Anode mud

Tellurium is occasionally found native, but is more often found as the telluride of gold (calaverite), and combined with other metals. It is recovered commercially from the anode muds that are produced during the electrolytic refining of blister copper. The U.S., Canada, Peru, and Japan are the largest Free World producers of the element. 

Selenium is recovered from anode muds or slimes in electrolytic refining of copper. Anode mud is treated with sulfuric acid and roasted. Selenium is converted to its dioxide, which vaporizes and is collected in a wet scrubber system. 

The unsoftened lead obtained after the softening operation contains silver in small but significant quantities. Such unsoftened lead is cast into anode and subjected to electrolytic refining. The anode mud that is formed adhering to these anodes is removed by scraping. It contains bismuth, silver, gold, and other impurity metals. Silver is obtained from this anode mud by methods similar to the extraction of anode mud from the copper refining process discussed earher. 

The anode mud is now a much more important source of tellurium than any of the minerals. The mud is generally roasted in a current of air, when tellurium dioxide is formed. This condenses, together with oxides of arsenic and antimony, as small white crystals in the flues and cooling chambers, and is reduced to tellurium by smelting with charcoal at a low temperature.1... 

Native copper ore is crushed, concentrated by washing with water, smelted, and cast into bars. Oxide and carbonate ores are treated with carhon in a smeller. Sulfide ore treatment is complex, hut. in brief, consists of smelling to a matte of cuprous sulfide, ferrous sulfide, and silica, which molten matte is treated in a converter by the addition of lime and air is forced under pressure through the mass. The products are blister copper, ferrous calcium silicate slag, and SO . Refining is conducted by electrolysis, and the anode mud is treated to obtain the gold and silver. See Fig. I. 

Selenium occurs as selenide in many sulfide ores, especially those of copper, silver, lead, and iron, and is obtained as a by-product from the anode mud of copper refineries. The mud is (li fused with sodium nitrate and silica, or (2) oxidized with HNOj. and the H20 extract is then treated with HG and SO2. whereupon free selenium is separated. 

Tellurium occurs chiefly as telluride in gold, silver, copper, lead, and nickel ores in Colorado. California, Ontario, Mexico, and Peru, and infrequently as free tellurium and tellurite (tellurium dioxide, Te02) The anode mud from copper and lead refineries, or the flue dust from roasting telluride gold ores is treated by fusion with sodium nitrate and carbonate and the melt extracted with water. The resulting solution is acidified carefully with H2S04, whereupon tellurium dioxide is precipitated, and die dioxide reduced to free tellurium by heating with carbon. 

FIGURE 18.19 Electrorefining of copper metal, (a) Alternating slabs of impure copper and pure copper serve as the electrodes in electrolytic cells for the refining of copper, (b) Copper is transferred through the CuS04 solution from the impure Cu anode to the pure Cu cathode. More easily oxidized impurities (Zn, Fe) remain in solution as cations, but noble metal impurities (Ag, Au, Pt) are not oxidized and collect as anode mud. 

At the impure Cu anode, copper is oxidized along with more easily oxidized metallic impurities such as zinc and iron. Less easily oxidized impurities such as silver, gold, and platinum fall to the bottom of the cell as anode mud, which is reprocessed to recover the precious metals. At the pure Cu cathode, Cu2+ ions are reduced to pure copper metal, but the less easily reduced metal ions (Zn2+, Fe2+, and so forth) remain in the solution. 

Anode mud — During the anodic dissolution of an impure electrode (e.g., in an - electrorefining or - electrodeposition process) a residue may form which is called anode mud or anode slime. It contains elements which are insoluble under the conditions of the - electrodissolution process and thus sediment near the anode. Usually the anode mud contains precious metals (Ag, Au, Pt, etc.) which are recovered from it. 

A sludge called anode mud collects under the anodes. It contains such valuable and difficult-to-oxidize elements as Au, Pt, Ag, Se, and Te. The separation, purification, and sale of these elements reduce the cost of refined copper. 

Selenium is quite rare (9 X 10 % of the earth s crust). It occurs mainly as an impurity in sulfur, sulfide, and sulfate deposits. It is obtained from the flue dusts that result from roasting sulfide ores and from the anode mud formed in the electrolytic refining of copper. It is used as a red coloring in glass. The gray crystalline allotropic form of selenium has an electrical conductivity that is very light-sensitive, so it is used in photocopy machines and in solar cells. 

Tellurium is even less abundant (2 X 10 % of the earth s crust) than selenium. It occurs mainly in sulfide ores, especially with copper sulfide, and as the tellurides of gold and silver. It, too, is obtained from the anode mud from refining of copper. The element forms brass-colored, shiny, hexagonal crystals having low electrical conductivity. It is added to some metals, particularly lead, to increase electrical resistance and improve resistance to heat, corrosion, mechanical shock, and wear. 

The method based on the complex of Pd with dibromo-o-carborylchlorophosphonazo was developed for the determination of Pd in catalysts and anode mud samples [5]. Derivative spectrophotometry of the complexes with oxazolidine derivative makes a basis of the simultaneous determination of Pd and Cu in alloys and activated charcoal [6]. 

Occurrence, — Selenium must be considered a rare element, although it is found widely distributed in nature. The distinctive selenium minerals are rare, and they are usually selenides, of such metals as lead, mercury, copper, bismuth, and silver. The element is also found in the free state associated with sulfur and as a selenite. The most common occurrence of selenium is in ores in which the element has partially displaced sulfur. Generally the selenium is present in very small proportions, but on account of the fact that enormous quantities of sulfide ores are used, this represents a considerable amount of selenium. It occurs also in small amounts in meteoric iron, in volcanic lavas, and in certain deposits of coal. Traces of selenium have been detected in rain and snow. Even though present in mineral ores in mere traces, it is readily concentrated either in the flue dusts or in the anode mud of the electrolytic refineries. Considerable quantities are known to exist in Hawaii, Japan,... 

Figure 21.27 The electrorefining of copper. A, Copper is refined electrolytically, using impure slabs of copper as anodes and sheets of pure copper as cathodes. The Cu ions released from the anode are reduced to Cu metal and plate out at the cathode. The anode mud contains valuable metal by-products. B, A small section of an industrial facility for electrorefining copper.

Copper and the more active impurities (Fe, Ni) are oxidized to their cations, while the less active ones (Ag, Au, Pt) are not. As the anode slabs react, these unoxidized metals fall off as a valuable anode mud and are purified separately. Sale of the precious metals in the anode mud nearly offsets the cost of electricity to operate the cell, making Cu wire inexpensive. 

Figure 2 shows the silver coulometer used by Richards (16), first described in 1902. The anode, C, is a massive piece of silver, preferably coated with electrolytic silver, suspended by its connecting wire. A, and surrounded by a porous cup, D, which is also suspended from above, B. The cathode consists of a platinum bowl or crucible, E. The porous cup serves to catch "anode mud" from dropping onto the cathode. 

Table 4 8 Typical composilion (% wt) of anode mud and Dori metal obtained from copper eketroreftning celts...

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