MINERAL PROCESSING KNOWLEDGE · SILICA SAND PROCESSING
Glass sand is bought on chemistry and foundry sand is bought on grain shape and size, and that difference decides which flowsheet a deposit can serve.
Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.
Silica sand is not one product. A deposit may suit flat glass, container glass, photovoltaic glass, foundry moulds and cores, or several of these at once, and the specification that decides which is different in each case. Glass makers buy chemistry: the sand must be pure enough for the finished glass to meet its optical and colour requirements. Foundries buy physical behaviour: the sand must pack, bond and withstand molten metal in a way that depends on grain size and grain shape. Understanding that split before a flowsheet is drawn is often worth more than any single piece of equipment.
Technical note: The figures below are typical industry specifications quoted for orientation. Actual limits are set by the individual customer and by the product being made, and any deposit should be tested against the specification of the intended buyer before a process route is committed.
1. Glass sand: the specification is chemical
For glassmaking the decisive numbers are chemical. High purity and photovoltaic glass sands require SiO2 of at least 99.5% combined with Fe2O3 at or below 0.01%, which is 100 ppm. Ultra-white photovoltaic glass is tighter still, typically requiring Fe2O3 below 80 ppm. Iron is restricted because it colours the glass and absorbs light in the body of the product, and at these levels the impurity is being controlled in parts per million rather than in percent.
Meeting such numbers is not a matter of washing. A glass-grade flowsheet is a sequence of purification steps, each aimed at a particular impurity form: attrition scrubbing to strip surface films, magnetic separation to remove liberated iron-bearing minerals, flotation to remove feldspar and mica, and acid leaching with oxalic or hydrofluoric acid to address what remains. Only the full sequence, run in the right order, will hold a 100 ppm iron limit consistently.
- SiO2 of 99.5% or more, with Fe2O3 at or below 0.01% (100 ppm), for high purity and photovoltaic glass sand.
- Below 80 ppm Fe2O3 for ultra-white photovoltaic glass.
- Full deep-purification route: scrubbing, magnetic separation, flotation for feldspar and mica removal, and acid leaching.
- Chemistry is the acceptance criterion, so every stage is justified by the impurity form it removes.
2. Foundry sand: the specification is physical
Foundry sand inverts the priority. A silica content of 95% to 98% is normal, and the tolerance for iron is far wider, with levels up to 0.5% to 1.0% accepted. What the foundry cares about is how the grains behave when they are mixed with binder, rammed into a mould and poured against liquid metal. Three properties dominate: particle size concentration, grain shape, and refractoriness.
Size concentration is usually expressed as the proportion of sand retained on three or four adjacent sieve fractions, and a figure above 85% is commonly required, because a sand spread across many sizes packs less predictably than one concentrated in a narrow band. Grain shape is described by an angularity coefficient, and a shape closer to spherical is preferred. Refractoriness must exceed 1700 degrees Celsius so that the mould face survives contact with molten metal.
| Property | High purity and photovoltaic glass sand | Foundry silica sand |
|---|---|---|
| Primary acceptance criterion | Chemical purity | Physical grain properties |
| SiO2 | At least 99.5% | 95% to 98% |
| Fe2O3 | At or below 0.01% (100 ppm); below 80 ppm for ultra-white photovoltaic glass | Up to 0.5% to 1.0% is accepted |
| Particle size | Distribution controlled for melting behaviour | Concentration above 85% on three or four adjacent sieve fractions |
| Grain shape | Not the governing specification | Angularity coefficient as close to spherical as possible |
| Refractoriness | Not a purchase criterion | Above 1700 degrees Celsius |
| Typical process emphasis | Scrubbing, magnetic separation, flotation, acid leaching | Washing, scrubbing, sizing and dewatering, with iron removal only where needed |
3. Where the two products diverge in the plant
The two specifications pull the flowsheet in different directions. A glass-grade circuit is built around impurity removal, adds stages and therefore cost, and is judged on the final chemical analysis of a product whose size distribution is secondary. A foundry-grade circuit is built around sizing and cleaning, and is judged on how tightly the delivered sand sits in its target size band and how uniform the grains are.
This has a practical consequence for deposit appraisal. A sand with an inclusion iron content above the glass ceiling may still be an excellent foundry sand, and a sand that is chemically outstanding may fail as a foundry sand if its size distribution is too broad or its grains too angular. The product decision should therefore be taken first, on the basis of chemical and physical testing, and the flowsheet designed backwards from it. Treating the flowsheet as fixed and looking for a buyer afterwards usually leads to a plant that satisfies neither specification well.
Where both products are possible, some operators split the market by size: the coarser, well-shaped fraction to foundry customers and the finer or higher-grade fraction to glass, each routed through its own finishing circuit. That decision depends on a mass balance by size and chemistry, not on a single head assay.
Conclusion
Glass sand and foundry sand are separated by what the buyer measures. Glass sand is defined by SiO2 at 99.5% or above with Fe2O3 at 100 ppm or below, and by the purification chain needed to hold that limit. Foundry sand is defined by a tight size concentration, a rounded grain shape and refractoriness above 1700 degrees Celsius, with much wider tolerance on chemistry. Decide which product the deposit is being sold as before designing the circuit: the two answers lead to different equipment and different economics.
How LiCheng Tech can help
LiCheng Tech supplies the washing, scrubbing, screening, classification, magnetic separation and dewatering equipment used in both glass-grade and foundry-grade silica sand lines. Send us your size distribution, chemical analysis and target product specification, and our engineers will help decide whether the deposit suits a chemical purification route or a physical sizing route, and what testwork is needed to confirm it. Start from the silica sand processing overview, or compare the vibrating screen, dry magnetic separator and rotary dryer used in these circuits.
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