Objective: Appreciate the basic process of flotation deinking and understand the concept of dirt counting in paper.

Background: The deinking of wastepaper includes several operations. These include pulping, screening, centrifugal cleaning, washing, flotation, bleaching and kneading. The focus of this lab will be the flotation process. In the flotation process, air is bubbled through the low consistency pulp stock. Hydrophobic particles, such as ink, attach to the bubbles and are lifted away from the stock. A foaming agent is added to create foam. The foam is scraped away as a reject stream, producing cleaner fibers in an accept stream. The process in industrial practice is continuos, having a constant feed, accepts and rejects flow.

1. Put 1000 ml of hot tap water in the blender cup.
2. Tear about 4.5 grams of air dry copy paper (with moderate printing) into 2 inch squares and place in the blender.
3. Add 5-10 drops of foaming agent to the blender.
Cover the blender and blend for 2 minutes.
4. Carefully fill the blender cup with cold tap water about 0.5 inches to the top.
5. Put the blender cup only (not the blender motor) in the plastic tray.
6. Connect the air diffuser with the tubing and the air pump and turn on the pump. You should make sure that the fiber has not risen to the top before turning on the pump. If the fiber has risen to the top, vigorously mix the contents of the blender cup so that the fiber is dispersed thoughout the container.
7. Place the air diffuser into the bottom of the blender cup for 5 minutes. Manually scrape foam off the top into the plastic tray. You may add water to the blender to keep the liquid surface near the top.
8. Remove the bubbler from the blender cup. The material remaining in the blender is the "ACCEPTS" sample. The material in the plastic tray is the "REJECTS" sample.
9. Filter the ACCEPTS stock using filter paper and the filtering apparatus provided. Label the sample as "ACCEPTS".
10. Add about 200 ml of water to the plastic tray and swish the tray to collect the "REJECTS". Then filter the REJECTS sample onto filter paper and label as REJECTS.
11. Repeat steps 1-4 (pulping but no flotation) to make a sample labeled as "FEED" and filter on filter paper. This sample is used as a comparison to the ACCEPTS sample, if desired.

The above is sufficient for qualitative comparison of samples. The following will provide a more quantitative evaluation:

1. Mark off a 1 cm by 1 cm square (approximately) on the surface of both the ACCEPTS and FEED sample. Count the number of ink spots in the 1 cm² area. Record the results in the table on the next page.
2. Repeat step 13 twice for both samples, recording data below.
3. Calculate the average number of spots/cm² for both samples.
4. Calculate the contaminant removal efficiency of the flotation process as: %EFF=100%*(FEED-ACCEPTS)/FEED, in which the values are # spots/cm².

%EFF = _________________ %

Question 1. Can you think of ways to modify the flotation experiment to increase the %EFF? Describe them.

Question 2. Some pulp fiber is scraped away with the foam in the rejects. Why is this undesirable in an industrial process?

Question 3. Look at the three values of # spots/cm² for the ACCEPTS and FEED samples. What is the range of the samples (i.e., the difference between the high and low value for each sample)? The range gives you an idea about how precise your measurements can be. Relative to the range values calculated, do you think there is a significant difference between the ACCEPTS and FEED # spots/cm² averages? Why? (Note, there are rigorous statistic methods to test for significance.)

Question 4. In the paper industry it is common to report the parts per million of dirt (PPM), which is calculated as one million times the area covered by dirt divided by the analyzed area. Assuming that the spots you detected had an average size of 0.01 mm², and using the average # spots/cm² values for the ACCEPTS and FEED samples, calculate the PPM values for each sample. Does the ACCEPTS sample meet a 50 PPM criteria for cleanliness?

Question 5. Why would the foaming agent concentration be important? What would be the disadvantage of having too much or too little foam in an automatic continuous system?

Further Experiments:

1. Yield of fibers is very important. Can you develop an experiment that exposes the relationship between yield and removal efficiency?
2. Can you explain the sensitivity of efficiency to time. What is the relationhsip?
3. How do different types of wastepaper respond to flotation?
4. What is the effect of pulping time on the toner particles and the resulting flotation efficiency?

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