HIGH PURITY SULFUR FROM SMELTER GASES Reduction with Natural Gas EDWARD P. FLEMING AND T. CLEON FITT American Smelting and Refining Company, San Francisco, Calif. Inadequate local markets for the disposal of large tonnages of by-product sulfur, either ae sulfuric acid or liquid sulfur dioxide, was the motive behind the efforts of American Smelting and Refining Company to develop a process to produce elemental sulfur. The latter offered advantages in the matter of stockpiling and delivery to distant markets. This paper notes the difficulties encountered in operating the coke reduction process and emphasizes the advantages to be derived by the substitution of natural gas as a reducing agent, when this i s available. The development of the methane process over a period of 3 years of operation of a semicommercial plant at Garfield, Utah, is discussed. B y reference to flow diagram and drawings of essential
operating unite, the process is described in detail. In general, this consists of high temperature combustion of approximately 7% sulfur dioxide gas with an automatically controlled volume of natural gas. By heat exchange and by-pass valve regulation, the entire system is kept under the close temperature control necessary for efficient conversion to elemental sulfur and electrical precipitation of same. Features include: automatic control of the hydrogen sulfide-sulfur dioxide ratio in converters, treatment of pure gas resulting in production of 99.99% sulfur, and close control of temperature and gas flow in Cottrell precipitators permitting efficient precipitation with a minimum time in the electrical field.
I
tion early in 1940. This unit operated for 3 years on 7.25% sulfur dioxide gas from copper roasters, then for 18 months on 6.6y0 gas from copper converters. During theae operations approximately 7500 tons of 99.9% sulfur were shipped. From a chemical stmdpoint the process may be considered as a combustion operation in which air, with the oxygen content depleted to 9 to 12% and containing from 8 to 5 park by volume of sulfur dioxide, is burned with natural gas. Sufficient gas is metered into the sulfur dioxide mixture to consume all the oiygen to carbon dioxide, as well as a regulated excess of methane to react with the sulfur dioxide to form sulfur, carbon dioxide, and water vapor. The combustion takes place in a large vertical steel chamber lined with refractory and insulating bricks. In order to obtain surface for complete combustion, the chamber is packed with a checker work of highly refractory brick. The combustion operation should generate a temperature of at least 1260' C. for satisfactory consumption of all the carbon in the methane, otherwise the resultant sulfur will be off-color. This temperature is controlled by the operation of what is termed the primary heat exchanger, this functioning as a preheater for raw sulfur dioxide gas. The required amount of natural gaa is metered into the hot sulfur dioxide gas just prior to entering the combustion chamber where ignition takes place instantly. While the over-all reduction reaction is:
N CERTAIN localities ill the Intermountain region of the United States it is desirable to curtail materially the amount of sulfur dioxide discharged into the atmosphere from copper smelting operations. Since the local consumption of sulfuric acid is rather limited and delivery costa to distant marketa are almost prohibitive, several methods for the conversion of sulfur dioxide to elemental sulfur have been investigated by the American Smelting and Refining Company. By such a procedure the problems of stockpiling and marketing would be materially reduced. Previous work a t Trail, British Columbia (81, and Boliden, Sweden, was reviewed and a pilot plant operating along these general linea was erected at Garfield, Utah. Roaster gas containing about 7% sulfur dioxide was absorbed and desorbed, using the ammonium sulfite-bisulfite cycle, and the concentrated sulfur dioxide, after dilution with air to approximately 30% sulfur dioxide, was passed through incandescent coke and the reduced sulfur recovered by condensation. Numerous complications arose which interfered with the successful operation of the cokereduction process. Mechanical upkeep of reduction and condenser units was high; the quality of the sulfur produced wm poor; and the cost of coke was steadily rising. Also the expense of sulfur dioxide ooncentration by ammonia absorption cycle was a major element of cost. Everything considered, high operating costs appeared to prohibit a satisfactory commercial outcome. A review af the literature indicated that Young's dry thiogen process ( 4 , 7) might have possibilities. Although Young had reduced sulfur dioxide with methane and other hydrocarbons on a laboratory scale at Stanford and conducted some pilot work at Balakalla Smelter, Calif., during the period 1909 to 1911, resulta were not encouraging, and no further work was done along these linea until Asarco resumed investigations at Garfield in March 1938. The Asarco work soon indicated that pure relatively low grade sulfur dioxide gas might be directly converted to high grade sulfur by reduction with natural gas. Experimental and small scale pilot operations were gradually expanded until a semicommercial unit of 6 tons daily capacity was put into opers,
2SOs
+ CHd +2H20 + COz + &
(1 1
There are several side reactions that take place owing to the high initial temperature employed. The chief side reaction products are hydrogen sulfide and carbonyl sulfide. In order to hold the process in equilibrium and obtain an efficient conversion of all the sulfur compounds to elemental sulfur, two Conversion stages are necessary. The fist step is for the purpose of combining the carbonyl sulfide formed, with sufficient residual eulfur dioxide to form carbon dioxide and sulfur vapor according to the reaction:
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2cos + so*+2co* + '/&
(2)
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This reaction works best at a temperature of 425 " to 450' C. using high grade bauxite aggregates about 0.75 inch in diameter as a catalyst. After the carbonyl sulfide conversion step, the mixture of gases and sulfur vapor is cooled down by heat exchange and radiation to approximately 130' C. At this temperature the sulfur vapor has been condensed to a liquid mist and the vapor pressure of sulfur reduced to a negligible figure. The gases which contained mist are then passed through a Cottrell precipitator where,
CONTROL TO N 4 N R A L
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head of the plant, whereas an excess of sulfur dioxide shows that there is a deficiency of natural gas. With a constant volume flow of a uniform grade of sulfur dioxide in head gas, it is relatively easy to keep the ratio in balance by frequent analysis of tail gas and manual control of natural gas valve. However, when operating on converter gas it was found that not only did the volume change, but the grade might vary rather rapidly between 4 and 8% sulfur dioxide. Under these conditions manual control was inoperative and a successful method was developed by
AUTOMETER
045
HzS-SO2
KEY-
TEMPER~TLRE iNCRE4SNG TEM-ATbRE DECREISYG
-------------Figure 1.
Flow Diagram of Asarco Sulfur Plant
under carefully regulated temperature, they are subjected to an intermittent direct current of 65,000 volts. This precipitates, as a liquid, whatever sulfur has been reduced to the elemental form in the reduction furnace and carbonyl sulfide converter. The recovered sulfur has a viscosity similar to that of water and flows from the base of the Cottrell at a temperature of approximately 125"C. or slightly above its freezing point. The gases leaving the first Cottrell are a mixture of carbon dioxide, water vapor, nitrogen, sulfur dioxide, and hydrogen sulfide. At this stage the reactions between the sulfur compounds should approach equilibrium, and the relation between the volume of hydrogen sulfide and sulfur dioxide should be in the ratio of two to one. Before the main reaction between hydrogen sulfide and sulfur dioxide can take place, the gas stream must be raised in temperature from 125" to 210" C.; this is accomplished by passing the gas mixture through a secondary heat exchanger, countercurrent to gas coming from the carbonyl sulfide converter. The heated gas is then passed through a large insulated steel chamber filled with activated alumina aggregates approximately 6 / ~ inch in diameter. The reaction between the hydrogen sulfide and sulfur dioxide is: 2HzS
+ SOz +2Hz0 +
3/&
(3)
For a high efficiency of conversion of the gases to sulfur, the volume of hydrogen sulfide should be just double that of sulfur dioxide. Any excess of either gas shows up in the tail gas and lowers the recovery accordingly. An excess of hydrogen sulfide in exit gas shows that too much natural gas is being fed at the
Asarco for continuous and automatic analysis for both hydrogen sulfide and sulfur dioxide in the gas stream (6). This analyzer was actuated through an electrical controller and relayed to the natural gas control valve, in brder to maintain the requisite feed of natural gas. This device not only reduced analytical work but increased conversion under all conditions of gas volume and grade. With the natural gas feed under precise control, approximately 90% of the sulfur dioxide entering the plant was converted into elemental sulfur corresponding to an over-all recovery of 8570. The reaction in the hydrogen sulfide converter is exothermic and, on the &ton scale of operations, the temperature of the gas stream passing through the catalyst bed increased from 210' to 235" C.; at these temperatures conversion was relatively good. The gas stream was then cooled by radiation to 130' C . and passed through a second Cottrell precipitator, identical in construction and operation to the first unit, for precipitation of sulfur. When operating large size commercial units the temperature rise in the first stage of hydrogen sulfide conversion, where the sulfur burden is heavy, might reach 260" C . , at which point reversion begins to take place. Although at Garfield the small scale of the operations permitted production with a single hydrogen sulfide conversion step, the flow diagram (Figure 1) visualizes two conversion steps. This system of operation is analogous to the two-stage sulfur dioxide to trioxide conversion used in producing sulfuric acid by the contact process. This system would add to capital cost, but the increased recovery of sulfur and lower tail gas would warrant the expense. Under these conditions recovery should approach 95% as the resulting temperature would be under close control because of the weak gas treated in the second converter.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
In the 5-ton plant the hydrogen sulfide was converted in two catalyst chambers, set in parallel, operating under identical conditions as to temperature and volume of gas. Various catalysts were tested in pairs under these conditions, the inferior one being discarded and another substituted in its place. This investigation showed that activated alumina was the most efficient, with high grade bauxite the second choice. Activated alumina, however, is more sensitive to critical temperature, requiring clo3er control than bauxite. Both activated alumina and bauxite have an indefinite life as far as conversion ability is concersed, provided they were regenerated occasionally by purging any condensed sulfur; this is done by increasing the temperature of the bed. Owing to some decrepitation the resultant fines increase the back pressure through the bed. This necessitates the removal of catalyst about once every 2 years for sizing and discard of fines. The gas originating at either the roasters or converters was freed from all metal impurities to the same degree as that practiced for the production of contact sulfuric acid. It is not necessary, however, to remove the water vapor as is done in the latter process by drying. Since neither the purified sulfur dioxide gas nor the natural gas contains anything other than water vapor, oxygen, nitrogen, carbon, and sulfur, the resultant sulfur is remarkably pure. Spectrographic analysis shows 99.99% sulfur. The entire plant is a welded steel structure and, being under pressure, has to be gastight. Where the steel is subjected to high temperatures, it is protected on the inside with refractory brick and, in general, is insulated on the outside to conserve heat and maintain the proper temperature control. Efficient operation of the process depends largely on close temperature control of all units. This is obtained by the use of heat exchangers and radiatiori coolers, which are regulated through the medium of by-pass valves. Since there are no erosive or corrosive substances in the gas stream and gas volume is normally constant, close temperature control is relatively simple. As the reduction furnace operates at a white heat and the recovered sulfur and waste gases leave the plant a t about the temperature of boiling water, the process has to be relieved of a large amount of heat energy. Even on a &ton unit the main problem was to dissipate excess heat by radiation. With a large commercial unit the logical step would be to utilize a waste heat boiler similar in construction to those used in acid plants for the recuperation of heat from sulfur burners. In relation to the amount of natural gas burned in the reduction furnace, the energy that could be recuperated as steam in the waste heat boiler would correspond to over 40% of that which could be generated if an equivalent amount of natural gas were burned directly under the boiler. This would furnish a material credit against the cost of natural gas consumed in process.
ASARCO BRIMSTONE PROCESS The flow sheet of the Asarco process (1-3) for the production of elemental sulfur is shown in Figure 1. This flow diagram shows the preferred system of operation althdugh in the semicommercial plant at Garfield, Utah, the two sulfur dioxide converters in series were not used. In the operation of the plant the gas flow was of the forced draft principle and the blower, operating at a pressure of approximately 45 inches of water, forced the gas stream through the entire system. Clean raw flue gas was introduced into the primary heat exchanger where i t flows in heat exchange relationship to the hot combustion products leaving the reduction furnace. The preheated raw flue gas enters the top of the reduction furnace just after the proportional amount of natural gas at atmospheric temperature has been added. Combustion takes place instantly. The hot combustion products leave the furnace a t the bottom, pacls through a brick-lined conduit, and enter the top of the primwy heat exchanger. Fimre 2 shows in some detail the construction of the reduction furnace, primary heat exchanger, and radiation coder. The fur-
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nace was lined with three courses of 4.5-inch brick, the innei course being Kruaite, the next firebrick, and the outer course Insulbrick. The Kruzite checker work nearly filled the furnace. The gas stream left the reduction furnace at a temperature of approximately 1200' C. The heat exchanger was of the Fitch recuperator type. The tubes were silicon carbide, 4 inches inside diameter, with silicon carbide core busters. They were set in firebrick tube walls by caulking with rope asbestos to which a small amount of sodium silicate had been added. This made a gastight wall, and raw gas could be preheated up to 500 C. O
PRIMARY
KEY-
FURNACE
HEAT' EXE~ANGER -------------
TEMPERATURE INCREASING TEMPERATURE DECREASING
COOLER
Figure 2. Detail of Reduction Furnace, Heat Exchanger, and Radiation Cooler The temperature of the partially cooled combustion gas mixture was still too high (600' C.) for the reactions to proceed in an orderly manner. Temperature control of the gas stream is quite critical at this point, and the radiation cooler shown in Figure 2 was introduced into the conduit connecting the primary and secondary exchangers. Pressure on the gas was increased by an orifice, and a valve on the cold leg of the cooler regulated the volume flowing through it; thus the temperature of the gas was controlled prior to entering the secondary heat exchanger which was similar in construction to the primary heat exchanger. From here the gas passed to the first catalyst chamber or carbonyl sulfide converter. Here the temperature was maintained at about 440" C., and the carbonyl sulfide present waa converted to elemental sulfur according to Reaction 2. There was substantially no heat liberated in the carbonyl sulfide converter, and the gas stream left a t about the same temperature at which it entered. It was then cooled to about 130' C . before entering the first precipitation unit. On small scale operations, this cooler was of the radiation type; with large units, a low pressure waste heat boiler followed by a feed-water heater should be employed. This latter system is indicated in Figure 1. Figure 3 shows in some detail the general construction of the radiation cooler followed by the sulfur mist precipitator. The temperature in the tubes of the latter was stabilized by passing the major part of the 130' C. gas stream into the top of the shell and causing it to flow downward around the tubes. I t then entered the electrostatic field by passing through openings in the lower tube sheet. The sulfur, which is precipitated on the inner walls of the tubes, flows down and runs into the catch basin a t the base of the Cottrell. Figure 3 also shows the method of supporting the high tension wires in the precipitator; these are insulated from the body of the precipitator by a solid sulfur seal. The weights on the ends of the discharge wires are hung in a bath of molten sulfur to prevent them from swhging. A voltage of from 60,000 to 65,000 volts was thus maintained without arcing. Although the time dura-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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tion in the electrical field was less than half a second, efficient precipitation wm obtained in a very small unit. This is explained by the extremely low viscosity of the precipitated sulfur. Because of the high purity of the sulfur and close temperature control, the surface of the collecting electrodes was continuously cleaned by gravity, thus preventing resistivity and back ionization. Any sulfur that condensed in the gas cooler flowed through the orifice into the main catch basin, as shown in Figure 3.
8 1 1 II /I CATALYST CHAMBER KEY-
II
COOLER
-------.
TEMPERATWE INCREASING TEMPERATURE DECREASING
Figure 3.
I/
COTTRELL
----
Catalyst Chamber, Radiation Cooler, and Sulfur Mist Precipitator
The cooled gas stream leaving the first Cottrell precipitator (Figure 1) is barren of elemental sulfur and should at this stage have reached equilibrium as regards the hydrogen sulfide to sulfur dioxide ratio. Here a sample of gas w a s withdrawn through an nutometer, which gave a continuous record of both hydrogen sulfide and sulfur dioxide. If the ratio of hydrogen sulfide to sulfur dioxide was not in the proportion of 2: 1, the autometer functioned through an electrical controller and relay so that the feed of natural gas n-m adjusted accordingly By means of a by-pass valve installed on the conduit connecting the first Cottrell and secondary heat exchanger, the temperature of the gas stream entering the hydrogen sulfide converter was maintained under close control at approximately 210' C. The
Figure 4.
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reaction between the hydrogen sulfide and sulfur dioxide for single-stage conversion was described earlier in this paper. Figure 1 shows the two-stage hydrogen sulfide conversion procedure, which would be necessary if a high recovery of sulfur were desired. The gas from the No. 2 converter or the first hydrogen sulfide converter is cooled in a heat exchanger, passing in heat rclationahip to the cool gas coming from the No. 2 Cottrell. Thus, the gas leaving the first hydrogen sulfide converter is cooled prior to precipitation in the No. 2 Cottrell, and the cool gas from the No. 2 Cottrell is heated prior to conversion in the second hydrogen sulfide converter. Both hydrogen sulfide converters are of the same design and gas enters each of them at the same temperature. The temperature increase in the first converter is much greater than in the second. All three of the Cottrells are of the same capacity and design, and all operate at the same temperature and voltage. In operating the final hydrogen sulfide converter a t low temperatures suitable for high conversion to sulfur, some of the sulfur will deposit on the catalyst and gradually lower its conversion efficiency. To regenerate the catalyst it is merely necessary to operate a by-pass valve that allows hot gas from the primary exchanger to pass through either catalyst mass as desired. The sulfur is boiled out, condensed in coolers, and recovered in Cottrells. Figure 4 is a view of one of the earlier pilot plants showing the carbonyl sulfide converter, first Cottrell precipitator, the two hydrogen sulfide converters in parallel, and also the final cooler and Cottrell, The operation of the natural gas reduction process was very simple. One attendant conducted all operations including analytical work, weighing the recovered sulfur, and placing it in stock bins. Because of uniform temperatures and absence of any corrosive products, maintenance costs were low. By far the most important factor in the economim of the process is the cost of natural gas. This item will vary with the grade of sulfur dioxide gas treated. With a 6% gas, about 9.570 by volume of natural gas would be required to operate the process. A t ordinary industrial rates, the cost of gas, before taking any waste heat credit, will amount to over 50% of total direct costs With moderately lorn priced natural gtls) the process is applicable for the treatment of sulfur dioxide gas between 5 and 8% by volume. When treating low grade gas, the plant, to a considerable extent, could be considered as a power-generating installation. Eight per cent gas is about the upper limit that could be utilized owing to the deficiency of oxygen necessary for high tem-
Early Stage of Process Development
November 1950
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
perature combustion. In this case, a preheat of over 600’ C.in raw gas would be necessary to maintain the required temperature in the reduction furnace. Asarco was considering the installation of a commercial 35-tonper-day sulfur unit at Garfield in 1944. By that time, however, the household requirements of Salt Lake City for natural gas during the winter months became so heavy that existing Pipe lines could not supply this demand and a t the mme time carry the industrial load. The project was therefore shelved.
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LITERATURE CITED (1) Fleming, E. P., and Fitt, T. C. (to American Smelting and Refining U. Patent 2,270,427 (Jan. 20, 1942). ( 2 ) Zbid., 2,388,259 (Nov. 6, 1946). (3) Z & f , 2,431,236 ( N ~ 18, ~ .1947). (4) Fulton, Chaa. H.,U. S. Dept. Interior, Bull. 84 (1915).
a.),s.
( 5 ) Leede & Northrup Co., Philadelphia, Pa., private communi-
cations.
cHIM., 30, 92 (1938);32, 910 (1940). (6) bpaoe, E.,lNn. (7) Young, s. W.,Trans. Am. Inst. Chem. Engrs., 8, 81 (1915). RECEIVED March 27, 1950.
LIQUID SULFUR DIOXIDE F R O M WASTE SMELTER GASES Use of Dimethylaniline as Absorbant EDWARD P. FLEMING AND T. CLEON FITT American Smelting and Refining Company, Sen Francisco, Calif.
T
The principal novel fesH E American Smelting This paper reviews the various pilot plant investigations tures of the Asarco process and Refining Company conducted by the American Smelting and Refining Comusing dimethylaniline rea has, for the past 30 years, pany during the past 30 years with the object of perfecting gent consist of: been engaged in inveatigae a process that could produce liquid sulfur dioxide commering various methods for recially from waste smelter gases. Processes developed along 1. Incorporating the recovering sulfur dioxide from these lines in Europe are briefly discussed and reasons given generation unit for t r e a t its waste smelter gases for the selection by American Smelting and Refining of ment of recaptured diwith the twofold objective anhydrous dimethylaniline as its preferred reagent. methylaniline sulfate as an integral part of the absorpof minimizing smoke comThe novel features of process system and plant construction-desorption cycle; this plaintsand perfectiigaproction are described in detail by reference to the flow diagram saves labor and steam and eas that could compete with of the 20-ton plant installed September 1947, in the comreduces the possible loss of brimstone in the sulfite pany’s lead smelter at Selby, Calif. The benefits of the imreagent, 2. Limiting the absorppaper pulp industry. Over proved system of operatian, as compared with the procetion-desorption cycle and this period, pilot plants of dure followed in European plants, are emphasized; among dimethylaniline recovery from 1 to 15 tons’ capacity these are high punty of product and facility of operation. system to two main bubhave been operated, using Basic data as indicated by 1949 operations at Selby are ble-cap tray towers where all solutions and gases come t h e Haenisch- S ch r oeder given, covering such i t e m as reagents, steam, power. coolin contact with lead only; water-absorption process ing water, labor, and supervirion. The posaibilitien of the this eliminates corrosion. (6),direct compression of process’ being applied to the recovery of sulfur dioxide from 3. Installing just above low-grade converter gas in a low-grade industrial gases by the use other reagents having the absorption section of tower, two bubble-cap trays 1-ton-per-day pilot plant (61, a high absorptive capacity for the gas are briefly noted. where the sodium carbonate the ammonium sulfite-bisulsolution is converted to sofite cycle (4), the Lurgi dium sulfite or bisulfite by “sulphidine” procesa using the residual sulfur dioxide in ths exit gases; this ermits regeneration of dimethylaniline in xylidine in water suspension (9), and finally the Asarco procesa the closed absorption-&sorption cycle, allowing the concentrated using dimethylaniline (9, a). Laboratory experimental work was sulfur dioxide to enter the compressor free of carbon dioxide. also conducted on other reagents, particularly Imperial Chemi4. Recaptur’ practically all the dimethylaniline vapor, cal’s basic aluminuna sulfate (1, 7)and Lurgi’s toluidine-in-water which has e s c a 2 from the absorption tower, i a bubble-cap, nme-tray extension of the same tower where the vapor is scrubbed suspension (9). with dilute sulfuric acid flowing countercurrent to gas flow. The final choice of process lay between Lurgi’s sulphidine in 5. Recovering and returning a considerable portion of the water suspension and absorption in anhydrous dimethylaniline. dimethylaniline to the abgorber without the necessity of heating The latter was selected because it was simpler to operate and also it in the main regeneration unit. because the process cycle of operation, patented by American A comparison of dimethylaniline and xylidine, as absorbants for Smelting and Refining Company, permitted material savings in sulfur dioxide, shows that each has some advantages over the reagent loss, steam consumption, and labor cost as compared other. Dimethylaniline is used substantially dry whereas xyliwith the Lurgi cyde aa operated in Europe by Metallgesellschaft. dine is used in a mixture of one-half xylidine and one-half water. The sulphidine system as developed by Metallgesellschaft The ability of dimethylaniline and xylidine to absorb sulfur diconsisted of separate unih for absorption of sulfur dioxide, recovering of xylidine vapor in acid scrubber solution, regeneration of oxide from flue gas is shown on Figure 1, curves A and B . The absorption isotherms show that xylidine, curve B, is a better abxylidine sulfate with soda ash and cooling to 2’ C., and desorption sorbant for sulfur dioxide in the lower percentages by volume, and of sulfur dioxide. These various operations involved considerdimethylanilino, curve A , is a better absorbant for sulfur dioxide able reagent loss, excessive labor, and high steam consumption.
