The STORY of ZINC.
H. R. HANLEY t School of Mines and Metallurgy, University of Missouri, Rolla, Missouri
This instalment, the third i n "The Story of Zinc," embraces the electrolytic method of production. A history of the process is given and reasons are presented for the long delay i n commercial afiplicatwn after the process was k n o w . Fundamental concepts as well as operating characteristics are described embracing leaching, settling, filtration, purification, and deposition.
++++++ THE ELECTROLYTIC PROCESS
Commercial methods of purification were slow of development even though the chemistry of the act was known for a long time. The earlier processes did not have dependable methods for the purification of the solution. There are many other factors of vital importance, but they are secondary to those of purification, which if not effective will cause a defeat of the process. The cost of power is one of the largest single items in the economics of operation. There have been revolutionary economies developed in the burning of coal in
HE fmt published account of the experimental work that produced cathodic zinc was presented in 1881 by M. Letrange, of France. He stated that to operate the process, the ore must be roasted, then leached with dilute sulfuric acid. The primary solution thus obtained was purified and then subjected to electrolysis which produced metallic zinc a t the cathode and sulfuric acid a t the anode. The cell-acid thus formed was used for leaching a fresh lot of roasted ore. These sequences embrace what is known as a cyclic process. Following initial effort, other men contributed materially to the process: Nahnsen, CowperColes, Ashcraft, Lasczynski, and Siemens-Halske. The electrolytic process of the present day comprises the principles known to these men. The commercial process was developed in 191416 and produced zinc on a commercial scale a year or two later. The long interval between the inception of the principles and the commercial development was due to the following causes: the inability of ore-dressing methods to produce a high-grade major mineral concentrate; the unsatisfactory status of the art of economically separating slime from solution; the ab- WATER sence of dependable methods of solutiop purification; and the high cost of electric power. A high-grade zinc concentrate will contain a low percentage of iron. Inasmuch as this metal oxide comb'mes with zinc oxide to form an insoluble ferrite during roasting, the lower the iron in the concentrate, the higher will be the solubility of the zinc in roasted concentrates. Furthermore, the weight of the residue resulting from leaching will be low if the roasted concentrate contains a high percentage of zinc. The low weight of the residue permits more effective washing of the entrained solution from it without causing undue dilution of the solution. The mechanizing of settling and thickening made vacuum filtration feasible. The solution contains approximately 100 g. of zinc per liter; consequently effective washing of the filter cake becomes necessary if high losses of zinc are to be avoided. * Part I appeared in the October issue; Part I1 in November. t Mining and metallurgical engineer, professor of metallurgy.
E R F
J. COML'YPURE SOL.
J. ZINCDUSTPPT. Cd. Cu, Zn, etc.
powdered form and this has materially lowered the weight of the coal burned per kilowatt hour developed. Power can be generated from coal and profitably sold in large blocks a t 4 to 4l/%mills per Kw.-hr a t 100% load factor when coal costing $2.00 per ton is used. Dependence on hydroelectric power, therefore, is no longer necessary. The four causes that contributed to the elapse of time between the inception of the hypothetical process and the development of the commercial production of electrolytic zinc embrace only the foundations of the process. The perfect coordination of all of these contributing causes became necessary for the successful operation, and accordingly became the governing factor in the process. The flow of materials through an electrolytic zinc plant is shown in Figure 8. Zinc sulfide concentrate in a finely divided state (flotation concentrate) is delivered to a roasting furnace where this concentrate is oxidized a t approximately 750°C. The roasted ore, termed calcine for brevity, contains zinc, principally in the form of an oxide and to a small extent as sulfate.
Manganese dioxide is added to the contents of this tank to oxidize the iron to the ferric state while the solution is still acid. The acid solution is gradually neutralized with additional calcine or in some cases powdered limestone. It is essential that a certain amount of iron be present in solution before complete neutralization. This iron is to effect the complete absorption of arsenic and antimony as it hydrolyzesan action that occurs upon complete neutralization. Insoluble ferric arsenate and antimonate are the compounds formed a t neutrality but it is impossible to precipitate these metals when present in low concentrations unless there is a very large excess of soluble iron present. Otherwise true chemical compounds are formed with the equivalent weight of soluble iron. The neutral pulp is next delivered to a Dorr thickener in which a continuous separation of the liquid and solids is made. The thickened slime is delivered to a vacuum type filter, such as the Moore, Oliver, or American, where the solids are further separated from the solution. Effective washing of the filter cake is essential. If the latter does not become cracked in the
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FIGURE 9.-FLOW-SHEET, GREATFALLS, MONTANA, 150-TON ZINC PLANT(AUGUST, 1919) The electrolytic zinc cell produces the final product, cathode zinc, and the product &st used in the process, sulfuric acid. The acid solution, containing 25 to 30 g. Zn and 100 g. sulfuric 'acid per liter, is delivered to the leaching tank where it is agitated with the calcine.
filter operation, the Moore vacuum leaf may be employed with effect. The Oliver or American are continuously revolving filters with a limited time for washing, but the economy of operation permits the discharge, re-pulping with water or weak solution and a
second filtration. The washed residue which in weight might be 40% to 50% of the calcine, may contain such metals as lead, gold, silver, and copper in addition to the insoluble zinc and are usually shipped to a lead smelter for reduction. The filtrate joins'the clear overflow from the Dorr iil-
ter and is delivered to a purification tank)o which zinc dust is added. The addition of zinc dust causes the precipitation of copper and cadmium when properly effected. The zinc dust precipitate containimg metallic copper and cadmium is delivered to the cadmium department of the plant where i t is treated for the production of this metal. The solution from the zmc dust treatment is settled and passed through a hardwood plate and frame filter press and then to a storage tank which supplies the electrolytic cells. The flow-sheet (Figure 9) gives a general idea of the operations without reference to important problems that have to be met in various departments or to the economic side of the operations. The Anaconda Copper Miming Co. (Zfi), and the Consolidated Mining Co. of Canada use the continuous system of leaching and purification, a system which is more economical, especially for large plants. The general scheme involved for successful continuous leaching is embraced in the following description. There are two leaching circuits, one acid and one "neutral."
All of the calcine is added to the neutral circuit a t a time when the iron is completely oxidized to the ferric state. The pulp suspension is agitated by means of air in Pachuca tanks (see Figure 10) placed in series. Agitation is carried on for a sufficient length of time to develop potential coagulation or granulation of the substances possessing a colloidal nature. The end Pachuca in the series delivers the pulp suspension to suitable classifiers to separate the sandy or crusted particles from the slime. The slime containing a large excess of unextracted zinc oxide is then delivered to the neutral Dorr thickener (see Figure 11) circuit where a continuous separation of primary solution from the slime goes on. The sand particles are ground, again classified, and delivered to the same circuit. The solution is commerciaUy pure with respect to iron, arsenic, and antimony but contains copper and cadmium as impurities. The slime is now delivered to the acid circuit where i t is selectively leached in order to dissolve the zinc oxide without dissolving, except to a slight extent, the precipitated impurities occasioned in the neutral circuit. After customary thickening and filtration of the acid-treated slims, the slightly acid solution is delivered to the neutral leaching circuit where i t receives conditioning and the calcine as previously described. The primary solution from the neutral Dorr circuit then flows to the zinc dust purification agitators where the copper and cadmium are prEcipitated. The agitation provided in these tanks is effected by mechanical means because the efficiency of this precipitation is not so high if air agitation is used. The precipitation is accompIished in a selective manner; that is, by removing the precipitate before complete removal of the copper and cadmium from the solution. The metals in this precipitate will he a t a higher concentration and consequently lower in zinc dust than would be the case if complete removal of the Impurities were made a t this time. The solution, separated from the rich precipitate, now receives a large excess of zinc dust to precipitate the balance of the impurities. This precipitate containing essentially unused zinc dust is used for the first precipitation of the primary solution, while the
Sulfuric acid is lost in the system to a small degree through the solution of substances other than zinc. Loss is sustained in the form of zinc sulfate in the filter DETAILS O F ROASTING cake and also in spills, overflows, etc. These losses Zinc oxide is completely soluble in dilute sulfuric are in most cases compensated by developing an equivaacid but zinc oxide in calcine is never 100% soluble. lent weight of zinc sulfate iu the roaster. This is acThis is due to the formation of compounds of ZnO and complished a t temperatures not exceeding 750°C., and other metallic oxides which are insoluble in weak acid. with a large excess of air in the presence of the natural These compounds, termed ferrites, consist most fre- catalytic substances present in the concentrate. All quently of iron and zinc oxides which may be repre- zinc plants endeavor to operate with a balanced acid sented by the formula: ZnO.Fe~08. One part by system to avoid the purchase of further amounts of sulweight of Fe comb'mes with 0.584 part of Zn. Consid- furic acid. This acid cannot be shipped economically erable experimental work has been done on the influence for long distances because of high freight rates. of iron on zinc insolubility during roasting (27). The ARSENIC AND ANTIMONY REMOVAL greatest detrimental influence of iron in this respect The precipitation of arsenic and antimony in the has been found when zinc sulfide is oxidizing in the presence of ferric oxide. The roasting of the mixture of leaching tank is the most important purification step ZnS and F e s a t 625'C. for eight hours renders only a in the process. If these substances are not removed, nominal proportion of zinc insoluble but increases with electrolytic deposition of Zn is made impossible further additions of FeS, and also with increased tem- due to corrosion of the deposits. When these metals perature. Marmatites, on the other hand, composed are present in extremely small amounts, the ratios of of compounds of Zn and iron sulfide (e. g., 3ZnS.FeS; Fe/Sb and Fe/As in solution become very great. Mr. 5ZnS.FeS, etc.) behave differently. AU the iron in this C. A. Hansen (28) has determined this ratio for antisulfide compound is found to be combined as ferrite mony. It is shown in Table 5, together with figures (Zn0.Fe20s) in the roasted product regardless of the for arsenic by Biltz. TABLE 5 temperature employed in roasting. Accordingly, - . -pure m-atite having the compositi& of 5ZnS.FeS, when HANSEN B%=Z g. of Sb/l. Rnlio FeISb g. of Asil. Rnlio &/As roasted a t any temperature (600' or 900°C.) will pron olnn 1 9 n 0 1 ~ 2 7 duce a calcine in which the zinc is only 90% soluble 0.0050 5.7 0.0050 4.a 5.1 8.5 0.0020 0.0020 in sulfuric acid solution containing 60 g. HeSonper liter. 0,0010 13.0 00010 5.8 0.0005 6.1 Similarly, the roasting of this mineral containing 3ZnS 0.0005 21.4 0.0001 43.2 0.0001 9.2 FeS will produce a calcine in which the Zn is only 83% Commercial electrolysis of zinc sulfate solution in soluble. Although the operator bas no control over the solu- the presence of cobalt or nickel in concentrations of 200 bilitv of the zinc in the calcine produced from roasting. and 50 mg./l., respectively, is uneconomical when the pure marmatites, he does have control over concentrates electrolyte is high in acidity. The large plants have containing free iron sulfide merely as an admixture. had very little trouble due to nickel because i t is seldom In the latter case the solubility will vary approximately present in the zinc ores in sufficient quantities to cause inversely with the temperature maintained. Table 4 toxic concentrations by accumulation in the electrolyte. shows the relative effects of roasting molecular ratios Cobalt, however, is frequently'present and has caused of zinc and iron compounds in the free state and also in more difficulty than any other impurity, with the excepthe combined state (marmatite, 5ZfiS.FeS) a t 625°C.. tion of germanium (29). It may be removed by a t least three methods, namely: [email protected]
; zinc for eight hours. dust in the presence of copper and sodium arsenate a t TABLE 4 80°C. (30); and zinc. dust in the presence of a minute Sddonccs Ramlad ZnO Sdublc in N a C I ond NFLOH amount of soluble tellurium. Nickel may also be reZnS + 2PeS 91% total Zn 85.5%total Zm Z n 0 + 2FeSt moved by the last method: 34 5570 total Zn ZnS + Pel01 5ZnS.FeS (msrmatife) 90% total zn ELECTROLYTIC CELLS 10 g. b l a d e 58.23%Zn 97.1%(cslcine = 4.66%Pe) 1g. pyrite 44.40%Fa A commercial cell consists of a rectangular tank, There are iduences present in commercial roasting lined with lead or sulfur-sand cement, in which are which are not in evidence on a laboratory scale of opera- suspended lead or lead-alloy anodes and aluminum tion; nevertheless the trend of results should be simi- cathodes. Ralston has developed zinc starting sheets similar to those in copper refining practice. A current lar. Zinc compounds insoluble in weak acid are to an density of 25 to 100 amperes per sq. ft. is applied a t a extent soluble in acid of greater strength a t 80°C. potential of 3.3 to 3.8 volts, depending upon the acidity, Consequently i t is a common pracbce in electrolytic distance between the electrodes, temperature, etc. zinc plants to re-leach the primary leached residue con- The zinc sulfate solution is decomposed, forming zinc taining approximately to 14% insoluble zinc with a t the cathode and SO; a t the anode. A secondary acid of greater strength a t 80°C., for several hours. reaction then occurs in which the SO4=' splits up, forming solution, now commercially pure, flows to the electrolytic cell circuit.
sulfuric acid and oxygen. The latter is continuously discharged from the anode in fine bubbles, which cany small amounts of the electrolyte into the atmosphere in the form of an attenuated mist. The cathode zinc, made from commercially pure solutions, is very resistant to corrosion by the acid electrolyte. There is, however, a low rate of corrosion taking place, resulting in the formation of hydrogen bubbles which are discharged into the atmosphere. These bubbles also take a slight amount of solution with them in the form of mist. The presence of impurities in the solution increases the corrosion of the zinc and hence the mist. A current of from 7000 to 10,000 amperes is applied to the cells in series a t a potential of approximately 3.5 volts per cell. With an assumed current density of 35 amperes per sq. ft., a current efficiency of 90%, with cathodes, the dimensions of which are 2' X 3'/%', and allowing 8340 amperes per cell, the following factors represent quantities per cell: 8340/35 = 238 sq. ft. cathode surface
238/14 = 17 cathodes
Assume the spacing to be 4" between electrodes. The 18 anodes will require 112" in the cell and allowing 12" for a cooling coil, the inside length would be about 10.5'. The width of the cell would be about 28". The 8340 amperes will deposit 20.18 lb. of cathode zinc per hour a t 90% current efficiency. This will result in the deposition upon the cathodes of 483 lb. per day of 24 hours and each cathode will gain 28.4 1b. in that period of time. The deposits become rougher with increasing age and the current efficiency falls with an increase in roughness. Hence the stripping is usually limited to a period of operation that will give a current efficiency of 90% and this is approximately 24 hours. Alteration in the weight of deposits may be made by increasing or lowering the current density. The acidity of 100 to 125 g. H2S04per liter is common in the cell electrolyte, which also contains 25 to 30 g. Zn per liter. The commercial electrodeposition, of zinc may be accomphshed under a wide variation of conditions with the exception of purity of solution; hereqhe limits of concentration are narrow with respect to some metals and approach zero with others. Good electrolytic deposition is contingent upon the effectiveness of the purification, particularly for such metals as arsenic, antimony, cobalt, nickel, and germanium. Practically all the other common metals are easily removed and those which are not are harmless to the electrolytic cell. All zinc sulfide ores contain manganese in small quantities. This element, to some extent, is changed to the sulfate during roasting and hence dissolves with the zinc in the leaching tank. In the electrolytic cell this sulfate is oxidized by the anodic oxygen to manganate or permanganate, and hydrated manganese dioxide. This deposition, however, is made with very low efficiency of the anodic oxygen. Under plant conditions the efficiency is approximately 1% (31). Rolled lead anodes increases the efficiencyof the oxygen, espe-
cially when the plates are clean. The physical condition of the surface of lead anodes alters the efficiency markedly. Lead containing 4% thallium occasions a greater precipitation of MnOl than pure lead. There are no plants operating on solutions free from manganese; therefore the effect of the absence of this salt on the products of electrolysis, must be determined on solutions espec~allytreated for its removal. This is done by the introduction of potassium permanganate into the neutral solution together with sufficient base to inhibit the formation of free sulfuric acid. The entire subject of the effect of manganese on the products of electrolysis is related to its concentration in the solution. This thought suggests that it may be tolerated until its concentration becomes adverse to efficient operation. In a solution that has been especially purified to a degree beyond that permitted in commercial work, manganese in rather high concentrations has hut little effect on the cathode. In a solution that has been purified commercially, the combmed effect of the residual impurities with high manganese concentration lowers the current efficiency. Any substance or condition which lowers this factor cannot he tolerated in practice. There is a reasonable latitude in the factors of electrolysis that will produce commercial deposits. It is very important that this latitude prevail because irregularities to a certain degree are always present in the operation of a large number of cells. Beyond this latitude, an unstable condition results which approaches and may reach a condition of dislocation of cell equilibrium. Electrodeposition of zinc in acid electrolyte is a race between corrosion and deposition of the cathode zinc. When conditions approaching dislocation.,occur, corrosion is increasing, the temperature is ,rising, which further increases the corrosion; finally this influence predominates and deposition ceases. The endeavor to maintain high current efficiency is based partly on the lower power cost resulting therefrom and partly on the lower labor cost of operation by virtue of the cell-room conditions being in balance. Dislocation not only increases the labor cost but directly lowers production and thereby increases the general expense or overhead per pound of zinc. In general, the prevention of a high concentration of manganese is necessary not because of its specific toxicity but because this concentration narrows the latitude of safe operation. Specifically, manganese in high concentration may be decidedly objectionable when some particular element is present with which i t joins to increase or accelerate the forces of corrosion. Manganese and cobalt furnish an example. Manganese in solution does not lower the potential of the cell; on the contrary i t raises it if it is present in sufficientconcentration. The following is the explanation: the lowering of the potential due to depolarization is insignificant while the increased drop in the electrolyte due to an increase of the ohmic resistance is the significant factor. The net result is to increase the terminal voltage.
POLARIZATION Zinc has a greater solution pressure than hydrogen as is represented by values given in the electromotive series, namely: H = 0.0, Zn = -0.76. Hence i t should be more difficult to deposit zinc from its solution than it is to deposit hydrogen from solutions containing hydrogen ions. However there is another force a t work; this is overvoltage. The lowest voltage required to liberate hydrogen on a platinum black electrode is taken as zero. On this scale it requires 0.7, 0.8, or 0.9 of a volt to liberate hydrogen on zinc. Because these values are equal to or exceed the single potential, i t is possible to deposit zinc from sulfate solution containing a high concentration of hydrogen ions. It is necessary to maintain a high hydrogen overvoltage during electrolysis. If the overvoltage falls in value, hydrogen is released and the deposit dissolves. High current density, smooth surfaces, high purity of solution, short periods of deposition and a low temperature of the electrolyte raise the overvoltage and thereby widen the range for successful cell operation. A satisfactory current density applicable to electrolytes containing 90 to 120 g. HzS04per liter is found in the range from 25 to 35 amperes per sq. ft. ADDITION AGENTS
Certain substances are added to zinc electrolytes to increase the smoothness of the de~osit. The analysis
of the cell potential shows that the cathode is increased after the addition agent has been introduced. The effect of colloids is closely associated with conditions a t or near the surface of the cathode. Blum and Rawdon (32) have presented a hypothesis that is useful in the study of the mechanism of the effects of colloids in electrolytes. Blum, in a colloid symposium monograph, states that to understand fully any electrolytic process, it is necessary to know what is taking place in: (1) the metal cathode itself, (2) the cathode film of the solution adjacent fo it, (3) the actual interface between the metal and solution. It is here that c our knowledge is lacking. A definite basis for considering the colloidal effects was made on the theory of Blum and Rowden. According to their hypothesis the union of one or more electrons with a positively charged ion, takes place on the cathode surface. at a ~ a i n tdetermined bath by the orientation of the -metal atoms on the cathode surface and hy the effective cancentration (i. e., activity) of the metal ions in the cathode a m . The latter concentration is approximately measured by the dynamic potential of the cathode. When the ion concentration is high. the cathode polarization will be low and conditions will be favorable for the growth of existing crystals. Conversely when the metal-ion concentration is low and the polarization high, the conditions are less favorable for crystal growth and more favorable for the formation of new crystals. This assumption is based on the well-known fact that small crystals of metal have a higher solution pressure and a more negative potential than large crystals and that hence a more negative potential will be required for the production of fine crystals. According to this theory, any change in conditions which increases cathode polarization must tend to make changes in the type of crystal structure in the order from fibrous to conical.
to broken and finally spongy or powdery deposits. . . . It seems reasonable therefore to assume that at least the principal effect of colloids on the crystal structure is associated with the increase in the cathode polarization which they produce
The general idea embraced in this theory is the desirability of dislocating the normal crystal growth with the consequent formation of new crystals. Many addition agents are found in the cathode deposits while others which may be used will not he found there. The colloid to be effectivemust migrate toward the cathode. Frolich (33) has suggested that it is possible for a colloid to migrate toward hut not to the cathode surface, as its charge is reversed when i t reaches the relatively alkalme film a t the cathode. The mechanism of electrodeposition has been studied by Mr. L. B. Hunt (34). His thesis brings to the foreground the proposition that the changes in our ideas on the subject of the degree of dissociation of salts in water and other solvents have not been applied to the study of electrolytic deposition. In this application, ions are not regarded as independent entities but as closely connected bodies having mutually operative forces acting on them. A given ion will be surrounded by more unlike ions. This ionic atmosphere must bear a charge equal and opposite to that on the central ion. On the application of an electromotive force, the ion and its atmosphere will be impelled in opposite directions by this current; the atmosphere continually building up in front of the ion and falling off in the rear. This retarding influence is exerted on an ion in its movement toward an electrode and the metal ion must be removed against the retarding force in order to enter the crystal lattice on deposition. According to this concept, polarization accompanying deposition is bound up with the resistance offered to.the process by the interionic forces. These forces vary with the following factors: (1) the charge on the ion, (2) the distance separating it from oppositely charged ions, (3) size of the ion, (4) its electrical arrangement. The crystal structure of an electrically deposited metal, according to this theory, will be governed by the relation of the concentration of the other constituents in that film. If the proportion of metal ions to inert particles is comparatively high, there will be little interference with crystal growth and coarse crystalline deposits will result and vice versa. The lead contamination of cathode zinc is caused by entrapment of particles 'of anode sludge containing lead dioxide. The influence of lead sulfate dissolved in the electrolyte on the lead content of the cathode was tested in a three-compartment Filtros diaphragm placed in a glass cell. Lead sulfate was introduced into the anode compartment and the cell was operated for a week. The cathode deposit contained 0.0005% lead. This lead content is so low as to warrant the conclusion that lead contamination of the cathode is not caused to an appreciable extent by the electrolytic deposition of the dissolved lead ion hut by entrapment of anode sludge particles containing lead. A Filtros diaphragm permits diffusion of dissolved salt but prevents the passage of solids.
The common addition agent employed in the electrolysis of zinc electrolytes is glue and this is used to the extent of 2 to 3 lb. per ton of cathode zinc produced. Glue (35) definitelyhas an influenceinproducing smooth cathode deposits. A smooth deposit will entrain less of the anode sludge particles and hence there will be less lead in the cathode produced in solutions to which glue has been added.
Dr. U. C. Tainton (37) and his associates have developed a process in which a current density of 100 amperes per sq. ft. is used. The roasting of marmatite ores renders part of the zinc insoluble in weak acid. This zinc is highly soluble in hot acid electrolyte containing 250 g. H2S04per liter. In order to produce this high acidity in the cell, a high current density must be ANODES . used, otherwise there would be corrosion of the cathode. In general, addition substances that are oxidized by High current density maintains a higher over-voltage anodic oxygen should cause the free residual oxygen to and by virtue of this, hydrogen is less easily evolved escape from the electrode a t a diminished pressure (36). from the cathode. Dr. Tainton has built two higbThe surface of the anode is less disturbed in the oxygen density plants-one near Kellogg, Idaho and one in discharge a t low pressure and consequently the anode East St. Louis, Illinois. There have been many probscale is not dislodged to the same degree. Many salts, lems presented in the construction and operation of particularly organic salts, were tested for this property these plants similar in character to those encountered but were found to be ineffectual. Oxalic acid and also in other electrolytic plants but in some cases more intenferrous sulfate were found to possess the property of sified. The successful operation of these plants bears lowering the oxygen potential of the anode, and also testimony to the skill and genius of Dr. Tainton. Cathode zinc is not a commercial form of the metal. minimized the dislodgment of anode sludge particles containing lead. The cathodes produced in the pres- It is melted in a reverbatory furnace fired in a manner ence of these addition agents are very low in lead. The to produce a reducing atmosphere. The cathodes are Consolidated Mining and Smelting Co. of Canada has dropped through openings in the furnace arch in sufalso done considerable work on this subject, particularly ficient amounts to cause them to be immersed to a great extent in the molten bath, thereby minimizing on ferrous sulfate. There is for practical purposes a constant temperature oxidation. Ammonium chloride, to the extent of 2 to 3 throughout the cell. The measurement of the metal lb. per ton of cathodes, is used to "unlock" particles temperature of the anode, by placing a thermometer in of metal surrounded by zinc oxide and lessen the amount a hole drilled in the top, shows the metal one-half a of drossy oxide. The recovery of bar zinc from subdegree C. higher than the adjacent electrolyte. The stantial cathode sheets amounts to 96% to 97% and sometimes more. The molten metal is not tapped from cathode does not show any temperature difference. The energy consumption in the production of electro- the furnace as is the customary practice with lead hut lytic zinc will approximate 3600 kw.-hr./ton under the dipped from an open end by means of a large ladle following conditions: current efficiency 90%; 3.5 which holds two or three 50 lb,. bhrs. The ladle is volts/cell; 2" spacing of electrodes; acidity of electro- properly balanced and supported by a chain attached lyte, 100 g. H2S04/1; 92% efficiency of conversion of to a bal-bearing crawl mounted on a monorail which A.C. to D.c., including transformer and line losses; extends parallel to the molds. After pouring, the surmechanical power being 10% of total power and the face of the zinc in the mold is skimmed with a wooden paddle to provide a smooth surface. melting efficiency being 96%.
(26) F. LAIST, F. F. F R I ~J.,0.ELTON. AND R. B. CAPLES, "Electrolytic zinc plant of Anaconda Capper Mining Co., at Great Falls, Montana." Trans. Am. Inst. Mining Met. Enpr., 64, 699 (192'3). (27) H. R. HANLEY, C. Y. CUTON,AND D. F. WAWH,"Formation of insoluble compounds during roasting," ibid., 85, 2 1 S 2 7 (1929). (28) C. A. HANSEN, "Dirussion-Puification of zinc sulphate solutions," Trans. Am. Electrochem. Soc., 44, 4.52-90 (1923). (29) U. C. TAINTON, "Germanium in relation to electrolytic zinc production." i m . , 57,279 (1930). (30) W. E. MITCHELL, "Electrolyttc cadmmm plant of Anaconda
Copper Mining Co. at Great Falls, Montana," Trans. Am. Inst. Mining Met. Engrs., 91, 239 (1930). (31) 0. C. RALSTON, "Manganese in electrolytic zinc practice." Eng. Mining J., 130, 511 (1930).
(32) WM. B L ~"Colloids , in the electra-deposition of metals," Colloid Symposium Monograph, 1923, pp. 301-12. (33) P. K. FROLICE. "The amphoteric character of gelatin and its
bearing on certain electrochemical phenomena." T a n s . Am. Elmrochem. Soc., 46, 67 (1924). (34) L. B. HUNT."The study of the structure of electro-deposited metals," J. Phys. Chene., 36, 1006 (1932). (35) J. H. ROESSER, thesis in metallurgy, Mo. School of Mines, 193031. (36) E. A. GODAT, thesis in metallurgy. Mo. School of Mines. 1931-32. (37) U. C. TAINTON AND D. BOSQUS."The electrolytic zinc plant of the Evans Wallower Zinc Co., at East St. Louis, Ill.," Trans. Am. Electrochem. Soc., 57, 241 (1933); U. C. TAINTON AND L. T. LEYSON, "Electrolytic zinc from cornplex Ores," Trans. Am. Inst. Mining Met. Enps., 70, 86 (1924).
(Part I V , the concluding instalment of this series, will appear in the Februnry irruc.)