Pulp Screening Reject Recirculation for CSF and Shive Control
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Solution Overview
Problem
Current pulping methods generate a significant amount of reject fraction during screening, which requires separate treatment and increases energy consumption, and often result in lower Canadian Standard Freeness (CSF) levels and higher shive content in the pulp composition.
Innovation Solution
The method involves impregnating wood chips with a composition containing NaOH, Na2SO3, or H2O2, refining them to achieve low shive content and high CSF, and then screening to separate accept and reject fractions, where the reject fraction is directed back to the impregnating step or a preceding step, allowing for dewatering using a screw press or disc filter to increase consistency and maintain high CSF levels throughout the process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If wood chips are refined to achieve low shive content and high CSF, then pulp quality is improved, but energy consumption increases
Solution Approach 1:
The patent applies preliminary chemical impregnation treatment to wood chips before mechanical refining. The impregnating composition (containing NaOH, Na2SO3, and/or H2O2) is applied to the chips prior to refining, which softens the wood structure and facilitates easier fiber separation. This preliminary action reduces the mechanical energy required during subsequent refining while still achieving the desired low shive content and high CSF levels in the final pulp.
2Measurement precision
If reject fraction is sent to separate treatment line, then screening efficiency is maintained, but device complexity and energy consumption increase
Solution Approach 1:
The patent merges the reject fraction treatment back into the main process stream by directing the reject from the screening step to the impregnating step. Instead of using a separate reject treatment line, the reject material is combined with fresh wood chips and reprocessed through the impregnation and refining stages. This integration reduces device complexity and eliminates the need for separate reject handling equipment while maintaining screening efficiency.
Solution Approach 2:
The patent recovers the reject fraction by redirecting it back to the impregnating step rather than discarding it or sending it to a separate treatment line. The reject material, which contains partially processed fibers and unrefined chips, is recovered and reprocessed through the chemical impregnation and mechanical refining stages, allowing for complete utilization of the wood material and reducing overall process complexity.
3Measurement precision
If reject fraction is stored in silos and treated separately, then screening performance is optimized, but loss of time and productivity occur
Solution Approach 1:
The patent establishes continuity of useful action by creating a closed-loop process where the reject fraction is continuously redirected back to the impregnating step without interruption. This eliminates the discontinuous storage and separate treatment operations that would otherwise be required. The continuous recirculation of reject material through the impregnation and refining stages maintains steady-state operation and maximizes pulp production rate while preserving screening performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the amount of reject generated, allows for higher CSF levels, potentially eliminates the need for a disc filter, and facilitates energy savings by minimizing equipment usage and maintaining pulp quality.
Implementation Method 1
impregnating the wood chips with an impregnating composition to obtain impregnated wood chips
Implementation Method 2
The impregnating composition is an aqueous composition comprising one or more of the following: NaOH, Na2SO3
Implementation Method 3
refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition
Implementation Method 4
screening the refined pulp composition to obtain an accept fraction and a reject fraction
Implementation Method 5
dewatering the accept fraction output from the screening by a screw press
Data Source
AI summary
According to an example aspect of the present invention, there is provided a method comprising: providing wood chips; impregnating the wood chips with an impregnating composition to obtain impregnated wood chips; refining the impregnated wood chips in at least one refiner to obtain a refined pulp composition; screening the refined pulp composition to obtain an accept fraction and a reject fraction; and directing at least part of the reject fraction from the screening step back to said impregnating step or to a step preceding said impregnating step.