Identification and classification of spent molecular sieve and spent catalyst generated in petrochemical complexes and assessing their recycling feasibility

Introduction:
Pars Special Economic Energy Zone in Asaluyeh has several petrochemical units. These units produce a significant amount of different types of wastes) hazardous and non-hazardous (annually. Spent molecular sieve and catalyst wastes are the most important waste produced in petrochemical complexes. Based on available statistics, in the next 25 years, annually an average of 3000 tons of spent molecular sieve and 2000 tons of catalyst waste will be produced in these areas. Because of the necessity of optimal management of industrial wastes, the primary objective of this study was to identify and classify these two types of waste products and then, to investigate the ability to recover these wastes and in particular the feasibility of using them in the manufacture of concrete.
Material and methods:
First, identification and classification of these two wastes was done using elemental analysis experiments, measurements of heavy metals and determination of organic compounds and compare them with the EPA toxicity standard. In order to assess the feasibility of using these two types of waste in the manufacture of concrete, concrete samples were made based on concrete mixing plan, which includes ways of replacing these two wastes with cement. The 7-day compressive strength measurement of the concrete and leakage toxicity analysis (TCLP) were also done in order to assess the feasibility of recycling these wastes from a technical and environmental point of view.
Results and discussion:
Both residues had two major combinations of Silicon dioxide (SiO2) and Aluminium oxide (Al2O3), which makes it possible to replace them with the cement in concrete. Based on the analysis of heavy metals, due to the significant amounts of Chromium (Cr) element in the spent molecular sieve (about 11ppm), this waste can be classified into industrial hazardous waste categories. Also, the analysis of organic compounds showed that the amounts of all organic compounds in these two types of waste are less than the standard amounts for hazardous waste. The results of the 7-day compressive strength measurement of the spent molecular sieve showed that using this waste in the manufacture of concrete improves the initial bonding of concrete, and this is due to the significant amounts of Al2O3 in this waste. So, replacing 10% of this waste can be considered as an optimal amount to increase the initial bonding of the concrete. However, the catalyst waste did not have a positive effect on the primary fix of the concrete. Finally, the TCLP analysis showed that the concretes made with these two wastes are classified as non-hazardous materials, which is due to the stabilizing properties of concrete.
Conclusion:
The preliminary study of spent molecular sieve and catalyst waste characteristics indicates that using these two types of waste in concrete production is feasible from a technical and environmental point of view. Therefore, we suggest that further tests need to be done.