Technical Bulletin. Continuous wet agglomeration by instant mixing technology

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1 Technical Bulletin Continuous wet agglomeration by instant mixing technology Abstract written by : Berthram Mak date : department : Schugi The process of instant mixing agglomeration is categorised as a wet agglomeration method. This means that at the beginning of the process, materials in the form of a powder and a liquid need to be present. Small particles are combined to larger, relatively permanent masses in which the original particles are still identifiable. Under the proper mixing conditions, predictable control of particle growth is possible. Perfect, efficient mixing conditions should ideally yield perfectly uniform granules. Although, obviously, this is not possible, instant mixing agglomeration is a process capitalising on the phenomenon, maximising control of appropriate mixing conditions. This is really the hart of the process, and is what differentiates instant mixing from other agglomeration methods. This paper will present details regarding one particular method of instant mixing agglomeration. This method is a continuous process utilizing high speed mechanical agitation and short residence times using the Schugi FLEXOMIX principle. Before a discussion of how these units operate, typical product qualities generated by this process are characterized. This will be followed by a discussion on how operating parameters affect these final product qualities. Application results will be presented based on utilizing this particular method. 1. Introduction Agglomeration can be defined as a process of particle size enlargement where small, fine particles, such as dusts or milled powders combine to form agglomerates with a larger size in which the original particles are still identifiable. These agglomerates have a coarse, open structure and a mean particle size ranging from 0,2 to a few millimeters. The process uses agitation in the presence of the required proportion of a liquid phase and perhaps other binding agents and is normally followed by evaporative drying and size classification. The dry agglomerates are for use as end products or in a further processing step. Before detailed discussion of the instant mixing method of agglomeration, it will prove beneficial to place this method within the context of particle size enlargement as a whole field of methods. This will further clarify use of this method relative to specific applications. The goal of any particle size enlargement process is to reliably produce particles exhibiting required end use properties. Particle size enlargement methods are selected, and ultimately evaluated, based on these properties and their associated benefits. Typical selection properties of granulated products and associated benefits range from simple dedusting, to the need for an engineered particle of specific size, shape, density and dissolving rate or compressibility.

2 2. Basic agglomeration methods Techniques for size enlargement to improve or adapt the bulk properties of particles can be classified into four basic processes (table 1). The processes are related to the principal method used to bring particles together into agglomerates. The categories are: thermal, compression, liquid and agitation methods. Process Subdivision Machinery Thermal Sintering Traveling Grate Nodulizing Extrusion Compression Compaction Roll Briquetter, Roll Compactor, Tablet Press Extrusion Roll (Gear) Pelletizer, Ring Die Pellet Mill Radial & Axial Extruders, Ram Extruder Liquid Systems Spray drying Spray Dryer with & without Fluid Bed Mechanical Mixing with Dry Recycle & Drying Hot Melt Cooling Spray Chiller, Prilling Tower, Flaking Drum Rotary Drum, Filter Belts Batch/Continuous Mixers Flocculation or Coagulation Agitation Mechanical Mixing High Speed Batch/Continuous Mixer Low Speed Batch/Continuous Mixer Tumbling Drums, Cones & Inclined Pans Pneumatic Mixing Table 1: Agglomeration methods Batch/Continuous FluidBed, Spouting Bed Thermal methods depend on the heat transfer to form particles into larger form. Agglomeration occurs through one or more of the following mechanisms: drying of a concentrated slurry, fusion, high temperature chemical reaction or solidification/crystallization of a melt/concentrated slurry. Heat may be transferred directly or indirectly. Using compression methods, powders are densified and compacted by using external force in a certain space. Forces involved to produce a stable agglomerate include solid bridges, immobile liquid bonds, surface forces and mechanical interlocking. Important matters for successful size enlargement by compression is the effective utilization and transmission of the applied external force and the physical properties of the product. The different compression methods can be classified according to the physical system used to apply the compression force: piston-type, roll-type and extrusion type pressing. Powder in agglomerated form can also be produced directly from a liquid or a semi-liquid phase by utilizing spray and dispersion methods. Agglomerates are formed in a highly dispersed suspension in equipment like spray dryers, prilling towers, fluidized and spouted beds, spraychillers or pneumatic conveying dryers. The feed material, which can consist of a solution, gel, paste, emulsion, slurry or melt, is injected as a spray, dispersed in a gas and converted into solid material by heat and/or mass transfer. Mechanisms of agglomerate formation include the hardening of feed droplets into solid particles, the layering of solids deposited from the feed onto existing nuclei and the coalescence of small particles into agglomerates by deposition of binding solids from the spray. In general, liquid methods can be classified into: spray drying, hot melt cooling and flocculation. Agitation methods are described as follows: For the production of agglomerates, the primary particles first have to be brought into contact with each other by agitation. Most commonly, a liquid phase is used to develop binding forces. A widely used liquid phase is water or an aqueous solution. This first step is indicated as the nucleation stage. In a second step, permanent adhesion forces stronger than any possibly existing disruptive forces must be established between these particles. After nucleation has occurred, the predominating growth mechanisms are coalescence and layering. On the other hand, destruction can occur by shatter, breakage, attrition and/or abrasion transfer.

3 The so-called build-up agglomerates are actually defined by interaction and balance between growth and destruction. The duration and intensity of these forces have an important influence upon agglomerate porosity and stability. An agglomeration process in which high agitation forces act on the particles and agglomerates will result in dense, smooth, and stable agglomerates. Agglomeration by agitation can be subdivided into mechanical and pneumatic mixing agglomeration. Typical agglomeration methods based on agitation are fluidbed, drum, pan, disc, falling curtain, shear mixing. See for typical agglomerate formation figure 1 (right column). Shear mixing agglomeration combines the mixing process and the agglomeration step. The method of agitation by mixing elements defines the shearing action and is therefore an important factor in defining the balance between particle growth and destruction. 3. Bonding mechanisms The interactive forces between small particles are difficult to measure directly due to their very low magnitude and limitations resulting from the small size. Nevertheless, a good understanding of the interaction between particles in an assembly is fundamental for understanding the relationship between agglomerate properties and agglomeration methods. A fundamental approach based on the nature of the particle-particle interaction and independent of the process step producing the interaction, was introduced by Rumpf and co-workers and is summarized in figure 1 (left two columns). without material bridge Van der Waals forces Electrostatic forces Surface roughness with material bridge Sinter bridge Crystallized bridge Liquid bridge Liquid filled capillaries Build-up agglomerate Primary particles enter the agglomeration zone Particle surfaces are wetted by collision with droplets and wetted particles collide Particles adhere to each other and liquid bridges develop In the drying zone, solubles crystallize out of the liquid bridges, forming solid bridges Fig. 1: Binding mechanisms between solid particles Primary classification of binding mechanisms is binding without material bridges between particles and through formation of solid or liquid bridges between particles. Powders, subject to agglomeration usually consist of primary particles whose size is smaller than 200 µm in order to facilitate solubility. In such powders, van der Waals forces are strong enough to cause the formation of dry agglomerates. Dry agglomeration may be caused also by electrostatic forces and by mechanical interlocking. These randomly developed agglomerates without material bridge formation, however, have undefined properties and are unstable. This form of dry agglomeration is the main cause for reduced flowability and dosing problems, and is therefore usually unwanted. Therefore further treatment is required to form stable agglomerates formed by material bridges. Particle size Mixing Agglomeration Less liquid particle liquid Small agglomerates % Liquid More liquid Larger agglomerates Fig. 2: Influence liquid addition on particle size Fig. 3: raspberry like structure

4 The liquid to powder ratio is an important parameter in the design of the particle size. Starting with 0% liquid addition, the liquid will be distributed over the powder particles and partly absorbed. After a certain liquid addition rate, a sufficient amount of liquid is available on the outside of the powder particle and liquid bridges are starting to build between the particles (figure 2). With even more liquid addition, liquid bridges between small agglomerates occur (figure 3). The maximum liquid to powder ratio is defined by the specific product characteristics, the binding agent, and the process conditions. Agglomeration is performed due to different reasons: dedusting, improving flow, dosing and storage properties, minimizing the risk for segregation, influencing the particle size and bulk density. Apart from that, the humidity of the endproduct can be defined. Also, by agglomeration, properties like dispersibility, wettability, sinkability and dissolving time are improved. These properties typically belong to the so-called instant food products. Typically, uncontrolled solid/liquid mixing operations result in a wide range of particle sizes due to a wide range in localised liquid loading levels. By using a process with controlled mixing conditions, however, an efficient, homogeneous, reproduceable agglomerate can be achieved. An optimal powder/liquid mixing system can be defined as an installation with which from the first moment on the dry components to be mixed are directly combined with the liquid components in exactly the correct proportion. This definition was taken into account in the development of the underneath described concept. 4. Instant mixing agglomeration The process of instant mixing agglomeration is categorised as a wet agglomeration method. This means that materials in the form of a powder and a liquid are required by the process. The raw feed material does not necessarily need to be in powder form, and the liquid does not necessarily need to be water or other solvent based solution or slurry. However, at the beginning of the process, a powder and liquid need to be present. Any agglomeration process is really a composition of multiple unit operations. The instant mixing agglomeration process is no exception, typically consisting of the unit operations of mixing, drying, conditioning, size reduction, size classification and material transport. Under the proper mixing conditions, predictable control of particle growth is possible. Perfect, efficient mixing conditions should yield perfectly uniform granules. Although, obviously, this is not possible, instant mixing agglomeration is a process capitalising on the phenomenon, maximising control of appropriate mixing conditions. This is really the hart of the process, and is what differentiates instant mixing from the other methods. Basic operations that qualify a process as instant mixing encompass the following criteria: fimixing intensity To achieve any degree of uniformity in final particle size, a high level mixing intensity is required. Mixing intensity here is defined in terms of rate of shear. Main criteria of the process is in velocity of particles in the mixing chamber. Velocities of 15 to 40 meter per second are typical. This mechanical mixing intensity provides the energy necessary to overcome resistance to coating, or wetting of the particles, i.e. surface tension. By application of sufficiently intense energy, particles are relatively uniformly wetted, maximizing uniformity of particle growth and therefore size. firesidence time One characteristic of a newly formed, wet agglomerate, is its malleability. These particles are easily deformed, compressed and otherwise changed by applied mechanical forces. Under the high level of mechanical mixing energy described above, then, once wetted, mechanical shear on the agglomerate will affect final product quality. Additional shear also consumes additional energy. For these reasons, actual contact time in the intense mixing environment is limited to a very short time interval, resulting in Instant Mixing processing. This time interval can be as short as 0,2 seconds and up to a maximum of a few seconds. Residence times longer than this are more typical of other mixing agglomeration processes, in most cases resulting in substantially different final product characteristics.

5 Short contact time provides the following benefits: * minimal time to line out process; minimal offspec product losses. * Maximum throughput capacity per unit volume of mixer and therefore, smaller size mixer for a given capacity required. * Minimized total energy required per unit mass processed. fiworking volume The combination of mixing intensity and short residence time require a very low working volume. Occupied mixer volume normally does not exceed 15% of total available mixer volume. Low percentage working volume provides maximum uniformity of applied work, particle particle comparison. Instant mixing is therefore defined as: The application of an intense level of turbulent mixing, on a small volume of material for a very short period of time. For the mechanical design the following critical process conditions are very important and have to be taken into account: fipositive, controllable conveyance In dealing with the fully saturated, wet granules, the design of the mixer is critical for both stable, continuous operations, as well as reliable and reproducible agglomeration results. Positive, controllable conveyance through the mixer is crucial. Saturated powders exhibit extremely poor flow characteristics. With insufficiently positive conveyance, mixer blockage is likely. fiself-cleaning Mixer should be designed with self-cleaning, especially of the mixer walls. Insufficient cleaning of material from the wall can result in overdensified material forming a compacted layer of product against the outer wall and finally, high risk for mixer blockage. With continuous processing, this compacted layer can break off and mix with the more loosely formed clustered granules. This phenomena results in nonuniform, non-homogeneous product characteristics. In end use applications these parts will show up as sinkers or undissolved, undispersed product. 5. Schugi Flexomix ª Principle Processing with the FLEXOMIX combines the instant mixing effect with controlled mixing conditions towards a unique process for realizing optimal product characteristics together with maximum efficiency. The FLEXOMIX (figure 4) is totally unlike any equipment and consists of a vertical cylindrical chamber enclosing a vertical rotating shaft. Several protruding knives are secured to the shaft, which rotates at adjustable speeds. Single or multiple liquids of varying viscosity or steam may be introduced into this turbulent stream through atomizing nozzles mounted in the upper portion of the chamber just above the upper mixing blades. FXD-160 Fig. 4: Schugi FLEXOMIX Mixer/Agglomerator Any number of powders are fed to the unit via top inlets and are brought into a highly turbulent, circular flow creating an aerosuspension. Figure 5 is giving a qualitative impression of the different effects. On this way, high collision rates are realised. The surfaces of the solid particles are evenly wetted by the dispersion of the atomised liquids. With increasing amounts of admixed liquid, wetted particles will grow in size by clustering together in the aerosuspension. A significant part of the mixing/agglomeration sequence has already taken place in the upper part of the mixing chamber. The remainder of the sequence is used to extend the turbulent flow.

6 The degree of turbulence is controlled by the shaft speed, number of knives, and by the relative position, angle and shape of the knives. Knives off-set at an angle force the material to the centre of the chamber, whereas horizontal knives impart a centrifugal action, forcing the material toward the chamber wall. Depending on subsequent knive Fig. 5: Turbulent Mixing effect positions, apart from radial and gravimetric forces, additional vertical forces can be introduced. The above parameters allow adjustments of the shear effects from low to medium. The adjustment of the knives and the rotational speed of the shaft determine the residence time of the material within the chamber, which is usually about 1 second or less (figure 6). Usually, 5-15% of the available chamber volume is occupied with material. The remainder is turbulent air and the temperature rise is usually less than 2 C. Apart from the turbulence, other mechanical forces are acting on the particles due to contact with the mixing elements. However, due to the low filling rate only a small part of particles come into contact with the knives. When liquids are injected in significant amounts, the product has a tendency to adhere to the chamber wall. The FLEXOMIX overcomes this build-up with a unique self-cleaning design consisting of a flexible mixing chamber which is continuously deformed by pneumatically or mechanically operated rollers moving vertically along the chamber, which consequently stays almost clean. Fig. 6: typical residence time in the FLEXOMIX On this way, the mixing/agglomerating condition will be kept constant which benefits the regularity and quality consistency of the product. Products, agglomerated with the use of the FLEXOMIX usually have a grain band of which the major portion lays between 0,2 mm and 2 mm. Fig. 7: influence shaft speed on particle Fig. 8: influence liquid addition size distribution on particle size distribution

7 The fast-rotating shaft, between 1500 and 4500 RPM, and the corresponding rotational speed of the knife-tips up to 40 m/s are factors limiting the typical particle size mentioned. By the definition of a typical granulometry, apart from the shaftspeed (figure 7), also the following parameters are essential: liquid addition rate (figure 8), type of product and binding agent, filling rate of the mixing chamber and knife arrangement, geometry and angle of attack of the subsequent knives. 6. Design of the process By using the FLEXOMIX principle for the processing of powdery materials, the following main process sequences can be defined: mixing agglomeration The unique FLEXOMIX mixing process sequence can especially be used for the humidification and dedusting of fine(-milled) powders and is described as follows (figure 9). Normally, the powdery components are automatically dosed towards the mixer, independent from each other. The dosing of water or watery solution is automatically controlled. The rpm of the dosing pump is set on a constant, defined rate. Powder dosing systems are carefully selected fitting to the properties of the raw material and the required accuracy for the end product. Accurate, uniform feed is essential and therefore gravimetric powder dosing systems are used. Perfect dispersion of water or watery solution and the homogenity of the endproduct is realised by the FLEXOMIX itself. The liquid-to-powder ratio is defined by the accuracy of the dosing systems. Fig. 9: mixing process sequence, typical for humidification After the mixing sequence and by having reached the required endproduct humidity, the product is ready for further processing, storage or packing. In the above described process, relatively low amounts of water is used. This means that relatively low amounts of small powder clusters are formed and the humidity of the endproduct is below the maximum allowed moisture level. Drying is not required. The FLEXOMIX agglomeration process sequence is typically designed according to the flowsheet, indicated in figure 10. Similar to "mixing", the powdery and liquid components are dosed independent from each other, towards the FLEXOMIX. By passing the mixer/agglomerator, a perfect dispersion of liquid over powdery material is realised, and consequently, wet agglomerates are formed with narrow size distribution. Experience has learned that these wet agglomerates can perfectly be dried by fluidbed technology. The wet agglomerates fall directly from the FLEXOMIX in the fluidbeddryer. The product height in the first section of the fluidbed can vary between 200 and 350 mm which allow relatively heavy and overwetted agglomerates to fall directly in a well mixed, pre-dried mass. This prevents the fresh agglomerates from touching and potentially blinding the fluidisation plate. The surface area of this first section is designed such that the average moisture content of the agglomerates is well below the agglomeration point of the base material. In this way, post-agglomeration in the fluidbed is minimized.

8 Fig. 10 Agglomeration process sequence, typical for a.o. instantization. To realise down-stream plug-flow, the fluidbed is devided into several sections, the number of which is determined by the way a particular product is to be dried and cooled. The product passes from one section to the next one by underflow. The product outlet from the last compartment is arranged such that part of the product leaves by overflow and the rest by underflow on fluidisation plate level. The overflow maintains a certain product level in the dryer and thus a constant residence time. By adjusting the height of the overflow weir, optimum residence time can be determined. The fluidisation plate slopes down to the product exit so that larger particles, which are less fluidized, travel regularly and constantly to the outlet. The product side of the fluidbed is tapered so that the air velocity in the expansion chamber falls below the terminal setting velocity of the particles carried by the drying/exhaust air, thus minimizing carry-over. The exhaust air is cleaned by a cyclon/filter system or by a build-in filter in the fluidbed. Dried and cooled agglomerates are classified by sieving. Oversize is crushed and fed back to the classifier. Onsize material is stored or directly packed. 7. Experimental work Experiments and extensive trials have proved that applications for instant mixing agglomeration with Flexomix Technology are for instance found where rapid dissolution, dispersion and wetting are important in the product end use (See fig. 11 for the typical particle shape). Another application area is for direct compressible powders. Loose clustered granules are very well suited to compression under low pressure and yet producing final pressed forms with good final strength. Powders suitable for processing with this process include most water soluble powders, including sugars, milk powder blends, water soluble chemicals. These products are generally most easily agglomerated using water or steam only as a binder. Partial dissolution at the surface followed by recrystallisation following drying result in durable yet dispersible granules. In other cases, binding agent has to be used. Fig. 12 Flexomix FXD 335 Instant mixing agglomeration may also be applied to and combined with multiple ingredient formulations. The combination of efficiency of mixing with agglomeration, in many cases, negates the need for a separate mixing step. Multiple powder ingredients can be fed to the agglomerator, in conjunction with incorporation of multiple liquid binders, which may also be part of the formulation. Fig. 11 Typical particle shape

9 Based on lots of experimental work and analysis of applications it is evident that typical applications for Schugi FLEXOMIX Technology range from the processing of chemicals such as super absorbing polymers, phosphate-based products, horticultural fertilizer, crop protection agents, laundry and dishwasher detergent (compounds), food such as instant drinks, -soups, -sauces, thickeners, flours, starches, bakery- and milkproducts, to veterinary- and bulk pharma. 8. Conclusion This paper introduces and reiterates the concept of the instant mixing method of agglomeration. Some of the requirements have been discussed in defining the process. FLEXOMIX Technology can be applied wherever small quantities of liquid are to be added to powders, or agglomerates have to be produced in which large quantities of liquid are absorbed, and where the size of particles needs to be enlarged. Although one of many methods of agglomeration, instant mixing with Flexomix Technology is proving a wide range of flexibility across industries and applications. However, for each specific application, Pilot Plant trials have to be performed for defining best process parameters for optimal aspects of the endproduct and for lowest energy consumption. References 1. Capes, C.E., Particle Size Enlargement, Handbook of Powder Technology, Volume 1, Elsevier, Amterdam-Oxford-New York (1980). 2. Retsina, T., Coucoulas, L., Agglomeration of powders, Scientific & Technical surveys no. 164, August 1988, Leatherhead Food R.A. 3. Polke, R., Hermann, W., Sommer, K., Charakterisierung von Agglomeraten, Chem.-Ing.-Tech. 51(1979) Nr. 4, page Schubert, H., Grundlagen des Agglomerierens, Chem.-Ing.-Tech. 51(1979) Nr. 4, page Hermann, W., Verfahren und Kosten der Agglomeration, Chem.-Ing.-Tech. 51(1979) Nr. 4, page Schubert, H., Handbuch der Verfahrenstechnik 12 (1978) 7. Pietsch, W., Das Agglomerierverhalten feiner Teilchen, Staub Reinhalt. Luft 27 (1967) Nr. 1, page Fischer, J.J., Liquids-Solids Blending, Chem. Eng. February 5, 1962, page Rumpf, H., Mechanische Verfahrenstechnik, Karl Hanser Verlag, München, Heinze, G., Handbuch der Agglomerationstechnik, Wiley-VCH, Weinheim, Schubert, H., Instantization of powdered foods, Int. Chem. Eng., 30, 1993, page