Silica, in concentrates

Several silaceous materials have been used successfully to induce flow. Among those quoted in the literature are pyrogenic silica in concentrations as low as 0.25% and hydrated sodium silioaluminate in concentrations of around 0.75%. The former has the additional property of being able to scavenge moisture, which might otherwise contribute to restricted flow characteristics. 

Barker, P., Fontes, J-C., Gasse, F. Druart, J-C. (1994) Experimental dissolution of diatom silica in concentrated salt solutions and implications for palaeoenviron-mental reconstruction. Limnology and Oceanography 39, 99-110. 

From Na2SiO3 and AlCl. I dm of Na.SiO solution was prepared by dissolving 13.5 g of silica in concentrated NaOH. The final pH was 13. This solution of Na2SiO3 and 1 dm of 0.22 M AlCl. solution in HCI were added simultaneously to lOOcm of water with stirring, and the pH = 7 was held by addition of NaOH or HCI. The dispersion was aged for 1 d at room temperature, and then washed with 1 M NaCl and then with water. The solid was dried at 100 C, and then calcined in air for 3 h at 4()() C. 

It is clear that when concentrated solutions of silicates are diluted, more monomer is released almost instantly into the solution. Thus the nature of silica in concentrated silicate solutions cannot be determined from diluted samples. 

Silica in concentrates produces Zn2Si04 in the roaster following the reactions below. 

It is apparent that these differences will be noted mainly in dilute sols containing only a few percent of silica. In concentrated mistures one can distinguish a gel, which is rigid, but not between a coagulate and a flocculate. 

Chemical methods to determine the crystalline content in silica have been reviewed (6). These are based on the solubility of amorphous silica in a variety of solvents, acids or bases, with respect to relatively inert crystalline silica, and include differences in reactivity in high temperature fusions with strong bases. These methods ate qualitative, however, and fail to satisfy regulatory requirements to determine crystallinity at 0.1% concentration in bulk materials. 

Soak silica beads in concentrated nitric acid at ca. 90°C overnight to remove organic impurities. Rinse the beads thoroughly with deionized water until neutral. If the silica beads are colored, repeat this step until they become colorless. 

If silica (in the form of volatile silicic acid) is present in steam at concentrations greater than 0.02 ppm Si02, turbine deposit problems result. In some cases, volatile silica appears not to significantly affect superheaters but certainly will always form deposits in turbines. Silica appears in different forms, depending on the steam pressure, and affects all turbine surfaces, It is most noticeable on the blades, which eventually results in rotor unbalancing. 

Because of the problems relating to silica and silicates, silica levels in the FW and BW must be strictly controlled. This normally is not a problem in higher pressure boilers because various methods of pretreatment are typically employed to reduce the concentration of silica in the FW as much as possible. Automatic monitoring and alarm systems are also provided. 

The concentration of solids, alkalinity and silica in the FW should not be so high as to require BD of BW in quantities exceeding the boiler design value or exceeding the FW pump capacity. 

NOTE The usual standard for maximum silica in steam used for modern turbines is 0.002 mg/kg. To ensure that this limit is not exceeded, the maximum concentrations of silica in BW is as follows ... 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

FIGURE 12.10 Tapping mode atomic force microscopy (AFM) images of the section analyzes of ethylene-propylene-diene monomer (EPDM) rubber-melamine fiber composites. A, composite containing no dry bonding system B, composite containing resorcinol, hexamine, and silica in the concentrations 5, 3, and 15 phr, respectively. 

FIGURE 12.18 Stress-strain curves of rubber-fiber composites developed for solid rocket motor insulator A, ethylene-propylene-diene monomer (EPDM) rubber-carbon fiber composites B, EPDM mbber-melamine fiber composites C, EPDM mbber-aramid fiber composites and D, EPDM rubber-aramid pulp composites. 1 and 2 stands for unaged and aged composites respectively. Carbon fiber- and melamine fiber-reinforced composites contain resorcinol, hexamine, and silica in the concentrations 10, 6 and 15, respectively and aramid fiber- and aramid pulp-based composites contain resorcinol, hexamine, and silica in the concentrations 5, 3 and 15, respectively. (From Rajeev, R.S., Bhowmick, A.K., De, S.K., and John, B., Internal communication. Rubber Technology Center, Indian Institute of Technology, Kharagpur, India, 2002.)... 

Solubilities of quartz and amorphous silica in aqueous solutions increase with increasing of temperature (Holland and Malinin, 1979). Solubility of barite depends on salinity and temperature (Blount, 1977). The solubility of barite in hydrothermal solution having more than 1 molal NaCl concentration increases with increasing temperature, while a solubility maximum exists in the solution with NaCl concentration less than ca. 0.2 molal (Blount, 1977). 

As shown in Fig. 1.143, the system is divided into four reservoirs. Each reservoir corresponding to alteration zone IV, III, II and I is assumed to be homogeneous with respect to temperature and concentrations of dissolved silica in aqueous solution. 

Formation permeability damage caused by precipitation of dissolved minerals such as colloidal silica, aluminum hydroxide, and aluminum fluoride can reduce the benefits of acidizing (132-134). Careful treatment design, particularly in the concentration and amount of HF used is needed to minimize this problem. Hydrofluoric acid initially reacts with clays and feldspars to form silicon and aluminum fluorides. These species can react with additional clays and feldspars depositing hydrated silica in rock flow channels (106). This usually occurs before the spent acid can be recovered from the formation. However, some workers have concluded that permeability damage due to silica precipitation is much less than previously thought (135). 

This method was developed as a second independent method to complement the usual colorimetric procedure in the determination of a certified concentration of dissolved silica in a seawater reference material. Ion exclusion affords a separation of the dissolved silica not only from major seawater cations but also from potentially interfering anions. The detection unit limit, conservatively estimated at 2.3 ng/g Si (0.08. im), is superior to that achievable by direct analysis using inductively coupled plasma emission spectrometry. 

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