Metals refining from ores

The metals Alexander Calder used to make Mobile were refined from ores using some of the processes discussed in this chapter. 

In the metal refining of ores, the metal is solubilized in an aqueous solution. The optical control of metal refining requires quick, accurate analysis of the major chemical species present in solution. Raman spectroscopy and resonance Raman are used to identify the amine complexes of Co, Ni, and Cu species, as well as ammonia sulfate and sulfamate, present in these industrial solutions. The Raman spectra of an industrial plant solution from mine tailings are shown in Fig. 7-15. Each solution contains one or more metal species, sulfate, sulfamate, ammonia, ammonium sulfate, and water. From a comparison with model ammine complexes, the vibrations in the spectra are identifiable. Bands were observed at 615, 980 and 1,110 cm-1 and were assigned to the sulfate ion. No bands were observed for free ammonia. A band at 376 cm-1 was assigned to the Ni(NH3)j + specie. A band at 490 cm-1 was assigned to the Co(NH3)g+ specie. 

Analysis of the precious metals is very important when the economics of metal recovery from ore or scrap is considered. The purity of refined metal is also critical because trace impurities can be responsible for a product s lack of performance. For these reasons, it is important to have dependable analytical methods. 

In the United States, about 90% of gold production originates from ores and placer deposits. The remainder is recovered primarily as a by-product of the refining of base metals, chiefly copper. The principal gold producing states are Nevada (60%) and Califomia (10%) followed by Montana, Utah, S. Dakota, Washington, Colorado, Alaska, Idaho, Arizona, and New Mexico (7). 

Lead is usually processed from ore to refined metal in four stages. These are ore dressing, smelting, drossing, and refining (see Mineral recovery and processing). 

Manufacture and Recovery. Electrolytic copper refinery slimes are the principal source of selenium and its sister element, tellurium, atomic numbers 34 and 52, respectively. Electrolytic copper refinery slimes are those constituents in the copper anode which are not solubilized during the refining process and ultimately accumulate in the bottom of the electrorefining tank. These slimes are periodically recovered and processed for their metal values. Slimes generated by the refining of primary copper, copper produced from ores and concentrates, generally contain from 5—25% selenium and 2—10% tellurium. 

Many metals are extracted from their compounds, as found in ores, by electrolytic processes. By far the most important is the Hall-Heroult process, invented in 1886, for producing aluminium from alumina, itself refined from bauxite ore. Alumina is dissolved in molten cryolite, Na3Alp6, and electrolysed, using carbon anodes and the aluminium itself as cathode. While various details are being steadily improved, the basic process is still the same today. 

Despite low collection efficiencies, settling chambers have been used extensively in the past. The metals refining industries have used settling chambers to collect large particles, such as arsenic trioxide from the smelting of copper ore. 

Figure 20-20 shows in schematic fashion some of the alternative paths leading from ores to pure metals. These paths include four general processes of which the essential chemical process is reduction to yield the neutral metal. First is separation. Generally, a metal ore obtained from a mine contains a particular compound of some desired metal mixed with various other materials. The mineral must be separated from these other contaminants. Separation often is followed by conversion, in which the mineral is treated chemically to convert it into a form that can be easily reduced. The third step is reduction. After a suitable compound has been obtained, it is reduced to free metal by chemical reaction with a reducing agent or by electrolysis. The metal obtained by reduction often contains small amounts of impurities, so the final step is refining to purify the metal. 

When nonrenewable mineral resources are processed for metal extraction and the metals extracted are utilized in various ways and in diverse forms, they become potentially available for reclamation and recycling. Recycled metals produced by the extraction and refining of metallic wastes are known as secondary metals, whereas metals produced from primary ores are termed primary metals. A classification of recyclable metalliferous resources such... 

Zinc is a bluish-white, lustrous metal which tarnishes in air. It is present in the earth s crust as sulfide (sphalerite), carbonate, or silicate ores, to the extent of only 78 ppm, making it the 23rd most abundant element.2 The metal is obtained from its ores by roasting and subsequent reduction with coke or by electrolysis. Approximately 8.36 million metric tons of zinc were produced worldwide in 2002 of this amount, two-thirds were from ores, while one-third was obtained from recycled zinc.3 The ease of mining and refining of the ore and the subsequent low price of the metal (ca. 1.2 kg-1 in 1998)3 have made zinc the third most popular non-ferrous metal (after aluminum and copper). 

It is not found in its pure metallic form in nature but is refined from the mineral (compound) zinc sulfide (ZnSO ) known as the ores sphalerite and zincblende. It is also recovered from minerals and ores known as willemite, hydrozincite, smithsonite, wurtzite, zincite, and Franklinite. Zinc ores are found in Canada, Mexico, Australia, and Belgium, as well as in the United States. Valuable grades of zinc ores are mined in Colorado and New Jersey. 

It was not until about 4000 bce that humans learned how to obtain these base metals from ores by using heat. Both the refining of and the uses for silver as jewelry are described in ancient Egyptian writings as well as the Old Testament of the Bible. During these ancient times, silver was more valuable than gold primarily because it was more difficult to find in its natural state as well as to extract from its ores. 

Although lead can be found as a metal in the Earth s crust, it is usually mined and refined from minerals and ores. Lead is one of the most common and famihar metalhc elements known. Although it is somewhat scarce, found at proportions of 13 ppm, it is stiU more prevalent than many other metals. Lead is noncofflbustible. and it resists corrosion. 

Copper is widely distributed in Nature as metal, in sulfides, arsenides, chlorides, carbonates, and so on. It is extracted from ores, usually by wet processes, for example, by leaching with dilute sulfuric acid, or by solvent extraction using salicylal-doximes and similar ligands. Copper is refined by electrolysis. 

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