How to Test the Purity of Silica Sand After Processing

After multi-stage purification including scrubbing, magnetic separation, flotation, acid leaching and high-temperature calcination, silica sand needs systematic purity testing to judge whether it meets glass-grade, photovoltaic or semiconductor-grade standards. Based on the quartz processing technical system from quartz-mill.com, this article classifies detection methods into rapid on-site screening tests, routine laboratory quantitative analysis and ultra-trace impurity testing for high-purity electronic silica, covering testing principles, detection targets and applicable product grades.

1. Preliminary Visual & Physical Rapid Inspection (On-Site Quick Screening)

This batch pre-inspection is carried out immediately after finished silica sand drying, with no complex instruments required, used to eliminate unqualified products at low cost.

1.1 Whiteness Test

A whiteness meter measures surface reflectance of silica sand compacts. Iron oxide, clay and residual feldspar will reduce whiteness significantly.

• Glass/ceramic grade silica sand: whiteness ≥ 90
• Photovoltaic high-purity silica sand: whiteness ≥ 94
  Low whiteness indicates incomplete removal of iron-titanium films and clay slimes, requiring re-scrubbing or magnetic separation.

1.2 Visual Observation & Particle Impurity Inspection

Spread dried silica sand evenly on clean white ceramic plates under strong light; pick out visible colored particles (black ilmenite, red iron oxide, off-white feldspar aggregates). Excessive colored grains mean insufficient flotation or magnetic separation efficiency.

1.3 Conductivity Test of Washing Liquid

Take silica sand sample, soak with ultra-pure deionized water and stir fully, then test liquid conductivity. High conductivity represents excessive soluble alkali metal impurities (Na, K, Li) from unremoved feldspar or residual acid salt. This test quickly judges the effect of countercurrent rinsing after acid leaching.

2. Conventional Quantitative Element Analysis for Industrial-Grade Silica Sand

For glass, casting and ordinary filler silica sand, XRF (X-ray Fluorescence Spectrometry) is the mainstream routine testing method, supporting fast full-element quantification of major impurities.

2.1 XRF X-ray Fluorescence Analysis

Principle

Grind silica sand into fine powder, press into uniform test sheets, and use X-ray excitation to measure spectral intensity of each element to calculate mass percentage of components.

Detected Targets

Main composition: SiO₂, Al₂O₃, Fe₂O₃, TiO₂, K₂O, Na₂O, CaO, MgO

Advantages

Fast detection (10–15 minutes per batch), non-destructive, suitable for factory daily batch sampling inspection.

Judgment Standard Reference

• Ordinary glass silica sand: SiO₂ ≥ 99.0%, Fe₂O₃ ≤ 0.02%, Al₂O₃ ≤ 0.5%
• Photovoltaic raw silica sand: SiO₂ ≥ 99.9%, Fe₂O₃ ≤ 0.001%, Al₂O₃ ≤ 0.05%

Limitation: XRF cannot detect ultra-trace impurities below ppm level, so it cannot be used for semiconductor-grade silica evaluation.

2.2 Wet Chemical Titration (Traditional Benchmark Method)

Chemical wet digestion serves as a calibration standard for XRF data, widely adopted by small mineral processing plants without spectral equipment.

1. Melt silica sample with flux at high temperature to fully dissolve all mineral components;
2. Configure standard titration solutions to separately quantify silicon, aluminum, iron, titanium and alkali metals;
3. Calculate mass content via consumption of titrant.

Advantage: low equipment cost; Disadvantage: long testing cycle, complicated operation, risk of reagent contamination affecting data accuracy.

3. Ultra-Trace Impurity Testing for High-Purity & Semiconductor-Grade Silica

When producing solar silicon raw materials, quartz crucible silica and semiconductor-grade ultra-high-purity silica (4N–5N purity), trace impurities at ppm/ppb levels must be tested, relying on high-precision mass spectrometry.

3.1 ICP-MS Inductively Coupled Plasma Mass Spectrometry (Core Ultra-High Purity Test)

Testing Process

1. Digest silica sand completely with high-purity hydrofluoric acid and nitric acid in a clean PTFE digestion tank;
2. Heat to volatilize silicon components, leaving only metal impurity ions dissolved in acid;
3. Atomize the solution and send into mass spectrometer to detect single metal element content at ppb level.

Key Detected Harmful Impurities

Al, Fe, Ti, Na, K, Li, Ca, Mg, B, P, Cu, Cr, Ni, Zr

Application Standard

Semiconductor-grade silica requires most metal impurities controlled below 10 ppb, boron below 1 ppb. This test verifies the purification effect of reverse flotation, hot acid leaching and high-temperature chlorination roasting.

3.2 ICP-OES Inductively Coupled Plasma Optical Emission Spectroscopy

Complementary to ICP-MS, suitable for quantifying impurities in the ppm range. It has stable performance for medium-concentration metal elements and is used to cross-check XRF data for photovoltaic silica sand.

4. Special Functional Purity Index Tests for High-End Silica

Beyond elemental impurity content, downstream industries require additional characteristic testing to confirm comprehensive silica purity performance.

4.1 XRD X-Ray Diffraction Test

Identify mineral phase composition of processed silica sand. Detect residual feldspar, mica, clay and other gangue crystal peaks. If gangue characteristic peaks appear, flotation separation is incomplete and reprocessing is required.

4.2 FTIR Infrared Spectroscopy Test

Detect hydroxyl (-OH) groups inside quartz crystal lattice. Excessive hydroxyl impurities will reduce high-temperature resistance of quartz crucibles and optical components, a critical indicator for photovoltaic and semiconductor quartz products.

4.3 Heavy Metal & Radioactive Element Detection

For food filler and optical-grade silica sand, test lead, arsenic, cadmium and radioactive elements such as uranium and thorium to meet environmental and safety access standards.

4.4 Particle Size & Grain Morphology Analysis

Laser particle size analyzer tests particle size distribution; scanning electron microscope (SEM) observes surface residue of impurity particles. Even if elemental content meets standards, excessive fine slime or surface adsorbed impurities will still degrade product purity performance.

5. Standard Sampling Rules to Ensure Accurate Test Results

Unstandardized sampling leads to large purity test errors. The standardized sampling process matched with quartz-mill processing lines is as follows:

1. Multi-point sampling at different positions of finished material silo or conveyor belt to avoid single local sample deviation;
2. Mix all sampled silica sand evenly, use quartering method to shrink samples into representative test portions;
3. Fully clean crushing and grinding tools with pure silica before sample preparation to prevent external metal contamination;
4. Set parallel samples for each batch testing to eliminate instrument and operational errors.

Silica sand purity testing forms a tiered inspection system matching different product grades:

1. On-site rapid physical tests (whiteness, conductivity, visual inspection) for preliminary batch screening;
2. XRF and wet chemical titration for routine elemental detection of industrial glass/ceramic silica sand;
3. ICP-MS and ICP-OES for ppb-level ultra-trace impurity analysis of photovoltaic and semiconductor high-purity silica;
4. Auxiliary XRD, FTIR and SEM testing to verify mineral phase, lattice hydroxyl and surface impurity morphology.

Combining multi-dimensional testing data can fully evaluate the overall purification effect of scrubbing, magnetic separation, flotation and chemical leaching, providing reliable quality basis for silica sand classification and factory delivery.