Furnace and a method of controlling a furnace

Abstract

The invention provides a furnace having a frozen lining inbetween a furnace lining and a furnace charge, the furnace having means to control the operation of the furnace, including means to measure the temperature in a wall of the furnace adjacent the frozen lining, and to estimate the thickness of the frozen lining as a function of the temperature in the wall, and means to control the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined value. Variables including side wall thermocouple measurements, gas plant instrument measurements, cooling system measurements and electrical, in-feed and chemical composition recordings are monitored, analysed and manipulated. There is also provided means to estimate a future furnace charge composition, perform chemistry control, estimate a material balance of the furnace, and perform inventory control over the furnace.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to application no. ZA 2001/1817, the entire contents of which is expressly incorporated herein by reference thereto.

FIELD OF THE INVENTION

[0002] The present invention relates to a furnace and a method of controlling a furnace.

BACKGROUND OF THE INVENTION

[0003] THIS invention relates to a furnace that can be used for the production of a metal value from a metal value bearing material and specifically to such a furnace where the refractory lining is protected by a frozen lining.

[0004] In processes for the production of metals from metal bearing ores, furnaces frequently form an indispensable part of the process. The ore is subjected to heat and certain reagents to unlock the metal contained therein and to transform it to a form from where it can be worked further.

[0005] Because of the high heat necessary for most processes and the need to contain the heat energy and the charge, furnaces almost always need an insulating lining known as a refractory lining.

[0006] Refractory linings form a high cost component in the production process due to their specialised nature, and the downtime needed to install a refractory lining also contributes to the cost factor.

[0007] There is therefore a need to maintain a refractory lining for as long as possible. Refractory linings wear away by a number of different mechanism including mechanical thermal and chemical wear.

[0008] Mechanical wear occurs through abrasion of charged material against the refractory lining material, such as may occur during charging of a furnace with hard material.

[0009] Thermal wear occurs when the temperature of the refractory lining material rises above a certain refractory-specific safe limit. Above such a temperature, the refractory lining material loses its strength and may start to dissolve into the charge.

[0010] Chemical wear occurs when the refractory lining is exposed to chemical compositions that tend to remove certain elements of compounds from the refractory lining material, thereby weakening its structure. This frequently occurs through what is commonly known as a slag-attack, where a layer of slag on top of a charge of liquid steel will attack the refractory lining material of a steel-making furnace.

[0011] One way of preventing or reducing refractory lining wear is by establishing and maintaining a layer of frozen charge between the charge and the refractory to serve as a barrier against mechanical, thermal and chemical wear.

SUMMARY OF THE INVENTION

[0012] In accordance with this invention there is provided for a furnace having a furnace lining and a charge therein to have a frozen lining at least partly between the furnace lining and the charge, and for means to control the operation of the furnace, the control means including means to measure the temperature in a wall of the furnace adjacent the frozen lining, and means to estimate the thickness of the frozen lining as a function of the temperature in the wall and means to control the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.

[0013] There is also provided for the means to control the rate of heat production in the furnace to include control over the rate of addition of carbonaceous reductant to the furnace, for the rate of heat production to increase with an increase in the rate of addition of carbonaceous reductant to the furnace to thereby urge the thickness of the frozen lining to decrease; or for the rate of heat production to decrease with a decrease in the rate of addition of carbonaceous reductant to the furnace to thereby urge thickness of the frozen lining to increase.

[0014] There is also provided for the control means to include means to measure furnace gas plant variables means to measure furnace cooling system variables, means to measure furnace in-feed variables, means to measure furnace electrical system variables, and means to measure furnace charge chemical composition variables.

[0015] There is also provided for a process of error detection and validation to be conducted on the measurements, for the process of error detection and validation to include analysis of the range of the measurements and the rate of change of the measurements to validate the measurements, and for invalid measurements to be replaced by pre-recorded measurements according to a set of logical rules.

[0016] The invention further provides for means to estimate the frozen lining thickness and hot face temperature as a function of the wall temperature measurements and gas plant measurements.

[0017] There is also provided for means to estimate heat losses in the furnace as a function of estimated frozen lining thickness and hot face temperatures, the gas plant measurements, and the cooling system measurements.

[0018] There is also provided for means to measure sensible heat changes of spray cooled roof panels, spray cooled off gas ducts, film cooled shell panels, air cooled hearth panels, hot gasses and dust, and charge removed from the furnace.

[0019] There is also provided for means to estimate heat losses in the furnace as a function of the estimated frozen lining thickness and hot face temperatures, the gas plant measurements, the cooling system measurements, and measured sensible heat changes.

[0020] The invention also provides for means to estimate a material balance of the furnace as a function of the estimated frozen lining thickness and hot face temperatures, the gas plant measurements, the in-feed measurements, the electrical system measurements, and the furnace charge chemical composition measurements.

[0021] There is further provided for means to perform inventory control over the furnace using the material balance of the furnace.

[0022] The invention further provides for means to estimate a future furnace charge chemical composition as a function of the estimated frozen lining thickness and hot face temperatures, the estimated heat losses, and the estimated material balance.

[0023] There is also provided for means to perform chemistry control of the furnace using the estimated material balance, the estimated future furnace charge chemical composition, the in-feed measurements, the electrical system measurements, and the furnace charge chemical composition measurements.

[0024] The invention further provides for start-up control of the furnace to be performed using the in-feed measurements, the electrical system measurements, the furnace charge chemical composition measurements, and the estimated heat losses.

[0025] The invention also provides a method for controlling a frozen interface between a furnace lining and a charge in the furnace, the method comprising the steps of:

[0026] (i) establishing the frozen lining;.

[0027] (ii) measuring at least the temperature in a wall of the furnace adjacent the frozen lining;

[0028] (iii) estimating the thickness of the frozen lining as a function of the temperature in the wall; and

[0029] (iv) controlling the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.

[0030] There is also provided for step (ii) of the method to include measuring gas plant variables, furnace cooling system variables, furnace in-feed variables, furnace electrical system variables, and/or furnace charge chemical composition variables.

[0031] The invention also provides for the method to include performing a process of error detection and validation on the measurements, the process of error detection and validation including the steps of.

[0032] (v) analysing the range of the measurements and the rate of change of the measurements;

[0033] (vi) validating the measurements; and

[0034] (vii) replacing invalid measurements by pre-recorded measurements according to a set of logical rules.

[0035] The invention further provides for step (iii) of the method to include a step of estimating the thickness of the frozen lining as a function of the temperature in the wall and the furnace gas plant measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 is a schematic representation of the logic of a control process for a furnace according to the invention.

[0037] FIG. 2 is a schematic representation of the feed-back used in the control process.

[0038] FIG. 3 is a schematic representation of the multi-use of input information in the chemistry control.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0039] A process control layout for a furnace (not shown) with a refractory lining and a charge is generally depicted by reference numeral 1 in FIG. 1. The furnace has a frozen lining (not shown) between the refractory lining and the charge.

[0040] Instrument readings from the furnace include sidewall thermocouple measurements (2), gas plant instrument measurements (3), cooling system instrument measurements (4), and in-feed, electrical system and charge chemical composition measurements (5).

[0041] A process of error detection and validation (6) is conducted on the instrument measurements. The process includes analysis of the range of the measurements and the rate of change of the measurements to validate the measurements, the replacement of invalid measurements by prerecorded measurements according to a set of logical rules.

[0042] Once the measurements have been validated, or if invalid, replaced by substitute values, the measurements are utilised for the control of the process.

[0043] The frozen lining thickness and hot face temperatures are estimated (7) as a function of the sidewall thermocouple measurements (2) and gas plant instrument measurements (3). The estimated frozen lining thickness and hot face temperatures (7) are used to control the frozen lining thickness (11).

[0044] The heat losses in the furnace is estimated (8) as a function of the estimated frozen lining thickness and hot face temperatures (7), the gas plant measurements (3), and the cooling system measurements (4).

[0045] The sensible heat changes of spray cooled roof panels (not shown), spray cooled off gas ducts (not shown), film cooled shell panels (not shown), air cooled hearth panels (not shown), hot gasses and dust (not shown), and charge removed from the furnace (not shown) is also measured and used in the determination of the heat losses in the furnace (8).

[0046] A material balance of the furnace (not shown) is determined and is used in the inventory control (9) of the furnace. The material balance (not shown) is determined as a function of the estimated frozen lining thickness and hot face temperatures (7), the gas plant measurements (3), the in-feed measurements, electrical system measurements, and furnace charge chemical composition measurements (5).

[0047] The future furnace charge chemical composition (10) is estimated as a function of the estimated frozen lining thickness and hot face temperatures (7), the estimated heat losses (8), and the estimated material balance (not shown). The estimated future furnace charge chemical composition (10), together with the estimated material balance (not shown), the in-feed measurements, electrical system measurements, and furnace charge chemical composition measurements (5) are used to perform chemistry control over the furnace (12).

[0048] Start-up control over the furnace (13) is performed using the in-feed measurements, the electrical system measurements, the furnace charge chemical composition measurements (5) and the estimated heat losses (8).

Ilmenite Smelting Furnace

[0049] The process control as described above is used for the control of an ilmenite smelting process in a DC arc furnace. Ilmenite mineral sand is smelted using anthracite as a reductant in the furnace. The furnace is refractory lined with magnesite bricks. Cold ilmenite, preheated ilmenite and anthracite are fed into the furnace through a hollow electrode. High titania slag and metallic iron are periodically tapped from the furnace. Hot gas containing dust is removed from the furnace through a single off gas duct where it is subsequently cleaned in a gas scrubbing plant. A film of flowing water cools the furnace shell. The roof panels and off gas panels are spray cooled and the hearth of the furnace is air-cooled.

[0050] The furnace frozen lining and chemistry are controlled by the amount of energy and carbon reductant input.

Gross Error Detection and Validation

[0051] Plant instruments can fall or drift, thereby giving invalid or inaccurate readings. This would make any calculation or model useless. For this reason, all raw data readings used by the control system go through an error detection and data validation process. The quality of the readings is marked as either good or bad. The model components are marked as either enabled or disabled based, on the status of their input tags. In the gross error detection, the range of the reading and its rate of change are checked for abnormalities. The data is validated by either a set of logical rules or neural network models, where the important data that is bad for some reason can be reconstructed if necessary.

Temperature Profiles and Frozen Lining Thickness Estimations

[0052] Dual sidewall thermocouples are used to read the temperature of the sidewalls. Together with knowledge of the thermal conductivity of the frozen lining and the refractory lining, an internal node calculation is performed to determine the temperature at any point between a sidewall thermocouple and the hot face, which is the interface between the refractory and frozen lining. This information is used to calculate the hot face temperature and frozen lining thickness.

[0053] The value of the frozen lining thickness is used in the frozen lining thickness control. This value is of more use in the frozen lining control than just the thermocouple readings, because it takes non-steady state conditions and time lapses between thermocouple readings and the frozen lining thickness into account.

Inventory Control

[0054] The total amount of material, including the dust losses, and power added to the furnace between taps is determined for use in inventory control, The analyses of certain elements in the feed materials, slag and iron are used in the material balance to determine the relative amounts of slag and iron produced The amount of frozen slag is taken into account via the frozen lining thickness calculation. Bath heights are calculated through the relationship between mass and volume. The relative amounts of slag and iron to be tapped are then determined using the heights of the tap holes as reference points. During the addition of electrodes, sounding measurements are taken through a hollow electrode. The actual measurements of the slag and iron bath heights are used to “zero” the control process calculation on almost a daily basis.

Heat Loss Calculations

[0055] Heat lost through the cooling system and exiting streams from the furnace is calculated by means of the sensible heat gain or loss of the cooling medium and exiting stream. The spray cooled roof panels, spray cooled off gas ducts, film cooled shelf panels, air cooled hearth panels, hot gasses and dust, and charge removed from the furnace are all used to take readings for the heat loss calculations.

Chemistry Predictors

[0056] Neural network models with high correlation coefficients are used to predict the current % TiO2 in the slag, % C in the iron and % Fe2O3 in the ilmenite, as well as those percentages 2 hours ahead. This data is used for feed forward control in the material and energy balance of the decision support module. The neural network models are extensive. There are approximately 42 inputs to each of the iron and slag models and 6 to the ilmenite model. Inputs include the data derived from the other modules (frozen lining thickness, inventory control, heat loss). The models auto train as the plant conditions change.

Frozen Lining Control

[0057] The philosophy used in the control of the freeze lining is that the frozen lining is viewed as an additional layer of “bricks”. As long as the frozen lining is maintained, the magnesite bricks will remain intact and should not have to be replaced for many years. The maintenance of even and uniform frozen lining means that the bath size is kept constant which makes for better operational control. Tight control of the frozen lining thickness is achieved by making regular changes to the C reductant addition rates in both the positive (frozen lining getting thinner because of an increased rate of heat production) and negative (frozen lining getting thicker because of a decreased rate of heat production) directions.

Chemistry Control

[0058] The reaction governing the process is given by:

FeTiO3+C+heat→Fe+TiO2+CO

[0059] The control objectives are to maintain the % TiO2 in the slag of 86% with minimal deviation and to maintain the freeze lining. This is achieved through manipulation of the C reductant addition rate (AIR, anthracite to ilmenite ratio) and the energy by input (IPR, ilmenite to power ratio). The system is interactive in that both of the manipulated variables influence both of the controller variables, as is shown in FIG. 3.

[0060] There are two portions to the control strategy, namely a feed forward portion (ff) and a feed back portion (fb). The feed forward portion attempts to absorb the disturbance introduced by varying feed material composition (ilmenite and anthracite analyses) and the feed back portion reacts on measurements of the controlled variables (% TiO2 in slag and freeze lining thickness).

[0061] Eff+Efb=Etot (IPR—specific energy, kg ilmenite per (MWh)

[0062] Cff+Cfb=Ctot (AIR—specific carbon, kg anthracite per ton of ilmenite)

[0063] The Eff portion is determined from a hard coded energy balance and the Cff from a hard coded material balance.

[0064] The Efb and Cfb portions are equivalent to the changes that were conventionally made by the shift supervisors based on the % TiO2 in the slag as-tapped and the sidewall thermocouple readings respectively. In the decision support system, these portions are determined by a fuzzy logic rule set that was derived from the experiences or operational staff and on line tuning.

[0065] The feed back portion consists of two loops, one fast and the other slow, as is shown in FIG. 2.

[0066] The fast loop is run every 15 minutes and uses the estimated frozen lining thickness. The slow loop is run after each tap and uses the % TiO2 in the slag.

Start-up Module

[0067] During a furnace stoppage, the length of the stoppage and energy lost is integrated. A given percentage of the lost energy is then recovered through a specified power ramp, IPR and AIR schedule. Once the start-up module is completed, the system switches back to the chemistry and freeze lining control modules.

[0068] The invention is not limited to the precise constructional details as herein described.

[0069] The applicant believes that the invention is advantageous in that it provides a furnace with means to control a frozen lining in the furnace, wherein the bath size is kept constant for better operational control and the wear on the refractory lining is reduced.

Claims

1. A furnace including: a furnace lining and a frozen lining which is positioned at least partly between the furnace lining and a charge within the furnace; and control means to control the operation of the furnace, the control means including means to measure the temperature in a wall of the furnace adjacent the frozen lining, means to estimate the thickness of the frozen lining as a function of the temperature in the wall, and means to control the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.

2. A furnace according to claim 1, wherein the means to control the rate of heat production in the furnace includes control over the rate of addition of carbonaceous reductant to the furnace, so that the rate of heat production is increased with an increase in the rate of addition of carbonaceous reductant to the furnace to thereby urge the thickness of the frozen lining to decrease, or the rate of heat production is decreased with a decrease in the rate of addition of carbonaceous reductant to the furnace to thereby urge the thickness of the frozen lining to increase.

3. A furnace according to either one of claim 1, wherein the control means further comprises one or more of the following measurement means: means to measure furnace gas plant variables; means to measure furnace cooling system variables; means to measure furnace in-feed variables; means to measure furnace electrical system variables; and means to measure furnace charge chemical composition variables.

4. A furnace according to claim 1, which further comprises a process for conducting error detection and validation of the measurements, the process comprising analysis of the range of the measurements and the rate of change of the measurements to validate the measurements, and a process for replacing invalid measurements with pre-recorded measurements according to a set of logical rules.

5. A furnace according to claim 3, which further comprises means to estimate the frozen lining thickness and hot face temperatures as a function of the wall temperature measurements and gas plant measurements.

6. A furnace according to claim 3, which further comprises means to estimate heat losses in the furnace as a function of estimated frozen lining thickness and hot face temperatures, the gas plant measurements, and the cooling system measurements.

7. A furnace according to claim 1, which further comprises means to measure sensible heat changes of spray cooled roof panels, spray cooled off gas ducts, film cooled shell panels, air cooled hearth panels, hot gasses and dust, and/or charge removed from the furnace.

8. A furnace according to claim 7, which further comprises means to estimate heat losses in the furnace as a function of the estimated frozen lining thickness and hot face temperatures, the gas plant measurements, the cooling system measurements, and measured sensible heat changes.

9. A furnace according to claim 3, which further comprises means to estimate a material balance of the furnace as a function of the estimated frozen lining thickness and hot face temperatures, the gas plant measurements, the in-feed measurements, the electrical system measurements, and the furnace charge chemical composition measurements.

10. A furnace according to claim 9, which further comprises means to perform inventory control over the furnace using the material balance of the furnace.

11. A furnace according to claim 9, which further comprises means to estimate a future furnace charge chemical composition as a function of the estimated frozen lining thickness and hot face temperatures, the estimated heat losses, and the estimated material balance.

12. A furnace according to claim 11, which further comprises means to perform chemistry control of the furnace using the estimated material balance, the estimated future furnace charge chemical composition, the in-feed measurements, the electrical system measurements, and the furnace charge chemical composition measurements.

13. A furnace according to claim 6, which further includes means for controlling start-up of the furnace to be performed using the in-feed measurements, the electrical system measurements, the furnace charge chemical composition measurements, and the estimated heat losses.

14. A method for controlling a frozen interface in a furnace between a furnace lining and a charge in the furnace, the method comprising the steps of: (i) establishing the frozen lining; (ii) measuring at least the temperature in a wall of the furnace adjacent the frozen lining; (iii) estimating the thickness of the frozen lining as a function of the temperature in the wall; and (iv) controlling the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined reference value.

15. A method according to claim 14, wherein the step of measuring at least temperature in the wall of the furnace comprises measuring one or more of gas plant variables, furnace cooling system variables, furnace in-feed variables, furnace electrical system variables, and furnace charge chemical composition variables.

16. A method according to claim 14, which further comprises the step of performing a process of error detection and validation on the measurements, the process of error detection and validation including the steps of: (v) analysing the range of the measurements and the rate of change of the measurements; (vi) validating the measurements; and (vii) replacing invalid measurements by pre-recorded measurements according to a set of logical rules.

17. A method according to claim 14, wherein the thickness of the frozen lining is estimated as a function of the temperature in the wall and the furnace gas plant measurements.