Material analysis of recycled zinc from incinerated waste ash Analysis of recycled zinc from fly ash of a waste to energy plant with the help of powder X-ray diffraction
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Examensarbete för masterexamen
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Master's Thesis
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With the EU increasing costs for landfill and incentivising recycling it has become more and
more common to try to recycle as much as possible from waste products such as fly ash. This
is especially the case for waste-to-energy (WtE) plants that have fly ash with high contents of
valuable metal ions. A WtE plant company in southern Sweden has started a project regarding
the recycling of zinc (Zn) from fly ash. Recently together with other partners a production
chain was designed and built to recycle Zn from the fly ash was designed. The principle is
based on two main reactions, the first is mixing fly ash from the electrostatic precipitators
with acidic water from the open scrubber in the flue gas treatment system. This is to dissolve
metal ions in the fly ash into the water via leaching. The remaining solid is then filtered and
reentered into the furnace. The second step is meant to produce the solid Zn in the form of zinc
hydroxide (Zn(OH)2) via precipitation. By mixing the acidic water from the first reactor with
sodium hydroxide (NaOH) the pH increases, which theoretically should promote the formation
of Zn(OH)2. However, investigations on the Zn product have shown that the extracted form of
Zn is not purely Zn(OH)2 even if the facility operates under optimal conditions. The focus of
this project has therefore been to investigate what compounds the extracted product is made
out of and if parameters such as pH for the precipitation reaction and the liquid to solid ratio
in the first leaching step have effects on the composition. The main method that has been used
during this thesis is powder X-ray diffraction (P-XRD), more exactly Bragg-Brentano analysis
which is heavily based on Bragg’s law. It turned out that the original wet Zn product (about
30% DS) consisted of mostly amorphous material which can’t be detected via P-XRD. The only
compound identified was gypsum. By drying the sample the amorphous part of the sample
decreased and a more crystalline structure appeared. In the dried sample sodium chloride
(NaCl) could also be identified in the sample together with gypsum. When drying the samples
at 105°C the gypsum was converted into anhydrite but there were still no zinc compounds
found in the sample. By drying the sample at 250°C a much clearer diffractogram could be
extracted that contained the missing Zn, however it was not possible to differentiate between
the different Zn compounds such as Zn(OH)2, ZnO and different metallic zinc oxides such
as zinc iron oxide since they all form similar diffractogram patterns. The total composition
analysis of the zinc product showed that about 40% of the product was zinc and the rest was
other elements such as sodium and chlorine. Chlorine is an element that easily forms corrosive
compounds and is therefore unwanted in the final product. To test if the chlorine is easily
removable an extra cleaning step was tested to try to remove chlorine from the sample. The
diffractogram showed no presence of sodium chloride and the liquid contained high amounts
of chlorine. To further investigate what different zinc compounds that were present in the
product another analysis method must be used. It could also be beneficial to investigate if the
amorphous zinc compounds are the same as the crystalline ones. The fact that the Zn is not in
the form of Zn(OH)2 might not be a big problem since compounds such as ZnO also can be
treated and converted into pure Zn. The fact that the Cl easily can be removed is very good
for the further treatment of the Zn product.