Sodium Metabisulfite in Gold Mining and Flotation

Sodium metabisulfite (Na₂S₂O₅, CAS 7681-57-4) is a workhorse reagent in mineral processing. In gold plants it destroys the cyanide left after leaching; in flotation circuits it depresses unwanted sulphide minerals and controls pulp chemistry. This article explains where it is used, how much is typically needed, and what to specify when buying for a mine.

1. Where sodium metabisulfite fits in a mine

Cyanide destruction in gold tailings (the SO₂/air process).
Flotation – depression of sphalerite and pyrite, and pulp-potential control.
Water treatment – dechlorination and redox control in process and tailings water.

One reagent covering all three means a mine can often consolidate its metabisulfite demand into a single grade, packaging format and COA stream.

2. Gold: cyanide detoxification by the SO₂/air process

After cyanide leaching, the tailings still contain free cyanide and weak-acid-dissociable (WAD) cyanide. These have to be destroyed before the water can be recycled, and before tailings are stored, so that the site meets its permit and the tailings facility is safe. The standard route is the SO₂/air process: sulphur dioxide and air, with a copper catalyst, oxidise cyanide to the much less toxic cyanate (OCN-).

Sodium metabisulfite is a convenient way to introduce SO₂ into the pulp. It is usually dosed with air and a copper source, with lime for pH control.

Parameter	Typical practice
Reagent	Sodium metabisulfite as SO₂ source, with air
Catalyst	Copper (about 20 – 50 mg/L in the pulp; system-specific)
pH	Approx. 8 – 9, controlled with lime
SO₂ to CN molar ratio	Approx. 4 – 5 to 1
Target	WAD cyanide reduced to the level set by the site permit

The ratio, dose and end point must be set on site – ore, cyanide concentration, pulp density and permit limits all differ. Always confirm the target with the site’s metallurgical and environmental teams rather than applying a book value.

3. Flotation: depressant and pulp conditioning

In flotation, sodium metabisulfite is a reducing reagent that shifts pulp chemistry and depresses selected minerals so they stay out of the concentrate.

Function	Typical use
Depression of sphalerite (ZnS)	Copper-zinc and lead-zinc separation, often with zinc sulphate, to keep zinc out of the copper or lead concentrate
Depression of pyrite	Suppression of iron sulphide in selective circuits
Pulp-potential control	Lowers redox potential where oxidised mineral surfaces reduce selectivity

Dosage is ore-specific and usually screened in laboratory flotation tests first – often in the range of hundreds of grams per tonne of ore before a plant trial. Because metabisulfite also consumes oxygen and oxidants, it is normally added at a controlled point in the circuit and monitored by grade and recovery.

4. Dosing and control

Cyanide destruction: control on measured WAD cyanide and the SO₂:CN ratio; watch pH and copper residual.
Flotation: control on concentrate grade, recovery and pulp potential; confirm the addition point.
Water treatment: control on residual chlorine (for dechlorination) or dissolved oxygen (for scavenging).
Feed form: metabisulfite is usually added as a solution made up from bagged or jumbo-bag solid, or as dry feed to a mixing tank.

5. Water and tailings in the circuit

Beyond cyanide destruction, metabisulfite is used to remove residual chlorine from process water (which would otherwise damage reagents, membranes and some equipment) and to lower the redox potential of water that returns to the circuit. This is one reason mining and water treatment often buy the same grade from the same supplier.

6. Grades, packaging and supply

Form	Typical use on site
25 kg PP/PE bags	Smaller dosing points; palletised for container shipment
1000 kg jumbo bags	Plant-scale dosing and reagent make-up
Full container loads	Regular mine supply, port-to-port

Industrial grade is the normal choice for mining; food grade is available where a site also runs food or beverage operations. Each batch is supplied with a certificate of analysis (COA) and safety data sheet (SDS). Tell us the annual tonnage and the discharge port for a quotation.

7. Handling and safety

Acid reaction. Metabisulfite releases sulphur dioxide on contact with acids; keep it away from acids and never mix the two.
Irritant. Dust irritates eyes and the respiratory tract; use dust protection and ventilation.
Moisture. Keep bags closed and store dry; the solid is hygroscopic and degrades with moisture and air.
Transport. Classified as a corrosive solid; the exact UN entry and class are given on the SDS.

Frequently Asked Questions

Why is sodium metabisulfite used in gold mining?

Mainly to destroy residual cyanide in tailings by the SO₂/air process, and as a depressant and pulp-conditioning reagent in flotation.

How much sodium metabisulfite is needed to destroy cyanide?

The SO₂ to CN molar ratio is typically about 4-5 to 1, but the actual dose depends on the site. It must be set against measured WAD cyanide, pH and the permit limit.

What does metabisulfite do in flotation?

It depresses sphalerite and pyrite and lowers pulp potential, improving selectivity in copper-zinc and lead-zinc circuits. Dosage is ore-specific and screened in the lab first.

Do you supply mining-grade metabisulfite in bulk?

Yes – 25 kg bags and 1000 kg jumbo bags, in full container loads. Supply is with per-batch COA and SDS.