silicothermic reduction of dolomite ore …library.nmlindia.org/fulltext/cmq2002.pdf ·...

15

CANADIAN METALLURGICAL QUARTERLY

SILICOTHERMIC REDUCTION OF DOLOMITE OREUNDER INERT ATMOSPHERE

I.M. MORSI, K.A. EL BARAWY, M.B. MORSI and S.R. ABDEL-GAWAD

Pyrometallurgical Lab, CMRDI, P. O. 87 Helwan, Cairo, Egypt

(Received September, 2000; in revised form June, 2001)

Abstract — Magnesium metal was produced from dolomite ore under inert atmosphere using ferrosili-con as a reducing agent. The kinetics of the silicothermic reduction process was studied at a temperaturerange from 1150-1300 °C. The charges studied are in the form of briquettes. The parameters studied aresilicon stoichiometry, temperature and time of preheating, calcium fluoride additive, molar ratios of cal-cium oxide/magnesium oxide, flow rate of inert gas, briquetting pressure, temperature and time of reduc-tion process. The maximum reduction extent of about 92% was achieved when using charges in the formof briquettes containing 2.5 wt.% CaF2, CaO/MgO molar ratio of 1.6 and using silicon as reductant withits weight ratio/MgO equals 1.45 at 1300 °C for 5 hours. The discussion takes into considerations thetype of intermediate compounds formed during the reduction and the mechanism of the silicothermicreduction process. It was found that the silicothermic reduction process of dolomite ore is a solid-statereaction controlled by diffusion of reacted species and the apparent activation energy obtained equals306 KJ.mol-1.

Résumé — On a produit du magnésium-métal à partir de minerai de dolomite en atmosphère inerte enutilisant du ferrosilicium comme agent de réduction. On a étudié la cinétique du procédé de réductionsilico-thermique dans une gamme de température de 1150-1300 °C. Les charges étudiées avaient la formede briquettes. Les paramètres étudiés incluent la stœchiométrie du silicium, la température et la durée depréchauffage, l’addition de fluorure de calcium, les rapports molaires oxyde de calcium/oxyde demagnésium, le taux d’écoulement du gaz inerte, la pression de mise en briquette, la température et ladurée du procédé de réduction. La réduction maximum d’environ 92% a été obtenue lorsqu’on utilisaitdes charges en forme de briquettes contenant 2.5% en poids de CaF2, avec un rapport molaire CaO/MgOde 1.6 et avec du silicium comme agent de réduction, avec un rapport de son poids/MgO égal à 1.45,à 1300 °C pendant 5 heures. La discussion prends en considération le type de composés intermédiairesformés lors de la réduction et le mécanisme du procédé de réduction silico-thermique. On a trouvé que leprocédé de réduction silico-thermique du minerai de dolomite était une réaction à l’état solide contrôlée parla diffusion des espèces ayant réagi; on a obtenu une énergie d’activation apparente égale à 306 kJ.mol-1.

INTRODUCTION

Today magnesium is produced industrially either fromdolomite ores by thermal reduction of magnesium oxide[1-3] using reactive metals (such as aluminum, silicon ortheir alloys) as a reductant [4-14] or from magnesium chlo-ride by an electrolytic process [15-18].

The recovery of the magnesium from dolomite ores byusing silicon (silicothermic process) was investigated bymany authors. It was reported that completion of thisprocess occurs via the formation of intermediate com-pounds such as calcium silicide [19-23] or calcium magne-

sium silicate Ca3Mg(SiO4)2 [19,21] where their formationsare dependent on temperature.

The silicothermic process was reported as a solid-statereaction controlled by the rate of diffusion of the solid reac-tants. It was found that this reaction could be carried outunder inert atmosphere instead of a vacuum [24-26]. Theeffect of a calcium fluoride addition has been studied bymany authors [9,11,27-30].

The present work aims to study the different parametersaffecting the kinetics of the silicothermic process of mag-nesium production from dolomite ore under inert atmos-

Canadian Metallurgical Quarterly, Vol 41, No 1 pp 15-28, 2002© Canadian Institute of Mining, Metallurgy and Petroleum

Published by Canadian Institute of Mining, Metallurgy and PetroleumPrinted in Canada. All rights reserved


phere. The discussion will take into consideration the typeof intermediate compounds formed during the reaction andthe mechanism of the process.

EXPERIMENTAL

Raw Materials

A sample of dolomite ore (Um Bogma, Sinai) was provid-ed by the Sinai Manganese Company (Egypt). The ore wascrushed in a jaw crusher to 3 cm and ground in a roller millto 1 mm and finally pulverized to 74 mm by using a vibrat-ing mill. A representative sample was taken from the pul-verized ore and used throughout this investigation. Thecomplete chemical analysis of this sample and the dolimeproduced from its calcination at 1000 °C for one hour aregiven in Table I.

A sample of ferrosilicon was provided by the EgyptianFerroalloys Company (EFACO) to be used as a reductant.It was ground to 1 mm size in a disk mill and pulverized to74 mm in a vibrating mill. The complete chemical analysisof this sample showed that it contains in weight percentage:74.8 Si, 23.36 Fe, 0.09 C, 0.31 Ca, 0.003 S, 0.031 P and1.41 Al. Pure grade calcium oxide was used as a fluxingagent and calcium fluoride of reagent grade was used as thecatalyst in this study.

Reduction Technique

The reduction process was carried out in the retort madefrom a heat resistant high Ni-Cr steel tube. The retort wasexternally heated by using a regulated resistance furnacehaving a maximum temperature of 1300 °C. The retort isclosed at one end and open at the other. The open end of theretort is closed by using a rubber stopper with a tube for theinput of argon gas and two terminals of U-shaped coppercooling tube. The outlet of the inert gas from the retort iscarried out through a fixed tube near the open end of the

retort. The cooling copper tube is used to condense the pro-duced magnesium vapour near the reaction zone.

The reduction experiments were carried out by usingdolime. The required weight ratios of dolime, calciumoxide, ferrosilicon and calcium fluoride were mixedthouroughly. The mixed charges were homogenized byrotating for 20 minutes in a porcelain ball mill. The bri-quetting of the charge was carried out under different pres-sures of up to 700 MPa using a bench scale hydraulic press.The briquettes have a diameter of 1.4 cm and a thickness ofabout 0.5 cm. The produced briquettes were used directlyfor the reduction experiments.

About 10 gm of charges (briquettes) were put in a boatmade from heat resistant high Ni-Cr steel alloy. Thecharged boat was inserted in the retort at room temperature.The flow of inert gas argon was adjusted over the charge atdifferent rates ranging from 0.08-0.25 L/min. The chargeswere subjected to a preheating cycle at temperature rangesfrom 600 - 1000 °C for different periods of up to 3 hrs. Thecharge was cooled to room temperature under argon gasatmosphere and the temperature of the furnace was raisedto the required reduction temperature ranging from 1150-1300 °C. After approaching the required reduction temper-ature, the preheated charge was inserted again in the fur-nace to perform the reduction process for different periodsup to 8 hours. At the end of the reduction experiments, theretort moved away and the reduced charge was lifted tocool to room temperature under argon atmosphere. Aftercooling, the condensed magnesium product was collectedand weighed. The residue obtained was also weighed andanalyzed for magnesium.

The extent of reduction process was calculated accord-ing to the following formula

Extent of reduction % = [(Mgo - Mgr) /Mgo] ¥ 100

where Mgo is the initial quantity of magnesium in the sam-ple and Mgr is the residual magnesium in the reductionresidue. The raw materials and the reduction products wereinvestigated physically and chemically.

Method of Calcium and Magnesium Determinations

Calcium and magnesium were determined in the fusion fil-trate of samples (original or residues) by complexometrictitration against a standard EDTA solution. Calcium wasdetermined by adjusting the pH 13.3 in the presence ofsolo-chrom dark blue, where the colour changes from pur-ple violet to blue. The total hardness (calcium and magne-sium) was determined at pH @ 10 adjusted by a buffer solu-tion (ammonia/ammonium chloride) and using EriochromeBlack T as an indicator, the magnesium content was calcu-lated by the difference. The interference of iron and othermetals were avoided by adjusting the pH of the solution in

16


Table I – Chemical analysis of dolomite ore and calcined dolomite (dolime).

Component Wt.%

Dolomite Dolime

CaO 29.20 53.85 MgO 18.89 34.9 SiO2 2.50 4.8 Fe2O3 0.98 1.9 Al2O3 1.20 2.22 Na2O 0.18 0.34 Moisture 0.89 - L.O.I* 46.00 -

*loss of ignition

each case using traces of solid potassium cyanide andhydroxylamine hydrochloride.

RESULTS AND DISCUSSION

Effect of Silicon Stoichiometry

The variation of the reduction extent of magnesiumevolved as a function of silicon stoichiometry added tothe charge is given in Figure 1 for reduction experimentscarried out at 1250 °C for 3 hours under argon flow rateof 0.08 L/min. The stoichiometry of the silicon in thecharge (X) was calculated according to the followingequation:

2MgO(S) + 2CaO(S) + Si(S) Æ 2Mg(g) + Ca2 SiO4(S) (1)

The results show that the reduction extent increasesfrom 15% to a value of about 30% as silicon stoichiometryincreases from 1X to 2X, respectively. After this, thereduction extent decreases to a lower value of 16% at 8X.

X-ray analysis of the reduction residues for chargescontaining sample 2XSi and sample 8XSi revealed thepresence of calcium magnesium silicate phaseCa3Mg(SiO4)2 in excess in the charge containing 8XSi [31].The presence of excess silicon may promote the dissolutionof magnesium oxide in excess calcium silicate formed as aresult of the reduction process in the form of calcium mag-nesium silicate [26]. The formation of this phase may pre-vent the direct contact of silicon with magnesium oxide andit may enhance the consumption of magnesium oxide andcalcium oxide needed to complete the reduction process.As a result the reduction extent decreased.

Effect of the Preheating Temperature

The variation of the reduction extent of magnesium as avariable of the preheating temperature for reduction exper-iments of charge containing 2XSi preheated at differenttemperatures for 1 hour and reduced at 1250 °C for 3 hoursunder the rate of argon gas of 0.08 L/min is given in Figure 2.The preheating of charges enhanced the reduction extentfrom 30 to 41% as the preheating temperature increasedfrom 700 to 800 °C respectively. At a higher preheated tem-perature of 1000 °C, the reduction extent decreased to 24%.

X-ray diffraction (XRD) analysis of the sample pre-heated at 1000 °C (Figure 3) revealed the formation of thephases Ca3SiO5 , Fe2Si and CaSi2 as well as CaO, MgO andSi phases. The reaction taking place at 1000 °C may be

2Si(S) + 2FeSi2(S) + 5CaO(S) Æ Fe2Si(S)(2)

+ 2CaSi2(L) + Ca3SiO5(S)

The occurrence of such a reaction during this preheat-ing step can consume the free calcium oxide available forthe reduction process and the formation of the calcium sil-icate (Ca3SiO5 ) phase. This phase can prevent the directcontact of CaSi2 and magnesium oxide leading to an incom-plete reduction process. A similarity between the intensitiesof lines belonging to the CaSi2 phase in both preheatedsamples at 1000 °C and its residue after reduction processwas also noticed. This indicates that CaSi2 probably did nottake place in the reduction process. On the other hand, aweak intensity of silicon peaks in the reduced sample indi-cates its reaction with MgO during the reduction process toform silicates and magnesium metal.

SILICOTHERMIC REDUCTION OF DOLOMITE ORE UNDER INERT ATMOSPHERE 17


Fig. 1. Effect of silicon amount on the reduction extent of dolime.

Fig. 2. Variation of reduction extent of magnesium as function of pre-heating temperature.

Silicon Stoichiometric Amount, X

Red

uctio

n E

xten

t, %

40

30

20

10

0

108642

Preheating Time 1 hour

Reduction Temperature

(1250 °C-3 hours)

Temperature, °C

Red

uctio

n E

xten

t, %

60

40

20

0

1000900800700

Calcination (1000 °C- 1 hour)

Reduction (1250 °C-3 hours)



Effect of Preheating Time

Figure 4 shows the variation of magnesium reductionextent with the time of preheating for charges preheated at800 °C. The increase of preheating time from 30 to 60 min-utes enhanced the reduction extent from 30 to 44%, respec-tively. By increasing the time of preheating by one hour, thereduction extent lowered again to approach a minimumvalue of 20% after three hours.

The XRD results of the preheated samples at a constanttemperature of 800 ∞C for 1 and 3 hours and their residuesafter reduction process are shown in Figures 5 and 6,respectively. It was noticed that the Fe2Si phase was presentwith lower intensity in the preheated sample for 1 hour butwith higher intensity in a sample preheated for 3 hours. Itis also observed that Ca3SiO5 phase is present in the pre-heated sample for 3 hours. These results indicated that thedecrease in the reduction extent is mainly due to the for-mation of the Ca3SiO5 phase. Also, increasing time of pre-heating enhances the dissociation of FeSi2 as the followingreaction [32]

2FeSi2(S) Æ Fe2Si(S) + 3 Si(S) (3)

The increase of silicon formation is confirmed by itshigher intensity lines for the samples preheated for 3 hoursand the presence of Fe2Si lines, (Figure 6). Excess offormed silicon in the preheated sample for 3 hours hasprobably reacted with CaO to form CaSi2 [22] and Ca3SiO5during the reduction process according to the followingreaction

5Si(S) + 5 CaO(S) Æ 2 CaSi2(L) + Ca3SiO5(S) (4)

DG°1300 k = -11.8 kJ

This reaction is confirmed by the lower intensity oflines belonging to the CaO phase in the reduction residue of

18

Fig. 3. X-ray diffractogram of preheated sample at 1000 °C and after reduction at 1250 °C for 3 hours.

Diffraction angle (2q)

Phase Identified Preheated Sample Sample ReducedSymbol Phase at 1000 °C-1 hour at 1250 °C- 3 hours

a CaO + +

b MgO + +

c Si + +

d FeSi2 + +

e Fe2Si + –

i CaSi2 + +

g Ca2SiO3 + +

k Ca2SiO4 – +

5560657075 50 45 40 35 30 25

Reduced Sample(1250 °C-3 hours)

Preheated Sample(1000 °C-1 hour)

Fig. 4. Variation of reduction extent of magnesium with time of preheat-ing at 800 °C.

Preheating Temperature

(800 °C)

Reduction Temperature

(1250 °C- 3 hours)

Time, hour

Red

uctio

n E

xten

t, %

60

40

20

0

43210

the 3 hour preheated sample than the 1 hour preheated sam-ple. As well, an increased silicon content in the preheatedsample before its reduction and the presence of Ca3SiO5phase can prevent the direct contact of either CaSi2 formedand silicon with magnesium oxide to complete its reductionprocess. As result, the reduction extent lowered as the pre-heating period increased by 1 hr.

Effect of Calcium Fluoride

Figure 7 shows the variation of the magnesium reductionextent with calcium fluoride percentage added to thecharges containing 2XSi preheated at 800 °C for 1 hour and

reduced at 1300 °C for 4 hours under argon atmosphere(0.08 L/min). The reduction extent increased from 44% to64% as the calcium fluoride weight percentage increasedup to 2.5%. A reverse effect was observed after this valuewhere reduction extent decreased from 64 to 43% as thecalcium fluoride increased from 2.5 to 3.5%, respectively.

The results obtained from X-ray analysis of the reduc-tion residues of samples containing 2.5% and 3.5% calciumfluoride and without calcium fluoride addition are given inFigure 8. The main phases identified are calcium oxide,magnesium oxide and calcium silicate (Ca2SiO4). A calci-um silicide (CaSi2) phase was identified in the residue of



Fig. 5. X-ray diffractograms of preheated sample at 800 °C and after its reduction at 1250 °C for 3 hours.


55606570 50 45 40 35 30 25


Preheated Sample(1000 °C-1 hour)


a CaO + +

b MgO + +

c Si + +

d FeSi2 + +

i CaSi2 – +

k Ca2SiO4 – +


the charge containing 2.5% calcium fluoride but existed astraces in the residue of the charge containing 3.5% calciumfluoride. However, the silicon phase disappeared in theresidue of the charge without calcium fluoride, occurred astraces with 2.5% calcium fluoride and was well identifiedin the residue of the charge containing 3.5% calcium fluo-ride. Therefore, the role of calcium fluoride is apparentlyaccompanied with the formation of the calcium silicidephase during the reduction process. It is noticed experi-mentally that the residues obtained at the end of the reduc-tion process were more dense by increasing the calciumfluoride addition which indicated sintering of the reactionproduct.

The promotion of magnesium reduction extent up to2.5 % calcium fluoride may be due to the fact that anincrease in the calcium fluoride concentration increases theactivity of both calcium oxide and silica which in turn pro-motes the formation of calcium silicate during the calciumsilicide formation according to the following reaction

20



5560657075 50 45 40 35 30 25


Preheated Sample(800 °C-3 hours)


a CaO + +

b MgO + +

c Si + +

d FeSi2 + +

e Fe2Si + –

i CaSi2 – +

g Ca2SiO3 – +

k Ca2SiO4 – +

Fig. 6. X-ray diffractograms of preheated sample at 800 °C for 3 hours and after its reduction at 1250 °C for 3 hours.

Preheating (800 °C - 1 hour)

Reduction (1300 °C - 4 hours)

CaF2, %

Red

uctio

n E

xten

t, %

80

60

20

0

2.52.01.51.0

Fig. 7. Effect of calcium fluoride additive on the reduction extent of mag-nesium.

40

3.0 3.5



4 CaO(S) + 5 Si(S) Æ 2 CaSi2(L) + Ca2SiO4(S)(5)

DGo1500 K = -53.02 kJ

At higher ratios of calcium fluoride, the excess forma-tion of the calcium silicate phase prevents the direct contactbetween the reacted calcium oxide and silicon whichdecreases the calcium silicide formation. This state isproved by the presence of high intensity lines belonging tothe silicide phase and very weak intensity for lines belong-ing to the silicon phase in charge containing 2.5% calciumfluoride. However, the intensity of the X-ray lines belong-ing to calcium silicide phase is weak and the silicon linesare strong for charge containing 3.5% calcium fluoride(Figure 8). The reaction belonging to the formation of cal-cium silicide can be written as follows:

2 CaO(S) + 5 Si(S) Æ 2 CaSi2(L) + SiO2(S)

(6)

DGo1500 K = + 25.48 kJ

2 CaO(S) + SiO2(S) Æ Ca2SiO4(S)

(7)

DGo1500 K = -78.2 kJ

From a thermodynamics viewpoint, Reaction 7 is morelikely to occur. It may be that calcium fluoride can promoteReaction 6. It should be mentioned here that our results arein agreement with other investigators [9,19] who suggestedthat the promoting effect of calcium fluoride added up to2.5% during the reduction process is due to its mineralizingeffect by the formation of low melting point phases whichincrease the contact between the calcium silicide phase andthe magnesia.

Effect of Calcium Oxide Addition

Different experiments were carried out on charges contain-ing 2XSi, 2.5 wt.% calcium fluoride and different molarratios of CaO/MgO ranging from 1.1 to 1.7. The chargeswere preheated at 800 °C for 1 hour followed by reduction


5055606570 45 40 35 30 25 20

Without CaF2

Phase Identified Without 2.5 3.5Symbol Phase CaF2 % CaF2 % CaF2 %

a CaO + – +

b MgO + – +

d Si – – +

i CaSi + + –

k Ca2SiO4(1) + + +

g Ca2SiO5(2) – + +

Fig. 8. XRD of reduced samples with calcium fluoride additive, 2.5 and 3.5%.

2.5% CaF2

3.5% CaF2

I.M. MORSI, K.A. EL BARAWY, M.B. MORSI and S.R. ABDEL-GAWAD 22


at 1300 °C for 4 hours under argon atmosphere at a constantflow rate of 0.08 L/min.

The results in Figure 9 show that the reduction extentincreases from 64 to 85% as the molar ratio CaO/MgOincreased from 1.1 to 1.6. Increasing this ratio by more than1.6 is accompanied by a decrease in the reduction extentreaching about 78% when using 1.7 molar ratio of CaO/MgO.

The results of X-ray diffraction for the reductionresidues of selected samples are given in Figure 10. It isclear that the relative intensities of calcium silicide CaSi2and magnesium oxide lines of reduction residue of a sam-ple containing CaO/MgO molar ratio 1.6 are lower thanthat obtained for the residue of a sample having CaO/MgOmolar ratio 1.7. This proves that the reaction between cal-cium silicide and magnesium oxide is promoted by addedcalcium oxide up to 1.6 molar ratio. On the other hand,excess addition of calcium oxide retards the magnesiumreduction extent due to the formation of calcium silicatephase (Ca2SiO4) as a result of maximum distribution of cal-cium oxide in the charges and so decreases the direct con-tact of calcium silicide with magnesium oxide. This resultis confirmed by the presence of high relative intensity linesof calcium silicide phase in the X-ray diffraction forresidue of CaO/MgO molar ratio 1.7.

Effect of Flow Rate of Inert Gas

A series of experiments was performed on samples con-taining 2XSi, 2.5% weight calcium fluoride and CaO/ MgOmolar ratio 1.6 preheated at 800 °C for 1 hour and reducedat 1300 °C for 1 hour. Figure 11 shows the variation of

magnesium reduction extent with variation of flow rate ofargon gas. It is clear that the magnesium extent increasesfrom 36% to about 56% as the flow rate increases from0.08 L to 0.250 L/min, respectively. The flow rates higherthan 0.250 L/min have no effect on the reduction extent ofmagnesium. This indicates that flushing of the magnesiumvapour from the reacted charge reaches its optimum valueat a rate of flow ≥ 0.250 L/min. Also, it indicates that thediffusion of the magnesium vapour through the boundarylayer around the reacted particles can play a role during thereduction process. Kinetically, the boundary layer formedduring the reduction process should be removed in order toobtain a maximum reduction extent.

Effect of Briquetting Pressures

This series of experiments was carried out using the sameexperimental conditions as mentioned above at a gas flowrate of 250 mL/min. Figure 12 shows the relation obtainedbetween the magnesium reduction extent as a variable ofpressing pressure used to form the briquettes. It is obviousthat the reduction extent increases from 36% to 60% byincreasing the pressing pressure from 80 to 450 MPa,respectively after which it becomes constant by increasingpressing pressure above 450 MPa. This increase may beexplained by the fact that the increase of the pressing pres-sure enlarges the contacting areas between the reacted par-ticles inside the briquettes which in turn is reflected inhigher magnesium reduction extent. It is possible that thecomponents of the charge approach the maximum contactafter a pressing pressure ≥ 450 MPa; hence a maximummagnesium reduction extent is constant.

It is known that when increasing the pressing pressureduring the preparation of briquettes, the microporesbetween the reacted particles should be decreased. Thedecrease of the micropores can create an obstacle for thediffusion of magnesium vapour through it to outer surfacesof briquettes. In contrast, our results show that an increasein the pressing pressure increases the magnesium reductionextent. This indicates that the diffusion of magnesiumvapour through the micropores has no effect on the reduc-tion extent of magnesium. Indeed, the reaction is affectedby the enlargement of reacted contact areas between thereactants indicating that the reduction is probably con-trolled by the diffusion of magnesium between the reac-tants in the solid state.

KINETICS OF SILICOTHERMIC REDUCTIONPROCESS

Reduction Isotherms.

The kinetics of dolime reduction process was studied underall optimum conditions obtained, which are summarized inTable II. Figure 13 shows the reduction isotherms obtained

Preheating (800 °C - 1 hour

Reduction (1300 °C - 4 hours)

CaO / MgO Molar Ratio

Red

uctio

n E

xten

t, %

100

80

40

20

1.41.31.21.1

Fig. 9. Effect of the variation of CaO/MgO molar ratio on the reductionextent of magnesium (with 2.5% CaF2).

60

1.5 1.60

1.7




5055606570 45 40 35 30 25 20

Phase Identified Without 2.5 3.5Symbol Phase CaF2 % CaF2 % CaF2 %

a CaO + – +

b MgO + – +

d Si – – +

i CaSi + + –

k Ca2SiO4(1) + + +

g Ca2SiO5(2) – + +

Fig. 10. XRD of reduced sample containing 1.1, 1.6 and 1.7 CaO/MgO molar ratio.

1.7CaO/MgO

1.6CaO/MgO

1.1CaO/MgO

Table II – Optimum conditions for reduction ofdolime.

Parameter Value

Reductant stoichiometry* 2XSiReductant weight ratio with respect to

MgO in charge 1.45Calcium fluoride in weight %** 2.5 % CaO/MgO molar ratio 1.6Rate of inert gas flow rate (Argon) 0.25 L/minPressing pressure of briquettes 450 MPaPreheating temperature 800 °CPreheating time 1 hour

* Using ferrosilicon (75 %).** With respect to the total charge.

Reduction Temperature 1300 °CReduction Time 1 hour

Flow Rate, mL/min

Red

uctio

n E

xten

t, % 80

40

800600400200

Fig. 11. Effect of flow rate of argon gas on the reduction extent of mag-nesium.

1000

0


for the variation of magnesium reduction extent at differentperiods of up to 5 hours and at temperatures of 1150, 1200,1250 and 1300 °C. The magnesium reduction extentincreases as reduction temperatures increase and graduallywith time approach nearly constant value. The maximumreduction extent of about 92% was obtained at 1300 °C for5 hours.

Figure14 shows photomicrographs for reduction residuesobtained after different reduction periods at 1300 °C. Itshows the metallic phase formed in a matrix of the reduc-tion product calcium silicate. The metallic phase quantitydecreases as the reduction period increases. Figure 15

shows a photomicrograph for residue obtained after 2hours reduction time at 1200 °C. It shows the same struc-ture as that obtained for the sample reduced at 1300 °C for30 minutes.

24


Reduction Temperature 1300 °CReduction Time 1 hour

Briquetting Pressure, MPa

Red

uctio

n E

xten

t, %

80

40

20

3002001000

Fig. 12. Effect of briquetting pressure on the reduction extent of magne-sium (using 2.5% CaF2 and CaO/MgO 1.6 and gas flow rate 250 mL/min.

60

400 5000

600

Time, hours

Red

uctio

n E

xten

t, %

100

80

40

20

4321

Fig. 13. Effect of time and temperatures on the reduction extent of mag-nesium at optimum conditions.

60

5 60

Fig. 14. Photomicrographs for residues obtained at 1300 °C for differentreduction periods 0.5, 2 and 3 hours showing the metallic phase “white”and calcium silicate matrix “light and grey”, ¥ 500.

3 hrs

2 hrs

0.5 hr

The types of elements and their quantities in themetallic phase observed in mineralogical investigationof residues obtained at 1300 °C were ascertained byusing a scanning electron microscope. Figure 16 showsthe emission diffraction spectrum obtained for the dis-tribution of different elements existing in the metallicphase with their percentages. The main elements after a30 minute reduction period are Mg, Ca, Si and Fe inmolar ratios of 1.61 : 1.11 : 0.57 : 0.01, respectively. Onthe other hand, the main elements after 2 and 3 hourreduction periods are Ca, Si and Fe in molar ratios of1.73 : 0.80 : 0.14 and 1.68 : 1.00 : 0.09, respectively.The results of the investigation on matrix are given inFigure 17 which shows the distribution of the differentexisting elements and their percentages for the samplereduced at 1300 °C for 2 hours. The main elements areCa, Mg, Si, O and Fe in molar ratios of 1.7 : 0.08 : 0.6 :2.0 : 0.1, respectively. From these molar ratios theexpected main phases in the matrix are probably calci-um silicate (Ca2SiO4) and iron.

Applicability of Solid State Diffusion Models.

Some solid state diffusion models were investigated tofind which kinetic equation can fit the reductionisotherms obtained in Figure 13. It is found that themodel postulated by Ginstling and Brounstein [1-(2/3)a - (1-a)2/3] = Kt, is the most applicable model for allreduction isotherms up to 85% reduction extent where ais the reaction extent, t is the time in minutes and K is thereaction rate constant. The mean value of the reaction rateconstants at different reduction temperatures were calcu-lated and their logarithmic values were plotted against thereciprocal of absolute reduction temperatures accordingto Arrhenius equation to give a straight line (Figure 18).The apparent activation energy is 306 kJ/mol as calculat-

ed from the slope of this line. This value of the activationenergy is slightly higher than that obtained by otherinvestigators [5]. Also, it may be confirmed that the reac-tion is controlled by the diffusion process existing in thesolid state.



Fig. 15 Photomicrograph for reduced sample at 1200 °C showing themetallic phase “white” and calcium silicate matrix “light and dark grey”,¥ 500.

Elmt. Element %

Mg 39.15Si 15.84Ca 44.45Fe 0.56

100

80

40

20

151050

60

20

0

a

Elmt. Element %

Si 22.59Ca 69.42Fe 7.98

2000

1500

500

0

1000

500

b

Elmt. Element %

Si 28.10Ca 67.15Fe 4.75

c

400

300

200

100

0

Fig. 16. SEM investigation for reduced sample at 1300 °C for 0.5, 2 and3 hours (metalic phase). a) Sample reduced at 1300 °C for 0.5 hours. b)Sample reduced at 1300 °C for 2 hours. c) Sample reduced at 1300 °Cfor 3 hours.


Mechanism of the Silicothermic Reduction Process UnderAtmospheric Pressure

The following facts can be deduced from the present inves-tigations:

1. The reduction process is diffusion in solid state.2. Presence of a metallic phase in the reduction residueindicates its liquification during the reduction process. This

metallic phase is not purely calcium silicide (CaSi2) only,as previously reported [9,19,33] but, according to the scan-ning electron microscopic investigation, it is a mixture ofmetallic phases of magnesium, iron, silicon and calciumformed during the reduction process. 3. The matrix consists of calcium silicate and remnants ofmagnesium oxide phase.4. Iron that initial exists in ferrosilicon is distributed inmetallic and matrix phases.

Depending on these facts the mechanism of the sili-cothermic reduction process of magnesium oxide present indolime to magnesium metal may be visualized as follows :

1. At the beginning of the reduction process, the calciumoxide reacts at phase boundaries with silicon to form liquidcalcium silicide and solid calcium silicate phase accordingto the following equation

4 CaO(S) + 5 Si(S) Æ 2 CaSi2(L) + Ca2SiO4(S)(8)

DGo1400 K = -6.7 KJ

2. The liquid calcium silicide subsequently reacts withmagnesium oxide to give magnesium vapour according tothe following equation

5 MgO(S) + CaSi2(L) Æ 5 Mg(g) + CaO(S) + 2 SiO2(S)(9)

DGo1400 K = +539.7 kJ

It should be mentioned that the magnesium metal pro-duced during the reduction process must pass with liquidstate before its evapouration. Indeed, the liquid magnesiumdissolves in the reductant calcium silicide phase formingthe intermetallic phase (Ca - Mg - Si - Fe) which wasdetected by the scanning electron microscope. Removal ofmagnesium from this metallic phase is essential to com-plete the reduction process.

3. The excess calcium oxide reacts with silica to obtain cal-cium silicate according to the following equation

4 CaO(S) + 2 SiO2 (S) Æ 2 Ca2SiO4 (S)(10)

DGo1400 K = -155.12 kJ

4. The overall reaction of the reduction process can be writ-ten as follows

2 CaO(S) + 2 MgO(S) + Si(S) = 2 Mg(g) + Ca2SiO4(S)(11)

DGo1400 K = +152.5 kJ

It is clear that steps 1 and 3 may occur spontaneously.Therefore, the reaction in step 2 may be considered as a ratecontrolling step.

26


Counts

1500

151050

Fig. 17. SEM investigation for reduced sample at 1300 °C for 2 hours(matrix phase).

200

(104/T) °k-1

Log

K

2.8

3.2

4.0

4.4

6.46.26.05.8

Fig. 18. Logarithm of reduction rate constant vs reciprocal reaction tem-perature “Arrhenius Plot”.

3.6

6.6 6.8

4.8

7.0

1000

500

Energy (keV)

Elmt. Element %

O 33.37Mg 1.93Si 16.34Ca 42.65Fe 5.72

7.2

CONCLUSIONS

1. Adjustment of molar ratio of calcium oxide/magnesiumoxide in the silicothermic reduction process of dolime isessential (1.6).

2. The suitable ratio of silicon in the reductant ferrosiliconcalculated with respect to magnesium oxide in the sili-cothermic charges is 1.45.

3. Preheating of charges used in the silicothermic reduc-tion process (at 800 °C for 1 hour) promotes the process.

4. Using calcium fluoride in weight percentage of 2.5 inthe charges for silicothermic process accelerates the reduc-tion process.

5. Using charges in form of briquettes (pressed at 450MPa) gives higher reduction rates.

6. The magnesium metal formed during the silicothermicprocess is partially dissolved in the calcium silicide phasebefore its evapouration.

7. The silicothermic reduction process is a solid-statereaction controlled by diffusion of reacted species in solidstate. The apparent activation energy of the reductionprocess is about 306 kJ/ mol.

REFERENCES

1. B.B. Clow, JOM, 1987, May, pp. 24-25.

2. L. Daniel, Light Metal Age, 1982, Aug., pp. 29-31.

3. P.A. Fisher, International Metals Reviews, 1978, No.6, pp.269-285.

4. O. Kubaschewski and E.L. Evans, MetallurgicalThermochemistry, 1958, Pergamon Press, London, p. 238.

5. R.V. Misra, V.S. Sampath and P.P. Bhatnagar, Trans.IndianInst.Metals, 1964, vol. 17, pp. 145-54.

6. S.K. Barua and J.R. Wynnyckyj, Can. Met. Quart., 1981,vol. 20(3), pp. 295-306.

7. B. Ellingsaeter and T. Rosenqvist, JOM, 1956, Aug., pp.1111-12.

8. L.M. Pidgeon, Trans. Met. Soc., AIME, 1963, Aug., vol. 227,pp. 821-34.

9. J.M. Toguri and L.M. Pidgeon, Can. J. Chem, 1962, vol. 40,pp. 1769-76.

10. J.M. Toguri and L.M. Pidgeon, Can. J. Chem, 1961, vol. 39,pp. 540-547.

11. L.M. Pidgeon and J.A. King, Trans. Farady Soc., 1948, vol.40, pp. 197-206.

12. L.M. Pidgeon, Trans. Can. Inst. Mining Met., 1946, vol. 49,pp. 621- 35.

13. L.M. Pidgeon, Trans. Can. Inst. Mining Met., 1944, vol. 47,p. 16.

14. L.M. Pidgeon and W.A. Alexander, Trans. AIME, 1944, vol.159, p. 315.

15. O. Sivilotti, Light Metal Age, 1984, Aug., pp. 16-18.

16. P. Ficara, E. Chin, T. Walker, D. Laroche, E. Palumbo, C.Celik and M. Avedesian, “A Noval Commercial Process forthe Primary Production of Magnesium” Light Metals,(Proc.Conf.), 1996, Montreal, Quebec, Aug., pp. 25-29.

17. N. Sevryukov, B. Kuzmin and Y. Chelishchev, (Translated byB. Kuznetsov,) General Metallurgy, 1969, Mir Publishers,Moscow, pp. 468-503.

18. R.A. Sharma, JOM, 1996, Oct., pp. 39-43.

19. R.B. Winkler, Vacuum Metallurgy, 1971, Elsevier, London,1971, pp. 193-208.

20. R.V. Misra, V.S. Sampath and P.P. Bhatnagar, “The Scope forthe Development of the Magnesium”, Light Metal Industryin India, (Jamshedpur: National Metallurgical Lab. 1961),1961, pp. 200-5.

21. W.T. Hughes, C.E. Ransley and E.F. Emley, “ReactionKinetics in the Production of Magnesium by Dolomite-fer-rosilicon (Pidgeon) Process”, Advan. Extr. Met. Proc. Symp.,London (Pub. 1968), pp. 429-54 (Eng.)., Inst.Mining Met,London, U.K., 1967.

22. J.R. Wynnyckyj and L.M. Pidgeon, Met.Trans., 1971, vol.2(4), pp. 979-986.

23. J.R. Wynnyckyj and E.N.Tackie, Light Met., 1988, pp. 807-15.

24. I.U. Frishberg and A.S. Miknlinskii, Izv. Sibirsk. old. Akad.Nauk. SSSR, 1962, No.2, pp. 63-6.

25. G.D. Oliveira, Met.ABM, 1971, vol. 27(167), pp. 715-19.

26. J.R. Wynnyckyj, E.N. Tackie and G. Chen, Can. Metall. Q.,1991, vol. 30(3), pp. 139-143.

27. I. Blahut, L. Rabatin, K. Tomasek and J. Kocur, Uhli-Rudy-Geol.Pruzkum, 1994, vol. 1(6), pp. 207-10.

28. A.S. Burnazyan and M.V. Dabrinyan, Sov. Mar. Khoz. Arm.SSR, 1961, vol. 2, pp. 103-11.

29. N.M. Nikolaishvil and I.I. Torotadze, Tr. Kavkaz. Inst. Miner.Syr’ya, 1965, No.6, pp. 119-20.




30. G.D. Oliveira, Met. ABM, 1969, vol. 25(135), pp. 125-41.

31. T.R. Bandopadhyay, Proceedings of the Industrial Conf.Advances in Chemical Metallurgy, India, 1991, Jan., pp.195 - 203.

32. K.B. Wafers, Metall., 1963, vol. 17, May, pp. 446- 51.

33. J.R. Wynnyckyj, O.B. Rao and G.S. Muller, “ReactionKinetics in the Silicothermic Magnesium Process”,Metallurgical Society of CIM Annual Volume FeaturingMolybdenum (Proc.Conf.), 1977, The Canadian Institute ofMining and Metallurgy, Montreal, Quebec, 1978.

28


silicothermic reduction of dolomite ore …library.nmlindia.org/fulltext/cmq2002.pdf ·...

Documents