- Email: [email protected]
Improving copper matte grade control in a concentrate flash furnace
IFAC Workshop on Automation in the Mining, Mineral and Metal Industries September 10-12, 2012. Gifu, Japan
Improving copper matte grade control in a concentrate flash furnace Luis Bergh*, Iván Cornejo*, Fernando Romero+ and Cristian Sulzer° *CASIM, Automation and Supervision Centre for Mineral Industry, Chemical Engineering Department, Santa Maria University, Valparaiso, Chile, (e-mail: [email protected]) +
Process Control Manager, Anglo American Chile, Santiago, Chile °Chagres Division, Anglo American Chile, Santiago, Chile
Abstract: The copper matte grade (CMG) variation in a flash furnace depends on bath temperature, phase levels, ratio of oxygen to sulfur, iron and copper content on the feed, and tonnage ratio of silica flux and feed, among others. The CMG is obtained from laboratory analysis of samples taken when the matte is tapped out from the furnace. Then the CMG is known with a time delay of 1-2 hours, almost every one hour. Since feed characteristics usually change during this period, any feedback control of CMG cannot reduce enough the grade variability. On the other hand, it was shown that main disturbances are coming with the composite feed tonnage and composition, being the powder tonnage control system the main contribution. A new calibration system was proposed and implemented. During an evaluation period of 60 days, it was found that the standard deviation of the CMG was reduced in 25%. This improvement can be explained considering two effects. First, the stabilization of the total feed tonnage that affect the mass balance in the furnace. Second, even when the powder tonnage is only 7% of the total tonnage, its special chemical composition contributed to increase the copper content in the matte. The stabilization of the furnace operation also improved the phase separation, as well as reduced the variance of the gas SO2 composition and flow rate. The operators took advantage of this reduced variability to increase the fusion rate and thus the mean generated gas flow rate in the furnace, without violating the plant constraints. One important conclusion is that this result will permit to increase the average value of CMG, when the development of the virtual sensor of feed composition is completed and when this estimation becomes part of a FF-FB control. Copyright © 2012 IFAC Keywords: flash furnace, control, industrial data, pyrometallurgy frequency measured variables with targets. These high frequency target predictions permit the effective use of feedback control during the absence of target measurements. Some complications may arise when the models are built on and uses inaccurate measurements. Better estimation of some key variables, as characteristics of the process feed, can be used to add a feed forward compensation. More details on control and optimization of flash smelting can be found in Davenport et al. (2003).
1. INTRODUCTION Feedback control of local objectives is a powerful tool to modify an input variable to compensate process disturbances or to closely follow set point changes. However, the successful implementation of this control mainly depends on process dynamics and on the accuracy of measurements and actuators. When process variables are frequently observed, and process disturbances are highly correlated, predictive control is a better alternative than conventional PID control, especially when the process presents significant time delay. Furthermore, if process dynamics is time variant, adaptive predictive control is the appropriate control tool. Knowledgebased expert control systems are an alternative when several objectives should be simultaneously met and process dynamics is complex. The use of complementary tools to improve control is discussed by Hodouin (2009).
In this paper, the copper flash smelting process control is analyzed in the previous perspective. Typical historical CMG, obtained from Chagres smelting plant for a period of 90 days, are shown in Figure 1. The dashed lines represent the upper and lower limits for this target. The mean CMG was 59.4% with a standard deviation of 2.2%. One can see that the target is at least 60% of the time outside the desired band.
The variability of the targets cannot be significantly reduced by any feedback control algorithm if significant time delay is present and target variables are observed at low frequency, as compared with process disturbances changes over time. One solution is to use models (virtual sensors) relating high 978-3-902823-12-0/12/$20.00 © 2012 IFAC
A second objective is the matte temperature, since the efficiency of the phase separation is influenced by its viscosity in the settler. The target temperature is 1255 °C, and an acceptable band is between 1230 and 1280 °C. Historical 13
10.3182/20120910-3-JP-4023.00035
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
Copper matte grade (%)
data was centered in 1270 °C with deviations between 1250 and 1300 °C.
A typical flash furnace is described in Figure 3. In flash smelting the fine, dried copper sulphide concentrate and silica flux with air, oxygen enriched air or pure oxygen blast and recycled flue dust are fed through a concentrate burner into the flash furnace reaction shaft. In the reaction shaft the suspension of gas, solid concentrate particles and solid flux particles, are formed and one important reaction is:
70 68 66 64 62 60 58 56 54 52 50
2CuFeS2 + 2.5O2 + SiO2 = Cu2S.FeS + FeO.SiO2 + 2SO2 + energy (1) Parallel reactions are the oxidation of FeS and FeS2:
Date [day-month]
FeS + 1.5O2 = FeO + SO2
(2)
FeS2 + 2.5 O2 = FeO + 2SO2
(3) Concentrate
Fig. 1. Historical copper matte grade.
O2
N2
Flux
Magnetite concentration [%]
A third objective is to maintain the magnetite concentration in the slag phase in 6%, and not to exceed 12%. In Figure 2 some typical magnetite concentrations are shown, with a mean value of 12% and deviations between 8 and 16%.
Dryer
22 20 18 16 14 12 10 8 6 4 2 0
TI AI AI
Fig. 3. A typical flash furnace. The formed FeO then migrates from matte to slag phase due to a change in density. The extension of both reactions is controlled by the amount of oxygen, as a limiting reactive. The powder is composed of oxidized sulfur, copper and iron, sulfated and reduced copper, and other materials that do not participate in oxidation reactions, and they only have the effect of cooling the bath.
Date [day-month] Fig. 2. Typical magnetite concentration in slag.
When the suspension leaves the reaction shaft, the reacted molten concentrate particles and inert flux particles are separated from the gas stream and hit towards molten slag and matte. In the molten bath, the reactions are completed and matte and slag will be settled at the settler of the furnace as respective layers due to their different density.
In the next sections the process characteristics, the main disturbances, measurements and control systems are described to understand the causes of this poor control. Some solutions are first proposed and then successfully implemented to reduce the observed variance and to permit displacing the mean value inside the control band.
The matte and slag are tapped out through the holes located at the side or at the end walls of the settler. The waste gases generated at the process are ducted through the uptake shaft into waste-heat boiler and electrostatic precipitator, and then processed on the sulphuric acid plant.
2. COPPER FLASH SMELTING IN CHAGRES Outokumpu flash smelting is a pyrometallurgical process for smelting metal sulphide concentrates (Jamsa-Jounela et al., 2003). The Outokumpu flash smelting process consists of a flash furnace, waste-heat boiler and electrostatic precipitator. A flash smelter usually includes the following auxiliary units: feed mixture preparation and drying, converters, slag treatment system, SO2 fixation system, anode furnace and anode casting.
2.1 Flash furnace operation and control The flash furnace operator must smelt concentrate at a steady, specified rate while: a) producing matte of specified Cu grade b) producing slag of specified Fe3O4 content c) producing slag at specified temperature, d) producing waste gases of
14
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
(b) keeping enough Fe and S in the matte so that subsequent convertions can operate autogenously while smelting some other recycled materials.
specified SO2 content and e) maintaining a protective coating of magnetite-rich slag into the furnace. The most important phenomenon that affects process control is the quality of feed material. The overall quality of the feed mixture is influenced by quality and amount of the feed (dry concentrate, recycled slag, revert and powder) and the amount of silica flux. A schematic of the feed material preparation is shown in Figure 4. Dry concentrate and silica flux
Revert
T40
2.2. Problems in CMG control
Powder
T100
Feedback CMG control can work when the dry feed is a prepared mixture of a given and known composition that last for several hours. In Chagres, a time variant composition feed is produced mainly because of constraints in space and concentrates coming from different plants. The matte grade is obtained by laboratory analysis from samples taken every one hour in average, when the matte is tapped out of the furnace. The sample analysis can take 1-2 hours. When this delayed information is available the ratio between the total oxygen and concentrate can be adjusted. Most of the time, the feed quality has changed during this period and the new ratio do not help to reduce the variance in CMG. A large ratio gives extensive Fe and S oxidation and high CMG. Physically, the ratio is controlled by adjusting the rates at which air and oxygen enter the furnace, at constant concentrate feed rate.
T50 7 tph
D
The sampling points are located, one at the exit of the drying system, and two in each of the powder belts. The revert is sampled before it is charged to the silo. All samples are taken and analysed for copper, iron, sulfur and other contents, 5-6 times per day. Total dry feed load and composition are not measured on line.
3 tph
D 70 tph
80 tph FF
Fig. 4. Schematic of feed materials to the furnace. Dry copper concentrate is mixed with a dosed amount of silica flux, in a 400 tons silo, discharging in a dosing system. The total amount of dry feed is set by screws discharging on a conveyor. Test performed contrasting the tonnage delivered against the set point of the dry feed control, showed that the maximum tonnage error was less than 3%.
Similarly, when the slag contents are available the silica flux can be adjusted. The iron oxide formed by concentrate oxidation is fluxed with SiO2 to form liquid slag. The amount of SiO2 is based upon the slag having (a) a low solubility for Cu and (b) sufficient fluidity for easy tapping and a clean matte/slag separation. A SiO2/Fe mass ratio of 0.7 to 1.0 is used. It is controlled by adjusting the rate at which silica flux is fed.
Revert is transported from a 100 tons silo by a series of lift mats and conveyor belts to a 3 m3 silo that discharges into a weight meter conveyor set to a precise load. The revert loading control system started its operation in 2010, and test performed to contrast the tonnage delivered against the revert tonnage set point showed that the maximum tonnage error was less than 0.5 tons/h.
Matte and slag temperatures are measured as matte and slag flow from the furnace. Disposable thermocouple probes and optical pyrometers are used. Matte and slag temperatures are controlled by adjusting the air to concentrate and to oil burners. Slag temperature is adjusted with the oil burner operation. Matte and slag temperatures are typically around 1250°C. Excessive temperatures are avoided to minimize refractory and cooling jacket wear.
A pair of conveyor drags, from the electrostatic precipitators and the waste heat boiler, feed a 50 tons silo with powder. The load of powder is adjusted by setting the drags velocity. Some tests were performed to the calibration system employed since 2006. Significant differences were found between the tonnage set point value and the powder tonnage delivered. When the control system required 2.6 tons/h the drags really delivered 3.9, and when the control system required 2,9 tons/h the drags really delivered 5.6. Furthermore, if the drags velocity was set to 0 rpm, the SCADA system displayed 1.9 tons/h. Consequently, the powder tonnage control system must be reviewed.
2.3. Improvements in CMG control CMG control can be improved if a virtual sensor of the objective replaces the lack of direct measurements on time. However, due to the sparse, delayed, and inaccurate measurements available, even PLS methods were not able to obtain a useful predictive model of CMG as a function of the measured variables. An alternative is the incorporation of feed forward control to adjust on-line the oxygen/concentrate ratio, based on stoichiometric calculations. To implement this, the actual composition of the total feed to the furnace must be estimated by using a virtual sensor based on delayed measurements from each kind of solid inputs.
Chagres flash furnace strategy is to charge dried concentrate mix to the furnace at a prescribed rate and to base all other controls on this rate. Having chosen concentrate feed rate, the flash furnace operator must next select the matte copper grade by estimating the extent of Fe and S oxidation. It is selected as a compromise between: (a) maximizing SO2 evolution in the flash furnace (to be efficiently captured) and 15
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
This new system reduced the average powder tonnage set point deviations from 2 to 0.25 tons/h, as it is illustrated in Figure 6.
Powder tonnage (ton/h)
For a constant setting of total dry feed a large variance has been observed. A study (Cornejo, 2012) shows that the large variability contribution came from the powder loading system (72%), and then from the dry feed loading system (21%) and from the revert loading system (7%). Then the first step in this project was to reduce the feed bias and variance, by checking each of the load estimation procedure. The main results are presented and discussed in the next section of this paper. The next step will be the estimation of time delays from the local measurements to the furnace injection burner. When these results were obtained a virtual sensor to predict the feed composition will be built. The improved predictions of feed composition will be used to adjust the oxygen/concentrate ratio.
14 12 10 8 6 4 2 0
Using old calibration
3. RESULTS AND DISCUSSIONS
Date [day-month]
The powder loading system was studied and some pitfalls were detected. The feed to the powder hopper was interrupted and the velocity of the dragging system was set for a given period of time to different values. The initial and final hopper weights were registered for each period. Using this data the tonnage calculated for each drugging system velocity was compared to the value registered in the SCADA system. The results are shown in Figure 5. The main explanation for the observed calibration errors was that the last calibration procedure was executed before some physical changes in the dragging system were performed and that the correlation between tonnage and drags velocity did not included the point zero tonnage at zero drags velocity. Any time the set point of the powder tonnage was changed the control system over reacted producing large overshoots and offsets. The effect of the powder hopper level was also investigated. A new correlation between tonnage and drags velocity was found, as it is shown in Figure 5.
Fig. 6. Effect of changing the powder tonnage control.
Copper matte grade (%)
One can also see the effect of stabilizing the same average total feed on the CMG in Figure 7. A 25% reduction in variability was observed. Analyses of gases showed a reduction in oxygen concentration variation. One question arises at this time, why the smelting process is so sensitive to this part of the feed which represents only 3 tons/h for a total feed rate of 80 tons/h. The improvements in the new control system are to deliver the right amount of powder to the burner and to significantly reduce its variation.
8 Effective powder tonnage [t/h]
Using new calibration
7
70 68 66 64 62 60 58 56 54 52 50
After
Before
6 Date [day-month]
5
Fig. 7 Effect on copper matte grade.
4
This unexpected result is explained in part because the total feed tonnage variation was considerably decreased and because the fraction of powder in the feed was reduced. The particular effect of powder addition can be explained by the chemical reactions occurring in the flash furnace. The powder contains mainly copper sulfate and hematite, and therefore the powder is decomposed, as follows:
New
3
Old
2 2
3
4 5 6 7 Powder tonnage set point [t/h]
8
Fig. 5. Comparison of new and old calibration systems. Since this model is effective only when the powder hopper is working normally, an algorithm to detect discharge problems from the hopper was developed, by using the change in tonnage available in the hopper.
CuSO4 = 1/2Cu2O + SO2 + 1/4O2
(4)
Fe2O3 = 2/3Fe3O4 + 1/6O2
(5)
Both reactions liberate oxygen to be used in the oxidation of iron, and thus in the concentration of the copper in the matte. 16
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
Additionally, the copper oxide produced in Eq. 4 reacts with the iron sulfur transferring the iron from the matte to the slag: Cu2O + FeS = Cu2S + FeO
A secondary effect of this result is that the standard deviation of the gas flow rate generated at the flash furnace was also reduced in almost 20%, as it is shown in Figure 10.
(6) 45
Then by reducing the amount and variability of powder addition, the copper matte grade average remained in 59.6%, but its variability is significantly reduced in 25%. Finally, the first goal of reducing CMG variability was achieved. This small variability will allow to finding a better reactive dosage, for example increasing the powder rate, to push the CMG inside the target band.
Gas flow rate [KNm3/h]
40
Magnetite concentration [%]
The slag magnetite concentration changes are shown in Figure 8. One can see that the concentration mean value was decreased from 13% to 10%, while its standard deviation was reduced in 50%.
30 25 20 Date [day-month]
Fig. 10. Effect on generated gas flow rate in flash furnace.
22 20 18 16 14 12 10 8 6 4 2 0
The recommended maximum target for the gas flow rate is 35 KNm3/h. This became an important result since one bottleneck is the capacity of the acid plant to process the gases. The small variability observed allow the operators to push the flow rate close to the limit most of the time. The mean gas flow rate was increased from 31.9 to 33.3 KNm3/h. The increased stability of the generated gas flow rate favored also the heat recovery at the boiler and the increment of the fusion rate, in order to take advantage of the new space to generate more gases, as it is shown in Figure 11. The mean fusion rate was increased from 68.2 to 73.6 ton/h. Then the next step is to shift the average CMG to a better value by proper adjustment of the oxygen enrichment coefficient by feed forward control, based on the virtual sensor of composition of the total feed. This work will be done in the near future. The matte temperature control and the control of the magnetite concentration in the slag can also take advantage of the initiated stabilization process.
Date [day-month] Fig. 8. Effect on slag magnetite concentration.
Fusion rate (ton/h)
Since the oxygen/concentrate ratio is used to manually control the CMG, the stabilization of the CMG was used by operators to reduce the variation of this ratio, as it is shown in Figure 9. The standard deviation of the ratio was reduced from 9.2 to 2.9 Nm3/ton, while its mean value remained around 153 Nm3/ton. 190 Oxygen/concentrate ratio [Nm3/ton]
35
180
Before
After
170
110 100 90 80 70 60 50 40
160 150
Date [day-month]
140
Fig. 11. Effect on concentrate fusion rate.
130 CONCLUSIONS Date [day-month]
Feed back control of CMG has found to be inefficient because of significant variation in the load preparation and
Fig. 9. Effect on oxygen/concentrate ratio. 17
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
direct measurement of targets with long time delays. Since efforts to build a virtual sensor to predict the CMG were unfruitful, due to sparse and inaccurate measurements, two proposals are made. First, to reduce the bias and variance of the total furnace load, and second, to develop a virtual sensor to have a better estimation of the composition of the total feed entering into the furnace, to be used in a feed forward control of CMG. A preliminary study showed that the powder loading control system was the main contribution of the bias and variability in the total feed entering into the furnace. The bias and variance reductions in the powder loading system allowed a reduction in CMG variability of 25%. This improvement can be explained considering two effects. First, the stabilization of the total feed tonnage that affect the mass balance in the furnace. Second, even when the powder tonnage is only 7% of the total tonnage, its special chemical composition contributed to increase the copper content in the matte through the described reactions. The stabilization of the furnace operation also improved the phase separation, as well as reduced the variance of the gas SO2 composition and flow rate. The operators took advantage of this reduced variability to increase the fusion rate and thus the mean generated gas flow rate in the furnace, without violating the plant constraints. One important conclusion is that this result will permit to increase the average value of CMG, when the development of the virtual sensor of feed composition is completed and when this estimation becomes part of a FF-FB control. ACKNOWLEDGEMENTS The author would like to thanks Santa Maria University (Project 271203), NEIM, Project P07-087-F, ICM-Mideplan and Fondecyt (Project 1100854) for their financial support, and Anglo American Chile for the work support and allowing the presentation of this paper. REFERENCES Davenport W., D. Jones, M. King and E. Partelpoeg, 2003. Flash Smelting Analysis, Control and Optimization, Second Edition, John Wiley and Sons. Hodouin D., 2009. Automatic Control in Mineral Processing Plants: An Overview, Pre-prints IFACMMM2009 Workshop, (Ed. L.G. Bergh), Viña del Mar, Chile, 14-16 October, pp. 1-12. Jämsä-Jounela S. -L., M. Vermasvuori, P. Endén, S. Haavisto, 2003. A Process Monitoring System Based on the Kohonen Self-organizing Maps, Control Engineering Practice, 11, 1, pp. 83-92. Cornejo, I., 2012, Improvements on the control system of a flash smelting furnace (in spanish), M.Sc. Thesis, Chemical Engineering Department, Santa Maria University, Valparaiso, Chile.
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Improving copper matte grade control in a concentrate flash furnace Luis Bergh*, Iván Cornejo*, Fernando Romero+ and Cristian Sulzer° *CASIM, Automation and Supervision Centre for Mineral Industry, Chemical Engineering Department, Santa Maria University, Valparaiso, Chile, (e-mail: [email protected]) +
Process Control Manager, Anglo American Chile, Santiago, Chile °Chagres Division, Anglo American Chile, Santiago, Chile
Abstract: The copper matte grade (CMG) variation in a flash furnace depends on bath temperature, phase levels, ratio of oxygen to sulfur, iron and copper content on the feed, and tonnage ratio of silica flux and feed, among others. The CMG is obtained from laboratory analysis of samples taken when the matte is tapped out from the furnace. Then the CMG is known with a time delay of 1-2 hours, almost every one hour. Since feed characteristics usually change during this period, any feedback control of CMG cannot reduce enough the grade variability. On the other hand, it was shown that main disturbances are coming with the composite feed tonnage and composition, being the powder tonnage control system the main contribution. A new calibration system was proposed and implemented. During an evaluation period of 60 days, it was found that the standard deviation of the CMG was reduced in 25%. This improvement can be explained considering two effects. First, the stabilization of the total feed tonnage that affect the mass balance in the furnace. Second, even when the powder tonnage is only 7% of the total tonnage, its special chemical composition contributed to increase the copper content in the matte. The stabilization of the furnace operation also improved the phase separation, as well as reduced the variance of the gas SO2 composition and flow rate. The operators took advantage of this reduced variability to increase the fusion rate and thus the mean generated gas flow rate in the furnace, without violating the plant constraints. One important conclusion is that this result will permit to increase the average value of CMG, when the development of the virtual sensor of feed composition is completed and when this estimation becomes part of a FF-FB control. Copyright © 2012 IFAC Keywords: flash furnace, control, industrial data, pyrometallurgy frequency measured variables with targets. These high frequency target predictions permit the effective use of feedback control during the absence of target measurements. Some complications may arise when the models are built on and uses inaccurate measurements. Better estimation of some key variables, as characteristics of the process feed, can be used to add a feed forward compensation. More details on control and optimization of flash smelting can be found in Davenport et al. (2003).
1. INTRODUCTION Feedback control of local objectives is a powerful tool to modify an input variable to compensate process disturbances or to closely follow set point changes. However, the successful implementation of this control mainly depends on process dynamics and on the accuracy of measurements and actuators. When process variables are frequently observed, and process disturbances are highly correlated, predictive control is a better alternative than conventional PID control, especially when the process presents significant time delay. Furthermore, if process dynamics is time variant, adaptive predictive control is the appropriate control tool. Knowledgebased expert control systems are an alternative when several objectives should be simultaneously met and process dynamics is complex. The use of complementary tools to improve control is discussed by Hodouin (2009).
In this paper, the copper flash smelting process control is analyzed in the previous perspective. Typical historical CMG, obtained from Chagres smelting plant for a period of 90 days, are shown in Figure 1. The dashed lines represent the upper and lower limits for this target. The mean CMG was 59.4% with a standard deviation of 2.2%. One can see that the target is at least 60% of the time outside the desired band.
The variability of the targets cannot be significantly reduced by any feedback control algorithm if significant time delay is present and target variables are observed at low frequency, as compared with process disturbances changes over time. One solution is to use models (virtual sensors) relating high 978-3-902823-12-0/12/$20.00 © 2012 IFAC
A second objective is the matte temperature, since the efficiency of the phase separation is influenced by its viscosity in the settler. The target temperature is 1255 °C, and an acceptable band is between 1230 and 1280 °C. Historical 13
10.3182/20120910-3-JP-4023.00035
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
Copper matte grade (%)
data was centered in 1270 °C with deviations between 1250 and 1300 °C.
A typical flash furnace is described in Figure 3. In flash smelting the fine, dried copper sulphide concentrate and silica flux with air, oxygen enriched air or pure oxygen blast and recycled flue dust are fed through a concentrate burner into the flash furnace reaction shaft. In the reaction shaft the suspension of gas, solid concentrate particles and solid flux particles, are formed and one important reaction is:
70 68 66 64 62 60 58 56 54 52 50
2CuFeS2 + 2.5O2 + SiO2 = Cu2S.FeS + FeO.SiO2 + 2SO2 + energy (1) Parallel reactions are the oxidation of FeS and FeS2:
Date [day-month]
FeS + 1.5O2 = FeO + SO2
(2)
FeS2 + 2.5 O2 = FeO + 2SO2
(3) Concentrate
Fig. 1. Historical copper matte grade.
O2
N2
Flux
Magnetite concentration [%]
A third objective is to maintain the magnetite concentration in the slag phase in 6%, and not to exceed 12%. In Figure 2 some typical magnetite concentrations are shown, with a mean value of 12% and deviations between 8 and 16%.
Dryer
22 20 18 16 14 12 10 8 6 4 2 0
TI AI AI
Fig. 3. A typical flash furnace. The formed FeO then migrates from matte to slag phase due to a change in density. The extension of both reactions is controlled by the amount of oxygen, as a limiting reactive. The powder is composed of oxidized sulfur, copper and iron, sulfated and reduced copper, and other materials that do not participate in oxidation reactions, and they only have the effect of cooling the bath.
Date [day-month] Fig. 2. Typical magnetite concentration in slag.
When the suspension leaves the reaction shaft, the reacted molten concentrate particles and inert flux particles are separated from the gas stream and hit towards molten slag and matte. In the molten bath, the reactions are completed and matte and slag will be settled at the settler of the furnace as respective layers due to their different density.
In the next sections the process characteristics, the main disturbances, measurements and control systems are described to understand the causes of this poor control. Some solutions are first proposed and then successfully implemented to reduce the observed variance and to permit displacing the mean value inside the control band.
The matte and slag are tapped out through the holes located at the side or at the end walls of the settler. The waste gases generated at the process are ducted through the uptake shaft into waste-heat boiler and electrostatic precipitator, and then processed on the sulphuric acid plant.
2. COPPER FLASH SMELTING IN CHAGRES Outokumpu flash smelting is a pyrometallurgical process for smelting metal sulphide concentrates (Jamsa-Jounela et al., 2003). The Outokumpu flash smelting process consists of a flash furnace, waste-heat boiler and electrostatic precipitator. A flash smelter usually includes the following auxiliary units: feed mixture preparation and drying, converters, slag treatment system, SO2 fixation system, anode furnace and anode casting.
2.1 Flash furnace operation and control The flash furnace operator must smelt concentrate at a steady, specified rate while: a) producing matte of specified Cu grade b) producing slag of specified Fe3O4 content c) producing slag at specified temperature, d) producing waste gases of
14
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
(b) keeping enough Fe and S in the matte so that subsequent convertions can operate autogenously while smelting some other recycled materials.
specified SO2 content and e) maintaining a protective coating of magnetite-rich slag into the furnace. The most important phenomenon that affects process control is the quality of feed material. The overall quality of the feed mixture is influenced by quality and amount of the feed (dry concentrate, recycled slag, revert and powder) and the amount of silica flux. A schematic of the feed material preparation is shown in Figure 4. Dry concentrate and silica flux
Revert
T40
2.2. Problems in CMG control
Powder
T100
Feedback CMG control can work when the dry feed is a prepared mixture of a given and known composition that last for several hours. In Chagres, a time variant composition feed is produced mainly because of constraints in space and concentrates coming from different plants. The matte grade is obtained by laboratory analysis from samples taken every one hour in average, when the matte is tapped out of the furnace. The sample analysis can take 1-2 hours. When this delayed information is available the ratio between the total oxygen and concentrate can be adjusted. Most of the time, the feed quality has changed during this period and the new ratio do not help to reduce the variance in CMG. A large ratio gives extensive Fe and S oxidation and high CMG. Physically, the ratio is controlled by adjusting the rates at which air and oxygen enter the furnace, at constant concentrate feed rate.
T50 7 tph
D
The sampling points are located, one at the exit of the drying system, and two in each of the powder belts. The revert is sampled before it is charged to the silo. All samples are taken and analysed for copper, iron, sulfur and other contents, 5-6 times per day. Total dry feed load and composition are not measured on line.
3 tph
D 70 tph
80 tph FF
Fig. 4. Schematic of feed materials to the furnace. Dry copper concentrate is mixed with a dosed amount of silica flux, in a 400 tons silo, discharging in a dosing system. The total amount of dry feed is set by screws discharging on a conveyor. Test performed contrasting the tonnage delivered against the set point of the dry feed control, showed that the maximum tonnage error was less than 3%.
Similarly, when the slag contents are available the silica flux can be adjusted. The iron oxide formed by concentrate oxidation is fluxed with SiO2 to form liquid slag. The amount of SiO2 is based upon the slag having (a) a low solubility for Cu and (b) sufficient fluidity for easy tapping and a clean matte/slag separation. A SiO2/Fe mass ratio of 0.7 to 1.0 is used. It is controlled by adjusting the rate at which silica flux is fed.
Revert is transported from a 100 tons silo by a series of lift mats and conveyor belts to a 3 m3 silo that discharges into a weight meter conveyor set to a precise load. The revert loading control system started its operation in 2010, and test performed to contrast the tonnage delivered against the revert tonnage set point showed that the maximum tonnage error was less than 0.5 tons/h.
Matte and slag temperatures are measured as matte and slag flow from the furnace. Disposable thermocouple probes and optical pyrometers are used. Matte and slag temperatures are controlled by adjusting the air to concentrate and to oil burners. Slag temperature is adjusted with the oil burner operation. Matte and slag temperatures are typically around 1250°C. Excessive temperatures are avoided to minimize refractory and cooling jacket wear.
A pair of conveyor drags, from the electrostatic precipitators and the waste heat boiler, feed a 50 tons silo with powder. The load of powder is adjusted by setting the drags velocity. Some tests were performed to the calibration system employed since 2006. Significant differences were found between the tonnage set point value and the powder tonnage delivered. When the control system required 2.6 tons/h the drags really delivered 3.9, and when the control system required 2,9 tons/h the drags really delivered 5.6. Furthermore, if the drags velocity was set to 0 rpm, the SCADA system displayed 1.9 tons/h. Consequently, the powder tonnage control system must be reviewed.
2.3. Improvements in CMG control CMG control can be improved if a virtual sensor of the objective replaces the lack of direct measurements on time. However, due to the sparse, delayed, and inaccurate measurements available, even PLS methods were not able to obtain a useful predictive model of CMG as a function of the measured variables. An alternative is the incorporation of feed forward control to adjust on-line the oxygen/concentrate ratio, based on stoichiometric calculations. To implement this, the actual composition of the total feed to the furnace must be estimated by using a virtual sensor based on delayed measurements from each kind of solid inputs.
Chagres flash furnace strategy is to charge dried concentrate mix to the furnace at a prescribed rate and to base all other controls on this rate. Having chosen concentrate feed rate, the flash furnace operator must next select the matte copper grade by estimating the extent of Fe and S oxidation. It is selected as a compromise between: (a) maximizing SO2 evolution in the flash furnace (to be efficiently captured) and 15
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
This new system reduced the average powder tonnage set point deviations from 2 to 0.25 tons/h, as it is illustrated in Figure 6.
Powder tonnage (ton/h)
For a constant setting of total dry feed a large variance has been observed. A study (Cornejo, 2012) shows that the large variability contribution came from the powder loading system (72%), and then from the dry feed loading system (21%) and from the revert loading system (7%). Then the first step in this project was to reduce the feed bias and variance, by checking each of the load estimation procedure. The main results are presented and discussed in the next section of this paper. The next step will be the estimation of time delays from the local measurements to the furnace injection burner. When these results were obtained a virtual sensor to predict the feed composition will be built. The improved predictions of feed composition will be used to adjust the oxygen/concentrate ratio.
14 12 10 8 6 4 2 0
Using old calibration
3. RESULTS AND DISCUSSIONS
Date [day-month]
The powder loading system was studied and some pitfalls were detected. The feed to the powder hopper was interrupted and the velocity of the dragging system was set for a given period of time to different values. The initial and final hopper weights were registered for each period. Using this data the tonnage calculated for each drugging system velocity was compared to the value registered in the SCADA system. The results are shown in Figure 5. The main explanation for the observed calibration errors was that the last calibration procedure was executed before some physical changes in the dragging system were performed and that the correlation between tonnage and drags velocity did not included the point zero tonnage at zero drags velocity. Any time the set point of the powder tonnage was changed the control system over reacted producing large overshoots and offsets. The effect of the powder hopper level was also investigated. A new correlation between tonnage and drags velocity was found, as it is shown in Figure 5.
Fig. 6. Effect of changing the powder tonnage control.
Copper matte grade (%)
One can also see the effect of stabilizing the same average total feed on the CMG in Figure 7. A 25% reduction in variability was observed. Analyses of gases showed a reduction in oxygen concentration variation. One question arises at this time, why the smelting process is so sensitive to this part of the feed which represents only 3 tons/h for a total feed rate of 80 tons/h. The improvements in the new control system are to deliver the right amount of powder to the burner and to significantly reduce its variation.
8 Effective powder tonnage [t/h]
Using new calibration
7
70 68 66 64 62 60 58 56 54 52 50
After
Before
6 Date [day-month]
5
Fig. 7 Effect on copper matte grade.
4
This unexpected result is explained in part because the total feed tonnage variation was considerably decreased and because the fraction of powder in the feed was reduced. The particular effect of powder addition can be explained by the chemical reactions occurring in the flash furnace. The powder contains mainly copper sulfate and hematite, and therefore the powder is decomposed, as follows:
New
3
Old
2 2
3
4 5 6 7 Powder tonnage set point [t/h]
8
Fig. 5. Comparison of new and old calibration systems. Since this model is effective only when the powder hopper is working normally, an algorithm to detect discharge problems from the hopper was developed, by using the change in tonnage available in the hopper.
CuSO4 = 1/2Cu2O + SO2 + 1/4O2
(4)
Fe2O3 = 2/3Fe3O4 + 1/6O2
(5)
Both reactions liberate oxygen to be used in the oxidation of iron, and thus in the concentration of the copper in the matte. 16
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
Additionally, the copper oxide produced in Eq. 4 reacts with the iron sulfur transferring the iron from the matte to the slag: Cu2O + FeS = Cu2S + FeO
A secondary effect of this result is that the standard deviation of the gas flow rate generated at the flash furnace was also reduced in almost 20%, as it is shown in Figure 10.
(6) 45
Then by reducing the amount and variability of powder addition, the copper matte grade average remained in 59.6%, but its variability is significantly reduced in 25%. Finally, the first goal of reducing CMG variability was achieved. This small variability will allow to finding a better reactive dosage, for example increasing the powder rate, to push the CMG inside the target band.
Gas flow rate [KNm3/h]
40
Magnetite concentration [%]
The slag magnetite concentration changes are shown in Figure 8. One can see that the concentration mean value was decreased from 13% to 10%, while its standard deviation was reduced in 50%.
30 25 20 Date [day-month]
Fig. 10. Effect on generated gas flow rate in flash furnace.
22 20 18 16 14 12 10 8 6 4 2 0
The recommended maximum target for the gas flow rate is 35 KNm3/h. This became an important result since one bottleneck is the capacity of the acid plant to process the gases. The small variability observed allow the operators to push the flow rate close to the limit most of the time. The mean gas flow rate was increased from 31.9 to 33.3 KNm3/h. The increased stability of the generated gas flow rate favored also the heat recovery at the boiler and the increment of the fusion rate, in order to take advantage of the new space to generate more gases, as it is shown in Figure 11. The mean fusion rate was increased from 68.2 to 73.6 ton/h. Then the next step is to shift the average CMG to a better value by proper adjustment of the oxygen enrichment coefficient by feed forward control, based on the virtual sensor of composition of the total feed. This work will be done in the near future. The matte temperature control and the control of the magnetite concentration in the slag can also take advantage of the initiated stabilization process.
Date [day-month] Fig. 8. Effect on slag magnetite concentration.
Fusion rate (ton/h)
Since the oxygen/concentrate ratio is used to manually control the CMG, the stabilization of the CMG was used by operators to reduce the variation of this ratio, as it is shown in Figure 9. The standard deviation of the ratio was reduced from 9.2 to 2.9 Nm3/ton, while its mean value remained around 153 Nm3/ton. 190 Oxygen/concentrate ratio [Nm3/ton]
35
180
Before
After
170
110 100 90 80 70 60 50 40
160 150
Date [day-month]
140
Fig. 11. Effect on concentrate fusion rate.
130 CONCLUSIONS Date [day-month]
Feed back control of CMG has found to be inefficient because of significant variation in the load preparation and
Fig. 9. Effect on oxygen/concentrate ratio. 17
IFAC MMM 2012 September 10-12, 2012. Gifu, Japan
direct measurement of targets with long time delays. Since efforts to build a virtual sensor to predict the CMG were unfruitful, due to sparse and inaccurate measurements, two proposals are made. First, to reduce the bias and variance of the total furnace load, and second, to develop a virtual sensor to have a better estimation of the composition of the total feed entering into the furnace, to be used in a feed forward control of CMG. A preliminary study showed that the powder loading control system was the main contribution of the bias and variability in the total feed entering into the furnace. The bias and variance reductions in the powder loading system allowed a reduction in CMG variability of 25%. This improvement can be explained considering two effects. First, the stabilization of the total feed tonnage that affect the mass balance in the furnace. Second, even when the powder tonnage is only 7% of the total tonnage, its special chemical composition contributed to increase the copper content in the matte through the described reactions. The stabilization of the furnace operation also improved the phase separation, as well as reduced the variance of the gas SO2 composition and flow rate. The operators took advantage of this reduced variability to increase the fusion rate and thus the mean generated gas flow rate in the furnace, without violating the plant constraints. One important conclusion is that this result will permit to increase the average value of CMG, when the development of the virtual sensor of feed composition is completed and when this estimation becomes part of a FF-FB control. ACKNOWLEDGEMENTS The author would like to thanks Santa Maria University (Project 271203), NEIM, Project P07-087-F, ICM-Mideplan and Fondecyt (Project 1100854) for their financial support, and Anglo American Chile for the work support and allowing the presentation of this paper. REFERENCES Davenport W., D. Jones, M. King and E. Partelpoeg, 2003. Flash Smelting Analysis, Control and Optimization, Second Edition, John Wiley and Sons. Hodouin D., 2009. Automatic Control in Mineral Processing Plants: An Overview, Pre-prints IFACMMM2009 Workshop, (Ed. L.G. Bergh), Viña del Mar, Chile, 14-16 October, pp. 1-12. Jämsä-Jounela S. -L., M. Vermasvuori, P. Endén, S. Haavisto, 2003. A Process Monitoring System Based on the Kohonen Self-organizing Maps, Control Engineering Practice, 11, 1, pp. 83-92. Cornejo, I., 2012, Improvements on the control system of a flash smelting furnace (in spanish), M.Sc. Thesis, Chemical Engineering Department, Santa Maria University, Valparaiso, Chile.
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