Preparation of Lamellar Mg(OH)2 with Caustic Calcined Magnesia

Aug 30, 2013 - A process using plentiful and inexpensive caustic calcined magnesia (CCM; main component is MgO) to prepare lamellar MH of high purity ...
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Preparation of Lamellar Mg(OH)2 with Caustic Calcined Magnesia through Apparent Hydration of MgO Hongfan Guo,†,‡ Jiayang Xie,† Han Hu,† Xue Li,†,‡ Tianbo Fan,†,‡ He Nan,† and Yunyi Liu*,†,‡ †

College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, P. R. China Key Laboratory of Applied Technology for Chemical Engineering of Liaoning Province, Shenyang 110142, P. R. China

‡

ABSTRACT: A process using plentiful and inexpensive caustic calcined magnesia (CCM; main component is MgO) to prepare lamellar MH of high purity is described. The process begins with the reaction of MgO in CCM with NH4NO3 at ∼100 °C to produce ammonia gas and Mg(NO3)2 solution and the subsequent filtration of Mg(NO3)2 solution to remove impurities (e.g., SiO2). Then, the obtained ammonia is introduced into the Mg(NO3)2 solution to produce MH precipitate and NH4NO3. After the MH precipitate is separated by filtration, the filtrate containing NH4NO3 is returned to the initial step. The overall reaction of this process is the hydration of MgO to MH. Therefore, this process is called the apparent-hydration method. This apparenthydration method overcomes the disadvantages of the true hydration method, such as the inability to remove impurities. A crucial finding is that, because of the presence of trace SO42− in the obtained Mg(NO3)2 solution, a MH product with a narrow particle size distribution cannot be produced. After removal of the SO42− with Ba(NO3)2, lamellar MH with a narrow particle size distribution can be synthesized at 120 °C in one step. This work provides a promising route to the use of CCM to prepare MH. flow contains two main units, namely, an ammonia-evaporation unit and a MH-precipitation (ammonia-adsorption) unit (see Figure 1). In the first unit, MgO in the CCM reacts with (NH4)2SO4 at 98−115 °C to produce ammonia gas and MgSO4 solution. After the reaction solution has been filtered to remove insoluble impurities, such as Fe2O3 and SiO2, refined MgSO4 solution is obtained. In the second unit, the refined MgSO4 solution is introduced into a bubbling reactor. The ammonia gas obtained in the first unit bubbles through the bubbling reactor to react with MgSO4 to produce MH precipitate and (NH4)2SO4. The MH precipitate is separated from the reaction solution by filtration as the MH product. The filtrate containing (NH4)2SO4 is returned to the first unit as the circulation medium, and thus (NH4)2SO4 or ammonia is recycled, rather than being emitted. The overall reaction of this process is the hydration of MgO [MgO + H2O → Mg(OH)2↓], but MH is actually produced by the reaction of MgSO4 with ammonia. No external base as the precipitant of Mg2+, such as NaOH or Ca(OH)2, is introduced. Using NaOH or Ca(OH)2 not only produces Na- or Ca-containing byproducts, but also contaminates the MH product. Even industrial aqua ammonia also contains many kinds of impurities. Therefore, the recycling of (NH4)2SO4 also improves the purity of the MH product. Based on laboratory-scale and pilot-scale experiments, we realize a production scale of 6000 tons/year per set of equipment. Despite the many advanges of the apparent-hydration method described above, the MH product prepared by using (NH4)2SO4 as the circulation medium has a very large particle size.12 This limits the application of the prepared MH. For

1. INTRODUCTION Magnesium hydroxide (MH) has a wide range of applications, such as in the neutralization of acid waste streams and treatment of flue gases (SO2) and as a fertilizer additive, adsorbent of heavy metals in wastewater, and precursor for preparing MgO.1−7 Among these various applications, its use as a smoking- and toxic-free flame retardant is particularly outstanding because of its high decomposition temperature, nontoxic and noncorrosive properties, and ability to undergo endothermic dehydration in fire conditions.2−5 Because of the rising safety standards worldwide and the increasing use of flammable materials, MH flame retardant will gain a growing market. The raw materials frequently used for preparing MH include sea bittern (MgCl2), bischofite (MgCl2), magnesite (MgCO3), and dolomite (MgCO3·CaO3).4,8−10 Magnesite (MgCO3) is a plentiful mineral wordwide that can be calcined to an inexpensive MgO product called caustic calcined magnesia (CCM; MgO content of 80−93%). Compared to the commonly used MgCl2 obtained from seawater and brine, CCM is particularly suitable for preparing chlorine-free MH. The direct hydration method [MgO + H2O → Mg(OH)2] can be used to prepare MH from CCM.3 However, this method cannot remove the impurities in the CCM (such as SiO2), so only low-end MH product is produced. Another usable method is as follows: The MgO in CCM is first converted into a soluble magnesium salt with acid, for example, into MgCl2 or MgSO4 with HCl or H2SO4, respectively, and then Mg2+ is precipitated with an alkali, such as NaOH, Ca(OH)2, or ammonia. However, the cost is obviously increased because of the use of acid. In addition, new impurities are introduced, such as Cl− from HCl and SO42− from H2SO4. To overcome these disadvangtages, we developed the apparent-hydration method to prepare MH in a large ammonia-bubbling setup designed by our group.11,12 The © 2013 American Chemical Society

Received: Revised: Accepted: Published: 13661

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Figure 1. Flowchart of the apparent-hydration method for the preparation of MH using CCM as the magnesium source and (NH4)2SO4 as the circulation medium.

applications in some fields have strict requirements on the Cl− and SO42− contents.13 This work provides an environmentally friendly process for utilizing CCM to prepare MH.

example, high-grade MH used as aflame retardant requires a small average particle size (below 2 μm) with a narrow particle size distribution. The presence of surfactant during MH precipitation can decrease the particle size from ∼14 to ∼2 μm;12 however, use of surfactant increases the cost and decreases the purity of MH. The type and amount of added surfactant also greatly affect MH precipitation so that the controllability becomes very poor. Moreover, the use of surfactant in the apparent-hydration process would prevent the (NH4)2SO4 solution produced in the MH-precipitation unit (denoted as the mother solution) from being reused in the ammonia-evaporation unit, because surfactant is very hard to separate from the mother solution so that it would be taken to the ammonia-evapration unit where it would not only increase the solubility of impurities but also froth the ammoniaevaporation reaction solution. Aside from using a surfactant, the morphology of MH can also be improved by using a two-step synthesis route: first deposition of Mg2+ with base [e.g., NaOH or Ca(OH)2] to produce MH at normal temperature and then hydrothermal treatment of the as-prepared MH at high temperature (160−220 °C) in the presence of a mineralizer (e.g., NaOH or KOH).11 Obviously, hydrothermal treatment increases the energy comsumption and introduces impurities into the MH product. Therefore, developing a surfactant-free process with mild reaction conditions is particularly meaningful. Because of the limitation of (NH4)2SO4 as a circulation medium, use of NH4NO3 as the circulation medium in the apparent-hydration method was investigated in this work. The results showed that NH4NO3 can be used in the ammoniaevaporation unit to produce ammonia and Mg(NO3)2 solution. Precipitating MH in the refined Mg(NO3)2 solution can produce lamellar MH desired for use as a flame retardant. This is very different from the precipitation of MH in the refined MgSO4 solution obtained by using (NH4)2SO4 as the circulation medium,11,12 in which intergrown flowerlike structures with large particle sizes are produced.11 However, although lamellar MH can be prepared from the refined Mg(NO3)2 solution, its particle size is very large. Control experiments showed that the trace SO42− present in the refined Mg(NO3)2 solution has a very significant negative influence on the particle size even when its concentration is as low as 0.6 mmol·L−1. After pretreatment of the refined Mg(NO3)2 solution with Ba(NO3)2 to remove SO42−, lamellar MH with a narrow particle size distribution and small average particle size (0.83 μm) can be prepared at 120 °C in one step in the absence of surfactant, not requiring a further hydrothermal treatment step at higher temperature. Using NH4NO3 as the circulation medium also provides other unique advantages. For example, if NH4Cl were used as the circulation medium, Cl− could erode the reactor badly. Moreover, if (NH4)2SO4 or NH4Cl were used as the circulation medium, Cl− or SO42− would inevitably be introduced into the MH product. High-grade MH and its

2. EXPERIMENTAL SECTION 2.1. Preparation of MH Using CCM as the Magnesium Source and NH4NO3 as the Circulation Medium. The composition of CCM used as the raw material for preparing MH is reported in Table 1. Among the components of CCM, SO42− is mainly from the coal used to calcine magnesite into CCM. Table 1. Composition of the CCM Used component

content (wt %)

component

content (wt %)

MgO SiO2 CaO

85.0 6.0 4.0

Fe2O3 SO42− H2O and others

2.0 0.2 2.8

The preparation of MH by the apparent-hydration method using CCM as the magnesium source and NH4NO3 as the circulation medium (see Figure 1) was divided into two units: an ammonia-evaporation unit and a MH-precipitation unit. In the ammonia-evaporation unit, MgO in the CCM was reacted with NH4NO3 to produce ammonia gas and Mg(NO3)2 as follows: NH4NO3 aqueous solution and CCM were added to a 1000 mL flask with a condenser. The reaction mixture was then stired at different temperatures under reflux until the ammonia-evaporation reaction ended. During the reflux, the produced ammonia gas flowed out of the flask through the condenser. After the ammonia-evaporation reaction had ended, the resulting Mg(NO3)2 solution was cooled and filtered to remove insoluble impurities, and then refined Mg(NO3)2 solution for use in the preparation of MH was obtained. The reaction occurring in this unit is MgO + 2NH4NO3 → Mg(NO3)2 + 2NH3 ↑ + 2H 2O

In the MH-precipitation unit, Mg(NO3)2 was reacted with ammonia to produce MH precipitate and NH4NO3 as follows: The refined Mg(NO3)2 solution obtained in the preceding unit was placed in an autoclave. The autoclave was then sealed, and the solution was heated to 120 °C under stirring. After that, ammonia was introduced into the autoclave from a steel cylinder. Then, the reaction solution was aged at 120 °C for a further 2 h. After the reaction mixture had cooled, the resulting MH precipitate was separated from the reaction solution by filtration. The obtained MH precipitate was washed with water and then dried in a vacuum at 100 °C for 10 h. The reaction occurring in this unit is 13662

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2.2.4. Measurement of the Ca2+ Concentration. The procedure was as follows: (1) Vx mL of Ca2+-containing solution with an unknown concentration of Ca2+ was pipetted into a vessel, and then add 20 mL of deionized water and 5 mL of triethanolamine solution were added. (2) Next, NaOH solution (100 g/L) was added dropwise until a precipitate appeared. (3) Approximately 0.1 g of calconcarboxylic acid indicator was added. (4) Then, NaOH solution was added dropwise until the color changed from blue to mauve, after which an additional 0.5 mL of NaOH solution was added in excess. (5) The solution was titrated with EDTA until the color changed from mauve to blue. Then, the concentration of Ca2+ was calculated as

Mg(NO3)2 + 2NH3 + 2H 2O → Mg(OH)2 ↓ + 2NH4NO3

2.2. Analytical Methods. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-6360LV scanning electron microscope (Tokyo, Japan) and a LEO 1530 FEG field-emission scanning electron microscope (Carl Zeiss, Oberkochen, Germany). X-ray diffraction (XRD) patterns were measured with a D8 diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation at a step length of 0.05°/s. The average particle size and the particle size distribution of MH were determined on a BT-9300S laser particle size analyzer (Dandong Bettersize Instruments Ltd., Dandong, China). Thermogravimetric/differential thermogravimetric (TG/ DTG) analysis was carried out on an EXSTAR TG/DTA 6300 instrument (SII Nanotechnology, Tokyo, Japan). The concentrations of Mg2+, NH4+, Ca2+, and SO42− ions were determined as follows:13 2.2.1. Measurement of the SO42− Concentration. The concentration of SO42− was determined by the following spectrophotometric method: (1) A certain amount of SO42−containing solution with an unknown concentration of SO42− was pipetted into a vessel, and then 0.5 mL of HCl solution and 10 mL of absolute ethanol were added. (2) Water was added until the volume reached 40 mL. (3) Next, 5 mL BaCl2 solution was added. (4) The absorbance was measured at λmax = 465 nm using a UV-9600 spectrophotometer. (5) The concentration of SO42− was determined through comparison with the standard curve for BaSO4. 2.2.2. Measurement of the Mg2+ Concentration. The concentration of Mg2+ was determined by titration with ethylenediaminetetraacetic acid (EDTA) titrant. NH3−NH4Cl solution was used as the buffer solution (pH 10), and eriochrome black T was used as the indicator. At the end point, the color changed from red to a pale blue. Then, the concentration of Mg2+ was calculated as

C Mg 2+ =

CCa 2+ =

where CEDTA is the concentration of EDTA standard solution, V is the volume of EDTA standard solution consumed during the titration, and Vx is the volume of Ca2+-containing solution whose concentration of Ca2+ needs to be measured.

3. RESULTS AND DISCUSSION 3.1. Ammonia-Evaporation Unit. MgO in the CCM can react with NH4NO3 to produce ammonia gas and Mg(NO3)2 under certain conditions, namely, the ammonia-evaporation reaction. The influences of the reaction temperature, the reaction time, and the molar ratio of NH4NO3 to MgO on the ammonia-evaporation reaction were investigated. From Figure 2, it can be seen that, as the reaction temperature increased, the

C EDTAV Vx

where CEDTA is the concentration of EDTA standard solution, V is the volume of EDTA standard solution consumed in the titration, and Vx is the volume of the Mg2+-containing solution whose concentration of Mg2+ needs to be measured. 2.2.3. Measurement of the NH4+ Concentration. The NH4+ concentration was measured as follows: (1) Vx mL of NH4+containing solution with an unknown concentration of NH4+ was pipetted into a vessel, and then 20 mL of deionized water, 10 mL of formaldehyde solution (prepared by mixing formaldehyde and water in a volume ratio of 1:1), and two drops of phenolphthalein indicator were added. (2) Three minutes later, the solution was titrated with 0.1 mol/L NaOH solution until its color became rosy. (3) When the color did not fade within 30 s, the titration was finished. Then, the concentration of NH4+ was calculated as C NH4+ =

C EDTAV Vx

Figure 2. Influence of the reaction temperature on the ammoniaevaporation reaction (reaction time = 120 min, nominal concentration of MgO = 1 mol/L, NH4NO3 mol/MgOmol = 2.2:1).

conversion of MgO to Mg2+ also increased, possibly because a high reaction temperature makes ammonia escape as a gas from the reaction solution more easily so as to accelerate the reaction toward the production of Mg(NO3)2 and ammonia gas. In this work, 105 °C was used as the ammonia-evaporation reaction temperature. Figure 3 shows the dynamic curves of the ammoniaevaporation reaction at different molar ratios of NH4NO3 to MgO. From Figure 3, it can be seen that the conversion of MgO to Mg2+ increased with increasing molar ratio of NH4NO3 to MgO. When NH4NO3 was present in 10% excess (i.e., NH4NO3 mol/MgOmol = 2.2:1), the MgO in the CCM was basically converted to Mg2+. Further increasing the molar ratio of NH4NO3 to MgO was not very effective. After 120 min, the

C NaOHV Vx

where CNaOH is the concentration of NaOH standard solution, V is the volume of NaOH standard solution consumed during the titration, and Vx is the volume of the NH4+-containing solution whose concentration of NH4+ needs to be measured. 13663

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could be indexed as the hexagonal structure of MH with lattice constants comparable to the values for JCPDS 7-239.14−22 No peaks from other phases can be observed. As the synthesis temperature increased, the intensity ratio of reflection (001) to reflection (110), that is, I001/I101, increased, indicating preferential orientation toward the (001) plane. The (001) plane has a weaker polarity than the (110) plane does. The increased I001/I101 ratio indicates that improving synthesis temperature can decrease the surface polarity and the internal microstress of the MH crystal to make the structure more stable. The SEM results show that, as the synthesis temperature increased, the morphology of MH changed toward hexagonal lamellae (Figure 5). The MH prepared at 120 °C had a Figure 3. Dynamic curves of the ammonia-evaporation reaction at different molar ratios of NH4NO3 to MgO (reaction temperature = 105 °C, nominal concentration of MgO = 1 mol/L).

concentration of Mg2+ essentially did not change. Thus, the ammonia-evaporation reaction conditions used in this work were 105 °C, 120 min, and NH4NO3 mol/MgOmol = 2.2:1, under which the concentration of Mg2+ can reach ∼0.85 mol/L. After the ammonia-evaporation reaction, the reaction mixture was filtered. The obtained Mg(NO3)2-containing filtrate (i.e., refined Mg(NO3)2 solution) was used for the precipitation of MH, whereas the solid filter residue containing magnesium can be processed into magnesium fertilizer. 3.2. Studies of the Properties of the Precipitation of MH Using Mg(NO3)2 as the Magnesium Source. In the obtained refined Mg(NO3)2 solution, in addition to Mg2+ and NO3−, ∼0.5 mol·L−1 NH4+, 1.0 mmol·L−1 SO42−, and 0.02 mol· L−1 Ca2+ were also obtained. Before using the refined Mg(NO3)2 solution to synthesize MH, the properties of the precipitation of MH using Mg(NO3)2 as the magnesium source were investigated. First, the properties of the precipitation of MH using Mg(NO3)2 as the magnesium source were studied with pure Mg(NO3)2 solution. Figure 4 shows the XRD patterns of the MH products prepared with pure Mg(NO3)2 at different synthesis temperatures. All diffraction peaks of each sample

Figure 5. SEM images of the MH products obtained using pure Mg(NO3)2 as the magnesium source and synthesized at (a) 75, (b) 100, (c) 120, and (d) 140 °C.

morphology similar to that of the MH prepared at 140 °C. On the basis of these results combined with those from XRD shown in Figure 4, 120 °C was used as the reaction temperature for MH precipitation in this work. In contrast to the pure Mg(NO3)2 solution, the refined Mg(NO3)2 solution contained, in addition to Mg2+ and NO3−, NH4+, SO42−, and Ca2+ ions as well. When using the refined Mg(NO3)2 solution to prepare MH, the presence of NH4+, SO42−, and Ca2+ ions can affect the particle size of the produced MH. To elucidate the influence of these ions on the particle size of MH, NH4+-, Ca2+-, and SO42−-containing Mg(NO3)2 solutions were separately used to prepare MH, respectively. NH4+-, Ca2+-, and SO42−-containing Mg(NO3)2 solutions were prepared by adding certain amounts of NH4NO3, Ca(NO3)2, and MgSO4, respectively, to pure Mg(NO3)2 solution. Panels a and b of Figure 6 show the influences of NH4+ and Ca2+, respectively, on the particle size of MH product. As shown in Figure 6a, as the NH4+ concentration increased from 0 to 0.8 mol·L−1, the average particle size of MH changed between 0.85 and 1.00 μm, and as shown in Figure 6b, as the Ca2+ concentration increased from 0 to 0.3 mol·L−1, the average particle size of MH increased from 0.85 to 1.12 μm. These results indicate that neither NH4+ nor Ca2+ affects the particle size of MH greatly within the studied concentration ranges. The influence of SO42− on the particle size of the prepared MH is shown in Figure 7. As can be seen, when the

Figure 4. XRD patterns of the MH products obtained using pure Mg(NO3)2 as the magnesium source and synthesized at (a) 75, (b) 100, (c) 120, and (d) 140 °C. 13664

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Figure 6. Effects of the concentrations of (a) NH4+ and (b) Ca2+ on the particle size of the MH product.

Figure 7. Influence of the concentration of SO42− on the particle size of the MH product.

SO42−

−1

Figure 8. XRD pattern of the product obtained using MgSO4 as the magnesium source (reaction temperature = 120 °C).

SO42−

concentration of was below 0.2 mmol·L , did not affect the particle size greatly; however, when the concentration of SO42− increased from 0.2 to 3.0 mmol·L−1, the average particle size of MH increased markedly from 1.30 to 12.8 μm, indicating that SO42− affects the particle size of MH significantly when the concentration of SO42− is more than 0.2 mmol·L−1. The crystal structure of crystalline MH is a layered CdI2-type arrangement with successive hexagonal Mg2+ ion layers and OH− ion layers stacked on each other.6,7 The Mg2+ ion is 6-fold-coordinated by OH− to form a Mg(OH)6 octahedron. Such a layered crystal structure is an advantage for platelet-shaped crystallization of the compound. The presence of SO42−, even a trace amount, can increase the particle size of MH when Mg(NO3)2 is used as the magnesium source, possibly because, during the growth of the MH crystal, a SO42− ion, a bivalent anion, can interact simultaneously with the Mg2+ ions of different growth units consisting of Mg(OH)6 octahedra and plays a bridging role in promoting the combination of different growth units so that the particle size of MH is increased. Indeed, the precipitation of MgSO4 is different from that of Mg(NO3)2. As shown in Figure 8, when MgSO4 replaces pure Mg(NO3)2 as the magnesium source, the precipitate obtained under the same reaction conditions is magnesium oxysulfate [MgSO4·5Mg(OH)2·3H2O],23−26 not pure MH. Moreover, when using pure MgSO4 as the magnesium source, the obtained MH particles were also easily subjected to agglomeration and intergrowth as compared to that observed using pure MgCl2 or Mg(NO3)2 as the magnesium source.7,11,27 Henrist et al.7,27 suggested that the change in the supersaturation level by sulfate ions is an important reason, probably because of the weak base property of sulfate ions.

3.3. Removal of SO42− from the Refined Mg(NO3)2 Solution. A concentration of 1.0 mmol·L−1 SO42− was present in the refined Mg(NO3)2 solution. The above studies showed that the presence of SO42− can significantly increase the particle size of MH when Mg(NO3)2 is used as the magnesium source, so before the precipitation of MH, the removal of SO42− from the refined Mg(NO3)2 solution was performed. Several methods are commonly used for the removal of SO42−,28,29 such as membrane filtration, the CaCl2 method, and the barium method. Membrane filtration is expensive and energyconsuming, and the membrane can be poisoned by impurities.28,29 The CaCl2 method is simple and inexpensive; however, it not only introduces new impurities, but also cannot remove SO42− thoroughly because of the relatively high solubility of CaSO4 (694 mg L−1 at 25 °C).28 Thus, the barium method was used in this work. The barium method mainly uses BaCO3 or BaCl2 as the agent for removing SO42−. To avoid introducing a new impurity, Ba(NO3)2 was investigated as the removal agent in this work. The influence of reaction conditions on the removal of SO42− was studied. As shown in Figure 9, ∼90% of of the SO42− was removed within in 20 min, which is attributed to the fact that BaSO4 is almost insoluble in water (solubility ≈ 3 mg·L−1).28 Further extending the reaction time does not increase the removal of SO42− markedly. When the reaction temperature and time were fixed at 50 °C and 20 min, respectively, increasing the Ba(NO3)2 mol/SO42−mol ratio from 0.8:1 to 1:1 significantly increased the removal of SO42− from ∼65% to ∼90% (Figure 10). Further increasing the amount of added Ba(NO3)2 did not give a marked enhancement of the removal 13665

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SO42− used as the magnesium source, the prepared MH product is denoted as Presence-SO42-MH. As shown in Figure 11, the particle size distribution of Presence-SO42-MH was very abroad. In addition, the average particle size was also very large (3.42 μm). In contrast, for the refined Mg(NO3)2 solution with the removal of SO42− used as the magnesium source, the prepared MH product is denoted as Absence-SO42-MH. Compared to Presence-SO42-MH (Figure 11), Absence-SO42-MH shows a much narrower particle size distribution (Figure 12). The average particle size of Absence-SO42-MH (0.83 μm) was also much smaller than that of Presence-SO42-MH (3.42 μm). In the XRD pattern of Absence-SO42-MH (Figure 13a), all of the diffraction peaks can be indexed as the hexagonal structure of MH, with lattice constants comparable to the values of JCPDS 7-239.14−22 No peaks from other phases can be observed. Composition analysis13 showed that the purtity of MH was >99%. The I001/I101 ratio in the XRD patterns shown in Figure 13a is 1.23, indicating that Absence-SO42-MH had a low surface polarity. Figure 13b shows that Absence-SO42-MH was lamellar. The thermal properties of the MH products were investigated by TG analysis. The relevant TG profiles are presented in Figure 14. Both Presence-SO42-MH and AbsenceSO42-MH exhibited a pronounced weight-loss step from ∼320 to 415 °C that is attributed to the decomposition of MH to MgO and H2O.30−35 Absence-SO42-MH started this weight-loss step at 327 °C (curve II in Figure 14), ∼15 °C higher than Presence-SO42-MH did (curve I in Figure 14). For both samples, the observed weight loss for the MH → MgO transformation (∼27%) was slightly lower than the theoretical weight loss (30.87%). The difference is ascribed to the incompleteness of the decomposition reaction in this temperature range and indicates complete crystallization and less deficiency in the crystal lattice, which is consistent with previous reports.30,31 After MH precipitation, the filtrate containing NH4NO3 (denoted as the mother solution) was reused in the initial ammonia-evaporation unit. After the mother solution had been reused five times, the contents of Fe, SO42−, and other impurities basically did not change; only the content of Ca2+ increased gradually. When the Ca2+ content becomes too high and can affect the purity of the MH product, the influence of Ca2+ can be eliminated easily by adding ammonium bicarbonate to the mother solution.

Figure 9. Influence of the reaction time on the removal of SO42− [Ba(NO3)2 mol/SO42−mol = 1:1, reaction temperature = 50 °C].

Figure 10. Influence of the amount of Ba(NO3)2 added on the removal of SO42− (reaction conditions: 50 °C and 20 min).

of SO42−. In addition, adding excess Ba(NO3)2 would introduces barium as an impurity in the MH product. Thus, the reaction conditions used to remove SO42− from refined Mg(NO3)2 solution in this work are 20 min and SO42−mol/ Ba(NO3)2 mol = 1:1, under which the concentration of SO42− is far below 0.2 mmol·L−1. 3.4. MH-Precipitation Unit: Precipitation of Lamellar MH Using Refined Mg(NO3)2 Solution as the Magnesium Source in One Step. Based on the preceding studies and observations, the refined Mg(NO3)2 solution without and with the removal of SO42− by Ba(NO3)2 were investigated as the magnesium source for the preparation of MH. According to the studies mentioned in section 3.2, MH was prepared at 120 °C. For the refined Mg(NO3)2 solution without the removal of

Figure 11. Particle size distribution of the MH prepared from refined Mg(NO3)2 solution without the removal of SO42− (denoted as Presence-SO42MH). 13666

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Figure 12. Particle size distribution of the MH prepared from refined Mg(NO3)2 solution with the removal of SO42− (denoted as Absence-SO42MH).

Figure 13. (a) XRD pattern and (b) SEM image of Absence-SO42-MH (scale bar in the image = 1 μm).

Figure 14. (a) TG and (b) DTG curves of (I) Presence-SO42-MH and (II) Absence-SO42-MH.

precipitation of MH. Without the removal of SO42−, the particle size distribution of the prepared MH is very board, and the average particle size is also very large (3.42 μm). Ba(NO3)2 can serve as a suitable agent for removing SO42−. After the removal of SO42− by Ba(NO3)2, lamellar MH with a narrow particle size distribution and small average particle size (0.83 μm) can also be synthesized at 120 °C in one step in the absence of surfactant, and further hydrothermal treatment at higher temperature is not required. The apparent-hydration method overcomes the disadvantages of the real hydration method of MgO, such as the inability to remove impurities. It also overcomes several disadvantages of the process using MgCl2 (obtained from seawater or brine) as the raw material, such as introducing Cl− into the product MH, eroding the reactor with Cl− especially at high temperature, and producing chloride byproducts (NaCl, CaCl2, or NH4Cl according the used alkline precipitant). In addition to providing

4. CONCLUSIONS In summary, a process for preparing lamellar MH using CCM as the raw material has been developed. The overall reaction of this process is the hydration of MgO to MH, and thus, this process is called the apparent-hydration method. The actual reactions of the apparent-hydration method are as follows: First, the MgO in the CCM reacts with NH4NO3 at 105 °C to produce ammonia gas and Mg(NO3)2 solution. After the resulting Mg(NO3)2 solution has been filtered to remove insoluble impuriries, such as SiO2, the ammonia obtained previously is returned to the Mg(NO3)2 solution and reacts with Mg(NO3)2 to produce MH precipitate and NH4NO3. The MH precipitate is separated by filtration. The NH4NO3containing filtrate is sent back to the initial unit, and thus ammonia evaporation and adsorption form a circle without the emission of ammonia or NH4NO3. The removal of trace SO42− present in the obtained Mg(NO3)2 solution is crucial for the 13667

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an environmentally friendly process for preparing MH, this work also provides a route to purify CCM to produce MgO of high purity, because MH is often used to prepare MgO by calcination. Studies on the preparation of MgO by the apparent-hydration method are ongoing.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 02489383760. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Program for Liaoning Innovative Research Team in University (LNIRT; Project LT2013010) and the Education Administration of Liaoning Province (Project L2012143) is gratefully acknowledged. Dandong Jinyuan Magnesium Industry Co., Ltd., and Liaoning Yew Magnesium Chemical Co., Ltd., are also thanked for their assistance in this work.

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REFERENCES

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dx.doi.org/10.1021/ie401669g | Ind. Eng. Chem. Res. 2013, 52, 13661−13668