Silica content

Not all rocks shatter well by impact. Impacl breaking is best suited for the reduction of relatively nonabrasive and low-silica-content materials such as hmestone, dolomite, anhydrite, shale, and cement rock, the most popular application being on limestone. 

Figure 1 from GPSA shows how the silica content of The American Boiler Manufacturers Association... 

Figure 1. Relationships between boiler pressure, boiler water silica content, and silica solubility in steam.

Bulk Samples Bulk samples must be submitted for all silica analyses. They have two purposes (1) For laboratory use only, to confirm the presence of quartz or cristobalite in respirable samples, or to assess the presence of other substances that may interfer in the analysis of respirable samples. (2) To determine the approximate percentage of quartz (or cristobalite) in the bulk sample. A bulk sample submitted "for laboratory use only" must be representative of the airborne free silica content of the work environment sampled otherwise it will be of no value. The order of preference for an evaluation is ... 

Diatomaceous earth, widely-known and long-used as a filteraid in process and waste filtrations, has a high microcrystalline silica content. As well as being a respiratory hazard in the workplace, the silica is being scrutinized in some jurisdictions as a potentially hazardous dust in landfills in which spent filter cakes are deposited. 

The silica content of water only becomes important if it is a large proportion, i.e. more than 10 per cent, of the TDS. This is unusual in normal water supplies, but can result from external water treatment (see below). 

Another mineral constituent of water is silica, present both as a colloidal suspension and dissolved in the form of silicates. The concentration varies very widely and, as silicates are sometimes applied as corrosion inhibitors, it might be thought that the silica content would affect the corrosive properties of a water. In general, the effect appears to be trivial the fact that silicate inhibitors are used in waters with a high initial silica content suggests that the form in which silica is present is important. 

Procedure. The method to be described is especially suitable for ceramic materials such as fireclay, firebrick, or silica brick. The finely ground sample should be dried at 110 °C. The weight of sample to be employed depends largely upon the silica content of the material, since not more than 35 40 mg of silica should be present in the aliquot employed for the determination. For samples... 

Figure 8-3 is explained as follows. With an increasing content of alumina, the intensity of silicon Ka decreases progressively below the value to be expected from Equation 7-1. The deviation is a negative absorption effect that can be calculated according to Equation 7-6. The intensity of aluminum Ka, on the other hand, behaves quite differently as the silica content increases. This intensity then increases... 

Both hot and cold processes are employed, although the hot process, which takes place at or above 212 °F (100 °C), is usually preferred for boiler FW applications, because it produces water of lower hardness levels and usually a lower silica content as well. Also, less lime is needed because the carbon dioxide with which it would normally react is driven off at the higher temperatures. Sometimes caustic soda (sodium hydroxide) is used in place of soda, depending on the alkalinity of the water and the chemical costs however, irrespective of the process or chemicals used, the major precipitants are always calcium carbonate and magnesium hydroxide. 

Unfortunately, the same authorities also do not provide any recommendations for FW silica content, sodium content, or electrical resis-tivity/conductivity, all of which are important constituents of high-quality water and are used as indicators of purity. (Silica and conductivity recommendations are, however, provided for BW quality.)... 

Total alkalinity is generally the most relevant alkalinity test, especially in FT boilers where the FW has a low silica content. Testing for T alkalinity provides the best correlation with pH, corrosion inhibition, and tendencies for carryover. 

Silicate scales can form readily in all boilers. Maintenance of a silica content to caustic alkalinity ratio of less than 0.4 1 prevents this. 

It is interesting to note that there is only slight difference in silica contents within chemically interactive (ENR) and noninteractive (ACM) mbber matrices at similar TEOS concentrations. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

Generally glasses with a high silica content are more resistant to abrasion than low silica content glasses. The hardness therefore increases with increase of softening temperature. Lead glasses can be scratched quite easily. 

The partitioning of La itself was found by Tiepolo et al. (2000a) to be a linear function of melt silica content ... 

Uranium and thorium partitioning into amphibole were also studied experimentally by Tiepolo et al. (2000a). Under the redox conditions of their experiments (FMQ-2 log units) U was dominantly tetravalent. They find no correlation between Aj, Djh and crystal composition, and, but again find a linear correlation with silica content ... 

The average crustal abundance of molybdenum is 1 to 2 parts per million. It occurs most readily in conjunction with silica - as the silica content in the igneous rocks goes up, so does the quantity of molybdenum. It is sometimes also found associated with uranium, as well as with coals and petroleum residues. In its host rock, molybdenum tends to occur in thin, tabular, and hexagonal plate forms, or simply as fine specks. 

Figure 14.3 Effect of silica content on the hardness of borosilicate optical glass. The ratio of B203 to K20 is 0.54 in these glasses. The data are from Schmidt and Reichardt...

Chars having a substantial silica content were formed by fire exposure of siloxane-carbonate polymers, and were shown by Kambour (17) to be excellent fire barriers, resistant to oxidation and with good insulating ability (Fig. 2). 

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