Patent Publication Number: US-2013243673-A1

Title: Separation of tungsten from ammonium molybdate solutions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Stage application under 35 U.S.C. §365 of PCT Application No. PCT/US2011/23852 filed Feb. 7, 2011, which claims priority under 35 U.S.C. §120 to U.S. Non-provisional patent application Ser. No. 12/875,385 filed on Sep. 3, 2010; which priority Under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/302,378 filed on Feb. 8, 2010, the subject matter of which are hereby incorporated by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention was not developed with the use of any Federal Funds, but was developed independently by the inventors. 
    
    
     BACKGROUND 
     Field and Background of the Invention 
     Tungsten was discovered and isolated in the late 18 th  century. Tungsten is found in the minerals wolframite (iron-manganese tungstate, FeWO 4 /MnWO 4 ), scheelite (calcium tungstate, (CaWO 4 ), ferberite (FeWO 4 ) and hithnerite (MnWO 4 ). In 2000, these minerals were mined and used to produce about 37,400 tons of tungsten concentrates. China produced over 75% of this total, with most of the remaining production coming from Austria, Bolivia, Portugal, and Russia. 
     Tungsten is extracted from its ores in several stages. The ore is eventually converted to tungsten(VI) oxide (WO 3 ), which is heated with hydrogen or carbon to produce powdered tungsten. It can be used in that state or pressed into solid bars. Tungsten can also be extracted by hydrogen reduction of WF 6  or pyrolytic decomposition. In its raw form, tungsten is a steel-gray metal that is often brittle and hard to work, but, if pure, it can be worked easily by forging, drawing, extruding or sintering. Of all metals in pure form, tungsten has the highest melting point, 3,422° C. (6,192° F.), lowest vapor pressure (at temperatures above 1,650° C. (3,000° F.)) and the highest tensile strength. 
     Because it retains its strength at high temperatures and has a high melting point, elemental tungsten is used today in many high-temperature applications, such as light bulb, cathode-ray tube, and vacuum tube filaments, heating elements, and rocket engine nozzles. Its high melting point also makes tungsten suitable for aerospace and high-temperature uses such as electrical, heating, and welding applications, notably in the gas tungsten arc welding process. 
     Because of its conductive properties and relative chemical inertia, tungsten is also used in electrodes, and in the emitter tips in electron-beam instruments that use field emission guns, such as electron microscopes. In electronics, tungsten is used as an interconnect material in integrated circuits, between the silicon dioxide dielectric material and the transistors. It is used in metallic films, which replace the wiring used in conventional electronics with a coat of tungsten (or molybdenum) on silicon. 
     The electronic structure of tungsten makes it one of the main sources for X-ray targets, and also for shielding from high-energy radiations (such as radioactive samples). Tungsten powder is used as a filler material in plastic composites, which are used as a nontoxic substitute for lead in bullets, shot, and radiation shields. Since this element&#39;s thermal expansion is similar to borosilicate glass, it is used for making glass-to-metal seals. 
     The hardness and density of tungsten are applied in obtaining heavy metal alloys, such as high speed steel, Tungsten high melting point makes tungsten a good material for applications like rocket nozzles. Superalloys containing tungsten are used in turbine blades and wear-resistant parts and coatings. Applications requiring its high density include heat sinks, weights, counterweights, ballast keels for yachts, tail ballast for commercial aircraft, and as ballast in race cars for NASCAR and Formula One. 
     High-density alloys of tungsten with nickel, copper or iron are used for fishing lures (tungsten beads allow the fly to sink rapidly). Some types of strings for musical instruments are wound with tungsten wires. Its density, similar to that of gold, allows tungsten to be used in jewelry as an alternative to gold or platinum. Its hardness makes it ideal for rings that will resist scratching, are hypoallergenic, and will not need polishing, which is especially useful in designs with a brushed finish. 
     Tungsten played a significant role in World War II in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore. Tungsten&#39;s resistance to high temperatures and its strength in alloys made it an important raw material for the weaponry industry. In armaments, tungsten, usually alloyed with nickel and iron or cobalt to form heavy alloys, is used in kinetic energy penetrators as an alternative to depleted uranium, in applications where uranium&#39;s additional pyrophoric properties are not required (for example, in ordinary small arms bullets designed to penetrate body armor). Similarly, tungsten alloys have also been used in cannon shells, grenades and missiles, to create supersonic shrapnel. 
     Tungsten compounds are used in catalysts, inorganic pigments (e.g. tungsten oxides), and as high-temperature lubricants (tungsten disulfide). Tungsten carbide is used to make wear-resistant abrasives and cutters and knives for drills, circular saws, milling and turning tools used by the metalworking, woodworking, mining, petroleum and construction industries and accounts for about 60% of current tungsten consumption. Tungsten oxides are used in ceramic glazes and calcium/magnesium tungstates are used widely in fluorescent lighting, while tungsten halogen bulbs are frequently used to light indoor photo shoots, and special negative films exist to take advantage of tungsten&#39;s unique disentangling properties. Crystal tungstates are used as scintillation detectors in nuclear physics and nuclear medicine. Other salts that contain tungsten are used in the chemical and tanning industries. 
     Molybdenum minerals have been known from approximately the same time as tungsten. Molybdenum is mined as a principal ore, and is also recovered as a byproduct of copper and tungsten mining. Molybdenum has similar physical properties as tungsten with the sixth highest melting point of 2,623° C. (4,753° F.). 
     The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening makes it useful in applications that involve intense heat, including the manufacture of aircraft parts, electrical contacts, industrial motors and filaments. Most high-strength steel alloys contain 0.25% to 8% molybdenum. More than 43,000 tons of molybdenum are used as an alloying agent each year in stainless steels, tool steels, cast irons and high-temperature superalloys. 
     Molybdenum is also used in steel alloys for its high corrosion resistance and weldability. Molybdenum contributes further corrosion resistance to “chrome-moly” type-300 stainless steels (high-chromium steels that are corrosion-resistant already due to their chromium content) and especially in the so-called superaustenitic stainless steels. Molybdenum acts by increasing lattice strain, thus increasing the energy required to dissolve out iron atoms from the surface. 
     Because of its lower density and more stable price, molybdenum is sometimes used instead of tungsten. An example is the ‘M’ series of high-speed steels such as M2, M4 and M42 as substitution for the ‘T’ steel series which contain tungsten. Molybdenum can be implemented both as an alloying agent and as a flame-resistant coating for other metals. Although its melting point is 2,623° C. (4,753° F.), molybdenum rapidly oxidizes at temperatures above 760° C. (1,400° F.) making it better-suited for use in vacuum environments. 
     Other uses of molybdenum include radioisotopes in medical procedures. Molybdenum disulfide (MoS 2 ) is used as a solid lubricant and a high-pressure high-temperature antiwear agent. Molybdenum disilicide (MoSi 2 ) is an electrically conducting ceramic with primary use in heating elements operating at temperatures above 1500° C. in air. Molybdenum powder is used as a fertilizer for some plants, such as cauliflower. The element is also used in analyzers in power plants for pollution controls acting as a catalyst for consistent readings by infrared light. Ammonium heptamolybdate is used in biological staining procedures. Lead molybdate (wulfenite) co-precipitated with lead chromate and lead sulfate is a bright-orange pigment used with ceramics and plastics. 
     Technical grade molybdenum oxide (TMO) is the most widely traded molybdenum product worldwide. Molybdenum trioxide (MoO 3 ) is used as an adhesive between enamels and metals. MoO 3  is produced industrially roasting molybdenum disulfide, the chief ore of molybdenum: 
       2MoS 2 +7O 2 →2MoO 3 +4SO 2  
 
     Molybdenum trioxide is used to manufacture molybdenum metal, which serves as an additive to steel and corrosion-resistant alloys. The relevant conversion entails treatment of MoO 3  with hydrogen at elevated temperatures: 
       MoO 3 +3H 2 →Mo+3H 2 O
 
     Technical grade molybdenum trioxide may contain from 50 to 1000 ppm of tungsten. High purity molybdenum powder requires that the amount of tungsten be no greater than approximately 150 weight ppm. Purification of tungsten from molybdenum solution is also necessary in order to produce pure ammonium dimolybdate. The separation of tungsten from molybdenum in aqueous solutions presents a challenge due to the similarity of the chemical properties of these two metals. The current invention presents a method for separating tungsten from molybdenum trioxide in order to obtain highly pure molybdenum and tungsten values. 
     U.S. Pat. No. 4,525,331 to Cheresnowsky et al. describes a conventional purification method for technical grade molybdenum trioxide. An aqueous solution of nitric acid and ammonium nitrate is contacted with impure molybdenum concentrate to solubilize a major portion of the impurities. The resulting molybdenum concentrate is digested in ammonium hydroxide under conditions that maximize iron precipitation and removal. The resulting ammonium molybdate solution is separated from the sludge and further purified by chelating cation exchange resin in the ammonium form. Although the process provides ammonium molybdate having low impurity levels of various metals, because the chemistries of tungsten and molybdenum are very similar, tungsten is not separated from the molybdenum during such process. 
     Several methods are known for the separation of tungsten from molybdenum. U.S. Pat. No. 4,303,622 to Huggins et al. is a process for recovering tungsten and molybdenum values from tungsten concentrates by dissolving the concentrate in hot sodium hydroxide solution and adding a sulfide precipitating agent to precipitate molybdenum trisulfide (and some tungsten). 
     U.S. Pat. No. 3,969,478 to Zelikman et al. in and U.S. Pat. No. 4,275,039 to Ozensoy et al. disclose a process for the separation of tungsten and molybdenum comprised of adding nitric or hydrochloric acid to obtain a pH from 0.5 to 4.3, introducing hydrogen peroxide as a complexing agent, and then selectively extracting molybdenum with an organic solvent phase. These processes cannot be applied for the separation of tungsten from ammonium molybdate solutions. 
     A publication entitled “Use of Hydrated Oxides of Multivalent Metals for Effective Removal of Tungsten from Molybdenum Compounds”, by M. I. Semenov et al published in the Journal of Applied Chem. USSR (Leningrad, 1984, 57(7), 1501-6) relates to the separation of tungsten from molybdenum by adding hydrated oxides of multivalent metals to a molybdate solution containing tungsten to cause sorption of the tungsten. The procedure was performed according to the publication by the inventors hereof and it was found that the process took as long as seven days, making any industrial application of the process impractical and undesirable. 
     A publication entitled “Ammonium Molybdate of High Purity” by Papageorgios, Panajotis, Plonka, Marian, Walczak, Wladylsaw, (Przedsiebiorstwo Przemyslowo-Handlowe “Polskie Odczynniki Chemiczne”) Pol. 54,639 (Cl. C 01 g), 20 Jan. 1968, Appl. 28 Mar. 1966; 2 pp, relates to obtaining spectrally pure ammonium molybdate by absorption on a freshly prepared suspension of products of hydrolysis of tin salts. However, preliminary purification is recommended and methyl alcohol, which is highly toxic, is used to precipitate the ammonium molybdate. 
     U.S. Pat. No. 4,999,169 also to Cheresnowsky discloses a process of separating tungsten from ammonium hydroxide solutions containing molybdenum by sorption of the tungsten on tin (IV) oxide hydrate. The sorption is selective and the molybdenum is left in the solution. The process is difficult to commercialize because it requires significant amounts of tin (IV) oxide hydrate. It is difficult to imagine a practical way of rejuvenation of tin oxide and further utilization of tungsten. 
     Still desired is an efficient process for the separation of tungsten from molybdenum, specifically from ammonium molybdate solutions. The present invention addresses the need by teaching a simple and effective metallurgical process for separating tungsten from molybdenum and thereby providing metal forms with improved purity. The invention enables a continuous process. These and other objects of the invention will be more clearly defined when taken in conjunction with the following disclosure, the accompanying figures and the appended claims. 
     The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process for the separation of tungsten from molybdenum and more particularly from ammonium molybdate solutions. The method comprises dissolving technical grade molybdenum trioxide in an aqueous ammonium hydroxide solution and further adding certain metal generating compounds to the aqueous solution thereby generating a tungsten-containing precipitate. Calcium, iron and manganese are the preferred metal generating compounds of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
         FIG. 1  is a Pourbaix diagram of a calcium, tungsten, molybdenum aqueous system at 25° C. 
         FIG. 2  is a Pourbaix diagram of a calcium, tungsten, molybdenum aqueous system at 50° C. 
         FIG. 3  is a Pourbaix diagram of a calcium, tungsten, molybdenum aqueous system at 75° C. 
         FIG. 4  is a Pourbaix diagram of a calcium, tungsten, molybdenum aqueous system at 10° C. 
         FIG. 5  is a Pourbaix diagram of an iron, tungsten, molybdenum aqueous system at 25° C. 
         FIG. 6  is a Pourbaix diagram of an iron, tungsten, molybdenum aqueous system at 50° C. 
         FIG. 7  is a Pourbaix diagram of an iron, tungsten, molybdenum aqueous system at 75° C. 
         FIG. 8  is a Pourbaix diagram of a manganese, tungsten, molybdenum aqueous system at 25° C. 
         FIG. 9  is a Pourbaix diagram of a manganese, tungsten, molybdenum aqueous system at 10° C. 
         FIG. 10  is a Pourbaix diagram of a manganese, tungsten, molybdenum aqueous system at 50° C. 
     
    
    
     DETAILED DESCRIPTION 
     Technical grade molybdenum trioxide usually contains from approximately 50 to 1000 ppm of tungsten, but any molybdenum trioxide compound that contains tungsten may be used in the process of the invention. Table 1 represents an example of the typical composition of technical grade molybdenum trioxide. In methods known to recover tungsten from wolframite and scheelite concentrates, the ore is usually concentrated by gravity or flotation methods and the concentrate is thereafter treated to recover tungsten that is substantially pure and free of the impurities listed in Table 1, except that due to the similarity of the physical properties between tungsten and molybdenum, they remain unseparated, as discussed in the background of the invention. For purposes of definition in this specification, “technical grade molybdenum trioxide” shall mean the technical grade molybdenum trioxide that contains some or all of the impurities listed in Table 1 and/or other minor metal impurities not listed therein but that may be recovered or contained in the ore. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Composition of Technical Grade Molybdenum Trioxide 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Metal 
                 % 
                 Metal 
                 % 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Mo 
                 53.38 
                 O 
                 36.80 
               
               
                   
                 S 
                 1.085 
                 W 
                 0.08 
               
               
                   
                 C 
                 0.012 
                 P 
                 0.026 
               
               
                   
                 Cu 
                 0.048 
                 Fe 
                 1.04 
               
               
                   
                 Si 
                 4.0 
                 Ca 
                 1.97 
               
               
                   
                 Pb 
                 0.035 
                 Mg 
                 1.0 
               
               
                   
                 Mn 
                 1.33 
                 K 
                 0.07 
               
               
                   
                 Al 
                 0.51 
                 Ni 
                 0.007 
               
               
                   
                   
               
            
           
         
       
     
     According to the process of the invention, the technical grade molybdenum trioxide is added to an ammoniacal solution, most preferably a solution of ammonium hydroxide. The technical grade molybdenum trioxide ore is preferably added in a finely divided state as this promotes dissolution of the compound. The ammonium hydroxide is formed by the addition to water of either gaseous or liquid anhydrous ammonia. Gaseous ammonia is the preferred form of ammonia in the process of the invention. The ammonia can be pre-dissolved in water. Dilute or concentrated solutions of ammonium hydroxide are utilized, depending on the subsequent process requirements. In a particularly preferred embodiment of the process of the invention, a target of about 200 g to 280 g, more preferably from 220 g to 260 g, and most preferably 240 g molybdenum per liter of ammonium hydroxide is desired. To obtain 240 g molybdenum per liter of ammonium hydroxide, 175 g to 185 g ammonium hydroxide per liter (85 g to 90 g ammonia per liter) is required. Once the technical grade molybdenum trioxide is added to the ammonium hydroxide solution, the ammonium hydroxide serves to dissolve the molybdenum in the aqueous solution. 
     The content of the molybdenum in the ammonium molybdate solution can be anywhere from 100 to 250 grams per liter. In such solutions, the content of the tungsten which is to be separated from the molybdenum in the solution is in the range of approximately 5 to 250 ppm. 
     Once the ammonium molybdate solution is prepared, the solution is then contacted with certain metal compounds that are capable of reacting with the tungsten and causing precipitation of the tungsten out of solution. In the preferred embodiments of the invention, calcium, iron, and manganese compounds are used to precipitate the tungsten from the ammonium molybdate solution. Combinations or mixtures of the metal compounds can be utilized, for example a calcium compound can be added together with an iron compound. Once the metal compound is added, the components can be further stirred, mixed or agitated in order to enhance the speed of reaction. 
     It has been identified that the temperature and pH of the system have an impact on the precipitation of the tungsten from the ammonia molybdate solution. Though in general, an increase in temperature increases the rate of any particular reaction, it has been discovered that in the process of the invention, an increase in temperature serves to effect the selectivity of the reaction and therefore particular parameters as disclosed herein are necessary in order to effectuate the operability of the method of the invention. 
     A preferred tungsten precipitate generating metal of the invention is calcium. Calcium ion forms a very strong compound with the tungstate ion.  FIGS. 1-4  are Pourbaix diagrams that demonstrate the selective precipitation of tungsten with calcium ions at various temperatures, 10° C., 25° C., 50° C. and 75° C., respectively. It was determined that the preferable temperature range for precipitation with calcium compounds is from 10° C. to 50° C., more preferably from 15° C. to 40° C. and more preferably from 20° C. to 30° C. In a preferred embodiment, the selective precipitation of tungsten is attained using calcium acetate or calcium hydroxide at room temperature (approximately 25° C.). Using calcium compounds in the process of the invention, the pH of the system will be maintained at greater than approximately 2.0, preferably greater than 7.0 and increasing slightly depending on the temperature of the system with the preferred pH range being from 7.5 to 10. Other calcium compounds useful in the process of the invention include, but are not limited to, calcium chloride, and calcium nitrate. 
     Iron is another operable metal in the process of the invention. It can be seen from  FIGS. 5 ,  6  and  7  that iron may be used to selectively precipitate tungsten from ammonium solutions in a wide range of temperatures and pH values. Iron tungstate (FeWO 4 ) is the major precipitate formed during the process demonstrated in  FIGS. 5-7 . Beyond operable conditions, other unwanted precipiates will form, such as ferrous molybdate (FeMoO 4 ). Tables 2 and 3 demonstrate that although the molybdenum content in the solution is much higher than that of tungsten, selective precipitation of tungsten is possible with iron compounds within certain temperature and pH conditions. Based on the data derived from  FIGS. 5-7 , the operable range of the process has a pH range of between 1.0 and 11.8 and temperature between 10° C. and 70° C.; more preferably the pH is between 6 and 10 and temperature between 40° C. and 70° C.; and more preferably the pH range is 7 to 9 and the temperature is 60° C. to 70° C. In a preferred embodiment, the temperature of the system using ferric sulfate is at approximately 50° C. having a pH of between 7 and 10. Other iron compounds useful in the process of the invention include, but are not limited to ferrous molybdate, ferric molybdate, ferric nitrate, and ferric chloride. 
     Manganese is another metal compound useful in the process of the invention.  FIGS. 8 ,  9  and  10  demonstrate that manganese ions form both molybdate and tungstate species in a wide pH range, preferably from 3 to 11, and can be used to precipitate tungsten from ammonium molybdate solutions. Decreasing temperature improves selectivity of tungsten precipitation with manganese compounds. The preferred temperatures for the process using manganese compounds is from 10° C. to 60° C. Some useful manganese compounds include, but are no limited to, manganous chloride, and manganous nitrate. 
     The tungsten containing precipitate may be separated from the solution in any convenient way known in the art, such as filtration. The tungsten can then be recovered from the filter cake, washed, and utilized in ferroalloy processes. In a continuous production industrial process of the invention, clean technical grade molybdenum trioxide is generated. The resulting clean technical grade molybdenum trioxide is substantially free of tungsten, containing tungsten in an amount of less than approximately 125 ppm, and depends greatly on the efficiency of the removal procedure. The iron, calcium and manganese metal compounds can also be separated and recovered after the separation of the tungsten from the solution and further used as alloys and/or formed into useful compounds of such metals. 
     EXAMPLES 
     In order to identify the possibility of tungsten removal, thermodynamic simulations were carried out using HSC CHEMISTRY® for Windows Thermodynamic Software 6.0 by Outokumpu Research Oy of Finland. Two software modules were used: Reaction Equations and Pourbaix Diagrams. The results are presented in the tables and figures herein. The Pourbaix diagrams, also known as a potential/pH diagrams, map out the potential stable or equilibrium phases of the particular aqueous electrochemical system. The predominant ion boundaries are represented by the lines indicated on the diagrams. 
     Thermodynamic Analysis 
     Iron molybdate precipitation was obtained based on the data of thermodynamic stability of iron tungstate in water based solutions. Table 2 presents the thermodynamic stability of iron molybdate (FeMoO 4 ) in an aqueous solution. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Thermodynamic Analysis of FeMoO 4  Formation 
               
               
                 Fe(+2a) + MoO4(−2a) = FeMoO 4   
               
            
           
           
               
               
               
               
               
               
            
               
                 T 
                 deltaH 
                 deltaS 
                 deltaG 
                   
                   
               
               
                 C. 
                 kcal 
                 cal/K 
                 kcal 
                 K 
                 Log (K) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0.000 
                 1.799 
                 40.803 
                 −9.346 
                 3.010E+007 
                 7.479 
               
               
                 10.000 
                 2.965 
                 44.996 
                 −9.776 
                 3.519E+007 
                 7.546 
               
               
                 20.000 
                 3.908 
                 48.274 
                 −10.243 
                 4.336E+007 
                 7.637 
               
               
                 30.000 
                 4.731 
                 51.033 
                 −10.740 
                 5.538E+007 
                 7.743 
               
               
                 40.000 
                 5.497 
                 53.519 
                 −11.263 
                 7.263E+007 
                 7.861 
               
               
                 50.000 
                 6.242 
                 55.863 
                 −11.810 
                 9.723E+007 
                 7.988 
               
               
                 60.000 
                 6.979 
                 58.110 
                 −12.380 
                 1.324E+008 
                 8.122 
               
               
                 70.000 
                 7.720 
                 60.299 
                 −12.972 
                 1.830E+008 
                 8.262 
               
               
                 80.000 
                 8.479 
                 62.481 
                 −13.586 
                 2.561E+008 
                 8.408 
               
               
                 90.000 
                 9.265 
                 64.676 
                 −14.222 
                 3.626E+008 
                 8.559 
               
               
                 100.000 
                 10.087 
                 66.906 
                 −14.879 
                 5.193E+008 
                 8.715 
               
               
                   
               
            
           
         
       
     
     Iron molybdate precipitation values for negative delta G were obtained based on the data of thermodynamic stability of iron tungstate (FeWO 4 ) in an aqueous solution, demonstrated in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Thermodynamic Analysis of FeWO 4  Formation 
               
               
                 Fe(+2a) + WO4(−2a) = FeWO 4   
               
            
           
           
               
               
               
               
               
               
            
               
                 T 
                 deltaH 
                 deltaS 
                 deltaG 
                   
                   
               
               
                 C. 
                 kcal 
                 cal/K 
                 kcal 
                 K 
                 Log (K) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0.000 
                 −7.042 
                 40.861 
                 −18.203 
                 3.677E+014 
                 14.565 
               
               
                 10.000 
                 −6.182 
                 43.956 
                 −18.628 
                 2.394E+014 
                 14.379 
               
               
                 20.000 
                 −5.540 
                 46.185 
                 −19.079 
                 1.679E+014 
                 14.225 
               
               
                 30.000 
                 −5.019 
                 47.932 
                 −19.550 
                 1.245E+014 
                 14.095 
               
               
                 40.000 
                 −4.557 
                 49.434 
                 −20.037 
                 9.663E+013 
                 13.985 
               
               
                 50.000 
                 −4.116 
                 50.819 
                 −20.538 
                 7.788E+013 
                 13.891 
               
               
                 60.000 
                 −3.686 
                 52.129 
                 −21.053 
                 6.489E+013 
                 13.812 
               
               
                 70.000 
                 −3.256 
                 53.402 
                 −21.581 
                 5.569E+013 
                 13.746 
               
               
                 80.000 
                 −2.809 
                 54.686 
                 −22.121 
                 4.909E+013 
                 13.691 
               
               
                 90.000 
                 −2.337 
                 56.002 
                 −22.675 
                 4.437E+013 
                 13.647 
               
               
                 100.000 
                 −1.834 
                 57.370 
                 −23.241 
                 4.105E+013 
                 13.613 
               
               
                   
               
            
           
         
       
     
     Tables 2 and 3 demonstrate that the formation of both species, iron molybdate and iron tungstate, is thermodynamically favorable, indicated by the negative delta G values. However, the equilibrium constant for the formation of iron tungstate is seven (7) orders of magnitude greater than that of iron molybdate. Therefore, in principle, the selective precipitation of tungsten seems to be thermodynamically feasible. In order to establish the correct values of temperature and pH it was necessary to build Pourbaix diagrams ( FIGS. 1 through 8 ). These diagrams were built using the ratios of the species contents from Table 1 that represents the typical composition of technical grade molybdenum trioxide. 
     Example 1 
     Calcium acetate and calcium hydroxide were used to experimentally verify the general concept of the feasibility of using calcium to precipitate tungsten from ammonium molybdate solutions. 
     Required aliquots of either calcium acetate solution or calcium hydroxide (as set forth in Table 4) were added to 100 mL of the ammonium molybdate solution at room temperature (25° C.). The pH was kept at about 10.1. The slurry was agitated for three hours and the precipitate was filtered and washed. The solutions were submitted for analysis. The results are set forth in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Precipitation of Tungsten with Calcium Compounds 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Vol- 
                   
                   
                   
                   
               
               
                   
                 ume, 
                 Mo 
                 W 
                 % W 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Ml 
                 g/L 
                 mg 
                 g/L 
                 mg 
                 Removed 
                 Mo/W 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Feed 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Solution 
               
               
                 Filtrate 
                 100 
                 263 
                 26300 
                 0.349 
                 34.9 
                 0 
                 754 
               
               
                 Calcium 
               
               
                 Acetate 
               
               
                 Addition = 
               
               
                 4 mL 
               
               
                 Filtrate 
                 174 
                 115 
                 20010 
                 0.089 
                 15.5 
                 42 
                 1290 
               
               
                 Calcium 
               
               
                 Acetate 
               
               
                 Addition = 
               
               
                 5 mL 
               
               
                 Filtrate 
                 174 
                 510 
                 88740 
                 0.098 
                 17.1 
                 86 
                 5189 
               
               
                 Ca(OH) 2   
               
               
                 Addition = 
               
               
                 0.3 g 
               
               
                 Filtrate 
                 170 
                 478 
                 81260 
                 0.077 
                 13.1 
                 88 
                 6203 
               
               
                 Ca(OH) 2   
               
               
                 Addition = 
               
               
                 0.3 g 
               
               
                 Filtrate 
                 186 
                 193 
                 35898 
                 0.100 
                 18.6 
                 61 
                 1930 
               
               
                   
               
            
           
         
       
     
     Example 2 
     In this example, ferric molybdate was used for tungsten precipitation. A sample of ferric molybdate was prepared by dissolving 100 grams of ferric sulfate (20 grams iron) in water and then adding to an ammonium molybdate solution. The slurry was agitated for an hour and the precipitate was filtered and washed. The weight of the wet cake containing 5% iron was 400 grams. An ammonia digestion of 440 grams of leached cake was started by sequential additions of cake and ammonium hydroxide maintaining the pH at 8.5 to 8.7 at approximately 60° C. Thirty minutes after the last addition, 100 grams of the ferric molybdate cake was added (about 5 g Fe/L) and the slurry was agitated for another 30 minutes. The slurry was filtered and the cake was washed. Afterwards, 200 mL aliquots were heated to 60° C. and ferric molybdate additions of 2, 4, 8, and 16 grams were made (0.5, 1.0, 2.0, and 4.0 g Fe/L). After one hour of agitation, the slurries were filtered. The solutions and the solids the solids were submitted for analysis. Major filtrate analysis data are set forth in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Precipitation of Tungsten with Ferric Molybdate 
               
            
           
           
               
               
               
            
               
                   
                 Mo 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Volume, 
                   
                 Weight, 
                 W 
                   
                   
               
               
                   
                 mL 
                 g/L 
                 g 
                 g/L 
                 Mo/W 
                 pH 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Fe Addition = 0.5 g/L 
                   
                   
                   
                   
                   
                   
               
               
                 Filtrate 
                 ~200 
                 228.9 
                 45.78 
                 0.067 
                 3416 
                 7.8 
               
               
                 Fe Addition = 1.0 g/L 
               
               
                 Filtrate 
                 ~200 
                 216.2 
                 43.24 
                 0.046 
                 4700 
                 7.4 
               
               
                 Fe Addition = 2.0 g/L 
               
               
                 Filtrate 
                 ~200 
                 207 
                 41.4 
                 0.026 
                 7962 
                 7.8 
               
               
                 Fe Addition = 4.0 g/L 
               
               
                 Filtrate 
                 ~200 
                 212 
                 42.41 
                 0.016 
                 13253 
                 7.8 
               
               
                   
               
            
           
         
       
     
     Analysis of the data presented in Tables 4 and 5 confirmed the thermodynamic calculations and demonstrated the feasibility of tungsten removal from ammonium molybdate solutions. 
     Although the present invention has been described in conjunction with preferred embodiments, it is to be understood by those skilled in the art that modifications and variations thereto may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims. 
     Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other ones of the embodiments. 
     Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, various variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.