Ind. Eng. Chem. Res. 2003, 42, 777-783
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MATERIALS AND INTERFACES De Wet Process for the Beneficiation of Zircon: Optimization of the Alkali Fusion Step Arao Manhique, Zola Kwela, and Walter W. Focke* Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa
Milled zircon, d50 ≈ 9 µm, was fused with caustic soda pearls in open reaction vessels at temperatures between 650 and 850 °C. Fusion times of 1, 2, 4, 24, and 336 h (with intermittent milling) were investigated. The fusion products were characterized by X-ray diffraction and by wet-chemical analysis. For prolonged fusion times, the fusion products approached equilibrium compositions. However, the phases Na2ZrSiO5 and Na4Zr2Si3O12 observed at 850 °C did not form at 650 °C. Because these compounds do not hydrolyze completely in water, they limit the recovery of alkali in the sodium silicate product stream. With short fusion times (2 h), a nonequilibrium product spectrum is obtained. It is dominated by sodium zirconates and sodium silicates, allowing reasonable zirconia yields and high alkali recovery in a sodium silicate product stream. This allows efficient fusion of two moles of NaOH per mole of zircon at 850 °C with a zirconia yield of ca. 57% and an alkali recovery of ca. 78%. Introduction Zircon is the naturally occurring form of zirconium silicate, ZrSiO4. It is the most abundant and widely distributed commercial zirconium mineral.1-5 It is found as an accessory mineral in silica-rich igneous rocks such as granite, pegmatite, and nepheline syenite.1-3 Sedimentary and metamorphic rocks also contain zircon but only in small amounts.4 Because of its high specific gravity of 4.6-4.8, zircon is found concentrated with other heavy minerals, e.g., rutile, in river and beach sands with iron as the main contaminant.1-5 Zircon is commonly produced as a byproduct in the mining and processing of heavy-mineral sands such as the titanium minerals rutile and ilmenite.4 Zircon is chemically and thermally very stable. This is due to the high coordination of bisdisphenoid ZrO8 in a tetragonal structure with SiO4 tetrahedra.6,7 Thus, zircon requires aggressive reaction conditions for decomposition.1,4,8 Decomposition of zircon with alkali at high reaction temperatures is a well-known procedure.1,8-13 Alternative plasma furnace processes generally require high temperatures exceeding 1750 °C.1,8,9 Plasma-dissociated zircon still requires milling (d50 < 0.1 µm) to release zirconia and allow rapid leaching. Further wet-chemical processing to achieve a commercially viable product is also required.8,9,14 Zircon on its own has a number of industrial applications.1-4,8 The largest uses are as foundry raw material and as opacifier for ceramic tile glazes and porcelain enamels.2,3 Other uses include refractories and plasma spraying.1-4 The presence of radioactive impurities is a potential problem associated with zircon use.8 For * To whom correspondence should be addressed. Fax: +27 12 811 1174. E-mail:
[email protected].
Figure 1. Simplified block diagram for the De Wet zirconium chemical recovery process.
some applications, concerns about and legislation with respect to radioactive impurities will eventually force a refinement of the raw material. Removal of such impurities from zircon is only possible via chemical decomposition of the zircon lattice.1,8 Owing to dwindling world supplies of baddeleyite (natural zirconia), a greater proportion of zirconiumbased products will in the future have to be obtained from the more abundant zircon sands. Figure 1 shows a simplified block diagram of the alkali fusion process for zirconia from zircon. Scheme 1 outlines the De Wet modification of this zirconium chemical recovery process. The main reactants are caustic soda and milled zircon sand. Sodium silicate solution and zirconia are the two product streams. Unreacted zircon is separated from silica waste using gravity-based separation techniques and is recycled to the process. The waste stream contains sodium salts and other impurities. Not shown are water and acid feed streams used in the process. The main inventive step in this process is the in situ formation of zirconium basic sulfate (ZBS) as a solid phase. This compound is very stable and insoluble in
10.1021/ie020140c CCC: $25.00 © 2003 American Chemical Society Published on Web 01/22/2003
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Scheme 1. Outline of the De Wet Alkali Fusion Process for AZST
dilute mineral acids other than sulfuric. This facilitates the removal of impurities such as iron and radioactive elements by leaching with hydrochloric acid. Compared to conventional processes, advantageous features of the De Wet process are as follows: (a) Fewer process steps are necessary to manufacture acid zirconium sulfate tetrahydrate (AZST). (b) A comparatively low-cost pigment-suitable zirconia with reduced radioactivity content can be made. (c) The alkali reactant can be recovered in the form of a saleable, radioactivity-free, sodium silicate product stream. (d) Sulfuric acid economy in the manufacture of AZST. The overall process requires less than 5% stoichiometric excess of sulfuric acid. (e) Radioactivity is leached out of a solid phase instead of using precipitation steps that require chemical additions and generate much waste.
(f) There are fewer effluent streams and less waste to discard. In this study we report on the optimization of the alkali-fusion step of this process. The experiments described below were designed to provide an indication of the effect that fusion conditions have on the overall product yields in terms of the simplified block diagram shown in Figure 1. The zirconia yield and alkali (NaOH) recovery in the sodium silicate are important process parameters that impact the commercial viability of the overall process. The effect of the operating parameters such as fusion time and temperature and also reagent stoichiometry (NaOH/ZrSiO4 mole ratio) on these variables was determined. Experimental Section Commercial-grade zircon, mined at Richards Bay, South Africa, was obtained from Ferro Industrial Prod-
Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 779 Table 1. Observed Phases in the XRD Spectra of the Products Obtained for 336-h Fusions temp mol ratio of time [°C] NaOH/ZrSiO4 [h] 850
6:1
850
4:1
850
2:1
850
1:1
750
4:1
750
2:1
750
1:1
700
2:1
650
6:1
650
4:1
650
2:1
650
1:1
600
2:1
phases observed in XRD
336 major phase: Na2ZrO3 minor phase: Na2SiO3 336 major phase: Na2ZrO3, Na2ZrSiO5, Na2SiO3 336 major phase: Na2ZrSiO5 minor phase: ZrSiO4, Na2ZrO3 trace phase: Na4Zr2Si3O12 336 major phase: Na4Zr2Si3O12, ZrSiO4 minor phase: Na2ZrSiO5 336 major phase: Na2ZrO3 minor phase: Na2SiO3 trace phase: Na2ZrSiO5 336 major phase: Na2ZrO3, ZrSiO4, Na2ZrSiO5 minor phase: Na2SiO3 336 major phase: Na2ZrSiO5, ZrSiO4 minor phase: Na2ZrO3 336 major phase: ZrSiO4, Na2ZrO3, Na2ZrSiO5 minor phase: Na2SiO3 336 major phase: Na2SiO3, Na2ZrO3 minor phase: Na4SiO4, ZrSiO4 336 major phase: ZrSiO4, Na2SiO3, Na2ZrO3 minor phase: Na4SiO4 336 major phase: ZrSiO4, Na2ZrO3 minor phase: Na2SiO3 strong amorphous halo at ca. 2θ ) 13 336 major phase: ZrSiO4 trace phase: Na2SiO3 336 major phase: ZrSiO4 minor phase: Na2ZrO3, Na2SiO3
ucts. A grade with a median particle size of d50 ≈ 9 µm and with the following composition, as determined by X-ray fluorescence, was used: ZrO2 + HfO2, 65.7%; SiO2, 33.6%; Ti, 738 ppm; Fe, 683 ppm; Ca, 666 ppm; P, 455 ppm; U, 331 ppm; Th, 144 ppm. Analytical-grade caustic soda, 98% sulfuric acid, and 32% hydrochloric acid were used in all experiments. The fusion reactions were carried out in a model TPF 12/2 high-temperature oven. In each fusion experiment, 36.6 g of zircon was fused with caustic. The NaOH/ ZrSiO4 mole ratio was varied from 1 to 6, and fusion temperatures of 600, 650, 700, 750, and 850 °C were investigated. After the required fusion time, the oven product was allowed to cool to room temperature. The fusion product was milled using a mortar and pestle to homogenize the mixture and was fused again. All fusion samples were characterized by X-ray diffraction (XRD) to identify the main compounds formed. The final fusion product was cooled and then weighed before leaching with water to remove the soluble silicates and unreacted sodium hydroxide. The liquids were separated from the solids using centrifugation. The solid was dried in an evaporating oven. The water leach was titrated with a standardized 32% (m/m) analytical-grade HCl solution. Bromomethyl orange was used as an indicator and a calibrated pH meter (Mettler Delta 340) to determine the equivalence point at pH ≈ 3.8. This value provides an indication of the alkali recoverable in the sodium silicate product stream and was expressed as a percentage of the alkali used in the fusion. The resulting hydrated silica was precipitated with an ammonia and ammonium chloride solution by adjusting the pH to 7. The sodium salts were then separated from the hydrated silica by washing and
centrifugation. The hydrated silica was dried overnight in a drying oven at 90 °C and calcined at 950 °C to silica. The silica yield was calculated with respect to the silica in the zircon feed and reported as a percentage. The zirconium-containing residue remaining, following removal of the sodium silicate, was titrated with a standardized 32% (m/m) analytical-grade HCl solution to a pH ≈ 3.8. Repeated washing and centrifugation with a total of 250 mL of distilled water in 4-5 portions removed water solubles, including NaCl. The residue consisted essentially of hydrous zirconia, silica, and unreacted zircon. This was reacted with an excess amount of sulfuric acid (ca. 5% excess) to form AZST. Water and excess sulfuric acid were removed by evaporation by first heating at 150 °C and then at 350 °C. The solids were allowed to cool to room temperature. AZST was dissolved in water and separated from the residue by centrifugation. AZST was then crystallized overnight at 90 °C by water evaporation and calcined to zirconia at 950 °C. The yield was expressed as a percentage of the zirconia in the zircon feed. The solids remaining after the AZST dissolution were dried at 90 °C and then calcined at 950 °C. All of the calcined products (zirconia, silica, and residue) were collected, weighed, and submitted for elemental analysis. All of these fusion experiments were duplicated, and in some instances there were with up to eight replications. The validity of the results was checked using internal mass balances. XRD measurements between 0.8 and 10° 2θ were obtained with a Siemens D-501 automated diffractometer Cu KR (0.15418 nm) with the Soller slits at 2° (diffracted beam side), divergence slits at 1°, and receiving slits at 0.05°. A scintillation counter detector was used, over the range of 3-65° 2θ and at a step width of 0.02°, at a scan rate of 30 s/step. Results and Discussion Extended Fusion Times. Long fusion times (336 h) were used in order to get an overview of the equilibrium fusion products that are likely to form. Table 1 lists the main products identified in the XRD spectra. These data were used to construct the simplified “phase” diagrams shown in Figure 2. The vertical dotted line in these ternary diagrams shows the locus of the NaOH/ZrSiO4 reactant ratio used in this study. The phase diagram for 850 °C is similar to the one accepted in the literature16,17 that was constructed from results obtained by fusing zircon with soda ash at temperatures above 1000 °C. (a) Fusions at 850 °C and 336 h. For fusions at 850 °C, the reaction products observed for a reagent ratio of 2:1 are consistent with the conversion of zircon according to the following reaction:
ZrSiO4 + Na2O f Na2ZrSiO5
(I)
From a purely stoichiometric viewpoint, one could expect that the following reaction would be favored at a mole ratio of 4:1:
ZrSiO4 + 2Na2O f Na2ZrO3 + Na2SiO3
(II)
However, the yields obtained and the XRD spectra recorded show that the products are better explained in terms of both reactions (I) and (II) occurring to approximately the same extent. This implies that higher
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Figure 2. Phases observed in the XRD spectra of fusion products fused for 336 h.
silicates must have formed as well:
ZrSiO4 + 3Na2O f Na2ZrO3 + Na4SiO4
Figure 3. Effect of stoichiometry and fusion temperature on yields. Fusions conducted for 336 h with intermediate milling.
(III)
Thus, the reaction at 850 °C, 336-h fusion time, and a mole ratio of 4:1 may be considered to be a “blend” of reactions (I)-(III):
ZrSiO4 + 2Na2O f xNa2ZrO3 + (2x - 1)Na2SiO3 + (1 - x)Na2ZrSiO5 + (1 - x)Na4SiO4 (IV) From the silica yields reported below, we estimate x ≈ 0.56. Our results indicate that reaction (IV) is applicable for all fusion temperatures above 700 °C with mole ratios above 2:1. The value that x assumes in the resultant product spectrum has important implications for the zirconia yield and the alkali recovery. Na2ZrSiO5 is insoluble in and does not hydrolyze in water.16 Its formation, therefore, prevents full recovery of alkali in the sodium silicate product stream. It must be hydrolyzed using a mineral acid, and this gives rise to a corresponding sodium salt waste stream. On the other hand, it facilitates high recovery of zirconia per mole of sodium hydroxide added. Compared to reaction scheme (II), scheme (I) requires only 2 mol, instead of 4 mol, of sodium to liberate a mole of zirconium as zirconia. Figure 3 shows zirconia yields and alkali recoveries as a function of the stoichiometry used. For a fusion temperature of 850 °C, the zirconia recovery approaches 100% for a 4:1 stoichiometry. The 90% zirconia yield at a 2:1 stoichiometry is attributed to the side reaction (II) that leads to the formation of Na2ZrO3 and Na2SiO3. The observed sodium recoveries for prolonged fusions at 850 °C can be explained as follows. All of the sodium
Figure 4. Effect of fusion temperature on the yield for NaOH: zircon ) 2:1. Fusions conducted for 336 h with intermediate milling.
silicates formed are highly soluble and can be removed by simple water washing. The compounds Na2ZrSiO5 and Na4Zr2Si3O12 are insoluble and remain with the solid phase. The zirconium and silica are only released
Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 781 Table 2. Observed Phases in XRD Spectra Obtained for the 2:1 Stoichiometry Fusions temp mol ratio of time [°C] NaOH/ZrSiO4 [h]
Figure 5. Effect of fusion time on yields. Fusions conducted at 850 °C.
upon hydrolysis with a strong acid. Exhaustive leaching with water hydrolyzes sodium zirconate to hydrated zirconia.1,4,16,17 (b) Fusions at 650 °C and 336 h. No evidence was found for the formation of either Na2ZrSiO5 or Na4Zr2Si3O12 at any stoichiometry for fusions conducted at 650 °C. Thus, reaction scheme (II) describes the main decomposition reaction at this temperature. Figure 3 also shows the zirconia yields and alkali recovery for the 650 °C fusion temperature. Because reaction (I) does not occur at this temperature, high zirconia yields require a higher NaOH/ZrSiO4 mole ratio (Figure 3A). At the same time, however, alkali recovery is always high (Figure 3B). Alkali recovery exceeding 50% is expected for a 4:1 stoichiometry and assuming complete conversion according to reaction (II) as well as partial hydrolysis of the compound Na2ZrO3. The experimentally observed value is ca. 60% and independent of stoichiometry over the range investigated here. In contrast, at a fusion temperature of 850 °C, the sodium recovery is very low (
