Abstract:
A materials processing apparatus for the wet attrition of particulate material includes at least one mixing chamber having one or more impeller blades or paddles rotatably mounted therein, and a drive device for driving the impeller blades or paddles. The apparatus includes a fluidizing device for adding a fluid to the at least one mixing chamber to fluidize the material contained therein prior to operation of the drive device.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a material processing apparatus and method for the wet attrition of particulate material and in particular to an attrition scrubber for cleaning contaminated particulate material and in particular for removing surface contamination from sand particles and breaking up friable material and clays to facilitate separation of such materials from the sand product, liberating clean particles from the contaminated feed material. 
       BACKGROUND OF THE INVENTION 
       [0002]    Attrition scrubbers are used for cleaning contaminated particulate material, in particular for delaminating clay from sand particles. The particulate material is typically delivered to the attrition scrubber as a liquid slurry having water content of between 20% and 25%. Typically attrition scrubbers comprise several attrition cells, each cell having two or more sets of impellers mounted on a common shaft driven by a respective drive motor, typically an electric motor, such that the movement of the blades of the impellers cause intense scrubbing, polishing and disintegration of the particulate material located within each cell. 
         [0003]    A problem with known attrition scrubbers is that solids material in the feed slurry tends to settle out of suspension within the cells when the attrition scrubber is not in operation, for example if the motor of one or more of the cells is stopped due to a fault or other problem. Therefore a very high starting torque is typically required to initially fluidize the solids material, requiring large drive motors. Also the blades and other moving components of the attrition scrubber must be made designed to withstand the initial high starting torque applied to the blades by the motors during start up, making the apparatus very heavy and expensive. 
       SUMMARY OF THE INVENTION 
       [0004]    According to a first aspect of the present invention there is provided a materials processing apparatus for the wet attrition of particulate material, the apparatus comprising at least one mixing chamber having at least one pair of opposing impeller blades or paddles arranged to direct material in opposing directions to facilitate the attrition process, and a drive device for driving the impeller blades or paddles, the apparatus including a fluidizing device for adding a fluid to the at least one mixing chamber to fluidize the material contained therein prior to operation of the drive device. 
         [0005]    The step of fluidizing material within the at least one mixing chamber may comprise adding a liquid to the at least one mixing chamber via one or more nozzles or jets located within the at least one mixing chamber. Alternatively the step of fluidizing solids material within the at least one mixing chamber may comprise adding a gas to the at least one mixing chamber via one or more nozzles or jets located within the at least one mixing chamber. 
         [0006]    In a further aspect, the present invention provides a method of operating a materials processing apparatus for the wet attrition of particulate material, the apparatus comprising at least one mixing chamber having at least one pair of opposing impeller blades or paddles arranged to direct material in opposing directions to facilitate the attrition process, and a drive device for driving the impeller blades or paddles, the method comprising the step of fluidizing material within the at least one mixing chamber prior to operation of the drive device of the impeller blades or paddles. 
         [0007]    The plurality of cells may be coupled in series such that material passes through the cells consecutively, the method comprising starting the drive devices of the cells consecutively, starting from the most downstream cell and finishing with the most upstream cell such that the impellers of the downstream cells start rotation before those of the upstream cells. 
         [0008]    In a further aspect, the present invention provides a method of operating an attrition cell cluster comprising a plurality of cells, each cell comprising a chamber for holding an aqueous slurry of particulate material, each cell having at least one pair of opposing impeller blades or paddles arranged to direct material in opposing directions to facilitate the attrition process, the blades or paddles being rotatably driven by a respective drive device, the method comprising adding a fluid to each cell in turn to fluidize the material contained therein at consecutive intervals prior to operating the drive device of the impeller blades or paddles of each cell. 
         [0009]    The plurality of cells may be coupled in series such that material passes through the cells consecutively, the method comprising adding a fluid to each cell consecutively to fluidize the material therein, starting from the most downstream cell and finishing with the most upstream cell. 
         [0010]    These and other objects, advantages and features of the invention will become apparent upon review of the following specification in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    An attrition scrubber in accordance with an embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a perspective view of an attrition scrubber in accordance with an embodiment of the present invention; 
           [0013]      FIG. 2  is a side view of the apparatus of  FIG. 1 ; 
           [0014]      FIG. 3  is an end view of the apparatus of  FIG. 1 ; 
           [0015]      FIG. 4  is a plan view of the apparatus of  FIG. 1  from above; 
           [0016]      FIG. 5  is a plan view of the apparatus of  FIG. 1  from below; and 
           [0017]      FIG. 6  is a part sectional detailed view of the apparatus of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    As can be seen from the drawings, an attrition cell cluster  2  in accordance with an embodiment of the present invention comprises a plurality of cells  4 , eight cells in the example shown arranged in two banks of four, each cell comprising a chamber for holding an aqueous slurry of particulate material, each cell  4  having an electric drive motor  6  mounted at an upper end thereof, the drive motor  6  being coupled to a vertically extending drive shaft  8  extending into the respective cell  4  and having three sets of impeller paddles  10 , 12 , 13  mounted thereon, two of the sets of paddles  10 , 12  being arranged to direct material in an upwards direction while an intermediate third set of paddles  13  is arranged to direct material in a downwards direction to facilitate attrition if the material. 
         [0019]    The drive motor  6  of each cell  4  can be powered to rotate the respective drive shaft  8  and thus move the respective sets of impeller paddles  10 , 12 , 13  through the sand or other particulate material slurry contained within each cell  4 , causing particles of the particulate material to act against one another, whereby intense scrubbing, polishing and disintegration of the particulate material located within each cell  4 . 
         [0020]    Openings (not shown) are provided between the cells  4  in each bank at alternating locations between upper and lower ends of adjacent cells so that material must pass through all of the cells of each bank of cells  4 A, 4 B, 4 C, 4 D in series, preferably passing vertically though each cell between the openings. 
         [0021]    Typically, with known attrition scrubbers, should one or more of the attrition cells  4  become choked with material, stalling the motor thereof, or should one or more of the motors fail, or if the attrition scrubber is stopped due to some other fault or emergency, the solid material within each cell  4  tends to fall out of suspension. Thus a greatly increased torque may need to be imparted by the motors to the impeller paddles  10 , 12 , 13  to re-fluidize the solid material within each cell. This may prevent one or more of the cells  4  from restarting without the need to remove components and remove the solid material manually. 
         [0022]    Even during normal start up of the attrition cell cluster  2 , it is normally necessary to apply a significant starting torque to the impeller paddles  10 , 12 , 13  in each attrition cell to initially fluidize the particulate material as the solid material in the aqueous slurry tends to settle out of suspension within each cell. Thus the drive motors  6  normally need to be large enough to generate this starting torque and the drive shafts  8 , impeller paddles  10 , 12 , 13  and their mountings need to be substantial enough to withstand this starting torque. 
         [0023]    The attrition cell cluster  2  in accordance with the present invention solves this problem by providing a starting regime which avoids the need for such large starting torque by providing an alternative method for initially fluidizing the slurry. 
         [0024]    Each attrition cell  4  includes one or more water outlet nozzles  14 , preferably located in a lower region thereof, connected to a water supply, whereby water may be supplied into each cell  4  under the control of a controller. The (or each) water outlet nozzle  14  may be located tangentially within the respective cell at the level of the bottom most impeller paddle  12 , preferably directed in the normal direction of rotation of the paddles. Additionally, or alternatively, one or more water outlet nozzles may be located in a base of the respective cell  14  to direct water upwardly into the cell to fluidize material therein. 
         [0025]    Preferably the (or each) outlet nozzle  14  in each cell is connected to the water supply via a respective valve  16 , whereby the water supply to the (or each) nozzle  14  in each cell  4  can be controlled independently by the controller. The controller is also programmed to control the operation of the drive motor  6  of each cell  4 . 
         [0026]    In a preferred embodiment the controller is programmed to supply water to the (or each) water outlet nozzle  14  in each cell  4  of the attrition cluster  2  and to initiate operation of the drive motor  6  of each cell  4  under a control algorithm adapted to avoid the need for a high start up torque, as will be described below. 
         [0027]    The controller is programmed to supply water under pressure to the (or each) water outlet nozzle  14  of each cell  4  to fluidize particulate material within the respective cell  4  before the respective drive motor  6  is energised to rotate the impeller paddles  10 , 12 . 
         [0028]    Preferably the controller is programmed to initiate the supply of water to the (or each) water outlet nozzle  14  of each cell in turn, preferably starting from the downstream most cell  4 D and finishing with the upstream most cell  4 A (in terms of the direction of flow of the slurry, through the attrition cluster) to fluidize the material therein consecutively starting with the downstream most cell  4 D. 
         [0029]    The controller may also be programmed to start the drive motor  6  of each cell  4 A, 4 B, 4 C, 4 D in turn consecutively, starting from the most downstream cell  4 D and finishing with the most upstream cell  4 A such that the impellers of the downstream cells start rotation before those of the upstream cells. The controller may be programmed to monitor the torque applied by each motor  6  and to repeat the water supply step if the motor torque exceeds a predetermined maximum. In one embodiment the controller may monitor the current applied to the respective motor  6  and may switch the motor  6  off and repeat the step of supplying water to the respective outlet nozzle  14  of the cell if the current applied to the motor  6  exceeds a predetermined maximum during start up of the motor  6 . 
         [0030]    Such process may be repeated up to a maximum of four times, whereafter an error state may be flagged to indicate that there is a fault with the respective cell. Alternatively, if the motor is still not able to turn the impeller paddles of one or more of the cells  4 , a drain outlet  18  of the one or more cells  4  may be opened and the water outlet nozzle  14  thereof may be used to flush material out of the one or more cells  4 . The drain outlets  18  may also be adapted to supply water into each cell to fluidize material therein, as well as flushing material from the respective cell. 
         [0031]    The controller may comprise a PLC (programmable logic controller), controlling the operation of the drive motors, monitoring the torque applied by the drive motors  6  and controlling motorised or pneumatically operated valves  16  to control the supply of water to the (or each) water outlet nozzle  14  and/or drain outlet  18  in each cell as required. 
         [0032]    In use, particulate material (e.g. sand) in an aqueous slurry is passed into the attrition cell cluster  2 , entering the upstream most cells  4 A. Within the cells  4 , the impeller paddles  10 , 12 , 13 , driven by the respective drive motors  6 , cause intense scrubbing, polishing and disintegration of the sand, delaminating clay, graphite and other contaminants from the sand grains. 
         [0033]    The water content of the product entering the cells  4  of the attrition cell cluster  2  is preferably controlled to obtain a water content of 20% to 25% (adding water to the product to achieve the desired water content) during normal operation of the cluster  2  to ensure optimum operation of the attrition cell cluster  2 . This may be achieved by monitoring the torque load applied to the impeller paddles  10 , 12 , 13  by the drive motor  6  of the upstream most cells  4 A of the attrition cell cluster  2 , or the current applied to the respective motor  6 , during operation of the attrition cell cluster  2  and adding water as necessary, either via the water outlet nozzles  14  or via other water supply means, for example a water supply associated with the feed means of the attrition cell cluster, to achieve the desired optimum water content, resulting in maximum attrition of the sand. 
         [0034]    The slurry discharged from the attrition cell cluster  2  may be fed into a sump or tank adjacent and downstream of the attrition cell cluster  2 . Fresh water may be added to the slurry in the sump to achieve the correct concentration for a subsequent pumping process (typically 350 g/l). A centrifugal slurry pump may be then used to feed the slurry into a set of hydro-cyclones provided downstream of the attrition cluster for removing the very fine material (clay and other contaminants) separated from the sand grains in the attrition process. 
         [0035]    The invention is not limited to the embodiment(s) described herein but can be amended or modified without departing from the scope of the present invention, which is intended to be limited only by the scope of the appended claims as interpreted according to the principles of patent law including the doctrine of equivalents.