1. 1 Effect of Tailings Properties on Paste backfill performance M. fall, M. Benzaazoua, S. Quellet ( University of Qucbec in Abitibi- Temiscamingus, Canada)

2. 2  Abstract : The increasing use of paste backfill in underground mining makes it necessary to quantify the effect of tailings properties,such as physical and chemical properties, on the performance properties of cemented paste backfill (CPB).

3. 3  Hence, this paper presents the results of an experimental investigation carried out to evaluate the influence of physical (tailings particle size and density) and chemical properties (tailings sulphide content ) of tailings on the performance properties of the paste backfill.

4. 4  The studied performance properties included: mechanical and economical performance,water demand, pulp density and transportability. The gained results have shown that the tailings fineness and density influence significantly the performance properties of paste backfill.

5. 5  The sulphide content of the tailings has also significant effect on the performance of paste backfill. The results of this study will contribute to better understand the behaviors of the paste backfill and optimize its mixtures.

6. 6 1 Introduction  The use of paste technology for underground backfill has been accepted as a cost-effective alternative to rock and hydraulic backfill worldwide in the mining industry. Its using is extensive in Canadian underground hard rock mines and follows an increasing trend as well as in many parts of the world.

7. 7  The application of paste backfill leads to a significant reduction of cyclical nature of mining, improves ground conditions, ensures the stability of the underground excavations, speed up production and greatly reduces environmental costs.Additionally, paste backfill applications, known to

8. 8  allow enhanced disposal of large fractions of fine tailings that have traditionally required permanent surface disposal and management, offer other significant environmental and cost benefits for mines. However, despite extensive use of this relatively new technology, all effects of tailings properties on paste backfill are not fully known.

9. 9  Only a few works have shown that the tailings particle size can influence the strength of the hardened paste backfill. But these works only briefly described the influence of tailings particle size on CPB uniaxial compressive strength (UCS).

10. 10  They had no information on the effect of tailings particle size on all of the most important quality criteria for backfill. They also did not account for the influence of tailings density and sulphide contents.  Hence, cooperative research studies were conducted by the above authors with several Canadian underground hard rock

11. 11  mines currently using cemented paste backfill to investigate the effect of the physical and chemical properties of tailings on paste backfill as well as to optimize the desliming of mill tailings. This paper presents the results of the experimental part of these research studies.

12. 12  Thus, the main purpose of this experimental study was to evaluate the influence of tailings fineness and density on the performance properties of the paste backfill, and to study the effect of the amount of sulphide minerals contained in the tailing on the quality of the paste backfill.

13. 13  The studied performance properties included the strength, cost and microstructure of the hardened paste backfill and the water demand for the fresh paste backfill. The reactivity of the sulphide minerals present in the tailings was also evaluated.

14. 14 2 Results and Discussions  2.1 Effect of physical properties of tailings on paste backfill performance  2.1.1 Effect of tailings fineness on the microstructure of paste backfill  The main results of the effect of tailings fineness on the microstructure of paste backfill are presented in Figure 2 to 3.  From this figure, it is clear that the proportion of fine tailings (<20um), i.e.,

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17. 17  the fineness of the tailings materials used in the sample mixture strongly influence both the overall porosity of the cemented backfill material( Figure 2)and the pore size distribution within it (Figure2 and 3).  The finer the tailings materials used, i.e., the greater the proportion of fine tailings particles, the greater the overall porosity of the backfill becomes.

18. 18  Figure2 shows that the total porosity of all of the paste backfill samples decreases as the proportion of fine particles (<20um) in the tailings material decrease. However, the extent of porosity decrease is variable,i.e., depends on fines content.  The decrease in porosity with the decrease in the fines content is greater

19. 19  for paste backfill made of fine (fines<60%) or medium (fines: 60%~35%) tailings than those made from coarse tailings.  Figure 2 shows also the microstructure (total porosity and void ratio) of the paste material is strongly influenced by the drainage ability of the fresh backfill.

20. 20  The drained paste backfill samples show both less porosity and smaller void ratios, As shown in Benzaazoua et al.(2003), paste backfill mixes made from coarse tailings loose more water (by drainage) than those made from fine tailings material. This water loss leads to the settling of the paste backfill (increasing

21. 21  of the packing density) and the consequent reduction of total porosity and void ratio of the backfill material. These observations are in substantial agreement with earlier experimental investigations about CPB drainage ability performed by Belem et al.(2002).

22. 22  Figure3 also shows that the paste backfill specimens are characterized by two distinct pore size distribution, notably pore diameters between 0.05~1um and pore diameters between 1~10um. The contribution of pore diameters smaller than 0.05um or greater than 10um to overall porosity is low.

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24. 24  The distribution of the two main pore diameters is significantly influenced by the proportions of fine tailings particles.  While the pore diameters between 0.05 and 1 um are relatively well represented in the samples made from fine and medium tailings, their volume is small in backfill samples made from coarse tailings (Figure3).

25. 25  Indeed, it can be observed in Figure 3, that decreasing of proportion of fines in the tailings is associated with a decreasing of the volumes of pores with diameter smaller than 1 um (Figure 3).  The volume of macropores >1um is greatest in paste backfill samples made from tailings containing only 25% of fines

26. 26  particles (coarse tailings). This may have effect on paste backfill strength gain.  2.1.2 Effect of tailings fineness on paste backfill strength development  Figure4 show the compressive strength development of the paste backfill related to the size of the tailings fineness. It points out that the proportion of fines

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28. 28  (<20um) in the tailings materials has a strong influence on the strength gain of the CPB.It can also be noted that coarse and medium tailings are more favorable for paste backfill strength gain. A CPB made of fine tailings generates lower strength.Figure4 also indicates that, for the undrained paste backfill samples,

29. 29  the UCS increases as the grain fineness decrease until approximately 35~55% of the fines content was reached, at which point it remains constant (Figure4a) or begins to slowly decrease (Figure4b) with decreasing grain fineness.  These observations can be attributed to the influence of the tailings fineness on the overall porosity of the paste backfill

30. 30  as well as to the effect on the pore size distribution within it (Figure2 and 3),and to the influence of the tailings particle size on the specific surface of the tailings material.  Indeed, from a fines content of 60~90 wt.% to coarse tailings with a fines content of 25 and 35 wt.%,

31. 31  there is a grading improvement of the tailings particle size distribution( Table 2 and Fifgure1).  This leads to a decrease of the void spaces between the tailings particles and consequently, to lower porosities or void spaces within paste backfill as shown in Figure2.

32. 32  This decreasing porosities or void spaces thus causes an increase in paste backfill strength.  However, decreasing the porosity of the backfill with a decrease in the proportions of the fines tailings particles (<20um) is not the only parameter responsible for the variation in backfill strength.

33. 33  Indeed, it can not explain the slight decrease of backfill strength for a fines proportion of 30%~25%.  Analysis of Figure 3 shows that the pore size distribution within the paste backfill, particularly the proportions macropores with diameter between 1~10um, seems to play a significant role in the strength gain

34. 34  of paste backfill. As observed in figure 3 and 4, at 25% fines content, the proportions of macropores 1~10um within the paste backfill increases drastically. This higher proportion of macropores (1~10um) may have caused the small decrease of the strength for paste backfill made from coarse tailings

35. 35  (25%fines) compared to backfill specimens made from tailings containing 50% to 40% fines particles. Thus, it can be concluded that not only does the overall porosity influence the strength of the paste backfill, but the pore size distribution, which is largely governed by the proportions of fine particles present in

36. 36  the tailings material, also plays a decisive role in the strength development of the cemented backfill.  In addition to the effect of overall porosity and pore size distribution on the backfill strength gain, the finer particles increase the specific surface of the tailings materials and thus increase the surface area that must be cemented,

37. 37  since the cement coats the surface of the tailings particles. This also contributes to strength decrease of the paste backfill (Figure4) with increased tailings fineness. From Figure 4a, it can be observed that the drained paste backfill samples show higher strength than the undrained samples.

38. 38  This maybe attributed to the fact that drainage of the excess water in the fresh paste backfill leads to the settling of and higher packing density of the backfill.  This causes a reduction in the total porosity and void ratio of the backfill material and consequently, to higher strength.

39. 39  Additionally, the drainage of the excess water affects positively the cement hydration (Benzaazoua et al. 2003b)  From Figure4, it can also be clearly seen that the fineness of the tailings strongly influences the rate of backfill strength gain at early stage (up to 28 days curing). Paste backfill mixes made from coarse

40. 40  tailings (low proportion of fines) gain strength faster than those made from fine tailings (high proportion of fines). This is caused by the fact,that for a given W/C, the volume of void spaces between the tailings particles to be filled by the cement hydration product is smaller in the paste backfill specimens made from

41. 41  coarser tailings (lower porosity) than those made from finer tailings material.  2.1.3 Effect of tailings fineness on water requirement of the fresh paste backfill  Figure5 shows the effect of the tailings fineness on the water/cement ratio and pulp density of cemented paste backfill. It can be noted that W/C of the paste

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43. 43  backfill increase with the grain fineness of the CPB. For a given slump, the pulp density of the CPB decreases as the fineness of the tailings increases.  This means fine tailings require more water for a given consistency than medium and coarse tailings.  The influence of tailings particle size on water demand comes from the fact

44. 44  that the cement paste in its function as an adhesive, coats the surface of all tailings particles.  Finer particles mean that there is more surface area to be wetted, which in turn yields both higher moisture levels and lower densities for a given consistency. Additionally, since the overall porosity and

45. 45  void ratio of the paste backfill decreases from fine to coarse tailings, for a given total weight of tailings and cement,coarse tailings require less water than medium or fine tailings to reach the same consistency or pulp density.  The comparison of Figure4 and 5 indicates that the optimal W/C ratio to produce paste backfill with high strength is dependent on the fineness of the used tailings materials.

46. 46  2.1.4 Effect of tailings density on strength and binder consumption of the paste backfill  Figure 6 shows that, for a given curing time, there is a relationship between the strength and the density of the tailings materials used. It can be observed that increasing of the tailings density (from Gs=3.2) gives the paste backfill a higher strength for the same

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48. 48  binder proportion (4.5% in weight).This increasing in U with the tailings density is due to higher binder consumption in volume, as shown in Figure 7. The latter puts into relief that, the higher the density of the tailings, the higher the binder consumption (in volume), i.e. the more expensive the backfill becomes.

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50. 50  These results clearly demonstrate that the evaluation of the binder content of the CPB by weight percent, as commonly used in the mining industry does not show the real binder consumption of the paste backfill,in cases where the density of the mill tailings strongly vary. They also demonstrate the economic significance of

51. 51  using tailings with low sulphide content (tailings sulphide content can be reduced by desulphurization) for paste backfill design, since high sulphide contents lead to higher tailings density and consequently, to more expensive paste backfill.

52. 52 2.2 Effect of chemical properties of tailings on paste backfill performance  The results of the effect of tailings sulphur content on paste backfill performance properties have shown that the sulphur has double effect (physical and chemical effect) on paste backfill properties.  The first effect is physical. Indeed, increasing of the sulphur content in the tailings give the latter higher density.

53. 53  This leads to paste backfill with higher strength ( Figure 8) due to the higher binder consumption in volume.  The second effect is chemical. As shown in Figure7, high sulphur content ( 39% sulphur) in the paste backfill leads after 120 days to strength loss of the paste backfill. This is caused by sulphate attack.

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55. 55  This raised the following question: Is the sulphides oxidation within the cemented paste backfill possible? The results of oxygen consumption tests (Figure9 and 10)bring some elements of response to this question. Indeed, Figure9,which represents the oxygen consumption (ie reactivity of the sulphide

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58. 58  minerals in CPB) of cemented paste backfill, shows that the amount of consumed oxygen depends on the sulphur content, and the reactivity of the paste backfill specimens (except CPB made from tailings with 39%sulphur) does not significantly increase after 28 days of curing time. The reactivity of the sulphide

59. 59  minerals in the cemented paste backfill are low compared to this in the non cemented tailings( Figure10). This low reactivity can be attributed to the fact, that the cemented paste backfill is water saturated. This high water saturation limits the diffusion of oxygen through the cement matrix and consequently its

60. 60  availability for oxidation reactions.  Figure 10 puts in relief the relationship between the degree of water saturation and the sulphide reactivity in non cemented tailings. It can be observed, that at high water saturation (Sr.>85%), the reactivity of the sulphides mineral in the non cemented tailings become as low as in cemented paste backfill.

61. 61 3 Conclusions  The objective of this study was to study the effects of the physical properties (particle size and density) and chemical properties (sulfur content) of tailings on the performance properties of cemented paste backfill.  The presented results have shown that the tailings particle size and density has a

62. 62  considerable effect on the properties ( strength, cost, water demand, microstructure) of the paste backfill. It was demonstrated that the tailings particle size, particularly the proportions of fine tailings particles (<20um) significantly affect the porosity of the paste backfill and the pore size distribution within it,

63. 63  its water drainage ability and consequently, its strength development and the water requirement for a given consistency. It was also shown that not only does the overall porosity influence the strength of the paste backfill, but the pore size distribution plays a decisive role in the strength development of the cemented backfill.

64. 64  The paste backfill water demand increases with the fineness of the tailings material used. Increasing the tailings density is associated with volumetrically higher binder consumption, i.e. more expensive backfill. In general, higher binder consumption gives the paste backfill higher strength.

65. 65  It has been demonstrated that higher sulfur contents in the tailings increases tailings density and consequently give the cemented paste backfill higher strength.  This increased strength is caused by a higher binder consumption. However, high sulfur content can lead to strength loss of cemented paste backfill through

66. 66  sulphate attack. This sulphate comes principally from the oxidation of the sulphide minerals in the non cemented tailings.  This work brings new light on the effect of tailings on CPB properties that can contribute to better optimization of paste backfill mixtures and to a better underst-

67. 67  anding of the behavior of cemented paste backfill.  Additionally, these experimental results agree well with those of modeling of the effect of paste backfill components on its properties (Fall and Benzaazoua 2003a,Fall and Benzaazoua 2003b,Fall and Benzaazoua 2004a).

68. 68  Acknowledgements  References :