In order to obtain the aqueous phase button type batteries were leached: Sony SR626SW with nitric acid (1 mol/L) at ambient temperature (22˚C ± 1˚C). The selected extractants are: triisobutylphosphine sulfide (CYANEX 471X) [8] [9] and the bis (2-ethylhexyl) dithiophosphoric acid (D2EHDTPA) [10]. Table 2 shows the formulas and properties of these extractants. The organic phase is prepared diluting the extractant in kerosene.

For the extraction equal volumes (10 mL) of each phase are in contact during 30 minutes at ambient temperature with constant magnetic stirring, varying concentration of the different selected extractants. At the end of the stirring the phases are divided, the aqueous phase is analyzed by atomic adsorption spectroscopy (AAnalyst 200, Perkin Elmer) and the organic phase by matter balance. The extraction yield is defined as follows:

% Extraction = ([Ag]_org/([Ag]_aq + [Ag]_org) * 100 (1)

where [Ag]_org is the total concentration of silver in organic phase and [Ag]_aq the total concentration of silver in aqueous phase, with a rate of aqueous phase (A) with regards organic phase (O) of 1 (A/O = 1).

In order to define the minimum contact time between phases, an extraction with phases ratio (A/O = 1) is performed at obtained conditions, and in a given time a sample is collected to analyze and obtain the extraction yield with regards time.

To obtain the isotherms, the parameters found by experimental procedures are used at different rates of aqueous

^aThe extractant CYANEX 471X was supplied by CYTEC Canada Inc. ^bThe D2EHDTPA has been obtained either by stripping from a commercial zinc salt (Rhein Chemie).

phase in relation to organic phase (A/O), the concentration of silver in organic phase is calculate as follows:

[Ag]_org = A/O ([Ag]_{aq initial} − [Ag]_aq) (2)

where [Ag]_{aq initial} is the initial silver concentration before extraction, [Ag]_aq the final silver concentration after extraction, while A and O are the volumes of aqueous and organic phases respectively. Aiming for industrial application, the number of stages required for continuous process is computed by using the McCabe-Thiele method [11]. Figure 1 shows the system of mixers-settlers with size of 3.8 × 23.3 × 6.5 cm, stirring section of 3.8 × 3.8 × 6.5 cm. Stirring is made with magnetic stirring and to control flows, pumps and valves are used.

In the leaching process of battery cells using nitric acid (1 mol/L), the concentration of silver is in average 1800 mg/L, this value is reference to perform the extraction studies. Table 3 lists the extraction yields of each extractant.

This study allows to select D2HDTPA and an extraction study is performed taking into account the extract concentration with an initial Ag concentration of 1992 mg/L, ratio A/O = 1, ambient temperature (22˚ ± 1˚C) and 20 minutes of contact, these results are shown in Figure 2. The concentration of 0.02 mol/L of D2EHDTPA is selected, which allows obtaining yield of 80%, in a single extraction stage.

The kinetic study of extraction is made with an aqueous solution of 60 mL of silver concentration: 1803 mg/L in nitric medium (1 mol/L) with 60 mL of an organic solution of extractant 0.02 mol/L. 1 mL of each phase is removed at different times and the concentration is measured by atomic adsorption. Figure 3 shows this study.

From these results it can be considered that balance is achieved at 15 minutes of contact. With these studies the best extraction conditions are found at a concentration of 0.02 mol/L of D2EHDTPA and contact time of 15 minutes.

Figure 4 shows the experimental isotherm plotted by the dotted line and the continuous line plots the modeled isotherm.

For an initial concentration of 1743 mg/L of silver in nitric medium (1 mol/L) in the aqueous phase, there are obtained two theoretical extraction stages, with a rate of A/O = 0.65 and a final concentration of silver of 4.7 mg/L, the calculation of these steps is shown in Figure 5.

In the extraction study in the continuous mixer-settler a flow of 5.7 mL/min is used for organic phase and of 3.7 mL/min for the aqueous phase (A/O ≈ 0.65), after 40 minutes, the residual silver concentration in aqueous

phase is of 17.5 mg/L, a value consistent with the expected value of 4.7 mg/L, obtaining yield of global extraction of 99%, allowing to validate results obtained by McCabe-Thiele method. For desextraction it is proposed to use thiourea in sulfuric medium at a concentration rate ranging from 0.2 to 0.5, and contact time of 5 minutes.