Recycling metal values

Swarfs, machining waste from steel -50 Recycling metal values... 

Scrap that is unsuitable for recycling into products by the primary aluminum producers is used in the secondary aluminum industry for castings that have modest property requirements. Oxide formation and dross buildup are encountered in the secondary aluminum industry, and fluxes are employed to assist in the collection of dross and removal of inclusions and gas. Such fluxes are usually mixtures of sodium and potassium chlorides. Fumes and residues from these fluxes and treatment of dross are problems of environmental and economic importance, and efforts are made to reclaim both flux and metal values in the dross. 

The Advanced Recovery Systems, Inc. (ARS) developed the patented, ex situ DeCaF hydrometallurgical technology to decontaminate fluoride by-products and to recover recyclable metals. The technology uses a proprietary acid mixture to digest the fluoride matrix, freeing radioactive contaminants (e.g., uranium, thorium, or radium) and hazardous contaminants (e.g., lead, arsenic, or chromium). Radioactive elements are recycled or disposed. Metals are also recycled, and fluoride is recovered as a high-value salt for aluminum smelting. 

Concentrating methods, such as solvent extraction for copper and carbon absorption for gold, will selectively strip just the metal of interest. Loaded solvents and carbon columns are stripped of their metal value in a concentrated solution that can now be electrowon to recover the metal value. The spent electrolytic solutions stripped of the copper or gold can be recycled back to recover more metal value from the solvents or carbon. Some contaminant metals may also pass through this process and continually build up in the electrolyte. For these operations, the solutions can be diluted into the original leach solutions as make up lixiviant, or a process similar to the one just described can be used to strip contaminant metals from the electrolytes. 

Small nickel/cadmium and lead-acid batteries generally experience the same fate. With some exceptions, the larger rechargeable batteries, automotive batteries in particular, are returned to the vendors to a large extent for subsequent recycling by the manufacturers, or for processing by scrap metal operators for recovery of their intrinsic metal values. 

The solid residues from the filter and from various other plant sources contain some cadmium as well as iron, nickel, and cobalt. Since they may not be discarded as waste, they are processed further by a sequence of selective dissolution steps, electrolysis, and differential precipitation to recover metallic cadmium and separate solutions of cadmium sulfate, nickel sulfate, and cobalt sulfate that are sold to metal refiners for recycling. Depending on their compositions, the rinse and neutralizing liquors may be discarded as waste or reprocessed to recover their metal values and to reduce the plant emissions to acceptable levels. 

An interesting recycling process has been developed the Recytec company in Switzerland in cooperation with ETH in Zurich (25). It combines an initial thermal treatment with a subsequent electrochemical process to recover separated metal values from spent household batteries. A virtue of the process is that it accepts unsorted mixtures of just about all types of batteries likely to be found in household wastes, and it can probably tolerate admixtures of some non-battery wastes as well. Although it is not designed to handle lead-acid batteries, it may well be capable of doing so after some process modifications. 

The last phrase is particularly important since, as has already been observed, it is technically much easier to recycle metals, paper and glass than plastics and more energy is vested in the primary production of aluminium, steel, glass and paper than in plastics (Tables 2.1). Moreover, the first three have no value as fuels whereas polymers do (next section). It might be anticipated then that the proportion of plastics ultimately recycled will settle out at the lower end of the range. 

Hammond and Craig (2012). Value for recycled metal is shown in parenthesis. Asif et al. (2005) for each material. 

Recovery of metals from used catalysts [34]. Electro-deposition can be used to recover the metal values from catalysts, such as Co-Mo, Ni-Mo and Ni used in hydroprocessing and edible oil hydrogenation, respectively. The catalysts are supported on alumina and silica substrates. The process for recycling the catalysts comprises of the following steps ... 

Saprolite leach discharge is partially neutralized with recycled Secondary Neutralization Precipitation residue slurries, simultaneously recovering the metal values associated with those streams. The residual acid is then further reduced to approximately 5 g/L H2SO4 using ground limestone slurry. Temperature is maintained at 95°C. 

Total merchant shipments of DRI and HBI in 1993 reached 5.1 x 10 t. The primary DRI exporting countries were Venezuela, Russia, Malaysia, Trinidad, and India. The price of merchant HBI in 1993 was in the range of 125 to 167/1 on a deUvered basis. Although there are expectations that the value of merchant DRI should some day stand on its own, the historic price has been tied to the price of ferrous scrap. A general mle of thumb has been that the value of merchant DRI is comparable to prime scrap (No. 1 Bundles or No. 1 Bushelings) in industrial countries, and comparable to imported shredded scrap in developing countries (see RECYCLING, FERROUS METALS). 

The selection of a particular type of reduction depends on technical feasibiUty and the economics of the process as well as on physicochemical considerations. In particular, the reducing agent should be inexpensive relative to the value of the metal to be reduced. The product of the reaction, RX, should be easily separated from the metal, easily contained, and safely recycled or disposed of. Furthermore, the physical conditions for the reaction should be such that a suitable reactor can be designed and operated economically. 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

Primary consumers for ferrous scrap are the iron and steel mills and foundries. Minor consumers iaclude ferroalloy producers, copper producers for use ia copper precipitation (see Recycling, nonferrous metals), and the chemical iadustry. The steel iadustry consumes about three-fourths of the total. Scrap consumption for ferroalloy production, copper precipitation, and the chemical iadustry total less than one million t. The United States is the leading exporter of ferrous scrap, exporting almost nine million t ia 1994, valued at about 1.3 biUioa. Total value of domestic scrap purchases and exports ia 1994 was 8 biUioa (2). 

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