FIFTEENTH LECTURE ELECTROCHEMISTRY LECTROCHEMISTRY is one of the most important applications of electric power, and possibly even more power is used for electrochemical work than for rail- roading. In electrochemical industries the most expensive part is electric power; material and labor are usually much less. Such industries therefore are located at water powers, where the cost of power is very low. The main classes of electrochemical work are : A. Electrol3rtic. B. Electrometallurgical. A. Ei^ECTROivYTic Work. . The chemical action of the current is used, by electrolyz- ing either solutions of salts or fused salts or compounds. Electrolysis of solutions in water is possible only with such metals which have less chemical affinity than hydrogen. For instance, Cu, Fe, and Zn can be deposited from salt solu- tions in water, but not Al, Mg, Na, etc. Electrolyzing, for instance, NaCl (salt solution) the sodium (Na) which appears at the negative terminal immediately dissociates the water and gives Na + HjO = NaOH -f H, or: sodium plus water = caustic soda plus hydrogen. It takes 1.4 volts to electrolyze water; any metal requiring more than 1.4 volts for separation therefore is not separated, but hydrogen is produced. Therefore the highest voltage used in an electrolytic cell containing water is 1.4 + the tr drop in the resistance of the 200 GENERAL LECTURES c«ll ; which latter, for reasons of economy, is made as low as possible. Even fused salts require fairly low voltage, at the highest from 3 to 4 volts. Since the voltage required per cell is very low, a large number of cells are connected in series, and even then large low voltage machines are required. Some of the important applications of electrolysis are : Blectr opiating; that is, covering with copper, nickel, silver, gold, etc. Electro typing; that is, making of copies, usually of cop- per; and especially Metal refining. A very large part of all the copper used is electrically refined. The crude copper as cast plate is used as anode or positive, and a thin plate of refined copper is used as cathode, or negative (terminal in a copper sulphate solution. The anode is dissolved by the current and the fine copper is deposited on the cathode ; while silver and gold go down into the mud, lead goes into the mud as sulphate, tin as oxide; sulphur, selenium and tellurium, arsenic and other impurities also go in the mud ; and zinc and iron remain in solution as sulphates if the current density is sufficiently low. If the current density is high, some zinc and iron may deposit : zinc and iron have a greater chemi- cal activity than copper, since they precipitate copper from solution. Therefore it takes more power, that is, more voltage, to deposit zinc and iron, than it takes to deposit copper. If the current density is low, the voltage required to deposit the copper plus the ir drop, that is, the total voltage of the cell, is less than the voltage required to deposit zinc or iron, and they do not deposit, but dissolve at the anode and remain in solution. ELECTROCHEMISTRY 20 1 At higher current density the ir drop in the cell is higher; thus the total voltage of the cell is higher, and may become high enough to deposit iron or even zinc. If the anode is crude copper, -the cathode pure copper, the voltage at the anode is higher than at the cathode and the cell takes some voltage. The voltage required for copper refining is the higher, the more impure the copper is; but is always very low, usually a fraction of a volt, and therefore very many cells are run in series. The solution gradually becomes impure and has to be replaced. Other metals are occasionally refined electrolytically, but only to a small extent. Metal Reduction. Metals are reduced from their ores electrolytically, especially such metals which have so high chemical affinity that they are not reduced by heating with carbon. In this way aluminum, magnesium, sodium, calcium, etc., are made electrcn lytically. Since their chemical affinity is greater than that of hydrogen, they cannot be deposited from solutions in water, but only from fused salts, or solutions in fused salts. So cal- cium is produced now by electrolyzing fused calcium chloride, CaCU. Aluminum is made by electrolyzing a solution of alumina in melted cryolite (sodium aluminum fluoride). Secondary Products. Frequently electrolysis is used to produce not the sub- stances which are directly deposited, but substances produced by the reaction of these deposits on the solutions. For instance, electrolyzing a solution of salt, NaCl, in water, we get sodium, Na, at the negative, chlorine, CI, at the positive terminal. 202 GENERAL LECTURES If we use mercury, Hg, as negative electrode, it dissolves the sodium and so we get sodium amalgam. Otherwise the sodium does not deposit but immediately acts upon the water and forms sodium hydrate or caustic soda, NaOH. The chlorine, CI, at the anode also reacts on the water, one chlorine atom taking up one hydrogen and another chlorine atom the remaining OH of the water H2O; that is, we get 2CI + H2O = CIH + ClOH, that is, hydrochloric + hypo- chlorous acid. With the sodium hydrate from the other cathode these acids form NaCl and ClONa, that is sodium chloride and hypochlorite, or bleaching soda. If the solution is hot, the reaction goes further and we get 6C1 + 3H2O = 5CIH +C108H, that is hydrochloric and chloric acid, and with the sodium hydrate from the other side (these form NaCl and ClOsNa, that is, sodium chloride and sodium chlorate. In this way considerable industries have developed, pro- ducing electrolytically caustic soda, bleaching soda, and chlorates. Alternating current is used very little for electrolytic work, as with organic compounds to produce oxidation and reduction at the same time; that is, act on the compound in rapid succession by oxygen and hydrogen, the one during the one, the other during the next half wave of current. Very active metals like manganese and silicon dissolve by alternating current; that is, one-half wave dissolves, but the other does not deposit again. Very inert metals like platinum are deposited by alternat- ing current; that is, the negative half wave deposits by alter- nating current, but the positive half wave does not dissolve. ELECTROCHEMISTRY 203 B, ElvECTROMETAI,I,URGICAI. WORK. In electrometallurgical work the heat is used to produce the chemical action; thus it is immaterial whether alternating or direct current is used. The voltage required is still low but not as low as in elec- trolytic work : The carborundum furnace takes from 250 to 90, mostly about 100 volts; that is, it starts cold with 250 volts. While heating up the resistance drops, and the voltage decreases down to 100 volts when the furnace is hot and remains there until towards the end. Then the inner layer of carborundum begins to change to graphite and the resistance, and therefore the voltage falls. The carbide furnace and arc furnaces in general take from 50 to 100 volts; the graphite furnace takes from 10 to 20 volts. To get very high temperatures a very large amount of energy has to be concentrated in one furnace; and with the moderate voltage used, this requires very large currents, thousands of amperes. Alternating currents are almost exclu- sively used, since it is easier to produce very large alternating currents by transformers, and since it is easier to control alter- nating than direct currents. Electric heat necessarily is very much more expensive than heat produced by burning coal, and so the electric furnace is used mainly: I St. Where very perfect control of the temperatures and freedom from impurities is essential. 2nd. Where temperatures higher than can be produced by combustion are required. I. Very accurate temperature reguHation and freedom from impurities, for instance, are important in making and 204 GENERAL LECTURES annealing high grade tool steels, etc. By using coal or oil as fuel, contamination by the gases of combustion, and by the metal taking up carbon or (if an excess of air is used, oxygen) is difficult to avoid. By electric heating, by resistance at lower temperature and by induction furnace at higher temperature, contamination can be perfectly avoided and even the air can be excluded. 2. The temperature of combustion is limited. Four-fifths of air is nitrogen which does not take part in the combustion, but which has to be heated, thus greatly lower- ing the temperature; therefore combustion in air, even if the air is preheated, gives a lower temperature than when using oxygen. But even the temperature of the oxy-hydrogen, or the oxy-acetylene flame is only just able to melt platinum. The temperature which can be reached by combustion, is limited, since at very high temperature the chemical affinity of oxygen for hydrogen and carbon ceases : water dissociates, that is, spontaneously splits up in hydrogen and oxygen at 2000 degrees Centigrade and no temperature higher than 2000° can therefore be reached b)' the oxy-hydrogen flame; carbon dioxide, CO2, already dissociates at about I500°C into carbon monoxide, CO, and oxygen, O. Carbon monoxide, CO, splits up into carbon and oxygen not much above 2000° C. (In all high temperature reactions of carbon, as in the formation of carbides, CO therefore always forms and not CO2, since CO2 cannot exist at a very high temperature ; and the CO when leaving the furnace then burns to CO2 with blue flame) . Higher temperatures than those generated by the com- bustion of carbon and hydrogen can be produced by the com- bustion of those elements whose oxides are stable at very high temperatures, as aluminum and calcium. Tn this way, many metals, as chromium and manganese, which cannot be reduced ELECTROCHEMISTRY 205 from the oxides by carbon (due to the lower temperature of carbon combustion) can be reduced by aluminum in the "ther- mite" process. That is, their oxides are mixed with powdered aluminum and then ignited : the aluminum burns in taking up the oxygen of the metal, and so produces an extremely high temperature, which melts the metal and the alumina (corun- dum) which is produced. Since, however, all the aluminum is made electrolytically, the thermite process still requires the use of electric power. The temperature of combustion of aluminum, however, is still far below that of the electric carbon arc, since in the car- bon arc, alumina boils. For temperatures above 2000° to 2500°C, and up to the arc temperature or about 3500°C, electric energy is therefore necessary. Electric furnaces are of two classes : Arc Furnaces and Resistance Furnaces. In the resistance furnace any temperature can be produced up to the point of destruction of the resistance material, that is, up to 35oo°C, when using carbon. The arc furnace gives the arc temperature of 3500°C, but allows the concentration of much more energy in a small space and thus produces reactions requiring the very highest temper- atures. Some of the electrometallurgical industries are : (a). Calcium carbide production. Arc furnaces are used and the reaction is CaO + 3C = CaC^ + CO. A mixture of coke and quick lime is used in the process. (b). Carborundum production. A resistance furnace is used, containing a carbon core about 24 feet long, around which the material is placed and heated by the current passing 2o6 GENERAL LECTURES through the core. The furnace takes looo HP and the reac- tion is : SiO^ -f 3C = SiC + 2CO. The material is a mixture of sand, coke, sawdust and salt. (c). Graphite furnace. A resistance furnace somewhat similar to the carborundum furnace is used, but with lower voltage and larger currents ; the material is coke or anthracite, which by the high temperature is converted into graphite, probably passing through an intermediate stage as a metal car- bide. (d). Silicon furnace. Either arc or resistance furnace is used ; the reaction is : SiOa + 20 = Si + 2CO. or, SiOa -f 2SiC = 3Si + 2CO. (e). Titanium carbide furnace. Arc or resistance fur- nace is used which requires a very high temperature; that is, a greater temperature than that of the calcium carbide furnace. T1O2 + 3C = TiC + 2CO. Other products of the electric furnace are siloxicon, sili- con monoxide, etc., and numerous alloys of refractory metals, mainly with iron; as of vanadium, tungsten, molybdenum, titanium, etc., which are used in steel manufacture. The use of the electric arc for the production of nitric acid and mtraite fertilizers ; of the high potential glow discharge for the production of ozone for water purification, etc., also are applications of electric power, which are of rapidly increas- ing industrial importance.