**4. Conclusions**

A facile mechanochemical method for the synthesis of Mg-Al, Ca-Al, Zn-Al, and Cu-Al LDH was successfully developed. Implementation of ageing resulted in more complete conversion of raw materials to LDH product. Starting materials selected were a mixture of oxides, hydroxides, and basic carbonates, promoting 'green' synthesis of LDH materials. The synthesis method eliminates the use of pH control and avoids the production of salt rich effluent. The selected mill offers potential for commercial up-scaling with a variety of iso mills available, capable of processing large volumes of liquid reagent. Mill operation is versatile offering the potential for batch, semi-batch, or continuous synthesis of LDH materials. Traditional methods have predominantly made use of ball mills, mixer mills, or a mortar and pestle [2]. Wet milling allows for ease of transfer of reagents to downstream processes with little to no process interruption. Inclusion of the ageing step has allowed for an increase in conversion of raw materials to LDH product and improved sample morphology. This offers potential for converting the one-step wet mechanochemical route to that including a second step. Previous studies conducted have indicated that a grinding step is necessary for LDH formation to occur more readily [9,17,18]. The grinding activity allows for mechanochemical activation of raw materials, creating active sites. The selected horizontal bead mill has successfully resulted in the

formation of multiple types of LDH product; however, raw material peaks were still observably present under the selected synthesis conditions. Samples S1, S2, S3, and S5 resulted in successful LDH formation with high intensity peaks observed after ageing. Samples S4 resulted in LDH formation prior to ageing; however, katoite formation occurred during the ageing step. The presence of a carbonate source was therefore necessary for LDH formation to occur and continue successfully. Cu-Al LDH did not form readily, with no clear indication of its presence prior to ageing. LDH peaks were observed to have formed upon ageing with precursor materials still notably present. The mill therefore o ffers potential for a successful one-step wet mechanochemical technique. Further investigation of a change in milling parameters such as retention time, jacket water temperature, bead size, solids loading and bead loading is warranted.

**Author Contributions:** B.A.B.: Methodology, Formal Analysis, Investigation, Data Curation, Writing—Original Draft and Final Version, Visualization, Conceptualisation. F.J.W.J.L.: Supervision, Project administration, Funding Acquisition, Writing—Review and Editing, Resources, Conceptualisation, Moral support. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research project was funded by TeckSparks Pty Ltd. and THRIP.

**Conflicts of Interest:** The authors declare no conflict of interest.
