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Friends often ask how accurate the work of our little stamp-mill is, or express the opinion that a little mill cannot do good work. As a reply we would like to place on record the results of a few tests.
There are two periods of work in our laboratory; the fall term, when the class collectively goes through all our chief laboratory-processes, and the spring term, when the students work up their theses, each electing some one process or investigation and devoting himself wholly, or almost wholly, to that.
Some of the tests, the results of which are recorded in this paper, were made in the fall term; the preparation of the apparatus, the sampling and the assaying being done largely by assistants, with the aid of the students. Others were made in the spring term as thesis-work, and the results are due wholly to the students.
The complete tree of our process is patterned after the most approved California mill-practice. It is modified only where the small-scale method must differ from the large, in order to secure results from a brief test lasting a few hours, followed, perhaps, by an interval of many months between tests, which seek to rank in accuracy with those of the large-scale work, running over months and years of continuous work, broken only by brief intervals for cleaning-up.
We use silver amalgam on our apron-plate, painting it on before the run, and scraping it off with the gold-contents afterwards, for two reasons: It renders the plate very quick to catch gold, and it renders the absorption-loss by the copper-plate insignificant, or prevents it altogether.
Our canvas and fine vanner-plant for treating the extremely fine tailings is that known as the Gates canvas-plant, originally designed by G. G. Gates and installed at the Kennedy mine, Jackson, California. Since it has been demonstrated clearly that this canvas-plant can pay only when the assay of concentrates is high, or when the percentage of concentrates is large, we do not always choose this run as the one to illustrate to students the wonderful catching-power of canvas for extremely fine concentrates. We do, however, make it our practice to separate, by classifying, the coarse Frue-vanner tailings from the fine, thereby preparing for canvas should we decide to use it.
Sampling.Many of the ores we run would come under the head of rich spotty ores, as the Colorado phrase is. To crush and cut down such an ore, according to best Colorado rules of sampling, would require breaking the whole batch of from 500 to 1,000 lb. down to 10 or even 30-mesh. This would ruin our gold stamp-mill run. We are thus put in the odd position of having to choose between sampling the lot according to the best rules for safe work and spoiling the stamp-run, or of making a stamp-mill run, using only a very crude sample for valuation, which we could not possibly defend on the score of accuracy. It is needless to say we choose the latter method.
Apron-plate of copper 6 ft. long, 2 ft. wide, covered with silver amalgam ; number of stamps, 3; weight of stamps, 225 lb.; diameter of shoe, 5.75 in.; number of drops, 98 per min.; height of drop, from 4.25 to 5.75 in.; height of discharge, 4.75 in.; screen, horizontally slotted, openings 1/40 in. wide; rate of crushing, 57 to 94 kilos, per hour; plate-slope, 1.5 in. per ft.; feed-water to the battery, 15 kilos, per min.; wash-water on vanner, from 4 to 8 kilos, per min.; Frue vanner-slope, 3.5 to 4.5 in. in 12 ft.; number of shakes, 198 per min.; belt-travel, from 40 to 60 in. per min.; canvas-table, 10 ft. long, 4 ft. wide; canvas-table slope, 1.5 in. per ft.; canvas-table feed, 10 kilos, water per min. and from 0.7 to 0.8 kilos, dry ore per min.; Embrey vanner, 1.5 in. per ft. slope, 80 in. per min. travel of belt, 1240 vibrations of 0.5 in. per min.
The mill is a little 3-stamp Fraser and Chalmers prospecting-mill of 1882. It is very satisfactory in all respects except the number of stamps. A school-battery should have either two or five stamps; the former is preferred by us. Three stamps cannot have an order of drop which satisfactorily distributes the sand.
The canvas-table is supplied with an accurate distributer. The canvas is No. 6 cotton duck, with the warp laid across the table for the greatest roughness, and is cut from a roll woven 10 ft. wide. The feed-water is the quantity, and the pulp is two to three times the quantity used by the Gates canvas-plant. The copper-plate is 1/8 in. thick, of softest copper.
The amalgam is made by dissolving 95 grammes silver in 380 c.c. nitric acid (1.2 sp. gr.) with addition of 817 c.c. water after solution. Pure mercury is then added at the rate of 16 parts mercury to 1 part silver. It should be stirred at short intervals during the formation of the amalgam. The result is a soft amalgam, clean and free from crystals or mercurous nitrate. Failure to follow this rule exactly is liable to give a very impure amalgam, which will require time and trouble to clean.
The apron-plate is cleaned preparatory to the application of silver amalgam as follows: It is washed off with water, scoured wet with infusorial earth or very fine sand, and then washed off with water again. Obstinate black spots can be removed by scouring wet with a flat piece of pumice-stone, adding mercury to plate the cleaned copper. It is then washed all over with a strong solution of cyanide, adding mercury, again rinsed off with water, and then rubbed dry with clean cloth or cotton- waste which must be free from oil. A moderate quantity of mercury is now rubbed on without water, after which the plate is ready for the amalgam. No water is allowed upon it yet.
The clean amalgam is squeezed in chamois skin to a moderately hard ball, and rubbed without water on the clean, bright, amalgamated copper-surface of the apron-plate. (A flat, bristle paste-brush may be useful to obtain an even distribution.) This coating is quick to catch gold, and prevents the gold from coming in contact with the copper. No water is allowed upon it until stamping begins.
After the run, the plate is rinsed off with water, and the amalgam scraped off as clean as possible with a rubber-scraper. More mercury is rubbed on, and the plate is again scraped. This is repeated twice more, and all the amalgam is put together. If there is any hard amalgam which has not been removed, it is gently rubbed wet with a flat piece of pumice-stone in the presence of mercury, and so removed without injury to the plate. The amalgam from the plate, combined with that from the traps, is retorted; the residue is melted with borax in a crucible, and the bullion is parted for gold.
Since the battery is fed with pure mercury, while the apron-plate is coated with silver amalgam, the battery-amalgam enables us to get a valuation on the fineness of the gold caught at this point. We have no means to get this ratio on the portion of the gold caught on the apron-plate.
The mercurous nitrate and a little residual silver nitrate from making silver amalgam are worked over for mercury and silver by immersing strips of iron in the liquid, which precipitates the metals as amalgam.
The method here used in tabulating the results of a test has been found to be a most concise and satisfactory one. As the value of the ore obtained by the mill-run is much more accurate than that based on the assay of the ore, the percentages found in the column headed Per cent, of the total gold are based on the total gold in the ore as found by the mill-run, and not on the assay of the feed; in fact, one of the objects of this work is to show the student the impossibility of valuing a free-milling gold-ore by sampling and assaying.
The degree of accuracy attained here is, we believe, up to that of the very best mill-practice. It has been attained by locating the causes of error, and then by removing them as completely as possible, even by departing, if necessary, from large-scale mill-methods.
Many problems on free milling gold ores have been solved by the 911MPETesting Laboratory. One free milling gold ore received for testing the application of the Mineral Jig in the Ball Mill Classifier circuit gave the following results:
On this ore, the Mineral Jig, with its selective action, recovered 89.4% of the gold in the ore, producing at the rate of only one ton of concentrates from every 98 tons of ore. Calculations from these results, based on gold payment at a smelter, and at the mint, deducting a minimum smelter treatment charge per ton of concentrates, shows that the jig concentrates shipped without amalgamation would return per ton of head ore; whereas shipping bullion to the mint and amalgamation residue to a smelter would return $ some per ton of head ore, or a difference of a few $per ton of ore in favor of amalgamation. At a lower ratio of concentration, tests showed that the recovery could be increased to as high as 94% of the gold in the ore.
More and more Industrial operations are turning to reclamation of products formerly considered waste, and manufacture of additional by-products. Test work on such problems is continually going forward.
One company which uses a large amount of silver was reclaiming a considerable portion of this valuable metal from waste product by smelting. Recovery by this method was relatively good but there was a large quantity of slag from the original smelting operation on hand which contained much silver in the form of fine beads.
The 911MPEOre Testing Laboratory was called upon to attempt a solution to the recovery of this silver. Test work indicated that the material could be crushed, ground and then treated with the Mineral Jig. A high recovery of this silver in a concentrate assaying over 50% silver was found possible.
Similar methods can often be used in recovering valuable metals from old retorts and crucibles. Valuable minerals contained in floor sweepings in industrial plants can often be profitably recovered using standard ore dressing equipment. In their work the engineers in the Ore Testing Division keep constantly before them not only the question of can the metallurgical results be obtained, but can they be obtained in a manner that will make a profitable operation?
Such work has not been confined to ores from the United States alonebut alsoon samples from all parts of the world; tin from Bolivia, China and the Malay States; gold and copper from Mexico, Africa and South America; silver and manganese from the East Indies; complex ores from Australia and New Zealand; gold and silver ores from Alaska; potash from France and many other ores from Bolivia, Yugoslavia, Canada, South America, Alaska, Central America, Turkey, Mexico, Australia, the East Indies and elsewhere.
More and more the desirability, and the actual necessity, of conducting exhaustive test work on ores and industrial products at the very beginning of such operations is becoming recognized. It is for this purpose and also as a service to the Ore Dressing and other Industries, that the 911MPEOre Testing Laboratory offers. The charges for this test work are very nominal. Just sufficient to cover the actual operating costs including the analysis and assay (both fire and chemical) of the test products made.
Pilot plant testing charges are based on laboratory pilot plant rental, power, water, lights, assays, chemicals, labor, supervision, and any special equipment which may be required. Batch testing should always be performed first in order to establish pilot plant flowsheet requirements, ore treatment characteristics, and reagents necessary to intelligently proceed with the work on a pilot plant basis.
A minimum of 5 tons of ore is necessary for a pilot plant test using the facilities available at the Denver Equipment Company Laboratory. The pilot plant is capable of treating 200 to 300 lbs. of ore per hour and usually a two-week period is required to complete a pilot plant test on 5 tons of ore.
Pilot plant tests are only recommended where it is not possible to determine from batch or cvcle tests the disposition of all products or middlings. Often these tonnage lot tests are necessary to secure several hundred lbs. of concentrates or products for further evaluation in establishing markets, etc. This is particularly true on non-metallics such as talc, clay, phosphate, potash, fluorspar, feldspar, barite, etc.
A free milling gold ore is leached using copper-citrate-thiosulfate solutions.Similar gold extraction is obtained for Copper-citrate-thiosulfate system and cyanidation.Leaching of gold ore is controlled by the product layer diffusion.
For the last two decades, a considerable amount of research has focused on developing a completely safe and non-toxic leaching process for gold to replace cyanidation. In this research, a free milling gold ore is leached using copper-citrate-thiosulfate solutions at elevated temperatures, where citrate serves as a stabilizer of cupric ion. Experimental factors such as agitation speed, temperature, initial pH, concentration of copper, citrate and thiosulfate have been investigated. Meanwhile, effects of these parameters on the thiosulfate consumption and Cu2Cit2H24 complex stability are also examined in the leaching process. The experimental results show that the copper-citrate-thiosulfate system has a similar extraction capacity for gold compared to the traditional cyanidation. The results of kinetic analysis of dissolution rate of gold indicate the leaching process gold from free milling ores may be controlled by the product layer diffusion under the experimental conditions, with an activation energy of 34.48kJ/mol. The copper-citrate-thiosulfate system has some advantages over cyanidation, such as green, nontoxic and shorter leaching time, although a high leaching temperature above 70C is usually required to an acceptable gold extraction. Obviously, the system still needs to be further developed to ensure its commercial viability.
The principal gold minerals that affect the processing of gold ores are native gold, electrum, Au-Ag tellurides, aurostibite, maldonite, and auricupride. In addition, submicroscopic (solid solution) gold, principally in arsenopyrite and pyrite, is also important. The main causes of refractory gold ores are submicroscopic gold, the Au-Ag tellurides, and very fine-grained gold (<10 m) locked in sulfides. Other types of problem gold ores include copper-gold ores and preg-robbing carbonaceous ores.
S.L. Chryssoulis, L.J. Cabri, and R.S. Salter, Direct Determination of Invisible Gold in Refractory Sulfide Ores, Proceedings of the International Symposium on Gold MetallurgyRefractory Gold, ed. R.S. Salter, D.M. Wyslouzil, and G.W. McDonald (Toronto, Canada: Pergamon Press, 1987), pp. 235244.
L.J. Cabri and S.L. Chryssoulis, Advanced Methods of Trace-Element Microbeam Analyses, Advanced Microscopic Studies of Ore Minerals, ed. J.L. Jambor and D.J. Vaughan (Ontario, Canada: Mineralogical Association of Canada, 1990), pp. 341377.
J.P. Vaughan, S. Bacigalupo-Rose, and R. Dunne, Mineralogy and Processing Characteristics of Arsenical Gold Ores, Report 51 (East Perth, West Australia: Minerals and Energy Research Institute of Western Australia, 1989).
J.P. Vaughan and I.J. Corrans, Mineralogy and Processing Characteristics of Arsenical Gold OresPhase 2, Report 87 (East Perth, West Australia: Minerals and Energy Research Institute of Western Australia, 1992).
M.J. Nicol, C.A. Flemin, and R.L. Paul, The Chemistry of the Extraction of Gold, The Extractive Metallurgy of Gold in South Africa, Monograph M7, ed. G.G. Stanley (Johannesburg, S. Africa: South African Institute of Mining and Metallurgy, 1987), pp 831905.
D.M. Hausen and C.H. Bucknam, Study of Preg-Robbing in the Cyanidation of Carbonaceous Gold Ores from Carlin, Nevada, Applied Mineralogy, Proceedings of the Second International Congress on Applied Mineralogy, ed. W.C. Park, D.M. Hausen, and R.D. Hagni (Warrendale, PA: TMS, 1985), pp. 833856.
J.P. Vaughan is associate professor and head of the Mining Geology Program at the Western Australian School of Mines, Curtin University of Technology, and carries out research in process mineralogy for the A.J. Parker Cooperative Research Centre for Hydrometallurgy, Perth, Western Australia.
This research characterises and evaluates liberation of gangue from a gold ore.Liberation for various size fractions is evaluated by sink-float as a proxy.Results establish gold deportment given as a recovery versus mass pull relationship.Liberation behaviour was dependent on crush type and varied by size class.The GRAT can determine the ideal grade and mass splits for a crushed product.
Coarse particle gangue rejection is highly topical due to the potential benefit in removing a significant fraction of gangue prior to fine grinding, thereby creating significant potential savings in energy, water, reagents (and consumables) and significantly reducing fine tailings deposition requirements. The research presented here focusses on characterising and evaluating liberation and separation of gangue from a gold bearing sulfide ore in the 2mm +0.3mm size range (associated with post tertiary crushing). This particle size range was chosen from a materials handling perspective, focussing on material that can be transported by pumping an ore slurry. This paper will review the procedure and results of an in-depth assessment of the amenability of a gold ore to gangue rejection in the pumpable size range. The liberation of the non-sulfide gangue of crushed gold ore is evaluated by proxy using sequential sink-float analysis for various size fractions. SPT (sodium polytungstate) and LST Fastfloat (comprising of lithium heteropolytungstates) were used as heavy liquids. The results establish the deportment of gold to the sink and float products yielding a recovery versus mass pull relationship. Liberation patterns were investigated as produced by various crushing technologies such as vertical shaft impact (VSI) crushing, cone crushing, SelFrag-based comminution, and high pressure grinding roll (HPGR) crushing. The gangue liberation behaviour, using a sink-float proxy, was dependent upon the mode of crushing and varied for different size intervals. The results of the Gangue Rejection Amenability Test allow for the determination of the grade and mass splits that are achievable for a given crushed product. The method is applicable to a wide range ores with varying mineralogies. Significant variation in the amount of barren rejectable gangue for a given gold ore and a given gold loss to rejects was observed for the various crushing technologies at the same 100% pass size, even though the particle size distributions were fairly similar.
The objective of milling gold ores is to extract the gold for the highest financial return. To recover the maximum amount of gold, the ore must be finely ground in order to liberate the gold particles for gravity separation and/or chemical extraction. Ores yielding acceptable gold recovery (more than 88%) when normally ground (6075% 200 mesh), by direct cyanide leaching and defined as amenable (in contrast to the refractory ores, which require extremely fine grinding and/or pretreatment before cyanidation). A general flowchart of the processing steps required to extract gold from its ores is presented in Figure 4-1.