Re-use of Bleed Water in Sulfur '
T
Mining
HE underground-fusion or Frasch process is used ex-
clusively for the production of sulfur from the cap rock of salt domes in the Gulf coast area of the United States. A salt dome is the upward intrusion of a salt plug which may or may not have a cap rock. These salt plugs may be from a few feet to several thousand feet below the surface of the ground and do not always cause the surface formation directly above them to rise above the surrounding terrain. Only a small percentage of salt domes discovered to date have beea,found to contain sulfur in areas of the cap rock, in amounts that would justify mining on a commercial scale. I n all cases where sulfur is found disseminated through the cap rock, it exists as rhombic crystals in that portion of the cap rock which is composed mainly of porous limestone. This sulfur-bearing formation is found over salt plugs, the depths of which are usually from 500 to 2500 feet. The upper formations consist of sedimentary deposits, and in some cases there is a layer of barren limestone directly above the ore. Between the sulfur-bearing limestone and the sodium chloride plug there is an impervious layer of anhydrite that varies in thickness and does not contain elemental sulfur in commercial quantities. The sodium chloride plug beneath this anhydrite is of unknown depth.
Reclamation and Treatment D.T.McIVER, J. B. CHATELAIN, AND B. A. AXEZRAD Freeport Sulphur Company, New Orleans, La.
then equipped with a set of three concentric pipes (Figure 1). Water raised to the desired temperatures, usually from 300° to 320" F., by heating it with steam under pressure is forced down the outer pipe and through perforations into the porous sulfur-bearing limestone. This superheated water melts the sulfur which accumulates around the well and enters the outer pipe through a set of perforations below and sealed off from those through which the water is discharged. The hydraulic pressure causes the molten sulfur to rise a certain distance in the middle pipe of the concentric series; the sulfur is then raised from its static level to the surface of the ground by an air lift. The air is supplied under pressure through the inner pipe of the well equipment.
Frasch Process I n the mining of sulfur by t h e Frasch process a hole is drilled through the upper unconsolidated formation and the porous sulfur-bearing limestone, and is usually terminated in the impervious anhydrite of the cap rock. The well is 752
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JULY, 1938
The Frasch process of mining sulfur is made possible by the porous characteristics of the sulfur-bearing limestone, khich allow the hot water to reach the sulfur to be melted. The pores existing throughout the limestone cap rock are filled with formation water. As mining progresses, the hot water which is forced into the cap rock dilutes the formation water present and fills the interstices left by the melting and removal of sulfur. The ratio of the volume of hot mine water to the volume of sulfur removed increases as the deposit is depleted by mining, finally becoming so large that for economical reasons the area has to be abandoned without totally depleting the deposit. Volumes of water, many times in excess of the volume of sulfur removed, are injected into the mining area; therefore, to prevent the pressure in the formation from becoming excessive, large volumes of water have to be drawn from the formation. The water exhausted from the formation for this purpose is known as bleed water and is composed of the formation water originally present diluted with hot mine water. The Frasch process for mining sulfur is a thermal process. The thermal inefficiency of this process is readily apparent when it is realized that 5 per cent or less of the total heat put into the sulfur-bearing formations is consumed in melting sulfur. The inefficiency is due to many factors; the two principal ones are the enormous amount of heat required to elevate the temperature of the entire formation above the melting point of sulfur and the heat wasted in large volumes of bleed water which have to be exhausted from the formation. Wells for removing bleed water are known as bleed wells. The location of bleed wells with respect to the mining area varies for different domes. Bleed wells are usually located at points where the coldest water can be exhausted and the formation pressure controlled within the desired limits without effecting adverse channeling of water in the mining areas. The temperature of bleed water is always greater than atmospheric but varies a t different domes, owing to differences in the geological characteristics of the cap rock. The chemical constituents found dissolved in this water are also highly
~~
~~
I n mining sulfur by the Frasch process, large volumes of superheated water are pumped underground to melt the crystalline sulfur. T o prevent the formation pressure from becoming excessive, large volumes of water have to be exhausted from the cap rock. This water, known as bleed water, has a temperature of 220" to 250" F. and a pH of 6.5, is very corrosive, and contains objectionable dissolved chemicals that have to be removed before it can be wasted. Wasting this hot water formerly involved great thermal losses. Methods of treatment to prevent corrosion of the steel and cast-iron equipment have been developed which allow this water to be re-used in mining. This is accomplished by treating the water with chemicals that result i n the deposition of a layer of scale on the equipment, thereby sealing it off from the corrosive water.
753
SULFUR L I N E
SULFUR AND AIR
.
HOT WATER
HOT WATER L I N E
SURFACE CASING
UNCONSOLIDATED C?.~...-,^.,
FIGURE 1. EQUIPMENT IN SULFUR-PRODUCINQ
WELL
variable but are always such as to result in a very corrosive water which is harmful to marine life. Therefore this water must always be treated to render it harmless before it can be disposed of ( 2 ) . The disposal of bleed water and the possibility of increasing the thermal efficiency of sulfur mining by reclaiming the heat contained in it have been problems occupying the interest of the industry since its beginning. I n the past, commercial recovery of the heat contained in bleed water was confined to a closed type of heat exchanger constructed of corrosion-resistant materials. This method allows only a small proportion of the total heat content of the bleed water to be reclaimed. Previous attempts to reclaim the total heat in bleed water failed as a result of the corrosive action of the acidic bleed water or as a result of the excessive scale deposited when the acidic gases were allowed to escape.
Conditions at Hoskins Mound The geologic structure of Hoskins Mound is ideally adapted to bleed water reclamation. Its characteristics are such that large volumes of bleed water a t temperatures ranging from 220" to 250" F. can be taken from the top of the barren cap rock without causing adverse channeling of hotter water directly from the mining area. This is not true of the Grande Ecaille Dome. The following discussion, with reference to Figure 2, will point out the reasons for this fact. The cap rock of Hoskins Mound is highly arched, and the sulfur occurs in a lens-shaped horizon located relatively low on the steeply dipping flanks. The center or apex of the dome is covered with a thick layer of porous barren limestone which, during mining operations over a period of 12 years, has served
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as a large collecting reservoir for the dissipated hot water injected into the mining areas. This large reservoir has been tapped a t considerable distances from the mining areas to supply the bleed water used in the reclamation process. Grande Ecaille Dome is comparatively flat, with the sulfur horizon existing on this flat portion of the dome and the much gentler dipping flank. Only a very thin barren strata of limestone overlies the sulfur-bearing horizon, with the sulfur-bear-
meates. Such water has a severe corrosive action on steel and cast iron, as a result of the formation of iron sulfide by t h e hydrogen sulfide present. TABLEI. CHEMICAL ANALYSIS OF TYPICAL HOSKINSMOUXD BLEED W.4TER SI02
Fe
Ca
Mg
NO L I M I T S TO OPERATING ARE4 ,
I T S OF O P E R A T I N G AREA 4 LASI MDETERMINED BY SURFACE FILL
,
FIGURE2. TYPICAL CROSSSECTIONOF HOSKINS MOUND(Above) AND OF GRANDE ESCAILLEDOME (Below)
P. p . m. 142.2 2.9 256.5 17.4
Ns, SO4
Pol sulfide S Sulldde S
P . p . m. 381.1 336.7 52.0 248.5
C1
HCOa SzOa
pH
P. v. m 923.0 224.6 43.3 6.5
SURFACE
FUR-BEARING
l
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Three possible methods of treating bleed water to reduce its corrosive action to a negligible amount were arrived a t in a study of the problem (1). The first two methods which have been used satisfactorily on a commercial scale to date are (a) addition of alkaline chemicals in amounts sufficient to cause the required change in the solubility of dissolved compounds to effect a deposition of scale on the metal surfaces, thereby sealing them off from the corrosive water; and (b) treatment with chemicals that would combine with ions already present in the water to form an insoluble precipitate which would form as scale on the equipment. The third method, which has not been tried on a commercial scale, is (c) to flash off enough of the dissolved acidic gases contained in the water to render it noncorrosive. At Hoskins Mound, bleed wells to supply water for re-use are drilled into the porous, barren limestone a t distances of 500 to 3000 feet from the sulfur-producing wells. Through a steel casing set in the well, the bleed water rises as a result of the formation pressure and flows through cast-iron lines to the relay station. The relay station is a central control station located in close Qroximity to the mining area. The bleed water is usually delivered to the relay station a t a pressure between 30 and 60 pounds per square inch, where the pressure is raised by centrifugal pumps, constructed of admiralty metal, to allow it to be mixed (3) under pressure with plant water a t 355’ F. in the required amounts to give a mixed water having the temperature desired for mining (Figure 3).
ing strata extending to the top of the cap rock in certain areas. The slope of the dome does not allow large volumes Hydrated Lime Treatment of water to accumulate. I n order to obtain sufficient water a t Theoretical calculations (4) based on bleed water analyses a desirable temperature to justify its being re-used, hot (Table I) and necessary assumptions showed that raising the channels, that would convey water directly from the mining pH of the water to 7.1 would effect the conversion of sufficient area, would have to be tapped. Also bleed wells for furnishcalcium bicarbonate already present to calcium carbonate, ing hot water for re-use could not be centrally located but t o exceed the solubility of calcium carbonate in this water would have to be drilled a t different localities as mining progressed from one area to another. The re-use of bleed water would be possible if the acidic gases were not allowed to escape before its temperature was raised to that required for mining, and if all equipment coming in contact with the corrosive water were constructed of metals highly resistant to corrosion. The cost of using such material, however, is prohibitive. To recover economically the total heat contained in bleed water, methods would have to be developed for protecting the steel and castiron pipe lines and equipment used in sulfur mining from corrosion. An analysis, typical of bleed water being re-used ST at Hoskins Mound, is given in Table I The mine L? water supplied by the power plant has a pH of 8.0 to STATION 10.0 and contains small amounts of calcium, magneFIGURE3. FLOW SHEETFOR UTILIZINGBLEEDWATERIN SULFUR sium, and sodium chlorides, sulfates, carbonates and bicarbonates, and silica. The corrosive constituents MINING found in bleed water are picked up by dilution of and effect the precipitation of this compound. As soon as the mine water mrith the water originally present in the formableed water reclamation was started, hydrated lime was intion and from the reaction of the hot mine water with the caljected into the water under pressure in amounts sufficient to cium carbonate, calcium sulfate, sodium chloride, sulfur, and raise the pH to 7.1. This method of treatment was used other chemicals found in the salt dome through which it per-
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75 5
posit a calcium silicate scale by the addition of sodium silicate to the water were unsuccessful. Theoretical calculations showed that uneconomical amounts of sodium sulfate would have to be added to effect the deposition of calcium sulfate. Examination of theoretical data did show, however, that it would be possible to cause the precipitation of calcium phosphate by the addition of small amounts of alkaline phosphates to the bleed water. Previous experience in boiler water treatment showed that tricalcium phosphate tends to precipitate as sludge instead of as a scale. As a result of the low p H a t which the precipitation of calcium phosphate would take place in bleed water in comparison to those existing in boiler water it was thought probable that deposition of this compound as a scale from bleed water would be possible. It was immediately found that the addition of small amounts of dior trisodium phosphate to bleed water would result in the deposition of a protective scale. This scale was not as dense and hard as the calcium carbonate scale deposited by lime treatment but had sufficient density and hardness to protect the equipment from the corrosive action of bleed water. Scales deposited by this method of treatment have a composition similar to that given in Table 111.
SAMPLES OF SCALEDEPOSITED INSIDE OF LIWE CARRYING BLEEDWATER because hydrated lime is the cheapest suitable alkaline chemical. It was proved on a commercial basis that the required amount of scale could be deposited on the equipment to seal it off from the corrosive water. If insufficient lime to raise the p H of the water to 6.9 or above is added, no scale will be deposited. If sufficient lime to raise the pH to values in excess of 7.5 is added, the calcium carbonate will be precipitated in the form of a sludge. Scales deposited by this method of treatment have a composition similar to that given in Table 11. OF SCALE DEPOSITED BY TREATIKG BLEED TABLE11. ANALYSIS WATERWITH HYDRATED LIME
% '
Probable Combination, % Si03 0 R2 ... Fez03 Trace Trace Cas04 2.98 38.20 n CaC03 93.23 " .~3 MgCOs 1.95 Cos 57.30'I MK(OH)Z 0.62 so4 2.05 ~. Determined by evolution and absorption. Analysis,
sin.
n
82
--
0,
Successful protection of the*gathering lines carrying 100 per cent bleed water can be secured by this method; however, when a rriixture containing 40 per cent treated bleed water (pH 7.1 and 240" F.) and 60 per cent plant water (pH 9.0 and 355" F.) is made, an excessive amount of scale is deposited. In order to operate continuously and. prevent excess deposition of scale in the booster lines carrying a mixture of plant water and bleed water, the maximum treatment with lime is that required to raise the pH of the bleed water to slightly less than 6.9. As previously mentioned, this rate of treatment will result in a gradual solution of deposited scale in the gathering lines that carry 100 per cent bleed water, and in the absence of scale there will be corrosion.
Alkaline Phosphate Treatment Investigations on a commercial scale were undertaken to develop methods whereby a scale could be deposited with a composition such that it would not rapidly redissolve in bleed water having a pH below 6.9. I n this study it seemed practical to add reagents which would result in the precipitation of a scale of low solubility in bleed water. Attempts to de-
TABLE111. ANALYSISOF SCALEDEPOSITED BY TREATING BLEEDWATERWITH TRISODIUM PHOSPHATE Analysis, %
Probable Combination, % 69.99 Caa(PO4)z 8.01 Cas04 CaCOs 16.17 Si02 0.62 0.44 FerOs, A1203 Hz0, organic 3.34 matter
organic c 3.49 matter a Determined by evolution and absorption. H20,
Alkaline Phosphate-Barium Chloride Treatment After the deposition of a calcium phosphate scale proved feasible, a commercial test was made in an attempt to deposit a scale containing appreciable amounts of barium sulfate by adding barium chloride to the bleed water. It was found that a scale could be deposited, but the rate of deposition was too slow. I n order to increase the rate of deposition, combination treatments using alkaline phosphates in conjunction with barium chloride were tried and proved practical. Scales deposited by this method of treatment have a corn position similar to that given in Table IV. TABLEIV. SCALEDEPOSITEDFROM BLEEDWATERBY DISODIUM PHOSPHATE AND BARIUM CHLORIDE TREATMENT Probable Combination, % 43.2 Bas04 13.1 CaSOa 30.6 Cas(PO4)z 8.2 CaCOa 1.5 Mgs(P0a)r 0.2 Pi06 Ms(OH)z Cora 0.1 SiOr 2.9 HzO, organic 3.0 Hz0, organic matter matte? Determined by evolution and absorption. Analysis, %
Si02 BaO cao
roo
a
0.10 28.4 26.6 0.9 22.5 14.8 3.6
Scales deposited by the alkaline phosphate treatment or the combination alkaline phosphate-barium chloride treatment are practically insoluble in bleed water having a pH of 6.5 or above.
Method of Adding Chemicals The practice of controlling the composition of municipal water in order to cause this water to deposit a calcium car-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
bonate scale over great distances for protecting distributing mains against corrosion by dissolved oxygen has been followed for a number of years. The continuous treatment of bleed water by the injection of chemicals a t one point to change the equilibrium conditions to a degree favorable to scale deposition is not possible, and it has been found necessary to inject chemicals a t numerous points throughout the system. The difference is due to the large concentration of dissolved solids and the higher temperatures of bleed water, which result in more rapid rates of reaction that give excessive scale deposits near points of injection if high rates of treatment are used. Even with high rates of treatment that cause too great a deposition near injection points, it has not been possible to effect a deposition of protective scale throughout the system by injecting the total treatment a t any single point. The addition of treating chemicals a t a controlled rate to the bleed water and to the mixtures,of plant water and bleed water required considerable work before satisfactory methods were obtained. All chemicals must be injected into the water which exists under pressure varying from 30 to 250 pounds per square inch. This necessitated adding all treating chemicals in solution or slurries which could be injected by pumps constructed to develop the required pressures. I t is not desirable to add large volumes of cold solutions in treating, because the result would be an undesirable lowering of the temperature of the water; therefore, the concentration of the solution of chemicals is regulated to require the addition of the minimum amount of the cold solution to the hot water being treated. Centrifugal pumps that would deliver from 2 to 10 gallons per minute a t the point of injection with the required pressures were not satisfactory for continuous service. Positive-displacement reciprocating pumps have proved most satisfactory for this service. In treating bleed water it is desired to control the rate of addition of chemicals between much narrower limits than is usually followed in boiler water treatment. KO type of automatic proportioner control tested to date has been found entirely satisfactory for controlling the addit:on of treating chemicals within the desired limits; therefore, semiautomatic
BLEEDWATER TREATING EQUIPMENT
t ; r *
757
manual control has to be used. The present methods employed are of two types. One is to make up the solution of the treating chemical to a definite concentration in a large solution tank. The rate at which chemicals are fed into the water is controlled according to the water flow by varying the speed of the reciprocating pump. This is done by means of a variable-diameter pulley drive direct-connected to a constant-speed electric motor. The amount of chemical fed is determined by measuring the rate a t which the chemical solution is drawn from the tank. In the second type of equipment used, the chemical solution is drawn from the large solution tank by means of a centrifugal pump that delivers the desired small volumes a t a low pressure. The rate of feed is measured by a recording Venturi meter and controlled by a lubricated plug valve in direct proportion to the flow of water to be treated. This controlled feed discharges into a receptacle from which the reciprocating pump takes suction and pumps the solution into the bleed water. A float control on this receptacle regulates the flow of a small volume of dilution water to the container to eliminate the necessity for adjusting the speed of the reciprocating pump to give a pumping rate exactly equivalent to the volume of treating solution required.
Deposition of Scale The usual practice now followed in protecting the equipment is first to lay down a protective scale in the gathering system which carries 100 per cent bleed water, either by the alkaline phosphate or by the alkaline phosphate-barium chloride treatment. This is accomplished by injecting the treating chemicals into the bleed water from 300 to 800 feet down in the bleed wells. When sufficient protective scale is obtained in this part of the system, treatment with lime a t rates which give a pH of 6.9 or slightly lower is started. Bleed water receiving such a treatment will not deposit additional scale in the gathering system and will not dissolve the cakium phosphate or mixture of calcium phosphate and barium sulfate scale previously deposited. These low rates of lime treatment will, however, cause a protective scale to be
Solution tank, Venturi tube section, automatic control valves, recording meters, etc., are shown. The narrow tank in the center is calibrated and used daily to check the &ccuracy of the Venturi tube measurements.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
deposited over distances of 500 to 3000 feet in the steel lines which carry water to the producing wells. This water is composed of approximately 60 per cent of 355' F. plant water and 40 per cent of 240' F. treated bleed water. The scale deposited from this water is composed mainly of calcium carbonate. The distance over which the deposition takes place and the rate a t which it takes place are proportional to the treatment to the bleed water, percentage of bleed water in the mixture, and composition of plant water used. The entire length of a line carrying mixed plant water and bleed water from the relay station to the bottom of a sulfur well is sometimes as great as 4000 feet. In some instances it has not been practical to effect the desired deposition of protective scale over the entire line to producing wells and on the equipment in producing wells. For this purpose it is necessary to inject an additional chemical treatment a t points along the lines. Since these lines and wells are not permanent, a scale which is mainly calcium carbonate is satisfactory. Because of this fact sodium carbonate is usually injected along the lines. This paper so far has dealt with the treatment of bleed water to be mixed with a hotter plant water to obtain a mixed water of the desired mining temperature. This allows the total volume of water used in mining to be made up of approximately 40 per cent reclaimed bleed water. Work recently started on a commercial scale has proved that it is practical to raise the 220-250" F. bleed water to the desired mining temperature by condensing steam into it under pressure in a suitably constructed heater. It is possible by this method to use approximately 90 per cent reclaimed bleed water in mining operations. Satisfactory protection from corrosion can be obtained by depositing scale on the equipment; however, because of the increase in the temperature of the bleed water in the heater, very close regulation of treatment is necessary to prevent the heater from becoming plugged with scale.
Protection of Equipment Certain pieces of equipment used in bleed water reclamation, such as centrifugal pumps, regulating valves, orifice plates, thermometer wells, etc., must be maintained free of scale to ensure satisfactory operation. Therefore the equipment cannot be protected with the required amount of deposited scale, and the surfaces exposed to the corrosive action of the bleed water must be constructed of corrosion-resistant metals. Admiralty metal and bronzes of similar composition have proved very resistant to corrosion in this service. Large tanks are installed in the bleed water gathering lines to settle out any scale that flakes off from the gathering line and that would otherwise be carried into the impellers of the centrifugal pumps and thereby impair their operation; these
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tanks are called "scale traps" and are constructed of steel metallized with a thin film of tin. Protection of large receptacles of this nature can be obtained by scale deposition, but because of the large circumference of these pieces of equipment the deposited scale is likely to flake off from spots when the system is cooled. Therefore it is desirable to protect these surfaces with a corrosion-resistant coating. Metallizing such surfaces with tin has proved the most satisfactory method. Metallizing with other corrosion-resistant metals or alloys such as admiralty brass or stainless steel has not proved nearly as satisfactory; because of the brittleness of the coating it cracks and allows the corrosive water to reach the nonresistant steel surfaces. I n order to protect the working parts of the recording instruments used in metering from being corroded, receptacles that are kept filled with oil are installed in the leads to the orifice sections. This practice seals off the recording instrument from the corrosive water and allows only oil to reach it. This practice has greatly reduced the repairs to these instruments and has appreciably increased the accuracy of measurement obtained. It is important that the thermometer wells set in the water lines be kept free from scale in order that accurate temperatures of the water be recorded and accurate control of the temperatures of the mine water be obtained. Before the development of an instrument allowing these wells to be freed from scale with the water line remaining in service, it was necessary to cut a line out of service to accomplish this purpose.
Conclusions The re-use of bleed water for sulfur mining in order to reclaim its heat can successfully be practiced at Hoskins Mound but is not universally applicable to all properties mining sulfur by the Frasch process because of differences in structures of the cap rock. Protection of equipment from the severe corrosive action of bleed water can be economically obtained by treating the water in such a manner as to cause i t to deposit a protective scale on metal surfaces with which it comes in contact. Satisfactory explanation of all the complex reactions cannot be given a t present because of the absence of the required theoretical data.
Literature Cited (1) Axelrad, B. A., U. S. Patent 2,109,611(March 1, 1937). (2) Butterworth, C.E., IND. ENQ.CHEM.,27,548-55 (1935). (3) Lundy, W. T., and Drachenberg, W., U. S. Patent 1,878,158 (Sept. 20, 1932). (4) McKinney, D. S., IKD. ENG.CHEM.,Anal. Ed., 3,192-7 (1931). RECEIVED March 28, 1938.
Presented before the Division of Industrial and Engineering Chemistry a t the 95th Meeting of t h e American Chemical Society, Dallas, Texas, 4prll 18 t o 22, 1938.
