Improvement Of The Evaluation Method Of Scale Inhibitor And Influence Of Additives On The Process Of CaCO₃ Precipitation

Although the scaling inhibition technology is improved greatly in industries, scaling still exists in petroleum industry, chemical industry, and electric power industry, because the reduced water resource makes working conditions more deteriorative. Scale inhibitors are widely applied to control or alleviate the scaling. However, the deficiency of evaluation methods and non-well known inhibition mechanisms bring about a lot of difficulties to the application of the scaling inhibitor technology. In order to resolve problems occurring in industry when inhibitors are applied, it is necessary to establish some rapid, practical methods to evaluate the efficiency of scale inhibitor, and make a deep research on the inhibition mechanism. This kind of work is helpful for developing new highly efficient inhibitors. The static-method is extensively applied to evaluate the inhibition efficiency and do some relative researches. The concentrations of metal ions are required in this method, which can be measured by using ion chromatography and other analytical methods. Ion chromatography requires high grade instruments and takes a long process time. Thus, the complexometric titration is often adopted to determine the concentration of metal ions. However the accuracy of complexometric titration is greatly limited by the quality of waters to be analyzed. This is an essential shortage of the complexometric titration. In Chapter 2, an improved complexometric titration was proposed, and systemic research has done for its accuracy to determine a single type of scaling cations and mixed types of scaling cations. The analysis results were found to be comparable with those obtained by using the methods specified in the relative National standards. The results showed that application of residual titration improved not only the accuracy but also the delicacy, in the presence of Mg-EDTA. This improved complexometric titration could also extend the application area of the static-method. The complexometric titration will be carried out at a buffered pH = 10. Thus, the adjustment of pH of the titration solution will meet an additional difficulty when the concentrations of scaling ions are very high and the efficiency of scale inhibitor is very high. This difficulty arises from the poor stability owing to the high supersaturation of the solution. At a high pH, slightly soluble salts will precipitate from highly supersaturated solution, leading to negative errors in the results of complexometric titration. Therefore, a novel method was further established by emphasizing scaling anions. In the system where calcium carbonate scale could form, the relationship between concentration changes of scaling cation and anion was found to be 2:1 during the process of precipitation. So the acid-base titration for determining the content of bicarbonate ions was applied to static-method and the inhibition efficiencies of HEDP and a PAA-HPA tri-copolymer were measured. In high hardness water, the efficiencies of scale inhibitor measured by acid-base titration were higher than those by complexometric titration, providing more accurate results. By monitoring the pH of the solution along the process of calcium carbonate precipitation, the complicated process of calcium carbonate precipitation and inhibition mechanism of scale inhibitor were studied systemically in Chapter 3. The process of calcium carbonate precipitation was reported to be divided to nucleation, growth and Ostwald ripening periods in many research papers. But the period of ACC form was clearly observed in our work by using pH-drift method. In this period, the pH value rapidly decreased with and without the presence of the additive, but the decrease range of pH went up with the increase of calcium ion concentration and temperature of solution. Scale inhibitor had no effect on the period of ACC form, but it could greatly influence the periods of nucleation and growth. The addition of the additive evidently retarded the nucleation time and decreased the growth rate of calcium carbonate. The influence of surfactant on inhibitor was also studied. It is found that there was an atactic effect between HEDP and SDBS. It is not fit to add surfactant to inhibitor solution simultaneous.