Is a lower silica content always better for chromite sand?
In general, a lower silica content is better for chromite sand, but it is not absolute and needs to be comprehensively considered in combination with specific casting processes and cost – effectiveness. The following is a detailed analysis:
- Advantages of low silica content
- Improving refractoriness: Chromite sand is mainly used in casting due to its high refractoriness. Silica in chromite sand comes from gangue. If the silica content is high, it will deteriorate the refractoriness of chromite sand and cause “glazing”, that is, at high temperatures, silica reacts with metal oxides to form low – melting – point compounds, which affects the quality of castings. When the silica content is low, the refractoriness of chromite sand can be effectively improved, and it can maintain stable performance at high temperatures, avoiding reactions with molten metal and preventing defects such as sand – sticking.
- Reducing thermal expansion risk: Silica will undergo phase transformation during the heating process, accompanied by volume expansion. If the silica content in chromite sand is too high, the sand mold or core is prone to cracks due to uneven expansion at high temperatures, which will lead to casting defects such as sand inclusions and sand holes. A lower silica content can reduce the structural instability caused by this thermal expansion and improve the dimensional accuracy of castings.
- Limitations of excessively low silica content
- Increased production costs: Natural chromite sand usually contains a certain amount of silica, and it needs to go through processes such as magnetic separation and flotation to remove it. If an extremely low silica content is required, for example, reducing the silica content from 5% to less than 1%, the complexity and energy consumption of the purification process will increase significantly, resulting in a multiple – fold increase in the purchase price of chromite sand, which will increase the production cost of castings.
- Unnecessary performance surplus: For some ordinary cast steel and cast iron parts with lower pouring temperatures (such as 1300 – 1500°C), as long as the silica content of chromite sand is controlled within 3% – 8%, it can effectively avoid obvious defects, and there is no need to pursue an extremely low silica content. Because in this case, an excessively low silica content will not bring obvious performance improvements, but will only cause a waste of resources.
In actual production, the silica content of chromite sand is usually required to be lower than 5%, and it is best to be below 3%. However, specific requirements should be determined according to the casting material, pouring temperature, and quality level.