Seawater magnesium from

In preparation of MgCl2 from seawater, magnesium hydroxide, Mg(OH)2, is first precipitated from seawater by the addition of dolime or lime. This is then treated with hydrochloric acid to produce a neutralized magnesium chloride solution. The solution obtained is evaporated and converted into soHd magnesium chloride hexahydrate (60,61). 

Table 1 gives the average metal content of the earth s cmst, ore deposits, and concentrates. With the exceptions of the recovery of magnesium from seawater and alkaU metals from brines, and the solution mining and dump or heap leaching of some copper, gold, and uranium (see Uranium and uranium compounds), most ores are processed through mills. Concentrates are the raw materials for the extraction of primary metals. 

Chesney A process for extracting magnesium from seawater. Developed by British Periclase in 1937. 

Dow seawater A process for extracting magnesium from seawater. Calcined dolomite (CaO-MgO, dololime), or calcined oyster shell, is added to seawater, precipitating magnesium hydroxide. This is flocculated, sedimented, and filtered off. For use as a refractory it is calcined for the manufacture of magnesium chloride for the manufacture of magnesium metal, it is dissolved in hydrochloric acid. Developed by the Dow Chemical Company and later operated by the Steetly Company in West Hartlepool, UK, and Sardinia. 

Ion-Exchange and Sorption Methods for Recovering Magnesium from Seawater... 

Magnesium with a share of 2.1% is the eighth most abundant element in the Earth s crust and the third most abundant metal in seawater, in which it is present as its chloride. Magnesium from seawater is an inexhaustible... 

The final step in the production of magnesium from seawater is the electrolysis of molten magnesium chloride, in which the overall reaction is... 

The magnesium hydroxide that is produced in this process includes not only the magnesium from the seawater but also that from the dolomite. 

Describe the Hall-Heroult process for the production of aluminum and the methods used to recover magnesium from seawater (Section 17.6, problems 57-58). 

Certain ocean precipitates incorporate the Mg2+ ion in their makeup. In calcite (CaC03), it has been shown that the magnesium content enhances calcite solubility, which, in turn, slows crystal growth. This raises concerns about increasing the magnesium content of seawater (as from oil production), which can have adverse implications for calcareous marine organisms such as plankton. 

The precipitation is generally controlled so as not to remove all Mg2+ from seawater, as doing so would result in the possibility of co-precipitating calcium sulfate. The theoretical solubility of calcium sulfate is reached when about 75% of the magnesium has been precipitated from seawater. Fortunately, supersaturated solutions of calcium sulfate are quite stable and precipitation does not normally represent a problem of contaminating the Mg(OH)2 precipitate and extraction of magnesium from seawater can be safely carried out to 95% removal. 

The thickener overflow is sampled for excess Mg2+. This test measures the quantity of magnesium ion (expressed as lb MgO/1000 gal) in the reacted seawater. This test is critical for ensuring that adequate removal of magnesium from seawater is occurring see Appendix for test method. 

Figure 5.5 Variation of crystallite size and surface area for magnesium oxide produced from seawater magnesium hydroxide.

Anhydrous magnesium chloride in the molten state has been produced extensively from MgO obtained either from calcined magnesium hydroxide from seawater or from calcined magnesite, as shown in reaction (14.7) ... 

Mechanically agitated thickeners are usually large, typically 10 to 100 m (30 to 300 ft) in diameter and 2,5 to 3.5 m (8 to 12 ft) deep. In a large thickener the rakes may revolve once every 30 min. These thickeners are especially valuable when large volumes of dilute slurry must be thickened, as in cement manufacture or the production of magnesium from seawater. They are also used extensively in sewage treatment and in water purification. The feed pulp is admitted at the centerline of the unit at a depth of 1 m or so below the surface of the liquid. Above the feed level is a clarification zone in which the liquid is almost free of solids. Below the feed level is a zone of hindered settling and, near the bottom, a... 

Using data in Table 6.4, determine the relative solubilities of Ca(OH)2 and Mg(OH)2 and explain the relevance of your answer to the extraction of magnesium from seawater. 

Electrolysis of molten MgCL is the final production step in the isolation of magnesium from seawater. Assuming that 35.6 g of Mg metal forms,... 

The sea is so vast that if the concentration of a substance in seawater is 1 part per billion (IX 10 g per kilogram of water), there is 1 x 10 kg of the substance in the world ocean. Nevertheless, because of high extracting costs, only three substances are obtained from seawater in commercially important amounts sodium chloride, bromine (from bromide salts), and magnesium (from its salts). 

A first-stage recovery of magnesium from seawater is precipitation of Mg(OH)2 with CaO ... 

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