The Corn Wet Mill Process
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DESTONING
Fluid-Quip Stone Cyclones are designed for the Corn Wet Milling industry to protect downstream process equipment by removing stones, pebbles, nuts, bolts, or any other heavy contaminants that enter the process stream or come in with the fresh corn.
STEEPING
The corn after arriving to the plant, must be cleaned to meet the standards of the U.S. Yellow Dent #2 corn. The cleaned corn is conveyed and metered into the steep tanks. Steeping the corn prior to milling is done by soaking the corn in a solution of sulfur dioxide and water at controlled temperature for a length of time between 30 and 45 hours. The purpose of steeping is to soften the kernel, allowing for separation of the germ without cracking during milling, to partially breakdown the protein matrix in which the starch is embedded allowing for separation in subsequent milling stages, and to remove the soluble impurities contained within the corn. Good steeping is a necessity for achieving a good quality starch product.
Freshly made steep acid is added to the steep tank where the corn has been in the steeping process the longest. The steep acid is circulated through the steep tanks towards the tank where the newest corn is being added. From this point, some of the steep liquor must be removed from the system. The amount of steep water removed is critical for producing a quality starch product as this is the only point in the system where soluble impurities can leave the system. The dissolved solids content of this stream is usually in the range of 10%.
Water in this light steep water stream needs to be evaporated until the solids content reaches about 50%. Doing this will allow the "heavy" steep water to be mixed with the end fiber product to increase the nutritional content of the feed product. The condensate from this evaporator, being high in impurities, can not be utilized back into the process and therefore makes up the majority of the waste water leaving the plant.
After steeping, the corn is conveyed to the milling area via sluice water. This water flows between the outlet of the steep tank to the dewatering screen prior the first stage milling where it is continuously recycled back. In between the steep tanks and the dewatering screen, the slurry is then fed to a Stone Cyclone.
CORN DEWATERING
Before the first grind process step, the corn slurry from the destoning cyclone is dewatered by a gravity screen. Dewatering the corn slurry prior to milling reduces the hydraulic load on the grind mill and improves the milling efficiency.
Fluid-Quip Gravity Screens are designed for ease of operation and for simplicity of maintenance to reduce down-time in liquid-solid separation and de-watering applications such as wet corn milling, pulp and paper processing, chemical polymers, waste water treatment and food processing.
FIRST GRIND MILL
The steeped corn, after being dewatered by the corn screens, enters into the first grind. The purpose of the first grind is to crack the corn kernels and free the germs.
The devils tooth grind plates commonly used in the first grind mill have a pattern of large interlocking teeth that the corn kernels must pass through before reaching the machine discharge. In the grind mill, one set of the grind plates are rotating, while the other set of plates is fixed.
The gap between these two sets of plates are adjusted so that first grind mill cracks most of the kernels and frees up most of the germs without damage to the soft germ particles. Any corn not cracked in the first stage, will be cracked in the second stage.
Dilution water is added to the first grind mill so that the starch freed from the kernel can enter into slurry without pasting up in the grind mill.
Factors affecting the capacity and efficiency of the first grind mills include the density of the feed slurry, the plate clearance, the applied horsepower, and the steep processes employed.
After leaving the first grindmill, the slurry is gravity discharged into the first grind tank where it is mixed with recycle streams from the germ separation and other millhouse recycle streams.
Fluid-Quip’s disc mills are designed for optimum performance. Our engineers focused on durability, structural stability and solidifying tram control to minimize vibration, reduce wear and maintenance costs. The result is one of the finest wet corn disc mills in the world today.
PRIMARY GERM SEPARATION
The slurry from first grind tank is fed to the primary germ separation system, where the main separation of the germs from the slurry occurs.
The primary germ separation system consists of a two stage system, where the underflow of the first stage is fed to a second stage. Usually the two stages are directly connected with only one feed pump for the entire system.
Typically, the baume in the first grind tank is around 8. At this baume, the germs, with lighter specific gravity than the starch slurry, will start to float on the top of the slurry. The centrifugal force inside the cyclone, driven by the pressure drop across the cyclone, accelerates the floating of the germs, so that the overflow of the first stage cyclone contains many of the germs in the slurry.
The overflow of the first stage cyclone is controlled with pressure to ensure that most of the germs are leaving the system with a minimal amount of fiber.
The underflow of the first stage cyclone is then fed to a second stage to recover more of the germs remaining in the slurry. The overflow of this second stage is directed back to the first grind tank, and the underflow of this stage proceeds to the second grind step.
SECOND GRIND DEWATERING SCREEN
Before entering the second grind mill, the slurry from the underflow of the primary germ separation system is dewatered by a gravity screen. Dewatering the corn slurry prior to milling reduces the hydraulic load on the grind mill and improves the milling efficiency.
Fluid-Quip Gravity Screens are designed for ease of operation and for simplicity of maintenance to reduce down-time in liquid-solid separation and de-watering applications such as wet corn milling, pulp and paper processing, chemical polymers, waste water treatment and food processing
SECOND GRIND MILL
The purpose of the second grind is to crack any of the kernels that were missed in the first grind. Like the first grind, the slurry feeding the second grind is dewatered just prior to milling. From the second grind mill, the slurry is again diluted and then dumped into the second grind tank, which feeds the secondary germ separation system.
A handful of discharge from the second grind mill should not contain any whole kernels, on average. Factors affecting the efficiency of the second grind include the steep processes, the efficiency of the first grind, the feed slurry density, the grind plates used, the clearance between the plates, and the applied horsepower.
Like the first grind, devil tooth plates are used in the second grind. The clearance between the plates should be set closer than in the first grind. Setting the plates too close, would result in cracking germs and tearing fibers more than necessary. Setting the plates too far would result in a lower overall oil recovery, as any germ not recovered in the second grind will be lost later in the process.
Fluid-Quip’s disc mills are designed for optimum performance. Our engineers focused on durability, structural stability and solidifying tram control to minimize vibration, reduce wear and maintenance costs. The result is one of the finest wet corn disc mills in the world today.
SECONDARY GERM SEPARATION
The slurry from the second grind tank is fed to the secondary germ separation system, where additional recovery of the germ from the slurry occurs.
The secondary germ separation system also consists of a two stage system, where the underflow of the first stage is fed to a second stage. Usually the two stages are directly connected with only one feed pump for the entire system.
The secondary germ separation system is usually the same size as the primary system, despite a large portion of the mill slurry has been removed via the primary germ separation system overflow.
Typically, the baume in the first grind tank is around 8. At this baume, the germs, with lighter specific gravity than the starch slurry, will start to float on the top of the slurry. The centrifugal force inside the cyclone, driven by the pressure drop across the cyclone, accelerates the floating of the germs, so that the overflow of the first stage cyclone contains many of the germs in the slurry.
The overflow of the first stage cyclone is controlled with pressure to ensure that most of the germs with a minimum amount of fiber is leaving the system with the germs.
The underflow of the first stage cyclone is then fed to a second stage to recover more of the germs remaining in the slurry. The overflow of this second stage is directed back to the first grind tank, and the underflow of this stage proceeds to the third grind step.
GERM WASHING SYSTEM
The overflow of the first stage of the primary germ separation system contains all of the recovered germ, some fiber, and some starch. The purpose of the germ washing system is to wash as much of the starch as possible from the germ, and separate fiber from the germ.
The system typically consists of a three stage counter current system, with the feed entering into the first stage and the wash water entering into the feed of the third stage. The overflow from the third stage is the germ which proceeds to further dewatering and drying stages. The first stage underflow is a starch/gluten stream which proceeds back to the first grind tank for dilution.
The amount of wash water added to the third stage should be about 9 gallons for every 1000 bushels of grind rate. This amount can be adjusted to either increase the starch recovery or control the baume in the first grind tank.
Fluid-Quip gravity screens with 31” long screens and 1mm screen spacing are common for germ washing screens. Germ washing screens are designed special to accommodate wash headers and stacking so screens can be stacked in the millhouse so the product can cascade from one screen to the next without the need for pumps in between stages.
GERM PRESS/GERM DRYING
After the germ stream is washed in the germ washing system, the slurry, which still contains some fiber, then proceeds to a dewatering press to remove as much free water as possible. At the outlet of the germ dewatering press, the germ product is dried in the germ dryer.
THIRD GRIND MILL
After the germ has been successfully removed during the first and second grind, the goal of the third grind is to remove the bound starch from the fiber. This should be done with as little damage to the fiber as possible. An increase in the amount of milling of the fiber lowers the bound starch but also increases the amount of fine fiber.
For the third grind application, Fluid-Quip offers both disc mills and Impact mills. For disc mills, like the other grind positions, devil tooth plates are used for the third grind application. The clearance between the plates should be closer than for second grind. Setting the plates too close, would result in an unnecessary increase in the amount of fine fibers. Setting the plates gap to wide would result in high amount of bound starch in the fiber product.
For impact milling, slurry enters rotor through the feed inlets and is accelerated to pins around the circumference of the rotor by centrifugal force Rotating pins impact the fiber and starch particles, breaking starch free from the fiber and reducing the particle size of starch grits. The initial impact from the rotating pins flings the particles into the stationary pins inside of the machine housing further freeing the starch from the fiber and reducing the particle size.
Impact mills are available in various sizes for different process needs. Impact mills offer several process benefits as described on the impact mill product page. Fluid-Quip offers the FQ-IM40 and FQ-IM40H horizontal rotor Impact Mills for the corn wet milling third grind application. By combining gentle action on the delicate corn fibers with powerful crushing power on the bound starch particles, the Fluid-Quip Impact Mills do make quite an Impact on corn wet mill bound starch numbers and overall starch yields.
The Fluid-Quip 52” third grind disc is the standard for 3rd grind. Fluid-Quip’s disc mills are designed for optimum performance. Our engineers focused on durability, structural stability and solidifying tram control to minimize vibration, reduce wear and maintenance costs. The result is one of the finest wet corn disc mills in the world today.
FIBER WASHING SYSTEM
The fiber wash system consists of a series of 120 degree pressure screens configured to counter-currently wash the fiber stream coming from the third grind mill. The goal of the fiberwash system is to wash all of the free starch off of the fiber pieces.
Typically, a 6 stage system is used where the feed enters into the 1st stage. The wash water enters in the final stage. The unders of each stage becomes the wash for the previous stage, and the overs of each stage become the feed for the next stage. The unders of the first stage becomes the feed starch slurry heading for the centrifuges while the overs of the last stage is fiber heading to the fiber dewatering step in the process.
There is some starch that leaves the system with the fiber and can not be recovered. This starch can either be bound starch (unrecoverable pieces of starch physically attached to the fiber) or free starch (starch in the water). An efficient wash system will minimize the amount of free starch leaving with the fiber to about 5-10%. Much of this free starch can be recovered prior to the fiber dewatering press with a fiber press dewatering screen. If the bound starch is high, the problem is most likely in the steeping and grinding processes.
In stages 2 to 6, 75 micron screens are used, and they are fed with 0.75” nozzles. In the first stage, 0.50” nozzles and a 50 micron screen is used to prevent fine fibers from entering the starch slurry. All stages can be fed at close to 60 psi pressure for maximum capacity.
To recover much of the free starch leaving with the fiber in the 6th stage overs, a fiber dewatering screen, with a 150 micron screen fed with 0.75” nozzles can be used. Using this screen to dewater prior to the press can increase the efficiency of the press and recover some of the free starch. The overs of the screen are further dewatered, and the unders of the screen are recycled back to the main fiberwash screens.
For the first stage fiber wash, one Fluid-Quip triple pressure screen should be used for every 15,000 bpd of grind rate. For stages, 2 through 6, one Fluid-Quip triple pressure screen should be used for every 45,000 bpd.
For the fiber dewatering screen, one Fluid-Quip triple pressure screen should be used for every 45,000 bpd of grind rate.
DEGRITTING SYSTEM
The mill starch stream, comprised of the combined filtrates from the third grind dewatering screen and the first stage fiber wash screen, will be further processed by centrifuges and starch washing cyclonettes to separate the starch and gluten fractions of the slurry.
To protect the down stream equipment, the mill starch stream is fed to a degritting system to remove and small heavy particles such as sand, grit, rust, or pipe scale. These materials, when passing through the high speed centrifuges, would cause rapid wear and lead to premature replacement of expensive machine parts. This heavy material also contributes to premature wear of the cyclonettes in the starch washing system.
The degritting system consists of a two stage system, with the underflow of the first stage being fed to a second stage.
The overflow of the first stage proceeds to the centrifuge separation steps. The overflow of the second stage is recycled back to the feed of the first stage.
Reject pots on the underflow of the second stage cyclones collect the contaminants, which are purged periodically with timed automated purge valves connected to the reject pot.
Fluid-Quip supplies degritting systems with 8” cyclones in the first stage and 6” cyclones in the second stage. The first stage cyclonettes are equipped with ceramic lower cones to extend cyclone life. Systems are sized based on capacity requirements. For smaller plant capacities, single stage, or single cyclone systems are also effective.
MST STRAINER
To offer protection against plugging of centrifuges and starch washing system cyclonettes, rotary strainers are used in the feed line to each of these process steps.
The feed to the strainers enter the inside of a cylindrical screen that is continuously cleaned by a rotating brush. The oversized particles and contaminants leave the strainer through the drain, while most of the feed stream continues through the screen and to the centrifuge or starch washing system.
The Fluid-Quip Rotary Strainer is a separation device designed for continuous delivery of a strained liquid or slurry stream, free of oversize particles. With a Fluid-Quip rotary strainer installed ahead of your process equipment, you can rest assured knowing that your process is protected against contamination, and your process equipment is protected against plugging, excessive wear, or failure due to contamination. Our improved design allows for consistent operation and minimal maintenance.
MST CENTRIFUGE
The combined filtrate streams from the third grind screen and first stage fiberwash screens will contain about 10 to 11% solids (5 to 6 Be). This slurry is comprised of the starch and protein components of the corn, along with some soluble impurities released from the corn during the steeping process.
This slurry is fed to the Mill Stream Thickener (MST) centrifuge to thicken the starch slurry and provide a process water stream. The centrifuge has high rotational speed, creating significant G forces inside the machine which drives the separation of the water, solubles, and higher specific gravity starch and gluten particles. The solids are continuously discharged through nozzles around the outside/circumference of the bowl, while the water and solubles head toward the center of the bowl and out the overflow of the machine.
The MST centrifuge increases the baume of the mill stream slurry providing a reduced flow rate and consistent feed baume to the downstream primary centrifuge, and also separates out solubles early in the starch protein separation process. This reduces the washing requirements to remove the solubles in the down stream primary centrifuge and starch washing system.
The overflow of the MST centrifuge is primarily used for the steeping system makeup water. Great care is taken in the operation of the MST centrifuge to ensure that a minimum amount of starch and gluten solids are sent back to the steeping system through the MST overflow.
Fluid-Quip offers 30” and 36” centrifuges with respective MST grind rate capacities up to 1000 mtpd and 2000 mtpd.
PRIMARY STRAINER
To offer protection against plugging of centrifuges and starch washing system cyclonettes, rotary strainers are used in the feed line to each of these process steps.
The feed to the strainers enter the inside of a cylindrical screen that is continuously cleaned by a rotating brush. The oversized particles and contaminants leave the strainer through the drain, while most of the feed stream continues through the screen and to the centrifuge or starch washing system.
The Fluid-Quip Rotary Strainer is a separation device designed for continuous delivery of a strained liquid or slurry stream, free of oversize particles. With a Fluid-Quip rotary strainer installed ahead of your process equipment, you can rest assured knowing that your process is protected against contamination, and your process equipment is protected against plugging, excessive wear, or failure due to contamination. Our improved design allows for consistent operation and minimal maintenance.
CLARIFIER CENTRIFUGE
The clarifier separates the starch washing system overflow into a process water stream, and a thickened starch/protein stream.
Part of the overflow of the clarifier centrifuge is used as wash water for the primary centrifuge. The remaining overflow not used as primary wash water is used for wash water in the fiber washing or germ washing systems.
Since any fine fiber making it through the third grind dewatering screen or the first stage fiberwash screens ends up in the underflow of the clarifier, a portion of this stream is recycled back to the fiber wash system. The remainder of the clarifier underflow is added to the primary centrifuge feed stream.
Fluid-Quip offers 30” and 36” centrifuges with respective Clarifier capacities up to 1000 mtpd and 2000 mtpd.
CLARIFIER STRAINER
To offer protection against plugging of centrifuges and starch washing system cyclonettes, rotary strainers are used in the feed line to each of these process steps.
The feed to the strainers enter the inside of a cylindrical screen that is continuously cleaned by a rotating brush. The oversized particles and contaminants leave the strainer through the drain, while most of the feed stream continues through the screen and to the centrifuge or starch washing system.
The Fluid-Quip Rotary Strainer is a separation device designed for continuous delivery of a strained liquid or slurry stream, free of oversize particles. With a Fluid-Quip rotary strainer installed ahead of your process equipment, you can rest assured knowing that your process is protected against contamination, and your process equipment is protected against plugging, excessive wear, or failure due to contamination. Our improved design allows for consistent operation and minimal maintenance.
PRIMARY CENTRIFUGE
The primary centrifuge is fed from the MST centrifuge underflow and any direct portion of the mill starch stream that bypasses the MST. The primary centrifuge is the main separation point between the starch and the water, gluten, and solubles.
Wash water from the clarifier/MST centrifuge overflow is introduced into the primary where it washes the solubles from the starch via displacement washing. Fluid-Quip primary centrifuges are designed to accommodate high rates of displacement washing, to lower the residual soluble protein levels in the starch leaving through the underflow of the machine.
The overflow of the primary centrifuge becomes the light gluten stream, which is further dewatered and dried to become the gluten meal product. Since any starch in the overflow of the primary centrifuge will be lost to the gluten meal product, and lower the important protein concentration of the gluten meal, it is very important to operate the machine with minimal starch loss to the overflow.
The underflow of the primary centrifuge, which still contains some insoluble and soluble proteins, is further purified in the starch washing system.
Fluid-Quip offers 30” and 36” centrifuges with respective Primary separation capacities up to 500 mtpd and 1000+ mtpd depending on plant configuration.
SW STRAINER
To offer protection against plugging of centrifuges and starch washing system cyclonettes, rotary strainers are used in the feed line to each of these process steps.
The feed to the strainers enter the inside of a cylindrical screen that is continuously cleaned by a rotating brush. The oversized particles and contaminants leave the strainer through the drain, while most of the feed stream continues through the screen and to the centrifuge or starch washing system.
The Fluid-Quip Rotary Strainer is a separation device designed for continuous delivery of a strained liquid or slurry stream, free of oversize particles. With a Fluid-Quip rotary strainer installed ahead of your process equipment, you can rest assured knowing that your process is protected against contamination, and your process equipment is protected against plugging, excessive wear, or failure due to contamination. Our improved design allows for consistent operation and minimal maintenance.
GT STRAINER
To offer protection against plugging of centrifuges and starch washing system cyclonettes, rotary strainers are used in the feed line to each of these process steps.
The feed to the strainers enter the inside of a cylindrical screen that is continuously cleaned by a rotating brush. The oversized particles and contaminants leave the strainer through the drain, while most of the feed stream continues through the screen and to the centrifuge or starch washing system.
The Fluid-Quip Rotary Strainer is a separation device designed for continuous delivery of a strained liquid or slurry stream, free of oversize particles. With a Fluid-Quip rotary strainer installed ahead of your process equipment, you can rest assured knowing that your process is protected against contamination, and your process equipment is protected against plugging, excessive wear, or failure due to contamination. Our improved design allows for consistent operation and minimal maintenance.
GLUTEN THICKENER
From the overflow of the primary centrifuge, the light gluten slurry is fed to a Gluten Thickener (GT) centrifuge. This light gluten stream contains all of the gluten to be recovered in the gluten product stream, water, and soluble proteins. The gluten thickener centrifuge concentrates the gluten stream prior to dewatering and also provides clear overflow for process water to be used upstream in the millhouse. Since the overflow of the GT machine has far less solubles than the overflow of the MST machine, this stream is used as fiber wash or germ wash process water. This improves the counter current washing efficiency of the entire wet mill process.
The underflow of the GT machine becomes the heavy gluten stream and contains all of the recoverable gluten that will leave with the gluten product. This heavy gluten continues in the process to the gluten dewatering step.
Fluid-Quip offers 30” and 36” centrifuges with respective GT capacities up to 500 mtpd and 1000 mtpd.
STARCH WASHING SYSTEM
To produce a high quality starch product, the underflow of the primary is sent to a multiple stage washing system. In this system, fresh water is added to the final stage to wash the starch slurry counter currently across 12 or more stages. The starch reports to the underflow of each stage, getting purer and purer as it progresses towards the end of the system. The protein and soluble impurities are carried out the overflow of the first stage of the system with some starch. Sending some starch out the overflow is necessary for ensuring a high quality starch product. The underflow of the starch washing system proceeds to further processing for modification into specialty starch products, conversion to syrup for sweeteners/ethanol, or dewatering/drying to make dry starch product.
The overflow of the starch washing system is sent back to the clarifier centrifuge.
GLUTEN DEWATERING
After the GT machine has thickened up the gluten stream by removing as much water as possible, vacuum belt filters are commonly used for additional dewatering. The water removed from the gluten slurry at the vacuum filters is sent back to the feed of the GT machine to recovery and remaining gluten.
The dewatering Gluten cake is then conveyed to the Gluten Dryer, typically a flash dryer system.
FIBER PRESS / DRYER
After the fiber stream is washed in the fiber washing system and dewaterd, the slurry is further dewatered in a fiber press. At the outlet of the fiber dewatering press, the fiber product is dried in the fiber dryer.
The dried fiber product is often mixed with heavy steep water and palletized for use as animal feed.
The corn wet milling process
Fluid-Quip supplies process equipment for most of the physical separation steps of the corn wet mill, whose individual processes shown on the Corn Wet Milling Process Flow Sheet are discussed in general terms above.