Chemical neutralization of industrial by-products from the secondary aluminum industry: re-use as foaming agents for the synthesis of geopolymers and monitoring of the hydrogen-rich gas production.

The project, financially supported by the Regione Marche (POR Marche FSE 2014/2020), was assigned to the Ph.D. student Roberto Ercoli (UNIURB) with the supervision of Prof. Alberto Renzulli (UNIURB) and Prof. Eleonora Paris (UNICAM). Several universities and research groups such as the University of Urbino, University of Camerino, Institute of Geosciences and Georesources (IGG-CNR, Florence), University of Florence, Technical University of Liberec, and Warsaw University of Technology collaborated to create a European network working in this innovative and brand-new field. Moreover, the primary aspect of this network is the collaboration between the scientific parts, the regional system, and the industrial system. The involvement of the National Technological Clusters is a proposal of the MIUR in 2012 (Decree No. 257 "Notice for the development and reinforcement of national technological clusters"), as well as national research agencies, such as CNR, INFN, IIT, ENEA, to develop highly qualified scientific research and to support the national development. Considering the policies for the economic development identified by the resolutions of the regional council of the Regione Marche (DGR n.1511/2016, and n. 1035 of 30/7/2018), the present research promotes a more sustainable manufacturing sector. The research focuses on the recovery, re-use, and valorization of industrial by-products from the secondary aluminum industry through their chemical neutralization and synthesis of geopolymer foams. Valorization of the so-called "industrial by-product" is an actual and discussed theme. The industry stakeholders support the achievement of the environmental targets about resource management efficiency, reducing waste output and gas emissions, and constant industrial processes optimization. The development and activation of demanufacturing logistics are essential for improving relative residual values by specific operations and treatments of raw material derived from end-life products and industrial waste. The chemical neutralization of highly reactive materials that come from the treatment processes of scraps (beverage cans and domestic appliances) was investigated through experiments in aqueous alkaline solutions. These metallic aluminum-rich by-products were classified, according to EU law, as dangerous waste, as they can potentially develop flammable gases capable of forming explosive mixtures with air. In this way, they cannot be disposed of in landfills for non-hazardous wastes if chemical neutralization is not planned and performed beforehand. The first experiments were mainly aimed at unraveling the oxidation rate and quantifying the production of hydrogen-rich gases from the reactions of the metallic aluminum-rich by-products in a water-rich alkaline (liquid or vapor) environment. Reactions were carried out in a stainless-steel batch mini-reactor with metering and sampling valves, with the resulting gases analyzed by gas-chromatography (GC). The experimental runs performed in the mini-reactor proved to be effective for eliminating the reactive metallic aluminum, reaching a maximum hydrogen production of 96% of the total gases. All the obtained results can be transferred and applied to (i) the possible industrialization of the method for the chemical neutralization of these dangerous by-products, increasing sustainability and workplace safety, (ii) the use of the resulting hydrogen as a source of energy for the furnaces of the secondary aluminum industry itself, and (iii) new technological materials, e.g., "geopolymer foams (GFs)," by using hydrogen coupled with aluminosilicate materials as a foaming agent. The special hazardous wastes derived from the entire recycling process in the secondary aluminum industry were trapped into these geopolymers. The experimental study affects a vast reality concerning waste management through the recovery, chemical neutralization, and incorporation of these hazardous substances into the GFs. A geopolymeric matrix composed of metakaolin (MK), silica sand (100 wt.% of MK), and chopped carbon fibers (1% wt.% of MK) was doped, adding the industrial by-products from the screening, pyrolysis, de-dusting, and fusion processes with specific contents (1; 2; 3; 4; 5 wt.% of MK). Several experimental tests were carried out to characterize the GFs by the mechanical (flexural, compressive, and Charpy impact strengths) and thermal properties (thermal conductivity, diffusivity, and specific heat).