Heap Leaching, as practiced at Rio Tinto, Spain, while one of the oldest, and probably one of the cheapest, methods of extracting copper from its ores, has not had, until recently, other than experimental application in this country. This has been due partly to a mistaken idea that the Rio Tinto ores possess some obscure and mysterious quality that renders them alone suitable to the process, partly to the fact that tests properly made meant the expenditure of large sums of money and several years time, and partly to the fact that favorable commercial and other conditions are not often found.
This paper gives some views on the chemistry and mechanism of the method, an account of the preliminary experiments, and the final plans adopted for a large-scale installation of the method by the Phelps Dodge Corpn. for treating low-grade ore from Sacramento Hill, Bisbee. If results comparable with those obtained from the test heap are to be had from the large plant, this plant will operate on the lowest-grade sulfide ore now being treated commercially in the country.
The method has several advantages: The installation cost is low. Interest charges must, of course, be made both on the cost of the ore delivered to the plant and for the plant itself, but as these are low, there is considerable advantage in the ability to regulate output by more or less complete shutdowns in accordance with market and other conditions. The amount of labor needed is small, and the cement copper produced may be considered nearer a finished product than the sulfide concentrates from a mill, and certainly means less installation cost per ton to put into final form by present methods.
At Bisbee, conditions were favorable for a trial of the method for, in the preparation of the Sacramento Hill orebody for steam shoveling, a large amount of material below concentrating grade had to be moved to a suitable dumping ground, so the cost of delivering this to leaching beds meant practically no extra expense. The only risk was the cost of the plant which, it was thought, could be amortized by an actual extraction sufficiently less than that obtained experimentally to make the proposal reasonably safe. Up to October, 1921, approximately 380,000 tons, from stripping operations, averaging 0.92 per cent, copper had been delivered to the first heap, but since that time, because of the suspension of stripping and other conditions, no deliveries have been made, and plant construction has been postponed. That part of the paper, therefore, dealing with proposed plant construction represents recommendations and plans made up to that date; whether they will be followed will depend on the future policy of the corporation.
The plant was designed to take care also of the mine water formerly treated at the cementation plant at Bisbee; it embodies certain improvements locally worked out as the result of experience there, which are expected to result in economy.
Factors Involved in Heap-Leaching Process
While the process of heap leaching is a simple one to operate, so little is known accurately of the chemistry involved that it is reasonable to hope for modifications in operating methods that will shorten materially the time required for treatment.
Sponge iron, if it is produced commercially at a sufficiently low cost, will be a much more satisfactory precipitant than the tin cans and scrap iron now used; either this product or pig iron will have to be used eventually, for the present supply of tin cans and scrap is not sufficient to cover much extension of cementation in the southwest.
We can easily define heap leaching in terms of its flow sheet and operations, which are possibly simpler and require less expensive and complicated apparatus than any other method of getting copper from ores. It cannot be said, however, that we know exactly, or even approximately, the reactions in the heaps by which the copper is converted into forms soluble in water.
There is no doubt that the kind of copper mineral, its physical character, and the manner of its original deposition are important factors. We can be quite sure that some ores cannot be leached, if it is known that they are quite free from porosity. This follows from the fact that it seems certain that extraction of copper by this method depends on each piece of rock having pores, or microscopic channels, either open or filled with material that can be acted on by the solutions, and which will permit the solutions both to penetrate and to leave the interior of the piece. It would seem probable, therefore, that ores in which the copper is of secondary origin (that is, enriched by the precipitation of copper from solution in microscopic channels, pores or fractures) will be amenable to heap leaching; and that if an ore, in which the copper is of primary origin, is lacking in pores leading from the outside of a lump to each particle of copper mineral, it cannot be treated by the method. We are, however, dealing in reactions and operations that require at least several years; and while small-scale tests can be made to show definitely that an ore can be treated, negative results in a small way are not conclusive evidence of large-scale results because of the time factor and the difficulty of reproducing large-scale conditions.
From the foregoing, there can be deduced one necessary factor in the method of large-scale operation, namely, that each piece of ore in the piles should alternately be wet completely and dried thoroughly. If any individual piece having the necessary pores or channels and thoroughly dry, is wet on the surface, in a reasonable time each passageway, by reason of capillary action, will be filled with the solution wetting the surface, providing the channels are open at both ends. No reasonable amount of washing will remove the solution from the interior; but if the surface is dried, the reversal of the capillary action will bring the solutions to the surface of the piece, bringing with them any salts dissolved on the way. This action can be observed easily on the outside of a pile or an individual piece, which when dry becomes coated with quite pure bluestone.
If the foregoing is correct, it will give an idea of the amount of solution theoretically necessary for application at each cycle. Assuming that a pile or an individual piece of rock is dry, all that can be accomplished in the way of extraction by a single application of solution or water is to wash off the soluble salts on the surfaces, and no greater quantity than is required for this purpose need be used; a larger amount means simply more dilute effluent liquors. Also, as the volume of the pores is comparatively very small, no large amount of solution is needed for filling them; the amount of solution remaining on the surfaces after washing is probably enough. It is certain that complete immersion of the ore or the use of an excessive amount of solution is unnecessary and undesirable.
In heap leaching, it is probable that carbonates, cupric oxide, cuprous oxide, and metallic copper will be attacked in the order named. Copper silicate is an indefinite compound, with widely varying character and solubility, so no definite statement can be made with regard to it. It seems probable, although not certain, that if an ore carries any considerable amount of its copper as oxide compounds, a certain amount of sulfuric acid will have to be used.
Chalcocite and bornite are readily attacked and dissolved, but chalcopyrite is more refractory. It is probable that an ore containing chalcopyrite as its principal copper mineral will be much more difficult to treat by the method.
The reactions are exothermic, and where, as at Rio Tinto, heaps consist entirely of heavy sulfides, careful attention must be paid to keep the resulting temperature below the ignition point.
A very important matter practically is the character of the gangue and its behavior in the heaps under the influence of the solutions.
As the method depends on alternate wetting and drying to provide for penetration of solutions into a lump of ore and their subsequent removal, the size of lump and proportion of fines in relation to depth of pile are important. In addition, some rocks, under the action of the solutions and alternate wetting and drying, may crumble more or less completely. If the resulting products are coarsely granular, this crumbling is not serious; it may even be advantageous in allowing quicker and more complete penetration of solutions. But when a rock breaks down into very fine or claylike particles, it will be more difficult to leach and the uneven distribution of the effects of such breaking down will result in uneven leaching. Data are not available to show how an area in a pile consisting entirely of such completely broken down rock will behave. The first experimental test of the method at Douglas, however, consisted of 25 tons of sand tailings. This small heap allowed complete penetration of solution which, when the heap was allowed to dry, showed marked tendency to return to the outside surface by capillary action, bringing dissolved salts to the surface. The factor of ease of drying comes in here, however, and it is possible that the depth of pile with a rock that breaks down easily will have to be reduced; but so many factors are involved that at present the depth of pile is purely experimental.
The practice at Rio Tinto is of little help in this or other matters, as the heavy pyrite and the siliceous ores leached there vary widely in composition and behavior from our southwestern porphyries.
These statements concerning some of the physical factors involved show that the method is not so simple as the operation would indicate, and the same is true of the chemistry. It will also prove to be true that both successful operation and any possible improvements can only be had by as complete a knowledge of the physical and chemical factors as possible. This sounds trite enough, but frequently expensive failures have been made in apparently simple proposals through neglect of this obvious consideration.
Chemistry of Process
Turning to the chemistry of the process, we can be quite sure of the occurrence of some reactions. So-called oxidized copper compounds will be present in appreciable amounts in the ores under consideration. Cupric oxide, carbonate, or silicate will be probably acted on as follows:
- CuO + H2SO4 = CuSO4 + H2O
- 2CuO + Fe2(SO4)3 + H2SO4 = 2FeSO4 + 2CuSO4 + H2O
- 3CuO + Fe2(SO4)3 + 3H2O = 3CuSO4 + Fe2(OH)6
- CuO + FeSO4 + H2O = CuSO4 + Fe(OH)2
depending on the composition of the solutions and the amounts of ferric and ferrous iron, and free acid present.
Soluble basic constituents of the ore, other than copper, will be acted on similarly, resulting in reduction of acidity and precipitation of iron compounds. It is therefore evident that maintaining the balance of iron and acid in the solutions is important practically, and experiments have shown that some ores require the addition of extraneous acid to maintain this balance, while others do not.
Theoretically, so far as iron is concerned, the reaction
3. 3CuO + Fe2(SO4)3 + 3H2O = 3CuSO4 + Fe2(OH)6 will be exactly balanced by the precipitation reaction
5. CuSO4 + Fe = Cu + FeSO4
This may or may not be the case practically. Both free acid and soluble iron salts are produced in the piles by reaction of the sulfides present, and may be sufficient to make up the losses, caused mainly by the usual methods of precipitation, by which basic salts of iron are formed in considerable amounts.
In large-scale testing of the method at Tyrone, acid was added to the liquors, but at Bisbee this was not done.
Another important difference between Rio Tinto practice and the application of the method to our ores is that there will be a smaller quantity of waste liquors to be disposed of in normal operation. This is obvious, as iron is the leaching reagent and it is not produced in excess as at Rio Tinto.
We have assumed the delivery to the plant of 300,000 gal. of mine water per day, which has to be treated in any case, and the pumping capacity needed for the leaching heap has been based on using 1¼ gal. per ton per day. Treating 400,000 tons of ore for the first year would require 500,000 gal. of water for leaching per day. Soakage and evaporation is estimated at 50,000 gal. per day so that in order to equalize the water problem the following amounts will be needed per day (soakage is the amount of water taken up by the ore):
From these figures it appears that after the second year the amount of waste solution will not exceed about 13 per cent, of the total from the heap, and is less than the amount of mine water added to the system.
There will be an advantage in returning solutions from one part of the heap to other parts, in order to decrease the ferric-iron content of the effluent liquors before sending these to the cementation plant. This method should increase the extraction from the heap and decrease the quantity and increase the grade of the solutions. Furthermore, it should postpone the addition of further units of the cementation plant for the second and third year of operation for the quantity of solution to be depleted of its copper will be approximately the same. If this plan works out successfully, it will also delay the building of reducing heaps.
The reactions by which the sulfides of copper are converted into water- soluble sulfates in the heaps are not known with certainty. The equations given in the latest description of operations at Rio Tinto are as follows:
6. 33FeS2 + 28O + 4H2O = 29FeS2 + 4FeSO4 + 4H2SO4
7. 4FeSO4 + 4H2SO4 + 2O = 2Fe2(SO4)3 + 2H2O + 2H2SO4
8. Cu2S + Fe2(SO4)3 = 2FeSO4 + CuSO4 + CuS
9. CuS + Fe2(SO4)3 + H2O + 3O = 2FeSO4 + CuSO4 + H2SO4 or
10. 33FeS2 + Cu2S + 3H2O + 33O = 29FeS2 + 2CuSO4 + 3H2SO4 + 4FeSO4
showing the oxidation of pyrite to form ferrous sulfate and free acid; the oxidation in solution of ferrous to ferric iron; and the solvent action of ferric sulfate on Cu2S and CuS.
Similar equations have been published for years, but they do not really explain what takes place, and it is difficult to see why they should be written in the forms given.
It seems certain that an important reaction accounting for the solution of copper from a sulfide mineral is
CuS + Fe2(SO4)3 = CuSO4 + 2FeSO4 + S
but unquestionably other reactions take place, and it cannot be said that the chemistry of the process is known with any degree of certainty or completeness.
In the absence of direct evidence, a definite explanation cannot be given, but some of the observed facts point toward what follows.
We have to explain that starting with sulfide of iron and sulfide of copper, in the presence of excess of air and moisture, there are produced in the heaps ferrous and ferric sulfate, free sulfuric acid, and copper sulfate. The theory of capillary action seems quite clearly to explain how copper can be removed from the interior of a piece of ore without showing any macroscopic changes, and if the progress of leaching is followed by a proper microscopic study, this will probably be verified and the actual way in which the solutions get in and out followed. This has been done to a certain extent. It is simple enough to explain the formation of ferric iron from ferrous iron in the presence of excess of air.
The direct oxidation of pyrite by oxygen to produce SO2 and iron oxides is a reaction that starts slowly at quite low temperatures, but the velocity increases rapidly with rising temperature. This reaction may, or may not, have a measurable velocity at ordinary temperatures, but at the lowest temperatures at which it will occur it is probable that moisture will, as it does for many other reactions, act catalytically to increase the reaction velocity. A determination of the lowest temperature at which this reaction occurs appreciably should be made, and the influence of moisture on it; but as there is no evidence to the contrary and this explanation fits the facts it may be the correct one. As the reaction is strongly exothermic and as there is very inefficient radiation in the heaps, the heat produced at the beginning will accumulate and act as an accelerator; we would expect in a pile of closely packed pyrite with small amounts of moisture present, a constantly rising temperature.
This is the “heating up” that has been so often described for Rio Tinto. This accumulation of heat may be sufficient to start the heaps burning, to prevent which, care must be exercised. This is exactly what we would expect if the above were the correct explanation of the reaction taking place.
The velocity of the reaction at low temperatures is probably very small, but we are dealing with reactions that take several years for completion.
The minute amounts of SO2 produced at any one time are under favorable conditions for conversion into SO3, as they are in the presence of relatively enormous surfaces capable of acting as contact surfaces. If oxidized to SO3, any oxide of iron produced, being in intimate contact with the acid, will be readily dissolved in the presence of moisture.
The foregoing, if true, will be as valid for sulfides of copper as for sulfides of iron. It is well known that some copper sulfides, under the influence of air and moisture, oxidize readily to form sulfates and that other sulfides do so much less readily; this probably explains the particular amenability of the Rio Tinto ore to the process, although the experimental results at Bisbee would indicate that the reactions take place easily with the forms of copper sulfide present in these ores.
The relative amounts of copper dissolved by direct oxidation of copper to sulfate and by direct solvent action of ferric salts cannot be determined; the latter reaction is no doubt important, but careful records kept of the ferric and ferrous iron entering and leaving the experimental piles did not disclose any relation between the amount of ferric iron entering and the copper dissolved.
If the foregoing is correct, it is evident that, at Rio Tinto, the most favorable conditions for rapid oxidation cannot be maintained without danger of spontaneous ignition of the heavy sulfides; but this is not the case for a pile of siliceous ore, for which the best conditions can be selected. The investigation has not been carried on sufficiently to determine what these conditions may be. It may therefore be possible that the conditions under which the test heaps at Bisbee and Tyrone were run were not the most favorable, and that further work will result in an appreciable shortening of the time required for extraction.
The recorded history of the method at Rio Tinto goes back many years, as stated by Mr. DeKalb. The precipitation of copper by immersion of iron in solutions of copper salts is one of the oldest known chemical facts, and as, generally speaking, iron has always been cheaper than copper, it is probable that the method was applied at Rio Tinto for many years before the recorded development of efforts at systematic use.
In this country precipitation of copper from mine waters has been carried on for many years, at various localities, but until recently no serious attempts have been made to apply heap leaching. The first work done on the method by the Phelps Dodge Corpn. was about 1900. After a visit to Rio Tinto, when Doctor Douglas was impressed with the possibilities of the process for Bisbee, some experiments were made to determine, first, whether heavy pyritic ore carrying copper from the Copper Queen mine could be treated; second, if at the same time the excess of acid and ferric salts produced could be used to leach the copper from low-grade oxidized ores in “reducing beds” similar to those at Rio Tinto. From the records of these tests, the main criterion seemed to have been as to whether or not the test pile would “heat up;” and as no “heating up” was observed when water was applied to the heap, the experiments were pronounced a failure. The addition of chlorine, as common salt, to the liquors appeared to give favorable results, but was too expensive to be practicable. The role of iron as a reagent and the necessity for its presence in the leach liquors was not recognized in these early tests.
It was shown later, by laboratory work, that the presence of iron salts in the water used for wetting the ore was an essential factor, and it was proved that, by attention to this and other necessary conditions, low-grade ores of various kinds could be leached in the laboratory by the method.
During the large-scale leaching and electrolytic tests at Douglas, authorization was obtained for a trial of heap leaching. The material used for this test was sand tailings from the Tyrone mill, about 25 tons of which were placed on a platform and treated for several months. The results obtained were sufficiently encouraging to recommend a large scale test at Tyrone. A brief description of this experiment, which was carried on by A. W. Hudson, is as follows:
Tyrone Experimental Heap
The ore for the test had mostly been produced from, development work and had been dumped adjacent to the various tunnels or shafts. Some of the ore had been in the dumps for over seven years, while some was from recent operations.
The leaching site was selected on account of its proximity to the ore. It consisted of a hillside with a natural slope of between 12 and 15 per cent. This was too steep to retain the slimes, so surface benches were graded with a gradual slope following the contours of the hill. The area was about 250 by 250 ft.
To assist in waterproofing the ground, mill slimes were spread to a depth of about 6 in.; culverts were made from the largest rocks.
About 20,000 tons of ore were elevated to the site and distributed with wagons, making a heap with an average depth of 6 ft. The following is an approximate analysis of the material: Cu, 2.71 per cent.; SiO2,66.0 per cent.; Fe, 6.0 percent.; CaO, 0.3 per cent.; S, 5.0 per cent.; Al2O3, 14.5.
Scrap-iron precipitation launders were built ahead of the heap so that all solutions leaving the heap were pumped to the plant, where they were depleted of the copper, enriched in iron content, and flowed by gravity to the heap again for washing purposes.
Leaching operations were commenced during February, 1917, and continued intermittently for three years, at which time further work was discontinued because the concentrating mill required all the water from the mine.
During the three years, a total of 38.6 per cent, of the copper was extracted, figured from measurements and assays of solutions on and off the heap. The addition of acid was found necessary.
Following the Douglas leaching tests, considerable experimental work was done at Bisbee by various methods on low-grade ores, mostly carrying acid-soluble copper. After the discovery of the extensive disseminated deposits of Sacramento Hill, and their exploration, it became evident that these contained large amounts of ore too low in grade for concentration, so recommendations were made by the research department of the Phelps Dodge Corpn. that a systematic and complete test of heap leaching be made on this material. This work at Bisbee has been described by Joseph Irving, who was in charge of the operation of the experimental heap.
It is an interesting fact that the proceeds from this small heap paid all expenses of operation, and that the actual cost of the copper produced compared favorably with that of other much larger scale operations during the same period. Experimental work paying for itself is sufficiently rare to make this worth noting.
Bisbee Experimental Heap
A dump containing a quantity of low-grade sulfide ore that had been mined from an air shaft two or three years previous provided the ore for the test.
The leaching site selected for the heap was in a gully, part of the floors being covered with old lumber and part dressed with slimes. The bed of the creek was used as the main drain into which all other culverts drained. The surface area of the heap was about 12,000 sq. ft. About 10,000 tons of ore were moved to the prepared site with mine cars, making a heap with an average depth of approximately 20 ft. The following is an analysis of the ore: Cu, 1.3 per cent.; SiO2, 60.7 per cent.; Fe, 10.5 per cent.; CaO, 1.2 per cent.; Al2O3, 12.1 per cent.; S, 9.9 per cent.
Leaching operations were commenced during April, 1917, with the use of mine water, which was later replaced by waste liquors from the scrap-iron precipitation plant. The solutions were measured daily going to and from the heap, the latter being passed through a series of scrap-iron launders for the removal of the copper. Solutions depleted of copper but enriched with iron were returned to the heap for washing purposes.
This heap was treated for three years with various resting and leaching periods, at the end of which time further work had to be discontinued on account of stripping operations by steam shovels on Sacramento Hill. During the three years, a total extraction of 45.2 per cent, of the copper was effected, figured from measurements and assays of solutions both on and off the heap.
The heap was systematically sampled with a series of drill holes and pits, as well as a steam shovel cut through one end. The general average assay was 0.36 per cent. Cu, showing the actual extraction of copper to have been 72.3 per cent.
This large difference between calculated and actual results was doubtless due to unaccounted for losses of liquor, which drained directly through into the ground, and emphasizes the necessity for proper preparation of the site for commercial work.
Large-Scale Installation for Treating Low-Grade Ores from Sacramento Hill
The material to be mined from Sacramento Hill is divided into four classes, as follows:
- Waste………………………………….0.0 to 0.5 per cent, copper
- Low grade………………………0.5 to 1.0 per cent, copper
- Concentrating………………1.0 to 3.0 per cent, copper
- Smelting…………………..3.0 per cent, copper and over
The second grade is classified as leaching ore. It is estimated that the deposit contains 8,500,000 tons of this material, averaging 0.72 per cent, copper.
During March, 1920, the first of the leaching ore from Sacramento Hill was placed on the leaching site, this ore being encountered during stripping operations with steam shovels. From that time until October, 1921, when operations at the Hill were suspended, approximately 380,000 tons have been mined and placed ready for leaching. The proposed general layout of the first unit of the plant and heaps is shown in Fig. 1.
The place selected for the leaching heaps was chosen because of its proximity to the ore, the contour of the ground, and the nature of the floor. The site selected for the first 2,000,000 tons of ore is approximately 1800 by 750 ft. The average slope of the ground is between 3½ to 4 per cent, thus allowing easy drainage of solutions from the heap. The floor consists of caliche and conglomerates.
The ore mined by the steam shovels was loaded into steel dump cars, built by the Western Wheeled Scraper Co., having capacities of 20 and 25 cu. yd., which dump either side of the track. These cars are dumped by compressed air.
Before operations were commenced, a track was built along the north end of the leaching site, and dumping operations were started on the ground sloping from this side to the south. Each trainload, consisting of six cars, was sampled at the heap after dumping. As the dump increased, the track was moved to the edge, the coarser material rolling to the foot of the heap. The large lumps were broken, with powder, to from 8 to 12 in, and some of these were used for building the culverts or drains. Fine ore was kept on the surface of the heap as required for a covering and for building the basins necessary for irrigation purposes.
The culverts were built ahead of the ore being dumped, the hardest and most suitable rock being used for the purpose. These culverts are used both as ventilating flues and as drains for the solutions. The rock must be packed tightly in place to resist the force of the lumps rolling down the face of the heap, otherwise the culvert would collapse and its purpose be defeated. While the cross culverts are continuous across the site, those connecting at right angles are staggered, so that the system is an interlaced network of flues. The culverts are not always built in regular order, for advantage is taken of all depressions or drainage channels on the ground surface. The opening in the culvert is 12 in. in the clear.
Preparation of Site
Before the ore was piled, the site was cleared of cactus, brush, etc. Waterproofing the surface of the ground under the heaps is important, for it was clearly shown, by sampling the Bisbee test heap, that the actual percentage of extraction calculated from an average analysis of the washed ore was much greater than that calculated from the records of quantities and analyses of solutions during operation. Assuming that the latter were correct, the only explanation for any great difference would be in loss of solution by seepage into the ground under the piles.
The question of a suitable and sufficiently cheap waterproofing method for the site of the large plant has not been settled satisfactorily. As long as it is kept wet, a layer of clay or of slime tailings will probably be sufficiently waterproof. As soon as slime tailings from the new mill are available, they will probably be used for additional ore heaps. This will be the cheapest available waterproofing method, as it should cost but little to flume these slime tailings from the mill to the site; and the cost of waterproofing per ton of ore piled under these conditions should be nearly negligible. At present, no slime tailings or clay are available at a possible cost.
Another possible method of waterproofing is by spraying the surface with oil. If this is done, the surface should first be thoroughly dry, and it should be dried between successive coats of oil, and thoroughly dried before the ore is laid down. While an oil-coated surface will be fairly waterproof, the culverts, etc., must be built upon it, which would cause it to be badly broken, unless it was thoroughly dried beforehand.
A good part of the ground under the heaps will consist of caliche, which should have formed upon it a crust of calcium sulfate from the leaching solutions and thus arrest the percolation of solutions into the ground, but it does not seem probable that too much reliance can be placed on the water-repelling character of such a layer, and a positive method of waterproofing should be adopted if possible.
Plant
There will be two reservoirs—one above and the other below the leaching heap. The reservoir above will be used for settling out suspended matter from the mine water previous to irrigating the heaps; it will also act as a storage in case of need. The one below will be need as a storage for the liquors coming from the heaps before going to the cementation plant for the recovery of copper. These reservoirs are not yet constructed.
In Fig. 2 is shown the proposed general arrangement of the first unit, of the cementation plant, which has yet to be erected. This plant is
designed so that practically all of the operations will be performed automatically. There will be twelve redwood tanks, each 24 ft. in diameter by 10 ft. high, placed in two rows of six in series. They will contain false bottoms for supporting the scrap iron. Underneath the false bottom will be an acid-proof stirring arm worked from a shaft in the center of the tank and suspended from the top of the tank. This will be used to agitate the solutions when necessary and to move the precipitated copper to the central discharge on the bottom of the tank, which will be partly conical to assist in this operation.
The liquor entering the tank will be introduced alongside the agitator shaft and delivered underneath the false bottom, flowing up through the iron, where it will discharge over a peripheral launder to the next tank in series. Each tank in the unit will be connected to the next tank in parallel by a launder so that any tank may be cut out for inspection with-out interrupting operations.
The iron will be distributed to the various tanks by a gantry crane, which will take the scrap from the railroad cars at the head of the plant or from storage.
The liquor, after being depleted of its copper, will flow to an equalizing tank, where the required amount will be returned to the heaps for washing purposes. Provision is made for the installation of a scrap-iron launder plant should it be found necessary to remove the last traces of copper from the solution.
The classifier will be of the Dorr type, built of acid-proof material, as will also the thickening tanks of which there will be two, 24 ft. in diameter by 10 ft. high. These will be equipped with acid-proof diaphragm pumps, which will remove the thickened cement copper to the drying floors, the clear overflow being pumped back into the cementation-tank circuit.
The drying floors will be built of concrete and so arranged that all surplus water can be drained to a sump and either returned to the system or go to waste.
Flow Sheet
Subject to variation from data gained from subsequent operation, the flow sheet and method of plant operation that will be followed are as follows; Fig. 3 shows the present plan.
The mine water, after leaving the reservoir, will be measured, sampled, and passed on to the leaching heaps, where it will be enriched and flow to the reservoir at the foot of the heaps. From there it will be measured, sampled, and go to the head of the cementation plant, passing through two rows of six tanks in series containing scrap iron. The solution, now depleted of its copper, will flow to a sump, where the required quantity will be returned to the heap, the remainder being sampled and run to waste. The precipitated copper will be removed, at intervals, from the cementation tanks and will pass through a classifier where the coarse copper will be removed and deposited on to drying floors. The fine or suspended copper will go to thickening tanks, from which the clear solution will be returned to the cementation tanks, while the thickened product will be pumped to and deposited on the drying floors. When sufficiently dry, the product will be shoveled into mine cars and dumped into railroad cars for shipment to the smelter.
As the operations in the heaps at Bisbee and Tyrone were experimental, the procedure there may not be closely followed as experience is gained in the larger heaps.
The first of the ore was laid down before the cementation plant was available. If a part of this was installed, the logical method would be to start wetting the heaps as soon as possible after they were laid down, applying only enough water to moisten the ore thoroughly, and then allow them to wait until ready to start operation. If the first part of the ore was wet in this way, a considerable part of the copper would be made soluble by the time the heap was finished.
Without sufficient precipitating capacity first, however, most of this soluble copper would be lost when the heaps were wet during the rainy season.
The following assumptions have been made in the design and flow sheet:
In normal times, a delivery to the leaching heaps of about 400,000 tons yearly up to the total quantity of ore graded as leaching material. The cementation plant to be increased by units as required. Grade of ore 0.72 per cent. Cu. Period of extraction, 6 years to make a total recovery of 70 per cent, divided as follows:
The period of extraction is an arbitrary assumption and the possible profit will be reduced in proportion to extension of time of treatment.
While all assumptions are believed to be conservative, the operation should be considered as a large scale experiment, the results of which will be of interest in view of the bearing it may have on the future treatment of stripping ore and other low-grade material, which in some cases may mean for some mines the addition of considerable tonnage to what is now estimated as ore.
The main experimental factor is the required time of treatment, and in operation the adoption of a method of treatment of site which will absolutely prevent solution losses.
Discussion
J. Parke Channing, New York, N. Y.—When the Tennessee Copper Co. started smelting in the Ducktown district the ore was first heap-roasted. These heaps or piles were all protected by sheds which were kept in good condition and I feel quite sure that none of the copper was ever lost by leaching. When pyrite smelting was started and the roasting of ore abandoned, the roast yards were carefully cleaned up and B. B. Gottsberger made a final calculation of the total amount of copper that went into the roast yards and the total amount that was taken out. My recollection is that there was an unaccountable loss of 2 lb. of copper per ton of ore. Inasmuch as we were absolutely sure that this was not leached by rain water, the only explanation that I could give was that this copper was volatilized even at the low temperature of the roast heaps, but we have no scientific evidence to this effect.
R. C Canby, Wallingford, Conn.—The thought that impressed me while reading this paper is the statement that it is not the washing action of the solutions but the capillary action of the solutions to and from the inside of the ore that produces the results. So these periods of oxidation might perhaps more properly be called periods of capillary action.
C. S. Witherell, New York, N. Y.—I think capillary attraction also performs another role; not only does it serve to make the leaching solution penetrate the lumps of ore through numerous small cracks, but when the salts crystallize, an expansion takes place, thus disintegrating the lumps and opening other passages for the leaching solution.
Edward L. Blossom, New York, N. Y. (written discussion).— For successful heap leaching there are two outstanding requirements: (1) Every step of the operation must be carried on at low cost. (2) The copper, if not already in water- or acid-soluble form, must be capable of oxidizing to that form in a reasonable period of time. The authors of this paper seem to have fixed 6 years as the maximum but hope to make the time shorter.
Carbonates and oxides are the most readily leachable copper minerals. Of the sulfides, chalcocite and bornite are the easiest dissolved. Chalcopyrite is more refractory. The material should be broken (cost permitting) sufficiently to expose all copper mineral to contact with the solution, but as few fines as possible should be made.
Water, with or without sulfuric acid, and with or without mine waters carrying iron salts, is the solvent employed. Access of moisture and oxygen to interior of heaps is indispensable for producing the desired results. The authors stress the importance of alternately wetting and drying each piece of ore in the heap, by this means they depend on reversed capillarity to bring the dissolved copper salts from the interior of each piece to the surface whence it can be removed by the next wash. This is the same physical factor as that which brings desert alkalis to the surface of the soil, and its recognition as an important factor in leaching is highly creditable to the experimenters. So far as I know, this factor lias not previously been mentioned in the literature on the subject.
The authors recommend waterproofing the ground on which the heaps are to be spread and present figures showing that losses up to 37 per cent, of the extracted copper have resulted from neglect of this precaution. The authors’ propose to precipitate their copper from solution in tanks resembling Dorr thickeners, each tank having a false bottom above the stirring arm to carry the scrap iron. This also is a novelty. Scrap-iron launders of the old type will be used, if at all, only for removing the last traces of copper from the solution. A portion of the depleted liquor will be returned to the heaps for washing purposes.
The chemistry of the process—how insoluble copper compounds are converted into soluble salts—deserves careful attention. The authors give a number of reactions which probably contribute to the desired end, but of the relative importance of these reactions little is known with certainty. Some of the copper sulfide is probably converted into sulfate by direct oxidation. More of it is certainly dissolved by the action of ferric sulfate, which also attacks any metallics which may be present. Oxides, carbonates, and silicates of copper are more or less completely dissolved by free acid. Ferrous and ferric sulfates, together with free sulfuric acid, are continuously produced in the heaps by oxidation of iron sulfide minerals. These reagents, freshly generated in close proximity to the copper mineral, doubtless play a major role in the extraction. But the same reagents may be introduced with the wash liquors. The efficacy of sulfuric acid is presumably independent of its source, but for some reason the iron salts introduced when mine water or depleted liquor from the precipitation tanks are led onto the heaps do not produce any definitely ascertainable increase in copper extracted. This, I have been told, is also the experience at Rio Tinto. Yet the authors state that in laboratory work the presence of iron salts was shown to be an essential factor and anyone who has observed the powerful solvent effect of ferric sulfate on the minerals in question cannot but be surprised that addition of this reagent in heap leaching should not improve the results.
For this apparent conflict between theory and practice two explanations suggest themselves:
- Mine water and tank liquors, having been in contact with reducing agents, contain a, relatively small proportion of ferric, as compared with ferrous, sulfate. The latter does not become an active solvent unless it is oxidized in the heaps.
- Mine water and tank liquors (in contrast with freshly made laboratory solutions) usually contain basic iron compounds ready to fall out of solution when the acidity and the concentration of ferric sulfate are reduced. Both are reduced by contact of the solution with reactive copper minerals, and in consequence a film of iron compounds is likely to be precipitated just where it will do most harm, viz., on the surface of partly dissolved particles of copper ore.
Once precipitated, this film is difficult to remove even if the subsequent washes contain free acid, and in consequence the needful contact between mineral and solvent is lessened or destroyed. Oftentimes mere dilution of a neutral iron-bearing solution will bring down the precipitate. Cases are on record where application of mine waters has so cemented a heap as to ruin it for leaching purposes.
The remedy, if there be a remedy, is not easy. A very considerable addition of acid to the wash waters would be required to insure maintenance of acidity in all portions of the heap. In presence of acid-soluble minerals other than copper the cost of this would be altogether prohibitive.
Were it possible to oxidize all the iron in the wash water before applying it to the heaps, interesting results might follow, but too much experimental work has been done on these lines to leave much hope that such oxidation can be economically accomplished.
We are thus brought back to the conclusion that improvements in heap leaching must lie in the increase of oxidation effects within the heap. Mention is made of the catalytic effect of the relatively enormous surface of the ore in converting SO2 to SO3. May not some cheap accelerator of catalytic action be found that can be mixed with the ore?
