PERFORMANCE OF ABRASIVE DISK DEHULLERS M. W. ...
PERFORMANCE OF ABRASIVE DISK DEHULLERS
M. W. BASSEY* and H.M. MBENGUEf
*1nrernationai Developmeti Research Centre (Canada),
BF’ 11007 CD Annexe, Dakar, StWgal
fl:entre hrarionale a!e Recherches Agronomiques,
tiP. 53 Bambey, Sénégal
AASTRACT
dehulling and the physical charüctcristics of thc in-
temal surface of the dehuller barrel. In the absence
Traditional dehulling of cercal grains is a timc
of detailed studies, in somc cases, certain hypothescs
cansuming, labour intensive, but important task per-
arc used to explain thc operation of thc dchuller.
fomed b:y women in rural areas in developing coun-
It is concluded that evcn though the perform-
triis. Dchulling separates the outer envelopc, which
ance of abrasive disc dehullcrs :trc satislâctory. sig-
contains bath non-nutritional and organoleptically
n i f i c a n t improvcment may 1%: ohraincd through
unacceptable factors, from the edible portion of thc
s t u d i e s aimed a t understandin:: fhc mcchaniçs oi’
gciin.
disk/grain, grain/grain and grainlwall mtcractions,
This: paper discusses the performance of dehul-
during thc dchulling proccss.
lers which use vertically rotating abrasive disks em-
bedded in the grains being processed. Results of
va:ious studies are used to explain the effects of se-
INTRODUCTION
veral parameters on the dehulling performance of thc
machines.
Nced for Dehulling
The performance of the dehullers depends on
the speed of the rotation of the disks, the disk’s sur-
Semi-arid regions in developing countries su&
face characteristics, the quantity of grain in the dc-
as Africa, for example, cari only support the cu!ii-
hu ler, spacing between disks, the disk diameter, thc
vation of cereals such as sorghum, millet and maize.
physical characteristics of the grain, the duration of
As a result, a signifïcant pcrcenragc of thc population
- _ , _ -
- . - - - - -
l . - - - . - - ”
- - p
< . “ l
Page 28
Journal of Engineering for Intematiorial Development
:
in many countries in Western, Central, Eastem and
types such as the Bavaria Record, the Wondergrain
Southerr, Africa, depend on sorghum and millet for
Jaybee, the FAO (fondateur de l’atelier de l’ouest)
their daily meal. Other crops such as barley and qui-
and Decomatic dehullers have been described [5].
noa contribute to the daily diets of populations in
Some of the disadvantages of various dehullers
other parts of the world.
for village use are: their large capacities, often greater
Rural women have traditionally improved the
than 1000 kg/day; lack of spam parts; inability to
organoleptic quality of foods prepated from cereals
dehull different grains without adjustment or mixtures
buy protzssing the grains to mmove the outer layers
of grains; and, the need to wet the grains.
(the pcricarp and testa). The pericarp @an) contains
mainly ,Fibre whereas the testa contains anti-nutri-
Aim of Paper
tional substances, such as polyphenols, which give a
bitter taste to the grain and inhibit the digestion of
The purpose of this paper is to discuss the per-
protein Iiom the grain [l]. Phytic acid, which is pre-
formance of the abrasive-disk dehuller which has
sent in the bran and germ of sorghum and millet,
been developed at various msearch institutions in
combines with minerai elements such as calcium,
Canada and in Africa, for use on cereal grains and
iron, sodium, zinc, and magnesium, to form insoluble
grain legumes. Both laboratory and field results and
compounds (phytates) thus making them unavailable
observations are used to explain the effect of various
for human nutrition [2]. The presence of high con-
parameters on the performance of this dehuller. It is
centraticns of C-glycosylflavones in undehulled mil-
anticipated that the discussion will serve as a basis
let has been linked to the incidence of goitre in Sudan
for further work aimed at improving the understand-
[3,4]. AU of these various anti-nutritional factors cari
ing of the operation of the dehulle?, ultimately lead-
be substantially reduced by dehulling.
ing to an improvement of its performance.
Description of Traditional Dehulling
DESCRIPTION OF ABRASIVE
Tmditional dehulling, which is done by women,
DISK DEHULLERS
involves pounding the wet grain in a mortar with a
pestle. Wetting reduces damage to the grain and fa-
General Features and Operating Principle
cilitates the removal of the bran. Once the required
degree of dehulling has been achieved, the grain is
The basic features of the abrasive disk dehuller
washed to remove excess bran, dried to reduce its
are shown in Fig. 1. It consists of several abrasive
moistun: content and then pounded into flour. This
disks, rigidly mounted on a horizontal shaft rotating
moist fll~ur cannot be kept for more than 24 hours as
on bearings mounted on the ends of a trough-shaped
it ferments.
barre1 made of mild steel sheet. In operation, the
disks are mtated in the grain at speeds mnging from
Dehulling Equipment for Grains
800 to 2500 rpm depending on the type of disk and
abrasive surface beiig used.
Technologies that cari be used for dehulling in-
Several models of this dehuller have becn de-
clude rcller milling equipment, rice dehulling equip-
veloped and have been fully described [6]. They have
ment, attrition-type dehullers and abrasive-type
been developed to take into account specific dehull-
dehullers [5]. Roller milling equipment does not ap-
ing needs and available technical skills in various
pear to be very useful in dehulling sorghum or millet;
countries. They are: PRL and h4ini-PRL dehullers,
the high degme of dehulling and colour of the flour
developed at the Prairie Regional Laboratoxy of the
obtained are unacceptable to the consumer. Rice
National Research Council in Canada (now the Plant
milling equipment does not generally work well on
Biotechnology Institute); the’ RIIC dehuller manu-
these cemal gmins, Abrasive-type dehullers use ro-
factured by the Rural Industries Innovation Centre in
tating abrasive surfaces to abrade the grain. Various
Botswana and being used extensively in that country;
Journal of Engineering for International Development
Page 29
Dehldlcr
Disc
DiSC
No. of
ElCCtliC
I TYF
Type
SpeedS
Discs
Mccor
RPM
POWtX
HP
PRL
stona
!No-1300
$13
8
RUC
stona
2aM
13
8
NUHULL
Stona
1500-2ooo
4
1 0
Mùli-PRL
Reshoid
MO-2OCXl
4
3
hfinKRs
Rcsinoid
1500-UMO
6
3
SISMARI
Resinoid
1500~2ooo
1 0
3
ISRA 1
SlSMAR/
Resinoid
15ca2ooo
8
3
ISRA II
Figure 1. Basic features of the abrasive disk
Mini-ENDA
SlOlleS
1500-2cm
1 0
3
dehuller:
A, spacers; B, end plate;
k
C, shaft; D, abrasive disk; E, barrel.
Table 1. Typical disc types, speeds and
power for the abrasive disc dehul-
lers.
the NUHULL dehuller manufactured by a Canadian
manufacturer, Nutana Machine Ltd.; the Mini-CRS
metry of the machine; it cari dehull an a!;soned mix-
dehuller developed by the Catholic Relief Services in
ture of grains in the same batch.
the Gambia and manufactured by local attisans; the
Mini-SISMAR/ISRA
1 and II dehullers developed by
the Centre Nationale de Recherches Agronomiques
ASPECTS OF DEHULLER’S PERFO:RMANCE
and a large manufacturing company SISMAR in
Scnegal; and the Mini-ENDA dehuller developed by
Laboratory and fïeld studies [6] indicate that the
ENDA in Zimbabwe.
performance of abrasive disk dehullers depend on:
The PRL, RIIC and NUHULL dehullers are
larger machines with aspiration systems for bran re-
speed of rotation of the disks;
moval, whemas the Mini dehullers have lower ca-
physical characteristics of the disk surface such
pacities and operate in the batch mode. Typical
as roughness and hardness;
features of these two sizes of machines are shown in
number and diamcter of disks in the barre]. rc-
Rgs. 2 and 3. Abrasive surfaces used in the dehullers
flecting the total surface area available for dc-
hulling;
arc in the form of carbonmdum stones or lightweight
spacing betwcen disks and thcir angle of incli-
resinoid discs. Some of the main features and speci-
nation to the shaft;
fil:ations of the dehullers are shown in Tables 1 and
distance between the end disks and the end plates
2 .
of the barrel;
Some of the advantages of the abrasive disc
clearance between the periphery and the barrel;
dehuller over others mentioned earlier are: its oper-
presence or absence of aspiration during dehull-
ating principle is simple; it is potentially lower in
ing;
price; it cari be easily operated by persons with little
roughness of the inner surface of thc barrel;
technical skills; it cari effectively dehull the grains in
the rate at which grains are fed into the dehuller
their dry state; it cari dehull a wide variety of cereal
in the continuous mode, and the quantity of grain
grains and grain legumes without adjusting the geo-
in the dehuller in the batch mode;
Page 30
Journal of Engbtxring for~%qmnati&al DweloPment
.,.’
.
Figure 2. Schematic showing specific fea-
tures of the Mini-SISMAWISRA
II dehuller: A, dehullers; B, feed
box; C, separator; D, aspirator, E, to
cyclone; F, two trap doors; G, sieve.
Solid arrows, movement of grains;
broken q-rows; bran (courtesy of
SISMAR, Senegal).
--
.
residence or retention time of the grain in the
dehuller;
. ’ physical characteristics of the grain being de-
hulled.
Available results are discussed in this section.
Effect of Speed of Rotation of Disks
Results of studies of t.hme models of abrasive
disk dehullers, using barrey. sorghum and millet, have
been reported [7]. The dehullers used were: a Graham
Hill IW%he:r containing 13 Carborundum stones 30.5
cm in diameter and 3.2 cm thick; an experimental
Resinoid Disk Dehuller containing 27 resinoid steel
cut&f disks, 30.5 cm in diameter sud 0.64 cm thick;
Jomnal of Engineering for International Development
Page 31
~---l--__-----.
-
0
1 0
20
30
40
DEGREE OF DEHULLING, 2;
Figure 4. Effect of disk speed on Resinoid
Disk Dehuller performance.-
-
that influence dehulling have been discussed 161.
They are the pericarp thickness and hardness of thc
Figure 3. Exploded view of the PRL: A,
grain.
bran to cyclone; B, fan; C, grain
happer, D, air inlet; E, feed gate; F,
Effect of Mass of Grain in Dehuller
abrasive disks; G, adjustable gates;
l-l, overflow outlet for dehuller
In practice the mass of grain dehulled in mills
grains (from 161).
varies from 0.5 to over 20 kg. The speed of rotation
of the disks, which is usualiy fixed for a given mill,
Laboratory [7] and field [9] observations indi-
depends on whether grinding stoncs or rcsinoid disks
cate that in’clining the lightweight disks at the end of
are used. The retention time is thus thc parameter
the dehulle:r to the vertical, impmves the unifonnity
used to control the degree of dehulling of the grains.
of dehulling. The angle of inclination of the disks
Detailed studies on the effect of mass of grain
have ranged from 9 degrees [7], to 5 degrees which
used on dehuller performance arc availablc for se-
eliminated disk breakage [9], pmbably due to the re-
veral Mini dehullers [8, 10, Il]. Results for quantities
duction of the rotationaI stresses. Inclining all of the
of millet between 2 and 8 kg, which is similar for
disks is expected to reduce dehulling time and im-
other grains, is shown in Fig. 5 for various disk
prove the ‘degree of dehulling but results arc no1
speeds. Holding all other parameters constant. the
available to confïrm the benefits of this arrangement.
degree of dehuiling increases as thc mass of the grain
Impmvement of the uniformity of dehulling has also
in the dehuller barre1 is increascd.
been achieved by reducing the gap between the disks
Typical results for quantities ranging from 0.5
at the cnd of the shaft and the end plates of thc de-
to 8 kg in the Mini-SISMAR/ISRA
1 machine using
hul1e.r (101.
resinoid discs [ 101 is shown in Fig. 6. The degree of
dehulling decreases with increasing mass of grain for
Effect of Grain Type
loads less than 2 kg.
The following explanation cari be givcn for the results
The performance of the dehuller depends on the
in Fig. 6 using Fig. 7. At very low loads thc grain at
type and variety of the grain being dehulled. Factors
lcvel 1 in Fig. 7 is not in contact with thc disc; there
-y,, .,
:
.._. ,’
Page 32
8
0
R.P.M.
15QO --54
l
Y- 4
2I
Retention Time =:3 min. I
0
10
15
DEGREE OF DEHULLiNG, %
--
Journal of Engineering for &.emational tivelopment
Page 33
yde resin bond (14%), Fluorspar (10.6%) and
reinforced with fibreglass (7%) to improve their
strength.
The comparative performance of Iightweight
disks anql grinding stones cari summarized as follows:
”
Coarse grit stones abrade grains much faster than
the smootier- faced resinoid disks but the colour
of the resulting fleur indicates that the resinoid
disks produce more unîform dehulling as de-
monstrated by reflectance measurements.
‘1
Grinding stones with coarsc grit should lu m-
tated at lower speeds in order to produc’e de-
hulling comparable to resingid disks.
#I
Grinding stones consume more energy compared
to lighter weight disks.
Degree of Dehulling
Results for the dehulling of maize using a Mini-PRL
dehuller using 5 carbonmdum disks (3 kg each) and
8 resinoid disks (0.15 kg each) [ 101 are shown in Fig.
Figurr! 7. Diagram used to explain the effect
8. The carborundum stones consume more fuel to
of mass of grain on the degree of
achieve a given degree of dehulling due to their
dehulling.
weight and surface friction effect.
processed (in batch sizes less than 3 kg), compared
Effect of Interna1 Lining in Barre1
to the village where 24 tonnes were being processed
(in batch sizes greater than 6 kg).
The interaction between disks and grain, grain
and grain, and grain and the barre1 wall is cornplex
Effect of Grinding Surface
and not well understood. Observations made during
tic development of the RIIC dehuller, for example,
Studies in SenegaI [lO] and in Canada [7] indi-
indicated differences between results for a bare wall
cated that resinoid disks, which are smoother than
and one lined with rubber. The degree of dehulling
carborundum stones, give better performance as in-
!was improved when the wall was lined with a rubber
dicated by degree of dehulling, uniformity of dehull-
material; the dehulling was less uniform, and power
ing and power consumption. Mechanical aspects of
consumption was increased [7] as shown in Table 4.
the dehulling #action between grinding stones and
Power consumption is increased due to the re-
grain h;ive been discussed [6]. Important charac-
stricted motion of the grains by the lining, and the
teristics of grinding stones for the dehullers appear
resulting increased resistance to the rotation of the
to be gtit size, the grade (pmhrtion of bonding ma-
disks. This is because the lining restricts the motion
terial) and the surface dressing. Examples of abrasive
of the grains near the wall, causing higher resistance
discs that have been used are:
to the rotation of the disks. In general the grains near
the wall are less dehulled than thosc near the dïsks.
.
carbomndum stones made of coarse grit silicon
carbide abrasives (85% of the total composition)
Areas for Further Studies
with vitrifïed clay bonds
.
resimid disks made of medium-grit aluminum
Although the mechanical performance of abra-
oxitie abrasive (67%), cured phenol formaldeh-
sive disc dehullers is satisfactory, the socio-economic
Page 34
+al Development
8
‘.3.
.
5
Retenbon Time, min
0
,i
0
5, . ..j. 0
,15
20 25
pmper sizing of dehullersfor given loads, mduction
ization of .disk
Table ;3. Wear experienced by resinoid disks
at village mills in.Senegal. : ’
_.<
,‘_
erating cos$.:. ._
~~g$g&
f thé dehuller is
** Average vhe for all 10 disks,
the .abrasive.:~&t5&$&@&nz&to
* Percentage wear with res@ct to newdisks: diam-
..% ,:r\\.ts r* .Y, , ,_,. 1
investigate the
eter = 25.4 cm, thickness = 3.6 cm, m.ass = 370.3 g.
effects-of :va@us~~& d@c&on the dehulling~ of
TEach batch dehulled greater than 6 kg.
grains. Emphasis’- sl$uidbe placed on longevity of
$Each batch dehulled between 2 and 3,kg.
disc surfaces and techniques for iocal manufacture.
,
viability of their opemtion is
main@ due to the inability of rural
o
;:.
.i;:.
.‘q.,i-
, .
de.hulling services, which is less
’ of grain. Thus any changes in
erating conditions of the dehuller aime&at,,mducing
its capitil and operating costs would allo.+‘, it to In
mor&i;v&ly used. These technical changesGUr be
obtained 'by improving the understanding of the per-
<.
formance of the dehuller.
.,
+There is a need to describe the mecha&s of the
inLeractiom hrween he abrasive di&@&&., and
the internal wall of the barml. In ‘&$!’ &?inve.s-
tigation, it would be informative ,to de,@nine the
optimum spacing between discs and ho; to maximize
the mixing of the grain so that uniform dehulling is
obtained. Results of such a study should lead to the
Journal d Engiriecring for International Development
Page 35
’;, 1. C O N C L U S I O N S ::
3. OSMAN, AK. 1981, “Bulrush MilNet (Pennise-
.A
..
,::{a:.:: <
.-- 1* :
tum typhoides) - a Contributory Factor to the
.‘*
Abrasive; disk dehullers have several featums
Endemicity of Goitre in Western Sudan”. Ecology
r
which make, themasupexior to: other dehullem for use
i ’
ofFmdandNutrition, Vol. 11, pp. 121-128.
’
in ;developing: cori&$& $heir Performance is depen-
4. GAI’T’AN E. et al, 1986, “C-glycosyflavones:
. .
dent on the;types;~gmin%the-physical arrangement
’ Goitrogens in Millet”, Frontie!;s ïn Throidology,,
and speed ofrot$on-of$he disks, the retention time,
eds. Medeiros-Neto, G., and Gaiian, G., Vol. 2,
the intemal surface :.of: the. dehuller banal, mass of
Plenum Medical Book Company, N. Y.
-. grain .beiig&$
5. REICHERT, R.D., 1981, “Sorghum Dry
aspiration,of’thi$b
k&i$$‘, Proceedirigs Internatioml S~&siti o n
:. $&$.&, Nov. 2-7, ICRISAT, F’atanchem, A. P.
f the dehull-
india, pp. 547-563.
een rotating
6 . BASSEY, M.W. and SCHMIDT, Q.G., 1989,
disks, grains @d&.hh,@et$_,surface of the barrel.
“Abrasive-Disk dehullers in Africa.: fmm Re-
T%e developmentl of appropriate abrasive surfaces
search to Dissemination”, /nternatiowl Develop-
that are durable and,.ch$&r .would encourage the
ment Research Centre, Ottawa, Canada.
wide spread use of the dehullers.
7 . OOMAH, B . D . , REICHBRT, R . D . and
YOUNGS, C.G., 1981, “Quantitative Compar-
ison Between Carborundum Stones and Resinoid
ACKNOWLEDGEMENTS
Disks in Dehulling Cereal Grains”, Cereal Chem-
istry, Vol. 58, No. 6, pp. 492-496.
Results djscussed in this paper were obtained
8 . MBENGUE, H.M., 1986, ‘“Cmation d’un
by research i&titu&ns:in Canada and Afïica with
decortiquer au SCnégal, Premier Rapport
tîmding pmvided ) by the Intemfitional Development
Int&inaire”, Projet 3-P-84-0016-02, Septembre.
Research Cent& (C&di).
Centre Nationale de Recherches Agronomiques
de Bambey, Senégal.
9 . NANCE, J.P.. 1987, “Gambian Grain Dehuiler,
REFERENCJXS
,
Final Report Phase One”, Catholic Relief Ser-
.,.
_.,
vices, The Gambia.
10. MBEMGUE, H.M., 1987, “Crt?at.ion d’un
, arjd PEARSON,
decortiqueur au SCnéegal, Deuxieme Rappon
ets, Their Compo-
Int&inaire”, Projet 3-P84-0016-02, Centre Natio-
Academic Press,
nale de Recherches Agronomiques de Bambey,
Sénegal.
11. MBENGUE, H.M., 1988, “Cr&ation d’un
decortiqueur au Sénegal, TroisiEme Rapport
ceedings, International
Int&inaixe”, Projet 3-P-84-0016-02, Centre Natio-
nale de Recherches Agronomiques dc Bambey.
Sénégal.
.’
:
-_
_ _c_ ._.
.._
M. W. BASSEY* and H.M. MBENGUEf
*1nrernationai Developmeti Research Centre (Canada),
BF’ 11007 CD Annexe, Dakar, StWgal
fl:entre hrarionale a!e Recherches Agronomiques,
tiP. 53 Bambey, Sénégal
AASTRACT
dehulling and the physical charüctcristics of thc in-
temal surface of the dehuller barrel. In the absence
Traditional dehulling of cercal grains is a timc
of detailed studies, in somc cases, certain hypothescs
cansuming, labour intensive, but important task per-
arc used to explain thc operation of thc dchuller.
fomed b:y women in rural areas in developing coun-
It is concluded that evcn though the perform-
triis. Dchulling separates the outer envelopc, which
ance of abrasive disc dehullcrs :trc satislâctory. sig-
contains bath non-nutritional and organoleptically
n i f i c a n t improvcment may 1%: ohraincd through
unacceptable factors, from the edible portion of thc
s t u d i e s aimed a t understandin:: fhc mcchaniçs oi’
gciin.
disk/grain, grain/grain and grainlwall mtcractions,
This: paper discusses the performance of dehul-
during thc dchulling proccss.
lers which use vertically rotating abrasive disks em-
bedded in the grains being processed. Results of
va:ious studies are used to explain the effects of se-
INTRODUCTION
veral parameters on the dehulling performance of thc
machines.
Nced for Dehulling
The performance of the dehullers depends on
the speed of the rotation of the disks, the disk’s sur-
Semi-arid regions in developing countries su&
face characteristics, the quantity of grain in the dc-
as Africa, for example, cari only support the cu!ii-
hu ler, spacing between disks, the disk diameter, thc
vation of cereals such as sorghum, millet and maize.
physical characteristics of the grain, the duration of
As a result, a signifïcant pcrcenragc of thc population
- _ , _ -
- . - - - - -
l . - - - . - - ”
- - p
< . “ l
Page 28
Journal of Engineering for Intematiorial Development
:
in many countries in Western, Central, Eastem and
types such as the Bavaria Record, the Wondergrain
Southerr, Africa, depend on sorghum and millet for
Jaybee, the FAO (fondateur de l’atelier de l’ouest)
their daily meal. Other crops such as barley and qui-
and Decomatic dehullers have been described [5].
noa contribute to the daily diets of populations in
Some of the disadvantages of various dehullers
other parts of the world.
for village use are: their large capacities, often greater
Rural women have traditionally improved the
than 1000 kg/day; lack of spam parts; inability to
organoleptic quality of foods prepated from cereals
dehull different grains without adjustment or mixtures
buy protzssing the grains to mmove the outer layers
of grains; and, the need to wet the grains.
(the pcricarp and testa). The pericarp @an) contains
mainly ,Fibre whereas the testa contains anti-nutri-
Aim of Paper
tional substances, such as polyphenols, which give a
bitter taste to the grain and inhibit the digestion of
The purpose of this paper is to discuss the per-
protein Iiom the grain [l]. Phytic acid, which is pre-
formance of the abrasive-disk dehuller which has
sent in the bran and germ of sorghum and millet,
been developed at various msearch institutions in
combines with minerai elements such as calcium,
Canada and in Africa, for use on cereal grains and
iron, sodium, zinc, and magnesium, to form insoluble
grain legumes. Both laboratory and field results and
compounds (phytates) thus making them unavailable
observations are used to explain the effect of various
for human nutrition [2]. The presence of high con-
parameters on the performance of this dehuller. It is
centraticns of C-glycosylflavones in undehulled mil-
anticipated that the discussion will serve as a basis
let has been linked to the incidence of goitre in Sudan
for further work aimed at improving the understand-
[3,4]. AU of these various anti-nutritional factors cari
ing of the operation of the dehulle?, ultimately lead-
be substantially reduced by dehulling.
ing to an improvement of its performance.
Description of Traditional Dehulling
DESCRIPTION OF ABRASIVE
Tmditional dehulling, which is done by women,
DISK DEHULLERS
involves pounding the wet grain in a mortar with a
pestle. Wetting reduces damage to the grain and fa-
General Features and Operating Principle
cilitates the removal of the bran. Once the required
degree of dehulling has been achieved, the grain is
The basic features of the abrasive disk dehuller
washed to remove excess bran, dried to reduce its
are shown in Fig. 1. It consists of several abrasive
moistun: content and then pounded into flour. This
disks, rigidly mounted on a horizontal shaft rotating
moist fll~ur cannot be kept for more than 24 hours as
on bearings mounted on the ends of a trough-shaped
it ferments.
barre1 made of mild steel sheet. In operation, the
disks are mtated in the grain at speeds mnging from
Dehulling Equipment for Grains
800 to 2500 rpm depending on the type of disk and
abrasive surface beiig used.
Technologies that cari be used for dehulling in-
Several models of this dehuller have becn de-
clude rcller milling equipment, rice dehulling equip-
veloped and have been fully described [6]. They have
ment, attrition-type dehullers and abrasive-type
been developed to take into account specific dehull-
dehullers [5]. Roller milling equipment does not ap-
ing needs and available technical skills in various
pear to be very useful in dehulling sorghum or millet;
countries. They are: PRL and h4ini-PRL dehullers,
the high degme of dehulling and colour of the flour
developed at the Prairie Regional Laboratoxy of the
obtained are unacceptable to the consumer. Rice
National Research Council in Canada (now the Plant
milling equipment does not generally work well on
Biotechnology Institute); the’ RIIC dehuller manu-
these cemal gmins, Abrasive-type dehullers use ro-
factured by the Rural Industries Innovation Centre in
tating abrasive surfaces to abrade the grain. Various
Botswana and being used extensively in that country;
Journal of Engineering for International Development
Page 29
Dehldlcr
Disc
DiSC
No. of
ElCCtliC
I TYF
Type
SpeedS
Discs
Mccor
RPM
POWtX
HP
PRL
stona
!No-1300
$13
8
RUC
stona
2aM
13
8
NUHULL
Stona
1500-2ooo
4
1 0
Mùli-PRL
Reshoid
MO-2OCXl
4
3
hfinKRs
Rcsinoid
1500-UMO
6
3
SISMARI
Resinoid
1500~2ooo
1 0
3
ISRA 1
SlSMAR/
Resinoid
15ca2ooo
8
3
ISRA II
Figure 1. Basic features of the abrasive disk
Mini-ENDA
SlOlleS
1500-2cm
1 0
3
dehuller:
A, spacers; B, end plate;
k
C, shaft; D, abrasive disk; E, barrel.
Table 1. Typical disc types, speeds and
power for the abrasive disc dehul-
lers.
the NUHULL dehuller manufactured by a Canadian
manufacturer, Nutana Machine Ltd.; the Mini-CRS
metry of the machine; it cari dehull an a!;soned mix-
dehuller developed by the Catholic Relief Services in
ture of grains in the same batch.
the Gambia and manufactured by local attisans; the
Mini-SISMAR/ISRA
1 and II dehullers developed by
the Centre Nationale de Recherches Agronomiques
ASPECTS OF DEHULLER’S PERFO:RMANCE
and a large manufacturing company SISMAR in
Scnegal; and the Mini-ENDA dehuller developed by
Laboratory and fïeld studies [6] indicate that the
ENDA in Zimbabwe.
performance of abrasive disk dehullers depend on:
The PRL, RIIC and NUHULL dehullers are
larger machines with aspiration systems for bran re-
speed of rotation of the disks;
moval, whemas the Mini dehullers have lower ca-
physical characteristics of the disk surface such
pacities and operate in the batch mode. Typical
as roughness and hardness;
features of these two sizes of machines are shown in
number and diamcter of disks in the barre]. rc-
Rgs. 2 and 3. Abrasive surfaces used in the dehullers
flecting the total surface area available for dc-
hulling;
arc in the form of carbonmdum stones or lightweight
spacing betwcen disks and thcir angle of incli-
resinoid discs. Some of the main features and speci-
nation to the shaft;
fil:ations of the dehullers are shown in Tables 1 and
distance between the end disks and the end plates
2 .
of the barrel;
Some of the advantages of the abrasive disc
clearance between the periphery and the barrel;
dehuller over others mentioned earlier are: its oper-
presence or absence of aspiration during dehull-
ating principle is simple; it is potentially lower in
ing;
price; it cari be easily operated by persons with little
roughness of the inner surface of thc barrel;
technical skills; it cari effectively dehull the grains in
the rate at which grains are fed into the dehuller
their dry state; it cari dehull a wide variety of cereal
in the continuous mode, and the quantity of grain
grains and grain legumes without adjusting the geo-
in the dehuller in the batch mode;
Page 30
Journal of Engbtxring for~%qmnati&al DweloPment
.,.’
.
Figure 2. Schematic showing specific fea-
tures of the Mini-SISMAWISRA
II dehuller: A, dehullers; B, feed
box; C, separator; D, aspirator, E, to
cyclone; F, two trap doors; G, sieve.
Solid arrows, movement of grains;
broken q-rows; bran (courtesy of
SISMAR, Senegal).
--
.
residence or retention time of the grain in the
dehuller;
. ’ physical characteristics of the grain being de-
hulled.
Available results are discussed in this section.
Effect of Speed of Rotation of Disks
Results of studies of t.hme models of abrasive
disk dehullers, using barrey. sorghum and millet, have
been reported [7]. The dehullers used were: a Graham
Hill IW%he:r containing 13 Carborundum stones 30.5
cm in diameter and 3.2 cm thick; an experimental
Resinoid Disk Dehuller containing 27 resinoid steel
cut&f disks, 30.5 cm in diameter sud 0.64 cm thick;
Jomnal of Engineering for International Development
Page 31
~---l--__-----.
-
0
1 0
20
30
40
DEGREE OF DEHULLING, 2;
Figure 4. Effect of disk speed on Resinoid
Disk Dehuller performance.-
-
that influence dehulling have been discussed 161.
They are the pericarp thickness and hardness of thc
Figure 3. Exploded view of the PRL: A,
grain.
bran to cyclone; B, fan; C, grain
happer, D, air inlet; E, feed gate; F,
Effect of Mass of Grain in Dehuller
abrasive disks; G, adjustable gates;
l-l, overflow outlet for dehuller
In practice the mass of grain dehulled in mills
grains (from 161).
varies from 0.5 to over 20 kg. The speed of rotation
of the disks, which is usualiy fixed for a given mill,
Laboratory [7] and field [9] observations indi-
depends on whether grinding stoncs or rcsinoid disks
cate that in’clining the lightweight disks at the end of
are used. The retention time is thus thc parameter
the dehulle:r to the vertical, impmves the unifonnity
used to control the degree of dehulling of the grains.
of dehulling. The angle of inclination of the disks
Detailed studies on the effect of mass of grain
have ranged from 9 degrees [7], to 5 degrees which
used on dehuller performance arc availablc for se-
eliminated disk breakage [9], pmbably due to the re-
veral Mini dehullers [8, 10, Il]. Results for quantities
duction of the rotationaI stresses. Inclining all of the
of millet between 2 and 8 kg, which is similar for
disks is expected to reduce dehulling time and im-
other grains, is shown in Fig. 5 for various disk
prove the ‘degree of dehulling but results arc no1
speeds. Holding all other parameters constant. the
available to confïrm the benefits of this arrangement.
degree of dehuiling increases as thc mass of the grain
Impmvement of the uniformity of dehulling has also
in the dehuller barre1 is increascd.
been achieved by reducing the gap between the disks
Typical results for quantities ranging from 0.5
at the cnd of the shaft and the end plates of thc de-
to 8 kg in the Mini-SISMAR/ISRA
1 machine using
hul1e.r (101.
resinoid discs [ 101 is shown in Fig. 6. The degree of
dehulling decreases with increasing mass of grain for
Effect of Grain Type
loads less than 2 kg.
The following explanation cari be givcn for the results
The performance of the dehuller depends on the
in Fig. 6 using Fig. 7. At very low loads thc grain at
type and variety of the grain being dehulled. Factors
lcvel 1 in Fig. 7 is not in contact with thc disc; there
-y,, .,
:
.._. ,’
Page 32
8
0
R.P.M.
15QO --54
l
Y- 4
2I
Retention Time =:3 min. I
0
10
15
DEGREE OF DEHULLiNG, %
--
Journal of Engineering for &.emational tivelopment
Page 33
yde resin bond (14%), Fluorspar (10.6%) and
reinforced with fibreglass (7%) to improve their
strength.
The comparative performance of Iightweight
disks anql grinding stones cari summarized as follows:
”
Coarse grit stones abrade grains much faster than
the smootier- faced resinoid disks but the colour
of the resulting fleur indicates that the resinoid
disks produce more unîform dehulling as de-
monstrated by reflectance measurements.
‘1
Grinding stones with coarsc grit should lu m-
tated at lower speeds in order to produc’e de-
hulling comparable to resingid disks.
#I
Grinding stones consume more energy compared
to lighter weight disks.
Degree of Dehulling
Results for the dehulling of maize using a Mini-PRL
dehuller using 5 carbonmdum disks (3 kg each) and
8 resinoid disks (0.15 kg each) [ 101 are shown in Fig.
Figurr! 7. Diagram used to explain the effect
8. The carborundum stones consume more fuel to
of mass of grain on the degree of
achieve a given degree of dehulling due to their
dehulling.
weight and surface friction effect.
processed (in batch sizes less than 3 kg), compared
Effect of Interna1 Lining in Barre1
to the village where 24 tonnes were being processed
(in batch sizes greater than 6 kg).
The interaction between disks and grain, grain
and grain, and grain and the barre1 wall is cornplex
Effect of Grinding Surface
and not well understood. Observations made during
tic development of the RIIC dehuller, for example,
Studies in SenegaI [lO] and in Canada [7] indi-
indicated differences between results for a bare wall
cated that resinoid disks, which are smoother than
and one lined with rubber. The degree of dehulling
carborundum stones, give better performance as in-
!was improved when the wall was lined with a rubber
dicated by degree of dehulling, uniformity of dehull-
material; the dehulling was less uniform, and power
ing and power consumption. Mechanical aspects of
consumption was increased [7] as shown in Table 4.
the dehulling #action between grinding stones and
Power consumption is increased due to the re-
grain h;ive been discussed [6]. Important charac-
stricted motion of the grains by the lining, and the
teristics of grinding stones for the dehullers appear
resulting increased resistance to the rotation of the
to be gtit size, the grade (pmhrtion of bonding ma-
disks. This is because the lining restricts the motion
terial) and the surface dressing. Examples of abrasive
of the grains near the wall, causing higher resistance
discs that have been used are:
to the rotation of the disks. In general the grains near
the wall are less dehulled than thosc near the dïsks.
.
carbomndum stones made of coarse grit silicon
carbide abrasives (85% of the total composition)
Areas for Further Studies
with vitrifïed clay bonds
.
resimid disks made of medium-grit aluminum
Although the mechanical performance of abra-
oxitie abrasive (67%), cured phenol formaldeh-
sive disc dehullers is satisfactory, the socio-economic
Page 34
+al Development
8
‘.3.
.
5
Retenbon Time, min
0
,i
0
5, . ..j. 0
,15
20 25
pmper sizing of dehullersfor given loads, mduction
ization of .disk
Table ;3. Wear experienced by resinoid disks
at village mills in.Senegal. : ’
_.<
,‘_
erating cos$.:. ._
~~g$g&
f thé dehuller is
** Average vhe for all 10 disks,
the .abrasive.:~&t5&$&@&nz&to
* Percentage wear with res@ct to newdisks: diam-
..% ,:r\\.ts r* .Y, , ,_,. 1
investigate the
eter = 25.4 cm, thickness = 3.6 cm, m.ass = 370.3 g.
effects-of :va@us~~& d@c&on the dehulling~ of
TEach batch dehulled greater than 6 kg.
grains. Emphasis’- sl$uidbe placed on longevity of
$Each batch dehulled between 2 and 3,kg.
disc surfaces and techniques for iocal manufacture.
,
viability of their opemtion is
main@ due to the inability of rural
o
;:.
.i;:.
.‘q.,i-
, .
de.hulling services, which is less
’ of grain. Thus any changes in
erating conditions of the dehuller aime&at,,mducing
its capitil and operating costs would allo.+‘, it to In
mor&i;v&ly used. These technical changesGUr be
obtained 'by improving the understanding of the per-
<.
formance of the dehuller.
.,
+There is a need to describe the mecha&s of the
inLeractiom hrween he abrasive di&@&&., and
the internal wall of the barml. In ‘&$!’ &?inve.s-
tigation, it would be informative ,to de,@nine the
optimum spacing between discs and ho; to maximize
the mixing of the grain so that uniform dehulling is
obtained. Results of such a study should lead to the
Journal d Engiriecring for International Development
Page 35
’;, 1. C O N C L U S I O N S ::
3. OSMAN, AK. 1981, “Bulrush MilNet (Pennise-
.A
..
,::{a:.:: <
.-- 1* :
tum typhoides) - a Contributory Factor to the
.‘*
Abrasive; disk dehullers have several featums
Endemicity of Goitre in Western Sudan”. Ecology
r
which make, themasupexior to: other dehullem for use
i ’
ofFmdandNutrition, Vol. 11, pp. 121-128.
’
in ;developing: cori&$& $heir Performance is depen-
4. GAI’T’AN E. et al, 1986, “C-glycosyflavones:
. .
dent on the;types;~gmin%the-physical arrangement
’ Goitrogens in Millet”, Frontie!;s ïn Throidology,,
and speed ofrot$on-of$he disks, the retention time,
eds. Medeiros-Neto, G., and Gaiian, G., Vol. 2,
the intemal surface :.of: the. dehuller banal, mass of
Plenum Medical Book Company, N. Y.
-. grain .beiig&$
5. REICHERT, R.D., 1981, “Sorghum Dry
aspiration,of’thi$b
k&i$$‘, Proceedirigs Internatioml S~&siti o n
:. $&$.&, Nov. 2-7, ICRISAT, F’atanchem, A. P.
f the dehull-
india, pp. 547-563.
een rotating
6 . BASSEY, M.W. and SCHMIDT, Q.G., 1989,
disks, grains @d&.hh,@et$_,surface of the barrel.
“Abrasive-Disk dehullers in Africa.: fmm Re-
T%e developmentl of appropriate abrasive surfaces
search to Dissemination”, /nternatiowl Develop-
that are durable and,.ch$&r .would encourage the
ment Research Centre, Ottawa, Canada.
wide spread use of the dehullers.
7 . OOMAH, B . D . , REICHBRT, R . D . and
YOUNGS, C.G., 1981, “Quantitative Compar-
ison Between Carborundum Stones and Resinoid
ACKNOWLEDGEMENTS
Disks in Dehulling Cereal Grains”, Cereal Chem-
istry, Vol. 58, No. 6, pp. 492-496.
Results djscussed in this paper were obtained
8 . MBENGUE, H.M., 1986, ‘“Cmation d’un
by research i&titu&ns:in Canada and Afïica with
decortiquer au SCnégal, Premier Rapport
tîmding pmvided ) by the Intemfitional Development
Int&inaire”, Projet 3-P-84-0016-02, Septembre.
Research Cent& (C&di).
Centre Nationale de Recherches Agronomiques
de Bambey, Senégal.
9 . NANCE, J.P.. 1987, “Gambian Grain Dehuiler,
REFERENCJXS
,
Final Report Phase One”, Catholic Relief Ser-
.,.
_.,
vices, The Gambia.
10. MBEMGUE, H.M., 1987, “Crt?at.ion d’un
, arjd PEARSON,
decortiqueur au SCnéegal, Deuxieme Rappon
ets, Their Compo-
Int&inaire”, Projet 3-P84-0016-02, Centre Natio-
Academic Press,
nale de Recherches Agronomiques de Bambey,
Sénegal.
11. MBENGUE, H.M., 1988, “Cr&ation d’un
decortiqueur au Sénegal, TroisiEme Rapport
ceedings, International
Int&inaixe”, Projet 3-P-84-0016-02, Centre Natio-
nale de Recherches Agronomiques dc Bambey.
Sénégal.
.’
:
-_
_ _c_ ._.
.._