Fluidized Bed Roasting Ovens ALBERT S. HESTER, Associate Editor and ADOLF JOHANNSEN and WILL1 DANZ Badische Anilin- &. Soda-Fabrik, Ludwigshafen am Rhein, Germany
N I EUROPEand most of the world outside the United States pyrites is the most important source of sulfur. I n 1956 some 55% of the world’s production of sulfuric acid was based on pyrites and other metallic sulfides, while 40% came from elemental sulfur. As the United States is fortunate enough to have an abundance of elemental sulfur, the roasting of sulfide ores is not as important as a source of sulfur dioxide for sulfuric acid plants as it is in Europe ( 3 ) . However, there are places where it is economical to obtain sulfur dioxide this way in the U. S. At Berlin, N. H., for example, a unit built by the Dorr Go. supplies sulfur dioxide for a sulfite pulp mill ( 8 ) .
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Another plant roasts low-grade sulfur ores in Nevada (2). Pyrites consumption in Europe will probably continue to hold its own despite strong competition from the U. S. and Mexican Frasch sulfur and elemental sulfur from the French natural gas fields at Lacq. Generally, pyrites-based sulfuric acid plants cost about twice as much as elemental sulfur plants to build, and operating costs are somewhat higher as well. However, in West Germany sulfuric acid can be made cheaper from pyrites than anywhere else, because of the credit given for the cinders by the German metallurgical industry, especially the Duisburger Kupferhutte. This or-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ganization treats the cinders to extract copper, cobalt and other metals, and finally iron. The leached cinders, socalled “purple ore,” are an important raw material for blast furnaces in the steel industry. Pyrites Roasting Pyrites roastings methods have been reviewed some years ago ( 4 ) . ‘The simplest roasting plants are “lump burners.” These are merely deep bedded grates, similar to much enlarged coal grates used in home fireplaces.
T h e pyrites, iron sulfide. is oxidized by air to iron oxide and sulfur dioxide. The reaction is exothermic, and proceeds of its own accord once the pyrites has been heated above the ignit ture. Ordinarily, lump made of fire brick, faced on the outside with cast iron. The disadvantage of this system is that gas production is not steady, nor does the gas contain a uniform amount of sulfur dioxide. Also, it is unsuitable for handling very finely divided material. Fines must be burned i n an environment which ensures that gases can flow freely around the particles. Care must be taken that the burning mass does not reach too high a temperature, .or the particles will fuse together to form a solid mass. A number of mechanical fines burners have been developed. A typical mechanical roaster consists of a n upright cylinder containing a number of shelves: or hearths. Pyrites is fed onto the top shelf, and is moved around the shelf by metal arms called rabbles. After one circuit around the shelf, the pyrites drops through a hole onto the next ,shelf, where the process is repeated, and so on until the bottom is reached. Air enters a t the bottom and the gaseous combustion products leave a t the top. By the time the solid material reaches the bottom and is discharged, the sulfur has been burned away and iron oxide cinders are left. A mechanical roaster might have several shelves and may be over 20 feet in diameter. I n one model the common vertical shaft for all the rabble arms is hollow and air-cooled. The arms themselves may also be hollow and air cooled. In another model the shaft is big enough for a man to enter, and is designed so that he can change broken rabgle rabble a r m blades from the shayt shaft without shutting down the roaster. Rotary kilns are also used for pyrites roasting. These were used in Germany before the war, especially in the larger sulfuric acid plants-because they were very suitable for large capacity units. Internal projections or scoops let the burning solids fall through the gas stream. Sometimes cool cinders are used to dilute the feed in
order to keep the burning temperatures below the sintering point. A flash roasting process was developed and used in Canada about 1929. Pyrites tially a large room, empty except for gases and a relatively small amount of dust. Combustion only requires a fraction of a minute, so the dust is swept
The sulfuric acid plant.
BASF Fluidized Processes After World War 11, Badische Anilin& Soda-Fabrik, BASF. was faced with the ,task of rebuilding its pyrites roasting facilities. Mechanical ovens and kilns are primarily for granular pyrites. The flash burner could be used for very fine material, such as flotation concentrates. BASF wanted something that could be
Roasting ovens are enclosed in the section to the right
through the chamber a t a rapid rate. Recycle of cooled exit gas cools the mixture. Heat is recovered in waste heat boilers. The dust burner has been used not only for. pyrites, but also for zinc sulfide. Consolidated Mining and Smelting has a large plant for the latter a t Trail, B. C.
used efficiently for both. The flash burner requires feed of a particle size under 0.1 mm. This often means that thorough grinding is needed-an expensive operation for hard ores. T h e feed must also be dried, which adds still more to the cost. So BASF turned to a fluidized bed technique,
Here’s How Others Do It About the same time BASF developed its roasting process, somewhat similar processes were developed independently b y the Dorr Co. in the United States and Canada-in conjunction with the Cochenor Willan Gold Mines Ltd., Red Lake, Ontario-and by the New Jersey Zinc Co. Fluidized bed roasting plants have been put up all over the world under license from one or the other o f these companies, not only for pyrites but also for roasting other sulfide ores, calcining limestone, and for other processes. A number of the Dorr installations have been de-
scribed (8). Besides the single compartment roasters, Dorr has engineered installations with two or more compartments. Their first installation of this type was a plant for calcining limestone. It has five compartments superimposed one on another, and is 13l/2 feet in diameter and 45 feet high. Feed enters the upper chamber, and is preheated in the first three compartments b y s i n g up from the calcining compartment below them. At the bottom, below the calcining compartment, solids are cooled and incoming air is preheated. Heat is supplied b y burning oil in the fourth compartment.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
STAFF-INDUSTRY R E P O R T BASE’S experience with fluidized processes started in 1921 when Fritz Winkler began his work leading to “Winkler generators,” which came into wide use for making synthesis gas from coal (5). Later these generators were used for many other processes. Preliminary tests for roasting sulfide ores at BASE’S Leuna works, now in East Germany, had been made in 1943. The first large-scale unit of 30-tonper-day roasting capacity corresponding to 36 tons of acid, was started up in Ludwigshafen in 1950. This was followed by several 120-ton-per-day units in 1952, and in 1957 by the 500-ton-perday plant described in this article (7, 6), comprising two roasters each with a capacity of 200 tons of pyrites fines or 175 tons of flotation concentrates per day.
The Fluidized 6ed Technique The fluidized bed technique has become very popular in processes involving contact between solids and gases, because of the high rates of heat and mass transfer possible. In a fluidized bed, a stream of gas i s bl mass of solid particles at a high rate, The particles become suspended in the gas stream and highly agitated. The mass of particles as a whole takes on many of the properties of a fluid. The rapid movement of the particles in the fast moving gas is what giv.es the high heat and mass transfer rates. One of the best known applicatiqns of fluidized bed technique in the United States is in the giant catalytic crackers in petroleum refineries. The stream of hydrocarbon gases being cracked flows through a fluidized bed of solid catalyst particles. Part of the catalyst may be drawn off continuously in a fluidized stream to a chamber where accumulated carbon is burned off in an air stream. However, before it was ever used in these processes, the fluidized bed approach was taken for coal gasifiCbtion in “Winkler” generators, developed at Badische Anilin- & Soda-Fabrik in Germany. Later it was adapted to other noncatalytic processes. B A S has recently put on stream a 500-tonper-day sulfuric acid plant with two fluid-bed, or “turbulent layer,“ roasting units.
The Process Although some pyrites is found in Germany, the deposits are not near waterways and it is generally cheaper for BASF to import from abroad. Pyrites is shipped from Spain, Portug Greece, Yugoslavia, Scandinav Canada, and elsewhere to North Sea ports such as Rotterdam or Antwerp, transferred to barges and taken up the Rhine to BASE’S docks at Ludwigshafen. Most of the pyrites i s placed in covered storage, but during the summer extra stocks must be laid in just in case the Rhine freezes over in winter, so some of the materia1 must be kept out of doors. Particle size varies considerably among
the various shipments.
Material con-
It is of stainless steel, covered with rubber about 1/2 inch thick. I t is about 200 yards long, and is inclined of 21’. The conveyor is h inclined covered way, and top of the roasting plant.
The roasting and sulfuric acid plants are in the same building. The roasting plant is completely enclosed. The acid plant is covered by the building’s roof, but one side is open. The conveyor belt dumps the pyrites into one of the hoppers for the two roasters, separate systems, operating in parallel. The sides of the hopper slope so that the bottom is narrow. It is closed loosely by an extractor belt, which can feed pyrites into the dosing unit by gravity. The extractor belt is a conveyor belt made of overlapping steel plates, about 25 inches by 12 inches in size. Pyrites catches in the depressions where the
Dosing units are loc a t e d b e s i d e the upper part of the roasting oven
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plates overlap and is brought out of the bottom of the hopper. Each furnace has two dosing units, but only one is normally used. The other is kept as a reserve unit. The dosing unit is a steel container almost cylindrical, but slightly smaller at the top than a t the bottom. It is supported by a revolving disk, upon which it sits. I t is not attached to the disk, but is centered on it by a fitting around the cylinder bottom. A variable speed motor turns the disk. The cylinder turns
depending upon the nature of the lot of feed-a warning light comes on and a bell sounds. This operation could be made entirely automatic, but it was thought better to have an operator to watch the process. Gross changes in feed rate, such as occur when an entirely different type of pyrites is to be processed, are made by adjusting a knife blade arrangement on the rotating disk. One of the two roasters ( 3 E ) , with its heat recovery system, is shown in Figure
4 Roasting oven showing part of the heat recovery fines collector units on the right
one of these approximate reactions: Fe& f 11/4(02) = 1/2 (FetOd
+
2S02
+ 205.5 kcal.
2S02
+ 195.2 kcal.
or
If the reaction were allowed to proceed uncontrolled, the temperature would rise to the point that the mass of pyrites and cinders might sinter and destroy the fluidity of the bed. So the mass must be cooled. I n the Dorr Co.'s process temperature is kept low by mixing water with the feed to form a slurry, and by spraying additional water ovcr the burning mass. A typical feed slurry
Lower portion of the roasting oven showing the cooling devices of the turbulent layer and fines collector with super heater to the right
with it, although at a slower rate. The cylinder is aboit 4 feet in diameter and 6 feet tall; it holds about 6 tons ofpyrites. Pyrites flows between the bottom of the cylinder and the revolving disk and falls through a steeply inclined chute into the roasting furnace. The path of the pyrites out of the cylinder is surrounded by a covering shield which keeps the assemblv as dust tight as possible The rate of feed to the furnace is regulated according to variations in temperature of the furnace. Air flow is kept constant. If the temperature rises, feed rate is lowered. Temperature is indicated on a recorder and feed rate is changed manually by varying the rate of revolution of the disk beneath the cylinder of the dosing unit. The disk is driven by a variable speed drive motor. An alarm system is provided as a safeguard. When the temperature movesoutside a preset range--700' C. to 850" C..
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1 Each roaster has a daily capacity of 200 tons of pyrites, which corredponds to 250 tons of sulfuric acid. The roaster itself, made of mild steel lined with firebrick. is vertical and roughly cylindrical. I t is about 25 feet tall with a grate area of 10 square yards. The grate, which supports the burning layer, is made of cast iron. Air from a blower enters the wind-box beneath the grate, is distributed uniformly over the whole underside of the grate, and passes up through the grate and the burning pyrites. Flow rate is about 25,000 cubic meters per hourmore than sufficient to fluidize the mass. The fluidized layer is 20 to 24 inches thick. The space above the bed serves as a burning chamber for particles escaping from the fluidized layer. Each roaster is equipped with two oil burners to heat the pyrites up to the burning point. This takes several hours. Then the reaction proceeds according to
INDUSTRIAL AND ENGINEERING CHEMISTRY
might contain 20 to 25Yc water by weight. The BASF i-oaszer uses cooling coils, fed with boiling water under pressure and made of the normal boiler steel used in Germany, to control temperature. No water however is introduced into the reaction mixture itself. Heat transfer in the layer is high: 200 kcal. per square meter per hour per C For this reason hear exchange surface can be kept very low-only 35 square meters for a 200-ton roaster for pyrite. Nearly 507, of the steam produced comes from the cooling coils in the layer, the rest comes from the waste heat boiler. The high specific capacity of the oven per square meter of grate surface is due to the intensive rate of cooling. About 20 tons of pyrites per day per square meter can be processed. A centrifugal pump provides forced circulation of water through the cooling
STRY REPORT d in a cylindrical lead, ceramictower about 9 yards tall and 3
coils. The steam and water mixture leaving the coils goes to the steam drum. The drum is connected with the boilers of the heat recovery system in Figure 1. The gases from the burning layer pass out at the top of the furnace to the waste heat boiler. Like the roaster itself, this is of mild steel lined with fire brick. In the first part of the waste heat boiler, the superheater of 65 atm. is superheated to 450" C. of steam. This drives the turbine of an electrical generator. The superheater is followed by further heat recovery apparatus. The entire heat recovery system brings the temperature of the gases from 850' to 900" C. down to 300'to 350' C.
Blast box located below the oven. left
m of the tower, and flows out at the a t the roaster is about 1 2 to 13%. Concentration is measured here by electrical conductivity (5E). I t is recorded automatically. Air is added to the stream later on in the process-just before the contactor. The gas is then cooled by the indirect gas coolers. Four of these, made of lead, are arranged in parallel for each of the
Blowers are to the
,About 35% of the electricity produced is used in operating the sulfuric acid factory. The remainder is fed into factory lines as 6000-volt current. Exhaust steam from the turbine: at 4.5-atm. pressure, is fed into the factory heating steam system. ,Cinders from the fluidized bed leave the system through the exit pipe and drop to a conveyor belt. A manometer in the wind box shows a pressure which corresponds to the height of the bed above. If the pressure gets too high a sliding damper is activated and the flow of the cinders increases so that the bed level drops. A varying portion of the cinders leave the roaster in th'e hot gases. Part of them fall to the bottom of the waste heat boiler, and then drop to the conveyor belt and join the stream leaving the roaster as overflow from the bed.
Turbogenerator utilizes steam from process to supply power to the acid plant and other facilities
two systems. Water in the heat exchange tubes cools the gas from 75" to
50' C.
PALL rings, a BASF development, are used
After cooling, the gas goes to electrostatic precipitators for mist removal. These are 80,000-volt units arranged in two stages. The first stage, constructed of lead, consists of three units in parallel. The second, of poly(vinl1 chloride), has two parallel units. The gases next pass into the dryer, a steel vessel 14 feet in diameter filled with PALL rings to a depth of 16 feet. I t is of steel, lined with brick, and contains 96 to97'%sulfuric acid. ThePALLringsare a BASF development, and have been found to be superior for this purpose to other rings which might be used. Resistance to gas flow is very small. A "drop catcher" packed with coarse particles of porous ceramic removes liquid from the gas before the blower which takes them to the contactor. VOL. 50, NO. 10
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of their small space requirements, low water consumption, and the fact that they give no vapor exhalation-with its attendant problems. Acid is pumped to the storage tanks through steel pipes by submerged pumps. Separate control rooms are provided for the roasting plant and for the acid plant. A crew of nine men per shift operates the entire plant, including the pyrites transportation and handling of product and cinders. Future
A converter under construction
Some facts about
BASF
......
)The Ludwigshafen plant is one of the largest chemical plants in the world-site is 4 miles long and 1 mile wide. )Employment is 39,000 plus several thousand outside contractor employees who work within the grounds. )Sulfuric acid is only one of the basic chemicals made here for a highly integrated complex for making a whole series of nitrogen and mixed fertilizers, plastic materials, synthetic fibers, colors, pharmaceuticals, and industrial chemicals. bBASF owns coal and limestone mines and has a half interest (with Shell) in Rheinische Olefin Werke, which makes petrochemicals. FBASF is one of the “big three” successors to the I. G. Farben combine. Total sales in 1957 were $430,000,000.
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One of the first articles (7) in the StaffIndustry Collaborative Report described a contact sulfuric acid plant. While the processes are similar from an over-all point of view, they differ in detailespecially in equipment. The vanadium oxide catalyst, made by BASF, is arranged in five layers. Gas enters the first layer a t about 450”C. and leaves a t about 600’ C. Conversion is 75 to 80%. The gas then passes through the exchanger system \vhere it is cooled to 450” C. by incoming gas. At the second stage temperature rises to about 500” C. Conversion after this stage has reached 90%. Cooling of this gas is by injection of furnace gas just below the bed. Cooling after the third and fourth stages is also by injection, but with air. Conversion after the fifth stage is 98% or better. Gases then go through the contact gas coolers and the absorbers. Absorption is in tlvo stages, the first for 24% oleum, the second for 98% acid. One acid cooler ( I E ) is provided for the oleum absorber, three for the driers, five on the acid absorbers. This includes two reserve units. These units are of the so-called “Roca” type, stainless steel spiral coolers similar to two hollow sheets rolled up together. Flow is countercurrent, and the heat exchange surface is 75 square meters BASF selected these coolers because
INDUSTRIAL AND ENGINEERING CHEMISTRY
The BASF roasters have been successful with ever). form of sulfur bearing material tried. These include pyrites, flotation concentrates, and spent oxide from town gas plants (still very important in Europe, containing 50% sulfur), other materials containing elementary sulfur-e.g., brimstone containing 30YG sulfur-and elemental sulfur per se or in mixture with pyrite. For example, some of the new Mexican sulfur is not very pure. The process can also be used for metallurgical processing of copper concentrates and zinc blend. Decomposition of iron sulfate and weak acid is possible. The many advantages of the fluid bed technique seem to ensure that its use will grow-both for established processors and for new ones. Literature Cited
(1) British Sulphur Corp., Ltd., “Sulfur,” special publication, 1957. (2) Chem. Eng. 62,288-91 (August 1955). (3) Chemical Industry Economics Group, Stanford Research Institute, Chem. Eng. ik‘ews 35, 40-2 (July 29, 1957). (4) Fairlie, A. M., “Sulfuric Acid Manufacture,” ACS Monograph, Reinhold, X e w York (1936). (5) Johannsen, Adolf, Chem.-Ing.-Tech. 24, NO. 2, 104-9 (1952). (6) Johannsen, A,, Danz, Willi, Zbid., 29, NO. 9, 563-72 (1957). (7) Kastens, M. L., Hutchinson, J. C., IKD. ENG.CHEM.40,1340-9 (1948). (8) Thompson, R. B., in “Fluidization,” Donald P. Othmer, ed., chap. 9, pp. 212-25, Reinhold, New York, 1956. Processing Equipment
(1E) Carl Canzler, Duren, West Germany. Acid coolers, Rosenblad patented type. (2E) Fried. Krupp Maschinen und Stahlbau, Rheinhausen, West Germany. Symons crusher. (3E) Lurgi-Gesellschaft fur Chemie und Huttenwesen mbH, Frankfurt, West Germany. Roasters, heat recovery system, and plant construction. (4E) Sandvikens Jernzerks AB, Sandviken, Sweden. Stainless steel conveyor belt, rubber cevered. (5E) Siemens and Halske AG Wernerwerk fur Messtechnik, Karlsruhe, West Germany. Electrical conductivity recorder for sulfur dioxide concentration.
