3,770,414 Recovery of rhenium and molybdenum values from molybdenite concentrates

Patent Number/Link:

United States Patent [19]

Lake et at

[IIJ 3,770,414

[45] Nov. 6, 1973

[52] U.S. CL 75/1, 75/7, 75/84,

75/121,423/53, 423/592

[51] Int. Cl. C22b 1/02

[58] Fieldof Search 75/1, 2, 6, 7, 9,

75/21, 23, 26, 36, 84, 121, 117; 423/592, 22,

[54] RECOVERY OF RHENIUM AND

MOLYBDENUM VALUES FROM

MOLYBDENITE CONCENTRATES

[75] Inventors: James L. Lake; John E. titz; Robert

B. Coleman, all of Lakewood, Colo.;

Marcel Goldenberg, Tarrytown,

N. Y.; Milos Vojkovic, Libertyville,

Ill.

[73] Assignee: Continental Ore Corporation, New

York, N.Y.

[22] Filed: Dec. 28, 1970

[21] Appl. No.: 101,784

[56]

2,809,092

2,345,067

2,234,378

3,455,677

3,117,860

3,458,277

3,196,004

References Cited

UNITED STATES PATENTS

10/1957 ZimmerIey 75/121 X

3/1944 Osann 75/7

3/1941 Loring " 75/7

7/1969 Litz 75/117

1/1964 Bjerkerud 75/121

7/1969 Platzke 75/121

7/1965 Kunda 75/121

3,348,942 10/1967 Davenport 75/63

3,579,328 5/1971 Aas 75/53

2,579,107 12/1951 Bertolus 423/592

1,970,467 8/1934 Mayr. 75/1

FOREIGN PATENTS OR APPLICATIONS

1,265,486 3/1972 Great Britain 75/1

Primary Examiner-L Dewayne Rutledge

Assistant Examiner-Peter D. Rosenberg

Attorney-Sheridan, Ross & Burton

[57] ABStRACT

An improvement in the recovery of molybdenum and

rhenium values by roasting molybdenite which comprises

preheating finely divided molybdenite concentrate

and passing it downwardly through a vertically

oriented reaction zone countercurrent to an upwardly

traveling stream of high temperature oxygen, oxygen

enriched air or oxygen-SUlfur dioxide mixture heated

by its passage through a roasting !hearth at the bottom

of the reaction zone. The rate of oxidation of sulfides

is controlled by various means to keep the temperature

in the reaction zone below that at which molybdenum

oxide volatilizes with resultant inhibition of the volatilization

of rhenium oxide. The process is attractive from

the standpoint of pollution control as by-product sulfur

dioxide gas ordinarily released to the atmosphere is

produced in the exhaust gases in concentrations high

enough to make its recovery ecolllomically feasible.

14 Claims, 2 Drawing Figures

MOLYBDENUM SULFIDE

CONCENTRATE

LIQUID OXYGEN

SULFUR

DIOXIDE

AMMONIUM HYDROXIDE

OXYGEN

GASES~GASES

SOLUTION

UNDERFLOW

IFI+ER CSOLUTION

SOLIDS ----'-------'-__

AMMONIUM MOLYBDATE

AMMONIUM PERRHENATE

SOLUTION t TO MOLYBDENUM

AND RHENIUM RECOVERY

ID'PER

PRECIPITATE

SOLUTION

TO WASTE

MOLYBDIC OXIDE CALCINE

GASES --1SCRUBBERh*~~~~~E

• . I

SOLUTION

'----_+_ RAFFINATE

PATENTED NOV 6 /973 3.770.414

SHEET 1 OF 2

INVENTORS

JAMES L. LAKE

JOHN E. LlTZ

ROBERT B. COLEMAN

MARCEL GOLDENBERG

MILOS VOJKOVIC

~~,,~

ATTORNEYS

tJ.q:. 1

//~

35 -~=_._=-=""'-"'''''

34 32

22 35

/7-

18 17

52.--..Ir-J"i1-N

...TO SCRUBBERS

-."

c.H

0- r-rl ::z

--<

,..."

I::::'

::IE =-<

-..! o

..l:J.

.t>.

:""-:l

~ ,

i wi

r'~

=..."

...,

--<

OXyGENJ

LIQUID

SULFUR

DIOXIDE

AMINE

SOLVENT I---AMMONIUM HYDROXIDE

STRIPPING

UNDERFLOW

~-SOLUTION

~SOLIDS I

AMMONIUM MOLYBDATE 2

AMMONIUM PERRHENATE T.l.q:

SOLUTION

TO MOLYBDENUM

AND RHENIUM RECOVERY

OVERFLOW

WASH

WATER

COPPER

PRECIPITATE

SOLUTION '

COPPER

,CEMENTATION

SOLUTION

TO WASTE

, I .. RAFFINATE

TECHNICAL GRADE

MOLYBDIC OXIDE

WATER

MOLYBDENUM SULFIDE

CONCENTRATE J tr------,------------

["PRIHEATER I OXYt_EN --.

~AsH1oAsTER GASES ~GASES

OXIDE CALCINE -rt SOLUTION

AIR~ RE-ROASTER I ~ GASES ~ ISCRUBBERh~ASES TO

I J f A MOSPHERE

SOLUTION

r:-<

~~::oc...c...

. r-)::,OO)::,

O::oOJ::r:~

(f)\)~<:n,

, ~.., (f)

~ ..,.,fTlr-

~G)~ .

c... 0 r-rh

1- ., r~b8=i)::, d OSn<r:r-~!'.JF~1)2

::0 (") OJ )::, '"

<: ~<: n,

n-<, G) .<.,:

(I) (°::f0)

3,770,414

The process will now be described with reference to

60 the accompanying drawings in which:

FIG. 1 is a partial schematic &ectional view of the

combined roaster and vertical column comprising the

flash roaster of the invention.

FIG. 2 is a flow diagram illustrating the operation of

65 the flash roasting process of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, the numeral 10 indicates a conand

countercurrent to an upwardly traveling stream of

pure hot oxygen, oxygen-enriched air or an oxygensulfur

dioxide mixture emanating from a concentrate

roasting furnace, the result being a highly effective oxi-

5 dation of the sulfur content of the concentrate to sulfur

dioxide, of rhenium and molybdenum to their higher

oxides, and volatilization of rhenium oxide.

The process begins with roasting a preheated molybdenite

concentrate in an oxygen or mixed oxygen-

10 sulfur dioxide atmosphere in a vertical flash roaster.

Sulfur dioxide is formed from the oxidation of sulfur

and will mix with the oxygen introduced or it may be

added with the oxygen to control oxidation rate. The

oxidizing gases and the molybdenum sulfide particles

15 pass countercurrentiy in the vertical section of the flash

roaster to provide maximum surface contact between

the particles and gases. The length of time of the concentrate

particles in the vertical section in countercurrent

contact with oxidizing gases is regulated to initiate

20 the oxidation of each of the particles of molybdenum

sulfide, such that each of the particles will have a layer

of molybdenum trioxide on its outer surface. The particles

pass downwardly through the vertical section onto

a rotating horizontal hearth where they are annealed

25 for a period of time at approximately 550° to 680°C. in

an oxygen or oxygen-sulfur dioxid.e atmosphere. This

annealing step permits complete oxidation of molybdenum

to molybdenum trioxide. The major portion of the

rhenium is volatilized while the concentrate particles

30 are roasted and annealed, and pass out of the roasting

and annealing furnace with the off-gases which contain

the excess oxygen, the sulfur dioxide formed and added

during roasting, and a small quantity of dust which is

incompletely roasted.

35 An improvement of the invention is the partial control

of the rate of oxidation of sulfides in the reaction

zone and thereby the temperature of the zone to keep

it below that at which molybdic oxide volatilizes or

melts with resultant inhibition of the volatilization of

rhenium oxide. Part of the heat is due to the oxidation

of the sulfides, the rate of this oxidation being controlled

by the gas stream composition and the stoichiometric

ratio of available oxygen to concentrate in the

reaction zone. This oxidation rate and the heat generated

thereby are controlled by varying the stoichiometric

ratio of oxygen to concentrate using the ratio of sulfur

dioxide to oxygen gases in the exhaust gas as a measure

of this ratio existing in the reaction area. Other parameters

affecting the operation of the process may be

varied, these being preheat temperature, height of reactor

column and heat dissipation from the column.

In the practice of the process it has been found that

less than 10% of the molybdenum and up to 95% of the

55 rhenium report to the off-gas as fUlmes or dusts and are

collected in a suitable scrubber.

RECOVERY OF RHENIUM AND MOllYBDENUM

VAUJES FROM MOlLYBDENI1l'E CONCENTRATES

BACKGROUND OF THE INVENTION

SUMMARY OF THE INVENTION

In accordance with the process of the present invention

up to 95% of the rhenium in molybdenite is consistently

recovered along with a high grade molybdic

oxide calcine. A principal feature of the process is the

provision of a reaction zone in which finely divides molybdenite

is effectively dispersed and moves vertically

The scarcity of rhenium in nature and its rapidly increasing

importance in industry emphasize the need for

a highly efficient and economical process for recovering

it from its ores. Molybdenum and rhenium are usually

found together in the molybdenite (MoS2 ) mineral

associated with the so-called "porphyry" copper ore

deposits. The molybdenite is usually separated from the

bulk of the copper minerals and is recovered as a flotation

concentrate containing from 40% to 55% molybdenum.

Then the molybdenum sulfide concentrate is

roasted to produce an oxide product containing a minimum

of sulfur, the chief objective of prior art processes

being the ultimate recovery of molybdenum. The rhenium

is volatilized during roasting and generally was

not recovered.

In presently used processes large volumes of air are

passed through the system for temperature control and

other reasons resulting in the by-product sulfur dioxide

being contained in such large volumes of exhaust gases,

mainly air, that itsrecovery is not economically feasible

and it is released to the atmosphere creating serious

presentday pollution hazards. Sulfur dioxide gas ordinarily

does not occupy much more than 1-2% of the

volume of the exhaust gases in these processes.

A process directed to the recovery from molybdenite

concentrate of rhenium along with molybdenum is disclosed

in U.S. Pat. No. 2,579,107 to Bertolus. The present

invention is directed to the recovery from molybdenite

of a high yield of rhenium either as a high grade intermediate

product or as a high grade metal along with

a metallurgical quality molybdic oxide calcine containing

a minimum of copper and sulfur impurities, all with

an efficient use of heat and materials.

The normal practice for the recovery of rhenium

from molybdenite concentrates consists of roasting the

concentrate in air utilizing a multiple hearth-type 40

roaster. A portion of the rhenium as rhenium oxide is

volatilized and is collected as a fume with the dust in

a scrubber where it is dissolved in water. Recoveries of

rhenium from the concentrate normally are in the 55%

to 65% range. An authoratative opinion by those 45

skilled in the art is that a conventional plant operated

primarily for the recovery of rhenium in accordance

with prior art procedure could effect a net recovery of

77% of the rhenium. 50

The samll concentration of rhenium oxide in the

large volume of exhaust gas resulting from the use of air

is an extreme disadvantage.

Accordingly, it is a principal objective of the invention

to accomplish a high recovery of the molybdenum

and rhenium content of molybdenite concentrates and

to produce byproduct sulfur dioxide in the exhaust gas

in high enough concentrations to warrant its recovery

for use so that it will not pass to the atmosphere as a

polluting agent.

3,770,414

heater into the reaction chamber 40. A feed screw 50

is vertically mounted in the main passageway 52 of the

upper section of the vertical column to aid in introduction

of preheated concentrate into the reaction zone

40. The feed screw is driven by conventional drive unit

54 mounted at the top of the vertical column. A feed

disperser 55 is mounted on the feed screw 50. A preheat

port 56 leads into the bottom of the reaction area

40 for introducing hot gases into the reaction zone during

start-up.

The preheater unit 44 is a conventional heater and

need not be mounted above the reaction zone as heated

concentrate from the preheater located elsewhere can

be transferred to the upper end of delivery pipe 46.

15 The operation of the process of the invention in conjunction

with the apparatus just described for a typical

operation is as follows.

A finely divided molybdenite concentrate is introduced

into the preheater 44 in an inert atmosphere and

20 brought to a minimum temperature in the neighborhood

of about 500"C. At the same time, hot gas is introduced

into the preheat port 56 and circulated through

the reaction zone 40 as a start-up procedure. Oxygen

gas may also be introduced at this time through inlet

ports 32 and circulated through the area above rotating

hearth bed 22 and out the outlet port 42.

When the reaction zone 40 has reached the required

telllperature, preferably between about 550°_ JWI

650°C., the feed screw 50 and the disperser 55 are

30 started up and preheated concentrate --is introduced

into the reaction zone 40. The feed disperser 55 effectively

disperses the s'mall concentrate particles so that

maximum surface contact of the particles with countercurrent

upwardly traveling hot gas is achieved.

The hearth bed 22 is rotating and the calcinecontinuously

leaves the hearth through an outlet, not shown,

preceded by a scraper, where it is collected and subsequently

processed to recover metal values from it. The

water cooling system 28 is used as necessary to control

the temperature of the material on rotating hearth 22.

In the reaction zone 40, sulfur dioxide is formed by

oxidation of sulfides present in the molybdenite concentrate.

This hot sulfur dioxide gas is mixed with the

upwardly traveling hot oxygen and passes out the exit

42 into the scrubbers along wi.ththe other gases, fumes

and dust where some of it unites with water in the

scrubbers to form sulfurous acid and the remainder is

collected. The sulfur dioxide produced normally occupies

from about 30-50% of the volume of the exhaust

gases. Excess oxygen which has not been consumed in

the reaction zone passes through the outlet 42 and may

be collected for recirculation through the system.

As is well known, the higher the. o'xide of rhenium the

more soluble the oxide is in water, so that maximum oxidation

of rhenium is desired for recovery in the scrubbers.

The higher oxides are more volatile. As is also

well known, rhenium oxide is formed by roasting rhenium

sulfide in the presence of oxygen at a temperature

60 between 200° and 300°C.; however, this reaction does

not readily proceed in the presence of molybdenum

sulfide. After the major part of the sulfur has been

driven off as sulfur dioxide in the reaction zone, rhenium

and molybdic oxides are formed in the temperature

range of about 500° to 650°C. The reaction zone

temperature during the oxidation of sulfides must be

kept below that at which molybdic oxide volatilizes or

melts with resultant inhibition of the volatilization of

ventional rotating hearth furnace used for roasting concentrates.

The furnace is illustrative of the type concentrate

roasting furnace which is constructed to provide

for the introduction and passage of hot gases over

the material in the roasting area. Other conventional 5

furnaces of this type are moving horizontal conveyor

(sintering machine), circular rabbled hearth (single or

multiple), longitudinal rabbled hearth (Edwards style

roaster) and horizontal kilns. Beyond this feature of the

invention is not limited to any type furnace. A furnace 10

in which a water cooling system is used for temperature

control is preferred. If the furnace is air-cooled, the air

is not passed through the reaction zone of the flash

roaster. The furnace may be operated under negative

or positive pressure.

Although the illustrated furnace is of conventional

construction and is represented partially in schematic,

its principal operating parts will be described. The furnace

10 comprises the rotating hearth 12 rotatably supported

by wheels 14 on platform 16 which moves vertically

along the interior surfaces of main support legs

17. Central support member 18, which supports platform

16, is mounted for raising and lowering by hydraulic

cylinder 20 to correspondingly raise and lower

the rotating furnace 12 and the platform 16. The bed 25

of the rotating hearth upon which the concentrate rests

is indicated at 22. A water cooling system for the rotating

hearth by which water enters the inlet 24 and leaves

through the outlet 26 is indicated generally at 28. The

furnace is provided with the usual cutter assembly 30

to periodically break crusts which form as the concentrate

is roasted. Circumferentially-spaced inlets 32 permit

the introduction of gases, such as oxidizing gases,

into the hearth area formed by the bed of the hearth

and roof 34. The gases may be introduced under posi- 35

tive pressure, or reduced pressure generated at the outlet

pipe 42. The hearth 12 rotates in Iiq uid seals indicated

at 35 to provide a gastight hearth area. Aconventional

drive unit 36 is provided for rotating the hearth.

The structure just described is well known in the art 40

and forms no part of the invention other than in combination

with the structure now to be described.

In order to provide a vertically oriented reaction

zone for countercurrent contact of concentrate with

gas, a hollow reactor column 38 is mounted vertically 45

on the furnace 10 so that its bottom end opens into the

hearth area as shown. The vertical reactor column may

be constructed of refractory or insulated material or

heat conducting material provided with a cooling or

heat transfer media. The reactor column may be made 50

vertically adjustable by constructing it in spool sections

Or otherwise. For support, it is secured to the top of 1beam

39 by means of a conventional flange and bolt arrangement

as shown, the I-beam being attached to sup- 55

port 17. The outer casing of the vertical reactor column

38 forms an inner chamber 40 which is the reaction

zone of the vertical column. The reaction zone 40 is

provided with an outlet 42 for gases, fumes, dust, etc.,

including rhenium oxide. Since its bottom end opens

into the enclosed area above the rotating hearth 12, gas

introduced into inlets 32 passes over the hearth bed 22

and upwardly through the reaction zone 40 and out the

outlet 42.

A concentrate preheater unit 44 is mounted above 65

the reaction zone 40 by supports, not shown, and is

connected to delivery pipe 46 so that preheated concentrate

may be introduced downwardly from the pre5

3,770,414

rhenium oxide. The rhenium oxide passes to the scrubbers

while the molybdic oxide particles fall by gravity

to the rotating hearth 22. Some of the unoxidized sulfides

will also reach the rotating hearth as well as impurities

in the form of compounds of copper, iron, etc. 5

Some of these latter impurities, along with some molybdenum

sulfide, also pass to the scrubbers.

The flow diagram of FIG. ngives a condensed showing

of the process described. After the scrubber solution

containing molybdate and perrhenate ions leaves 10

the thickener, the molybdenum and rhenium may be

separated and recovered from solution by conventional

means. A preferred method is disclosed in co-pending

patent application Ser. No. 94,268 filed Dec. 2, 1970,

now U.S. Pat. No. 3,681,016. 15

As shown in the flow diagram, the calcine from the

roaster is leached to remove impurities and the leach

solution filtered with technical grade molybdic oxide

being recovered.

The efficiency of the system in converting a large 20

percentage of the molybdenum and rhenium in the moIybdenite

· to oxides is partially achieved through the

high, exposed particle surface area at elevated temperatures

which comes in contact with oxygen. This is pro- 25

moted largely· by the efficient dispersion of the vertically

traveling sulfide particles and the maximum surface

contact of the particles with the countercurrent,

upwardly traveling oxygen. Oxygen and preheated concentrate

particles are introduced at rates to provide a 30

preferable stoichiometric ratio of oxygen to metal silrfides

in the reaction zone of 120% or more. An acceptable

ratio was found to be about 170-240%. This ratio

can be as low as stoichiometric; however, the process

proceeds quite slowly at stoichiometric and a much 35

longer vertical column would be required. Excessive

amounts of oxygen beyond the. above range could be

used as oxygen can be recycled after separation of sulfur

dioxide.

An atmosphere free of oxygen is maintained in the 40

preheater at all times. Preferably an atmosphere of nitrogen

is used. Preheating of the concentrate before it

reaches the reaction chamber greatly accelerates oxidation

between the hot sulfides and hot oxygen. !talso

shortens the required time in the reaction zone 40 for 45

completion of the chemical reactions involved and, accordingly,

enables a shorter vertical column to be used.

This also reduces the volume of gas necessary in the reaction

zone 40 at all times.

Pure oxygen is used as the oxidizing medium; how- 50

ever, oxygen-enriched air can be used but this creates

the problem of removing introduced nitrogen from the

system. The more available oxygen per volume of gas,

the more efficient the system is and the shorter the required

height of the vertical column. Accordingly, pure 55

oxygen is the preferred gas for introduction into the

furnace.

All of the oxidation reactions do not occur in the reaction

zone 40 as they are also occurring on the rotat- 60

ing hearth until the final calcine leaves the furnace. The

gases introduced at inlets 43 pass over the concentrate

on the rotating hearth and absorb heat before they

travel upwardly into the reaction zone. Accordingly;

oxidation reactions are occurring continuously through 65

contact of oxygen with the hot calcines on the rotating

hearth. The gaseous oxidation products are carried by

the oxygen up through the reaction zone and to the

scrubbers. These products are principally sulfur dioxide,

rhenium oxides and molybdenum oxides.

Vertical orientation of the reaction zone is preferable

in that it makes feasible complete dispersion of the molybdenite

concentrate particles above the reaction zone

and before the particles enter the reaction zone. The

oxygen updraft further disperses the falling particles.

As stated previously, the effective dispersion of the hot

particles in the hot reaction zone gives maximum surface

contact of the particles with the oxygen to provide

complete oxidation thereof. This feature is referred to

as flash roasting.

A further important feature of the invention is the

use of sulfurous acid formed in the scrubbers for leaching

of the molybdic oxide concentrate in the recovery

of the remaining rhenium from the molybdic oxide calcine.

This feature contributes to the economy of the

system. Other leaching agents may be used.

The efficiency of the system may be increased by reroasting

the calcine. T.he molybdenum trioxide produced

in the flash roaster may not be completely oxidized

and may contain some sulfur in addition to any

copper which may have been in title feed. The sulfur is

more completely oxidized by re-roasting the calcines in

air at 600°C. for thirty minutes. This second roast also

insures almostcompiete OXidation of the copper. It was"

found most convenient to perform the reroast in an externallyheated

kiln using a small air flow to complete

the oxidation. An addition portion of rhenium is also

volatilized during the re-roast and can be collected by

scrubbing the off-gas in a second scrubber.

The process described above consistently provides

up to 95% recovery of rhenium from molybdenite concentrate

and a molybdic oxide calcine of increased purity.

A refinement in the control of the process will now

be described.

It is necessary to maintain proper temperature con-"

trol for rhenium volatilization while maintaining conditions

that will prevent excessive volatilization of molybdic

oxide. The most efficient operation of the system is

dependent upon the controlled rate of reaction and

partial oxidation of each particle: of molybdenum sulfide

as it descends through the vertical column. This

control is required to prevent complete oxidation in a

confined zone which would result in high temperature

and possible volatilization or fusion of molybdenum oxide.

The rhenium volatilization may be depressed because

of the formation of a non-volatile oxide or the

possible surface sintering or fusion of molybdenum

compounds which entrap the rhenium.

The oxidation process in the vertical column is to a

degree self-regulating by the variation occurring in the

sulfur dioxide to oxygen ratio of the gas stream. As the

molybdenum sulfide oxidizes, the sulfur dioxide concentration

increases and the oxygen concentration decreases,

resulting in a suppression of the reaction rate.

Accordingly, the ratio of sulfur dioxide to oxygen in the

exhaust gas is, in effect, a measure of the efficiency of

the process when the most desirable ratio is known. To

demonstrate this, the process was operated at various

ratios of sulfur dioxide to oxygen in the exhaust gas,

and high volatilization of rhenium oxide with high recovery

of molybdenum in the calcine occurred when

operating at sulfur dioxide percentages by volume of 30

and 3S in the exhaust gas. These volumes are, of

course, related to the ratio of sulfur dioxide to oxygen,

3,770,414

Percent

solubilized

Re Cu S

62 87 63

50 89 83

74 97 26

55 99 36

(%J

Leached residue

Re' Cu

(ppm) (%)

(%)

Feed calcine

(ppm)

TABLE 2.-BALANCE OF RHENIUM AND MOLYBDENUM

.IN SCRUBBER SOLUTION AND SOLIDS

Solution Solids Distribution percent

Re Mo

Re Mo Re Mo

Test (ppm) (gil) (ppm) (%) Soln Solids Soln Solids

1.. .... 30 1.0 360 54 90 10 15 85

2...... 17 0.2 760 57 92 8 16 84

TABLE 3. - RE-ROASTING FLASH ROASTER CALCINES

IN AIR AT 600'C.

Feed calcine Product calcine Percent

volatilized

Re S Re S

Sample (ppm) (%) (ppm) (%) Re S

1.. ......... ;...... 128 1.37 51 1.33 61 6

2.................. 218 1.63 86 0.69 61 58

3............... ;.. 141 0.49 58 .12 60 76

4.................. 192 .72 43 .18 79 78

I. ..... 86 0.11 0.69 36 0.02 0.2.8

2.. .... 51 .12 1.33 28 .01 .25

3.. .... 58 .75 0.12 17 .02 .10

4 ...... 43 .90 .18 22 .01 .13

similar conditions. With careful control, a molybdenum

product may be produced which contains only 5% of

the feed rhenium, demonstrating that the process is effective

to provide high yields of both molybdenum and

rhenium.

The high sulfur dioxide content of the off-gas rapidly

saturated the scrubber solution with sulfur dioxide. As

shown in Table 2 below, about 90% of the rhenium reporting

to the scrubber was found to be soluble,

10 whereas less than 20% of the molybdenum was soluble

(see data in Table 2). Therefore, under normal operating

conditions of the process, at least 80% of the rhenium

and less than 2% of the molydenum report to the

scrubber solution. The insoluble portion of the dusts,

containing the residual molybdenum and rhenium, are

separated from the scrubber solution and recycled

back to the roaster feed for retreatment.

After the off-gas has been scrubbed of its loading of

30 dust and fume, it consists mainly of sulfur dioxide and

oxygen. The gas may be dried and compressed to liquefy

the sulfur dioxide. The oxygen remains in the gaseous

form and is recycled to the flash roaster.

Table 3 shows the results from re-roasting a number

of calcines:

After re-roasting of the calcine, portions of the copper,

the remaining rhenium not collected in the scrubbers,

and a portion of the sulfur are soluble in mineral

50 acid solutions. Since the process produces a dilute mineral

acid -sulfurous acid- in the scrubbers, it is used

to leach the copper and the remaining rhenium from

the calcine (Table 4).

55 TABLE 4.-REMOYAL OF COPPER AND RESIDUAL RHENIUM

AND SULFUR FROM RE-ROASTED CALCINES

Molybdenum balance BY LEACHING WITH SULFUROUS ACID

(percent distribution)

88 94 6

84 94 6

76 ..

95 .

125

140

218

Rhenium balance

567

707

736

TABLE I.-ROASTING TEST RESULTS

Preheat temperatures: 650-750'C. range

Hearth temperature: 550-650'C. range

Percent stoichiometric oxygen: 170-240

the latter being the only other gaseous component in

the exhaust stream· pertinent to this control feature.

The sulfur dioxide-oxygen ratio in the exhaust stream

can be partially controlled to provide the optimum

value, if necessary, by the introduction of sulfur dioxide 5

gas with the oxygen. The ratios reflected by 30-35%

volume of sulfur dioxide are by no means critical, but

its use to provide favorable reaction zone conditions

illustrates the effectiveness of this method of control.

There are three other principal parameters affecting

the temperature control and/or the oxidationvolatilization

process, one or all of which may be used

to control these factors in varying degrees. These parameters

are: (1) preheat temperature, (2) the height

ofthe reactor column, and (3) heat dissipation from 15

the column. The first of these, like the oxygen-sulfur

dioxide ratio, is applied during the operation of the process..

The latter two are built-in to the construction of

the apparatus.

The preheat temperature is readily controlled by ad- 20

justing the heat input to the indirect-fired preheat furnace.

The height of the reactor column determines the

dwell time of the sulfide particles in the reaction zone

for complete oxidation and for formation and volatil- 25

ization of rhenium oxide. The optimum height for a

given operation is developed by calculations and measurements

derived from pilot plant operation. For example,

in a continuous pilot plant operation excellent

results were obtained using a vertical column 44 inches

in height and 6 inches in diameter with a rotating

hearth 3 feet in diameter. These dimensional relationships

are not critical and would change with change in

other variables, such as, concentrate characteristics,

composition of feed gases, rate of gas injection, etc. 35

The heat dissipated from the vertical column is controlled

by design, and construction materials used. The

construction can be varied from highly insulated construction

to high conductivity construction with a cooling

media. The radiation and convection loss of heat 40

generated for a metal conducting material and a given

feed rate can be readily calculated. Additional heat

may be removed from the column by water cooling or

other heat exchange media.

The results given below are illustrative of those obc 45

tained by application of the above-described process in

conjunction with the apparatus described.

Table 1 shows some material balances obtained in

roasting tests performed on molybdenite concentrate.

1.. ..

2 ..

3 ..

4 .

Rhenium Product Yolatil- Dust and

T_es_t ___--f,e-ed_--,(p-p_m)--,iz-ed_(_%)__P_ro_du_ct__sc_ru_bb_er 60 Spiaem-

----------------------

The data in Table 1 shows the variability of the rhenium

content of the product produced at somewhat

65 About 7 percent of the molybdenum contained in

the calcines is also solubilized in the sulfurous acid

leach.

3,770,414

JlO

zone,

c. controlling the temperature in the first oxidation

zone durin~ the introduction thereto of said preheated

particles and thereafter to maintain a temperature

therein above the volatilization temperature

of rhenium oxide and below the volatilization

temperature of molybdic oxide to form rhenium

oxide, sulfur dioxide and molybdic oxide, which

latter oxide along with other solids passes to a second

oxidation zone where any unoxidized molybdenite

is completely oxidized, said second oxidation

zone being hec;ted by exothermic heat of the

reactions occurring therein,

d. passing oxygen through said second oxidation zone

to oxidize molybdenite contained therein,

e. passing at least some of the oxygen travelling to

said first oxidation zone through said second oxida,

tion zone to heat the oxygen before it reaches the

first oxidation zone,

20 f. recovering rhenium oxide by collecting it in a recovery

zone outside the first oxidation zone and

dissolving it in water,

g. recovering rhenium from the water solution of rhenium

oxide, and

25 h. recovering insoluble molybdic oxide from the second

oxidation zone.

2. The process of claim 1 in whi'ch said concentrate

is preheated to about 500°C.

30 3. The process of claim 1 in which molybdenum values

are recovered.

4. The process in claim 1 in which rhenium values are

recovered.

5. The method of claim 1I in which the temperature

of the first oxidation zone resulting from exothermic

heat of reaction is controlled by controlling the reaction

rate of the oxidation reactions occurring therein.

6. The method of claim 5 in which said reaction rate

is controlled by adjusting the relative feed rate of oxy40

gen and molybdenite concentrate to the first oxidation

zone to control the stoichiometric ratio of oxygen to

metal sulfides introduced therein.

7. The method of claim 6 in which said stoichiometric

ratio is at least one.

S. The method of claim If) in which said stoichiometric

ratio is at least 120%.

9. The method of claim 6 in which sulfur dioxide is

introduced to the first oxidation zone.

10. The method of claim 6 in which the sulfur diox50

ide-oxygen ratio in the exhaust gases from the first oxidation

zone is used to determine the relative rate of addition

of oxygen and concentrate.

1I1. The method of claim 1 in which the exhaust gas

contains up to about 50% by volume of sulfur diOldde.

55 12. The method of claim 1 in which the dwell time of

concentrate particles in the first oxidation zone is controlled

by varying the diameter and height of said zone.

1I3. The process of claim 1I in which oxygen in the exhaust

gases is recycled for reuse in the method.

60 14. The process of claim 1 in which sulfur dioxide in

the exhaust gases is dissolved in water to form sulfurous

acid and the sulfurous acid used to leach impurities

from the molybdic oxide calcine recovered from the

second oxidation zone.

* >I< >I< * *

About 7% of the molybdenum contained in the calcines

is also solubilized in the sulfurous acid leach.

The leached residue is separated from the leach solution

by filtration and after drying is ready for packaging 5

for sale. The leach solution joins the solutions from the

scrubbers on the flash roaster and re-roaster.

The effectiveness of the above-described process is

graphically illustrated by the high recovery of rhenium

and molybdenum achieved. it provides for the recovery 10

of up to 95% of rhenium and high recovery ofmolybdenum

in molybdenite with a minimum of process time

and a minimum of oxygen and added heat. The economic

advantages of these features are apparent. The

process is adaptable to either a batch or continuous op- is

eration.

It is an attractive side advantage of the. process that

a small volume of exhaust gas containing a high percentage

by volume of sulfur dioxide is produced. The

process is normally operated with an exhaust gas volume

discharge rate of 1,350 cubic feet per minute

(CFM) with up to 220% excess oxygen and 30-50% by

volume of sulfur dioxide in the exhaust gas. This high

volume percentage of sulfur dioxide makes its recovery

economically feasible for various commercial uses. In

contrast, present-day processes utilizing air for cooling

and for supplying oxygen are of necessity operated with

an exhaust volume discharge rate of 40,000 CFM, 16

volume percent excess oxygen and 1-2 volume percent

of sulfur dioxide. This volume percentage of sulfur dioxide

in the exhaust gas is so low that its recovery is not

economically feasible because it involves processing

such large volumes of gas. As a result the sulfur dioxide

is exhausted to the atmosphere creating a serious pollution

problem in heavily populated areas. The process of 35

this invention eliminates this problem.

The reduced volume of exhaust gas also results in a

much higher concentration of rhenium oxide in the exhaust

gas than is obtained in conventional processes.

As a result, recovery of substantially all of the rhenium

is far more feasible and economical than in present processes

using air with resultant large volumes of exhaust

gas to be processed for recovery of the rhenium oxide.

Reduction of the volume of gas processed through

the system by a factor of about 30resultsin a dr1!§jic: 45

reduction in the size of equipment require-d~ith~jgnificant

savings in equipment cost and floor space.

What is claimed is:

n. A method for recovering rhenium and molybdic

mdde from molybdenite concentrate which comprises:

a. pre-heating particles of said concentrate in an oxygen-

free atmosphere to a temperature not in excess

of about 750"C to raise the temperature of the particles

to promote flash oxidation of the molybdenite

when the particles are introduced into a flash

oxidation zone,

b. causing said pre-heated particles to fall through a

first oxidizing zone of heated oxygen with said particles

and heated oxygen moving countercurrent to

each other to disperse said pre-heated molybdenite

particles in said heated oxygen to provide maximum

particle surface contact with heated oxygen

for effective oxidation, said first oxidation zone

being heated substantially by the exothermic heat

of the reactions occurring in said first oxidation 65