Calcium-Exchanged Silica

Other types of inhibitive pigments include calcium-exchanged silica, barium metaborate, molybdates, and silicates. 

The dual action of entrapment of aggressive cations and release of inhibitor gives calcium-exchange silica two advantages over haditional anticorrosion pigments  

The inhibitor ion is only released in the presence of aggressive cations, which means that no excess of the pigment to allow for solubility is necessary. 

No voids are created in the film by the ion-exchange the coating has fairly constant permeability characteristics [38,93-95]. 

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Figure 5.12 demonstrates the corrosion protection afforded by using calcium-exchanged silica [5.92]. 

In the brine electrolysis system, silica is also contained in raw salt. Silica will precipitate on to membranes in the presence of calcium, strontium, aluminium and iodine resulting in the loss of current efficiency [8-10]. Silica can also be removed in a column filled with ion-exchange resin containing zirconium hydroxide, just like the iodide ion. 

Multimedia filters, which consist of a top layer of coarse and low density anthracite, layers of silica, and then dense finest medium vitreous silicate, remove about 98% of particulates >20 tm. These filters are regularly back-washed to avoid buildup of particulates. Finer filters (S-lO tm) are used to remove suspended matter and colloidal materials. To prevent scaling due to water hardness, sodium ions generated from brine are exchanged with calcium and magnesium ions in the water. Activated carbon or metabisulfite is used to remove chlorine. 

One also sees that at room temperature the two peaks of the quadru-pole split pattern are different in intensity. This difference disappears at 77°K. It is an example of what is known as the Gordanskii effect 10) and is caused by the fact that the amplitude of vibration is different, parallel to the surface and normal to the surface, so that the effective resonant fraction is different for the two halves of the quadrupole splitting. This effect has been extensively studied by the Russians. In a recent paper, Suzdalev and others report a study of tin in the surface of silica gel 18), They put calcium ions in the surface of silica gel, then replaced them with divalent tin by ion exchange. The result was a mixture of stannous and stannic ions, and of course, the use of the Mossbauer effect made it possible to determine accurately the relative amounts of the two. They studied the amplitude of vibration of the two kinds of atoms and found, for example, for the stannous ions, the r.m.s. amplitude of vibration parallel to the surface was 0.07 A., and the r.m.s. amplitude of vibration perpendicular to the surface was about twice as great—about 0.13 A. Karasev and others have also worked on the chemistry of adsorbed... 

Sugar analysis by hplc has advanced gready as a result of the development of columns specifically designed for carbohydrate separation. These columns fall into several categories. (/) Aminopropyl-bonded silica used in reverse-phase mode with acetonitrile—water as the eluent. (2) Ion-moderated cation-exchange resins using water as the eluent. Efficiency of these columns is enhanced at elevated temperature, ca 80—90°C. Calcium is the usual counterion for carbohydrate analysis, but lead, silver, hydrogen, sodium, and potassium are used to confer specific selectivities for mono-, di-, and... 

Ion-exchange pigments were developed as nontoxic alternatives to the chromate pigments. They consist of a silicate carrier (zeolite [5.141] or amorphous silica gel [5.142]) to which calcium ions are bound. Commercial ion-exchange pigments have the following properties [5.143]—[5.145] ... 

Hoi-Lime Zeolite Softening. In this process hydrated lime is employed to react with the bicarbonate alkalinity of the raw water. The precipitate is calcium carbonate and is filtered from the solution. To reduce silica, the natural magnesium of the raw supply can be precipitated as magnesium hydroxide, which acts as a natural absorbent fur silica. These reactions are carried out in a vat or tank that is located just head of the zeolite softener tank. The effluent from this tank is filtered and then introduced into the zeolite softener. There is always some residual hardness leakage from the hot-process softener to be removed in the final zeolite process. The hot lime process operates at about 220T (I04°C). At this temperature the potential for the exchange of sodium for hardness ions is greater than at ambient temperature, and the result is a lower hardness effluent than is achieved at ambient temperatures. This system is shown schematically in Fig. 2. 

Most chemical treatment programs have not in the past specifically focused on controlling silica levels in cooling water, and as a consequence almost all analyses of scales and deposits taken from the waterside of cooling systems, especially from heat exchangers, have shown the presence of small percentages of silica. Research over the last five years or so has led to the introduction of silica-specific deposit control polymers and has also led, with some success, to the reevaluation and promotion of some established calcium carbonate polymer products for effective silica control. 

Yamamoto et al. [7] and Matsuchita [92] have described a method for the determination of chloride, phosphate, nitrate, nitrite, sulphate, calcium and magnesium in rainwater. They used lm mol L 1 EDTA as an eluent and a silica based anion exchange column. 

Calcium, and to a somewhat lesser extent, silica, can cause scaling problems for the cooling tower, particularly if the RO reject is a major portion of the cooling tower make-up. Scaling of the heat exchanger equipment results in a loss in productivity on the process side of the... 

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