Silicon dioxide (CAS 7631-86-9): storage and disposal

Silicon dioxide covers the bags of sand and tubs of fine silica powder that gather in school, college and university stores. It is not normally classified as hazardous, although fine crystalline silica dust can damage the lungs with repeated inhalation, and it stays a chemical waste that needs an accurate description. The things that go wrong are mix ups: sand that has been used in an experiment, tubs that lost their labels and silica stored near the wrong acid.

CAS number: 7631-86-9
Form: Solid
Storage class: General
Usual EWC code, laboratory stock: 16 05 09

What to do this week

Without moving anything that looks damaged, check the label is readable and write down the opened date if it is not already there.
Check it is stored as general stock, and apart from anything listed under "Do not store with" on this page. If nothing is listed, that does not prove it is safe with everything.
If you no longer need it, add it to your disposal list so it can be included in a collection.

Safe storage in a school or lab

Sand is usually held in a heavy bag or a plastic tub, and silica powder in a screw cap jar. Both want a dry, labelled shelf in a general store, dated on arrival and kept separate from food grade supplies. The one neighbour to avoid is hydrofluoric acid, which attacks glass and silicates and gives off a harmful gas, so a shelf of silica should not sit near it. Fine powder can leak from a loose lid and spread through a cupboard, so stand jars on a tray and avoid raising dust, because fine crystalline silica dust can damage the lungs with repeated inhalation and respirable crystalline silica has a Workplace Exposure Limit in EH40. A split bag should stand inside a labelled lidded outer container rather than be left open.

How to tell it has deteriorated

Silica gives few visible signs of age, because the material itself is very stable. What a technician sees first on the shelf is a damaged container: a split bag, a sticky cap or a label that no longer reads. Damp sand clumps and may grow mould where organic matter has been mixed in, while powder that has caked has usually had water in it. The change that matters more is use. Sand that has carried a dye, a metal salt or an organic residue is no longer plain silicon dioxide, and a jar that has been topped up from several sources cannot be described with confidence.

How it is disposed of

We arrange this by specialist collection. We do not publish a drain or general waste route for any chemical. It is normally packed in a non-regulated or low hazard laboratory pack with compatible chemicals.

Waste code 16 05 09 is the usual entry for chemical waste that is not hazardous, with a waste transfer note, though the producer classifies the waste. It only fits when the material really is clean silica or sand. If experiments have contaminated it, the description follows whatever is in it, so list that at booking. Record the form, the quantity and the container. The only route is specialist collection. A school should not put it in general waste, a skip or a drain, and nobody should try to treat it first.

Waste codes for Silicon dioxide
Usual EWC code, laboratory stock	16 05 09
Usually classed as hazardous waste	No

The code depends on where the waste came from and what it contains, and classifying it is the producer's duty. Use the waste code finder to see why.

Do not store with

Named chemicals to keep apart

Hydrofluoric acidHydrofluoric acid attacks glass and silicates, releasing toxic gas.

Common quantities

Usually held as: sand, powder.

Tell us the container size and how many you hold. An estimate is enough to start with, and some items cannot be priced until someone has looked at them.

Where it is commonly held

Questions

Is clean sand from a science department really chemical waste?

If it is held as laboratory stock, yes, and it should be described as silicon dioxide. It is not normally classified as hazardous, but it still goes through specialist collection with an accurate description.

Why is hydrofluoric acid a problem for silica stock?

It attacks glass and silicates and releases a harmful gas. Silica jars and sand stored beside it, or in the same cupboard, are better moved well away before that stock is used.

We used sand in a reaction demonstration. Is it still silicon dioxide?

Not once it has picked up another chemical. Used sand is described by what it has absorbed, which could change the waste code. Keep it in a labelled container, separate from clean stock, and list the contaminants at booking.

Does silica powder go out of date?

The material is stable and gives few visible signs of age. What expires is the packaging and the paperwork. A split bag, a lost label or an unknown source is the reason to book a collection, not the age of the silica.