The Role of Operating Conditions in the Precipitation of Magnesium Hydroxide Hexagonal Platelets Using NaOH Solutions

Magnesium hydroxide, Mg(OH)2, is an inorganic compound extensively employed in several industrial sectors. Nowadays, it is mostly produced from magnesium-rich minerals. Nevertheless, magnesium-rich solutions, such as natural and industrial brines, could prove to be a great treasure. In this work, synthetic magnesium chloride and sodium hydroxide (NaOH) solutions were used to recover Mg(OH)2 by reactive crystallization. A detailed experimental campaign was conducted aiming at producing grown Mg(OH)2 hexagonal platelets. Experiments were carried out in a stirred tank crystallizer operated in single- and double-feed configurations. In the single-feed configuration, globular and nanoflakes primary particles were obtained, as always reported in the literature when NaOH is used as a precipitant. However, these products are not complying with flame-retardant applications that require large hexagonal Mg(OH)2 platelets. This work suggests an effective precipitation strategy to favor crystal growth while, at the same time, limiting the nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)2 hexagonal platelets. The influence of reactant flow rates, reactant concentrations, and reaction temperature was analyzed. Scanning electron microscopy (SEM) pictures were also taken to investigate the morphology of Mg(OH)2 crystals. The proposed precipitation strategy paves the road to satisfy flame-retardant market requirements.


S.1 ENLARGED SEM PICTURES
PSDs obtained by the two equipment are almost overlapping.However, the SH well characterizes particles of bigger sizes, while the ZN, particles sizes lower than 1 μm.These results are expected due to the different particle size range of the two adopted techniques.Table S1 reports the d(0,1), d(0,5) and d(0,9) for the above discussed cases.

S.3 SEM MICROGRAPHS
Two SEM micrographs at different locations of the same sample were provided for Cases 1.f1, 1, 3 and 3.f4 to confirm the observations discussed in the main manuscript.Figures S6, S7

S.4 REPEATABILITY AND REPRODUCIBILITY
The repeatability and reproducibility of the experimental results were addressed by considering Case 3. Concerning the repeatability, Figure S10 shows PSDs collected before PAA-US, 5 measurements using Sympatec HELOS granulometer, and after PAA-US, 3 measurements using Zetasizer.Measurements were carried out two times on fresh Mg(OH)2 suspensions (produced 3 weeks after).PSDs very well overlap each other, indicating high repeatability of the experimental data.
Test Table S2.Characteristic diameters, i.e. d(0.1), d(0.5) and d(0.9), obtained from PSDs measured employing Sympatec HELOS (SH) and Zetasizer Nano (ZN) for Tests 1 and 2 of Case 3 before and after PAA-US.The COV was calculated as the ratio between the average diameter and its standard deviation among each measurement.
The coefficient of variation (COV) of the characteristic diameters is always below 6 % showing an excellent repeatability of the experimental data.SEM images confirmed the high reproducibility of the experimental data.

Figure
Figure S1 reports enlarged SEM images of Cases 1 (a) and 1.f1 (b) in single-feed configuration.

Figure
Figure S4 reports enlarged SEM images of Cases 3 (a) and 3.t2 (b) in double-feed configuration.

Figure S6 .
Figure S6.SEM pictures collected for Case 1.f1 in two different sites of the same sample.MgCl2

Figure S7 .
Figure S7.SEM pictures collected for Case 1 in two different sites of the same sample.MgCl2

Figure S8 .
Figure S8.SEM pictures collected for Case 3 in two different sites of the same sample.Feeds flow

Figure S9 .
Figure S9.SEM pictures collected for Case 3.f4 in two different sites of the sample.Feeds flow

Figure S10 .
Figure S10.Repeatability analysis.On the left, 5 measurements (SH) before PAA-US; on the

Figure
Figure S10 and data inTable S2 also clearly show excellent reproducibility of the experimental

Figure S11 .
Figure S11.Reproducibility analysis on Case 3 conducted at two different times: Test 1 and Test

Figure S12 .
Figure S12.Reproducibility analysis on Case 3 replicates: SEM images of (a) Case 3-Test 1 and