Synthesis of red mud derived M-type barium hexaferrites with tuneable coercivity


Hexagonal ferrites can be employed in a multitude of applications, the most common hexaferrites are the M ferrites such as BaFe12O19 (barium hexaferrite, BaM). It is known that if Fe3+ is substituted with a combination of Ti4+/Co2+ the coercivity of BaM can be reduced to produce soft M ferrites with easily switchable magnetisation. They can be utilised as powders, films or bulk ceramics, and can be manufactured from a wide variety of synthesis methods. The production of hexaferrites usually requires commercial raw materials, but if an industrial waste can be utilised, this will help to ease waste disposal and storage costs, valorise a waste material and encourage circular economy. In this study, bauxite residue (red mud) from the production of alumina was used to synthesise M-type hexaferrites, using a simple ceramic process. BaCO3, or BaCO3 + Co3O4, were added to the red mud, blended and heated at 1000 degrees C to produce the M-type hexaferrites. Without cobalt addition up to 81.1 wt% M ferrite was produced, and with Co addition up to 74.3 wt% M ferrite was formed. Without cobalt, the M ferrite phase closely resembled BaFe9Al3O19, and was a hard ferrite with a magnetisation of 12-19 A m(2)/kg for the whole powder (up to 23.6 A m(2)/kg for the M ferrite phase) and a coercivity of similar to 290 kA/m. When cobalt was added, secondary titanate phases vanished, and Ti4+/Co2+ partially substituted very soft M ferrite was formed with a low coercivity of similar to 16 kA/m but a higher magnetisation of 24.5 A m(2)/kg for the whole powder (up to 34.9 A m(2)/kg for the M ferrite phase). Therefore, not only can good quality magnetic materials be easily produced from this common waste material, but its magnetic properties can be tuned by varying the 2 + ions added during the process.



subject category

Materials Science


Carvalheiras, J; Novais, RM; Mohseni, F; Amaral, JS; Seabra, MP; Labrincha, JA; Pullar, RC

our authors


This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES. R.C. Pullar wishes to thank FCT grant IF/00681/2015, and R.M. Novais wished to thank FCT grant CEECIND/00335/2017.

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