Drying
Rotary Drying
In a Rotary Dryer the wet material travels along a rotating drum through which hot gases are drawn. The hot gases are directed through the drum either in the same direction as the wet material (co-current), or in the opposite direction to the material (counter-current). Internal “lifters” cause the material to cascade through the hot gas stream which promotes better heat transfer and improves drying efficiency.
The majority of the dry material is discharged by gravity at the end of the drum with any airbourne dust being collected in a cyclone/filter system. An exhaust fan generates the airflow through the dryer ensuring that the system operates under suction.
The hot gas for drying is typically heated using a gas or oil burner which either fires directly into the dryer or into a separate combustion chamber from which the gas is drawn into the drum.
Flash Drying
In a Flash Dryer the wet material is introduced into the drying system at a controlled rate and is dispersed into a high velocity air-stream, dried and simultaneously conveyed to the product cyclone/filter. Material retention time in the system is very short, hence the term “flash” drying.
With the appropriate selection of feed handling equipment Flash Dryers can be used to process difficult materials such as filter cakes, pastes and even slurries. When processing these type of materials in some cases it is necessary to “back mix” a proportion of the dry product with the incoming wet feed in order to improve materials handling properties.
Fluid Bed Drying
In a Fluid Bed Dryer the material is fluidised by an upward-moving flow of hot gas with the mechanical movement of the fluid bed ensuring that the hot gas and product are intimately mixed, resulting in excellent heat transfer.
With lower specific energy costs, in recent years Fluid Bed Dryers have replaced Rotary Dryers on industrial mineral applications such as limestone and silica sand.
Fluid Bed Dryers are typically used to process powders and granules up to approximately 5 mm. They are particularly well suited to the drying of granules and agglomerates where degradation is to be avoided.
Simultaneous Drying and Grinding
With the use of hot gas in an airswept milling system it is possible to simultaneously dry and grind a product containing up to 10% moisture.
Compared to separate drying and milling processes the ability to dry and grind a product offers advantages in both investment and operating costs. By using the heat generated by the size reduction process together with the circuit fan it is possible to reduce the quantity of fuel fired to perform a given drying duty by 10 – 15%.