Drying of temperature sensitive and often hygroscopic bulk solid products often goes on for many hours on a low temperature profile. A protective drying process can therefore well go along with a gentle product agitation and homogenization, whilst intensive macro-mixing by fast rotating mixing tools or massy product displacement very often will destroy the crystallized structure of the particles or overheat the product in a unwanted way. Dryer types with a very short clearance of the agitator to the vessel wall additionally can very easily and rapidly over-heat the product particles (at least partially) and further irreparably destroy the crucial character and mode of action of the valuable ingredients.
Indeed an intensive product circulation within a single shaft dryer such as the traditional horizontal paddle dryer or the vertical shovel- and helix dryer will certainly offer a perfect mixing in the sense of convective mixing. But due to the wetness inside the product single parts of the batch will stick to each other, and so the micro-mixing - the exchange of individual particles itself- is very moderate and not sufficient.
For sticky, adhesive and very wet products this could easily result in a very poor mixing efficiency, hence the entire batch may only circulate with the mixing tool as the flow along the polished wall is not interrupted. To maintain the desired and necessary mixing effect at all, these dryer types will require a high level of drive energy. This will needless increase the operational cost compared to the conical screw dryer and will put additional mechanical stress onto the sensitive product.
Especially on intially very wet product, on a bulk with extremely fine particles or if the wetness encapsulated in the particles is driven to its surface by extremely high starting temperatures very rapidly a certain risk of agglomeration may be observed. Mechanically high forces by intensive mixing and agitation in this case may facilitate the creation of different sized, however compacted spherical balls (“balling”) or build-up of a solidified and crusty layer on the heated vessel wall. This for example could be observed with foto-chemicals, color pigments or toner material. The granulated spherical particles do enclose the residual moisture inside the balls, the wall layer solidifies by the wall temperature and very much limits the achievable heat transfer rates. All this will increase the total drying time and limit the product quality as the hard balls must be broken into homogeneous powder by a downstream milling process.
Remaining product layers on the wall and agitator do not only mean lost of valuable solid, but are also difficult to be removed by a standard spray cleaning process. The conical screw dryer with its unique and gentle mixing principle in this respect can offer some remarkable advantages.