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Foundry Chrome Sand: Part 1 - By Nino Ciani

South Africa is a country rich in mineral resources, one such mineral resource is foundry chrome sand. Optichrome foundry sand is a product designed to specifically to meet the stringent standards within modern foundries.
Optichrome Sand is a shiny black, angular to semi angular sand.  It is highly refractory and chemically unreactive, has good thermal stability and offers excellent chilling properties. This is particularly beneficial when experiencing hot spots in a casting. Optichrome sand is ideally suited as a chill medium and assists greatly in reducing migrating hot spots.
In a foundry, certain types of metals are cast at higher temperatures.  Standard foundry sands such as silica sand (which is commonly used in the moulding process) cannot offer enough heat resistance against these increased casting temperatures. This is where a specialised sand like Optichrome sand is ideally suited, as it has a much higher fusion point than traditional silica sand, that fuses at around 1600c – 1700c.  

The reason why Optichrome sand is chosen as a moulding medium is due to the specific properties that only Optichrome sand can provide:  Typically, Optichrome sand can resist up to 2150c, and because of this, Optichrome sand is used in Manganese Steel Castings, such as in the manufacturing of manganese mill liners, screens and grids commonly used in the gold mining industry. It is also used as Manganese crushing wear parts for gyratory crushers and jaw crushers. 

When using silica sand, some foundries use up to several coats of mould paint to protect the molten metal from the silica sand face, this is due to silica sands’ slightly alkaline nature, which causes a reaction to Manganese steel which is acid in nature, causing a sand-to-metal reaction. Adding more coats to the mould to stop this reaction will add to the cost per ton to produce a casting, as well as increases the chances of casting-related defects.

The benefits of using Optichrome sand in Manganese steel casting is Optichrome sands’ ability to resist acid attack (sand-to-metal reaction, caused by an acid / alkaline reaction in silica sand).  Because Optichrome sand is pH-neutral, it minimises this reaction. Optichrome sands’ higher heat resistance reduces metal penetration and sand burn on.  As a result, one would require fewer coats of mould paint to be applied to the mould, which in turn reduces operational costs in the foundry.

When using Optichrome sand with a self-setting resin bonding system like an Alkaline Phenolic or Acid Set Phenolic, the sand requires less resin to bond the material, due to Optichrome sands’ microporous structure and angular to semi angular shape (and in some cases, semi-round shape). These benefits will offset the cost to run this system effectively and efficiently.

Optichrome sand comes in two specific grades:
  1. Above 46% Cr203; and
  2. Min 45% Cr203
Each grade offers specific benefits for various industries, including foundries, refractory manufactures, glass manufactures, etc. The typical AFS range of Optichrome sand is a 45 – 55 AFS and a 40 – 50 AFS.

When selecting a quality Chrome sand, here are some key qualities to look out for:
  • Sand shape and geometry
  • Sand sieve distribution
  •  Quantity of clay and volatiles present in the sand
  • Silica content
  • CR: FE ratio
  • Fines content (typically, the global standard for foundry Chrome sand is below 2%)
Optichrome sand offers all the above qualities and therefore Optichrome sand is the first choice of foundries and manufactures worldwide.
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