The Kennecott smelter is a captive smelter and thus must respond to changes in mine and concentrator output including wide swings in concentrate production rates and grade over relatively short periods to maximize the Enterprise Value to Rio Tinto. The original flash smelting furnace maximum design capacity was 150 tonnes per hour of total charge (not including recycled dust) based on an average concentrate grade of 26.8%. The Smelter now operates routinely at 240 tonnes per hour of feed with periods in excess of this tonnage. The true limits of the flash smelting furnace are not known but gas handling, sulfur capture and slag handling, are likely the most important bottlenecks.
Changes in the concentrator operation and evolution of the mining at Bingham Canyon has led to much lower grade concentrate with high levels of either pyrite or gangue. The Smelter now treats concentrate ranging from a low of 16% copper to a high of over 30% copper with an average grade of `22% copper. Treating this very low grade concentrate required development of new strategies to manage furnace heat balance while still using high levels of oxygen enrichment. Multiple bottleneck constraints had to be considered including the maximum acid plant sulfur capacity of 43 tonnes per hour.
Over the years the RTK Smelter has improved its capability to smelt more complex feed stocks such as lower grade copper concentrate, lower Cu/S ratio in feed, and more elevated impurity levels such as SiO 2 , As, Pb and Bi. This has been achieved through improvements in process control, chemistry control, and improved blending of feed stocks, and also adoption of new coolant blends such as crushed flash furnace (FS) slag (in addition to C-Slag that has traditionally been used as coolant) and purchased secondary material. At times the total coolant load has been >25% of the total charge, dispelling the myth that flash furnaces cannot process secondary materials without a Pierce-Smith converter.
The FCF consists of 10 blister tap-holes and 2 slag tap-holes, and was completely rebuilt in 2012 and included modifications to tap-block windows, an evolution to a more robust design. Work continues to mitigate potential safety concerns around copper tapping and this has led to productivity improvements and added cost benefits across the Smelter. The frequency of tap-hole repairs have reduced from a twice a month schedule to once every two months without compromising integrity.
Tap-hole rotation and proper tapping techniques have always been an integral part of the FCF operation best practices, and recent efforts have been focused around further operational standardization and consistency through continuous operator training, standard work assessments, and regular audits. Several different types of tap-hole brick qualities have also been tested since April 2012, as an alternative to traditional silicon carbide bricks. Among the qualities tested, alumina chrome bricks have yielded encouraging results, with an insert life improvement from an average of 2,200 tonnes of blister tapped, to over 8,000 tonnes of blister tapped before an insert change-out is required.
Improvements to operation and control of the FCF continue to be made, particularly as relates to slag chemistry. In recent years the Smelter has deliberately implemented measures to dramatically reduce entrained %SiO 2 (as a contaminant) in FSF matte, which in turn has dramatically improved FCF slag accretion control, and required lower CaO/Fe flux, as well as improved furnace tapping conditions.
Control of magnetite in slag has also improved, with lower %Fe 3 O 4 in slag due to better lime/flux control; which in turn helped improve tapping conditions and also maximize available furnace blister capacity. Improved control of %S in blister is also a direct result of improved copper in slag control in the FCF and is generally in the 0.2% range.
Chemical impurity levels of anodes a RTK are among the highest in the world, yet production of ASTM Grade 1 cathode is possible due to precipitation of bismuth and antimony as a complex of arsenates in the porous anode slime layer (formed with elevated lead in anode). Specifications have evolved over years, and impurity in anode specifications are shown in Table 3. These are among the highest impurity anodes produced by any major smelter. The variability in the impurity levels also is a function of the ore mined and can vary widely below the range listed below.