Geotech Geol Eng DOI 10.1007/s10706-016-0094-7

ORIGINAL PAPER

Improving Collar Zone Fragmentation by Top Air-Deck Blasting Technique Eugie Kabwe

Received: 10 February 2016 / Accepted: 25 September 2016  Springer International Publishing Switzerland 2016

Abstract Air gap in an explosive column has long been applied in open-pit blasting as a way of reducing explosive charge, vibration, fly rock and improve fragment size. In conventional blasting a greater amount of explosive energy is lost in the generation of oversize fragments. Oversize fragments reduces loading and hauling efficiencies of equipment which requires secondary blasting. Recurring oscillation of shock waves in the air gap increases the time over which it acts on the adjacent rock mass by factor of 2–5. Top air deck blasting technique trial conducted with an application of gas bags at Chimiwungo pit resulted in an improved fragmentation of about 94 % less than 950 mm. Results obtained from the analysis of muckpile images using split-desktop exhibited that the mean fragment size was 264.81 mm and F20, F80 and top-size were 41.99, 683.18 and 1454.69 mm respectively. Optimum crusher feed size was as large as 1200 mm and crushed down to the 40 mm and only a small percent of the material was above 1200 mm. Gas bag application resulted in a significant reduction in explosives load in production holes without loss in fragmentation or movement of the collar zone. This reduced total cost of charging as compared to conventional blasts with a variance of $20, powder factor was dropped to an average of 0.86 kg/bcm. The

E. Kabwe (&) Department of Mining Engineering, School of Mines, University of Zambia, Lusaka 32379, Zambia e-mail: [email protected]

technique reduced the cost of bulk blend explosive by 15 %, reduced overall cost of charging per hole by 12 %, enhanced premature ejections. The overall blast results were satisfactory, 443,624 tonnes of blasted material from the block which represented 90 % of the total muckpile material was within 900 mm size. The overall muckpile blasted was well fragmented. Keywords Air-deck  Blasting  Collar zone  Fragmentation  Gasbag

1 Introduction Achieving optimum rock fragmentation is one of the main objective of production blasting, a significant part is to control and minimize the overall production costs. Blasting is the first step of the size reduction in mining, proceeded by crushing and grinding operations. Operation efficiency of these units is directly linked to the size distribution of blasted material. Ideal rock fragmentation in blasting operation reduces the workload of primary crushers and improves the energy efficiency of crushing and grinding processes (Siddiqui et al. 2009). In rock blasting, explosives deliver an extreme source of energy, which regularly exceeds the level that is required to cause adequate breakage in the adjacent rock mass. Charge configurations play a substantial role in attaining required blasting performance (Jhanwar 2012). In charge blasting, as a full

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column of explosive detonates, the great initial pressure that arises in explosion products greatly exceeds the strength of the rock mass, so that a strong shock wave begins to propagate into the medium, breaking it into very small particles (Jhanwar 2011). Because of this intense, excessive crushing of the rock, a large portion of the explosive energy is wasted in an area near the charge (Chiappetta and Mammele 1987). To maintain the desirable fragment sizes, air deck blasting has been trailed and applied. Air decking is an air gap space in a blast hole, it could be located at the bottom, middle or top of the charge column. Initially air-deck was used in surface blasting as a means of distributing the explosives energy more evenly throughout the rock mass being blasted. Today, application of this system is being used successfully to allow reductions from 10 to 30 % in total explosives required for production blasting. Other applications of air-decking include wall control and pre-splitting techniques. In all these cases, air decking can result in significant savings and improved efficiencies over conventional methods (Sharma 2015). The experimental blast was conducted at the Chimiwungo open pit, observation of the conventional blast practice revealed problems related to poor blast performance which have been the major challenges at the pit: 1. 2. 3.

Poor fragmentation in the collar zone of blasted material. Tendency of uncontrolled fly rock and premature ejections. Elevated cost of blasting consumables.

The main aim of the trial was to evaluate the influence of top air deck blasting technique using gasbags on collar zone fragmentation, and sub objectives where to: 1. 2.

Improve on the containment of each blast, and hence reduce ejections and the risk of fly-rocks. Reduce stemming material quantities and other blasting consumables.

2 Geology The Chimiwungo pit, is part of the Lumwana project located in the North-Western Province of Zambia (Stroud 2010). The Lumwana copper, cobalt, gold,

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uranium deposits of Malundwe and Chimiwungo are held inside the Mwombezhi Dome, a bi-lobate basement dome in the western limb of the Neoproterozoic Lufilian Arc thrust fold belt, that trends northeast. This thrustfold belt comprises of a north directed arcuate array of void antiformal basement inliersor domes bounded by Katanganmeta sediments, which host the Central African Copperbelt, straddling across the Copperbelt and Northwest Provinces of Zambia and the Katangan Province of the Democratic Republic of Congo. In Zambia, the Lufilian Arc comprises erratically deformed and metamorphosed Late Proterozoicmeta sediments and volcanics of the Katangan Lower and Upper Roan, and the Lower and Upper Kundelungu Supergroups, unconformably overlying the Basement. The Basement comprises of older metamorphosed gneisses, schists, migmatites, amphibolites and granitoids. Successive to the deposition of the Katangan categorisations the basin was inverted, deformed, metamorphosed and uplifted by mostly north directed thrusting and folding creating the late Neoproteozoic Lufilian Arc (Richards and Nisbet 2003).

3 Theory of Air Decking Explanation is that when an explosive charge in a blast hole is detonated, the blast hole expands due to high pressure, the pressure crushes the bore hole walls. A shock wave with a high peak pressure propagates outwards in all directions as a compressive stress wave. According to Melnikov and Marchenko (1970) an airdeck explosive column develops extra compressional shock waves after the main compression wave formed in the rock mass due to blasting. The extra compressional wave produced is an outcome of collision between the two gas flows in the center of the air gap. When the gases collide they generate excessive pressure at the summit point, but the gases are also redirected back and enter in the fissures hence assisting fragmentation. It was also established that once an airdeck is positioned in the explosive column, the peak bore hole pressure decreases due to wave collision in the air gap. Nonetheless, at the same time, several effects of shock wave inside the medium are created due to impact and reflection of gases in the airdeck area. This may result in more energy transferred to the medium as compared to blasting a

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continuous charge. Hence, an improved rock fragmentation can be achieved by providing air gap in the explosive column of a blast hole. Trials to study the theory of air decks led by Fourney et al. (1981) on Plexiglas model also maintained Menikov’s theory and verified that a shock wave reaching the stemming is reflected back to fortify the stress field. Jhanwar and Jethwa (2000) in their work on airdeck blasting determined that airdeck blasting results in better fragmentation and improved utilization of explosive energy. Jhanwar et al. (2000) found that the degree of fragmentation resulting from airdeck blast holes is better than that of conventional blast holes. Airdeck blasting was also established to be more effective in very low to low strength moderately jointed rocks as compared to medium strength highly jointed rocks. The drill hole with airdeck consumed 17 % less explosive than that of conventional charge. According to Moxon et al. (1993) no significant effect on the degree of fragmentation was observed with the airdecks which occupied 40 % or less of the maximum volume of explosive. The types of airdeck positions normally used are top, middle and bottom of the explosive column. Generally, airdeck when employed at the top of explosive column created a good rock breakage in the stemming area (Jhanwer and Jethwa 2000). Jhanwar (2011) proposed that airdecks were best if positioned at the middle point of an explosive column. Jhanwer and Jethwa (2000) also alleged that bottom airdeck can only be used for blasting of holes with softer bottoms. Moxon et al. (1993) concluded that the middle position of the airdeck resulted in an improved rock fragmentation due to the interaction of two concurrent shock wave fronts from the top and bottom of an explosive charge. However, Chiappetta (2004) stated that the bottom airdeck could be used more effectively because it produces more pressure at the bottom of the hole when properly practiced. Liu and Katsabanis (1996) suggested that only the top position of airdeck improved the rock fragmentation than that of other two positions. Since the shock waves oscillate repeatedly within an air-gap, their velocities and pressure at the wave front are governed by the length they travel within the air column. Air-deck length as a result is critical to the fragmentation, efficiency of this technique is also controlled by the rock mass structure and its strength (Jhanwar 2011). Air and gasbag employment, indirectly reduces the amount of explosives used per hole

thus reducing the explosive costs up to 10–30 % while keeping the benefits on both fragmentation and movement. Top column air decks are the most suitable where improved collar fragmentation is required without gross movement (Fig. 1a). In this technique, about 60 % of the total charge should be in the bottom. Charge weights range 3–10 % of the total charge in a normal production hole. Holes should be stemmed for noise reduction, gasbags’ serve as stemming enhancers where they lock and prevent energy and gasses from escaping (Fig. 1b).

4 Materials and Methods Top air deck with the application of air bags blast trial was conducted on shot (1) at the same time with a conventional blast (2). General ground conditions (geo-technical) of the trial area are given in (Table 1). Drilling (Table 2) and preparation were done, drill area cleaned, visible survey stake out, new hole markers put in place. The blast design parameters, bench height, hole diameter, burden and spacing, in the conventional blast and air deck blast trials were kept similar. 4.1 Blast Parameters The original blast design was based on 3.5 m stemming height and 0.92 kg/m3 powder factor, but after introducing gas bags, the planned powder factor dropped to 0.74 kg/m3 with a stemming height of 2.5 m. Blast parameters are given in (Table 3). Figure 2 illustrates blast hole explosive loading for both Top air-deck and conventional blasts. In bench blast design, theoretically subdrill height should be; Subdrill ¼ 9  0:165  1:5 m

ð1Þ

The designed subdrill was 0.5, which was way short in relative to the hole diameter, thus gave rise to uneven floors. The burden stiffness ratio predicts the degree of fragmentation if all blast and bench parameters are to standard, it’s given by: Burden stiffness ratio ¼

Bench height ¼ 2:9 Burden

ð2Þ

where good fragmentation: 2–3.5, Very good fragmentation: [3.5.

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Geotech Geol Eng Fig. 1 Top column air deck (A) and Gasbag mechanism (B)

Table 1 Blast site geo-technical conditions Location

Chimi south*

Type of rock mass

Value (rating) Rock density (t/Cu m)

UCS (Mpa)

RQD (%)

Joint spacing

Joint conditions

Ground water

Joint orientation

RMR

Gneiss

2.8

193

80

1.5

Fresh walls/staining

Wet

Favorable

62

* Chimiwungo south pit

Table 2 Accuracy (drilling accuracy) Drill rig

Drilling accuracy (%)

DR05

90

DR06 DR07

79 82

DR08

82

DR10

83

DR11

78

DR12

100

Average drilling accuracy (%)

85

Average DR penetration rate

22 m/h

% Re-drills

15

where B = Burden, S = Spacing, BH = Bench height There were less holes drilled as compared to marked out holes but the pumped emulsion was more (Table 5), result of: •

•

Challenges during the trial included; •

Cost of charging hole (A) was higher than hole (B) (Fig. 2). Overall cost of blast consumables where reduced upon the application of gasbags (Table 4). Volume blasted per hole ¼ B  S  BH  226:8 bcm

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Over-drilled holes were back filled with drill chippings instead of aggregate, the emulsion basically mixed with chippings hence overcharged the holes. Charging not stopping at 5 m from the collar (as per procedure), resulted in an overcharged hole.

ð3Þ

•

•

Over-drilled holes not to desired level, potentially gave rise to uneven floors. Inability to use polypipe as per gasbag insertion procedure (Fig. 3), resulted in improper insertion in wet holes. Hole blockages occurring in water logged areas made it extremely difficult to insert gas bags to planned depth.

Geotech Geol Eng Table 3 Blast parameters

Top air-deck (gas bag)

Conventional

Parameters

Description

Hole diameter (mm)

165

165

Bench Height (m)

12

12

Sub-drilling (m)

0.5

0.5

Burden

4.2

4.2

Spacing

4.5

4.5

Stemming

2.5

3.5

Blasting pattern Initiation system

Square Nonel or shock-tube

Square Nonel or shock-tube

Powder factor (kg/m3)

0.72 kg/m3

0.92 kg/m3

Fig. 2 Illustration of conventional (A) and gasbag (B) charged holes

5 Modelling and Analysis Using AEL Tie-up software, the blast timing pattern was analysed to determine the number of holes firing within a 8 ms time frame. Figure 4 shows tie up plan, timing model and relief model.

The top air deck blast was fired simultaneously with a conventional blast for assessment, (Fig. 5) it was observed that, the conventional blast experienced premature ejections and fly rocks as compared to the top air-deck blast. Effective stemming improves efficiency of explosives, it

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Geotech Geol Eng Table 4 Cost and blast measurable per hole

Cost per hole

Conventional blast hole (A)

Air-deck blast hole (B)

Explosives cost ($)

187.75

165.75

Stemming cost ($)

2.28

Gasbag cost ($)

–

Total cost per hole ($)

190.03

170.46

Volume blasted per hole (bcm)

226.8

226.8

Powder factor (kg/bcm)

0.97

Table 5 Trial blast measurable

1.82 2.89

0.86

Blast measurable Marked out

Actual (charged holes)

Number of holes blasted

756

722

Pumped emulsion (kg)

136.477

146.583

Powder factor (kg/bcm)

0.77

0.89

Blasted material (tonnes)

473,063

443,624

Fig. 3 Gasbag insertion using a polypipe

was observed that airbags reduced ejections and fly rocks. The blast area was inspected and it was observed that the top air-deck blasted muckpile was more evenly fragmented as compared to the conventional blast muckpile (Fig. 6).

indirect methods are applicable, direct methods include screen analyses, boulder counting and most accurate method is sieving which of course is more complicated (Hustrulid 1999).

5.2 Digital Image Processing 5.1 Fragmentation Analysis Critical element in fragmentation system optimization is the development of practical methods for determining the degree of fragmentation. Both direct and

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Split desktop an image-processing program designed to calculate the size distribution of rock fragments through analysing digital grayscale images. Digital grayscale images can be acquired through the use of a

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Fig. 4 Tie up plan, timing and relief model

Fig. 5 Gasbag and conventional shots a charged and b blasting

digital camera, digitized photographs (Siddiqui et al. 2009). Photos of the muck pile taken included the 150 mm plastic ball for scale reference (Fig. 7). The photos were then processed with split desktop to

provide the cumulative results of size distribution (Table 6) and (Fig. 8). Based on the crushers at the mine, the allowable size classification is given below (Table 7).

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Fig. 6 Blasted muckpile a top air-deck and b conventional blast

Fig. 7 Digitization of sample photos using split desktop

5.3 Kuz-Ram Fragmentation Model The Kuz-Ram model provides an effective way of determining the effect of the blast design on fragmentation. The Kuz-Ram model was propounded by Cunningham and is a simple and effective method for estimating fragmentation results (Cunningham 1983). It provides a method for calculating the mean fragment size and rock uniformity. 5.4 Air-Deck Blast Assessment Kuznetsov (1973) suggested the following empirical equation to predict the mean fragmentation size resulting from rock blasting (Eq. 4):  19 115 20 1 Xm ¼ AK0:8 Q6 ð4Þ RWS Hence; Xm ¼ 32:4 cm where Xm is the mean fragment size in cm, A is the Rock factor (between 4 and 12), K is the powder

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factor, kg explosive per m3, Q is the mass explosive in the hole kg, RWS is the explosive weight strength relative to ANFO expressed as a percentage. Rosin– Rammler equation (Rosin and Rammler 1933) given below to estimate the complete fragmentation distribution resulting from rock blasting. 



X Yx ¼ exp 0:693 Xm

n  ð5Þ

Y = Proportion of the material larger than X, Xm = characteristic size = X50 and n = uniformity index. The final equation of the Kuz-Ram model is an expression for uniformity index (Kulatilake et al. 2012); rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   B SD S n ¼ 2:2  0:014 1 0:5 1 þ d B B  0:1   j Lb  L c j Ltot  þ 0:1 Ltot H 

Thus;

ð6Þ

Geotech Geol Eng Table 6 Air-deck muckpile cumulative size distribution results obtained from split desktop Size (mm)

Yx ¼ 0:073 Basically about 7 % of the material is greater than 900 mm, 90 % of fine material (Table 7), and optimum fragmentation.

Split-desktop size analysis results Combined % Passing

% Passing

% Passing

2000

100

100

100

1000

93.38

100

86.5

750

83.68

98.94

67.79

500

68.36

86.81

49.14

375

58.93

75.31

41.87

250 187

48.72 42.33

61.15 52.3

35.79 31.96

78.7

27.47

35.02

19.61

5.5 Conventional Blast Assessment Applying the Kuz-Ram model on the conventional blast, the mean fragmentation size ðXm Þ was calculated and is given below; Xm ¼ 34:83 cm   n  X Yx ¼ exp 0:693 Xm

% Passing

Size (mm)

Size (mm)

Size (mm)

F10

10.69

5.69

23.97

F20

41.99

24.17

81.46

F30

93.69

56.75

167.15

F40 F50

167.73 264.81

104.29 170.93

339.53 513.62

F60

389.37

241.56

652.12

F70

523.09

324.22

776.95

F80

683.18

424.19

904.2

 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   B SD S n ¼ 2:2  0:014 0:5 1 þ 1 d B B  0:1   Ltot jLb  Lc j  þ 0:1 Ltot H ð9Þ Thus;

F90

898.66

542.61

1059.21

Top size (99.95 %)

1454.69

808.66

1459.57

Fines cut off (mm)

137.25

137.25

214.5

n ¼ 1:076 100Drilling accuracy SD ¼  Spacing 100 SD ¼ 0:675 m

Fines factor

50

50

50

Yx ¼ 0:111

n  1:30 100Drilling accuracy  Spacing SD ¼ 100 SD  0:675 m

ð8Þ

ð7Þ

where B = Burden, S = Spacing, d = hole diameter, Lb = bottom charge length, Lc = column charge length, Ltot = charge length, H = bench height and SD = standard deviation of drilling precision, Accuracy = Average drilling accuracy (Table 2). The n value, is dependent on drilling pattern, hole deviation, hole depth, charge, length. Varies between 0.8 and 1.5. Higher value indicates uniform sizing while lower value indicates proportions of fines and coarse material. Proportion of material larger than 900 mm is given by:

ð10Þ

From the conventional blast muckpile about 11.13 % of the material is greater than 900 mm. The overall results from the blast audit are given in (Table 8). The values of the mean fragmentation are slightly different in the cumulative size distribution results obtained from split desktop and the Kuz Ram fragmentation model analysis, could be as a result of site factors.

6 Results The application of Top air deck resulted in a significant reduction in explosives load in production holes without loss in fragmentation or movement of the collar zone. Substantial reduction in explosive load in conventional holes to Top air deck holes was 221–195 kg/hole. Stemming height reduced from 3.5 to 2.5 m hence drop in quantity and costs. Powder

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Geotech Geol Eng Fig. 8 Air-deck muckpile cumulative size distribution chart

Table 7 Classification Crusher material classification Particle size (mm)

Classification

Less than 50

Over blasted material

50–800

Very fine material

800–1100

Coase material good crusher feed

Table 8 Blast results Conventional blast

Top air-deck blast

Explosives cost ($)

187.75

165.75

Stemming cost ($)

2.28

1.82

Total cost per hole ($)

190.03

170.46

Powder factor (kg/bcm) Air blast (dB)

0.97 128.8

0.86 69.3

Mean fragmentation (mm)

340

264

factor dropped from 0.92 to an average of 0.72 kg/m3, decrease in cost of explosive per hole from $187.75 to $165.75. Stemming cost reduced from $2.28 to $1.82 per hole, the overall cost per hole reduced from $190.03 to $170.46 a variance of $20 per hole. Thus the total reduction on the 722 holes blast by Top air deck if the blast was charged conventionally was about

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$14,129.54 this accounted for 10–20 % in cost reduction. Top air deck blast revealed a potential reduction in premature ejections and fly rocks due to the fact that the gasbag retained the explosive energy for a longer period of time, this accumulated the energy in the air gap and reduced venting and stemming ejection. However fly rocks and ejections can be managed through a combination of blast design and operational controls these include use of adequate stemming lengths and materials to manage fly rock generation. Results obtained from the analysis of muckpile images using split-desktop exhibited that the mean fragment size was 264.81 mm and F20, F80 and top-size were 41.99, 683.18 and 1454.69 mm respectively. Optimum crusher feed size was as large as 1200 mm and crushed down to the 40 mm and only a small percent of the material was above 1200 mm. The overall blast results were satisfactory, 443,624 tonnes of blasted material from the block which represented 90 % of the total muckpile material was within 900 mm size. 7 Conclusion To improve charge productivity, once a stable pre-gas level has been attained for explosives, gasbag can be inserted, inflated and stemmed immediately without

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waiting for the 45 min gassing phase to take place, prior to stemming in the standard charge practice. The practice reduced the quantity of explosives and stemming used. There was a reduction in premature ejections, fly rocks were restricted within the blast area. There was no blasted material spillages on the ramp area next to the blast shot, while the ramp was still intact after the blast. There was great decrease in proportion of oversize fragment generated from the stemming area, recorded more uniform fragmentation across the collar zone. This led to potential improvement in crusher efficiency by reducing bridging. The Rosin–Rammler uniformity index of the entire muckpile was 1.3. This index is generally used to approximate the size distribution of rock in blasted muckpile. So the obtained index value confirmed uniform size distribution. Challenges are to be expected with implementing Top air deck, and this technique may not be applicable to some sites or areas. However, the advantages outweigh the disadvantages, so it would be prudent to conduct trials over a period of time. With the correct execution the mine could benefit in terms of cost savings, greater efficiencies and environmentally sound operations.

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Fourney WL, Barker DB, Holloway DC (1981) Model studies of explosives well stimulation techniques. Int J Rock Mech Min Sci 18:113–127 Hustrulid W (1999) Blasting principles for open pit mining, vol 1. A.A. Balkema Publishers, Rotterdam Jhanwar J (2011) Theory and practice of air—deck blasting in mines and surface excavations—a review. Geotech Geol Eng 29(5):651–663 Jhanwar JC (2012) Investigation into the influence of airdecking on blast performance in opencast mines in India: a study. Blast Min New Trends. doi:10.1201/b13739-14 Jhanwer JC, Jethwa JL (2000) The use of air deck in production blasting in an open pit coal mine. Geotech Geol Eng 18:269–287 Jhanwar JC, Jethwa JL, Reddy AH (2000) Influence of air deck blasting on fragmentation in jointed rocks in an open pit manganese mine. Eng Geol 57:13–29 Kulatilake PHSW, Hudaverdi T, Wu Q (2012) New prediction models for mean particle size in rock blast fragmentation. Geotech Geol Eng 30(3):665–684 Kuznetsov VM (1973) The mean diameter of the fragments formed by blasting rock. J Min Sci 9(2):144–148 Liu L, Katsabanis PD (1996) Numerical modelling of the effects of air decking/decoupling in production and controlled blasting. In: Proceeding 5th international conference on rock fragmentation by blasting. AA Balkema, Rotterdam, pp 319–330 Melnikov NV, Marchenko LN (1970) Effective methods of application of explosion energy in mining and construction. In: The 12th US symposium on rock mechanics (USRMS). American Rock Mechanics Association Moxon NT, Mead D, Richardson SB (1993) Air-decked blasting techniques: some collaborative experiments. Trans Inst Min Metall (Section A: Mining Industry) 102:A25–A30 Richards M, Nisbet B (2003) The geology of the Mwombezhi dome and Malundwe and Chimiwungo copper deposits Northwest Province Zambia Equinox, Kitwe Rosin P, Rammler E (1933) Regularities in the distribution of cement particles. J Inst Fuel 7:29–33 Sharma PD (2015) Application of air-deck technique in surface blasting. http://miningandblasting.wordpress.com/, http:// miningandblasting.wordpress.com/. Retrieved 4 Oct 2015 Siddiqui FI, Shah SA, Behan MY (2009) Measurement of size distribution of blasted rock using digital image processing. J King Abdulaziz Univ 20(2):81–93 Stroud R (2010) 2010 life of mine plan Malundwe/Kanga and Chimiwungo pits. Optiro Pty Limited, Perth

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