۱- Introduction

۲- Materials & methods

۳- Results & discussion

۴- Process analysis

۵- Conclusions

References

Abstract

The present paper describes the experimental tests for the recycling of fluid catalytic cracking catalysts (FCCCs). The process aims at the recovery of cerium (Ce) and lanthanum (La) as well as the reuse of the leaching solid residue that represents the actual problem in terms of global amount landfilled every year. Landfilling is still the main choice for the handling of such catalysts. This novel process proposes an alternative recycling approach that leads to the production of synthetic zeolites, that have several industrial applications. FCCC was leached by 1.5 mol/L of HNO3, HCl and H2SO4 solutions at 80 °C, for 2 h with a solid to liquid ratio of 20 %wt, and the two rare earth elements were recovered by precipitation with an overall yield in the range 70–۸۰%. The solid residues from the leaching stage were used as the base material for the synthesis of the zeolites by means of a combined thermal-hydrothermal treatment. The characterization of the zeolites demonstrated that the Na-A phase was predominant over the Na-X phase. The zeolites were tested as sorbent material for CO2 separation from CH4, in order to simulate the upgrading of biogas to biomethane. The maximum adsorption rate of CO2 was 0.778 mol CO2/kg of zeolite at 3 bar, with a resulting CH4 recovery of 62% and 97 %vol as purity. Since the results in adsorption of CO2 were not satisfying, the same zeolites were used to remove heavy metals from a synthetic wastewater solution containing three metals. Equilibrium and kinetic models were also developed in order to describe the adsorption process. The maximum adsorption load was calculated by the Langmuir isotherm and resulted to be 24–۳۲ mg/g for Ni, 52–۶۰ mg/g for Zn and 122–۱۸۱ mg/g for Cu. The results also showed that the kinetics of the adsorption process is almost fast, as after 1 h at least 95% of zinc and copper were removed, whereas the kinetics of nickel was slower for all the three zeolites. As a conclusion, the zeolites are more efficient in metal adsorption than CO2 capture, but other applications will be tested in the future.

Introduction

In the last fifty years, the significant expansion of the industrial, commercial and agricultural sectors was accompanied by a huge increase in production of petrochemical products and intermediates, as well as refined fuels like gasoline, diesel fuel, kerosene, jet fuel, naphtha, and gas oil. Hence, this led to massive use of catalysts for typical refinery processes like hydrotreating (HT), fluid catalytic cracking (FCC), catalytic reforming (CR), hydrodesulphurization (HDS), alkylation, isomerization. Many of them, once exhaust, can be regenerated by thermal treatments in presence of nitrogen, air or oxygen at a controlled temperature, but after a certain number of regenerations, the catalytic activity is irretrievably compromised. Other kinds of catalysts, like those for FCC, are poisoned by heavy metals such as nickel and vanadium, so that they cannot be regenerated and have to be replaced from time to time with fresh catalyst. The use of rare earths in FCC catalysts was driven by the need for more active and hydrothermally stable products with better yield performance. Rare earth oxides (REO) achieved these goals by enhancing catalytic activity and preventing the loss of acid sites during operation. REO concentration gradually increased over the years, and the average is currently in the range 3e5 %wt. China produces 95% of the world’s rare earths elements (REEs) supply, thus the recovery of such elements from industrial waste will play a crucial role in the future economy of the European countries (BASF, 2018).