U.S. patent application number 09/983507 was filed with the patent office on 2002-04-11 for collector of dissolved metal from sea water having an amidoxime group and a hydrophilic group, a method for production thereof, a collecting cassette comprising laminated collectors, and a method for adsorbing and collecting dissolved metal from sea water by the cassette.
Invention is credited to Katakai, Akio, Seko, Noriaki, Sugo, Takanobu.
Application Number | 20020041936 09/983507 |
Document ID | / |
Family ID | 26580008 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020041936 |
Kind Code |
A1 |
Sugo, Takanobu ; et
al. |
April 11, 2002 |
Collector of dissolved metal from sea water having an amidoxime
group and a hydrophilic group, a method for production thereof, a
collecting cassette comprising laminated collectors, and a method
for adsorbing and collecting dissolved metal from sea water by the
cassette
Abstract
A collector is disclosed that is made of a polyolefin fiber
having amidoxime groups and that is capable of efficient adsorptive
recovery of useful metals such as uranium, vanadium, cobalt and
titanium which are dissolved in small quantities in seawater. In
the presence of a polymerizable monomer having a hydrophilic group,
a polymerizable monomer having a cyano group is grafted to a
polyolefin fiber by radiation-initiated graft polymerization to
form both a hydrophilic group and a cyano group in the same graft
side chains, and the cyano groups in the graft side chains are
reacted with hydroxylamine to be converted to amidoxime groups,
thereby producing a collector capable of recovering dissolved
metals from seawater.
Inventors: |
Sugo, Takanobu; (Gunma-ken,
JP) ; Katakai, Akio; (Gunma-ken, JP) ; Seko,
Noriaki; (Gunma-ken, JP) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Family ID: |
26580008 |
Appl. No.: |
09/983507 |
Filed: |
October 24, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09983507 |
Oct 24, 2001 |
|
|
|
09460511 |
Dec 14, 1999 |
|
|
|
Current U.S.
Class: |
427/532 |
Current CPC
Class: |
B01J 20/26 20130101;
Y10T 442/20 20150401; C02F 2101/006 20130101; Y02P 10/234 20151101;
Y10T 428/31504 20150401; C02F 1/285 20130101; B01J 20/28023
20130101; C02F 2101/20 20130101; Y02P 10/20 20151101; C22B 3/24
20130101; D06M 14/28 20130101; C22B 60/0204 20130101; B01J 20/28052
20130101; B01J 20/3242 20130101; B01J 20/28033 20130101; C02F
2103/08 20130101 |
Class at
Publication: |
427/532 |
International
Class: |
B05D 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 1998 |
JP |
354194/1998 |
Dec 14, 1998 |
JP |
354197/1998 |
Claims
What is claimed is:
1. A collector of dissolved metals from seawater that is made of a
polyolefinic fiber having both an amidoxime group and a hydrophilic
group in the same graft side chains.
2. The collector according to claim 1, wherein the molar ratio of
the amidoxime group to the hydrophilic group is in the range of
70:30 to 30:70.
3. The collector according to claim 1, wherein the molar ratio of
the amidoxime group to the hydrophilic group is in the range of
60:40 to 40:60.
4. The collector according to claim 1, wherein the molar ratio of
the amidoxime group to the hydrophilic group is 50:50.
5. A process for producing a collector of dissolved metals from
seawater which comprises the steps of grafting a polymerizable
monomer having a cyano group onto a polyolefin fiber in the
presence of a polymerizable monomer having a hydrophilic group and
then reacting the cyano groups in the graft side chains with
hydroxylamine to convert them to amidoxime groups, whereby both
amidoxime and hydrophilic groups are introduced into the same graft
side chains.
6. A collector for adsorptive recovery of dissolved metals from
seawater that is produced by grafting a monomer having a cyano
group onto a substrate fiber in the form of either a nonwoven or
woven cloth that is made of a polyolefinic fiber and converting the
cyano groups in the resulting graft side chains to amidoxime
groups.
7. A collector for adsorptive recovery of dissolved metals from
seawater that is produced by grafting a polymerizable monomer
having a hydrophilic group onto a substrate fiber in the form of
either a nonwoven or woven cloth that is made of a polyolefinic
fiber, then grafting a monomer having a cyano group, and converting
the cyano groups in the resulting graft side chains to amidoxime
groups, thereby introducing both a hydrophilic group and an
amidoxime group in different side chains.
8. A collector for adsorptive recovery of dissolved metals from
seawater that is produced by grafting a monomer having a cyano
group onto a substrate fiber in the form of either a nonwoven or
woven cloth that is made of a polyolefinic fiber in the presence of
a polymerizable monomer having a hydrophilic group, and then
converting the cyano groups in the resulting graft side chains to
amidoxime groups, thereby introducing both a hydrophilic group and
an amidoxime group in the same side chains.
9. A collector for separation and recovery of dissolved metals from
seawater that is produced by grafting a monomer having a cyano
group onto a substrate fiber in the form of either a nonwoven or
woven cloth that is made of a fiber of a core/sheath structure that
has a polyolefinic fiber coated with a different polyolefin and
converting the cyano groups in the resulting graft side chains to
amidoxime groups.
10. A collector for separation and recovery of dissolved metals
from seawater that is produced by grafting a polymerizable monomer
having a hydrophilic group onto a substrate fiber in the form of
either a nonwoven or woven cloth that is made of a fiber of a
core/sheath structure that has a polyolefinic fiber coated with a
different polyolefin, then grafting a monomer having a cyano group,
and converting the cyano groups in the resulting graft side chains
to amidoxime groups, thereby introducing both a hydrophilic group
and an amidoxime group in different side chains.
11. A collector for adsorptive recovery of dissolved metals from
seawater that is produced by grafting a monomer having a cyano
group onto a substrate fiber in the form of either a nonwoven or
woven cloth that is made of a fiber of a core/sheath structure that
has a polyolefinic fiber coated with a different polyolefin in the
presence of a polymerizable monomer having a hydrophilic group, and
then converting the cyano groups in the resulting graft side chains
to amidoxime groups, thereby introducing both a hydrophilic group
and an amidoxime group in the same side chains.
12. The collector according to any one of claims 6-11, wherein the
polymerizable monomer having a hydrophilic group is at least one
member of the group consisting of acrylic acid, methacrylic acid,
2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, allyl
alcohol, polyethylene glycol acrylate, polyethylene glycol
methacrylate, polyethylene glycol diacrylate, polyethylene glycol
dimethacrylate, N-vinylpyrrolidone, acrylamide and mixtures
thereof.
13. The collector according to any one of claims 6-11, wherein the
polymerizable monomer having a cyano group is at least one member
of the group consisting of acrylonitrile, vinylidene cyanide,
crotononitrile, metacrylonitrile, chloroacrylonitrile,
2-cyanomethyl acrylate, 2-cyanoethyl acrylate and mixtures
thereof.
14. A cassette of the collectors according to any one of claims
6-13 which are stacked and fixed at suitable spacings.
15. A method for adsorptive recovery of dissolved metals from
seawater, in which cages each containing the cassette according to
claim 14 are attached to a rope at given spacings and anchored in
seawater.
Description
BACKGROUND OF THE INVENTION
[0001] (a) This invention relates to a collector made of a
polyolefinic fiber having an amidoxime group and a hydrophilic
group and which is capable of efficient adsorptive recovery of
useful metals such as uranium, vanadium, cobalt and titanium that
occur dissolved in small quantities in seawater. The invention also
relates to a process for producing the collector.
[0002] Seawater has various metals (see Table 1) dissolved in it
and the present invention aims at recovering these dissolved metals
by adsorption using a collector.
1TABLE 1 Total estimated Concentration dissolved Dependency Rare
metal in seawater, quantity, on overseas, sources (mg/ton) (x
10.sup.8 tons) (%) Cobalt (Co) 0.1 1 100 Yttrium (Y) 0.3 3 100
Titanium (Ti) 1 15 100 Manganese (Mn) 2 30 90 Vanadium (V) 2 30 100
Uranium (U) 3 45 100 Molybdenum (Mo) 10 150 100 Lithium (Li) 170
2,330 100 Boron (B) 4,600 63,020 100 Strontium (Sr) 8,000 109,600
100
[0003] (b) The invention relates to a collector that is produced by
introducing an amidoxime group, either alone or in combination with
a hydrophilic group, into side chains grafted to a polyolefinic
fiber substrate and which needs only to be anchored in seawater to
accomplish efficient recovery of useful metals such as vanadium,
cobalt, uranium and titanium that are dissolved in the seawater.
The invention also relates to a cassette of such collectors and a
method of collecting the above-mentioned useful metals from
seawater using the cassette.
[0004] To produce the collector of the invention, a polymerizable
monomer such as acrylonitrile (CH.sub.2.dbd.CHCN) that contains a
cyan group (--CN) is grafted onto a polyolefinic fiber substrate by
radiation-initiated graft polymerization so as to form grafted side
chains and the cyan groups in these side chains are reacted with
hydroxylamine (NH.sub.2OH) or the like to be converted to amidoxime
groups.
[0005] A plurality of the thus produced collectors may be
sandwiched between nets and a plurality of the resulting assemblies
are stacked in position at suitable spacings to construct a
collector cassette. The cassette may be placed in a number of cages
that are anchored in seawater to recover useful dissolved metals
from it by adsorption. (a) Conventionally, amidoxime groups are
introduced into a polymer structure in accordance with the
following scheme (1) by reacting the cyano group (--CN) with
hydroxylamine (NH.sub.2OH): 1
[0006] To synthesize a satisfactory amidoxime resin by introducing
amidoxime groups into a polymer structure, the introduction of
amidoxime groups into substrates typically made of the
general-purpose polyacrylic fiber or polyacrylic beads produced by
emulsion polymerization. However, these acrylic resins have
suffered from deterioration in skeletal strength of the polymer on
account of the introduction of hydrophilic amidoxime groups into
the cyano groups in the polymer skeleton. With a view to preventing
this problem, a review has been made to form crosslinks in the
polymer structure. In fact, however, the increase in the degree of
crosslinking is accompanied by a decrease in the rate of metal
adsorption and this tradeoff has been an obstacle to the solution
of the problem.
[0007] It is known that a collector that is capable of selective
adsorptive recovery of dissolved metals from seawater can be
produced by grafting acrylonitrile onto a polyethylene fiber under
exposure to radiation and then reacting it with hydroxylamine to
introduce amidoxime groups (see the following newspaper articles on
May 27, 1998 based on the announcement by one of the inventors of
the present invention; page 1 of the morning edition of Kagaku
Kogyo Nippo, page 5 of the morning edition of Nikkei Sangyo
shinbun, and page 21 of the morning edition of Nikkan Kogyo
Shinbun).
[0008] It is also known that a selective adsorbent of uranium
dissolved in seawater can be produced from a substrate of a desired
shape that is made of an inorganic material, an organic material or
a composite thereof and into which both an amidoxime group and a
hydrophilic group are introduced by radiation-initiated graft
polymerization (see Japanese Patent Publication No. 56775/1987
filed by one of present inventors).
[0009] Under the circumstances, there has been a pressing need to
improve the existing collectors and develop a material that is
strong enough to withstand prolonged exposure to hostile weather
conditions in ocean and which maintains high performance in
collecting vanadium, uranium and other useful metals in seawater.
(b) In seawater, vanadium, uranium and many other rare metals that
scarcely occur in Japan are contained dissolved but their
concentrations are extremely low, only about 1.9 mg of vanadium per
ton of seawater and about 3.3 mg of uranium.
[0010] Heretofore, uranium has been recovered from seawater by the
following methods using an adsorbent; seawater is brought into
contact with the particles of titanic acid to adsorb uranium from
the seawater and fine air bubbles are attached to the particles of
titanic acid, which are then floated on the seawater and separated
therefrom to recover the uranium (Japanese Patent Public Disclosure
No. 61018/1979); calcium or carbonate ions are removed from
seawater before uranium in the seawater is recovered by adsorption
onto a hydrous metal oxide adsorbent (Japanese Patent Public
Disclosure No. 79111/1979); a collector produced by reacting a
polyethyleneimine derivative with hydroxylamine is used to achieve
adsorptive recovery of metal ions dissolved in seawater (Japanese
Patent Public Disclosure No. 48725/1987); and using a kalixarene
derivative to recover uranium in seawater by adsorption (Japanese
Patent Public Disclosure No. 136242/1987).
[0011] Dissolved metals can also be recovered using chelate resins
and conventional methods based on this approach include the
following: a specified group is introduced into a chloromethylated
crosslinked polystyrene, which is then reacted with hydroxylamine
to produce an adsorbent resin that is used to recover dissolved
metals from seawater by adsorption (Japanese Patent Public
Disclosure No. 84907/1984); a chelate resin having malonyl
dihydroxamate residue is used as an adsorbent to recover dissolved
metals by adsorption (Japanese Patent Public Disclosure No.
83730/1984); and a chelate resin having functional groups of a
specified structure in the molecule is used to recover dissolved
metals by adsorption (Japanese Patent Public Disclosure No.
11224/1985).
[0012] To date, the conventional methods of recovering uranium from
seawater using adsorbents or those for recovering dissolved metals
using chelate resins have not been implemented in practice since
they are incapable of cost-effective collection of uranium and
other rare metals. However, for Japan which is by no means rich in
mineral resources, it has been long desired to exploit the metals
that are dissolved in the surrounding sea.
SUMMARY OF THE INVENTION
[0013] (a) The present has been accomplished with a view to
developing a material that is strong enough to withstand hostile
weather conditions in ocean and which has high performance in
collecting dissolved metals from seawater. To attain this object,
the fiber of a polyolefin such as polyethylene or polypropylene
that is a highly durable polymer is used as a substrate, side
chains are grafted to the substrate polymer by radiation-initiated
graft polymerization, and then an amidoxime group and a hydrophilic
group are introduced into the same graft side chains.
[0014] The collector of the invention is produced by a process
comprising the following steps: (1) to generate a reaction
initiating species (radicals), a substrate comprising the fiber of
a polyolefin such as polyethylene or polypropylene is exposed to
electrons; (2) grafting a polymerizable monomer having a cyano
group such as acrylonitrile (CH.sub.2.dbd.CHCN) onto the polyolefin
fiber in the presence of a polymerizable monomer having a
hydrophilic group; and (3) then reacting the cyano groups in the
graft side chains with hydroxylamine (NH.sub.2OH) to convert them
to amidoxime groups, whereby both amidoxime and hydrophilic groups
are introduced into the same graft side chains.
[0015] If desired, grafting of the polymerizable monomer having a
cyano group onto the polyolefin fiber in the presence of the
polymerizable monomer having a hydrophilic group is performed at a
properly adjusted molar ratio of the two polymerizable monomers
and, thereafter, the cyano groups in the graft side chains are
reacted with hydroxylamine (NH.sub.2OH) to be converted to
amidoxime groups, whereby the amidoxime and hydrophilic groups are
introduced at a molar ratio of 70:30-30:70, preferably 60:40-40:60,
more preferably 50:50.
[0016] (b) The collector of the invention needs only to be
submerged and anchored in seawater such as the Kuroshio current so
that slightly dissolved useful metals such as vanadium, uranium,
cobalt, titanium and molybdenum are efficiently recovered from the
seawater. The collector is characterized in that the fiber of a
polyolefin such as polyethylene that is commonly used in oil fences
is irradiated to introduce a chemical structure capable of
selective trapping of metals.
[0017] Specifically, the collector of the invention can be produced
by one of the following methods: (a) acrylonitrile is grafted to a
substrate fiber in the form of either a nonwoven or woven cloth of
the fiber of a polyethylene such as polypropylene or polyethylene
and amidoxime groups are introduced into the graft side chains; (b)
acrylonitrile and a polymerizable monomer having a hydrophilic
group are co-grafted to the substrate fiber in either nonwoven or
woven cloth form and amidoxime groups are introduced into the graft
side chains made of acrylonitrile; and (c) the fiber of a
polyolefin such as polypropylene is coated with a different
polyolefin such as polyethylene to form a fiber of a core/sheath
structure, a substrate is formed of this fiber in the form of
either a nonwoven or woven cloth, either acrylonitrile or a
polymerizable monomer having a hydrophilic group or both are
grafted to the substrate fiber, and amidoxime groups are introduced
into the graft side chains made of acrylonitrile.
[0018] For actual use, a plurality of such collectors are
superposed and sandwiched between nets and a plurality of the
resulting assemblies are stacked in position at suitable spacings
to construct a cassette of collectors.
[0019] To collect dissolved metals from seawater, the cassette is
placed in a number of corrosion-resistant cages, which are bound to
a rope at suitable spacings; an anchor is attached to the submerged
end of the rope and a buoy is attached to the other end so that the
cassette is anchored in seawater either depthwise or laterally as
long as the collectors are kept in contact with the seawater to
collect dissolved metals from it.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a graph showing the ratio of grafting onto a
nonwoven cloth as a function of time in accordance with the
invention;
[0021] FIG. 2 is a graph showing the relationship between the
reaction time for conversion to amidoxime groups and the rate of
uranium trapping in accordance with the invention;
[0022] FIG. 3 shows the functional groups generated in the reaction
for conversion to amidoxime groups;
[0023] FIG. 4 shows two types of functional group distribution in
the fractured surfaces of fibers produced by graft polymerization
in the invention;
[0024] FIG. 5 is a graph comparing three methods of graft
polymerization in terms of the rate of uranium trapping;
[0025] FIG. 6 shows three different cross-sectional shapes for the
fiber used in the collector of the invention;
[0026] FIG. 7 is a graph showing how the rate of uranium trapping
by the collector of the invention varies with its specific surface
area;
[0027] FIG. 8 is a graph showing the durability of the collector of
the invention;
[0028] FIG. 9 illustrates how the collector of the invention is
anchored in seawater to collect dissolved metals;
[0029] FIG. 10 is a graph showing how the results of uranium
trapping by the collector of the invention vary with the method of
graft polymerization;
[0030] FIG. 11 is a graph showing how the adsorbing performance of
the collector of the invention is affected by the depth to which it
is submerged in seawater;
[0031] FIG. 12 is a graph showing the performance of the collector
of the invention in trapping vanadium and uranium;
[0032] FIG. 13 illustrates the structure of the collector cassette
of the invention; and
[0033] FIG. 14 shows two different distributions of uranium
adsorption as observed in the collector cassette of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] (a) To introduce amidoxime groups into a substrate of a
polyolefin fiber made of highly endurable polyethylene (PE) or
polypropylene (PP) by radiation-initiated graft polymerization, the
following scheme (2) may be employed: 2
[0035] When AN (acrylonitrile) or AN/MAA (methacrylic acid) were
grafted to a nonwoven cloth made of a polyolefin fiber, the graft
ratio (%) was related to the reaction time (h) as shown in FIG. 1.
Obviously, the rate of AN grafting was high and a graft ratio of
150% was reached in one hour of the reaction. According to scheme
(2), the cyano groups in graft side chains were converted to
amidoxime groups in an efficiency of 70-80% and their concentration
reached 7-8 mmol/g at a graft ratio of 100%.
[0036] In the presence of MAA or various other vinyl monomers, AN
can be co-grafted with such vinyl monomers; the curve connecting
open circles in FIG. 1 shows the result of co-grafting of AN and
MAA. By applying this co-grafting technique, both an amidoxime
group (metal trapping functional group) and a hydrophilic group
(carboxyl group) can be introduced into the fiber substrate. The
time of reaction for converting the cyano groups in graft side
chains to amidoxime groups and the rate of trapping dissolved
uranium in seawater by means of the collector can be correlated as
shown in FIG. 2. When the time of reaction for converting the cyano
groups in the graft side chains to amidoxime groups was no longer
than 30 minutes, the collector could trap an increasing amount of
uranium; however, beyond 30 minutes of the reaction, the uranium
trapping capability of the collector showed a tendency to drop
sharply.
[0037] This is because depending on the reaction conditions, the
conversion to amidoxime groups involved side reactions as shown in
FIG. 3. More specifically, as the generation of amidoxime groups
proceeds, the reaction for ammonia removal occurs between two
adjacent amidoxime groups to create a cyclic imidoxime group. The
creation of a cyclic imidoxime group can be inhibited by adding
methyl alcohol to the aqueous solution in the reaction system for
conversion of cyano groups to amidoxime groups. Since the cyclic
imidoxime group readily decomposes in an alkali solution,
determining conditions for efficient formation of amidoxime groups
is a critical factor to the synthesis of an effective collector of
dissolved metals in seawater.
[0038] FIG. 4 shows two different distributions of a functional
group (amidoxime group) in the fractured surfaces of fibrous
amidoxime resins synthesized by the method shown in scheme (2). To
obtain data, copper ions forming a complex with the fibrous
amidoxime resin were saturated and adsorbed on the resin and their
characteristic X-ray intensity distribution was determined with an
X-ray micro-analyzer and shown graphically. In the pictures shown
in FIG. 4, white spots represent the functional group and its
distribution differed greatly depending on the specific method of
grafting acrylonitrile. When irradiation of the fiber substrate
with electron beams was followed by the grafting of acrylonitrile
which was brought into contact with the substrate in a vapor phase,
graft chains grew at high density on the fiber surface; when the
reaction was performed in a liquid phase, the functional groups
distributed uniformly into the bulk of the fiber.
[0039] The concentration of dissolved uranium in seawater is
extremely low (3 mg per ton of seawater) and, hence, increasing the
efficiency of contact between the amidoxime group and the seawater
is important for enhancing the rate of uranium trapping. This is
why a molecular structure is needed that has hydrophilic groups
present in the neighborhood of amidoxime groups. The two methods of
graft polymerization (two-step grafting and co-grafting) for
creating the stated molecular structure were compared with
homo-grafting in terms of the rate of uranium trapping and the
results are shown in FIG. 5.
[0040] In the presence of an acrylic acid having a hydrophilic
group (carboxyl group), a molecular structure having hydroxyl
groups in the neighborhood of amidoxime groups can be created by
either co-graft polymerization or two-step graft polymerization.
The rate of uranium trapping is markedly increased in the presence
of a hydrophilic group and as is clear from FIG. 5, this effect is
more conspicuous in co-grafting than in two-step grafting. In the
two-step grafting method, an amidoxime group and a hydrophilic
group are attached to different graft side chains; on the other
hand, in the co-grafting method, both an amidoxime group and a
hydrophilic group are introduced in the same graft side chain,
permitting the hydrophilic group to act more effectively in
combination with the amidoxime group.
[0041] The polymerizable monomers having a cyano group that can be
used in the invention are acrylonitrile, vinylidene cyanide,
crotononitrile, metacrylonitrile, chloroacrylo-nitrile,
2-cyanomethyl acrylate, 2-cyanoethyl acrylate and mixtures thereof.
The polymerizable monomers having a hydrophilic group that can be
used in the invention are 2-hydroxyethyl methacrylate,
2-hydroxyethyl acrylate, allyl alcohol, polyethylene glycol
acrylate, polyethylene glycol methacrylate, polyethylene glycol
diacrylate, polyethylene glycol dimethacrylate, N-vinylpyrrolidone
and acrylamide. the polyolefinic fiber to be used in the invention
may be in the form of a woven cloth, a nonwoven cloth, a membrane
or a tube.
[0042] (b) Three specific examples of the substrate fiber in the
collector of the invention for trapping dissolved metals in
seawater are shown in FIG. 6; they comprise sheath types A and B
each comprising a polypropylene (PP) core and a polyethylene (PE)
sheath in fiber cross section, and a segmented type in which the
cross section is divided into PE segments and PP segments. To
produce the collector of the invention, acrylonitrile is grafted to
the surface of the above-described substrate fiber by
radiation-initiated graft polymerization and the resulting graft
side chains are reacted with hydroxylamine to introduce amidoxime
groups in the graft side chains. Collectors were produced from
various substrate fibers and their diameter, AO (amidoxime) resin
diameter (graft side chain diameter), graft ratio, AO group's
concentration and U (uranium) adsorption are shown in Table 2, from
which one can see how the performance of the collector in trapping
uranium is affected by the cross-sectional shape of the fiber
substrate.
2TABLE 2 Collector Fabrication from Various Substrate Fibers (AO:
amidoxime group) Substrate fiber graft AO group cross-sectional
constituent diameter AO resin ratio concentration U adsorption
shape material (denier) (.mu.m) diameter (%) (mmol/g) (mg/kg)
sheath type A PE-PP 2 16 28 130 5.7 120 sheath type A PE-PP 10 36
80 110 5.8 76 sheath type B PE-PP 3 20 40 130 5.5 100 sheath type B
PE-PP 18 46 100 120 6.2 70 Segmented type PE-PP 2 36 120 5.3 100
circular type PP 6 56 130 5.7 92 triangular type PP 18 140 140 6.2
72
[0043] The collector to be used in the invention may use a
composite fiber substrate or a single fiber substrate; the former
is classified as sheath type A, sheath type B or segmented type
(see above) depending upon the cross-sectional shape of the fiber,
and the latter is either circular or triangular in cross-sectional
shape. In each of these cases, the substrate is composed of
polyethylene and/or polypropylene. To this substrate, acrylonitrile
is grafted by radiation-initiated graft polymerization and
amidoxime groups are introduced into the resulting graft side
chains, thereby producing the collector of the invention. FIG. 7
shows how the performance of five types of collector in trapping
dissolved uranium in seawater varies with the specific surface area
of the collector. Obviously, the rate of uranium trapping is
independent of the collector type and shape but is greatly
influenced by its specific surface area. FIG. 7 makes it clear that
the collector can trap an increasing amount of uranium as its
specific surface area increases.
[0044] To introduce both an amidoxime group and a hydrophilic group
into the substrate fiber by radiation-initiated graft
polymerization, a polymerizable monomer having a hydrophilic group
is first grafted and then a polymerizable monomer having a cyano
group is grafted to the polyolefinic fiber, and the cyano groups in
the graft side chains are reacted with hydroxylamine so that they
are converted to amidoxime groups, which occur in different graft
side chains than those where the hydrophilic groups are present
(this process is called "two-step grafting").
[0045] According to another method of introducing both an amidoxime
group and a hydrophilic group into the substrate fiber by
radiation-initiated graft polymerization, a polymerizable monomer
having a cyano group is grafted to the substrate fiber in the
presence of a polymerizable monomer having a hydrophilic group and,
thereafter, the cyano groups in the resulting graft side chains are
reacted with hydroxylamine so that they are converted to amidoxime
groups, which occur in the same graft side chains as those where
the hydrophilic groups are present (this process is called
"co-grafting").
[0046] Various radiations may be used in graft polymerization and
they include .alpha.-rays, .beta.-rays, .gamma.-rays, X-rays and
accelerated electron beams. A polymerizable monomer may be grafted
to the substrate by either simultaneous irradiation in which the
substrate and the polymerizable monomer are simultaneously
irradiated or pre-irradiation in which the polymerizable monomer is
brought into contact with the already irradiated substrate.
[0047] After trapping dissolved metals in seawater by adsorption,
the collector of the invention is regenerated by an acidic solution
such as hydrochloric acid, with which the trapped metal is desorbed
and eluted from the collector. The durability of the collector is
expressed by the number of adsorption and desorption cycles that
can be performed before its performance drops to an impractical
level and FIG. 8 shows the durability of the collector of the
invention as evaluated by this criterion. The service durability of
the collector is affected by the conditions for its fabrication,
the conditions under which the adsorbed metal is eluted by the
acidic solution and the duration of time for which the acidic
solution is kept in contact with the collector; as is clear from
FIG. 8, the performance of the collector of the invention in
trapping uranium from seawater is such that it can withstand up to
10 adsorption/desorption cycles.
[0048] The collector of the invention is capable of selective
trapping of metals other than uranium that are dissolved in
seawater. Table 2 lists the names and amounts of dissolved metals
that can be trapped by contacting the collector of the invention
with seawater for 20 days. Vanadium, cobalt, uranium and titanium
were the principal metals that could be trapped.
3TABLE 3 Collectable Useful Metals Concentration Concentration in
seawater, in collector, Useful metals (.mu.g/kg) (g/kg) Uranium (U)
3 .about.3 Titanium (Ti) 1 .about.2 Vanadium (V) 2 .about.6 Cobalt
(Co) 0.1 .about.6 Note: Contact with seawater continued for 20
days.
[0049] The dissolved metals trapped by the collector of the
invention have different energies of binding to the amidoxime group
and, hence, they can be selectively eluted by controlling the
concentration of hydrogen ion (pH) in the eluting acidic solution
such as hydrochloric acid. As a result, the dissolved metals
adsorbed on the collector can be efficiently recovered by selective
eluting.
[0050] The following examples are provided for the purpose of
further illustrating the present invention but are in no way to be
taken as limiting.
Example 1
[0051] Co-Grafting; See FIG. 5
[0052] Fiber filaments (40 .mu.m in diameter) of a
tetrafluoroethylene/eth- ylene copolymer were exposed to 10 Mrad of
electron beams from an electron accelerator (acceleration voltage,
1.5 MeV; beam current, 1 mA) in a nitrogen atmosphere. The
irradiated filaments were placed in a reactor which was evacuated
to 10.sup.-3 mmHg and charged with a solution of 2-hydroxyethyl
methacrylate (HEMA) in methanol, the concentration of dissolved
oxygen in which had been adjusted to no more than 0.1 ppm by
nitrogen bubbling; thereafter, a 50 wt % solution of acrylonitrile
(AN) in methanol that had been similarly adjusted by nitrogen
bubbling was charged into the reactor and reaction was performed at
25.degree. C. for 6 hours with the filaments being immersed in the
reaction mixture. The resulting graft ratio was 15% for HEMA and
47% for AN.
[0053] The filaments thus acquiring HEMA and AN grafts were
immersed in a solution of 3 wt % hydroxylamine hydrochloride in a
1:1 mixture (by weight) of water and methanol after it was
neutralized with potassium hydroxide and reaction was performed at
40.degree. C. for 2 hours to produce a collector having amidoxime
groups. The anion exchange capacity of the collector which was
equivalent to the concentration of amidoxime groups was 5.0
meq/g.
[0054] A portion (0.1 g) of the collector was immersed in 50 ml of
seawater the vanadium concentration of which had been adjusted to 1
mg/L; the seawater was shaken at 30.degree. C. for 1 hour to adsorb
vanadium onto the collector. The result is shown in FIG. 5.
Comparable Example 1
[0055] Two-Stage Grafting; See FIG. 5
[0056] Fiber filaments (40 .mu.m in diameter) of a
tetrafluoro-ethylene/et- hylene copolymer were exposed to 10 Mrad
of electron beams from an electron accelerator (acceleration
voltage, 1.5 MeV; beam current, 1 mA) in a nitrogen atmosphere. The
irradiated filaments were placed in a reactor which was evacuated
to 10.sup.-3 mmHg and charged with a solution of 2-hydroxyethyl
methacrylate (HEMA) in methanol, the concentration of dissolved
oxygen in which had been adjusted to no more than 0.1 ppm by
nitrogen bubbling; thereafter, reaction was performed at 25.degree.
C. for 6 hours with the filaments being immersed in the HEMA
solution. The resulting graft ratio of HEMA was 15%.
[0057] By the same method as described above, the filaments with
HEMA grafts were exposed to 10 Mrad of electron beams, immersed in
a 50 wt % solution of acrylonitrile (AN) in methanol and reaction
was performed at 25.degree. C. for 6 hours. The resulting graft
ratio of AN was 47%.
[0058] The filaments thus acquiring HEMA and AN grafts were
immersed in a solution of 3 wt % hydroxylamine hydrochloride in a
1:1 mixture (by weight) of water and methanol after it was
neutralized with potassium hydroxide and reaction was performed at
40.degree. C. for 2 hours to produce a collector having amidoxime
groups. The anion exchange capacity of the collector which was
equivalent to the concentration of amidoxime groups was 5.0
meq/g.
[0059] A portion (0.1 g) of the collector was immersed in 50 ml of
seawater the vanadium concentration of which had been adjusted to 1
mg/L; the seawater was shaken at 30.degree. C. for 1 hour to adsorb
vanadium onto the collector. The result is shown in FIG. 5.
Comparable Example 2
[0060] AN Homo-Grafting; See FIG. 5
[0061] Fiber filaments (40 .mu.m in diameter) of a
tetrafluoroethylene/eth- ylene copolymer were exposed to 10 Mrad of
electron beams from an electron accelerator (acceleration voltage,
1.5 MeV; beam current, 1 mA) in a nitrogen atmosphere. The
irradiated filaments were placed in a reactor which was evacuated
to 10.sup.-3 mmHg and charged with a 50 wt % solution of
acrylonitrile (AN) in methanol, the concentration of dissolved
oxygen in which had been adjusted to no more than 0.1 ppm by
nitrogen bubbling; thereafter, reaction was performed at 25.degree.
C. for 6 hours with the filaments being immersed in the AN
solution. The resulting graft ratio of AN was 47%.
[0062] The filaments thus acquiring AN grafts were immersed in a
solution of 3 wt % hydroxylamine hydrochloride in a 1:1 mixture (by
weight) of water and methanol after it was neutralized with
potassium hydroxide and reaction was performed at 40.degree. C. for
2 hours to produce a collector having amidoxime groups. The anion
exchange capacity of the collector which was equivalent to the
concentration of amidoxime groups was 5.0 meq/g.
[0063] A portion (0.1 g) of the collector was immersed in 50 ml of
seawater the vanadium concentration of which had been adjusted to 1
mg/L; the seawater was shaken at 30.degree. C. for 1 hour to adsorb
vanadium onto the collector. The result is shown in FIG. 5.
[0064] Obviously, a molecular structure having hydrophilic groups
present in the neighborhood of amidoxime groups is indispensable to
improve the rate of trapping dissolved metals. Of the three methods
of graft polymerization, homo-grafting, two-step grafting and
co-grafting, the last-mentioned co-grafting process is the most
suitable for adsorbing dissolved metals.
Example 2
[0065] Trapping Performance of the Collector of the Invention
[0066] A collector cassette was placed in a stainless steel cage
measuring 440 mm in diameter and 160 mm thick and anchored in
seawater at depths of 10 m, 20 m and 30 m. Every 20 days, the
amount of dissolved uranium trapped in the cassette was measured
and the results are shown in FIG. 10.
[0067] The cassette contained the following three collectors:
[0068] (1) acrylonitrile (AN) was homo-grafted to a substrate fiber
in the form of a polypropylene (PP) nonwoven cloth and amidoxime
groups were introduced into the graft side chains; the collector
prepared by this method is indicated by .circle-solid.;
[0069] (2) AN and methacrylic acid (MAA) were co-grafted to the
same substrate as in (1) to form AN and MAA graft side chains, and
amidoxime groups were introduced into the AN graft side chains; the
collector prepared by this method is indicated by .largecircle.;
and
[0070] (3) AN and MAA were co-grafted to the surface of a substrate
fiber in nonwoven cloth form having a core/sheath structure created
by coating polypropylene (PP) filaments with polyethylene (PE),
thereby forming AN and MAA graft side chains, and amidoxime groups
were introduced into the AN graft side chains; the collector
prepared by this method is indicated by .quadrature..
[0071] As is clear from FIG. 10, the speed of uranium trapping by
the collector (.largecircle.) was about twice as fast as that of
uranium trapping by the collector (.circle-solid.). This is due to
the combined effect of the amidoxime group and the hydrophilic
carboxyl group derived from MAA. The speed of uranium trapping by
the collector (.quadrature.) was about twice as fast as that of
uranium trapping by the collector (.largecircle.). This is due to
the effect of the core/sheath structure of the fiber substrate.
Thus, compared to the collector produced by homo-grafting, the
speed of trapping uranium dissolved in seawater could be increased
by a factor of about 4 by means of selecting the appropriate
grafting method and substrate.
[0072] Using the collector (.largecircle.) of sheath type A
illustrated in FIG. 6, the amounts of uranium and vanadium trapped
by the collector were measured at varying depths in seawater where
it was anchored and the results are shown in FIG. 11, from which
one can see that the amounts of uranium and vanadium trapped by the
collector were substantially the same at varying depths from the
sea level. Obviously, the adsorbing performance of the collector
was independent of depth.
[0073] FIG. 12 shows the performance of the same collector
(.largecircle.) in trapping different useful metals (uranium and
vanadium) dissolved in seawater. The collector had only to be
immersed in seawater to trap not only uranium but also equally rare
vanadium. According to FIG. 12, two grams of uranium were trapped
per kilogram of the collector which was immersed in seawater for 60
days whereas as many as 3.2 grams of vanadium could be trapped in
the same test (1.6 times as much as the uranium that could be
trapped). The trapped uranium and vanadium were desorbed from the
collector and purified to uranium oxide and vanadium pentoxide,
respectively, for recovery.
Example 3
[0074] Structure of the Dissolved Metal Collector Cassette of the
Invention
[0075] The structure of the collector cassette used in Example 1 to
trap dissolved metals in seawater is shown in FIG. 13. A plurality
of collectors are superposed to a thickness of about 17 mm and
sandwiched between two nets each having a thickness of about 1.5 mm
to make a collector unit. A plurality of such collector units, say,
in ten blocks, are stacked at suitable spacings and fixed by means
of a plurality of polyvinyl chloride bolts to make a collector
cassette measuring about 150 mm.times.200 mm and 272 mm in
height.
[0076] This collector cassette was used to trap dissolved uranium
in seawater and the distribution of uranium adsorption in the
cassette is shown in FIG. 14. After uranium trapping by the
cassette anchored in seawater, one of the collector units was cut
into 5.times.3 sections and the adsorbed uranium was eluted to give
the adsorption profile shown in the top right of FIG. 14, from
which one can see that the amount of uranium adsorbed on the
collector was little different between the periphery and the center
of the cassette that was immersed in seawater. This uniformity in
uranium adsorption profile across the surface of the cassette would
be due to the provision of the 1.5 mm thick net between adjacent
collector units (see FIG. 13) as a spacer that served as a
passageway of seawater.
[0077] Another collector unit that was cut out of the cassette was
vertically divided into nine equal parts and the uranium adsorption
profile across the thickness of the unit is shown in the bottom
right of FIG. 14; obviously, notwithstanding slight variation, the
uranium adsorption by the cassette was fairly uniform across its
thickness.
[0078] Effect of the Invention (a) According to the invention,
highly durable polyethylene or polypropylene is subjected
radiation-initiated graft polymerization and both an amidoxime
group and a hydrophilic group are introduced into the same graft
side chains without compromising the characteristics of the
polymer; this contributed to develop a collector that was strong
enough to withstand hostile weather conditions in ocean and which
had high performance in trapping dissolved metals in seawater.
[0079] The collector of the invention uses as the substrate the
nonwoven cloth commonly used as an oil fence to prevent the spread
of oil from a wrecked tanker; since the hydrophobic skeleton of the
substrate and the grafted branches having functional (hydrophilic)
groups exhibit different characteristics, the collector can
withstand hostile ocean environments.
[0080] The radiation-initiated grafting performed in the invention
can impart functional groups without impairing the characteristics
of existing substrate shapes such as nonwoven cloths. Therefore,
according to the invention, a collector can be fabricated that
achieves highly efficient contact with the large quantity of
seawater that has been difficult to handle by the conventional
technology. (b) Unlike the conventional adsorbents, the collector
of the invention features high capability for selective adsorption
of dissolved metals in seawater, which can be recovered in
concentrations 10.sup.6-10.sup.7 times higher than those found in
the seawater; in addition, the collector has sufficient corrosion
resistance to seawater and can consistently be used for many times
after regeneration.
[0081] In the collector of the invention, amidoxime groups are not
attached to the skeleton of the substrate polymer that is made of a
polyolefinic fiber but they are attached to graft side chains that
extend like branches from the skeleton of the polymer; hence, the
collector as anchored in seawater makes such a close contact with
dissolved metals that it can adsorb them efficiently.
* * * * *