270401 Experimental and Statistical Optimization of Chelant Assisted Extraction of Heavy Metals From Bimetallic Spent Catalyst
Extraction of heavy metals from the spent catalyst is of great concern these days to overcome the scarcity of available resources and to minimize the metallic waste. Disposal of metallic waste directly to the environment may cause severe health issues due to environmental pollution. Conventional technologies viz. leaching, roasting, smelting etc., which been have been developed for the recovery of heavy metals, are not environmental friendly.
In the present study, an ecofriendly approach has been adopted for the extraction of heavy metals from bimetallic spent catalyst (CoMox) of a hydroprocessing unit. The catalyst procured from secondary reforming unit contained Mo (11-13%), Co (2-3%), with the balance being the support material γ-Al2O3. A chelating agent ethylene diamine tetraacetic acid (EDTA) was employed for the extraction of Co and Mo. All the experiments were performed at atmospheric pressure under reflux condition and reaction parameters (temperature, solid to liquid ratio, time, molar ratio of EDTA and metal, pH, stirring speed, and particle size) were optimized to maximize the extraction. Cobalt was separated in form of precipitate at alkaline pH while molybdenum was recovered in form of MoS2at pH 3. Direct and derivative spectrophotometric determination of molybdenum (VI) was performed using cinnamaldehyde-4-hydroxy benzoylhydrazone (CHBH). 85% molybdenum and 76% cobalt was extracted from spent catalyst at optimum reaction conditions. Characterization of spent catalyst, residue and precipitated metals was also performed to understand the structural and phyisco-chemical properties. Residue contains a large amount of alumina. This alumina content was reused as a support in the preparation of other catalyst.
The effect of process parameters on the extraction of Co and Mo were quantitavely evaluated by the analysis of variance (ANOVA). Box-behnken design with various process parameters at three different levels (-1,0,1) was coupled with response surface methodology for designing of experiment. Statistical analysis of the results showed that the linear and quadric terms of the variables had significant effects to the response. However, interactions terms did not show a substantial impression on Co extraction. 3D response surface plots and corresponding 2D contours were examined to study the effect of process parameters on extraction of Ni. The optimal conditions of process parameters were predicted by the proposed models. Verification of the optimization showed a small percentage error between the predicted and the experimental values of Ni extraction.
The process is significant demonstration of the use of chelation chemistry for metal extraction from spent catalyst. Use of sulphuric acid for dechelation and metal precipitation is limited to low temperatures making it a non-corrosive and energy-efficient process. Bimetallic extraction from spent catalyst and recoverability of EDTA make it an eco-friendly approach. Efforts are still ongoing to setup a process with zero waste generation. Statistical optimization of process parameters for extraction of Ni using the box-behnken design appeared to be a valuable tool for the extraction of Ni. It has been demonstrated that box-behnken design coupled with response surface methodology allows the optimization of the experimental variables.
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