Impurities in Gold Ore: Types and Removal Methods

Gold ore typically contains more than just gold. In addition to the target metal, the ore also contains gangue minerals and impurity elements that often interfere with the recovery process or reduce the grade of the final product. Identifying which impurities are present and their form of occurrence is the first step in determining how to treat them.

Identifying Impurities and Their Occurrence Forms

The primary distinguishing factor lies in the form in which harmful elements are present. If they exist as discrete minerals, they can be treated using physical beneficiation methods; if they exist as solid solutions (i.e. displaced into the mineral lattice), physical methods cannot separate them, and chemical or pyrometallurgical processes must be employed. This key issue determines the type of process to be applied, and as it cannot be resolved by chemical analysis alone, mineralogical testing is essential.

Common interfering elements in gold ore include arsenic, sulphur, and carbon- or clay-bearing materials. Antimony and copper also have a significant impact, as they interfere with the cyanide leaching process; however, it is worth noting that they do not interfere with thiourea leaching.

Arsenic and Sulphur in High-Sulphur Concentrates

High-arsenic, high-sulphur flotation concentrates are typical examples of difficult-to-process gold ore materials. When flotation is applied to cyanide tailings, sulphur, iron and arsenic become enriched in the product, forming difficult-to-process concentrates that cannot be subjected to direct cyanide leaching. There are two approaches to addressing this.

High-acid washing following roasting. The flotation concentrate is subjected to oxidative roasting to remove arsenic and sulphur. The resulting roasted concentrate has a loose, porous structure, which facilitates the leaching of gold and silver. This is followed by high-acid pre-treatment washing, which dissolves and washes away most of the residual impurities, thereby ensuring cyanide recovery rates and effectively improving the gold grade.

Thiourea leaching. The concentrate is leached using an acidic thiourea solution. Acidic thiourea solutions rapidly dissolve gold and silver, whilst being low in toxicity and easy to regenerate. Crucially, whilst minerals containing antimony, arsenic, copper and sulphur may interfere with the cyanidation process, they are insensitive to thiourea; consequently, the thiourea leaching method is capable of extracting gold and silver from ores that are difficult to process using cyanidation. The leaching time is typically much shorter than that required for cyanidation. Carbon-bearing and clay-bearing materials

For vein-type gold ores with high carbon or clay content, the use of flotation is also advantageous; the gold-bearing sulphide concentrate produced by this process can subsequently be treated using thiourea leaching. The principle is the same: flotation concentrates the gold and interfering minerals into a smaller volume of material stream, and the subsequent chemical treatment targets only this stream, rather than the entire ore.

Sulphide impurities in concentrate products

The issue of impurities is not confined to the leaching stage; it is equally crucial in concentrate quality control. For example, during fluorite flotation, small amounts of sulphide inhibitors, such as cyanide, must be added to suppress residual sulphides and ensure concentrate quality. In experimental studies, concentrate analysis serves precisely this purpose: to identify the forms in which impurities are present and the causes of poor quality. For polymetallic rough concentrates, analysing the intergrowth relationships between minerals provides a basis for subsequent purification steps.

Mud and other difficult-to-treat substances

Mud is a widespread problem in mineral processing, and gold ore beneficiation is no exception. Taking manganese ore as an example, mud not only reduces the grade of useful minerals but also severely impairs the effectiveness of flotation, gravity separation and magnetic separation; therefore, pre-washing is usually required. Gold ore beneficiation processes follow the same logic: the properties of the slurry must be controlled prior to separation; test programmes will explicitly examine the impact of clay and other harmful substances (including the return of intermediate products to the process).

Conclusion

The removal of impurities from gold ore is not a single unit operation, but rather a series of decision-making processes: first, determining which elements are harmful; assessing whether they exist as discrete minerals or in the form of solid solutions; where applicable, employing physical separation; if impurities are encapsulated within sulphides or carbonaceous matrices, they are concentrated via flotation, followed by ‘roasting plus high-acid leaching’ or ‘thiourea leaching’ to release the gold. Determining the optimal process combination relies on mineralogical studies and closed-circuit tests conducted on specific ores, rather than relying solely on experience.