A Agrawal

REMEDIATION OPTIONS FOR THE TREATMENT OF ELECTROPLATING AND LEATHER TANNING EFFLUENT CONTAINING CHROMIUM—A REVIEW

Chromium used in the electro plating and tanning industries causes environmental pollution through the generation of effluent. Various methods such as precipitation–flocculation coupled with pre/post-oxidation, reduction, and concentration are often employed to control environmental pollution. Though these techniques, referred to as “removal–disposal,” serve the purpose of satisfying water pollution norms, they produce solid residues containing Cr(OH)3 as the sludges, which are usually dumped as landfill. Besides the possibility of mobility of the metal as Cr(VI) by the biological and chemical oxidation, the dumping of sludges also leads to the loss of metal values exerting pressure on the corresponding primary reserves. Therefore, processes based on “recovery–reuse” are now being increasingly projected and used. In this article, streams/wastes containing chromium relevant to electroplating have been identified and the applicability of conventional and promising techniques to treat such substances have been reviewed. Earlier developments and recent modifications on the most common routes, such as precipitation, evaporation, bioremediation, etc., are highlighted. Other methods such as electrolysis, solvent extraction, membrane separation, ion exchange, etc. are discussed with respect to their applicability, status, and scope.

Pressure sulpuric acid leaching of a sulphide concentrate to recover copper, nickel and cobalt

Ore from the Jaduguda mines, India, includes significant contents of important metals,1 such as Cu (0.1%), Ni (0.1%), Co (0.006%) and Mo (0.02%), together with appreciable amounts of other valuable materials, such as apatite (3–4%), rare earths (0.1% including Y) and magnetite (5.5%). The purpose of the by-product recovery plant of the uranium mill is to recover as much of these minerals as possible to augment India’s supply. It currently produces a complex sulphide concentrate at the rate of about 1200 t/year, which yields a high-grade copper concentrate and a low-grade silicate tailing containing Cu and Ni, together with a salable molybdenum concentrate. The copper concentrate was formerly sold to the Indian Copper Complex (ICC) at Ghatsila, but of late the ICC has discontinued processing of the concentrate because of its high nickel content. As a result, the copper concentrate has remained stockpiled for want of a suitable process. This product, which was previously designated a copper concentrate, but is now referred to as a sulphide concentrate, comprises 15% copper, 10.85% nickel and 0.37% cobalt (Table 1). Several processing options present themselves and some have been tried at various institutes in India. Mukherjee et al.2 peformed tests at kilogram scale in which roasting in the presence of NaCl in the temperature range 400–500°C was followed by water leaching and solution purification. The three metals, copper, nickel and cobalt, were separated from the leach liquor of salt-roasted material as their sulphate salts by solvent extraction3 with D2EHPA for subsequent recovery in the desired form. The salt roast–leach–solvent extraction– electrowin route was also followed at NML4 for recovery of the metals as electrolytic-grade cathodes. Because of excessive corrosion during roasting, however, the process was considered unfavourable. Other important process routes for the extraction of metals from sulphide concentrates are ferric chloride leaching5 and cupric chloride–O2 leaching6 in acid conditions. Although high recoveries of the metals are often achieved, ferric chloride leaching is also known to present corrosion problems and, therefore, has not been considered by Indian investigators for this concentrate. Preliminary experiments with pressure leaching in acidic conditions are reported to have yielded metal recoveries of 70–75% Ni and 10–20% Cu.7,8 Despite these developments, it has long been considered that smelting is likely to remain the principal processing route for sulphide concentrates. This view is being challenged through pressure-leaching developments by Dynatec and Cominco Engineering Services, Ltd. (CESL), in Canada, the installation at Mt. Gordon in Queensland, Australia, and the very recent announcement by Phelps Dodge.9 The CESL process10 for copper concentrate, which uses an oxidative pressure leach with a mixed sulphate–chloride solution, is reported to have been piloted successfully. Chloride ions play a catalytic role and are required to prevent excessive sulphur oxidation that would otherwise make the process uneconomic. Pressure leaching is followed by solvent extraction and electrowinning. The Dynatec process11 for chalcopyrite concentrate treatment also uses chloride additions to the leach liquor to minimize sulphide oxidation to sulphur. In addition, fine coal is used as a dispersant for the molten sulphur formed under the leaching conditions in the autoclave. This is much cheaper than the sodium or calcium lignosulphonate or quebracho used as additives in the zinc concentrate pressureleach process. Solvent extraction is the process of choice for copper recovery from the leach liquor. The Activox process11 is claimed to offer the potential to treat sulphide concentrate directly without the need for intermediate pyrometallurgical treatment. The process involves fine grinding and low-pressure oxidative leaching. The autoclave leaching conditions are typically 100°C at an oxygen pressure of 100 kPa with a residence time of 4 h for chalcopyrite oxidation. The solution generated by leaching is finally treated by solvent extraction and electrowinning. Recently,12 a two-stage acid pressure leach at 140°C and pO2 of 600 kPa was applied to a low-grade copper–nickel concentrate containing 2% Ni, 0.43% Cu and 6.2% Mg, when it was recognized that a preleach step was required to recover more than 90% of metals.12 Acid pressure leaching was deemed to merit consideration as a potential means of extracting the valuable metals from the Jaduguda sulphide concentrate.