2.1 RAW MATERIALS 2. EXPERIMENT AL 2

Kinetics of Gaseous Reduction of Manganese Ore

Tor Lindstad (1,2) and Lars-Arne Stalheim (2) (1) SINTEF Materials Technology,

(2). Norwegian University of Science and Technology, N-7491 Trondheim, Norway Phone: 47 7359 6885/ Fax: 47 7359 2786, (2) Present: Elkem ASA, Kristiansand, Norway

O.ABSTRACT

The reduction of Mn30₄ to MnO has been investigated in a thermo balance apparatus.The charge was composed of Mn₃0₄ and coke, in about the same proportion as in an industrial scale reduction furnace. Except Argon supplied at the start before heating to reaction temperature, no extra gas was furnished. The reaction between the two solids proceeded through CO and C02

as gaseous intermediates. The reaction can be described by the shrinking core reaction model for the manganese oxide, and seems to be rate-controlled by chemical reaction at the reaction front or a mixed resistance of chemical reaction and gaseous diffusion through the product layer.

1. INTRODUCTION

The research group at SINTEF/NTNU working on Manganese processes is conducting research both on the solid state pre-reduction and the smelting process.

When a manganese ore lump descends through the shaft in the furnace, the ore often with an inlet composition corresponding to Mn02, it decomposes or is reduced by the countercurrent CO-containing gas first to Mn20.3 and then to Mn304. The reaction mechanisms for these two steps depend on the mineralogical phases present and are usually explained by the shrinking unreacted core model, Berg and Olsen 1 2 ' . After the ore has been exposed to temperatures up to 1 OOO °C we might have a core of Mn203 surrounded by a layer of Mn304 and an outer layer of MnO. When the temperature increases above 1000 °C the oxide will be reduced to MnO. These reactions seem to be controlled by pore diffusion through the product layer. The size of the pores, and the CO diffusivity, increases with temperature up to a certain point, where it starts to decline rapidly due to heavy sintering, Berg and Olsen ¹ '2

.

The present paper is based on results obtained by Stalheim 3

. In this work we wanted to study only the last

223

gaseous reduction step from Mn304 to MnO. Further- more we did not want to have an outer layer ofMnO at

start. To accomplish this it was decided to produce Mn304 by decomposing manganese ore.

2. EXPERIMENT AL 2.1 RAW MATERIALS

Chemical analyses of the ores and coke that were used are shown in Table 1 and Table 2. The ores and the coke were taken from samples we received from Norwegian producers.

I COMILOG _II BHP Component

1Wt%

I ~t%

Mn 62.7

Fe 1.5 1.5

Si02 2.2 5

Al203 4.7 5

KzO 1.0 2 ^~

Table 1. Chemical analysis of ores, after calcmation at 1100 °

c

Wt%

Fix-C 86.9

Volatiles 2.0

Ash 11.1

-In ash:

I

Si02

I

^{43.9% MnO 5.9%}

Al203 27.9% KzO 3.8%

FeO 6.2%

I I

^{Cao 2.6%}

Table 2. Chemical analysis of coke

We have not, however investigated if the samples were representative for the ores they used.

2.2 EQUIPMENT

The equipment used for the experiments was a thermobalance apparatus, with which it was possible to

I

weigh a crucible with weight up to 1.2 kg. The crucible was made of Kanthal APM, an FeCrAl alloy.

The crucible can be placed in a furnace, which can be moved up and down by a pulley system. Figure 1 shows the crucible and Figure 2 the experimental setup.

Thermocouple tube Gas outlet tu

T

33cm

Figure 1 Crucible

Lid

Inside 0=5,3cm

Wall thickness

=0,2cm

2.3 CHARGE COMPOSITION

To prepare the charge, the ore was heated in the furnace in an argon atmosphere at 1100 °C. The coke was similarly calcined, and we observed a weight loss of 1,8

%, which is equal to the content of volatiles.

Balance

The charge for each experiment was mixed of 100 g c calcined ore and 20 g of coke. For the experiments wit Comilog ore this gives a molar ratio of 0.25 mol Mn304/mole C, which is a typical ratio for a FeM process.

2.4 EXPERIMENTAL PROCEDURE The furnace is heated to the set temperature for th experiment (1000 to 1100 °C). The mixed charge of or and coke is poured into the crucible. The gas tubes an thermocouples are set in place and the crucible is hun in the wire attached to the weight. Argon gas is blow trough the crucible for 5 minutes before the furnace i elevated to the position where the lid is in height wit the furnace top. The argon gas is switched off. When th temperature in the crucible has reached 850 °C, th weight is tared.

The gas produced during the experiment leaves th crucible through a tube and goes through a silicon tub to an IR gas analyzer measuring the content of CO an C02.

After the reduction experiment has run the wanted tim (in the present experiments preferably to the time whe1 further weight loss is negligible), argon is again blow through and the furnace is lowered. After the crucible i cooled, the resulting charge is weighed, and also spli manually in a reduced ore fraction and a coke fractior which are separately weighed.

3. EXPERIMENTALRESULTS 3.1 CALCULATIONS Datalogger (PC)

Gas ^-·---·-·-·-·-·-·-·-·-·-·-···-···-·-·-·-·-·-·-·-·-·"

0

...

_Q)

'"C

c

>.

(.)

Crucible

'

r·····-··-···-···i l

!

Gas analyzer Gas '---3>-outlet L. ... 1

The experimentally measured values of weight loss ,

represent the total weight loss, Wtot; of both ore and coke. The weight losses of ore ^Wm are calculated by help of the analyses of the outgoing gas, and the calculated values are also checked with the results of weighing the manually separated ore fractions.

Ore reduction:

(1)

Boudouard reaction:

ZC0₂ + ZC = 2ZCO (2)

Adding these two equations we obtain:

Mn₃0₄ +ZC=3Mn0+(1-Z)C0₂ +(2Z-l)CO (3) Coke consumptiopn per mole of removed oxygen is calculated by :

Z = 1 /(1 + %C0₂ 1100) (4)

The relation between the derivatives of W101 and Wm at any time is calculated by:

k = dW101 I dt = 3Z + 1

dWm I dt 4 (5)

The reaction rate (mole Mn304 per unit time) is given by the equation:

dnMn304 dt

1 dW101 - -·- - l6k(t) dt

The weight loss of ore is given by:

(6)

(7)

, The weight losses are measured as discrete points. The integral is calculated by the trapezoid method:

where:

225

f

(t) = _1_. dW101 (t)

k(t) dt (9)

and

(10) The coke consumption is calculated by:

dnc =

z.

dnMn304

=_i.

^{(k -} l). dnMn304

& &

3

&

⁽¹¹⁾

4. RESULTS AND DISCUSSION

4.1 DATA REGISTERED

Figure 3 shows temperature, total weight loss, % CO and

% C02 as function of time for Experiment No. 3, which was conducted with 2-4 mm Comilog at 1100 °C.

0.5

-0.5 .1 -1.5

·2

·2.5 .3 -3.5

~ -4

s

"' -4.5

"'

~ -5 .c .!2'·5.5

..

3: -6 -6.5 .7 -7.5 -8 -8.5 .g -9.5 -10 -10.5

·11

\

I

I

I

COM_2-4mm_Coke_4-6,3mm_1100°C Time [min]

4 8 12 16 20 24 28 32 36 40 44

h-.

(1)

\ v

⁴⁾ \

y

_\

,;

_\

I

\

I

I

^A. \

I ,\

I \ '

^{(2) (3)}

I \ \

/

-

I \

\

^v

\

\

I I

\ I

\

~

\

^{I \}

\ \

~

\ I

^{\ \}

I

"

^\

J ~

I

~-

v

I

I/ _\~

- (1)Weightloss -(2)C02 -(3)CO - (4)Temperature

Figure 3. Registered data in exp. No. 1

1200

1100

1000

900

800 5' ~

700

.

^~

ii)

"'

(!) 600

"

^c:

"'

u

500 re:

E

..

I- 400

300

200

100

6.5

5.5

4.5

i ⁴

~

~

~ 3.5 .c O>

~ 3 2.5

1.5

0.5

(2)

- - -

I

^{/ / (4) (1j_ ~} ""i3i""

-

,/ /'

^{" /}

l /

II I

If

j

I j

I

I /

/

I j

^I/

I

u

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60

Time [min]

- (1 )COM_2-4mm_ Coke_ 4-6,3mm_ 1 ooo•c -(2)COM_2-4mm_Coke_ 4-6,3mm_ 11oo•c - (3)COM_2-4mm_ Coke_2-4mm_ 1 ooo•c -(4)COM_2-4mm_Coke_2-4mm_ 11 oo•c

Figure 4. Weight loss of COMILOG ore 2-4 mm and two coke sizes

65

55

45

"

"

035

E

~ Cl 3

25

15

05

... -·-·-^... ···•···•••· ····-·· ···-···· ... ·--··· .

v

^{i--- l/ 121 I...-' v}

v

^{I~ t/ l,? I/ !sli (~}

v

^v v v

; I / I

^{I/ [/}

WJv

/ /

v

y

_{I I}

Vvv

I II

vv

J j / /

I ; I / I v

ff

J; /,

/

I

/I

I I

II P

^Jj

II v

'!/ I

~

h0

0 4 8 12 18 20 24 28 32 36 40 44 48 52 56 60 64 68 Tlme[min]

- (1)COM_ 4-6,3mm_ 1ooo•c -(2)COM_4·6,3mm_1 oso•c - (3)COM_ 4-6,3mm_ 11oo•c - (4)BHP _ 4-6,3mm_ 1ooo•c -(5)BHP _ 4·6,3mm_1oso•c -(&)BHP_ 4-6,3mm_11oo•c

Figure 5. Weight loss of COMILOG and BHP ore; Coke size = 4-6,3 mm

4.2 WEIGHT LOSS OF ORE

With the COMILOG ore a weight loss of 6.7<

corresponds to 100 % conversion from Mn304 to Mn(

With the BPH ore a weight loss of 6.0 % corresponds 1

100 % conversion.

Figure 4 shows the weight loss for Comilog ore, 2- mm, reduced with two different size ranges of coke; 2- and 4-6.3 mm at temperatures 1000 and 1100 °C. W observe that the size· of coke has no effect at 1 OOO °C. } 1100 °C the reduction actually has gone faster with tl greater coke size, but the difference is small. This is ni

unexpected; as long as we have a big surplus of C, tl coke size does not matter.

The temperature has a great effect. When tl temperature is increased from 1 OOO to 1100 °C, the tim needed for 90 % conversion decreases 65 %.

In Figure 5 the weight losses with Comilog ore ai

compared to weight losses with BHP ore. We obsen that the times needed for a conversion of 80 % or m01 are approximately the same for the two different ores (8

% conversion is equivalent to 5.4 % weight loss for tl COMILOG ore and 4.8 % for the BHP ore).

4.3 REACTIONRATE

Figure 6 shows reaction rate as function of time for 4-6.

mm Comilog ore at temperatures 1000-1100 °C.

Figure 7 shows reaction rates calculated at 40% and 8

% conversion as functions of temperature for diff ere1 ore sizes (Comilog). We observe that:

• Reaction rate increases with increasing temperature

• Reaction rate decreases with increasing ore particl size.

4.4 REACTION MECHANISM

The total system consists of two solid phases and a g~

phase. To analyze such a system, we can use the mode presented by Szekely, Evans and Sohn 4 . As a fir:

approximation, we have in the present µi.per analyze separately the reactions in the ore and in the coke.

The fraction of unconverted ore (1-X01) is traced function of reaction time in Figure 8. Assuming shrinking-core model, Figure 9 show the dimensionle~

radius of the unreacted core (rcfR) as function ofreactio

time. Comparing these curves, with the theoretical

::urves, given by Szekely, Evans and Sohn ⁴ or by

Levenspiel⁵ it seems that several mechanisms are ietermining the rate of the reduction reaction. The results given as fraction of unconverted ore indicate that either chemical reaction control or diffusion through product layer or both is rate controlling. When the results are plotted as dimensionless radius of the unreacted core, they indicate chemical reaction control or mixed control.

Figure 10 shows the reaction rate (ln[ r]) as function of 1/T, calculated from the results given in Figure 7. From these results, Table 3 gives the activation energies calculated for the temperature ranges 1000-1050 °C and 1050-1100 °C

co

Temp. range

o c

_kJ/mole

1000-1050 207 1050-1100 121 Table 3 Activation energy

0.03

0.025

0.02

~ 0

~ 0.015 :;:

..

' 0

.s

0.01

0.005

0

J

I

r\

\

I

^\

\

1

,,.... _...

\

I \ ' ~

^{~ \ ... '\,}

'

COM 6,3-9,5 X=0,4 kJ/mole

194 111

...

-

^{... 1 J}

2) !'.... r-....

... r-....

I'-

- - -

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 . Time [min]

- ( 1 )COM_ 4-6,3mm_ 1 ooo•c -(2)COM_4·6,3mm_1050°C -(3)COM_4·6,3mm_11oo•c

Figure 6. Reaction rate as function of time; Coke size=

'4-6,3 mm

227

0.04 ···-·-·-··-···-· -··-·-·-·---·-· .... ··· -·---;:;;;;~~x~o.4

c ~

0

~ 0.02 t---t-r--t---r-t---,ol"'----7""1~--<

:;:

..

0

.s

0.015 -t--+---t~,.._-+-__.,,,.--,,...,.__ _ _ + -_ !Hnt·

1000 1020 1040 1060 1080 1100 1120

Temperature [°C]

... 2-4 mm_Xcom=0,4 -o--4-6,3 mm_Xcom=0,4 _...6,3-9,52 mm_Xcom=0,4 - 2 - 4 mm_Xcom=0,8

... 4-6,3 mm_){com=0,8 -o-6,3-9,52 mm_Xcom=0,8

Figure 7. Reaction rate as function of temperature;

X=0,4 and 0,8; Coke size= 4-6,3 mm

... ...

0.9

,~

:-.

,

0.8

\

0.7

~~~

\ l~~ I\ ' I\

0.6

·' '

^'\

\

^I\

I\

'

I

\

' \ '\,

w

~ 0.5

~

0.4

~ \ "

'

^{"\ '\,}

"" "

^{... (7}

0.3

\~ \ \ '~ "' !'-..

^{~4 1'}

0.2 l

\ ~ !\ ' ~

""'!'-

(6 r-.... ⁽⁾ ... ...

0.1

~ ~

_I"-...._ !'-,, ^~

N

^{~) ~}

"' N

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 Tlme[mln]

-(1)COM_2-4mm_1ooo•c - (2)COM_2-4mm-1 oso•c -(3)COM_2-4mm_ 11oo•c _ (4)COM_ 4-6,3mm_ 1ooo•c -(5)COM_4·6,3mm_105o•c -(6)COM_ 4·6,3mm_11oo•c -(7)COM_6,3-9,52mm_ 1ooo•c - (8)COM_6,3-9,52mm_ 1 oso•c -(9)COM_6,3-9,52mm_ 11oo•c

Figure 8. Conversion of Mn₃04 as function of reaction time, coke = 4-6,3 mm

~K···

1~1\

0.9

~~

^\

'~\ \I\

^\

' I ~\ I \

_I\

I \ I\

\

^\

\

^I\

0.8

0.7

\

^\ () ^I\

I \

^{\ I"\}

~

g

^0.6

0.5

0.4

0.3

0.2

~ \

^{\ I\ ()}

1 \[\I\ ^\

\

_I

~"

^\

1\1"\

13)

\K

\ \tn

\

\ \

I

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88

Time [min]

-(1)COM_2-4mm_ 1DOO'C -(2)COM_2-4mm-1050°C -(3)COM_2-4mm_ 11DO'C -(4)COM_ 4-6,3mm_ 100D'C -(5)COM_ 4-6,3mm_ 1050°C -(6)COM_ 4-6,3mm_ 1100'C

Figure 9. Progress of reaction; Mn304 reduction

-3,--- -··· -·--···---·----··· ·---- - -···-- - - ,

Z"

~ 0

~ -4.5 +---~

~ c

~ .:

',,,

-5;---.,--~,-,·~,-,,-""<---1

"'--"··

' , , "~ ⁽⁸⁾

-5.5 ;---'~~-~6,,_..

' , , (9)

-6;---~----~-~--~---l

0.72 0.73 0.74 0.75 0.76 0.77 0.78 0.79

1 ooorr [1/KI -(1) 2-4 mm_Xcom=0,4

-(3) 6,3-9,52 mm_Xcom~,4

-···-·· (5) 4-6,3 mm_Xcom=0,6

··· ··· (7) 2-4 mm_Xcom=0,7

···(9) 6,3-9,52 mm_Xcom~,7

-(2) 4-6,3 mm_Xcom=0,4 - (4) 2-4 mm_Xcom~,6

···--· (6) 6,3-9,52 mm_Xcom=0,6

··· (8) 4-6,3 mm_Xcom~,7

Figure 10. Logarithmic reaction rate as function of 1/T

The activation energy is about 200 kl/mole in the 1001 1050 °C range, which clearly indicates chemical reactic control, and the difference between the finest and tl largest fractions is small. But the activation energy substantially lower in the 1050-1100 °C rang1 indicating perhaps a change in mechanism for high temperatures.

The fraction of unreacted coke (1-Xc) is plotted <

function of reaction time in Figure 11. Analyzing the:

curves we must bear in mind that there is a large surph of coke, as in the industrial process. 100 % conversion <

ore corresponds to consumption of about 20 % of tl coke. Actually the curves in Figure 11 shows the sarr trends for the individual experiments as Fig. 8 do. Th shows that for these experimental conditions, and wi1 the type of coke used, the Boudoard reaction seem 1 have little influence.

0.98

0.96

0.94

0.92 Q;' .><

3 ^0.9

>::

0.88

0.86

0.84

0.82

0.8

' ~~ ~ \~ ~ ' ~ \ \

'

\ \

_\

'

\ ~

' \

"

I\ "-

^...

~ '

!'..

\ !'...

'

'

' '

^...

,

r\ _\

'

_~ ^I'\

'

^...

, '

^{(4) ~ (7 .....}

"'

⁽⁸⁾ r--..._

~

^....

, I °"'

^{(1) ...} ...

r\.. ~ ^{I"-.. to--} ,...

- -

I"

'" (;,Jr--

~

^'-Ill _~

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 nme[min]

- (1)COM_2-4mm_1DOO'C -(2)COM_2-4mm-1 oso•c - (3)COM_2-4mm_ 11oo•c -(4)COM_ 4-6,3mm_1 OOO'C - (S)COM_ 4-6,3mm_ 1050°C -(6)COM_ 4-6,3mm_11 oo·c - (7)COM_6,3-9,52mm_ 1 OOO'C ....;....(S)COM_6,3-9,52mm_1 oso·c - (9)COM_6,3-9,52mm_ 110D'C

Figure 11. Consumption of coke;

Coke size

=

4-6,3 mm

5. CONCLUSION

It has been an aim for the present work, to analyze th reaction system in a FeMn process at the last gas-soli reduction (to FeO). It was chosen not to feed CO gas t the reactor. These experiments can not be direct!

:ompared with the industrial process. But by performing

·eduction experiments in laboratory scale at different :onditions; with and without feed gas, with different ores md cokes, varying coke/ore ratios, size ranges and emperatures, we hope to obtain more general reduction ate parameters. So far the present results show that the :hemical reaction in the ore to a great extent controls the otal reaction rate for the reduction of Mn30 4 to MnO.

[o evaluate the Boudoard reaction, and thus coke :onsumption in an industrial process, experiments where

¥e also feed CO-gas to the reactor shall be conducted.

ACKNOWLEDGMENTS

[he authors thank the Research Association of

'-Jorwegian Ferroalloy Producers (FFF) and the Reearch :::ouncil of Norway for their financial support.

REFERENCES

l. Berg KL, Olsen SE. Kinetics of Manganese Ore

~eduction, Electric Furnace Conference Proceedings, SS, 54 (1996), 217-226.

~. Berg KL, Olsen SE. Kinetics of Manganese Ore f?.eduction by Carbon Monoxide, Metall. Trans. B, Vol 31 B (2000), 4 77-490.

L Stalheim LA. Kinetics for Pre-reduction of

\!fanganese Ores (In Norwegian), MSc Thesis,

\Jorwegian University of Science and Technology )998).

t Szekely J, Evans JW, Sohn HY. Gas- Solid Reactions, t\cademic Press, New York (1976).

;. Levenspiel 0. Chemical Reaction Engineering, John Wiley, New York (1999).

NOTATION

(=k(t) Ratio between the derivatives ofW tot and Wm

'IA Moles of substance A (A=Mn304, C, ·:·) R Radius of ore particle, [cm]

Reaction rate, [molls]

"c Radius of unreacted core, [cm]

I' Temperature, [K]

Time, [s]

W= Wm Weight loss of ore, [g]

Wtot Total weight loss, [g]

¥=Xm Fractional conversion ofMn30 4 to MnO

Xc z

Fractional conversion of C

Moles coke consumed per mole of removed oxygen