tailieunhanh - Particles in Water Properties and Processes - Chpater 6

Đông máu và keo tụ 6,1 Thuật ngữ "đông máu" và "keo tụ" Chương này là có liên quan với các quá trình trong đó các hạt nhỏ trong nước tạo thành tập hợp lớn hơn có thể được loại bỏ dễ dàng hơn bởi quá trình phân tách vật lý như lắng đọng trầm tích, nổi, và lọc. Giả sử rằng các hạt ban đầu ổn định trong ý nghĩa keo (xem Chương 4), sau đó có hai bước cần thiết trong quá trình tập hợp, được thể hiện sơ đồ trong hình : • bất ổn của các hạt •. | chapter six Coagulation and flocculation Terminology Coagulation and flocculation This chapter is concerned with processes in which small particles in water form larger aggregates that can be more easily removed by physical separation processes such as sedimentation flotation and filtration. Assuming that the particles are initially stable in the colloidal sense see Chapter 4 then there are two essential steps in the aggregation process which are shown schematically in Figure Destabilization of particles Collisions of particles to form aggregates We shall be dealing mainly with particles that are stable by virtue of their surface charge and hence electrical double-layer repulsion. In that case destabilization involves either an increase in ionic strength or a neutralization of the particle charge as discussed in Chapter 4. Adding salts simply to increase ionic strength is almost never a practical option and other additives have to be used as described later in this chapter. The purpose of the destabilization step is to make the collision efficiency a as high as possible ideally with a 1 so that every collision leads to aggregation. Even when particles are fully destabilized so that the a 1 collisions are essential if aggregates are to be formed. We saw in Chapter 5 that the particle collision frequency is greatly dependent on the particle concentration and on the collision mechanism. For dilute dispersions where the collision frequency may be very low it is possible for particles to be fully destabilized but to show very little aggregation over appreciable time scales. Because of the short-range nature of colloidal interactions it is usually possible to treat the destabilization and collision processes as independent. In other words 121 2006 by Taylor Francis Group LLC 122 Particles in Water Properties and Processes Stable Destabilized Collisions Aggregated Add coagulant flocculant Figure Destabilization and aggregation of particles. it is nearly