The availability of large volumes of catalytic converters from automobiles (autocats) has led to the development of smelting technologies based on iron and copper collection (Mishra and Reddy, 1987; Hoffmann, 1988). Engelhard has developed pyrometallurgical and hydrometallurgical technologies to concentrate and refine a variety of materials containing low grades of precious metals, including gold (Benson et al., 2000). This is a departure from typical autocat smelters, where gold is not considered as an input to the furnace.
The smelter is a 2.5-MVA submerged carbon arc with a circular three-electrode (AC) and is operated as a slag-resistance furnace. The power density of this specialized furnace is relatively high at 320 kW/m2 to deal with the high-alumina feeds. The furnace is refractory lined and chilled with three water-cooled copper plates for the development of a freeze liner. The operation is semicontinuous; slag is tapped every 3 h though a water-cooled cinder monkey, while alloy is tapped once a day through a tapping hole in an alumina block. A mud gun is used to open and plug the alloy tap-hole, and the slag tap-hole is opened and closed by hand.
The off-gas stream is passed through a thermal oxidizer to oxidize CO to CO2, and mixed with cooling air and filtered using a static bag-house for primary cleaning. The off-gas is then scrubbed with caustic and passed through an electrostatic precipitator before final discharge to the atmosphere.
A rather large variety of materials are available for smelting and includes refinery residues generated from internal hydrometallurgical refining circuits; autocatalysts (also termed autocats) from internal manufacturing and after market sources, and spent catalysts from the chemical industry. Refinery residues are insoluble materials, typically leach resides containing significant PGM content, including gold and silver together with significant amounts of sodium and chloride.
Autocat manufacture generates a considerable volume of waste materials with small but significant PGM contents. These ceramic substrates are high melting-point aluminosilicates, namely, cordierite [Mg2Al4Si5O18] and mullite [Al6Si2O13], with varying amounts of alumina. The after-market autocats vary considerably in PGM content, with contaminants that include iron, nickel, chrome, lead, phosphorus, zinc, and rare-earth metals like CeO2.
Spent catalysts are refractory materials with a large spread of compositions ranging from alumina, aluminosilicates, zeolites, and silicates to silicon carbides. Metal content ranges from 0.1% to 5% PGMs, and compositions range from single PGMs (Pt on Al2O3), and single PGMs plus base metal (Pt/Fe on Al2O3), to mixed PGMs (Au/Pd on Al2O3). These materials usually have relatively small PGM contents and high-surface areas and do not respond well to leaching owing to the considerable loss of PGMs that takes place by reabsorption.
The more traditional sweeps are also added to the smelting circuit and include jewelers’ sweeps that are typically less than 0.1% gold as well as polishing rouge, which are mixtures of refractory abrasive materials such as iron oxides, corundum [Al6Si2O13], and alumina [Al2O3]. The smelting of such complex mixtures requires good chemical analysis for calculation of lime and other flux additions for the formation of fluid slags in the range of 1500–1600 °C. To achieve this, reference is made to the ternary phase diagrams for CaO–Al2O3–SiO2 and CaO–FeO–SiO2 when compounding smelter blends.
The collection mechanism in essence uses the carbothermic reaction between hematite and carbon to form finely dispersed iron particles that act as the collector. Smelting conditions are considered to be oxidizing, where most of the iron is deported to the slag as FeO, but some of the iron oxide is reduced to metal, forming a dense, finely dispersed metallic phase. The finely dispersed iron collector rains through the molten slag, colliding with gold and PGMs, and once a critical particle size is reached gravitational forces cause the particles to settle on the hearth.