276004 Adsorption Behaviour of a Calcium Carbonate Particle to Solid Walls Coated by Different Hydrophilic Films
When hard water is heated, the scale (deposit) of calcium carbonate builds up due to the decrease of solubility in heat-transfer devices like heat exchangers and evaporaters. This phenomenon is called fouling. The scale of calcium carbonate forms the low heat transfer layers on walls of heat exchangers. It causes heat transfer reduction. Furthermore, the reduction of the amount of pass of a heat medium, and the increase of pressure drop are caused, and the pumping power increases. At the final stage, it results the pipe blocking. Thus, it is important to control such fouling phenomena.
The primary material causing the fouling in a heat exchanger is calcium carbonate. Calcium carbonate has two kinds of crystallographic structures of calcite and araginite in nature. Especially the aragonite crystal is considered as a main scale material because an aragonite crystal has more adherability than a calcite crystal. In order to prevent the fouling of the calcium carbonate, some reseaches have reported many studies. Recently, Kim et al. (2002) observed the fouling process of CaCO3 with a microscopic imaging technique. The validity of an electronic anti-fouling technology in a single tube heat exchanger was investigated (Cho et al., 1999). It was found that aragonite was predominantly precipitated in the presense of NaOH (Konno et al., 2002). Škapin et al. (2010) reported that the presence of ethylene glycol leads to the formation of aragonite, while the presence of diethylene glycol and tetraethylene glycol leads to the formation of calcite. Catalytic material used for the mitigation of mineral fouling (Lee et al., 2006). However, the mechanism of the fouling has not been clarified sufficiently.
In this study, the adsorption characteristics of the calcium carbonate onto solid walls were investigated in order to investigate the fouling mechanism.
The adsorption forces of the calcite and of the aragonite to several kinds of walls were directly measured by use of an atomic force microscope (AFM). Heavy calcium carbonate (calcite) and aragonite particles were tested in calcium carbonate saturation solution. The particle with 6.7µm of averaged diameter was bonded onto the tip of the cantilever. The particle approaches and releases onto wall by use an AFM. The minimum absolute value of the interaction force observed in the releasing curve was defined as the adsorption force on the particle onto the solid wall. The contact area between the particle and wall was calculated from Hertz’s elastic contact theory.
Hydrophilic characteristics of walls were evaluated from contact angle measurement. As walls, the glass, aluminum, copper and copper with several coatings were tested at room temperature of 20oC. Coatings of copper plate were silane coupling, TiO2 coating and silica coating.
The crystallization characteristics of calcium carbonate onto plates of the copper, the copper with coatings and glass were also investigated by fouling experiments. The substrate was soaked into the solution of 5mM CaCl2 with 5mM NaHCO3 in a bottle. After the bottle was heated from 20 minutes to 9 hours at 80oC by a constant temperature bath, these sample plates were picked up from the bottle, dried, and the weight of the calcium carbonate crystals adhering to these surfaces were measured. The structures of the crystals floating in the solution and adhering to walls the structures were analyzed by an X-ray diffraction system (XRD).
From the results of the adsorption measurements and contact angle measurements, it was found that the adsorption forces both of calcite and of aragonite decrease with hydrophilic characteristics of walls. From the results of fouling experiments, it was found that the needle-like crystals are deposited onto the wall and that the number of crystals increases with time in the copper case. This crystal is a typical aragonite crystal. On the other hand, not only the needle-like but also the petaline-like crystals can be observed on the surface of glass.
The masses of the CaCO3 crystal adsorbing onto the walls increase with heating time. The crystal masses on glass and on copper with coatings took low values than those in the case of the uncoated copper. Moreover, the ratio of the adsorbing crystal to floating crystal is smaller in the cases of glass, silane coupling coated and silica coated copper plates than that in the case of the uncoated copper.
This result is considered to correspond to the adsorption force characteristics. It was also found from X-ray diffraction measurements that the structure of calcium carbonate adsorbing to the copper surface was aragonite. On the other hand, not only an aragonite crystal but also vaterite was observed on the glass and silica coated copper surface. Thus, it was concluded that the fouling of calcium carbonate particles in heat exchangers occurs due to the adsorption of aragonites on the hydrophobic substrate from the solution where the aragonites are formed. It is expected that the fouling of the calcium carbonate can be controlled by use of the hydrophilic wall for the heat-transfer surface.