Abstract:
A recovery process and apparatus employing a pressure water washer to process wood chip wash water, to separate pin chips from sand and dirt contained in the wash water. Wash water from a chip wash system is supplied to a pressure washer having a cylindrical screening basket and a rotor positioned within the basket. The screen basket has holes of 1-3 mm which allows dirt and sand to pass but retains pin chips. The wood fiber pin chips on the way to the accepts outlet thus do not pass through the screen basket. Additional water may be added to the pressure washer to balance the flow of water along and through the screen basket.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to systems for washing chips and to systems for recovering wood fibers from wastewater streams.  
         [0002]     Mechanical, chemical, and semi-chemical wood pulps are created from wood chips which allows mechanical, or chemical action to uniformly act on the fibers contained in the raw wood starting material. The wood chips often become contaminated with sand, dirt, rocks, scrap metal, and broken glass. The contamination can arise because waste materials are used to generate the wood chips or because of storage of wood chips on the ground, or due to wind and water depositing sand and dust on the chips during storage or handling. Such contamination of the wood chips is undesirable because it affects the quality of the finished product made from the wood fibers and because it causes excessive wear and tear on equipment handling the chips and the fibers produced therefrom. Therefore wood chips are often washed, typically with water, before they are further processed, and the wash water is purified and the contaminants are discarded. The washing process often takes place in an unpressurized chip washer which uses a screen with holes through which dirt and contaminants are washed. Because of the requirements of chip washing, the screen holes in the chip washer are often of sufficient size that chip pins, i.e. small slivers of wood of about a sixteenth of an inch in diameter or less and about a quarter-inch long, are lost with the wash water.  
         [0003]     Although only a small percentage of the wood fiber contained in the wood chips is lost with the wash water, over the course of a day in a large modern pulp or fiber making facility, 10 to 15 tons of dry weight fiber per day can be lost with the wash water from the chip washing system. This can represents a cost of $500 to $1,000 a day depending on fiber recovery and value. What is need is a method of recovering clean pin chips from wood chip wash water.  
       SUMMARY OF THE INVENTION  
       [0004]     The recovery process and apparatus of this invention employs a chip wash system to clean wood chips, and a pressure washer to process the wash water, to separate pin chips from the sand and dirt contained in the wash water. Used water from a chip wash system such as used in the pulp and paper and fiberboard industry, is supplied to a pressure washer having a cylindrical screening basket and a rotor positioned within the screen basket which moves hydrofoil shaped blades or foils over the surface of the screen basket.  
         [0005]     The recovery water is supplied at a pressure of, for example, of about 1-40 psi and the rotor causes positive and negative pressure pulses. The positive pressure pulses aid the passage of the rejected sand and dirt through the screen basket, and the negative pressure pulses periodically back flush holes formed in the screen basket, thus keeping the holes from becoming plugged. A screen basket with holes of 1-3 mm rather than slots is used because the dirt and sand easily pass through the screen basket holes but the pin chips do not. The wood fiber in the form of pin chips, i.e., the accept, do not pass through the screen basket, but the rejects consisting of sand and dirt or mud do. Contrary to normal practice, the accepts, i.e. the pin chips, do not pass through the screen basket. Therefore additional water may be added to the pressure washer to balance the through screen water velocity, with a flow volume and velocity along the screen basket so as to move the pin chips along the screen basket surface and out of the pressure washer.  
         [0006]     It is a feature of the present invention to reduce the cost of washing wood chips by recovery of wood fiber.  
         [0007]     It is a further feature of the present invention to recover wood fiber from the wash water of a chip washing system.  
         [0008]     It is another feature the present invention to configure a pressure washer following a chip washing system to recover pin chips, and to reject sand and dirt.  
         [0009]     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic view of a chip washer system, in combination with a pressure washer of this invention.  
         [0011]      FIG. 2  is a side elevational schematic cross-sectional view of the pressure washer of  FIG. 1 .  
         [0012]      FIG. 3  is a side elevational schematic cross-sectional view of an alternative embodiment of the pressure washer of this invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Referring more particularly to  FIGS. 1-3 , wherein like numbers refer to similar parts, a wood chip wash system  22  is shown in  FIG. 1 . The wood chip wash system  22  forms a part of a chip wash and recovery system  20 , and is supplied with wood chips  24 , and water  26 , and outputs clean wood chips  28  and wash water  30  containing pin chips  31 , mud, and sand  33 . The wash water  30  then flows into a slurry tank  32  which is equipped with an agitator  34 , and a pressure washer feed pump  36 . The retention time within the slurry tank is about one to ten minutes following which the wash water  30  is fed under pressure produced by the pump  36  into a pressure washer  38  as shown in  FIG. 2  or the pressure washer  64  as shown in  FIGS. 1 and 3 . The general attributes of the pressure washers  38 ,  64  such as the use of holes instead of slots in the screen basket, the addition of supplemental feedwater, and the general arrangement of a rotor for causing pressure pulses to prevent clogging of the screen are well-known. However, the particular application and arrangement of components perform a new function of recovering fiber which would otherwise be discarded. Further, the pressure washers  38 ,  64  are particularly configured so as to perform the new function of separating pin chips from the sand and dirt in the wash water  30 .  
         [0014]     The pressure washer  38  has a pressure vessel  40 , and a screen basket  42  which is arranged to form a cylinder which has screening holes  44 , as shown in  FIG. 2 . The screen holes  44  are shown exaggerated in  FIG. 2  for illustrative purposes and have an actual diameter of approximately 1 to 3 mm, and substantially fill the surface  58  on the screen basket cylinder. The screen basket  42  defines a pre-filter interior volume  46  and a post filter volume  48 . The pre-filter interior volume  46  is supplied with wash water  30  through an inlet pipe  50  and is connected to an accept outlet  52  through which pin chips  31  are recovered. A rotor  54  is positioned to rotate about an axis  57  defined by the cylindrical screen basket  42 , and driven by a motor  55 . The rotor  54  may take the form of any conventional rotor which moves rotor foils  56  in a circular motion closely spaced from the inside surface  58  of the screen basket  42 , for example 1 to 6 mm.  
         [0015]     The foils  56  produce a positive pressure pulse followed by a negative pressure pulse sufficient to keep the holes in the cylindrical screen basket  42  from clogging. A negative pressure pulse of sufficient magnitude causes a back flushing of the screen basket  42  so as to periodically clear the holes of the screen basket. A pressure screen such as that manufactured by Metso Paper, Inc. in 2003, such as FS-560LC/HC under the trademark Optiscreen SF type, or pressure screens manufactured by other suppliers such as Hooper may be modified to form the pressure washers  38 ,  64 . These screens, while not having the specific screen arrangement of the invention, provide an example of the basic design which can be configured and used in accord with the invention.  
         [0016]     Normally the accept output of a pressure washer passes through the screen basket, however in the pressure washer filter  38  it is the rejected sand and dirt which passes through the screen basket  42 , and the accept flow of pin chips which are retained by the cylindrical screen basket  42  move along between the screen and the rotor  54  always remaining in the prefilter interior volume and exiting through the accept outlet  52 . On the other hand, the reject flow passes through the screen&#39;s 1 to 3 mm holes  44  and is passed through the post filter interior volume  48  to the reject flow outlet  62 .  
         [0017]     The majority of the flow from the pressure washer  38  passes through the screen basket  38  to the reject outlet  62 . The flow within the pressure washer  38  is controlled so that the water velocity through the holes  44  of the screen basket  42  are such as to lead to effective passage of dirt and sand  33  contained in the wash water  30  through the screen basket  42 , at the same time so as to allow sufficient flow to move the retained pin chips along the screen basket surface  58  to the accept outlet  52 . As shown in  FIG. 3 , an alternative embodiment pressure washer  64  is shown which in addition to the parts described with respect to  FIG. 2  includes a supplemental water inlet  66  which allows a flow of clean wash water  68  to pass to the prefilter interior volume  46  and increases the water flow available to move the pin chips to the outlet  52 .  
         [0018]     Typical water and material flows for a representative chip wash and recovery system  20  are for example  625  bone dry metric tons per day (BDMT/D) and over 14,000 tons of water. The output of the chip wash and recovery system  20  is 5,000 to 10,000 liters per minute of water and 10 to 20 BDMT/D of pin chips and contaminants, which flow into a tank  32  with a volume of 5-100 cubic meters. Thus the average time that the flow is acted upon in the tank by the agitator  34  is about 1 to 10 minutes. From the slurry tank  32  the pump  36  passes the wash water  30  under a pressure of about 1 to 40 psi to the pressure washer  38 ,  64  where the pressure drop across the screen of the screen basket is typically about 5 psi. The pressure washer reject flow to the outlet  62  is about 4,500 to 9,000 liters per minute containing about 2.5 to 5 BDMT/D of sand and mud  33 . And the pressure washer  38 ,  64  flow to the accept outlet  52  is about 500 to 1,000 liters per minute and 7.5 to 15 BDMT/D of pin chips. In addition, a certain amount of clean wash water  68  may be supplied to the supplemental water inlet  66  to increase the water flow with the pin chips, if necessary, for proper functioning of the pressure washer  64 .  
         [0019]     It should be understood that the chip wash and recovery system  20  will not recover all fiber in the wood chip wash water, but can be expected to recover the majority of such fiber, particularly the fiber contained in pin chips of approximately 1/16 inch in diameter and in the neighborhood of a quarter inch long. It should also be understood that the holes formed in the screen basket will have a diameter approximately that of the diameter of the pin chips, however for optimal sizing it may be necessary to try screen holes of varying sizes to obtain the best recovery from a particular wash water stream.  
         [0020]     It should be understood that the holes  44  are preferably circular but may deviate from circular so long as the holes easily pass the sand and dirt but retain the pin chips. It should also be understood that the screen basket is preferably cylindrical but could be constructed with any shape forming a surface of revolution whose axis is the axis  57  about which the rotor  54  turns. The pressure developed across the screen of the screen basket is a pressure effective to obtain selected flow rates through the screen basket  42  and which at the same time can be overcome by the negative pressure caused by the rotor  54  and the foils  56  attached thereto, and for example may be about 5 psi.  
         [0021]     It should be understood that the pressure washer may be fed by pressurizing the wash water with a pump or by using gravity to establish the necessary pressure head to thus connect the pressure washer in pressure increasing relation to the wash water outlet of the chip washer system.  
         [0022]     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.