The uses of sodium phosphite are not yet very widespread, mainly in surface treatment and flame retardant production. The raw materials, production processes, and equipment for sodium phosphite production vary considerably depending on the process. Below, we will briefly explain the common production methods and main equipment.

I. Production Methods

1. Neutralization Method: Neutralizing an aqueous solution of phosphorous acid with a stoichiometric amount of sodium hydroxide solution, followed by concentration and crystallization, yields Na₂HPO₃·5H₂O. Alternatively, neutralizing the phosphorous acid solution with a stoichiometric amount of sodium hydroxide solution, controlling the pH to 4.6, and then evaporating and crystallizing, yields sodium hydrogen phosphite.

2. Metathesis Method: Treating calcium phosphite, a byproduct of sodium hypophosphite synthesis, with sodium sulfate (or sodium carbonate), the filter cake is calcium sulfate (or calcium carbonate). The filtrate, after concentration and crystallization, yields sodium phosphite. Excess sodium carbonate in the reaction solution can be removed with a small amount of phosphorous acid, the reaction formula being: Na₂CO₃ + H₃PO₃ → Na₂HPO₃ + CO₂ + H₂O

II. Main Equipment The main equipment for sodium phosphite production includes a neutralization reactor, filter, evaporator, crystallizer, and centrifuge. The neutralization reactor is a cylindrical device made of corrosion-resistant stainless steel. The reactor has a flat top cover and a sloped bottom plate with a discharge pipe at the lowest point. The top cover is equipped with a speed reducer, feed pipe, and exhaust pipe. The reactor contains a highly dispersing agitator. During the neutralization reaction, the alkali solution is first added through the feed pipe. After the agitator is turned on, phosphorous acid is added through the feed pipe, and the generated gas is exhausted through the exhaust pipe. After the reaction is complete, the material is discharged through the discharge pipe. It should be noted that the agitator selected for the neutralization reactor must have a high dispersing function. Only an agitator with a high dispersing function can accelerate the reaction process, reduce entanglement during the reaction, and ensure stable product quality.