Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020) CI&CEQ

395

JESÚS M. CASAS

JOSUÉ LAGOS

Universidad Técnica Federico Santa María, Valparaíso, Chile

SCIENTIFIC PAPER

UDC 546.33:546.77:66:661.722

DROWNING-OUT CRYSTALLIZATION OF SODIUM MOLYBDATE IN AQUEOUS– –ETHANOL SOLUTIONS

Article Highlights • Sodium molybdate crystallization from aqueous solutions was studied • Sodium molybdate solubility was studied in water-ethanol solutions at 300 K • Na2MoO4 crystallization promoted by ethanol shows low energy and low water

consumption Abstract

The drowning-out crystallization process of sodium molybdate (Na2MoO4) was studied in water-ethanol solutions at room temperature. Sodium molybdate was separated from the solution in well-crystalline particles using less water and energy compared with the industrial processes that use evaporative crys-tallization. Results showed that crystallization of sodium molybdate di-hydrate was achieved after 20 or 40 min in one or two operating stages for super-satur-ation control with the addition of 25 or 50 vol.% ethanol, respectively. Crystal-lization with ethanol could reduce the operating costs for about 32% with res-pect to the conventional evaporative crystallization method, and the exhausted ethanol in the aqueous solution could be recovered by distillation and then recycled into the process.

Keywords: crystallization, drowning-out, ethanol, molybdenum, sodium molybdate.

Molybdenum is an element classified as a tran-sition metal with low concentration (10 mg/t) in nature. Commercial molybdenum recoveries are obtained from some massive deposits of molybdenite (MoS2) ores or from molybdenite concentrates obtained from copper-moly sulphide ores in which the molybdenum grades range from 0.005 to 0.18% [1,2]. Molybdenum is being increasingly used in industry [3,4], especially in alloys and high-performance steels applications. Molybdenum can be alloyed with other metals to form corrosion-, thermal-, and wear-resistant materials and also is used as a protective coating on metals.

Molybdenum compounds with multiple valences (-2 to +5) and mainly molybdates (valence +6) are also used as reagents in analytical chemistry and the

Correspondence: J.M. Casas, Universidad Técnica Federico Santa María., Avda. España 1680, Valparaíso, Chile, Postal Code 2340000. E-mail: jesus.casas@usm.cl Paper received: 29 December, 2019 Paper revised: 25 March, 2020 Paper accepted: 6 May, 2020

https://doi.org/10.2298/CICEQ191029017C

chemical industry, as catalysts, pigments, thermally stable colouring agents, engine coolants, corrosion inhibitors, fertilizer micronutrients, flame and smoke suppressants, and in semiconductor and electronic manufacturing. Sodium molybdates (Na2MoO4) are used in the production of fertilizers, catalysts, fire ret-ardants, pigments, and for corrosion inhibition in water-cooling systems [1,2,4–6].

Molybdenum and molybdates are produced by metallurgical processes after a series of concentration and refining operations. Ammonium molybdate has been the main compound produced, which is syn-thesized from the leach solutions obtained in copper, lead or molybdenum industries, and also from solut-ions obtained from the recycling of molybdenum-con-taining wastes. In Chile, the Molymet company pro-duces ammonium heptamolybdate, AHM, [(NH4)6Mo7O24.4H2O] and ammonium dimolybdate, ADM, [(NH4)2Mo2O7] by ammoniacal leaching of cal-cines (technical grade molybdenum trioxide, MoO3). Then, the purified molybdenum trioxide is obtained from calcination of molybdates. Metallic molybdenum is obtained in Germany from molybdenum trioxide by

J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020)

396

a high temperature reduction using gaseous hydro-gen [1,2,4,6,7].

Sodium molybdate can be produced by an eva-porative crystallization process (90−100 °C), mixing sodium hydroxide and molybdenum trioxide in an aqueous solution. Then, sodium molybdate is crystal-lized by thermal evaporation of water with a high energy cost (about 6.74 MJ/kg Na2MoO4 [8]). The molybdenum product generated by evaporative crys-tallization is a powder with fine particle sizes and pre-sents low crystallinity and different particle morpho-logies [8]. Rumble 2018 [9] had reported that sodium molybdate is very soluble in water; its solubility inc-reased from 38.8 to 41.7 wt.% as the temperature increased from 10 to 60 °C.

The drowning-out crystallization of dissolved metals, promoted by the addition of organic com-pounds has been studied for a long time [10–12]. Ethanol has a ready market availability, it is a cheap reagent with low toxicity, and has been selected in this work to improve the crystallization of highly sol-uble sodium molybdate. A new drowning-out molyb-date crystallization process, proposed in this study, is expected to reduce the water and energy cost and consumption, compared to conventional industrial processes, and also permits reagent recovery, result-ing in a better cost-effective process.

EXPERIMENTAL METHOD

A sample of commercial molybdenum trioxide MoO3 (purity 57%) was provided by Molymet SA; the main impurities contained in this sample were: Cu 0.5, S 0.1, P 0.05, Pb 0.05, K2O 0.2 (values in g/t).

The sodium molybdate compound used in this study was synthesized by evaporating, at 80 °C, 1 L of an aqueous solution containing 40 g NaOH and 118 g MoO3. Then, the synthesized compound was contacted with acetone to remove the impregnated solution, dried at 40 °C and then characterized by SEM (JEOL JSM-5410 scanning electron micro-scope) and XRD (CuKα radiation of a Siemens® D5000 diffractometer), which confirmed that sodium molybdate di-hydrate was the only crystallized phase.

Distilled water, 99% NaOH p.a. pellets and 99.5% C2H5OH p.a. from Merck were used.

The molybdenum concentrations of the filtered samples were determined by a colorimetric method (8189, HACH).

The solubility of Na2MoO4 was measured in water-ethanol solutions (5−50 vol.% ethanol) by immersing, for a period of one hour, sealed tubes in a thermo-regulated water bath (PolyScience) at 27±2

°C. The equilibrium time was verified by preliminary assays and measurements after 10 min did not exhi-bit changes in the mass of sodium molybdate crystals formed.

Equilibrium conditions were obtained readily, after 10 min, according to preliminary assays.

The final crystallization experiments were car-ried out at room temperature in a stirred small glass reactor using two operating batch stages, with 25 and 50 vol.% ethanol, respectively. After 40 min of resi-dence time, the final solution was filtered through a GV 0.22 µm Millipore filter and then analysed. The collected solids were washed with acetone in order to remove the impregnated solution and then dried at 40 °C in an electric oven. Then, the prepared solid sample was homogenized and characterized by SEM and XRD.

RESULTS AND DISCUSSION

Solubilities of sodium molybdate in aqueous-ethanol solutions at 27 °C are presented in Figure 1. This figure also presents previous results of crystal-lization experiments obtained after 1 h at 27 °C from 5 to 50 vol.% ethanol.

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40 45 50 55

Mol

ybde

num

Con

cent

ratio

n (g

/L)

Ethanol Concentration, (vol.%) Figure 1. Solubility of Na2MoO4 in water-ethanol solutions 27 °C.

The solubility of sodium molybdate in the assayed solutions decreased markedly as the ethanol concentration was increased. Molybdenum solubility in 5 and 50 vol.% ethanol were 256 and 37.8 g/L, respectively. The solubility of Na2MoO4 in aqueous solutions firstly was diminished by about 21% in 5 vol.% ethanol and finally was reduced by 89% in 50 vol.% ethanol.

Ethanol acts as a mixed-solvent in Na2MoO4-H2O solutions and produces a decrease in the inter-action forces between water molecules and dissolved species, due to the decrease in the dielectric constant and density of the solution. This promotes ionic asso-ciation between Na+ and MoO4

2- species, favoring the sodium molybdate crystallization [13].

J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020)

397

The obtained phases of sodium molybdate were analysed and showed a high purity and no other rel-evant substance or element was found. Figure 2 pre-sents the micrographs of sodium molybdate phases generated by evaporative crystallization and by crys-tallization assisted by ethanol, at 0 and 50 vol.% ethanol, respectively.

The crystallized phase showed more consistent morphology and larger particle sizes compared with molybdate produced from evaporative crystallization. This difference could be by attributed to evaporation temperatures, that produces differences in the eva-poration and its respective crystallization rates. Crys-tals formed with large particle sizes facilitates the solid liquid separation. Elemental and DRX analyses show that the crystallized phase was Na2MoO4⋅2H2O.

Figure 3 presents an operating diagram pro-posed for the crystallization of sodium molybdate from aqueous solution. Starting from a solution containing 225 g/L Mo, two crystallization stages are required in order to obtain a performance higher than 80%, after 40 min of total crystallization time. In the first stage, adding ethanol to 25 vol.%, molybdenum precipitates up to 121 g/L Mo. Further addition of ethanol to 50 vol.% (second stage) reduces the concentration up to 44 g/L Mo.

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40 45 50 55

Mol

ybde

num

Con

cent

ratio

n (g

/L)

Ethanol Concentration, (vol.%)

1st Stage

2nd Stage

Figure 3. Operating diagram of two crystallization stages for sodium molybdate in H2O-EtOH, at room temperature. Expe-rimental results after 20 min of crystallization in each stage.

The crystallization process generated from res-ults presented by Figure 3 could help to define a conceptual process for producing sodium molybdate by crystallization with ethanol. The final ethanol sol-ution could be distilled, in the range of 80–86 °C, for recovering the solvent.

Table 1 presents a summary of operating cost differences estimated for evaporative crystallization and crystallization with ethanol of sodium molybdate. These calculations considered mass and energy bal-ances and unitary costs of purified ethanol (US$/m3 1.000), power (US$/MWh 100), and water (US$/m3 12).

Figure 2. SEM micrographs with identification of elements for sodium molybdate solids obtained: a-b) by evaporative crystallization; c-d)

by crystallization assisted with ethanol.

J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020)

398

Table 1. Operating cost differences between crystallization with ethanol and evaporative crystallization (values were calculated from mass and energy balances, available as supplementary material from the authors, and the unitary prices for electrical energy (US$/MWh 100), demineralized water (US$/m3 12) and ethanol (US$/m3 1000) were obtained from the Chilean mar-kets [14–16]

Cost Item (US$/kg Na2MoO4)

Crystallization with ethanol

Evaporative crystallization

Power (for heating) 0.25 0.51

Purified water (evaporation) – 0.02

Ethanol (losses) 0.03 –

Total 0.28 0.53

The crystallization process with ethanol at room temperature exhibited about 50% lower operating cost of water and energy, relative to the conventional evaporative crystallization process performed, for example, at 92 °C using vacuum conditions.

CONCLUSIONS

Solubility of sodium molybdate in aqueous sol-ution decreased markedly with the presence of etha-nol. Experimental solubilities of Na2MoO4⋅2H2O, mea-sured at 27 °C, in 5 and 50 vol.% ethanol were 256 and 37.8 g/L Mo, respectively. Aqueous molybdenum solubility can be reduced by about 89% at 50 vol.% ethanol and this reagent was an effective mixed-sol-vent that induced the crystallization.

The crystallized phase had more consistent morphology and a larger particle size than the molyb-date produced from evaporative crystallization. Mass balancing the system and DRX analysis show that crystallized phase was Na2MoO4⋅2H2O.

Finally, the proposed process for sodium molyb-date recovery consists of 1 or 2 crystallization stages at room temperature, where the super-saturation and molybdenum recovery could be controlled by the addition of ethanol (drowning-out crystallization pro-cess). The crystallization promoted by ethanol results in about 50% lower operating costs for water and energy, compared with the conventional evaporative crystallization method. The spent ethanol aqueous solution obtained after the separation of crystals could be distilled in the range of 81–86 °C, in order to rec-over the ethanol for recycling within the process.

Supplementary material

Additional data are available from corresponding author upon request.

Acknowledgments

This work was funded by the CONICYT Chilean Government agency via FONDECYT project 106 1160. Thanks goes to MOLYMET S.A. for providing the valuable MoO3 used in the present work and to the Metallurgy and Materials Department of the Uni-versidad Federico Santa María, as well as the Mining Engineering Department of the Universidad de Chile, for their support.

REFERENCES

[1] A. Sutulov, International Molybdenum Encyclopaedia, Products, Uses and Trade, Vol. 3, Intermet Publications, Santiago de Chile, 189-200 (1980), pp. 13-22,149

[2] F. Habashi, Principles of Extractive Metallurgy, Amalgam and Electro Metallurgy. Laval University, Métallurgie Ext-ractive Québec, Vol. 4, Quebec, Canada, 1998, pp. 1361- –1402

[3] S.J. Kropschot, Molybdenum — A key component of metal alloys: U.S. Geological Survey Fact Sheet, 2009–3106, 2010, 2p., available at http://pubs.usgs.gov/fs/2009/3106 (accessed 25 October 2019)

[4] P.C.H. Mitchell, Chemical Applications of Molybdenum, 2004, www.imoa.info (accessed 25 October 2019)

[5] K.-H. Tytko, W.-D. Fleischmann, D. Grasm, E. Warkentin, in Gmelin Handbook of Inorganic Chemistry: Mo Molyb-denum Supplement Vol. B 4, 8th ed., Hartmut Katscher, Wolfgang Kurtz, Friedrich Schröder, Springer-Verlag GmbH, Berlin, 1985, pp. 51-74

[6] C. Gupta, Extractive Metallurgy of Molybdenum, CRC, Press, Boca Raton, FL, 1992, pp. 4-11

[7] Molymet S.A., Applications of Molybdenum; Chemical Industries, www.molymet.cl (accessed 25 October 2019)

[8] J. Lagos, Thesis of Environmental Chemistry, Univer-sidad de Chile, 2009 (unpublished results)

[9] J.R. Rumble, Aqueous solubility of inorganic compounds at various temperatures, in CRC Handbook of Chemistry & Physics, 99th ed., CRC Press, Boca Raton, FL,, 2018, pp. 8-116

[10] A. Seidell, W.F. Linke, Solubilities: Inorganic and Metal- -Organic Compounds: A Compilation of Solubility Data from the Periodical, Vol. 1, Am. Chem. Soc., Washington DC, 1958, p. 1272

[11] T.A. Graber, J.W. Morales, P.A. Robles, H.R. Galleguil-los, M.E. Taboada, Cryst. Res. Technol. 43 (2008) 616- –625

[12] J.M. Casas, E. Sepúlveda, L. Bravo, L. Cifuentes, Hydro-metallurgy 113–114 (2012) 192–194

[13] M.A. Abolghassemi-Fakhree, D.R. Delgado, F. Martínez, A. Jouyban, AAPS PharmSciTech 11 (2010) 1726-1729

[14] National Energy Council, Market price of electrical energy, www.cne.cl (accessed 25 October 2019)

[15] J. Balbontín, Water interconnection study: opportunities and challenges for Chile, Report, SMICEChile, 2019, https://consejominero.cl (accessed 25 October 2019)

[16] Ethanol Prices, GlobalPetrolPrices, https://es.global-petrolprices.com (accessed 25 October 2019)

J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020)

399

[17] CRC Handbook of Chemistry and Physics, D.R. Lide (Ed.), 85th ed., CRC Press, Boca Raton, FL, 2004, pp.

380,986-987,2427.

JESÚS M. CASAS

JOSUÉ LAGOS

Universidad Técnica Federico Santa María, Valparaíso, Chile

NAUČNI RAD

KRISTALIZACIJA NATRIJUM-MOLIBDATA PRECIPITACIJOM U VODENO-ETANOLNIM RASTVORIMA

Proučavan je proces kristalizacije natrijum-molibdata (Na2MoO4) precipitacijom u rastvorima vode i etanola na sobnoj temperaturi. Natrijum molibdat je odvojen od rastvora u obliku kristalnih čestica koristeći manje vode i energije u poređenju sa industrijskim procesima koji koriste evaporativnu kristalizaciju. Rezultati su pokazali da je kristalizacija natrijum-molibdat-dihidrata postignuta nakon 20 ili 40 min, u jednoj ili dve operativne faze za kontrolu super-zasićenja sa dodatkom 25 ili 50 vol% etanola. Kristalizacija etanolom može smanjiti operativne troškove za oko 32% u odnosu na konvencionalnu metodu evaporativne kristalizacije, a iscrpljeni etanol u vodenom rastvoru može se predestilisati i ponovo koristiti.

Ključne reči: kristalizacija, precipitacija, etanol, molibden, natrijum-molibdat.