The innovative method of producing sodium tripolyphosphate as described in this invention involves the countercurrent extraction of wet process phosphoric acid comprising 28 to 32% P2O5 content, utilizing a polar organic solvent like n-butanol. This technique effectively mitigates the drawbacks associated with the multistep purification typically employed in traditional methods. The resulting organic phase, made up of n-butanol, phosphoric acid, and water, undergoes neutralization either directly or after further purification to eliminate SO42− ions. This is achieved using CaO or CaCO3 at a pH of 7.2 to 7.5 and a temperature range of 60 to 70 °C, employing a Na2CO3 or NaOH solution, applied directly or after saturation at 50 °C, yielding a molecular mixture of Na1HPO4 : Na2HPO4 in a 1:2 ratio. The crystal suspension is segregated from the n-butanol via centrifugation, consequently dried, homogenized through grinding, and calcined.
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The end product’s quality is comparable to sodium tripolyphosphate derived from electrothermic phosphoric acid.
For additional details, see the copending application 615,927, issued Sept. 9, now Pat. No. 3,466,141.
This invention pertains to a procedure designed to produce high-purity sodium tripolyphosphate.
The production of sodium tripolyphosphate initially utilized electrothermic phosphoric acid, known for its high purity which does not present technological challenges. This acid is neutralized with Na2CO3 in a ratio that ensures the final solution features a 1:2 mixture of monosodium and disodium phosphate. Following this, the mixture is dried and the resultant crystals are calcined, yielding high-purity sodium tripolyphosphate.
As demand for sodium tripolyphosphate has surged, the rising costs tied to electrothermic phosphoric acid have prompted the exploration of wet process phosphoric acid. Despite its growing use, challenges persist concerning the elimination of impurities naturally found in wet-process phosphoric acid, such as fluorine, iron, aluminum, calcium, and sulfuric acid, which have yet to be entirely resolved.
In conventional production of sodium tripolyphosphate, the purification of wet-process phosphoric acid requires multiple stages. Initially, free sulfuric acid is extracted, typically using barium salts or phosphate rock, followed by the filtration of the resulting suspension. The subsequent phase involves precipitating fluorine ions with sodium carbonate, leading to a clear solution that undergoes neutralization to generate the aforementioned phosphate mixture. During the neutralization process, residual impurities precipitate as calcium, aluminum, and iron phosphates, which must also be filtered out before drying and calcination.
An alternate purification method for wet-process acid involves removing impurities prior to neutralization with NaOH, primarily through crystallization. In this initial phase, about 30% P2O5 wet-process phosphoric acid is neutralized with NH4OH at a pH that favors the formation of monoammonium phosphate, precipitating most impurities. Following centrifugation, the resulting monoammonium phosphate solutions are treated with sodium hydroxide at a molar ratio of Na2O : P2O5 of 1:1, leading to the crystallization of the mixed salt NaNH4HPO4. After their separation, these crystals dissolve in a heated NaOH solution, with the ratio calibrated to yield both the crystal mixture and NH3, which is subsequently distilled and reintroduced into the initial purification phase.
This invention addresses and rectifies the previously noted issues that stemmed from lengthy multi-step purification procedures by introducing a method that allows for the extraction of wet-process phosphoric acid using organic solvents (for instance, butanol) followed by direct neutralization of the organic extract (either directly or post-purification) at pH levels between 7.4 and 7.5, with temperatures between 60 to 70 °C, in a singular step while utilizing a saturated Na2CO3 or NaOH solution. The objective is to achieve a molecular mixture of 1:2. The subsequent crystal suspension is then separated from the butanol, dried, and calcined using established methods.
Two examples illustrating the application of this invention are detailed below:
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EXAMPLE 1: Wet-process phosphoric acid with approximately 30% P2O5 content is extracted using n-butanol in a solvent-to-acid ratio of 1.5:1 in a mixer-settler with nine extraction units. 100 g of the organic extract, either as is or after purification to eliminate SO42− and F− ions, comprising the following constituents: H3PO4 - 16.3%, SO42− - 0.1%, F− - 0.17%, Fe2O3 - 0.016%, Al2O3 - 0.008%, n-butanol - 60.3%, and H2O - 22.9%, is neutralized with stirring, using 45.8 g of a 32% saturated Na2CO3 solution at 50 °C, aiming for pH = 7.4, and a temperature range of 60 to 70 °C within a solvent refluxing system. The solvent, comprising 18% H2O, is separated from the crystal suspension through decantation, culminating in 107 g of solvent and a saturated solution yielding 22.4 g of the crystal blend along with 33.6 g of water and 1 g of butanol. This mixture is dried using a hot gas current at a temperature up to 95 °C until it achieves 23% moisture, with the prior addition of 1 g of NH4NO3 to stabilize the final product. The hot gases are subsequently cooled to recover the solvent, which is then directed along with the resultant from decantation to water removal, conducted via established methods. The dried crystals exhibit a mixed phosphates content of 99.5%, SO42− - 0.3%, and Fe2O3 - 0.002%, and are calcined at 500 °C to produce sodium tripolyphosphate.
EXAMPLE 2: The same procedure is repeated as in Example 1, with the only variation being the neutralization, executed using 18.5 g of a 60% NaOH solution, saturated at 50 °C, under identical conditions. Following neutralization, the organic extract is further processed in a similar manner.
The method introduced by this invention offers several benefits:
- It facilitates the production of a monosodium-disodium phosphate mixture exhibiting purity comparable to products obtained from electrothermic phosphoric acid.
Neutralization of the extract to achieve the aforementioned mixture is accomplished in just a single step, contrasting with the commonly required three steps.
What we claim is:
1. In a process for manufacturing sodium tripolyphosphate of high purity by neutralizing an organic extract composed of phosphoric acid, butanol, and water, obtained by the extraction of wet-process phosphoric acid with butanol, the enhancement wherein neutralization takes place in a single step at temperatures ranging from 50 to 60 °C and at pH values between 7.4 and 7.5 to obtain crystals featuring a molecular mixture in a 1:2 ratio of NaH2PO4 : Na2HPO4, followed by separating, drying, and calcining the crystals at temperatures ranging from 250 to 450 °C to yield the sodium tripolyphosphate.
4 References Cited - UNITED STATES PATENTS - OTHER REFERENCES Van Wazer: Phosphorus and Its Compounds, vol. 2, 10, pp.-.
OSCAR R. VERTIZ, Primary Examiner G. A. HELLER, Assistant Examiner US. Cl. X.R.
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