Gold Metallurgy and Leaching Process

Gold Metallurgy and Leaching Process

Leaching, often gold, is the process of extracting a soluble constituent from a solid by means of a solvent. In extractive metallurgy, of gold, it is the process of dissolving a certain mineral (or minerals) from an ore or a concentrate, or dissolving certain constituents from materials such as a calcines, mattes, scrap alloys, anodic slimes, etc., to achieve either one or two purposes:

  • Opening of ores, concentrates or metallurgical products to recover the metal values.
  • Leaching easily soluble constituents (usually gangue minerals) in an ore or concentrate in order to have it in a more concentrated form, e.g. leaching of tungsten flotation concentrate with hydrochloric acid to dissolve away calcite and apatite.

Leaching is a heterogeneous process, involving three reaction steps:

  • Diffusion of solvent through the pores in the solid particles, and of the dissolved substance outwards after solution.
  • Chemical solution at the reaction site in the particle.
  • Transport of the dissolved substance in the solvent away from the particle surface.

Factors controlling the rates of the above steps are temperature, agitation, concentration of leachant, particle size, leaching time and pH.

Commonly leached metals are:

  • Naturally occurring metals in the native state, e.g. Au, Ag, Platinum, Cu.
  • Produced as a by-product of a metallurgical process, e.g. Au, Ag, Pt, Se.
  • Produced by metal oxide reduction, e.g. Cu and Ni.

Gold and Silver Leaching

For Gold and Silver, the most common leachants used are Na or K cyanides in the presence of oxygen, Thiourea (NH2. CS.NH2), also in the presence of oxygen, or Ammonium polysulphide (NH4) 2Sx, especially for Gold.

Optimum physical conditions for cyanidation are:

  • Maximum temperature of 85oC,
  • Operating ph range of 9.5 – 11.5
  • A (CN)/(O2) molar ratio of between 4.6 and 7.4

Minerals characteristically associated with Au and Ag are commonly pyrite, galena, sphalerite, arsenopyrite, stibnite, pyrrhotite and chalcopyrite and occasionally various selenium minerals, magnetite and uraninite. Carbonaceous matter, if present in Au ores usually cause high cyanide consumptions since they absorb gold cyanide complex. Metallic minerals that dissolve in cyanide solution can have either an “accelerating” or “retarding” effect on cyanidation.

“Accelerating agents” are small amounts of Pb, Hg, Bi and Thallium salts, the rapid dissolution of gold in the presence of these ions being possibly due to alteration in the surface character of gold by alloying with the displaced metals.

“Retarding” effects may be due to any of the following:

  1. Consumption of oxygen from solution, e.g. Pyrrhotite accompanying gold in its ores decomposes in an alkaline medium forming ferrous hydroxide (Fe(OH)2) and sodium sulphide each of which oxidize readily, depleting 02 levels in solution.
  2. Formation of complex cyanides, eg. Cu, Zn and iron minerals may dissolve preferentially in cyanide solution, thereby depleting the solution of its cyanide content.
  3. Formation of thiocyanate: the sulphide ion liberated when the sulphide mineral reacts with cyanide and oxygen forms a thiocyanate which has no effect on Au.
  4. Adsorption on gangue mineral: Gold ores containing aluminosilicates or other silicates when finely divided in an aqueous alkaline medium often form colloidal Silica and Alumina to which ferric hydroxides with a strong adsorptive capacity for sodium cyanide get attached.
  5. Film formation on the surface of the metal
  • As a consequence of build-up of a thin layer of sulphide ions

  • Formation of a layer of calcium peroxide at pH > 11.5 or

  • Films of insoluble cyanide, e.g. Pb(CN)2 or of flotation reagent like potassium ethyl xanthate.

The effects of retardants can be ameliorated somewhat by addition of Lead salts like lead oxide, nitrate or acetate which, as insoluble lead salts, eliminate the sulphide ions as soon as they are formed. Additions of small amounts of Potassium permanganate also help eliminate difficulties due to sulphides by apparently oxidizing the sulphide ion to sulphate. Additionally, agitating the ore pulp in an alkaline medium helps decompose sulphide minerals, after which the solution is discarded. The sulphide-free pulp is then subjected to cyanidation.

Native Copper and Nickel containing ores are generally leached in aqueous ammonia solutions, important parameters being ammonium ion concentration and Oxygen partial pressure.