Removal of Silica from Water A. S. BEHRMAN AND H. GUSTAFSON International Filter Co., Chicago, Ill.
The importance of silica in an industrial water supply is now generally appreciated. This is especially true when the water is to be employed for boiler feed purposes in modern high-pressure steam boilers operating at high rates, since any siliceous scale formed on boiler tube surfaces interferes so effectively with rapid heat transfer that local overheating and tube failure may result. The present paper is in part a critical summary and comparison of various meth-
ESS than twenty years ago sodium silicate was still
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used in a number of boiler compounds for internal water treatment. Today silica is probably the mostfeared single constituent of a boiler water in a modern steam power plant. The last few years in particular have witnessed a succession of extensive and expensive efforts to reduce to a minimum the amount of silica in a boiler water, and to avoid by any means possible the formation of a siliceous scale. The reason for the present attitude is so well known to the chemist and engineer in the power plant field that it need be given only passing mention here. I n the days of the practically universal use of low-pressure boilers (rarely more than 200 pounds per square inch), scale of any sort was to be avoided primarily because of the waste of fuel it entailed. Today boilers in modern steam-generating stations seldom operate a t pressures below 400 pounds per square inch and not infrequently a t more than 1000 pounds, and a t rates commonly in considerable excess of their normal capacity; the concomitant high temperatures and intense heating of the boiler tube surfaces make necessary such rapid and unimpeded heat transfer that the absence of scale is imperative in order to avoid local overheating and consequent tube failure. Because of its exceptional effectiveness in retarding heat transfer, silica scale, even in comparatively minute amount, has come to be the btte noire of modern boiler plant operation. The nature and formation of siliceousboiler scales have been studied and described in considerable detail in recent years. Depending principally on the composition of the boiler water and on conditions of operation, the composition of such scales may vary widely; i t may range from practically pure silica through simple magnesium and calcium silicates to more complex compounds such as analcite (sodium aluminum silicate), and to rather indeterminate mixtures and aggregates in which the siliceous components appear t o function as cementitious agents. Generally speaking, methods of conditioning silica-containing waters within the boiler to make the silica innocuous have not been highly effective. It is this fact which has placed the emphasis on removing silica from the water before it enters the boiler. I n consequence, many efforts have been made in the last few years to devise effective methods for silica removal. Several of these methods have attempted to combine silica removal with some degree of water softening. This has been 468
ods of silica removal. These methods comprise chiefly lime-soda softening and the use of compounds of iron and aluminum. Typical data are given. The economic and technical advantages and limitations of the methods are discussed, and their proper fields indicated. New methods of silica removal are suggested which involve principally the use of aluminum and magnesium compounds. Test data are presented, and their significance is discussed. a logical approach, since external water softening is now almost universally employed in the preparation of boiler feed water for modern high-pressure boiler plants. As these silica removal methods function through the agency of a precipitate, it is natural that they should be employed, as far as practicable, in conjunction with lime-soda softening which itself depends upon precipitation reactions. It may be recalled that no silica is removed in the zeolite or base-exchange method of water softening; on the contrary, the possibility of imparting silica to water from siliceous zeolites has led to an increasing interest in the carbonaceous zeolites for softening boiler feed water.
Methods of Silica Removal No attempt will be made here to give a complete or historical bibliography of proposed methods of silica removal. Schwartz (8) has already published a paper with a bibliography of 107 references. In general, methods which have been proposed specifically for silica removal have been for the most part unsuccessful except in certain isolated special cases. The one general and outstanding exception to this statement comprises the metallic oxides and hydroxides, particularly those of iron and aluminum The method of silica removal that has received most attention in the past few years and is now being employed to a considerable extent is that described by Schwartz (8), who treats the water with ferric sulfate; optimum silica removal by the precipitated hydrous ferric oxide is attained a t pH 9 and the treatment must be held rather close to this pH environment. Powell, Carpenter, and Coates (6) reported recently, in connection with a comprehensive discussion of silicate scales, the results of their study of modified methods for employing the silica-removal property of precipitated ferric oxide; the best results in the case of the water studied were obtained by flocculation with ferric sulfate and acid a t a pH of 3.5 to 4.5 with the release of colloidal ferric oxide, followed by alkalinization to a pH greater than 7.5. The ferric sulfate method has been effective for silica removal in a number of operating plants. It has several limitations, however, which prevent its being considered as the ultimate solution of the silica-removal problem. In the first place, the necessity of maintaining a pH environment of 9.0 usually precludes the possibility of securing silica removal and
APRIL, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
complete lime-soda bofteinng in one precipitation; although it is possible to precipitate the calcium of raw water rather completely as calcium carbonate a t pH 9, precipitation of magnesium as magnesium hydroxide does not begin until a pH of about 10.2 is attained. I n consequence, it will usually be found necessary to accomplish the desired softening and silica removal in a two-stage process and thus increase the complexity and cost of the treating equipment. Again, since the removal of silica by the precipitated ferric oxide is apparently an adsorption or quasi-adsorption phenomenon, it follows that, while an appreciable part of an initially high silica content of a water may be removed with moderate dosages of ferric sulfate, disproportionately large doses will be required to reduce the remainder to the desired minimum. Reports of actual operation show that dosages of the order of 5 to 7 grains per gallon (85 to 12l0-p,-gm2)-a;e
