Excess Silica Research Articles

Synthesis and characterization of mesoporous silicon carbide

A modified sol–gel method is proposed for the preparation of mesoporous silicon carbide. In this method, tetraethoxysilane (TEOS) and phenolic resin are used for preparing a binary carbonaceous silicon xerogel, and nickel nitrate is employed in the sol–gel process as a pore-adjusting reagent. SiC was produced in the carbothermal reduction of the binary xerogel at 1250 °C in an argon flow (40 cm 3 min −1) and purified by removing excess silica, carbon and other impurities. The purified SiC sample was characterized by XRD, SEM, TEM, and N 2 adsorption, and the results showed that the SiC sample had a surface area of 112 m 2 g −1 (BET) and an average pore diameter of about 10 nm.

Silica recovery from Sumikawa and Ohnuma geothermal brines (Japan) by addition of CaO and cationic precipitants in a newly developed seed circulation device

With the objective of preventing silica scaling, several types of chemical compounds such as silica gels, clay minerals, chlorides of Ca, Mg, Al and Fe, and calcium compounds were first examined to remove the excess silica in the Ohnuma geothermal brines. The total silica concentrations of silica in the brines after reaction by addition of these chemicals were observed to decrease linearly with an increase in the reagent concentration. Quicklime (CaO), added at dosages of 0.5 and 1.0 g/l, can remove excess and most silica at 90 °C, respectively. These results indicate that the silica concentration of Ohnuma brines (520 mg/l initial concentration) can be controlled by the amount of quicklime added, and that low-temperature resources can be used without scaling. Continuous flow tests were then performed at Ohnuma and Sumikawa fields by addition of CaO or cationic precipitants in a newly developed seed circulation device (flow rate of 10 l brine/min). At Ohnuma, the total silica concentrations of brines after reaction with CaO showed almost constant values: 10, 200 and 300 mg/l for additions of 1.0, 0.65 and 0.50 g CaO/l, respectively. At Sumikawa, 11 experiments were run, adding cationic precipitants. The total silica concentrations decrease with the increase in reagent concentrations (25–100 mg/l) and with an increase in the retaining time (15–120 min). With the addition of CaO (2 g/l) to Sumikawa geothermal brines with a retaining time of 120 min at 90 °C, the total silica concentration was almost non-detectable after 10 h from the start of the reaction. After reaction for 118 h, addition of CaO was suspended and only silica deposits were reused through the circulation pipe to desilicate brine. After this treatment, the total silica concentration quickly increased to 400 mg/l and remained constant for 210 h before increasing to 650 mg/l after 265 h. There are good correlations between the zeta potentials, a measure of colloidal stability, and the Si/Ca atomic ratios of the silica deposits and amounts of silica removed from brines at Ohnuma and Sumikawa. The CaO addition and circulation method (SACS method) is a very useful way of removing excess silica in brines for silica scale prevention.

地熱水から回収したシリカの高強度セメント用混和材適用評価試験

Silica recovered from Sumikawa geothermal brines by reaction with cationic precipitant was examined to the industrial application as admixtures to high strength cements. The silica deposits from brines consist of > 90 % silica and H2O with minor amounts of Al, Ca and Na. They were converted to silica fume (spherules of 1 to 10 m in diameter) by heating at 180°C in a spray-dryer. Then, the products were heated up to 900°C and sintered in an oven for 2 hours. During this process, the zeta potentials of the products drastically changed from positive to negative values at 300°C in calcination temperature. The sintered products were mixed with cement and the strength of the hardened body exceeds > 100 N/mm2. These results show that the fumed silica converted from excess silica in geothermal brines can be applied as admixtures to high strength cements.

Structure and composition of silicon-stabilized tricalcium phosphate

Silicon stabilized tricalcium phosphate [Si-TCP] is formed within the calcium hydroxyapatite (HA)—tricalcium phosphate (TCP) system when a stoichiometric precipitate of hydroxyapatite is fired at 1000° in the presence of SiO 2. This paper proposes a composition range and crystallographic structure for Si-TCP. Reitveld XRD powder diffraction, transmission electron microscopy, infrared and proton nuclear magnetic resonance measurements show that crystalline Si-TCP is associated with the displacement of OH from an initial hydroxyapatite structure. The resulting calcium phosphate is modified by the incorporation of silicon into its structure with excess silica contributing to an amorphous component. Si-TCP has a monoclinic structure with a space group P2 1/a akin to α-TCP with estimated lattice constants of a=12.863±0.004 Å, b=9.119 ±0.003 Å, c=15.232±0.004 Å, β=126.3±0.1°. It is proposed that Si 4+ substitutes for P 5+in the TCP lattice with the average chemical composition of Si-TCP set primarily by the mechanisms available for charge compensation. While the formation of OH vacancies in HA initiates the transformation to Si-TCP, two mechanisms of charge compensation in the Si-TCP structure are plausible. If O 2− vacancies provide charge compensation, the composition of Si-TCP is Ca 3(P 0.9Si 0.1O 3.95) 2 derived for the addition of 0.33 mol SiO 2:mol HA. If excess Ca 2+ compensates, the composition is Ca 3.08(P 0.92Si 0.08O 4) 2 derived for the addition of 0.25 mol SiO 2:mol HA. The reaction occurs most effectively when SiO 2 is added as a colloidal suspension rather than by the in-situ thermal decomposition of a silicon metallorganic compound. The material is a bioceramic of major biological interest because of its osteoconductivity and unique influence on skeletal tissue repair and remodeling.

Development of ceramic tiles from common clay and blast furnace slag

With an objective of making ceramic tile, granulated blast furnace slag (GBFS) and common clay were mixed whereby calcia:silica ratio (w/w) varied from 0.04 to 0.86. Calcia and silica combined to form wollastonite as the major phase in the sintered microstructure. Excess silica combined with magnesia, iron oxide and alumina to form enstatite, fyalite and alumino silicates respectively during sintering. Optimum compositions were found to be where calcia-silica ratio were in the range of 0.1–0.3 where mechanical strength and water absorption of the fired specimen were in the range of 28–38 MPa and 2.5–0.1%, respectively. Physical properties of the sintered specimen are explained on the basis of XRD and SEM analysis. Formation of wollastonite in the sintered compacts with finer grain size was found to be an important parameter for increase in strength.

Some Problems of Clastogenic Mica Transformation during Anchimetamorphism (Metagenesis)

The transformation of clastogenic muscovite and biotite from the Tyumen superdeep well was studied with the SEM and microprobe techniques. It was shown that under conditions of anchimetamorphism (metagenesis), the white mica becomes unstable and may be replaced by several newly formed minerals. In particular, the brammallite, a sodic analogue of illite, is formed at the expense of sodic component of muscovite. The chloritization of mica provokes the removal of excess silica, which is present in the anchizone as a readily soluble in water modification rather than quartz. The release of excess silica as a result of clastogenic mica destruction may be a source of rock silification. As follows from the review of experimental data on silica solubility, the replacement of many silicate minerals leads to the formation of excess silica. This process can be highly extensive during the breakdown of clastogenic minerals.

Smectite Diagenesis in Neogene Marine Sandstone and Mudstone of the Niigata Basin, Japan

AbstractIllitization of smectite during progressive burial diagenesis occurs differently in sandstone and mudstone, which are interbedded in the sedimentary sequence of the Niigata basin. Reaction progress of illitization of smectite via mixed-layer illite-smectite (I-S) in the mudstone is more complete than in the sandstone. In sandstone, smectite converts to (Reichweit, R ≥ 3) I-S and illite via random (R = 0) I-S to ordered (R = 1) I-S, and authigenic chlorite and quartz form as products of the illitization of smectite.The original composition of detrital smectite and the occurrence of Na+, K+, Ca2+, Mg2+, and Fe2+ in pore fluids partly control both illitization of smectite and the resulting authigenic mineral products in the diagenetic process. In mudstone, detrital smectite is K- and Si-rich in composition, and the illitization of smectite indicates that the original composition is mostly inherited. Excess silica owing to illitization is released to produce authigenic quartz. In sandstone, smectite forms primarily by precipitation. The evolving compositions produced by early illitization form smectite, then random (R = 0) I-S, and then ordered (R = 1) I-S. These transitions are related to compositions of pore fluid. Changes in K/(K + Ca + Na) vs. K + Ca + Na imply that the increase of interlayer cations occurs by absorption and smectite transforms to random (R = 0) I-S, followed by the exchange of interlayer cations to ordered (R = 1) I-S with increasing diagenetic grade. Late illitization from (R = 1) I-S to R ≥ 3 suggests decomposition of smectite and early I-S with an increase in the number of illite layers. Dissolution experiments of host rocks with pure water and 0.01 M HCl solution reflect the differences in chemistry of the original pore fluid and authigenic carbonate in the process of diagenesis of clastic rocks. These results explain how chemical composition produces large variations in transformation temperature of smectite to illite in the diagenetic process.

Fabrication and microstructure of sialon-bonded silicon carbide

The sintering behaviour of ceramic-matrix composites consisting of a sialon matrix and silicon carbide particulate reinforcement has been studied in order to elucidate the densification behaviour and the formation of the microstructure. The materials are produced from a mixture of clay, silicon nitride and yttria sintering additive mixed with 60% of silicon carbide grit. The results lead to a description of the mechanism of phase formation which consists of the following steps: 1. Decomposition of china clay and formation of mullite and silica on heating. 2. Formation of liquid phase from these reaction products, together with excess silica from silicon carbide and yttria sintering additive. 3. Dissolution of silicon nitride in the liquid and precipitation of sialon X and O′ phases. 4. Formation of a high-yttrium, nitrogen glass on cooling. Composites of this kind with porosity of about 14% and zero linear shrinkage on firing, due to the rigid skeleton of carbide particles, are promising candidate materials for high-temperature applications where the microstructure offers the prospect of superior high-temperature strength through the continuous interlocking texture of the X-phase and O′-silicon oxynitride crystalline phases, the carbide skeleton and the high viscosity yttrium-nitrogen glass matrix. ©

Modeling the Durability of Silica Additives Proposed to Protect Nuclear Waste Glass Packages in A Geological Repository

AbstractSilica additives have been proposed to protect nuclear waste glass packages in deep geological repositories by presaturating silicon adsorption sites* in the media surrounding the package (engineered barrier, canister and overpack corrosion products). The durability of such additives was estimated using a one-dimensional dissolution-transport model developed at the CEA's Rhône Valley Research Center for the French reference glass in typical disposal sites. The silica additive would be placed between the wasteform and the overpack and thus dissolved before the waste glass. Its lifetime is proportional to Madd2(where Madd is the added silica mass) up to a critical value Mcrit at which it increases considerably (from 103 or 104 to 108 years) due to saturation of the adsorption sites. Excess silica consumption is then limited by the typically very low (3 × 10−3 l·m−2y−1 ) groundwater flow in the geological site. After consumption of the added silica, the evolution of the altered waste glass fraction is comparable to the evolution without added silica if Madd « Mcrit, but much lower if Madd ≈ Mcrit or Madd > Mcrit. These findings were confirmed by some experimental evidence, and establish that a few kilograms of added silica for a 400 kg package can considerably enhance the waste glass durability.

The ubiquitous presence of silica‐rich glass inclusions in mafic minerals: Examples from Earth, Mars, Moon and the aubrite parent body

Abstract— Highly silicic glass inclusions are commonly present in mafic minerals of xenolithic terrestrial upper mantle rocks (Schiano and Clocchiatti, 1994). They are believed to be the products of volatile‐rich silicic melts for which several sources have been proposed (Francis, 1976; Frey and Green, 1974; Schiano et al, 1995), but their origin(s) and, consequently, that of the glasses, remains unknown. However, in situ formation by very low‐degree partial melting seems to be possible as has been shown by experiments (e.g., Baker et al, 1995; Draper and Green, 1997).Glass inclusions of silicic chemical composition are also present in some mafic minerals of achondritic meteorites (e.g., Fuchs, 1974; Okada et al, 1988; Johnson et al, 1991). The enstatite achondrites (aubrites) Aubres and Norton County, which record early planetesimal and planet formation in the solar nebula, and the olivine achondrite (chassignite) Chassigny, a rock believed to originate from Mars, contain abundant glass inclusions in their main minerals enstatite and olivine, respectively. Glasses of glass‐bearing inclusions have a highly silicic and volatile‐rich chemical composition similar, but not identical, to that of glass inclusions in terrestrial upper mantle peridotite minerals. Furthermore, glass inclusions in olivines from the Moon (e.g., Roedder and Weiblen, 1977) are also silica‐rich.Because different physicochemical conditions prevail in the source regions of these rocks, the process of melting is, perhaps, not generally applicable for the generation of silica‐rich glasses. Alternatively, the glasses could have been formed via precipitation from silicate‐loaded fluids (Schneider and Eggler, 1986) or vapors. Another possible mechanism, not previously identified, could be dehydrogenation of nominally nonhydrous mafic minerals by heating or depressurization that should be accompanied by expulsion of excess silica and incompatible elements. This process will mimic low‐temperature, very low‐degree partial melting. It could account also for the highly variable glass/bubble ratios observed in glass inclusions in aubrite enstatites. We suggest that such a process could have been operating in the solar nebula, the Moon and Mars, and could be operating still on Earth.

Partitioning of Water Between High Pressure Phases in the System MgSiO3 + 0.5 wt.% H2O

Recent experimental studies indicate that a substantial amount of water can be dissolved as hydroxyl in most of the nominally anhydrous minerals of the Earth's mantle. Among these phases, magnesium silicate perovskite is believed to constitute about 80% by volume of the lower mantle. Consequently, the physical and chemical behaviour of this phase is of major importance when assessing the properties of the deep mantle. But still, its water content is very poorly known, although trace amounts of OH could greatly affect mantle properties such as melting, transport properties and rheology. The amount of water that can be recycled via subduction zones is also essential to understand the global cycle of water in the Earth. Therefore, we conducted water partitioning experiments in the system MgSiO3 + 0.5 wt.% H20 + excess silica using a multi-anvil apparatus. Bayerisches Geoinstitut, Universitaet Bayreuth, D-95440 Bayreuth, Germany

Removal of Silica from Geothermal Brine by Seeding Method Using Silica Gel.

In order to apply a seeding method to the removal of silica from geothermal brines, the effects of temperature, initial seed concentration and initial silica concentration on the silica removal were experimentally examined. When silica gel seeds are added to a supersaturated silica solution, amorphous silica is precipitated on the pore surface of the seeds and the silica concentration in the solution decreases. The silica removal ratio, which means the ratio of the removed silica to the excess silica, increases with increasing initial seed concentration and increasing temperature. A high silica removal ratio is obtained for relatively low initial silica concentrations (≤0.7 kg-SiO2·m–3). When the initial silica concentration is higher, the silica removal ratio is expected to be enhanced by increasing initial seed concentration. The seeding method is suggested to be effective for the removal of silica in actual geothermal brines.

Formation of WSi<sub>2</sub> Powder by Carbothermal Reduction Method

The preparation of WSi2 powder by a carbothermal reduction process was investigated using ammonium paratungstate, quartz and active carbon in Ar+H2 atmosphere. Excess silica and carbon were required for producing WSi2 without W5Si3, WC and SiC. Increase only in the amount of carbon led to increase in WC and SiC contents. WSi2 single phase was obtained by adjusting the amounts of silica and carbon at W:Si:C=1:4:8. The formation of WSi2 became rapid at 1400°C. It was found that WSi2 was formed via WC, and SiO vapor took part in the reaction. Wet-mixing of tungsten source by using ammonium salt solutions was effective to produce fine WSi2 powder with a particle size of about 1μm.

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples. The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of γ-Al2O3 spinel and mullite, or simple γ-Al2O3 spinel, which subsequently transformed to mullite at 1260°C. Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of γ-Al2O3 spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.

Optimal parameters for the synthesis of the mesoporous molecular sieve [Si]-MCM-41

Highly crystalline MCM-41 with a very narrow pore-size distribution (FWHM=1.5 A), high surface area (1185 m2 g -1 ), large grain size and thick channel walls (ca. 17 A) was prepared in alkali-free media. The properties of the product depend on the source and concentration of the reactants, the gel aging time, the temperature and the duration of the synthesis. There is no induction period in the course of the synthesis and Ostwald's rule of successive transformations applies. The initially produced hexagonal phase is transformed into the lamellar phase and then into an amorphous phase. In the 150°C synthesis the most stable product is amorphous silica. The course of the synthesis is conveniently monitored by pH measurement. Gel aging, during which a spatial distribution of silicate polyanions and micellar cations is established, is essential for preparing high-quality MCM-41. Surfactants with the same cationic organic group but different counter-anions alter the course of the synthesis. The degree of polymerization of silica is also important. Highly basic gels favour the lamellar product; when the gel is weakly basic the quality of MCM-41 is lower as insufficient TMAOH is available to dissolve the silica. The result of excess silica is similar but even more pronounced. Purely lamellar products are made at low SiO 2 concentrations, when the gel is more strongly basic. The best quality MCM-41 is prepared from a gel of molar composition SiO 2 :0.19 TMAOH:0.27 CTABr:40 H 2 O (with CTABr/SiO 2 =0.27, similar to the ratio in the solid product) aged at 20°C for 24 h and synthesis lasting for 48 h.

Porous mullite honeycomb by hydrothermal treatment of fired kaolin bodies in NaOH

A conventional hydrothermal treatment with various concentrations of NaOH was used at 150° and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin body. The effect of hydrothermal treatment time on the dissolution of the glass was examined. At 150°C, the dissolution of glass was almost complete after treatment for 8 hrs in 5N-NaOH solution and about 40–43 wt% of the glass was removed from the fired kaolin body leading to porous mullite. However, when the fired kaolin body was treated for more than 5 hrs in 5N-NaOH at 190°C, a composite of mullite and a nonporous crystalline phase of unknown symmetry resulted. These crystals formed from the dissolution and recrystallization of the glass. After the dissolution of glass in 2N-NaOH solution at 190°C for 5 hrs, a porous mullite body of 52.8% porosity with an average pore diameter of 0.57 μm could be obtained, and this was only composed of mullite whiskers. Growth of unidentified nonporous crystals in the body which was treated in 5N-NaOH solution at 190°C led to a decrease in specific surface area and therefore, these crystals should be avoided.

Reduction of iron ore with coal by microwave heating

Microwave heating was investigated because it would largely solve the problem of slow heat transfer during reduction inherent in conventional reduction processes. Microwave heating and reduction tests were run on composite green balls made from coal and magnetite concentrate. Bench scale tests were run using a 15 KW microwave generator on both powdered and pelletized feeds. A significant increase in the reduction rate was obtained. Over 90% reduction of the iron ore can be achieved in about 10 minutes at temperatures about 1,000 C with 20% excess stoichiometric carbon in the mix. The reduction of iron ore with coal by microwave heating produces a pure, dust-free, and high Btu flue gas that can be used in related thermal operations. Depending upon the purity, the reduced iron product could be fed directly to electric arc furnaces for melting. Removal of a slag phase to remove excess silica and alumina may be required in some cases.

Conventional- versus microwave-hydrothermal leaching of glass from sintered kaolinite to make porous mullite

A microwave-hydrothermal treatment with 1 and 5N-NaOH solution was used at 150°C and 190°C to dissolve excess silica glass and thus make porous mullite ceramics from a fired New Zealand kaolin honeycomb. The effect of microwave-hydrothermal (M-H) treatment time on the dissolution of the glass was examined and compared to the result of conventional-hydrothermal (C-H) treatment. As expected, the rate of the dissolution of glass was faster with M-H treatment compared to C-H treatment. The dissolution of 40–43% glass was almost complete after M-H treatment in 1N-NaOH for 6 hrs at 150°C, in1N-NaOH for 3 hrs at 190°C, in5N-NaOH for 1.5 hrs at 150°C, and in 5N-NaOH for 35 minutes at 190°C. When the M-H treatment in 5N-NaOH solution at 190°C was more than 45 minutes, nonporous prismatic crystals were formed on the honeycomb and these crystals decreased the specific surface area. There were no changes in the morphology of mullite whiskers or in the structure of porous mullite body after M-H treatments in NaOH solutions compared to that of C-H treatment. The M-H treatment has been shown to be a rapid technique to prepare porous mullite ceramics from fired kaolin honey comb.

Growth of Mullite Whiskers on Alumina Particle by Thermal Decomposition of Clay Minerals

Whisker formation on alumina particles was attempted through growth of a number of fine mullite whiskers by the thermal decomposition of clay for porcelain at 1400 to 1600°C. After firing, the excess silica phase was hydrothermally dissolved in 2N NaOH solution at 180°C. The effect of firing temperature, and compositions of alumina and clay on the formation of mullite whiskers on alumina particles was investigated. In case of 18.8mass% alumina and 81.2mass% calcined clay, the growth of whiskers enhanced by the decomposition of clay was promoted, and alumina particles were coated with a number of mullite whiskers 50 to 200μm in length and 1 to 3μm in diameter just like sea urchin. Furthermore, alumina particles reacted with the excess amorphous silica in the fired clay to form small pillarlike mullite crystals in an alumina particle. But growth of whiskers was inhibited with increasing amount of alumina.

Silicon nitride ceramics with celsian as an additive

Four different Si3N4 ceramics using 10 wt% celsian (BaAl2Si2O8) as an additive, have been prepared by hot pressing. Different celsian sources and hot-pressing conditions were used and their effects on densification, in situ crystallization of celsian, α to β phase transformation, microstructures and properties, were examined. The use of a presynthesized celsian as a raw material was found to enhance the rates of densification and phase transformation due to the excess silica from raw Si3N4 powders which, however, did not change the microstructure and crystallization behaviour of celsian. Increases in hot-pressing temperatures and times increased the total number of large elongated β grains and resulted in coincidental increases in strength and toughness until the α to β transformation was complete. An intermediate quench-and-reheat step during hot pressing made the microstructure finer and more uniform by greatly reducing the large-sized elongated β grains. The quenching step also disrupted complete crystallization of celsian which led to more grain-boundary glassy phase and compromised the material's high-temperature properties.