Paper making and fiber liberation – Processes and products – Reclamation – salvage or reuse of materials
Reexamination Certificate
1993-04-15
2001-03-27
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Reclamation, salvage or reuse of materials
C162S190000, C162SDIG009
Reexamination Certificate
active
06207015
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the recovery of raw materials for the manufacture of paper, pulp board and cardboard from the residual waste water slurry of a mechanical clarification apparatus or separation apparatus by separation into a fiber-rich portion and a filler-rich portion.
Processes for the recovery of paper fibers, fillers and coating pigments from cycles of the paper industry are well known.
The German publication, “Handbuch der Papier- und Pappenfabrikation (Papierlexikon)”, Dr. Martin Sändig oHG, Niederwalluf, 1971, reports in detail under the key words: Waste Water Purification, Fiber Recovery, Sludge Processing and Pulp Collectors. Of the recovery processes performed with pulp collectors, three different methods of operation have long been in use: settling in so-called funnel or scraper clarifiers, filtration with or without filter aids on drum filters, and lastly flotation. The first two methods are used for the recovery of fibers and fillers, while flotation, with few exceptions, serves mainly for the recovery of useful fiber components.
All recovery processes are aimed at removing materials from not very contaminated cycles, since the reuse of the so-called trap stuff or thick stuff in the paper manufacturing process forbids a higher dirt content. This is especially true of so-called fine papers, which call for a high white content and a minimum number of dirt spots.
In the state of the art it is therefore common to feed so-called paper machine cycles II or III to a stuff catcher, while all leakage drains, bottom drains and other diffuse losses of a paper factory are fed by way of the so-called drain to the residual waste water purifier where the mechanical and possibly biological purification takes place. The sludge produced thereby is considerably contaminated with dirt particles, so that it cannot be reused for high-quality papers.
If the paper mill is properly operated, it is possible to reduce the undesirable losses of fibers, especially high-quality primary fibers and fillers to a minimum, but in case of disturbances or breakdowns, losses through the drain occur, which amount to as much as 20% of the daily gross machine production and have to be discarded on account of the heavy contamination.
DE-A-29 46 160 lastly describes the recovery of useful fiber materials from the wastes of a waste paper treatment plant with a flotation stage. The floated waste is separated by means of a hydrocyclone separator into a fiber-rich fraction and a hydrophobic light fraction. The fiber fraction is then, after an additional screening, returned to the primary flotation, while the hydrophobic light fraction and that which passes through the screen are discarded. In this process no recovery of basically useful fillers and pigment particles takes place.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to make available a process that will permit recovering the usable fibers and fillers contained in the residual waste water sludge of the mechanical clarification plant without having to accept the limitations described above.
DESCRIPTION OF THE INVENTION
The above object is achieved according to the present invention by a process for the recovery of raw materials for the manufacture of paper, pulp board and cardboard from the residual waste water slurry of a mechanical clarification apparatus or separation apparatus by separation into a fiber-rich portion and a filler-rich portion, which comprises
a) adjusting the solids content of the thin slurry to a value of 1 to 5% by weight by adding a mechanically clarified waste water and preparing a homogenized suspension,
b) separating the coarse contaminant component with a screen of 1 to 2 mm mesh size,
c) separating the black particle components of less than 10 micrometers by centrifugation in one or more hydrocyclones connected in series or parallel,
d) fractionally fine screening of the accept component from the hydrocyclone or hydrocyclones with separation into the fiber component, agglomerate component and filler and pigment components, and
e) returning the fiber component and the filler and pigment components into the raw material processing of the paper mill.
The waste water fed to the residual waste water clarification plant by way of the drain is first subjected in conventional manner to a mechanical sedimentation, optionally with the aid of flocculants. The thin sludge is then drawn continuously or batchwisely from the sludge collecting chamber by means of known apparatus, and its consistency is determined with the aid of conventional density measuring apparatus. If it is greater than 5 wt.-% dry content, the thin sludge is adjusted by the addition of mechanically clarified residual waste water to a consistency of 1 to 5 weight-%. Lower consistencies require very large volumes, while consistencies above 5 wt.-% lead to rheological problems with the danger of clogging in the treatment units that follow. In addition to automated consistency determination, the consistency can also be determined by conventional waste water testing methods. This is also recommended for the checking and calibration of the automated consistency determination.
After the consistency adjustment, in order to separate coarse dirt consisting of splinters, sand grains and other impurities, the homogenized suspension is passed through a screen with a mesh size of 1 to 2 mm. Self-cleaning flat screens or mechanically powered vibrating screens have proven useful for this purpose. The separated coarse dirt component is discarded. What passes through the sieve consists of fibers, fillers, pigments, fine sand, black particles and agglomerates of filler and pigments or pigments, fibers and filler. The term filler, as used in this connection is to be understood to mean the fine particles used in the paper mass; pigment is understood to be the fine particles used in the brush coating.
The unusable black particles have a particle size >10 &mgr;m. They consist mainly of gray to black-colored sand, bottom detritus, machine detritus, coked lubricants, acid-attacked organic particles, rust and agglomerated dust, or a mixture thereof.
The separation of these black particles is performed by centrifugation in one or more hydrocyclones connected in series or parallel. Since a hydrocyclone sorts both according to differences in density as well as according to a shape factor, the precipitation of large-volume, stable pigment agglomerates, fiber lumps and fine wood splinters which are undesirable for the recycling is additionally performed.
By connecting several hydrocyclones in series and feeding the bottom run to an additional hydrocyclone, the loss of good substance is minimized and the good substance content is concentrated in the hydrocyclone overflow.
Any commercially available types of hydrocyclone can be used. Due to the abrasiveness of the black particle content, hydrocyclones of hard, corrosion-resistant material, such as ceramic oxide, have proven useful.
Preferred are so-called mini hydrocyclones with a nominal diameter of 10 to 100 mm, a good substance nozzle diameter of 7 to 14 mm, and a bottom run nozzle diameter of 2 to 8 mm.
The charging pressure of the hydrocyclones is selected between 0.5 and 6 bar. Higher pressures offer no advantage.
In a preferred embodiment of the process the black particle content precipitated in the hydrocyclones is subjected to an additional fine screening with a mesh size of greater than 25 &mgr;m, especially 63 &mgr;m, and what passes the screen is added to the good substance content. What passes the screen, however, can alternatively be fed back again ahead of the hydrocyclone or hydrocyclones in order to increase the precipitation rate of the black particles which are still in the passage through the screen. By this means it is possible to further increase the content of recovered good substance of fiber, filler and pigment.
As already indicated above, the hydrocyclone sorts not only by the shape factor but also by the density difference. If the content of black particle
Jacobs Günther
Tegethoff Wolfgang
Templer Klaus
Chin Peter
Fulbright & Jaworski LLP
Stora Feldmuhle AG
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