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In a world full of differing dry powder sieving technologies, how do you determine the most appropriate equipment for your specific application? What follows is a simple, short descriptive guide to sifter selection for the most popular sieving technologies for dry bulk solids. Please note the following is intended to provide an overview of some of the most popular sifter designs and their features and should not be considered to be an exhaustive study of the subject. Before we even start to look at which machine to use, we first need to consider three important factors relating to the process at hand: A) what are the material characteristics? B) What is the sieving application that we are aiming to achieve? C) How is the Sifter going to be installed into the proposed process? With material characteristics, we typically need to consider: particle size – fine powder thru to coarse granules and lumps; particle shape – spherical, irregular, flake, etc.; friable – is the material easily damaged; abrasive – will the material cause rapid wear of the sieving media; flow characteristics – free-flowing thru to cohesive and difficult; moisture and fat content – is the material likely to smear; temperature sensitive – will the material degrade if subject to heat during the process. With typical sifting applications, we need to consider: policing – check sifting of feedstock to remove extraneous materials such as packaging, string, and lumps of hard agglomerated product; scalping – removal of a small percentage of oversize material from the feedstock; classification – one or more separations at specific screen sizes; de-dusting – removal of ‘fines’ from the feedstock; de-agglomeration – sifting and breaking of soft agglomerates to recover usable feed product that otherwise might be considered scrap; conditioning – a gentle process to enliven stored product or break down unmixed agglomerates; As far as use and installation of the sifter, we typically need to consider factors such as: gravity installation – feed to sifter is in a free-fall condition; inline pressure installation – connected directly into a pneumatic conveying line (positive or vacuum); controlled feed of material – an external device controls the feed rate to the sifter; flood feed – the sifter needs to be capable of handling a flood-feed condition; process and environment conditions – explosive materials or environment, product and ambient temperatures, special electrical requirements, etc.; legislative and other industry requirements – ATEX, 3A Dairy, NFPA, etc. Having identified the process and installation conditions for the application, then we need to turn our attention to the selection of a suitable sifter or screener. Popular sifter types and advantages include: Centrifugal or rotary sifters that feature: internal rotating paddle – provides energetic sifting action; easy access – hinged and interlocked doors for ease of cleaning & maintenance; high capacity – ideal for high throughput scalping and policing applications; compact design – fits into small spaces; solid flanged connections – provides dust-tight sealing; strong design – can be integrated directly into pneumatic conveying lines; de-agglomeration – energetic action breaks up soft agglomerates; single separations only. Vibratory screeners that feature: flat-deck – normally round screens; multiple decks – ideal for multiple separations; vibrator motor drive – provides two-dimensional high-frequency, low-amplitude sifting; gentle vibratory action – good for friable or easily damaged materials; minimal energy input – good for temperature sensitive materials. Tumbler screeners that feature: flat-deck – both round and rectangular screens; multiple decks – ideal for multiple separations; vibrator motor drive – provides three-dimensional low-frequency, high-amplitude sifting; high throughput – more efficient for high-volume applications; finer screening – ability to sift at smaller mesh sizes; gentle vibratory action – good for friable or easily damaged materials; minimal energy input – good for temperature sensitive materials. Taking all the different aspects of the proposed process and installation into consideration, we now move to establishing the most suitable sifting technology for the specific application. Three examples are: 1) High throughput policing of flour inline in a positive pneumatic conveying system. Selection = centrifugal sifter. Why: single separation; inline pressure rated machine; ideal for high-volume scalping. 2) Multiple classification of agglomerated dairy powder. Selection = vibratory screener. Why: multiple decks for multi-separations; gentle action doesn’t degrade fragile product; no heat input into temperature-sensitive product. 3) High-volume screening of a cohesive mineral powder. Selection = tumbler screener. Why: low-frequency, high-amplitude action; three-dimensional action is efficient for screening cohesive powders; ideal for high-capacity applications; gentle action minimizes wear issues due to abrasive nature of material. Rob Dallow is president and managing director of Kemutec Group Inc. (Bristol, PA). He has worked in both the UK and U.S. in technical, sales and administrative positions in the powder handling and processing industry for more than 35 years, with the last 12 years spent providing custom powder processing solutions to the North and South American markets. For related articles, news, and equipment reviews, visit our Screening & Separation Equipment Zone
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