Article pubs.acs.org/IECR
Preparation of Some Useful Compounds of Zirconium from Bangladeshi Zircon Ranjit K. Biswas,* Mohammad A. Habib, Aneek K. Karmakar, and Mohammad R. Islam Department of Applied Chemistry and Chemical Engineering, Rajshahi University, Rajshahi-6205, Bangladesh ABSTRACT: A number of useful zirconium compounds, such as hydrated zirconyl chloride, hydrated zirconium sulfate, zirconium dioxide, γ-zirconium ammonium phosphate, γ-zirconium phosphate, γ-zirconium phosphate phosphite, and γzirconium phosphate hypophosphite have been prepared from Bangladeshi zircon following the NaOH-fusion, acid (HCl and H2SO4)-leaching, precipitation, calcination/pyrolysis, condensation polymerization, ion exchange and the tap tactic reaction methods. The products have been characterized by chemical and thermogravimetric analyses in all cases, together with the XRD patterns and sodium exchange capacities in some cases. According to da Silva,20 the optimum conditions in the NaOH-fusion−H2SO4-leaching for 91.5% zirconium recovery from zircon are as follows: (i) for fusion, NaOH/ZrSiO4 (wt/ wt) = 1.5, temperature = 848 K, time = 30 min; and (ii) for leaching, [H2SO4] = 3 Kmol/m3, time = 20 min, temperature = 363 K. On the other hand, Biswas et al.21,35 reported that the optimum conditions in NaOH-fusion−HCl-leaching for 100% dissolution of zirconium from zircon are (i) NaOH/ZrSiO4 (wt/wt) = 1.8, temperature = 973 K, and time = 15 min for fusion and (ii) [HCl] = 4 Kmol/m3, temperature = refluxing temperature (∼381 K), and time = 5 min for leaching. In another work, Abdelkader et al.36 have reported the NaOH− KOH (equimolar, eutectic mp = 458 K) fusion of zircon and found out the optimum conditions for 96% zirconium recovery from zircon are (i) NaOH−KOH (equimolar)/zircon (wt/wt) = 1.30, temperature = 823 K, time = 90 min for fusion and (ii) [H2SO4], [HCl] or [HNO3] = concentrated, but the temperature and time not specified, for leaching. Therefore, the method of Biswas et al.21,35 may be considered as the best method, so far, to decompose zircon as it can recover 100% zirconium on using the shortest fusion and leaching times, though it requires slightly elevated fusion temperature and 20% excess NaOH in fusion. According to the past work by the present authors,21 the fused mass can be water washed for most of its silicate removal, dried, dry-ground, and sieved to collect (44−53) × 10−6 m sized conditioned fused mass (CFM). The optimum condition for leaching of CFM was evaluated to be the conditions already cited above together with stirring at 300 Hz keeping the solid to liquid (S/L) ratio at 141:1 w(kg)/ v(m3). On applying stage-wise leaching, it is possible to obtain a solution containing as much as 112 kg Zr(IV)/m3. Among the useful compounds of zirconium, zirconyl chloride, zirconium(IV) sulfate, zirconia, γ-layered zirconium ammonium phosphate, γ-zirconium phosphate, γ-zirconium phosphate phosphite, and γ-zirconium phosphate hypophos-
1. INTRODUCTION Zircon (ZrSiO4) is a heavy mineral (sp. gr. 4.6−4.8), chemically inert and resistant to erosion. As a result, it is concentrated naturally in placer deposits (e.g. beach sand) with other heavy minerals such as ilmenite, magnetite, monazite, garnet, leucoxene, epidote, apatite, etc. or cassiterite and stannite. It can be separated from the heavy mineral sands by gravity concentration followed by high field magnetic and electrostatic steps.1−3 A typical zircon sample contains 62−64% wt % ZrO2 + 3−1 wt % HfO2, 32−33 wt % SiO2, 0.2 wt % TiO2, and 0.15 wt % Fe2O3.4 As the standard free energy of formation of zircon o = −1 489 100 kJ/Kmol), and its is very high (ΔGf,1400K structure consists of high coordinated bisdisphenoid ZrO8 in a tetragonal structure and SiO4 tetrahedra, a drastic condition is required to decompose it.5,6 Among the various methods used to decompose ZrSiO4, (i) the thermal dissociation in the presence of carbon within a plasma arc furnace,7−9 (ii) chlorination,10 (iii) fluorosilicate fusion,11 (iv) substoichiometric Na2CO3 fusion,12 (v) slight excess NaOH or Na2CO3 fusion,13,14 (vi) high stoichiometric NaOH fusion,15−21 (vii) alkaline earth metal oxide or carbonate fusion,22−26 (viii) formation of ZrC in an electric furnace (2273−2473 K) followed by chlorination at 623−723 K,27 (ix) mechano-chemical treatment followed by acid attack28−34 etc. are mentionable. The slight-excess NaOH and the high stoichiometric NaOH fusion methods represent good versatility and are simple requiring both low capital and operation costs because the expensive equipment and reagents are not required. Consequently, the alkali-fusion method has become a unique step to decompose zircon in order to get zirconium compounds. However, in the alkali-fusion method, the fusion temperature as well as the subsequent leaching temperature and leaching acid concentration for maximum zirconium recovery depend on the time of fusion and leaching. Normally, HCl21,35 and H2SO415,16,19,20 solutions are used as leaching agent for leaching of the alkali fused mass of zircon with an aim to obtain ZrOCl2 and ZrOSO4 or Zr(SO4)2 solutions, respectively. In some cases,20,25 these solutions have been alkali treated to precipitate ZrO(OH)2, followed by pyrolysis of ZrO(OH)2 to ZrO2. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 13552
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standard volumetric methods were applied to estimate the chloride54d and sulfate54e contents in aqueous solutions. Dissolved silicon, sodium, and Zr(IV) in fluoride medium were estimated by AAS (Shimadzu AA 6000). XRD patterns were recorded by a Philips PW 3040 X′ part PRO XRD system (λ = 1.5405 Å, 40 KV, 30 mA). Nitrogen was estimated by the micro-Kjeldahl method.54f The weight loss curves of the compounds as function of temperature were obtained by an automatic thermal analyzer (Stanton Redcroft STA 780) in a constant supply of air (4 × 10−5 m3/min). About 1.5 × 10−5 kg samples against an alumina standard in alumina crucibles were used with a heating rate of 5 K/min. 2.3. Procedures. 2.3.1. Procedure for Fusion. An aliquot of 0.20 kg NaOH (∼10% excess) was taken in a steel boat of 5 × 10−4 m3 capacity, melted, and heated to 973 K in a tube furnace (Carbolite, England) and to this molten mass, preheated 0.10 kg of zircon of (44−53) × 10−6 m particle size was added in such a way that all powder remained immersed in the molten mass. The whole mass was immediately inserted in the furnace and heated for 15 min at 973 K. The following reaction took place during fusion:21
phite are some very useful compounds of zirconium. Zirconyl chloride can be used for preparation of other zirconium(IV) salts or compounds. It is also used in textile dying, oil-field acidizing, cosmetics, and greases, and as antiperspirants, water repellents, and TiO2 pigment coating.37 It can be converted just by heating to ZrO 2 possessing manifold applications. Zirconium(IV) sulfate is an analytical reagent and can be converted to ZrO2 by simple heating. Zirconia has widespread applications for its high melting point, corrosion-erosion resistance, low thermal conductivity, high strength, high fracture toughness, good thermal shock resistance, high refractive index, high temperature ionic conductivity, low friction and high wear resistance (negative properties include high density and thermal instability at elevated temperature for morphological change). It can be used to make knives, bottle opener, golf club faces, golf shoe spikes, watch cases, fish hooks, ball point pens, scissor blades (for cutting plastic film, magnetic tape and abrasive wheels), extrusion dies, grinding media, ceramic filters.38 It can also be used in making seals, valves, pump impellers, orthopedic implants, dentistry, resistive heating elements, oxygen sensor, piezoelectric and electrooptical devices, capacitors, crucible, furnace cores, etc.38−40 Moreover, it can be used as the electrolyte in solid oxide fuel cells, synthetic gem stones, pigments, catalysts for hydrogenation, oxidation, esterification, polymerization, etc.38,41−43 Its suggested future uses include38 (i) insulation and wear resistant parts and coatings for advanced heat engines, (ii) fuel cells, (iii) MHD electrodes, and (iv) oxygen pumps, etc. On the other hand, γ-zirconium ammonium phosphate is an intermediate for the preparations of γ-zirconium phosphate, γzirconium phosphate phosphite, other γ-Zr-phosphate phosphonates, γ-Zr-phosphate hypophosphite and γ-Zr-phosphate phosphinates, etc. 4 4 − 4 6 γ-Zirconium phosphate (γZrPO4·H2PO4·2H2O) is the precursor of the compounds noted above excluding itself.44−47 It can be used, as ionexchanger,48 intercalating agent,49 molecular sieve, catalyst,50 proton conductor,44 soap additive etc. γ-zirconium phosphate phosphite and related phosphate phosphonates can be used as ion exchanger,45,51 intercalating agent,45,52 molecular sieve, catalyst and proton conductor etc. like γ-zirconium phosphate. But its ion-exchange and intercalation capacities and the proton conductivity will be less than those possessed by γ-zirconium phosphate. Besides, γ-zirconium phosphate hypophosphite lacks acidic group in the interlayer region; and so it is neither an ion-exchanger nor an intercalating agent.46 It can be used as an insulator and may find other applications. In this paper, the above-mentioned useful compounds of zirconium have been prepared from the caustic fused mass of zircon obtained by the method of Biswas et al.21 after leaching with HCl as well as H2SO4. The products have been characterized in various ways.
ZrSiO4 + 8NaOH = Na4ZrO4 + Na4SiO4 + H 2O
(1)
It was then removed from the furnace, cooled to 313 K, and the fused mass was etched out on a polythene sheet. Two other batches for fusion were performed. 2.3.2. Procedure for Preparing Conditioned Fused Mass (CFM). An aliquot of 0.90 kg fused mass was put in 5 × 10−3 m3 distilled water, constantly stirred for 5 min, allowed to settle overnight, and decanted. The decanted flow did not contain Zr(IV), but contained most of the sodium silicate formed during fusion according to the following hydrolytic reaction in the aqueous solution:21 Na4ZrO4 + Na4SiO4 + 4H 2O → Na4ZrO4 + 4NaOH + H4SiO4 Insoluble Soluble Gel
(2)
The residue left after decantation was water-washed and decanted several times, and separated finally by filtration under suction using a sintered glass crucible. The residue was first dried at ambient temperature for 5 d, then at 378 K overnight, dry-ground, and sieved to collect (44−53) × 10−6 m particles which was termed as the conditioned fused mass (CFM). The weight of the CFM per kilogram of zircon was found to be 1.41 kg (should be 1.35 kg; the increment was due to the presence of some unwashed sodium silicate). 2.3.3. Procedure for Leaching CFM by Acids. To a boiling aliquot of 5 × 10−4 m3 of 4 Kmol/m3 HCl, 0.0705 kg of CFM was added and heated under reflux for 5 min when all CFM was found to be dissolved. Dissolution occurred via the following reaction:35
2. EXPERIMENTAL SECTION 2.1. Reagents. Zircon sample is described earlier.21 It was dry-ground and sieved to collect (44−53) × 10−6 m particles for use in NaOH fusion. Chemicals used in this study were the analytical grade products of E. Merck (Germany or India). 2.2. Analytical. Zirconium(IV) (in nonfluoride medium), Fe(III) Ti(IV) and PO3− 4 in aqueous solutions were estimated colorimetrically using a WPA S104 spectrophotometer by the pyro-catechol violet method at 590 nm,53 HNO3−NH4SCN method at 480 nm,54a H2SO4−H2O2 method at 420 nm,54b and the molybdenum blue method at 830 nm,54c respectively. The
Na4ZrO4 + 6HCl = 4NaCl + ZrOCl 2 + 3H 2O
(3)
The solution was again charged with 0.0705 kg of CFM and refluxed for 5 min. In this stage, CFM was partially dissolved. The solution was separated from the unreacted mass by filtration to get 5 × 10−4 m3 filtrate plus washing. It was again refluxed for 5 min with 0.0705 kg CFM and filtered to obtain 5 × 10−4 m3 leached solution containing 112 kg/m3 Zr(IV), ∼3 kg/m3 Fe(III) (mostly mixed from the steel boat used in fusion; only 0.36 kg/m3 from zircon), 0.20 kg/m3 Ti(IV), ∼2 kg/m3 silicon, and ∼115 kg/m3 Na+. 13553
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Figure 1. Flow sheet for the preparation of Product 1 (hydrated zirconyl chloride) from zircon. Abbreviations: FM = fused mass; CFM = conditioned fused mass; F = filtration; W = washing; S = solution; R = residue; L = leaching; SEX = solvent extraction; ∗ = solution B.
Figure 2. Flow sheet for preparation of Product 2 (hydrated zirconium(IV) sulfate) from CFM. Abbreviations are as in Figure 1.
In H2SO4-leaching, 1 × 10−4 m3 of 3 Kmol/m3 of acid was boiled in a beaker, and CFM was added gradually into this solution until saturation was obtained following the reaction:
The phases were settled and disengaged. On this treatment, the brown colored solution became colorless. The organic phase could be stripped by mild HCl solution of pH 3 and recycled.56 The iron(III)-free HCl-leach solution (Solution B) was used for the preparation of some compounds of Zr(IV). 2.3.5. Procedures for Preparation of Some Useful Compounds of Zr(IV). a. Product 1 (Hydrated Zirconyl Chloride). The flow sheet for the preparation of Product 1 starting from zircon is given in Figure 1. An aliquot of 1 × 10−4 m3 of Solution B (∗) containing 112 kg/m3 Zr(IV) was distilled to reduce its volume by 50% and also to recover constant boiling HCl (6 Kmol/m3). After being cooled to 280 K, 1 × 10−4 kg of ZrOCl2·8H2O crystal as seed was added to the solution and left overnight at the same temperature. Crystals of hydrated zirconyl chloride appeared in large quantities accordingly. It was filtered. After dissolving the residue in a minimum quantity (∼5 × 10−5 m3) of 0.1 Kmol/m3 HCl, an equal volume of concentrated HCl was added to decrease the solubility of zirconyl chloride. On adding 1 × 10−4 kg of seed, the solution was allowed to recrystallize again at 280 K overnight. Crystals were separated from the liquid and washed several times with ethanol, dried in ambient condition and
Na4ZrO4 + H 2SO4 = 2Na 2SO4 + Zr(SO4 )2 + 4H 2O (4)
It was found that 0.06 kg CFM could be dissolved to yield a solution (Solution A) containing ∼208 kg Zr(IV)/m3. 2.3.4. Procedure for Removal of Fe(III) from HCl Leach Solution. An aliquot of 5 × 10−4 m3 HCl-leached solution containing 112 kg Zr(IV)/m3 was evaporated by 50%. An aliquot of 2.5 × 10−4 m3 concentrated HCl (12 Kmol/m3) was added to it. This solution was equilibrated with 1 × 10−4 m3 methyl isobutyl ketone (MIBK) for 10 min to extract most of the Fe(III) present in solution with log D > 2 (while Zr(IV) remained in the aqueous phase with log D < −2) according to following reactions:55 +MIBK
Fe3 + + 3Cl− + HCl → HFeCl4 ⎯⎯⎯⎯⎯⎯⎯→ MIBK·HFeCl4(o) ion ‐ pair(a)
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Figure 3. Flow sheet for the preparation of Products 3a, 3b, and 3c (zirconia) from Product 1, Product 2, and iron-free HCl leach solution of CFM (Solution B is marked as an asterisk (∗) in Figure 1).
Figure 4. Flow sheet for preparation of Products 4 (γ-zirconium ammonium phosphate), 5 (γ-zirconium phosphate), 6 (γ-zirconium phosphate phosphite), and 7 (γ-zirconium phosphate hypophosphite) from HCl-leach solution (Fe(III)-free) of CFM.
finally at 368 K overnight. The dried mass (Product 1) was then stored in a saturated BaCl2 desiccator (RH = 90%). b. Product 2 (Hydrated Zirconium(IV) Sulfate). The flow sheet for the preparation of Product 2 is depicted in Figure 2. An aliquot of 1 × 10−4 m3 of solution A was diluted to 2.50 × 10−4 m3, cooled to 280 K and left overnight at that temperature for crystallization of Na2SO4·10H2O. After filtration, the filtrate was evaporated to 1 × 10−4 m3 and left overnight at 280 K. On crystallization, the whole mass appeared as solid. An aliquot of 1 × 10−4 m3 of ether was added to it, stirred, and filtered. The residue was washed several times with ether, dried in air
followed by in an oven at 368 K for 24 h (Product 2) and desiccated over saturated BaCl2 solution for 30 d. c. Product 3 (Zirconium Dioxide or Zirconia). It was prepared by three methods as shown in Figure 3. In the first method, 0.02 kg of Product 1 was heated at 973 K in the presence of air for 1 h to get Product 3a. In the second method, 0.01 kg of Product 2 was decomposed at 1323 K for 1 h to get Product 3b. In the last method, 2 × 10−5 m3 of solution B (∗) was diluted with 1 × 10−4 m3 water, and 0.006 kg of NaOH pellet was added. The mixture was stirred to dissolve the pellet with a concomitant precipitate. The whole mass was diluted to 13555
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Industrial & Engineering Chemistry Research
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2.50 × 10−4 m3, filtered, and washed several times with water. The residue was dried at ambient temperature for 5 d. The dried mass was calcined at 773 K for 1 h to get Product 3c. d. Product 4 (γ-Zirconium Ammonium Phosphate). The flow sheet for the preparations of Product 4 is given in Figure 4. An aliquot of 1.10 × 10−4 m3 Solution B (∗) was evaporated to 6.80 × 10−5 m3 to increase the Zr(IV) concentration to ∼181 kg/m3 (∼2 Kmol/m3). On taking this solution in a 1 × 10−3 m3 plastic beaker, 3 × 10−5 m3 of 26.5 Kmol/m3 HF and 9 × 10−4 m3 of 2 Kmol/m3 NH4H2PO4 were added, mixed, and digested for 5 d in a water bath at 353 K. The volume of the solution in the beaker was kept constant by a device of constant water addition. A considerable amount of crystal was formed. The product mass was filtered and washed with water until complete elimination of Cl−. It was dried at ambient condition for 5 d and desiccated over saturated BaCl2 solution for at least 30 d to obtain Product 4. e. Product 5 (γ-Zirconium Phosphate). The preparation step for this compound is also included in Figure 4. A portion of Product 4 (0.025 kg) was suspended in 5 × 10−4 m3 of 1 Kmol/m3 HCl solution for 24 h at
