Comparative Study of α-Hemihydrate Preparation from Phosphogypsum Before and After Flotation Purification

doi:10.21203/rs.3.rs-888156/v1

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Comparative Study of α-Hemihydrate Preparation from Phosphogypsum Before and After Flotation Purification

https://doi.org/10.21203/rs.3.rs-888156/v1

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Phosphogypsum (PG) is a massive industrial solid waste. In this paper, PG was purified by flotation method, and α-hemihydrate gypsum (α-HH) was prepared by the autoclaving method. The morphology of α-HH was adjusted by adding different doses of Maleic acid and Aluminium sulfate. The results showed that after flotation purification, the impurity content in PG was significantly reduced, the soluble phosphorus content decreased from 0.48% to 0.07%, the PG purity increased from 73.12% to 94.37%, and the PG whiteness risen from 19.4 to 40.5. Then the performance of α-HH prepared from PG before and after purification was compared. Fixing the amount of Aluminium sulfate at 0.2wt%, the reaction temperature at 140°C, and the reaction time at 120min, the average length/diameter ratio of α-HH crystals decreased from 7.2 to 0.6 as the amount of Maleic acid increased from 0 to 0.17wt%. When the amount of Maleic acid was 0.13wt%, the α-hemihydrate gypsum reached the best mechanical properties. The mechanical strength of high strength gypsum prepared from PG concentrate was significantly better than that of raw PG, indicating that flotation purification can effectively improve the performance of PG. In this study, a new method of PG purification and resource utilization was proposed.

Environmental Policy

Phosphogypsum

Flotation

α-hemihydrate gypsum

Mechanical strength

Phosphogypsum (PG) is a massive industrial solid waste generated by the wet process of phosphoric acid preparation, which typically produces 5 tons of PG for every 1 ton of phosphoric acid produced. The cumulative global emissions of PG are estimated to be about 6 billion tons and are increasing at a rate of 150 million tons/year. It is estimated that the total amount of PG stockpiles will grow to twice the current level by 2025 to 2045 ^1–3. The main component of PG is CaSO₄·2H₂O, which also contains a variety of impurities, such as SiO₂, soluble phosphorus, fluorine, organic matter, and some containing heavy metal ions and radioactive elements. The disadvantages of PG such as poor water resistance and uneven particle size distribution lead to its inferiority to natural gypsum in terms of usability, hardness, and whiteness, thus limiting its application areas and making it challenging to be resourcefully utilized ^4–6. Currently, only 15% of PG is utilized, while the remaining portion can only be stockpiled in large quantities ⁷. The piled PG occupies a large amount of land resources and the long time piling will lead to dust flying to pollute the air and the leaching of soluble phosphorus and fluorine to pollute the soil and water bodies ^8,9. Therefore, it is of environmental importance and urgency to choose a reasonable method to purify and resourcefully utilize PG.

The currently commonly used PG pretreatment methods can partially solve the impurity problem, such as using high-temperature calcination to obtain hemihydrate or anhydrous gypsum for use as cement and slag composite cementitious materials ^10,11; using calcium hydroxide to neutralize the acid in PG ¹²; reducing the retarding time of PG by ammonia pretreatment ¹³; and using citric acid solution pretreatment to convert phosphorus and fluorine in PG into citrate that can be removed by water washing ¹⁴. However, all these methods may increase the treatment cost and cause secondary pollution. Moreover, due to the differences and complex composition of phosphate ores in different regions, it is difficult to remove harmful impurities from PG by a single method effectively. Flotation is an efficient and environmentally friendly method that is now widely used in mineral purification. The organic matter in PG can be partially removed by flotation equipment, which is hydrophobic and much less dense than water. The strong agitation in the flotation process can also effectively remove harmful impurities such as soluble phosphorus and soluble fluorine from PG ¹⁵. For the consideration of treatment cost, process complexity, and treatment effect, flotation was proposed as a method for the purification of PG.

At this stage, much research has been done on the resource utilization of PG. For example, PG is used as construction materials such as hard gypsum board ¹⁶, foam concrete ¹⁷, calcium sulfate whiskers ¹⁸, no-burn bricks ¹⁹, ceramics ²⁰, and lime-fly ash-phosphogypsum binder ²¹. However, the amount of PG in the above-mentioned products is relatively small and cannot achieve the purpose of consuming it in large quantities. In addition, there are problems such as the relatively low added value of the product and the high cost of processing technology, which prevent it from being widely used. The α-hemihydrate gypsum (α-HH) has the advantages of high strength, lightweight, has no pollution, and good biocompatibility ^22,23. Currently, it has been widely used as a high value-added gelling material in the construction field ²⁴. Therefore, the use of PG as a raw material to prepare α-HH not only realizes resource recycling but also protects natural gypsum resources and achieves sustainable development of the environment, economy, and society.

In this study, a combination of ball milling and forward and reverse flotation was used to purify PG. Then the PG raw ore and concentrate were converted to α-HH using the autoclave method. The effects of different doses of Maleic acid on the crystal morphology and properties of α-HH were discussed, and the optimal amount of crystal modifier was obtained. The performance differences between the α-HH products prepared by PG before and after purification were compared. The results of the study have important guiding significance for the resource utilization of PG.

2.1 Materials

PG used in this study was obtained from a large phosphorus chemical company in Deyang, Sichuan, China. The samples were naturally dried at room temperature, mixed, shrunk, and weighed 200 g per bag for the test. Methyl isobutylcarbinol (MIBC) and Dodecylamine were produced by Shanghai RHAWM Technology Development Co., Ltd., and 1231 was produced by Tianjin Kermel Chemical Reagent Co. The crystal modifier used in the test were Maleic acid and Aluminium sulfate, both produced by Chengdu Chron Chemical Co. All the reagents in the test were analytically pure.

2.2 Flotation experiment

The flotation test first weighed 200 g of PG raw ore for ball milling (GSDM-003A fine grinding machine, Beijing GOSDEL POWDE & TECHNOLOGY Co., Ltd., China). It used a 1 L single flotation cell (XFDIV, Jilin Province Prospecting Machinery Factory, China) to stir the ball-milled raw ore into a slurry evenly. Then The organic matter and fine slime in PG were counterflotted by adding frother. Finally, the PG concentrate with higher whiteness and purity was positively flotated by adding collector. The test water was laboratory tap water. The test samples were dried at a temperature of 42°C and used to check their chemical composition, purity, whiteness, physical phase, morphology, etc.

2.3 Preparation of α-HH

The α-HH was prepared by autoclave method. The main steps included hydrothermal synthesis reaction, high-temperature drying, and characterization. The specific procedure was as follows: 2 g of PG concentrate (solid-liquid ratio 1:1) was weighed and hydrothermally reacted in a 50 ml autoclave at 140°C under fully closed conditions, and the sample was quickly removed after 2 h of reaction, the upper liquid layer was poured out and dried at 120°C for more than 12 h. The dried sample was stored in a desiccator and observed under a scanning electron microscope (TM-1000 ; HITACHI Co. Ltd., Tokyo, Japan) at a magnification of 1000 times to study whether there was any change in the crystal morphology. The reaction was completed by adjusting the amount of crystal modifier to achieve the best macroscopic properties of the prepared product when the SEM image showed that it contained 95% and more hexagonal prismatic crystals. To further verify the differential changes of PG concentrate in the hydrothermal reaction with PG raw ore, PG raw ore was used as a control group to prepare α-HH under the same experimental conditions and compared with the characteristics of α-HH prepared from PG concentrate.

2.4 Characterization

The chemical composition of PG was analyzed using a sequential wavelength dispersive X-ray fluorescence spectrometer (XRF; Axios advanced, PANalytical B.V, Netherlands) with a tungsten target, tube pressure 60 KV, tube current 10 mA. A scanning electron microscope (TM-1000; HITACHI Co. Ltd., Tokyo, Japan) was used to observe the PG crystal morphology. The thermal stability of PG was analyzed using a simultaneous thermal analyzer (TG-DSC; SDT Q600, TA Instruments, America) to characterize PG further and determine the water of crystallization content. Chemical functional groups were characterized using a Fourier transform infrared spectrometer (FTIR; Spectrum One, Perkin Elmer Corporation, America) with a scan range of 500–4000 cm^− 1. A whiteness tester (SBDY-1P, Shanghai Yuefeng Instrumentation Co. China) was used to determine the PG whiteness. The soluble phosphorus content in the samples was determined by the phosphorus-vanadium-molybdenum yellow double wavelength photometric method in the People's Republic of China Building Materials Industry Standard "Method for the determination of phosphorus and fluorine in phosphogypsum" (JC/T 2073). The content of attached water and the purity of calcium sulfate dihydrate of PG were determined according to GB/T 5484 − 2012.

The water requirement of the standard consistency of the prepared products is determined with reference to the standard of "Determination of the physical properties of net gypsum slurry for construction" (GB/T 17669.4–1995). The setting time of the product is measured with reference to the standard of "Determination of physical properties of net gypsum slurry for construction" (GB/T 17669.4–1995). The flexural and compressive strengths of high strength gypsum in different periods (2h, 3d) are prepared, maintained, and measured with reference to the standard "α-type high strength gypsum" (JC/T 2038 − 2010). Weigh the appropriate amount of α-bassanite powder and mix it according to the w/c of 0.4, pour it into three 40 mm × 40 mm × 160 mm test molds, shake and compact them, let them stand for 30 min, take off the molds, and cure them for 2 h. The flexural strength of the three test blocks was tested separately, and the final average value was the 2-h flexural strength of the test blocks. The specimens broken after the flexural test were maintained for 3 d and tested for 3 d compressive strength after baking to constant weight in an oven at (40 ± 4)℃. The compressive strength was determined by a microcomputer-controlled electronic pressure tester (305F-2, Shenzhen WANCE Test Equipment Co., Ltd., China) with a loading area of 0.0016 m². The flexural strength was determined by an electric flexural tester (DKZ-6000, Wuxi JIANYI Instrument & Machinery Co., Ltd., China).

3.1 Flotation test results

3.1.1 Frother screening

The addition of a frother can remove the organic matter and fine slime in PG and remove part of the phosphorus enriched in the fine slime. Firstly, several commonly used frother were screened, and the effects of Pine oil, Tributyl phosphate, MIBC, P86, on the flotation behavior of easy-to-float organic impurities in PG were compared. The test procedure is shown in Fig. 1, and the dosage of the agents is tentatively set at 300 g/t. The test results are shown in Table 1.

From the results of the screening test, it can be seen that several agents can flotation remove part of the easily floatable organic impurities. However, after comparison, the PG concentrate obtained by MIBC reverse flotation reached 27.7 whiteness and 92.54% purity, which is the best agent for reverse flotation slime removal, so MIBC was used as the frother to remove the floatable organic impurities and fine slime from PG in the subsequent test. And from the comparison of the data obtained from several other flotation agents in Table 1, it is concluded that the whiteness of PG is positively related to the purity, and the higher the purity, the higher the whiteness.

Table 1

Frother type screening test results
Type	Concentrate			Tailings
Type	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%
Pine oil	94.8	90.26	23.2	5.2	60.93	11.6
MIBC	94.9	92.54	27.7	5.1	61.42	11.4
Tributyl phosphate	95.8	89.71	22.7	4.2	57.73	8.9
P86	93.2	90.39	23.4	6.8	65.81	14.8

3.1.2 Frother usage

After the frother screening test, MIBC was determined as the optimal frother for the removal of floatable organic impurities and slime, and the subsequent MIBC dosage test was conducted. The results are shown in Table 2. It can be seen that with the increase of MIBC dosage, the removal rate of organic impurities and slime, the whiteness and purity of PG concentrate increased. When the dosage of MIBC was 300g/t, the PG concentrate index reached the best, and the change of PG flotation index was not significant when the dosage of MIBC continued to increase. Therefore, in the follow-up test, the MIBC dosage was set at 300g/t.

Table 2

Frother dosage test results
Dosage /(g/t)	Concentrate			Tailings
Dosage /(g/t)	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%
50	92.5	91.21	26.2	7.5	62.36	12.9
150	91.4	90.18	25.9	8.6	65.91	14.7
250	90.8	91.30	26.2	9.2	65.48	14.5
300	94.9	92.54	27.7	5.1	61.42	11.4
350	85.7	91.74	26.7	14.3	67.83	15.2
400	83.9	91.86	26.3	16.1	70.10	16.3

3.1.3 Grinding test before desliming

From the phenomenon of MIBC reverse flotation desliming, we can see that there are still a large number of flaky crystals in the PG concentrate aggregating with each other. Some of the gypsum and apatite are wrapped and not physically separated. A large number of aggregated crystals will affect the removal of organic matter, so it is believed that the flotation purification process should first lightly grind the PG so as to break up the aggregates so that more surface impurities can be separated from the gypsum. According to the pre-exploration test, the fine mill was selected, the material to ball ratio of the ball mill was 3:1, and the water volume was 500 ml, and the effect on the flotation behavior of the floatable organic impurities in PG was observed by varying the grinding time. The test flow is shown in Fig. 2, and the test results are shown in Table 3.

As can be seen from Table 3, the yield, whiteness, and purity of PG concentrate showed an increasing trend with the increase of grinding time. After 15min grinding, the particle size content − 0.074mm reaches 31.6%, the growth trend reached its peak, and the whiteness of PG concentrate obtained by reverse flotation reached 29.8, and the purity reached 93.96, which was the optimal condition for reverse flotation desliming, so the particle size content of samples − 0.074mm was set at 31.6% in the subsequent test.

Table 3

Grinding test results before desliming
Time/min	-0.074mm percentage/%	Concentrate			Tailings
Time/min	-0.074mm percentage/%	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%	Productivity /%	CaSO₄·2H₂O purity/%	Whiteness /%
5	26.3	83.5	91.65	26.2	16.5	71.02	16.7
10	30.8	84.6	93.41	28.3	15.4	70.17	16.0
15	31.6	87.6	93.96	29.8	12.4	62.89	13.9

3.1.4 Direct flotation dodecylamine collector dosage

After grinding, reverse flotation to remove easy to float organic impurities and fine slime, PG still contains some impurity minerals, mainly some coarse size of calcium phosphate stone, these impurities still affect the PG whiteness, and phosphorus elements, etc. will be enriched in these impurities, so it is necessary to separate gypsum from these residual impurities. Since the surface of CaSO₄·2H₂O is negatively charged in most of the pH range, the cationic collector has good collecting ability for it ²⁵. In this experiment, dodecylamine was selected for the separation and purification of PG, and the effect of dodecylamine dosage on the whiteness and purity of gypsum was investigated. The experimental procedure is shown in Fig. 3, and the results are shown in Table 4.

From Table 4, it can be seen that the yield of PG concentrate increased with the increase of dodecylamine dosage. When the amount of dodecylamine reached 150g/t, the continued increase of dosage led to the PG concentrate rate, whiteness and purity no longer changed and had a decreasing trend, which can be inferred that the increase of collector dosage led to the increase of CaSO₄·2H₂O up-floating in PG, which is easy to entrain more impurities in the up-floating process. Considering the cost of dodecylamine, the dosage of dodecylamine was selected as 150g/t in the flotation test.

Table 4

Dodecylamine dosage test results
Dosage/(g/t)	Product	Productivity/%	CaSO₄·2H₂O purity/%	Whiteness/%
25	Concentrate	28.6	93.58	28.5
	Tailings 1	18.1	70.26	16.9
	Tailings 2	53.3	91.39	28.6
50	Concentrate	49.1	93.96	29.8
	Tailings 1	19.1	71.99	17.8
	Tailings 2	31.8	91.82	29.2
100	Concentrate	71.6	93.67	28.9
	Tailings 1	15.7	65.29	15.9
	Tailings 2	12.7	88.74	22.9
150	Concentrate	78.9	94.16	34.5
	Tailings 1	15.4	70.92	17.2
	Tailings 2	5.7	72.37	19.7
200	Concentrate	76.3	93.85	29.9
	Tailings 1	16.8	68.29	16.6
	Tailings 2	6.9	73.36	18.3

3.1.5 Comparison of PG properties before and after purification by flotation closed-circuit test

After the open circuit test, the PG closed-circuit flotation process is shown in Fig. 4, and the test results are shown in Table 5. Compared with the raw PG, the PG concentrate treated by the closed-circuit flotation condition has less impurity content and therefore has a higher utilization value. The total phosphorus content of the purified PG concentrate was 1.17%, and the soluble phosphorus content was reduced from 0.48–0.07%. Most of the soluble phosphorus and fluorine went into the beneficiation tailing water. The whiteness of PG was increased from 19.4 to 40.5, and the purity was increased from 73.12–94.37%. The purified PG concentrate has met the national standard PG (GB/T 23456 − 2018) first-class product standard for gypsum building materials.

Table 5

Chemical composition, purity and whiteness of PG Raw Ore and PG Concentrate (wt%)
	PG Ore	PG Concentrate	PG Tailings 1	PG Tailings 2
SO₃	49.33	51.64	45.27	48.70
CaO	41.31	42.88	34.13	38.12
SiO₂	5.03	2.76	11.93	4.51
P₂O₅	1.49	1.17	2.07	2.81
Al₂O₃	1.42	0.72	3.39	2.51
Fe₂O₃	0.58	0.30	1.77	0.89
SrO	0.35	0.20	0.42	1.63
K₂O	0.19	0.09	0.32	0.17
TiO₂	0.10	-	0.28	0.28
BaO	0.07	0.08	0.11	0.09
Na₂O	0.07	-	0.04	0.03
Y₂O₃	0.02	0.02	0.01	0.02
ZnO	0.01	-	0.02	-
Whiteness	19.4	40.5	17.4	20.5
Purity	73.12	94.37	69.31	72.67

The SEM analysis of the concentrate and the original ore is shown in Fig. 5. Figure 5(b) indicates that the PG concentrate has a rhombic, plate-like structure, and the crystalline surface is flatter than the PG original ore, with significantly fewer surface impurities and no apparent defects. From Fig. 5(a), it is seen that there are a large number of impurity particles on the surface of PG raw ore, and most of the plate structure is fractured and defective.

3.2 Preparation and characterization of α-hemihydrate gypsum

Firstly, the preparation of α gypsum using PG concentrate without the addition of the crystal modifier was investigated. Figure 6 show the thermogravimetric analysis of α gypsum prepared from PG concentrate and α-hemihydrate gypsum, respectively. From Fig. 6(a), it can be seen that the TG mass change for the PG concentrate is 21.38%. The DSC curves showed heat absorption peaks at 160.125°C and 171.13°C, indicating that the water in PG crystals changed from 2 molecules to 1.5 molecules and then to 0.5 molecules. An obscure exothermic peak was detected at 459.1°C, indicating that the hard gypsum underwent a crystalline transformation. The results of the thermal analysis can verify that the main phase of PG is CaSO₄·2H₂O. From Fig. 6(b), we can see that the heat absorption peak appears around 167.3°C, followed by the exothermic peak around 206.3°C, which is the unique characteristic peak of α-HH ^26–28, and no characteristic peak of CaSO₄·2H₂O phase was detected. The results verified that the CaSO₄·2H₂O crystals had been completely transformed into α-HH crystals.

Figure 7 shows the SEM images of α-HH crystals transformed without the addition of crystal modifier, as shown in the figure, the length of the prepared α-HH crystals was in the range of 54–98 µm (average value of 76 µm), and the diameter was 5–10 µm (average value of 8 µm). The morphology of the transformed α-HH crystals was irregular hexagonal prisms. Since α-HH has various crystalline forms (plate, rod, column, etc.), and the different crystalline forms lead to different mechanical strengths. Usually, the short column α-HH with a complete crystalline form has higher mechanical strength ^29,30. However, α-HH will only form long columnar and needle-like crystals without the modifier, resulting in its low strength. Therefore, the unmodified α-HH crystals should be adjusted to smaller length and diameter crystals to improve their strength and performance.

3.3 α-HH crystal shape modulation

The other experimental conditions for the preparation of α-HH were kept consistent. The amount of Aluminium sulfate was fixed at 0.2wt% of the sample mass. The α-HH crystals were adjusted by adding different amounts of Maleic acid (0.01wt%, 0.03wt%, 0.05wt%, 0.07wt%, 0.09wt%, 0.11wt%, 0.13wt%, 0.15wt%, 0.17wt%) on α-HH crystals were modulated. The effect of Maleic acid dosage on the morphology of PG crystals was analyzed using SEM, and the results are shown in Fig. 8. As seen from the figure, when the amount of Maleic acid was increased from 0.01wt% to 0.13wt% (Fig. 8a-g), the length of α-HH crystals became shorter, the diameter increased, and the L/D ratio decreased. The crystal morphology gradually changed from long columnar to shortly columnar, and finally, the L/D ratio was 0.7 on average at the dosage of 0.13wt%. When the dosage of Maleic acid exceeded 0.13wt% (Fig. 8h-i), the α-crystal length gradually became longer and smaller in diameter, and the L/D ratio increased.

The change of crystal morphology is determined by the relative growth rates of different crystalline planes, so the addition of crystal modifier to the reaction system can change the external shape of the crystal ²⁶. This theory is confirmed by the different morphologies of α-HH crystals shown in Fig. 8, where the change in crystal morphology can be interpreted as the adsorption of modifier on a specific crystal face, thus changing the relative growth rate of that face. Since the Ca²⁺ content of the top surface of α-HH crystals is higher than that of the prismatic surface, the crystal surface parallel to the c-axis is usually positively charged and has higher surface energy, and has a higher relative growth rate during the growth process ^27,31. When Maleic acid was added, the hydroxyl group adsorbed with Ca²⁺ on the top surface and formed a complex, which inhibited the growth of crystalline surfaces along the c-axis and coordinated the growth rates of different crystalline surfaces, and finally obtained α-HH crystals with relatively small L/D.

When the amount of Maleic acid was from 0.01wt% to 0.09wt%, the top surface of some α-HH crystals was defective. When the amount of Maleic acid was from 0.11wt% to 0.17wt%, the top surface of the crystals was basically flat, which indicated that the modifier mainly affected the growth of the top surface of α-HH crystals. Further observation of the defective part of the top of the crystal shows that most of the defective area is the middle area of the top surface. Therefore, it can be assumed that when the Maleic acid concentration is relatively low (0.01wt%-0.09wt%), which belongs to the initial stage of crystal growth, the modifier partially adsorbs on the top surface of the crystal, inhibiting the growth of the crystal along the c-axis direction and accelerating the growth of the crystal diameter, resulting in a radial expansion of the top surface. With the consumption of the modifier, the protruding part of the top surface of the crystal could not be adsorbed by the modifier, and the growth rate of these areas not adsorbed to the modifier was gradually larger than the central area, which eventually led to the defect of the top surface of the crystal. However, when the concentration of Maleic acid is relatively high (0.11wt%-0.17wt%), the amount of modifier is sufficient to adsorb on the whole top surface of the crystal, so there is no obvious defect on the surface.

After analysis by SEM image, it was found that the crystal morphology of α-HH had reached the best state when the amount of Maleic acid was 0.13wt% (Fig. 8-g), so the α-HH was prepared with the amount of crystal modifier of 0.13wt% was chosen. The results of the modified sample compared with the blank sample by FTIR analysis are shown in Fig. 9. The modified sample showed new absorption peaks at 1005 cm^− 1 and 672 cm^− 1, while the characteristic peak of the C-H group existed at 866 cm^− 1 for Maleic acid, indicating that Maleic acid interacted with the crystal to fix the C-H group on the crystal. And the new absorption peak at 1005 cm^− 1 may be due to the stretching vibration peak of SO₄²⁻ of Aluminium sulfate at 1099 cm^− 1. Further observation of the spectra of the modified samples revealed that the peaks at 3611 cm^− 1 and 3546 cm^− 1 were shifted to the left by 65 cm^− 1 and 135 cm^− 1, respectively, compared with the wave numbers of the blank samples. It can be inferred that the carboxylate anion in Maleic acid located at 3440 cm^− 1 formed a complex with Ca²⁺ on the top surface of the crystal, which further verified the interaction between the modifier and the α-HH crystal.

3.4 Comparison of the performance of PG raw ore and concentrate preparation of α-HH

The α-HH was prepared using PG raw ore and concentrate under the same conditions to compare the differences in the properties of the α-HH prepared from the two raw materials. The results of the study are shown in Table 6. The whiteness of α-HH prepared from PG raw ore and concentrate was increased from 27.8 to 46.3. The initial setting time and final setting time of the concentrate were extended by 3min10s and 7min40s respectively, compared with the original ore. 2h flexural and compressive strength of purified PG was increased by 46.15% and 79.46% compared with the original ore, and 3d dry compressive strength was increased by 39.6% compared with the original ore.

Table 6

Comparison of α-HH Mechanical Strength of PG Raw Ore and PG Concentrate
	α-HH (Raw Ore)	α-HH (Concentrate)
Average L/D ratio	7.2	0.7
W/H ratio/%	68	70
Initial setting time/(min:s)	6:50	10:00
Final setting time/(min:s)	10:20	18:00
2h flexural strength (Mpa)	4.16	6.09
2h compressive strength (Mpa)	12.1695	21.8395
3d compressive strength (Mpa)	29.3424	40.9614

Comparative analysis of the SEM images of the crystals of the two products. Compared with Fig. 8-g, the α-HH crystals prepared from PG raw ore (Fig. 10-a) are smaller in size, longer in length and diameter, heavily agglomerated, and with more impurities on the surface. It can be assumed that some impurities in the PG raw ore accelerate the crystal transformation in the hydrothermal reaction, thus limiting the crystal size, and the smaller size crystals are more likely to agglomerate with each other. And some of the α-HH crystals prepared from PG raw ore showed fractures or defects on the surface (Fig. 10-b), which may be due to the presence of impurities such as soluble phosphate on the surface of raw ore crystals, resulting in the deterioration of crystal morphology during the reaction ³².

The two α-HH products were prepared as specimens of the same size, and the differences in the fracture surfaces of the specimens were further observed (Fig. 11). It is apparent that the fracture surfaces of the α-HH specimens prepared from PG concentrate are relatively dense, while the fracture surfaces of the α-HH specimens prepared from PG raw ore are loose and have a large number of voids. From the comparison of the fracture surface cavity size of α-HH specimens prepared from two different raw materials in Fig. 12, it can be seen that the pore size in α-HH specimens purified by flotation shrinks from about 38 µm to about 13 µm. This is the fundamental reason for the improvement of the mechanical strength of α-HH specimens.

In this study, the PG raw ore was purified using the flotation method, and then α-HH with different crystal shapes was prepared by hydrothermal reaction. The differences between the α-HH products prepared from PG raw ore and PG concentrate were discussed, and the following conclusions were obtained:

Adding frother MIBC can effectively remove organic matter and floatable slime from PG and then use dodecylamine to carry out positive flotation on PG. After treatment, the whiteness of PG concentrate reaches 40.5%, the purity of CaSO₄·2H₂O reaches 94.37%, and the soluble phosphorus content is reduced to 0.07%, which meets the national standard (GB/T 23456 − 2018) first-class product standard for gypsum building materials.

As the amount of Maleic acid increased from 0 to 0.17wt%, the L/D ratio of α-HH crystals decreased from 7.2 to 0.7. When the amount of Maleic acid was 0.13wt%, the transformed α-HH products showed the best mechanical strength, the 2h compressive and flexural strengths reached 21.8395Mpa and 6.09Mpa, respectively, and the 3d dry compressive strength was 40.9614Mpa.

Compared with the raw PG ore, the whiteness of α-HH product prepared from PG concentrate increased from 27.8 to 46.3. 2h flexural and compressive strength increased by 46.15% and 79.46% compared with the raw ore, and 3d dry compressive strength increased by 39.6% compared with the raw ore.

Acknowledgment

This research was financially supported by the open project of State Key Laboratory of Solid Waste Reuse for Building Materials (SWR-2019-003), the China Postdoctoral Science Foundation (Project No.2019M653837XB); the National Natural Science Foundation of China (Grant No.51874106).

Competing interests

The authors declare no competing interests.

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Comparative Study of α-Hemihydrate Preparation from Phosphogypsum Before and After Flotation Purification

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Abstract

Figures

1. Introduction

2. Experimental Method

2.1 Materials

2.2 Flotation experiment

2.3 Preparation of α-HH

2.4 Characterization

3. Result And Discussions

3.1 Flotation test results

3.1.1 Frother screening

3.1.2 Frother usage

3.1.3 Grinding test before desliming

3.1.4 Direct flotation dodecylamine collector dosage

3.1.5 Comparison of PG properties before and after purification by flotation closed-circuit test

3.2 Preparation and characterization of α-hemihydrate gypsum

3.3 α-HH crystal shape modulation

3.4 Comparison of the performance of PG raw ore and concentrate preparation of α-HH

4. Conclusion

Declarations

References

Additional Declarations

Status:

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