Grade: Senior
Subject: Science

#3270. Designing a Flotation Strategy for the Purification of Recyc

Science, level: Senior
Posted Tue Oct 26 09:01:42 PDT 2004 by Frank Virzi (
North Middlesex Regional High School, Townsend, MA 01469
Materials Required: Plastics, Isopropyl alcohol, sugar, salt, triple beam balances, graduated cylinders
Activity Time: 2-3 class periods of about 50 minutes each
Concepts Taught: Density, plastics, recycling, measurement

In this lab, your team will be trying to figure out a way to purify recycled plastic materials. Your purification strategy will be based on the fact that different plastic materials have different densities, and you will be using various liquid solutions to separate batches of mixed plastics by flotation.

The Problem

The Happy Plastics Recycling Company will be hiring a firm to solve a problem for them. Happy Plastics now receives about 100 tons of mixed, impure recycled plastics waste every week, and this material needs to be sorted out and purified in order to be used for manufacturing. That's where you come in.


Different kinds of plastics have different densities. You will be using this physical property to purify batches of impure plastic pieces by flotation of the mixed plastics in different solvents, so that only one kind of plastic will float (or remain submerged).

Selection Criteria

Happy Plastics Recycling Company will award this contract based on three criteria:
(1) Minimum Cost. You will have at your disposal the following materials: sugar, table salt, water, and isopropyl alcohol. Try to accomplish your purification using as little of these materials as possible. Keep your costs down!

(2) Speed. Here at Happy Plastics, we're looking for the purification process that will enable us to handle the greatest amount of raw material in the shortest amount of time.

(3) Purity of Final Product.


(1) Collect 5 plastics samples. With your lab partner(s), collect FIVE different kinds of plastic. Chop, break, cut, tear, or crumble the plastic into small pieces approximately the size of Rice Krispies. For each kind of plastic, cut up enough plastic to fill about half of a Dixie Bathroom Cup (about 44 ml of plastic pieces). Label the cups with your names and the kind of plastic in the cup. You should have 5 Dixie Cups, each containing one kind of chopped up plastic material. Pile the Dixie Cups one inside the other to make a stack of them, and put your stack of plastic pieces in a safe place. Save enough of each plastic sample for yourself to use in your density measurement experiments and in your flotation testing.

(2) Measure the density of each plastic material.

(3) Prepare your solutions, and do some flotation experiments. Make up solutions of salt water, sugar water, or isopropyl alcohol / water that will enable you to separate and purify mixtures of your plastic pieces. Try adding one piece of two different kinds of plastics to a small test tube, and then separate them by adding a solution with a density which will make only one of the pieces float. After you have successfully separated mixtures of 2 plastics, try your luck at sorting out combinations of 3, 4, and all 5. Keep in mind that the plastic pieces MUST be separated on the basis of flotation ONLY; you will not be allowed to hand-sort any plastic pieces during your demonstration.

In order to keep solvent costs to a minimum, you will be given 10 mL of isopropyl alcohol and a small test tube with which to do all of your flotation testing experiments. You may prepare any sugar-water or salt-water solutions that you may need for your demonstration ahead of time.

For your demonstration, you will be using a larger test tube that can hold approximately 60 mL of liquid. This will enable the Happy Plastics Quality Control Inspector to judge the purity of the final product. Time will officially begin when you are handed the test tube containing the pieces of three different kinds of plastic, along with a rubber stopper. Time will officially end when you have scooped out and placed all of your plastic pieces into three watch glasses which will be provided. Any plastic pieces that you are not able to deliver successfully for one reason or another from the test tube to the watch glass will be taken into account in figuring your "purity" grade.

Half of your grade for this lab will be determined by demonstrating your separation system as follows:

Your teacher will randomly pick 3 out of 5 of your Dixie Cups which contain your plastic pieces. About 5 - 10 pieces of each plastic will be mixed together, and you will purify the mixed plastics, while timed.

Remember, your team is working toward three goals:

(1) Use minimum amounts of materials (water, salt, sugar & ethanol).

(2) Speed.

(3) Purity of your final product.





Plastics Purification Demo Summary

Time required to purify _________________________ minutes

Approximate purity of final product _________________ %


water ____________________ L

Salt _____________________ g

Sugar ____________________ g

Ethanol __________________ L

Grading Scorecard

____Written Lab Report (50 points)__________

Separation Demo (50 points)__________

TIME (10 points)__________
____ Under 3:00 minutes (10 points)
____ 3:01 - 5:00 minutes (6 points)
____ 5:01 - 9:00 minutes (4 points)
____ over 9 minutes (0 points)

PURITY (10 points)__________
____ 100 % (10 points)
____ 90 - 99 % (8 points)
____ 70 - 89 % (5 points)
____ 0 - 69% (0 points)

SCIENTIFIC APPROACH (30 points) __________
____ Measured volumes of liquids carefully
____ Record-keeping during demonstration will enable reliable cost estimate
____ Consulted density data and flotation experiment data to plan purification strategy
____ Scale-up of liquid volumes permitted easy scoop-outs
____ Logical sequence of steps in purification method
____ Volumes of liquids were not measured
____ Cost estimate of procedure not reliable, due to poor or no record-keeping during purification
____ One or more steps of purification method follow no logical sequence
____ No scale-up of liquid volumes, leading to difficulty with scoop-outs