Scale Formation in Sea-water Distilling Plants and its Prevention

Abstract

The scales found in sea-water evaporators are formed of calcium sulphate, calcium carbonate, and magnesium hydroxide. Calcium sulphate scale can be avoided by using a sufficiently dilute brine concentration to maintain the sulphate in solution. The conditions are indicated under which the formation of this scale can be avoided. Calcium carbonate and magnesium hydroxide scales are shown to be due to the carbonate alkalinity in the sea water, which is limited and reasonably constant. These scales are due to the break-up of the bicarbonate ions in the sea-water as heating and boiling occur. Up to a well-defined range of operating temperatures, a scale that is predominantly of calcium carbonate is experienced. At higher temperatures, a scale predominantly of magnesium hydroxide is formed; but both types of scale may occur in the region of the change-over zone. It is shown that the break-up of the bicarbonate ions produces carbonate ions, which give rise to the calcium carbonate scale and that, with further heating and boiling, break-up of the carbonate ions occurs with the formation of hydroxyl ions, which combine with magnesium ions to form the magnesium hydroxide scale. It is shown that some of the alkalinity leaves the evaporator with the blow-down; as calcium carbonate in solution, or calcium carbonate and/or magnesium hydroxide in suspension. The amount of scale formed is found to be proportional to the amount of sea-water used, and no benefit is obtained by operating at low brine densities. It is shown that the rate of scale formation is greater for low temperatures than high temperatures, and increases with the temperature difference across the heating surface. Tests show that the use of organic dispersive compounds gives rise to a weaker scale structure, which allows some of the scale to be shed by cracking. The calcium carbonate and magnesium hydroxide scales can be completely prevented if the appropriate quantity of hydrogen ions is supplied by the injection of acids such as hydrochloric acid or sulphuric acid, or an acid salt such as sodium bisulphate. These scales can also be prevented by the injection of ferric chloride, which provides a supply of ferric ions. The ferric ions combine preferentially with the hydroxyl ions formed by the break-up of the bicarbonate and carbonate ions— thus allowing the formation of ferric hydroxide, which is very insoluble and is maintained in suspension in the brine without forming scale. Long-period tests on large commercial plants have confirmed the research tests.