Recovery of Elemental Sulfur

Elemental sulfur can be produced from H2S-laden waste gases by either of the following two methods  

Later on, the process was subdivided into two stages - both of them catalytic 

The first stage consists of a combustion chamber in which all the hydrogen sulfide is reacted with the stoichiometric air rate to obtain elemental sulfur, sulfur dioxide and residual H2S. The elemental sulfur is condensed out and the sulfur dioxide and residual H2S are catalytically reacted in two stages with decreasing temperatures to produce elemental sulfur and water according to the equation 

The reaction equilibrium and thus also the achievable sulfur yield are heavily dependent upon simultaneous side reactions caused by such other sour gas components as CO2, NH3, HCN and, above all, by hydrocarbons producing COS, CS2, CO and similar substances. As Fig. 5.9 shows that temperatures of less than 300° C are already close to the dew point of vaporous sulfur, it is not possible to reach a 100 % sulfur yield. The limit for Claus units with two catalytic stages is 96 % it can be increased to something like 98 % by adding a third stage. Newly developed titanium oxide catalysts are expected to increase the sulfur yield to approximately 99% already in Claus units with only two catalytic stages [5.7]. 

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The treating of sour gases produces a purified gas stream and an acid gas stream rich in hydrogen sulfide. The H2S-rich stream can be flared, burned as fuel, or processed for recovery of elemental sulfur. 

Removal of H2S and acid gases from the hydrogenation reactor process stream and also from the raw gases obtained from gasification of the heavy residual is required. The incentive for H2S removal and recovery of elemental sulfur in this case is environmental. 

Techniques for purification of acid gas streams by removal of H2S, COS and carbon dioxide are standard technology. Recovery of elemental sulfur from these acid gas streams by use of Claus or Stretford units is also conventional technology. These technologies are being practiced on a large scale by both petroleum refiners and natural gas processors. 

Of the removal processes that have attained commercial status, the current favorite employs a slurry of lime or limestone. The activity of the reagent is promoted by the addition of small amounts of carboxylic acids such as adipic acid. The gas and the slurry are contacted in a spray tower. The calcium salt is discarded. A process that employs aqueous sodium citrate, however, is suited for the recovery of elemental sulfur. The citrate solution is regenerated and recycled. (Kohl and Riesenfeld, Gas Purification, Gulf, 1985, p. 356.)... 

The waste streams in standard metallurgical processing are good potential sources of important elements. The U.S. Bureau of Mines has developed processes for the recovery of elemental sulfur from stack gases discharged by base metal smelters (G4) and for the recovery and production of alumina from waste solutions of mining operations (G6). A potential of 1,750,000 tons of sulfur per year and an estimated 2000 tons of alumina per day are recoverable just from 14 copper mines included in the study. 

The sulfide minerals are at present the major source of the base metals. Associated with most of the sulfide ores are the minerals pyrite and pyrrhotite. If the hydrometallurgical processing of ores becomes the predominant method of metal extraction, the recovery of elemental sulfur as a by-product is a very promising possibility. The formation of elemental sulfur has been observed by many investigators as a reaction product of sulfide minerals under certain experimental conditions. 

Randey, R. A. and S. Malhotra, Desulfurization of gaseous fuels with recovery of elemental sulfur an overview. Critical Reviews in Environmental Science Technology, 29(3), 1999, pp. 237-238. 

From the standpoint of sulfur recovery, recent experience has reemphasized the desirability, in most cases, of processes that permit recovery of elemental sulfur. Sulfuric acid is the useful by-product that is usually most readily and economically produced from sulfur dioxide, but it cannot be economically shipped for long distances or be economically and safely stored for long periods. Elemental sulfur can be shipped long distances or stored indefinitely with minimal environmental problems. 

With the gaseous sulfur-bearing species being predominantly hydrogen sulfide and sulfur dioxide, further recovery of elemental sulfur depends on the Claus reaction ... 

T he citrate process for the recovery of elemental sulfur from sulfur dioxide emissions in waste gas was conceived by Bureau of Mines investigators at the Salt Lake City Metallurgy Research Center in their initial laboratory research reported in 1970 (I). This work led to a scale-up of the process to a 400 cu ft/min (CFM) pilot unit which began treating reverberatory furnace gas at a copper smelter in Arizona in November 1970. While a series of mechanical difficulties allowed only... 

Residual oils might be gasified or cracked over calcined dolomite with recovery of elemental sulfur in a cyclic process incorporating Reaction 1. A version of this process may be useful in providing sulfur-free fuel to existing power station boilers in communities which impose restrictions on S02 content of stack gases. 

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