**.
4Q
Water Problems in Sulfur Mining C. E. BUTTERWORTH Texas Gulf Sulphur Company,
Newgulf, Texas
The mining of sulfur by the Frasch process is largely dependent upon water. Water in large volumes is heated to 320’ F. under pressure and pumped underground for melting the sulfur. A description of the process used in the treatment and heating of this water is given, and problems of corrosion and scale formation are discussed. To prevent building up high underground pressures, water is bled from the sulfur-bearing formation simultaneously with the introduction of the hot water. For disposal to waste, this “bleed water” is treated for the removal of soluble sulfides. A description is given of the plant and process used for the treatment of bleed water.
H E sulfur b e i n g mined in the United States along the Gulf Coast is obtained in every case from an elementary sulfur deposit found in the so-called “cap rock” of certain “salt domes.” Numerous articles have been published about salt domes (1). The Gulf Coast is a sedimentary deposit of great depth, and these salt domes are plugs of salt (sodium chloride), with or without a cap rock, which seem to have been intruded upward through overlaying beds and mantle rock. G e o l o g i s t s h a v e presented numerous theories to account for the origin and formation of salt domes (11). The surface of the ground directly over a salt dome may or may not be higher than the surrounding country. The salt domes are located now largely by such instruments as the seismograph and torsion balance. The distance from the surface to the top of a salt dome may be a few feet or several thousand feet, and the area covered may be many acres. Exploration for salt domes has been largely by the petroleum industry. Oil is sometimes found in the cap rock but more often in the disturbed sands around the flanks of the domes. The salt industry mines the fairly pure salt found in these domes, and a recent development in the South is the utilization of this salt by the alkali industry for the production of soda and allied products. Native sulfur occurs in sufficient amount as rhombic crystals in the calcite and limestone layer of the cap rock thus far of only a few of the explored domes to justify commercial mining operations. The domes in which sulfur has been found are of intermedia t e depth (500 to 2000 feet). The cap rock layer may be several hundred feet thick, while the salt below is of unknown depth. The cap rock is generally composed of two layers; the top, or calcium carbonate layer, contains the sulfur, but the bottom, or calcium sulfate, contains as a rule no elemental sulfur. Numerous theories have been presented to account for the origin of the sulfur in this cap rock, and there is a diversity of opinion as to whether the calcium carbonate and sulfur have resulted from the reduction of the calcium sulfate by the petroleum (8).
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water or Frasch process (S), by which all sulfur in the Gulf Coast is now mined. Brief descriptions of the new sulfur mines have appeared in the literature (9). With the F r a s c h p r o c e s s a well is drilled by a “rotary rig” in fashion similar to d r i l l i n g for oil. Upon r e a c h i n g c a p rock, a surface casing may be set and cemented. Drilling is continued within t h i s c a s i n g to the calcium sulfate layer. Three sets of pipes are strung. The first (6-inch) goes to the bottom of the hole and is perforated a t the bottom for adm i t t i n g m o l t e n sulfur and higher for the discharge of hot water, The 3-inch pipe is string within the 6-inch, is not perforated, and goes nearly to the bottom of the hole. A 1inch air line is strung within the 3-inch pipe and does not go quite to the end of the 3-inch pipe. Hot water is admitted between the 6-inch and 3-inch pipes; the sulfur melts and, being insoluble in water and twice as heavy as water, gathers a t the bottom. E l e n the liquid sulfur is deep enough a t the bottom of the hole to seal the 3-inch pipe, air is admitted through the 1-inch line and the sulfur is forced to the surface. During this operation of getting sulfur from the well, hot water is constantly admitted between the &inch and 3-inch pipes, thus melting more sulfur to replace that being removed. Should the sulfur level fall below the bottom of the 3-inch pipe, then the hot water will be returned to the surface instead of sulfur. This is known as “blowing.” When this happens, the air is turned off and the well is given a “boost” by admitting hot water through both the 6-inch and 3-inch pipes until sufficient sulfur has accumulated around the 3inch pipe to give a sulfur “seal.” Sulfur production is then again resumed. The production of sulfur from these deposits is made possible by the fact that the calcium carbonate formation is porous. The pores of the formation are f d e d with a sulfidecontaining brine. The solids dissolved in the brine are, for the most part, sodium chloride, although they also contain appreciable amounts of calcium, magnesium, bicarbonate, sulfide, and sulfate ions. This water is known as “formation water.” As the hot, fresh water enters the formation t o melt sulfur in volumes far exceeding the volume of melted sulfur removed, great underground pressures are soon built up. To prevent excessive pressures during sulfur mining, it is general practice to “bleed” water from the bottom of the formation a t the flanks of the dome. Since the cold brine is heavier, it
Hot Water Process for Mining Sulfur After the discovery of sulfur in one of these domes, many discouraging attempts were made t o mine it by shaft. There was too much quicksand and gas t o permit of completion of a shaft. Between 1890 and 1900 Frasch developed the hot 548
MAY, 1935
INDUSTRIAL, AND ENGINEERING CHEMISTRY
549
siderable extent as well as remove suspended matter. Susseeks these lower levels, while the warm fresh water tends to pended matter of a colloidal nature, if left in the water, seems rise in the formation to melt sulfur, or at least warm it. Thus to increase in size with heat and, unlike a large flocculent the first "bleed water" in a sulfur mining operation is formaprecipitate, adheres tenaciously to a pipe. The softening tion water. -4s mining continues, it slowly becomes diluted and deaeration are carried out in a Cochrane hot process with the fresh water and with this dilution there is generally softener. some rise in temperature. At first thought i t would seem possible to reheat this bleed water and return it to the formation for melting more sulfur. Power Plant for Steam Generating and Water This has been done in the past and difficulties soon arose. Heating -4s the temperature of this water is increased, the bicarbonThe water for the whole mine is softened and heated in a ates and sulfides decompose and scale formation results. The scale-forming ingredients are present in this brine in far central power plant which, on account of subsidence of the greater quantities than in ordinary well water. The presground over the mining area, is necessarily located a t some ence of the sulfides and salt also renders this bleed water very distance from the sulfur field. The Xewgulf power plant is corrosive. On account of the quantities of chemicals inequipped with ten Babcock and Wilcox 1.500-horsepower volved, softening this water is out of the question Thus it boilers of the Stirling water tube type. The steam pressure is general practice in the sulfur industry to use a t the sulfur used is from 90 to 100 pounds per square inch gage. The m l l for melting the sulfur a "mine water" derived by softenboiler settings are cooled with air, and the air being heated in ing fresh water followed by superheating to a temperature of these settings is used as a force draft under the boilers. They 300" to 320" F. with live steam. It really amounts to conare fired with natural gas, although the settings are designed densing steam into mater under pressure. Although this so that fuel oil or even lignite may be used. In addition to condensation results in dilution of the total solids originally heating water, the steam power is used for pumping all the present in the water, the increase in the temperature is ofteri water, compressing the air, and generating the electrical accomplished with a decrease in solubility of calcium and magenergy needed about the mines. The steam from this power nesium salts, especially those present as carbonates, bicarequipment is exhausted a t about 5 pounds per square inch bonates, and silicates. At this high temperature magnesium gage and is used in the jet heaters of the Cochrane water salts seem t o hydrolyze, and magnesium hydroxide precipisofteners for raising the temperature of the water to 220" F. tates because of the increase in temperature alone. Small From 13 to 15 per cent of the water to these softeners is this amounts of calcium sulfate are soluble since there is no conlow-pressure steam. centration as in evaporation of boiler n-ater. Corrosion is very actil-e a t these high temperatures unless the pipe is proRaw Water Supply tected with a thin layer of scale or has been completely freed The raw water supply is from two sources, wells and the of dissolved oxygen. Thus the softening of a suitable "mine San Bernard River. The water wells are drilled off the salt water" for sulfur mining presents a condition perhaps pecudome and are 460 to 500 feet in depth. Each well will deliar to itself. liver 500 or more gallons per minute. Well water a t this Based on experiment and experience gained through mindepth on the Texas Gulf Coast often contains some sodium ing sulfur at Gulf, Texas, the process of softening the mine bicarbonate. It is thought that the water sands through water now used a t S'ewgulf, Texas, seems the most promiswhich it passes contain zeolites. When treated with lime, ing. This consists in softening with lime, and with little or, this sodium bicarbonate is converted to sodium carbonate. at times, no sodium carbonate, a t a temperature slightly At times the amount of sodium carbonate present is too high above the boiling point of water-for example, 220" F. At for use as a boiler feed water. On the other hand, the San this temperature with lime nearly all the magnesium precipiBernard River water contains no sodium bicarbonate and tates as the hydroxide. The calcium carbonate after a 60often contains some noncarbonate hardness. Thus, by mixto 80-minute retention period is precipitated to such an extent that there is not much further precipitation upon heating ing these two waters in such proportions as to give a final water c o n t a i n i n g , this water to 320" F. after softening, 2.5 to C a l c i u m sulfate is 1 f I N E \vATER H E 4 T E R S \%'HERE T H E \vATER TEMPERATURE 3.0 grains per gallon soluble in water a t Is RAISEDTO ABOUT320" F. excess soclium car2 2 0 " a n d 3 2 0 " F. bonate, a very good Condensation of live boiler feed water can steam in the softened be s e c u r e d without 22O"water a. a means the purchase of any of h e a t i n g t o 320" sodium c a r b o n a t e . results in a certain On the other hand, d i l u t i o n effect. At noncarbonate liar d the same time one is ness removal and such attempting to make c o m p l e t e carbonate a scale-free m i n e hardness removal is water. he is reducing the solubility of all n o t n e c e s s a r y with the mine water, and the di-qolved g a s e s , the San Bernard such as oxygen and River water has carbon dioxide, and by venting them is proved a suitable doing away with corsource of supply. r o s i o n . The water The analysis of well s h o u l d b e filtered, water changes very Eince filters stabilize slowly from week to the water to a conweek, and the wells
IKDUSTRIAL AND EKGINEERING CHEMISTRY
550
VOL. 27, NO. 5
TABLE I. ANALYSESO F HOT-PROCESS LIME-SODA SOFTENED WATER (Softener temperature, 220' F.; pH, 9 . P results in grains per gallon) Date of Analysia 11-1-30
12-1 1-32 1-15-31 11-20-30 8-8-31 10-21-30 11-8-31 11-4-30 11-30-30 6-5-32 1-20-33 12-14-31 (1
Total Alkalinity Minus Total Hardness as NslCOa 3.00
2.98
2.46 2.53 2.04 2.01 1.50 1.05
0.57 0.51 0.03 0.06
Determined colorimetrically.
Total Hardness as CaCOs 0.54 0.55 0.69
0.73
0.86 0.89 1.14 1.56 1.78 1.84 2.18 2.39 b Determined by difference.
OH None
0.12 None 0.04 Xone 0.08 None None None 0.08 None 0.20
are pumped only as the water is needed. On the other hand, the river water analysis varies with the weather. I n dry weather it is fairly hard with very little suspended matter present; in wet weather the hardness is low, but the suspended matter is very high. Water whose analysis fluctuates rapidly is not a desirable source of supply for any type of softening operation. During dry weather there is not sufficient volume in the river to permit of operation. For these reasons a 260-acre reservoir has been built for the storage of river water. This reservoir is filled during the wet periods, which insures water during dry weather; and on account of the softer water obtainable a t these times, considerable saving in chemicals for softening is effected. The reservoir provides for some settlement of the suspended matter. However, if the water becomes too soft, there is not enough floc formed during softening to coagulate properly the finely divided suspended matter.
Cochrane Hot Process Softeners At Newgulf there are six Cochrane hot process softeners, each of 70,000 gallons per hour capacity (12); thus it is the largest installation of its kind in this country. Each unit consists of a sedimentation tank of such size as to give, a t full capacity, a 60-minute retention period for the water, a jet heater by means of which exhaust steam a t 5 pounds pressure heats the raw water to 220' F. and provides for almost complete deaeration of the water, a vent condenser for the venting of the permanent gases without the loss of heat, a chemical solution tank and chemical proportioner for feeding the chemicals to the water, and two calcite filters for filtration of the water. The sedimentation tanks of these softeners are made of boiler plate and are for the most part vertical cylinders 25 feet in diameter and 40 feet in height. They are located outside the power plant and are well insulated for loss of heat. For a few feet only the top a,wumes a cone shape, and the jet heaters are attached a t the apex of this cone. As the low-pressure steam enters a vertical cylinder 36 inches in diameter on its way to the interior of the softener, it is met a t right angles with the cold water delivered through jets, and a good mixture of steam and water results. The resulting hot water then drops through a gas- and steamfilled space to a sheet-iron splash plate which is located a t about the elevation where the sedimentation tank assumes its cone shape. The water level in the sedimentation tank is constantly carried just below this splash plate by means of a float valve control and overflow device. Through 1.25-inch pipes, which enter
Ion
HCOi
cos
Ca
0.43
1.81 1.81 1.53 1.81 1.53 1.53 1.39 1.25 1.12 1.25 1.25 1.12
0.18 0.18 0.22
None 0.57 None
0.28
None
0.28 0.57 0.57
None
0.14
None
0.25 0.30 0.30 0.40 0.58 0.66 0.69 0.82 0.91
Mg 0.02
0.03
0.03 0.03 0.03
0.05 0.04
0.03 0.04
0.03 0.04 0.03
-
Na b
so4
c1
4.93
2.58
3.69 5.38 3.28 3.95 6.03 3.63 6.02 3.98
6.20 4.43 4.82 5.92 4.43 5.98
3.75 3.34 3.98 4.55 5.03
2.98 2.59 2.54
2.34 2.53 2.54 1.48 1.40
1.76
2.12 2.37
3.75
4.50 5.44 5.97
on the sides of the sedimentation tank, the necessary chemicals for softening and coagulation meet the mater a t this point. This is the only mixing action the water receives with the chemicals. Outside the softener and on this coneshaped top the vent condenser is located. By means of this device, which is cooled by the cold, incoming raw water, all of the vented steam is condensed and returned as hot water to the softener while the permanent gases, such as carbon dioxide and oxygen, are vented to the atmosphere. Also on the exterior of the cone-shaped top there are several large spring-set safety valves which open in case a vacuum should form within the sedimentation tank and which provide protection against internal collapse, Near the inside bottom of the sedimentation tank a sheet-iron cone or funnel has been erected whose base is slightly less in diameter than the diameter of the sedimentation tank itself. At the apex of this cone the softened water leaves the sedimentation tank by means of a pipe through the side of the sedimentation tank. Thus the water in passing through the sedimentation tank flows over the outside of this funnel, upon reaching the base turns through an arc of B O 0 , and is conducted from the tank. The sludge resulting from the action of the chemicals on the hardness of the water, instead of making this 180' turn with the water, tends to continue its course downward. The bottom of the sedimentation tank is used for collecting this sludge and is made in the shape of an inverted cone to expedite its disposal. At the apex of this cone which is, of course, the lowest part of the sedimentation tank, there is a large valve. Through this valve a t regular intervals the sludge may be b l o m off. While most of the suspended matter has been separated by sedimentation in the sedimentation tank, there is, under the
POWER HOUSEWITH HOTWATER LINES TO SULFURWELLS IN FOREGROUND AND MINEWATER SOFTENING PLANT AT LEFT
INDUSTRI.4L AND ENGINEERING CHEMISTRY
MAY, 1935
best conditions, still some suspended matter remaining and filters are provided for its separation from the water. Two filters, located under each sedimentation tank, are of the horizontal cylindrical type, each about 25 feet long and 8 feet in diameter. They are made of boiler plate and filled with calcite rather than sand, as it is said that hot alkaline water tends to dissolve silica. Means are provided for backwashing these filters, the wash water being returned to the sedimentation tank a t a point above the internal funnel for heat economy. The filter bed may be given a slight backward agitation with air before the backwash water is turned through. This tends to loosen the lodged suspended matter from the calcite particles and to permit of better backwashing. On the raw water line to the softener an orifice is installed. The differential pressure created by the presence of this orifice is carried by means of two water pipes to the chemical proportioner and actuates it. The proportioners are attached to the chemical solution tanks which, in turn, are housed in a large building used also for storage of chemicals and testing of treated water. The chemicals are dissolved in solution tanks. These tanks are of circular sheet-steel and hold 2000 to 3000 gallons of solution. The lime is held in suspension by means of a propeller type agitator driven with an electric motor. Two lime pumps are used in connection with each lime solution tank. One pump delivers an excess of lime solution to the proportioner. A proportioned quantity of the lime solution is delivered by the other lime pump to the splash
gradual. Changes in the rate of feeding the lime are based on alkalinity titrations made at the power plant hourly for each
