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Cost-effective Drilling Comes at a Price—But Pays Big Dividends
By Russell A. Carter, Managing Editor
The success or failure of an individual blasthole to achieve its intended effect doesn’t carry much statistical clout in a blast pattern comprising up to 1,500 holes, or at a mine that routinely conducts multiple daily blasts—or across a global industry that measures total daily blasthole production in the five-figure range or higher. It’s only when the reasons for consistent drilling and blasting (D&B) success or failure become systemic to an operation that a noticeable change in productivity becomes apparent, occasionally leading to spirited discussions within and between mine departments about “what are we doing wrong?” or much less frequently, “What are we doing right?”
As described by an experienced applications engineer for one of the major drill-equipment suppliers, D&B is “all about putting the right amount of energy in the right place at the right time at minimum cost to achieve maximum control over the shot rock volume and the resulting particle size distribution in the muck pile.” The benefits of a well-designed blast—or the repercussions of a poorly executed D&B plan—reverberate far away from the actual blast site, as shown in the accompanying diagram that depicts how various elements of D&B practice can influence downstream operations.
Although the physics of sinking a simple hole into the ground seem straightforward, the path to consistently effective D&B strategy meanders through a thicket of thorny issues that demand attention, ranging from an understanding of local geological conditions, proper drilling equipment selection and climate considerations, to the type of explosives required or locally available, for example. Accompanying those considerations are other factors such as volume of material to be excavated according to mine plan, hole diameter, optimum bench height, stemming material source, fragmentation requirements, and desired level of equipment utilization and availability, among others.
Adding to the technical difficulty is the quick pace of daily job duties, technological progress and product introductions that can make it hard for mine personnel to stay current on best practices for D&B success. In October, Sandvik Mining & Construction convened its first Mining Forum, aimed at bringing participants up to date by focusing on fundamentals as well as recent technological developments in surface-mine drilling and blasting operations. The three-day event, which included 112 participants from 25 mineral producers and mining contractors, was held in Johannesburg, South Africa, against the backdrop of the African continent’s vast mineral potential—and equally immense needs for drilling and blasting equipment, techniques and management strategies to effectively cope with its widely varying mine-site conditions.
Realizing Regional Potential
Although most forum presentations addressed specific aspects of D&B practice, leadoff speaker Chris Brindley, president of Sandvik Mining & Construction Region Africa, began by highlighting Africa’s strengths and weaknesses as they relate to the global mining industry. Noting that it’s somewhat difficult to mentally grasp the sheer size of the continent, Brindley displayed a slide showing how outline maps of the United States, Western Europe, China, India and Argentina could all be superimposed upon a map of Africa—with room to spare. Its 30.3-million km2 of land area contain mineralization currently representing about 90% of the world’s known platinum resources, 80% of its chromite, 65% of diamonds and 40% of gold. Its 2010 estimated population of 1.013 billion people account for 14.8% of world population—a share that is expected to grow to 24% by 2050—and 60% of the current population is under the age of 24.
However, the challenges facing African economic development are equally expansive, including the threat of nationalization of private assets, political corruption and instability, the prospect of increased taxation on mining, a shortage of skilled workers and difficult logistics.
“Africa is probably the wealthiest continent in the world when it comes to minerals,” said Brindley. “But as you can see, it faces a lot of challenges. Until [these challenges] are resolved, it will be difficult to get major capital funding for projects in this part of the world.”
On the other hand, he noted, “Africa is one of the best places in the world to find new orebodies.”
Brindley said Sandvik currently has business operations in 12 African countries, employing roughly 3,250 workers; and eight distributors throughout the continent.
In addition to the regional macro-economic trends and political issues, Africa-based mineral producers share mine-site operational challenges that are common throughout the global industry. Among these, the search for improvement in D&B economics may not be paramount in the list of cost-cutting concerns but is definitely rising rapidly in importance. Several of the forum’s presentations dwelt on fundamental aspects of identifying and selecting the most appropriate drilling equipment and methods for a given application.
Charles Deacon, Sandvik’s vice president of marketing for the Africa region, explained that D&B activities may account for as much as 15% of total production costs, and are actually the most controllable of these costs. Across an entire operation, D&B can affect excavation rates, cost of loading, secondary breakage requirements, ore grade dilution, processing rates, slope stability concerns and mine site safety.
One of the basic informational needs for determining the best drilling approach for the application, said Deacon, is knowledge of rock mass “drillability”—defined by three factors: drilling rate (penetration), bit wear rate (time elapsed between regrinds), and bit life (distance drilled before reaching end of economic bit life). The most well-known indicator of drillability is the Drilling Rate Index, a relative measure of penetration rates in a given rock type. DRI is determined by two common tests that measure rock toughness and rock surface hardness. In general, the lower the DRI, the lower the drilling rate that can be expected, and vice versa.
Armed with knowledge of local rock characteristics, the customer still faces a long list of factors that must be considered when choosing the proper drilling method. These involve both technical and commercial issues, according to Deacon, and include:
- Hole diameter
- Hole depth/bench height
- Production rate
- Size of operation
- Special techniques required
- Legal requirements – dust, noise, etc.
- Rock hardness
- Hole angle
- Power availability
- Fleet size
- Economic life
- Technical support
- Parts supply
- Operating cost
Putting Together the Right Rotary String
For those operators considering rotary drilling methods, Mark Baker, Sandvik’s global product line manager for rotary tools, highlighted the physical limits of the equipment and the importance of using the proper drill string and bit setup. He emphasized that effective control of the feed and rotation applied by a rotary drill rig are essential to productive and cost effective operation of the drill. Excessive loading by either parameter will significantly reduce consumable life and increase mining costs.
In addition, careful selection of every drill string component is vital to achieve accurate holes, optimal fragmentation and operational efficiency. A complete rotary drill string assembly can include the following:
- Shock sub (optional) – Recommended for use in applications with high levels of axial and lateral vibration (>10g) such as drilling in fractured formations. Benefits include increased drill availability, reduced mast maintenance and less rotary drive head repairs, smoother on-bottom running and improved torque control.
- Top sub – The connection between the drill pipe and rotary motor or shock sub.
- Drill pipe – Based on the outer blasthole diameter, a proper drill string OD should be selected that will provide the necessary column support to reduce flexing, as well as sufficient annular area for cuttings evacuation.
- Deck bushing – Guides the drill string, reduces risk of wobbling, prevents reduction of rotary head torque and supports drill string configuration in producing straight holes.
- Bottom sub or stabilizer – Allows for connection of the bit to the pipe. Roller stabilizers are used for improved hole stability in hard and broken formations, where hole caving is prevalent. Blade stabilizers are used in softer formation where the gauging and scraping of the hole wall improves hole quality.
- Rotary bit – Proper drill bit selection is vital for achieving desired results. Pay attention to factors such as ground conditions (rock hardness, abrasiveness, competency and ground water); study product specifications and local availability; determine correct cutting structure, bearing configuration (sealed or air-cooled) and air-nozzle sizing for site conditions.
Baker cited several case studies in which changeover to a properly configured drill string produced significant results, including:
- A copper-gold mine at which average drill pipe life without a shock sub was 25,000–30,000 m with eventual breakdowns usually due to thread failure from vibration. With shock subs in place, drill pipe life increased to 42,000 m, with end failure resulting from eventual surface erosion.
- In another application, drill pipe conversion from 40/20 ft to 33 ft (x2) resulted in less handling, improved ease of rotation and longer service life. Savings amounted to $270,000 per year, primarily from improved efficiency.
Turning Money into Air—and Vice Versa
Compressed air requirements differ among drilling methods. Rotary drilling requires low-pressure, high-volume air fed through the center of the drill pipe to the bit for hole cleaning (cuttings removal) and bearing cooling. Similarly, top hammer drill-rig compressor capacity is calculated according to hole-cleaning requirements, but with DTH drilling the rating of the hammer defines the required compressor capacity. Whether a customer chooses rotary or percussion drilling, it’s important for them to understand and know how to determine the right compressed-air volume and pressure for the selected drilling application, explained Karl Ingmarsson, vice president of marketing for Sandvik Mining & Construction. At a minimum, the user should be familiar with the following concepts:
- The purposes of compressed air in drilling.
- How to make a quick and simple calculation of correct up-hole velocity.
- Why sufficient volume is required for a DTH hammer to perform well.
- How to match rod or tube size to tophammer bit sizes.
- How to estimate cutting settling velocity and target exit velocity.
- Why air nozzle selection is important for rotary tools.
- How to interpret in-cab pressure readings.
- How to compensate for high altitude.
Stating that “air is money,” Ingmarsson provided examples of how much fuel a typical, small DTH drill rig would burn in its lifetime (at 70 l/hr and average load factor of 74%, roughly 2.8 million l or 743,000 gal), or a large rotary blasthole drill (at 140 l/hr with same load factor, about 5.6 million l or 1.48 million gal)—of which about 2.3 million l and 4.5 million l, respectively, would be consumed to run the rig’s compressor alone. And with so much fuel being burned to provide compressed air, is that air being used economically?
Not usually, explained Ingmarsson. In recent years, as diesel engine OEMs built better monitoring systems into their products, a rig’s nondrilling fuel-burn rate has become much more noticeable. Traditional rigs, when in drilling mode, provide maximum air volume regardless of actual drilling conditions; when not drilling, they maintain maximum pressure, thus loading the engine for no particular benefit.
After an extended effort to find ways to alleviate this problem, Sandvik recently introduced its Compressor Management System (CMS), designed to reduce fuel consumption, extend engine life and reduce associated drilling costs by electronically managing compressor operation to provide the necessary amount of air required at all times, ensuring the compressor runs at full volume only when needed (See E&MJ, May 2011, “New System Manages Main Compressor on Rotary Drills,” pp.30-34). It also provides continuous feedback to the operator on downhole conditions and indicates how CMS is responding to current drill demands.
According to Ingmarsson, CMS is currently available as a retrofit for Sandvik’s rotary drill rig models, and will be available for DTH rigs in 2012.
Playing it Straight
No matter what drilling method is selected, overall D&B performance will suffer unless holes are drilled straight and according to plan, from collar to bottom. When an operation “drills holes that look like spaghetti,” according to Arne Lislerud, surface applications manager for Sandvik, it can expect:
- Floor humps, hindering efficient loading due to uneven pit floors;
- Unstable pit walls and difficult first-row drilling;
- Safety concerns from flyrock;
- Stemming material blowouts that generate safety, excessive dust and “bad toe” concerns;
- Poor blast direction, affecting quality of floors and walls;
- Shothole deflagration and/or misfires that produce safety hazards and poor muckpile diggability.
The keys to achieving consistent straight hole drilling, said Lislerud, are simple: Be aware of the numerous issues that lead to drillhole deviation; operate with a technically sound drill rig, drill string and instrumentation; and motivate drillers to strive for best results. Good practice dictates only 2%-3% maximum drillhole deviation in regular production drilling operations.
For collar position error control, Lislerud recommends:
- Using tape, optical squares or alignment lasers for measuring-in collar positions; or
- Using GPS or total stations to measure collar positions;
- Marking collar positions using painted lines, not movable objects such as rocks, etc.;
- Protecting completed drillholes with shothole plugs to prevent holes from caving in (and filling up);
- Using GPS guided collar positioning devices, such as Sandvik’s TIM3D drill rig navigation system.
Similarly, to control drill-hole deflection:
- Select bits less influenced by rock-mass discontinuities;
- Reduce drill string deflection by using guide tubes, etc.;
- Reduce drill string bending by using less feed force;
- Reduce feed foot slippage since this will cause a misalignment of the feed and lead to excessive drill string bending;
- Avoid gravitational effects that lead to drill string sag when drilling inclined shotholes (>15°);
- Avoid excessive bench heights.
Choosing the proper bit face design can enhance drill-hole straightness, he also noted. When a percussion bit first starts to penetrate through a rock-joint surface at the hole bottom, for example, the gauge buttons tend to skid off this surface and thus deflect the bit. More aggressively shaped gauge inserts (ballistic / chisel inserts) and bit face gauge profiles (drop center) reduce this skidding effect by enabling the gauge buttons to “cut” through the joint surface quickly, thus resulting in less overall bit deflection.
The right bit-skirt design also helps: As the bit cuts through a joint surface, an uneven bit face loading condition arises; resulting in bit and drill string axial rotation that is proportional to bit impact force imbalance. A rear bit skirt support (retrac type bits) reduces bit and string axial rotation by “centralizing” the bit.
Other deviation countermeasures include using a longer bit body, adding a pilot tube behind the bit, using lower impact energy, or employing a drilling control system that can rapidly react to varying torque, feed and percussion or pulldown demands based on hole conditions.
Additional information regarding the 2011 Mining Forum can be obtained at www.theminingforum.com.
Drilling Format, Fleets Differ at Zambia’s Largest Copper Mines
Mining is one of two dominant drivers of Zambia’s economy, the other being agriculture. In 2010, the mining industry paid more than $500 million in taxes, customs duties and other regulatory fees, and accounted for more than $3 billion in export earnings for this Central African nation of 13 million people. With competition for the country’s mineral resources heating up between major Western and Eastern mining interests, Zambia’s Chamber of Mines estimates that from 2000 to the end of 2009, recent entrants in the country’s mining industry have invested more than $5 billion in plant rehabilitations, plant expansions, new mines and new processing facilities.
Within the mining sector, copper is king. Two of Zambia’s largest copper mines—Kansanshi and Lumwana—are separated by distance, ownership and other differences but still share some common characteristics, ranging from ambitious expansion plans to an avid interest in innovative mining techniques and technology that could assist in meeting their future production goals. Both are located in the Northwest Province, where anticipated mine startups and expansions are spawning references to the ‘new’ Zambian Copperbelt; and both use similar, conventional surface-mining methods and equipment for drilling, blasting, loading and haulage.
Two additional shared traits for these operations include some tough drilling and blasting conditions at both sites due to local geology and pit configuration, and an ongoing reliance on Sandvik Mining & Construction drilling rigs for the bulk of their production drilling requirements.
Lumwana, now owned by Canada-based Barrick Gold after its C$7.3-billion acquisition of former owner Equinox Minerals earlier this year, produced almost 147,000 mt of copper in concentrate in 2010 and is targeting 145,000 mt of copper in concentrate this year as mining operations move away from a higher-grade area active in 2010. To date, it also has stockpiled 4.5 million mt of copper ore with uranium levels that exceed current customer contract specifications, but has no current plans to process this material. Lumwana began production in 2008.
Kansanshi Mining PLC, owned by Canada’s First Quantum Minerals, began operation in 2005 and produced more than 231,000 mt of copper last year, including 184,183 mt of copper cathode and almost 47,000 mt of copper in concentrate, and plans to raise production to 259,000 mt in 2011. It also produced about 110,000 oz of gold. Kansanshi mined slightly more than 23 million mt of ore at an average grade of 1.3% Cu during 2010.
For Sandvik, the trend toward expansion of Zambian surface mines and an associated need for larger, more versatile drilling equipment converges neatly with its recent purchase of Canadian drill builder Cubex’s QXR line of track-mounted DTH surface rigs. The result has been increased interest in and sales of the three QXR models (now identified in the Sandvik line as DR540, DR560 and DR580). Kansanshi, for example, has been gradually replacing its aging fleet of Sandvik Pantera crawler drills—now identified as DP1500s—with larger DR560 rigs to take advantage of the higher drilling capacity and versatility offered by these units.
Kansanshi: Big Plans Demand Bigger Drills
E&MJ recently visited with D&B managers and engineers at both mines to gain a better understanding of typical practices and problems in this region.
Kansanshi is located approximately 10 km north of the town of Solwezi, 18 km south of the Democratic Republic of Congo border, and 180 km northwest of the town of Chingola. Kansanshi extracts mixed sulphide and oxide ores from two pits with similar mineralization and rock characteristics; the pits will join at a future date but currently one—the older Main pit—is approximately 170 m deep and the smaller Northwest pit is about 100 m deep. Drill and Blast Supervisor Etienne Haasbroek oversees a fleet of 17 Sandvik rigs, including two DK45s, four DK25s, seven 1500s and five DR560s.
A typical blast at Kansanshi may include up to 1,300 blastholes, drilled over the course of seven to 10 days, and loaded with a total of 230-250 mt of emulsion explosive. Blasthole depth varies from 5 to 10 m, hole diameter (depending on whether it’s sulphide or oxide ore) ranges from 165 mm down to 115 mm or even 102 mm. The shallower, 5-m-deep holes are stemmed to a 2-m height, with the deeper 10-m holes receiving 3-4 m of stemming.
Pattern spacing is normally 5x5 but is tightened to 4x4 when drilling in areas that include ‘floating rock’—large igneous boulders interspersed throughout the more common softer material. According to Haasbroek, even with the tighter pattern, many of the boulders remain intact and require secondary drilling. Also, because of persistent groundwater flows in the active mining areas, holes occasionally collapse and consequently, redrilling requirements can vary from around 9% during the dry season to as much as 40% during the rainy season.
With the first phase of a three-step expansion program currently under way to raise annual copper production from 250,000 mt/y to 400,000 mt/y by 2015, efficient drilling, blasting, loading and haulage will become even more crucial for meeting production demands, and Kansanshi has taken steps to ensure its equipment capabilities are up to the task—including installation of a haul-truck trolley-assist system in the Main pit, bolstering the loading and haulage fleet with new equipment and machines transferred from another operation, and gradual phaseout of the older DP1500 drill fleet drill fleet from production drilling duties in favor of the larger DR series rigs.
The DP1500 rigs, according to Haasbroek, have been used since the mine began operations and have performed admirably. “We have one rig with 37,000 hours, and four of them have more than 30,000 hours,” he said. (Sandvik’s guidelines suggest the economic service life of these rigs begins to fade at about 28,000 hours.) “Plus, the 1500s can only efficiently drill blasthole diameters up to 127 mm. We need larger rigs to handle our 165-mm-diameter holes,” Haasbroek said.
The DR560s are powered by a standard Cummins QSK23 turbocharged and aftercooled diesel producing 860 hp (641 KW); a Caterpillar C27 diesel also is available as an option. The rig’s Sullair two-stage screw compressor provides 1,150 cfm at 500 psi (543 l/s and 34.5 bar) at 1,800 rpm; at 2,100 rpm, the compressor provides a maximum output of 1,350 cfm at 330 psi. Capable of drilling 4-in. (100 mm) to 8-in.-diameter holes, the rigs also are highly versatile: with the feed mast and cab mounted on independent booms, setup from hole to hole can be quicker and easier with the cab able to swing away from a nearby face when necessary, for example, while the rig’s rod changer enables easier rod handling when two rods are needed to sink the mine’s 10-m-deep blastholes.
Expansion Ahead for Lumwana
Barrick’s Lumwana mine, located 65 km west of Solwezi, produces copper concentrates that are trucked to the Chambishi Metals smelter near Chingola, Zambia, operated by China Non-ferrous Metal Mining and Yunnan Copper, and Vedanta’s Konkola Copper Mines Plc smelter at Nchanga, for processing. Recent information released by Barrick indicates that the company is bullish on finding and developing additional sizable copper resources at Lumwana, as well as on optimizing pit and plant operations.
For example, Barrick is carrying out an 18-month exploration program to increase measured and indicated resources at the property’s nearby Chimiwungo deposit—next in line to be mined and considered the keystone to extending the mine’s useful life. The drilling program is part of an expansion study, scheduled for completion in late 2012, that could lead to a potential doubling of the mine’s processing rates. Other areas of focus highlighted by Barrick include mill debottlenecking, pit re-optimization, changes to mine sequencing, dilution control, equipment availability and leveraging Barrick’s supply chain agreements.
As previously mentioned, Lumwana shares some common traits with Kansanshi, including coping with pervasive pit water problems and slabby, troublesome rock; installation of a trolley-assist system to increase haulage efficiency while cutting costs; and a reliance on Sandvik drilling equipment. However, the point of departure between the two mines also starts with the drill fleet—Lumwana employs 10 Sandvik D45KS rigs, and just two Pantera rigs that are mostly used for pre-split drilling. The mine layout at Lumwana also dictates a somewhat different approach to D&B operations.
“We have a long, narrow pit with a lot of equipment inside, and because of that we have to keep our blast patterns quite small, so we’ll blast almost every day, or every other day. Because of water seepage into the pit, we also have to do quite a bit of redrilling,” said Peter Rule, senior production engineer at Lumwana.
Benches in Lumwana’s 6-km-long pit are generally kept at 8 m in height to minimize ore dilution and allow single-pass drilling, but occasional circumstances require double-height benches; all are sunk with 1 m of subdrilling to ensure good toe breakage and pit floor evenness. Patterns are spaced with 5- to 6-m burden in most rock conditions but are tightened to 4.5–5.5-m burden in harder material. Because of the tendency of the local schist material to blow out in slabs at the top of the blasthole, the mine will reduce its normal 172-mm blasthole diameter to 165 mm to maintain a ‘reasonable’ powder factor.
Overall drill penetration rate is targeted at an average 28 m per hour, ranging from a low of 26 m/hr in hard material to 31 m/hr in softer areas. Daily totals per rig average about 400 m; for the drill fleet, between 4,500 and 5,500 m per shift total with a recent high of 6,000 m.
Drill availability averages about 75%. Overall performance of the Sandvik fleet has been highly satisfactory, according to mine personnel, despite working in highly abrasive material. “We get only about half the expected service life from our drill undercarriages because of the abrasive nature of the rock here,” said Rule.
Sandvik, which supports the rigs at Lumwana through a full MARC (maintenance and repair) contract as well as a separate contract to supply all drilling consumables, has 63 permanent employees stationed at the mine.