At the beginning of our investigation we seriously considered what I called the “brutal method” of leaching, namely, the manufacture of sulphuric acid, the solution of the copper from the ore with such acid, and the precipitation of the copper by metallic iron, with the resultant complete, or nearly complete, destruction of both acid and iron and the production of an impure cement copper that will have to go through the process of smelting and refining. The results of Mr. Croasdale’s tests and Mr. Wedge’s cost figures indicated that such a course was commercial, and we concluded that the 1½ per cent. Ajo ore, with no overburden, mined by steam shovel and crushed coarse, would yield a profit on a 12c. copper market if a cheap grade of Alabama pig iron were used.
But it occurred to me, as it had occurred to many engineers, that oxide of iron could be reduced to metallic iron without fusion, and if so we might bring Bisbee sulphide s to the mine, calcine them for the manufacture of sulphuric acid and then metallize the available iron in the impure calcine, already containing values requiring recovery, and use this for a precipitant. I found at the start that metallic iron was being produced by this method with an inferior fuel from high-grade iron ore by an intermittent process; that is, by first heating to the desired temperature by internal combustion of a part of the excess coal in the charge and then allowing the heated charge to stand without the admission of air, but allowing the gases formed to escape freely.
After consultation with friends, we concluded to try the experiment of a multi-hearth furnace, and at first my idea was that possibly calcining could be done on the upper hearths and the hot calcine could then pass to the lower hearths, where it could be further heated and submitted to a reducing action. We erected a small Wedge furnace, but only tried to metallize, and not to calcine in it. Some of the iron oxide was reduced to metallic iron, but on the whole our experiment was a failure because in keeping the charge hot the percentage of carbonic acid gas produced was sufficient to retard and reverse the action of carbon monoxide in reduction. In parallel with this, we also made experiments in revolving cylinders, aiming to reduce the iron to the metallic form with illuminating gas and with fuel oil by a continuous process. These also were failures.
Mr. Laist conducted similar experiments on a continuous process for making sponge iron at Anaconda, and ended, with a like failure. He has, however, demonstrated that he can make sponge iron by an intermittent process in a revolving drum. It had become evident to me, even in my limited environment, that the simple use of iron, with the destruction of both iron and acid, to the production of an unfinished product, while apparently commercial, was in our extremely simple case preposterous, and investigation showed the splendid work done on other lines and convinced me that if considered from a commercial viewpoint better methods were bound to develop a process that would fit our requirements, and very much cheaper copper would result.
At the start we decided to investigate the electrolytic deposition of copper from solution, but a little later on we took up the subject of the direct precipitation of the copper by sulphurous acid gas. I prefer to attack the latter subject first for the purposes of this paper.
In the precipitation of copper by sulphurous acid gas, temperature and consequent pressure must be considered. If the copper is precipitated in metallic form this temperature and pressure are very high, but if a sub-chloride of copper would satisfy us, the temperature and pressure may be moderate. In both of these processes, the reactions we are familiar with in the formation of sulphurous acid by treating copper turnings with concentrated sulphuric acid are reversed by heat. For the production of metallic copper the action depends upon getting sulphurous acid dissolved in the sulphate solution in sufficient amount to precipitate the copper and then heating the mixture under proper control to about 150° C. The question of the use of an abnormally rich gas is involved. Under proper regulation pure copper is produced, which only needs melting for the market, and a large excess of sulphuric acid is formed. Mr. Van Arsdale has done valuable original work on this process, followed by the work of Weidlein and others in its commercial development. Laist has studied precipitating sub-chloride of copper. A soluble chloride is added to the sulphate solution of copper and the proper amount of sulphurous acid is added. Moderate heat under pressure—90° C. of heat—precipitated almost all the copper as sub-chloride. The subsequent process of obtaining the copper without volatilization and the recovery of the chlorine offered difficulties. Messrs. Van Arsdale and Bacon have valuable experience on this subject.
These processes need further development and are most interesting it is with no disparagement of the process that we have abandoned it in our case. I simply feel that the method would be a complicated and an expensive one, and I fear working with heat and pressure where fuel is dear and a corrosive solution is used. To me and my friends the electrolytic method seemed best adapted to our condition and the simplest and more certain of commercial application.
Of course, the first difficulty was the solution of other oxides than copper and the danger of some loss of acid thereby, and, what was more important, the susceptibility of some of these dissolved neutral salts to oxidation by nascent oxygen to the production of acid salts, which in turn will dissolve copper from the cathode. It seems that such salts are most valuable as depolarizers, but must be held under control.
After investigating the subject I advised Mr. Green way on behalf of his company, and we employed F. L. Antisell to make and direct a series of tests and experiments covering the precipitation of copper in the presence of iron and aluminum sulphates. For some time previous I had been familiar with experiments tried by Messrs. Pope and Hahn, and these gentlemen were employed to investigate in parallel the practicability of removing enough of the iron and alumina from the solutions as they foul to keep them below the danger point in electrolytic work. Both Mr. Antisell and Messrs. Pope and Hahn began their experiments upon a laboratory scale at the Raritan Copper Works. Messrs. Pope and Hahn found the leaching could be so conducted that the rate of increase of iron and alumina was slow. They therefore decided to leach in a certain way and to treat a certain high iron and aluminum neutral solution with copper oxides by agitating, heating, and injecting air. This is a rejuvenation of the old Hoffman process applied to a new purpose. The iron and aluminum sulphates in such foul solution, together with arsenic and other impurities, are precipitated as insoluble oxides and the acid previously combined with these salts is taken over by the copper oxide and forms sulphate of copper. The clarified solution, with these impurities removed, is then mixed with the balance of the solution, thus keeping an iron content of, say, less than ¾ per cent.
Mr. Antisell began his experiments by using a patent anode. This anode was a narrow, rectangular wooden or lead frame with thin wooden sheets as diaphragms, forming a long and deep but very narrow box. It was packed with a clean coke in lumps crushed to pass a 1-in. ring, and contact was given by carbon rods imbedded in the coke. A solution of sulphate of iron and aluminum in great excess was made up and sulphuric acid added. This was used as a solvent for the copper. After the solution came from the ore it was at first passed to an absorption tower, where sulphurous acid gas was absorbed, and then it passed to the electrolytic tank. I go into no detail in describing Mr. Antisell’s flow sheet and his process, as he himself is present. After promising results had been had in the laboratory tests at Raritan showing the possibility of high extraction and low power costs, Mr. Antisell sent Mr. Jamieson to Douglas, where tests were made with full-sized anodes. Excellent results were obtained here, but difficulty was had with the complete absorption of sulphurous acid. Also, the anode was cumbersome and not adapted to operating conditions, but, notwithstanding, excellent results were obtained experimentally and he ran for a long time with a recovery of nearly 2 lb. of copper per kilowatt-hour, an extraction of 80 per cent, of the copper contents of the ore, and the production of nearly the theoretical amount of sulphuric acid from the SO2 gas.
Messrs. Pope and Hahn later went to Douglas and began experiments, and also later conducted a most valuable set of experiments on the manufacture of acid-proof coatings for concrete that will neither crack nor soften under the high temperatures of the Arizona desert, and which will adhere closely to the sides of the tank. This work is very important and needs mention.
Mr. Greenway discovered that if a certain copper solution containing salts of triad elements is treated in a simple way, the iron or aluminum triad salts may be removed. Dyad salts are not affected. He was led to the belief that such oxides thus precipitated were in large part insoluble in dilute acid and that therefore he had a very simple method of removing iron and aluminum from solution. Against my prediction, laboratory tests seemed to show his contention was correct. Having used up about 300 or 400 tons of Ajo ore at Douglas for the Croasdale, Antisell, and Pope and Hahn experiments, and having promising results, the engineers moved to the mine at Ajo. Mr. Greenway erected a small plant of his own, and a Pope and Hahn plant was installed and the use of sulphurous acid was temporarily discontinued. As for the Greenway process, I am not yet at liberty to discuss it further. But I may say over 100 charges were run with this process. The iron content in the solution was kept down to less than 1 per cent., and we obtained an average of slightly over 1 lb. of copper per kilowatt-hour, but the extraction was low. We then extended the time of the leach and obtained a higher extraction, and finally we began to crush the ore, which at that time had been crushed only to ½-in. ring, finer, and found that if crushed to pass ¼-in. mesh percolation was not retarded and an extraction of over 80 per cent, was practicable. The iron has reached as high as 1.2 per cent., but we are still getting a pound of copper per kilowatt-hour from the electrolytic tanks.
These experiments have been so successful that we are proceeding with the erection of a 40-ton plant, where tanks will be used that will hold 60 tons, if necessary, and will be abundantly large to permit of a study of the comparatively even distribution of crushed ore, and we shall proceed with these most interesting experiments, using first, as we have in the smaller tanks, the Greenway process. In the meantime, the 1-ton plant will be liberated and we will immediately proceed with the continuance of a modified form of the Antisell system of leaching. He finds that with the construction of better absorption towers we can absorb the necessary amount of SO2 gas, and he has demonstrated that with the use of SO2 gas, using dyad iron as a depolarizer and triad iron as an oxidizer, he can manufacture sulphuric acid. We can, if successful, obtain 1½ lb., and possibly more, of copper per kilowatt-hour, and at the same time manufacture sufficient acid to replace wastage. The electrolytic copper in all cases has been fully up to the trade standards.
My deductions from a study of the work of the able engineers engaged in these extensive operations are as follows:
- Dilute sulphuric acid will give a very high extraction on oxidized ores of the Ajo type. Time is a more important element in extraction than strength of acid. A maximum size of 6-mm. cube is required. Such ore crushed to the maximum size permits percolation and the production of a clear solution because the slime content in the ore so crushed is very small.
- Where such ore is mined at a cost of between 25c. and 50c. a ton, it is commercial on a 12c. market, even if leached with sulphuric acid and the copper precipitated with pig iron to the waste of the acid and iron and the production of an impure cement copper that requires further treatment. Metallized iron from calcines can be made and will prove a little cheaper than pig iron if it is marginal.
- An electrolytic method for the recovery of pure copper from sulphate solutions is far preferable and will produce cheap copper. In all cases a low current density is required. Hard-lead anodes or composite anodes of hard lead and coke of uniform thickness will be used.
- Such electrolytic method, with control of the amount of iron and aluminum in the electrolyte, and without the use of sulphurous acid gas, is practicable. An acid loss of about 1.5 lb. per pound of copper will be sustained. Chamber acid will be required.
- I believe there is a decided probability that the cheapest and best method will be the electrolytic deposition from sulphate solutions without iron control, and the use of the requisite quantity of sulphurous acid. The sulphurous acid will be absorbed without the tank house and in quantity that will not cause annoyance. Iron and aluminum will act both as depolarizers and catalytes. This process promises a high yield of cathode copper per unit of power and the manufacture of sufficient sulphuric acid to replace the waste.
- While these remarks apply to the Ajo oxidized ore and ores physically and chemically similar, they need not apply to ores of a different composition. Mixed oxide and sulphide ores, be the sulphide chalcocite or chalcopyrite and bornite; ores containing great quantities of clay, or presenting other difficulties, have to be considered separately, and there is beyond question a great field for other processes and for other methods, both for obtaining the copper in solution and obtaining the copper from the solution.
And now, gentlemen, while thanking you for your attention, I end my theme as I began. Notwithstanding the importance of this subject, it covers but a corner in the metallurgy of one metal. There are many other subjects which if discussed in committee and with an open mind will lead to the advance of ourselves and of our institutions.