Segregation methods, reduction

The recovery of aluminum metal is divided into two steps, i. e., the production of pure alumina (Bayer Process) and its molten salt electrolysis. Raw aluminum obtained by reduction electrolysis already has a high purity (99.5-99.7%). Refining methods for raw aluminum to obtain higher purities include the segregation process (99.94-99.99% Al) and three-layer electrolysis (99.99-99.998% Al) [142, 236]. Besides these, processes are available whereby the aluminum is anodically dissolved in an organic electrolyte and then cathodically deposited [37, 118, 217, 221]. The dissolution as well as the deposition process contribute to the electrolytic refining of aluminum. 

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

Even if care is taken to minimize waste, there will still be waste to treat. Currently used wet finishing processes produce waste containing organic as well as inorganic compounds. The efflnents are rich in chemicals of which some are persistent or resistant to water treatment methods. Table 2.1 lists typical examples of finishing waste that resists biodegradation. Removal of these substances from wastewater is expensive and difficult to achieve. Because of this, the effiuent segregation and source reduction methods are preferred as economically attractive alternatives. 

Before starting an end-of-pipe treatment, effiuent segregation is required this separates contaminated fluid from the cleaner streams. The final wastewater is less voluminous and can be treated more effectively and by more appropriate treatment methods, whereas the more clean stream can be reused in the factory with limited or no treatment needed. Vajnhandl and Valh (2014) reviewed the wastewater reuse programs used over the last 10 years covering the European textile finishing industry, with special emphasis on the FP7 project Aquafit4use. The water treatment methodologies presented enable more than a 40% reduction in freshwater consumption. 

All of the above processes, in principle, afford the benefit of controlling the particle sizes of bimetallic systems that are eventually formed, whereas it may be more difficult to synthesize bimetallic colloids via traditional methods. Depending on the thermodynamic properties of the two metals, conventional catalyst preparation techniques (i.e., co-impregnation, co-precipitation) may result in the formation of a homogeneous alloy, segregation into pure monometallic phases, or a combination thereof. On the other hand, templating by dendrimers at least ensures that both metals are in close proximity with each other during complexation and after reduction. 

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