Temperature control on hot pots in the aluminium production

Diplomarbeit von Karsten Sommer

Introduction:

The process of aluminium production is, even nearly 150 years after its discovery, not totally understood. Such a large quantity of factors influences the production process that no standard can be applied on it. In an aluminium reduction plant there are no two identical electrolysis cells. Differences in the start-up and operational disturbances alter the thermal and electrical behaviour of each cell individually. So every cell has to be monitored individually through the continuous changing bath composition and temperature variation. Every cell needs an individual dynamic optimisation of the chemical and thermal input, e.g. the AlF3- and CaF2-addition and the regulation of the anode-cathode distance. Self-regulating mechanism are the melting and freezing of the sidewall ledge, external mechanisms the lifting and lowering of the anodes.

In this work a cell control system at Alumar (Brazil) was analysed and improved. The focus laid on controlling the cell temperature through a variation of the anode-cathode distance. The main objective was to analyse the effect and influence of a so-called temperature resistance modifier on hot pots (electrolysis cells).

When the temperature of an electrolysis cell exceeds a certain limit, the pot operates outside its optimal working conditions, thus it has to be cooled down. This can happen by reducing the anode-cathode distance and therefore the resistance of the pot. The reduction is controlled by the so-called temperature resistance modifier. The concept of the temperature modifier was introduced less than one year ago at Alumar and is also within the Alcoa group a quite unexplored field.

This work is structured in two parts, first an introduction to theory, then the practical part. Chapter 2 gives some general information, from the history of aluminium to a brief overview over the entire production process. Chapter 3 focuses on the general functioning of the aluminium electrolysis. Chapter 4 details the properties of the cryolite bath in the reduction cell and their impacts on the pot operating characteristics.

Chapter 5 contains the experimental part of the work. In a first time the quality of the measurement system was tested. Then the relations between temperature resistance modifier and time as well as two bath parameters, the temperature and the ratio were inquired. Thereafter substantial modifications on the program that calculates the temperature modifier were analysed and tested:

The program logic includes several restrictive conditions whose task it is to prevent that the temperature modifier action could harm the pot and destabilise the reduction process. All these conditions were analysed and tested one by one to check their sense and their effect. The objective was to augment the percentage of pots on which the temperature modifier can be applied, i.e. to cool more hot pots down. The formula that calculates the temperature modifier value was analysed and a new formula was tested. The objective was to adapt the temperature modifier better to the pots needs and turn its action more flexible.

In chapter 6 the final conclusions are drawn including the benefits and disadvantages of all the realised tests. Chapter 7 gives suggestions for future researches and improvements.

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