CA1094781A - Hydrometallurgical processing for molybdenite ore concentrates - Google Patents

Abstract

ABSTRACT

An improved process is described for the hydrometal-lurgical liquid phase oxidation of molybdenum disulfide ore concentrates, in which the reactants include in the range of from about 0.12 to about 1.6% mole of an alkali metal hydroxide, preferably sodium hydroxide, per mol of molybdenite and the time of the reaction, the amount of the alkali metal hydroxide, and the reaction temperature and pressure are coordinated to achieve at least a 95% oxidation conversion of the molybdenum disulfide.

Description

1. This invention is directed to the oxidation in a liquid I!aqueous slurry of molybdenum disulfide to hexavalent molyb- ¦
~ldenum~ predominantly molybdic trioxide.
!, In 1956. Dresher, et al., J. Metals, pp. 794-800 (June 1956) disclosed a process for the oxidation of molyb-!'denite with molecular oxygen in an aqueous potassium hydroxide~
jlsolution at elevated temperatures (100-175C) and pressures ¦(up to 700 ps ) to form a solution of water-soluble molybdate !
ions (MoO4=), according to the reaction:
~ MoS2 + 6KOH + 9/202 + K2~O4 + ~2SO4 + 3H2).
jtIn order to obtain a solid molybdenum-containing product, the )mother liquor from the reaction had to be treated with, for instance, a calcium salt to precipitate calcium molybdate, for, ~hich there is somewhat limited commercial utility.
~¦ U.S. Patent No. 3,656,888 discloses a process for joxidizing a molybdenite ore concentrate to molybdic trioxide ~by employing a ~i~uid phase slurry maintained at an elRvated te~perature and pressure and employing gaseous molecular oxy~en 1! i , .. .
.

, 10947Bl ¦l as the oxidizing agent. The reaction is:

, MoS2 + 9/202 + 3H20 ~ MoO3.H2O + 2H2SO4, with ii the molybdic trioxide (monohydrate) forming a solid phase Ij and being separable by filtration. Further, this patent 5il em~hasizes the need of fine grinding. A particle size ¦ of less than 200 mesh preferably less than 20 microns and better still less than S microns, is necessary to promote the efficiency of the oxidation reaction and also to facili-tate the formation and maintenance of a uniform aqueous lOi, dispersion. However, a substantial amount of the molybde-'' num remains in solution as slightly soluble molybdic acid ; ~ evèn when the reaction is continued to a terminal pH of li the slurry of about zero or less. ~ecause of the economic value of the molybdenum in the mother liquor, in commercial 15¦! practice the mother liquor must be further processed for recovery of it. Furthermore, the process has a relatively ~ I poor yield, as stated in the examples of the patent.
I It has now been discovered that the total oxidative :~ I
conversion of tetra-valent molybdenum, as molybdenum 20l disulfide, to hexavalent molybdenum, and also the yield , of solid molybdic trioxide recovered in the solids fraction (both based on the total molybdenum disulfide in the charge ~l, to the reaction zone), may be increased by adding to the Il reaction zone a minor amount, in the range of about 0.12 25! to about 1.68 mols, of a stronq hydroxide per mol e~uiva-lent of molybdenum disulfide introduced into the reaction zone. The reaction is terminated when the pH of the mother liquor of the reaction is in the range from about O.OS to about 0.5.

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i¦ The oxidation conversion step of the present ¦! invention is advantageous in that it obviates the fine 1 grinding operation. That is, the molybdenum ore or con-j~ centrate typically is separated from other minerals, 51 usually copper, as by flotation, and the molybdenum 'i particles can be utilized as received without the additional , step of f ine grinding. As a consequence, concentrate ~I having a particle size of 65 mesh, or finer, may be ¦¦ treated directly in the oxidation conversion operation, 10¦¦ whereas the prior art requires the fine grinding.
It has also been discovered that the yield of molybdic trioxide in the solid phase and the total oxida-tive conversion of molybdenum disulfide (to molybdic tri-~¦ oxide and molybdate ion) may be increased by scrubbing 15~¦ the molybdenite ore concentrate with a caustic base in ~¦ a stirred slurry containing a high content of solids, ¦¦ and then dewatering, by decanting and/or filtering, the Ii liquid phase from the solids before commencing the oxida-1~ tion reaction.

~094781 .j I
,l In preferred embodiments of the process, the oxidative ,~conversion is 99+ mol percent, and the contaminating sulfur 'content of the product solids fraction is not greater than 0.2 'weight percent of the product molybdic trioxide. (Unless I.,otherwise noted, all percentages herein are by weight).
The charge to the process is typically an ore concentrate ~comprising predominantly ground or finely divided molybdenite (which is the mineral term for molybdenum disulfide), and wherein the molybdenum content of the ore concentrate is gen-~lerally greater than 35 weight percent. It is to be understood ~that ore concentrates from various mines and beneficiating .plants vary substantially in their characteristics, even if their chemically analyzed molybdenite content is substantiallyl 'the same, and hence the optimization of the process variables ¦
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,of the invention described herein will depend upon the specifi ~source and characteristics of the ore concentrate being ,processed.
In conducting the process, the ore concentrate is slurried with water in a reaction zone, the solids content of the slurr ! i `,being in the range of from about 5 to about 25 weight percent, 'advantageously about 12-15% of the slurry. The slurry is then heated to the desired reaction temperature, in the range of `~from about 150 to about 230C, and preferably to the range of f about 185-205C. The reaction is initially exothermic and 'generally requires cooling by indirect heat transfer means.
;After about 30-60 minutes, the exothermicity declines, and heat may need to be supplied by indirect means.
' The strong hydroxide, as referred to herein, to be employ~d .in the reaction zone is intended to mean sodium hydroxide, preferably, or potassium or ammonium hydroxi~. It may be _ ,; .
!

-`` 1094781 i .1 added to the slurry at any point in time up to and including the occasion when the slurry reaches the desired reaction temperature, but preferably the strong hydroxide is added to the slurry prior to increasing the temperature thereof above 5l ambient. The amount of strong hydroxide to be employed is in the range of from about 0.12 to about 1.68 mols per mol of I molybdenum disulfide, preferably in the range of from about j 0.48 to about 0.90 mols per mol of molybdenum disulfide. The i strong hydroxide may be added as a solid or as a liquid con-10ll centrate. It should be understood that the stoichiometricmol ratio of a strong hydroxide to molybdenum disulfide re-' quired for the process disclosed by Dresher, et al., is 6.
! The weight amounts of sodium hydroxide, per weight equivalent molybdenum in the ore concentrate (i.e. NaOH/~qo weignt ratio), 15,1 which may be employed are in the range of about 0.05 to about¦
0.70, which corresponds to the above-stated 0.12-1.68 stoi-chiometric molar amounts.
Gaseous molecular oxygen is introduced into the slurry in the reaction zone during the reaction as necessary to main' 20l tain as oxygen partial pressure of in the range of from about !
50 to about 500 psia, preferably in the range of from about j 200 to about 400 psia. The total pressure maintained on the ¦
reaction zone is such as to provide for the above-mentioned l oxygen partial pressure. Additional pressure in excess of 25~ that required to satisfy such criteria is not necessary.
During the reaction, the slurry is stirred, preferably by mechanical means.
The reaction is continued for a desired period of time, I
typically in the range of about 2-4 hours in batch processing,j !

, ,' 109~781 ior the equivalent in continuous processing, until the conver-sion of the molybdenum disulfide is essentially complete, or l~no further reaction is occurring, and the pH of the slurry is jlin the range from about 0.05 to about 0.5, preferably about 5!¦0.15 to about 0.45. The time of reaction will vary substan-tiallY depending upon the characteristics of the ore concen-lltrate, and is not a primary variable. However, it has been ''discovered that the reaction time needed to obtain maximum jeconomic conversion of the molybdenum disulfide and on-stream lQl¦utilization of the apparatus is shorter wh~n a strong hydrox-ide is added in the amount defined herein to the reaction zone than if it is not employed, all other variables being constant When the reaction is terminated, the slurry is dumped ',from the reaction zone and the solids are separated from the 15~Imother liquor by decanting and/or by filtering. The mother liquor may be employed in further processing, for instance, to ,irecover the molybdenum content thereof or the rhenium-contain-ing compounds in the mother liquor. The solids comprise pre-~dominantly the product molybdenum trioxide monohydrate. This 20product may be further purified and/or dried by known means or jemployed directly without further purification as an ingredien~
in some processes, such as an alloying addition in the steel industry.
! Molybdenite ore concentrates generally contain up to 25,about 7~ of the hydrocarbon flotation oils employed in the flotation extraction process by which they are derived. It jhas been found advantageous with some ores and ore concentrate$
to remove the predominant portion of the flotation oils to pro vide for increased efficiency in the oxidation reaction. This 30deoiling process generally involves heating tne concentrate to _5_ Ij I
1, ~

- ,' 1094781 il ~emove these oils. This procedure, however, requires relative i~ly expensive equipment, plus being an energy consuming step.
urthermore, the volatilized oils must be controlled by com- ¦
bustion, condensation or some other means to avoid air ¦~ollution.
~¦ It has been found that the flotation oils can be satis-~factorily removed by scrubbing the concentrate with a strong ~alkali solution prior to the oxi~ation step. One method for accomplishing this is to pre-treat the concentrate with a ~Istrong alkali solu~ion, using a high solids content slurry ¦~ith agitation SUCh that the molybdenite particles are caused to rub against, or scrub, one another. SuCh a procedure is generally referred to as attrition scrubbing. Employing the ~attrition scrubbing process further increases the conversion ~of molybdenum disulfide to molybdic trioxide.
In the attrition scrubbing process, the ore concentrate particles are slurried in an aqueous solution of a strong alkali, such as sodium, potassium or ammonium hydroxide, or sodium carbonate, and vigorously stirred. Unlike the oxidation Ireaction process, the solids content of the slurry should be high, over 40~ and advantageously in the range of about 60 to ~about 75%. The amount of strong alkali employed is not chem-liically related to the molybdenum content of the ore concentrat~
~and may suitably be in the range of about 1 to about 10 lbs. of, ~for instance, sodium hydroxide per ton of ore concentrate, ~advantageously sufficient to maintain the pH of the high solidcontent slurry above about 11, up to a maximum of 12-13, durin~
the attrition scrubbing process. A minor amount of surfactant ~of the type classed as wetting agents, may be added to the slurry during the attrition scrubbing process The process --6 - i ., I

1, ,may be conducted at ambient temperature and pressure.
After the attrition scrubbing process, the scrubbed solids are separated (i.e., dewatered) from the liquor by de-~ canting and/or filtering. The solids may then be introduced ~into the reaction conversion zone described above. Alterna-tively, and preferably, the solids are washed again by being slurried, in ordinary water, to a solids content of about 20-30%, and separated from the wash water by decanting and/or ,filtering before being introduced into the conversion '!reaction zone.
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EXAMPLES
The following examples are illustrative of the invention ¦
and are not intended to be limiting in scope. I
The reactions in the following examples were carried out ¦
l'in a two-liter autoclave. At the start of each test approxi-;,mately one liter of tap water or sodium hydroxide solution was~
~added to-the autoclave reactor along with the appropriate weight of concentrate. The autoclave was sealed and the heat-ing cycle initiated. When the desired reaction temperature was reached, the autoclave was vented for a few seconds to ex-'~pel air. Oxygen was then introduced into the reactor to a 'particular pressure and held at this pressure with a regulatorlThe reaction time was determined from the time of oxygen addi-¦
tion. A temperature regulator was set to supply heat when thej temperature fell below the desired reaction temperature. ~x-cess heat, produced by the exothermic oxidation, was removed by cooling water circulated through coils inside the reactor.
jWhen the reaction was completed, the reactor was then opened 29 and the reaction products removed for subseq~i~nt analysis.

jl In the first two examples, a molybde~nite ore concentrate was reacted, as received, both with and without the addition jof sodium hydroxide to the autoclave. Molybdenite conversion ~'in the presence of the proper amount of NaOH was 96.2~ while 5 ¦Iwithout NaOH, but under the same autoclave oxidation condi-~!tions (time, temperature, etc.) it was only 62.7%.

! I EXAMPLE 1 I!
ji 154.0 grams of a molybdenite ore concentrate (53.7% Mo) ~ias received, without any pretreatment (i.e., without grind-10¦¦ing, drying, or deoiling), were slurried with 1000 grams of water and 22.4 grans of sodium hydroxide. The NaOH/Mo weight ratio was 0.270 (NaOH/Mo molar ratio = 0.65) and the llinitial slurry pH was 13.5. The slurry was reacted in the iautoclave at 195C and a total pressure of 500 psig. An 15lloxygen atmosphere was maintained in the autoclave with an jloxygen partial pressure of about 310 psig. The reaction time was four hours.
At the completion of the reaction the pH was 0.29.
~Analysis of the products, solution plus solids, showed that 20 96.2~ of the molybdenite had been converted to molybdic Itrioxide.

I , EXAMPLE 2 A second 154.0 gram smaple of the same molybdenite Iconcentrate was reacted under identical conditions, but 25 without the addition of NaOH to the autoclave. The molyb-denite concentrate was reacted under identical conditions, ~but without the addition of NaOH to the autoclave. The molybdenitc conversion was only 62.7%.
~xam~les 3 through 12 demonstrate the advantage of '~0 adding sodium hydroxide within the range defined in the -8- !

i! 1094781 ¦patent in processing molybdenite ore concentrates that have ¦bèen treated to remove flotation oils. In examples 3 through 7 the oils were removed by heating, or drying, at 240C. In examples ~, 10 and 11, the oils were removed by the more conveninent and economical process of attrition scrubbing the ore concentrate with a sodium hydroxide solution as described herein.

10 ¦ 154.0 grams of another molybdenite ore concentrate ¦~54.4% Mo), previously dried at 240C to remove flotation ~oils, but not sround, were slurried with 1000 grams of water lland 21.5 grams of sodium hydroxide. The NaOH/Mo weight j!ratio was 0.250 (NaOH/Mo molar ratio = 0.60) and the initial 15 ¦~slurry pH was greater than 13. The slurry was reacted in an autoclave at 195C and a total pressure of S00 psig. An ~oxygen atmosphere was maintained in the autoclave with an oxygen partial pressure of 310-315 psi. The reaction time llwas four hours.
20 ~l At the completion of the reaction the pH of the slurry was 0.15. The slurry was filtered and the filter cake ~washed. The filtrate and washings were combined and analyzed l~for molybdenum, as was the filter cake. The solution volume i was 1.225 liters and contained 6.6 grams/liter molybdenum.
25 j It was found that 99.2~ of the molybdenite had reacted to form hexavalent molybdenum, of which 9.5% was dissolved in '¦the solution phase as molybdate ions and 30.5~ remained in the leach solids as molybdic trioxide. The filter cake was I! dried to convert the solids to technical grade molybdic 30 l¦trioxide. ~nalysis of th~ product showed it to contain , ' 'I

~¦0~05% sulfur. The sulfur specification for technical grade llmolybdenum trioxide is generally 0.2~ sulfur or less.
i!
, EXAM2LE 4 ~' Another 154.0 grams of the same molybdenite concentrate, 5 ~lpretreated the same as the sample in Example 3, were slurried with 1000 grams of water. In this example, however, there was no addition of sodium hydroxide. The initial !
,jslurry pH was 5.8. The slurry was reacted in an autoclave ¦iunder the same process conditions as the sample in ~xample 10 113- At the completion of the reaction the s1urry pH was less than 0Ø The slurry was filtered and the filter cake llwashed with deionized water. The volume of the combined filtrate and washings was 1.225 liters and contained 8.0 grams/liter molybdenum.
; 15 il Complete analysis of the product showed that 97.2% of ¦Ithe molybdenite had reacted to form hexavalent molybdenum, ,jof which 11.8% was dissolved in the solution phase and 88.2%
,remained in the leach solids as molybdic trioxide. Analysis ,of the dried solids product showed it to contain 0.50%
;
20 isulfur, and it therefore did not meet the specifications llfor a technical grade molybdenum trioxide.
i.~
'~ EXAMPLE 5 ~ A third identical test sample was treated in the same ,fashion except that 71.6 grams of sodium hydroxide were 25 ,~added giving a NaOH/Mo weight ratio of 0.833 (~aOH~Mo molar ratio = 2.0). The pH of the reacted slurry was 0.55. ~ow-iever, under these conditions only 93~ of the molybdenite ~h(~d reacted. Furthermore, 45% of the conversion product was dissolved in the aqueous phase. The combined filtrate and washings contained 25.5 grams/liter of molybdenum as , 11 ,.,, i ! !molybdate .

'I In the following Examples 6 and 7, a low grade molyb-ldenite ore concentrate was treated in the same manner as the ,concentrate in Examples 3 and 4 above, that is, after drying 5 lito remove flotation oils it was reacted in an autoclave with ¦jand without, respectively, the addition of sodium hydroxide.
jjWhen the concentrate was reacted with the addition of sodium Ihydroxide (Example 6) as described herein, 100% conversion ilof the molybdenite was achieved and therefore a product 10 limeeting the sulfur specification of a technical grade molyb-¦Idenum oxide could be prepared by then drying to remove ab-sorbed and combined water.
t Under the same process conditions, but without the ~jaddition of sodium hydroxide (Example 7) in the range speci-lS l'fied herein, only 97.3~ of the molybdenite in the concen-,trate was converted to molybdic acid. Further purification would therefore be reuqired to prepare a molybdenum oxide Iproduct meeting the commercial specifications for a technical j~grade molybdenum trioxide. This purification might involve, -~ 20 Ifor instance, extracting the molybdenum oxide with ammonia, 'recovering ammonium dimolybdate by evaporative crystalliza-,jtion, heating the ammonium dimolybdate to drive off ammonia i~! and convert it to molybdenum trioxide, and recovering the l!ammonia so that it can be recycled in the process. The 25 ''economic advantage of being able to obtain 99% or greater molybdenite conversion under relatively mild autoclave con-ditions is significant.

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'I 10~4q81 !

A further sample of 154.0 grams of a low grade molyb-denite ore concentrate (39.6~ Mo) previously dried at 1¦240C to remove flotation oils, but not ground, was slurried 5 ¦¦with 1000 grams of deionized water containing 16.4 grams of sodium hydroxide. The NaOH/Mo weight ratio was 0.258 and l the initial slurry pH was greater than 13. The slurry was i reacted in an autoclave at 185C and a total pressure of l 500 psig for four hours. An oxygen atmosphere was main-10 ¦ tained in the autoclave with an oxygen partial pressure of350 psi.
At the completion of the reaction the pH of the slurry as~o.43. The slurry was filtered as before. Analysis l¦showed that 100% of the molybdenite had been converted, of 15 Iwhich 16.3~ was dissolved in the aqueous phase and 83.7% was ¦precipitated in the leach solids as molybdic trioxide.

i; i ll EXAMPLE 7 ~ Il.
Another 154.0 grams of the same deoiled molybdenite concentrate as used in Example 6 were slurried with 1000 20 ¦ grams of water. In this example, however, there was no ! addition of sodium hydroxide. The initial slurry pH was l 4.26. The slurry was reacted in an autoclave under the same I ¦ conditions as the sample in Example 6. The slllrry was i filtered and the products analyzed as in Example 6. Only 25 ¦ 97.3~ of the molybdenite had been converted, 17.5~ of which liwas dissolved in the aqueous phase.
In the absence of sodium hydroxide, the proportion of ¦reacted molybdic acid that reported to the aqueous phase llincreased by 7.4~. Furthermore, 2.7% of the nolybdenite was 30 ''unreacted at the end of the reaction. The calculated sulfur 1'1 , -12-10~347~
.,1 1 content of the residue, due to the unrea~ted molybdenite con- ¦
~tained therein, would be 0.8%, substantially above the sulfur ¦
Ispecification of 0.2~ maximum.
, In the following Examples 8-11, the molybdenite concen-,trates were deoiled by attrition scrubbing with sodium hydrox-ide rather than by heating at 240C. Although the removal of the oils was not quite as effective in the examples cited '!below as that obtained by heating or drying at 240C, the economical and environmentai advantages of attrition scrubbing 10 jas opposed to driving off or volatilizing the oils by heat-¦ing should be apparent.

,l 400 grams of the same molybdenite concentrate as in '~~examples 3, 4, and 5 above (54.4% Mo) as received without any 15 ''pretreatment (i.e., without grinding, drying, or deoiling) ~were scrubbed at 75~ solids for five minutes with sodium hydroxide addition to pH 12.2. The pulp was filtered, repulped to 2~v solids and refiltered. 169.7 grams of the wet pulp (90.75% solids) were added to the autoclave along 20 i,with 20.9 grams I~aOH (a NaOH/Mo weight ratio of 0.25) dis- ¦
'solved in 1000 grams of water. The slurry was reacted at 195C and a total pressure of 500 psig for four hours. The ,oxygen partial pressure was 310-315 psi.
' At the completion of the reaction the pH of the slurry 25 ''was 0.15. Analysis of the products showed that 98.3~ of the molybdenite had been converted.

. ' .

~, -13- , A second portion of the same molybdenite concentrate twithout prior removal of the flotation oils was reacted under ~jidentical conditions, but without the addition of NaOh to 5 l¦the autoclave. The molybdenite conversion was only 92.5%.

¦ EXAMPLE 10 400 grams of the same molybdenite concentrate as in examples 6 and 7 above (39.6~ Mo) as received without any Ipretreatment (i.e., without grinding, dryin~, or deoiling) were scrubbed at 75% solids for five minutes with sodium hydroxide addition to pH 12.2. The pulp was filtered, re-¦Ipulped to 25% solids and refiltered. 192.0 grams of the wet cake were added to the autoclave along with 16.8 grams NaOH/
¦MO weight ratio = 0.25) dissolved in 1000 grams water. The slurry was reacted at 195C and a total pressure of 500 psig for four hours. The oxygen partial pressure was 310-315 psi.
At the completion of the reaction the slurry pH was 0.45. The slurry was filtered and the filter cake washed l¦with deionized water. The volume of the combined filtrate 20 lland washings was 1.235 liters and contained 7.42 grams/liter !l o f molybdenum .
Complete analysis of the products showed that 98.4~ of the molybdenite had reacted to form molybdic acid.

), 25 il A second sample of the same molybdenite concentrate, ¦! attrition scrubbed with NaOH as above was reacted under iden-l tical conditions but without the subsequent addition of ! NaO~I to the autoclave. The total molybdenite~conversion was 93.6~.

,j 10~478~

i! EXAMPLE 12 '~ A third sample of the same molybdenite concentrate, this ¦Itime without prior removal of the flotation oils, was reacted llunder the same conditions as above, but without the addition 5 i¦of NaO'I to the autoclave. The total molybdenite conversion ~was 88.7%.
!I For maximum effectiveness of the process of this inven-~,!tion, prior deoiling of the concentrate by attrition scrub-'bing with a base such as sodium hydroxide is to be preferred.
10 .IIt should be understood that increasing the reaction time or reaction temperature slightly would be sufficient to in-sure essentially 100~ conversion of molybdenite to molybdenum ¦Itrioxide in Examples 8 and 10 above. However, in the absence ~ .
'l~f sodium hydroxide, such as in Examples 9, 11 and 12, much 15 ¦~longer reaction times and/or temperatures would be required ¦~to obtain the desired results to the point of being commer-'cially impractical and/or uneconomical. For instance, at i230C severe corrosion is experienced in the autoclave even liwhen it is constructed of rather exotic materials (e.g., zir-20 l¦conium, nickel and molybdenum alloys). -Whereas there are here specifically set forth certain preferred embodiments which are presently regarded as the lIbest mode of carrying out the invention, it should be under-,jstood that various changes may be made and other procedures 25 ~adopted without departing from the inventive subject matter ¦particularly pointed out and hereinafter claimed.
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i Having thus described the invention, what is claimed is: I
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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for the conversion in a reaction zone of a molybdenum disulfide to molybdic trioxide by maintaining an agitated aqueous liquid slurry of particles of molybdenum disulfide-bearing material at a temperature in the range of from about 150 to about 230°C in contact with gaseous molecular oxygen having a partial pressure in the reaction zone in the range of from about 50 to 500 psi for a period of time sufficient to effect conversion of molybdenum disulfide to molybdic trioxide and thereafter recovering product solid molybdic trioxide from said slurry, the improvement of adding to said reaction zone in the range of from about 0.12 to about 1.68 mols of a strong hydroxide per mol of molybdenum disulfide and continuing said conversion reaction and thereafter separating from said slurry a solids fraction containing predominately molybdic trioxide.

2. The improved process of claim 1 wherein said strong hydroxide is sodium hydroxide.

3. The improved process of claim 1 wherein the amount of said strong hydroxide is in the range of about 0.48 to 0.90 mols per mol of molybdenum disulfide.

4. The improved process of claim 1 wherein said process is continued until the pH of said slurry is in the range of from about .05 to about 0.5.

5. The improved process of claim 1 wherein the tempera-ture of the reaction zone is maintained in the range of about 185-205°C.

6. The improved process of claim 1 wherein the total conversion of molybdenum disulfide to hexavalent molybdenum is not less than 95%.

7. The improved process of claim 1 wherein the conver-sion of molybdenum disulfide to hexavalent molybdenum is not less than 99 mol percent, and the sulfur content of the solid product is not more than 0.2% of the molybdic trioxide therein.

8. The improved process of claim 1 wherein said moly-bdenum disulfide-bearing material is pre-treated prior to being introduced into said reaction zone by scrubbing agita-tion in an aqueous alkali solution slurry comprising at least 40% solids and the liquid phase separated therefrom before said materials are introduced into said reaction zone.

9. The improved process of claim 8 wherein said material is scrubbed, dewatered, reslurried, and again dewatered before introduction into said reaction zone.

10. The improved process of claim 8 wherein a sur-factant is added to said aqueous alkali solution.