This paper presents the results of monitoring data analysis for experimental areas producing primary aluminum based on the RA-300 and Soderberg technologies. The authors considered the events of technological disorders, such as anode effects and formations on anode face, carried out the statistical analysis of technological parameters and process disruptions in relation to the extreme values of the parameters. The results of analysis allowed us to obtain new knowledge about technological disorders and features of abnormal working states of aluminum reduction cells, detect the technological patterns, conditions and causes for the occurrence of technological disorders.
technological parameters of aluminum reduction cells [
Anode effect is a phenomenon characterized by decreasing in the alumina
dissolution and a significant increase in the cell voltage. Distortions of the anode
geometry are classified into formations on the anode face (“spikes”, “laggings”,
“chunks”), and anode destruction (“corner shedding”). “Spike” is a formation of a
cylindrical or conical shape [
“Corner shedding” is a physical loss of large carbon pieces from the anode surface
[
2
Analysis of the Monitoring Data in the Context of Extreme Values of Technological Parameters
aluminum smelter (KhAZ), Boguchansky aluminum smelter (BoAZ), that use the
Soderberg technology. Technology determines the design features of aluminum
reduction cells and plants and, as a result, the scope of controlled technological
parameters and types of process disruptions [
values was estimated for each cell, and the ranges of reliable values were found as μ ± σ, where μ is the mean value of the data series, σ is the standard deviation. Parameter values outside this range are considered as extreme values. The result of the analysis of the statistical characteristics showed that the cells, in the context of the same parameters, differ significantly both in the average values of the parameters and in their deviation intervals. For each experimental area, the parameters with the largest percent of anomalies over entire observation period were determined, among them: for KhAZ – Back EMF, Dose of alumina, Coefficient of anode-cathode distance; for BoAZ – Cryolite ratio, Coefficient of anode-cathode distance, CaF2 concentration, for BrAZ – AlF3 dose, Minimum distance, MgF2 concentration.
values of the parameters, we considered the dates of disorders detection (in cases of anode effects) and the five-day period preceding the registration of the disorder (in cases of any formations). As a result, for each type of disorder, the distribution of cases accompanied by extreme values of the parameters was obtained (Tables 1, 2).
Electrolyte temperature or Coefficient of anode-cathode distance or Number of AlF3 doses were extremes. At BoAZ most of the “spikes” were accompanied by extreme values of Coefficient of anode-cathode distance or Electrolyte temperature or
anode-cathode distance or Electrolyte temperature or AlF3 dose were extremes. Most of both the “corner sheddings” and the “chunks” occurred at BrAZ while Min. distance or AlF3 dose or Electrolyte level were extremes.
0.08 BrAZ
accompanied by extreme values of Counter EMF or Coefficient of anode-cathode distance or Aluminium level, at BrAZ most of the anode effects were accompanied by extreme values of AlF3 dose or Min. distance or CPC temperature.
In most cases, technological disorders occur when several parameters have extreme values at the same time. The distribution of disorders events in the experimental areas by the number of parameters with extreme values is shown below in Figures 1, 2, 3.
Spikes 2000 t 1500 n u co1000 e s aC 500 0 1 2 3 4 5 12 13 14 15 16 6 7 8 9 10 11
Fig. 1. Distribution of disorders events by the number of parameters with extreme values at KhAZ.
1400 1200 t 1000 n uo 800 c se 600 aC 400 200
0 12000 10000 tn 8000 u co 6000 e sa 4000 C 2000 0 5 6 7 8 9
a rule, outliers of values are observed for several parameters, but there are cases when technological disorders occurred when the values of the parameters were within the statistical norm. At the same time, the number and composition of parameters are different for different types of disorders.
Moreover, regardless of the aluminum production technology, in the case of anode effects, the number of parameters with extreme values is less than in the case of formations on the anodes. So, KhAZ is characterized by extreme values of 2-4 parameters for the anode effect and 6-8 parameters for “spikes”; BoAZ is characterized by extreme values of 1-2 parameters for the anode effect, 5-7 parameters for “lagging” and 4-6 parameters for “spikes”; BrAZ is characterized by extreme values of 4-6 parameters in case of anode effects, 9-11 parameters in cases of “corner shedding” and “chunks”.
Also, this research included the correlation analysis of parameters in part of extreme values. The result demonstrated a quite strong relationship between the following parameters: for KhAZ: Duration of pouring and Velocity ratio with correlation coefficient -0.69, Amperage and Bus voltage with correlation coefficient 0.74; for BoAZ: Electrolyte temperature and Cryolite ratio with correlation coefficient 0.91, Cell Voltage and Electrolyte level with correlation coefficient 0.82,
Number of Alumina doses and Time of Alumina doses with correlation coefficient 0.9,
of technological disorders for each cell showed that in most cases (75%-95% depending on the disorder type) a large number of disorders are accompanied by a large number of extreme values of technological parameters. Additionally, it was possible to identify periods of everyday disorders with varying duration and to rank the cells of the experimental areas by the number of technological disorders.
The analysis of detailed data made it possible to detect the dependence of the number of technological disorders on the location of the anodes in the cells (Figures 4, 5). For instance, at KhAZ we can observe significantly more "spikes" on the anodes of the front side of the cells – in its central part (anodes No. 9 and No. 10) and along the edges (anodes No. 1 and No. 18). At BoAZ, in contrast, we can observe more “spikes” and “laggings” on the anodes of the backside of the cells – mostly around the edges (anodes No. 22, 23 and 33, 34).
500 400 300 200 100
0 500 400 300 200 100 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
Fig. 5. Distribution of “spikes” and “laggings” cases by the cell anodes at BoAZ.
Conclusion
disruptions in terms of the extreme values of the technological parameters for three experimental areas: KhAZ, BoAZ and BrAZ.
The analysis of the statistical characteristics showed that the cells, in the context of the same parameters, differ significantly both in the average values of the parameters and in their deviation intervals. Within the study, for each experimental area, the parameters with the largest percent of anomalies were determined. The distribution of disorders events (in the cases of anode effects and formations on the anode face) by the number of parameters with extreme values was obtained. The results showed that in most cases (75%-95%) a large number of disorders are accompanied by a large number of extreme values of technological parameters. The analysis of detailed data revealed the dependence of the number of technological disorders on the location of the anodes in the cells.