Objective: Be able to perform coarse screening of recycled fiber and calculate the yield of the 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 but 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 of 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 of a handsheet (in which small particles are "washed" through the papermachine wire). By weighing the dry envelope before processing and the generated pulp after, a process yield can be determined.
*Weigh the rejects from screening after drying them. Does this weight explain all of the yield losses? If it doesnt, 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 papermachine wire can be considered a type of "washing process". For the envelope material adjust the experiment so that after draining the water from 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 and plot the yields versus washing stages. Can you explain why the data follows the observed behavior?Back to other paper science lesson plans.