kinetics and reaction mechanisms of high-temperature flash oxidation of molybdenite

Kinetics and Reaction Mechanisms of High-TemperatureFlash Oxidation of Molybdenite

IGOR WILKOMIRSKY, ALFONSO OTERO, and EDUARDO BALLADARES

The kinetics and reaction mechanism of the flash oxidation of +35/–53 lm molybdenite par-ticles in air, as well as in 25, 50, and 100 pct oxygen higher than 800 K, has been investigatedusing a stagnant gas reactor and a laminar flow reactor coupled to a fast-response, two-wavelength pyrometer. The changes in the morphology and in the chemical composition ofpartially reacted particles were also investigated by scanning electron microscopy (SEM), X-raydiffraction (XRD), differential thermal analysis (DTA), and electron microprobe. High-speedphotography was also used to characterize the particle combustion phenomena. The effects ofoxygen concentration and gas temperature on ignition and peak combustion temperatures werestudied. The experimental results indicate that MoS2 goes through a process of ignition/com-bustion with the formation of gaseous MoO3 and SO2 with no evidence of formation of amolten phase, although the reacting molybdenite particles reach temperatures much higher thantheir melting temperature. This effect may be a result of the combustion of gaseous sulfur frompartial decomposition of molybdenite to Mo2S3 under a high gas temperature and 100 pctoxygen. In some cases, the partial fragmentation and distortion of particles also takes place. Thetransformation can be approximated to the unreacted core model with chemical control andwith activation energy of 104.0 ± 4 kJ/mol at the actual temperature of the reacting particles.The reaction was found to be first order with respect to the oxygen concentration. The rateconstant calculated at the actual temperatures of the reacting particles shows a good agreementwith kinetic data obtained at lower temperatures. The ignition temperature of molybdeniteshows an inverse relationship with the gas temperature and oxygen content, with the lowestignition temperature of 1120 K for 100 pct oxygen. Increasing the oxygen content from 21 to100 pct increases the particle combustion temperature from 1600 K to more than 2600 K. Ahigh oxygen content also resulted in a change of the reaction mechanism from relatively con-stant combustion temperatures in air to much faster transient combustion pulses in pure oxygen.

DOI: 10.1007/s11663-009-9313-4� The Minerals, Metals & Materials Society and ASM International 2009

I. INTRODUCTION

IN the flash smelting process for copper and nickelconcentrates, fine particles of dry sulfides and flux areflash smelted by injecting the gas–solid suspensionthrough a burner located on top of a vertical reactionchamber. The particles are dispersed in the highlyturbulent flow generated, igniting at some distance fromthe burner.

The temperature at which ignition occurs under theprevailing heat, mass, and momentum transfer condi-tions inside the upper section of the reaction chamberdetermines its length. Because almost no reaction takesplace before ignition occurs, it is important to determinethe ignition temperature of the particles to optimize thedimensions of the reaction chamber.

Previous studies on particle ignition temperature ofcopper, nickel, iron, and lead sulfides under flashconditions have been conducted by Jorgensen,[1,2] Oteroet al.,[3] Chaubal and Sohn,[4] Tuffrey et al.,[5,6] andMorgan and Brimacombe.[7] Because molybdenumdisulfide (molybdenite) is not smelted but oxidized(roasted) at lower temperatures, no study has beenreported in the literature of its ignition temperature,although a process based on high-temperature flashoxidation-vaporization of MoO3 has been proposed.[8]

Other more efficient roasting processes than the con-ventional one also have been proposed by Sohn andKim[9] and more recently by McHugh et al.[10]

Molybdenum trioxide, which is the standard productobtained from molybdenite oxidation, has a high vaporpressure with a boiling temperature of 1373 K.[11] Thisproperty, together with the fast oxidation kinetics thatshow molybdenite above 1000 K in pure oxygen,suggest that high-temperature oxidation to producegaseous molybdic oxide (MoO3) rather than the solidoxide could be a better form to process molybdeniteconcentrates. This technological alternative could be areplacement of the traditional roasting process inmultiple hearth furnaces, and it also could allow theprocessing of low-grade concentrates, because most

IGOR WILKOMIRSKY, Professor, and EDUARDOBALLADARES, Assistant Professor, are with the MetallurgicalDepartment, University of Concepcion, Casilla 160-C, Correo 3,Concepcion, Chile. Contact e-mail: [email protected] ALFONSOOTERO, Associate Professor, is with the Mining Center, CatholicUniversity, Casilla 306, Correo 22, Santiago, Chile.

Manuscript submitted October 1, 2007.Article published online November 7, 2009.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 41B, FEBRUARY 2010—63

impurities present in molybdenite concentrates arenonvolatile.

The process developed[8] using these properties indeedshowed that above 1000 K, the oxidation kinetics arefast and a high degree of volatilization of MoO3 can beachieved, obtaining a virtually pure condensed productof molybdic oxide.

II. OBJECTIVES AND SCOPE

To have a better understanding of the kinetics andtransfer phenomena that take place during the high-temperature flash oxidation of molybdenite particles, abasic study was conducted. The study covered thekinetics and mechanism of molybdenite particles, theflash oxidation and volatilization of the molybdenumtrioxide formed, the morphology of partially trans-formed particles, and the measurement of the ignitiontemperature of the particles. Ignition temperaturesof sulfides are normally determined experimentally,because a theoretical value is not possible to obtain orthe calculated value has a large degree of uncertainty.

A stagnant gas reactor equipped for high-speedphotography was used to observe the ignition/combus-tion reaction phenomena and the morphology of par-tially reacted particles, as well as to determine theignition temperature of molybdenite particles. A lami-nar flow reactor allowed the determination of theparticles’ combustion temperature as a function of theparticle size, gas temperature, and oxygen potential.

Although both the stagnant gas and laminar flowreactors are similar in construction and the resultsobtained are equivalent, in the stagnant gas reactor,individual particles ignite and combust under free fallalong the reaction tube, whereas in the laminar flowreactor, they are pneumatically transported by thedownflow concurrent reactant gas.

III. EXPERIMENTAL

A. Molybdenite Particles

The molybdenite used was a high grade concentratewith 55.35 pct Mo, equivalent to 92.3 pct MoS2. Theonly significant impurity was silica. To obtain highgrade molybdenite, the initial concentrate was sulfatedwith pure H2SO4 (98 pct) at 180 �C for 4 hours,followed by a water leach of the pulp and a thoroughwash with hot water, which reduced the copper and ironcontent from 1.5 pct and 0.8 pct to less than 0.001 pctand 0.01 pct, respectively. The purified concentrate wasscreened to obtain a narrow particles size fraction of+35/–53 lm. Its chemical analysis is given in Table I.

The optical microscopy of the purified concentrateshowed that silica particles were separate entities fromthe molybdenite particles. No occluded or partiallyliberated molybdenite attached to silica was found.Although the silica can be removed from the molybde-nite concentrate by hydrofluoric acid leaching, it was leftin the concentrate as an indication of the extent of the

molybdenite oxidation. For the complete oxidation ofthe molybdenite, the residue will be virtually pure silica.The differential thermal analysis (DTA) measurement

on unreacted molybdenite particles showed no evidenceof thermal decomposition. Because pure MoS2 has atheoretical Mo/S mass ratio of 1.496, the 2.86 wt pctexcess of molybdenum in the sample with respect to pureMoS2 could be associated to small quantities of otherslower molybdenum sulfides, although the X-ray diffrac-tion (XRD) analysis showed no evidence of othermolybdenum compounds than MoS2.

B. Experimental Apparatus

Ignition temperature measurements and flash oxida-tion kinetic experiments were made in a 7-cm diametervertical reaction quartz tube or stagnant gas reactorfitted inside of an electrical resistance furnace (Figure 1),which has an axial thermal gradient of ±25 K. Thefurnace was hinged along its axis, which permits a directobservation of the reacting particles. The experimentalapparatus is similar to the equipment described in detailby Tuffrey et al.[5]

Table I. Chemical Analysis of Purified Molybdenite

Concentrate

Element Wt Pct

Mo 55.35S 35.90Cu <0.001Fe <0.01SiO2 8.80Mo/S weight ratio 1.542

Fig. 1—Schematic diagram of the stagnant gas reactor and ancillaryequipment used for the particle ignition temperature and particlesmorphology study. (1) Compressed gases cylinders, (2) Gas filters,(3) Flowmeters, (4) Gas inlet ring, (5) Molybdenite bin, (6) Electro-magnetic vibrator, (7) Particle feed channel, (8) Particles inlet, (9)Quartz reaction tube, (10) Water-cooled lance, (11) Reacting parti-cles, (12) Electric furnace, (13) Particles discharge, (14) Gases exit,(15) Adjustable jack, (16) Control thermocouple, (17) Water thimble,(18) Controller, (19) Gas temperature thermocouple, and (20) High-speed or still-frame photography.

64—VOLUME 41B, FEBRUARY 2010 METALLURGICAL AND MATERIALS TRANSACTIONS B

Particles were fed through an adjustable height water-cooled lance from the top of the reaction tube to avoidpreignition before entering the reaction tube. Thefeeding system permits the addition of particles at alow rate, which allows observation of the combustionduring their reaction period. The oxygen and nitrogengases used were dried and metered separately beforemixing and injected into the reaction tube. A concentricpipe ring around the feed tube permitted the injection ofthe reacting gas into the reaction tube.

Partially reacted particles were quenched and col-lected in a lower water thimble for subsequent chemical,XRD, scanning electron microscopy (SEM), and elec-tron microprobe analysis. High-speed and still photo-graphs were also taken to provide physical evidence ofthe ignition phenomena.

Ignition, particle temperature, and kinetic experi-ments were performed as a function of the followingvariables:

Gas temperature, K: 873 to 1323Oxygen vol pct: 21, 25, 50, and 100Particle size, lm: +35/–53Feed rate, g/h: 2.0 ± 0.2

For the peak combustion temperature measure-ment experiments, the radiant heat generated from thecombustion was continuously monitored by a fastresponse two-wavelength pyrometer, which sampledand recorded simultaneously the signal at a rate of50 kHz as the reacting particles passed in front of thepyrometer lens.

The peak particle temperature measurements wereperformed using a Laminar Flow Reactor and a high-speed two-color pyrometer at the Centre for Metallur-gical Process Engineering of the British ColumbiaUniversity by one of the authors. The pyrometer hasan estimated accuracy of ±50 K to ± 100 K.[5] Both theignition temperature and the kinetic experiments wereperformed at the Department of Metallurgical Engi-neering of the University of Concepcion.

IV. RESULTS

A. Morphology of the Transformation

The overall oxidation reaction of molybdenite tomolybdic oxide can be written as follows:

MoS2ðsÞ þ 3:5O2 !MoO3ðgÞ þ 2SO2 ½1�

Figures 2(a) and (b) show typical unreacted and par-tially reacted molybdenite particles, respectively. It canbe observed that the initially smooth surface of theparticles becomes more irregular by the oxidationphenomena, although the platelet shape that character-izes the hexagonal crystalline structure of molybdeniteremains approximately constant during the oxidationphenomena.

An electron microprobe analysis performed over thesurface of partially reacted particles confirmed thepresence of only molybdenum and sulfur, with noevidence of oxygen that indicated the presence of

molybdic oxide or other molybdenum oxides on thesurface. This finding indicates that the oxidationproceeded with the formation of gaseous MoO3

directly from MoS2 rather than forming a solid orliquid MoO3 product over the unreacted core of solidMoS2.Under both high oxygen content and high tempera-

ture, the fragmentation, exfoliation, and distortion ofsome particles was found, possibly a result of thegeneration of gaseous MoO3 and SO2 between molyb-denite crystalline planes, because it exfoliates easilyalong the sulfur unions of its crystalline structure. Nomelted or partially melted molybdenite particles werefound, although individual tearshaped droplets of silicawere found in several samples.

Fig. 2—SEM photomicrographs of molybdenite particles of +35/�53 lm. (a) unreacted particle (9800), (b) partially reacted particlein air at 1173 K (9600).


B. Kinetic Experiments

Partially reacted particles collected in the waterthimble of the stagnant gas reactor were analyzed fortotal residual elements. Because at a high temperaturethe oxidation reaction generates only gaseous products(MoO3 and SO2), the extent of the reaction wascalculated according to the composition of the residualsolids collected, assuming that all remaining sulfur wasassociated only to molybdenum. This result occursbecause the weight ratio Mo/S in the partially reactedparticles remains approximately the same as in theconcentrate (1.542), with the exception of the last twotests in less than 100 pct oxygen and high gas temper-ature, as it is shown in Table II.

The degree of oxidation of molybdenite to molybdicoxide as a function of the silica, molybdenum, andsulfur content in the partially reacted particles is shownin Figure 3. The best fit between the calculated and thetheoretical conversion was found to be correlated withthe sulfur content, which was used for the kineticscalculations. Experimental results obtained are shown inTable II. These results were used to evaluate the rateconstant, activation energy, and order of reaction withrespect to the oxygen for reaction [1].

The fractional conversion of MoS2 to MoO3 wascalculated considering a constant residence time of theparticles inside the reaction tube of 0.24 seconds for the+35/–53 lm particle size fraction. The reaction timewas measured because the theoretical calculation gives alarge error for such small particles. Nevertheless,because of the platelet shape of the particles and theirtendency to form clusters, the falling time varies between0.15 to 0.35 seconds, with an average of 0.24 seconds.This effect generates a rather large scatter in the

experimental data, which is reflected in the valuesshown in Table II.The calculated rate constant at the reacting gas

temperature, as it usually calculated, shows a fair fit inthe Arrhenius plot as it is shown in Figure 4, with anapparent activation energy of 74.8 ± 2 kJ/mol. The

Table II. Conditions and Results of Kinetics and Ignition Temperature Experiments

Gas Temperature(K)

Oxygen inGas (vol pct)

Sulfur in Residue(wt pct)

Fractional ConversionMolybdenite

ðdMoS2dt Þ � 10�6

(mole/s)Rate Constant

(cm/s)Mo/S Wt Ratio

in Residue

873 21 34.22 0.048 0.677 0.431 1.55973 31.35 0.127 1.830 1.141 1.571073 28.60 0.203 2.920 1.824 1.381123 27.31 0.239 3.460 2.147 1.461173 26.47 0.263 3.798 2.363 1.521223 30.28 0.157 2.263 1.410 1.511273 26.87 0.252 3.637 2.264 1.61873 50 31.82 0.114 1.861 1.024 1.54973 30.68 0.145 2.102 1.303 1.531073 28.27 0.213 3.073 1.913 1.411123 20.12 0.440 6.355 3.953 1.431173 24.13 0.328 4.740 2.946 1.501223 23.80 0.337 4.873 3.027 1.701273 20.05 0.442 6.383 3.911 1.431323 1.41 0.961 13.890 8.633 1.17873 100 29.28 0.184 2.666 1.653 1.41973 31.28 0.129 1.861 1.159 1.561123 28.68 0.201 2.908 1.806 1.591173 18.85 0.475 6.867 4.267 1.561223 17.78 0.505 7.298 4.536 1.331273 6.68 0.814 11.768 7.312 2.111323 7.82 0.782 11.309 7.025 1.82

Average 1.53

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.00 5 10 15 20 25 30 35 40

10 20 30 40 50 60 70 80 90 100

10 504030200

Molybdenum in calcine (wt-%)

Silica in calcine (wt-%)

Sulphur in calcine (wt-%)

Fra

ctio

nalc

onv

erti

onof

MoS

toM

oO3

Theoretical (sulphur)

Theoretical (molybdenum)

Theor

etica

l (Silic

a)

MolybdenumSulphurSilica

Fractional convertionaccording to :

2Fig. 3—Fractional conversion of molybdenite to molybdic oxide cal-culated according to the silica, sulfur, and molybdenum content inthe partially reacted residue.


calculated value for the order of reaction was1.0 ± 0.04 with respect to the oxygen.

C. High-Speed Photography of Reacting Particles

Figures 5(a) and (b) show two high-speed (1/1000 sec-onds) photographs of reacting molybdenite particles in100 pct oxygen. The particles that react under pureoxygen and high gas temperature show a bright spotwith short streaks of light, as is shown in Figure 5(b).The streaks seem to be small fragments of molybdenitegenerated by partial disintegration; this phenomenoncould be attributed to the exfoliation at the edges foundin some partially reacted particles or by the disintegra-tion of clusters of several particles.

For gas temperatures less than 900 K, a diffusecloud was noticeable around the igniting particles(Figure 5(a)), with a slight product fume of condensedMoO3 trailing the reacting particles.

D. Ignition Temperature

The results shown in Table II were used to determinethe ignition temperatures of +35/–53 lm particles in airand at 50 and 100 pct oxygen for gas temperatures from873 K to 1323 K. The results are given in Figure 6. Theabrupt change in the slope of the curves can beassociated with the start of ignition. From about1100 K, the particles start to react rapidly; the extentof the reaction becomes linear with gas temperature.Table III shows the values for the ignition temperaturesobtained from Figure 6.

The ignition temperature shows a significant influenceof the oxygen content. The lowest ignition temperatureof 1100 K was found for 100 pct oxygen, whereas thehighest value corresponds to the lowest oxygen content(air), of 1220 K.

E. Combustion Pulse Experiments

The direct observation of reacting particles and peakcombustion temperatures measurements indicates thatthe combustion of molybdenite particles in pure oxygenwas much more intense than in air in terms of radiationemitted. The intensity of the spark was a function ofthe oxygen content and temperature of the gas, whereasthe span of reaction was inversely proportional to theoxygen content.The pulses recorded in the data acquisition equipment

showed in general only one peak, with a duration ofabout 10 to 25 ms, which represents the span ofmaximum temperature attained by the reacting particle.

10.0 9.0 8.0

7.0

6.0

5.0

4.0

3.0

2.0

1.0 0.9 0.8

0.7

0.6

0.5

0.4 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Oxygen in gas(vol - %)

21 50100

1/T x 10 (K )-3 -1

k,r

ate

cons

t ant

,(cm

/s)

r

Fig. 4—Arrhenius plot for reaction [1].

Fig. 5—High-speed photographs of reacting +35/–53 lm molybde-nite particles in 100 pct oxygen (1/1000 s). (a) 973 K gas temperatureand (b) 1273 K gas temperature.


In some cases, two temperature peaks were observed,which could be produced by separate combustionpulses, as it was also found by Tuffrey et al.[6] and byMorgan and Brimacombe[7] for pyrite and galena. Thetypical measured temperatures from the pulses recordedare shown in Figures 7 through 10 for 25, 50, and100 pct oxygen for 973 K and 1173 K gas temperatures.

In all the experiments performed, the higher the gastemperature, the higher the measured combustion tem-perature for the same oxygen partial pressure. Thiseffect could be related to the significant amount ofthermal radiation emitted by the reacting particle,because for low gas temperature the heat loss from theparticles to the surrounding gas is proportionally higherthan for high gas temperature.

F. Maximum Combustion Temperature

The results of pyrometry measurements are shown inTable IV for two gas temperatures. For 25 and 50 pctoxygen and low gas temperature, only one maximumenergy output was recorded, whereas for 100 pct oxy-gen, more than one maximum energy and temperaturepeaks were measured (Figures 9 and 10).

1.0

0.5

0.015001000500

Gas temperature, K

21

100

50

(vol - %)Oxygen in gas

Fra

cti o

nof

mol

ybde

nite

inth

ere

s idu

e

Fig. 6—Residual fraction of +35/–53 lm molybdenite particlesin the solid residue as a function of the oxygen content and gastemperature.

Table III. Estimated Ignition Temperature of MolybdeniteParticles +35/–53 lm (from Figure 6)

Oxygen (pct) Ignition Temperature (±20 K)

21 122050 1140100 1100

1400

1600

1800

2000

2200

2400

0 2 4 6 8 10 12

time (ms)

Mea

sure

d t

emp

erat

ure

(K

)

Gas temp. 1173 K25% oxygen

Fig. 7—Measured temperature during the combustion of +35/–53 lm molybdenite particles in 25 pct oxygen at 1173 K.

2000

2200

2400

2600

2800

3000

3200

0 5 10 15 20

time (ms)

Mea

sure

d T

emp

erat

ure

(K

)



1400

1600

1800

2000

2200

2400

2600

0 5 10 15

time (ms)

Mea

sure

d t

emp

erat

ure

(K

)




Both the oxygen content and the gas temperatureshow a significant influence on the maximum tempera-tures measured, as it is shown in Figure 11. For 25 pctoxygen, the difference in the maximum temperaturemeasured for the gas temperatures of 923 K and 1173 Kwas 740 K, whereas for 50 pct oxygen it was 1630 K,and for 100 pct oxygen it was 1330 K. The value for50 pct oxygen has a larger uncertainty as a result of thelower than expected temperature measured for 973 Kgas.

The influence of the oxygen content is more signifi-cant than the gas temperature, as is shown in Table IV.For a gas temperature of 973 K, the maximum mea-sured temperature increase was 960 K between 25 and100 pct oxygen content, whereas for a gas temperatureof 1173 K, the temperature increase was 1550 K for thesame gas compositions.

V. DISCUSSION

The ignition/combustion behavior of molybdeniteparticles was found to be different from other sulfidessuch as pyrite, chalcopyrite, chalcosite, and galena,which in most cases fragment and explode to form smallindividual particles surrounded by a diffuse brightcloud (galena) or a molten cenosphere such as for

chalcopyrite, pyrite, and chalcosite, as has been foundby Otero et al.,[3] Tuffrey et al.,[5,6] and Morgan andBrimacombe.[7]

Low-temperature oxidation of molybdenite in air attemperatures of 850 K to 920 K, as it is practiced inconventional multiple-hearth furnace, proceeds approx-imately according to the unreacted core model withchemical control in the initial stage, followed by anincreasing diffusional control once a solid layer of MoO3

grows progressively over the unreacted core.[12] Incontrast, at high temperatures, the flash oxidation ofmolybdenite generates only gaseous MoO3 and SO2.SEM of partially reacted particles shows that they reactmainly from both sides retaining approximately con-stant their platelet shape, which decreases in thicknessuntil disappearance.Because the kinetic experiments were performed with

a fixed residence time of the particles and a narrowparticle size, the experimental data are related only tothe temperature and gas composition but not to thereaction time. To determine the rate-controlling step, itcould be compared with the theoretical reaction timerequired to transform a molybdenite particle under gasfilm mass transfer control and under chemical reactioncontrol at the reacting interface.To estimate the mass transfer coefficient through the

gas film, the Wilkes relationship may be applied[13] for

2500

2750

3000

3250

3500

3750

4000

0 5 10 15 20

time (ms)

Mea

sure

d t

emp

erat

ure

(K

)Gas temp. 1173 K

100% oxygen


Table IV. Maximum Combustion Temperature of Molybdenite Particles of +35/–53 lm

GasTemperature (K)

Numberof Pulses

Oxygen(vol pct)

Pulse Duration(ms)

Maximum MeasuredTemperature (K)

Average MaximumMeasured Temperature (±100 K)

973 8 25 7.0 1610 133019 50 18.2 1410 128514 100 6.5 2570 2086

1173 10 25 9.0 2350 216514 50 7.2 3040 256411 100 5.0 3900 2995

Gas temperature :

973 K1173 K

Molybdenite particles+35 / -53 m

4000

3000

2000

1000

00 20 40 60 80 100

1173K

973 K

Max

.mea

sure

dte

mp e

ratu

re(K

)

Oxygen content in reacting gas (vol-%)

Fig. 11—Maximum temperature measured for the oxidation of+35/–53 lm molybdenite particles as a function of gas temperatureand oxygen content.


vertical flat particles in natural convection (which is themost unfavorable condition in this case):

Sh ¼ 0:673Gr0:25 Sc0:25 ½2�

At 2500 K and for 100 pct oxygen, Gr = 4.1 9 10�5,Sc = 0.20, and Sh = 3.6 9 10�2, with a mass transfercoefficient value of 58.9 cm/s.

The total reaction time under gas film mass transfercontrol could be estimated by the unreacted core modelfor flat particles[14]:

st ¼qs e

o=2ð Þ3:5 hmCO2

½3�

The calculated time for complete conversion underthese conditions is st = 0.007 seconds. In the sameform, for the unreacted core model for flat particlesunder chemical reaction control at the interface, thetotal transformation time can be estimated by theexpression[14]:

sq ¼qs e

o=2ð Þ3:5 krCO2

½4�

For the same temperature and oxygen concentration,and using the experimental rate constant determined forthe actual temperature of the particles of 1.9 cm/s fromTables II and V, the time required for complete con-version is sq = 0.22 seconds; this time is three orders ofmagnitude larger than the time required under masstransfer control. Therefore, the flash oxidation ofmolybdenite should be controlled essentially by thechemical reaction at the gas–solid interface rather thanthe mass transport through the gas film.

For the unreacted core model with chemical reactioncontrol, the theoretical fractional conversion of molyb-denite can be compared with the experimental values, asis shown in Figure 12. Although there is considerablescatter, it seems that this model represent reasonablywell the flash oxidation of molybdenite at a hightemperature.

The calculated apparent activation energy of74.8 ± 2 kJ/mol based on the gas temperature is lowerthan the value of 147.6 kJ/mol obtained in static bed

experiments by Ammann and Loose[15] and the value of104.9 kJ/mol obtained from fluidized bed experi-ments,[12] but it is higher than the value reported byMarin et al. of 22 kJ/mol[16] obtained in static bed.Because the experimental results for the flash oxida-

tion of molybdenite show that the reacting particlesreach a much higher temperature than the gas temper-ature, by using the mean temperature obtained bytaking 25 readings in the time span of 0 to 15 ms fromeach of the pyrometry measurements, Figure 13 was

Table V. Estimated Actual Temperature of the Reacting

Particles

GasTemperature(K)

Actual Temperature of Particles,(±150 K)

Oxygen in Gas (vol pct)

21 50 100

873 960 1150 1500973 1390 1800 21501073 1850 2350 —1123 2100 2700 30501173 2120 2740 31301223 2550 2990 34001273 2640 3300 35201323 — 3380 3650

Oxygen in gas(vol - %)

21 50100

00

X , Fractional convertion (model)

X,F

ract

ion a

lcon

vert

ion

(exp

erim

e nta

l)

1.00.5

0.5

1.0

MoS

MoS2

2

Fig. 12—Experimental versus theoretical fractional conversion ofmolybdenite between 873 K to 1323 K assuming an unreacted coremodel of reaction with chemical control.

25

100 50

(vol - %)Oxygen in gas

5000

4000

3000

2000

1000

900800 900 1000 1100 1200 1300

Max

.te

mpe

ratu

reof

part

icle

s,(K

)

Gas temperature, (K)

Fig. 13—Estimated average maximum temperature attained by thereacting +35/–53 lm molybdenite particles as a function of the gascomposition and temperature.


traced, assuming a parabolic relationship of the averagetemperature attained by the particles for the three gastemperatures tested. From Figure 13, the temperaturesof the reacting particles and other gas molecules wereestimated. The results are given in Table V.

By using the calculated actual temperatures of thereacting particles of Table V, the Arrhenius plot wasretraced as it is shown in Figure 14, where the kineticdata obtained at lower temperatures in fluid bedexperiments has been included. The recalculated valueof the apparent activation energy from Figure 14 gives avalue of 104.0 ± 4 kJ/mol. This value is close to the oneobtained in a fluid bed of 104.9 kJ/mol. It can beobserved in Figure 14 that for temperatures of theparticles higher 1700 K, the correlation between bothdata is reasonably strong, with the exception of airbelow 1000 K. This finding indicates that for theoxidation of molybdenite at low temperatures such asat 823 K to 873 K in air in fluid bed, the displacement ofthe reaction kinetics by thermal effects is negligible,whereas for flash oxidation at high temperature (abovegas temperatures of about 1000 K) and high oxygenconcentration, the thermal effect is significant.

The reaction was found to be of first order withrespect to the oxygen value, which is in agreement withprevious data reported for the range of temperature of823 K to 873 K.[12] According to this result, the flashoxidation of molybdenite particles at high temperaturecan be expressed as follows:

dXMoS2

dt¼ 1

3:5kr _sCO2ð Þ ½5�

Considering that molybdic oxide has a melting point of1068 K and boiling point of 1373 K, no solid or liquidMoO3 should be formed above this temperature.Because the oxidation reaction of molybdenite releasesa large amount of heat (DH�R ¼ �697:9 kJ/mol at2500 K), the particles should rapidly reach a tempera-ture higher than its melting point. Also, becauseparticles are small and their thermal conductivity israther large (Biot number ~0.2), their temperature canbe considered homogenous with negligible internalgradient.The thermal cycle of a reacting particle can be

estimated by assuming that the heat generated byoxidation take place at both sides of a flat particle ofinitial thickness eo. Once it enters into the reaction tube,the increase of its temperature as a result of the externalheat sources (convection+ radiation) and internal heatsource (heat of reaction) as a function of time can beexpressed by the following:

qpCpvpdTp

dt¼�hsp Tg � Tp

� �þ sped T4

g � T4p

� �

þ qpsp �DHoR

� �dedt

½6�

and the decrease of thickness due to the chemical reac-tion can be written as:

de

dt¼ � M

3:5 q

� �hm �kr

hm þ �kr

� �½7�

The simultaneous solution of these two differentialequations with B.C. t ¼ to, Tp ¼ T0, e ¼ eo and t ¼ t,Tp = Tp, e = e, shows that the particles heats up veryfast in all cases, reaching the ignition temperature in lessthan 0.01 second, as shown in Table VI. In addition, itreaches its estimated melting point at 0.02 to 0.03 sec-onds after ignition. The melting point of MoS2 isuncertain, being reported as 1923 K to 1973 K byZelikman and Belayaeskaya,[17] whereas Cannon[18]

measured approximately 2073 K. After being correctedby applying Tammann’s rule, this temperature is mea-sured at 2646 K.These values, however, do not agree with the exper-

imental results, which in the partially reacted samplescollected no evidence of melting or cenosphere forma-tion was found. One possible explanation for this

100

Gas

21

temperature

Rate constant calculated at :Average measured

temperature of particles

Rate constant calculatedfrom fluid bed experimentsat 823 - 873 K in air (10)

50

Oxygen (vol - %) Oxygen (vol - %)

100

21 50

102

10.0

1.0

-110

-210

-310

-410

-5100.2 0.4 0.6 0.8 1.0 1.2 1.4

1/T x 10 (K )-3 -1

k,r

ate

cons

tan t

,(cm

/s)

Fig. 14—Arrhenius plot for the flash oxidation of molybdenite parti-cles assuming that the reaction takes place at the calculated actualtemperature of the particles.

Table VI. Calculated Time of Particles to Reach IgnitionTemperature

Gas Temperature(K)

Oxygen in Gas(vol pct)

Time to Reach IgnitionTemperature (ms)

973 21 9.250 7.6100 6.8

1173 21 6.450 5.6100 5.2


discrepancy may be that at high temperature, thermaldecomposition of MoS2 could takes place. McCabe[19]

and Zelikman and Belayaeskaya[17] found that at1923 K to 1973 K molybdenite melts with simultaneousdecomposition to molybdenum sesquisulfide (Mo2S3)generating gaseous sulfur according to:

2MoS2ðsÞ !Mo2S3ðsÞ þ 0:5S2ðgÞ ½8�

Because the experimental results show that increasingthe oxygen content and the gas temperature generates ahigher combustion temperature, this effect may beattributed to the combustion of gaseous sulfur sur-rounding a decomposing solid core of MoS2.

No data have been published on the kinetics ofthermal decomposition of MoS2 to Mo2S3, but theexperimental results indicate that it may be slower thanthe flash oxidation kinetics of MoS2 to MoO3, becausethe Mo/S wt ratio in the residues remain near constantat 1.53, which is close to the molybdenite ratio of 1.496,whereas the Mo/S ratio in the Mo2S3 is 1.995. Onlyunder 100 pct oxygen and high gas temperature(1273 K and 1323 K) is a significant increase in theMo/S ratio observed in the residue, with 2.11 and 1.82,respectively, as it is shown in Table II. Under thesecircumstances, the pyrometer measurement could beaffected by the cloud of burning gaseous sulfur thatsurrounds the particles, and it could explain the hightemperature measured of 3900 K for 100 pct oxygenand 1173 K gas temperature. In addition, the decom-position of MoS2 to Mo2S3, being endothermic, couldreduce the temperature of the MoS2 particles below itsmelting point.

Other possible mechanisms may involve the hightemperature dissociation of molybdenite with genera-tion of gaseous sulfur and metallic molybdenum.Zelikman and Belayaeskaya[17] calculated the equilib-rium partial pressure of sulfur for the following disso-ciation reaction:

MoS2ðsÞ � MoðsÞ þ S2ðgÞ ½9�

in the temperature range of 1073 K to 1373 K. Byextrapolating these results to a higher temperature(2500 K), the estimated partial pressure of sulfur wouldbe approximately 27 mmHg; therefore, the combustionof gaseous sulfur simultaneously with oxidation ofmolybdenum to molybdic oxide may occur. However,this dissociation–oxidation mechanism could be limited.Because the dissociation heat is large (418.8 kJ/mol at2500 K), it would absorb a significant amount of theoxidation heat of MoS2 to MoO3 (–697.9 kJ/mol at2500 K), decreasing the temperature of the particle andmaking the overall mechanism self-extinguishing unlessexternal heat is supplied. This condition cannot takeplace because in all experiments, the gas temperaturewas lower than the temperature of the reacting particles.

The high temperature measured of the reactingparticles can be compared with the calculated adiabaticcombustion temperature, as is shown in Table VII,where it can be observed that the adiabatic temperatureof the reacting particles can reach values higher than5000 K for 100 pct oxygen.

VI. CONCLUSIONS

The oxidation of molybdenite particles under flashoxidation conditions indicate that at high temperature,the ignition–combustion proceeds with the generation ofgaseous products after approximately the unreacted coremodel, with the particles maintaining their shape anddecreasing in thickness until disappearance, with nomolten phase or cenosfere detected. In some cases,partial fragmentation and distortion of the particles alsooccurs.The corrected apparent activation energy calculated

at the actual reacting temperature of the particles is104.0 ± 4 kJ/mol. The calculated order of reaction withrespect to the oxygen is 1.0 ± 0.04. For oxidation at ahigh temperature, the products formed are gaseousMoO3 and SO2 rather than the solid or liquid molyb-denum trioxide, with the chemical reaction controllingthe transformation.For gas temperatures higher than 1000 K, and for 50

and 100 pct oxygen, the calculated rate constant at theestimated actual temperature attained by the particlesshows a good agreement with previous data obtained at823 K to 873 K in air in a fluidized bed reactor.The ignition pulse measurements show that the high-

est oxidation temperature measured approaches 3000 Kfor +35/–53 lm particles under pure oxygen gas at1173 K; this temperature decreases to 1370 K for 21 pctoxygen. The measured ignition temperature of molyb-denite concentrate shows an inverse relationship with thegas temperature and oxygen content, with the lowestignition temperature of 1120 K for 100 pct oxygen.Although the maximum temperatures of the reacting

particles are much higher than the melting point ofmolybdenite, in the partially reacted particles no evi-dence of melting was found, which may be attributed tothe combustion of gaseous sulfur from partial decom-position of MoS2 to Mo2S3 under high gas temperatureand 100 pct oxygen, which could decrease the temper-ature of the reacting particles and affect the lecture ofthe pyrometer.

ACKNOWLEDGMENTS

This work was supported by the Chilean Scienceand Technology Council (CONICYT) under FondecytGrant Number 1930477. The assistance of the lateDr. J. K. Brimacombe and the pyrometric data analysis

Table VII. Calculated Adiabatic Temperature

of the Reacting Particles

Gas Temperature(K)

Oxygen in Gas(vol pct)

ParticleTemperature (K)

1073 21 273150 3375100 5168

1273 21 292850 3566100 5340


of Dr. G. J. Morgan of U.B.C. is greatly appreciated,as well as the additional experimental data obtainedby E. Patino and H. Sandoval at the University ofConcepcion.

NOMENCLATURE

Bi Biot number�Cp Average heat capacity of molybdenite = 0.18

(cal/g 9 �C)CO2

Oxygen concentration, (mol/cm3)�dp Average particle diameter, (cm)eo Initial thickness of a particle, (cm)e Thickness of a particle, (cm)Gr Grashoff number, (–)�h Overall heat transfer coefficient,

(cal/cm2 9 s 9 K)hm Mass transfer coefficient, (cm/s)k Average thermal conductivity of

molybdenite = 2.4 9 10�3 (cal/cm 9 s 9 �C)kr Rate constant, (cm/s)M Molecular weight of molybdenite, (gr/mol)_s Specific surface area of particles = 2.12 9 105

(cm2/mol)sp Surface area of a particle, (cm2)Sc Schmidt number, (�)Sh Sherwood number, (�)t Reaction time, (s)T0 Initial temperature of a particle = 298, (K)Tp Temperature of a reacting particle, (K)XMoS2

Fractional conversion of molybdenite (–)a Thermal diffusivity of molybdenite = �k=q�Cp=

2.78 9 10�3 (cm2/s)e Emissivity (–)d Stefan–Boltzmann constant = 5.669 9 10�8

(w/m2 9 K)

q Sp. gr. of molybdenite = 4.80 (g/cm3)sq Total reaction time in chemical regime, (s)st Total reaction time in gas film mass transport

regime, (s)DHo

R Heat of reaction (kJ/mol)

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kinetics and reaction mechanisms of high-temperature flash oxidation of molybdenite

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