Rotary Dryers

The rotary dryer is one of the most used in the industrial processes.
It’ s simply way of operation,  the low maintenance cost,  the possibility to
achieve large throughput make it often preferred to other more perfect types.
Typical applications: drying of minerals in general, sand, clays, kaolin,
limestone, metal shavings, fertilizers, various chemical products, bentonite,
gypsum, sewage sludge’s, bones,   leather   manufacturing   wastes. The rotary
dryer may be designed in various arrangements, according to the features of  
the material to be treated.  First  of  all  the  drying  process  may  be 
carried  out ίn COUNTERFLOW  or PARALLEL FLOW. ln  the  first  case,  the  hot 
gases  meet  the  material near  the  discharge  outlet  of  the  dryer.
This  solution  enables,  to achieve high drying  degrees  and  adequate  when
the  material  may with¬ stand  high  temperatures  without  any  alterιnation.
The drying in parallel flow is instead preferable for heat-sensitive materials or
for materials that are in¬ tally very wet and so have the tendency to stick to the walls.
The material may be DIRECTLΥ or JNDIRECTLΥ heated.
ln the dryers  with  direct  heating the wet  material gets ίn contact  with  hot  gases,
which may be combustion  products  (direct flue  gases drying) or  heated  air, (hot  air  drying).
The direct flue gases dryer is the simplest and most economical Solution and it is
generally adopted when the material may get in contact with the combustion products,
which depends also on the kind of the available fuel.
lt is possible, in many cases, to save fuel by reco¬vering flue gases, which may be available
from other processes, heating them up to the necessary tempe¬rature by suitable burners.
Drying by heated air is used when the material cannot be exposed directly to the products
of combu¬stion. Finally,  when it is necessary to reach rather high temperatures  without  the
material  getting  in contact with  the combustion  products, the  indirect  heating may
be  employed. ln this case rotary dryer is mounted within a sta¬tionary furnace lined with
refractory bricks, and it is heated from the outside.
Α typical application of this solution is  the rotary kiln for the baking of gypsum.
This kind of gypsum kilns haνe the advantage, com,  pared to the static bowie kilns,
of requiring less maintenance and giving the possibility,  for the largest units,
of operating, with continuous  process.