Crystallization of Hydrated Ferric Arsenate. Process Design Using

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Ind. Eng. Chem. Res. 2009, 48, 10522–10531

Crystallization of Hydrated Ferric Arsenate. Process Design Using METSIM Pıá C. Hernańdez, Marıá E. Taboada,* Teo´filo A. Graber, and Hećtor R. Galleguillos Department of Chemical Engineering, Laboratory of Process, UniVersidad de Antofagasta and CICITEM, Angamos 601, Antofagasta, Chile

An industrial process for extracting arsenic from solutions through scorodite crystallization at 70 °C was designed and simulated using METSIM software. Using equilibrium information from the phase diagram of the system As(V)/Fe(III)/H2O at 50 and 70 °C, experimental tests were carried out of crystallization of scorodite at 50, 70, and 95 °C, as well as crystallization at 170 °C in an autoclave. This information provides the preliminary operational conditions for the process. The tests show that temperature influences the quality of crystals and the velocity of scorodite crystallization (residence time between 3 h and 3 days). The simulated process allows for the analysis of different evaporation ratios (42.5, 71.97, and 86.7%), determining the material and energy balances and the quality of the products and residues, thus providing important information for potential industrial applications. The energy requirements for each evaporation ratio are 21 568, 33 747, and 39 836 Mcal/h, and the sums of the total flows are 11 588.60, 8876.28, and 7520.11 tons/day, respectively. This last variable is related to the size of the equipment used in the process. Molar Fe/As ratios of 1, 0.9, and 0.8 result in yields of 93, 88, and 78%. Introduction A common problem for copper-producing countries, such as Chile, Canada, and Australia, is arsenic contained in sulfide ores. These ores are readily concentrated by froth flotation and are treated in a variety of ways. Whichever method of treatment is used, arsenical waste remains that necessitates stabilization and storage over the long term. In this process, crystallization of arsenic salts in stable solids is important, in particular hydrated ferric arsenate production. Arsenic is a toxic and carcinogenic element and one of the main contaminants of industrial processes, particularly in copper concentrates. Its presence and compulsory treatment result in increased production costs, interfere with extraction, reduce the purity of final products, and present environmental and disposal problems. A number of published works suggest that the solution to this problem may lie in stabilization of insoluble salts and subsequent disposal. Arsenic in the pentavalent state can be removed from a solution by ferric arsenate precipitation. Scorodite (FeAsO4 · 2H2O) is a common hydrated iron arsenate mineral that has been proposed as a solution for depositing arsenic as a stable compound, mainly because it shows low solubility in water and mild acid solutions. Ferric arsenate can be easily produced by hydrothermal precipitation at temperatures above 150 °C. However, hydrothermal precipitation of scorodite requires the use of autoclaves. Ugarte1 has shown that the Fe(III)-As(V) system has several phases. At temperatures below 90 °C, the phases tend to be amorphous because the arsenic ions are absorbed in a ferric hydroxide gel. With an initial molar ratio of Fe/As g 1.5 and at temperatures between 150 and 200 °C, crystalline hydrated ferric arsenate forms, identical to mineral scorodite, verified by TCLP (Toxicity Characteristic Leaching Procedure) < 5 mg/L arsenic. At temperatures higher than 200 °C, nonhydrated ferric arsenic hydroxide sulfate predominates (Fe3(AsO4)2(SO4)(OH)), a phase called type two, with a TCLP of 5 mg/L arsenic as well. Demopoulos et al.2 obtained scorodite at standard pressure and temperatures ranging from 95 to 80 °C from concentrated chloride solutions applying a controlled oversaturation procedure. Without seed, crystalline scorodite was obtained at 95 °C via heterogeneous nucleation in the reactor walls. In addition, scorodite was obtained from sulfate-type effluent at standard pressure and at 95 °C applying the same procedure of controlled oversaturation.3 Droppert et al.4 has shown that higher yields and shorter residence times can be achieved using Mg(OH)2 instead of NaOH as an Arrhenius base and using as much seed as possible (50 g/L). Filippou and Demopoulos5 have determined that the precipitation of arsenate at standard pressure is affected by direct neutralization for a terminal pH >4, which is produced by an acute increase in the oversaturation of iron and arsenic in the solution and a high velocity of nucleation, forming amorphous iron-arsenic compounds that are not stable enough for crystallization. Control of oversaturation through a neutralization technique can avoid the precipitation of amorphous arsenical compounds. In this case, the precipitation of crystalline scorodite is induced by seeding scorodite over a rich arsenic solution. Singhania et al.6 studied the seeding effect on scorodite synthesis at standard pressure using crystals produced hydrothermally in an autoclave at 160 °C and adding them at 95 °C in solutions of As(V)-Fe(III)-H2SO4 at temperatures of 85-100 °C. The effect of seeding with hematite seeds (Fe2O3) and gypsum (CaSO4 · 2H2O) was also studied. It has clearly been shown that the precipitation of crystalline scorodite at atmospheric pressure is a viable process. The scorodite produced by different seeding is environmentally stable (according to the TCLP test). The same authors have stated that fine hydrothermal scorodite is the best material to be used as seed for precipitation of crystalline scorodite under atmospheric pressure conditions. Dutrizac and Jambor7 synthesized crystalline scorodite with ferric nitrate at pH 0.7 and found that a temperature above 125

10.1021/ie900639e CCC: $40.75  2009 American Chemical Society Published on Web 10/07/2009

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009

°C was required (ideally 160 °C) to ensure good crystallinity. Scorodite is soluble in water in the arsenic range of 0.1-3.6 mg/L.6,8,9 In a more recent work, Dutrizac and Jambor10 carried out scorodite crystallization using an autoclave in a temperature range of 150-175 °C by mixing Fe(SO4)1.5, H2SO4, and As2O5, showing the feasibility of producing scorodite at temperatures above 150 °C. Swash and Monhemius11 found that amorphous ferric arsenates have higher solubility than crystalline scorodite, by approximately 2 orders of magnitude. They also observed that ferric arsenates begin to precipitate at pH values between 1 and 2, while the formation of calcium arsenates begins at pH between 3 and 4. Le Berre et al.12 studied the transformation of ferric arsenate of poor crystallinity and crystalline scorodite, working at different temperatures (40, 60, 80 °C). As a result, they found that the length of induction is totally dependent on temperature and pH. While pH remains at 2, the induction period is around 9 h at 80 °C, increases to 40 h at 60 °C, and increases to 240 h at 40 °C. On the other hand, if temperature is maintained at 80 °C and pH is increased from 2 to 4, the induction period increases by almost 10 times. Besides, a color change of the precipitate is verified as a function of time, indicating residence time dependence. Diffractograms of each test were compared to the crystalline scorodite pattern, finding the characteristic peaks for the first time at 10 h, thus proving that the longer the residence time, the higher the crystallinity. Fujita et al.13 studied the effects of reaction temperature (95, 70, 50 °C) and the oxidizing agent (air, pure oxygen gas) to optimize scorodite formation in a practical process. Their results indicate that controlled synthesis at 70 °C achieved sufficiently stable scorodite particles and that air oxidation produced scorodite, even at 50 °C. The final concentration of arsenic at 70 °C was same as that at 95 °C. A reaction temperature of 50 °C was not sufficient. The results indicated that very fine crystals formed at 50 °C agglomerate to produce larger particles, while at higher temperatures, the smaller particles are more crystalline and discrete. The present study has the objective of designing a process for extraction of arsenic from a residual liquid with concentrations to industrial levels. The arsenic was extracted through crystallization of ferric arsenate (scorodite, FeAsO4 · 2H2O) at temperatures of less than 100 °C. The methodology employed was to use experimental studies of the crystallization, reaction kinetics, and phase equilibrium of the system As-Fe-H2O at (50 and 70) °C,14 which provided the operational conditions of the process. The effect of temperature in scorodite crystallization and the stability of crystals as residue (TCLP test15) were studied. The crystals obtained were chemically analyzed. With the aforementioned information, a process was simulated using METSIM software that allowed for reducing the number of experiments in the laboratory and projecting experimental results to an industrial process. METSIM16 software computes mass and energy balances in a steady or dynamic state, including process control and chemical reactions, along with a great number of unitary operations. Algorithms can be incorporated using APL, a concise computer programming language. A fictitious feeding solution was used for designing the process. The point that represents the feeding solution is located in the phase diagram and determines the necessary processes to enter the crystallization field and obtain scorodite.

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To simulate the process, three cases were evaluated with different flows. Yields, equipment size, and required energy were studied. As well, the conversion variables of the reaction for forming scorodite and the Fe/As molar ratio in crystallization were studied. Experimental Procedure Materials. The analytical grade reagents used were the following: arsenic pentoxide hydrate (As2O5 · 3H2O), Sigma Aldrich, 98%; sodium hydroxide (NaOH), Merck, 99%; ferric sulfate pentahydrate (Fe2(SO4)3 · 5H2O), Riedel-De Hae¨n, 21-23% Fe; and ultrapure water (conductivity e0.05 µS/cm) obtained by passing distilled water through a Millipore ultrapure device. Ferric hydroxide was prepared in the laboratory just before each experiment by the reaction of NaOH and Fe2(SO4)3 · 5H2O. This was done because active Fe(OH)3 is required and the commercial variety does not meet this requirement. Equipment and Experimental Procedure. Crystallizations were carried out in a 250 cm3 Pyrex crystallizer with a heating jacket. The crystallizer included a thermoregulated Lauda RM-B bath, with a 6-L capacity and temperature control from -20 to 100 °C ( 0.1 °C. For scorodite crystallization, Fe(OH)3(s), As2O5 · 3H2O(s), and water were mixed. The reaction temperatures were 50, 70, and 95 °C. These temperatures were chosen based on equilibrium phase diagrams reported in a previous study.14 As well, it was desired to obtain scorodite crystallization at temperatures of less than 100 °C. At 95 °C, the phase diagram is assumed to be similar to that obtained at 70 °C. The reagent concentrations for scorodite crystallization were determined according to the proportions given by the phase diagrams14 in the crystallization zone. At temperatures of 50, 70, and 95 °C, an initial concentration of 9.7% As and 7% Fe was used, in the line of the maximum yield of the scorodite crystallization field (point 1, Figure 4). The solutions reacted in the crystallizer under well-controlled heating while being magnetically agitated (300-400 rpm). The residence time began once the temperature was reached and the reactives were mixed until precipitating scorodite was achieved. To stop the reaction, the suspensions were vacuum filtered, the pH of the liquids was measured, and the solid was washed with distilled water and dried at room temperature until reaching a constant weight. The solids and liquids obtained were analyzed by atomic absorption spectrometry (AA), using a Varian Model SpectrAA 220 instrument. As well, the solids were characterized by X-ray diffraction using a Siemens D5000 X-ray diffractometer and were submitted to a TCLP test and size distribution analysis. The particle size distribution was determined using an automatic analyzer of centrifugal particle size distribution, HORIBA CAPA-300. The TLCP test was carried out on the crystals in order to determine the stability of salts and to evaluate if they were potentially interesting as alternative waste materials in industrial processes. The TCLP test established standards in terms of concentration limits. This procedure consisted of leaching a solid sample for 18 h in a weak solution of acetic acid in a liquid/ solid proportion of 20:1, with agitation of 30 rpm and a temperature of 22 °C. Once the test was completed, the filtered solution was analyzed, considering a stable solid when arsenic did not exceed 5 mg/L. The solutions obtained from the TCLP test were characterized by atomic absorption spectrometry. (Note: For very low arsenic concentration, the generation of hydrides is necessary.) With the objective of knowing the effect of temperature on product quality, crystallization tests were carried out at 50, 70,

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Table 1. Results of Scorodite Crystallization at Different Temperatures % w/w

temp (°C)

residence time

As

Fe

TCLP lixiviation of As (mg/L)

average size of particle (µm)

50 70 95 170

3 days 5h 3h 5h

31.05 32.23 32.56 33.13

21.77 23.02 22.90 24.21

400 100 100 21

10.0 21.0 18.1 9.8

and 95 °C and crystallization occurred in an autoclave, using the same procedure at 170 °C and the pressure at 130 psi. For 170 °C, an initial concentration of 8% As and 6% Fe was used. The autoclave was from Parr Instrument Co., Model Bench Top Reactors 4520, made of titanium, with 1-L capacity and temperature control from 0 to 800 °C. Results and Discussion Scorodite Crystallization Study. All the experiments were conducted with a molar ratio of Fe/As e 1 and an agitation velocity of 300-400 rpm. Slower agitations permit the crystals to settle and faster agitations cause crystal damage. The experimental results are shown in Table 1. Table 1 summarizes the results of the tests for obtaining scorodite for all working temperatures. The final pH of the solution obtained after the crystallization was 1 for all the experiments carried out. Effect of Temperature and TCLP. The system at atmospheric pressure was studied at three different temperatures: 50, 70, and 95 °C. According to the experimental results, temperature and residence time present a proportionally inverse relationship, as can be seen in Table 1. Our results are in a good agreement with those of Le Berre12 and Singhania.6 The coloring of the crystals also varied with temperature. At 50 and

70 °C, the crystals have an ivory color, and at 95 °C they are beige-green. Temperature also plays a very important role on scorodite stability against the TCLP test. As Table 1 shows, the higher the temperature, the more stable the scorodite that is obtained. No major difference in the solubility of the crystal is noted at 70 and 95 °C. Scorodite Produced in an Autoclave and TCLP. The crystals obtained in this experiment have a pale green color, rather different from the crystals synthesized at atmospheric pressure. These crystals are also more stable according to the TCLP test ([As] ) 21 mg/L), compared to the experiments carried out at 95 and 70 °C ([As] ) 100 mg/L; see Table 1). The crystals produced in the autoclave are the most stable, and will therefore be used as seed in future works. It is worth noting that the TLCP test for this solid is well over the permitted limit of arsenic solubility (5 mg/L), perhaps because the formation of Fe2(HAsO4)3 · xH2O, called type one phase by Ugarte,1 forms at molar ratio Fe/As ) 1 and temperatures above 150 °C . Particle Size. The average size of crystals was determined for different temperatures as shown in Table 1. No clear effect of crystallization temperature was observed on the average size because crystallizations were reached without control of the degree of oversaturation. Consequently, the sizes obtained were small and susceptible to agglomeration. The results show that the crystals produced in the autoclave are the smallest, likely due to either mechanical agitation or less agglomeration of crystals because of temperature. Nevertheless, this size is appropriate for use as seed.6 Diffractogram Analysis. Figure 1 shows diffractograms of the solid residues experimentally obtained. The black line represents the scorodite synthesized in the experiment. The red

Figure 1. Diffractograms of the scorodite synthesized at (a) 50, (b) 70, (c) 95, and (d) 170 °C.


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Figure 2. Preliminary designed flow sheet for scorodite production.

line represents the scorodite pattern from the database. According to the analysis, the crystals produced are scorodite. Model Development Process Design and Scorodite Crystallization Modeling from Acid Solutions Using METSIM Software. The process was based on the stability diagram for the As-Fe-H2O system at 70 °C,14 seeking scorodite crystallization with the lowest solubility possible, which would make disposal in waste dumps feasible. The temperature of 70 °C was chosen for the crystallization process given that in the industrial context it is more manageable and economic to work at temperatures of less than 95 °C. It is considered that the results obtained at 70 °C are comparable to those obtained at 95 °C and better than those obtained at 50 °C. Models were developed using METSIM software, acquiring all the available industrial experience and the experimental information from our research program. The software computed the mass and energy balances, providing valuable information about a potential industrial plant. All of the thermodynamic data used were obtained from the database of the HSC17 and METSIM software, as well as from the literature.18 As well as scorodite, sodium sulfate decahydrate (Na2SO4 · 10H2O) was obtained as a byproduct, which can be marketed for multiple uses (medicinal, decanting, as well as cellulose, glass, and detergent production). The phase diagram considers As, Fe, and H2O. Sodium hydroxide and sodium sulfate are present in the process, as well in a lower proportion in comparison to the other components, which justifies the ternary diagram. The flow sheet of the proposed process is shown in Figure 2. The feed stream is a synthetic solution prepared according to estimated average industrial values (15 g/L As and 217 ppm Fe approximately). Feed (stream 1) enters the evaporator at room temperature and pressure, containing As 0.73% w/w (as H3AsO4(aq)) and Fe

0.01% w/w (as Fe2(SO4)3(aq)), thus producing a stream of concentrated arsenic ranging from 1.25 to 2.5, and 5% w/w (stream 3). Fe2(SO4)3(s) (stream 4) and NaOH(aq) (stream 5) enter mixer 1 in order to produce Fe(OH)3(s), according to the following reaction: Fe2(SO4)3(s) + 6NaOH(aq) f 2Fe(OH)3(s) + 3Na2SO4(aq) (1) Some 20% of NaOH stoichiometric excess (stream 5) was added according to reaction 1. The conversion of reaction 1 was considered complete (experimentally tested). Fe2(SO4)3(s) (stream 4) was added according to the Fe(OH)3(s) requirements in the crystallizer unit. Stream 6, containing Fe(OH)3(s), Na2SO4(aq), and H2O(aq), was filtered. The solid Fe(OH)3(s) was mixed with stream 3 in crystallizer 1, where reaction 2 was carried out with a 93% conversion (laboratory tested): H3AsO4(aq) + Fe(OH)3(s) f FeAsO4 · 2H2O(s) + H2O(aq) (2) It should be noted that, in order to produce scorodite, the concentrations of arsenic and iron must be within the scorodite crystallization zone given by the phase diagram.14 The discharge stream 10 containing solution and solid advanced to the thickener for phase splitting; the clarified solution stream 11 and the solids (stream 12) were washed and dried (stream 22). Stream 22 was sampled to carry out the TCLP test. These test results have been incorporated to the model. According to the experimental results, 0.43% of scorodite produced at 70 °C dissolved in the acid solution of the TCLP test, passing As to the solution. This solubility is added to the model as a conversion value of a dissolution reaction. Stream 11 was mixed with washing stag filtrates (streams 19 and 20) producing stream 28, which still contained a high concentration of arsenic (ranging from 1.96 to 7.89 g/L As).

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the filtrate (stream 15) was split in the separator, sending only 8% to the scorodite washing stage (stream 17). Na2SO4(aq) + 10H2O f Na2SO4 · 10H2O(s)

(4)

The byproduct Na2SO4 · 10H2O was produced using data from Figure 3, constructed by the authors with solubility values obtained from Seidell.19 The dotted lines represent the process simulated using METSIM. Stream 7 entered at 53.4 °C (11% w/w Na2SO4) and cooled to 10 °C to crystallize Na2SO4 · 10H2O (stream 14). The scorodite production process was analyzed with three different evaporation ratios in order to establish the potential trade-offs regarding capital investment (equipment size) and operational cost (mainly energy costs). The simulation for the three cases was with a Fe/As molar ratio equal to 1. Case 1 had an evaporation ratio of 42.5%, which implies a discharge (stream 3) at a total arsenic concentration of 1.25% w/w. Case 2 had an evaporation ratio of 71.97%, which implies a discharge (stream 3) at a total arsenic concentration of 2.5% w/w. Case 3 had an evaporation ratio of 86.7%, which implies a discharge (stream 3) at a total arsenic concentration of 5% w/w. Tables 2, 4, and 6 show the mass balance, temperatures, and arsenic concentrations of each stream, for each case respectively. Tables 3, 5, and 7 show the energy balances for each unitary operation under the case 1, 2, and 3 assumptions, providing

Figure 3. Phase diagram of the Na2SO4-H2O system. The dotted lines show the process developed using METSIM.

This stream was mixed with CaCO3(s) in mixer 4 for calcium arsenate precipitation, haidingerite (CaHAsO4 · H2O),14 according to reaction 3. This solid was stored in appropriate containers for residue. The final discharge solution (stream 31) was in the range of 0.11-0.44 g/L As, and therefore needed to be further treated or recycled. H3AsO4(aq) + CaCO3(s) f CaHAsO4 · H2O(s) + CO2(g) (3) On the other hand, stream 7, rich in Na2SO4, was crystallized by cooling in crystallizer 2, according to reaction 4. Crystals of Na2SO4 · 10H2O thus obtained were filtered (stream 14), and Table 2. Mass Balance for the Scorodite Production Process: Case 1

% w/w stream

% solid

T (°C)

flow rate (tons/day)

1 2a 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21a 22 23 24 25 26 27 28 29 30a 31 total flow yield of the process

0.0 0.0 0.0 100.0 0.0 5.1 1 × 10-2 85.0 1.7 3.6 0.00 60.0 6.2 85.0 0.0 0.0 0.0 83.0 0.0 0.0 0.0 97.1 97.1 97.1 0.00 4.6 97.1 0.0 100.0 0.0 0.2

25.0 100.0 100.0 25.0 25.0 53.4 53.4 53.4 99.9 70.0 70.0 70.0 10.0 10.0 10.0 10.0 10.0 31.4 31.4 70.0 60.0 60.0 60.0 60.0 25.0 22.0 59.9 69.1 25.0 64.3 64.3

1867.20 782.27 1084.93 35.51 336.09 371.60 349.29 22.31 1107.24 1107.24 1040.68 66.56 349.29 25.49 323.80 297.90 25.90 48.10 25.92 18.45 6.96 41.14 4.1 × 10-6 41.14 8.22 × 10-5 0.0001 41.14 1085.05 1.18 0.49 1085.73

a

Gaseous state.

As

Fe

0.73 0.01 0.00 0.00 1.25 0.02 0.00 27.93 0.00 0.00 0.00 2.67 0.00 2.84 × 10-3 0.00 44.42 1.22 0.91 1.22 0.91 0.08 0.02 19.08 14.90 0.00 2.84 × 10-3 0.00 0.04 0.00 0.00 0.00 0.00 0.00 0.00 26.36 20.60 0.03 0.01 0.08 0.02 0.00 0.00 30.82 24.09 29.27 24.17 30.82 24.09 0.00 0.00 1.40 1.15 30.82 24.09 0.08 0.02 0.00 0.00 0.00 0.00 0.08 0.02 11 588.60 tons/day 93%

Ca

Na

H2O

[As] (g/L)

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 40.05 0.00 0.04

0.00 0.00 0.00 0.00 4.37 3.96 4.17 0.63 0.01 0.01 0.01 5.23 × 10-3 4.17 12.65 3.50 3.50 3.50 0.59 2.40 0.01 0.00 0.69 0.70 0.69 0.14 0.17 0.69 0.07 0.00 0.00 0.07

98.58 100.00 97.56 0.00 92.39 83.56 88.05 13.21 95.87 96.14 99.74 39.90 84.60 13.53 90.18 90.18 90.18 15.31 93.19 99.74 100.00 0.98 0.98 0.98 99.15 94.55 0.98 99.58 0.00 0.00 99.52

14.80 0.00 24.60 0.00 0.00 0.00 0.00 0.00 24.64 1.92 1.92 1.92 0.00 0.00 0.00 0.00 0.00 0.01 0.69 1.92 0.00 0.05 0.00 0.05 0.00 0.10 0.05 1.96 0.00 0.00 0.11


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Table 3. Energy Balance: Case 1

Table 5. Energy Balance: Case 2

feed × enthalpies × heat required × discharge × process step 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h)

feed × enthalpies × heat required × discharge × Process step 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h)

evaporator mixer 1 filter mixer 2 crystallizer 1 thickener crystallizer 2 filter 2 separator filter/washer dryer sampler TCLP mixer 3 mixer 4 mixer 5 a

0 2 367 3315 3315 2003 364 -184 -182 44 3 8 0 1985 1985 8

-19 028 365 0 0 -968 0 53 0 0 0 -169 0 0 0 -217 0

22 339 0 0 0 -344 0 -601 0 0 0 174 0 0 0 0 0 21 568a

-3311 -367 -367 -3315 -2003 -2003 184 184 182 -44 -8 -8 0 -1985 -1768 -8

evaporator mixer 1 filter mixer 2 crystallizer 1 thickener crystallizer 2 filter 2 separator filter/washer dryer sampler TCLP mixer 3 mixer 4 mixer 5 a

Net Q.

details of feed and discharge temperatures, as well as enthalpies and heat requirements. Figure 4 shows the process of each case in the phase diagram. “L” denotes the saturated solution, “I” represents Fe(OH)3, “II” represents FeAsO4 · 2H2O (scorodite), and “III” is for Fe(H2AsO4)3 · 5H2O (kaatialaite). For each case, the figure shows that the streams were displaced according to the evaporation ratio. Case 3 shows a wide area of operation.

-32 223 365 0 0 -968 0 53 0 0 0 -169 0 0 0 -217 0

0 2 367 1618 1618 987 364 -184 -182 44 3 8 0 969 969 8

33 837 0 0 0 337 0 -601 0 0 0 174 0 0 0 0 0 33 747a

-1615 -367 -367 -1618 -987 -987 184 184 182 -44 -8 -8 0 -969 752 -8

Net Q.

The yield was obtained by the quotient between the quantity of arsenic that leaves the system as scorodite and the arsenic that enters the system as feed (stream 1). For the three cases, the yield was the same (93%) because the quantity of arsenic that enters through feed (stream 1) and the quantity of Fe that enters stream 4 did not vary for the three cases given that they maintained the Fe/As ) 1 molar ratio. Upon changing the evaporation rate, the saturated solution generated by the reaction

Table 4. Mass Balance for the Scorodite Production Process: Case 2 % w/w stream

% solid

T (°C)



0.0 0.0 0.0 100.0 0.0 5.1 1 × 10-2 85.0 3.4 7.1 0.0 60.0 6.2 85.0 0.0 0.0 0.0 83.0 0.0 0.0 0.0 97.1 97.1 97.1 0.0 4.6 97.1 0.0 100.0 0.0 0.4

25.0 100.0 100.0 25.0 25.0 53.4 53.4 53.4 99.7 70.0 70.0 70.0 10.0 10.0 10.0 10.0 10.0 31.4 31.4 70.0 60.0 60.0 60.0 60.0 25.0 22.0 59.9 68.3 25.0 58.62 58.62

1867.20 1324.73 542.47 35.51 336.09 371.60 349.29 22.31 564.78 564.78 498.21 66.56 349.29 25.49 323.80 297.90 25.90 48.10 25.92 18.45 6.96 41.14 4.1 × 10-6 41.14 8.22 × 10-5 0.0001 41.14 542.58 1.18 0.49 543.27

a

Gaseous state.

As

Fe

0.73 0.01 0.00 0.00 2.5 0.04 0.00 27.93 0.00 0.00 0.00 2.67 0.00 2.84 × 10-3 0.00 44.42 2.40 1.79 2.40 1.79 0.17 0.04 19.12 14.90 0.00 2.84 × 10-3 0.00 3.89 × 10-2 0.00 0.00 0.00 0.00 0.00 0.00 26.36 20.60 0.05 0.01 0.16 0.04 0.00 0.00 30.82 24.09 29.27 24.17 30.82 24.09 0.00 0.00 1.40 1.15 30.82 24.09 0.16 0.04 0.00 0.00 0.00 0.00 0.16 0.04 8876.28 tons/day 93%

Ca

Na

H2O

[As] (g/L)

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 40.05 0.00 0.09

0.00 0.00 0.00 0.00 4.37 3.96 4.17 0.63 0.02 0.02 0.03 0.01 4.17 12.65 3.50 3.50 3.50 0.59 2.40 0.03 0.00 0.69 0.70 0.69 0.14 0.17 0.69 0.11 0.00 0.00 0.14

98.58 100.00 95.13 0.00 92.39 83.56 88.05 13.21 91.90 92.44 99.47 39.80 84.60 13.53 90.18 90.18 90.18 15.31 93.10 99.47 100.00 0.98 0.98 0.98 99.15 94.55 0.98 99.17 0.00 0.00 99.04

14.80 0.00 49.78 0.00 0.00 0.00 0.00 0.00 49.71 3.90 3.90 3.90 0.00 0.00 0.00 0.00 0.00 0.01 1.40 3.90 0.00 0.11 0.00 0.11 0.00 0.10 0.11 3.93 0.00 0.00 0.22

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Table 6. Mass Balance for the Scorodite Production Process: Case 3 % w/w stream

% solid

T (°C)



0.0 0.0 0.0 100.0 0.0 5.1 1 × 10-2 85.0 6.5 13.6 0.0 60.0 6.2 85.0 0.0 0.0 0.0 83.0 0.0 0.0 0.0 97.1 97.1 97.1 0.0 4.6 97.1 0.0 100.0 0.0 0.8

25.0 100.0 100.0 25.0 25.0 53.4 53.4 53.4 99.5 70.0 70.0 70.0 10.0 10.0 10.0 10.0 10.0 31.3 31.3 70.0 60.0 60.0 60.0 60.0 25.0 22.0 60.0 66.5 25.0 47.0 47.0

1867.20 1595.97 271.23 35.51 336.09 371.60 349.29 22.31 293.54 293.54 226.98 66.56 349.29 25.49 323.80 297.90 25.90 48.10 25.92 18.45 6.96 41.14 4.1 × 10-6 41.14 8.22 × 10-5 0.0001 41.14 271.35 1.18 0.50 272.03

a

Fe

0.73 0.01 0.00 0.00 5.00 0.07 0.00 27.93 0.00 0.00 0.00 2.67 0.00 2.84 × 10-3 0.00 44.42 4.62 3.44 4.62 3.44 0.35 0.08 19.19 14.92 0.00 2.84 × 10-3 0.00 3.89 × 10-2 0.00 0.00 0.00 0.00 0.00 0.00 26.36 20.60 0.11 0.02 0.35 0.08 0.00 0.00 30.82 24.09 29.27 24.17 30.82 24.09 0.00 0.00 1.40 1.15 30.82 24.09 0.32 0.07 0.00 0.00 0.00 0.00 0.32 0.07 7520.11 tons/day 93%

Ca

Na

H2O

[As] (g/L)

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 40.05 0.00 0.17

0.00 0.00 0.00 0.00 4.37 3.96 4.17 0.63 0.05 0.05 0.06 0.02 4.17 12.65 3.50 3.50 3.50 0.59 2.42 0.06 0.00 0.69 0.69 0.69 0.15 0.17 0.69 0.28 0.00 0.00 0.28

98.58 100.00 90.26 0.00 92.39 83.56 88.05 13.21 84.41 85.45 98.90 39.56 84.59 13.53 90.18 90.18 90.18 15.31 92.93 98.90 100.00 0.98 0.98 0.98 99.15 94.55 0.98 98.33 0.00 0.00 98.08

14.80 0.00 101.90 0.00 0.00 0.00 0.00 0.00 101.13 8.10 8.10 8.10 0.00 0.00 0.00 0.00 0.00 0.03 2.90 8.10 0.00 0.22 0.00 0.22 0.00 0.10 0.22 7.89 0.00 0.00 0.44

Gaseous state.

Table 7. Energy Balance: Case 3 feed × enthalpies × heat required × discharge × process step 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h) 10-3 (kcal/h) evaporator mixer 1 filter mixer 2 crystallizer 1 thickener crystallizer 2 filter 2 separator filter/washer dryer sampler TCLP mixer 3 mixer 4 mixer 5 a

As

0 2 367 770 770 479 364 -184 -182 44 3 8 0 461 461 8

-38 821 365 0 0 -968 0 53 0 0 0 -169 0 0 0 0 -367

39 586 0 0 0 677 0 -601 0 0 0 174 0 0 0 0 0 39 836a

-766 -367 -367 -770 -479 -479 184 184 182 -44 -8 -8 0 -461 -461 -8

Net Q.

varied (eq 2), which is influenced by the size of the equipment, the amount of energy used and generated in the process, and the quantity of residue that is discharged. The temperature did not change. Comparison of the three cases under study (Table 8) shows that arsenic concentration in the discharge streams is directly dependent on the evaporation ratio, as expected. The greater the evaporation, the greater the concentration of arsenic in the residual solutions (streams 27 and 31), which requires a subsequent treatment of aqueous residues. On the other hand,

lower concentration of arsenic in residual streams required higher flows (see Table 9). On the other hand, the higher the evaporation ratio, the smaller the equipment and the lower the cost of construction of the equipment, but the higher the heat requirements, which implies higher energy costs. The lowest heat requirement (case 1) is equivalent to 54% of the highest one (case 3). Accordingly, the total mass flow for the smallest equipment (case 3) represents 65% of that for the largest one (case 1). As the evaporation percentage increases, the energy requirement increases, as can be seen clearly in case 3. The energy balance for the crystallizer based upon data from Tables 2-7 is FinputHinput + Q ) FoutputHoutput + Qreaction

(5)

where F is the mass flow [tons/h], H is the enthalpy [kcal/mol], and Q is the required heat [kcal/h]. For stream numbers, check Figure 2. The heat requirements in crystallizer 1 for cases 1, 2, and 3 are -344, +337, and +677 Mcal/h, respectively. In case 1 the heat necessary to the cooling is higher than the precipitation heat. This is because the flow entering to the crystallizer has the highest value. The results of the energy balance are the opposite for both cases 2 and 3, where the crystallizer requires heat. For the three cases, the heat requirement for crystallizer 2 is the same because there is no variation in the flows involved. Analyzing the net heat flow for the process, the results show that, as the percentage of evaporation increases, the net heat required increases, and consequently energy costs increase.


10529

Figure 4. Phase diagram of the As-Fe-H2O system (% w/w) at 70 °C for (a) case 1, (b) case 2, and (c) case 3. The points in the diagram denote As and Fe compositions, where the colored solid diamonds (red, green, blue, and purple) represent the concentrations of streams 1, 3, 9, and 11, respectively, along the solubility. “L” denotes the saturated solution, “I” represents Fe(OH)3, “II” represents FeAsO4 · 2H2O (scorodite), and “III” is for Fe(H2AsO4)3 · 5H2O (kaatialaite). Table 8. Comparative Summary

case

evaporation ratio [%]

[As] at evaporator outlet [% w/w]

[As] in outlet streams 27 and 31 [g/L]

total required heat × 10-3 [kcal/h]

1 2 3

42.5 71.97 86.7

1.25 2.5 5

0.05-0.11 0.11-0.22 0.22-0.44

21 568 33 747 39 836

case

As concentration in discharge solution [g/L]

1 2 3

1083.5 541 269.8

0.11 0.22 0.44

evaporator

crystallizer 1

total mass flow [tons/day]

22 339 33 837 39 586

-344 337 677

11 588.60 8 876.28 7 520.11

Experimentally, it was found that, for Fe/As molar ratios less than 1, ferric arsenate was obtained equally. As the Fe/As molar ratio decreases, the yield of the process also obviously decreases, with 93, 88, and 78% for molar ratios equal to 1, 0.9, and 0.8, respectively. Net heat and total flows of the process also decrease with the molar ratio. Inversely, the arsenic concentration in solution increases with the decrease in the molar ratio, because it does not have sufficient iron for the scorodite reaction, which results in obtaining liquid residue with greater arsenic concentration. To analyze the conversion effect, it was modeled with the values of 96 and 86% (see Table 10). The results show a slight variation in the flows and the energy requirements, with these increasing to increase conversion. The greater the conversion, the less the concentration of arsenic in the liquid residuals of the process, which implies an optimal process for the treatment

Table 9. Residual Solution Characterization residual discharge solution [tons/day]

heat requirement × 10-3 [kcal/h]

In terms of the arsenic confinement capacity, there were no significant differences in the arsenic precipitated, either as scorodite or as calcium salt comparing the cases under study. Arsenic concentration in the final discharge streams is higher than accepted by Chilean regulations (0.11, 0.22, and 0.44 g/L As). This process could be improved with recycling or seeding. To complete the study, the effect of the Fe/As molar ratio and the conversion of the reaction of scorodite formation were evaluated (eq 2) for case 2, using METSIM. The results are shown in Table 10 and Figure 5. Table 10. Analysis for Case 2

molar ratio Fe/As

total flow (tons/day) yield of the process net heat required × 10-3 (kcal/h) As concentration in solution (g/L) stream 11 stream 27 stream 31

conversion of reaction (eq 2)

1

0.9

0.8

96%

86%

8876.28 93% 33.747

8632.96 88% 33.736

8388.74 78% 33.675

8876.79 96% 33.775

8874.77 86% 33.661

3.90 0.11 0.22

6.79 0.23 0.40

11.92 0.42 0.74

2.52 0.07 0.13

7.86 0.18 0.47

10530


Figure 5. Phase diagram of the As-Fe-H2O system (% w/w) at 70 °C for case 2. (a) Molar ratio Fe/As ) 0.9, (b) molar ratio Fe/As ) 0.8, (c) conversion 96%, and (d) conversion 86%. The points in the diagram denote As and Fe compositions, where the colored solid diamonds (red, green, blue, and purple) represent the concentrations of streams 1, 3, 9, and 11, respectively along the solubility. “L” denotes the saturated solution, “I” represents Fe(OH)3, “II” represents FeAsO4 · 2H2O (scorodite), and “III” is for Fe(H2AsO4)3 · 5H2O (kaatialaite).

of arsenical residues. For these four analyses, no significant displacement is noted in the streams of the process, in the phase diagrams (Figure 5). Conclusions The possibility to experimentally obtain scorodite at temperatures of 95, 70, and 50 °C, with distinct residence times has been demonstrated as shown in the diffractograms. The process takes 3 h for the highest temperature and 3 days for the lowest. Scorodite crystal stability increases as the temperature of the process increases, but none of them complies with TCLP standards. Changes in crystal coloring are verified as well, denoting the changes in the crystal purity of each solid. Further experiments are needed in order to establish optimal conditions for complying with the maximum permitted concentration of arsenic. The mean size of crystals obtained is low, owing to the fact that crystallization is a spontaneous product of the reaction without control of oversaturation and varied between 9.8 and 21 µm. The experimental conversion of Fe(OH)3 production and the obtaining of scorodite are high (100 and 93%, respectively) and were used for the simulation. On the other hand, the simulation using METSIM allows for evaluating an industrial process based upon experimental data and reasonable assumptions. For the three cases studied at different evaporation ratios, the overall yield was 93%. A low evaporation ratio implies a lower energy requirement. However, the trade-offs are also well established, as the lower the evaporation ratio, the larger the total mass flow, which means the larger the equipment and the higher the capital investment. The operational expenditure can be reduced by marketing the Na2SO4 · 10H2O, obtained as a byproduct. For Fe/As molar ratios of 1, 0.9, and 0.8, the process yields obtained are 93, 88, and 78%. Lower heat flows are required

for lower ratios; nevertheless, the residual solution increases its arsenical content. Acknowledgment The authors wish to thank Conicyt for financing this study, Fondecyt Project 1050869, and the collaboration of CICITEM and Xstrata CoppersNorth of Chile. Literature Cited (1) Ugarte, G. La problema´tica del arseńico y su solucioń en los nuevos procesos hidrometalu´rgicos para la produccioń de cobre. In Hydrocopper 2005, Santiago, Chile, November 23-25, 2005; Menacho, J. M., Casa de Prada, J. M., Eds.; Universidad de Chile: Santiago, Chile, 2005; pp 403412. (2) Demopoulos, G. P.; Droppert, D. J.; Van Weert, G. Precipitation of crystalline scorodite from chloride solutions. Hydrometallurgy 1995, 38, 245–261. (3) Demopoulos, G. P. Effluent treatment by crystallization. Clean Technology for the Mining Industry; University of Concepcion: Concepcion, Chile, 1996. (4) Droppert, D. J.; Demopoulos, G. P.; Harris, G. B. Ambient pressure production of crystalline scorodite from arsenic-rich metallurgical effluent solutions. EPD Congress, 1996; The Minerals, Metals and Materials Society: Warrendale, PA, 1996; p 227. (5) Filippou, D.; Demopoulos, G. P. Arsenic immobilization by controlled scorodite precipitation. JOM 1997, 52–55. (6) Singhania, S.; Wang, Q.; Filippou, D.; Demopoulos, G. P. Temperature and seeding effects on the precipitation of scorodite from sulfate solutions under atmospheric-pressure conditions. Metall. Mater. Trans., B 2005, 36B, 327–333. (7) Dutrizac, J. E.; Jambor, J. L. The behaviour of arsenic during jarosite precipitation: arsenic precipitation at 97 °C from sulphate and chloride media. Can. Metall. Q. 1987, 6, 91–101. (8) Swash, P. M.; Monhemius, A. J. Synthesis, characterization and solubility testing of solids in the Ca-Fe-AsO4 system. In Sudbury ’95. Mining and the EnVironment; CANMET: Ottawa, Canada; 1995.

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009 (9) Krause, E.; Ettel, V. A. Solubilities and stabilities of ferric arsenates. Crystallization and Precipitation; Pergamon Press: New York, 1987; pp 195-210. (10) Dutrizac, J. E.; Jambor, J. L. Characterization of the iron arsenatesulphate compounds precipitated at elevated temperatures. Hydrometallurgy 2007, 86, 147–163. (11) Swash, P. M.; Monhemius, A. J. The Scorodite Process: A technology for the disposal of arsenic in the 21st century. Effluent Treatment in the Mining Industry; University of Concepcion: Concepcion, Chile; 1998; pp 119-161. (12) Le Berre, J. F.; Gauvin, R.; Demopoulos, G. P. A study of the crystallization kinetics of scorodite via the transformation of poorly crystalline ferric arsenate in weakly acidic solution. Colloids Surf., A 2008, 315, 117–129. (13) Fujita, T.; Taguchi, R.; Abumiya, M.; Matsumoto, M.; Shibata, E.; Nakamura, T. Novel atmospheric scorodite synthesis by oxidation of ferrous sulfate solution. Part II. Effect of temperature and air. Hydrometallurgy 2008, 90, 85–91. (14) Taboada M. E.; Hernańdez P. C.; Flores E. K.; Galleguillos H. R.; Graber T. A. Equilibria of aqueous system phases containing arsenic +

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iron and arsenic + calcium at (323.15 and 343.15) K. J. Chem. Eng. Data, DOI: 10.1021/je900151s. (15) USEPA Method 1311. Toxicity Characteristic Leaching Procedure. Code of Federal Regulations, 40 CFR part 261, Appendix II; July 1991. (16) METSIM Process Simulator (http://www.metsim.com). (17) Outokumpu HSC Chemistry. Chemical Reaction and Equilibrium Software with extensive thermochemical database, version 4.0, Outokumpu Research Oy, Finland. (18) Perry, R. H. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997. (19) Seidell, A.; Linke, W. Solubilities, Inorganic and Metal-Organic Compounds, 4th ed.; American Chemical Society: Washington, DC, 1965; Vol. II.

ReceiVed for reView April 21, 2009 ReVised manuscript receiVed September 2, 2009 Accepted September 18, 2009 IE900639E

Crystallization of Hydrated Ferric Arsenate. Process Design Using

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