Rice milling

Paddy Cleaning -Essential for removal of undesired foreign matter, paddy cleaning is given utmost importance to ensure proper functioning of the Rice Milling machinery. Rough rice is passed through a series of sieves and closed circuit aspiration system is provided to remove dust and light impurities through positive air suction.. Undesired material, heavier than rough rice (but of similar size) is removed through a de-stoner/gravity separator. This machine works on the principle of specific gravity. Stones and other heavy impurities, being heavier, stay on the screen surface whereas rough rice, being lighter, fluidizes into the positive air gradient created by an external source.
1. Paddy De-husking –A streamlined paddy flow is directed into a pair of rubber rolls, rotating at different speeds, in opposite directions. A horizontal inward pressure is applied on the rubble rollers, pneumatically. Due to the difference in the seed of rotation, a shear force is generated on the surface of hull (with two sides being rubber by tow rubber rolls) that breaks apart of the surface/hull.
Husk, being of lower specific gravity, is then separated form brown rice by a closed circuit aspiration system.
This process leads to breakage of brown rice. Although a proper horizontal inward pressure is most important factor for breakage or rice, de-husking efficiency is equally important and should be maintained between 75 to 85%.
2. Paddy Separation -Rice surface is smooth as compared to rough paddy surface. This difference in surface texture is utilized to separate brown rice from paddy through paddy separator.
Grain surface with smooth texture, being of higher width, is removed off along with red grains by precision sizes.
3. Rice Whitening -Brown rice is rubbed with a rough surface, created using emery stones of specific grid size. The rough emery removes off the brown bran layer. The radial velocity of the stone wheels, grid size of the stones, clearance between stone surface & the other screen and the external pressure on the outlet chamber of the whitening machines determine the extent of whiteness.
The bran layer removed from the surface if pneumatically conveyed to a separate room for further processing /storage.
4. Rice Polishing -The surface of whitened rice is still rough and is smoothened by a humidified rice polisher. The process involves rubbing of rice surface against another rice surface with mystified air acting as lubricant between the two surfaces. Usually a modified version of this process is used to produce superfine silky finish on rice surface.
The bran layer removed from the surface if pneumatically conveyed to a separate room for further processing/storage.
5. Rice Grading -Broken rice is removed from whole rice by passing the lot through a cylindrical indented screen rotating at a particular speed. The broken/small grains, fit into the indents of the rotating cylinder, are lifted by centrifugal force and gravitational pull falls the grains into a trough. Adjusting the rotational speed and angle of trough can vary the average length of grains.
6. Rice Colour Sorting -Discoloured rice grains are removed off from the like coloured grains by Rice colour sorting machines. Photo sensors/CCD (Charged Coupled Device) sensors generate voltage signal on viewing discoloured grains, which are then removed off by air jet generated through solenoid valves.

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