Article pubs.acs.org/IECR
Effects of Metal Ions on Crystal Morphology and Size of Calcium Sulfate Whiskers in Aqueous HCl Solutions Xiulong Mao, Xingfu Song,* Guimin Lu, Yuzhu Sun, Yanxia Xu, and Jianguo Yu National Engineering Research Center for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: The effects of metal ions such as Na+, K+, Mg2+, Cu2+, Al3+, and Fe3+ on the crystal morphology and size of calcium sulfate whiskers in aqueous HCl solutions were investigated. The average diameter, aspect ratio, length, crystal water content, phase composition, and elemental composition of the products were determined and compared. When the concentration of monovalent Na+ or K+ ions was low, with the increase in the ion concentration, the average diameter increased and the aspect ratio decreased. However, with further increase in the ion concentration, the average diameter decreased. The presence of divalent Mg2+ or Cu2+ ions increased the crystal diameter and length. All the products were calcium sulfate hemihydrate in the presence of Na+, K+, Mg2+, or Cu2+ ions within the studied concentration range. The presence of trivalent Al3+ or Fe3+ ions at low concentrations resulted in bigger and shorter crystals; furthermore, at higher ion concentrations, partial phase transformation from calcium sulfate hemihydrate to calcium sulfate dihydrate was induced. Among the metal ions investigated, only Na+ ions were found be incorporated into the crystals. presence of Al3+ ions. Moreover, the presence of Al3+ (0.13− 0.54 M) or Fe3+ (0.06−0.35 M) ions increased both the average crystal size and thickness-to-length ratio, consequently changing the morphology from thin plates to thick crystals.20 However, the previous studies mainly focused on the effects of impurities on the nucleation and crystal growth of calcium sulfate and the qualitative analysis of crystal morphology instead of quantitative analysis. Furthermore, there were contradictions among the studies conducted under different conditions. The presence of Al3+ or Fe3+ ions affected the crystal morphology and size.20 However, in the later studies, Li et al.21 and Kruger et al.22 found that the presence of Al3+ or Fe3+ ions did not change the crystal morphology, and the morphological observations were in general agreement with the latest experimental results obtained in concentrated HCl−CaCl2 solutions.17 As discussed above, the previous studies generally focused on the conditions related to specific industrial processes such as HH preparation,11,14,15 phosphoric acid production,12,20−22 and wet FGD.23 The objective of this study was to investigate the effects of impurities under the reported conditions of calcium sulfate whisker preparation. In this study, calcium sulfate whiskers were prepared in the presence of some common metal ion impurities including K+, Na+, Mg2+, Cu2+, Al3+, and Fe3+ ions. Because the performance of calcium sulfate whiskers in different applications is closely associated with the crystal morphology and size (i.e., crystal diameter and aspect ratio),24 the effects of impurities on the crystal morphology and size were investigated. The average diameter, length, and aspect ratio of the products served as the
1. INTRODUCTION Calcium sulfate whiskers, single crystals of calcium sulfate with a high aspect ratio, exhibit excellent performance in mechanical strength, thermal stability, chemical resistance, and compatibility; furthermore, they are inexpensive compared to other types of whiskers. Therefore, calcium sulfate whiskers have great potential as reinforcing agents in rubbers, plastics, papers, etc.1−3 Several methods have been used to prepare calcium sulfate whiskers including hydrothermal reaction,2−4 reactive crystallization,5 recrystallization,6 and the use of organic media.7,8 From the perspectives of environmental protection and economy, it is very attractive to prepare calcium sulfate whiskers using industrial byproducts such as distiller waste, flue-gas desulfurization (FGD) gypsum, and phosphogypsum.3,5,6,9 However, many impurities are present in these industrial byproducts,10,11 and the presence of metal ion impurities significantly affects the nucleation and crystal growth of calcium sulfate, thus changing the crystal morphology.12−15 In addition to the selective adsorption of the impurities on the crystal surface,14,15 the incorporation of the impurities into the crystals has been suggested to play an important role in the morphology modification of calcium sulfate hemihydrate (HH) crystals.16,17 Furthermore, the presence of metal ions significantly changes the crystal size by influencing the phase transitions between the calcium sulfate phases.17−19 Therefore, the effect of impurities should be investigated to control the morphology during the production of calcium sulfate whiskers. The effects of some metal ions such as K+,14 Mg2+,13−15 Cu2+,13 Al3+,12,17,20,21 and Fe3+13,17,20,22 have been well investigated under different conditions. The presence of K+ or Mg2+ ions in concentrated CaCl2 solutions (3.74 M) afforded HH crystals with smoother surfaces and less regular ends.14 Rashad et al.12 studied the effect of Al3+ ions on the crystallization of gypsum under simulated conditions for phosphoric acid production; the gypsum morphology changed from a needlelike shape in the absence of impurities to thick rhombohedron in the © XXXX American Chemical Society
Received: July 29, 2014 Revised: October 11, 2014 Accepted: October 20, 2014
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dx.doi.org/10.1021/ie5030134 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
quantitative indicators of the effects of metal ion impurities on crystal morphology and size.
(Mastersizer 2000, Malvern Co., UK) by dispersing the crystals in anhydrous ethanol.
2. EXPERIMENTAL SECTION 2.1. Materials. Analytically pure chemicals, CaCl2, sulfuric acid (H2SO4), hydrochloric acid (HCl), anhydrous ethanol, NaCl, KCl, and MgCl2·6H2O, were purchased from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. CuCl2·2H2O, AlCl3·6H2O, and FeCl3 were of analytical grade and purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd., Shanghai, China. Deionized water was used to prepare solutions. 2.2. Preparation of Calcium Sulfate Whiskers. Calcium sulfate whiskers were prepared by reactive crystallization. A 500 mL, four-neck, round-bottom flask was used as the reactor equipped with a reflux condenser and stirrer. First, 50 mL of 1 M HCl with or without impurity was added to the reactor. The amount of impurity added was expressed as the molar concentration of the impurity in solution after the reagent addition. An overview of the studied impurity concentration level is shown in Table 1. These were chosen to cover the possible concentration range of the impurities in industrial byproducts.5,10,11
3. RESULTS AND DISCUSSION 3.1. Influences of Na+ and K+ Ions. 3.1.1. Effects of Na+ and K+ Ions on Crystal Morphology and Size. In general, Na+ ions are the main metal ion impurities in industrial byproducts, particularly in the distiller waste from the Solvay process.10 In this study, the studied concentration range of Na+ ions was taken from 0 to 381 mM to simulate the variation of the Na+ ion concentration in the industrial process. Figure 1 shows the variations
Table 1. Concentration of Metal Ion Impurities in Experimental Solutions impurity species
concentration (mM)
Na+ K+ Mg2+ Cu2+ Al3+ Fe3+
3.81, 7.62, 11.4, 19.0, 26.7, 38.1, 76.2, 114, 152, 190, 267, 381 3.81, 7.62, 11.4, 19.0, 38.1 3.81, 7.62, 11.4, 19.0, 38.1 3.81, 7.62, 11.4, 19.0 3.81, 7.62, 11.4, 19.0, 22.9, 26.7, 38.1 1.90, 2.67, 3.81, 7.62, 11.4, 19.0
Figure 1. Effect of Na+ ion concentration on the average diameter and aspect ratio of calcium sulfate whiskers.
in the average diameter and aspect ratio of calcium sulfate whiskers with Na+ ion concentration. The crystals prepared without impurity were whisker-shaped with an average diameter of 10.3 μm and an average aspect ratio of 106.8 (Figure 3a). When a low concentration of Na+ ions (i.e., 3.81−19.0 mM) was added to the solution, the average diameter slightly increased to 10.6 μm and the average aspect ratio decreased to 96.8. The crystals prepared without Na+ ions and with 19.0 mM concentration of Na+ ions did not show any clear diversity in the morphology (Figure 3a,b). With the increase in Na+ ion concentration from 19.0 to 152 mM, the average diameter of the crystals increased up to 13.1 μm, while the average aspect ratio decreased to 64.9. However, with further increase in Na+ ion concentration (≥190 mM), the average diameter of the crystals decreased. These phenomena can be clearly observed in Figure 3c−e. Because the effect of Na+ ions at low concentrations was not significant, from the economic perspective, the raw materials containing a small amount of Na+ ions (e.g.,
