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Drowning-Out Crystallization of Sodium Molybdate In Available on line at Association of the Chemical Engineers of Serbia AChE Chemical Industry & Chemical Engineering Quarterly www.ache.org.rs/CICEQ Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020) CI&CEQ JESÚS M. CASAS DROWNING-OUT CRYSTALLIZATION OF JOSUÉ LAGOS SODIUM MOLYBDATE IN AQUEOUS– Universidad Técnica Federico –ETHANOL SOLUTIONS Santa María, Valparaíso, Chile Article Highlights SCIENTIFIC PAPER • Sodium molybdate crystallization from aqueous solutions was studied • Sodium molybdate solubility was studied in water-ethanol solutions at 300 K UDC 546.33:546.77:66:661.722 • 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- chemical industry, as catalysts, pigments, thermally sition metal with low concentration (10 mg/t) in nature. stable colouring agents, engine coolants, corrosion Commercial molybdenum recoveries are obtained inhibitors, fertilizer micronutrients, flame and smoke from some massive deposits of molybdenite (MoS2) suppressants, and in semiconductor and electronic ores or from molybdenite concentrates obtained from manufacturing. Sodium molybdates (Na2MoO4) are copper-moly sulphide ores in which the molybdenum used in the production of fertilizers, catalysts, fire ret- grades range from 0.005 to 0.18% [1,2]. Molybdenum ardants, pigments, and for corrosion inhibition in is being increasingly used in industry [3,4], especially water-cooling systems [1,2,4–6]. in alloys and high-performance steels applications. Molybdenum and molybdates are produced by Molybdenum can be alloyed with other metals to form metallurgical processes after a series of concentration corrosion-, thermal-, and wear-resistant materials and and refining operations. Ammonium molybdate has also is used as a protective coating on metals. been the main compound produced, which is syn- Molybdenum compounds with multiple valences thesized from the leach solutions obtained in copper, (-2 to +5) and mainly molybdates (valence +6) are lead or molybdenum industries, and also from solut- also used as reagents in analytical chemistry and the ions obtained from the recycling of molybdenum-con- taining wastes. In Chile, the Molymet company pro- duces ammonium heptamolybdate, AHM, Correspondence: J.M. Casas, Universidad Técnica Federico [(NH ) Mo O .4H O] and ammonium dimolybdate, Santa María., Avda. España 1680, Valparaíso, Chile, Postal Code 4 6 7 24 2 2340000. ADM, [(NH4)2Mo2O7] by ammoniacal leaching of cal- E-mail: [email protected] cines (technical grade molybdenum trioxide, MoO3). Paper received: 29 December, 2019 Then, the purified molybdenum trioxide is obtained Paper revised: 25 March, 2020 Paper accepted: 6 May, 2020 from calcination of molybdates. Metallic molybdenum https://doi.org/10.2298/CICEQ191029017C is obtained in Germany from molybdenum trioxide by 395 J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020) a high temperature reduction using gaseous hydro- °C. The equilibrium time was verified by preliminary gen [1,2,4,6,7]. assays and measurements after 10 min did not exhi- Sodium molybdate can be produced by an eva- bit changes in the mass of sodium molybdate crystals porative crystallization process (90−100 °C), mixing formed. sodium hydroxide and molybdenum trioxide in an Equilibrium conditions were obtained readily, aqueous solution. Then, sodium molybdate is crystal- after 10 min, according to preliminary assays. lized by thermal evaporation of water with a high The final crystallization experiments were car- energy cost (about 6.74 MJ/kg Na2MoO4 [8]). The ried out at room temperature in a stirred small glass molybdenum product generated by evaporative crys- reactor using two operating batch stages, with 25 and tallization is a powder with fine particle sizes and pre- 50 vol.% ethanol, respectively. After 40 min of resi- sents low crystallinity and different particle morpho- dence time, the final solution was filtered through a logies [8]. Rumble 2018 [9] had reported that sodium GV 0.22 µm Millipore filter and then analysed. The molybdate is very soluble in water; its solubility inc- collected solids were washed with acetone in order to reased from 38.8 to 41.7 wt.% as the temperature remove the impregnated solution and then dried at 40 increased from 10 to 60 °C. °C in an electric oven. Then, the prepared solid sample The drowning-out crystallization of dissolved was homogenized and characterized by SEM and XRD. metals, promoted by the addition of organic com- pounds has been studied for a long time [10–12]. RESULTS AND DISCUSSION Ethanol has a ready market availability, it is a cheap reagent with low toxicity, and has been selected in Solubilities of sodium molybdate in aqueous- ° this work to improve the crystallization of highly sol- ethanol solutions at 27 C are presented in Figure 1. uble sodium molybdate. A new drowning-out molyb- This figure also presents previous results of crystal- ° date crystallization process, proposed in this study, is lization experiments obtained after 1 h at 27 C from 5 expected to reduce the water and energy cost and to 50 vol.% ethanol. consumption, compared to conventional industrial 300 processes, and also permits reagent recovery, result- ing in a better cost-effective process. 250 200 EXPERIMENTAL METHOD 150 A sample of commercial molybdenum trioxide 100 MoO3 (purity 57%) was provided by Molymet SA; the main impurities contained in this sample were: Cu 50 Molybdenum Concentration (g/L) 0.5, S 0.1, P 0.05, Pb 0.05, K2O 0.2 (values in g/t). 0 The sodium molybdate compound used in this 0 5 10 15 20 25 30 35 40 45 50 55 study was synthesized by evaporating, at 80 °C, 1 L Ethanol Concentration, (vol.%) of an aqueous solution containing 40 g NaOH and Figure 1. Solubility of Na2MoO4 in water-ethanol solutions 27 °C. 118 g MoO3. Then, the synthesized compound was contacted with acetone to remove the impregnated The solubility of sodium molybdate in the solution, dried at 40 °C and then characterized by assayed solutions decreased markedly as the ethanol SEM (JEOL JSM-5410 scanning electron micro- concentration was increased. Molybdenum solubility scope) and XRD (CuKα radiation of a Siemens® in 5 and 50 vol.% ethanol were 256 and 37.8 g/L, D5000 diffractometer), which confirmed that sodium respectively. The solubility of Na2MoO4 in aqueous molybdate di-hydrate was the only crystallized phase. solutions firstly was diminished by about 21% in 5 Distilled water, 99% NaOH p.a. pellets and vol.% ethanol and finally was reduced by 89% in 50 99.5% C2H5OH p.a. from Merck were used. vol.% ethanol. The molybdenum concentrations of the filtered Ethanol acts as a mixed-solvent in Na2MoO4- samples were determined by a colorimetric method H2O solutions and produces a decrease in the inter- (8189, HACH). action forces between water molecules and dissolved The solubility of Na2MoO4 was measured in species, due to the decrease in the dielectric constant water-ethanol solutions (5−50 vol.% ethanol) by and density of the solution. This promotes ionic asso- + 2- immersing, for a period of one hour, sealed tubes in a ciation between Na and MoO4 species, favoring the thermo-regulated water bath (PolyScience) at 27±2 sodium molybdate crystallization [13]. 396 J.M. CASAS, J. LAGOS: DROWNING-OUT CRYSTALLIZATION… Chem. Ind. Chem. Eng. Q. 26 (4) 395−399 (2020) The obtained phases of sodium molybdate were 300 analysed and showed a high purity and no other rel- 250 evant substance or element was found. Figure 2 pre- 200 sents the micrographs of sodium molybdate phases 1st Stage generated by evaporative crystallization and by crys- 150 tallization assisted by ethanol, at 0 and 50 vol.% 100 ethanol, respectively. 2nd Stage The crystallized phase showed more consistent 50 morphology and larger particle sizes compared with Molybdenum Concentration (g/L) molybdate produced from evaporative crystallization. 0 0 5 10 15 20 25 30 35 40 45 50 55 This difference could be by attributed to evaporation Ethanol Concentration, (vol.%) temperatures, that produces differences in the eva- Figure 3. Operating diagram of two crystallization stages for poration and its respective crystallization rates. Crys- sodium molybdate in H2O-EtOH, at room temperature. Expe- tals formed with large particle sizes facilitates the rimental results after 20 min of crystallization in each stage. solid liquid separation. Elemental and DRX analyses show that the crystallized phase was Na2MoO4⋅2H2O. The crystallization process generated from res- Figure 3 presents an operating diagram pro- ults presented by Figure 3 could help to define a posed for the crystallization of sodium molybdate from conceptual process for producing sodium molybdate aqueous solution. Starting from a solution containing by crystallization with ethanol. The final ethanol sol- 225 g/L Mo, two crystallization stages are required in ution could be distilled, in the range of 80–86 °C, for order to obtain a performance higher than 80%, after recovering the solvent. 40 min of total crystallization time. In the first stage, Table 1 presents a summary of operating cost adding ethanol to 25 vol.%, molybdenum precipitates differences estimated for evaporative crystallization up to 121 g/L Mo.
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