Extractant-coated magnetic particles for cobalt and nickel recovery from acidic solution

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Abstract

Waste minimization and recycling practices can often constitute a significant fraction of industrial operating costs. Magnetically assisted chemical separation (MACS) is a simple, cost-effective process that utilizes micrometer-sized magnetic composite materials containing a sorbed layer of chelating or ion exchange material. This paper presents the use of MACS particles for recovering cobalt and nickel from acidic solution.

Introduction

Hazardous wastes are generated by industries and can often represent a large percentage of the plant operating costs. With environmental laws regulating disposal expected to become more stringent, industries will investigate more closely waste minimization options. However, many industries are deterred from practicing waste minimization and recycling routinely because of the undeniable shortcomings of current technologies. Companies are often reluctant to adopt new technologies because the new technologies are uneconomical compared to direct disposal and often require grace periods to alleviate process uncertainties that accompany immature technologies [1]. Current technologies such as precipitation, ion exchange, and solvent extraction suffer from major drawbacks. These range from large secondary wastes, significant chemical additives, solvent losses, complex equipment, and bulky design. These shortcomings translate to an inability to effectively remove low concentrations of hazardous metals at acceptable costs.

The magnetically assisted chemical separation (MACS) process was developed at Argonne National Laboratory as a compact, simple process for selectively separating transuranics and fission product radionuclides from radioactive liquid waste streams [2]. It employs small ferromagnetic composite particles coated with a selective solvent extractant or ion exchange material. The coated particles are mixed with the waste solution in situ or in a reactor vessel. The hazardous metals are selectively extracted onto the particle surface because of the chelating or exchange properties of the particle coating. The particles are removed by magnetic filtration or simply recovered with a magnet. The clean water can be reused by the facility or sent to sewerage. The target metals can be concentrated into a small volume by stripping the metals off the surface using an appropriate stripping agent. The concentrated product can be disposed properly, reused by the facility, or sent for resale and the MACS particles can be reused. There is an on-going effort to evaluate the MACS process for the recovery of hazardous metals from industrial wastes for waste minimization and recycling efforts.

A variety of magnetic particles (uncoated) from commercial vendors were evaluated for stability toward acid hydrolysis and radiolysis [3]. Electron microscopy was employed to evaluate the surface characteristics and internal structure of the particles. Optical microscopy was employed to distinguish macroscopic surface characteristics. In the uncoated form, the particles are dull-black spherical conglomerates of millimeter size. When suspended in solution, the mixing action is sufficient to segregate the individual particles. The coated particles are very similar to dry particles at low solvent loadings but at higher loadings the particles appear shiny and saturated with solvent and form much larger conglomerates. The inter-particle attraction is evident as the dispersibility of these conglomerates in solution is less pronounced as the solvent loading is increased.

High gradient magnetic separation is a filtration option for these magnetic microparticles. These microparticles proved to be paramagnetic, with a volume magnetic susceptibility of 0.593×10−5 in a 1000 ppm particle solution. At this magnetic susceptibility, even a relatively modest magnetic field of 0.1 T in an HGMS unit is capable of removing 98% of the uncoated magnetic microparticles. At a higher field (1.5 T) and a low flow rate (0.6 l/h), experiments have shown that breakthrough did not occur [4]. Instead, the microparticles were retained in the filter and eventually completely blocked the flow with a 26 psi pressure drop. As expected, a lower magnetic field (0.1 T) resulted in an earlier breakthrough of <10 min opposed to slightly greater than 20 min for a magnetic field of 0.8 T. At 0.8 T, the breakthroughs occurred at approximately 50 min., 20 min., and after a few minutes for flow rates of 11.5, 24, and 51 l/h, respectively. The leakage rates (prior to breakthrough) were <2% in all cases and will improve with the increased magnetic field expected for scale-up processing. These magnetic microparticles were successfully modeled to predict the magnetic filtration using a mathematical approach without empirical fitting coefficients [5].

With the information that has been gathered to date on the performance of the MACS process, a preliminary cost analysis was completed. For a single stage recovery scheme, the MACS process costs are estimated to save $36,500 – $236,500 yearly over ion exchange for a large plating facility treating 50 ppm of Cr (III).

This paper presents recent data on the partitioning of Ni (II) and Co (II) onto coated MACS particles. In earlier efforts [4], [6], this magnetic separation system had proven to be very efficient for recovering hazardous metals. According to Buchholz et al. [4], data were presented showing the recoverability of Zn (II) and Cd (II) from acidic wastes using MACS particles coated with bis(trimethylpentyl) phosphinic acid (Cyanex 272 or C272) and bis(trimethylpentyl) dithiophosphinic acid (Cyanex 301) diluted in bis(2-ethylhexyl) phosphoric acid (D2EHPA). According to Kaminski et al. [6], the MACS particles were evaluated for the recovery of metals from a commercial plating waste containing suspended solid material. The results were very promising, demonstrating removal of Cr, Cd, Cu, Ni, Pb, and Zn to below RCRA limits. In this study, the MACS particles were coated with C272 and/or D2EHPA, and these results were compared with solvent extraction measurements using C272.

Section snippets

Experimental

The Co (II) and Ni (II) stock solutions were prepared by dissolving the appropriate amount of the sulfate salts (Aldrich Co. ACS grade) in deionized water. Stock solutions were prepared at 2 mg/ml Co (II) and 70 mg/ml of Ni (II). The pH was adjusted using microliter quantities of dilute NaOH or H2SO4. The bis(2,4,4 trimethylpentyl) phosphinic acid (Cyanex-272 or C272) was purchased from Cyanamid Co. and the bis(2-ethylhexyl) phosphoric acid (D2EHPA) and ethanol were purchased from Aldrich

Results and discussion

Metallurgical processing for nickel and cobalt often requires the separation of the chemically similar elements to produce a pure substance. Quantitatively, the separation factor, αCo, is defined as the ratio of the distribution coefficients for each metal, DM, in a solvent extraction system orαCo=DCoDNi=[Co]org[Co]aq[Ni]org[Ni]aq,where the bracketed terms refer to the activities of the metals but can be accurately approximated by the metal molar concentrations for dilute solutions. For

Summary

The MACS particles have shown promise in bridging the gap between ion exchange and solvent extraction in economically treating liquid hazardous wastes for waste minimization and recycling efforts. The MACS particles coated with C272 and D2EHPA displayed excellent partitioning of Co (II) and Ni (II) from acidic solution but did not display the pronounced selective recovery of Co (II) that is possible for similar solvent extraction systems. The highest partitioning of Co (II) and largest

Acknowledgements

The authors would like to thank LaTerra Holden for her assistance in the partitioning experiments.

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