Alkali chlorides, dissolution process

Rocket propulsion oxidizers, 18 384-385 Rocks, weathering of, radiation and, 3 299 Rocksalt, crystal structure of, 2 6, 29 Rock-salt-type alkali halide crystals, dissolution process, 39 411 19 alkali chlorides, 39 413, 416 alkali fluorides, 39 413-415... 

Results obtained from studies on alkali fluoride and chloride crystals show that alkali metal ions (cesium in this case) dissolve first in the former, whereas chloride ions dissolve first in the latter. The differences are accounted for by the ionic sizes of the cation and anion, which are significantly different in the two cases. The dissolution of the second ion occurs soon afterward, although we cannot predict when. If the size of the cation and anion is about the same, the dissolution process is retarded. 

The dissolution process of HC1 in molten alkali metal halides is accompanied by the formation of H-bonds, whose energies (kJ mol-1) increase in the sequence Na (21.9)—K (25.1)—Rb (27.2)—Cs (31.0) [269]. The strength of H-bonds in molten chlorides is dependent on the polarizing... 

The simplified hole model was shown to describe the data on C02 solubility in the alkali metal halide melts with good accuracy. The entropy changes in the process of dissolution are close to — 1 J mol-1 K-1, which agrees with the data of Novozhilov [311], and the solubility data obtained for the molten chlorides are in good agreement with Ref. [311]. An interesting fact was revealed— the solubility of C02 increased by four times upon the addition of a small concentration of Ni2+ ( 10-3 mol kg-1), introduced into molten NaCl as an admixture. A study of the kinetics of the dissolution process showed that the rate of C02 dissolution in alkali metal halide melts was defined by the rate of transfer of C02 from the gaseous phase into the liquid, but not by the diffusion and convection of the dissolved molecules in the melt. 

In the present work in situ high-temperature electronic absorption spectroscopy was employed to identify the corrosion products of metals and alloys in fused salts. Corrosion of metals in molten salts under an inert atmosphere has an electrochemical origin and anodic dissolution was used here to facilitate the process studied. Spectroscopic investigation of stainless steel anodic dissolution in alkali chloride melts allows determining the sequence in which the steel components are dissolved. To interpret the observed phenomena, the spectra recorded after steel dissolution were compared with the absorption spectra of the melts containing the products of anodic dissolution of pure metals constituting the stainless steels studied. 

Under applied anodic potential the corrosion of stainless steels in molten chlorides is electrochemical in nature. At the initial period the exchange reaction between steel components and alkali metal cations takes place in parallel with the electrochemical process. It was found that titanium in steels forms stable carbonitride species that do not dissolve during anodic oxidation. Preliminary thermal treatment of austenite steels has an effect on anodic dissolution processes. 

Molybdenum pentachloride dissolved in high-temperature alkali chloride-based melts, and concentrations of Mo(V) up to 0.2mol/dm were obtained. Its dissolution process was studied in LiCl-KCl, NaCl-KCl, NaCl-KCl-CsCl and NaCl-CsCl melts between 450 and 850 °C and in all the systems resulted in the formation of molybdenum(V) chloro-ions, which we show to be MoCl ". The symmetry of this species complies with ligand field theory as octahedral but some distortions may be present. 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

In anodic dissolution of mercury in a solution of nitric acid, where both mercurous and mercuric salts are asumed to be completely dissociated, both the formed ions enter the solution in the ratio of their respective activities hKo+/ h1 ++ = 76. When alkali cyanide is used as electrolyte the bivalent ions formed on dissolution are predominantly consumed for the formation of the complex Hg(CN). As a result of the formation of this complex the concentration of free Hg++ jpns decreases considerably in accordance with the neghgible degree of dissociation of the above-mentioned complex, and consequently the dissolution potential of the system Hg/Hgt+ also decreases. For this reason, mercuric ions converted to mercuricyanide complex can be considered to be practically the sole product of the anodic process while the amount of univalent mercury ions is quite negligible. Contrary to this, on dissolving mercury in a solution of hydrochloric acid mercurous ions are predominantly formed due to the slight dissociation of mercurous chloride, the main product of the reaction. 

Based on available information, we believe that transfer of the uranate to a molten chloride system with electrolytic reduction is the most feasible method. Electrolytic deposition from molten alkali metal chlorides was an integral step in the pyro-chemical process known as the Hanford Salt Cycle. Documentation of this phase of the process was extensive and also represents one of the very few pyrochemical processes that has been carried through pilot-plant scale on irradiated fuel. Unknowns exist, such as the rate and conditions of uranate dissolution, but considerable use could be made of previously documented results. 

The coefficients of the obtained thermal plots and the thermodynamic characteristics of the process of C02 dissolution calculated according to equation (2.5.24) in molten alkali metal chlorides are presented in Table 2.5.4. 

Damage is generally associated with the combined presence of vanadium (arising as a porphyrin in the fuel), sodium (derived from sodium chloride), sulfur (from the fuel), and oxygen. During combustion, vanadic oxides and sodium sulfate may condense as low melting vanadyl vanadates, which permit the rapid dissolution of surface oxides and metal alkali. The processes occur very rapidly. Oxide dissolution can appear to be synonymous with metallic loss (Johnson et al. 1975). 

Chlorine and sodium hydroxide are the main products of the industrial chlor-alkali electrolysis that is described as a process example in Section 6.19. Hydrochloric acid is produced by reaction from the elements H2 and CI2 or by the reaction of chloride salts such as, for example, NaCl or CaCl2, with sulfuric acid. Other important sources of HCl are industrial chlorination processes using CI2 as chlorination agent (e.g., chlorination of benzene to form chlorobenzene and HCl or the chlorination of methane to give chloromethane and HCl) or industrial dehydrochlorination processes (e.g., production of vinyl chloride and HCl from 1,2-dichloroethane). The main uses of hydrochloric acid are addition reactions to unsaturated compounds (by hydrochlorination or oxychlorination), formation of chlorine in the Deacon process, production of chloride salts from amines and other organic bases, dissolution of metals, regeneration of ion exchange resins, and the neutralization of alkaline products. 

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