Environmental Analysis

Determination of silicate in process and boiler water using rapid photometric tests


Economic efficiency is becoming an ever more important aspect of everyday life, with the efficiency of industrial plants and equipment constituting one of the basic preconditions for sustainable economic operations. One avoidable problem that can result in losses of efficiency is posed by the undesirable build-up of deposits, i.e., scale – in pipes, boilers, and turbines.

A principal cause of scale in such equipment is silicate. Especially at high pressures – as in high-pressure turbines – silicate is deposited on the internal surfaces. This problem occurs mainly as a result of silicate dissolved in the steam.

The expansion of the steam results in a reduction of the solubility capacity of silicate, which in turns leads to the formation of solid silicon dioxide on the surrounding surfaces, for example the turbine blades, reducing the efficiency of the plant.

One measure that can help to minimize the need for time-consuming cleaning operations that interrupt the operation of the machine is to regularly inspect the boiler and boiler feed water for their silicate concentrations.

The guidance values depend on a variety of operating conditions (e.g., steam capacity, heating-surface load, and working pressure) of the boiler. In high-pressure turbines, even the slightest concentration of silicate in the steam can lead to deposits. To avoid such deposits, in most cases it is recommended to prevent steam silicate concentrations from exceeding 20 µg/L SiO2. Depending on the operating conditions, the limit for silicate may even be as low as 10 µg/L SiO2 or less.

The determination of concentrations of silicate in such a low range requires an extremely sensitive detection method. Graphite furnace atomic absorption spectrometry (GF-AAS) is frequently the method of choice here, capable of detecting concentrations of silicate down into the lower ppb range. Besides the element-analytical methods, classic photometry has also proven to be a reliable method. This method is based on the reaction of silicate ions in acidic solution with molybdate ions to produce yellow silicomolybdic acid. The addition of a suitable reduction agent then produces deep blue silicomolybdenum blue, which is subsequently determined photometrically.

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