Cyanide Leaching

Small Gold Leaching Circuit Design

The gold cyanide leaching mill site is centrally located with respect to the mine, water supply and improved highway. This selection reduced both the length of the haulage road from mine to mill and main water supply line to a minimum. The location provided a sloping mill site, adequate tailings disposal area, housing space for both mine and mill supervisory employees and operating labor, plus a suitable area for assay office, machine and maintenance shops, lime kiln, power plant, in addition, strip for small planes for emergency usage. A tramway was installed to deliver ore from the mine portal to a point accessible for truck haulage for six miles to the mill. The water supply was developed by wells in the gravel of a river-bed and pumped four miles through a 4-inch line to the mill site.

Small Gold Leaching Circuit Design Local conditions as to labor, materials, mine location, water supply, and accessibility were controlling factors in the planning and design of this 200-ton continuous counter current gold cyanidation plant.

Cyanide Leaching Flowsheet

The flowsheet was developed from laboratory tests conducted by the 911Metallurgist, and is briefly as follows: Ore from ….Read more

Electrolytic Oxygen in Cyanide Solutions

There are two conditions generally prevailing upon the earth—those within atmospheric influence, tending towards oxidation, and those away from atmospheric influence, tending towards reduction. Practically all mineral substances from mines of any depth are in a reducing condition.

Since the cyanide process, in order to dissolve silver or gold, requires that the prevailing conditions under which it operates shall be oxidizing, and the materials usually acted upon being of a reducing character, it becomes necessary to supply oxygen to the solution carrying the cyanide. This oxygen is usually supplied through the medium of dissolved air in the solution, or through the medium of various chemical compounds, which upon combining with the solution or the ore give off a part of their oxygen.

Strange as it may seem, practically all mineral substances are partly soluble in water, especially water carrying alkali or cyanide. The greater the surface exposed and the liner the material is ground, the greater will be the rate of dissolving of the reducing-agents from the ore into the cyanide solution. In most cases, if the solution carrying the ore particles is agitated with air, the air will dissolve into the solution faster than will the reducing-agents; ….Read more

Fine Grinding in Cyanide Plant

Fine_Grinding_in_Cyanide_PlantThis plant, near Marysville, Montana, was planned to treat the ore from the Piegan and Gloster mines, the latter being one of the early and famous producers of the Marysville district. When the mill was closed, treatment consisted of stamp milling, followed by pans and settlers for pan amalgamation. The extraction was evidently poor, because, a few years later, thousands of tons of tailings were re-treated by the ranchers in the valley below the mill, by the then new cyanide process, and gave a handsome profit.

After laboratory tests and an experimental mill run, decided upon the following treatment: crushing to 40 mesh in cyanide solution, concentration, amalgamation, classification, and cyanidation of sands by leaching, and of slime by agitation and decantation in charges. The ore averaged about $7 per ton, gold and silver, the ratio of weight being 1 oz. gold to 7.56 oz. silver. The ratio of gold and silver in tailings was 1 oz. Au to 36.6 oz. Ag; the quartz was sharp and sandy, however finely ground, and rather difficult to slime.

The mechanical equipment was a No. 5 Symons gyratory crusher; three ….Read more

Silver Flotation Concentrate Leaching and Filtration Circuits

silver flotation Silver Flotation

Producing a Silver Flotation concentrate from this EXAMPLE Argentite (cubic silver sulfide (Ag2S)) Flotation Circuit enters the first of four successive Argentite leach tanks. The purpose of this leach circuit is to dissolve the silver and gold recovered in the Flotation Circuit. A high strength solution of sodium cyanide, air, and lime are added to the first tank to provide the excess cyanide, oxygen, and protective alkalinity required for successful leaching. Dissolved precious metals are recovered in the small Argentite drum filter, the tails are sampled and fed to the tails box for disposal, and the pregnant solution is delivered to the primary filters vacuum receiver, where it is combined with the pregnant solution from the other filtration circuits. This section is intended to be read in conjunction with Flowsheet and Piping and Instrument Diagram.

The Argentite, this silver mineral, Leaching and Filtration Circuit is designed to the following criteria.

Silver Leaching Retention Time96 hours
Cyanide Concentration10 gm/litre
Solids Concentration20% solids
Leaching pH10.5
Ag Leach Residue Filtration Rate120 kg/m2/hour
Filter Wash Ratio1.5:1

Detailed Leaching and Filtration Process Description and Control

….Read more

Dissolve Gold and Silver with Cyanide

The addition of gold or silver to an alkaline sodium cyanide solution will cause the gold and silver to react with the cyanide and dissolve into the solution in a process called cyanidation. This process is more frequently referred to as leaching. As well as reacting with the precious metals, the cyanide will react with iron, copper or zinc that may be in the ore. In order to leach the precious metals efficiently, the following leaching conditions must exist:

  1. Excess residual cyanide. More than enough cyanide than what is required to dissolve the gold and silver.
  2. Enough air and therefore oxygen in the leaching solution to react with the cyanide and the gold and silver.
  3. High pH or “protective alkalinity” to prevent the generation of lethal hydrogen cyanide gas.
  4. Sufficient time for the cyanide and oxygen to react with the gold and silver.

Since cyanide reacts with the gold and silver, it is consumed during leaching. The cyanide can and will also react with other undesirable metals and is essentially wasted. Therefore, if excess cyanide is not added to the leach circuit, the amount of available or free cyanide’ decreases, limiting gold and silver dissolution. For this reason, free cyanide is added continuously at the ball mill and ….Read more

Why Oxygen is Important in Cyanidation

The Rule and Function of Oxygen in Cyanidation.—Oxygen appears to be an indispensable factor, either directly or indirectly, in the dissolution of gold and silver by cyanide solutions, except in the case of the haloid compounds of silver. Whether the action of oxygen in the dissolution of gold be a direct one, as illustrated by the Eisner equation, or an indirect one, in the sense of acting merely the part of a depolarizing agent, as maintained by Julian and Smart, will not affect the general statement that it is a necessary adjunct for the dissolution of the precious metal.oxygen use in cyanidation

The most generally useful agent for Cyanidation is atmospheric oxygen, and in many instances sufficient oxygen is absorbed by the solutions in their circulation through the plant to accomplish all that is necessary. Ores that contain reducing constituents may need more oxygen than is obtained in this way, and the additional amount is most easily supplied in the case of percolation by draining dry between washes, and by the use of air lifts or centrifugal pumps, in agitation.

The addition of chemical oxidizing agents, such as potassium ferricyanide, permanganate, sodium ….Read more

Sodium Cyanide Safety – Poisoning – HCN Vapor

Almost all solutions in the plant contain cyanide, and in the absence of any other information it should be assumed that all solutions contain cyanide.

Because of the toxicity of sodium cyanide, it is important that all persons coming in contact with it be completely familiar with and observe the established safety practices.

Sodium cyanide is a rapidly fatal poison when taken internally. Poisoning may occur if sodium cyanide dust is inhaled. Prolonged contact with the skin may cause irritation and possibly poisoning, particularly if there are open wounds or skin abrasions. Sodium cyanide causes eye burns.

Sodium cyanide in contact with acids or weak alkalies liberates highly toxic and flammable hydrocyanic acid (HCN) gas.
Also toxic amounts of HCN can be liberated from water solutions of sodium cyanide. View a cyanide MSDS.sodium-cyanide-safety-poisoning-hcn-vapor

The basic NaCN safety precautions are:

  1. Do not breathe dust or gas. Wear an approved dust respirator when there is danger of inhaling cyanide dust (such as when handling cyanide containers or mixing cyanide).
  2. Avoid contact of cyanides with the skin. Never permit it to contact open wounds or skin abrasions. Wear protective gloves when handling solid cyanides. Wear rubber gloves when handling cyanide ….Read more

Diagnostic Gold Leach Testing

A Gold Diagnostic Leach Test is used to evaluate the distribution or deportment of gold in various minerals is determined by a series of selective leaches, usually by increasingly stronger oxidative acid leaches. Between each stage, cyanide leaching is used to extract the released gold. In this study a total of ten analysis stages were carried out for the 8 composite at a grind size of 150µm P80.

The diagnostic leach results indicate that, on average, about 64 percent of the gold content occurred as free cyanidable gold. Lithology composite ore 4 and 5 were less than 50%. The remaining gold was associated with acid digestible minerals, such as iron sulphides and iron oxides, being, on average, about 27%. Lithology composite on Ore 4 had a significant proportion of the gold content locked in gangue minerals, being about 55% of the gold in the feed.

Diagnostic Leach Test Work
Gold Diagnostic Leach Test Work example

More on Diagnostic Gold Leach Test Work.

Diagnostic Gold Analysis

In order to determine the nature of the gold occurrence and associations within the ore; we do a multi-stage diagnostic analysis where a 1 kg portion was ground to a ….Read more

Cyanide Bottle Roll Leach Test – Leaching Procedures

List Equipment Required to Perform of a Standard Bottle Leach TestBottle Leach Procedure for Cyanidation

  • Winchester bottles, with tops
  • Plastic tape to seal tops of bottles
  • Flexible air lines for possible aeration requirements
  • Magnetic or mechanical stirrer for agitation
  • Timer, pH meter, thermometer and weighing device
  • Leachants, i.e. acid, alkaline or neutral solutions, or specific solutions
  • Filter: vacuum or pressure
  • Titration systems

Procedures

  • Dry the ore and crush it to minus 2380 pm (or as specified, otherwise).
  • Prepare the solution of required concentration (e.g. 1M H2SO4 requires 52 ml of con. H2SO4/litre of water).
  • Place 50 gm sample in the leach bottle and add 250 cc of prepared solution.
  • Allow leaching to take place for 1 hour under stirred conditions, ensuring the right conditions of pH, temperature, % solids, etc.Bottle Leach Procedure for Cyanidation
  • After leaching, discharge contents of leach vessel into clean pressure or vacuum filter without washing the vessel.
  • Filter pulp, and collect pregnant leach liquor undiluted.
  • Wash ad dry the residue and record it weight.
  • Analyse residues and pregnant liquors for relevant metal content.

Precautions

Sulphide Leaching

Methods of leaching sulphide ores can be:

  1. in absence of oxidizing agents
  2. in presence of oxidizing agents
  3. Bacterial leaching

    sulphide_leaching Sulphide Leaching system

Absence of oxidizing agents

Acid leaching:

Dilute acids dissolve some metal sulphides with liberation of H2S, e.g.

ZnS + 2H+ ^ Zn2+ + H2S

Three types of sulphides can be encountered.

  1. Completely soluble sulphides: Zns, CoS, NiS, FeS
  2. Slightly soluble, e.g. FeS2.
  3. Insoluble: PbS, CuS, AS2S3, Sb2S3, CaS
  • Alkali cyanides
    Sodium cyanide has a solvent action on sulphides.
  • Alkali sulphides:
    Sodium sulphide solution reacts with sulphides of arsenic, antimony, tin and mercury to form soluble thiosalts, additions of NaOH helping prevent hydrolysis of Na2S.
  • Alkali hydroxides:
    NaOH may be used to leach PbS and ZnS forming soluble plumbite (Na2Pb02) and zincate solutions respectively (Na2Pb02) respectively.
  • Double decomposition
    Copper sulphate can be used to separate Copper from Nickel in a sulphide ore or Copper from Fe in Cu -Fe sulphide ore. CuSO4 solution can also be used for removing Pb, Zn, and Fe from copper sulphide ores.

In presence of oxidizing agents:

  1. Ferric ion:
    Ferric sulphate or chloride react with metal sulphides liberating elemental sulphur, e.g.

Zns + 2Fe3+ ^ Zn2 + 2Fe2+ + S
And in the case of cuprous sulphide
Cu2S + 2Fe3+ ^ CuS + Cu2+ + 2Fe2+
CuS + 2Fe3+ ^ Cu2+ ….Read more

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