What does SCD mean in the reaction sedimentation tank?

SCD refers to a flowing current detector; some call it PCD (Particle Charge Detector). 1 Measurement principle of SCD. 30 years ago, under environmental pressures, papermakers increased water recycling; it was at that time that dissolved charges attracted considerable interest. Reused water causes an increase in the amount of dissolved and suspended colloidal solids in the plant’s water system. Many of these substances carry a negative charge; they react with positive additives, not only reducing the effectiveness of those additives but also that of many other chemical additives as well. We call these negative substances in these cycles “anionic waste” or “interfering substances”. Clearly, we need a method to detect the amount of these interfering substances, and to predict their impact on the positive additives. Although various detection methods have been developed, the most widely used method is still based on the principle of colloid titration. SCDs are no exception. The colloid titration method involves using a standard polymer to titrate the sample – the standard polymer reacts with the groups carrying opposite charges in the sample, forming charge-for-charge complexes. The amount of charge of the titrant consumed to reach the isoelectric point is equal to the amount of dissolved charge with an opposite charge in the sample. The titration results are expressed as the charge per unit weight of the sample or the charge per unit volume of the sample. The most common method for determining the endpoint is to use the point at which the reading of the AC current detector becomes zero as the endpoint for colloidal titration. When the charge of the system is altered using a polyelectrolyte, the isoelectric point of the suspension corresponds to the point at which the charges on the surface of the piston and the tank reverse. Flow current detection is widely used as an endpoint indicator for detecting cation demand. 2 Structure of the SCCD The SCCD is a flowing current detector; it is also called a PCD (Particle Charge Detector). It was invented in the 1960s and was initially used in the water treatment industry; it is only recently that it has been widely used as a end-point detector for charge titration in paper mills. This instrument is compact and easy to use. The typical SCD structure is shown in the figure above: the sample is placed in a barrel-shaped container (the measurement chamber) —— this container is usually made of polytetrafluoroethylene (PTFE), with a smaller inner diameter at the bottom and a larger one at the top. A PTFE piston that fits into the narrower part of the barrel can move up and down vertically, thereby repeatedly compressing and drawing back the liquid in the measurement chamber. This results in a very high flow rate between the piston and the wall of the container. This high-speed flow separates the charged substances adsorbed on the wall surfaces and piston from their counterions (as shown in the lower right corner of the figure), thereby generating a flow current. The two gold electrodes on the container generate a current, which is converted into a corresponding potential and displayed on the instrument’s screen. The front side of the instrument can display the charge status of the liquid being tested during the titration process: if there are an excessive number of cations in the liquid being tested, it will show positive (cationic); if there are an excessive number of anions in the liquid being tested, it will show negative (anionic). 3 Premises for analyzing SCD measurement results Several assumptions are involved in analyzing the titration results of SCD. First, we assume that it is those components with a significant impact that adsorb on the walls of the instrument and generate a flow potential. Although all substances must be electrically neutral, in colloidal systems the charge is unevenly distributed between the large particles and the small ones. What has a significant impact in the system are the charged macromolecules. For example, dispersions of typical extender pigments are negative in charge, as the surface of the pigment particles carries negative charges, while positive charges are distributed on individual counterions – whose charge magnitude is several orders of magnitude smaller than that of the negative charges. Similarly, polyallyl dimethyl ammonium chloride (poly-DADMAC) is positive in charge, because all of the positive charges are adsorbed on relatively stable large polymer molecules. In contrast, a sodium chloride solution appears neutral because its positive and negative components, namely Na+ and Cl-, are both small; their sizes and migration properties are roughly the same. Generally speaking, if a solution or suspension has the following characteristics, it can be described as a positive or negative fluid: it contains charged polyvalent sols or macromolecular components, and their charges are balanced by simpler ions with relatively smaller charges and higher migration properties. In fact, there are also some exceptions. For example, aluminum sulfate solutions generally show a positive result. Although they do combine to form polymeric aluminum compounds at certain pH values, the solution turns positive not necessarily because aluminum ions are larger than sulfate counterions. It becomes charged because when it mixes with colloidal particles carrying a negative charge, it strongly adsorbs these sols, thereby causing their coagulation. Therefore, whether a suspension or solution is positive or negative must be determined based on the properties of the molecules, ions, or particles it contains, as well as the influence of these components on other sol suspension systems. Therefore, when using an SCCD to determine the charge of a sample, we must assume that it is those components with a significant impact that adsorb on the walls of the instrument and generate a streaming potential. Thus, the factors that affect whether the substance to be tested adsorbs onto the walls of the instrument become the key determinants of the effectiveness of this detection method. Due to its neutral surface, low surface energy, and ease of cleaning, as well as its lack of selectivity toward the substances it adsorbs, PTFE is generally chosen for making titration containers. Thus, the adsorption that occurs during this measurement process depends on the interactions between the components in the solution, and is independent of the surface of the container. The charge of any single component does not determine whether it can adsorb onto a naturally neutral surface (such as PTFE). Both positive and negative substances can adhere to the surface of the container. Another important assumption in interpreting titration results is that positive and negative macromolecules or colloids do not coexist. Under normal circumstances, when these substances come into contact with each other, they combine, causing the bound counterions to be released, thereby reducing the entropy of the system. For example, charged polymers will combine with each other, undergoing a charge-stoichiometric reaction until no more counterions are released. For certain substances or polymers with high rigidity, a stoichiometric reaction cannot occur because the charges carried by these substances cannot get close enough to each other. Other non-ionic reactions can also cause the reaction to deviate from stoichiometry. But in fact, the Zeta potential method has shown that positive and negative components can coexist. If such a situation occurs, the signal generated by the SCCD will be determined by the substance with stronger adsorption properties. The results obtained from the titration of polyelectrolytes also depend on the properties of the polymer titrant used, as well as on its ability to come into contact with all the surface-charged groups and colloidal charged groups in the system. For this reason, flexible polymers with high charge and low molecular weight are commonly used as titrants. They can be in maximum contact with the relevant charges. Generally speaking, it is assumed that the charges carried by the substances in the suspension can make full contact, and that there is only one type of polarized charged colloid present in the sample. In this way, the stream potential we measure can represent the charge of all colloids in the sample. The polarity of the stream potential will only reverse when all charges are neutralized by the polymeric titrant. Hope this can help you