Capture of metallic copper by high gradient magnetic separation system

Abstract

Valence copper was recovered from wastewater by chemical reduction and use of a high gradient magnetic separation (HGMS) system. Ammonia (NH3) and sodium dithionate (Na2S2O4) at a molar ratio of [Cu]:[NH3]:[Na2S2O4] = 1:4:3 at pH = 9.5 were used first to chemically reduce copper ion to metallic copper; the resultant metal solids were captured in an upflowing reactor space equipped with a permalloy matrix net under a high gradient magnetic field. The captured solids were predominantly 6-20 microm in diameter, with Cu2O and CuO present among the solids. Four treatment configurations with and without the use of magnetic field and metal alloy as the matrix net were tested and their effects evaluated: (1) no magnetic field or matrix, (2) no magnetic field but with matrix, (3) with magnetic field but no matrix, (4) with both magnetic field and matrix. At flow rates of 40, 60, 80 and 100 cm3/min, capture efficiencies for metallic copper in the absence of magnetic field were 87%, 86%, 63%, and 39%, respectively, and in the presence of magnetic field were 99%, 98%, 95%, and 93%, respectively. The HGMS was critical for a high capture efficiency, whereas a matrix net only marginally enhanced it. Additional tests with a larger reactor confirmed similarly high efficiencies of > 85%. The use of an alloy matrix appeared to be important when high flow rates are most likely to be employed in practical applications.

Recovery of Cu(II) by chemical reduction using sodium dithionite

“…Based on these information, it is reasonable to assume that any ligands that could form complexes with copper, resulting in high solubility of copper at alkaline pH values, will act the same role as did ammonia in the studies by Wu et al (2011aWu et al ( , 2011b. Therefore, in this study we take the advantage of ligands, such as EDTA, that are ubiquitous in metal-containing wastewater and use sodium dithionite as reductant to reduce and recover copper.…”

“…For example, Wu et al (2011aWu et al ( , 2011b) integrated a high gradient magnetic separation system to remove metallic copper particles formed by chemical reduction. The size of copper particles ranges from less than 0.2 lm to around 0.5-1 lm depending on dithionite concentration (Wu et al, 2011a).…”

“…The pH value as high as 11 was suggested by Chen and Lim (2002) who studied chemical reduction of precious metals by hydrazine. Wu et al (2011aWu et al ( , 2011b investigated recovery of copper ions as metallic copper by chemical reduction using dithionite. Without elaborating the reason why reduction reaction has to be conducted at alkaline condition, the authors stated that addition of ammonia to increase copper solubility at alkaline pH values was essential for improving copper reduction efficiency (Wu et al, 2011a).…”

“…However, current efficiency is quite low ($30%) with ionic exchange membrane installed, and very low operation pH of 2.2 has to be maintained for efficient copper recovery. Meanwhile, anolyte has to contain enough concentration of sodium salts to prevent pH rising in the cathodic chamber and to reduce EDTA or NTA precipitation on to the membrane surface (Juang and Lin, 2000 Several researchers (Gomez-Lahoz et al, 1992a, 1992bChen and Lim, 2002;Geoffroy and Demopoulos, 2009;Wu et al, 2011aWu et al, , 2011b had explored chemical reduction for removal of heavy metals. The most of these studies suggested that alkaline pH values should be maintained to optimize the metal removal efficiency.…”