Method for determining calcium sulfate content in PVC pipe

Abstract

This invention discloses a method for determining the calcium sulfate content in PVC pipes, mainly solving the technical problem of inaccurate quantification of phosphogypsum in existing PVC drainage and sewage pipes, power pipes, and communication pipes containing phosphogypsum (mainly calcium sulfate). The technical solution of this invention is as follows: a method for determining the calcium sulfate content in PVC pipes, comprising the following steps: sample pretreatment, where the PVC pipe sample is crushed using a low-temperature crusher to obtain particles smaller than 1 mm; the particles are then placed in a microwave calcination furnace and calcined at 500°C in an oxygen-rich environment until constant weight is achieved, yielding a residue; qualitative analysis, where the crystal structure of the residue is analyzed by XRD, and the diffraction peaks are compared with those of the phosphogypsum standard spectrum to clarify the calcium sulfate phase composition in the pipe; and quantitative analysis, where the Ca and S ion content in the residue is analyzed by ICP-OES, and the calcium sulfate content is calculated. This invention's method is simple to operate, highly accurate, and widely applicable, suitable for determining the calcium sulfate content in PVC pipes with different phosphogypsum content.

Description

Technical Field

[0001] This invention relates to the field of polymer material composition analysis technology, specifically to a method for determining the calcium sulfate content in PVC pipes, which is particularly suitable for quality testing of PVC drainage and sewage pipes, power pipes, communication pipes, and other products containing phosphogypsum filler. Background Technology

[0002] Phosphogypsum, a major industrial byproduct of the phosphate chemical industry, has an annual increase of over 100 million tons. Its storage not only occupies land resources but also poses environmental risks such as groundwater pollution and soil salinization, highlighting the urgent need to improve its comprehensive utilization rate. Research indicates that calcium sulfate (including modified phosphogypsum products), when added to PVC materials as a filler, can significantly optimize the material's mechanical properties (such as tensile strength and impact toughness), and its cost is lower than that of traditional calcium carbonate and talc fillers. It has already been applied in products such as porous and multi-faceted pipes for communications, drainage and sewage pipes, and power protection pipes.

[0003] The addition of phosphogypsum to plastics can effectively improve the structural properties and enhance the toughness of materials, making them more adaptable to various applications. However, analyzing the phosphogypsum content in plastics is quite difficult. Publicly available group standards such as T / GZHG01-2020 "Phosphogypsum-Reinforced Polyvinyl Chloride Porous and Multifaceted Pipes for Communication Use," T / GZHG022-2021 "Phosphogypsum-Modified Rigid Polyvinyl Chloride Drainage and Sewage Pipes," and T / GZHG027-2021 "Polyvinyl Chloride (Modified Anhydrous Phosphogypsum) Biaxially Oriented Power Pipes" clearly specify the amount of modified phosphogypsum to be added, but do not mention relevant testing methods. The product standards published by relevant companies also limit the amount of phosphogypsum added. Some standards only limit the sulfur trioxide content, and the measurement is carried out according to GB / T176-2017 "Chemical Analysis Methods for Cement". This method mentions iodometric titration, coulometric titration, ion exchange, and inductively coupled plasma atomic emission spectrometry, which involve acid hydrolysis or combustion decomposition of the sample to detect sulfur trioxide content. However, due to the stable chemical properties of phosphogypsum-modified plastic products, especially their acid and high-temperature resistance, the sulfur trioxide content in these plastic products cannot be detected. Furthermore, the calcium sulfate detection method in standard GB / T5484-2012 "Chemical Analysis Methods for Gypsum" mainly targets the analysis of calcium sulfate content in natural gypsum, anhydrite, and industrial by-product gypsum, using the barium sulfate gravimetric method. For natural gypsum, anhydrite, and industrial by-product gypsum that does not contain calcium sulfite, the sample is decomposed with hydrochloric acid, filtered, and then precipitated with barium chloride solution in an acidic solution. After ignition, the precipitate is weighed as barium sulfate. The results were calculated as sulfur trioxide, and the final calcium sulfate content was converted from the sulfur trioxide content. In preliminary experiments using this method to detect calcium sulfate content in plastics, it was found that hydrochloric acid could not dissolve the plastic products at all. Furthermore, when the plastic was calcined at high temperatures and then dissolved again, it was found that the calcination was incomplete, and PVC material, with its strong flame retardancy, could not dissolve the sample with hydrochloric acid. Therefore, this method is not suitable for the detection and analysis of calcium sulfate content in plastic products. Currently, there is no method for detecting and analyzing the phosphogypsum content in plastic products. This difficulty seriously affects the development of phosphogypsum in the polymer materials industry and hinders the application of solid waste phosphogypsum in the plastics industry. Summary of the Invention

[0004] The purpose of this invention is to provide a method for determining the calcium sulfate content in PVC pipes, mainly to solve the technical problem that the phosphogypsum in existing PVC drainage and sewage pipes, power pipes, communication pipes and other products containing phosphogypsum cannot be accurately quantified.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for determining the calcium sulfate content in PVC pipes includes the following steps:

[0007] (1) Sample pretreatment: The PVC pipe sample is crushed to obtain a granular sample; the granular sample is spread in a crucible, placed in an oxygen-rich environment, and burned at 450℃~550℃ until constant weight is obtained to obtain the residue.

[0008] (2) Qualitative analysis: The crystal structure of the burn residue was analyzed by X-ray diffraction (XRD), and the diffraction peaks of the standard phosphogypsum spectrum were compared to clarify the calcium sulfate phase composition of the burn residue and confirm whether it contains calcium carbonate components.

[0009] (3) Digestion treatment: Accurately weigh the residue and transfer it to a sealed container. Add concentrated hydrofluoric acid and concentrated nitric acid, and place it in an oven at 180℃~190℃ for 24h~30h in a sealed environment until the solution is clear and completely digested. Then, heat it to dryness on a hot plate at 140℃, adding a small amount of nitric acid to assist in removing the acid. After evaporation, add nitric acid and ultrapure water, and reconstitute it at 140℃ in a sealed environment for 4h~5h. After cooling, dilute to volume with ultrapure water to obtain the sample test solution. At the same time, prepare a blank test solution.

[0010] (4) Quantitative analysis: Using the prepared standard working solution, an elemental standard curve is constructed, and the Ca and / or S ion content in the sample test solution is determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and then the mass percentage of calcium sulfate in the PVC pipe is calculated.

[0011] In the XRD analysis of step (2), if the characteristic diffraction peak of calcium carbonate is detected in the burnt residue, the calcium sulfate content must be calculated using the S ion content in step (4).

[0012] Preferably, in step (1), the specific method of the crushing process is as follows: first, the PVC pipe sample is cut to obtain a block sample with a length of less than 2cm, and then the block sample is crushed into a particle sample with a particle size of less than 1mm by a low temperature freeze crusher.

[0013] Preferably, in step (1), the calcination temperature in the oxygen-rich environment is 500°C, the calcination is completed in a microwave calcination furnace, and the crucible used is a crucible that has been preheated to constant weight at 500°C.

[0014] Preferably, in step (3), 0.1g (accurate to 0.0001g) of the residue is weighed and placed in a polytetrafluoroethylene liner, and 2mL of concentrated hydrofluoric acid and 4mL of concentrated nitric acid are added.

[0015] In step (4), during the ICP-OES determination, the characteristic spectral wavelength of Ca ions is 317.933 nm (abbreviated as Ca 317), and the characteristic spectral wavelength of S ions is 181.975 nm (abbreviated as S 181). Preferably, when the XRD analysis in step (2) reveals the presence of calcium carbonate diffraction peaks in the burnt residue, the quantitative analysis selects to determine the sulfide ion content to calculate the calcium sulfate content, in order to avoid interference from calcium carbonate. In step (4), when calculating the mass percentage of calcium sulfate in the PVC pipe, if it is converted based on Ca content, it is calculated according to Formula 1; if it is converted based on S content, it is calculated according to Formula 2:

[0016] Formula 1

[0017] Formula 2

[0018] Where W is the calcium sulfate content in the sample, in %; C is the ion concentration measured by the instrument, in mg / L; V is the volume of the test sample, in mL; m is the mass of the residue used for the test, in g; M is the total mass of the sample used for the test, in g; 3.3967 is the conversion factor between calcium and calcium sulfate; and 4.4264 is the conversion factor between sulfur and calcium sulfate.

[0019] Furthermore, in step (3), the contents of impurity elements such as arsenic, lead, cadmium, chromium, mercury, aluminum, iron, magnesium, silicon, potassium, sodium, and phosphorus in the solution can also be determined simultaneously.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] (1) Highly targeted and accurate: This invention is specifically designed for PVC composite material systems. Through optimized oxygen-enriched calcination pretreatment at 500℃, the PVC matrix can be completely removed while minimizing interference with calcium sulfate filler. The subsequent XRD qualitative and ICP-OES quantitative methods are accurate and reliable, with spiked recovery rates of 95.9%~99.6% and RSD of 1.3%~1.4%.

[0022] (2) Strong anti-interference ability: The presence of interfering substances such as calcium carbonate can be qualitatively determined by XRD, and the sulfur content can be quantified by flexibly selecting the determination of sulfur element content, which effectively avoids the interference of coexisting calcium carbonate filler on the determination results.

[0023] (3) High efficiency and wide application: The method and process are clear and the operation is relatively simple. The use of microwave calcination and ICP-OES improves the processing and analysis efficiency. It is suitable for the detection of various PVC pipes with different formulations and different phosphogypsum content, and can simultaneously detect multiple heavy metals and impurity elements, making it a comprehensive product. Attached Figure Description Figure 1This is a schematic diagram of the thermal oxidative degradation mechanism of PVC.

[0024] Figure 2 Comparative X-ray diffraction (XRD) spectra of inorganic powder residues obtained after treating samples with different pretreatment methods (high-temperature calcination, burning, and solvent dissolution). Figure 3 FT-IR spectra of PVC composite powder and samples obtained by different pretreatment methods; Figure 4 The images show the scanning electron microscope (SEM) and elemental distribution of the samples; (a) shows the microstructure of the PVC composite material, and (b) shows the microstructure of the residue after calcination. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments.

[0026] The PVC sewage pipe containing phosphogypsum was cut into blocks less than 2 cm in length. The block samples were then crushed into particles less than 1 mm in diameter using a low-temperature cryogenic crusher. 20 g of each particle was accurately weighed (accurate to 0.0001 g) and spread evenly in two crucibles that had been fired to constant weight at 500 °C. The crucibles were then completely ignited at 500 °C in an oxygen-rich environment until constant weight was achieved, yielding the residue.

[0027] Two samples of the residue from the parallel experiments were prepared and analyzed by XRD. CaCO3 diffraction peaks were found. The S content was determined by ICP-OES, and then the calcium sulfate content was calculated.

[0028] Accurately weigh 0.1 g (accurate to 0.0001 g) of the residue after combustion, transfer it to a polytetrafluoroethylene (PTFE) liner, add 2 mL of concentrated hydrofluoric acid and 4 mL of concentrated nitric acid, and place it in a digestion vessel. Digest in a sealed oven at high temperature (180-190°C) for 24-30 hours until the solution is clear and completely digested. Then place it on a hot plate at 140°C, add a small amount of nitric acid (<1 mL), and evaporate to dryness. Add 2 mL of nitric acid and 3 mL of ultrapure water to the evaporated sample, seal, and heat in an oven at 140°C for 4-5 hours. After cooling, dilute to 100 mL with ultrapure water for analysis. Perform a blank test simultaneously.

[0029] For the preparation of standard solutions, appropriate amounts of Ca, K, Si, Ti, Fe, Al, Na, P, and S standard solutions were taken and mixed standard working solutions were prepared by stepwise dilution with 2% dilute nitric acid. The target concentration gradients were 1 µg / mL, 10 µg / mL, 20 µg / mL, 30 µg / mL, and 50 µg / mL, respectively.

[0030] The contents of Ca and S ions in the standard working solution and the sample solution are determined by ICP-OES, and then the calcium sulfate content is calculated. If the contents exceed the curve range, appropriate dilution should be performed. Alternatively, the contents of other elements such as K, Si, Ti, Fe, Al, Na, and P in the sample can be measured to obtain the standard working solution curve.

[0031] To verify the accuracy of the method for determining sulfur (S) content in phosphogypsum, phosphogypsum in PVC reference samples was evaluated. Anhydrous calcium sulfate was added to blank PVC samples to prepare reference samples with three concentration gradients: low (5%), medium (15%), and high (25%). Each group was measured in triplicate. After sample pretreatment, the sulfur content was determined by ICP-OES (S 181.975 nm).

[0032]

[0033] As shown in Table 1, the recovery rate of sulfur spiked in the sample was 95.9%–99.6%, and the relative standard deviation (RSD) was 1.3%–1.4%, which meets the requirements of GB / T 27417-2017 "Guideline for Conformity Assessment and Validation of Chemical Analysis Methods" (recovery rate 95%–105%, RSD ≤ 2%), indicating that the method is accurate and reliable and suitable for the analysis of calcium sulfate content in PVC pipes.

Claims

1. A method for determining the calcium sulfate content in PVC pipes, characterized in that, Includes the following steps: Sample pretreatment: The PVC pipe sample is crushed to obtain a granular sample; the granular sample is calcined in an oxygen-rich environment at 450℃~550℃ to constant weight to obtain the calcination residue; Qualitative analysis: The crystal structure of the burn residue was analyzed by X-ray diffraction (XRD) and compared with standard spectra to clarify the calcium sulfate phase composition in the burn residue; Digestion treatment: Weigh the residue, add concentrated hydrofluoric acid and concentrated nitric acid, and digest in a sealed container until the solution is clear. Then evaporate to dryness, and add nitric acid and ultrapure water to make up to volume to obtain the test solution. (4) Quantitative analysis: The content of Ca and / or S ions in the test solution is determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), and the content of calcium sulfate in the PVC pipe is calculated based on the content of Ca and / or S ions; wherein, if the diffraction peak of calcium carbonate is detected in step (2), the content of calcium sulfate is calculated based on the content of S ions in step (4).

2. The determination method according to claim 1, characterized in that, In step (1), the PVC pipe sample is cut into blocks with a length of less than 2 cm, and then crushed into particles with a particle size of less than 1 mm by a low-temperature freeze crusher.

3. The determination method according to claim 1, characterized in that, In step (1), the calcination temperature is 500°C, and the calcination equipment is a microwave calcination furnace.

4. The determination method according to claim 1, characterized in that, In step (3), the conditions for the sealed digestion are: sealed digestion at 180℃~190℃ for 24h~30h.

5. The determination method according to claim 1, characterized in that, In step (3), the amount of concentrated hydrofluoric acid used for every 0.1g of the residue is 2mL and the amount of concentrated nitric acid used is 4mL.

6. The determination method according to claim 1, characterized in that, In step (3), the specific steps of evaporation are as follows: after digestion, heat at 140°C and add less than 1 mL of nitric acid until evaporation is complete.

7. The determination method according to claim 1, characterized in that, In step (3), the specific steps for volume adjustment are as follows: add 2 mL of nitric acid and 3 mL of ultrapure water to the sample after evaporation, heat at 140°C for 4 to 5 hours, cool, and then adjust the volume to 100 mL with ultrapure water.

8. The determination method according to claim 1, characterized in that, In step (4), the analytical spectral line of Ca ions in the ICP-OES analysis is 317.933 nm, and the analytical spectral line of S ions is 181.975 nm.

9. The determination method according to claim 1, characterized in that, In step (4), inductively coupled plasma atomic emission spectrometry is used to determine the content of one or more of the elements arsenic, lead, cadmium, chromium, mercury, aluminum, iron, magnesium, silicon, potassium, sodium, and phosphorus in the test solution.

10. The determination method according to claim 1, characterized in that, The PVC pipes are PVC drainage and sewage pipes, power pipes, or communication pipes containing phosphogypsum.

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