Agglomeration, secondary

When wastes containing valuable components are processed for recycling, it is necessary to not only identify the need for and method of use of the agglomerated secondary raw material, but also an actual user and its relative location. The cost for transportation from the source to the potential consumer often becomes prohibitive. 

In addition to the fundamental property of particle si2e (and surface area), carbon black possesses a secondary characteristic of stmcture, best described as the tendency of individual particles to agglomerate or associate with one another. These two properties or characteristics of the carbon control the degree and nature of the reinforcing character of the black in mbber. The stmcture of the carbon black is deterrnined by dibutyl phthalate absorption and surface area is estimated by N2 absorption (Table 10). 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

While primary agglomeration can occur originating from a single crystal, a second form of agglomeration occurs because of the presence and motion of more than one crystal in a suspension, leading to secondary crystal aggregation. Two types of secondary agglomeration occur ... 

Each process can give rise to collisions, hence has the potential of causing aggregation leading to secondary agglomeration. These processes are illustrated in Figures 6.6(a) and (b) respectively. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

Critical relative humidity The primary value of the critical relative humidity denotes that humidity below which no corrosion of the metal in question takes place. However, it is important to know whether this refers to a clean metal surface or one covered with corrosion products. In the latter case a secondary critical humidity is usually found at which the rate of corrosion increases markedly. This is attributed to the hygroscopic nature of the corrosion product (see later). In the case of iron and steel it appears that there may even be a tertiary critical humidity . Thus at about 60% r.h. rusting commences at a very slow rate (primary value) at 75-80% r.h. there is a sharp increase in corrosion rate probably attributable to capillary condensation of moisture within the rust . At 90% r.h. there is a further increase in rusting rate corresponding to the vapour pressure of saturated ferrous sulphate solution , ferrous sulphate being identifiable in rust as crystalline agglomerates. The primary critical r.h. for uncorroded metal surfaces seems to be virtually the same for all metals, but the secondary values vary quite widely. 

The same applies to crystallizers, in which particle sizes and particle number concentrations not only depend on nucleation and growth from supersaturated mother liquid, but are also affected by shear-dominated agglomeration and by secondary nucleation as a result of particle particle and particle-impeller collisions. Some of the subprocesses involved may be limited to specific and different parts of the vessel e.g., nucleation may be restricted to a flame-like region around the outlet of a feed pipe (Van Leeuwen, 1998). In addition, in... 

Milnes and Mostaghaci [5.5] compared the consequences of different drying methods on the density, the sinter rate and micro structures of sublimated TiO-, suspensions. Evaporation of water in a micro-oven and by radiation heating leds to strongly bound agglomerates, while freeze drying resulted in softly bound secondary clusters. The freeze dried powder reached in 2 h of sintering 98 % of the theoretical density, while differently dried powders needed twice as much time and had a less fine microstructure. 

The degree of contamination governs the performance and product quality and affects the utility value of the end product. Secondary contaminants such as additive agglomerates are included in the term contamination . Particularly in case of film polymers, such particles diminish the quality of audio or video tapes due to the vulnerability of films to this drop-out phenomenon. 

Separation depends on the selection of a process in which the behaviour of the material is influenced to a very marked degree by some physical property. Thus, if a material is to be separated into various size fractions, a sieving method may be used because this process depends primarily on the size of the particles, though other physical properties such as the shape of the particles and their tendency to agglomerate may also be involved. Other methods of separation depend on the differences in the behaviour of the particles in a moving fluid, and in this case the size and the density of the particles are the most important factors and shape is of secondary importance. Other processes make use of differences in electrical or magnetic properties of the materials or in their surface properties. 

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