Electrolysis metal purification

Potassium removal is required because the presence of potassium during electrolysis reportedly promotes the formation of the a-Mn02 phase which is nonbattery active. Neutralization is continued to a pH of approximately 4.5, which results in the precipitation of additional trace elements and, along with the ore gangue, can be removed by filtration. Pinal purification of the electrolyte Hquor by the addition of sulfide salts results in the precipitation of all nonmanganese transition metals. 

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

Preparation. The simplest method of preparation is a combination of the elements at a suitable temperature, usually ia the range of 1100—2000°C. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides usiag aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor-phase reaction or electrolysis. 

Other industrial applications of electrolysis include extraction/purification of metals from ores, electroplating, and the manufacture of certain chemicals such as sodium hydroxide. In the latter, sodium chloride solution when electrolysed is converted to sodium hydroxide to produce chlorine at the anode and hydrogen at the cathode. Both of these gaseous by-products are collected for industrial use chlorine is used in the production of bleach and PVC hydrogen is used as a fuel, to saturate fats, and to make ammonia. 

Production of A1 metal involves two stages (a) the extraction, purification and dehydration of bauxite, and (b) the electrolysis of AI2O3 dissolved in molten cryolite Na3AlF6. Bauxite is now almost universally treated by the Bayer process this involves dissolution in aqueous NaOH, separation from insoluble impurities (red muds), partial precipitation of the trihydrate... 

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. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

Electrorefining purification of a metal using electrolysis Electrostatic charge at rest Element see Chemical Element Elementary Particle collectively the smallest units of matter, electrons, protons, quarks, and so on. 

Electrolytic purification of metals is considered at length in Chapter 17. In essence, metals can be deposited in high purity from solution on a cathodic surface, by careful control of the voltage and other parameters. The anode can be a billet of the impure metal, and the impurities will either stay in solution or form an insoluble anode slime here, both dissolution and reprecipitation of the desired metal are accomplished in a single electrolytic step. Alternatively, a crude solution of the metal ion might be prepared by some other means, and the pure metal deposited on a cathode with an anode of some inert material the product of electrolysis at the anode will normally be oxygen gas. 

Inasmuch.as che usual method of purif ication of water by distillation is expensive, it was proposed that the impurities be removed by electrolysis. For this, water is placed in a cell, divided by means of porous diaphragms into three compartments, a large one in the middle and two small ones on either side. Each outer compartment contains an electrode, connected to terminals of DC current. When the current is switched on, the electrolyte substances which are dissolved in the water, decompose, the positively charged metallic ions (such.as Ca,... 

It should be noted that in most cases wastestreams are composed not only of process chemistry but also of by-products of chemical reactions and electrolysis. This fact becomes important when attempting to recover and return those escaped" solutions (e.g., dilute metal-bearing rinse streams) which are often the focus of process and operation modification. It is usually not sufficient to stop generating waste simply by returning it to its source. Usually some type of purification or separation, themselves sources of waste generation, will eventually be required. This quickly puts the lie to the myth of "closed-loop" operations. While source reduction is powerful, it is only reduction, not elimination. 

Electrolysis is used in the manufacture of many important chemicals and in numerous processes for purification and electroplating of metals. Let s look at some examples. 

Electroplating movie The purification of a metal by means of electrolysis is called electrorefining. For... 

When metals are produced by any method, the extent to which they must be purified depends on the applications intended. For many purposes, the crude metals containing appreciable quantities of impurities may be employed for other applications, extensive refining may be necessary. Since this problem involves so many variables, it seems sufficient merely to indicate the general character of the refining processes most commonly utilized. Purification by electrolysis has already been discussed, and this is unquestionably one of the most important and useful methods. Some metals, notably iron and lead, may be purified by oxidation of the impurities by gaseous oxygen (from air), followed by the removal of the oxidized impurities. Still other metals, such as mercury and zinc, are sufficiently volatile that they may be purified by distillation. 

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