Analysis of quartz sand high-efficiency beneficiation process technology

As the core material of shale gas fracturing proppant, the physical and chemical properties of quartz sand have a direct impact on the flow capacity and mining efficiency of oil and gas wells. However, natural quartz sand is often accompanied by clay minerals and trace metal oxides, which need to be purified and optimised by precise beneficiation process. 

 I. Ore pretreatment and physical sorting

1. Crushing and screening

Raw quartz sand needs to be roughly crushed and finely crushed by jaw crusher, cone crusher and other equipment, and the particle size is controlled within the range of 0.1-0.8mm (common specification of proppant). The quartz sand in Yunnan mining area has high hardness (4241.58MPa), which needs to adopt high-pressure roller mill to reduce energy consumption; the ore in Chuannan is agglutinated and loose, which can simplify the crushing process. Screening link adopts high-frequency vibrating screen, grading by particle size (e.g. 20/40 mesh, 30/50 mesh), removing ultra-fine powder (<100 mesh) and coarse-grained impurities.

2. Re-election and flotation to remove impurities

Re-election process: using the density difference between quartz and clay minerals (quartz density 2.65g/cm³, clay is about 2.2g/cm³), using a spiral chute or shaking bed to separate light clay. Yunnan sand is poorly sorted (the particles are angular), and the hydraulic retention time needs to be extended; northern Sichuan sand is well sorted (the particles are sub-circular), and the processing efficiency can be improved 

Flotation process: For iron and aluminium oxide impurities, reverse flotation method is adopted. Under acidic medium (pH=3-4), dodecylamine cationic collector selectively adsorbs clay minerals, quartz is sunk as tailings, and the purity of SiO₂ concentrate can reach over 99%.

Ⅱ. chemical purification and stability optimisation

1. Acid leaching to remove impurities

Mixed acid (HCl+HF) was used to dissolve the surface iron staining and microcrystalline impurities. Experiments show:

Acid leaching parameters: acid concentration 12% HCl + 3% HF, temperature 80 ℃, reaction time 2h, Yunnan sand acid corrosion rate of 6.34%, need to shorten the reaction time to 1.5h in order to reduce the loss; Chuanbei white sand acid corrosion rate of 3.68%, can be moderately increase the concentration of acid in order to enhance the effect of impurity removal.

Acid recycling: Recycle waste acid through neutralisation and precipitation to reduce environmental pollution and cost.

2. Acid resistance enhancement

Reduce the dissolution rate of quartz sand in fracturing acid by surface passivation treatment (such as silane coupling agent coating). After treatment, the acid corrosion rate of Yunnan sand can be reduced to less than 4%, which significantly improves its applicability in acidic reservoirs.

III. Control of key process parameters

1. Optimisation of particle size and sortability

Target particle size: 20/40 mesh (0.45-0.85mm) is commonly used for proppant, which needs to be precisely controlled by multi-stage screening;

Sorting coefficient: Chuanbei white sand has a high sorting coefficient (uniform particles), and can reach the standard by direct sieving; Yunnan sand needs to increase the air classifier to assist sorting.

2. Density and hardness matching

Density control: the density of quartz sand should be ≤2.6g/cm³ (2.47g/cm³ is the best) to ensure the suspension in the fracturing fluid;

Hardness guarantee: Yunnan sand has a hardness of grade 9 (for hard strata), and excessive crushing should be avoided; Chuannan sand has a lower hardness (grade 4), and the compressive strength should be enhanced by resin coating.

3. Microstructure control

Particle rounding degree: adopt rod mill to shape the angular Yunnan sand, and enhance the rounding degree to sub-rounded, so as to reduce the particle crushing during the fracturing process; 

Collodion removal: the yellow sand from northern Sichuan contains film-like clay, which needs to be mechanically stripped by scrubbing machine to reduce the risk of impurity wrapping.

IV. Examples of customised processes

Case 1: Yunnan high hardness quartz sand purification

Rough crushing + high-pressure roller mill: particle size control to 1-3mm;

Two-stage re-election: spiral chute to remove clay, shaking table to select;

Short time acid leaching: acid reaction for 1.5h, SiO₂ purity increased to 99.5%;

Surface passivation: silane treatment to reduce acid corrosion rate;

Grain shaping: 20/40 mesh fine sand sieved after rod milling.

Achievements: fine sand crushing rate ≤5%, inflow capacity up to 70D-cm, meeting the demand of deep shale gas fracturing.

Case 2: Chuannan low hardness sand performance improvement

Mild crushing: cone crushing to 0.5-1mm;

Flotation iron removal: dodecylamine reverse flotation, Fe₂O₃ content ≤ 0.1%;

Resin coating: epoxy resin coating, hardness upgraded to grade 6;

Density regulation: adding hollow glass beads, density reduced to 2.4g/cm³.

Achievements: proppant suspension is improved by 30%, suitable for fracturing medium and shallow wells.

V. Technology Outlook

Intelligent sorting: introduce AI image recognition technology, real-time regulation of screening and flotation parameters;

Green chemistry: develop fluorine-free flotation chemicals to reduce the environmental pressure of acid leaching;

Composite modification: nano-coating technology to enhance the temperature resistance and acid corrosion resistance of quartz sand.

Through process innovation and parameter optimisation, quartz sand beneficiation can significantly reduce the cost of proppant (localised production saves more than 50% of transportation cost), and help efficient shale gas development.