The carbon is added to the foundry sand in order to minimize metal penetration. Also, it reduces gas production which causes a poor finish to the casting. The carbon additives range from coke to wood based, and can be used in steel casting. However, many of these additives require high temperatures to reach a carbon atom concentration suitable for use in the casting process. In addition, most of these carbon additives have a large quantity of volatile organic compounds that need to be removed before they can be used in a metal casting.
In the process of casting a metal molten into a chemically-bonded sand mould, rapid heat transfers produce steep thermal gradients which causes the sand grain to expand. In this process, sand is broken down into carbonaceous gases and released in the melt. Carbonaceous gases can be released into the melt and cause unwanted defects in casting such as metal penetration, carbon pickup or lustrous luster.
The preferred carbon additive for foundry sand is an oxidized lignite called FLOCARB or SEACOAL, which is produced through a patented process that converts a portion of the volatile organic content to carbon monoxide and methane. These hydrocarbons, while less hazardous than benzene in terms of toxicities and vaporization risks (which can be deadly when used in casting plants), are still far more effective. The conversion process to methane, carbon monoxide, and humic oxide releases significant amounts of these natural compounds.
A sand add-on's ability to reduce carbon dioxide produced during the heating process of sand with carbon monoxide is another important benefit. It reduces carbon dioxide emission from sand, and therefore the associated atmospheric pollution. This sand addition is highly beneficial for foundries which use fossil fuels as fuels.
Comparing the carbon-based additives described in this document, it is possible to achieve similar or even better mold tensile strengths while also reducing gases, volatile organic compound and harmful air pollutants generated during casting. This reduction in gas production is particularly beneficial for foundries operating on tight emission limits.
SphereOX is superior to all other sand-additives in eliminating common defects such as veining, steel and iron penetration, and an unacceptable surface finish. SphereOX, in fact is the only sand additon that has demonstrated its ability to provide superior results with a casting while still maintaining a level of high mechanical properties. The combination of its thermal stability, and ability to absorb carbon monoxide or other toxic gases is what makes this possible. The additive can either be used on its own or with another type of silica. It is particularly useful with roasted carbon sand, such as that described in Assignee's U.S. Patent No. 5,094,289, incorporated by reference, which has the additional advantage of being designed primarily for low melting temperature metals.
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