Utilizing Screening Equipment

One of the biggest mistakes I see operators make is thinking that the best way to maximize profits is to purchase the biggest grinder they can find in order to produce a final product on the first pass.  Although I have heard various manufacturers promote this idea over the years, this is almost always the wrong approach to maximizing profit in a mulch operation.

For most operators the math seems simple, the less time I spend handling the material, the lower my costs will be right?  Yes your handling costs will be lower, but fuel and wear item costs will increase dramatically.  Grinders are notorious for consuming large amounts of fuel and wear parts.  Anyone who has been around grinders can tell you that it is nearly always cheaper to regrind material than to try and force it down to the final size in the first pass; despite that regrinding material means handling all of your material at least twice. 

This concept seems a little backwards to most first time customers, but the logic is actually fairly simple.  In all grinders, you have a grinding box that houses the hammermill with screen grates below and maybe behind the hammermill which allow material to pass through once it achieves the correct size.  The problem with trying to make a final product in the first pass is that you are keeping material in this grinding chamber longer, forcing the engine to run at a high rpm to keep the hammermill turning.  Although much of the material may already be the right size it has trouble getting out of the grinding chamber due to the small holes in the screen grate, and so the material remains in the grinding chamber and burdens the mill and engine burning more fuel and causing increased wear.  More fuel + more wear parts = more $$$.

Fuel and wear parts aren’t the only reasons to regrind your material, and often not even the most expensive. CONTAMINANTS can cost you loads of money.  A large piece of steel can set you back thousands in repairs alone, not to mention lost production time.  Using large 4” or 6” screens for your primary grind will help provide a larger outlet for contaminants to exit the hammermill as quickly as possible and minimize damage.

Ok, so now you have processed your material with large screens in your grinder and you are getting ready to regrind all of the material.  Here is the big money saving advice, Don’t .  You can save big if you can separate the material so that you only have to regrind the portion of the material that is too large. 

In order to separate the material you will need to invest some additional capital and purchase a screening machine (such as a trommel screen or star screen).  As the saying goes, “you need to spend money to make money”.  Fortunately, if you are processing very much material, this investment will generally pay off quickly. 

Trommel screens are the most common type of screening machine utilized in mulch and compost operations.  They are basically a large drum with screens that separate material into two or three sizes.  Most trommels are just a two sort though and produce two piles, the “fines” and the “overs”.  The fines are the portion of the pile that is already the desired size.  The overs are the portion that you run through the grinder again with the smaller screen grates installed.    Star screens are also very high production screening machines and they tend to work better than trommels on wetter material.  However the cost of most star screens puts them out of reach for many operations.

Why does screening save so much money?  The math here is simple.  Grinders burn somewhere between 25 and 45 gallons of fuel an hour.   The smaller the product you are trying to make, the more fuel your grinder is going to use.  They also have several wear parts which are not cheap either. Screens on the other hand burn something like 8 to 15 gallons of fuel an hour and have no high-wear parts.  As you can see, the savings are primarily in the fuel and wear items.  Also, if you are only regrinding a smaller portion of the material, you won’t need a grinder with as high of a production rate in order to get through your material in the same amount of time.

Screening your material will bring up a few issues that you will need to resolve, however; first, although the overs are easily reground, the fines must be addressed. If you don’t have too much dirt in the fines, you might be able to send them for boiler fuel.  Otherwise the fines make good soil amendment and are a great additive for composting. Secondarily, by screening the product you will be removing the weight as well as volume.  Consequently, you will need to increase your input in order to produce the same volume of finished product.  Be sure to plan on this when determining transportation costs as well as volume of your finished material.

If you have difficulty disposing of your fines, then reduce the amount of fines that you produce.  Increasing the screen sizes in your grinder is an easy way to decrease the % of fines you produce, while simultaneously reducing grinder fuel costs and extending wear items.  You will also increase the amount of overs that will need to be reground, but regrinding should already be a staple part of your operation and typically a high percentage of the product on the first pass will already be significantly smaller than the screen sizes in your grinder. 

Just because your grinders have screens in them does not mean that they are capable of truly screening material and making a consistent finished product.  A big mistake many first time buyers make is assuming that a 2” hole is going to make a 2” x 2” material.  2” screen are more likely to product 2” x 4” and minus material.  If you have strict size requirement, plan according and plan to add a screening machine to your operation.

If you have any questions or would like help selecting the right screening machine for your application, please contact us.