A flotação por ar dissolvido como alternativa ao tratamento de efluente mineral visando ao reuso da água e à melhoria do processo de flotação de apatita

Abstract

Increasingly stringent restrictions on the disposal of effluent associated to the scarcity and high cost of water require actions to improve its reuse. The water reuse in the industries became urgent and nowadays the water is recognized as a strategic resource for the mining industry due to economic issues and environmental restrictions. However, despite this scenario, investments and research are incipient to optmize its reuse. The mineral sector industries face a major difficulty related to obtaining yield indices economically viable since they are increasingly using primary sources of water with high levels of salinity or high proportions of recycled water from tailing ponds. The latter, in turn, may contain considerable amounts of dissolved species, residual reagents as collectors, depressants, activators, flocculants, particulate matter, etc., which can significantly affect the cost and efficiency of the flotation process. Particularly, the apatite flotation is strongly affected by the presence of certain contaminants in the water such as calcium, magnesium, phosphate and fluoride, directly influencing recovery and P2O5 content obtained. The removal of these ions from the water before returning it to the process is needed in order to ensure the quality and yield of the apatitic concentrate. The dissolved air flotation (DAF) has been shown to be an efficient technique in the water treatment. Nevertheless, in the DAF application it is important to perform steps of coagulation and flocculation aiming a satisfactory removal of ionic charge. This main goal of this work was to obtain a better quality reuse of water through physical-chemical treatment by dissolved air flotation (DAF) aimed at improving the subsequent apatite flotation process. It was collected 2 batch of process water from the tailings dam of Vale Fertilizantes (Araxá-MG) named B5. It was tested coagulants such as alum (aluminum sulphate), ferric chloride and polyaluminum chloride (PAC). An anionic polyacrylamide was used as flocculant. The treated water by DAF from the 1st batch of water dam resulted in a good reduction in turbidity and ionic charge mainly of fluoride and magnesium ions by setting the variables in 500 mg/L of alum, 2 mg/L of flocculant and 20% recycle rate. However, while the removal of calcium was favored only at a pH value of 6, an efficient removal of phosphate (P) only occurred at pH value of 11. Apatite flotation tests when carried out with water from the dam (1st batch) resulted in a decreased in the apatite recovery of approximately 38% and 7% in the P2O5 content. These results make clear the worsening in the productivity and the quality in the concentration of apatite process with the water reuse without treatment. The apatite flotation tests performed with treated water by DAF showed to be quite satisfactory since the apatite recovery values have been restored providing mean values above 60%. Remember that test conducted with fresh water yielded a mean value of 63.6% in the apatite recovery. In the 2nd batch of dam water collected, it was made by the company a previous treatment adding sulfuric acid. The treatment with this 2nd provided very favorable results from removal of the ionic charge, including calcium and phosphate (P), ion that most affect the process of apatite flotation. With the exception of fluoride, which only achieved good removal with PAC, good results removal of calcium, magnesium, and phosphate (P) were achieved with aluminum sulphate (50 mg/L of Al3+), ferric chloride (5 mg/L Fe3+) and PAC (50 mg/L Al3+) at pH values of 11 and 12. It was possible to obtain 90.3% removal of calcium and phosphate 95.4% (P). The apatite flotation tests with the 2nd batch of water from the dam (without treatment) resulted in the recovery mean values of only 36.4%, representing a drop of 54% in productivity flotation. This drastic decrease is strongly related to ionic charge present in the dam water which was twice that observed in the 1st batch. Again, the flotation tests carried out with the treated water produced apatite recovery values that can be considered statistically similar to that achieved with fresh water (79.1%). The system coagulation/flocculation/DAF showed to be a promising alternative for improving water quality, whether for disposal in water bodies or for reuse in the concentration of phosphatic ore from Araxá (MG). Thus, the results highlight the importance of adequate treatment of recycled water in mineral processing plants.