*3.5. Multistage Process for Technical Scale Separation of Cu(II) from Fly Ash Extract*

A PAUF-based process for the selective separation and purification of Cu(II) was investigated, as shown in Figure 5. In the retention stage, fly ash extract and HB-PEI solution were fed to an ultrafiltration plant. Cu(II) was bound by the polymer and retained in the filtration circuit. A preconcentrate containing Cu(II) loaded HB-PEI in fly ash extract was (continuously) ejected from the filtration circuit, and the Cu(II) depleted permeate was discharged as wastewater. In order to improve the selectivity of Cu(II) toward interfering ions in the fly ash extract, the preconcentrate was purified in another ultrafiltration stage. This can be done either by thickening the preconcentrate, where the concentration of Cu(II) loaded polymer and therefore Cu(II) in the filtration circuit is enriched or by rinsing the filtration circuit with water in order to displace fly ash extract from the polymer solution. Depending on the technical realization, the steps may be done in a different order or at the same time. For Cu(II) recovery and regeneration of HB-PEI, the pH is decreased and Cu(II) is released from the polymer. Cu(II) is rinsed from the filtration circuit, producing a polymer-free Cu(II) concentrate. The regenerated HB-PEI solution is recirculated into the retention stage.

**Figure 5.** Multistep polymer-assisted ultrafiltration (PAUF) process for selective separation of Cu(II) from MSWI fly ash extract. A polymer-free Cu(II) concentrate is obtained in the process.

The technical implementation of the different steps (retention, enrichment/purification, regeneration) is investigated in the following sections. The cumulative volume of solutions fed into the pilot plant is given in ˾ (multiples of the filtration circuit volume). The concentration of elements in the filtration circuit is referenced to their concentration in the feed solution (in continuous operation) respective to their concentration in the filtration circuit at the beginning of the rinsing experiments (in batch operation).

### 3.5.1. Retention of Cu(II) in Continuous Operation

The retention of Cu(II) was investigated with two fly ash extracts containing 0.13 g L−<sup>1</sup> and 0.7 g L−<sup>1</sup> Cu(II). The pilot plant was filled with 4 g L−<sup>1</sup> pretreated HB-PEI in water. After startup, fly ash extract was fed to the plant and treated at pH 3.0. Figure 6 shows the concentrations of Cu(II), Pb(II), Zn(II), and Ca(II) in the filtration circuit during both experiments.

**Figure 6.** Enrichment of Cu(II), Pb(II), Zn(II), and Ca(II) in filtration circuit during continuous Cu(II) retention treating fly ash extracts from MVA Ingolstadt. The process was started with water in the filtration circuit, resulting in an arithmetically negative enrichment. Reference: concentration in the fly ash extract. HB-PEI = 4gL<sup>−</sup>1, pH 3.0, T = 40 ◦C.

At the beginning, the concentration of elements in the filtration circuit was lower than in the fly ash extract, resulting in an arithmetically negative enrichment factor. After the feed was switched to fly ash extract, the water in the filtration circuit was continuously displaced and the concentrations of Cu(II), Zn(II), Pb(II), and Ca(II) increased. After ˾ = 3, Ca(II) concentration in the filtration circuit was equal to the Ca(II) concentration in the feed solution and therefore the enrichment was zero. This implies that the displacement of water was completed and the filtration circuit from then on was only filled with fly ash extract and HB-PEI.

Ca(II) enrichment remained zero in further operation of the pilot plant. In accordance with the laboratory experiments, Ca(II) was not retained in PAUF. It represents further elements that were not bound by HB-PEI at pH 3.0 which also showed zero enrichment (including chloride).

For Zn(II) and Pb(II), a slight enrichment in the filtration circuit was observed, reaching a steady state at ˾ = 4. This slight enrichment was limited to 10% for Zn(II) and 30% for Pb(II). The latter already showed a slight binding by HB-PEI in laboratory experiments. This originates from Pb-chloro complexes interacting electrostatically with HB-PEI (see Section 3.1). At pH 3.0 Zn(II) can also form Zn-chloro complexes (weaker compared to Pb-chloro complexes) [6]. Both metal ions do not bind to

HB-PEI via metal complex formation at pH 3.0 and therefore showed no linear increase in the filtration circuit in continuous operation.

In contrast, the concentration of Cu(II) in the filtration circuit correlated linearly with the volume of fly ash extract treated, irrespective of whether a feed with 0.13 g L−<sup>1</sup> or 0.7 g L−<sup>1</sup> Cu(II) was used. With 4 g L−<sup>1</sup> of HB-PEI, the effective Cu(II) concentration in the filtration circuit was limited to 0.8 g L−<sup>1</sup> Cu(II) when loading of 200 mg Cu(II)/g polymer was accepted. Cu(II) exceeding this concentration was not sufficiently bound by HB-PEI and was lost in the following stages.

For Cu(II) retention in continuous operation, addition of sodium hydroxide solution to the filtration circuit is required, as otherwise the pH value drops and Cu(II) is no longer retained. The intense mixing due to the tangential flow and high buffer capacity of the polymer solution allowed perfect pH control in the filtration circuit. The process was able to selectively retain Cu(II) in the filtration circuit, even if strong fluctuation of the Cu(II) concentration occurred in the feed.

Though Cu(II) was enriched considerably, the achievable selectivity of Cu(II) toward Zn(II), Pb(II), and Ca(II) in a preconcentrate ejected from the filtration circuit was quite low (Table 4). In steady state, the Cu(II) loaded polymer was in the fly ash extract the filtration circuit was filled with. To improve Cu(II) selectivity, the fly ash extract had to be rinsed out from the preconcentrate in a second step.

**Table 4.** Concentration of Pb(II), Zn(II), and Ca(II) in feed solution and filtration circuit in steady state. Composition of concentrate ejected from retention step if loading of 200 mg Cu/g polymer is accepted.

