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

Abstract

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.