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Impermeable Dump Leaching Chemistry

A collection of exams have been run on recent Becker drill samples from a drill hole in Kennecott’s Chino “J” dump, close to Silver City, New Mexico, to acquire knowledge on the leaching chemistry of a comparatively impermeable sulfide copper waste dump. Vertical profiles have been manufactured from the moisture and soluble-salt content material; the content material of Fe+³, Fe+², Cu+², and H+ in the interstitial options; and the content material (each complete and non-water soluble) of iron, copper, and sulfur within the residues. Conclusions have been drawn from these knowledge in regards to the leaching chemistry and hydrological nature of this dump.

This research was just like a previous research carried out on different, more permeable leach dumps at Chino. Briefly, the conclusions of the primary research are as follows:

  1. Options are transported by way of major parts of more-or-less permeable dumps at variable and quite sluggish circulate charges based on the permeability of the strata. Some short-circuiting of options by means of the dump, nevertheless, is indicated.
  2. The oxidation and hydrolysis of iron-bearing options happens within the higher 40 ft of these dumps. This phenomenon seems to be a results of bacterial, air, and ground-water oxidation of a near-surface zone of retained iron and copper salts. The cyclic addition of leach fluids and the way through which they’re added (sprinkling or ponding) look like essential elements within the accumulation of soluble salts on this iron oxidation-hydrolysis zone.
  3. An inexpensive diploma of copper dissolution occurs inside the wetted zones of those dumps because of contact with the oxidized leach solutions. Some evidence, nevertheless, indicates there to be relatively inefficient removing of solubilized copper from some areas of the dumps by the leach options.

Determine 1 diagramatically exhibits the hydrology and chemistry of permeable-dump leaching, as decided by this early research.

Experimental Procedure

Eight-foot interval, pneumatic, Becker drill samples have been analyzed for his or her moisture contents, soluble-salt contents, the chemical compositions of the interstitial options, and the chemical compositions of the rinsed and unrinsed solids within every week after they have been collected at Chino.

The following process was used to acquire these knowledge: (1) Each of the Eight-foot-interval samples was crushed to -3 mesh and cut up into

two 300-g fractions. (2) Moisture contents have been decided by weighing, drying and reweighing one among these 300-g fractions. Copper, iron, and sulfur analyses have been run on these residues. (3) The opposite 300-g sample was rinsed on a Buchener filter with 50 ml of answer adjusted to a pH of 2.80 with H2SO4 simply earlier than rinsing. In a separate rinse, the solids have been additional rinsed with 900 ml of water, dried, reweighed, and analyzed for iron, copper, and sulfur. The difference in the dried, rinsed weight of this pattern, and the dried weight of the unrinsed sample was thought-about to be the load of the soluble salts. The 50-ml rinse options have been analyzed for copper, complete iron, ferrous iron, and (pH).

Experimental Knowledge

Check knowledge (see desk 1), tabulated within the order of their vertical profiles, present the content material of Fe+³, Fe+², Cu+², and H in the interstitial solutions, moisture contents, the odds of soluble salts within the interstitial options, and the iron, copper, and sulfur contents of the rinsed and unrinsed residues.

Dialogue of Experimental Results

Moisture Content – The check knowledge present the typical moisture content of the upper 176 ft of the “J” dump to be 7.40 %. Vast variances are evident, nevertheless, within vertical zones of the dump. There are several zones the place the dump is nearly dry (about 2.5 to

leaching chemistry analyses made on sample

5 % moisture) and a number of other the place the samples have been both partially or utterly saturated with solutions (8 to 12 % moisture) and a number of other the place the samples have been both partially or utterly saturated with solutions (Eight to 12 % moisture).

The moisture content material of the extra permeable dumps, studied beforehand, averaged in the 8 to 9 % range and assorted inside a relatively slender range with dump depth.

Soluble Salts – There was very little soluble salt buildup in the interstitial solutions of the “J” dump. The typical value, exclusive of the excessive worth in the 48-56 foot zone, was Eight.10 weight %. Seven of the 22 samples, all of which got here from dry zones, showed values of more than 10 %. The higher 8 foot zone of this dump contained just one. 5 % dissolved solids in the interstitial solutions, whereas leach options coming onto the dump have averaged about 5.6 %.

These findings are in sharp distinction to these obtained within the previous research on the extra permeable leach dumps. The earlier knowledge showed very giant buildups of soluble salts inside the higher zone of the dumps and relatively excessive and uniform concentrations through-out the remainder of their depth.

Chemical Composition of Interstitial Solutions – Observations on the distribution and quantity of ferrous and ferric iron, copper, and H+ within the interstitial solutions are as follows:

  1. There was no buildup of ferric salts inside the interstitial options of this dump, and in only one or two situations was there a big quantity of ferrous iron. The ferrous iron to ferric iron ratio was high in all interstitial-solution samples.
  2. The pH of the interstitial options was above Three.1 (average of 50-ml, 2.80-pH wash options equaled 3.30) all through the 176 ft of dump tested. Undoubtedly, this lack of acid in the interstitial options was chargeable for their very low ferric iron content material. The pH values of the interstitial solutions within the extra permeable Chino dump (common of the 50-ml, Three.Zero-pH rinse options have been 2.94 and a couple of.86) have been much lower than these in this dump.
  3. The copper content material of interstitial options was low (the typical 50-ml wash answer was 0.124 gpl) throughout the upper 64 ft of this dump and really low (the typical was 0.047 gpl within the 50-ml wash solutions) throughout the remainder of its drilled depth. One exception to the above was the 48-foot to 56-foot zone the place the copper content material of the 50-ml rinse answer was Zero.318 gpl. The typical copper contents of 50-ml wash solutions from the more permeable Chino leach dumps have been 0.129 gpl and Zero. 478 gpl.
  4. The typical ratios of Fe+³ :Fe+² :Cu+² have been Zero.16:1.20:1.00 for the first 64 ft, 0.92:Four.70:1. 00 for the subsequent 112 ft, and Zero. 62:2. 80:1. 00 for the whole 176 ft drilled. These ratios have been 0. 45:Zero. 67:1. 00 and Zero. 35:Zero. 13:1. Zero under the 32-foot depths for the interstitial options of the extra permeable dumps.
  5. The typical ferrous, ferric, copper, and acid concentrations (about Zero.40 gpl Fe+², Zero.08 gpl Fe+³, Zero.14 gpl Cu+², and 3. 60 pH) of those interstitial solutions have been very low compared to the typical assays of the whole combined dump effluent solutions which might be obtained at Chino (approximately 1. 0 gpl Fe+², 1.3 gpl Fe+³, 1.0 gpl Cu+², and a couple of. 5 pH).

Composition of Rinsed and Unrinsed Residues – Rinsed and unrinsed residue samples have been assayed for copper, complete iron, and sulfur. These samples averaged Zero. 068 % complete copper and Zero. 064 % non-water-soluble copper; 2.54 % complete iron and a couple of.54 % non-water-soluble iron; and 1. 26 % complete sulfur and 0.95 % non-water soluble sulfur.

The distribution of copper, iron, and sulfur inside the dump was as follows:

Complete copper was considerably greater in the upper 64 ft (average = 0.111) of the dump than in the 64- to 176-foot
zone (average = Zero.047).

Sulfur assays, although somewhat inconsistent, showed some evidence of segregation at the prime of the dump. The inconsistent nature of those assays, lack of copper at depth, and the shortage of other proof of iron oxidation and hydrolysis, would point out that the anomalous amount of sulfur within the prime of this dump is in the form of copper and iron sulfides and never as a primary iron sulfate product, shaped because of oxidation and hydrolysis of iron solutions.

Interpretation of Knowledge

The three areas during which these knowledge might contribute to an under-standing of waste-dump leaching chemistry are: (1) ore-solution contact or the hydrological nature of the dump (2) iron oxidation and hydrolysis and (3) the dissolution and removing of copper from the dumps. Interpretations made on these knowledge are diagramatically shown in figure 2.

Ore-Answer Contact – Options look like transported at relatively high move charges by way of select zones or layers of the dump. Proof of this is (1) the variable moisture profile of the dump, (2) the high ratio of Fe+² to Fe+³, Cu+², and H+ in the interstitial solutions of moist zones, (Three) the low content of ferric iron, copper, and H+ within the interstitial solutions, (4) the small amount of retained soluble salts, and (5) the low charges of copper dissolution within both dry and moist strata. Apparently the more permeable layers of this dump act as channels for the ingress and egress of relatively fast-flowing leach liquors. There

leaching chemistry hydrology

additionally seems to be but slight penetration of solutions or redeposition of soluble salts inside the dry or impermeable zones.

Correlative gamma logging and tritrium tracer research on this dump, by the U. S. Bureau of Mines, Kennecott Copper Company, and the New Mexico Bureau of Mines and Mineral Assets, have also borne out these observations.

Iron Oxidation and Hydrolysis – There’s robust evidence that this dump does not have an iron sulfate oxidation and hydrolysis zone near the floor, as was found to be the case with extra permeable Chino dumps. The info show the following: (1) There isn’t any buildup of soluble salts in the upper layer of the dump. In reality, rain water seems to have rinsed the soluble salts from this zone to a degree considerably under that of the leach liquors flowing onto the top of the dump. (2) There isn’t any buildup of soluble ferrous iron, ferric iron, copper, or acid within the upper zone of the dump. (3) There isn’t a proof of ferric iron being shaped within the interstitial options of the dump. These options consist principally of ferrous iron with little or no acid and very little copper. (Four) Residue assays don’t help the concept there has been a precipitation of an insoluble primary iron sulfate product within the upper portion of this dump because of ferrous iron oxidation and hydrolysis reactions.

This lack of oxidation and hydrolysis of the leach liquors suggests that bacteria are either not current within this dump, or if they’re present, they don’t seem to be lively in changing ferrous iron to ferric iron and acid.

Dissolution and Removing of Copper from the Dumps – There appears to be very little dissolution and removing of copper from the Chino “J” dump. This statement is supported by the truth that (1) the interstitial solutions are very low of their copper content material in each the wet and dry zones of the dump and (2) the stagnant nature of the dry zones.

The reasons for these low rates of copper dissolution and removing are: (1) there’s very little, if any, ferric iron and acid out there for dissolving the copper minerals, and (2) the penetration of solutions into the dry areas of the dump is poor, and therefore there’s little rinsing or flushing of mineral values from the dump. The truth that the decrease two-thirds of this dump could be very low in grade may contribute to the poor extractions of copper from this dump.

Within the earlier research it was said that several means can be found for dissolving copper sulfides in waste dumps, i.e. dissolution with acid-ferric sulfate options, alternate wetting and drying cycles, and electrolytic reactions between minerals and impartial salt-acid reactions. The low fee of copper dissolution inside the “J” dump signifies that crucial means of dissolving copper minerals in dump leaching is oxidation and dissolution of copper minerals with acid-ferric sulfate options. Very little leaching seems to occur when acid and/or ferric iron are absent from the interstitial options.

Conclusions

This research indicates the nature of leaching in the Chino “J” dump to be as follows:

  1. This dump is wetted in select horizontal zones and vertical channels by relatively fast-flowing ferrous sulfate solutions.
  2. There isn’t a iron sulfate oxidation and hydrolysis zone inside this dump. Therefore, there isn’t a means of producing acid and ferric sulfate inside the leach liquors.
  3. Due to the short-circuiting of solutions by means of the dump and the shortage of a way of producing acid-ferric sulfate, there’s little or no leaching occurring inside this dump.

These outcomes are in sharp contrast to these obtained in a earlier, comparable research on some extra permeable Chino dumps. Within the earlier studies, answer movement was found to be slower and more uniform and to penetrate extra of the dump; the influent ferrous sulfate solutions have been oxidized and hydrolyzed to acid and ferric sulfate in the higher parts of the dump by a soluble-salt retention-oxidation mechanism and relatively good copper dissolution and removing from the dump was affected.

Acknowledgments

The authors sincerely categorical their appreciation to the New Mexico Bureau of Mines and Mineral Assets, a division of the New Mexico Institute of Mining and Know-how, for permission to current this report of investigation. Appreciation can also be prolonged to Mrs. Lynn Brandvold and numerous undergraduate and graduate scholar assistants for their in a position assist in analytical work and metallurgical testings; to Mr. William Arnold for drafting; and to the secretarial employees of the Bureau for typing and reproducing the manuscript. Particularly, the authors categorical their gratitude to the administration and employees of the Chino Mines Division, Kennecott Copper Corp. , Hurley, New Mexico for their cooperation and help throughout the venture and for furnishing the samples for laboratory studies.

During emplacement of mine waste for dump leaching, compacted layers of low permeability are produced by heavy vans and bulldozers that transport and unfold the rock. Migration of fines and precipitation of salts after leaching begins type different impermeable layers. As part of a cooperative’ research involved with improved leaching of compacted and cemented dumps, percolation leaching checks have been made at our laboratory on drill core samples from a previously leached dump (“J” dump at the Kennecott operation close to Santa Rita, N. Mex.) to determine what copper extraction could possibly be expected if the dump have been made permeable. This report presents the outcomes of our mineralogic examination and leaching exams.

Descriptions of Samples

The “J” dump was systematically sampled by the use of a Becker pneumatic drilling machine. Nine holes, from 130 ft to 207 ft deep, have been drilled to bedrock from the highest of the dump. Samples of the Three-inch-diameter drill cores have been collected at 8-foot intervals.

The core splits acquired at Salt Lake City have been primarily all minus-1 inch in measurement. A mineralogical research was made on an 8-foot interval (168- to 176-foot section) of a core pattern (LDH-164) obtained from near the center of the drill gap array to determine the predominant copper and clay minerals. The pattern contained, in %, Zero.22 complete Cu, 3.1 Fe, 0.21 sulfide S, and 0.9 sulfate S. Examination with binocular and petrographic microscopes revealed that the part was composed of quartz, altered feldspar, sericite, clay, chlorite, magnetite, limonite, pyrite, chalcopyrite, covellite, chalcocite, and remoted grains of metallic copper. No discrete oxide copper minerals have been found. The sulfate sulfur is within the type of water-soluble iron, magnesium, and manganese moderately than any distinct mineralogical species. Nearly all the sulfide minerals occur as small inclusions in the gangue material. Some chalcopyrite happens as locked grains with the pyrite.

Differential thermal analysis was performed on the slime fraction of the sample to find out the clay minerals. The ready clay pattern, when heated, showed endothermic peaks at 100° to 200° C, representing loss of loosely held or absorbed water; at 325° C, representing impurities within the sample; at 550° to 610° C, akin to loss of (OH)-; and a small peak at 900° C, comparable to disruption of the remaining structure. This curve is in shut agreement with curves produced from illite samples. The chemical components most commonly assigned to illite is (OH)4K2(Si6Al2)Al4O20 and the theoretical composition is, in %, 45.2 SiO2, 38.5 Al2O3, 11.Eight K2O, and Four.5 H2O.

Laboratory Percolation Leaching Exams

Laboratory-scale percolation leaching exams have been made on the drill core samples in Four-inch-diameter by 36-inch-long glass tubes. Glass wool and small quartz pebbles have been placed at the bottom of the ore column to stop plugging of the drainage holes in the Buckner funnel which supported the loaded tube. Cut up samples of each 8-foot drill core interval from an individual drill gap have been placed in a glass column in the same order by which the interval occurred in the dump. One liter of dilute sulfuric acid leach answer at pH 2 was fed to the highest of every ore column. Leach answer move fee was regulated to equal the speed of effluent discharge. A couple of inches of answer have been maintained on the surface of the nine ore columns to simulate ponding on a dump. Copper-bearing liquor that drained from the columns was collected in flasks and recirculated constantly by way of the ore bed for 2 weeks. Throughout this time, water was added as wanted to make up for evaporation loss. Each time the pH of the recirculating solutions exceeded 2.Three acid was added to decrease answer pH to 2.0. On the conclusion of a 17-day leach cycle, each column was allowed to drain and dry for about 2 days to simulate the alternate wetting and drying procedure generally employed in business follow. The pregnant copper answer was sampled for evaluation, and the surplus answer was discarded. The leaching and drying cycles have been repeated three extra occasions with recent acid answer at pH 2 launched firstly of each cycle. The exams have been terminated on the finish of 4 cycles (about 75 days). Summarized results of the acid leaching exams on the drill core samples from the “j” dump are given in desk 1.

leaching-tests-percolation

Addition of ferric sulfate to assist in oxidation and dissolution of the copper sulfide minerals was not mandatory because the acid leaching answer dissolved a number of the iron salts that had precipitated in the dump. The ferric iron content material of the recycled leach liquors ranged from Zero.2 to 2 grams per liter. Copper content of the leach liquors ranged from 0.1 to 2.7 grams per liter.

Analyses of the leach answer confirmed that 45 to 50 % of the entire copper extracted was dissolved within the first -cycle, 30 to 35 % in the second cycle, 15 to 20 % in the third cycle, and less than 5 % within the fourth cycle. Leaching was discontinued after four cycles. Copper extractions ranging from 15 to 38 % have been obtained by percolation leaching of the core samples from the nine drill holes. The check results indicate that about 27 % of the copper remaining in the “j” dump could possibly be extracted underneath circumstances by which compacted zones have been damaged up and made pervious to acid leaching options.

Leaching traits of the nine drill core samples have been comparable, apart from the slower flowrate for samples 164-S and 166, which contained extra high quality materials than the other samples. The sluggish flowrate didn’t adversely affect copper extraction.

A consultant portion of the laboratory-leached residue from drill core 165 was examined to find out how the unleached copper occurred. A sample of the residue, which assayed 0.05 % Cu, was ground to minus-65 mesh and fractioned with a heavy liquid of 2.9 particular gravity into sink and float products. A elegant briquette of the sink product was ready for metallographic examination. The residual copper occurred as chalcocite and covellite coatings on pyrite particles locked within the gangue minerals. Sporadic inclusions of chalcopyrite also have been noticed within the locked pyrite grains. Grinding to minus-65 mesh didn’t liberate the pyrite with which the copper minerals are associated.

Conclusions

Laboratory leach exams indicated that about 27 % of the copper might be extracted from the “J” dump beneath excellent percolation leaching circumstances utilizing sulfuric acid options. Mineralogical examination of the residue from one of the laboratory-leached drill core samples confirmed that the residual copper occurred primarily as fine-grained chalcocite and covellite coating pyrite grains locked inside gangue minerals. The residual copper on this mode of prevalence can’t be effectively reached by leach answer; therefore, further leaching is unlikely to dissolve vital quantities of further copper.