Remoção de íons sulfato aplicando a hidrocalumita calcinada como agente precipitante

Abstract

The hydrocalumite (Ca2Al(OH)6Cl.2H2O), a layered double hydroxide (LDH), was synthesized and calcined at different temperatures in order to evaluate its application as a precipitating agent in the sulfate removal. For the characterization of the used materials, the techniques of X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The precipitation tests were carried out by evaluating the reaction time (up to 300 minutes), dosage (2, 4 and 6 g L-1) and thermal treatment of the precipitating agent (without calcination and calcined at 100, 200, 300 and 400°C). In addition, the influence of nitrate and phosphate anions on the sulfate removal and the reuse of the sludge generated in the process were analyzed. In precipitation tests, higher hydrocalumite dosages resulted in shorter reaction times to achieve process stability, due to the greater amount of precipitating agent to react with the anion. With the lowest dosage of precipitating agent analyzed, the largest sulfate removal was 64.8±4.0%, in 120 minutes of reaction with hydrocalumite calcined at 100°C, considering the initial concentration in all tests close to 950 mg (SO4-2) L-1. Higher dosages resulted in shorter reaction times to achieve stability in the sulfate precipitation, due to the greater amount of precipitating agent to react with the anion. The application of calcined hydrocalumite showed higher values for anion removal, in relation to the value obtained with the synthesized hydrocalumite without thermal treatment; however the increase in the calcination temperature did not influence the final sulfate concentration. The sulfate removal on increasing order in tests with interfering anions was: 30.3% (assay with all anions), 33.8% (assay with sulfate and phosphate), 82.7% (assay with sulfate and nitrate) and 90.1% (test with only sulfate). Regarding the evaluated anions, the presence of phosphate showed greater interference in the precipitation of sulfate than the presence of nitrate. The reuse of the thermally treated sludge in the sulfate precipitation did not show significant results in view of the low value for sulfate removal obtained, so it was only possible two reapplication cycles. With the characterization of the synthesized samples, the formation of hydrocalumite and the collapse of the lamellar structure of HDL at 400°C was confirmed. In the samples obtained after the sulfate removal test, the presence of the ettringite and gypsum phases was observed by the DRX, which confirms the anion precipitation. Yet in the samples of the treated sludge, was observed the presence of phases containing sulfate, calcium and aluminum. Thus, the application of calcined hydrocalumite as a precipitating agent in the sulfate removal proved to be effective due to the satisfactory results obtained.