Thermal decomposition of synthetic ammonium jarosite

Journal of Molecular Structure 744–747 (2005) 295–300 www.elsevier.com/locate/molstruc

Thermal decomposition of synthetic ammonium jarosite Mira Ristic´*, Svetozar Music´, Zvonko Orehovec Division of Materials Chemistry, RuCer Bosˇkovic´ Institute, P.O. Box 180, HR-10002 Zagreb, Croatia Received 6 September 2004; accepted 11 October 2004 Available online 8 December 2004

Abstract Ammonium jarosite (NH4Fe3(OH)6(SO4)2) was produced by forced hydrolysis of a NH4Fe(SO4)2 solution. Upon heating of ammonium jarosite at 400 8C, the IR band at 1426 cmK1 corresponding to n(N–H) vibrations almost disappeared, thus indicating decomposition of the K1 NHC can be assigned to sulphate groups in ammonium jarosite. Upon heating of ammonium 4 ion. The IR bands at 1197, 1081 and 1003 cm jarosite at 400 8C the IR bands showed positions at 1200, 1092 and 1011 cmK1, with a shoulder at 1026 cmK1. The FT-IR spectrum did not change by heating ammonium jarosite at 500 8C. Assignations of IR bands are given. The changes in the FT-IR spectra recorded for the samples produced up to 500 8C were not sufficient for the phase analysis of these samples. 57Fe Mo¨ssbauer spectroscopy was very sensitive to the phase changes in the thermal decomposition products of ammonium jarosite. Upon heating of ammonium jarosite at 400 8C, the corresponding Mo¨ssbauer spectrum was deconvoluted into two doublets which were assigned to Fe(OH)SO4 and Fe2O(SO4)2. The Mo¨ssbauer spectrum recorded for the sample produced by heating of ammonium jarosite at 500 8C, was deconvoluted into three doublets which were assigned to Fe2(SO4)3, Fe(OH)SO4 and Fe2O(SO4)2. Hematite (a-Fe2O3) was produced upon heating of ammonium jarosite at 600 8C, as found by Mo¨ssbauer and FT-IR spectroscopies. Quantitative data obtained by deconvolution of the Mo¨ssbauer spectra are summarized. q 2004 Elsevier B.V. All rights reserved. Keywords: Ammonium jarosite; a-Fe2O3; Fe(OH)SO4; Fe2O(SO4)2; Fe2(SO4)3; FT-IR; Mo¨ssbauer; Electron microscopy

1. Introduction Hematite (a-Fe2O3) has a long history of use a red pigment, starting with the time of cave man and his paintings. The red pigment was traditionally prepared by milling of naturally occurring mineral hematite. The red pigment can be industrially produced by thermal decomposition of synthetic goethite (a-FeOOH). The size and shape of a-Fe2O3 particles can be well controlled by varying physico-chemical parameters of ‘wet’ precipitation process [1–7]. Thermal decomposition of Fe-bearing salts can also be utilized in the production of iron oxides. This procedure is important in the industries where Fe-bearing salts are produced as technological by-products. For example, a-Fe2O3 can be produced by thermal decomposition of some jarosites.

* Corresponding author. Tel./fax: C385 1 456 1111. E-mail address: [email protected] (M. Ristic´). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.10.051

Precipitation of iron in the chemical form of jarosite is widely used in hydrometallurgy [8]. This process exhibits excellent liquid–solid separation properties, minimum losses of Zn2C, Cd2C and Cu2C ions, as well as simultaneous control of sulphate and alkali ions. Researchers synthesized and analysed various jarosites of monovalent and divalent metals; however, end-member jarosites CsFe3(OH)6(SO4)2 and LiFe3(OH)6(SO4)2 could not be synthesized in spite of the fact that optimum conditions for alkali jarosite precipitation were ensured [9]. Tozawa and Sasaki [10] investigated removal of iron in the form of a-Fe2O3 from leach solutions containing significant amounts of sulphates. a-Fe2O3 precipitated at a fairly high concentration of a free sulphuric acid in the presence of metal sulphates at elevated temperature. Ammonium jarosite and hydronium jarosite (H3O)Fe3 (OH)6(SO4)2 are typical solid products of the forced hydrolysis of NH4Fe(SO4)2 or Fe2(SO4)3 solutions [11, 12]. At relatively lower concentrations of NH4Fe(SO4)2 and Fe2(SO4)3 solutions the formation of a-FeOOH

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particles is favoured. Kandori et al. [13] investigated the forced hydrolysis of Fe2(SO4)3 solutions and they noticed that acicular a-FeOOH particles precipitated at pHR1.6, whereas jarosite particles precipitated at pH%1.5. The removal of iron by jarosite precipitation is an elegant hydrometallurgical process; however, this is not cheap technology. With a view to compensate the high cost of iron removal by jarosite precipitation, researchers and engineers focused on the utilization of jarosite precipitates as a raw material in the production of pigments. Sˇolc et al. [14] investigated thermal decomposition of sodium jarosite as a raw material in the production of the ceramic pigment ZnFe1KxCr1CxO4. The kinetics of thermal decomposition of hydronium jarosite was monitored by the TGA technique [15]. Kunda and Veltman [16] investigated the chemical conversion of ammonium jarosite into iron oxide, with separate recovery of ammonia and sulphur dioxide or ammonium sulphate as by-products. According to their results the thermal decomposition of ammonium jarosite proceeded in three steps: (a) dehydration below 300 8C, (b) removal of ammonia at 360 8C and formation of a water-soluble ferric sulphate, and (c) decomposition of ferric sulphate into hematite and sulphur dioxide at 600 8C. The authors [16] showed that decomposition of sodium jarosite took place at 500–600 8C, and the solid reaction products were found as a mixture of hematite and sodium sulphate. In the present investigation we have focused on the monitoring of solid products formed by thermal decomposition of ammonium jarosite, in order to obtain more data about the nature of this process. The present investigation was undertaken because our preliminary findings showed that the formation of the solid products by thermal decomposition of ammonium jarosite is a process more complex than described in the literature.

the recorded spectra. The specimens were pressed into small discs using a spectroscopically pure KBr matrix. 57 Fe Mo¨ssbauer spectra were recorded using a constant acceleration spectrometer of standard design in conjunction with a multichannel analyser. Mathematical deconvolutions of the Mo¨ssbauer spectra were performed using the SIRIUS program (Central Physical Research Institute, Budapest, Hungary). Isomer shifts are given relative to a-Fe. Electron microscopic monitoring of the particles were performed using the Morgagni 268 and TESCAN instruments.

3. Results and discussion Fig. 1 shows the FT-IR spectra of (a) ammonium jarosite, (b) upon its heating at 400 8C for 6 h, and (c) additional heating at 500 8C for 3 h. The characteristic IR bands of ammonium jarosite and their assignations are given in Table 1. Upon heating of ammonium jarosite at 400 8C, the IR band at 1426 cmK1 corresponding to the n(N–H)

2. Experimental NH4Fe(SO4)2$12H2O of analytical purity and twice distilled water were used. If provided NH4Fe(SO4)2$12H2O is a fresh chemical, there is no problem with the dissolution in water in accordance with data in the analytical handbook. If the NH4Fe(SO4)2$12H2O chemical is laboratory stored for a long time it may undergo hydrolysis in the solid state that affects the solubility of this chemical. The conditions for the preparation of ammonium jarosites were given in a earlier paper [11]. At high concentrations of NH4Fe(SO4)2 solutions there is a formation of ammonium jarosite as a single phase. Ammonium jarosite was heated in a laboratory furnace in contact with air. Fourier transform infrared (FT-IR) spectra were recorded at room temperature (RT) using the Perkin-Elmer spectrometer (model 2000). The FT-IR spectrometer was coupled with IRDM (IR Data Manager) program to process

Fig. 1. Characteristic parts of the FT-IR spectra of (a) ammonium jarosite, (b) upon its heating at 400 8C for 6 h, and (c) additional heating at 500 8C for 3 h.

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Table 1 Characteristic infrared bands of ammonium jarosite observed in the present investigation in comparison with Ref. [17] NH4Fe3(OH)6(SO4)2 (present investigation)

NH4Fe3(OH)6(SO4)2 [17]

Assignation

1426 1197 1081 1003a 1003a 655 sh 628 581 sh 508 470 452 sh 344 315 267

1428 1195 1078 1006 1000 655 630 560 505 472 450 345 320 265

n(N–H) n3(S–O) n3(S–O) dOH n1(S–O) n4(S–O) n4(S–O) gOH O–Fe O–Fe n2(S–O) O–Fe O–Fe O–Fe

a

The IR bands corresponding to dOH and n1(S–O) are not separated.

frequency almost disappeared indicating the removal of NHC 4 ions. In the same sample, a very weak intensity band centered at 1420 cmK1 was also observed due to NHC 4 traces remaining in the sample. The IR bands at 1197, 1081 and 1003 cmK1 can be assigned to sulphate groups. Upon heating of ammonium jarosite at 400 8C, the IR band at 1081 shifted to 1092 cmK1, and the IR band at 1011 cmK1 with a shoulder at 1026 cmK1 appeared. Additionally, a very weak intensity band at 595 cmK1 appeared instead of the shoulder at 581 cmK1, whereas the shoulder at 452 cmK1 was not well visible. a-Fe2O3 produced at 600 8C (Fig. 2) was characterized by a very strong and broad band with two shoulders at 597 and 547 cmK1, in addition to the bands centered at 478, 382 and 337 cmK1. For comparison, in Fig. 2 the FT-IR spectrum of a commercial a-Fe2O3 is shown. The FT-IR spectrum of the sample produced by additional heating at 900 8C showed an increase in relative intensity of the band at 544 cmK1 and a corresponding decrease in relative intensity of the band at 602 cmK1. In the same spectrum, a broad band with shoulders at 473 and 437 cmK1 was also observed. The IR band at 337 cmK1 was shifted to 345 cmK1. The results of our FT-IR measurements of a-Fe2O3 are in general agreement with reference literature [18–20]. Iglesias and Serna [21] reported for a-Fe2O3 spheres the IR bands at 575, 485, 385 and 360 cmK1, whereas a-Fe2O3 laths showed the IR bands at 650, 525, 440 and 300 cmK1. Yariv and Mendelovici [22] investigated thermal decomposition of synthetic a-FeOOH between 200 and 1000 8C in air. The IR spectrum of a-Fe2O3, formed as a thermal decomposition product of a-FeOOH, did not change, significantly up to 700 8C. At higher temperatures however, three O2K displacement bands were shifted to higher wave numbers (543, 468 and 333 cmK1). Shoulders at w600 and w440 cmK1 were observed in the IR spectrum of a-Fe2O3 produced at 1000 8C.

Fig. 2. FT-IR spectra of (a) a-Fe2O3 obtained by thermal treatment of ammonium jarosite at 600 8C, (b) a-Fe2O3 by additional heating at 900 8C, and (c) commercial a-Fe2O3.

Fig. 3a shows the TEM photograph of ammonium jarosite particles used as the starting material, whereas Fig. 3b shows the TEM photograph of a-Fe2O3 particles produced at 600 8C. Upon heating at 900 8C, the sintering effect on a-Fe2O3 particles is highly pronounced as shown in the SEM photograph (Fig. 4). It is evident that the changes observed in FT-IR spectra of a-Fe2O3, as shown in Fig. 2, are the consequence of microstructural changes (crystal ordering and sintering effect) which occurred by heating the sample at 900 8C. In order to obtain more data about the thermal decomposition products of ammonium jarosite, specifically of those which form up to 500 8C, we have also performed 57 Fe Mo¨ssbauer experiments. The characteristic results of these experiments are shown in Figs. 5 and 6 and Table 2. The spectrum of the starting material at RT showed quadrupole doublet (Fig. 5a) with Mo¨ssbauer parameters, dFeZ0.40 mm sK1 and DZ1.10 mm sK1, which can be assigned to ammonium jarosite. The sample produced by heat treatment of starting material at 400 8C for 6 h yielded a spectrum which was resolved into two quadrupole doublets (Q1, Q2), as shown in Fig. 5b. The doublet Q1 is assigned to Fe(OH)SO4 with DZ0.86 mm sK1, whereas the quadrupole

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Fig. 3. TEM photographs of (a) ammonium jarosite, (b) a-Fe2O3 obtained by its thermal decomposition at 600 8C.

splitting of 1.44 mm sK1 for Q2 is assigned to Fe2O(SO4)2, although this value is somewhat higher than that cited in literature [23]. The Mo¨ssbauer spectrum of the sample, which was prepared by additional heat treatment of a previous sample at 500 8C for 3 h, showed a more complex spectral feature (Fig. 6a). A fitting procedure was performed for three doublets (Q1, Q2 and Q3). Mo¨ssbauer parameters of the Q1 component are assigned to Fe2(SO4)3. It is known from literature [24–26] that the Mo¨ssbauer spectrum of iron(III)-sulphate shows a single line or a doublet with a small quadrupole splitting. The values for the isomer shift

Fig. 4. SEM photograph of a-Fe2O3 obtained by thermal treatment of ammonium jarosite at 900 8C.

Fig. 5. 57Fe Mo¨ssbauer spectra of (a) ammonium jarosite, (b) after its thermal treatment at 400 8C for 6 h.

(relative to a-Fe) of Fe2(SO4)3 are in the range 0.325– 0.55 mm sK1, while values for quadrupole splittings D are close to 0.30 mm sK1. The doublet Q2 corresponds to residual Fe(OH)SO4. The decrease of D for Fe(OH)SO4 is a result of the loss of –OH groups in the bulk of the crystal and a difficulty of structural rearrangement. The doublet Q3 can be assigned to Fe2O(SO4)2. The results of Mo¨ssbauer analysis also shed more light on the origin of the IR band at 1003 cmK1 recorded for ammonium jarosite, and the bands at 1026 and 1011 cmK1 recorded after heating of ammonium jarosite at 400 and 500 8C. The FT-IR spectrum of ammonium jarosite (Fig. 1) showed the IR band at 1003 cmK1, an overlap of two bands corresponding to dOH and n1(S–O) vibrations. Since Mo¨ssbauer spectroscopy showed the presence of Fe(OH)SO4 in thermal decomposition products produced at 400 and 500 8C, it can be concluded that the dOH vibration at 1026 cmK1 is due to this phase, whereas the IR band at 1011 cmK1 originated from n1(S–O) vibrations in the phases containing sulphate groups. Sasaki et al. [27] recorded the FT-IR spectra of several jarosite compounds. For ammonium jarosite the authors recorded the IR bands at 1002

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Table 2 57 Fe Mo¨ssbauer parameters of ammonium jarosite and its thermal decomposition products at RT History of sample

Spectral lines

d (mm sK1)

Precipitated ammonium jarosite NH4Fe3(OH)6(SO4)2 heated for 6 h at 400 8C Additional heating for 3 h at 500 8C

Q

0.40

Q1 Q2 Q1 Q2 Q3 S

0.40 0.42 0.65 0.30 0.40 0.49

Additional heating for 3 h at 600 8C

D or Eq (mm sK1

G (mm sK1)

A (%)

Identification

1.10

0.32

100

NH4Fe3(OH)6(SO4)2

0.86 1.44 0.36 0.61 1.49 K0.13

0.60 0.45 0.43 0.57 0.56 0.39

32.5 67.5 18.7 30.3 51.0 100

Fe(OH)SO4 Fe2O(SO4)2 Fe2(SO4)2 Fe(OH)SO4 Fe2O(SO4)2 a-Fe2O3

HMF (T)

52.3

Key: d, isomer shift given relative to a-Fe; D, quadrupole splitting of doublet; Eq, quadrupole splitting of sextet; H, hyperfine magnetic field; G, line-width; A, area under the spectral lines. Errors: dZG0.01 mm sK1; D or EqZG0.01 mm sK1; HMFZG0.3 T.

and 1000 cmK1 which were assigned to dOH and n1 ðSO2K 4 Þ vibrations, respectively. These assignations can be taken as tentative due to their adjacent positions. On the other hand, in other jarosite compounds of KC, NaC, AgC and 1/2 Pb2C the separation of dOH and n1 ðSO2K 4 Þ bands was much greater, in line with our observation of the thermal decomposition products of ammonium jarosite at 400 and 500 8C. The thermal treatment of ammonium jarosite at 600 8C yielded a-Fe2O3 as a single phase (Fig. 6b). Evidently the sulphate group incorporated in ammonium jarosite and its thermal decomposition products, such as Fe(OH)SO4, Fe2O(SO4)2 and Fe2(SO4)3 strongly suppressed the formation of a-Fe2O3 at lower temperatures. Music´ et al. [28] recently showed a strong effect of specifically adsorbed sulphate groups on the thermal decomposition of b-FeOOH particles. Upon heating of sulphated b-FeOOH particles between 300 and 500 8C the formation of an amorphous phase and a small fraction of a-Fe2O3 were observed. Needle-like morphology of amorphous particles was preserved at these temperatures, whereas a-Fe2O3 particles produced at 600 8C were much smaller than those produced by heating a pure b-FeOOH.

–

57

Fe Mo¨ssbauer spectrum of ammonium jarosite was characterized by a quadrupole doublet at RT. Upon heating of ammonium jarosite at 400 8C, the Mo¨ssbauer spectrum of a thermal decomposition product was resolved into two doublets which were assigned to Fe(OH)SO4 and Fe2O(SO4)2. The Mo¨ssbauer spectrum of the thermal decomposition product obtained at 500 8C

4. Conclusion – Precipitated ammonium jarosite (NH4Fe3(OH)6SO4)2) was subjected to heating at 400, 500, 600 and 900 8C in air. Solid decomposition products were monitored by FTIR, Mo¨ssbauer and electron microscopy. – Ammonium jarosite showed characteristic IR bands at 1426 cmK1 due to a n(N–H) vibration, and at 1197, 1081 and 1003 cmK1 typical of sulphate group. The IR band at 1003 cmK1 could not be distinguished between dOH and n1(S–O) vibrations. Upon heating of ammonium jarosite at 400 8C the IR band at 1426 cmK1 almost disappeared, indicating decomposition of the starting material. The new IR positions at 1200, 1092 and 1011 with a shoulder at 1026 cm K1 were recorded. Upon heating of ammonium jarosite at 500 8C the corresponding FT-IR spectrum did not change.

Fig. 6. 57Fe Mo¨ssbauer spectra of (a) sample obtained by thermal treatment of ammonium jarosite at 400 8C for 6 h and at 500 8C for 3 h, and (b) additional heating at 600 8C for 3 h.

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was resolved into three doublets which were assigned to Fe2(SO4)3, Fe(OH)SO4 and Fe2O(SO4)2. a-Fe2O3 was obtained by thermal decomposition of ammonium jarosite at 600 8C, whereas at 900 8C the a-Fe2O3 particles showed a sintering effect. Acknowledgements We wish to thank Prof. N. Ljubesˇic´ and Prof. V. Bermanec for their valuable assistance in the electron microscopic work.

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