3.2.2.1. Results of solvent extraction method

Results from the liquid extraction of [Co(CN)6]3- are shown in Fig. 48. The primary amine, Primene JM-T, extracted only minor quantities at pH1. At higher pH values, the extraction was not detectable. The secondary amine, Amberlite LA-2, without H2SO4 pretreatment shows as expected a reasonably high extraction at pH1, but has a strongly falling tendency towards higher pH values while the same resin with H2SO4 pretreatment maintains a high extraction yield until pH5 but decreases at higher pH values. Only the tertiary amine extractant, Alamine 336, extracted the Co complex strongly (near 100%) over the whole pH1-7 range.

Since Alamine 336 contains a basic nitrogen atom in the amine group, it may react with a variety of inorganic and organic acids to form amine salts, which are capable of undergoing ion exchange reactions with a host of other anions. As such, Alamine 336 is a liquid ion exchanger operated in a solvent extraction system. The general reactions, which are shown in Table 9, illustrate the two steps protonation and anion exchange:

After extraction, the organic phases containing Amberlite LA-2 and Alamine 336 were tested for stripping by the agents listed in Table 8. Ammonia could be used for stripping the Co complex from the Amberlite phases, while

TABLE 9. PROTONATION AND ION EXCHANGE: GENERAL REACTIONS

1. [R3N]org + [HA]aq 5 [RgNH+A-lorg

2. [RgN]org + [HCl]aq 5 [RgNH+Cl-]org

3. [RgNH+A-]org + [B-]aq 5 [RgNH+B-]org + [A-]aq

4. 3[R3NH+Cl-]org + [Co(CN)6]3-aq 5 {^NH+yCo^NL]3-^ + 3[Cl-]aq

K2(COO)2 (potassium oxalate) could be used for stripping Co complex from both for the Amberlite and Alamine phases. For Alamine 336, for instance, the type of stripping agent to be recommended depends on the overall recovery process. In general, basic stripping agents, which reverse the protonation reaction, show the best stripping efficiency.

Two alternative mechanisms for stripping [Co(CN)6]3- are:

(1) 2{[R3NH+]3[Co(CN)6]3-}org + 3[K2(COO)2]aq 5 З^КЦ

+ 3[H2(COO)2]aq+org + 2K3[Co(CN)6]aq

(2) 2{[R3NH+]3[Co(CN)6]3-}org + 3[K2(COO)2]aq 5 3{[R3NH+]2[(COO)22-]}org

+ 2{K3[Co(CN)6]}

3.2.2.2. Results of anion exchange method (1) [Co(CN)J3-

Table 10 gives the ion exchange yields for tracer concentration of [Co(CN)6]3-on various ion exchange resins with a feed volume of 100 mL.

TABLE 10. THE ION EXCHANGE YIELDS FOR THE RESINS Anion resin Anion exchange yield (Tabs (%)) Amberlite IR45 40.7 Lewatit MP60 2.6 Dowex 1 x 2 (50-100 mesh) 95.9 Dowex 1 x 2 (200-400 mesh) 100 Dowex 2 x 8 (100-200 mesh) 100

FIG 49. Functional group of Dowex 1 (a) and 2 (b).

It is obvious from the data in Table 10 that the strong base resins are superior to the weak base ones. Although both Dowex 1 and Dowex 2 are strong bases and quaternary amines, there is a difference in the functional groups:

• The functional (or ionogenic) group of Dowex 1 (Fig. 49(a)) is — CH2-N+(CH3)3.

• For Dowex 2 the ionogenic group (Fig. 49(b)) is

-CH2-N+(CH3)2-C2H4OH.

• This latter structure implies that the base strength is somewhat lower than for the first structure. This is not seen in the ion exchange yield but became evident in the succeeding stripping process.

Table 11 shows that elution of [Co(CN)6]3- from the Dowex 2 resin is easier than from the Dowex 1 resin both for strong HNO3 and HCl elution agent solutions reflecting the somewhat weaker basicity of Dowex 2.

(2) SCN-

A method for isolation and upconcentration of SCN- from sea water (produced water) has previously been published and is based on the use of the

Elution agent		Elution yield (ГеЫ (%))
Dowex 1	Dowex 2
12M HCl	24.7	63.4
14.5M HNO3	52.3	80.3

Optimized elution volume (m!) FIG. 50. Absorption yield of radiolabelled SCN on BioRad AG1 is ~98.5% for a sample volume of1000 mL of tracer-containing brine (seawater salinity).

anion exchange resin BioRad AG1, which is, in principle, equivalent to Dowex 1. Figure 50 shows the stripping yield (green curve) with 2.8M NaClO4 as the stripping agent and the total chemical yield (red curve) of the SCN — separation and enrichment process, both as a function of the collected volume of the eluate. The elution peak is shown in the inset.

The performance of SCN — on the Dowex 2 x 8 resin with the column dimension described earlier was checked and the separation factor from [Co(CN)6]3- with various elution agents investigated. Results of elution yields and separation factors are given in Table 12.

Figure 51 illustrates the absorption characteristics for [Co(CN)6]3- and SCN — on the Dowex 2 x 8 column and compares these data with the sorption characteristics of SCN — on the BioRad AG1 column.

О

„20 ■ 1 ■ I ■ I_________________ ■ I ■ I

0 200 400 600 800 1000

Raffinate volume (mL)

FIG. 51. Absorption yield of [Co(CN)6]3 and SCN — on the 0.5 mL Dowex 2 x8 (100-200 mesh) column, and for SCN — on the 8.5 mL BioRad AG1 column, as a function of raffinate volume (or original sample size) for tracer-containing seawater samples.

It is obvious from the data that SCN — experiences a rather fast breakthrough on the 0.5 mL Dowex 2 x 8 column, so the separation from [Co(CN)6]3- will be substantial in the absorption process.