Coagulants for water purification: selection of products and design of the manufacturing process

Abstract

Coagulants are inorganic salts, mainly based on iron or aluminium, widely used in water treatment as colloids destabilizers in order to clarify and remove organics and suspended solids. Current work seeks to develop a production process for a new type of coagulant for water purification consisting on a polymerized blend of iron and aluminium chloride (PAFC). Initially the study focuses on the characterization of a new third-generation coagulant, a blend salt of aluminium and iron, its mechanism of action as well as a comparison with the different current market options. Later, the development of a new production process is detailed, including the determination of the necessary stages. At this point, among other aspects, the different alternatives available in reference to the process are studied, such as the raw material to be used or the needed unit operations. The new manufacturing process will mainly consist in 3 stages. The first stage will be the chemical reaction for dissolving iron and aluminium from bauxite tailings. The second stage will be the clarification of solved salts and separation of mud and suspended particles. Finally, the last stage will consist on the polymerization of the simple mixture of aluminium and ferric chloride in order to achieve a more complete product with better performance. By last, following the guidelines established in the previous points, the sizing of the equipment for the new production plant has been carried out. This point includes both the sizing of the different equipment involved in the process and other aspects as design of the plant with the different control loops, the plant piping and the lay out. The main characteristics of the designed plant are the implementation of vitrified reactors in order to cope with the high corrosion resulting from free HCl. Therefore, it will be necessary to consider this limitation throughout the manufacturing process, both in the design of the main equipment and the secondary elements such as pipes or pumps. It has been also considered designing a continuous operation of the plant. As will be seen in detail both reactors and polymerizers will necessarily work by batch. Therefore, considering the reaction times and the loading and unloading times, the plant has been designed as similar as possible to a continuous plant through the implementation of reactors in parallel. The final product resulting from the new plant would be competitive within the current coagulants market and would provide better performance than classic coagulants