An atomic force microscope study of calcium carbonate adhesion to desalination process equipment: effect of anti-scale agent

Whilst carbon dioxide is water soluble the system is somewhat complex and results in the presence of carbonate anions which interact with cations such as Ca2+ and Mg2+ present in seawater to form insoluble carbonates, especially at high temperatures. In multistage flash (MSF) desalination plants CO2 gas becomes less soluble in the brine as a result of the brines high temperature and high salinity which causes the pH to be in the range of 8–9. The presence of these conditions causes the release of CO2, simultaneous to the formation of scale deposits since its solubility is a function of the solution pH. The formation of scale deposits, such as CaCO3 causes fouling in the MSF distillers which has previously been studied by many researchers. A great amount of work has been carried out and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. The deposits may serve as an adsorbing film raising the speed of the loss of crystals or promoting the formation of scale deposits and therefore further adhesion on the wall surfaces of the MSF distillers and other process plant equipment leading to deterioration in the performance and efficiency of the whole desalination plant. This paper shows direct quantification of the adhesion forces between CaCO3 crystals and different process equipment surfaces under different conditions. This was carried out using an atomic force microscope (AFM) with an attached CaCO3 crystal as a colloid probe to bring the CaCO3 directly into and out of contact with the surfaces and measuring the resultant adhesion. This involved using surfaces different grades of roughness and carrying out measurements in synthetic sea water solutions of differing ionic strengths as well as with real seawater samples. Furthermore, the effect on measured adhesion of adding anti-scalant to the solutions was examined.

In this work AFM has been utilized to demonstrate the direct adhesion of CaCO3 crystals to stainless steel surfaces under different conditions. To the best of our knowledge no other adhesion data has been reported for the interaction between CaCO3 and surfaces in fresh water or salt water. The AFM technique used in this study has allowed direct quantification of adhesion between CaCO3 and steel surfaces and how it is affected by the addition of industrially used anti-scalant chemicals. When measuring the adhesion between CaCO3 and stainless steel surfaces of different roughness it was found that the roughest surface had a lower adhesion than the two less rough surfaces. In general surfaces with a greater roughness tend to have smaller contact areas. This is the most likely explanation for this decrease in adhesion. However, for particles of a size comparable to the length scale of the roughness artefacts on the surface, contact are can actually be greater than against a smoother surface. This suggests that the trend seen here will not necessarily be repeated for all possible different particle sizes. Adhesion was measured in different liquid environments. It was found that the adhesive force between CaCO3 and stainless steel was greater in water plus CaCO3 than in the synthetic sea water samples with dissolved mineral contents of 15 and 25 ppm. When comparing this adhesion in real sea water with and without a 2 ppm concentration of anti-scale it was found that the anti-scale reduced the adhesive forces between the CaCO3 and the steel to approximately a fifth of that measured without anti-scale. In addition, of the force curves measured with anti-scale 57% did not show any features indicating adhesion, whereas when no anti-scale was present this was observed for only 1% of force curves. This is the first time that AFM has been used to directly measure the adhesion between CaCO3 and stainless steel and has directly demonstrated the efficacy of adding anti-scale chemicals for water treatment.