The present invention pertains to salvaging of sodium anthraquinone disulphonate and vanadium salts from Stretford Process solutions and recycling of said anthraquinone (ADA) values and vanadium values.
The Stretford Process is well established in the art of gas purification for the removal of H.sub.2 S from "foul gas" or "tail gas" streams of various compositions and origins, such as gas streams evolved from refining of petroleum or coal products. The Stretford Process employs, fundamentally, an alkaline aqueous solution comprising a salt of an anthraquinone disulfonic acid (ADA), such as Na.sub.2 -ADA, and a vanadium salt, such as sodium vanadate. During its use the Stretford solution, acting on the H.sub.2 S and other gas components, builds up various levels of side-products such as sodium thiosulfate, sodium sulfate, and various "alkalinities" such as sodium carbonate and sodium bicarbonate due to reaction with the alkali (pref. NaOH) employed to maintain the solution alkaline.
In particular, the present invention pertains to a process whereby a portion of the Stretford solution is removed as a side-stream in order to maintain the build-up of various side-products to a controllable, allowable concentration, the process being particularly effective in substantially removing sodium sulfate and sodium thiosulfate in the side-stream, while salvaging a substantial amount of the ADA values and the vanadium values, both of these values being recycled to the Stretford solution. Thus, as the Stretford Solution is continuously cycled through its use step and through its recovery, clean-up, and replenishment steps, the level of deleterious sulfate and thiosulfate is effectively controlled, the loss of expensive ADA and vanadium salt is substantially minimized, and the alkalinity or basicity of the solution is maintained. This process effectively avoids the need for acidification of the Stretford solution to destroy the thiosulfate, which then requires re-alkalizing; the re-alkalizing forms soluble salts of the acid used for acidification.
Various patents relating to the Stretford Process and to handling, using, or treating the Stretford solution are, e.g., U.S. Pat. No. 2,997,439; U.S. Pat. No. 3,035,889; U.S. Pat. No. 3,097,926; U.S. Pat. No. 3,752,877; U.S. Pat. No. 3,810,833; U. S. Pat. No. 3,904,734; U.S. Pat. No. 3,959,452; U.S. Pat. No. 3,975,508; U.S. Pat. No. 4,017,594; U.S. Pat. No. 4,118,467; U.S. Pat. No. 4,206,194; and European Patent Application Publ. No. 0 002 880 A3 having an Application No. 78300471.6 filed 09.10.78.