The Block Cave Mining Method

The Block Cave Mining Method Block caving is a large-scale underground mining method applicable to the extraction of low-grade, massive ore bodies. With the amount of literature available on block caving this report identifies the need to provide a simple understanding of the process. Understanding a production process of a block cave mine is an important aspect before getting involved with technical aspects of the mine. This report attempts to give an introduction into the production process of a block cave mine and also an understanding about block caving. The document has been split into four chapters, Chapter One gives a basic understanding of the method and highlights the considerations that have to be made before the implementation of a block cave mine. Chapter Two gives an introduction into the production process involved in a block cave mine by taking into account four major levels involved in production. The production process has been described in the form of a flow chart for simple understanding of the process. Chapter Three outlines the significance of production control and production management in order to increase productivity of the mine. Chapter Four outlines some of the safety and risks involved in a block cave mine and the necessary precautions to be taken in order to increase safety. This report has been intended to provide a simple understanding of the block cave mining method and the production process involved. This report is advocated towards a layman in block caving in view of getting an impression about the block cave mining method. Chapter One Introduction 1.1 Block Caving Block caving is an underground mining method applicable to the extraction of low-grade, massive ore bodies with the following characteristics: large vertical and horizontal dimensions, a rock mass that will break into pieces of manageable size, and a surface that is allowed to subside. These rather unique conditions limit block caving to particular types of mineral deposits. Block caving is used for extracting iron ore, low-grade copper, molybdenum  deposits, and diamond-bearing kimberlite pipes. 1.1.1 Block Caving Method A large slice of material is blasted at the base of the ore body which creates an instability within the orebody, inducing the breakdown and mobilization of ore to the production level through the breakdown of ore and waste due to the natural pattern of breakages, development of stresses in the active caving area, and the low strength of the rock mass. The size and shape of the undercut depends on the characteristics of the rock mass. Excavations are created at the production level at base of the orebody to draw out the broken material. A large amount of development expenditure is required to set up the facilities to break the lowest level of the ore body, and all the broken rock is extracted out of the block cave through a system of drawbells. Once the caving is initiated, operating cost of the block cave is very low comparable to the operating costs in open pit mining. Once caving is initiated, production can be ramped up until the production rate is almost equal to the caving rate. The undercut is advanced in the horizontal plane to create greater areas of caving for increasing the production.   Rock breakage occurs only in the caving areas, induced by undercutting, and has low drilling and blasting cost; some amount of blasting may be required at the drawpoints1  to break some of the large rocks coming through the drawbell, especially during the initial stages of draw. Most block caves these days are highly mechanized with large number of large LHDs (load-haul-dump machines) working at the lower levels, though smaller orebodies can also be caved and extracted using gravity draw systems with orepasses2  and slushers3. The development of a conventional gravity flow system of block caving involves Figure Conventional Gravity Flow System http://technology.infomine.com/reviews/BlockCaving/assets/images/BlockCaving1.jpgSource: Infomine Block Caving A spot where gravity fed ore from a higher level is loaded into hauling units A vertical or inclined passage for the downward transfer of ore A mechanical drag shovel loader an undercut where the rock mass underneath the block is fractured by blasting; drawbells beneath the undercut that gather the rock into finger raises4; finger raises that draw rock from drawbells to the grizzlies; a grizzly level where oversized blocks are caught and broken up; a lower set of finger raises that channel ore from grizzlies to chutes for train loading the finger raises are arranged like the branches of a tree, gathering ore from a large area at the undercut level and further channeling material to chutes at the haulage level; and a lowermost level where ore is prepared for train haulage and chute loading. When LHDs are used, the development required is considerably less complex and involves Undercut Levelhttp://www.edumine.com/xcourse/xblock101/docs/figures/images/10002x.jpg Extraction LevelSource: Infomine Block Caving 4 Steeply sloping openings permitting caved ore to flow down raises through grizzlies to chutes on the haulage level an undercut where the rock mass underneath the block is fractured by blasting; drawbells constructed between the undercut and extraction levels; an extraction level with drawpoints at the base of drawbells; and an ore haulage system to collect, crush and transport the ore out of the mine. Underground Mining Methods Unsupported Artificially Supported Pillar SUpported Shrink Stoping Bench and Fill Stoping Room and Pillar Sublevel Mining Longwall Mining Sublevel and Longhole Open Stoping Block and Panel Caving VCR Stoping Cut and Fill Stoping 1.1.2 History of Block Caving Late 19th century:  precursor to modern block caving developed in the Pewabic iron ore mine, Michigan, USA Early 20th century:  the block caving method developed in the USA for iron ore and then copper mining in the western states 1920s:  block caving started in Canada and Chile Late 1950s:  block caving introduced into southern African diamond mines and then chrysotile asbestos  mines Late 1960s:  LHD vehicles developed for underground mining 1970:  LHDs used with block caving at El Salvador mine, Chile 1981:  mechanised panel caving introduced in the primary ore at El Teniente mine, Chile 1990s:  planning of the new generation of block caves with larger block heights in stronger orebodies (e.g. Northparkes, Palabora) 2000s:  planning and development of super block caves under existing open pit mines (Grasberg, Chuquicamata, Bingham Canyon) and at great depth (Resolution Copper) http://www.edumine.com/xcourse/xblock101/docs/figures/images/10003x.jpg Source: Infomine Block Caving 1.2 Management Organizational Chart Mine Manager Technical Services Superintendent Technical Services Superintendent Mine Superintendent Human Resource Logistics Electrical Mechanical Cave Development Cave Production Ventilation Projects Geo-Technology Geology Survey Long Term Planner Short Term Planner Design The organizational chart might differ based on the requirements of a specific mine. 1.2.1 Managerial Responsibilities: Mine Manager is responsible for the overall management, direction and coordination of the mine and related operations. Mine Managers are also intended to provide the technical leadership in the area of underground mine engineering.   The focus of the Mine Manager should be on the following subjects Ensuring underground mining activities are conducted in accordance with the Occupational Health and Safety Act and Regulations and environmental standards Complying with all safety requirements Observing all company policies and procedures Assisting with the development of production targets Ensuring production targets are met or exceeded Developing schedules, budget and ensuring these are controlled and managed effectively Monitoring production results on a progressive basis and preparing monthly progress and variance reports Maintaining effective working relationships with Contractors, Suppliers and Service Providers, and ensuring adherence to contractual requirements Developing a sense of continuous improvement Ensuring appropriate training programs are in place to meet safety and production requirements Maintaining knowledge of current statutory requirements and industry best practices and ensuring compliance at all times Interphases with other managers and superintendents as part of the management team Reviewing mining methods Implementing optimisation programs where appropriate Managing manpower levels to achieve their performance 1.3 Parameters to be considered before the implementation of cave mining Twenty five parameters that should be considered before the implementation of any cave mining operation are set out in Table 1. Many of the parameters are uniquely defined by the orebody and the mining system. No. Parameters Considerations 1 Cavability Rockmass Strength Rockmass Structure In situ stress Hydraulic radius of orebody Water 2 Primary Fragmentation Rockmass strength Geological structures Joint/fracture spacing Joint condition ratings Stress or subsidence caving Induced stress 3 Drawpoint Spacing Fragmentation Overburden load and direction Friction angles of caved particles Practical excavation size Stability of host tockmass Induced Stress 4 Draw Heights Capital Orebody geometry Excavation stability 5 Layout Fragmentaion Drawpoint spacing and size Method of draw 6 Rockburst Potential Regional and induced stresses Rockmass Strength Structures Mining Sequence 7 Sequence Cavability Orebody geometry Induced stresses Geological environment Influence on adjacent operations Rockburst potential Production requirements Water inflow No. Parameters Considerations 8 Undercutting Sequence Regional stresses Rockmass strength Rockburst potential Rate of advance Ore requirements 9 Induced Cave Stresses Regional stresses Area of undercut Shape of undercut Rate of undercutting Rate of draw 10 Drilling Blasting Rockmass strength Powder factor Rockmass stability Required fragmentation Height of undercut 11 Development Layout Sequence Production Drilling and blasting 12 Excavation Stability Rockmass strength Regional and induced stresses Rockburst potential Excavation size Draw height Mining Sequence 13 Primary Support Excavation stability Rockburst potential Brow stability 14 Practical Excavation Size Rockmass strength Insitu stress Induced stress Caving stress Secondary blasting 15 Draw Method Fragmentation Practical drawpoint spacing Practical size of excavation 16 Draw Rate Fragmentation Method of draw Percentage hangups Secondary breaking requirements 17 Drawpoint Interaction Drawpoint spacing Fragmentation Time frame of working drawpoints No. Parameters Considerations 18 Draw Column Stresses Draw-column height Fragmentation Homogenity of ore fragmentation Draw control Height-to-base ratio Direction of draw 19 Secondary Fragmentation Rock- block shape Draw height Draw rate-time dependent failure Rock-block workability Range in fragmentation size Draw control program 20 Secondary Blasting Secondary fragmentation Draw method Drawpoint size Size of equipment and grizzly spacing 21 Dilution Orebody geometry Fragmentation range of unpay ore and waste Grade distribution of pay and unpay ore Mineral distribution in ore Drawpoint interaction Secondary breaking Draw control 22 Tonnage Drawn Level interval Drawpoint spacing Dilution percentage 23 Support Repair Tonnage drawn Point and column loading Secondary blasting 24 Extraction Mineral distribution Method of draw Rate of draw Dilution percentage Ore losses 25 Subsidence Major geological structures Rockmass strength Induced stresses Depth of mining Source: Laubsher Chapter Two -Production Process 2.1 Block Cave Mining System In a Block Cave Mine there are four major levels that contribute to the production of the mine. The levels that have been taken into account here are Extraction Undercut Haulage Ventilation In a natural progression of a block cave mine the infrastructure that need to be built before the start of caving includes Primary access to the production levels (ramps and shafts) Extraction level excavations Haulage and Ventilation level excavations; and Crushing and ore transport facilities. While most of these excavations need to be created before the start of caving operations, construction of some extraction, haulage and ventilation level drifts can be planned just in advance of actual caving operations. Each of these levels is given a brief introduction and the production process for each level are outlined from collecting data from different sources. The information flow in the form of a flow chart is provided for ease of understanding the process. The information flow chart provided is implemented from personal experience and its objective is to provide an impression on the production process of an underground block cave mine. 2.2 Extraction Level The extraction level is the main production level in a block cave operation. All the ore from the block cave is drawn through draw points at the extraction level and then transferred to haulage level through a system of ore passes or a fleet of LHDs. Since this is the main production level, it is developed and supported to counter the stresses and displacements that can be expected during the life of the drawpoints at the level. The arrangement of drawpoints, drawbells and other excavations on the extraction or production level is referred to as the extraction level layout. The development of the extraction level and the drawbells creates two types of pillars. The major apex is the shaped structure or pillar above the extraction level formed between two adjacent drawpoints but separated by the extraction or production drift. The minor apex is the shaped structure or pillar formed between two adjacent drawbells on the same side of the extraction drift. The drawpoint spacing, the drawpoint width, and the distance between the undercut and extraction levels are all designed based on the fragmentation expected within the block cave. The ground support installed in the excavations at the extraction level is based on the characteristics of the rock mass and the expected stress levels at different locations. 2.2.1. Drawbells The ideal shape of the drawbell is like a bell, so that ore can flow to the drawpoint. However it is a compromise between strength and shape. The major and minor apexes must have sufficient strength to last out the life of the draw. It needs to be established how much influence the shape of the drawbell has on interaction. It has always been an empirical point that shaped drawpoints improve ore recovery as the ore should have better flow characteristics than a drawbell with vertical faces and a large flat top major apex. The time consuming operation is creating the drawbell. The undercut technique also determines the shape of the major apex and importantly the shape of the drawbell. The draw rate from the drawbells is an important factor in that it must provide space for caving; also it must not be too fast to create a large air gap and possible air-blasts. If the draw rate is too fast seismic activity will occur. Production must be based on this value and not rely on economic factors such as short term return on investment that ignores long term consequences. There is also the fact that a slow draw rate will mean improved fragmentation. 2.2.2 Extraction Level Production Process Planning Design Equipment/People Decision Making Ground Support Drawbells Drifts Ground Support Development Pathways Ventilation Ventilation Blast Hang ups Drawpoint Undercutting Secondary Blasting Ore Removal LHDs Ore pass full Ore Pass Haulage Level Secondary Ore pass Crusher 2.3 Undercut Level The process of undercutting creates instability at the base of the block being caved. Block cave mining is based on the principle that when a sufficiently large area of a block has been undercut by drilling and blasting, the overlying block of ore will start to cave under the influence of gravity. The process will continue until caving propagates through the entire block surface or to the open pit above, unless a stable shape is achieved. The purpose of the undercut level is therefore to remove a slice of sufficient area near the base of the block to start the caving of the ore above. The undercut level is developed at the base of the block to be caved. The caving of the block is initiated by mining an undercut area until the hydraulic radius of the excavation reaches a critical value. As the broken ore above it will collapse into the void so created. Vertical propagation of the cave will then occur in response to the continued removal of broken ore through the active drawpoints. The horizontal propagation of the cave will occur as more drawpoints are brought into operation under the undercut area. 2.3.1 Undercutting Undercutting is the most important process in cave mining. As not only is a complete undercut necessary to induce a cave, but the design and the sequencing of the undercut is important to reduce the effects of the induced abutment stress. It is essential that the undercut is continuous and it should not be advanced is there is a possibility that pillars will be left. This rule which is often ignored owing to the problems in re-drilling holes, results in the leaving of pillars resulting in the collapse of large areas and consequent high ore losses. The undercut technique also determines the shape of the major apex and importantly the shape of the drawbell. Care must be taken that there is no stacking of large blocks on the major apex as this could prevent cave propagation. 2.3.2 Undercutting Techniques Conventional The conventional undercutting sequence is to develop the drawbell and then to break the undercut into the drawbell. Henderson Technique The Henderson Mine technique of blasting the drawbell with long holes from the undercut level just ahead of blasting the undercut reduces the time interval in which damage can occur. They have also found it necessary to delay the development of the drawbell drift until the drawbell has to be blasted. Advance Undercut The advance undercut technique means that the drawpoints and drawbells are developed after the undercut has passed over, so that the abutment stresses are located in the massive rock mass with only the production drift. 2.3.3 Undercut Level Production Process Design Planning Development Equipment/People Decision Making Ground Support Ventilation Drifts Pathways Undercutting Ore Removal Haulage Level LHDs Crusher Muck Removal LHDs Ore Pass Waste Dump 2.4 Haulage and Ventilation Level The haulage and ventilation levels lie below the extraction level. They need to be developed with adequate excavations to handle the quantity of broken ore and ventilating air streams required for the designed production rates, equipment and manpower employed within the block cave. Facilities for storing, crushing and conveying the broken ore to the mill need to be developed at the haulage level. The larger excavations required for the crushers, ore bins and conveyor transfer stations need to be located outside the zone of influence of the stresses due to the block cave, and adequate ground support will need to be installed to ensure that the excavations are stable during their expected life. The excavations and levels must be placed far enough apart so that there is limited interaction between numerous excavations created to move the ore from the production level to the milling facilities at the surface. 2.4.1 Haulage Level Much of the development of the infrastructure for a block cave operation is completed during the pre-production stage though some haulage lines and ventilation drifts and raises may be deferred to later in the life of the block cave. Scheduling the development of haulage and ventilation drifts needs careful planning so that the required facilities are in-place well in advance of their requirement. Though there is some flexibility in the development of these levels since they are different elevations and lie below the extraction level, the preliminary layouts need to be prepared so that the flow of materials, ore and ventilating air can be integrated without interruption as the block cave progresses. 2.4.2 Ventilation Level Ventilation Levels are normally developed between the haulage and the extraction levels. During the development phase air is streamed through the undercut and extraction levels to the working faces and exhausted through the raises to the ventilation level. During production, air is coursed through the extraction level and exhausted through the ventilation raises to the exhaust side of the ventilation level. Additional air is provided at the working areas through ventilation raises which connect to the intake of the ventilation level 2.4.3 Haulage Level Information Chart Scoop Ore Removal Haulage Level Haul Distance Optimization LHDs Crusher Figure Haulage Level Information Chart 2.4.4 Ventilation Level Information Chart Auxillary Ventilation Intake Raise Exhaust Raise Fresh Air Exhaust Air Drifts Pathways Fans/Vent Ducts Chapter Three Production Control 3.1 Departments in a block cave mine involved in Production Control Design Planning Geology Geo-technology Ventilation Maintenance Cave Development/Production Survey Construction Electrical Mechanical Human Resource Safety In a Mine Environment each and every department plays a crucial role to keep the Mine running and to meet the production targets. Problems associated with these departments no matter how small they may be contribute damage in their own way to dampen the production. Production planning for block cave operations can be complex. The factors to be considered include geotechnical constraints, cave shape, draw point development sequence, draw point productivity, production block limits such as loader capacity and ore pass capacity and variable shut-off grade mining costs. The nature of the problem also changes during the life of a cave from initial production build up to final closure. Overall objective for production planning should be to maximize productivity, some of the aspects of production planning include Minimum/Maximum tonnage per period Maximum total tonnage per draw point Ratio of tonnage from current drawpoint compared with other drawpoints. Height of draw of current draw point with respect to other drawpoints Percentage drawn for current draw point with respect to other drawpoints Maximum tonnage from selected groups of drawpoints in a period. 3.2. Production Control Major Concerns 3.2.1 Fragmentation Rock fragmentation is the fragment size distribution of blasted rock material, in caving operations fragmentation has a bearing on Drawpoint spacing Dilution entry into the draw column Draw control Drawpoint productivity Secondary blasting/breaking costs Secondary blasting damage Primary Fragmentation Caving results in primary fragmentation which can be defined as the particle size that separates from the cave back and enters the draw column. The data to be considered for the calculation of the primary fragmentation is In situ rock mass ratings Intact rock strength Mean joint spacing and maximum and minimum spacing Orientation of cave front Induced stresses Secondary Fragmentation Secondary fragmentation is the reduction in size of the primary fragmentation particle as it moves down through the draw column. The processes to which particles are subjected to, determine the fragmentation size distribution in the drawpoints. The data to be considered for the calculation of the primary fragmentation is The effect of fines cushioning Draw strategy and draw rate Rock block strength Shape of fragments Frictional properties of fragments Column height Fragmentation is the major factor that determines productivity from a drawpoint. Fine material will ensure high productivity. 3.2.2 Draw control Draw control is one of the major concerns that need to be optimized in order to increase productivity of the mine. Geomechanical issues related to draw control have played a dominant role in efforts to reduce stress and improve fragmentation and reduce dilution. Draw control is the practice of controlling the tonnages drawn from individual drawpoints with the object of Minimising dilution and maintaining the planned ore grade. Ensuring maximum ore recovery with minimum dilution. Avoiding damaging load concentrations on the extraction horizon. Avoiding the creation of conditions that could lead to air blasts or mud-rushes. The following have to be considered for draw control strategy in order to maximize productivity, Any factors observed during the start of caving that will influence the planned caving and drawdown processes. Control the draw from the first tonnage into the drawpoint. Define the potential tonnages and grades that will be available from each drawpoint. The draw control system must be fully operational. Confirm that the planned draw strategy is correct. The recording and analysis of the tonnages drawn, this important aspect is often not treated with the required respect. Managing the draw by following the adopted draw strategy. Define how the control is to be monitored, maintained and audited. Planning for how the draw column would behave with time. An estimation of the remaining tonnages and grade for future production scheduling and planning. Personnel must be aware of the definition of isolated drawpoint. Ensure the drawpoints are clearly and correctly identified underground. There must be reporting system to record and describe why allocated drawpoints have not been drawn. Ensure secondary breakings are done effectively and efficiently. Develop standard procedure for close drawpoints. Draw control is what block caving is about, the reasons for and the principles of draw control must be clearly understood by all operating personnel. Preparation of orebody must be done in a sound way so that preventable problems do not hamper the draw control. 3.2.3 Secondary Breaking Irrespective of the method of primary blasting employed, it may be necessary to reblast a proportion of the rock which can then be handled by the loading, hauling and crushing system. There are four types of problems that cause a need for secondary breaking, High hang-ups are where a large fragment lies across the entrance to the draw bell up to 19m above the footwall. This type of hang up is very rare though, and it is more common that this will only occur up to a distance of 5 m above the draw point floor. Rock jumble is where several ore fragments of rock smaller than two cubic meters form an arch in a drawbell. This is found to occur especially in the troat of the drawpoint. Low hang up is a large fragment of over two cubic metres hanging in the troat or on the floor of a draw point clocking the flow of ore. Draw point oversize is any large fragment over two cubic metres on the floor of a draw point and effectively prevents loading by LHDs. Some of the techniques that are in use for secondary breaking are as follows, Concussion blasting Drill and blast Emulsion secondary blasting Robust hydro fracturing breaking system There are many products on the market today that promise effective secondary breaking of both hang-ups and boulders, including cone packs, the quick draw system, the boulder buster and the penetrating cone fracture technique. In order to choose a secondary breaking method with respect to productivity the following need to considered and evaluated, Explosive quantities Labour and Equipment requirements Fragmentation Safety 3.3 Significance of Production Management