Comminution aggregates

Once the comminution process is completed, the succeeding operations in mineral processing are taken over by what is known as separation. Regardless of the method or methods used, the aim is always the same-to take a natural aggregate of minerals (an ore) and separate it into two or more mineral products. In general, the products of separation are (i) the concentrate which contains the valuable minerals and (ii) the tailings which contain primarily materials of little or no value. It may be borne in mind that minerals have been liberated, either by grinding or by chemical means, must usually be sized prior to... 

In industrial and laboratory settings the subdivision process more commonly involves the comminution of large particles or aggregates into smaller sizes, either dry with subsequent dispersion (size reduction to the order of a few pm) or directly in a slurry (size reduction to as small as a few tenths of pm). Examples of comminution machines include agitator ball mills, colloid mills, cutting mills, disk mills, homogenizers, jet mills, mechanical impact mills, ring-roller mills, and roll crushers. 

Dea lomeration can take place by two methods comminution and ultrasonification. Comminution is the subject of Chapter 4, so it will not be discussed in detail here. In the comminution of aggregates, the popvdation balance can be used to predict the size distribution as a function of time in a batch mill or as a function of mean residence time in a continuous mill. Aggregates have the same type of birth and death fvmctions for particle breakage as polycrystalline particles but the rate constants are much hi er and the size selectivities for aggregates are different than those for the comminution of crystalline materials. The... 

Any particular mill and mill charge combination has a limiting particle size, beyond which no further comminution will occur. For most industrial aggregates, this is in the range of 0.5-1 pm [9.25]. Only in optimized mills is further comminution possible, finally reaching a size limit of about 100-150 nm [9.24]. This limit is inherent to the WC (i.e., to its mechanical properties). This kind of milling is used only by specialists. 

There are a large number of methods (Table 2) to prepare nanoparticulate systems. These depend to a large extent on the material (polymer, protein, metal, ceramic) that will form the basis of the carrier. One can, in essence, consider three approaches to their production (0 by comminution (in the case of solids, milling, and in the case of liquids, high pressure emulsification) (ii) molecular self-assembly, such as that occurs with polymeric surfactants to form polymeric micelles or with dendrons to form dendrimeric aggregates and (iii) precipitation from a good solvent as shown in Figure 6. 

For the formulation of suspensions, the hydrophobic or hydrophilic solid is dispersed in a aqueous or nonaqueous medium to produce a system that covers a wide particle size range, typically 0.1-5 pm. This process requires the presence of a surfactant (dispersant) that satisfies four criteria (i) wetting of the powder by the Uquid (if) the dispersion of aggregates and agglomerates into single units (iii) comminution of the large particles into smaller units and (iv) stabilisation... 

Particle Shape. Shape Factors. The method of formation plays a major role in shaping the resultant particles. Particles generated by comminution, attrition, or disintegration resemble the parent material. However, if the method of formation is condensation from vapor or precipitation from solution, the smallest unitary particle may be spherical or cubical. Often condensation is followed immediately by solidification and the formation of chainlike aggregates (e.g., iron oxide fumes, carbon black). In liquid suspensions, similar particle aggregation or flocculation is important in determining suspension behavior. 

The dispersion of aggregates and agglomerates and subsequent reduction of particle size by wet milling (a process referred to as comminution) also requires... 

F. Garcia, N. LeBolay, C. Frances, Rheological behaviour and related granulometric properties of dense aggregated suspensions during an ultrafine comminution process. Powder Tech., 130, 407-414 (2003). 

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