Dissolved Metal Species Research Articles

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 cAl = cNa(diss) + cAlF2- + cAl2F3- + cAl3F4- + cAl4F5- 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).

Determination of dissolved heavy metal gradients at the sediment-water interface by use of a diffusion-controlled sampler

Laboratory and field evaluations were made of a diffusion-controlled sampler which incorporates a 0.2-μm polycarbonate membrane to filter pore water samples in situ. Sample ports at 1-cm intervals were exposed both above and beneath the sediment-water interface enabling determination of geochemical gradients and dissolved metal profiles at the interface. Gradients obtained for ions whose concentrations are controlled by redox processes in the sediment column were not consistent with those determined from interstitial water by squeezing of adjacent sediment. The diffusive flux of dissolved metal species across the sediment-water interface can be assessed more accurately using this sampler than by data collected from conventional benthic cores.

Influence of Nitrilotriacetate on the Elimination of Metal Ions by Coprecipitation with Ferric Hydroxide

The elimination of dissolved copper, zinc, cadmium and lead by coprecipitation with iron(III) hydroxide was investigated. The reaction conditions of the batch experiments were adopted from water treatment. Conditional distribution coefficients show the order Cd &amp;lt; Zn &amp;lt; Cu &amp;lt; Pb for increasing interaction of the metals with the ferric hydroxide. The presence of nitrilotriacetate (NTA) decreases the efficiency of the metal elimination. NTA is also able to remobilize metals from the metal enriched precipitation. There is a significant effect, even at low concentrations of a few µmol/l. Remobilization of Cu, Zn and Cd increases, remobilization of Pb decreases with reaction time. The “ageing” of the precipitation plays an important role for the yield of the reactions. From a broad application of NTA in detergents an increase of the concentration of dissolved metal species has to be expected.

Dissolved Metal Species Mechanism for Initiation of Crevice Corrosion of Aluminum: I . Experimental Investigations in Chloride Solutions

Experiments on initiation of crevice corrosion were carried out on Al specimens in dilute aqueous solutions at ambient temperature. Transient current distributions within crevices were measured by a sectioned electrode technique, and microsampling was used to monitor dissolved aluminum, pH, and [Cl−] composition changes within the crevice prior to breakdown. Auxiliary (noncrevice) experiments were carried out in solutions having various pH, [Cl−], and dissolved aluminum concentration. Initiation was found to occur upon exceeding a minimum concentration of dissolved aluminum inside the crevice. Mechanisms for initiation based on acidification within the crevice were excluded.

Dissolved Metal Species Mechanism for Initiation of Crevice Corrosion of Aluminum: II . Mathematical Model

A mathematical model for the initiation of crevice corrosion was developed based on the hypothesis that, for Al in , initiation occurs when the concentration of dissolved metal exceeds a certain minimum critical value. This hypothesis was tested experimentally in the preceding qualitative paper. The model accounts for crevice geometry; for electrode reactions of aluminum, oxygen, and hydronium ion; for homogeneous hydrolysis equilibria; and for transport by unsteady‐state diffusion and migration. The predictions of the model, made with use of literature data for all parameters, were found to compare favorably with experimental observations reported in the previous paper. The model predicts rapid depletion of and rapid acidification of the crevice solution, followed by the gradual buildup of dissolved metal species which eventually trigger breakdown. The model also predicts that there is negligible accumulation of Cl− within the crevice prior to breakdown.The model accurately predicts the influence of crevice geometry upon breakdown time.