J. Cent. South Univ. Technol. (2009) 16: 0285-0291

DOI: 10.1007/s11771-009-0049-8

Cemented backfill technology based on phosphorous gypsum

WANG Xin-min(王新民), ZHAO Bin(赵 彬), ZHANG Qin-li(张钦礼)

 (School of Resources and Safety Engineering, Central South University, Changsha 410083, China)

Abstract: Physical-chemical properties of phosphorous gypsum, proportion and cemented mechanism of slurry with gypsum as aggregate were studied to remove the harms of gypsum pile, combining with difficult problems of excessive mined-out gobs, enormous ore body under roadway and low recovery ratio of Yongshaba Mine, Kaiyang Phosphor Mine Group, Guizhou Province, China. An appropriate backfill system and craftwork were designed, using shattering milling method to crush gypsum, double-axles mixing and strong activation mixing way to mix slurry, cemented slurry and mullock backfill alternately process. The results show that gypsum is fit for backfilling afterwards by adding fly ash, though it is not an ideal aggregate for fine granule and coagulate retardation. The suggested dosage (the mass ratio of cement to fly ash to gypsum) is 1?1?6-1?1?8 with mass fraction of solid materials 60%-63%. Slurry is transported in suspend state with non-plastic strength, and then in concretion state after backfilling. The application to mine shows the technology is feasible, and gypsum utilization ratio is up to 100%. Transportation and backfill effect is very good for paste-like slurry and drenching cemented slurry into mullock, and the compressive strength and recovery ratio are 2.0 MPa and 82.6%, respectively, with the maximum subsidence of surface only 1.307 mm. Furthermore, the investment of system is about 7×10⁶ yuan (RMB), only 1/10 of that of traditional paste backfill system.

Key words: phosphorous gypsum; self-flowing transportation; cemented backfill; cemented mechanism; backfill system and craftwork

1 Introduction

Phosphorous gypsum is a serious pollution source and industrial flotsam, from the production of phosphorous fertilizer and phosphoric acid [1]. Recently, because the quantity of discharged has increased by    2×10⁷ t/a and the utilization ratio is low, phosphorous gypsum has exhausted lots of lands, polluted environment acutely and brought enormous burden to corporation. So, the synthesized utilization of phosphorous gypsum has become an urgent problem for environment and safety [2-4]. It could not only remove the disasters of phosphorous gypsum accumulating on the surface of the earth, but also reduce the surface deformation of the earth, improve the recovery ratio and mining safety, if phosphorous gypsum is backfilled to mined-out gobs in mines.

Ground Calcium Carbonate Plant (GCCP), Kaiyang Phosphor Mine Group (KPM), Guizhou Province, China, has to invest lots of money to build phosphor storeroom for the large quantity of phosphorous gypsum discharged 5×10⁵ t/a and (2.5-3.0)×10⁶ t/a after 2-3 years. The accumulated gypsum not only holds a great deal of farmland, but also brings a worse effect to ambience. Meanwhile, the Yongshaba Mine (YM) of KPM, whose good mineral can be sold straightly without mineral processing, however, has formed lots of mined-out gobs for using open-stope mining method, resulting in the roof collapse in some sections, the subsidence of the covered terrane, the serious waste of mineral (the recovery ratio is only 70%) and the geological disasters such as the hill slide and ground collapse etc. So, YM has to layout lots of high quality safety mineral pillars (more than 2×10⁷ t) to guarantee safe mining and Jinyang roadway. Consequently, a safe mining method with high recovery ratio must be put forward taking into account the overstock resources and waste of mineral. The backfill mining method has been selected for YM for high recovery ratio with the phosphorous gypsum from GCCP as aggregate after prudent pondering. This technology has gained favorable benefit in YM, not only using the flotsam as treasure, protecting environment, improving the recovery ratio of mineral and safety of mining, but also starting a precedent of cemented backfill with phosphorous gypsum and providing academic gist for the similar mines.

2 Physical-chemical evaluation of phosphorous gypsum as aggregate

The phosphorous gypsum as aggregate was stockpiled in GCCP, which must be shattered, milled and sifted to remove the agglomerate with granularity larger than 20 mm, No.425 cement was used as cement material, and, the low-cost coarse dry discharged fly ash from Guiyang Power Plant was used as part replacement of cement. The physical-chemical properties of phosphorous gypsum must be studied for no instance according to literatures.

2.1 Physical-chemical properties of phosphorous gypsum

Chemical compositions are mainly CaSO₄·2H₂O, accounting for more than 90%, followed by little inorganic and organic phosphor, as well as microelement such as As, Cu, Zn, Fe, Mn, Pb, Ge, Hg and F, so phosphorous gypsum is hard to be used directly. It has been popularly used as the amendment of saline and alkali soil, construction material replacing part natural gypsum after removing F and free P₂O₅, coagulation retarder for cement and the material for sulfuric acid, cement and ammonium sulfate [5-6]. The properties of minerals vary from the producing area. Tables 1-3 list the physical-chemical properties of the selected gypsum and fly ash (the coarse dry discharged fly ash).

Table 1 Main chemical compositions of phosphorous gypsum and fly ash (mass fraction, %)

Table 2 Main physical-mechanical properties of gypsum and fly ash

Table 3 Granularity composition of phosphorous gypsum and fly ash (mass fraction, %)

2.2 Evaluation of properties

The following conclusions can be given by physical-chemical evaluation.

(1) Phosphorous gypsum is not an ideal aggregate. It goes against dehydration and rigidification of backfill body for its fine granule (smaller than 0.1 mm) accounting for 93%, mediate size of 43 μm, void ratio of 1.1-3.4 and permeability coefficient of 2.94×10^-6 m/s. Furthermore, the content of CaSO₄·2H₂O accounts for 90%, which is usually used as coagulation retarder delaying the initial strength of backfill body. However, the high content of CaSO₄·2H₂O makes it easy to backfill afterwards for the high evening strength.

(2) As fly ash possesses certain potential cementation property for the high content of SiO₂ and Al₂O₃ (43.70% and 20.52% respectively), fly ash can replace part cement, thus saving money, improving transportation property, reducing pipeline wearing and advancing the evening strength of backfill body, though it is also not an ideal aggregate for its fine granule with mediate size of 72 μm and small permeability coefficient of 6.48×10^-6 m/s.

(3) The high content of CaO in phosphorous gypsum (30%), which is propitious to the activation of materials, can supply the low activation of fly ash for the low content of CaO (only 7.32%). Furthermore, both gypsum and fly ash, being well-proportioned clay with asymmetry index of 3.71 and 2.31 respectively, are easy to mix, transport and reduce the degree of cement isolation.

In a word, phosphorous gypsum is hopeful to be a good aggregate by the addition of fly ash and appropriate activation material, though backfilling only with gypsum is not ideal. So, it is feasible to backfill the gypsum into mined-out gobs, when there is no other appropriate aggregate.

3 Optimized proportion of phosphorous gypsum cemented backfill

3.1 Proportion test

The slurry was made in normal temperature circumstance, and 9 samples for every suggested dosage were formed using the standard tri-unit model of 7.07 cm×7.07 cm×7.07 cm, then, went through the uniaxial compressive strength test after maintenance period with the Instron 250 kN rigid hydraulic pressure servo machine. The mode pressed in axes with the formula: σ_c=P/S, in which P represents press load (N) and S represents press area (m²). And the mullock such as fuchsia shale and little dolomite during the mining process were used as the assistant aggregate to reduce cost. The results of laboratory test are listed in Table 4.

Table 4 Results of laboratory test of phosphorous gypsum backfill

3.2 Analysis of results

(1) The single gypsum is not an ideal aggregate with the 90 d uniaxial compressive strength lower than 1.6 MPa. However, the evening strength of cemented sample increases evidently by adding fly ash, though the middle strength does not increase. For example, for the samples with the dosage of 1?1?8, the 90 d uniaxial compressive strength is about 1.5 (No.11) to 2.5 (No.8) times as the 28 d strength.

(2) The strength of backfill body increases with the slurry mass fraction increasing. The 90 d uniaxial compressive strength increases by 20% as the mass fraction of slurry increases from 60% to 63%. However, the best mass fraction should lie between 60% and 63%, but not exceeding 65%, to make fluent transportation.

(3) The force—transfiguration curves of gypsum samples, representing elastic and plastic properties, means that the gypsum sample possesses high residual strength after terminal breakage, which is very easy to backfill afterwards (Fig.1, where σ_t denotes micro- fracture compaction limit load, P₁P₂ denotes elastic stage, σ_β denotes yield load and σ_b denotes limit load).

(4) The coarse fly ash contributes to the construction of backfill body for bigger granule and framework function. The strength of cemented backfill body with dolomite is higher than that of shale evidently for the dolomite’s high strength and good framework function.

(5) The rheological property of gypsum slurry shows that the slurry is good for pipeline transportation due to satisfied viscous property, good theory stability and similar structure flow. The retarded coagulation property makes it slow curing and easily transported. Furthermore, the properties of strong workability, low bleeding rate and difficult settlement, result in full pipeline flowing easily. The phenomena of pipeline jam, pipeline wearing and isolation can be avoided.

Fig.1 Force—transfiguration curve of sample (No.8, 90 d)

(6) Taking into account both test results and backfill requirements, the best method is self-flowing transportation fly ash-phosphorous gypsum cemented backfill with the suggested dosage of 1?1?6-1?1?8 and mass fraction of 60%-63%, accompanied with dolomite cemented backfill.

4 Mechanism of cemented backfill

Fly ash is an active eruption material whose mainly compositions are vitreous Al-Si, quartz (α-SiO₂), crystal mineral such as mullite (3Al₂O₃·2SiO₂), as well as some unburned carbon. The vitreous Al-Si accounting for more than 70% is the most one for action, whose content increasing results in the high active property for fly ash [7-8]. Fig.2(a) shows that there are slippery roundness vitreous body, anomalistic particles and loosen porous unburned carbon. The active property of slurry increases as the roundness vitreous body increases for its lubricated function.

Fig.2(b) shows that the crystal of gypsum is big and emerges six faceplates shape [9]. This crystal makes it possess large water requirement during solidification, long time requirement to coagulate, plank shape of hydrated production and weak combination between crystals even low initial strength during solidification. However, because of the large water requirement, cemented slurry dehydrates very little in stope, about 2/3-3/4 of that of the cemented tailing slurry backfill.

Fig.2 SEM images of fly ash (a) and phosphorous gypsum (b)

The plastic strength of mutative properties is partitioned to three states during the construction formation: non-plastic strength suspended state, initial coagulation state, and consolidation state [10-12].

(1) Non-plastic strength suspended state. The mixture of fly ash, cement and phosphorous gypsum is suspended slurry after adding water in the initial phase. A layer of rich silicon is formed on the surface of the mineral particles of fly ash and cement after adding water. And, a superficial electric double layer structure is constructed on the surface of the mineral particles by absorbing calcium from solution to maintain electric equilibrium. Consequently, the slurry during the process of initial hydration is a dispersoid of solid particles with superficial electric double layer structure, and the properties and change of slurry lie on the reaction between these solid particles.

At the beginning of adding water to the mixture, slurry is in suspended state with non-plastic strength that is easy to pipeline transportation, for the big space between particles and no represented force each other.

(2) Initial coagulation state. The force of particles is embodied when the space decreases to a certain value, and the long-distance coagulation is represented at last when the Vander Waals force of attraction and the electrostatic repulsive-force are equivalent. Because the electric potential and charge reduce and the Vander Waals force of attraction keeps unchanged, two particles can adhere together. Then, the initial coagulation state is emerged for the loosen reticulation framework by reaction coagulation. This state shows that phosphorous gypsum possesses retard coagulation property, and the initial strength is low.

(3) Consolidation state. The force of particles, changing from the Vander Waals force of attraction to chemical bond force or hypo-chemical bond force, makes the strength higher, along with the yield of hydration particles and decrease of space between particles. Because of the property change of the force, the subsequent hydration particles form cross-link or connect each other by nucleation on the surface of particles, the inner chemical function represents crystallization, and the outer of slurry enters consolidation state. Meanwhile, strength is high, being easy to backfill afterwards.

5 Backfill system and craftwork

5.1 Backfill system

The preparation plant for the phosphorous gypsum cemented backfill system, including five production lines, was established on a mild hillside. The product line of phosphorous gypsum, cement, water and the preparation-transportation line of slurry are shown in Fig.3.

The selected gypsum is transported by loaders from gypsum pile to 1^# gypsum storehouse, then to 2^# gypsum storehouse through vibrated feeder, belt conveyor and muller, furthermore to mixer at last through another vibrated feeder and belt conveyor. Muller 6 is formed by tubbiness shell and screws to crush the stockpiled gypsum agglomeration during the free-fall stage of agglomeration, and the crushed fine gypsum drops into 2^# storehouse.

Fly ash is also transported by loaders from fly ash pile to fly ash storehouse, then to mixer through vibrated feeder and belt conveyor. Cement is unloaded by compressed air automatically to cement storehouse from cement tank car, then to mixer through spiral feeder and electronic steelyard. Water is measured by electromagnetic flow meter, and pumped to mixer from pool.

Phosphorous gypsum, fly ash, cement and water are mixed by two mixers in series. At first, the mixed slurry is mixed by double-axles mixer 17 to commingle these materials fully. Secondly, slurry is stirred actively by strong activation mixer 18 with high circumrotate speed to produce homogeneity slurry with good flow-ability and high mass fraction. The eligible slurry is flowed by gravity through pipeline in backfill drill to stopes.

Meanwhile, the mullock is backfilled to mined-out gobs accompanied with the phosphorous gypsum-fly ash slurry alternately, through another store and transportation system.

5.2 Backfill craftwork

The backfill afterwards method with phosphorous gypsums slurry and mullock, is used to the large samdwich-like space with height of 8 m, according to the medium-deep hole mining method. At first, backfill pipeline is fixed and backfill block-wall is constructed. Then, phosphorous gypsum slurry is transported by pipeline, through the drift of last phase, ramp road of section, slice drift, crosscut, drilling entry to stope. Meanwhile, mullock is transported by loaders to the same stope through the route above. The route with arrowhead is shown in Fig.4.

At first, phosphorous gypsum cemented slurry is used to backfill the samdwich-like stope, until the backfill height to 1-2 m. Then, some mullock with appropriate volume is backfilled after the cemented slurry. Then, the cemented slurry is backfilled again. Such cycle process will be continued until the mined-out gobs are backfilled fully. During the process, the pressure of slurry must be taken completely to mix the mullock and slurry well, slurry should fill both sides of stope to permeate and drench the mullock in mined-out gobs. After the process, the mixer and pipeline should be washed by water. The backfill work will stop until 3-5 min after the water flowing out of pipeline. At last, the dreggy water must be discharged out of the stopes.

6 Application

During the application experiment, the mass fraction of solid materials of slurry is adjusted to 57%, lower than the suggested value, to meet pipeline elf- flowing transportation property for the greater pipe length-backfill depth ratio (more than 5.7). The results of application in mine are shown as follows.

Fig.3 Phosphorous gypsum cemented backfill system and craftwork flow: 1—Phosphorous gypsum pile; 2—Loader; 3—Vibrated feeder; 4—1^# Phosphorous gypsum storehouse; 5—Belt conveyor; 6—Muller; 7—2^# Phosphorous gypsum storehouse; 8—Fly ash pile; 9—Fly ash storehouse; 10—Cement tank car; 11—Cement storehouse; 12—Spiral feeder; 13—Electronic steelyard; 14—Pool; 15—Water pump; 16—Electromagnetic flow meter; 17—Double-axles mixer; 18—Strong activation mixer; 19—Slurry filler

Fig.4 Backfill craftwork in stope: 1—Drift of last phase; 2— Ramp road of section; 3—Slice drift; 4—Crosscut; 5—Drilling entry; 6—Backfill body

(1) The technology meets the production requirement for the average backfill capability of system of 40-42 m³/a, and the throughput of the whole section reaches 200-250 kt/a with 500 t/d of a single stope.

(2) The shatter-milling technology is used to crush gypsum agglomeration, making gypsum utilization ratio reach 100%, which not only makes use of gypsum 120 kt/a (saving 1.2×10⁶ yuan(RMB) for storehouse and maintenance), but also protects environment. The vibration technology is used to ensure the uniformly continuous feeding of gypsum and fly ash, avoiding the arching phenomena. The double-axles mixer and strong activation mixer are used in series to prepare equable slurry with paste-like rheological property, good flow-ability, no-isolation and low bleeding ratio [13-14].

(3) The technology of backfilling with gypsum and mullock alternately makes very good effect (Fig.5). The whole backfill body is good for the effect of drenching slurry into mullock, which forms a cemented crust with a certain thickness around stope to surround the inner mullock [15-16]. Furthermore, the effect of roof-contacted backfill is good and the average of compressed strength reaches 2.0 MPa, which improves the safety of mining.

(4) The high quality phosphor resource is reclaimed with recovery ratio high to 82.67% from 70% formerly and dilution ratio lower than 6%, removing the difficult problem of 2.26×10⁷ t safety pillars resource under roadway. The subsidence of roof is only 1.307 mm, which avoids the geological disaster such as the collapse of surface and roadway.

Fig.5 Picture of backfill effect

(5) The investment of system is about 7×10⁶ yuan (RMB), only 1/10 of that of traditional paste backfill system. This technology is very profitable for YM with about 1×10⁸ yuan (RMB) straightly.

7 Conclusions

(1) Phosphorous gypsum is easy to backfill afterwards for the high evening strength by adding fly ash, though it is not an ideal aggregate for fine particle and retard coagulation. The suggested dosage (mass ratio of cement to fly ash to phosphorous gypsum) is 1?1?6-1?1?8 with mass fraction of solid materials 60%-63%, accompanied with dolomite cemented backfill.

(2) The study of cemented mechanism shows that fly ash possesses high active property and phosphorous gypsum possesses high evening strength and low bleeding ratio. The cemented process goes through three states: non-plastic strength suspended state, initial coagulation state, and consolidation state. Moreover, the pipeline transportation process lies in the non-plastic strength suspended state, and the consolidation takes place after backfilling into mined-out gobs.

(3) Five production lines are designed for the system whose investment only 1/10 of that of traditional paste backfill system. The technologies of shatter-milling, combination of double-axles mixing and mightiness activation mixing, as well as synthesized backfilling with phosphorous gypsum slurry and mullock alternately, result in the utilization ration 100% of gypsum, good pipeline self-flowing ability and the average backfill body compressive strength of 2.0 MPa.

(4) During the application process, the slurry is prepared with the dosage (mass ratio of cement to fly ash to phosphorous gypsum) of 1?1?6-1?1?8 with mass fraction of solid materials 57%. The resource under roadway is reclaimed successfully with the high recovery ratio of 82.67%, and, the maximum subsidence of roof is only 1.307 mm.

(5) The backfill technology based on phosphorous gypsum is not only easy to protect environment, reduce exhausted land and maintenance cost for gypsum pile, but also to improve recovery ratio and fulfill the sustainable development of resources. It also starts a precedent of backfill technology with phosphorous gypsum, providing valuable experiences for environmental protection and mining of the similar mines.

[1] SEOG K K, YONG J P, SANG L, YUNMI K L. Efficiency of gypsum as a capping material for the capturing of phosphorus release from contaminated lake sediments [J]. Materials Science Forum (Switzerland), 2007, 544/545: 561-564.

[2] JI J. Study on dephosphorization the technology for high-phosphorus iron ore [J]. Mining & Metallurgy, 2003, 12(2): 33-37. (in Chinese)

[3] China Environment Protection Bureau. Investigation report on damage and rebuild of ecology & environment in mine development [R]. Beijing: China Environment Protection Bureau, 2004. (in Chinese)

[4] KURAHASHI T, SHIOMI H, KITAGUTI S, SHIRAKAWA M. Effect of addition to the gypsum on phosphorous removal properties of wollastonite-type adsorbent [J]. Journal of the Society of Materials Science, Japan, 2007, 56(6): 516-520.

[5] WANG Xin-min, YAO Jian, TIAN Dong-mei, ZHANG Qin-li. Test research on performances of phosphorous gypsum as aggregate for cementation filling [J]. Metal Mine, 2005(12): 14-16. (in Chinese)

[6] DENG Jian, BIAN Li. Investigation and characterization of mining subsidence in Kaiyang Phosphorus Mine [J]. Journal of Central South University of Technology, 2007, 14(3): 413-417.

[7] SHEN Wei-guo, ZHOU Ming-kai, ZHAO Qing-lin. Study on lime-fly ash-phosphogypsum binder [J]. Construction & Building Materials, 2007, 21(7): 1480-1485.

[8] HOU Hao-bo, HE Xing-hua, ZHU Shu-jing, ZHANG Da-jie. The cement solidification of municipal solid waste incineration fly ash [J]. Journal of Wuhan University of Technology: Material Science Edition, 2006, 21(4): 137-140.

[9] BAO Lin-lin, LI Dong, LI Xiang-kun, HUANG Rong-xin, ZHANG Jie, L? Yang, XIA Guang-qing. Phosphorus accumulation by bacteria isolated from a continuous-flow two-sludge system [J]. Journal of Environment Sciences, 2007, 19(4): 391-395.

[10] YAN Pei-yu, ZHOU Yong-xiang, YANG Zhen-jie, QIN Jian. Microstructure formation and degradation mechanism of cementitious plugging agent slurries [J]. Journal of Wuhan University of Technology: Material Science Edition, 2007, 22(1): 61-65.

[11] JIANG Zeng-guo, ZHAO Yuan. Mechanism and optimal application of chemical additives for accelerating early strength of lime-fly ash stabilized soils [J]. Journal of Wuhan University of Technology: Material Science Edition, 2005, 20(3): 110-112.

[12] QIAN Jue-shi. Fly ash characteristic and fly ash concrete [M]. Beijing: Science Press, 2002. (in Chinese)

[13] KESIMAL A, YILMAZ E, ERCIKDI B. Evaluation of paste backfill mixtures consisting of sulphide-rich mill tailings and varying cement contents [J]. Cement and Concrete Research, 2004, 34(10): 1817-1822.

[14] ZHAO Cai-zhi, ZHOU Hua-qiang, QU Qun-di. Preliminary test on mechanical properties of paste filling material [J]. Journal of China University of Mining & Technology, 2004, 33(2): 159-161. (in Chinese)

[15] WANG Xin-min, XIAO Wei-guo, ZHANG Qin-li. Deep mining backfill theory and technology [M]. Changsha: Central South University Press, 2005. (in Chinese)

[16] PENG Xin, LI Xi-bing, ZHANG Qin-li, WANG Xin-min. Quality evaluation of layer-like backfilling and flow pattern of backfill slurry in stope [J]. Journal of Central South University of Technology, 2007, 14(4): 580-583.

Foundation item: Project (2006BAB02A03) supported by the National Key Technology Research and Development Program; Project (08MX16) supported by Mittal Scientific and Technological Innovation Projects of Central South University during 2008

Received date: 2008-06-10; Accepted date: 2008-09-29

Corresponding author: WANG Xin-min, Professor, PhD; Tel: +86-731-8879821; E-mail: wxm1958@126.com

(Edited by YANG You-ping)