Supercritical fluid extraction of cobalt

Cosolvated supercritical fluid carbon dioxide has been investigated for its potential for the kinetically controlled extraction of coordinated cobalt ion s with 2-ethyl hexy l 2-ethyl hexyl phosphonic acid as the -complexing agent. Hexamine has been used as the coordinating agent and heptane as th e cosolvent. A mixture of cobaltous chloride and hexamine with a small quantity of water was made to react dynamically with the complexing agent in heptane in the medium of supercritical fluid carbon dioxide (SCF CO2) . Extraction efficiencies have been found as a function of time and the optimum pressure­ temperature regime established. The study has implications for the end purification of cobalt from nickel and may serve as a role model for the refinin g and purification of one rare earth element from it s other rare earth impurities. Traditional solvent extraction methods for the separation of metals usually result in large overall volumes of solvents and wastes that are toxic and difficult to dispose off. Supercritical fluid extraction (SFE) with CO2, is attractive as a separation technique because it avoids the use of toxic organic solvents. Even where cosolvated supercritical fluid, CO2 [SCF CO2], is used, the amounts of toxic solvents used are significantly lesser than the amounts used in solvent extraction. In the case of SFE the required component is obtained as a solute in either SCF CO2 or in cosolvated SCF CO2. The gas on depresssurisation escapes leaving the solute in concentrated form. Carbon dioxide is non-toxic, inexpensive, non­ flammable, non-corrosive and readily available and has a critical pressure of 7.89 MPa and a critical temperature of 31°C. Another difference with solvent extraction systems is that the formation of the metal complex may be mass transfer related. SCFs have relatively low viscosities and high diffusivities, which might enhance complex formation increasing extraction efficiencies. Also, solvent properties of a SCF can be changed greatly by small temperature and pressure changes in the region near critical point which can be used to enhance selective extraction. The research to date on metal chelates in SCF CO2 has focussed primarily on the use of two broad families of chelating agents, viz., l3-diketones' ·2 and dithiocarbamates 3 . There have also been some investigations of phosphates 4 ,5 and crown ethers 6 . In the present case the reagent used is purified 2-ethyl hexyl 2-ethyl hexyl phosphonic acid [PC-88A] manufactured by Daihachi Chemical Industry Co. Ltd. This reagent is insoluble in water, but has a high solubility in heptane and kerosene 7 • This reagent has been used to separate cobalt and nickel from mixtures by solvent extraction with pH as a determining factor. The final refining of cobalt requires a large number of steps. The purpose of the present work is to explore the possibilities of refining cobalt using SFE with cosolvated carbon dioxide using the present reagent, thereby reducing the number of steps and the amount of toxic solvents. Ion-pairing has been used as a strategy for extraction of radioactive metal ions using SCF C02 8 . In the present study, it is proposed to use coordination as a strategy for the SFE of cobalt in order to kinetically control the extraction efficiencies and to attain selectivities in separation. With these aims in view an investigation of the basic parameters for the controlled extraction of cobalt using cosolvated SCF CO2 has been undertaken.