What is Chrome Flour and How is it Used in Daily Life?
Highly pure chrome flour is an essential part of modern refractory and pigmentation arsenals - offering high thermal tolerance and impressive stability across a variety of uses - and is available through African Pegmatite.
What is Chrome Flour?
Also known as iron chromite, chromite powder, chrome flour 325, and chromite flour, it is an inorganic compound that is used as a pigment and as a vital component in some of the highest performing refractory materials. Its chemical composition is Cr2O3. Chromite is the sole naturally occurring ore of chromium, occurs with iron oxide, and It has a melting point of 2,040 °C. It is referred to as being “almost chemically inert”(1), lending itself to a variety of applications where long term stability is required. Abundant in South Africa, chromite ore is considered to be more usable than silica sand owing to the former producing less fines and therefore requiring less new sands to be added to a casting system overall(2). Because of the size and quantity of the escaping fines, chrome sand is viewed as superior to silica sand in terms of less potential respiratory damage in the foundry.
What are its Properties?
Chromite flour is used because it is:
- Stable at high temperatures
- Thermal-shock resistant
- Resistant to corrosive glasses and slags
- High heat resistance
What are its Uses?
As a pigment for glasses, chrome did not find much use owing to chemists preferring to capitalise on its primitively understood refractory qualities at the time over its colourant ones, most likely because old methods of producing finely divided pigments were significantly less efficient than modern day ones - despite chrome being used as a glaze pigment as early as the start of the 19th Century. Chrome flour
has considerable long term stability, which makes it perfect for use as a pigment in paints, inks, and, most notably, glasses(3). Long being the pigment of choice to produce green glass, chrome flour is not used to pigment plate glass, but container glass only. Beers, wines and sparkling water are the food products most associated with green glass bottles, with green being chosen for its better ability than colourless glass to resist penetration of ultraviolet radiation which could cause a loss of shelf life by spoiling the product, even before it can reach the consumer. One of the reasons spirits are mostly sold clear bottles on the shelves is that they are far less susceptible to spoilage by ultraviolet light, while beer on the other hand is always sold in brown or green glass bottles; beer is particularly sensitive to UV light.
Chrome flour as a pigment is responsible for instantly recognisable green glasses such as emerald green and Georgia green, as well as dead leaf (feuille morte) green, dark olive green, champagne green, UV green and antique green. Suggesting that chrome flour alone is responsible for these colourations tells only half the story. Green chromophores are achieved by not only using chromite but by using iron pyrite - the use of these materials in various concentrations in concert, along with modulation of the reducing or oxidising nature of the melt, will determine final glass colouration.
The differences between oxidised and reduced glasses are, in terms of their chemistries, subtle. These are dealt with in another article.
Most reduced glass is not pure green but a yellow-amber colour or a dark green, owing to its enhanced iron pyrite content. Iron pyrite alone is known for creating amber coloured glass and it is indeed the amber coloured glass that is well known for reducing the penetration of UV rays. Amber coloured glass, however, is considered less attractive and so most brands prefer to choose a green bottle, hence the need to mix iron chromite flour with iron pyrite to create the more visually appealing shades of green, while still retaining most of the effectiveness of pyrite’s UV reflecting qualities.
As an oxidant, the strongest green colours are achieved with large quantities of chromite. Emerald coloured green glass uses ten times the amount of chromite per tonne than does Georgia green glass (the famous colour associated with bottles of Coca-Cola), which itself uses up to six kilograms of chromite per tonne of sand used in glass manufacture(4) - that is to say that emerald glass can require the use of up to 60 kg of chromite per tonne of sand to achieve the desired colour. The use of chrome flour is greatly preferred over the archaic method of achieving such high oxidation levels, potassium dichromate. Potassium dichromate is toxic to humans and handling should be avoided wherever possible. Feuille morte glass uses a 2:1 ratio of iron pyrite to chromite to achieve its unique visual appearance.
Refractory Bricks and Refractory Cement
The use of chromite/chrome flour as a refractory material as we understand refractory materials today dates back well into the 19th Century. Early chrome-based refractories had been used in iron and steelmaking, however their use was somewhat hampered by their lack of physical and mechanical strength at the highest temperatures. Iterative improvements to more traditional magnesia and silica-type bricks started to be realised in European foundries around the late 1800s, where significant performance enhancements, particularly in the areas of thermal/chemical resistance, and a resistance to shock and spalling, were noted when chrome flour had been added to the bricks(5).
Chrome flour is also optimal for use in the production of chrome magnesite refractory bricks and cement, which are used for high-heat environments such as metal smelting furnaces, electric arc furnaces, cement rotary kilns, glass kilns, and other high-temperature industrial furnaces, owing to its superior heat resistant qualities. The addition of chrome flour means that the refractory bricks and cements can withstand the extreme heat necessary in smelting furnaces, for example, and won’t crack under heating and cooling stress.
With the increasing demand for metal products at lower costs in the mid-20th Century, foundries began introducing oxygen directly into their furnaces, which immediately resulted in higher operating temperatures. Chrome-magnesite bricks began replacing more traditional silicate type refractory bricks so as to better handle the significantly elevated furnace temperatures(6). Modern refractory bricks are composed of magnesia-chrome and make extensive use of chrome flour, where such bricks have been routinely demonstrated to be highly effective in the 1,900 °C region. In addition to brick form, chromite-magnetite can be produced in a casted form, where it becomes denser and less porous(7) - further opening up the applicability of chrome flour as a refractory material.
One point of contention with the use of chromite as a refractory is that it can oxidise from chromium(iii) to chromium(vi) under certain highly oxidising conditions. Chromium(vi) is a known carcinogen and thus allowing its formation should be avoided at all costs. Aluminium oxide can be added to the otherwise predominantly chromite refractory to prevent such oxidation from occuring in the first instance(8).
In terms of cements, chromite has found great use as a vital component in refractory cements, with the high chromia content ensuring the production of a highly stable material upon curing that is resistant to wetting(9).
As A Pigment For Standard Bricks, Pavings, Roof Tiles And Coatings
This material can also be added to ivory filing clays in the production of bricks, paving, and roof tiles to give them various attractive shades of grey, which have become increasingly popular in architecture in recent years. The adage ‘a little goes a long way’ rings true with chromite as a pigment for decorative ceramics and coatings. Scanning electron microscopy experiments have shown that when utilised as a liquid phase pigment, chromite adopts a “spongy” structure after heating and when prepared with glycine, a reddish colour is attainable(10). This and other similar studies point towards post-production heating of the chromite can produce a small range of hues beyond the default grey/black. They belong to a broader class of ‘mixed metal oxide’ pigments(11). Such pigments are characterised by their ability to easily and homogeneously mix within the host material.
Similarly, chromite flour can offer the same benefits to the ceramics industry and create grey bodies. It’s also of use in the creation of glazes for use on the ceramics, with such glazes tending to be green in colouration, following a similar principle as when chrome flour is used as a pigment in glass. Black pigments, for example, are easily attainable with chromite - being resistant to firing at temperatures in excess of 1,200 °C whilst causing no defects(12). Chromite is reliable as a pigment for ceramic glazes up to around the 25% by weight mark. Studies have shown that the inclusion of mineral chromite as a ceramic glaze has impacts on crystallisation (i.e. the formation of the glaze, by way of modulating the kinetics of the reactions) and on machinability (which can additionally be thought of as improving durability)(13). The study suggested that chromite, in terms of crystallisation ability, acted as a source of iron oxide, with the activation energy for crystallisation reducing with a greater chromite content despite the material’s exceptionally high melting point. The use of chromite therefore has potential as a glaze to improve the quality of low-end ceramics.
In Greensand Castings
Greensand castings are widely used for the routine and specialised production of casted metal products. Chrome flour is a vital component in the greensand, providing to it the enhanced refractory capabilities required to handle molten metal. In greensand, up to 85% of the mass can be sand, such as chromite. Chrome-type sands are oftentimes used for the production of heavy, sectioned, ferrous-type castings. Owing to chromite’s cost relative to silica sand, it is only used for higher end castings where supreme performance is required. Of particular note in the casting space is that chromite is not easily wetted. Its chemical resistance is also prized, making it particularly useful in the production of high-manganese steels such as Hadfield steel. It is also a commonly chosen refractory for aluminium casting(14).
In the contemporary casting field, some 40 to 60% of casting defects are attributable to poor quality moulding compounds and fillers(15). Quartz sand - the most popular choice - begins allotropic transformations at around 575 °C and therefore can be stated that it is subject to thermal shock, despite its otherwise good levels of performance in the refractory casting space (high hardness, good mixability(16)). Specifically for ferrous castings, chromite on the other hand, is inert to high temperature iron oxides and is poorly wettable to molten metal(17). Surface defects are reduced by less mixing at the metal-sand interface. Furthermore, solidification irregularities are prevented through the use of chromite owing to its superior thermal storage and conductivity properties, leading to more uniform directional solidification in the casting overall.
Stainless Steel Production
Chrome flour is also used in stainless steel production, where it helps prevent the molten steel from settling inside tap holes and solidifying there. This is a particularly attractive quality as it ensures a much more stable long-term production pathway, without the need to stop to relieve the tap holes of solidified iron or steel. Steel alloys such as Fe-Cr-Ni-N can be achieved with chromite flour(18). Researchers have shown that by moderating upwards the iron and chromium content (ie. by using more chromite), austenitic stainless steel can be produced using significantly less nickel. This is advantageous owing to nickel’s relative toxicity. When chromite moulds have been used in large section industrial steel production, computational studies have suggested that double skin penetration can occur under certain conditions, although these can be predicted and planned for ahead of time, and the primary cause was mechanical penetration and not chemical or physical effects(19). Therefore, it can be suggested that chromite is suitable for large section steel casting moulds.
Along with other oxides, chromite is used as a compound for polishing the edges of knives, razors, blades, and the surfaces of optical devices on a piece of leather or cloth. When it is produced in this context, it is available as a powder or wax called “green compound”. In this sense, chromite and related compounds are prized for their hardness and durability(20).
Where Would We Be Without Chromite Flour?
Chrome flour is responsible for some of the most visible (glass bottles) and invisible (refractory materials for the production of globally significant materials such as stainless steel and aluminium) aspects of modern life. Its broad applicability in such applications comes down to its long term resistance to change, be that chemically or physically, under exposure to significant heat in a refractory or ultraviolet radiation in the case of glass. Where chrome flour is used, it makes the process easier. A refractory means better temperature control in a foundry; in glass it is an easily incorporated material in the melt; in casting it reduces the possibility of wetting - all of these features reduce the energy requirements for their respective processes.
The high purity and wide applicability of chrome flour make it an ideal choice for pigmentation, refractory production and a wide array of other applications. African Pegmatite is a leading processor, miller and supplier of high quality chrome flour and other premium chromite-based refractory and pigmentation compounds, for a wealth of uses where excellence, reliability and service are paramount.
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