Over the past 15 years, we have been contacted occasionally about metal in containerboard and corrugated boxes. Below is a compendium of published responses to members and Associates that have appeared in
BoxScore, the
Ask Ralph blog, and white papers.
Here is a recurring scenario: “I have a food customer who must send product through a metal detector to ensure metal isn’t in the food. Intermittently, the detector has gone off, and when they send my corrugated box through without product, it still goes off, leading them to the conclusion that metal is in the boxes. We are using recycled liners, and the mill has let us know that they cannot 100% guarantee that no metal will be embedded in the box’s fibers. The mill also provided me with a document from the Fibre Box Association (FBA), saying that during the paper-making process, there is the slight possibility of small metal fibers getting into the paper. Is this consistent with how you understand it to be? Do we have any other options for guiding our customer and ensuring no metal will be in the box? Is there a way to prescreen the boxes to determine which ones may have a trace of metal in them before I ship them? Is there a way to test the boxes to determine where the metal exactly is?”
So, you may ask, how do containerboard mills control and remove contaminants in virgin and recovered fiber as sources for engineering their linerboards and mediums? The information below relates to preventing metal and other unwanted contaminants from coming forward into the finished roll stock. Yes, it is a lot of detail, but that’s what it takes to build an acceptable corrugated component today.
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- Origin of the metal fragments: The vast majority of the offending contaminants originates from the old corrugated container streams that are converted into clean recycled furnish. A very small amount can also occur when process equipment in contact with the furnish fails in operation.
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- Furnish cleaning process and contaminants barriers: Contaminants and metal pieces larger than 0.625″ (15.875 mm) are removed in the pulper detrashing system. Two continuous light detrashers, one grapple hoist, and one ragger perform that duty. The pulper extraction plate acts as the barrier.
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- Glass particles, staples, and coarse sand are removed by density difference in the high-density hydro cyclones.
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- Contaminants larger than 0.024″ (0.61 mm; slotted baskets) are rejected by the coarse screens.
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- Fine sand and metal particles are removed by density difference
in a five-stage forward cleaning system.
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- Contaminants larger than 0.006″ (0.15 mm) and 0.008″ (0.2 mm; slotted baskets) are rejected
by the fractionation and fine-screening system.
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- The long fraction is further cleaned with reverse cleaners to remove stickies that may have been accepted by the 0.008″ (0.2mm) fine screens.
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- The above equipment does an excellent job of removing the contaminants. However, if foil or metal flakes are smaller than the fine slots or if they are generated after the furnish cleaning, they can end up in the linerboard sheet.
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- The paper machine approach screen acts as a final barrier rejecting any contaminant larger than 0.062″ (1.52 mm; hole basket) that may have been introduced in the process after stock preparation or let through due to equipment failure.
I must concur with the position of FBA and the nature of the more than 140 domestic paper machines that manufacture containerboard. I have attended enough Technical Association of the Pulp and Paper Industry Corrugated Technical Committee meetings over my 40 years to know this concern comes up at every meeting three times a year. Even the sophistication levels at the different recycled mills vary in their ability to remove metals from the recovered fiber.
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Ralph Young is the principal of Alternative Paper Solutions and is AICC’s technical advisor. Contact Ralph directly about technical issues that impact our industry at askralph@AICCbox.org.
