Integrated production of fresh water, sea salt and magnesium from sea water

Abstract

Seawater desalination is becoming an important source of fresh water in several countries all around the world. One of the main drawbacks of desalination processes, however, is related to the disposal of large quantities of concentrated brine, which is an always-present by-product of the process. An integrated production of fresh water and salts may be achieved using the discharge brine from a desalination plant as a feed for conventional salt ponds, with the advantages of using brine more concentrated than sea water and, in the case of thermal desalination plants, warmer than sea water. By doing so, the process is faster as a consequence of the enhancement of evaporation rate on the surface of ponds. The above concept has been proposed already several years ago, but only rare examples exist of real applications. A pilot test has been performed in the last 4 years in Trapani (Italy), where a 36,000-m3/d multiple effects desalination with thermal vapour compression plant is operating very close to a traditional salt pond normally fed with sea water. Furthermore, the use of fractionated crystallisation process, typically adopted in conventional salt ponds, allows for the easy separation of salts like calcium carbonates and sulphates, sodium chloride and a final saturated brine which is extremely rich in magnesium as a sole bivalent cation. Thus, the possibility of a further exploitation of such saturated brine has been experimentally analysed by laboratory tests in order to produce high-purity magnesium to be commercialised in the pharmaceutical, food and metal industries. Results have shown a very promising enhancement of the salt pond production capacity, keeping at the same time the very high quality standards required for the production of food-grade salt from sea water. On the other side, laboratory experiments indicated the actual possibility of producing high-purity magnesium salts, thus encouraging towards further investigation for the development of a pilot process development and installation. © 2012 Desalination Publications. All rights reserved.