How is it Used?
Casting of metals is a six-step process.
- A pattern is placed into sand,
- The pattern shape is incorporated into the sand as a mould using a flask,
- The pattern is removed,
- The mould is filled with molten metal,
- Allowed to cool/solidify,
- Finally the mould is removed leaving the finished casting.
During the process of casting, the latent heat from the molten metal cures the mould in situ. The molten metal/alloy is poured into the mould directly after it has been compacted into shape. After cooling of the casting, the mould can be removed(2). Spent moulds are typically ground down and re-used, especially if based on one of the more expensive sand types, though it is noteworthy that the US and UK send on average 500kg and 250kg respectively of sand to landfill for every tonne of cast metal(1). Studies have shown that spent foundry sand can be re-used in concrete production(3). Green sand casting is used for a wide variety of casting applications, from small and detailed, to large moulds up to 500 kg in size.
Coal Dust
Coal dust, or anthracite, is fast becoming a common additive in the green sand moulding process. It is a carbonaceous additive that under foundry conditions combusts and oxidises. Historically, the leading additive in green sand casting, as a carbonaceous material, was highly volatile bituminous coal, often referred to as sea coal. Bituminous coal combusts in the mould upon heating and releases hazardous pollutants such as benzene, xylene and toluene. It is imperative that bituminous coal is replaced with an equally well performing carbonaceous material, but with a less environmentally damaging profile. An experimental study showed that under laboratory conditions, anthracite and lower-grade lignite-type coals(7) emitted significantly less hazardous pollutants than did bituminous coal(8), therefore it can be stated that the incorporation of coal dust into a green sand mould is advantageous from an environmental perspective. It is noted that the casting industry wishes to move away from bituminous coal because of its poor environmental performance (9) and fewer hazardous pollutants mean fewer resources need to be dedicated to scrubbing of the foundry’s exhaust overall.
The use of ground anthracite decreases burned-on defects, improves the finish on the surface of the product, and decreases metal penetration. Perhaps its major use, however, is to prevent wetting. Wetting is the process by which the liquid metal/alloy sticks to the sand particles in the green sand mould, leaving them present on the surface of the casted product. Whilst in many cases, mechanical polishing or further machining of the casted product can be performed to remove errant sands, prevention is better than cure. Coal dust provides an effective and inexpensive method of achieving this.
When the mould is heated by the presence of the melted metal/alloy, the coal present decomposes (burns) and gives off carbon dioxide and a variety of volatile organic compounds. The production of such reducing gases inhibits the production of iron oxide (in iron and steel casting applications) which have been hypothesised as an intermediate in the production of ‘burn on’ - impurity deposits on the surface of the casting(10). In addition, due to the reducing environment, a layer of highly lustrous carbon is deposited on the mould surface, affording a refractory barrier between molten metal and sand, thus improving the surface finish(11) and making the casting easier to remove from the mould.
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