Study On Evaluation Method Of Scale Inhibitors Based On Conductivity Measurement And Kinetics Of Formation Of CaCO₃ Crystal

It is particularly a problem in the cooling-water system where the formation of calcium carbonate is a common and main concern of endangering security in the cooling-water system. In most cases, scale is mainly composed of infusible material with abnormal solubility. In the cooling water system, the most common scale is carbonate, sulphate, phosphate, silicate. In some certain condition, when the scale assume supersaturation state, they are likely to precipitate on the heater surface. Calcium carbonate is the most usual and unsafe scale in the cooling water system. We commonly research into the problem of controlling calcium carbonate scale. At present, adding scale inhibitors is the main method of controlling scale in cooling water system. A method of evaluation scale inhibitors rapidly and exactly is therefore crucial. And an understanding of the crystallization kinetics of calcium carbonate is essential.Firstly, the common evaluation methods of scale inhibitors and their advantages and disadvantages are summarized. Then the crystallization mechanics of calcium carbonate is presented.A new evaluation method of scale inhibitors has been developed and tested. This method is based on solution conductivity measurement, In the given water, at the constant temperature and airtight conditions, the conductivity values of the solution were measured, the point of the precipitation located. By calculating the critical supersaturation value, we can determine the stabilizing property of scale inhibitors and determine the optimum concentration. The experimental results showed that the conductivity method has the merits of simple experimental devices, convenient operation, rapid, accurate ect. We can evaluate the efficiency of different scale inhibitors at different concentration.In the kinetic studies of calcium carbonate crystallization, the process is divided into two processes: nuclei formation and crystal growth. This method is based on the conductivity measurement of the supersaturation solution. It showed that the rate of nuclei formation and crystal growth can be denoted as first order reaction rate equation and the rate constants and activation energy values can be calculated. In the study, the results indicated that a certain amount of scale inhibitors and temperature have influence on the rate of crystallization. The rate constants increase when temperature gets higher. Scale inhibitors can postpone the induction period and retard the scale formation. Temperature and scale inhibitors influence mainly on nuclei formation in the whole reaction process. It is because of the small rate constant of crystal growth that the crystal growth is irreversible reaction when nuclei appear in the supersaturation solution. That is, the condition of crystal growth is the existence of nuclei. Temperature and scale inhibitor have little influence on crystal growth. In the experiment, the reaction order can't change for temperature and scale inhibitors.