Your guide to pelleting

For centuries people allowed their pigs and chickens to forage and occasionally tossed them some food scraps. The industrial revolution changed that.

Farmers now needed horsepower. Horses that supplied power needed high energy feeds. Horses that provided transportation needed feeding stations along the way. Horses that went to war had to carry their feed with them.

The British developed the first compound feed as transportable nourishment for war horses. The feed, resembling a large baked biscuit, was a combination of meal from oat, peas, rye, flax, wheat or maize. According to feeding experiments done by the Prussian army, 1.5 kg (3 3/8 lb) of this compound horse feed could substitute for 5 kg (11 pounds) of oats.

Concentration of food processing, particularly large flour mills, created a new environmental problem: These mills (conveniently built close to water) began dumping waste into rivers and streams.

As tonnes of wheat midds fouled the waters, the government intervened and prohibited dumping by law. The midds had nutritional value for animals but did not flow well, had low bulk density, dusty texture and, therefore, were not pleasant to eat. Whey generated by cheese production, and meat and bone meal from the packing plants were equally problematic.

In 1928, the needs for efficient animal feeds, and use of industrial food wastes were both brilliantly satisfied when Purina began pelleting flour mill waste. Wheat midds were mixed with animal by-products or soya cake, plus ground corn and minerals, and compressed into convenient pellets. The obvious advantages were:

• Less dust
• Increased bulk density
• Improved flowability
• Improved palatability
• Reduced feed wastage
• Increased consumption rate
• Less energy expended in consumption
• Dense minerals did not segregate out

When we say “pelleting”, we refer to the process of compressing materials through a die to form a cylindrical shape with a length that is generally 2–4 times its diameter.

In the simplest terms, you can make pellets by pressing a feed mixture through a circular hole in a piece of metal. If you drill a hole in a piece of metal, put some feed on it, then drive over it with your car, you would make a pellet. If you drill many holes, you can make many pellets. If you drill the holes in a circle, put the wheel on a shaft enabling it to drive around, and continuously place meal in front of the rolling wheel, you would create the flat die press.

However, you are left with a challenge: The wheel, or roller, travels faster on the outside of the die than on the inside. This may impact the quality of the pellet and cause the press to wear unevenly. To improve the process, form the die into a ring with the roller set inside it. Initially, the roller was powered, and the ring just coasted underneath it. This technology was similar to the flat die, except now both the roller and the die were moving, leading to a more positive extrusion. Today, the ring die is the driving power and the roller(s) coast inside.

1. Formulate a recipe. This is usually done by a nutritionist to ensure the target animal will get the nutrients it needs.
   
   
2. Weigh out the ingredients according to the recipe. You need several different size scales: large scales for macro-ingredients that make up more than 10% of the formulation, and small scales/manual hand-weighing to add micro-ingredients which make up less than 1% of the formulation.
   
   
3. Grind the ingredients to less than 2 mm. Consider grinding individual ingredients before mixing (pre-grinding), or grind the entire mixture (post-grinding) after weighing.
   
   
4. Mix the formulation thoroughly. Use a ribbon blender or a paddle mixer that can mix up to 10 tonnes at a time. You may add a pelleting aid and liquid-based ingredients at this stage.
   
   
5. Move the mix to the pelleting press. Use a bucket elevator to carry the feed to the top of the mill and then let it drop to a bin above the pellet press.
   
   
6. Condition the mix with steam immediately before it enters the press. You do this by adding steam directly into a horizontal mixing/transport chamber. Conditioning adds heat and moisture to the feed, which softens it and makes it easier to press the pellets.
7. Conditioned meal falls into the press where it is pelleted. Sometimes the pellets are cut to a specific length by a knife on the outside of the die.
   
   
8. Hot pellets drop into a counter-flow cooler where air is blown through them in the opposite direction. This evaporative cooling returns the pellet to within 3-5°C of ambient temperature, and at the same time removes the moisture that was added by condensing steam.
   
   
9. Transport the cooled pellets for storage or packaging. This usually involves another lift to the top of the mill by a bucket elevator and gravity flow to their destination.

 

Watch our video 'The Pelleting Process'

Pellets should maintain their shape until the animal eats them. There is a lot of handling and transport on the way to the farm. Pellets with poor physical quality will break apart or abrade during transport and generate dust or fines. This is a problem for several reasons:

• Nutrient segregation that unbalances the diet.
• Dust can congest the nostrils of ducks and rabbits.
• Fines will build up in robot milker/feeder systems and prevent their operation.

Pellets with good physical quality are hard and durable.

Durability describes the pellet’s ability to remain intact during transport and not produce fines. You can measure it by blowing 100 grams of pellets in a pneumatic tester (New Holmen Pellet Tester) for 30-60 seconds and then calculate the percentage of intact pellets that remain. Another method is to place 500 grams of pellets in a rotating chamber (KSU tumbling can) and tumble them for 10 minutes, then measure the percentage of remaining pellets.

Hardness describes the pellet’s ability to withstand a crushing force. Instruments that measure hardness include the Acme Penetrometer and the Kahl Hardness tester.

The ingredients that go into the mixture are a primary factor. At least some of them must act as glue to bind the particles together. The grind of the ingredients is also important; they must be able to close-pack with no internal voids. Conditioning the mixture with steam provides heat and moisture, which softens the particles and activates natural binders. Compression of the feed as it passes through the die is critical. Finally, cooling the pellet and reducing the moisture sets the bonds and strengthens the pellets.

To anticipate problems due to the use of certain raw materials, Borregaard has developed the Feed Pellet Quality Factor (FPQF), which indicates how well a feed formulation will pellet.

You should choose ingredients that meet your nutritional and economic criteria. Some ingredients may lack binding properties. Others can even be negative. One example is fat, which is very harmful to pellet durability. Ingredients may also be difficult to press through the die and cause blockage. A small amount of pelleting aid can often correct these problems.

 

• Clays: These binders are generally used at a level of 1–2%. You may use them in feeds that are not nutritionally dense to provide a filler as well as a binder.
  
  
• Lignosulfonates/Lignin-based products: These are sustainable products derived from wood. They are typically applied at 0.5–1.0% and are effective over a wide range of temperatures and moistures.
  
  
• Urea-formaldehyde (UF) or polymethylolcarbamide (PMC): These binders are used primarily when the pellets must remain stable when submerged in water. Dosage is typical 0.1–0.5%. These binders are thermoset polymers and require heat to activate their bonds.
  
  
• Lubricants: These are often formulated products (composed of at least two ingredients). They facilitate the pelleting process without specific regard to pellet durability.
  
  
• Scrubbers: These products are hard bits of grit that can scrub or polish the die hole as it passes through. Sand might be used.

 

Binders should reduce the amount of fines by 25–50%. If a binder is added to a feed that has a pellet durability of 90.0, that durability should increase to 92.5–95.0. If the starting pellet durability is 80.0, it should increase to 85.0–90.0. The lower the initial pellet durability, the larger the improvement will be. However, the percentage of fines reduction, off the pellet tester or at the farm, should be in the range of 25-50%.

Lubricants and scrubbers should reduce amperage by at least 20%, or increase production rate by 25%. In some cases, it may be possible to double the production rate, but that can also reduce physical pellet quality.

 

Some pellet manufacturers try to keep pelleting aid costs low by limiting the dosage. At some point, the dosage becomes so low that the pelleting aid is no longer effective, and even the small amount spent is wasted. It is, therefore, important to determine the most cost-effective dosage for a pelleting aid in the system where it is being used.

For example, the most cost-effective dosage for a lignosulfonate/lignin-based product could be 0.5%, which might reduce fines by 40%. If the dosage is increased to 1.0%, fines might be reduced by 55%. This is indeed an improvement, but the cost is doubled and the increase in improvement less than half. However, if that improvement causes a farmer to purchase those pellets, the cost is covered and worth it. Besides gaining sales through excellent quality, other benefits of using a pelleting aid include:

• No need to recover poor quality pellets from the farm
• Reduced recycle of fines
• Ability to make pellets with sub-optimal conditions, e.g., boiler problems, worn die
• Ability to formulate with “opportunity” ingredients without loss of pellet durability
• Increased production rate

Although Borregaard's pelleting aids offer a large number of potential benefits, our experience is that different people within a feed company may have different reasons for using one. Learn more about typical benefits that should be of interest if you are a Production manager, nutritionist, sales manager or feed mill manager/owner.

First, consider the problem you need to solve. Do you want to improve pellet durability? Do you need your press to produce at a higher rate? Do you need to achieve a certain level of water stability in your pellets? Articulating your need will guide your choice towards a binder, lubricant or water stabiliser.

The next consideration might be available “space”. Do you have a bulk bin to hold the pelleting aid, or is there room in the formulation for inclusion of a high-volume pelleting aid? If not, clay binders may not be appropriate. Available “space” will guide the choice between clay (higher dosage levels), a lignosulfonate/lignin-based product, or polymethylolcarbamide (PMC).

Finally, pick two different pelleting aids and see which is the most cost-effective or otherwise appropriate for your needs. Before you begin to test;

• Know the outcome you are seeking – write it down;
• Determine how you can measure this outcome;
• Measure how things are before adding the pelleting aid, i.e., durability, amperage, tonnes per hour (TPH);
• Add the pelleting aid, using a “generous” level, and record the outcome;
• If the outcome is positive, continue testing with lower addition rates.

All pelleting aids are either powder or liquid. Powders should be added to the mixer at the same time as other mid-level ingredients. Liquids can be added to the mixer or applied at the conditioning chamber. There is much debate which method is the best.

Consider a commercial feed mill that makes pellets for a variety of species. This mill may have several presses with production rates ranging from 2–10 tonnes per hour (TPH), and the same 3-tonne mixer may supply all. In that case, addition at the mixer could supply the pelleting aid to any of the three presses with a single application system. If the dosage is 1%, it might be easy to add 30 kg of a liquid pelleting aid during the mixing cycle. However, if the liquid is very viscous, it can be difficult to achieve effective mixing. Another advantage of application at the mixer is the certainty that the correct dosage has been included.

Now consider an integrated pellet mill that operates two presses at 50 TPH, both supplied by a 10-tonne mixer. Addition of 1% pelleting aid to the mixer would require 100 kg per batch. This might be difficult to apply and mix in a short mixing cycle. Addition of 1% liquid pelleting aid to the conditioner would mean a rate of less than 1 litre per minute, which requires a much smaller pump, which needs to be integrated into the control system. Also, when the liquid pelleting aid is added in conjunction with steam and mixing in the conditioner, its distribution is continuous and even. Addition to the conditioner with a variable speed pump allows the dosage to be easily changed as needed. Of course, there might be some pushback from the formulators about any variation of this type.

Pelleting aids can be useful when manufacturing conditions are less than perfect, which is most of the time. In some cases, their use is critical. There are a variety of needs, and a corresponding variety of pelleting aid options. A well-considered choice and a documented outcome can make a real difference.