3.1.1. CaSO4

SSA contains calcium sulphate. HCl digests this initial SSA component. This material is commonly found around the edges of angular silicate particles, forming narrow rims (max ~30 μm) around grains. In terms of particle formation within the thermal process, this is interpreted as a result of condensation onto already formed particles. According to the mineral grain size distribution function of the MLA dataview software, this material has a typical D50 of ~54 μm (grainsize at 50% of the cumulative passing grain size distribution curve).

#### 3.1.2. Quartz and Other Silicates

Quartz is used as a fluidized bed in sewage sludge combustion, and is the only mineral reliably detected by XRD in these SSA. Since the acid used is unable to digest quartz, the initial amount of quartz should be preserved. For HCl digested residue, the compositional variance is within the normal

variation of this material—a mode between 26 and 29 area-% (original SSA: mode of quartz 27 area-%). The spectra grouped as "other silicates" all contain Si as the major component, meaning no other measured element is more abundant within this spectrum. Still, within this group, the measured Si content varies between 7 and 32 wt% and averaging 23 wt% of the mode. "Other silicates" reach a slightly lower D50 (~98 μm) than quartz (~140 μm). "Other silicates" tend to be porous, rounded, amoeboid particles whereas quartz particles are more angular and lack pores.

## 3.1.3. Ca-Dominant Material

Material, in which Ca is the main component, is reduced from 7 area-% in SSA to 1 area-% in digestion residues. Spectra in this group contain an average of 27 wt% Ca. This material reaches a D50 of about 50 μm. It is found both as part of narrow rims around larger quartz and other silicate particles as well as porous, amoeboid, particles in conjunction with other silicates or phosphates. This material rarely forms particles with Al- or Fe-dominant material, an observation which encourages the interpretation that the thermal reorganization of SSA material is incomplete: phosphates are precipitated using Al- and Fe-salts during the wastewater treatment process. Since Al- and Fe-ions are unwanted in the recovered phosphoric acid, one goal of sewage sludge incineration for P-recovery is reorganizing the P-content into calcium-phosphates. However, the majority of spectra in this group are not only Ca-dominant but also contain Si, Al, and Fe.

#### 3.1.4. High P-Material (>14 wt% P)

The residue still contains some remnants of P-rich material. Nonetheless, the content was greatly decreased in comparison to the original material. Less than 1.4 area-% of these phases remain, mostly the spectrum named as FePMgO. This residual P-content is rendered as acid-accessible by thermochemical pre-treatment, as was concluded by its absence in thermochemically pre-treated residue. Using the initial generic labelling, this material would have ended up in other groups, the aforementioned spectrum in Fe-rich material, for example. At a D50 of 90 μm, these particles are among the larger ones within the material, and they do form particles on their own, but often with the fully recoverable 3 key phases, silicates and Ca-dominant material. One of the spectra in here is Fe-P-O, a product of the precipitation of phosphates using Fe-salts during the wastewater treatment process. This material on its own has a D50 of 40 μm and usually forms particles with one of the 3 key spectra or other high P-material, a product of the incomplete thermal reorganization of phosphates during incineration.

#### 3.1.5. Fe- and Al-Rich Material

These material components are formed during phosphorus removal in sewage treatment plants. Using iron and/or aluminum salts, phosphorus compounds precipitate and are preserved during incineration. Some of these only slightly soluble compounds even survive acid treatment, which is why these components appear relatively enriched in the residues. The residue shows an enrichment in Fe-phases compared to SSA. Residues will always represent an enrichment in phases which are hard to digest. As already mentioned, this SSA is contaminated with heavy metals and does not comply with the German fertilizer ordinance [19]. To address this issue, thermochemical pre-treatment was carried out. It was found that this treatment also improved P-recovery [9], which is why this process step was studied in the present experiments testing another acid. Less than 0.1 area-% of the three P-rich (14+ P) spectra remain. Fe-rich material was removed during pre-treatment, thus, reduced contents of this group are not surprising (by calculated assay, Fe content was roughly halved, 13.4 to 6.35 wt%). All apparent increases in components in these residues are relative increases as more material has been digested. Particle sizes for these groups vary; at a D50 of ~100 μm, the Al-rich material is much coarser than Fe-oxides (D50 of ~45 μm). Fe-rich material typically appears as grains within larger, host particles of almost all other groups, the two exceptions being CaSO4 and high P-material. Al-rich material follows the same trend.

#### 3.1.6. HCl Digestion of Thermochemically Pre-Treated SSA

Comparing the pre-treated residue to the untreated residue, the Al-rich material and silicate material proportions rise. Fractions of Fe-rich, Ca-rich and residual high P-material decrease, the latter almost disappearing (0.02 area-%). P-recovery was more successful and pre-treated digestion residue contains less P (by calculated assay, about 0.5% less—1.08 wt% P remain). This highlights the need for an improved data evaluation—if as much P as possible is to be recovered, the current approach does not necessarily help process evaluation.

#### *3.2. Insights Gathered by Target Grouping of EDX-Spectra*

When applying the target grouping (P), compositions appear slightly di fferent (Figure 3). All spectra are now sorted according to their P-content. If there was no P detected in the spectrum, it was labelled as before, by dominant (most abundant element within the spectrum). Thus, values for CaSO4 are not a ffected by the new grouping procedure, since it does not contain P.

**Figure 3.** Composition of SSA before and after digestion with di fferent inorganic acids evaluated with the target phase grouping procedure.

#### 3.2.1. Quartz and Silicates

The distribution of quartz and silicates (quartz + silicates) remains in the same range as quartz in generic grouping. Here, generic labelling is expected to yield lower values, as the generic strategy distinguishes between "other silicates" and "quartz". The target grouping combines the two, wherever they are P-free. Hence, whenever values for target quartz + silicates are lower than generic "quartz" and "other silicates" values, the Si-dominant material is P-enriched.

## 3.2.2. P-Containing Material

Residue content of P-phases digestion with HCl is only significant for the <5 wt% P spectra (groups < 1 and 1–5 wt% P). For spectra with a P-content of more than 15 wt%, the contents are reduced in residues (as compared to original SSA content). Although it looks like the fraction has the same extent as in the original material, taking into account that at least ~40 area-% of SSA were removed, reveals that these values represent a decrease beyond proportion. These material phases must have been particularly acid-accessible. In other words, among the P-containing groups, the original material

has the largest share in 5–10 wt% P. Digestion shifts proportions, and in residues, the largest group among the P-containing groups is 1–5 wt% P. While the conventional grouping showed that high P-phases are severely depleted, target phase grouping highlights that the mid-level P-containing groups (quantitative relevance in the material) are also shifted towards lower P-values. Therefore, digestion acts on more P-containing species than just the high P-containing ones.

Phosphate content in SSA was depleted by 86 wt% [11]. This is shown by XRF analyses, since MLA studies cannot supply this information. There is the "Calculated Assay" operation within the MLA software in complex materials such as SSA, however, these values are better understood as a proxy.

The 10 to 15 wt% P-phase group is very accessible by the acid; they have been reduced from ~34 to <1.8 area-%.
