Extraction and stripping of rare earths using mixtures of acidic phosphorus-based reagents

Studies were carried out on the extraction characters of trivalent rare earths from chloride solutions using organophosphorus acids 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP) combined with [di-(2-ethylhexyl)-phosphoric acid (HDEHP),isopropylphosphonic acid 1-hexyl-4-ethyloctyl ester (HHEOIPP),bis(2,4,4-trimethylpentyl)-phosphinic acid (Cyanex 272),bis(2,4,4-trimethypentyl)-monothiophosphinic acid (Cyanex 302) or bis(2,4,4-trimethypentyl)-dithiophosphinic acid (Cyanex 301)] as extractants. The effect of the equilibrium aqueous acidity on the extraction was studied. According to the corresponding separation factors for adjacent pairs of rare earths,it could be concluded that HEHEHP and Cyanex 272 could be employed for the separation of Tm(Ⅲ),Yb(Ⅲ),Lu(Ⅲ) from the other rare earths. Taking Yb(Ⅲ) as an example,based on the different stripping acid,the potential of the stripping was estimated.     

Solvent extraction characteristics of thiosubstituted organophosphinic acid extractants

Organophosphorus reagents are well known in solvent extraction. Commercial operations for the separation of cobalt from nickel have been successfully carried out using organophosphoric, -phosphonic, and -phosphinic acid extractants. Two new reagents in this class are the mono and dithio analogs of the commercial dialkylphosphinic acid, Cyanex 272. The replacement of oxygen by sulfur in these reagents enables extraction to be carried out at much lower pH.Characterization of Cyanex 272, Cyanex 302 (bis-(2,4,4-trimethylpentyl)-thiophosphinic acid), and Cyanex 301 (bis-(2,4,4-trimethylpentyl)-dithiophosphinic acid) has been undertaken. A comparison of the solvent extraction behavior of metallurgically important first-row transition metal ions from acidic sulfate solution by these reagents is reported. Distribution coefficients shift to lower pH with increasing sulfur substitution and decreasing pK_a of the extractant, the greatest effect being observed for nickel. Stoichiometry of the extraction reactions, and the nature of the metal complexes formed have been determined using slope analysis techniques and spectroscopic measurements.     

Extraction of chromium (III) from spent tanning baths

Extraction of chromium(III) from a model spent tanning bath of the leather industry has been investigated using ammoniated and non-ammoniated di(2-ethylhexyl)phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex® 272). Chromium extraction of 95% by 15% (v/v) D2EHPA, 10% (v/v) isodecanol in kerosene and 86.1% by 15% (v/v) Cyanex® 272, 10% (v/v) p-nonylphenol in kerosene was obtained at equilibrium pH values of 4.0 and 5.0, respectively. The separation of small amounts of iron (III) and aluminium (III) present in the solution along with the Cr(III), was also examined and it was found that Cyanex® 272 was a better reagent than D2EHPA. The slow kinetics of extraction and stripping observed in the case of AI(III) was advantageous for its separation from Fe(III) at low pH values. Difficulties faced in the stripping of loaded metals were also studied because only about 80% chromium recovery by 8 M HCl was obtained from both solvents. The incomplete stripping of the metal may be a result of the formation of a stable species in the organic phase and needs further investigation. The Cr(III) can be recovered as chloride from the strip liquor and recycled for retanning purposes.     

Studies on synergistic solvent extraction of rare earth elements from nitrate medium by mixtures of 8-hydroxyquinoline with Cyanex 301 or Cyanex 302

Synergistic solvent extraction of rare earth elements (REEs) from nitrate medium was investigated with mixtures of 8-hydroxyquinoline (HQ) and acidic organophosphorus extractants,bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) and bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex 302).The extraction behavior of Cyanex 301/Cyanex 302 and their binary mixtures with HQ towards several lanthanoids (La,Nd,Sm,Tb,Ho,Tm) and yttrium (Y) was investigated.The separation ability of REEs was studied according to the various extraction effects.The extraction mechanisms for yttrium were studied with the methods of slope analysis and constant moles in the synergistic systems.The extracted compounds,the equilibrium constants,and thermodynamic functions were also determined.     

Mass transfer kinetics of yttrium(III) using a constant interfacial cell with laminar flow. Part II extraction with Cyanex 272

The yttrium(III) extraction kinetics and mechanism with bis-(2,4,4-trimethyl-pentyl) phosphinic acid (Cyanex 272, HA) dissolved in heptane have been investigated by constant interfacial cell with laminar flow. The data has been analyzed in terms of pseudo-first order constants. Studies on the effects of stirring rate, temperature, acidity in aqueous phase, and extractant concentration on the extraction rate show that the extraction regime is dependent on the extraction conditions. The plot of interfacial area on the rate has shown a linear relationship. This fact together with the strong surface activity of Cyanex 272 at heptane–water interfaces has made the interface the most probable location for the chemical reactions. The forward, reverse rate equations and extraction rate constant for the yttrium extraction with Cyanex 272 have been obtained under the experimental conditions. The rate-determining step has been also predicted from interfacial reaction models. The predictions have been found to be in good agreement with the rate equations obtained from experimental data, confirming the basic assumption that the chemical reaction is located at the liquid–liquid interface.     

Extraction technology of purification of sulfate nickel solutions of impurities with the use of dialkylphosphine acid

The data of laboratory investigations of the removal of impurities (Fe, Cu, Zn, Mg, Ca) from sulfate nickel solutions are presented. It is shown that Cyanex 272 [di(2,4,4-dimethylpenthyl)phosphinic acid] is the most efficient extractant. For two extraction steps by a nickel form of Cyanex 272 from a solution (after iron purification) that contained: Ni 120–130, Cu 0.8–1.0, Zn 3.0–4.0, Mg 1.5–2.0, Ca 0.4–0.8, and Fe 0.01–0.02 g/l, raffinates of the following composition are obtained: Ni 120–130, Cu, Zn, and Fe ≤ 2 × 10^?3 each, Mg ≤ 0.1, and Ca 0.2–0.3 g/l. Purification of raffinates of organic impurities (extractant, diluent) after extraction is performed on the DAK-activated charcoal. Reextraction of metals and, correspondingly, extractant regeneration, is easily performed by solutions of mineral acids (H₂SO₄ and HNO₃). Extraction of nickel from the solution at the extraction step is ≥99.8%.     

Hydrometallurgical methods of recovery of scandium from the wastes of various technologies

The recovery of scandium from the wastes of the production of uranium, titanium, iron–vanadium, and alumina is studied. The applied acid schemes of scandium transfer to a solution followed by ion-exchange recovery and extraction concentration of scandium ensure the precipitation of crude scandium oxides containing up to 5% Sc₂O₃. Scandium oxides of 99.96–99.99% purity are formed after additional refining of these crude oxides according to an extraction technology using a mixture 15% multiradical phosphine oxide or Cyanex-925 + 15% tributyl phosphate in kerosene.     

Extraction Mechanism of La^3 ＋ from Hydrochloric Acid Solution Using Cyanex 302

The solvent extraction of La3 from hydrochloric acid solutions was investigated using bis (2, 4, 4-trimethylpentyl) monothiophosphinic acid (Cyanex 302, HL) as an extractant. The effect of .equilibrium of aqueous acidity on extraction of La^3 using Cyanex 302 in different diluents was discussed. The effects of extractant concentration and chloride ion on the extraction reaction were also studied. Stoichiometry of the extraction reactions and the nature of metal complexes formed were determined using slope analysis technique and IR measurement.     

Extraction equilibria of ammonia with acidic organophosphorus compounds

Extraction equilibrium of ammonia from aqueous ammonium nitrate-ammonia mixture by various acidic organophosphorus extractants was investigated at 30°C in terms of the distribution of ammonia between two phases, water content in the organic phase and viscosity of the organic phase. The acidic organophosphorus extractants used were di(2-ethylhexyl)phosphoric acid, 2-ethylhexyl 2-ethylhexylphosphonic acid and di(2,4,4′-trimethylpentyl)phosphinic acid; toluene was used as a diluent.It was found that ammonia is extracted into the organic phase not only through the chemical interaction with the extractants but also through its physical partition into the diluent itself.The distribution through the chemical interaction with the extractants was quantitatively interpreted by the formation of the ion-pair complex, NH₄R(HR)₄, in the low concentration region of ammonia in the organic phase. In the higher concentration region, it was qualitatively interpreted by the transformation of this complex into other complexes of the types NH₄R(HR) or NH₄R. They were inferred to exist as micelles from the experimental results that the water content and viscosity largely increase in the high pH region.     

Stripping of copper from CYANEX® 301 extract with thiourea–hydrazine–sodium hydroxide solution

The stripping of copper from the organic extract of bis(2,4,4-trimethylpentyl) phosphinodithioic acid, CYANEX® 301, using an aqueous mixture of thiourea, hydrazine and sodium hydroxide has been investigated. The optimal concentrations of the aqueous solution were found to be 1.0 M, 5 × 10^− 2 M and 5.0 M, respectively which led to 95% stripping of copper from CYANEX 301 with concomitant regeneration of the extractant. The characterization of the stripped copper product was done using a combination of microanalyses, cyclic voltammetry and X-ray diffractometry (XRD). The product was made up of two non-stoichiometric copper sulfides, Cu_xS with x = 1.60 and 1.77. Cu_1.60S was the major product. Hydrazine appears to play the role of reducing the disulfide species of CYANEX 301, R₂P(S)S–S(S)PR₂, formed during the extraction step, back into CYANEX 301, whereas thiourea provides a source of sulfur in the formation of the stripping products.     

A novel extractant bis(2-ethylhexyl) ((2-ethylhexylamino)methyl) phosphine oxide for cerium(IV) extraction and separation from sulfate medium

By introducing the amine group into phosphorus extractant, a novel aminophosphine compound bis(2-ethylhexyl) ((2-ethylhexylamino)methyl) phosphine oxide (DEHAPO, abbreviated as A) was synthesized for the extraction of cerium (IV) (Ce(IV)) from sulfate medium (H₂SO₄). The influence factors including extractant concentration, H₂SO₄ concentration and temperature on the Ce(IV) extraction were investigated and discussed. It is found that the extraction ability of Ce(IV), thorium (IV) (Th(IV)) and rare earths (REs(III)) (La, Gd, Yb) decreases in sulphate medium in the following order: Ce(IV) > Th(IV) > REs(III). The extraction process is an exothermic reaction and the thermodynamic parameters were calculated. The extracted complex of Ce(HSO₄)₂SO₄·A in loaded organic solution was identified by the slope methods and further proved by FT-IR spectral analysis. Stripping studies indicate that Ce(IV) can be effectively stripped from the organic phase. The results of separation factors (β) and saturation loading capacity demonstrate that DEHAPO could be used to selectively extract Ce(IV) from sulphate medium with high separation efficiency and good extraction ability.     

Extraction of molybdenum from weak acid rhenium-containing solutions

The possibility of extracting molybdenum from weak acid (pH 2 to 3) rhenium-containing solutions with the non-specific extractant Aliquat 336 is studied. A mechanism for the temperature influence on extraction and in particular for the extraction of molybdenum and rhenium is proposed. It is shown that the selectivity coefficient (β = D_Mo/D_Re) depends on the concentrations of both elements and also on the NO₃^? concentration in solution. When the concentration of molybdenum and nitrate ions increases, β increases. Increasing temperature has the same effect. A modifier is essential. At low rhenium concentrations it is better to use high molecular weight alcohols (as modifier), and at high concentrations it is better to use acetophenone. The authors believe that the investigations can be generalized and that analogous results can be expected if trialkylamine (for example, trioctylamine) is used.     

Study on Extracting Rare Earth from Sulfate System by Long-Chain Fatty Acid

The extraction of rare earths by long-chain fatty acid in kerosene from sulphate system was described.It was demonstrated from the experimental results that the ratio of kerosene: fatty acid: isooctanol = 55 : 30: 15 ( V/V),By the saturation capability method and the slope method, the extracted reaction mechanism of the extraction of rare earth was studied.It is shown that the extraction reaction conform to the cation exchange reaction mechanism.The extracted sequence of rare earth was determined in this system and it is shown that there is no tetrad effect and the position of yttrium is between lanthanum and cerium.     

Colorimetric estimation and some physicochemical properties of purified Cyanex 272

A technique for the colorimetric estimation of purified Cyanex 272 has been developed. The technique consists of the digestion of pure sample or its aqueous solution with concentrated nitric acid (70%)–perchloric acid (70%) mixture for 1 h. The oxidizing mixed acid quantitatively converts Cyanex 272 to a clear solution of orthophosphate that can be easily estimated by the molybdenum-blue colorimetric method at 830 nm. The method is sensitive with a molar extinction coefficient of ∼2.6×10⁴ and reproducible within ±2%. Applying this technique of analysis, the dimerization constant (K₂), distribution or partition coefficient (K_d) and ionization constant (K_a) of the purified Cyanex 272 (bis-2,4,4-trimethylpentylphosphinic acid, BTMPPA) have been estimated to be 190, 53 and 5.52×10⁻⁴, respectively.     

Liquid extraction of rhenium(VII) and molybdenum(VI) with trialkylphosphine oxide from acidic solutions

The liquid extraction of rhenium(VII) and molybdenum(VI) ions from sulfuric, hydrochloric, and nitric acid media is studied in the temperature range from 20 to 40°C using trialkylphosphine oxide in kerosene as an extracting agent. The maximum separation of these metals is attained when they are extracted from solutions of 1.0–2.0 M H₂SO₄ (the duration of intense phase mixing was 3–5 min). The enthalpy of the studied process is estimated to be ΔH = ?32.32 kJ/mol for molybdenum and ?51.52 kJ/mol for rhenium. The chemical aspects of the extraction process studied are discussed.     

Extraction of chromium(III) from sodium chloride solutions by means of carboxylic acids

Studies of Cr(III) extraction with carboxylic acids showed that the extraction process takes place at a pH of the aqueous phase ranging from 4 to 5. It was shown that sodium chloride is active with respect to chromium(III) as a salting-out agent. For extraction of Cr(III) with hexanoic acid it was shown that in the organic phase trinuclear complexes of the [Cr(OH)R₂·HR]₃ formula are formed.     

Extraction and separation of yttrium from other rare earths in chloride medium by phosphorylcarboxylic acids

Two phosphorylcarboxylic acids, 3-((bis(2-ethylhexyloxy))phosphoryl)propanoic acid (PPA) and 3-((bis(2-ethylhexyloxy))phosphoryl)-3-phenylpropanoic acid (PPPA), were synthesized for separating yttrium from other rare earths in the chloride feed of ion-adsorption type rare earth concentrate. The effect of the factors such as pH_1/2, temperature, saponification degree and phase modifiers was investigated. The separation efficiencies of PPA and PPPA are obviously better than the typical extractants such as sec-octylphenoxy acetic acid (CA-12) and naphthenic acid (NA). The extraction process of rare earths by PPA and PPPA is a cation exchanging reaction, which is similar to those of CA-12 and NA. The loaded rare earths in both PPA and PPPA systems can be effectively back-extracted by 0.5 mol/L HCl or higher concentration. A cascade extraction process for separating yttrium from other rare earths was developed using PPPA as the extractant. The yttrium product with the purity of 97.20 wt% was obtained by 35 stages of extraction and 12 stages of scrubbing.     

Solvent extraction and separation of light rare earth elements (La,Pr and Nd) in the presence of lactic acid as a complexing agent by Cyanex 272 in kerosene and the effect of citric acid,acetic acid and Titriplex III as auxiliary agents

At present, the use of rare earth elements (REEs) has become an inevitable necessity in many modern industries. In general, liquid extraction is the best commercial method for extracting REEs due to its ability to control high volumes of liquids with electrical load. With the aim of improving a separation technology that would be superior to the existing extraction systems, the extraction behaviors of La(III), Pr(III), and Nd(III) from an HCl medium with Cyanex 272 in the presence of the complexing agent lactic acid (HLac) and auxiliary agents citric acid (H₃Cit), acetic acid (HAc), and Titriplex III have been reported. The effect of pH and lactic acid concentration has been examined. The use of lactic acid as a complexing agent leading to a high extraction of REEs with Cyanex 272 at pH = 5 was compared with systems without lactic acid. The results show that the use of acetic acid along with lactic acid leads to an increase in the extraction percentage of LREEs. While use of citric acid and Titriplex III reduces the extraction percentage of LREEs. Finally, the presence of Titriplex III together with lactic acid could lead to an increase in the separation factor of Pr and Nd.     

Separation of molybdenum from tungsten by di-2-ethylhexyl phosphoric acid extractant

The efficiency of extracting molybdenum(VI) from a weakly acidic solution containing a large amount of tungsten and a small amount of molybdenum using di-2-ethylhexyl phosphoric acid (D2EHPA) is low. However, when ethylenediaminetetraacetic acid (EDTA) complexing agent is present, the efficiency of molybdenum extraction is significantly increased. In this paper the effect of various factors (including the D2EHPA and EDTA concentration, pH of the feed solution, contact time, phase ratio) on molybdenum extraction has been studied. The effect of the composition of the strip solution on stripping molybdenum from the organic phase is reported. Counter-current extraction has also been investigated. The results of multiple-stage extraction show that molybdenum can be satisfactorily separated from sodium tungstate solution obtained by leaching tungsten ore, with little loss of tungsten. A possible mechanism of the extraction of molybdenum in the presence of EDTA is discussed. The reaction for extracting molybdenum is: