Kinetics of the reaction between Pb-Sb complex sulphide concentrates ... · Kinetics of the reaction between Pb-Sb complex sulphide concentrates and water vapour by F. Zhu*, Y. Hua†,

The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 106 REFEREED PAPER OCTOBER 2006

Introduction

Pb-Sb complex sulphide concentrates (majorphase is jamesonite (FePb4Sb6S14) and PbS,ZnS, SiO2 and FeS are minor phases), mainlydistributed in the Guangxi Zhuangantonomous region, are important resources oflead and antimony in China. Presently they aretreated by pyrometallurgical process1–5, asshown in Figure 1.

Although it has some advantages, itsmajor disadvantages are inferior Pb-Sbseparation, low direct recoveries of Pb (68%)and Sb (40%), and meaningless recycling ofPb and Sb in the smelting process. Theexisting pyrometallurgical flowsheet usuallyproduces low-quality antimony or even lead-

antimony alloy. The Pb-Sb separation is a keyproblem in treating this kind of Pb-Sb complexsulphide concentrates. Moreover, the problemwith this process is that the sulphur dioxidegenerated during fluid bed roasting can not beefficiently recovered due to its low concen-tration. In the present investigation, the lead-antimony separation technology for the Pb-Sbcomplex sulphide concentrates by oxidativevolatilization roasting with water vapour isproved to be highly effective6. The directrecoveries of Pb and Sb are >94% and >93%,respectively. The emission of sulphur-bearinggases during the volatilization roasting processof the Pb-Sb complex sulphide concentrateswill be considerably reduced. However, thekinetics and mechanism of oxidation of thePb-Sb complex sulphide concentrates withwater vapour have not been studied. It isworthwhile to study the kinetics andmechanism of oxidation of the Pb-Sb complexsulphide concentrates with water vapour.

Experimental

The schematic diagram of experimentalapparatus is illustrated in Figure 2. The Pb-Sbcomplex sulphide concentrates are obtainedfrom the Guangxi Huaxi Group CoporationLtd.(Guangxi, China). The chemicalcomposition of Pb-Sb complex sulphideconcentrates is given in Table I, whichindicates that the primary components of theconcentrates are Pb, Sb, S, Fe, Zn and SiO2_app. 85%_. It is indicated that the majorphase observed in the concentratesisjamesonite (Pb4Sb6FeS14), and that PbS,

Kinetics of the reaction between Pb-Sbcomplex sulphide concentrates andwater vapourby F. Zhu*, Y. Hua†, and Y. Meng†

Synopsis

The kinetics on the separation of antimony from lead by oxidativevolatilization roasting of Pb-Sb complex sulphide concentrates withwater vapour is investigated in this paper. The experimental resultsshow that the reaction rate of the separation process is controlledby chemical reaction and internal diffusion simultaneously whenthe temperature <1073 K and by internal diffusion when thetemperature >1073 K. The apparent activation energies werecalculated to be 85.92 kJ/mol for the chemical reaction and 10.27kJ/mol for the internal diffusion, respectively. X-ray diffraction(XRD) analysis of the products demonstrates that the reaction ofthe Pb-Sb complex sulphide concentrates with water vapour canpromote the Pb-Sb separation, because steam can decompose thePb-Sb complex sulphide concentrates and the complex intermediateproducts into simple sulphides of lead, antimony and iron. Incontrast, in nitrogen atmosphere, Pb-Sb complex sulphide concen-trates can be decomposed into single sulphides to a limited extent.The remaining Pb-Sb complex sulphide concentrates hindersvolatilization of Sb2S3. In air atmosphere, the Pb-Sb complexsulphide concentrates is transformed into stable Pb4Sb3FeO13,making volatilization of antimony more difficult. The condensedproduct consists of Sb2O3 with a small quantity of PbS and S byXRD analysis. This result indicates that only in steam atmosphereSb2S3 can be selectively oxidized to Sb2O3 while PbS cannot beoxidized. As a result, the Pb-Sb separation will be promoted.

Keywords: Kinetics, reaction, Pb-Sb complex sulphide concen-trates, water vapour

* College of Materials Science and Technology,Lanzhou University of Technology, Lanzhou,China.

† Kunming University of Science and Technology,China.

© The Southern African Institute of Mining andMetallurgy, 2006. SA ISSN 0038–223X/3.00 +0.00. Paper received Apr. 2006; revised paperreceived Aug. 2006.

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Kinetics of the reaction between Pb-Sb complex sulphide concentrates and water

ZnS, SiO2 and FeS as minor phases are also observed by XRDanalysis of the concentrate, as shown in Figure 3.

Experiments were carried out in a typical tubular furnace,where the corundum crucible containing the sample (10 g)was placed in the centre of a tube reactor (6.8 cm i.d. and100 cm long). The even temperature zone is 10 cm long. Inexperiments the apparatus was sealed, flushed with highpurity argon (>99.99%) for 30 min. to eliminate the oxygenin the reaction system and then heated in an argonatmosphere. Temperature was measured by a Pt-Rd thermo-

couple (error ±1°C). After reaching the desired temperatures,the crucible was quickly put into the even temperature zonefrom the left of the furnace and water vapour was introducedinto the furnace tube from the right. The gaseous reactionproducts were carried away from the hot zone by watervapour and then condensed on the wall of the furnace tube.After completion of the reaction, the reactor was pulled outfrom the hot zone to the cold zone and cooled under the flowof high purity nitrogen (>99.99%). The weight of the roastthus obtained was determined by electronic balance.

The flow rate of water vapour was controlled by a steamrotor flow meter. The sulphur-bearing gas produced duringreaction process was determined by standard iodine solution.The volatile species and roast were collected for XRDanalysis.

Results and discussion

Theory of kinetics

The reaction between the Pb-Sb complex sulphide concen-trates and water vapour is represented by the following:

[1]

According to the law of additive reaction times7–14, when theprocess was controlled by chemical reaction, internaldiffusion and the external diffusion simultaneously, thekinetics for the gas-solid reaction can be expressed as:

[2]tn

bkC M

F V

Ag X

n

bD C

F V

Ap X

n

bk C M

V

AX

B

H O b b

P P

P

B

e H O bM

P P

P

B

g H O b B

P

P

B

=

( ) +

( ) +

ƒ

ƒ

ƒ

,

,

,

2

2

2

6

2

FePb Sb S s H O g Sb O g

PbS s FeS s H S g

4 6 14 2 2 3

2

9 3

4 9

( ) + ( ) = ( ) +

( ) + ( ) + ( )

▲

714 OCTOBER 2006 VOLUME 106 REFEREED PAPER The Journal of The Southern African Institute of Mining and Metallurgy

Figure 1—Pyrometallurgical flow chart for Pb-Sb complex sulphideconcentrates

Figure2—Schematic diagram of experimental apparatus

Table I

Chemical analysis of Pb-Sb complex sulphide concentrates

Constituent Pb Sb S Fe Zn Cu As CaO SiO2 Al2O3

Wt Pct 23.73 19.70 23.21 11.00 5.85 0.30 1.20 1.30 2.50 1.00

[3]

[4]

[5]

[6]

where X is the fractional conversion of the solid reactant. Xcan be expressed as follows:

[7]

Where Sb% is the Sb content of the roast (%); Sb0% is the Sbcontent of the Pb-Sb complex sulphide ores (%); W is theweight of the roast (g); W0 is the weight of the Pb-Sbcomplex sulphide ores (g); t is time(min); σ̂ is the generalizedgas-solid reaction modulus; Sh* is the modified Sherwoodnumber; AP is the external surface area of the individualpellet(cm2); FP is the shape factor for the pellet; VP is thevolume of the individual pellet (cm3); De is the effectivediffusion coefficient of gaseous H2O through the productlayer (cm2.min-1); kg is the gas phase mass transfercoefficient (cm.min-1); b is the stoichoimetric coefficient; k isthe chemical reaction rate constant (cm.min-1); CH2O,b is theconcentration of H2O in the gas phase (mol.cm-3); MB the ismolecular weight of the solid reactant; _B is the density of the solid reactant (g.cm-3); R0 is the initial radius of thegrain (cm).

Let[8]

[9]

[10]

where k1 (min-1), k2 (min-1) and k3 (min-1) are apparent rateconstants for the chemical reaction, internal diffusion andexternal diffusion, respectively. Substituting Equations [8],[9] and [10] into Equation [2], one has

[11]

A multi-variable least-squares linear regression methodwas used to find the values of the time constants of the threereaction steps, using the package ORIGIN 7.5, and fittingEquation [11] (as given above) to the data.

For the fitting procedure, the independent variables (X1to X3) and model parameters (_1 to _3) were as follows

[12]

Effect of steam flow rate on the overall reaction rateThe relationship between steam flow rate and overall reaction

β

β

β

1 1 11 3

2 2 22 3

3 3 3

1 1

1 3 1 2 1

= = − −( )[ ]= = − −( ) + −( )[ ]= =

f X X

f X X X

f X X

, ,

, ,

,

/

/

t f X

f X X f X

= − −( )[ ] +

− −( ) + −( )[ ] +

11 3

22 3

3

1 1

1 3 1 2 1

/

/

fk

n

bk C M

V

AB

g H O b B

P

P3

3

1

2

= = ( )ƒ

min,

fk

n

bD C M

F V

AB

e H O b B

P P

P2

2

21

62

= =

( )

ƒmin

,

fk

n

bkC M

F V

AB

H O b B

P P

P1

1

1

2

= =

( )

ƒmin

,

XSb W

Sb W= −1

0 0

%%

p X X X( ) = − −( ) + −( )[ ]1 3 1 2 12 3

g X X( ) = − −( )1 1 1 3

)o

k

D

V

Ae

P

P

2

2=

Shk

D

F V

Ag

e

P P

P

* =

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Figure 3—XRD analysis of the Pb-Sb complex sulphide concentrates

Figure 4—Effect of QH2O on X, for t= 30 minutes and 60 minutes, at

1023 K

rate for 30 minutes and 60 minutes at 1023 K is shown inFigure 4. From Figure 4, the volatilization rate of antimony iskept fixed when the steam flow rate is above 2500 ml/min. Itis demonstrated that the external diffusion resistance (f3) ofH2O in the gas phase is very small.

The effect of steam flow rate in the range 567 to 3500ml/min on the overall reaction rate at 1023 K is shown inFigure 5. It is clear that the reaction rate is enhanced withincreasing steam flow rate. Substitution of the data obtainedfrom Figure 5 into Equation [11] and making a regressionanalysis, the values of f1, f2 and f3 can be obtained atdifferent steam flow rates, as shown in Table II.

From Table II, it can be seen that the external diffusionresistance (f3) of H2O in gas phase is much smaller than theinternal diffusion resistance (f2) and chemical reactionresistance (f1) when the flow rate is above 2500 ml/min.Therefore, Equation [11] may be simplified as

[13]

Effect of temperature on the overall reaction rate

In order to eliminate the effect of external diffusion on theoverall reaction rate, the flow rate of steam was controlled at

2500 ml/min. The effect of temperature on the overallreaction rate is shown in Figure 6. It is demonstrated that theconversion (X) appreciably increases with raising reactiontemperature. Substituting the data obtained from Figure 6into Equation [13] and making a regression analysis, thevalues of f1 and f2 can be calculated at different temper-atures, as shown in Table III. It is obvious that the chemicalreaction resistance was decreased with the increase oftemperature. At a lower temperature (T <1073 K), theprocess is controlled by chemical reaction and internaldiffusion simultaneously9–12, due to the values of σσ̂2 beingin the range of 0.1~10. Both the chemical reaction rate andinternal diffusion rate can be accelerated by increasing thereaction temperature. When T >1073 K the values of σ̂2 isgreater than 10, the chemical reaction resistance is smallerthan that of internal diffusion. Under this condition, theprocess is mainly controlled by internal diffusion.

The values of f1 and f2 obtained previously were found tobe in agreement with Equation [13] when 1–(1–X)1/3+f2/f1[1–3(1–X)2/3+2(1–X)] was plotted against t at differenttemperatures(Figure 7). It is clear from Figure 7 that straightlines are observed when temperature ≤ 1023 K. Thisindicates that the volatilization rate of Sb2S3 is in good

t f X f

X X

= − −( )[ ] +

− −( ) + −( )[ ]1

1 32

2 3

1 1

1 3 1 2 1

/

/


▲


Figure 5—Effect of X on t for different steam flow rate at 1023 K

Table II

Values of f1, f2 and f3 at different steam flow rates

Flow rate f1 f2 f3 correlation(ml/min) coefficient

567 15.16 171.83 156.65 0.9968869 15.20 169.45 99.31 0.99821408 15.23 166.58 56.25 0.99281723 15.22 162.70 38.37 0.99612000 15.19 159.82 28.77 0.98862500 15.19 156.37 4.47 0.98233500 15.21 151.96 2.40 0.9742

Figure 6—Effect of temperature on the overall reaction rate forQH2O=2500 ml/min

Table III

Values of f1 and f2 at different temperatures

Temp. (K) f1 f2 σ̂ 2 (f2/f1) Correlation coefficient

773 592.12 232.70 0.39 0.9977873 227.31 201.71 0.89 0.9971973 48.05 187.36 3.90 0.98981023 22.53 170.50 7.57 0.99921073 13.28 143.65 10.82 0.99401173 9.17 137.13 14.95 0.9773

agreement with Equation [13]. When the temperature > 1023K and the fractional conversion X < 0.95, approximate linearrelationships are obtained. However, when X > 0.95, theexperimental results evidently deviate from the linearrelationships. It may be explained by the existence of a smallquantity of Pb-Sb complex compounds. Comparing thevolatilization of antimony from the residual Pb-Sb complexcompounds and single compounds, it is evident that thevolatilizations of antimony from the Pb-Sb complexcompounds become more difficult, causing deviations fromexperimental results. According to the values of f2 and f1, thevalues of k1 and k2 can also be calculated. Plotting ln k1 or lnk2 against 1/T (as shown in Figure 8) gives the temperaturedependence of k1 or k2 as follows:

[14]

[15]

The apparent activation energies were calculated to be85.92 kJ/mol for the chemical reaction and 10.27 kJ/mol forthe internal diffusion, respectively.

Effect of atmosphere on the overall reaction rate

The effect of atmosphere on the overall reaction rate isshown in Figure 9 for t = 150 min at 1023 K. It is seen thatthe conversion(X) in the steam atmosphere is much greaterthan those in the air or nitrogen atmosphere.

In steam atmosphere, we consider the reaction:

[16]

[17]

Reaction [17] has been discussed in detail in ourprevious work published in Journal of Materials Sceince&Technology (Reference15).

In air atmosphere, the process can be described by thefollowing reactions:

[18]

[19]

The FePb4Sb3O13 produced in Reaction [18] wasenhanced with increasing air flow rate. Therefore, theconversion(X) was decreased with increasing air flow rate.The XRD pattern of the roast in the different atmospheresfor 2000 ml/min flow rate at 1023 K is shown in Figure 10.In the steam atmosphere, the roast obtained is composed ofPbS with small quantities of Fe3O4, FeS, SiO2 and ZnS, andno Sb-containing species is found in the roast, making gooda good Pb-Sb separation result. In the air atmosphere, quitedifferently, the roast consists of Pb4Sb3FeO13, FeS, Sb andPbSO4.

In the nitrogen atmosphere, the roast contains complexsulphides (Pb4Sb6FeS14, PbSb2S5), FeS and PbS. The needlecrystal Sb2S3 is observed in the dust and tube wall. Thesecomplex compounds cannot be decomposed further into

PbS O PbSO+ =2 2 4

4 91

4 56 64 6 14 2

4 3 13 2 2 3

FePb Sb S O

FePb Sb O SO Sb O

+ =

+ +

Sb S H O Sb O H S2 3 2 2 3 23 3+ = +

FePb Sb S H O FeS PbS Sb S4 6 14 2 2 34 3 + +

ln . / ± .k T2 1235 1 3 887= −

ln . / .k T1 10334 4 6 801= − +



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Figure 7—Plot of 1–(1–X)1/3+f2/f1[1–3(1–X)2/3+2(1–X)] against t at differenttemperatures

Figure 8—Arrhernius plot for chemical reaction rate (QH2O=2500 ml/min)

Figure 9—Effect of atmosphere on the overall reaction rate


single compounds; consequently, the Pb-Sb separation willbe inhibited. It is suggested that the oxygen partial pressureof the system is a significant factor for Pb-Sb separation. TheXRD pattern of condensed product in the steam atmosphereat 1023 K is shown in Figure 11. The condensed productobtained is composed mainly of Sb2O3 with small quantitiesof PbS and sulphur. The primary reason for different Pb-Sbseparation results is that steam can decompose the Pb-Sbcomplex sulphide concentrates and the complex intermediateproducts into simple sulphides of lead, antimony and iron. Atthe same temperature, the saturated vapour pressure of theSb2S3 is higher than that of PbS17,18. The detailed discussionabout the volatilization of Sb2S3 was published in MineralProcessing and Extractive Metallurgy18.Therefore, Sb2S3 willbe preferentially volatized into vapour phase and PbS

enriched in the roast. Besides Sb2S3 in the vapour phase canbe selectively oxidized by steam into Sb2O315. As a result, thepartial pressure of Sb2S3 in the vapour phase will be reducedand the volatilization of Sb2S3 will be promoted. In contrast,in nitrogen atmosphere, Pb-Sb complex sulphide concentratescan be decomposed into single sulphides to some extent only.The remaining Pb-Sb complex sulphide concentrates hindersto volatilization of Sb2S3, although it is easily volatile. In airatmosphere, the Pb-Sb complex sulphide concentrates aretransformed into stable Pb4Sb3FeO13, making volatilizationof antimony more difficult. The condensed product consistsof Sb2O3 with a small quantity of PbS and S, as shown inFigure 11. This result indicates that Sb2S3 can be selectivelyoxidized to Sb2O3 by steam while PbS cannot be oxidized. Inthis case, the Pb-Sb separation achieves the anticipated result.

▲


Figure 10—X-ray diffraction pattern of the roast in the various atmospheres (2000 ml/min)

Figure 11— X-ray diffraction pattern of the condensed product in steam atmosphere

Rel

ativ

e in

tens

ity

2θ/deg.

Sb2O3

PbSS

ConclusionThe kinetics of the separation of antimony from lead for Pb-Sb complex sulphide concentrates by oxidation andvolatilization roasting with water vapour is investigated.When the temperature <1073 K, the process rate ofseparation of antimony from lead is controlled by chemicalreaction and internal diffusion. However, when thetemperature >1023 K, the process rate is mainly controlled byinternal diffusion. The apparent activation energies werecalculated to be 85.92 kJ/mol for the chemical reaction and10.27 kJ/mol for the internal diffusion, respectively.

The mechanisms for treating Pb-Sb complex sulphideconcentrates have been investigated. XRD analysis of theproducts in steam atmosphere demonstrates that the reactionof Pb-Sb complex sulphide ores with steam can promote thePb-Sb separation. The direct recoveries of Pb in roast and Sbin dust, which were examined by chemical analysis, are>94% and >93% for Pb-Sb complex sulphide concentrates byoxidative volatilization roasting with water vapour, respec-tively. Comparing the direct recoveries of 68% Pb and 40%Sb in the existing process, the Pb-Sb separation result isbetter than that of the existing process. In contrast, in anitrogen atmosphere, the jamesonite (FePb4Sb6S14) can bedecomposed into single sulphides to some extent only. Theremaining jamesonite hinders to volatilization of Sb2S3,although it is easily volatile. In air atmosphere, thejamesonite(FePb4Sb6S14) is transformed into stableFePb4Sb3O13, making volatilization of antimony moredifficult.

Although hydrogen sulphide may be formed in theprocess, it is easily removed by the Claus process from thegas phase. When water vapour instead of air is used for thevolatilization roasting of antimony-bearing concentrates andconcentrates, the emission of sulphur-bearing gases will bereduced.

Acknowledgement

This work was supported by the National Natural ScienceFoundation of China under grant No. 59964001.

References

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the Presence of Water Vapour, Metallurgical Transactions B,1987, vol. 18,

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Metallurgical Industry Press (in Chinese).

15. HUA, Y., YANG, Y., and ZHU, F. Volatilization Kinetics of Sb2S3 in Steam

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The winners of the Nedbank Capital Green MiningAwards, which seek to acknowledge and celebrate thecontribution that responsible mining makes to theeconomic development of Southern Africa, wereannounced at a ceremony at Nedbank's head office inSandton.

The winners are: Anglo Coal for its IsiboneloWetlands project in the Environmental Category;Palabora Mining Company for the Palabora Foundationin the Socio-Economic Category and Anglo Platinum forthe company's sustainability programme in theSustainability Category. No award was made in theLimited Resource Category.

Mark Tyler, head of Mining & Resources atNedbank Capital says: ‘Nominations received for thisyear's award were encouraging, bearing in mind this isthe first official year where nominations were called for.No award was granted in the Limited ResourceCategory, as nominations did not satisfy all of theaward criteria. This demonstrated that stringent judgingplayed a significant role in determining the winners.

‘Sustainability is a philosophy, something that allmining companies should be striving for. Yet it is acomplex and multi-faceted issue to measure, andtherefore genuine success is difficult to determine. Thebar for demonstrating sustainability is constantly beingraised, whilst at the same time the ground upon whichthe principles of the philosophy stand, is constantlymoving. It is against this background that the winnerswere chosen.’

Anglo Coal’s Isibonelo Wetlands Project isinnovative and extends beyond compliance. Anglo Coaland its partners pioneered ‘off-site’ wetland rehabili-tation in South Africa as mitigation action for on-sitewetland loss. Anglo Coal recognized that due to itsIsibonelo opencast operations, a portion of theSteenkoolspruit wetlands would be destroyed. As amitigation measure for this loss, the companycommitted to rehabilitate a wetland area elsewhere inthe Upper Oliphants River Catchment. They created aprecedent, which other mining companies are following.

Palabora Mining Company's Palabora Foundation,winner of the Socio-Economic Award, was recognizedfor its significant contribution in terms of social andeconomic upliftment of the communities surroundingthe mine. Noteworthy characteristics of theFoundation’s work include, amongst others, thebenefits of the programme, especially in the fields ofmathematics, science and environmental education,

educator development, schools governanceprogrammes, skills development training, SMMEprogrammes and healthcare. While the concept of minesforming trusts to benefit communities that surroundthem is not unique, Palabora’s win demonstrates a clearintent to continuously ‘push the boundaries’ to ensurethe benefits are sustained way beyond the life of themine.

Anglo Platinum, winner in the SustainabilityCategory, has made a significant effort in convergingeconomics, the environment and society for the benefitof present and future generations with its SustainableDevelopment Programme. The programme is strategic,has executive buy-in and is mainstreaming sustain-ability at Anglo Platinum’s operations. The programmecomprises a range of projects, including the AngloPlatinum Converting Project (ACP). It is acknowledgedfor its innovative approach to reducing sulphuremissions to well below the legal limit and, in doing so,setting a new standard for the platinum industry, andfor working with the community in resolving keyissues. In particular, RPM Union Section's CommunityEnvironmental Outreach Programme has fostered anappreciation for the environment among learners andencourages them to make a positive contribution to theenvironment. The project has demonstrated a fruitfulpartnership between the environmental and corporatesocial responsibility departments of the mine, wherebya shared vision has reaped tangible benefits. Other finalists for the Awards included:

* Issued by: Gayle Rodrigues, Tel: +27 11 294 0372,Cell: +27 83 307 6484, E-mail:[email protected] further information: Mark Tyler, Tel: +27 11 294 3424

Winners of the Nedbank Capital Green MiningAwards announced*

Finalist Project

EnvironmentalRichard Bay Minerals Rehabilitation

Socio-economicAnglo Coal Landau Colliery Rebuild SchoolsKumba Kgalagadi Charcoal and Firewood

Project

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Kinetics of the reaction between Pb-Sb complex sulphide concentrates ... · Kinetics of the reaction between Pb-Sb complex sulphide concentrates and water vapour by F. Zhu*, Y. Hua†,