The present invention relates to a method for scraping filter cake and a rotary drum type filter using this method.  More particularly, the present invention relates to a method for scraping filter cake by expanding the same and a rotary drum type filter equipped with a means for expanding the filter cake and a mechanism for scraping it. 
In general, a rotary drum type filter, which is equipped with a rotary filter drum inside of its suspension vessel to filtrate the suspension through a filter medium laid on the surface of that drum or through a filter aid layer or a filter cake layer deposited on that filter medium, is widely known.  A scrape blade with a sharp edge is provided beside the filter drum of such a rotary drum type filter.  As the filter drum rotates, the scrape blade scrapes a sludge layer deposited on the surface of the filter medium of the filter drum. 
In the case where the suspension to be filtered is a suspension which contains fine and hard particles such as a grinding fluid after use, however, the filter cake layer becomes stiff and hardened so that smooth scraping with the scrape blade is likely to be difficult.  When pushing the scrape blade to the filter cake layer continuously, it is possible to scrape the layer little by little.  But, the scrape blade gets worn so quickly that the filter operation is disturbed and the operating cost rises. 
On the other hand, there is another method for scraping the filter cake layer by pushing the scrape blade into the layer only when a block of the filter occurs while keeping the blade away from the layer usually.  However, this method involves the following two problems. 
1 When pushing the scrape blade into a hard filter cake layer forcibly, an overload of the blade feeding motor or the drum rotating motor occurs and this causes the operation of the filter to stop. 
2 The second problem arises even if the blade could be pushed into the filter cake layer.  At the moment when the block is eliminated by the scraping, the filtration pressure decreases rapidly, which causes an expansion of the filter medium layer (if it is elastic) or a filter cake layer.  As the result, the scrape blade would cut unnecessarily deep into the filter medium layer.  This unnecessary deep cutting leads to a wear of the filter medium or an excessive scraping of the precoated filter aid layer.  The latter results in a waste of filter aids and an increased frequency of precoating operation i.e. a drop of filtration efficiency.  These results are greatly undesirable. 
These problems are not peculiar only to rotary drum type filters.  The developments of a method to facilitate the removal of filter cakes have been long-awaited in all fields of filtration technique.  As for such method, there are given an ultrasonic method which loosens the filter cake layer by ultrasonic vibrations and a back-washing method which back-washes the filter medium using air or water.  However, there are shortcomings in those methods.  The ultrasonic method requires expensive and complicated equipment.  The back-washing method also requires complicated equipment and operation, and at the same time, it requires time for back-washing so that the time efficiency of filtration drops.