Recovery of Mg from H₂SO₄ Leaching Solution of Serpentine to Precipitation of High-Purity Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O

Abstract

This paper describes a leaching-purifying-precipitation process to recover magnesium from serpentine acid-leaching solution and to synthesize high purity Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O. Fe, Al, and Cr in the leaching solution were removed using the oxidation precipitation method with active MgO as a precipitant and H₂O₂ as an oxidant. Ni, Co and Mn were removed by Na₂S precipitation to obtain a pure MgSO₄ solution. Mg²⁺ ions were first precipitated with NH₃·H₂O to synthesize Mg(OH)₂, followed by NH₄HCO₃ precipitation to obtain 4MgCO₃·Mg(OH)₂·4H₂O. A small part of MgSO₄ coprecipitates with Mg(OH)₂ to form MgSO₄·5Mg(OH)₂·3H₂O. The Mg(OH)₂ was aged with a diluted NaOH solution to remove the sulfur; the two-stage precipitation percentage of Mg is 96.3%. Mg(OH)₂ has a purity of 98.48% with a sulfur content of 0.28%. This process provides a promising method for the high-efficiency recovery of Mg and the large-scale production of the high purity of Mg(OH)₂ from the serpentine.

1. Introduction

Serpentine is known as an important nonmetallic mineral resource in China with a monoclinic structure and chemical composition of [(Mg,Fe)₃Si₂O₅(OH)₄] [1,2,3]. Serpentine can be divided into three types: lizardite, antigorite, and chrysotile. China has abundant serpentine resources with reserves of more than 15 billion tons [4]. Serpentine is a silicate mineral formed through the hydrothermal alteration of super bedrock. Its main components are silicon dioxide and magnesium oxide, which account for approximately 80%~90% of the total mass of serpentine. It also contains a small amount of valuable metal elements, such as iron, nickel, manganese and cobalt [5]. Serpentine minerals in China are mainly distributed in Jiangxi, Sichuan, Henan, and Inner Mongolia [6]. Wide distribution, large reserve and good texture are the characteristics of serpentine minerals in China. Serpentine is widely applied for the production of calcium magnesium phosphate and Mg [7]. Serpentine contains more than 20 wt% Mg and it is of great significance to extract Mg to prepare Mg-containing products such as MgO and Mg(OH)₂. Due to its high melting point, high dielectric constant, and mechanical strength, MgO acts as a good additive in the fields of ceramics, catalysis, and refractory materials [8,9,10,11,12]. Other minerals that are similar to serpentine include magnesite and dolomite. Magnesite is mainly composed of MgCO₃ and is widely used. Magnesite is leached by hydrochloric acid and the anhydrous magnesium chloride powder is obtained through the spray drying of the leaching solution [13]. Dolomite is a double carbonate, often expressed with the chemical formula CaCO₃*MgCO₃; magnesium can also be extracted by leaching [14].

Mg(OH)₂ has the advantages of high thermal stability, low smoke, non-toxic, no corrosion, etc. [15,16,17]. It is a kind of environmental protection inorganic flame retardant and smoke suppressant. Many studies have been conducted on the application of Mg(OH)₂ in the field of environmental protection. Mg(OH)₂ has been used as a flame retardant filler for polymer materials and passive fire prevention [18]. Below its dehydration temperature, Mg(OH)₂ undergoes endothermic decomposition, releasing water vapor and magnesium oxide. Magnesium hydroxide is also used as an acid waste neutralizer, pharmaceutical excipients, paper preservation, a component of ethanol chemical sensors, and the most important precursor for the preparation of (nanostructured) magnesium oxide [19]. MgO produced by the calcination of Mg(OH)₂ is used as the refractory material in the process of steelmaking.

In recent years, the development of serpentine ore has attracted wide attention, such as in hydrometallurgy, pyrometallurgy and bio-metallurgy [20]. Hydrometallurgy and pyrometallurgy are widely used in industry. In the leaching of valuable metals in minerals, the most commonly used leaching solutions are hydrochloric acid, sulfuric acid, nitric acid, and nitric acid [21,22,23,24]. Chemical plants mainly use sulfuric acid for economic reasons. For example, the preferred methods for extracting nickel and cobalt from minerals are the atmospheric leaching process [25] and high pressure leaching processes, which have the advantage that iron can be precipitated as hematite. However, compared with atmospheric pressure leaching, the high pressure leaching process has a relatively high cost, large investment, and high energy consumption at the commercial level. Wang, B. et al. [26] leached a limonitic laterite from hydrochloric acid at atmospheric pressure, Da Costa et al. [27] leached A lateritic nickel ore Indonesia at atmospheric pressure with ammonium oxalate. Although they have successfully extracted Ni and Co through atmospheric leaching processes, these processes generally consume more acid and leach more Fe and Mg, which hinders their further application.

There are many reports about the preparation of magnesium products from serpentine [28,29]. Magnesium products prepared from serpentine leaching solution mainly include Mg(OH)₂, MgO, etc. Erlund, R. et al. [29] used ammonium bisulfate as an additive to extract magnesium from serpentinite through carbonation to obtain Mg₅(OH)₂(CO₃)₄·4H₂O. Lin P C et al. [30] obtained Mg(OH)₂ by hydrothermal treatment with HCl followed by NaOH precipitation. This process is high-cost and suffers the corrosion problems imposed by HCl. Gladikova, L.A. et al. [31] decomposed serpentine directly with H₂SO₄ to obtain a MgSO₄ solution; after removing impurities, precipitation, and calcination, MgO was obtained with 96.5% purity and containing 0.42% SO₄²⁻. For the sulfuric acid treatment method, SO₄²⁻ usually enters into Mg(OH)₂ and inevitably increases the sulfur content. The entry of the S element into Mg(OH)₂ and MgO will reduce the purity of Mg(OH)₂ and MgO.

In the NH₄⁺-NH₃-Mg-SO₄²⁻ system, MgSO₄·5Mg(OH)₂·3H₂O coexists with Mg(OH)₂. The presence of MgSO₄·5Mg(OH)₂·3H₂O inevitably increases the sulfur content and decreases the purity of Mg(OH)₂. Element sulfur could induce the brittle crack and corrosion of the furnace if Mg(OH)₂ acts as a refractory material in steel making. Sierra, C. et al. [28] took serpentine tailings as their research object; serpentine tailings were leached with a certain concentration of H₂SO₄ solution, and Mg²⁺ was precipitated in the form of Mg(OH)₂ with a certain concentration of NaOH solution in the resulting leaching solution, and Mg was recovered. It is effective to use NaOH to precipitate Mg²⁺ because of its simple process and low energy consumption. However, the Mg(OH)₂ prepared with NaOH as the precipitator has high viscosity, resulting in a slow filtration rate and low taste of Mg(OH)₂ precipitated by this process, which affects the current price of Mg(OH)₂. The cost of preparing Mg(OH)₂ with NaOH as the precipitator is too high, which limits the economic feasibility of the process. However, the removal of sulfur in Mg(OH)₂ has seldom been reported. Furthermore, the purity of Mg-containing products resulting from serpentine is low, which limits the range of magnesium products used.

In this paper, high-purity and low-sulfur Mg(OH)₂ and MgO were synthesized and Mg in serpentine was efficiently recovered. Fe, Al, Cr, Ni, Co, Mn, and Mg in serpentine was efficiently leached by a counter-current leaching process with a low concentration of sulfuric acid. Then, the leaching solution was purified via two-step precipitation to obtain a pure magnesium sulfate solution. Finally, this solution was precipitated with NH₃·H₂O. followed by NH₄HCO₃. Mg(OH)₂ was aged with the NaOH solution to remove sulfur. The optimum process parameters of purification, precipitation, and aging were obtained through a single-factor experiment. This process is low-cost and has a high Mg recovery rate, which provides a promising method for the exploitation and utilization of magnesium resources in serpentine.

2. Materials and Methods

2.1. Materials

The serpentine was supplied by Jiujinzi-arukorqinqi, Inner Mongolia (antigorite, Huaxia Jianlong, Mongolia, China). The chemical composition of serpentine and the sulfuric acid leaching solution are listed in

Table 1. The main component of serpentine and solution.

Element	Mg	Si	Al	Fe	Ca	Ni	Co	Cr	Mn
Content (%)	21.3	17.76	0.44	6.57	0.14	0.21	0.01	0.23	0.05
Concentration (g·L−1)	69.82	--	1.28	9.10	0.62	0.57	0.03	0.31	0.23

. The H₂O₂, Na₂S, NH₃·H₂O, NH₄HCO₃ and NaOH were from Aladdin Industrial Co. Ltd (AR reagents, Aladdin Industrial Co. Ltd, Shanghai, China) and were used as received. All of the chemicals used are AR reagents. In each leaching experiment, the mass of the serpentine mineral was 500 g, and the volume of leach solution was 1.5 L. Via the two-stage counter current leaching, the concentration of the elements in the leach solution is shown in Table 1, the leaching percentage of Mg was 98.35%, and the pH of the leaching solution was 1.0~1.8.

2.2. Methods

The complete process is as follows: Fe, Al, and Cr are first removed from the serpentine leaching solution, and then Ni, Co, and Mn are removed. In this way, a pure MgSO4 solution is obtained. With NH₃·H₂O as the precipitator, Mg(OH)₂ is obtained by sinking magnesium, and high purity and low sulfur Mg(OH)₂ is obtained by removing sulfur from Mg(OH)₂. 4MgCO₃· Mg(OH)₂·4H₂O is formed by adding NH₄HCO₃ into the primary precipitation solution. The flowchart of the recovery of Mg and the preparation of high purity Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O from the leaching solution of sulfuric acid is shown in Figure 1.

2.2.1. Purification of H₂SO₄ Leaching Solution

First, 10 mL H₂O₂ was added into the leach solution to oxidize Fe²⁺ into Fe³⁺, and a suspension containing 10 wt% active MgO was added drop-wise to adjust the pH value to 4.0. After stirring for 1.0 h, Fe³⁺, Al³⁺, and Cr³⁺ were precipitated from the leach solution, and the mixed solution was filtered. Then, 6 g/L Na₂S solution was added to the filtrate and reacted for 1.0 h with stirring to remove Ni²⁺, Co²⁺, and Mn²⁺ in the form of NiS, MnS, and CoS, respectively. Finally, the suspension was filtered to obtain a pure MgSO₄ solution. The concentrations of the various metal ions in the solution (before and after impurity removal) were analyzed to calculate the impurity-removing efficiency.

2.2.2. Synthesis of High Purity Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O

After purification, the leaching solution was diluted with water by a factor of 1.8. High-purity Mg(OH)₂ was synthesized via the NH₃·H₂O precipitation of the resulting MgSO₄ solution, and the molar ratio of NH₃·H₂O to MgSO₄ was 2.4~4.0:1. Next, 27% NH₃·H₂O was drop-wise added into the resulting MgSO₄ solution and kept stirring at 40~70 °C. Afterward, the suspension was subject to solid/solution separation. The solid was aged with NaOH solution following filtering and washing with deionized water. When Mg(OH)₂ was aged, the amount of NaOH solution was 600 mL and the concentration was 0.25 mol/L. The pregnant solution of precipitation was further reacted with NH₄HCO₃ to form basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O), and the molar ratio of NH₄HCO₃ to Mg²⁺ was controlled at 1.0. For the magnesium hydroxide precipitation section, 1 L purified solution was used; in the second-stage precipitation, 1 L pregnant solution resulting from magnesium hydroxide precipitation was used.

2.2.3. Characterization

The element concentration in the solution was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES7400, Shanghai Yuzhong Industrial Co., LTD, Shanghai, China). The phase component of the sample was detected by X-ray diffraction (XRD, Rint-2000, Rigaku Corporation (XRDRint-2000, Shenzhen Keshida Electronic Technology Co., LTD, Shenzhen, China), using Cu K radiation = 1.5406 Å), at a scanning speed of 5 °/min in the 2θ range of 10°–80°. The morphologies of the samples were detected by scanning electron microscopic (SEM, JSM-6330 LV, Japan). The N₂ adsorption-desorption isotherm was detected at 77K, using Quantachrome Quadrasorb SI type adsorbometer utilizing Barrett-Emmett-Teller (BET) calculations for the surface area and BJH calculations for the pore size distribution for the desorption branch of the isotherm.

3. Results

3.1. Removing Fe, Al and Cr from H₂SO₄ Leaching Solution

The serpentine acid-leaching solution had a pH of 1.0~1.8. Iron in the leach solution may exist as Fe²⁺ and Fe³⁺. Fe²⁺ was oxidized to Fe³⁺ when 10 mL H₂O₂ was added to 1 L of leach solution. Active MgO was added to adjust the pH value to 4.0, and then Fe³⁺ was precipitated from the leaching solution. Cr³⁺ and Al³⁺ were coprecipitated together. The chemical reactions are represented by Equations (1)–(4).

$${3\mathrm{Fe}}_2{{(\mathrm{SO}}_4)}_3{+9\mathrm{H}}_2{\mathrm{O}+5\mathrm{MgO}=2\left[\right(\mathrm{H}}_3{\mathrm{O})\mathrm{Fe}}_3{{(\mathrm{SO}}_4)}_2{\left(\mathrm{OH}\right)}_6]\downarrow {+5\mathrm{MgSO}}_4$$

(1)

$${\mathrm{Fe}}_2{{(\mathrm{SO}}_4)}_3{+3\mathrm{H}}_2{\mathrm{O}+3\mathrm{MgO}=2\mathrm{Fe}\left(\mathrm{OH}\right)}_3\downarrow {+3\mathrm{MgSO}}_4$$

(2)

$${\mathrm{Cr}}_2{{(\mathrm{SO}}_4)}_3{+3\mathrm{H}}_2{\mathrm{O}+3\mathrm{MgO}=2\mathrm{Cr}\left(\mathrm{OH}\right)}_3\downarrow {+3\mathrm{MgSO}}_4$$

(3)

$${\mathrm{Al}}_2{{(\mathrm{SO}}_4)}_3{+3\mathrm{H}}_2{\mathrm{O}+3\mathrm{MgO}=2\mathrm{Al}\left(\mathrm{OH}\right)}_3\downarrow {+3\mathrm{MgSO}}_4$$

(4)

The effect of the pH value on the precipitation percentage of different metals is shown in

Table 2. Removal of Fe, Al and Cr from the leach solution (%).

pH	Fe	Al	Ni	Co	Mn	Cr
3.2	99.67	61.90	28.13	31.34	29.22	93.45
3.5	99.94	89.99	33.96	27.60	30.84	94.71
4.0	99.94	98.57	27.63	28.16	28.10	99.98
4.5	99.98	99.66	29.65	30.11	29.95	99.99

. The results show that all of the Fe species could be completely removed at a pH value of 4.0. The precipitation percentage of Fe, Al, and Cr are 99.94%, 98.57%, and 99.98%, respectively.

3.2. Removing Mn, Ni, and Co from Solution

After the removal of Fe, Al, and Cr, the solution contains a small amount of Mn (0.134 g·L⁻¹), Ni (0.405 g·L⁻¹) and Co (0.022 g·L⁻¹). They are removed through Na₂S precipitation based on Ksp(MnS) = 2.8 × 10⁻¹³, Ksp(NiS) = 2.8 × 10⁻²¹, and Ksp(CoS) = 1.8 × 10⁻¹⁸.

A series of experiments were carried out to optimize the conditions. such as the amount of Na₂S, pH value, reaction time, and reaction temperature. The effect of the excess coefficient (actual amount/theoretical amount) of Na₂S on the precipitation percentage was studied on the conditions of 6 g/L Na₂S, reaction at 60 °C for 60 min. As shown in Figure 2a, the precipitation percentages of Mn, Ni, and Co increase with an increase in the excess coefficient. When the excess coefficient is 2.6, the precipitation percentages of Mn, Ni, and Co reach 66.13%, 74.91%, and 73.68%, respectively. Figure 2b indicates that the pH value has a minor effect on the precipitation percentages of Ni²⁺ and Co²⁺, while the Mn²⁺ precipitation percentage reaches the maximum of 86.65% at a pH value of 5.5. A part of Na₂S hydrolyzes to produce volatile H₂S; a high pH value is unfavorable to hydrolysis. Therefore, in order to precipitate Mn²⁺, the pH value in the solution should be adjusted to 5.5, and the precipitation percentages of Ni²⁺, Co²⁺, and Mn²⁺ could reach 96.45%, 98.86%, and 86.65%, respectively.

$${\mathrm{Mn}}^{2+}{+\mathrm{H}}_2\mathrm{S}=\mathrm{MnS}\downarrow {+2\mathrm{H}}^{+}$$

(5)

Figure 2c shows that when the reaction time reaches 60 min, the precipitation percentage of Co²⁺ remains stable, while that of Ni²⁺ and Mn²⁺ increases very slightly, so the reaction time should be set as 60 min. Figure 2d shows that continuously raising the reaction temperature was conducive to the forward reaction and the precipitation percentages of Ni²⁺, Co²⁺, and Mn²⁺ were also increased. However, when the temperature was higher than 60 °C, the precipitation percentages decreased, which is caused by the hydrolysis of Na₂S and the volatilization of H₂S with the increase in reaction temperature, leading to the decrease in the S²⁻ concentration in the solution. Therefore, the reaction temperature is set at 60 °C. For two-stage purification, the Mg²⁺ concentration reaches 73.5 g/L because MgO suspension transformed into MgSO₄ solution during the prior Fe, Al, and Cr precipitation. The total Mg loss rate is 1.95% in this purification section.

3.3. Precipitation of Mg(OH)₂

After removing Fe³⁺, Fe²⁺, Al³⁺, Cr³⁺, Mn²⁺, Ni²⁺, and Co²⁺ from the solution, the MgSO₄ solution was diluted to 1.8 times liquid volume to obtain a concentration of 40.82 g/L Mg²⁺. NH₃·H₂O was used to precipitate Mg²⁺ for the formation of Mg(OH)₂, as presented by Equation (6).

$${2\mathrm{NH}}_3\cdot {\mathrm{H}}_2\mathrm{O}{+\mathrm{MgSO}}_4{=\mathrm{Mg}\left(\mathrm{OH}\right)}_2\downarrow {+(\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4$$

(6)

A series of experiments were carried out to optimize the conditions. The effect of the molar ratio of NH₃·H₂O to Mg²⁺ on the precipitation percentage and S content in Mg(OH)₂ under the conditions of a reaction temperature of 85 ℃ and a reaction time of 2.0 h was explored. As can be seen from Figure 3a, with the increase in the amount of NH₃·H₂O, the precipitation percentage of Mg²⁺ is improved. When the molar ratio of NH₃:Mg reaches 2.8, the precipitation percentage of Mg²⁺ can reach 63.6%. However, when the NH₃/Mg molar ratio is more than 3.0, the precipitation percentage of Mg²⁺ increases slightly with the increase in the NH₃/Mg molar ratio.

However, with the increase in the amount of NH₃·H₂O, the content of S in Mg(OH)₂ slightly decreases; when the NH₃/Mg molar ratio reaches 2.8, the content of S decreases to 1.25%, and further increasing the NH₃/Mg molar ratio has little effect on the content of S. Furthermore, excessive NH₃·H₂O increases the cost of production, and causes environmental pollution and other problems. Therefore, the NH₃/Mg molar ratio should be controlled at 2.8 after comprehensive consideration.

The effect of the reaction temperature was studied under the conditions: Mg²⁺ 40.82 g/L, NH₃/Mg molar ratio 2.8, and reaction time at 2.0 h. It can be seen from Figure 3b that when the reaction temperature reaches 70℃, the precipitation rate of Mg reaches 69.44%; however, the content of S in Mg(OH)₂ has little change with the temperature. However, when the temperature is more than 70 °C, the precipitation rate of Mg gradually decreases because of the severe NH₃ volatilization, and the rapid reduction in the pH value of the system leads to a downward trend of the yield of Mg, and with the increases of the temperature, the content of S in Mg(OH)₂ also increases. Therefore, the optimal reaction temperature should be chosen at 70 °C.

3.4. Removing Element S in Mg(OH)₂

According to the relevant literature, element S may exist in the form of the MgSO₄·5Mg(OH)₂·3H₂O in NH₄⁺-NH₃₋Mg²⁺-SO₄²⁻ system. In order to reduce the content of the S element in Mg(OH)₂, the Mg(OH)₂ was aged with the NaOH solution. The chemical reactions are shown as Equations (7) and (8).

$${\mathrm{MgSO}}_4\cdot 5\mathrm{Mg}{(\mathrm{OH})}_2\cdot 3{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{MgSO}}_4+5\mathrm{Mg}{(\mathrm{OH})}_2\downarrow {+3\mathrm{H}}_2\mathrm{O}$$

(7)

$${\mathrm{MgSO}}_4+2{\mathrm{NaOH}=\mathrm{Mg}\left(\mathrm{OH}\right)}_2\downarrow {+\mathrm{Na}}_2{\mathrm{SO}}_4$$

(8)

The effect of the aging temperature was studied in the conditions: 0.25 mol/L NaOH, aging time of 2.0 h. As shown in Figure 4a, the increase in the aging temperature accelerates the reaction of MgSO₄·5Mg(OH)₂·3H₂O with NaOH. When the aging temperature reaches 85 °C, the S content in Mg(OH)₂ is 0.28%. Further increasing the temperature does not change the content of S. Hence, the optimized temperature is 85 °C.

The effect of the aging concentration of NaOH was studied in the optimal conditions: temperature of 85 °C, time of 2.0 h. As shown in Figure 4b, increasing the NaOH concentration from 0.25 to 1.00 mol/L increases the S content from 0.28 to about 0.31%. This may be due to the undeveloped magnesium hydroxide crystal dissolving as the concentration of sodium hydroxide increases, making the mass of magnesium hydroxide smaller and increasing the S content. Therefore, 0.25 mol/L NaOH is sufficient. Through a series of conditional experiments, the optimal aging conditions are obtained, and the content of S in the Mg(OH)₂ is controlled at 0.28%.

3.5. Precipitation of 4MgCO₃·Mg(OH)₂·4H₂O

It was found that the magnesium precipitation percentage reached 69.44% with the NH₃/Mg molar ratio 2.8 at 70 °C; therefore, a small portion of Mg²⁺ was still left in the solution. In order to fully extract Mg²⁺ in the solution, a second-stage precipitation method to prepare the basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O) was adopted. The chemical reaction occurred, as shown in Equation (9).

$$\begin{array}{l}{6\mathrm{NH}}_3{+5\mathrm{MgSO}}_4{+4\mathrm{NH}}_4{\mathrm{HCO}}_3{+6\mathrm{H}}_2\mathrm{O}=\\ {4\mathrm{MgCO}}_3\cdot {\mathrm{Mg}\left(\mathrm{OH}\right)}_2\cdot {4\mathrm{H}}_2\mathrm{O}\downarrow {+5(\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4\end{array}$$

(9)

When NH₃·H₂O is the theoretical amount, 1.0 h of reaction time at 85 °C, NH₄HCO₃ is 1.5 times the theoretical amount, the magnesium precipitation percentage reaches 88%, and the concentration of Mg²⁺ in the solution is 1.25 g/L. However, with further increasing the dosage of NH₄HCO₃, the precipitation percentage does not change significantly. The optimum amount of NH₄HCO₃ is 1.5 times the theoretical amount.

4. Discussion

4.1. Product Characteristic Analysis

By combining the two-stage precipitation of the Mg process, the total precipitation percentage of Mg can reach 96.3%. The XRD patterns of the Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O samples were obtained, as shown in Figure 5 and Figure 6, respectively. All of the peaks in Figure 5 are indexed to Mg(OH)₂ (PDF 44-1482) and the intensity is high, indicating that the Mg(OH)₂ crystal is pure and well-developed. The peaks in Figure 6 match well with the ideal 4MgCO₃·Mg(OH)₂·4H₂O (PDF70-1177).

The obtained Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O are calcined in a muffle furnace at 850 °C for 2.0 h to obtain MgO. Kastiukas et al. [32] pointed out that MgO with strong activity can be obtained when magnesium hydroxide is calcined at 900 °C. The results of the ICP-OES analysis are shown in

Table 3. Element analysis of MgO after calcination of Mg(OH)₂.

Element	Na	Al	Fe	Ca	Ni	Co	Cr	Mn	S
content (%)	0.0017	0.0014	0.0004	0.0029	0.0001	-	0.0002	0.0069	1.55

,

Table 4. Elemental analysis of MgO after aging by NaOH.

Element	Na	Al	Fe	Ca	Ni	Co	Cr	Mn	S
content (%)	0.051	0.0067	0.0010	0.0060	0.0002	0.0003	<0.0001	0.0001	0.20

and

Table 5. The total element analysis of MgO after calcination of 4MgCO₃·Mg(OH)₂·4H₂O.

Element	Al	Fe	Ca	Ni	Co	Cr	Mn	S
content (%)	0.0044	0.0019	0.0007	0.0010	0.0017	0.0002	0.0015	0.46

. As shown in Table 3, Mg(OH)₂ is not aged by NaOH and the content of S in the MgO is 1.55%. The content of the other impurity elements is very low. The XRD pattern of MgO is shown in Figure 7, the peaks of the synthesized MgO are basically the same as those of standard MgO (PDF 71-1176), except for the peaks at angles of 24°–26°. When Mg(OH)₂ is calcinated at 1200 °C, the two peaks disappear. It is suggested that those peaks should be from MgSO₄, which conforms to the high content of S. When Mg(OH)₂ is aged by NaOH, the content of S decreased to 0.28%, and the purity of Mg(OH)₂ reaches 98.48%. Correspondingly, the S content in MgO obtained through the calcination of Mg(OH)₂, shown in Table 4, is 0.2%, which can satisfy the industrial high purity MgO standard. The MgO obtained by calcination of 4MgCO₃·Mg(OH)₂·4H₂O has an S content of 0.46%, as shown in Table 5. The other impurity elements have very low content, indicating that the MgO is high purity.

4.2. SEM of Mg(OH)₂, 4MgCO₃·Mg(OH)₂·4H₂O

The morphologies of Mg(OH)₂, 4MgCO₃·Mg(OH)₂·4H₂O and MgO are observed by SEM, which are shown in Figure 8 and Figure 9. The Mg(OH)₂ crystal has the shape of the hexagonal sheet, with a thickness of less than 100 nm. Those sheets agglomerate into a big spherical particle with petals. The sample takes the shape of spherical petals. MgO shows a similar image to Mg(OH)₂. As shown in Figure 9, the morphology of 4MgCO₃·Mg(OH)₂·4H₂O is a sheet with a rough surface. The sheet-shaped crystal also agglomerates into a sphere particle. The calcination of 4MgCO₃·Mg(OH)₂·4H₂O releases H₂O and CO₂, and the sphere particle cracks and turns into lamellar aggregates.

4.3. N₂ Adsorption Analysis

Figure 10 shows the N₂ adsorption-desorption isotherm and pore size distribution curve of MgO. The pore size distribution curve is calculated using the desorption branch, according to the BJH method. The two isotherms belong to the IV type in the IUPAC classification, and the adsorption amount increases gently in the low-pressure section. At this time, N₂ molecules are adsorbed on the inner surface of the pore, from a single layer to multiple layers, and the adsorption amount increases sharply when P/PO = 0.7~0.9, forming H1-type hysteric ring. This indicating that the magnesium oxide sample has a typical mesoporous structure. The specific surface area of the MgO obtained by calcination of Mg(OH)₂ is 84.934 m²/g, and that of the MgO obtained through the calcination of 4MgCO₃·Mg(OH)₂·4H₂O is 93.258 m²/g, the pore volumes are 0.676 cm³/g and 0.277 cm³/g, and the average pore diameters are 17.611 nm and 7.816 nm, respectively. Therefore, these two kinds of MgO samples have large surface areas and mesoporous structures, which are good additives for catalysis and ceramics.

5. Conclusions

The Fe, Al, Cr, Ni, Co, Mn, and Mg in the serpentine mineral are efficiently leached to the solution by a counter-current leaching process, and the leaching rate of magnesium is close to 98.35%. Through a two-stage purification process, a purified MgSO₄ solution is obtained. After oxidation by H₂O₂, Fe, Al, and Cr are removed by a precipitation method, with active MgO as the precipitant. Mn, Ni, and Co are precipitated by Na₂S. Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O are synthesized by a two-stage precipitation of the purified MgSO₄ solution. The total precipitation percentage of Mg is 96.3%. The total Mg recovery rate is 92.8%. Element sulfur enters into Mg(OH)₂ in the form of MgSO₄·5Mg(OH)₂·3H₂O. After aging in 0.25 mol/L NaOH solution for 2 h, the content of the S element in Mg(OH)₂ decreases from 1.25% to 0.28%. The purity of Mg(OH)₂ reaches 98.48%. The specific surface areas of MgO obtained through the calcination of Mg(OH)₂ and 4MgCO₃·Mg(OH)₂·4H₂O are 84.63 m²/g and 93.26 m²/g, respectively. This process provides a reference for the comprehensive utilization of serpentine magnesium resources and improves the added value of magnesium products.