por He iron(III) oxide Iron oxide is an inorganic solid formed by the reaction of iron (Fe) and oxygen (O2), in which an oxidation state of iron of +3 is obtained. Its chemical formula is Fe2O3.
In nature it is found mainly in the form of the mineral hematite, which gets its name from the red color of its stripes. Hematite is the main iron ore for industrial use.
The color and appearance of Fe2O3 depend on the size and shape of its particles, as well as the identity and amount of impurities and water present. Yellow, orange and red pigments are known. It does not have a metallic shine.
It does not conduct electricity, but mixed with other oxides it makes it possible to manufacture semiconductor glasses. The alpha crystalline form is antiferromagnetic and the gamma crystalline form is ferromagnetic.
It is used as a red pigment in paints, rubber, ceramics, and paper. Also in protective coatings for steel and other metals. Its versatility is due to its tinting and covering power, its resistance to ultraviolet light and alkalis.
It is used in the preparation of garnets or fine stones of various metal oxides. It is used to polish glass, diamonds and precious metals (jewelry grade). It is also used as a catalyst in various reactions. It has been used for wastewater treatment.
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Structure
Alpha
The crystalline form α-Fe2O3 has the structure of corundum (Al2O3 mineral), where the oxide ions (O-2) form hexagonal packed layers, with the Fe+3 ions occupying two thirds of the octahedral sites.
In other words, each Fe+3 is surrounded octahedrally by 6 O-2 ions. Its color changes with increasing particle size from bright red to dark purple.
gamma
γ-Fe2O3 presents a spinel-like structure with a cubic packing arrangement of oxide ions, with Fe+3 ions randomly distributed between the octahedral and tetrahedral interstices. This crystalline variety, when heated in air to more than 400 ºC, changes to the alpha structure. It has a brown color.
Beta and Epsilon:
They are rare crystalline forms of this oxide. β-Fe2O3 crystallizes in a rhombohedral system. This structure is metastable and when heated above approximately 500 ºC it changes to the alpha variety.
ε-Fe2O3 crystallizes orthorhombically. It is also metastable and at temperatures between 230 and 500 ºC it changes to the alpha structure.
Nomenclature
Hematite: natural mineral of Fe2O3 that crystallizes in the alpha form. It is also known as specularite or oligist.
maghemite or magnetic hematite: gamma form of Fe2O3, not very abundant in nature.
ferric oxide: Fe2O3.
Natural iron (III) oxides: are those found in nature. They were used since prehistoric times, for example, in the paintings of the Altamira caves.
Synthetic iron (III) oxides: They are prepared synthetically, obtaining a composition that corresponds to that of natural minerals. They are preferred to naturals for their pure hue or hue, consistent properties, and tintability.
Properties
physical state
Solid, whose color can be light red, reddish brown and dark purple depending on the crystalline structure and the particle size.
Molecular weight
159.69 g/mol.
Melting point
1566ºC.
Density
5.24 g/cm3
Solubility
Insoluble in water, soluble in hydrochloric acid (HCl) and sulfuric acid (H2SO4).
other properties
– Iron (III) oxides are characterized by their low color intensity, their excellent resistance to ultraviolet light, their tinting capacity and excellent hiding power.
– They are non-toxic, do not fade and are economical.
– They are resistant to alkalis. They do not react with weak acids or weak bases. If they are not contaminated with manganese (Mn), they do not react with organic solvents.
– The alpha form is paramagnetic (it is attracted to magnets, but does not become a permanently magnetized material) or antiferromagnetic. It is an electrical insulator.
– The gamma form is ferromagnetic. This means that when subjected to a magnetic field, ordering of the magnetic dipoles of the material occurs, which remains for a certain time after removing the magnetic field.
Applications
In the construction industry
Iron(III) oxide pigments are largely used to color cement and other construction materials: concrete tiles, paving bricks, fibrous cement, bitumen or mortar, among others.
Said use is based on the fact that they do not affect the setting time, the compressive strength, or the tensile strength of cement or other materials.
They can be incorporated into many binders due to their pure color shade, good hiding power, good abrasion resistance, and low tendency to settle.
In paints and coatings
Due to their resistance to acids and bases, they are used as pigments in paints and varnishes. Their resistance to high temperatures makes them good in enamels.
Synthetic hematite-based pigments are used in corrosion protection coatings, especially marine ones. Its crystalline structure delays the penetration of humidity and corrosive substances present in the saltpeter.
Protects well on coatings for interiors, exteriors and metal parts. In the maintenance and repainting of bridges, its use leads to protection against humidity, dew or dense fog, and easy drying at low ambient temperatures.
It is also used in paper to cover walls.
In the plastic and rubber industry
Iron(III) oxides are used to color plastics and rubber. Synthetic iron(III) oxides are preferred in this application. Although natural iron (III) oxides are cheaper, their use has decreased compared to synthetic ones.
In glass and jewelry
They are also used in polishing glass, precious metals, diamonds and precious stones.
They also serve as dyes in the manufacture of glass.
On magnetic recording material
The gamma form has been used as a magnetic material in the production of magnetic recording media, for example, in information storage systems such as audio and video cassettes, in broadcast studios, floppy disks, computer tapes, and computer hard or floppy disks. .
In such an application the particle size is extremely important to ensure good magnetic properties. The noise level in magnetic tapes decreases with decreasing particle size.
Its resistance to friction is also important, since diskettes have a magnetic layer where the maghemite particles are found, and their useful life depends on the ability of said layer to withstand friction.
Magnetic polymeric compounds with γ-Fe2O3 nanoparticles have been prepared for use in electromagnetic interference and microwave absorption devices.
In the food, pharmaceutical and cosmetic industry
Pigments based on synthetic iron(III) oxides that have been produced from pure starting materials are considered non-toxic.
For this reason they can be used as colorants for food, pharmaceuticals and cosmetics.
In catalysis of chemical reactions
Iron(III) oxides are used as catalysts or catalyst bases in various chemical processes.
Together with cellulose acetate, they have been tested as a support for metal nanoparticles to be used as catalysts in the degradation of toxic organic compounds that pollute wastewater.
Due to their ability to absorb light from the visible spectrum, they have been proposed for photocatalysis in the photodegradation of organic contamination.
In reducing global warming
Hematite has been studied as a sorbent in carbon dioxide (CO2) capture reactions. It is being investigated whether this would help to solve the problem of the effects of global warming produced by the high concentration of CO2 in the atmosphere.
Other uses
– Due to its adsorbent capacity, Fe2O3 is used in the manufacture of fluorine or other gas sensors, and in humidity detectors.
– Mixed with other oxides, it is used to make semiconductor crystals.
– It has been used as an enhancer of the electrochemical properties in rechargeable lithium batteries.
References
American Elements (2019). Iron(III) Oxide. Retrieved from americanelements.com.
Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. John Wiley & Sons.
Kirk-Othmer (1994). Encyclopedia of Chemical Technology. Volume 14 and 19. Fourth Edition. John Wiley & Sons.
Ullmann’s Encyclopedia of Industrial Chemistry. (1990). Volume A20. fifth edition. VCH. Verlagsgessellschaft mbH.
Castaño, JG and Arroyave, C. (1998). The functionality of iron oxides. Metal. Madrid, 34 (3), 1998. Retrieved from revistademetalurgia.revistas.csic.es
Esraa M. Bakhsh, Shahid Ali Khan, Hadi M. Marwani, Ekram Y. Danish, Abdullah M. Asiri, Sher Bahadar Khan. (2017). Performance of cellulose acetate-ferric oxide nanocomposite supported metal catalysts toward the reduction of environmental pollutants. International Journal of Biological Macromolecules. DOI: 10.1016/j.ijbiomac.2017.09.034
Mora Mendoza, EY et al. (2019). Iron oxides as efficient sorbents for CO2 capture. Journal of Materials Research and Technology. 2019, 8(3):2944-2956. Retrieved from sciencedirect.com.
Piao Xu, et al. (2012). Use of iron oxide nanomaterials in wastewater treatment: A review. Science of the Total Environment 424 (2012) 1-10. Retrieved from sciencedirect.com.