Whey or permeate concentrate from the crystallization tanks can also be passed through a decanter, where the main portion of the lactose crystals is separated. The crystals are then washed in order to remove residuals of mother liquor, after which it is dried in a specially designed fluid bed, see Fig. 141. The fluid bed is divided into three sections. The first section is a stationary back-mix bed into which the wet crystals with 8% moisture are fed by a rotating disc. To avoid lump formations this section is equipped with a rotating rake. From the back-mix section the powder enters the plug-flow fluid bed section for the final drying. Hot air of about 100ºC is used for the drying.´
Fig. 141 Combined back-mix/plug-flow fluid bed for drying lactose
The average crystal size (200-250μ) is of importance for the fluidization velocity that can be used, and it therefore influences the performance of the fluid bed, as smaller crystals would require lower fluidization velocity, or too big carry-over of product to the bag filter would be the result. Pre-crystallization during 48 hours is therefore used to "grow" the size of lactose crystals.
From the second drying section the powder enters the cooling section which like the last drying section is designed as a plug-flow fluid bed. The air used for cooling should be dehumidified. All the exhaust air is passed through a bag filter for separateion of fines, which are returned to the bag mix section. Lactose produced this way is termed crude lactose. A more refined type pharmaceutical lactose - is produced by redissolving the washed crystals from above mentioned decanter and then crystallizing a second time by cooling followed by another decanting/washing prior to the final fluid bed drying.
Lactose powder is very often classified with different crystal sizes. Milling of the powder after the fluid bed and final sifting is therefore used.