Dissolution, selective

Chen S J, Sanz F, Ogletree D F, Hallmark V M, Devine T M and Salmeron M 1993 Selective dissolution of copper from Au-rich Au-Cu alloys an electrochemical STS study Surf. Sc . 292 289... 

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. 

Conventional Refining Process. The conventional refining process is based on complex selective dissolution and precipitation techniques. The exact process at each refinery differs in detail (12—14), but a typical scheme is outlined in Figure 2. 

Several processes are available for the recovery of platinum and palladium from spent automotive or petroleum industry catalysts. These include the following. (/) Selective dissolution of the PGM from the ceramic support in aqua regia. Soluble chloro complexes of Pt, Pd, and Rh are formed, and reduction of these gives cmde PGM for further refining. (2) Dissolution of the catalyst support in sulfuric acid, in which platinum is insoluble. This... 

Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, permeable sohd phase with which it is associated. The separation usually involves selective dissolution, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid and it may be incorporated within, chemically combined with, adsorbed upon, or held mechanically in the pore structure of the insoluble material. The insoluble sohd may be massive and porous more often it is particulate, and the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated. 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

In contrast, the selective dissolution or leaching-out by corrosion of one component of a single-phase alloy is of considerable practical importance. The most common example of this phenomenon, which is also referred to as parting , is dezincification, i.e. the selective removal of zinc from brass (see Section 1.6). Similar phenomena are observed in other binary copper-base alloys, notably Cu-Al, as well as in other alloy systems. 

In principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

There is considerable metallographic and electrochemical evidence in support of each theory and it is of interest to note that two of the most authoritative works on corrosion appear to support opposite views — Uhlig favours the selective dissolution of zinc theory, whereas Fontana and Greene favour the dissolution-precipitation theory. 

Decrease in velocity will favour all forms of localised attack in which an occluded cell is involved in the mechanism, and will also favour selective dissolution of alloys that are susceptible to this form of attack. 

Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich compound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S -containing media . Au-Cu-rich lamellae have similarly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of oxygen . 

For environments in which tin is less readily corroded than lead, corrosion resistance of the alloy decreases as the lead content increases the decrease may, in some circumstances, be sharp at a particular composition. In the more corrosive media, such as nitrite solution, a sharp increase of corrosion rate is observed as the lead content increases beyond 30waters with low contents of dissolved salts, the corrosion rate increases slowly with lead content up to about 70% and then rises more steeply, but in the general run of supply waters the ability of lead to form protective insoluble anodic products is helpful to the durability of solder. Selective dissolution of tin has been... 

Parting selective dissolution of one metal (usually the most electro-reactive) from an alloy leaving a residue of the less reactive constituents. 

In favourable systems, selective dissolution of components from reactant—product mixtures, that have been heated for appropriate time intervals, coupled with chemical analyses, have been used to obtain kinetic data for solid—solid reactions [419], Thus MgO is the only component which will dissolve in dilute HC1 and, therefore, can be specifically extracted from mixtures reacting as follows... 

In principle, the process is quite simple. The input is cut and reduced in size. The key of the process is selective dissolution of the PVC and its additives in a special solvent. The pure PVC is recovered by means of precipitation and dried and is ready for a new life (Figure 2). 

The Vinyloop process is based on the selective dissolution of PVC used in composites applications like cable insulation, flooring, tarpaulins, blisters, etc. After removal of insoluble parts like metals, rubber or other polymers, the PVC is reprecipitated with all additives by introduction of a non-solvent component whieh will form with the seleetive solvent an azeotropie mixture. By using typical conditions, the process is able to reeover a pure PVC eompound powder ready for use without any additional treatment like melt filtration or a new pelletisation (speeific characteristics of the powder are average diameter of 400 microns and bulk density above 600 kg/ eub.m). All the solvents used are eompletely reeyeled and reused. PVC compounds recovered in the Vinyloop process can be reused in a closed-loop recycling scheme... 

Kunststoffe German Plastics 83,No.ll,Nov.l993,p.21-3 SELECTIVE DISSOLUTION Schurr U Schneider M... 

Different views exist as to the reasons for selective dissolution of the asperities. According to older concepts, convection of the liquid is hindered in the solution layers filling recesses hence, reaction products will accumulate there and raise the concentration and viscosity in these layers. Both factors tend to lower a metal s anodic dissolution rate relative to that at raised points. According to other concepts, a surface condition close to passive arises during electropolishing. In this case, the conditions for passivation of the metal at raised points differ from those in recesses. 

In this chapter, photoelectrochemical control of size and color of silver nanoparticles, i.e., multicolor photo-chromism [1], is described. Silver nanoparticles are deposited on UV-irradiated Ti02 by photocatal5dic means [2]. Size of the nanoparticles can be roughly controlled in the photocatalytic deposition process. However, it is rather important that this method provides nanoparticles with broadly distributed sizes. The deposited silver nanoparticles are able to be dissolved partially and reduced in size by plasmon-induced photoelectrochemical oxidation in the presence of an appropriate electron acceptor such as oxygen. If a monochromatic visible light is used, only the particles that are resonant with the light are dissolved. That is, size-selective dissolution is possible [3]. This is the principle of the multicolor photochromism. 

In an industrial application dissolution/reprecipitation technology is used to separate and recover nylon from carpet waste [636]. Carpets are generally composed of three primary polymer components, namely polypropylene (backing), SBR latex (binding) and nylon (face fibres), and calcium carbonate filler. The process involves selective dissolution of nylon (typically constituting more than 50wt% of carpet polymer mass) with an 88 wt % liquid formic acid solution and recovery of nylon powder with scCC>2 antisolvent precipitation at high pressure. Papaspyrides and Kartalis [637] used dimethylsulfoxide as a solvent for PA6 and formic acid for PA6.6, and methylethylketone as the nonsolvent for both polymers. 

Selective dissolution of the polymer may be used industrially to separate polymer from additives for recycling purposes. However, separation of PPE from its additives (CB, talc, mica) in integrated circuit board scrap by means of trichloroethylene would not seem to meet industrial requirements (toxicity, cost) [10]. 

Significant progress has been made in the application of ionic liquids (ILs) as alternative solvents to C02 capture because of their unique properties such as very low vapour pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities and selective dissolution of certain organic and inorganic materials. ILs are liquid organic salts at ambient conditions with a cationic part and an anionic part. 

The kinetics of dissolution of pure CaC03 and soil CaC03, as indicated by the volume of C02 released and Ca dissolved during extraction, are presented in Fig. 4.3. It shows that dissolution of both pure and soil CaC03 by the NaOAc-HOAc solutions at various pHs reached a plateau after two hours. This indicates that a certain acid dose reacts completely with the proper content of soil carbonate within two hours. Tessier et al. (1979) reported that after five hours of leaching sediments, there was no increase in the calcium concentration, thus indicating that it is unnecessary to allow 16 hours for extraction of the CARB fraction, as was originally done in this sequentially selective dissolution procedure. 

---