Magnesium precipitation reactions

Use Figures 10-5 and 10-6 or Table 10-1 to decide which of the following soluble salts would permit a separation of magnesium and lead through a precipitation reaction sodium iodide, Nal sodium sulfide, Na2S sodium carbonate, Na2C03. 

Carbon dioxide carryover also occurs following the deliberate addition of soda ash (sodium carbonate) directly to the boiler. Where boiler designs provide for a significant internal drum or shell, the use of soda ash and caustic soda to prevent calcium and magnesium scales by precipitation reactions (internal softening) may be employed. 

Finally, common ion effects link many mineral precipitation reactions, so the reactions do not operate independently. In the seawater example, dolomite precipitation consumed magnesium and produced hydrogen ions, significantly altering the saturation states of the other supersaturated minerals. 

Fig. 6.1 shows the basics of an electrolysis plant. The brine that is used must be purified for this electrolytic process. Calcium, sulfate, and magnesium ions are removed by precipitation reactions. 

Co-gel formation. In addition to the co-precipitation of the two hydroxides we have found that a very homogeneous mixture of Mg2+ and Al3+ species can be obtained by co-gel formation (8). This co-gel, usually prepared by combining aqueous slurries of psuedoboehmite alumina, high surface area MgO, and an acid, is dried and calcined at 700 to 800°C to produce both stoichiometric and high magnesium spinels (reaction 4). 

Measurements of the specific surface area, SSA, of the products grown at various times indicate that the initial formation of a microcrystalline or amorphous precursor leads to a rapid increase in SSA. The development of these phases is also observed by scanning electron microscopy, and dissolution kinetic studies of the grown material have indicated the formation of OCP as a precursor phase ( , 7). The overall precipitation reaction appears to involve, therefore, not only the formation of different calcium phosphate phases, but also the concomitant dissolution of the thermodynamically unstable OCP formed rapidly in the initial stages of the reaction. In the presence of magnesium ion the overall rate of crystallization is reduced and lower Ca P ratios are observed for the first formed phases (51). 

The most common route for the s5mthesis of magnesium silicate is via a precipitation reaction between a soluble metal silicate (e.g., sodium orthosilicate, sodium metasilicate, or potassium silicate) and a soluble magnesium salt (e.g., magnesium sulfate, nitrate, or chloride). The aqueous suspension of the precipitate is filtered and the collected solid is washed and dried (Fig. 7.2) [6,7]. 

FIGURE 7.2 Schematic of the synthesis of magnesium silicate via a precipitation reaction. 

Congruent and incongruent dissolution and precipitation reactions, other than for halite, which probably control the major cation compositions of formation waters include dolomitization of limestone, resulting in a major increase of calcium and a major decrease of magnesium, as in reaction (2) ... 

In industry, precipitation reactions are used in the manufacture of many chemicals. For example, the first step in the extraction of magnesium from sea water is to precipitate Mg2+ as Mg(OH)2(s). 

Another important precipitation reaction involves the formation of insoluble carbonates (solubility guideline 7). Limestone deposits are mostly calcium carbonate, CaC03, although many also contain significant amounts of magnesium carbonate, MgC03. 

The direct method of potentionietric titration has been applied to various precipitation reactions, among which may be mentioned the determination of magnesium in dolomite 23 the precipitation of zinc with ferrocyanide 24 and the titration of chloride, bromide and thiocyanate 25 with mercurous perchlorate 20... 

While caustic soda and lime have traditionally been used for many years to achieve the above precipitation reaction, magnesium hydroxide is gaining popularity as a replacement for these two alkalis. Magnesium hydroxide has a number of advantages over lime and caustic soda, and these are further discussed below. 

Hypothetical reaction pathways chosen to model the L2 leachate-Uinta Sandstone system are illustrated in Figure 5. As a first approximation, dissolution/precipitation reactions affecting the mass balance of Na, K, Mo, SO4, and Cl were not considered. Instead, based upon the solubility controls discussed in the previous sections of this paper, the working hypothesis for the simulations is that the recarbonation of L2 leachate drives the reactions toward equilibrium. Along the path toward equilibrium, recarbonation is accompanied by the precipitation and dissolution of sepiolite, calcite, and an inferred hydrated magnesium carbonate mineral such as hydromagnesite. 

The simplest method for precipitating the scale-forming elements (used in preceding process) is to add phosphoric acid to the sea water and neutralize with ammonia. The precipitation reactions for magnesium and calcium (assumed present in the sea water as chlorides) are shown by Equations 2 and 3. 

Cathodic inhibitors either selectively precipitate on cathodic areas or slow the cathodic reaction by increasing hydrogen overvoltage. Other cathodic inhibitors utilize alkalinity increase at cathodic sites to precipitate insoluble compounds on the metal surface. Hydrogen evolution causes the metal-concrete interface to become more alkaline and ions such as calcium or magnesium precipitate as oxides to form a protective layer. 

Ores tested were predominantly limonitic in character, although some contained in excess of 2% magnesium. Leaching and precipitation reactions took place over an extended period of up to 70 hours from addition of acid. Nickel extractions were in excess of 80%, at acid addition rates of around 500 kg/t. Terminal iron solution concentrations of less than 0.5 g/L were typically achieved. Both sodium and potassium additions were made to promote iron precipitation. The potential for using seawater to slurry the ore was noted. Potassium, in the form of a carbonate was also added on occasion to raise the solution pH to around 1.5 to 2. 

Scale formation on steel in contact with natural water offers another example of inhibition by film formation due to precipitation reactions. Natural water contains many species, including the bicarbonates of magnesium and calcium. Their content in the water is expressed by the hardness (on the French hardness scale, a value of 1 ° is equivalent to 10 mg of dissolved CaC03 per liter). The hardness of natural waters is normally situated between 5° (soft water) and 25° (hard water), in the French units. 

Separation of clement traces is also possible with precipitation reactions. This technique permits rapid and extensive concentration in a relatively uniform matrix, but it is not very selective. Aluminum hydroxide, magnesium hydroxide, iron hydroxide, and hydrogen sulfide have all been used for trapping trace amounts of various elements[101], [118]-[I20], as have such organic precipitating agents as thionalide, cupferron. and dithiocarbamate [101]. [121]. [122]. A subsequent determination is carried out either directly on the separated precipitate or after restoring it to solution [118]. 

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