1. Field of the Invention:
The present invention pertains to brine feeds for use in chlor-alkali cells. More particularly, the present invention pertains to the treatment of brine prior to the electrolytic decomposition thereof. Even more particularly, the present invention pertains to the further treatment of a conventionally treated brine feed prior to the electrolytic decomposition thereof.
2. Prior Art:
As is known to those skilled in the art to which the invention pertains, a sodium chloride brine of relatively high purity is required to operate an electrolytic diaphragm cell for the production of chlorine and sodium hydroxide therefrom. This is because the hardness ions in the brine, e.g. calcium, magnesium and strontium ions, tend to precipitate out of the brine as hydroxides or carbonates during electrolysis. These precipitates form in the diaphragm thereby plugging same. Plugging of the diaphragm reduces cell efficiency and neccessitates the shutting of the cell to either renew or replace the diaphragm.
The prior art has appreciated this problem and has sought to alleviate it. For example, in U.S. Pat. Nos. 1,259,683 and 1,308,509 there is taught the addition of sodium chloride to the brine to maintain a high concentration of sodium chloride therewithin in comparison to the hardness ions. Other art teaches the operation of a chlor-alkali cell at currents below the necessary to decompose the calcium salts present in the brine. Still other pertinent prior art is found in U.S. Pat. No. 1,386,244 which teaches a pre-treatment of the feed by adding thereto alkali materials which react with the hardness ions to form precipitates which are then separated out from the brine. The reference specifically teaches the stage-wise precipitation of the hardness ions by the addition of caustic soda to the feed, coupled with the filtration of the solution to provide a purified brine. Because of the cost associated with these prior art processes they are, generally, not commercially employed.
Conventionally, today, the brine is treated by the addition thereto of sodium carbonate and sodium hydroxide to precipitate, simultaneously, magnesium hydroxide and calcium carbonate. The so-treated brine is, then, clarified and filtered. Generally, by so-treating the brine the total hardness ion level (based on calcium) is reduced to about ten parts per million, by weight, based on the weight of the brine. Also, the brine is at a pH level of about eight to ten. However, even this treatment of the brine does not provide sufficient reduction in the hardness ion content to avoid the problems of rapid plugging of the diaphragm.
The present invention, on the other hand, reduces the hardness ion level in the brine while being economically feasible and acceptable.