Minerals Engineering xxx (xxxx) xxx–xxx

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Minerals Engineering
journal homepage: www.elsevier.com/locate/mineng

Short communication

Nanobubbles generation in a high-rate hydrodynamic cavitation tube

⁎
H. Oliveira, A. Azevedo, J. Rubio
Laboratório de Tecnologia Mineral e Ambiental, Departamento de Engenharia de Minas, PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500,
Setor 6 (Centro de Tecnologia), Prédio 43819, Bairro Agronomia, 91501-970 Porto Alegre, RS, Brazil

A R T I C L E I N F O A B S T R A C T

Keywords: Gas dispersion parameters (air holdup-ɛg, superficial air velocity-Jg and bubble surface area flux-Sb) and
Flotation especially the concentration of formed nanobubbles in a hydrodynamic cavitation tube were measured. Best
Cavitation tube results were obtained at 30% air/liquid volume ratio; 49 mN m−1 air/liquid interfacial tension resulting in an air
Micro and nanobubbles holdup of 16%, a Jg of 0.87 cm s−1, a Sb of 85 s−1 and a nanobubbles (230–280 nm) concentration of
Gas dispersion values
6.4 × 108 NBs mL−1. The lower the air/liquid interfacial tension, the higher the air holdup; this proceed by
assisting cavitation and formation of micro and nano-sized bubbles. Data obtained are discussed in terms of
solution, hydrodynamics and interfacial phenomena. Main mechanisms involving the role of the nanobubbles
and their interactions with solids and bigger bubbles were envisaged. It is believed that these findings are
important because of the need for techniques and devices producing nanobubbles at low cost and high rate. This
work shows that the cavitation tube studied has a high potential, due to the wide size of bubbles formed, high
bubble surface flux obtained and especially the generation of a high concentration of nanobubbles. These
bubbles are very important in modern flotation of mineral fines and in pollutant separation in wastewater
treatment and some examples are shown and discussed.

1. Introduction and a need to scale up the so-called flotation assisted by nanobubbles

(Azevedo et al., 2016a,b). Main available generation procedures are
Flotation with fine bubbles is today an efficient process in the re- depressurization and cavitation of air dissolved in water (Azevedo
covery of mineral fines (< 13 µm diameter) and in water/wastewater et al., 2016a; Calgaroto et al., 2014), fluidic oscillation flux through
treatment (Calgaroto et al., 2015; Gontijo et al., 2008; Rubio et al., nanomembranes (Zimmerman et al., 2011) and hydrodynamic cavita-
2002). The smaller the bubbles are, the better the flotation performance tion in multiphase pumps (Etchepare et al., 2017c).
is because the fine or colloidal (micro and nano) sized particles are The CavTube® is an Eriez’s patented sparger employed for slurry
effectively captured by fine and not by large bubbles (Gontijo et al., aeration and mineral flotation processes that generate fine bubbles (Fan
2008). Recently, studies revealed that the presence of microbubbles and et al., 2012). Herein, water and air mixtures are pumped through a
especially nanobubbles enhance the flotation separation of these small sudden contraction and expansion in the tube causing a high flux ve-
entities (Calgaroto et al., 2016, 2015, Etchepare et al., 2017a,b). locity in the throat of the device, provoking bubbles nucleation and
Nanobubbles are gas cavities with diameters less than 1 µm, gen- formation. Yet, the manufactures, neither authors, have measured fully
erated jointly with bigger bubbles following hydrodynamic cavitation the concentration, bubbles size distribution of the nanobubbles gener-
phenomena (Etchepare et al., 2017c). Due to their large surface area, ated. This constitutes the aim of this note, envisaging future applica-
high concentration, long stability and high hydrophobic affinity, the tions of this hydrodynamic cavitation device.
nano-sized bubbles adsorb rapidly at surfaces, aggregate the dispersed
fines particles and serve as the nuclei for bigger (micro and macro- 2. Experimental
bubbles) attachment (Azevedo et al., 2016a; Calgaroto et al., 2015).
These mechanisms are unique which make the nanobubbles extremely 2.1. Materials
important in modern flotation of the very small particles. In this con-
text, the discovery and development of gas spargers able to generate Deionized (DI) water having an air/liquid interfacial tension (γ) of
nano-sized bubbles, at high rate and low cost is a technical challenge 72.5 ± 0.2 mN m−1 and a pH of 5.5 was used in the experiments at

⁎
Corresponding author.
E-mail address: jrubio@ufrgs.br (J. Rubio).
URL: http://www.ufrgs.br/ltm (J. Rubio).

https://doi.org/10.1016/j.mineng.2017.10.020
Received 14 August 2017; Received in revised form 27 October 2017; Accepted 27 October 2017
0892-6875/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Oliveira, H., Minerals Engineering (2017), http://dx.doi.org/10.1016/j.mineng.2017.10.020
H. Oliveira et al. Minerals Engineering xxx (xxxx) xxx–xxx

Fig. 2. Air dispersion parameters of bubbles generated by the CavTube: Air holdup and
nanobubbles concentration as a function of the liquid/air interfacial tension, γ, at
22° ± 1. Conditions: Liquid flow = 1.6 m3 h−1; Air flow rate = 0.5 Nm3 h−1;
P = 3.5 bar. The error bars correspond to the standard deviation of the triplicate tests.

3.5 bar.

3. Results and discussion

The concentrations of nanobubbles ranged between 4.2 and

6.4 × 108 NBs mL−1 and the Sauter mean diameter (volumetric) fluc-
tuated between 220 and 280 nm, with the bubbles size ranging from
100 to 550 nm in all surfactant solutions, after 30 min of recirculation.
Fig. 2 presents the values of εg and concentration of the nanobubbles as
a function of γ. Results show an increase in air holdup with the decrease
in γ, following the lower energy ΔF required for microbubble formation
Fig. 1. Experimental rig employed for the bubbles generation with the CavTube. (1) (Takahashi et al., 1979; Rodrigues and Rubio, 2003).
Cooler; (2) water tank; (3) recirculation pump; (4) CavTube; (5) pressurized air; (6)
Best results were obtained with 100 mg.L−1 pine oil solution
pressure sensors; (7) bubble generation column; F: flowmeter; M: manometer. On top, a
detail of the CavTube® (4).
(γ = 49 mN m−1). Air holdup values varied between 13 and 16%; Jg of
0.87 cm s–1 and Sb of 85 s−1 (Table 1).
The properties of bubbles (gas) dispersion or hydrodynamic condi-
22 °C. Pine oil was employed to change the interfacial tension. tions clearly play an important role in froth flotation and applied flo-
tation to effluent treatment (Matiolo et al., 2011). Here, the gas dis-
2.2. Methods persion values while using this cavitation tube qualify it as a high rate
bubble generator. A wide size distribution; high concentration of na-
2.2.1. Bubble generation nobubbles plus micro and macrobubbles (visually observed) were va-
The experimental rig (Fig. 1) consisted of a column (10 cm in dia- lidated by the high εg obtained. In addition, because the concentration
meter and 240 cm in height) associated with a centrifugal pump (Ni- of the nanobubbles formed is very high, the CavTube® should certainly
kuni®, KTM20ND) for liquid recirculation. The CavTube® (CT 100, broaden its applications in mineral (slimes) flotation and specially in
connection type 1″ NPT, 316 SS) was installed at the bottom entrance of wastewater treatment by flotation or aeration.
the column and air (pressurized) was injected upstream. A water re- Herein, the bulk nanobubbles, in the absence of solids, remain
cycle tank was equipped with a heat exchanger, adjusted to maintain dispersed (and stable for weeks) and as such do not contribute to any of
22 °C, by circulation of refrigerated fluid (Maqtermo®, LS03AR). gas dispersion parameters including the Sb, the bubble surface area
flux. Thus, there is no bubbling flux with nanobubbles, because they do
2.2.2. Nanobubbles characterization not have rising movement (Azevedo et al., 2016).
Bubbles were measured employing nanoparticle tracking analysis In fact, a method of separation of the nanobubbles by leaving the
(NTA-Zeta View®, Particle Metrix, Germany) as described in Etchepare bigger bubbles to abandon the system after their rising, has been
et al. (2017c). Results were expressed by the number of nanobubbles
per milliliter (NBs mL−1) and by the Sauter mean diameter (volu- Table 1
metric). Air dispersion parameters of bubbles generated by the CavTube as a function of γ.
Conditions: liquid flow = 1.6 m3 h−1, air flow rate = 0.5 Nm3 h−1, P = 3.5 bar,
2.2.3. Determination of gas dispersion parameters Temperature = 22° ± 1. The experimental error corresponds to the standard deviation
(in parenthesis) of triplicate tests. The Sb values were calculated as 5.5 εg.
The air holdup (εg) was calculated by measuring the differential
pressure (ΔP) between two points of the column, using hydrostatic Pine oil concentration, Air/liquid interfacial εg,% Jg, cm s−1 Sb, s−1
pressure sensors (Pressure Transmitter SP98, Sitron®). The superficial mg L−1 tension (γ), mN m−1
air velocity (Jg) was considered to be the ratio of air volume passing
0 72 3.9 0.22 (0.06) 21 (6)
through the column cross section and the and bubble surface area flux (1.1)
(Sb) is related to ɛg and was estimated according to Finch et al. (2000), 15 68 5.2 0.29 (0.06) 29 (6)
as Sb = 5.5 × ɛg. This relation holds over a range of conditions that can (1.1)
be interpreted as normal for flotation, namely Sb up to about 130 s−1 50 58 13.2 0.75 (0.07) 73 (7)
(1.3)
and ɛg up to about 25%.
100 49 15.4 0.87 (0.08) 85 (8)
Experiments were performed with 40 L of solution, a liquid flow of (1.4)
1.6 m3 h−1, air flow of 0.5 Nm3 h−1 and an operating pressure of

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H. Oliveira et al. Minerals Engineering xxx (xxxx) xxx–xxx

Fig. 3. Illustrative scheme on solution and interfacial

Macrobubble Hydrophilic particle Hydrophobic particle phenomena involving big bubbles, nanobubbles and
fine particles.

Nanobubble

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Acknowledgements
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FAPERGS and UFRGS and to all students for their kind collaboration.