Magnetite: Uses and Applications in Recording Media, Pigments/Dyes and the Fischer-Tropsch Process
Brief Introduction To Magnetite
Magnetite, iron(ii,iii) oxide, is a major ore of iron, and thus finds its primary use as a source of iron and for the production of steel. It is black, opaque and has the chemical formula Fe3O4, possessing iron in oxidation states 3+ (ferric) and 2+ (ferrous). It is found widely distributed as large scale deposits, in igneous and metamorphic rock, in addition to in black sand and in fossils(1).
Aside from iron and steel, magnetite is used widely in water purification, the Haber-Bosch process, in medicine and for contaminant removal from industrial processes. Here, we will look at three applications of magnetite: in recording media, the Fischer-Tropsch process for producing synthetic hydrocarbons, and in the coatings, pigments and dyes space.
Magnetic tape is one of the oldest methods of storing data (analogue voice/music and digitised computer back ups, for example) and despite its perceived obsolescence, many businesses still rely on a magnetic tape based system within their overall data management regime. As many as 77% of companies surveyed(2) stated that they used magnetic tape as part of their data management operation. Known for its longevity under optimal storage conditions, magnetic tape is still a key part of many archiving services(3). As recently as 2014, Sony announced a new magnetic tape, ostensibly for server operations, with a capacity of 185 GB. Magnetite, as a prime source of iron oxide, was an early material used for production of magnetic tape(4).
How does it work?
As magnetite (and this iron(iii) oxide) is ferrimagnetic, passing it through a magnetic coil will align the magnetic moments of the iron oxide in a single direction. In the case of recording media, the magnetic coil is a ferromagnet and an electromagnet, and is called a recording head. To record, a current of the signal to be recorded is pulsed to the ferromagnet coil, which in turn magnetises the tape via an induced magnetic field proportional to the signal. For playback (or decoding), an already magnetised tape is passed through the same coil, this induces a voltage in the coil, which can be transmitted onwards. The same basic idea is used across all magnetic tape media, with variations only in whether the recording method is linear or scanning-based(2).
Why magnetite? How is it used?
Magnetite is known as an inexpensive and high purity source of iron(iii) oxide, and its nature as a ferrimagnetic material mean that it is useful as a component of a storage medium. Briefly, an emulsion of iron oxide is deposited onto a plastic film with a binder. This unmagnetised iron oxide is stable and the tape will progress on to a recording head. Magnetite has been employed not alone, but doped with other elements as early as the 1950’s, such as cobalt(5) afford tapes with much more consistent signal output. Preparation of the magnetite film involves depositing amorphous magnetite (Fe2O3) onto a film, heating it until it reaches the alpha-crystalline phase, and then reducing to magnetite(6), this results in a single, continuous film of pure magnetite, highly favourable for high quality recording media applications, such as in a data centre environment.
Pigments, Dyes and Coatings
As a naturally occurring and highly resistant material, magnetite has found several uses in the pigments/dyes/coatings sector. Prized for its relative hardness (circa. 6 mohs) and resistance to heat, pressure and weathering, magnetite is widely used especially for coating steel and iron structures, mechanical equipment and more.
Why magnetite? How is it used?
In terms of coatings, magnetite’s ability to absorb light is higher than that of many other common inorganic pigments(7), it’s high performance is especially notable due to its low cost and high availability. In pigmenting and dyeing situations, magnetite has been shown to have high tinting strength and good oil absorption(8). This second quality is particularly important considering the major component of paint is based on oils and/or hydrocarbon-based chemicals. Dispersion of the iron oxide particles was on the micron scale. Magnetite is a pigment that affords a black colour. It has been used as a pigment at least as early as in Ancient Greece, where the characteristic figures on terracotta pottery were at least partially produced with magnetite pigment(9).
The fact that such ornaments survive today in such good condition is a testament to magnetite’s stability. In printing inks, magnetite has been used by adding it to a siccative oil(10). Building on the perceived stability and relative inertness of magnetite, anti-corrosive paints containing magnetite have been used to protect steel structures and machinery(11), with coatings of steel of between 50 and 80 microns. It is reported that magnetite-based anti-corrosive treatments outperform their haematite-based commercially available counterparts. Using magnetite with epoxy-type resins has been shown to be a useful hybrid paint coating for marine applications(12). Overall, pigments and coatings containing magnetite are highly praised for their resistance to penetration by water and mild acids and bases.
The Fischer-Tropsch Process
The Fischer-Tropsch (F-T) process is an essential component of the modern global petrochemical industry. It is an industrial process that converts low value carbon monoxide and hydrogen (together referred to as synthesis gas ‘syngas’) into higher value hydrocarbon products, which can be further processed via cracking, isomerisation and reforming into essential products such as diesel and aviation fuels. The F-T process ensures that synthetic oils and fuels are always available to the market, providing the global economy an insurance policy against problems with crude oil production. F-T relies on high temperatures and pressures - and crucially a metal catalyst - to convert syngas into usable fuels. An often-cited perspective suggests that some naturally occurring hydrocarbon deposits originated due to a magnetite-catalysed F-T-like process at tectonic plate boundaries in the Middle East(13). The F-T process can also utilise carbon dioxide in the production of fuels(14).
How does it work?
The F-T process is a series of chemical reactions, too convoluted to discuss here, but essentially is the transition metal catalysed reaction between hydrogen and carbon monoxide producing, typically, short chain hydrocarbons and water as a byproduct. The identity of the catalyst is usually nickel, cobalt, ruthenium or iron-based. Magnetite is often used as a catalyst as it is a high-purity and inexpensive, due to its relative abundance, source of iron. Iron catalysts are significantly cheaper and of comparable activity to ruthenium ones(15). In the reactor, powdered magnetite is partially reduced by the hydrogen in the syngas, producing a combined iron-iron oxide catalyst in situ. The catalyst produced is characterised by its low porosity and small pore size - with diameters in the region of 100 microns. Magnetite is added to the reactor alongside silica which acts as a promoter of the reaction. Magnetite-based catalysts are known for their stability over time, and thus aid in ensuring a stable overall process.
Why magnetite? How is it used?
As mentioned, the ubiquity and price of magnetite is a key reason as to why it is employed as a catalyst. In a typical large scale reactor, tens or hundreds of kilograms of catalyst can be used, and it is important to prevent costs escalating. Iron based catalysts have been shown to be useful in a variety of F-T conditions, including lower temperature reactors to produce liquid hydrocarbons and even waxes. High temperature F-T typically produces very short chain hydrocarbons such as propane. ethane and methane - which are realised as gases. The water-gas shift reaction is a crucial part of the overall F-T process and magnetite is known to be active in this(16), and iron-type catalysts such as magnetite are known to be more resistant to sulfide poisoning than their cobalt counterparts(17) - hydrogen sulfide is a common contaminant in syngas. The use of F-T to produce diesel fuel is particularly advantageous as it often produces a lower sulfur content fuel than would be available from conventional production. Studies have shown that iron-based catalysts are more selective for olefin production than other transition metals(18).
Many studies have looked into supplementing magnetite in the reactor to fine tune the reaction outcome - to provide a selectivity bias for a particular type of fuel for example. Traditional powdered iron oxide has been treated via impregnation with up to 6 wt% of potassium, cobalt or molybdenum(19), with the potassium and cobalt-doped experiments demonstrating a substantial selectivity bias for kerosene-range hydrocarbons (as used in aviation fuels) of up to 30%. When sodium was used as a promoter, selectivity for methane decreased, however its impact on the overall efficiency of the F-T reaction is only notable when the iron catalyst is supported on alumina(20). Additionally, copper has been used as a promoter, increasing F-T rates(21). In the biomass-to-liquid process of producing sustainable fuels from waste products using the F-T process, iron oxide catalysts can be used(22), but it is noted that large crystals of magnetite should be avoided in favour of smaller examples due to risk of carbide formation(23).
- Magnetite is a widely available and inexpensive source of iron oxides which can be used in a variety of operations
- Magnetite has been used in recording media for the production of magnetic tapes, and still finds use today in high-quality tapes for data centre applications
- In coatings, pigments and dyes, magnetite is used as an effective black colourant and as parts of coating to protect steel, iron and industrial machinery
- The Fischer-Tropsch process for the synthetic production of hydrocarbons extensively uses magnetite and magnetite-based catalysts, providing stable and resilient production
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