Metal dissolution separation

In the broadest sense, coordination chemistry is involved in the majority of steps prior to the isolation of a pure metal because the physical properties and relative stabilities of metal compounds relate to the nature and disposition of ligands in the metal coordination spheres. This applies both to pyrometallurgy, which produces metals or intermediate products directly from the ore by use of high-temperature oxidative or reductive processes and to hydrometallurgy, which involves the processing of an ore by the dissolution, separation, purification, and precipitation of the dissolved metal by the use of aqueous solutions. 4... 

An examination of the theoretical models proposed for metal dissolution and for the general Impedance behavior of electrodes enables the rate-determining step of the corrosion reaction to be Identified. It Is then possible to separately study the rate determining step In order to find a suitable Inhibitor or a suitable surface coating. 

Complex chlorides of Ce4+ may be prepared by dissolution of cerium(IV) hydroxide in methanolic hydrochloric acid followed by addition of a base B, giving (BH)2[CeCl6]. (NH4)2CeCl6 can be used as a selective precipitant for Cs+ ions in presence of other alkali metals, with separation coefficients ranging from 5300 (Na) to 60 (Rb).718... 

The basic mechanism for the instability of ultrapure metals was suggested by Wagner and Traud in a classic paper in 1938.1 The essence of their view is that for corrosion to occur, there need not exist spatially separated electron-sink and -source areas on the corroding metal. Hence, impurities or other heterogeneities on the surface are not essential for the occurrence of corrosion. The necessary and sufficient condition for corrosion is that the metal dissolution reaction and some electronation reaction proceed simultaneously at the metal/environment interface. For these two processes to take place simultaneously, it is necessary and sufficient that the corrosion potential be more positive than the equilibrium potential of the M, + + ne M reaction and more negative than the equilibrium potential of the electronation (cathodic) reaction A + ne — D involving electron acceptors contained in the electrolyte (Fig. 12.8). 

The processing of metal oxides is essential for metal extraction, waste recycling, and catalyst preparation [12]. One of the major difficulties with metal oxides is that metal oxides are insoluble in most molecular solvents and generally strong aqueous mineral acids are used for their dissolution. Separation has usually been achieved... 

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

Figure 1-29 has shown the superposition of metal dissolution and a cathodic process which leads to zero total current density i and a corrosion rate I c at the rest potential Sr. If the Nernst potential of the metal/met-al-ion electrode and the involved redox system are sufficiently separated and the related I-E characteristic is sufficiently steep, the polarization curve only contains the anodic metal dissolution and the cathodic reduction of the redox system. The related opposite reactions are neglected. 

Rates of corrosion can also be measured using an electrochemical technique known as potentiodynamic polarization. The potential of the test metal electrode relative to a reference electrode (commonly the saturated calomel electrode SCE) is varied at a controlled rate using a potentiostat. The resultant current density which flows in the cell via an auxiliary electrode, typically platinum, is recorded as a function of potential. The schematic curve in fig. 2 is typical of data obtained from such a test. These data can provide various parameters in addition to corrosion rate, all of which are suitable for describing corrosion resistance. The corrosion potential F corr is nominally the open circuit or rest potential of the metal in solution. At this potential, the anodic and cathodic processes occurring on the surface are in equilibrium. When the sample is polarized to potentials more positive than Scon the anodic processes, such as metal dissolution, dominate (Anodic Polarization Curve). With polarization to potentials more negative than Scorr the cathodic processes involved in the corrosion reaction such as oxygen reduction and hydrogen evolution dominate (Cathodic Polarization Curve). These separate halves of the total polarization curve may provide information about the rates of anodic and cathodic processes. The current density at any particular potential is a measure of the... 

The time resolution for the detection of a product at the disk by the ring current is in the range of 0.1 s, which is required for the transport due to the laminar flow of the electrolyte in front of the electrode. Therefore, this method is suited for measurements of potentio-static dissolution transients in this time frame. One example for corrosion is the separation of currents measured during passivation transients at the disc into a part for metal dissolution and another for oxide formation. The ring current allows calculating the dissolution rate i at the disc according to Equation 1.130, whereas the difference to the total disc current foi - ic = K yields the current density of layer formation These investigations may be done with the time resolution of the method of ca. 0.1 s, i.e., x c and may be followed as a function of time. 

Corrosion in open circuit conditions requires a counter reaction to consume the electrons, which are produced by metal dissolution. Important reactions are hydrogen evolution and oxygen reduction. Fe + reduction is another. [Fe(CN)6] reduction is a simple outer sphere reaction, which may be used for basic corrosion studies. In most cases, one tries for simplicity to study metal dissolution and the redox reactions separately with the use of a potentiostat. In the following, the hydrogen evolution and the oxygen reduction reaction are discussed separately. 

The formation of the metallic salts is a pyrometaHurgical process, and is commonly referred to as the dry process. The separation of the salts from each other is accompHshed by selective dissolution in water, and is named the wet process. 

---