Otimização da remoção de fluoreto e alumínio de efluente de barragem de mina de cobre

Abstract

This work comprises the study of the methodology for treating the raw liquid effluent from a dam of a copper mine in the region of Carajás (Pará) to discharge this treated effluent into a water stream that passes in the mine area. The levels of total fluoride and dissolved aluminum were above the maximum limit allowed by the applicable standard (CONAMA Resolution No. 357/05 - Fresh Water - Class 2 with maximum limit for total fluoride of 1.4 mg/L and for dissolved aluminum of 0.1 mg/L). The presence of fluoride and aluminum in high levels, and in water that can be used for human consumption, represent health risks. The effluent treatment process includes the steps of fluoride complexation with an aluminum salt, coagulation, flocculation, sedimentation and filtration. The Effluent Treatment Station (ETE) operation indicated that the raw liquid effluent was frequently arriving for treatment with turbidity below the minimum design condition of 2 NTU and the treated liquid effluent was not meeting the applicable standard for total fluoride and aluminum dissolved. It was verified that the inefficiency of the process could be due to the large number of flocs suspended in the decanter that were being carried to the filters, clogging them and impairing the removal of contaminants. The proposed solution was to increase the initial turbidity of the raw liquid effluent with the use of bentonite (a clay composed by smectite minerals) so that the flakes could have more weight and better sedimentation. Bench tests in Jar Test were carried out to evaluate the behavior of the effluent with the bentonite and to optimize the dosage of the reagents, as well as to try to better understand the mechanism of fluoride and aluminum removal. Bench tests in Jar Test showed that the ETE methodology for treating the dam effluent to remove fluoride and aluminum is effective when using bentonite combined with an anionic polymer powder (polymer B). The levels of total fluoride (1.01 mg/L) and dissolved aluminum (0.08 mg/L) in the treated and filtered effluent were below the maximum allowed values. The fluoride content in the treated and decanted effluent was 1.65 mg/L, above the limit, but close to it. The lower the fluoride content in the decanted effluent, the better it is for the process, as it indicates that the sedimentation is efficient and not many flocs are being carried to the filters. Using the same anionic polymer in emulsion (polymer A) that is used in the industrial ETE, the same efficiency was not achieved. It is important to note that the maximum aluminum content allowed for in the standard is very low and there is a risk that the residual aluminum from the process will exceed it, even with polymer B. The predicted mechanism of fluoride and aluminum removal is coagulation with complexation, in which fluoride is first complexed with aluminum sulfate and the Al-F complex acts as a precursor to coagulation, generating the Al-F-OH coprecipitate, and promoting the removal of the desired elements, which are retained in the sedimentation and filtration stages.