Double Post Mining. A New Mining Method: Written By:

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1 A New Mining Method: Double Post Mining Written By: Industrias Penoles; Ing. Octavio Alvidrez Ing. Javier Berumen DPM Mining Inc; Charles Gryba, P.Eng EDC Mining Ltd; Ivan Arriagada, P.Eng Michael Arriagada, P.Eng 0

2 Abstract As part of Penoles strategic vision a decision was made to search the globe for alternative mining methods which would vastly improve production and safety at their mines. A dinner meeting at the PDAC in Toronto in March 2010 between Ivan Arriagada, then with Wardrop (Tetra Tek), and Armando Sanchez of Penoles led to a discussion of Double Post Mining (hereafter referred to as DPM ). This led to a DPM presentation for senior members of Penoles mining team followed by a positive cost and technical study, which ultimately led to the current phase of DPM test mining at the Madero Mine in Zacatecas, Mexico. DPM is a patented mining method that generates room and pillar productivity in midsized underground orebodies. Mine planning starts by generating a 7.5m x 7.5m x 6m high block model, with mining progressing from the top down in 6m lifts. The initial lift (the Top Slice ) is standard drift and fill mining, but prior to CRF backfilling concrete posts are inserted into the ground and concrete floors are poured. On completion of top slicing one ends up with a continuous reinforced concrete floor, which then becomes the roof of the next lower lift, supported by a 7.5m grid of posts. A combination of inserted and filler posts allows for a continuous system to set up as part of the mining/backfill cycle; mining and backfilling proceed in parallel. With DPM, 100% of the orebody is mined. Subsequent mining is similar to working in a car parkade with a concrete roof and posts. DPM substantially reduces the amount of scaling, rock bolting, cable bolting, shotcreting, long hole drilling, slot raising etc, thus safety statistics will improve. Economic factors such as 50% less capital development, earlier production, better dilution control, 100% ore recovery, higher labor and equipment productivity plus improved safety more than offset the cost of posts, concrete and rebar. Golder FLAC 3D modeling indicates that 15m wide panels (stopes) can be set up; primary panels are mined and backfilled to form pillars so the secondary panels can be mined and backfilled. All aspects of mining and backfilling over the life cycle of the DPM area can be monitored with load cells in real time, thus ensuring safety and the possibility for future optimization as the method is being mine-proven at the Madero mine. This paper will explain the DPM mining method, the Golder modeling of the concept, the instrumentation to be used in the mine and the advantages of the DPM process. This will be followed by a description of the research and development process Penoles followed that led to the decision to mine prove the DPM mining system at the 7000 tpd Madero base metal mine. 1

3 Introduction: DPM Mining Method DPM mining is based on mining 6m lifts of 800 to 1200 ton blocks of ore generated by a Gemcom or Datamine 3D geological block modal. Each DPM room is mined by 2 drift rounds or a combination of drift rounds and slashes that dimensionally match the geological block model; the model becomes the stoping plan for the orebodies with 100% ore recovery. DPM mines the orebody from the top down. The initial lift utilizes standard drift and fill mining except a grid of 7.5m concrete posts and a continuous concrete floor is installed prior to backfilling with cemented rock fill (CRF). Lower lifts are similar to room and pillar mining but carried out under a concrete roof temporarily supported by a grid of concrete posts. As with any new technology there are a few new terms that have been developed to explain the system e.g. DPM top slicing, DPM rooms, double posting, pre breaking around posts and filler posts. Figure 1: Typical DPM Rooms and Terminology DPM is a very flexible mining method that can use drill blast muck techniques for hard ore and roadheaders for softer ores. Mining can be done in any direction under the concrete floor and it can extend out past the concrete to follow the ore this new area then becomes a top slice. Every DPM room within the orebody will have exactly the same standard design. The outer perimeter rooms have the addition of wall pins and rebar hangers to support the perimeter of the concrete floor slab. 1

4 Figure 2: DPM Post Layout Plan View Figure 3: DPM Post Layout Section View E Elev L 1932 L 1926 L Elev L 1914 L Elev.1900 Elev.1875 Elev.1850 The backfill cycle is very standardized; install the posts, prepare and pour the concrete floors, then fill with CRF. Posting starts with drilling a grid of post holes surveyed to match the corner location of each ore block. A precast concrete post is than installed into each hole, followed by drilling pre-shearing holes around the post. Preparation for installing the concrete floor starts with spreading a layer broken followed by a layer of plastic; the ore acts as a cushion to prevent blast damage to the concrete roof while the layer of plastic keeps wet concrete from leaking into the cushion material. At this time filler posts are installed in the DPM lifts they are bolted to the bottom flange of the post from the previous lift forming the double posting system. Rebar and welded concrete mesh can now be installed, followed by special concrete forms that are backfilled with sand. Removing the sand after the adjacent room is mined allows the rebar to be over lapped, thus forming a continuous concrete floor. Standard 3000psi concrete is pumped to complete the reinforced slab. Once the concrete floor sets the CRF is tight filled using a push blade on an LHD plus a Paus Slinger truck for the nooks and crannies. The DPM mining and backfill cycles use only standard mine proven equipment, concrete and CRF. Subsequent DPM mining is then carried out under the pre-posted composite roof beam comprised of reinforced concrete plus tightly-packed CRF. Ivan Arriagada and his current company EDC Mining Ltd. has been involved in developing DPM from the beginning and has now been retained to provide the detail engineering required to bring DPM to completion. However, before production could begin, there remained two large question marks: what is the loading on the concrete posts; and does post loading increase with each additional mining lift? Golder Modeling Joe Carvello, PhD of Golder Toronto was retained to model the test mining area using FLAC 3D. Based on previous 2D modeling 0.4m diameter concrete posts and a 7.5m x 7.5m x 6m room size was fixed. An 8 room wide x 12 room long by 5 lift high (or 400,000t) area was selected to allow for maximum load development within the backfill; excavation is via primary and secondary panels 2 rooms (15m) wide accessed from a 2

5 central entry drift. The concrete floor was modeled only as a tension member as the concrete floor plus cemented rock fill act as a composite beam. Figure 4: Golder Modeling Plan A total of 10 computer runs were performed using various stiffness for the backfill, posts and floors; each run taking about 120 to 150 hours to completely mine the 480 blocks. Snapshots of data results were captured every 15 minutes for analysis. Figure 5: Golder FLAC 3D Model Results Some of the results were: 1. Normal 6% cemented rock fill generated post loading mainly between 100t and 250t and the loads stabilized after 4 lifts. Posts were designed for 400t thus post loading is about 50% of the design strength of the posts in compression. 3

6 Figure 6: Golder FLAC 3D Post Loading Results 2. To mobilize the backfill strength of typical 6% CRF the posts had to be compressible; weaker fills have to move further to arch loads to the walls thus causing more post compression. DPM has designed 400t capacity compression springs that can be adjusted to match the required movement. 3. The concrete floors act only as a tensile member to confine the CRF and the loads arched as predicated. Backfill arching is seen on 2 scales initially it remains within the DPM rooms; as additional lifts are mined it expands to cover the lift. 4. Surprisingly with weaker fills the tensile loads on the posts in the backfill reduced to 300t. The concrete posts in effect become large friction rockbolts in the composite CRF beam. To take advantage of this anchoring phenomenon the posts were redesigned with flanges to attain a continuous 150t tensile strength for individual posts and 300t for double posting. Wardrop was then retained to provide an initial civil design for the posts and floors. As actual mine data is collected the Golder model will be updated to verify results and to find improvements in the DPM system. Instrumentation Through the years many attempts have been made to fully instrument a mine to provide useful, real-time feedback with regards to loads, stresses, etc. DPM provides the framework for this type of instrumentation coverage. Dr. Andrew Hyett of Yield Point is on board to design the instrumentation package in conjunction with DPM Mining Inc, EDC Mining Ltd and Golder. The main item to be instrumented is the concrete post loading as one goes through the mining and backfill cycle. However this alone will not provide a snapshot of what is happening within the backfill and concrete floors for example is the fill separating from 4

7 the stope back while the backfill arches? This type of technical questioning soon lead to list of the various items that had to be monitored with unique instrumentation to provide the necessary answers. Figure 7: Instrumentation Layout A summary of the instrumentation installed in a quadrant of the test mine area or 9 sets of posts is as follows: 1. Instrumented cable bolts installed in the back above 9 post locations to measure the movement of the hanging wall or the convergence of the HW into the backfill thus loading the backfill. Similarly cables could be installed in the perimeter walls to see if the walls converge into the backfill 2. Similar instrumented cables anchored at the back, extending through the CRF and bolted to the top of the 9 posts supporting the top concrete floor will measure the elevation of the concrete floor vs. the back to see if there is any separation of fill from the back. 3. Instrumented cables will measure a range of tensile loads in key areas of floor slab loading to monitor the tension in the rebar. Cables can also be installed around the perimeter of the floor slab to see what stresses are encountered near the edge of the floor. Similarly by draping cables over a 2 inch diameter wall pin with the ends anchored in the floor slab the loading along the walls can be measured. 4. The concrete post compression movement and post loading will be measured by the reduction in height of the compression members below the posts. The concrete posts have been designed with a conduit pipe to allow instrumentation wires to run though the post and through conduit imbedded in the concrete floor slabs. Post compression pads bolt to the post bottom flange and are reusable. 5. The tensile loading of the post can be measured in several ways, instrumented cable bolts cast in the concrete parallel to the rebar or a standard mine extensometer could be installed into a conduit in the post and anchored to the top and bottom steel flanges. 6. Instrumented 3/4inch dia. flange bolts will be used between the instrumented posts to monitor tensile loads from one post to the next. 5

8 7. The Golder 3D modal shows the backfill loads arching to the walls. Custom instrument packs are being developed to monitor the loads within the backfill to ensure the arching is developing as predicted, to check if the backfill is separating from the floor or back, and to monitor in real-time what is happening as the backfill is being compressed (packed) into place. 8. Tilt meters will be located in various areas of the concrete floor to see how the floor is bending near the concrete posts or how the floor edges bend as one goes through the mining or backfill cycle. All of the instrumentation that leaves the Yield Point factory is calibrated with it s own on board computer and battery power supply. Each instrument has its own custom data file thus downloading data from a number of instruments automatically feeds into the proper data file. Results can be transmitted to Golder at regular intervals as each lift is mined to update the 3D model. DPM Changes Mine Planning DPM mining provides a new mining method that has the potential to totally revolutionize underground mine planning of midsized orebodies. The key breakthrough comes from the small stope size - 7.5m x 7.5m x 6m - that has a reinforced concrete roof held up by 4 large concrete posts. The individual blocks in the initial geological block model now becomes the stoping plan! DPM was developed 25 years ago but until recently computer modeling wasn t powerful enough to recalculate the redistribution of loads every time a drift round was removed in an individual DPM room. The Golder FLAC 3D modeling answered many of the what if questions: what is the loading on the posts? Does the loading increase with each lower lift? How strong does the backfill have to be? How thick do the concrete floors have to be? With these questions now answered, DPM and all the advantages associated with it can now move forward. DPM Mining Inc. is approaching the initial mine in a very methodical way to eliminate risk in the process as much as possible. We are in the early stages but the benefits to the mine owner are immediately apparent these are listed below: 1. DPM mine planning - The mine plan for DPM mining is the geological block model; all that is required is access to the top 6m high mining lift and a second access for ventilation and egress. Mining and backfilling of 100% of the 6m lift proceeds in parallel. A safe planning rule of thumb is that an orebody can support a 1000tpd mining rate per 100 ore blocks with the number of blocks known the mining rate can be estimated and then the mine infrastructure designed to suite. Parallel mining and backfilling plus 100% of the ore lift in production gives a much higher mining rate per million tons of orebody compared to other mining methods such as blasthole or cut and fill stoping. 2. Following the Ore - the normal mine planning process of designing and scheduling stopes and pillars is an iteration process; planning various scenarios takes time and a change in orebody size or shape or a change in metal prices 6

9 requires a complete redesign. The versatility of DPM means that mining can halt at any point under the concrete floor if the orebody ends or the grade diminishes. Similarly mining can continue past the concrete to follow the ore, in effect becoming a new top slice. This means that a change in the shape of the ore body or grade will not affect production or require a redesign. Also, if ore values increase, a road header can drive through the backfill to reach now profitable ore at the far end of the ore body. 3. Elimination of Work DPM eliminates most ground control functions such as rock bolting, cable bolting and shotcreting (except for the top slicing). Other mining functions like cut lose raises, long hole drilling and the equipment to carry out the functions are reduced. DPM also eliminates a lot of higher cost mining functions primary, secondary and sill pillar recoveries, fill fences or bulkheads etc. Most mines spend 30% of their labor and material on ground control. Ground control work also reduces development advance rates by 30 to 50% - more development footage or headings, more delays. By eliminating development work, both productivity and safety statistics improve by that percentage. 4. Ore Recovery - The initial geological block model with conventional mining methods is usually chopped by 20% or so by the mining engineers as the size of stopes and pillars don t necessarily follow the orebody. Room and pillar or post pillar mining methods leave an additional 20 to 30% of the orebody behind. DPM recovers 100% of the ore identified by the geological block. DPM mining can also remove internal dilution as well, thus the mining grade can be higher than the original block model average geological grade. Room grades are confirmed by mapping, face sampling and post hole chip sampling. The orebody can be mined selectively with minimum of internal and wall dilution. Economic mine life of Madero could be extended 4 or more years, plus DPM ore grade may substantially increase mill revenue. Other Penoles mines may have a similar increase in mine life or revenue per ton milled. 5. Capital Development Cost DPM mines the orebody from the top down; pre production waste development is limited to providing access to the top 6m lift or multiple locations depending on the size or shape of the orebody. Two other factors come into play less development leads to quicker ore production plus a higher mining rate is achieved earlier. Operating revenue reduces the capital cost dollar for dollar thus the ROI of the project is substantially increased. 6. Mechanized Mining DPM provides room to maneuver large road headers and the concrete roof eliminates falls of ground. Ground that is soft enough to cut with a roadheader usually limits the safe size of openings; the DPM concrete roofs and posts eliminate most ground imperfections. If there is a combination of weak and hard ore the hard sections can be drilled and blasted. 7

10 7. Cemented Tailings Fill - Future development of DPM will examine other opportunities for improvement, such as using paste fill to replace CRF. Using paste fill the posts may have to compress 250mm and post spacing may have to be reduced to 6m x 6m. Once the Golder 3D model is calibrated by mining with stiff fill, weaker fills can be modeled. 8. Safety Reducing accidents is a complex operation; the largest source of accidents is development work, scaling, rock bolting and other ground control functions. Falls of ground, falls of backfill or unexpected pillar or back failures, working on broken ore, runs of fill, driving raises etc are all source of injuries. In base metal mines large stope blasts often cause dust explosions. DPM creates a shop like work environment that can be monitored, uses large equipment with high productivity and reduces the number of miners underground. New hazards such as tripping on rebar or chemical burns from working with concrete will have to be identified and managed. Penoles Approach to Research and Development Penoles has been underground mining in Mexico for 130 years. With multiple mines in production, Penoles has continuously purchased all of the modern mining equipment as it became mine proven, including raise bore machines, roadheaders, electric hydraulic drills, automated shotcrete and cable bolting machines etc. Modern equipment has proven to be only a partial mining solution. Productivity and cost savings have plateaued for all conventional mining methods such as room and pillar, cut and fill and blast hole stoping. Rather than just funding R&D Penoles started searching globally for mining systems that had the potential to make a fundamental change on how they mined. Javier Berumen, a very experienced Penoles mining engineer, remembered DPM mining from the CIM 2000 Conference in Toronto. Armando Sanchez approached DPM mining at the 2009 PDA in Toronto via Ivan Arriagada who was Senior Mining Engineer for Wardrop Tetra Tech. A DPM Wardrop presentation was made to 20 senior Penoles technical and operating staff in Torreon. Penoles, Wardrop and DPM mining had a series of meetings to pick the test mine location and secondly to design a program to independently 3D modal and cost DPM mining using the Madero Mine cost components. Subsequently several meetings were held with Golder to design FLAC 3D program parameters. Once the 3D Golder modeling and Wardrop preliminary civil engineering was complete; Wardrop mining engineers were able to estimate costs and recommend a test mining project to confirm the 3D modeling parameters. Penoles chose the Madero Mine in Zacatecas for the test mine location. Madero is Manto type low grade 3% zinc, 2% Pb and 1 opt silver mine. The NSR dollar value of the ore at the time test mining location was chosen was $47 per tonne. The mine has been in production for 10 years, and mills at 7,000 tpd. The main mining method is room and pillar mining with 6m wide rooms and a grid of 6m x 6m post pillars; development waste is used for fill when available. 8

11 The initial test mine location covered a 7 room by 12 room plan area with separate ramp access to each of 5, 6m high mining lifts or about 470 rooms. Updating the ore reserves plus extending the test mine along strike has tripled the number of rooms to The DPM test mine and the Golder 3D model have similar plan dimensions thus allowing rapid updating of the FLAC 3D model. Mini Test Mine From both a safety and cost viewpoint it is critical to have hard engineering data before top slicing a 1500 room block of ore. One major advantage of DPM is that it is scalable. Based on the Golder 3D modeling the maximum backfill loading is on the 1 st lift and based on the post pattern the maximum area loading a single post is 4 rooms. All aspects of mini test mine can be instrumented thus obtaining real data for a final set of civil calculations. The test mine also allows for field proving all operational and safety aspects of both the mining and backfill cycles. The initial post can be a small diameter steel post with a top flange that is the same diameter as the production posts. The concrete floor in the 4 rooms would have an initial 1m x 1m pattern of 25mm rebar with a couple of layers of concrete mesh. The edges of the concrete slab will be suspended from 2 inch diameter pins with hangers made from 25mm rebar. The floor rebar, edge rebar, post compression pad and backfill can all be instrumented to see what happens as more and more concrete floor is exposed. Results from the calibration of this small scale test mine can be used to eliminate risk from the large scale 5 lift mine. Wardrop Cost Study Once the Golder modeling was complete, the main parameters fixed and a preliminary civil design completed, the mining and backfill productivities were estimated. DPM mining is one of the easiest mining methods to cost because each DPM room is exactly the same. Madero actual costs for labor and material were used for the Wardrop cost estimates. Typical components for estimating the cost of top slicing and the cost of mining a DPM room are summarized in the tables below: Table 1: DPM ROOM PARAMETERS Room Width 7.5m Room Length 7.5m Room Height 6.0m Area 56.3 m 2 Volume 337.5m 3 Ore Specific Gravity 3.00t/m 3 Tonnes per Room t 9

12 Post to Room Ratio (Top Slice + 4 DPM Levels) 1.11 Table 2: COST SUMMARY PER DPM ROOM Task Description Top Slice DPM Top Slice Drift 5 m x 6 m H. x 3.75 m round (3 rounds) $9,051 DPM Drift 6 m x 6 m H. x 3.75 m round (2 rounds x 2 slashes) $5,553 Drill Post Holes 0.60 m diameter x 5.5 m length $645 $645 Insert Post 0.50 m diameter x 6.5 m length $690 $690 Drill & Blast Helper Holes 30 mm diameter x 6m $369 $369 Stood Post 0.50 m diameter x 5.5 m length $567 Ventilation Manway and Services $87 $87 Concrete Floors 0.25 m, 19m 3 (materials and labor) $3,483 Concrete Floors 0.25 m, 19m 3 (materials and labor) $3,483 Tight Fill Room 6.0% cement, 700 tons fill $1,751 $1,751 Total Cost per Room $16,076 $13,145 Cost per Ton $15.88 $12.98 Weighted Cost per Ton (Top Slice + 4 DPM Levels) $13.43 TABLE 3: AVERAGE MINING CYCLE PRODUCTION CYCLE Man Shifts Man Shifts /Room Drill- Blast Muck 2 rds per DPM room BACKFILL CYCLE {POSTING = UNIT WORK X POST ROOM RATIO) Drill post holes 1 man 3 holes per shift Insert Posts 3 men 3 posts per shift I Drill & Blast Helper Holes 1 man 3 posts per shift Stand posts 3 men 3 posts per shift Vent Manway and Services Every 16 rooms plus extensions Pour concrete floor Rebar screen, level concrete etc Tight fill room Tonnes/mans hift: In summary, DPM mining and backfill costs will be approximately the same as Penoles current mining cost per tonne. The main economic factor is that the DPM ore recovery increases 30%. A second major advantage is that while top slicing the post location can be test drilled to a 30m depth by deepening a helper hole. This allows rerunning the block model to identifying sub ore grade blocks that can be mined, used as backfill or left in situ; whichever is more cost effective. The price of cement and rebar are two of the major cost components. Once CRF test work is done a 4 room by 2 lift DPM test stopes will be set up and test mined to confirm the steel design, the Golder 3D assumptions, safely check critical components and 10

13 operational procedures by actual test mining on a small scale. DPM room costs can then be re calculated using accurate field data for cement and steel. The capital cost of the specialized equipment required for DPM mining and backfilling is about $4,000,000. With current metal prices the profit margin of mining and milling; an extra 150,000t of ore more than pays for the extra DPM test mining costs. Applying the 30% extra recovery to the current Madero proven and probable reserves indicates mine life would be extended by 4 years at the current 7000tpd milling rate. The decision was made to go ahead with the DPM test mine. DPM Summary DPM offers many benefits when compared to standard methods used today. These advantages to DPM are highlighted in the following list: Improved Safety Majority of work is performed under a pre-posted, continuous concrete floor which eliminates most ground control issues. Combined with minimizing development work and real-time feedback, these result in a much safer work environment. 100% Ore Recovery Easier Mine Planning The block model becomes the mine plan. Block Model Updating as post holes/drift rounds are drilled, the ore can be analyzed and block model updated to ensure accuracy. Reduction in Dilution DPM allows for chasing the ore so the mine can halt if ore grade drops, which leads to; Increased Ore Recovery if a higher grade is found the mine can continue to follow the ore in whichever direction is required. Minimal Development Work Since DPM works from the top down, the ore is reached much quicker. Also, the use of internal ramps can further reduce development. Improved Productivity Wide spans (made possible by the concrete ceiling) and multiple faces allow the drill/blast/muck, posting and backfilling cycles to be carried out in parallel which leads to improved productivity. Control of Big Muck DPM produces only drift muck. Real-Time Feedback with DPM the mine can be fully instrumented; posts, floors and backfill all contain sensors providing real-time loading/stresses to the mine engineer ensuring a safe work environment. While the two main benefits to DPM mining are 100% ore recovery and much improved safety, the combination of all the above factors results in quicker start-up/production, improved productivity, improved quality, and reduced costs, all while maintaining a safe work environment. 11