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Study on the flotation behaviour of low energy materials at low surface tensions
Minerals Engineering 16 (2003) 1193–1196 This article is also available online at: www.elsevier.com/locate/mineng
Study on the flotation behaviour of low energy materials at low surface tensions V. Kirjavainen *, H. Lehto, K. Heiskanen Laboratory of Mechanical Process and Recycling Technology, Helsinki University of Technology, P.O. Box 6200, 02150 Helsinki HUT, Finland Received 11 April 2003; accepted 15 July 2003
Abstract Surface tension is one of the most important factors in a flotation process, normally being understood to mean surface tension at the pulp surface. In most cases this is enough, but when we try to process materials which have low surface energies, we also have to take into account the surface tension between the air bubbles and the liquid (water) in the flotation cell. This paper reports preliminary study results on this subject. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Column flotation; Flotation bubbles; Flotation frothers; Flotation froths
flotation of hydrophobic plastic particles at low surface tensions.
1. Introduction A prerequisite for flotation is that the surfaces of the floatable particles are hydrophobic and the Gibbs free energy change (DG) in bubble attachment is negative. Thus DG ¼ cSV cSL cLV < 0
ð1Þ
where cSV , cSL and cLV are the solid/vapour, solid/liquid and liquid/vapour interfacial tensions, respectively (Laskowski, 1992). This means that the floatability of particles depends, not only on the adsorption of collector molecules onto the floatable particles, but also on the surface tension between the liquid and gas. In the case of materials, such as plastics, which have low surface energy, the surfaces are usually hydrophobic and collectors are not necessarily needed. When the separation of materials is carried out by controlling the liquid/ vapour surface tension (cLV ) the process is called gamma flotation (Yarar, 1988). In conventional flotation, frother dosage is relatively low and frother tends to concentrate at the surface of the pulp. Therefore, liquid– vapour surface tension may vary in different parts of the pulp. The purpose of this work was to consider the
* Corresponding author. Tel.: +358-9-4512787; fax: +358-94512795. E-mail address: vesa.kirjavainen@hut.fi (V. Kirjavainen).
0892-6875/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.07.015
2. Experimental The plastic materials were first crushed with a laboratory shredder (Fritsch Pulverisette) and then divided into size fractions using vibrating sieving (Fritsch). The preliminary tests were made with shredded bottles (2–4 mm) which were probably polyethyleneterephthalate (PET), but the material in these tests may have varied. Three plastics, the densities of which were higher than that of water, were chosen as the actual test materials. These were polyvinyl chloride (PVC), polycarbonate (PC) and polymethylmethacrylate (PMMA). PVC is also a raw material used in industry, which must be separated in recycling processes. The materials were crushed with the shredder and sieved to 0.5–1, 1–1.4, 1.4–2 and 2–2.8 mm fractions for testing. Flotation tests were carried out using a column type cell. The cell comprised two cylindrical columns cemented on the circular bottom plate and the inner column was the actual cell (Fig. 1). The froth from the inner column was collected in the space between the columns so that floated and non-floated products could be separated after the tests. Pressurized air was introduced into the cell through a ceramic sparger in the bottom. The diameter of the inner column was 80 mm
1194
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
Fig. 1. Column type flotation cell.
and the height 340 mm. Agitation was not used in the tests. After the preliminary testing, flotation experiments were made with 10 g samples and n-amyl alcohol was the only reagent used. Amyl alcohol was chosen as frother because it allowed control of surface tension at the liquid/vapour interface by dosing, and also to ensure that the value would be constant in different parts of the pulp. Before each test tap water and frother solution were prepared in a beaker. The pH of the water was approximately 8.2 and the temperature of the liquid 21 °C. One liter of the liquid was then poured into the cell onto the sample. After 5 min, flotation commenced for 4 min. When frothing diminished liquid was added to the cell. At the end of the test a liquid sample was taken from the cell and the floated product was dried and weighed to determine the recovery. The surface tension of the liquid samples was measured with a ring tensiometer (Sigma 70, KSV instruments). In the calculation of surface tensions, the correction factors of Huh and Mason (1975) were used.
3. Results and discussion The work was started by performing flotation tests on shredded plastic bottles (2–4 mm) using commercial frothers (Flotol, Flotanol, Montanol, Dowfroth) and ethanol as reagents. As the formation of a froth bed was negligible in the flotation of coarse plastic particles, due to the absence of the stabilizing effect of fine particles, the froth product was collected by other techniques. In the preliminary testing particles were mostly picked
from the pulp surface. For the same reason, the cell design was such that the floated particles could be collected at the outer cell compartment if both of the compartments were filled with water, and the liquid level was lifted above the inner cell. In principle, this solution does not necessitate the usage of a frother at all. Using both of these methods plastic particles were collected in the first tests. All the frothers floated particles already at low frother concentrations, the floatability of particles being better in acid than in alkaline liquid, this obviously being a consequence of higher surface tension at acid pH. The floatability was also dependent on the characteristics of the frother and complete flotation recovery was achieved at slightly different dosage. As is well known, ethanol is not a very surface active compound, being more evenly distributed in the liquid. Therefore, ethanol additions increased the formation of small air bubbles and clearly improved the flotation of particles, especially with frother. It seems that in conventional flotation, frother is added to form the froth bed but its role as a surface active reagent is often forgotten. When commercial frothers are the only surface active reagents used, the surface tension between air and liquid is obviously different deep in the pulp and at the surface of the pulp. The effect of these changes on flotation has obviously been ignored so far in practice, but it could be studied, for example, by comparing samples from large cells or columns at different depths. On the basis of the preliminary tests, it was concluded that it would be important to have constant surface tension between liquid and air in plastic flotation. Therefore, n-amyl alcohol was chosen as frother for further tests. The first task with amyl alcohol was to determine the reagent dosage that would give appropriate surface tension in the cell. Testing showed that an amyl alcohol concentration of 30 ml/l produced a surface tension that was slightly less than 30 mN/m. In the plastic flotation, however, the maximum dosage was 20 ml/l. Normally alcohol addition was 10, 15 or 20 ml/l, such that the differences between the dosages were reasonably large compared with the corresponding differences of surface tensions using amyl alcohol as frother. Testing with amyl alcohol also showed that low surface tension has a significant effect on pulp properties. In conventional flotation, air bubbles usually rise through the pulp and, due to the high concentration of frother molecules, form a froth bed on the surface of the pulp. There are thus two different phases in the cell, pulp phase and froth phase. With amyl alcohol, however, the froth phase filled the whole cell, so that at the start the concentration of amyl alcohol molecules was higher in the upper part of the liquid, but after froth phase formation the molecules were more evenly distributed in the whole cell. Thus, flotation of particles with this procedure actually takes place in the froth. The forma-
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
tion of the froth phase is dependent on surface tension so that at the highest concentration the froth phase was formed in a few seconds but at the lower concentrations it took about 1 min. The first tests have already shown that the flotation procedure is different compared to conventional flotation and may have an effect on the floatability of particles.
1195
surface tension in the flotation tests approaching the socalled critical surface tension of PVC, where DG is no more favourable for the attachment of air bubbles. It should be remembered, however, that the surface tension measurements were made with a ring tensiometer and the values are lower than in the flotation froth. 3.2. Tests with polycarbonate
3.1. Tests with polyvinyl chloride PVC is a common material used in building and construction, for example, for pipes, fittings and various profiles. It was chosen as a test material due to its high density (1.4) compared with many plastics, to assess the floatability of heavy plastic particles. The flotation of PVC was studied in all the four size fractions between 0.5 and 2.8 mm and the results are presented in Fig. 2. The tests showed that the coarse PVC particles (2–2.8 mm) floated relatively well at the highest surface tension with this technique. This is to some extent surprising because the floatability of plastic particles has been found to deteriorate fast with increasing particle size in conventional flotation (Huiting et al., 2001). The difference can be explained as the fact that in conventional flotation frother tends to be concentrated at the surface of the pulp. This decreases surface tension at the pulp surface and also the hydrophobicity and floatability of particles as shown by Eq. (1). Therefore, particularly coarse particles with heavy mass are not properly recovered in conventional flotation but tend to remain in the pulp due to the effect of gravity force. With the applied method amyl alcohol is more evenly distributed in the froth and coarse particles are floated efficiently in spite of the absence of the pulp phase. As can be seen, low surface tension decreased the floatability of particles in all the size fractions, but the floatability of particles in the two finest fractions was quite similar. The low recoveries are clearly indicative of
PC is used for many purposes, for example, in lenses, goggles, bottles, computer parts and automotive plastics. It is also used in blends and alloys with other plastics (ABS, PMMA, PBT). The test material was shredded mobile phone covers, which were sieved to 1– 1.4, 1.4–2 and 2–2.8 mm fractions. The flotation results are presented in Fig. 3. Coarse particles (2–2.8 mm) also floated well with this material at the highest surface tensions, confirming the observations made with PVC. PC is a more hydrophobic material than PVC and the size fractions between 1 and 2 mm were readily floatable with the method. Considering the results, it is obvious that the critical surface tension of PC was not approached in the tests. Due to the floatability of the PC particles the finest fraction was not tested. 3.3. Tests with polymethylmethacrylate PMMA products are also known as acrylic parts and can be used to replace glass in lenses and covers. The test material was shredded Plexiglas that was divided to 1– 1.4 and 1.4–2 mm fractions. As the thickness of the Plexiglas plates was about 1 mm, the shape of the particles was obviously more spherical in the finer fraction due to the shredding. However, the shape of the particles in the size fractions was not determined. The flotation results are presented in Fig. 4. As the results show, PMMA particles also floated quite well with the method, but it can be concluded that
100
100
90 80
70
RECOVERY, %
RECOVERY, %
80
60 50 40 30 20 10 0
25
0.5 - 1 mm 1 - 1.4 mm 1.4 - 2 mm 2 - 2.8 mm 30 35 40 45 SURFACE TENSION, mN/m
60 1 - 1.4 mm 1.4 - 2 mm
40
2 - 2.8 mm 20
50
Fig. 2. Recovery of PVC particles in different size fractions with amyl alcohol.
0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 3. Recovery of PC particles in different size fractions with amyl alcohol.
1196
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
shapes in the test materials, but it seems also possible that the adsorption of amyl alcohol molecules and /or ions were different on PVC and PMMA, changing their floatabilities. There may be other relevant explanations, which also require more research.
100 90 RECOVERY, %
80 70 60 50 40
4. Conclusions
30
1 - 1.4 mm
20
1.4 - 2 mm
10 0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 4. Recovery of PMMA particles in different size fractions with amyl alcohol.
100 90 RECOVERY, %
80 70 60 50
PMMA
40
PVC
30
PC
20 10 0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 5. The floatability of PMMA, PVC and PC in size fraction 1–1.4 mm.
the recoveries also reflect the effect of particle shape on flotation. The floatability of PMMA particles decreased strongly in the coarse fraction at low surface tension, obviously being affected by both particle size and shape. Flaky particles were not as floatable as more spherical particles. Therefore, it is necessary to consider the recoveries in the finer fraction when comparing the test materials. In Fig. 5 the recoveries of the test materials are presented for the size fraction 1–1.4 mm for comparison. PC particles were very hydrophobic even at low surface tension. PVC and PMMA recoveries clearly decreased at low surface tensions, and the recovery curves cut each other. This could be a consequence of different particle
The floatabilities of PVC, PC and PMMA were assessed in different size fractions with amyl alcohol using a column type cell. The flotation method used is different to the conventional procedure. As amyl alcohol was evenly distributed, the froth phase was formed in the cell. As a consequence, DG in bubble attachment was more negative and all the particles were closely surrounded by air and air bubbles in the froth. Therefore, the flotation of particles became efficient. Attachment and detachment of particles also depend on other factors, such as mixing and washing but they were not studied in the work. The floatability of the plastics was dependent on particle size and shape but the flotation properties of plastics are generally known to be very similar, so the role of regulating agents is also obviously important with this method. Although the formation of the froth phase is considered here in a column type cell in dilute suspensions, a similar mechanism may be effective in conventional flotation. In most cases minerals, such as sulfides, are floated with low dosages of collectors and frothers, so that the formation of both pulp and froth phases is obvious. However, with some ores high dosages of emulsifying reagents (fuel oil) are also used and their effect on pulp behaviour can be similar. This would explain the improved floatability, and particularly the increased recovery, of coarse particles in conventional flotation. References Huh, C., Mason, S.G., 1975. Colloid and Polymer Science 253, 266– 580. Huiting, S., Forssberg, E., Pugh, R.J., 2001. Selective flotation separation of plastics by particle control. Resources, Conservation and Recycling 33 (1), 37–50. Laskowski, J.S., 1992. An introduction: physicochemical methods of separation. In: Laskowski, J.S., Ralston, J. (Eds.), Colloid Chemistry in Mineral Processing. Elsevier Science Publishers B.V, Amsterdam, pp. 225–241. Yarar, B., 1988. Gamma flotation: a new approach to flotation using liquid–vapor surface tension control. In: Castro, S.H., Alvarez, J. (Eds.), Froth Flotation. Elsevier Science Publishers B.V, Amsterdam, pp. 44–64.
Study on the flotation behaviour of low energy materials at low surface tensions V. Kirjavainen *, H. Lehto, K. Heiskanen Laboratory of Mechanical Process and Recycling Technology, Helsinki University of Technology, P.O. Box 6200, 02150 Helsinki HUT, Finland Received 11 April 2003; accepted 15 July 2003
Abstract Surface tension is one of the most important factors in a flotation process, normally being understood to mean surface tension at the pulp surface. In most cases this is enough, but when we try to process materials which have low surface energies, we also have to take into account the surface tension between the air bubbles and the liquid (water) in the flotation cell. This paper reports preliminary study results on this subject. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Column flotation; Flotation bubbles; Flotation frothers; Flotation froths
flotation of hydrophobic plastic particles at low surface tensions.
1. Introduction A prerequisite for flotation is that the surfaces of the floatable particles are hydrophobic and the Gibbs free energy change (DG) in bubble attachment is negative. Thus DG ¼ cSV cSL cLV < 0
ð1Þ
where cSV , cSL and cLV are the solid/vapour, solid/liquid and liquid/vapour interfacial tensions, respectively (Laskowski, 1992). This means that the floatability of particles depends, not only on the adsorption of collector molecules onto the floatable particles, but also on the surface tension between the liquid and gas. In the case of materials, such as plastics, which have low surface energy, the surfaces are usually hydrophobic and collectors are not necessarily needed. When the separation of materials is carried out by controlling the liquid/ vapour surface tension (cLV ) the process is called gamma flotation (Yarar, 1988). In conventional flotation, frother dosage is relatively low and frother tends to concentrate at the surface of the pulp. Therefore, liquid– vapour surface tension may vary in different parts of the pulp. The purpose of this work was to consider the
* Corresponding author. Tel.: +358-9-4512787; fax: +358-94512795. E-mail address: vesa.kirjavainen@hut.fi (V. Kirjavainen).
0892-6875/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.07.015
2. Experimental The plastic materials were first crushed with a laboratory shredder (Fritsch Pulverisette) and then divided into size fractions using vibrating sieving (Fritsch). The preliminary tests were made with shredded bottles (2–4 mm) which were probably polyethyleneterephthalate (PET), but the material in these tests may have varied. Three plastics, the densities of which were higher than that of water, were chosen as the actual test materials. These were polyvinyl chloride (PVC), polycarbonate (PC) and polymethylmethacrylate (PMMA). PVC is also a raw material used in industry, which must be separated in recycling processes. The materials were crushed with the shredder and sieved to 0.5–1, 1–1.4, 1.4–2 and 2–2.8 mm fractions for testing. Flotation tests were carried out using a column type cell. The cell comprised two cylindrical columns cemented on the circular bottom plate and the inner column was the actual cell (Fig. 1). The froth from the inner column was collected in the space between the columns so that floated and non-floated products could be separated after the tests. Pressurized air was introduced into the cell through a ceramic sparger in the bottom. The diameter of the inner column was 80 mm
1194
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
Fig. 1. Column type flotation cell.
and the height 340 mm. Agitation was not used in the tests. After the preliminary testing, flotation experiments were made with 10 g samples and n-amyl alcohol was the only reagent used. Amyl alcohol was chosen as frother because it allowed control of surface tension at the liquid/vapour interface by dosing, and also to ensure that the value would be constant in different parts of the pulp. Before each test tap water and frother solution were prepared in a beaker. The pH of the water was approximately 8.2 and the temperature of the liquid 21 °C. One liter of the liquid was then poured into the cell onto the sample. After 5 min, flotation commenced for 4 min. When frothing diminished liquid was added to the cell. At the end of the test a liquid sample was taken from the cell and the floated product was dried and weighed to determine the recovery. The surface tension of the liquid samples was measured with a ring tensiometer (Sigma 70, KSV instruments). In the calculation of surface tensions, the correction factors of Huh and Mason (1975) were used.
3. Results and discussion The work was started by performing flotation tests on shredded plastic bottles (2–4 mm) using commercial frothers (Flotol, Flotanol, Montanol, Dowfroth) and ethanol as reagents. As the formation of a froth bed was negligible in the flotation of coarse plastic particles, due to the absence of the stabilizing effect of fine particles, the froth product was collected by other techniques. In the preliminary testing particles were mostly picked
from the pulp surface. For the same reason, the cell design was such that the floated particles could be collected at the outer cell compartment if both of the compartments were filled with water, and the liquid level was lifted above the inner cell. In principle, this solution does not necessitate the usage of a frother at all. Using both of these methods plastic particles were collected in the first tests. All the frothers floated particles already at low frother concentrations, the floatability of particles being better in acid than in alkaline liquid, this obviously being a consequence of higher surface tension at acid pH. The floatability was also dependent on the characteristics of the frother and complete flotation recovery was achieved at slightly different dosage. As is well known, ethanol is not a very surface active compound, being more evenly distributed in the liquid. Therefore, ethanol additions increased the formation of small air bubbles and clearly improved the flotation of particles, especially with frother. It seems that in conventional flotation, frother is added to form the froth bed but its role as a surface active reagent is often forgotten. When commercial frothers are the only surface active reagents used, the surface tension between air and liquid is obviously different deep in the pulp and at the surface of the pulp. The effect of these changes on flotation has obviously been ignored so far in practice, but it could be studied, for example, by comparing samples from large cells or columns at different depths. On the basis of the preliminary tests, it was concluded that it would be important to have constant surface tension between liquid and air in plastic flotation. Therefore, n-amyl alcohol was chosen as frother for further tests. The first task with amyl alcohol was to determine the reagent dosage that would give appropriate surface tension in the cell. Testing showed that an amyl alcohol concentration of 30 ml/l produced a surface tension that was slightly less than 30 mN/m. In the plastic flotation, however, the maximum dosage was 20 ml/l. Normally alcohol addition was 10, 15 or 20 ml/l, such that the differences between the dosages were reasonably large compared with the corresponding differences of surface tensions using amyl alcohol as frother. Testing with amyl alcohol also showed that low surface tension has a significant effect on pulp properties. In conventional flotation, air bubbles usually rise through the pulp and, due to the high concentration of frother molecules, form a froth bed on the surface of the pulp. There are thus two different phases in the cell, pulp phase and froth phase. With amyl alcohol, however, the froth phase filled the whole cell, so that at the start the concentration of amyl alcohol molecules was higher in the upper part of the liquid, but after froth phase formation the molecules were more evenly distributed in the whole cell. Thus, flotation of particles with this procedure actually takes place in the froth. The forma-
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
tion of the froth phase is dependent on surface tension so that at the highest concentration the froth phase was formed in a few seconds but at the lower concentrations it took about 1 min. The first tests have already shown that the flotation procedure is different compared to conventional flotation and may have an effect on the floatability of particles.
1195
surface tension in the flotation tests approaching the socalled critical surface tension of PVC, where DG is no more favourable for the attachment of air bubbles. It should be remembered, however, that the surface tension measurements were made with a ring tensiometer and the values are lower than in the flotation froth. 3.2. Tests with polycarbonate
3.1. Tests with polyvinyl chloride PVC is a common material used in building and construction, for example, for pipes, fittings and various profiles. It was chosen as a test material due to its high density (1.4) compared with many plastics, to assess the floatability of heavy plastic particles. The flotation of PVC was studied in all the four size fractions between 0.5 and 2.8 mm and the results are presented in Fig. 2. The tests showed that the coarse PVC particles (2–2.8 mm) floated relatively well at the highest surface tension with this technique. This is to some extent surprising because the floatability of plastic particles has been found to deteriorate fast with increasing particle size in conventional flotation (Huiting et al., 2001). The difference can be explained as the fact that in conventional flotation frother tends to be concentrated at the surface of the pulp. This decreases surface tension at the pulp surface and also the hydrophobicity and floatability of particles as shown by Eq. (1). Therefore, particularly coarse particles with heavy mass are not properly recovered in conventional flotation but tend to remain in the pulp due to the effect of gravity force. With the applied method amyl alcohol is more evenly distributed in the froth and coarse particles are floated efficiently in spite of the absence of the pulp phase. As can be seen, low surface tension decreased the floatability of particles in all the size fractions, but the floatability of particles in the two finest fractions was quite similar. The low recoveries are clearly indicative of
PC is used for many purposes, for example, in lenses, goggles, bottles, computer parts and automotive plastics. It is also used in blends and alloys with other plastics (ABS, PMMA, PBT). The test material was shredded mobile phone covers, which were sieved to 1– 1.4, 1.4–2 and 2–2.8 mm fractions. The flotation results are presented in Fig. 3. Coarse particles (2–2.8 mm) also floated well with this material at the highest surface tensions, confirming the observations made with PVC. PC is a more hydrophobic material than PVC and the size fractions between 1 and 2 mm were readily floatable with the method. Considering the results, it is obvious that the critical surface tension of PC was not approached in the tests. Due to the floatability of the PC particles the finest fraction was not tested. 3.3. Tests with polymethylmethacrylate PMMA products are also known as acrylic parts and can be used to replace glass in lenses and covers. The test material was shredded Plexiglas that was divided to 1– 1.4 and 1.4–2 mm fractions. As the thickness of the Plexiglas plates was about 1 mm, the shape of the particles was obviously more spherical in the finer fraction due to the shredding. However, the shape of the particles in the size fractions was not determined. The flotation results are presented in Fig. 4. As the results show, PMMA particles also floated quite well with the method, but it can be concluded that
100
100
90 80
70
RECOVERY, %
RECOVERY, %
80
60 50 40 30 20 10 0
25
0.5 - 1 mm 1 - 1.4 mm 1.4 - 2 mm 2 - 2.8 mm 30 35 40 45 SURFACE TENSION, mN/m
60 1 - 1.4 mm 1.4 - 2 mm
40
2 - 2.8 mm 20
50
Fig. 2. Recovery of PVC particles in different size fractions with amyl alcohol.
0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 3. Recovery of PC particles in different size fractions with amyl alcohol.
1196
V. Kirjavainen et al. / Minerals Engineering 16 (2003) 1193–1196
shapes in the test materials, but it seems also possible that the adsorption of amyl alcohol molecules and /or ions were different on PVC and PMMA, changing their floatabilities. There may be other relevant explanations, which also require more research.
100 90 RECOVERY, %
80 70 60 50 40
4. Conclusions
30
1 - 1.4 mm
20
1.4 - 2 mm
10 0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 4. Recovery of PMMA particles in different size fractions with amyl alcohol.
100 90 RECOVERY, %
80 70 60 50
PMMA
40
PVC
30
PC
20 10 0 25
30
35 40 45 SURFACE TENSION, mN/m
50
Fig. 5. The floatability of PMMA, PVC and PC in size fraction 1–1.4 mm.
the recoveries also reflect the effect of particle shape on flotation. The floatability of PMMA particles decreased strongly in the coarse fraction at low surface tension, obviously being affected by both particle size and shape. Flaky particles were not as floatable as more spherical particles. Therefore, it is necessary to consider the recoveries in the finer fraction when comparing the test materials. In Fig. 5 the recoveries of the test materials are presented for the size fraction 1–1.4 mm for comparison. PC particles were very hydrophobic even at low surface tension. PVC and PMMA recoveries clearly decreased at low surface tensions, and the recovery curves cut each other. This could be a consequence of different particle
The floatabilities of PVC, PC and PMMA were assessed in different size fractions with amyl alcohol using a column type cell. The flotation method used is different to the conventional procedure. As amyl alcohol was evenly distributed, the froth phase was formed in the cell. As a consequence, DG in bubble attachment was more negative and all the particles were closely surrounded by air and air bubbles in the froth. Therefore, the flotation of particles became efficient. Attachment and detachment of particles also depend on other factors, such as mixing and washing but they were not studied in the work. The floatability of the plastics was dependent on particle size and shape but the flotation properties of plastics are generally known to be very similar, so the role of regulating agents is also obviously important with this method. Although the formation of the froth phase is considered here in a column type cell in dilute suspensions, a similar mechanism may be effective in conventional flotation. In most cases minerals, such as sulfides, are floated with low dosages of collectors and frothers, so that the formation of both pulp and froth phases is obvious. However, with some ores high dosages of emulsifying reagents (fuel oil) are also used and their effect on pulp behaviour can be similar. This would explain the improved floatability, and particularly the increased recovery, of coarse particles in conventional flotation. References Huh, C., Mason, S.G., 1975. Colloid and Polymer Science 253, 266– 580. Huiting, S., Forssberg, E., Pugh, R.J., 2001. Selective flotation separation of plastics by particle control. Resources, Conservation and Recycling 33 (1), 37–50. Laskowski, J.S., 1992. An introduction: physicochemical methods of separation. In: Laskowski, J.S., Ralston, J. (Eds.), Colloid Chemistry in Mineral Processing. Elsevier Science Publishers B.V, Amsterdam, pp. 225–241. Yarar, B., 1988. Gamma flotation: a new approach to flotation using liquid–vapor surface tension control. In: Castro, S.H., Alvarez, J. (Eds.), Froth Flotation. Elsevier Science Publishers B.V, Amsterdam, pp. 44–64.