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The synthesis of crystalline scorodite, FeAsO4 · 2H2O
Hydrometallurgy, 19 (1988) 377-384 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
377
The Synthesis of Crystalline Scorodite, FeAs04" 2H20 J.E. DUTRIZAC and J.L. JAMBOR
Mineral Sciences Laboratories, CANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, Ontario KIA OG1 (Canada) (Received March 13, 1987; accepted in revised form June 26, 1987)
ABSTRACT Dutrizac, J.E. and Jambor, J.L., 1988. The synthesis of crystalline scorodite, FeAs04.2H20. Hydrometallurgy, 19: 377-384. Crystalline scorodite, FeAs04- 2H20, was synthesized by precipitation from 0.3 M Fe (NO~) 3-25 g/L As5+ solutions at pH ~0.7. Temperatures greater than 125°C are needed to ensure good scorodite crystallinity, and a temperature of 160°C was used for the current syntheses. Although initial solution pH values in the 0.2 to 1.8 range have little effect on either the structure or composition of the scorodite, As~+ concentrations greater than 15 g/L are needed to ensure a pure product. About 1% SO~- could be incorporated in the scorodite structure, and the S042- content decreased with increasing solution pH. Upon heating, scorodite decomposes at 100-200°C with the simultaneous release of both of its H20 molecules to form anhydrous FeAs04.
INTRODUCTION
Arsenic is a common impurity in many metallurgical ores and concentrates. During the hydrometallurgical processing of such feeds, significant amounts of arsenic pass into solution, often in the pentavalent (arsenate) form. The concentration of arsenic must be closely controlled in the processing solutions to avoid product contamination and/or the pollution of nearby water systems. For practical reasons, arsenate often is precipitated as ferric arsenate [ 1 ] although the precipitation of other compounds such as barium arsenate [ 2 ] or calcium arsenate [3] has been used. Because the industrial precipitates usually are discarded in storage ponds, the solubilities of the iron arsenate compounds are of considerable importance as they affect directly the quality of the water overflowing the pond into the local water system. The solubility of arsenate depends on the compound precipitated, its composition, and, especially, its crystallinity. It is well established that amorphous or metastable phases have higher solubilities than the thermodynamically sta0304-386X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
378 ble species [ 4 ]. During the course of a recent study on the behaviour of phosphate during the precipitation ofjarosite-type compounds, it became apparent that phosphate precipitated in a variety of mineral forms depending on the precipitation conditions and the cations present. In contrast, the corresponding arsenate systems invariably yielded only scorodite, FeAsO4" 2H20 [ 5,6 ]. The precipitation experiments suggested, however, that the crystallinity of the scorodite varied greatly with the experimental conditions, and that sulphate substituted for arsenate to a limited extent. Scorodite has been synthesized by a number of workers, but all syntheses seem to have been carried out at temperatures below 105 ° C. Syntheses by Chukhlantsev [ 7], Nishimura and Tozawa [8], and Makhmetov et al. [9] were carried out at low temperatures. Although the precipitates had the composition of scorodite, they were completely amorphous. Later syntheses carried out at ~100°C by Robins [1] and Dove and Rimstidt [10] yielded precipitates which had the scorodite composition and gave a scorodite powder diffraction pattern. Based on the behaviour of arsenate during iron precipitation as jarosite [ 5,6 ] and on the absolute intensities of the powder diffraction patterns of precipitates made at ~ 100 ° C, however, it is thought that such precipitates contain a significant percentage of amorphous FeAsQ- 2H20 in addition to crystalline scorodite. The presence of such an amorphous phase would result in arsenate solubilities many times higher than those of well-crystallized scorodite [ 11 ]. Because arsenic stability in residue ponds is a subject of current metallurgical concern and because of the need for well-crystallized scorodite for such investigations, experiments were carried out to define the conditions for the precipitation of well-crystallized scorodite and to identify some of the properties of this material. EXPERIMENTAL Reagent-grade chemicals were used for all the syntheses. Scorodite usually was synthesized from 0.3 M Fe ( NO~ ) 3 solutions containing 25 g/L As 5+ at the natural pH of ~ 0.7. The arsenic was added to the solution as As205 to avoid the coincidental introduction ofjarosite-forming alkalis. Experiments were also done in the present of Li2S04; the pH was raised for test purposes with Li2CO3. Lithium salts were used for this purpose, as lithium does not form an endmember jarosite-type compound [12], and because lithium arsenates do not precipitate under the conditions used. The precipitates were formed by dissolution of the appropriate reagents in water, adjusting the pH if necessary, and then heating to the appropriate temperature. The preferred temperature for scorodite synthesis was found to be 160 ° C. Twenty-four hour reaction periods were used for all the experiments, and preliminary tests showed that such times yielded near-equilibrium conditions. The experiments were done by heating 1 L of solution in a Parr 2-L autoclave with a glass liner and titanium internals;
379 the experiments were done in a sealed reactor under the solution pressure. At the end of the reaction period, the autoclave was cooled quickly ( < 10 min), and the slurries were filtered immediately. The precipitates were washed well in ~ 4 L of hot water and subsequently were dried at 110 ° C before analysis. All the precipitates were analyzed chemically, and all were examined by Guinier-deWolff X-ray diffraction patterns to confirm the presence of scorodite and to identify possible impurity phases. Guinier precision focussing cameras are particularly sensitive to impurity phases, and ~ 0.25 wt% of an impurity usually can be detected by this technique. Additional X-ray powder patterns of the precipitates were obtained using a Debye-Scherrer camera of 114.6 mm diameter, CoKal radiation (~ = 0.17889 nm ), and MgO as an internal standard. RESULTS AND DISCUSSION Preliminary efforts to synthesize scorodite from 0.3 M Fe (SO4) 1.~ solutions containing 5-10 g/L As ~+ at pH = 1.3 and 97 ° C resulted in amorphous precipitates containing several percent sulphate [ 5 ]. Further work [ 6 ] showed that crystalline scorodite was formed above 125 ° C, and subsequent studies therefore were carried out at a considerably higher temperature, 160 ° C, to ensure both rapid rates and well-crystallized products. Also, preliminary testwork showed that ferric nitrate solutions resulted in crystallized scorodite free of sulphate contamination, and subsequent experiments generally were done using the nitrate salt. Figure 1 illustrates the effect of the As 5+ concentration of the solution on the yield and composition of the precipitates formed from 0.3 M Fe (NO3)3 solutions at the natural pH of ~ 0.7 and heated to 160 ° C for 24 h. At low Ass+ concentrations (As s+ < 10 g/L), the products are deficient in arsenate but enriched in iron. Such products were commonly red, despite the fact that only scorodite was detected by Guinier X-ray diffraction. The colour presumably is due to an amorphous iron oxide or hydroxide compound. For Ass + concentrations greater than 20 g/L, light greenish-yellow precipitates having the approximate composition of scorodite are formed; i.e., Fe ~ 24% and AsO4 ~ 62 %; scorodite contains 24.20% Fe, 60.20% AsO4 and 15.60% H20. All these precipitates yielded sharp powder diffraction patterns of scorodite. Also, the amount of precipitate is maximized for Ass + concentrations greater than 20 g/L, and consequently, all additional syntheses were done using an As5+ concentration of 25 g/L. Figure 2 illustrates the effect of the sulphate concentration of the 0.3 M Fe (NO3)3-25 g/L As5+ synthesis medium on the amount and composition of the precipitates formed at 160 °C and at the natural pH of the solution. As the sulphate concentration of the solution increases, the sulphate content of the product increases to ~ 1% SO 2- at 0.5 M Li2SO4 and thereafter levels off. The iron and arsenate contents do not vary significantly although there is consid-
380
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erable scatter of the data for arsenate. The product-yield curve indicates that all the syntheses yielded ~ 65 g of precipitate. All the precipitates possessed the light greenish-yellow colour of scorodite, and Guinier X-ray diffraction showed all to be well-crystallized scorodite. Consequently, it seems that ~ 1% SO~- can be structurally incorporated in scorodite. The precipitate made from solutions containing 2.0 M Li2S04 contained 1.2 wt% SO~-, and this composition yields the formula: Feo.92 [ ( AsO4 ) 0.97 ( SO4 ) o.oa ] 1.oo" ( H 2 0 ) 1.29. In addition to a ~ 3 mol% substitution of SO~- for AsO a- , the formula indicates a deficiency of Fe 3 + that would be necessary to maintain charge neutrality as a consequence of the sulphate substitution. The above experiments were repeated under similar conditions but using an As 5+ concentration of 10 g/L to ascertain the effect of sulphate in more dilute arsenic solutions typical of some metallurgical processes. As noted above ( Fig. 1 ), low As s+ concentrations result in contaminated scorodite, and all the precipitates prepared from 10 g/L As 5+ solutions either were red or contained Xray-detectable impurity phases. Nevertheless, increasing Li2SO4 concentrations in the synthesis solutions resulted in significant sulphate contents of the products. The sulphate content increased in a nearly linear manner to ~ 11% SO42- at 0.3 M Li2SO4 and then declined gradually with further increases in the Li2SO 4 concentration. Although these results are industrially interesting in that they suggest high sulphate in some commercial precipitates, they are
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Fig. 2. Influenceof the concentrationof Li2S04 on the amountand compositionof the scorodite precipitatedat 160°C and pH = 0.73. of little structural significance as much of the sulphate in these products likely is present in the associated impurity phases. The effect of the initial solution pH on the amount and composition of the scorodite precipitate formed at 160°C from 0.3 M Fe (NO3)3-25 g/L As 5+ solutions at the natural pH of 0.73 is shown in Fig. 3. These synthesis solutions also contained 0.3 M Li2S04, and this allowed the effect of pH on the incorporation of sulphate in scorodite to be determined. There is an indication that the amount of product formed increases slightly with increasing pH, as would be expected from pH-solubility considerations, but the effect is very small over the pH range 0.2 to 1.8. Both the arsenate and iron contents are independent ofpH, and both are near the theoretical values for scorodite (Fe, 24.20%; AsO4, 60.2% ). The low sulphate contents are consistent with the 0.3 M Li~SO4 concentration of the solution (Fig. 2); furthermore, the sulphate content seems to decrease with increasing solution pH. The above experiments suggest the following preferred conditions for the synthesis of scorodite: 1 L of 0.3 M Fe (NO3) 3 solution containing 25 g/L As 5+ at the natural pH of ~ 0.73 is heated to 160 ° C for 24 h with good stirring. Each synthesis yields ~ 70 g of crystalline scorodite having the orthorhombic cell parameters: a, 1.033 nm; b, 1.002 nm; c, 0.894 nm. These cell parameters are nearly identical to those obtained recently by Kitahama et al. [ 13 ] for natural scorodite: a, 1.0325 nm; b, 1.0038 nm; c, 0.8953 nm. The synthetic product has
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Fig. 3. Effect of initial solution pH on the amount and compositionof the ferric arsenate precipitates made at 160°C in the presence of 0.3 M Li2S04. the composition FeAsO4-2H20 and is free of sulphate, chloride, nitrate, etc. Solubility studies carried out by Inco Limited [ 11 ] showed that this synthetic scorodite behaved very much as the mineral. By contrast, precipitates made at ~ 100 °C gave significantly higher arsenic solubilities, and this is likely due to the presence of some amorphous iron arsenate which has a higher solubility [1,11].
Thermal decomposition of scorodite Figure 4 is a weight loss trace of the synthetic scorodite heated at 5 ° C/min in flowing N2. A single, low temperature (100-200 °C ) weight loss is observed that amounts to 15.5% of the original mass. The theoretical weight change associated with the loss of the two H20 molecules of FeAs04" 2H20 is 15.60%. The close agreement between theoretical and experimental values indicates that both water molecules are lost simultaneously and at low temperature. This observation is consistent with the structure of scorodite t h a t shows both water molecules to fill wide channels in the scorodite structure [ 13 ]. The water molecules are held only by relatively weak hydrogen bonds, and are, as a result, evolved at relatively low temperatures. X-ray diffraction examination of the decomposition products formed at 275 °C and at 400 °C indicated a mixture of
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two FeAsO4 polymorphs of unknown symmetry, one of which is known to form at moderate temperatures, and the other at high temperatures [14,15]. Material heated to 750°C showed that only the higher temperature phase was present. CONCLUSIONS
Scorodite, FeAsO4"2H20, is readily synthesized by reacting 1 L of 0.3 M Fe(NO3)a-25 g/L As 5+ solution at pH ~0.7 for 24 h at temperatures above 125 ° C, and ideally at 160 ° C. The product has both the scorodite composition and crystal structure. Although solution pH values in the range 0.2 to 1.8 have little effect on the synthesis of scorodite, As 5+ concentrations greater than 15 g/L are required to ensure a pure product. About 1% SO 2- can be structurally incorporated in scorodite, and the extent of sulphate incorporation seems to decrease with increasing solution pH. Thermal decomposition of scorodite proceeds by the simultaneous loss of both water molecules at 100-200 ° C; the final product of the thermal decomposition reaction is FeAsO4. ACKNOWLEDGEMENTS The assistance of O. Dinardo with the synthesis experiments and P. Carriere with the X-ray diffraction investigation is recognized.
384 REFERENCES 1 Robins, R.G., The aqueous chemistry of arsenic in relation to hydrometallurgical processes, in: Oliver, A.J. (Ed.), Impurity Control & Disposal, CIM, Montreal, 1985, pap. 1. 2 Robins, R.G., The solubility of barium arsenate: Sherritt's barium arsenate process, Metal. Trans., 16B {1985) 404-406. 3 Nishimura, T., Ito, C.T., Tozawa, K. and Robins, R.G., The calcium-arsenic-water-air system, in: Oliver, A.J. (Ed.), Impurity Control & Disposal, CIM, Montreal, 1985, pap. 2. 4 Darken, L.S. and Gurry, R.W., Physical Chemistry of Metals, McGraw-Hill, New York, NY, 1953, pp. 326-341. 5 Dutrizac, J.E. and Jambor, J.L., The behaviour of arsenic during jarosite precipitation: Arsenic precipitation at 97 ° C from sulphate or chloride media. Can. Metall. Q., (1987) in press. 6 Dutrizac, J.E. and Jambor, J.L., The behaviour of arsenic during jarosite precipitation: Reactions at 150°C and the mechanism of arsenic precipitation, Can. Metall. Q., (1987) in press. 7 Chukhlantsev, V.G., The solubility products of metal arsenates, Zh. Anal. Khim., 11 (1956) 529-535. 8 Nishimura, T. and Tozawa, K., On the solubility products of ferric, calcium and magnesium arsenates, Bull. Inst. Miner. Dress. Met., Tohoku University, Sendai, Japan, 34 (1977) 202-206. 9 Makhmetov, M.Z., Sagadieva, A.K. and Chuprakov, V.I., Studies of solubility of iron arsenate, Zh. Prikl. Khim., 54(5) (1981) 1009-1011. 10 Dove, P.M. and Rimstidt, J.D., The solubility and stability of scorodite, FeAsO4"2H~O, Amer. Mineral., 70 (1985) 838-844. 11 Krause, E. and Ettel, V.A., Solubilities and stabilities of ferric arsenates, Proc. Int. Symp. Crystallization and Precipitation, Saskatoon, CIM, Montreal, 1987. 12 Dutrizac, J.E. and Jambor, J.L., Behaviour of cesium and lithium during the precipitation ofjarosite-type compounds, Hydrometallurgy, 17 (1987) 251-265. 13 Kitahama, K., Kiriyama, R. and Baba, Y., Refinement of the crystal structure of scorodite, Acta Cryst., B31 (1975) 322-324. 14 d'Yvoire, F. and Ronis,M., Preparation and thermal decomposition of stoichiometric scorodite, FeAsO4"2H20 and non-stoichiometric, Fe~_xH:~xAsO4.2HeO, C.R. Acad. Sci. Paris, 267 {1968) C827-C830. 15 d'Yvoire, F. and Ronis, M., On the polymorphism of FeAs04, C.R. Acad. Sci. Paris, 267 (1968) C955-C958.
377
The Synthesis of Crystalline Scorodite, FeAs04" 2H20 J.E. DUTRIZAC and J.L. JAMBOR
Mineral Sciences Laboratories, CANMET, Energy, Mines and Resources Canada, 555 Booth Street, Ottawa, Ontario KIA OG1 (Canada) (Received March 13, 1987; accepted in revised form June 26, 1987)
ABSTRACT Dutrizac, J.E. and Jambor, J.L., 1988. The synthesis of crystalline scorodite, FeAs04.2H20. Hydrometallurgy, 19: 377-384. Crystalline scorodite, FeAs04- 2H20, was synthesized by precipitation from 0.3 M Fe (NO~) 3-25 g/L As5+ solutions at pH ~0.7. Temperatures greater than 125°C are needed to ensure good scorodite crystallinity, and a temperature of 160°C was used for the current syntheses. Although initial solution pH values in the 0.2 to 1.8 range have little effect on either the structure or composition of the scorodite, As~+ concentrations greater than 15 g/L are needed to ensure a pure product. About 1% SO~- could be incorporated in the scorodite structure, and the S042- content decreased with increasing solution pH. Upon heating, scorodite decomposes at 100-200°C with the simultaneous release of both of its H20 molecules to form anhydrous FeAs04.
INTRODUCTION
Arsenic is a common impurity in many metallurgical ores and concentrates. During the hydrometallurgical processing of such feeds, significant amounts of arsenic pass into solution, often in the pentavalent (arsenate) form. The concentration of arsenic must be closely controlled in the processing solutions to avoid product contamination and/or the pollution of nearby water systems. For practical reasons, arsenate often is precipitated as ferric arsenate [ 1 ] although the precipitation of other compounds such as barium arsenate [ 2 ] or calcium arsenate [3] has been used. Because the industrial precipitates usually are discarded in storage ponds, the solubilities of the iron arsenate compounds are of considerable importance as they affect directly the quality of the water overflowing the pond into the local water system. The solubility of arsenate depends on the compound precipitated, its composition, and, especially, its crystallinity. It is well established that amorphous or metastable phases have higher solubilities than the thermodynamically sta0304-386X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
378 ble species [ 4 ]. During the course of a recent study on the behaviour of phosphate during the precipitation ofjarosite-type compounds, it became apparent that phosphate precipitated in a variety of mineral forms depending on the precipitation conditions and the cations present. In contrast, the corresponding arsenate systems invariably yielded only scorodite, FeAsO4" 2H20 [ 5,6 ]. The precipitation experiments suggested, however, that the crystallinity of the scorodite varied greatly with the experimental conditions, and that sulphate substituted for arsenate to a limited extent. Scorodite has been synthesized by a number of workers, but all syntheses seem to have been carried out at temperatures below 105 ° C. Syntheses by Chukhlantsev [ 7], Nishimura and Tozawa [8], and Makhmetov et al. [9] were carried out at low temperatures. Although the precipitates had the composition of scorodite, they were completely amorphous. Later syntheses carried out at ~100°C by Robins [1] and Dove and Rimstidt [10] yielded precipitates which had the scorodite composition and gave a scorodite powder diffraction pattern. Based on the behaviour of arsenate during iron precipitation as jarosite [ 5,6 ] and on the absolute intensities of the powder diffraction patterns of precipitates made at ~ 100 ° C, however, it is thought that such precipitates contain a significant percentage of amorphous FeAsQ- 2H20 in addition to crystalline scorodite. The presence of such an amorphous phase would result in arsenate solubilities many times higher than those of well-crystallized scorodite [ 11 ]. Because arsenic stability in residue ponds is a subject of current metallurgical concern and because of the need for well-crystallized scorodite for such investigations, experiments were carried out to define the conditions for the precipitation of well-crystallized scorodite and to identify some of the properties of this material. EXPERIMENTAL Reagent-grade chemicals were used for all the syntheses. Scorodite usually was synthesized from 0.3 M Fe ( NO~ ) 3 solutions containing 25 g/L As 5+ at the natural pH of ~ 0.7. The arsenic was added to the solution as As205 to avoid the coincidental introduction ofjarosite-forming alkalis. Experiments were also done in the present of Li2S04; the pH was raised for test purposes with Li2CO3. Lithium salts were used for this purpose, as lithium does not form an endmember jarosite-type compound [12], and because lithium arsenates do not precipitate under the conditions used. The precipitates were formed by dissolution of the appropriate reagents in water, adjusting the pH if necessary, and then heating to the appropriate temperature. The preferred temperature for scorodite synthesis was found to be 160 ° C. Twenty-four hour reaction periods were used for all the experiments, and preliminary tests showed that such times yielded near-equilibrium conditions. The experiments were done by heating 1 L of solution in a Parr 2-L autoclave with a glass liner and titanium internals;
379 the experiments were done in a sealed reactor under the solution pressure. At the end of the reaction period, the autoclave was cooled quickly ( < 10 min), and the slurries were filtered immediately. The precipitates were washed well in ~ 4 L of hot water and subsequently were dried at 110 ° C before analysis. All the precipitates were analyzed chemically, and all were examined by Guinier-deWolff X-ray diffraction patterns to confirm the presence of scorodite and to identify possible impurity phases. Guinier precision focussing cameras are particularly sensitive to impurity phases, and ~ 0.25 wt% of an impurity usually can be detected by this technique. Additional X-ray powder patterns of the precipitates were obtained using a Debye-Scherrer camera of 114.6 mm diameter, CoKal radiation (~ = 0.17889 nm ), and MgO as an internal standard. RESULTS AND DISCUSSION Preliminary efforts to synthesize scorodite from 0.3 M Fe (SO4) 1.~ solutions containing 5-10 g/L As ~+ at pH = 1.3 and 97 ° C resulted in amorphous precipitates containing several percent sulphate [ 5 ]. Further work [ 6 ] showed that crystalline scorodite was formed above 125 ° C, and subsequent studies therefore were carried out at a considerably higher temperature, 160 ° C, to ensure both rapid rates and well-crystallized products. Also, preliminary testwork showed that ferric nitrate solutions resulted in crystallized scorodite free of sulphate contamination, and subsequent experiments generally were done using the nitrate salt. Figure 1 illustrates the effect of the As 5+ concentration of the solution on the yield and composition of the precipitates formed from 0.3 M Fe (NO3)3 solutions at the natural pH of ~ 0.7 and heated to 160 ° C for 24 h. At low Ass+ concentrations (As s+ < 10 g/L), the products are deficient in arsenate but enriched in iron. Such products were commonly red, despite the fact that only scorodite was detected by Guinier X-ray diffraction. The colour presumably is due to an amorphous iron oxide or hydroxide compound. For Ass + concentrations greater than 20 g/L, light greenish-yellow precipitates having the approximate composition of scorodite are formed; i.e., Fe ~ 24% and AsO4 ~ 62 %; scorodite contains 24.20% Fe, 60.20% AsO4 and 15.60% H20. All these precipitates yielded sharp powder diffraction patterns of scorodite. Also, the amount of precipitate is maximized for Ass + concentrations greater than 20 g/L, and consequently, all additional syntheses were done using an As5+ concentration of 25 g/L. Figure 2 illustrates the effect of the sulphate concentration of the 0.3 M Fe (NO3)3-25 g/L As5+ synthesis medium on the amount and composition of the precipitates formed at 160 °C and at the natural pH of the solution. As the sulphate concentration of the solution increases, the sulphate content of the product increases to ~ 1% SO 2- at 0.5 M Li2SO4 and thereafter levels off. The iron and arsenate contents do not vary significantly although there is consid-
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erable scatter of the data for arsenate. The product-yield curve indicates that all the syntheses yielded ~ 65 g of precipitate. All the precipitates possessed the light greenish-yellow colour of scorodite, and Guinier X-ray diffraction showed all to be well-crystallized scorodite. Consequently, it seems that ~ 1% SO~- can be structurally incorporated in scorodite. The precipitate made from solutions containing 2.0 M Li2S04 contained 1.2 wt% SO~-, and this composition yields the formula: Feo.92 [ ( AsO4 ) 0.97 ( SO4 ) o.oa ] 1.oo" ( H 2 0 ) 1.29. In addition to a ~ 3 mol% substitution of SO~- for AsO a- , the formula indicates a deficiency of Fe 3 + that would be necessary to maintain charge neutrality as a consequence of the sulphate substitution. The above experiments were repeated under similar conditions but using an As 5+ concentration of 10 g/L to ascertain the effect of sulphate in more dilute arsenic solutions typical of some metallurgical processes. As noted above ( Fig. 1 ), low As s+ concentrations result in contaminated scorodite, and all the precipitates prepared from 10 g/L As 5+ solutions either were red or contained Xray-detectable impurity phases. Nevertheless, increasing Li2SO4 concentrations in the synthesis solutions resulted in significant sulphate contents of the products. The sulphate content increased in a nearly linear manner to ~ 11% SO42- at 0.3 M Li2SO4 and then declined gradually with further increases in the Li2SO 4 concentration. Although these results are industrially interesting in that they suggest high sulphate in some commercial precipitates, they are
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Fig. 2. Influenceof the concentrationof Li2S04 on the amountand compositionof the scorodite precipitatedat 160°C and pH = 0.73. of little structural significance as much of the sulphate in these products likely is present in the associated impurity phases. The effect of the initial solution pH on the amount and composition of the scorodite precipitate formed at 160°C from 0.3 M Fe (NO3)3-25 g/L As 5+ solutions at the natural pH of 0.73 is shown in Fig. 3. These synthesis solutions also contained 0.3 M Li2S04, and this allowed the effect of pH on the incorporation of sulphate in scorodite to be determined. There is an indication that the amount of product formed increases slightly with increasing pH, as would be expected from pH-solubility considerations, but the effect is very small over the pH range 0.2 to 1.8. Both the arsenate and iron contents are independent ofpH, and both are near the theoretical values for scorodite (Fe, 24.20%; AsO4, 60.2% ). The low sulphate contents are consistent with the 0.3 M Li~SO4 concentration of the solution (Fig. 2); furthermore, the sulphate content seems to decrease with increasing solution pH. The above experiments suggest the following preferred conditions for the synthesis of scorodite: 1 L of 0.3 M Fe (NO3) 3 solution containing 25 g/L As 5+ at the natural pH of ~ 0.73 is heated to 160 ° C for 24 h with good stirring. Each synthesis yields ~ 70 g of crystalline scorodite having the orthorhombic cell parameters: a, 1.033 nm; b, 1.002 nm; c, 0.894 nm. These cell parameters are nearly identical to those obtained recently by Kitahama et al. [ 13 ] for natural scorodite: a, 1.0325 nm; b, 1.0038 nm; c, 0.8953 nm. The synthetic product has
382 7O
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60
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I
I
I
0 . 3 M Fe( N O 3 ) s . 2 5 9 / L AsS*,O.3 M Li=S04 160%
- - --qlt~ 0
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I
--150
--120
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PRODUCT YIELD
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80
-- 70
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0.4
0,8
1.2
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i
1.6
2.0
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Fig. 3. Effect of initial solution pH on the amount and compositionof the ferric arsenate precipitates made at 160°C in the presence of 0.3 M Li2S04. the composition FeAsO4-2H20 and is free of sulphate, chloride, nitrate, etc. Solubility studies carried out by Inco Limited [ 11 ] showed that this synthetic scorodite behaved very much as the mineral. By contrast, precipitates made at ~ 100 °C gave significantly higher arsenic solubilities, and this is likely due to the presence of some amorphous iron arsenate which has a higher solubility [1,11].
Thermal decomposition of scorodite Figure 4 is a weight loss trace of the synthetic scorodite heated at 5 ° C/min in flowing N2. A single, low temperature (100-200 °C ) weight loss is observed that amounts to 15.5% of the original mass. The theoretical weight change associated with the loss of the two H20 molecules of FeAs04" 2H20 is 15.60%. The close agreement between theoretical and experimental values indicates that both water molecules are lost simultaneously and at low temperature. This observation is consistent with the structure of scorodite t h a t shows both water molecules to fill wide channels in the scorodite structure [ 13 ]. The water molecules are held only by relatively weak hydrogen bonds, and are, as a result, evolved at relatively low temperatures. X-ray diffraction examination of the decomposition products formed at 275 °C and at 400 °C indicated a mixture of
383 104
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96-z
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80--
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I
50
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L
100 ISO TEMPERATURE (°C)
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Fig. 4. T h e r m a l decomposition curve of synthetic scorodite.
two FeAsO4 polymorphs of unknown symmetry, one of which is known to form at moderate temperatures, and the other at high temperatures [14,15]. Material heated to 750°C showed that only the higher temperature phase was present. CONCLUSIONS
Scorodite, FeAsO4"2H20, is readily synthesized by reacting 1 L of 0.3 M Fe(NO3)a-25 g/L As 5+ solution at pH ~0.7 for 24 h at temperatures above 125 ° C, and ideally at 160 ° C. The product has both the scorodite composition and crystal structure. Although solution pH values in the range 0.2 to 1.8 have little effect on the synthesis of scorodite, As 5+ concentrations greater than 15 g/L are required to ensure a pure product. About 1% SO 2- can be structurally incorporated in scorodite, and the extent of sulphate incorporation seems to decrease with increasing solution pH. Thermal decomposition of scorodite proceeds by the simultaneous loss of both water molecules at 100-200 ° C; the final product of the thermal decomposition reaction is FeAsO4. ACKNOWLEDGEMENTS The assistance of O. Dinardo with the synthesis experiments and P. Carriere with the X-ray diffraction investigation is recognized.
384 REFERENCES 1 Robins, R.G., The aqueous chemistry of arsenic in relation to hydrometallurgical processes, in: Oliver, A.J. (Ed.), Impurity Control & Disposal, CIM, Montreal, 1985, pap. 1. 2 Robins, R.G., The solubility of barium arsenate: Sherritt's barium arsenate process, Metal. Trans., 16B {1985) 404-406. 3 Nishimura, T., Ito, C.T., Tozawa, K. and Robins, R.G., The calcium-arsenic-water-air system, in: Oliver, A.J. (Ed.), Impurity Control & Disposal, CIM, Montreal, 1985, pap. 2. 4 Darken, L.S. and Gurry, R.W., Physical Chemistry of Metals, McGraw-Hill, New York, NY, 1953, pp. 326-341. 5 Dutrizac, J.E. and Jambor, J.L., The behaviour of arsenic during jarosite precipitation: Arsenic precipitation at 97 ° C from sulphate or chloride media. Can. Metall. Q., (1987) in press. 6 Dutrizac, J.E. and Jambor, J.L., The behaviour of arsenic during jarosite precipitation: Reactions at 150°C and the mechanism of arsenic precipitation, Can. Metall. Q., (1987) in press. 7 Chukhlantsev, V.G., The solubility products of metal arsenates, Zh. Anal. Khim., 11 (1956) 529-535. 8 Nishimura, T. and Tozawa, K., On the solubility products of ferric, calcium and magnesium arsenates, Bull. Inst. Miner. Dress. Met., Tohoku University, Sendai, Japan, 34 (1977) 202-206. 9 Makhmetov, M.Z., Sagadieva, A.K. and Chuprakov, V.I., Studies of solubility of iron arsenate, Zh. Prikl. Khim., 54(5) (1981) 1009-1011. 10 Dove, P.M. and Rimstidt, J.D., The solubility and stability of scorodite, FeAsO4"2H~O, Amer. Mineral., 70 (1985) 838-844. 11 Krause, E. and Ettel, V.A., Solubilities and stabilities of ferric arsenates, Proc. Int. Symp. Crystallization and Precipitation, Saskatoon, CIM, Montreal, 1987. 12 Dutrizac, J.E. and Jambor, J.L., Behaviour of cesium and lithium during the precipitation ofjarosite-type compounds, Hydrometallurgy, 17 (1987) 251-265. 13 Kitahama, K., Kiriyama, R. and Baba, Y., Refinement of the crystal structure of scorodite, Acta Cryst., B31 (1975) 322-324. 14 d'Yvoire, F. and Ronis,M., Preparation and thermal decomposition of stoichiometric scorodite, FeAsO4"2H20 and non-stoichiometric, Fe~_xH:~xAsO4.2HeO, C.R. Acad. Sci. Paris, 267 {1968) C827-C830. 15 d'Yvoire, F. and Ronis, M., On the polymorphism of FeAs04, C.R. Acad. Sci. Paris, 267 (1968) C955-C958.