Greenockite

Abstract: Cadmium, Cd, a Group 12 (IIB) element occurring between zinc and mercury, is a soft, ductile, silver-white metal having a distorted hexagonal close-packed structure The crustal abundance of cadmium is somewhere between 01 and 05 ppm, and several cadmium minerals have been identified, the most common being greenockite, CdS Cadmium is generally encountered in zinc ores, zinc-bearing lead ores, or complex copper-lead-zinc ores, where, however, it forms an isomorphic impurity in the zinc mineral sphalerite, ZnS For this reason, cadmium is almost invariably recovered as a by-product from the processing of zinc, lead, and copper ores Its oxidation state in almost all of its compoumds is + 2, although a few compounds have been reported in which cadmium exists in the + 1 oxidation state There are eight natural isotopes Cadmium forms a fume of brown-colored cadmium oxide, CdO, when heated in air Other elements which react readily with cadmium metal upon heating include the halogens, phosphorus, selenium, sulfur, and tellurium Cadmium is rapidly oxidized by hot dilute nitric acid Cadmium occurs primarily as sulfide minerals in zinc, lead-zinc, and copper-lead-zinc ores Beneficiation of these minerals, usually by flotation or heavy-media separation, yields concentrates which are then processed for the recovery of the contained metal values Cadmium follows the zinc with which it is so closely associated Air pollution problems and labor costs have led to the closing of older pyrometallurgical plants, and to increased electrolytic production Cadmium production is dependent on the processing of zinc ores, which often contain 02 to 04% cadmium Cadmium is classified as a toxic metal Acute industrial poisoning by cadmium dust or fume can occur during the melting or pouring of cadmium metal; the welding, burning, or heating of cadmium-plated steel; or spraying, brazing, and overheating of cadmium metal Protection should be provided by a properly designed exhaust ventilation system or by a suitable individual filter or air-supplied respirator Industrial exposure to cadmium fumes and dust has been reported to result in emphysema, hypertension, kidney failure, osteomalacia, and perhaps an increased incidence of cancer To help maintain the balance between supply and demand for cadmium, efforts can be made to recycle such cadmium-containing materials as spent nickel-cadmium batteries as well as dust and other residues from the pigment industry Consumption of cadmium is in batteries, coating and plating, pigments, plastics and synthetic products, and alloys and other uses Cadmium is an important component in brazing and low melting alloys, used in bearings, solders, and nuclear reactor control rods Keywords: Occurrence; Cadmium; Sources; Recycling; Properties; Manufacture; Economics; Environmental Concerns; Specifications; Analytical Methods; Nealth & Safety; Uses; Minerals; Batteries; Nickel; Contamination; Pigments; Stabilizers; Coatings; Alloys; Cadmium Telluride; Cadmium Sulfide

Abstract: This study reports the feasibility of recovering metal precipitates from a synthetic acidic wastewater containing ethanol, Fe, Zn, and Cd at an organic loading rate of 2.5 g COD/L-day and a COD to sulfate ratio of 0.8 in a sulfate reducing down-flow fluidized bed reactor. The metals were added at increasing loading rates: Fe from 104 to 320 mg/L-day, Zn from 20 to 220 mg/L-day, and Cd from 5 to 20 mg/L-day. The maximum COD and sulfate removals attained were 54% and 41%, respectively. The biofilm reactor was operated at pH as low as 5.0 with stable performance, and no adverse effect over COD consumption or sulfide production was observed. The metals precipitation efficiencies obtained for Fe, Zn, and Cd exceeded 99.7%, 99.3%, and 99.4%, respectively. The total recovered precipitate was estimated to be 90% of the theoretical mass expected as metal sulfides. The precipitate was mainly recovered from the bottom of the reactor and the equalizer. The analysis of the precipitates showed the presence of pyrite (FeS2), sphalerite (ZnS) and greenockite (CdS); no metal hydroxides or carbonates in crystalline phases were identified. This study is the first in reporting the feasibility to recover metal sulfides separated from the biomass in a sulfate reducing process in one stage. Biotechnol. Bioeng. 2009;102: 91–99. © 2008 Wiley Periodicals, Inc.