Patent Application
20070045157
The present invention relates to systems for washing chips and to systems for recovering wood fibers from wastewater streams.
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.
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.
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.
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.
It is a feature of the present invention to reduce the cost of washing wood chips by recovery of wood fiber.
It is a further feature of the present invention to recover wood fiber from the wash water of a chip washing system.
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.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.
Referring more particularly to
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
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.
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's 1 to 3 mm holes 44 and is passed through the post filter interior volume 48 to the reject flow outlet 62.
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
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.
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.
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.
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.
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.
