Utilization of Induction-Furnace Slag as a Partial Silica-Sand Replacement in CO 2 –Sodium Silicate Molding: Effects on Mold Properties and Casting Quality
This study demonstrates that substituting 25–30% of silica sand with induction-furnace slag in CO₂-sodium silicate molding offers a viable, cost-effective solution that maintains acceptable mold properties and casting quality while valorizing industrial waste.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Every year, steel foundries face a double burden. On one side, they must pay rising costs for high-purity silica sand, the gritty, white material that forms the molds into which molten metal is poured. On the other side, they must pay to dispose of the heavy, rocky waste left behind after melting steel, known as slag. Usually, this slag is hauled away to a landfill, a costly and environmentally taxing process. Meanwhile, the demand for silica sand is tightening as much of the world's supply is diverted to other industries, and the health risks of breathing silica dust are driving factories to seek safer alternatives. The question facing engineers is simple but difficult: can these two problems be solved at once by turning the waste slag into a useful part of the mold?
To answer this, researchers turned to a specific type of mold-making process common in medium-sized factories. In this method, a mixture of sand and a liquid binder called sodium silicate is packed tightly around a pattern to create a hollow shape. Carbon dioxide gas is then blown through the mixture, triggering a chemical reaction that turns the liquid binder into a rigid, glue-like network, hardening the mold in seconds. This rapid hardening allows for quick production of steel parts. The challenge is that the sand used must be strong enough to hold the shape and porous enough to let gases escape when the molten metal hits it. If the sand is too weak, the mold breaks; if it is not porous enough, the metal traps gas and forms bubbles or holes in the final product.
A team of researchers decided to test whether they could replace some of the expensive silica sand with crushed induction-furnace slag, the waste product from melting a specific low-alloy steel. They did not just guess at the right amount; they created a series of test molds where the slag replaced the sand in increasing amounts, starting with a small fraction and going up to nearly half the total weight. For each mixture, they packed the sand into cylinders, hardened them with gas, and then subjected them to a battery of standard tests. They measured how easily air could pass through the packed material, how well the mold resisted breaking when dropped or squeezed, and how hard the surface was. They also cast actual steel parts in these molds to see if the changes in the sand mixture caused any visible defects on the metal surface.
The results were clear and consistent. As the researchers added more slag, the performance of the mold dropped steadily. The ability of the mold to let gas escape decreased, and the strength of the mold—how much force it could take before crumbling—fell significantly. By the time the mixture contained forty percent slag, the mold was noticeably weaker and less porous than the pure sand version. The researchers traced this decline to two main reasons. First, the chemical makeup of the slag was different; it contained less silicon and far more iron, manganese, and other metals than the pure silica sand, which meant the liquid binder did not stick to the slag particles as effectively. Second, the physical size of the slag particles was a problem. Even though the researchers had crushed the slag to look similar to the sand, the final material was much finer and contained far more tiny dust-like particles. These fine particles packed together too tightly, blocking the tiny gaps where gas usually escapes, and they required more of the liquid binder to coat their surfaces, leaving less binder available to hold the larger grains together.
Despite these declines in strength and porosity, the study found a practical middle ground. When the slag made up between twenty-five and thirty percent of the mixture, the mold remained strong enough to handle the rigors of production and porous enough to let gases escape safely. To confirm this, the team poured molten steel at temperatures of 1600 degrees Celsius into molds with varying amounts of slag, from none at all to the highest levels. After the metal cooled and the molds were broken away, the resulting steel parts were inspected. Remarkably, none of the castings showed signs of surface defects like pinholes or cracks, regardless of how much slag was in the mold. The surface texture of the metal remained smooth and consistent across all tests, showing no worsening trend as the slag content increased.
This work suggests that foundries can safely replace a significant portion of their silica sand with this specific type of steel slag without ruining the quality of their castings. By using twenty-five to thirty percent slag, a factory could reduce its reliance on expensive, scarce sand and simultaneously cut the cost of disposing of its waste. The study does not claim this works for every type of slag or every metal, as the chemical composition of waste varies widely, but for the steel they tested, the solution is viable. It offers a straightforward path to turning a waste product into a resource, proving that with careful testing, industrial byproducts can find a new life in the very process that created them.
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