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Step Grinding For Improved Efficiency Of Grain And Meal Products

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 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" usi 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 tot mixed feed ration. This type of system is commonly referred to as a "post mixer" grinding syste or perhaps just "post grind" system but differs from the European "post grind" concept of batc 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).

Step Grinding, The European Approach
More than ten years ago, a step grinding system approach was being presented by European manufacturers of feed milling equipment as a means of reducing operating costs. An integral part of the European approach was sieving before grinding and sieving between grinding stages. Because the European feed manufacturer uses such a wide range of ingredients received in the form of a meal, there is a potential for a high percentage of the raw materials to already be an acceptable particle size for the feed manufacturing process.

By removing these sized materials, the load on the grinding equipment could be reduced considerably. It appears from research and testing in actual applications that the reduction in energy consumption is roughly equal to one half of the amount of the materials removed. In other words, removing 30% of the materials to be ground (as fines) and by-passing the grinder reduces the energy required to grind by about 15% (Figure 4 and Figure 5).

Step Grind For Efficiency in the U.S. Since corn, wheat, sorghum and barley are the basis for most complete feeds in the U.S., the primary economic benefit of step grinding is an actual reduction in the specific energy required to grind feed materials rather than efficiency gains from sifting. As a rule of thumb for a two shift operation, one horsepower costs approximately $1.00 per day. An energy reduction of 50 horsepower will save about $50.00 per day in energy expenditures. Where does this savings come from? Power consumption can be expressed in terms of work accomplished over time.

                  WORK
POWER = TIME

The step grind approach yields this energy reduction (less power required) since the materials are reduced more gradually, through a slightly longer period of time. By doing the same amount of work (grinding) over a longer period of time (two or three gradual reductions instead of one instantaneous reduction) the total power requirement is reduced.

Because the roller mill offers such significant energy savings over a hammermill when processing grain in the coarser particle size ranges, the use of the roller mill as a pre-break device can offer substantial savings in a typical grain grinding circuit. By substituting a single pair roller mill in place of the conventional hammermill rotary feeder, the feed rate can be accurately controlled and a significant increase in hammermill capacity can be realized. Because the materials are reduced in size prior to being introduced to the hammermill grinder, larger screens may be employed with no significant increase in finished particle sizing.

This combination of finer feed, increased hammermill efficiency, and the possible use of larger screens not only reduces the energy cost when grinding, but reduces other operating costs (maintenance, parts) as well. Since a rotary feeder is not required the cost of a roller mill for use as a feeder is substantially offset in new installations. In other cases where existing hammermills require a boost in capacity, the application of a roller mill as a feeder can boost hammermill capacity by as much as 40 to 50% with no loss in the fineness of the grind.

Tags · Step Grinding · Efficiency · Grain · Meal Products · Feed Processing

17.02.2008. 13:16

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