Membrane crystallization for recovery of valuable compounds from waste streams

Abstract

Sustainable development and Process intensification strategy are guidelines for industrial processes in perspective. It is becoming more and more common that industry wants to fully exploit their resources due to environmental regulations, economic gain, sustainable standpoint, etc. In this perspective, waste streams have to be turned into resources in the most environmental friendly, economic and sustainable way. Membrane Engineering is already a key-figure to realize this objective. Novel membrane technologies such as membrane distillation (MD), membrane crystallization (MCr), pressure retarded osmosis (PRO), reverse electrodialysis (RED) and forward osmosis (FO), are evolving and are being suggested for a better exploitation of waste streams. This Ph.D. study focusses, particular, on Membrane crystallization (MCr), which is a novel technology for simultaneously production of water and minerals. It has several advantages with respect to conventional crystallizers in terms of purity, controlled kinetics and crystal morphology. Moreover, MCr is able to treat high concentration solutions, which are challenging for other traditional membrane operations. The current Ph.D. work emphasizes on various aspects of membrane crystallization for approaching zero-liquid discharge in industrial processes. Improved membranes, specifically developed for MCr applications, have to be manufactured. In this study, preliminary suggestions on membrane features are given for the requirements in MCr. Lab-made PVDF membranes with different characteristics have been tested and evaluated for their performance in MCr. This study, suggests that membranes with symmetric sponge layer structure and low thickness are favorable. Membrane of asymmetric structure with many macrovoids seems more pronounced to suffer from wetting. Moreover, it has been shown that, membrane crystallization is able to treat several kinds of feed solutions including RO brine, produced water and wastewater containing high amounts of sodium sulfate. The recovered crystals exhibit high purity, good size distribution and controlled growth. Na2SO4 can be recovered as different polymorphs and in this study it has been crystallized in the anhydrous form (Thenardite). Moreover, the process has shown excellent stability in terms of transmembrane flux and maintenance of hydrophobicity of the membrane. In some cases the treatment has been continued for more than 90 hours by only slight cleaning with distillate water. Membrane crystallization, in the direct-contact membrane distillation configuration, can normally treat solutions with very high concentrations. However, its limitations in the recovery of lithium from single salt solutions have been highlighted in this study. Vapor pressure, due to increase in concentration, is reduced significant, that it is not possible to reach LiCl saturation by this configuration. Likewise, combined direct-contact and osmotic distillation configuration have not been able to increase the driving force enough in order to exceed saturation. Instead vacuum membrane distillation has been introduced to eliminate the osmotic phenomena. This configuration has been able to recover LiCl in two different polymorph structures depending on the utilized operative conditions. Furthermore, integrated membrane system, including membrane crystallization, has shown excellent capability to treat orange juice. The quality of the juice has been maintained through ultrafiltration, membrane distillation and membrane crystallization treatment. In this study, the MD/MCr feed temperature is kept below 30 °C causing a relatively low flux. However, it has still been possible to reach from a concentration of 9 °brix to 65 °brix using MD/MCr. The advantages of MD/MCr with respect to isothermal osmotic membrane distillation configuration, is the elimination of the reconcentration stages of the draw solution. All the carried out case studies show that MD/MCr is able to reduce the volume of the waste stream significantly. The obtained results might be used as guidelines for practical application. Moreover, the low temperatures and atmospheric pressures utilized, makes it possible in real industrial processes to use waste or low-grade heat. Unlike other processes, MCr is able to produce two high quality products (i.e. water and salts) and will therefore not produce any additional waste. Hereby, the extended treatment by means of MCr will only positively influence the overall “sustainability” of the entire industrial process.