Miller Magazine Issue: 132 December 2020

53 COVER STORY MILLER / december 2020 plosion is an important point for bag-filter system. 1.1.5. Storage After pre-cleaning operation (separation of coarse for- eign material), grain is transferred into silo or warehouse. In modern industry, vertical steel silo (galvanized by zinc) is today used. They can be designed as conical or flat bottom. The shape depends on capacity and solid han- dling requirement. In modern silo, aeration, temperature control and relative humidity control are used. Grain is transferred from silo to main cleaning process. 1.1.6. Cleaning operations In the cleaning operation, there are some important parameters to make efficient separation of foreign ma- terials i.e. dust, stem, straw, soil, mud, seeds, colored kernels, stone, insects, rodents, big particles, glass, met- als, plastic, toxic, toxin, microorganisms etc. Unmill-able materials should be removed from main product in the cleaning system by multiple methods based on different principles because a load of grain usually contains het- erogeneous materials differing substantially in their char- acteristics. In the mill cleaning system, various machines, techniques and principles are used to separate impurities. As first, these principles and techniques as base- are ex- plained as follow to understand the working principles of the cleaning machines. In the cleaning explanation, Posner and Hibbs (2005) format is used as follow. i) Cleaning by using Magnetic Properties The impurity; metals comes with raw material is very dangerous for equipment and food safety. Especially, iron/ ferric type hard metals are dangerous for milling equip- ment, therefore it must be separated first in the cleaning unit. Regular magnets are widely used; however, they sep- arate only ferric/iron. Rarely found metals such as stain- less steel, copper and aluminum etc. are not separated by regular magnet, therefore metal detectors can be used at this stage. For today in the industry, metal detectors are used in further stage before and after packaging. A magnetic separator can be installed as a first machine in the system, to separate metal pieces in raw material. Regular magnets are needed not only to remove metal from grain but also to pro- tect machines from damage and generation of sparks that might trigger an explosion (especially for dust explosion). Rocket, belt, rotary and drum magnet designs are used in the flow- ing line. Sometimes electromagnets are used to separate the metals. ii) Cleaning by using Size and Dimensions Size and dimension as solid properties are good sepa- ration attributes for the cleaning operation and they are widely used in grain industry operations. For separating grain based on size and dimension, machines with dif- ferent mechanical arrangements of sieves are used. The separating media, the sieves, can be made from perforat- ed sheet metal, grid and wire cloth. To achieve a separat- ing action with a sieve, static, dynamic (vibro, oscillated, eccentric, centrifugal, go-back types) motions are used. Machines are constructed with rotary motion, oscillating or reciprocating motion, and a combination of headend rotary motion and tail-end reciprocating motion. The rel- ative motions of grain and the screen can be obtained by following methods: i) the movement of particles caused by their gravity along an inclined and ii) the movement of particles caused by the movement of the screen. In advanced design, the screens or sieves always move. The material on the sieves moves at a slower velocity than the sieve (relative motion). The relative velocities depend on the sieve surface and the materiel’s shape and surface. For ellipsoidal grain e.g. wheat, reciprocating sieves can run at 300-600 rpm with a back and forth stroke of 1.25-1.75 inch. When the reciprocating move- ment is in the direction of the inclination, the sieve can handle a larger capacity but the separation is not very efficient. However, when the reciprocation is at right an- gles to the inclination of the sieve, capacity is lower but with higher separation efficiency. A gyrating screen operates at a speed of 150-300 rpm with a throw diameter of 1.5-4 inch. The capacity of gyrat- ing screens is below that of length, whereas the reciprocat- ing screen separates according to the width of the kernels. New developments in the design of machines with vi-