Abstract:
A process is disclosed wherein essentially all of the rhenium can be removed from an aqueous solution of molybdenum without the extraction of molybdenum using a particular solvent extraction solution. The extraction solution consists essentially of a quaternary ammonium compound and at least about 50% by volume of a highly aromatic solvent which dissolves a quaternary ammonium-rhenium complex to the extent of at least 0.1% by volume.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of Ser. No. 510,067, filed Sept. 27, 1974, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 1. Field of the Invention 
     This invention relates to an improved process for the recovery of rhenium values from aqueous molybdenum solutions containing rhenium. In particular, it relates to a process for the recovery of rhenium from sodium molybdate solutions prepared from molybdenite concentrates. 
     2. Prior Art 
     Rhenium is often present in small amounts in molybdenite concentrates. For example, molybdenite concentrates formed as a by-product from the recovery of other metals from ores such as copper contain small amounts of rhenium, generally in order of from 0.4 to 4 pounds per ton of concentrate. Recovery of even these rhenium values can aid appreciably in lower overall manufacturing costs because of the high cost of rhenium. Usually these concentrates are roasted to recover the molybdenum as molybdenum oxide and the rhenium values are volatilized. The rhenium values are then recovered from the flue ducts and gases by contacting them with water or some other solvent, such as an acidic or basic medium. The rhenium values are then extracted from such rhenium bearing solutions by ion-exchange. According to U.S. Pat. No. 2,876,065, the impure clarified rhenium solution is passed through ion-exchange equipment in intimate contact with a strongly basic alkylamine-type synthetic resin. Examples of commercially available resins are Amberlite IRA-400 and Dowex 1×8. After eluting to remove molybdenum and other impurities, the rhenium is eluted with a strong mineral acid such as perchloric acid. Rhenium sulfide (Re 2  S 7 ) is precipitated from the rhenium-bearing acid solution by H 2  S gas. The rhenium sulfide is then oxidized and converted to NH 4  ReO 4  by the addition of NH 4  OH. The NH 4  ReO 4  is crystallized by evaporation. 27 A similar process is disclosed U.S. Pat. No. 3,244,475 except that a liquid ion-exchange material is used instead of an ion-exchange resin to recover the rhenium. The rhenium values are recovered from the aqueous solution by means of a quaternary ammonium compound carried in kerosene with primary decyl alcohol as the solubilizer. The foregoing process is also described in the Bureau of Mines Report of Investigation No. 6246 entitled &#34;Sources and Recovery Methods for Rhenium&#34; by the inventors of the foregoing patent. The amine is stripped with dilute perchloric acid and NH 4  ReO 4  is recovered in much the same manner as is disclosed in U.S. Pat. No. 2,876,065. In the foregoing report 190 6246, it is disclosed that various acids and salt solutions were tried and only perchloric acid and perchlorate salt solutions were effective in removing rhenium from the organic. Under the conditions disclosed in the patent, the extraction efficiency was lowered upon recycle of the extraction system. For example, as shown in Example I in the original pass 142 mg. of rhenium was extracted out of 152 mg., or 94.5% of the rhenium, and upon recycle, the recycle extracted only 85% of the rhenium. The foregoing Bureau of Mines Report also states that reuse of the organic results in lower extraction efficiencies particularly when perchloric acid was used as the stripping agent. 
     More recently, however, the U.S. Pat. No. 3,558,268 discloses a process similar to the above processes in which ammonium thiocyanate (NH 4  SCN) is used as the stripping agent and crystallization of NH 4  ReO 4  directly from the NH 4  SCN solution is claimed, thereby eliminating the sulfide precipitation and oxidation steps. 
     There are several disadvantages with the foregoing processes which result in recovery appreciably less than all of the rhenium initially present in the solutions. For example, the molybdenum must be separated from the rhenium by roasting prior to the recovery of rhenium and that technique generally results in losses of up to about 50% of the rhenium present due to volatilization. Additionally, after the recovery of rhenium by the extraction processes mentioned, the raffinate discarded contains 0.01 grams Re/liter. This concentration is often higher than that found in some concentrates. Also, reuse of the organic solution as the salt from perchlorate, thiocyanate, etc. which result after stripping with the agents previously disclosed, sharply lowers the extraction coefficients. The decrease in the extraction capacity with use is believed to be associated with the insolubility of the loaded amine in the decylalcohol-amine-kerosine system. 
     It is believed, therefore, that a process that enables recovery of essentially all of the rhenium and an organic extraction solution that can be regenerated without an appreciable loss in extractive capacity is an advancement in the art. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an improved process for the extraction and recovery of rhenium from molybdenum solution containing relatively lower concentrations of rhenium. 
     It is a further object of this invention to provide an improved organic selective extraction solution which can be regenerated and recycled. 
     It is still a further object of this invention to provide a process capable of recovering essentially all of the rhenium from molybdate solutions. 
     It is an additional object of this invention to provide a process capable of recovering rhenium from solutions having a low concentration of rhenium. 
     These and other objects are achieved in one embodiment of the invention wherein a process extracting rhenium from an aqueous molybdate solution is achieved by extracting with an organic extractant consisting essentially of at least about 0.05% by volume of a quaternary ammonium compound, water-insoluble hydrocarbon having predominantly high aromatic content which dissolves a quaternary ammonium-rhenium complex to the extent of at least about 0.1% volume. The quaternary ammonium compounds which are particularly useful are those with the general formula [R 3  (CH 3 ) N]  +  A  - , in which R is an alkyl group of from about 8 to about 10 carbon atoms and A is an anion, derived from mineral acids. Particularly useful extractants are tricapryryl monomethyl ammonium chloride and tricapryryl monomethyl ammonium nitrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a flow sheet of one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawings. 
     As previously mentioned, the active extractant is a quaternary ammonium compound represented by the formula: 
     
         [R.sub.3 (CH.sub.3) N] .sup.+ A .sup.- 
    
     wherein R is an alkyl group having an average of from about 8 to about 10 carbon atoms and A is an anion derived from a mineral acid. Although R is shown as having a definite chain length, it is to be understood that the extractant can be a mixture of hydrocarbons having an average carbon chain length of from 8 to 10. It is preferred that R have an average chain length of about 8, thus tricapryryl monomethyl ammonium compounds are preferred. A in the foregoing formula can be any anion derived from a mineral acid and will be dependent in a continuous system upon the stripping agent used. For example, if perchlorate is desired for A, then perchloric acid can be used as the stripping agent. If nitrate is desired for A, then nitric acid is the stripping agent. Nitric acid is the preferred stripping agent and will convert the commercially available tricapryryl monomethyl ammonium chloride to the nitrate. Tricapryryl monomethyl ammonium chloride is sold by General Mills Corporation under the trade name Aliquat 336. 
     The solvents used are high aromatic content solvents and seem to overcome the difficulties associated with the aliphatic hydrocarbon solvent used in the prior art. The prior art solvent systems lost extraction efficiency and new extractant had to be added because rhenium extraction efficiencies would drop appreciably. Suitable solvents include benzene, toluene, xylene, and the like and mixtures thereof and mixtures such as benzene and remainder kerosene can also be used. A particularly preferred solvent which has desired characteristics from a handling standpoint is a high aromatic content solvent sold by Buffalo Solvents and Chemical Corporation under the trade name S.C. 28.  A major criteria for the solvent is that it will dissolve at least 0.1% by volume of the quaternary ammonium-rhenium complex. 
     The concentration of quaternary amine in the organic extraction solution will be dependent upon the concentration of rhenium in the feed and the ratio of rhenium to molybdenum in the feed. The concentration of the amine active extractant is low, that is, in the range of from about 0.05% to about 1% by volume when the concentration of rhenium is low such as below 50 mg/liter and the weight ratio of rhenium to molybdenum is from 1:2000 to 1:3000. Higher concentrations above about 1% tend to extract some of the molybdenum values in the foregoing illustration. Concentrations lower than about 0.05% by volume requires excessively large volumes of extractant solutions to be utilized. If the rhenium concentration is significantly above 50 mg/liter or significantly less molybdenum is present in relation to rhenium, higher concentration of active extractant can be used. If there is not a significant amount of molybdenum present, then only the limitation imposed upon the maximum concentration of the amine results from the viscosity of the solution. 
     The organic to aqueous volumetric ratios can be varied within the practical limits of from about 1:10 to about 10:1. The number of extraction stages used will be primarily dependent upon the ratio of organic to aqueous and the concentration of rhenium in the aqueous phase. 
     To more fully illustrate the invention, the following non-limiting examples are given. All parts, proportions and percentages are by volume unless otherwise indicated. 
     EXAMPLE 1 
     To aid in the description reference is made to the drawing in which the extraction circuit 10, consists of three units 12, 14, and 16 connected in series. The aqueous feed stream 18 (containing about 30 mg/liter of rhenium and about 66 grams/liter of molybdenum) enters unit 16 at the rate of about 41 ml/minute while the organic phase 20 enters unit 12 providing counter current flow at the rate of about 12.5 ml/minute. The extractant solutions consists essentially of 0.1% by volume tricapryryl monomethyl amine and 99.9% by volume of S.C. 28. The raffinate 22 containing the molybdenum values exits from unit 12. The organic phase exits from unit 16 and enters the stripping circuit comprised of generally at least three units illustrated in the drawing as units 24, 26, and 28 at unit 24. The stripping solution 30 enters units 24 and 26 and the stripped organic 32 exits from 28 and is recycled. Although a 4N nitric acid stripping solution at the rate of about 8 ml/minute is suitable, other stripping agents can be used, such as perchloric acid, perchlorate salt solutions, ammonium thiocyanate and the like. The rhenium values 34 exit from units 24 and 28. Although in the drawing there is illustrated only three extraction units in the stripping circuits, more or less units in either circuit can be employed. Additionally, the organic can be washed with water to remove water soluble impurities if desired. The product stream 34 is sent to a recovery system to recover the rhenium values such as by conversion of the perrhenic acid contained in stream 34 to ammonium perrhenate or other known recovery techniques. 
     EXAMPLE 2 
     An organic extraction solution is prepared containing varying amounts of amine and decyl alcohol as a solubilizer and kerosene as a solvent. An aqueous solution containing sodium molybdate and rhenium at concentrations of 100 g. to 120 g. MoO 3  /liter and 40 to 90 mg Re/liter brought into contact with the organic solutions. The following table gives the results of the experiment. 
     
         ______________________________________% Aliquat 336      Volume Ratio -- O:A% Decyl Alcohol      1:1     1:2    1:3  1:4  1:5  1:10 1:50______________________________________ %10.00* Re Ext.  100*** %0.00  Mo Ext.   10 %10.00* Re Ext.  100 %5.00  Mo Ext.   10 %5.00* Re Ext.  100 %3.00  Mo Ext.   10 %1.00** Re Ext.  100     100  100  100  100 %1.00  Mo Ext.   --     --   --   --   -- %0.50** Re Ext.  100     100  100  100  100  100  100 %0.50  Mo Ext.   --     --   --   --   --    10   20 %0.10** Re Ext.  75.12    70   70   70   65 %0.10  Mo Ext.   --     --   --   --   -- %0.01** Re Ext.   --     --   --   --   -- %0.01  Mo Ext.   10     --   --   --   --______________________________________ *Aqueous 40-50 mg Re/1 **˜90 mg Re/1 ***Organic separated into 2 phases after extraction. 
    
     As is clearly shown, unless the Aliquat 336 decyl alcohol are both at the 0.5% to 1% level and the Re concentration is higher than 90 mg. Re/liter, then either the recovery of Re is lower or else some molybdenum is extracted. 
     EXAMPLE 3 
     An aqueous solution containing about 40 mg/liter of rhenium is contacted with five of the organic extraction solutions. In each instance 0.1% Aliquat 336 is the extractant. The organic solutions are stripped to remove the rhenium, then are recycled for a number of times. The five organics used are: 
     1. 0.1% Aliquat 336 in benzene. 
     2. 0.1% Aliquat 336, 0.2% decyl alcohol, balance kerosene. 
     3. 0.1% Aliquat 336, 0.1% decyl alcohol, 2.5% benzene, balance kerosene. 
     4. 0.1% Aliquat 336, 0.1% decyl alcohol, 10% benzene, balance kerosene. 
     5. 0.1% Aliquat 336, 0.1% decyl alcohol, 25% benzene, balance kerosene. 
     The amount of rhenium extracted on the first pass is essentially the same for organics 1 through 5. After being recycled only Organic #1 remains at its original level. Organic No. #2 after about 17 recycles has about 60% of its original extraction capacity. After 24 recycles Organic #still remains essentially at the original level. 
     EXAMPLE 4 
     Essentially the same procedure as in Example 3 is used. The following organics are prepared: 
     Organic 
     1. 0.1% Aliquat 336 Balance benzene 
     2. 0.1% Aliquat 336 0.2% decyl alcohol, Balance kerosene 
     3. 0.1% Aliquat 336 Balance HAN* 
     4. 0.1% aliquat 336 25% benzene, balance HAN* 
     5. 0.1% aliquat 336 50% benzene, balance HAN* 
     6. 0.1% aliquat 336 in S.C. #28. 
    
     The foregoing examples are believed to conclusively illustrate the advantage of a highly aromatic solvent system over the parafinic kerosene system. 
     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.