MINERAL PROCESSING KNOWLEDGE · SILICA SAND PROCESSING
High-solids attrition scrubbing makes the sand clean itself, grain against grain at 60-70% solids, stripping surface films that ordinary washing leaves in place.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Attrition scrubbing is the standard high-energy cleaning step in a silica sand flowsheet. Its principle is deliberately simple: rather than relying on dissolution or on a water jet, the machine puts the pulp into a condition where the particles rub against one another with enough energy to shear away anything loosely attached to their surfaces. Almost every design decision follows from that idea. The pulp has to be as dense as the machine can handle, the impeller has to deliver shear rather than circulation, and the circuit downstream has to capture the fine material the scrubber has just released.
Technical note: Scrubber performance is a property of the whole circuit, not of the machine alone. Pulp density, impeller speed, residence time, feed size distribution and the efficiency of the following classification step all influence the final iron result, and should be fixed through testwork on representative sand rather than by rule of thumb.
1. Why the scrubber runs at 60-70% solids
The defining condition of attrition scrubbing is pulp density. In silica sand practice the scrubber is fed with a slurry at 60% to 70% solids by weight, which is far denser than the pulp used for washing, classification or flotation. At that density the mixture behaves as a dense, viscous mass in which grains are in near-continuous contact with their neighbours.
That contact is the whole point. In a dilute pulp, grains are separated by water and the impeller tends to shear the water rather than the particles, so particles travel with the fluid and rarely meet with any force. In a dense pulp, the water volume between grains is small, energy is transferred from particle to particle, and the surface of each grain is repeatedly struck by the surfaces of others. The scrubbing action is therefore not supplied by the impeller directly; the impeller supplies energy, and the particle bed converts it into attrition.
- Density is the mechanism, not a process preference: grains must touch for attrition to occur.
- Water is a poor transmitter of shear at low solids, so dilution destroys the effect being paid for.
- There is an upper practical limit, above which the pulp stops flowing and the drive is loaded without useful work.
- Feed preparation matters: oversize lumps, tramp metal and heavily clay-bound material should not reach the scrubber untreated.
2. Where the energy is converted into cleaning
The equipment used for this duty is a vertical, high-power attrition scrubber: a tank with one or more impellers on a central shaft, driven at high speed and selected for high power per unit of volume rather than throughput. The impeller tips create local high-velocity zones in which grains are accelerated, thrown against each other and sheared. Two effects act at the same time. The first is true attrition, in which the weathered film on a grain is ground away by contact with neighbouring grains. The second is shear, in which clay and loosely bound weathering products attached to the surface are torn off and dispersed into the pulp.
The result of both is the same from a flowsheet point of view: contamination that was fixed to the sand becomes detached fine material suspended in the slurry. Nothing has been dissolved at this stage; the iron has only been moved from a saleable grain surface into the water.
| Operating variable | What it controls | Design consideration |
|---|---|---|
| Pulp density | Particle contact and energy transfer | Run at 60-70% solids by weight; dilution cushions the grains and wastes power |
| Impeller speed | Tip velocity and collision frequency | Higher speed raises energy input per tonne and wear rate together |
| Residence time | Completeness of film removal | Best fixed by bench or pilot testing on the actual sand |
| Feed size distribution | Whether grains can abrade one another | Very fine feed cushions contact and reduces scrubbing efficiency |
| Water addition | Pulp density at the inlet | Controlled addition is easier than correcting a wet feed afterwards |
3. Capturing what the scrubber has released
A scrubber detaches contamination; it does not remove it from the circuit. Immediately after scrubbing the pulp is diluted and passed to hydraulic classification or desliming, where the fine fraction carrying the stripped iron leaves with the overflow while the cleaned coarse product reports to the underflow. The two steps have to be designed as one. If dilution is late, or if the released slime recirculates, the iron settles back onto clean grain surfaces and the scrubbing benefit is lost in the thickener rather than captured in the product.
Reagent addition can raise the efficiency of the mechanical work. Adding a suitable acidic scrubbing reagent, such as oxalic acid, helps to loosen and stabilise the iron that the grain-to-grain action has exposed, so that more of it reports to the fines stream instead of remaining attached. Reagent dosage and the associated water balance need to be established in testwork before the reagent is written into the operating procedure.
Watch the size balance as well: attrition produces some fine material from the grain surfaces, and a stage that improves the iron analysis while shifting the size distribution outside the product window has only moved the problem.
Conclusion
Attrition scrubbing is a dense-pulp, high-energy process whose value lies in converting surface contamination into removable fines. Run the scrubber at the high solids range the duty requires, give the impeller enough power to load the particle bed, classify the released slime out promptly, and treat the reagent as an assist rather than as the mechanism. The combination of dense pulp, adequate energy and immediate desliming is what separates a scrubbing stage that changes the iron analysis from one that merely consumes power.
How LiCheng Tech can help
LiCheng Tech builds silica sand washing and scrubbing lines and the equipment that surrounds them, including screening, classification, fine sand recovery and dewatering units. Share your raw sand analysis, the target iron level and the required size distribution, and our engineers will help define the scrubbing duty, the pulp density and the desliming arrangement that suit your material, then confirm the design through representative testwork. Compare the sand washer, high-frequency screen and classifier typically used around a scrubbing stage.
Explore the Mineral Processing Solutions or send your project information.