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MAIZE MILLING TECHNICAL ADVICE - PART 1
MAIZE MILLING TECHNICAL ADVICE, PART ONE
INTRODUCTION SHAMROCK MILLING SYTEMS
Shamrock Milling Systems design, engineer, erect and commission the highest technical standard maize mills in South Africa. We can offer our customers either new, or a combination of new and used milling equipment, as per their requirements. We are recognized as being amongst the leaders in the construction of state of the art maize and wheat mills in South Africa.
MAIZE MILLING TECHNICAL ADVICE
Maize is one of the most important crops produced in the world with direct links to food production, animal feed, bio fuels, starches, alcohol production, snack food manufacturing and literally hundreds of products produced with maize in one or another form in the product.
In any given year, there are grain crop production variations affected by market conditions, droughts and carryover stocks from previous years. In 2006, Maize production was approximately 700 million tons, with the US producing 330 million tons or 40% of the world crop. Important maize producing countries are,
SOUTH AFRICAN MAIZE PRODUCTION
A mild to moderate El Nino prediction in November 2009 luckily did not transpire and the entire growing season in South Africa, this past six months, has been exceptionally good. Good October rainfall, which was well distributed, especially in the maize triangle production area, saw 2.7 million hectares planted. The latest crop production forecast for 2009-10 season, is 12.96 million tons, with 7.8 million white maize and 5.1 million yellow maize expected. This harvest will be South Africa's second highest, coming in just below the record 14.42 million tons delivered in the 1981-82 season. Also of importance is that the crop quality seems to be exceptionally good.
Maize milling falls into two very distinct categories
1) Wet maize milling
2) Dry maize milling with a number of different process methods.
WET MAIZE MILLING
The purpose of this article is to discuss the dry milling industry, in which Shamrock Milling Systems has considerable technical expertise. Shamrock Milling Systems are experts in the design, engineering and construction of dry maize milling plants and our numerous customers bear testimony to our expertise. However in passing - a broad overview of the wet milling process, is as follows:
This process resembles the dry maize milling industry only in that the common feed stock is maize and the initial grain receiving systems, mechanical transport elements for the delivery and reclaiming of maize from storage silos, is the same. Once the maize is withdrawn from the silos and passes through a typical maize cleaning plant, this is where the process similarity ends and the highly complex wet milling process starts.
For the most part, mechanical engineering companies supplying process plant and equipment to the dry milling industry, are not involved in the wet milling industry, beyond supplying the mechanical conveying and cleaning equipment mentioned above. The wet milling process in very basic terms, is as follows:
An overview of a wet milling plant resembles that of a refinery or chemical engineering plant. There are so many processes involved with considerable operational involvement required from chemical engineers and cereal chemists to control the very exact mechanical and chemical processes. Some of the hundreds of products using components produced from the wet milling process are:
In conclusion, besides the modification to starch by chemical alteration, the primary difference in the two major categories of milling, is that in wet milling, the protein is separated from the starch and almost pure starches result with protein contents as low as 0,3 %. This separation of starch and protein is only possible through a wet milling process. Due to the enormous capital and operation costs, each plant seeks the optimal use and maximum financial realization of each part of the maize kernel. Continual research and development of existing and new products to meet every expanding customers requirements, are the order of the day.
DRY MAIZE MILLING
It is somewhat of a contradiction to call it "dry maize milling", as the highest quality products are only produced when water is added to the whole, cleaned kernels, prior to the milling process, as many as three times. Besides some novel approaches to dry milling of maize, the following would be the primary methods of dry maize milling.
Whole maize grinding on a hammermill, micro and small maize mills
Beall or high compression degermination
Maize Milling for human consumption
WHOLE MAIZE GRINDING, SMALL CAPACITY MAIZE MILLS
In some cases, in the rural areas of the Southern African region, one finds a very simple hammermill driven by a diesel motor, generator when electricity is not available or PTO from a tractor. The process involves the manual feeding of a bag of whole maize into a receiving hopper with dosing slide to control the feed rate into the hammermill. The maize may be cleaned and there may or may not be a simple magnet device.
The process would involve varying diameters of screen size in the hammermill and this could be followed by a direct bagging off of the whole ground maize stock, exiting the hammermill without sieving. A further extension of the system, would be a rudimentary type reel or centrifugal sifter to remove the coarsest fractions of germ and bran for animal feed. The remaining meal, which could be anything from 100% in whole maize meal, to a 90 % extraction, following a rudimentary sieving, could well form the basic dietary needs of a rural community. In these situations there would almost certainly be no vitamin additions and packing materials would consist of anything from a bag to a wheel barrow.
These mills are sometimes taken a step further in technical ability, with refinements to the sieving method and a basic maize cleaner and simple conditioning bin placed in front of the mill. There are a number of milling engineers who have specialized in the manufacturing of a simple micro or small milling plant. Very often these mills have a multiple double high rollermills with small diameter rolls and short roller length and these directly feed the ground maize onto reciprocating sieves below the rollers and form one machine. The entire milling machine can fit on the back of a pick-up truck and weighs just over 1000 kg .Further refinement of the system would be a standalone mini sifter or number of them, for various grades and fractions to be separated. In some cases, the coarser fractions will be mechanically or pneumatically conveyed to a further roll or series of rolls for further refinement. The products produced on these micro or small milling plants, are usually Special and Sifted maize meal and in some cases on an extended process flow, small quantities of Super maize meal could be extracted. These mills have a place in the market and allow low cost entry into the milling industry and supply niche markets. These milling systems are short flow milling systems and cannot produce the highest grade products at a low fat content, at the maximum extraction rates, as obtained in large scale commercial milling plants. Obtaining the highest extraction rates of premium quality products requires the highest application of milling technology.
In 1901 the Beall degerminator was patented by the Beall Manufacturing Co in Illinois USA. Its arrival in South Africa seems to have been around the mid nineteen sixties, when urbanization and its inherent lifestyle, brought about higher disposable income and demand for more refined maize products. Prior to the introduction of the degerminator, with a detailed explanation following in the next section, the standard maize mill in South Africa was a so-called rollermill degermination plant with whole cleaned conditioned maize delivered onto first break rollermill. The process flow was as follows:
The normal layout of these mills, and there are many of them still existing today, was a single row of rollermills at ground floor level, typically three to five, and a line shaft drive in a basement below. This basement was wide enough, usually only on the one side, to accommodate the elevator boots and a second floor above the rollermills, accommodated an assortment of reels and mini sifters. These buildings usually had a mono pitch roof with the high point accommodating the elevator heads and as the years progressed in the nineteen sixties and seventies these mills would typically lift the roof to accommodate cyclones and airlocks to replace the elevator system with pneumatics. Further upgrading would result in plansifters replacing the reels and centrifugal sifters. As height was a problem, one always found low head room sixteen box high plansifters in these mills. These mills usually had three or four aspirators and with the conditioning just prior to first break roll, they could produce very high quality products. In the best equipped mills, as much as 20 % super maize meal, could be extracted. The germ extraction in these mills, was anywhere between 10 and 22 % and this variation was as a direct result of the percentage of super and special extracted as opposed to sifted and unsifted meals. There are still today, a number of these mills in operation and are usually locally bound in their marketing and sales. The germ and bran in these mills, is very clean and lacks the ground starch or fine meal found in the germ bag from a degermination plant. The typical fluting found in such a mill, would be:
First break with 8 FPI,
Second break with 12 FPI,
Third break with 16 FPI,
Reduction or Sizing 1 with 16 FPI,
Reduction or Sizing 2 with 20 FPI
Reduction or Sizing 3 with 24 FPI. (sometimes referred to as Tails roll)
When a germ roll was employed, a fluting of 9 FPI with a differential of 1 to 1.5 was employed to squeeze and flatten the germ without cutting.
There were quite a number of standard mills installed throughout the country with four or five rollermills producing 8 or 10 individual passages, and for a five ton mill, a single six section plansifter was more than sufficient, with a number of the end passages only requiring half a plansifter section.
The mills that were better equipped would have three or four aspirators on the breaks and first reduction and possibly a bran finisher to clean up the tail end offal streams. These mills had two maize meal collection screw conveyors for special and sifted and normally a flap with fine germ would be diverted into the sifted conveyor to produce unsifted maize meal. A germ collection conveyor would be directed to a far corner of the warehouse to segregate the offal packing operation from the maize meal.
BEALL OR HIGH COMPRESSION DEGERMINATION-MODERN MILLING TECHNOLOGY
As briefly mentioned in the rollermill degermination paragraph above, we understand the Beall degerminator first appeared in the South African maize milling industry in the mid nineteen sixties. In those early mills there was a considerable belief that the maize should be treated with steam and hot water prior to degermination and that this enhanced the maize meal taste and assisted in the following cooking process. Over the years, the general mess and maintenance associated with steam conditioners and costs of running boilers, has all but seen the disappearance of steam as a conditioning agent. The original Beall degerminator patented in 1901, had many modifications made to it over the 90 years that it existed as the core determination machine in the maize milling industry.
Over the years all the various milling engineers made modifications to the original basic conical rotor machine. Some adjusted the rotor in relation to the stator and others visa versa to adjust the grinding gap. All and every means of adjusting and modifying the stock retarding/retention gate, were tried and tested to control retention time. In the mid nineteen eighties a cylindrical version of the Beall type degerminator was introduced with adjusting stationary side walls forming the conical shape within the grinding chamber, rather than the standard Beall Rotor, which was conical. The typical degerminator capacity was 3000-3500 KG/hour and the overtails/thru?s ratio was 70%-30%.The standard motor was 55 kW and normal amps drawn was 60-80 amps depending on how hard the grinding requirement was. There are always exceptions to the rule and in some cases millers fed higher capacities thru the machine with 75 kW motors. At higher capacities the machine becomes a conveyor and results with a high percentage of un-effected kernels exiting the machine. As the machine is relatively crude in design and grinding gap tolerances of no importance give or take 1mm some millers sprayed the rotors with tungsten to extend grinding life of the knobs on the rotor. As this machine is longer the prominent degerminating machine in the South African maize milling industry our technical overview will focus on the latest maize mills using high compression degermination.
In early 2000, maize mills in South Africa started to replace the Beall degerminator with new technology so-called high compression degerminators. There are two main models on the market, one a horizontal operating machine and the other vertical. Both of these machines are equipped with a main drive 75 kW motor and an on board screen cleaning fan requiring a 3 kW motor. The big difference between these new degerminators and their predecessors, is that the Beall degerminator which was horizontal in operation typically had a screen from 4.75 mm to 6.35 mm (1/4 inch) in diameter. The latest high compression degerminators have screens with narrow 1.3 x 15 mm long slots or 1.1mm x 12mm.
The principle difference between the two machines, is that with the Beall type degerminator, quite a considerable fraction of endosperm broken pieces and almost all the germ and bran particles, passed through the screen and required considerable grading and aspiration systems and in some cases concentrators to grade this mixed fraction, prior to stock reaching the rollermills. With the new degerminator and its considerably small screen aperture, there is only the smallest chips of endosperm along with some of the germ and bran going through the screen and this fraction receives no further treatment and goes directly to offal. There is an on board fan which blows air from inside the screen to the outside and thereby assists the screen to stay open and the thru's stock to pass through. The thru's stock along with its 30-40 m³/min of air from the machine on board fan, has to be evacuated from the machine and therefore these new degerminators, by necessity, have higher pneumatic requirements than their predecessor. The thru's stock require a strong pneumatic system with a high air to stock ratio. There were some attempts made in the early years of these new degerminators operating to recover some of the tiny broken chips of endosperm within this thru's fraction. However the quantity of equipment required and the complicated, wet, high fat nature of the stock, resulted in this effort being abandoned. In all the mills we are involved with, the thru's stock from these new high compression degerminators, goes directly to offal. Differing from one mill to another, the stock may or may not be dried. The stock does have a very high moisture content, as high as 20 % in some cases. However when its pneumatically conveyed in mill suction pneumatic conveying systems and pressure pneumatic transport blowlines to bulk out-loading bins, and mixes with the dryer germ, bran stocks and screenings coming from the milling process itself, the overall moisture can be acceptable without drying. As everyone involved with milling will know, germ can be stored in bulk in preparation for loading, but there are two simple facts that determine whether the stock can be discharged trouble free from those bins or not. They are moisture content and storage time. If wet offal is allowed to rest over a weekend in even the best designed bulk germ out-loading bins, serious problems can be encountered discharging. The percentage of thru's stock from these new degerminators which exit the milling process directly from the degerminator differs from one mill to another. Depending on grinding effect it is between 14-18 % in conventional maize meal plants and in grits mills it can run up to 20 % as excessively hard grinding of specially selected vitreous kernels can be required.
INTAKE, CLEANING AND CONDITIONING.
Considering these sections of the plant, the following would be an ideal process flow and the equipment necessary to achieve best practice.
The milling process proper starts on the discharge of the second conditioning bin and it is under this bin that accurate capacity control equipment is fitted to ensure an accurate and constant feed rate onto the degerminators. The process flow in this section is as follows:
On a 7000 kg/hour degerminator feed, the overtails will be 85 %, i.e. 5950 kg and each pneumatic lift to handle that capacity, will consume approx. 9 kW of the total pneumatic system handling this section.
An alternative is to place the degerminators above the plansifters and feed directly into the plansifters, however there are plant layout and spouting angles to be considered and the plansifter will require considerable aspiration, typically with a double inlet into the sifter, so there is good, effective aspiration. Even with the best aspiration, this hot humid stock coming directly from the degerminators, will greatly reduce the expected life span of the sieve boxes in this section.
A third alternative is to have a collection screw conveyor under the degerminators and feed a bucket elevator for mechanical transport to the plansifter section or sections as the case may be. A problem with this solution is the accurate splitting of the stock onto multiple plansifter sections and if height is permitting, a rotary distributer valve is an ideal way to accurately split the stock. All the mechanical elements should have strong aspiration connections.
THE GRADING SYSTEM
MAIZE MILLING SYSTEM
The maize milling section comprising of rollermills, plansifters, purifiers, aspirators and numerous auxiliary pieces of equipment, are determined by the required mill capacity and the granulation, fat and fibre content of the finished products. Other considerations are the possible requirement for very fine granulation flours and whether there is an off take of coarse products required, i.e. samp, rice, grits. For each milling system, after considering the above requirements, the resultant equipment can be represented in terms of specific values that allow us as milling engineers, to ensure there is adequate equipment in the mill to perform properly and meet capacity and design specifications. In the South African maize milling industry, there is a never ending demand for higher grade products and the past 10 years has seen the almost total disappearance of Sifted and Unsifted Maize Meal and it is becoming increasingly difficult to sell Special Maize Meal in certain market areas.
BEST PRACTICE - HIGHEST LEVEL OF TECHNICAL DESIGN
To meet these requirements, Shamrock Milling Systems believe in the approach of highest technical design and generous application of equipment that pays for itself over and over. The price differential today between maize germ meal and the lowest grade maize meal, is so considerable, that in large scale commercial milling operations, every possible effort to refine and produce the highest quality products, with the minimal presence of germ, bran particles or black specs, requires the highest levels of technical engineering. Today's mills must endeavor to produce the maximum extraction of high grades, with the lowest possible germ percentage. Further driving this highest technical approach, is the fact that there are huge fluctuations in the supply and demand for germ meal from feedlots. High/low rain fall, seasonal demand for meat products, abnormal milling volumes due to supplying world food program production requirements and exports to neighbouring countries, all contribute to the abnormal supply and demand conditions for germ meal. The price can be 100 % higher or lower over a given 6 month period.
Further adding to the supply of germ meal, is the fact that over the last ten years, the increasing demand for higher grade products has seen the average germ extraction in the large scale commercial milling plants, going from a norm of 23 to 25 %, to anything from 27 to 33 %, depending on extraction of high grade products and maize quality. This in effect, has added 5% of the total milling production over the past 10 years from human consumption to the animal feed industry and is a direct result of the ever increasing demand for low fat content Super Maize meal or simply further refined, purer products. Unsifted and sifted maize meal, the main stay of the milling industry in South Africa in the sixties, seventies and early eighties, have all but disappeared from the large scale commercial millers' production statements.
MEANING OF SPECIFIC VALUES
The term specific roller length, specific sieving surface, specific purifier width, specific aspiration length, are all design criteria which allow the milling engineer to plan, design or evaluate an existing milling system, relative to the capacity and production requirements. There are cases we know of, where these best-practice criteria are not adhered to and such milling systems are compromised in extraction and product quality.
SPECIFIC ROLLERMILL LENGTH
Over the past 20 years, South African white maize has become less vitreous than what it used to be. Reasons advanced for this by producers and plant breeders, are climatic conditions, GM varieties, faster growing cultivars, etc. The maize meal market today, calls for generally more granular meals, and there are very small markets for fine maize flours. Grinding on short roller lengths, results in a greater production of fines at each grinding passage, and therefore we favour a longer more gradual reduction system. This results in a more controllable milling process and closer granulation ranges being sent to individual passages. A further benefit of narrower granulation ranges on rolls, is that the stock more closely suits the specific fluting profile on that particular rollermill passage. It is for this reason that we design modern roller lengths on anything from 5 ? 6.5 mm. The wide variation is explained in a 6 and 12 tons rollermill layout below and an example of how differences in specific values occur, is the following example. It is always easier to equip bigger mills than smaller capacity plants, as the 6 and 12 ton mill example below, illustrates. A six ton mill, requires four rollermills to produce the necessary individual number of passages (8 passages).
The following two examples are of best practice allocation of rollermills, for a 6 tph (1 Degerminator) and a 12 tph (2 degerminator) milling plant. In the 6 ton mill, four rollermills with 8 passages is adequate and in the 12 ton mill, 6 rollermills provide sufficient roller length.
SIX TONS PER HOUR MILL ? 144 TONS PER 24 HOURS (4 ROLLERMILLS)
TWELVE TONS PER HOUR MILL ? 288 TONS PER 24 HOURS (6 ROLLERMILLS)
Part 2 to follow -
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