Reduction of Segregation

Segregation occurs primarily as a result of size difference. The difficulty of mixing two components can therefore be reduced by making the size of the components as similar as possible and by reducing the absolute size of both components. Segregation is generally not a serious problem when all particles are less than 30 pm (for particle densities in the range 2000-3000 kg/m ). In such fine powders 

If it is not possible to alter the size of the components of the mixture or to add liquid, then in order to avoid serious segregation, care should be taken to avoid situations which are likely to promote segregation. In particular pouring operations and the formation of a moving sloping powder surface should be avoided. 

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Reduction of segregation. When mixtures of powders and granules are mixed, the powder will percolate through the interstitial voids of the granules. By granulating the powder, the segregation can be reduced. 

Step 1 convection by mean velocity Step 2 turbulent dispersion by large eddies Step 3 reduction of segregation length scale Step 4 laminar stretching of small eddies Step 5 molecular diffusion and chemical reaction... 

However, the models described above are purely phenomenological. What is the underlying mechanism for reduction of segregation and how do micromixing parameters depend on mechanical and physicochemical data ... 

Reduction of segregation length scale by breakdown of large eddies... 

Unfortunately, it seems that segregation seldom can be avoided completely, but it seems that, when the reasons why it occurs are understood, it will be possible to reduce it to a level where the adverse effects are acceptable in most cases. In principle there are three main possibilities for reduction of segregation problems. First, one can try to change the particulate solid in such a way that its tendency to segregate is reduced. Second, the equipment can be modified to reduce segregation, and third, the process conditions can be modified. 

Another technique is to change the particle size distribution. There are, however, disadvantages. If segregation is occurring by the sifting mechanism, the particles must be almost identical in size before sifting is prevented. Alternatively, the mean particle size can be reduced below 100 p.m, but this size reduction (qv) increases the probabiUty of segregation by the too fine powder mechanisms. 

Thus the addition of an inert gas which does not intervene chemically in the uansport reaction but adds to the density of die gas, reduces the segregation due to thermal diffusion. An example of this is the reduction of tlrermal separation in a mixture of H2 and H2O by the addition of Hg vapour (Dastur and Chipman, 1948). 

In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

Figure 13 Scheme of the influence of the dose rate on the competition between the inter-metal electron transfer and the coalescence processes during the radiolytic reduction of mixed metal ion solutions. Sudden irradiation at high dose rates favor alloying, whereas low dose rates favor coreshell segregation of the metals because of metal displacement in the clusters. 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

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