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Step Grinding for Improved Efficiency of Grain and Meal Products: Part 1

In many parts of the world attention is being focused again on a concept known as “Step Grinding”. What is “Step Grinding”, why is there such interest returning, and is this a concept that may hold a benefit for you?

Step grinding in the simplest terms, is size reduction accomplished in steps or stages, usually incorporating two grinding machines (hammermills, roller mills, pulverizers, or some combination thereof). The primary objective of step grinding is to reduce the cost to produce a ton of fine ground finished product. Additional benefits may include improved control of the particle size distribution (more uniform grind with less oversize and fewer fines), reduced product heating and subsequent moisture loss, a reduction in the maintenance cost per ton of ground material, finer finished products, and greater flexibility in the grinding circuit.

As noted above, step grinding may be accomplished in circuits utilizing two machines, though it is certainly possible to “step grind” using a single machine, or more than two machines. With a single machine, a step grind circuit will either involve batch processing (grind a batch coarse, readjust the grinding machine finer and process again) or a continuous operation with a screening stage returning oversize materials for reprocessing (circulation grinding). The potential benefits of circulation grinding were explored in the March 1994 edition of Feed Management in an article authored by William L. Ritchie titled Increasing the efficiency of particle reduction. This type of system does offer the same potential of reducing energy and improving particle size control, but does not significantly add to the flexibility of the grinding system (Figure 1).

A second approach, and one that is employed in a number of U.S. feed manufacturing plant is the utilization of two grinders in “series”, one performing a pre-break, and the second grinding the total mixed feed ration. This type of system is commonly referred to as a “post mixer” grinding system, or perhaps just “post grind” system but differs from the European “post grind” concept of batching directly to the grinder.

The advantages of this kind of circuit include lower grinding costs, finer finished products, more uniform particle sizing, more uniform finished product mix (lower C.V.), and greater grinding system capacity. The primary disadvantages of this kind of system are the potential for the destruction of some micro ingredients and vitamins, and the higher capital costs to install the system. In most cases, the cost of additional capital equipment is offset in 6 to 12 months in the energy savings of the grinding circuit alone. Additional benefits such as increased (pellet mill) die and roller or (extruder) die life and increased pelleting or extrusion efficiency are bonuses on top of the energy savings.

This two grinder system may employ two hammermills, one roller mill and one hammermill, or two roller mills. Additionally, sieving between breaks may be added to further enhance the energy efficiency of the system and reduce operating costs by removing sized materials before the secondary grinder, or by returning oversize materials to the pre-break machine (Figure 2 and Figure 3).

Tags · Hammer mill · Step Grinding · Efficency · Feed · Technology · Process · Grinding

03.03.2009. 19:33

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