Magnesium extraction from seawater

Case Study 3 Magnesium Extraction from Seawater... 

CASE STUDY 3 MAGNESIUM EXTRACTION FROM SEAWATER 10.4.1 Introduction... 

Seawater. Salt extraction from seawater is done by most countries having coastlines and weather conducive to evaporation. Seawater is evaporated in a series of concentration ponds until it is saturated with sodium chloride. At this point over 90% of the water has been removed, and some impurities, CaSO and CaCO, have been crystallized. This brine, now saturated in NaCl, is transferred to crystallizer ponds where salt precipitates on the floor of the pond as more water evaporates. Brine left over from the salt crystallizers is called bitterns because of its bitter taste. Bitterns is high in MgCl2, MgSO, and KCl. In some isolated cases, eg, India and China, magnesium and potassium compounds have been commercially extracted, but these represent only a small fraction of total world production. 

In the process used by Norsk Hydro, magnesium hydroxide extracted from seawater with the aid of calcined dolomite is mixed with charcoal and magnesium chloride brine and is heated to 1000-1200°C in the presence of chlorine produced during subsequent electrolysis of magnesium chloride. The main reactions are [266]... 

The world s oceans hold 1.37x10 of water (97.2% of the total amount of water of the hydrosphere). They cover 71% of the earth s surface, are actually the biggest reservoir on our planet, and contain many important minerals. The overall content of mineral matter in the oceans is estimated to be about 5 x 10 tons [1,2]. The seas contain virtually all of the naturally occurring elements and are the only universal source of mineral wealth that is available to most nations. For some of them it is the only source. Yet, most of the elements, the microelements, are available in very low concentrations, i.e., in parts per billion (ppb). The products being extracted from seawater with economic profit at present are sodium chloride, magnesium compounds, and bromine [2-4]. During the last two decades there has been growing interest in the possibility of commercial recovery of additional minerals from seawater [5] and brines [6]. 

Magnesium chloride is extracted from seawater or brine, of which it is a component, and from minerals, such as carnal-lite (KCl-MgClj-HjO) and bischofite (MgCl2,6H20). The usual procedure is to treat seawater, brine, or the mineral with lime (CaO), calcined dolomite (CaO-MgO), or caustic soda (sodium... 

Dow process /dow/ An industrial method whereby magnesium is extracted from seawater via the precipitation of Mg(OH)2 by Ca(OH)2, followed by solvation of the precipitated hydroxide by hydrochloric acid. The process is named for the American industrial chemist Herbert Henry Dow (1866-1930), who founded the Dow Chemical Company (1897). 

Magnesium is extracted from seawater. It is used for alloys for auto and aircraft parts. 

The extraction of magnesium chloride from seawater is done first, using solar energy to vaporize a dynamic stream of preconcentrated seawater flowing along an inclined preferential salt separator (PSS). Magnesium chloride salts soluble in seawater will separate as the very end product. Distilled water will be produced as a by-product in this phase. It represents a typical feed of water for hydrogen production by electrolysis. 

The proposed process offers a scheme that provides three products as compared to one product only by the Solvay process. Magnesium chloride is an important product for the manufacture of magnesium metal. As a matter of fact, one can claim that the proposed process could compete with the well-known Dow process for the extraction of magnesium chloride from seawater. 

Some metal salts, such as NaCl, are quite soluble in water, while others, such as AgCl, are not very soluble at all. In fact, so little AgCl dissolves in water that this compound is generally considered to be insoluble. Precipitation reactions occur when certain cations and anions combine to produce an insoluble ionic solid called a precipitate. One laboratory use of precipitation reactions is in identifying the ions present in a solution, as shown in Figure 5-7. In industry, precipitation reactions are used to manufacture numerous chemicals. In the extraction of magnesium mefal from seawater, for instance, the first step is to precipitate Mg " " as Mg(OH)2(s). In this section, the objective is to represent precipitation reactions by chemical equations and to apply some simple rules for predicting precipitation reactions. 

AH metals come originally from natural deposits present in the earth s cmst. These ore deposits result from a geological concentration process, and consist mainly of metallic oxides and sulfides from which metals can be extracted. Seawater and brines are another natural source of metals, eg, magnesium (see Chemicals frombrine Magnesium and magnesium alloys Ocean raw materials). Metal extracted from a natural source is called primary metal. 

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. 

Dissolved Minerals. The most significant source of minerals for sustainable recovery may be ocean waters which contain nearly all the known elements in some degree of solution. Production of dissolved minerals from seawater is limited to fresh water, magnesium, magnesium compounds (qv), salt, bromine, and heavy water, ie, deuterium oxide. Considerable development of techniques for recovery of copper, gold, and uranium by solution or bacterial methods has been carried out in several countries for appHcation onshore. These methods are expected to be fully transferable to the marine environment (5). The potential for extraction of dissolved materials from naturally enriched sources, such as hydrothermal vents, may be high. 

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. 

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