A New Measure for Active Clay in Green Sand
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Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00514-2
Abstract An alternative approach to measure active clay in green sand has been developed. While active clay determination using methylene blue adsorption is a comparison of the cation exchange capacity (CEC) of greensand to the CEC of a reference bentonite, the new technique allows direct CEC measurement by adsorption of highly selective Cu(II)triethylenetetramine dye and subsequent quantification in a spectrophotometer. The difference in concentration of the dye before and after reaction to the greensand allows to calculate the number of adsorption sites in the molding sand, which are predominately attributed to present smectite clay minerals. The result obtained is the CEC of the molding sand, which can be used to calculate the active clay content or number of smectite clay minerals, respectively, by literature data or the CEC of a reference sample.
The test procedure can discriminate among various levels of active clay content, regardless if sodium or calcium bentonite is available in silica sand. Results in this study with the new approach show repeatability with acceptable test-to-test variability when measuring green sand samples from 6 to 12% active clay. Increasing sample weight allows to discriminate active clay levels even below 4 weight-%. The new proposed method eliminates systematic errors of the methylene blue test and offers advantages in usability and accuracy.
Introduction
Due to the weak surface charge of smectite clay minerals, adsorbed cations can be replaced by other cation species. This cation exchange reaction is controlled by concentration, charge, ion-radius, weight, and specific interactions of cations or cationic molecules with the constituents of the clay mineral lattice.
The AFS 2210-00-S methylene blue clay test (MBT) has been used as a part of foundry control programs to determine the active clay percent in green sand since 1967.1 Active clay content is an approach to quantify the number of smectite clay minerals in a bentonite or molding sand. These minerals control bentonite properties and act as greensand binder. Smectite clay minerals can be characterized as silicate platelets providing weak negative surface charges. These charges are balanced by counterions (Na?, Ca2?, Mg2?) which can hydrate within the interlayer of stacked smectite platelets. If Na?-ions dominate, they are hydrated to the extent that individual platelets exfoliate, and cohesive electrostatic forces develop a bonding film to bind sand particles.
International Journal of Metalcasting
Keywords: cation exchange capacity, dye adsorption, active clay, Methylene blue, spectrophotometer
Active clay determination by methylene blue adsorption is a cation exchange reaction where all available anionic sites at the mineral surfaces will be occupied by methylene blue molecules (Figure 1). Methylene blue adsorption does not quantify the number of smectite clay minerals (aka active clay) in the molding sand or bentonite, but the number
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