Top Submerged Lance Furnace Lining Cooling System Upgrade

Abstract

Freeport-McMoRan Inc. operates an ISASMELT^TM furnace for the smelting of copper concentrate at the Miami Smelter in Claypool, Arizona. In May 2017, the furnace was restarted after incorporating newly designed copper cooling elements from just above the tap hole to the underside of the roof. The objective of the improved cooling design was to extend campaign life by eliminating existing failure mechanisms with the wall refractory . In addition to upgraded bathline coolers and complete coverage of the gas offtake transition, or ‘kettle’, with plate coolers, a novel arrangement of coolers was installed in the remaining freeboard area. The shell required significant cutting and re-stiffening, as well as extensive modification of the cooling water piping. Based on performance observed to date, it is expected that smelter throughput will be increased due to extended ISASMELT^TM refractory campaign life.

Introduction

Freeport-McMoRan Inc. (FCX) owns and operates a copper smelter in Miami, Arizona that treats copper concentrates produced at the North American mine sites. The smelter utilizes ISASMELT^TM technology for the smelting furnace , an ELKEM electric slag settling furnace , four Hoboken converters, two anode refining vessels, and a sulfuric acid plant. The smelter typically processes 680,000 dry metric tons (DMT) per year of copper concentrate.

A recent environmental upgrade at the smelter has allowed for an increase in permitted concentrate tonnage. There have been multiple constraints identified in the smelter and acid plant, one of the most significant is the 21 days of down time required to replace the refractory in the ISASMELT^TM furnace , during which concentrate cannot be treated.

The refractory life of the ISASMELT^TM furnace depends on a number of factors and as a result has varied throughout the life of the furnace . The refractory life was still lower than desired; despite the best efforts of the Miami Smelter staff to improve furnace control and optimize refractory selection. As a result, a project to design and install an intense refractory cooling system to enhance the refractory life of the furnace was initiated.

Project Background

The Miami Smelter implemented ISASMELT™ technology as the primary smelting process in 1992 as an alternative to the electric smelting furnace . This technology is one variation of a technology referred to as Top Submerged Lance, or TSL smelting , that utilizes a lance pipe to inject oxygen-enriched air directly into a molten bath. A basic diagram of the furnace and associated equipment is included in Fig. 1 for reference.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig1_HTML.gif — Fig. 1
ISASMELT^TM Furnace component diagram

ISASMELT^TM refractory replacement is the reason for the frequency of the major smelter turnarounds as the refractory thins from heat, chemical dissolution, and the violent turbulence of the smelting reactions. The refractory life in the vessel has varied over the past 26 years and is affected by many operating variables. The recent campaign lengths have been approximately 9 months as shown in Fig. 2.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig2_HTML.gif — Fig. 2
ISASMELT^TM campaign length

The campaign life of an ISA furnace is a function of many factors: (1) temperature control , (2) relative intensity of the process, (3) slag chemistry, (4) refractory quality and (5) thermal cycling. The Miami Smelter staff have addressed many of these factors and made many improvements in the furnace operation:

Gradually lowered the furnace operating temperature to reduce thermal stress on the refractory .
Replaced the boiler-tube style splash block with a water-cooled copper splash block; as the old style was a source of leaks that caused downtime.
Performed extensive refractory testing to understand the mechanisms of failure and improve refractory specifications for subsequent brick purchases.
Made extensive improvements in lance control to reduce thermal cycling.
Gradually reduced blast volume down the lance to minimize bath turbulence and erosive brick wear.
Improved overall downstream plant equipment availability to reduce thermal cycling.

In addition to the operational control improvements, there were several design changes to the furnace cooling system to enhance refractory performance and minimize downtime. Upgrades to the vessel cooling were not completed in a single step and consisted of the following:

Elimination of cooling blocks in the roof with boiler panels, which is outside the scope of this paper
Redesign of the splash block
Replacement of flat billet copper bathline cooling bocks with copper castings
Installation of horizontal plate coolers in the sloped portion of the kettle
Installation of new bathline cooling blocks
Installation of new horizontal plate coolers in previously uncooled regions of the freeboard and the vertical half cylinder upper portion of the furnace

The original boiler tube splash block would last less than one year, due in part to the relatively high gas velocities in the FCX vessel. Large chunks of boiler accretion would also fall onto the block and cause premature failure. A new thick copper block was designed with intensive cooling , copper nickel pipe coils (MTI patent), pockets to retain castable or rammed refractory , and a weld overlay on the ‘nose.’ Installation was problematic as the vessel shell had distorted after installation of the original splash block due to the large horizontal hole in the steel shell. The shell was stiffened as part of the design for the new splash block, and a ‘table’ and trolley installed outside the furnace to slide the block in and out. The current design is expected to last for many years, thereby eliminating a significant maintenance issue.

The original bathline cooling blocks were 4500 mm tall flat copper billets with drilled water passages. Steam was generated intermittently at the top of the water passages due to the poor flow regime with drilled passages. This led to significant vibration and soldered copper joints failing. Several of these blocks were replaced with copper castings with internal pipe coils during an earlier campaign to both eliminate steam generation and the potential for leaking plugs inside the furnace . Internal pipe coils were used in subsequent designs due to their success here.

The flat shape of the bathline blocks did not fit well to the cylindrical steel shell and pattern of the brick. A gap of about 40 mm was present due to the mismatch, which was filled with mortar. As the mortar crushed, pressure on the brick became uneven which led to shorter refractory life. Further, the mortar is a relatively poor thermal conductor, which causes uneven heat removal from the brick, resulting in thermally induced strains in the refractory .

Heat loads are not uniform above the bath, which leads to uneven vertical growth and vertical pressure on the refractory . Such uneven pressure can lead to premature failure of a brick lining. Differential thermal expansion between the tall copper bath cooling blocks and the brick in front of it was the primary cause of premature failure of the refractory just above the coolers. This required a 21-day shutdown to completely reline the furnace .

Traditionally, there were also 10-day mid-campaign outages to reline the offtake area (i.e. kettle) area of the furnace . This was because large accretions would form in the offgas hood and fall onto the refractory in this area thus accelerating the wear.

To solve the shortened kettle refractory life, FCX collaborated with MacRae Technologies (MTI) in 2014 to develop a refractory cooling system for this area. The results of the cooling system installation were very favorable and, after collecting three campaigns worth of data, FCX was confident that the concept was applicable to the rest of the furnace . Causes of premature wear included corrosion due to slag , impact from accretions, and wear from high velocity dust laden process gases. MTI designed a system of horizontal plate coolers for the slope portion of the vessel, which incorporated the following:

A three-dimensional (3D) model of the furnace shell, refractory , and cooling blocks (copper , pipes, thermowells) was developed for the preparation of all design drawings, analysis, and extraction of quantities.
The copper plate coolers utilized a ‘tulip’ shaped groove on the hot face to retain refractory . The grooves had radii to reduce stress concentrations associated with sharp edges in the refractory .
The number of shapes was minimized with a novel pattern to accommodate the complex shape of the slope profile in the kettle.
Hold backs were added on the copper tabs which extended through the shell to prevent any block from falling into the furnace either during operation or maintenance . Installation, operation, and maintenance all need to be included in the design of any furnace component.
Openings were cut in the steel shell as the furnace was bricked to accommodate the copper tabs with the inlet and outlet piping and thermowell.
The shell was stiffened on the outside due to the considerable number of holes. A check of the shell stresses was performed using finite element analysis (FEA).
A thermal analysis was performed on the cooled portion of the kettle; including the shell, refractory , and cooling blocks. Performance curves were developed for each cooler to establish appropriate thermocouple alarm limits.

As a result of these modifications, the kettle area then survived as long as the remainder of the vessel lining. The critical failure point then became to the top of the existing bath zone copper coolers. The issue was that the refractory at the top of the coolers would fall out and expose the cooler to the process, as illustrated in Fig. 3. The design of the original copper coolers allowed for boiling of the cooling water, which would damage the piping system and force the smelter into a shutdown.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig3_HTML.gif — Fig. 3
Missing refractory on bath zone copper cooler

Even though the cooled kettle area refractory was in good condition, it had to be removed three times during subsequent relines of the furnace as it created an overhead hazard when replacing brick below. This increased the reline time and lost production. Based on the measurements taken from the cooled kettle area refractory after 9-month campaigns, a 3-year campaign life appeared achievable if the intensive water cooling was expanded to the entire vessel. The design goals were to make the furnace cooling system modular to facilitate partial refractory replacement, eliminate the bath zone refractory failures, and incorporate heat loss/wear monitoring by zone.

The timing of the proposed refractory cooling project was such that it would coincide with a scheduled 24-day outage for a heat exchanger replacement at the sulfuric acid plant. After an extensive investigation of the timeline with the refractory installation contractor, MTI, and copper cooler manufacturers, it was determined that the cooling system could be designed, procured, and installed by the end of the planned outage.

In order to complete the project in time for the 24-day outage it was imperative that any unnecessary installation difficulties be eliminated. The actions taken included:

Utilizing the existing furnace shell and bath zone cooling water connections
Incorporating an existing heat exchanger that wasn’t in service
Utilizing existing main cooling water manifolds to supply the new cooling system via smaller manifolds for each zone
Optimizing shell stiffening steel additions
Miscellaneous piping fabrication/installation prior to the outage
Optimizing cooler design for minimal shell cutting

Design of 2017 Cooling System Upgrade

The furnace at FCX posed several technical challenges to the design. It was also necessary to minimize both capital costs and shutdown duration.

The following remaining issues needed to be resolved:

Premature failure of brick at the top of the bathline cooling blocks, as it was a significant cause of short campaign life.
Rapid wear of brick in front of the bathline coolers.
Refractory wear above the bathline coolers, except for the sloped portion of the kettle where the recently designed system was performing very well.
Uneven pressure on the brick due to uneven vertical expansion and constraint from above.

To minimize technical risk, the following design aspects were employed:

Replace the flat vertical cooling blocks in the bathline with curved panels, to make heat removal from the brick uniform, and reduce stresses due to uneven radial grown between the brick and the cooler . The thick layer of mortar with the faceted flat panel layout was thus eliminated.
Retain the design for the splash block, including the system of mechanical fastening to the shell and expansion allowance.
Develop a pattern for plate coolers above the bathline based on expected heat loads and proven MTI designs used elsewhere.
Retain the recently installed horizontal plate coolers in the kettle and employ this pattern as the basis for the remainder of the upper portion of the vessel.
Employ a pocket pattern where possible to hold protective accretions; based on the successful performance on the splash block.
Install a minimum of dual cooling circuits in high wear areas.
Use existing openings in the steel shell wherever possible.

The design of an individual cooling block to remove the required heat load is a relatively straight forward task. Well established existing designs could be relied upon for most of the detailed design. However, the nagging issue of vertical expansion remained, both at the top of the bathline and for the remainder of the furnace .

A novel design approach with lintel coolers was adopted (MTI patent pending) to partition the furnace into three zones: the bath line, freeboard, and upper barrel/kettle. Each zone is supported independently to balance vertical expansion and pressure on the brick. Brick, cooling blocks and accretions within a zone can be supported by a lintel ring below with a backup steel shelf in close proximity to the lintel for redundancy. Brick in front of the bathline coolers are still supported by refractory below. The lintel ring design mates to the splash block, so that the same amount of expansion allowance material could be used below and above the ring. Further, the lower lintel ring is designed to accommodate the step change from the bathline coolers to the plate coolers.

The lintels and bathline coolers were designed to withstand high heat loads in case of refractory loss at the top of the bathline coolers.

The existing shell was retained to minimize the time for the shutdown. New holes needed to be cut for the additional plate coolers and lintels. A sizeable number of stiffeners were added to compensate for the new shell cutouts and as well as the need to carry forces from the lintel rings.

Refractory drawings were updated to reflect the changes in required expansion allowance within each ring of brick.

An image of the layout of the shell stiffening and new cooling blocks is included in Fig. 4. Close proximity of the shell and tabs of the copper blocks to existing building steel necessitated the introduction of a few special coolers shapes.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig4_HTML.jpg — Fig. 4
Model of Modified ISASMELT^TM furnace with surrounding building Steel

A 3D model of the shell incorporated several changes in shell thickness, the complex shape of the furnace , and a system of new stiffeners to balance the new holes. A plot of the 3D model and the shell stresses are included in Fig. 5.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig5_HTML.gif — Fig. 5
3D model and shell stress analysis

A thermal analysis was performed for each zone to approximate wear patterns and to obtain values for the preparation of cooler performance curves. An image of part of the thermal analysis for the freeboard is included in Fig. 6.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig6_HTML.gif — Fig. 6
Sample thermal analysis for part of the freeboard area

Installation

The project engineering was completed in phases to deliver items that could be fabricated or procured in advance of the actual outage. The cooling elements had the longest lead time and as a result took precedence; followed by piping, structural steel modifications, and lastly, instrumentation.

During the 2014 kettle cooling project, a benefit was realized by running small manifolds close to the furnace to supply the individual cooling circuit supply and return lines, as opposed to running them individually from the main manifold. This allows for less piping overall, which is a benefit because the individual circuit pipe runs were field fit. These manifolds were fabricated and installed prior to the outage as well. An example of these assemblies is shown in Fig. 7.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig7_HTML.jpg — Fig. 7
Plate cooler supply and return manifold assemblies

The piping for the plate cooler manifolds included temperature , pressure, and flow measurements with a bypass that would allow them to be serviced without shutting down the furnace . Each individual circuit services 8–11 plate coolers in series. These circuits include manual globe valves, flow indication, ball valves, drains, pressure indication, temperature indication, and a pressure relief valve. This allows for flow adjustment and pressure leak testing for individual circuits.

Another piping feature that expedited the installation was the circular manifolds that serviced the bath line cooler supplies and returns. These manifolds were custom built to utilize the existing holes in the furnace shell and were installed prior to the outage; this greatly reduced the required piping runs. Each bath line circuit included manual globe valves, ball valves, drains, pressure indication, and temperature measurement similar to the plate cooler circuits. The bath zone cooler manifolds are shown in Fig. 8.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig8_HTML.jpg — Fig. 8
Bath zone cooler supply and return manifolds and circuits

The bath line coolers were relatively straightforward to install as they were the same height as the original units and the cooling water piping used the same shell cutouts. The coolers are very large and difficult to maneuver into place, but there was an additional design feature on the new curved coolers to facilitate easier installation and removal . After setting them into place with the overhead crane, the coolers were bolted to the shell from the outside, which was another added benefit over the original units. The bath line cooler installation is shown in Fig. 9.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig9_HTML.jpg — Fig. 9
Bath line cooler installation

Installing the plate coolers required measuring out the location inside the shell and cutting out the hole. This was done with one course of brick early so the bottom of the hole could be cut to have the required clearance for any expansion and contraction of the refractory during operation. The plate cooler was laid into the hole with the overhead crane after the next brick course was laid and a safety holdback was bolted to the cooler externally.

The challenges associated with installing these coolers included ensuring the cooler was laid in the right direction, the correct cooler shape was used, and the vertical expansion was installed in the correct refractory courses and not in direct contact with a cooler . There were also two water-cooled copper inspection doors incorporated in the design to inspect coolers in areas that can’t be seen from openings in the roof. The plate cooler and inspection door installation processes are shown in Fig. 10.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig10_HTML.jpg — Fig. 10
Plate cooler and inspection door installation

The shell stiffening steel was a point of complication as it was known that the furnace shell had been distorted over its life; however, there was no reasonable method to quantify it. All of the stiffening steel design and fabrication was based on a straight furnace shell so there was a fair amount of trimming anticipated prior to the outage. The stiffening steel could not be installed prior to the outage even though it was procured. Some of the steel was in close proximity to coolers protruding through the shell. Their exact positions could fluctuate as governed by refractory course work. Coordinating the cooler , stiffening steel , and flex hose installations in that order was a challenge.

The steel around the lintel cooler rings was the most problematic aspect of the stiffening steel installation. The lower lintel coolers were unintentionally installed 9° off in the clockwise direction, which caused interference with the vertical stiffening layout. As a result, another horizontal stiffening ring had to be fabricated in the field and installed above the lower lintel ring, in addition to rotating vertical gussets between the coolers. The upper lintel ring steel required extensive trimming due to the large amount of shell deformation in this area. Some stiffening steel examples are shown in Fig. 11.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig11_HTML.jpg — Fig. 11
Stiffening steel installation and fabrication

As the stiffening steel was completed, the individual circuit field piping and stainless steel flex hoses were installed, which was the most challenging aspect of the whole installation. Due to the large number of hoses, dual circuit coolers, and existing obstructions in the field, this process required a great deal of coordination and planning. Connecting the hoses was not intuitive—it was possible to land them in the wrong location and have an incomplete circuit. Welding around the installed hoses was also challenging. Some field piping and hose installations are shown in Fig. 12.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig12_HTML.jpg — Fig. 12
Plate cooler circuit piping runs and hose connections

The final aspect of the project was installing and connecting any remaining instrumentation to the new remote I/O racks and junction boxes built prior to the outage. This was done last because the hot work and congestion in the area would have damaged the wiring and instruments. The instrumentation is only intended to monitor the system so the furnace heat up could begin without it, as long as water flow was confirmed through all circuits. In addition to the cooling water instrumentation mentioned previously, every plate and bath cooler had a dedicated thermocouple with alarm set points based on the FEA results to prevent damage to the coolers.

Operation

At the time of this writing, the cooling system has been in operation for 9.5 months. There have been no cooler leaks or operational issues associated with the cooling system during this time. A visual inspection of the furnace interior was completed during a scheduled outage in November 2017. The refractory lining and cooling system were found to be in good condition. Figure 13 shows the different furnace zones referred to in later sections.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig13_HTML.gif — Fig. 13
Cooling system zones

The plate cooler zones (kettle, upper freeboard, and freeboard) had worn approximately 1”–4” depending on the zone. The freeboard zone showed the least amount of wear. This fact was interesting because this area is closest to the agitated molten bath and showed the highest heat losses during operation. The upper freeboard area showed the most extensive wear between coolers and generally had the lowest heat load during operation. The observations showed a positive relationship between heat removal and cooler density. Several plate cooler hot faces were inspected and no erosion or oxidation was found. Pictures of the kettle/upper freeboard and freeboard wear profiles are shown in Figs. 14 and 15, respectively.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig14_HTML.gif — Fig. 14
Upper freeboard and kettle condition

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig15_HTML.jpg — Fig. 15
Freeboard condition

During furnace operation, the heat losses in the various cooler zones are highly dependent on the slag coating thickness, which is a function of the operation. Operating parameters during steady operation can be optimized to decrease the heat losses in the plate cooler zones. During upset conditions, the furnace temperature can be very challenging to control and the slag coating on the coolers can be compromised. This is especially true if the startup of the furnace is not well managed or there is a large amount of solid material in the vessel to melt. It is worth noting that the heat losses in the furnace do not appear to be proportional to concentrate throughput as long as the operating temperature is controlled. Figure 16 represents the heat losses in the freeboard zone, which is the most reactive of all the zones, for the first 100 days of operation.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig16_HTML.gif — Fig. 16
Freeboard heat loss variation during operation

The heat loss graph above illustrates that a stable slag coating can be formed and heat losses decreased during consistent operation. This is illustrated between startup and the first offtake blockage. Conversely, the heat losses can be erratic and a slag coating difficult to maintain during unstable operation. This is illustrated after the first offtake blockage, when the furnace feed rate was constantly fluctuating due to issues at the sulfuric acid plant and compounded by a malfunctioning oxygen valve actuator. After the acid plant and oxygen valve actuator issues were rectified, the furnace operation stabilized and the slag coating was reestablished, although there was likely some permanent refractory loss during that time. The appearance of the refractory lining and cooling system also reflects the slag lining integrity as shown in Fig. 17.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig17_HTML.jpg — Fig. 17
Cooling system conditions during low (left) and high (right) heat losses

This is the first campaign operating with the new cooling system; hence, it is difficult to quantify the long-term performance. While the kettle zone cooling design did not change, the existing bath zone coolers were upgraded so the heat loss data for that zone can be compared. The lower heat loss depicted in Fig. 18 indicates that the new cooler design likely slowed the bath zone refractory wear. The data is also less erratic due to the addition of new flow and temperature measurements for this zone.

../images/468727_1_En_37_Chapter/468727_1_En_37_Fig18_HTML.gif — Fig. 18
Bath block cooling heat loss comparison for new cooler design

Summary

The FCX Miami Smelter and MTI designed and installed an intense refractory cooling system in the ISASMELT^TM primary smelting furnace . The intent of this cooling system was to prolong the refractory life of the furnace , which was the cause of frequent outages and lost production. The design was fashioned after the previously successful refractory cooling system in the ‘kettle’ area of the furnace .

The cooling system has been in operation for 9.5 months and has operated without any significant issues or cooler leaks. An inspection was conducted during an outage in November 2017 and the refractory and cooling system was found in good physical condition. The refractory in the plate cooler zones (kettle, upper freeboard, and freeboard) showed wear ranging from 25 to 100 mm. The bath zone refractory had some wear in the previous failure area at the top of the coolers, but the remaining refractory was in good condition and likely thicker than at the same time into the previous campaign as shown by Fig. 18.

The main factor to lower heat losses and wear rates is good operational control that establishes a stable slag coating. There has been no indication that higher concentrate throughput increases heat losses as long as temperature is held constant.

Based on the observations during the inspection and the FEA data, the plate cooler areas should find an equilibrium wear point and drastically slow down to provide a long refractory campaign. The bath zone refractory will likely wear back to the coolers later in the campaign. The bath zone coolers are designed for large heat loads and should be able to establish a slag coating and continue safe operation.

Acknowledgements

The authors would like to thank the FCX Miami management team, as well as Western Refractory Construction Inc. and FCX Miami Maintenance for installing the cooling system components. We would also like to thank the FCX Miami Instrumentation and Process Control groups for building and installing all associated electrical equipment, as well as making the final instrument connections. Lastly, thanks to Robert Brandt and Patrick Essay of the FCX Miami Smelter Metallurgy Department for their project management assistance.