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

Abstract

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 °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 m²/kg for the whole powder (up to 23.6 A m²/kg for the M ferrite phase) and a coercivity of ~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 ~16 kA/m but a higher magnetisation of 24.5 A m²/kg for the whole powder (up to 34.9 A m²/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.

Introduction

Iron-rich bauxite waste, red mud, is a well-known waste from alumina production by the Bayer process, consisting mainly of Fe, Al, Ti, Na and Si oxides. It is estimated that up to 1.5 tonnes of red mud are generated to produce 1 tonne of alumina [[1], [2], [3]], in an extremely polluting process, resulting in a global red mud stockpile of around 4 Gt and it is expected that the amount of this waste will increase by a further 146 million tonnes every year [4]. Red mud is considered a hazardous material, because of the toxic metals present, and the Bayer process uses large quantities of sodium hydroxide, making bauxite wastes extremely alkaline. Before the 1970's this type of waste was dumped directly into the sea or stored in land reservoirs [3,5], which is clearly unsustainable and raises severe environmental problems. Consequently, there is currently a great deal of interest in, and a need for, the reuse and valorisation of this waste stream. As a result of the failure of red mud dams/reservoirs, tragic accidents have occurred in the recent past in Hungry and China, drawing even more attention to the necessity to recycle such wastes [6].

In recent years a paradigm change in the way wastes are viewed, from unwanted by-products to precious raw materials, has led to extensive research attempting to reuse red mud, including applications as red mud as colouring agent in glazes [7,8], as fine aggregate and as aluminosilicate source in concretes and alkali activated materials for structural applications [[9], [10], [11]], as secondary source of Al₂O₃ in porous alkali activated materials for pH regulators [12,13], and as a potential source of metals for metallurgical industries [14,15].

Hexaferrites are a group of iron-based magnetic oxides, and one of the most common magnetic materials used nowadays, with around 300,000 tonnes per year manufactured globally [16], usually from processed oxides, carbonates and minerals. They are used in a widespread variety applications, such as permanent magnets, memories and data storage, electric motors, electronics, microwave and wireless communications devices, stealth technology and radar absorbing materials (RAM), and electromagnetic interference (EMI) shielding at GHz frequencies in electronics and telecommunications [17]. The hexagonal ferrites were first discovered in the 1950's by workers at the Philips Laboratories [18], and they found that M-type ferrites such as BaFe₁₂O₁₉ (BaM) have the hexagonal magnetoplumbite structure, and are very hard ferrites, with typical saturation magnetisation (M_s) of 72 A m²/kg and a coercivity (H_c) of up to 600 kA/m for BaM ceramics [19], although it can be as little as half of this maximum value in non-optimised ceramics. The charge compensated pair of Ti⁺⁴/Co⁺² ions can replace Fe³⁺ in cobalt–titanium substituted M ferrites (BaCo_xTi_xFe_12-2xO₁₉) [17], in which coercivity reduces considerably with substitution, to give very soft ferrites with increasing x [20], while maintaining high magnetisation. The substitution reduces the axial anisotropy until it becomes in-plane at x = 1.3, with reported coercivity values in ceramics as low as 16.0–5.6 kA/m for x = 0.5–1.0 [21,22]. Often, a non-stoichiometric ratio of Fe:Ba between 10 and 11.5 (excess barium) is required to form the single phase BaM ferrite from oxides [17], although this is not always the case [23].

A few previous studies have been performed on the use of wastes as a precursor for hexaferrites. Steel pickling is a surface treatment used to remove impurities and rust from ferrous metals, producing toxic and hazardous wastes containing acids and heavy metals. In the 1990's the ferrites goethite, hematite and magnetite were made from iron oxides recovered from waste steel pickling liquors by Dufour et al., as a cheap source of raw material in a sulphuric acid liquor, and with the high Fe²⁺ content required to produce the spinel ferrites [24]. The same authors also produced BaM with good magnetic properties from these recycled steel pickling liquors [25,26], which were oxidised during oxicoprecipitation at pH 11–12 with a barium salt. They also oxidised and mixed iron rolling scale (steel production) waste with BaCO₃ to form a BaM precursor [27], and electroplating wastes (electrolytic slime) have also been processed to produce BaM [28]. However, these were all complicated processes - the highly acidic iron waste had to be oxidised first, and subsequently granulated, mixed with BaCO₃ and fired to form the M phase at high temperatures. In the 1980's, electroplating waste slurries contain large amounts of iron hydroxides were mixed with BaCO₃ and heated to 1200 °C to form BaM, but as a mixed phase with non-magnetic orthoferrites and spinels [29].

Between 2012 and 2016 Pullar et al. reported on the valorisation of a steel wire drawing waste to make a range of M-type ferrites, such SrFe₁₂O₁₉ (SrM) with an addition of SrCO₃ [[30], [31], [32]], the cobalt-manganese doped SrM ferrites SrCo_0.5Mn_0.5Fe₁₁O₁₉ and SrCoMnFe₁₀O₁₉ with additions of SrCO₃, Co₃O₄ and MnCl₂·4H₂O [31,32], and BaM with an addition of BaCO₃ [33]. All these M ferrites were made from dried sludges by simple solid-state reactions, with no further processing or treatment apart from addition of the 2 + ions, and they formed at relatively low temperatures of around 1000 °C. It was found that with stoichiometric additions of SrCO₃ and BaCO₃ single phase M ferrites were not formed, but with a nonstoichiometric addition of BaCO₃ at a ratio of Fe:Ba of 11:1 the optimum amount (86%) of M ferrite was formed [33]. These M ferrites were investigated as black pigments for colouring glazes and clay bodies, and as magnetic materials, and despite their mixed phases and content of many ions other than Fe³⁺ and Ba²⁺ from the wastes used, they possessed good magnetic properties, suitable for use as permanent magnets despite these impurities. The cobalt-manganese SrM ferrites were very soft magnets, with M_s = 50–60 A m²/kg and H_c = 12–20 kA/m [32], while the BaM was a hard magnet with M_s = 48 A m²/kg and H_c up to 300 kA/m [33].

The only other reports of wastes-based hexagonal ferrites are in the last years (2018–2019), of BaM made from unspecified iron oxides wastes from the steel industry mixed with BaCO₃ heated at 1100 °C [34], soft magnetic glass ceramics containing a portion of BaM from a mixture of 50% iron oxide sintering wastes in a glass with a large amount of added BaCO₃ [35], and BaM made from dewatered acid mine drainage sludges with added BaCO₃ [36], forming at 1100 °C with secondary alumina silicate and calcium sulphate phases (no magnetic data given).

Here, for the first-time red mud was used as a source of Fe to form hexaferrites. The red mud also contains other ions, such as Al³⁺, Ti⁴⁺ and Si⁴⁺ that can substitute Fe³⁺, and Ca²⁺ which can substitute Ba²⁺, in the hexaferrite structure. The addition of cobalt to the structure was also studied, as Co²⁺ can compensate for the excess charge when Ti⁴⁺ substitutes Fe³⁺. Clearly, the production of such widely used magnetic materials from the valorisation of wastes could be a major advantage, from both economic and sustainability aspects, as well as removing a potential contaminant from the environment.