Screening Experiment to Determine Yield of a Recycling Process:

Background: One of the most important aspects of paper recycling is the fiber yield from the process. The fiber yield is defined as the % of the solids in the incoming wastepaper that are in the accepts of the recycled pulp. For instance, if 100 tons per day of solid wastepaper entered a process and 75 tons per day of recovered fiber solids were produced, the yield of the process would be 75%. The yield depends on the wastepaper furnish, the types of operations in the recycling process (e.g., screening, washing, flotation, etc.,) and the conditions under which those recycling sub-operations are run. In particular, removal efficiencies of contaminants are typically higher if the yield of the sub-operation is decreased by rejecting more material. The reject streams of all sub-operations typically has a substantial amount of fiber as well as contaminants in them.

Screening is a typical sub-operation in which the yield is always less than 100%. In screening, openings in baskets allow good fiber to go to the Accepts side buy block large contaminants. The industrial screening operation can be simulated in the lab by simply screening a dilute pulp stream with a simple colander with openings about 1-2 mm (typically used to wash foods). In this lab, a paper envelope will be pulped, screened and then made into a handsheet. Solid material can be lost in the screening operation and also in the making a handsheet (in which small particles are "washed" through the papermachine wire). By weighting the dry envelope before processing the generated pulp after, a process yield can be determined.

Find a small envelope with a see-through window in it.

Weigh the envelope to at least 0.1 gram precision. This is the air-dry weight of the envelope; all paper picks up some moisture from the atmosphere.

SAFETY: Always use caution with a laundry iron. Do not leave a hot iron where it could be incidentally contacted of dropped off a table. Make sure the electric cord is not in a position to be snagged accidentally. Make sure to turn off the iron when finished using. To obtain a simple measurement of the oven-dry weight of the envelope put the envelope between two high quality kitchen tissues with the window down and put a hot laundry iron (on high setting) over the sandwich. Let the iron dry the envelope for about 5 minutes. Change the position of the iron so that all parts of the envelope are heated every 30 seconds.

Quickly take the envelope and put it on a balance and record the weight. The envelope will begin to increase weight on the balance due to the absorption of moisture. Therefore, record the lowest weight detected by the balance. This is the FEED oven dry weight.

Tear the envelope into about 2 inch squares and put the squares into a blender cup with 1000 ml of hot water.

Cover the blender a blend for 30 seconds. There may be lumps in the blender contents that can be parts of the envelope or adhesive particles or unpulped fiber bundles, this is ok.

Have about 4000 ml of clean cold dilution water ready.

Put the colander on top or a clean, empty handsheet mold (that is corked).

Carefully and slowly pour one quarter of the blender contents into the colander. Make sure that the liquid level is such that none of the water/fiber spills outside of the handsheet mold. Anything that passes through the colander should be captured in the top of the handsheet mold.

Pour 750 ml of dilution water through the contents of the colander. Scrape the contents of the colander with your finger tips gently to improve the passage of fibers through the colander.

Repeat steps 9 and 10 until all of the pulped material have been put in the colander. Then pass the remaining dilution water through the colander again while scraping the contents of the colander gently with your fingers.

The residual material in the colander is the REJECTS for screening. Fill the handsheet mold with water, stir and them make handsheet. See handsheet preparation lab for more details. The handsheet may drain slowly because of the sample size.

Dry the handsheet completely with an iron. Then quickly weight the handsheet alone recording the lowest weight. This is the overall ACCEPTS oven dry weight.

Calculate the yield as 100%* (ACCEPTS oven dry weight/ FEED oven dry weight)?

Can you calculate the oven dry grams of rejects for the overall process using the data provided?

What is the consistency of the air dry envelope using the data collected above?

What do you think the effect of pulping time might have on the yield?

How might size of the openings in the colander affect the yield?

How might the type of wastepaper affect the yield for this process?

Weigh the rejects from screening after drying them. Does the weight explain all of the yield losses? If it doesn’t, then what part of the yield loss is from the screening operation and what part is from the handsheet making process (a crude washing step)?

Determine the yield of this process for other types of wastepaper including corrugated box material, newspaper, magazine, copy paper, etc. Can you explain why the yields are higher or lower for the different wastepaper grades?

For a single wastepaper grade, determine the yield as a function of the pulping time. Try pulping for 10, 20, 30, 45, 60, 120, 180, 360 seconds.

Try a colander or other type of barrier with different size openings and determine the yield of the process.

Making a handsheet with a paper machine wire can be considered a type of "washing process". For the envelope material adjust the experiment so that after draining the water form the handsheet mold but before opening the mold, replace the cork, re-add water to fill the mold and stir to disperse the fibers and drain again. This would simulate a second washing operation. Determine the yield and compare to the "single washing" original experiment. Do the experiment for a three, four, and five stage washing a plot the yields versus washing stages. Can you explain why the data follows the observed behavior?