Sulfur continued recovery

Recovered sulfur proved to be the downfall of the Mexican Frasch industry as well. In August 1992, APSA declared bankruptcy. The debt of the company was 220 million. APSA closed its three mines in November 1992, and CEDI closed its mine in May 1993. Total sulfur production from the Frasch industry in Mexico was 55 million tonnes (see Table 4.8, and Figure 4.13). The assets of APSA and control of Mexican sulfur exports were assigned to Pemex (becoming their Texistepec Mining Unit) by the Mexican government in lieu of prior sulfur sales owing. Sulfur continued to be produced from their oil refineries at Salina Cruz and Tula. Pemex operates nine sulfiir recovery units, and produces over one million tonnes of recovered sulfur per year. 

Small amounts of propionitrile and bis(cyanoethyl) ether are formed as by-products. The hydrogen ions are formed from water at the anode and pass to the cathode through a membrane. The catholyte that is continuously recirculated in the cell consists of a mixture of acrylonitrile, water, and a tetraalkylammonium salt the anolyte is recirculated aqueous sulfuric acid. A quantity of catholyte is continuously removed for recovery of adiponitrile and unreacted acrylonitrile the latter is fed back to the catholyte with fresh acrylonitrile. Oxygen that is produced at the anodes is vented and water is added to the circulating anolyte to replace the water that is lost through electrolysis. The operating temperature of the cell is ca 50—60°C. Current densities are 0.25-1.5 A/cm (see Electrochemical processing). 

The process options reflect the broad range of compositions and gas volumes that must be processed. Both batch processes and continuous processes are used. Batch processes are used when the daily production of sulfur is small and of the order of 10 kg. When the daily sulfur production is higher, of the order of 45 kg, continuous processes are usually more economical. Using batch processes, regeneration of the absorbant or adsorbant is carried out in the primary reactor. Using continuous processes, absorption of the acid gases occurs in one vessel and acid gas recovery and solvent regeneration occur in a separate reactor. 

Uranium ores are leached with dilute sulfuric acid or an alkaline carbonate [3812-32-6] solution. Hexavalent uranium forms anionic complexes, such as uranyl sulfate [56959-61-6], U02(S0 3, which are more selectively adsorbed by strong base anion exchangers than are other anions in the leach Hquors. Sulfate complexes are eluted with an acidified NaCl or ammonium nitrate [6484-52-2], NH NO, solution. Carbonate complexes are eluted with a neutral brine solution. Uranium is precipitated from the eluent and shipped to other locations for enrichment. Columnar recovery systems were popular in South Africa and Canada. Continuous resin-in-pulp (RIP) systems gained popularity in the United States since they eliminated a difficult and cosdy ore particle/leach hquor separation step. 

The gaseous ammonia is passed through electrostatic precipitators for particulate removal and mixed with the cooled gas stream. The combined stream flows to the ammonia absorber where the ammonia is recovered by reaction with a dilute solution of sulfuric acid to form ammonium sulfate. Ammonium sulfate precipitates as small crystals after the solution becomes saturated and is withdrawn as a slurry. The slurry is further processed in centrifuge faciHties for recovery. Crystal size can be increased by employing one of two processes (99), either low differential controUed crystallization or mechanical size enlargement by continuous compacting and granulation. 

Sulfur can be produced direcdy via Frasch mining or conventional mining methods, or it can be recovered as a by-product from sulfur removal and recovery processes. Production of recovered sulfur has become more significant as increasingly sour feedstocks are utilized and environmental regulations concerning emissions and waste streams have continued to tighten worldwide. Whereas recovered sulfur represented only 5% of the total sulfur production ia 1950, as of 1996 recovered sulfur represented approximately two-thirds of total sulfur production (1). Recovered sulfur could completely replace native sulfur production ia the twenty-first century (2). 

In the United States calcium carbide-based acetylene is mainly used in the oxyacetylene welding market although some continues to be used for production of such chemicals as vinyl ethers and acetylenic alcohols. Calcium carbide is used extensively as a desulfurizing reagent in steel and ductile iron production allowing steel mills to use high sulfur coke without the penalty of excessive sulfur in the resultant steel (see Sulfurremoval and recovery). Calcium cyanamide production continues in Canada and Europe (see Cyanamides). 

In a modem carbon disulfide plant, all operations are continuous and under automatic control. On-stream times in excess of 90% are obtainable. The process is in three steps melting and purification of sulfur production and purification of carbon disulfide and recovery of sulfur from by-product hydrogen sulfide. A typical process appears in the flow diagram of Figure 1 (50). 

Continuous releases of concentrated HjS streams must be segregated in a separate flare system to limit the extent of fouling and plugging problems. Releases of HjS such as diversion of sour gas product to flares during shutdown or upset of a downstream sulfur recovery unit are considered to be continuous, but safety valve releases are not included in this category. However, if a special HjS flare system is provided for continuous releases, the concentrated HjS safety valve releases should be tied into it rather than into the regular flare system. Due to the nature of HjS one should plan on frequent inspection and flushing of HjS flares to remove scale and corrosion products. 

D. 2,6-Dibromoaniline. In a 2-1. flask, equipped with a two-holed stopper carrying an exit tube to a condenser and an entrance tube for steam, 50 g. of crude 3,5-dibromosulfanilamide (Notes 17 and 18) and 250 ml. (5 ml./g.) of 70% sulfuric acid are heated in an oil bath when the temperature of the bath reaches 175— 180°, steam is passed rapidly through the mixture (Note 19). The hydrolysis is continued in this way for 2 hours small amounts of the dibromoaniline distil (Note 20). The bath is then allowed to cool to 105-110°. At this temperature the main mass of the product is steam-distilled. The slightly colored 2,6-dibromoaniline melts at 84-86° and weighs 25-30 g. (66-79% based on 3,5-dibromosulfanilamide) (Note 21). It may be purified by recrystallization from 70% alcohol (7 ml./g.) after recrystallization the product is obtained as long colorless needles which melt at 87-88°. The recovery is 85-90%. 

Swain, E. Coke, Sulfur Recovery from U.S. Refineries Continues to Increase. Oil Gas Journal, January 2, 1995. 

Recovered sulfur has grown steadily in importance as a world source of brimstone since the mid 1950 s. Between 1965 and 1977 recovered sulfur s share of world supply grew from 18% to 30% (D and continues to grow in relation to Frasch mined and other forms of native elemental sulfur. All of this has meant a rapid growth in the number of sulfur recovery facilities and new developments in the various techniques and processes associated with the industry. 

Although the Claus catalytic conversion is a highly efficient process as presently employed in sulfur recovery plants the continuing efforts to reduce sulfur emissions to atmosphere demand that the last possible ounce of efficiency be squeezed from the process. Whether further small but critical improvements in the already high sulfur recovery efficiency can be achieved by more fine tuning of the converters and their catalyst charge remains to be seen. What cannot be accomplished in the catalytic converters will be achieved in the tail gas desulfurization processes. 

The presence of the liquid sulfur phase in the Richard process may favor the formation of the product liquid sulfur by aiding the formation of the intermediate hydrogen polysulfide building block. The success of a 5LTD demonstration plant may well determine whether future sulfur recovery from H2S feed continues to be by... 

The recovered sulfur industry has come of age in the last twD decades and is now a major world source of this essential element Much progress has been made in the improvement of various process steps, particularly those related to overall recovery efficiency. These improvements continue to reduce undesirable process losses to atmosphere and the environmental impact of the industry. As... 

It is therefore certain that research to find improved methods of sulfur removal will continue, with emphasis being given to those products for which consumer interests are most pressing. Recent evidence of diminution of sulfur supplies in the United States, as well as the desire of other countries to minimize purchase of dollar sulfur, may add impetus to the search for new methods of sulfur removal and recovery. The tonnages of sulfur in present production of crude petroleum, excluding natural gas, are given in Table VII. It shows that the potential availability is very high. 

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