Utilization of the MgO-Rich Residue Originated from Ludwigite Ore : Hydrothermal Synthesis of MHSH Whiskers

Abstract: In this work, from the MgO-enriched residue originated from ludwigite ore, value-added magnesium hydroxide sulfate hydrate (MHSH) whiskers were prepared via sulfuric acid leaching and leachate purification, followed by hydrothermal synthesis. During sulfuric acid leaching, 98.2% magnesium and 99.6% silica were removed under the optimal leaching conditions: sulfuric acid concentration of 40%, leaching temperature of 80 ◦C, leaching time of 90 min, and liquid to solid ratio of 4 mL/g. After purification of the acidic leachate via oxidation and precipitation to remove impurities including iron and aluminum, the Mg2+-rich solution was used to prepare magnesium hydroxide sulfate hydrate whiskers (5Mg(OH)2·MgSO4·2H2O, abbreviated as 512MHSH) via hydrothermal synthesis. Finally, 512MHSH whiskers were obtained with 30–100 μm in length, 0.5–1.0 μm in diameter and aspect ratio of approximately 100. The physicochemical characteristics of whiskers were further characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermogravimetry-Differential Scanning Calorimetry (TG-DSC).

Ludwigite ore is rich in boron, iron and magnesium.Its comprehensive utilization has been attempted by using various technologies [19][20][21], with an emphasis on the recovery of boron and iron rather than magnesium [22,23].Therefore, it's more beneficial to make use of magnesium component for the production of value-added products, in addition to the recovery of boron and iron [24].In our previous studies, a process for stepwise recovery of metallic iron and borate from a ludwigite ore based on coal-based direct reduction has shown its potential and promising future [25,26].After reductive roasting in the presence of soda ash, 72.1% boron and 95.7% iron were extracted in the forms of sodium metaborate and powdery metallic iron, respectively.As boron and iron could be recovered efficiently, magnesium could also be enriched in the nonmagnetic material to a great extent (about 40 wt % MgO).Due to the transformation of magnesium-containing minerals during soda-ash roasting, the non-magnetic material has excellent solubility in acidic liquor.Therefore, the no-magnetic material with high MgO content could be used as a potential magnesium source for the synthesis of MHSH whiskers.
In this study, MgO-enriched residue derived from ludwigite ore was used as the starting material to prepare MHSH whiskers via acid leaching, leachate purification and hydrothermal synthesis.Acid-leaching characteristics of the MgO-enriched residue were evaluated.After removing the impurities of acid leachate, hydrothermal synthesis was then performed to prepare MHSH whiskers.

MgO-Enriched Residue
The MgO-enriched residue was obtained from ludwigite ore after coal-based reduction roasting, ball mill grinding-water leaching, and magnetic separation [27].The production process is schematically described in Figure 1.
Minerals 2017, 7, 138 2 of 16 ludwigite ore based on coal-based direct reduction has shown its potential and promising future [25,26].After reductive roasting in the presence of soda ash, 72.1% boron and 95.7% iron were extracted in the forms of sodium metaborate and powdery metallic iron, respectively.As boron and iron could be recovered efficiently, magnesium could also be enriched in the nonmagnetic material to a great extent (about 40 wt % MgO).Due to the transformation of magnesium-containing minerals during soda-ash roasting, the non-magnetic material has excellent solubility in acidic liquor.Therefore, the no-magnetic material with high MgO content could be used as a potential magnesium source for the synthesis of MHSH whiskers.
In this study, MgO-enriched residue derived from ludwigite ore was used as the starting material to prepare MHSH whiskers via acid leaching, leachate purification and hydrothermal synthesis.Acid-leaching characteristics of the MgO-enriched residue were evaluated.After removing the impurities of acid leachate, hydrothermal synthesis was then performed to prepare MHSH whiskers.

MgO-Enriched Residue
The MgO-enriched residue was obtained from ludwigite ore after coal-based reduction roasting, ball mill grinding-water leaching, and magnetic separation [27].The production process is schematically described in Figure 1.The chemical composition of the MgO-enriched residue is shown in Table 1, which was characterized by high magnesium (43.54 wt % MgO) and silicon (16.07 wt % SiO2) contents.The X-ray diffraction (XRD) pattern in Figure 2 indicates that the major MgO-enriched residue components were magnesium oxide (MgO), magnesium ferrous oxide (Mg0.91Fe0.09O)and disodium magnesium silicate (Na2MgSiO4).The chemical composition of the MgO-enriched residue is shown in Table 1, which was characterized by high magnesium (43.54 wt % MgO) and silicon (16.07 wt % SiO 2 ) contents.The X-ray diffraction (XRD) pattern in Figure 2 indicates that the major MgO-enriched residue components were magnesium oxide (MgO), magnesium ferrous oxide (Mg 0.91 Fe 0.09 O) and disodium magnesium silicate (Na 2 MgSiO 4 ).

Chemical Regents
Sulfuric acid, hydrogen peroxide, magnesia and sodium hydroxide were used in the experiments.All of these chemicals were of analytical reagent grade.

Experimental Procedure
The flow sheet for preparing MHSH whiskers is shown in Figure 3

Sulfuric Acid Leaching of MgO-Enriched Residue
Leaching experiments were carried out in a 250-mL beaker immersed in a temperature-controlled electrically heated water bath installed with a plastic stirrer.At the beginning of each test, the MgO-enriched residue was added into the beaker and mixed with sulfuric acid solution with the liquid-to-solid ratio ranging from 3 to 7 mL/g.Then, the beaker was soaked in

Chemical Regents
Sulfuric acid, hydrogen peroxide, magnesia and sodium hydroxide were used in the experiments.All of these chemicals were of analytical reagent grade.

Experimental Procedure
The flow sheet for preparing MHSH whiskers is shown in Figure 3

Chemical Regents
Sulfuric acid, hydrogen peroxide, magnesia and sodium hydroxide were used in the experiments.All of these chemicals were of analytical reagent grade.

Experimental Procedure
The flow sheet for preparing MHSH whiskers is shown in Figure 3, including: (1) Leaching MgO-enriched residue with sulfuric acid to obtain leachate containing magnesium sulfate after separation of silica; (2) Removing impurities of acidic leachate by oxidation and precipitation; and (3) Preparing MHSH whiskers via hydrothermal synthesis.

Sulfuric Acid Leaching of MgO-Enriched Residue
Leaching experiments were carried out in a 250-mL beaker immersed in a temperature-controlled electrically heated water bath installed with a plastic stirrer.At the beginning of each test, the MgO-enriched residue was added into the beaker and mixed with sulfuric acid solution with the liquid-to-solid ratio ranging from 3 to 7 mL/g.Then, the beaker was soaked in

Sulfuric Acid Leaching of MgO-Enriched Residue
Leaching experiments were carried out in a 250-mL beaker immersed in a temperature-controlled electrically heated water bath installed with a plastic stirrer.At the beginning of each test, the MgO-enriched residue was added into the beaker and mixed with sulfuric acid solution with the liquid-to-solid ratio ranging from 3 to 7 mL/g.Then, the beaker was soaked in the bath at a given temperature and agitated for a specified period of time.After filtration, the residue was washed with hot water and dried at 105 • C in a drying oven.The leaching ratio of constituents (µ) was calculated as follows: where µ is the leaching ratio, %; C t is the constituents content of leaching solution; V t is the volume of leaching solution, mL; α t is the constituents content of MgO-enriched residue; and m t is the mass of MgO-enriched residue.

Purification of Acidic Leachate
Sulfuric acid leachate was charged into a beaker before adding a few drops of potassium ferricyanide as a reaction indicator.Then, hydrogen peroxide was slowly added to oxidize Fe 2+ to Fe 3+ until the solution's color changed from light green to yellow.The magniferous solution was added to adjust the pH value to 5.7.The whole process was conducted at a certain temperature and stirring speed.The residue (hydroxides of Fe and Al) was separated from leaching solution and washed with hot water.The removal ratio of impurities (θ) and retention ratio of magnesium (γ) were calculated as follows: where α p and α q are the content of impurities and magnesium of the precipitate respectively; m p is the weight of the precipitate; C t and V 0 are the constituents content and volume of leachate used in the impurities removal experiments, respectively.

Hydrothermal Synthesis
Hydrothermal synthesis was conducted in an electrically heated autoclave equipped with a one-liter stainless steel tank and plastic stirrer.The synthesis temperature was controlled by a thermostat.For preparing 512MHSH whiskers, the purified MgSO 4 solution was added with NaOH (1 mol/L) with various mole ratio in the autoclave.The autoclave was then heated to 120-200 • C and kept under isothermal condition for 0.5-7 h.A white slurry was obtained after the autoclave was cooled down to the room temperature naturally.After filtration, the precipitate was washed with distilled water and then dried in a vacuum oven at 100 • C for 10 h to obtain the whiskers.

Instrumental Analyses
The chemical compositions of the samples were examined using an X-ray fluorescence spectrometer (XRF, PANalytical, Axios mAX, Almelo, The Netherlands).The mineral constituents were determined by X-ray diffraction spectrometer (XRD, Rigaku, D/Max 2500, Tokyo, Japan), using a Cu-anode target with the wavelength of 1.54056 Å, scan mode for the step scan, step length of 0.02 • .The morphologies of MHSH whiskers were characterized by using scanning electron microscopy (SEM, JEOL, JSM-6360LM, Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS).The infrared spectrum of MHSH whiskers was characterized by using Fourier transform infrared spectroscopy (FT-IR, 830, Shimadzu, Kyoto, Japan).The thermal decomposition behavior of MHSH whiskers were characterized using thermogravimetric analysis (TG-DSC, NETZSCH, STA449C, Selb, Germany).The temperature range covered was 30-1400 • C, with a heating rate of 10

Sulfuric-Acid Leaching Behavior of Magnesium from MgO-Enriched Residue
Effects of leaching temperature, leaching time, sulfuric acid concentration and liquid-to-solid (L/S) ratio on the magnesium dissolution of MgO-enriched residue were examined at a fixed stirring speed of 300 r/min, and the corresponding results are shown in Figure 4.

Sulfuric-Acid Leaching Behavior of Magnesium from MgO-Enriched Residue
Effects of leaching temperature, leaching time, sulfuric acid concentration and liquid-to-solid (L/S) ratio on the magnesium dissolution of MgO-enriched residue were examined at a fixed stirring speed of 300 r/min, and the corresponding results are shown in Figure 4.The leaching ratio of magnesium increased with leaching temperature, time, sulfuric acid concentration and L/S ratio (Figure 4).However, for each variable, the leaching ratio leveled off after a certain value.For temperature, liquid solid ratio, leaching time and acid concentration, these values were determined to be 80 °C, 4, 90 min and 40% sulfuric acid respectively.
From the above results, the optimal parameters for the conversion of magnesium oxide into magnesium sulfate were obtained.The leaching ratio of magnesium reached about 98% under the following leaching conditions: sulfuric acid concentration of 30-40 wt %, leaching temperature of 70-90 °C, leaching time of 90 min, and L/S ratio of 4 mL/g.By comparing Figures 2 and 5, it was found that magniferous minerals were dissolved by sulfuric acid, leaving amorphous silica gel and calcium sulfate in the leaching residue.Table 2 shows that the leaching residue contains 67.7 wt % SiO2 and 13.98 wt % CaSO4, which could be further used to prepare white carbon black or lithium silicate material.The leaching ratio of magnesium increased with leaching temperature, time, sulfuric acid concentration and L/S ratio (Figure 4).However, for each variable, the leaching ratio leveled off after a certain value.For temperature, liquid solid ratio, leaching time and acid concentration, these values were determined to be 80 • C, 4, 90 min and 40% sulfuric acid respectively.
From the above results, the optimal parameters for the conversion of magnesium oxide into magnesium sulfate were obtained.The leaching ratio of magnesium reached about 98% under the following leaching conditions: sulfuric acid concentration of 30-40 wt %, leaching temperature of 70-90 • C, leaching time of 90 min, and L/S ratio of 4 mL/g.By comparing Figures 2 and 5, it was found that magniferous minerals were dissolved by sulfuric acid, leaving amorphous silica gel and calcium sulfate in the leaching residue.Table 2 shows that the leaching residue contains 67.7 wt % SiO 2 and 13.98 wt % CaSO 4 , which could be further used to prepare white carbon black or lithium silicate material.

Sulfuric-Acid Leaching Behavior of Magnesium from MgO-Enriched Residue
Effects of leaching temperature, leaching time, sulfuric acid concentration and liquid-to-solid (L/S) ratio on the magnesium dissolution of MgO-enriched residue were examined at a fixed stirring speed of 300 r/min, and the corresponding results are shown in Figure 4.The leaching ratio of magnesium increased with leaching temperature, time, sulfuric acid concentration and L/S ratio (Figure 4).However, for each variable, the leaching ratio leveled off after a certain value.For temperature, liquid solid ratio, leaching time and acid concentration, these values were determined to be 80 °C, 4, 90 min and 40% sulfuric acid respectively.
From the above results, the optimal parameters for the conversion of magnesium oxide into magnesium sulfate were obtained.The leaching ratio of magnesium reached about 98% under the following leaching conditions: sulfuric acid concentration of 30-40 wt %, leaching temperature of 70-90 °C, leaching time of 90 min, and L/S ratio of 4 mL/g.By comparing Figures 2 and 5, it was found that magniferous minerals were dissolved by sulfuric acid, leaving amorphous silica gel and calcium sulfate in the leaching residue.Table 2 shows that the leaching residue contains 67.7 wt % SiO2 and 13.98 wt % CaSO4, which could be further used to prepare white carbon black or lithium silicate material.Under the optimal conditions of sulfuric acid leaching of MgO-enriched residue, the leaching of different constituents is shown in Table 3.As can be seen from Table 3, impurity ions such as Fe 3+ , Fe 2+ and Al 3+ existed in the leachate.The presence of these impurity ions will deteriorate the quality of the MHSH whiskers during hydrothermal synthesizing.Therefore, it is necessary to remove Fe 3+ , Fe 2+ and Al 3+ in advance.
Table 4 shows that the pH value range for the precipitation of Mg 2+ is different from those of Fe 3+ , Fe 2+ , Al 3+ , but the complete precipitation pH of Fe 2+ is close to the starting precipitation pH of Mg 2+ .By adding hydrogen peroxide to oxidize Fe 2+ to Fe 3+ , the differences of precipitating pH values between Mg 2+ and impurity ions could be enlarged.In this way, Fe 3+ , Fe 2+ and Al 3+ ions are removed according to the following reactions [28]: Under the conditions of pH of 5.7, reaction time of 30 min and reaction temperature of 60 • C, the retention ratio of magnesium was 98.1%, while the Fe 3+ and Al 3+ removal ratios reached 99.6% and 99.9%, respectively.The elemental concentration of the purified solution after oxidation and precipitation is shown in Table 5. Obviously, the purified solution is more suitable for the downstream preparation of value-added products.The chemical composition of precipitate is shown in Table 6, showing that the precipitate mainly constitutes of Fe 2 O 3 (60.63wt %).XRD patterns of the precipitate before and after calcination (at 600 • C for 60 min) are shown in Figure 6.The chemical composition of precipitate is shown in Table 6, showing that the precipitate mainly constitutes of Fe2O3 (60.63 wt %).XRD patterns of the precipitate before and after calcination (at 600 °C for 60 min) are shown in Figure 6.

Preparing MHSH Whiskers from Acidic Leachate
During hydrothermal synthesis, Mg 2+ in the solution initially reacted with OH − from NaOH to form Mg(OH)2 (Equation ( 10)).At high temperatures, the Mg(OH)2 precursor dissolved and underwent hydrolysis to produce Mg (OH)6 4− (Equation ( 11)).Then the reaction of Mg(OH)6 4− with SO4 2− of the solution resulted in MHSH (Equation ( 12)).Due to the low solubility of Mg(OH)2, an excess amount of MgSO4 was necessary for the growth of whiskers.
The corresponding formation of 512MHSH whiskers can be expressed as:

Effect of MgSO4 Concentration
The effect of MgSO4 concentration on the morphology and phase composition of the obtained whiskers was evaluated under the conditions of n(MgSO4):n(NaOH) = 2:1, NaOH concentration of 1 mol/L, hydrothermal processing at 180 °C for 4 h, and stirring speed of 300 r/min.
As shown in Figures 7 and 8, when the MgSO4 concentration was 1.5 mol/L, the synthesized product was Mg(OH)2 and no whiskers were formed.However, when the MgSO4 concentration reached 2.5 mol/L, the morphology of 512MHSH whiskers became more regular, and the maximum aspect ratio exceeded 100.The lengths of whiskers changed slightly with the increasing

Preparing MHSH Whiskers from Acidic Leachate
During hydrothermal synthesis, Mg 2+ in the solution initially reacted with OH − from NaOH to form Mg(OH) 2 (Equation ( 10)).At high temperatures, the Mg(OH) 2 precursor dissolved and underwent hydrolysis to produce Mg (OH) 6 4− (Equation ( 11)).Then the reaction of Mg(OH) 6 4− with SO 4 2− of the solution resulted in MHSH (Equation ( 12)).Due to the low solubility of Mg(OH) 2 , an excess amount of MgSO 4 was necessary for the growth of whiskers.The corresponding formation of 512MHSH whiskers can be expressed as:

Effect of MgSO 4 Concentration
The effect of MgSO 4 concentration on the morphology and phase composition of the obtained whiskers was evaluated under the conditions of n(MgSO 4 ):n(NaOH) = 2:1, NaOH concentration of 1 mol/L, hydrothermal processing at 180 • C for 4 h, and stirring speed of 300 r/min.
As shown in Figures 7 and 8, when the MgSO 4 concentration was 1.5 mol/L, the synthesized product was Mg(OH) 2 and no whiskers were formed.However, when the MgSO 4 concentration reached 2.5 mol/L, the morphology of 512MHSH whiskers became more regular, and the maximum aspect ratio exceeded 100.The lengths of whiskers changed slightly with the increasing concentration of MgSO 4 to 3.0 mol/L.Overall, the MgSO 4 concentration of 2.5-3.0 mol/L was suitable for the hydrothermal synthesis of 512MHSH whiskers.

Effect of Mole Ratio of MgSO4/NaOH
The effect of the mole ratio of MgSO4/NaOH on the morphology and structure of the whiskers was studied under the conditions of the MgSO4 concentration of 2.5 mol/L, NaOH concentration of 1

Effect of Mole Ratio of MgSO4/NaOH
The effect of the mole ratio of MgSO4/NaOH on the morphology and structure of the whiskers was studied under the conditions of the MgSO4 concentration of 2.5 mol/L, NaOH concentration of 1

Effect of Mole Ratio of MgSO 4 /NaOH
The effect of the mole ratio of MgSO 4 /NaOH on the morphology and structure of the whiskers was studied under the conditions of the MgSO 4 concentration of 2.5 mol/L, NaOH concentration of mol/L, hydrothermal duration of 4 h, hydrothermal temperature of 180 °C, and stirring speed of 300 r/min (Figures 9 and 10).
When the mole ratio of MgSO4/NaOH reached 3, the synthesized product was Mg(OH)2 and no whiskers were formed.However, when the mole ratio of MgSO4/NaOH was decreased to 2, high-quality 512MHSH whiskers with a maximum aspect ratio of about 100 and smooth surface were obtained.Nevertheless, 512MHSH whiskers gradually converted to Mg(OH)2 with the further decrease of MgSO4/NaOH mole ratio.Thus, the optimum mole ratio of MgSO4/NaOH was 2.   mol/L, hydrothermal duration of 4 h, hydrothermal temperature of 180 °C, and stirring speed of 300 r/min (Figures 9 and 10).
When the mole ratio of MgSO4/NaOH reached 3, the synthesized product was Mg(OH)2 and no whiskers were formed.However, when the mole ratio of MgSO4/NaOH was decreased to 2, high-quality 512MHSH whiskers with a maximum aspect ratio of about 100 and smooth surface were obtained.Nevertheless, 512MHSH whiskers gradually converted to Mg(OH)2 with the further decrease of MgSO4/NaOH mole ratio.Thus, the optimum mole ratio of MgSO4/NaOH was 2.   When the mole ratio of MgSO 4 /NaOH reached 3, the synthesized product was Mg(OH) 2 and no whiskers were formed.However, when the mole ratio of MgSO 4 /NaOH was decreased to 2, Minerals 2017, 7, 138 10 of 16 high-quality 512MHSH whiskers with a maximum aspect ratio of about 100 and smooth surface were obtained.Nevertheless, 512MHSH whiskers gradually converted to Mg(OH) 2 with the further decrease of MgSO 4 /NaOH mole ratio.Thus, the optimum mole ratio of MgSO 4 /NaOH was 2.

Effect of Hydrothermal Temperature
Effects of hydrothermal temperature on the morphology and phase composition of whiskers were studied under the conditions of n(MgSO 4 ):n(NaOH) = 2:1, MgSO 4 concentration of 2.5 mol/L, NaOH concentration of 1 mol/L, hydrothermal duration of 4 h, and stirring speed of 300 r/min.
From Figures 11 and 12, there were no 512MHSH whiskers at 120 • C. The morphology of whiskers became regular with increasing temperature.512MHSH whiskers with high aspect ratio of 100 and smooth surface in the absence of Mg(OH) 2 impurities were successfully synthesized at 180 • C, but the 512MHSH whiskers disappeared when the temperature was further increased to 200 • C. Thus, the optimal hydrothermal temperature range was 160-180 • C. From Figures 11 and 12, there were no 512MHSH whiskers at 120 °C.The morphology of whiskers became regular with increasing temperature.512MHSH whiskers with high aspect ratio of 100 and smooth surface in the absence of Mg(OH)2 impurities were successfully synthesized at 180 °C, but the 512MHSH whiskers disappeared when the temperature was further increased to 200 °C.Thus, the optimal hydrothermal temperature range was 160-180 °C.Figures 13 and 14 show morphologies of whiskers synthesized by varying the hydrothermal duration from 0.5 to 7 h.The morphologies became more complete with the extension of hydrothermal time.The 512MHSH whiskers only presented a few sporadic whiskers with short fiber length in 0.5 h.When the hydrothermal duration was extended to 4 h, the whiskers gradually grew to be smooth with a uniform aspect ratio of about 100.Whiskers, however, were destroyed once again when the hydrothermal duration further increased to 7 h.Figures 13 and 14 show morphologies of whiskers synthesized by varying the hydrothermal duration from 0.5 to 7 h.The morphologies became more complete with the extension of hydrothermal time.The 512MHSH whiskers only presented a few sporadic whiskers with short fiber length in 0.5 h.When the hydrothermal duration was extended to 4 h, the whiskers gradually grew to be smooth with a uniform aspect ratio of about 100.Whiskers, however, were destroyed once again when the hydrothermal duration further increased to 7 h.

Effect of Hydrothermal Duration
Effects of hydrothermal duration on the morphology and phase of whiskers were studied under the conditions of n(MgSO4):n(NaOH) = 2:1, MgSO4 concentration of 2.5 mol/L, NaOH concentration of 1 mol/L, hydrothermal temperature of 180 °C, and stirring speed of 300 r/min.
Figures 13 and 14 show morphologies of whiskers synthesized by varying the hydrothermal duration from 0.5 to 7 h.The morphologies became more complete with the extension of hydrothermal time.The 512MHSH whiskers only presented a few sporadic whiskers with short fiber length in 0.5 h.When the hydrothermal duration was extended to 4 h, the whiskers gradually grew to be smooth with a uniform aspect ratio of about 100.Whiskers, however, were destroyed once again when the hydrothermal duration further increased to 7 h.Based on the above results, the optimal hydrothermal synthesis conditions for preparing 512MHSH whiskers are as follows: 2.5-3.0 mol/L MgSO4 in 1.0 mol/L NaOH solution for a fixed MgSO4/NaOH mole ratio of 2 at 160-180 °C for 2.0-4.0 h.Under these conditions, the 512MHSH whiskers with a length of 30-100 μm, an average diameter of 0.5-1.0μm and an aspect ratio of ~100 were produced.

XRD
The XRD pattern of the hydrothermal product, prepared under the optimal hydrothermal synthesis conditions, is shown in Figure 15.All the diffraction peaks can be indexed to the orthorhombic structure of 5Mg(OH)2•MgSO4•2H2O (PDF No. 86-1322).

SEM-EDS
It can be observed that the hydrothermal product has needle-like morphology with sector-like ends (Figure 16).The whiskers have a length of 30-100 μm and an average diameter of 0.5-1.0μm.

SEM-EDS
It can be observed that the hydrothermal product has needle-like morphology with sector-like ends (Figure 16).The whiskers have a length of 30-100 µm and an average diameter of 0.5-1.0µm.
The EDS spectrum indicates that the ratio of Mg to S is very close to the stoichiometric value of 5Mg (OH)   The EDS spectrum indicates that the ratio of Mg to S is very close to the stoichiometric value of 5Mg(OH)2•MgSO4•2H2O.

FT-IR
The infrared spectrum of the hydrothermal product is shown in Figure 17.The peaks at 3648.78 and 3611.43 cm −1 are assigned to the -OH stretching vibration.The peak at 3416.40 cm −1 is the hydrogen bond formed by reactions of hydroxyls.The peak at 1628.19 cm −1 is due to the bending vibration of the -OH bond.The peak at 815.91 cm −1 is the coordination bond between H2O and Mg 2+ .Two sharp absorption bands at 1116.89 and 639.52 cm −1 are ascribed to the O-S-O bond.

TG-DSC
TG-DSC curves (Figure 18) of the hydrothermal product exhibit three stages of pronounced weight loss and endothermic effect, respectively.Table 7 compares the weight losses of the hydrothermal product and theoretical values.The first stage of weight loss (7.9%) in the temperature range of 306-377 °C corresponds to the loss of crystal water (Equation ( 13)).The second stage of weight loss (20.0%) in the temperature range of 386-641 °C corresponds to the loss of hydroxyl water (Equation ( 14)).The last stage of weight loss (17.8%) in the temperature range of 897-1029 °C corresponds to the desulfation of MgSO4•5MgO (Equation ( 15)).
The thermal decomposition reactions of the hydrothermal product can be expressed as:   The EDS spectrum indicates that the ratio of Mg to S is very close to the stoichiometric value of 5Mg(OH)2•MgSO4•2H2O.

TG-DSC
TG-DSC curves (Figure 18) of the hydrothermal product exhibit three stages of pronounced weight loss and endothermic effect, respectively.Table 7 compares the weight losses of the hydrothermal product and theoretical values.The first stage of weight loss (7.9%) in the temperature range of 306-377 °C corresponds to the loss of crystal water (Equation ( 13)).The second stage of weight loss (20.0%) in the temperature range of 386-641 °C corresponds to the loss of hydroxyl water (Equation ( 14)).The last stage of weight loss (17.8%) in the temperature range of 897-1029 °C corresponds to the desulfation of MgSO4•5MgO (Equation ( 15)).
The thermal decomposition reactions of the hydrothermal product can be expressed as:

TG-DSC
TG-DSC curves (Figure 18) of the hydrothermal product exhibit three stages of pronounced weight loss and endothermic effect, respectively.Table 7 compares the weight losses of the hydrothermal product and theoretical values.The first stage of weight loss (7.9%) in the temperature range of 306-377 • C corresponds to the loss of crystal water (Equation ( 13)).The second stage of weight loss (20.0%) in the temperature range of 386-641 • C corresponds to the loss of hydroxyl water (Equation ( 14)).The last stage of weight loss (17.8%) in the temperature range of 897-1029 • C corresponds to the desulfation of MgSO 4 •5MgO (Equation ( 15)).
The thermal decomposition reactions of the hydrothermal product can be expressed as:
, including: (1) Leaching MgO-enriched residue with sulfuric acid to obtain leachate containing magnesium sulfate after separation of silica; (2) Removing impurities of acidic leachate by oxidation and precipitation; and (3) Preparing MHSH whiskers via hydrothermal synthesis.

Figure 4 .
Figure 4. Extraction of magnesium as the function of (a) leaching temperature; (b) liquid-to-solid ratio; (c) leaching time; and (d) sulfuric acid concentration.

Figure 4 .
Figure 4. Extraction of magnesium as the function of (a) leaching temperature; (b) liquid-to-solid ratio; (c) leaching time; and (d) sulfuric acid concentration.

Figure 4 .
Figure 4. Extraction of magnesium as the function of (a) leaching temperature; (b) liquid-to-solid ratio; (c) leaching time; and (d) sulfuric acid concentration.
to 3.0 mol/L.Overall, the MgSO4 concentration of 2.5-3.0 mol/L was suitable for the hydrothermal synthesis of 512MHSH whiskers.

3 .
Effect of Hydrothermal Temperature Effects of hydrothermal temperature on the morphology and phase composition of whiskers were studied under the conditions of n(MgSO4):n(NaOH) = 2:1, MgSO4 concentration of 2.5 mol/L, NaOH concentration of 1 mol/L, hydrothermal duration of 4 h, and stirring speed of 300 r/min.

Figure 12 .
Figure 12.XRD patterns of whiskers obtained at different hydrothermal temperatures H-Mg(OH)2, S-5Mg(OH)2•MgSO4•2H2O, M-MgSO4•xH2O.3.2.4.Effect of Hydrothermal DurationEffects of hydrothermal duration on the morphology and phase of whiskers were studied under the conditions of n(MgSO4):n(NaOH) = 2:1, MgSO4 concentration of 2.5 mol/L, NaOH concentration of 1 mol/L, hydrothermal temperature of 180 °C, and stirring speed of 300 r/min.Figures13 and 14show morphologies of whiskers synthesized by varying the hydrothermal duration from 0.5 to 7 h.The morphologies became more complete with the extension of hydrothermal time.The 512MHSH whiskers only presented a few sporadic whiskers with short fiber length in 0.5 h.When the hydrothermal duration was extended to 4 h, the whiskers gradually grew to be smooth with a uniform aspect ratio of about 100.Whiskers, however, were destroyed once again when the hydrothermal duration further increased to 7 h.

Figure 15 .
Figure 15.XRD pattern of the hydrothermal product.

Figure 15 .
Figure 15.XRD pattern of the hydrothermal product.

Figure 16 .
Figure 16.EDS spectra of the hydrothermal product.

Figure 17 .
Figure 17.Fourier transform infrared spectroscopy FI-IR pattern of the hydrothermal product.

Figure 16 .
Figure 16.EDS spectra of the hydrothermal product.

3. 3 . 3 .
FT-IRThe infrared spectrum of the hydrothermal product is shown in Figure17.The peaks at 3648.78 and 3611.43 cm −1 are assigned to the -OH stretching vibration.The peak at 3416.40 cm −1 is the hydrogen bond formed by reactions of hydroxyls.The peak at 1628.19 cm −1 is due to the bending vibration of the -OH bond.The peak at 815.91 cm −1 is the coordination bond between H 2 O and Mg 2+ .Two sharp absorption bands at 1116.89 and 639.52 cm −1 are ascribed to the O-S-O bond.Minerals 2017, 7, 138 13 of 16

Figure 16 .
Figure 16.EDS spectra of the hydrothermal product.

3. 3 . 3 .
FT-IRThe infrared spectrum of the hydrothermal product is shown in Figure17.The peaks at 3648.78 and 3611.43 cm −1 are assigned to the -OH stretching vibration.The peak at 3416.40 cm −1 is the hydrogen bond formed by reactions of hydroxyls.The peak at 1628.19 cm −1 is due to the bending vibration of the -OH bond.The peak at 815.91 cm −1 is the coordination bond between H2O and Mg 2+ .Two sharp absorption bands at 1116.89 and 639.52 cm −1 are ascribed to the O-S-O bond.

Figure 17 .
Figure 17.Fourier transform infrared spectroscopy FI-IR pattern of the hydrothermal product.

Figure 17 .
Figure 17.Fourier transform infrared spectroscopy FI-IR pattern of the hydrothermal product.
1LOI is the mass loss on ignition.
1LOI is the mass loss on ignition.

Table 3 .
Result of sulfuric acid leaching of MgO-enriched residue.

Table 4 .
Metal hydroxides precipitation pH value and solubility product constant Ksp value.