Citrate process, Bureau

The citrate process, in which much development work has been invested by the U.S. Bureau of Mines and by Pfizer, Inc., uses an aqueous solution of citric acid to capture sulfur dioxide (Eqs. 3.28 and 3.29). 

FIGURE 13.1 I The citrate process for sulfur dioxide capture and conversion to elemental sulfur developed by the U.S. Bureau of Mines. Details available from U.S. Bureau of Mines [40]. 

One subclass of sulfur dioxide recovery processes incorporates a liquid-phase variation of the Claus reaction for regenerating the absorbent and directly producing elemental sulfur. Processes of this type are the Stauffer Aquaclaus process (88), which was developed specifically for Claus plant tail gases, and the Bureau of Mines Citrate process (89). In each, the absorbent is the sodium salt of a stable, nonvolatile weak acid, which forms a basic solution by hydrolysis. The anion of the acid buffers the solution as acid is formed by the absorption of sulfur dioxide. The spent absorbent, which consists of a solution of sodium sulfite and bisulfite and of the weak acid, is contacted directly wtih hydrogen sulfide. The hydrogen sulfide reacts with the sulfite and bisulfite to yield elemental sulfur, and the regenerated basic salt solution is recirculated to the absorption step. 

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... 

The citrate process was developed specifically for the removal of sulfur dioxide from smelter gases by the Salt Lake City Metallurgy Researdi Cmiter of the U.S. Bureau of Mines (Rosenbaum et al., 1973). The absorbent is an aqueous solution containing tqiproximately 190 g of citric acid and 80 g of sodium carbonate per liter, and is capable of absorbing 10 to 20 g of sulfur dioxide per liter. 

Sodium citrate was recognized as a potential aqueous absorbent for absorption/stripping as early as 1934 (3, 4). It has recently reappeared in work by the U. S. Bureau of Mines (5), in process development sponsored by Peabody, Inc., and in a process offered by Flakt, Inc. (6). This paper reports on work which is part of a development program on absorption/stripping sponsored by the Electric Power Research Institute. 

The U.S. Bureau of Mines has tested a pilot-scale sulfur dioxide to sulfur conversion process which involves initial absorption of sulfur dioxide in an aqueous solution of citric acid, H0C(CH2C02H)2C02H, and sodium citrate (Fig. 13.11). This solution is then contacted with hydrogen sulfide to reduce the sulfur dioxide to elemental sulfur, which is then readily filtered from the resulting slurry (Eqs. 13.41-13.43). 

In the Bureau of Mines process the absorbent is sodium citrate that used in the Aquaclaus process has been identified as sodium phosphate (90). As is common in similar processes, some of the sulfur dioxide is oxidized to sulfate, which is not readily regenerated, and thiosulfate and polythionates are also formed (88, 89). Consequently, it is necessary to draw off purge streams of the absorbents, recover the citrate and phosphate for reuse, and discard the sulfate and thionates. Losses of citrate and phosphate in this operation can greatly affect the economics of the processes. 

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