3. Results and discussion
Cleaning by calcination. The thermogravimetric and thermodifferential analyses in the samples of regenerated and recovered sands (Fig. 3) showed that the mass loss, originated from the decomposition of the phenolic resin, essentially occurs
at the 450–550 ◦C temperature range. Higher calcining temperatures mean unnecessary energy expenses, besides leading to a more intense formation of agglomerates. This agglomeration arises from the sintering mechanism through the formation of a liquid phase, formed by means of the reaction between the residual sodium and potassium and the fine silica grains.
Fig. 4 presents the heating curves of the recovered and regenerated sands, which were observed during the simulations at different heating rates.
The particle size distributions, related to before and after calcinations, are represented by the curves presented in
Figs. 5 and 6 for the recovered and regenerated sands, respectively. These results showed that the recovered sand, as well as the regenerated sand presented an increase in the coarse fraction (above 0.5 mm) and a reduction in the fine fraction (below 0.15 mm). This evidences the agglomeration effect of the fine sand during the calcination process, which is probably associated to the reaction of the residual alkalies, from the phenolic resin (Na and K), with the fine silica grains, leading to the formation of a liquid phase and grain agglomeration.
The thermogravimetric and thermodifferential analyses of the recovered and regenerated sands, performed after the calcination (Fig. 7), evidenced the efficiency of this process. After the calcination, no further mass losses were verified.
However, the results of compressing resistance of the regenerated sand agglomerated with alkaline phenolic resin (Fig. 8) showed that the calcination process is not efficient,regarding the regeneration of the initial properties of the material.
According to the obtained results, the calcination process significantly degrades the mechanical properties of the sand. This is probably due to the reaction of triacetine with the agglomerating alkaline solution of phenol–formaldehyde, once the salts originated from such reaction adhere to the surface of the sand grains, reducing therefore the agglomerating capacity