Recycling in a Paper Mill

You would think recycling in a paper mill would be very simple, but it can be difficult at times.  Most waste paper from the paper machine is recycled through the broke system.  However, there are areas where it is not feasible to feed the paper back to the broke system. 

I had the opportunity to optimize production in the converting area of a mill.  During this optimization I noticed that there was lots of waste generated in this area that could be recycled.  However, due to the mix of the paper and the location it was not ideal to sort the paper and feed it back through the broke system.  The trash bins contained a few layers of paper from the rolls that were to be converted, cores, core plugs, cardboard, and other general trash.

The core plugs can be easily recylced by simply sending them back the line to be reused.  The cores generally had some paper left on them and could not easily be sent back up the line to be used.  So I contacted a recycler to come and do a study to see if there was enough waste for him to install a baler and provide a trailer to load the paper waste.  His study concluded that the paper waste was of relatively good quality to him, and that he could install a baler and trailer at his cost and also pay the company for the paper waste that he was taking away to be recycled.  This was not a large sum of money, but we would have been happy with him simply installing the baler and taking the waste away.  The managers and employees were very happy with the result of the project.  After the baler was installed the employees no longer had to take the waste to the compacter.  The waste was compacted and baled on the floor which reduced the number of trips the forklift driver had to make from five or six per shift to just one or two.  The trip was also shorter since the trailer was placed closer than where the baler location. 

So look around for these opportunities.  Companies are looking to be more green and sustainable, why not get paid a little bit for it.  Just a thought.

Troubleshooting Tip: Sealed Sheet

I know it has been a while since my last post, but I'm still here.  I've just been busy.  Anyhow, without further ado, sealing the sheet can be a very big problem.  This is basically when the sheet is not draining properly.  It seals itself and does not allow the water to drain.  This problem can be quickly diagnosed by just looking at the sheet as it moves along the table. 

Once the situation has been assessed, what do you do?  There are several options.  The simplest thing to do is to remove some broke.  Broke contains lots of fines so reducing broke can alleviate the sealing.  Another thing you can do is refine less mainly with the hardwood refiners.  You can also adjust your furnish to increase softwood.

Those were the simple solutions.  If none of those options helped then you've got to dig deeper.  These are more machine specific options.  You can increase the amount of water going to the headbox.  This seems counter intuitive but in many cases this added water will create channels in the sheet to allow for improved drainage.  Another option is to adjust the jet to wire ratio.  This will change the fiber orientation which may help.  One more option is to adjust the slice position which will change the delivery angle.  This will effect other parameters such as the jet to wire ration, so be cautious when adjusting the slice.  This will change where the jet lands on the forming board as well.   Just be aware of the adjustments.  Remember these are machine specific and may not apply to all machines.

I hope that helps.  I know some of it is a little vague and general.  Consult your process engineer and wet end operators before making these changes.  They may have some insights that could prove useful in troubleshooting.

Keep Vendors in Check

The quality of the products purchased from anyone should be checked periodically. This keeps the suppliers and vendors honest. It shouldn't be that way but it is. I'll give you an example of a very big headache that occurred due to a supplier/vendor.

I was working on a fine paper machine and shortly after an outage a mysterious diagonal curl started appearing in the sheet. The thing with diagonal curl is that it's easy to diagnose but difficult to cure. The curl had not been there prior to the outage so it must have come from something that was changed out during the outage. The problem is that there are lots of the things changed out during an outage. So after a week or so of investigating the cause of the curl still couldn't be identified. The problem was that the curl was moving. It wasn't always in the same position. So whatever was causing the curl had to be a moving part. On a paper machine everything moves. So the next step was to send in the corporate engineers. The were as puzzled as the rest of us. Another week passes and still no answer. Next some paper machine experts are brought in, they are the ones that built most of this particular machine. They also supplied some parts that were routinely replaced. By this time it was about time for another outage. The paper machine is taken apart and things are be changed out as normal. Then an old machine tender is looking around at some of the headbox parts. He picks up the lexan sheet and notices something. He says, "these sheets feel flimsier than normal." He tells the machine manager who then tells the paper machine experts. They experts say no it can't be the lexan sheets, but for argument sakes lets put in a new set. Note that they supply the lexan sheets. So the machine is started back up and the diagonal curl is still there. A few days goes by and the machine tender keeps saying he thinks its the lexan sheets, there's something wrong with them. Finally the machine manager finds the old lexan sheets which had a few pieces broken off and says to put them back in. This takes about an hour and a half or so. The machine starts back up and the curl is gone. Hmmm.... The paper machine experts call back to their home office and are told that some changes to the lexan sheet production process had been made but that shouldn't have affected the sheets. After hearing of this change the mill manager makes a call and lets just say that they went back to the old way of making lexan sheets.

So trust your operators when they say something isn't right. And just because they say they're experts it doesn't mean they're always right.

Refining

Refining in the paper making processing refines the fiber. Sounds easy enough right? It's a little more complicated than that. Refining the fibers means cutting or brushing the fibers. Too much cutting and the fibers produce too many fines which is lost during the drainage process. So we want to keep the fibers long enough to provide the strength properties we are looking for while also cutting the fibers to a length that will produce an aesthetically pleasing sheet. Too long of fibers will produce knotting and blobs in the sheet, not very uniform. The brushing part of the refining process fibrillates the fiber to produce more surface area for bonding. The more bonding that takes place the stronger the sheet. Imagine a fiber starting as a long stick. After refining the stick may be cut in half or thirds. It also will have the outside bark stripped off and look like its been chewed up a bit. That's basically what refining does.

There are different types of refiners from cones to cylinders to discs. The most common seem to be the disc models. Pulp is pumped into the refiner through a set of discs. One disc is stationary while the other spins. The plates on the disc have bars which differ depending on the refiner model and the desired results. So plates cause more cutting while others provide more brushing. As the bars from the opposing plates cross the squeeze and cut the fibers. Operators adjust the load or amount of pressure on the plates to attain a desired freeness which is the measurement used to gauge refining. The freeness is usually tested by a lab person or a utility hand.

Different plate patterns are available for different purposes, as well as different fiber species. Hardwood plates are designed for more brushing since the fibers are already relative short. Hardwood refiners are also generally set in parallel since they usually only require one pass through the main refiners. Softwood fibers on the other hand are generally set in parallel since the fibers are longer. This gives the operator more flexibility in controlling the freeness.

This is a very brief overview of refining. There's much more to it. So if there is anything in particular you would like to know, just let me know.

Gravity Flow

Using gravity can be economical in some circumstances. It is also very reliable since it's pretty much constant. The place where gravity flow is most often used on a paper machine is the stuff box. The stuff box provides a constant head of pressure to the basis weight valve which meters in the appropriate amount of stock.

There are other places where gravity can be used to provide pressure. In order to utilize gravity one must utilize the Bernoulli equation. One of the few equations I learned in school that I was able to use in industry. Paper machines are constructed in such a way that there are various levels that can be used to provide pressure, from the basement to the mezzanine floor. If you go into enough paper mills you will also learn the term mezzaline, which is the proper way to say mezzanine in some mills. There is usually another floor above the mezzanine which is the deculator floor or some other floor. Not all paper machines have a deculator, but they could have a floor to provide access to dryer hood vents. Of course there is also the roof. I would not recommend putting anything on the roof unless it's relatively small.

I was able to utilize gravity flow in a project that required less fluctuation in a dye flow. The dye had a relatively small flow and the pump supplying the dye had been oversized. I was given a small budget of $20,000. The reason the flow was fluctuating was due to the small orifice size on the control valves and the oversized pump. The valves worked better when they were more open in the 50% range. I calculated the pressure I needed to open the valves in this range for the necessary flow. I was able to salvage a stainless tank and set it on the mezzanine floor, the control valves were in the basement. I used the pump to fill the tank to a level setpoint. I realized that the head pressure would change when the level in the tank changed, but it was within the operating range of the control valves.

The end result was a much happier stock tender. He didn't have to worry about keeping an eye on the dye flow. The valves were stroked regularly during breaks to ensure there was no build up at the orifice. In addition to the pressure reliability, the system did not go down if the pump went down. The tank could be filled manually with totes if necessary which added more reliability to the system. So when working with a limited budget and needing a reliable pressure source, look to gravity. The pressure is only limited by how high the tank or line can go.

Acid

This post is somewhat grade specific. Acid is used in some mills to control pH. In multiply linerboard it can also be used to improve drainage. However, too much will damage the fiber and lead to reduced strength. The reward of improved drainage may allow for the machine to be sped up, resulting in increased production.

Using acid on the base sheet is generally not an issue since there is plenty of flow to dilute the acid. On a secondary or top sheet controlling the amount of acid can be an issue due to the low acid addition rates. The acid addition should be controlled with a pH setpoint. The control response should be fairly quick. Some bump tests should be performed to determine how the control should be set up. This should be done with the assistance of the controls engineer and E&I personel. To improve the control, the pressure going to the control valve can be reduced to allow the valve to operate at a larger opening range.

Acid is relatively cheap. Increasing acid usage can not only increase production, but also reduce alum usage. This should not be a high priority project but is definitely worth looking at.

Ropes

Ropes are used to thread the paper machine. The rope run goes through all the dryer sections and are setup so that the tail, a narrow sheet, goes through the dryers properly and reaches the reel drum. Paper machine ropes, or carrier rope, is made from nylon and other synthetic materials which are abrasion resistant.

Ropes are very dangerous. When I first started working on a paper machine the first thing the machine manager told me was if I ever heard a rope break, run. If a rope breaks and catches anyone's limb it would surely catch and pull the person through the rope run which would more than likely kill the person. This is why it is important to correctly splice the ropes. This is where the ends of the rope come around to close a loop. The splice should be overlapping at least 18 inches. Glue should also be applied to the splice for extra strength. If a rope begins to fray it should be carefully monitored. If the fray worsens, then a decision should be made to replace the rope. If a rope breaks it can not only result in a break, but also take out other ropes. If the rope is in the first section and it breaks and takes out all the other ropes down the machine this can lead to several hours of downtime. Some rope runs are fairly complicated and take some time to re route. It is much simpler to replace a rope by tying a new rope to the old rope than to have to manually guide the new rope by itself through the rope run.

It's better to be safe than sorry, so do not take unnecessary risk with the ropes, you could get burned.