Acidic organophosphorus extractants—XIX

J. inorg, nucl. Chem., 1974, Vol. 36, pp. 189-192. PergamonPress. Printed in Great Britain.

ACIDIC ORGANOPHOSPHORUS E X T R A C T A N T S - - X I X EXTRACTION OF Cu(II), Co(II), Ni(II), Zn(II) AND Cd(II) BY DI(2-ETHYLHEXYL) PHOSPHORIC ACID R. GRIMM and Z. KOLAI~fK Institut fiJr Heisse Chemie, Kernforschungszentrum Karlsruhe, German Federal Republic (Received 9 February 1973)

The extractability by di(2-ethylhexyl) phosphoric acid in n-dodecane from 1 M (Na, H)NO 3 and 1 M (Na, H)CI decreases in the orders Znill) > Cd(ll) > Cu(ll) > Co(lI) > Ni(II)and Zn(ll) > Cu(II) > Cd(II) ~ Co(II) > Ni(ll) respectively. With HA denoting the monomeric molecule of the extractant, the composition of the extracted complexes is ZnA 2 . HA, ZnA 2 . 2HA, CuA 2 . 2HA, CoA 2 . 2HA, CdA 2 . 3HA and NiA2.4HA. Zn can be effectively separated from the other metals. Abstract

INTRODUCTION DI(2-ETHYLHEXYL) phosphoric acid (henceforth H D E H P ) has been shown to be a promising extractant for large scale hydrometallurgical processing of divalent transition metals like Cu, Mn, Co, etc.[l]. Solutions of H D E H P in organic diluents can be highly loaded by divalent metal ions without formation of a third phase or becoming unacceptably viscous, while a favourable metal to H D E H P ratio of 1/2 is reached[l, 2]. There have appeared rather numerous papers dealing with the extraction of divalent transition metals[1 10]. They have shown that the metals are extracted by H D E H P in an extractability order different from carboxylic acids or oximes which extract preferentially Cu(I I). However, the published data differ in the conditions of the work so that there is no possibility of comparing directly the extractability of single metals and estimating the separation factors. Moreover, very little is known about the effects of compounds like tri-n-butyl phosphate (henceforth TBP) or inorganic anions. Thus it appeared interesting to study the extractability of important bivalent transition metals, namely Cu, Co, Ni, Zn and Cu. EXPERIMENTAL

The procedure has been described previously[l l]; the shaking time was 1 hr and the initial metal concentrations in trace experiments were < l0 4 M. The ionic strength was adjusted to 1.0 by sodium nitrate or chloride, if not otherwise given. The pH-meter 26 (Radiometer, Copenhagen) was used and pH was measured as described before[12~. The beta radioactivity of 63Ni was measured by a liquid scintillation method, while the gamma radioactivity of the other radioisotopes was measured by NaI(T1) scintillation crystals.[ 13].

Reagents HDEHP was purifed as given in[t3, 14]. n-Dodecane (Fluka, olefine free), TBP (Fluka, puriss.) and common inorganic reagents (Merck, reagent grade) were used as received. The radioactive isotopes 6°Co, 6SEn, 63Ni and 1°9Cd were purchased from Amersham and 64Cu was obtained by irradiating Cu metal by thermal neutrons. RESULTS AND DISCUSSION

The logarithmic dependencies of the distribution ratio, D, of trace metals on the hydrogen ion concentration have a linear form at - l o g [ H +] < 2.7 (Fig. 1); the slope is 2 as expected. The deviation of the log D vs - l o g [ H +] dependencies from straight lines at lower hydrogen ion concentrations is due to a partial conversion of H D E H P in the organic phase to a sodium salt: pH values in this region were adjusted by the addition of sodium hydroxide before shaking. The formation of a salt of the type N a A . nHA (with HA denoting the monomeric molecule of H D E H P ) can be expected under the conditions of our work, i.e. at less than 25 per cent H D E H P neutralized[15]. This lowers significantly the concentration of free H D E H P even at a low degree of neutralization. The logarithmic dependencies of the distribution ratio on the H D E H P concentration, CA,in the organic phase are shown in Fig. 2. Straight lines with the slope 2 were obtained with Cu(II) and Co(II) and, consequently, the composition of the extracted complexes can be written as M A 2 . 2 H A (with M = Cu or Co). This agrees with the published data on the extraction of Cu(II)[2, 4] and Co(II)[2, 4, 6] by H D E H P in aliphatic diluents. The use of benzene diluent in the extraction of Cu(lI) lowers the extraction efficiency, but does not 189

190

R. GRIMMand Z. KOLAI~I'K

c~

0

I

2

3

4

-log IH ÷]

Fig. 1. Distribution ratio, D, of trace metals as a function of the hydrogen ion concentration. Pb(II)--data taken from [8], Mn(II)--data taken from [9]. Organic phase: 0-3 M HDEHP in n-dodecane (Zrg Cd, Cu, Co, Ni), n-heptane (Pb) and n-octane (Mn) diluents. Aqueous phase: 1.0 M (Na, H) NOa(Zn, Cd, Cu, Co, Ni), 1.0 M (Li, H)NO3(Mn) and dilute HNO3(Pb). change the composition of the extracted complex[5]. In the extraction of Zn(II) the log Dvslog CA dependency, if considered as linear, has a slope of 1.7 at CA ~< 0.1 M, i.e. a value slightly higher than that of 1.5 reported for aliphatic diluents[10]. Thus according to our results both the complexes ZnA2.HA, and ZnA 2.2HA are formed in the organic phase. The slightly curved theoretical log D vs log CA dependence corresponding to the simultaneous formation of two extracted complexes fits well the experimental points at CA <<-0.1 M (Fig. 2). We have no explanation for the deviation of the distribution ratios to lower values at CA > 0.1 M. An eventual higher self-association of HDEHP than dimerization, as suggested in n-hexane diluent[Ill, whould also influence the logD vs log CA dependencies for the other metals, but this is not the case (Fig. 2). In carbon tetrachloride, o-xylene, benzene and chloroform diluents only the complex ZnA2 . 2HA has been reported to be formed[10]. The slope 3 of the log D vs log Ca dependence obtained with Ni(II) suggests the formation of the complex NiA2.4HA in n-dodecane. This is in contradiction to the reported extraction of Ni(II) as the complex NiA2.2HA by HDEH P in a very similar diluent, namely kerosene [2, 4]. With Cd(II) the slope of the log vs log Ca dependency has been found to be 2-5, indicating so that the complex

CdA 2 . 3HA is extracted. No published data about the composition of an extracted complex of Cd(II) with HDEHP are available. In closing the discussion of the log D vs log CA dependencies, it should be noted that they have been measured at very low degrees (< 3 ~o) of conversion of HDEHP to a sodium salt. TBP lowers the distribution ratios of all the metals studied (Fig. 3). The extent of the TBP effect becomes more pronounced in the order of increasing extractability of the metals by HDEHP. The nonuniformity of the log D vs log CTBpdependencies can be explained by the lowering of the thermodynamic activity of the HDEHP by interacting with TBP being partially compensated in the extraction of Cu(II), Co(II) and Ni(II) by the formation of synergic mixed metalHDEHP-TBP complexes. The curve obtained with Ni(II) is perhaps not fully comparable because it was measured at about 10 per cent HDEHP neutralized in the organic phase in order not to work at low D values. No essential lowering of the separation factors must be expected if the TBP concentration is kept at a level necessary to prevent the formation of a third phase during an alkaline solvent wash: according to our experience the ratio of molar concentrations of TBP and HDEHP need not be higher in such a case than two-thirds. The effect of chloride ions on the extraction is shown in Fig. 4. The position of Cd(II) in the extractability sequence is significantly changed by replacing the 1"0 M (Na, H)NO 3 medium with 1'0 M (Na, H)C1. All

-~

t~ ~ -a

-a

-4

I -3

-2

-I

0

log cA

Fig. 2. Distribution ratio, D, of trace metals as a function of the HDEHP concentration, Ca, in the organic phase. Aqueous phase 1.0 M (Na, H)NO3, n-dodecane diluent.

191

Acidic organophosphorus extractants--XIX

o..2 zx

('e-

Co(~)~

I

L

i

I

-log CTBp

Fig. 3. Distribution ratio, D, of trace metals as a function of the TBP concentration, CTBP, in the organic phase. Aqueous phase 1.0 M (Na, H)NO 3, - l o g [ H +] = 2-43 (Zn), 2.42(Cd), 3.25(Ni), 2.95(Co) and 2.10(Cu). n-Dodecane diluent. other metals studied exhibit a m u c h smaller difference between the stability of chloride and nitrate complexes. A chloride containing aqueous phase is thus suitable for an effective separation of Zn from Cd, Cu, Ni and Co. To show the practical applicability of H D E H P for the separation of large a m o u n t s of Zn, some saturation experiments have been undertaken. The organic phase has been 0 . 5 M (Na, H ) D E H P + 0 . 3 4 M TBP in ndodecane. The use of at least partially neutralized solvent should simulate a technological application, where the use of H D E H P in a pure acid form would cause difficulties in adjusting the equilibrium hydrogen ion concentration[l]. The results of the saturation experiments from chloride containing aqueous phases Zn CII)

are gathered in Table 1. The m a x i m u m a m o u n t of Zn(II) extracted is larger than corresponds to the formation of a complex with the Zn to H D E H P ratio of 1/2. This can be ascribed to the presence of T B P in the organic phase, because T B P alone extracts Zn(II) rather effectively from HC1 + NaCl solutions[16]. If a 0 . 5 M N a D E H P + 0.34M T B P solution in dodecane is contacted repeatedly with aqueous 0-2 M ZnCI 2 + 0'2 M CdC12 + 0.2 M CuC12, the organic phase is loaded preferentially with Zn(II), while Cd(II) and Cu(II) are displaced; their concentration in the organic phase after the third contact is of the order 10 -~ M. Table 1. Loading of 0.5 M (Na, H)DEHP + 0.34 M TBP in n-dodecane by Zn(lI). Aqueous phase: variable ZnCI 2 ;EH + 1 and ionic strength were not kept constant. C is molar concentration --0.475 M NaDEHP + 0-025 M HDEHP Czn.aq Czn.org

0.5 M NaDEHP + no HDEHP Czn.aq Czn.org

0.0014 0-042 0.206 0.43

0.0003 0-025 0-18 0.41

0-200 0.258 0-294 0.32

0-200 0.275 0-32 0-34

O u r results can be completed with published data on the extraction of Pb(II)[8] and Mn(II)E9] by H D E H P in aliphatic diluents from nitrate solutions. Then the extractability sequence from 1-0 M (Na, H)NO3 can be written as Zn(II) > Pb(II) ~ Mn(II) > Cd(II) > Cu(II) > Co(II) > Ni(II). With 1.0 M(Na, H)C1 as the aqueous phase the sequence changes to Z n ( I I ) > (Mn(II)) > Cu(II) > Cd(II) ~ Co(II) > Ni(II). The extractability of Ca(lI) by 0 . 3 M H D E H P in an aliphatic diluent is similar to that of Zn(II)[17]. As already mentioned, carboxylic acids exhibit a quite different selectivity order. For example the extraction by a mixture of C7 C9 fatty acids in kerosene decreases

I --

0.4 Cu (If)

o

i

~

NI

(If)

0-3

Zn (11)

F 0.2 -I

0"I

-2 ~

C

o

(ll)

c.

o O

-lo~ [ c i ' ]

Fig. 4. Distribution ratio, D, oftrace metals as a function of the C1 concentration in the aqueous phase which has been 1.0 M (Na, H)(NO 3, CI); - l o g [ H +] = 2-00 (Zn and Cd), 2.44 (Cu), 3-55 (Ni) and 1-85 {Co). n-Dodecane diluent.

~

i

2

3

n

I

4

l

5

Fig. 5. Molar concentrations of metals in initially 0-5 NaDEHP + 0.34M TBP in n-dodecane after repeated contacting with fresh 0-2 M ZnC12 + 0.2 M CdC1 z + 0.2 M CuCI2 ; n is the number of contacts.

192

R. GRIMMand Z. KOLAI~I'K

in the order Pb(II) > Cu(II) >> Cd(II) > Zn(II) > Ni(II) > Co(II)[18]. Similarly Versatic 911, a mixture of branched alkyl carboxylic acids manufactured by Shell. extracts divalent metals in the order Cu(II)>> Ni(II) > Co(lI) > Mn(II)[19, 20). The extractability of metals by carboxylic acids and the stability of metal acetate complexes in aqueous solutions[21] decreases in qualitatively parallel series. We have tried to find a similar parallel for H D E H P ; this could be useful for predicting extractabilities from known values. However, of the data available both solubility products of phosphates ([21] p. 180) and stability constants of pyrophosphate complexes ([21] p. 190) decrease in an order of metals more similar to that found with carboxlic acids than with H D E H P . REFERENCES

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