Potassium alum crystal derived from aluminum salt in water treatment sludge by nanofiltration

The major method used for the recovery of aluminum from water purification sludge is through acidification or alkalization, however, the cost of the acid and alkali limits the amount available for use, and thus results in the low concentration of dissolved aluminum salt; moreover, the metal ions and organic matter dissolved simultaneously in the process will also increase the difficulty in reuse. Therefore, this study used sulfuric acid to dissolve aluminum in the sludge from the water treatment plant, followed by the low price nanofiltration membranes to increase the concentration of aluminum ions, and then adding potassium sulfate to produce the potassium alum crystal, which has less impurity. The influences of the molar ratio of potassium to aluminum and the temperature on the recovery of crystals are investigated. The result showed that there was good crystal recovery when the molar ratio of potassium to aluminum is above 1.6, but the recovery rate decreased as the temperature increased.