RU2263899C1 - Method of quantitatively determining iron in phosphoric acids obtained via sulfuric acid-assisted decomposition of phosphate feedstock followed by tributyl phosphate-assisted purification thereof - Google Patents

Abstract

FIELD: analytical methods.

SUBSTANCE: method comprises diluting test sample of acid with orthophosphoric acid reagent at volume ratio of diluent to solution being analyzed equal to 1:(0.002-1) followed by photometric measurement within wavelength range 200-270 nm. A diluent, aqueous solutions of orthophosphoric acid reagent with P₂O₅ concentration 10-40% are used.

EFFECT: reduced consumption of reagents, number of operations, and analysis time.

2 cl, 1 tbl

Description

The invention relates to analytical chemistry, to analysis of the iron content of extraction phosphoric acids (EPA) obtained by the sulfuric acid decomposition of phosphate raw materials, as well as phosphoric acids formed during the purification of EPA using tri-n-butyl phosphate (TBP). These include unpaired and stripped-off EPA, desulfurized phosphoric acid entering the purification process using TBP, purified phosphoric acids formed during purification, and raffinate, which is a phosphoric acid solution in which the main impurities are concentrated.

A known method for determining the iron content in phosphoric acid solutions by the complexometric method, in which Fe ⁺³ is titrated with a solution of ethylenediaminetetraacetic acid (EDTA) at pH = 1.5: 1.8 in the presence of sulfosalicylic acid ("Methods of analysis of phosphate raw materials, phosphoric and complex fertilizers, feed phosphates. "M.," Chemistry ", 1975, p.134). The method is suitable for the analysis of phosphoric acids for iron with a content of at least 0.1%.

For lower concentrations of iron in phosphoric acid solutions (0.001-0.1% wt.), The method of photometric determination of iron is used, based on the formation of iron sulfosalicylate, which is yellow in ammonia ("Reagents and highly pure substances. Colorimetric methods for determining the content iron ". GOST10555-75). This method, which we took as a prototype, is multi-stage and, as a result, is quite lengthy.

The technical result of the claimed invention is to reduce the consumption of reagents, reducing the number of operations and reducing the duration of the analysis.

The method for the quantitative determination of iron in phosphoric acids obtained by the sulfuric acid decomposition of phosphate raw materials and their subsequent purification involves diluting the analyzed acid sample with reactive phosphoric acid with a volume ratio of diluent (V _p ) to the analyzed solution (V _an ) equal to V _p : V _an = 1 : (0.002-1), and photometry in the wavelength region of 200-270 nm. As a diluent, aqueous solutions of reactive phosphoric acid with a concentration of 10-40% P ₂ O _{5 are used} .

The essence of the proposed method boils down to the use of light absorption of iron complexes formed in acids during the opening of phosphate raw materials with sulfuric acid. The resulting iron complexes have characteristic absorption spectra in the region of 200-270 nm.

Complex compounds of iron after the formation of phosphate raw materials at the stage of opening, together with EPA, are transferred to the residue, and after evaporation they retain the ability to absorb light in the indicated wavelength range. These properties are retained after passing through the desulfurization stage and are found in solutions after passing through the stage of purification of EPA using TBP. The value of optical absorption in the range 200-270 nm is proportional to the concentration of iron. It also depends on the nature of the phosphate feedstock, the stage of processing the acid in the process chain: unpaired EPA → evaporated EPA → desulfurized FC → acid purified with TBP → purified evaporated acid and raffinate - a product in which all impurities are concentrated, including Fe .

The properties of absorbing light in the region of 200-270 nm are preserved when a sample or an aliquot of the analyzed acid is dissolved in solutions of reactive phosphoric acid. The concentration of the solvent - an aqueous solution of reactive phosphoric acid is 10-40% P ₂ O ₅ .

According to the proposed method, a sample or an aliquot of the analyzed acid is dissolved in solutions of reactive phosphoric acid with a ratio of solvent to the analyzed acid V _p : V _an = 1: (0.002-1) depending on the iron content in the analyzed solution. Then the solution is mixed, placed in a cuvette with an absorbing layer thickness of 0.1 cm, and the absorption spectrum of the solution is measured on a Spectord M-40 spectrophotometer in the region of 200-270 nm. In this case, orthophosphoric reactive acid with a concentration of 10-40% P ₂ O ₅ is used as a comparison solution.

The concentration of iron in the analyzed sample is determined by the calculation method according to the formula (1).

where C is the mass fraction of Fe,%,

And the optical density of the solution at λ _max , measured on a spectrophotometer, in the range of 200 - 270 nm,

56 - molecular weight of iron,

V is the volume of the analyzed solution, cm ³

ε _λmax - molar coefficient of light absorption, l · mol ^-1 · cm ^-1 ,

l is the thickness of the cell, cm

g - sample of the analyzed acid, g.

ε _{λmax is} determined by the formula (2):

where: C is the mass fraction of iron in wt.% in controlled solutions is determined once analytically for each type of phosphoric acid solution using the methods described as an analogue and prototype.

The frequency of checking the coefficient ε _λmax depends on the type of acid being analyzed.

So, for unpaired, evaporated and desulfurized EPA, the installation ε _{λmax is} carried out 1-2 times a year, for the evaporated acids purified using TBP 1-2 times a year, for purified unpaired acid it is necessary to establish ε _λmax 1 -2 times a quarter, depending from the operating mode of the installation. For the raffinate, ε _λmax depends on the degree of extraction of phosphoric acid from desulfurized EPA; therefore, the coefficient is set for each extraction mode.

The iron content can be determined by the calibration graph, built on standard solutions. As a standard solution, controlled solutions are used in which the iron content is determined once analytically using methods taken as an analog or prototype. The frequency of construction of the calibration graph coincides with the frequency of determination of ε _λmax .

The volumetric ratio of dilution of reactive phosphoric acid (V _p ) and the analyzed acid (V _en ) depends on the concentration of iron complex compounds in the sample. Thus, the concentration of complexes in phosphoric acid samples of stripped, unpaired, desulfurized EPA and raffinate is so high that they use a ratio of 1: 0.002 to measure the optical density of solutions, i.e. 0.05-0.25 cm ^{3 of the} analyzed solution is diluted in a flask with a capacity of 25 cm ³ reactive phosphoric acid. When analyzing acids that have passed the purification stage using TBP, the content of contaminants in which decreases, the volume ratio changes and becomes equal to 1: (0.04-1), i.e. 1-12.5 cm ^{3 of the} analyzed solution, depending on the degree of purification, is introduced into a 25 cm ³ volumetric flask and adjusted to the mark with reactive phosphoric acid.

Solutions of phosphoric acids - EPA unpaired, stripped, desulphated, obtained from Khibiny apatite, as well as purified phosphoric acids (stripped and unpaired) have a characteristic absorption maximum in the range of 221-230 nm, raffinate has a maximum absorption in the range of 221-238 nm.

Solutions of phosphoric acids obtained from other phosphate feedstocks, for example, apatite from the Kovdorsky deposit or Kara-Tau phosphorites, have two absorption maxima, one in the region of 205–208 nm, the other in the region of 236–238 nm; any of them can be used for photometry.

Reactive orthophosphoric acid with a concentration of 10-40% P ₂ O ₅ is used as a dilution. At a higher concentration (more than 40% P ₂ O ₅ ), measurements are difficult due to the high viscosity of the acid; at a concentration of less than 10%, the nature of the spectrum of the solutions changes.

The proposed method for determining iron, in contrast to the prototype, is suitable for a wide range of concentrations of iron in solutions (0.001-0.7% wt.). In this case, one reagent is used - reactive orthophosphoric acid, the method consists of a minimum number of operations: one dilution, one stirring and photometry on a spectrophotometer in the region of 200-270 nm. Duration of determination is 5-10 minutes, in contrast to 30-40 minutes spent using the method taken as a prototype. The advantage of the method is the ability to utilize the solutions formed after analysis by transferring them to the production acids formed during the purification of EPA using TBP.

Example 1. Determination of iron content in desulfurized acid

0.2019 g of desulfurized extraction phosphoric acid (with a specific gravity of 1.622 g / cm ³ and a P ₂ O ₅ content of 53.85%) was added to a 25 cm ³ volumetric flask and adjusted to the mark with orthophosphoric acid, grade “chda”, diluted with distilled water to a concentration of 40% P ₂ O ₅ . The contents of the flask are mixed, the solution is filled with a quartz cuvette with an absorbing layer thickness of 0.1 cm and photometered on a Spectord M-40 spectrophotometer in the range of 200-270 nm. As a comparison solution, “chda” grade orthophosphoric acid is used, diluted with water up to 40% P ₂ O ₅ and placed in a quartz cuvette with an absorbing layer thickness of 0.1 cm. The absorption maximum is at 221.2 nm, while the optical absorption density is 0.8486. The volume ratio of V _p : V _en = 25: (0,2019: 1,622) = 25: 0,124 = 1: 0,005.

To determine ε _λmah used averaged over 5 days of the plant obessulfachennaya EPC (γ = 1,619 g / cm ^3, the concentration of 53,7% P ₂ O _5), Fe content is determined by EDTA titration with acid in the presence of sulfosalicylic acid and of 0.271%. 0.1937 g of acid is placed in a volumetric flask with a capacity of 25 cm ³ and adjusted to the mark with reactive phosphoric acid with a content of 40% P ₂ O ₅ . The contents of the flask are mixed, filled with a solution of a cuvette with an absorbing layer thickness of 0.1 cm and photographed on a Spekord M-40 spectrophotometer in the range of 200-270 nm. As a comparison solution, orthophosphoric acid of the “chda” brand is used, diluted with distilled water to 40% P ₂ O ₅ . The maximum of the absorption spectrum occurs at a wavelength of 223.2 nm, while the optical density is 0.8116. According to the formula (2)

According to the formula (1)

we find the concentration of iron in the analyzed sample of desulfurized acid:

Example 2. Determination of the iron content in the raffinates formed at the stage of purification of EPA tributyl phosphate.

The degree of extraction of phosphoric acid from EPA 79-81%.

0.0625 g of raffinate (with a specific gravity of 1.240 g / cm ³ and a P ₂ O ₅ content of 19.1%) was added to a 25 cm ³ volumetric flask and adjusted to the mark with “chda” grade phosphoric acid diluted with distilled water to a concentration of 10 % P ₂ O ₅ . The volume ratio is V _p : V _en = 25: (0.0625: 1.24) = 25: 0.05 = 1: 0.002.

The contents of the flask are mixed, the solution is filled with a quartz cuvette with an absorbing layer thickness of 0.1 cm and photometered on a Spectord M-40 spectrophotometer in the range of 200-270 nm. Chda grade orthophosphoric acid diluted with water up to 10% P ₂ O ₅ , placed in a quartz cuvette with an absorbing layer thickness of 0.1 cm, is used as a comparison solution. The absorption maximum is at 230.8 nm, while the optical absorption density is 0.2575.

To build a calibration graph as a standard solution using an average sample of raffinate for 5 days of installation (with a specific gravity of 1.247 g / cm ³ and a P ₂ O ₅ content of 21.33%). The Fe content in the raffinate is determined by titration of EDTA in the presence of sulfosalicylic acid [1] and is 0.679%. The following weights of a standard solution are placed in four volumetric flasks with a capacity of 25 cm ³ : 0.2262 g, 0.1723 g, 0.1084 g and 0.0518 g. The volumetric flasks are adjusted to the mark with a solution of orthophosphoric acid of the “chda” grade (diluted with water to 10% P ₂ O ₅ ) and photometer on a spectrophotometer, as described above, measure the optical density at the absorption maxima and plot in coordinates A - C _Fe mg / cm ³ .

The value of 0.2575 optical density of the analyzed solution on the calibration graph corresponds to an iron concentration of 0.014 mg / cm ³ that when converted to the initial raffinate solution will be

Other examples of the method are presented in the table. In all examples, cuvettes with an absorbing layer thickness of 0.1 cm were used.

Sources of information

1. Methods of analysis of phosphate raw materials, phosphorus and complex fertilizers, feed phosphates. M., "Chemistry", 1975, p.134.

2. GOST 10555-75 "Reagents and highly pure substances. Colorimetric methods for determining the content of iron."

Claims (2)

1. A method for the quantitative determination of iron in phosphoric acids obtained by the sulfuric acid decomposition of phosphate raw materials and their subsequent purification with tri-n-butyl phosphate (TBP), including photometry of complex iron compounds in solutions, characterized in that the phosphoric acids formed in the process of opening sulfuric acid phosphate raw materials and subsequent purification of tributylphosphate, diluted with a reactive phosphoric acid at a volume ratio of diluent (V _p) to the assay solution (V _en) equal to V _p: V 1 = _en: (0,002-1) photometry in the wavelength range 200-270 nm. 2. The method according to claim 1, characterized in that as a diluent of phosphoric acids use aqueous solutions of reactive phosphoric acid with a concentration of 10-40% P ₂ O ₅ .