Agglomeration can be defined as a process of particle size enlargement where fine powder particles are bonded to form large, relatively permanent masses in which the original particles are still identifiable.
These agglomerates have a coarse, open structure and a mean particle size ranging from 0.1 to a few millimetres. The process uses agitation in the presence of the required proportion of a liquid phase or other binding agents and is normally followed by evaporative drying.
Applications for instant mixing are, for example, found where rapid dissolution, dispersion and wetting are important in the product end use. Powders suitable for processing with this process include most water soluble powders, including sugars, milk powder blends and water soluble chemicals.
Benefits
The goal of any particle size enlargement process is to reliably produce particles exhibiting required end use properties – simple de-dusting, to engineered particle of specific size, shape, density and dissolving rate or compressability.
Instant Mixing Agglomeration
Instant mixing agglomeration is a wet agglomeration method. This means powders and a liquid are required by the process which typically includes mixing, drying, conditioning, size reduction, size classification and material transport.
Mixing
Instant mixing agglomeration maximises control of mixing conditions which give rise to yielding ‘perfectly’ formed uniform granules. With high level mixing intensity, instant mixing agglomeration delivers uniform wetting of particles and uniform particle growth.
Because newly formed wet agglomerate is malleable, particles are easily deformed. It is important therefore that residence in the mixer is limited; 0.2 seconds to a maximum of a few seconds.
Saturated powders exhibit extremely poor flow characteristics which can cause mixer blockage. A self cleaning mixer offers the answer. However insufficient cleaning of material from the wall can give rise to non-uniform, non-homogenous product characteristics which in application show up as ‘sinkers’ or undissolved, undispersed product.
Liquids are introduced typically near the feed end of the agglomerator. The liquid feed stream can be atomised using air; also steam can be injected. The mixer itself can provide the remaining mechanical atomisation to disperse the fluid.
Conditioning
Mixed, agglomerated product is discharged directly to a conditioner, typically a dryer, to stabilise the agglomerate. Because of the malleable nature of the wet granule, physical handling will affect ultimate product quality, size, shape, density, etc. Further mechanical handling methods need to be carefully considered in light of desired end product qualities.
Options for further processing after the wet agglomeration step include:
Drying
When no further sizing is required and a dispersible granule with maximum surface area is required for products including chemicals or instant beverages fluidbed drying is preferred, providing minimal mechanical force and maximum drying rate. Wet granules are suspended in and surrounded by drying air.
The fluidbed dryer effectively classifies the particle returning fines to the agglomerator for rewetting resulting in a dust free product.
Cooling
Cooling stabilises the malleable granules.
Forming
The instant mixing process is most efficient in producing a fine agglomerate in the range of 100 µm to 1000 µm. Due to the instant forming process, the agglomerates are clusters of smaller particles.
Because of the irregular surface, the instant agglomerate will have more surface area than a solid ‘regular’ granule – properties which aid dispersibility, fast dissolving and compressibility of the agglomerate. If, however, a more defined shape is required, a secondary forming process prior to conditioning can be done.
APPLICATION: CHOCOLATE DRINKS
The consumption of cocoa in chocolate drinks has increased considerably over the past decades. This is mainly because products have been developed that make it easier to transform cocoa powder into hot and cold drinks.
These products must have instant properties. The final product is judged by the following criteria:
- Bulk density
- Colour
- Flowability
- Wettability
- Sinkability
- Dispersibility
- Homogeneity of product, no segregation
- Dissolving time in hot or cold liquids
- Appearance
Agglomeration process
Before the actual agglomeration takes place, all powdery components will have to be blended. This can be done in different ways, the choice of which depends on the production capacity. For a production capacity between 500 and 1000 kg/h the most practical way is to pre-blend all powdery components in a Vrieco Nauta Conical Mixer.
After preblending, the mixture is fed to a buffer hopper. From this hopper, the blend is fed to the Flexomix agglomerator, in the correct proportion by a constant weight feeder.
For agglomeration with water, 2 – 8% water is generally needed. This is an important parameter to influence the grain size of the product. The grain size of instant cocoa is generally between 300 and 600 µm.
The use of water in combination with steam gives special quality characteristics on the end product. Emulsifiers and flavours could be dissolved in water using a liquid premix tank. Part of the sugar can also be dissolved in the water, which influences the instant effect.
The wet granules fall directly from the Flexomix into the Fluidbed Dryer. Warm drying air is blown through the perforated base plates. The temperature of the drying air and the maximum temperature of the product during drying are very important factors for the quality and the taste of the end product.
After passing the drying section of the fluidbed, the product transfers to the cooling section in the same bed. Dependent on the behaviour of the end product, cooling is done by ambient air or conditioned air.
The blow-over air is cleaned by a (cyclone and a) filter and exhausted. The separated dust is either collected in a small hopper or transported directly to the packing line, together with the on-size product.
After cooling the product is discharged from the Fluidbed Dryer and passes a sieve to eliminate any oversize particles which may be present. The oversize is continuously crushed and added to the end product stream. The total stream of end product is fed to a storage hopper from where it is transported to the packing lines.
The total Agglomeration Plant is controlled by a SCADA system as Operator interface.
