The difference between foundry chromite sand and silica sand
Chromite sand and silica sand are commonly used refractory materials in molding sand casting. However, as casting sand for foundry, chromite sand and silica sand have many differences, with the following specific differences:
1. Source and composition:
Chromite sand is a naturally occurring mineral in the form of iron chromium oxide FeCr2O4. It’s a kind of natural chrome ore sand. China’s chrome ore reserves are very small and almost all rely on imports. The main source is chromite sand from South Africa and Zimbabwe. Silica sand is a mineral containing high-purity silica, mainly composed of silica. China has abundant reserves of quartz sand, mainly distributed in Hebei, Shandong, Jiangsu, Yunnan, and Inner Mongolia. In recent years, the supply of alluvial silica sand along the coast of Hainan has also been significant. At present, silica sand in China is not allowed to be exported.
2. Particle morphology:
After crushing the chrome ore, the chromium ore sand is in a sandy shape, with a sub-spherical shape and no sharp edges. The broken particles of silica sand have distinct edges and sharp corners, and the particle morphology of coastal sedimentary silica sand is relatively rounded after seawater scour. For sand used in the casting industry, especially for resin-coated sand processes, rounded particles mean a smaller angular coefficient. The advantage of a small angular coefficient is that it has a smaller specific surface area, reduces the amount of resin added, and at the same time, the particles are not easily broken, have high strength, and good fluidity, and are easy to collapse.
3. Specific gravity and hardness:
The specific gravity of chrome ore sand is 4.5-4.8g/cm3, and the bulk density of sand particles is 2.5-3.0g/cm3. The specific gravity of silica sand is 3.6g/cm3, and the bulk density of sand particles is 1.5-1.6g/cm3. The Vickers hardness of chromite sand is 1278-1456 kg/mm2, and the Mohs hardness is 5.5-6. The Mohs hardness of silica sand is 7-7.5.
4. Fire resistance temperature:
The melting point of chrome ore sand is 2180 degrees, and the fire resistance temperature can reach over 1800 degrees. The melting point of silica sand is 1750 °, and the refractory temperature is 1300-1450 °. For steel castings, the pouring temperature is higher than 1500 °. Obviously, silica sand cannot withstand such a high pouring temperature, and steel liquid is prone to penetrate into quartz sand, causing casting defects. Chromite sand not only has a high refractory temperature but also has good thermal conductivity, which is conducive to the rapid cooling of castings.
5. Thermodynamic performance:
The thermal conductivity of chromium ore sand can be improved due to the lower thermal expansion coefficient of silica sand. That is beneficial for improving the accuracy of castings.
6. Usage:
Due to its good fire resistance, chromite sand is usually used for casting under high-temperature conditions, such as the production of stainless steel, alloys, and cast iron. It can serve as a surface layer of sand that directly contacts steel liquid. Silica sand is widely used in ordinary sand cast iron casting, and can also be used for sand that does not directly come into contact with molten steel.
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In summary, chromite sand has higher refractory temperature, alkaline slag resistance, and thermodynamic properties, therefore it has obvious advantages for processes such as molding sand casting, resin no-bake sand, and shell mold casting.
