Aplicação de caulim como ligante em moldes de areias a verde de fundição

Abstract

Green sand molding techniques are widely used in the production of castings. Foundry green sand is a mixture of a refractory base sand, bonded with clay and water. Industrially, silica-based sand is the most widely used in the world, especially in countries that have these raw materials at low cost. Sodium bentonite, on the other hand, is the clay of greatest application due to its superior properties as a binder. Brazil imports natural sodium bentonite, but it has good reserves of calcium bentonite in the north and northeast of the country from which they are converted to activated sodium bentonite. The application of sodium bentonite in the foundry sector is consolidated, however, there are more than 140 industrial applications described in the literature, in Brazil the mining and oil production industry stands out. The growth of these sectors puts great pressure on the demand for bentonite and is the motivation for the present study. In this work, the integral replacement of sodium bentonite by kaolin as a binder in the molding sands is evaluated. In fact, few recent studies have pointed out alternatives for the components of foundry green sands. Kaolin was chosen for its ceramic properties, especially its refractoriness, but on the other hand it has less plasticity. In choosing this clay, the logistical aspect, the widespread occurrence of this mineral in Brazil, the technological advances in its production chain, and the local supply in various regions of the country were also decisive. The research evaluated several mixtures of silica sand, kaolin and water in the balance of the three basic properties for foundry molds: compactability, mechanical resistance and permeability. The properties were evaluated according to humidity, following recommendations by the AFS (American Foundry Society). Kaolin did not undergo any concentration or grinding process, being used in-natura. Being characterized by X-ray diffraction techniques (XRD), scanning electron microscopy (SEM) with microanalysis (EDS), thermogravimetry (TG), differential calorimetry (DSC) and X-ray fluorescence (FRX). The silica-based sand was analyzed according to the AFS recommendations for granulometric distribution, fineness modulus and AFS clay content. As a result, mechanical strengths close to 9.8 N / cm² and permeability above 200 cm⁴.gˉ¹.minˉ¹ were obtained, generating a wide working range even with fresh kaolin. The results showed that the use of kaolin in partial or total replacement of sodium bentonite is feasible and indicated the process limit parameters for its application in molding green sands.