Initially, the zinc hydroxide is filtered off and reprocessed to zinc dust. Then,

After concentration of the liquor by evaporation, sodium dithionite is precipitated

By the addition of sodium chloride. The salt, which is dried before shipment, contains

Up to 300 p.p.m. zinc.

The dominant process is the reaction of sodium formate with sulfur dioxide

and caustic soda under pressure. The reaction is described by:

HCOONa + 2SO2 + NaOH → NaS2O4 + CO2 + H2O

Commercial grades of sodium dithionite powder typically have a content of

about 88% sodium dithionite, with the sodium salts of bisulfite, sulfite, sulfate

And carbonate as the byproducts. Solutions of dithionite in water must be prepared

On-site. In pulp bleaching, the use of returnable steel containers with up to

Kg of technical-grade product is common. Typically, a small amount of a chelant

(e.g., ethylene diamine tetra-acetic acid; EDTA) is added. This avoids scaling

(precipitation of calcium carbonate) caused by the hardness of the dilution water.

An analysis of the stability of dithionite solutions in the presence of iron and manganese

[33] contradicted the speculation about the metal ion (Fe, Mn) -induced

decomposition of dithionite [34]. The fine powder reacts exothermically with atmospheric

Oxygen; the heat of oxidation can lead to ignition. Sodium dithionite is

therefore classified as “spontaneously combustible goods”, and the corresponding

Transport and storage regulations must be applied.

Alternatively, some mills operate an on-site process using a solution of sodium

borohydride (~12% by weight) with caustic soda (~40%). This mixture is reacted

with sulfur dioxide to yield dithionite solutions:

NaBH4 + 8NaOH + 8SO2 → 4Na2S2O4 + NaBO2 + 6H2O

Some pulp mills are supplied with an alkaline solution (pH > 12) of sodium

dithionite. Transportation requires cooling to a temperature below 10 °C to maintain

the content at ca. 11–12% dithionite. Cooling of the storage tank is not required

When there is a rapid turnover of the product. Exclusion of air is required

To avoid product losses, as dithionite reacts rapidly with oxygen in the air to yield

Sulfite. In the absence of air, decomposition reactions take place, with one of the

Products being thiosulfate; this, in turn, may accelerate corrosion reactions.

Bleaching with dithionite typically is conducted at moderately acidic pH, between

PH 4.5 and 6.5. The temperature in tower or tube bleaching is maintained

at 60–80 °C, although a higher temperature produces a faster response to the

Chemical addition. The reduction of chromophores occurs very quickly; so the

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