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Analytical Study of the Formation Process of Hemimorphite-Part I -Analysis of the Crystallization Process by the Co-Precipitation Method

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Analytical Study of the Formation Process of Hemimorphite-Part I -Analysis of the Crystallization Process by the Co-Precipitation Method Zairyo-to-Kankyo, 42, 225-233 (1993) 論文 Analytical Study of the Formation Process of Hemimorphite-Part I -Analysis of the Crystallization Process by the Co-precipitation Method- Hideki Nagata*, Morio Matsunaga** and Kunisuke Hosokawa** * Department of Biotechnology & Water Treatment, TOTO Ltd. ** Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology In order to verify the mechanism of the formation process of hemimorphite (Zn4Si2O7(OH)2.H2O) in galvanized steel pipe for water service, the synthesis of hemimorphite was performed through the application of a co-precipitation reaction, which was named the co-precipitation method. Sodium metasilicate (Na2SiO3.9H2O) solution was mixed with Zn(OH)2 colloidal solution. The precipitate obtained from the mixture was aged in aqueous solution to crystallize it into hemimorphite. The samples which were aged for the prescribed periods were analyzed using X-ray diffraction, infrared spectroscopy and thermal differential analysis. It became clear from the analysis that at the initial stage of hemimorphite formation, dissolved silicate was adsorbed onto the colloidal particles of Zn(OH)2 and precipitated with Zn(OH)2 particles; at the next stage, the adsorbed silicate broke the structure of Zn(OH)2 to form an amorphous compound as a precursor of hemimorphite; and at the final stage, rearrangement of atoms proceeded over a long period to form the framework of the hemimorphite. Furthermore, it was suggested that the OH bond of the Zn-OH-Zn bridge in the hemimorphite crystal was hard to form compared with the other bonds in the crystal. Key words: galvanized steel pipe, water supply system, corrosion product, hemimorphite, dissolved silicate, adsorption, zinc hydroxide, amorphous compound, crystallization, synthe- sis, co-precipitation. 1. Introduction change of the crystal structure to 13-Zn2SiO4. It is known that hemimorphite (Zn4Si207(OH)2 This peak was sharp and strong and the tem- H20) is naturally found with other zinc miner- perature ranged from 630°C to 790°C. The als in zinc ore under the supergene environment exothermic peak corresponds to the transition and is stable below 250°C1' . Thermal analysis of the crystal structure from jl-Zn2SiO4 to a- studies of hemimorphite2have shown that the Zp2SiO4, willemite. This peak was fairly broad differential thermal analysis curve for hemi- and moderately strong, from 835°C to 971°C. morphite exhibited two kinds of endothermic Several diffraction crystallographic studies6 peaks and one exothermic peak between 100°C have revealed the crystal structure of hemi- and 1000°C. The first endothermic peak morphite and its structural changes with the corresponds to the loss of water of crystalliza- dehydration process. The framework of the tion. This peak was flat and weak. The tem- hemimorphite crystal consists of the combina- perature at which this peak appeared varied tion of an Si04 tetrahedron and/or a Zn(OH)03 in each paper and ranged from 175°C to 657°C tetrahedron. In the framework, two Si04 in all the papers. The second endothermic tetrahedra form pyrosilicate, Si207, and two peak corresponds to dehydroxylation and partial Zn(OH)03 tetrahedra form the zinc polyhedra, Zn2(OH)06. The water molecule iss located in * 2-1-1 , Nakashima, Kokurakita-ku, Kitakyushu, 802 Japan the cavity which is surrounded by 6- and 8- ** 1-1 , Sensuicho, Tobata-ku, Kitakyushu, 804 membered tetrahedra rings, Zn4Si2(OH)2016 and Japan Zn4Si4(OH)4020, respectively. The first dehydra- 226 Zairyo-to-Kankyo (A) (B) Fig. 1 IR spectra of aged samples. (A) aging time: 0 hr-6 days, (B) aging time: 15 days-120 days. tion occurs by expelling the water molecules crystallization process was elucidated using through the channelways which are formed by Fourier-transform infrared spectroscopy(FT-IR), interconnected cavities along the c-axis of the differential thermal analysis(DTA) and X-ray cell. The second dehydration induces the diffraction(XRD). rearrangement of ZnO4 and SiO4 tetrahedra and leads to the structural change to the j3- 2. Experimental Zn2SiO4 crystal. The amorphous compounds which were crys- Spectroscopic studiesiohave shown that tallized to hemimorphite were prepared based the specific bands, which were attributed to on literature data13' . Zinc sulfate aqueous the vibrations of the pyrosilicate groups, the solution (0.33 mol dm-3) was added drop by OH groups, or the molecules of water of drop with continuous stirring into 750 cm3 of crystallization, were observed between 4000 sodium hydroxide aqueous solution (0.6 mol cm-' and 250 cmrl in the Raman or infrared dm-3). A white precipitate of Zn(OH)2 formed (IR) spectrum of the hemimorphite crystal. when the zinc sulfate solution was added but We reported in a previous paper12' that disappeared slowly on stirring. When the hemimorphite was observed as a layer-like entire solution became translucent white, the compound on the interior surface of the addition of zinc sulfate solution was stopped. galvanized steel pipe in water service and the Subsequently, 50 cm3 of sodium metasilicate formation of the hemimorphite layer on the (Na2SiO3.9H2O) aqueous solution (0.33 mol dm-3) zinc coating prevented the anodic oxidation was slowly added to the Zn(OH)2 colloidal of zinc. In the present study, we obtained a solution with continuous stirring. The solution sufficient amount of the homogeneous pre- became milky white. Zinc sulfate solution cipitate mainly by the technique of Visser and was added further to the milky solution until van Aardt13' and aged it in aqueous solution the added zinc sulfate solution reached a total to crystallize it into hemimorphite. The of 100 cm3. The mixed volume ratio of zinc Vol. 42, No. 4 227 (A) (B) Fig. 2 DTA curves of aged samples. (A) aging time: 0 hr-6 days, (B) aging time: 15 days-120 days. sulfate solution to sodium metasilicate solution of the solution with that of the Zn/Si=2 mix- was determined to be Zn/Si=2, which value is ture. Each mixture contained five times the equal to the stoichiometric ratio of hemimor- concentration of the Zn or Si compared with phite. The pH of the milky solution, which the Zn/Si=2 mixture. The precipitates obtain- was about 13, was not adjusted. After the ed from the mixtures were aged in the same precipitate settled, the supernatant liquid was manner. All chemicals used in experiments discarded by decantation. The residue was were reagent grade and were dissolved in dis- transferred to a water bath after three cleaning tilled water. operations by decantation and was kept at 85°C. The white powder obtained before aging as A white powder was obtained by filtration well as after aging was analyzed using FT-IR, after the prescribed aging period and was dried DTA, and XRD. The DTA measurement was at 50°C in an air oven. In order to evaluate carried out in an N2 atmosphere with an a- the influence of the mixed volume ratio of zinc A1203 reference at 10°C/min from room tem- sulfate solution to sodium metasilicate solution perature to 1000°C. The FT-IR measurement upon the crystallization of hemimorphite, an was done with 13 mm~b KBr pellets at a wave- excess Zn mixture (Zn/Si=10) and an excess Si number resolution of 4 cm-1 between 250 cm-1 mixture (Zn/Si=2/5) were prepared, and those and 4000 cm-1. The XRD measurement was solutions were adjusted to pH 13.1 by addition done with CoKa radiation at a scan rate of of NaOH or H2SO4 solution to equalize the pH 2°/min between 28=10° and 90°. In order to 228 Zairyo-to-Kankyo interpret the DTA curves of the aged sample , some of the samples were analyzed by FT-IR and XRD after the DTA measurements . The filter cake obtained from the Zn(OH)2 colloidal solution before addition of silicate was analyzed using DTA, FT-IR, and XRD to confirm the formation of zinc hydroxide in the colloidal solution. 3. Results The precipitates obtained from the Zn/Si=2 Fig. 3 IR spectrum of -Zn(OH)2. mixture were aged for the prescribed periods between 0 and 120 days. Fig. 1 shows the representative IR absorption spectra of the aged Fig. 2 shows the representative DTA curves samples with the IR absorption spectrum of for the aged sample and the hemimorphite stand- the standard mineral (STD spectrum) of hemi- ard mineral. The DTA curve of the hemimor- morphite obtained from Mina Ojuela, Mexico. phite standard (STD curve) exhibited a sharp, The precipitate before aging and its aged strong endothermic peak at 710°C and a broad samples exhibited similar IR spectra up to 18 exothermic peak at 925°C. No endothermic hrs of aging time. The spectrum of the 18 hr peak was observed below 600°C. For the unaged aged sample showed the three different broad sample, no distinct peak was observed except bands which have peak tops at 3355 cm-1, 925 for a broad endothermic peak around 115°C. cm-1, and 555 cm-1, and two very weak, broad After 18 hrs, the broad endothermic peak bands at 1655 cm-1 and 375 cm-1. It was ob- shifted to a slightly higher temperature and served after 24 hr aging that the 925 cm-' two exothermic peaks appeared, the first one broad band of the spectrum of the 18 hr aged at 700°C and the second one at 840°C. After samples split into another three bands at 1100 24 hrs, the broad endothermic peak spread out cm-1, 930 cm-1, and 870 cm -1, which bands are further around 160°C, and the two exothermic some of the specific bands of hemimorphite.
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