Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02262186 1999-02-17
Docket No. 028948-257
PATENT
COPPER DISSOLVER PROCESS
BACKGROUND OF THE INVENTION
This invention relates to a process for dissolving
copper metal in a dilute solution of monoethanolamine (MEA)
and to the apparatus for efficiently carrying out the
process. The invention is particularly useful as a source
of copper in the treatment of wood.
The treatment of wood against decay and insect
infestation and to provide color preservation of wood is
well known. A particularly effective wood preservative is
copper dimethyldithiocarbamate. Commonly assigned U.S. Pat.
Application No. 08/786,823, filed January 21, 1997
i(presently allowed) is directed to a process for the in situ
formation of copper dimethyldithiocarbamate directly in the
wood matrix by reacting carbon disulfide, dimethylamine and
a water-soluble source of copper, such as copper hydroxide
solubilized by a complexing agent such as monoethanolamine,
in the wood being treated. One step of the process involves
rinsing the wood with an aqueous solution containing about
2.5 weight percent monoethanolamine (MEA). A portion of the
rinse solution is charged with cupric ion to produce a
solution enriched in copper content for use in another
process step. Typically, the source of the copper ion is
copper hydroxide. However, the cost of copper hydroxide is
relatively high and an alternative source of copper ion is
therefore desirable.
. U.S. Pat. No. 4,808,407 to Hein discloses a multi
step process for preparing soluble copper salts of
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carboxylic acids wherein a source of copper, such as copper
metal, is reacted with a carboxylic acid and an alkanolamine
is a first step and the mixture is subsequently treated with
oxygen at an elevated temperature of about 70-95°C (158-
203°F) until the desired copper salt is obtained.
SUMMARY OF THE INVENTION
The present invention involves a process wherein
copper metal is dissolved in an aqueous solution of
monoethanolamine (MEA) at ambient temperature to provide a
copper-monoethanolamine complex wherein the complex consists
of at least one mole monoethanolamine per mole of copper.
Typically, the complex contains two moles monoethanolamine
per mole of copper. In accordance with the process, the
monoethanolamine solution and air are passed through a
reactor bed containing copper metal, such as shredded copper
wire, at a solution-circulation rate and an aeration rate
sufficient to effect the formation of copper-MEA complex.
The apparatus for carrying out the process comprises a
column which contains the bed of copper metal, a tank which
holds the monoethanolamine solution, a centrifugal pump for
pumping the monoethanolamine solution through the bed of
copper metal, and an air compressor for forcing air upward
through the bed of copper metal.
Accordingly, it is an object of this present
invention to provide a process for dissolving copper metal
in an aqueous solution of monoethanolamine in a single step.
It is another object of the present invention to
provide an apparatus for use in dissolving copper metal in
an aqueous solution of monoethanolamine.
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It is yet another object of the present invention
to provide a relatively inexpensive source of copper ions
for use in a process for preserving wood.
These and other objects and advantages of the
present invention will become apparent from the following
detailed description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow diagram of a pilot plant
illustrating the process of the present invention;
FIG. 2 is a graph illustrating the effect of
temperature on the dissolution of copper in the present
invention;
FIG. 3 is a graph illustrating the effect of
circulation rate of monoethanol-amine solution on the
dissolution of copper in the present invention;
FIG. 4 is a graph illustrating the effect of flow
rate of air on the dissolution of copper in the present
invention;
FIG. 5 is a graph illustrating the effect of
oxygen content on the dissolution of copper in the present
invention; and
FIG. 6 is a graph illustrating the effect of the
amount of copper in the bed on the dissolution of copper in
the present invention.
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DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention an aqueous
solution of monoethanolamine and air are passed through a
bed of copper metal to provide a solution of copper-mono-
ethanolamine. In a preferred aspect of the invention, the
process for dissolving copper metal in monoethanolamine is
effective for providing a source of copper ion useful in the
preservation of wood, particularly, in the process described
in the above mentioned commonly assigned, U.S. Pat.
Application No. 08/786,823, where copper hydroxide is
disclosed as the source of cupric ion. The copper metal of
the present invention is economically more attractive as the
source of copper ion than the relatively expensive copper
hydroxide. The present process is especially favorable
since the copper ions may be obtained from scrap copper.
Copper dissolution in the MEA solution typically
is a three step process, mechanistically involving:
(1) oxygen dissolution at the gas-liquid interface;
(2) oxygen transfer to the copper surface; and
(3) a reaction at the copper surface with oxygen and
MEA.
At any point in time, the rates of the three processes are
equal. The rate of copper dissolution per unit volume of
bed = kla [Poz/Hoz-Coz] =Kiwi [Coz-Cozsl =KrapCozs CMEA (1) where:
Coz - Oz concentration in the bulk solution
(moles/liter)
C02s - Oz concentration at the copper surface
(moles/liter)
Poz - Partial pressure of Oz in air
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Hoz - Henrys Law constant for Oz in solution
aP - surface area of copper packing per unit volume
of bed
al - effective area of packing for liquid - solid
mass transfer
klal - volumetric mass transfer coefficient at air -
liquid interface (1/time)
klsal - volumetric mass transfer coefficient between
solution and copper
kr - rate constant for copper dissolution. If the
expression krapCozSCM~, has dimensions of (gm-
atoms) /liter-time) , and aP = (SP/VP) (1-a) has
dimensions of cm2/cm3 (where 1-E is the packing
volume fraction in the bed) , then kr = (cm-
liter)/gmmole-time)
El iminating Coz and Cozs f rom equat ion ( 1 ) in terms of Poz/Hoz
the rate - [k apCM~p,Po2/Hoz] / [1+kraPCMEA/klsal+krapCMEA/kla] (2)
r
~y material balance, the rate of disappearance of MEA per
unit volume of copper bed equals twice the rate of copper
dissolution per unit volume of copper bed. Therefore: The
rate=-dCMEA/dt=2KrCMeA(Poz/Hoz) (Ub/US)
(3)
where : Kr - kraP/ [1+kraPCM~,/klsal+krapCM~"/kla]
Vb - volume of the copper bed (including
voidage)
VS - volume of the solution
If one can assume that all terms except CMEA are essentially
constant throughout the reaction process, then one can
easily integrate (3) to obtain the following expression:
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In [CMEAo/CMEA~t>~ =2krap (Poz/Hoz) (Vb/VS) t-krap [1/ki5ai+1/kia]
[CMEFIo/CMEA~t~ ~ (4 )
where: CMEAo = initial MEA concentration of the
solution
CMEA~t~ - MEA concentration of solution at time t
t - time (hr)
Equation (4) can be rearranged to solve for t:
t = In [CMEAo/CMEA~t) ] + krap [1/klsal+1/kla] [CMSao-CMer.~c) ] (5)
[2krap (Po2/Ho2) (vb/vs) ]
Equation (5) describes how various parameters affect the
rate of dissolution of copper in the system. To increase
the rate of dissolution of copper (minimize t), one can
increase the temperature of the solution (increase Kr),
increase the surface area of the copper metal (increase ap),
increase the flowrate of the solution through the copper bed
( increase klsal ) , increase the f lowrate of air ( oxygen)
through the column (increase kla), increase the content of
oxygen in the air passing through the bed (increase Poz),
increase the amount of copper in the column (increase Vb),
and decrease the amount of solution in the batch (decrease
VS ) .
The copper-dissolving reaction of the present invention
is carried out using a monoethanolamine solution temperature
of about ambient to about 120°F. Typically, the solution
temperature is at ambient temperature (about 65-75°F).
The aqueous monoethanolamine solution typically has a
monoethanolamine concentration of about 1% to 50%, and
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preferably about 15% to 25%, and most preferably about 20
weight percent. The circulation rate of the
monoethanolamine solution upward through the copper bed is
about 20 to 40 gpm/sq. ft. (gallons per crossectional sq.
ft. of the column) and preferably about 35 to 40 gpm/sq. ft.
The flow rate of air upward through the copper bed is
about 5 to 30 cfm/sq. ft., and preferably about 20 to 25
cfm/sq. ft. Generally, atmospheric air is sufficient for
use in the present invention; however, air enriched with
oxygen tends to produce a slight increase in the rate of
dissolution of the copper.
The copper metal useful in the invention may be new
copper or scrap copper and is present in a reaction column
as a packed bed of copper metal. The bed is supported by a
porous structure such as a screen. Preferably, the copper
is in the form of shredded copper wire, typically about 10
to 20 gauge is effective to provide the desired results.
The amount of copper metal employed in the process is
dependent upon the concentration of the monoethanolamine
solution, the circulation rate of the MEA solution, the flow
rate of air, and the temperature of the reaction.
While the process is most preferably directed to the
use of aqueous solutions of monoethanolamine, aqueous
solutions of other amines, such as alkylamines,
alkanolamines, particularly other monoalkanolamines, may be
used, to the extent of their solubility in water and their
ability to complex with copper metal, in carrying out the
invention.
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EXPERIMENTAL
A pilot plant was constructed to measure the rate of
dissolution of copper in accordance with the present
invention. As illustrated in Fig. 1, the pilot plant 10
consisted of a glass column 12 (2" in diameter) filled with
shredded scrap copper wire 14 (10-14 gauge). The copper
wire 14 was supported by a heavy gauge stainless steel
screen 16. An aqueous solution of 2.5 wt.% monoethanolamine
was pumped from a holding tank 18 to the column 12 via
conduit 20 using a Wilden m. 025 diaphragm centrifugal pump
22 where the solution passed upwards through the bed of
copper wire 14. The solution containing copper-
monoethanolamine overflowed from the column 12 back to the
holding tank 18 via conduit 24. Simultaneously, air was
passed into column 12 and upwards through the bed of copper
wire 14 via conduit 28 using compressor 26. After passing
through the bed of copper wire 14, the air was released to
the atmosphere through opening 30.
Several runs were made in the pilot plant for
determining the effects of (1) temperature of the solution;
(2) flow rate of the monoethanolamine solution; (3) oxygen
content of the air passed through the copper bed; and (4)
the amount of copper in the bed. The results of the runs
accord with equation (5).
Runs 1, 2 and 3 were made with solution temperatures of
70°F, 100°F and 120°F, respectively. Runs 4 and 5 were
made
' at temperatures of 70°F and 95°F, respectively and, in runs
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4 and 5, 50 SCFH air was passed through the copper bed. As
seen in Fig. 2 an increase in temperature of the solution
from ambient only slightly increased the rate of dissolution
of copper. Any increase in Kr attained by increasing the
temperature of the solution, however, is largely negated by
a decrease in the solubility of oxygen (Po2/Ho2) since the
solubility of oxygen decreases with increasing temperature.
Furthermore, it was noted that, as the temperature of the
solution was increased, a black insoluble precipitate formed
on the elements of the heater.
Runs 6 and 7 were made varying the circulation rate of
the monoethanolamine solution. As seen in Fig. 3, an
increase in the circulation of the monoethanolamine solution
through the copper bed increases the rate of dissolution of
copper by increasing the mass transfer coefficient klSa1-
Runs 8, 9, 10 and 11 were made varying the flow rate of
air through the copper bed. As seen in Fig. 4, an increase
in the flow rate of air passing upward through the copper
bed significantly increases the rate of dissolution of
copper by increasing the mass transfer coefficient kla. As
the flow rate of air increases through the copper bed for a
given circulation rate of the solution, the hydrodynamic
conditions in the copper bed approach pluse-flow regime in
which the copper bed is near fluidization.
Runs 12 and 13 were made varying the oxygen content of
air passed through the copper bed. As seen in Fig. 5, the
addition of oxygen to the air passing through the copper bed
at atmospheric pressure produces only a slight increase in
the rate of dissolution of copper.
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Runs 14, 15 and 16 were made varying the amount of
copper in the bed. As seen in Fig. 6, increasing the amount
of copper.
Based on these experimental runs, the most significant
process parameters of a copper dissolution process are the
circulation rate of the monoethanolamine solution upward
through the copper bed, the air flow rate upward through the
copper bed, and the amount and size of the copper metal in
the bed.
In a typical reaction scheme, circulation of the
monoethanolamine solution (about 2.5o wt. % of MEA) through
a packed bed of shredded copper wire (10 to 20 gauge) at a
circulation rate of about 37 gpm/sq. ft. of packed copper
metal in the bed and with an aeration rate of about 23
cfm/sq. ft. of packed copper, will provide conditions
suitable for dissolving the copper metal to provide about
1.0 to 1.2 wt. % of copper-MEA in the monoethanolamine
solution at about 70°F.
Having described the invention in detail by reference
to preferred aspects thereof, it will be apparent that
modifications and variations are possible without departing
from the scope of the invention defined in the appended
claims.
What is claimed is:
