Application of Sodium Hexametaphosphate in Gold Mining And Processing

In general, sodium hexametaphosphate (SHMP) is primarily used as a highly effective dispersant and inhibitor in the gold mining industry, playing a crucial role in processing complex and difficult-to-process gold ores containing argillaceous, high-calcium-magnesium gangue.

Its primary applications and principles can be understood from the following perspectives:

1. As a dispersant

This is the core function of sodium hexametaphosphate in gold flotation.

Problem it solves: Gold ores often contain fine, argillaceous gangues such as kaolin and illite. These fines are extremely fine, possessing a large surface area and high surface energy. During the grinding and flotation processes, they can exhibit the following detrimental behaviors:

Slime Coating: Fine slime non-selectively coats the surface of coarse gold minerals (such as gold-bearing pyrite), encapsulating them and hindering the collector's interaction with the gold minerals, rendering them unfloatable.

Increased reagent consumption: Fine mud indiscriminately absorbs large amounts of flotation reagents (collectors and frothers), leading to a sharp increase in reagent consumption and increased costs.

Deteriorated foam properties: Fine mud can make the foam overly stable and sticky, resulting in a decrease in concentrate grade and making subsequent concentration and filtration operations difficult.

Mechanism of Action: Sodium hexametaphosphate is a polyphosphate that carries a negative charge when dissolved in water. It adsorbs onto the surface of fine mud particles, significantly increasing their negative surface charge density.

Electrostatic Repulsion: Because all fine mud particles have a strong, identical negative surface charge, they generate a strong electrostatic repulsion based on the principle of "like charges repel like," preventing them from agglomerating and maintaining a stable dispersion within the slurry.

Steric Hindrance: The long molecular chain structure also provides a certain degree of steric hindrance, preventing particles from approaching each other.

Result: The dispersed fine mud is no longer able to cover the surface of the gold minerals and cannot absorb large amounts of reagents, thus "freeing" the gold minerals for effective capture by the collector, thereby improving gold recovery.

2. As a Depressant

Sodium hexametaphosphate has a depressant effect on certain gangue minerals.

Targets: Primarily calcium-magnesium carbonate gangue minerals such as calcite and dolomite.

Mechanism of Action: Hexametaphosphate ions ((PO₃)₆⁶⁻) form stable, water-soluble complexes with metal ions such as Ca₂⁺ and Mg₂⁺. They preferentially adsorb on the surfaces of these minerals or react with Ca₂⁺ and Mg⁺ ions dissolved from the mineral surfaces, thereby hydrophilizing the surfaces of these gangue minerals and preventing collector adsorption, thereby inhibiting their buoyancy.

Benefits: This helps to improve the grade of gold concentrate, reduce the influx of carbonate gangue into the concentrate, and reduce subsequent smelting costs (because CaO and MgO are harmful impurities)

3. As a Water Softener

Function: Flotation water or ore may contain high levels of calcium and magnesium ions (hard water). These ions react with collectors (such as xanthate) to form insoluble calcium/magnesium xanthate precipitates, resulting in unnecessary reagent consumption and interference with the flotation process.

Mechanism: Sodium hexametaphosphate strongly complexes Ca²⁺ and Mg²⁺ ions, forming soluble complexes. This softens the water, prevents harmful precipitation, and ensures effective concentration and selectivity of the flotation reagent.

Key Points in the Application Process

Addition Location: Typically added in the grinding circuit or pre-flotation slurry mixing tank.

Reason: Early addition disperses fine sludge immediately after it is generated, preventing it from forming a cap and creating favorable conditions for subsequent flotation operations.

Dosage Control: Dosage is crucial. Too little SHMP will not effectively disperse and inhibit sludge; too much SHMP will also inhibit certain sulfide minerals (including gold-bearing pyrite), resulting in excessively fragile froth, impacting recovery rates and increasing costs. Therefore, the optimal dosage must be determined through detailed ore selectivity testing (both laboratory and industrial). Typically, the dosage ranges from tens to hundreds of grams per ton of ore.

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