Title of Invention

"CONTROL OF REFRACTORY WEAR"

Abstract

Controlling the wear rate of refractory (102) within a furnace (100) by measuring the temperature (104) of the refractory layer (102), measuring the viscosity of a melt (106) and adjusting the melt temperature to an optimum set point temperature determined by a pre-defined relationship between the temperature of the refractory and the viscosity of the melt wherein, the optimum set point temperature is a minimum temperature that will sustain a desired rate at which the melt (106) is tapped from the furnace (100).

Full Text

FIELD OF THE INVENTION The present invention relates to refractory linings and refractory walls used to contain a melt in the production of iron and non-ferrous metals. BACKGROUND OF THE INVENTION Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. Refractory linings and walls are used in the production of iron, steel and non-ferrous metals. The purpose of the refractory lining or walls is to contain "a melt" within a furnace. The melt may include molten metal, oxides, slag and molten sulphides. The furnace is typically a steel shell lined with refractory material. Refractories are typically high melting point crystalline solids, carbides, carbon or graphite mat have been engineered to withstand the high temperatures and the corrosive environments within the furnace and contain the melt While refractory materials have a good corrosion resistance, the extreme environment within a furnace will eventually cause the refractory to wear out The operative lifetime of the refractory is referred to as a "campaign". The replacement of refractory represents a significant cost as the refractory material is expensive, the installation is difficult and there is lost production while the furnace is out of operation. In light of this, there is an ongoing effort to extend the campaign life of the refractory through the development of new refractory materials that offer improved performance with reduced wear rates or lower installation times. The refractory wear rate decreases with decreasing temperatoe within the furnace. However, in order to tap the melt from the furnace at an adequate flow rate, the melt must be maintained at a temperature that is greater than its freezing point to keep it sufficiently fluid. The freezing point temperature of the melt is dependent on the particular composition of the melt For a known melt composition, the freezing point is also known from calculation, experiment or plant experience. Therefore it is possible to determine the minimum.temperature at which the viscosity of the melt is low enough (or conversely, the fluidity is high enough) to provide the desired tapping flow rate. By keeping the furnace temperature as close as possible to the minimum, the corrosion of the refractory is minimised thereby extending the campaign of the lining. Unfortunately, the composition of the melt within the furnace will vary over time and hence the freezing point is likewise subject to variation. In order to account for this, the furnace is operated to maintain the melt at a temperature set point calculated such that an adequate tapping rate will be maintained if the freezing point is at the upper limit of the typical deviations from the theoretical freezing point. Therefore, the melt temperature is usually higher than it needs to be for most of the time during the operation of the furnace. As a result, the refractory wear rate is greater than it needs to be, thereby reducing the campaign to less than the theoretically ideal campaign for that lining. It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. SUMMARY OF THE INVENTION According to a first aspect, the present invention provides a refractory wear control assembly for installation in a refractory furnace, trie assembly including: a refractory sensor for measuring the temperature of the refractory; a viscosity sensor for deterrnining the viscosity of the melt in the furnace; melt temperature adjustment means responsive to the refractory sensor and the viscosity sensor, such that, the adjustment means adjusts the melt temperature to an optimum set point temperature determined from a predefined relationship between the refractory temperature and the melt viscosity, wherein the optimum set point temperature is a minimum temperature that will sustain a desired rate of tapping the melt from the furnace. According to a second aspect, the present invention provides a method of controlling refractory wear rates within a refractory furnace, the method including: measuring the temperature of the refractory layer; measuring the viscosity of a melt within the furnace: adjusting the melt temperature to an optimum set point temperature determined from a predefined relationship between the temperature of the refractory and the viscosity of the melt; wherein, the optimum set point temperature is a minimum temperature that will sustain a desired rate at which the melt is tapped from the furnace. Preferably, the viscosity of the melt is determined using a measurement of the flow' rate as it is tapped from the furnace. In another preferred form, the refractory temperature and the tapping flow rate are continuously monitored. This provides real time adjustment of the optimum set point temperature. However, it will be appreciated that the refractory temperature and the tapping flow rate may be periodically monitored to provide periodically calculated optimum set point temperatures which are then compared to periodically measured melt temperatures. Preferably, the refractory temperature is measured in a high wear area of the furnace. In a particularly preferred form, the melt temperature sensor is at least one thermocouple is used for continuously measuring the temperature of the melt and the refractory sensor is at least one other thermocouple is used for continuously monitoring the refractory temperature. Preferably, the rate of change of the refractory temperature is compared to a theoretically calculated ideal rate of change in the refractory temperature based on the theoretical nunimum possible refractory wear rate for the particular operating conditions of the furnace, whereby the comparison between the actual rate of change in the refractory temperature and the monitored value of the melt viscosity against the theoretically ideal rate of change in refractory temperature and the melt viscosity is used to determine the adjustment required in the melt temperature. Preferably, the theoretically ideal rate of change in the refractory temperature is calculated from the expected wear rate of a refractory lining that maintains an ideal accretion layer on its internal surface. It will be appreciated by those skilled in this field of technology, that the ideal accretion layer will vary in thickness depending on the particular melt. In some cases, the ideal situation is not to have an accretion layer at all. In another embodiment of the invention, the height of the melt within the furnace is measured during tapping using an air-cooled lance whereby the rate of change of the height of the melt is used to detennine the tapping flow rate and thereby determine the melt viscosity. In some embodiments, the refractory temperature, and/or the melt temperature, is measured using an optical pyrometer. In a further preferred form, forced cooling is applied to the refractory in order to further retard the refractory wear rate. In some embodiments, the melt viscosity is measured using an inclined slope viscometer. In another preferred form, a relationship between viscosity and temperature of the melt is derived from an analysis of the feed prior to entering the furnace. Alternatively, a relationship between viscosity and temperature of the melt is derived from an analysis of the melt. The freezing point of the melt may also be estimated using these techniques. According to a third aspect, the present invention provides an instantaneous refractory wear rate monitoring system for predicting the instantaneous refractory wear rate in a refractory furnace, the system including: a refractory temperature sensor adapted to measure the temperature of the refractory as well as the rate of change of the temperature in the refractory; a conversion unit responsive to the temperature sensor for determining an instantaneous rate of change of the thickness of the refractory using a predefined relationship between the heat transfer characteristics of the refractory, the refractory temperature and the rate of change of the refractory temperature. According to another aspect, the present invention provides a method of predicting the campaign of a refractory lining in a refractory furnace, the method including: measuring the refractory temperature and the rate of change of the refractory temperature; determining the instantaneous rate of change of the thickness of the refractory from a predefined relationship between the rate of change of thickness of the refractory, the refractory temperature, the rate of change of the refractory temperature and the heat transfer characteristics of the refractory material. Preferably, the temperature of the refractory is measured with an embedded refractory temperature sensor. Preferably, the refractory temperature sensor is a type N thermocouple. In a further preferred form, the thermocouple is embedded at a predetermined position within the refractory. This particular technique for measuring the actual refractory wear rate enables an accurate prediction of the length of a campaign. This allows operators to get maximum usage from the refractory lining before shutting down the furnace to replace the refractory material. It also allows the operators to detect any departure from normal operation, such as accelerated refractory wear and react to it. This means they can investigate the cause of the departure, correct any improper operation and return to more stable operation. Using the present invention, the melt temperature is continually adjusted to an actual minimum value above the freezing point that will provide a melt viscosity suitable for the desired tapping rate. By keeping the melt temperature to a minimum, the refractory wear rate is likewise kept to a minimum, which in turn extends the campaign of the refractory lining. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vertical section through a refractory furnace suitable for use with the present invention; Figure 2 is section A-A of the refractory furnace shown in Figure 1; and Figure 3 is a graphical comparison of actual refractory wear rates taken from subsequent refractory campaigns in the same furnace, the furnace being operated in accordance with prior art techniques during the first campaign, and in accordance with the present invention during the second campaign. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, the refractory furnace 100 consists of a steel casing 101 enclosing a refractory lining 102. The temperature of the melt 106 is measured using the thermocouple 105 which extends from the casing 101 to the interior of the furnace in order to "see" the melt 106 The temperature of the refractory layer 102 is monitored using the "in-brick" thermocouples such as those shown at locations 104a, b and c. The in-brick thermocouple are embedded at a set depth within the refractory. The temperature sensors used are generally type N thermocouples, however, other thermocouple types and optical pyrometers may also be used. The in-brick thermocouple is carefully positioned as the distance from the exterior surface influences the heat transfer calculations for the material. The melt is tapped from the furnace 100 through the tapping block 107. An air-cooled lance 103 lowered into the furnace is used to measure the tapping rate. Using the flow rate of the melt as it is tapped from the furnace, it is possible to estimate the relative viscosity of the melt. By monitoring the refractory temperature and combining this with a measure of the melt viscosity, the present invention allows the system to continuously provide real time control of refractory wear rates. Traditionally, the thickness of the refractory layer was measured every month or so using a manual method. However, a monthly measurement of the refractory thickness (also known as the "brick length") is inferior to real time control. Real time control of the refractory wear rate allows the campaign of the refractory lining to be maximised without the use of forced cooling systems for the refractory. The invention uses the relationship between the tapping rate and the freezing point of the melt to keep the furnace temperature at a minimum. It is known in the industry that the refractory wear rate increases when the furnace temperature is higher. However, in order to ensure that the melt can be tapped from the furnace at a sufficient flow rate, the melt needs to have a sufficiently high fluidity (or conversely, a sufficiently low viscosity). Given the theoretical composition of the melt, it is possible to determine the theoretical freezing point and the theoretically ideal set point temperature which is the minimum temperature at which the melt has sufficiently low viscosity. In reality, the composition of the melt is constantly varying and therefore the freezing temperature of the melt also varies. Therefore, it has been common practice in the industry to maintain the melt at a set point temperature that is higher than it needs to be in order to take into account the fluctuations in melt composition. This practice reduces the campaign of the refractory which increases production costs as discussed above in the Background of the Invention. The present invention has provided the ability to monitor the refractory wear by measuring the refractory temperature and the rate of change of the refractory temperature then comparing these values to theoretically optimum values for minimum refractory wear calculated using the known heat transfer characteristics of the material. By combining this information with the melt viscosity data calculated from the tapping flow rate, it is possible to continuously adjust the melt temperature to an optimum set point temperature. The optimum set point temperature is the actual minimum temperature at which the melt viscosity is low enough for an effective tapping rate. Therefore the temperature in the furnace is continually at the actual minimum thereby reducing refractory wear rate and extending the campaign of the refractory lining. Figure 3 shows a comparison between the actual refractory wear rates from subsequent campaigns in the same furnace. The brick length is a measurement of the refractory thickness and the campaign is measured in months. During campaign no. 7 the furnace was operated in accordance with the techniques discussed in the Background of the Invention. The plot of the refractory wear during campaign no. 7 is shown along line 109. During campaign no. 8, the furnace was operated in accordance with the present invention and the plot of refractory wear against time is shown along line 108. Line 110 is the statistically calculated trend line for refractory wear during campaign no. 8 calculated at 79.3 weeks into the campaign. The trend line indicates that the average wear rate during campaign no. 8 will be 1.7 mm per week. In contrast, the refractory wear during campaign no. 7 was roughly double that of campaign no. 8. At equivalent times during campaigns nos. 7 and 8 (79.3 weeks), the brick length during campaign no. 7 was 184 mm while in campaign no. 8 it is 314 mm. As a further reference, dotted line 111 shows a wear rate of 1 mm per week and dotted line 112 shows a wear rate of 2.24 mm per week. The data presented in Figure 3 confirms that the present invention provides an effective method of reducing refractory wear and increasing refractory campaigns. The relatively close alignment between the actual wear rate 108 during campaign no. 8 and the statistical trend line 110 is a further indication of the degree of control the present invention provides over wear rates and the increased accuracy of the predicted campaign. The invention has been described herein by way of example only. Skilled workers in this field will readily recognise many variations and modifications which do not depart from the spirit and scope of the broad inventive concept. WE CLAIM: 1. A refractory wear control assembly for installation in a refractory furnace (100), the assembly including: a refractory sensor (104) for measuring the temperature of the refractory (102); a viscosity sensor for determining the viscosity of the melt (106) in the furnace; and characterised by a melt temperature adjustment means responsive to the refractory sensor and the viscosity sensor, such that, the adjustment means adjusts the melt temperature to an optimum set point temperature determined from a predefined relationship between the refractory temperature and the melt viscosity, wherein the optimum set point temperature is a minimum temperature that will sustain a desired rate of tapping the melt (106) from the furnace (100). 2. An assembly according to claim 1, wherein the melt temperature adjustment means includes a melt temperature sensor (105) for monitoring the temperature of the melt (106) and the viscosity sensor has a tapping flow rate sensor, wherein the viscosity of the melt is determined using the tapping flow rate measurement. 3. An assembly according to claim 2, wherein the refractory temperature and the tapping flow rate are continuously monitored. 4. An assembly according to any one of claims 1 to 3, wherein the refractory temperature is measured in a high wear area of the furnace (100). 5. An assembly according to any one of claims 1 to 4, wherein the melt temperature sensor has at least one thermocouple (105) for continuously measuring the temperature of the melt and the refractory sensor has at least one other thermocouple (104) for continuously monitoring the refractory temperature. 6. An assembly according to any one of claims 1 to 5, wherein the rate of change of the refractory temperature is compared to a theoretically calculated ideal rate of change in the refractory temperature based on the theoretical minimum possible refractory wear rate for the particular operating conditions of the furnace (100), whereby the comparison between the actual rate of change in the refractory temperature and the monitored value of the melt viscosity against the theoretically ideal rate of change in refractory temperature and the melt viscosity is used to determine the adjustment required in the melt temperature. 7. An assembly according to any one of claims 1 to 6, wherein a theoretical relationship between viscosity and temperature of the melt (106) is derived from an analysis of the feed prior to entering the furnace (100). 8. An assembly according to any of claims 1 to 6, wherein a theoretical relationship between viscosity and temperature of the melt (106) is derived from an analysis of the melt (106). 9. An assembly according to claim 6, wherein the theoretically ideal rate of change in the refractory temperature is calculated from the expected wear rate of a refractory lining (102) that maintains an ideal accretion layer on its internal surface. 10. An assembly according to any one of claims 1 to 9, wherein the height of the melt (106) within the furnace (100) is measured during tapping using an air-cooled lance whereby the rate of change of the height of the melt (106) is used to determine the tapping flow rate and thereby determine the melt viscosity. 11. An assembly according to claim 1, wherein the refractory temperature, and/or the melt temperature, is measured using an optical pyrometer. 12. An assembly according to claim 1, wherein the melt viscosity is measured using an inclined slope viscometer. 13. An assembly according to any one of claims 1 to 12, wherein forced cooling is applied to the refractory in order to further retard the refractory wear rate. 14. An assembly according to claim 2, wherein the refractory temperature and the tapping flow rate may be periodically monitored to provide periodically calculated optimum set point temperatures which are then compared to periodically measured melt temperatures. 15. A method of controlling refractory wear rates within a refractory furnace (100), the method characterised by: measuring the temperature of the refractory layer (102); measuring the viscosity of a melt (106) within the furnace (100); adjusting the melt temperature to an optimum set point temperature determined from a predefined relationship between the temperature of the refractory and the viscosity of the melt (106); wherein, the optimum set point temperature is a minimum temperature that will sustain a desired rate at which the melt is tapped from the furnace. 16. A method according to claim 15, wherein the viscosity of the melt is determined using a measurement of the flow rate of melt (106) as it is tapped from the furnace (100). 17. A method according to claim 16, wherein the refractory temperature and the tapping flow rate are continuously monitored. 18. A method according to any one of claims 15 to 17, wherein the refractory temperature is measured in a high wear area of the furnace. 19. A method according to any one of claims 15 to 18, wherein the rate of change of the refractory temperature is compared to a theoretically calculated ideal rate of change in the refractory temperature based on the theoretical minimum possible refractory wear rate for the particular operating conditions of the furnace (100), whereby the comparison between the actual rate of change in the refractory temperature and the monitored value of the melt viscosity against the theoretically ideal rate of change in refractory temperature and the melt viscosity is used to determine the adjustment required in the melt temperature. 20. A method according to any one of claims 15 to 19, wherein a theoretical relationship between viscosity and temperature of the melt (106) is derived from an analysis of the feed prior to entering the furnace (100). 21. A method according to any one of claims 15 to 19, wherein a theoretical relationship between viscosity and temperature of the melt (106) is derived from an analysis of the melt. 22. A method according to claim 19, wherein the theoretically ideal rate of change in the refractory temperature is calculated from the expected wear rate of a refractory lining (102) has an ideal accretion layer on its internal surface. 23. A method according to any one of claims 15 to 22, wherein the height of the melt (106) within the furnace (100) is measured during tapping using an air-cooled lance whereby the rate of change of the height of the melt (106) is used to determine the tapping flow rate and thereby determine the melt viscosity 24. A method according to claim 15, wherein the refractory temperature, and/or the melt temperature, is measured using an optical pyrometer. 25. A method according to claim 15, wherein the melt viscosity is measured using an inclined slope viscometer. 26. A method according to any one of claims 15 to 25, wherein forced cooling is applied to the refractory (102) in order to further retard the refractory wear rate. 27. A method according to claim 16, wherein the refractory temperature and the tapping flow rate are periodically monitored to provide periodically calculated optimum set point temperatures which are then compared to periodically measured melt temperatures.

Documents:

2857-delnp-2004-abstract.pdf

2857-DELNP-2004-Claims-(07-07-2011).pdf

2857-delnp-2004-claims.pdf

2857-DELNP-2004-Correspondence Others-(07-07-2011).pdf

2857-DELNP-2004-Correspondence Others-(09-08-2011)..pdf

2857-DELNP-2004-Correspondence Others-(09-08-2011).pdf

2857-delnp-2004-correspondence-others.pdf

2857-delnp-2004-description (complete).pdf

2857-delnp-2004-drawings.pdf

2857-delnp-2004-form-1.pdf

2857-delnp-2004-form-18.pdf

2857-delnp-2004-form-2.pdf

2857-DELNP-2004-Form-3-(09-08-2011).pdf

2857-delnp-2004-form-3.pdf

2857-delnp-2004-form-5.pdf

2857-DELNP-2004-GPA-(07-07-2011).pdf

2857-delnp-2004-gpa.pdf

2857-delnp-2004-pct-210.pdf

2857-delnp-2004-pct-306.pdf

2857-delnp-2004-pct-409.pdf

2857-DELNP-2004-Petition-137-(09-08-2011).pdf

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