Ind. Eng. Chem. Res. 1993,32, 1801-1804
1801
KINETICS, CATALYSIS, AND REACTION ENGINEERING Ferric Hydroxide Oxide from the Goethite Process: Characterization and Potential Use Jbrdme Pradel, Simone Castillo,’ and Jean-Pierre Traverse Laboratoire Matkriaux et Energie, UniversitB Paul Sabatier, 31062 Toulouse cedex, France
Raoul Grezes-Besset Laboratoire de Minkralogie et MatBriaux, UniversitB Paul Sabatier, 31062 Toulouse cedex, France
Michel Darcy Laboratoire de Recherche de la Sociktk Vieille Montagne, Viuiez, 12 Aubin, France
Iron compounds obtained as by-products in the removal of iron from acid leach liquors in the electrolytic zinc industry exhibit high catalytic activity in the hydroprocessing of coal. They were analyzed and compared to model compounds prepared on the laboratory scale. The industrial product is mainly composed of amorphous 8-FeOOH instead of goethite as previously considered in the name “goethite process”. Correlatively with catalytic activity, high reactivity toward H2S was demonstrated.
Introduction In the electrochemical extraction of zinc, treatment of the ores involves dissolving out the zinc and this leaves a residue of zinc ferrite (Andre and Masson, 1973;Davey and Scott, 1975). To recover the zinc from the ferrite, the residue is dissolved in acid solution, and by hot oxidation, the iron is precipitated in the form of goethite (a-FeOOH), whereas zinc remains in solution. In the aim of valorizing the goethite precipitate, we tested its efficiency as a catalyst of coal hydroliquefaction using, as a reference, the “red mud” obtained on treating aluminum ores (Morita et al., 1983). The hydroliquefactionresults obtained encouraged us to carry out a detailed physicochemical analysis of the iron-based goethite product in order to clarify the ambiguities surrounding its structure (Davey and Scott, 1975) and give a better idea of the range of products susceptible to arise from the process. With this aim, reference compounds were synthesized in the laboratory. Our objective was to present and compare the characteristics of the various products.
Experimental Section and Results I. Catalytic Properties of the Product for the Hydroliquefaction of Coal. Coal is generally hydroliquefied in the presence of a catalyst. Iron-based compounds are often used as reported by various researchers (Morita et al., 1983;Satriana, 1982;Prbgermain, 1982;Charcosset et al., 1983,1986;Castillo et al., 19921, especiallyindustrial residues of the “red mud” type (Morita et al., 1983;Charcosset et al., 1986). Comparative tests of catalysts were carried out in the following experimental conditions: 10 g of coal dust (particle size < 80 pm) from a widely used deposit in Freyming (for analysis, see Pregermain, (1982)and Charcosset et al. (1983))was mixed with 10 g of tetralin (and catalyst when included) and placed in a 500-cm3autoclave under an initial pressure of 11 MPa of hydrogen. It was mechanically agitated by a
rocking system at the rate of 108min-l within an amplitude of f15O. After 2 h of heating, the maximum temperature reached 470 “C and was maintained for 30 min. The autoclave coolingtime was roughly 3h. The contents were recovered in 50 cm3 of benzene. Soxhlet extraction for 1 h in 300 cm3 of benzene, and then evaporation of the benzene under partial vacuum, gave a quantity of oil and a residue, the weight of which allowed the yield of the whole operation to be determined. The results reported in Table I concern the following catalysts: the industrial residue, one of the ”reference” compounds prepared in the laboratory (see details below), and a commercial compound (Ni-Mo on alumina), frequently used as a known standard catalyst, for comparison. The accuracy on the data was fl % . Three replicates at least were made in each case. The difference between oil % and coal converted 76 corresponds to heavy products and gases. All calculations were made after crystal weight (considered as a constant) had been subtracted. Table I also includes results obtained by other authors (Morita et al., 1983;Andrik et al., 1983)to give a more general picture. The experiments at low (0.2%)and high (5%) catalyst concentration were performed by ourselves and by other authors, respectively. The yield differences can be referred to the experimental conditions. It is seen that the industrial residue from zinc extraction gives a conversion yield similar to that of the “red mud”. Its physicochemical characteristics were therefore determined. 11. Preparation Conditions. A. The Industrial Residue. It is one of the by-products of a process for the extraction of zinc used by Vieille Montagne Co. Ltd (Andre and Masson, 1973;Davey and Scott, 1975). The ore, which is mainly composed of zinc blende, is roasted and part of the zinc dissolved out. The residue, mainly zinc ferrite, is dissolved in acid medium; ferric ions are reduced by blende before oxidation and precipitation in a cascade of reactors in which the solution is stirred strongly and maintained
0888-588519312632-1801$04.00/0 0 1993 American Chemical Society
1802 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Table I. Comparative Results of Coal Conversion (Catalyst. Coal Converted. and Oil: w t %, daf Coal Basis) coal experimental catalyst converted oil conditions without catalyst 68 43 Freymingcoal, Pi(H2) = 11MPa, T- = 470 OC (30 min) 68 NiMo/AlzOS 0.2 89 93 69 industrial residue 5 68 model producta 5 95 50 Freymingcoal, NiMo/Al20Sb 5 83 Pi(H2) = 15 MPa, T- = 450 O C (3 h) red mudC 5 93 69 Japanesecoal, Pi(H2) = 10 MPa, T,, = 420 O C (10min)
*
a For preparation conditions,see text. And& et al.,1983. Morita et al., 1983.
at 85 "C. Gaseous oxygen is injected at a variable flow rate and dispersed in each reactor. Ferrous ion is oxidized to ferric leading to the formation of an iron hydroxide oxide precipitate:
+
+
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2FeS0, 1/202 3H20 2FeOOH + 2H,SO, The acid formed during the reaction is neutralized with zinc oxide from the roasted ore and the pH is maintained at 3.5. The zinc solution is then separated by filtration from the iron hydroxide oxide (FeOOH) and remaining impurities. Liquid containing the precipitate was sampled and filtered in the laboratory to give what is referred to here as the industrial product. B. The Standard Reference Compound. The reference compound was prepared as to present characteristics as close as possible to those of the industrial product but without the impurities. The main parameter which we varied was the oxidation rate. The solution used was composed of iron sulfate and zinc sulfate (iron 25 pL-l; zinc 120gL-9. The solution, heated to 85 "Cwith constant stirring, was oxidized with a 3% solution of hydrogen peroxide, and the pH was maintained at 3.5 by addition of 1 M NaOH. The reaction was considered to be over when the pH no longer varied with the addition of H202. The residue, obtained by filtration, was thoroughly washed to eliminate the soluble sulfates. The product was ovendried at 60 O C for 48 h and then ground to obtain a fine homogeneous powder. 111. Analysis of the Materials. The powders were analyzed by X-ray diffraction using the K a line of iron (A = 1.9373A); the scanning rate was 1"C min-l. Thermogravimetric analysis (TGA and DTA) was carried out on samples of about 50 mg; the temperature reached 1000 "C at a heating rate of 10 "C min-'. The specific surface area was determined by the BET method. The proportion of iron contained in the amorphous industrial product was determined from the amount dissolved (initial mass 2.5 g) after 1h in 1or 2 M hot HC1 solution (55 "C, 250 cm3). The amount of iron present in the form of Fe3+ in the solutions after acid attack was determined by compleximetric titration with EDTA. A. Standard Reference Products. Two rates of oxidation were distinguished according to the rate of input of H2Oz solution: slow, VI = 1.78 X lo3 mol of 0 2 (g of mol of 02 (g of iron)-' h-l. iron)-l h-l; fast, V2 = 7.12 X (a)Product of Slow Oxidation: Its specific surface area was about 70 mg2 g-l. The X-ray diffraction diagram presented wide peaks typical of a poorly crystallized product. It was, however, possible to identify goethite (a-FeOOH) and zinc ferrite (FezZnO4). After heating in air at 400 "C for 6 h, crystallization was much improved
0
100 200 300 400 500 600 700 800 900 T
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A 0.1
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10,0 200 300 400 500 600 700 800 900.
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Figure 1. Thermogravimetric analysis (TGA and DTA) of the materials. (a) model compound from low-rate oxidation; (b) model compound from high-rate oxidation; (c) industrial product.
and it was possible to identify hematite (a-FezO3) and zinc ferrite-stable to at least 900 "C. The differential thermal analysis diagram (Figure la) shows two endothermal peaks a t 260 and 715 "C. The total loss of mass of the product heated to 900 "C was about 20%. The endothermic peak at 260 "C can be attributed to the dehydration of a-FeOOH and ita transformation into a-FezO3. The peak at 715 "C corresponds to the decomposition of sulfates adsorbed into the iron hydroxide oxide. ( b )Product of Fast Oxidation: The specific surface area was 200 mg2gl. The product was found to be completely amorphous and did not show any change of structure even after heating in air for 24 h at 400 "C. When heated to 660 "C for 1h, the product became well crystallized and the X-ray diagram shows peaks characteristic of a-Fe203 and FezZnO4. The thermal analysis diagram (Figure lb) shows a broad endothermic peak around 140-150 "C accompanied by a loss of mass of about 20% and an exothermic peak at 540 "C. These resulta indicate the presence of j3-FeOOH, which dehydrates at around 150 "C and crystallizes into a-Fe2O3 at 540 "C (Naono et al., 1982); Fe2ZnO4 crystallizes a t the same temperature. B. The Industrial Product. The industrial product studied was obtained at an oxidation rate of 3.62 X 1V mol of 0 2 (g of iron)-' h-1. The elementary chemical analysis of a sample taken from the industrial plant in normal working conditions is given in Table 11. The specific surface area was found to be of the order of 120 m2 g-1. X-ray diffraction showed poor crystallinity but did allow the identification of a few well-crystallized phases present in low proportions: zinc ferrite (FezZnOr), zinc silicate (ZnzSi04),beudantite (Fe3Pb(AeOd(S04)(OH)s), and potassium jarosite (FesK(SO4)(OH)s). After heating in air at 650 "C for 6 h, the diffraction diagram presented
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1803 Table 11. Elemental Analysis (wt %) of an Industrial Sample Fe Zn cu Si02 SO,” Ca 396 8b 0.5 3.8 3.2 0.3
‘lFe
a The balance corresponds to oxygen and hydrogen: the product is mainly a hydrated ferric oxyhydroxide: FeOOHaH20. Uncertainty of these values is about &1% , due to the variability of the industrial product.
very intense peaks: zinc ferrite, zinc silicate, and hematite (a-FezOs) were clearly identified. Solubility in acid medium shows that 85% of the product was in the amorphous form and that 82% of the iron was dissolved. The results of the thermogravimetric analysis are given in Figure IC. The endothermal peak at 150 O C corresponds to a dehydration; the peak at 375 O C corresponds to the destruction of beudantite and jarosite, which, moreover, are absent from the X-ray diagram of the product heated to 650 OC. The exothermal peak at 620 “C corresponds to the appearance of crystallized a-FezOs. For temperatures above 700 “C,the endothermal peaks observed originate from the decomposition of sulfates. Total mass loss after heating to 800 “C was about 12 % . These results, and their similarity with the thermograms of the standard reference compound prepared at high oxidation rate, indicate the presence of iron hydroxide oxide mainly in the form of amorphous 8-FeOOH. This hydrated phase constitutes about 85 % of the mass of the product. IV. Product Reactivity to Sulfurization. The reactivity was assessed by determining the quantity of H2S taken up per gram of iron in the form of FeOOH. At ambient temperature, the product was subjected to the action of excess hydrogen sulfide introduced by connection to a flask containing slightly pressurized H2S. The resulting pressure was then measured; the results are reported in Figure 2. For the reference products, the curves show high reactivity with respect to H2S: sulfurization was very rapid during the first few minutes; it then slowed down but still continued after 8 min. The quantity of bound sulfur was greater than 0.6 g/g of iron corresponding to an S/Fe atomic ratio above 1. For the sake of comparison, tests were carried out on products obtained from solutions containing ferrous sulfate only (see Figure 2): the amounts of bound iron were muchlower. However, we have not performed further study and more particularly we have not determined the specific area of pure ferric hydroxide oxide because we focused our interest on the industrial or industrial-like product and because studies of pure compounds correspond to an intricate problem as demonstrated in numerous references (Atkinson et al., 1968; Dousma and De Bruyn, 1979; Feitknecht and Michaelis, 1962; Knight and Sylva, 1974; Wang and Hsu, 1980). For the industrial product, binding to H2S was fast (3-4 min) and was practically finished after 8 min. I t led to an S/Fe atomic ratio less than 1. The diffractogram of the sulfurized industrial product shows the presence of pure sulfur alongside the previously identified compounds (zinc ferrite, zinc silicate, beudantite, and jarosite). I t was seen that the P-FeOOH was more reactive than the a-FeOOH in both the presence and absence of FezZnOr. The hydroxide oxide reactivity was improved in the presence of zinc ferrite. However, in spite of the presence of this compound, the industrial product presented a lower reactivity.
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Figure 2. Sulfuration reaction of the materials ( f i e d H2S in gram per gram of iron in the form FeOOH). Model compound, starting solution with iron sulfate and zinc sulfate: (- * -), bold) low-rate oxidation; (- - -) high-rate oxidation. Model compound, starting solution with iron sulfate: (- -) low-rate oxidation;(- - -) high-rate oxidation. (-) Industrial product.
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Discussion and Conclusion One of our aims was to investigate if the industrial product could be used as well as red mud for coal hydroliquefaction. Consequently, we tested the product in the rather severe conditions employed with red mud. The positive results we obtained incited us to investigate the characteristics of the product in close relation with ita fabrication process. However,the chosen severe conditions are not suitable for a comparative study between pure aand @-ferrichydroxide oxide. Although we can hypothesize that 8-iron hydroxide oxide is more active than goethite, we consider that such a study is out of the scope of the paper and would have to be realized in temperature and pressure conditions far from those used here. Some authors have claimed that the term “goethite process” can be found to be slightly misleading (Davey and Scott, 1975);they identified the precipitation product obtained from synthetic solutions on the laboratory scale and they found that the “Vieille Montagne Process” gave predominantly a-FeOOH when precipitation occured from sulfate solution and a mixture of a-FezOa and subordinate a-FeOOH from a chloride solution. It was concluded that a-FeOOH was by no means the only product obtained during the precipitation of iron by variation of the goethite process. Furthermore, the complexity of the mechanism of ferric hydroxide oxide formation is widely recognized (Atkinson et al., 1968; Dousma and De Bruyn, 1979; Feitknecht and Michaelis, 1962; Knight and Sylva, 1974; Wang and Hsu, 1980). The product we tested, however, was obtained from industrial solutions. The major component was amorphous 8-FeOOH, but there were also secondary wellcrystallized products. Our results do confirm the ambiguous character of the denomination since the main product
1804 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
was clearly identified as P-FeOOH. Moreover, we were able to define the subordinate compounds formed in realistic conditions of analysis. Laboratory preparation involving rapid oxidation leads to compounds analogous to the industrial one. However, the water content of the laboratory product was found to be higher and the crystallization temperatures toward a-FezO3lower. The method using the slow oxidation rate leads to production of a-FeOOH; a higher water content than that of the industrial product is also noticed. These results demonstrate the sensitivity of the process to the oxidation rate and the large differences produced in the reactivity of the "model products" depending on the oxidation rate and composition of the product. In every case, the presence of Zn enhances adsorption of sulfur in two ways: both the rate of adsorption and the amount of adsorbed sulfur increase. The amorphous state of 0-FeOOH is favorable to absorption: S/Fe ratio values as highas 1.1 canbeobtained(0,6gofHZS/gofFe). However, the only crystallized compound identified after H2S absorption was S. In the case of the industrial product, the sulfurization rate in the early stages of sulfurization was as high as those observed with the most reactive model compound. However, the plateau value was lower. These differences can be related to water content, to porosity, and perhaps to the presence of minor compounds in the industrial product. The hydroliquefaction experiments using the industrial product as catalyst were performed in conditions allowing high yields of conversion to be obtained (about 93% coal conversion). Thus, the catalytic effect of the sulfurized product only slightly improves coal conversion (94-95 % ) and the beneficial effect of sulfurization is not very significant. However,the very large affinity of the products toward HzS is coherent with the enhancement of the catalytic effect of iron-oxide-basedcatalysts in the presence of sulfur compounds and Hz as established earlier (And& et al., 1982; Aitchison et al., 1986; Kamiya et al., 1988). The analysis of the residue obtained after coal hydroliquefaction in the presence of industrial sulfur catalyst shows the presence of crystallized ZnS, Fel,S, and FeaOl. Some analogy can be found with red mud catalysts. In any case, iron hydroxide reacts with sulfur present in the coal or added in sulfurization operation and leads to the production of nonstoichiometric iron sulfide Fel,S, which is regarded by some authors as the active center in the hydroliquefaction reaction (Djega-Mariadassou et al., 1986).
Conclusion. The goethite process leads essentially to the production of 0-FeOOH. A high sensitivity of the process to oxidation rate is demonstrated in the production of ferric hydroxide oxide. A lower oxidation rate leads to an a-FeOOH-based product which is less effective toward H2S reactivity. The high reactivity of the products toward H2S is probably related to the presence of 0-FeOOH.
Andrb, J. A.; Masson, N. J. J. The goethite process iri retreating zinc leaching residues. Paper presented at AIME 102nd annual meeting, Chicago, 1973. AndrBs, M.; Charcosset, H.; Davignon, L.; Djega-Mariadasaou, G.; Joly, J. P. Evolution de la texture de prbcurseurs oxydes de fer au cours de leur sulfuration en relation avec I'hydroliqubfaction Chim. Fr. 1982,ll-12,427catalytique du charbon. Bull. SOC. 432. AndrBs, M.; Charcosset, H.; Chiche, P.; Davignon, L.; DjegaMariadassou, G.; Joly, J. P.; Prbgermain, S. Catalysis of coal liquefaction by synthetic iron catalysts. Fuel 1983,62, 69-72. Atkinson, R. J.; Posner, A. M.; Quirk, J. P. Crystal nucleation in Fe(II1) solutions and hydroxide gels. J. Znorg. Nucl. Chem. 1968, 30,2371-2381. Castillo, S.; Lauret, A.; Yaouancq, A.; AriBs, L.; Traverse, J. P. Novel Catalystsapplied to hydroliquefaction of coal. Fuel 1992,71,243245. Charcosset, H.; AndrBs-Besson, M.; Bacaud, R.; Chiche, P.; DjegaMariadassou, G.; Tran Huu Vinh. Recherche de nouveaux catalyseurs au fer pour l'hydroliqubfaction du charbon. Entropie 1983, 113-114, 44-56 and references therein. Charcosset, H.; Bacaud, R.; Besson, M.; Jeunet, A.; Nickel, B.; Oberson, M. On the chemical effects on catalysts in the direct liquefaction of coal. Fuel Process. Technol. 1986, 12, 189-201. Davey, P. T.; Scott, T. R. Formation of PFeOOH and aFezOs in the goethite process. Australian Inst. Min. Metall. 1975, 83-86. Djega-Mariadassou,G.; Besson, M.; Brodzki, D.; Charcossat, H.; Tran Vinh Huu; Varloud, J. Evolution of highly dispersed catalysts during hydroliquefaction of coal. Fuel Process. Technol. 1986, 12, 143-153. Dousma, J.; De Bruyn, P. L. Hydrolysis-precipitation studies of iron solutions. 111. Application of growth models to the formation of colloidal a-iron (111)oxyhydroxide from acid solutions. J . Colloid Interface Sci. 1979, 72, 314-320. Feitknecht, W.; Michaelis, W. Hydrolysis of iron (111) perchlorate solutions. Helu. Chim. Acta 1962,45, 212-214. Kamiya, Y.; Nobusawa, T.; Futamura, S. Catalytic effects of iron compounds and the role of sulphur in coal liquefaction and hydrogenolysis of SRC. Fuel Process. Technol. 1988, 18, 1-10. Knight, R. J.; Sylva, A. M. Precipitation in hydrolysed iron (111) solutions. J. Znorg. Nucl. Chem. 1974, 36, 591-597. Morita, M.; Ikezoe, M.; Hashimoto, T.; Sato, S.; Imaizuni, T.; Nihei, H. Catalytic activities of iron ores in direct liquefaction of coal. Energy Deu. Jpn. 1983,6, 187-206. Naono, H.; Fujiwara, R.; Sugioka, H.; Sumiya, K.; Yanazawa, H. Micropore formation due to thermal decomposition of acicular microcrystals of PFeOOH. J . Colloid Interface Sci. 1982,87,317332. Prbgermain, S. Etude de l'hydroliquefaction d'un charbon lorrain. Chemical and Physical Valorization of Coal, Round Table, Brurelles, November 1980; paper published by the DirectorateGeneral Information Market and Innovation, Commission of the European Communities, 1982. Satriana, M. J. Hydroprocessing catalysts for heavy oil and coal; Noyes Data Corporation: Park Ridge, NJ, 1982. Wang, M. K.; Hsu, P. H. Crystallization of goethite and hematite at 70 OC. Soil Sci. SOC.Am. J . 1980,44, 143-149.
Literature Cited Aitchison, D. W.; Clark, P. D.; Fitzpatrick, E.; Hawkins, R. W.; Lee, T. L. The liquefaction of Albert subbituminous coals with ironsulphur catalysts systems. Fuel 1986,65, 603-607.
Received for review December 29, 1992 Revised manuscript received May 28, 1993 Accepted June 8, 1993
