Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REMOVAL OF WATER-SOLUBLE COMPOUNDS FROM
WOOD CHIPS PRIOR TO COOKING
FIELD OF THE INVENTION
The present invention relates generally to the field of wood chip
treatments. In especially preferred embodiments, the present invention
relates to treatment of wood chips prior to cooking in a pulp manufacturing
process.
BACKGROUND AND SUMMARY OF THE INVENTION
The presence of some water-soluble compounds can have
1o deleterious effects in pulping processes. For example, compounds which
disassociate in water to form potassium and/or chloride ions, lower the
melting point of fly ash in the recovery boiler of a wood pulp mill. This
reduced melting point of the fly ash in turn requires that the recovery boiler
be operated at a lower temperature, thereby reducing the efficiency of the
boiler. To overcome this problem, it is highly desirable that water-soluble
compounds be removed from wood chips prior to being cooked.
Similarly, water soluble metal compounds such as manganese,
calcium, potassium and the like should be removed prior to cooking of wood
chips so as to ensure that the resulting pulp may be more easily bleached.
In this regard, manganese is especially harmful to the bleaching process
and is known to cause poor bleaching results. Moreover, the removal of
calcium results in reduced scaling in the digester and evaporators.
Broadly, the present invention is embodied in processes whereby
wood chips are treated prior to cooking so as to remove substantially (e.g.,
at least about 30 wt.%, more preferably at least about 50 wt.% and most
preferably at least 70 wt.%) water-soluble compounds, especially those
compounds which disassociate in water to form potassium and/or chloride
ions.
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More specifically, as described briefly above, wood chips entering the
cooking system contain water-soluble compounds, such as those
compounds which disassociate in water to form potassium and/or chloride
ions. According to the present invention, wood chips are treated prior to
cooking by being brought into contact with an aqueous treatment stream
(e.g., water or steam condensate) so as to reduce substantially (e.g., at
least about 30 wt.%) such potassium- or chloride-containing water-soluble
compounds. By treatment of the wood chips in accordance with the present
invention, water-soluble compounds, specifically potassium- or chloride-
1o containing water-soluble compounds, present in the wood chips are
displaced from the interior of the chips and accumulate in the aqueous
treatment liquid present. The aqueous treatment liquid containing the water-
soluble compounds may then be drained from the chips prior to the chips
entering the cooking system. These wood chips can be treated (washed)
multiple times so as to sequentially increase the amount of water-soluble
compounds, specifically potassium- and/or chloride-containing compounds,
removed therefrom.
These and other aspects and advantages will become more apparent
after careful consideration is given to the following detailed description of
the
preferred exemplary embodiments thereof.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Reference will hereinafter be made to the accompanying drawings,
wherein like reference numerals throughout the various FIGURES denote
like structural elements, and wherein;
FIGURES 1-3 each represent a schematic view of a possible wood
chip treatment technique in accordance with the present invention;
FIGURES 4A and 4B are graphical plots in accordance with the
Examples below of potassium and chloride ion concentrations (mg/L) in chip
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filtrate versus leach time for 2-3 mm unsteamed and steamed wood chips,
respectively;
FIGURES 5A and 5B are graphical plots in accordance with the
Examples below of potassium and chloride ion concentrations (mg/L) in chip
filtrate versus leach time for 4-6 mm unsteamed and steamed wood chips,
respectively;
FIGURES 6A and 6B are graphical plots in accordance with the
Examples below of potassium and chloride ion concentrations (mg/L) in chip
filtrate versus leach time for 8-10 mm unsteamed and steamed wood chips,
lo respectively; and
FIGURE 7 is plot of the percentage of total potassium removed from
three different wood chip size categories in accordance with the Examples
below.
DETAILED DESCRIPTION OF THE INVENTION
The aqueous wash stream that may be employed in the practice of
the present invention can be virtually any stream which is capable of being
processed in the pulp mill's existing wastewater treatment system. Such
streams include, but are not limited to, water, evaporator (and/or other mill)
condensates, collected rainwater, and bleach plant effluent. The wash may
2o advantageously be carried out at temperatures between about 20 C to
about 120 C (e.g., advantageously between about 30 C to about 90 C), at
pressures up to the operating pressure of the downstream digester. The
residence time in which the wood chips may be in contact with the aqueous
wash liquid can be from about 2 to about 200 minutes. The wash is most
preferably conducted at a pH of between about 2 to about 12, and more
preferably between about 4 to about 10. It is especially preferred that the
wash be conducted at elevated pressures which, as noted above, can be up
to the pressure condition of the downstream digester.
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Various techniques may be employed in accordance with the present
invention. For example, as shown in accompanying FIGURE 1, one system
includes a chip bin 12, such as disclosed in U.S. Patent No. 5,500,083 and
available commercially from Andritz Inc. of Glens Falls, New York as
5 DiamondbackO Steaming Vessel. Steam may be injected into the wood chips
contained within the chip bin 12 via line 12-1. The chip bin 12 is provided
with
a meter screw 14, such that condensate is capable of being drained therefrom
via line 16. Such drained condensate may then be sent to the pulp mill's
existing wastewater treatment system (not shown). The chip bin 12 shown in
10 FIGURE 1 could, if desired, be provided without steaming, but with a meter
screw and a horizontally or vertically disposed wash vessel (not shown)
downstream of the meter screw where wash liquid may be introduced. The
wash effluent may thus be drained from the wash vessel and processed in the
pulp mill's existing wastewater treatment system.
The system 20 shown in FIGURE 2 includes a similar chip bin 12 as
depicted in FIGURE 1, in that it is steam may be injected into the wood chips
contained therein by line 22-1 and that it is provided with a meter screw 24.
In
addition, the chip bin 22 is provided with a plurality of serially arranged
horizontally or vertically disposed wash vessels 25, 26 downstream thereof
where wash liquid may be introduced. The chips may be washed with water
or other suitable aqueous stream via line 27. The liquid may be drained from
an initial one of the wash vessels 25 via line 25-1 and then directed to a
second downstream wash vessel 26 where the chips are washed a second
time. Liquid drained from the second wash vessel may be divided into
respective portions, with one portion thereof being employed as wastewater
treatment via line 26-1, and another portion thereof used to flow counter-
current via line 26-2 with the wood chips in the first wash vessel 25. Thus,
with the series of wash vessels 25, 26 as depicted in FIGURE 2, each one
has the ability to gather the drained wash liquid and return it to the
preceding
wash vessel or be sent to the pulp mill's existing wastewater treatment
system.
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The last wash vessel in the series could be, for example, an in-line
separator, while an intermediate wash vessel could be a retention vessel to
increase the retention time of wash material in contact with wood chips. Such
a system 30 is depicted in the embodiment depicted in accompanying
FIGURE 3. Alternatively, instead of a dedicated wash vessel, the final chip
wash could occur physically in the top-separator of the digester.
More specifically, as shown in FIGURE 3, the chip bin 32 includes a
meter screw 34 and is provided with a line 32-1 to allow steam injection into
the wood chips contained therein. A series of wash vessels 35, 36 and 37 are
provided to allow the wood chips to be sequentially washed in countercurrent
fashion via fresh wash water supplied via lines 39-1, 39-2 and 39-3,
respectively. As noted briefly above, the last wash vessel 37 in the series
could be, for example, an in-line separator. A retention vessel 40 is
preferably
interposed in the chip flow between the initial was vessel 35 and the
intermediate wash vessel 36. The retention vessel serves to increase the
retention time of wash material in contact with wood chips. The liquid may be
drained from wash vessel 35 via line 35-1.
Liquid drained from the intermediate and final wash vessels 36, 37 may
be divided into respective portions, with one portion thereof being employed
as wastewater treatment via lines 36-1 and 37-1, respectively, and another
portion thereof used to flow counter-current via lines 36-2 and 37-2 with the
wood chips in the first and intermediate wash vessels 35 and 36, respectively.
Thus, as was the case with the series of wash vessels 25, 26 as depicted in
FIGURE 2, the wash vessels 36, 37 in the system 30 shown in FIGURE 3 has
the ability to gather the drained wash liquid and return it to the preceding
wash vessel or be sent to the pulp mill's existing wastewater treatment
system.
It has been found that the removal of water-soluble compounds can be
accomplished. In one specific embodiment of the invention, it has been
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found that water-soluble potassium can be removed from wood chips. More
specifically, in laboratory experiments carried out at atmospheric pressure
and at temperatures ranging from 30 C to 90 C with wash liquid contact
time of about 30 minutes, potassium-containing compounds may be
reduced substantially in an amount of at least about 30 wt.% (e.g., between
about 30 wt.% to about 40 wt.%). The operational conditions used in the
laboratory experiments were selected due to the practical limitations existing
in the laboratory and are thus non-limiting to the present invention. That is,
other operational conditions may be employed within the context of the
1o present invention. Based on the results of the laboratory experiments, it
is
expected that potassium removal will increase at higher temperatures
and/or operating pressures. It is also expected that the commercial
operating conditions will be different from those used in the laboratory,
specifically higher temperature, higher pressure and longer retention time.
The present invention will be further understood from the following
non-limiting Examples.
EXAMPLES
Wood chips were tested for moisture content before any treatment
was performed. Chips were separated into two portions - that is, one
portion for those chips to be pre-treated with steam prior to the leaching
treatment and another portion for those chips to be treated only by the
leaching treatment (i.e., without any steam pre-treatment). The condensate
formed during the steaming treatment was drained and tested for potassium
content. Chips to be pre-treated with steam were segregated into three
different chip size categories - that is, chips having a size, on average, of
2-
3 mm, 4-6 mm and 8-10 mm, respectively. Numerous bags containing 50g
of wood chips for each chip size category were prepared. Respective bags
of wood chips were then subjected to each of three different temperature
regimes (i.e., 30 C, 60 C and 90 C) and five different retention time regimes
(i.e., 1 min., 2 min., 5 min., 10 min. and 30 min.). A total of 45 bags of
chips
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was prepared for the steamed wood chips. The same procedures as
described above were also followed for preparation of wood chip-containing
bags for the non-steam pre-treated chips.
A container of water was heated to the desired temperature (i.e.,
30 C, 60 C and 90 C). Once at temperature, a bag of chips was introduced
and held at temperature for the desired retention time. After reaching the
desired retention time, the heating was terminated and the filtrate tested for
chloride and potassium content. Standard Method 4500 (Standard
Methods, 18th Ed., Meruric Nitrate Method 4500-Cl, Section C, 1992) was
1o used for chlorides and Standard Method 3111 was used for potassium
(Standard Methods, 18th Ed., Metals by Flame Atomic Absorption
Spectrometry 3111 A & B, 1992)
The concentrations of potassium and chloride present in the filtrate
versus the leach time were plotted for each of the unsteamed and steamed
wood chips size categories and appears as FIGURES 4A-4B, 5A-5B and
6A-6B, respectively.
The percentage of potassium removed by steaming and leaching was
also calculated and such data appear in FIGURE 7.
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While the invention has been described in connection with what is
presently considered to be the most practical and preferred embodiment, it
is to be understood that the invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover various modifications
and equivalent arrangements included within the spirit and scope of the
appended claims.
