Process characteristics and process of quartz stone pickling

Quartz pickling is the core process of quartz sand purification. It dissolves metal oxides, silicates and other impurities with acid to significantly improve the purity of silicon dioxide (up to 99.7% or more), meeting the needs of high-end fields such as photovoltaic glass, semiconductors, and high-purity quartz.

Process selection suggestions
● Ordinary impurity removal (SiO₂＞94%): simple pickling with hydrochloric acid/sulfuric acid (24-hour immersion);
● High purity requirements (SiO₂＞99.7%): mixed acid (HF system) + adsorbent circulation technology + PPH equipment;
● Environmental protection priority: oxalic acid-ammonium bifluoride system + zero waste liquid discharge design.

Current technology has broken through the bottleneck of traditional pickling efficiency. The future direction focuses on low-temperature catalytic acidolysis (such as Fe²⁺-catalyzed oxalic acid) and fully enclosed automated production lines to further reduce energy consumption and risks.

1. Efficient impurity removal
● Acid combination: Commonly used mixed acids (such as hydrochloric acid-hydrofluoric acid, oxalic acid-hydrofluoric acid) are used to remove metal impurities such as iron and aluminum. For example: hydrofluoric acid (HF) dissolves silicate inclusions; Oxalic acid specializes in iron oxides (such as hematite), and the iron dissolution rate can reach 40% at 90-100°C.
● Auxiliary technology: Adsorption materials: such as the patented separator tube filled with adsorbents (such as modified PP particles), real-time adsorption of dissolved metal ions, extending the life of the acid; impurity removal particles: PP particles with surface modified sodium alginate/chitosan to enhance impurity capture capacity.

2. Circulation and energy-saving design

● Acid regeneration: The acid discharged after pickling is recycled after concentration detection and supplementation with new acid, reducing costs by more than 30%;
● Heating optimization: Use PPH material reaction tank (resistant to 110°C) with electric heating (cost only RMB 3-5/ton of sand), replacing steam/biomass heating (> RMB 10/ton).

3. High corrosion resistance requirements for equipment

●Hydrofluoric acid is highly corrosive, and the equipment needs to use *graphite lining or be coated with ZS-1033 hydrofluoric acid resistant anti-corrosion coating (poly-modified phenolic fluorine resin + flake graphite), which increases the service life by 3 times.

The following is a general process for industrial pickling, combining pretreatment, pickling, and post-treatment:

Stage 1: Pretreatment (physical impurity removal)
1. Crushing and screening: jaw crusher coarse crushing → cone crusher fine crushing → vibrating screen water washing (high-pressure water jet) → 15-100 mesh screening.
2. Magnetic separation and scrubbing: Ultrasonic scrubbing: 9-10 hours at 6-12 W/m² intensity, dispersant (1.45-2.05 kg/ton sand) to assist in stripping surface impurities; high gradient magnetic separation: vertical ring pulsating magnetic separator (0.8-1.6 T increasing magnetic field), remove 99% of mechanical iron.
3. High temperature roasting (optional): roasting at 900-950℃ for 3 hours to remove organic matter/water and make quartz more acid-reactive.

Stage 2: Core process of pickling

1. Acid preparation and feeding

● Acid formula example:

Recipe Type	composition	Applicable scenarios
Oxalic acid-hydrofluoric acid system	0% oxalic acid + 1% HF (aqueous solution)	General purpose iron and aluminum removal
Nitric acid-hydrochloric acid-HF mixed acid	30% HNO₃ + 40% HCl + 35% HF	Deep cleaning
Oxalic acid-ammonium bifluoride	Oxalic acid + 96% ammonium bifluoride (1:12 with water)	Environmentally friendly alternative to hydrofluoric acid

●Solid-liquid mixing: quartz sand and impurity removal particles (volume ratio 0.4-0.48) are loaded into the pickling tank, and the volume of acid accounts for 70-75%.
2. Dynamic reaction control
●Temperature: 60-75℃ (general sand) → 90-100℃ (high purity sand);
●Time: 3-12 hours (depending on the quality of the original ore);
●Mixing method: Rotating pickling tank (3-4 h, 70-75℃); Air compressor bubbling or mechanical stirring to ensure uniform acid exposure.
3. Acid separation and regeneration
●After standing for 40-50 minutes, the acid is discharged, and the floating impurity removal particles are filtered and recovered;
●The acid is recycled after concentration adjustment (the oxalic acid system can be recycled 5-8 times).

Stage 3: Post-treatment (deacidification and drying)
1. Water washing and deacidification: rinse with deionized water until pH is neutral, and neutralize residual acid with alkaline solution (such as NaOH);
2. Dehydration and drying: vacuum filtration (water content ≈ 10%) → natural drying/hot air drying (water content < 5%);
3. Screening and packaging: particle size classification (such as 5-20 mesh), quality inspection of purity/whiteness before packaging.

1. Formula adaptability: The acid ratio needs to be adjusted according to the iron content/morphology of the original ore; High hematite: Oxalic acid is preferred (Fe²⁺ is enhanced at pH=3); Complex inclusions: HF concentration needs to be increased to 40–55%.

2. Wastewater treatment challenges: Fluorine/heavy metal wastewater requires lime milk precipitation → neutralization tank agent treatment (such as NH₄PF₆);
Acid mist is purified and discharged through an alkaline water spray tower.

3. Equipment innovation: PPH integrated reactor: welded pipe + injection flange to prevent leakage; Fluidized bed pickling: Oxalic acid and sand react dynamically in the fluidized bed, increasing efficiency by 30%.

●Operation protection: HF corrosion resistant equipment (mask/rubber gloves) + closed workshop ventilation;

●Circular economy: acid regeneration rate>80%; flushing water concentration tank reuse.