Acid-assisted leaching of iron and manganese from Sri Lankan laterite: a potential source of alumina production

Laterites are of great interest for industrial applications. This study is focussed on determining optimum conditions (i.e. pH, temperature, and sonication time) for leaching Fe and Mn from Sri Lankan laterite. Physicochemical parameters, atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopic analyses were performed. Different solutions were prepared for a pH range from 1 to 10, temperature from 40 oC to 100 oC, and sonication time from 10 min to 60 min. Elemental concentrations of the filtrates were used to determine the optimum conditions. Fe and Mn leaching efficiencies decreased with increasing pH from 1 to 5. The solution of pH 1 indicated the maximum leaching capacities of Fe (17.69 ppm) and Mn (2.05 ppm). Fe and Mn leaching efficiencies almost exhibited a positive correlation with temperature. The maximum leaching concentrations were observed after 15 min of sonication. Therefore, the optimum conditions for leaching both Fe and Mn were determined as pH 1 at 60 oC temperature after 15 minutes of sonication. Precipitates of treated samples were characterised using XRD and FTIR to determine changes compared to the raw sample. XRD results identified the crystalline phases, and thereby chemical composition of raw laterite can be discovered by the software as goethite and hematite as main Fe-rich minerals. The raw laterite was also associated with gibbsite, kaolinite, and quartz. However, weak reflections of hematite were observed in the XRD spectra of treated samples. FTIR observations suggested alteration of functional groups (e.g. the disappearance of Fe―O bond) in acid leached and sonicated samples. Consequently, FTIR and XRD results also confirmed the acid-leaching capacity of Sri Lankan laterite.


INTRODUCTION
Laterite is a common surficial/near surficial product of long-lasting and intensive tropical rock weathering and oxidation present in tropical countries such as Sri Lanka (Dahanayake, 1982;Pham et al., 2020). According to Persons (2012), 'laterite is a stone of a thousand uses'. The most fundamental application of laterites is as a building material. Many economically important metallic elements (e.g. Al, Fe, Mn, Ni) are present in laterites (Maiti et al., 2012;Çetintaş & Bingöl, 2020). Therefore, the extraction of these valuable components has been the focus of many researchers. For example, minor elements such as Zn, Cu, Ni, Cd, and Pb are present in Sri Lankan laterite (Nayanthika et al., 2018). Furthermore, Sri Lankan laterites have been used in the synthesis of pure hematite nanoparticles as superior adsorption dyes (Dissanayake et al., 2019).
Metals and metal oxides present in laterites can be leached using acids, and thus enhance the Al content (Nasab et al., 2020). Bauxite and sillimanite are the globally prominent raw materials for the extraction of alumina and Al metal (Perks & Mudd, 2019). Alumina has many primary and advanced applications (The

June 2021
Journal of the National Science Foundation of Sri Lanka 49 (3) Aluminium Association, 2020). However, the availability of bauxite and sillimanite resources are limited. The industries utilising alumina as raw materials (e.g. cement industry) are urged to go for alternatives such as laterites and lateritic soils that are composed of Al 2 O 3 (Moutei et al., 2018). Consequently, laterites have been leached to obtain metals such as nickel and cobalt, and to enhance the workability of laterite as a source of alumina (Goswami & Mahanta, 2007;Liu et al., 2010;Ayanda et al., 2011;Ilyas et al., 2020). Leaching and charging behaviour of laterites are strongly dependent on the pH (Pham et al., 2020). However, complex mineralogy and limited application of existing technologies have caused difficulties in the processing of laterites for value addition (Nasab et al., 2020).
Sri Lankan laterite is typically yellow, deep mottled red or reddish-brown ferruginous earth with many vesicular and botryoidal structures (Dahanayake, 1982). The mineralogical composition of Sri Lankan laterite is mainly composed of goethite, hematite, gibbsite, quartz, and clay minerals (Herath & Pathirana, 1983). However, the value addition potential of Sri Lankan laterites has rarely been focussed in the literature. Therefore, the current study is mainly focussed on investigating the metal leaching characteristics of Sri Lankan laterite. In this study, the authors investigated the optimum conditions such as pH, temperature, and sonication time for leaching iron and manganese from Sri Lankan laterite.

Sample collection and powder sample preparation
The representative laterite samples were collected from a quarry site in Horana-Millaniya (global position system coordinates: 6°38'59"N 80°00'48"E) in the Southwestern Highland Complex of Sri Lanka. Samples were collected from about 1.50 m depth from the surface. Laterite samples were milled using a vertical planetary ball mill (Tencan XQM-0.4A, Japan) and sieved using a British standard sieve shaker to obtain the particles below 63 µm.

Physicochemical analysis of raw laterite
The wet and dry weight of raw laterite was measured at room temperature, and after heating at 105 ºC for 24 h, respectively. Moisture content was calculated by the difference between wet and dry weights to the initial wet weight.
About 5 g of the sample was ignited at 550 ºC for 24 h using a Protherm-PLF 140/5 muffle furnace to remove organic matter. The sample was then reheated at 1050 ºC for 2 h in the muffle furnace to evolve carbon dioxide from carbonate. Loss on ignition (LOI 1050 ºC) was calculated by the difference between dry weight and ignited weight at 1050 ºC to the dry weight (Heiri et al., 2001).
Powdered raw laterite (< 63 µm) was oven-dried at 80 ºC for 120 min. About 5 g of raw laterite was suspended in distilled water with 1:10 soil suspension ratio, and pH value was measured using an Edge ® Multiparameter pH Meter -HI2020.

Determination of optimum conditions (pH range, temperature and sonication time)
(a) Raw laterite (5 g, particles < 63 µm and oven-dried at 80 ºC for 120 min) was suspended in 50 mL of distilled water. Suspension solutions were prepared from pH 1 to 10 at room temperature. The pH of the solution was adjusted by adding aqueous HCl or aqueous NaOH dropwise. The mixture was filtered after 30 min. The filtrates and precipitates were collected. The precipitates were washed with distilled water to remove unconsumed acid/base and oven-dried at 80 ºC for 60 min.
(b) Powdered and dried laterite (5 g) samples were suspended in 50 mL of distilled water to determine the optimum temperature. The pH values of the solutions were adjusted by adding 1 M HCl drop wise. After the determination of optimum pH range, pH 1 solutions were considered for further evaluation. The solutions were heated for 60 min at different temperatures of 40, 60, 80, and 100 ºC in a water bath. The filtrate and the precipitate were collected after filtration.
(c) Powdered and dried laterite (5 g) samples were suspended in 50 mL of distilled water at room temperature (27 ºC) to determine the optimum sonication time. The pH values were adjusted by drop wise addition of 1 M HCl. The pH 1 solution series was used to determine sonication time. The mixtures were sonicated for 10, 15, 30, 45 and 60 min using Rocker ultrasonic cleaner sonicator (Soner 206) at room temperature at a frequency of 53 kHz. The mixtures were filtered, and the filtrate and precipitate were collected.
Journal of the National Science Foundation of Sri Lanka 49 (2) June 2021

Atomic absorption spectroscopic (AAS) analysis
Raw laterite (5 g, particles < 63 µm and oven-dried at 80 ºC for 2 h) was dissolved in 50 mL of distilled water and stirred for 30 min. Then, the mixture was filtered and the filtrate was collected to determine the water solubility of iron (Fe), cadmium (Cd), and manganese (Mn) in raw laterite.
Similarly, the filtrates of treated laterite were also collected to determine the leaching concentrations of Fe, Cd, and Mn. Concentrations of Fe, Cd, and Mn in the raw and treated samples were measured using Varian AA240 atomic absorption spectrometer, after calibrating the instrument linearly using relevant standards of known concentrations.

Fourier-transform infrared (FTIR) analysis
KBr pellet method was used to analyse the samples. The powdered samples were mixed with potassium bromide (KBr) at 1:10. The FTIR grade dried KBr (assay ≥ 99%) was used. A pellet (1 mm in thickness and 7 mm in diameter) was prepared for KBr/powdered sample mixture using a hand press pelletiser. The pellets of raw and treated (precipitate) laterite were placed directly in the beam of Bruker Alpha spectrophotometer over the range of 500-4000 cm -1 .

X-ray diffraction (XRD) analysis
The untreated and treated precipitates of laterite samples were finely powdered. These samples were analysed via powder X-Ray Diffraction (XRD) method using Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ=1.54) at 40 kV and 30 mA. The diffractogram was recorded with a scanning rate of 0.02°/s in a 2θ range from 10° to 60°.

Physicochemical parameters of raw laterite
The average pH of powdered laterite suspension in water was 5.69. Therefore, 1 M HCl and 1 M NaOH were used to prepare solutions of pH below 5.69 and pH above 5.69, respectively. The moisture content and LOI 1050 ºC values of raw laterite samples were 15.80 % and 16.56 %, respectively. LOI 1050 ºC value indicates relatively high carbonate values for the raw laterite. In addition, raw laterite shows a higher abundance of Cd in filtrates relative to Fe and Mn (Figure 1). Consequently, Cd can be easily leached from laterite than Fe and Mn under normal soil pH conditions. The geochemistry of laterites and lateritic soils significantly varies based on the location, climatic conditions, and depth. Layer thickness, geochemistry, and ore mineralogy of lateritic soils are determined by the degree of prolonged chemical weathering process. Laterites and their derivative soils contain SiO 2 , Al 2 O 3 , Fe 2 O 3 , and metals such as K, Mg, Ca, Ni, Co, Mn, Cd, and Ti (Dissanayake, 1980;Gleeson et al., 2004;Fan & Gerson, 2011;Nayanthika et al., 2018).

Determination of optimum conditions for the leaching of Fe and Mn from laterite
Effective pH range Figure 2 shows variations in ion leaching capacities of selected elements with pH. Accordingly, the Fe leaching concentration decreases suddenly with increasing pH values from 1 to 2 ( Figure 2). Then, Fe leaching concentration slightly decreases with increasing pH from 1 to 5. However, Fe concentrations are almost constant in pH values above 6 ( Figure 2). Therefore, pH of 1 provides the highest leaching potential for Fe (17.69 ppm) with an enrichment factor of 17,690 compared to water solubility. The Aluminium Association (2020). Available at https://www.aluminum.org/indu refining, Accessed August 2020. Mn leaching efficiency also decreases gradually with increasing pH from 1 to 5 (Figure 2). Clear variations of Mn concentrations were not observed in pH values above 6 ( Figure 2). Therefore, Mn can be leached under low pH conditions ( Figure 2). However, Cd dissolution concentration gradually increases from pH 1 to 10 ( Figure  2). This suggests that Cd can be effectively leached under basic conditions, and the effective pH is 7 (0.18 ppm in Figure 2). Therefore, Cd does not leach well with 1 M HCl at any pH value, though it leaches in water.
The peaks corresponding to iron oxides such as Fe-OH (801 cm -1 ) and Fe-O (536 cm -1 ) are recorded in acid leached samples (pH 1 and pH 2 solutions) (Figure 5b), and acid leached and heated (pH 1/60°C) samples ( Figure  5c). Therefore, it suggests that iron oxides remain in the precipitate after low pH acid leaching, and acid leaching at elevated temperatures (Figure 5b, c). Similarly, the peak corresponding to iron hydroxide (Fe-OH) can be observed in the acid leached and sonicated (pH 1/15 min) sample ( Figure 5d). However, the peak corresponding to Fe-O bond in hematite has disappeared in the acid leached and sonicated (pH 1/15 min) sample ( Figure  5d). Therefore, the combination of acid leaching and sonication can be identified as the most effective method for Fe leaching from laterite samples. Figure 6a shows that raw laterite indicates the presence of Fe-rich secondary minerals such as goethite substituted with Al ((~Fe 0.9 Al 0.1 )O(OH)), hematite (Fe 2 O 3 ), and Alrich secondary minerals such as gibbsite (Al(OH 3 )). Kaolinite indicates the presence of clay minerals in laterite ( Figure 6). Besides, goethite can occur in laterite as a weathering product of Fe-bearing minerals and as a direct precipitated agent from Fe-containing solutions (Mohapatra et al., 2008). Goethite is metastable under a wide range of environmental conditions and temperatures (Cornell et al., 1975) and has lower dissolution with 1M HCl than hematite (Cornell & Giovanoli, 1993). Consequently, the minor changes in peak positions and the increment of intensity in treated samples indicate that the Al-rich goethite in raw laterite has been converted to  gibbsite after leaching, thus enhancing the Al content in the samples (Figure 6).

XRD observations
The occurrence of quartz peaks in raw laterite suggests the presence of silicate minerals as impurities ( Figure 6). Quartz in laterite can be coated with Fe-oxides such as hematite. In addition, the reduction of the number of hematite peaks after treatment indicates that raw laterite has been successfully leached, liberating Fe ions into the solution ( Figure 6). However, the remaining hematite peaks in the treated samples suggest the presence of small amount of Fe even after treatment. Thus, acid leached (pH 1 solutions), acid leached and heated (pH 1/60°C), and acid leached and sonicated (pH 1/15 min) samples recorded more gibbsite peaks suggesting successful Al enhancement (Figure 6b, c, d).

CONCLUSIONS
XRD results show goethite and hematite as main Ferich secondary minerals in Sri Lankan raw laterite. The Fe and Mn leaching efficiencies decrease with increase in pH from 1 to 5. The Cd leaching efficiency increase with pH from 1 to 10. Therefore, the Fe and Mn leaching efficiencies are controlled by hydrogen ion concentration.
In contrast, Cd leaching was controlled by hydroxyl ion concentration. The Fe in the leachate obtained after treating at pH 1 showed a Fe enrichment factor of 17,690. Fe and Mn leaching efficiencies increased with temperature from 40 ºC to 100 ºC. However, pH 1, temperature 60 °C, and 15 min sonication time were determined as optimum industrial applicable conditions for Fe leaching from Sri Lankan laterite. Sonication for 15 min at 60 °C in pH of 1 was determined as the optimum condition for Mn leaching from Sri Lankan laterite, with an enrichment ratio of 2,695 in the leachate. The goethite found in laterite used in this study is (~Fe 0.9 Al 0.1 )O(OH)) according to XRD analysis and this goethite would react with concentrated acid to form gibbsite and FeCl 3 . Leaching Fe and Mn enhance the Al content present in laterite. Consequently, the upgraded laterite could be used in applications, such as an additive to adjust Al content in clinker during cement production. The leachate could be a potential source of Mn and Fe.