Using polymers for deposit control in once-through utility systems
Michael Standish of Radical Polymers presents an alternative to HEDP for calcium carbonate control
Mineral scale inhibitors based on polymer and phosphonate technologies have been used in once-through utility applications since at least the 1970s, mainly to control CaCO 3 and trace levels of transition metals. In recent years, the industry has predominantly employed phosphonates such as hydroxyethylidene diphosphonic acid( HEDP) for calcium carbonate( CaCO 3
) control, along with secondary levels of sulfonated copolymers to help manage transition metals like iron and manganese.
Recently, supply chain issues, such as COVID-related problems, labour shortages, government regulations and restrictions, domestic and international transportation bottlenecks, port congestion and energy costs have limited the availability of HEDP and raised costs by two to three times historical levels. Users have therefore sought for alternatives. This paper describes the viability of a combination of a patented enhanced polymaleic acid( PEMPA) 1 and a high purity sulfonated polymer( HPSP) as an alternative.
Figure 1- Untreated calcite( a) & calcite treated with PEPMA( b)
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Functions of polymers & phosphonates
In once-through and recirculating cooling applications, mineral scale control is mainly achieved with phosphonates, polymers or, more commonly, a combination, due to their increased functionality and the ability to use them at sub-stoichiometric levels with exceptional results.
Traditional chelating agents, such as ethylenediaminetetraacetic acid( EDTA) and other discrete or nonpolymeric small molecules, require
Figure 2- Example of blank QCM experiment
dosages based on a stoichiometric relationship with the metal ions present in the treated water. This can lead to very high dosages of chelating agents. These are therefore typically only employed in applications like boiler water treatment, where hardness levels are low and make-up water volume is minimised.
To evaluate polymers and phosphonates for water treatment purposes, we first must understand how these materials function. Polymers and phosphonates provide mineral scale control using five primary functionalities.
Threshold inhibition involves extending the solubility of an otherwise insoluble salt beyond normal saturation limits using an additive that functions at sub-stoichiometric levels. This differentiates treatment additives from materials that function according to strict stoichiometric ratios.
44 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981