CHEMISTRY AS APPLIED TO THE OIL FLOTATION OF COPPER ORES* ESTHER LAINE. BISBEEHIGH SCHOOL, BISBEE, ARIZONA Developing steadily through the centuries, the copper industry has changed from a crude practice into a complicated system of interwoven processes. Agricola, with his descriptions of crude metallurgical processes, could not vision upon what a stupendous scale our modern copper industry would be balanced. The force that has created this wonderful transformation is chemistry, the backbone of industrv. Simole charcoal effected the first chemical change, causing the reduction of copper oxide:
From this modest beginning, the copper industry has become so complex and so important that we are fairly amazed at its extent. In our own United States, untold wealth has been made by the indispensable aid rendered the copper industry by chemistry. When the great copper camps in the West were established, only oxidized ores were smelted, while rich suhide ores were left untouched. With the invention of pyritic smelting, however, these great sulfide deposits became valuable. In countless other ways, chemical science has improved the copper industry; she has made so many improvements in its production that today copper is the only raw material of importance that sells below pre-war prices. The story of copper is characterized by the constant presence of chemistry. From youth t o age, copper is fostered by the chemist. Copper's metamorphosis from a peeuish tinted ore into a beautiful reddish colored metal is the result of his work. Chemistry has always solved the puzzles of industry. Yet, twenty years ago, it looked as if even chemistry could not combat a thing that seemed inevitablethe exhaustion of the copper supplies. At that time
* Prize-winning high-school essay, 1927-28.
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it was thought that the copper supplies, the consumption of which was increasing sixty per cent each decade, would be exhausted within sixty years. Prospectors were no longer finding rich ore bodies. Those already found were, by the unceasing operations upon them, fast being exhausted. Yet there were huge porphyry mountains containing low-grade copper deposits. But these were worthless to metallurgists. No smelter could profitably smelt ore of such low grade. If this ore were to be treated in a smelter, the minute particles of copper sulfide which spotted the porphyry ore here and there would have to be put into a more suitable and concentrated form. Despite this perplexing situation, chemistry found a solution. This time the talisman i t offered was the oil flotation process. Let us take the Sacramento Hill project of the Phelps Dodge Corporation a t Bisbee,Arizona,as one example of the many projects made possible by oil flotation. Before the discovery of this process, Sacramento Hill, although known to contain low-grade ore averaging 1.60% copper, was considered worthless. Now, however, Sacramento Hill is being mined in full force; and already it has dwindled into a deep pit. About the same amount of material has been removed from i t as that removed from the famous Culebra cut of the Panama Canal. The chemist not only made possible the profitable mining of this hill, but he also installed efficiency and economy in the mining itself. After the ore has been steam-shoveled into the ore cars, an employee prods off a sample. This sample is hurriedly taken to the chemist whose analysis determines the route to he taken by the ore cars. This analysis is very important, for each grade of ore requires a speaal treatment. If the sample shows 0.3% copper or below, the ore is sent to the waste dump; if 0.3 to 0.55%, to the number two leaching heap; if 0.55 to 0.8%, to the number one leaching heap; if 0.8 to 6.0%, to the concentrator; and if above 6.0%, to the smelter. Chemistry allows no waste of time. This analysis is quickly made; and before the ore-laden cars have reached the junction a t which the railroad divides, the chemist has telephoned the train dispatcher who, by the information given him, switches the cars to the proper track. This may lead to the waste dump, the leaching heaps, the concentrator, or the smelter. One of these destinations, the concentrator, is a great and successful experiment of the chemist. This concentrator is built upon the oil flotation principle and may he taken as an example. The ore arrives a t the concentrator in fifty-ton side-dump cars. Each car is in turn rolled up beside an eight-inch grizzly. A whistle blows; a rod of the car disconnects; and with a deafening roar, the grayish colored ore tumbles over the grizzly. This marks the beginning of many processes which reduce the ore from boulders five and six feet in diameter to particles small enough to pass a 200-mesh screen, the fineness required by oil flotation. To accomplish
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this reduction, a series of crushers, rod mills, and classifiers are used. Fist, a succession of relentless crushers reduces the ore into pieces of ll/nin. maximum. Then, the ore, to which lime and water have been added, enters rod mills. These are strongly built, barrel-shaped, steel rotators containing tons of steel rods. The rotation of the mill causes the steel rods to fall upon each other, thus grinding the ore into minute particles. For efficient operation, classifiers work in closed circuit with the rod mills. They prevent wasteful grinding of the undersized particles which they send directly to the pulp distributing box while sending the coarser sands back to the rod mills for further grinding. From the distributing box, the pulp flows into the flotation machine. Here the true romance of the concentrator begins. Flotation is a process which can truly be said to have caused more heart palpitation than any other chemical or metallurgical wonder of the twentieth century. Litigations involving millions have been fought over flotation patent rights. Yet, to Mrs. Carrie Everson, who reaped no reward for the discovery that has earned millions for others, goes the credit for the true achievement. Through her work with her husband, she became interested in chemistry and metallurgy. By experimentation, she discovered the possibility of flotation; and in 1886, she took out a patent for the process. Flotation is b a ~ e don a strange phenomenon. The heavier mineral floats; the lighter gangue sinks. This paradox is caused by the judicious use of chemicals or reagents aided by air bubbles. Oils, by some strange attraction, coat the minerals, leaving untouched the gangue matter. But, as the chemist desires only the copper mineral and not the iron pyrite to be oiled, he adds a flotation controlling agent which causes the preferential filming of the copper mineral. Air bubbles introduced into the pulp show an affinity for the oiled, finely ground, copper particles which they elevate to the surface of the pulp. The undesirable iron pyrite and gangue matter are depressed to form the tailings; while millions of little bubbles bespeckled with copper mineral float off as a concentrate. This wonderful method of recovering the copper mineral was made possible only by the patient experimentation of chemists who discovered the process and who tested out and found the reagents which would bring about the desired flotation. In this mill, pine oil and potassium xanthate (KS&H,O) are used as the flotation and flotation-controlling agents, respectively. The lime added a t the rod mill serves as a reagent preventing any detrimental soluble matter from forming in the pulp. The flotation machine is a long, oblong apparatus. The froth, carrying with it the valuable mineral, overflows on the sides of the machine. The first twelve to sixteen feet of the length of the machine holds a rich concentrate in the form of dark greenish colored froth. This is the finished concentrate; while the less valuable overAow of the remaining section is
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pumped back into the flotation distributing box for redistribution. The flotation machine has two products, concentrates and tailings. The tailings, which contain only 0.22'30 copper, are sent to the tailings dams, while the concentrates are classified into two products: slime concentrate and sand concentrate. To remove moisture, dewatering tanks take care of the sand concentrate while thickeners and filters dry the slime concentrate. After this treatment, the concentrates enter waiting cars. These carry their contents to the smelter-there to undergo numerous chemical changes and issue forth as "blister" copper, ready for the final refinement. Oil flotation has concentrated the copper from 0.26 tons of ore into one ton. From this synopsis of the concentrator, we see how a complex assemblage of machines, dependent upon flotation, has come into existence through the work of the chemist. But his work did not stop here; i t encompasses more. He is, in fact, the master of the mill, because the concentrator is a chemically controlled apparatus. All stages of grinding and the operation of the machines are controlled by knowledge gained from chemical analysis of the products. Head, tail, and concentrate samples are taken; and from the results of the assays of these, the superintendent knows exactly what extraction of copper has taken place. This shows the condition of the apparatus. A high percentage of copper in the tailings indicates the need of a change. Thus, if repairs are needed, the chemist will warn the mill man; if the process is unsatisfactory and a better method could be substituted, the chemist will submit his recommendations which will offset the trouble. The flotation apparatus itself is the one upon which the scrutinizing eye of the chemist has been especially placed. The mill originally used the Callow flotation machine. However, the deficiencies of the machine caused poor recoveries which were detected by the chemist. The Forrester type was substituted. Even now, the Macintosh flotation machine is being tried. In the great 5000-ton concentrator of the Phelps Dodge Corporation a t Bisbee, three sampling men, three assayers, and one or two testing engineers are employed while only six men operate the entire mill. These numbers show the importance of the chemist in the control of the mill. When new reagents are tested as to their applicability to the ores, the chemists experiment in certain sections of the mill and there determine whether or not they are suitable. In these experiments, the chemist found that potassium xanthate could he used as a selective flotation reagent. Before this discovery, the use of gravity tables was necessary to further recover the copper lost in the tailings of the flotation machine. But as a result of the use of potassium xanthate, these tables which occupied an entire floor of the mill have been eliminated. This change in the flow sheet of
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the mill has meant the elimination of all repair work on the tables and has effected a saving in the power cost of the mill. Moreover, where formerly eleven men were required with the use of gravity tables, only six operators are now needed. Thus, by selective flotation, the chemist has advanced another step toward his goal, the perfection of every phase of flotation. However, one problem now confronts him. There are certain obstinate copper compounds which do not yield readily to flotation. Yet, with chemists delving into the mysteries of reagents, these compounds cannot long offer a barrier against the overwhelming powers of flotation. Some day this copper will be recovered from the enormous tailings dumps. Although there are innumerable organic substances, comparatively few have been tested out as flotation reagents. Preferential flotation offers vast possibilities. With the constant finding of new reagents, the chemist is called upon again. Upon him rests the responsibility of determining the exact amount of reagents needed for each ton of ore. Although an error would be negligible for one ton, think of the waste it would involve if applied to the fortyone million tons of copper ore treated by flotation in the United States in 1925! Furthermore, an excessive amount of reagents adds to the floatability of the worthless gangue while an insufficient amount neglects some of the valuable mineral content. This determination is a very delicate responsibility ably shouldered by the chemist. Thus, flotation is seen to involve a vast field of activities. Moreover, it has stimulated another great enterprise, leaching. With oil flotation taking out one grade of porphyry ore, the profitable treatment of still lower grade ore was made possible by the chemist's leaching process. As this process involves a slow gradual evolution of the ore into copper, its use would be unprofitable were it not that oil flotation made possible the extraction of the slightly richer ore. But now, with the mining of the porphyry ores, millions of tons of the less blessed ore are being sent to the leaching heaps. Here, through an enormous plateau-shaped mass of ore, water is spread; and in trickling down through this ore body, it leaches out the valuable copper content in the form of a sulfate solution. And this, carried by a miniature river flowing nearby, is pumped into a system of tanks filled with the waste iron of the city. Copper, by a chemical reaction, replaces the worthless iron and thus is recovered. The chemical reaction is represented by the equation: Fe
+ CuSOc = FeSOl + Cu.
A million pounds of copper a month is extracted a t a very low cost from ore which had long lain in the deep recesses of a mountain. Leaching, the beneficiary of oil flotation, is a product of the chemist. Oil flotation, the worker of wonders, accounts for more than three-
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fourths of the present copper production. Every American should have a special pride in this development because Americans designed the great concentrators in this country and in South America. An American firm has just moved its offices from Arizona t o London to design and build concentrators and smelters in Russia, Africa, and India. Thus, American genius and capital, aided by chemistry, go to foreign countries as ambassadors of good will and dominate the copper industry throughout the world. I n place of forts on foreign soil, America's monuments shall be structures of industry not to curse but t o bless the world.
Bibliography 1. Personal interviews with Mr. Charles G. Wood, chemist and teacher, and the Messn. Papin, Einneking, and Fugate of the Copper Queen Concentrator. 2. "The Story of Copper," Watson Davis. 3. "Wonders Worked by Selective Flotation," Gail Martin, Compressed Air Magazine. June, 1927. 4. "Consumption of Reagents Used in Flotation, 1925," Thomas Valrey. 5 . Mining and Metallurgy, December. 1927. 6. Flow Sheet of the Copper Queen Concentrator, November. 1927. 7. "Present Status of Differential Flotation," A. W. Fahrenwald.
