The apparent solubility of aluminum in cryolite melts

The apparent solubility of aluminum in cryolite melts saturated with A1₂O₃ has been determined by titration with electrolytically generated O₂. The results may be expressed by wt pct Al = − 0.2877 + 0.0268 (NaF/AlF₃ wt ratio) + 2.992 × 10⁻⁴ (temp °C) − 0.00192 (% CaF₂) −0.00174 (% Li₃AlF₆) −0.00288 (% NaCl) with a standard deviation of ±0.017. Ranges covered were ratio 0.8 to 2.3, temperatures 969° to 1054°C, CaF₂ ≤ 14 pct, Li₃AlF₆ ≤ 20 pct, and NaCl ≤ 10 pct. There was no significant effect of adding 0 to 38. pct K₃A1F₆ or 0 to 10 pct MgF₂. It was found that solubility was approximately proportional to activity of aluminum when Al-Cu alloys were used. Possible mechanisms of solution are discussed. Monovalent aluminum is ruled out on the basis of the variation of solubility with NaF/AlF₃ ratio and a_Al. The favored, but not proven, mechanism involves formation of both sodium atoms and a colloidal dispersion of aluminum.     

On the solubility of aluminum in cryolitic melts

The solubility of aluminum in NaF-AlF₃-Al₂O₃ melts with various additives was found to increase with increasing NaF/AlF₃ molar ratio (CR) and increasing temperature and to decrease with additions of A1₂O₃, CaF₂, MgF₂, and LiF to the melts. With the use of literature data for the activities of NaF and A1F₃ in cryolitic melts, three dissolution reaction models were found to give a good fit to the experimental solubility data. According to the most probable of these models the total concentration of dissolved aluminum (aluminum and sodium species) is given by c_Al = c_Na(diss) + c_AlF2- + c_Al2F3- + c_Al3F4- + c_Al4F5- In NaF rich melts, aluminum will dominantly dissolve as sodium, while at cryolite ratios commonly used in aluminum electrowinning (CR = 2.25 to 2.7) the AlF _-2 ^- -ion is the predominant dissolved metal species. Other species (A1₂F₃ ^-, A1₃F₄-, A1₄F₅-) were found to be of some significance only in melts with high excess A1F₃ (CR < 2).     

On the solubility of aluminum in cryolitic melts

The solubility of aluminum in NaF-AlF₃-Al₂O₃ melts with various additives was found to increase with increasing NaF/AlF₃ molar ratio (CR) and increasing temperature and to decrease with additions of A1₂O₃, CaF₂, MgF₂, and LiF to the melts. With the use of literature data for the activities of NaF and A1F₃ in cryolitic melts, three dissolution reaction models were found to give a good fit to the experimental solubility data. According to the most probable of these models the total concentration of dissolved aluminum (aluminum and sodium species) is given by c_Al = c_Na(diss) + c_AlF2- + c_Al2F3- + c_Al3F4- + c_Al4F5- In NaF rich melts, aluminum will dominantly dissolve as sodium, while at cryolite ratios commonly used in aluminum electrowinning (CR = 2.25 to 2.7) the AlF _-2 ^- -ion is the predominant dissolved metal species. Other species (A1₂F₃ ^-, A1₃F₄-, A1₄F₅-) were found to be of some significance only in melts with high excess A1F₃ (CR < 2).     

On the solubility of aluminum carbide in cryolitic melts

The solubility of aluminum carbide in cryolitic melts was determined as a function of NaF/A1F₃ molar ratio (CR), temperature, and the concentrations of A1₂O₃, CaF₂, MgF₂, and LiF. At 1020 °C a maximum concentration of 2.1 wt pct aluminum carbide was found at CR = 1.80. The following model for the aluminum carbide dissolution reaction based on activity data for NaF and A1F₃ was found to fit the experimental solubility data: Al₄C₃(s) + 5AlF₃(diss) + 9NaF(l) = 3Na₃Al₃CF₈(diss). From the solubility data for aluminum carbide an empirical equation giving the equilibrium carbide concentration was derived for CR > 1.80.     

On the solubility of aluminum carbide in cryolitic melts

The solubility of aluminum carbide in cryolitic melts was determined as a function of NaF/A1F₃ molar ratio (CR), temperature, and the concentrations of A1₂O₃, CaF₂, MgF₂, and LiF. At 1020 °C a maximum concentration of 2.1 wt pct aluminum carbide was found at CR = 1.80. The following model for the aluminum carbide dissolution reaction based on activity data for NaF and A1F₃ was found to fit the experimental solubility data: Al₄C₃(s) + 5AlF₃(diss) + 9NaF(l) = 3Na₃Al₃CF₈(diss). From the solubility data for aluminum carbide an empirical equation giving the equilibrium carbide concentration was derived for CR > 1.80.     

A novel method for the determination of the hydrogen solubility in aluminum and aluminum alloy melts

The CHAPEL method, which was developed for the direct and continuous determination of the hydrogen activity in aluminum melts by measuring the equilibrium hydrogen pressure, has been modified for the determination of the hydrogen solubility in aluminum and aluminum alloy melts. The change of the hydrogen equilibrium pressure due to addition or removal of a given amount of hydrogen from a melt of known mass yields directly the Sieverts constant. Such experiments provide reliable data only if no additional gas exchange takes place between the melt and the gas phase. In the present work, a quasi-impermeable interface between the melt and the surrounding hydrogen atmosphere has been realized by maintaining the hydrogen pressure above the melt continuously at the level of the hydrogen equilibrium pressure. By this technique, which is performed by a relatively simple experimental setup, fast determination of the hydrogen solubility is possible. The main advantage of this novel method is the fact that it can be applied also for aluminum alloys without protective oxide layer on the surface. Preliminary results on pure aluminum and Al-Cu alloy melts show good agreement with the data obtained by the classical method of Sieverts that is not well suited for routine determinations on a wide range of alloy composition and temperature since it is very time-consuming. The method can also be applied for the investigation of other metal and alloy melts.     

Solubility of aluminum in cryolite-alumina electrolytes

The complex influence that additives of aluminum, calcium, lithium, and potassium fluorides have on the total solubility of aluminum in the cryolite-alumina melt is studied. The investigations were performed at a constant overheating of the melt saturated by alumina (15°C relative to the liquidus temperature). The aluminum solubility was determined via the prolonged holding of metal in the melt at a specified temperature with the subsequent sampling of the specimen, which was rapidly frozen, and the metal content in it was determined by a gas-volumetric analysis. It was seen that, as the cryolite ratio (CR) decreases from 2.5 to 2.2, the aluminum solubility in the electrolyte drops by a factor of 1.7, while the further decrease in CR to 2.1 does not cause its substantial variation. The introduction of 3 wt % LiF and 5 wt % KF into the melt with a constant overheating of the melt substantially decreases the metal solubility, namely, by a factor of 2.9 and 1.5, respectively.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part II. Solubility of TiO2

The solubility of TiO₂ in cryolite-alumina melts at 1020 °C was measured; it decreased with increasing alumina concentration up to ∼3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an alumina-saturated melt. Modeling indicated that the most probable titanium species are TiOF₂ and Na₂TiO₃, which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 °C to 2.8 wt pct Ti at 1035 °C, the apparent partial molar enthalpy of dissolution of TiO₂ being 88±4 kJ mol⁻¹.     

The equilibrium solubility of nitrogen in aluminosilicate melts

The equilibrium solubility of nitrogen in aluminosilicate melts, similar in composition to many blast furnace slags, has been measured, although the silica contents under investigation have been limited by experimental problems to between 30 and 38 mole pct. The solubility is a chemical rather than a physical phenomenon, it being necessary to impose severe reducing conditions to obtain significant quantities of nitrogen in solution. The dependence of solubility on slag basicity has been investigated, and a simple dependence upon nitrogen and carbon monoxide contents in the gas phase established. From the results, it is postulated that an exchange reaction takes place between nitrogen gas and bridging oxygen atoms in the aluminosilicate structure, and the results are discussed in relation to the structure of the melts.     

The prediction of the nitrogen solubility in binary iron melts

Statistical model of nitrogen solubility in iron melts. Equations. Calculation of nitrogen solubility of the systems Fe–Co–N, Fe–Cr–N, Fe–Mn–N and Fe–Ni–N in the liquid state at 0.101 MPa. Comparison between model and experiments.     

Oxygen solubility in silicon-containing Fe-Co melts

Thermodynamic analysis of the oxygen solutions in silicon-containing Fe-Co melts is performed. The equilibrium constant of silicon deoxidation of iron-cobalt melts, the activity coefficients for infinite dilution, and the interaction parameters for melts differing in composition are determined. The dependences of the oxygen solubility in the melts under study are calculated for different cobalt and silicon contents. The deoxidizing capacity of silicon increases substantially as the cobalt content in a melt increases. The curves of oxygen solubility in Fe-Co melts have a minimum; the minimum oxygen solubility shifts to a low silicon content as the cobalt content in the melts increases. The silicon contents for the minima in the curves of oxygen solubility and the minimum oxygen concentrations corresponding to the silicon contents are determined.     

The dissolution of aluminum oxide in cryolite melts

Several modifications of Al₂O₃ were prepared. Al₂O₃ samples pressed to tablets were dissolved in cryolite melts at temperatures between 975 and 1090°C. The dissolution rates obtained are consistent with the consecutive occurrence of two rate laws. The dissolution rate changes at an Al₂O₃ content of 5 to 6 pct in the cryolite melts. The results are discussed with consideration of other experimental and theoretical work. It is concluded that the alteration of the rate law may be explained by structural changes in the melts.     

The influence of basicity on the solubility of platinum in oxide melts

The solubility of platinum in molten BaO-CuO _x , BaO-MnO _x , CaO_satd-SiO₂-FeO _x , KO_0.5-SiO₂, NaO_0.5-SiO₂, and NaO_0.5-PO_2.5 fluxes has been measured in order to seek a measure of the basicity of highly basic fluxes containing transition metal ions and to clarify the chemical behavior of platinum in those melts. The solubility of platinum increases with increasing content of basic oxide in highly basic fluxes, suggesting that it may be a good indicator of the basicity of highly basic fluxes containing transition metal ions. The solubility of platinum in the KO_0.5-SiO₂, NaO_0.5-SiO₂, and NaO_0.5-PO_2.5 melts has a minimum value, and it is suggested that a platinum is amphoteric: it exists as a platinum cation in an acidic flux and does as a platinate ion in a basic flux.