Nanofiltration of nitric acidic solutions from Picture Tube production

In this paper the pilot tests of a recycling process for removing lead form nitric acid solutions have been presented. At first batch tests were performed. Permeate yields up to 80% could be achieved in these tests. Corresponding to these yields the lead concentrations rose up to 70 g/l. Due to the saturation limit of lead salts at about 75 g/l no higher yields could be achieved without precipitation of the lead salts. Altering a washing step of the production avoided the input of free silicic acid into the nitric acid solution. This step ensured that there was no fouling of the membranes due to the precipitation of silicon dioxide. Continuous tests showed that the permeate flows and the lead rejection rates remained stable over a long period of time. After successful completion of the pilot tests the concept has been realized on technical scale. Using the nanofiltration it is now possible to recycle approximately 80-90% of the nitric acid, which formerly had to be exchanged. In addition to the reduced need for fresh acid for the etching process, the alkali needed for neutralizing the waste acid stream could also be drastically reduced. As an added benefit the nitrate load of the remaining waste water was also decreased.