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Effect of substituent on the Performance of Cupferron as a Collector for Uranium.

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Effect of Substituent on the Performance of
Cupferron as a Collector for Uranium
1. Nirdosh* and S.V. Muthuswami
Deparfmenf of Chemical hgineeing, Lakehead Univers&
Thunder Bay, Onfado P7B 5E7, CANADA
and
R. Natarajan and R. Jeyaraman
Deparfmenf of Chemisfm Bharafidasan UniversQ
7imchirapalli, Tamil Nadu 620 020, INDIA
Cupferron derivatives were synthesized and wed as collectorsfor uranium in the benchscale flotation of a uranium ore from CluffLake, Saskatchewan, Canada. All the
derivatives were found to be more effective than cupferron itself. For alkyl-substituted
cupferronr, uranium recovery increased with an increase in alkyl chain length, with p nonylcupferron having a uranium recovery of 82% in 25% floated maw. Among the
halogen-substitutedcupferrons,para-substituted compowrdr petformed better than metasubstituted compounds. Although uranium recovery was only 50%, in the case of p methoxycupferron, the mass floated was as low as 8% of the feed. Separation
effikncies were correlated with Hammett sigma values.
Introduction
Many naturally occurring metal-complexes, such as those found in the living
organisms, contain metal ions complexed with organic chelating agents that are
specific to certain metals. Such a specificity is rare in most synthesized chelating
agents. Attempts have been made to synthesize metal-selective chelating agents [I].
Most of the synthetic reagents form complexes with a variety of metals and they are
not very selective, the absolute specificity found in biological systems is
unattainable. However, the specificity of a synthesized chelating agent can be
improved either by altering the conditions of complexation (taking advantage of the
differences in the stability constants of the complexes) or by modifying its molecular
structure [24]. Therefore, metals can be separated from a solution using specific
chelating agents, which are used extensively not only to effect the Separation but also
for qualitativeand quantitative analyses 15-81.
Flotation is an important method of ore concentration and makes use of the
selective adsorption of surfactants and polymers onto mineral surfaces. Chelaring
agents capable of forming chelates with metal ions in a mineral lattice have been
found to be good collectors in mineral flotation. They are selective in the flotation
of complex mineral systems because they act by chemisorbing on mineral surfaces,
unlike conventional collectors which interact with minerals by physical
202
Eflect OfSubstitueM on the PerjiimMce of Cupfmron 0s a Collectorf o p Uranium
adsorption [9,10]. The use of chelating agents as float aids is increasing due to their
metal selectivity, the use of chelating agents in mineral processing has been reviewed
D11.
The introduction of an ore preconcentration stage prior to leaching reduces the
mass of solids to be leached, and eventually the amount of leachant required.
Because most radionuclides would be concentrated in this smaller separated mass,
this would also produce bulk solids as disposable tailings with acceptable levels of
radionuclides. Flotation has been widely tested as a means of preconcentrating the
uranium ores [12-171. The flotation of uranium from Elliot Lake ores has been
studied [12,13]. The objective was to preconcentrate the ore by removing uranium
and its long-lived radio-daughters into a small mass of float-concentrate, thus
yielding the bulk of the solids without any significant amounts of radionuclides.
Extensive work was performed on the evaluation of various flotation agents and
conditions for uranium flotation, and it was found that cupferron showed the
maximum promise. However, the consumption of cupferron was 4 kg/Mg ore for
each stage of flotation, which was several orders of magnitude larger than the
amounts required conventionally,and was cost prohibitive.
The substituentspresent in an organic chelating compound influence the electron
densities at the chelating donor atoms (the two oxygen atoms in cupferron) by
inductive and mesomeric effects [3,4]. Cupferron is a resonance hybrid of the
canonical forms (la-ld) shown in Figure 1. The IR spectral data [18] and the X-ray
crystallographic data [19-211 of metal cupferrates confirmed that the contribution
due to structure la occurs more in the formation of chelates. For all these canonical
forms (including la) the electron density at the donor atoms will be strongly
influenced by the nature of the substituent present in the benzene ring, thus
influencing the chelating capacity.
This paper describes work performed in an attempt to modify the structure of
cupfenon, and to synthesize specific collectors which would not only give adequate
uranium recovery in a smaller mass but also have negligible adsorption onto the
gangue minerals. Methods of synthesizing various cupferron derivatives, and the
effectsof the nature and position of substituent molecules on collector efficiency, are
described for the flotation of a high-grade ore (0.34% U) from Cluff Lake,
Saskatchewan,Canada.
10
lb
IC
id
Figure I. Canonical structures of cupferron.
203
1.Nirdosh, S.V. Muthuswami,R. Ndarajan and R . Jeymaman
Review Of Flotation With Cupferron As Collector
Cupferron is the ammonium salt of N-hydroxy-N-nitrosobenzenamine,
and the name
was suggested since it was observed to precipitate both copper and iron, However,
this name is a misnomer, since the reagent precipitates many metal ions in addition
to copper and iron. It forms a five-membered chelate ring with the metal ions and is
used extensively in the quantitative analysis [22] and solvent extraction of many
metals 123-263 and radioisotopes [27]. Surface tension measurements [12] of
cupferron solutions are similar to those of water. When the conductivities of
cupfemn solutions of various concentrations were plotted against concentration, no
inflexion was observed which would be a characteristic of micelle formation by a
surfactant in water [28]. Hence, cupferron is not a surfactant and its collector
property is only due to chemisoption.
Cupferron has been used [29] as a collector for cassiterite, obtaining a recovery
of 91% of tin in 25% mass by using a low collector concentration of 41 g/Mg ore.
Cassiterite has been floated from calcite with cupferron at pH 5-7.5 1301. Some of
the early flotation work using cupfemn as collector was perfmed in Russia 1311.
Titanium minerals (titanto-magnetite) from the agerine tailings obtained in apatite
production were floated and 78% of titanium was recovered in the collective
concentrate. The best results with cupferron were obtained when a sulfate soap,
kerosene and a foam-forming agent were added. Cupferron floated sphene and
ilmenite, and vanadium adsorbed onto titanium was also recovered. A hydrophobic
film formed on metal surfaces by many chelating agents including cupferron has
been studied 1321. The results showed that chelating agents such as cupferron
change the wettability of the transition metal surfaces. Rinelli and Marabini [33]
developed a collector system which usually employed a chelating agent and fuel oil.
The chelating agent formed a protective film on the mineral surface by
chemisorption, and fuel oil rendered this complex more hydrophobic and facilitated
the flotation. From several chelating agents they found cupferron to be effective for
flotation of pitchblende and hematite from quartz 1341. The recovery of pitchblende
was 100% in the pH range 0-3, with the cupferron concentration of 0.5 g/l (32
mmol/l) and fuel oil concentration of 0.1 g/l. A study of the flotation of chalcopyrite
ores by cupfemn found that fuel oil was required to float chalcopyrite [351. The
zeta potential measurements confmed the highly electrostatic nature of the collector
adsorption [36]. Fhbhakar et al. 1371 also studied chelation-flotation of other sflide
minerals, galena and sphalerite, using cupferron-fuel oil collector system and found
that cupferron was suitable. Dianzuo, Genquiang and Yuehya 1381 floated
wolframite from fluorspar and quartz using fuel oil in the presence of cupferron and
other chelating agents. All the flotation experiments on oxide minerals with
cupferron as collector were done in a Hallimond tube flotation cell with pure
minerals. Adsorption studies [13] confmed selective adsorption of cupfemn on
uranium in preference to quartz and illite.
Experimental Details
Materials
The Cluff Lake uranium ore from Saskatchewan,Canada was used. The ore has 0.3
to 3.0 wt% of uranium, about 10%clay, and minor amounrs of quartz and graphite.
204
Effkct of Substituent on the Performance of Cupferron as a Collectorfor Uranium
It has been shown that a8U, UDrh and =Ra exist in secular equilibrium in the ore
[39]. The average ore composition is given in Table 1.
Table I. Average Composition OfClq7Lake Uranium Ore (*valuesin Bqlg).
ElClDGBb
Composition (wt '%)
A1
8.05
As
Ba
0.07
0.03
0.32
0.02
1.22
1.43
0.01
0.0 I
0.01
0.02
0.0 I
0.03
0.01
0.33
0.35
0.02
Ca
cu
Fe
Mg
Mn
Mo
Ni
P
Pb
S
Th
Ti
U
V
Zn
=Ra
=rh
0.02
'45
'45
Aerofroth 65 obtained from Cyanamid Company was used as frother. Distilled
deionized water was used for all the flotation tests. For pH adjustments, analytical
grade HCI or KOH was used. Pure cupferron and various cupferron derivatives were
used as collectors. A stock collector solution containing 32 mmol/l was prepared by
dissolving the corresponding collector in distilled water. For low reagent solubility
in water, a minimum amount of methanol was used with water. All cupferron
derivatives were synthesizedby the procedures described below.
Synthesis of Cupferron Derivatives
A total of 19 cupferron derivatives were synthesized. For convenience of
presentation and discussion of results, they are divided into two groups, namely
substituted-cupferrons(p-chloro,p-bromo,p-fluom, mchloro, m-fluoro. pphenyl, p methoxy, p-methyl and rn-methyl cupfmns) and alkylcupferrons (methyl to nonyl
substituents, all in thep-position).
Reagents for Syntheses
Cupferron (analytical grade) was purchased from the Fisher Scientific Co. and was
recrystallized fiom methanol before use. All other reagents were obtained from
Aldrich Chemical Company. All substituted nitrobenzenes, alkyl benzenes,
hydrazine hydrate and 50% wet 5% Rh (rhodium) on carbon were used as purchased.
205
I. Nirdosh, S.V. Muthuswami, R. Ndarajan and R. Jeymaman
Butyl nitrite (95%) was distilled and used for nitrosation. Solvents used were all
ACS reagents. Zinc dust (-325 mesh) was washed with dilute hydrochloric acid to
remove any zinc oxide on the surface,and then vacuum dried.
Synthesis
Three schemes following standard procedures [40] were used to prepare cupfemn
derivatives, as shown in Figure 2.
x = p - CH,,m
- 9.
p - 0 M c . p - F,m - F,p - Cl, m - Cl.p - Br,p - C,H,
Scheme 1. Synthesis of Substituted Cupferrons
@
H-N-OH
($ 5;H;,50a/
Rh THF
on C
R
R
R = CH,OCH,
Scheme 2. Reduction Using Rh on Carbon
H-N-OH
I
R
i
R
R
R = Methyl, Ethyl, Propyl, iso-Propyl, Butyl, t-Butyl, iso-Butyl, n-Pentyl,
n-Hexyl, n-Heptyl, n-Octyl, n-Nonyl
Scheme 3. Synthesis of Alkyl Cupferrons
Figure 2 . Synthesis schemesfor preparing various cupferrons.
206
Effect OfSubstituent on the Petformame of Cupfcron ap a Collectorfor Urm'um
Procedure For Scheme 1
The general procedure is described using the p-bromocupferron synthesis as
example.
Ammonium chloride (1.5 g) in water (10 ml) was added to a solution of 1bromo-4-nitrobenzene(10.0 g) in methanol (100 ml). The solution was heated to
4OoC and zinc dust (6.0 g) was added slowly over a period of 20 minutes with
vigorous stirring. The progress of the reaction was monitored by analytical TLC for
maximum reduction of the starting compound. After completion of the reaction, the
solution was cooled and about 50 ml of cold water was added. The sotid was filtered
and washed with about 150 ml of solvent ether. The filtrate was extracted with 100
ml portions of ether, repeated three times. All the ether extracts were collected and
dried over anhydrous sodium sulfate. The ether solution was then transferred to a 1
liter three-necked round-bottomed flask fitted with a thennometer, a separatory
funnel and a gas inlet. The flask was cooled in an ice-salt bath. When the
temperature had fallen to OOC, anhydrous ammonia from a compressed gas cylinder
was bubbled through the solution for 10 minutes. The addition of ammonia was
continued and butyl nitrite (7 ml) was added slowly for about 5 minutes. The
solution was stirred for another 10 minutes for completion of the nitrosation reaction.
The colourless solid (p-bromocupferron) formed was filtered, washed with cool
ether, and weighed. The solid weighed approximately 2.5 g. The p-bromocupferron
was recrystallized from methanol.
Procedure for Scheme 2
The yields of cupferron derivatives synthesized by the above method varied from 20
to 60% of the theoretical value. For pmethoxycupfemn and p-rnethylcupferron,
the yields were low and hence the reduction of the corresponding nitro compounds
were tried with 50% wet 5 % Rh on carbon as catalyst and hydrazine hydrate as
reductant [41]. The yield was improved by about 10%. The catalyst used in this
method was washed and reused for further syntheses which is an advantage of this
procedure.
In a typical procedure,p-nitrotoluenewas reduced to hydroxylamine using Rh on
C as follows. To a solution of p-nitrotoluene (13.7 g) in tetrahydrofuran (50 ml)
taken in a 100 ml three-necked round-bottomedflask fitted with a thermometer and a
pressure equalized addition funnel, was added wet 5 % Rh on carbon (0.3 g). The
mixture was cooled to 15°C and hydrazine hydrate (5.4 ml) was added slowly
through the addition funnel for about 15 minutes. The mixture was then stirred for
another 2 hours and kept at 2530°C. The mixture was filtered and the catalyst
washed with 10 ml tetrahydrofuran. The p-tolylhydroxylamine formed in solution
was extracted by shaking the solution 'with 30 ml methylene chloride, and repeated
twice. The methylene chloride extracts were collected, dried over anhydrous sodium
sulfate, and concentrated to a low volume under reduced pressure. The ptolylhydroxylamine was precipitated by adding petroleum ether and the solid was
separated. The ptolylhydroxylamineobtained was dissolved in anhydrous ether and
nitrosatedusing butyl nitrite.
207
I. Nirdosh, S.V. M u t h w a m i , R.Naarajan and R.Jeymaman
Procedure for Scheme 3
The alkylcupferrons were prepared folIowing the general Scheme 3. This method is
a slightly modified procedure of Shimidzu and Okushita [42]. In a typical
procedure, p-butylcupferron was synthesized as follows. Nitric acid (45 ml, 70%)
was placed into a 500 ml round-bottomed flask. Sulfuric acid (50 ml, 96%) was
added slowly with constant stirring, and the flask was cooled in a cold-water bath.
This acid mixture was used for the nitration of allrylbenzenes. The p-butylbenzene
(25 g) was placed in a 250 ml round-bottomed flask, and the acid mixture (40 ml)
was added to it for one hour with stirring (maintained at about 3OOC). The solution
was then added to cold water and the organic layer of nim compound formed was
separated and washed several times with water, then with dilute sodium carbonate
solution, and finally with hot water. It was dried over anhydrous calcium chloride.
The nitrocompound was purified by fkactional distillation under vacuum (5 mm Hg).
The purified nitrocompound was then converted to the corresponding cupferron
derivative by Scheme 1 described above.
All the cupferrons were stored in a desiccator containing ammonium carbonate,
and held below 10°C in a refrigerator. The purity of the samples was confirmed by
C, H and N analyses. The calculated values showed good agreement with the
experimental values (see Table 2). The 'H NMR chemical shift values for the
cupferron derivativesconfirmed the identities of the compounds.
Table 2 . Carbon, hydrogen and nitrogen analyses of cupferron derivatives
(calculated values are given in brackets).
%C
%H
pMethyl
49.20 (49.68)
49.82 (49.68)
44.72 (45.42)
41.19 (41.61)
41.38 (41.61)
37.92 (38.00)
37.98 (38.00)
30.90 (30.78)
62.19 (62.19)
51.21 (52.43)
53.80 (54.79)
53.87 (54.79)
56.36 (56.84)
56.85 (56.84)
56.55 (56.84)
58.04 (58.64)
61.60 (61.63)
62.72 (62.89)
64.09 (64.01)
6.29 (6.56)
6.53 (6.56)
4.38 (4.41)
4.62 (4.66)
4.51 (4.66)
4.03 (4.26)
4.05 (4.26)
3.29 (3.45)
5.61 (5.68)
6.94 (7.16)
7.63 (7.68)
7.31 (7.68)
7.94 (8.12)
8.12 (8.12)
8.19 (8.12)
8.45 (8.50)
9.02 (9.15)
9.38 (9.42)
9.67 (9.69)
m-Methyl
p-Methoxy
p-Fluoro
ni -Fluor0
p-Chloro
m-Chloro
pBromo
p-Phenyl
PEthYI
pn-Propy 1
p-i-Propyl
p-n-Butyl
pt-Butyl
p-i-B~tyl
p-n-Penty1
p-n-Heptyl
p-n-Octyl
p-n-Nony I
208
-
Subtitueot
Oh
N
24.58 (24.84)
25.27 (24.84)
20.04 (20.04)
23.84 (24.27)
24.01 (24.27)
20.63 (22.16)
22.12 (22.16)
17.02 (17.96)
18.34 (18.17)
22.32 (22.94)
21.45 (21.81)
20.77 (21.81)
19.96 (19.89)
19.95 (19.89)
19.97 (19.89)
18.60 (18.64)
16.52 (16.58)
15.78 (15.81)
14.87 (14.93)
Effect of Substituent on the Performance of Cupfmron as a Collectorfor UrMiwn
Sample Preparation for Flotation
A 4 kg rod-mill discharge from the Cluff Lake Mill was passed through 12 mesh
screen. The oversize was ground using a pestle and mortar. The screened ore was
60% minus 50 mesh. The solids were mixed thoroughly, coned, and quartered
repeatedly, providing representative samples for flotation.
Flotation Procedure
For each flotation test, a 75 g sample of ore was wet ground for 45 minutes to
liberate fresh surface. This was done with burundum cylinders in a 1 liter porcelainfortified-with-aluminaball mill, both obtained from Fisher ScientifE, and 95% of the
wet ground ore passed through 50 mesh screen. The ground ore was immediately
transferred into a 2 liter beaker, 20 ml of the collector solution was added to it, and
the pH was adjusted to between 6.5 and 6.8. The ore was conditionedfor 45 minutes
by agitating with a mechanical stirrer. The pulp was then transferred to a Denver
flotation cell and about 8 1 of water was added. The pH was again adjusted to
between 6.5 and 6.8. Aerofroth 65 (2 drops) was added and the pulp was stirred for
5 minutes. Air was then introduced and the ore was floated to exhaustion. Two
additional floats were collected by adding 40 ml of collector solution and
conditioning for 15 minutes each time. The tail was then flocculated and filtered off.
The floats and tails were oven dried, weighed and analyzed for U and Al.
Analysis
1. The C, H and N analyses were carried out in a Conlrol Equipment Corporation,
Elemental Analyzer (Model 240-XA).
2. N M R spectra were recorded at 200 MHz on a Bruker, Aspect 2000 model
system. *HNMR were recorded in CDC1, with 0.03% TMS as internal standard
or in D20 containing DSS as internal standard.
3. The floats and tails were analyzed for uranium and aluminum by neutron
activation at the Slow Poke Reactor facility of the University of Toronto.
4. The filtrates of the flotation experiments were analyzed for uranium, aluminum
and sulfur by emission spectrum in a Jarrel-Ash, ICAP 9OOO instrument.
Results and Discussion
The performance of a cupferron derivative as a uranium collector will depend on its
hydrophobicity and also on the electron releasing/withdrawing nature of the
substituent. The %mass, %uranium and %aluminumrecoveries in float-concentrates
are given in Table 3. In general, all the cupferron derivatives recovered more
uranium than cupferron itself (see Table 3). In previous work [121on the flotation of
uranium from Elliot Lake ores, cupferron was found to recover 90-96 % of uranium
in 25-308 mass in contrast to the recovery of 35% in 8% mass in the present work.
The better performance in the case of the Elliot Lake ore may be attributed to the
branneritic and sulfidic nature of this ore as opposed to the pitchblende in Cluff Lake
ore. Because clay was the only aluminum-containing mineral in the ore, the
percentage of clay floated was taken as equal to the percentage of aluminum floated
The solutions were found to contain no detectableamount of uranium. This indicates
that the c h e w formed due to the interaction of the collectors with uranium in the
209
I. Nirabsh. S.V.M ~ h u s w a m iR.
. Natarajan and R. Jeymaman
mineral lattice are insoluble, within the pH range of 6.5-6.8 used in these flotation
tests.
Table 3. Results offitation tests with cupferron derivatives.
continued on oppositepage. . ..
Performance of Substituted-Cupferrons
Table 3 shows that the mass recovery in floats is quite low and lies between 10 and
20% of the feed. For the pmethoxycupferron, the mass of the float is as low as 8%
with a uranium recovery of 50%, and the clay floated is only 7%. This indicates that
uranium is floated in preference to aluminum, and the reagent has better selectivity
for uranium. The methoxy group is inductively electron-withdrawingdue to the
electronegativity of the oxygen. However, when it is in the para position on the
benzene ring, it exerts a strong electron-releasing effect due to resonance. This
results in the formation of an ion with high electron density on the donor (oxygen)
atoms due to cross conjugation (see Figure 3). Therefore, the basicity of the
compound increases and p-methoxycupferron will be the most basic of all the
cupferron derivatives. The enhanced basicity will increase the resonance bond
stability, however, factors such as back bonding from the metal to the vacant dorbitals of the ligand do not occur as the oxygen atoms are the donor atoms in
cupferrons. This may explain the improved selectivity of pmethoxycupferron, as
suggested by Marabini 1431.
210
Efect qf Substituent on the PerformMce of Cupferrm ap a Collectorfor Uranium
Table 3. continued
For the p-phenylcupferron, the uranium recovery was higher compared to
methoxy, methyl and ethyl substituents in the para position. The higher efficiency
may be attributed to the strong electron donating nature of the phenyl group, and
also to the increased hydrophobicity and the molecular cross section of the
compound. Introduction of a phenyl ring at the para position increases the chain
length of the hydrophobic carbon chain. Though addition of a six-membered ring
does not increase the carbon chain by six atoms and is equivalent to an ethyl group
14-41, the hydrophobicity of pphenylcupfenon appears to be greater than that of the
ethylcupferron. This was shown by the low solubility of the compound in water
during the prepation of the collector solution. A molecule of p-phenylcupfemn
has a greater cross-section at the hydrophobic chain than any of the other collectors
used. This causes an increase in the area occupied by the molecule when it attaches
to the mineral surface, and a larger area of the mineral surface becomes hydrophobic.
Hence the performance of p-phenylcupferron is influenced by a combination of
electronic interaction of the phenyl group at the para position, increased
hydrophobicity due to an increase in the chain length of the hydrophobic chain, and
greater cross-section of the molecule. The para halogenated compounds performed
better than meta substituted analogues. The better uranium recovery may be
atmbuted to the resonance interaction from the para position.
211
I. Nirdosh, S.V. M u t h w a m i , R. Natarajan and R. Jeymanum
Figure 3. Structure of p-methoxycupferron.
The chelating capacity affecting the flotation efficiency is related to the basicity of
the collector molecules, which in turn depends upon the electron withdrawing nature
of the substituent in the benzene ring of the cupferron molecules. The effect of
substituent on the pK, of benzoic acid was studied by Hammett [45], and the
electron releasing or electron withdrawing capacity of the substituent was quantified
from the dissociation constants of the substituted benzoic acids. These values are
known as the substituent constants, or the a-values, and they indicate whether the
substituent is electron withdrawing or electron releasing with respect to hydrogen.
Higher (s values mean a greater electron withdrawing effect by the substituent If
substituted H is assigned a a value of zero, substituents with positive a values will
be more electron withdrawing than H, and vice versa. Much research has and is
60
-
'P
p-ocn3
0
40-
P-Lw
0
d
m-F
0
rn-013
-
P-+
z
OI
n0
C
0
.-
m-CI
c
20v)
0
I
Figure 4. Dependence of separation eflciency on the Hammett substituent constant,
o.(Separation mciency values are given in Table 3.)
being directed towards the Linear Free Energy Relations, and there are many
physical organic chemistry books that explain these LFXR. NMR chemical shift
values and IR absorptions have also been correlated with substituent constants [MI.
There must exist a linear relation between the flotation efficiency and the Hammett a
values in the case of para and meta substituted cupferrons. Therefore, the four
compounds, namely the three para-halogenocupferrons and the pphenylcupfmn,
212
Effect ofSLlbstituent on the Pe$wmance of Cupfenon as a Collectotfor Uranium
are not included for the reasons discussed above, and their flotation efficiencies are
affected not only by mesomeric and inductive effects of the substituent but also by
other factors. Instead of uranium recovery, the separation efficiencies are plotted
against o values in Figure 4, showing a straight line with a correlation coefficient of
0.7. As chelation is the main process in flotation, charged species are involved in
flotation kinetics and hence Q+ values, which take into account the resonance effect
of the substituent, are preferred over Q values [46]. Separation efficiencies and Q+
values gave a straight line (see Figure 5)with an improved correlation coefficient of
0.8. It is unusual to get a straight line with a correlation efficient of 0.8 for a set of
data in an experiment such as flotation, where there are many variables and control is
difficult.
m-CI
0
ti
-0.8
-0.6
-0.2
-0.4
0
0.2
0.4
subslnwrd consml (Q*)
Figure 5. Dependence of separation efficiency on substituent constant with
resonance (a+).
0
2
4
6
8
u)
Number d CubonAtoms
Figure 6 . Dependence of separation efficiency on chain length of linear
alkylcupfemons. ( Point not included in least-squares analysis).
213
I. Nwdosh, S.V. Muthuswami, R.Natarajan and R.Jeyaraman
Performance of Akylcupferrons
The uranium recovery for all the compounds is high (see Table 3), and the improved
uranium recovery appears to be due to the increase in hydrophobicity of the
collectors The increase in chain length has affected the selectivity, and is shown by
the higher mass of the float concentrate when compared to the performance of the psubstituted cupferrons. There is a linear relationship between the separation
efficiency and the chain length of the akyl group (see Figure 6). The branched a k y l
groups such as isostructures have recovered more uranium, but this is at the expense
of increased mass of concentrate. Hence the separation efficiency of these
compounds indicates that branching does not significantly alter their performance.
Conclusions
General
1. Every substituent added to cupfemn has improved U recovery, compared to the
unsubstituted cupfemn. The separation efficiency of substitu?ed-cupferronris a
function of the substituent constants, and increases with an increase in the
electron-releasing ability (lower Q values) of the substituent. For compounds
such as p-phenyl and p-nonyl cupfmons, the better performance may also be due
to the greater cross-section of the molecule.
Linear Alkyl Substituents
2. Due to the improved hydrophobicity, alkylcupferrons perform better than
cupferron. As cupferrons are capable of forming a chelate with aluminum, an
initial removal of clay (or the use of a clay depressant) will improve the uranium
recovery and 99% uranium recovery in 10-15%mass appears to be feasible.
3. Mass recovery increases from 0 to 4 carbon atoms in the substituent, and then
decreases. A chain length with more than 7 carbon atoms appears to stabilize the
mass recovery.
4. Uranium recovery increases with chain length without going through a
maximum.
Linear vs Branched Alkyl Substituents
5. Recoveries from n-pmpyl, iso-propyl, n-butyl, iso-butyl and ten-butyl cupferrons
indicate that branched chains give better recovery than linear chains.
6. Recoveries from iso-propyl, iso-butyl and ten-butyl cupfemns appear to indicate
that a methyl branch, if it is close to the benzene ring, gives a better grade (low
mass, high uranium). Most of the extra mass floated when the methyl branch is
away from the benzene ring (as in p-isobutyl cupfemn) appears to be from the
flotation of clays.
Position of the Substituent
7. Both mass and uranium recoveries appear to decrease in the order: p > m > 0 .
8. Halogen substituents do not show any better results than alkyl substituents.
Uranium recovery does not change significantly with halogen in the p-position,
but the mass recovery decreases from F > C1> Br. The mass floated appears to
be related to the interfacial tension (0,0273, 0.0336 and 0.0358 N/m for fluoro-,
214
Effect of Substihcent on the Peflormance of Cupfuron as a Collectorfor Uranium
chlaro-, and bromo-benzenerespectively) [47]. However, in the rn-position there
is a marked decrease in both mass and uranium recoveries h m fluom to chloro
substituents.
9. Phenyl substituent is as good as n-heptyl even though its chain length is similar
to the ethyl substituent.
Acknowledgements
Financial support from the Natural Sciences and Engineering Research Council of
Canada (NSERC)is acknowledged. Thanks are due to Dr. TJ.Griffith, Mr. Keith
Prignik and Mr. Ian Wtsaka of the Lakehead University Instrument Laboratory for
elemental analyses, NMR spectral measurements, and analyses of uranium, iron and
sulphur by ICAP. R. Natarajan is grateful to NSERC for a Visiting Fellowship, and
to Bharatidasan Univetsity for permission to carry out part of his post-graduate work
at Lakehead University.
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Received: 13 July 1993; Accepted after revision: 20 April 1994.
217
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