Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PRODUCING VERY LOW COD UNBLEACHED PULP
FIELD OF THE INVENTION
The present invention relates to cellulosic pulps, and more particularly, to
unbleached cellulosic pulps having a low COD that are useful in cementitious
products.
BACKGROUND OF THE INVENTION
The internal structures of houses and other buildings are commonly protected
from environmental elements by exterior siding materials. These siding
materials are
typically planks or panels composed of wood, concrete, brick, aluminum,
stucco, wood
composites, or fiber-cement composites. A common fiber-cement composite is
fiber-
cement siding, which is generally coinposed of cement, silica sand, unbleached
wood
pulp, and various additives. Fiber-cement siding offers several advantages
over other
types of siding materials, such as wood siding: it is weatherproof, relatively
inexpensive
to manufacture, fire-resistant, and invulnerable to rotting or insect damage.
Commercial fiber-reinforced cement siding planks or panels are made using the
Hatsheck process. The Hatsheck process was initially developed for the
production of
asbestos composites, but it is now used for the manufacture of non-asbestos,
cellulose
fiber reinforced cement composites. In this process, unbleached cellulose
fibers are re-
pulped in warm water at an alkaline pH of 11 to 12.5; the re-pulped fibers are
refined and
then mixed with cement, silica sand, and other additives to form a mixture.
The fiber-
cement mixture, is deposited on a felt band substrate, vacuum dewatered, and
cured to
form a fiber reinforced ceinent matrix in sheet form.
The curing of the cement matrix is hindered by the presence of sugars or other
organic materials. These materials retard the hydration reaction of cement and
thereby
retard the setting or hardening of a mortar or concrete. Cement is purposely
retarded in
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ready-mix concrete during long hours of transportation, for mitigation of
stress due to
temperature (heat) when used in a large-sized concrete structures, and for
decorated
washing finishes. When these organic materials are measured, the manufacturers
of fiber-
cement siding have observed an inverse relationship between the amount of
these
materials in an unbleached pulp and the strength properties of the final
product. The
amount of these materials is commonly measured using the chemical oxygen
demand
(COD) test. When considering the detrimental effect of these materials on
strength
properties, it is apparent that there are a needs in the art for very low COD
unbleached
pulp. The present invention fulfills these needs and provides further related
advantages.
SUMMARY OF THE INVENTION
The present invention provides a process for making a low COD pulp,
comprising:
(a) repetitively soaking and washing unbleached pulp containing sugars and
other organic materials in water that is maintained at alkaline conditions
such that said pulp is
soaked in steps for a total of at least 400 minutes wherein the soaking in the
first step is
conducted at- a first elevated temperature of at least 60 C to produce a pulp
product having
sugars and other organic products reduced such that the pulp product has a COD
of less than
or equal to 3.0 kg per 100 kg; and
(b) adding alkali to said pulp while soaking and washing; and
(c) dewatering and drying said pulp while maintaining said alkaline
conditions.
BRIEF DESCRIPTION OF THE DR.AWINGS
The foregoing aspects and many of the attendant advantages of this invention
will
become more readily appreciated as the same become better understood by
reference to
the following detailed description and the accompanying drawings, wherein:
FIGiJRE 1 illustrates the steps of the caustic washing process for the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a low chemical oxygen demand (COD) pulp that is
particularly useful for reinforcing fiber cement products. The low COD
cellulosic pulp
that is useful in the present invention is most preferably an unbleached pulp
from a kraft
pulping process. However, a wide variety of pulped cellulosic fibers can be
used, which
are derived from wood and non-wood sources. Of all the pulp sources, wood pulp
is the
most commonly employed because of its availability and price.
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To obtain suitable wood pulp fibers, the sulfate pulping process is the most
preferred method. Using this process and considering the desired composite
properties,
the preferred wood fiber source is long-fibered coniferous wood species.
Examples of
these species include the following: Southern pine, Douglas fir, spruce,
hemlock, and
Radiata pine. In addition to these wood fiber sources, other chemical pulps
can be used
that include pulps made from short or long fibered wood species or recycled,
wood pulp
fibers. Short wood fibers, which are typically produced from hardwood species
such as
eucalyptus, can also be used. The processes to produce these wood pulp fibers
are well-
known to those skilled in the art of pulp manufacturing. These fibers are
commercially
available from a number of companies, including the Weyerhaeuser Company. In
contrast to wood pulp fiber sources, there are other natural cellulosic fiber
sources which
include straw, flax, kenaf, hemp, or similar materials. Like wood-based
fibers, these non-
wood fibers may also be pulped and subsequently used in fiber cement-based
composites.
Referring to FIGURE 1, unbleached pulp, preferably unbleached kraft pulp, is
first passed through conventional brown stock washers 10 and a brown stock
decker 20 at
a consistency of approximately 10% when it exits from the decker. Prior to
washing, the
unbleached pulp that is used in the present invention must have a relatively
low Kappa
number. Preferably the Kappa number is less than or equal to 30, but it is
more
preferably less than or equal to 25 3. The unbleached pulp from the decker
is at an
alkaline condition and must be maintained at an alkaline condition (pH equal
to or greater
than 7.0) while it is processed in accordance with the present invention. The
unbleached
pulp from the decker, which is at a consistency of approximately 10%, is
initially soaked
in the first diffusion tower 30 at an alkaline condition for a predetermined
length of time
and a predetermined elevated temperature. This first soaking step is
preferably conducted
at a temperature of at least 60 C, and more preferably at approximately 65 C
or higher.
The pulp is soaked in tower 30 preferably for at least 120 minutes. After
tower 30, the
pulp is washed and dewatered in press 40. However, this dewatering step is
optional, but
if it is employed, then the pulp may preferably be dewatered to a consistency
of
approximately 30%.
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After the initial step in which the pulp in tower 30 has been soaked and
dewatered, the pulp is again diluted to a 10% consistency it is then sent to a
set of
successive diffusion and washing steps. Optionally, the pulp can be placed in
a high
density storage tank 50 in which it can reside anywhere from 0 to 430 minutes.
However,
if the high density storage tank is employed during normal operation, the pulp
will reside
in the tank for approximately 100 minutes. The purpose of the high density
storage tank
is to prevent diffusion tower flow variations from passing to the downstream
processes
that are after the storage tank.
After the high density storage tank, the pulp is then introduced into the
first of a
series 60n of diffusion tanks at a consistency of approximately 10%. Hot water
and
steam or only steam are added to the pulp in a second of the series 60n of
diffusion tanks
to raise the temperature to a value that is preferably equal to or greater
than 80 C. The
pulp is then repetitively soaked in the remaining series 60n of diffusion
towers. After
soaking in each tower 60n, the pulp is subjected to washing in successive
washers 70n.
Preferably the pulp is soaked and washed five times in towers 60n and washers
70n
before it is forwarded to a storage tank 80. Each of the successive soaking
and diffusion
steps may take a minimum of 30 minutes but may take up to 100 or more minutes.
These
time periods include the washing steps that follow each soaking step. It is
preferred that
the repetitive soaking steps in diffusion towers 60n occur for a total of at
least
280 minutes but preferably 335 minutes and in either case at the elevated
temperature of
at least 80 C. It is preferred that the total soaking time in diffusion tower
30, storage
vessel 50, and diffusion towers 60n ranges from a minimum of 400 minutes (120
minutes
in diffusion tower 30, 0 minutes in vessel 50, 280 minutes in diffusion towers
60n) to a
maximuin of approximately 1,050 ininutes (120 minutes in diffusion tower 30,
430 minutes in vesse150, 500 minutes in diffusion towers 60n). Conventionally,
however, the soak will occur for approximately 555 minutes (120 minutes in
diffusion
tower 30, 100 minutes in vessel 50, 335 minutes in diffusion towers 60n).
After the pulp has been repetitively soaked and washed, it is forwarded to the
storage tank 80. In this storage tank, the pulp remains at a consistency of
about 10%.
Thereafter, the pulp is fed in a conventional manner to a conventional pulp
machine 100
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and dryer 110. After the pulp is dried into sheets, it is cut to size, sent to
storage 120, and
ultiinately shipped to a customer.
The unbleached and washed pulp produced by the process of the present
invention
has a COD of less than or equal to 3.0 kg per metric ton (1,000 kg), and
preferably less
5 than 1.7 to 2.8 kg/1000 kg. This level of COD is well below that achieved in
ordinary
pulp mills and particularly in kraft pulp mills.
A critical feature of the present invention is that the pulp must be
maintained at
alkaline conditions from the time it begins the initial soaking in tower 30
until the pulp is
dried. It is preferred that the pH be maintained at or above 7.0 throughout
the entire
process, from the soaking in the initial diffusion tower 30 through the
repetitive soaks in
towers 60n. Preferably in these steps, the pH is maintained in the range of
from 10.0
to 11Ø The pulp is then run through the pulp machine 100 and the dryer 110.
In these
steps, the pH may be reduced, e.g., to 8.0 to 8.5, but can be run lower. Under
certain
circuinstances, it may be necessary to add caustic solution (20% by weight
aqueous
sodium hydroxide) to the first diffusion tower 30. It has been found that the
addition of
caustic at the rate of at least 2 kg per metric ton, and more preferably 3 kg
per metric ton,
will be sufficient to maintain the alkalinity of the pulp above pH 7.0
throughout the
process. It is only necessary to add caustic during the initial portion of the
run of the
process. For example, at a throughput of about 40 to 50 tons per hour, the
caustic needs
to be added at the rate of 3 kg per metric ton for the initia124 to 48 hour
period that the
pulp is run through the initial tower 30. This will assure that the alkalinity
will be
maintained above pH 7.0 throughout the entire process, which includes the
drying stage.
In the most preferred embodiment, the present invention is carried out in a
converted oxygen delignification and bleaching plant that is normally
associated with a
Kraft pulp mill, which is used to convert unbleached pulp to bleached pulp. A
typical
bleach plant comprises an oxygen delignification reactor that is followed by a
series,
typically five, of bleach reactors, in which various bleaching agents such as
chlorine
dioxide are added. In accordance with the present invention, the bleach plant
is converted
to use with the present invention by first cutting off the supply of oxygen to
the oxygen
reactor, and thereafter, the supply of bleaching agents to the bleach reactors
is
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sequentially shut off as the pulp (with no oxygen added) sequentially enters
the bleach
reactors.
In this preferred embodiment, the oxygen reactor becomes the first diffusion
tower 30. At the same time that oxygen is cut off from the diffusion tower,
caustic is
introduced into the pulp via line 32 at the rates set forth above. In the
preferred mode of
operation, the pulp is resident in the oxygen reactor (tower 30) for
approximately
120 minutes. The pulp is then run through the press that is typically
associated with the
oxygen reactor (tower 30) to increase the consistency from approximately 10%
to
approximately 30%. The pulp is then diluted to 10% consistency and introduced
into a
high density storage vessel (vessel 50). The vessel is nonmally operated at a
partial
capacity so that the pulp residence time in the storage vessel is
approximately
100 minutes.
The pulp is then introduced into the first bleach reactor vessel (towers 60n)
that is
one of five total. Steam and hot water are added to the pulp when the pulp
enters the
second of the series 60n of reactor vessels to raise the temperature of the
pulp to 80 C or
higher. Depending on the size of the vessel, the residence time for the pulp
will vary. In
one particular plant, the residence time in the five bleach reaction vessels
was on the
order of 60 minutes, 30 minutes, 45 minutes, 100 minutes, and 100 minutes,
respectively.
This residence time is dependent on the size of the successive reactors. It is
very
important in this cycle that the temperature be maintained at or above 80 C.
This is
accomplished by adding steam to the vessels, as necessary, to maintain
temperatures.
After soaking in each bleach reactor vessel, the pulp is run through
conventional washing
unit (washers 70n) that normally following each of the bleach reactors.
Preferably, fresh
water is used to wash the pulp; however, white water from the associated pulp
plant may
be used, if necessary.
After the pulp from the fifth bleach reaction vessel is washed, it is placed
into the
vessel that is normally used for bleached pulp storage (vessel 80). It is
thereafter diluted
in successive steps to a consistency of about 1.5% and fed into a pulp head
box on the
pulp machine 100. The dewatered sheet produced on the pulp machine is then run
through the pulp dryer 110, for example, a Flakt~dryer. After drying, the
sheet is
subsequently cut, stacked, stored, and, thereafter sent to shipping
operations.
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If desired, the pulp machine and pulp dryer can be eliminated and the pulp can
be
introduced directly into a jet dryer. The jet dryer produces a dried
si.ngulated fiber,
particularly useful in some cementitious products. One suitable jet dryer for
use in the
present invention is a fluid energy Aijet Model Thermajet,~X0870L,
manufactured by
Fluid Energy Processing and Equipment Company. It is also possible to
completely sldp
the drying stage and use the pulp in a never-dried state.
EXAMPLE
The following example is intended for illustrative purposes only and is not
intended to in any way delimit the invention. Chemical oxygen demand (COD) is
determined by the following method. Pulp sheets produced in accordance with
the
preferred embodiment of the present invention just described are torn or cut
into small
pieces (approximately 4 cm square). The small pieces are mixed and the
moisture is
measured in accordance with Tappi procedure T412 om-94. Forty grams of pulp,
oven
dried weight, are then carefully weighed. A 2,000 ml solution of 0.01 N sodium
hydroxide solution is prepared using distilled or deionized water and
analytical grade
sodium hydroxide. Thereafter, the pulp is placed in 2,000 ml of the 0.01 N
sodium
hydroxide solution and placed in a disintegrater and disintegrated for 15
minutes at
3,000 rpm in a British Pulp Evaluation Apparatus (or British disintegrater)
described in
Tappi 505 sp-95. The pulp slurry is then vacuum filtered immediately after
disintegration
TM
using a'Whatman No. 3 filter paper. The filtration time is long enough so that
a majority
of the filtrate is passed through the filter. Two hundred and fifty ml of
filtrate is
separated for COD analysis. A sample is preserved with 2.5 ml of 50% sulfinic
acid.
Thereafter, the COD of the filtrate is measured using the titration method
described in
Standard Methods for the Examination of Water and Wastewater, 20th Edition,
1998,
Method #5220C, "Closed Reflux, Titrimetric Method". The COD content is then
calculated as kilograms per metric ton of pulp based on the oven dried weight
of the pulp.
The pulp samples were randomly selected from several production runs through
the converted oxygen delignification and bleach plant described above.
Unbleached pulp
was run at the rate of 45 tons per hour through the converted oxygen
delignification and
bleach plant. Residence time in the oxygen reactor vessel was on the order of
120 minutes. Residence times in the successive bleach vessels was 60, 35, 45,
100, and
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100 minutes respectively. Pulp samples were taken during production runs
downstream
from the Flakt pulp dryer and tested for COD as set forth above. Samples A, B,
C, D, and
E were taken over a three-day treatment period. Samples C5, C7, C9, Cl 1, and
C13 were
taken over a nine-day treatment period. The results are set forth below.
SAMPLE ID COD (kg/metric ton)
E 2.34
C13 2.36
C 2.53
B 2.54
Cl l 2.55
A 2.63
D 2.73
C7 2.75
C5 2.78
C9 2.85
The pulp produced in accordance with the foregoing example was incorporated
into cementitious products at about 8% by weight pulp using the Hatsheck
process. (The
pulp content may be varied from 6% to 10% by weight if desired.) Cement panels
and
planks were produced for use in residential and commercial construction.
Tilebacker
boards used for ceramic tile underlayment were also produced. All of these
products
exhibited excellent strength characteristics.
While the preferred embodiinent of the invention has been illustrated and
described, it will be appreciated that various changes can be made therein
without
departing from the spirit and scope of the invention.
