FIELD OF THE INVENTION

This invention relates to a metal smelting system with a control system arranged to control a cooling system and electric power input to the electrolytic reduction process to reduce or increase power consumption and which at least in preferred embodiments dynamically modulates together the cooling system and the electric current and/or power input to vary plant output with variations in market demand.

BACKGROUND

Aluminium is produced industrially by electrolytic reduction of aluminium ore in a molten cryolite, in a 'cell' or 'pot'. The cell or pot is heated by resistance heating from the same current, to a temperature typically about 960°C. However it is necessary to reduce the temperature of the shell of the pot to protect it from corrosion by the cryolite, and heat is intentionally dissipated to maintain a solid or frozen cryolite insulating layer or "ledge" at the sidewalls of the cell. The economic viability of a metal smelter is dependent at least in part on the

international demand and price for the metal produced.

With the traditional smelting system, large reductions in smelter output during periods of lower demand is possible only by shutting down a potline or the smelter altogether. This is undesirable as subsequent restart procedures are major and costly.

It is an object of the present invention to provide an improved or alternative system or method for the production of metal, and/or to at least provide the public with a useful choice.

SUMMARY OF THE INVENTION

In broad terms in a first aspect the invention comprises a metal smelting system which includes:

a pot or potline in which metal is produced by an electrolytic reduction process at elevated temperature,

a cooling system arranged to convey cooling fluid to the pot or potline; and a control system arranged to control together both the cooling system and the electric current and/or power input to the electrolytic reduction process, including at least reduce the flow of cooling fluid and reduce electric current and/or power input to a relative minimum in an insulating mode of the cooling system.

In at least some embodiments the control system arranged to terminate the flow of cooling fluid in the insulating mode of the cooling system.

In at least some embodiments the invention also includes adjusting the pot chemistry and input feed rate. In at least preferred embodiments the control system is arranged to control together the cooling system and the electric power input to the electrolytic reduction process between :

• a maximum mode in which the cooling system operates at a relatively high or maximum cooling, and

· said insulating mode in which the cooling system adjacent the potline acts as an insulator to reduce heat dissipation from the potline.

For example in the insulating mode air having a relatively low flow rate or stationary air in the cooling system may act as an insulator.

Preferably the control system is arranged to also operate the cooling system and the electric current and/or power input to the electrolytic reduction process in an

intermediate and/or minimum mode or modes between the maximum and insulating modes, in which the cooling system operates at intermediate or minimum cooling and the electric current and/or power input is at an intermediate and/or minimum level.

Preferably the control system is arranged to dynamically modulate together the cooling system and the electric current and/or power input to the electrolytic reduction process. In the preferred embodiment, the control system is further arranged to:

receive input data indicative of a combination of one or more of a price of the metal, a cost of the power required to produce the metal, and energy dynamics,

adjust one or more control settings associated with the electrolytic reduction process or the cooling system in response to the input data, and

operate the cooling system and the electrolytic reduction process in accordance with the adjusted control settings.

Preferably the control settings include an electric current and/or power input setting for the electrolytic reduction process. Preferably the control system Is further arranged to release surplus Input power to a power grid during operation of the electrolytic reduction process in accordance with the adjusted electric power input setting.

Preferably the control settings include a mode of operation and/or flow rate setting for the cooling system.

Preferably the control settings further include bath chemistry settings associated with the electrolytic reduction process.

The control settings may further include electrode separation settings associated with the electrolytic reduction process. Preferably the energy dynamics include information relating to any combination of power supplied by a power grid associated with the metal smelting system, or power demand of a population associated with the power grid, or an emergency power grid condition.

Preferably the control system is further configured to receive input data indicative of seasonal conditions, such as weather change.

Preferably the control system is configured to:

decrease the electric current and/or power input to the electrolytic reduction process and operate the cooling system in the insulating mode when the received input data is indicative of any combination of one or more of a relatively low metal price, a relatively high power price and/or a relatively high energy demand, and/or

increase the electric current and/or power input to the electrolytic reduction process and operate the cooling system in the maximum mode when the received input data is indicative of any combination of one or more of a relatively high metal price, a relatively low power price and/or a relatively low energy demand.

In broad terms in a second aspect the invention comprises a metal smelting method which includes reducing a metal ore to metal by an electrolytic reduction process at elevated temperature in a pot or potline, which method includes in a maximum output operating mode conveying cooling fluid to the pot or potline and maximum electric current and/or power input to the electrolytic reduction process, and in a minimum output operating mode reducing cooling fluid flow so that fluid in a cooling system acts as an insulator to reduce heat dissipation from the pot or potline and reducing electric current and/or power input to a relative minimum. In broad terms in a third aspect the Invention comprises a method of controlling a metal smelting system which includes a pot or potline in which metal is produced by an electrolytic reduction process at elevated temperature and a cooling system arranged to convey cooling fluid to the pot or potline, the method comprising the steps of:

receiving input data indicative of a combination of one or more of a price of the metal, a cost of the power required to produce the metal, and energy dynamics, and adjusting one or more control settings associated with the electrolytic reduction process or the cooling system in response to the input data.

Preferably the control settings associated with the electrolytic reduction process include an electric current and/or power input to the electrolytic reduction process.

Preferably the control settings associated with the cooling system includes a mode of operation of the cooling system.

Preferably the method further comprises the step of determining a desired rate of metal production from the input data, and the step of adjusting the one or more control settings is based on the desired rate of metal production.

Preferably when the determined desired rate of production is a minimum rate of production, the step of adjusting includes adjusting the control setting for the cooling system to an insulating mode where the flow of cooling fluid is reduced to a relative minimum or terminated, and adjusting the electric current and/or power input to a relative minimum.

Preferably when the determined desired rate of production is a maximum rate of production, the step of adjusting includes adjusting the control setting for the cooling system to a maximum cooling mode where the flow of cooling fluid is at a maximum, and to adjust the electric current and/or power input to a relative maximum.

In a fourth aspect the invention may broadly be said to consist of a pot of a metal smelting system in which metal is produced by an electrolytic reduction process at elevated temperatures, the pot design having a heat transfer dynamic and a range of throughput, wherein the range of throughput includes a sub-range in which the dynamic is maintained by passing air through, a shell heat exchanger capable of insulating the pot and wherein airflow through the shell heat exchanger negates the insulation of the pot, and whereby the range of throughput includes ranges of throughput outside the sub- range In which the shell heat exchanger provides cooling and ranges of throughput outside the sub-range in which the shell heat exchanger provides insulation.

In a fifth aspect the invention may broadly be said to consist of a metal smelting system which includes:

a pot in which metal is produced by an electrolytic reduction process at elevated temperature, the pot having a heat transfer dynamic and a range of throughput including a sub-range in which the dynamic is maintained,

a shell heat exchanger operatively coupled to the pot and capable of insulating the pot, wherein airflow through the shell heat exchanger negates the insulation of the pot, and whereby the range of throughput includes a range of throughput in which the shell heat exchanger provides cooling to the pot and a range of throughput in which the shell heat exchanger provides insulation to the pot. In a sixth aspect the invention may broadly be said to consist of a power grid for the accumulation and distribution of electricity to an associated population, and for the supply of power to an associated metal smelting system including a pot or pots In which metal is produced by an electrolytic reduction process at elevated temperature, and a cooling system arranged to convey cooling fluid to the pot or potline, and wherein the power grid comprises a control system arranged to control together both the cooling system and the electric power input to the electrolytic reduction process to regulate the production of metal in response to a condition of the power grid.

Preferably the condition of the power grid includes energy dynamics associated with the power grid. Energy dynamics may include, for example, demand of electricity by the population associated and available/accumulated power.

In the system of the invention, because when the potline is operating with maximum electric current and/or power input and the output of metal produced is maximum, an associated cooling system is also operating at a maximum, the electric current and/or power input to the smelter may be reduced and cooling reduced, to reduce the smelter output, when market demand for metal reduces for example. For example, at maximum capacity, the current input to the electrolytic process may be in the range 230-250kA, at which operating level the cooling system is operating at maximum capacity. When the cooling system is operating at minimum capacity or turned off in the insulating mode referred to above, then the current input to the electrolytic reduction process may be reduced, for example, to 170kA, which will in turn significantly reduce the rate of electrolytic reduction and metal production but without freezing of the potline. Thus the invention provides a metal smelting system in which the output may be reduced significantly, during periods of lower demand for the metal produced (without requiring shutting down of the potline) and/or during periods of higher electrical power demand from a power grid associated with the potline. The term "potline" as used in this specification means two or more pots in any configuration operating together to produce metal within a metal smelting system or process.

The term "comprising" as used in this specification means "consisting at least in part of". When interpreting each statement in this specification that includes the term

"comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner. BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are now described with reference to the accompanying diagrammatic drawings in which :

Figure 1 shows a flow diagram of a preferred form metal production system of the invention;

Figures 2A and 2B together set out a control algorithm for use in a preferred form metal production system and method of the invention;

Figure 3 shows a schematic of a heat exchanger system of the invention in use between two furnaces;

Figures 4-6 show three dimensional view of heat exchangers in accordance with a first embodiment of the invention ;

Figure 7 shows a schematic, three dimensional view of a part of a heat transfer arrangement of the heat exchanger in accordance with a further embodiment of the invention;

Figure 8 shows an exit opening of the part of the heat transfer arrangement of figure 6;

Figure 9 shows a schematic, three dimensional view of a first variation of the part of the heat transfer arrangement of figure 6;

Figure 10 shows a schematic, sectional plan view of the part of the heat transfer arrangement of figure 9;

Figure 11 shows a schematic, three dimensional view of a second variation of the part of the heat transfer arrangement of figure 7;

Figure 12 shows a schematic, sectional plan view of the part of the heat transfer arrangement of figure 11 ; Figure 13 shows a schematic, three dimensional view of a third variation of the part of the heat transfer arrangement of figure 7;

Figure 14 shows a schematic, sectional plan view of the part of the heat transfer arrangement of figure 13.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Overview

In a state of the art smelter producing 200,000 tpa and consuming 300MW of electricity for example, approximately 150MW is converted to heat and passes into the atmosphere within the potroom and beyond. This heat loss however is part of the intended design of an aluminium reduction cell because being able to extract heat from the sidewalls plays an important role in forming a frozen cryolite ledge to protect the sidewalls of the cell. The molten cryolite (electrolyte) is extremely corrosive and will dissolve most materials readily and this includes the silicon carbide sidewall material and the steel shell if it is exposed to it; therefore a protection layer is required and the best material to do this currently is a frozen form of the cryolite material itself. The formation of this frozen ledge is a function of heat transfer out of the cell and the superheat of the electrolyte and therefore the design and the operation of the cell is extremely important to maintaining a constant ledge on the sidewalls.

Due to the need for the protective cryolite ledge, cells are designed with specific heat balance in mind, and because of this reduction cells are very inflexible in their operation in terms of deviating from their designed heat balance state. This means that changes in power input, through say amperage increase or decrease is very limited, what is currently allowable is deviation of approximately +- 5%. In the past, running at a constant amperage was acceptable as power cost in relation to the aluminium price were at an acceptable ratio that mean that making aluminium was a profitable business.

An aluminium reduction cell cannot be just switched to a new operating point and expected to run perfectly. As stated, in a metal smelting system of the invention the control system is arranged to control a cooling system and electric current and/or power input to the electrolytic reduction process to vary power consumption and at least in preferred embodiments dynamically modulate together the cooling system and the electric current and/or power input to vary plant output, with variations in market demand for example. When the potline is operating with maximum electric current and/or power input and the output of metal produced is at a maximum, the cooling system also operates at a maximum. The electric current and/or power input to the smelter may be reduced and cooling reduced, to reduce the smelter output. The cooling system may be turned off or operated at minimum capacity in an insulating mode and the current input to the electrolytic reduction process reduced, which will in turn significantly reduce the rate of electrolytic reduction and metal production but without freezing of the potline. A preferred form cooling system capable of being operated by the control system in at least a cooling mode and an insulating mode will be described in further detail below.

The control system of the invention will now be described with reference to an aluminium smelting system. However, it will be appreciated that the control system may

alternatively be utilised to control a similar or related production process for the same or other metals.

Control System

Referring to figure 1, a flow diagram representing a preferred form aluminium production system 200 of the invention is shown. The system 200 is configured to control the operation of an electrolytic reduction process and/or the cooling system associated with an aluminium smelting system 240 in response to one or more input parameters 210.

The input parameters 210 are directly or indirectly related to the production of aluminium and include any combination of one or more of economic inputs, such as the market price of aluminium 211 and the cost of production of aluminium 212, and energy dynamics such as the status/dynamics of the power grid 213 associated with the production of the aluminium. A control module 220 is configured to receive the inputs 210 and update various settings associated with the aluminium production process in response. These settings include but are not limited to any combination of one or more of, current input settings 231, cooling system settings 232, aluminium reduction chemistry settings 233 and other operational control settings 234 such as anode-cathode distance control. The control system generates the necessary control outputs 230 in association with the updated settings 231-234 to control the appropriate equipment of the aluminium smelting system. In particular, the control outputs 230 are used to control the electrolytic reduction process and the operation of the associated cooling system of the smelting system 240 to alter the rate of production of aluminium in accordance with the inputs 210. Figures 2A and 2B together set out an embodiment of a control algorithm for implementation in the control module 220. As mentioned above, to alter the rate of production of aluminium (i.e. the output of the smelting system), both the electrolytic reduction process and the cooling system need to be regulated accordingly. The electrolytic reduction process can be controlled by varying the current input and/or the chemistry of the process in accordance with the settings 231 and 233. To facilitate dynamic regulation of the electrolytic reduction process, the state of operation of the cooling system is adjusted accordingly. For example, to achieve maximum yield/production of aluminium the rate of electrolytic reduction is increased to a maximum by increasing the current input to the smelter and/or varying the chemistry of the process within the smelter accordingly. This has the effect of increasing the operating temperature of the smelter. To prevent melting of the protective frozen cryolite ledge, the cooling system is operated in a cooling mode to extract heat from smelter pot. Conversely, to achieve minimum yield/production of aluminium the rate of electrolytic reduction is decreased to a minimum by decreasing the current input to the smelter and/or varying the chemistry of the process within the smelter accordingly. This has the effect of decreasing the operating temperature of the smelter. To prevent freezing of the smelter chemistry, the cooling system is operated in an insulating mode to retain heat within the smelter pot.

Typical aluminium smelting systems are operated with substantially constant high input power. The ability to dynamically regulate the rate of production of aluminium smelters enables the conservation of power during required/desired periods as may be dictated by certain economic drivers. The control system 200 of the invention is further configured to release any surplus power that is not utilised by the aluminium smelter during production to the associated power grid. The decision to release excess power and the amount of power to be released to the grid 250 is based on either the economic drivers such as the price 211 or the production cost 212 (related to power price) of aluminium, and/or energy dynamics 213 including the demand of electricity (by the population associated with the grid) vs. the available power/condition of the grid 260. The control system 200 in the preferred form received feedback in relation to the grid condition 260 to update the grid information and energy dynamics 213 at the input end.

In the above described manner, the control system 200 achieves a large scale and efficient energy distribution system. The energy consumption of smelters can be significantly reduces to not only allow the smelter to save costs when power prices are high and/or aluminium price is low, but also enables the conserved energy to be fed back into the associated power grid, thus providing a mechanism to cope with periods when population energy demand exceeds supply (locally, nationally or across a continent for example). Circumstances in which this situation may apply include, for example, weather change, domestic demand change and input availability for renewable energy (such as wind or solar). These circumstances make up a combination of input factors that affect the energy dynamics 213 received by the control system 200. The mechanism therefore provides a means of stabilising a power grid and alleviates the need for costly peak power stations, A supply and demand balance and network capacity model is achieved by the control system 200 that determines the optimum quantity and duration of load reduction or load increase that maximises smelter efficiency and benefits the general electricity consumer.

The control system 200 in the preferred embodiment is configured to operate based on a predetermined supply/demand and network capacity model. The control module 220 will utilise a model that is initially customised to the associated smelters' local market and energy conditions. During operation of the smelters, the control system 200 feeds the inputs 210 into the model to predict optimal times for reducing or increasing the energy load to the smelters. As mentioned above, the model will determine the optimal time to increase smelter efficiency and benefit the associated power grid via grid stabilisation. The model may further predict the cost savings of load reduction at the smelters and the effects on the local energy market (e.g. average power price).

In its various aspects, the control system 200 can be embodied in a computer- implemented process, a machine (such as an electronic device, or a general purpose computer or other device) that provides a platform on which computer programs can be executed, or readable storage medium containing computer program instructions or computer readable data stored thereon. Input information 210 can be received from an external and/or remote device or system either via a telecommunications network, directly from an electronic source, and/or may be received through manual input by an operator. The input information may be provided periodically and/or based on certain events such as a significant drop in market demand of aluminium.

Chemistry Control

In at least some embodiments the invention the control system 200 is configured to adjust the pot chemistry and input feed rate of the electrolytic reduction process to alter the production rate of the aluminium. The electrolyte chemistry of a modern reduction cell is designed to target a delicate balance which minimises the energy input (electrolyte resistivity) whilst maintaining heat balance to allow for sufficient alumina dissolution. This tight operating window is poorly suited to energy modulation, as the short term changes in power input may cause significant process disturbances including sludge build up and anode effects. The control of an aluminium reduction cell is essentially driven by the interaction between the energy input (current/voltage) materials additions (alumina, aluminium fluoride) and the molten salt electrolyte chemistry (resistivity, liquidus temperature). With regard to the latter, a cell is typically operated around 10°C above the temperature at which the electrolyte will freeze, and the routine additions of alumina will typically take much of this superheat (temperature above the liquidus) in order to dissolve. Note that this superheat is only around 1% of the cells operating temperature and is limited by avoiding energy wastage through excess heat and the requirement to maintain a protective frozen electrolyte layer against the sidewall refractory. Some smelters have at times operated with additions of LiF to the electrolyte to lower operating temperature and thus energy input, at the expense of subsequent metal processing in order to remove lithium from the product aluminium.

The composition of the almost universally used current electrolyte is shown in Table 1. The excess AIF ₃ lowers the liquidus temperature, thus saving energy needed to heat the cell, and lowers the solubility of metallic aluminium, thus limiting the back reaction of metal dissolution and raising current efficiency. However the 10-12% excess AlF ₃ also lowers the solubility of alumina; the primary feedstock into the reduction cell. Note that alumina concentration is cycled as alumina is fed to the cell and reduced to from aluminium. With this electrolyte chemistry, alumina solubility may be limited to around 5% while minimising electrolyte resistivity favours operation at around 2%.

Compromising solubility causes sludge (mostly undissolved alumina) build up on the cathode, with a range of impacts on temperature and current efficiency of the cell. A deficiency in dissolved alumina also induces anode effects where the electrolyte itself is electrolysed, with accompanying emission of potent greenhouse species CF ₄ and C ₂ F ₆ . In C0 ₂ equivalents, emissions of these gases currently make up around 37% of the industry's greenhouse footprint.

Table 1: Typical electrolyte composition

Electrolyte component Wt%

Na ₃ AIF ₆ 80

AIF ₃ 11

CaF ₂ 5.6

Al ₂ 0 ₃ 2.5 LiF Trace

The electrolyte chemistry of a modern reduction cell is thus designed to target a delicate balance which minimises the temperature and thus energy input (by operating at high AIF ₃ ), minimises dissolved alumina and thus electrolyte resistivity, but maintains sufficient superheat and electrolyte circulation to dissolve the alumina feed and thus avoid sludge build-up on the cathode, anode effects and in the extreme, freezing of the electrolyte. Improved process control has steadily tightened this operating window and together with improved cell design, is responsible for much of the historic improvement in specific energy consumption in aluminium smelting.

However there are many reasons why this increasingly tight, conventional operating window is poorly suited to the operation of an energy modulated cell. The critical parameter in the operation of any reduction technology is the ability to dissolve alumina. This is currently the limitation in many modern technologies as electrolyte chemistry and anode-cathode distance are squeezed to minimise energy consumption. It is critical to a power modulated cell that a sufficient energy window to dissolve the feed alumina is sustained, even though superheat will vary as energy flow is modulated and the dynamics of freezing and ledge formation against the cell sidewall will also vary as heat flow through the sidewall is modulated.

Even though operating temperature will be higher, it is likely that operation at lower excess AIF ₃ and thus higher temperature will be favoured, simply through the wider alumina solubility window that this enables. This measure increases the actual operating temperature of the cell thus increasing energy consumption although this is more than offset by enabling a much wider power modulation window.

Pot Design

When a pot is designed, it is designed to have a specific heat balance and therefore a specific heat transfer dynamic. This means cells are intentionally designed to lose heat from the top, sides and bottom of the cell. The heat loss from the sides is particularly important because it allows the protective cryolite based side ledge to form which prevents degradation of the cell lining materials. However, if too much heat is lost then excess freezing will occur which will cause the cell to run into operational problems. On the contrary, if not enough heat is removed then the ledge can melt and expose the side wall of the pot to degradation issues. Because of this delicate balance and the difficulties associated with manual control of heat loss, tolerated deviations to power input to the cell may be typically less than +-5%. For example, a cell operating at 200 kA, may only be able to go up or down by a maximum of 10 kA before cell performance diminishes.

With the inclusion of a controllable cooling system, the heat transfer dynamic at the sidewall can be adjusted to allow a greater operating window, for example +-40kA on a 200kA designed pot. When the cooling system is installed there will be a minimum air flow rate required to go through the system to allow it to operate at its original designed state. For example, for a 200kA cell without the cooling system, the average sidewall shell temperature may be 300°C and this temperature reflects the normal heat transfer dynamic for the cell running at 200kA (in one embodiment the shell temperature is what is used to control the air flow rate). With the cooling system installed and the power input not decreased (remains at 200kA), if no or minimal air flow is passed, the cooling system will act as an insulator and the average sidewall shell temperature will increase, for example to above 500°C. This will cause the ledge to melt and expose the sidewall lining to degradation. Therefore, air must be passed through to maintain pot

temperature. The air flow rate will need to be enough to get the average sidewall temperature back down to 300°C (the designed sidewall temperature) which when achieved means the heat transfer dynamics have been restored to the original designed state. On the contrary when power input is decreased, the electrolytic reduction process within the cell will generate less heat, increasing the chances of freezing. With the cooling system installed, and substantially no air or minimal air flowing through, the cooling system will act as an insulator and increase the temperature of the cell back up towards the designed operating temperature. The invention therefore can be embodied in a pot of a metal smelting system in which metal is produced by an electrolytic reduction process at elevated temperatures, where the pot design has a heat transfer dynamic and a range of throughput. The range of throughput includes a sub-range in which the dynamic is maintained by passing air through a shell heat exchanger/cooling system capable of insulating the pot. Airflow through the shell heat exchanger negates the insulation of the pot. The range of throughput further includes ranges of throughput outside this sub-range in which the shell heat exchanger provides cooling and ranges of throughput outside the sub-range in which the shell heat exchanger provides insulation. Cooling System

As stated above, the control system's ability to achieve appropriate regulation of the electrolytic reduction process of the aluminium smelter is facilitated by the ability to regulate the operational state of the associated cooling system. The preferred form cooling system of the invention is a heat exchanger operatively connected to the aluminium smelting pot. The heat exchanger is configured to operate in at least a cooling state and an insulating state. In the cooling state, the heat exchanger extracts heat from the smelting pot and in the insulating state the heat exchanger retains heat within the smelting pot. The heat exchanger includes at least one conduit that is closely associated with the smelting pot in use. An inner passage of the conduit is fluidly and/or thermally coupled with an exterior surface of the smelting pot. In the cooling state, the cooling system is operated to convey a cooling fluid through the passage sufficient to extract heat from the smelting pot. In the insulating state, the cooling system is operated to substantially reduce or terminate flow of the cooling fluid within the passage of the conduit to insulate the smelting pot and retain heat therein. Insulation is provided by the stationary fluid (preferably air) within the conduit or by a minimum flow of fluid sufficient to insulate about the smelting pot. In the preferred embodiment, the cooling system can be operated to vary the level of flow of the cooling fluid within the conduit to vary the level of cooling between a minimum insulation level (for example substantially zero flow) to a maximum cooling level (dependent on the smelter).

Flow control through the heat exchanger is based on a target temperature determined by the control system 200. The preferred smelting system may further comprise thermo- couples located on the walls 30 of the furnace 14 to measure the temperature. The temperature of the furnace 14 is fed back to the control system which is capable of adjusting the flow rate of cooling fluid to achieve the target temperature. The cooling fluid is preferably air provided by either a variable speed extractor fan or an adjustable compressed air supply for example.

A preferred embodiment heat exchange system will be described with reference to figures 3-14. Referring to figure 3, reference numeral 10 generally designates a heat exchanger system, in accordance with the preferred embodiment of the invention. The heat exchanger system 10 includes a conduit 12 which, in use, is arranged between two furnaces, illustrated schematically at 14. The conduit 12 defines a passage 16.

Referring to Figures 4 to 6 of the drawings, a preferred embodiment of the heat exchanger system 10 is illustrated and described. In the example shown in Figure 4 of the drawings, the heat exchanger system 10 comprises two banks 60 of heat exchanger sections 62. The heat exchanger sections 62 are connected via duct branches 64 and duct connectors 66 to the conduit 12 defining the passage 16. In the version shown in Figure 3 of the drawings, the conduit 12 is maintained at basement level and exits outside a furnace building out of a work zone of operators of the furnace. Thus, air heated in the heat exchanger system 10 is discharged, as indicated by arrow 68, through the passage 16 of the conduit 12.

A heat transfer arrangement in the form of a plurality of heat exchanger sections 62 connected via duct branches 64 and duct connectors 66 to the conduit 12 is referred to below, for ease of explanation, as a duct 20 (see figure 3),

The passage 16 of the conduit 12 of the duct 20 is connected, at an egress end, to a fluid extraction arrangement of a smelter in which the furnaces 14 are contained. More particularly, the passage 16 is connected to an extractor fan (not shown) to create a low pressure region in the heat exchanger system 10 to encourage fluid flow through the passage 16 at least in the cooling state of the heat exchanger system 10. A flue-like effect is therefore created in the passage 16 of the duct 20. In use, sections of the duct 20 of the heat exchanger system 10 are positioned in end-to- end, connected relationship between two furnaces 14 to be cooled. The downstream end of the passage 16 of the duct 20 is connected to the extractor fan of the smelter.

During cooling of the smelter, due to the extractor fan drawing air through the passage 16 of the duct 20, a low pressure area is created in the passage 16 in comparison with the exterior of the duct 20. As a result, the air heated by the walls 30 of the furnaces 14 is accelerated up the furnace walls 30 and is drawn in, through the conduit 12, into the passage 16 as indicated by arrows 28. Prior to the air entering the interior of the conduit 12 of the duct 20, the air must pass through the heat exchanger sections 62 which absorb radiant heat emitted from the walls 30 of the furnace 14.

When the air enters the passage 16 of the conduit 12 of the duct 20, it is entrained in the draft and is drawn towards the exit end of the passage 16. As it passes through the passage 16, the air cools the conduit 12 convectively.

During the insulation state, operation of the extractor fan is ceased and/or altered to stop the airflow through the passage 16 and/or any airflow adjacent the walls 30 of the furnace 14. In this manner, the layer of stagnant air creates a layer or insulation adjacent the walls 30 of the furnace 14. Alternatively the extractor fan is reduced in speed to substantially reduce airflow through the passage 16 and/or any airflow adjacent the walls 30 of the furnace 14 sufficient to achieve a layer of insulation adjacent the walls 30 of the furnace.

Referring to Figure 5 of the drawings, once again, the heat exchanger 10 is made up of two banks 60 of heat exchanger sections 62. In this version of the embodiment, each bank 60 is bifurcated to have two stacks 70, one at each end of the bank 60, through which heated air is expelled above the operators' work zone.

Similarly, in the version of the heat exchanger shown in Figure 6 of the drawings, the banks 60 are bifurcated to have a stack 70 at each end through which air is expelled as indicated by the arrows 68. In the case of the versions in both Figures 5 and 6, therefore, the air heated in the heat exchanger system 10 is expelled at a region above the operators' work zone. In all three versions, exposure of the operators to heat stress arising from operation of the heat exchanger 10 is reduced.

Referring now to Figures 7-10, yet a further variation of the preferred embodiment of the heat exchanger 10 is described. With reference to the previous drawings, like reference numerals refer to like parts, unless otherwise specified.

In this variation of the second embodiment of the invention, each section 62 of the heat transfer arrangement of the heat exchanger system 10 comprises at least one channel shaped duct 100 (one of which is shown) having a pair of outwardly extending flanges 102. These flanges 102, in use, are placed against an outer surface of the wall 30 of the furnace 14 to be cooled as shown in Figure 6 of the drawings. In so doing, a passage 104 is formed. In the cooling state, the cooling fluid or air passes through the passage in the direction of arrow 106.

To encourage heat exchange between the wall 30 of the furnace 14 and the duct 100 during the cooling state, internal surfaces of the duct 100 are prepared or coated to provide a high emissivity surface to encourage heat absorption from the furnace wall 30. Typically, the duct 100 is of a suitable metal and is coated with black heat absorption paint to encourage heat transfer.

Radiant heat exchange occurs between the furnace wall 30 and, particularly, the wall 108 of the heat exchanger duct 100 spaced from the furnace wall 30. Convective heat exchange occurs due to the passage of air through the passage 104, through an exit opening 110 (Figure 8) and into the manifold which connects the passage 104 to the duct branches 64 which, in turn, are connected via the connectors 66 to the conduit 12. The air from the manifold is drawn into the passage 16 of the conduit 12 for expulsion from the structure in which the furnaces 14 are arranged.

Once again, convective heat exchange occurs due to assisted flow of the air through the ducts 100, the manifolds and the conduits 12 by connecting an egress end of the conduit 12 to a suitable extractor fan. Additionally, natural convective flow may be enhanced due to the flue-like effect created by the stacks 70.

A single exit opening 110 for each duct 100 may be provided as shown in Figure 8 of the drawings for connection to a manifold so that all the cooling air passes into the manifold. A single exit has the advantage that all the heated air from the sections 62 is removed from surrounds of the furnace 14 including the operator working zone during the cooling state. This has the potential for reducing operator heat stress.

In one configuration, each duct 100 comprises a substantially planar inner wall.

In other configurations, to enhance heat transfer between each section 62 and the furnace wall 30 during the cooling state, each duct 100 contains heat transfer enhancing surfaces 118. In Figures 8 and 9 of the drawings, the heat transfer enhancing surfaces 118 comprise vortex generators 124 secured to an inner surface of the wall 108 of each duct to lie within the passage 104, in use. The vortex generators 124 impede fluid flow through the passage 104 and cause vortices to develop. These vortices, once again, reduce the build up of thermal boundary layers enhancing convective heat transfer. As a further enhancement, orifices can be cut into the wall 108 of each duct.

Yet a further variation of the heat transferring surfaces 118 is shown in figures 11 and 12 of the drawings. In this variation, the vortex generators 124 are arranged at vertically spaced intervals on the fins 120. The vortex generators 124 assist in transferring heat from the fins 120 to the cooling fluid and serve to maintain the fins 120 at a lower temperature. This allows both radiant heat transfer from the furnace wall 30 to occur as well as convective heat transfer from the heat transfer enhancing surfaces 118 to the cooling fluid flowing through the passage 104. In the variation shown in figures 13 and 14 of the drawings, the heat transfer enhancing surfaces are defined by corrugated fins 128. In addition, the fins 128 are perforated. The fins 128 are so arranged to form alternating wider and narrower passages between adjacent fins 128. The cooling fluid moves through these alternating wider and narrower sections creating localised pressure differentials which promote fluid flow through the perforated fins 128. The combination of the extended surfaces defined by the fins 128, the alternating narrower and wider sections which reduce thermal boundary layers and fluid flow through the perforations of the fins 128 all enhance heat transfer.

To enhance the insulation properties of the heat exchanger 10 during the insulation state, one or more layers of insulation may be provided adjacent the duct 100 of each section 62 of the heat transfer arrangement. The insulation layer may be wool or any other material suitable for providing the necessary insulation properties. In some embodiments, during the insulation state, the outlet of the passage 16 is partially or fully occluded to reduce or eliminate natural convection through the system. An advantage of the preferred embodiment of the heat exchanger 10 is the use of natural convective flow outside of the heat exchanger tubes. As indicated above, should forced convective flow in the passage 16 stop for any reason, the natural convective flow will, the Applicant believes, reduce the temperature rise of the wall 30 of the furnace 14 enabling remedial action to be taken with the likelihood of damage to the furnace due to overheating being reduced.

It is a particular advantage of the invention that a heat exchanger 10 is provided which uses a single heat exchange fluid. Heat exchange between the heat exchanger 10 and the furnaces 14 occurs both connectively and radiantly to enhance heat transfer during the cooling state of the heat exchanger system 10. A further major advantage of the preferred embodiment is that a heat exchanger 10 is provided which can be mounted in situ without the need for any modification of the furnaces 14. Thus, the heat exchanger system 10 can be mounted in position relative to the furnaces 14 without shutting down the furnaces 14. Thus, down time of the furnaces 14 is reduced, if not altogether eliminated, which has major economic benefits. In addition, the provision of the heat exchanger system 10 in lengths or sections facilitates the installation of the heat exchanger system 10. No significant modification of the smelter is required apart from, where applicable, the installation of a fan system for the heat exchanger system 10, which may optionally include a connection of the exit end of the conduit 12 to the extractor fan of the smelter. In regard to the preferred embodiment it is yet a further advantage that heat loading on operators in the smelter is reduced as heat is drawn through the conduit 12 and exits remote from the working zone of the operators.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

CLAIMS

1. A metal smelting system which includes:

a pot or potline in which metal is produced by an electrolytic reduction process at elevated temperature,

a cooling system arranged to convey cooling fluid to the pot or potline; and a control system arranged to control together both the cooling system and the electric current and/or power input to the electrolytic reduction process, including at least reduce the flow of cooling fluid and reduce electric current and/or power input to a relative minimum in an insulating mode of the cooling system.

2. A metal smelting system according to claim 1 wherein the control system is arranged to terminate the flow of cooling fluid in the insulating mode of the cooling system.

3. A metal smelting system according to either claim 1 or claim 2 wherein the control system is arranged to control together the cooling system and the electric current and/or power input to the electrolytic reduction process between :

• a maximum mode in which the cooling system operates at a relatively high or maximum cooling, and

• said insulating mode in which the cooling system adjacent the potline acts as an insulator to reduce heat dissipation from the potline.

4. A metal smelting system according to claim 3 wherein the control system is arranged to also operate the cooling system and the electric current and/or power input to the electrolytic reduction process in an intermediate and/or minimum mode or modes between the maximum and insulating modes, in which the cooling system operates at intermediate or minimum cooling and the electric current and/or power input is at an intermediate and/or minimum level. 5. A metal smelting system according to any one of claims 1 to 4 wherein the control system is arranged to dynamically modulate together the cooling system and the electric current and/or power input to the electrolytic reduction process.

6. A metal smelting system according to any one of claims 1 to 5 the control system is further arranged to:

receive input data indicative of a combination of one or more of a price of the metal, a cost of the power required to produce the metal, and energy dynamics,

adjust one or more control settings associated with the electrolytic reduction process or the cooling system in response to the input data, and