Nature of serpentinite interactions with low-concentration sulfuric acid solutions

: The nature of serpentinite interaction with low-concentration sulfuric acid solutions is studied. Using IR-Fourier spectroscopy, X-ray, and serpentinite leaching with sulfuric acid solutions containing 10 – 60% of stoichiometri-cally required amount (SRA) of H 2 SO 4 (taken relative to the molar magnesium content in serpentinite), it is shown that the appearance and in ﬂ uence of silica on the course of serpentinite dissolution is detected at concentrations of 30 – 40% of SRA of H 2 SO 4 . On the basis of the obtained results, this article points out the technological, economic, and ecological advantages of use of sulfuric acid solutions of low concentrations in the process of acid processing with the aim of producing magnesium salts.


Introduction
Recently, many researchers have focused on extracting magnesium from serpentinite using hydrometallurgical techniques.Various leaching reagents such as sulfuric acid, hydrochloric acid, nitric acid, and organic acids have been systematically studied [1][2][3][4].Experiments conducted at ambient temperature showed that sulfuric acid is most effective in extracting magnesium from serpentinite, followed by hydrochloric, nitric, and formic acids [5].
Serpentinite belongs to layered silicate minerals and has a stable structure with the chemical formula [(Mg,Fe) 3 Si 2 O 5 (OH) 4 ] [6,7].It represents a group of antigorites, chrysotile, and lizardite and consists of silica terahedra and magnesium hydroxide octahedra in a molar ratio of 1:1 [8,9].With the exception of magnesium, silicon, and iron, serpentinite contains a certain amount of chromium, nickel, and other elements.
Magnesium is considered the most valuable component in serpentinite, since its content, for example, from the Zhitikara deposit (Kostanay region, Kazakhstan) reaches up to 25-26 wt%.Pure magnesium compounds can be obtained by acid leaching of serpentinite.Leaching results in the formation of a soluble magnesium salt, which accumulates in the solution and can be separated from the insoluble residue and subjected to further purification [10,11].Mg 2+ (MgSO 4 , MgCl 2 , Mg(NO 3 ) 2 , etc.) salt products themselves can be used in various industries (metallurgy, agriculture, medicine, etc.) and are starting materials for the production of industrially important magnesium compounds such as magnesium hydroxide, magnesium oxide, and others.
Despite the numerous studies carried out in the field of acid treatment of serpentinite, in order to use it as a source of magnesium and its compounds, it is necessary to state the fact that to date there are no implemented technologies in the world for the production of magnesium and its compounds from serpentinite and serpentinite waste (chrysotile production).So, there is no need to even mention any other useful components (amorphous silica, Fe, Ni, Cr, etc.) present in serpentinite.Silica is an insoluble residue of acid dissolution, which can be used in various applications [11].
There are many methods for processing serpentinites (including from chrysotile production waste) [12][13][14][15][16][17][18]; however, along with the advantages of each method, they also have disadvantages that limit their use.If we highlight and point out the general shortcomings of the aforementioned studies, in general, the following features can be noted: "the treatment process is long, labor-intensive, involves significant materials and energy costs, and inappropriate from an economic point of view."Most of these disadvantages are due to the problem associated with the occurrence and subsequent formation of polysilicic acid gels when using acid methods for extracting magnesium from serpentinite, which greatly affects the manufacturability of the developed methods.Unfortunately, the scientific literature does not always touch upon (or reveal) problems related to this important issue, which requires a more detailed study.
An analysis of scientific and technical works devoted to the study of dissolution processes in acids (H 2 SO 4 , HCl, and HNO 3 ) of various types of serpentinites shows that most serpentinite processing schemes were aimed at more complete extraction of magnesium from it.The difference between the schemes and each other is mainly due to the nature of the acid and the modes of process [19][20][21][22][23][24].In addition, serpentinite decomposition technologies used high concentrations of acids in order to extract magnesium to the fullest extent possible, which complicated the process of separating the magnesium solution (solid/liquid) due to the formation of gel-like silicic acid in the system.The development of leaching of magnesium from serpentinite without technological complications and with acceptable economics is currently an urgent issue.
In this regard, in this work, much attention was paid to the study of the transformation of silicate components of serpentinites into silica when treated with sulfuric acid solutions.The studies were carried out using X-ray diffraction, IR-Fourier spectroscopy, and the method of leaching serpentinite using low-concentration sulfuric acid solutions.The research results may be useful in the search for new economically viable methods for processing serpentinites (as well as serpentinite-containing waste) in order to obtain magnesium and its compounds.
The purpose of the work is to study the nature of serpentinite interactions with low-concentration sulfuric acid solutions, to determine the beginning of transformation of silicate components of serpentine into silica in the "serpentinite-sulfuric acid" system, and to identify effective cases of application of low concentration of acid in the production of magnesium salts based on serpentinite.

Materials and methods
Serpentinite samples were presented by Kostanay Minerals JSC (Zhitikara, Republic of Kazakhstan) producing chrysotile from the serpentinite ore of the Zhitikara deposit, Republic of Kazakhstan.About 20 g of serpentinite was taken, crushed, and sieved, and a fraction of particles with a size of <0.14 mm was selected.For the experiment, we took 10 g, containing 26.6 wt% Mg, 18.8 wt% Si, 2.7 wt% Fe, and 0.49 wt % Ca.The amount of magnesium was 0.11 mol, and the amount of iron was 0.005 mol.
The volume and mass of H 2 SO 4 for preparing a 200 cm 3 solution containing the estimated amount of sulfuric acid (from 92% H 2 SO 4 , d = 1.824) of the chemically pure grade was calculated using formulas (1) and ( 2): where m is the mass of H 2 SO 4 (92%); C is the molar concentration of H 2 SO 4 ; M r is the molar mass of H 2 SO 4 ; V is the volume; d is the density of H 2 SO 4 (92%).
The stoichiometrically required amount (SRA) of sulfuric acid for interaction in the "serpentinite-acid" system was calculated using the following equation: (3) Solutions containing different SRAs of H 2 SO 4 as a percentage of 100% of SRA of H 2 SO 4 (calculated from the actual magnesium content in a 10 g sample of serpentinite) were prepared by diluting from the initially prepared solution containing 100% of SRA of H 2 SO 4 .
Treatment of serpentinite with solutions of sulfuric acid in the concentration range (10-60%) by fractions of SRA of H 2 SO 4 was carried out in a thermostated glass reactor with a volume of 300 ml, equipped with a propeller stirrer and a sampler (Figure 1).Experiments were carried out at a solid/liquid (S/L) weight ratio of 1:10.The flask with the sulfuric acid solution was preheated to a temperature of 90°C, then 10 g of serpentinite was poured through the sampler, and the stopwatch was turned on.When serpentinite is added to a H 2 SO 4 solution heated to 90°C, the temperature rises slightly up to 94°C.The color of the filtrate is pale green-blue.After 5 min, the suspension was quantitatively transferred into a folded paper filter with white tape while hot.After filtering the suspension, the total volume, weight, and pH of the filtrate were determined.
The degree of extraction of Mg (P Mg ) and other elements (Re) from 10 g of serpentinite with H 2 SO 4 solutions was calculated by the ratio of the analytical amount of the element in the filtrate to its amount in the initial sample of serpentinite, by the following formula: where P Mg is the mass of extracted magnesium; m is the mass of dry filtrate residue, g; Mg, % is the magnesium content in the dry residue of filtrate according to chemical analysis; 2.538 is the magnesium content in the initial 10 g sample of serpentinite according to chemical analysis.The degree of extraction of Fe, Ca, and Si was determined similarly.
The filtrate and insoluble precipitate, after preliminary preparation (drying at 100°C), were subjected to chemical analysis and examined by X-ray and IR spectroscopic methods.Chemical analysis of the samples was performed on a scanning electron microscope JSM-6490LV, JEOL (Japan), complete with an INCA Energy 350 energy-dispersive microanalyzer system.X-ray phase analysis was performed using a D8 Endeavor diffractometer manufactured by Bruker (Germany) (Cu-K α , Ni-filter).IR spectral analysis was recorded on an IR-Fourier spectrometer "IR-21 Prestige" (Japan).The acidity (pH) of solutions was determined using the pH-150 MI ion meter.

Results and discussion
The acid-base interaction in the "serpentinite-sulfuric acid" system depends on many factors and can be complex.However, there are several factors determined by the peculiarities of the structural and molecular structures of serpentinite that affect the speed and nature of interactions in the system.
The study assumed that the interaction rate would be determined depending on the nature of the acid-base reactions occurring in the serpentinitis-H 2 SO 4 system and the properties of direct and indirect products of interactions.At the same time, the location and distribution of magnesium in the structure and molecular structure of serpentinite were adopted similar to the structure of chrysotile presented in Figure 2 [25].
In this case, the structural formula of chrysotileserpentinite can be formally represented as Furthermore, one-third of Mg is presumably located in the brusite (octahedral) and two-third of Mg in tetrahedral layers of a serpentinite packet.Serpentinitie is likely to dissolve in acid by the reactions presented in Figure 3.
In terms of kinetics, reaction (Figure 3a) can be considered as an acid-base interaction, which occurs at the initial stage through the formation of a magnesium salt and water.The reaction rate in this case can depend both on the acid concentration and on the surface concentration of brucite [Mg(OH) 2 ].The kinetics of the process can be described by the first-order reaction equation relative to the concentration of sulfuric acid v = k[H 2 SO 4 ], where v is the reaction rate, k is the rate constant, and [H 2 SO 4 ] is the concentration of sulfuric acid.
The [MgOH] 2 fragment is characterized by a less alkaline property than brucite -Mg(OH) 2 ; therefore, although the interaction of the [MgOH] 2 fragment with an acid is also acid-base in nature, the reaction (Figure 3b) proceeds at a slower rate than the reaction (Figure 3a).Nature of serpentinite interactions with low-concentration sulfuric acid solutions  3 As the quantity of magnesium extracted from the structure of the serpentinite crystal lattice increases and the pH of the medium decreases, the probability of reactions (Figure 3c) forming a silicic acid gel increases.

X-ray examination
Changes in the phase composition of serpentinite after its treatment with SRA of H 2 SO 4 solutions were traced by X-ray analysis.A comparative analysis of X-ray diffraction patterns, Figure 4 (initial serpentinite), and the same serpentinite after treatment with 30% of SRA of H 2 SO 4 solution (Figure 5), shows that the prescribed peaks for serpentinite (chrysotile and antigorite) Mg      [31][32][33][34], and when treated with H 2 SO 4 solutions up to 30% of SRA, they practically do not undergo any special changes.These data show that complete destruction of the structural structure of the silicate component of serpentinite does not occur.

IR spectroscopic study
In the IR spectrum of serpentinite (Figure 6), a high-frequency band with a hump at ν OH = 3,660-3,680 cm −1 is probably the result of the merger of two narrow bands of asymmetric vibrations of the OH groups of silica in the intraglobular and surface modes [31][32][33][34].The small absorption band at ν sOH = 2,931 cm −1 is a symmetrical stretching vibration of the OH group of skeletal tetrahedral silica [35][36][37][38].
When serpentinite is treated with solutions of 20%, 30%, and 50% of SRA of H 2 SO 4 , the peak with a hump ν as OH = 3,684-3,660 cm −1 and the surface mode ν as OH = 3,645 cm −1 increases.In addition, in a wide range of 3,150-3,500 cm −1 , a growing absorption wave appears, which also indicates an increase in surface-adsorbed H + -bound hydroxyls of chain acid SiOH + silonol groups on the surface of serpentinite particles.The observed changes in the spectrum of serpentinite after treatment with acid solutions may indicate the dependence of the amount of weakly dissociating monosilicic acids Vibrations in the low-frequency region in the range of 2,000-450 cm −1 (Figure 7) ν as (Si-O-Si) = 937 cm −1 , together with the shoulder, is the stretching asymmetric vibration of silica coordinated by the octahedral brucite layer, a small hump at ν as (Si-O-Si) = 1064.7 cm −1asymmetric stretching vibration of skeletal tetrahedral silica [34][35][36][37][38]. Vibrations in the low-frequency region at 620, 601, 551 and 455 cm −1 belong to the bending vibrations of Si-O-Si, Si-O-Mg+, Si-O-Ca+, and Si-O-Fe+ bonds, respectively [32][33][34][35][36].
A decrease, after treating serpentinite with solutions of 20, 30, and 50% of SRA of H 2 SO 4 , of stretching asymmetric vibrations of silica by a coordinated octahedral brucite layer at ν as (Si-O-Si) = 937 cm −1 , indicates the beginning of destruction of the integrity of the tetrahedral-octahedral layered structure pairs of serpentinite and the appearance of various surface H + -bound hydroxyls.The appearance of a shallow broad band in the region of 1,100-1,250 cm −1 is further evidence of the appearance of various SiO 4 groups, dimers, trimers, and surface chain acid SiOH + groups [32,[34][35][36][37].
A decrease in the intensity of the asymmetric stretching vibrations ν as (Si-O-Si) of the silica bond at 937 cm −1 , highlighting a shoulder at ν as (Si-O-Si) = 1,072 cm −1 of the asymmetric stretching vibrations of tetrahedral silica, and a deepening of the intensity of a broad band in the range 1,100-1,250 cm −1 may serve as additional confirmation of the presence of various "silica fragments" formed on the surface layers of serpentinite particles [34][35][36][37][38][39].This is probably preceded by the accumulation of monosilicic acids on the surface layer, which in an acidic environment (pH = 2-3) gradually enter a polycondensation reaction with the formation of colloidal particles.

Treatment of serpentinite with lowconcentration sulfuric acid solutions
Solutions of sulfuric acid were used that contained 10%, 20%, 30%, 40%, 50%, and 60% of SRA of H 2 SO 4 for complete (100%) dissolution of the molar amount of magnesium contained in serpentinite, determined by its analysis.Figure 8 (curves 1, 2, and 3) separately shows the dependence of the degree of leaching of magnesium and other elements (Fe, Ca, and Si) on the concentrations of used SRA of H 2 SO 4 and the pH of the environment established at the same time when changing the acid concentrations in the working solution used.
The deviation from the linear dependence of the degree of magnesium leaching on the amount of H 2 SO 4 (curve 1) shows the completion of reaction (a) (Figure 3), starting from a concentration of 30-40% of SRA of H 2 SO 4 , and the process of magnesium dissolution begins to slow down   due to changes in the surface states of serpentinite particles.The rate of acid-base interactions between magnesium-containing fragments in the structure of the serpentine molecule and H 3 O + slows down, which leads to incomplete consumption of the amount of acid introduced into the system (SRA of H 2 SO 4 ) for interaction.Obviously, first of all, this may be due to the appearance of colloidal silica particles on the reaction surface layers, which have a relatively high viscosity.
The experimental results obtained show that the degree of magnesium leaching (of its total amount in serpentinite) when using solutions of up to 30% of SRA of H 2 SO 4 practically corresponds to the equivalent amount of SRA of H 2 SO 4 taken, after which this correspondence is interrupted in the direction of excess of acid consumption, that is, the degree of magnesium leaching decreases.This circumstance, in turn, directly affects the efficiency of sulfuric acid in the process of extracting magnesium from serpentinite (Figure 9).
When processing the results, we deliberately paid little attention to the nature of behavior of other elements, considering them in this case to be secondary.However, we note that when using solutions of up to 60% of SRA of H 2 SO 4 , the degree of leaching of iron from serpentinite increases to P = 20% of its total content in the initial sample, calciumup to 68%, and siliconup to 2.8%.From other observations, it can be noted that the filtration rate (under the same conditions) becomes noticeably slow when using a solution containing 50% of SRA of H 2 SO 4 .It is also noteworthy that up to 30% of pH of H 2 SO 4 , the change in the pH of the medium (Figure 8) is linear; then, its decrease becomes unnoticeable.Apparently, the excess free H 3 O + , which did not enter the acid-base interaction with the magnesium-containing components of serpentinite due to diffusion restrictions, is spent on the formation of low-dissociating polysilicic acid.
The extraction of useful components, especially magnesium, from serpentinite is often associated with the technological and economic aspects of problems of the hydrometallurgical process at the initial stage of serpentinite leaching.Experimental evidence has shown some advantages in using low-concentration sulfuric acid solutions in the extraction of magnesium from serpentinite, as shown in Table 1.

Conclusions
There is a huge quantity of serpentinites in the form of mineral ores and waste from the extraction and enrichment of chrysotile in the world.Typically, they contain 35-43 wt% MgO, i.e., they contain a quite acceptable amount for industrial use as a source for the production of magnesium and its compounds.The use of solutions of lower concentrations may allow leaching of serpentinite with a lower activation energy than in the case of high concentrations of H 2 SO 4 , where a higher activation energy will be associated with a decrease in the rate of interaction due to more intense gelation in the system.Therefore, the extraction of magnesium from serpentinite in an amount of 30-40% of its initial content in serpentinite using low-concentration sulfuric acid solutions (up to 40-50% of SRA of H 2 SO 4 ), without complicating technological processes associated with the appearance of colloidal silica, can be considered as one of the alternative effective ways to extract magnesium from serpentinite.Author contributions: Chaizada Yeskibayeva: conceptualization, methodology, writingoriginal draft preparation, Table 1: Benefits of using low-concentration sulfuric acid solutions in extracting magnesium from serpentinite

No.
Leaching efficiency The use of a concentration of 30-40% of SRA of H 2 SO 4 makes it possible to leach, on average, 85-90% of magnesium from serpentinite with a sulfuric acid utilization factor of 88-90%, without technological complications associated with the formation of colloidal silica particles 2 Further, this circumstance results in lower consumption of the neutralizing agent (NaOH, NH 4 OH, etc.) for the neutralization of the medium for the subsequent hydrometallurgical process operations 3 The reduction of the quantities of consumable reagents (H 2 SO 4 and the neutralizing agent) at the serpentinite leaching stage in the general hydrometallurgical process is beneficial to achieving a positive economic balance 4 Low-concentration solutions are less aggressive and less dangerous for equipment and personnel, which reduce the risk of accidents, and accidents have less negative environmental impact from leaks and emissions 5 The use of cheaper low-concentration solutions with a high efficiency can be economically advantageous, especially when processing large volumes of material containing a high content of a useful, easily extractable portion of magnesium from serpentinite Nature of serpentinite interactions with low-concentration sulfuric acid solutions  7

Figure 3 :
Figure 3: Probable scheme of magnesium dissolution and formation of colloidal silica particles in the "serpentinite-sulfuric acid" system.

Figure 4 :
Figure 4: X-ray diffraction pattern of the original serpentinite.

Figure 5 :
Figure 5: X-ray diffraction pattern of serpentinite treated with 30% of SRA of H 2 SO 4 (washed once with water) after treatment.

Figure 8 :
Figure 8: Dependence of the degree of leaching of magnesium and other elements (Fe, Ca, and Si) into the solution and the pH of the medium on the SRA of H 2 SO 4 : 1 -Mg; 2 -Fe; 3 -Ca; 4 -Si.

Funding information :
This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP19676952).