Filtration - filters and filter media

Abstract

This report presents a detailed review of the application and operational aspects of the filtration process and outlines criteria that should be applied for selecting filters and filter media for various system and slurry characteristics. Chapter I classifies filters according to their mode of operation and the driving force employed to effect separation between solid and liquid. They are therefore grouped according to whether used in batch or continuous operation, whether the driving force is gravity, pressure, vacuum, or centrifugal, and according to feeding arrangement and the mechanism of cake removal. Generally, the most suitable filter for a given operation is the one which will fulfill the requirements at minimum overall cost. Since cost is related to filtering area, high rates of filtration are normally provided by high pressures and continuous operation. Maximum pressures, on the other hand, are often limited by mechanical design considerations and batch operation thus may be necessary. The illustrated report provides essential characteristics of the most common types of filters, stating advantages and disadvantages of each. Plate filter presses are the most common batch filters. In continuous filters, filtration, washing, dewatering and cake discharge take place simultaneously. Continuous vacuum filters have wide application, particularly in rotary drum and rotary disc configurations, and are described in detail. Continuous pressure filters are similar in many respects to their vacuum counterparts but are more complex and expensive to build and operate. Factors which determine filter selection include cake formation characteristics, production level, required process results and construction materials. Filter selection is also based on slurry characteristics. Chapter II discusses the various types of material used as filter media. Criteria are presented for making a selection according to application on the basis of physical properties strength, blinding, or cake discharge, chemical properties resistance to chemicals, to heat, etc. and constructional properties types of yarn, fabric geometry, weave pattern, etc. Optimum filter medium choice is a compromise between the maximum service life, recovery, cake release, permeability of flow, production rate, gasketing, resistance to blinding, and clarity of filtrate. Ranking of the numerous types of filter cloth as to suitability for various applications is presented in tabular form.