Evaluation and Development of Physical Water Treatment Processes for the Reduction of CaCO3 Scale
1. BACKGROUND AND MOTIVATION
Physical water treatment (PWT) for the reduction of scale has been actively promoted as alternative for chemical treatment of water since the first PWT patent was registered in 1945. This patent was for magnetic water treatment (Vermeiren, 1958). Other techniques that gained in popularity, in particular during the last decade, include non-intrusive devices based on varying frequency electric fields induced through wrap-around coils and intrusive "catalytic conversion" devices where water is treated electrochemically by exposure to a metal alloy surface. The potential reward to industry and the environment should physical water treatment prove to produce consistent scale reduction effects, is enormous. This is the reason why, despite the fact that more than 50 years of intensive research into the effects of electromagnetic fields on scaling processes did not produce a generally accepted theory to explain the mechanisms involved, research and technical development continues unabated. Although PWT technology has changed substantially in some respects since 1945, the basic claims and effects (Baker and Judd, 1996) remained essentially the same. These include: the formation of a soft scale with weak adhesion properties, reduced kinetics of crystallisation, a memory effect of up to 3 days, changes to crystal morphology, reduced or increased particle sizes, descaling and dissolution of existing scale, and decreased zeta potential and surface tension.
The current status is, however, that these claims and effects cannot always be demonstrated experimentally under controlled laboratory conditions in a reproducible way. The reasons why PWT technology sometimes produces excellent results and then seem to stop working altogether, are therefore not clear and will not become clear until the mechanisms involved in these processes are fully understood. In this project the approach was to develop laboratory methods in conjunction with an industrial size testing facility to provide the means for the acquisition of reliable data that could be used to substantiate mechanistic models.
The project follows up on the recommendations for further research expressed in a previous WRC sponsored project (Coetzee and Haarhoff, 1997) on the reduction of scaling by magnetic water treatment: The main finding of this report was the discovery that small amounts of free metal ions, in particular Zn2+, released from magnet surfaces during water treatment, playa major role in the scale reduction properties of PWT devices in cases where the water is in physical contact with the device surface during treatment. This finding, including its potential use in scale reduction processes, was patented by the WRC in 1996 (Coetzee, 1996). The report, did not find conclusive evidence of scale reduction effects that could be attributed to electromagnetic fields. It was, however, recommended that the search for evidence of electromagnetically induced effects on scaling processes be continued using new state-of-the-art analytical technology such as atomic force microscopy.
The main objectives of the project were addressed in three parts. The objectives for each of the parts are summarized below.