Key Takeaways & Executive Findings
- •• A novel repeated approximation method accurately quantifies oolitic content in green sand by monitoring mass changes during controlled KOH reactions. • Complete removal of oolitic deposits achieved after three KOH treatments at 300°C for 20 minutes, validated by SEM and EDS. • The method overcomes limitations of existing KOH and H3PO4 techniques that overreact with SiO2, providing reliable and consistent results. • Industrial implementation at Huangshi Dongbei Casting Co., Ltd. confirms its practical utility for monitoring and improving green sand reuse efficiency.
Abstract
The reuse of green sand in casting production is hindered by the accumulation of oolitic deposits, primarily composed of clay binder with surface degradation, which may adversely affect the the moulding sand performance. Currently, there is a lack of standardized methods for quantifying the oolitic content. Accurate measurement of oolitic content is of great significance to the reuse of green sand. Attempts to determine oolitic content using potassium hydroxide (KOH) and phosphoric acid (H3PO4) methods encounter challenges due to their excessive reactions with SiO2 in the sand. In this study, an improved method for measuring the oolitic content of green sand with repeated approximations was proposed. This method judges the chemical activity of the sample surface through the change of its mass to accurately obtain the mass of the reaction oolitic deposits. The test result of the used sand samples from the foundry shows that the oolitic deposits are completely removed after reacting with KOH solution three times at 300 °C for 20 min. SEM and EDS also show that after three times of reactions, the surface of green sand becomes smooth and the content of Al-containing oolitic deposits is very low. This indicates that the method can accurately control the extent of the reaction. Implementation of this method at Huangshi Dongbei Casting Co., Ltd. has yielded consistent and reliable test results, effectively mirroring variations in green sand oolitic content on the production line. This new method is expected to be widely adopted to improve the efficiency and quality of reused green sand in casting operations.
1. Introduction
Due to its low cost of raw materials, easy treatment, strong reusability, and strong ability to adapt to molding conditions, the green sand is currently the most widely used casting sand [1-2]. The green sand is made of bentonite, silica sand, water, and other additives such as coal or starch [3]. Bentonite, as a binder, is a non-metallic mineral with montmorillonite as the main mineral component. Its main chemical components are SiO2, Al2O3, and H2O, as well as Fe, Mg, Ca, Na, K, and other elements [4-8]. Bentonite expands when absorbing water, disperses and covers the surface of sand particles to form a viscous film after sand mixing. This film gathers the sand particles together through a hydrogen bond to make the mold obtain enough strength and maintain the required shape during molding process and metal pouring [9-11].
The recycling performance of bentonite is significantly affected by the mold’s temperature field during pouring [12]. When the temperature is below 300 °C, bentonite will only lose free water and can be completely recycled [13]. Schiebel et al. [14] made the green sand undergo 22 times of drying and rewetting cycles below 225 °C, and its water absorption performance and tensile strength did not change significantly. At above 300 °C, the binding ability of bentonite was gradually losing. At 500 °C to 700 °C, montmorillonite undergone a dehydroxylation reaction. Holtzer et al. [15] observed that a bentonite could undergo two endothermic dehydration processes at 519 °C and 672 °C, respectively. When the bentonite was heated to 600 °C, the montmorillonite content gradually decreased. After the bentonite was kept at 700 °C for 1 h, the montmorillonite content was almost zero, and the bentonite lost binding property. When the temperature reached above 700 °C, Khan et al. [16] believed that the dead clay, which had lost its cohesive ability would be sintered in large quantities. The sintered dead clay closely combined with the sand particles to form the oolitic deposits, which were difficult to remove. When the temperature was higher than 900 °C, the phase structure of montmorillonite was completely destroyed and transformed into amorphous phase. At 1,100 °C, the amorphous phase was recrystallized to form a new cristobalite phase [17].
The recovery and reuse of foundry green sand has great environmental and economic value. However, in the recycled green sand, the bentonite layer deactivated by high temperature will adhere tightly to the surface of the sand grains and form oolitic deposits during multiple accumulations. It may influence the properties of moulding sand and the quality of casting. Hofmann et al. [18] proposed that the oolitic process increased the particle size and the roundness of the related sand. It is pointed out that moderate oolitization is conducive to reducing thermal expansion and avoiding sand expansion defects. However, when the ooolitic content is too high, due to the insufficient fire resistance of the thicker wrapped oolitic deposits, the sand mold will be penetrated.
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Yu-yang Qi, Jian-xin Zhou, Xin Peng, Si-yuan Pan, Ya-jun Yin, Xiao-yuan Ji, Yuan-cai Li, Peng-fei Lin, Peng Wan (2025). A new oolitic content test method for green sand by repeated approximation. China Foundry. https://doi.org/10.1007/s41230-025-4043-z
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Frequently Asked Questions
What is the main challenge in measuring oolitic content in green sand?
Existing methods using KOH or H3PO4 react excessively with SiO2 in the sand, leading to inaccurate measurements. The new repeated approximation method overcomes this by controlling the reaction extent through mass change monitoring.
How does the repeated approximation method work?
The method involves reacting green sand samples with KOH solution at 300°C for 20 minutes, repeated three times. The mass change of the sample indicates the amount of oolitic deposits removed, allowing accurate quantification.
What are the benefits of accurate oolitic content measurement?
Accurate measurement helps in controlling the quality of recycled green sand, preventing defects like sand expansion and penetration, and improving the efficiency and sustainability of casting operations.
Has the method been validated in industrial settings?
Yes, implementation at Huangshi Dongbei Casting Co., Ltd. has shown consistent and reliable results, effectively reflecting variations in oolitic content on the production line.
What are the environmental and economic implications of this method?
By enabling better reuse of green sand, the method reduces waste and raw material consumption, contributing to environmental sustainability and cost savings in foundries.
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