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High-Grade Silica Refined from Diatomaceous Earth for Solar-Grade Silicon Production Masahiko Bessho, Yasuhiro Fukunaka,* Hiromu Kusuda, and Takashi Nishiyama Department of Energy Science and Technology, Kyoto UniVersity, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan ReceiVed April 22, 2009. ReVised Manuscript ReceiVed June 26, 2009
A refining processing of high-purity silica from biogenic diatomaceous earth is newly proposed to exploit the steady and stable resource to the solar-power generation industry. The specimens collected from various representative diatomaceous earth deposits, including both marine and freshwater origin, were chemically analyzed. Trace element distribution in diatomaceous earth was influenced by the biotope habitat of diatoms, when compared to that in quartz of igneous origin. Al, K, and Fe were mainly terrestrially derived, while Si and B were from diatom shells. B content in diatomaceous earth specimens from lacustrine sources was less than that in marine origin. Diatomaceous earth was then dissolved into caustic alkaline solution. With a decreasing pH value, amorphous silica precipitated with impurities. Al and Fe were concentrated in silica precipitated in the pH range of 12.5-10.5, while B was more soluble than silica at pH less than 9. Silica can be precipitated in the pH range of 10.5-9.0, followed by acid leaching to reduce Al and Fe content. A simple chemical operation consisting of extraction, precipitation, and acid leaching has been proposed. Repetition of chemical processing 3 times provides more than 5 N silica from diatomaceous earth samples from freshwatersource rocks.
Introduction A solar-hydrogen energy system is nowadays the most stimulus target for young researchers.1-5 Many technologies in various fields must be closely linked together to make it practical in the energy network system. The further development of photovoltaic power generation based on Si solar cells will requires a huge amount of high-grade quartz, which usually occurs in vugs or pockets in deeply weathered quartz veins and blanket deposits. The reserve of ore deposit is not sufficient because the distribution of deposits in the earth’s crust is physically limited. Other resources steadily supplied to the photovoltaic power generation industry must be considered. Diatomaceous earth is ubiquitous throughout the world and consisting mainly of opaline diatom frustules. It is composed of diatom shells (amorphous silica) associated with a small amount of clay and detrital rocks stemmed from low-cost open pit mining. This light-colored soft friable siliceous rock is porous, with a high absorptive capacity and relatively good chemical stability. It is therefore used primarily for filtration, absorbents, filler, and insulation. By the way, the diatom shell is characterized by easy dissolution in caustic alkaline solution. It suggests that highquality silica may be produced from diatomite using an aqueous * To whom correspondence should be addressed. Telephone: +81-75383-3207. Fax: +81-75-383-3400. E-mail:
[email protected]. (1) Goetzberger, A.; Hebling, C.; Schock, H. W. Mater. Sci. Eng., R 2003, 40, 1–46. (2) Nowotny, J.; Sorrell, C. C.; Sheppard, L. R.; Bak, T. Int. J. Hydrogen Energy 2005, 30, 521–544. (3) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834–2860. (4) Lewis, N. Science 2007, 315, 798–801. (5) Lewis, N.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729–15735.
chemical process. This paper examines a possibility of diatomaceous earth as a new resource of high-quality silica. Experimental Section Samples. Diatomaceous earth is derived from marine or lacustrine diatom frustules.6 A total of 31 diatomaceous earth samples of seawater origin were collected from 4 ore deposits located in Akita Prefecture, and a total of 28 samples of freshwater origin were collected from 2 deposits in Okayama and Oita Prefectures in Japan. a grayish white collection of fossilized skeletal diatoms remains in the range from 3 µm to 100 mm with a variety of perforated shapes. It contains 50-75% water, and the dried samples contained 70-96% amorphous silica (diatom shell) and with small amounts of clay and rock chips. A scanning electron microscopy (SEM) picture of a representative diatomaceous earth sample is shown in Figure 1. Chemical and X-ray Diffraction (XRD) Analyses. Each sample was ground and stirred into a homogeneous mixture prior to chemical analysis. Two kinds of chemical analyses were engaged. One is the total analysis with acid leaching. A total of 1 g of each sample was treated with a mixture of 10 mL of aqua regia, 10 mL of hydrogen fluoride, and 1 mL of hydrogen peroxide. The dissolved solution was evaporated to dilute to 50 mL with distilled water. A total of 25 elements were detected. The other is with alkaline solution. A total of 1 or 10 g of each sample was leached with 2.5 N NaOH in a covered beaker at 90-100 °C for 1 h. The leached solutions were filtered through a 1 µm mesh membrane filter and diluted to 500 mL with distilled water. Si and 12 trace elements were qualitatively detected using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Atomic adsorption spectroscopy (AAS) and ICP-AES techniques must be used on the basis of a careful consideration on their sensitivity analyses. Si, Al, Fe, and K were then quantitatively (6) Okuno, H. Atlas of Fossil Diatoms from Japanese Diatomite Deposits; Botanical Institute, Faculty of Textile Fibers, Kyoto University of Industrial Arts and Textile Fibers: Kyoto, Japan, 1952; pp 50-51.
10.1021/ef900359m CCC: $40.75 2009 American Chemical Society Published on Web 07/16/2009
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Figure 1. Scanning electron micrograph of diatomaceous earth.
Figure 2. XRD patterns of diatomaceous earth sample, No. F4: (a) original sample and (b) residue by dissolving in 2.5 N NaOH. Cr, cristobalite; Fls, feldspar.
analyzed with a Jarrel-Ash AA-8500 atomic absorption spectrograph, and B was quantitatively analyzed with ICP-AES (SPS4000, Seiko Instruments, Inc.). The weight percentages of silica extracted from diatomaceous earth reached 86-98% in the bulk samples. The content of Al, Fe, and K was relatively high in the range of 200-25 000 ppm. B was less than 150 ppm. XRD patterns for the bulk samples as well as the residue after dissolving in 2.5 N NaOH are shown in Figure 2. A broad XRD pattern probably corresponding to poorly ordered silica 2.8-4.5 Å
in size was found in the bulk samples. Cristobalite and feldspar were identified in the leached residue.
Results and Discussion Distribution of Minor Elements. These diatomaceous earth samples stem from two different types of ore deposits, seawater and freshwater origins. Figure 3 compares the minor element concentrations in dissolved diatomaceous earth of each origin.
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Figure 3. Difference of the content of elements in dissolved diatomaceous earth between marine (O) and lacustrine (4) origins. (b and 2) Average content.
Figure 4. Difference of the amount of boron dissolution between seawater and freshwater diatomaceous earth.
The averaged content of Al is about 10 000 ppm, and no difference is seen between both samples. The K content is ranging from 1500 to 8000 ppm in the marine origin sample, while a much larger variation is seen for the lucustrine origin. It is relatively low in the upper parts than from lower in the ore deposits. Sedimentation phenomena may participate in this
result. Fe content covers from 100 to 1500 ppm, and no significant difference of the averaged value is apparently noticed between both samples. A striking difference is found for B distribution. The content is evidently higher in marine-type ore deposits (approximately 120 ppm) than in lacustrine-type ore deposits (approximately
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Figure 5. Dependence of the normalized dissolution ratio of (a) silica and (b) Al, Fe, and B in the biogenic silica solutions. The amount of dissolution of the element at pH 12 corresponds to 1.
20 ppm). Dissolved minor elements should be attributed to either diatoms or rock-forming minerals included in diatomaceous earth. The positive correlation between the abundance of silica and the amount of minor elements is clearly demonstrated, especially for the case of B in the marine origin sample in Figure 4. It suggests that B is mainly dissolved from diatom frustules through a leached operation, while the other minor elements, having no evident correlation with silica content, stem from clay fractions and/or detrital rocks. Moreover, this different distribution certainly reflects the biotope habitat of diatoms. Precipitation Separation. Solution chemistry has been described in many references.7-13 The sharp increase in total dissolved silica over pH 8 is due to the presence of SiO(OH)3in addition to silicic acid. Additionally, the presence of hydrolyzable electrolytes or polyvalent cations allows for a decrease of the solubility of colloidal silica or the formation of insoluble silicates. Iler12 reported that silica particles aggregated three-dimensionally and formed gels with salts in the pH range of 7-10. It is caused by H2O confined in the network structure. A fairly satisfactory separation of the silica precipitate from the
suspension liquid can be performed by filtration. Moreover, silica precipitates dissolve again at pH less than 4. However, the behavior of coexisting minor elements in silica precipitates has not been thoroughly examined. Figure 5 illustrates the measured content of silica and coexisting minor elements, such as Al, Fe, and B, in saturated silica solutions supplied after the leaching operation. The vertical axis is normalized with the measured content at pH 12. More than 90% of Al and Fe contaminants in the solution precipitated by decreasing pH value from 12 to 10.5 in the first precipitation operation and could be disposed as a residue, while about 80% of silica and B were still soluble in the solution. A further decrease of the pH value from 10.5 to 9 in the second operation made it possible to reduce the B content to about 20% of the starting solution at pH 12. No silica was substantially left in the solution. That is, about 80% of silica and 25% of B content associated with a trace amount of Al and Fe precipitate in the pH range between 10.5 and 9.0. Consequently, the introduction of a two-stage operation is fairly effective to decrease the impurity level, as demonstrated in Table 1.
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Table 1. Al and Fe Content in Silica Precipitated by Adjustment of pH diatomaceous earth (sample F4) precipitate in the pH range of 12.5-9.0 precipitate in the pH range of 10.5-9.0
Al (ppm)
Fe (ppm)
20000 3200 1100
10000 170 40
Table 2. Al and Fe Content in Aquagel after Acid-Leaching Treatment Al (ppm) Fe (ppm) precipitated silica aquagel after acid-leaching treatment by 5 N HCl aquagel after acid-leaching treatment by 5 N HNO3 aquagel after acid-leaching treatment by 5 N H2SO4
1900 110 210 500
150 59 34 34
Further Removal of Impurities by Acid Leaching. The impurity level of silica precipitate mentioned above is still rather high. A strong acid-leaching technique was applied. Three kinds of strong acid solution were examined. HCl solution (5 N) is more effective than nitric or sulfuric acid. Moisturized silica (aquagel) was thus leached with 5 N HCl for 1 h at 60 °C. The results are demonstrated in Table 2. The effectiveness of acid leaching is apparent. Refining Process to 5 N Silica. Preliminary experiments demonstrate the effectiveness of solution chemistry to refine the diatomaceous earth to higher grade silica. A repetitive procedure in combination with dissolution, precipitation, and acid leaching 3 times is proposed, as illustrated in Figure 6. Diatomaceous earth of lacustrine origin from Shonai deposits in Oita Prefecture was supplied to the refining process with chemical treatment. Purified silica contains 2.6 ppm Al and 4.3 ppm Fe. The other 22 elements, which were found in the range
Table 3. Minor Element Content in a Starting Material of Sample F4 and Its Refined Silicaa element
starting material content (µg/g)
refined silica content (µg/g)
Li B Na Mg Al P K Ca Ti V Cr Mn Fe Co Ni Cu Zn Sr Y Zr Ba La Ce
5 200 2000 500 20000 100 500 4000 1000 50 5 300 10000 5 2 10 10 50 5 20 50 5 5
NDb
