Electrical precipitation of solids from smelter gases

Abstract

The fundamental principles of the process in its simplest form are set forth. It is shown why the Cottrell process during recent years has in a large measure supplanted the bag-house and dust chamber in treating smelter gases. The commonly accepted theory concerning the manner in which the dust particles is charged and precipitated is given. It is pointed out that the thing most important to the operating man is how the particles may be enabled to give up their charge to the electrodes under all conditions, rather than the manner in which they receive it. The various types of treaters in common use are described and discussed. The advantages of straight line treaters over those in which the gases are by-passed are emphasized. It is shown that the gas is ionized much more efficiently for a given power consumption and the construction simplified and reduced by arranging the electrodes in the flue so that their electric fields are in series with each other. It is shown that this is accomplished by causing the gas to flow parallel to the electrostatic lines instead of at right angles to them, as in all other types. The factors of lead and copper metallurgy are given which control the amount of sulphuric acid and water vapor in the gases. It is also pointed out that these things are a measure of the successful operation when treating smelter gases. The physical rather than the chemical structure of the dust in suspension is shown to be the all-important matter. Several theories are given as to why flue dust is so much easier precipitated than fume. A number of photo-micrographs are given to illustrate the difference in physical structure between fume and dust. Two methods are featured of obtaining sufficient conductivity in a dry precipitated coating to permit the electric charge to leak through it to the electrode. The theory of selective absorption is advanced as an explanation of how aqueous vapor added to the gas stream functions in this respect. The method of adding very finely atomized sulphuric acid is shown to be the most practical, it not having certain disadvantages of the water and its higher boiling point permitting a wider field of application. Its action is shown to be due to the fine acid particles being precipitated with the dust particles thereby imparting conductive film to the particles by diffusion. The amount of free sulphuric acid in grams per 1000 cu. ft. of gas which permits a good precipitation on gas without conditioning is given from tests made on a large installation. The theory of back ionization and phenomena of discontinuous dielectric are discussed. Electrical matters are shown to be secondary to treater design and to the conditioning gas. All that can be expected of electrical equipment is to stress the space between electrodes to the economical limit. Local conditions must govern choice of electrical equipment. The tendency to regard electrical phenomena, such as surging which as a rule accompanies poor precipitation as causes rather than effects, is cited. Effects of conditioning gas in reducing surges is shown. Internal reactance in transformer best suited to precipitation work is discussed, also possibility of exceeding the practical limit and wasting power on treatable gas by carrying voltage up to the point of disruption; necessity of knowing electrical values in the treater, and these are best obtained with milliammeter and electrostatic voltmeter; finally, the subject of proportioning the plant investment between electrical equipment and treater is covered.