Note: Descriptions are shown in the official language in which they were submitted.
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BA~K~GROUND OF THE INVEN'rlON
ield of thQ Invention
~ his invention is in the field of refining a thermo-mechan-
ical p~llp immediately prior.to passing the same to the head box of a
paper making maclline and involves re~ining the pulp at a high tempera-
ture level and low intensity levels to reduce the bullc of the fibers and
to reduce the amount of b.reakage of the fibers in the refiner.
DES~RlPr~ON OF T~E PRIOR ~RT
The thermo-mechanical pulping process has become wid~ly
accepted in tlle paper industry for producing mechanical pulp from chips.
To a large extent, it has replaced the purely mechanical process in
which the chips were ground by means of stone grinding wheels and
cl~Qmical processes involving the use of sulfates or sulfites. Pulps
produced by ~he thermo-mechanical pulping process exhil~it substantially
higher strength properties than pulps produced by the stone groundwood
prc)cess. While the chemical pulping methods produce a higher quality
pulp,. they require extensive capital investment for high pressure treating
vessels and the like.
One of the difficulties with thermo-mechanical pulp~, how-
ever, is their higher bulk (lower d~nsity) in papers produced from such
pulps. Tllis high bulk causes problems in printing and converting
operations and may itsQlf be a pro~lem if'finished sheet tllickness, for
a given basis weight, is crltical. Additionally, higher strength sl~eets
can usually be obtained from a given pulp if the bulk is reduced, for
example, by higher pressing pressures.
The processing of mechanicaI pulp suspensions by con-
ventiona~ "post refining methods" in a refiner at a tow consistency of
a~out 3 to 57c' has not proven completely satisfactory because of the
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~070536
tendency to cut the fibers during refining. Although some
reduction in bulk can be achieved in this way, the reduced fiber
length caused by the broken fibers results in excessive freeness
drop, and possible loss in sheet strength properties.
SUMMARY OF THE INVENTION
In accordance with the present invention, the fiber length
reduction usually associated with post refining of mechanical
pulps is reduced by increasing the refining temperature to the
range of about 200 to 300F at which temperature the fibers are
much more flexible and have less tendency to be cut in the
refiner. In addition to the increased temperature, the refining
is carried out under conditions of minimum practical intensity.
Specifically, the refiner is operated at an intensity of less
than 40x10 applied horsepower per inch contacts per minute.
With conventional sized refiners, this means that the refiner
operates at a peripheral speed in excess of 6000 feet per minute.
Additionally, a very fine bar pattern is used on the confronting
stator and rotor plates so that high speed operation with this
type of bar pattern on the plates results in maximum practical
levels of inch-contacts per minute.
In a preferred form of the invention, the stator plates are
composed of a synthetic resin such as a polysulfone resin, while
the rotor plates are composed of steel. The widths of the alter-
nating bars and grooves on both the rotor and stator surfaces
should not exceed about 1/8 of an inch.
BRIEF DESCRIPTION OF T~E DRAWINGS
Other objects, features and advantages of the invention will
be readily apparent from the following description of certain
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~07~)536
preferred embodiments thereof, taken in conjunction with the
accompanying drawings, although variations and modifications may
be effected without departing from the spirit and scope of the
novel concepts of the disclosure, and in which:
Figure 1 is a partial block diagram illustrating some of
the steps which may precede the post-refining operation with which
the present invention is concerned;
Figure 2 is a fragmentary view with portions broken away to
illustrate the interior of a refiner which can be used for the
purposes of the invention; and
Figure 3 is a greatly enlarged fragmentary cross-sectional
view of the confronting rotor and stator plates which form the
preferred embodiments of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 of the drawings illustrates a flow diagram of a
typical sequence which can include the post-refining step of the
present invention. Specifically, the stock may be treated in one
or more refiners lQ and then passed to one or more mixing chests
11 where suitable additives such as clay and the like may be added
through a line 12. The thus treated stock is then passed to one
or more machine chests 13 and ultimately to a post refiner 14,
the details of which will be subsequently described. The stock
entering the post refiner 14 is a suspension having a consistency
of 3 to 5~. The thermo-mechanical pulp is characterized by the
presence of softened lignin and by substantial freedom from
chemical residues. In the post refiner stage 14, the stock is
treated at a temperature of 200 to 300~F, and the refiner operates
at an intensity of less than 40xlO 4 horsepower/inch contacts/
minute. Preferably, the intensity is less th~n 15xlO horse-
power/inch contacts/minute.
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107~53~
Conventional refi1-ers all act OIl tl~e fil)ers J~y means of at-
trition. 1`1~re arc various types of refine-r config~Iratiolls, but most
of tIlem make use of metal attrition members in tIle form of bars.
The intensIty of rcfining is defined as the net refining power divided by
the number of b,lr crossings per unit time. Since the refining action
of the fil~er takes place at the leading edge of the filling clement, the
numher of inch contacts/millute can be delermilled from the following:
IncIl contacts/minute ~ Total length of
bars in rotor x
total length of
b~rs in stator x
refiner speed in
rpm
A tyI~icaI refiner which can be used for the purpos~s of
the present invention is illustrated in Figure 2 of the drawings. It
includes a ~ase 15 on whicII there is mounted a shaft 16 supported for
rotation in a beariIlg 17. The shaft is driven from a coupling 18 con-
nected to a suitable motor (not shown). I`he stocl; is introduced illto
an inlet 19 and then flows into a refining cavity 20 whereupon the
~tock flow is~plit up by passing through a pair of perforated rotor
clements 21 and 22 secured to a rotating head 23. Details of such
rotor ~lements will be found described in U.S. Patent No. 3,438,586.
The rotor elements 21 and 22 are in confronting relation
with stator plates 24 and 2S on opposed sides tllereof. The spacings
between the rotor plates and the stator pl~tes is ~djusted by means
of a worm gear 2~ which operates in conjunction with an adjllsting scrPw
27 W}1iCh in turn is connected to a gear motor 2~.
~ more clet~iIe~ view of the refiner plates is set forth
in I~igure 3 of the drawil1gs. ~s illustrated in th~t fi~ure, Ih~
107~536
the rotor plate 22 is provided with a series of alternating
bars 29 and grooves 30, the widths of the bars and grooves not
exceeding 1/8 inch. Similarly, the stator 25 is provided with
ribs 31 and grooves 32 in which the widths of the bars and the
grooves do not exceed 1/8 inch. In the preferred form of the
present invention, the rotor plate 22 is composed of stainless
steel and the stator plate 25 is composed of a synthetic resin
such as a polysulfone resin.
Fiber length reduction which is achieved in normal low
consistency "post refining" of mechanical pulps can be
attributed to the relatively high stiffness of the fiber. At
normal refining temperatures of 110 to 140F, the brittle fibers
are fractured when impacted by the impinging edges of the
refiner. In accordance with the present invention, the refiner
is operated at an increased temperature so that the lignin
softening temperature is approached or exceeded. At these
higher temperatures, the fibers are much more flexible and have
a tendency to be cut in the refiner.
To further reduce fiber damage, the high temperature
processing is carried out under conditions of minimum practical
intensity in the refiner. To minimize the intensity, the
refiner is operated at relatively high peripheral speeds, in
excess of about 6,000 feet/minute. This high speed, coupled with
the very fine pattern of bars in the stator and rotor results in
maximum practical levels of inch-contacts/minute. Consequently,
when relatively low power is applied to the refiner, the
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1070536
intensity will be at t~le lowest practical level.
Since the refiner operation is at relatively high speed,
a considerable circulatillg load due to hydraulic losses will exist. This
load is reduce(l sul)stantially by reducing the draft of the rotor plates,
and this reduced draft becomes practical because of the -lower wear rate
of the steel rotor whell operated against a synthetic resin stator. This
reduction iJl load, tl~ere~ore, contributes to power savings.
~ s an i1lustration of the differences involved between the
operation of the refiner according to the present invention and con-
ventional practice, it might be noted that the normal operation of a
42 inch diameter xefiner makes use of bars alld grooves which are
3/16 inch in width, a rotational speed of about 514 rpm, and a net
applied horsepower of 600. ln the refining operation of the present
invention, the bar and groove widtll is reduced to 1/8 inch or less,
the rotational speed of at least 600 rpm is used, and the net applied
hor~epower is 300 or less.
It should l~e evident that v~rious modifications can be
made to the described embodiments without departing from the- scope
o~ the present invention.
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