Method of controlling the alumina feed into reduction cells for

Details Method of controlling the alumina feed into reduction cells for Method of controlling the alumina feed into reduction cells for

1986-10-16

1988-08-23

Ruggiero, Joseph

Boots, shoes, and leggings

204 67, 204245, 364149, 364164, G05B 1702, G06F 1546, C25C 306

Patent

active

047665521

DESCRIPTION:

BRIEF SUMMARY
Reduction cells with pre-baked anodes for producing aluminium have become increasingly automated during the last years. The development has passed from cells having side break and feed systems with large and infrequent addition of alumina, to cells having a center break and feed system giving a more frequent addition of alumina and smaller amounts for each addition, and then to cells having point feeders giving frequent and small additions of alumina. Close control of the alumina concentration in the electrolytic bath is important in order to obtain high current efficiency and good operational regularity of the cell. For obtaining this a good process control system is needed.
If there is a deficiency of alumina in the bath, an anode effect will occur (the voltage drop increases from appr. 4 V to 20-40 V), and if there is an excess of alumina in the bath, there will be formed sludge in the cell (undissolved alumina and bath deposits at the bottom of the cell). In order to obtain a high current efficiency there is employed a bath of low liquidus temperature. Such electrolytic baths having a low liquidus temperature are less capable of dissolving alumina and therefore it is very important to have close control of the feeding of alumina.
The process control of reduction cells for producing aluminium is a very difficult task because one has to do with a complicated interaction of many factors. In addition to the alumina concentration and the bath temperature there are, inter alia, important magnitudes such as the anode-cathode distance and the electrical resistance across the cell. Except for the resistance these magnitudes are not easily available for a more or less continuous and sufficiently accurate measurement during operation, for use in the process control. Thus, there have been proposed various control methods based upon known theories within control engineering.
The present invention is directed to all types of alumina reduction cells, including old cells in which the cell resistance is subject to external disturbances because of magnetic fields or the heat balance. This will be the case in many of the pot lines being today in operation.
During many years attempts have been made to feed alumina into reduction cells for producing aluminium with a certain degree of regularity in order to maintain the alumina content in the bath within certain limits of concentration. In Published British Patent Application No. 2080830 there is described a method for maintaining the alumina content in the bath within the range 1-3,5%. However, this known method is not so flexible and robust that it may be employed with a good result in plants being subjected to disturbances and noise. Apparently the method is intended for a specific type of cells which inherently have a higher degree of stability than present-day common alumina reduction cells.
In this known control method there is added at least one of the following additives to a bath of molten cryolite: 5 to 20% aluminium fluoride, lithium salts at a concentration equal to or smaller than 1% in terms of Li, magnesium salts at a concentration equal to or smaller than 2% in terms of Mg, and alkali metal or alkaline earth metal chloride at a concentration equal to or smaller than 3% in terms of C1. The rate of alumina feeding is modulated depending upon the variation in the bath resistance during predetermined time intervals in alternating cycles of equal duration, for adding alumina at a slower rate and a faster rate, respectively, than what is corresponding to the consumption in the cell. This method makes it necessary to operate in the lowest part of the alumina concentration range, because it is required with the method that the bath resistance varies quickly with a change in the alumina concentration.
The above known control method takes little account of noise, such as unpredictable variations in the electric voltage, and therefore is sensitive to external disturbances.
In contrast to the above and other known control methods the present invention is directed to the

REFERENCES:
patent: 4197576 (1980-04-01), Sanchez
patent: 4245201 (1984-01-01), Wilson et al.
patent: 4368510 (1983-01-01), Anderson
patent: 4451878 (1984-05-01), Shigemasa
"Adaptive Control of Alumina Reduction Cells with Point Feeders", Peter Borg et al., Modeling, Identification and Control, 1986, vol. 7, No. 1, 45-56.

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