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Rice Milling SystemRice Milling System
NitatNitat TangpinijkulTangpinijkul
PostPost--harvest Engineering Research Groupharvest Engineering Research Group
Agricultural Engineering Research InstituteAgricultural Engineering Research Institute
Department of AgricultureDepartment of Agriculture
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Rice MillingRice Milling
The processes of converting paddy into rice
Removing the husk from the paddy
Removing all or part of the bran layer
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The basic objective of a rice milling
system is to remove the husk and the bran
layers, and produce an edible, white rice
kernel that is sufficiently milled and free of
impurities. Depending on the requirements
of the customer, the rice should have a
minimum of broken kernels.
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Factors Affecting Milling Recovery
Paddy quality
Type of machinery, efficiency, system
Operator
Others milling degree
ambient condition
Milling Recovery = Weight produced milled rice x 100%
Weight processed paddy
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Course : Grain Quality
Module 4 : Rice Mil ling
Lesson 1 : Introduction to the milling process and its outputs
Good
Equipment
Skilled
Miller
Good Paddy
Rice
the use of good
milling equipment
and good quality
paddy does notguarantee a highquality product
the miller is not
skilled,
the mill will always
produce poor quality
milled rice, despite
the skill or the miller
or maintenance ofthe mill.
you use poor quality
paddy,
the mill will produce
high quality head
rice.
you have good
quality paddy in a
well-maintained mill
that is operated by a
skilled miller,
ThenIf
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Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
Quality characteristics of paddyQuality characteristics of paddy
These characteristics are
determined by the
environmental weather
conditions during
production, cropproduction practices, soil
conditions, harvesting,
and post harvest
practices
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Moisture content has a significant
influence on all aspects of paddyquality. To obtain high yields, it is
essential that paddy be milled at
the proper moisture content.
Paddy is at its optimum milling
potential when its moisture
content is 14%
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: MOISTURE CONTENT
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Immature rice kernels are
very slender and chalky and
result in the production of
excessive bran, broken
grains and brewers rice
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: IMMATURE
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A mixture of varieties in a sampleof paddy causes difficulties in
milling and usually results in
reduced milling capacity,
excessive breakage, lower milled
rice recovery, and reduced head
rice yields.
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: VARIETAL PURITY
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Dockage includes chaff,stones, weed seeds, soil,
rice straw, stalks and other
foreign matter. These
impurities generally come
from the field or from the
drying floor.
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: DOCKAGE
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Water, insects and heatexposure can cause paddy to
deteriorate through biochemical
changes in the grain which may
result in the development of off-
doors and changes in physical
appearance
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: DISCOLORED
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Overexposure of mature paddy
to fluctuating temperature and
moisture conditions leads to
the development of crack in
individual kernels.
Course : Grain Quality
Module 3 : Determining the Physical Characteristics of Paddy Rice
Lesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: CRACKED
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Weight per volume or density, gram/litre
Weight per 100 or 1000 grains
Thai rice variety: 100 grains weight 2.25 - 3.67 g
Color of Husk
Affecting color of parboil rice Color of Brown rice
Affecting color of parboil rice
Milling recovery and quality of milled rice grade
OTHERS QUALITY CHARACTERISTIC:
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Grain DimensionGrain Dimension
Grain size and shape (length-width ratio) is a varietal
property. Long slender grains normally have greaterbreakage than short, bold grains and consequently have a
lower milled rice recovery
Type of Paddy : Length of brown riceExtra long 7.5
Long 6.5 < 7.5Medium 5.5 < 6.5
Short < 5.5
Sub-type of Paddy : Length/Width Ratio
Slender 3.0
Bold 2.0 < 3.0
Round < 2.0
Thai rice variety: Length 7.1 - 7.6 mm
Slender 3.1 3.4
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ChalkinessChalkiness
grain with opaque areas in the endosperm, caused by
the loose packing of the starch and protein particles,breaks more readily during milling than clear grain,
greatly reducing its market value
controlled by genetic and environment
white center white belly white back
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Good Quality Paddy
uniformly mature kernels
uniform size and shape (vareital purity) free of fissures and cracks
free of empty or half filled grains
less chalky, red kernel
free of contaminants such as stones and weed seeds
moisture content ~ 14%
high milling yield variety
Good variety, good crop management
Harvest at optimum period Drying immediately after harvest to safe level m.c. Proper drying to prevent cracking
Good aeration during storage to prevent grain deterioration Prevent birds & rodents during storage
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Purity of paddy
Milling Recovery = Weight produced milled rice x 100%Weight processed paddy
Purity % Impurity % Milled rice %
100 0 68.00
99 1 67.3298 2 66.54
97 3 65.96
96 4 65.28
95 5 64.60
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Embryo
Bran Ricegrain
BranEmbryo
+ =
Rice flourSplinters
Total bran producedwhen whiteningcracked grains
Cracks
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75.065.5 63.568.0Recovery
0.51.5 1.51.0Brewer Rice
75.567.0 65.069.0Milled Rice
7.510.0 10.08.0Bran
83.077.0 75.077.0Brown Rice
17.023.0 25.023.0Husk
100.0100.0 100.0100.0Paddy
JaponicaCrack ImmaturedGood
Paddy Quality
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Size of Rice MillSize of Rice Mill
Small 12 ton/day
earning from bran, broken or money as milling wage no grading of milled rice
Medium 13-59 ton/day
commercial mill for domestic market
Large > 60 ton/day
for domestic market and export
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43,2752,7313,2328,77928,5331997
44,4364,3865,11410,04924,8871996
44,4724,3905,12910,06524,8881995
44,5604,3955,15610,10424,9051994
45,0194,4005,22310,18325,2131993
KingdomPlainEastern
WholeSouthernCentralNorthernNorth-Year
Source : Factory Control Division, Ministry of Industry
Number of rice mill by region, 1993 - 1997
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Number of village 962Number of Rice mill 194
< 5 ton/day 90 (46.4%)
> 5 < 20 ton/day 42 (21.6%)
> 20 ton/day 62 (32.0%)
Number of rice mill in Suphanburi province (2004)
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The Aim of Rice Milling
To attain the highest
yield of white rice
With the best quality
Remove least amountof hull and bran
Minimum brokens
Little foreign matter
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Rice milling system
0ne step process
Two step process
Multi stage process
In a one step milling process, husk and bran
removal are done in one pass and milled or white rice isproduced directly out of paddy.
In a two step process, removing husk and
removing bran are done separately, and brown rice is
produced as an intermediate product.
In multistage milling, rice will undergo a number ofdifferent processing steps.
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Rice Milling Process
Cleaning
Husking
Paddy Separation
Whitening & Polishing
Grading and Blending
Impurity
Husk
Bran
Head rice
Broken
Paddy
Paddy
Paddy, Brown rice
Brown rice
Milled rice
Paddy into rice mill
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Paddy Intake
Quality input = quality output
Garbage in = garbage out!
Paddy intake chute
The quality of milled white
rice will be dependant onthe quality of the paddy or
rough rice coming into the
mill as well as the milling
process. Good paddy into
the mill means quality
milled rice output.
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Paddy Cleaning
After harvest, foreign matter can be as
high as 5-10%
Rice pre-cleaner
While most paddy has been
cleaned after harvesting, some
foreign material is still present.This can be as high as 5-10%.
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Paddy Precleaner
Sieve: separate large/ small impurities
Aspirator: separate light impurities
De-stoner: separate same size impurities but
heavier
Trieur (Length Grader) : separate impurities
having same width but different in length
Magnet : separate irons
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Open double-sieve precleaner
Examples of
perforated sheets
and wire mesh
Closed type Cl d t
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Closed-type
single-action aspirator
precleaner
Closed-type
double-action aspirator
precleaner
Double drum type of Single drum type of
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Double-drum type of
Precleaner
(West Germany)
Single-drum type of
Precleaner
(Japan)
Magnetic separator
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Magnetic separator
Magnet
Brass
A, B: Simple permanent magnet
C: Rotating brass cylinder and
permanent magnet
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De-stoner
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Paddy inPaddy in
PaddyPaddy
StoneStoneAir streamAir stream
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Stone
Wind direction
rice
Vibration direction
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De h sking
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De-husking
Underrun disc huller
Rubber roll huller
Centrifugal disc huller
Underrun disc huller
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Underrun disc huller
PaddyPaddy
Brown riceBrown riceHuskHusk
Particulars of the under
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Ratio of W/D ~ 1/6 or 1/7
Composition:
Emery grit 14, 16 Silicium carbide grit 16
Peripheral speed ~ 14 m/s
Particulars of the under-
runner disc huller:
V = peripheral speed
(recommended 14 m/s);
W = width of coating;
D = stone diameter;
and W/D = 1/6 or 1/7.
U d di h ll
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Under-runner disc huller
peripheral speed-curve for
V = 14 m/s.
D = stone diameter = 700
mm (0.7 m);
and V peripheral speed =
14 m/s.
V = x D x n (m/s)
60
n = 60 x V = 60 x 14
x D 3.14 x 0.7
= 380 rpm.
Capacity and power requirement of
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Diameter Capacity Motor
(mm) (kg of paddy/hr) (hp)
750 450-600 3
1000 700-1000 3.5
1250 1000-1400 4
1400 1600-2100 5.5
Capacity and power requirement of
underrun disc huller
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AdvantageOperational simplicity, low running cost
The abrasive covering can be remade at the
site
Disadvantage
Given grain breakage
The abrasions to outer bran layers
Underrun disc huller
Rubber Roll Huller
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Rubber Roll Huller
Rice Huller
Rollers
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Dehusking principle of rubber roll huller: The rubber roll huller consists of two rubber rolls. Both rolls have the same diameter.
One has a fixed position, the other is adjustable to obtain
the desired clearance between the two rolls.
The rolls rotate in opposite directions. The adjustable roll speed running about 25% slower than
the fixed one.
Size and speed of the rubber roll
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74010001025010250
90012003768.5220
90013202.5646150(rpm)(rpm)(Inch)(mm)(Inch)(mm)
Lowspeed
HighspeedWidthDiameter
Size and speed of the rubber roll
When the rolls are new, their peripheral speed is about 14 m/s
so that a smaller roll runs faster than a larger one.
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Durability of Rubber Rolls
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Durability of Rubber Rolls
15302.5
20353
40754
601106
1402408
17030010
Long grain(ton)
Short grain(ton)
Type(Inch)
Capacity and power requirement for
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Diameter
6.03.82.225425410
4.01.91.22201506
2.51.250.92201004
(hp)Shortgrain
Longgrain(mm)(mm)(Inch)
MotorCapacity (ton/hr)WidthSize
different rubber roll hullers
Rubber Roll Huller
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Rubber Roll Huller
AdvantageHulling efficiency is higher.
Reduce grain breakage and loss of small
broken.
The silver skin of brown rice is undamaged.Sieving the husked products is unnecessary.
Disadvantage
The cost of replacing rubber rolls.
Husk Separation
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Dehusker
Sieve
Paddy
Brown rice, Broken, Paddy, Bran, Husk
Coarsebran
Husk
Brewerrice
Husk Aspirator
Brown rice, Broken, Paddy
Paddy Separator
Husk Aspirator
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Closed Circuit Husk Aspirator
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1 !"#$%"&'"#$()!"*
2 !"#$%"&'"#$(+",-./.0
3 !"#$%"&'"#$(12'%!"2.(/
12'%3"4.!
4 )5/0367*"+
5 )5/0367*"+
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Huller with
Husk Aspirator
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Husk Aspirator
(BUHLER: DRHC/DRSC)
Paddy Separation
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y p
Removal of paddy from brown rice
After husk separation, a mixture of 85-90% brown rice
and 10-15% paddy is fed into the paddy separation
stage. The paddy must be separated before the brown
rice goes to the bran removal stage. The separated
paddy is returned to the husker for dehusking.
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Weight per volume of paddy is less than that of brown rice, Specific gravity of paddy is lower than that of brown rice,
Dimensions of paddy are longer, wider and thicker,
Coefficient of friction is different.
Paddy and brown rice have different characteristics
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Compartment-type Separator
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The main part of this paddy separator is the oscillating compartment-
assembly where the actual separation of paddy and brown rice takes place.
The compartment-assembly consists of a number of compartments in one,two, three, or sometimes four decks.
The number of compartments depends on the capacity of the rice mill,
high
Paddy, brown ricePaddy, brown rice
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PaddyPaddy
low
high
Brown riceBrown rice
Paddy, brown ricePaddy, brown rice
Brown riceBrown rice
PaddyPaddy
P ddP ddHi hHi h HUSKERHUSKER
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Brown riceBrown rice
PaddyPaddy
aa
HighHigh
LowerLower
AdjustmentAdjustment
SpeedSpeed 9090 -- 120 strokes/min120 strokes/min
SlopeSlope
StrokeStroke
HUSKERHUSKER
WHITENERWHITENER
Tray type separator
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First rubber roll huller
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Second rubber roll huller
Whitening Machine
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Removes bran layer to produce white rice.
Amount of bran removed (usually 8-12%)
Whitening
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Whitening Actions
Abrasive type
High speed / Lowpressure
Griding action
Impact action
Friction type
Low speed / Highpressure
Tearing action
Cutting action
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Milled rice & bran
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Friction type
Smooth
surface
Large
Abrasive type
Rough
surface
Fine
Whitening Machine
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Vertical abrasive whitening cone
Horizontal abrasive whitener
Horizontal friction-type whitener
Bottom-Up Vertical friction-type whitener
Vertical Abrasive
Whitening ConeAbrasive
cone
Air
Feeding
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Whitening Cone
Level
adjust
Wiremesh
Rubberbrake
Bransweeper
Bran
Rice
Number of Brakes = Cone Diameter - 2100
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100
Peripheral Speed 13 m/s
Eg. Cone Diameter = 600 mmNumber of Brakes = 600/100 2 = 4
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Capacity of Cone-type Whitener(kg of brown rice / hr)
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Diameter Power
mm hp Long grain Short grain Long grain Short grain Long grain Short grain
500 4 350 420 570 680 680 800
600 7.5 550 650 890 1100 1050 1250
800 10 750 900 1230 1450 1460 1725
1000 15 1000 1200 1700 2000 1900 2300
1250 20 1350 1600 2200 2600 2600 3000
1500 25 1700 2000 2700 3200 3200 3800
1 Cone 2 Cones 3 Cones
( g )
Composition
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Composition
Whitening Cone No. 1 and No. 2
Emery grit 16 25%
Silicium carbide grit 16 50%
Silicium carbide grit 18 25%
Whitening Cone No. 3
Emery grit 18 25%
Silicium carbide grit 18 75%
Horizontal abrasive whitener
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Horizontal abrasive whitener
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Aspirator
Counter
Weight
Bran
Abrasive roll
Screw
Resistance pieces
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ScreenAbrasive grit
#30, 36
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Short
grainLong
grain
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Characteristics of
Abrasive Action Milling(Speed 12 - 13 m/s)
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Shaping Action Factors
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LowMediumHighPressure
LargeMediumSmallRoll Grit
HighMediumLowPeripheral Speed
Round ShapeSlim shapeFlat ShapeCondition
Horizontal friction-type whitener
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Characteristics of
Low-Speed Machine
(Friction-type)
Effect of Ventilation on Low Speed Machine
(Friction-type Whitener)
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Bottom-Up Vertical friction-type whitener(SATAKE)
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Polisher
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Humidifying Rice Milling Machine
Vertical Polishing Cone Brush type
Vertical Polishing Cone Leather type
Horizontal Polisher Leather type
Humidifying Rice Milling Machine
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Nozzle
Bran
Water addition rate 0.3 - 0.4 %
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Mold Growth on Rice from Non-Humidifying
and Humidifying Milling
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Humifying rice-milling machine is used in the
last step of whitening system
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Characteristics of
Humidifying Milling
Machine
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Polisher
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(BUHLER: DRPE)
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Vertical Polishing
Cone
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Brush type
Vertical Polishing Cone - Leather type
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Horizontal
Polisher
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Whitening System
System 1 2 3 4 5
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Long grain (Indica)
1 Abrasive Abrasive Abrasive - -
2 Abrasive Abrasive Abrasive Vent Friction -
3 Abrasive Abrasive Abrasive Hu Friction -
4 Abrasive Abrasive Abrasive Vent Friction Hu Friction
Short grain (Japonica)
1 Abrasive Vent Friction Vent Friction Vent Friction Hu Friction
Milling System for Indica-type variety
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Abrasive Friction
ABRAS ABRAS ABRAS ABRAS ABRAS ABRAS FRICT FRICT
Combined System vs
Low Speed System
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Combined System
(High + Low speed)
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1st Vertical whitening roll
2nd Vertical whitening roll
3rd Hor Humidifying friction
polisher
Size of brokens according to Thai Rice Standard (1997)
Grading
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g ( )
General standard
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Japanese standard
A from B : By thickness
A from C : By thickness / length
B from C : By length
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A from B : By length
Bold grain
A from B/C : By thickness / length
B from C : By length
Slender grain
A from B/C : By length
B from C : By length
Grading of White Rice
After polishing,
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white ricecontains: Head rice
Large brokens
Small brokens
The white rice isseparated using Sifter
Length grader
Rotary Seiver
Rotary Sifter
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White Rice Grading
One double layers sifter
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and two trieurs inparallel.
One double layers sifter
and three trieurs in
series.
Rice Mixing
An efficient rice mill will produce:
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50% head rice (whole kernels)
5-15% large broken and
5-15% small broken kernels
Depending on the countrys standards, ricegrades in the market will contain from 5-25%broken kernels.
Rice Mixing
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Whole
kernels
Largebrokens
Smallbrokens
Variable speed motors
allow the mill operator to
mix the appropriate
amount of whole kernels
with large and small
brokens.
Rice Weighing
Rice is normally sold in
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50 kg sacks whichmust be accurately
weighed and labeled.
Bag filler andweighing
scales
Color SorterColor Sorter
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Conveying SystemConveying System
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Duct Collecting SystemDuct Collecting System
Scaling and PackagingScaling and Packaging SystemSystem
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Milling Quality AssessmentMilling Quality Assessment
Sampling / dividing
M i t t t
Assessment the milling quality of paddy
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Measure moisture content Cleaning sample
Milling test
Grading
Sample divider
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Scale Sieve
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Aspirator
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Moisture Meter
Resistance type
Capacitance type
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Capacitance type
Oven for m.c. determination130oC 1 hr
103oC 72 hr
105oC 5 hr
A, B : Weight per Volume Tester
C : Volume Measuring Cylinder
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C : Volume Measuring CylinderD : Counting plate
Rubber roll huller
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Abrasive-type Whitener
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Length Grader, Trieur
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Friction-type Whitener
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Friction-type Whitener with Length Grader
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Using Abrasive-type Whitener
Weigh clean paddy 250 g
Procedures for Milling Quality Assessment
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Weigh clean paddy 250 g
De-husk paddy with the rubber roll husker
Weigh brown rice
Whiten brown rice w the abrasive type whitener for 90 s
Weigh milled rice Separate broken from milled rice w the trieur for 90 s
Weigh head milled rice
Using Friction-type Whitener (De-husking method)
Weigh clean paddy 100 g
Procedures for Milling Quality Assessment
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Weigh clean paddy 100 g
De-husk paddy with the rubber roll husker
Weigh brown rice
Whiten brown rice w the friction-type whitener for 25 s
Weigh milled rice Separate broken from milled rice w the length grader for 90 s
Weigh head milled rice
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Moisture Content = Wt of Water x 100Wt of Sample
After Drying Weight = (100 Initial MC ) x Initial Weight
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After Drying Weight = (100 - Initial MC.) x Initial Weight(100 - Final MC.)
Eg. Paddy 500 t with initial MC 26%
Final MC after drying is 14%
Then After Drying
Weight = (100 - 26) x 50000 = 430,23 t(100 - 14)
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Thank you
8/10/2019 Rice Milling System
151/151