**Christine Hettenkofer \*,**†**, Stephan Fromm** † **and Michael Schuster**

Division of Analytical Chemistry, Department of Chemistry, Technical University of Munich, Lichtenbergstraße 4, 85748 Garching, Germany; stephan.fromm.sw@t-online.de (S.F.); michael.schuster@tum.de (M.S.)


Received: 29 November 2020; Accepted: 14 December 2020; Published: 16 December 2020

**Abstract:** Urban mining from fly ash resulting from municipal solid waste incineration (MSWI) is becoming more and more important due to the increasing scarcity of supply-critical metals. Metal extraction from acid fly ash leaching has already been established. In this context selective Cu recovery is still a challenge. Therefore, our purpose was the separation of Cu(II) from MSWI fly ash extracts by polymer-assisted ultrafiltration (PAUF). We investigated three polyethyleneimines (PEIs) with regard to metal retention, Cu(II) selectivity, Cu(II) loading capacity, and the viscosity of the PEI containing solutions. A demanding challenge was the highly complex matrix of the fly ash extracts, which contain up to 16 interfering metal ions in high concentrations and a chloride content of 60 g L<sup>−</sup>1. Overcoming that, Cu(II) was selectively enriched and separated from real fly ash extract at pH 3.0. At pH 1.0, a PEI-free Cu(II) concentrate was obtained and PEIs could be regenerated for reuse in further separation cycles. The PAUF conditions developed at laboratory scale were successfully transferred to pilot scale, and hyperbranched PEI (HB-PEI) was found to be the most suitable reagent for PAUF in a technical scale. Moreover, HB-PEI enables photometric control of the Cu(II) enrichment.

**Keywords:** selective Cu(II) separation; sustainable waste treatment; municipal solid waste; polymer-assisted ultrafiltration; real fly ash extracts; urban mining; pilot installation

### **1. Introduction**

For decades, prosperity has increased worldwide, which is reflected in a strong rise in resource consumption. To cover the resulting resource demand, the use of secondary raw materials is becoming more and more important. Processing of residues derived from municipal solid waste incineration (MSWI) contributes to a sustainable circular economy. Beneath the widely used bottom ash [1], a mixture of the finer boiler and filter ash, called fly ash, represents an attractive source for raw materials in urban mining. The chemical composition and resource potential of MSWI fly ash was intensively investigated [2–4], with Zn, Cu, and Pb identified as being particularly suitable due to the high concentration of these metals in MSWI fly ash and their good extractability [5].

Acid washing is an established method for extracting heavy metals from MSWI fly ash. The fly ash is treated with a hydrochloric acid solution, preferably stemming from wet flue gas cleaning in MSWI plants. Under slightly acidic conditions (pH 3.0–4.0), heavy metals are extracted with an efficiency > 90% for Cd and Pb and 70–80% for Zn and Cu [2,3,6–8]. Vacuum belt filtration is used for dewatering, resulting in a chloride-rich fly ash extract containing the extracted metals and a heavy metal-poor filter cake. Acid fly ash washing is widely established in Swiss MSWI plants, but currently only two plants outside of Switzerland are treating fly ash this way (MVA Ingolstadt, Germany, and Termizo Liberec, Czech Republic).

For the recovery of heavy metals from fly ash extract, the so-called FLUREC process is a proven method [9]. Pb, Cu, and Cd are precipitated by cementation with Zn powder and collected as metal sludge. Subsequently, solvent extraction is used for selective extraction and enrichment of Zn. From a sulfuric acid concentrate, high-grade Zn is then recovered by electrowinning. This process is performed in the Zuchwil MSWI plant (Switzerland).

In most MSWI plants, including MVA Ingolstadt, the extract from acid fly ash washing is processed by hydroxide precipitation during wastewater treatment. The drained sludge contains, among other components, hydroxides of Zn, Pb, Cd, and Cu. For metal recovery, the sludge may be treated outside of the MSWI plant by applying the Waelz process [10], a pyrometallurgical technique primarily used for steelwork dusts. Here, Zn and Pb are recovered as Waelz oxide, used as secondary raw material in hydrometallurgical Zn production, while Cu is slagged and therefore lost for recovery.

Cu, however, became one of the most valuable metals through the 20th century, ranking after iron and aluminum in importance for infrastructure and technology [11]. It is commonly used in a broad range of industrial applications, becoming even more important hand-in-hand with the electrification of transport technologies. According to Elshkaki et al. [12], the Cu demand could rise from 275 to 350% by the year 2050 and exceed the projected Cu mineral resources. Consequently, preventing resource scarcity by improving the efficiency of the Cu cycle and enhancing Cu recycling rates is crucial.

In the so-called SESAM project funded by the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung), the resource potential of German MSWI fly ash and the recovery of valuable metals from MSWI fly ash was investigated [4,5,13]. Special attention was given to the acid fly ash leaching process as applied in the MSWI plant in Ingolstadt (Germany). In particular, the separation of valuable metals from the fly ash extract prior to hydroxide precipitation was targeted. In order to extend the spectrum of available techniques, the application of polymer-assisted ultrafiltration (PAUF) for selective separation of Cu from fly ash extract was studied.

This ultrafiltration technique is also called polymer-enhanced ultrafiltration (PEUF), polymersupported ultrafiltration (PSU), liquid-phase polymer-based retention (LPR), and polymer filtration (PF) [14]. It is based on the pH-dependent, reversible binding of metal ions to functionalized water-soluble polymers. In a membrane ultrafiltration step, the polymer–metal complexes are retained by the membrane and remain in the retentate solution, whereas non-bound ions pass the membrane and are found in the permeate solution. Polymer-assisted ultrafiltration is a single-phase process examined for the complexation, enrichment, and separation of metal ions from wastewater.

So far, many different polymers have been studied regarding PAUF applications [15]. An outstanding and extensively investigated water-soluble polymer class are polyethyleneimines (PEIs). Their amino groups are able to form ammine complexes with various (heavy) metal ions, such as Zn(II), Ni(II), and Cu(II). In recent years, Cu(II) separation has been performed using, inter alia, branched PEI and partially ethoxylated PEI; selective retention of Cu(II) using PEI was carried out at pH ≥ 3.0 [16,17], pH ≥ 5.0 [18], and pH ≥ 6.0 [19,20]. By decreasing the pH value, the respective metal ion can be released from the polymer.

A challenge in PAUF is the technical implementation of the filtration step in continuous operation. The polymer held back in the ultrafiltration retentate forms a layer on the membrane surface, severely decreasing the permeate flux. In order to control the thickness of this so-called gel layer, cross- or tangential flow filtration is applied. The polymer containing retentate is passed over the membrane surface tangentially to the permeate flow at high velocity. This causes turbulence in the retentate and reduces the gel layer formation. When assessing the technical application of water-soluble polymers, determining their hydrodynamic behavior in tangential flow filtration is an important factor [21–23].

Yet despite a substantial body of research polymer-assisted ultrafiltration still has not reached wide industrial application. Up to now, only a few PAUF studies have been carried out with operation in continuous mode with a pilot installation [22,24–27]. Schulte-Bockholt et al. [28] investigated the processing of industrial phosphating rinsing baths using a PAUF pilot plant. Apart from a few other applications like metal removal from chlorine free pulp and paper industry wastewater [29] and from mine drainage water [30] with metal concentrations in the mg L−<sup>1</sup> range, PAUF has primarily been investigated using synthetic metal ion solutions without testing the applicability with real wastewater. To the best of our knowledge, PAUF has not yet been applied for highly complex matrices, such as extracts of municipal solid waste or other applications in terms of urban mining.

In our study, the selective enrichment and separation of Cu(II) from MSWI fly ash extracts by PAUF was investigated. The high salinity of fly ash extracts with competing ions in concentrations up to the g L−<sup>1</sup> range, is extremely challenging not only for PAUF applications. Three commercially available PEIs were examined in laboratory-scale experiments with fly ash extracts from two MSWI plants. Based on these results, the hydrodynamic behavior of hyperbranched PEI was studied in the operation of a membrane ultrafiltration pilot plant. The technical scale retention, enrichment, purification, and regeneration of Cu(II) was thoroughly investigated and is discussed in the following sections.
