Aluminum thermodynamic data

Table 3. Thermodynamic Data for Crystalline Aluminum Hydroxides at 298.15K and 0.1 MPa ...

Aluminothermic reduction is one among the few pyrometallurgical processes where the actual process closely follows the route theoretically predicted from thermodynamic data. This characteristic, coupled with the simplicity associated with the whole process, makes it well suited for demonstration experiments. The reduction of magnetite by aluminum is a suitable example in this context. 

The ionic species present in the electrolyte appear to be Na+, A1F63 , A1F4 , F and certain A1—O—F complexes (see above). The fact that sodium is present as free ions, whereas aluminum is bound in complexes, and the fact that the sodium ion is the carrier of current, have led many authors to the assumption that sodium is the primary discharge product at the cathode. However, the thermodynamic data favor primary aluminum deposition on aluminum in cryolite melts. 

Clay minerals are present in almost all surface-water and ground-water systems, and in many instances may be controlling the concentration of aluminum, silica, iron, magnesium, or other cations in solution. The thermodynamic data necessary to evaluate the state of reaction (saturation) are not available for some clay minerals, and for those minerals with published values, the data are in disagreement by as much as 10 kilocalories per mole for the same clay mineral. A critical review of the available data for kaolinite and sepiolite, incorporating both the most recent thermodynamic data for the components in the reaction schemes and a more complete computation for the solubility data, yields the values of -907.7 +1.3 and 1105.6 +0.4 kilocalories per mole for the free energy of formation of kaolinite and sepiolite, respectively. 

Figure 10.2. Acid-base speciation of (a) citrate, (b) aluminum, and (c) iron(III) as a function of solution pH at 25°C and in 0.01 mol L NaNOs solutions. The distributions were computed using a total soluble citrate (citys) concentration of 10 mol and total soluble aluminum (Aljs) and iron(in) [Fe(ni)Ts] concentrations controlled by gibbsite (A1(OH)3(x)) and goethite [FeOOH(x)]. (Relevant thermodynamic data from May and Murray, 2000 Baes and Mesmer, 1986 Liu and Millero, 1999.)...

A central issue in the attempt to establish a reliable database is the requirement of critically evaluated thermodynamic data for several key species. One such pivotal element is aluminum, which has an extensive literature of solubility and thermochemical data from which to choose, for each of the aqueous species or complexes. The aluminum species are fundamental to the calculation of solubility and reaction state with respect to many silicates and aluminum oxides and hydroxides and are principal components in numerous surface chemical reactions in the environment. Two key chapters in this volume address this fundamental problem Apps and Neil give a critical evaluation of the data for the aluminum system and Hem and Roberson present the kinetic mechanisms for hydrolysis of aluminum species. 

Table I. Chemical Thermodynamic Data for Aluminum Species...

Equilibrium constants involving each compound were evaluated using the partial pressures by the third law method. Accepting the heats of formation of WF and WFg obtained from bomb calorimetry, the values for WF (ra = 1 to 4) could be extracted by iterative fitting to partial pressure data. The W/Og/Fg and W/S/Fg systems were also examined to give heats of formation of tungsten 0x0- and thiofluorides. This experimentally simple technique yields thermodynamic data on high-temperature species inaccessible to conventional calorimetry. An example of transport relevant to aluminum production... 

The thermodynamic data for fluoride complexes of aluminum, from published literature (12), were recalculated to zero ionic strength by means of the Debye-Hiickel equation. These, with stability data from our experiments with aluminum hydroxide species, provide a basis for deciding which complexes will be predominant when pH and dissolved fluoride concentrations are known. Conclusions drawn concerning the hydroxide species show that the polymeric aggregates of Al(OH)3 should be considered colloidal and they are not included in this calculation as equilibrium solute species. Hydroxide complexes of significance are the monomer A10H- and the anion Al(OH)4". Fluoride complex species constitute the series AlFn " where n ranges from one to six. 

We know that corrosion is a kinetic phenomenon. Aluminum is a fine material in water and in oxygen thanks to its corrosion kinetics and notwithstanding its instability and lead, a material that is known to be inert in oxygen, is pyrophoric in air when finely divided. Thus when one is asked to develop a material that is corrosion-resistant, it is useless to consult the Ellingham diagrams, to look for low solubilities, to find the redox potentials, or to consult other thermodynamic data. Kinetics is the key to processes, and this is true not only for corrosion, but also for other chemical reactions (except those near equilibrium). Salient examples in chemical engineering are crystal growth and catalysis. 

Aluminum hydroxide was used as an adsorbent in a number of publications. Coprecipitation of Cd(OH)2 and Al(OH)3 was measured by Simon et al. in the absence [69] and presence [70] of NH3. Shiao et al. [50] demonstrate that cadmium distribution coefficients between liquid and surface are practically uninfluenced by the NaNOs concentration, while NaCl has a remarkable influence. According to Shimada et al. [71] and Packter and Derry [1], coprecipitation leads to the formation of Zn aluminates. Besides investigating various ions [62], Mustafa and coworkers describe sorption on a-Al(OH)3 [72,72], especially that of Zn, at different temperatures. They observed Langmuir isotherms and derived thermodynamic data. Kinetic studies were carried out by Simon et al. [74] using polaro-graphic techniques. In their experiments equilibrium was reached after 3h with two consecutive first-order reactions, in contrast to the findings of Lo and Leckie... 

We have shown that pyrrole can be electropolymerized on various oxidizable metals such as iron, mild steel, aluminum, copper, tin and zinc, and that this can be achieved in one or two steps. During our investigations, the strategy adopted was to circumvent the unfavorable thermodynamic data by the optimization of kinetic conditions. To this end, we have developed various processes, in which the metal is initially passivated with respect to corrosion... 

The present section illustrates how calculations from basic thermodynamic data can lead to open-circuit cell potential in any condition of temperature and pressure. Chemical power sources, with the exception of fuel cells, are all based on the corrosion of a metal connected to the negative terminal. The aluminum-air power source, that owes its energy to the corrosion of aluminum in caustic, was chosen for this example because of the relative simple chemistry. 

Figure 4.2 shows a general schematic of a typical aluminum-air system. Tables 4.3 and 4.4, respectively, contain thermodynamic data... 

Table 4.3 Thermodynamic Data of Pure Species Involved in the Operation of an Aluminum-Air Power Source...

Electrochemistry of the Al-Oj couple. Figure D.5 shows a general schematic of a typical Al-air system. Tables D.4 and D.5, respectively, contain thermodynamic data for pure species and soluble species involved in the equilibria associated with aluminum, water, and oxygen. Table D.6 contains the chemical and electrochemical reactions possibly occurring in a typical Al-air corrosion cell. 

Actually, the drop of pH is related to more complex reactions and species. Thus, in more sophisticated models, several hydrolysis reactions and metal chloride formation are taken into account but the selection of species and reactions is somewhat different from model to model. Oldfield and Sutton [94] and Watson and Postlethwaite [2] considered only hydroxides as the product of cation hydrolysis. Sharland [96] introduced simple metallic chlorides. The most complete set of species and reactions has been used by Bernhardsson et al. [4], which made available the thermodynamic data of a large number of species, including several iron, nickel, chromium, and molybdenum polycations as well as metal chlorides and hydroxychlorides. Gartland [19] used a more limited set of species (Table 10.3) selected among the Bernhardsson data. According to their experimental results, Hebert and Alkire [95] included Al(OH) " as the hydrolysis product in their model of the crevice corrosion of aluminum alloys. 

A critical evaluation of the thermodynamic data in table 13 does not yield any accurate systematics. Within the errors involved for the different experimental techniques it can be concluded that at least for the systems with aluminum chloride (RCI3-AICI3), the thermodynamics of formation have rather common values for the various rare-earth chlorides involved. Thus for the values of = i, 1 and the thermodynamic quantities of reaction (2) are ... 

Fig. 2.15. Release wavespeeds at very high pressure can be determined by experiments in which the sample thickness is varied for fixed thickness of a high velocity impactor. Data on aluminum alloy 2024 are shown. As indicated in the figure, shear velocity (C ) and Poisson s ratio (cr) at pressure can be calculated from the elastic and bulk speeds if thermodynamic equilibrium is assumed (after McQueen et al. [84M02]).

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