New technology at Cortez.
Many open pit operators around the world are unaware, or are unwilling to admit to themselves, that misclassification of haul truck loads occurs at their operations. But it often does, resulting in significant losses to operating revenues. The most obvious misclassification would be sending ore to waste dumps, losing valuable product, or sending waste to the process plant, wasting time and money in that facility. Trucks with a payload capacity of 216 t are common today: in a gold mine with a head grade averaging 8 g/t Au, sending one truck load of ore to the waste dump results in the loss of some $15,000 (at $250/oz gold). Other misclassification might be wrongly classifying ore type, such as oxide as sulphide, or sending leach pad ore to the concentrator and so on, all reducing processing efficiency and lowering profits.
While it is a relatively high grade operation at present, with a head grade currently averaging 12 g/t, Cortez Gold Mines' Pipeline open pit makes every effort to avoid misclassification. It employs a Modular Mining Dispatch system, but for added security also uses Caterpillar's CAES -- Computer Aided Earthmoving System (MM, May 1998, p.312) -- on its loading fleet. This further reduces the potential for misclassification as well as greatly improving elevation grade control during earthmoving operations and provides other benefits (see also MM, November 1998, p.MNA12 on).
There are four primary loading units, all equipped with CAES. These are two 33 [m.sup.3] P&H 2800 electric shovels, mining overburden, one 18 [m.sup.3] Demag 285 hydraulic shovel loading ore and one 18 [m.sup.3] Caterpillar 994 wheel loader, which serves as a backup unit and undertakes ongoing pit development tasks. These units load 15 Komatsu 830E haul trucks of 216 t capacity and one 290 t Komatsu 930E. This fleet is supplemented by four 90 t capacity haul trucks. Drilling is by four Driltech D75K and one Ingersoll-Rand DM45 blasthole drills. These generally drill 171 mm or 222 mm diameter holes. Some of the overburden comprises alluvium, in which 270 mm diameter holes might be used. Overburden is mined from 12.2 m benches, while ore benches are 6.1 m high. The mine's support equipment includes a variety of tractor and wheel dozers (including a Komatsu 475 and a Caterpillar D10, which share the use of the fifth CAES system), water trucks, graders, wheel loaders, maintenance equipment and a 270 t/h portabl e crushing plant to provide road base and material for the over liner to be used on the leach pads.
Blasthole drill control
In addition to CAES, Pipeline uses two other technologies for precise mine control; the Aquila high precision GPS guidance package on the blasthole drills and Maptek's Vulcan software for mine planning and ore control, both implemented before CAES. Vulcan offers extremely powerful database and visualisation (3-D) capabilities. Aquila's DM-5 system employs two GPS receivers and antennas, onboard computers and displays, hand held data entry keypads and two-way radios for data transfer. Drill patterns are designed using the Vulcan package and each hole is assigned a number and co-ordinates. Once the Vulcan data file has been transferred to Aquila format a map file is sent to each drill from the office by radio. A hard copy of the map is provided to the drill operator. Hole locations (as drilled) are retrieved from each drill's computer via the two-way radios to check that the drill patterns have been drilled correctly.
At the time the Aquila system was acquired six surveyors had been needed, with some extra contract surveying required from time to time on specific projects. When two surveyors left the department the mine felt it was time to purchase DM-5 rather than hiring two replacement surveyors. It was also believed that the Aquila system would improve total costs. Performance and accuracy guarantees were built into the purchase agreement between Aquila and Cortez.
Britt Buhl, Cortez's chief engineer, notes that the payback on the Aquila system was calculated, conservatively, to be 5.4 years and the purchase was approved in August 1997. All five drill systems were operational by December and apart from some early radio communication problems the installation went as planned. Mr Buhl says the systems performed well above expectations over the first eight months: "satellite availability was above 90% and operator acceptance was high." Then, however, some problems arose which were discovered to be grounding problems on the drills (dirty power) and the results of little preventive maintenance. Since then the systems have performed well, with Aquila conducting preventive maintenance visits every six months. Satellite availability in the upper benches of the pit is generally above 98%, and in the pit bottom it ranges between 80% and 95%.
Mr Buhl explains that lessons were learnt by Cortez in the installation of the Aquila systems, including:
* Preventive maintenance, which has proven to be essential to system performance, was not thought of in the original purchase;
* Surveyors and drillers still have to lay out some holes;
* To enable drilling when satellites are down, surveyors stake end row holes on patterns;
* It is often possible to predict when satellites will be lost, in which case the drills will quickly tram through the pattern, marking each hole's location, allowing the full depth to be drilled at leisure after the satellites have been lost; and
* Before installation, ensure electrical systems are tested and understood. In addition, electrical maintenance staff need to understand the Aquila systems.
Cortez conducted a 'back analysis' of system performance and economics which indicated that the systems have worked better than originally thought. Three, rather than four, surveyors spend approximately 40% of their time on drill pattern related activities. Contract surveying has all but been eliminated. Cortez calculates that annual savings (compared with blasthole drilling without Aquila) are of the order of $140,000. Other benefits are that holes are drilled as designed, improving fragmentation, and that ramps and floors are more accurately excavated to grade, thanks to the Aquila systems, giving better truck cycle times. The amount of time spent redrilling holes has been considerably reduced, as has the use of consumables (paint and stakes).
Mr Buhl cautions however, that at Cortez the main driver for the economic benefits of the Aquila system is reduced labour cost. At other mines it may be something else. Also, he feels that countries with lower labour costs will realise fewer benefits. He also notes that the indirect benefits will vary from mine to mine.
Mr Buhl also notes that, "in efficiency alone, the Vulcan system has more than paid for itself," and that it "has worked extremely well integrating with other software," such as Aquila and CAES. He stresses its importance as a link to Aquila and CAES, being "instrumental in their success."
The main function of the CAES system is to display, to the machine operators, maps of their working area in relation to the machine under their control, in real time. The system also displays unit elevation in real time.
Ore blockout/design maps are produced in Vulcan and a dxf file is converted to CAES format. The maps are sent to the CAES-equipped machines via two-way radio. In this way, the display screens in the cabs of the shovels, the 994 loader and one of the two dozers are constantly updated as material is excavated from benches or moved by the dozers and 994.
Similarly, the office computers are also constantly updated. The screen shows the shovel operators what material is in the bucket and truck loads are dispatched accordingly. Cuts and fills are displayed for dozer applications. Hard copies of maps are still made available. The surveyors still stake ore control boundaries and ore and waste flags are still set out so that management can quickly assimilate what is happening when visiting the pit. However, the density of flags is not as great as was employed before the use of CAES, so the surveying time is still significantly reduced.
Cortez foresaw three main needs for CAES when considering purchasing the system. These were:
* That misclassification occurred when digging on boundaries between material types. It was obvious that reducing misclassifications would improve profitability;
* Considerable surveying time is required for dump reclamation. In addition, the amount of material to be rehandled can increase significantly if surveying is neglected; and
* Bench floors were not always level because of the limited information supplied by the surveyors, and, of course, through working at night.
Cortez came to an agreement with Caterpillar's local dealer, Cashman Equipment, to lease the system for a six-month test period. Over this period, the mine conducted a detailed evaluation of the system effectiveness on the loading tools and dozers. During the loading tests more than 6.5 Mt of material was moved. For half the time, the system was available to the operators, while the rest of the time the system ran in background to allow a comparison with excavating using traditional surveying controls. Misclassifications were identified by tracking all data available (from CAES, Modular Mining's Dispatch system and Vulcan) in a database. A buffer zone of 3.8 m was used to identify misclassified loads. GPS was used to confirm that a misclassification had occurred, by tracking the truck route.
The dozer test comprised a small-scale dump reclamation project. Again the system was available to the operator for half the test, with no surveyors, and for the other half the system was off and traditional reclamation surveying was used. The dozer and surveyors time were monitored and the benefits tabulated.
The results of the tests showed that misclassified loads were cut tenfold. Final reclamation costs were reduced by 35% per acre and dozer productivity improved by 35%. The purchase of the CAES system was approved in March 1999.
Installation was straightforward, taking just one day per machine. Configuring the system took some time because bucket positions are determined based on dimensions and the equipment parameters have to be input into the system. However, once the system parameters had been correctly established, initial performance was excellent, according to Britt Buhl. Operator and office personnel training took about one month. "Training operators on computer file manipulation was probably the most difficult task," says Mr Buhl.
The operators and shift foremen have adapted well to the use of the systems, and satellite availability has been good. The lessons learned in implementing the system were:
* Consistent operating techniques should be in place before a CAES system is installed;
* The evaluation study would have been very difficult without Modular's Dispatch system; and
* Thanks to lessons learned from Aquila, the mine was much better prepared for installing the CAES system.
The CAES evaluation test led Cortez to estimate a payback time for the system of 2.14 years. Sometime in 2000, after a year or more of operation, and with work on the database complete, a full evaluation of the system benefits will be conducted. However, the following benefits are already apparent from using
* Misclassifications have been reduced;
* Bench floors are much more level;
* The operators are enthusiastic about CAES as it provides them with a great deal more information and "takes the guesswork" out of their work;
* At night, and in winter conditions, the systems provide the assurance that the material being mined is being correctly loaded;
* The amount of supplies used to stake ore in the pit has been reduced; and
* Reclamation is more efficient.
Britt Buhl concludes that: "Even at significantly lower grades than Pipeline, the system has robust economics. This system and ones in development by Caterpillar are going to lead the way in technology for mining in the future."
CAES will be one part of the new Caterpillar/Mincom MineStar system that will incorporate Cat's existing GPS-based positioning and machine monitoring systems, to be available soon (MM, January 2000, p 53).
The mine is currently working on a database to link the Dispatch and CAES data with the block models to track stockpile grades and misclassified loads.
Low cost Cortez
Cortez is situated 120 km southwest of Elko in Nevada, US. It is 60%-owned, and operated, by Placer Dome, and 40%-owned by the Rio Tinto's Kennecott Exploration subsidiary. In 1998 it produced 1.1 Moz of gold at a cash production cost of $58/ oz including royalties, a total cost of $147/oz, and was expected to repeat orexceed that performance in 1999. Production in 2000 and 2001 is forecast to be 750,000 oz/y at a cash production cost of $80/oz. In total, Cortez is expected to produce 7 Moz for the remaining 11 years of its planned life, between 1999 and 2010.
The Pipeline ore body was discovered in March 1991 (MM, September 1996, pp.MNA6-15) and construction of the new mine and mill started in early March 1996. Once the $289 million project had been permitted and approved, it was 'fast tracked'. Commissioning began 11 months after the start of construction and the project was completed $40 million under budget. The primary cost saving item was in prestripping the orebody, which was budgeted to be done by a contractor. However, it was undertaken by Cortez operations using the new mining fleet.
The Cortez Joint Venture has operated in northeast Lander County, Nevada, since the late 1960s. Over the years production has been from a number of small open pits - Cortez, Horse Canyon, Gold Acres - but has now moved to a larger scale with Pipeline, moving 60-65 Mt/y of which about one third is ore. The reserve is a simple oxide orebody (95% oxide and 5% carbonaceous), amounting to 162.3 Mt of ore at a grade of 1.7 g/t.
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|Date:||Feb 1, 2000|
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