Chrome flour is used in the production of chrome magnesite refractory bricks and refractory cement, which are used in the construction of certain furnaces and kilns. Due to its highly corrosion-resistant nature, chrome flour is also used to produce magnesia chrome refractories that are used in non-ferrous metals like copper, lead, and zinc.
In stainless steel production, filler sand made with chrome flour prevents molten steel from settling inside tap holes and solidifying. The chrome flour allows this solution to be effective at high temperatures with long processing times.
- Stable at high temperatures
- Thermal-shock resistant
- Resistant to corrosive slags and gasses
- High refractoriness/heat resistance
- Cr203 of typically 44%
- Al2O3 of 15%
- Chrome content between 15% and 35% Cr203 (Magnesite chrome refractory bricks require 8% to 18% Cr203)
Refractory bricks made with chrome flour are suitable for use in:
- Open earth furnaces
- Electric arc furnaces
- Metallurgy furnaces
- Cement rotary kilns
- Glass kilns
- High-temp industrial furnaces
Chrome flour is a finely ground powder of iron chromite. African Pegmatite’s chrome is trivalent chromium (Chrome III), not hexavalent (Chrome VI). Hexavalent chrome is a hazardous carcinogen and is heavily regulated.
Chromium’s main industrial use is as a pigment, but chromite, a naturally-occurring ore of chromium, is used extensively in the manufacture of magnesia chrome refractory bricks and refractory cements for furnaces and kilns. Sources directly from the ore contain iron, but the iron will oxidise to inert iron oxide in furnace conditions. Chromite has a melting point of 2040° C and is almost chemically inert. Its high chromium content lends itself to the creation of highly stable materials that are resistant to wetting.
Bricks of chromite, alumina, and magnesium oxide are stable up to 1900° C. This is lower than the melting point of chromite alone, but the combination of materials is considered superior because it’s easier to handle and use – the addition of aluminium oxide increases chromite’s mechanical strength – and because the other materials are available at a lower cost than chromite.
Chrome has been used in refractory applications for over 100 years. Because of its high melting point, it was used alone in the production of iron and steel at one point, but this method often failed under a high weight load and has since been abandoned.
Over time it was discovered that silicate-bonded magnesia bricks were greatly improved by the addition of chromite and adopted a resistance to shock and spalling. When foundries began introducing oxygen directly into their furnaces to reach higher operating temperatures in the 1960s, the silicate bricks were replaced with magnesite and chromite bricks, which performed much better at the elevated temperatures.
When subjecting materials to oxidising conditions, it is vital to consider the potential oxidation of all compounds present. In highly-oxidising conditions, chromium III might become chromium VI. Chromium VI is a known carcinogen and a health hazard. This reaction can generally be avoided by the addition of a high quantity of aluminium oxide.
Chrome Flour in Refractory Applications
At least 18% of all chromite is used for refractory purposes. Chrome flour provides an essential thermal barrier inside the foundry furnaces and is used to join together other refractories for various purposes, bringing its high thermal stability to each mixture. The primary application of chrome flour in refractory applications is as an ingredient in refractory cements and concretes.
These cements and concretes are made through a similar process as Portland cements but are made using heat-tolerant materials so the cement can then be used in high-heat environments. Chrome flour can be combined with alumina and calcium carbonate, commonly known as lime, to create a cement that can be dry-pressed, cast, or formed into the required product. These refractory products are then used in a variety of furnaces to produce iron, steel, and aluminium.
Refractory materials are generally classified by their oxide compounds into groups of chemical and physical type. Physical classifications are shaped refractory materials, unshaped or monolithic refractory materials, and fibrous refractory materials. Chemical classifications are basic, acidic, and neutral.
These materials are used in the manufacture of thermal-resistant cements and bricks used in the foundry industry, as well as in ladle and filler sands. The average consumer is probably familiar with applications of refractory materials like fireplaces, ovens, and kilns. Industrial applications include the manufacture of glass and metals. High corrosion resistance prolongs the life of the furnace or kiln in which the bricks or cement are used.
Chrome flour is used in combination with other compounds to create magnesia chrome refractories that are used in the manufacture of non-ferrous metals like copper, lead, and zinc. It is also used in filler sand to prevent molten steel from settling in tap holes during the production of stainless steel – the high heat tolerance of chrome flour makes the filler sand effective even during long processing times at high temperatures – and is a crucial component of furnaces and masonry ovens. Other common refractory materials include chrome sand, coal dust, and glass powder.
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