Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~S~403
MECHANICAL PUI,P PUMPING
AND HYDP~OSULFITE BLEACHING
BACKGROUND AND SUMMARY OF THE INVENTION
_ .. . . _
With ~hortages in raw materials for
manufacture o paper pulps, mechanical pulping
processes have become of more int:erest. Mechanical
pulping processes generally have higher yields than
chemical pulping processes. ~owever, since i~
mechanical pulpin~ processes ~including refiner
mechanical pulping ~RM~), chemimechanical pulping
(CMP), thermomechanical pulping ~TMP), and
chemithermomechanical pulping lCTMP) ), most of the
lignin and polys~ccharides are left in the pulp,
rather than being dissolved and modified as i~
chemical pulping, bleaching must be accomplished by
decoloration, instead of by solubili~ation, as in
chemical pulpin~.
One major process for bleaching of mechanical
pulps is a reductive bleaching process utilizing
20 hydrosul~ite, such as ~odium hydrosulfite or zinc
hydrosulfite. While hydrosulfite bleaching can
achieve th~ desired decoloration of the mechanical
pulp, there are a number of drawbacks associated with
its use. ~ydrosulfite is oxidized extremely rapidly
25 by air, therefore ~he entrainment of air in the pulp
can negate the bleaching efects of the hydrosulfite.
This makes hydrosulite impractical for use with
'~''JP~' ~
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mechanical pulps having a consistency of much greater
than 4 percent since as the consistency rises the pulp
tends to entrain air. However, it is d~sirable to
. effect bleaching at the same eonsistency as for other
processing of the pulp te.g. 5-25 percent, preferably
8-15 percent), although successive diluting and
thickening stages are to ~e avoided because of the
equipment costs and ener~y expense associated
therewith.
According to the present invention, a method
of treating mechanical pulp is provided that allows
reductive bleaching of the pulp in an effective manner
at pulp consistencies of about 5-25 percent, and
preferably B-15 perrent~ during the entire treating
procedure. ~ccording to one aspect of the present
invention, the method comprises, or consists
essentially of, the following sequential steps~
(a~ Simultaneous pumping and degassing the mechanical
pulp. This is preferably accomplished utilizing a
~o fluidizing centrifugal pump. (b) Immediately a~ter
after (a), mixing, substantially without air
introduction, the mechanical pulp with an effective
charge of hydrosulfite bleaching chemical. The mixing
preferably is aecomplished in a fluidizing mixer, and
the hydrosulfite bleaching chemical includes
seques~ering, chelating, and buffering agentsO A
typical charge is 0.5-1.5 percent hydrosulfite. And
(c) retaining the pulp at predetermined pH (e.g. about
4.5-6.53 and temperature ~e.g. about 35-80C)
conditions for a predetermined period of time ~e.g.
about 0.25-2 hours) to effect the desired bleaching.
By practicing the invention oxidation of the reductive
bleaching agent is avoided, a minimal amount of
equipment is utilizedt and the processing can be done
at the desired consistency range of 8-15 percent.
54~3
Alternativelyl the mixer may be eliminated,
and the hydrosulfite added directly to the fluidizing
pump. For instance, the chemical may be added to the
discharge, or the suction, of the pump.
It is the primary object of the presen~
invention to provide a method for trea~ing mechanical
pulp to provide efficient reductive ~leaching thereof.
This and other objects of the invention will become
clear from an inspection of the detailed description
of the invention and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic view illustrating
exemplary apparatus utilized in practicing the method
according to the present invention;
FIGURE 2 is a schematic detail cross-
sectional YieW 0~ the exemplary fluidizing centrifugal
pump illustrated in ~IGURE l; and
FI~URES 3 and 4 are schema~ic views of two
different alternative embodiments of apparatus for
practicing the method of the invention.
DETAILED DE~CRIPTION O~ THE DRA~INGS
The method according to the present invention
is applicable to mechanical pulps. That term, 2S
commonly used in the industry and as used in the
2S instant ~pecification and claims, encompasses refiner
mechanical pulps ~RMP), chemimechanical pulps (CMP),
thermomechanical pulps ITMP), chemithermomechanical
pulps (CTMP~, and stone groundwood pulps.
~ILZ~54~3
Typically, the mechanical pulp is produced
utilizing any desired conventional technique, and then
is fed by line 11 to a storage tower 12 or the like.
According to the present invention it is preferable to
5 mount a fluidizing centrifugal pump 13 at the bottom
of the storage tower 12, to draw the mechanical pulp
downwardly therefrom. The pulp typically has a
consistency of about 5 25 percent, and a consistency
of 8-15 percent is preferred.
The fluidizing centrifugal pump 13, and an
apparatus and method for controlling the head there-
from, are illustrated and described in Applic~nt's
United States Patent No. 4,435,193 issued March 6,
1984. The pump 13, as illustrated in Figure 2, may
15 comprise an inlet channel 14, a xotor 15 including
a plurality o E axially extending blades 16 having
an interior openin~ 17 therebetween, and impeller
blades 18. The blades 18 rotate in a spiral housing
section 19, and gas can pass through openings 21, 22
20 from the housing section 19 to a gas chamber 20,
the gas chamber 20 being connected up to a gas
discharge line 23.
~ ulp passes from the pump 13 into the
discharge line 25/ with a throttling valve 25 dis-
25 posed therein. The head of the pump 13 i5 controlled,by adjusting the throttling valve 26 in a manner
explained in the aforementioned U.S. Patent No.
4,435,193, so that a variety of heads within a wide
xange may be achieved. The valve 26 preferably is
30 controlled, according to the present invention, so
that the pump head is maximized, to thereby maximize
the degassing action of the pump (i.e. maximizing the
amount of air withdrawn through conduit23).
59L~3
After simultaneous pumping and degassing of
the mechanical pulp in the pump 13, the pulp is
immediately passed to a mixer 28. The pulp must be
passed to the mixer 28 so that substantially no air
introduction occurs, and the mixer 28 must effect
mixing without air introduction. At the mixer,
hydrosulfite bleaching chemical is added from line 29
to the mixer, and is thoroughly mixed with the pulp.
- For the practice of the present invention, the mixer
28 preferably comprises a fluidizing mixer such as
shown in U.S. Patents 4,0~3,506 and 4,339,206,
The amount of hydrosulfite bleaching chemical
added to provide an effective charge will vary
depending upon the properties of the mechanical pulp,
the amount of bleaching desired, etc. Typically,
however, the sodium hydrosulfite or zinc hydrosulfite
would be added so that the total charge was about
0.5-1.5 percent. Buffering, sequestering, and/or
chelating agents, such as sodium tripolyphophate and
sodium diethylenetriaminepetaacetate, are also
preferably added with the hydrosulfite.
After mixing of the hydrosulfite bleaching
chemical into the pulp using mixer 28, the pulp, still
at the same consistency (i.e. 5-25 percent), is
retained at pred~termined pH and temperature condi-
tions for a predetermined period of time to effect the
desired bleaching. ~his retention may be merely in
conduits passing the pulp onto a further processing
station, or may be in a vessel particularly designed
for that purpose, such as a conventional up-flow
bleaching tower 31. Typical, although not limiting,
conditions are: pH, about 4.5-6.5; temperature, about
35-80 C; retention time, about 0.25-2 hours.
~454~3
Since the pulp is essentially completely
degassed before mixing with the hydrosulfite bleaching
chemical, the bleaching action can be effectively
controlled, and the desired amount of bleaching
chemical can be added to effect the desired
bleachin~. The degassing action is accomplished,
according to the present invention, in a simple and
effective manner, and with a minimum amount of
equipment. The pulp, at a consistency of between 5-25
percent, and preferably 8-15 percent, i5 withdrawn
from storage tower 12, si~ultaneously pumped and
degassed by fluidizing centrifugal pump 13, the head
of which is controlled by valve 26 to maximize
degassing action, immediately passed to fluidixing
mixer 28 at which the hydrosulfite bleachin~ chemical
is added via line 29, and subsequently passed to a
conventional retention tower 31, or the like, where it
is retained at predetermined pH and temperature
conditions for a predetermined period of time to
effect desired bleaching.
In the exemplary embodim~nts of the present
invention illustrated schematically in FI~URES 3 and
4, the mixer 28 is eliminated, and instead the hydro-
sulfite is added directly to the pulp within the pump
13. In the FIGURES 3 and 4 embodiments, the control
mechanisms for the pump 13 have not been shown, for
clarity of illustration.
In the FIGURE 3 embodiment, the hydrosulfite
bleaching chemical is added through line 33 directly
to the discharge of the pump 13. At the discharge of
the pump 13, the pulp has been efectively degassed,
and when the bleaching chemical i5 added through a
nozzle, or like other introduction structure, ~ue to
the turbulence of the pulp at that point, it will
~s~
become intermixed with the pulp~ and effective
bleaching will occur.
In the embodiment illustrated in FIGURE 4,
the pump 13 is shown with the axis of rotation of the
rotor thereof horizontal. This merely illustrates
that the present inven~ion may be practiced
irrespective of the orientation (e~gO, horizontal or
vertical) of the pump rotor.
In the FIGURE 4 embodiment, the hydrosulfite
bleaching chemical is added via :Line 35 at the suction
of the pump 13. Again, in~roduction can be accom-
plished utilizing a suitable nozzle, or the like.
While the pulp is not completely degassed at this
position, adding the bleaching chemical at the suction
provides for complete intermixing of the bleaching
chemical with the pulp as it is pumped by the pump 13,
and the time from ehemical addition at the suction
side of the pump, to effective degassing by the pump,
is short enough that under most circumstances
entrained air will nbt siqnificantly negate the
bleaching effects of the hydrosulfite. ~owever, if
entrained air does nega~e such effects for a particu-
lar pulp, then the embodiments of FIGURES 1 and 3 will
be utilized instead of the embodiment of FIGVRE 4.
While the invention has been herein shown and
described in what is presently conceived to be the
most practical and preferred embodiment thereof, it
will be apparent to those of ordinary skill in the art
that many modifications may be made thereof within the
scope of the invention, which scope is to be accorded
the broadest interpretation of the appended claims so
as to enrompass all equivalent methods and procedures.
