Eco-Friendly Mining Practices: Field Test of Phosphogypsum Filling Based on Slag Powder in Dayukou Phosphate Mine

Abstract

Filling with phosphogypsum is one of the important ways to realize the sustainability development of phosphate mines. This study is based on the extensive on-site experiments conducted at the Dayukou phosphate mine. Over a period of 60 days, different proportions of phosphogypsum, cement, and mineral powder were used to fill the voids in the No. 1 and No. 2 test ore pillars. The results of strength testing during the experimental process indicate that the strength development of the filling material at various stages is normal, meeting all the requirements for mining production. The environmental protection monitoring station in the city conducted water quality analysis during the filling process, indicating that the concentrations of major elements such as Cu, Zn, Mn, Pb, Cd, and Hg in the water samples meet the industrial wastewater discharge standards. The fluoride content ranges from 3.28 to 6.90 mg/L, which is below the first-level standard of 10 mg/L specified in the “Comprehensive Wastewater Discharge Standards” (GB8978-1996). This suggests that the filling process has a minimal impact on the groundwater environment. After the completion of the filling, the pre-embedded pressure boxes function normally, and the data are generally stable, experiencing pressures ranging from 0.11 to 2.53 MPa. This on-site expanded trial indicates the feasibility of using cement, mineral powder, and phosphogypsum for underground filling. It demonstrates the potential for reutilizing the solid waste phosphogypsum as filling aggregate.

1. Introduction

“Reuse of industrial wastes” is the requirement for the sustainable development of the mining industry [1]. Phosphogypsum (PG) is an industrial solid waste generated during the production of phosphoric acid in the wet process by phosphate chemical companies [2]. Typically, the production of 1 ton of phosphoric acid results in the generation of 5 tons of phosphogypsum, with its main component being CaSO₄∙2H₂O [3,4]. China’s annual production is close to 80 million tons, with a comprehensive utilization rate of less than 40%. The accumulated stockpile has reached 870 million tons [5]. Most of the phosphogypsum is directly stored without treatment in nearby facilities’ storage yards, causing significant environmental impact during the weathering process [6,7]. The comprehensive utilization of phosphogypsum still relies primarily on traditional methods, with the main approach being the production of cement retarders [8], making ecological restoration materials [9], producing construction gypsum, and gypsum boards construction [10,11]. The utilization and proportions of various applications of phosphogypsum in China in 2022 are shown in Figure 1, and the data refer to In-depth Market Assessment and Investment Strategy Advisory Report of China Phosphate Industry 2023–2028 [12]. The quantity used for backfilling is less than 10%, primarily due to transportation constraints and the presence of toxic substances in phosphogypsum, making it less suitable as backfilling aggregate.

The primary use of phosphogypsum in backfilling is as an aggregate for cemented filling. Once solidified, it serves to support surrounding rocks and roof, control ground pressure, and effectively enhance ore recovery rates [13,14,15]. However, phosphogypsum possesses the following characteristics: high viscosity, prone to agglomeration, which hinders effective mixing; fine particle size and low permeability coefficient, unfavorable for dewatering and rapid solidification of filling slurry; and a certain amount of phosphoric acid and hydrofluoric acid, thereby significantly limiting its application in backfilling. To enhance the application of phosphogypsum in mine backfilling, numerous researchers have conducted a series of studies. Leveraging the fine particle size and high reactivity of phosphogypsum, when added to the filling slurry in specific proportions, it can effectively reduce hydration heat and suppress alkali-aggregate reactions [16,17]. We proved blended ladle slag with phosphogypsum to create a novel binding material and investigated its mechanical properties and durability after solidification. The results indicate the effect of NaOH content on the feasibility of preparing a novel industrial by-product-based binder material using phosphogypsum and finely ground blast furnace slag [18]. Many mining researchers in China have carried out research work on the harmlessness and reuse of phosphogypsum [19,20,21,22]. The aforementioned studies have provided a solid foundation for the application of phosphogypsum in underground mine backfilling. However, constrained by the limitations of the wet-process phosphoric acid production, phosphogypsum contains impurities such as phosphorus, fluorine, and heavy metals. Prior to utilizing phosphogypsum for mine backfilling, extensive research and on-site scale-up experiments are essential. This ensures that the backfilling material meets strength requirements without causing environmental pollution.

In the preliminary backfill material test project at Dayukou phosphate mine, researchers conducted a series of experiments to explore the feasibility of phosphogypsum back filling. They determined the viability of using slag micro powder + cement as the binding material and phosphogypsum as the filling aggregate. The study included an analysis of the hydration characteristics, strength evolution, and pore structure of the backfill material. Toxicity indicators in the leachate from the backfilling material met national standards. Simultaneously, through slump and L-shaped pipeline transport tests, the transport concentration of the filling slurry was determined to be approximately 56% to 59%. The self-flow transport multiplier for the pipeline ranged from 2 to 6, with a transport pipe diameter of about 120 mm to 140 mm, and an internal flow velocity of 1.0 m/s to 3.0 m/s [23]. In theory, phosphogypsum backfilling technology based on slag cement is deemed essentially feasible. These preliminary studies have already provided partial foundations for the company’s backfilling method and backfilling system design.

Given this, the current large-scale on-site experiment, building upon previous research, selected two unmined zones with a combined volume of approximately 540 m³ in the +155 section as the experimental pillars for backfilling. The experiment includes testing the strength of the backfilled material, monitoring stress changes in the pillars, and observing variations in the underground water environment. These measures aim to validate the feasibility of the phosphogypsum backfilling technique based on slag micro-powder and provide a foundation for the subsequent design of the backfilling process.

2. Problem Statement and Field Test Design

2.1. Engineering Background

The on-site mixing and slurry preparation test for underground filling were carried out in No. 1 and No. 2 test pillars, where the mining layers are approximately 7–8 m high. The No. 2 test pillar and No. 1 test pillar have similar shapes, with a slope of about 15 degrees, an elevation ranging from 155 m to 162 m, an oblique length of approximately 20 m, a width of about 6 m, and a height of about 4.5 m. The total volume for each mining pillar void space is approximately 540 m³.

2.2. Field Test Design

2.2.1. Materials

Using phosphogypsum (Dayukou phosphate mine) as backfilling aggregate, and the cementitious materials are ordinary Portland 425 silicate cement (Hubei Huaxin Cement Co., Ltd., Huangshi, China) and S95-grade slag micropowder (Hubei EgangJiahua New Building Materials Co., Ltd., Ezhou, China). For the backfilling of Dayukou phosphate ore, the specific gravity of phosphogypsum is 2.386, with an average particle size of 83.27 μm, and 50% of particles are below −75.03 μm. The material has a high moisture content and a flattened shape. By adding appropriate cementitious materials, it can be used as a filling aggregate. The main chemical components of the slag powder (SP) and phosphogypsum (PG) are provided in

Table 1. Results of main chemical components of the slag powder (SP) and phosphogypsum (PG).

		Proportions (%)
Mineral		SiO2	Al2O3	P2O5	SO3	CaO	MgO	MnO	Others
Sample	(S P)	26.24	14.13	–	2.72	36.35	8.74	0.27	11.55
	(P G)	0.06	0.31	1.31	50.76	35.83	0.11	–	11.62

, and the particle size composition is provided in

Table 2. Results of particle size composition of the slag powder (SP) and phosphogypsum (PG).

Unit/(μm)		$\overline{\mathbf{d}}$ (bar)	d10	d30	d50	d60	Cu	Cc
Sample	(S P)	9.68	2.01	5.56	8.89	11.94	5.94	1.29
	(P G)	83.27	20.99	50.93	75.03	94.24	4.59	1.31

Note: $\overline{\mathrm{d}}$ (bar) refers to the average diameter of lithium slag and fine tailings; C_{u =} d₆₀/d₁₀; C_c = (d₃₀/(d₁₀ × d₆₀)).

.

2.2.2. Experimental Design

(1) Experimental Stope Backfill Wall Design

Based on the condition of the trial mining column goaf and considering the sampling and collection of seepage water after backfilling in the trial mining column goaf, the layout diagram of backfilling barricades in the trial mining column goaf is shown in Figure 2. Among them, the No. 1 and No. 2 backfilling barricades are the main load-bearing barricades during backfilling; the construction of the No. 3 and No. 4 backfilling barricades is mainly for facilitating the capping of the goaf in the later stage of backfilling in the trial mining column; the No. 5 and No. 6 barricades are designed to intercept and collect seepage water, permeation water, and soaking water from the No. 1 trial mining column goaf backfilling body; the No. 7 and No. 8 barricades are designed to intercept and collect seepage water, permeation water, and soaking water from the No. 2 trial mining column goaf backfilling body. The main material consumption and cost estimation for each backfilling barricade are provided in

Table 3. Results of consumption and cost statistics for various backfill wall materials.

No.	Size (L × B × H) (m)	Hollow Brick (m3)	Cement Mortar (m3)	Concrete (m3)	Budgeted Costs (CNY)
1#	5 × 2.5 × 5	62.5	15.6	15.6	6250
2#	5 × 2.5 × 5	62.5	15.6	15.6	6250
3#	5 × 1.5 × 4.7	35.3	8.8	8.8	3000
4#	5 × 1.5 × 4.7	35.3	8.8	8.8	3000
5#	3.2 × 1 × 1	3.2	0.8	0.8	320
6#	3.2 × 1 × 1	3.2	0.8	0.8	320
7#	3.2 × 1 × 1	3.2	0.8	0.8	320
8#	3.2 × 1 × 1	3.2	0.8	0.8	320
Sum		197.8	49.5	49.5	19,780

. The backfilling barricades are constructed using red bricks or hollow bricks, with the No. 1 and No. 2 backfilling barricades built to the height of the goaf roof, approximately 5.0 m, the No. 3 and No. 4 backfilling barricades built to a height of 4.7 m, and the No. 5, No. 6, No. 7, and No. 8 backfilling barricades built to a height of 1 m.

(2) Backfilling scale and interval time

For the protection of the lower backfill retaining wall, pre-filling is conducted before the backfill slurry passes through the retaining wall, with each filling increment being 0.5 m in height. The shapes and sizes of two experimental goaf areas are essentially similar. Based on the flat and cross-sectional profiles of the goaf, the volume of each filling increment during the preliminary filling process is estimated. A total of 7 filling increments are required before passing through the retaining wall, with an intermittent duration between each filling corresponding to the initial setting time of the filling slurry, as specified in

Table 4. Results of preliminary backfilling volume estimation.

Times	Section (m2)	Filling Quantity (m3)
1st	0.82	4.9
2nd	1.874	11.2
3rd	2.957	17.7
4th	4.04	24.2
5th	5.123	30.7
6th	6.206	37.2
7th	7.289	43.7

. Once the filling slurry has passed through the retaining wall, it will no longer cause damage to the wall, and the filling scale is unrestricted. Filling operations continue according to the on-site filling schedule until the experimental goaf is completely filled.

(3) Design of Water Seepage Treatment Measures

During the backfilling process, efforts should be made to minimize the entry of washwater into the mining area, as it not only affects the quality of backfilling but also increases seepage. For the treatment of seepage, a preemptive measure involves placing a filtration pipe in the roof of the void, wrapped in geotextile fabric and securely suspended. The other end of the pipe is directed out through the barrier wall. In the later stages of backfilling, if there is still a significant accumulation of water in the upper void, the installation of a submersible pump in the upper section can be considered to pump out the excess water.

(4) Backfilling Roof Support Measures Design

The main solutions to backfilling roof issues are as follows: firstly, the empty area itself has a slope, and the mixing station is positioned higher than the highest point of the test void; secondly, progressively constructing an elevated upper backfill barrier, gradually raising the backfill discharge point; thirdly, compensating for voids generated due to slurry settlement through multiple backfilling cycles and promptly discharging seepage water from the upper part.

3. Field Test Results and Discuss

3.1. Backfilling Material Consumption

Based on preliminary research, researchers and mining management have determined that the abandoned space in the No. 1 experimental mining pillar will be filled using a slurry mixture of phosphogypsum, 425 cement, and S95 slag micro-powder in a ratio of 1:1:8, with the slurry being stirred and conveyed for self-flow filling. Similarly, the No. 2 experimental mining pillar abandoned space will be filled with a slurry mixture of phosphogypsum, 425 cement, and S95 slag micro-powder in a ratio of 1:1:12, with the slurry being stirred and conveyed for self-flow filling. The entire filling period is approximately 3 months, with a daily normal filling capacity of 10 barrels/d, equivalent to about 35 m³. The consumption of filling materials for the No. 1 and No. 2 experimental mining pillar abandoned spaces is detailed in

Table 5. No. 1 and No. 2 experimental mine pillar void filling material consumption.

Material Consumption	Filling Quantity (keg)	425 Cement (t)	S95 Slag Micro-Powder (t)	Phosphogypsum (m3)
1# (1:1:8)	216	75.6	72.00	607
2# (1:1:12)	176	52.8	44	586
Sum	392	128	116	1193

after the on-site experiments are completed.

3.2. Backfilling Material Strength

Research findings indicate that during the process of using phosphogypsum for backfilling, the presence of phosphates, acids, heavy metals, etc., in phosphogypsum can interfere with the hydration reaction, affecting the ultimate strength of the backfill. Existing studies suggest that this phenomenon is attributed to the adsorption of phosphates on the surface of cement particles, hindering the initial hydration reaction. Additionally, the residual acids in phosphogypsum reduce the alkalinity of the backfill slurry, impacting the hydration product content in the backfill and consequently leading to a decrease in final strength [24,25].

In this study, to determine the ultimate strength of the backfill, backfill specimens were prepared according to the selected mix ratio, and their uniaxial compressive strength values at different curing periods (7 days, 14 days, and 28 days) as well as tensile strength (28 days) were tested and calculated by a Rock Mechanics Test System (Wuhan Institute of Geotechnical, Chinese Academy of Sciences. Wuhan, China), as shown in

Table 6. Results of phosphogypsum backfilling mix ratio test strength.

No.	Ash–Sand Ratio (Cement–Fly Ash–Phosphogypsum)	Compressive Strength (MPa)			28d Tensile Strength (MPa)
		7d	14d	28d
1	1:1:8	1.26	2.62	3.24	0.32
2	1:1:12	0.92	2.13	2.69	0.21

.

According to the results of the backfill mix ratio test, the 28-day strength of the No. 1 void area’s six blocks of backfill is consistently above 2.6 MPa, with an average strength of 3.24 MPa. Similarly, the No. 2 void area’s six blocks of backfill achieve a 28-day strength of at least 1.8 MPa, with an average strength of 2.69 MPa. Based on the comprehensive analysis of the backfill strength results from the current expansive testing of the filling materials on-site, it is feasible to use the materials from this experiment for backfilling.

From the on-site test results of the stope filling, it is observed that when the backfill slurry fills the void for 2–3 days, the backfill reaches final setting and can support personnel to walk freely on it, even under working conditions. The situation after 48 h of slurry backfilling can be seen in Figure 3.

3.3. Stress Variations

After the solidification of the grouting agent, the filling slurry gradually gains strength and transforms into a filling mass [26]. The backfill material needs to support the surrounding rock to provide a safe mining space for the next step of extraction [27,28]. To ensure that the stress experienced by the backfill mass remains within its compressive strength, a field experiment was conducted with the installation of pressure sensors on-site. These sensors were employed to monitor the stress variations in the secondary stress field within the backfill mass during the mining process in other chambers. The stress box used for this study is the TGH-4 model. Prior to backfilling, it was positioned in the goaf, and the stress display screen was connected via a cable and suspended on a supporting pillar. Once backfilled, the stress box was enveloped by the backfill mass. During mining operations, stress variations can be directly read from the display screen (GSJ-2A intelligent monitoring instrument). The layout of the stress boxes is depicted in Figure 4, and the on-site installation and monitoring of the stress boxes are illustrated in Figure 5.

Researchers recorded the initial frequency values of stress gauges during the installation of stress boxes. After the completion of backfilling, stress measurements were conducted for the first void once, ranging from 0.21 to 1.36 MPa. The second void underwent stress measurements twice, with pressure readings displayed by the gauge ranging from 0.11 to 2.53 MPa. Notably, among the two voids, the pressure boxes for the second and fifth voids experienced the highest compressive forces.

3.4. Backfill Permeability Analysis and Detection of Water Infiltration

In the process of wet phosphoric acid production, the fluorine (F) content in the phosphate ore is converted into gaseous HF and SiF₄ and volatilized, while the remaining approximately 1.7–3.6% is retained in the form of soluble fluorides (NaF, KF) and insoluble fluorides (Na₂SiF₆, Na₃AlF₆, Na₃FeF₆, and CaF₂) in phosphogypsum [10,29]. Once these fluorides enter the surrounding soil and water from the fill material in large quantities and eventually enter the food chain, they can cause neurological defects and damage to the human nervous system [30,31]. Therefore, this on-site experiment analyzes and detects the impact on water quality during the phosphogypsum filling process.

During the entire experimental process, a total of six water samples were collected, each obtained from the 155th and 142nd levels, respectively. The collected water samples were promptly delivered by Dayu Kou Company to the Jingmen Environmental Protection Testing Station for analysis. The testing parameters included fluoride, cyanide, suspended solids, total phosphorus, total arsenic, total selenium, total copper, hexavalent chromium, and 20 other components, as detailed in

Table 7. Testing results of water samples.

No.	Analysis Parameters and Detection Limits (mg/L)		Sample Name and Identification Results (mg/L)
			1#	2#	3#	4#	5#	6#
1	PH	-	9.35	8.85	9.42	10.27	10.30	10.27
2	Total Phosphorus	0.01	0.078	0.062	0.07	0.043	0.05	0.062
3	Suspended solids	-	35	34	39	10	26	24
4	Sulfate	8	827	797	969	1604	1313	1666
5	Cr6+	0.004	0.046	0.08	0.307	0.319	0.3	0.3
6	Fluoride	0.05	3.28	3.74	6.9	5.21	6.16	5.09
7	Cyanide	0.004	0.005	0.004	0.005	0.011	0.011	0.013
8	An-ionic surfactant	0.05	0.12	0.12	0.14	0.21	0.15	0.16
9	Chemical Oxygen Demand	15	15	15	18	23	18	24
10	Manganese (Mn)	0.01	0.01	0.01	0.01	0.01	0.01	0.01
11	Cadmium (Cd)	0.005	0.005	0.005	0.005	0.005	0.005	0.005
12	Copper (Cu)	0.02	0.02	0.02	0.02	0.02	0.02	0.02
13	Zinc (Zn)	0.005	0.005	0.005	0.005	0.005	0.005	0.005
14	Lead (Pb)	0.1	0.1	0.1	0.1	0.1	0.1	0.1
15	Nickel (Ni)	0.005	0.005	0.005	0.016	0.016	0.016	0.005
16	Chromium (Cr)	0.03	0.036	0.069	0.311	0.335	0.306	0.315
17	Mercury (Hg)	0.00001	0.00001	0.00001	0.00001	0.00001	0.00001	0.00001
18	Arsenic (As)	0.0002	0.00022	0.00164	0.0008	0.00177	0.00005	0.00178
19	Selenium (Se)	0.0003	0.0124	0.0069	0.0098	0.0079	0.0071	0.0059

.

4. Discussion

(1) Based on the previous indoor test results, the pH value of the leachate from the raw phosphogypsum is 2.98. However, in the on-site backfilling test, the percolation and permeate water pH values range from 8.85 to 10.3, which complies with the Class V sewage discharge standard [12].

(2) In accordance with the (GB/T14848-93 [32]), several indicators for the test and detection of phosphogypsum cemented filling seepage and permeation water have been examined. The fluoride content ranges from 3.28 to 6.90 mg/L, which is below the first-grade standard of 10 mg/L specified in the (GB8978-1996 [33]). This categorizes it as Groundwater Quality Classification Indicator Standard Class III. Based on the (GB5085.3-1996 [34]), the standard limit of 50 mg/L is within the safe range.

(3) The comprehensive analysis of water quality components in backfill seepage and permeate water samples indicates that the concentrations of major elements such as Cu, Zn, Mn, Pb, Cd, and Hg in the backfill seepage and permeate water meet the industrial wastewater discharge standards [23].

(4) After conducting expansive trials on the fill material at the site, the mining operation has adopted a filling method using phosphogypsum, cement, and finely ground mine slag. The seepage and permeation water from the fill exhibit compliance with industrial wastewater discharge standards for the majority of elements. Although a few elements slightly exceed the permissible limits, the impact on the overall water quality is negligible due to the minimal amount of water genuinely seeping out during the filling process. The water is efficiently reclaimed and reused, minimizing any significant influence on the entire water system.

5. Conclusions

The expansive testing at the Dayukou phosphate mine primarily investigated whether the use of phosphogypsum combined with (cement + slag micro-powder) as backfilling material underground would cause pollution to water. The on-site testing focused on the mechanical characteristics of the filled body, including its strength features and long-term stress–strain behaviors. The reliability of the small-scale phosphogypsum backfilling system was assessed. Additionally, the study examined whether the backfilled test pillars provided safety assurance for subsequent mining operations in the test mining chamber. Based on the experimental results:

(1)

Both empty spaces in the two experimental mine columns have been completely filled through the self-flow transport of filling slurry, and the filling at the interface is in good condition. Phosphogypsum + cement + mine slag micro-powder is used for filling. The strength development at various stages of the filling body is normal, and the strength is relatively high. This filling method is reasonable and feasible.

(2)

Through monitoring the seepage and permeation water, the concentrations of major elements such as Cu, Zn, Mn, Pb, Cd, and Hg in the water samples meet the industrial wastewater discharge standards. The fluoride content complies with the national standard of China and falls within the groundwater quality classification index standard Class III. According to the standard limit of 50 mg/L in the (GB5085.3-1996 [34]), it is within the safe range.

(3)

The pH value of the backfilling seepage and permeation water is in the range of 8.85 to 10.3, but its yield is much smaller than that of natural spring water. It can be used to lower the pH value of mine drainage or spring water from 8.5 to 7.0. Additionally, a pH value greater than 7 mainly originates from the alkalinity in cement, which is a common issue in concrete construction. Its impact is limited, and the duration is short.

(4)

After the completion of backfilling in the experimental pillar void, all pressure chambers are functioning normally. According to the data received from the monitoring device, the fill body pressures in the two test mining areas are still undergoing changes, with pressures ranging between 0.11 and 2.53 MPa.

(5)

The experimental results demonstrate that the expansibility test of the backfilling material for the DaYukou phosphate mine is successful. The small-scale phosphate gypsum-like structure flowable backfilling self-flow conveying backfilling system used in the process is smooth and reliable.