J. Cent. South Univ. Technol. (2007)02-0216-04
DOI: 10.1007/s11771-007-0043-y
Performance of cemented coal gangue backfill
ZHANG Qin-li(张钦礼), WANG Xin-min(王新民)
(School of Resources and Safety Engineering, Central South University, Changsha 410083, China)
Abstract: Possibility of cemented gangue backfill was studied with gangue of Suncun Coal Mine, Xinwen Coal Group, Shandong, and fly ash of nearby thermal power plant, in order to treat enormous coal gangue on a large scale and to recovery safety coal pillars. The results indicate that coal gangue is not an ideal aggregate for pipeline gravity flow backfill, but such disadvantages of gangue as bad fluidity and serious pipe wear can be overcome by addition of fly ash. It is approved that quality indexes such as strength and dewatering ratio and piping feature of slurry can satisfy requirement of cemented backfill if mass ratio of cement to fly ash to gangue is 1?4?15 and mass fraction of solid materials reaches 72%-75%. Harden mechanism suggests that the cemented gangue fill has a higher middle and long term comprehensive strength.
Key words: coal gangue; fill; strength; rheological properties; mechanism
1 Introduction
Coal gangue, a kind of waste material produced while coal is mined and processed, accounts for about 15%-20% of coal production in the same year. It is reported that coal gangue makes up 25% of total waste materials with total amount of 7 Gt and occupation area of about 70 km2 in China. Furthermore, it is increased by 150 Mt every year[1]. Numerous gangue hills not only occupy large amount of valuable land but also pollute air and underground water. Overall utilization percentage of gangue is only about 43% by the end of 2000 because of poor equipment and low technical level in China[2], although gangue has been used to generate electricity, fabricate building materials, recover useful minerals, produce chemical products and fertilizers, and improve quality of soil in some cases[3-7]. The percentage is far less than that in the developed countries (about 90%)[8-9]. Therefore, how to treat huge stocks of gangue is an urgent task faced coal mines. Treatment difficulties will be solved completely if gangue is used to fill underground emptied stopes as aggregate. At the same time, recovery rate of coal can be improved, safety pillar can be extracted, movement of layer can be restrained and ground surface can be protected.
Suncun Coal Mine (SCM), Xinwen Coal Group, Shandong, China, is the deepest coal mine in China with mining depth of 1 300 m. Gangue site with area of 1.334×105 m2 has exceeded its limit and the gangue is still increased by 200 kt every year, but no spared space can accommodate gangue newly produced. Cemented gangue backfill is the best way to recover land occupied by old gangue hill and to treat new gangue.
Residual coal, organic substance and soft rock within gangue will exert a great influence on its engineering characteristics. Density and structure of gangue will be changed by weathering and breakage of soft rock, self-combustion of residual coal and metamorphism of organic substance[10]. Consequently, gangue is compressed and its shear strength and burden capacity are reduced year by year. Therefore, influence of such factors on quality of backfill must be clarified by experiment when gangue is used as filling material on a large scale. Laboratory cemented backfill experiment was carried out and mechanism of cemented ash fly-gangue backfill was studied in this paper.
2 Physical and chemical evaluation of coal
gangue as aggregate
Gangue is a mixture of inorganic substances and a few organic substances. Deep grey gangue will be kaolinized in air and become light red[11]. The most common minerals include kaolin, smectite, illite, quartz, feldspar, mica and chlorite. One of the characteristics of the coal gangue is that minerals are layered structure silicates except for quartz and feldspar. Chemical compositions are mainly SiO2, Al2O3 and C, followed by Fe2O3, CaO, MgO, Na2O, K2O, SO3, P2O5, N and H, as well as such metal elements as Ti, V, Co and Ga in some cases[12]. Proportions of chemical compositions are not fixed and vary in location. Table 1 lists the chemical compositions of fresh and kaolinized gangue of SCM.
Table 1 Chemical compositions of gangue of SCM (mass fraction, %)
Pipeline transportation and backfill quality depend greatly on size distribution of gangue when it is used as a backfill material[13]. Three kinds of gangues were adopted in laboratory experiment. They were crushed kaolinized gangue (KG), coarsely crushed fresh gangue (FG1) and finely crushed fresh gangue (FG2) respectively. Main physical and mechanical features, size distributions and compression coefficients of KG, FG1 and FG2 are listed in Tables 2-4, respectively.
Following conclusions can be given by the analysis of physical and mechanical properties and chemical compositions of the gangue.
1) Particles of coarsely crushed gangue (FG1) whose size is more than 2 mm make up 77%. Mediate size corresponding to 50% of accumulative percentage is about 3.5 mm, and nonhomogeneous index defined as a ratio of particle size corresponding to 90% of accumulative percentage to that corresponding to 10% of accumulative percentage reaches 50%. The size distribution suggests that pipeline transportation of FG1 is difficult.
2) For both KG and FG2 (85%), most of particles are less than 2 mm in size. Mediate sizes are 0.40 mm and 0.45 mm, respectively. Although crushing cost is higher than that of FG1, both can be easily transported by pipeline.
3) Permeability coefficient of FG1 is obviously greater than that of KG and FG2, suggesting that dewatering and hardening rates of cemented FG1 backfill are faster than those of cemented KG or FG2 backfill. It should be noted that permeability of cemented KG or FG2 backfill could meet the needs of underground backfilling activities if mass fraction is more than 70% and fly ash is added.
4) Fly ash is not an ideal aggregate in backfill owing to fine particles and low permeability coefficient, but it has a good potential binding ability due to high contents of SiO2 and Al2O3. It can be used, therefore, to replace partially cement so that the cost of backfill can be reduced and long term strength can be raised. Moreover, the unit mass of backfill becomes low and consumption of filling materials is reduced consequently by addition of fly ash.
In conclusion, finely crushed gangue (KG and FG1) can be easily transported by pipeline. Disadvantages such as low permeability coefficient can be overcome by addition of fly ash.
3 Quality evaluation of cemented coal gangue backfill
The laboratory experiment was carried out in order to determine optimal contents of gangue, cement and fly ash, and to clarify strength features and dewatering proportion. The results are listed in Table 5. The following conclusions can be obtained from Table 5.
Table 2 Main physical and mechanical properties of gangue of SCM
Table 3 Compositions of gangue of SCM with different sizes (mass fraction, %)
Table 4 Compression coefficients of gangue of SCM (MPa-1)
Table 5 Results of laboratory test of cemented gangue backfill in Suncun Coal Mine
1) Initial strengths of cemented KG backfill are much higher than those of cemented FG1 and FG2 backfill. For example, 7d compressive strength of cemented KG backfill increases by 32% and 68% compared with that of cemented KG2 and KG1 backfill, respectively. This means that kaolinized gangue has certain potential binding capacity, and that backfill with fine gangue is much dense.
2) Fly ash can effectively retrain settlement of aggregate, improve flowing property of slurry and reduce dewatering amount of backfill. It is measured that dewatering proportion of cemented backfill with fly ash is only about 3%, much lower than that of the backfill without fly ash (usually about 10%)[14]. Low dewatering amount is helpful to solidification of backfill and reduction of underground drainage volume.
3) It is found that solid particles are completely suspended, transportation is smooth and pipe wear is little when fly ash accounts for 20% of total solid amount.
4) Following dosage of filling materials is suggested by economical and technical analysis of experimental results. Aggregate: kaolinized or fresh gangue with size of less than 5 mm; mass ratio of cement to fly ash to gangue (m1?m2?m3) is 1?4?15; mass fraction of solid materials varies between 72% and 75%. 7 d compressive strength of backfill with suggested dosage is not less than 0.7 MPa.
4 Evaluation of rheological properties of slurry with suggested dosage
Rheological properties of slurry with suggested dosage, such as slump, diffusibility and denseness are listed in Table 6. The slurry is proved to be perfect in flow property by both slump and diffusibility-slump ratio. The denseness value implies that solid particles could be easily suspended while transported by pipeline
5 Binding mechanism of cement-fly ash- gangue admixture
Solidification of cement-fly ash-gangue backfill is actually a multi-stage and multi-level hydration process as follows.
1) Hydrating stage of cement
This stage lasts for two weeks after slurry formation. It includes hydrating process of about 3 d when slurry cannot flow freely, and calcifying process of about 10 d when a large amount of Ca(OH)2 crystals are formed.
Fly ash particles are soaked and surrounded by alkali substances produced in hydration process of cement. Neither SiO2 nor Ca(OH)2 begins hydration and surface of fly ash particles has almost no any change in this stage[15]. The structure of slurry is kept primarily by Ca(OH)2 .
2) Solidifying stage of backfill
This stage ranges from 14 to 90 d after slurry formation. It includes silicifying process when fly ash particles are eroded and xCaO?SiO2?yH2O coagulation is produced on the surface of fly ash particles by combination between negative silicate ions and Ca2+, and diffusing process when Ca2+ within xCao?SiO2?yH2O coagulation diffuses into inner of fly ash particles and gangue particles are cemented by diffused xCao?SiO2?yH2O coagulation. xCao?SiO2?yH2O coagulation can be easily seen in SEM image of backfill with suggested dosage(see Fig.1).
Table 6 Rheological properties of slurry with suggested dosage
Fig.1 SEM image of backfill with suggested dosage
3) Increasing and stabilizing stage of strength
This stage also includes two sub-stages. One is cementing sub-stage, which lasts for about one year when xCao?SiO2?yH2O coagulation is produced and backfill strength is increased greatly. The other is stabilizing sub-stage, which follows the cementing sub-stage until backfill strength is almost not increased.
6 Conclusions
1) One of the best ways for treating enormous coal gangue is to fill it into underground emptied areas as aggregate, which is helpful not only to environmental protection but also to recovery rate of coal resources and sustainable development of coal mines.
2) Fluidity of cement-gangue slurry can be obviously improved by addition of enough fly ash. Compressive strength and dewatering of backfill can meet the requirement if mass ratio of cement to fly ash to gangue is 1?4?15 and the mass fraction of solid materials reaches 72%-75%.
3) Although residual coal, organic substance and soft rock within gangue may make backfill weathered in open space, weathering can be greatly prevented and consequently damage of backfilli can be lessened greatly under condition of relatively closed space and moist atmosphere in underground stopes.
4) Solidification of cement-fly ash-gangue system includes three stages, i.e. hydrating of cement, solidifying of backfill and stabilizing of strength. Long term strength of suggested admixture is much higher than initial strength because hydration of fly ash begins lately.
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Foundation item: Project(50490274) supported by the National Natural Science Foundation of China
Received date: 2006-05-07; Accepted date: 2006-07-27
Corresponding author: ZHANG Qin-li, Professor; Tel: +86-731-8879612; E-mail: zhangqinli@hotmail.com
(Edited by LI Xiang-qun)