Background and overview[1][2]
Sodium phosphate is the product with the largest output and largest consumption among fine phosphorus chemical products. Its technology development and market development are relatively mature. It mainly includes sodium orthophosphate, sodium pyrophosphate, sodium polyphosphate, and metaphosphoric acid. sodium salt. Among them, sodium hexametaphosphate is the most widely used functional phosphate in the market and can be used in industrial production such as washing, water treatment, food, mineral processing, and metallurgy.
Sodium hexametaphosphate is also called Graham salt. It is one of the polymers of sodium metaphosphate (NaPO3). Colorless and transparent glassy powder. Relative molecular mass 611.54. It is deliquescent and easily soluble in water. Solubility in water (g/l): 973.2 at 20°C and 1744.0 at 80°C. When added to an aqueous solution, it slowly decomposes to generate orthophosphate. It has a great chelating effect on calcium and various metals in aqueous solution and generates precipitation. The relative density of this product is about 2.5. The melting point is about 616℃ (decomposition). Insoluble in organic solvents. As industrial products, there are not only hexamers, but also chain-structured metaphosphates with a polymerization degree of 10 to 23. Their general formula is: (NaPO3)n( In the formula, n=10～23). Rat oral LD500mg/kg.
Quality standards[3]
Functions and features[4]
(1) Sodium polymetaphosphate used in vegetable tanning can block the alkaline base of collagen (fixation), reduce the combination of vegetable tannins and collagen proteins, and is beneficial to accelerating tanning.
(2) It can form soluble complexes with Ca2+, Mg2+ and other ions in water, and it can also prevent Ca3(PO4)2 and other boiler scale deposits.
(3) Its anion has a blocking effect.
Apply[2]
1. Application in washing and water treatment
Because sodium hexametaphosphate has extremely strong complexing and scale inhibition capabilities for metal ions, especially alkaline earth metal ions such as calcium and magnesium. Therefore, it can prevent the iron and manganese dissolved in the water from oxidizing and causing the water to turn red and black, and at the same time prevent the precipitation of calcium and magnesium ions to form scale and reduce the heat transfer efficiency.
2.Application in food industry
Sodium hexametaphosphate can be used as food quality improver, pH regulator, metal ion chelating agent, adhesive and swelling agent in the food industry. Sodium hexametaphosphate is used in meat products, fish sausages, ham, etc., to improve water holding capacity, increase binding properties, and prevent fat oxidation; when used in bean paste and soy sauce, it can prevent discoloration, increase viscosity, shorten the fermentation period, and adjust the taste; Used in fruit drinks and refreshing drinks, it can increase the juice yield and viscosity, and inhibit the decomposition of vitamin C; used in ice cream, it can improve expansion capacity, increase volume, enhance emulsification, prevent paste damage, and improve taste and color; used in dairy products , beverages can prevent gel precipitation; added to beer, it can clarify the wine liquid and prevent turbidity; used in canned beans, fruits and vegetables, it can stabilize natural pigments and maintain food color. In addition, spraying sodium hexametaphosphate aqueous solution on cured meat can improve the preservative performance.
3. Antioxidants should be commonly used in mineral processing and oil field industries
Sodium hexametaphosphate is a commonly used inhibitor in flotation. It is mainly used to inhibit quartz and silicate minerals, as well as carbonate minerals such as calcite and limestone. The main inhibition mechanisms of sodium hexametaphosphate are: first, when sodium hexametaphosphate inhibits the flotation of albite, zircon, nepheline, pyrochlore and halloysite, according to tracer atomic tests, six Sodium metaphosphate can non-selectively reduce the adsorption of sodium oleate in the above minerals and form stable complexes with multivalent metal ions on the mineral surface. For example, in nickel ore flotation, sodium hexametaphosphate and MgOH+ on the surface of serpentine generate MgNa4 P6 O18, thereby inhibiting serpentine. Secondly, sodium hexametaphosphate is easily ionized in aqueous solution and can react with Ca2+ on the mineral surface and in the liquid phase. For example, when calcite and cassiterite are separated by flotation, sodium hexametaphosphate generates the complex CaNa4 P6 O18 on the surface of calcite. However, CaNa4 P6 O18 cannot completely stay on the surface of calcite. Instead, it is adsorbed on the surface of cassiterite, so cassiterite will also is inhibited, making its inhibition selectivity worse and reducing the adsorption amount of the collector on the mineral surface.
In addition, sodium hexametaphosphate also acts as a dispersant during the flotation process. During the flotation process of nickel ore, serpentine sludges and covers the surface of pentlandite, which is very detrimental to the flotation process. Therefore, adding a certain amount of sodium hexametaphosphate is very beneficial to the flotation recovery of pentlandite. This is mainly because the addition of sodium hexametaphosphate disperses the flotation slurry and reduces the coverage of serpentine on the surface of pentlandite. Its main dispersion mechanism is as follows: reducing the surface potential of minerals and increasing the electrostatic repulsion between mineral particles. Because sodium hexametaphosphate is a linear polymer compound with a chain length of at least 20 to 100 units, when it is adsorbed on the mineral surface, it will increase the steric hindrance effect between particles. In the oilfield industry, sodium hexametaphosphate is used as a drilling mud additive to avoid the precipitation of multivalent metal ions, improve the salt resistance of the mud, and reduce mud water loss. In addition, dissolving a certain amount of sodium hexametaphosphate and injecting it into the water pipe can form a thin film on the pipe wall to prevent pipe corrosion.
4. Application in metallurgy and metal anti-corrosion
Adding sodium hexametaphosphate into some additives and spraying the steelmaking converter at a certain temperature and force can greatly extend the life of the converter. six deviationsSodium phosphate chelates with divalent metal ions in solution to form a positively charged polyelectrolyte, which forms a dense, continuous film when it is adsorbed on the metal surface. This film can completely cover the corrosion micro-battery, reduce or prevent the passage of corrosion current, and play an anti-corrosion role.
5. Application in color film development and printing
Sodium hexametaphosphate is an excellent water softener, which must be added when preparing color developers to reduce the hardness of water, prevent the formation of calcium reticulation, ensure clear and bright colors, and has no effect on the oxidation and reduction of the developer. .
6. Application in fiber and paper industry
Sodium hexametaphosphate is used in the fiber industry. In the refining process, it can inhibit the formation of metal soap, making it easy to remove the colloid in raw cotton, and improve the refining efficiency; in the bleaching process, it can prevent the decomposition of hydrogen peroxide and reduce the amount of bleach. It can prevent the color change of the dyed goods, maintain the original color of the dye, and prevent dye spots, handfeel deterioration, yellowing of white cloth and reduction of fiber strength during dyeing. In papermaking, the mixture of sodium hexametaphosphate and sulfonated benzaldehyde can be used as a dispersant in steel plate paper coatings to disperse lead carbonate pigments.
Preparation[2]
At present, the production process of sodium hexametaphosphate mainly includes one-step method and two-step method. The production scale is generally 1 to 3 kt/a. The representative one is the two-step method of producing sodium hexametaphosphate by sodium dihydrogen phosphate. method.
1. Two-step method
This method is also called liquid phase neutralization, drying (crystallization) melt polymerization method. The generated sodium dihydrogen phosphate solution is first concentrated and crystallized or spray dried to remove free water to produce anhydrous sodium dihydrogen phosphate and The sodium dihydrogen phosphate is crystallized and then sent to the melting furnace for dehydration and melting to produce sodium hexametaphosphate. This method can accurately control the molecular ratio of Na2O and P2O5, so it can produce stable sodium hexametaphosphate with a certain chain length. Since what enters the melting furnace is anhydrous sodium dihydrogen phosphate or crystallized sodium dihydrogen phosphate, less water enters the furnace, which can greatly reduce the partial pressure of water vapor on the surface of the melt and the moisture in the material, thereby making the material Products with a higher degree of polymerization are produced in a shorter time. After the melt comes out of the furnace, it is cooled by a continuous double-roller internal cooling quenching machine. The cooling effect is good and can ensure that the product does not crystallize and become glassy. It is also a continuous production with large production capacity and low labor intensity. Since the materials are reacted and transported in closed reactors and pipelines, the labor environment and sanitary conditions are good. Specific production is divided into the following methods.
1) Sodium dihydrogen phosphate method
First, yellow phosphorus is used as raw material to prepare thermal phosphoric acid, and then caustic soda is used as neutralizing agent to prepare sodium dihydrogen phosphate. First, sodium dihydrogen phosphate is spray-dried and dehydrated to obtain sodium dihydrogen phosphate dry powder. The dry powder is then heated in a graphite crucible for high-temperature polymerization, then taken out with a stainless steel spoon and quenched in a cooling disc to obtain flake sodium metaphosphate, which can be crushed to obtain powdery sodium hexametaphosphate product. This method has the advantages of easy control of material ratio, continuous production, and low labor intensity.
2) Phosphorus iron method
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The ferrophosphorus method uses ferrophosphorus, a by-product of electric furnace yellow phosphorus, as raw material. After crushing and grinding, it is mixed with soda ash for roasting. The resultant is a sintered solid. It contains soluble trisodium phosphate, which is leached with hot water to obtain a trisodium phosphate solution, which is filtered to remove insoluble impurities. The purified trisodium phosphate liquid is neutralized to obtain disodium hydrogen phosphate. The disodium hydrogen phosphate solution is concentrated in a vacuum, crystallized to remove soluble impurities, and then further neutralized to prepare a sodium dihydrogen phosphate solution. The sodium dihydrogen phosphate solution is spray-dried to produce anhydrous monosodium, which is then dehydrated and polymerized to form sodium hexametaphosphate. The disadvantages of this process are many processes, long process, and high fuel and power consumption.
3) Phosphoric acid and water glass method
Using the neutralization reaction of phosphoric acid and water glass to prepare silica, and then reusing the waste liquid of silica production to obtain the finished product six times through multiple filtration, pH adjustment, volume expansion evaporation, polymerization reaction, roller cooling, crushing, and screening. Sodium metaphosphate. This method uses phosphoric acid and water glass to simultaneously prepare two products, silica black and sodium hexametaphosphate, which achieves various beneficial effects such as utilizing waste liquid, reducing production costs, and protecting the ecological environment.
4) Phosphoric acid and salt method
Study the polycondensation reaction of phosphoric acid and salt. The reaction equation is as follows:
Using the polycondensation reaction of phosphoric acid and table salt to produce sodium hexametaphosphate can not only save soda ash and caustic soda, recover the by-product hydrochloric acid, but can also be used to decompose phosphate rock and other purposes.
2. One-step method
The one-step process of sodium hexametaphosphate uses liquid yellow phosphorus as raw material, which is directly combined with soda ash at high temperature after oxidation and polymerization. The molten sodium hexametaphosphate thus obtained flows directly into the furnace hearth into the cooling disc and is quenched. That is, sodium hexametaphosphate is obtained. This method is also called solid phase reaction and crucible melting method. In this method, yellow phosphorus is heated and melted, then sent to an oxidation combustion furnace, and oxygen in dry air is used for oxidation and combustion reaction to generate the intermediate product phosphorus pentoxide. Then mix phosphorus pentoxide and soda ash, undergo a high-temperature polymerization reaction, and quench the flakes to obtain sodium hexametaphosphate in flakes. After crushing, sodium hexametaphosphate can be obtained. This process has the characteristics of few steps, short process, and continuous production. The heat energy of yellow phosphorus combustion can provide the heat source required for the drying and polymerization process of sodium hexametaphosphate in the production process, which can effectively save energy. However, the production control of this process is difficult and the stable quality of sodium hexametaphosphate products cannot be guaranteed.
Main reference materials
[1] Practical Fine Chemical Dictionary
[2] Application and preparation technology of sodium hexametaphosphate
[3] Chemical additives
[4] Leather Chemical Handbook
Exam information
[1] Practical Fine Chemical Dictionary
[2] Application and preparation technology of sodium hexametaphosphate
[3] Chemical additives
[4] Leather Chemical Handbook
