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Gold and silver metallurgist

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ECOLOGICAL CONSIDERATIONS IN CYANIDATION PLANT PRACTICES

4/17/2016

 
Ecological controls of tailing storage pond effluents are mainly concerned with mercury and residual cyanide and are critical problems for gold and silver recovery plant operators. For example, mercury for direct amalgamation of gold ore was used at Homestake until 1970 when the Federal Government acted to prevent further use thereby forcing the abandonment of this practice. However, mercury is still used at Homestake, in South Africa, and also in some Philippine and Canadian cyanide plants for barrel amalgamation of free gold gravity concentrates. The amount of mercury involved in South African practice is about 150 ounces for every 100, 000 tons of ore milled and which reaches the tailings ponds. In South Africa, it is obligatory that no pond effluent be discharged beyond the mining property boundaries and waters containing mercury and cyanide must be evaporated either on the tailings pond or in specially constructed evaporation dams.

In some cyanide plants such as Carlin and Cortez in the United States, all supernatant solution from the tailings ponds are returned to the plant for reuse. When all solution cannot be reused, the excess may be evaporated in shallow ponds, or by spray systems inside of the tailings dam. Such plant reuse of tailings effluent depends strictly upon the degree of contamination or "fouling" of the barren solution and tailing water with cyanicides. Foul solutions that cannot be returned from the tailings pond may be effectively cleaned up by cyanide regeneration. Cyanogens in plant solutions exist chiefly as free alkali cyanides, as zinc and copper double cyanides, as sulphocyanides, and as ferrocyanides. From the free cyanides and zinc double cyanides, substantially complete recovery of the cyanogen is easily effected. Part of the cyanogen combined with copper is also recovered; regeneration of the remainder with the cyanogen existing as sulphocyanide and ferrocyanides requires special treatment. At this stage, it would be generally more practical to eliminate the remaining cyanogen by chlorination.

Ecologists have also been concerned with pollution of ground waters from cyanidation plant tailings. An accute problem is solution seepage down stream from tailings dams and strict attention to prevent or collect any seepage is a real necessity.

The tolerance levels of the total cyanide content in potable water set by several U. S. and State health agencies are 10 ppb, and 20 ppb discharged into ground waters. The cyanide content so established is, strangely enough, based on the survival tolerance of fingerling trout. Decanted waste water from tailings dams may be effectively treated by the use of alkaline chlorination for the purpose of destroying all cyanide compounds. The alkaline hypochlorite used to oxidize the cyanide to cyanate is applied as such or generated within the waste solution by adding caustic (NaOH) and chlorine gas. A second chemical reaction involves complete destruction of the sodium cyanate.

RECOVERY METHODS AS RELATED TO PROPERTIES OF SILVER ORES

4/17/2016

 
The earliest metallurgical process for treating silver ores was amalgamation with mercury which was in use in the early 1500's. Closely following was the development of the Patio process for treating ores at Pachuca, Mexico.

Silver often occurs as the native metal and in deposits associated with other metals such as gold, copper, lead and zinc. Principal silver minerals include compounds of sulfur, antimony, arsenic, and copper. Silver chloride, and argentiferous galena are also prime sources.

Native silver and the chloride characterize the oxide zone of most deposits. Native silver can be concentrated, amalgamated, or cyanided. Silver chloride rapidly dissolves in cyanide without oxygen. Oxidized silver ores containing the higher oxides of manganese are generally refractory to metallurgical treatment. A refractory compound of manganese and silver is formed, probably a manganite, which is insoluble in cyanide solutions.

Argentite is the predominant silver mineral. Other important economic minerals are native silver, argentiferous galena, cerargyrite, pyrargyrite and tetrahedrite. Argentite dissolves slowly, the reaction being reversible requires an excess of cyanide. Argentite with disseminated fine grained gold occurring in quartz veins with minor amounts of chalcopyrite and galena may be cyanided directly after grinding, with high silver and gold recoveries. The flowsheet adapted to this type of ore is the Minas de San Luis, Tayoltita cyanide plant, which treats a high grade argentite silver ore containing gold. It is a standard cyanidation circuit but employs long periods of agitation contact for the high silver sulfide content ore.

Lead-zinc base metal semi-oxidized-complex ores in pipes and chimneys are replacement deposits in limestone at El Mochito Mine, Honduras. Mineralization comprises galena, cerussite, anglesite, sphalerite, calamine, and smithsonite. The silver content of about 16 ounces occurs as native silver globs, wire and the mineral argentite. After flotation of lead and zinc concentrates, the tailings are cyanided for gold and silver recovery.

The great Huelva pyritic copper deposits of Spain carry a little gold and silver. At Cerro Colorado, these metals were concentrated above the chalcocite zone into the so-labeled "gossan". This material with a content of 0. 08-0. 09 oz Au and 1.50 oz Ag per ton is amenable to cyanidation.

See Cerro Colorado flowsheet.

In southern Hidalgo, about 60 miles north of Mexico City, are the great silver--gold veins of one of the foremost silver mining districts of the world. Real del Monte, Pachuca. The mineralized area consists of flow rocks and intrusives. The oxidized ores carry pyrites with oxides of iron and manganese and other minerals, silver and gold. The first zone contains auriferous iron oxide, chlorides, and bromides of silver. These ores can be cyanided directly. The lower zone contains pyrite, galena, sphalerite, argentite, and chalcopyrite. The Del Monte flowsheet uses selective flotation to produce a lead, zinc, and pyrite concentrate. The tailings are cyanided.
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refractory, Carbonaceous, and Graphitic Ores

4/17/2016

 
Carbonaceous is a term loosely applied to those ores containing black graphitic material which causes dissolved gold to adsorb on the carbon thus causing premature precipitation. The gold adsorbed on the carbon is lost with the tailings. Refractory carbonaceous material in gold ores has presented metallurgical problems since cyanidation was adopted in the late 1800's. Some carbonaceous material (unactivated) may not be an adsorbant for gold. Many schemes have been tried such as blanking the carbon with kerosene or fuel oil thereby inhibiting adsorption of gold from solution. Kerr-Addison employs this practice.

Carbonaceous gold ores in the State of Nevada are essentially hydrothermally altered silty dolomitic limestones. The carbonaceous materials are graphitic or activated carbon and long chain organic compounds similar to humic acids. Oxidation by roasting at 500 to 550°C is effective, but costly. Another treatment is chlorine oxidation in pulp as used at Carlin.

Gold Tellurides, Copper, Lead and Zinc Minerals

4/17/2016

 
Following the native metal, the tellurides are the most important gold minerals. The tellurides include calaverite and krennerite which contain about 40 percent gold, and sylvanite and hessite with about 25 percent.

The Kalgoorlie, Australian, ores contain free gold and tellurides which occur in Pre-Cambrian rock consisting essentially of schists and quartz-dolerite-greenstones. Auriferous pyrite is also present and the gold is occasionally associated with chalcopyrite, tetrahedrite and arsenopyrite.

The gold in the pyrite is finely divided and requires grinding to about 75 percent passing 200 mesh. The ground product is floated and the concentrate, after cyanidation and filtration, is roasted and recyanided. Flotation tailings are also cyanided.

The ores of the Emperor Mine in Fiji contain gold associated with the telluride minerals sylvanite and hessite. A chemical oxidation step is used in place of roasting to liberate the gold for cyanidation.

Gold with Copper Minerals

Gold is often associated with chalcopyrite in porphyry deposits. When recovered into the copper concentrate, it travels through the smelter and to the refinery where it reports with the anode slimes from electrolytic refining and is subsequently recovered as gold bullion. Gold losses in copper concentrating are about the same as for copper, but are negligible in smelting and refining. Gold occurring in pyrite associated with chalco-pyrite can sometimes be separated by flotation into an auriferous pyritic concentrate for cyanidation as at Benguet Exploration. At the Itogon Suyoc, Palidan Mill, the auriferous pyrite and chalcopyrite are recovered into a bulk flotation concentrate which is then separated into two flotation products; a pyrite concentrate for cyanidation of the gold and a copper concentrate for shipment to a smelter.

At San Manuel, Arizona, the gold follows the molybdenite and this concentrate is treated by a standard type of cyanidation flowsheet.


Gold with Lead and Zinc Minerals

Gold occurring with lead-zinc sulfide ores or copper-lead-zinc ores usually is recovered into the flotation concentrates and shipped to a smelter where gold recovery is high, particularly at lead smelters. Occasionally, free gold may be recovered by amalgamating the concentrate from a jig in the grinding circuit. Gold contained in the flotation tailing is recovered by cyanidation as any residual galena or sphalerite is not harmful to cyanidation.

Gold with Pyrite, Pyrrhotite, Marcasite and Arsenopyrite

4/17/2016

 
In this ore classification, the gold occurs both in the free state and disseminated in the sulfides. (Pyrite is found to some degree in most of the world's gold deposits. ) Sulfides tend to decompose in cyanide solutions. Pyrite is the most stable but when pyrrhotite is present trouble is usually experienced both in regard to cyanide consumption and gold extraction. Pyritic flotation concentrates are often reground for gold liberation before cyanidation as at Itogon-Suyoc Itogon, and Pamour. After fine grinding, long periods of agitation are often required to dissolve the gold. Gold-bearing pyrite concentrates are sometimes roasted and cyanided in separate circuits as at Kerr Addison. Also, high grade gold-pyrite flotation concentrates can be shipped to the smelter as is Knob Hill practice. Pyrite and pyrrhotite often occur together creating an overlap in treatment methods.

Gold with Pyrrhotite

Pyrrhotite readily reacts with cyanide to form cyanates and thiocyanates and it readily consumes oxygen. Aeration with lime ahead of cyanidation is usually used on ores in this classification. Aeration for preconditioning is used at Dome, Homestake, Kerr-Addison, and Pamour.

Gold with Arsenopyrite - Arsenic Minerals

Gold is occasionally associated with arsenic minerals as well as pyrite, stibnite, chalcopyrite, etc. Direct cyanidation in these cases is seldom possible. Additionally, when gold is associated with readily soluble arsenic compounds, there is the hazard, in precipitation, of forming arsine, AsH3. In plants where this extremely toxic gas is evolved, special ventilation techniques are required.

Giant Yellowknife produces a refractory flotation concentrate carrying gold in association with arsenopyrite, stibnite, and sulphantimonides of copper, lead and iron. Roasting liberates the sulfide-enclosed gold allowing the calcine to undergo conventional cyanidation. Campbell Red Lake roasts a flotation concentrate ahead of cyanidation.



RECOVERY METHODS AS RELATED TO PROPERTIES OF GOLD ORES

4/17/2016

 
The properties of gold in ores from the standpoint of recovery are its extremely high specific gravity (15.5 to 19. 3 depending upon amount of alloying metal admixed); the fact that mercury wets it readily in the presence of water (amalgamation); its solubility in dilute aqueous solutions of alkaline cyanides to form relatively stable compounds of the form NaAu(CN)2; and its response, particularly as naturally alloyed, to flotation collectors.

Native Gold Ores

Free milling lode ores are those in which the gold is relatively coarse and amalgamable, the sulfide content is low and nonarsenical, oxidized compounds of bismuth and antimony are absent, and the gangue is substantially free from talc, clay and graphitic constituents. With these ores, there are advantages in extracting as much free gold as possible in the grinding circuit by gravity concentration. Concentration of free gold by gravity is a relatively simple method of recovery and when used in cyanide plants is applied ahead of cyanidation. On lode gold ores, launder traps, hydraulic traps or pulsating jigs are sometimes used in the grinding circuits for recovery of as much as 60 percent of the total gold in the mill feed. The jig hutch product may be continuously discharged onto a shaking riffled table with the concentrate fed in batches to barrel amalgamation. Homestake recovers 20 to 25 percent of the gold in launder traps. Other recovery methods have not been successful because of cable splinters, blasting wire, etc. , in the ore. Woolen blankets have long been used for trapping fine gold particles and particularly for tellurides. Blankets are generally laid overlapping on wide inclined tables.

​From this practice of using blankets came the development of corduroy to entrap gold and a South African version of corduroy is sheet rubber having "V" shaped riffles molded into its surface. The Johnson concentrator, an inclined rotating cylinder, the plane tables, and belt concentrators are lined with this material.

Amalgamation depends upon the wetting and alloying of metallic gold with mercury. Direct amalgamation in which the entire ore stream flows over mercury-covered copper plates has now been generally abandoned to prevent stream pollution. It has been replaced by a concentration step which subjects only a relatively small quantity of high grade concentrate to barrel amalgamation. This method eliminates the tedious cleaning and recoating of the copper plates and reduces the chances for loss through theft. The gravity concentrate is ground for several hours in a small mill or barrel with steel balls or rods before the mercury is added. This form of amalgamation is the simplest and most common method of treating an enriched gold-bearing concentrate. Examples of free gold concentration and amalgamation are shown in the flowsheets of Dome, Homestake, Itogon-Suyoc Palidan, Kalgoorlie, Campbell Red Lake, Blyvooruitzicht, and Vaal Reefs.

Following the recovery of the coarser free gold particles by gravity and barrel amalgamation, the grinding of the ore in cyanide solution with ball or pebble mills is generally practiced. Separate cyanidation of sand and slimes has diminished with the development of closed-circuit fine grinding for "all-slime11 treatment by agitation.

Other Free Milling Ores

Gold mineralization in these ores may occur in a limey siltstone containing intermittent shale beds. Sulfides are seldom seen, but pyrite, galena, sphalerite, chalcopyrite, antimony, mercury and arsenic occur in minute amounts. Gold occurring in micron size is readily amenable to cyanidation as at Carlin and Cortez. Other free milling ores are Benguet, Camflo, Kinross, Kloof, and the new Pueblo Viejo.

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