Screening for success

Editor’s note: The following article is an edited transcript of the presentation by Mark Henke, senior recycling manager of Republic Services’ midwestern operations, from the MRF & Recycling Plant Operations Forum held in Chicago, Oct. 17-18, 2016.

Generally I have no preset script when I enter a material recovery facility (MRF) to do a screen adjustment. I usually start out by asking questions about configuration for screens. What is the in-feed set at? How many tons per hour does the plant process? When was the last time you validated that? What are conditions of the screens and stars? When was the last time you changed out the stars? How is the impact zone? How many sorters do you have on the line? If some are missing, how many are missing and what positions did you sacrifice to compensate for that? Lastly, are the screens performing well and are you getting good a quality material that your end user will use?

The purpose of the screens are for size and material separation. Fine screens, for example, are used to take out the glass, and material separation screens, such as your paper screens, are used to separate the paper from the plastics. We also look at an elliptical sorter, which is meant to do the same thing. Lights and papers go over the top, and containers bounce backward, while dust, dirt and fines are collected in the holes of the paddles themselves.

A CHANGED ENVIRONMENT

The composition of the recycling stream has changed. A lot less paper is in the stream, and workers have more plastic bottles to contend with. But even the bottles have changed. They have become much thinner. The result is that, as the plastic bottles get thinner, during the collection process they lose their shape. So, the three-dimensional plastic bottle turns into a two-dimensional plastic bottle that doesn’t fall backward with the other plastics. Rather, it rides up the screen with the paper and requires lot more QC (quality control) work on the other end. Plant operators also must collect more plastic to get a ton of recyclables.

More old corrugated containers (OCC) is generated from residences through the growth of online sales. It is smaller cardboard that falls through the OCC screens and ends up with paper, which causes more QC work to be done on the back end. As a result, you have less newspaper and more corrugated in the mixed paper, and you must find a market for that material.

You are pumping a lot more material through, volumewise, to get the same weight because you are handling a lot more containers. The screening package you may have purchased four or five years ago is the same, but it is dealing with material characteristics that have changed quite significantly.

MEETING THE SPECIFICATIONS

To fully appreciate the screen, plant operators need to understand how it functions in the whole process. The screen package defines the optimal throughput of the entire processing system. It comes down to the number of screens, the sequence of screens and the size of the screens—that tells you if it is a 20-, 25- or 35-ton-per-hour system. The length of the screen helps you with the quality; the more contact the screens have with the material, the better quality you get, the better the separation.

You want to make sure the tons per hour matches the design capacity that was specified by your manufacturer. Are you operating the system in a correct manner? If you have a 25-tons-per-hour system but are pushing 35 tons and have quality issues, that is not a hard problem to correct. It was designed for a certain tonnage for a certain reason.

The next thing to consider is if the QC process matches processing. If so, many people are on the line, and the system is performing well. But if the sorting people can’t meet the demands of the nonpaper material on the line, [the question is], Am I operating the system [in a way] to give the sorters a fighting chance to perform their jobs?

There are a couple different ways to check. One way would be a pick-per-minute survey. Go up to the line and see at certain positions how many picks per minute that person can get.

If I am on QC on a paper line, I should get 30 to 50 picks per minute pretty easily. If they are doing less, some coaching might be needed. If they are doing more than that, they are star players. Are they performing with the expectation that you are looking for in that position? Each position may have a different level of picks per minute. On average, 35 to 50 picks per minute is good hand-to-eye coordination and a good pick-per-minute count.

The other way to find out if you are doing a good job operating the screening system is to take a sample of the material and have sorters not pick anything. Count the number of nonfiber materials in that sample and divide that by the number of sorters. If the number exceeds 30 to 50, you know you have too much material on the line and you are not allowing the sorting staff to do its job.

MAKING THE ADJUSTMENT

After validating the tons per hour, finding we are well within the capacity of the system and the sorters are doing a good job, we can look to see if we should make adjustments to the screens. Screens operate on variable speed drives (VSD). Most screens are divided into two or three different sections, depending on the type of screen. Preceding conveyors operate on a VSD as well.

There are good ways to regulate what goes to that screen. You can speed it up or slow it down to help adjust the entire screening package. The QC line on the opposite end of the screen can run on variable speed, which allows you to adjust the speed to help sorters when they are doing the QC work.

Where is a good spot to look at the system to make sure it is meeting the quality specs of the end market? If you check the bales and none of them meet the spec, it is too late. You can look at the bunker, but it is too late. I recommend looking at the end of the line. If you see lots of plastic on a paper line falling into the bunker, you had better check the bales right away.

Each motor controls a different zone and each motor is on a VSD that can be changed independently. A paper screen generally has three zones—the drop zone or impact zone is usually around the center of the screen. That is where you want to get your most churn and is a good start at separation, so the paper goes upward and containers fall backward.

Take a look at the screen angles. Generally, the screen angle can be anywhere from the mid to low 30s up to around 50 degrees, depending on the manufacturer. The optimal angle is dependent on the ambient conditions.

The stars work on a friction coefficient. As they wear over time, they lose some of their mass. When they lose mass, the friction coefficient is lessened and the material doesn’t move up the screen as well. So you lower the screens, but you end up allowing more containers to flow over the screen.

A key for the OCC screen is a good presort. You first want to have a good presort at the front to pick off anything that can ride with the OCC. Otherwise, you must get it on the backside. Remember two things: OCC screens also are set up with variable speed, and the majority of the material entering the OCC screens falls through the first deck.

Now you have a lot of opportunity to do a good job with the second or third deck if you have the variable speed. Keep it fast to move material, and you are getting a good bounce or agitation to clean out the paper.

One of the first settings you want to look at is the proper distribution of the material to all of your screens. If you are running a screen package with two, three or more screens in succession, you want to make sure each screen is working as efficiently as the others to avoid overloading one screen and starving the next screen. A good distribution of the material can be done by lowering or raising the screens to see what happens and how that flow goes.

With the polishing screen, also called a bilateral screen, an operator can not only adjust the screen up and down like the other screens, but a feature of the bilateral screen allows it to go from about 8 degrees to 13 degrees maximum.

As the material falls into far corner and the paper meanders up the screen, the containers naturally flow toward the middle, fall through and are collected that way. It is another way to adjust your system to increase the separation of paper and containers. It takes a little more practice to get it right, but it creates good separation.

KEY POINTS

It is very frustrating when you think you have changes made, and there isn’t better quality at the end. But what happens?

The operator takes a break on the tip floor or nothing is coming through. They feed drum feeders or the lines inconsistently, and you get some ebbs and flows off the drum feed. Maybe the material on the tip floor is a mix of residential and commercial and they didn’t do a good job of blending the material.

If you run residential and commercial recyclables together, you should blend on the tip floor to get a homogeneous material and keep the settings correct. It is easier to maintain your settings.

This will help you with the recipes. Most systems are set up to save the settings under a destination. For example, if you were running a dry residential stream, you should mark that.

In the beginning, I would suggest you make one big change at a time until you are familiar with the system. Once you get familiar with it and know how it reacts, then you can make several changes.

If I make a change on the infeed, it might be 10 minutes before it influences the screening package. If I make a change on the screening package, I want to turn around, look at the container line and make sure I don’t have too many containers on my paper line or that I am flooding it with paper. With every change you make, you have to see what the counter move is going to be.

Note what the drum feed is set at and what the incline is set at and how many hertz you are running on the drum feed. What are the changes made? And what happens the first time I tilt that first screen up to 45 degrees?

When people invest in new systems, they are afraid to make a change. I say no; make as many changes as you can. Experiment with it. If you make a few bad bales, you can always reprocess them. It’s the only way you can learn. That is the good thing about recipes—whatever changes you make, you can always go back to the original settings.

Mark Henke is the senior recycling manager of Republic Service’s midwestern operations.