Efficient Leaching Process of Valuable Elements from Gibbsite Ore Materials in Talet Seleim, Southwestern, Sinai, Egypt Using Green and Ecofriendly Lixiviant Agent: Optimization, Kinetic and Thermodynamic Study

Abstract Here, valuable elements in Talet Seleim, Southwestern Sinai, Egypt, were leached using methanesulfonic acid (MSA) as an environmentally friendly lixiviant agent. The effects of methanesulfonic acid (MSA) concentration, time of leaching reaction, temperature, liquid/solid ratio (L/S) and stirring speed on dissolution of valuable elements in MSA were studied. The optimized conditions for dissolution of various metals were: methanesulfonic acid concentration; 150g/L, dissolution time; 300 min, temperature; 363K, liquid to solid ratio; 4 ml/g and stirring speed; 400 rpm. The dissolution efficiency of Zn, Cu, Ni and Pb are 81.8%, 79.47%, 76.6%, and 70.7%, respectively under the optimal conditions. Different mathematical kinetic models for the dissolution process in MSA were studied since it is very important for economic metal dissolution. The experimental data were best fitted by pseudo-second-order homogeneous reaction model for zinc (Zn), copper (Cu) and nickel (Ni), whilst lead (Pb) was best fitted by the diffusion through the product or ash layer. The activation energies of the dissolution processes of different metals were evaluated. Overall, the MSA leaching system provides an efficient, clean and green method for dissolution of valuable metals. It can be a promising alternative to inorganic acids. GRAPHICAL ABSTRACT


Introduction
There is increasing demand for recovering of valuable metals (nickel, zinc, lead, and copper) from the mining residues or secondary sources in the world. Aluminum and copper followed by lead and zinc are the most widely used non-ferrous metals. They are used in different industrial fields and all aspects of modern life such as chemical engineering acid batteries, alloys, electroplating, rubbers galvanization, radio-active shielding, alloy, medical industry, and pigments, etc.
Copper is the world's third most used valuable metal (after iron and aluminum). There are several techniques that can be used in the extraction of metals from various ore minerals 1,2 . Metals can be produced through hydrometallurgical, bioleaching and pyrometallurgical processes, respectively [1][2][3] . A lot of hydrometallurgical approaches studies have been made to develop novel methods to exploit the decreasing of resources. Different inorganic acids and organic acids were used for dissolution of metals from different ores sources such as sulphuric, hydrochloric, nitric and formic acids as well as, ammonium sulphate, ammonium acetate, ammonium carbamate and ammonium nitrate [1][2][3] . Sulfuric acid leaching was the most common leaching agents for dissolution the metals from low-grade resources due to its merits such as; the ease of the process, the simplicity and economic. The chemical reaction of metal with different acid. Metal (II) oxide is considered a basic oxide, so it dissolves in mineral acids according to the following general equations 4  Recently, many researchers have focused on the enhancement of hydrometallurgical processes that can replace the pyrometallurgical methods used for metal production 5 .
Hydrometallurgical method routes aim to get rid of some of the disadvantage of pyrometallurgical methods to environmental and economical such as low extraction of targeted metals for low-grade resources, high energy needs, high initial costs of investment and the liberation of hazardous SO x gases when treated the sulphide ores. So, the hydrometallurgical processes are more favorable than pyrometallurgical methods. With declining grades of copper and nickel deposits, there is a need for finding economical and environmentally responsible methods of metal extraction from alternative sources. Recently, hydrometallurgical processes are produced more than 20% of the world. Particularly, acidic heap leaching, heap bioleaching, solvent extraction and electrowinning processes for low grade ores. Hydrometallurgical processes are based on the dissolution of metals from their ores using different lixiviant agents, microorganisms and followed by metals recovery from the leaching liquor solutions 6,7 . Most of inorganic leaching agents are either too corrosive to the reactors 8 , too low in metal extraction 9 , too slow in leaching 10 , or generate toxic gases such as NO x 11 . Metal dissolution and recovery from different ore or wastes by environmentally friendly techniques is becoming important as we move towards a more sustainable future 12,13 . So, with declining grades of metals deposits, there is a need for finding economical and environmentally responsible methods of metal extraction from alternative sources. So, the development of ecofriendly process and more economical is of great interest and importance between the scientific research in metallurgy.
Methanesulfonic acid (MSA) is considered green acid with a high solubility for different base metals. Methanesulfonic acid (MSA) is a strong non-oxidizing organic acid that has high saturation solubility of metal salts, excellent conductivity, low toxicity, good disposability and good electrochemical stabilization. MSA is safer and less toxic than the inorganic acids (H 2 SO 4 , HCl, HNO 3 ) recently used for leaching metals from different sources (primary and secondary). Methanesulfonic acid (MSA) can replace sulfuric acid in a certain condition as reported by many researchers [14][15][16][17][18] . It has merits in characteristics of high solubility of metals with low corrosiveness and toxicity. MSA was used in dissolution of lead (Pb) and silver (Ag) from zinc (Zn) 19 and dissolution of lead and zinc from iron rich jarosite residue 20 . MSA has also been used as a leaching medium to extract metals from ores 21 or secondary sources 22 . MSA can be considered as a natural product; it is biodegradable and finally forms carbon dioxide (CO 2 ) and sulfate (SO 4 2-), contributing to the natural sulfur (S) cycle 23 .
Therefore, the objectives of this work are to analyze valuable metals dissolution from Gibbsite ore materials in Talet Seleim, Southwestern, Sinai, Egypt in MSA and find the proper model to explain the reaction mechanism.

Materials and methods
All chemicals and reagents used in this are analytical grade. Physical and chemical properties of methanesulfonic acid are represented in table 1. The ore sample were collected from gibbsite ore materials of Talet Selim area. It was crushed by jaw-crusher followed by grinding to 200 mesh particle size then the ore was quartered to have a representative sample for the chemical analyses of the ore.

Leaching tests
All batch leaching tests were performed using 2.00 g of ore solid sample in known volume of methanesulfonic acid (MSA) lixiviant agent solution and stirred at constant temperature and allowed to react for specified times and subjected to solid-liquid separation. A watch glass cover was put on the beaker during the dissolution experiments to prevent the evaporative loss of the solution at high temperature. The effect of MSA on the dissolution from ore concentrate was evaluated. Filtrate solution were collected from all leaching tests and concentrations were analyzed by atomic absorption spectrophotometer (Unicam 969, England) The dissolution efficiency (%) was calculated by Eq. (6): Dissolution efficiency (%) = Amount of metal in the leach liquor / Total amount of metal in the ore sample x 100 (6) The expressed dissolution efficiency represents the average yield of the metals in the ore, as dissolution percentages.

Analytical procedures
The major oxides in Talet seliem ore were measured using the conventional wet chemical technique of Shapiro et al. 24 . TiO 2, SiO 2 , and P 2 O 5 were measured using their spectrophotometric methods whereas the total iron as Fe 2 O 3 was measured by titrimetric methods using sulfo-salicylic against EDTA solutions. While REEs was measured by using Arsenazo-III 25 . Uranium was evaluated by oxidimetric titration with ammonium metavanadate (NH 4 VO 3 ) after reduction of hexavalent uranium (U(VI) with ferrous sulfate (FeSO 4 ) in the presence of sodium diphenylamine-4-sulfonate as indicator 26,27 . The titration was repeated several times for each sample, and the experimental error was <3%. While the purity of the final residue was qualitatively determined using ESEM-EDX analysis.

Geologic setting
Talet Seleim area is located between 33°20′ and 33°25′ E longitudes and 29°00′-29°05′ N latitudes in southwestern Sinai, Egypt. It is primarily covered by early Carboniferous Um Bogma Formation as shown in Fig. 1. This formation is consisted of a sequence of three lithological units. The middle unit is made up of ferruginous shale with marly lenses and extends more than 2 km and of 4-6 m thickness. The presence of a variety of valuable metals, including zinc, manganese, aluminium, cobalt nickel and copper, distinguishes this rock block 28 .

The working sample characterization
The chemical composition of the working ore is given in Table 2 that reveal the predominance of Fe 2 O 3 , Al 2 O 3 , silica, and with a content of 17.16%, 19.45%, 35.42%. The working ore sample also contains concentration of copper, zinc, nickel, vanadium, uranium, cadmium, lead, and total rare earth elements with concentration of 1500 ppm, 5000 ppm, 1761 ppm, 1006 ppm, 600 ppm, 200 ppm, 188 ppm and 2250 ppm respectively 27 .

Optimization process for leaching process
Optimization of the dissolution process through various experimental conditions such as, concentration of MSA, time, liquid solid ratio, stirring speed and temperature as follow:

Effect of the MSA concentration
The dissolution of copper, zinc, lead and nickel from the working ore sample was performed as

Effect of the dissolution time
A lot of experiments were done in the following condition; MSA concentration of 150 g/L, temperature of 298K, stirring speed of 200 rpm, liquid-solid ratio of ratio of 5 ml/g, particle size of 200 mesh and leaching time in each experiment was changed to be 60 min, 90 min, 120 min, 180 min, 240 min and 300 min, respectively. The obtained results in Fig. 3 revealed that the metal dissolution efficiency increased with increasing the contact time from 60 min to 300 min. The dissolution efficiency

Effect of temperature
The effect of temperature on the dissolution of the four different valuable elements was studied in the range of 298K-363K, while the other dissolution parameters fixed to evaluate its effect on metal dissolution. The results presented in Fig. 4 demonstrate that the temperature remarkably affects the dissolution since when the temperature increase from 298K to 363K improves the dissolution from 46.32% to 74% for Ni and from 50.75% to 75.34% for copper, 53% to 77% for Zn and 35.85% to 59.94% for Pb which shows that the rate of dissolution is very fast at higher temperature. This is due to the enhanced reaction kinetics at elevated temperatures since at elevated temperatures, the frequency of collisions between the molecules raises, which leads to a higher reaction rate and higher dissolution of metals 29 . The maximum dissolution of Cu, Zn, and Ni was obtained at 363K and therefore, it was considered as the optimum temperature of the process.

Effect of liquid /solid ratio
The liquid/solid ratio effect on dissolution of the working ore was studied at different liquid/solid ratios (L/S) from (2 ml/g to 7 ml/g) and the other conditions are fixed at MSA concentration of 150 g/L, leaching time of 300 min, leaching temperature of 363K and stirring speed of 200 rpm. The obtained results are seen in Fig. 5 which showed that the three metals have the same attitude when increasing the liquid/solid ratio from 2 ml/g to 4 ml/g but lead has a different attitude. The higher liquid/solid ratio resulted in an increase in extraction dissolution efficiency. The mass transfer driving force of metals from the solid to liquid phase is increased because when the solid/liquid ratio increase caused decrease in the concentration of leachate in the liquor so the total metals recovered from the solid phase is enhanced 30 . Hence, the liquid/solid ratio 4 ml/g is chosen for optimum dissolution of copper, nickel and zinc but 5 ml/g for lead dissolution.

Effect of stirring speed
The mechanical stirring speed effect on metals dissolution efficiency was investigated using different mechanical stirring speed (100 rpm to 500 rpm), MSA concentration of 150 g/L, time of 300 min, temperature of 363K and Liquid / solid ratio of 4 ml/g. Experimental results seen in Fig. 6 showed that the mechanical stirring speed has a remarkable influence on the dissolution of valuable elements at 400 rpm. The metal dissolution was increased with increasing the stirring speed. This may be contributed to the fact that increasing agitation of the dissolution solutions enhanced the diffusion of the reactants from bulk solution to the surface of mineral, increased the mass transfers and accelerated the dissolution of metals prevent the formation of agglomerates 31,32 . The dissolution efficiency of metals was decreased when stirring speed is more than 400 rpm therefore, 400 rpm is chosen for optimal stirring speed for the optimum dissolution of metals in this study.
According to the above dissolution process conducted on the gibbsite ore mineralized sample, Table S1 in the supplementary files provides an overview of the maximum dissolution conditions for metals in ore by MSA.

Kinetics of the dissolution process of ore with MSA analysis Effect of time at different temperatures on dissolution of different valuable elements
The temperatures effect on the dissolution rate of four different elements were investigated at different temperatures (298-363K) using conditions of 200 mesh particles size, 150 g/L with 4 ml/g liquid /solid ratio. Results indicate that dissolution increases as the time increases and the dissolution increases with more increase in the temperature from 298K to 363K. The experimental data seen in Fig. 7 was correlated to different kinetic models for solid-liquid reactions to determine the best kinetic equation for the dissolution of different metals.

Kinetics analysis
The dissolution of metals in the working gibbsite ore by MSA lixiviant agent is considered a liquid to solid heterogeneous reaction. The following reaction is used to illustrate the dissolution reaction's stoichiometric equation symbolically: Q(fluid) + iM(solid) → products (7) where Q, M, and i refer to the fluid of reactant, the solid undergoing dissolution and stoichiometric coefficient, respectively.
The kinetics analysis is very important in hydrometallurgical processes since it is the basis for scale-up of experiments and the design of The kinetics of heterogeneous systems involving non-porous materials are commonly described using the shrinking core model (SCM). The shrinking-core model assumes that the dissolution process is controlled by a single step of the following steps: diffusion through the fluid film, chemical reaction at the surface of unreacted particles, or diffusion through the product or ash layer. The kinetic of dissolution equation of a liquid-solid heterogeneous system was mainly controlled by one of the following mathematical equations 33,34 : The chemical controlled reaction can be represented in equation (8): The diffusion controlled rection can be represented in equation (9): The experimental data were applied to homogenous and heterogenous models to know kinetics of dissolution of different metals.
To know the rate-controlling steps during the dissolution of different metals with MSA, the experimental data of 0-300 min seen in Fig.  8-11 were fitted into the four kinetic equations, and the evaluated the regression coefficients R 2 are shown in Table 3-6 for different metals. The reaction mechanism was determined by selecting the model with the highest determination coefficient (R 2 ). According to the statistical analysis for Cu, Zn and Ni at different temperatures, the average R 2 for the secondorder pseudo-homogeneous model 0.98787, 0.98098 and 0.98884, respectively were found to be higher than that of the other mathematical models. This means that second-order pseudohomogeneous model is suitable for describing the dissolution kinetic of copper during the dissolution process. However, the dissolution of lead is diffusion-controlled reaction through the product since the average regression coefficient were higher than that of other models 0.963813. This means that diffusion through the product layer model could be the suitable model for describing the dissolution kinetics of lead.

Activation energies evaluation
Activation energy is very important factor that can be used to justify the rate determining factor in different hydrometallurgical processes 30 . The relationship between reaction constant and temperature can be determined from activation energy on Arrhenius equation. Arrhenius equation (Eq. (12)) was used to evaluate the activation energies.
ln k = ln A -(E A /RT) (12) k is the constant of reaction, A is the frequency  factor, E A (kJ/mol) is the activation energy of the dissolution process, R is the universal gas constant (8.314 J/K/mol), and T (Kelvin) is the absolute temperature. The Arrhenius plot is shown in Fig. 12 for different metals and different reaction controlled the dissolution process, and the activation energy was determined from the slope of plotting lnk versus 1/T (E A = R x slope). The activation energy value in Fig. 12was calculated from Fig. 8-11 are presented in Table  S2-S5 in the supplementary files. Activation energy for a diffusion-controlled mechanism is <40 kJ/ mol , whilst the activation energy a chemically controlled mechanism is >40 kJ/mol 30 . The fact that the activation energies for the dissolution of different metals are 10-20 kJ /mol means that the dissolution of different metals were controlled by diffusion controlled 30 .

Thermodynamics characteristic of dissolution process
The dissolution of different metals from the ore sample is thermodynamically evaluated to know the spontaneity and the feasibility of the dissolution and interpreting thermodynamic behavior of the dissolution process 30 . Thermodynamic parameters are evaluated by the following equations [Eqs. (13) and (14)]: ΔG° = -RT ln k d (13) ln k d = (ΔS°/R) -(ΔH°/RT) (14) where ∆G° is the gibbs free energy change, ∆S° is entropy change and ∆H° is enthalpy change. T is the temperature during dissolution process (Klven), R is the gas constant = 8.314 J/ (mol . K) . k d is equilibrium constant, which could be evaluated from the following equation:  (14) versus inverted time (1/T) give a straight line whose slope = -ΔH°/R and an intercept = ∆S°/R was seen in Fig. 13. The thermodynamic parameters (∆S°, ∆H° and ∆G°) were evaluated and summarized in Table 7. The positive charge values of enthalpy change ΔH° for different metals ascertained that the dissolution process in this study is endothermic in nature and the positive charge values of ΔS, for the different metals, suggested an increase in randomness when different metals were leached in MSA. The process for the dissolution by methanesulfonic acid was spontaneous, which was ascertained by the Gibbs free energy change value is lower than zero (∆G0 < 0, ∆H0 > 0 and ∆S0 > 0). Table  7 summarizes the thermodynamic parameters for copper, nickel, lead and zinc dissolution. The positive values of enthalpy change, ΔH, for metals ascertained the endothermic nature of the acidic dissolution of the working ore. In addition to the spontaneous reaction at any temperature from the negative values of gibbs free energy and the increase in the negative value of ΔG° value with increasing the temperature implies that the reaction is favorable at elevated temperature. The values of TΔS° are higher than the absolute ΔG° so the dissolution process of different metals is controlled by entropic changes rather than enthalpic changes for dissolution process. The endothermic nature of dissolution reaction   for different four metals is also confirmed by the increase in the reaction rate constant (k) versus time (t) as represented in Fig. 9-11.

Mechanism of dissolution process
Methanesulfonic acid can be completely ionized into proton (H + ) and methane sulfonate anions (CH 3 SO 3 -) at aqueous solution as seen in Eq. (1) (16): groups is attached directly to hydroxyl group (OH -) are quite strong electron withdrawing groups due to the presence of oxygen atom with highly electronegativity and the moderately sulfur atom. The polarity of hydroxyl bonds (O-H bonds) was increased as a result of the inductive effect, resulting in the liberation of protons (H + ) easily. Moreover, the resonance effects also increase in methanesulfonic acid (MSA). Besides, the inductive and resonance effects lead to the stability of the polarity. The strong polarity of hydroxyl bonds (OH) can explain the strong acidic properties of methane sulfonate (CH 3 SO 3 -) ions due to the stabilization of the negative charge on the oxygen atom after the liberation of proton ions 35 . This can be seen in the solubility of their metal salts (Fig. 14). Dissolution reactions of metals can be represented in equation (17): So, MSA will be used in extractive metallurgy as well as electrochemical processes in the production of metals from their solutions due to its merits. The dissolution percentage of different metals in MSA acid depends on the differences in the nature of the metals, such as their crystalline structure, reduction potentials, and stable complexes in the solution. The crystal structures of methane-sulphonates of several metals were solved. The methanesulphonate anion was considered to be a bidentate ligand 35 .

Recovery of metal values
For recovering Cu, Ni, Zn, and Pb, the acidic leach liquor of MSA was probably prepared by applying the optimum dissolution conditions shown in table S1 in the supplementary file to 1 kg of the provided working ore sample. This resulted in 0.298 g/L, 0.337 g/L, 1.023 g/L and 0.033 g/L within dissolution for Cu, Ni, Zn, and Pb, respectively.

Characterization of the residue
The solid leach residue obtained after dissolution process of the ore under optimum dissolution conditions was analysed using SEM-EDX technique which illustrated in Fig. 15. The characterization of the residue after the dissolution process, its morphology and the chemical composition provide significant inputs and confirmations for the dissolution process in the previous results in this paper. The morphology of the leach residue (Fig. 15) shows corroded nature and porous in nature. The pits on the sample indicate portions of the surface which had undergone dissolution 36 .

Conclusion
The dissolution reactions of copper, nickel, lead, and zinc in ore and methane sulfonic acid using various reaction conditions were studied. Dissolution efficiency values of 81.8%, 79.47%, 76.6%, and 70.7% of Zn, Cu, Ni, and Pb present in the ore were evaluated under the following conditions: particle size 200 mesh, temperature 363K, reaction time 300 min, acid concentration 150 g/L, and liquid/solid ratio 4 ml/g. The results of the kinetic model study indicated that the diffusion control reaction model can be used to describe the leaching process of lead and that the leaching rate is controlled through second order reactioncontrolled mechanism for copper, nickel, and  Overall, the MSA leaching system provides clean, an efficient, and environmental-friendly method for dissolution of valuable metals. It is expected to be a promising alternative for inorganic acids.

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