Metal Recovery from End-of-Life Hydrotreating Catalysts by Selective Precipitation: Laboratory Tests and Preliminary Process Analysis Alessio Cibati, Francesca Pagnanelli, and Luigi Toro Department of Chemistry, Sapienza University of Rome, 00185 Rome, Italy; cibale1983@gmail.com (for correspondence) Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12053 Hydrotreating catalysts (HTCs) are a waste byproduct of the petroleum refining industry. The recovery of intrinsic and sorbed metals from spent catalysts is desirable for meeting environmental disposal regulations, as well as for the metal resell value. In this study, innovative process schemes for metal recovery from end-of-life (i.e., spent) HTCs were tested in labo- ratory experiments and compared by process analysis. Syn- thetic leach liquors containing Al, Mo, Ni, and V were prepared resembling both chemical leaching and bioleaching methods for metal extraction from spent catalysts. Subse- quently, metals in the leach liquor were recovered individually according to two pH-dependent process schemes: (i) Mo and Ni were separated first and Al was removed last, and (ii) Al was removed first. The second metal separation process gave the best results in terms of selectivity, whereby 65% of Al was recov- ered by precipitation at pH 4.0, 87% of Mo was precipitated by sulfide addition at pH 0.5, 52% of Ni was precipitated by sul- fide addition at pH 3.5, and 65% of V was recovered by hydroxide complexation at pH 6.0. Experimental data of metal precipitation were used to perform a process analysis compar- ing chemical leaching and bioleaching for different input flow rates and product depreciation in a simulated commercial- scale plant. Simulation results indicate that chemical leaching is of superior performance over biological leaching in terms of metal recovery, which decrease the payback time for the capi- tal investment to build the plant. V C 2014 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2014 Keywords: end-of-life catalysts, metal recovery, selective precipitation, process simulation, secondary raw materials, technical–economical feasibility INTRODUCTION Catalysts are widely used in several refinery operations such as hydroprocessing for demetalization, desulfurization of feedstock, and treatment of heavy fractions [1–3]. Hydro- treating catalysts (HTCs) are generally made up of a matrix of Al 2 O 3 supporting Mo and Co or Ni as catalytic centers. During hydroprocessing, sulfides and metal oxides (e.g., V and Ni from the feedstock) tend to deposit on the catalyst along with coke. Thus, the activity of the catalyst is lowered, allowing for only a limited number of regeneration cycles prior to their inferior performance and subsequent disposal [4,5]. The global load of spent HTC generated is estimated at 150,000–170,000 ton/yr [4]. This amount is expected to increase due to the growing energy consumption and shift in supply of low-quality oil rich in metal impurities. Characterization of end-of-life HTC in terms of metal con- tent (as weight percent) has reported in the literature as fol- lows. Zhang and Zhao [6] found 4% of Ni, 18.5% of Mo, 1–15% of V, and 1.9% of Co; Kar et al. [7] reported 21% of Mo, 0.8% of Ni, and 1.5% of Co; Biswas et al. [8] found 1–12% of V, 10–30% of Mo, 0.5–6% of Ni, and 1–6% of Co; and Mishra et al. [9] reported 19.5% of Al, 2% of Ni, 11.6 of V, 1.4% of Mo, and 0.8% of Co. In addition to the reported metals, end-of-life HTC usually contains 15–25% of coke, 7–15% of sulfur, and 5–10% of residual oil [10]. Because of the metal content, spent HTC has been classified as hazardous solid waste by the United States Environmental Protection Agency. The combined need to meet environmental regulations and potential profits from resell of recovered metals (particularly V, Co, Ni, and Mo) drive the need to develop novel and sustainable processes to treat industrial hazardous waste and recover a commercial product simultaneously. Pyrometallurgical and hydrometallurgical processes used for recovering metals from HTC [10,11] generally involve an initial thermal pretreatment to remove sulfur and coke and transform Mo, Ni, Co, and V compounds into metal oxides. Then, the cata- lyst is subjected to roasting with alkali or basic aqueous solutions to recover Na 2 MoO 4 and NaVO 3 in the pH basic leachate. Resi- dues are lastly treated by acid leaching to extract Ni and Co. Alternatively, acid leaching can be performed directly on exhausted HTC to extract all leachable metals (e.g., Mo, Ni, Co, V, and Al) in the liquor simultaneously [12]. Bioleaching tests using Fe/S-oxidizing bacteria have also been reported in the liter- ature. For example, Beolchini et al. [13] investigated metal recov- ery from HTC by bioleaching and achieved extraction yields of 83% for Ni, 90% for V, and 40% for Mo. In a different study, Mis- hra et al. [9] performed a similar extraction method and obtained 88.3% recovery yield for Ni, 94.8% for V, and 46.3% for Mo. It is important to note that metal recovery from leach liquors can be achieved in a variety of ways [14], including selective precipitation as hydroxides [7,15,16], solvent extrac- tion [8,16–20], synthetic resins [6,21], and adsorption onto activated carbon [7,22,23]. Logically, metal recovery efficiency is linked to the method used and the metals to be extracted. Although metal precipitation as hydroxides is a common practice, it presents a series of disadvantages, involving Current address of Alessio Cibati: Civil Engineering Department, Howard college campus, University of KwaZulu-Natal, Durban, South Africa. V C 2014 American Institute of Chemical Engineers Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2014 1