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Mining and metals refining
Jul 3, 2017

Rio Tinto Kennecott smelter - 20 years strong

The Rio Tinto Kennecott (RTK) Smelter near in Salt Lake City, Utah is entering its 20 th year of operation since the Smelter Modernization Project in the early 1990’s when flash smelting and Kennecott-Outotec Flash Converting was first implemented.

During this period, the Smelter has seen a number of innovative technical developments, year on year improvements in safety, health, environmental emissions reductions, greatly improved asset management and productivity improvement practices, and successful planning and execution of major maintenance shutdowns. This article summarizes the key safety and operational performance over the last 20 years, treatment of very low grade copper concentrate, blister copper production, power production from waste heat/co-generation, and large scale sulfuric acid production. Technology developments include improved furnace integrity and condition monitoring, automated tapping, and increased capability in copper scrap melting. During Sept-Nov 2014 the Smelter executed its largest maintenance shutdown, which included a complete rebuild of the Flash Smelting Furnace and replacement of the concentrate dryer, furnace waste heat boiler and sulfuric acid plant modifications, as well as significant upgrades to the anode casting plant.

Celebrating 20 years of flash smelting and converting!

Since the start-up of the modernized Smelter in 1995, the facility has smelted ~22.6M tonnes of dry feed 
to the FSF, producing ~4.6M tonnes of blister copper. The Smelter has also produced ~16.1M tonnes of 
sulfuric acid since startup in 1995, and remains one of the largest sulfuric acid producers in North 
America. 

The Smelter was designed to recover waste heat in the form of steam and produces ~66% of its own 
power needs through co-generation (35MW steam turbine generator). Tables 1 & 2 describe historical 
production totals since the 1995 start-up of the Smelter, and also various production records achieved 
(monthly and daily); respectively.

Table 1: Cumulative production by RTK Smelter since 1995 start-up

Table 2: Proudction records set by RTK Smelter [Note: all are monthly records except FSF dry feed over 24hrs]

Health Safety and Environment

In 2012 and 2013 the RTK Smelter set a new safety record, achieving 960 days without a Lost Time
Injury. In total, 3.6 million man-hours were worked without an LTI during 2012/2013; and a new All Injury Frequency Rate (AIFR) record low of 0.27 was established. Note that in the US the All Injury Frequency Rate (AIFR) is based on incidents per 200,000 hours which is equivalent to 100 people working full time for one year. The environmental performance of the Smelter continues to improve on an already excellent performance(see Figure 1). Annual sulfur dioxide emissions over the last 20 years show a downward trend in spite of the Smelter treating ever lower concentrate grades with higher sulfur content. Over the last 6 years the sulfur dioxide emissions have varied between 26 kg/hr and 45 kg/hr SO 2 against a permitted limit of 96 kg/hr. The sulfuric acid plant tail gas is now normally below 50 ppm SO 2 and often in the 30 ppm range; a third of the original design emission rate and accomplished without tail gas scrubbing. Overall sulfurcapture is in excess of 99.93% of the input sulfur and the RTK Smelter remains one of the very cleanest smelters in the world, if not the cleanest.

RTK Smelter historical SO2 emissions
Figure 1: RTK Smelter historical SO2 emissions (note plant modernization was in 1995)

OPERATIONAL PERFORMANCE

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.

Table 3: Chemical specifications of copper anodes

FCF side-wall accretion thickness monitoring

Installation of a furnace sidewall diagnostic (PCA based) system now allows real-time magnetite side-wall thickness prediction, which brings new capability to furnace integrity monitoring for the FCF. This technology was developed by Hatch and the system improves furnace safety by providing real-time information of the sidewall accretion condition – enabling more informed operating decisions. It can also provide early detection of abnormal process measurements due to sensor failures or abnormal process drifts - adding confidence to existing methods of furnace integrity monitoring.

Remote tapping of FS matte

The RTK Smelter has historically relied on very manual intensive methods to tap matte from the FSF. It typically involved 1-2 operators at a time, use of oxygen burning lances and requiring close proximity to the working tap-hole, resulting in higher exposure to heat stress and potential for burns from molten matte. Although RTK has proven the practice to be safe over the years there has always been interest in taking the step to a more automated process, therefore reducing the workload on the operators, removing them from the direct line of fire, and decreasing exposure to off-gas and metal splash from tap-holes. There is also value from improved safety, hygiene and reduced heat stress, as well as more consistent burning and longer life of tap-hole inserts (resulting in higher furnace online times). Lewis Australia, an engineering firm specializing in automated equipment, worked with RTK engineers to develop a compact matte tapping machine. Initial trials were encouraging (Figure 2) and two mechanized tap-hole opening and closing machines were installed during the 2014 Shutdown.

Prototype testing of matte tapping system
Figure 2: Prototype testing of matte tapping system. Full scale design utilizes wireless remote controls

Rhenium recovery from acid plant blow-down by continuous ion exchange

RTKC smelts copper concentrate containing trace amounts of the rare metal rhenium which is associated with molybdenum in the Bingham Canyon orebody. There is a potential yield of ~ 1,000 kg/annum of recoverable rhenium. When copper concentrate is smelted the rhenium volatilizes in the flash smelting process off-gas process and is recovered in the Acid Plant Blow-down (APB). APB is treated in the Hydromet Plant along with refinery bleed solutions to recover copper while fixing bismuth and other impurities in a form suitable for discharge to tailings. Rhenium is recirculated to the smelter and can build to high levels approaching 50 mg/l (averages 20 mg/l) in the APB. Rhenium is, without a recovery plant, ultimately lost to the Hydromet Plant tails. The Smelter recently commissioned a new plant to recover Rhenium and the plant has been integrated with the existing Hydromet Plant (HMP).

Copper anode fire-refining and scrap melting using Praxair Co-Jet™ technology

The Smelter continues to improve copper scrap melting capability through application of Co-Jet technology, and in collaboration of technical developments with Praxair. The Praxair Co-Jet technology continued to add value to RTKC through increase secondary scrap treatment and flexibility to operating practice. The challenge in moving forward is to maintain this capability and flexibility while improving Anode Furnace campaign life. Additional burner development since 2012 related to burner design and flame setting control have led to positive results on maintaining the furnace refractory integrity. Additional process developments in this area, such as ‘tuyere-less’ refining, are anticipated.

Shutdown scope & execution

The longest and largest ever planned Smelter Shutdown occurred in Sept-Nov 2014, and was innovative and also critical to the future operation, and involved more than 2,000 employees and contractors. RTKC Smelter “reset” the asset health of critical, larger equipment like the FSF and concentrate dryer. The innovative FSF rebuild (which included elimination of roof refractory brick) is a unique design to manage ‘hotter’ feed at higher throughput rates and processing of lower grades of copper concentrate.

Improvements being made to the acid plant, ensuring worlds-best practice/environmental assurance around sulfur capture, as well as production assurance in sustaining throughput rates going forward. The Smelter is also working to increase the amount of time before acid plant shutdowns are required (moving from 2 years, to every 3 years).

New FSF under construction
Figure 3: New FSF under construction (October 2014)

CONCLUSIONS

The Rio Tinto 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. Since the start-up of the modernized Smelter in 1995, the facility has smelted ~22.6M tonnes of dry feed to the FSF, producing ~4.6M tonnes of blister copper. Improvements to operation and control of the FCF continue to be made, particularly as relates to slag chemistry, and tap-hole insert life. Flash converting process technology is also increasingly being internationally recognized, with several new plants constructed (for instance in China) now utilizing FCF technology. The Smelter recently “reset” the asset health of critical, larger equipment such as installation of a new FSF (with new burner and air-slide systems) and a new gas-fired concentrate dryer. The innovative FSF design (which included elimination of roof refractory brick) will further enable processing of ‘hotter’ and lower grade feed at sustained higher throughput rates. The Smelter also continues to demonstrate a capacity for innovation such as a new plant to recover Rhenium from off-gas/acid streams, remote tapping of FSF matte, enhanced melting of copper scrap utilizing Co-Jet technology, and FCF sidewall accretion thickness monitoring in real-time. All of these operational practices and innovative technologies, coupled with the experience and operating know-how gained over the last 20 years, will help drive even further improvements at RTKC for the next 20 years!

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