Washing and attrition scrubbing remove clay slimes, surface iron oxide coatings, silt and organic contaminants bonded on silica sand grains. This physical pretreatment is widely used before magnetic separation, flotation or acid leaching for high-purity quartz production.
Step 1: Pre-Screening & Coarse Washing
Raw silica sand first passes through vibrating screens or drum washers to remove large debris, plant roots, big clay lumps and oversized rock fragments. High-pressure water sprays break loose clay aggregates. Coarse waste is discharged, and sand slurry flows into the sump. This step prevents large blocks from damaging downstream scrubbing equipment.
Step 2: Slurry Concentration & Pulp Preparation
Adjust the slurry solid concentration to 60–70% for attrition scrubbing. Too dilute pulp reduces particle friction; too high viscosity limits movement. Add clean process water. For heavily stained sand, a small amount of dispersant can be added to disperse clay slime and prevent slime coating on quartz surfaces.
Step 3: Attrition Scrubbing (Core Stage)
Feed prepared slurry into an attrition scrubber. High-speed rotor paddles create intense particle-to-particle friction and shear force. Sand grains rub against each other to peel off thin iron oxide films, tightly bonded clay and surface stains that simple water washing cannot remove.
- Typical retention time: 10–30 minutes
- Multiple scrubbing stages are adopted for low-iron quartz targets
- No heavy grinding; only surface friction to preserve original grain shape
Step 4: Desliming & Secondary Washing
After scrubbing, impurities are detached from silica grains. Pump the slurry to hydrocyclones or desliming cones. Fine slime, clay and suspended iron-rich micro-particles are separated as overflow tailings. The underflow clean silica sand goes to spiral washer or vibrating rinsing screen for further rinsing. This critical step stops fine impurities from re-attaching onto sand surfaces.
Step 5: Final Rinsing
For high-purity silica sand, use deionized water for final rinsing to wash away residual dissolved ions and fine slime. Monitor overflow water turbidity; finish rinsing until the water becomes clear.
Step 6: Dewatering
Dewatering screens, filter presses or thickeners remove free water from cleaned silica sand. Reduce surface moisture to 12–18% for next purification processes (magnetic separation, flotation or acid leaching).
Step 7: Water Recycling & Waste Sludge Treatment
Wash water is sent to thickeners for clarification. Recycled clean water returns to the washing circuit. Settled clay sludge is dewatered and disposed separately.
Key Process Notes
- Scrubbing only removes surface-bound impurities; it cannot eliminate lattice iron or embedded mineral inclusions.
- Avoid steel contact in scrubber liner and rotor for ultra-low iron silica sand; use polyurethane or ceramic lining.
- If slime remains after scrubbing, slime coating will lower iron removal efficiency in subsequent magnetic separation.
The full sequence: pre-screening coarse washing → slurry concentration → attrition scrubbing → hydrocyclone desliming → fresh water rinsing → dewatering → water recycling. This combination mechanically strips surface clay and iron stains, serving as essential pretreatment for high-purity silica sand purification.
