Kennecott Utah Copper Smelter Modernization Mining Engineering magazine, July 1999, Volume 51, Number 7 By C. J.Newman; T. I. Probert; A. J. Weddick Before 1995, the smelting process at the Kennecott Utah Copper Corp. (KUCC) smelter consisted of Noranda reactors and Pierce-Smith converters. When considering expansion, it was decided that the existing process would not lend itself to a program geared toward processing the total output of copper precipitate and copper concentrate from KUCC mining and concentrating operations. The smelter facility that used the Noranda technology was shut down in May 1995. The flash smelting furnace operation began in June [1995] with the flash converting furnace startup a few weeks later. Initial expansion of the KUCC Bingham Canyon Mine and Copperton concentrator facilities was completed in 1988. Production rates increased to 852,000 tons per year of copper concentrate. In 1992, a fourth milling circuit was added to increase concentrate production to 1.1 million tons per year. At that time, KUCC's smelter could process only about half of the concentrate production because of insufficient gas-handling and acid-plant capacity to meet air emission regulations. As a result, the rest of the concentrate production was sold. In 1989, KUCC began a study of smelting requirements to process all of the available concentrate resulting from the expansion of mining and milling facilities and also future expansions. In 1990, the Outokumpu flash-smelting process was chosen as the primary smelting vessel, to be combined with Kennecott-Outokumpu flash-converting technology to process solid matte through to blister copper. These two processes best met the primary criteria of achieving the identified annual throughput, complying with environmental requirements and positioning the smelter for future anticipated, more restrictive, emission requirements. In the spring of 1991, full-scale tests were carried out using 3,850 tons of KUCC concentrate to confirm impurity deportments. These tests were performed at the Outokumpu smelter at Harjavalta, Finland. Matte produced was granulated and ground to the required size for subsequent conversion to blister copper in Outokumpu's pilot furnace at the Pori Research Center. This test work confirmed the selection of flash smelting and flash converting for the modernized KUCC smelter. Engineering was started and the basic layout and design were completed in early 1992. The smelter expansion required a modernization of the KUCC copper refinery. This included installing the Kidd Process, using stainless steel electrodes. An innovative hydrometallurgical process was also installed to recover precious metals from tankhouse slimes. The strategy developed for the modernized smelter was to install the best available emission-control equipment to meet stringent environmental regulations and minimize the need for progressive modifications to the facility in the future. The permit limits for the modernized smelter are low by contemporary standards for smelters. The modernized KUCC smelter was designed and constructed to be the "cleanest" smelter in the world, with a sulfur capture of 99.9%. The new air-permit approval was received in April 1993, less than five months after the initial submission. Construction began in early 1993 and was completed by May 1995. Plant description Materials handling facilities. The flash smelting furnace (FSF) processes a combination of copper concentrate, copper precipitate, silica flux and recycled materials. Recycled materials include concentrate produced in a slag flotation plant. It also includes material from a new hydrometallurgical process handling smelter dust and the refinery bleed streams. A 150-mm (6-in.) stainless steel slurry pipeline delivers copper concentrates to the smelter from KUCC's concentrators. Copper precipitate, from an iron cementation operation at the mine site, is blended with concentrate before delivery to the Flash Smelting Furnace (FSF). The copper concentrates are subsequently dewatered in the filter plant, next to the smelter. The filtered concentrates, at 8% to 9% moisture, are conveyed to a concentrate storage building where two blended concentrate beds are made. A variety of crushed secondary materials (screened to -10 mm or 0.4 in.) are added to the bed as it is being built. A linear reclaimer reclaims the bedded material to the wet concentrate system, feeding two 440-ton capacity storage bins. Silica flux is delivered in trucks and transferred by tube conveyor to the flux bin. Granulated slag from the flash converting furnace (FCF) is recycled to a separate storage bin. Load cells on each bin allow continuous monitoring of bin loading. The feed rates to the dryer are controlled by variable-speed belt feeders and belt scales. Silica flux and recycled slag are withdrawn from their respective bins and fed to the dryer feed-conveyor system at a set ratio-to-concentrate feed. This ratio is periodically adjusted to maintain furnace slag and matte chemistry. The wet-feed materials are conveyed to a 12.5-ft diameter by 121-ft long rotary dryer. There, the moisture content is reduced to less than 0.2%. Dryer control is achieved by monitoring off-gas and product temperatures. Nitrogen is added to the combustion chamber gas to control the temperature and to maintain a low-oxygen atmosphere. This has prevented concentrate combustion in the associated bag filter and pneumatic conveying system. The particulate-laden dryer off-gas is vented through a four-compartment bag filter to remove the dried concentrate before being ducted to a single-stage sodium hydroxide scrubber. Flash smelting furnace. Dried furnace feed material is transported by a three-pod, dense-phase pneumatic conveying system to a charge bin above the FSF. The furnace feed is directed to a single concentrate burner using two drag conveyors. Air enriched to 75% to 85% oxygen is also fed through the concentrate burner. The reaction shaft has more than 200 water-cooled copper blocks in 13 equally spaced rings. The FSF has six slag tapholes and four matte tapholes. The 68% to 70% copper matte is tapped along refractory-lined launders, granulated using water to a product size of about 2 mm (0.08 in.). It is then conveyed to a covered, blended storage facility. The silica-based furnace slag is tapped down water-cooled copper launders into pots and slow-cooled before being processed through a dedicated slag concentrator. The slag concentrate produced from this flotation circuit is blended with other copper concentrates in the filter plant. Furnace off-gas is cooled and cleaned in a waste-heat boiler (WHB) and electrostatic precipitator (ESP). It then passes through primary and secondary gas scrubbers and then to the acid plant. All of the dust collected in the WHB is recycled to the FSF. The ESP dust is normally recycled through the FSF. If the heat balance of the furnace cannot process all of the generated dust, or the impurity content of the dust is above prescribed upper limits, the ESP dust can be processed through the hydrometallurgical plant. This recovers copper and precious metals and provides bismuth and arsenic control to the operation. Flash converting furnace. A mechanical reclaimer recovers granulated matte from blended storage and feeds it to a vertical roller mill where it is simultaneously ground and dried. The ground matte is then pneumatically conveyed to a storage bin located above the FCF. Matte, burned lime flux and a portion of the FCF off-gas dusts are fed to a single burner on the reaction shaft, together with air enriched to 70% to 85% oxygen. Matte is converted to blister copper and a calcium-ferrite slag, controlled to 18% copper and 16% calcium-ferrite. Slag is tapped down water-cooled copper launders to a water granulation system and recycled to the FSF. Blister copper, containing 0.1% to 0.5% sulfur, is laundered to one of two anode furnaces for further refining before being cast into anodes. As with the FSF, FCF off-gas is cooled and cleaned in a WHB and an ESP before passing through primary and secondary gas scrubbers, and then to the acid plant. All of the dust collected in the WHB and ESP is recycled through the FSF or back through the FCF Anode plant. Remaining sulfur in the blister copper is removed by injecting air through four tuyeres located symmetrically on each side of the anode furnace charge opening. The oxygen content is then reduced to target levels by injecting a mixture of steam and natural gas. The refined anode copper is cast on twin 16-mold casting wheels using automatic weighing. Product anodes are loaded onto railcars and shipped to KUCC's refinery. Spent anodes are returned to the casting plant for remelting through a shaft furnace and then cast into anodes. Acid plant. Variable-speed, wet-gas booster blowers are provided downstream of each of the two scrubbing systems. They exhaust furnace gases through the WHB, hot ESPs and the scrubbers. The gases are combined and flow through the wet ESPs. The cleaned gas then passes to the double absorption acid plant with a daily production capacity of 3,860 tons of acid. Excess heat from the acid-plant converter and intermediate absorption step is used to generate 150 psi steam in WHBs that are part of a proprietary heat recovery system (HRS). A superheater, located after the first converter pass parallel with the hot exchanger, superheats the steam produced in the HRS before it is sent to the powerhouse. The powerhouse can produce up to 31 MW of electrical power. The designed operating power production of 24 MW is 65 % of the smelter's electrical requirements. Hydrometallurgical plant. The hydrometallurgical plant processes dust from the FSF ESP for impurity control. It also decopperizes and neutralizes refinery electrolyte and FSF and FCF scrubber blowdown. Copper and precious metals are recovered as a residue and reprocessed through the FSF. The deleterious elements such as bismuth and arsenic, are removed from the system. All feed-handling equipment is vented through high-efficiency bag filters. Dryer gas, after cleaning in a bag filter, passes through a desulfurization gas scrubber before being discharged to the atmosphere through the main stack. Secondary gas, primarily from the furnace launder ventilation systems, is exhausted through a four-compartment bag filter, scrubbed using sodium hydroxide and then discharged through the main smelter stack. Anode furnace combustion and refining gases are processed through desulfurization scrubbers. These scrubbers control emissions during the furnace oxidation process and remove the fine particulate in the furnace gas. The cleaned gas is discharged to the atmosphere through the main smelter stack. Desulfurization scrubbers are also used to process gas from the hydrometallurgical plant before being discharged through the main stack. The exhaust gases from the matte and slag granulation systems are contained and processed through the secondary gas-handling system. Operating history Flash smelting furnace. The FSF began commercial smelting operations of copper concentrate on June 13,1995 under the direction of smelter operations. Steady progress was made during the first three-and-a-half years of operation. The concentrate reclaimer required many modifications to increase reliability to an acceptable level. The rotary dryer has operated effectively at its rated capacity of 264 wet-tons per hour. Unlike most flash smelting operations, the rotary dryer is equipped with a bag filter instead of an ESP. To date, this system has worked effectively. No incidence of roasting concentrate has been encountered. An oxygen content of less than 6% is maintained in the off-gas to the baghouse by using nitrogen. The pneumatic transport system had to support increasing furnace feed. Generally, the FSF has performed well. The concentrate burner included the latest improvements from Outokumpu but proved to be less than satisfactory throughout the range of operating parameters required during normal operation. This resulted in buildup forming at the tip of the burner, on the burner block and on the reaction shaft roof, requiring regular cleaning. Modifications have been made in this area that have improved the operation and reduced maintenance. During ramp-up, the ability to maintain effective bath-level control in the FSF became more difficult as feed rates increased. Common launders were used to carry matte or slag away from two tapholes. As such, any difficulty on one set of launders immediately isolated two tapholes from service. This proved to be difficult with matte tapping because there were only four tapholes and two launders. This problem was eliminated in early 1996 when all systems were redesigned to provide individual launders to each taphole on the furnace. The ventilation and burner systems associated with matte tapping were also upgraded. FSF matte and FCF slag granulation have proceeded without diffculty. The bucket excavator systems were upgraded because the tapping rates were more than 148 tons per hour at times, or almost twice the original design. A key parameter in maintaining control of FCF slag metallurgy is to control the silica content in FSF matte. If the silica in FSF slag from slag contamination is greater than desired, the silica content in FCF slag increases to more than 3%. At this point, the slag viscosity significantly increases with the formation of di-calcium silicate and can result in tapping difficulties. With flash converting, additional care must be taken to minimize the amount of slag tapped with matte from the FSE. Blending of up to three days of matte production in the storage building assists in diluting any slag if any should be tapped with matte. Process dust, recovered in the waste heat boiler, is transported from the boiler drag conveyor to a roll crusher. The crushed dust is then pneumatically conveyed to a bin above the FSF by a dense-phase conveying system. During the May 1997 shutdown, a backup crushing and conveying system was installed to minimize downtime due to dust handling problems and allow more time for maintenance. Flash converting furnace. The FCF began commercial converting operations on July 4,1995, under the direction of smelter operations. Granulated matte is reclaimed and conveyed to a vertical grinding mill. The mill has been generally trouble-free and is operated at rates between 55 and 100 tons per hour. The handling of slag and blister copper from the furnace initially hampered operations. The length of the launders from the blister tapholes to the two anode furnaces and the design of the original launder burners and covers contributed to problems in maintaining the flow of blister copper to the anode furnaces. The launder configuration was modified and new, ventilated launder covers were installed, together with an improved launder burner system. Downtime caused by frozen copper in the launders is no longer a problem. The choice of calcium ferrite slag chemistry was primarily a function of minimizing the risk of foaming, together with reduced impurity recovery to blister copper. This slag tends to be more corrosive than a silica-based slag, particularly in the sidewall area beneath the reaction shaft. With calcium ferrite slag chemistry, it is practical to maintain a protective layer on the furnace sidewall. This is done by controlling the magnetite level in the slag by control of lime, silica and copper levels in the slag. Any reduction in the protective layer can be detected by monitoring the heat losses to the settler sidewall cooling system. After processing more than 715,000 tons of matte during one campaign, cooling losses through the settler sidewall were stable. After some initial startup difficulties, the FCFs operation has steadily improved and now exceeds design production rates. The design of the sidewall cooling element to extend the time between furnace rebuilds has resulted in continuous improvements to the operation. During the scheduled furnace repair in April 1997, improved skew and sidewall coolers were installed based on a proprietary composite design. As with the FSF, the handling systems for process dust from the WHB and ESP were upgraded in 1997. Process gas handling. The furnace gases are cooled in WHBs. Excess oxygen is injected into the furnace uptake and reacts with dust to form nonsticky sulfates. Both of the boilers have operated effectively since startup. Two additional convection banks were installed in May 1997 to improve the heat recovery of the FSF WHB at feed rates of more than 220 tons per hour. The design of the boiler had allowed for this and their installation was completed in a few days. Problems were experienced in the scrubbers that were caused by selenium precipitation in the scrubber liquors. While the selenium content of the concentrate feed was known, the problems were unexpected. Selenium deposited in the weir bowl of the FSF scrubber restricted the liquid flow. This resulted in a burn-through of the fiberglass. The system has been redesigned and the weir bowl and associated FRP ductwork replaced with an alloy unit. Selenium has also plugged pumps, screens, flow meters and other instrumentation. Selenium compounds are now removed from the system by periodically allowing the pH of the scrubbing system to become more acidic and the use of dispersion chemical additives. Steam-driven compressors pull the gases from the booster blowers and through the rest of the acid plant. Plant design was to have two compressors operating. However, surging problems encountered when a second compressor was put online limited off-gas handling capacity and furnace operation. Many attempts were made without success to resolve this issue by changes to the control system. Eventually, in July 1996, process gas recycle ducting was installed around each compressor, to ensure that there would always be sufficient gas flow to the compressors to prevent surging. This solved the problem. Acid plant. The acid plant operated consistently during the early part of smelter operation. In early December 1995, the steam superheater in the acid plant failed. The escaping steam diluted the acid in much of the plant. This resulted in severe corrosion and failure of one of the two HRS systems, much of the acid piping and the final absorbing tower. In April 1996, the plant was shut down to complete repairs and restore the plant to original capacity. With high feed rates on both furnaces and higher-than-design sulfur levels in concentrate feed rates, acid has been produced at the acid plant design rate of 160 tons per hour for extended periods. Sulfur dioxide emissions at this full rate were maintained at less than the design of 100 ppm. This represented a 99.95% collection efficiency of the 14% sulphur-oxide in the feed gas. Under normal operating conditions, sulphur-oxide emissions are in the 50 to 70 ppm range. Anode plant. Each anode furnace had a design capacity of 600 tons. In early 1997, modifications were made to the location and size of the mouth and tuyeres to increase the capacity to 750 tons. Originally, ammonia was used for deoxidization in the anode furnaces. Although the chemistry worked well, the use of ammonia resulted in the production of much higher levels of nitrogen oxides in the offgases than the permit allowed. As a result, the conversion was made to a steam/natural gas system. This has proved successful and less expensive to operate. The original casting wheel was a dual pour, dual takeoff system. It incorporated two concentric rings of 20 molds per ring and was designed for a casting rate of 110 tons per hour. This equipment was difficult to operate and maintain and could not produce quality anodes at a rate that would support design production levels. The decision was made to replace the casting equipment with twin 16-mold casting wheels. In this system, two wheels operate in concert to yield a total 110 tons per hour casting rate. Demolition and installation took place during a 43-day period during April and May 1997. The new wheels came up to capacity within two months and are now routinely casting at an average rate of 84 tons per hour. Recent operation A steady improvement in FSF operation has been achieved. The FSF has demonstrated the ability to sustain feed rates of more than 225 dry-tons per hour. The maximum daily throughput of dry feed to date is 5,306 tons. A steady improvement in FCF operation has been achieved. The FCF has also demonstrated that there is excess processing capability, as feed rates of up to 85 tons per hour of matte have been sustained for extended periods. The maximum throughput to date is 1,987 tons per day of matte. Successes While there have been areas in the smelter where improvements to equipment have been made, there are many areas where actual performance has exceeded expectations. The loss-in-weight feeders on all streams to each furnace have proven to be effective in accurately controlling material feed rates. This has enhanced metallurgical control on both furnaces. The flash converting process has been proven as an effective commercial method of converting matte directly to blister copper. Furnace feed rates of 50% above nominal design rates have been demonstrated. The furnace operates comfortably between 50 and 85 tons per hour with no adverse effect on process control. The granulation systems on the FSF and FCF have proven to be effective and reliable. Considerable effort was put into the design of the WHBs by analyzing problems experienced at other smelters. The result of this has been virtually troublefree operation. The acid plant was intended to operate at design capacity with a tail gas containing 100 ppm of sulphur-oxide. Tail gas concentrations of 50 to 70 ppm are consistently obtained from the feed gas stream, demonstrating a conversion efficiency of more than 99.9%. The hydrometallurgical plant has demonstrated that it is a versatile and effective processing route for impurity control in the smelter and treatment of bleed streams from the smelter and refinery. Conclusion The KUCC smelting facility was designed to minimize its environmental impact and thus set the best environmental practice for the industry. It has met its goal while demonstrating the ability to reliably process above-design tonnages of copper concentrates and matte to anode copper. Author Affiliation C.J. Newman, T.I. Probert and A.J. Weddick are manager of technical smelting and refining, smelter operations manager and technical superintendent respectively, with Kennecott Utah Copper Corp.