Journal of Hazardous Materials 176 (2010) 1113–1117

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Short communication

Leaching zinc from spent catalyst: Process optimization using response surface methodology
Zhengyong Zhang a,b , Jinhui Peng a,b,∗ , C. Srinivasakannan c , Zebiao Zhang a,b , Libo Zhang a,b , Y. Fernández d , J.A. Menéndez d a Key Laboratory of Unconventional Metallurgy (Kunming University of Science and Technology), Ministry of Education, Kunming 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China Chemical Engineering Program, The Petroleum Institute Abu Dhabi, 2533, United Arab Emirates d Instituto Nacional del Carbón, CSIC, Apartado 73, 33080 Oviedo, Spain b c

a r t i c l e

i n f o

a b s t r a c t
The spent catalyst from vinyl acetate synthesis contains large quantity of zinc. The present study attempts to leach zinc using a mixture of ammonia, ammonium carbonate and water solution, after microwave treatment. The effect of important parameters such as leaching time, liquid/solid ratio and the ammonia concentration was investigated and the process conditions were optimized using surface response methodology (RSM) based on central composite design (CCD). The optimum condition for leaching of zinc from spent catalyst was identified to be a leaching time of 2.50 h, a liquid/solid ratio of 6 and ammonia concentration 5.37 mol/L. A maximum of 97% of zinc was recovered under the optimum experimental conditions. The proposed model equation using RSM has shown good agreement with the experimental data, with a correlation coefficient (R2 ) of 0.95. The samples were characterized before and after leaching using X-ray diffraction (XRD), nitrogen adsorption and scanning electron microscope (SEM). © 2009 Elsevier B.V. All rights reserved.

Article history: Received 23 September 2009 Received in revised form 22 November 2009 Accepted 24 November 2009 Available online 13 January 2010 Keywords: Spent catalyst Ammonia leaching Response surface methodology Optimization Zinc

1. Introduction Increasing concerns of environment and ore shortage has led to considerable efforts being made to recover valuable metals from various waste materials [1–3]. Fresh catalyst used in vinyl acetate synthesis production contains up to 35 wt% zinc acetate in the activated carbon [4]. The catalyst losses its effectiveness progressively due to poisoning of the active sites, rendering ineffective for further utilization. The spent catalyst still contains considerable amount of zinc occurring in the form of acetate, oxide or metallic form or as organic compound which were by-products of vinyl acetate synthesis [5]. In China, around 9000 tons of the spent activated carbon is generated from vinyl acetate synthesis per year as solid waste. These spent catalysts do not have commercial value and are usually dumped as landfills. The hazardous nature of the spent catalyst is a potential threat to the environment, as various chemicals can possibly leach to surrounding areas from the dump site. Few of the possible ways for regeneration of the spent catalyst have been attempted in the recent years [5], which include

∗ Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China. Tel.: +86 871 5191046; fax: +86 871 5192076. E-mail addresses: jhpeng@kmust.edu.cn, jhpeng ok@yeah.net (J. Peng). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.11.125

removing organic substances by organic solvents or with carbon dioxide in the supercritical phase, leaching of zinc with a solution of hydrochloric acid with solutions of HCl and HNO3 assisted by microwave energy. The maximum % of zinc removal using the above methods was reported to be less than 94.7%. As more than 5% of the zinc is still left with the activated carbon it significantly influences the absorption capacity of regenerated activated carbon. Our earlier publication [4] reported the enhanced recovery of zinc from the spent catalyst, using a two step being pretreatment by microwave irradiation, followed by leaching by a mixture of ammonia, ammonium bicarbonate and water. The % leaching was reported to improve on account of organic substances removal, achieving an effective separation of activated carbon from zinc. The present study attempts to identify extraction conditions that could possibly maximize the zinc removal by optimizing the process conditions, by designing the experiments using response surface methodology (RSM). RSM is one of the relevant multivariate techniques which can deal with multivariant experimental design strategy, statistical modeling and process optimization [6–11]. It is used to examine the relationship between one or more response variables and a set of quantitative experimental variables or factors. This method is often employed after the vital controllable factors are identified and to find the factor settings that optimize the response. Designs of this type are usually chosen when a curvature in the response

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Z. Zhang et al. / Journal of Hazardous Materials 176 (2010) 1113–1117 Table 2 Independent variables and their levels used for central composite rotatable design. Variables Symbol Range and levels −1.682 Ammonia concentration (mol/L) Leaching time (h) Liquild/solid ratio
1

surface is suspected. The process optimization of the microwave induced zinc recovery from the spent catalyst of vinyl acetate process has not been reported in literature. Hence the present work intends to assess the effects of variables such as ammonia concentration, leaching time and liquid/solid ratio to identify the optimum conditions using a central composite design (CCD). The characteristics of sample before and after leaching are assessed using the advanced analytical instruments such as X-ray diffraction (XRD), nitrogen adsorption and scanning electron microscope (SEM). 2. Materials and methods 2.1. Materials and apparatus The spent catalyst of vinyl acetate synthesis was obtained from a chemical plant (Yunnan Yunwei Group Co., Ltd.) in Yunnan province, China. The composition of spent catalyst is presented in Table 1. Prior to leaching, the samples were treated with microwave and heated to a temperature of 900 ◦ C for 30 min using a self-made microwave oven. The zinc content of pretreated sample decreased from 8.76 to 5.55% due to lose of zinc during the pretreatment process. SEM (Philips XL30ESEM-TMP) analysis of the sample was utilized to assess the microstructure of sample before and after leaching. XRD (Rigaku diffractometer [D/max 2500] using Cu K radiation) analysis was performed to assess the changes in composition of samples, while (Brunauer–Emmett–Teller) BET analysis was used to assess the surface area and pore structure. 2.2. Experimental methods The leaching solution was prepared by dissolving ammonium carbonate in an ammonia solution. A desired liquid to solid mass ratio was prepared by mixing distilled water to the ammonium carbonate solution. A total of 20 experiments were conducted with 20 g of sample in a batch mode using a glass reactor of volume 250 mL. The contents of reactor were well stirred using a digital and thermostatic magnetic stirrer at an agitation speed of 800 rpm, so as to keep the contents of the reaction well stirred and suspended. The contents of reactor were filtered, upon completion of the experiment and the filtrate was analyzed for the zinc content using EDTA titration [12]. The amount of zinc leached was estimated using Eq. (1). % leaching = m1 × 100 m0 (1)

−1 2.00 1.00 3.00

0 4.00 2.00 4.50

+1 6.00 3.00 6.00

+1.682 7.36 3.68 7.02

0.64 0.32 1.98

2 3

according to Eq. (2), i =

i

−

0

(2)

where i was the dimensionless coded value of the ith independent was the variable, 0 was the value of i at the center point and step change value. The % leaching of zinc was the response variable of the experimental conditions in the design of experiments. The leaching time was varied from 0.32 to 3.68 h, the liquid/solid ratio was varied from 1.98 to 7.02, the ammonia concentration was varied from 0.64 to 7.36 mol/L. A total of 20 experiments consisting of 8 factorial points, 6 axial points and 6 replicates at the central points were performed [13]. Experimental results obtained from the CCD model were described in the form as given in Eq. (3), n n

Y = ˇ0 + i=1 ˇi

i

+ i=1 ˇii

2 i

+ i F