[0001] This invention is concerned with ion-exchange resins which are capable of very selective
removal of heavy metal ions from aqueous solutions and methods for the synthesis of
these resins. The resins of the invention are designed to selectively chelate mercury
and copper ions and are particularly useful where the concentration of metal is-low.
They have a further advantage that the metal can be recovered easily from the resins
under appropriate conditions.
[0002] Use of, ion-exchange resins for the removal of metal ions from aqueous solutions
is commercially wide spread; especially in water softening applications and for the
removal of toxic products from effluents. Other areas of importance are in the processing
of radioactive wastes and the purification of rare earth metals. Many of these commercial
ion-exchange resins do have some selectivity for certain metal ions but as this is
usually only by a factor of 2 or 3 it is not often of economic significance.
[0003] In an effort to improve this. selectivity a considerable amount of research has been
carried out in the area of chelating resins which because of their multifunctional
nature have greatly improved metal selectivity for the transition metal ions over
the alkali metal ions. Many chelating polymers have been synthesised and are commercially
available, e.g. imidoacetic acid resins of the Dowex A-1 and Chelex 100 type ("Dowex"
and "Chelex" are registered trade marks). However, most of these resins are only marginally
selective for a particular heavy metal ion.
[0004] Ion-exchange resins of sufficient selectivity and cheapness have many potentially
important applications. These include the removal of polluting heavy metal ions in
the effluent from electroplating or metal pickling baths, Another possible application
is the recovery of metals from dilute mine dump leachings where selective solvent
extraction using low molecular weight chelating agents is being used commercially
but problems of solvent and complexing agent loss prevent wider use of solvent extraction
(G. Ciggett and W. Hopkins, Chem. & Ind. 23, 1019 (1976)). The possibility of medical
use, e.g., for selective removal of poisoning metal ions, could be a welcome benefit
of selective resins.
[0005] As a result of research carried out on the synthesis of highly selective ion exchange
resins, a number of resins with improved selectivity for copper and mercury have been
reported. These materials have generally been made by carrying out a series of chemical
transformations on crosslinked polystyrene, [I.V. Sarbaskii et. al., U.S.S.R. 507,
588 (1976); L.R. Melby, J. Amer. Chem. Soc., 97, 4044 (1974); R.R. Grinstead et. al.,
in "Extractive metallurgy of Copper" vol. 2 Yannopolous and Agarwal Eds., 1976, AIME
New York pp. 1009-1024] polyethyleneimine, [A.K. Kusaiva et. al., Izv. Akad. Nauk.
Kaz. SSR, Ser. Khim., 26 (3), 33 (1976); Tsuchida et. al., J. Polym. Sci., Polym.
Chem. Ed., 14, 1557 (1976)] or poly(4-vinyl pyridine), [Hiroyuki et al., Makromol
Chem., 177, 2295 (1976)]. Any reaction carried out on a preformed polymer leads to
the formation of some non selective or non-absorbing defect structures on the chain,
which cannot then be removed. Multiple reactions compound the formation of defects
considerably and hence limit the selectivity of a resin. The costs of such chemical
transformations also limit greatly the applicability of the chelating resins formed.
[0006] Another type of chelating polymer has been made by direct synthesis, using the condensation
of phenol, formaldehyde and iminodiacetic acid [Japan Kokai, 75, 92, 991 (1975) and
75, 107, 092 (1975)]. Also the condensation of polyamines, formaldehyde and 8-hydroxyquinoline
[V. Balakin and V.V. Glukahikk, Iz. Vyssh. Uchebn. Zaved, Khim, Tekhnol, 18 (9), 1466-9
(1975); F. Vernon, Chemistry and Industry, 634 (1977)] gives a similar chelating polymer.
However, in both these cases (and in most other attempts to form chelating polymers)
well-known, strong chelating structural units are used in the polymers. These powerful
chelating units have precluded the development of high selectivity. Another common
type of phenol/formaldehyde resin is that formed with various mono- or di-primary
amines especially under acid catalysed conditions [e.g. A. Rio U.S. 3,976,571]. In
these cases many different reactions can and do occur at the one amine group. Because
of this these resins do not have high metal selectivity.
[0007] The resins of this invention are polymers produced by the condensation of a phenolic
component comprising one or more phenols, formaldehyde, and a diamino component comprising
one or more di-(secondary amino) compounds, under neutral (Mannich reaction) conditions.
[0008] The preferred polymers are formed from diamino compounds in which the secondary amino
groups are present as part of a saturated heterocyclic ring. The two groups may be
combined in a single ring, as for example in - piperazine or in separate linked rings,
as for example in 4,4'-bipiperidine.
[0009] The most preferred polymers are those prepared using piperazine and have the following
structure, as confirmed by nuclear magnetic resonance and infrared measurements.

where R' and R"' are independently lower (C
1-C
3) alkyl groups or hydrogen atoms and R" is a hydrogen atom or a crosslinking group.
[0010] The groups R' and R"' on adjacent atomatic rings may be the same or different. Usually
only a few of the groups R" will be a crosslinking group, the majority being hydrogen.
[0011] The nature of the crosslinkinggroup will vary according to the phenol(s) selected
and the degree and type of crosslinking required. The methylene crosslinking groups
typical of phenol-formaldehyde resins will be present, but in addition the cross-linking
groups may include methylene-piperazine or methylene-piperazine- methylene bridges.
Bisphenols (including bisphenol sulphides or sulphoxides) can also be used to provide
further types of cross-linking.
[0012] The strongest and most selective chelating resins are obtained when the phenol used
is phenol itself (i.e. hydroxybenzene). However, the addition of small amounts of
a less useful chelating but better crosslinking phenol, such as bisphenol A gives
resins which are more stable to acid breakdown and mechanicl attrition. Hydroxybenzenes
having C
1-C
3 alkyl substituents in the 3 and/or 3 and 5 positions, e.g. 3-ethyl-l-hydroxybenzene,
can also be used.
[0013] In accordance with another aspect of the invention, selective copper and mercury-chelating
resins are prepared by reacting approximately equimolar quantities of phenolic and
diamino components with approximately twice the molar quantity of formaldehyde under
Mannich reaction conditions, for example in aqueous ethanol at reflux for 24 hours.
[0014] A more complex procedure is needed to form useful chelate resins in a highly crosslinked
spherical form. In this preferred method of the invention, the phenolic and amino
components, with some formaldehyde, are precondensed under Mannich conditions to give
a linear polymer, which is then reacted under suspension polymerisation conditions
with an additional quantity of the same or another phenolic component and formaldehyde,
to complete polymerisation and cross-linking.
[0015] By way of example, the phenolic and amino components in approximately 1:1.2 molar
quantities may be precondensed with slightly less than 1.5 moles of formaldehyde under
neutral conditions, e.g. in aqueous alcohol at 70°C for 2 hours to give a linear polymer.
The linear precondensate polymer so produced is then condensed with an additional
quantity of the same of another phenolic component and a further 1 mole of formaldehyde
under suspension polymerisation conditions. Preferably, chlorobenzene and carbon tetrachloride
are added with more phenol and 1 mole extra of formaldehyde and the suspension, stabilised
with polyvinyl alcohol or other suitable surfactant, is heated with strong stirring
for 3 hours at 120°C.
[0016] Acid-catalysed formaldehyde crosslinking of the linear polymer can also be used in
the second stage to give fast crosslinking and hence good bead formation.
[0017] The polymers of the invention are cream to light brown acid insoluble resins which
show strong and specific absorption for copper and mercury only, even in the presence
of large excesses of other ions. In the case of copper, the strong adsorption of copper
commences at about pH 3.5 on the acid side and the resin turns deep green to almost
black when fully saturated with up to 1.5 milli-moles of copper/gram. The adsorption
is reversible below pH 3.5 (the rate of desorption of copper increased as the pH decreased
below 3.5) or at high pH in the presence of ammonia or cyanide ions. The only other
ion which shows any noticeable adsorption on these resins is the mercuric -ion at
all pH's below 8 except in very strong acid where some desorption occurred along with
resin degradation. Mercury ions could be recovered at high pH or by the use of strong
chelating agents.
[0018] The invention is further described and elucidated in the following examples.
EXAMPLE 1
[0019] This example illustrates the preparation and properties of a copper and mercury chelating
resin prepared from phenol, bisphenol A, formaldehyde and piperazine.
[0020] Phenol (42.3 g, 0.45 mole) and bisphenol A (11.4 g, 0.05 mole) were dissolved in
ethanol (1 1/2 litre) and piperazine hexahydrate (108 g, 0.55 mole) was added slowly
with stirring. After complete addition the mixture was cooled in an ice bath and stirred
while formaldehyde solution (37% formaldehyde; 100 ml, 1.2 mole) was added slowly.
The mixture was then heated slowly to increase the temperature over 2 hours to a vigorous
reflux with strong stirring. Some solid product started to form after about 3 hours
heating but after 12 hours a further (0.3 mole) of formaldehyde solution was added
slowly and the reflux was continued a further 12 hours. The solution was decanted
off and the solid product washed with ethanol and then water. The resin was then soaked
in O.lN hydrochloric acid for 24 hours which caused it to break up into small flexible
particles. The resin particles were washed with O.lN hydrochloric acid until the washings
remained transparent after basification with ammonia. This removed soluble organic
material. The resin was then washed with distilled water, ammonia and distilled water
again to give the free base product.
EXAMPLE 2
[0021] The selectivity of the above ion-exchange resin was measured in the following general
manner. A 1 g portion of the wet resin particles were equilibrated by stirring the
material in sodium acetate/acetic acid/ hydrochloric acid buffer solutions of pH 2.5-7.0,
containing 100 ppm of copper, zinc, cobalt and nickel salts and 50 ppm of cadmium
chloride. The concentrations of the various ions were monitored by atomic absorption
spectroscopy. The only significant ( > 1ppm) quantity of ion uptake over 24 hours
was for cupric ions. Above pII 4.5 this was almost complete (i.e. less than 2 ppm
remaining unchelated by the dark green resin) but the uptake of copper dropped to
almost nothing below pH 3.5 and copper previously adsorbed could be completely recovered
at these low pH values. Similar selectivities for the mercuric ion against zinc, cobalt,
nickel and cadmium were obtained, except that the range of mercury adsorption was
pH 1-7.
EXAMPLE 3
[0022] Column operation, demonstrating the selectivity of the above resin, was carried out
as follows. Approximately 15 gm of the resin particles were equilibrated by stirring
in an aqueous sodium acetate/ acetic acid buffer solution at pH 6 and then packed
in a column. A buffered (pH 6) solution containing 50 ppm each of copper, nickel and
zinc sulphates and 25 ppm of cadmium sulphate was passed through the column and the
effluent continuously monitored by atomic absorption spectroscopy. Nickel, cadmium
and zinc salts started to emerge in the 1st bed volume of eluent (25 ml) and the first
two ions were emerging at full concentration (50 ppm) by the 3rd bed volume, zinc
by the 6th. Significant quantities (> 1 ppm) of copper did not emerge until the 68th
bed volume (1.7 litres of solution). The copper capacity of the resin was shown to
be 1.5 millimole/gram of dry resin. The copper extracted by the resin was recovered
in one bed volume of basic (pH 10) sodium cyanide.
EXAMPLE 4
[0023] This example illustrates the preparation and properties of an ion exchange resin
made in spherical beads.
[0024] Phenol (75.3 g, 0.8 mole), piperazine hexahydrate (196 g, 1 mole) in 50:50 aqueous
ethanol (2 litres) were precondensed with 37% aqueous formaldehyde solution (120 ml,
1.5 mole) and stirred at 70°C for 2 hours. Then further phenol (18 g, 0.2 mole) and
formaldehyde (80 ml, 1 mole) were added in 250 ml of water and the mixture stirred
and poured into 50/50 carbon tetrachloride/chlorobenzene (5 litre) and suspension
polymerized with vigorous stirring at 120°C under pressure. The resulting resin spheres
were washed with alcohol and water and then soaked with dilute acid until no further
soluble organic base remained, then washed with distilled water, ammonia and distilled
water again.
EXAMPLE 5
[0025] Selectivity of the resin of Example 4 for mercury even in high salt concentrations
was determined by the following test. A solution containing 1000 ppm of potassium,
calcium and barium chlorides and 3,300 ppm of sodium as acetic acid/sodium acetate
buffer (pH 4.50) was made up with 50 ppm of mercuric chloride. A 1 gm sample of the
wet resin beads was added to 100 ml of this solution and equilibrated by stirring.
Under these conditions the resin adsorbed 46 ppm of mercury and no other ion.
EXAMPLE 6
[0026] This example illustrates the preparation and properties of an ion exchange resin
made in spherical beads crosslinked by an acid catalysed formaldehyde system.
[0027] Phenol (94.1 g, 1 mole), piperazine hexahydrate (196 g, 1 mole) in 50:50 aqueous
ethanol (2 litre) were precondensed to linear polymer with formalin (37% aqueous formaldehyde
solution) (120 ml, 1.5 mole) under Mannich reaction conditions (reflux for 3 hours).
The linear polymer was then poured into 50:50 tetrachlorethylene/petroleum ether (B.p.
100-120°) mixture containing polyvinyl alcohol (100 ml of 7.5% solution) as a surface
active agent to form a well stirred suspension. Sulphuric acid catalyst (200 ml, 10%)
and formalin (40 ml, 0.5 mole) were added and the suspension was heated to reflux
while stirring and refluxed for 4 hours. The resulting resin spheres were treated
as in Example 4.
EXAMPLE 7
[0028] This example illustrates the preparation and properties of a resin prepared from
a substituted mono phenol (3-ethylphenol) and crosslinked with a different multifunctional
di-phenol (4,4'-dihydroxydiphenylsulphide).
[0029] 3-Ethylphenol (12.2 g, 0.1 mole) and 4,4'-dihydroxydiphenylsulphide (4.4 g, 0.02
mole) were dissolved in ethanol (300 ml) and piperazine hexahydrate (22 g, 0.11 mole)
was added slowly with stirring. After complete addition formalin was added (24 ml,
0.3 mole) and then stirred strongly for 24 hours at 110°C in a pressure vessel. The
resulting resin was washed with alcohol and water and then soaked in hydrochloric
acid for 24 hours before being converted back to the base form.
[0030] This resin although more selective for copper than that in Example 1 had a capacity
of only 0.3 millimole/gram.
EXAMPLE 8
[0031] This example illustrates the preparation and properties of a resin prepared from
4,4'-Bipiperidine and phenol crosslinked with 4',4'-Dihydroxydiphenyl- sulphone.
[0032] Phenol (42.3 g, 0.45 mole) and 4,4'-Dihydroxy- diphenylsulphone (11.7 g, 0.05 mole)
were dissolved in ethanol (1 1/2 litres) and 4,4'-Bipiperidine (92.4 g, 0.55 mole)
added with stirring. Formalin (120 ml, 1.5 mole) was then added slowly and the solution
heated slowly to reflux with strong stirring. After 24 hours reflux the resulting
resin was worked up as in
Example 1.
[0033] This resin again was very selective for copper and mercury. The capacity for copper
was 0.5 millimoles/gram.
1. A metal-chelating phenolic resin characterised in that it consists of a polymer
produced by the condensation of a phenolic component comprising one or more phenols,
formaldehyde, and a diamino component comprising one or more di-(secondary amino)
compounds under neutral conditions.
2. A resin as claimed in claim 1, characterised in that the diamino component comprises
one or more di-(secondary amino) compounds in which each secondary amino group forms
part of a saturated heterocyclic ring.
3. A resin as claimed in claim 1 or 2, characterised in that the phenolic component
comprises one or more compounds selected from the group consisting of hydroxybenzene
and hydroxybenzenes - having C1 to C3 alkyl substituents in the 3 and/or 3 and 5 positions.
4. A resin as claimed in claim 1, characterised in that the phenolic component is
hydroxybenzene or 3-ethyl-l-hydroxybenzene and the diamino component is piperazine
or 4,4'-bipiperidine.
5. A resin as claimed in any one of claims 1 to 4, characterised in that the phenolic
component includes a bisphenol, bisphenol sulphide or bisphenol sulphoxide as a crosslinking
agent. ,
6. A resin as claimed in claim 1 or 5, characterised in that the diamino component
is piperazine and the polymer has the structure

where R' and R"' are independently lower (C
l-C
3) alkyl groups or hydrogen atoms and R" is a hydrogen atom or a cross-linking group.
7. A method for preparing a selective metal-chelating phenolic resin characterised
by condensing approximately equimolar quantities of a phenolic component comprising
one or more phenols, and a diamino component comprising one or more di-(secondary
amino) compounds with approximately twice the molar quantity of formaldehyde under
Mannich reaction conditions.
8. A method for preparing cross-linked spherical beads of a selective metal chelating
phenolic resin characterised by condensing about 1 mole of a phenolic component comprising
one or more phenols with about 1.2 mole of a diamino component comprising one or more
di-(secondary amino) compounds and about 1.5 moles of formaldehyde under neutral conditions,
and condensing the linear precondensate polymer so produced with an additional quantity
of the same or another phenolic component and a further 1 mole of formaldehyde, under
suspension polymerisation conditions.
9. A method as claimed in claim 8, characterised in that the first condensation is
carried out by heating the reactants in aqueous alcohol at about 70°C for about 2
hours and second condensation is carried out by dispersing the linear polymer in chlorobenzene/carbon
tetrachloride in the presence of a surfactant and heating the mixture with vigorous
stirring about 120°C for 3 hours.
10. A method for the selective removal of copper and/or mercury from an aqueous solution
by contact with a resin characterised in that the said solution is contacted with
a resin as claimed in any one of claims 1 to 6.