Montmorillonite: a Comparison of Methods for Its Determination in Foundry Bentonites

Currently, approximately 70-80 % of casting production of ferrous alloys is done with sands containing bentonite (green sands). Information on an amount of active bentonite in moulding sands, after shakeout of casting, is necessary in the rebounding process of bentonite sands – under real casting house conditions. It decides how much of fresh bentonite should be added to moulding sands, thereby influencing their technological properties and cost of the rebounding process 1 . A standard method of an active bentonite estimation in moulding sands normalised in the Polish foundry plants and in the majority of European ones is based on the methylene blue adsorption 2 . This is a time-consuming method and therefore the results are available for a technologists some hours after the introduction of rebound sands into the circulation. This is specially important in the case of a variable casting production (it mainly concerns casting sands), which causes different sand overheating degree, thus causing various amounts of inactive bentonite in a whole sand volume. Therefore several companies which are dealing with a bentonite or mixtures bentonite – lustrous carbon carrier production for the foundry industry, perform intensive investigations in order to develop a reliable and simultaneously fast method of the active bentonite (montmorillonite) estimation in moulding sands. In order to do that, the spectrophotometric determination method, which is applying triethylenetetramine complex (Cu(II)-TET) used in geology for checking clays activity, was also introduced. This method is much faster and much simpler from the analytical point of view, than the methylene blue adsorption method. Tests of applying the infrared spectroscopic method (FTIR) are also undertaken. This method allows for a very fast determination of active bentonite, which is essential under production conditions. However, it has certain limitations related to some admixtures present in bentonite, causing distortions of the final result 3–6 . Investigations in this field aimed at the development of the proper analytical method as well as the correct in-