The initial fragmentation protocol at Dos Pobres drilled and blasted ore to ensure that shovels and loaders were loading and not digging. The material size was easy to achieve due to the highly weathered and fractured condition of the ore near the surface. As the pit progressed deeper into the ore body, less weathered and different rock types were encountered. The top size was the main concern, as the crusher pocket was easily plugged, and several tons of production could be lost while shooting over the vibrating grizzly to free the jam. A “quantum leap” in powder factor and energy used in the ore was undertaken to assist the crusher with limited success. As this was done on a complete bench basis, several areas were blasted at a high energy factor or drilled on a tight burden and spacing – not optimizing cost and/or energy distribution, as some areas still produced over size material.
The engineering department initiated a program to improve fragmentation by optimizing drilling and blasting techniques and utilizing Drill Energy Index, image analysis and crusher circuit data to relate explosive energy distribution and use to crusher feed size and throughput. This paper will describe the inter-departmental fragmentation program undertaken at Dos Pobres and summarize the results of improved crusher throughput from better blasting fragmentation and teamwork.
Dos Pobres is a porphyry copper deposit with an oxide zone consisting predominantly of chrysocolla, black oxides, copper-bearing clays, and copper iron oxides. Ore reserves reported in the 2008 10-K report for crushed leach material at Safford are 496 million tons (450 million metric tonnes), averaging 0.38% copper. The operation is mining at a rate of 180,000 tons per day (163,293 tonnes per day), of which 60,000 tons (54,431 tonnes) is ore.
AUTOMATIC DRILLING SYSTEM
Installing an automatic drilling system on three drills at Dos Pobres provides GPS navigation to each hole through the operator, auto-levelling and auto-drilling from collaring the hole to finishing the hole and machine monitoring and fault notification. The system also produces a quantitative Drill Energy Index value. This value is determined from an algorithm accounting for rotary torque and speed as well as hoist downforce and speed which is then averaged over the amount of time it takes to drill 1 ft (0.3 m), scaled for typical drilling conditions and displayed onboard the drill as well as stored in a database.
The database is populated with all the values reported from the equipped drills. The Drill Energy Index value averaged over the entirety of the drill hole produces a Hole Profile value. The Hole Profile value is used as a quantitative measure of the rock hardness for that particular drill hole, and is often interchangeable with the Drill Energy Index in this report, as the focus is on the average hole value, not the individual 1-foot interval. The relationship and range of Hole Profile values to rock hardness will vary for every site and has been determined for Dos Pobres as described in the following section.
Read full white paper > FMI Safford – Better Fragmentation through Teamwork at Dos Pobres