Sulphate melts

Sulphate melts are referred to melts with the own acid-base dissociation equilibrium  

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Thompson Argent 2002) predicts the formation of an alkali sulphate-based melt, with the majority of the other elements also forming sulphates, implying a complex melt and solid solution(s) based on sulphates. At the present, information on the activity of cations in mixed sulphate melts is lacking and identification of actual phases is difficult because of the problem of concentrating the sulphate fraction. It is therefore difficult to predict with certainty exactly how a fly ash sample will behave in water and hence the need for standardized leaching tests. 

Pentammino-cuprie sulphate melts below 200° C., darkens in colour, and at 400° C. completely decomposes, leaving1 a residue of metallic copper. It is stable up to 99° C., and when heated between 99° and 141° C. loses one molecule of ammonia and passes into tetrammino-cupric sulphate.1... 

A mixture of the above compound (11 g) and ethylenediamine (27.8 g) is stirred at laboratory temperature for 3 days and the excess of ethylene diamine is removed by evaporation under reduced pressure. The residue is dissolved in methanol, the solution is cooled to 5°C and concentrated sulfuric acid is added until the pH of the solution is 2. A filter-aid (Celite, 10 g) is added and the mixture is stirred for 1 hour and then filtered. The filtrate is evaporated to dryness under reduced pressure and the residue is stirred with ethyl acetate. The mixture is filtered and there is thus obtained as solid residue 4-(N-beta-aminoethylcarbamoyl)morpholine hydrogen sulphate, melting point 168-169°C. 

The dissociation pressure of calcite reaches 0.101 kPa (1 atm) at 894°C (S20) and the decarbonation reaction is highly endothermic (Section 3.1.4). The rate of decarbonation becomes significant at 500-600°C if a sufficiently low partial pressure of COj is maintained or if the calcite is intimately mixed with materials, such as quartz or clay mineral decomposition products, that react with the calcium oxide. Even in a precalciner, such mixing occurs, aided by agglomeration caused by the presence of low-temperature sulphate melts. 

C3S is sufficiently intimately mixed with either C2S or CaO. Some substituent ions affect the rate Al has little effect, Fe markedly accelerates, Na" slightly retards, and Mg markedly retards decomposition. Where both and Fe are present, the accelerating effect of the Fe is dominant. The decomposition is also accelerated by water vapour and, very strongly, by contact with sulphate melts with a CaS04-Na2S04 melt, decomposition was complete in 2 h at 1025 "C. 

The reactions of Na2S0. Sodium sulphate melts at 884 °C and in the pure state does not considerably decompose below 1500 C. With Si02, it begins to react from about 1200 °C upwards, higher by several hundred degrees than NajCOa ... 

Gas oxygen electrodes [Pt(02) being the most widely used] are often exploited for the investigations of oxide-ion chemistry in most nitrate, chloride and sulphate melts. The construction of this kind of electrode is simple enough [62] they are made of a noble (or in a more general case, inconsumable) metal plate (Au, Pt, Ag, Pd), placed in an atmosphere with a definite, known constant partial pressure of 02 and partially immersed in the studied melt containing oxide ions. 

Deanhardt and Stern investigated the stability of 02 ion in higher-temperature chloride and sulphate melts, such as NaCl (at 830 °C) and Na2S04 (at 920 °C) [244, 245]. They showed that peroxide ions could be formed in appreciable concentrations only in the atmosphere of free oxygen over the melt, whereas an inert atmosphere resulted in complete destruction of 02 to oxide ions. The values of the equilibrium constants of (0.42) in molten NaCl and Na2SC>4 at 1100K were estimated in Refs. [244, 245] to be within 0.05 -0.07 for the NaCl melt and 0.03-0.05 for the Na2S04 one. 

The so-called kinetic methods of melt acidity determination, which will be considered below, are based just on this reaction. The reaction [10.4.9] leads to the decrease of nitronium concentration and the rate of NO2 emission decreases until essentially constant acid concentration is determined by a sequence of consecutive measurements. This concentration is the upper limit of acidity of the melt. The thermal dependence of upper limit of acidity in nitrate melts can be easily explained on the basis of the increase of the melt temperature, which leads not only to the reduction of nitronium stability but also to the elevation of process [10.4.9] rate. Sulphate melts have the upper limit of acidity too, it seems connected with limited and low solubility of SO3 at elevated temperatures, such assumption may be confirmed by results. ... 

Proton migration in a glass was demonstrated as long ago as 1927 using ammonium hydrogen sulphate melt as a proton source Hydrogen is present in these glasses in the form of water and OH ions, as shown by characteristic infrared bands near 3300 and 3600 cm respectively - . 

THE OXIDATION OF ALLOYS Fe-Si-(Ta, Nb, W, Sn) BY SODIUM AND CALCIUM SULPHATE MELTS... 

Oxidation of iron-silicon alloys by sulphate melts 245... 

Corrosion phenomena beneath molten salts were initially investigated for sulphate systems [e.g. 2]. A review of fundamental work on sulphate melt-induced corrosion is given by Rapp [3]. Increased corrosion is excited by dissolution of the passivating oxide layer in the salt melt. Two major dissolution mechanisms have to be considered basic dissolution and acidic dissolution. Basic dissolution is caused by a basic melt, meaning a high activity of 0 , for example in the form of Na20. By reaction with the oxide, this leads to the formation of a complex oxide ion (e.g. FeO ) according to Eq. 30.1. 

Aggravated corrosion was also observed on metals and alloys covered by molten chloride deposits. A solubility study on protective oxide fQms in molten chlorides at 727°C [4] has shown that its behaviour is similar to the observed dissolution of metal oxides in sulphate melt. Metal oxides can be dissolved in molten NaCl-KCl also by acidic and basic dissolution. 

Hot corrosion is a complex phenomenon involving sulphidation, oxidation or both. Hot corrosion is a form of accelerated oxidation which affects the surfaces exposed to high-temperature gases contaminated with sulphur and alkali metal salts. These contaminants combine in the gas phase to form alkali metal sulphates. If the temperature of the alloy or coating surface is below the dew point of the alkali sulphate vapours and above the sulphate melting points, molten sulphate deposits are formed. Molten sodium sulphate is the principal agent in causing hot corrosion. 

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