries of both Ni and Co of 94.40% and 83.57 were achieved, respectively. Those were the optimal conditions. The iron recovery rate was low in all experimental trials, not exceeding 7.18%. The low extraction of Fe is due to the existence of the metal as hematite, the lattice of which cannot be easily dissolved by HNO₃. Low Fe content in the PLS has a positive role as the element is considered an impurity and must be rejected during the processing of the pregnant solution.