CHAPTER 10

DISSOLVED MINERAL SPECIES PRECIPITATION DURING COAL FLOTATION

P. SOMASUNDARAN' AND D. LlLJ2'HENRY KRUMB SCHOOL OF MINES, COLUMBIA UNIVERSITY, NEW YORK, NY 10027; 2CERAMICS DIVISION,

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, GAITHERSBURG, MD 20899

ABSTRACT 2.2 Grinding and Flotation

Beneficiation by froth flotation, which exploits the difference insurface properties of minerals, has been a promising method forcoal cleaning. However, dissolved mineral species present in coalflotation systems can interact with particles and other species lead-ing to drastic effects on flotation. Particularly, precipitation or ad-sorption of such species on the particles can alter their surface prop-erties and thus influence the efficiency of coal cleaning. In this work,the bulk and surface precipitation of the dissolved mineral speciespresent in Pittsburgh No. 8 coal was investigated under controlledexperimental conditions. Changes in the surface properties of coaldue to the precipitation were monitored by following zeta potential.Solution potential data ~ used to elucidate the mechanism of theprecipitation. The effect of the precipitation of the dissolved specieson the floatability of coal was found to be marked.

A laboratory rod mill was used to prepare the minus 200 meshwet ground feed for flotation. The ground product from the rod millwas divided into four fractions using a mechanical splitter. Each ofthese fractions contains about 125 grams of solids and 375 ml ofwater. The coal slurry was aged in air for about one hour in a Den-ver D-1 flotation machine and then agitated for two minutes afterdilution to 1800 ml with distilled water. The desired amounts of col-lector was then added and the slurry agitated for one minute afterwhich a specified amount of frother was added. Conditioning withthe reagents was continued until the total conditioning time (indud-ing the preconditioning time) of twelve minutes was reached. Flota-tion was then carried out in a Denver D-1 cell for five minutes.

2.3 Zeta Potential and Solution Potential Measurements

Zeta potential of the coal particles was determined by micro-electrophoresis technique using a Pen Kem 501 Lazer lee Meter.Electrode potential (Eh) of the coal slurry was measured under flo-tation conditions Oust after the conditioning stage) using a platinumelectrode with a reference (silver/silver chloride) electrode. All po-tentials quoted are in reference to the standard hydrogen electrode

(SHE).

1. INTRODUCTION

Utilization of coals as environmentally acceptable form of fuelrequires cleaning of coal to remove sulfur and ash-forming miner-als. Deep cleaning of coal by flotation requires enhancing thefloatability of coal relative to that of the gangue minerals. Variousmultivalent cations such as Fe2+/Fe3+, AI3+ and Ca2+ have been foundto depress coal flotation in the pH region of the precipitation of theirhydroxides'-3. Also. adsorption of humic substances on coal andcoal-oontaining minerals has been reported to result in a significantdecrease in the coal flotation response4.5. However. the earlier in-vestigations had focussed mainly on the precipitation or adsorptionof externally ackIed organic and inorganic reagents. The precipita-tion of multivalent cations and humic substances. although veryimportant. has not been adequately understood. In this work, thetype and the concentration of the dissolved mineral species presentin Pittsburgh No.8 coal slurry were studied along with the effect oftheir precipitation in bulk and the surface. Zeta potential and solu-tion potential measurements were made and the data used to de-termine the mechanisms of the precipitation.

2.4 Precipitation Analysis

Precipitation tests were conducted both in the presence and inthe absence of the solid. NaOH was used to raise the pH to thedesired value. The coal slurry was centrifuged and filtered to sepa-rate the solids. Concentrations of dissolved multivalent cations inthe supernatants were measured by a Perkin-Elmer ICP/6500 In-ductively Coupled Plasma spectrophoto-meter while dissolved or-ganic species by a Dohrmann DC-90 Total Organic Carbon Ana-lyzer.

3. RESULTS AND DISCUSSION

3.1 Release of Mineral Species du.ring Coal Flotation2. EXPERIMENTAL

The coal slurry prepared as flotation feed was found to containa considerable amount of dissolved ions. These dissolved ions canundergo reactions when pH is subjected to any change. The effectof pH on the release of mineral ions was examined using two differ-ent pH controlling methods: increasing pH mode (NaOH addition inthe flotation cell) and dBCr8asing pH mode (NaOH addition in themill). Figure 1 shows the concentrations of the predominant dis-solved mineral species in the ground skJrry under the two differentmodes as a function of pH. It can be

2.1 Coal Samples

Coal used in this study is a Pittsburgh No.8 seam sample sup-plied by R and F Coal Co., Ohio. The flotation feed was preparedby wet grinding (either in distilled water or in caustic solution) thesample to 95% passing 200 mesh in a rod mill. "Washed coal"sample was prepared by washing the wet ground (in distilled water)product three times with distilled water at a solids concentration of1 0 percent by weight.

Fbtation results ~ under the inCIeasing pH arxt decreas.ing pH modes, arxt also with washed 0081, suggest that the reduc-tion W1 the btabIity of 0081 under precipitation oonditions is M to~on/adsorption of the metal ions and/or their hydroxides onthe ooaI surf~. To urKierstaOO this further, zeta potential studesof coal were oonducted ul'Kler seleded conditions. Figure 5. Effed of ~ on the Concenntioo of Dissolved

Ions as a F~ of pH Obtained from Precipitatioo Either WI thePresence or the Ab6ence of Coal.

..: 6 -"-1/1E0-0

~I')

0.-

x(,)z0(,)U)wUwa-U)0w

~0U)U)0

Zeta JX)tentiai behavior of washed and unwashed ooaJs underbott1 the incI8asing pH and the decreasing pH modes, is shown inFigure 4 as a fundion of pH. The isoefedric pok1t is around pH 4.5for bott1 the natural and the washed coal. In the pH range of 5 to 10,the zeta potential of washed coal under the incr8asing pH mode istt1e most negative followed by that of unwashed coal under the de-cr88Sing pH mode. The zeta JX)tentiaJ of unwashed coal under theincreasing pH mode is the least negative. Zeta JX)tential of the pre-cipitate, obtained by raising the pH of the coal supernatant from thenatural pH of about 4 to the desired pH, is found to be positivethroughout the entire pH range tested. Coating of the negative ooalsurface with the positively charged precipitates and/or with themetal ions or their relative ion complexes accounts for the lessnegative zeta potential obtained with unwashed coals compared tothat ~ned for the washed coal. The less negative surface of coalunder the increasing pH oonditions is attri)uted to the pr~of Fe, Ca and Mg species.

5

. .Increasing pH (NaOH In cell)

Coal Coalslurry supernalant .

. 0 r.

. A AI

. C Wg. 0. Ca -.

3

2

Figure 4. Zeta Potential of Coal as a Function of pH MeasuredUnder V~s Condtions.

30 - -

20

10>E 0J~ -10~~ -200L.. -30I-...N -~

-50

-60

-70

., .' '~r . . .0

4 6 8 10

EQUILIBRIUM pH~-8A.o-- -A

-~

~3.6 Effects of the Precipitation of Different Mineral Species on theF~lity of Coal

""" The relative change in the fIoatablity of coal as a function of pHis oompared in Figure 6 with the extent of overall precipitation aswell as those of iOOivDJaJ species. It can be seen that Fe speciesis the predominant component of the precipitate in the entire pHrange tested. Also. above pH 7 there is no further increase In Feprecipitation. A comparison of the Fe concentration curve wItt1 thefIoat;M)ility CUnle irdicates that ~ of Fe species is a ma-jor reason for the deaeased hymophcmicity of the coal. and the &f-fed of Ca. Mg am AI species is possibly minor. It shoukj be notedthat the effect of Fe ~ on the floatability of coal is snft8dslightly wItt1 respect to pH (Figure 6). This .s attributed to the bulk~ of iron hydroxMM ~ is r»sstiy not con1IIet8Iy de-posited on the surface of coal immediately. The role of Fr- on coalflotation is examined further by testing the effect of added F~ onthe ~Iity of washed coal under precipitation conditions (at aflotation pH of 8). As shown in Figure 7. coal floatability decreasedsignificandy with Fr- addiOOrVpr~. The actfition of 5 x 1o.aM Fr. (concentration in the natural coal slurry) resulted In a nota-tion dea'8ase WI coal recovery ~ 8)()Ut 82% to 420/0. ~of this resuh with those of the effect of the precipitation of dissolved

.- .,'-"': 0.03 """/m --~0 Wo- - - pH '""'0 No - - ,IIeNolurol_-o oM,.pHA Praclpllole frwm - ~. . . . . . . . .

3 4 5 e 7 8 , to 11 12

EOUIU8RIUW pH

3.5 Precipitation In the Preserx:e of and the Absence of SoIkis

Precipitation studes were ronducted both in the presence am

in the absence of soIm. In the former case, 8IkaJi was ~ to the

coal slurry to raise the pH k) the desired value. After ~,the coal slurry was centrifuged and filtered to obtain the superna-

tant. In the latter case, the ooaI skJrry groond at the natural pH wasfirst centrifuged and filtered to separate the solids, and then alkali

was added to raise the J*I to the desired vakJe. After rx-edpitaOOn,

DISSOLVED MINERAL SPECIES PRECI

pH range is atlrbJted mainly to the ionization of surface carboxylicgroUps$-13. The flotation behavior of natural coal observed in thisstudy has not been reported before to our knowledge, and ~deserves further study.

3.4 Eledrokinetic Behavk)r of Coal

PITA TlON DURING COAL FLOTATION 69

the solution with the precipitate was centrifuged again for a clearsupernatant for elemental analysis. Figure 5 shows the concentra-tions of dissolved species as a fundk>n of pH in the presence arwJabsence of solids under the increasing pH conditions. It can beseen that the P'8Sera of ~ has 00 ~ eff8d on the (X)r1-centration of Fe, Mg and AI species, whereas there is a sianificant

70 PROCEEDtNGS OF THE XIX IMPC

mineral species (Figure 3) suggests the precipitation of dissolved Figure 8. Coal Slurry Equilibrium EhopH Shown on the Eh-pH Equi-FeZ. species to be a major reason for the drastic depression of coal librium Diagram for Fe-H2O System.flotation.

Figure 6. Effect of Different Dissolved Mineral Ions on theFloatability of Coal.

0~ 8 ---<!:: 7Li:i 6a:L 5III% e .Q Po .

~ Po :5

i 2J0 I~-<

I ' ' ;" '::;' -. r.+A/+Mg+Co0 r.A"

--- ~:::::=: D Mg ~ Co /-e--e---$--E>

---

~

-'-<0u~

~z~u...>

~...~

pH

Eh-pH for coal slurry (Pt. ya. Ag/AgCI)0 NaOH added In c.n - Increasing pH. NoOH added In milt - Decreasing pH5 . t 8 . 10

Therefore, this decease in potential can considered to be the resultof oxidation of FeZ. to Fe'+ during the precipitation. The reaction in-volved in Fe species precipitation is as following:

Fe2+ + 3 OH- = Fe(OH)3 + e-

Therefore, it is proposed that the dissolved Fe species in coalslurry are mainly in the form of Fe2+ and the precipitate of the Fe2+is in the fOffl1 of Fe(OH)3' It should be mentioned that although theprecipitation of Fe species could be characterized as tMJlk precipi-tation, surface precipitation/adsorption of the species on the surfaceof coal is also expected to occur under the coal flotation conditions.In ackjition to surface precipitation, the bulk precipitate of Fe(OH)3can attach to the surface of coal, and render coal particles hydro-

philic.

4. CONCLUSIONS

1 ) The coal flotation feed prepared in distilled water reached anequilibrium pH of 4 and contained a considerable amount of dis-solved ions of Fe. Ca. Mg and AI. The concentrations of dissolvedminefai k>ns were very much dependent on the equilibrium pH. If thepH is adjusted to increase (from the natural pH of 4) precipitation ofFe. Ca and Mg species occurs.

0 2 4 6 8 102+ 3r. CONC. X 10 . 9 atoms/lit.

3.7 Med1anisms ~ it the ~~ c1 Fe species

2) The effed of the precipitation of the dissolved species on thefIoataDlity of coal was examined in detail by conducting studies withunwashed and washed coals and controlling the path of awroachto the flotation pI-!. Upon adjusting the pH to increase in the pi-! re-gion of 4 to 10. a drastic depression of coal floatation was obseIVed.which corresponded to precipitation/adsorption of Fe, Ca. Mg andAI species with the effed of Fe species being predominant.

The results obtained for the potential of coal slurry under theprecipitation conditions are shown in an Eh-pH equilibrium diagramfor Fe- H2O system (Figure 8). Since the precipitation of dissolvedFe k>ns is reoognized as bulk precipitation, the Eh-pH diagram forFe-H2O system, based on thermodynamic calculations1., is used todetermine the state of dissolved Fe ions as well as their precipitatesin coal flotation systems. As shown in Figure 8, the potential of coalslurry decreased dramatically with pH increase in the pH range of5 to 6, which is just the precipitation pH range for F~. species. 3) Zeta potential studies suggested that metal hydroxide preapi-

~

a:uw~G-

o0-W~0

~:Jm~9...

..,

,",0.

C,

-,

~

0

10

20

50

40

50

603

DISSOLVED MINERAL SPECIES PRECI

tates can coat the surface of coal and thereby reduce its hydropho-bicity. Analysis of dissolved species carried out in the presence andabsence of solids showed that in the case of Fe species the mecha-nism of coating was not only surface precipitation, but also possiblyattachment of bulk precipitates. whereas in the case of Ca speciesit is proposed to be entirely surface precipitation. Solution potentialmeasurements suggested the dissolved species to be mainly in theform of Fe2+ and the precipitate to be Fe(0I-I)3. Both surface precipi-tation and bulk precipitation followed by precipitate attachment toooaI particles can result in the obsefVed depression of coal flotation.

8. M.S. Celik and P. Somasundaran, "The Effect of Multiva-lent Ions on the Flotation of Coar, Ssp. Sci. Ted)fJOI., 21(10), (1986)393-402.

ACKNOWLEDGMENTS

Authors acknowledge the support of the Department of Energyand valuable discussions with Professor D.W. Fuerstenau. 9. A.F. Baker and K.J. Miller, -Zeta Potential Control, Its Ap-

plicatk>n in Coal Preparation", Min. ~r. J. 54 (1968) 43-49.REFERENCES

10. J.A.L Campbell and S.C. Sun, "Bituminous Coal Eledro-kinetics., Trans. A/ME, 247, (1970) 111-114.1. Y. Ye, S.M. Khandrika and J.D. Miller, "Induction-Time

Measurements at a Partide Bed", Int J. Miner. Process., 25, (1989)221-240. 11. W. W. Wen aOO S.C. Sun, "An Electrokinetic Study of the

Amine Flotation of Oxkfized Coal", Trans. A/ME, 262, (1977) 174-180.2. A.F. Baker and K.J. Miller, "Hydrolyzed Meta/Ions as Py'

rite Depressants in Coal Flotation: A Laboratory Study" Report ofInvestigations 7518, Bureau of Mines, U. S. Department of Interior,Washington, D. C., (1971).

12. R.C. Jessop and J.l. Stretton, -Electrokinetic Measure-ments on Coal and a Criteria for Its Hydrophobicity-, Fuel, 48,(1969) 317-320.

3. M.S. Celik and P. Somasundaran, "Effect of Pretreatmenton Aotation and Electrokinetic Properties of Coal', Colloids andSurfaces, 1, (1980) 121-124.

13. T. Hamieh and B. Siffert, "Oetennination of Point of ZeroCharge and Acid-Base Superficial Coal Groups in Water", Collomand Surfaces, 61, (1991) 83-96.

4. B.A. Firth and S.K. NkX>I, "The Influence of Humic Materi-als on the Rotation of Coal-, Int. J. Miner. Process., 8, (1981) 239-248.

14. W. Stumm and J.J. Morgan, AQuatic ChemistrY, JohnWiley & Sons, New Yori<, (1981).

PITATION DURING COAL FLOTATION 71

5. R. W. Lai and W. Wen, "Effect of Humic Substances on theRotation Response of Coal", Coal Preparation, 7, (1989) 69-83.

6. R.E. Zimmermann, "Flotation of Bituminous Coal", Trans.AIME, 117, (1948) 338-356.

7. C.J. Brown, Coal Flotation, in Froth Flotation. 50th Anni-V8[H[X Volume (C.W. Fuerstaneu, ed.), AIME, New York, (1962)518-538.