Introduction
Yellow ochre is a hydrated form of iron oxide hydroxide, FeO(OH).nH2O, commonly referred to as limonite. It is one of the three principal ores of iron, alongside hematite and magnetite, and is found naturally across the world. Its main use over time has been as a dye, owing to the bright yellow colour it is often presented with, with some reports stating its use as a dye in Africa goes back almost 300,000 years. As a commercial product, it found fame after Jean-Atelier Astier developed a process for extracting it at scale from the brightly coloured red and yellow cliffs of Provence in the late 18th century, and became the pigment of choice for yellow, red and orange paints and dyes.
Ochre may be found in multiple colours; red ochre is the product of an anhydrous iron oxide, whereas the yellow colour is imparted from hydrated iron oxide hydroxide. Mixtures of ferrous and ferric iron will produce a brown ochre. Overall, the colouration is due to the presence of oxides of iron. Limonite itself is formed from the hydration by way of oxidation of the other iron ores, magnetite and hematite. In addition, it can be formed from weathering processes on other minerals rich in iron. When found in a deposit, limonite is an amorphous solid, appearing in shades of yellow or brown, with a moderate hardness level of 4 - 5.5 on the Mohs scale(1). When mined, it can be broken into shards, or ground to a powder for use. Yellow ochre is non-toxic.
Over time, yellow ochre/limonite has found uses in primitive adhesives(2) in early hand tools, in religious artworks and sun protection - though the latter has been displaced by titania-based methods.
In contemporary times, ochre remains an important material. It is still used as a dye, but has found other applications such as in catalysis, as a cement additive and in the synthesis of iron nanoparticles. This is in addition to its use as an ore of iron; limonite can contain up to 59.8% iron(3). Whilst this value isn’t as high as magnetite or, in particular, hematite(4), it remains a viable ore of the highly economically important metal.
The terms yellow ochre and limonite will be used interchangeably throughout this article.
Yellow ochre as supplied by African Pegmatite
Yellow ochre as supplied by African Pegmatite contains iron(iii) oxide (27.0%), alumina (14.0%) and silica (47.0%) by mass, with the balance being trace minerals. Standard milling is to a particle size where 90% are smaller than 75 μm and drying is to a maximum of 3% water by mass.
Dyes and Pigments
The most ubiquitous use of ochre is as a pigment. Its lustrous yellow colouration is highly desirable. Studies on ancient paintings and wall coverings suggest that yellow ochre was used in both Ancient Rome(5) and Egypt(6), though in South Africa, abstract designs made with ochre have been dated back as much as 75,000 years. As a pigment in modern paints, it is responsible for earthy, yellow hues(7). It has been employed as a long-term stable dye for sails, natural fibres and even is effective on synthetic polyacrylonitrile fibres(8).
Permineralization
Permineralization is the process by which mineral deposits collect and form internal casts of organisms, as a method of fossilisation. Limonite is one of the leading minerals found in fossilised organisms(9) and it has been noted that where an organism has been fossilised with limonite, it is often better preserved than other methods(10).
Materials for the Built Environment
One of the oldest uses for ochre is as a pigment in cements/render, partly explaining the richly coloured houses in parts of Latin America and around the Mediterranean Basin. In many cases, ochre was used only as a colourant(11), but there have been documented uses of it where it provides more of a structural role in conjunction with other compounds.
Cements and Concretes
As a pigment in cements, ochre is responsible for a strong yellow colouration and has been reported to be highly stable over long terms(11). This ‘chromatic effectiveness’ has proven its worth over many years. It has been noted that in general, pigmented concretes and cements have lesser mechanical properties than conventional concretes - but not sufficiently so to prevent them from being used as structural concretes(12).
One interesting proposal for ochre was the use as a component in radiator cement(13). That is, a quick setting cement that forms a seal around a pipe containing liquid. Alongside gelatin and plaster of paris, the cement proposed used ochre as one of its co-equal largest ingredients, providing a low cost fixing seal that is resilient to water, including at high temperatures. As a component of concrete, alongside ilmenite, limonite as part of the aggregate has proven to be a highly performing heat resistant concrete, with applications including in nuclear reactors on small and large scale, where ilmenite-limonite concrete has been found to be highly attenuating against primary- and secondary-gamma rays and slow moving neutrons(14,15), this radiation shielding effect is primarily due to the iron content of the concrete slab afforded by the limonite.
Within reactor radiation screening, a general rule of thumb seems to be that the greater the iron content (i.e. via inclusion of limonite within the concrete mixture) in the concrete, then the better the TVL value is(16). The TVL is the ‘tenth value layer’ referring to the average amount of shielding material (limonite doped concrete) is required to reduce the expelled radiation to 10% of its initial intensity. TVL can be thought of as a measure of how good a material is at containing nuclear radiation. With limonite-containing reactor concrete insulation, the thicker the deployment of the concrete, the better the containment of the radiation(17).
Further to this, in terms of more conventional concrete, high ochre-containing aggregates have been used in concrete production in South East Asia at up to 30% of total aggregate, producing a concrete as strong as regular concrete(18).
Lateritic Soils
In certain parts of the world, soil is described as lateritic. This means that they are largely clay-based, and are porous. Oftentimes, these also contain large quantities of ochre. Rudimentary bricks have been made from these lateritic soils and accounted for many early structures, particularly in India. Developing on this idea, and applying modern building methods, a concrete brick made now using local lateritic soil requires 50% less cement as a similar one would in a temperate climate(19). In addition, ochres can be used as components in highway building, providing infrastructure at an economically attractive rate(20).
Soil And Water Remediation
Many iron containing materials can be used in a remediation capacity - typically removing unwanted contaminants or pollutants from wastewater streams or soil. Chromium(vi) is a toxic pollutant and the highest oxidation state of chromium metal - even modest exposure is sufficient to cause health problems. Removal of hexavalent chromium from soils and water supplies is crucial if it ever penetrates the supply - and can be achieved using limonite. Limonite combines reduction (to chromium(iii) from chromium(vi), chromium(iii) is not as significant a problem) and sorption to remove it. In a mildly acidic aqueous solution, limonite was able to remove 55% ±1% of Cr(VI) using limonite processed to 0.15 to 0.075 mm (100 to 200 mesh)(21). The only hindrance came after vastly extended periods of time where constant exposure caused morphology changes to the limonite - therefore reductions in sorption and reduction efficiency.
Catalysis
Iron catalysis is, itself, a large and varied field. Some of the major concerns with conventional catalysts are that they can be expensive to produce or lack long-term stability. Ochre/limonite has the potential to overcome these issues. In some cases, limonite can even be sacrificial, being converted readily into other compounds such as nanoparticles.
For the Synthesis of Iron Nanoparticles
Iron nanoparticles have been employed for myriad tasks in the last two decades, with applications across the catalytic spectrum, they are prized for their surface area to volume ratio. Iron nanoparticles have been synthesised directly from limonite(22), and have been shown as effective for the removal of toxic hexavalent chromium from waste streams. Limonite can be an inexpensive source of high purity iron oxide from which to make iron nanoparticles - comparable to magnetite or hematite in many situations.
Limonite can be reduced and formed into zero-valent iron nanoparticles (ZVNP) by a relatively simple process, and such ZVNPs have found applications in water purification and in industrial waste treatment pathways such as the removal of para-nitrophenol(23). As early as 1972, ZVNPs have been used in pesticides and chlorinated compounds in aqueous media(24).
Decomposition and Reforming Processes
Limonite/yellow ochre, as mentioned, is a useful source of iron and can be utilised as a catalyst for several decomposition/reforming processes. These take typically toxic or otherwise waste materials and convert them into something useful or easier to handle. For example, it has been reported that when volatile organic compounds from biomass processes are passed over a bed of limonite at relatively low temperatures are reformed into a hydrogen-rich gas (similar to synthesis gas); this approach also works with the gasified tarry residues left in biomass processes, and is claimed to be as effective as a commercial nickel-aluminium oxide catalyst. The advantage here is clear, the use of a non-toxic and cheap catalyst is advantageous over a toxic and expensive one(25).
Gasification processes often employ a whole suite of catalysts for the removal of various compounds from the crude gas. Ammonia is a contaminant that can be found as part of the mixed gas resulting from biomass gasification and research has shown that it is removable via the use of limonite(26). Limonite resists poisoning by sulfur which is the downfall of many other catalysts used for ammonia removal, however, limonite requires elevated temperatures to achieve this. Virtually complete conversion by the limonite of NH3 to N2 was reported. Gasification as catalysed by limonite of highly woody biomass is highly successful, with hydrogen-rich biogas produced at a 25% greater rate than would be achieved with an olivine catalyst(27) when used in a fluidised bed reactor setup. High-H2 containing gases are much more akin to synthesis gas and therefore open up greater scope for the gas produced to be used for chemical synthesis as opposed to just being burned as a fuel.
In a process similar to gasification, coal can be converted to liquid hydrocarbons via a process known as direct coal liquefaction. Particularly useful for lower quality coals, the technique provides viable fuels and feedstocks for other chemical and industrial processes. Such a catalyst for the direct coal liquefaction is limonite. Research shows that low-hematite content limonite is a superior catalyst for the conversion, with optimal conversion efficiencies observed when limonite bearing a water to iron ratio of 0.60 was used(28).
Building on the classical petrochemical industry-style reactions with tarry hydrocarbons, limonite has been used to catalyse the cracking of exhaust gases from the pyrolysis of low-quality coal at low temperatures. In this reaction, the cracking favours small, aromatic hydrocarbons which are synthetically useful as feedstocks(29). Australian-mined high ɑ-FeOOH content ochre has been used for hot glass cleanup. The ɑ-FeOOH is placed in a reducing atmosphere at 500 °C and was shown to remove pyridine from a gas flow and convert it to benign nitrogen gas in excess of an 80% conversion rate(30). Particularly impressive is that the conversion also worked well at the same temperature, but without the reducing atmosphere.
When used as a supporter for other catalysts, limonite has proven utility in the decomposition of carbon disulfide, a gas that through reactions in the atmosphere is one of the leading causes of acid rain. The combination of limonite and a BiVO4 catalyst effectively removed the disulfide at moderate temperatures(31). Treating ochre thermally converts it from limonite to hematite, which can be used for the thermal catalytic cracking of toluene into small hydrocarbons, in excess of 90% efficiency(32). It was noted that such activity could not be realised from hematite alone as mined. As mentioned earlier, limonite/ochre is not regarded as the ‘best’ source of iron, and by alkali roasting ochre followed by hydrothermal treatment, it can be converted into a higher Fe2O3-containing material, which has a wider application(33).
Moving towards biological-type applications, it has been shown that limonite is catalytically active for the hydrolysis of microcystin peptides, outperforming its other mineral peers(34), due to a highly Lewis acidic character at its surface. With this knowledge, insights can be drawn into the natural decay and decomposition of microcystins.
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