Direct Chlorination of Nickel-Containing ... - ACS Publications

8184

Ind. Eng. Chem. Res. 2008, 47, 8184–8191

Direct Chlorination of Nickel-Containing Materials. Recovery of the Metal from Different Sources Fabiola J. Alvarez*,† and Ana E. Bohe´†,‡,§ Centro Ato´mico Bariloche, Chemical Kinetics DiVision, Comisioń Nacional de Energıá Ato´mica, AV. Bustillo 9500, CP 8400, San Carlos de Bariloche, Rıó Negro, Argentina, Consejo Nacional de InVestigaciones Cientı´ficas, RiVadaVia 1917, CP 1033, Ciudad Autónoma de Buenos Aires, Argentina, and UniVersidad Nacional del Comahue, Quintral 1250, CP 8400, San Carlos de Bariloche, Río Negro, Argentina

In the present work, the chlorination of nickel-containing materials is proposed: using NiO and mixtures that simulate the conditions of 12 wt % NiO-Al2O3 catalyst, prepared by mechanical mixing and incipient wetness impregnation (IWI), the reaction with gaseous chlorine proceeds from 500 to 800 °C. Non-isothermal experiments from 20 to 950 °C and isothermal measurements at 725, 800, and 850 °C were performed under Ar-Cl2 atmosphere in a thermogravimetric (TG) system. Direct chlorination of the mixtures was carried out isothermally in a tubular reactor under a flow of Cl2 from 500 to 800 °C. The chlorination started at 725, 473, and 428 °C for NiO, the mechanical mixture, and the IWI mixture, respectively. The high temperature for NiO can be explained by its BET area, which is small enough to reduce the reactivity of the sample. The recovery factor of Ni from Al2O3 attained at 800 °C was 85% for the mechanical mixture and 96% for the IWI mixture. All the species were characterized by X-ray diffraction (XRD), scanning-electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) techniques. They were quantified by X-ray fluorescence spectrometry (XRF), and BET surface area (BET) was measured in the samples. 1. Introduction The use of catalysts of the Ni/Al2O3 and NiO/Al2O3 type in the re-forming of methane to obtain hydrogen and synthesis gas is quite widespread.1 They are also used in low-temperature oxidations (473 °C) of ethanol2 and in the reduction of sulfurous oxide to elemental sulfur.3 As the catalyst usually deactivates temporarily due to deposition of coke or permanently due to the formation of NiAl2O4, different methods of recovery were studied: oxidation under air and reduction with hydrogen;4 hydrometallurgy methods, using sulfuric acid 50%5 or nitric acid 60-70%6 or by leaching with ammonium carbonate;7 and others. However, these methods present operating costs higher than those of processing the spent catalysts by selective chlorination at 500 °C.8 As the affinity of the valuable elements (Ni) toward chlorine is higher than that of the support (Al2O3), it was possible to chlorinate these elements without chlorinating the support. Besides, the difference between the boiling points of the refractory metal chlorides and those of the transition metals allowed the separation of these two groups. This last method gave amazing results because it was possible to recover up to 98% of Ni, from the rest of the impurities, while nonreactive alumina remained in the residue.8 In a previous work, it was shown that the chlorination allows the separation of nickel from aluminum, and the formation of NiCl2 was detected from 200 °C in binary mixtures Ni-Al 50 wt %, for isothermal conditions in chlorine flux.9 Other researchers carried out studies on the corrosion of nickel in chlorine gas at temperatures above 950 °C and suggested that nickel is not corroded below 538 °C in chlorine gas.10 McKinley and Shuler measured the reaction rate of the gaseous chlorine at a constant pressure between 0.01 and 0.05 * Corresponding author. Tel/Fax: +54 2944 445293. E-mail: [email protected]. † Comisioń Nacional de Energıá Ato´mica. ‡ Consejo Nacional de Investigaciones Cientı´ficas. § Universidad Nacional del Comahue.

kPa, with metallic nickel in the temperature range from 927 to 1427 °C. They found that the reaction of formation of highly volatile NiCl2 takes place on the exposed metal surface, and the process whereby nickel was removed from the surface was a first-order reaction with respect to chlorine pressure.11 Downey et al. studied the chlorination kinetics of nickel in vacuum, finding that the starting temperature of the reaction was 333 °C and the volatilization was over 525 °C.12 Murase et al. carried out a selective chlorination above 650 °C of a fly ash containing 1.4 wt % of Ni and other metals (V, Mg, Al, and Fe). The Ni was separated from the Mg, and NiCl2 started to vaporize at 550 °C and was thereafter condensed in a zone of the reactor above 380 °C with 90% of purity.13 With regard to the chlorination of nickel oxide, there are kinetic studies of the reaction, and the formation of the chloride was detected at 350 °C with complete volatilization above 750 °C.14 In the present work, chlorination of nickel-containing materials by reaction with gaseous chlorine in the range of temperature from 500 to 800 °C is proposed. The three types of nickel source chlorinated were NiO, a mechanical mixture constituted by 12 wt % NiO-Al2O3 (mixture 1), and a 12 wt % NiO-Al2O3 mixture prepared by the incipient wetness impregnation (IWI) technique (catalyst). The experiments were carried out in a thermogravimetric system (TG) and in a tubular reactor assembled with an electric furnace. The influence of temperature and microstructure of the sample on the kinetic was studied. Thermal treatments at 950 °C were performed at periods of time of different length with the aim of detecting if any important phase change occurs in the alumina-containing samples (Alfa Aesar alumina, CINDECA alumina, mixture 1 and catalyst). In order to determine the effect of thermal treatments and chlorination on the evolution of the alumina microstructure and in the appearance of the products, BET surface area was measured in the unreacted samples of alumina and over the products. The recovery factor of nickel from aluminum oxide after the chlorinations was calculated.

10.1021/ie8006598 CCC: $40.75  2008 American Chemical Society Published on Web 09/30/2008

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8185

The discussion comprises a section dedicated to thermodynamics evaluation and two sections describing the experimental systems. In the first, the thermogravimetric measurements under non-isothermal and isothermal conditions are discussed, and in the second, the experiments in the tubular reactor are outlined. The last method deals with the separation of nickel from alumina and the comparison between thermal treatments under different atmospheres. 2. Experimental Section 2.1. Materials. The gases used were argon (99.999%; AGA) and chlorine (99.8%; Indupa). The solid powders were γ-aluminum oxide (99.97 wt %, Alfa Aesar), γ-aluminum oxide (provided by CINDECA, University of La Plata, CONICET) and NiO (99 wt %, -325 mesh powder; Alfa Aesar). One mechanical mixture was prepared by using a Y-shaped mixer in the following concentration: 12 wt % NiO + 88 wt % Al2O3 (mixture 1), the proportion of each constituent was calculated by considering the values found for similar catalysts.2,3 A 12 wt % NiO + 88 wt % Al2O3 mixture (catalyst) that was prepared by incipient wetness impregnation technique was provided by CINDECA. The samples were constituted by nickel in the Ni2+ state, as nickel oxide. Figure 1a-c shows the morphologies of the agglomerates and the surfaces of the different reactives used for the preparation of the mixtures. They correspond to agglomerates of Al2O3 from Alfa Aesar (Figure 1a), the surface of the particles of Al2O3 from CINDECA (Figure 1b), and the clusters of NiO (Figure 1c). The alumina sample in Figure 1a is constituted by grains of about 0.6 µm, while the aluminum oxide in Figure 1b has a quite sintered surface, with small particles covering its surface. In Figure 1c, nickel oxide is distributed like large chains of particles forming clusters with grains of about 3.8 µm. 2.2. Procedure. The thermogravimetric measurements were carried out in a Cahn 2000 electrobalance adapted for working with corrosive gases. The system has been described elsewhere.15 Non-isothermal and isothermal experiments from 20 to 950 °C were performed under Ar-Cl2 atmospheres. The powdered samples of about 15 to 30 mg were placed on a flat quartz crucible adopting a loose-packed bed inside a vertical reactor. Chlorine was admitted into the system to give a partial pressure of 36.47 kPa under an overall pressure of 101.30 kPa. Non-isothermal measurements were done from room temperature to 950 °C and the starting temperature of the reaction for the different species with chlorine was determined, being of 725, 473, and 428 °C for NiO, mixture 1, and catalyst, respectively. Next, isothermal experiments were done at 725, 800, and 850 °C with the purpose of finding out the reaction rates. Direct chlorination of the mixtures was carried out in a tubular reactor heated by an electric furnace, and the samples were placed in quartz crucibles. The thermal treatments were done isothermally in a flow of 2 L/h chlorine from 500 to 800 °C for periods of 1 h. The products were removed from different temperature zones of the reactor and from the crucible in the reaction zone. Thermal treatments of the two kinds of aluminas (Alfa Aesar and CINDECA) and the two mixtures (mixture 1 and catalyst) were performed in a vertical electric furnace at 950 °C under air atmosphere and the periods of heating that varied from 1 to 72 h. All reactants, products, and residues from TG and tubular reactor systems were characterized by X-ray diffraction (XRD). The evolution of the morphology was followed by scanning-

Figure 1. SEM images of the reactants: (a) powder of γ-Al2O3 used for the preparation of mixture 1, (b) powder of γ-Al2O3 used for the preparation of the catalyst, and (c) powder of NiO.

electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) techniques. Afterward, they were quantified by X-ray fluorescence spectrometry (XRF). BET surface area (BET) was measured in the unreacted samples of alumina, mixture 1, and catalyst and also over the products. 3. Results and Discussion 3.1. Thermodynamics Evaluation. The formation of nickel chloride from the direct chlorination of the oxide is represented by the following equilibrium:

8186 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

1 NiO + Cl2(g) a NiCl2 + O2(g) (1) 2 According to thermodynamic studies, when NiO is directly contacted with chlorine in the interval of temperatures under study, the formation reaction of NiCl2 occurs spontaneously, as it is shown by the values of standard Gibbs free energy for reaction 1: -7.45, -3.41, and -0.75 kJ/mol Cl2 at 725, 800, and 850 °C, respectively.16 Furthermore, there is a solid-gas equilibrium of the formed nickel chloride, NiCl2 a NiCl2(g)

(2)

The vapor pressures of NiCl2 in the equilibrium given by reaction 2 at the working temperatures are 0.58, 3.65, and 10.90 kPa.16 From these pressure values it can be assumed that, as far as the chloride is formed, it is promptly extracted by the inert gas flow, leaving the reaction zone free from this product. Otherwise, the standard Gibbs free energy for reaction 2 is always positive: 42.85, 29.60, and 20.82 kJ/mol NiCl2 at 725, 800, and 850 °C, respectively.16 The previous behavior demonstrates that though thermodynamics favored reaction 1 in the interval of temperatures of the experiments, the evaporation of the chloride shown by eq 2 offers resistence to the whole process. This equilibrium is the controlling stage giving a positive standard Gibbs free energy for the reaction. However, as the experiments were done in a continuous flow of chlorine, the oxygen formed is cleared away from the system and the equilibrium of reaction 1 is moved toward the right. The formation of aluminum chloride from the chlorination of the oxide can be seen in the following reaction: 2 1 1 Al O + Cl2(g) f AlCl3 + O2(g) (3) 3 2 3 3 2 The values of standard Gibbs free energy for reaction 3 are 115.7, 114.8, and 114 kJ/mol Cl2 at 725, 800, and 850 °C, respectively.16 These values evidence that reaction 3 is not thermodynamically favored. Other researchers found that transition aluminas react at 850 °C with gaseous chlorine, but as the synthesis of Al2Cl6(g) is thermodynamically unstable, the chloride recrystallizes into R-Al2O3 at 900 °C17 and the chlorination is only possible at 700 °C and in the presence of carbon.18 From these results it can be assumed that AlCl3 was not formed; consequently, the interactions NiO-AlCl3 or NiCl2-AlCl3 would not occur in the experimental conditions under study. 3.2. Thermogravimetry. 3.2.1. Non-Isothermal Experiments. The TG curve in Figure 2 represents the heating of the pure reactives and the mixtures, under Ar-Cl2 atmosphere, from room temperature to 950 °C. In this figure, the mass change of the sample over the initial mass of reactive substance against temperature is illustrated. The reactive substance was considered to be the relevant compound of the sample that would react with chlorine in the different systems, which is Al2O3 in the aluminum oxides and NiO in the samples of nickel oxide, mixture 1, and catalyst. No considerable mass change was detected in the alumina samples in the temperature interval of the chlorination; there was only a slight mass gain of 0.6 wt % due to the heating effect, followed by a mass loss of 0.7 wt %, attributed to the reaction of superficial hydroxyls with chlorine.19 The reaction of NiO started at 725 °C and a continuous volatilization occurred until 853 °C without accumulation. Mixture 1 demonstrated a first mass gain stage from 473 to 577 °C, because of formation and accumulation of solid NiCl2,

Figure 2. Non-isothermal TG curves of the pure reactants and the mixtures. pCl2 ) 36.47 kPa, pAr ) 64.83 kPa.

followed by a mass loss stage due to volatilization. The catalyst started to react with Cl2 at 428 °C with mass gain, and at 653 °C volatilization started till completion. According to the previous results, different starting temperatures in the reaction of the samples with Cl2 were found: 725, 473, and 428 °C, for NiO, mixture 1, and catalyst, respectively. This behavior shows that the formation of NiCl2 depends on the source of nickel and could be explained by considering the BET areas of the samples, which are 0.34, 71.99, and 111.56 m2/g, for NiO, mixture 1, and catalyst, respectively. From these results it could be appreciated that the lowest starting temperature for the reaction is in agreement with the largest BET area. It can be assumed that, if the supporting bed is porous enough, diffusion of chlorine through it will be enhanced, due to the dependence of this process on this feature. The beds of the mixtures are similar and their reactivity was found to be comparable, while in the nickel oxide there is no support and the diffusion in this material was expected to be quite different. 3.2.2. Isothermal Experiments. Figure 3 shows the isothermal curves at 725, 800, and 850 °C for the different reactants, for the first 2000 s. The figure represents the mass change of the sample over the initial mass of reactive substance against time. The conversion was calculated as the difference between initial and final mass of the sample over the initial content of nickel oxide in the specimen, the value of 100% being the state reached when complete reaction is obtained. It was of 7% (725 °C), 98% (800 °C), and 100% (850 °C) for NiO; 61% (725 °C), 91% (800 °C), and 100% (850 °C) for mixture 1; and 35% (725 °C), 86% (800 °C), and 100% (850 °C) for the catalyst. The reactions continued further on, and that is why at the first two temperatures complete reaction was not attained. At this point of the discussion, and based on the measurements of BET area previously shown, it could be supposed that the first step in the chlorination depended strongly on the structural characteristics of the samples. Moreover, as the reaction temperature increased, the achieved conversion was improved. From the analysis of the isothermal curve of NiO, it can be clearly seen that this compound showed a continuous mass loss, and the slope of the line at 725 °C is minor compared to that at the other two temperatures. It seems that the reaction of nickel oxide at 725 °C has an induction time, which is visible because its mass loss occurred amazingly too slowly. Regarding mixture 1, there is a short first mass gain stage, followed by a mass loss period with a constant rate (whose values are shown in Table 1 for several temperatures), and finally a nonlinear decreasing stage.


Figure 3. Isothermal TG curves at 725, 800, and 850 °C: (a) NiO, (b) mixture 1, and (c) catalyst. pCl2 ) 36.47 kPa, pAr ) 64.83 kPa

In the case of the catalyst, it can be observed that the behavior of this system at 725 °C has a quite brief stage of mass gain followed by a linear mass loss, and despite their same range of

order, three different rate values can be differentiated. This behavior can be appreciated only if we enlarge the scale of the figure, and the rate values can be read in Table 1. On the other hand, at the other two temperatures considered, there is a mass gain period followed by a continuous rate diminution that goes on with a sharp slope. As can be observed from Table 1, the isothermal experiments have different stages, and when there is a linear period, its rate value is shown. If we compare the mass loss expressed as ∆m/mi between the three systems considered in Figure 3, the whole curve shows that at 725 °C mixture 1 has the highest reaction rate value, followed by NiO and afterward by the catalyst. According to the second stage shown in Table 1, from 2600 s and forward the reaction of NiO became faster than for mixture 1 and for the catalyst. At 800 and 850 °C, the difference in the velocities is about 1 order of magnitude, so the reaction of NiO is always faster than that of the other two species. In Table 2 are exhibited the XRD results and composition of the residues left in the crucible. It can be observed that partial phase transformation γ-Al2O3|δ-Al2O3 occurred for the experiments of the catalyst that were performed at the highest temperatures, that is, isothermal TG at 850 °C and nonisothermal TG from 20 to 950 °C. In the rest of the experiments, γ-Al2O3 remained unchanged. Moreover, the compositions shown for the non-isothermal experiments for mixture 1 and the catalyst determined by EDS are almost the same, and even though traces of NiO were found by XRD in all the experiments of the catalyst, nickel could not be detected by EDS. Furthermore, it could be observed that aluminum content increased with temperature increase, from 42.0 to 51.3 wt % for mixture 1 and from 48.0 to 52.9 wt % for the catalyst, while chlorine percentage diminished or remained constant. In Figure 4 is shown the morphology of the residues from mixture 1 and catalyst left in the crucible after chlorination carried out in the TG system. Figure 4a,b corresponds to the non-isothermal experiments, which ended at 950 °C, while the samples of Figure 4c,d were obtained after the isothermal experiments done at 725 °C. The general appearance of the particles did not exhibit noticeable changes from 725 to 950 °C, but the size of particle grains at 950 °C is smaller than that at 725 °C. This feature is better appreciated for mixture 1 than for the catalyst. Comparing both sources of NiO, it can be seen that, after the non-isothermal TG, mixture 1 produces a residue of γ-Al2O3 with the finest particle grains. 3.3. Reactor Experiments. 3.3.1. Effect of Thermal Treatment, under Air or Chlorine Atmosphere, on the Materials Surface Area. Lippens and De Boer found that the transition γ-Al2O3|δ-Al2O3 occurred at 750 °C, and near 1000 °C the transition to θ + R-Al2O3 took place.20 Lopasso et al. performed air treatments of bohemite (AlOOH) at various

Table 1. Different Stages of the Isothermal TG at 725, 800, and 850 °C, for the Reactants, with the Corresponding Values of Reaction Ratesa 725 °C NiO 1st stage reaction rate 2nd stage reaction rate 3rd stage reaction rate 4th stage reaction rate a

mixture 1

800 °C catalyst

ML

MG

MG

ML 1.6 × 10-8

ML 7.2 × 10-9 ML

ML 3.5 × 10-9 ML 1.2 × 10-9 ML 5.9 × 10-10

0 ML 4.8 × 10-10

NiO ML 6.1 × 10-8 ML

mixture 1

850 °C catalyst

MG

MG

ML 2.6 × 10-8 ML

ML 1.5 × 10-8 ML

NiO ML 9.2 × 10-8 0

mixture 1 MG

MG

ML 4.2 × 10-8 ML

ML 1.9 × 10-8 ML

0

ML: mass loss. MG: mass gain. Reaction rate is in Cl2 mol/s.

catalyst

ML 3.2 × 10-10

8188 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 2. XRD and EDS Composition of the Residues Left in the Crucible, after the Experiments Carried Out in the TG System experimental conditions

XRD

isothermal TG, 725 °C mixture 1

γ-Al2O3


γ-Al2O3


γ-Al2O3

non-isothermal TG, 20-950 °C mixture 1

γ-Al2O3

isothermal TG, 725 °C catalyst

γ-Al2O3 NiO


γ-Al2O3 NiO


γ-Al2O3 δ-Al2O3 NiO γ-Al2O3 δ-Al2O3 NiO

non-isothermal TG, 20-950 °C catalyst

composition (wt %) Al: 42.9 O: 56.5 Cl: 0.6 Al: 46.0 O: 53.6 Cl: 0.4 Al: 46.9 O: 52.8 Cl: 0.3 Al: 51.3 O: 47.2 Cl: 1.5 Al: 48.0 O: 50.8 Cl: 1.2 Al: 50.6 O: 48.5 Cl: 0.9 Al: 52.7 O: 46.4 Cl: 0.9 Al: 52.9 O: 46.1 Cl: 1.0

temperatures and times, and they found that the transition γ|δ arose at 800 °C only after 61 h of heating and at 900 °C after 11 h.21 In Table 3 are exposed the XRD results and BET measurements of both types of aluminas, the mixtures, and their products after thermal treatments at 950 °C under air atmosphere for different intervals of time. It can be seen in Table 3 that for the experiments with the Alfa Aesar alumina there was a phase transition of γ-Al2O3

toward δ-Al2O3, which is a more stable phase at 950 °C, and a 58% reduction of the surface area happened. Afterward, mixture 1 was prepared and there was an evident decrease in the area of 28% referred to the initial alumina, and then thermal treatments at 950 °C were performed, and the longer the period of heating was, the larger the diminution in the surface area of the sample was. In the case of the aluminum oxide provided by CINDECA, it exhibited a surface area of 260.50 m2/g, which doubles that of Alfa Aesar oxide. This Al2O3 was heated for 24 h under air atmosphere at 950 °C and showed no phase transition and only a slight reduction in the surface area. After preparing the catalyst the area diminishes 57% (the final value was 111.56 m2/g). Concerning the phase transition demonstrated by the catalyst after the thermal treatment under air atmosphere, from the data exposed in Table 3 it could be seen that there is only partial transition of the alumina, because some γ-Al2O3 remained unchanged in the crucible, together with the δ phase. As expected, the BET area of the sample changes as the time of the heating increases from 1 to 24 h. There is some research on the effect of cations added to γ-Al2O3. Ozawa et al. studied the rare earth modification introduced on γ-Al2O3 as regards the thermal stability of the oxide. After impregnation of alumina with the respective nitrate and heating the samples in air from 800 to 1400 °C, they found that Ln, Ce, Gd, and others inhibited the grain growth and the formation of R-Al2O3. Besides, the surface area of these modified oxides increased.22 Other researchers followed the effect of various cation added to γ-Al2O3. On the one hand, the phase transition temperature was increased and there was a linear relationship with the ionic radius of the added monovalent cations, which means that the specific surface area of the mixture after heating at 1400 °C increased the larger the cation was.23

Figure 4. SEM images of the residues left in the crucible: (a) non-isothermal TG of mixture 1, (b) non-isothermal TG of the catalyst, (c) isothermal TG of mixture 1 at 725 °C, and (d) isothermal TG of the catalyst at 725 °C.

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8189 Table 3. Results of XRD, XRF, and BET Area of Both Types of Aluminas, Mixture 1, Catalyst, and Their Residues after Thermal Treatments at 950 °C under Air Atmosphere at Different Intervals of Time experimental conditions Al2O3 a Al2O3a 950 °C, 24 h mixture 1 mixture 1 950 °C, 1 h mixture 1 950 °C, 6 h mixture 1 950 °C, 12 h mixture 1 950 °C-24 h mixture 1 950 °C, 72 h Al2O3b Al2O3b 950 °C, 24 h catalyst catalyst 950 °C, 1 h catalyst 950 °C, 24 h a

XRD

composition (wt %)

γ-Al2O3 δ-Al2O3

surface area (m2/g) 100.02 41.96

γ-Al2O3 NiO δ-Al2O3 NiO δ-Al2O3 NiO δ-Al2O3 NiO δ-Al2O3 NiO δ-Al2O3 NiO γ-Al2O3 γ-Al2O3

Al: 84.2 Ni: 15.8

γ-Al2O3 NiO γ-Al2O3 δ-Al2O3 NiO γ-Al2O3 δ-Al2O3 NiO

Al: 77.1 Ni: 22.9

71.99 53.11 51.48 46.80 45.83 49.79 260.50 227.90

Table 4. Results of XRD, XRF, and BET Area of the Residues Left in the Crucible from Pure Aluminas and the Mixtures after Chlorination in the Reactor at Different Temperatures experimental conditions

XRD

Al2O3 a 800 °C, 1 h of Cl2

γ-Al2O3

Al2O3b 800 °C, 1 h of Cl2 mixture 1 500 °C, 1 h of Cl2

γ-Al2O3

mixture 1 500 °C, 1 h of Cl2 725 °C, 1 h of N2 mixture 1 600 °C, 1 h of Cl2

γ-Al2O3 NiCl2 NiO γ-Al2O3 NiO γ-Al2O3 NiCl2

mixture 1 650 °C, 1 h of Cl2

γ-Al2O3 NiCl2


γ-Al2O3


γ-Al2O3


γ-Al2O3


γ-Al2O3

catalyst 725 °C, 1 h of Cl2

γ-Al2O3

catalyst 800 °C, 1 h of Cl2

γ-Al2O3 δ-Al2O3 NiO

111.56 102.50 89.30

Provided by Alfa Aesar. b Provided by CINDECA.

On the other hand, depending on the kind of divalent cation used, the effect on the γ to R transition was acceleration for Mn2+ and Cu2+, slight change for Ni2+ and Mg2+, and retardation for Ba2+ and Sr2+. In the second group, the cation incorporates into the alumina, increasing its crystallinity near 1100 °C.24 Pijolat et al. agreed with the last results about the stabilization effect of Mg2+and Ca2+ when they are present in low concentrations in the γ-Al2O3, avoiding sintering and phase change. Moreover, water vapor had an enhancing effect on the surface area diminution.25 The previous results exposed by other researchers are of special importance, because the initial loss in surface area before transformation may represent a substantial part of the deactivation of the catalyst.26 As we are approaching a method for the recovery of nickel from simulated catalysts without substantially affecting the microstructure and properties of the supporting alumina, it is expected that this behavior does not happen. In Table 4 the surface areas of the residues from pure aluminas and the mixtures after thermal treatments under chlorine at different temperatures are summarized. As regards the changes noticed in pure Al2O3, the Alfa Aesar source exhibits a diminution from 100.02 in the initial sample (see Table 3) to 64.10 m2/g, while the CINDECA powder only diminished 5 m2/g at 800 °C after 1 h of Cl2 flow. Related to mixture 1, it can be seen that as temperature rises, BET area decreases from the initial value of 71.99 to 49.96 m2/g at 725 °C. In the case of the catalyst, the diminution goes from 111.56 to 104.55 m2/g at 725 °C (see Table 3). If we compare the changes in the surface area for the thermal treatments under the different atmospheres, it can be observed that, for mixture 1, the value decreased from 71.99 to 53.90, 53.11, and 45.83 m2/g, for the experiments at 800 °C for 1 h in Cl2, at 950 °C for 1 h in air, and at 950 °C for 24 h in air, respectively. These results showed that the effect of Cl2 over

a

composition (wt %)

surface area (m2/g)

Al: 96.8 Cl: 3.2

64.10

Al: 86.5 Cl: 13.5 Al: 64.8 Cl: 16.7 Ni: 18.5 Al: 75.2 Cl: 5.3 Ni: 19.5 Al: 77.3 Cl: 6.6 Ni: 16.1 Al: 70.4 Cl: 12.3 Ni: 17.3 Al: 76.6 Cl: 8.2 Ni: 15.2 Al: 80.3 Cl: 9.2 Ni: 10.5 Al: 91.1 Cl: 5.9 Ni: 3.0 Al: 92.1 Cl: 5.4 Ni: 2.5 Al: 78.1 Cl: 10.2 Ni: 11.7 Al: 91.3 Cl: 8.0 Ni: 0.7

255.40 61.47 58.22 50.11

49.96

53.90 104.55 100.00

Alfa Aesar. b Al2O3 provided by CINDECA.

BET area is almost the same of that produced in air atmosphere, but the effect of the time of the heating over BET area is considerable, due to a sintering effect that became more noticeable as the heating lasted longer. In the case of the catalyst, the diminution went from 111.56 to 100.00, 102.5, and 89.30 m2/g, under the experimental conditions described above. Then it can be said that thermal treatments in air at high temperatures and for long periods of time have the strongest effect over surface area diminution and, therefore, are the least recommended conditions to be used for the catalyst recovery. 3.3.2. Separation of the Reactive Species. According to reaction 1, NiCl2 formation occurred with a favorable Gibbs free energy variation at the temperatures under study. As regards the chlorination of mixture 1, NiCl2 was formed in the whole temperature interval, but for temperatures below 650 °C, it remained together with the alumina in the reaction zone, promoted by the low vapor pressure of NiCl2 at this temperature (see reaction 2). At 500 °C some NiO did not react, and starting from 650 °C, the product volatilized and deposited in a zone of low temperature, but some NiCl2 still remained with Al2O3. When the temperature reached 700 °C, the condensation of the chloride occurred far from the reaction crucible, leaving Al2O3 free from NiCl2. In Table 4 is shown the XRF analysis of the residues from the crucible, which is in accordance with the composition detected by XRD that was discussed before. Nickel content in mixture 1 diminishes from 18.5 wt % at 500 °C to 2.5 wt % at 800 °C, ensuring that the chloride leaves the reaction zone and deposits in a cooler region of the reactor.

8190 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

Figure 5. SEM images of the condensed products from the chlorination of mixture 1 at different temperatures: (a) 650, (b) 700, (c) 750, and (d) 800 °C.

Figure 6. SEM images of the condensed products from the chlorination at 725 °C of different reactants: (a) NiO and (b) catalyst.

Considering the compositions determined by XRF and the initial composition of mixture 1 (9.4 wt % Ni), the recovery factor of nickel from the original sample was calculated. The values were 2, 35, 82, and 85%, at 700, 725, 750, and 800 °C, respectively. On the other hand, at 725 °C for the same mixture and in the TG system this value was of 54%. These results showed a more effective recovery of the metal chlorinated in the TG system than in the reactor experiments. In the case of the catalyst, the nickel composition decreased from 11.7 wt % at 725 °C to 0.7 wt % at 800 °C and the recovery factor was 27 and 96%, respectively. At this point it can be said that for temperatures below 800 °C the separation of Ni from the mixture is easier than from the catalyst, which can be attributed to the different interactions that exist in both samples. On one hand, a mechanical mixture was prepared, without the participation of an acqueous phase, while on the other, there was a liquid environment intervening on the dissolution of the compounds, which involve the

formation of chemical bonds, stronger than the links that maintain the reactants together in mixture 1. When temperature was raised to 800 °C, these interactions became weaker and the best recovery of Ni was achieved by the catalyst. 3.3.3. Morphology of Condensed Gaseous Products. In Figure 5 the morphology of the yellowish crystals of nickel chloride formed from mixture 1 at four different temperatures of reaction, 650, 700, 750, and 800 °C, is shown. The crystals were deposited on the walls of the reactor in a zone at temperatures below 650 °C but above 400 °C. The length of the platelets is between 66 and 116 µm, and their thickness is about 1-8 µm. The morphology of the product is in accordance with that described elsewhere.27,28 As the temperature increases, the crystallization of the solid is the major occurrence and the plates of NiCl2 are larger with sharp edges. In Figure 6 is shown the condensed nickel chloride that was obtained at 725 °C and came from NiO (Figure 6a) and catalyst (Figure 6b). The composition of the condensed product from


mixture 1 and catalyst was analyzed by EDS, and a high purity NiCl2 with no traces of Al was found. From the thermodynamic aspect, the vapor pressure of NiCl2 increases from 0.06 to 4.72 kPa in the temperature interval 650-800 °C, showing that chloride crystals were condensed from the gaseous phase, producing well-defined crystals. In the case of the catalyst, the vapor pressure of NiCl2 could have been reduced by the occurrence of superficial interactions created when the sample was prepared by the IWI method, while in NiO there is no such effect and, therefore, the vapor pressure is enough to allow a better volatilization of the chloride, and larger platelets of the chloride are obtained. 4. Conclusions The beginning of the chlorination reaction was determined with non-isothermal TG, and the temperatures measured were 725, 473, and 428 °C, for NiO, mixture 1, and catalyst, respectively. The lowest starting temperature corresponds to the largest BET area of the sample, which contributes to the enhancement of the path for the diffusion of Cl2. Isothermal experiments proved that the conversion of the reaction depended strongly on the structural characteristic of the sample. Moreover, the higher the temperature was, the better the values of conversion that were achieved. In mixture 1, the phase transition from γ- to δ-Al2O3 was only detected under air flow. On the other hand, for the catalyst the transition γ to δ transition occurred partially under both atmospheres and above 800 °C. The changes observed in the solid surface area were the same for the experiments done under air or chlorine atmosphere. However, as the heating lasted longer, the sintering of the surface became more noticeable. The reaction of chlorination produced well-defined platelets of NiCl2. The high-purity crystals condensed from the gaseous phase were collected from the walls of the reactor from regions between 650 and 700 °C. The recovery of nickel from aluminum oxide was calculated, and the best conditions were found for the experiments done in the tubular reactor at 800 °C, with values of 85 and 96% for mixture 1 and catalyst, respectively. Acknowledgment The authors thank Carlos N. Cotaro and Ernesto Scerbo, Diana C. Lago, Gustavo Pastrana, and Silvina Pe´rez Fornells, for performing the SEM/EDS, XRF, BET, and XRD measurements, respectively. They also thank Dr. Francisco Pompeo from CINDECA, University of La Plata, CONICET Argentina, for the samples prepared by IWI. Finally, the authors express their gratitude to Agencia Nacional de Promocioń Cientı´fica y Tecnolo´gica (ANPCyT), Consejo Nacional de Investigaciones Cientı´ficas (CONICET), and Universidad Nacional del Comahue for their financial support. Literature Cited (1) Song, C.; Pan, W. Tri-reforming of methane: A novel concept for catalytic production of industrially useful synthesis gas with desired H2/ CO ratios. Catal. Today 2004, 98, 463. (2) Noronha, F. B.; Duraõ, M. C.; Batista, M. S.; Appel, L. G. The role of Ni on the performance of automotive catalysts: Evaluating the ethanol oxidation reaction. Catal. Today 2003, 85, 13. (3) Chen, C. L.; Wang, C. H.; Weng, H. S. Supported transition-metal oxide catalysts for reduction of sulfur dioxide with hydrogen to elemental sulfur. Chemosphere 2004, 56, 425.

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ReceiVed for reView April 22, 2008 ReVised manuscript receiVed August 11, 2008 Accepted August 19, 2008 IE8006598

Direct Chlorination of Nickel-Containing ... - ACS Publications

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