Electrolytic purification of copper

Preparation. Industrially, silver is usually a by-product of processes of extraction of other metals such as copper, lead, zinc. The so-called anode slimes from the electrolytic purification of copper contain silver and the involved process is often finished by an electrolysis of a nitrate solution containing silver. 

Selenium and tellurium are much less abundant than sulfur and no ores are rich in these elements. They are recovered from the anode slime deposited in the electrolytic purification of copper (having been present as impurities in the copper sulfide ores), as by-products in other sulfide ore processing, and in sulfuric acid manufacture. 

TABLE 13.2 Standard Reduction Potentials for a Series of Elements of Importance to Electrolytic Purification of Copper"... 

Gkrld is also recovered from the anode sludge from electrolytic purification of copper Section 21-7. Gktld is so rare that it is also obtained from very low-grade ores by the cyanide process. Air is bubbled through an agitated slurry of the ore mixed with a solution of NaCN. This causes slow oxidation of the metal and the formation of a soluble complex compound. 

In the electrolytic purification of copper, the anodes are blister copper bars and the cathodes are made of pure copper. As electrolysis proceeds, copper is oxidized at the anode, moves through the solution as Cu ions, and is deposited on the cathode. The voltage of the cell is regulated so that more active impurities (such as iron) are left in the solution, and less active ones are not oxidized at all. The less active impurities include gold and silver, which collect as anode slime, an insoluble residue beneath the anode. The anode slime is subsequently treated to recover the valuable metals. 

Q Outline the chemical steps of a process for separating the rare metals Ru, Os, Rh, Ir, Pd, Pt, Ag and Au present in the wastes ( anode slimes ) formed during the electrolytic purification of nickel or copper. 

Electrolytic purification of smelted copper removes contaminants which adversely affect electrical conductivity, malleability, and other properties. It also permits recovery of the precious metal content of the fire-refined product. 

FIGURE 13.8 Diagram of a section of an electrolytic cell for the purification of copper. 

Only weakly electropositive metals can, obviously, be purified by electrorefining, since the concerned metal must not react with water and must be easily oxidized (at anode) and reduced (at cathode) relative to hydrogen. Beside copper, some other metals for which electrorefining has been employed are Silver, gold, tin and lead. In the electrolytic purification of these metals the relevant electrolyte is the one listed below against each metal ... 

Purification of copper is achieved by electrolysis, as illustrated in Figure 23.11 . Large slabs of crude copper serve as ihe anodes in tire cell, and thin sheets of pure copper serve as the catiiodes. The electrolyte consists of an acidic solution of CUSO4. Application of a suitable voltage to flie electrodes causes oxidation of copper metal at the anode and reduction of Cu to form copper metal at the cathode. This strategy can be used because copper is both oxidized and... 

The commercial purification of copper metal is carried out in electrolytic cells. The anode is composed of impure ( blister ) copper, and the electrolyte is a mixture of aqueous CUSO4 and H2SO4. During purification, copper is effectively transferred from the anode to the cathode, and pure copper is thereby produced, (a) How does an electrolytic cell differ from a galvanic cell (b) Write half equations for the cathode and anode reactimis. (c) Is the overall cell reaction spontaneous If not, how does it occur ... 

The cementation of gold and the purification of the ziac electrolyte ate usually carried out ia cylindrical vessels usiag mechanical agitation. The cementation of copper is carried out ia long narrow tanks called launders, ia rotating dmms, or ia an iaverted cone precipitator (see Copper). 

Metals less noble than copper, such as iron, nickel, and lead, dissolve from the anode. The lead precipitates as lead sulfate in the slimes. Other impurities such as arsenic, antimony, and bismuth remain partiy as insoluble compounds in the slimes and partiy as soluble complexes in the electrolyte. Precious metals, such as gold and silver, remain as metals in the anode slimes. The bulk of the slimes consist of particles of copper falling from the anode, and insoluble sulfides, selenides, or teUurides. These slimes are processed further for the recovery of the various constituents. Metals less noble than copper do not deposit but accumulate in solution. This requires periodic purification of the electrolyte to remove nickel sulfate, arsenic, and other impurities. 

The precipitated copper from this reaction is an important constituent of the slime that collects at the bottom of the electrolytic cells. The accumulation of copper as well as of impurities such as nickel, arsenic, antimony, and bismuth is controlled by periodic bleed-off and treatment in the electrolyte purification section. 

Electrorefining has been used for the purification of many common as well as reactive metals. It has been seen that the emf or the potential required for such a process is usually small because the energy needed for the reduction of the ionic species at the cathode is almost equal to that released by the oxidation of the crude metal at the anode. Some metals, such as copper, nickel, lead, silver, gold, etc., are refined by using aqueous electrolytes whereas molten salt electrolytes are necessary for the refining of reactive metals such as aluminum,... 

Other references in Table in discuss applications in precipitation of metal.compounds, gaseous reduction of metals from solution, equilibrium of copper in solvent extraction, electrolyte purification and solid-liquid equilibria in concentrated salt solutions. The papers by Cognet and Renon (25) and Vega and Funk (59) stand out as recent studies in which rational approaches have been used for estimating ionic activity coefficients. In general, however, few of the studies are based on the more recent developments in ionic activity coefficients. 

Polonium may be purified by various processes. Such purification methods include precipitation of polonium as sulfide and then decomposing the sulfide at elevated temperatures spontaneous decomposition of polonium onto a nickel or copper surface and electrolysis of nitric acid solutions of polonium-bismuth mixture. In electrolytic purification polonium is electrodeposited onto a platinum, gold, nickel, or carbon electrode. 

Purification of metals is another important application of electrolysis. For example, impure copper from the chemical reduction of copper ore is cast into large slabs that serve as the anodes for electrolytic cells. Aqueous copper sulfate... 

Copper is refined electrolytically, the silver and gold present separating as an anode sludge. The last two metals are also recovered during the purification of both lead (p. 310) and nickel (p. 502). Silver and gold are extracted by aqueous sodium cyanide which reduces the oxidation potential of the metals so that atmospheric oxygen brings them, or their salts, into solution as soluble complexes ... 

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