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Improving Recovery In Granite Mining Operations Through Scientific Approach

Abstract

India possesses substantial reserves of granites of very attractive colours and physico-mechanical properties to carve a niche in the world market lack of knowledge of the rock mass quality and failure to adopt the modern quarrying techniques and scientific methods of exploitation have led to poor recovery and huge wastage of the valuable material. In this paper, an attempt is made to highlight the need for 'scientific approach' for proper exploitation of granite resources. Two flow charts are given which will serve as guidelines for the scientific design of the quarries and profitable exploitation of the reserves. A case study is also presented incorporating the problems faced, the ways and means adopted to resolve them and the results achieved.

I. Introduction

India is endowed with vast resources of granite of attractive colours and studded with black granite dyke swarms. It possesses over 20% of the world's granite resources but despite an impressive increase in production of granites, the country's contribution of finished stones is less than 3% in the world market. The economic viability of granite mining operations is dependent directly on the quality of stones, the methods of extraction of blocks and the yield of usable material for direct export or value addition through the quarrying and processing phases. The mines in the organised sector of India are at the moment able to recover between 20 to 30% as usable products for direct export/internal consumption or for value addition and then export or sell here, Low recovery of saleable blocks of between to 30% from the rock mass in small scale and organised sectors is attributable to in lack of understanding of the geotechnical features of the rock mass, small lease holdings and other unique technical problems associated with separation/extraction of large size of blocks from the bed rock.

The Government of India has declare Granite as one the thrust sectors to boost the production capacity and Promote its export in a big way. The new policies of the central government emphasises on higher efficiency, competitiveness and utilisation of resources through, technological upgradation of every industry. Since this will mean a quantum jump in terms of the contribution of the granite mining industry towards export earnings, it is imperative that the scientific and technical aspects of the exploitation of granites are examined in proper perspective to ensure better recovery and conservation.

II. The Scientific Approach

A. GEOTECHNICAL STUDIES

Many problems arise while quarrying for the granite blocks because of the following :

  • Unscientific selection of the faces without taking into consideration the major geological and geotechnical factors.
  • Neglecting the stability associated problems of the cut slopes.

In quarrying, the size of blocks extracted usually depends on the frequency of joints. If the frequency is too high, the deposit is useless because availability of certain sizes of blocks will be impossible. The block structure will be determined by jointing parameters like a) the number of joints b) their orientation in space c) distance between joints d) the length of such joints. The knowledge on some of the important parameters like the orientation, spacing, persistence, roughness, wall strength, aperture and filling of discontinuities linked of the geometry of the slopes in the quarries will allow a definition of the shape and the mean volume of solid rock blocks that can be made, an analysis of the possible sliding of blocks from the excavation faces and their kinematic movements.

For releasing any granite block from sheet lock, it is essential to develop free faces at least on two sides besides the top portion. Once the free faces on either side are established the sheet joints will determine the process and level of exploitation. The jointing pattern has to be studied to get an idea fro systematic development of quarries. Normally, the granite deposits are associated with either closely or moderately spaced joint planes. Depending on the orientation of the main set of discontinuity, it will be possible to classify the exploitation into three main configurations :

  • Sub-vertical main joint set
  • Main joint set with an average dip
  • Sub-horizontal main joint set.

The influence of different means of exploitation on these three types of configuration has to be examined thoroughly.

B. STUDY OF THE PHYSICO-MECHANICAL PROPERTIES :

Inspite of the abundant use of granites, no systematic approach has been made on most of the occasions to study their properties before commencing exploitation. Keeping this in mind, the geological and some of the geotechnical properties of granites like colour, texture, composition, grain size, specific gravity, moisture content, density, water absorption and porosity, compressive strength, tensile strength, shear strength, ultrasonic pulse velocity, Schmidt hammer number, slake durability etc. will have to be studied in the laboratory. The increasing occurrences of defects after installation of the ornamental stones have brought about the necessity of standard norms regarding selection of the particular technical requirements of granites to be used for a purpose.

C. CONTROLLED BLASTING :

Drilling and blasting with due caution still remain to be the most effective and cheapest method of splitting hard and abrasive rocks from the bedrock. Splitting of hard and abrasive rocks by conventional means is accomplished by drilling of coplanar holes that are closely placed and pressurised through the minimal usage of very low strength explosive to tear away a desired volume of rock from the bedrock.

The blasting and more appropriately splitting needs of dimensional stone are entirely different from other conventional methods of blasting. The principal objective of such blasting or the splitting techniques is either to expand the natural cracks present in the rock so as to dislodge it or to create an uniform split to separate a block from the surrounding rock with no or minimal damages. The development of these cracks is always in a desired direction. Since these stones are to be cut to thin plates, the crystalline structure should be free from fractures. The explosive type and blasting/ splitting technique for use in such cases should be such that they avoid/eliminate stresses which may lead to fractures or micro fractures in a direction which is not desirable.

In order to achieve the best results through blasting, it is necessary that a standard drilling and blasting design is established for each site. It may be noted that blasting for separation of bigger blocks involve very precise drilling and the use of right quality and quantity of charges. The discontinuities present in the rock mass should be advantageously used in the blast design. The following flow chart (figure 1) shows the parameters to be considered to arrive at a standard drilling and blasting procedure.


Figure 1. Flow chart showing the blast design procedures in a Dimensional Stone Quarry

III. Block Splitting

Apart from blasting, the granite is separate from the main bedrock by various techniques that are so designed as optimise the block recovery. The prevailing methods in Indian quarries are :

A. FEATHERS AND WEDGES METHODS :

This one of the oldest methods of granite mining. In this method, a series of holes using jack hammer are drilled to a depth of 30-50 cm, mostly along the existing bedding planes, joints etc. Feathers and wedges are inserted and carefully hammered in a calculated sequence so that the block splits along the line.

B. LINE DRILLING :

This method involves drilling of a series of closely spaced holes in line, charged with low explosive cartridges like gun powder and instantaneous detonators and blasted. The use of delay detonators cause fragmentation and cracks in rocks. Alternatively, detonating cords may be used. The detonating cords are inserted into the holes and the holes are water stemmed and plugged at the mouth and fired to separate the blocks from the mother rock.

C. DIAMOND WIRE SAW :

The principle of wire sawing is to pull spinning continuous loop of wire mounted with diamond steel beads through the granite rock to provide cutting action.

D. HELICOIDAL WIRE SAW :

The equipment consists of an endless helicoidal wire with single, double or triple stand, running overs heaves and is fixed under tension into the stone. The wire is made to rotate to cut the block.

E. FLAME CUTTING :

The rock is cut by creating fractures into layers with a thermic jet. Huge quantity of heat produced by the flame causes damage on the surface of the rocks.

The demand for ornamental and decorative stones have increased so much during the last two decades that conventional quarrying techniques were considered inefficient for production to meet the demand. As a result, new techniques, explosives, chemicals and equipment have been developed/evolved to substantially increase the production. Some of these are :

  • Smooth blasting techniques
  • Special explosives like Donarite 2 E (Austria), Dynashear (USA). k-pipe & Forcit tube (Finland), S.B.2 (Italy) and Gurit & Nabit tube (Sweden),Pyronex/Cotonex (UK), B-Gel (USA)
  • Finish method with the application of advanced perforation techniques
  • Water jet cutting
  • Chain cutting
  • Silent non-explosive demolition agents like ACCONEX (ACC, India), S MITE (Japan), BRISTAR (Japan),ASTAC (Japan) etc.
  • Methods using air-decks
  • Methods using detonating cord.

The applicability of each of or a combination of these techniques is, however, dependent on the site-specific parameters and the success of each venture will depend upon the conduct of the techno-economic study for each deposit. The basic parameters considered for proper exploitation of the granite deposits through scientific approach is given in Figure No. 2.


Figure 2. Flow chart showing the Scientific design of the quarries

IV. Glimpses Of The Field Investigations

A. FIELD INVESTIGATIONS :

Systematic field investigations were carried out in three quarries located in the Krishnagiri area of Tamil Nadu. All of them are mining the paradiso granite having different qualities and hence the market value. Various problems were reported from these quarries leading to very poor recovery and productivity. A brief account of the field investigations conducted in one of the quarries and the results achieved are presented below :

The location of the quarry is about 20 km from the Krishnagiri town. The major rock types encountered in the area are the Khondalite, group of rocks represented mainly by quartzite and followed by Charnockite group consisting mainly of lenses of chamockite and bands of magnetite quartzite, amphibolite and pyroxenite. The migmatic group of rocks include the coarse grained dark grey migmatite, pinkish grey and greyish pink Paradiso migmatite. The general trend of the rock types range from NNE-SSW to NNW-SSE and again to NNE-SSW direction from south to north in the area with steep dips on either side indicating a series of tightly packed antiforms and synforms.

Many attempts were made earlier by the operators at this quarry to maintain the benches to improve the recovery and productivity. The recovery percentage was as low as 5% and there was no continuity in the production due to want of a proper technique of exploitation. A team of scientists from the National Institute of Rock Mechanics (NIRM) conducted the field investigations to identify the problems and to improve the recovery and productivity.

B. THE FOLLOWING PROBLEMS WERE IDENTIFIED.

  1. Improper layout of the quarry posed problems in handling of the blocks and transportation.
  2. Unfavourable face orientations caused undesirable cracks during main splitting of the blocks leading to very less recovery or no recovery at all.
  3. Non-uniform hole depths,unsuitable hole spacing, undesirable charge concentrations and hole deviations made blasting a very difficult operation. Many a time, repeated charging and blasting of the same holes created unwanted cracks and left behind considerable amount of toe.
  4. Absence of an organisation for the procedures and practices of block-splitting;
  5. Lack of knowledge on the quarrying methods/systems;
  6. Lack of competent persons and skilled labour;
  7. Improper selection of equipment and their underutilisation.

C. SOLUTIONS AND RESULTS

a) The quarry layout and the present practices were studied in detail. It was observed that the wrong layout of the quarry caused problems while advancing the faces in the desired direction and handling of the blocks. Loading and transportation of bigger blocks were tedious and risky due to lower capacity of the equipment and steep road gradient.

The most suitable quarry layout was suggested in tune with the targeted production. Though it was not possible to immediately adopt the recommended quarry layout, the idea was well received and efforts are underway to slowly reorient it. Suggestions were made to procure suitable loading and transporting equipment and to maintain proper road gradient to match with the production.

b) Undesirable cracks developed while using jet burners for creating the free faces on the sides. Although due care was taken in drilling and blasting operations for splitting the blocks, invariably the breakage followed these cracks thus rendering the whole block useless.

Geotechnical studies were carried out and the details of discontinuities were recorded. Ground penetrating radar was used to locate/identify the sub surface joints on specific sites. In most of the cases, these cracks were found along the wavy structure of the rock mass which is making an angle of 30-45 degrees with the existing face. It was found that cutting with jet burners along the strike direction proved to be faster and produced less cracks than while cutting against the strike direction.

Based on the above data, proper face orientations were suggested and the quantity of recoverable blocks was estimated. In some cases, the presence of natural joints were favourably used as free faces mainly to avoid cutting with jet burners. It was suggested to execute the cut along strike wherever possible and reorient the faces to the favourable direction.

c) The rock samples from different locations of the quarry were taken to the laboratory at NIRM and tested for their physico-mechanical properties like the compressive strength, tensile strength, density, P-wave velocity etc. as per the ISRM standards. The tests values are presented in Table 1.

d) The conventional drilling and blasting operations were carried out without any touch of scientific and professional acumen. As a result, the rockmass suffered severe damage on several occasions producing unwanted cracks and considerable toe in every blast. Hand held Jackhammer drills were used for drilling the blast holes. The length of the holes did not correspond to the desired depth of cut. The results of the detonating cord blast were affected significantly due to unsuitable hole spacing, undesirable charge concentrations and hole deviations.

Based on the above observations and the physico-mechanical properties, the designs for block splitting by drilling and blasting were developed to suit the site conditions and the available equipment, explosives/accessories. The presence of joints and other cracks/fractures were advantageously used in the blast designs. The hole spacing was maintained at 30 cm along the strike and 25 cm across respectively. The drilling operations were supervised to maintain the hole depths to the desired positions and proper care was taken to minimise the hole deviations. Though suitable low density explosives were not available at the site, tailor made charges were used at the predetermined positions in the holes to maintain the charge concentration within limits. The charge concentration varied between 10 to 40 grams per linear meter. Excellent results in terms of recovery from the insitu blocks obtained in all the five blasts were taken leading to increased and continuous production. The required displacement of 10-20 cm was achieved with clean breakage along the holes. The toe problem was eliminated. The formation of toe not only minimised the recovery but also caused severe damage to the surrounding block due to repeated charging and blasting of the same holes. Sometimes additional holes were drilled and blasting as in waste rock to get rid of the toe. On the site, training was imparted to the supervisors and workers for systematic drilling and blasting practices.

V. Conclusions

India possesses vast resource of granite of different varieties having very good commercial value. At present, majority of the quarries adopt unscientific method exploitation leading to very low recovery and productivity in terms of saleable blocks. In order to optimise the recovery from the in-situ reserves and improve the export earnings from granites, it is necessary to adopt the scientific and technical aspects of exploitation. The methodology suggested for the scientific exploitation will help the entrepreneurs to optimise their quarrying operations. The case study presented indicates that through scientific and systematic approach, the recovery and productivity can be improved substantially within a short period.