SULFUR. I1 WILLIAM A. CUNNINGHAM, Chemical Engineer San Ang&, Texas
T
HERE are at present four companies mining The third Texas company is the Duval Sulphur Comsalt dome sulfur deposits by the modified pany, which has a plant at Palw&ma Dome, in Duval and improved Frasch process, three of which County. The production at this dome has never been have mines in Texas and two in Louisiana. The oldest large and, until recently, the plant was shut *own of these is the Freeport Sulphur Company which is awaiting increased demands for sulfur. producing at Bryan Mound and at Hoskins Mound, The latest entry into the sulfur mining field is the both near Freeport, Texas, and in Plaquemines Parish, Jefferson Lake Oil Company which started the operaLouisiana. The Bryan Mound deposit is about de- tion of a mine in Iberia Parish, Louisiana, in October, pleted, but large amounts of sulfur kave been re- 1932. Although present production is small, this commoved from it, particularly during the World War when pany deserves much credit for this development beit was practically the only American source of sulfur. cause its deposit lies under water, and all work has been The Hoskins Mound deposit has been mined for about done from scows, piers, or pontoons especially designed ten years, and the capacity of the plant a t that place for this specificpurpose. has been recently materially increased in order to The mechanical equipment -required by a Frasch counteract the decreased production at Bryan Mound. process sulfur mine is largely equipment for acquiring, A $5,000,000 plant has been installed at the Louisiana treating, heating, and transporting water. Instead location, and production there started early in 1934. of having boilers installed at the individual wells, The Texas Gulf Sulphur Company started operations modern sulfur mining methods make use of a central at Big Hill, or Gulf as it is now called, in Matagorda water heating plant in which from 5,000,000 to 10,000,County, but that deposit has been exhausted and the 000 gallons of water are heated daily to approxilocation is now being abandoned. This company is mately 340°F. and then distributed to the individual also operating plants a t New Gulf, in Wharton County, wells. This is more water than a modern city of and at Long Point, in Fort Bend County. The deposit 60,000 persons will use. The utilization of this water at New Gulf is said to be the largest sulfur deposit requires huge storage reservoirs to insure a never-failing in the world, and in normal times has a production water s u p p l y a lack of water would mean disaster; of 4000 tons per day. The Long Point deposit has only it requires an efficient and closely controlled system of recently been opened and production there is com- water treatment to soften the water; it requires many paratively small. huge boilers to heat the water to the desired tem83
peratures; it requires literally hundreds of pumps and miles of pipe lines, over half of which must be well insulated, t o transport the water. In addition there must be machine shops, blacksmith shops, forge shops, electric shops, ice plants, and up-to-date laboratories to maintain and control the endless production of pure sulfur. The actual application of the Frasch process as it is now operated may be divided quite logically into two departments-the plant department, which handles the production, treating, and heating of the water, and the field .department, which handles the distribution of the hot water and the drilling, steaming, and pumping of the individual wells. The sulfur minmg of the Freeport Sulphur Company a t Hoskins Mound near Freeport, Texas, which is described below, furnishes a good illustration of the commercial application of the process. The power plant proper a t Hosk'ms Mound contains twelve 750-horsepower Sterling type boilers which are operated a t about 200 per cent. rating, thus making a total of approximately 16,000 t o 18,000boiler horsepower available a t all times. Under normal operating conditions, natural gas is used for fuel, but in an emergency this may be replaced by fuel oil within five minutes. The daily consumpti8n of fuel is enormous, more than enough to supply a modern city of 100,000 persons. Within the plant building there are two 500-kw. and one 1000-kw. General Electric turbo-generators for furnishing electrical energy to the entire system. Three low-pressure (250 pounds per square inch) air compressors and five high-pressure (1000 pounds per square inch) air compressors having a total rated capacity of about 4500 cubic feet of free air per minute supply the compressed air for pumping the molten sulfur out of the wells. In addition to this equipment, there are sixty-three steam pumps which are required t o handle the enormous amount of water, boiler feed water pre-heaters, exhaust-steam mine water heaters, high-pressure mine water heaters, and other auxiliary equipment. The daily water consumption of the mines is about 7,500,000 gallons. Part of this is surface water taken through canals from Austin Bayou and part of it comes from deep wells. Since Hoskins Mound is within a
mile or two of the Gulf of Mexico, the surface water becomes quite salty a t times and hence cannot be used. During such times and during periods of drought, the surface water is taken from a 350,000,000-gallon storage reservoir which is filled during periods when good water is available. Frequently, during the summer, evaporation losses alone from this reservoir amount to as much as 1,000,000 gallons per day. Of the 7,500,000 gallons of water required each day, approximately 2,750,000, are used for boiler feed water and the remaining 4,750,000 gallons are heated by steam to a temperature of 330" t o 340' Fahrenheit and pumped directly to the wells. The boiler water receives a closely controlled chemical treatment by the lime-soda-sodium-aluminate method. Actually, very little or no soda ash is ever required because the well water, like a large number of Gulf Coast well waters, has a high bicarbonate content which is utilized by judicious mixing and controlled lime treatment. Whenever necessary, sodium sulfate is added t o the boiler water to maintain the correct carbonate-sulfate ratio to prevent caustic embrittlement of the boilers. The dosed boiler water passes through a long, well-baffled, wooden flume into the settling basin. After a three-day sedimentation period the water is passed through rapid sand filters to remove all suspended matter. The filters are automatically controlled by the demand for boiler water and are equipped with automatic by-pass valves which allow unfiltered water to be taken up by the boiler feed pumps in the event of a radical decrease in the amount of water passing through the Glters. The water passes from the 6Iter to the "bleed-water" heat exchangers where i t . 4 ~partially heated by the spent water returning from the sulfur stratum. The feed water then enters the pre-heaters where it is heated to 21S0 Fahrenheit by exhaust steam from the generators and prime movers in the plant. The heaters are vented and all dissolved oxygen is removed from the water before it enters the boilers. Vent condensers are used so that there is very little heat loss. As the heated water enters the main boiler feed header.
which supplies feed water to all the boilers, it is given a light dosage of di-sodium phosphate, which reduces scale formation in the boilers to a minimum. The blow-down water from the boilers is used to aid in heating the mine water, and hence there is little or no heat loss. As a result, a continuous boiler blowdown of often as high as 25% to 30% is maintained. This amount is flexible, and can be varied in such a manner as to be of maximum benefit in the maintenance of the most economical heat balance. The mine water ordinarily receives a cold lime-soda treatment, supplemented by the addition of ferrous sulfate coagulant whenever required. All lime used in water treatment is bought as quick-lime and is slaked at the plant. It is fed to the water in the form of a milk-of-lime slurry. As in the case of the boiler water, the treated mine water is delivered to the settling basin through a baffled, wooden flume and is allowed a three-day sedimentation ueriod. The mine water is take; direct from the sedimentation basin to so-called "flue gas heat reclaimers" where it is heated to about 135O Fahrenheit by direct contact with the hot flue gases. These heat reclaimers were perfected by engineers of the Freeport Sulphur Company and effect a saving of approximately 80% of the heat which is ordinarily lost up the stack. The mine water then passes into the exhaust-steam gine water heaters where it is heated to 21Z0 Fahrenheit bv the. continuous boiler blow-down and by exhaust steam from the generators and prime movers. Whenever necessary live steam may be used to maintain the desired temperature. The heaters are equipped with vent condensers whereby the dissolved gases are removed without causing an appreciable heat loss. The pre-heated mine water is then pumped into the high pressure heaters where it is heated to 325-340° Fahrenheit by direct contact with live steam. These heaters operate a t boiler pressure and are practically 100% efficient in the utilization of aU the available heat in the steam. The heated water is then pumped through well-insulated ten-inch lines to t h h field for use by the field department. The field department handles all affairs concerned with the actual production of the sulfur. In the early days of the industry, the wells were "punched" with slightly improved water-well drilling equipment using an "A-frame" derrick. Compare the picture of this old-style equipment with that of a modern well. At the present time, the sulfur wells are d r i e d with the electrically operated standard rotary well-drilling equipment which is used in drillmg for oil. The practice is to mount the derrick and equipment on skids so that they may be moved from one location to another without having to be dismantled. The well is drilled to the top of the cap rock which forms the roof of the sulfur-bearing limestone. The depth varies in the different fields, but a t Hoskins Mound it is usually from 1000 to 1600 feet. A teninch casing is then set and fumly cemented into the
cap rock in order to seal off the overlying formations. The drilling is then resumed and the well is extended to the top of the anbydrite stratum which lies immediately underneath the sulfur-bearing formation. The well is then equipped with an &inch hot water line, a 4-inch molten sulfur line, and a 1%-inch air line, all located concentrically within the larger pipe. The 8-inch line rests on and is firmly cemented to the anhydrite floor. Approximately thirty-five feet of the lower end of this string of pipe is perforated to permit the hot water to pass out into the sulfur-bearing formation and to allow the molten sulfur to enter the well. A flange is welded inside of the perforated pipe a short distance above the bottom, and the 4-inch sulfur line rests on this flange. The air line is suspended 1,
1'SULFUR AND BOOSTING L I N E \
I+ A I R L I N E
from the top of the well and extends to within two hundred feet of the bottom. The accompanying drawing, although not to scale, gives a good picture of the general equipment of a well. When a well has been fully equipped and "tied in" with the water, air, and sulfur distributing system, the hot water is started into the well. At k t the water is pumped down both the 8-inch and the 4inch lines in order to melt the sulfur more quickly. The hot water flows out into the sulfur-bearing stratum through the perforations in the pipe, the heat in the water melts the sulfur, and the molten sulfur flows down through the formation and collects in a pool a t the base of the well. This melting process is made possible by the f a d that the sulfur is not a solid deposit, but is intermixed with a matrix of limestone.
After a while, the water is shut off, and if the amount of the molten sulfur is suJ3cient for i t to be forced by hydrostatic pressure up into the 4-inch pipe as high as the lower end of the air line the well is said to "seal" and to be ready for pumping. When a new sulfur well has sealed and is ready for pumping, air a t a pressure of 500 pounds per square inch is turned into the ll/r-inch lme and the molten sulfur is brought to the surface by air l i t . At first the periods of pumping are rather short because there is a comparatively small amount of molten sulfur, but the duration of the pumping period is increased as more sulfur becomes available. The optimum condition is reached when, by continuous pumping, there is removed just that amount of sulfur which is melted by the hot water being pumped into the well. Once a well has started to produce it is never allowed to get cool, because whenever a well "freezesM-that is, the sulfur solidifies in the sump a t the bottom of the well-it is lost to production and must be abandoned. This is due to the fact that the solidified sulfur is nonporous and is a good insulator, hence the hot water cannot circulate through the mass to remelt the sulfur. I t is estimated that a normal well will remove the sulfur from an area of about a half acre. The water being pumped into the wells must be kept a t approximately 320-335'F. If it is cooler than this there will not be enough heat carried per unit amount of water, and if it is above this temperature the molten sulfur will become too viscous to be pumped. S i c e the optimum operating temperature of all wells is not the same, an ingenious system has been devised by plant and field officials of the company whereby "tempering water" a t about 210°F. is mixed a t each individual well with the hot water in such quantities as to give the best operating temperature in that particular well. The pumping of seven to eight million gallons of water per day into a limited underground area cannot go on indefinitely without $he removal of some of the cooled water. Consequently, "bleeder wells" are located a t selected points over the dome whereby the internal or "formation" pressure can be regulated. The "bleed water" thus removed has a temperature of about 180-200°F. but contains such large quantities of hydrogen sulfide and other dissolved substances that it is very corrosive and cannot be used again. However, i t is taken through large wooden or cast-iron pipes to the "bleed water heat exchangers" where it is utilized to pre-heat the boiler feed water. The purification of bleed water has received much study and attention. Several schemes have been devised for the removal of the sulfides, the most obnoxious impurity, and have been put into operation by both this company and the Texas Gulf Sulphur Company, but none has proved entirely satisfactory. The sulfur produced by all the wells in a limited area is pumped by air to central collecting stations known as "relay stations." All lines through which the molten sulfur passes must be well insulated and equipped with steam "gut lines" to prevent the freezing of the sulfur
in them. In the relay stations are located the remote controls of the air and water supply of each well so that two men can control the pumping and steaming of all wells within that particular area. As the molten sulfur enters the relay station it flows into steamjacketed separators from which the air and other gases are released to the atmosphere and the sulfur passes into a steam-heated constant-level flow tank. From the level tank the sulfur is discharged through orifice meters into the steam-heated cast-iron relay pit. Whenever the relay pit becomes full, i t is accurately sampled and then pumped by large, fully submerged centrifugal pumps to the storage vats. The samples thus obtained are sent to the laboratory for analysis, affordingan accurate check on the quality of the sulfur being produced. The storage vats to which the molten sulfur is pumped are huge wooden enclosures averaging 800 to
A VAT IN THE EARLY STAGESOP FORMAILON Note the large cones or "icicles" of sulfur which are built up at every point of discharge. The dark areas in the foreground are molten sulfur.
1000 feet long, 200 feet wide, and 40 to 60 feet high and containing 400,000 to 750,000 tons of sulfur. The vats are built up gradually as the sulfur is pumped into them; the walls are a t no time over three or four feet above the level of the sulfur. As the molten sulfur is pumped into the vats it has a dark, reddish brown color but it turns into a brilliant golden yellow when it solidifies. When a vat is full enough i t is usually allowed to remain intact for from six months to a year. As a rule, all the molten sulfur is solidified within a very short time after entering the vat, but the insulating qualities of the solid sulfur are so great that pockets of molten sulfur have been encountered in vats which have been standing for eight months. The crude sulfur thus solidified in the vats and ready for shipment is from 99.5% to 99.95% pure. Any sulfur having over 0.5% impurities must be sold as off-grade sulfur a t a reduced price.
When the sulfur in a vat is ready for shipment, the wood retaining walls are removed and a standardgage railroad track is laid alongside the huge yellow block. Holes are then drilled into the sulfur by means of miniature rotary well drills similar to the rotary rigs used to sink the sulfur wells. Dynamite charges are placed a t the bottoms of these holes and the solid sulfur is broken into lumps small enough to be easily
A VAT
handled by clamshell buckets or steam shovels. All dynamite and explosives used are covered with a brilliant red paper which, in contrast to the yellow sulfur, tends to induce greater safety in handling. The broken sulfur is loaded into box cars or gondolas for inland shipment or for transportation to the coast where it is loaded into ships which take it to ports all over the world.
FROM ~ I I I C HSULFUR IS
BEINGLOADED
