Paper making and fiber liberation – Processes and products – Reclamation – salvage or reuse of materials
Patent
1994-11-03
1996-10-22
Czaja, Donald E.
Paper making and fiber liberation
Processes and products
Reclamation, salvage or reuse of materials
162189, 162264, 162335, 162337, D21F 166
Patent
active
055672787
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and apparatus for the circulation of papermaking machine backwater. Particularly the invention relates to a process, which provides a fast and exact control of the papermaking process and which significantly reduces the time needed for changing the paper grade produced. The process according to the invention especially presents measures by which the recycling time for backwater is significantly reduced. The apparatus according to the invention is designed to serve the principle of a controlled and fast recycling of back water.
In a conventional papermaking process the paper stock, prepared in a separate stock preparation department, goes through the following phases, which constitute the primary process: after consistency control the stock is fed as a constant flow to the papermachine approach system, where it is brought close to a mixing pump, in which it is diluted and mixed to a consistency suitable for cleaning n centrifugal cleaners; the mixing pump pumps this thin stock to primary centrifugal cleaners, where debris is separated by means of the centrifugal force; the accept is brought either directly or via a second mixing pump to one or more primary pressurized screens, from where it is forwarded to the papermachine headbox via an appropriate dilution system; the headbox distributes the diluted stock evenly on an endless forming wire, or in some cases between two such wires, through which the major part of the water contained in the thin stock is drained, leaving a consolidated fibre web on the wire, from which it is transferred to the following phases of paper making, typically pressing and drying.
In the conventional primary fibre process parts of the stock are deviated and circulated in secondary loops. This is particularly the case with the rejects of cleaners and screens which, due to a poor selectivity of these devices, contain good fibres, typically 10 to 30% of the material handled in the primary stage. Said rejects are diluted in mixing pumps and recovered in secondary cleaning stages which may be numerous, each handling the reject of a previous stage and recycling the accepts upstream to a previous stage or into the primary fibre process.
A significant part, typically 5 to 50% of the solids of the thin stock follows the water drained through the forming wire and is circulated back into the fibre process with the backwater. In a traditional papermaking process said backwater passes through backwater pans and channels into a backwater tank, where said water is collected and which feeds the mixing pumps mentioned above. The backwater contains a significant amount of air which would disturb the fibre process and web forming and which therefore has to be removed. This is achieved by letting the flow speeds in the backwater tanks and channels be low, or by letting the entire thin stock flow pass through separate deaeration tanks.
The process in which backwater is recycled from web forming into the fibre process immediately prior to paper forming, and thick stock supplied from the stock preparation is diluted to forming consistency, fed to the headbox and drained as explained above, constitutes the "short circulation". Due to the consistency difference between thick stock and the fibre web leaving the forming part of the paper machine, and various other additions of water into the process, an excess of backwater results and is circulated to the stock preparation as a "long circulation". Mostly the solid material in water flowing to said long circulation is recovered and returned to the short circulation or the fibre process by means of savealls.
At a change in the composition of the thick stock fed from the stock preparation, or in other process conditions influencing the composition of backwater, the great amount of material circulating in the backwater will delay reaching of an equilibrium state. Each time the water of the short or the long circulation passes the web forming zone a certain share of the circulating material, corresponding to a so called
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Meinander, Paul Olof, "Easy grade changes are in the pipe line", Pulp & Paper International, May 1993, pp. 61, 63.
Henrik Nisser: The Formator Method--a Road to More Homogenious Formation, Summary of Patent Application: Process and Apparatus for Circulating Backwater in a Papermaking Machine.
H. J. Shultz: Practice and Theory of Paper Production on the Example of the Sheet Formation, Summary of Patent Application: Process and Apparatus for Circulating Backwater in a Papermaking Machine.
Czaja Donald E.
Padgett Calvin
POM Technology Oy AB
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