Chlorination of Commercial Molybdenite Concentrate in a Fluidized Bed Reactor

K.U. NAIR, D. SATHIYAMOORTHY, D.K. BOSE, M. SUNDARESAN, and C. K. GUPTA

Studies on recovery of molybdenum from commercial grade molybdenite using the technique of fluidized bed chlorination in the presence of oxygen are presented. Molybdenum recovery above 99 pct at a chlorine utilization efficiency of 84 pct has been achieved for a fluidizing gas flow-rate of 3 L/min of the gases Clz, 02, and Nz mixed in the proportion of 2: 5 : 23, respectively, at 300 ~ The investigations on kinetics showed that the overall oxychlorination reaction is controlled by chemical reaction and is of first order with respect to particle surface area.

I. INTRODUCTION

MOLYBDENITE, the principal source of molybdenum, occurs in nature as disseminations in igneous rocks. It is also recovered as a byproduct in the processing of copper min- erals. The latter source contributes about 80 pct of world production.

Naturally-occurring molybdenum ore is beneficiated by a simple froth flotation technique to commercial grade molyb- denite concentrate (98 pct MoS2). In the conventional pro- cess of molybdenum extraction, the concentrate is roasted to molybdic oxide which forms the starting material for almost all other molybdenum products. Roasted molybdic oxide as such, or ferro-molybdenum smelted from it, are used as steel additives, and constitute about 85 pct of the world production of molybdenum. For the production of pure molybdenum, roasted oxide is further purified by vacuum distillation followed by ammonia leaching and calcination. Pure MoO3 thus obtained is reduced to metal by hydrogen at 1100 ~

Chlorination of sulfide ore as an altemative to roasting has drawn considerable interest for the processing of low grade sulfide concentrate. The high reactivity of chlorine, relatively low temperature operation, and possibilities of recovering sulfur in a nonpolluting form are some of the advantages that can be harnessed from this process. In re- cent years, a considerable amount of work has been reported on chlorination of common metal sulfides. Senderoff and Labrie I were the first to chlorinate molybdenite concentrate at 823 K to form molybdenum pentachloride for use in the fused salt electrorefining of molybdenum. Hayer et al . 2 in their attempt to recover sulfur by chlorination at 675 K ended up with a product consisting of MoC15 and $2C12. In a recent work from this laboratory, the authors 3 have shown the feasibility of recovering molybdenum and other associ- ated metal values from a low-grade Indian molybdenite- concentrate by static bed chlorination in the presence of oxygen at 573 K. However, as fluid beds have many merits compared to static beds, especially for exothermic reac- tions, it was considered worthwhile to develop them for scaling-up the process. The design parameters evolved from these studies have been applied to recover molybdenum

K.U. NAIR, D. SATHIYAMOORTHY, and D.K. BOSE, Scientific Officers, and C. K. GUPTA, Head, are with Metallurgy Division, Bhabha Atomic Research Centre, Bombay 400 085, India. M. SUNDARESAN is Head, Electro Analytical Chemistry Section, Analytical Chemistry Division, BARC, Bombay 400 085, India.

Manuscript submitted September 3, 1986.

from low-grade Indian molybdenite concentrate, and the results will be presented in a separate paper.

II. REACTION PRINCIPLES

The reaction between MoSz and C12 in the presence of oxygen is

MoS2 + C12 + 302 ) MOO2C12 + 2SO2 [1]

while with MoS2 and C12 alone,

2MOS2 + 7Clz > 2MOC15 + 2S2Clz [2]

The standard free energy change of chlorination (AG~98) of the former reaction is more negative than that of the latter. Therefore the reaction between MoS2 and C12 in the presence of oxygen is more readily feasible than the reaction without oxygen. However, the kinetics of the reaction should also favor the feasibility and it should be established by experi- ments. The vapor pressure vs temperature data 4,5 favor the selective separation of oxychloride formed by distillation in a single stage operation. Thus, considering the ease of for- mation of MoOzC12, its stability, low temperature volatility, etc . , the processing of molybdenite concentrate with a mix- ture of C12 and O2 in a fluid bed reactor seems to be tech- nically very promising. The oxychlorination reaction ~ may be expected to follow firstly the oxidation of MoSz to MoOz and then the chlorination. The formation of any MoO3 in the presence of MoS2 is ruled out.6 According to Galateanu 7 the roasting stage is the rate determining step and is controlled by chemical reaction.

IlL EXPERIMENTAL

A schematic representation of the experimental set-up is shown in Figure 1. A 40 mm diameter, 600 mm-long pyrex glass column fitted at the bottom with a sintered glass gas distributor and at the top with an enlarged disengaging sec- tion (80 mm diameter, 300 mm length) was used as the reactor. The disengaging column fitted at the top was useful to decrease the elutriation rate of fines from the reactor. A split tube furnace with a Nichrome wire element was used to heat the reactor to temperature which was monitored with a 30 SWG iron-constantan thermocouple sheathed in a thin-bore Pyrex tube extending from the top to the center of the bed.

METALLURGICAL TRANSACTIONS B VOLUME 18B, JUNE 1987--445

|

I. GAS PURIFICATION TRAIN

2. GAS FLOW M E T E R

:3. GAS MIXING CHAMBER

4. GAS PRE HEATER

5. D ISTRIBUTOR P L A T E

6. DISENGAGING COLUMN

7. T H E R M O C O U P L E S

8. M O N O M E T E R

9 . CYCLONE CONDENSER

I0. RECIVER I1. COOLING COIL

12,13 SCRUBBERS

f / c - FURNACES

Ct 2 - CHLORINE GAS

N 2 - N I T R O G E N GAS

0 2 - O X Y G E N GAS

f b s - F L U I D I Z . E D BED SECTION

Fig. I - -Apparatus for molybdenite oxychlorination.

N2 _ t

Technical grade molybdenite concentrate of average par- ticle size 75 /xm, procured from Climax Molybdenum Co., USA, was used in the experimental work; chemical analysis is given in Table I. A gas mixture containing the requisite proportions of nitrogen, oxygen, and chlorine was used as fluidizing and reacting gas. The gases from the cylinders were dried by passing through silica gel and CaCI2 columns, and then metered by rotameters.

The first set of experimental runs was carried out on a 50 gm batch (bed height 8.5 cm) with the fluidizing gases C12, 02, and N2 mixed in the proportion 2: 5 : 23 at 3 L/min, well above the minimum fluidization rate of 0.2 L/min. The temperature range was 200 ~ to 350 ~ To start each run N2 alone was used to fluidize the bed till the desired tempera- ture was reached. Once the reaction was initiated, the power supply to the furnace was lowered as the heat released was

Table I. Chemical Composition of Technical Grade Climax Molybdenite (92 to 98 Pct MoS2)

Molybdenum 53.0 pct w/w Silica 3.7 pct w/w Sulfur 35.6 pct w/w Nickel 0.005 pct w/w Copper 0.04 pct w/w Iron 0.34 pct w/w Balance moisture and oil

|

/ c 0 o (

|

o ,

f /c

! I

Hg

I@i

t

@

sufficient to sustain the reaction. The experiments were carried out for periods of 10 to 110 minutes.

Once the chlorination was over, the unreacted solids in the column were kept in a fluidized state by the flow of N2 alone. The oxychloride of molybdenum, a yellowish-white powder, was collected in a cyclone condenser, dissolved, and analyzed gravimetrically 8 for its molybdenum content.

Other series of experiments investigated are the effects of gas flow rate and composition, and the particle size of the concentrate.

IV. RESULTS AND DISCUSSION

The degree of conversion of MoS2 achieved for various reaction periods and temperatures ranging from 250 to 350 ~ is shown in Figure 2. The conversion of MoS2 in- creases rapidly with temperature, and almost 100 pct con- version is achieved for a reaction time of 50 minutes at 350 ~ Because the complete conversion of MoS2 to MoOzC12 is achieved about 50 minutes from the start-up of the reaction, over the temperature range of 285 to 350 ~ we selected the intermediate temperature 300 ~ for further experiments.

The effect of gas flow-rate was studied at a C12: O2 ratio of 2:5. It can be seen from Figure 3 that the conversion of MoS2 steadily increased with gas flow rate from 0.75 to 2.5 L/min, remained almost constant up to 3 L/min,

446--VOLUME 18B, JUNE 1987 METALLURGICAL TRANSACTIONS B

I 0 0 1 I

9 0

8 0

70

60

_g '~

N 2~ I

I

1:3 - 3 5 0 " C

& - 3 0 0 " C

�9 - 2 8 5 " C

A - 2 7 5 2 C

�9 - 2 5 0 C

o I I O 3 0 5 0 7 0 9 0

{ - TIME IN MINS.

Fig. 2 - - C o n v e r s i o n of MoS2 with respect to t ime.

I I I 1 0 130

I 0 0

a 9 0 El I - w 8 o h i

z 7 o 0 u

6 0 e,i

(/) o 5 0

h 4 0 0

3oi ~ 2 0 Q_

I o

0

P A R T I C L E SIZE ; 7 5 /..tin

T E M P E R A T U R E : 3 0 0 " C

R E A C T I O N T I M E = 5 0 min.

C ~ 2 : 0 2 : N 2 : 2 : 5 : 2 3

I I I I I I

I 2 3 4 5 6

TOTAL GAS FLOW RATE ( L P M ) Fig. 3--Effect of gas flow rate on MoSz conversion.

and then decreased abruptly beyond 4 L/min. The fall in the conversion of MoS2 at higher gas flow rate may be attributed to slugging in fluidization and also insufficient heating of the gases. From these studies a gas flow-rate of 3 L/min was found to be optimum for maximum recovery of molybdenum.

The influence of gas composition was studied at 300 ~ by varying the 02 to C12 ratio from 0.5 to 4.0 while keeping total gas flow-rate at 3 L/min for a period of 50 minutes.

The details on the volume fractions of gases are shown in Figure 4. The conversion (Figure 4) increased steadily up to a ratio of 1.5, then shot up to 99 pct at 2.5, and remained steady thereafter, giving a chlorine utilization of 84 pct.

A series of runs was conducted at 300 ~ with particle sizes from 75 /zm to 400/xm. The results are presented in Figures 5(a) and 5(b). Experiments with particles outside this range were not tried as the fluidization characteristics were not satisfactory. From Figure 5(a), it is clear that the maximum conversion of MoS2 was achievable with a par- ticle size of 75 tzm. Since the average particle size of the as-received commercial grade MoS2 concentrate was in this size range (70 to 80/zm), it could be used directly.

The conversion of MoS2 dropped to 65 pct when the particle size was increased from 75 to 200/xm. In the par- ticle size range of 200 to 400 /zm the drop was not very appreciable. Thus the conversion of MoS2 varied inversely with increasing particle size. However, a distinct change in the falling trend (Figure 5(a)) occurred near 200/xm par- ticle size. In general one might expect the fall in conversion to continue along the extrapolated dotted line as in Fig- ure 5(a) without a marked change at 200/xm. The increased level of conversion in the size range 200 /zm to 400 /zm as against the expected values may be attributed to the quality of fluidization which would improve when the solid par- ticles in this size range tend to behave more like Group A powder (i.e., aeratable type) of Geldart 9 classification. The test for the quality of fluidization during the reaction is a practical problem. Hence, our prediction needs further clari- fication in the future by investigating the mechanism of the reaction also.

I 0 0

9 0

8 0

70

o4

~o 6 0 =E

u. o

5 0 z 9

.~ 4 o z u

N 3 0

20

IO

0 0 . 5 I

02/Ct2 RATIO 02 : Ct2 : N2

"0,5 5 : I0 : 135 1 . 0 I0 : tO : 130 1 .5 15 : I0 ; 125 2 .0 20 : I0 : 120 2 .5 25 : IO ; 115 5 .0 30 : I0 : IlO 3.5 35 : I0 : 105 4.0 40 ; I0 : IO0

CHARGE : .50 g

PARTICLE SIZE ; 75 /J m

TEMPERATURE : 300~ REACTION T I M E ; 5 0 min. FLOW RATE ;3 LPM

1.5 2 2 . 5

02 / C L 2 R A T I O

!

5 3.5

Fig. 4--Effect of 02 to C12 ratio on MoS2 conversion.

METALLURGICAL TRANSACTIONS B VOLUME 18B, JUNE 1987--447

1001 f ~ CHARGE ; 50 O ~ TEMPERATURE : 300*C (5 q) ~ ' ~ REACTION TIME: 50 rain

90 U.~ Ct2; 02 :H2 : 2 - 5 : 2 3 FLOW RATE ; 3 L P M

u 60

N i

5O

I I \ 1 I 801" I00 200 300 400

60

r-I

--? 40 X 2.

= 2 0 / I o t_l

I I I I l I 20 40 60 80 I00 120 140 160

- I I / ap (g )

Fig. 5 - - E f f e c t o f conversion of MoS2 on particle size (dp).

If the moles of MoS2, z, reacted is dependent on surface area of the particles A, then

where

dz - - = - K I A " 131 d t

A = (TrNp) '~3 ( 6 M / p , ) z'~ zZ~J/C, 141 For a first order reaction, ( i .e . , n = 1), substitution of Eq. [4] in [3] results in

so Z z/~ - K d t [5]

where

K = A / Z 2J3

The solution of Eq. [5] is

1 Z j/3 = Z~ n - - ~ K t [6]

The equation is similar to the one proposed by Maniah et al. lo The plot obtained as in Figure 6 fits well with Eq. [6], confirming the proposed model.

When chemical reaction is the controlling resistance, the gas-solid reaction model" which relates the conversion of solid, X, with reaction time t is

t = (p, dp)[1 - (1 -X)I/3]/2K2CAg [7]

The plot on the variation of the parameter [1 - (1 - X) v3] with t in Figure 7 follows Eq. [7], establishing the fact that the oxychlorination of MoS2 is controlled by chem- ical reaction.

I CHARGE : ,50 g 4 - -

PARTICLE SIZE : 75 ~m TEMPERATURE 300~ CL2: 02: N Z

--~N 21 ]_ :3LPM

o I 1 i I I I " ~ I I0 20 30 40 50 60 70

t - rain

Fig. 6 - -Var ia t ion of Z "3 with time.

I 80

1

I

,o!

0.7 ~ ' / CHARGE `50 g

0.5 / -PARTICLE SIZE 7`5~m TEMPERATURE 300"C

0.3 Ct2102 ~ N2 2 : ~ : 23 0,1 ~ ~ P ~ ! ! RZ[ 3 L PM

I L t i I 1 t .. I I0 20 30 40 50 60 70 80

t - rain.

Fig. 7 - - V a r i a t i o n o f I - ( I - .t) t'~ wi th time.

NOMENCLATURE

CA~ concentration of reacting gases (gm �9 mole �9 cm 3) C, shape factor ( - ) dp particle diameter (/zm) K surface area parameter as in Eq. [5] (cm 2) K~ rate constant with respect to surface area

(gm" mol/cm 2. S) K2 rate constant as in Eq. [7] (cm/S) M molecular weight of MoS2 (gm) Np number of particles ( - ) t time (s) X fraction of MoS: reacted ( - ) Z weight of MoS2 remaining in the bed at time t

(gm. mole) Z0 weight of MoS2 remaining in the bed at time t = 0

(gm �9 mole) Ps molar density of MoS2 (gm �9 mole �9 cm -3)

REFERENCES

1. S. Senderoff and R. Labrie: J. Electrochem Soc., 1955, vol. 102, pp. 77-80.

2. F.P. Hayer, K. Uchida, and M. M. Wong: U.S. Bur. Mines, RI 7185, 1968.

3. K .U . Nair, D. K. Bose, and C. K. Gupta: Mining Engineering, 1978, vol. 30 (3), pp. 291-96.

4. J .H . Canterford and R. Colton: Halides of Second and Third Row Transition Metals, John Wiley, New York, NY, 1968.

5. J .H . Canterford and R. Colton: Halides of the First Row Transition Metals, John Wiley, New York, NY, 1969.

448--VOLUME 18B, JUNE 1987 METALLURGICAL TRANSACTIONS B

6. A.N. Zelikman, O. E Kesin, and G. V. Samsanov: Metallurgy of Rare Metals, Translated from Russian, Israel Programme for Scientific Translation, 1966.

7. I. Galateantt: Rev. Chim. Buc., 1957, voL 8, pp. 363-68; Chem. Abstr., 1957, vol. 51, p. 16062.

8. A.I. Vogel: A Textbook of Quantitative Inorganic Analysis, 3rd ed.,

The E.L.B.S. and Longman's Green and Co. Ltd., London, 1962. 9. D. Geldart: Powder Technol., 1973, vol. 7, pp. 285-92.

10. A.A. Maniah, D.S. Scott, and D.R. Spink: Can. J. Chem. Eng., 1974, vol. 52, pp. 507-14.

11. O. Levenspiel: Chemical Reaction Engineering, 1st ed_, Wiley Eastern Pvl. Ltd., New Delhi, 1969.

METALLURGICAL TRANSACTIONS B VOLUME 18B, JUNE 1987--449