Abstract:
This invention provides a method to recover valuable DTPA from DTPA solvent extraction organic solution that avoids the need to operate at either high or very low temperatures, with extended reaction times and having to carry out solid/liquid separation on organic solution. Furthermore, the invention provides the flexibility of selecting appropriate base solution to carry out the DTPA recovery process and further provides the ability to directly treat metal-containing DTPA organic solution.

Description:
FIELD OF THE INVENTION 
     The proposed invention refers to a method for recovering DTPA bis(2,4,4-trimethylpentyl)dithiophosphinic acid from other organophosphorus impurities while avoiding the need to operate at extreme temperatures or producing, and having to deal with, organic solids. 
     BACKGROUND OF THE INVENTION 
     Hydrometallurgical processing of lateritic ores for nickel and cobalt recovery, based on high-pressure sulfuric acid leaching (HPAL), has been gaining technological and commercial acceptance over the last 20 years. 
     Once nickel and cobalt values are transferred from the laterite ore into the sulfuric acid leach solution in the HPAL autoclave, a number of different refining routes have been developed for recovering the nickel and cobalt from the leach solution into saleable products. Solvent extraction (SX), employing metal extractants dissolved in suitable hydrocarbon diluents, is often used in one or more steps within these refining routes. 
     One of the extractants used in the hydrometallurgical processing of lateritic ores for nickel and cobalt recovery is Cyanex 301, a solvent extractant developed by Cytec Industries (EP 021387). The use of Cyanex 301 allows for the selective transfer of nickel and cobalt from the sulfate leach solution into an upgraded hydrochloric acid leach solution, from which nickel and cobalt are separated and refined to final products (U.S. Pat. No. 5,378,262). 
     Bis(2,4,4-trimethylpentyl)dithiophosphinic acid (referred hereto as dithiophosphinic Acid or DTPA) is the active ingredient of the Cyanex 301. Being an organic thiol ([C 8 H 17 ] 2 P(S)SH), DTPA can undergo oxidation. A common oxidation path generates disulfide: 
     
       
                 
         
             
             
         
      
     
     The disulfide can be regenerated back to DTPA and Vale has developed regeneration processes to achieve that (U.S. Pat. No. 5,759,512 and U.S. Pat. No. 6,022,991). These regeneration processes support the commercial viability of using Cyanex 301 for metal extraction. 
     The oxidation of DTPA can result in the formation, over time, of oxidation products other than disulfide. These oxidation products can be generally described as organophosphorus compounds that contain sulfur and/or oxygen. These compounds do not have the extractive and metal selectivity properties of DTPA and cannot be readily reduced back to DTPA. Hence, as Cyanex 301 (DTPA) is added in order to maintain the metallurgy-required DTPA concentration in the organic solution, these compounds will accumulate over time. 
     Their presence in the organic solution can be followed by suitable methods of analysis, including phosphorus nuclear magnetic resonance ( 31 P-NMR), chemical analysis for phosphorus and sulfur as well as acid titration determination for acids such as DTPA. 
     The accumulation of these compounds in the SX organic solution can impact the desirable metallurgy of the Cyanex 301 based SX system by impacting the physical properties of the organic solution, such as viscosity, interfacial tension and the like and/or by affecting the ability of the extractant system to maintain the desired degree of metal extraction selectivity. 
     Thus, removal for some of the SX organic solution (organic bleed) from the operating organic circuit becomes necessary. The obvious disadvantage of this approach is the loss of DTPA with the organic bleed. Furthermore, disposal of the organic bleed itself will add further costs. 
     Therefore, a process that would enable the recovery of DTPA from the organic bleed and its separation from the accumulating oxidation products would be metallurgically and operationally advantageous as well as economically desirable. 
     Given that the DTPA content of Cyanex 301 is about 75-80% [Cyanex® 301 Extractant—Technical brochure, Cytec] and that it also contains other organothiophosphorus compounds, a purification technique was developed by Zhu et al [Zhu, Y., Chen, J., Jiao R., 1996, Extraction of Am(III) and Eu(III) from Nitrate Solution with Purified Cyanex 301 , Solvent Extraction and Ion Exchange,  14(1), 61-68] aimed at isolating high purity DTPA, primarily for the purpose of fundamental metal extraction chemistry research. 
     In the Zhu purification technique, Cyanex 301 is contacted with ammonium carbonate solution at 70° C. for one hour to produce the ammonium salt of DTPA (ammonium dithiophosphinate) and then the mixture is left overnight at 0° C. to allow for the crystallization of the ammonium salt; the crystalized solids are eventually converted back to DTPA by contact with 4 N hydrochloric acid. This purification technique has been subsequently used by other researchers, e.g., [Groenewold, G. S, Peterman, P. R., Klaehn, J. R., Delmau, L. H., Marc P., Custelcean, R., 2012,  Oxidative degradation of bis (2,4,4- trimethylpentyl ) dithiophosphinic acid in nitric acid studied by electrospray ionization mass spectrometry , Rapid Commun. Mass Spectrom., 26, 2195-2203.] for similar purposes. 
     Applying this technique on an industrial scale will be very difficult and costly for the following reasons:
         use of elevated temperatures and prolonged time for the contact with the ammonium carbonate solution, which could result in further oxidation of DTPA;   need to cool the mixture to 0° C. for a long period of time in order to produce the ammonium dithiophosphinate solids; and   need to carry out filtration of the ammonium dithiophosphinate solids from the organic solution, while likely having to maintain temperatures near 0° C.       

     Therefore, there is a need for a method that will be able to effectively separate DTPA from other organophosphorus impurities while avoiding having to operate at extreme temperatures or producing, and having to deal with, organic solids. 
     It has now been discovered that DTPA can be effectively and efficiently recovered from DTPA-containing organic solutions and various other organophosphorus compounds, as described earlier, in which solutions DTPA is present in its acid form and/or in its base metals-salt form, where the group of base metals include nickel, cobalt, zinc, copper, chromium and the like, without the formation of solids or the necessity of maintaining low temperatures over long period of time. 
     The method avoids the need for elevated temperatures (above 70° C.), the need for operating at very low (near 0° C.) temperatures, the prolonged reaction times and the generation and handling of organic solids. 
     This is accomplished by contacting the DTPA-containing organic solution with a suitable aqueous base solution supplied in excess of the stoichiometric requirement for the acid-base neutralization or other metal cation-exchange reactions for the contained DTPA. This prevents the formation of solids that would otherwise form from the treated DTPA-containing solution. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a typical  31 P-NMR spectrum of field organic solution. 
         FIG. 2  shows a typical  31 P-NMR spectrum of DTPA-recovered product organic solution. 
         FIG. 3  shows a typical  31 P-NMR spectrum of light (DTPA-lean) organic solution. 
     
    
    
     SUMMARY OF THE INVENTION 
     The present invention refers to a method for recovering bis(2,4,4-trimethylpentyl)dithiophosphinic acid (DTPA) from organic solution containing DTPA and various other organophosphorus compounds that have been generally produced from the oxidation of DTPA and are different from disulfide and may also contain base metals, such as nickel, cobalt, zinc, copper and chromium, chemically bonded to DTPA, comprising the following steps: (a) contacting the DTPA-containing solution with a suitable aqueous base solution supplied in excess of the stoichiometric requirement for the acid-base neutralization or other metal cation-exchange reactions for the contained DTPA to produce a DTPA-rich and a DTPA-lean organic phases; (b) separation the two organic phases; (c) contacting the DTPA-rich organic phase with suitable acidic solution to produce a base-free DTPA-rich organic solution; and (d) contacting the DTPA-lean organic phase with suitable acidic solution to produce a base-free DTPA-lean organic solution. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention overcomes the drawbacks of the prior art by providing a method to recover DTPA from DTPA-containing organic solution and various other organophosphorus compounds that have been generally produced from the oxidation of DTPA and are different from disulfide and may also contain base metals, such as nickel, cobalt, zinc, copper and chromium, chemically bonded to DTPA. 
     The method comprises the following steps: (a) contact of the DTPA-containing solution with a suitable aqueous base solution supplied in excess of the stoichiometric requirement for the acid-base neutralization or other metal cation-exchange reactions for the contained DTPA to produce a DTPA-rich and a DTPA-lean organic phases; (b) separation of the two organic phases; (c) contact of the DTPA-rich organic phase with suitable acidic solution to produce a base-free DTPA-rich organic solution; and (d) contact of the DTPA-lean organic phase with suitable acidic solution to produce a base-free DTPA-lean organic solution. 
     The treatment of step (a) is carried out at moderate temperatures within the range between 25 and 70° C., more preferably at temperatures between 35 and 60° C. and most preferably at temperatures between 45 and 55° C. 
     Suitable aqueous base solutions for step (a) include aqueous solutions of alkali-metals hydroxides, such as sodium hydroxide and potassium hydroxide, ammonium hydroxide and the like. More advantageously, the base is selected from the group of sodium hydroxide and ammonium hydroxide. Most advantageously, the base is ammonium hydroxide solution. 
     The base solutions can be used over a range of concentrations, preferably greater than 1 mol/L. More advantageously, the base solutions with concentration greater than 5 mol/L are used. Most advantageously, a 50 wt % sodium hydroxide or a 28 wt % ammonia solution is used. 
     In the present invention, the organic solution of Cyanex 301, dissolved in suitable hydrocarbon diluent, which contains DTPA together with various other organophosphorus compounds that have been generally produced from the oxidation of DTPA (further referred to as ‘field organic’), and may also contain base metals, such as nickel, cobalt, zinc, copper and chromium, chemically bonded to DTPA, is contacted with suitable aqueous base such solution as alkali-metal hydroxide or ammonium hydroxide, added in excess of the stoichiometric equivalent for the amount of DTPA present in the organic solution in an acid- and or metal-bonded form. 
     This contact, carried out in a suitable mixing vessel and at moderate temperatures, produces a solids-free mixture of at least two distinct solutions (DTPA-rich organic phase and DTPA-lean organic phase) that can be readily separated (step b). 
     The separation in step (b) can be carried out by a variety of means, based on the different specific densities of the distinct solutions formed; these means include separation by gravity, separation by centrifuging and the like. 
     The heavier solution is the DTPA-rich organic phase which contains DTPA and its respective base salt, such as sodium, potassium or ammonium dithiophosphinate. 
     The lighter solution is the DTPA-lean organic phase which comprises the hydrocarbon diluent containing the majority of the organophosphorus compounds that have been generally produced from the oxidation of DTPA. 
     Depending on the particular conditions, such as the concentration of base solution and/or its addition dosage, a third phase, comprising an aqueous base solution, may be formed as well. 
     Following the separation, the recovered DTPA-rich organic solution obtained in step (c) is suitable for re-use for metal extraction operation. 
     Depending on the particular operating circumstances, it may be advantageous, to contact the DTPA-rich organic solution obtained in step (c) with a suitable acid aqueous solution in order to convert the base salt form of DTPA, such as sodium, potassium or ammonium dithiophosphinate, into the acid form of DTPA and/or dilute the DTPA-rich organic solution obtained in step (c) to lower the DTPA concentration by using, for example, DTPA-containing organic solution from the solvent extraction operation or organic diluent used when preparing that organic solution. In this way the properties of the DTPA-rich solution, such as its specific gravity and viscosity, in addition to the DTPA concentration, can be suitably brought closer to those of the organic solution present in the solvent extraction circuit operation such that to avoid wide operational swings in the properties of the solvent extraction organic solution. 
     A variety of suitable acid aqueous solutions can be used to convert the base salt form of DTPA into the acid form of DTPA. Advantageously, the acid aqueous solutions contain at least one mineral acid. More advantageously, the mineral acid is selected from the group of hydrochloric and sulfuric acid. Most advantageously the mineral acid is sulfuric acid. 
     The DTPA-lean organic phase comprising the hydrocarbon diluent and the majority of the organophosphorus compounds that have been generally produced from the oxidation of DTPA, can be further treated by a variety of known means, such as solvent distillation, steam stripping and the like, to separate and recover the majority of the hydrocarbon diluent from the higher boiling point organophosphorus compounds. 
     This allows for the majority of the hydrocarbon diluent to be recovered and returned into the solvent extraction operation while greatly reducing the volume of the remaining organic solution, now containing in much more concentrated form the organophosphorus compounds that have been generally produced from the oxidation of DTPA, for disposal. 
     The present invention is illustrated by the following examples: 
     Example 1 
     A 200 mL sample of nickel-free field organic Cyanex 301 solution, having a specific gravity of 0.81 and analyzing 0.211 mol/kg (0.171 mol/L) DTPA, 19.0 g/kg P, 33.1 g/kg S, is mixed with 12 grams of 50 wt % NaOH solution (6 g of NaOH) at 65° C. for 20 minutes. A heavier organic phase is formed and then separated by gravity. 
     Analysis of this organic phase shows that it contained 1.31 mol/kg DTPA, 38.5 g/kg P and 90.3 g/kg S. 
     Considering that each mole of DTPA represents one mole of P and two moles of S, it is readily evident that the DTPA (0.211 mol/kg) in the field organic accounts only for 6.54 g/kg P and 13.5 g/kg S, representing about 34% of the total P (19.0 g/kg) and 41% of the total S (33.1 g/kg) content of the field organic. In contrast, in the recovered organic phase, the 1.31 mol/kg DTPA represents 105% of the total P (40.6 g/kg P equivalent to DTPA) and 93% of the total S (83.8 g/kg S equivalent to DTPA). 
     Example 2 
     A 200 mL sample of the same nickel-free field organic Cyanex 301 solution, as described in Example 1, is mixed with 24.3 grams of 28 wt % ammonia solution (6.8 g of NH 3 ) and under the same conditions. Formation of solids was observed and the recovered DTPA-rich organic phase contained 0.86 mol/kg DTPA, 35.3 g/kg P and 67.4 g/kg S. From the analysis of this phase, it is evident that the DTPA separation from other organophosphorus species was inferior to what was achieved in Example 1: the 0.86 mol/kg DTPA accounts for 76% of the total P (26.7 g/kg P equivalent to DTPA) and 82% of the total S (55.0 g/kg S equivalent to DTPA). 
     The ammonia dosage (6.8 g of NH 3  equivalent to 0.4 moles of NH 3 ) compared to the 0.034 moles of DTPA in the sample (200 mL of 0.171 mol/L DTPA) represents  ˜ 12 times the stoichiometric requirement for acid-base neutralization. 
     Example 3 
     The test described in Example 2 was repeated under otherwise the same conditions but with a higher dosage of the base solution, 40.7 grams of the same 28 wt % ammonia solution (11.4 g of NH 3 ), only the heavier, DTPA-rich organic phase was formed. There was no formation of solids observed. The recovered DTPA-rich organic phase contained 1.66 mol/kg DTPA, 40.7 g/kg P and 79.9 g/kg S, indicating the essentially complete removal of the non-DTPA organophosphorus compounds. 
     The ammonia dosage (11.4 g of NH 3  equivalent to 0.67 moles of NH 3 ) compared to the 0.034 moles of DTPA in the sample (200 mL of 0.171 mol/L DTPA) represents  ˜ 20 times the stoichiometric requirement for acid-base neutralization. 
     This example illustrates the importance of base dosage for preventing solids formation as well as achieving the high degree of DTPA separation. 
     Example 4 
     The test described in Example 1 was repeated under otherwise the same conditions but using a field organic solution containing chemically extracted nickel at several gram per liter concentration. 
     Even with a higher dosage of 50 wt % NaOH solution, a precipitate of nickel hydroxide intermixed with an organic phase was produced. Effective physical separation of such mixture of fine solid precipitate and organic solution will be challenging. 
     This example illustrates that while sodium hydroxide is effective for DTPA recovery, its application would be restricted to nickel-free DTPA containing organic solution. 
     Example 5 
     One liter samples of field organic solution, containing different amounts of nickel, were each contacted for 5 minutes with 220 mL of 28 wt % ammonia solution, at 55° C. 
     In every case, three distinct phases were formed and allowed to separate. There was no formation of nickel precipitate solids; it is known from prior art that nickel remains soluble in ammoniacal solutions due to the formation of aqueous soluble nickel amine complexes. 
     The lighter organic phase was collected and then contacted with 6 N HCl solution to remove residual ammonia. Similarly, the heavier DTPA-rich organic phase was separated and then also contacted with 6 N HCl solution to remove residual ammonia. The residual third phase, representing unreacted ammonia aqueous solution was removed. 
     The organic phases from all three tests were sampled for DTPA and chemical analysis as well as  31 P-NMR analysis. The results are summarized in the table below. 
     The  31 P-NMR analysis confirms the high degree of separation for DTPA from the other organophosphorus species, indicated through the DTPA acid titration and organic chemical analyses. The overall DTPA recovery to the heavier organic phase is in the 90% range. 
     
       
         
               
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                 Field 
                 Field 
                 Field 
               
               
                   
                 Organic I 
                 Organic II 
                 Organic III 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 DTPA in the field sample (mol/L) 
                 0.174 
                 0.197 
                 0.217 
               
             
          
           
               
                 Nickel in the field sample (g/L) 
                 2.6 
                 ~5.7-6.0 
               
             
          
           
               
                 DTPA in the recovered product  
                 1.646 
                 1.584 
                 1.554 
               
               
                 organic (mol/L) 
                   
                   
                   
               
               
                 % of P with DTPA as % of total  
                 32% 
                 34% 
                 43% 
               
               
                 P in field sample 
                   
                   
                   
               
               
                 % of P with DTPA as % of total  
                 83% 
                 86% 
                 85% 
               
               
                 P in product sample 
                   
                   
                   
               
               
                 DTPA in field sample as % of all 
                 27 
                 32 
                 39 
               
               
                 organophosphorus compounds (from  
                   
                   
                   
               
               
                   31 P-NMR) 
                   
                   
                   
               
               
                 DTPA in the light organic sample  
                 3.4 
                 1.5 
                 1.3 
               
               
                 as % of all organophosphorus 
                   
                   
                   
               
               
                 compounds 
                   
                   
                   
               
               
                 (from  31 P-NMR) 
                   
                   
                   
               
               
                 DTPA in heavy organic  
                 72 
                 76 
                 75 
               
               
                 (product) sample as % 
                   
                   
                   
               
               
                 of all organophosphorus 
                   
                   
                   
               
               
                 compounds (from  31 P-NMR) 
               
               
                   
               
             
          
         
       
     
     This example illustrates the ability of the method of invention to treat also nickel-containing Cyanex 301 organic solutions. 
     The extent of DTPA separation is also illustrated by the figures, showing typical  31 P-NMR spectra for field organic, recovered DTPA organic and of the light organic phase. The peak at  ˜ 63 ppm represents DTPA, the peaks in the 76-80 ppm range represent recoverable disulfide compounds.