Lupine Publishers| Journal of Robotics and Mechanical Engineering
Abstract
Complex multivariable
processes are generated in multi-hearth furnaces, and their modeling contains a
high index of uncertainty. The main variables that characterize the
post-combustion subprocess were identified and data were taken that comprise a
period of three months of operation of the installation, to which a regression
analysis was carried out step by step backwards. This analysis allowed us to
determine that the linear correlation coefficient for hearth temperature four
was 0.79 and 0.65 for hearth temperature six, in addition to identifying the
independent variables that most influence these process output variables.
Keywords: Furnaces,
Post-Combustion subprocess, Regression analysis
Introduction
Nickel-producing
companies are characterized by continuous processes of great complexity; that
require automation to achieve greater efficiency in their productions. The
Company under study operates according to the carbonate-ammoniacal leaching
scheme for reduced ore. This company has a multi-hearth reduction furnace
plant, which constitutes a key stage in the production process. The reduction
furnaces are large metal cylinders, where basically the reduction of nickel
oxide and cobalt is carried out to their corresponding metallic forms [1]. In
these equipments it is required to maintain a profile of temperature and
reducing gases (carbon monoxide and hydrogen), for each hearth, its
noncompliance produces significant losses due to the formation of crystalline
structures of iron spinels, olivines and pyroxenes that trap nickel and to
cobalt in the form of oxides and to a lesser extent in a metallic state, and to
the appearance of high contents of metallic iron in the reduced mineral. This
results in a decrease in nickel and cobalt extraction in the leaching process
[2]. To contribute to the establishment of the thermal profile required by the
furnace, secondary air is introduced into hearths four and six
(post-combustion), with the purpose of guaranteeing the complete combustion of
residual carbon monoxide and other combustible gases that come from incomplete
combustion in lower hearths. In this exothermic reaction, an amount of heat is
generated that contributes to the preheating and drying of the mineral. The
control loop of hearth four operates automatically and in hearth six manually,
as a consequence the chemical physical process that takes place in these
hearths is not carried out efficiently; observing temperature oscillations,
which affect the thermal and aerodynamic processes that take place in the furnace.
The literature consulted shows linear mathematical models for the furnaces of a
company with similar characteristics, which operated under different operating
conditions [3]. These models were achieved through experimental identification,
for mean square fit values between 0.72 and 6.1. Also defined as input
variables: the air flow to hearths four and six, and as output variables: the
temperature corresponding to these hearths. Montero [4], obtained dynamic
mathematical models, with adjustment between 62 and 72%, which characterize the
reduction furnaces of the company in question; where they were selected as
input variables: the flow of ore fed to the furnace; Air flow to hearths four
and six. As output variables: temperature of these hearths and concentration of
residual carbon monoxide. To design an effective control strategy for the
post-combustion subprocess, it is necessary to know the behavior of the
variables in different situations and to obtain a process model. The objective
of the work is to obtain a statistical model that represents the behavior of
the post-combustion thread.
Materials and Methods
Description of The Reactor
Herreshoff type furnaces
[5], are composed of a metal cylinder, in an upright position, lined internally
with chamotte bricks or high alumina, protected externally by a metal housing,
agitation facilities, feed and discharge of ore and combustion chambers. They
are formed internally by 17 hearths or hearths that are shaped like spherical
vaults (Figure 1). The furnace has a central rotating shaft to which 68 arms
are articulated, four for each hearth. Each arm has, depending on the hearth,
eight to 12 vanes or inclined teeth. Depending on the area of the furnace, they
will be withholding or sweeping and depending on the odd or even hearths, they
will allow the discharge from one hearth to another in the form of zigzag. In
peer hearths, the discharge is carried out through 30 holes located equidistant
from the periphery, in odd hearths by a hole located in the center around the
central axis. The combustion chambers are equipped with high pressure oil
burners, which are located two for each hearth in: hearth six, hearth eight,
hearth 10, hearth 12; except hearths 14 and 15 that only have one camera. In
each chamber: the oil distributor consists of a main valve, a filter to
separate impurities, a solenoid valve, a thermometer, a manometer, a flow meter
with bypass, a pressure regulator and the burner [6].
Figure 1: Schematic diagram of the reduction furnace seen from the SCADA (CITECT).
Influence of Temperature in The Reduction
Process
Temperature is a
fundamental parameter in pyrometallurgical processes, because it facilitates
the weakening of the crystalline structures of the mineral and therefore the
development of reduction reactions. During the operation, a certain prescribed
profile must be maintained, increasing from the top to the bottom, in order to
guarantee a gradual heating of the mineral. Special attention is paid to the
temperature values of hearths four, 10 and 15. The temperature stability in
hearth four is extremely important because of the influence it has on the
temperature in the other furnace hearths. If there are temperature values below
the norm, there is a displacement of the thermal zones of the furnace, which
entails effects on the extraction of nickel and cobalt [7].
Description of The Post-Combustion
Installation
The system consists
basically of a centrifugal fan (Table 1) installed on the upper floor (ceiling)
of the furnace, with hot air suction intake (150 to 2000C) from the chimney
from the central axis and an air duct from the fan to hearths four and six, with
flow regulation system through butterfly valves. The post-combustion air duct
at the fan outlet , it has an internal diameter of 0.407 m and falls parallel
to the furnace body to the four hearth, where it branches into two ducts of
equal diameter, one goes to hearth four and the other to hearth six (Figure 1).
Figure 2 shows the characteristics of the fan for the constant conditions in
which it is operating. The fan curve intercepts the system characteristic curve
at the operating point (A); for an approximate air flow that circulates before
the fork of 6 796 m3/ h, with a pressure of 3 kPa. Considering that the total
air flow that is guaranteed before branching is constant and corresponds to a
fixed duct air system; are presented in Table 2 airflow hearths four six after
the split, according to the valve opening.
Table 1: Technical data of the post-combustion fan.
Table 2: Equivalent air flow based on valve opening.
Figure 2: Characteristics of the afterburner fan
Statistical Analysis of The Data
The presentation of the
data allows any researcher to easily interpret them. This presentation can be
done in two ways:
a) Frequency Tables: It consists of grouping the data into classes or
categories with their respective frequencies. It is applicable to any type of
variable.
b) Graphics: example (Histogram). It is the representation of the data by
rectangles that are based on a horizontal axis and their area proportional to
the frequency of the class interval. They are used primarily for continuous
variables.
Regression Analysis
Through a step-by-step
regression analysis, the main variables that influence the process-dependent
variables are determined, as well as the linear correlation coefficient.
Results and Discussion
As an auxiliary tool to
select the variables to be used in the control, a statistical analysis was
carried out based on a set of operating data, measured appropriately and
continuously. The statistical analysis was carried out with the objective of
determining the variables that have the greatest influence on the temperature
behavior of hearths four and six of the furnace. For this analysis, five
furnace operation data were taken during the months of May to July of a recent
year and processed with Microsoft Excel and Statgraphics Plus 5.1 software.
These data were obtained from the reports issued by the CITECT Supervisory
System. Table 3 shows the average values represented by (X) and the standard
deviations by (S) for each of the variables measured in the furnace, which are:
Table 3: Behavior of the furnace variables for three months of work.
1. ApH4, ApH6 [Opening
of the air flow regulating valve to hearths four and six (%)].
2. TH0, TH2, TH4, TH6, TH7, TH9, TH11, TH13, TH14, TH15 [Hearth temperature
zero, two, four, six, seven, nine, 11, 14 and 15 respectively (° C)].
3. PH0, PH16 [Pressure in hearths zero and 16 (Pa)].
4. TC6S, TC8N, TC8S, TC10N, TC10S, TC12N, TC12S, TC15S [Temperature of
combustion chambers, hearths six, eight, 12 and 15, north and south side (°
C)].
5. CO [Residual carbon monoxide concentration (%)].
In the months indicated above, the post-combustion air flow meter was not
installed, so that the openings of the air flow regulating valves to hearths
four and six were taken, as proportional measures to the air flow.
The ore processed during this period was of very good characteristics, with a
high iron content. A descriptive statistical analysis of the general trend of
the thermal profile of hearths four and six during these three months of work,
the results of which are presented in Table 4. For this case it is observed
that the values of Kurtosis and the Asymmetry Coefficient allow us to state
that the dependent variables (temperature of hearths four and six) behave like
normal distributions. The frequency histograms for TH4 and TH6 are also
presented during the three months of work in Figures 3-8. histogram. With the
data for the month of May, a step-by-step regression analysis was carried out
to determine the independent variables that most influence TH4 and TH6 (see
equations 1 and 2). 1) TH4=-166.9 + 0.3TC8S - 0.5TH0 + 0.13TH13 + 1.4TH2 -
0.8ApH4 - 0.1TH6
2) TH6=171.6 + 0.3TC6S + 0.8TH13 - 1,1TH14 + 0.7TH15 - 0.13TH4 - 0.2ApH4 +
0.6ApH6 + 0.9TH7 - 0.9TH9
Table 4: Summary of the descriptive statistical analysis of the sample for three months.
Figure 3: Characteristics of the post-combustion fan
Figure 4: Hearth temperature histogram six (TH6). May
Figure 5: Hearth temperature histogram four (TH4). June
Figure 6: Hearth temperature histogram six (TH6). June
Figure 7: Hearth temperature histogram four (TH4). July
Figure 8: Hearth temperature histogram six (TH6). July
In Table 5, the
statistic R2 indicates that model 1 explains 0.62 of the variability in hearth
temperature four, while model 2 explains 0.42 of the variability in hearth
temperature six. The statistic R2 adjusted, which is more convenient for
comparing models with different numbers of independent variables, is 0.62 for
TH4 and 0.42 for TH6. The standard error of the estimate shows the standard
deviation of the residuals, which is 50.62 for TH4 and 47.35 for TH6. Tables 6
& 7 show the analysis of variance for the dependent variables TH4 and TH6.
It is noted that the percentages are less than 0.01, so there is a
statistically significant relationship between the variables for a 99%
confidence level.
Table 5: Summary of the regression analysis for TH4 and TH6.
Table 6: Analysis of variance for hearth temperature four.
Table 7: Analysis of variance for hearth temperature six.
Conclusion
As a result of the
statistical analysis, the influence of six variables can be seen for hearth
temperature four and nine for hearth temperature six. The openings of the air
flow regulating valves, which can be manipulated by a final action element, are
highlighted. The multivariable nature of the thermal profile of hearths four
and six was verified, with respect to the flow of air supplied to these
hearths.
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