.'(.:£;'
Int. J. Miner. Process.41 (1994) 201-214
~
:'~~~EVIER
;?;t1~'
Roleof pH and dissolved mineral speciesin
PittsburghNo.8 coal flotation systemI. Floatability of coal
D. Liu, P. Somasundaran*, T. V. Vasudevan, C.C. Harris
Henry Krumb School of Mines, Columbia University, New York, NY 10017, USA
(Received 15 May 1992; accepted after revision 21 October 1993)
A~tracl
The rolesof pH and of dissolved mineral specieson the floatability of Pittsburgh No.8
aJaIwasinvestigated. Isolation of the direct effect of pH from that of the interaction of the
dissolvedmineral specieswith OH - was achieved by the appropriate design of the experimentalprocedure.The effect of pH on flotation, determined using a washed coal sample,
wasfound to be significant, especially above pH 8, with the floatability faDing sharply with
pH increase.The effect of interaction of the dissolved mineral specieswith OH - depended
on the path of approach to the flotation pH. Under the conditions of increasing pH, pregpitation/ adsorption of Fe, AI, Ca and Mg speciesoccurred, and the effect of interaction
wasmarkedwith the floa'tability decreasing with pH increase.This was shown to be mainly
dueto precipitation! adsorption on the coal surface of dissolved Fe specieswhich were the
Predominantinorganic speciesin the supernatant of the coal slurry. Under the conditions
of decreasingpH, dissolution of Fe, Ca and Mg species and precipitation of Al species
!JCCUfred
as the pH is lowered, and the effect of the interaction between pH and the dis~ved mineral specieswas marginal.
I. Introduction
~fficient utilization of coals as environmentally acceptableform of fuel requiresdeepcleaningof coal to removal pyrite and other ash-forming minerals.
Frothflotation, which exploits the surfacepropertiesof minerals, is recognized
as.an efficient method for coal cleaning.Deep cleaningof coal by flotation reqUiresenhancingthe floatability of coal relative to that of the gangueminerals.
--r 0 ~om
correspondenceshould be adressed.
OJOI-7SI6/94/S07.00
e 1994ElsevierScienceB.V. All ripts reserved
~Dl0301- 7S16(93 )EOO63-Q
202
D. Liu et aI. / Int. J. Miner. Process.41 (1994) 101-114
The floatability/hydrophobicity of bituminous coal hasbeenshownto bedependent on pH, generallyexhibiting a maximum in the neutral pH region (Zimmermann, 1948;Brown, 1962;Celik and Somasundaran,1986;Ye et al., 1989).Various multivalent cations suchas Fe2+/Fe3+, Al3+ and Ca2+havebeenfoundto
depresscoal flotation in the pH region of metal hydroxide precipitation (Ye et
al., 1989;Bakerand Miller, 1971;Celik and Somasundaran,1980).Also,adsorption of humic substanceson the surfacesof coal and coal-containingmineralswas
reportedto result in a significant decreasein the coal flotation response(Finh
and Nicol, 1981;Lai and Wen, 1989). However, the earlier investigationshad
focussedmainly on the effect of externallyaddedorganicand inorganicreagents.
The effectof multivalent cationsand humic substances
dissolvedfrom coalminerals, although very important, has not been adequatelyunderstood.Also,the
effect of interaction of thesespecieswith H+ /OH- from the direct pH effect
itself hasnot beenisolated.
In this study,the role of pH and of dissolvedmineral specieson the floatability
of coal was investigatedusing a samplefrom Pittsburgh No.8 seam.Dissolved
mineral speciesanalysisand particle zeta potential and solution potential measurementswereconducted,and the infonnation obtainedwasusedto explainthe
mechanismof depressionof coal causedby the interaction betweenpH andthe
dissolvedmineral species.
2. Experimental
2.1. Materials
Coal
Coal usedin this study is a PittsburghNo.8 seamsamplesuppliedby Rand F
CoalCo., Ohio. The proximate,ultimate and sulfur forms analysesof this sample
are given in Table 1. The 2 to 4 inch samplesupplied was crushedto nominal
1/4 inch sizein a laboratoryroll crusherunderargonatmosphereand theproduct
wasstoredunder argon.The flotation feedwas preparedby wet grinding (either
Table I
Analysisof the coalsample dry basis
Ultimate analysis(%)
C
H
N
0
S
Ash
Heatingvalue,
8tu/lb
Proximateanalysisand
sulfur-formsanalysis(%)
87
10
I.- 4S
6. 40
4. S8
12.'60
69.
S.
12,420
Moisture
Volatile matter
Fixed carbon
Ash
SulfaticS
Pyritic S
OrganicS
Total sulfur
2.30
35.55
51.86
12.59
0.28
2.12
1.64
4.64
D. Liu et aI. lInt. J. Miner. Proce.ss.
41 (1994)101-214
203
~~,~distilled wate~or in causticsolution) the 1/4 inch sampleto 95%passing200
i ~:~
in a rod mill.
~';"::'"("Washed
coat" samplewas preparedby washingthe wet ground (in distilled
f;";'"ter) productthree times with distilled water at a solids concentrationof 10%
}Q,CoaI
supernatantusedin the testswas obtained from the wet ground (in dis-
water)productwhichwaspreparedasflotationfeed.The coalslurrywas
-"
";'i'
-,,-
-'
- j and flltered to separatethe solids and then alkali wasaddedto raise
,~;;:~~ pH to the desired value.
~~~~c'
""."",,~en
~~.:
Is
~~:Y;;NaOHand HO were usedas pH modifiers and were of ACS Fisher certified
"'~~::Jrade.
Distilled waterwasusedfor preparingsolutions.
.c'-';' ,
'1.1..Methods
Grinding and flotation
A laboratoryrod mill wasusedto prepareminus 200 meshwet groundfeedfor
Rotation.Rod mill dimensionsand details of various experimentalparameters
Used
aregivenin Table2. The ground product from the rod mill wasdivided into
fourfractionsusinga mechanicalsplitter. Eachof thesefractions containsabout
Table2
Grindingandflotationexperimentalconditions
Grinding
Roomill dimensions
Roodimensions
Nltmber of rods
Feed
Grinding medium
Mill speed
Grinding time
Product
Flotation
flotation machiM
Flotation cell dimensions
Impellerspeed
NUmberof paddles
Paddlespeed
Air Rowrate
Solidconcentration
COnditioningtime
flotation time
-
I
25.4cm lengthX 22.8cm i.d.
22.8cm lengthX 1.9cm i.d., #1
22.8cm lengthX 1.6cm Ld., #2
22.8cm lenatbX 1.3cm i.d., #3
42 (14eacbof#I.#2and#3)
500gramcoat (size: minus 1/4-) +
700cc distilled water
Air
56 rpm (60%critical speed)
18 min.
Coal slurry (95~ passing200 mesh)
DenverDI
12.5cmX 12.5cmX 15cm
1200 rpm
2x2
38 rpm
4Iit./min.
6.25~wt.
12 minutes
5 minutes
204
D. UN naJ. / In/. J. Miner. Prcx:ess.
41 (1994)101-114
125gramsof solids and 375 ml of water.This samplewasstored/agedin air for
about one hour, and thereafterwasusedas flotation feed.
Flotation testswereconductedin a Denver D-I flotation machinewith a two
liter ceOand provisions for mechanicalfroth removal and air flow control.The
agedcoal slurry was diluted with distilled water into 1800mi. The slurryagitatedfor two minutes prior to the addition of the reagents.FollowingthePre.
conditioning, a known amount of frother wasaddedand the slurry wasagitated
for oneminute after which a specifiedamountof collectorwasadded.Conditioning with the reagentswill be continued until the total conditioning time, includ;.
ing the preconditioningtime, of twelve minutesis reached.Flotationwill thenbe
carried out for five minutes. Details of flotation machineparametersandother
pertinent variablesare alsogiven in Table 2.
Particle zeta potential and solution potential
Zeta potential of coal particles was determined by microelectrophoresis tech.
niQue using a Pen Kern 50 I Lazer Zee Meter. Eh of the coal slurry was measured
using a Pt electrode with silver/silver chloride as the reference electrode. For
comparison purpose, the Eh of coal slurry was also measured by employing stan-dard calomel electrode (SCE) as reference. All potentials Quotedare gi yenon the
standard hydrogen (SHE) scale.
Solutionpotential measurements
Electrodepotential (Eh) of the coal slurry wasmeasuredunder flotationconditions Oust after the conditioning stage) usinga platinum electrodewith a silver/silver chloride electrodeas the reference.All potentialsquotedare in referenceto the standardhydrogenelectrode(SHE).
Dissolved mineral speciesanalysis and pH control
A Perkin-Elmer ICP /6500 Inductively Coupled Plasma spectrophotometerw,as
used to measure the concentrations of dissolved multivalent cations, For dissolved organic species,a Dohrmann DC-90 Total Organic Carbon Analyzer was
used. The desired pH was achieved by adding NaOH solution either in fl°.tatioD
cell during conditioning or adding NaOH solution with dry coal sample In the
mill before grinding.
3.Resultsanddiscussion
[t is found, during the preliminary testsof this study, that the coal slurryP.reparedas flotation feedhasan equilibrium pH of about 4, and containsa considerableamountof dissolvedions.Thesedissolvedionscanundergoreactionswhen
pH is subjectedto any change.[n order to identify the direct effect of pH f~m
that of the interaction of pH with the dissolvedions, two different pH cont.rolllng
methods, increasingpH method (NaOH addition in cell) and decreasIng
pH
method (NaOH addition in mill), wereusedin this study.
~;(:
~
D. Liu et aI. Ifnt. J. Miner. Process.
41(1994)101-114
205
.\~;
~i,?;'i;Jj;Effect of pH and the point of sodium hydroxide addition on coal flotation
iyt;;..~~-.;"
Floatability of natural coal
:,,-Theeffectof pH on coal flotation recovery,under conditions of sodium hy..'aroxide
additions in mill and in cell, is shownin Fig. I. The sodium hydroxide
'funsumptions
for eachpH under both the NaOH addition conditions are given
in Table3. When sodium hydroxide is added in the mill. there is a marginal in~e of flotation recovery with pH in the acidic pH range (pH 4 to 7) and
..
90
.
~
~
80
R
1i:"
w
>
0
U
w
«
-C
0
U
70
60
50
40
\
Noturol
cool
0.13 g/kg
30
""
MISC
~
0.9 g/k9
dodecane
6 NaOH In cell - precipitation
0 NoOH in mill - non-precipitation,
20
-A-
4
5
~
"
8
t
10
pH
F1I-I. Comparison of effects of sodium hydroxide addition to the mill and to the cell on coal flotation
~Vety .
206
D. Liu et ai. / Int. J. Miner. Process.41 (1994) 201-214
relatively sharpdecreasebetweenpH 8 and 10.However,it wasdiscovereddur.
ing this study that the flotation behavior changesdrastically when the point of
addition of sodium hydroxide was changedfrom the mill to the cell. Whens0dium hydroxide is addedinto the cell, the flotation recoverydecreases
with pH
increasethroughoutthe entire pH rangetested.Thus, in this casethe coalsurface
is renderedmore hydrophilic with pH increasethan whenalkali wasaddedinto
the mill. This suggeststhat a factor, possiblyrelatedto the presenceof diSSolved
mineral speciesin the flotation systems,could be responsiblefor the differences
betweenthe two reagentaddition schemes:sodium hydroxide addition into the
cell, and into the min.
Floatability of washedcoal
In this case, a washed coal sample was used as flotation feed to eliminate the
effect of dissolved mineral species.The concentrations of Fe, AI, Mg and Ca ions
in the flotation system, determined by ICP analysis, were all less than 10-4 g
atoms/liter in the whole pH range investigated. The effect of pH on the flotation
recovery of washed coal under the conditions of NaOH additions in cell is given
in Fig. 2; the sodium hydroxide consumptions for each pH are given in Table 3.
For comparison purposes, the coal flotation recovery obtained for unwashedcoal
under similar conditions is also presented in Fig. 2. It can be seen that the coal
recovery versus pH curve obtained for washed coal differs significantly from that
for the natural coal obtained under similar conditions (addition of the alkali in
cell). Interestingly, it was found that the recovery curve obtained by adding the
alkali into cell for washed coal is similar to the curve obtained by adding the alkali
~
>Q:
&.I
>
0
U
&.I
Q:
-'
-0(
0
u
pH
Fig. 2. Effect of pH on the floatability of washed and unwashed coals for sodium hydroxide addition
in the ceO.
D. Uu~al.11nt. J. Miner.~.
41(1994)201-214
207
,iinto mill for natural coal (Fig. 1); these effects will be discussed later. It should
"f;~,~:"be
mentionedthat the observed flotation behavior of washed coal agreeswith the
\{{t:\'\"resu1ts(Zimmermann, 1948;Brown, 1962;Celik and Somasundaran,1986) ob-
~~..i;,:tained
earlierfor the pH effecton coal floatability. The decreasein coal flotation
'~~::"rea>verywith pH increase in the alkaline pH range is mainly attributed to the
,i~,: ionization of surface carboxylic groups «Baker and Miller, 1968; Campbell and
~":
:
~:-
Sun,1970;Wenand Sun, 1977;Jessopand Stretton, 1969;Hamieh and Siffert,
1991).Theflotation behaviorof natural coal observedin this study hasnot been
reportedbefore, and hence it deserves to be studied further.
3.2.Interaction betweenpH and dissolved mineral ions
Releaseof mineral speciesas a function of pH and point of NaDH addition
To investigate whether the dissolved mineral species present in the systems
playa role in determining the floatability of natural coal, the relationship betweenpH and dissolved mineral species in the supernatant of the coal slurry was
investigated.It was found that the pH of the natural coal slurry reached an equilibrium at pH about 4, and that a significant amount of dissolved multivalent
metalions were present in the natural coal slurry. Fig. 3 shows that the concentrationsof the predominant dissolved mineral species in the ground slurry prepared
asflotation feed under the two different schemesof alkali addition are function
of pH. It can be seen that the concentrations of dissolved Fe, AI, Mg and Ca
speciesare very much dependent on equilibrium pH, but not much on the point
Noturol pH
.
~
~
.
6
E
0
-0
0
.¥
~o
X
(j
Z
0
U
~
W
U
W
A.
~
3
2
0
w
>
-J
~
~
0
0
4
6
8
10
12
EQUILIBRIUM pH
~ig. 3. Concentration of dissolved mineral speciesas a function of pH for sodium hydroxide addition
In mill and cell.
208
D. Liu et aJ.lInt. J. Miner. Process.
41 (1994)101-114
of addition of sodium hydroxide. Also, the concentrationsof dissolvedFe,ca
and Mg species,which arepredominant in the supernatant,do decrease
with pH
increase.This suggeststhat in the pH rangeof 4 to 1O,if the pH is adjusted~
increasethere will be precipitation of metal ion specieswhereasif the pH is ad~
justed to decreasethere will be dissolution of mineral species.It shouldbementioned that the amount of dissolvedorganicspecies,especiallyhumic substance
which has been reported to be releasedfrom oxidized coals (Firth and Nicol.
1981;Lai and Wen, 1989), is found to be negligibleunder the experimentalconditions usedfor this study.
Point of sodium hydroxide addition on pH-time curve
The changes in pH of the coal slurry, under conditions of sodium hydroxide
addition in the mill and in the cell, during the sequenceof grinding, aging of the
ground slurry, conditioning (with alkali and reagents) and flotation are shown
as a function of time in Fig. 4. This fIgUre shows the pH versus time curve for a
specific case in which the flotation pH was 8, but the trend was the same for all
pH values. It can be seen from this figure that the path of approach towards the
flotation pH strongly depends on the point of sodium hydroxide addition (mill
or cell).
When sodium hydroxide is added in the mill, the pH of the slurry decreases
during grinding and remains constant during aging of the slurry, conditioning
(with reagents) and flotation. By varying the amount of sodium hydroxide added,
different final pH's were achieved. In this case,the slurry is subjectedto pH change
only in the decreasingpH direction. On the other hand, when grinding is carried
out in distilled water the pH of the slurry drops from 6.2 to about 4 which is the
lowest value in the pH range tested in this study (pH 4 to 10). Therefore, when
"i.
TIME. min
Fia. 4. pH vmus ti~
curves for sodium bydro.tide addition in mill and cell.
D. Liu et al. lInt. J. Miner. Process.41 (1994) 201-214
209
f{§;;,
t;.:.~~J,
Sodium hydroxide is added in the cell the slurry will be subjected to a pH change
.It;:;:, in the increasing pH direction which, as explained earlier, can cause precipitation
7~~.:t:to occur, and this can affect mineral surface properties and their flotation behavior.
;J.3.Electrokinetic behavior of coal
c~{
flotation resultsobtained under increasingpH and decreasingpH conditions,
~~~:~:cand
also those with washed coal, suggestedthat reduction in the floatability of
"'c'~ooaI
under precipitation conditions is due to precipitation/adsorption of the metal
: ionsand/or their hydroxides on the coal surface. To investigate this further, zeta
:'potential studies of coal were conducted under different conditions.
Zeta potential behavior of washed coal and, of unwashed coals under both increasingpH and decreasingpH conditions, is shown as a function of pH in Fig. 5.
The isoelectric point is around pH 4.5 for both the natural and the washed coal.
In the pH range
5 tonegative
10, the zeta
potential
of washed
coal under
increasing
pH
. conditions
is theofmost
followed
by that
of unwashed
coal under
decreasingpH conditions. The zeta potential of unwashed coal under increasing pH conditions is the least negative. Zeta potential of the precipitate, obtained by raising
thepH of the coal supernatant from the natural pH value of about 4 to the desired
value, is found to be positive throughout the entire pH range tested (Fig. 5).
Coatingof negative coal surface with the positively charged precipitates and/or
with the metal ions or their relative ion complexes accounts for the less negative
zetapotential obtained with unwashed coals compared to that of the washed coal.
The lessnegative surface of coal under increasing pH conditions is attributed to
the precipitation of Fe, Ca and Mg species.
>
E
J
~
;::
-z
w
00
Go
.c
0w
N
EQUILIBRIUM pH
fiB. S.Zetapotentialof coal asa function of pH measuredundervariousconditions
~
210
D. Liu et aI. / 1nt. J. Miner. Process.41 (1994) 201-214
3.4. Nature of precipitation/adsorption of the dissolved mineral ions
Precipitation in the presenceof and the absenceof solids
Precipitation studies were conducted both in the presenceand in the absence
of solids. In the latter case, the slurry from the rod mill was centrifuged and filtered to separate the solids and then alkali was added to raise the pH to the desired value. Fig. 6 shows the concentration of dissolved speciesas a function of
pH in the presenceand absenceof solids under increasing pH conditions (sodium
hydroxide added in cell). It can be seen that the presence of solids has no detectable effect on the concentration of Fe, Mg and AI species, whereas there is a significant decreasein calcium concentration above pH 6. This suggeststhat, in addition to surface precipitation/adsorption, adsorption of bulk precipitates OCcurs
in the caseof Fe, Mg and AI species, while in the case of Ca speciessurfaceprecipitation is the mechanism by which the solids are coated.
Precipitation/adsorption of Fe species
The results obtained from measurements of the potential of coal slurry under
the precipitation conditions are shown in an Eh-pH equilibrium diagram for FeH2O system (Fig. 7). Since the precipitation of dissolved Fe ions is recognized
as a bulk precipitation, the Eb-pH diagram for Fe-H2O system, establishedbased
on thermodynamic calculations (Stumm and Morgan, 1981), is used to determine the states of dissolved Fe ions as well as their precipitates present in coal
flotation systems. As shown in Fig. 7, the potential of coal slurry decreaseddramatically with pH increase in the pH range of 5 to 8, which is just the precipita=-=
"vi
E
0
'0
0)
..
0
...
)(
<J
z
0
(J
1/1
101
U
101
CL
VI
0
101
>
-'
0
1/1
1/1
is
EQUILIBRIUM pH
Fig. 6. Effect of precipitation on the concentration of dissolved ions as a function of pH obtainc(
from precipitation either in the presenceor the absenceof coal.
.
D. Liu et aI. / Int. J. Miner. Ptoc-e;r.s.
41 (1994) 201-114
211
1.0
0.8
0.1
0..
>
'W 0.2
J
~
0.0
..z
w
to -0.2
Go
-0.6
-0.8
-1.0
pH
Fig. 7. Coal slurry equilibrium Eb-pH shown on the Eb-pH equilibrium diagram for Fe-H2O system.
Pt vs. Ail A,a. (0) NaOH added in cell- in~&
pH; (.) NaOH added in mill- decreasing
pH.
tion pH rangefor Fe2+species.Therefore,this deceasein potential could be explainedasa result of oxidation of Fe2+to Fe3+during the precipitation, which
tookchargesfrom the liquid phase.The reaction involved in Fe speciesprecipitationis givenas foUowing:
Fe2++30H-=Fe(OH)3 +eTherefore,it is concludedthat the dissolvedFespeciesin coal slurry are mainly
in theform of Fe2+and the precipitate from the Fe2+is in the fonn ofFe(OH)3.
It shouldbe mentioned that although the precipitation of Fe speciescould be
characterized
as bulk precipitation, surfaceprecipitation/adsorption of the specieson the surfaceof coal is also expectedto occur under the coal flotation conditions.In addition to surfaceprecipitation, the bulk precipitateofFe(OH)3 can
adsorbon the surfaceof coal, and rendercoal particleshydrophilic.
3.5.Effectsof various dissolved mineral specieson coal flotation
Effectof different mineral specieson the floatability of coal
The relative change in the floatability* of coal as a function of pH (Fig. 8A) is
compared with the extent of overall precipitation as well as those of different
species(Fig. 8B). It can be seen that Fe species is the predominant component
-
-.Relativechangein floatability due to precipitation=floatability of washedcoal (precip. conditloos)- floatabilityof unwashedcoal (precip. conditions).
212
D. Liu et aI. / Int. J. Miner. Procm. 41 (1994) 101-114
EOUILIBRIUW
pH
Fia. 8. Effectof different dissolvedmineral ionson the floatability of coal
of the precipitatein the entire pH rangetested (4 to 10). Also, abovepH 7 there
is no further increasein Fe precipitation (concentrationof Fe abovepH 7 is below the detectablelimit). A comparisonof the Fe concentrationcurve with the
floatability curve suggests
that precipitation of Fe speciesis the major reasonfor
the decreasedhydrophobicity of coal in the pH rangeof 4 to 10,and the effectof
Ca. Mg and AI speciesis minor. It should be noted that the effect of Fe precipitation on the floatability of coal is shifted slightly with respectto pH (Fig. 8).
This is attributed to iron hydroxide precipitation in the bulk fonn which is possibly not completelydepositedon the surfaceof coal immediately, as discussed
before.
Effect of precipitation of dissolved Fe specieson coal flotation
The role of Fe2+ ions precipitation on coal flotation is examined further by
testing the effect of added FeCl2 on the floatability of washed coal under precipitation conditions (at a flotation pH of 8). As shown in Fig. 9, coal floatability
decreased
significantlywith Fe2+addition/precipitation. The addition of 5X 10- 3
M Fe2+ (concentration in the natural coal slurry) resulted in a decreasein coal
recovery from about 82% to 42%. Comparison of this result with those of the
effect of the precipitation of dissolved mineral species (Fig. 2) suggeststhe precipitation of dissolved Fe2+ species to be a major reason for the drastic depres~ion of coal flotation.
I
D. Lill et aI. /lnL J. Miner. Process.
41 (1994)201-214
213
~
>-"
~
w
>
0
u
w
~
-'
.c
0
u
FiJ. 9. Eff«t offerrous chloride addition on flotation recovery ofwasbed coal.
4. Conclusions
By conductingstudieswith unwashedand washedcoals,the direct effect of
pH was isolated from that of the OH - interaction with dissolved mineral
species.
The effect of pH, detennined using a washedcoal sample,was found to be
not verysignificantbetweenpH 4 and 8, increasingonly marginallywith pH
increase.Above pH 8, the floatability decreasedsignificantly.
3. Theeffectof interactionof OH - with the dissolvedmineralspecieswasfound
to bedependenton the path of approachto the flotation pH. Under decreasingpH conditionsthe effectof the interaction, which resultedin precipitation
of AI hydroxidesand dissolution of Fe, Ca and Mg species,causedonly a
marginaldecreasein floatability. Under increasingpH conditions, the effect
of interaction was found to be substantialin the pH regionof 4 to 10 which
correspondedto precipitation/adsorption of Fe, Ca, Mg and Al specieswith
theeffectof Fe speciesbeingpredominant.
4.
Zeta potential studiessuggestedthat metal hydroxide precipitatescan coat
the surfaceof coal and thereby reduce its hydrophobicity. Analysis of dis~Ived speciescarried out in the presenceand absenceof solids showedthat
In the caseof Fe speciesthe mechanismsof coatingwerenot only the surface
precipitation,but alsothe adsorptionof bulk precipitates,whereasin the case
of Ca speciesit wassurfaceprecipitation.
2.
214
D. Liu et al. / Int. J. Miner. Process.
41 (1994)201-214
Acknowledgement
The authorsacknowledgethe United StatesDepartment of Energyfor the fi.
nancial support (DE-AC22-88PC88878)and Prof. D.W. FuerstenauofUniversity of California at Berkeleyfor helpful suggestions.
References
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Congr. J., 54: 43-49.
Baker, A.F. and Miller, K.J., 1971. Hydrolyzed Metal Ions as Pyrite Depressants in Coal Flotation: A
Laooratory Study Report of Investigations 7518. Bureau of Mines, U.S. Department of Interior,
Washington, DC.
Brown, DJ., 1962. Coal flotation. In: D. W. Fuerstenau (Editor), Froth Flotation - 50th Anniversary Volume. AlME, New Yor~ pp. 518-538.
Campbell,. J .A.L and Sun, S.C., 1970. Bituminous coal electrokinetics. Trans. AIME, 247: 111-114.
Celik, M.S. and Somasundaran, P., 1980. Effect of pretreatment on flotation and electrokinetic prop.
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I: 121-124.
Celik, M.S. and Somasundaran, Po.,1986. The effect of multivalent ions on the flotation of coal. Sep.
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Firth, B.A. and Nicol, S.K., 1981. The influence of humic materials on the flotation of coal. Int. J.
Miner. Process.,8: 239-248.
Hamieh, T. and Siffert, B., 1991. Determination of point of zero charge and acid-base superficial coal
groups in water. Colloids Surf., 61: 83-96.
Jessop, R.C. and Stretton, J.L., 1969. Electrokinetic measurements on coal and a criterion for its
hydrophobicity. Fuel, 48: 317-320.
Lai, R.W. and Wen, W., 1989. Effect of humic substanceson the flotation responseof coal. Coal Prep.,
7: 69-83.
Stumm, N. W. and Morgan, JJ., 1981. Aquatic Chemistry. Wiley, New York.
Wen, W. W. and Sun, S.C., 1977. An electrokinetic study of the amine flotation of oxidized coal. Trans.
AIME, 262: 174-180.
¥e, Y., Khandrika, S.M. and Miller, J.D., 1989. Induction-time measurements at a particle bed. Int.
J. Miner. Process.,25: 221-240.
Zimmermann, R.E., 1948. Flotation of bituminous coal. Trans. AlME, 117: 338-356.