The production of steel castings that use the alkali-phenolic Alphaset process of moulding sand preparation has been recognised as the most convenient and environmentally friendly casting process. Mechanical reclamation of spent molding sand is a process that removes residual binders, fines, and other contaminants to restore sand for reuse in foundry applications.
Difficulties in Mechanical Reclamation
Tough Resin Film: The resin film on the surface of alkali-phenolic resin used sand has certain toughness at room temperature, especially when the water content is above 1.0%. It is difficult to achieve the desired reclamation effect by general mechanical reclamation methods.
Difficulty in Alkali Removal: The potassium in the resin film on the surface of used sand grains is difficult to be completely removed by general thermal reclamation methods (800 – 900°C). The potassium melts at about 360°C and gasifies after 1320°C.
Accumulation of Impurities: Organic esters and other substances accumulate gradually with the increase of the number of times the used sand is used, resulting in the shortening of the available time of the reclaimed sand, the decrease in bonding strength, and the deterioration of the technological properties of the molding sand.
Common Mechanical Reclamation Methods and Their Principles
Mechanical Grinding: Use mechanical equipment such as grinders and crushers to apply mechanical force to the spent molding sand, so that the alkali-phenolic binder on the surface of the sand grains is peeled off, rubbed and crushed, thereby achieving the purpose of separation from the sand grains.
Air Jet Reclamation: Use high-pressure air to spray the spent molding sand, and the kinetic energy of the high-speed airflow makes the binder on the surface of the sand grains fall off. The high-pressure air can also take away the fine powder and impurities produced during the reclamation process.
Vibration Reclamation: The spent molding sand is placed on a vibrating device. Through the vibration of the equipment, the sand grains collide and rub against each other, so that the binder on the surface of the sand grains is loosened and removed.
Influencing Factors of Mechanical Reclamation
Equipment Parameters: Different types of mechanical reclamation equipment have different structures and working principles, and their reclamation effects will also vary. Parameters such as the rotation speed, vibration frequency and amplitude of the equipment have a great influence on the reclamation effect.
Sand Properties: The particle size distribution, shape and surface characteristics of the spent molding sand have an impact on the mechanical reclamation effect. For example, sands with different particle sizes and shapes will have different degrees of collision and friction during mechanical reclamation, thus affecting the removal efficiency of the binder.
Reclamation Environment: Environmental factors such as temperature and humidity also affect the mechanical reclamation process. For example, in a humid environment, the surface of the sand grains is prone to moisture, which may increase the viscosity of the alkali-phenolic binder and affect the reclamation effect.
Mechanical Reclamation Steps
1. Lump Breakdown
Large sand lumps are broken down using a lump crusher or impact mill.
2. Attrition & Scrubbing
Sand grains undergo high-speed attrition to remove adhered resin.
Equipment: Attrition mills, rotary scrubbers, or vibrating reclamation units.
3. Screening & Particle Size Classification
Fine dust, degraded resin, and oversized grains are separated using sieves or air classifiers.
4. Cooling & Conditioning
Reclaimed sand is cooled and mixed with additives (if needed) before reuse.
Mechanical Reclamation Advantages
- Cost-effective and energy-efficient
- Suitable for multiple reclamation cycles
- Removes a significant amount of binder (~70-80%)
Combination with Other Reclamation Methods
Combination with Thermal Reclamation: Thermal reclamation can burn off most of the organic binder in the spent molding sand at high temperatures. However, for alkali-phenolic resin used sand, simple thermal reclamation is difficult to completely remove the alkali metal. Therefore, it can be combined with mechanical reclamation. The high-temperature softened binder after thermal reclamation is further removed by mechanical force to improve the reclamation effect.
Combination with Chemical Reclamation: Chemical reclamation involves adding chemical reagents to the spent molding sand to make the alkali-phenolic binder undergo chemical reactions to achieve the purpose of decomposition and removal. Chemical reclamation can be used in combination with mechanical reclamation. The chemical reagents first act on the binder to weaken its bonding force with the sand grains, and then the mechanical reclamation further separates the binder from the sand grains.
Conclusion
Alkali-phenolic resin-bonded sand is commonly used in foundries due to its good collapsibility, low emissions, and excellent casting performance. However, after use, the sand must be reclaimed to remove residual resin and restore its properties for reuse. The reclamation process can be mechanical, thermal, or a combination of both, depending on the level of binder removal required.
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