Acid leaching is an effective purification method for quartz ore, but it cannot remove all metal oxides completely. Its removal efficiency depends on the mineral occurrence state, crystal embedding form and acid type.
1. Metal oxides that acid leaching can effectively remove
Free metal oxides exposed on the quartz particle surface or existing as independent mineral grains are readily dissolved by mineral acids (hydrochloric acid, sulfuric acid, oxalic acid, mixed acid).
Typical targets: hematite, limonite, goethite, manganese oxide, loose iron hydroxide coatings on quartz surface.
These impurities are the main cause of low whiteness of quartz powder, and acid leaching can reduce their content to a very low level.
2. Metal oxides that acid leaching cannot fully eliminate
(1) Lattice-substituted metal ions
Metal atoms (Al³⁺, Fe³⁺, Ti⁴⁺) replace silicon atoms inside the quartz crystal lattice. These metal ions are tightly bonded within the silica crystal structure. Acids cannot break the strong Si-O crystal bonds under normal leaching temperature and pressure, so these embedded metal impurities remain.
(2) Refractory silicate mineral inclusions
Impurity minerals such as feldspar, mica, kaolinite, amphibole contain Al, K, Na, Ca, Fe, Ti oxides.
Common inorganic acids have limited etching ability for these silicates. Only hydrofluoric acid can corrode silicate structures, yet HF will also dissolve quartz itself, which is not acceptable for quartz purification.
(3) Fine encapsulated mineral inclusions
Tiny metal oxide particles fully locked inside intact quartz grains. Acid cannot contact these encapsulated impurities unless the ore is ground to liberate the inclusions first. Without sufficient dissociation, acid solution cannot reach the impurity phase.
3. Key influencing factors
- Ore grinding degree: adequate grinding liberates embedded impurities and improves leaching effect.
- Acid formula, temperature, leaching time and solid-liquid ratio. Oxalic acid shows good selectivity for iron oxides with less corrosion to quartz.
- Pre-treatment: magnetic separation and flotation before acid leaching remove most separable impurity minerals and reduce acid consumption.
4. Combined process recommendation
To produce ultra-high purity quartz, acid leaching is only one stage. The full purification chain normally includes crushing, grinding, magnetic separation, flotation, acid leaching and high-temperature calcination / chlorination.
Chlorination at high temperature is the industrial route to remove lattice-bound metal impurities for semiconductor-grade quartz.
Acid leaching efficiently removes surface and free metal oxide impurities, especially iron oxides. However, it cannot remove lattice-substituted metal ions or encapsulated silicate impurities. It is not a standalone solution to eliminate all metal oxides from quartz ore.
