Evaluación global de las mejores prácticas de relleno de minas subterráneas. Presented by: Dr. David Stone, P.E. President MineFill Services, Inc.

Transcription

1 Evaluación global de las mejores prácticas de relleno de minas subterráneas Presented by: Dr. David Stone, P.E. President MineFill Services, Inc.

2 Presentation Overview Backfill in the Mining Cycle Common Roles for Mine Backfills History of Mine Fills Early years for Paste Fills The science of Paste Fills Backfilling Best Practices Adoption of Paste Fills globally Role of the Mining Stakeholder Recent Events The new reality and future of Mine Fills

3 A Quick History of Modern Backfills 1970 s - in the beginning we had hydraulic fill 1980 s - there was a need for a better fill early attempts to make paste Late 1980 s to early 1990 s - the birth of paste rheology 1990 s Rapid evolution of thickening and dewatering equipment Improvements in pumping Improvements in the science of rheology 2000 s to now paste backfill accepted globally

4 Backfill in the Mining Cycle Cure Drill Fill Blast Barricade Muck

5 Common Types of Backfill in use Today ROCKFILLS HYDRAULIC FILLS PASTE FILLS

6 Backfill as a Void Filler In many instances backfill is simply used for filling voids, e.g. room and pillar mining without pillar recovery, overhand cut and fill mining. Reduce need for surface impoundments Easier project permitting Lower water treatment costs Reduced closure costs and liabilities Can selectively place high sulfur or ARD tailings underground Fill generally does not carry much load only provides a working floor Uncemented hydraulic fills commonly used

7 Fill for Pillar Recovery Most mines use backfill for increased ore recovery Works best in primary stopes in open stoping operations (e.g. large blasthole stopes, longhole, vertical crater retreat, sub-level stoping etc) Cement must be added to provide stable vertical faces during exposure of fills Uncemented fills in secondaries unless plan to undercut Typical strengths > 0.5 Mpa Can encompass all types of fills: CRF, CHF, CPF Fills generally not designed to carry external loads (other than self-weight)

8 Fills for High Stress Ground Paste works well in high stress ground because it is not as stiff as CRF Paste fills not brittle even at 4MPa UCS the yield strain ~ 0.5% -- equivalent to a closure strain of 500mm in a 10m span Paste fills can absorb more strain than CRF Design is complicated need to build numerical models in FLAC3D

9 The History of Mine Backfills Backfill has been used in mining since the 1500 s. Several mines in Mexico go back to the days of Spanish conquistadors. Historical fills included any kind of trash, timbers, or mine waste to fill mine voids and provide a working floor for overhand mining Between 1950 and 1970 the use of hydraulic (sand) fill (HF) expanded rapidly due to the availability of tailings Low cost Portland cement introduced in the 1960 s allowed cemented HF which allowed recovery of pillars However HF did not come without problems Massive volumes of dirty water Very high cement consumption As comminution technology improved grinds got finer and there was less sand to make HF

10 History (continued) Very early paste systems can attribute their beginnings to the Tailspinner manufactured by Joy Manufacturing Company of Cambridge, ON Centrifuge produced a filter cake no more than 25% c/w water Solids were pneumatically stowed or pumped into stope Patent granted in July 1978 to W.R. Wayment First placed into use in South Africa in Wits basin as a prototype Tailspinners formed the basis of the first paste plants in Canada at Dome, South Porcupine However its adoption was limited by its capacity ~ 15 tph

11 The Birth of Paste Backfills 1980 s Prior to 1985 the rheology of paste mixes was very poorly understood. Verkerk in 1988 was the first to examine and publish rheograms for tailings mixes and open the door to understanding paste flow and paste pumping Verkerk also understood that paste behaves as a non-newtonian fluid governed by viscosity and yield stress Early paste systems such as Lucky Friday (1988) had to learn by trial and error By early 1990 a number of mining companies and researchers were working on paste rheology -- Mt Isa in Australia Inco in Canada Universities such as McGill, Laval in Canada

12 Paste Rheology becomes a Science The starting point for understanding the rheology of paste mixes that could be pumped came from the concrete industry (INCO circa 1979 Dave Landriault): Definition of paste a solids concentration with point to point particle contacts. Non-settling, non-segregating, non-newtonian Rule-of-thumb paste must have at least 15% passing 20 microns to make a pumpable paste -- based on mix for concrete which is 1 part cement 2 parts sand 3 parts gravel = 1/6 part cement (fines) Measuring rheology - ASTM slump cone to measure rheology later modified by David Boger with his 5c rheometer now improved with digital viscometers Plug flow

13 Paste Rheology becomes a Science Tailings thickening continuum by Richard Jewell (ACG)

14 The Thickening Continuum Consistency De-watering Equipment Slurry Conventional Thickener High Density Slurry High Rate Thickener High Slump Paste Deep Bed Thickener Low Slump Paste Disk Filter

15 Source: Westech-Inc. The Thickening Continuum

16 The Evolution of the Hydro Cyclone First patented in the 1890 s First units were wooden!! Development driven by the coal industry for removal of sand from coal Commercialized by Dorr Company in 1948 Large scale production by Krebs starting in 1955

17 The Evolution of the Thickener Earliest thickeners date back to patents by John Dorr in 1906 Earliest design included the concept of rakes in a circular tank Alcoa and Alcan both developed deep bed technology for producing alumina pastes In 1996 Alcan licensed this technology to Eimco to take the technology to non-alumina industries first Eimco units installed in 1998 Modern paste thickeners are a significant improvement over those early designs increased capacity, larger tanks, better operational control The need for paste thickened tailings put additional pressure on the designers to deliver improvements

18 The Evolution of the Thickener Better rake designs due to increased torque Deep bed designs to prevent bed buildup Better dewatering picket designs Feed wells and feed dilution Flocculent chemistry Today there are roughly 300 units installed ~100 minesites surface paste + backfill 1987 Kwinana red mud thickener 75m dia

19 Deep Cone Thickener Performance ph of carrier fluid Particle size distribution Clay content and mineralogy Temperature Dissolved salts and other minerals Mineralogy of the solids Specific gravity of the solids Particle shapes round vs platey

20 Deep Cone Thickener Performance

21 Improvements in Rake Technology

22 Filtration Technology In use in mine mineral process plants since the turn of the century: for removal of cyanide laden water from gold tailings dewatering of coal products dewatering of mineral concentrates Not much has changed in the design but Productivities have increased Better diaphragm materials Capital costs have come down

23 Types of Filters

24 Pumping Paste Backfill Verkerk was the first to note the conundrum of aggregate paste systems In laminar flow conditions the YS increases as the coarse fraction increases In the non-newtonian range pressure losses go down as the coarse fraction in increased Many of the early paste systems contained aggregates e.g. Bad Grund, Lucky Friday, Boliden Garpenberg In 1992 it was not possible to pump paste Landriault In 1994 it was possible to pump paste 1 km - Brackebusch In 1998 it was possible to pump paste 3 km Brackebush Today several mines pump paste 3-4 km Nowadays the practical limit is taken to be about 2 km due to economics Lucky Friday and many early paste plants learned hard lessons about pumping paste freefalls, abrupt bends, air entry in dropholes, design of couplers, hangers, pipe supports

25 Pumping Paste Backfill 1970 first truck mounted Putzmeister concrete pump with M16 boom and 100mm delivery line Putzmeister Elephant concrete pump at Bad Grund in These pumps could deliver fill horizontally 2,000m

26 Modern Paste Pumps

27 The Early Adopters of Paste Backfill Cannington 1998 First paste plant built by Pastec Stillwater 1997 First DCT licensed under the Alcan agreement

28 The Early Adopters of Paste Backfill Ewarton Bauxite - Jamaica First bauxite paste plant built by Alcan Photo courtesy of FLSmidth Pajingo Gold Australia m diameter with bolted tank design Photo courtesy of Outotec 150 paste plants built at least a dozen plants here in Peru Adding 5 new plants per year

29 Paste Control Technology has Evolved Then Garson plant Now modern paste plant control room

30 Paste Rheology then and Now A modern minesite paste laboratory versus those that like the old way..

31 Paste Backfill Gains Global Acceptance Minefill Symposia Symposia Paste Related Papers 1973 Mount Isa Sudbury Montreal Sweden South Africa Brisbane Seattle Bejiing Montreal Cape Town Perth 22 Paste Symposia Symposia Backfill Related Papers 2014 Vancouver Brazil South Africa Perth Toronto Chile Botswana Perth Ireland Santiago 4

32 Backfill Roles Void filler for disposal of mining wastes tailings or waste rock Provides a working floor for overhand cut and fill mining Increases resource extraction through the recovery of pillars Increases safety in unstable or rock burst prone ground high stresses Decreases subsidence from mining due to filling of voids Specific to Peru Allows for the safe disposal of mine wastes in steep terrain Reduces/mitigates impact of acid drainage Allow underground disposal of sulfide tailings prone to acidity on surface

33 Recent Events -- Impact on Stakeholders Recent case of Mount Polley tailings spill in Canada. 10 million m 3 of water 4.5 Mm 3 of tailings slurry Complete inundation of Hazeltine creek Quesnel/Polley lake impacts Fortunately no heavy metals released In fact the water quality testing never revealed any impacts beyond drinking water standards

34 The Result - Increased Stakeholder Demands Who are the stakeholders: Include government regulators The general public NGO s and Activist groups Financial community that funds mining An unintended consequence of the Mount Polley breach was the quick reaction by Alaskan communities, first nations groups, and activist groups who united to condemn mining. First nations want a say in approval of project designs and project permits, as does Alaska. The failure of a well engineered facility run by a reputable company in a first world jurisdiction with regulatory oversight has unsettled many and provided ammunition for activist and NGO fights against mining projects.

35 The New Reality The Mount Polley Investigative Report has recommended: The elimination of water from tailings impoundments Adoption of Best Available Technologies The outcome is a strong push towards: dry stack surface disposal of filtered tailings Maximizing the use of mine backfills which use tailings The Future Stakeholders will push for zero impacts due to mining Conservation of water Minimal surface disturbance Minimal long term monitoring or remediation after closure Regulators will push for the elimination of wet tailings disposal Mining companies need to adopt technologies that maximize use of tailings based backfill systems such as paste backfill Surface disposal will focus on paste thickened tailings or dry stacking

36 Paste Thickened Tailings for Surface Disposal There has never been a reported liquefaction failure of a thickened tailings (TT) facility Even after several major earthquake events in Chile - no failures reported TT produces a stable facility because The void ratios are much higher than slurried tailings There is no water impounded Dessication and drying produces a self consolidation of the tailings

37 Beach Angle Beach Angles in Thickened Tailings Extremely difficult to predict Laboratory models eg flume tests, have proven to be grossly inadequate Numerical models have likewise proven to be extremely complex Bottom line most beach slopes average about 0.5% to 2% over the length of beach Can get steeper near discharge up to 8% locally but only over very short distances

38 Dry Stack for Surface Disposal MILPO Cerro Lindo

39 Best Practices for Mine Fills Farming of dried tailings from existing or closed tailings impoundments is very popular in Australia Saves on dewatering tailings capital and operating costs Smaller plant that can be mobile Saves on reclamation of old tailings sites

40 Best Practices for Mine Fills - Barricades Barricades serve many roles: CRF confinement of the rockfill no risk of flow HF containment of water and fine solids LQ risk CPF containment of the un-cured paste LQ risk Loading on a barricade is time dependent, material dependent, and barricade design dependent Selection of the ideal barricade is dependent on the short and long term loading: Short term HF or CPF as a fluid Long term drained HF or cured CPF Barricades are generally much stronger if arched up to 3x capacity of a flat fence

41 Best Practices for Mine Fills - Barricades Waste rock with timber cap Timber fence Cable Fence with Welded Wire Mesh Shotcrete Cable Fence with Welded Wire Mesh

42 Best Practices for Mine Fills - Barricades Liquefaction of paste poses a real hazard. There have been over a dozen reported failures. Risk is highest hrs after pouring There have been a number of bulkhead failures due to liquefaction of paste. One incident due to over-pressuring due to a plugged vent line Several incidents due to massive in-stope back and wall failures Several incidents due to excessive filling rates, or not allowing an initial rest period after covering the barricade Several incidents have been traced back to excess water in the paste during pouring due to groundwater inflow or dumping flush water into the paste. Paste mixes at 0.5% are clearly liquefiable, whereas mixes at 1% are considered marginal. Mixes at > 2% are liquefaction resistant The industry standard of >100kPa paste strength appears to be conservative.

43 HF Best Practices - Barricades Barricade loads can be controlled with proper controls on fill placement e.g controlled pouring rates (controlled rate of rise), rest periods, and visual monitoring by CCTV. For HF the initial pour should not exceed 2m above the brow, then allow rest period for fill to fully drain Critical with HF to allow rest periods during pours dependent on size of stope. Do not allow a large decant pool to develop. HF barricade design may need to withstand full hydrostatic pressure to height of stope.

44 Paste Fill Best Practices - Barricades Again barricade loading is dependent on a controlled fill schedule In paste the horizontal loading generally is < 200 kpa Controlled by an initial plug pour to 2m above brow of drawpoint and 24 to 72 hr rest period to allow paste to cure. Paste plug is designed for full vertical load of overlying paste Barricade should never be placed right at brow setback at least 1m Establish a non-entry zone around barricades during pouring

45 Come and see us in Denver in 2017

46 GRACIAS.