Sihcate

Bond Type. Most bonded abrasive products are produced with either a vitreous (glass or ceramic) or a resinoid (usually phenoHc resin) bond. Bonding agents such as mbber, shellac, sodium siHcate, magnesium oxychloride, or metal are used for special appHcations. 

The reaction is generally carried out at atmospheric pressure and at 350—400°C. A variety of catalysts, eg, bases and metal salts and oxides on siUca or alumina—sihcates, have been patented (86—91). Conversions are in the 30—70% range and selectivities in the 60—90% range, depending on the catalyst and the ratio of formaldehyde to acetate. 

The elimination of alcohol from P-alkoxypropionates can also be carried out by passing the alkyl P-alkoxypropionate at 200—400°C over metal phosphates, sihcates, metal oxide catalysts (99), or base-treated zeoHtes (98). In addition to the route via oxidative carbonylation of ethylene, alkyl P-alkoxypropionates can be prepared by reaction of dialkoxy methane and ketene (100). 

The inorganic flocculating agents are water-soluble salts of divalent or trivalent metals. For all practical purposes these metals are aluminum, iron, and calcium. Sodium siHcate is also used in some appHcations. The principal materials currently in use are described in the foUowing. 

A more recently introduced product is polyalurninum—siHcate—sulfate or PASS, made by Handy Chemical of Quebec. It is said to be more effective than alum in cold water, have only a slight effect on pH, and leave less residual aluminum in the water. It is sold as a solution (8). 

Sodium sihcate is usually added to slurries as a dispersant (see Dispersants). Small amounts of sodium siUcate are used as flocculants. The active species are polymeric siUcates formed by hydrolysis. 

Applications. Ammonium bifluoride solubilizes sihca and sihcates by forming ammonium fluorosilicate [16919-19-0] (NH 2 ip6 ... 

Aqueous hydrogen fluoride of greater than 60% maybe handled in steel up to 38°C, provided velocities are kept low (<0.3 m/s) and iron pickup in the process stream is acceptable. Otherwise, mbber or polytetrafluoroethylene (PTFE) linings are used. For all appHcations, PTFE or PTEE-lined materials are suitable up to the maximum use temperature of 200°C. PTEE is also the material of choice for gasketing. AHoy 20 or Monel is typically used for valve and pump appHcations. Materials unacceptable for use in HE include cast iron, type 400 stainless steel, hardened steels, titanium, glass, and siHcate ceramics. 

Multicomponent Silicate Systems. Most commercial glasses fall into the category of sihcates containing modifiers and intermediates. Addition of a modifier such as sodium oxide, Na20, to the siUca network alters the stmcture by cleaving the Si—O—Si bonds to form Si—0-Na linkages (see Fig. 3c). 

Li Si O enstatite [14681-78-8] MgSiO diopside [14483-19-3], CaMgSi O and woUastonite [14567-57-2], CaSiO. A number of sihcate glass-ceramic compositions are given in Table 1. 

Other thickeners used include derivatives of ceUulose such as methylceUulose, hydroxypropylmethylceUulose, and ceUulose gum natural gums such as tragacanth and xanthan (see Cellulose ethers Gums) the carboxyvinyl polymers and the poly(vinyl alcohol)s. The magnesium aluminum siHcates, glycol stearates, and fatty alcohols in shampoos also can affect viscosity. 

Silicates. In 1990, a continuous hydrothermal production plant was started up in Sulitjehna, Norway for manufacture of 9000 m /yr of calcium sihcate [10101-39-0] having average density of 0.255 g/cm. The calcium siUcate is of the xonodite [12141 -77-4] form, Ca Sig02y(0H)2, which can easily be... 

Meta.1 Conta.mina.nts and Ash. Alkali metals form basic oxides that are very reactive toward acidic species such as the acid gases, siHcates, and alurninates. These form stable salts with acid gases if the off-gas contains such gases. Sodium, the most common of these metals, prefers to form chlorides ahead of sulfates. Sodium carbonate only forms in the absence of haHdes and sulfur oxides, SO. There usually is too Htde NO present to form nitrates (see Sodium compounds). 

Tribasic Lead Silicate. Ttibasic lead siHcate [12397-06-7] 3PbO -Si02, is a reddish yeUow powder, spatingly soluble ia water. Its compositioa... 

Silica and Alumina. The manufacture of Pordand cement is predicated on the reaction of lime with siUca and alumina to form tricalcium sihcate [12168-85-3] and aluminate. However, under certain ambient conditions of compaction with sustained optimum moisture content, lime reacts very slowly to form complex mono- and dicalcium siUcates, ie, cementitious compounds (9,10). If such a moist, compact mixture of lime and siUca is subjected to steam and pressure in an autoclave, the lime—silica reaction is greatiy accelerated, and when sand and aggregate is added, materials of concrete-like hardness are produced. Limestone does not react with siUca and alumina under any circumstances, unless it is first calcined to lime, as in the case of hydrauhc lime or cement manufacture. 

Hydrated lime is also used to stabilize the calcium sulfite—sulfate sludge derived from thickeners at SO2 scmbbing plants that use limestone—lime. Hydrated lime (2—3%) is added to react with the gypsum sludge and flyash or other added siHceous material. Under ambient conditions the lime and siHca serve as a binder by reacting as calcium siHcates so that the material hardens into a safe, nonleaching, stable, sanitary landfill or embankment fill. 

PetaHte, also a monoclinic lithium aluminum siHcate, LiAlSi O Q, has a theoretical Li O content of 4.88%. Commercial ores usually contain 3.5—4.5% Li O without concentration and ate a preferred source of lithia for use in ceramics and specialty gla2es. PetaHte is monoclinic and has a density of 2.4—2.5 g/cm. Heating to high temperature results in an irreversible phase change to a P-spodumene—Si02 soHd solution that could provide an extractable source... 

---