The invention relates to a method for drying pasty materials and the production of a high density granulate by compacting.
The problem arises when drying pasty materials, such as e.g. sewage sludge, of producing a granulate, which has a very uniform granulate size and a good abrasion resistance in order to avoid dust formation.
U.S. Pat. No. 4,193,206 discloses a method of this type, in which the compacting takes place at the end of the drying process with a material dry fraction of more than 85%. As such a material cannot be extruded through nozzles, the addition of a plasticizer is proposed. However, the product compacted in this way has a non-uniform granulate size, so that it is subsequently necessary to separate and return dust and comminute granulate particles which are too large. The aim is to obtain a granulate with a bulk density of more than 480 kg/m3 (30 lb/ft3). An installation for performing this method is correspondingly complicated requiring different apparatuses for the drying method and the extruder and despite the use of plasticizers the extruder has to process a particularly firm, tough material.
EP-B-781741 discloses a method for drying sewage sludge in which a still relatively moist material with a dry fraction of 40 to 60% is extruded on leaving a thin film evaporator, so that as a result of the sudden change in the method conditions a porous product with a hardened surface area is obtained, whose enlarged specific surface is advantageous both for the subsequent, further drying and for the later use thereof as a fertilizer. As a result of the special heat control used, due to a relatively complicated drying in a thin film evaporator, the method has a good thermal efficiency. However, the relatively complicated installation used is disadvantageous, as is the bulk density of the granulate limited to approximately 500 mg/m3.
A method is also known in which drying of sewage sludge only takes place in a belt dryer. However, it suffers from the disadvantage that the product leaving the belt dryer has a very irregular granulate size and a low density, so that it is very crumbly and therefore has a limited abrasion resistance. This leads to correspondingly high capacity requirements during the subsequent handling, e.g. on conveying to silos, on filling into sacks and during transportation, associated with the problem of a significant dust production.
To avoid this disadvantage it is also known to pelletize the dry product after leaving the belt dryer. However, the equipment used for this is mechanically complicated and expensive, subject to high wear and requiring high motive energy.
The problem of the invention is to avoid the indicated disadvantages of the known methods and to find a method which, with relatively limited apparatus expenditure and low operating costs, permits the production of a high-grade granulate, particularly with respect to a high density and strength, such as could hitherto only be achieved in a particularly complicated manner by pelletizing the finally dried product.
In the case of a method of the aforementioned type, the invention solves this problem in that the compacting takes place as a function of the nature of the material with a dry fraction of 60 to 80%, so that the degree of compacting is substantially maintained during subsequent further drying.
In a preferred embodiment, the method of the invention permits the drying of material in a simple manner under constant method conditions both before and after compacting. Identical method conditions before and after compacting lead to a significant simplification of the installation used for the method, in that all the drying, including the compacting, can take place in a common housing of an apparatus, e.g. a belt dryer, without separate systems having to be created with respect to the pressure and temperature. In addition, all the drying can inexpensively take place with a mechanically unitary conveyor system, e.g. on a belt conveyor system with one or more conveyor belts.