Surface Agglomeration

In the surface agglomeration either steam or warm moist air with a high relative humidity is used as the moistening agent. The surface of the individual dry particles is wetted due to condensation, whereby the stickiness required for the agglomeration is created. The subsequent agglomeration will take place, if the particles are exposed to sufficient mechanical impact. The impact can for example be accomplished in a vortex tube into which the moistening medium is introduced tangentially and the powder to be agglomerated along the centre axis. It is very important that the humid air/steam used for the rewetting does not contain any droplets of water, as that, in combination with the intensive mechanical impact, will result in over whetted agglomerates being too compact for a quick rate of rehydration.

The warm humid air is usually made by spraying steam into warm air at a given temperature to obtain a relative humidity of 100%. Any water droplets are removed in a demister, after which the air is heated further to give the desired relative humidity. By changing the air temperature prior to the steam injection and the subsequent air temperature, it is possible to obtain air with a given temperature and relative humidity. See Figure 2.

Figure 2. Feed System for a rewet plant. Wetting by means of warm moist air.

The surface agglomeration is mainly used for skim milk powder when large agglomerates are aimed at. The final product properties depend to a great extent on the raw material used for the rewetting, and below is shown a specification for a recommendable basis powder:

The surface agglomeration can also be used for whole milk powder, but the agglomerates get too compact for obtaining a powder with good rehydrating properties.