Melt dissolution

Sum and Skyllas-Kazacos [44] studied the deposition and dissolution of aluminum in an acidic cryolite melt. The graphite electrode was preconditioned (immersed in cryolite melt) to saturate the surface of the electrode in sodium before aluminum deposition could be observed. Current reversal chronoamperometry was used to measure the rate of aluminum dissolution in the acidic melt. Dissolution rate was mass transport controlled [45] and in the order of 0.8 10 7 and 1.8 10 7 molcm 2s 1 at 1030 °C and 980 °C respectively [44]. 

Because the crystal density usually differs significantly from that of the melt, it is necessary to distinguish the crystal growth rate and the melt consumption rate (or melt dissolution rate). The latter equals pcryst/pmeit times the crystal growth rate. Because we are interested in the melt phase, u in the above equation is specified as the melt consumption rate. 

Figure 8.1. Visual observations of melting, dissolution, and reciystallization of tetrafluoroethylene polymer-solvent mixtures.

Secondary nucleation may also occur in static conditions under certain circumstances (11). In lipid systems, needle-like or dendritic crystals that form under certain conditions may lead to the formation of secondary nuclei. Heat dissipation and/ or concentration of noncrystallizing species in certain regions may lead to melting/ dissolution at the base of the branches of dendritic crystals and result in the formation of numerous nuclei centers. Although the exact mechanisms for this type of secondary nucleation are not fully understood, it is undoubtedly important for... 

In the authors opinion, the thermodynamic data presented show that the whole of the concentration region studied can be treated from the viewpoint of the formation of MgCl - coordination compounds in the melts. Dissolution of hydrogen chloride in the melts is not accompanied by an appreciable interaction with the melt components (KC1 and MgCl2). Therefore, the work of the formation of the void in the melts should be the main factor, which defines the behaviour of the temperature dependence. The changes in the deviation of the derived AH and AS values from the additive magnitudes possess an extremal character the maximum deviations are observed at an... 

The cluster mechanisms of crystal nucleation and growth as well as crystal melting/dissolution are viewed. The nature of nanoparticles and the processes of nanostructured materials formation are discussed from the point of view of the cluster concept. 

The mechanisms of crystal phase formation are a key problem in materials science that has not clear comprehension still now. At present, the study of this problem is especially important in connection with the development of nanostructured materials. There are two different approaches to consideration of crystal nucleation/growth as well as crystal melting/dissolution processes [1,2], In accordance with the first approach based on the atomic-molecular theory, the individual atoms or molecules take the leading part in these processes (the role of clusters is ignored). In accordance with the second approach based on the cluster theory, these processes are carried out mainly by means of clusters. Till recently the atomic-molecular theory was generally accepted. However, today many scientific data vote for the cluster theory. The aim of this paper is to analyze the main statements of the cluster conception of crystal phase formation and as a result to consider the nature of nanocrystal. 

Kinetic considerations. Studies of phosphate solubility reveal kinetic limitations to dissolution rates. Harrison and Watson (1984) and Rapp and Watson (1986) measured the dissolution rates of apatite and monazite, respectively, and found that the dissolution rate is limited by diffusion of P or LREEs away from the dissolving apatite or monazite. Furthermore, the diffusivity, and hence dissolution rate, is strongly dependent on the H2O content of the melt. In dry melts, dissolution is so slow that complete dissolution of even small crystals of apatite or monazite is unlikely. In melts produced by dehydration melting of muscovite or biotite, where the H2O content is in the range of 4-8 wt % H2O, apatite crystals on the order of 500 pm diameter will dissolve in 100-1000 years. 

Both techniques provide similar results the comparison of TREF and CRYSTAF has aheady been discussed [84] and the most significant difference is the temperature shift due to the undercooling, as analytical conditions are far from equilibrium CRYSTAF data are obtained during the crystallization whereas TREF data are obtained in the melting-dissolution cycle. Both techniques, however, can be calibrated and the results expressed in branches/lOOOC will be similar for PE copolymers. 

Because SCFs typically have lower density and higher compressibility than a pure polymer melt, dissolution of the SCF into the polymer melt results in swelling of the polymer. This in turn leads to an increase in free volume of the mixture, so transport properties such as viscosity and diffusion coefficient can be significantly enhanced. The semi-empirical Doolittle equation [128,129] predicts that the zero-shear rate viscosity, of a polymer is exponentially related to the fractional free volume,/, via ... 

When the intimately mixed batch is chaiged into the hot furnace, a series of melting, dissolution, volatization and redox reactions take place between the materials in a particular order and at the appropriate temperature. 

Dissolution of Silver. Silver is dissolved by oxidising acids and alkaU metal cyanide solutions in the presence of oxygen. The latter method is the principal technique for dissolving silver from ore. Silver has extensive solubiUty in mercury (qv) and low melting metals such as sodium, potassium, and their mixtures. Cyanide solutions of silver are used for electroplating and electroforming. The silver is deposited at the cathode either as pure crystals or as layers on a mandrel. 

The analytical chemistry of titanium has been reviewed (179—181). Titanium ores can be dissolved by fusion with potassium pyrosulfate, followed by dissolution of the cooled melt in dilute sulfuric acid. For some ores, even if all of the titanium is dissolved, a small amount of residue may still remain. If a hiU analysis is required, the residue may be treated by moistening with sulfuric and hydrofluoric acids and evaporating, to remove siUca, and then fused in a sodium carbonate—borate mixture. Alternatively, fusion in sodium carbonate—borate mixture can be used for ores and a boiling mixture of concentrated sulfuric acid and ammonium sulfate for titanium dioxide pigments. For trace-element deterrninations, the preferred method is dissolution in a mixture of hydrofluoric and hydrochloric acids. 

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