Magnetite: Uses and Applications in Recording Media, Pigments/Dyes and the Fischer-Tropsch Process, Water Purification and Soil Remediation
A widely occurring ore of iron, magnetite finds a great variety of uses from pigments to soil remediation. Providing services from mining to processing and custom milling, African Pegmatite is a leading supplier of magnetite for a wealth of industrial processes, achieving the best quality to exact specifications every time.
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.
Many of magnetite’s applications, in addition to its magnetic nature, revolve around its hardness and non-toxic nature. Small quantities of magnetite are not toxic to mammalian or aquatic life. In terms of hardness, it has a rating of 5 to 6 on the Mohs scale, which is in the same region as titanium, manganese and tooth enamel.
Magnetite occurs in many places across the world, often occurring in good to excellent natural purity. It often occurs in ‘black sand’ deposits.
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 In Soil Remediation
Iron rich soils are naturally occurring in many parts of the world, often providing for a deep red colouration in the soil. Soils rich in iron oxides are not considered a good source of iron as the iron is so dispersed relative to other components(24). In other places, iron compounds may be voluntarily added to the soil to enhance its properties, particularly in the area of soil remediation.
Studies have shown the impact of magnetite addition to soils, where it is often added to catalyse the decomposition of contaminants including industrial pollutants from runoffs, aromatic organic compounds, landfill site leachate pollutants and others. Oftentimes, industrial installations will be required to consider their outflows, how they may impact the local soil structure and how best they plan to remediate it.
Industrial waste outflows such as vinyl chloride and trichloroethylene enter the soil via contaminated water, and studies have shown that they can be removed from soil by magnetite via a decomposition process(25). Once the initial dechlorination stage has taken place, the subsequent hydrolysis steps are vastly accelerated by the presence of iron(ii) from magnetite. Other industrial runoffs can include nitrobenzene which can be hydrolysed and decomposed via increasing the magnetite content - even if this means that the iron(ii) content is directly increased(26). It has been suggested that magnetite has a strong affinity to heavy metal residues. Arsenic and selected other heavy metals from mine tailings can be eliminated, too(27), a process which can be further enhanced by the addition of zinc(28). Heavy metals such as cadmium, lead and uranium can be effectively immobilised and/or removed from solid when magnetite is present - ideally in nanoparticle form - in quantities as low as 1.5 weight%(29).
Polycyclic aromatic hydrocarbons, n-alkanes and refractory oil residues as soil contaminants can be eliminated via the use of magnetite enriched soils. Degradation via fenton-like peroxide and persulfate oxidation processes have been demonstrated as effective in soils where ground magnetite has been used as a catalyst(30).
Perhaps counterintuitively, magnetite can outperform more complicated iron(ii) catalysts, with one study reporting 90% of crude oil in a soil sample being decomposed by magnetite after one week compared with only 15% of the oil decomposed in the same time when a commercial catalyst was used(31).
It should be noted that the addition of iron to the soil is not a quick fix unless the iron is biologically available. Biologically available iron means that it is, in many cases, in a chelated form. Magnetite and hematite are not biologically available and therefore it is futile to use them for the treatment of soil with a view to enhancing the viability of iron deficient plants.
Water purification and clean up of industrial wastes is a use case for magnetite that is becoming ever more popular. Known for its excellent ability to catalyse the removal of phenol and other related hydrocarbons from waste streams when ultraviolet light is applied, further enhanced when magnetite is in a nanoparticle form; with the iron(iii) to iron(ii) reduction purported to be the reason for the catalytic efficacy(32).
Magnetic ion exchange resins have been produced for water purification applications that use magnetite alongside styrene and divinylbenzene. These resins have been used to remove cobalt and nitrites from water supplies whereupon potable supply was produced from contaminated groundwater(33), with similar cases in Australia using magnetite ground to the micron regime. The advantage to these polymers and composites is that when they are ‘full’ of contaminants, they can be removed in bulk using magnets - tapping into magnetite’s nature as a weak magnet(34), similar work has shown this to be effective for bacteria-catalysed chlorinated hydrocarbon removal from water, when the bacteria had been adsorbed onto the magnetite(35).
- 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
- Soil improvement
- Other uses
Magnetite is a widely useful mineral suited to many applications from media storage to fuel production. African Pegmatite is a leading miner, processor and supplier of magnetite and a wide array of other high quality minerals. Boasting in-house milling, a global reach and decades of experience, African Pegmatite is the go-to industrial partner for minerals.
1 B. J. Woodford et al., PNAS, 1992, 89, 7683
2 R. H. Dee, Proc. IEEE, 2008, 96, 1775
3 R. Bradshaw and C. Schroeder, IBM J. Res. Dev., 2003, 47, 373
4 S. Onodera et al., MRS Bull., 1996, 21, 35
5 US Patent US3031341A, 1958, expired
6 US Patent US3620841A, 1970, expired
7 K. Ghani et al., J. Coatings Tech. Res., 2015, 12, 1065
8 M. A. Legodi and D. de Waal, Dyes and Pigments, 2007, 74, 161
9 P. Maravelaki-Kalaitzaki and N. Kallithrakas-Kontos, Anal. Chim. Acta, 2003, 497, 209
10 US Patent US3826667A, 1972, expired
11 J. Calderón et al., Rev. Metal. Madrid Vol. Extr., 2003, 2003, 97
12 A. M. Atta et al., RSC Adv., 2015, 5, 923
13 P. Szatmari, AAPG Bull., 1989, 73, 989
14 S. Upadhyayula et al., J. Cleaner Prod., 2019, 228, 1013
15 H. G. Stenger Jr. and C. N. Satterfield, Ind. Eng. Chem. Process Dev., 1985, 24, 415
16 K. R. P. M. Rao et al., Hyperfine Interactions, 1994, 93, 1745
17 C. N. Satterfield et al., Ind. Eng. Chem. Process Dev., 1986, 25, 401
18 M. E. Dry, Catal. Lett., 1991, 7, 241
19 D. Martínez del Monte et al., Fuel Process. Technol., 2019, 194, 106
20 A. Y. Khodakov et al., Appl. Cat. A: Gen., 2015, 502, 204
21 S. Li et al., J. Phys. Chem. B, 2002, 106, 85
22 S. S. Ali and S. Dasappa, Renew. Sustain. Energy Rev., 2016, 58, 267
23 E. van Steen and M. Claeys, Chem. Eng. Technol., 2008, 31, 655