Gaseous ammonia: superior to aqua ammonia in the precipitation of Mg(OH)₂ under mild conditions

Abstract

Aqua ammonia (AA) is frequently used to precipitate various metal hydroxide or oxide particles. Here, we replace AA with gaseous ammonia (GA) to precipitate Mg(OH)₂ particles in MgCl₂ aqueous solution under mild conditions. The results show that GA can give a higher consistency in the growth of Mg(OH)₂ particles although both AA and GA essentially produce Mg(OH)₂ by the reaction of Mg²⁺ with OH⁻. This work provides results that suggest using ammonia as a precipitant.

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Introduction

Aqua ammonia (AA), also known as ammonium hydroxide (NH₄OH), is frequently used as an alkaline precipitant, because (1) its relatively weak basicity makes the reaction mild and easily controlled; (2) the residual ammonia in the product can be removed easily by drying; (3) the purity is high, particularly when compared to Ca(OH)₂. AA has exhibited the noticeable ability to synthesize ZrO₂ nanoparticles,¹ γ-Fe₂O₃@SiO₂ (SiO₂-coated γ-Fe₂O₃),² Ag@carbon@silica,³ Fe₃O₄ magnetic nanoparticles,⁴ iron-doped TiO₂,⁵ Co₃O₄–NiO composite,⁶ mesoporous Al–P–V–O catalysts,⁷ etc.

In industry, AA is also superior to Ca(OH)₂ and NaOH in the precipitation of magnesium hydroxide (MH), an important fine chemical with a wide range of applications,^8–11 among which the use as a smoking- and toxic-free flame-retardant filler in polymers is especially outstanding. Recent years, we developed using ammonia to precipitate MH from different magnesium sources.^8–10 From practical industrial application, we found that gaseous ammonia (GA; NH₃) is more suitable for producing uniform MH precipitates on an industrial scale than conventionally used AA (both AA and GA are referred to as ammonia in this work). After pilot-scale experiment, very recently we realized a production scale of 8000 tons per year per set of equipment using GA as the precipitant, wherein the GA is directly introduced into the Mg²⁺ solution by bubbling in a large reactor equipped with an ejector, a gas redistributor and a mechanical agitator.

Due to the above-mentioned advantage exhibited by GA and the fact that AA is widely used as a precipitant, herein GA and AA are compared by precipitating MH in MgCl₂ aqueous solution. The study is performed under mild reaction conditions (below 100 °C; normal pressure; absence of surfactant and hydrothermal conditions) which are much desired by the practical production.

Results and discussion

The comparison between GA and AA in the precipitation of MH is performed under the following conditions: both the total molar amount of the added ammonia (ammonia solute for AA) and the introducing time of ammonia were the same, which means that the adding rate of ammonia was also the same (see part one of the Experimental section of ESI† for the details). MH was first synthesized at different temperatures. The SEM results show that, at low temperature (below 55 °C), both GA (Fig. S1a and S1b†) and AA (Fig. S1c and S1d†) produce irregular MH particles with bad aggregation. This limits the application of the prepared MH. For example, high-grade MH used as flame retardant requires that the average particle size is below 2 μm, narrow particle size distribution and decreased micro internal stress being preferred.

As the reaction temperature is increased to 75 °C, the morphology becomes much more uniform (Fig. 1). It should be noted that all the reactions were performed in the absence of surfactant and hydrothermal conditions. When the reaction temperature is further increased to 90 °C, not only the particle size becomes much larger (Fig. S2†), but also the conversion of Mg²⁺ into MH becomes lower (Fig. S3†).

Fig. 1 SEM images of the MH products precipitated with (a) GA and (b) 25.0 wt% AA at 75 °C (scale bar in the images = 1 μm).

Although 75 °C is the optimum reaction temperature for both GA and AA routes, relatively, GA route produces a more uniform morphology (Fig. 1). Moreover, GA route gives a smaller average particle size and a much narrower particle size distribution (the detailed data will be shown and further discussed later). These results demonstrate a relatively high consistency in the growth of MH particles by GA. The MH produced by GA also shows a higher intensity ratio of reflection (001) to reflection (101), I₀₀₁/I₁₀₁, than the MH produced by AA (Fig. 2),^12–14 indicating an increased orientation toward (001) plane.^12,13 The (001) plane is a non-polar plane, while the (101) plane is a polar plane.¹³ The increased I₀₀₁/I₁₀₁ ratio indicates that using GA as the precipitant can decrease the surface polarity and the internal microstress of the MH crystal to make the structure more stable.

Fig. 2 XRD patterns of the MH products precipitated with (a) GA and (b) AA (reaction temperature: 75 °C; concentration of AA: 25.0 wt%).

In the GA route, the reactions are as follows:

NH₃(g) + H₂O(l) → NH₄OH(aq) ΔH = −34.5 kJ mol⁻¹ (1)

MgCl₂(aq) + 2NH₄OH(aq) → Mg(OH)₂(s) + 2NH₄Cl(aq) ΔH = −178.1 kJ mol⁻¹ (2)

Reaction (1) can be approximately expressed by the solubility of ammonia in water. According to the expression of solubility put forward by Vuuren et al. (eqn (3))¹⁵ and the solubility of ammonia at normal pressure,¹⁶ the expression of the solubility of ammonia at normal pressure can be attained (eqn (4)). From the corresponding curve of eqn (4) (Fig. S4†), it can be seen that the solubility of ammonia decreases markedly as the temperature increases. For example, the solubility is ∼53 wt% at 20 °C; it decreases fast to ∼9 wt% at 75 °C, and just ∼3 wt% at 90 °C. In the AA route, only reaction (2) happens, i.e., the formation of MH by the metathesis reaction between MgCl₂ and NH₄OH. However, both reaction (1) and (2) are exothermic. Thus, a low reaction temperature favours the nucleation and the growth of MH particles, leading to bad aggregation and intergrowth (Fig. S1†).

where S is solubility, A is pre-exponential factor, ΔH is the heat of solution, and T is temperature.

The difference between GA route and AA route mainly lies in the ammonia state when ammonia enters reaction solution. To elucidate the effect of ammonia state, MH was precipitated using the AA with different concentrations, respectively. Whatever the concentration of AA is used, both the total amount of the added ammonia (ammonia solute) and the introducing time of ammonia are kept unchanged and are the same as those in the GA route (see part one of Experimental section of ESI†). The SEM results (Fig. 3) show that as the concentration of AA increases, the morphology is gradually improved (see Fig. S5† for the SEM images with the same magnification of various MH products prepared by GA and AA). At low concentrations, e.g., 8.3 wt%, the MH particles intergrow and aggregate badly (Fig. 3a). While at high concentrations, e.g., 37.5 wt%, the consistency of the MH particles is markedly improved (Fig. 3d). Fig. 4 shows that GA produces the narrowest particle size distribution; the lower the concentration of AA is, the broader the particle size distribution is; the higher the concentration of AA is, the smaller the difference in particle size distribution between AA route and GA route is. Fig. 4 is consistent with Fig. S5.†

Fig. 3 SEM images of the MH prepared at 75 °C by (a) 8.3 wt% AA, (b) 16.7 wt% AA, (c) 25.0 wt% AA and (d) 37.5 wt% AA (scale bar = 1 μm in Fig. 3a and b; scale bar = 2 μm in Fig. 3c and d).

Fig. 4 Particle size distribution of the MH products prepared with different ammonia precipitants.

In the AA route, only reaction (2) happens, but the essence of the production of MH is actually the same as in the GA route, i.e., by the reaction of Mg²⁺ ions with the OH⁻ ions from NH₄OH. MH crystallizes with Mg(OH)₆⁴⁻ octahedron as the growth unit.^17,18 When AA is used as the precipitant, the local concentrations of OH⁻, Mg²⁺ or Mg(OH)₆⁴⁻ in some positions can be fluctuated by the water in the continuously introduced AA so that individual MH particles cannot grow equally and some MH particles possess relatively high growth rate. Possibly, this is a main reason for the inferiority of AA to GA. The higher the concentration of AA is, the less the water taken into the reaction solution is. GA route probably can be considered as the limit of AA route (the concentration of AA is 100 wt%). Different from AA route, GA route can avoid the big fluctuation in the concentrations of OH⁻, Mg²⁺ or Mg(OH)₆⁴⁻. Thus, individual MH particles tend to grow uniformly.

Conclusions

In conclusion, GA is a better precipitant for MH than AA is, especially when the concentration of AA is low. As the concentration of AA increases, the particle size distribution of the MH produced by AA is gradually improved toward that of the MH produced by GA. GA route probably can be considered as the limit of AA route (i.e., the concentration of AA is 100 wt%). The inferiority of AA to GA as a precipitant is possibly because in the AA route the local concentrations of OH⁻, Mg²⁺ or Mg(OH)₆⁴⁻ are fluctuated by the water in the introduced AA.

Moreover, industrial AA may contain some impurity ions, while GA doesn't contain impurity ions. Thus we speculate that AA is also more suitable for precipitating the products with high purity.

Acknowledgements

Financial support from the Program for Liaoning Innovative Research Team in University (LNIRT; Project no. LT2013010) 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.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra02998d

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