Gas Dispersion and de-Inking in Flotation Column

5/20/2018 Gas Dispersion and de-Inking in Flotation Column

Gas dispersion and de-inking in a flotation column

H. Hernaandez 1, C.O. Goomez, J.A. Finch *

Department of Mining, Metals & Materials Engineering, McGill University, 3610 University, Montreal, Qc., Canada H3A 2B2

Received 14 March 2003; accepted 2 May 2003

Abstract

The role of four gas dispersion parameters in ink particle collection was investigated in 4 00 and 2000 flotation columns. Gas holdup

(eg) and superficial gas velocity (Jg) were measured on-line and bubble size (db) was estimated using drift flux analysis that enabled

bubble surface area flux (Sb) to be calculated. Operating with approximately zero froth depth ink recovery as a function of retention

time (controlled by underflow rate) was determined. Using a mixing model, the collection zone flotation rate constant (kc) wasestimated from the recoverytime data. The rate constant was not related to Jg or db but was linearly dependent on eg and Sb,

similar to findings in mineral flotation studies.

2003 Elsevier Ltd. All rights reserved.

Keywords: Froth flotation; Flotation kinetics; Modeling

1. Introduction

Flotation involves contacting particles (typically


the gas holdup and to control the position of the pulp/

froth interface (i.e., level). The air rate was regulated by

a mass flowmeter/controller. A peristaltic pump (with I/

O card) and a centrifugal pump controlled the flow of

feed and underflow, 3 respectively. The feed and un-

derflow rates were measured by magnetic flowmeter with

signal conversion and were regulated using control

valves. Bubble generation used three porous stainless

steel spargers supplied by Mott Industrial with nominal

pore size 0.5, 2 and 20 lm. The temperature was mea-

sured with an ICTD probe. All instruments were mon-

itored using a serial communication interface OPTO and

a computer. The software package for data collection

and column operation was FIXDMACS version 6.1.

The general set-up and layout is shown in Fig. 2.

2.2. Procedure

The columns were fed continuously with pulp (45C, 1.2% w/w solids, pH 10 (NaOH) and surfactant,a blend of stearic and oleic fatty acid soaps) from the

feed end of the de-inking plant flotation circuit (ca. 1.2%w/w solids). The chemical conditions were not altered.

Level was controlled with the feed rate and set to within

12 cm of the column overflow lip, i.e., operation with

essentially zero froth depth. The feed and underflow

rates, gas rate and gas holdup were monitored during

each experiment. Samples of 1 L were taken from the

underflow and feed streams as a function of time when

steady-state conditions were achieved. The particle re-

tention time was controlled with the underflow rate. Ink

recovery was measured according to Bowaters proce-

dure and calculated as follows:

RinkCiSiQiqi CoSoQoqo

CiSiQiqi 1

whereSis the % w/w solids, Q the volumetric flowrate, q

the stream density and the subscripts refer to initial (i)

and final (o) values. The ink concentration C(ppm) was

measured using the reflectance of paper pads at a

wavelength of 950 nm in a Technidyne Micro TB-1C

machine. An average of 10 values (5 per side) was ob-

tained.

2.3. Pulp (collection) zone mixing model

Recovery in the collection zone (Rc) is a function of

the rate constant (kc), ink particle residence time (sp) and

degree of mixing, represented by the vessel dispersion

number (Nd). One extreme of mixing is plug flow

(Nd! 0), where all elements have the same residencetime and there is a concentration gradient of floatable

particles along the axis of the column. The other ex-

treme is a completely mixed reactor (Nd! 1), where adistribution of residence time exists and the concentra-

tion is the same throughout the vessel (Levenspiel,

1999). For a first-order rate process with plug flow and

perfect mixing transport Rc is given by Eqs. (2) and (3)respectively:

Rc Req1 expkcsp 2

Rc Req 1

"

1

1 kcsp

# 3

where Req is the equilibrium (or maximum) recovery at

long flotation time.

For vessels with levels of mixing between the two

extremes, the relationship between Rc and Nd can be

given by the axial dispersion model (Levenspiel, 1999):

Fig. 1. Column flotation test facility at Bowaters de-inking plant.

3 In de-inking the underflow is called the accepts.

Fig. 2. General experimental set-up.

740 H. Hernaandez et al. / Minerals Engineering 16 (2003) 739744


Rc Req 1

24 4A exp 12Nd

1 A

2exp A

2Nd

1 A

2exp A

2Nd

35

4

where: A 1 4kcsinkNd1=2

An expression for the vessel dispersion number Nd asa function of design and operating variables for the

system had been previously derived (Hernandez et al.,

2001) based on the form suggested by Luttrell et al.

(1990), namely:

Nd 1:3 Dc

Hc

Jg

Jl1

eg0:67

5

where Dc and Hc are column diameter and height, re-

spectively. With the estimate of vessel dispersion num-

ber from Eq. (5) substituted in Eq. (4) the rate constant

was calculated.

2.4. Bubble size

In the absence of direct bubble size measurement,

which is only now becoming available for flotation

systems (Hernandez-Aguilar et al., 2002; Grau and

Heiskanan, 2002; Randall et al., 1989), bubble size is

estimated. A method derived from drift flux analysis

using gas rate and gas holdup is employed (Banisi and

Finch, 1994).

3. Results

The rate equations require Req, which, from inspec-

tion of the data, was 87%, a consistent finding in test

work at this Bowater operation (Watson, 1996; Leichtle,

1998; Hardie, 1998). In the lab column where the height/

diameter ratio is 60 plug flow can be assumed (Finchand Dobby, 1990) where the flotation rate constant (kc)

is the slope of the plot ln1 Rc=Req against residencetime (sp). Fig. 3 shows this assumption is supported.

Nevertheless to be consistent the rate constants for alltests, lab and pilot scale, were determined using the axial

dispersion model given by Eq. (4).

The dependence of kc on the various gas dispersion

parameters was explored. Against gas rate (Fig. 4) the

data divide into a set for each column while against gas

holdup a reasonably linear dependence was found (Fig.

5). Against bubble size (Fig. 6) no relationship is ap-

parent (if anything, an inverse dependence is antici-

pated) while the dependence on Sb shows agreement

with a linear relationship (Fig. 7). The slope (i.e., P) is

ca. 104 (dimensionless).

Fig. 3. Test of plug flow assumption for lab column (sparger, 0.5 lm).

Fig. 4. Flotation rate constant as a function of superficial gas velocity.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25 30 35

Gas holdup, Eg (%)

Flotationrateconstant,

kc(1/min)

4" Lab 20" Pilot

Fig. 5. Flotation rate constant as a function of gas holdup.

H. Hernaandez et al. / Minerals Engineering 16 (2003) 739744 741


4. Discussion

To test the dependence of rate constant on the gas

dispersion parameters requires integration of several

measurements. The work of Gorain et al., Heiskanen

(2000) and Heiskanen et al. (2001) illustrate some of the

issues. The system employed here offered the advantages

of operation without a froth (thus collection rate con-

stant is determined directly rather than having to de-

couple from froth effects) and low % solids (thus

approaching the assumption of a first-order process,

which Fig. 3 indicates was met. The disadvantages are

the need to estimate the rate constant from a mixing

model and the need to compute db. Experience in col-

umns suggests the estimate ofdb is within 15% of the

Sauter mean (Yianatos et al., 1988). The particle size

range may be considered wide, ink particles are typically

1implies that bubble size has more impact on the rate

constant than a linear dependence onSb would indicate.

It may be that the variation in bubble size is too limited

(Fig. 6 shows a range of only 0.1 to 0.2 cm) and a linear

dependence on Sb emerges as an approximation. The

range of bubble size available in an operating machine

tends, however, to be limited and thus the approxima-

tion may be practical. Bubble size is controlled by ma-chine design and operating variables, most of which are

fixed except for gas rate and frother dosage. If frother

concentration is constant the dbJg relationship could

be calibrated, then Sb becomes a function of Jg only

(Xu et al., 1991; Gomez and Finch, 2002). Fig. 4 hints at

this (surfactant dosage was not varied in our tests): the

rate constant is reasonably correlated with gas rate for

the two columns individually.

The linear kcSb relationship has excited much at-

tention, forming the basis of one approach to flotation

modeling (Gorian et al., 2000; Harris and Harris, 2002).

Apart from the database being limited in bubble size

range it is also dominated by relatively fine particles

(again the case here). Heiskanen et al. (2001) found

linearity was approached only for the finest particles

(


have been used to remedy instrument and machine

malfunctions and define the cell operating range (Dahlke

et al., 2001). We have used measurement ofSb to help

unravel summer/winter metallurgy swings (communica-

tion in preparation); to evaluate changes in rotor speed

and location in a mechanical machine (as reported by

Kerr et al., 2003); and, in scaling up a de-inking column,to select from among candidate sparger types, finding

that while all operated on the same recoverySb line

some could not deliver sufficient Sb (Finch et al., 1999).

Surveys show Sb in the range 2050 s1 (Deglon et al.,

2000) with a maximum ca. 100120 s1 before the

cell floods (or boils) (Finch et al., 1999; Gorain

et al., 1997; Xu et al., 1991). Being well below the

maximum may imply the operating Sb seeks a compro-

mise among rate of flotation, entrainment and froth

transport. In a bank of cells, a certain profile of Sbmay be optimal (Gomez and Finch, 2002; Dahlke et al.,

2001).

5. Conclusions

De-inking in a flotation column was used to test the

dependence of the pulp zone rate constant on the gas

dispersion parameters. A linear relationship with bubble

surface area flux was found as reported in mineral flo-

tation in mechanical machines. A linear dependence on

gas holdup was also found supporting a previous sug-

gestion that these two gas dispersion parameters can be

interchanged.

Acknowledgements

The authors wish to thank Bowater Gatineaus mill

for providing equipment and test facilities. Special

thanks are also given to the Mechanical and Chemi-

mechanical Wood-Pulps Network (Natural Sciences and

Engineering Council of Canada Network of Excellence)

for their financial support, and the Consejo Nacional de

Ciencia y Tecnologa (CONACYT) of Mexico for a

scholarship to H.H.

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Gas Dispersion and de-Inking in Flotation Column

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