Magnesium extraction of ferronickel slag processed by alkali fusion and hydrochloric acid leaching

A research using ferronickel slag, the by-product of ferronickel production,
 as raw material for magnesium extraction has been carried out. It is
 essential to upgrade the value of ferronickel slag instead of used directly
 for reclamation materials. Moreover, accumulation due to increasing
 ferronickel demand as well as environmental contamination due to various
 potencially toxic elements contained in the ferronickel slag could be
 prevented. The general objective of this study is to utilize the ferronickel
 slag for magnesium materials. The specific objective is to determine the
 optimum conditions of magnesium extraction in the process of alkali fusion
 followed by hydrochloric acid leaching. A novel method for magnesium
 extraction from ferronickel slag was carried out through reducing silica
 content followed by acid leaching method. Alkali fusion of the mixture of
 ferronickel slag and Na2CO3 at 1000 ?C for 60 minutes followed by water
 leaching at 100 ?C for 60 minutes with solid to liquid percentage of 20 %
 were carried out to separate the silica. The leaching residue resulted from
 water leaching was then leached using hydrochloric aid solution to extract
 magnesium. The leaching temperature and time as well as the hycrochloric acid
 concentration were varied in the acid leaching process. Alkali fusion process
 proved can be generated the sodium silicate that can be separated in the
 water leaching to the leached solution. Meanwhile, the leaching residue was
 leached using hydrochloric acid to extract the magnesium. The highest
 magnesium extraction percentage is 82.67% that resulted from an optimum acid
 leaching condition with temperature of 80 ?C for 30 minutes using 2M HCl
 solution. Based on the kinetics study, the activation energy (Ea) for the
 leaching reaction of magnesium at atmospheric pressure between 32?C to 80?C
 is 9.44 kJ/mol and affected by diffusion and chemical reactions.

magnesium materials. The specific objective is to determine the optimum conditions of magnesium extraction in the process of alkali fusion followed by hydrochloric acid leaching.
Due to reach the objectives of this study, a novel method of magnesium extraction from ferronickel slag wass carried out in two stages consisted of reducing silica content by decomposition through alkali fusion with Na2CO3 addition followed by water leaching and upgrading of magnesium content by acid leaching using hydrochloric acid solution. Therefore, silica and magnesium leached solution were produced from this method.

Materials and Procedures of Experiment
The basic material used was the slag from a by-product of the ferronickel processing in Indonesia. Ferronickel slag was crushed by using a crusher and a disk mill to obtain a 200 mesh of particle size. The initial process was alkali fusion of 200 mesh of ferronickel slag with the addition of sodium carbonate (Na2CO3). It was intended to increase slag porosity and transform silica-bonded phases in the easy separated phase, sodium silicate (Na2SiO3). The operation condition of alkali fusion process is based on the previous study [12]. It was carried out in the muffle furnace carbolite at a temperature of 1000 °C for 60 minutes with rasio 1:1 of ferronickel slag and Na2CO3. Water leaching was performed to the product resulted from alkali fusion process at temperature of 100 °C for 60 minutes, solid to liquid ratio 20 % and stirring speed 400 rpm. It aims to eliminate sodium silicate that separated in the leached solution. Meanwhile, the leaching residue was then leached using hydrochloric acid solution to extract magnesium.
The leaching process was undertaken by varying the leaching temperature and time as well as leaching agent concentration. Those three variables generally have an important role in the upgrading of magnesium contents in the acid leaching process [17]. Leaching process was carried out in the three-neck flask with solid to liquid ratio 1:10 and stirring speed 300 rpm. Leaching temperature and time were varied at ambient temperature up tp 80 °C and for 15 -240 minutes respectively. Hydrochloric acid concentration was varied from 2 -8 M. Filtration was then performed to separate the leached solution and leaching residue.
XRF (X-Ray Fluoresence) analysis using XRF Bruker-S2 Puma was performed to the ferronickel slag and the product of alkali fusion process to determine its composition. The phase transformation of ferronickel slag, the product of alkali fusion process and leaching residue resulted from water leaching process was analysed using XRD (X-Ray Diffraction) Panalytical Epsilon 1. Moreover, the morphology and the mapping were characterize using SEM (Scanning Electron Microscope)-mapping analysis JEOL Jsm 6390A. Mg content in the leached solutions from acid leaching process as effect of leaching temperature, leaching time and acid concentration were determined using ICP-OES Agilent 725 series. In addition, kinetic study of magnesium extraction in the acid leaching process was investigated.

Results and Discussions 3.1. Thermodynamic Studies on Alkali Process
Alkali fusion process of the mixture of ferronickel slag and Na2CO3 in the present study is carried out at a temperature of 1000 °C for 60 minutes. The determination of temperature is engages with delta G calculation using HSC chemistry 6 software to find out the temperature of the spountaneous reaction start to occurr. Due to Mg2SiO4 is the dominant phase in the ferronickel slag, it used as phase representative that reacts with Na2CO3 in the alkali fusion process. The results of the ΔG calculation using HSC chemistry 6 software can be seen on Table 1. Based on the ΔG calculation using HSC chemistry 6 software, Table 1, the reaction between Mg2SiO4 and Na2CO3 can occur starting at a temperature of 900 °C. Since the ΔG has a negative value which indicates that spontaneous reaction occurred. Therefore, alkali fusion process at temperature of 1000 °C is in a good agreement with the thermodynamic study. The use of temperature at 1000 °C for 60 minutes is appropriate with previous studies that recommended the alkali fusion of ferronickel slag with Na2CO3 at a temperature above 900 °C [12].

XRF Analysis
The identification of chemical compositions and contents in the ferronickel slags was analyzed by XRF (X-Ray Fluorescence) with the Bruker-S2 Puma brand. The chemical composition of ferronickel slag, product of alkali fusion and the residue from water leaching process can be seen in Table 2.
As can be seen from Table 2, the dominant element found in ferronickel slag is silica dioxide (SiO2) with the content of 45.69%. Other elements contained in the ferronickel slag are magnesium oxide (MgO), iron (III) oxide (Fe2O3), and nickel (II) oxide (NiO) with the content of 29.332%, 16.503%, and 0.121%, respectively. The results of elemental contents in the ferronickel slag are not much different from some works of literature that show an analysis using XRF on ferronickel slag samples [12,15,17]. The XRF results after an alkali fusion shows that the contents of SiO2, MgO, Fe2O3, and NiO have decreased due to the addition of sodium carbonate as an additive.
Meanwhile, decreasing SiO2 content in the leaching residue of water leaching process indicates that water leaching process is able to dissolve decomposed SiO2.

XRD Analysis
The identification of minerals contained in slags was characterized using X-Ray Diffraction Panalytical Epsilon 1 brand instrument. This XRD analysis is a qualitative analysis determined by PANalytical Xpert Pro MPD XRD with Cu Ka radiation (λ = 1.5406 Å). The generator was operated with 40kV of voltage, 30 mA of an electric current, 2 o /min of measurement speed and 0.020 o of a sampling pitch. Figure 1(a) shows the results of the XRD analysis of ferronickel slags. The composition of silica dioxides binds with magnesium and iron in the form of forsterite (Mg2SiO4) and fayalite (Fe2SiO4), which are dominated the compounds. The results of the XRD analysis of ferronickel slag is in accordance with some previous studies [15,17].
The XRD analysis of the product of alkali fusion process, Figure 1 (b), shows the formation of Na2SiO3. It indicates that reaction (1) -(3) were occurred since the intensity of magnesium silicate decreases. Moreover, SiO2 is observed with high crystallinity, it might be cause of crystallization of silica in the ferronickel slag due to thermal treatment. The presence of SiO2 led to FeSiO3 formation, as reaction (5). Fe2SiO4 + SiO2  2FeSiO3 (5) This result is appropriate with the research conducted by Prasetyo that the success of an alkali fusion process is indicated by reducing the intensity of magnesium silicate bonds [12,17]. This result also corresponds to the HSC analysis previously. Figure 1(c) shows the presence of Na2MgSiO4, MgSiO3, FeSiO3 and Mg2SiO4. Na2SiO3 is not observed that indicates it was dissolved in the water, then Na ions contacts with Mg2SiO4 to form Na2MgSiO4. Meanwhile, MgSiO3, FeSiO3 and Mg2SiO4 remain in the residue of water leaching product that was then leached using hydrochloric acid subsequently.

SEM Analysis
The SEM analysis was undertaken to find out the distribution of each element. Figure 2 (a) shows that the distribution of silica and magnesium element covers almost every section of the ferronickel slag in concert with oxygen. From the SEM observations, the red-color-area found with silica, magnesium, and oxygen. This indicates that most silica is found in a-silica dioxidecompound form and is associated with magnesium. The depiction of SEM results is in accordance with XRD analysis which explains that silicate is associated with magnesium and iron, namely enstatite, forsterite and fayalite as seen in the XRD analysis towards ferronickel slag which shows that the most dominant compound is the bond between magnesium and silicate. Thus, these results are appropriate with studies conducted by Mubarok et al and Tangahu et al who has explained that the dominant distribution in the SEM analysis of ferronickel slag was magnesium, silica, and oxygen [2,17]. Figure 2 (b) shows the formation of sodium distribution which binds silica as a result of SEM mapping analysis of ferronickel slag alkali fusion. On the other hand, the bond between magnesium and silicate from its distribution is no longer in the same distribution spot, so it indicates that the magnesium and silicate bond is not as strong as the first ferronickel SEM results. This is also reinforced by the XRD results towards the ferronickel slag fusion process which adds the sodium carbonate addictive substance shown in Figure 1 (b). In Figure 1 (b) sodium silicate is formed and the detection of SiO2 and MgO as a single-compound. Figure 2 (c) shows the SEM mapping of the residual leach with water. The distribution of silica has decreased, shown in a less bright red color, while magnesium and oxygen have an increase in the distribution of reddening colors in one spot area. This result indicates the decreasing content of silica in the residue of water leaching in a good agreement with the XRF results of the residue of water leaching on Table 2.

Effect of Leaching Temperature toward Magnesium Extraction Percentage
The leaching process was done by varying the leaching temperatures at room temperature (32°C), 60°C, and 80°C with 300 rpm of stirring speed and a solid/liquid ratio 1:10. The leached solution was analyzed using ICP (Inductively Coupled Plasma)-OES, an Agilent Technologies 5000 brand to determine the soluble magnesium contents.
As shown in Figure 3, the percentage of magnesium extraction increases along with the rise of leaching temperatures. The leaching process with a temperature of 80°C produced the leached solution with the highest magnesium extraction percentage, 82.67%, at thirtieth minute. It is accordance with the previous research, that explained the leaching temperature for leaching of magnesium silicate should be done at 50-105°C [24], since low temperature of the leaching process becomes less effective due to silica gel formation that can reduce magnesium extraction. Moreover, the statement is in accordance with the present research that shows the highest magnesium extraction percentage in the leaching process at a room temperature (< 50 ° C) for 240 minutes was only 54.12%. In addition, the research conducted by A. Royani in extracting magnesium from dolomite shows the similar trend, the higher the temperature is, the higher the magnesium extraction percentage obtained. Eventhough, in some ways, temperatures above 75°C are considered giving less significant impact [26]. However, the economic factor should be perceived as the addition of temperature is less economical since it increases the energy and the loss of acid used substantially [26].

Effect of Solution Concentration towards Magnesium Extraction Percentage
The leaching processes with the variations of hydrochloric acid concentration were carried out at 80°C for 15 -240 minutes using 2M, 4M, 6M, and 8M of hydrochloric acid. Figure 4 shows that the highest value of magnesium extraction percentage was at 2M of hydrochloric acid concentration, 82.67%, for 30 minutes. Increasing hydrochloric acid concentration to 4M, 6M and 8M, decreasing the magnesium extraction percentage to 72.48%, 69.36%, and 62.89% respectively. The higher acid concentration in the leaching process, the more elements would dissolve, it is correspond with research by Raschman et al. That magnesium extraction percentage in the leached solution decreases with increasing HCl concentration [28].  Figure 5 shows that the highest magnesium extraction percentage, 82.67%, was reached at leaching temperature and time of 80 ° C and for 30 minutes using 2 M of hydrochloric acid solution. The magnesium extraction percentage tends to decrease for further leaching time, > 30 minutes, at all temperature variations. Extending the times into the process could cause dilution of other components. It is consistent with the research of Ozdemir et al. That shows an increasing magnesium recovery in the leaching process by increasing leaching time to 20 minutes, then the magnesium recovery in the leaching process decreases for further leaching time at all variations of HCl concentration [29].

XRF analysis of residue from acid leaching
Chemical composition of the residue of HCl leaching using a concentration of 2M and a leaching temperature of 80 ° C was carried out using XRF analysis. It can be seen on the Table 3. that the remaining content of magnesium (Mg), iron (Fe), aluminum (Al), chrome (Cr) and nickel (Ni) are 0.42%, 1.49%, 0, 56%, 1.53% and 0.03% respectively. It shows that the majority of magnesium can be extracted in the dissolved solution and have left over silica in the residue.

Kinetics Studies of Leaching
Kinetics Leaching can be learned using the kinetics Shrinking-core approach model [30][31][32][33] In this kinetics model, the speed of the reaction, whether it is controlled by chemical reactions on the surface or by diffusion through the liquid boundary layer, is determined quantitatively based on the suitability between the elements fraction data that reacts at each time using the following equations (6) and (7) [30][31][32][33]: α is the fraction of the reacting elements, t is the leaching time equation (6) assumes that the leaching speed is controlled by chemical reactions occur on the surface of mineral particles with the reaction-rate constant kc, while equation (7) assumes that the leaching rate reaction control is the diffusion on the surface of mineral particles with the reaction-rate constant kd. Furthermore, the activation energy is determined based on the Arrhenius equation: Where, A = pre-exponential factor, R = universal gas constant (8.314 J / (mol K)), T = absolute temperature (K) and Ea = activation energy (kJ mol) [34,35].
The data plot between α and t using equations (6) and (7) for the leaching reaction of Mg element from nickel slags with 2M of HCl solution concentration can be seen in Figure 6   The optimum linear regression is obtained in the data plot using equation (6) as shown in Figure 6 (a). The Arrhenius plot according to the equation (8) is based on the rate constants determined from the slope of the linear regression in Figure 6 (a). From the straight-line slope in Figure 7, it can determine the activation energy (Ea) for the leaching reaction of Mg element at atmospheric pressure between 32°C to 80°C is 9.44 kJ/mol. Based on the extraction percentage data in Figure 3 and the kinetics data in Figure 6, the reaction rates increase significantly at temperatures of 32°C to 60°C so the average extraction percentage increases by ± 30% for each reaction time. Subsequent temperature increases up to 80°C only give an extraction percentage of ± 6% on average. Furthermore, the concentration of hydrochloric acid solution also did not have a significant effect on the Mg extraction percentage as shown in Figure 4. This condition indicates that the Mg leaching process from nickel slag is affected by diffusion and chemical reactions.

Conclusion
Magnesium extraction of ferronickel slag processed by alkali fusion and hydrochloric acid leaching was carried out. Ferronickel slag is proved can be used as magnesium materials in the two-steps, reducing silica content and upgrading magnesium content. Sodium silicate is confirmed to be formed due to alkali fusion of ferronickel slag with sodium carbonate addition at temperature of 1000 ° C for 60 minutes. It was then can be separated by water leaching. XRD analysis of leaching residue resulted from water leaching process shows the disappearance of sodium silicate phase that beneficiate magnesium extraction in the acid leaching process. An optimum condition of magnesium extraction in the hydrochloric acid leaching reached at temperature of 80 ° C for 30 minutes using 2M of hydrochloric acid solution with stirring speed 300 rpm and solid to liquid ratio of 1:10. The reaction is affected by diffusion and chemical reactions with the activation energy (Ea) of 9.44 kJ/mol. The highest magnesium extraction obtained is 82.67 %.