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,
53
[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
38
39
56
//~
35 -~=_._=-=""'-"'''''
34 32
22 35
35
12
/7-
18 17
50
52.--..Ir-J"i1-N
42
...TO SCRUBBERS
en
a5
-."
c.H
>-
0- r-rl ::z
--<
,..."
I::::'
::IE =-<
w
~
-..! o
..l:J.
~
.t>.
:""-:l
~ ,
i wi
r'~
=..."
...,
--<
OXyGENJ
LIQUID
SULFUR
DIOXIDE
AMINE
SOLVENT I---AMMONIUM HYDROXIDE
STRIPPING
i
UNDERFLOW
~-SOLUTION
~SOLIDS I
AMMONIUM MOLYBDATE 2
AMMONIUM PERRHENATE T.l.q:
SOLUTION
t
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.
2
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.
1
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
3,770,414
4
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
6
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,
7
3,770,414
8
Percent
solubilized
Re Cu S
62 87 63
50 89 83
74 97 26
55 99 36
S
(%J
Leached residue
Re' Cu
(ppm) (%)
S
(%)
Cu
(%)
Feed calcine
Re
(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
40
Rhenium balance
567
707
736
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