3.5.2. Cu(II) Enrichment and Purification

Rinsing of the preconcentrate with water in the batch process was investigated. The purpose of this procedure is to displace fly ash extract from the preconcentrate gained in the retention step. Due to its high concentration in the preconcentrate, Ca(II) concentration serves as reference for further unbound species in the filtration circuit. As can be seen in Figure 7, the concentration of Ca(II) followed an exponential decrease during rinsing, which can be described by Equation (2). The concentration of an unbound species in the filtration circuit decreased from the initial concentration *ci*,0 according to

$$c\_{i, \mathfrak{o}} = c\_{i, 0} \times e^{-\mathfrak{o}} \tag{2}$$

with ˾ giving the volume of rinsing water used (in multiples of filtration circuit volume). An evaluation of this equation is given in Table 5. Though slightly bound by the polymer in laboratory experiments and in the retention step, Zn(II) and Pb(II) were successfully rinsed from the filtration circuit equally to unbound species.

**Table 5.** Reduction of unbound species in the filtration circuit by rinsing with water (calculated by Equation (2)).


**Figure 7.** Rinsing of filtration circuit with water.

An alternative way to increase Cu(II) selectivity is to thicken the preconcentrate. This is achieved by feeding Cu(II) loaded preconcentrate exceeding the volume of the filtration circuit to the pilot plant. The polymer is retained by the membrane and Cu(II) is enriched linearly with increasing polymer concentration in the filtration circuit, while the concentration of unbound species passing the membrane remains equal. Combining the thickening step with purification of the concentrate obtained by rinsing with water, a substantial enhancement of Cu(II) selectivity toward unbound species could be obtained (Table 6).


**Table 6.** Enhancement of Cu(II) selectivity toward unbound species by increased polymer concentration (thickening) and rinsing of filtration circuit (with water); 4 g L−<sup>1</sup> HB-PEI in the initial concentrate was considered.

When the preconcentrate was rinsed at an initial Cu(II) loading of 250 mg Cu(II)/g polymer, a significant loss of Cu(II) occurred. Therefore, rinsing was optimized by reducing the initial loading to 200 Cu(II)/g polymer. As can be seen in Figure 7, the Cu(II) concentration did not drop when rinsing was done at pH 4.0, and only slightly decreased when the pH value was lowered to 3.5 (96%) and 3.0 (94%) during rinsing (˾= 1.8).

The very high concentration of interfering ions in the preconcentrate requires intense rinsing of the filtration circuit. It was very efficient at the beginning but dilution of the fly ash extract in the filtration circuit resulted in a steady decrease of rinsing efficiency with increasing volume of rinsing water used. High purification of the Cu(II) preconcentrate was accompanied by a high consumption of rinsing water and, due to the application of tangential flow ultrafiltration, energy. Using PAUF for selective enrichment and separation requires processes for further metal recovery that tolerate remaining interfering ions in the PAUF concentrate in order to minimize the required rinsing.
