US3073779A

US3073779A - Method of preparing molybdenum disulfide powder

Info

Publication number: US3073779A
Application number: US43995A
Authority: US
Inventors: Ernest S Wheeler
Original assignee: American Metal Climax Inc
Current assignee: Cyprus Amax Minerals Co
Priority date: 1960-07-20
Filing date: 1960-07-20
Publication date: 1963-01-15
Anticipated expiration: 1980-01-15

Description

This invention broadly releates to a method of preparing molybdenum disulfide powder, and more particularly to an improved method for preparing an exceedingly fine sized molybdenum disulfide powder having the particles thereof coated with a controlled quantity of an oily substance.

The exposure of substantially dry and oil-free molybdenum disulfide powders. to the atmosphere causes oxidation of the particles forming oxides of molybdenum and sulfur which impart objectionable acidity to the powder. The susceptibility of substantially dry and oil-free molybdenum disulfide powder to oxidation increases as the particle size thereof decreases exposing a greater surface area to oxidation attack. The oxidation of molybdenum disulfide powder is accompanied by an increase in acidity which constitutes an objectionable characteristic particularly when the powder is to be used as a lubricant and in which use the powder is preferably for an extremely fine particle size.

It is now known that coating the surfaces of the particles of a molybdenum disulfide powder with an oily substance will inhibit oxidation of the powder for extended periods of time. Oil coatings in concentrations as low as 0.05% have been found to provide minimal protection against oxidation of molybdenum disulfide powders. Since the addition of a small amount of oil or oily molybdenum disulfide powder to a substantially oil-free molybdenum disulfide powder will not provide a satisfactory protective coating to the surfaces of all the particles of the substantially oil-free powder even though subjected to conventional mixing techniques, relatively elaborate and expensive methods have been heretofore employed to apply a substantially uniform coating of an oily substance on the surfaces of the powder particles. One such complex method is disclosed in United States Patent N0. 2,686,156 wherein an oil coating is applied to an oil-free powder by a solvent containing a low percentage of oil and thereafter subjecting the powder to a controlled solvent removal process.

It is a primary object of the present invention to provide an improved method of coating substantially oil-free particles of a molybdenum disulfide powder with an oily substance in a controlled amount so as to inhibit the oxidation of the powder and which method is simpler and more economical than methods heretofore known.

Another object of this invention is to provide an improved method of producing an extremely fine oil coated molybdenum disulfide powder containing a controlled quantity of an oily substance and which method concurrently provides for substantially uniformly coating substantially oil-free particles with an oily substance while simultaneously reducing the powder to a desired degree of fineness.

The foregoing and other objects of this invention are achieved by blending controlled proportions of a substantially oil-free molybdenum disulfide powder with an tet oil-containing molybdenum disulfide powder containing a known oil content ranging up to about 8% oil and charging the blended powdered mass to a fluid energy impact pulverization mill and pulverizing the blended powdered mass under controlled conditions to produce a substantially dry and exceedingly fine sized molybdenum disulfide powder having a substantially uniform coating of an oily substance on the particles thereof.

Other objects and advantages of the unique method comprising the present invention will become apparent from the following detailed description.

In the past, commercial molybdenums disulfide has been produced by grinding molybdenite ore which is comprised largely of granite and molybdenum disulfide and thereafter extracting the molybdenum disulfide from the gangue by an oil flotation extraction process. The amount of gangue in the molybdenum disulfide powder is reduced to the desired level by subjecting the ore to successive grinding and floatation operations. The gangue which is comprised predominantly of silica and is hereinafter referred to as such as usually identified as the portion insoluble in nitric acid and soluble in hydrofiuoric acid. Commercial milling and flotation extraction of molybdenum disulfide from molybdenite ore are usually continued until the silica content of the extracted molybdenum disulfide powder is reduced to a level below about 12%. However, when a higher degree of purity is desired, such as, for example, when the molybdenum disulfide powder is to be employed for lubrication purposes it is possible to reduce the silica content to a level ranging from about .3% to about .5 by successive grinding and oil flotation extractions dur. ing which the extracted molybdenum disulfide powder is reduced to an average particle size ranging from about Further reduction of the an elevated temperature such as, for example, a tempera-.

ture of about 1200 F. to evaporate and thus remove the oil and and water to contents of less than 0.03% oil and 0.03% water. Alternatively, it has been proposed to subject the wet oily powdered molybdenum disulfide mass to a controlled retorting operation wherein substantially all of the water is. removed and the oilysubstance is reduced toa desired level. Although the controlled retorting operation produces an oil coated powder which is resistant to oxidation the method requires relatively expensive retort and retort controlling equipment to control the residuary oil content to within the desired range.

The molybdenum disulfide powders derived from the aforementioned uncontrolled retorting operation containing less than about 0.03% oil, or other molybdenum disulfide powders which are substantially oil-free and derived from alternate molybdenite ore refining processes or other sources, are highly susceptible to oxidation when exposed to the atmosphere. tice of the present invention, the foregoing substantially oil-free powders can be blended in appropriate proportions with an oil-containing molybdenum disulfide powder of known oil content and thereafter pulverized in a fluid In, accordance. with the prac-- energy impact pulverization mill whereby pulverization and coating of the oil-free particles is achieved. Oil containing molybdenum disulfide powders suitable for this purpose include the wet oily powdered mass derived directly from the oil flotation extraction refining of molybdenite ore and which contain up to about 8% flotation oils and up to about 16% water. The residuary flotation oil in the extracted powder can be comprised of a wide variety of oily substances of a vegetable or petroleum origin and mixtures thereof which will wet the surfaces of the particles. Suitable oils for this purpose include refined petroleum oils, kerosene, pine tar oil, or any of the oily substances disclosed in United States Patent No. 2,686,156. It is beneficial under some conditions that the residuary oil content include some fairly high boiling point oils such as, for example, oils having an end boiling point of about 250 C. to facilitate control of the amount of oil removed by evaporation during the impact pulverization of the powder blend.

Suitable oil containing molybdenum disulfide powders also include flotation extracted powders which have been subjected to an intervening chemical treatment to reduce the silica content thereof without materially reducing the oil content of the powder. It is also contemplated that the wet oily molybdenum disulfide powder, whether or not subjected to the prior chemical treatment, can alternatively be treated in a preliminary drying step wherein all or a substantial portion of the moisture content is removed. The drying operation may be achieved by any one of a number of well known methods to evaporate the water without significantly reducing the oil content of the powder. In either case, either the wet silica free or substantially dry oil containing molybdenum disulfide powder of known oil content is preliminarily blended with the substantially oil-free powder in conventional mixing equipment to achieve a substantially uniform mixture thereof.

The appropriate portions of oil-free powder to oil containing powder of known oil content can be calculated so that the resultant blend will contain the desired residuary oil content. As heretofore mentioned, a coating of oil in an amount as low as .05% will provide minimal protection of the molybdenum disulfide particles against oxidation. Protection against oxidation increases as the oil content in the powder increases. Seemingly dry free-flowing molybdenum disulfide powders having an oil coating on the particles thereof can contain from .05 up to about .5% oil. These free-flowing powders are particularly desirable for use as solid lubricants. Oil contents above about .5 impair the free-flowing characteristics of the molybdenum disulfide powder but are satisfactory for compounding the powder with suitable lubricating oils and greases. Depending on the specific use of molybdenum disulfide powder, the proportions of the substantially oilfree and oil-containing powders in the blend can be controlled so as to produce a pulverized product containing from .05 oil up to a level approaching the oil content of the oil-containing powder, such as, up to about 8%. In view of the fact that a reduction in oil content usually occurs during the fluid energy impact pulverization of the powdered blend, depending on the milling conditions as will be subsequently described in detail, it is preferred that the proportions of oil-free and oil containing molybdenum disulfide powders be controlled so that the powder blend chargedtothe mill contains a quantity of oil surficient to assure that the resultant pulverized product contains residual oils, for example, in the range of from about 0.15% to about 0.3%, thus providing better than minimal protection against oxidation and still assure a free-flowing powder.

Pulverization of the molybdenum disulfide powder is achieved in an impact pulverization mill in which the powder is reduced in particle size through the action of impact and abrasion of the individual particles against each other at high velocities while entrained in a stream of circulating gaseous fluid. The gaseous fluid can be any compressed gas or vapor such as, for example, air, carbon dioxide, nitrogen, superheated steam, or a partially inert gaseous fluid such as results from the evaporation of a portion of the residual oils during impact pulverization of the powder at elevated temperatures. Impact pulverization mills suitable for this purpose are made by Fluid Energy Processing and Equipment Company of Philadelphia, Pennsylvania, and by Sturdevant Mill Company of Boston, Massachusetts. Impact pulverization, or mi cronization as it is usually referred to, can be controlled to achieve various particle size distributions. The heavier or larger particles are retained in the mill while the finer or desired sized particles are carried out in the fluid stream. The particle size of the molybdenum disulfide powder blend charged to the impact pulverization mill is usually in the order of an average particle size ranging from about 10 to about 250 microns and is thereafter materially reduced by micronization to an average particle size such as, for example, 5' microns or less. Oil coated molybdenum disulfide powders having an average particle size in the order of about 5 microns or less, are particularly desirable for dry lubricants or when compounding the powders with suitable lubricating oils and greases.

During the milling operation of the powder blend comprising oil-free and oil-containing molybdenum disulfide powders, the impact and abrasion between the oil coated particles and oil-free particles is eflective to transfer a portion of the oil to the oil-free particles, substantially uniformly coating the surfaces thereof in addition to coating the freshly exposed surface area created by the breakdown of the particles into smaller sizes. The oil coating mechanism, while not exactly understood, occurs quickly thereby inhibiting oxidation of the powder during impact pulverization. It is believed however, that at the point of impact between the particles, suficient residual oil is evaporated and condenses on the immediately adjacent newly formed oil-free surface. The effectiveness of this coating mechanism is surprising, in view of the fact that the direct coating of molybdenum disulfide powders with such small quantities of oil by ordinary mixing techniques is virtually impossible and moreover, that the surface area of the powder is increased many times during impact pulverization.

In addition to applying a substantially uniform oil coating to the oil-free particles and comminuting the powder blend to the desired degree of fineness, the volume and temperature of the gaseous fluid, as well as the feed rate of the powder blend to the mill can be controlled to achieve a controlled reduction of the water content of the feed mixture. Rapid and efficient drying of a powder blend containing up to about 16% water can be achieved during the impact pulverization milling by using heated gases such as compressed air, for example, at temperatures ranging from room temperature (about 65 F.) to about 700 F. As the temperature of the gaseous fluid is increased, and the volume thereof is increased relative to a fixed weight of feed material, the efliciency of drying is increased. Accordingly, these variables can be controlled to achieve any desired drying effect. The impact pulverization of wet and oily molybdenum disulfide powder blends is ordinarily effective to reduce the water content to a level below about .5 Water contents of about .5 in the pulverized product are not objectionable for most uses. However, it is preferred that the milling operation be conducted under controlled conditions so as to remove substantially all of the moisture or to subject the wet oilcontaining powder prior to blending to a preliminary drying step as hereinbefore described. Since impact pulverization conditions which are conducive to remove the water from the powder blend feed material are also conducive to removing a portion of the residuary oil content, it is necessary that the oil content of the powder feed material be of a level so as to compensate for any loss that may occur. In addition to the volume and temperature of the gaseous fluid and the rate of feed of the powdered blend to the impact pulverization mill, the quantity of oil removed during the milling operation is also dependent on the particular volatility characteristics and the quantity initially present. The amount of oil removed for any given powder blend feed material during the milling operation at specified conditions can readily be established by trial. As higher temperatures are required to control the oil content at the desired level, the tendency to oxidize the particles also increases when an oxygen-containing gas, such as air for example, is employed as the milling fluid. This oxidizing tendency in the mill, however, is lessened somewhat by the increased inhibiting affect of the volatilized portion of the residual oils.

The following examples are provided for further illustration of the effectiveness of the method comprising the present invention for coating substantially oil-free molybdenum disulfide powder and the oxidation stability characteristics of the resultant micronized powder blend relative to a typical oil-free powder. It will be understood that the examples are provided by way of illustration and are not intended to be limiting in any way:

EXAMPLE I A 1,000 lb. quantity of a dry retorted molybdenum disulfide powder containing .0 2% oil and having particle sizes predominantly in a range of from about to about 250 microns was charged to a fluid energy impact pulverization mill employing air at room temperature and was micronized to an average particle size in the order of about 1 micron or less. The resultant micronized powder had an oil content of 0.02%.

EXAMPLE II A 1,000 lb. lot of the dry retorted molybdenum disulfide powder feed material of Example I was blended with 23.5 lbs. of substantially dry molybdenum disulfide powder containing 6.7% oil in a blender until a substantially uniform powder blend was obtained. The blended powder mass was then charged to a fluid energy impact pulverization mill employing air at room temperature and micronized to an average particle size in the order of about 1 micron or less. The resultant micronized powder blend had a residuary oil content of 0.18%.

EXAMPLE III A 1,000 lb. lot of the dry retorted powder feed material of Example I was blended with 40 lbs. of a substantially dry molybdenum disulfide powder containing 6.7% oil in a blender until substantially uniform mixture was obtained. The powder blend was thereafter charged to a fluid energy impact pulverization mill employing air at room temperature and micronized to an average particle size in the order of about 1 micron or less. The resultant micronized powder contained 28% oil.

EXAMPLE iv A blend was prepared similar to that of Example III, which was charged to a fluid energy impact pulverization mill employing air heated to a temperature of about 195 F. and micronized to an average particle size in the order of about 1 micron or less. The resultant pulverized powdered blend had a residuary oil content of about .23%.

The oxidation stability characteristics of each of the powders of Examples I-IV Were tested by placing a portion of each sample in a closed container in which the samples had access to air at room temperature with normal or low humidity to simulate typical storage and shipping conditions. The samples were periodically analyzed for their molybdenum oxide contents. In addition, a portion of each of the samples of Examples I-IV were spread in an open pan and were exposed to air at room temperature providing accelerated oxidation test in which the 6 samples were subjected to greater humidity variations. The samples were periodically analyzed for their molybdenum oxide contents. The data thus obtained on the powders of Examples I-IV are tabulated below:

Oxidation Stability Characteristics SAMPLES STORED IN CLOSED CONTAINERS M003 Content, percent:

Before milling I. O. 02 0.02 0.02 0.02 After 10 days 0. 054 0. 067 O. 045 0. 045 After 32 days 0. 104 0. 075 0. 067 0. 067 After days 0. 156 0. 093 0. 101 0. 087

SAMPLES STORED IN OPEN PANS M00; Content, percent Before Milling- 0.02 0.02 0.02 0. 02 After 10 Days 0. 112 0. 064 0. 059 0. 063 After 32 Days 0. 142 0. 085 0. 070 0. 066 After 120 Days 0.201 0. 154 0. 120 0. 095

As will be noted from the data obtained on the test samples, the substantially oil-free molybdenum disulfide powder represented by Example I increased sharply in percentage of M00 after the impact pulverization thereof and continued to increase in percentage of M00 on aging while stored in the closed test containers. On the other hand, the percentage of M00 in the pulverized powder blends represented by Examples II-IV increased at a slower rate the higher the oil content.

Additional samples of mixtures substantially oil-free molybdenum disulfide powder and small amounts of molybdenum disulfide powder containing oil were micronized. In order to determine the increase in oxidation in terms of acidity, shelf life tests were run as follows:

1 M1. N/lO, KOH/lO gm. Sample.

The trend of increase in oxidation as shown by the increase in Acid Number is shown in the above table. The acidity increases with time.

In accordance with the method hereinbefore described, oil-free or substantially oil-free (containing less than 0.03% oil) molybdenum disulfide powders can be substantially uniformly coated with controlled quantity of oily substance by blending the oil-free powder with a powder containing a known residual oil content of up to about 8% oil providing a blend containing from 0.05% up to about 8% oil, preferably from about 0.15% to 6% oil, and impact pulverizing the powder blend in a fluid energy mill producing a pulverized powder product that contains from 0.05 up to about 8% oil, and preferably from 0.05 to about 6% oil. In addition, the method also enables the concurrent removal of any water from the feed material by controlling the milling conditions whereby water contents ranging up to about 16% are reduced to a level below about 0.5% in the pulverized product.

While it Will be apparent that the preferred embodiments herein illustrated are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. The method of preparing a molybdenum disulfide powder comprising the steps of providing a blended powdered mass of molybdenum disulfide containing from 0.05% to about 8% oil comprising a mixture of oil-contaim'ng molybdenum disulfide powder and a substantially oil-free molybdenum disulfide powder, and thereafter pulverizing said blended powdered mass in a fluid energy impact pulverization mill thereby concurrently comminuting said blended powdered mass and coating the particles of said oil-free powder with oil supplied from said oil-containing powder producing a substantially uniformly oil coated pulverized powdered product containing at least 0.05% oil.

2. The method of preparing a molybdenum disulfide powder comprising the steps of blending a substantially oil-free molybdenum disulfide powder with an oil-containing molybdenum disulfide powder in proportions so as to provide a powder blend containing an oil content ranging from 0.05% up to about 8%, pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coating the particles of said oil-free powder with oil supplied from said oil-containing powder and producing a substantially uniformly oil coated pulverized powdered product containing at least 0.05 oil.

3. The method of preparing a molybdenum disulfide powder comprising the steps of blending a wet, oil-containing molybdenum disulfide powder with a substantially oil-free molybdenum disulfide powder in proportions so as to provide a powder blend having an oil content ranging from 0.05% up to about 8% and containing up to about 16% water, and thereafter pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coating the particles of said oil-free powder with oil supplied from said oil-containing powder and reducing the water content to a level of less than about .5% by the action of said mill producing a substantially dry and uniformly oil coated pulverized powdered product containing at least 0.05% oil.

4. The method of preparing a molybdenum disulfide powder comprising the steps of blending a molybdenum disulfide powder containing up to about 8% oil with a substantially oil-free molybdenum disulfide powder in proportions so as to provide a powder blend having an oil content from about .15 up to about 8%, and thereafter pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coating the particles of said oil-free powder with oil supplied from said oil-containing powder producing a substantially uniformly oil coated pulverized powdered product containing at least 0.05 oil.

5. The method as defined in claim 4, further characterized by the fact that said powder blend contains up to about 16% water which is reduced to a level less than about .5% by controlling the volume and temperature of the gaseous fluid to said mill and the rate of feed of said powder blend to said mill.

6. The method of preparing a molybdenum disulfide powder comprising the steps of blending a molybdenum disulfide powder containing up to about 8% oil with a substantially oil-free molybdenum disulfide powder in proportions so as to provide a powder blend having an oil content ranging from 0.05 to about .5 and thereafter pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coating the particles of said oil-free powder with oil supplied from said oil-containing powder producing a substantially uniformly oil coated pulverized powdered product containing from 0.05 to about .5% oil.

7. The method as defined in claim 6, further characterized by the fact that said blended powdered mass contains up to about 16% Water which is reduced to a level less than about .5 by controlling the volume and the temperature of the gaseous fluid supplied to said mill and the rate of feed of said powder blend to said mill.

8. The method of preparing a molybdenum disulfide powder comprising the steps of blending a molybdenum disulfide powder containing up to about 8% oil with a substantially oil-free molybdenum disulfide powder in the proportions so as to provide a powder blend having an oil content ranging from about .15 to about .5 and thereafter pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coating the particles of said oilfree powder with oil supplied from said oil-containing powder producing a substantially uniformly oil coated pulverized powdered product containing from 0.05 to about .5% oil.

9. The method as defined in claim 8, further characterized by the fact that said powder blend contains up to about 16% water which is reduced to a level less than about .5 by controlling the volume and temperature of the gaseous fluid supplied to said mill and the rate of feed of said powder blend to said mill.

10. The method of preparing a molybdenum disulfide powder comprising the steps of blending a flotation extraction refined molybdenum disulfide powder containing from about 1% to about 8% flotation oil with a substantially oil-free molybdenum disulfide powder containing less than about 0.03% oil in the proportions so as to provide a powder blend having an oil content ranging from 0.05% up to about 6%, and thereafter pulverizing said powder blend in a fluid energy impact pulverization mill thereby concurrently comminuting said powder blend and coatingthe particles of said oil-free powder with oil supplied from said oil-containing powder producing a substantially uniformly oil coated pulverized powdered product containing from 0.05 to about 6% oil.

11. The method as defined in claim 10, further characterized by the fact that said powder blend contains up to about 16% water which is reduced to a level of less than about .5 by controlling the volume and temperature of the gaseous fluid supplied to said mill and the feed rate of said powder blend to said mill.

References Cited in the file of this patent UNITED STATES PATENTS 2,686,156 Arntzen et a1. Aug. 10, 1954

Claims

1. THE METHOD OF PREPARING A MOLYBDENUM DISULFIDE POWDER COMPRISING THE STEPS OF PROVIDING A BLENDED POWDERED MASS OF MOLYBDENUM DISULFIDE CONTAINING FROM 0.05% TO ABOUT 8% OIL COMPRISING A MIXTURE OF OIL-CONTAINING MOLYBDENUM DISULFIDE POWDER AND A SUBSTANTIALLY OIL-FREE MOLYBDENUM DISULFIDE POWDER, AND THEREAFTER PULVERZING SAID BLENDED POWDERED MASS IN A FLUID ENERGY IMPACT PULVERIZATION MILL THEREBY CONCURRENTLY COMMINUTING SAID BLENDED POWDERED MASS AND COATING THE PARTICLES OF SAID OIL-FREE POWDER WITH OIL SUPPLIED FROM SAID OIL-CONTAINING POWDER PRODUCING A SUBSTANTIALLY UNIFORMLY OIL COATED PULVERIZED POWDERED PRODUCT CONTAINING AT LEAST 0.05% OIL.