Recovery of metals from spent catalysts in a DC plasma furnace

Synopsis

The increased use of catalysts in the oil and petroleum industry has resulted in production of significant quantities of spent catalysts comprised of a porous ceramic substrate originally doped with heavy metals (e.g. cobalt, nickel, molybdenum) and further contaminated with heavy metal oxides and sulphides (e.g. nickel, vanadium) during use. These spent catalysts create an environmental threat because of their environmental leachability and pyrophoricity, which together preclude their disposal by landfill. Therefore, considerable effort has been spent in finding an alternative disposal method.

Most activities have centred on some form of immobilisation, in which the toxic components are physically bound in a low permeability medium, or on hydrometallurgical extraction techniques which, because of their complexity and number of process steps, are only viable for particular types of catalyst at relatively high throughputs (ca. 20,000 t/a). Requirement to operate at a large throughput is a disadvantage since it normally requires transport of large quantities of spent catalyst from an extensive catchment area, which itself causes a potential environmental problem. Therefore, a programme has been undertaken at Billiton Research to develop a process which is economically viable at a small scale (ca. 4,000 t/a) and which is flexible enough to process all major types of ceramic substrate based spent catalyst arising in a limited catchment area.

The developed process is carbothermic reduction of the metal oxides present on the spent catalysts in the presence of iron to collect the reduced metal and suitable fluxes to combine the carrier in a fluid slag. Products are an environmentally stable slag with low toxic metals content suitable for either dumping or use as a building material and a mixed ferroalloy containing the cobalt, nickel, molybdenum, etc. which can be sold for its alloy content to a special steel manufacturer or sold as scrap iron. The different uses of the products depend on the composition of the spent catalyst smelted. It is anticipated that the process will be operated in a semi-batch mode in which each different type (or group of types) of spent catalyst is processed to specific products.

To achieve the temperatures and energy densities required for catalyst smelting while minimising the quantities of offgas to be cleaned, the process technology selected was DC transferred arc plasma technology. Results of small scale (30 1.) and large scale (0.2 m³) trials are presented showing cobalt and molybdenum recoveries to alloy greater than 99% and residual analyses in the slag less than 0.05%.

A preliminary economic analysis for the process indicates that a 4,000 t/a plant will cost approximately U.S.$ 6.2 million. For processing a typical HDS catalyst, a net return of U.S.$. 392 per tonne can be calculated.