US2877100A - Sulphur recovery - Google Patents

Description

March l0, 1959 H. J. HARTLEY ETAL SUL'PHUR RECOVERY 5 Sheets-Sheet 1 Filed May 1, 1955 N .y new 0L TT A NR R wi s. WH E M N mw mwm J. m v. E R M Nl! 51 M Alnwwwhrw W m\/, mw .UT/v SON l 9., w., mw Q ATTORNEY MalCh 10, 1959 J. HARTLEY ETAL 2,877,100

SULPHUR RECOVERY 3 Sheets-Sheet 2 Filed May l. 1955 nsi INVENTORS HENRY J. HARTLEY BY CLARENCE s. RANK/N ATTORNEY H. J. HARTLEY ETAL 2,877,100

March 10, 1959 SULPHUR RECOVERY 5 Sheets-Sheet 3 Filed May 1. 1953 of 1/ n HENRY J. HARTLEY BY CLARENCE S. RAN/(IN ATTORNEY -Some of the sulphur burns vpartmented unit heated A 20day cycle is required for the treatment together with H28.

L-in greater detail below,

United States Patent() 2,877,100 SULPHUR RECOVERY Henry J. Hartley, Los Angeles, and Clarence S. Rankin, San Francisco, Calif., assignors to Pacific Foundry Company, Ltd., San Francisco, Calif., a corporation of California Application May 1, 1953, Serial No. 352,534 7 Claims. (Cl. 23-294) This invention is concerned with the recovery of elemental sulphur and provides improved methods and equipment to this end. More particularly, the invention is concerned with the recovery of elemental sulphur from solid material, such as relatively low grade sulphur-bearing rock mined from volcanic deposits.

This application is a continuation-in-part of our copnding application Serial No. 292,442, tiled June 9, 1952, and now abandoned.

Sulphur and sulphur compounds are required by so many industries that sulphur consumption is an accurate index of industrial activity throughout the world. For many years, about 85% of the worlds sulphur has been produced by the Frasch process from wells penetrating the deep-seated sulphur domes in the Gulf regions of Texas and Louisiana. The remainder has been obtained largely in the form of sulphuric acid produced from pyrite, smelter and petroleum relinery waste gases and other waste products.

Many native sulphur deposits are known throughout the world. The Sicilian deposits, which are typical of others 'in Spain, Japan, the Western United States, Mexico and Central and South America, consist of volcanic tulf impregnated with elemental (native) sulphur in varying proportions ranging from a few percent (say to as high as 80%. In the old Sicilian practice, the ore is mined and piled so as to leave passages for air. The piles are then covered to prevent free access of air and set aiire. and the heat thus furnished melts another part which runs down to the bottom of the pile, from which point it is tapped off from time to time. The degree of recovery of this process is low and the crude brimstone produced falls far short of the purity of 99.5% which has become the modern requirement.

More recent practices in the recovery of native sulphur from the so-called surface deposits employ a steam autoclave or a Gill furnace. In both of these the sulphur, or part of it, is melted and drained out of the gangue as in the older practice described above. But these practices have not been adopted widely. For economical operation the ore must be of reasonably high grade--above v50% sulphur. Recoveries are lowseldom above 80%, and more often as low as 30%. Operating costs are high.

The Gill practice is a batch treatment through a comby burning part of the sulphur. of a single charge of 30 tons in a four compartment Gill furnace. Moreover, the sulphur dioxide evolved represents both a loss of sulphur and a nuisance, unless it be employed as a vfeed gas for an adjacent sulphuric acid plant or equivalent,

which is seldom feasible.

The steam autoclave practice may also produce SO2 It is also a batch operation. Sulphur is a very poor conductor of heat and, as will be discussed there is an inversion of freely owing molten sulphur to a viscous state at elevated temperatures. These properties of sulphur make for didiculties in the steam autoclave because the sulphur does not drain from the gangue unless the whole charge is heated uniformly and to the proper temperature range.

Continuous processes proposed for the extraction of sulphur from low-grade deposits generally involve either flotation or solvent extraction. Neither has been notably successful. Attempts to apply flotation techniques borrowed from the metallurgical art to sulphur recovery result in a concentrate containing only about sulphurand requiring further treatment. Good solvents for sulphur are, in general, organic compounds which cost several times as much per pound as the recovered sulphur is worth, so that even small solvent losses are serious from a cost standpoint.

There are, of course, successful existing processes for making SO2 rather than elemental sulphur from the material mined from low-grade sulphur deposits, and this SO2 may be liquefied and shipped in pressure vessels or converted to sulphuric acid. But since the mines are usually in isolated regions far from the chemical market these practices seldom furnish a solution to the problem. Neither liquefied SO2 nor sulphuric acid have a high sulphur content and for both, because of the shipping hazards, etc. involved, the freight rates are several times that for the original mined material, which is too low grade to stand shipping charges in the first place.

All of the foregoing demonstrate that there is a distinct need for an economical continuous method capable of a high recovery of elemental sulphur of high purity from low grade native deposits and the like. Our invention provides this, together with novel apparatus in which the process may be practiced.

In accordance with our invention, sulphur-bearing rock or other solid material impregnated with elemental sulphur is crushed to size, say minus 11/2 or smaller, and passed continuously through a sealed furnace. The zone into which the ore enters the furnace is maintained at a temperature above the boiling point of sulphur, so that the sulphur passes from the solid to the vapor state so rapidly that only a small proportion of the sulphurv in the furnace exists in the viscous state. Under these conditions, the sulphur is, in eifect, sublimed and the bed of material being moved continuously through the furnace does not become sticky and retains its granular character without adhering to the furnace surfaces or to the members, such as rabbles, which bring about the movement through the furnace.

The charge is heated in the zone directly by a current of substantially non-reactive hot gas containing a substantial proportion of heteropolar molecules. The gas is passed continuously through the furnaceand fthe sulphur vapor coming off the charge mingles with this gas and is withdrawn. The withdrawn gaseous mixture passes to a condensing system, preferably one in which the sulphur is cooled by direct contact with a rain of molten sulphur.

Sulphur, under ordinary conditions, melts at C. to form a thin straw-colored liquid. But when heated above this temperature this liquid becomes darker and at the same time more viscous, and at 300 F.500 F. it is so viscous that a vessel in which it is containedmay be turned upside down without danger of its running out. Maximum viscosity is attained at 368 F., and a mixture of sulphur and gangue (say tuti or gypsum) in the temperature range of 300 F.-500 F. is a hard sticky adhesive mass which clings tenaciously to containers and has a high shear strength and a' poor heat conductivity. At 832.2 F. sulphur boils and becomes a brownish yellow vapor. It is in the temperature range of 300 13.4532" F. that sulphur causes trouble in furnacing. However, as indicated above, we have discovered that by suddenly jber so that no free oxygen enters the furnace.

Imay be `proportions of air ahead Vits melting and boiling points.

' tent from 20% vailing in the is more diicult to recover heating a comminuted mass of solid sulphur-bearing material through this range, preferably while rabbling or stirring it, the proportion of viscous sulphur, as compared with the amount of ore present is so small that it does not have a serious cementing action. Thus, if the lzone into which the charge is introduced is maintained at 850 F. or higher, and if the charge is stirred so as v.to expose surfaces on its particles continuously to this high temperature (thus preventing the formation of localized cool spots where excessive viscosity can develop and compensating at least in part for the poor heat conductivity of the sulphur) the charge can be raked almost as though it were rock with no sulphur present. There is no sticking or balling up and no accretions form on `interior furnace surfaces.

The maintenance of an oxygen-free atmosphere within the furnace is essential to prevent the oxidation of any sulphur. This involves more than merely insuring an excess of fuel over stoichiometric requirements of the oxygen in the air employed. By way of example, it is usually undesirable to burn the mixture of air and lfuel in the presence of the sulphur vapor in the furnace, even when there is more than enough fuel to combine with the oxygen in the air. Apparently the sulphur has a greater affinity for oxygen than does the fuel, and will combine preferentially with the oxygen at least to a small extent. Consequently, in our preferred practice the fuel and air are burned in a separate combustion chamber `adjacent the furnace and 'I' opening rdirectly into 1t, care being taken to insure that the burning is completed in the combustion cham- Any carbonaceous fuel, including fuel oil and pulverized coal, employed, but We prefer to use natural or gas, and to pre-mix it with stoichiometric of the burner, so as to prothe furnace a neutral oxygenproducer duce for introduction into 'free combustion product containing no excess fuel.

lThe minimum temperature to be maintained within lthe furnace is, as indicated above, a function of the "low heat conductivity of sulphur and its property of becoming viscous and sticky at temperatures between Maximum furnace temperatures are dictated by (l) the fact that at temperatures in excess of about l750 F., CO2 formed by the combustion of the carbonaceous fuel tends to dissociate into CO and free oxygen, making the latter Aavailable for oxidation of the sulphur vapor to SO2 and (2) the fact that the sulphur vapor if heated too highly or chilled too quickly tends to form aerosols -that are difficult to stop in a condensing system. Consequently, the temperature insiderthe furnace should not be permitted to exceed about 1475".

In testing a variety of ores ranging in sulphur conto 70%, it has been found that operations are satisfactory if the space above the top hearth is maintained at about 950 F., with the space above the lowest hearth at about 1475 F.

The cooling of the gases from the furnace to the condenser should be gradual and by indirect heat exchange, in order to avoid the formation of sulphur laerosols which are difficult to capture and which tend to pass through condensing equipment. The favored molecule for recovery is S3, a stable form at tower temperature. At high temperatures, such as those prefurnace, this molecule dissociates to S3, S4, S2, and perhaps S1. Each of these latter forms thanis S8 and higher forms,

andif the gases are cooled too quickly, they may con- 'tinue to exist at the lower temperature and bring -about a loss in the condenser system. By way of example, it has been found that too rapid cooling of the vapors maybring about the formation of S2 or S4 in the form of an aerosol or vsmoke which is virtually irrecoveryWith these concepts in mind, it will be plain that there is a disadvantage in heating the sulphur any higher than is necessary to prevent sticking in the furnace, for this not only Wastes fuel but may also cause loss in the condensing system.

Sulphur, nitrogen, oxygen and, in fact, most elemental gases have symmetrical molecules and the several bonds between the atoms composing such molecules are the same. Perhaps in consequence, such gases do not radiate heat (energy) to a great extent at high temperatures, say in the infra red range and up. .0n the other hand, heteropolar molecules vwhich lack symmetry and bond uniformity radiate heat readily and efliciently in the infra red range and above. Many normally gaseous compounds including CO2, CO, SO2, H28, HC1, NH3 and paraffin hydrocarbons such as CH, are heteropolar, and lose heat rapidly and readily in the infra red range and up.

The process of our invention makes use of the energy radiation of heteropolar gases in two ways, i. e., in the transfer of heat to the sulphur in the furnace to bring about evaporation and in the cooling of the sulphur vapor ahead of the condenser.

If one ,attempts to vaporize sulphur by passing a current of hot nitrogen over it, the eciency of heat transfer is very low and the percentage of sulphur in the resulting gaseous Amixture of nitrogen and sulphur `is low even when the temperature .is much above the boiling point of sulphur. This is because the symmetrical molecules of nitrogen do not radiate energy eectively and the symmetrical molecules of sulphur do not absorb radiant energy readily. But, if the nitrogen be diluted with a substantial proportion of a heteropolar gas, say CO2 or water vapor, kand the sulphur is in contact with a gangue `.which absorbs radiant energy more efficiently, the heteropolar molecules gain energy from the hot nitrogen molecules by impact and in turn radiate this energy to the gangue which -transfers it to the sulphur. In'addition the heteropolar molecules radiate their own heat directly tothe gangue for .transfer by conduction to the sulphur besides having the same heating effect on sulphur by direct contact therewith as the nitrogen. The result is a marked improvement in heat transfer, ,and this result is obtained inthe practice of our invention.

Conversely, if one attempts to cool a hot mixture of nitrogen and sulphur vapor, neither the nitrogen nor the sulphur molecules radiate heat effectively at high temperatures, and it will be found that tremendous volurnes of cooling equipment are required. If, however, the mixture contains a substantial proportion of heteropolar gas, such as CO2, the latter lderives Yheat from the symmetrical nitrogen and sulphur,molecules-through impact and in turn releases this heat to the surroundings by radiation.

It is important to .employ this latter phenomenon in ythe practice of our invention to cool thesulphur substantially before bringing ,it into contact witha rain of molten sulphur in the condenser. Otherwise, the contact of the superheated sulphur vapor and the molten sulphur results in the formation of a sulphur aerosol which resists recovery by scrubbing in sulphur or even by electrostatic precipitation. By way of example, if sulphur vapor at furnace temperature (say 850 F.) is brought into contact with a rain of molten ysulphur in a condenser, as little as 30% of the vapor may be recovered, the balanceV being carried out of the system as a mist.

Of course, pure sulphur vapor or a mixture of sulphur vapor and gaseous nitrogen-will cool eventually, vbut the time required is lso great that vthe cost of conduits to contain it while it is Vbeing cooled to atemperature at which it may 'be safely admitted to the condenser becomes prohibitive.

Our invention., therefore, contemplates the vaporization of sulphur by heating it Lto .atemperatureabovegts goes to the right at elevated temperatures and may lead to the loss of sulphur. The water may be derived either from moisture in the charge or from a hydrogenous fuel, say hydrogen gas or methane. In consequence, an ideal situation would be one in which the charge is bone-dry and the fuel is straight carbon (say powdered anthracite) so that no water vapor is formed in combustion. As a practical matter, however, the use of hydrogenous fuel, say natural or producer gas, does not bring about a serious sulphur loss, and total recoveries of 90% of the sulphur in the charge have been obtained even when the charge contained as much as 5% moisture and the fuel was natural gas. Recoveries as high as 95% can lbe expected when the moisture content of the charge is kept under 2%, and a charge this dry is ordinarily easy to obtain by the use of waste heat and an open drying hearth on the top of the furnace.

The presence of carbon monoxide in the furnace is also to be avoided because athigh temperatures the reaction CO+S=COS tends to go to the right, thus producing carbonyl sulphide which escapes through the condenser.

The process of the invention is best conducted on an enclosed hearth onto which the charge is fed substantially continuously and over which it is moved positively and continuously by raking. Good results have been obtained in a furnace with a single hearth, but those with multiple superposed hearths are preferred. The amount of sulphur evolved per foot 0f hearth area is unusually high, amounting to at least 60 pounds of sulphur per square foot of hearth area per hour, even with charges containing as little as 20% sulphur by weight. Within rather wide limits, the capacity of a furnace in terms of sulphur is substantially unaffected by the grade of the charge. In other words, the capacity of a furnace for the practice of the invention is rated in terms of sulphur, not in terms of ore, and the same furnace will treat while operating at capacity say 100 tons per 24 hours of 20% ore or 50 tons per 24 hours of 40% ore, the sulphur output of 20 tons being the same in each case.

Optimum particle size of the charge is to some degree dependent upon sulphur content, gangue porosity and treatment temperature. Generally speaking, the size .should be such as to allow rapid heat penetration and proper movement of rabbles, thus accelerating vaporization of the sulphur and minimizing its residence time in the molten and hot viscous states, both of which are troublesome in tending to cause the formation of large accretions. Good results have been obtained with ores containing sulphur in the range of to 50% crushed to maximum sizes ranging from 11/2" down to 68. Charges crushed to less than 3716 are generally objectionable because of excessive dusting in the furnace. In multi-hearth furnaces, optimum maximum particle size increases with furnace size. In other words, in a large multi-l hearth furnace, coarse particle size tends to hold down sulphur elimination on the upper hearth or hearths, keeping sulphur in the charge for elimination on lower hearths and thus utilizing the entire hearth area more efficiently.

The space over each hearth should be relatively high in order that there be suicient room in which the sul phur,l which evolves copiously, may mix with the furnace vapor by the air,

gases. As a general rule, the height of space above the upper hearth (where more sulphur is evolved) should be at least two-thirds the hearth radius.

Rake speeds should be relatively high, say 10 to 20 feet per minute at the outside end of the arms, and bed thicknesses relatively small-say 1A" to 1" on the average.

These and other aspects of our invention will be understood thoroughly in the light of the following description, taken in conjunction with the accompanying drawings in which:

Fig. l is an elevation, partly in section, of a simple form of the apparatus of our invention;

Fig. 2 is an elevation, partly in section, of a small scale installation constructed in accordance with our invention and successfully operated with a variety of ores;

Fig. 3 is an enlarged sectional view through a burner of Fig. 2; and

Fig. 4 is an elevation, partly in section, of a furnace adapted for full scale commercial operation of the in-l vention.

To consider an example of the practice of the invention illustrated by Fig. l, the ore 10 treated is a tuff impregnated with about 35% by weight of elemental sulphur and containing about 3% moisture. It is first crushed by conventional means (not shown) until substantially all of it is ne enough to pass a 1%" screen.v It is then carried on a belt conveyor 11 to a surge bin 12 having a hopper bottom terminating in a vertical pipe 13. Flow through the pipe is governed by a conventional star feeder 14 which is substantially gas tight.

The pipe passes through the roof 15 of a cylindrical' multiple hearth furnace 16 having two hearths 17, 18' which respectively underlie furnace chambers 19, 20. The furnace is gas tight and is provided with a conventionaly vertical shaft 21 passing centrally through the hearthsY and carrying four radial arms 22 at each hearth level. To assure good agitation of the charge on the hearth and thus continuously expo-se charge surfaces to heating, at least one arm on each hearth has blades with reversed pitch so that they backrab'blef i. e. tend to move the' charge opposite to the general direction across the hearth.A The arms are air cooled by conventional means (not shown) and the shaft is driven from the bottom through a'conventional ring gear 23 and pinion 24. i

Each furnace chamber has at least one burner 25' which fires horizontally into the chamber through the wall above the arms. Air and a stoichiometric equivalent or an excess of natural gas are supplied to the burner and burned in the chamber to maintain a neutral or reducing, i. e. substantially oxygen-free, atmosphere and a temperature well above the boiling point of sulphur, say 1000 F. The furnace isI thus heated before it receives any solid charge, and the ring is kept up at all times during treatment in order to maintain such a tem-` perature and prevent the sticking that accompanies the presence in the charge on the hearths of substantial' amounts of molten or of the hotter semi-solid sulphur.

The crushed charge drops continuously onto the first hearth near the outside and immediately 'begins to move inward in a spiral path toward a central drop hole 26 impelled and rabbled by a plurality of rakes 27 carried on the several arms. The charge is heated rapidly and its temperature is raised quickly well above the boiling f point of the sulphur so that the bulk of the latter sublimes and almost immediately becomes superheated sulphur vapor which mixes with the products of combustion from the burner and passes out of the furnace through a flue.

It will be observed that in this instance, the furnace is direct-fired, with the fuel-air mixture burning in the space overlying the hearth. For the reasons already' given, this practice results in some oxidation of sulphur which combines with the sulphur before it has a chance to oxidize the carbonaceous fuel. The"i proportion ef sulphur thus oxidized is relatively smell, .Say not te exceed of the total- Even so, the rtree tice illustrated by Fig. 2 is to be preferred.

Under ordinary circumstances, the bulk of the sulphur is removed on the upper hearth of the apparatus of Fig. 1. The remainder is evolved .on the second hearth which is maintained by its burner A at a temperature at least as high as that prevailing on the upper hearth.

The charge is moved outward on the second hearth in an expanding spiral, this being accomplished by the rakes 27A of appropriate pitch mounted on the arms. The hot residue from which the sulphur has been completely removed, and which is substantially purnice or tuff, is dis- `charged continuously through a lower drop hole 28 into a sealed bin 29, from which it is removed from time to time in batches.

The gas stream leaving the upper furnace chamber through the ue 30 contains sulphur vapor, the products of combustion of the air and the natural gas, some dust and small amounts of CO, HES, SO2 and COS. It passes immediately to a cyclone collector 31 made of stainless steel or other metal having high resistance to corrosion from sulphur. The cyclone collector serves a dual purpose in that it acts as a radiator and also separates out the bulk of any dust 32 passing over from the furnace.

Under some conditions, dusting of gangue material may be reduced by maintaining a slight positive gas pressure on the upper hearth, although this practice is unnecessary in most cases and only lnds application with charges containing a large proportion of fines.

The gas is cooled (by indirect heat exchange through a metal wall) in the ue from the furnace, in the collector and in a second flue 34 passing from the collector to a condensing tower 33. Due to the presence of heretopolar compounds such as CO2, CO, HZS, SO2, COS and water vapor in the gas, the heat lost by radiation at this stage is appreciable and the gas enters the condenser at a temperature o f about 650 F. The sulphur is thus below its boiling point, but does not drop out of the gas stream.

`The tower comprises a stainless steel shell 35 disposed vertically and having an inverted conical bottom 36. The top of the tower is closed and an exit iiue 37 projects from the tower side just below the top. This flue is connected to a centrifugal exhaust fan 38 which supplies the draft for the system, although, if desired, a fan for this same purpose may be disposed in the flue immediately before or after the dust collector. In practice it has been found that diiculties are avoided if the fan is placed ahead of the condenser at a point where the temperature is above that a which any sulphur can condense and form accretions in the fan.

The mid portion of the tower from a level just above the iiue entrance to a level just below the ilue exit is packed with suitable packing 39, say small Raschig rings. These rest on a perforated plate 40 which extends across the tower and provide a number of tortuous passages to facilitate contact of the rising gas and vapor with a descending shower of molten sulphur.

The molten sulphur is sprayed onto the top of the packing by a conventional distributor head or ldeiiector plate 41 which receives a jet of molten sulphur from a nozzle 42. This pipe is supplied with molten sulphur maintained in a tluid condition, i. e. in a temperature range of 250 F. to 295 F.

In the tower the descending rain of molten sulphur absorbs some sulphur fume particles formed ahead of the condenser and condenses the sulphur vapor. In so doing, the descending sulphur is heated, and care must be taken to assure that the volume of descending sulphur is suff.

cient to prevent it from rising in temperature substantially above 300 F., i. e. that temperature at which it begins to thicken, for if this occurs the tower will become plugged. On the other hand, the temperature of the circulating sulphur in the tower must not be allowed to drop below Z489 E, or. .it will treeaeand plus ,thetowep "111e molten sulphur which enters the bottom ofthe tower is drained olf into a sump 43 through a valved outlet pipe 44. The sump serves as a surge tank and accommodates changes in lvolume due to the continual condensation of sulphur in the tower. It also serves as a cleaner in that any tine fluffy ash d5 which passes through the cyclone collector, floats on the tank and may be skimmed olf. Heavy impurities 46, if any, tend to settle in the sump and if in substantial amount may be .cleaned out from time to time, for example at times when the equipment is shut down for repairs.

Molten sulphur to produce the rain is pumped out of the tank and returned Aby a pump 47 to the distributor at the top of the tower through a pipe 43 that also serves as a radiator. The excess sulphur collected in the tank, which represents the production or make of the system, is withdrawn either periodically or continually through a valved line 5l) and handled in the conventional manner. Usually this involves only running the sulphur into bins or molds and allowing it to freeze. To prevent plugging, the sump temperature, like that of the tower, should be held lbetween 250 F. and 295 F.

Fig. 2 represents a pilot plant which has been employed successfully in the practice of the invention with a great variety of native sulphur ores. Its furnace 51 embodies a number of improvements over that illustrated in Fig. l, and its condensing system is a highly efficient one which is described and claimed in co-pending application Serial No. 366,118, filed July 6, 1953, by MacAfee and Ruth now Patent 2,751,043.

The furnace 51 comprises an upright air tight steel shell with an annulus of heat insulating material 52 immediately Within it, this annulus being protected by another annulus of fire brick 53 which forms the inner furnace wall. The furnace has four relatively level refractory ceramic hearths 54, 55, 56, 57 overlain respectively by relatively high furnace chambers 58, 59, 64), 61. The furnace is covered by a refractory top 62 which is overlain by a heat insulating layer 63.

The furnace has a conventional central vertical shaft 64 passing through the hearths and the top and supporting four radial rabble arms 65 just above each hearth level. Each arm carries a series of conventional rakes 66 which are pitched to rabble the material on the hearths and at the same time move it in spiral paths across the hearths. The arms are air-cooled from the inside by conventional means (not shown), say the usual piping and a low pressure blower. The shaft is driven through a ring gear 67 and pinion 68 from a conventional motor 69 equipped with a speed reducer, which may (if desired) `be variable to take account of variations in the sulphur content of the charge.

The furnace charge is fed continuously from a hopper 70 that discharges through a vertical pipe 71 equipped with an air-tight sliding gate 72 into a conventional horizontal screw feeder 73 that is choke-fed and which discharges through the furnace wall onto the top hearth. This arrangement is substantially air-tight. The charge is rabbled in a converging spiral path across the top hearth to a central drop hole 74 adjacent the shaft, thence across the second hearth in a diverging spiral to drop holes 75 adjacent the furnace wall and so on until it drops through an outlet pipe 76 on the periphery of the lowest hearth into a sealed bin 77 from which it is removed from time to time in batches.

To keep the furnace gas-tight an annular lute or seal 78 filled with fine particles of charge is provided around the shaft where it passes through the lowest hearth, and aA similar seal 79 is provided where the shaft passes through the top of the furnace.

Each hearth is provided with a plurality of burners 30 (only one being shown per hearth). One burner is shownin detail in Fig. 3. It Qomprises a refractory cylinder 8,1

mounted horizontally the'furuace wall. @peeing dif 'rectly into the furnace there is a cylindrical combustion chamber 82 about 9" long. The rear of this chamber merges into a tapered ignition chamber 83. A nozzle 84 is sealed into the small end of the ignition chamber from outside the furnace. It comprises a anged tube 85 backed by a spherical pre-mixing chamber 86 into which metered amounts of air and natural gas are fed under pressure respectively through feed pipes 87, 88. A vertical bar 89 in the mixing chamber carries a rod 90 which extends concentrically through the burner pipe. This rod tapers toward the furnace and carries a ceramic tip 91 which projects from the pipe into the ignition chamber. This tip glows when the burner is operating and aids in igniting the mixture of air and natural gas ejected from the nozzle. Y

The pre-mixed volume of fuel is fed at very low pressure into the ignition chamber, the ratio of natural gas to air fed being calculated to burn all the oxygen in the air to CO2, with no CO or excess oxygen present. About 3 cubic feet of natural gas is required per pound of sulphur evolved, and this gure is changed but little by the grade of the ore, since 1.85 cubic feet of the gas is required to supply the heat of vaporization of the pound of sulphur, the balance being consumed in heating the othergases and the gangue present and as heat lost to the atmosphere by radiation, convection, etc. of the furnace.

Combustion of the fuel air mixture is substantially completed in the relatively short combustion chamber, so that there is no free oxygen and substantially no CO to react with sulphur vapor in the furnace. The desired substantially neutral atmosphere is thus assured.

Sulphur vapor evolved on the several hearths in the furnace mingles with the combustion gases and rises countercurrent to the feed to be withdrawn from the top hearth through a ue 92 provided with an auxiliary stack or vent 93 to atmosphere. This vent is opened during starting periods when the furnace is being heated preparatory to receiving the charge, but during normal operation is closed by a valve or gate 94, draft for the furnace being then provided by a fan 95 connected to the flue and dis charging into a cyclone collector 96. This position of the fan is preferred, for at the temperature of the exit gases from the furnace there is no danger that sulphur will condense and plug the fan or interfere with its operation. Moreover, such a position assures adequate draft on the furnace, so that it may be operated at negative or at worst only slightly positive pressure at the top hearth, while the entire sulphur recovery system which follows is maintained at positive pressure. Under these conditions any leakage in the recovery system is outward and there is no danger of tire within the condenser, etc.

The cyclone collector may be of conventional type, but preferably is constructed in accordance with the disclosure of copending application Serial No. 358,008, filed May 28, 1953, by Herman Ruth now Patent 2,780,307. Such a collector effectively removes calcine dust derived from the gangue down to minus micron size with no external power required and with but little pressure loss. The dust collected drops through a vertical pipe 97 into a relatively large sealed storage hopper 98 from which it is removed in batches from time to time. With very dusty ore as much as 0.5 pound per 100 pounds of ore may be collected.

A gas outlet pipe 99 from the cyclone collector has a control valve 100 for ilow regulation purposes and is connected to a gas cooler 101 comprising an inlet manifold pipe 102 and an outlet manifold pipe 103 joined by a plurality of cooler tubes 104 each with a shut-off valve 105 at its inlet. The outlet manifold is connected by a pipe 106 to the top of a condensing tower 107, but all or part of the cleaned gas may be sent around the cooler to the condensing tower through a valved by-pass 108.

The amount of cooling surface to be employed will depend-upon the temperature and volume of the furnace gas, its sulphur content and its content of heteropolar gases. But the effective cooling surface may be varied over wide limits by adjusting the gas ow through the by-pass and by cutting tubes in and out of service by means of their respective valves.

The gases from the cooler are fed into the top of the vertical condenser tower. This consists of an insulated cylindrical steel shell 109 having five superposed compartments 110. The compartments are separated by horizontal grids 111 held in flanges. The four upper compartments are partially filled with burl saddles which act as tower packing 112. The lowest compartment collects liquid sulphur which is drained out through a submerged pipe 113 into a sump 114. From the sump liquid sulphur is drawn by a positive-displacement centrifugal pump 115 and forced through an indirect heat exchanger 116, where the liquid sulphur gives up part of its heat to superheated water, as described later. From the heat exchanger the still liquid sulphur is forced back to the tower and may be introduced into any or all of its four upper compartments through a valved manifold 117. The liquid sulphur thus introduced trickles downward through tortuous paths in the tower packing concurrently with the flow of gas and condensing and picking up sulphur from the vapor, so that at the bottom of the tower substantially no sulphur vapor is left.

The production or make of sulphur in the tower is permitted to overflow from the upper portion of the sump into wooden molds 118 where it freezes into blocks. Other arrangements can, of course, be provided for handling the iinal product. Molten sulphur is self-insulating, and can be stored in an uninsulated tank car for as long as seven days without producing more than a thin iilm of solid sulphur as a lining on the inside of the car.

The gases issuing from the base of the tower through a side-pipe 119 are free of sulphur save for a small amount of mist. The great bulk of this mist is removed by passing the gas upward through a sealed mist eliminator 120 equipped with spiral bafes (not shown) which cause the gas to whirl and throw the mist particles outward to impinge on the eliminator wall and form a thin liquid film. This lilm trickles down through the eliminator and the side pipe into the bottom compartment of the tower, where it joins the rest of the make. The mist eliminator is described in greater detail and claimed in co-pending application Serial No. 368,892, filed July 20, 1953 by Merrill W. MacAfee and Herman A. Ruth now Patent 2,751,037.

Some sulphur, thought to be principally in the form of solid fume, is uncaptured in the mist eliminator. Part of this is trapped in a fume chamber 121 packed with porous coke and substantially all of the balance is picked up by a small electrostatic precipitator 122 of the Cottrell type. One of the electrodes of this device (the grounded one) consists of an upright metal exhaust stack 123. The other electrode is a chain 124 disposed concentrically in the stack and insulated from it. High voltage rectified current is maintained between the electrodes by a conventional source 125.

Returning now to the heat exchanger-system, it should be-borne in mindthat its function is to remove from the molten sulphur leaving the tower the heat which it picked up in its passage through the tower. The amount of heat thus removed must be regulated closely so that the sulphur` returned to the tower will be hot enough to ow readily but not so hot that it becomes viscous in the tower or on its way to it. This result is accomplished readily in the heat exchanger which consists of a water jacketed pipe 126. The sulphur is pumped through the pipeand superheated water is pumped countercurrent to it through' the jacket 127 by a conventional hot water pump 128 which draws the superheated water out of the bottom of an equalizer drum 129 and returns it to a lower portion `ofthe drum after it has passed through the heat exchanger. 'The drum is provided with a conventional sight glass 130 andthe water in the drum is held at an intermediate level during operation, leaving a space 131 for steam in the top of the drum. The drum has a conventional steam pressure gauge 132 and a blowdown line 133 at the top. This line has a pressure relief valve 134 which opens at a predetermined gauge pressure to release the steam generated from the heat captured in the exchanger.

The drum is also provided with a valved make-up Water line 135 for the introduction Iof water required to replace that released as steam. It is also provided with a valved steam line 136 through which high pressure steam from another source may be introduced, for example dur ing starting-up periods, so that the heat exchanger may be heated above the melting point of sulphur and thus avoid plugging Iof the tower circulation.

In starting the apparatus of Fig. 2, the furnace is heated to operating temperatures by the burners while it is exhausted to atmosphere and before any feed is introduced. After the furnace is hot the stack valve is closed; the fan is started, and hot furnace gases are driven through the cooler, the tower and the rest of the gas conduit system until the Cooler and the ilue system ahead of the tower are hot enough to prevent substantial sulphur condensation. As soon as possible, the feed is started through the furnace in order to provide some sulphur vapor in the ue system. The combustion gases alone lose their heat 4rapidly by radiation to the surrounding atmosphere without heating the Walls of the lues and the cooler to a marked degree, and it has been found that only a small amount of sulphur vapor in the gas aids greatly in getting the flue system up to safe operating temperatures.

While the gas conduit system is being heated, the circulating system for the molten sulphur is also brought up to temperature. If desired, the sulphur circulating lines 137, 13S to and from the condenser may be steam jacketed and in any event should be lagged with adequate heat insulation, say conventional magnesite steam pipe covering. It is also convenient to provide the sulphur sump with a steam jacket (not shown). The heat exchanger is heated up with superheated water to a temperature well above the melting point of sulphur, thus reversing the direction of heat exchange temporarily. The heating of the exchanger is accomplished by the introduction of live steam into the equalizer while circulating the water through it and while maintaining the pressure somewhat above normal so as to assure adequate superheat in the water. Thereafter, the circulation of molten sulphur is started from the sump, and as soon as it is well established, the furnace operation is raised to normal by increasing the charge rate and the tiring.

A practical example of normal operation of the apparatus of Fig. 2 after the starting period is as follows:

Charge-Native sulphur ore containing to 70% S by weight in a tuff gangue. Size-All crushed to pass V2 screen. Moisture contentabout 3% by weight. Furnace shaft speedabout 3 R. P. M. Feed rate-60 pounds S per sq. ft. total hearth area per hour. Natural gas consumption cu. ft. per lb. sulphur in feed- 3. Temperature:

Top hearth, F. 1000 2nd hearth, F. 1150 3rd hearth, F. 1335 Bottom hearth, F. 1450 Average charge residence time in furnace-15 minutes Sulphur in calcine-nil Percent sulphur vapor (by volume) in furnace gas-33 Percent carbon dioxide (by volume) in furnace gas-Approximately 6% Temperature, F.:

Gas entering cooler-930 Gas leaving cooler-645 Gas entering condenser-640 Top condenser compartment-295 2nd condenser compartment-275 3rd condenser compartment-270 4th condenser compartment-264 Bottom condenser compartment- 262 Sulphur sump-262 Sulphur entering heat exchanger- 262 Sulphur leaving heat exchanger-259 Water, entering heat exchanger-237 Water, leaving heat exchanger-241 Pressure, lbs./in.2 gauge-equalizer drum-9.08 Ecency-condenser system-98 -l- Overall sulphur recovery-% Purity sulphur recovered-99.5 -l- In the foregoing example, no sulphur was lost in the calcine, and little or none as mist. The bulk of the loss was in the exit gases as H28, SO2 and a small proportion of COS and CS2. Be reducing the moisture in the charge to 1% or less, sulphur recovery, all other things being equal, may be expected to rise to The furnace of Fig. 4 is in general like that of Fig. 2, like parts being designated by like reference characters. However, is it designed for large scale operation and for more eicient utilization of hearth surface. In such a is kept somewhat coarser than in the pilot operation Just described in order to retard sulphur upper hearths and carry more of it down for evolution on the lower hearths, from which it is drawn off directly. Thus dampered stub ilues 140, 141, 142, 143 lead from the space above each hearth into a manifold llue 144 extending up the side of the furnace,

- Another feature of the apparatus of Fig. 4 is the proviimpel the feed across the hearth in to a pipe 148 through which is drops onto a shelf 149 inside the upper hearth. The angle of repose of the charge on the shelf is such that the pipe is kept full of charge, and thus sealed. A scraper 150 xed yon each arm and rotating with it periodically cuts through the pile on the shelf and drops a proper amount of charge onto the hearth.

We claim:

1. In the treatment of particles of solid material containing elemental sulphur to recover the latter, the improvement which comprises introducing the solid material containing not to exceed about 5% particles to agitation, maintaining the heating zone at a temperature above the melting point of sulphur, heating the particles passing through the zone from below the melting point of sulphur to above the melting point of sulphur so rapidly that the proportion of the sulphur existing in hot viscous state is insuicient to bring about the formation of substantial aggregates of the particles in the bed by cementing due to the presence of the viscous sulphur, the heating of the particles being accomplished by passing through the zone in contact with the particles a hot stream of substantially non-reactive gaseous combustion products of carbonaceous fuel and air high in CO2 and containing substantially no CO and no oxygen, con.- tinuously withdrawing from the zone the resulting mix ture of gas and sulphur vapor at a temperature below l475 F., partially cooling the withdrawn mixture by indirect heat exchange to a temperature substantially below 850 F. but without removing any substantial quantity of sulphur from the partially cooled mixture and thereafter bringing the partially cooled mixture linto contact asvmoo with liquid sulphur to remove the sulphur from the gaseous mixture.

2. Process according to claim 1 in which the hot stream of substantially non-reactive gaseous combustion products is produced in a chamber immediately adjacent the heating zone and opening thereinto by burning the carbonaceous fuel and the air substantially completely in the chamber in stoichiometric proportions.

3. Process according to claim 1 in which dusting of the particles is reduced by maintaining the heating zone under a slight positive pressure.

4. Process according to claim l in which at least some of the particles are larger than 1%@ inch.

5. Process according to claim l in which the bed is of the order of l inch thick.

6. In apparatus for treating solid particles containing elemental sulphur to recover the latter, the combination which comprises a sealed furnace with a plurality of superposed hearths and heating chambers overlying the respective hearths, means for introducing the solid particles onto the uppermost hearth, means for rabbling the solid particles in thin beds successively over each hearth beginning with the uppermost, means for removing the solid particles from the lowermost hearth, a plurality of combustion chambers opening respectively into the uppermost heating chamber of the furnace and into at least one of the lower heating chambers of the furnace, means for introducing carbonaceous fuel and air into each of the combustion chambers in substantially stoichiometric proportions, the combustion chambers being large enough to permit the fuel to burn substantially completely in the chambers to produce gaseous combustion products containing carbon dioxide but substantially no carbon monoxide and no free oxygen and having temperatures substantially above the boiling point of the sulphur in the particles, whereby the gaseous combustion products entering the heating chambers in the furnace heat the solid particles on the hearths rapidly to a temperature above the boiling point of sulphur and prevent the formation of substantial proportions of viscous sulphur in the beds on the hearths while forming a mixture of sulphur vapor with the gaseous combustion products in the furnace, a conduit for withdrawing said mixture from said furnace, indirect cooling means connected to said conduit for cooling the withdrawn mixture to a temperature below the condensation point of the sulphur in the mixture, means for drawing the mixture through the cooling means so rapidly that substantially none of the resulting condensed sulphurV is deposited in the cooling means, a scrubbing means connected to the cooling means, and means for circulating liquid sulphur through the scrubbing means in contact with the mixture withdrawn from the cooling means to remove the sulphur from said mixture.

7. In apparatus for treating solid particles containing elemental sulphur to recover the latter, the combination which comprises a sealed furnace with a plurality of superposed round hearths and heating chambers overlying the respective hearths, means for introducing the solid particles onto the` uppermost hearth, the height of the uppermost heating chamber being at least two-thirdsI the radius of the hearth immediately underlying it, means for rabbling the solid particles in thin beds successively over each hearth beginning with the uppermost, means for removing the solid particles from the lowermost hearth, a plurality of combustion chambers opening respectively into the uppermost heating chamber of the furnace and into at least one of the lower heating chambers of the furnace, means for introducing carbonaceous fuel and air into each of the combustion chambers in substantially stoichiometric proportions, the combustion chambers being large enough to permit the fuel to burn substantially completely in the chambers to produce gaseous combustion products containing carbon dioxide but substantially no carbon monoxide and no free oxygen and having temperatures substantially above the boiling point of the sulphur in the particles, whereby the gaseous combustion products entering the heating chambers in the furnace heat the solid particles in the furnace rapidly to a temperature above the boiling point of sulphur and prevent the formation of substantial proportions of viscous sulphur in the beds on the hearths while forming a mixture of sulphur vapor with the gaseous combustion products in the furnace, a conduit for withdrawing said mixture from said furnace, indirect cooling means connected to said conduit for cooling the withdrawn mixture to a temperature below the condensation point of the sulphur in the mixture, means for drawing the mixture through the cooling means so rapidly that substantially none of the resulting condensed sulphur is deposited in the cooling means, a scrubbing means connected to the cooling means, and means for circulating liquid sulphur through the cooling means in contact with the mixture withdrawn from the scrubbing means to remove the sulphur from said mixture.

References Cited in the le of this patent UNITED STATES PATENTS 139,664 Eames et al June 10, 1873 652,672 Fleming et al June 26, 1900 873,812 Walter Dec. 17, 1907 1,083,246 Hall Dec. 30, 1913 1,083,250 Hall Dec. 30, 1913 1,083,251 Hall Dec. 30, 1913 1,083,252 Hall Dec. 30, 1913 1,083,253 Hall Dec. 30, 1913 1,133,636 Hall Mar. 30, 1915 1,409,338 Fenton Mar. 14, 1922 1,473,723 Fogh Nov. 13, 1923 1,574,987 Marx Mar. 2, 1926 1,972,884 Gleason et al Sept. 11, 1934 2,314,112 Tuttle Mar. 16, 1943 2,549,367 Clark Apr. 17, 1951 FOREIGN PATENTS 253,830 France July 8, 1905 UNITED STATES PATENT OFFICE CERTIFICATE OF 'CORRECTION Pe tent No, 23.877100 Merch lO, 1959 Henry J, Hartley et slr or appears 'in the -prnted specification It s hereby certified that err correction and that the said Letters Of the above "numbered patent requiring Patent should read as corrected below.

Column 5, line 58, for "of rabbles" reed by rebbles y.

Signed and sealed this 30th 'day of June 1959.

(SEAL) Attest:

Commissioner of Patents .Attesting Officer

Claims (1)

1. IN THE TREATMENT OF PARTICLES OF SOLID MATERIAL CONTAINING ELEMENTAL SULPHUR TO RECOVER THE LATTER, THE IMPROVEMENT WHICH COMPRISES INTRODUCING THE SOLID MATERIAL CONTAINING NOT TO EXCEED ABOUT 5% MOISTURE INTO A HEAT ING ZONE, PASSING A THIN BED OF THE PARTICLES CONTINUOUSLY OVER A HEARTH IN THE HEATING ZONE WHILE SUBJECTING THESE PARTICLES TO AGITATION, MAINTAINING THE HEATING ZONE AT A TEMPERATURE ABOVE THE MELTING POINT OF SULFUR, HEATING THE PARTICLES PASSING THROUGH THE ZONE FROM BELOW THE MELTING POINT OF SULPHUR TO ABOVE THE MELTING POINT OF SULPHUR SO RAPIDLY THAT THE PROPORTION OF THE SULFUR EXISTING IN HOT VISCOUS STATE IN INSUFFICIENT TO BRING ABOUT THE FORMATION OF SUBSTANTIAL AGGREGATES OF THE PARTICLERS IN THE BED BY CEMENTING DUE TO THE PRESENCE OF THE VISCOUS SULPHUR, THE HEATING OF THE PARTICLES BEING ACCOMPLISHED BY PASSING THROUGH THE ZONE IN CONTACT WITH THE PARTICLES A HOT STREAM OF SUBSTANTIALLY NON-REACTIVE GASEOUS COMBUSTION PRODUCTS OF CARBONCEOUS FUEL AND AIR HIGH IN CO2 AND CONTAINING SUBSTANTIALLY NO CO AND NO OXYGEN, CONTINUSOUSLY WITHDRAWING FROM THE ZONE THE RESULTING MIXTURE OF GAS AND SULPHUR VAPOR AT A TEMPERATURE BELOW 1475* F., PARTIALLY COOLING THE WITHDRAW MIXTURE BY INDIRECT HEAT EXCHANGE TO A TEMPERATURE SUBSTANTIALLY BELOW 850* F. BUT WITHOUT REMOVING ANY SUBSTANTIAL QUANTITY OF SULPHUR FROM THE PARTIALLY COOLED MIXTURE AND THEREAFTER BRINGING THE PARTIALLY COOLED MIXTURE INTO CONTACT WITH LIQUID SULPHUR TO REMOVE THE SULPHUR FROM THE GASEOUS MIXTURE.

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