Corn wet milling process for manufacturing starch

High-rate washing centrifuges are employed in a corn wet milling process to accomplish displacement washing of the starch and a sharp classification into starch and gluten.

FIELD OF THE INVENTION 
This invention is directed to improvements in the manufacture of starch by 
the wet method and, more particularly, to an improvement in the 
Starch-Gluten Separation Station in that method whereby heightened washing 
action and sharp classification is achieved. 
BACKGROUND OF THE INVENTION 
The modern process of starch wet milling is a series of independent 
classifications and separations integrated into a single balanced process 
in which a combination of centrifuges and hydrocyclones are commonly 
employed to separate product components including germ, fiber, gluten, 
starch and soluble solids. A great many flow arrangements have been 
employed over the years, but most users now have basically similar systems 
utilizing a countercurrent flow of water to the starch fraction. Such a 
starch wet milling system requires the following treatment stations: 
A. Steeping and Germ Separation 
B. Fiber Washing and Dewatering 
C. Starch-Gluten Separation 
D. Starch Washing and Thickening 
Each of the product components reports as the product of its respective 
station with the exception of the soluble solids which "fall out" during 
steep water evaporation. 
The starch consists of spherical granules about 10-15 microns in diameter 
having a density of about 1.5. The gluten is a flocculant structure of 1-3 
microns having a density of about 1.2. 
Evaporation of the steep water is carried out to obtain the soluble solids 
therein in the form of a heavy liquor which is mixed with other products 
of the process, such as fiber, and dried for use as animal feed. 
In the Steeping and Germ Separation Station the corn kernels are first 
steeped for softening and then cracked in a mill to release the germ. The 
germ contains the valuable corn oil and is separated from the magma of 
starch, hulls and fiber in the overflow of a hydrocyclone stage. The germ 
is washed in washing screens and then exits the system. The essentially 
germ-free hydrocyclone underflow reports to the Fiber Washing and 
Dewatering Station. 
The Fiber Washing and Dewatering Station includes a grit starch screening 
stage in which more than half the free starch is removed as undersize and 
forwarded to the Starch-Gluten Separation (Centrifugation) Station 
described below. The oversize from the grit starch screening stage is 
forwarded to a refining mill and then to a plurality of screen stages 
arranged for countercurrent washing of the fiber with the starch reporting 
as undersize at a density of 5.degree.Be to 6.degree.Be to the 
Starch-Gluten Separation Station. The fiber leaves the system at this 
point reporting to a centrifuge for dewatering and is subsequently dried. 
The Starch-Gluten Separation Station accepts the underflow from the grit 
starch screens and separates the starch from the gluten. The main 
starch-bearing stream is forwarded to the Starch Washing and Thickening 
Station while the gluten is forwarded to thickening and drying. Clarified 
water (free of insoluble solids) is provided by the Starch-Gluten 
Separation Station for the steeps as is process water (low in solubles) 
for use in various stages of the system. There is also a recycle stream 
containing solubles, insolubles and some starch that is sent to the Fiber 
Washing and Dewatering Station. 
The Starch-Gluten Separation Station commonly includes a plurality of 
centrifuges of the disc nozzle type, the first of which is the Mill Stream 
Thickener which accepts the feed stream from the grit starch screens 
underflow and thickens it to a concentration of 9.degree.Be to 
12.degree.Be. This thickened stream is forwarded as feed to the Primary 
centrifuge (a centrifuge accomplishing separation of starch and gluten) 
into which a wash liquid may be introduced in a wash liquid/draw-off 
volume ratio of 0.1 to 0.5, the draw-off being the amount of thickened 
slurry leaving the Primary centrifuge. A wash/liquid draw-off volume 
ration of 0.5 approximates the maximum ratio formerly attainable with 
available centrifuges in the corn wet milling process. The volume of wash 
liquid addition improves the protein (gluten) recovery from 35.+-.10% to a 
maximum of 50.+-.10%. 
The low protein recovery of the primary separator, especially when using 
larger size disc-nozzle centrifuges, causes protein to build up in the 
system and increases the runaround inside the loop including the fiber 
washing stages and the starch washing stages. A tight control on underflow 
density of the washing stages is a necessity in order to meet the finished 
starch quality requirement (less than 0.3% protein). 
In the corn wet milling process, there are many advantages from the 
process, economic and operational points of view to be derived from an 
improvement in protein recovery to as close to 100% as possible in the 
primary stage, and by avoiding protein accumulating and recycling in the 
process flows. 
The Starch Washing and Thickening Station is a multiple-stage 
countercurrent washing system using hydrocyclones which remove solubles 
together with the remaining insoluble protein and fine fiber in the feed 
stream as the final starch product is concentrated. The overflow stream 
from the Starch Washing and Thickening Station has the lowest soluble 
solids concentration of any stream in the process (except fresh wash 
water) and is returned to the Starch-Gluten Separation Station as process 
water. A system of the type described above is outlined in detail in an 
article entitled "Integrated Starch Wet Milling Process" by T. H. Bier, J. 
C. Elsken and R. W. Honeychurch, published in Die Starke 26 Jahrg. 
1974/No. 1, Pgs. 23-28. 
In co-pending U.S. patent application Ser. No. 612,044 for "High-Rate 
Washing Centrifuge", filed Nov. 13, 1990, there is disclosed an improved 
disc-nozzle centrifuge with structure permitting the introduction of very 
much larger amounts of wash liquid into the centrifuge than had been 
possible heretofore. Wash liquid in volume of from over 0.5 to 3 times the 
underflow draw-off volume is introduced directly into the rotor/separation 
chamber of the centrifuge simultaneously with the return flow of recycled 
underflow and results in displacement washing of the feed. The liquid 
originally associated with the feed is largely displaced to the overflow. 
In addition to the washing function, substantial improvement in 
classification of solids is effected due to the high rate of return flow 
and wash liquid into the separation chamber. The high upflow wash 
elutriates the bed of solids within the rotor/separation chamber and lifts 
the fines out of the fluid bed and sweeps them to the overflow. 
The disclosure of application Ser. No. 612,044 is incorporated herein by 
reference. 
It is an object of the invention to provide a starch wet milling process 
which incorporates an improved Starch-Gluten Separation Station and 
process. 
It is a further object of the invention to utilize the high-rate washing 
centrifuge in the Starch-Gluten Separation Station of the starch wet 
milling process to obtain improved washing of the starch and a sharp 
classification into starch and gluten. 
SUMMARY OF THE INVENTION 
In the present invention, the countercurrent flow of water and starch is 
maximized by routing the starch-gluten feed stream from the Fiber Washing 
and Dewatering Station through one or more high-rate washing centrifuges. 
Primary separation of the gluten and starch is effected by the high-rate 
washing centrifuges. The mill stream thickening centrifuge of the prior 
art is eliminated. The protein (gluten) recovery improves by about 20%. 
The wash water utilized in these high-rate washing centrifuges provides a 
wash liquid/draw-off volume ratio of substantially more than 0.5. In the 
process of this invention the wash liquid/draw-off volume ratio is at 
least 1.0 and in the range of 1 to 2, with a preferred ratio of 1.5. 
Providing this excess of wash water over the draw-off volume results in a 
high net flow of wash water inward toward the center of the centrifuge 
rotor. The inward wash stream "lifts" the insoluble gluten (protein) away 
from the starch by changing the sedimentation conditions so that the 
(relatively small) difference in density of the heavier starch and the 
lighter gluten is emphasized (magnified) by having a powerful liquid 
current flowing inward whereas a strong gravitational (centrigugal) force 
is exerted outward. The rotors spin at a high RPM, say 2700 RPM, 
generating a higher G-force (up to 2600 G) as opposed to 1500 G formerly 
available. The high RPM's and G-force generated compensate for the very 
high liquid flow volumes that must be handled for the process to have 
strong economic advantage over previous practice. 
In the process of the invention all of the wash water moves forward 
countercurrent against the flow of the starch solids and the gluten is 
decisively separated due to the strong upflow of the wash stream. 
Previously, the upflow elutriation phenomenon was not recognized as being 
practical in the corn wet milling process. The wash stream in the present 
invention elutriates the insoluble protein free of the starch granules and 
carries the gluten out the overflow. The soluble protein is similarly 
blocked and removed. As a result, the underflow will approach a 1.0% upper 
limit for insoluble protein, for example. 
The invention effects improved utilization of wash water to accomplish an 
important objective more efficiently; that is, less water is required to 
block the solubles.

DETAILED DESCRIPTION 
In the block diagram of FIG. 1 showing the starch wet milling process, the 
letter "A" marks the Steeping and Germ Separation Station in which shelled 
corn and steeping water are admitted and steeping is carried out to soften 
the kernels which are then screened and cracked in an attrition mill 
freeing the germ. Steep water is drawn off and routed to evaporators for 
recovery of soluble substances. The germ is separated and washed in this 
Station and then leaves this process for further treatment. A starch-rich 
underflow stream from the Germ Separation Station passes to the next 
Station in the process. 
The starch-rich stream from Station A is passed to the Fiber Washing and 
Dewatering Station (Station "B") where the starch milk (fiber starch) is 
separated from the coarse and fine fiber by multistage screening and 
countercurrent washing. The fibercontaining overflow from this screening 
and washing operation is dewatered and exits the process for fiber drying. 
The starch and gluten-containing underflow is forwarded to the 
Starch-Gluten Separation Station "C". 
In Station "C" the starch is centrifugally separated from the gluten. The 
gluten is thickened and exits the process. The starch slurry underflow of 
the centrifuges is forwarded to the Starch Washing and Thickening Station 
"D". 
In Station "D" countercurrent washing of the starch slurry takes place in 
multiple hydrocyclones to remove any remaining soluble and insoluble 
protein with the underflow constituting the starch product. 
FIG. 2 is a detailed representation of the prior art starch wet milling 
process disclosed in U.S. Pat. No. 4,207,118, owned by the assignee of the 
present invention. Attention is particularly directed to the Starch-Gluten 
Separation Station "C" illustrated. It should be noted that the mill 
stream from the Fiber Washing and Dewatering Station "B" is directed to 
the non-washing mill stream thickener centrifuge 94 for preliminary 
thickening before the underflow is subjected to the starch-gluten 
separation in the primary starch separator 96. While the primary starch 
separator does have a washing function, the ratio of wash liquid to 
draw-off volume never exceeds about 0.5, since that was the capability of 
available centrifuges. The gluten-containing overflow passes to a 
non-washing gluten thickening centrifuge 98 while the starch-rich 
underflow is forwarded to the starch washing system "D". 
FIG. 3 shows the high-rate washing centrifuge 100 used in the process of 
the invention. A rotor 107 is driven in rotation at high speed forcing the 
liquid/solid material in rotor/separation chamber 101 through nozzles 102 
into underflow pipe 103. A portion of the underflow is returned to 
rotor/separation chamber 101 through recycle line 104. A large volume of 
wash liquid is introduced into the rotor/separation chamber simultaneously 
with recycled underflow through wash line 105. The overflow moves upward 
to chamber 109 of the centrifuge and exits therefrom through pipe 108. 
In FIG. 4 the flow diagram represents the starch wet milling process of the 
invention comprising the Steeping and Germ Separation Station "A", the 
Fiber Washing and Dewatering Station "B", the Starch-Gluten Separation 
Station "C" and the Starch Washing and Thickening Station "D". The numeral 
10 designates one of the tanks of the steeping system which ordinarily 
consists of a plurality of steeping tanks arranged for countercurrent 
operation. The shelled corn is fed to tank 10 through line 12 and steeping 
water or acid is introduced into the steeping tank through line 14, and 
the steep water is drawn off through conduit 16 and sent to the evaporator 
(not shown) for recovery of soluble substances. The steeped corn from the 
tanks 10 is then passed via a conduit 18 to an attrition mill 20 to break 
up the steeped corn and to free the germ. From the attrition mill 20 the 
milled steeped corn is passed through conduit 24 to a germ washing and 
separation stage 22 where the germ is separated and passed by a conduit 26 
to a germ processing station (not shown) where it is screened, washed, 
dewatered, dried and the oil recovered. The underflow from the germ 
separation stage 22 is conducted via conduit 30 to the grit starch screens 
28 where it is screened to remove the starch, commonly called grit starch, 
released in the milling operation 20. The grit screen tailing from the 
grit starch screens 28 are conducted via a conduit 31 to Buhr mills 32 or 
other suitable disintegrators. From the mills 32 the ground grit screen 
tailings are passed by conduit 33 to a screening and washing station 34 
where the starch milk (fiber starch) is separated from the coarse and fine 
fiber by multistage screening and countercurrent washing. The 
fiber-containing overflow from the screening and washing station is 
conducted by conduit 36 to a processing station (not shown) for drying 
and/or further processing. 
Conduit 41, which conducts the grit starch from the grit starch screens 28, 
joins conduit 47, conducting the fiber starch from the Fiber Washing stage 
34, with the combined flows in conduit 49 forming the feed slurry to 
Station C. The feed slurry may contain from 5% to 15% protein (gluten) 
with approximately 8% protein on a dry basis. The feed slurry at a density 
of from 6.degree.Be to 12.degree.Be, and usually about 7.5.degree.Be, is 
introduced into the high-rate washing classification centrifuge 61 as a 
strong flow of wash liquid (a wash liquid/draw-off volume ratio in the 
range 1 to 2) is injected into the centrifuge through conduit 62. The 
purpose in having more wash water than draw-off is to have a high net flow 
of wash water inward towards the center of the rotor. This inward wash 
stream will "lift" the insoluble gluten away from the starch by changing 
the sedimentation situation so that the difference in density of the 
heavier starch and lighter gluten is emphasized (magnified) by having a 
powerful liquid current flowing inward whereas a strong gravitational 
(centrifugal) force is exerted outward. The sloping surfaces of the rotor 
bowl walls and the discs serve to positively reinforce this separating 
action on the two insoluble fractions with their widely different 
sedimentation characteristics. The net reversal in flow direction will 
prevent the hindered settlement action and improve the heavy media effect 
and give the beneficial result of a lower gluten concentration in the 
underflow water phase than the gluten concentration in the overflow. The 
centrifuge 61 has provision for recycle of a portion of the underflow 
through return line 63. A gluten-rich overflow stream leaves the 
centrifuge 61 through conduit 64 to enter a gluten-thickening non-washing 
centrifuge 81. With the introduction of the proper amount of wash water 
and control of other operating conditions, protein recovery close to 100% 
can be achieved. In centrifuge 81 the gluten-rich underflow leaves the 
process through conduit 82 for dewatering while the overflow stream is 
sufficiently low in solubles that it can serve as process water elsewhere 
in the process, passing through line 85. 
The starch-rich underflow from centrifuge 61 passes through conduit 66 to 
enter a second high-rate washing classification centrifuge 67 into which 
is injected through conduit 65 a strong flow of wash liquid amounting to 
more than 100% of the draw-off volume. The overflow stream from centrifuge 
67 constitutes the wash liquid which is injected into centrifuge 61 
through conduit 62. A portion of the underflow of centrifuge 67 is 
recycled through return line 68 for injection into the centrifuge with the 
wash liquid from conduit 65. The starch-rich underflow from centrifuge 67, 
at a density of from 14.degree.Be to 22.degree.Be, passes through conduit 
69 to the Starch Washing and Thickening Station D (with insoluble protein 
reduced to about 0.5%) where the final starch product is concentrated in a 
series of hydrocyclones. This hydrocyclone washing can be accomplished in 
from one to six stages (one stage indicated at 87), preferably three 
stages. This contrasts with contemporary systems in which twelve stages of 
hydrocyclone washing are customary. Using 10 mm hydrocyclones a further 
concentration of the starch to as much as 25.degree.Be is effected. The 
greatly reduced number of hydrocyclone washing stages required is due to 
the very effective washing accomplished in the high-rate washing 
centrifuges of the Starch-Gluten Separation Station C. The overflow stream 
of the hydrocyclones constitutes the wash liquid for centrifuge 67 and 
passes thereto through conduit 65. 
Alternatively, part of the hydrocyclone overflow can be routed back to the 
fiber washing system through line 70 (dotted) and, as a further alternate, 
this portion of the overflow (by diversion through line 72 (dotted line) 
can be thickened in a non-washing clarifier centrifuge 91 (dotted line 
showing) so that only a solids slurry moving through line 72 (dotted line) 
returns for rescreening and the full wash stream is still available for 
use in the high-rate washing centrifuge separation system. 
A further alternate is to send all or some of this slurry back (through 
line 74) to the second stage high-rate washing centrifuge separator. 
The overflow of the first stage high-rate washing centrifuge contains the 
entire gluten stream and it proceeds to a similarly sized non-washing 
centrifuge 81 that is configured for thickening only. 
The invention has been described above in an embodiment employing two 
stages of high-rate washing centrifuges with subsequent thickening and 
washing accomplished in hydrocyclone stages. It has been noted that, due 
to the excellent thickening and washing performance obtained with these 
centrifuges, the number of hydrocyclone thickening and washing stages can 
be reduced from the customary twelve stages to six or fewer stages. It 
will be understood that the thickening and washing functions may be 
entirely or almost entirely accomplished by three or more high-rate 
washing centrifuge stages. With three or more such high-rate washing 
centrifuge stages the need for additional thickening and washing in 
hydrocyclones is rendered very nearly or completely superfluous. 
FIG. 5 illustrates a Starch-Gluten Separation Station which, by including 
three stages of high-rate washing centrifuges, accomplishes such thorough 
washing and thickening that the underflow from the last centrifuge is the 
final starch product. The need for hydrocyclone washing and thickening 
stages is entirely eliminated. 
In FIG. 5, where possible, equipment and conduits have been identified by 
the same reference characters as in FIG. 4. The feed slurry enters the 
first high-rate washing classification centrifuge 61 from the Fiber 
Washing and Dewatering Station through conduit 62. The gluten-rich 
overflow from centrifuge 61 passes through conduit 64 for further 
processing while the starch-rich underflow moves through conduit 66 as 
feed for centrifuge 67. Overflow from centrifuge 67 is the wash liquid for 
centrifuge 61, conducted there through conduit 62. Underflow from 
centrifuge 67 is the feed for centrifuge 75 moving thereto through conduit 
69. The overflow of centrifuge 75 is the wash liquid for centrifuge 67 and 
passes thereto through conduit 65. The underflow of centrifuge 75 exits 
therefrom through conduit 78 as the final thickened starch product. Wash 
liquid, which may be fresh water, is provided for centrifuge 75 through 
conduit 79. Each of the centrifuges 61, 67 and 75 recycle part of the 
underflow that each generates through their respective recycle lines 63, 
68 and 77. 
In this three-stage Starch-Gluten Separation Station, just described, the 
effect achieved is to eliminate the necessity for additional treatment of 
the centrifuge product in hydrocyclone washing and thickening stages. Of 
course, it will be understood that the characteristics of the feed slurry 
under treatment will determine the number of centrifuge stations required 
to produce the final thickened starch product and, in some cases, more 
than three centrifuge stages may be needed. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations to be resorted to without departing from the spirit and scope 
of the invention as those skilled in the art will readily understand. Such 
modifications and variations are considered to be within the purview and 
scope of the invention and appended claims.