Abstract:
Apparatus and method for drying sludge, in particular sewage sludge, by creating a granulate from the dried product. A mixture of a carrier material consisting of recycled sludge which has already been dried and liquid sludge is fed to a dryer  6 . The dried product is fed to a solids separator  8  after the dryer  6  and is stored intermediately in the former in an intermediate container that is placed immediately downstream of the separator.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method for drying sludge, in particular sewage sludge, which produces granulate from the dried product. According to said method, a mixture of a carrier material, consisting of recycled sludge that has already been dried, and wet sludge is fed to a dryer. The invention also relates to an installation for carrying out said method. 
   Drum drying plants have been known for a number of years. This type of installation and method is described specifically for drying sludge, in particular sewage sludge, in e.g. EP 0 789 209 or U.S. Pat. No. 5,318,184. What these installations have in common is that they provide for carrier material, consisting of material that has already been dried, being mixed back into the liquid sludge in order to eliminate the so-called sticky phase. In order to do this, a large portion of the material that has already been dried is brought to a carrier material silo by a number of conveying elements. From here, it is added to the liquid sludge according to various controlling variables and after being mixed into it, fed to the dryer again. In order to be able to store the dried material in the silo, it has to be cooled using a special cooling device (e.g. a cooling screw). In addition, the silo for the carrier material must be of a special, expensive design for safety reasons. 
   SUMMARY OF THE INVENTION 
   The aim of the invention is thus to create a method and an installation with optimised safety features, high availability, as well as lower space requirements and manufacturing costs achieved by reducing the number of plant components. 
   The invention is characterised in that the dried product is fed to a solids separator after the dryer and is stored intermediately in the former or in an intermediate container that is placed immediately downstream of said separator, with the dry product used as carrier material being mixed into the liquid sludge without undergoing further storage. As a result, there is no further need for the carrier material silo, nor for the cooling device hitherto required. 
   A favourable further development of the invention is characterised by the filling level in the separator or intermediate container being controlled by an integrated conveyor, for example a conveying screw. As a result, it is always possible to achieve optimum addition to the liquid sludge on the one hand, and an optimum separation effect by the separator. 
   An advantageous configuration of the invention is characterised by the dry material undergoing intermediate storage in the inert sector. Since the dried material is stored in the inert part of the drying system, there is significantly less risk of fire or deflagration. 
   The invention also relates to an installation for carrying out said method. This is characterised by a solids separator with a storage volume or with a separate storage container for the dried material directly connected to the solids separator being provided after the dryer. As a result, there is no longer any need for the silo for carrier material required hitherto, nor for the cooling screw. 
   A favourable further development of the invention is characterised by the solids separator being a filter, where the solids separator can also be a cyclone. Thus, the plant can have a lower-cost design. 
   An advantageous configuration of the invention is characterised by a discharge device, for example a discharge screw conveyor, being integrated into the separator and/or storage container. This ensures optimum addition of liquid sludge on the one hand, as well as an optimum separating effect by the separator. 
   A favourable configuration of the invention is characterised by at least one conveying unit being provided between discharge device and mixer and which conveys the carrier material, consisting of sludge that has already been dried, from the separator to the mixer directly, i.e. without other units or containers being included in between. This conveying unit can be a tube or cubic chain conveyor, a vertical bucket conveyor, or a screw conveyor. A tube chain conveyor provides a facility for re-circulating the dried material that is to be back-mixed when the plant is shut down and for cooling it if necessary while it is being re-circulated, with the product always remaining in the inert sector. A vertical bucket conveyor is a favourable device for transporting the dried sludge and a screw conveyor is particularly good for controlling the amount added. 
   An advantageous further development of the invention is characterised by at least one more conveying unit, e.g. tube chain conveyor, cubic chain conveyor, vertical bucket conveyor or screw conveyor, being provided between the discharge device and the mixer. 
   This ensures optimum conveying of the dried material at all times, as well as optimum control of the amount to be back-mixed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described using the examples in the drawings, where  FIG. 1  shows a state-of-the-art plant,  FIG. 2  shows a plant according to the invention; and  FIG. 3  shows an alternative solids separator. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  illustrates a state-of-the-art drying plant, where pre-dewatered sludge  1  is fed to a container  2 , from which it is fed via a feed screw  3 , which is driven by an adjustable motor  4 , is brought to a mixer  5  and goes from there through a further mixing and conveying screw  5 ′ for example, into the dryer  6 . As used herein, “wet sludge” encompasses liquid and dewatered sludge. Besides the triple-pass drum dryer shown, it would also be possible to use a fluidised bed, belt, or disc dryer. The dried material is carried by the drying air supplied through a duct  7  to a separator  8  and then passed on through a rotary vane feeder  10  to a screw conveyor  11 . The fine solids particles that are left in the air after the pre-separator  8  are removed in a subsequent filter  9  and also fed to the screw conveyor  11 . The screw conveyor is designed as a cooling screw in order to maintain the temperatures allowed for storage in the carrier material silo  18 . The solids in the form of granulate are then fed through a lock  12  to a screening plant  13 , where oversized grains are brought to a crusher  14  and the granulate with the designated size is conveyed through a duct  15  to packing and transport equipment. As an option, part of the grain with the designated size can also be fed to the crusher. The material milled in the crusher  14  is mixed with the very fine solids from the screen  13  and then brought to a carrier material silo  18  by a screw conveyor  16  followed by a vertical bucket conveyor  17 . From here, the carrier material is fed to the mixer  5  through a carrier material feed screw  19 , which is driven by a speed-adjustable motor  20 . The energy for the drying process is generated according to  FIG. 1  using a burner  21 , which yields its heat to the circulating air in a heat exchanger  22 . Some of the exhaust gas from the burner is returned to the burner though a duct  24  by means of a fan  23  or is brought through a duct  25  to the exhaust gas chimney stack  26  and then discharged into the atmosphere. The circulating air heated in the heat exchanger  22  is fed to the dryer  6  through the duct  27 . As an alternative, the exhaust gas from the burner can also be fed directly to the dryer  6 . The drying air is conveyed by a fan  28  located after the filter  9 . This also guarantees that there is a vacuum in that part of the air system where material is conveyed, thus no dust can be released into the atmosphere. After the fan  28 , the hot air charged with moisture is brought to a scrubber/condenser  29 , where cooling water is injected. After this, the air that has now been cooled and dried is brought through a duct  30  to the heat exchanger again and thus, is re-used. A partial air current is brought from the scrubber  29  through a duct  31  to the burner for incineration. 
     FIG. 2  now shows a system according to the invention, where the same elements are marked with the same references. In contrast to  FIG. 1 , this illustration shows a system where the exhaust gas from the burner is fed directly to the dryer, which can also use an indirect heating system (with heat exchanger). The liquid sludge  1  is also dried here in a dryer  6  after being mixed with sludge that has already been dried. The dried material is separated from the drying gas in a separator  8 . The illustration shows a storage section  32  of the separator  8 , where the dried, granulated material is stored. Part of it is fed as final product through a discharge screw  33  to the rotary vane feeder  12  and then brought through duct  15  to the packing and transport devices. Thanks to the special arrangement of the discharge screw  33 , whereby the effective flow rate of the final product can be modulated (e.g., by simple on/off or by variable rotation speed of the screw, as described above with respect to feed screw  19  with associated variable speed motor  20 ) the filling level in the container  32  is controlled. Instead of being stored in a separate container, the dried material can also be stored directly in the separator  8 , where the discharge screw  33  then controls the filling level in the separator  8 . The dried material used as carrier is conveyed to a crusher  14 ′ and then directly to the mixer  5  by, for example, a tube chain conveyor  34 . In the mixer, it is mixed with fresh, liquid sludge from the liquid sludge silo  2 . In order to be able to re-circulate the dried material when the plant is shut down, an additional cooler  35  is provided here. 
   As a result, there is no need here for the cooling screw ( 11 ), nor for the carrier material silo ( 18 ) with screw ( 19 ) and drive ( 20 ) required by the state-of-the-art system. By storing the material in the separator  8  or a directly adjoining container  32 , the material remains in the inert sector, i.e. in the enclosed system, with the drying gas, thus greatly reducing the risk of deflagration caused by dust, as well as the risk of fire.