Agglomeration methods

For accurately shaped compacts with extreme demands on tolerance only the confined die approach of pressure agglomeration is applicable. Such requirements exist, for example, in the pharmaceutical industry where tabletting machines are used to carry out the task. None of the growth agglomeration methods can yield products with these specifications. 

The reciprocating movement of the piston(s) and the relatively small volume of the die cavity severely restrict the capacity of this type of equipment even if modern, multistation, rotary tabletting presses are considered. With the production of larger pieces in, for example, hydraulic presses or roller briquetting machines, the accuracy in weight, shape, and dimensions is no longer obtained. 

Shape is often also an important characteristic. In many cases spherical products of size enlargement are desired. This approximate shape can be obtained with all growth agglomeration methods. On the other hand, unless extremely accurate feed control can be established in some tabletting machines, spherical products cannot be produced with pressure agglomeration equipment. The nearest approximation would be lens- or almond-shaped compacts. 

Depending on the requirements of the application, methods for size enlargement by agglomeration may be operating as batch or continuous processes. Batch processes are normally characterized by low capacity but feature a high degree 

Space and energy requirements as well as investment costs are frequently factors that render an otherwise perfectly feasible and desirable process uneconomical and kill a project. Sometimes they also incorrectly direct the interest toward methods which, after superficial investigation, seem to offer cheaper alternatives. In this context it should be stressed that the entire process must always be considered. For example, for the granulation of fertilizers a granulation drum may seem to be the cheaper solution in comparison with a roller compactor if only investment costs are considered this may even be true if the entire system is investigated. However, if space requirements and energy (operating) costs are compared, a different conclusion may be obtained (see Section 5.4.4). 

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The agglomerating methods typically used in the iron ore industry are pelletizing, sintering and, to a limited extent, briquetting and nodulizing (see Size enlargel nt). 

Sirianni, A.F., Coleman, R.D., Goodhue, E.C. and Puddington, I.E., Separation studies of iron ore bodies containing apatite by spherical agglomeration methods, Trans. Can. Inst. Mining Met. 71 (1968) 149—153. 

Pellets. Pellets are mainly prepared by four different methods powder layering, rotating fluidized bed, extrusion/spheronization, and the agglomeration method. The oldest one is the so-called. 

Li LC, Peck GE. The effect of agglomeration methods on the micrometric properties of a maltodextrin product Maltrin ISO. Drug Dev Ind Pharm 1990 16 1491—1503. 

Typical in (iron) ore industry any agglomeration method involving growth agglomeration with subsequent heat induration. 

Particle shape is of the greatest importance for agglomeration. Typical characteristics are particle roundness or the general overall shape and surface roughness. For some binding mechanisms and/or agglomeration methods, the particle shape is the most decisive particle characteristic. 

Pressure agglomeration methods which use sometimes very high forces to consolidate the particulate matter can be applied for larger feed sizes. 

In some industries, special material characteristics, such as heat and/or pressure sensitivity, toxicity, reactivity, etc., are of importance. Particularly in the pharmaceutical industry and some sectors of chemistry these parameters must be considered. While pressure agglomeration methods have only limited applicability for heat- and/or pressure-sensitive materials, toxicity may limit the appeal of tumble agglomeration because of the difficulty to contain dust and avoid contamination of the environment. 

The above has been an attempt to discuss some of the requirements that must be taken into consideration when selecting the best suited agglomeration method for a particular project. Of course, this treatment of the subject is not complete and a new set of conditions must be developed for each application. It should be accepted as a rough guideline directing the investigator to the most important parameters to be considered. 

With the exception of very few applications where particles are so small that they naturally agglomerate in the dry state, tumble agglomeration methods utilize binders. Even in those materials that contain the binder component inherently, this constituent of the bulk mass to be agglomerated is so obvious that one cannot classify such processes as binderless. 

Because it is normally not feasible to sample the contents of batch mixers during operation to determine the progress and state of agglomeration, methods have been developed to control the performance of mixer-granulators, e.g. by continuously measuring the momentary energy consumption of the equipment. 

Another advantage of pressure agglomeration is that, in most cases, essentially dry solids are processed which do not tend to set and that the amount of material in the system is relatively small. Therefore, pressure agglomeration methods lend themselves particularly well to batch or shift operation and to applications whereby several products must be manufactured from different feed mixtures. At the end of a production run, the system can be easily and completely emptied in a relatively short period of time. 

Several materials naturally contain binders, e.g. the bituminous components in many coals. Since this binder was not added intentionally, pressure agglomeration methods processing such materials are considered binderless. 

Constant product quality requires an even feed rate, homogeneous bulk density of the material to be treated, uniform densification, and reproducible maximum pressure. This statement is true for all pressure agglomeration methods. However, while these conditions can be met relatively easily in die and roller presses with proper feed preparation and specific equipment parameters, it is rather difficult to achieve in extrusion. The reason for this is that densification and maximum pressure depend on the resistance to flow in the die channel or holes. Small variations in feed homogeneity or frictional properties can yield major differences in equipment performance and product quality. Wear or buildup in the extrusion die are among the most important parameters influencing the back-pressure which, in turn, is responsible for the amount of densification prior to extrusion. 

Other Agglomeration Methods for Special Applications 4.2.3.1 General... 

Therefore, the basis for all agglomeration methods can be found in the availability and/or selection of binding mechanisms. The technique or equipment used is only the vehicle to obtain the agglomerated product of desired shape, size, strength, density, etc. Consequently, agglomeration methods that can be considered special still employ similar effects and mechanisms, as mentioned before in the two main groups tumble (Section 4.2.1) and pressure (Section 4.2.2)... 

On the other end of the scale of sophistication are agglomeration methods needed for low cost applications in the field of recovery of small amounts of valuable materials by leaching and waste processing for disposal. Many finely divided particulate wastes cannot be deposited in landfills or similar open storage facilities because of the danger of recontamination by wind and water. Because, in this case, agglomeration is only an additional cost, the cheapest possible method must be selected. 

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