Recycling of Wastes from the Production of Alumina-Based Catalyst

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RECYCLING OF WASTES FROM PRODUCTION OF ALUMINA BASED CATALYST CARRIERS Anna N. Matveyeva, Nikolai Pakhomov, and Dmitry Yu. Murzin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00978 • Publication Date (Web): 12 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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Industrial & Engineering Chemistry Research

RECYCLING OF WASTES FROM PRODUCTION OF ALUMINA BASED CATALYST CARRIERS Anna N. Matveyeva,1,2, Nikolai A. Pakhomov1,2, Dmitry Yu. Murzin2,3* 1

Department of General Chemical Technology and Catalysis, St. Petersburg State

Institute of Technology (Technical University); 26, Moskovsky Ave., St. Petersburg 190013, Russia 2

Laboratory of Catalytic Technologies, St. Petersburg State Institute of Technology (Technical University); 26, Moskovsky Ave., St. Petersburg 190013, Russia

3

Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University; 8, Biskopsgatan, Turku/Åbo 20500, Finland *

e-mail: [email protected]

Abstract: Preparation of microspherical chromia-alumina KDM-type catalysts for alkane dehydrogenation is based on utilization of an amorphous nanostructured hydroxide-oxide Al2O3-х(OH)2·nH2O, (х=0-0.28, n=0.03-1.8) prepared by the centrifugal

thermal

activation (CTA) of gibbsite. The fine fraction of this carrier is considered as an industrial waste calling for efficient recycling options, which were explored in this work. A range of physico-chemical methods was applied to analyze the main properties (phase composition, morphology, reactivity, etc.) of pilot and industrial batches of gibbsite centrifugal thermal activation (CTA) products. It has been clearly shown that the properties of CTA-products depend on the nature of gibbsite used. The largest amount of the active amorphous phase of alumina is formed using gibbsite made from the nepheline concentrate. Regularities of rehydration of the CTA-products fine fraction in acidic/alkaline media and dissolution in sulfuric acid of the fine fraction of gibbsite and the industrial CTA-product were explored to prepare aluminum sulphate, a water purification coagulant.

Keywords: gibbsite, centrifugal thermal activation of gibbsite, amorphous alumina, CTA-product, rehydration, bayerite, pseudoboehmite. 1

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Introduction Dehydrogenation processes are extensively used in petrochemical industry. Unsaturated compounds, such as propylene, butadiene, isoprene, isobutylene are obtained from the corresponding alkanes by dehydrogenation. They are valuable monomers for production of synthetic rubber and plastics, as well as for synthesizing high-octane number components of gasoline (for example, MTBE and alkylate) and other important chemical products. There are several industrial options for C3-C5 paraffin dehydrogenation different in both technology and the catalyst type [1, 2]. Chromium oxide-based catalysts have been one of the most actively investigated formulations for dehydrogenation since the first report of Frey and Huppke in 1933 [3]. Dehydrogenation of butanes over a chromia/alumina catalyst was first developed in the 40-s at Leuna and independently at UOP by Ipatieff and coworkers [4, 5]. These catalysts have been applied to several processes for dehydrogenation of light olefins. Several challenges in utilization of these catalysts were repeatedly pointed out, namely a certain health risk, when the plant operators are exposed to Cr(VI) containing catalysts, and sintering at high temperatures. Despite these apparent drawbacks more than half of the global market of industrial catalysts for dehydrogenation of C3-C5 paraffins can be attributed to chromia-alumina systems – for example, the best known process is CATOFIN™, by CB&I Lummus [5-8]. In the Russian Federation industrial processes of isobutane dehydrogenation are carried out in fluidized bed reactors with microspherical chromia-alumina catalysts. This Fluidized-Bed Dehydrogenation (FBD) technology of Yarsintez, further developed and licensed by Snamprogetti, is similar to old fluid catalytic cracking units with a continuous catalyst circulation between a fluidized bed reactor and a regenerator [9, 10]. As a catalyst chromium oxide on alumina in a microspherical form with an average diameter 2


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of particles less than 0.1 mm is used. In the Russian Federation alone around 10 plants are operating with the nameplate overall capacity at least 700 000 t/y [11]. A commercial plant for the production of 450 kt /a of isobutene using microspherical chromia-alumina catalysts is also in operation for SADAF in Al-Jubail (Saudi Arabia) since 1997. Other projects were reported to be under negotiations. The FBD technology imposes several requirements for the catalysts not only in terms of activity, but also in terms of abrasion resistance. A chromia-alumina catalyst developed rather recently at Boreskov Institute of Catalysis (Novosibirsk, Russia) together with an industrial company «Sintez» (Barnaul, Russia) fulfills these requirements exhibiting high activity, selectivity and mechanical strength [12-14]. At the present time the catalyst under the trade mark KDM is applied in a number of industrial plants. First industrial experience, briefly summarized in [11], points out on a substantial decrease in the nominal catalyst consumption, which in fact is a serious issue. The technology of KDM synthesis (Figure 1) is based on using as a carrier an amorphous nanostructured hydroxide-oxide Al2O3-х(OH)2·nH2O, where х=0-0.28, n=0.03-1.8, which is prepared by the centrifugal

thermal activation (CTA) of gibbsite (GB) using a

CEFLAR™ technology [15, 16].

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Figure 1. The flowsheet of catalyst manufacturing.

Thermal activation of gibbsite on CEFLAR™ technology is one of the key steps of KDM catalyst preparation. The technology allows regulating the phase composition of the CTA product over a wide range and synthesizing on its basis a carrier with the desired structural and textural characteristics. The CTA-method is based on the transformation of the non-porous crystalline gibbsite – Al(OH)3 to an amorphous product by the pulsed (sudden) heating of gibbsite on a metallic preheated surface to the temperature of dehydration, which is followed by a rapid cooling step strengthening the support. Such method of gibbsite thermal processing allows incomplete decomposition by stopping it with the aid of rapid cooling at the stage of the hydroxide structure decomposition, when oxide, stable at low temperature, has not yet been formed [16]. The CTA-product is a gibbsite pseudomorph, having in contrast to gibbsite a developed pore structure, a large specific area and a high chemical reactivity. In general, such technology of carrier preparation is environmentally friendly not leading to harmful emissions and effluents. 4


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It is known that fluidization properties of microspherical catalysts strongly depend on their fractional composition and density. In order to ensure uniform fluidization of the KDM catalyst during operation a fractional composition of microparticles from 50 to 140

µm is needed [11]. Catalyst particles below this size have an irregular shape strongly deviating from a spherical one. As a consequence the mechanical strength is compromised. Alternatively larger particles, resulting from intergrowth of several smaller 3D particles, exhibit undesired cracks. Such cracking takes places between the layers of the crystal lattice of gibbsite during its dehydration. Unfortunately gibbsite from various suppliers, although having appropriate chemical purity do not fully meet these requirements of fractional composition. Such problems exist independent on the production methods of gibbsite either starting from nepheline concentrate using sintering technology with limestone or applying bauxite using the well-known Bayer process [17]. Nepheline concentrate, being a by-product itself in apatite concentrate production, contains on average 27-28% Al2O3, 44% SiO2, and approximately 20% R2O (Na2O + K2O). The Bayer process is based on dissolution of alumina in bauxite by means of treating it with caustic alkali in autoclaves. Bauxites have somewhat higher content of alumina and less silica than nepheline concentrate. Moreover alkali metals are absent, while content of iron oxide in bauxites can be in the range 14-23 %. Gibbsite contains unacceptable amounts of fine fractions, prohibiting its direct utilization in industrial operation of the chromia-alumina catalyst. In response to this challenge the production technology of the carrier for the KDM catalyst after drying or thermal activation steps includes classification of the material (Fig. 1). The desired fraction is used in further catalyst preparation, while particles smaller than 50 microns are considered as waste. The proportion of such waste is at least 15 wt. %. of the overall catalyst production. It is apparently clear that implementation of fine fractions recycling 5



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into useful products is important from the viewpoint of process technology and economics. The aim of the current work was to develop methods for recycling of the aluminum-containing carrier fine fractions of KDM catalyst, which will make the production technology completely waste free. While in the current work gibbsite from local suppliers was used, it should be noted that the feedstock in fact is produced according to two well established technologies, mentioned above (sintering technology with limestone or the Bayer process). Therefore, recycling of fine fractions has a general interest. The paper focuses on two ways of solving this problem, namely rehydration of CTA-products in alkaline and acidic media to produce precursors of active alumina – bayerite (BA) and pseudoboehmite (PB), and dissolution of gibbsite and CTA-product in sulfuric acid generating aluminum sulphate – the coagulant for water purification. An important initial stage in the first approach was to explore different rehydration modes of CTA-products using model samples obtained on CEFLAR™ pilot set-up installation at Boreskov Institute of Catalysis and scaling up the results to the industrial batches of the carrier. For the first time capability of rehydration of CTA-products, synthesized from gibbsites of different nature was investigated and it was shown also for the first time that products of gibbsite thermoactivation (CTA product) obtained using different technologies are different in terms of their re-hydration ability. An explanation for this behavior was put forward. Since CTA product applications are not limited to chromia-alumina catalysts, this knowledge could be used for production of alumina carries in general providing a more waste free technology.

Experimental Rehydration and dissolution 6


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Gibbsite samples for further application in CEFLAR™ set-up were derived from a nepheline concentrate using sintering technology and from bauxite using the Bayer process. The corresponding suppliers were Achinsk Alumina Refinery and Bogoslovsk Aluminium Smelter, thus the samples were coded as A-CTA and B-CTA, with A and B reflecting the first letter in the supplier name. The centrifugal thermal activation was carried out in the conditions [11] (activation temperature=550°C) providing a minimal amount of impurities namely undecomposed gibbsite and boehmite (BE) which is a byproduct of gibbsite dehydration. As an industrial waste fine fractions (dp ≤ 40 µm) of original gibbsite (I-GB), prepared using the Bayer process, and corresponding I-CTA-product, were used. The ICTA-product was obtained in an industrial rotary kiln by the CEFLAR™ technology. Rehydration of CTA-products was performed in a glass reactor equipped with a magnetic stirrer or in an ultra-thermostat at temperatures of 30-35°C, 70-90°C and 90°C. First a suspension of the sample (dp ≤ 40 µm) was prepared with a concentration of Al2O3 100 g/l. The alkaline rehydration was carried out with solutions of ammonia or sodium hydroxide (pH=10-11), while acid rehydration was performed with acetic or nitric acids solutions (pH=5-5,5). The rehydration time varied between 3 and 24 h. The products were separated and washed on a filter paper until neutral pH. Thereafter the products were dried at 100-110°C. Dissolution of I-GB and I-CTA-product (10 g calculated based on Al2O3) in sulfuric acid was done with a stoichiometric ratio (1:3) and an excess of acid with continuous stirring in a glass reactor placed into a thermostat for 0.5 and 2 hours at 95°C. Different concentrations of sulfuric acid from 20 to 39 wt. %

were used. After

dissolution the products were diluted with water to prevent crystallization of aluminum sulfate. The unreacted gibbsite was filtered and dried by acetone. The leftover of I-CTA-

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product were filtered in a Buchner funnel and subsequently dried in an oven at 100110°C. Characterization The samples were characterized using several techniques. The fractional composition of CTA-products was determined by sieving with sieves – 40 and 100 µm, and by the laser scattering method using SALD-2201 Laser Diffraction Particle Size Analyzer (Shimadzu, Japan). Morphology gibbsite and CTA-products samples was studied by scanning electron microscopy (SEM) with a JSM-6460 LV (Jeol) microscope with the resolution limit of ca. 30 Å within the magnification 100-30000 with primary electron beam energy of 25 keV. The phase composition of CTA-products and their rehydration derivatives was obtained by X-ray diffraction (XRD) with a DRON-3 diffractometer using Cu Kα radiation in the 2Θ range 10-75° by the points scanning method. Differential thermal (DTA) and thermogravimetric (TGA) analysis was carried out with the Paulik-Paulik-Erdei system device. The samples (350-750 mg) were heated with the ramp 10°/min from 25°C to 800°C in air. The accuracy of determining the weight loss is in the range 1-5 wt. %. Quantitatively the phase composition of the samples was determined from the weight loss upon heating (TGA curve) within respective temperature intervals for certain phases. The specific surface area (Ssp., m2/g) of the initial samples was determined by N2 physisorption with the BET method [19]. Powders were compressed into tablets (amount – 0.5 g; pressure – 200 bar; time – 7 minutes). Pretreatment was done at 100-150°C for 1 h. Reactivity of CTA-samples was determined by the degree of their dissolution in 50 ml of 5M HCl at 60°C for 1 h with constant stirring [16]. The samples (10 g) in all 8


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cases had the size between 40 and 100 µm. The reactivity (R, %) was calculated in the following way: R = (m1 – m2)·100/m0,

(1)

m1 – mass of the initial sample (without adsorbed moisture); m2 – undissolved portion of its mass. The amount of the undissolved residue was determined by filtration and washing with distilled water, followed by drying at 100-110°C before a constant weight was obtained. The reactivity of the various forms of aluminum hydroxide and oxides can be found in Table 1 [20].

Table 1. The reactivity of the various forms of aluminum hydroxide and oxides № Aluminum compound Reactivity (HCl), % 1 Gibbsite 10 2 χ-Al2O3 28 3 Boehmite 4 Amorphous hydroxide 98

Results and discussion According to X-ray analysis, the phase composition of the A-CTA-product has crystalline phases of undecomposed gibbsite and boehmite as impurities (Figure 2).

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Figure. 2. XRD patterns of CTA-products (* – gibbsite; + – boehmite; • – χ-Al2O3)

The B-CTA-product does not have such impurities. Zoomed X-ray diffraction pattern, (Figure 3), contains only weak peaks with significant broadening , which may belong to highly dispersed phases of χ- and γ-Al2O3. The I-CTA-product contains impurities of boehmite (BE) and a finely dispersed phase of χ-Al2O3.

Figure 3. XRD pattern of the I-CTA-product

Studies of the original samples by DTA and TGA are in line with X-ray data allowing making a quantitative assessment of the phase composition, which is presented in Table 2.

№

Sample

1

A-CTA

2

A-CTA

3 4

B-CTA I-CTA

Table 2. The phase composition of CTA-products Phase composition (DTA dp, µm Method of obtaining the andTGA, X-ray), wt. % gibbsite Al(OH)3 BE AmPh GB BA 0-200 From a nepheline 9 18 73 concentrate using sintering 0-40 7 11 82 technology 0-40 From bauxite using the +χ Bayer process 0-40 17 +χ 10


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Notation: dp – the equivalent diameter of particles; AmPh – the amorphous phase; χ – Al2O3-phase

Three endothermic effects with minima at 165, 310 and 525°C, appear in the DTA curve when heating the A-CTA-product (Figure 4). The first peak is associated with release of chemisorbed water, while the second and the third peaks correspond to decomposition of gibbsite and boehmite, respectively. A mild exothermic effect with a minima at 760°C reflects crystallization of amorphous alumina Al2O3·nH2O [20]. Based on the weight loss, it can be stated that A-CTA-product (sample 1) contains on average 9 wt. % gibbsite and 18 wt. % boehmite. The fine fraction (0-40 µm) of this sample contains 7 % less boehmite than its coarse fraction (sample №2). This difference can be attributed to higher water vapor pressure in larger particles during dehydration leading to more prominent formation of boehmite from gibbsite [22].

Figure 4. Thermograms of the A-CTA and B-CTA-products

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On the contrary, no thermic effects on the DTA curve were seen for the B-CTAproduct (sample № 3) demonstrates. However, a monotonous weight decrease can be observed for this sample starting from 100°С (Figure 3). Together with X-ray data, this fact allows to assume that in this sample γ-, χ-Al2O3 phases and, probably, an amorphous phase of alumina Al2O3·nH2O are present. For both B-CTA and I-CTA samples exothermic effects in 760-800°C range are absent pointing out on preferential formation of crystalline oxide phases, rather than the amorphous phase, during thermal activation of gibbsite, obtained from bauxite using the Bayer process. According to TGA the overall weight losses during calcination of the industrial IGB catalyst waste fraction are 34.3 wt. %, being in line with dehydration reaction stoichiometry: Al(OH)3 → χ-Al2O3 + 3H2O and pointing out that during drying of gibbsite in industrial conditions only physically bound water is released without any changes to the hydroxide crystalline structure. The specific area of the initial samples is shown in Table 3. After CTA of gibbsite the specific area increased substantially. Such increase, reaching more than two orders of magnitude, depends on the particles size and the amount of adsorbed water.

№

1 2 3 4 5 6 7

Table 3. The specific area of the initial samples Sample Fractional composition Pretreatment temperature, °C I-GB 100 I-CTA > 100 µm ≤ 40 µm A-CTA > 100 µm ≤ 40 µm > 40 ≤ 100 µm 150 B-CTA > 40 ≤ 100 µm 100

Ssp., m2/g

1 145 130 48 54 100 129

It is known [19], that amorphous aluminum compounds exhibit the highest reactivity, which decreases with increase in crystallinity. The most reactivity among the 12


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tested materials was A-CTA-product (R = 37.5 %). The I-CTA-product has 34.7 % of the reactivity and the B-CTA-product – 30.5 %. The reactivity provides an opportunity to determine quantitatively the phase composition of the CTA-product by solving a system of two equations with two unknowns x and y [19]: R = 0.98·x + 0.28·y + 0.1·a,

(2)

x = 100 – y – a – b,

(3)

where x contribution of amorphous alumina, wt. %; y – concentration of χ-like alumina phase, wt. %; a and b, respectively, amounts of gibbsite, wt. % and boehmite, wt. % determined from ТGA. In eq. (1) the reactivity is calculated as an additive of reactivity of different phase. The respective constants (0.98, 0.28 and 0.1) in eq. (1) are the reactivity values of pure phases – amorphous, χ-Al2O3 and gibbsite, which were determined after dissolution them in HCl. In eq. (1) contribution of boehmite was not taken into account since it does not dissolve in HCl. The phase composition based on reactivity data is shown in Table 4.

Table 4. The phase composition of CTA-products determined from reactivity data and TGA Phase composition, wt. % Sample Gibbsite Boehmite Amorphous phase χ-Al2O3 A-CTA B-CTA I-CTA

8 -

14.5 17

21.5 4 18.5

56 96 64.5

While some literature reports [23] state that there were no differences between boehmite and other AL(OH)3 polymorphs in terms of their solubility at acidic pH, it should be noted that in [23] the experiments were performed for several days (from 14 days upwards) to minimize the influence of reactive surface material. However, it was shown earlier [24] removal of the reactive surface material by contact with HCl significantly 13



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decreased solubility. It is difficult to compare data from these studies and the results of our work, mainly because of a large difference in the procedure for determining chemical activity of aluminum-containing compounds. In the current work, we have used another less time consuming method, according to which an experiment lasted for only an hour at elevated temperature. Such short experimental time allows seeing differences in performance between boehmite and other polymorphs, which is otherwise not visible in case of longer exposures. Experimental conditions used in our method were chosen in a way to maximize solubility of CTA and at the same time, to have lowest solubility of individual aluminum hydroxides. The method of reactivity permits determination of amorphous alumina contribution separately from χ-like alumina phase, which is otherwise impossible based on X-ray, DTA and TGA alone. Besides the phase composition, the CTA-products differ in the particle morphology. As seen from Figure 5, particles of A-CTA-product with the size below 50 µm have a random form. The finest particles with the size 10-20 µm are microcrystallites from which larger coarse particles are formed. The larger is the particle size the closer is their form to the spherical, the most optimal for fluidized bed processes [11].

Figure 5. A micrograph of A-CTA-product. The fraction < 70 µm. Reproduced from [11] with permission from Springer. 14


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B-GB and B-CTA particles are aggregates of smaller 3D particles of irregular shape (Figure. 6).

Figure 6. A micrograph of B-CTA-product. The fraction < 100 µm. Reproduced from [11] with permission from Springer.

B-CTA-product particles of the size below 100 µm contain cracks, located parallel to each other. The reason could be because of generation of high vapor pressure during dehydration of gibbsite in coarse assisting transformations of gibbsite to boehmite. Water vapors are rather freely released from nepheline gibbsite layers without destroying its crystallite structure, while during dehydration of gibbsite, obtained from bauxite, such release is significantly impeded causing particles cracking. Because of the layered gibbsite structure, cracking takes place along the layers of the crystalline packing [11].

The rehydration of CTA-products Experimental results of the phase composition for alkaline rehydration products are presented in Table 5.

№

5 6 7 8 9 10

Table 5. Characteristics of CTA-products rehydration of in alkaline medium Conditions of rehydration Phase composition (DTA Sample dp, µm and TGA, X-ray), wt. % Electrolyte pH tr, °С tr, Al(OH)3 BE AmPh h GB BA A-CTA-1a

Recycling of Wastes from the Production of Alumina-Based Catalyst

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