The quality of sodium hypophosphite is closely linked to quality control during the production process. Quality control for one-step production primarily encompasses raw material control, production process control, and product inspection and control. Each step in the one-step sodium hypophosphite production process must be strictly performed within controlled process parameters. Deviations from these parameters may result in substandard product specifications. Below, we provide a detailed description of ten aspects of quality control in the sodium hypophosphite production process.

1. Yellow Phosphorus Treatment

Usually, filtration is performed using a corundum tube filter. During the filtration process, a filter aid (diatomaceous earth phosphate) should first be evenly applied to the surface of the porous corundum tube. The pore size of the porous corundum tube is less than 1μm. The operating pressure for filtration should be controlled within 0.2MPa. Higher pressures can cause impurities in the yellow phosphorus to filter through and may even rupture the corundum tube. The yellow phosphorus initially filtered must be returned to the original yellow phosphorus tank and inspected before it can be used as a raw material for sodium hypophosphite production.

2. Lime and Caustic Soda Treatment

In addition to raw material quality inspections, the chloride ion content of the water used for caustic soda preparation must be strictly controlled. Lime must be protected from moisture, and lime that has been stored for a long time should not be used. Lime must be accurately weighed according to the process ratio. Excessive lime usage results in a lower sodium phosphite content in the product, but increases the amount of carbon dioxide used for carbonization, resulting in higher costs. Excessive caustic soda usage also increases phosphite content in the product. However, sodium phosphite is soluble and difficult to remove or convert. Clearly, an inaccurate caustic soda content in the mix significantly impacts product quality.

3. Reaction Synthesis

Because the synthesis reaction is a multiphase reaction, the dispersion of yellow phosphorus is a key factor affecting the reaction speed. If the dispersion of yellow phosphorus is poor, the reaction time will be prolonged, and long reaction times will increase side reactions. To achieve high dispersion of yellow phosphorus, high-speed stirring (or a highly dispersing impeller) can be used, or solid or liquid dispersants such as glass powder, activated carbon, or fatty alcohols (ethanol, amyl alcohol) can be added during stirring. This significantly accelerates the reaction speed, shortening the reaction time to several hours, tens of minutes, or even less. Reaction temperature is also a key factor influencing the reaction speed. Higher temperatures increase the reaction speed, but also accelerate side reactions. At temperatures between 40 and 50°C, the reaction is slow, and the product is difficult to filter. Furthermore, solid polyphosphine may be produced, which slowly decomposes into phosphine monomer in air, making the product toxic. At temperatures exceeding 95°C, the reaction is too rapid, and there is a risk of the reactants escaping from the reaction atmosphere. The temperature is generally controlled between 90 and 95°C. The reaction also produces phosphine gas. The reactor pressure should be kept slightly positive. Excessive pressure can cause phosphine gas to escape from the shaft seal and potentially explode. Negative pressure can also cause air to be drawn in and mix with phosphine, forming an explosive gas. For a 3000L reactor, the following amounts of materials are added per batch: Lime: 100-120 kg; Caustic soda: 400-500 kg; Yellow phosphorus: 150-180 kg; Deionized water: 1000-1500 kg.

4. Exhaust Gas Treatment

The exhaust gas from the sodium hypophosphite reaction is a mixture of phosphine and hydrogen, which can spontaneously ignite in air. To obtain pure phosphine gas, the reaction temperature must be controlled. Temperatures above 95°C can easily cause materials in the reactor to overflow from the phosphine gas pipeline and mix with the phosphine gas. This can lead to the inclusion of calcium and sodium ions in the phosphoric acid produced by the combustion of phosphine. This can also complicate purification and cause pipeline blockage in the production of organophosphorus flame retardants using phosphine as a raw material. Material overflow also increases material consumption.

5. Purification and impurity removal includes carbonization and filtration

Carbon dioxide or caustic soda is added simultaneously to the filtrate containing calcium hydroxide (magnesium) and calcium phosphite (magnesium) to remove calcium and magnesium ions. The amounts of carbon dioxide and sodium hydroxide added must be controlled during this process; excessive amounts can increase the amount of hypophosphorous acid used in subsequent steps. During production, it is recommended to analyze the calcium and magnesium ion content before adding them in a quantitative manner, or to monitor the calcium and magnesium ion content during carbonization. Once the calcium and magnesium ion levels reach the specified values, the carbon dioxide and caustic soda should be discontinued immediately. After carbonization, the pH of the feed solution is between 8.5 and 10. The calcium content of the feed solution is: 0-1.0 mL 0.02 mol EDTA/50 mL feed solution (a method of expressing calcium content using 0.02 mol EDTA consumption, the same below). Coarse filtration is typically performed using a plate and frame filter press, using filter cloth as the filter medium. Fine filtration can be performed using filter paper or reinforced polypropylene membranes. To achieve the desired filtration, the filtration pressure must be controlled during the production process.

6. Hypophosphorous Acid Production

Hypophosphorous acid is commonly produced by ion exchange. Hydrochloric acid is often used to activate the resin during ion exchange. However, improper activation process control can result in chloride ions entering the hypophosphorous acid. Because chloride ions are difficult to remove, the chloride in the hypophosphorous acid is directly transferred to the sodium hypophosphite product. When producing hypophosphorous acid by sulfuric acid decomposition, most of the sulfate must be removed from the produced hypophosphorous acid. Desulfurization with barium carbonate can also result in the inclusion of barium ions in the hypophosphorous acid. Frequent testing during production ensures that the sulfate and barium ion balance remains within a certain range.

7. pH Adjustment

Hypophosphorous acid is used to neutralize the OH-ions in the sodium hypophosphite solution. After the pre-carbonization step, a small amount of carbonate ions remains in the solution. The reaction between carbonate and hypophosphorous acid is reversible, so maintaining the acid adjustment temperature is crucial for the reaction to proceed completely. During production, pH adjustment is often performed at a higher temperature.

8. Evaporation and Concentration

Solution evaporation is often performed under negative pressure. Some sources suggest that heating the solution at atmospheric pressure (in a water bath or sand bath) can cause explosions. The concentration of the evaporation product significantly affects the particle size of the sodium hypophosphite crystals. Solutions with excessive saturation will result in smaller particles and higher impurity content when cooled and crystallized. The discharge density is generally maintained within 1.5 g/mL.

9. Cooling Crystallization

Solution saturation, stirring intensity, and cooling rate are key factors affecting product particle size and uniformity. Controlling the saturation and cooling rate of the sodium hypophosphite solution, as well as selecting an appropriate stirring device, vary among manufacturers.

10. Centrifugal Separation, Drying, and Packaging

The primary goal is to avoid the introduction of mechanical impurities. Drying is done to make the product content reach *~102% according to user requirements.