Examining the Effect of Powder Factor Variability on Granite Productivity

Abstract

This research seeks to examine the effect of powder factor variability on granite productivity during its quarrying. Schmidt hammer was used for the in-situ determination of rock hardness. Uniaxial compressive strength (UCS) of in situ rock was estimated from the values obtained from Schmidt hammer rebound hardness test and its density determined from laboratory test. After preliminary field studies, ten (10) blasts with varied powder factors were studied and their overall effects on granite productivity examined. Three (3) rock samples were carefully collected from the quarrying site and subjected to laboratory analysis for UCS and bulk density tests. With spacing and burden kept between 1.7 m and 1.8 m and stemming height also varied between 1.5 m and 2 m, charge columns of between 4.5 m to 6.5 m were maintained, while number of holes drilled per blast was between 64 and 88. Results obtained from the test revealed that the average UCS of the granite samples was 80.67 MN/m2 while the average bulk density was 2465.67 kg/m3. Therefore, considering ten (10) blasts with varied powder factors of between 0.77 kg/m3 and 0.97 kg/m3, total volumes of rock of between 1109.76 m3 and 2280.96 m3 was produced. Hence, varied powder factors have been found to have varying effects on rock fragmentation sizes and by extension, granite productivity.

1. Introduction

Efficiency of blasting operations in underground and surface mines determines, to a large extent, utilization of equipment, productivity and economics. Proper fragmentation of blasted rocks improves the efficiency of downstream operations by loading and crushing to desired sizes. An optimal blast not only results in proper fragmentation but also reduces undesirable effects in ground vibration, fly rock and formation of toe in quarry benches. Drilling and blasting are the first unit operations in the mining process and have a major impact on the performance and cost of subsequent unit operations [1, 2, 3].

According to Salati and Mark [4], powder factor can be defined as the quantity of explosives needed to fragment a unit cubic metre of rock (1m³). Hence, optimum powder factor results in good fragmentation, having less throw and less ground vibration. It can serve as an indicator for rock hardness, cost of explosives used or as a guide to shot firing plan [5].

Improved fragmentation gives loading equipment a higher rate of productivity; hence, it results in lower cost per tonne or cubic yard moved. The effect of wear and tear also decreases giving lower operating cost per hour under similar condition of haul, lift, size and type of truck. Haul/load road condition, truck production per hour also increase with greater degree of fragment due to faster shovel or loader longing rate and a decrease in bridging at the crusher. Therefore, there is a consequent decrease in cycle time. Fragmentation optimization involves breaking of rock to ensure quality control, safe, consistent and efficient blasting [6]. Subsequently, boulder or the opposite, excess fines, result from poorly selected drilling and blasting patterns. A well selected pattern would produce fragmentation that can be accommodated by available loading and hauling equipment and crushing plant with little or no need for secondary blasting. Therefore, it is a well acknowledged fact that the performance of mining operations such as excavation and crushing reeves on fragmentation has been pre-conditioned by blast designs [6, 7, 8].

The effectiveness of hard rock blasting is measured with two basics indices namely, oversize generation and blast hole productivity. Cost per tonne of rock blasted is another index that measures the effectiveness of blasting and is dependent on rockiness and blast design parameter such as hole diameter, burden, spaces among others [9, 10]. Such parameters differ from one mine to the other and some of the blast design parameters could be regulated to deliver the desired blasting effectiveness. The individual influence of the determinant parameter on blasting has been studied by several authors but their cumulative influence on the same is yet to be formulated. However, the huge statistical data generated from the well organized and documented large scale hard rock surface mines operating variables condition worldwide constitute the only readily available resources which could be used for the analysis and regression model of indices that determine effectiveness of blasting of rock blasted on uncontrollable and controllable blasting parameters [1]. Efficiency of drilling and blasting operations must contribute to the best overall economics of a quarry [11, 12]; therefore, variability of powder factor has potentials to improve surface mines’ productivity [13, 14]. Hence, there is need to study the effect of powder factor variability in the productivity of granite quarrying. The study is an attempt to achieve the following specific objectives:

1. To determine the relationship between the powder factor and uniaxial compressive strength at the quarry.

2. To determine the effect of varying powder factor in the fragmentation of rock; and

3. To recommend ways to improve the productivity of granite in the quarry.

The appraised parameters would give optimum blasting results through the regression model generated using indices such as oversize generation and geometric volume of the blasted rock on blast design parameters.

2. Materials, methods used

2.1 Climate, vegetation and relief of typical granite areas

A study carried out by Abaje and Oladipo [15] shows that the temperatures of granite mineralized areas could increase from 0.2 to 0.5°C. A case of rainfall trend and variation characteristics across Kaduna State, North-western Nigeria, using 11 selected stations in the Southern, Central and Northern parts of the State for a period of 50 years (1966–2015) was carried out and the climatic condition of the study revealed that its Southern part, has the highest total rainfall, yet there was no significant trend in the five decadal periods examined. From the early 1970s to the late 1990s the rainfall was below the long-term mean. The rainfall of the remaining years nearly approximates the long-term mean [16]. Local spatial patterning of most granitic areas partly describes their spatial heterogeneity and related to alternation between vegetated and bare areas which are commonly found in savanna systems [17].

Ladan [18] also shows that granitic areas which have natural rain forest vegetation often fall within the Savanna. Their two distinct characteristic seasons are rainy and dry, while their annual rainfall is between 80 cm and 100 cm. Their temperature variation throughout the year could vary from 20° to 27°. The vegetation in such area may be Guinea Savannah and can be characterized by long and short grasses as well as low trees and shrubs. Their sparse distribution is sometimes accompanied by dense vegetation along streams and river channels [19].

2.2 Brief geology granite areas

The areas consist of rocks that range in age from Pre-Cambrian to Lower Paleozoic and Quaternary period. Four groups of rocks can be distinguished for the Basement Complex Terrain in the area. The crystalline basement rocks which consist of gneisses and migmatites with different varieties of the gneisses like the banded gneiss, granite gneiss, biotite gneiss, hornblende gneiss and ortho genesis [20]. Figure 1 shows the exposure of a typical granite outcrop.

Figure 1.

Exposure of a typical Quarry’s granite outcrop showing its relief.

In the older granite underlain by the Precambrian Basement Complex rock, porphyritic granite could be the most common basement rock intruding both magnetite and metasediments. Pegmatites are widely distributed throughout the Precambrian Basement Complex of the area studied by Abere et al. [21]. However, the extremely coarse igneous bodies which are closely weathered and spaced to large masses plutonic rock often consist of quartz, feldspars and muscovite [22]. They are elliptical to elongated shape, which is seen to be elevated to the forsian emplacement.

2.3 Field studies and data collection

The operation of the selected granite site was studied in order to obtain information about the various explosives, drilling machines, blasting parameters such as burden, spacing and depth of hole. Their physical condition and quantity available were studied. Observation was also made to estimate the size of fragmented rocks. Three samples of blasted boulders were collected at three different faces of the quarry in the study area. The coordinates of each location were taken and recorded with the aid of global positioning system (GPS).

2.4 Drilling/blasting and powder factor variability procedures

As shown in Tables 1–10, the various drilling and blasting parameters used at Tutu quarry including the parameters for burden and spacing, depth of blast holes, column charge, base charge, tonnage factor, quantity of material blasted and their corresponding powder factors adopted for rock fragmentation are presented for ten (10) blasting operations.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	6.30	M
4	Diameter	76.00	Mm
5	Burden	1.70	M
6	Spacing	1.70	M
7	Stemming height	1.60	M
8	Charged column	4.70	M
9	Number of holes	64.00	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	4.00	—
12	High explosive	400.00	Kg/Hole
13	ANFO	3.72	Kg/m
14	ANFO	719.00	Kg/Hole
15	Total weight of explosive	1119.00	Kg
16	Volume of rock	1165.25	m^3
17	Powder factor	0.96	Kg/m^3
18	Tonnage factor	2.70	T/m^3

Table 1.

Blast number one.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	7.90	M
4	Diameter	76.00	Mm
5	Burden	1.80	M
6	Spacing	1.80	M
7	Stemming height	2.00	M
8	Charged column	5.90	M
9	Number of holes	88.00	—
10	High Explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	6.00	—
12	High explosive	825.00	Kg/Hole
13	ANFO	3.72	Kg/m
14	ANFO	1106.48	Kg/Hole
15	Total weight of explosive	1931.48	Kg
16	Volume of rock	2252.45	m^3
17	Powder factor	0.86	Kg/m^3
18	Tonnage factor	2.70	T/m^3

Table 2.

Blast number two.

S/N	Parameters	Numeric Value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	6.80	M
4	Diameter	76.00	Mm
5	Burden	1.90	M
6	Spacing	1.90	M
7	Stemming height	1.50	M
8	Charged column	5.30	M
9	Number of holes	74.00	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	4.00	—
12	High explosive	462.50	Kg/Hole
13	ANFO	3.72	Kg/m
14	ANFO	996.51	Kg/Hole
15	Total weight of explosive	1459.01	Kg
16	Volume of rock	1816.55	m^3
17	Powder factor	0.80	Kg/m^3
18	Tonnage factor	2.70	T/m^3

Table 3.

Blast number three.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	7.76	M
4	Diameter	165	Mm
5	Burden	4	M
6	Spacing	4	M
7	Stemming height	1.8	M
8	Charged column	5.5	M
9	Number of holes	92	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	4	—
12	High explosive	650	Kg/Hole
13	ANFO	18.73	Kg/m
14	ANFO	102.99	Kg/Hole
15	Total weight of explosive	10,137.00	Kg
16	Volume of rock	11,425.60	m^3
17	Powder factor	0.89	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 4.

Blast number four.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	13.28	M
4	Diameter	165	Mm
5	Burden	3	M
6	Spacing	4	M
7	Stemming height	1.3	M
8	Charged column	9.82	M
9	Number of holes	52	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	4	—
12	High explosive	408	Kg/Hole
13	ANFO	10.93	Kg/m
14	ANFO	107.34	Kg/Hole
15	Total weight of explosive	10,295.00	Kg
16	Volume of rock	8286.00	m^3
17	Powder factor	1.24	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 5.

Blast number five.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Staggered
3	Hole depth	14.41	M
4	Diameter	165	Mm
5	Burden	3	M
6	Spacing	4	M
7	Stemming height	1.8	M
8	Charged column	12.15	M
9	Number of holes	52	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	4	—
12	High explosive	760	Kg/Hole
13	ANFO	19.52	Kg/m
14	ANFO	107.34	Kg/Hole
15	Total weight of explosive	11,094.60	Kg
16	Volume of rock	8994.48	m^3
17	Powder factor	1.23	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 6.

Blast number six.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Square
3	Hole depth	16.44	M
4	Diameter	102	Mm
5	Burden	2.5	M
6	Spacing	2.5	M
7	Stemming height	1.52	M
8	Charged column	14.44	M
9	Number of holes	87	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	3	—
12	High explosive	4.68	Kg/Hole
13	ANFO	6.70	Kg/m
14	ANFO	91.53	Kg/Hole
15	Total weight of explosive	8237.95	Kg
16	Volume of rock	8938.06	m^3
17	Powder factor	0.89	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 7.

Blast number seven.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Square
3	Hole depth	12.07	M
4	Diameter	102	Mm
5	Burden	2.8	M
6	Spacing	2.8	M
7	Stemming height	1.8	M
8	Charged column	10.27	M
9	Number of holes	92	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	3	—
12	High explosive	4.68	Kg/Hole
13	ANFO	6.70	Kg/m
14	ANFO	60.30	Kg/Hole
15	Total weight of explosive	6046.37	Kg
16	Volume of rock	8707.57	m^3
17	Powder factor	0.69	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 8.

Blast number eight.

S/N	Parameters	Numeric value
1	Explosive used	High Explosive + ANFO
2	Drilling pattern	Square
3	Hole depth	15.59	M
4	Diameter	102	Mm
5	Burden	2.5	M
6	Spacing	2.5	M
7	Stemming height	1.5	M
8	Charged column	12.83	M
9	Number of holes	105	—
10	High explosive	1.56	Kg/Cartridge
11	Number of explosives used per hole	3	—
12	High explosive	4.68	Kg/Hole
13	ANFO	6.70	Kg/m
14	ANFO	60.30	Kg/Hole
15	Total weight of explosive	9176.62	Kg
16	Volume of rock	10,033.30	m^3
17	Powder factor	0.91	Kg/m^3
18	Tonnage factor	2.7	T/m^3

Table 9.

Blast number nine.

Sample	Maximum load (KN)	Specimen length (m)	Specimen width (m)	Cross sectional area (m^2)	Uniaxial compressive strength (MN/m^2)
Tutu Quarry	133.2	0.06	0.03	0.0018	74
F&P Quarry	145.8	0.06	0.03	0.0018	81
AAY Quarry	157.5	0.06	0.03	0.0018	87.5

Table 10.

Uniaxial compressive strength (UCS) MN/m³.

Details of the variability results are follows:

Drilling pattern adopted

Staggered.

Explosive type used

High explosive (Gelatin) + ANFO.

Hole depth

6.30 m: 16.44 m.

Hole diameter

76 mm, 102 and 165 mm.

Burden-spacing pattern

1.7 × 1.7: 2.8 × 2.8.

Stemming height

1.2 M: 2.7 m.

Charged column

4.0 M: 14.44 m.

Number of holes

58: 105.

Total weight of explosives

1119.00 kg – 11,094.60 kg.

Volume of rock blasted

1165.25 m³–11,425.60 m³.

Varied PF

0.77–1.24.

Constant TF

2.7.

2.5 Sample preparation and laboratory analysis

A circular saw with a diamond blade was used to cut the specimens to their final lengths. The surfaces were then ground after cutting in a grinding machine in order to achieve a high-quality surface for the axial loading. The measurement of the specimen dimensions was made with a sliding caliper and metre rule. Furthermore, the tolerances were checked by means of a dial indicator and a stone face plate. The specimen preparation was carried out in accordance with ASTM test procedure (ASTM, 39-71) and as adopted by Vandergrift and Schindler [23] in their experiment. The sample was cut using cutting machine to a dimension suitable for uniaxial compressive stress (UCS) test. The specimen was placed in horizontal direction but perpendicular to the direction of cutting edge of the blade. Then the vice was used to hold the specimen firmly to obtain a smooth surface as accurately as possible. The machine was switched on and the necessary shield applied. Water was allowed to lubricate the blade during the cutting process.

2.5.1 Procedure for uniaxial compressive strength test

The ASTM test procedure (39-71) was adopted. The specimen was placed in the ELE ADR 2000 compression machine. The load is continuously applied on the specimen until it failed. The failure mode was noted as well as the pressure or load at failure. The type of failure and the maximum load carried by the specimen were recorded. The unconfined UCS of the rock sample was obtained by dividing the maximum load carried by the cross-sectional area. Testing machine of standard recommended ASTM C 39-71 was used to load the squared sample until it failed.

2.5.2 Test specimens

Squared samples were used for this test. The four sides of each sample were ground flat, smooth and perpendicular to axis, that is they were parallel to each other 4 cm × 4 cm cube specimen were cut from block samples supplied (in the absence of core which are commonly used). The platens on the compression machine were altered to suit this configuration. The edges were cut to shape and smoothened by polishing them with carborundum powder.

3. Discussion of results

Findings from field studies and observations as shown in Table 11 have revealed that the trend in the spread of GPS coordinates of granite samples collected indicates that the samples were taken at relatively close intervals. Results shown in Tables 1–10 also indicate the various specifications of drilling and blasting parameters as used in the study site. From the tables it can be deduced that the stemming heights, burden spacing, column of charged holes and bulk density were varied as the depth of blast holes increases. It is also shown that the number of explosives charged per hole indicates an increase in the depth of hole drilled, thereby increasing more explosives consumption and equally implying that more volume of granite has been fragmented. Table 10 shows uniaxial compressive strength (UCS) MN/m³ of granite from the studied quarries, Table 12 on the other hand shows the bulk densities of granite from the selected quarries. Also as apparently shown in Table 13, the mineralogy of the granitic constituents of granite is partly influenced by either a corresponding increase of decrease in PF. Hence, some sort of correlation seems to be found between granite mineralogy and PF. In addition, the PF for the nine (9) blasts was not the same, which means that it was varied at different times of the blast as shown in Table 14. For all the selected quarries, bigger blasthole diameter also varies with increased PF.

Sample	Latitude	Longitude
1	N26^0 51′7.2”	E007^0 59′ 71.4”
2	N26^051’8.1”	E007^059’75.5”
3	N26^0 51′13.3”	E007^059’64.5”

Table 11.

Coordinates of samples collected.

Sample ID	Mass of specimen (g)	Height of specimen (m)	Width of specimen (m)	length of specimen (m)	volume of specimen (m^3)	Bulk density (Kg/m^3)
Tutu Quarry	2742.6	0.1	0.1	0.1	0.001	2743
F&P Quarry	2742.4	0.1	0.1	0.1	0.001	2742
AAY Quarry	2183.3	0.1	0.1	0.1	0.001	2183

Table 12.

Bulk density of selected quarries’ granite samples.

Mineral	Sample 1 (%)	Sample 2 (%)	Sample 3 (%)	Average (%)
Quartz Syn	39	40	30	36.33
Microcline	0	0	28	9.33
Albeit	34.6	31	10	25.20
Orthoclase	25	17	16	19.33
Anorthite	0.3	10	6	5.43
Muscovite	0.1	0.8	2	0.97
Osumilite	0.9	0.7	8	3.20
TOTAL	99.9	99.5	100	99.80

Table 13.

Result of mineralogical analysis of samples.

S/N	Powder factor (Kg/m^3)	Volume of rock produced (m^3)	Location
1	0.96	1165.25	Tutu quarry, Kaduna
2	0.86	2252.45	Tutu quarry, Kaduna
3	0.80	1816.55	Tutu quarry, Kaduna
4	0.89	11,425.60	F&P Quarry, Abuja
5	1.24	8286.00	F&P Quarry, Abuja
6	1.23	11,094.60	F&P Quarry, Abuja
7	0.89	8938.06	AAY Quarry, Kano
8	0.69	10,033.30	AAY Quarry, Kano
9	0.91	8707.57	AAY Quarry, Kano

Table 14.

Powder factors and volume of rock produced.

Results from this study have shown the level of relationship between the strength of rock and PF. For instance, it can be deduced from Figures 2–4 that the Uniaxial Compressive Strength (UCS) of rock increases as the PF increases. This is also true in the reverse case as PF reduces. In the same vein, keeping UCS as a constant, PF increases as volume of blasted materials increases which is also true for the reverse case.

Figure 2.

UCS across the study locations.

Figure 3.

Average powder factor (pf) across the study locations.

Figure 4.

Variability of the UCS and average powder factor (pf) across the study locations.

From the figures, it is evident that PF increases as the volume of blasted rock increases. Thus, more volume of rocks means higher PF to fragment the rock. Therefore, more explosives are charged in the holes to get the required results as the volume of rock increases. It can be deduced that to reasonably vary the PF of rock, the ratio of burden to spacing must be carefully selected with a view to increasing its productivity. Hence, PF variability becomes more effective with careful and staggered increment of burden and spacing.

As evident from Figures 5–7, more explosives are consumed when the volume of fragmented rock increases. Therefore, varied PF with carefully varied and selected drilling and blasting parameters are required for optimum blast and higher productivity.

Figure 5.

Variability of the stemming height across the blasts.

Figure 6.

Powder factor (PF) across the blasts.

Figure 7.

Variability of the stemming height and powder factor (PF) across the blasts.

4. Conclusions and recommendations

The PF used at the selected quarries has been examined for nine (9) blasts to ascertain its effects on the level of granite productivity in the three selected quarries. It can therefore be concluded that:

1. PF variability can lead to increase or decrease in the level of productivity of granite.

2. Blasthole diameter varies with increase in PF of granite.

3. PF selection and variability can lead to higher cost of productivity.

4. The apparent correlation between PF and granite mineralogy can be a factor for consideration in blast design and the desired productivity.

5. The strength of granite influenced the PF selected at the selected quarries.

Also, the quantity of explosives for some of the sizes of the fragmented rock at the quarries was found to be moderate, while for some, it was optimal for the blast. Hence, it can be concluded from the observed blasts that the productivity of granite at a quarry can be improved for optimum economic benefit, if the PF is varied as appropriate for any blast design and the quality and properties of the explosives selected are adequate for the strength of the rock to be blasted.

Having observed and examined the effects of PF variability on the level of granite productivity, the following measures are hereby recommended for optimum rock fragmentation:

1. The strength of granite should be examined before selecting a suitable PF.

2. Since the PF variability has great impact on the productivity of granite, its selection must be guided by expertise and experience.

3. Economy and productivity of granite quarries have potentials for improvement when serious attention is given to the blast design and study of explosive characteristics and properties.