Optimization of foundry sand characteristics of soda-activated calcium bentonite

Abstract

Following soda activation, the bentonite properties fluctuate and change more or less with time depending on the soda addition. Since these changes on activated bentonite properties are reflected in the further use of these products, controlling methods should be developed to optimize the product quality. In this study, optimization was achieved using the Response Surface Regression method (RSM) for activated calcium bentonites used in foundry sands. Evaluating the influence of soda addition and aging time on the bentonite properties, the activation conditions were optimized.

Introduction

Sand casting though the least accurate one is the most common and most widely used method of casting which involves the pouring of molten metal into a mould including typically 80% silica sand, 5–10% clayey binders (mostly bentonite), 2–6% tempering water and a variety of additives. Sand casting is mostly used for high volume production where surface finish and dimensional accuracy are not critical (Roskill Information Services Ltd, 2005).

Although bentonites are the most common binders in clay-bonded foundry sand, fireclays, kaolin and ball clays can also be used as binders but their use is generally restricted to dry sand casting of steel products.

There can be considerable variations in the properties of foundry sands containing different types of bentonite. Natural sodium bentonites generally have higher fusion temperatures and higher dry strength and thus are more durable and produce less scabbing defects on the finished casting. Many sodium-exchanged bentonites cannot meet the liquid limit requirement for steel castings, but exhibit good green strength. Calcium bentonites may facilitate the release of castings from the mould. As a result, there is a tendency to prefer certain types of bentonite for particular types of casting (Roskill Information Services Ltd, 2005).

Ca-bentonites are usually activated with soda using varying proportions of soda. Alkali or soda activation with sodium carbonate (Na₂CO₃) converts the calcium and/or magnesium bentonites into sodium–calcium forms. In this process which was first proposed by Hofmann and Endell (1936), bentonite moisturized to 35–40% (m/m) is usually kneaded or milled with 1–5% (m/m) Na₂CO₃ and homogenized (Abend and Lagaly, 2000, Bergaya et al., 2006). Freshly excavated bentonites usually have moistures in the range of 30–40%, and no additional moisturizing process is required in industrial applications. Activation is generally grouped into three categories; (i) activation with bentonite moisture < 22.5% (dry activation), (ii) with 22.5%—the moisture of the flow point of the mud and (iii) with the moistures higher than the moisture of the flow point of the mud. In industrial application, activation is mostly performed with a moisture level of 35–45% which provides suitable kneading and does not lend itself to additional drying costs.

The aging process starts with activation. In the first stages, some improvements on bentonite characteristics are observed followed by the degradation. Lebedenko and Plee (1988) reported that the activated bentonite characteristics fluctuate depending on the activation/curing time called aging time. Aging the soda activated bentonites can enhance or reduce the viscosity. This rheological behavior depends on the Ca/CEC ratio.

Since bentonite is used as a binder in foundry sand, changes on bentonite properties (swelling ability; filtration loss, etc.) will affect the molding properties of the silica sand–bentonite mixture (foundry sand) such as compactibility, mouldability and strengths of dry, green, wet tensile, splitting and twin transverse (shear) to form the mould with desired properties. Such ability will provide the means to handle it before and after operation and also during the casting process.

In this study, the aging of soda activated calcium bentonites and its influence on the properties of the foundry sands were examined by the response surface methodology. In this way, the activation conditions could be optimized.