Sand is uniquely effective as a moulding material for making metal castings, an application in which it has been used for centuries. Sand casting remains by far the most prevalent metal casting technique, despite the existence of numerous other casting methods such as die casting, investment casting and permanent mould casting. The global foundry and metal-casting industry uses around 100 million tonnes of sand every year which, in modern industry practice, is typically recycled and reused through many production cycles. Once used sand becomes unusable for further use in a foundry, it is often sold for re-use in other industrial applications.
Sand is used in foundries not only to mould the external shape of a casting, but also to form internal void spaces. Sand grains do not naturally adhere to each other, therefore binders need to be added to make the sand stick together and hold its shape for when molten metal is poured into a casting mould and left to cool.
Foundry sand must adhere to strict physical and chemical specifications to ensure that castings are kept free of defects. Both foundries and sand producers have extensive quality control systems to test and maintain consistent specifications for sand, ensuring that all foundry sand from a particular source will generally have a very consistent specification.
Various types of sand are used in foundries, each being particularly useful in certain castings due to their physical or chemical characteristics. One sand extensively used in foundries is chrome ore sand, which is used heavily in moulds for automotive castings. Castings for automotive applications such as engine blocks and camshafts require the high thermal conductivity which is a key characteristic of chrome ore foundry sand. Its high thermal conductivity ensures that the casting cools down quickly, minimising the potential for the molten metal to compromise and penetrate the surface of the mould (this phenomenon is known as "burn-on"). Chrome ore foundry sand also has low thermal expansion, which facilitates a high degree of dimensional stability. As its basicity is close to neutral, chrome ore foundry sand can be used together with a wide variety of resin bonding systems and inorganic binders.
Key properties of chrome ore foundry sand:
Melting point of 2150°C
Good thermal conductivity
Strong resistance to thermal shock
Excellent resistance to metal penetration or burn-on
Resists slag attack
High dimensional stability
Only around 1 million tonnes out of the 30 million tonnes of chrome ore produced annually is used as foundry sand. The overwhelming majority of the chrome ore produced globally is smelted into ferrochrome, for use as an alloy in stainless steel. In order to be suitable for use as foundry sand, chrome ore has to meet several strict criteria. Its chrome oxide (Cr2O3) content must be above 46% and its silica (SiO2) content must be below 1%, otherwise its chilling properties will be inadequate. The size of the sand particles is also of paramount importance – if the material is too fine it will severely inhibit the strength, performance and lifespan of the casting mould, and more resin and bonding agents will be required.
The vast majority of the world's chrome ore foundry sand is produced in South Africa, where more than 80% of the world's chrome ore resources are located.
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