Solubility silica

R. K. her. The Chemisty of Silica Solubility, Polymericyation, Colloid and Suface Properties, and Biochemisty,John Wiley Sons, New York, 1979. 

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

R. K. Iler, The Chemistry of Silica Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry, Wiley. New York. 1979, 866 pp. 

Minimum level of hydroxide alkalinity in boilers 1000 Ib/in must be individually specified with regard to silica solubility and other components of internal treatment. 

Silica soluble or reactive silica concentration in mg/l as SiOs. Suspended solids the concentration of insoluble contaminants in mg/l. 

Demineralization by SAC(H)/SBA(OH) Removes virtually all TDS, including carbon dioxide and silica. The TDS in treated water is normally below 2 to 4 ppm. Conductivity is below 10 to 15 pS/cm, but this depends on the degree of sodium slip from the cation. Any slip is converted to sodium hydroxide in the anion unit (salt splitting), causing a sharp increase in conductivity and increased silica solubility. The lack of a degasser drains the anion unit capacity and requires increased caustic for regeneration. 

Softening and raising the pH level of the RW tends to increase the rejection rate of silica, boron, and total organic carbon (TOC). It also tends to increase silica solubility, thus reducing the risk of silica deposition on the membrane. 

Treatment chemicals should preferably be fed to the FW tank to minimize sludge deposits in the coils. Hydroxide alkalinity in ppm (mg/1) CaC03 must be maintained at a sufficient concentration to keep silica soluble and avoid complex silicate deposits. These precautions are necessary because scale-control internal treatment chemicals usually are not employed to assist in the prevention of such deposits in coil-type steam generators. 

ASME proposes that the maximum hydroxide alkalinity that is consistent with the steam purity target and sufficient to maintain silica solubility should be employed. If necessary, this value should override conductance as the BD control parameter. 

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. 

Silica limits are based on the avoidance of silicate scale deposition and the limitation of silica solubility in steam. The BW silica levels in this table have been set to achieve a level of 0.002 mg/kg in the steam, the normal level specified for modem steam turbines. 

Silica reinforced rubber, 22 703 Silica sheets, 22 383-385 Silica-silane system, 22 377-378 Silica sol-gel fiber processing, 23 80 Silica sols, 22 383, 473-474 applications of, 22 394 modification of, 22 393-394 preparation of, 22 392-393 properties of, 22 391-392 purification of, 22 393 Silica, solubility in steam, 23 212-213 Silica-supported activated manganese dioxide, 76 568... 

Fig. 13. Quartz ai>d amorphous silica solubility vs. temperature along the vapour saturation curve. The dashed lines show the silica concentration in water initially in equilibrium with quartz during adiabatic boiling to 100 C and subsequent cooling. The increase in aqueous silica concentrations during boiling is the consequence of steam formation. Amorphous silica saturation (shown by the dots) is attained at 188 C in the case of the 300 C aquifer water, but at 94 C in the case of the 200 C aquircr water. It was assumed that the pH of the water is not raised sufficiently during boiling to cause significant ionization of the aqueous silica. If some ionization had occurred, amorphous silica saturation would be reached at lower temperatures than those indicated in Fig. 13.

At this point (the same pH as that of Wollast, t al., 1968) sepiolite begins to precipitate. In experiments maintaining pH at values above 9, montmorillonoids and talc were formed. Chemical analysis of the precipitates reveal a greater proportion of magnesium as the pH of the experiment is increased. Recalling the information on amorphous silica solubility, a two-fold increase in solubility of SiC>2 occurs between pH 8 and 10.5 (Krauskopf, 1959)—and thus at higher pH it could be expected that relatively less silicious phases would precipitate where the masses of Mg and Si are fixed. Final concentrations of Mg-Si in solution were not determined by Siffert and therefore thermodynamic calculations of mineral stabilities cannot be made. 

SIEVER (R.), 1962. Silica solubility, 0°-200°C, and the diagenesis of siliceous sediments. Journ. Geol. TO, 127-150. 

Siever, R., Silica Solubility 0-200°C, and the Diagenesis of Silican Sedi-... 

Interestingly, impurities such as aluminum, calcium, magnesium or zinc were reported to reduce both the rate of dissolution and the solubility of silica at equilibrium. Nitric acid-cleaned silica was immersed in solutions of Al, Be, Fe, Ga or Gd ions at pH from 2 to 9. A drastic reduction of the solubility of silica was observed. In particular, the introduction of Al ions rendered silica insoluble at pH 9. This effect was attributed to the formation of a monolayer of insoluble silicate which lowered the silica solubility to that of the surface compound [17]. Seemingly, silicates involving metal ions are formed even at pH values apparently non-aggressive for silica. However, the amount of metal silicate is not specified and the formation of only a silicate monolayer is purely speculative. 

Figure 7.2 Silica solubility as a function of pH. To determine silica solubility at a given pH, multiply the solubility as a function of temperature by the pH correction factor of the given pH of the concentrate solution.

Her, R. K., The chemistry of silica, solubility, polymerization, colloid and surface properties and biochemistry (John Wiley and Sons, New York, 1979) p. 248. 

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