The Coal Handbook: Towards Cleaner Production || Surface chemistry fundamentals in fine coal processing

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  • Woodhead Publishing Limited, 2013

    347

    12 Surface chemistry fundamentals

    in fine coal processing

    J. S. LASKOWSKI, University of British Columbia, Canada

    DOI : 10.1533/9780857097309.2.347

    Abstract : It is argued that the wettability which is fundamental for fl otation also determines the properties of fi ne coal aqueous suspensions and thus controls not only fl otation but also fl otation products dewatering and handling either as dry products or as suspensions (e.g. coal-water slurries). Typical fi ne particle technology problems appear also in gravity separation methods in which fi ne magnetite aqueous suspensions are used as a medium. In this chapter an attempt is made to look at these various unit operations in some unifi ed way based on the fact the main aspects of these unit operations result from the fundamental fact that all these are aqueous suspensions of fi ne particles characterized by the same rheological phenomena.

    Key words : fi ne coal, coal fl otation, settling, fi ltration, fl occulation, oil agglomeration, pelletization, fi ne coal handleability, coal-water slurries, magnetite dense media, rheology.

    12.1 Surface properties of coal

    Coal is an organic sedimentary rock whose composition changes with coal-ifi cation. Since metamorphic development of coal, also referred to as coal-ifi cation, is synonymous in chemical terms with progressive enrichment of the coal substance in organically bound carbon, all coals, regardless of their origin or type, can be arranged in an ascending order of carbon content (Fig. 12.1). As this fi gure shows, coal is a highly cross-linked polymer con-sisting of a number of stable fragments connected by relatively weak cross-links. Coal also contains heteroatoms, such as oxygen (which appears in coal in the form of phenolic, etheric, and carboxylic groups), nitrogen, and sulfur, and their presence in coal structure strongly affects coal surface properties.

    Coal surface properties, like the properties of any other solid, can be stud-ied via wettability measurements. This involves measurement of contact angle ( ) with the use of liquid with known surface tension ( L ).

    The work of adhesion of liquid to solid ( W SL ) is given by

    W W WSLWW SWW LLW SLWW AB( ) +WSWW LLW L (1 [12.1]

  • 348 The coal handbook

    Woodhead Publishing Limited, 2013

    where WSLWW LW and WSLWW AB stand for the Lifshitsvan der Waals contribution to the work of adhesion and the acid-base interactions energy contribution, respectively (please note that in older publications the term WSLWW d , the disper-sion forces contribution, was used instead of WSLWW LW as is common today).

    In order to evaluate the dispersion forces contribution to the wettabil-ity of coals, Gutierrez-Rodriquez et al . (1984) used methylene iodide and showed that the values of the contact angle measured with this compound do not depend on coal rank, or on its oxidation. These contact angle values for various coals were in the range of 28 9 irrespective of the experimen-tal technique (captive-bubble or sessile-drop).

    As shown by Fowkes (1964)

    W WSLWW d SLWW LW d Ld=WSLWW LW 2 sdd Ldd [12.2]

    For water L d 22 mJ/m 2 . Methylene iodide, as saturated hydrocarbons, is a useful reference liquid because its intermolecular attraction is entirely due to London dispersion forces. For methylene iodide L d = L = 50.8 mJ/m 2 , and for methylene iodide wetting coal surface, one can obtain:

    W WSLWW SLWW d d Ld L=WSLWW d 2 d Ld = L ssdd Ldd L ( c+1 os ) [12.3]

    Coal rank%CdafPeat

    Lignite60

    70

    87

    91

    Sub-bituminous

    High-volatile bituminous

    Medium-volatile bituminous

    Low-volatile bituminous

    Semi-anthracite

    Anthracite

    Graphite

    12.1 Variation in coal structure and carbon content with coal rank.

  • Surface chemistry fundamentals in fi ne coal processing 349

    Woodhead Publishing Limited, 2013

    This gives for coal s d 44 mJ/m 2 . For coal interacting with water, if it is assumed that coal is a homogenous

    hydrocarbon matrix that is unoxidized, is mineral matter free, and interacts with water only via dispersion forces:

    W WSLWW SLWW d =WSLWW d ( ) L ( ))1+ [12.4]

    and thus

    cos

    = +1 1+ = 2W d L dSLWW d

    L L [12.5]

    Putting for water L = 72 mJ/m 2 one can derive the contact angle on such a coal surface would have been about 98 . Any smooth coal surface having a water contact angle of less than 98 contains, therefore, various hydrophilic areas (polar functional groups, inorganic impurities, etc.) on the hydropho-bic hydrocarbon matrix (Laskowski, 1994, 2001).

    12.1.1 Effect of coal rank on wettability

    In the 1940s, Brady and Gauger (1940) observed that the contact angle values measured on Pennsylvania bituminous coals were larger than on anthracite, while North Dakota lignites were very hydrophilic. The results of comprehensive wettability studies on coal from the Donbass Basin (Ukraine) were published by Elyashevich (1941), while further details were provided by Horsley and Smith (1951) in the 1950s. The analysis of the wet-tability of coals as a function of coal rank was offered by Klassen in his coal fl otation monograph (Klassen, 1963) in which he used Elyaschevichs data. This relationship is shown in Fig. 12.2 using more recent data of coal analysis for oxygen content of Gutierrez-Rodriquez et al . (1984) and Bloom et al . (1957). As Fig. 12.2 shows, low-rank coals that possess a lot of oxygen are quite hydrophilic, while low-volatile matter bituminous coals are the most hydrophobic of all. Comparison of the contact angle values shown in Fig. 12.2 with the calculated value for pure coal organic matrix (about 98 ) indicates that while the contact angles measured on bituminous coals are not that different from this calculated value, the difference increases with decreasing coal rank. This is an obvious effect of increasing oxygen content in coal with decreasing rank (also shown in Fig. 12.2). The contact angle measured on bituminous coal is smaller than the calculated values for pure coal organic matrix because coal always contains some hydrophilic inor-ganic matter (ash).

  • 350 The coal handbook

    Woodhead Publishing Limited, 2013

    Coal is a very heterogeneous solid. Figure 12.3 is a schematic represen-tation of coal surface. Coal can be depicted as a hydrocarbon matrix that contains various functional groups (Fuersteau et al ., 1982). The composi-tion of the matrix varies with the coalifi cation (Fig. 12.1). Coal also contains mineral matter and is porous. As Fig. 12.1 shows, with increasing coalifi ca-tion degree hydrocarbons building coal become more aromatic. Rosenbaum and Fuerstenau (1984) assumed that coal may be modeled as composite material, the non-wettable portions of which are made up of paraffi ns and aromatic hydrocarbons, and whose wettable portions are represented by functional groups and mineral matter. To calculate the contact angle on such a composite surface they used the Cassie-Baxter equation and assumed that

    65 70 75 80 85

    Carbon (%, daf)

    90

    Oxygen

    Water-captivebubble

    Water-sessiledrops

    95 10000

    10

    20

    30

    40

    50

    60

    70

    80

    5

    10

    Oxy

    gen

    (%)

    Con

    tact

    ang

    le (

    )

    15

    20

    25

    30

    12.2 Relationship between coal rank and wettability by water measured by the captive-bubble and sessile-drop methods (Source: After Gutierrez-Rodriquez et al ., 1984 with permission of Elsevier), and the relationship between coal rank and the total oxygen content (Source: After Bloom et al ., 1957 with permission of Elsevier).

    Coal

    Min

    eral

    mat

    ter

    Coo

    h

    OH

    Por

    es

    12.3 Schematic representation of coal surface.

  • Surface chemistry fundamentals in fi ne coal processing 351

    Woodhead Publishing Limited, 2013

    the maximum values for contact angles on paraffi nic hydrocarbons can be as high as 110 , while those for aromatic hydrocarbons are only 85 . This explains why the wettability of very aromatic anthracites is lower than that of bituminous coals. This concept was further developed in the patchwork assembly model by Keller (1987).

    Such an analysis must also include coal porosity. For example, Horsley and Smith (1951) observed that some petrographic constituents (e.g. fusain), lose good natural fl oatability after prolonged immersion in water. More recent results (He and Laskowski, 1992) entirely prove the effect of porosity. However, while on less hydrophobic surfaces water is sucked into capillaries by capillary forces and this makes such a coal even more hydrophilic, the capillaries on the surface of a hydrophobic coal will stay fi lled with air and this will make such a surface more hydrophobic.

    12.2 Coal flotation

    Coal fl otation is the only fi ne coal cleaning process that is effective in treat-ing 0.15 mm size coal. Because of coal high natural hydrophobicity it may appear to be easy to fl oat, but the wide range of surface properties of coals from various ranks, and various degrees of liberation of the treated particles make the process very often diffi cult. Flotation of low rank/oxidized coals and desulfurizing fl otation are still challenging problems awaiting for solution.

    12.2.1 Effect of rank on fl otability

    In accordance with what has been said, coal fl oatability should strongly depend on rank as has been extensively discussed (Laskowski, 2001). In 1951, Horsely and Smith concluded that in order to obtain equal recoveries a larger quantity of reagents were required for anthracites and lignites than for bituminous coals. In practice, this requires the use of different combina-tions of reagents in fl oating different coals. Xu and Aplan (1993) demon-strated it in a very simple way. Figure 12.4 shows that while MIBC alone is suffi cient to fl oat the very hydrophobic bituminous coals, a combination of MIBC (frother) and an oil (collector) is needed to fl oat lower-rank coals. Aplan noted a semi-logarithmic relationship between the fuel oil consump-tion and the carbon content in coal.

    As has already been pointed out, coal is heterogeneous and it contains organic matter and mineral matter. The former appears in the form of mac-erals, and the latter as minerals. Macerals are classifi ed into three groups: vitrinite, exinite (liptinite) and inertinite. The vitrinite group comprises the most abundant macerals in coal. Macerals do not appear in isolation, but occur in associations in various proportions and with variable amounts of mineral matter to give rise to the characteristic banded or layered character

  • 352 The coal handbook

    Woodhead Publishing Limited, 2013

    of most coals. These associations are referred to as lithotypes and can be distinguished macroscopically. The lithotypes include vitrain (bright bands in coal), clarain (bright, lustrous constituent, which in contrast to vitrain has dull intercalations), durain (dull) and fusain (black or gray in color with fi brous structure similar to that of charcoal).

    Since macerals have different chemical compositions, their surface and fl otation properties also vary. As Fig. 12.5 taken from Klassens monograph (Klassen, 1963) shows, coal particles varying in size and petrographic com-position behave differently in the process. Fine bright particles dominate in the fi rst products and only with time coarse particles and dull constituents start fl oating. Large particles, including particles that are not liberated, fl oat only when the fi ne particles are removed from the cell. These data correlate very well with Horsley and Smiths observations (Horsley and Smith, 1951), which indicate that bright petrographic components (vitrain) are more hydrophobic and fl oat better than dull components (durain).

    Arnold and Aplan (1989) claim that hydrophobicity of coal macerals fol-lows the pattern:

    exinite>vitrinite>inertinite.

    70 80 90 100

    Carbon (%)

    0.01

    0.1

    1

    10

    100

    Min

    imum

    am

    ount

    of f

    roth

    er p

    lus

    colle

    ctor

    for

    optim

    umre

    cove

    ry (

    kg/t)

    MIBC only

    MIBC and oil

    MIBC and naphthenic acid

    12.4 Minimum amount of frother and collector for optimum recovery of coals of various carbon contents. (Source: After Aplan, 1993.)

  • Surface chemistry fundamentals in fi ne coal processing 353

    Woodhead Publishing Limited, 2013

    The conclusions regarding the behavior of coal macerals in fl otation are further complicated by mineral matter content. The effect of petrographic composition of coal particles on their fl otation properties can be studied only for fresh (unoxidized) and low ash samples (Holuszko and Laskowski, 1995). For samples containing more than 15% ash, the surface properties are predominantly determined by mineral matter.

    12.2.2 Flotation reagents

    The behavior of coal in the fl otation process is determined not only by a coals natural fl oatability (hydrophobicity), but also by the acquired fl oat-ability resulting from the use of fl otation reagents. The general classifi cation of the reagents for coal fl otation is shown in Table 12.1 (Laskowski, 2001).

    The use of liquid hydrocarbons (oils) as collectors in fl otation of coal is characteristic for the group of inherently hydrophobic minerals (graphite, sulfur, molybdenite, talc, coals are classifi ed in this group). Since oily collec-tors are water-insoluble, they must be dispersed in water to form an emul-sion. The feature making emulsion fl otation different from conventional fl otation is the presence of a collector in the form of oil droplets, which must

    2 4 6 8 10 12 14 16 18 20

    Flotation time (min)

    0

    20

    40

    60

    80

    100

    Yie

    ld (

    %)

    4

    2

    1

    3

    12.5 Effect of petrographic composition and particle size on coal fl otation kinetics. (1) bright coal; (2) dull coal; (3) shale interlocked with dull constituents; (4) gangue. (Source: After Klassen, 1963.)

  • Woodhead Publishing Limited, 2013

    Tab

    le 1

    2.1

    Co

    al fl

    ota

    tio

    n r

    eag

    ents

    Typ

    e Fl

    ota

    tio

    n u

    se a

    s Fu

    nct

    ion

    al g

    rou

    p

    Exa

    mp

    les

    Act

    ion

    No

    np

    ola

    r (W

    ater

    -in

    solu

    ble

    ) C

    olle

    cto

    rs

    _ K

    ero

    sen

    e Fu

    el o

    il S

    elec

    tive

    wet

    tin

    g a

    nd

    ad

    hes

    ion

    o

    f o

    il d

    rop

    s to

    co

    al p

    arti

    cles

    S

    urf

    ace

    acti

    ve

    (Wat

    er s

    olu

    ble

    ) Fr

    oth

    ers

    Hyd

    roxy

    l N

    itro

    gen

    ou

    s A

    liph

    atic

    alc

    oh

    ols

    Po

    lyg

    lyco

    ls

    Fro

    ther

    s w

    ith

    so

    me

    colle

    ctin

    g

    abili

    ties

    . Als

    o im

    pro

    ve

    emu

    lsifi

    cati

    on

    of

    oily

    co

    llect

    ors

    E

    mu

    lsifi

    ers

    (So

    lub

    le in

    oily

    co

    llect

    or)

    Pro

    mo

    ters

    H

    ydro

    xyl

    Car

    box

    yl

    Nit

    rog

    eno

    us

    Poly

    eth

    oxyl

    ated

    al

    coh

    ols

    , fat

    ty a

    cid

    s,

    etc.

    Faci

    litat

    e co

    llect

    or

    emu

    lsifi

    cati

    on

    an

    d

    spre

    adin

    g o

    ver

    coal

    In

    org

    anic

    (W

    ater

    so

    lub

    le s

    alts

    ) M

    od

    ifi er

    s _

    NaC

    l, N

    a 2 S

    O 4

    H 2 S

    O 4 ,

    Ca(

    OH

    ) 2

    Ca(

    OH

    ) 2

    Pro

    mo

    ters

    p

    H r

    egu

    lato

    rs

    Su

    lfi d

    e d

    epre

    ssan

    ts

    Pro

    tect

    ive

    Co

    lloid

    s D

    epre

    ssan

    ts

    Hyd

    roxy

    l C

    arb

    oxyl

    Po

    lym

    ers:

    sta

    rch

    , d

    extr

    in,

    carb

    oxym

    ethy

    l...

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