Alkali silicate detection

Detection of anionic impurities in alkali silicates has not been as fully developed as for cations. The anion of greatest concern is carbonate which is absorbed from the atmosphere. Potentially, carbonate could originate from the soda ash or potash raw material used in silicate manufacture but under normal furnace operation the ash should be thoroughly decomposed. The standard classical method for carbonate analysis involves acidification and boiling of the solution to release CO2 which is adsorbed on Ascarite. 

Alkali Sulfates. Since the silicate phases (C3S and C2S), present in total at up to 85 wt.%, normally dominate clinker samples, all minor phases are significantly concentrated in Residue 2. This includes the important alkali sulfate phases which can (i) affect setting times and final strength, and (ii) be used to assess kiln operating conditions. Since these are normally present at a total of about 0.5 wt.% in clinker, and are often distributed across several Na and K sulfate phases, they are not easily identified in raw clinker XRD patterns. However, their presence may be more easily detected in the XRD pattern of Residue 2. By optimizing the parameters of the alkali sulfates from Residue 2 data, and then constraining them in the on-line analysis system, these phases can be measured at the <0.5wt.% level (Madsen, Scarlett and Storer 2001, unpublished results) even when rapidly collected on-line data is used. 

The blue paramagnetic [82] ion is an analogue of the superoxide ion and has been detected in solutions of alkali metal sulfides in acetone or dimethyl sulfoxide. Simple salts containing [82] are not known, but the blue colour of the silicate mineral ultramarine is due to the presence of the radical anions [82] and [83] (see Box 15.4). 

The nitrogen-phosphorus detector (NPD) is also called the alkali flame ionization detector (AFID), or thermionic NPD if no flame is used. The flame NPD is similar to the FID, but with an additional unit, usually a rubidium silicate bead, which is heated by an electrical current (Figure 2.9). When a compound enters the detection compartment, the ion current for compounds containing N or P increases. The mechanism for N detection may briefly described by the following ... 

A simple potentiometric method for the determination of silica in solutions of alkali metal silicates is described. The precision of the determination of Na20 and SiQz in silicates was improved and the determination time decreased by developing a potentiometric titration method. Better determination results due to potentiometric detection of the endpoint pH are demonstrated by comparison with gravimetric determination of Na20 and Si02 in water glass containing 26-260 g/L. The potentiometric determination method can be easily automated. 

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