Patent Application
20140161715
This application is directed to methods for the production of molybdenum oxides from molybdenite, and in particular a looping sulfide oxidation process for the production of molybdenum oxides from molybdenite.
Molybdenum (Mo) metal is usually found as deposits of molybdenite (MoS2) in nature. Traditionally, the metal value is extracted from the sulfidic ore through various pyrometallurgical techniques. The primary application for Mo is in the steel industry, where it is used to produce high strength steel alloys. The Mo used in these applications is supplied to the steel manufacturers either as molybdenum(VI) oxide (molybdenum trioxide, MoO3) or ferromolybdenum, an iron-molybdenum alloy.
In producing molybdenum trioxide, or so called technical grade oxide, the molybdenite is oxidized in various multiple hearth roasting furnaces. For example, one conversion process, referred to as the Climax conversion process, uses a Nichols Herreshoff or Lurgi design multiple hearth furnace to slowly roast the MoS2 at temperatures in the range of 530 to 700° C. (L. F. McHugh, P. L. Sallade, Molybdenum Conversion Practice, Metec, Inc.: Winslow N.J., 1977; L. F. McHugh, J. Godshalk, M. Kuzior, Climax Conversion Practice III, The Metallurgical Society of CIM, 1977, 21-24). Over the course of roasting, the molybdenite is first converted to molybdenum(IV) oxide (molybdenum dioxide, MoO2) and then slowly converted to the trioxide, MoO3, in the lower hearths. In such a roasting operation, the lower hearths are maintained at lower temperatures to keep the highly volatile MoO3 from sublimating. The SO2 produced during conversion, which is highly diluted due to the significant amount of excess air used during roasting, is usually converted to sulfuric acid in a downstream process, which may result in some high pressure steam production.
The molybdenite host ore (unprocessed ore containing molybdenite) is usually concentrated prior to oxidation to upgrade the molybdenite concentration via an oil flotation method. A method has been described wherein the residual flotation oil can be removed in situ during roasting to produce technical grade oxide from the molybdenite without further purification (L. F. McHugh, D. E. Barchers, Roasting of Molybdenite Concentrates Containing Flotation Oils, U.S. Pat. No. 4,523,948, issued Jun. 18, 1985).
In alternative incarnations of MoO3 production, microwaves have been used to heat molybdenite host ores in the presence of oxygen to convert them to oxides, followed by separation and recovery (P. R. Kruesi, V. H. Frahm, Jr., Process for the Recovery of Molybdenum and Rhenium from their Sulfide Ores, U.S. Pat. No. 4,321,089, issued Mar. 23, 1982). Chlorine can be substituted for oxygen and the metals recovered as chlorides. The conversion can be conducted in a flash roasting set-up wherein the molybdenite is converted to gaseous MoO3 and any slag-forming constituents are converted to a liquid slag that separates from the molybdenum, wherein the flash roasting is performed at 1600±200° C., and the technical grade oxide is later condensed at significantly lower temperatures (B. J. Sabacky, M. T. Hepworth, Flash Roasting of Molybdenum Sulfide Concentrates in a Slagging Reactor, U.S. Pat. No. 4,555,387, issued Nov. 26, 1985). High pressure oxidation carried out in an autoclave followed by leaching and recovery has also been described (R. W. Balliett, W. Kummer, J. E. Litz, L. F. McHugh, H. H. K. Nauta, P. B. Queneau, R.-C. Wu, Production of Pure Molybdenum Oxide from Low Grade Molybdenite Concentrates, U.S. Pat. No. 6,730,279, issued May 4, 2004).
It is more attractive to the steel industry to use MoO2 rather than MoO3 because the lower oxygen content means a higher Mo content, and less reducing agent consumption during the production of molybdenum steels. Additionally, the dioxide is less volatile than the trioxide. The use of MoO2 also eliminates the need to produce ferromolybdenum. Several authors have attempted to produce the dioxide from molybdenite concentrates using various techniques.
For example, by mixing stoichiometric amounts of powdered or pelletized MoO3 and MoS2, MoO2 can be produced at temperatures of 600 to 700° C. and pressure slightly in excess of atmospheric pressure, while liberating SO2 (R. Cloppet, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 3,336,100, issued Aug. 15, 1967). The molybdenum product was then further treated in an SO2 lean atmosphere to produce the final dioxide product. In a similar process, particulate MoO2 and MoS2 (weight ratio 2:1) were roasted at 700 to 800° C. in the presence of enough oxygen to facilitate the conversion of the molybdenite to MoO2 (B. J. Sabacky, M. T. Hepworth, Molybdenum Dioxide-Molybdenite Roasting, U.S. Pat. No. 4,462,822, issued Jul. 31, 1984). A portion of the dioxide produced was recycled to convert the next charge of molybdenite. Some MoO3 is produced as a by-product. Pelletized MoO2 has been produced from MoO3 using a reducing H2 atmosphere in a reaction vessel that was able to control the exothermic reduction of the trioxide (H. W. Meyer, J. D. Baker, W. H. Ceckler, Direct Reduction of Molybdenum Oxide to Substantially Metallic Molybdenum, U.S. Pat. No. 4,045,215, issued Aug. 30, 1977). The dioxide was further reduced to metallic Mo, or the dioxide was collected as a final product.
It has also been proposed to convert molybdenite to MoO2 using water steam (K. Y. Hakobyan, H. Y. Sohn, A. V. Tarasov, P. A. Kovgan, A. K. Hakobyan, V. A. Briovkvine, V. G. Leontiev, and O. I. Tsybine, “New Technology for the Treatment of Molybdenum Sulfide Concentrates,” Sohn International Symposium Advanced Processing of Metals and Materials Vol. 4 to New, Improved and Existing Technologies: Non-Ferrous Materials Extraction and Processing, ed. by F. Kongoli and R. G. Reddy, TMS (The Minerals, Metals & Materials Society), pp. 203-216, 2006; H. Y. Sohn, Process for Treating Sulfide-Bearing Ores, U.S. Pat. No. 4,376,647, issued Mar. 15, 1983; K. Y. Hakobyan, P. A. Kovgan, A. V. Tarasov, A. K. Hakobyan, Eurasian Patent 002417, issued 2002; K. Y. Hakobyan, H. Y. Sohn, A. K. Hakobyan, V. A. Bryukvin, V. G. Leontiev, and O. I. Tsibin, “The Oxidation of Molybdenum Sulfide Concentrate with Water Vapor: Part I. Thermodynamic Aspects,” Mineral Processing and Extractive Metallurgy (TIMM C), 116, 152-154 (2007); K. Y. Hakobyan, H. Y. Sohn, A. K. Hakobyan, V. A. Bryukvin, V. G. Leontiev, and O. I. Tsibin, “The Oxidation of Molybdenum Sulfide Concentrate with Water Vapor: Part II. Macrokinetics and Mechanism,” Mineral Processing and Extractive Metallurgy (TIMM C), 116, 155-158 (2007)). In such a scheme, steam is reacted with the MoS2 feed to produce the dioxide and an H25 off-gas stream. Hydrogen sulfide is a toxic gas and is difficult to handle in downstream processes. A rotating furnace is used to treat molybdenite ores at 900 to 1,000° C. in a countercurrent flow of water vapor in an excess of 6 to 10 times the mass of the molybdenite. Residual sulfur is optimized to minimize the loss of MoO2 to MoO3 vapor formation.
Solid MoO3 has been used in a reaction between MoO3 and MoS2 to produce MoO2 (L. F. McHugh, D. K. Huggins, M. T. Hepworth, J. M. Laferty, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 4,552,749, issued Nov. 12, 1985). In this process the conversion to dioxide is carried out at temperatures low enough to favor dioxide formation and to produce an SO2-rich stream (750 to 950° C.). A portion of the MoO2 product was sent to a flash reactor where it was reoxidized to the trioxide at temperatures sufficient to sublimate the MoO3 (1,000 to 1,700° C.) and recycled to convert the next charge of molybdenite. In a similar process, the MoO2 is oxidized in a second step to MoO3 to recover rhenium from the final product; in this operation the MoO2 is not the final product (H. Y. Sohn, Process for Treating Sulfide-Bearing Ores, U.S. Pat. No. 4,376,647, issued Mar. 15, 1983).
Solid MoO3 has been proposed as an oxidant to molybdenite to achieve a high degree of desulfurization; the process required ca. 10% excess MoO3 to be mixed intimately with the molybdenite in order to achieve a low sulfur content product (L. F. McHugh, R. Balliett, J. A. Mozolic, The Sulfide Ore Looping Oxidation Process: An Alternative to Current Roasting and Smelting Practice, JOM, July 2008, 84-87; L. F. McHugh, Oxidation of metallic materials as part of an extraction, purification, and/or refining process, U.S. Patent Application 2008/0260612 A1, published Oct. 23, 2008). Some of the dioxide was then reoxidized in a flash furnace with oxygen to recycle MoO3 to the molybdenite charge (similar to as reported in L. F. McHugh, D. K. Huggins, M. T. Hepworth, J. M. Laferty, Process for the Production of Molybdenum Dioxide, U.S. Pat. No. 4,552,749, issued Nov. 12, 1985).
The methods described above all appear to use significant excess oxygen. While these methods produce MoO2 from molybdenite feeds, it has not been possible to produce a product without concomitantly producing some MoO3. When it has been used, oxygen has also always been in excess. A method that can produce exclusively MoO2 is desired.
There accordingly remains a need in the art for methods for the production of MoO2 from molybdenite. Producing MoO2 with lower amounts of MoO3 is considered particularly advantageous. It is further desired that the MoO2 productions methods allowed control of SO2 and energy consumption during production.
The present disclosure provides a pyrometallurgical method for production of molybdenum(IV) oxide, the method including contacting molybdenite feed with oxygen in a furnace including a high temperature zone, wherein an amount of the oxygen is substantially stoichiometric; and reacting the molybdenite feed with the oxygen to produce molybdenum(IV) oxide and sulfur(IV) oxide, wherein complete desulfurization of the molybdenite feed is accomplished in the high temperature zone at a temperature of about 1,000 to about 1,500° C. with a residence time of about 0.1 to about 40 seconds.
The present disclosure also provides a looping method for production of molybdenum(IV) oxide, the method comprising reacting a molybdenite feed with a substantially stoichiometric mixture comprising molybdenum(VI) oxide and oxygen in a first furnace to produce molybdenum(IV) oxide and sulfur(IV) oxide; removing a first portion of the molybdenum(IV) oxide from the first furnace; transferring a second portion of the molybdenum(IV) oxide from the first furnace to a second furnace; reoxidizing the second portion of the molybdenum(IV) oxide in the second furnace to molybdenum(VI) oxide; and looping the molybdenum(VI) oxide from the second furnace to the first furnace for use as an oxidizing agent.
Other embodiments will be apparent from the Drawings, Detailed Description, and Claims below.
These and other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:
Direct Oxidation of MoS2 with Gaseous O2
In an embodiment, a method for production of molybdenum(IV) oxide (MoO2) includes contacting a molybdenite feed with oxygen, wherein an amount of the oxygen is substantially stoichiometric, and oxidizing the molybdenite feed with the oxygen in a furnace to MoO2.
The term “molybdenite feed” as defined herein refers to molybdenum sulfide (MoS2) concentrate that primary includes molybdenum disulfide and small amounts of gangue components such as SiO2, Al2O3, FeO, or CaO. The molybdenite feed may include any particulated concentrate composed predominantly of molybdenum disulfide derived from any one of a number of commercial sources. For example, the molybdenite feed may include a concentrate derived from various ore beneficiation processes that are effective to reduce the gangue and other contaminating substances in the concentrate to levels below about 10%. The concentration of the molybdenum disulfide in the ore, as mined, is generally in the order of about 0.03% to about 0.06%, but this concentration may be increased through various beneficiation processes, such as an oil flotation extraction process, to levels of 50% or greater molybdenum, and even of 60% or greater molybdenum.
The oxygen used in the method for production of MoO2 may include pure oxygen, or a mixture of oxygen and another gas, for example, a mixture of oxygen and nitrogen, air, or oxygen-enriched air. The content of oxygen in air may be about 21%, and the content of oxygen in oxygen-enriched air may be from about 21% to about 100%.
The term “substantially stoichiometric,” as defined herein with regard to the method for production of MoO2 by oxidation with oxygen but without use of molybdenum(VI) oxide MoO3 as an oxidant, refers to a ratio of molecular oxygen (O2) to molybdenite feed (MoS2) that is 3:1 or about 3:1 (moles O2:moles MoS2). The stoichiometric amount is given as a percent of the stoichiometric 3 moles oxidizing agent (molecular oxygen): 1 mole MoS2 stoichiometry. A stoichiometric percent of 90% to 110% indicates a ratio of moles O2 to moles MoS2 of about 2.7:1 to about 3.3:1. The substantially stoichiometric amount may be about 85% to about 115% based on the total amount of molybdenite feed, for example, about 90% to about 110% based on the total amount of molybdenite feed, or about 95% to about 105% based on the total amount of molybdenite feed.
The oxidation of the molybdenite feed with the oxygen may be conducted in a furnace, which for example, can be a flash furnace, a shaft furnace, a multiple hearth furnace, a rotary furnace, a rotary kiln, or a fluid bed furnace.
The oxidation of the molybdenite feed with the oxygen may further be conducted at temperature of about 500° C. to about 1,500° C., for example, 800° C. to about 1,500° C., or 1,100° C. to about 1,500° C.
A flash type reactor is well suited to provide the short residence that ensures complete desulfurization and conversion to MoO2 without overoxidizing the molybdenum. The term “complete desulfurization” as defined herein means that less than 0.1 weight % of sulfur remains in the molybdenum oxide product(s), for example 0 to less than 0.1 weight %. More preferably, 0 to less than 0.05 weight % of sulfur remains in the molybdenum oxide product(s). Alternative embodiments may include fluid bed reactors. A self-propagating high-temperature synthesis (SHS) technique can be employed (USSR Patent No. 255221).
MoS2 +(3.5+x)O2→MoO3+2 SO2 Equation 1
When using MoO3 to produce alloy steels, manufacturers must reduce the oxide to metallic molybdenum. If selective production of MoO2 were possible, then MoO2 would be a more desirable material for production of metallic molybdenum than MoO3 since MoO2 has higher molybdenum content, and therefore, it requires less reducing agent to produce metallic molybdenum. All prior art attempts to selectively produce MoO2 from MoS2 were unsuccessful because the excess amount of O2 further oxidized MoO2 to MoO3. In conventional multiple hearth roasting, excess O2 is required to achieve complete desulfurization and complete oxidation to MoO3. In the present disclosure, limiting the amount of oxygen to a stoichiometric amount, selectively produces MoO2 from MoS2 without further oxidation of MoO2 to MoO3. The appropriate FactSage simulations summarized in Table 1 confirm this result (Bale, C. W., et al. FactSage™ 6.3.1, Thermfact and GTT-Technologies, CRCT, Montreal, Canada (2012)).
Oxidation of MoS2 with a stoichiometric amount of oxygen is shown by Equation 2.
MoS2+3 O2 (stoichiometric)→MoO2+2 SO2 Equation 2
As can be seen from Equation 2, the maximum amount of MoO2 produced from the input materials is 1 mole. As seen in
As shown in
Under conditions utilizing O2 in a much lower excess (0.5%), the product distribution is less adversely affected and the selectivity to MoO2 is much higher (
The data indicating adiabatic temperature of varying % O2 stoichiometry are listed in Table 2. These data demonstrate the feasibility of direct MoO2 production from MoS2 using O2 from the air as an oxidizer. Almost quantitative yields are achieved under proper stoichiometric and thermal control. Table 2 suggests that operating below, at, or even slightly above stoichiometry permits higher yields of MoO2 with reasonable adiabatic temperatures in 85% nitrogen and 15% oxygen mixture. Additionally, SO2 present in the off-gas may be captured and used for sulfuric acid production.
While direct oxidation of MoS2 with O2 produces MoO2 in one step, it is difficult to accurately control the O2 stoichiometry, so the resulting MoO2 may be further oxidized to produce MoO3. This issue can be resolved by introducing MoO3 as an oxidizing agent, combining direct oxidation and Looping Sulfide Oxidation (“LSO”) simultaneously in a common cycle oxidation process (L. F. McHugh, R. Balliett, J. A. Mozolic, The Sulfide Ore Looping Oxidation Process: An Alternative to Current Roasting and Smelting Practice, JOM, July 2008, 84-87; L. N. Shekhter, C. G. Anderson, D. G. Gribbin, E. Cankaya-Yalcin, J. D. Lessard, L. F. McHugh, Looping Sulfide Oxidation™ Process for Anode Copper Production, Proceedings of the 4th International Symposium on High-Temperature Metallurgical Processing, to be published).
In an embodiment, the looping method for production of molybdenum(IV) oxide may include: oxidation of the molybdenite feed with a mixture comprising MoO3 and oxygen in a first furnace to MoO2; removing a portion of MoO2 from the first furnace; transferring a second portion of MoO2 from the first furnace to a second furnace; reoxidation of the transferred portion of the MoO2 in the second furnace to MoO3; and looping MoO3 from the second furnace into the first furnace for use as an oxidizing agent.
As shown in
A total amount of the oxidizing agent including MoO3 and oxygen may be substantially stoichiometric to an amount of the molybdenite feed. The term “substantially stoichiometric” as defined herein with regard to the looping method for production of MoO2 refers to a ratio of the oxidizing agent [molecular oxygen (O2) +molybdenum(VI) oxide (MoO3)] to molybdenite feed (MoS2) that is [6×(1−a/3)+a]:1 or about [6×(1−a/3)+a]:1 [(moles O2+moles MoO3):moles MoS2] where a represents the moles of O2 (cf. Equation 4). The stoichiometric amount is given as a percent of the stoichiometric [6×(1−a/3)+a] moles of the oxidizing agent (molecular oxygen+molybdenum (VI) oxide): 1 mole MoS2 stoichiometry. A stoichiometric percent of 90% to 110% indicates a ratio of moles of the oxidizing agent to moles of MoS2 of about 0.90×[6×(1−a/3)+a]:1 to about 1.10×[6×(1−a/3)+a]:1. The substantially stoichiometric amount may be about 85% to about 115% based on the total amount of the molybdenum feed, specifically, about 90% to about 110% based on the total amount of the molybdenum feed, more specifically, about 95% to about 105% based on the total amount of the molybdenum feed.
Another benefit of the combined usage of MoO3 and O2 for the oxidation of the molybdenite is the reduction of the recycled (re-oxidized) amount of MoO2 due to the usage of oxygen. When no oxygen is used for the oxidation of MoS2, more than 85% Mo on molar basis will have to be recycled.
After the oxidation, a portion of MoO2 may be removed from Flash Furnace I as a product and stored or used as needed. Another portion of MoO2 may be carried to a second furnace (Flash Furnace II), which may be a flash furnace. In the second furnace, MoO2 may be converted to MoO3 by use of an appropriate oxidizing agent, such as oxygen. The oxygen may include pure oxygen, air, or oxygen-enriched air. The stream of the SO2 off-gas may be carried to the Boiler wherein water steam is generated due to the heat transfer. To facilitate localized condensation of any excess MoO3 in a given area of the reactor, a portion of the SO2 off-gas may be recycled to Flash Furnace I. The rest of the SO2 off-gas may be passed through Catalyst Bed and Heat Exchanger to Acid Absorption Tower for H2SO4 (sulfuric acid) production.
The regenerated MoO3 (for example, condensed particulate MoO3) may be looped back to the first furnace. The incoming reaction air to the furnace may be preheated to increase the efficiency in the overall system. The off-gas stream from the second flash furnace may be directed to a heat exchanger-boiler system for energy generation and reaction air preheating. Boilers downstream of the second flash furnace system and in the acid plant may be utilized to generate high pressure steam for energy production via turbines. In an embodiment, the oxidation may produce energy in an amount of about 385 to about 400 kiloWatt×hour based on 1,000 kilogram of the molybdenite feed.
As stated above, the presented looping method allows a selective production of MoO2 without its further oxidation to MoO3. While not wanting to be bound by a theory, it is understood that oxygen that is present in the oxidizing agent in less than a stoichiometric amount selectively converts some of the molybdenite feed into MoO2. The rest of the molybdenite is oxidized with MoO3 to produce MoO2. Thus, use of the oxidizing agent including both oxygen and MoO3, allows to obtain a nearly quantitative yield of MoO2 without any noticeable quantities of MoO3.
MoS2+6 MoO3→7 MoO2+2 SO2 Equation 3
In order to investigate the relationship between the MoO3 and O2 as oxidizing agents in combined cycle oxidation, the ratio of MoO3 to O2 was varied (Table 3). The reaction stoichiometry for these experiments can be represented by Equation 4, wherein the ratio represents the ratio between MoO3 and O2.
Under each unique oxidation condition, the equilibrium behavior was noted with attention paid to determining the optimal operating conditions for MoO2 production.
MoO2+½O2→MoO3 ΔH°=−87.1 kWh Equation 5
For the combined cycle oxidation of the molybdenite feed in the presence of a mixture of MoO3 and O2, a flash reactor can be used to create a high temperature zone in which the conversion of the MoS2 to MoO2 takes place. Since the residence time in this high temperature zone will be short, the flash temperature should exceed the boiling point of the looped MoO3 to ensure that it is fully vaporized, mitigating mass transfer limitations when it reacts with the MoS2. As previously discussed, the upper temperature of the flash zone is limited by the temperature at which the yield of MoO2 decreases due to in situ gaseous Mo3O9 formation. Additionally, the off-gas produced in the combined cycle oxidation flash reactor is rich in SO2 (Table 4), and provides another avenue for significant energy capture in a downstream sulfuric acid plant.
The MoO2 product that is not looped for reoxidation can be collected as a final product. The MoO2 to be recycled enters a second flash furnace at 600° C. where it is reacted at over 1,100° C. (due to the exothermic nature of the oxidation reaction, Equation 5) in the presence of oxygen in the high temperature zone of the furnace. These conditions lead to the complete oxidation of MoO2 to MoO3 (
This entire process in the second flash furnace operates free of sulfur (in either the gas or solid phase), so that condensation of sulfuric acid (due to SO2 or SO3) in downstream processes is eliminated. As such, energy capture from the MoO2 reoxidation with O2 to MoO3 for looping is maximal. This energy capture is realized as preheating reaction air for this second flash furnace and as high pressure steam generation.
From
It must be noted that the production of intermediate molybdenum suboxides is a process that takes place at low temperatures (relative to the production of MoO2 described in the LSO process). During this low temperature process, direct production of the suboxides from the molybdenite concentrate is impossible (
Oxidation of Molybdenite with Metal Oxides
To determine the extent to which sulfide minerals are oxidized with metal oxides, a testing was performed. The amount of desulfurization achieved in mixing the sulfide and metal oxide was determined. In one test, MoS2 was mixed with MoO3 to achieve 98.5% desulfurization producing MoO2 as a primary product. These results correlate well with the performed calculations, as can be seen below in
Since feeding stoichiometric amounts of two powders is easier than feeding stoichiometric amounts of a powder and a gas, the effect of running in both excess and lean conditions has been investigated (
With 5% excess MoO3 (
When operating at a 5% shortage of MoO3 (
While both of the foregoing reactions yield MoO2 as the product, they differ substantially from an energy generation perspective (Table 5). The reaction of MoS2 with O2 is extremely exothermic, allowing for high energy capture potential. In contrast, the reaction of MoS2 with MoO3 is endothermic until approximately 800° C. At or above this temperature, MoO3 exists as either a molten liquid or a gas and its reaction with MoS2 is exothermic. Accordingly, it is more desirable to operate at a higher temperature, wherein the kinetics are faster, provided the particles of the reagents are small enough in order to mitigate diffusion limitations. Thus, ΔHrxn is shown for both 25° C. and 1,200° C. in Table 5.
Oxidation of MoS2 with O2 has a greater potential for energy capture than the oxidation with MoO3. However, reacting MoS2 with MoO3 is a practically desirable reaction, because having a small excess or shortage of MoO3 does not detrimentally impact the yield of MoO2. Thus it was determined that oxidation by a combination of MoO3 and O2 could be beneficial for the production of MoO2. The first step in doing so was to determine what the adiabatic temperatures would be for various ratios of MoO3 to O2 (Table 4). Calculations were performed using the FactSage software (Bale, C. W., et al. FactSage™ 6.3.1, Thermfact and GTT-Technologies, CRCT, Montreal, Canada (2012)).
Table 4 demonstrates that for this range of reagent ratios, the adiabatic temperature is between 1,111° C. to 1,381° C. In this temperature range, MoO3 is completely vaporized (
Due to a decrease in yield of MoO2 with an increase in temperature and to ensure a stable adiabatic temperature, a reagent ratio of 2.2 moles MoO3:1.9 moles O2 was chosen for sample calculations. This ratio allows operation at high enough temperature to avoid MoO3 condensation on the walls of the furnace, but does not affect yield of MoO2. It should be noted that this reaction proceeds with all stoichiometric combinations of MoO3 and O2 but sample calculations were only performed for this particular ratio.
As can be seen in Table 6, this combination of oxidizing agents allows for both low and high temperature exothermic production of MoO2.
The heat and material balances for each processing step in
1. Oxidation of Molybdenite
A process which takes place in Flash Furnace I may be represented by Equation 6:
MoS2+2.2 MO2+1.9 O2→3.2 MoO3+2 SO2 Equation 6
ΔH°=−155 kWh
The ratio of MoO3:O2 37/63% was used in the furnace. The air was introduced at 80° C. which was preheated in the heat exchanger of the acid absorption tower. The adiabatic temperature was 1,170° C. and the products were cooled down to 600° C. with the SO2 recycle.
2. Reoxidation of MoO2 to MoO3
In Flash Furnace II the reoxidation of MoO2 to MoO3 took place (Equation 5). The air in this reaction was preheated to 500° C. and 10% excess air was used. The adiabatic/flash temperature was around 1,115° C. and the products were cooled down to 600° C. with cooling air at 25° C. The product MoO3 powder was transferred back in to Flash Furnace I to be used as an oxygen source.
Based on these calculations, an energy flow sheet diagram was prepared (
The potential for energy capture was evaluated for each of the MoO3:O2 ratios investigated above (Table 18). As can be seen, the amount of energy to be captured ranges from 385 to 400 kWh per 1,000 kg MoS2.
In the configurations presented here, the key process steps have been modeled; these steps are: (1) deoiling and the concentrate and blending of the concentrate and recycled molybdenum trioxide; (2) reaction of the molybdenum disulfide concentrate with molybdenum trioxide to selectively produce molybdenum dioxide in a rotary kiln; (3) splitting of the molybdenum dioxide into the product stream and the stream to be oxidized and recycled; (4) oxidation of the molybdenum dioxide to molybdenum trioxide in a downer furnace; (5) recycling of the molybdenum trioxide to complete the cycle.
This configuration of processing steps and unit operations is one possible configuration, alternative configurations (including, but not limited to, the use of alternative reaction vessels, e.g. multiple hearth roasters, flash furnaces, etc.) are possible.
Heat losses at the reaction steps have been estimated. Additionally, heat exchangers have been used to maximize heat recovery and minimize the requirement for outside heating. In this model, a heat exchanger is modeled as two units—the first unit cools the hot process gas from the inlet temperature to a specified outlet temperature (potentially constrained by a sulfuric acid dew point consideration); during this cooling, HSC calculates the enthalpy change required. The second unit “calls” the enthalpy change from the cooling unit and applies an enthalpy change of equal magnitude and opposite sign to the cooling air it is supplied; an objective function is used to iterate either the outlet temperature of the cooling air with a given flow rate, or its mass flow rate with a given outlet temperature. In this fashion, an iterative solution for the heat balance is calculated. Because the heat exchangers in this process are linked consecutively to maximize heat recovery and minimize the external heating (via natural gas combustion) required in the furnaces, the mass flow rates of the two linked heat exchanger systems (heat exchangers #1 and #2, and heat exchangers #3, #5, and #6) are specified based on the hot air load required by the furnaces they service. As such, in the iterative solutions for each heat exchanger the mass flow rate is specified and the only degree of freedom available for iteration is the outlet temperature at each unit. A heat transfer efficiency of 90% was assumed. The connectivity of the heat exchangers presented is only one possible configuration; alternative, perhaps more efficient, configurations are possible.
In the model presented below, a deoiler is used to remove water and flotation oil associated with the concentrate before it is fed to the rotary kiln. A blender then adequately mixes the deoiled and dried concentrate with the recycle stream of molybdenum trioxide.
A direct fired rotary kiln is used to perform the looping sulfide oxidation reaction. This rotary kiln has been modeled as two discrete units, though in reality it would function as a direct, countercurrent fired rotary kiln. By modeling the reactor in two discrete steps, the “hockey stick” temperature curve can be better approximated. In the first discrete unit the reaction is carried out to approximately 90% completion and 650° C., in the second discrete unit the reaction is completed at 700° C. This configuration provides a better indication of the natural gas usage and external heating required.
In this case, the amount of hot air required is dependent on the stoichiometry of the reactions taking place in the rotary kiln. As previously discussed, the looping sulfide oxidation reaction provides for a range of molybdenum trioxide to oxygen ratios; in this simulation a ratio of 80% stoichiometry for molybdenum trioxide and 20% stoichiometry for oxygen has been used. Additionally, the requisite amount of air for natural gas combustion has been included. The product gases from the rotary kiln are passed through a heat exchanger and sent for gas cleaning; due to the rich SO2 content in this gas stream the gas must be kept above the acid dew point. The solid products, now mostly molybdenum dioxide, per the looping sulfide oxidation reaction scheme, are further cooled and split based on the reaction stoichiometry. The fraction to be collected as product is cooled further; the fraction to be recycled into molybdenum trioxide is kept at elevated temperature and sent for processing in the downer furnace.
The downer furnace is the reactor in which the molybdenum dioxide is oxidized to molybdenum trioxide with excess air. In this simulation, 300% excess air is used. The air, along with the air required for natural gas combustion is preheated to elevated temperatures by heat exchangers to minimize the amount of natural gas that must be combusted. The downer furnace is designed to operate at 650° C. to minimize molybdenum trioxide volatilization. A heat loss unit is included in this simulation to model the rapid cooling that takes place in the lower sections of the downer furnace as the converted molybdenum trioxide rapidly cools as it falls the length of the reactor. The gases exiting the reactor are cooled and sent to gas handling. The solids are cooled in a heat exchanger and looped back to the blender to complete the reaction cycle.
To demonstrate the relative amounts of reagents consumed in this process, a range of operating throughputs is presented below. In the first case, a molybdenum concentrate feed containing 50 kg per hour of molybdenum disulfide is considered. This throughput is equivalent to approximately 30,000 lbs molybdenum per year of production. In the second case, 200 kg per hour of molybdenum disulfide is considered, which is equivalent to 120,000 lbs molybdenum per year of production.
An alternative configuration not shown here, but that would differ only slightly, is a rotary kiln that is indirectly fired. In this scheme, the rotary kiln would have a firebox, in which natural gas is burned to heat the contents of the kiln via conduction. The natural gas consumption would be slightly higher than that in the directly fired scheme because of the heat losses and inefficiencies associated with heat transfer through the kiln walls. However, the volume of kiln gases requiring treatment in the SO2 handling system would be smaller.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. The terms “a” and “an” and “the” herein do not denote a limitation of quantity, and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. “Or” means “and/or.” All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other (e.g., ranges of “up to 25 wt. %, or, more specifically, 5 wt. % to 20 wt. %”, is inclusive of the endpoints and all intermediate values of the ranges of “5 wt. % to 25 wt. %,” etc.). “Combination” is inclusive of blends, mixtures, alloys, reaction products, and the like. The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to denote one element from another. Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
The terms first, second, third etc. can be used herein to describe various elements, components, regions, layers and/or sections, but these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present embodiments.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this general inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.
This application claims the benefit of U.S. Provisional Application No. 61/736,114 filed Dec. 12, 2012, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61736114 | Dec 2012 | US |
