Analysis on characteristics of phosphogypsum

Analysis on characteristics of phosphogypsum
With the increasing demand of the society for the protection of land and water resources, the traditional stockpiling treatment method has restricted the development of phosphorus chemical enterprises. Only by solving the balance between the import and export of phosphogypsum can the development of enterprises be sustainable. In order to solve the problem of comprehensive utilization of phosphogypsum, the author made a superficial analysis of the characteristics of phosphogypsum.
Shijiazhuang Jingjin Mechanical Equipment Installation Co., Ltd. --- Zhou Yanhui
1 Physical characteristics of phosphogypsum
Phosphogypsum is an industrial by-product produced by the reaction of apatite and sulfuric acid in chemical plants to produce phosphoric acid by wet process. Phosphogypsum is yellow white, light gray white or black gray fine powdery solid, mainly composed of CaSO4 · nH2O, and its mass fraction is usually more than 85%. Theoretically, like natural gypsum, there are seven mineral phases in industrial waste phosphogypsum, including CaSO4 · 2H2O, – CaSO4 · 0.5H2O β– CaSO4 · 0.5H20, etc. Similar to natural gypsum, phosphogypsum generally contains 20% - 30% free water. According to the different production processes of wet process phosphoric acid, phosphogypsum crystals have the forms of dihydrate (CaSO4 · 2H2O) and hemihydrate (CaSO4 · 0.5H2O).
According to the routine laboratory measurement, phosphogypsum is grayish white, yellow and grayish yellow locally, with strong acidity, loose unit weight of 0.85g/cm3, void ratio of 1.0~3.5, and permeability coefficient of 2.8~3.2 × 10-4cm/s。 It is a multi-component complex crystal, containing 19~25% crystal water, 4%~12% adherent water, 52% liquid limit and 32% plastic limit.
2 Chemical and mineral composition of phosphogypsum
For the determination of phosphogypsum and other components, SIEMENS D500 X-ray diffraction analyzer is used for the determination. The working voltage and current of the instrument are 40kV and 30mA, and the scanning speed is (2 θ）=  40/min。
Phosphogypsum is mainly composed of calcium sulfate dihydrate, in addition to a certain amount of P2O3, F -, Fe2O3, organic matter, undissolved phosphate rock powder and acid insoluble matter. See Table 1 for its specific chemical composition.
Table 1 Chemical Composition of Phosphogypsum (%)
Composition SiO2 Fe2O3 MgO CaO SO3 P2O5 F-crystal water Others
Content (%) 3.52 0.25 0.89 30.68 41.64 2.03 0.63 19.95 0.41
3. Micro crystal structure of phosphogypsum
3.1 Needle crystal
The size of the crystal is 80-500 in length, 20-100 in width, 5-10 in thickness and 4-5 in aspect ratio. This crystal has a suspension tendency and is neutral when the slurry is dehydrated.
3.2 Monodisperse tabular crystals
The crystals are mostly parallelogram or rhombus, and the crystal size is: 40~200 long, 30~150 wide, 5~10 thick, and the length to width ratio is about 1.3~1.5. These crystals have a tendency to pile up with each other along the horizontal "layers", resulting in a phenomenon that is not conducive to the movement of interstitial liquid.
3.3 Dense crystals
It is transformed from the plate crystal. When the thickness of the plate crystal increases to tens of microns, it forms a dense crystal. Its formation conditions are similar to that of the plate crystal.
3.4 Polycrystalline nuclear crystals
It is a polymer of dense crystals with "burrs", which have good filterability. A large number of observations show that the appearance of phosphogypsum crystals in Kaiyang Phosphate Mine area mostly belongs to monodisperse parallelogram, rhombus, pentagon and hexagonal crystal columns. The crystals are crisscrossed and stacked with each other, forming membrane pores that are mostly unequal rectangular.
4 Particle size distribution of phosphogypsum
The particle size and distribution of phosphogypsum is one of the important factors to be analyzed when it is used as filling material, so it is necessary to conduct corresponding research. On the basis of SEM photos, the project has made statistics on the length distribution of 400 phosphogypsum crystal particles. Figure 1 is a trend chart of particle length distribution based on the principle of histogram in statistics.
Fig. 1 Distribution of Phosphogypsum Crystal Size
It can be seen from the figure that the length of phosphogypsum crystal is mainly distributed between 0.0025 and 0.0575mm. Table 2 shows the full size distribution of phosphogypsum. It can be seen from the table that phosphogypsum crystal with size less than 0.03mm has a great advantage, with the proportion up to 82.5%.
Table 2 Particle Size Distribution of Phosphogypsum (%)
Size range (mm) 0.06
Percentage (%) 27 32 23.5 7.8 5.6 3 1.1
Study on Hydration Characteristics of Phosphogypsum
X-ray phase analysis shows that the main hydration products of phosphogypsum are ettringite, hydrated calcium silicate, and calcium hydroxide in the early stage of hydration. The strength development of phosphogypsum hydrate hardened body mainly depends on ettringite and hydrated calcium silicate gel. With the progress of hydration, ettringite and hydrated calcium silicate gel continue to increase, and the strength of hardened body continues to increase. When the hydration tends to stop and ettringite and hydrated calcium silicate no longer form, the strength of hardened body tends to be stable. Ettringite crystal plays a skeleton role in the hardened body, while hydrated calcium silicate gel is distributed in the pores, which plays an important role in reducing porosity and improving the strength of the hardened body. Phosphogypsum dihydrate is filled in the hardened body as a micro aggregate.
6 Other impurities of phosphogypsum and their effects on its properties
There are two kinds of impurities in phosphogypsum: soluble and insoluble impurities. The former refers to impurities that cannot be removed during filtering and washing; The latter is the crystal formed by the transformation of impurities in phosphate rock and the undissolved insoluble impurities in phosphate rock.
Soluble impurities include free phosphoric acid and sulfuric acid; The other is monocalcium phosphate, fluosilicate, F -, etc; The third is sodium and potassium salts.
(1) Free phosphoric acid and sulfuric acid
Free phosphoric acid and sulfuric acid are the acid residues filtered in the production of wet process phosphoric acid. Free acid phosphogypsum is highly corrosive, causing environmental pollution on the surface and groundwater pollution underground. At the same time, the free acid will also prolong the setting time of gypsum and reduce the strength of gypsum.
(2) Monocalcium phosphate, fluosilicate, F -, etc
The content of soluble phosphorus (including free phosphoric acid, calcium phosphate, etc.) depends on the washing and filtering process of phosphogypsum, and the general content (mass fraction) is 0.4%~0.8%. Soluble phosphorus is adsorbed by calcium sulfate dihydrate crystal and distributed on its surface. During hydration, it can react with Ca2+in solution to generate insoluble tricalcium phosphate and adhere to the surface of phosphogypsum, thus preventing further dissolution and hydration of gypsum, extending the setting time of phosphogypsum, loosening its structure and reducing its strength.
(3) Sodium and potassium salts
Sodium and potassium salts will cause the dried gypsum products to appear "crystal flowers" and pulverization, reducing their strength.
There are two kinds of insoluble impurities, one is silica sand (SiO2) that exists in phosphate rock and will not be acidolysis, and undissolved minerals and organic matter; The other is dicalcium phosphate, insoluble phosphate and fluorine compounds that crystallize together with calcium sulfate during the acidolysis of phosphate rock.
Silica sand and unreacted fine phosphate rock have little influence on the quality of gypsum, but wear on the treatment equipment. The organic substances in phosphogypsum mainly come from the organic substances in phosphate rock and the organic additives added in the production of phosphoric acid. It mainly includes ethylene glycol methyl ether acetate, isothiocyanomethane, 3-methoxy-n-pentane, 2-ethyl-1, 3-dioxypentane, etc. It is mainly distributed on the surface of phosphogypsum, which can blacken gypsum products, slow down the solidification rate, and reduce the strength of products.
Co crystalline phosphate, insoluble phosphate and fluorine compound co crystalline phosphate (eutectic phosphorus for short, mainly dicalcium phosphate) is formed because HPO2-4 partially replaces SO2-4 into CaSO4 lattice. Its content is affected by the extraction process conditions in phosphoric acid production, and the general content (mass fraction) is 0.2%~0.8%. In the process of hydration, the co crystalline dicalcium phosphate dissolves from the lattice and reacts with Ca2+in the solution to generate insoluble tricalcium phosphate, resulting in coarsening of the phosphogypsum lattice, slower solidification rate and lower strength. Other insoluble phosphates are mainly tricalcium phosphate and a small amount of phosphate complexes (iron, sodium, potassium, aluminum, strontium, magnesium and other metal complexes). Their content (mass fraction) is about 0.6%~1.4%, which mainly exists in the coarse particles of phosphogypsum, but has little impact on the performance of phosphogypsum. The insoluble fluorine compounds are mainly CaF2, AlF3, Na3AlF6, etc., which will slow down the solidification rate and reduce the flexural strength of phosphogypsum.
The content of radioactive substances in phosphogypsum should be paid attention to. As phosphogypsum is mainly used in the construction industry, Rad-226, Thorium-232, Potassium - 40 and other radioactive elements will be released γ Radon, a radioactive gas, will also be released during the decay of Radium-226 and Thorium-232. Once these radioactive substances exceed the standard, they will do great harm to human health.
Through the action of alkaline activator, free phosphoric acid, sulfuric acid, monocalcium phosphate, fluorosilicate, F -, etc. in phosphogypsum can generate slightly soluble or water-insoluble calcium salt, which is equivalent to solidification treatment, and has no significant impact on the surrounding water quality.