Method of recovering molybdenum oxide

A method of recovering molybdenum oxide as in U.S. Pat. No. 4,379,127 wherein, however, the temperature in the autoclave and the pressure therein are controlled within narrow ranges by increasing the suspension density of the molybdenum sulfide suspension fed to the autoclave upon a fall in temperature and by adding water to the slurry of molybdenum sulfide concentrate formed before introduction into the autoclave upon an increase in temperature.

FIELD OF THE INVENTION 
Our present invention relates to a method of recovering molybdenum oxide 
and, more particularly, to an improvement upon the method described and 
claimed in our U.S. Pat. No. 4,379,127 issued Apr. 5, 1983 and based, in 
turn, upon German Patent Document No. 31 28 921. 
BACKGROUND OF THE INVENTION 
The aforementioned patent describes a method of recovering molybdenum oxide 
by oxidation of a molybdenum sulfide aqueous concentrate slurry containing 
impurities and in which the molybdenum concentrate has an average particle 
or grain size of 20 microns to 90 microns, preferably about 70 microns. 
In this method, the contaminated molybdenum sulfide concentrate is slurried 
in the slurrying stage with water to form an aqueous suspension which is 
introduced into an autoclave and subjected to an oxidation process at 
elevated temperature and with an elevated oxygen partial pressure. In the 
reaction, the molybdenum sulfide is transformed into molybdenum oxide and 
sulfuric acid by a reaction scheme outlined in the aforementioned patent. 
The reacted suspension is discharged from the autoclave and the molybdenum 
oxide is filtered from the reacted suspension and the first filtrate which 
is thus obtained and which contains sulfuric acid, is neutralized with 
lime or calcium carbonate to form gypsum (calcium sulfate dihydrate), 
whereupon the suspension is filtered to remove the gypsum therefrom and 
the resulting second filtrate is recirculated to the slurrying stage. 
It will be understood that the neutralization to form gypsum, to be strict, 
is only a partial neutralization and further that the molybdenum sulfide 
concentrate is prepared and introduced in consideration of the capacity of 
the apparatus and the amount of molybdenum oxide to be generated. 
The throughput of the molybdenum compounds is thus dependent upon the size 
and configuration of the apparatus. 
Make-up water is introduced to cover amounts which may be lost in the 
recovered molybdenum oxide and gypsum. 
In an earlier process described in German Patent No. 28 30 394, not only is 
the second filtrate recirculated but a two-stage recirculation is 
provided. In a first recirculation stage, the reacted suspension after 
oxidation and before filtering off the molybdenum oxide is recirculated to 
the slurrying stage and is combined with an additional quantity of 
molybdenum sulfide concentrate before the mixture of the recycled 
suspension and additional molybdenum sulfide concentrate is returned to 
the autoclave. 
This recirculation continues until the sulfuric acid content of the slurry 
is about 80 to 120 g/l. Only then is the molybdenum oxide filtered from 
the multiple recirculated suspension. 
The second recirculation stage recirculates a product following the 
precipitation of gypsum. More particularly, the first filtrate obtained 
from filtering off molybdenum oxide is neutralized with the formation of 
calcium sulfate to a pH value in the range of 0.9 to 1.5 and preferably 
close to 0.9. The gypsum which is thereby produced is filtered off leaving 
a second filtrate which is recirculated to the slurrying stage and 
therefore combined with additional quantities of molybdenum sulfide 
concentrate before the resulting mixture is again fed to the autoclave. 
This double recirculation continues until the impurity elements in the 
second filtrate build up to a sufficiently high level as to warrant their 
recovery and restarting of the process. 
In this earlier system, moreover, the impurity-enriched second filtrate is 
neutralized to a pH of about 2.5 with an alkali hydroxide and this second 
filtrate is returned to the autoclave where it is again subjected to 
oxidation with oxygen to precipitate out iron molybdate. The iron 
molybdate is filtered off and the filtrate is then subjected to further 
processing to recover the various impurity elements contained therein. 
The latter process has been found to be highly satisfactory because the 
recirculation generates a sufficiently high level of impurities that such 
impurities can be readily recovered. The first recirculation which 
involves the recirculation of molybdenum oxide contributes to the 
avoidance of encrustation in the autoclave. The recycled filtrate is 
sufficiently hot so that external heat need not be provided and 
contributes to the maintenance of the otherwise exothermic reaction in the 
autoclave (MoS.sub.2 +9O.sub.2 +2H.sub.2 O=MoO.sub.3 +2H.sub.2 SO.sub.4). 
In this system the suspension has a suspension density of about 50 to 75 
g/l. 
However, with this system, because of the multiple recycling operations, an 
apparatus for producing a given quantity of molybdenum oxide must be 
considerably larger than is desired and the energy which must be 
introduced, in spite of the exothermicity of the reaction, because of the 
recirculation of dense fluids, is comparatively large. 
In order to avoid these disadvantages, in our aforementioned U.S. patent we 
have provided a method in which for an apparatus of a given size a 
particularly high throughput can be obtained, i.e. the efficiency of the 
apparatus is far more pronounced. Consequently, for a given output, the 
apparatus can be simplified and the energy input can be reduced. 
In our prior patent, this is obtained by operating the oxidation stage with 
a suspension whose suspension density is in the range of 100 to 150 g/l 
and such that only the second filtrate obtained by neutralization of the 
first filtrate and filtering or separation of gypsum therefrom is 
recirculated. Important to the invention there described is that the 
second filtrate be recycled in such quantity that the suspension density 
of the suspension fed to the autoclave is maintained in the aforementioned 
range of 100 to 150 g/l. 
Experience with our earlier system, however, has shown that in practice the 
composition of the molybdenum sulfide concentrate varies with time, e.g. 
by changes in the gangue components, or by changes in the flotation-oil 
content. These variations in composition lead to undesired temperature 
variations in the autoclave which complicate the earlier system and 
prevent the meticulous control of the autoclave temperature which is 
required. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide a method 
which improves upon the system described in our U.S. Pat. No. 4,379,127 by 
simply and conveniently controlling the temperature in the autoclave. 
Another object of this invention is to provide an improved method for 
recovering molybdenum oxide wherein the efficiency of the apparatus can be 
improved or, for an apparatus of given size and energy consumption, the 
output in terms of molybdenum oxide can be improved. 
Still another object of this invention is to provide a method which 
overcomes the disadvantages of the earlier systems and, indeed, which in a 
simple and economical way permits control of crucical reaction parameters. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the present invention in a method which is 
based upon our discovery that a particularly convenient, simple and 
economical control for the temperature and therefore the pressure in the 
autoclave can be effected in response to the suspension density. More 
particularly, we provide that, with a fall of the temperature in the 
autoclave, the suspension density is increased and with an increase of the 
temperature in the autoclave, water is added to reduce the suspension 
density at the slurrying stage upstream of the autoclave. In other words 
the temperature in the autoclave is monitored and the suspension density 
is controlled in response thereto, preferably within the range given 
earlier but with a variability depending upon temperature in the manner 
described. 
Since the response that the autoclave to changes in the suspension density 
at the slurrying stage is comparatively slow, i.e. the lag time or 
response delay is comparatively long, depending upon the reaction 
requirement, a rapid response control can also be provided. 
In this case, to respond to sudden or brief changes in temperature, the 
invention provides for control of the temperature by adjusting the oxygen 
partial pressure so that, with a temperature drop within the autoclave, 
the oxygen partial pressure is briefly and commensurately raised while, 
with an increase in the temperature in the autoclave, the oxygen partial 
pressure is commensurately reduced. 
Best results are obtained when the temperature within the autoclave is 
maintained by these control systems at 230.degree. to 245.degree. C. while 
the oxygen partial pressure is maintained within the range of 1 to 5 bar, 
although variability is permitted within this range for the control 
purposes described. 
Apart from these modifications, the apparatus can be constructed and 
arranged in the manner described in our U.S. Pat. No. 4,379,127, the 
parameters of the method can conform to the parameters set forth in this 
patent and in general both the apparatus and the method of the patent can 
remain the same. The method of the invention operates with a single stage 
recirculation with autogenous temperature control, i.e. without 
temperature regulation by coolers, heaters or the like which are external 
to the autoclave. 
The suspension density can be varied by simply modifying the amount of 
water which is mixed with the slurry and the control of the oxygen partial 
pressure can be effected simply by throttling or increasing the rate at 
which oxygen is fed to the autoclave and the pressure at which oxygen is 
fed to the autoclave. 
Mention may also be had of the process for recovering molybdenum oxide from 
a molybdenum sulfide concentrate with single stage recirculation in German 
Open Application (DE-OS 20 43 874). 
In this system the particle or grain size may be up to 20 microns and is 
preferably about 5 microns for the molybdenum sulfide. In this process, 
moreover, the filter keg after the first filtration and containing the 
molybdenum oxide contains traces of molybdenum sulfide and inert 
impurities and is further treated. In addition, the neutralization of the 
first filtrate results in solubilization of molybdenum oxide and this must 
be recovered from the filter keg of the second filtration. Temperature 
control by regulating the suspension density is not described and, indeed, 
this systems requires external heating. Mention is made of this patent 
only to indicate that it is known to recycle a portion of the first 
filtrate for preparation of the suspension which is to be admitted to the 
autoclave. However, since the suspension which is thus admitted to the 
autoclave lies on the acid side and can bring about an excess acidity of 
the suspension to be reacted, this system has not found widespread use. It 
may be observed that the method of the invention does not permit the 
development of an excess acidity. 
The method of the invention has numerous advantages. Firstly, it permits 
the treatment of molybdenum sulfide concentrates which can have a large 
particle size component, i.e. particles of a size greater than 74 microns, 
without difficulty. It might be expected that such large particle size 
molybdenum sulfide concentrates could only be oxidized under pressure with 
difficulty whereas in the system of the invention it turns out that such 
particles are oxidized with ease. 
Secondly and, indeed, surprisingly, encrustation problems do not arise in 
the autoclave even though an extremely high suspension density is used in 
the system according to the invention. Even with such high suspension 
densities, a sufficiently precise temperature control is maintained by 
adjustment of the suspension density that the reaction is effected 
reproducibly even where there are changes in the composition of the 
molybdenum sulfide concentrate which is used. The reaction is carried out, 
moreover, at relatively low oxygen partial pressure and thus oxygen losses 
are reduced or, put otherwise, per unit volume, less oxygen is lost in the 
system. 
The autogenous temperature control operates effectively and without 
problems, especially when the oxygen partial pressure is maintained in the 
range of 1 to 5 bar and the temperature in the autoclave is maintained in 
the range of 230.degree. to 245.degree. C. Because only a single stage 
recirculation is carried out, at comparatively low energy costs, the 
continuous process can bring about a yield of 90% molybdenum and more.

SPECIFIC DESCRIPTION AND EXAMPLE 
In the drawing, we have shown a slurrying unit 1 in which the molybdenum 
sulfide concentrate from a hopper 25 via a metering device 26 is dispensed 
into the aqueous phase which may be formed by water introduced by a valve 
21 or recycled second filtrate as controlled by a valve 22, to form a 
suspension adapted to be fed to the autoclave. 
The slurrying stage 1 is also provided with a mixing motor 23 whose blades 
are represented at 24 to ensure a thorough dispersal of the solids in the 
aqueous phase and homogeneity of the suspension. 
The suspension is fed via line 2 and a pump 3 into the autoclave 4. The 
autoclave 4 is provided with a stirrer 27 and with baffles 28, 30 and 31 
defining an undulating path for the suspension. In addition, means is 
provided as is represented at 5 for feeding oxygen into the autoclave, the 
oxygen partial pressure being controlled by a valve which has been 
represented at 32. The partial pressure within the autoclave is preferably 
maintained at about 1 to 5 bar although a maximum oxygen partial pressure 
of up to 10 bar can be used. 
A vent is provided at 29 to hold back the oxygen pressure but to release it 
when an excess partial pressure is built up in the autoclave. 
The oxidized suspension is discharged from the autoclave via line 6 into a 
filter 7. In the filter 7 molybdenum oxide is separated out and discharged 
at 8 while the first filtrate is fed to a precipitation reactor 10 or a 
neutralization stage in which the solution is neutralized by the addition 
of limestone thereto. The limestone is fed from a hopper 11 by an 
appropriate metering unit. 
The calcium carbonate reacts with the sulfuric acid in the reactor to 
produce carbon dioxide which is discharged at 40 and a suspension of 
calcium sulfate dihydrate, i.e. gypsum, in the aqueous phase. This 
suspension of gypsum is fed at 41 to a further filter 12. In the filter 
12, the gypsum is filtered out as represented at 13 while the resulting 
second filtrate, via a pump 15 and as controlled by the valve 16 is 
recirculated to the slurrying stage 1. 
The operation of the device will be described in greater detail in 
connection with an example although it should be noted that a controller 
18 is provided to respond to a sensor 17 of the temperature within the 
autoclave 4. The sensor 17 is connected to the controller 18 via line 19. 
The controller 18, in turn, controls the valve 21 via the line 20a or the 
valve 32 via the line 20b to regulate either or both of the water addition 
or the oxygen feed to control the density of the slurry or suspension 
transferred to the autoclave or the oxygen partial pressure in accordance 
with the temperature. 
In the slurrying unit 1 the molybdenum sulfide concentrate suspension is 
made up first with water and thereafter with make-up water and 
recirculated second filtrate so that the suspension has a suspension 
density of about 150 g/l. 
This suspension is fed by the pump 3 continuously at the rate of 200 l/h at 
ambient temperature into the autoclave 4. After an initial heating to 
170.degree. C., the temperature in the autoclave is maintained between 
230.degree. and 245.degree. C. by the exothermicity of the reaction. Via 
the unit 5, 45 kg/h of oxygen is introduced so that the oxygen partial 
pressure in the autoclave is maintained at about 5 bar. 
Via line 6 and a valve not shown the oxidized suspension is fed to the 
filter 7 and 27 kg/h of molybdenum oxide are filtered off at 8 while the 
filtrate is introduced into the neutralizer 10 in which it is reacted with 
34 kg/h of calcium carbonate. 
The resulting gypsum is discharged at 13 at a rate of 45 kg/h of the 
calcium sulfate dihydrate. 
The partially neutralized second filtrate containing 20 g/l sulfuric acid 
is recirculated via line 14 by pump 15 and valve 16 at a rate of 200 l/h 
to the slurrying unit 1. 30 kg/h of fresh molybdenum sulfide concentrate 
is combined therewith. The suspension density thus established generates 
sufficient heat so that no external heating of the autoclave or the 
remainder of the system is necessary. 
As a result of expansion, venting of inert gases, filtration off of the 
molybdenum oxide and of the gypsum and the like, water is lost at a rate 
of about 100 l/h and the corresponding water deficiency can be returned to 
the system by adding 100 l/h of water by washing the molybdenum oxide and 
the gypsum and returning the washed liquid to the cycle with the second 
filtrate. Once steady state conditions have been reached, control of the 
temperature in the reactor and the pressure therein is maintained by the 
controller 18 by either controlling the make-up water added at 21 or the 
oxygen partial pressure via the valve 32 in the manner previously 
described. 
3000 kg of molybdenum disulfide concentrate of the following analysis is 
reacted in the manner described: 
______________________________________ 
Sieve analysis: 
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Mo = 53.7% + 100 mesh 0.8% 
Cu = 1.2% -100 + 150 mesh 
4.0% 
Fe = 1.7% -150 + 200 mesh 
15.2% 
S = 38.8% -200 + 270 mesh 
12.8% 
H.sub.2 O = 3.8% 
-270 + 325 mesh 
16.4% 
Oil = 2.1% -325 + 400 mesh 
15.6% 
-400 mesh 35.2% 
______________________________________ 
The product is 2528 kg of MoO.sub.3 with the following composition: 
Mo=63.1% 
Cu=0.015% 
Fe=0.3% 
S=0.04% 
This corresponded to a molybdenum yield or recovery of 99%. 
When the temperature in the autoclave 4 tended to fall by reason of higher 
gangue content of the molybdenum sulfide concentrate or reduction in the 
escape of water from the system, additional molybdenum sulfide concentrate 
was added to the suspension and/or the water supply was reduced to 
increase the suspension density. When, however, the temperature in the 
autoclave 4 increased by reason of an increased flotation oil content in 
the molybdenum disulfide concentrate, the feed of water to the molybdenum 
sulfide suspension was increased to reduce the suspension density. 
To compensate for brief fluctuations in the temperature, the oxygen partial 
pressure was controlled, e.g. by reducing the partial pressure to about 3 
bar and the oxygen feed rate to about 40 kg/h. This resulted in a 
reduction in the autoclave temperature almost instantaneously. 
Corrosion problems were avoided by maintaining the oxidizing conditions 
continuously. These oxidizing conditions could be maintained easily in 
part because of the continuous recirculation of the second filtrate with 
its increasing copper content. It thus was indeed surprising that the 
corrosion problems did not arise even though brief reductions in the 
oxygen partial pressure were effected by the control system.