Improvements to Stillwater Mining Company’s Smelting Furnace Yielding Increased Capacity and Productivity

N. Voermann, V. Vaculik, T. Ma, C. Nichols - Hatch Associates, 2800 Speakman Drive, Mississauga, Ontario, Canada L5K 2R7

G. Roset, W. Thurman - Stillwater Mining Co., Precious Metals Smelter, Columbus, Mt, USA, 59019

Abstract

Stillwater Mining Company operates an electric furnace at its smelter in Columbus, Montana to treat a Ni-Cu-platinum group metal concentrate along with secondary PGM bearing materials. Hatch Associates has designed several improvements to this furnace, which was originally built by others. Implementation of these relatively low cost improvements has resulted in substantial increases in furnace availability and capacity, along with reduced unit costs.

The improvements included:

Installation of water-cooled copper cooling elements to stabilize refractory wear and increase furnace life.

Improved furnace bindings, to enhance the furnace’s structural integrity by improving the contact between the refractory and cooling elements, and by promoting tight brick joints resistant to matte or slag leaks.

Adjustment of the electrode regulator to allow higher furnace power levels. Installation of an automated air-slide (aeration conveyor) feed system to evenly distribute

the furnace feed in an insulating layer on the bath surface. This “black-top” practice reduces unit energy costs and makes available additional smelting capacity.

Electrode seals to reduce ingress of air that absorbs heat before exiting the furnace in the off-gas stream. The saved energy can be used to smelt additional concentrate.

This paper describes the above furnace modifications, and the benefits their implementation provides.

Introduction Stillwater Mining Company (SMC) operates a precious metals smelter and base metals refinery in Columbus, Montana to treat a platinum group metal (PGM) bearing nickel-copper concentrate, along with some secondary PGM bearing materials. The concentrate is produced at the company’s mine and mill at Nye, Montana. The secondary materials processed are spent catalysts from petroleum refining and automobiles.

The plant flowsheet is shown in Figure 1. A 1680 kVA three-electrode-in-line electric furnace (EF) is fed with concentrate, secondary materials, granulated return converter slag, and limestone flux. The EF slag is discarded, and the matte is water granulated and dried before being fed to one of two top blown rotary converters (TBRC’s). Desulferized matte produced by the TBRC’s is also water granulated and dried. It is further processed in SMC’s on-site base metals refinery.

Figure 1: SMC Smelter Flowsheet

The start-up and early operation of the electric furnace was difficult, principally due to rapid refractory wear leading to short campaign lives [1]. SMC, in an effort to reduce unit costs and increase reliability and capacity, have made several modifications to the smelting furnace originally commissioned in 1990. Converting capacity has been increased to match the furnace production by adding a second TBRC identical to the first. The incremental electric furnace improvements have been made at low capital cost, and have yielded significant increases in furnace campaign life, smelting capacity and efficiency. Campaign life improved from 5-9 months to several years. By increasing furnace efficiency (i.e., reducing the kWh per ton feed requirement) furnace capacity was increased approximately 20%, as shown in Figure 4.

The table below lists the furnace upgrades along with their implementation dates. Details of each upgrade and the results obtained from them are given in following sections of this paper.

Table I: Electric Furnace Upgrades

Upgrade Purpose Results Date

Implemented Water-cooled copper elements (finger coolers) installed in slag zone of walls

Stabilization of refractory erosion

Increased campaign life Allows furnace to run at higher

power

1992

Binding springs added Improve contact between coolers and refractory to prevent leaks of molten bath

Prevents leaks of molten bath at brick joints

1994

Electrode regulator tuned Allow furnace to operate at maximum transformer power

Higher smelting capacity under automatic control

1995

Feed system upgraded Cover the molten bath with an insulating layer of feed, to reduce furnace heat losses through roof upper walls and off-gas

over 20% higher smelting capacity

Increased efficiency (reduced kWh/ton concentrate)

Longer roof refractory life Lower temperatures above

furnace for operators adding electrodes or taking bath depth soundings

1997

Effective electrode seals installed

Reduce furnace ingress air, and thereby reduce heat lost to heating this air before it exits furnace in off-gas stream

Higher smelting capacity Increased efficiency (reduced

kWh/ton concentrate) Lower electrode consumption

1997

Stabilizing Refractory Erosion With Water-Cooling SMC’s electric smelting furnace had suffered short campaign lives after its initial construction and start-up. Severe wear of the refractory lining occurred in the slag zone of the furnace walls, particularly at the slag/freeboard interface. This refractory erosion was the cause of several run-outs; one such run-out was near the matte taphole.

To remedy this situation, water-cooled copper cooling fingers were installed in the slag zone, and a water-cooled copper module was installed to intensely cool the matte tap hole area.

Typically, adding water-cooling to a furnace allows the power input, and thus feed rate, to be approximately doubled, while simultaneously improving refractory life significantly relative to the naturally air-cooled case [2, 3, 4]. Power densities of well over 200 kW/m2 of hearth area are possible with an effective water-cooled refractory wall design.

Heat Flux/Equilibrium Wall Thickness Calculations Thermal conditions in the molten slag bath result in the process heat flux that the furnace lining must withstand. This heat flux is caused principally by natural convection in the bath, and forced convection due to bath stirring from electro-magnetic effects. The heat flux is given by:

Equation 1

QA

h T TPROCESS

BATH BATH WALL= −( )

where, hBATH = bath heat transfer coefficient TBATH = bath bulk temperature TBATH = wall inside surface temperature

Thermal equilibrium is reached when the wall has eroded sufficiently for a frozen slag layer to form on the refractory hot face. Once this frozen layer forms, the refractory is protected from further chemical attack and erosion, since the refractory is no longer in contact with molten slag. At thermal equilibrium Equation 1 becomes:

Equation 2

QA

h T TPROCESS

BATH BATH LIQUIDUS= −( )

where, TLIQUIDUS = frozen build-up liquidus temperature

The slag liquidus temperature is the temperature at which high melting point solid components just begin to freeze out of the bath. At thermal equilibrium, the heat flux through the wall is given by:

Equation 3

QA

QAPROCESS WALL

= where T TWALL LIQUIDUS=

For the simple one-dimensional case, Equation 3 can be expanded as follows:

Equation 4

( ) ( )h T T

T Tx

kx

k h

BATH BATH LIQUIDUSLIQUIDUS WATER

BRICK COOLER WATER

− =−

+ +∆ ∆ 1

where,

TWATER = bulk cooling water temperature ∆xk BRICK

= brick and frozen slag thermal resistance

∆xk COOLER

= copper cooler thermal resistance

hWATER = cooling water heat transfer coefficient

Equation 4 can be approximated by Equation 5:

Equation 5

( ) ( )QA

h T Tk T T

xPROCESSBATH BATH LIQUIDUS

WALL LIQUIDUS WATER

WALL

= − ≈−

∆

where,

kWALL = average wall conductivity ∆xWALL = residual wall thickness

It should be noted from the equations above that the heat flux through the wall depends only on process parameters, such as slag superheat and convection coefficient, and is independent of furnace wall construction. Therefore, contrary to intuition, adding water cooling to a furnace does not increase the furnace heat loss at equilibrium.

From inspection of Equation 5 it can be seen that if the average wall conductivity is high (e.g., with copper coolers) the residual wall thickness is large; conversely, if the wall conductivity is low (e.g., with naturally air-cooled refractory wall) the equilibrium wall thickness will be small.

The bath convection coefficient depends on slag conductivity and viscosity among other variables. Usually this data is unavailable; therefore, the design heat flux is usually determined from measurements of an existing process and/or comparison to similar processes.

Extending the above equations to calculate residual worn wall thickness in three dimensions is significantly more complex, and requires the use of three dimensional finite element modelling. Output from a computerized version of such a 3-D finite element model for SMC’s furnace is shown in Figure 2.

Figure 2: Section of 3-Dimensional Finite Element Heat Transfer Model of Furnace Wall

Furnace Refractory Compression / Binding The purpose of furnace binding systems is to compress the refractory and prevent leaks or run-outs of molten bath from brick joints. Compression of the refractory lining also enhances performance of water cooled elements embedded in the refractory, by promoting good contact and thus good heat transfer between the bricks and cooling elements.

The binding system should be strong enough to overcome brick/brick and brick/shell friction and push the bricks back together in a furnace cooling cycle. As the furnace cools the bricks shrink, potentially causing cracks to open up between individual bricks. An effective binding system keeps the bricks pressed tightly together even as they shrink.

The SMC furnace has a unique shape; see Figure 5. In plan the furnace is a rectangle with semi-circular ends. The discontinuity between the curved ends and straight sides had been the location of bath leaks. Springs acting tangentially to the curved ends at the joints (i.e. parallel to the straight sides) were added to compress the bricks at this critical area. This design was successful in preventing leaks. A similar design was recently used to provide binding force on a circular furnace [5].

Electrode Control System

The furnace power on SMC’s EF is controlled by the position of the electrode tips in the resistive slag bath and transformer voltage tap. The electrode positions are regulated with three analog impedance controllers. These controllers compare the operator selected impedance set points on the control panel Load Adjust Rheostats with the actual impedance of each electrode. Any error between actual and set point values is corrected by the controller causing the electrodes to be raised or lowered.

Electrode impedance versus tip position tests were performed, using a “dip test”. This procedure entails immersing the electrode known distances into the slag and measuring the corresponding electrode voltage and current. The system resistance can then be calculated using the simple relationship voltage x power factor = current x resistance. In addition to control system optimization, such immersion versus resistance data is also useful in selecting transformer voltage and current ranges for new installations.

It was determined that the previous furnace maximum power limit of 1350 kW correlated to an resistance of 40mΩ, as shown in Figure 3. By increasing the maximum electrode immersion in the slag by only 1/2”, a 250 kW increase in furnace power to 1600 kW could be achieved.

Figure 3: Resistance vs. Electrode Immersion

The maximum furnace power for each electrode is set by a slide wire calibration resistor connected in series with each control panel Load Adjust Rheostat. These resistors together (RT) limit the maximum furnace current and secondary furnace impedance at each transformer tap. Reduction of the resistance RT enabled operation of the furnace at the full transformer rating under automatic control, as derived in Table II and illustrated in the PVI diagram of Figure 4.

Table II: Electric Furnace Upgrades

Calibration Resistor (ΩΩΩΩ)

Min. RT (ΩΩΩΩ)

Max RT (ΩΩΩΩ)

Min. Rbath (mΩΩΩΩ)

Max. Rbath (mΩΩΩΩ)

Max. Fce. Power Auto control (kW)

Previous 3.2 3.2 6.4 40 82 1350 Revised 2.6 2.6 5.8 30 77 1600

The maximum furnace power is achieved at the top Tap 6 at 30 mΩ resistance. The change from 40 mΩ to 30 mΩ bath resistance at top tap results in a negligibly lower electrode immersion. Transformer resistance/power range could be increased further by reducing the resistance of the Calibration Resistor sufficiently to enable a secondary bath resistance of 25 mΩ per electrode.

Figure 4: Power Voltage Current and Smelting Rate Diagram

Furnace Feed System Black Top Practice and Furnace Heat Losses The relatively high heat losses from SMC’s electric furnace are due, in part, to the relatively small size of the SMC furnace. The smelting capacity of a furnace is related to its volume, or hearth area, while the heat losses are more dependent on outside surface area or perimeter. For a rectangular box, doubling the volume increases the surface area only about 1.5 times; therefore, a larger furnace is intrinsically more efficient than a smaller one.

Notwithstanding the fixed size of the existing furnace, significant energy savings can be achieved by black-top charging. This practice insulates the hot bath by completely covering the slag with a layer of furnace feed, thus reducing heat lost through the furnace roof, upper walls, and off-gas. Black top practice reduced the roof heat loss to 40% of that of the open bath case, as shown below.

Figure 5: Plan on Furnace Roof Comparing Open Bath to Black-top Heat Loss

Maintaining a black-top over the entire bath has reduced the furnace freeboard temperature from its previous 1150°C to about 500°C. The lower freeboard temperature resulted in an energy saving of about 270 kW due to reduced roof and upper wall surface heat loss, and reduced off-gas temperature. Better sealing of the electrode ports, as described further on in this paper, also contributed to energy savings by reducing furnace air ingress and thus the volume of heated off-gas.

The reduced losses resulting form the new furnace black top feed system and electrode seals are detailed in the table below:

Table III: Furnace Heat Losses

Source of Energy Stillwater (1500 kW) Stillwater (1500 kW) Saving Loss Open Bath Operation with Black-top and

electrode seals

kW

kW/m2

kW

kW/m2

kW

% of power input

Off-gas 302 - 140 - 162 11 Roof Surface Losses

150 8.68 59 3.3 91 6

Upper Wall Surface Losses

23 1.5 8 0.6 15 1

Copper Coolers 96 17.0 96 17.0 0 0 Lower Wall Surface Losses

19 0.9 19 0.9 0 0

Bottom cooling air 91 5.3 91 5.3 0 0 Secondary bus and electrode arms

17 - 17 - 0 0

TOTAL HEAT LOSS 698 430 268 18

Achieving these energy savings allows approximately 20% more concentrate to be smelted in the existing furnace as shown in the power vs. feed rate graph of Figure 4. In addition to the energy savings, a cooler furnace roof makes working above the roof level (e.g., adding electrode sticks, measuring matte and slag levels) more comfortable, and is also expected to increase the life of the roof refractory.

The electric furnace feed is a mix of 78% concentrate and secondary materials (70% less than 150 mesh), 15% return TBRC slag (-1/8”) and 5% limestone (-1/4”). As the angle of repose of the mixed feed is 15°, four feed ports equally spaced along the furnace centerline are sufficient to completely cover the bath with charge. Figure 6 is a schematic of the new feed arrangement.

Figure 6: Feed System Schematic Using the existing pneumatic transfer system, the 3000 lb. mixed feed batch is conveyed to a single bin at the South end of the furnace. During the time required to pneumatically convey this batch into the bin, the furnace is not fed and the thickness of the black top decreases by about 1” to 5-7" when running at 1600 kW furnace power level. When the bin is full, feeding resumes, and the black top thickness is increased to its normal 6 - 8". A variable speed rotary valve (star feeder) at the outlet of the bin is used to control the overall feed rate to the furnace. This rotary valve discharges onto an air slide parallel to the furnace longitudinal axis. By aerating the feed, the air slide can convey it down a slope of approximately 8° to 15°. A cross-section of a typical air-slide is shown in Figure 7.

Figure 7: Air-Slide Detail

By changing the position of diverter gates at the air-slide outlets, the feed is directed down each feed pipe in turn. An automatic control system (described in more detail below) cycles through the four feed ports over time until the bin is nearly empty. Feed is directed to each feed point for an appropriate fraction of the cycle time to ensure that the bath is completely covered with charge.

Thermocouples installed in the furnace roof could be used to provide feedback on the black-top operation. If an area of open bath arises, the roof temperature above this area would quickly rise, indicating that more feed is needed in this location.

Layout Figure 8 is a photograph showing the new elements of the feed system.

The arrangement shown, with an air slide parallel to the furnace centerline passing over the electrode arms at the West side of the furnace, leaves the East side free for the addition of electrode sections. The lower end of the air slide (farthest from the bin) is still three feet above the upper travel limit of the electrode masts.

From the air slide, feed pipes with a slope of 65° take the feed down between the electrode masts to the furnace freeboard. Solenoid-controlled, pneumatic-cylinder operated diverter gates are used to select the feed point in use at any particular time.

Bin Air-Slide Feed Pipe (typical 4)

Figure 8: New Feed System

Test Arrangements In order to test all the major components of the new feed system, prior to final design of the new feed system, a 4 foot air slide section was installed under the existing rotary valve of the original feed system. This test air-slide discharged into 'A' phase electrode port. The arrangement is shown in the photographs of Figure 9.

Test air slide View inside furnace Discharge end of test air slide

Figure 9: Air Slide Testing

The test was successful in demonstrating: that the feed flows from the rotary valve and down a 15 degree air slide at maximum and

minimum feed rates without plugging that six inch nominal diameter pipes with 65 degree slopes would be adequate to convey the

feed from the air-slide into the furnace that feeding from a point on the furnace centerline covers the bath from one furnace sidewall

to the other.

These results have now been verified in practice.

Furnace Feed Control Control Overview The feed control system automatically matches the furnace feed rate to the furnace power, (using the feed rate versus power relationship shown in the lower graph of Figure 4) and distributes the feed among the four feed ports to maintain the desired covered bath, black-top furnace operation.

Feed Air Flowfrom Air-Slide

‘B’ Electrode

Feed Pile(Black Top)

A modular approach to the control system design was used to reduce the complexity. The system was partitioned into the three control modules: feed bin level control feed rate control feed distribution control.

Figure 10 shows the function blocks for the new features of the control system.

Figure 10: Feed Control Functional Block Diagram

Feed Bin Level Control The feed bin inventory is maintained by requesting additional feed form the batch transfer vessel (blow tank) whenever the level in the bin drops below the level switch. Furnace feeding is paused while the feed is transferred from the batch system to the feed bin. After the transfer is complete and the level in the bin is above the level switch, furnace feed automatically resumes. There is provision for a second feed bin, which would enable the process to be continuously fed, should this become necessary at future higher feed rates.

Feed Rate Control The feed rate control module is made up of three blocks: power monitor feedrate setpoint calculation feedrate PID controller (existing).

The power monitor acts basically as a filter on the measured furnace power. A one minute average is used to determine the feedrate setpoint.

The power consumed over the last interval from the power monitor block is divided by the feed consumption rate to generate a feed rate setpoint in lb/min. In addition to this, this module will

handle any adjustment to feedrate to make up for the rotary valve being stopped during refilling of the feed bin.

The feedrate Proportional Integral Derivative (PID) control is part of the original control system. The setpoint is provided by the feedrate setpoint calculation block.

Feed Gate Control The feed gate control module is made up of four blocks: feed distribution calculation gate delay compensation gate time calculation gate sequencer.

The feed distribution calculation processes the operator input to determine the proportion of feed to be distributed at each feed port. The gate delay compensation models the transport delay of the material in the air slide. The gate time calculation uses an operator specified total cycle time and calculates the individual feed times. The gate open and close times are determined using the cycle time, feed times, and delay times. The feed gate sequencer determines the feed sequence. The sequencer partitions the feed cycle into seven sequential cases. The durations are calculated by the gate time calculation block. During each of these cases, an output bit is set or reset for each of the diverter gates to open or close them depending on the desired conditions.

Implementation The control program was implemented on an Allen-Bradley PLC5/40 using a modular software structure mapped into the functional block structure described above. The integration of the air slide software was done by adding separate program files to provide the required functionality. Conditional program calls were added to the main control program to sequentially call the individual functions. This was done to separate the new software from the existing to facilitate testing and debugging while minimizing disruption to the existing control program. Operator screens where developed specifically for testing with the emulator. Figure 11 shows a typical testing screen.

Figure 11: Operator Screen

Commissioning Due to the extensive pre-installation testing, control system commissioning was completed within one week. The bulk of the effort during commissioning was spent modifying the existing software and integrating the new program files with the existing system. Relatively little time was spent commissioning the new functions in the control program.

Metallurgical Aspects The black-top (covered bath) furnace operation had some unforeseen metallurgical consequences that necessitated some changes in the furnace feed mix. With open bath operation there had been good contact between the bath and the air in the furnace freeboard. To counter the oxidation that occurred in this situation and to improve PGM recovery SMC added coke with the feed. Some carbon is also present in secondary materials. However, with black-top operation, the feed cover essentially eliminated contact between the bath and freeboard. The oxidation potential in the bath was thus greatly reduced. Consequently, coke (carbon) additions had to be reduced or eliminated to avoid forming a metallic, PGM rich frozen heel on the bottom of the furnace.

Electrode Seals The electrodes at Stillwater suffered severe pencilling (tapering towards the tips) due to burning of the graphite in the freeboard. Pencilling can be substantially decreased through a combination of reduced freeboard temperature (black-top smelting) and reduced air ingress (better electrode seals).

The electrode seals installed on SMC’s EF are simple, yet effective. A copper leveling plate grouted to the furnace roof electrode port refractory provides a smooth surface on the furnace roof to seal against. Since copper is very conductive thermally, the temperature difference between the inner and outer diameter of the plate will be small, and the plate will tend to stay flat instead of warping. To seal against the electrode, stainless steel weight rings compress a

refractory blanket packing, and in conjunction with the sloped outer surface of the housing, push this packing onto the electrode surface.

Conclusion The modifications to Stillwater’s electric furnace have been incremental and relatively low cost. Taken together, they have provided substantial benefits in terms of furnace reliability (copper coolers and bindings) and in terms of capacity and efficiency (black top feed arrangement, copper coolers and electrode seals). These projects illustrate how close, multi-disciplinary cooperation between operators and engineers on the retrofit of a metallurgical facility can provide cost effective results.

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