Molten alkali metal sulfates

Molten alkali metal sulfates are typical ionic liquids, which means that their dissociation to the constituent ions is practically complete. For example, Na2S04 breaks down as shown  

The first study of the oxoacidic properties of molten sulfates was performed by Lux [17] on an equimolar K2S04-Na2S04 mixture at 950 °C. The measurements of activity of oxide ions in the melt were made by the potentiometric method with the use of an Au(C 2) gas oxygen electrode. 

Additions of Na20 were equivalent to those of Na202. and Na2C03 was found to decompose completely with the formation of Na20 under the experimental conditions. The process of decomposition is efficiently retarded in concentrated solutions, since C02 is hardly removed from strongly basic media. The basic solutions were characterized by practically stable potentials, and only slightly shifted towards the neutral point of the ionic solvent studied. In contrast, acidic solutions obtained by the addition of NaPO to the pure sulfate melt are extremely unstable dissolution of the acid is accompanied with fast evolution of SO3, and in a short time the e.m.f. value achieves that of the neutral point. The equilibrium constant of the following reaction 

The papers of Flood et al. [18-20] are devoted to the investigations of acid-base reactions in molten pyrosulfate media. On the basis of the data obtained in Ref. [19], an obvious conclusion is that the strengthening of the constituent cation s acidity results in a reduction in the decomposition temperature of molten pyrosulfates. 

Kaneko and Kojima studied the oxoacidic properties of Na2S207 and Na202 in the molten eutectic mixture K2S04-Li2S04-Na2S04 at 550 °C [124, 125]. The difference of pO values in their 0.01 mol kg-1 solutions is equal to 10 pO units. This value allows us to determine that the width of the acid-base range for the standard solutions in this melt is equal to 14 pO units. The acid-base range of stable existence of V02+ ion is found to lie within the interval of pO = 8.5-10.6. 

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Fig. 8.1. Photograph of catalyst pieces, courtesy Haldor Topsoe A/S www.haldortopsoe.com Rings, daisy (star) rings and pellets are shown. The daisy rings maximize catalyst area and minimize resistance to gas flow. In operation (700-900 K), the catalyst consists of a molten vanadium-alkali metal sulfate/pyrosulfate layer on a porous solid silica substrate. The outside diameter of the largest piece (far left) is 20 mm.

Hot corrosion is a rapid form of attack that is generally associated with alkali metal contaminants, such as sodium and potassium, reacting with sulfur in the fuel to form molten sulfates. The presence of only a few parts per million (ppm) of such contaminants in the fuel, or equivalent in the air, is sufficient to cause this corrosion. Sodium can be introduced in a number of ways, such as salt water in liquid fuel, through the turbine air inlet at sites near salt water or other contaminated areas, or as contaminants in water/steam injections. Besides the alkali metals such as sodium and potassium, other chemical elements can influence or cause corrosion on bucketing. Notable in this connection are vanadium, primarily found in crude and residual oils. 

Ru metal is quite refractory. It is not significantly soluble in any single acid even aqua regia has little effect. At room temperature, the metal does not react with O2, but, when heated in air, a film of the dioxide appears. The metal is insoluble in fused sulfates. Molten alkali slowly dissolves the metal. The rate of attack is rapid under oxidizing conditions, and a molten mixture of NaOH and Na20s will readily dissolve the metal. 

In the molten carbonate process a molten eutectic mixture of lithium, sodium, and potassium carbonates removes sulfur oxides from power plant stack gases. The resulting molten solution of alkali metal sulfites, sulfates, and unreacted car bonate is regenerated in a two-step process to the alkali carbonate for recycling. Hydrogen sulfide, which is evolved in the regeneration step, is converted to sulfur in a conventional Claus plant. A 10 MW pilot plant of the process has been constructed at the Consolidated Edison Arthur KiU Station on Staten Island, and startup is underway. 

The alkaline earth metals show a wider range of chemical properties than the alkali metals. The IIA metals are not as reactive as the lA metals, but they are much too reactive to occur free in nature. They are obtained by electrolysis of their molten chlorides. Calcium and magnesium are abundant in the earth s crust, especially as carbonates and sulfates. Beryllium, strontium, and barium are less abundant. All known radium isotopes are radioactive and are extremely rare. 

The incineration is accomplished by injecting the hazardous material and air beneath the surface of a pool of molten salts. Typically, sodium carbonate with a small amount (1 to 10%) of sodium sulfate is used as the molten salt, however, other alkali metal carbonates or mixtures of alkali metal carbonates can be employed. Sodium carbonate is used because it reacts instantly with acidic gases to form sodium salts. The small amount of sodium sulfate is used to catalyze the combustion of carbon. Temperatures of the molten salts are usually in the 700° to 1000°C range. 

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

In the case of x-ray diffraction involving molten salts with polyatomic anions (nitrate, carbonate, sulfate) [69, 70] the information is obscured by the intra-anionic distances (central atom to oxygen) and little useful information on the stmcture of the melt is obtained. The same problem occurs with alkali metal thiocyanates [78], where the data confirm the structure of the linear SCN anion but does not position the cations with respect to it. 

Resistant to acids, including HF and molten metals not resistant to alkalis, molten sodium sulfate SiC produced by sintering is prone to oxidation and corrosion while single phase SiC is resistant oxidation results in formation of SiC>2 SiC is more sensitive to hot corrosion than Si3N4... 

Whatever the explanation for the color change, the interesting fact remains that in molten potassium or sodium thiocyanate the sulphur is highly reactive and displays reactions which are not realizable in aqueous solutions of alkali thiocyanates. Among such reactions are formation of silver sulfide from metallic silver formation of sodium thiosulfate with sodium sulfite conversion of metal oxides and sulfates (even lead sulfate)... 

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