Analysis of ore types applicable to gold cyanide heap leaching technology

As a highly efficient and low-cost gold extraction method, the applicability of cyanide heap leaching technology is highly dependent on the physicochemical characteristics of the ore.  The following is a systematic description of the types of ores suitable for cyanide heap leaching in terms of three aspects: classification of ores, key characteristics and specific deposit types.

I. Classification of ore suitable for cyanide heap leaching

From the metallurgical point of view, gold ores can be divided into six categories, but only the following three categories are suitable for cyanide heap leaching:

 Single oxide ore: such as quartz veins or limestone containing fine-grained natural gold in the ore;

 Single sulphide ores: gold co-occurring with small amounts of pyrite and phosphopyrites, and the sulphides are not over-oxidised;

 Alluvial gold ores: free gold is found in loose sandy or gravelly layers that are easily penetrated by cyanide solutions.

 The remaining three types (difficult to select, complex sulphide ores, associated heavy metal ores) are usually not suitable for direct heap leaching due to the presence of substances interfering with the cyanide reaction (e.g., antimony, arsenic, carbonaceous materials, etc.), or dense gold grains encapsulation.

 Ⅱ.the key characteristics of the ore suitable for heap leaching

Gold leachability: gold and silver need to be able to effectively dissolve through cyanidation, and does not contain adsorption of gold complexes or lead to premature precipitation of gold (such as activated carbon);

Gold grain size: Gold grains need to be sub-micron to micron in size, with fine-grained gold more readily contacting cyanide;

Permeability of the surrounding rock: the ore needs to be porous or breakable to a porous state (porosity ≥ 30%) to ensure uniform penetration of the solution;

Low interference content: no significant clay (impedes penetration), sulphides (depletes cyanide) or acid-forming minerals (requires additional pH adjustment);

Structural stability: the ore pile needs to be physically stable during leaching to avoid collapsing and causing the solution to short circuit.

III. Specific applicable deposit types

According to the global heap leaching practice, the following seven types of deposits are highly applicable:

Lime siltstone type: containing submicron gold grains, accompanied by a small amount of pyrite, toxic sand, etc., with loose and porous surrounding rocks;

Siliceous siltstone type: micron-sized gold grains coexist with residual iron oxide, and the siliceous matrix facilitates solution penetration;

Sandy dolomite ore: gold is endowed in the grain interface, dolomite is easy to break and dissociate the gold grains;

Limonite fissure type: gold embedded in limonite cavities or fissures in quartz veins, cyanide can reach the gold surface;

Schist type: rock layers contain free gold, and the schist structure promotes solution flow;

Hot spring quartzite type: siliceous quartzite is porous and gold grains are evenly dispersed;

Low-grade outcrop ore: particle size ≤15cm after crushing, the permeability of the ore pile meets the standard.

IV. Treatment optimisation of special ores

For ores containing clay or chalk, permeability can be improved by granulation technology:

Add lime or cement as binder to form a porous particle skeleton;

After granulation, the leaching rate of the ore can be increased from 20% to 75%, and the leaching time can be shortened by more than 80% (e.g. the case of New Root Mine in the United States).

V. Conclusion

Cyanide heap leaching technology is mainly applicable to oxidised ores, sulphide ores and alluvial gold ores with fine gold grains, porous surrounding rocks and few disturbing components. Its core lies in the permeability of the ore and the degree of gold dissociation, and the introduction of granulation technology further extends the applicable ore range. The application of this technology to more complex ore types will need to be promoted in the future in conjunction with environmentally friendly technologies, such as cyanide-free treatment. Cyanide heap leaching will continue to provide a cost-effective solution for the development of low-grade gold ores through accurate beneficiation and process optimisation.