Abstract:
Hydrometallurgy means extraction of metal from ore by preparing an aqueous solution of a salt of the metal and recovering the metal from the solution. The first applications of hydrometallurgy were based on the cementation reaction, where noble metal is recovered from solution by an exchange reaction involving dissolution of a less noble metal. Cementation was used in the 16th century to recover copper from pregnant heap leaching solutions. Modern hydrometallurgy started in 1880's when cyanidation for treating gold ores and the Bayer process for alumina production were invented. Several metals production technologies were developed during the 19th century using electrolysis, like production of zinc, copper and lead. New technologies were developed during the first half of 20th century including ion exchange, solvent extraction and pressure hydrometallurgy. Traditionally, the role of hydrometallurgy has been either in processing of raw materials not suitable for pyrometallurgy or in last refining step after pyrometallurgical processing.
The quality of the raw materials has decreased continuously. Metal content of primary materials decreases, the raw materials become more and more complex and energy consumption in the metals production increases. Processing of low-grade and complex primary raw materials is more demanding than processing of high-grade resources, but the metallurgical processes have flexibility to this. The number of metals and elements used to manufacture products and constructions has increased. During the 1700's some six elements were used, in the 1800's it had increased to ten, in the 1900's to well over twenty, and nowadays over sixty metals are used to manufacture a wide range of products. As the products become more sophisticated they have had to be manufactured using metal combinations not seen in primary sources. This will make recycling of high-technology products difficult.
Metals are not renewable resources like biological ones. The metals production processes can have undesirable environmental consequences if not properly controlled. Mining of primary sources can have adverse local impacts through mining wastes and the pollution of water. Metals production is energy intensive and affects global environment through greenhouse gases. The primary production of metals presently consumes 7 - 8 % of total global energy.
The global demand for metals is increasing. The role of hydrometallurgy in production of metals in the future depends on the available raw material sources. It is well known that high-grade primary sources are declining. Some metals will be produced by hydrometallurgy, like copper. Hydrometallurgy can have a stronger role in production of non-ferrous metals from low-grade sources, for example by using heap leaching with or without microbes. The recovery of specialty metals from end of life products will be the big challenge for hydrometallurgy. Less than one-third of some 60 metals used in consumer products have an end-of-life recycling rate above 50 per cent and 34 elements are below one per cent recycling. The important metals needed in electronics and renewable energy production methods are currently in the less than one percent group. To improve the recycling rate new production methods are needed to make the products recyclable.
|
