Electrolysis aluminum refining

Electrolysis and Polarity Passive Electrolysis in Refining Aluminum... 

Table 1 lists several electrochemical processes and their reactions, feeds, and products. The chlor-aUcali process consumes approximately 2 % of the electricity generated in the USA. The process involves electrolysis of a brine solution to produce CI2 at the anode and NaOH at the cathode. In the Hall process for aluminum refining, AI2O2 reacts with a carbon electrode to form A1 and CO2. Water electrolysis is a widespread technology for generating pure H2 and O2. While reforming of methane or other... 

We have already described the refining of copper and the electrolytic extraction of aluminum, magnesium, and fluorine. Another important industrial application of electrolysis is the production of sodium metal by the Downs process, the electrolysis of molten rock salt (Fig. 12.15) ... 

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). 

Electrolysis is used industrially to produce aluminum and magnesium to extract metals from their salts to prepare chlorine, fluorine, and sodium hydroxide to refine copper and in electroplating. 

The recovery of aluminum metal is divided into two steps, i. e., the production of pure alumina (Bayer Process) and its molten salt electrolysis. Raw aluminum obtained by reduction electrolysis already has a high purity (99.5-99.7%). Refining methods for raw aluminum to obtain higher purities include the segregation process (99.94-99.99% Al) and three-layer electrolysis (99.99-99.998% Al) [142, 236]. Besides these, processes are available whereby the aluminum is anodically dissolved in an organic electrolyte and then cathodically deposited [37, 118, 217, 221]. The dissolution as well as the deposition process contribute to the electrolytic refining of aluminum. 

Based on this observation, pure aluminum is obtained by a two-step refinement technique whereby electrolysis is performed with an insoluble anode, i. e. Cu [229, 230]. 

In the modern version of this process, aluminum metal is obtained by electrolysis of aluminum oxide, which is refined from bauxite ore (AI2O3 2H2O). The aluminum oxide is dissolved at 1000°C in molten synthetic cryolite (Na3AlFg), another aluminum compound. The cell is lined with graphite, which forms the cathode for the reaction. Another set of graphite rods is immersed in the molten solution as an anode. The following half-reaction occurs at the cathode. 

Several elements are produced commercially by electrolysis. In Sections 21-3 to 21-5, we described some electrolytic cells that produce sodium (the Downs cell), chlorine, hydrogen, and oxygen. Electrolysis of molten compounds is also the common method of obtaining other Group lA metals, HA metals (except barium), and aluminum (Section 22-3). Impure metals can also be refined electrolytically, as we will describe for copper in Section 22-8. 

In our natural environment, metals are most stable in an oxidized state, e.g., Fe203 or AI2O3. As a consequence, one step of metal ore refining is the reduction of the metal oxide to its zero oxidation state. An electrochemical reduction process, where the electrolysis medium is a molten salt, is preferred for very electropositive metals (e.g., aluminum, sodium, lithium, and magnesium) and for metal refining where the chemical route suffers from environmental problems. 

Sodium hydroxide Electrolysis of NaCl solution Chemicals, pulp and paper, aluminum, textiles, oil refining... 

Hall-Heroult process involves the electrolytic refining of aluminum from AI2O3. Carrying out the electrolysis in a bath of cryolite (NajAIF ) lowers the required temperature significantly, making the process economically feasible. 

When aluminum is refined by electrolysis from its oxide ores, is the process used active or passive electrolysis Explain your answer. 

Copper, however, is used in applications where purity is important. Copper, when pure, is ductile and an excellent electrical conductor, so it needs to be refined to be used in electrical wiring. Copper anodes (blister copper) are suspended in a water solution containing sulfuric acid and copper sulfate with steel cathodes. Electrolysis results in dissolution of copper from the anode and migration of copper ions to the cathode, where purified metal is deposited. The result is copper of 99.9 percent purity. A similar procedure may be used in recycling copper. Other metals that are electrorefined include aluminum. 

The electrolysis of molten salts is an important industrial process for the production of active metals such as sodium and aluminum. We have more to say about them in Chapter 23, when we discuss how ores are refined into metals. 

Zinc is an element of group 12 of the periodic table, of which the outer electrons are 3d 4s and zinc is in the same group of elements, such as Cd and Hg. Zinc exists as 70 ppm in the Earth s crust and exists in similar amounts to Cr (100 ppm) and Ni (80ppm) [1]. Zinc is produced from sphalerite (ZnS) as ores. There are two processes, a dry process and a wet process, for the refinement of zinc. However, zinc is largely produced by the wet process. The wet process is equally to be said as an electrolysis process. The sphalerite contains lead, iron cadmium and copper, etc. besides zinc and, by a flotation separation of the ores, the zinc concentrate (50-55 % Zn) and the lead concentrate are separated. The zinc concentrate is burnt and put into a vessel and dissolved in an electrolysis foul solution. The zinc sulfate solution and concentrated mud (the common name is red mud) are separated by a filter. Cadmium and copper, etc., as impurities dissolve in the zinc sulfate solution. Then zinc dust is added to the solution to precipitate these impurities to form a clean solution of zinc sulfate to be electrolyzed. In the electrolysis, lead containing 1 % of silver is used as a cathode and aluminum is used as an anode, and zinc is produced on the cathode [2—4]. The properties of zinc are shown in Table 5.1. 

---