Pyrometallurgical processes are generally highly energy intensive, and the emission of harmful gases is a major disadvantage ( Ihan, 2020). Hence, a number of studies attempted to recover valuable metals from spent catalysts through pyrometallurgical, hydrometallurgical and pyrohydrometallurgical routes ( Zeng and Cheng, 2009). If not correctly handled, the spent catalysts can cause serious environmental pollution through the dissolution of toxic heavy metals and are thus classified as hazardous waste by the environmental protection agency in the United States ( Ihan, 2020).ĭespite such environmental concerns, spent catalysts are regarded as an essential secondary metal source due to their high metal grades (e.g., 4%–12% Mo, 1%–5% Ni, 0%–4% Co, 0%–0.5% V, 15%–30% Al Park et al., 2006 Pinto and Soares, 2012, Ilhan, 2020). Deactivated catalysts (due to deposition of C, S and other heavy metals deriving from crude oil) are usually regenerated and reused multiple times before the end of the cycle. Hydrodesulfurization catalysts generally consist of the MoS 2 active phase with Co or Ni promoters supported on γ-Al 2O 3 ( Ihan, 2020). Nonetheless, the processing of heavier crude oil with higher sulfur, nitrogen and metal contents is growing, necessitating the increased use of hydrodesulfurization catalysts in petroleum refineries ( Marafi and Stanislaus, 2007 Akcil et al., 2015 Li et al., 2015). The demand for low-sulfur fuels is increasing due to the implementation of stricter environmental regulations in the last decade worldwide.
This study highlighted the potential utility of amino acids as non-toxic, alternative metal lixiviant as well as a metal precipitant for selective leaching of critical metals from spent hydrodesulfurization catalyst. Consequently, highly metal-selective, two separate Co-rich (<1% Mo and 79% Co dissolved, Al not detected) and Mo-rich (96% Mo, 19% Co, and 2.1% Al dissolved) leachates were obtained. 3) Effectively suppressing unwanted dissolution of Al throughout the reaction without needing pH control. The sequential 3-step leaching (Step-1: Alkaline Ala leaching at 45☌, Step-2: Hot water leaching at 70☌, Step-3: Second alkaline Ala leaching at 45☌) was conducted where the role of Ala was found to be at least three-fold 1) maintaining alkalinity by amino acid’s buffering capacity to assist Mo leaching, 2) selectively precipitating Co by forming Co-Ala complex with a distinctive pink color, which can readily re-dissolve in hot water to be separated from spent catalyst particles. Alanine (Ala) was first screened as the most effective type of amino acid to be used for the selective leaching of spent hydrodesulfurization catalyst (consisting of MoS 2 and Co 3S 4 supported on Al 2O 3, at 10% Mo and 2.4% Co grades). The formation of the amino acid-metal complex is often seen in nature, and its potential application in hydrometallurgy can be foreseen. While spent catalysts can cause serious environmental pollution, they can be considered an essential secondary metal source due to their high critical metal grades. Department of Earth Resources Engineering, Kyushu University, Fukuoka, Japan.įIGURE 56.7 Pourbaix diagram of the copper- chlorine-water system.Idol Phann, Yu Tanaka, Sae Yamamoto and Naoko Okibe* The Pourbaix diagrams relate to 25 ☌ but, as is known, it is often necessary to implement operation at elevated temperatures to improve reaction rates, and at elevated temperatures used in practice the thermodynamic equilibria calculated at 25 ☌ are no longer valid. Thus, the potential-pH diagrams for simple metal- water systems are not directly applicable in these cases. For example, in ammoniacal solutions, nickel, cobalt, and copper are present as complex ions which are characterized by their different stabilities from hydrated ions. One factor which has an important bearing on the thermodynamics of metal ions in aqueous solutions is the presence of complex ions. It is important to recognize some of the limitations of the Pourbaix diagrams. įigure 2.22 Pourbaix diagram for the copper-water system at 25° C. The oxide of copper, Cu+ and Cu " " are only. Copper is not passive in acid electrolytes. Copper (E° = 0.337 V) is more noble than iron ( p = -0.444V), however, it is more stable in water (SHE) than iron. Ī Pourbaix diagram for copper/water system is shown in Fig. įig- 7.36 Pourbaix diagram for the system copper-water. This greater nobility results in copper being thermodynamically stable in water that is, line 14 (-6) representing aCu2+ = 10-6 lies above line a. The more positive standard electrode potential of copper (+337 mV (SHE)) as compared to iron (-440 mV (SHE)) is evident.
The Pourbaix diagram for the copper/water system is shown in Fig. FIGURE 7.1 Pourbaix diagram of copper-water system (from Ref.
0 kommentar(er)
