HIGH-RATE SULFUR MELTER B. A. AXELRAD AND L. A. NELSON,JR. Freeport Sulphur Company, Freeport, Tex. Production of sulfur by the Frasch process, while yielding a product of high purity, results in an accumulation of e large tonnage of unsalable contaminated sulfur over the life of the mine. No commercial equipment had been developed to recover this sulfur, and pilot plant operation wae required to furnish the basis for the design of a highrate, high-capacity melter and auxiliary equipment.
Foaming of the sulfur during the melting step and corrosion of equipment because of the moisture and acid content of the contaminatedsulfur were other seriousproblems. While of special interest to the sulfur industry and industries requiring the melting of large quantities of eul. fur, the paper illustrates the value of pilot plant research in determining the unknown factors of a process.
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crude sulfur, thus making their operation on even an exception ally good grade of bin-bottom sulfur undesirable. Because of the speciaI circumstances at Hoskins Mound the tonnage of available bin-hottom was in excess of 100,000 tons and the contamination was severe. The ash, or solid gangue, content averaged 3% with a peak of lo%, the acid content averaged 0.2% as sulfuric acid with a peak of O.S%, and the moisture content averaged 4% with a peak of 10%.
OMEBTIC sulfur is mined by the well known Frasch process ( 4 ) which yields a basic raw material of exceptional punty. The liyuid sulfur as produced from wells contains only a few hundredths of 1% of solid impurities ( I ) , and special efforts are made, in operating the producing wells, in collecting the liquid sulfur, and in solidifying it in massive blocks, to avoid increasing the trace amounts of Contaminants. Considering the large tonnages handled at a sulfur mining propertry, it has been impractical to develop a method of solidifying and storing sulfur except in huge bins such as those originally used by Frasch (3,7). These bins may range from 100 to 300 feet in width, from 600 to 1200 feet in length, and to 55 feet in height. Improvements in the art of solidifying and storing sulfur, although numerous, have been confined, tint, to bin wall construction to eliminate wood and metal as contaminating materials in the shipped product and, secondly, to special types of distributors to control the cooling rate of the sulfur to form a product of improved appearance and physical structure ( 2 , 6, 0 ) .
ACCUMULATED OFF-GRADE SULFUR In the early days of the industry, the storage bins were floored with wood planka, but in blasting the sulfur for shipment the wood was splintered, causing troublesome contamination Therefore, a new storage bin was started on a well graded earth Boor and the bottom sulfur, which is contaminated by contact with the dirt, was not shipped. Although the original bin site can be re-used any number of times, the presence of several feet of sulfur remaining a t the end of shipping operations acts as a protective base for subsequent production, and prevents the continued formation of new contaminated, or “bin-bottom,” sulfur. However, at Hoskins Mound, Tex., a number of these bin sites had become obsolete, because in the early days of production they had been located within the outer boundaries of the ore body before those boundaries had been definitely defined. Obviously, re-use of these bin sites was impossible when mining operations estended into the areas of their location. In addition to the bin-bottom sulfur, a small tonnage of offgrade or “blow-box” sulfur (6) is produced in the various phasea of mining. For instance, the first and last sulfur produced during each pumping interval of a well is segregated because it is contaminated with water, acid, and small pieces of the cap-rock formations. In the past, some tonnage of bin-bottom and other off-grade sulfur had been salvaged for sale from mining properties, usually at the time of their abandonment due to depletion of the ore body. This salable off-grade sulfur in general was limited to that containing 1% or less of nonsulfur solids. In recent yeara, design changes in sulfur burners and various other acid plant equipment have taken advantage of the normal high purity of
RECLAMATION OF OFF-GRADE SULFUR In reviewing this problem in 1947, no information was found on a process or equipment that could be used economically to reclaim this sulfur. Basically, the desired process involved melting this bin-bottom sulfur and then separating the acid and solid contaminants from the molten fluid. Separation of the contaminants from the liquid sulfur was not considered a problem, because in a commercial installation a t the Freeport Sulphur Company’s mine at Grande Ecaille, La., Kelly-type, steamjacketed. pressure filters have been used successfully for sulfur filtration since 1934. Laboratory tests showed that the filtration step would provide a product of cxcellent quality. On the other hand, conventional commercial sulfur melters (8) and their associated sulfur burners and other acid manufacturing equipment, being designed to handle the high quality, run-of-the-mine crude sulfur, would not accommodate this off-grade sulfur. Initially an unsuccessful attempt was made to melt the sulfur by direct contact with hot flue gases. An attempt to melt the sulfur with hot water under pressure and to float off the contaminants also proved unsuccessful. Small scale tests had indicated that indirect heating, such as with steam coils, could be used to melt the sulfur, but heat transfer rates and, therefore, capacities would be prohibitively low, and excessive foaming would be experienced because of the high moisture content. These problems were solved, however, by pilot plant research in which the fundamentals for efficient melting of the wet sulfur were developed. PILOT PLIANT RESEARCH From experience in sulfur mining operations it was known that liquid sulfur has a high heat transfer rate as contrasted with that of solid sulfur. Therefme, it was surmised that liquid sulfur should be an efficient melting medium if good agitation across the heating coils was provided. Also, agitation would be necessary to disperse the particles of solid bin-bottom sulfur, and it was hoped that the intense agitation thus required would minimize the foaming problem. The pilot plant unit was built to permit the study of various types of agitation in combination with various positions of the heating coils. The experimental unit was designed for a melting capacity of approximately 1 ton per hour and consisted of a steel tank in which was maintained an agitated pool of liquid sulfur with heat supplied by a series of submerged steam coils.
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u!k REQULATOR
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50 P S I . AIR FOR CAKE BLOW
I
SULPHUR MELTER
1 FILTRATE RECYCLINO ,
4 CENT.
1 1
COLLECTING TANK
FEED SULPHUR RECIRCULATION
f ----_----"7LTER YELL LIL_-_______ J r
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DRAIN
ORLLOWJ
AQITATOq CENT.
LINE
KELLY FILTER
FILTER FEED
FEED TANK
Figure 1. Process Flowsheet for Reclamation of Off-Grade Sulfur at Hoskins Mound, Tex.
Wet, bin-bottom sulfur was fed to the heated pool, which was tapped periodically to maintain the proper operating level by pumping melted sulfur from it. The data accumulated in the operation of this unit were directly expanded to form the basis of design for the commercial melting unit, The pilot plant operations showed that intense agitation of the liquid pool of sulfur was required, such as that produced with a turbine mixer. This type of agitation, suppelmented by some form of surface agitation, not only was very important in minimizing and breaking the foam but was also necessary to prevent the feed solid sulfur from accumulating on the pool surface as a crust or island. Continuous feeding of the wet sulfur was found to be a necessity, because intermittent feeding resulted in excessive foaming and low capacities. It was also found that location of the heating coils to act aa baffles was beneficial in minimizing the foam problem. Operation of the pilot plant not only allowed the commercial plant design to be recommended with assurance that its operation would be successful but also permitted the melting unit to be one half the size which would have been used if accurate capacity data h8d not been determined.
COMMERCIAL SULFUR RECLAMATION PLIANT The commercial plant, fabricated to reclaim the contaminated sulfur described, was readily coordinated into the existing mining operations. A flowsheet of the process appears in Figure 1. The sulfur to be reclaimed waa dug and accumulated by tractor-doaer and dra line, loaded into dump trucks, and transported to the stock ile ofthe centrally located reclamation plant. From the stock pipe, the sulfur waa loaded by dragline into a 20-ton ho per discharging to the mechanism feedmg the lar e ca acity m3ter. The reciprocating-tray !ype feeder em eioyef was equipped with a variablespeed drive and adjustabre discharge gate. This system provided convenient regulation of the feed rate to compensate for variation in the moisture content of the sulfur, which in turn sffected both the depth of froth formation in the melting process and the rate of melting. The melting unit, a sketch of which appears in Figure 2, consisted of an open-top, steel-reinforced, Lumnite concrete pit, 15 feet square and 10 feet dee in which were located 20 banks of 3inch pipe steam coils p r o v i k g a total heating surfaoe area of 1348 square feet. During normal operation a pool of molten sulfur was maintained at a minimum depth of 5 feet, equivalent to about 40 tons of sulfur, at which point the coils were just submerged.
Agitation of the liquid sulfur pool was provided by two impellers mounted on a common vertical drive shaft located in the center of the pit and rotated at 63 r .m. One large impeller, 5 feet in diameter and consisting of 10 f a t blades set a t an angle of 45O, was located 8 inches above the bottom of the pit. The other impeller, a three-bladed propeller-t e unit, 30 inches in diameter, waa located about 1 foot below %e minimum sulfur level or approximately 4 feet above the bottom of the pit. The larger, bottom impeller carried the major part of the load and rotated in such a direction that the sulfur flow wm down through the center a t the oint of feed, across the bottom of the pit, and up through the coirs around the sides. The top propeller su plied additional surface agitation and also formed a vortex whici tended to pull the solid, wet, feed sulfur beneath the surface. A total of a p r i m a t e l y 38 hp. of agitation was provided by this system with 0th impellers submerged in the liquid pool. Approximately twice as much heat is required to melt a given weight of sulfur containing 5% moisture as to melt an equal weight of dry sulfur. Melting rates as high as 30 tons per hour of sulfur containing 2.5% moisture were attained in the commercial unit. In the melting process, the moisture was driven off a8 water vapor which was accompanied by some acidic vapor, since the acid content of the feed sulfur was found to be reduced by about 30% in the melter. The over-all heat transfer coefficients realized in the commercial unit were appro%imately 80 B.t.u./(hour) (sq. ft.) ( OF.)when using saturated steam at a pressure of 100 pounds per square inch gage and maintaining a liquid pool temperature of 260" F. This compared favorably with the 88 B.t.u./(hour) (sq. ft.)( F.) coefficient realized with 39.4 square feet of heating surface in the pilot plant unit when operated under the same conditions. Periodically to maintain the proper operating level in the pit, part of the melted sulfur was pumped from the melter into one of three 30-ton storage tanks which were used as surge tanks for the filtering cycle. That step of the process involving the removal of the contaminating materials consisted of pum ing the sulfur from the stora e tanks through a steam-jacketed fiEer. The filtrate, a liquid sulfur of exceptionally high purity, was collected in another steel tank, similar in design to the aforementioned storage tanka, but without an agitator. The filter used in the process was a Kelly-t e p r w u r e filter providing a total filtering area of 450 square by means of a set of ten screens covered with 100-mesh, Type 316 stainless steel wire cloth. The temperature in the filter was held above the melting point of sulfur by maintaining 50 pounds per square inch gage steam inside of several steel "blisters"
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water is added. This suspension is agitated, and after sufficient time for complete leaching, the acid is titrated with a standard hydroxide solution. Even assuming that complete neutralization of the acid does not take place in the stora e tanks, it should do so in the water solution during analysis wi& twice the amount, of theoretically necessary lime present. However, this does not occur, positive titrations showing that the lime is probably still occluded in the sulfur. Thus, because acid is detected in the limetreated sulfur, it is certain that neutralization did not take place in the storage tanks. It wm realized also that if no acid had been detected in the sulfur, it would stili not be certain that neutralization was effected in the tanks, a8 it may have occurred in the flasks during analysis at the laboratory.
PLAN
Figure 2.
Details of High-Rate Sulfur Melter
welded on the outside of the cylindrical filter shell. The shell and blisters were covered with a heavy layer of insulation. The filtration cycle was operated on a combination constant rate-constant pressure basis. Filtering rates in the range of 80 to 90 tons per hour were attained when filtering the majority of the bin-bottom sulfur. Naturally, the type of solid contaminant being removed was the controlling factor, but in general this material was relatively coarse. Occasionally some very h e material, such as a clay earth, was encountered in the sulfur, and the filtering rates were detrimentally affected. Although facilities were available for using a precoat and filter aid, it was found to be unnecessary to use either; the solids removed in the filtering operation resulted in a porous cake with good filtering rate characteristics. The residual acids, as well as the solid contaminants were effectively removed in the filtration step, as was predicteu by prior research and experience.
CONTAMINANT ACID NEUTRALIZATION Originally it was assumed to be necessary to add lime to the melted bin-bottom sulfur t o neutralize its high acidity in order to protect the tanks, pumps, pipe lines, filter, and filter screens from corrosion. In the initial operations, lime was added to the melted sulfur in the storage tanks in amounts equivalent to about twice the quantity required to neutralize the total acid content. However, after addition of lime and agitating for as much as 50 minutes, analyses for acid on samples of the sulfur showed that neutralization was not being accomplished. Repeated tests gave the same result and led to the following hypothesis: Assume that the solid gangue material with occluded acid is entirely wetted or coated by sulfur. When the lime is added, it also becomes wetted by the sulfur. Tbus the surface films of sulfur on the angue and lime prevent good contact for acid neutralization in t%e tanks. This theory is substantiated by the analytical method by which acid content is determined in the laboktory. In this analytical procedure samples of the solid sulfur are first pulvericed, alcohol is added a wetting agent, and then distilled
Therefore, the addition of lime was disrontinued and not used for the last 12 months' operation of the plant. At no time in the 16 months that the plant was operated was any detectable amount of acid found in the filtrate, even at the beginning of the filtering cycles. Because acid was always present and was always removed by the filtration step, it is logical to assume that the acid was adsorbed by the solid gangue material. In this connection when dark liquid sulfur, containing acid equivalent to 0.8% as sulfuric acid, is treated for color improvement with 16 pounds of activated clay per ton, a filtrate product free of acid is produced. It is also known that water-soluble salts such as sodium chloride, sodium hydroxide, and sodium carbonate are insoluble in liquid sulfur and can, therefore, be removed by filtration. This being true, caustic or soda ash could be used and might be more suitable than lime if it is desired that a neutralizing agent be added. The authors' experience indicates that the effectiveness of any neutralizing agent would be greatly increased by adding it to the sulfur before it is melted. Where the amount of nonsulfur solids to be removed is very low, small, continuous additions of filter clay or filter aid should also prove more desirable than lime additions in removing thc acid, because a cake providing improved filtering rates would result.
SULFUR PUMP OPERATION The greatest difficulty in plant operation was offered by the sulfur pumps used throughout the process. These were singlestage vertical centrifugal pumps, constructed similar to standard submerged types of sump pumps. They were steam-jacketed, sulfur-lubricated, and driven by vertical, hollow-shaft motors. These pumps had been used successfully in various places in mining operations to transfer liquid sulfur. However, pumping liquid sulfur with a high percentage of solids, as in this case, wa8 another question. The difficulty in the operation of the pumps arose from the fact that excessive wear in the bowl and plugging of thc vane openings in the impeller caused the pumps to fail rapidly. Changes in the method of operating the pumps together with modifications in pump design were necessary before satisfactory results were secured. Difficultieswith the pump in the melter pit were the most serious. Intermittent operation of this pump in transferring the sulfur from the melter to the storage tanks a t a rate of 100 tons per hour instead of continuous operation a t the lower continuous melting rate proved very beneficial. A recording mete. showing the level of the liquid sulfur in the melter was installed. The recorded rate of drop in the level during a pumping interval gave a measure of the pumping rate. At the first sign of decreasing pumping rate, which indicated plugging of the pump impeller, the pump was stopped and back-washed with clean sulfur. Operating all the pumps at full capacity to maintain maximum flow proved to be helpful in decreasing the tendency of the impellers to become plugged and, therefore, unbalanced. Frequent back-washing with clean sulfur contributed considerably to their life and efficiency. Among the modifications in pump design that were tried, those considered the most beneficial were: the use of shop-built impellers with wide clearances in the vane openings, reduction in operating speed from 1800 to 1200 r.p.m., and the use of hardened materials for wear rings in the pump bowl and on the impeller hubs. Two easily cleaned coarse straining units placed in the pumping system also aided pump operation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
CORROSION OF EQUIPMENT Corrosion of the commercial plant equipment (9, 9 ) waa much less severe than was anticipated when the plant waa designed. The dump trucks, dragline machines, and tray feeder on the melter s d e r e d most from corrosion attack because of their intimate contact with the highly acid, wet bin-bottom sulfur. Most of this attack waa confined to the easily repaired truck beds and dragline buckets. The reciprocating tray of the sulfur feeder waa lined with aluminum plate after its fmt failure and thereafter gave very good service. The %ton hopper on the sulfur feeder was lined with aluminum plate in its original installation and gave excellent service. The 3-iich steam coils in the melting pit showed very little, if any, indication of Corrosion attack. This is especially noteworthy because the original coils were fabricated of second-hand, galvanized steel pipe from which most of the galvanizing was gone. This experience indicates that aa long as the coils are hot, common black pipe will be satisfactory in this type of service. One equipment failure, although it was not especially serious, served to illustrate the conditions which are particularly contributive to corrosion attack on steel. A basket-type, coarse strainer, located in the discharge line of the melting pit pump, naa housed in a steam-jacketed pressure vessel. A small amount of air waa bled into the top of the vessel a t all times to facilitate the measurement of line pressure by an ordinary Bourdon gage. The air waa moist, and the sulfur flowing through the screen vessel contained some residual acid. Severe internal corrosion of the
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steel vessel occurred a t the air-sulfur interface near the top. Here, air, moisture, and sulfur in direct contact constituted extremely corrosive conditions. In the Kelly filter, steam-jacketed sulfur linea, and storage tanks, no detrimental corrosion occurred. The fact that there was no noticeable corrosion in the filter, which also had a small amount of air flowing into it through the pressure gaging system similar to that on the screen vessel, can best be explained by the fact that no air-sulfur interface was allowed to exist. The air flowing into the filter was continuously removed by a bleeder in the top of the filter through which.part of the sulfur feed was recirculated to the storage tank throughout the filtering cycle. By this means the filter was maintained full of sulfur at all times, and no static air-sulfur interface was ever formed.
LITERATURE CITED (1)Bacon, R. F.,and Fanelli, R., IND. ENO. CREM.,34, 1043-8 (1942). (2) Butterworth, C.E., and Sohwab, J. W., Ibid., 30, 746-51 (1938). (3) Chem. &Met. Eng., 39,392(1932). (4) Frasch, Herman, J. TND.
[email protected].,4, 134-40 (1912). (6)Lundy, W. T.,Trans. Am. Inst. Mining Met. Enprs., 109,354 (1934). (6) Mason, D. B., IND. ENG.CHEW,30,74C-6 (1938). (7) Pough, F. H., Ibid., 4, 143-7 (1912). (8) Schwab, J. W., and Duecker, W. W., Chem. & Met. Bng., 44, 441-2 (1937). (9) West, J. R., Ibid., 53,225 (October 1946). ,----I
REIC~IWD Msroh 27, 1050.
IMPROVEMENTS IN ROTARY SULFUR BURNER ALFRED LIPPMAN, JR. Bay Chemical Company, Weeks, La.
A description is given of four improvementa in a rotary sulfur burner which permit the burning of sulfur even with a high bituminous content, automatically and at much greater capacity, unirormity, and safety. Them improvements comprise: tilting the burner to establish a continuous overflow of sulfur to purge bituminous material;
fin^ within burner to increase interior surface area and to cause droplete of molten sulfur to fall through hot gas to ash bituminous matter; insulation on shell to utilise heat more effectively ; and a unique automatic level controller for smooth, economical, and safe operation. The combination has increased burner capacity over 2.5 times.
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in the sulfur dioxide gas concentration: ( a ) the adjustable ports N , on Bend of burner; (b) the length of gap 1; ( c ) the adjustable damper, L; and ( d ) the draft applied to the combustion chamber. The burning rate can be controlled accurately by regulation of the draft at the discharge end of the burner, indicated by a diaphragm-type draft gage reading up to 0.30 inch of water in 0.01inch divisions, with its tube M placed through gap 1 and extending slightly into end B‘.
HE well-known rotary sulfur burner consists of a horizontal steel cylinder, A, with cast-iron fnrstro-conical ends, B, B’,
Figure 1. The burner rotates about 1 r.p.m. on trunnion rings, C, C’, to keep the interior surfaces wet with molten sulfur from the pool, 2, maintained by either liquid or solid feed. Air drawn through the burner by 0.25 to 2 inches of draft a p plied at the discharge end of combustion chamber D, sweeps out the sulfur vapors evolved from the hot molten sulfur and oxidizes some of the sulfur vapor within the burner to provide heat necessary for the melting and volatilization of the sulfur feed.
CONVENTIONAL CONTROLS OF BURNER Air paasing through the burner determines the rate of sulfur volatilization and consumption; while air entering gap 1and combustion chamber D through port L,provides for the combustion of sulfur vapor evolved from the burner, and determines the concentration of the sulfur dioxide gaa product. Therefore, the foibwing controls allow a considerable range in the sulfur burning rate and
BURNING OF SULFUR WHICH CONTAINS BITUMINOUS MATTER Sulfur virtually devoid of bituminous matter burns well in an ordinary rotary burner, but some sulfur indigenous to the Gulf Coast contains about O.&% bituminous matter which makes a tarry scum over the surface of the sulfur pool, thereby seriously reducing the capacity. In time, large carbonaceous lumps may be found floating on top of the sulfur pool; these lumps reduce the capacity still further by impeding gaa flow through the burner.
