Separation Of Molybdenum Sulfide From Copper Sulfide With Depressants

Abstract

Molybdenum sulfide is separated from a molybdenite-containing copper ore concentrate by subjecting an aqueous pulp of the concentrate to froth flotation in the presence of a collector for molybdenum sulfide and a Nokes-type (e.g., arsenic trioxide/sodium sulfide) depressant for copper sulfide, the aqueous pulp being aerated with an inert gas to effect flotation of the molybdenum sulfide constituent of the pulp while maintaining the emf of the pulp above about minus 200 millivolts.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to separation and recovery of molybdenum sulfide from copper ore concentrates containing both molybdenum sulfide and copper sulfide by froth flotation in the presence of a collector for molybdenum sulfide and a depressant for copper sulfide to effect recovery of a molybdenum concentrate in the flotation overflow and recovery of a copper concentrate in the flotation underflow.

2. Prior Art

Sulfidic copper ores frequently contain minor but economically significant amounts of molybdenite (MoS.sub.2) which, when separated from the copper ore, comprises an important source of molybdenum. By way of example, a typical low-grade copper ore may contain about 1 percent copper sulfides and about 0.1 to 0.5 percent molybdenum disulfide. Such copper ores are beneficiated by conventional froth flotation to obtain an ore concentrate containing typically about 30 percent copper sulfides, if the ore is chalcopyritic, and about 1 percent molybdenum disulfide, the molybdenite following the copper minerals in the flotation circuit. The molybdenite content of the copper ore concentrate is then advantageously separated therefrom by differential flotation to obtain a molybdenum concentrate containing 90 percent or more MoS.sub.2. As noted, sulfidic copper minerals and molybdenite are normally floated by the same collectors. Therefore, in order to effect differential flotation of molybdenite-containing copper concentrates it is necessary either to depress the copper sulfides while floating the molybdenite content of the concentrate or to depress the molybdenite while floating the copper sulfide content of the concentrate.

A process widely used in the industry for effecting the aforementioned differential flotation of molybdenite comprises conditioning an aqueous pulp of the molybdenite-containing copper ore concentrate with a conventional collector for molybdenum sulfide and with a Nokes-type depressant for copper sulfide followed by froth flotation of the conditioned pulp with a conventional flotation machine to obtain a molybdenum concentrate as the flotation overflow (concentrate) and a copper concentrate as the flotation underflow (tailing). Nokes-type reagents are complex sulfidic compounds of phosphorus, arsenic or antimony and a caustic. They are available commercially from a number of suppliers, one such reagent being Anamol "D" which is a mixture of arsenic trioxide and sodium sulfide. Nokes-type copper depressants do not destroy the ability of conventional copper collectors to collect and float copper minerals, they merely selectively mask or hinder the collecting power of these collectors for copper minerals. It has been found that the amount of depressant in the pulp must be maintained above certain empirically determined levels to be effective. If the amount of copper depressant reagent present in the flotation pulp is insufficient to be effective, either because an insufficient amount was initially used or because some of the depressant initially present has been consumed or lost, the collector employed to float both sulfide minerals initially will allow the copper minerals to refloat and efficient separation of the two will not be obtained. Flotation plant operators have heretofore found that the amount of Nokes-type copper depressant required to insure efficient differential flotation is on the order of from 15 to 20 pounds of depressant per ton of copper ore concentrates being treated. As Nokes-type reagents are relatively expensive, the relatively large quantities of these reagents heretofore required to effect efficient differential separation add significantly to the cost of the molybdenum ultimately produced.

After an extensive investigation into the causes for high Nokes reagent consumption I have discovered that if certain essential operating procedures and criteria are observed a relatively small amount of Nokes-type copper depressant is required to effect efficient differential separation of molybdenite from sulfidic copper minerals. Specifically I have found that when a copper ore concentrate is subjected to conventional froth flotation in the presence of a Nokes-type reagent differential flotation of the mineral values proceeds efficiently and effectively until a point is reached at which the reagent appears to lose, rather abruptly, almost all of its ability to depress copper minerals. At the same time the electromotive force (emf) of the flotation pulp, as measured by reference to a standard half-cell, progressively declines, the aforementioned abrupt loss in depressant ability of the Nokes reagent becoming manifest when the emf of the pulp reaches the vicinity of minus 200 millivolts (-200 mv.). Both phenomena appear to be the result of the progressive destruction or inactivation of the Nokes reagent which, in turn, appears to be due to the use of air or other oxidizing gases as the aerating medium in the conventional flotation operation. Based on these findings and discoveries I have devised the improved process for the differential flotation of molybdenite-containing copper ore concentrates that is hereinafter described.

SUMMARY OF THE INVENTION

The improved process of the invention comprises conditioning an aqueous pulp of copper ore concentrates containing both molybdenum sulfide and copper sulfide with a collector for molybdenum sulfide and a Nokes-type depressant for copper sulfide. The conditioned pulp is then subjected to froth flotation in which an inert gas (or gases) is employed as the froth producing medium to effect flotation of a molybdenum concentrate in the flotation overflow and the recovery of a copper concentrate in the flotation underflow while maintaining the emf of the pulp above a predetermined value in order to maintain the effectiveness of the depressant for copper sulfide. The amount of the aforesaid copper depressant in the flotation pulp should be sufficient to maintain the emf of the pulp above about -200 mv. when measured with a platinum electrode with reference to a standard GP glass electrode (calomel electrode). The froth producing gas should be one which will not oxidize or react with the copper depressant and advantageously is selected from the group consisting of nitrogen, or the inert gases such as helium and argon, or a mixture of these gases. I presently prefer to employ nitrogen as the aerating gas, although specially treated combustion gases the flue gases containing only nitrogen and other inert gases may also advantageously be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the percentage of the copper content of the pulp that is depressed (i.e., that appears in the flotation underflow) as measured with respect to time when the pulp is aerated with an oxidizing gas (air) and with a non-oxidizing gas (nitrogen); and

FIG. 2 is a graph which illustrates the relationship between the percentage of copper depressed and the emf of the flotation pulp.

DETAILED DESCRIPTION

As previously described, sulfidic copper ores frequently contain minor amounts of molybdenum sulfides. Conventional ore beneficiation operations result in the preparation of a copper ore concentrate that may typically contain up to about 30 percent by weight copper sulfides (if the ore is chalcopyritic) and up to about 1 percent by weight molybdenite (MoS.sub.2). The molybdenite is advantageously separated from the copper ore concentrate by differential flotation in which an aqueous pulp of the concentrate is conditioned with a conventional collector for molybdenum sulfide and with a depressant for copper minerals, the conditioned pulp then being subjected to conventional froth flotation to obtain a molybdenum sulfide concentrate as the flotation overflow and a copper sulfide concentrate as the flotation underflow. The copper depressant employed is advantageously one of a variety of Nokes-type reagents. Nokes-type reagents were first described in U.S. Pat. No. 2,492,936, dated Dec. 27, 1949, and may be defined broadly as the reaction product of two or more inorganic compounds, the reaction product containing bivalent sulfur, a caustic cation (for example, sodium, potassium, calcium, ammonium or the like) and, combined therewith, an element selected from the group consisting of phosphorus, arsenic and antimony. These reagents are well known in the art and are available commercially from a number of sources.

Conventional froth flotation is usually carried out in agitator-equipped flotation cells which use air as the froth producing gas. The amount of Nokes-type reagent required to depress copper minerals effectively during such conventional froth flotation operations has heretofore been determined empirically by plant operators to be in the order of from 15 to 20 pounds of reagent per ton of copper concentrates being treated. As a result of my investigations I have found that when a non-oxidizing gas is used as the froth producing gas in place of air, and when the electromotive force (emf) of the flotation pulp is maintained above a predetermined value, the amount of Nokes-type reagent required to depress copper minerals effectively is reduced to about one fifth to one half of the amount heretofore required.

Specifically, I have found when a copper ore concentrate is conditioned with a Nokes-type reagent and is subjected to conventional froth flotation using air as the froth producing gas that, after a certain period of time, the Nokes reagent rather abruptly loses its ability to depress copper minerals and as a result these minerals now appear in the flotation overflow (the froth) as though no copper depressant were present in the pulp. I have also found when using air as the froth producer that the emf of the flotation pulp progressively decreases from a maximum initial value to a final value that closely approaches the emf of the unconditioned pulp. The length of time that the Nokes reagent retains its ability to depress copper minerals depends directly on the amount of reagent initially present in the pulp, and the rather abrupt loss in the ability of the reagent to depress these minerals appears to take place when the emf of the pulp reaches an empirically predetermined value. These findings indicate that, in the course of conventional froth flotation with air, Nokes reagents are progressively decomposed or consumed until insufficient reagent remains in the pulp to be effective.

When an inert gas that is non-reactive with respect to the highly alkaline Nokes reagent is used as the froth producing gas, the emf of the flotation pulp remains substantially constant, and the Nokes-type reagent retains its ability to depress copper minerals, throughout the duration of the flotation operation. Moreover, when an inert gas is used as the froth producing medium, a substantially smaller quantity of Nokes-type reagent may be used as a copper depressant than is the case when air or some other oxidizing gas is employed as the froth producer. The non-oxidizing gases useful in the practice of the invention include, but are not necessarily limited to, nitrogen; and other inert gases such as helium and argon; and various mixtures of these gases. One convenient source of this type of gas is flue gas or combustion gas which has been cooled and cleaned to remove dust particles and treated to remove free oxygen, carbon dioxide and/or carbon monoxide therefrom.

The emf of the flotation pulp is determined by techniques that are well known in the art. In brief, the electrical potential of the pulp is measured by reference to the potential of a conventional standard half cell. The absolute value, in millivolts, of the emf or potential of the pulp will depend on a number of factors which include the type and composition of the copper ore concentrate, the type and concentration of the Nokes reagent employed and the type of standard half cell with reference to which the emf of the pulp is being measured. However, for a given ore concentrate, Nokes reagent and type of standard half cell the emf of the pulp is essentially reproducible from sample to sample. Moreover, in such cases the emf of the pulp at which approximate value the Nokes reagent appears to lose its ability to depress copper minerals is approximately the same from sample to sample, and this approximate emf value can be empirically predetermined for any given ore concentrate, Nokes reagent and standard half cell by known techniques. By way of example, when a chalcopyrite copper concentrate produced from Twin Buttes ore is conditioned with Anamol "D," a commercially available Nokes-type reagent, the reagent appears to lose its ability to depress copper minerals when the emf of the flotation pulp decreases to about -200 to -250 mv. when measured with a platinum electrode with reference to a standard GP (calomel) glass electrode.

The following examples are illustrative but not limitative of the practice of the invention.

EXAMPLE I

A series of flotation experiments were performed on freshly produced Twin Buttes copper concentrates assaying approximately 31.9 percent copper and 0.51 percent molybdenum to extract a molybdenite rougher concentrate by the use of Anamol "D," a Nokes-type reagent, as the depressant for copper minerals. A single portion of the ore concentrate was divided into four equal samples having an average weight of about 740 grams. Each sample (herein referred to as Samples A, B. C and D) was conditioned with the same collector for molybdenite and with a different amount of the aforementioned Nokes-type depressant for copper. These conditioned flotation samples are set forth in Table 1. --------------------------------------------------------------------------- TABLE 1

Sample Molybdenite Copper Initial Weight Collector Depressant emf Sample (gms) (lbs/ton) (lbs/ton) (Mv) (negative) __________________________________________________________________________ A 750 0.520 5.34 -460 B 730 0.535 7.66 -520 C 750 0.52 10.7 -560 D 733 0.533 16.5 -610 __________________________________________________________________________

each conditioned sample was then subjected to froth flotation in a standard laboratory flotation machine for a total period of 16 minutes using air as the froth producing gas. The emf of the conditioned flotation pulp prepared from each sample was determined just before the start of each flotation experiment and thereafter at two minute intervals throughout the duration of the experiment. The flotation overflow (that is, the froth) was also collected at two minute intervals and the mineral content thereof weighed and assayed. At the conclusion of each experiment the flotation underflow (that is, the tails) was collected and the mineral content thereof weighed and assayed. The results of these four flotation experiments are set forth in Table 2: ##SPC1##

EXAMPLE II

A second series of flotation experiments were performed on similar freshly produced Twin Buttes copper concentrates, a single portion of the ore concentrate being divided into two equal samples (herein referred to as Samples E and F) having an average weight of about 510 grams. Each sample was conditioned with the same molybdenite collector and each with a different, relatively small, amount of Anamol "D," the Nokes-type copper depressant. These conditioned flotation samples are set forth in Table 3: --------------------------------------------------------------------------- TABLE 3

Sample Molybdenite Copper Initial Weight Collector Depressant emf Sample (gms) (lbs/ton) (lbs/ton) (Mv) __________________________________________________________________________ E 527 0.76 3.8 -370 F 493 0.81 6.5 -360 __________________________________________________________________________

each conditioned sample was then subjected to froth flotation in the same laboratory flotation machine for a total period of 16 minutes using nitrogen as the froth producing medium. The emf of the flotation pulp was measured, and the flotation overflow was collected and assayed, as in Example I. At the conclusion of each flotation experiment the underflow was collected and assayed as before. The results of these two flotation experiments are set forth in Table 4: ##SPC2##

The results of the flotation experiments described in Examples I and II are combined and are presented graphically in FIGS. 1 and 2 of the drawings. FIG. 1 shows the effectiveness of various concentrations of Anamol "D" as a copper depressant with respect to time (duration of flotation) when air and when nitrogen are used as the froth producing gas. FIG. 2 illustrates the progressive decrease in the emf and depressing effect of the Nokes reagent on copper minerals in the flotation pulp when air is employed as the froth producing gas as contrasted with the relatively constant emf and the effective depression of the copper minerals in the pulp when nitrogen is employed for this purpose.

Referring to FIG. 1, it is evident that, when the froth producing gas is air, the percent of copper depressed with respect to time is a function of the amount or concentration of Nokes reagent initially present in the flotation pulp. Moreover, it is evident that for a certain length of time, depending on the initial concentration of the Nokes reagent, the percent of copper depressed is very high until, rather abruptly, the reagent appears to lose its ability to depress copper minerals, whereupon the percent of copper minerals depressed drops sharply. In contrast to this, when the froth producing gas is nitrogen, the Nokes reagent retains its ability to depress copper minerals without appreciable deterioration throughout the flotation operation. Of particular significance is the fact that when an inert gas is used as the froth producer, the amount of Nokes reagent required to achieve and maintain a very high degree of copper depression is markedly less than the amount required when air is employed as the froth producing gas.

Referring to FIG. 2, the contrast between the rapid decrease in the emf of the flotation pulp when air is used as the froth producer and the relatively stable emf of the pulp when nitrogen is employed as the froth producing gas is readily apparent. Also, the relationship between the emf of the pulp and the aforementioned abrupt loss is depressant ability of the Nokes reagent is apparent.

Claims

I claim:

1. The process for the separation and recovery of molybdenum sulfide from copper ore concentrates containing both molybdenum sulfide and copper sulfide which comprises conditioning an aqueous pulp of the copper ore concentrates with a collector for molybdenum sulfide and a Nokes-type depressant for copper sulfide, and subjecting the conditioned pulp to froth flotation in which an inert gas that will prevent any oxidation from occuring in the conditioned aqueous pulp is employed as the froth producing medium to effect recovery of a molybdenum concentrate in the flotation overflow and recovery of a copper concentrate in the flotation underflow while maintaining the emf of the aqueous pulp above a predetermined value to maintain the effectiveness of said depressant for copper sulfide.

2. The process according to claim 1 in which the emf of the aqueous pulp is maintained above about -200 mv. when measured with a platinum electrode with reference to a standard GP (calomel) glass electrode.

3. The process according to claim 1 in which the depressant for copper sulfide is a reaction product of two or more inorganic compounds, said reaction product containing bivalent sulfur, a caustic cation and, combined therewith, an element selected from the group consisting of phosphorus, aresenic and antimony.

4. The process according to claim 3 in which the aqueous pulp contains a sufficient amount of said depressant for copper sulfide to maintain the emf of the pulp above about -200 mv. when measured with a platinum electrode with reference to a standard GP (calomel) glass electrode.

5. The process according to claim 1 in which the non-oxidizing aerating gas is selected from the group consisting of nitrogen, freon, the inert gases and mixtures thereof.

6. The process according to claim 1 in which the non-oxidizing aerating gas comprises nitrogen.