Arch. Min. Sci., Vol. 56 (2011), No 3, p. 477–488

Arch. Min. Sci., Vol. 56 (2011), No 3, p. 477–488
477
Electronic version (in color) of this paper is available: http://mining.archives.pl
MAREK LENARTOWICZ*, JERZY SABLIK**
ASSESSMENT OF OPERATION OF PNEUMO-MECHANICAL FLOTATION MACHINES USING FILM
FLOTATION METHOD
OCENA PRACY PNEUMOMECHANICZNYCH MASZYN FLOTACYJNYCH PRZY WYKORZYSTANIU
FRAKCJONOWANEJ FLOTACJI POWIERZCHNIOWEJ
Flotation of coal slurry is carried out on a mass scale in flotation machines. In the industrial practice
the concentrate output and separation selectivity, which is expressed in ash content in a concentrate and
in tailings (Sablik, 1980, 1998) decide about technical efficiency of the flotation process. Also the yield
of flammable substance in the concentrate is the measure of efficiency of washing process (Sablik, 1980,
1998).
It is possible to assess the flotation process and efficiency of the flotation machines by use of film
flotation method (Lenartowicz, 2007a, 2007b; Lenartowicz & Sablik, 2006; Sablik & Lenartowicz, 2007).
Use of the above mentioned method enables determining surface characteristics (surface tension of wetting, energetic nonhomogeneity of surface) of floating particles along entire flotation machine, a content
of hydrophilic particles in products and a content of hydrophobic particles in tailings. The method enables
to analyze flotation process in a selected impeller compartment of pneumo-mechanical flotation machine.
Assessment of applied flotation technology and efficiency of given machine is also possible. Besides, it
is possible to identify the factors, which cause that the machines operate properly or they operate without
satisfactory results (Lenartowicz, 2007a; Sablik & Lenartowicz, 2007).
The procedure in assessment of operation of pneumo-mechanical flotation machines with use of film
flotation method as well as operation of IZ – 12 flotation machine that beneficiates flame-and-gaseous
coal of 32.1 type was presented in the paper.
Keywords: flotation, flotation machine, assessment of operation, film flotation method
Proces flotacji pianowej mułów węglowych w sposób masowy prowadzi się w maszynach flotacyjnych.
W praktyce przemysłowej o technicznej efektywności procesu flotacji decyduje wychód koncentratu oraz
selektywność rozdziału miarą, której są zawartości popiołu w koncentracie i odpadach (Sablik, 1980,
1998). Miarą efektywności procesu wzbogacania, może być również uzysk substancji palnej w koncentracie (Sablik, 1980, 1998).
*
**
KOMAG INSTITUTE OF MINING TECHNOLOGY, DIVISION OF PREPARATION SYSTEMS, PSZCZYŃSKA 37, 44-101
GLIWICE, POLAND; EMAIL: [email protected]
RETIRED EMPLOYEE OF CENTRAL MINING INSTITUTE IN KATOWICE, POLAND
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Do oceny technologii flotacji i sprawności flotowników można wykorzystać badania procesu flotacji
przy pomocy metody frakcjonowanej flotacji powierzchniowej (Lenartowicz, 2007a, 2007b; Lenartowicz
i Sablik, 2006; Sablik i Lenartowicz, 2007). Zastosowanie ww. metody umożliwia określenie właściwości
powierzchniowych (napięcie powierzchniowe zwilżania, niejednorodność energetyczną powierzchni)
ziaren flotujących wzdłuż całej maszyny flotacyjnej, zawartości ziaren hydrofilowych w poszczególnych
produktach oraz zawartości ziaren hydrofobowych w odpadach. Dzięki tej metodzie możliwa jest analiza
przebiegu procesu flotacji w wybranym przedziale wirnikowym pneumomechanicznej maszyny flotacyjnej,
a także możliwa jest ocena zastosowanej technologii flotacji i sprawności technologicznej danej maszyny.
Ponadto możliwe jest określenie czynników, które powodują, że maszyny te pracują właściwie lub nie
uzyskują zadawalających wyników (Lenartowicz, 2007a; Sablik i Lenartowicz, 2007).
W pracy przedstawiono tok postępowania przy ocenie pracy pneumomechnicznych maszyn flotacyjnych za pomocą frakcjonowanej flotacji powierzchniowej oraz przedstawiono ocenę pracy flotownika IZ
– 12 wzbogacającego węgiel gazowo-płomienny typu 32.1.
Słowa kluczowe: flotacja, maszyna flotacyjna, ocena pracy, metoda frakcjonowanej flotacji powierzchniowej
1. Introduction
In the result of advance in coal mining mechanization and transportation much more fine
coal fraction that has to be beneficiated reaches coal processing plants. Hard coal particles of size
below 0.5 mm can be processed by flotation and oil agglomeration methods, in hydrocyclones
and in coil separators. In the Polish coal processing plants froth flotation is most frequently used
to beneficiate above mentioned particle size fraction (Lenartowicz, 2007a; Sablik & Lenartowicz,
2007). In Poland this method is used to process mainly coke coal fines, but also steam coal slurry
is more frequently beneficiated using that method (Sablik, 1998).
Froth flotation is a physical and chemical method consisting in a selective separation of
hydrophobic materials by taking advantage of differences in coal particles and waste rock surface
properties and first of all of differences in their surface wettability. Coal particles of not wettable
surface (hydrophobic) can flotate and waste rock particles of wettable surface (hydrophilic) do
not flotate. Surface properties of coal particles can be determine, among others by measuring of
their boundary wetting angle (Eliaszewicz, 1941; Sablik, 1980; Sablik & Makula, 1984), wetting
heat (Fowkes, 1964), zeta electro-kinetic potential (Fijał et al.,1980; Fuerstenau, 1980; Sobieraj
& Majka-Myrcha, 1980), adsorption energy (Bangham & Razouk, 1937; Wierzchowski, 1993) as
well as by use of film flotation method (Diao & Fuerstenau, 1991; Fuerstenau, 1980; Fuerstenau
et al., 1990, 1991; Fuerstenau & Williams, 1987; Sablik, 2005, 2007; Williams and Fuerstenau,
1987). Film flotation method enables characterizing quantitatively the surface of low-energy
solids (e.g. coal) as well as determining energetic nonhomogeneity of surface in a group of given
solid particles (coal).
Froth flotation of coal slurry is carried out on a mass scale in the flotation machines. A distinction is made among mechanical, pneumo-mechanical, pneumatic and other flotation machines
(Blaschke & Blaschke, 1990; Brzezina, 1972; Folder Instytutu Metali Nieżelaznych, 2007; Sablik, 1998). In the Polish minerals processing plants mainly trough pneumo-mechanical flotation
machines of IZ type are used for washing of hard coal slurry (Folder..., 2007; Sablik, 1998). The
flotation machines have a simple design, good air dispersion and low electric power consumption (Brzezina, 1972). Their principles of operation enables constructing flotation machines of
huge volumes and reaching high efficienties with a possibility to control many operational parameters of the machine in a wide range. However, the machines require supply of compressed
479
air from outside (an installation of additional blowers) (Brzezina, 1972; Brzezina et al., 1974).
Also a proper feed size composition is an important condition of proper operation of the flotation
machines (Brzezina, 1972).
In the industrial practice the concentrate yield and separation selectivity, which is expressed
in ash content in concentrate and in tailings decide about technical efficiency of the flotation
process. Also the yield of flammable substance in the concentrate is the measure of beneficiation
process efficiency (Sablik, 1980, 1998).
In Lenartowicz and Sablik studies (Lenartowicz, 2007b; Lenartowicz & Sablik, 2005, 2006;
Sablik & Lenartowicz, 2007) it was found that knowledge of surface characteristics of flotating
particles in the following compartments of IZ machine’s impeller can be used for more comprehensive assessment of proper selection of flotation technical parameters and technological
efficiency of IZ machine as well as for assessment of washing technology in those machines.
In the paper the procedure in assessment of operation of pneumo-mechanical flotation
machines with use of film flotation method was presented. The exemplary operation of IZ – 12
flotation machine that processes flame-and-gaseous coal of 32.1 type was also given.
2. Film flotation method
Film flotation method has been developed by D.W. Fuerstenau and co-workers (Fuerstenau
& Williams, 1987; Williams & Fuerstenau, 1987) in the result of development of earlier projects
of P.L. Walker (Walker et al., 1952), W.A. Zisman (Zisman, 1964) as well as D.T. Hornsby and
J. Leji (Hornsby & Leja, 1980, 1983, 1984).
The method consists in carrying out separation process on the fluid surface film of coal
particles, which sink or float on the surface supported by forces of the surface film. Use of series
of solutions with decreasing surface tension enables to separate the sample into a few sinking
fractions (lyiophilic ones) and floating fractions (lyiophobic ones).
A typical separation cumulative curve obtained from the fractional film flotation method
was presented in Fig. 1 (Fuerstenau & Williams, 1987).
In the result of analyze of the above curve D. W. Fuerstenau and co-workers (Fuerstenau
et al., 1990, 1991; Fuerstenau & Williams, 1987; Williams & Fuerstenau, 1987) defined four
parameters:
γc min — surface tension of the solution wetting all the particles,
γc max — surface tension of the solution which does not wet any particle,
—
γc — mean critical surface tension of particle wetting,
σγc — standard deviation (as the measure of an energetic nonhomogeneity of particle
surfaces).
For homogenous particles the following relationships has to be satisfied:
γc min = γc max = —
γc
and
σγc = 0
(1)
In all other cases for which equations (1) are not satisfied, we say about particles distribution according to their critical surface tension (surface energy). Particles distribution can be
determined by differentiation of the curve from Fig. 1 which is form mathematical point of view
a distribution function.
480
Portion of lyiophobic fraction, %
100
γc max
80
60
40
20
0
20
γc min
30
40
50
60
70
80
Solution surface tension, mN/m
Fig. 1. Cambria bituminous coal cumulative separation curve obtained by film flotation method
(Fuerstenau & Williams, 1987)
Average critical surface tension of each distribution can be calculated from the frequency
histogram according to equation 1 (Diao & Fuerstenau, 1991; Fuerstenau et al., 1991):
(2)
where:
γc — surface tension of wetting,
f (γc) — density function (histogram) of surface tension of particle wetting.
Standard deviation of density function of surface tension of particle wetting (Fuerstenau et
al., 1988, 1991) images the energetic nonhomogeneity of particle surfaces in a set of particles:
(3)
Small values of σγc refer to the materials of small differentiation in surface tension of wetting
and high values refer to the materials of significant differentiation in surface tension of wetting of
particles in a given set. The film flotation method enables determining distribution of γc, average
—
value of γc and assessment of deviation from the average value σγc. This is an important progress
in relation to the classical methods for assessment of the surface hydrophobility (wetting heat,
immersion time, measurement of wetting angles), which provide only average values (Sablik,
1998, 2000; Sablik & Wierzchowski, 2001; Wierzchowski, 1993; Wierzchowski & Sablik, 1993).
Possibility of taking measurements on particles of natural surface not processed during preparation of sample for measurements by e.g. grinding, polishing etc., is also the method advantage
(Sablik, 1998).
481
The film flotation method enables studying of coal surface properties (Diao & Fuerstenau,
1991; Fuerstenau et al., 1988, 1991; Fuerstenau and Williams, 1987, 1988). It can also be used
for determination of boundary wetting angles (Williams & Fuerstenau, 1987; Sablik, 2003, 2004,
2007), assessment of coal oxidation degree (Fuerstenau et al., 1988), determination of shares of
hydrophilic and hydrophobic centres on coal surface (Fuerstenau et al., 1990) as well as for assessment its flotation activity (Fuerstenau et al., 1988). The method can also be used for studying
changes in coal surface properties after covering its surface with different chemical substances
(Fuerstenau et al., 1991; Sablik & Wierzchowski, 1992, 1994, 1995, 2003; Wierzchowski &
Sablik, 1991). It also enables determining an impact of different agents on boundary wetting
angle of coal surface (Sablik, 2007; Sablik & Wierzchowski, 2004.
3. Methodology for assessment of operation of flotation
machines by use of film flotation method
3.1. Investigation of flotation process in IZ – 12 machine
in an industrial scale
Sampling of IZ flotation machine, operating in real conditions, is an initial stage of assessment of the machine operation. In Fig. 2 the method for sampling of IZ flotation machine IZ
(Lenartowicz, 2007a) was shown. In the result of such sampling of the flotation machine we get
one feed sample, six concentrate samples from the successive impeller chambers of the flotation machine (in the case of eight-impeller flotation machine we get 8 samples) and one sample
of tailings. The samples are then dewatered and dried in ambient temperature. The next step is
screening of the samples with separation of fraction 0.2-0.3 mm, then weighted amounts of the
samples are prepared for carrying out tests by film flotation method (Lenartowicz, 2007a; Sablik
& Lenartowicz, 2007).
The feed sample has to be taken before introduction of the agent to the feed (1) the concentrate samples are taken from both sides of the impeller chambers and they are combined (2 do 7).
The tailings sample is taken from the tailing box overflow (8).
Besides, samples of solid coal of high purity – of low mineral substance content, are taken
as the reference samples (Lenartowicz, 2007a; Sablik & Lenartowicz, 2007).
8
7
6
5
4
3
2
1
Feed
Tallings
Flotation
Reagent
7
6
5
4
3
2
Fig. 2. Method for sampling of IZ flotation machine with sampling points (Lenartowicz, 2007a)
1 – Point of feed sampling, 2 – Point of concentrate I sampling, 3 – Point of concentrate II sampling,
4 – Point of concentrate III sampling, 5 – Point of concentrate IV sampling, 6 – Point of concentrate V
sampling, 7 – Point of concentrate VI sampling, 8 – Point of tailings sampling
482
3.2. Laboratory tests with film flotation method
According to the methodology given in item 2 grain size fraction 0.2-0.3 mm separated from
each product is then separated with the film flotation method, which procedure was presented in
many papers (Diao & Fuerstenau, 1991; Fuerstenau et al., 1991; Sablik, 1998; Sablik & Wierzchowski, 2001; Wierzchowski, 1993). After completion of film flotation the mean critical surface
tension of particle wetting is calculated as well as standard deviation which is the measure of an
energetic nonhomogeneity of surface in a set of particles, according to equations 2 and 3 (Diao
& Fuerstenau, 1991; Fuerstenau et al., 1988, 1991).
3.3. Determination of hydrophilic particles share
in a set of particles
Distribution curves of surface tension of wetting obtained from the results of tests carried
out with film flotation method as well as surface tension of the zero contact angle γc (θ = 0) are
used to determine percentage share of hydrophilic particles in a feed and concentrates and share
of hydrophobic particles in tailings (Sablik, 2003, 2004). The method for determination of hydrophilic particles was described by Sablik and Lenartowicz (Lenartowicz, 2007; Sablik, 2003,
2004; Sablik & Lenartowicz, 2007).
4. Assessment of operation of the flotation machine
Assessment of operation of IZ-12 pneumo-mechanical flotation machine with use of film
flotation method according to the methodology described in item 3, was given on the example
of flotation of 32.1 gas-and-flame coal.
In the case of the machine flotating 32.1 gas-and-flame coal, the results from film flotation
show that first impeller compartments operate with high efficiency. Products obtained in these
compartments give evidence for that. The products have the lowest ash content, the lowest
mean critical surface tension of wetting and the lowest energetic nonhomogeneity (Table 1, Fig.
3 and 4). Test results proved that in the next impeller compartments decrease of efficiency of
flotation process and machine operation is observed (in these compartments). It can result from
the fact that in these impeller compartments a share of particles with higher surface tension of
wetting and with higher energetic nonhomogeneity increases. The concentrates obtained in these
compartments have greater and greater values of —
γc and σγc, and also they contain more hydrophilic
particles, what means higher content of ash (Table 1).
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TABLE 1
Surface tension of wetting of 32.1 gas-and-flame coal as well as ash content and share of hydrophilic particles
in the products of IZ flotation machine (Lenartowicz, 2007a)
Surface tension of wetting
[mJ/m2]
Av.
Max.
Min.
γc max
γc min
γ—c
Type of sample
Solid coal
Feed
Concentrate I
Concentrate II
Concentrate III
Concentrate IV
Concentrate V
Concentrate VI
Tailings
48.14
62.40
42.42
43.78
47.26
48.58
48.32
49.50
65.58
Ash
A a [%]
Share of hydrophilic particles
≥ γc(θ = 0), [%]
7.47
17.37
9.21
11.13
12.03
13.08
13.39
13.68
17.87
8.87
33.87
8.55
10.47
16.02
17.25
18.06
18.22
52.87
about 6.5
about 65
about 6.5
about 9
about 16
about 18
about 20
about 20
about 70
76
86*
76
81
81
81
81
82*
88*
Obtained by extrapolation of curves: 1 (Fig. 3) and 7,8 ( Fig. 4)
100 ī2,9
Content of lyiophobic fraction[%]
*
32
25
25
25
25
25
25
25
25
Energetic nonhomogeneity of particle
surface σγc [mJ/m2]
ī3
ī4
80
2
3
60
1
4
40
ī1
9
20
Ȗc(ș=0)
0
20
30
40
50
60
70
80
90
Surface tension of a solution [mN/m]
Fig. 3. Distribution curves for 32.1 coal surface tension of wetting in the products obtained
in IZ flotation machine (Lenartowicz, 2007a)
1– feed, 2 – concentrate I, 3 – concentrate II, 4 – concentrate III, 9 – solid coal,
(100 – Γn ) – yield of hydrophilic particles
484
Content of lyiophobic fraction[%]
100
6
ī5
80
ī6,7
7
60
5
40
ī8
8
20
Ȗc(ș=0)
0
20
30
40
50
60
70
80
90
Surface tension of a solution [mN/m]
Fig. 4. Distribution curves for 32.1 coal surface tension of wetting in the products obtained
in IZ flotation machine (Lenartowicz, 2007a)
5 – concentrate IV, 6 – concentrate V, 7 – concentrate VI, 8 – tailings,
9 – (100 – Γn ) – yield of hydrophilic particles
The reason is low density of feed (20 kg m-3), which causes difficulties in proper control of
turbidity level and a scraper. In the result hydrophilic particles (of high surface tension) come
up to froth in a mechanical way and are scrapped to the concentrate. Decrease of flotation efficiency in further impeller compartments of IZ flotation machine can also be caused by lower
hydrophobicity of flotating particles. Additionally there is no proper hydrophobizing component
in the flotation agent and there is not enough contact of these particles with the agent (as the
flotation agent was dosed only to the mixing tank before the machine). Besides, in those impeller compartments the ratio of hydrophilic particles to hydrophobic particles changes (in those
compartments there are more hydrophilic particles than in the first impeller compartments). Too
low concentration of solids in the feed, equal 20 kg m-3, caused not sufficient contact of particles
with the flotation agent. To ensure proper concentration of the flotation agent in a suspension as
well as to maintain constant output and ash content in the flotation product, amount of flotation
agent, at Kcs equal to 20 kg m-3, should be five times higher than at concentration 100 kg m-3
(Sablik 1980, 1998). Besides analysis of energetic nonhomogeneity of flotating particles in successive compartments (troughs) of the IZ flotation machine has proved that such nonhomogeneity
is greater and greater. It means that in successive impeller compartments conditions of flotation
deteriorate. Higher energetic nonhomogeneity of particle surface requires proper selection of the
flotation agent composition. From the tests it results that in the first flotation chambers of the IZ
machine we obtain the concentrates with small number of hydrophilic particles, what proves their
high flotation efficiency. Hydrophilic particles go to the concentrates in a result of mechanical
raising up of mineral particles, raising up of accretions of high content of mineral part as well
485
as in the result of creation of mud cover on the particles (Jowett, 1966) or they are scrapped by
the scrappers together with the flotation pulp. In further impeller compartments of the flotation
machine a decrease of operational effectiveness of the machine is observed as share of particles
with hydrophilic surface is greater in those compartments. Mechanical raising up of mineral
particles and scrapping them together with the flotation pulp is the reason.
Analysis of tailings from IZ-12 pneumo-mechanical flotation machine has proved that
flotation technology in that flotation machine is not proper. Too high number of particles with
hydrophobic surface of low ash content (about 30 %) goes to tailings. The theory shows that these
particles should have high flotation activity and should flotate at the initial part of the process.
The following can be a reason of transfer of those particles to tailings:
1. not proper concentration of solids in a suspension,
2. dimensions of particles (in this case coal particles of diameter over 100 μm have lower
flotation activity, so they can go to tailings (Sablik et al., 1991, 1992),
3. creation of mud cover made of tailings particles on useful particles can cause depressing of useful particles and their transfer to tailings. Transfer of those particles to tailings
causes deterioration of flotation effectiveness in the discussed flotation machine.
An assessment of operation of IZ pneumo-mechanical flotation machine using the film flotation method (according to methodology discussed in Item 3) has proved that accepted technology
for processing of coal of 32.1 type is not correct. Results of flotation in the abovementioned flotation machine are not such good as it can be expected. At properly selected flotation parameters
it is possible to obtain much better results of coal beneficiation in IZ flotation machine. The
mentioned method enabled to identify the reasons why the flotation machine is not so effective
causing coal loss as well as economic losses.
Ash content in tailings was the measure to prove that in the tested flotation machine flotation results were not satisfied (Sablik, 1980, 1998). Ash content in tailings during beneficiation
of steam coals (type 32.1) by a flotation method in IZ flotation machine should be greater than
60% (Brzezina et al.,1974; Brzezina & Sablik, 1978; Sablik et al., 1977). In the case of tested
machine ash content in tailings was below 55% what is the prove that flotation machine operates
with not proper effectiveness.
5. Sumary
The film flotation method can be used for assessment of flotation technology and effectiveness of flotation machines (Lenartowicz, 2007a, 2007b; Lenartowicz & Sablik, 2006; Sablik &
Lenartowicz, 2007). The abovementioned method enables to determine surface properties (surface
tension of wetting, energetic nonhomogeneity of particle surface) of flotating particles along
entire flotation machine, hydrophilic particles content in each flotation product as well as content
of hydrophobic particles in tailings. The method also enables to analyze the flotation process in
each impeller compartment of the pneumo-mechanical flotation machine. Also an assessment of
used flotation technology of given flotation machine and its effectiveness is possible. Besides, it is
possible to identify all the factors that cause not proper operation of the machine or that a reason
of not satisfying flotation results (Lenartowicz, 2007a; Sablik & Lenartowicz, 2007).
In the industrial practice, the yield of the concentrate and separation selectivity most often
decides about technical efficiency of the flotation machine. Ash content in the concentrate and
486
in tailings is a measure of machine efficiency (Sablik, 1980, 1998). Also yield of flammable
substances in the concentrate can be a measure of beneficiation process (Sablik, 1980, 1998). In
the case, when ash content in the concentrate is used to assess operation of flotation machine, it
is assumed that ash content should be at the same level as in pure coal. In practice flotation technology is not always aimed at the lowest ash content in the concentrate mainly due to economic
reasons. In flotation of hard coal it is important to obtain tailings with high ash content, so ash
content in tailings is the measure for assessment of operation of IZ flotation machine. Thus the
flotation is aimed at obtaining the tailings with highest possible ash content. But, if we take yield
of flammable substances in the concentrate as the measure to assess the machine’s operation, than
its value should be as high as possible (at assumed ash content in the concentrate).
The above mentioned parameters can be used only to assess technology and technological
effectiveness of the flotation machines. They cannot be used to identify reasons of not proper
operation of flotation machines. That fact confirms better usefulness of the film flotation method
in assessment of operation of pneumo-mechanical flotation machines than the classical methods
for assessment of effectiveness of flotation process.
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Received: 22 November 2010