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Introduction

Polishing and separation of rice

The output from the husker is a mixture of paddy, brown rice, husk, broken paddy, and sometimes bran.

Paddy separators separate the mixtures of paddy and brown rice making use of the physical differences, weight and surface coarseness.

Separated brown rice is whitened/polished and converted into white rice.

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Polishing

Polishing and separation of rice

Two types of polishing machines are used in the rice milling industry

Vertical cone type polisher

Horizontal polisher

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Polishing

Polishing and separation of rice

This machine basically consists of

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a cone-shaped cast iron cylinder with an abrasive coating.

The cone fixed on a vertical shaft that rotates either clockwise or counter clockwise.

A wire screen that has a mesh size depending on the variety of the paddy to be hulled is fixed around the entire cone

The average distance between the cone coating and the screen is about 10 mm.

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Polishing

Polishing and separation of rice

At regular intervals the wire screen is divided into segments by adjustable rubber brakes

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These brakes are 30 to 50 mm wide depending on the size of the machine.

The number of rubber brakes depends on the diameter of the cone and the preferences of the manufacturer.

A whitening machine removes the bran from the brown rice by friction

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Separation

Polishing and separation of rice

The output from the husker is a mixture of paddy, brown rice, husk, broken paddy, and sometimes bran.

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The husker aspirator removes the lighter material such as husk, bran and very small brokens.

The remainder passes onto the paddy separator where the unhusked paddy is separated from the brown rice. 

Separation

Polishing and separation of rice

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The amount of paddy present depends on the efficiency of the husker, and should not be more 10%. 

Paddy separators separates the mixtures of paddy and brown rice making use of the physical differences, weight and surface coarseness between paddy and brown rice.

Paddy has a lower specific gravity, higher buoyancy, and is physically bigger, longer and wider than brown rice.

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Separation

Polishing and separation of rice

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Paddy Separator is mainly designed for the complete separation of paddy from the brown rice.

The capacity of the separator should be at around 80% of the husker capacity (example: a 3 ton/hr husker requires a 2.4 ton/hr paddy separator).

When the mixtures enter the separator, they are divided into paddy, rice and the mixture of paddy and brown rice.

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Separation

Polishing and separation of rice

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The paddy is returned to the paddy husker and the mixture of paddy and brown rice to the inlet of paddy separator.

The brown rice which comes out of the machine is nearly 100 % pure .

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Screen

Polishing and separation of rice

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A very simple separator with no moving parts is the screen separator, which can be seen in smaller commercials mills located near rural markets.

Brown rice/paddy will move through a number of screens that are at 31-35º angle.  

The rough rice moves down the top screen, a mixture of rough rice and brown rice moves down the middle screen and finishing screen, and brown rice will pass through the finishing screen.

Screen

Polishing and separation of rice

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In operation, the mixture of brown rice and paddy are introduced in the upper end, and as the plate oscillates the paddy grain and brown rice mixture separates as it moves down.

The grain layer on the deck should be maintained at 7 mm to 9 mm thick, which can be controlled by the rate of feed and the inclination of the deck.

Too thick a layer will result in too much mixture, while too thin a layer will decrease separating capacity.

Screen

Polishing and separation of rice

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Basic screen arrangement for sorting grain

Screen

Polishing and separation of rice

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Efficient screen arrangement for most cleaning and sorting jobs

Screen

Polishing and separation of rice

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The top sieve is called the scalper and it scalp off materials larger than that to be retained.

It may also be considered as the cleaner since it removes undesirable larger materials.

Usually scalper has round holes and it should be just larger than the materials to be retained.

The second screen should have holes just small enough to retain the best materials.

If the materials are spherical, the round hole scalper and slotted hole bottom screen perform the best.

Screen cleaners that employ air blast to clean the materials are also known as FANNING MILLS.

Polishing and separation of rice

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Cross sectional view of a conventional fanning mill

Fanning mill

Polishing and separation of rice

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Screen type paddy separator

Compartment Separator

Polishing and separation of rice

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The compartment type of paddy separator uses the difference in specific gravity and the buoyancy to separate paddy and brown rice. 

Brown grains are smaller, heavier, rounder and smoother and will slide faster than paddy grains.

The processing capacity of the compartment separator is dependent on the compartment area.

For a 2-ton/hr capacity rice mill, a 45-compartment separator made up of 15 compartments on each of three decks is used.

Let us sum up

Polishing and separation of rice

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The output from the husker is a mixture of paddy, brown rice, husk, broken paddy, and sometimes bran. 

When paddy and brown rice move over an inclined plane, they move at different speeds depending on their specific gravity, their shape and contact area, smoothness of inclined surface and the co-efficient of sliding friction. 

Screen type separator

Tray type separator

Compartment type separator

Separated brown rice is fed into vertical cone type or horizontal type polishers for polishing of rice.