Recuperação da água de concentrado salino de eletrodiálise reversa por cristalização visando seu reuso na indústria

Abstract

The decrease of water availability in the last years has motivated researchers to investigate alternative ways to reduce water consumption and improve reuse of water in many industries. The main objective of this work was to understand the influence of commonly used treatment techniques on the purification of industrial saline waste and also to identify a suitable technique to recover pure water that meets standard requirements for reuse. Evaporative crystallization technique was used to recover water from electrodialysis reverse saline concentrate, at laboratory scale. This technique was able to selectively remove the main salts responsible for scaling. However, these were recovered as a mixed salt of calcium sulfate, barium sulfate and calcium carbonate. The quality of the water obtained was suitable for reuse on cooling towers and boilers with a typical composition of 8mg/L of CaCO3, 1.6mg/L of Ca, 0.04mg/L of Mg, 6mg/L of Cl-, less than 10mg/L of SO42- and 23mg/L of total solids dissolved. Thus, the technique significantly eliminated the volumes of wastewater to be discharged. However the solid residue produced is still classified as not dangerous waste based on the Brazilian standard NBR10004 requirement. This study also identified incrustations in the reactor and foam formation, which are some of the main problems associated with the presence of carbonated and sulphates solids during crystallization. Further investigations should be carried out to address these undesirable implications for pilot plant operations. The effect of phenol as a water soluble organic contaminant in the saline wastewater was also studied using eutectic freeze crystallization. A synthetic solution of composition similar to that of the electrodialysis concentrate was used. Although the presence of phenol significantly improved the crystal growth of sodium sulphate only slight changes on its dissolution and eutectic temperature were noted. Eutectic Freezing Crystallization and Alkaline precipitation were investigated as potential pre-treatment methods for electrodialysis reverse concentrate. Though eutectic freeze crystallization presented lower energy consumption than evaporative crystallization, the separation of solids was very poor resulting in a water stream that does not meet the reuse requirement. However, eutectic freezing crystallization presented to be more efficient in simultaneous solid separation of calcium sulphate and sodium sulphate. In alkaline precipitation experiments, sodium hydroxide and calcium hydroxide were studied. Complete removal of calcium and magnesium ions was achieved through precipitating saline concentrate with 400g/L sodium hydroxide. The main precipitated solids were characterized as calcium and magnesium carbonate, calcium carbonate and magnesium hydroxide.