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.
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(2). 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 colurations 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(3) - 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(4).
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(5). 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(6) - 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(7).
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(8).
As A Pigment For Standard Bricks, Pavings And Roof Tiles
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.
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.
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(9).
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.
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”.
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.
1 J. O. Nriagu and E. Nieboer (eds.), Chromium in the Natural and Human Environments, Wiley-Interscience, New York, 1988
2 I. C. Freestone and M. Bimson, J. Glass Stud., 2003, 45, 183
3 W. Vogel, Glass Chemistry, 2nd ed., Springer-Verlag, Heidelberg and Berlin, 1994
4 W. D. Kingery, H, K. Bowen and D. R. Uhlman, Introduction to Ceramics, 2nd ed., Wiley, New York, 1960
5 A. Muan and E. F. Osborne, Phase Equilibrium among Oxides in Steelmaking, Addison Wesley, Reading, United States, 1965
6 W. E. Lee and W. M. Rainforth, Ceramic Microstructures - Property Control by Processing, Chapman and Hall, London, 1994
7 J. H. Chesters, Refractories: Production and Properties, Institute of Materials, London, 1973
8 A. Muan and S. Somiya, J. Am. Ceram. Soc., 1959, 42, 603
9 J. E. Kogel, N.C. Trivedi, J. M. Barker and S. T. Krukowski (eds.), Industrial Materials and Rocks, 7th ed., 2006, SME Press, Littleton, United States