CA1217580A - Differential flotation reagent for molybdenum separation - Google Patents

Abstract

DIFFERENTIAL FLOTATION REAGENT FOR
MOLYBDENUM SEPARATION

ABSTRACT OF THE DISCLOSURE
A dry differential flotation reagent for molybdenum-bearing ore is disclosed comprising from about 1 to about 30 percent by weight of NaSH, 0 to 20 percent by weight of NaOH, 60 to 90 per-cent by weight of a mixture of sodium thiophosphates comprising, as major components thereof, Na3PS4, Na3PS30, Na3PS202, and Na3P03S, with Na3PS4 further comprising from about 15 mole per-cent to about 70 mole percent of the phosphate mixture. The flotation reagent can be prepared by reacting P4S10 with NaOH
and NaSH in a molar ratio of about 1:16. When added to a flo-tation vessel containing molybdenum concentrate, the flotation reagent promotes suppression of lead and copper sulfides, even without the addition of sodium cyanide, to permit the improved recovery of molybdenum with a lower level of impurities.

Description

~2:1~5~1 DIFFERENTIAL FLOTATION REAGENT FOR
MOLYBDENUM SEPARATION
BACKGROUND OF THE INVENTION
Sulfidic ores, prevalent in the Western United States, contain amounts of copper, molybdenum, lead, iron and other elements and are mined for these metals. Separation or refining operations for this type of ore are commonly achieved by physically grinding or pulverizing the crude ore, followed by separation of copper, Malibu-denim and other metals through differential flotation methods.
on This invention applies to various chemical techniques used in differential flotation.
Through the use of differential flotation, sulfidic ores, which contain small amounts of copper sulfide and molybdenum dip sulfide; may be separated into fractions or concentrates which us-timately contain 90% or more copper sulfide by weight in a copper concentrate or up to 90% or more molybdenum disulfide (Most) by weight in molybdenum concentrate. These methods of differential flotation separation are generally well known and are in widespread commercial use.
Various differential flotation techniques have been used to separate and concentrate minerals, and many chemical compositions haze been employed for this purpose. In some froth flotation tech-piques, the concentrate of a preferred metal salt floats to the surface of a flotation vessel while the non-preferred salts are suppressed and discarded. Other techniques concentrate one pro-furred metal salt by flotation, while another preferred metal salt is suppressed and later recovered from the tailings. In primary copper recovery, for example, a molybdenum suppressant, such as dextrin, can be added to a flotation cell to cause the molybdenum to sink while allowing the copper to float. The tailings contain-in suppressed material can then be added to another flotation cell for flotation (and recovery) of molybdenum. In an alternate process, or 75~

a copper suppressant, such as Notes Reagent, or alternatively sodium hydrosulfide, can be added to the primary flotation cell and the tailings added to a secondary cell for recovery of copper. This invention is primarily concerned, however, with the first approach.
Since many minerals occur as sulfides in nature, methods for separating metal sulfides from their environment are well known.
For instance, one commercial process grinds the molybdenum ore to a fine sand and disperses the ground ore in water. The ore slurry is then pumped to a flotation cell where it is mixed with a light-lo weight oil. Air bubbles passing through the flotation cell carry the molybdenum particles to the surface in the form of molybdenum concentrate which is collected for further processing.
Through the use of certain chemicals in the final stages of flotation processing, the physical properties of the metallic sulk fixes can be altered permitting fine particles of the selected metallic sulfide to be further concentrated in the processing foam, or to otherwise float and be removed, while other metallic sulfides are relatively unaffected and remain dispersed or suppressed in the aqueous phase. Inorganic chemicals which affect the physical pro-parties of sulfidic ores in this way and allow their separation include aqueous solutions of phosphorus, sulfur, arsenic, and an-ptomaine compounds to name but a few. Organic chemicals which are useful in sulfidic ore flotation include hydrocarbon oils, Jan-hates, alcohols, and silicones, as well as various surfactants and wetting agents. It is believed that chemicals which function to suppress flotation of metal salts, or metal sulfides in the case us sulfidic ores, do so by altering the surface caricaturist-tics of one or more of the metal salts (sulfides) so that they remain in suspension, while allowing a selected metal salt (sup-Fidel to float to the surface.
In the case of molybdenum, the most important domestic source of this mineral is molybdenite ore which contains molybdenum in ~LZ~75~30 the form of molybdenum disulfide (Most), as well as sulfides of copper, lead, and iron. Suppression of lead sulfide or copper sulfide) can be accomplished by the addition of a suppressant so-lotion to the molybdenum concentrate, commonly known as Notes Reagent, which is well known for this purpose. Typically, Notes Reagent is prepared by reacting P4Slo in an aqueous solution of sodium hydroxide at a molar ratio of about 1:14, respectively.
See, for example, US. Patent 2,492,336, issued December 27, 1949, to Notes et at., which describes the preparation of Notes Reagent in wet or dry form, and its use as a depressant for the froth flotation of molybdenum. This patent discloses the prepare-lion of a dry product by suitable means such as spray drying.
Where economics are favorable, the dry product can be prepared at a central location and shipped to the mining site for convent tent use. Various other flotation methods and chemicals are also described in US Patent 2,811,255, issued October 29, 1957, to Notes et at., US Patent 2,608,298, issued August 26, 1952 to Arbiter et at., and US Patent 3,785,488, issued January 15, 1974 to Werneke.
The preparation of such Notes Reagent solutions at the mining or refining site is frequently associated with numerous problems and difficulties, such as the evolution of poisonous gases, e.g.
hydrogen sulfide. In addition, mixtures of P4Slo and sodium ho-reaccede have been known to react with explosive violence if the reaction conditions are not properly or carefully controlled.
Thus, safety in handling, storage and use as well as difficulty in preparation, continues to be a major problem of great concern throughout the industry, and is a major factor in the search for alternate technology.
The suppression of copper sulfide, and to some extent, iron sulfide, is generally accomplished by the addition of sodium cry-aside to the molybdenum concentrate either concurrently with the ~2~7S~(~

Notes Reagent or in a separate processing step. However, sodium cyanide is also hazardous to use and must be handled with extreme care. Further, the effluent resulting from the use of sodium cry-aside in froth flotation is an environmental contaminate and govern-mint regulations require treating such effluents with an oxidizing agent, such as chlorine dioxide, which increases the cost of the flotation process.
Accordingly, the present invention seeks to provide a dip-ferential flotation reagent for molybdenum ore which is economical, JO convenient to use, and results in a cleaner flotation process due to the improved suppression of metallic impurities.
This invention also seeks to provide a differential flotation reagent which will permit elimination of sodium cyanide in the flotation process without substantial loss of performance.
The differential flotation reagent of the present invention comprises from about 1 to about 30 percent by weight of Nash from about 0 to about 20 percent by weight of Noah, and from about 60 to about 90 percent by weight of a mixture of sodium thiophosphates, the primary thiophosphate components being selected from the group g 3 So, Nips Nips and Nips. The No PUS
component additionally comprises at least about 15 mole percent to about 70 mole percent, and preferably from about 30 mole percent to about 70 mole percent, of the sodium thiophosphate mixture.
Such compositions can be prepared by reacting P4S1o with a mixture of Noah and Nash in a molar ratio of P4S10:NaOH:NaSH of from 1:16:0 to 1:8:8 and at a temperature suitably in the range of from about 50C. to about 70C. Excess water can be advantageously removed from the reaction mixture by flash-drying the product at a temperature in the range of from about 130C. to about 170C. to form a substantially dry product suitably having a moisture con-tent of less than about 5 weight percent.

..

7~80 When used in the froth flotation process for molybdenum recovery, the differential reagent of the present invention results in the improved suppression of lead sulfide, and also results in acceptable levels of copper sulfide suppression without the use of sodium aye-nude in the flotation process.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, an improved differ-entail flotation reagent for separation of molybdenum from Malibu-denum-bearing ore can be prepared by reacting P4Slo with an alkali mixture of Noah and Nash in a molar ratio of P4Slo to alkali of from about 1:15 to about 1:17, respectively, with the NaOH/NaSH
mixture containing at least about 50 mole percent of Noah. The reaction is advantageously carried out at a temperature in the range of from about 50C. to about 70C. and at about atmospheric pressure by the controlled addition of P4Slo to an aqueous solution of Noah, or Noah and Nash followed by the removal of excess water from the reaction mixture.
This procedure contrasts with the preparation of the standard Notes Reagent which is typically prepared by reacting P4Slo with Noah in a molar ratio of 1:14, respectively. The use of additional Noah in the present invention yields a product which contains a higher content of sodium tetrathiophosphate (Nips) in comparison to conventional Notes Reagent. The substitution of Nash in part for Noah, again at the higher molar ratio, increases the sodium tetrathiophosphate content even more. For example, the reaction of P4Slo with Noah in a molar ratio of about 1:16 produces a pro-duct which contains a mixture of phosphorus compounds such as 3 4 3 3 , Nips, Nips, Nope, Nope' and No HO S
The sodium tetrathiophosphate (Nips) content of this mixture is about 15% on a molar basis. The partial substitution of Nash for ~2~7S8~

Noah can further increase the level of sodium tetrathiophosphate to as much as 70 mole percent. This compares with a sodium twitter-thiophosphate content of only about 8% for standard Notes Reagent.
Whole not wishing to be bound to any particular theory of operabi-lily, it is believed that this higher level of sodium tetrathio-phosphate contributes to both (1) increase the suppression of lead sulfide and copper sulfide in the froth flotation process, pro-during a cleaner molybdenum concentrate, and (2) increase the sup-press ion of copper sulfide without the use of sodium cyanide, permitting the elimination of sodium cyanide in the flotation process.
The preparation of sodium tetrathiophosphate has been prove-ouzel demonstrated in the literature on an experimental basis. It is known that P4Slo and sodium sulfide hydrate can be reacted as follows:

Noah 9H20 + P4Slo -I nips + 9H20 After dissolving the reaction product in additional water and filtering off the insoluble, the filtrate is cooled to crystal-live Nips 8H20. This procedure is disclosed by E. Glatzel in Z. anorg. all. Chum. 44, 65-78 (1905), which also discloses that the crystalline product hydrolyzes readily unless purified very carefully, and thus has a poor shelf life. This procedure would not appear to be readily adaptable for use in a production scale process. In addition, the use of sodium tetrathiophosphate as a differential flotation reagent is not disclosed.
The reaction of P4Slo with either Noah or Noah and Nash according to the present invention is carried out in an aqueous medium. The water must be efficiently removed, since the sodium tetrathiophosphate can quickly hydrolyze to sodium orthophosphate and/or lower sodium thiophosphates under certain conditions.

~L7580 For instance, the retention of solution in the 100C. to 125C.
range at atmospheric pressure, where water is only slowly removed from the slurry, could cause extensive hydrolysis within 2 or 3 hours.
The removal of part of the water from the reaction mixture can be conveniently accomplished by distillation, preferably under vacuum at a temperature in the range of from about 70C to about 90C. and a pressure in the range of from about 50 mm to about 100 mm Hug to form a liquid slurry. Water remaining in the liquid slur-rye after distillation can then be dried using conventional tech-piques suitable for converting thick slurries to dry solids, such as by means of a vacuum tray drier under the conditions described above for distillation, or at above about 130C. at atmospheric pressure, spray drying or drying in a fluidized bed, to form a free flowing solid having a moisture content of less than about 5% by weight, preferably less than 1%. The term "dry" as used in the specification and claims to describe the present differential flow station reagent refers to compositions having such a moisture content.
Alternately, the reaction product can be dried directly after preparation, with or without an intervening distillation step, by flash drying using, for instance, a heated drum operated at a them-portray of from about 130C. to about 170C and at about atoms-phonic pressure. A suitable piece of equipment for flash drying would be a double drum dryer heated with 100 prig. steam having a 1-2 mm thick layer of product on the drum.
Although the present invention has been described above and in the examples in terms of various methods of preparation, the differential flotation reagent of this invention is not to be con-trued as being limited to any particular method or technique of preparation. As broadly conceived, the improved differential flow station reagent comprises from about 1 to about 30 percent by weight ~7580 of Nash from about O to about 20 percent by weight of Nash, and from about 60 to about 90 percent by weight of a mixture of sodium thiophosphates the primary components of which are selected from the group consisting of Nips, N3PS30, Nips, 3 3 Nips component additionally comprising from about lo mole per-cent to about 70 mole percent, and preferably from about 30 to about 70 mole percents of the sodium thiophosphate mixture.
The dried differential flotation reagent, typically in the form of a flaked or powdered product, is particularly hydrophilic and must be shipped or stored in moisture proof containers to prevent hydrolysis. Other than this consideration, shipping and handling of the product is relatively safe and no unusual precautions are needed at the mining or refining site. Alternately, the different trial flotation reagent can be prepared directly at the mining or refining site.
In commercial use, the flotation reagent, usually in the form of an aqueous solution, is added directly to a flotation vessel or cell containing the molybdenum concentrate which has been processed to remove coarser impurities. Typically, this concentrate will con-lain about 3 to lo by weight of molybdenum as compared with 0.2% by weight of molybdenum contained in the primary ore. The flotation reagent is advantageously added to the vessel in an amount of from about Owl lobs. to about lo lobs., and preferably from about 0.2 lobs. to about 0.5 lobs. (P4Slo equivalent) per ton of molybdenum concentrate. Such amounts are effective in suppressing the other metallic sulfides contained in the concentrate, particularly lead and copper sulfides, to produce a purified or clean concentrate containing about 90% by weight of molybdenum sulfide. The high degree of suppression of copper sulfide is surprising in view of the fact that the addition of sodium cyanide to the flotation ~Z~7S8~

vessel us not required. This results in a marked improvement in the economics of the flotation process since (1) the purchase of sodium cyanide is obviated, and (2) removal of cyanide from efflu-en streams is also obviated.
In another commercial use, the reagent is added to a concern-irate where copper sulfide is the major component, where it acts to suppress the copper minerals while molybdenum sulfide by-produce is floated off and recovered. The addition rate would be higher in this case, i.e. 1 lobs. or more (P4Slo equivalent) per ton of concentrate.
The following examples are intended to further illustrate the various embodiments and advantages of the present invention with-out limiting it thereby.
Examples 1 and 2 are comparative examples illustrating the preparation of a conventional Notes Reagent in wet and dry form, respectively.

60 Grams of P4Slo was dissolved in a solution of 75.6 grams of Noah in 464 grams of water. The P4Slo was added over a one hour period, and the reaction mixture was vigorously stirred and maintained at a temperature between 60C and 70C. The mole ratio of P4Slo to Noah was 1:14. The final solution contained 10% (wt.) P4Slo, where "% (wt.) P4Slo" represents total phosphorus content calculated from Pow input and material balance data. The disk tribution of phosphorus compounds includes the following major come pennants, the relative mole % being determined by nuclear magnetic resonance using a phosphorus-31 probe:

~2~L75B(~

COMPOUND NATIVE MOLE %
Nips 8 Na3PDS3 51 Nips 19 Nips 4 ape Nope 11 Other 6 1220 Grams of P4Slo was added over a 3-hour period to a solution of 1537 grams of Noah in 2686 grams of water, the react lion mixture being maintained at a temperature between 70C and 75C~ The resulting solution, containing 23% (wt.) P4Slo, was then stirred at the same temperature for one hour. Portions of this mixture were then dried using a double drum dryer to yield a flaked product. The 6x8 inch drums had a gap of 0.13 inches, were rotated at 9 rum, and were internally heated with steam at 84 prig. The mole ratio of P4Slo to Noah was 1:14. The dried product contained 43.1 weight percent P4Slo and had a disturb-lion of phosphorus compounds with the following major components:
COMPOUND RELATIVE MOLE %
Nips 10 Nips 51 Nips 17 Nips 7 Nope 3 Nope 12 Examples 3-6 illustrate the preparation of the dry differential flotation reagent of the present invention having various relative proportions of sodium tetrathiophosphate (Nips).

Ida 121~75~0 60 Grams of P4Slo was added over a 20-minute period to a solution of 86.4 grams of Noah in 154 grams of water, and the solution was stirred for an additional 15 minutes. The mole ratio of P4Slo to Noah was 1:16. The reaction mixture tempera-lure was maintained at 60C. to 70C., and the final concentration was 21% (wt.) P4Slo. The mixture was evaporated for one hour by vacuum distillation (93C. and 300 mm. of Hug) to 25% (wt.) P4Slo.
Final drying as a thin layer was done by retention in a vacuum oven at 110C. and 300 mm of Hug for two hours. The final dry product contained 40.6% (wt.) P4Slo and had the following composition:
71% (wt.) sodium thiophosphates 6% (wt.) sodium phosphates and phosphates 1% (wt.) Nash 17% (wt.) Noah 5% (wt.) H20 The distribution of phosphorous compounds, in terms of relative mole %, was as follows:
COMPOUND RELATIVE MOLE %
Nips 15 Nips 48 Nips 18 Nips 7 Nope 2 Nope 10 60 Grams of P4Slo was dissolved in a solution of 71.2 grams of flaked Nays (60% dry basis) and 43.2 grams of Noah in 90 grams of water, to form an initial mixture of 23% (wt.) P4Slo. The US

mole ratio of Na2S:NaOH:P4S10 was 4:8:1, which is equivalent to a ratio of NaSH:NaOH:P4S10 of 4:12:1 after the mixed alkali was dissolved.* The reaction conditions were the same as in Example 3. The final dry product contained 41.5% (wt.) P4Slo and had a distribution of phosphorous compounds with the following major components:

COMPOUND RELATIVE MOLE %
Nips 38 Nips 27 Nips 13 Nips 1 1 Nope 3 Nope 5 60 Grams of P4Slo was dissolved in a solution of 84.1 grams of flaked Nash (72% dry basis) and 43.2 grams Noah in 113 grams of water, to form a reaction mixture of 20% (wt.) P4Slo. The mole ratio of NaSH:NaOH:P4S1O was 8:8:1. The reaction conditions were the same as in Example 3. The final dry product contained 40.4%
(wt.) P4Slo and had a distribution of phosphorous compounds with the following major components:
COMPOUND RELATIVE MOLE %
Nips 66 Nips 18 Nips 7 Nips 5 Nope 2 Nope *Nays is converted to Nash and Noah upon reaction with water as follows:
Nays + H20 Nash + Noah lzl~s~lo A jacketed 50-gallon reactor equipped with an agitator was charged with a mixed alkali solution of 174.2 lobs. of 50% (wt.) Noah solution plus 305.2 lobs. of 40% (wt.) Nash solution. The mixed alkali solution was heated to 50C. During the addition of P4Slo, the reactor was maintained at 70C. After addition was complete, the reaction mixture was held at 70C for two hours with stirring to insure complete reaction of P4Slo with the alkali.
The solution was then fed to the double drum dryer described in Example 2, the drums being heated internally with steam at 100 prig. (164C). The feed was 50% (wt.) solids (dry basis) fed at the rate of about 15 lbs/hour to yield about 7.5 lbs./hour of dry product. The product was a free flowing coarse solid having the following composition:
70% (wt.) sodium thiophosphates 25% (wt.) Nash 3% (wt.) Nope 1% (wt.) Nope 1% (wt.) ~2 The distribution of phosphorus compounds, in terms of relative mole %, was as follows:
COMPOUND RELATIVE MOLE %
Nips 59 Nips 20 Nips 8 Nips 5 Nope 6 Nope --Other 2 ISLE

As shown in Examples 3-67 which illustrate various aspects of the present invention, the differential flotation reagent of this -invention has a minimum sodium tetrathiophosphate content of about 15% (see Example 3), while the sodium tetrathiophosphate of the prior art Notes Reagent is between 8% and 10%.
Example 7 illustrates the preparation of a rough molybdenum concentrate for testing differential flotation reagents in a Sims slated froth flotation process.

A slurry containing 20 Kg. of crude molybdenite ore was ground in a rod mill then conditioned with agitation in a Denver D-l flow station cell. During the agitation, lime, vapor oil as a collection, and Arctic Syntax as a detergent were added. A pine oil frothier was added, and a concentrate containing molybdenum values was floated off upon sparring with air. This rougher concentrate was recovered by filtration, typically 280 grams dry weight. Typical analyses were as follows:
Crude Ore Rougher Concentrate % My 0.2 11.0 % Cut 0.01 0.28 % Pub 0.005 0.1 % Fe 1.5 3.2 Metal values were present in the concentrate primarily in the form of sulfide minerals: molybdenite, chalcopyrite, pyrites and Golan.
Examples 8-10 are comparative examples illustrating the use of conventional Notes Reagent in a simulated froth flotation process to separate molybdenum values from copper iron, and lead sulfides.

The rougher concentrate of Example 7 was reslurried in water and subjected to a regrinding step. During this step, lime and ~217S80 vapor Gil collector were added. Sufficient conventional differ-entail flotation reagent of Example 1 was added to provide a weight ratio of 0.25 lobs. P4Slo (contained) per ton of rougher concentrate (dry basis). The slurry was then conditioned in the flotation cell with agitation, while lime was added to adjust the slurry to a pi of 9.0, and sodium cyanide was added to a weight ratio of 0.25 lobs.
Nan per ton of rougher concentrate. Air spurge was introduced, and the cleaner flotation step was allowed to continue for 4 minutes.
The cleaner concentrate was recovered from the froth and the tailings were recovered, weighed and analyzed. The % recovery of molybdenum in the cleaner concentrate and the % of copper, lead and iron found on the tailings was calculated for the cleaner flotation step. The results were as follows:
Metal % Recovery My 96.9 (cleaner concentrate) Cut 63.6 (tailings) Pub 74.2 (tailings) Fe 79.4 (tailings) The procedure of Example 8 was repeated except that the differential flotation reagent of Example 2 was used in the cleaner concentration step. The results were as follows:
Metal % Recovery My 96.9 cleaner concentrate) Cut 64.0 tailings) Pub Not Determined (tailings) Fe Not Determined (tailings) ~2i75~30 The procedure of Example 8 was repeated except that no sodium cyanide was added in the cleaner conditioning step. The results were as follows:
Metal % Recovery My 96.8 (cleaner concentrate) Cut 36.9 (tailings) Pub 73.4 (tailings) Fe 74.2 (tailings) lo Examples 11-13 illustrate the use of the differential flotation reagent of the present invention in a simulated froth flotation pro-cuss to separate molybdenum values from copper, iron, and lead sulfides.

The procedure of Example 8 was repeated except that the differ-entail flotation reagent of Example 4 was used in the cleaner flow station step. The results were as follows:
ME % RECOVERY
My 97.4 (cleaner concentrate) Cut 73.3 (tailings) Pub 82.6 (tailings) Fe 76.5 (tailings The procedure of Example 8 was repeated except that the differential flotation reagent of Example 5 was used in the cleaner flotation step. The results were as follows:
METAL % RECOVERY
My 97.8 (cleaner concentrate) Cut 70.0 (tailings) Pub 82.6 (tailings) Fe 79.0 (tailings) ~75~

The procedure of Example 12 was repeated except that no sodium cyanide was added in the cleaner conditioning step. The results were as follows:
METAL % RECOVERY
My 96.3 (cleaner concentrate) Cut 56.9 (tailings) Pub 84.6 (tailings) Fe 81.5 (tailings) A comparison of the results of Examples 8 and 9 with the results of Examples 11 and 12 illustrates the improved suppression of copper and lead obtained using the differential flotation no-agent of the present invention. A further comparison of Examples I and 13 illustrates the improved suppression of copper and lead obtained without the addition of sodium cyanide using the present differential flotation reagent.
While various embodiments and exemplifications of this invention have been shown and described in the specification, modifications and variations thereof will be readily appreciated by those skilled in the art. It is to be understood, therefore, that the appended claims are intended to cover all such modifications and variations which are considered to be within the scope and spirit of the pro-sent invention.

Claims (13)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
   OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   A dry differential flotation reagent for molybdenum-bearing ore comprising from about 1 to about 30 percent by weight of NaSH, from about 0 to about 20 percent by weight, NaOH and from about 60 to about 90 percent by weight of a mixture of sodium thiophosphates having primary components selected from the group consisting of Na3PS4, Na3PS30, Na3PS202, and Na3PO3S, with the Na3PS4 component of said thiophosphate mixture being present in an amount of from about 15 mole percent to about 70 mole percent.
2. The composition of Claim 1 which comprises from about 30 mole percent to about 70 mole percent Na3PS4.
3. A method for preparing a dry differential flotation reagent for molybdenum-bearing ore comprising the steps of:
   a) reacting P4S10 with an alkali mixture of NaOH and NaSH in a molar ratio of P4S10 to alkali of from about 1:15 to about 1:17, said mixture containing at least about 50 mole percent of NaOH, and b) drying the reaction mixture.
4. The method of Claim 3 wherein the alkali mixture of NaOH and NaSH contains at least about 75 mole percent NaOH.
5. The method of Claim 3 wherein the alkali mixture contains 100 mole percent NaOH.
6. 6. The method of claim 3, wherein water is removed from the reaction mixture by distillation prior to drying.
7. 7. The method of claim 6, wherein the distillation is con-ducted at a temperature in the range of from about 70°C. to about 50 mm to about 100 mm Hg.
8. 8. The method of claim 3, 4 or 5, wherein the reaction is conducted at a temperature in the range of from about 50°C.
   to about 70°C.
9. 9. The method of claim 3, 4 or 5, wherein the reaction mixture is flash dried at a temperature in the range of from about 130°C. to about 170°C.
10. 10. The method of claim 3, 4 or 5, wherein said molar ratio is about 1.16.
11. 11. The method of claim 6 or 7, wherein the alkali mixture of NaOH and NaSH contains at least about 75 mole percent NaOH and said mole ratio is about 1:6.
12. 12. The method of claim 6 or 7, wherein the alkali mixture contains 100 mole percent NaOH and said mole ratio is about 1:6.
13. 13. A method for recovering molybdenum from molybdenum-bearing sulfidic ore containing copper, lead and iron as impurities, said method comprising the steps of:
    a) reducing the size of the ore to particulate form, b) feeding the finely ground ore to a flotation vessel to obtain particles having a higher concentration of molybdenum than the original ore, c) feeding the particles obtained from step (b) to another flotation vessel containing differential flotation reagent comprising the reaction product of P4S10 and an alkali mixture of NaOH and NaSH
    in a molar ratio of P4S10 to alkali mixture of from about 1:15 to about 1:17, said mixture con-taining at least about 50 mole percent of NaOH, and d) recovering a concentrated molybdenum-containing material from step (c).
    
    The method of Claim 13 wherein the reaction product of step (c) contains about 50 mole percent of NaOH.
    
    The method of Claim 13 wherein sodium cyanide is fed to the flotation vessel in step (c).
    
    The method of Claim 13 wherein the reaction product of step (c) is present in an amount of from about 0.1 lbs. to about 1.0 lbs. of P4S10 equivalent per ton of material produced in step (b).
    
    The method of Claim 16 wherein the reaction product of step (c) is present in an amount of from about 0.2 lbs. to about 0.5 lbs. of P4S10 equivalent per ton of the material produced in step (b).